Note: Descriptions are shown in the official language in which they were submitted.
STANF-60B-Foleign
~6~
IN-LINE FIBER OPTIC MEM~Y
The ~ubject matter of this application is related to that
in U.S. Patent 4,708,421 issued Nove~ber 24, 1987.
Back~round of the Invention
The present invention relates generally to fiber optic
optical memvry devices, and particularly to in-line fiber
10 optic memories which recirculate optical signals, and
,! which utilize non-linear effects for amplification of the
recirculating signals.
Fiber optic recirculating memories typically comprise
a loop of optical fiber and a fiber optic directional
coupler for coupling light to and from the loop of optical
fiber. An exemplary fiber optic recirculating memory is
disclosed in U.S. Patent No. 4,473,27U, issued ~eptember
25, 1984, which is incorporated by reference herein. As
disclo~ed in that patent, a 8ingle signal pulse is
supplied a~ an input to the memory device. This pulse
recirculates in the loop and a por~ion of the pulse is
coupled out of the loop on each circulation to provide a
serie3 o~ output signal pulses identical to the input
~ignal pulse, although at ~maller, gradually decreasing
amplitudes. Such a device is particularly useful, for
example, to provide a short term optical memory in a
system where data is generated at a rate faster than ie
can be accepted by a data processor, or to adjust time
delay~ between sys~ems exchanging data (optical delay
line) or to repe~t data (data regeneratior.). This device
may also be u~i~i2ed as a p~Lse train generator, or as a
filter. ~oreover, recirculating memory devices may be
utilized as re-entrant fiber sensing loops for fiber optic
gyro~copes.
Although ~he recirculating memory disclo~ed in the
above-referenced patent represents a significant advance
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in the art, and is quite advantageous for many optical
memory applications, the optical memory capabilities of
such devices are limited due to the fact that the
circulating signal pulse is degraded by coupling a portion
of this pulse on each circulation around the fiber loop to
provide the train of output pulses. Further, fiber
propagation losses attenuate the optical signal pulse as
it propagates through the loop, causing additional
losses. These losses cause the intensity of the signal
pulse circulating in the loop to decay, which causes a
concomitant decay in the series of output pulses, thereby
limiting the number of useable output pulses and the
lifetime of the optical signal memory.
In order to prevent the problems associated with decay
of the circulating pulse, it has been proposed in the
prior art to insert a fiber optic amplifier in the loop so
as to amplify the pulse on each circulation to compensate
~or the loop losses. Such a system, for example, has been
disclosed in U.S. Patent No. 4,136,929, to Suzaki,
entitled "APPARATUS FOR GENERATING LIGHT PULSE TRAIN".
Fiber optic amplifiers, however, are typically discrete
components, which must be spliced into the fiber loop.
Such splicing will cause additional loop losses which must
be compensated for by the fiber optic amplifier.
~5 Accordingly, there is a need in the art ~or a low loss
in-line recirculating optical memory which utilizes an
amplification process capable of providing sufficient gain
to compensate for the total round trip loop losses.
Summary of the Invention
The present invention is an in-line fiber optic
recirculating memory having a length of optical fiber,
preferably splice-free, for receiving an optical signal
pulse. The optical fiber forms a loop which is optically
closed by means of an optical coupler, for example, a
fiber optic directional coupler. The coupler ~ouples the
optical signal input pulse to the loop for circulation
~2~ 5~
therein, and outputs a portion of the signal pulse on each
circulation to provide a series of output pulses.
The present invention further includes a light source
for generating an input pump signal comprising a series of
pulses. Each of these pulses is serially input to the
loop by the coupler~ The pulses circulate in the loop and
generate Stokes light therein for amplifica~ion of the
circulating optical signal pulse during each of plural
circulations of the signal pulse. The optical wavelengths
of the pump pulses and the optical signal pulse are
selected such that the Stokes light has the same optical
wavelength as the optical signal pulse. The coupling
ratio of the coupler is greater than zero for the pump
pulses so that each pump pulse makes plural circulations
through the 1oop7 The length of the loop provides a loop
transit time for the pump pulses such that each pump pulse
in a series of pump pulses makes plural circulations in
the loop before overlapping with any subsequent pump pulse
in the series o~ pump pulses. This allows the amplitude
of the pump pulses to decay prior to overlapping, thereby
reducing phase noise effects in the loop.
In a preferred embodiment, the pump pulses are equally
spaced and have the same time duration æo that the pump
signal is periodic. The period of the pump signal is less
than the amount of the loop transit time. Preferably, the
time duration of each pump pulse is sub~tantially less
than the loop transit time. In a preferred embodiment,
the period of the pump signal is substantially equal to an
integer multiple of the time duration of each pump
pulse. Preferably, the sum of the period of the pump
signal and the time duration of each pump signal pulse is
substantially equal to the loop transit time.
The invention further includes a method for generating
a series of output pulses from a single input pulse. A
loop, having a loop leng~h, is formed from a length of
optical fiber that is closed by a coupl~er. An optical
~;Z68~
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signal pulse having an optical signal wavelength is input
into the optical fiber to circulate in the loop to provide
a circulating optical pulse that makes plural circulations~
in the loop. The circulating optical pulse propaga~es
S around the loop during a signal pulse loop propagation
time which is determined by the optical signal wavelength
and the loop length. A portion o~ the circulating signal
pulse is tapped from the loop on each of the
recirculations to provide the series of output pulses.
Additionally, a series of input pump pulses having an
optical pump wave length are coupled into the fiber loop
to cause stimulated scattering in the loop. The
stimulated scattering generates Stokes light in the loop
for amplification of the circulating signal pulse during
each of the plural circulations by the signal pulsesO The
pump pulses propagate around the loop in a pump pulse loop
propagation time determined by the optical pump wavelength
and the loop length. Each input pump pulse has a pump
pulse time duration. The optical signal wavelength and
the optical pump wavelength are selected such that the
Stokes light has the same optical wavelength as the
optical signal wavelen~th. Each pump pulse is spaced
apart in time from the next succeeding pump pulse in a
series of pump pulses so that each pump pulse makes plural
circulations in the loop before overlapping with any
- subsequent pump pulse. This allows the amplitude of the
pump pulses to decay prior to any overlapping, thereby
reducing phase noise effects in the loop.
In a preferred embodiment, the pump pulses are spaced
apart in time at a selected time delay so that the pump
pulses are periodic. The time delay is selected so that
the pump pulse has a period less than the pump pulse loop
propagation time. Preferably, the pump pulse time
duration of each pump pulse is selected to be
substantially less than the pump pulse loop propagation
time. In a preferred embodiment, the period of the pump
~;:61~S~,
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pulses is selec~ed to be substantially equal to an integer
multiple of the pump pulse time duration. Preferably, the
pump pulse time duration and the pump pulse period are
selected so that the sum of the pump pulse period and the
S pump pulse time duration are substantially equal to the
pump pulse loo2 propagation time.
The pump signal has sufficient intensity to cause
gain, for example, Raman gain caused by stimulated Raman
scattering (SRS), in the core of the fiber. Preferably,
the optical fiber forming the loop is single mode fiber,
which typically has a core diameter on the order of 5-10
microns. The small core diameter of single mode fiber is
highly advantageous over the larger core diameter
multimode fiber, since its small size provides increased
optical power density in the fiber core for a given amount
of pump light, and thus increases the optical gain.
In the preferred embodiment, the pump source has a
coherence length which is short compared to the length of
the fiber loop to ensure that any of the pump ligh~ that
recirculates in the loop does not cause a resonant
effect. Further, the pulse width of the signal pulse is
preferably less than the loop length to ensure that the
signal pulse does not interfere with itself while
circulating in the loop.
Stimulated Raman scattering may be viewed as a three
level laser emission process in which molecules of the
active media (i.e., the fiber core) are excited from the
ground level to an excited virtual level by absorbing
input pump photons. Return of the excited molecules to an
intermediate level results in the emission of photons,
commonly referred to as "Stokes photons". These Stokes
photons have a frequency downshifted with respect to the
pump frequency by a characteristic amount, typically
referred to as the ~aman shift. In the case of SRS in a
fused silica fiber, the Stokes photons have a frequency
which is about 13-15 terahertz (THz) below the frequency
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of the pump light. In addition to the emission of Stokes
photons, the transition from the intermediate level back
to the ground level results in the emission of phonons.
~owever, such phonons are of little interest in the
present invention, as they are quickly absorbed by the
fiber and converted to heat.
In the present invention, the input signal pulse has a
wavelength which is equal to the Stokes wavelength. When
the signal pulse is injected into the fiber loop, such
pulse will stimulate relaxation of the excited molecules
and emission of photons at the Stokes wavelength. These
stimulated photons have the same phase characteristics and
frequency as the input signal pulse, and thus, provide
coherent amplification of the input signal pulse.
The optical gain, and thus, the amount of
amplification, depends upon the gain coefficient of the
fiber (g); the optical power Ppc of the input pump signal;
the effective interaction area (Af) of the optical fiber
(i.e., the area of mode or electromagnetic field overlap
between the signal pulse and the pump signal); and the
effective interaction length (Lf) of the fiber loop (i.e.~
the calculated length, less than the loop length, which
accounts for pump power attenuation along the fiber
loop). In addition, it has been found that for the above-
described recirculating memory configuration, the gain isalso dependent upon the attenuation (a) of the fiber, the
length (L) of the fiber loop, and the coupling ratio ~)
of the coupler. These recirculating memory device
parameters are coordinated in the present invention so
that the gain provided by pumping the fiber compensates
for the total round trip losses in the fiber loop. This
loss compensation, therefore, provides an output pulse
train of substantially constant amplitude pulses.
The invention may be implemented utilizing either an
ordinary "standard" coupler or a specifically adapted
"multiplexing coupler". The term "standard coupler" as
~Z~5~
used herein, refers to a coupler in which the coupling
ratio ~or the input signal pulse is close to that of the
pump signal, such that the coupling ratios differ by no
more than 0. 2. Conversely, a "multiplexing coupler" is a
coupler which exhibits a sizeable difference in coupling
ratio for the pump signal and signal pulse, such that the
difference in coupling ratios is greater than 0.2. For a
given input pump power, there is a set of coupling ratios
for the pump signal and signal pulse which supplies the
loop with the exact amount of pump power required to
produce just enough gain to offset total round trip losses
of the signal pulse such that the output pulses will have
a constant amplitude.
If a standard coupler is used and it is tuned to have
relatively high coupling ratios, the total round trip loop
losses will be advantageously low thereby decreasing the
amount of amplification or gain necessary to compensate
for them. However, very little pump power will be
launched into the loop, so that pump power utilization is
quite inefficient. If, on the o~her hand, a standard
coupler tuned to relatively low coupling ratios is used, a
sizeable fraction of the pump power will be launched into
the loop, so that the pump power i9 used efficiently to
provide gain. However, a major fraction of the
circulating pulse will be coupled out on each circulation,
so that the total round trip loop losses will be quite
high. Thus, if the coupler is set to yield high gain, the
loop losses will be high, while, if the coupler is set for
low losses, the gain will be low. Accordingly, there
exists an optimum set of coupling ratios corresponding to
a specific coupler tunin~ which minimizes the pump power
required for compensation of total round trip losses. It
has been observed however that with such mode of operation
(i.e., using a standard coupler with a relatively high
pump coupling ratio) the recirculation and interference of
pump power in the loop causes enhanced pump power
41
fluc~uations or noise which in turn cause gain
fluctuations and ~hus sizeable signal intensity noise.
These fluctua~ions are due to interference between the
recirculating pump electromagnetic fields of short
coherence time and are commonly referred to as a phase
noise effect.
As indicated above, a multiplexing coupler may be
utilized, rather than a standard coupler. Preferably, the
mutliplexing coupler is formed to have a large interaction
length. There exists then a tuning position for the
coupler for which the interaction length is substantially
equal to an even number of coupling lengths for the pump
signal and substantially an odd number of coupling lengths
for the signal pulse, so that the coupling ratio for the
pump signal is zero, or close to zero, while the coupling
ratio Xor the signal pulse is close to unity, but not
exactly unity. A coupling ratio of zero for the pump
signal makes it possible to utilize Che entire amount of
input pump power and thereby provides for maximum possible
gain, while a coupling ratio close to unity (but not
exactly unity) for the signal pulse provides for minimum
round trip loop loss. Such combination of coupling ratios
permits the invention to operate at very low input pump
signal power, while still providing the proper amount of
gain. In addition, the use of a zero or null pump
coupling ratio suppresses the effect of pump power
recirculacion, which, due to the phase noise, would cause
gain fluctuations in the loop.
The invention may be implemented in either a forward
configuration in which the pump signal and signal pulse
are both input into the same end of the fiber loop such
! that the signal pulse propagates in the same direction
around the loop as the pump signal, or it may be
implemented in a backward configuration in which the pump
signal and signal pulse are input into opposite ends of
the loop, so that the signal pulse propagates in a
~26~
direction opposite that of the pump signal. In the
preferred embodiment, a backward configuration is
utilized, since it has been found that this advantageously
averages out the effect of pump fluctuation frequencies
(i.e., those larger than the reciprocal of the loop
transit time) on the signal amplitude fluctuations.
The present invention suppresses the effects of phase
noise, and thus, minimizes the signal intensity noise
while using a coupler having a pump coupling ratio
substantially greater than zero, such that the pump power
is recirculated in the loop.
In the ~referred embodiment of the invention, the pump
signal is input to the recirculating loop as a series or
train of pump pulses having a selectable pulse width and a
selectable delay between the beginning of each successive
input pump pulse. The pulse width and delay are selec~ed
so that each input pump pulse makes plural circulations in
the fiber loop before overlapping with any subsequent pump
pulse in the series oE pump pulses. This allows the
amplitude of the recirculating pump pulses to decay before
overlapping occurs and thus reduces the effect of their
mutual interference. The pump power recirculating in the
loop is ~hereby stabilized causing a sizeable reduction of
gain noise, which in turn effects a drastic reduction of
the amplified signal noise. Therefore, the invention
improves the output signal stability when using a non zero
pump coupling ratio, such as exists in the above discussed
standard coupler.
Description of the Drawings
These and o~her aspects of the present invention may
be more fully understood ~hrough reference to the drawings
in which:
Figure 1 is a schematic drawing of an embodiment of
the optical memory of the present invention showing a
signal pulse generator for introducing a signal pulse into
the fiber loop, and a pump source for introducing a pump
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signal into the fiber loop to cause Raman gain to amplify
the signal pulse;
Figure 2 is a schematic drawing of an embodiment of
the signal pulse generator of Figure l;
Figure 3 is a sectional view of one embodiment of a
fiber optic directional coupler for use in the optical
memory of Figure 1;
Figure 4 is a graph showing relative coupled power
versus signal wavelength for a fiber optic coupler having
a minimum fiber spacing of four microns, an offset of the
oval facing surfaces of zero microns, and a fiber radius
of curvature of 25 centimeters;
Figure 5 is a graph of relative coupled power versus
signal wavelength similar to that of Figure 4, but for a
fiber radius of 200 centimeters;
Figure 6 is a graph of relative coupled power versus
signal wavelength for a fiber optic coupler having a
minimum fiber spacing of four microns, a fiber radius
curvature of 200 centimeters, and a selectable fiber
offset;
Figure 7 is a schematic drawing showing the loop and
fiber optic coupler of Figure 1 which illustrates the
operation of the invention;
Figure ~ is a graph of the input pump power required
to cause sufficient Kaman gain to make the train of output
pulses substantially constant in intensity as a function
of the coupling ratio of a standard coupler;
Figures 9a and 9b are grayhs of the signal coupling
ratio as a function of the pump coupling ratio for a
multiplexing coupler, with the input pump power required
for constant amplitude output pulses as a parameter, for
two different values of fiber attenuation.
Figure 10 is a schematic drawing of an alternative
embodiment which provides the pump signal as a series of
pulses to the recirculating loop to reduce phase noise.
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Figures lla, llb, 11c and lld are graphs of a pump
input pulse train, a train of once-delayed pump pulses, a
train of twice-delayed pump pulses, and a train of thrice-
delayed pump pulses, respectfully, showing the timing
S relationships between the pulse trains in the embodiment
of Figure lO.
Description of the Preferred Embo_iments
As shown in Figure 1, the optical memory of one
embodiment of the present invention comprises a
continuous, uninterrupted strand lO of optical fiber,
preferably single-mode optical fiber, having an input end
portion 12, a loop portion 14, and an ou~put end portion
16. At the ends of the loop portion 14, the single~mode
fiber 10 is optically coupled together by means of a fiber
optic, evanescent field, four port, directional coupler
20, having ports 1 and 2 on one side thereof, and ports 3
and 4 on the other side thereof. Tracing the fiber 12
from one end to the other, the fiber 10 first passes
through ports 1 and 3, and then through port~ 2 and 4, so
that the loop 14 extends from ports 3 and 2, while the
input portion 12 extends from port 1, and the output
portion 16 extends from port ~.
A signal pulse generator 22 is provided to selectively
introduce an input pulse 21 into the input fiber portion
12. The coupler 20 couples a portion 24 o~ this pulse 21
to the loop 14 for recirculation therein. Each time the
pulse 24 circulates in the loop, a portion is output
through the output fiber portion 16 to provide a series or
train of output pulses 26. These output pulses 26 are
directed through a lens 27 to a beam splitter 29 which
splits the output pulses 26 and directs a portion through
a monochromator 28 to suppress Fresnel reflections and
scattered pump light, so as to filter the output signal
pulses 26. Af~er filtering by the monochromator 28, the
output signal pulses 26 may be directed to a detector 30
which converts the optical signals to electrical signals
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for display on an oscilloscope 32. Those skilled in the
art will recognize that a spike filter at the wavelength
of the optical signal pulses 26 (or a dispensive element
such as a prism or grating) may be substituted for the
monochromator 28.
The embodiment of Figure 1 also includes a pump source
34 for producing a pump light signal 31. In the
embodiment shown in Figure 1, the signal 31 is a pulse
having a width which is at least as long as the desired
length of the output pulse train ~6. In other words, the
pump pulse 31 should have a duration which is at least as
long as the desired number of output pulses times the loop
transit time. In one particular embodiment of Figure 1,
the duration of the pulse 31 is selected to be at least 50
times the loop transit time so as to yield at least S0
substantially identical output pulses 26. The pump source
34 of the embodiment of Figure 1 comprises a continuous
wave Nd:YA~ laser having a CW power of 2 watts and a
wavelength of 1.064 microns. A chopper 35 is included at
the output of the pump source 34 to chop the pump signal
into relatively long pulses (e.g., 0.5 msec). As is well
known in the art, a chopper typically comprises a rotating
disk having apertures to alternately block and pass the
output light of a laser at a specific frequency. Those
skilled in the art will recognize that an acousto-optic or
electro-optic modulator ma~ be used instead of the chopper
35.
The pump signal 31 is directed from the pump source 34
to the beam splitter 29, where the light is split such
that one portion (not shown) is directed to the
monochromator 28, and the other portion 33 is directed to
the lens 27 for introduction into ~he output end portion
16 of the fiber 10 for propagation through the loop 14.
The signal portion 33 in the end portion 16 will be
referred to as the "input pump signal~'l The filter
characteristics of the monochromator 28 are selected so
~;~6~5~
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that the por~ion of the pump signal directed towards the
monochromator 28 will be blocked to prevent it from
reaching the detector 3~.
The pump source 34 and signal pulse generator 22 are
connected by respective lines 36, 38 to a synchronizer
40. The synchronizer 40 synchronizes the chopped pump
signal pulses to the signal pulses from the pulse
generator 22, such that the pump light enters the loop 14
just prior to the signal pulses so that the pump light is
present in the loop when the signal pulses arrive.
Preferably, the coherence length of the pump source 34
is relatively short compared to the length of the fi'ber
loop 14 to ensure that any of the pump light that
recirculates in the loop 14 does not interfere with
itself. Further, the width of the signal pulse 21
produced by the generator 22 should preferably be less
than the length of the ~iber loop to ensure that the
circulating pulse 24 does not interfere with itself as it
circuLates through the loop 14.
The wavelength and amplitude of the input pump signal
33 are selected to cause stimulated scattering in the
fiber loop 14 at the pulse signal wavelength. As
discussed in more detail hereinafter, Ruch stimulated
scattering produces photons at a wavelength referred
herein as the "Stokes wavelength". In the case of
stimulated Kaman scattering in silica glass ibers, the
Stokes wavelength i8 1.12 microns for a pump wavelength of
1.064 microns. The photons produced by this stimulated
scattering amplify the circulating pulse 24 as it
propagates around the fiber loop 14~ However, for such
amplification to occur, it is important that the signal
pulse ~1 (and the circulating pulse 24) be at a wavelength
equal to the Stokes wavelength.
To insure that the signal wavelength i5 the same as
the Stokes wavelength, the signal pulse generator 22
comprises a laser 42 which produces light at a waveleng~h
~61~5g~L
-14-
identical to that of the pump source 34 (e.g., 1.064
microns), and a signal generating loop 44, formed of the
same type of fiber as the loop 14, as shown in Figure 2.
By way of example, the laser 42 may be a ~-switched Nd:YAG
laser having an output wavelength of 1.064 microns, a peak
! power of 120 watts and a switching rate of 50 ~z, so as
to generate relatively thin (500 nsec) high power pump
pulses which are widely spaced (e.g., a spacing greater
than the pump signal duration). ~y way of example, the
loop 44 may have a length of about l km. The light from
the source 42 is input to one end of the loop 44 through a
lens 45 to generate stimulated scattering in the loop 44
such that a wave having the Stokes wavelength of e.g. 1.12
microns is output, together with the pump light, at the
other end of the fiber. The Stokes wave and ~he pump
light are then directed through a lens 46 to a diffraction
grating 48 which separates the two wavelengths into two
diverging rays. A diaphragm 49 with a slit therein is
positioned to block the 1,064 micron pump light, but pass
the 1.12 micron Stokes light to provide the signal pulse
21 (Figure l) which is input to the input end portion 12
of the ~iber lO (Figure 1) through a lens 41. Thus, the
signal pulse generator 22 provides signals at exactly the
same optical wavelength as the Stokes light generated in
the fiber loop 14 (Figure 1).
Those skilled in the art will recognize that a simpler
set up may be achieved by replacing the grating 48 with a
spike filter at the Stokes wavelength.
A preferred fiber optic directional coupler for use as
the coupler 20 (Figure l) in the optical memory of the
present invention is shown in Figure 3. As illustrated
therein, this coupler includes two exemplary strands 50A
and 50B of a single mode fiber optic material mounted in
longitudinal arcuate grooves 52A and 52B, respectively,
formed in optically flat, confronting surfaces of
rectangular bases or blocks 53A and 53B, respectively.
~6~
The block 53A with the strand 50A mounted in the groove
52A will be referred to as the coupler half 51A, and the
block 53~ with the strand 50B mounted in the groove 52B
will be referred to as the coupler half 51B.
S The arcuate grooves 52A and 52B have a radius of
curvature which is large compared to the diameter of the
fibers 50, and have a width slightly larger than the fiber
diameter to permit the fibers 50, when mounted therein, to
conform to a path defined by the bottom walls of the
grooves 52. The depth of the grooves 52A and 52B varies
from a minimum at the center of the blocks 53A and 53~,
respectively, to a maximum at the edges of the blocks 53A
and 53B, respectively. This advantageously permits the
fiber optic strands~ 50A and 50B, when mounted in the
grooves 52A and 52B, respectively, to gradually converge
toward the center and diverge toward the edges of the
blocks 53A, 53B, thereby eliminating any sharp bends or
abrupt changes in direction of the fibers 50 which may
cause power loss through mode perturbation. In the
embodiment shown, the grooves 5~ are rectangular in cross-
section, however, it will be understood that other
suitable cross-sectional contours which will accommodate
the fibers 50 may be used alternatively, such as U-shaped
cross-section or a V-shaped cross-section.
2S At the centers of the blocks 53, in the embodiment
shown, the depth of the grooves 52 which mount the strands
50 is less than the diameter of the strands 50, while at
the edges of the blocks 53, the depth of the grooves 52 is
preferably at least as great as the diameter of the
strands 50. Fiber optic material was removed from each of
the st~ands 50A and 50B, e.g., by lapping, to form
respective oval-shaped planar surfaces, which are coplanar
with ~he confronting surfaces of the bloc~s 53A, 53B.
These oval surfaces, where the fiber optic material
removed, will be referred to herein as the fiber "facing
surfaces". Thus, the amount of fiber optic material that
~Z6i3~
-16-
has been removed increases gradually from zero towards the
edges of the blocks 53 to a maximum towards the center of
the blocks 53. l'his ~apered removal of the fiber optic
material enables the fibers to converge and diverge
gradually, which is advantageous for avoi~ing backward
reflection and excess loss of light energy.
In the embodiment shown, the coupler halves 51A and
51B are identical, and are assembled by placing the
confronting surfaces of the blocks 53A and 53B together,
so that the facing surfaces of the strands 50A and 50~ are
juxtaposed in facing relationship~
An index matching substance (not shown), such as index
matching oil, is provided between the confronting surfaces
of the blocks 53. This substance has a refractive index
approximately equal to the refractive index of the fiber
cladding, and also functions to prevent the optically flat
surfaces from becoming permanently locked together. The
oil is introduced between the blocks 53 by capillary
action.
An interaction region 54 is formed at the junction of
the strands 50, in which light is transferred between the
strands by evanescent field coupling~ It has been found
that, to ensure proper evanescent field coupling, the
amount of material removed from the fibers 50 must 'be
carefully controlled so that the spacing between the core
portions of the strands 50 is within a predetermined
"critical zone". The evanescent fields extend into the
cladding and decrease rapidly with distance outside their
respective cores. Thus, sufficient material should be
removed to permit each core to be positioned substantially
within the evanescent field of the other. If too little
material is removed, the cores will not be sufficiently
close to permit the evanescent fields to cause the desired
interaction of the guided modes, and thus, insufficient
coupling will result. Conversely, if too much material is
removed, the pro~agation characteristics of the fibers
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will be altered, resulting in loss of light energy due to
mode perturbation. However, when the spacing between the
cores of the strands 50 is within the critical zone, each
strand receives a significant portion of the evanescent
field energy from the other strand, and good coupling is
achieved without significant energy loss. The critical
zone includes that area in which the evanescent fields of
the fibers 50A and 50B overlap with sufficient strength to
provide coupling, i.e., each core is within the ev~nescent
field of the other. ~owever, as previously indicated,
mode perturbation occurs when the cores are brought too
close together. For example, it is believed that, for
weakly guided modes, such as the HE~1 mode in single mode
fibers, such mode perturbation begins to occur when
sufficient material is removed from the fibers 50 to
expose their cores. Thus, the critical zone is defined as
that area in which the evanescent fields overlap with
sufficient strength to cause coupling without substantial
mode perturbation induced power loss.
The extent of the critical zone for a particular
coupler is dependent upon a number of interrelated factors
such as the ~arameters of the fiber itself and the
geometry of the coupler. ~'urther, for a single mode fiber
having a step-index profile, the critical zone can be
quite narrow. In a single mode fiber coupler of the type
shown, the required center-to-center spacing between the
strands 50 at the center of the coupler is typically less
than a few (e.g., 2-3) core diameters,
Preferably, the strands 50A and 50B (1) are identical
to each other; (2) have the same radius of curvature at
the interac~ion region 54; and (3) have an equal amount of
fiber optic material removed therefrom to form their
respective facing surfaces. Thus, the fibers 50 are
symmetrical, through the interaction region 54, in the
plane of their facing surfaces, so that their facing
surfaces are coextensive if superimposed~ This ensures
~2~
-18-
.
that the ~wo fibers 50A and 5~ will have the same
propagation characteristics at the interaction region 54,
and thereby avoids coupling attenuation associated with
dissimilar propagation characteristics.
S The blocks or bases 53 may be fabricated of any
suitable rigid material. In one presently preferred
embodiment, the bases 53 comprise generally rectangular
blocks of fused quartz glass approxîmately 1 inch long, 1
inch wide, and r).4 inch thick. In this embodiment, the
fiber optic strands 50 are secured in the slots 52 by
suitable cement, such as epoxy glue. ~ne advantage of the
fused quartz blocks 53 is that they have a coefficient of
thermal expansion similar to that of glass fibers, and
this advantage is particularly important if the blocks 53
and fibers 50 are subjected to any heat treatment during
the manufacturing process. Another suitable material for
the block 53 is silicon, which also has excellent thermal
properties for this application.
The coupler of Figure 3 includes four ports, labeled
A, B, C, and D, which correspond to the ports 1, 2, 3, and
4, respectively, of the coupler 20 in Figure 1 When
viewed from the perspective of Figure 3, ports A and B
which correspond to strands S~A and 50B, respectively, are
on the left-hand side of the coupler, while the ports C
2S and D, which correspond to the strands 5~A and 50B,
respectively, are on the right-hand side oi the coupler.
For the purposes of discussion, it will be assumed that
input light is applied to port A. This light passes
through the coupler and is output at port C and/or port D,
depending upon the amount of power ~hat is coupled between
the strands 5~. In this regard, the term "coupling ratio"
is defined as the ratio of the coupled power to the total
output power. In the above example, the coupling ratio
would be equal to the ratio of the power at port D to the
sum of the power output at ports C and D. This ratio is
also referred to as the "coupling efficiency", and when so
~Z6~
, g
used, is typically expressed as a percent. Thus, when the
term "coupling ratio" is used herein, it should be
understood that the corresponding coupling efficiency is
equal to the coupling ratio times 100. For example, a
coupling ratio of 0.5 is equivalent to a coupling
efficiency of 50%1The coupler may be "tuned" to adjust the
coupling ratio to any desired value between zero and 1.0,
by offsetting the facing surfaces of the blocks 53. Such
tuning may be accomplished by sliding the blocks 53
1~ laterally relative to each other, so as to increase the
distance between the fiber cores.
The coupler is highly directional, with substantially
all of the power applied at one side of the coupler being
delivered to the other side of the coupler. That is,
substantially all of the light applied to input port A is
delivered to the ports C and D, without contra-directional
coupling to port B. Likewise, substantially all of the
light applied to port B is delivered to the ports C and
D. Eur~her, this directivity is symmetrical, so that
substantially all of the light applied to either port C or
input port D is delivered to the ports A and B. Moreover,
the coupler is essentially non-discriminatory with respect
to polarizations, and thus, preserves the polarization of
the light. Thus, for example, if a light beam having a
vertical polarization is input to port A, the light cross-
coupled from port ~ to port D, as well as the light
passing straight through from port A to port C, will
remain vertically polarized.
The coupler is also a low loss device, having
insertion or throughput losses typically on the order of
2-3 percent. The term "insertion loss", as used herein,
refers to the real scattering losses of light passing
through the coupler, from one side to the other. For
example, if light is applied to port A, and 97% of that
light reaches ports C and D (combined), the insertion loss
would be 0~03 (3~). The ter~ "coupler transmission" is
~6
-20-
defined as one minus the insertion loss. Thus, if the
insertion loss is 0.03 (37O)~ the coupler transmission is
0.97 (97%)
Further details regarding this coupler are disclosed
5in U.S. Patent No. 4,536,058 issued on August 20, 1985,
and U.S. ~atent No. 4,4~3,52~, issued on January 15,
1985. In addition, the coupler is described in the March
29, 1980 issue of Electronics Letters, Vol. 16, No. 7,
pa~es ~60-261.
In general, the coupler depicted in Figure 3 ~xhibits
wavelength dependencies such that its coupling ratio may
be different for two signals having a large ~avelength
separation. ~owever, for the small wavelength separation
between the pump signal 33 (Figure 1) and the signal pulse
21, the coupling ratios for the coupler of Figure 3 are
ordinarily clo~e to each other (i.e., within 0.2).
The coupler of Figure 3 may, nevertheless, be
specially adapted to operate a~ a "multiplexing coupler",
~uch that the coupler exhibits significantly different
coupling ratios for diferent wavelengths, even if the
wavelength separation is quite small. ~y properly
selecting the radius of curvature of the fibers and the
core spacing therebetween the coupler can be made to
~S provide virtually any desired coupling ratio for
substantially ang pair of ~avelengths.
To further ~xplain ~hi3 aspect of the invention, it
will be recalled that the coupler 20 operates on
eva~es~ent ~ie~d coupling principles in which guided modes
of the str~d~ 5~ interact through their evanescent fields
to c~u~e l~ght ~o be transferred between the strands 50 at
the interaction region 54. The amount of light
transferred i~ dependent upon the proximity and
orieDtation of the cores 8S well as the effective length
35 o~ the in~ceract ion region 54 . The length of ~:he
~n~eraction region 54 is dependen~ upon ehe radius of
6 ~ 5
-21-
curvatu~e of the fibers 50, and, to a limited extent, the
core spacing, although it has been found that the
effective length of the interaction region is
substantially independent of core spacing~ However, the
"coupling length" (i.e., the length within the interaction
region 54 which is required Eor a single, complete
transfer of a light signal from one fiber to the other? is
a function of core spacing, as well as wavelength.
If the length of the interaction region 5~ is
increased, and/or core spacing decreased, so that the
coupling length is shorter than the e~fective interaction
length, a phenomenon referred to herein as "overcoupling"
will occur. Under these circumstances, the light will
transfer back to the strand from which it originated. As
the interaction length is further increased, and/or the
core spacing further decreased, the effective interaction
length becomes a greater multiple of the coupling length,
and the ligh~ transfers back to the other strand. Thus,
the light may make multiple transfers back and forth
between the two strands as it travels through the region
54, the number of such transfers being dependent on the
leng~h of the interaction region 54, the light wavelength,
and the core spacing.
This phenomena permits selection of virtually any two
coupling ratios for any two signals of different
wavelengths. ~or example, by properly choosing the
geometrical parameters for the coupler 20, one signal
wavelength may be substantially totally coupled, while a
second signal wavelength remains substantially
uncoupled.
To illustrate this wavelength dependence, Figure 4
provides a plot of coupled power versus signal wavelength
in the visible and near infrared spectrum for a particular
coupler geometry. For this coupler configuration, the
effective interaction length of the coupler is an odd
multiple cf the coupling length for the wavelength 720 nm,
-22-
but an even multiple of the coupling length for the
waveleng~h 550 nm. Thus, the wavelength 720 nm will be
100% coupled, while the wavelength 550 nm will be
effectively uncoupled, yielding a wavelength resolution of
170 nm. Other wavelengths exhihit different coupling
efficiencies~ For example, 590 nm has a coupling
efficiency of about 5-10% and 650 nm has a coupling
efficiency of about 80-85%.
As the number of coupling lengths within the effective
interac~ion length increases, the resolution of the
multiplexing coupler is enhanced. Thus, by increasing the
radius of curvature to increase the effective interaction
length so that it becomes a higher multiple of the
coupling length, resolution is improved. This result is
illustrated in Figure 5, which is comparable to the graph
of Figure 4, except that the radius of curvature has been
increased from 25 cm to 200 cm. As expected, Lhis
increase in radius improves the coupler resolution near
600 nm from approximately 170 nm in the 2S cm radius case
of Figure 4 to approximately 60 nm in the 200 cm case.
After the resolution of the coupler has been set in
accordance with the particular wavelengths of interest,
the coupler may be tuned to precisely adjust the coupling
length for the wavelengths to yield the desired coupling
efficiencies. This is accomylished by offsetting the
fibers by sliding the blocks 53A, 53B (Figure 3) relative
to each other in a direction normal to the axis of the
fibers 50A, 50B. ~uch an offset has the effect of
increasing the core spacing. If the required offset is
small, it will not upset the resolution.
To illustrate the tunability of multiplexing couplers,
Figure 6 provides a plot of relative coupled power versus
wavelength for three increasing values of fiber offset (~
microns, 055 microns, and 1.0 microns). The curve is seen
to shift toward increasing wavelengths as the offset
increases, while the period of oscillation (or resolution)
~6~
-23-
remains virtually unchanged. In this partlcular example
~n which the radius of curvature was 20~ cm and the
minimum core-to-core spacing was 4 microns, a one micron
offset shifted the curve by approximately 45 nm~
Additional details of the above-described multiplexing
coupler may be found in U~S. Patent No. 4,556,279 lssued
on December 3, 1~85. The wavelength dependencies of the
above-described coupler are further discussed in an
article by Digonnet, et al., entitled "ANALYSIS OF A
TUNABLE SINGLE MODE OPTICAL FIBER COU~LER", IEEE Journal
of ~uantu~ Mech~nics, Vol. ~-18, No. 4 (April, 1982).
The embodiment of Figure 1 of the present invention
can utilize the above-described multiplexing coupler and
can also utilize a standard nonmultiplexing coupler. If a
mul~Lple~ing coupler is utilized, the coupler is
preferably adapted ~o exhibit overcoupling for either the
pump wavelength (e.g., 1.064 microns) or ~he sLgnal pulse
wavelength (e.g., 1.12 microns), so as to provide a very
low coupling ratio for the pump signal 33 ~nd a very high
c~upling ratio for the signal pulce 21. Except for the
type o coupler used, bo~h embodiments of Figure 1 are the
~ame.
2S The above embodiments o~ Figure 1 will be discussed
further in reference to Figure 7, which shows the fiber 10
and coupler 20 of Figure 1 without the associated
component~, for clarity of illustration. The device of
Figure 7 will first be described in terms of operation
~ithout ~he p~,p &ou~ce 34 ~Fi~re 1).
~ ~he ~bsence of the pump 34, the inpue signal pulse
21 p~opagaees along the input fiber portion 12 to port 1
of the coupler, where this pu~se is split by the coupler
into the circulating pulse 24, which exits the coupler
through port 3, ~nd a pulse (noe shown) which exits the
coupler 20 thr~ugh por~ 4. Depending on the coupling
~1,~
~Z6BS41
-24-
ratio of the coupler 20, the portion of the pulse 24
exiting port 4 can be substantial. Accordingly, it would
be preferable to utilize a switchable coupler as the
coupler 20, if available, so that the signal pulse
5 coupling ratio can be switched to a lower value when the
signal pulse 21 is initially coupled to the loop 14, and
switched back to a high value before the circulating
signal 24 completes its first circulation. It should be
recognized, however, that the device will function in the
intended manner regardless of whether the coupler 20 is
switchable.
After the pulse 24 exits port 3 of the coupler, it
circulates through the loop 14 and arrives at port 2 of
the coupler, where a portion of the pulse 24 is coupled to
port 3, to again circulate in the loop 14, while another
portion is output from port 4 as the pulse 26a. For
purposes of explanation, it will be assumed that the
coupling ratio of the coupler 20 for the circulating
signal 24 is 0.9, and for simplicity, it will be assumed
that the loop 14 and coupler 20 are lossless. If it is
further assumed that the pulse 24 has a normalized value
of 1.0 on its first circulation abou~ the loop 14, it
follows that 10% of the pulse 24 will exit port 4 of the
coupler, yielding a normalized value of 0.1 for the first
output pulse 26a. The remaining 90~ of the light will be
coupled to port 3 for propagation about the loop a second
time, thus yielding a normalized value of 0~9 or the
pulse 24 on its second circulation. When the pulse
arrives back at port 2 of the coupler, 10% of the light
will again exit port 4 of the coupler to form the second
pulse 24b, which is delayed from the pulse 26a by an
amount equal to the round-trip transit time of the fiber
loop 14. The amplitude of the pulse 26b will be 10% of
the 0.9 normalized value of the second recirculating
pulse, or 0.09. On the third circulation, the pulse 24
will have a normalized value of 0.81 (90X x 0.9). After
~6~
completing the third circulation, 10% of the 0.81
normali~ed amplitude of the pulse 24 will be coupled to
for~ a third output pulse 26c, which is separated from the
second output pulse 26b by the round-trip transit time of
the loop~ and has a normalized value of 0.081. By
continuing these calculations the normalized amplitude
values of subsequent pulses may be found. For example,
the fourth pul~e 26d will have a normalized amplitude of
.073, while the fifth pulse 26e will have a normalized
amplitude of ~066. Accordingly, it can be ~een that each
ti~e ~he pulse 24 circulates in the loop 14, an output
pulse i~ generated such that a train of output pulses 26
is provided at the f iber output end portlon 16. Of
particular significance i8 the fact t~at the output pulses
26 gradually decreaQe in amplitude, thereby limiting the
lifetime of the optical memory (i.e., the number of
useable output pulses 26). Additional details a~ to the
lifetime of the decaying pul~e train 26 may be found in
U.S. Patent No. 4,473,270, issued September 25, 1984 to
Shaw.
The present inven~ion advantageously amplifies the
pulse 24 on each ~irculation through the loop 14 to
compen~ate for round-trip losses ~uch that the pulse 24
ha~ a normalized value of 1.0 each time it arrives back at
port 2 of the coupler, thereby providing an output pulse
train 26~1 ~o 26-5 having a constant amplitude. This is
acc~mpliQhe~ without splicing any discrete amplifying
components i~o the lo~p ~4, but ra~her by utilizing the
core of the fi~er 14 a~ ~he active ampl~fying medium.
A~ was di~cussed in reference ~o Figure l, the pump
source 34 (Figure 1) inputs the pump signal 33 into the
output end portion 16 of the fiber 10 for introduction
into the loop 14 through the coupler 20. The roupler 20
~plit~ the pump ~ignal in an a~ount depending on the
coupling rstio such ~hat a portion ou~puts the coupler
through port 1 and another portion outputs the coupler
~3
~26~5~
-~6-
through port 2 for propagation through the loop 14. For
example, assuming the coupling ratio for the pump signal
33 is the same as that previously assumed for the signal
pulse 21, namely 0.~, it follows ~hat 90% of ~he light
will exit port 1 and be lost, while the remaining 10% will
be available to propagate through the fiber loop and pump
the fiber loop 14. The portion of the pump signal which
propagates in the fiber loop 14 is depicted in Figure 7 as
the arrow labelled "loop pump signal". In the embodiment
shown, the loop pump signal propagates in a direction
opposite that of the circulating pulse 24. The pump
signal duration for this embodiment is at least equal to
the number (N) of desired constant amplitude output pulses
(e.g., 26-1 through 26-N) times the round-trip transit
time of the loop. In other word~, the input pump signal
pulse 33 has a sufficiently long duration such that it can
be considered as a continuous wave for the desired number
of circulations of the circulating pulse 240 The loop 14
is thus continuously activated by pump light throughout
the duration of the constant amplitude pulse train output
26-1, 26-2 . . . 26-N. The optical power of the pump
4ignal 33 is selected to provide sufficient optical power
for the portion of this signal 33 propagating in the fiber
loop 14 to cause stimulated scattering. As discussed
above, such stimulated scattering will generate photons at
the S~okes wavelength, 90 as to amplify the circulating
slgnal pulse 24 as it propagates through the loop 14.
Although various types of stimulated scattering
processes are available for use in the present invention,
the preferred embodiments utilize stimulated Raman
scattering. As is well known in the art, stimulated Raman
scattering is a phenomena in which coherent radiation is
generated by optically pumping the molecules of a
material, such as an optical fiber, into an excited
vibrational state. The process may be viewed as a three-
level laser emission process in ~hich molecules of the
active media are excited from the ground level to an
excited virtual level by absorbing input pump photons.
Return of the excited molecules to an intermediate level
results in ~he emission of photons, commonly referred to
as "Stokes photons" which have a characteristic
wavelength, commonly referred to as the "Stokes
wavelength". In the case of S~S, the Stokes photons have
a particular optical frequency relationship to ~he pump
light which depends upon the molecular structure of the
core of the optical fiber. For a preferred embodiment,
which utilizes silica glass fibers, this ~requency
relationship causes the Stokes photons to be shifted in
frequency relative to the pump signal by an amount,
referred herein as the "Stokes shift", which, for SRS, is
typically about 13 to 15 THz. The Stokes shift in SRS is
due to the difference in energy between the incident pump
photon and the vibrational level of the molecule, It
should be noted that the transition of the excited
molecules from the intermediate level back to the ground
level results in emission of phonons, although such
phonons are of little interest in the present invention,
as they are quickly absorbed by the fiber and converted to
heat.
As discussed above, the input signal light 21, 24 has
a wavelength which is equal to the Stokes wavelength.
When the circulating signal pulse 24 is injected into the
loop 14, such pulse 24 will stimulate relaxation of the
excited molecules and emission of photons at the Stokes
wavelength. The stimulated photons have the same phase
characteristics and frequency as the pulse 24, and thus,
amplify the circulating pulse 24.
Those skilled in the art will understand that
stimulated scattering processes are quite complex, and
that the above description of amplification using
stimulated Raman sca~tering is somewhat simplified. For
example, those skilled in the art will recognize that the
5~
28~
lifetime of the excited vibrational states of the
molecules is extremely short. Accordingly, a number of
the excited molecules will return ~o the ground state
spontaneously, resulting in the spontaneous emission of
Stokes photons. These spontaneously emitted Stokes
photons can themselves be amplified by the same process as
described above for the input signal pulse, such that a
secondary wave is created in the loop, referred to herein
as a "Stokes background wave". Since the fiber 10
provides an optical guiding structure, two such Stokes
background waves will be generated, and these two waves
will counterpropagate in the optical fiber loop 14. As
shown in Figure 7, one of the Stokes background waves
travels around the loop in the same direction as the pump
signal (referred herein as the "forward" Stvkes background
wave), while the other travels in the opposite direction
around the loop relative to the pump signal (referred
herein as the "backward" S~okes background wave). Since
the backward Stokes background wave propagates in the same
direction through the loop 14 as the circulating signal
24, the output 26 of the optical memory will be comprised
of a superposition of light from the amplified signal
pulse 24 and light from the backward Stokes background
wave. Thus, the backward Stokes background wave may be
~5 considered as background noise in the optical memory
- output 26. It should be noted that the forward Stokes
background wave has no significant effect on the output
26, since it propagates in a direction opposite to the
circulating signal 24.
If the optical gain provided by the stimulated
scattering in the fiber loop 14 is sufficiently high,
these Stokes background waves may grow rapidly in
amplitude, due to the fact that they will be amplified in
the same manner as the circulating pulse 24, thereby
increasing the noise level and adversely affecting the
signal to noise ratio. Fortunately, it has been found
-29-
that, by properly controlling the optical gain provided by
the stimulated scattering process in the loop 14, the
growth of the background Stokes wave can be main~ained at
a level, such that the background Stokes power is several
orders of magnitude below that of the circulating signal
24 for a large number of circulations (e.g. thousands).
The optical power of the input pump signal 33 should
be selected to provide sufficient pump power in ~he fiber
loop 14 such that the stimulated scattering process yields
just enough amplification (e.g., Raman gain) to compensate
for the total round-trip loop losses of the circulating
pulse 24. A portion of these round-trip loop losses is
due to fiber attenuation which causes propagation losses
in the opti~al fiber loop 14. These propagation losses
cause some degradation of the pulse 24 as it propagates
around the loop 14. The attenuation (a) of an optical
fiber is typically expressed in dB per kilometer. Thus,
the total propagation losses per circulation (in dB) due
to fiber attenuation are equal to aL, where L is length of
the fiber loop 14 in kilometers. Additional losses are
caused by the fact that a portion of the pulse 24 is lost
from the loop 14 on each circulation du~ to tapping by the
coupler 20 to form the outyut signals 26. For the
configuration disclosed, such coupling losses will be a
function of l - ns. where tls is the coupling ratio of the
coupler 2~ for the signal pulse 24. The total round-trip
loop losses (~) of the optical signal pulse 24 may be
expressed as follows:
~ = (1 - tls) [l-exp(-asL) ] (1
~2~6~
-30-
Where as is the attenuation coefficient at the Stokes
wavelength, expressed in km~l through the following
conversion: as(km 1) = -as(dB/km)/(lO log~Oe)O
Equation 1 thus indicates the fraction of the optical
5 power of the pulse 24 which is lost on each circulation
through the loop 14. The fraction of optical power
remaining in the loop 14 will be referred to as the loop
transmission (Tloop), and is given by.
Tloop = ns~s (2)
where Ts is the fiber transmission for the signal pulse 24
(i.e., exp(-asL)).
Those skilled in the art will understand that there is
an additional source of loss which was not discussed
above, namely, the coupler insertion losses. However, for
the coupler 20, these losses are qui~e small (e.g., 0.15
2Q db), and thus may be ignored. However, if a coupler
having high insertion losses i8 used as the coupler 20,
these coupler insertion losses should be taken into
account, for example, by adding the insertion losses to
the fiber propagation losses.
For a given input pump power Pp, the Raman optical
gain (G) in the fiber loop 14 may be expressed as:
G = exp [ ~ (I - `lp~p) ]
where ~p is the coupling ratio of the pump signal, g is
the ~aman gain coefficient, A~ is the effective
interaction area accounting for the pump signal and signal
pulse 24 mode overlap in the fiber 14, and Lf is the
-31-
effective fiber length of interaction area (explicitly Lf
= (1-Tp)/~p), where Tp is the fiber transmission at the
pump wavelength, and ap is the attenuation coefficient`at
the pump wavelength. In order to maintain the
recirculating pulse 24 at constant intensity, the gain (G)
should equal the total loop loss (~), such that:
GTloop = 1 (4)
It may thus be found that the amount of optical power
(Ppc) of the input pump signal 33 which is necessary to
maintain constant intensity of the recirculating pulse 24
is given by:
A~ e pL
Ppc ~ (asL - log ns) 1 - ~p
If the signal pulse coupling ratio (ns) and the pump
signal coupling ratio (~p) are substantially the same,
Equation 5 may be rewritten as:
P = ~ (a L - log n) -1 ~ ne p (6)
where ~ is the coupling ratio of both the pump signal and
the signal pulse (i.e- ns = np = n)
Equation 6 is a transcendental equation, and thus, the
coupling coefficient cannot be expressed directly as a
function of the pump power (Ppc)~ ~owever, for any given
set of fiber parameters (a, g, Lf, Af), the pump power Ppc
defined by Equation (6) may be plotted as a function of
the coupling ratio, as shown in the curves of Figure 8~
In developing the curves of Figure 8, it was assumed that
s~
-32
the fiber attenuation for the signal light (~s) and the
fiber attenuation ~or the pump light (ap) were equal
(i.e., aS=ap=a), which is a very good approximationO
The curves of Figure 8 illustrate the coupling ratio
as a ~unction of the input pump power, Ppc (in watts)
necessary to achieve an optical gain which will compensate
for to~al loop losses, for various values of fiber
attenuation (~) assuming, in accordance with Equation ~,
that the coupler ~0 exhibits substantially the same
coupling ratios for the pump light and the signal light.
It was also assumed that the loop length (L) was 810
meters. As shown in Figure 8, the optical input pump
power (Ppc) required for loop loss compensation varies
dramatically with the coupling ratio (n). Indeed, the
required pump power (Ppc) increases toward infinite values
for very low coupling ratios and for very high coupling
ratios. For intermediate coupling ratios, the required
pump power Ppc decreases dramatically. Further, there is
an optimum coupling ratio for e~ch value of fiber
attenuation . In this exemplary set of curves, the curve
ïO corresponds to a fiber attenuation of 1.8 dB per
kilometer, while the curve 72 and 74 correspond to
attenuations of 1.0 dB per kilometer and 0.5 dB per
kilometer, respectively. It may be seen that, in general,
the required pump power Ppc decreases as the fiber
attenuation decreases. Further, there is an optimum
coupling ratlo for each of the curves 70, 72, 74,
corresponding to the point of zero slope, at which the
required pump power Ppc is a minimum. Significantly, as
the fiber attenuation decreases, these optimum coupling
ratios increase, whereas the required pump power Ppc
decreases. Thus, by properly selecting the coupling ratio
for the particular fiber utilized, the required pump power
Ppc can be reduced and minimized.
In the yresent invention, it is preferable that the
pump power utilized be nc more ~han about 2 watts. It has
s~
been found, for example, that pump powers in excess of 2
watts tend to cause "burn out" of the coupler 20~ in that
the index matching oil between the blocks 53 tends to
burn, thereby damaging the coupler. Although such coupler
"burn out" might be alleviated by fusing the fiber
together without oil therebetween, high optical powers are
disadvantageous for other reasons. For exa~ple, they may
cause undesirable non-linear effects (e.g., multi~Stokes
effect) in the fiber 10~ Further, high power laser
instability may cause enhanced amplitude variations in the
output pulse train 26, as opposed to lower power lasers.
Moreover, such amplitude instability of high power lasers
tend to cause the circulating Stokes background waves to
build up very rapidly in the fiber loop. This buildup of
t5 the Stokes background can significantly reduce the signal
to noise ratio of the output pulses and/or cause coupler
burn out.
Accordingly, the coupling ratio of the coupler 20
should be selected such that the input pump signal 33
optical power Ppc required for loop loss compensation is
no greater than about 2 watts. This relationship may be
defined by rewriting Equation 6 as:
A (asL - log n) T-~ ~ (7)
It will be recognized that Equations 6 and 7 assume
that the coupling ratios for the pump light and the signal
light are the same. Thus, these equations are accurate to
a good approximation only in the event that a "standard"
coupler is utili~ed.
If the signal pulse coupling ratio and the pump signal
coupling ratio are not close to e~ch other, e.g., as in
the case of a multiylexing coupler, Equations 6 and 7 may
~26~
-34-
not be sufficiently accurate. In such event, the pump
power Ppc required for constant amplitude output pulses 26
should be determined in accordance with Equation 5, above,
which may be rewritten to express the signal coupling
ratio (~s) as a function of the pump coupling ratio (~p),
with the pump power Ppc as a parameter, as follows:
~s = exp [ asL - P
Equa~ion 8 defines a family of curves, as shown in
Figures 9a and gb. Figure 9a illustrates the signal
coupling ratio (~s) at the signal wavelength of 1.12
microns as a function of the pump coupling ratio (~p) at
the pump wavelength of 1.064 microns for various values of
Ppc~ with a fiber attenuation of 1.8 dB per kilometer.
Figure 9b shows the same relationships as Figure 9a, but
for a fiber attenuation of 0.5 dB per kilometer. As in
Figure 8, it was assumed in Figure 9a and 9b that aS=ap=a,
which is a very good approximation. It may be seen from
the curves of Figures 9a and 9b that the pump power Ppc
is, in general, at a minimum when the signal coupling
ratio is close to one and the pump coupling ratio is equal
to zero. Indeed, for any given signal coupling ratio, the
pump power Ppc for that coupling ratio is at a minimum
when the pump coupling ratio is equal to zero.
Conversely, for any given pump coupling ratio, the pump
power Ppc decreases as the signal coupling ratio
increases.
It will be understood that a high signal coupling
ratio advantageously decreases the round-trip loop losses,
since less optical power is coupled from the loop to form
the constant amplitude output pulses 26. Although it is
advantageous to select the signal coupling ratio so that
it i8 close to unity, it should preferably not be exactly
unity, otherwise there will be no light coupled from the
loop to form t~e outpu~ pulse train ~6. On the other
5~
-35-
hand, the pump signal ratio should preferably be zero, or
close to zero, so as to couple maximum pump power to the
loop, and thereby efficiently use such pump power to cause
Raman gain in the loop.
S From the foregoing, it will be understood that,
although the invention ~ay be implemented utilizing either
a standard coupler or a multiplexing coupler, a
multiplexing coupler is advantageous in that it provides
the gain necessary to achieve constant amplitude output
pulses 26 at a lower optical power of the input pump
signal 33. Ln the case of a multiplexing coupler, the
signal coupling ratio and pump coupling ratio may be
selected independently of each other so as to achieve low
loop loss while still coupling substantially all of the
pump light signal to the loop for maximum pumping
efficiency. In the case of a standard coupler, the pump
and signal coupling ratios are close to each other, and
thus, a compromise must be drawn between the loop losses
and the amount of pump power coupled to the loop. As
discussed in reference to Figure 8, there is a particular
coupling ratio compromise which makes it possible to
minimixe the pump power Ppc~
Regardless of whether a standard coupler or a
multiplexing coupler is used, the coupling ratios should
preferably be selected so that the pump signal 33 optical
power Ppc is no greater than 2 watts, for the reasons
discussed above. From Equation 8, it may be found that
this result is achieved when the signal coupling ratio is
related to the pump coupling ratio as follows:
~s ~ exp [ asL ~ A (1 -~pL)]
-36-
Although the pump power required for cons~ant
amplitude output pulses 26 may be readily calculated from
the foregoing equations, it may be advantageous to adjust
the pump power Ppc downwards very slightly to suppress any
build-up of the Stokes background waves. As discussed
previously, these Stokes background waves are amplified
with the signal pulse, and their growth can significantly
reduce the signal to noise ratio. It is believed that
growth of the Stokes background waves is related to the
stability of the pump laser, such that the greater the
pump signal fluctuations, the higher the rate of growth of
the Stokes background waves. For pump lasers having
amplitude fluctuations of no more than about 10% from a
nominal value, the amount of downward adjustment of the
pump power necessary to suppress build-up of the Stokes
background waves is typically very small, such that the
output pulses 26 are still substantially constant in
amplitude over a large number of circulations. It is
expected that use of highly stable pump sources at
appropriate wavelengths will substantially reduce any
problems relating to build-up of the Stokes background
waves, and will permit the memory of the present invention
to achieve millions of circulations without significant
degradation of the recirculating signal pulses and build-
up of background noise. E'urther, due ~o the low pumppower requirements of the present invention, it is
expected that the invention may be implemented using high
power laser diodes. Such laser diodes are advantageous in
that, through proper control of the driving current, the
amplitude of the laser output can be made highly stable.
In addition, laser diodes have the advantage of permitting
the invention to be compactly packaged in a relatively
small unit.
Although preferred embodiments of the invention
utilize stimulated Raman scattering, it will be recognized
that other types of scattering processes may be used
~2~
-37-
alternatively. For example, those skilled in the art will
understand that the invention may be implemented utilizing
stimulated Brillouin scattering, or four photon mixing.
Stimulated Raman scattering is preferred becawse (1) it
does not have any phase matching recluirements, (2) it can
be implemented in both forward and backward
configurations, and (3) the ~aman gain curve is relatively
large so that frequency matching betweert the signal pulse
and the pump generated Stokes light does not have to be as
precise, thereby making it possible to use a signal pulse
having a larger bandwidth~ Additional advantages of Raman
scattering will be apparent to those skilled in the art~
An exemplary embodiment corresponding to the
arrangement shown in Figure 1 was tested. In the
experimental set-up, the signal generating loop 44 (Figure
2) was formed from 1 km length of non-polarization holding
fiber. The pump pulses from the laser 42 (Figure 2) had a
peak power of 120 watts and a repetition rate of 50 Hz,
thus generating 500 n~ signal pulses 21. Each signal
pulse 21 had an intensity of about 50 mW.
The loop 14 (Figures 1 and 7) was formed of 810 meters
of 6 micron core diameter single mode non-polarization
holding fiber. The coupler 20 was a standard coupler
presenting coupling ratios of 0.48 and 0.66 at the pump
and Stokes wavelengths, respectively.
The pump laser 34 was a 2 watt cw Nd:YAG laser. The
output of this laser was chopped in 0.5 ms pulses
synchronized to the output of the Q-switched YAG laser
used for pulsed signal generation. The intensity of the
input pump signal 33 was about 708 mW.
The fiber attenuation was 2.26 dB per km, the
interaction area was 38.3 ~m2, and the interaction length
was 676 meters. The Kaman gain coefficient was
experimentally determined to be 8.5 x l0-14 meters per
watt.
~2~5~
-38-
A total of about 13~ pulses of substantially constant
amplitude were observed during the duration of the pump
signal (0.5 ms). This corresponds to an optical path of
nearly 105 km or a time delay of 500 microseconds between
the first output pulse and the last output pulse.
Accordingly, the embodiment of Figure 1 of the present
invention provides an optical memory having a
significantly longer lifetime than prior art optical
memories. Further, it is believed that this is the first
optical memory utilizing in-line amplification through the
non-linear properties of optical fibers.
As discussed above, it is important that the pump
power in the recirculating loop be maintained at or below
the cri~ical pump power in order to minimize the build-up
of ,Stokes noise. It has been established experimentally
that pump power fluctuations about the critical level
cause gain fluctuations in the recirculating loop that
result in the dramatic build-up of the Stokes
intensities. Although instability of the pump source is a
principal cause of such gain fluctuations, it has been
found that gain fluctuations in the recirculating loop can
also be attributed to phase noise in the pump power in the
loop caused by the interaction of the pump signals
circulating in the loop with pump signals entering the
loop from the pump source. This phase noise is associated
with a low coherence optical signal, such as is preferred
for the pump signal in the present invention, and results
in a s~ructured intensity noise in the pump power in the
loop. This intensity noise i9 minimal at low pump signal
coupling ratios, and thus, the noise can be avoided
through use of a multiplexing coupler having a pump
coupling ratio of zero. ~lowever, for pump coupling ratios
substantially greater than zero, particularly intermediate
pump signal couplin~ ratios, such as those used for some
embodiments of the present invention, pump power
fluctuations due to phase noise can result in broadband
~L26~
-3g-
modulation of the gain in the loop which causes
significant signal noise and build-up of Stokes noise.
Figure 10 illuscrates a preferred embodiment of the
above described invention which reduces the phase noise
and thus reduces the build-up of Stokes noise in the
loop. The phase noise reduction is accomplished by
utilizing a pump signal modulation scheme in which the
pump power is input in the form of pulses which have a
duration and are spaced in time in relation to the loop
transit time to reduce the in~eraction between the
recirculating pump light and the pump light input to the
loop.
The embodiment of Figure 10 comprises a signal
generator 100, shown in dashed lines, which corresponds
generally to the signal generator 22 described above in
connection with Figure 2. The signal generator 100
advantageously comprises a Q-switched Nd:YAG laser 102
which advantageously operates at a wavelength of 1~064
microns, for example, to provide pump light; an optical
fiber signal generating loop 106 that receives the pump
light from the laser 102; a lens 110 through which the
light generated by the laser 100 i8 input to the signal
generating loop 106; a lens 112 through which the light
from the signal generating loop 106 is output; and an
interference filter 120. The signal generator 100
operates in a manner similar to that described above in
connection with Figure 2 to produce a pulse signal. The
1.064 micron pump light entering the signal generating
loop 106 stimulates 1.12 micron Stokes waves which exit
the loop 106 through the lens 112. In an exemplary
embodiment, the Q-switched laser 102 is operated at a low
repetition rate, for example 31 Hz, and with a relatively
low duty cycle ~o generate relatively narrow pulses, for
example 280 nanosecond pulses.
The interference filter 120 has a 30 nanometer
bandwidth and suppresses the 1.064 micron pump signal
5~L
-40-
pulses and transmits only the 1.12 micron Stokes signal
generated in the signal generating loop 106. The
resulting 1.12 micron signal pulses from the interference
filter 120 are input through a lens 132 into a first end
of a fiber 134 which ~orms ~ recirculating loop 130. The
recirculating loop 130 generally corresponds to the
recirculating loop lO of Figure 1. The fiber 134
preferably comprises a continuous, uninterrupted length of
a single-mode optical fiber 134. A coupler 136 is
interconnected in the manner described above in connection
with Figure l so that the fiber 134 forms the loop 130.
Preferably, the fiber 134 is a nonpolarization-preserving
single-mode fiber having a 6 micron core and a 720
nanometer cutoff wavelength. The coupler 136, ~hich
generally corresponds to the coupler 20 in Figure 1 and
Figure 3, has four ports 1, 2, 3, and 4, and has a
coupling ratio greater than zero. The signal propagates
in the :~iber 134 from the interference filter 120 to the
coupler 136 and enters the coupler 136 via the port l. A
coupled portion of each 1.12 micron signal pulse entering
the coupler 136 via the port 1 from the interference
filter 120 exits the coupler via the port 4 as a first
1.12 micron signal output pulse~ An uncoupled portion of
the 1.12 micron signal pulse entering the coupler via the
port I exits the coupler 136 via the port 3 and travels
- around the loop portion of the fiber 134 and reenters the
coupler 136 via the port 2. An uncoupled portion of the
1.12 micron signal pulse entering the coupler 136 from the
loop portion via the port 2 exits the coupler 136 via the
port 4 as a second 1.12 micron signal outpuc pulse, and a
coupled portion of the 1.12 micron signal pulse entering
the coupler 136 via the port 2 exits the coupler 136 via
the port 3 and recirculates through the loop portion of
the fiber l30. Thus, each 1.12 micron input signal
entering the loop 130 via the port 1 of the coupler 136
will recirculate in the loop 130 as described above so
~6~5~
-41-
that a portion of the signal is output after the
completion of each transit of the loop 130 of the fiber
134. The output portions of the circulatin~ signal
comprise a series or train of 1.12 micron signal outpu~
S pulses that are spaced apart in time by an amount equal to
the loop transit time.
A loop pump source 140 is provided that generates a
pump signal at a wavelength which causes Raman gain,
which, for a fused silica fiber and a signal wavelength of
1.12 microns, is 1.064 microns. The pump signal from the
loop pump source 140 is input to the loop 130 via the port
4 of the coupler 136 so that the pump signal recirculates
around the loop 13~ in the direction opposite the
direction of recirculation of the 1.12 micron signal
pulses described above (i.e., the pump signal is
counterpropagating with respect to the 1.12 micron signal
pulses). The pump signal causes stimulated 'Kaman
scattering in the fiber 134 of the loop 130 which
amplifies the recirculating signal pulses as described
above. Those skilled in the art will understand that
Raman gain can be produced in fibers at a number of
frequencies in which the pump frequency is separated from
the signal frequency by the amount o~' the Raman shift
(e.g., 13-14 Terahertz). The choice of frequency for the
described em'bodiment was due in part to the ready
avai'lability of a laser pump source at 1.064 microns.
In the embodiment ~hown in Figure 10, the loop pump
source 140 comprises a polarized Nd:YAG continuous wave
(CW) laser 150 which operates at 1.064 microns. The
output o~ the CW laser 150 is passed through an acousto-
optic modulator 152 which modulates the amplitude of the
pump light. The acousto-optic modulator 152 is driven by
an enable pulse generator 154 which is controlled by a
gate generator 156. The gate generator 156 is controlled
by an amplifier 158 which is connected to the output of a
photodetector 160. The photodetector 160 receives light
~6 ~
-42-
from a beam splitter 162 which is positioned in the
optical path of the light from the ~-switched laser 10~ in
the generator 100 so that the photodetector 160 provides
an active output signal when the Q-switched laser 102
S generates the pump signal for the signal generating loop
106 . The active output signal ~rom the amplifier 158
causes the gate generator 156 to generate an active gating
pulse which is thus synchronized with the signal pulses
which are produced in the signal generating loop 106. The
gate generator 156 generates a gating pulse having an
adjustable pulse width. Thus, the gate generator 156
operates in a manner similar to a one-shot multivibrator
to produce a single gating pulse which begins in
synchronism with the pump signal generated by the Q-
switched laser 1~ and ends in accordance with the
selected pulse width. The gating pulse from the gate
generator 156 controls the enable pulse generator 154.
When the gating pulse is active, the enable pulse
generator 154 generates a series of periodic enabling
pulses having a selectable width (i.e., time duration) and
a selectable repetition rate or periodicity (i.e., time
duration between the beginning of a pulse and the
beginning of the next successive pulse). The periodic
enabling pulses from the pulse generator 154 control the
acousto-optic modulator 152. When the periodic enabling
pulses are active, the acousto-optic modulator 152 gates
the pump signal from the CW laser 150 to the output of the
pump source 140 to create a series of pump pulses which
enter the loop 130 via the port 4 of the coupler 13S.
When the periodic enabling pulses are inactive, the
acousto-optic modulator 152 blocks the pump signal from
the CW laser 150 so that no pump signals are output from
the pump source 14U. 'rhe series of enabling pulses and
thus the series of pump pulses will continue to occur so
long as the gating pulse generated by the gate generator
156 is ac~ive. These pump pulses are illustrated in
-~3-
Figure 11a. In the preferred embodiment of Figure 10, the
pulse widths and repetition rates of the periodic enabling
pulses from the pulse generator 154 and thus the widths
and repetition rates of the pump pulses gated through the
acousto optic modulator 152 are preferably selected so
that the pump pulses entering the loop 130 do not overlap
with the first few recirculations of pump pulses which
have previously entered the loop 130 as will be discussed
more fully below in connection with Figures 1la-1ld.
In the preferred embodiment of the inven~ion
illustrated in Figure 10, the pump source 140 is optically
isolated from power backscattered by the loop 130 with a
Glan polarizer 170 and a quarter-wave plate 172. A fiber
polarization controller 174 is advantageously placed on
the pump input end of the loop 130 and is used to adjust
the backscattered light polarization and thus maximize the
a~ount of backscattered power which is rejected by the
Glan polarizer 170. Since the optical fiber 134 is
preferably nonpolarization~preserving, and since the
coupler coupling ratio is substantially polarization-
independent, the polarization controller 174 does not
sub.stantially affect pump power input to the loop and thus
does not substantially affect the signal recirculations in
the loop 130. A lens 176 is also preferably included to
direct the output signal from the polarization controller
174 to the quarter-wave plate l72.
The Glan polarizer 170 also acts as a beam splitter
that directs a portion of the output signal from the loop
130 to a monochromator 180. The output of the
monochromator 180 is directed onto a photodetector 182
which provides an electrical representation of the output
signal from the loop 130. In the experimental set up, the
electrical output from the photodetector 182 is provided
to an oscilloscope 184 so that the signal can be
analyzed. When incorporated into a system, the electrical
output from the photodetector 182 can be provided to other
~ ~ 6
-4~-
portions of the system to provide a representation of the
pulses output from the loop. Alternatively, the optical
output pulses from the monochromator 180 can be provided
directly to optical fiber components of a system.
As mentioned above, phase noise reduction is
accomplished in the embodiment of Figure 1~ by modulating
the light produced by the pump source 140 to provide a
series of pump pulses having a duration and spacing
selected in relation to the transit ~ime of the fiber loop
130 to reduce the interaction between the pump light
recirculating in the loop 130 and the pump light input to
port 4 of the coupler 136. A specific example o a pulsed
wave form utilized to reduce phase noise, and thereby
suppress build up of the Stokes waves, is illustrated in
Figures 11a and 11b. Figure 11a depicts an exemplary
train of pump pulses referred to herein as "pump input
pulses" which are produced by the source 140 and input to
port 4 of the coupler 136. These pump input pulses, which
are shown in Figure 11a with respect to time, include a
first pump input pulse 202, a second pump input pulse 204,
a third pump input pulse ~06, and fourth, fifth, sixth and
seventh pump input pulses 208, 210, 212 and 214,
respectively. Each pump input pulse has a width (i.e.,
time duration) of approximately one-fourth the loop
transit time. l'he loop transit time is shown as tdelay~
Thus, each pump pulse has a width of approximately 1/4
tdelay~ The pump input pulses 202-214 have a periodicity
(i.e., time between the beginnings of successive pump
input pulses) selected to be substantially equal to 3/4
tdelaY'
When an input pump pulse reaches port 4 of the
coupler, a portion is coupled ~o the loop 130 to form a
circulating pump pulse which is delayed in time on each
circulation by an amount tdelay~ A circulating pump pulse
which has circulated once through the loop 130 will be
referred to as a "once-delayed" pump pulse. Similarly, a
-45-
pulse which has circulated twice through the loop 130 will
be referred to as a "twice-delayed" pump pulse, and so
forth,
Figure 11b illustrates a train of once-delayed pump
pulses with respect to time and with respect to the pump
input pulse train of Figure lla. The first pump pulse
entering the loop (e.g., the first pump input pulse 202)
circulates through the loop and arrives back at the
coupler 136 after a delay of tdelay as a once-delayed
first pump pulse 202'~ Since the delay time through the
loop 130 is greater than the time between pump input
pulses, the once-delayed first pump pulse 202' does not
overlap with the second pump input pulse 204 and instead
arrives at the coupler 136 with a delay o~ tdelay/4 with
respect to the second pump input pulse 204. Similarly,
the second pump input pulse 204 will be delayed by the
time tdelay and will arrive at the coupler 136 as a once-
delayed second pump pulse 204' with a delay o~ tdelay/4
with respect to the third pump input pulse 206.
2~ Additional once-delayed pump pulses 20~', 206', 208',
210', etc., in the train of once-delayed pump pulses will
arrive at the coupler 136 with corresponding delays with
respect to the pump input pulses in the train of pump
input pulses. None of the pulses in the train of once-
delayed pump pulses overlap with any of the pulses in the
train o~ pump input pulses.
Figure llc illustrates a train of twice-delayed pump
pulses with respect to time and with respect to the pump
pulse trains of Figures lla and llb. ~fter an additional
delay o~ tdelay with respect to the once-delayed first
pump pulse 202', a twice-delayed first pump pulse 202"
arrives at the coupler 136. The twice-delayed first pump
pulse 202" does not o-~erlap with any of the pump input
pulses (Figure 1la) or with any of the once-delayed pump
pulses (Figure 11b). Similarly, a twice-delayed second
pump pulse 204", a twice-delayed third pump pulse 206",
~2~
-46-
and so forth, do not overlap with any of the pump input
pulses or wi~h any of the once-delayed pump pulses.
Figure lld illustrates a train of thrice-delayed pump
pulses with respect to time and with respect to the pulse
trains of Figures 11a, llb, and 11c. After an additional
delay of tdelay with respect to the twice-delayed first
pump pulse 202", a thrice-delayed first pump pulse 202"'
arrives at the coupler 136. As illustrated, the thrice-
delayed first pump pulse 202"' overlaps the fifth pump
input pulse 210 in the pump input pulse train (Figure
l l a) . Subsequent thrice-delayed pump pulses 204"', 2~6"',
etc., overlap with the sixth, seventh, and so forth, pump
input pulses. Although the thrice-delayed pump pulse
train overlaps the pump input pulse train, the effec~ of
the thrice-delayed pump pulse train is minimized because
the pulses in the thrice-delayed pump pulse train of
Figure 11d are substantially attenuated by the
recirculations in the loop 130. For example, in one
embodiment, the pump coupling ratio ~p and the loop
attenuation for the pump signal may be such that each
pulse is reduced by one-half during each recirculation
(i.e., the intensity of the once-delayed first pump pulse
202' is one-half the intensity of the first pump input
pulse, the intensity of the twice-delayed first pump pulse
202" is approximately one-half the intensity of the once-
- delayed first pump pulse 202', etc.). Thus, for this
example, the thrice-delayed first pump pulse 202"' has an
intensity of one-eighth the intensity of the fifth pump
input pulse 210. Thus, the interaction between the pulses
in the pump input pulse train and the circulating pulses
in the thrice-delayed pump pulse train is substantially
reduced, and the pump intensity noise caused by the
interference of the recirculating pump power with the
input pump power, both of which have a high phase noise,
is also substantially reduced. The pump intensity noise
and power fluctuations can be fur~her reduced by
-~7-
increasing the number of pulses before overlapping occurs,
which can be done by reducing the pulse widths of the pump
pulses. For example, in an alternative embodiment (not
shown), each pulse can have a pulse width of ~delay/5 and
a repetition rate o~ 4/5 tdelay~ In this alternative
embodiment, there will be no overlap of a circulating
pulse with a pump input pulse until the sixth pump input
pulse arrives at the coupler 136. Using the above
example, at the time that the first circulating pulse
overlaps with the sixth pump input pulse the first
circulating pulse would have traversed the loop four
times, thereby reducing i~s intensity to approximately
1/16 of the intensity of the input pulses.
The effective pump power in the loop Peff is a
summation of the effects of each of the recirculating pump
pulses in the loopO It can be shown that for the example
described above, the steady-state pump power corresponding
to Ppc in ~he continuous pump embodiment i9
~1/3)peff/(1-np)~ In the embodiment of Figure 10, with a
pump coupling ratio of 0.42, the steady-state pump power
is 0.57 times Peff. In an experimental set up used to
test the present invention, the gate generator l 56 was
adjusted to provide a pulse width of between 1 millisecond
and 3 milliseconds. The pump pulses generated by the
pulse generator l 54 had widths of 930 nanoseconds and were
spaced apart by 2.78 microseconds corresponding to 1/4
tdelay and 3/4 tdelay for a delay time in the loop 130 of
approximately 3. 72 microseconds. Using this modulation,
the measured effective pump power PeiE was 1.08 watts,
corresponding to coupling ratios np = O. 42 and ~s =
0.6S. This corresponds to a theoretical value of
approximately 530 milliwatts for the steady state power
Ppc for the same coupler ratios. This is in general
agreement with the calculated required peak power (i.e.,
530 milliwatts X 1/0. 57 = 922 milliwatts. A 3 millisecond
delay, corresponding to more than 800 recirculations in
5~
-4~-
the loop 130 and thus corresponding to an optical fiber
path longer than 600 kilometers, was obtained with a pump
source having a 3 percent r.m.s. power stability. It is
believed that additional recirculations can be obtained by
S using a power supply with greater stability.
