Note: Descriptions are shown in the official language in which they were submitted.
CA 02317668 2000-09-06
FURUK.OOlA'US pA,~~
OPTICAL SIGNAL AMPLLtb"IER
The present invention relates to fiber optic communications systems, and more
specifically, to amplification of optical signals propagating in an optical
fiber.
Optical signals for conveying information in a fiber optic communication
system
experience attenuation as the optical signals are transmitted though an
optical $ber over
extended distances. The attenuated optical signal can be regenerated using
amplifiers
such as optical $ber Raman amplifiers, which rely on stimulated Raman
scattering to
transfer energy to the optical signal. The optical fiber Raman amplifier
comprises a fiber
that receives two input beams: a pump beam and the optical signal_ Energy in
the pump
beam is coupled ioato the signal beam through stimulated ltaman xattering, and
the
optical signal is thereby amplified upon passing through the fiber amp)i$er.
The extent of
amplification or gain depends on the relation between the polarization of the
pump beam
and that of the signal beam. If both the pump beam and the signal beam are
linearly
polarized and have electric fields oriented in the same direction, then the
gain is higher
than if the vlcctric fields are oriented perpendicular to each other.
Disadvantageously,
$uctuations in the polarization of the signal or pump beam that cause the
relative
orientations of the electric fields to vary produce $uetuations in the gain of
the amplifier.
For example, the gain will decrease for pump and signal beams that initially
have electric
fields oriented parallel but are reoriented so as to no longer be parallel.
Conversely, gain
will increase if the beams arc initially perpendicular but subsequently
contain parallel
eornponents. Such fluctuations ici the gain cause variations in the intensity
of the optical
signal, which introduces noise into the signal and thereby increases the
flceIihood of
errors in transmitting information over optical fibers.
In conventional systems designed to minimi~~C fluctuations in gain, the pump
beam is provided by two or more ,semiconductor lasers that output polarized
Light. The
polarized light is directed to a coupler that combines the Light from the
different
semiconductor lasers after lust separating the respective beams into
perpendicular
3Q polari~ations. For example, in the case where two semiconductors are
employed to
pump the fiber amplifier, light omitted from the two semiconductors is input
into the
_1_
CA 02317668 2000-09-06
coupler. The coupler causes the polarized light beams from the tvcro
semiconductor
lasers to have electric fields oriented perpendicular to each other and
produces a
combined beam that is then directed to the optical fiber Raman amplifier.
Although employing a plurality of semiconductor lasers can reduce the
fluctuations in gain, reguiring more than one semiconductor laser adds to the
compleadty
of the amplifier. 'What is needed is a design for an optical fiber Raman
amplifier that is
sinopler and less expensive yet that minimizes the fluctuation in gain caused
by variations
in polarization of the pump and signal beams.
SummarX
Methods and apparatus for optical signal amplification arc provided. In one
embodiment, an amplifier for amplifying optical signals comprises a light
source having
as an output a first beam of light characterized by a first degree of
polarization, a
depolarizes optically connected to the light source so as to receive the first
light beam as
an input and having as an output a pump beam characterized by a second degree
of
polarization wherein said second degree of polarization is less than said
first degree of
polarication_ A gain medium is optically connected to the depolarizes so as to
receive
the optical signal and the pump beam as inputs and is configured to transfer
energy from
the pump beam to the optical signal. The depolarizes advantageously comprises
one or
more bireficingent optical fibers.
2p A method of making an optical s canal amplifier in one embodiment of the
invention cvrnprises coupling a light source to an input of at least one
birefringcnt optical
fiber and coupling an output of said at least one birefringent optical fiber
to a gain
medium.
Methods of optical signal amplification include coDecting light from a light
source that emits at least partially polarized light divisible into light of
two orthogonal
linearly polarized statos_ This collected light is at least partially
depolarised by imparting
phase delay between the light of the two orthogonal linearly polari~.ed states
and is then
directed into a gain medium of an optical signal amplifier. In another
embodiment, a
method of m~izing polari»tion induced gain fluctuations in an optical signal
amplifier
comprises at least partially depolarizing a beam of light from a first fight
source without
combining the beam of light with a second beam of light from a second light
source.
-2-
CA 02317668 2000-09-06
'This at least partially depolarized beam of light is used as a pump beam is
the optical
signal amplifier.
Brief Deser~'t~tion o the Dr winos
FIGURE 1A is a xhematic diagram of an optical communication system
employing an optical amplifier.
FIGURE 7,B is a block diagram of a preferred embodiment of the optical
amplifier comprising a pump laser, a depolarizer, and a gain medium.
FIGURES 2A 2C are schematic views of preferred embodiments of the present
invention comprising a non-depolarizing buefringent optical fiber joined to a
depolarizing
bireftingent optical fiber so as to. provide a mismatch between respective
principal axes
of the two fibers.
FIGURES 3A 3C are schematic views of preferred embodiments of the present
invention comprising a pump laser that emits linearly polarized light having
an electric
field oriented in a fixed direction and a depolarizing bireGingent optical
fiber having
principal axes that are not aligned with the electric field of the polarized
Rght.
FIGURIr 4 is a xhcmatic view of a preferred embodiment of the present
invention sirnlar to that shown in FIGURE 2A additionally comprising a
polarization
controller.
FIGURE 5 is a schematic view of a preferred embodiment of the present
invention comprising a non-depolarizing birefringent optical fiber coupled to
two
depolarizing birefringent optical fibers.
FIGURE 6 is a scheri~atic view of a preferred embodiment o~ the present
invention similar to that shown in FIGURE S additionally comprising a fiber
Bragg
grating inserted in the non-depolarizing birefringent optical 5ber-
FIGURE 7 is a schematic view of a preferred embodiment of the present
invention similar to that shown in FIGURE 5 additionally comprising a
polarization
controller inserted in the non-depolarizing birefringeot optical fiber.
FIGURE 8 is a schematic view of a preferred embodiment of the present
invention similar to that shown in FIGURE 5 additionally comprising a fiber
Bragg
grating and a polarization controller inserted in the non-depolarizing
birefringent optical
fiber.
-3-
CA 02317668 2000-09-06
FIGURE 9A is a schematic view of a preferred embodiment of the present
invention wherein a plurality of semiconductor lasers and accompanying
depolarizers are
coupled to a mufti-wavelcngch optical coupler_
FIGURE 9B is a schematic view simdlar to that shown in FIGURE 9A with fiber
$ragg gratings inserted between the lasers and depolarizers_
FIGURE x0 is a schematic view of a preferred embodiment of the present
invention showing the plurality of semiconductor lasers coupled to a plurality
of non-
depolarizing birefringent optical fibers that are joined to a plurality of
depolarizing
birefringcnt optical fibers that lead to the mufti-wavelength optical coupler_
FIGURE 11A is a schematic view of a preferred embodiment of the present
invention wherein the plurality of semiconductor lasers are coupled to the
multi-
wavelength optical coupler, which is coupled to the depolarizer.
FIGURE 11B is a schematic view similar to that shown in FIGURE 11A with
fiber Bragg gratings inserted between the lasers and the mufti-wavelength
optical coupler
FIGURE x2 is a plot, on axes of fiber length, in centimeters (em), and degree
of
polarization (DOP), in percent, depicting how the degree of polarization is
reduced with
increasing length of the depolarizing birefringent optical fiber.
FIGURE 13 is a plot, on axes of degree of polarization, in percent, and
polarization dependence oI gain (1'DG), in decibels, illustrating how lowering
the degree
of polari2ation reduces the ~luetuations in gain caused by fluctuations in
polarization.
t~' ,gra9led D ccri ion
Embodiments of the invention w~l now be desenbed with reference to the
accompanying Figures, wherein fkc numerals refer to hlce elements throughout.
The
tennioology used in the description presented heroin is not intended to be
interpreted in any
Limited or restrictive manner, sizz~ply because it is being ui~lized in
conjunction with a detat-led
description of certain specific embodiments of the invention. Furthermore,
embodiments of
the invention may include several novel features, no single: one of which is
solely
responsib)e for its desirable attributes or which is essential to practicing
the inventions
herein descn'bed.
As shown in Figure lA, a fiber optical communication system 2 comprises a
transmitter 4 optically connected to a receiver 6 through an optical fiber 8.
An amplifier
-4-
CA 02317668 2000-09-06
such as an optical fiber Rarnan amplifier may be inserted between t~cro
segments at the
optical fiber 8. The transmitter 4 comprises an optical source such as a laser
diode which
emits an optical beam that is modulated to introduce a signal onto the beam.
The optical
signal beam is coupled into the optical fiber 8, which carries the beam to the
receiver 6.
5 At the receiver 6, the optical signal is converted info an electrical signal
via an optical
detector. To ensure that the optical signal is sufficiently strong such that
the modulation
can be accurately detected at the receiver 6, amplification is provided by the
optical fiber
Kaman amplifier 10. Such amplification is especially critical when the optical
signal is
transported orrer long distancES within the optical fiber 8.
10 A block diagram of the optical fiber Kaman amplifier 10 that is a preferred
embodiment of the present invention is shown in FiG~URE 1B_ The Kaman ampIiser
10
comprises a light source 12, a depolatizer 14, and a gain medium 16 and also
has an
input X8 for the optical signal that is to be amplified and an output 20 for
the amplified
optical signal. '1'hc light svuree 12 may comprise a single light generator or
a plurality of
light generators having the sarnc or different wavelengths_
The Light source 12 emits a beam of light represented by a line 22 extending
from
the light source 12 in 1~TCIURE 1B. Preferably, the beam of light 22 and the
optical
signal are separated in wavelength by about 50 to 200 nanometers (nm), and
more
preferably, by about 100 nanometers_ The light source 12 may, for example,
comprise a
semiconductor laser or laser diode_ As is well lrnown i!n the art,
semiconductor laser
diodes generally emit light that is substantially linearly polarised, i.e.,
electromagnetic
waves having an electric field oriented in a fixed direction. To provide a
constant level
of gain in the gain medium 16, as will be discussed more fully below, the pump
beam
preferably comprises substantially unpolarized light, not linearly polarized
light.
Accordingly, the beam 22 is directed to the depolarizer 14, which receives the
linearly
polarised light and at least partially depolarizes the light. In preferred
embodiments, the
output of the depolarizer 14 comprises at least partially depolarized light_
Most
preferably, this output comprises substantially unpolatizcd light; all or
substantially a1I of
the beam 22 emitted by the light source 12 is depolarized by the depolari~er
14.
The light beam 22, after passing through the depolarizer 14 is directed to the
gain
medium 16 as depicted by line 24 extending from the depolari~er to the gain
medium.
-5-
CA 02317668 2000-09-06
The beam entering the gain medium 24 is referred to herein as the pump beam.
The
optical signal is also sent to the gam medium 16 as illustrated by line 26 in
FIGURE 1B.
The optical signal enters the input 18, is amplified within the gain mcdiium
16, and exits
the output ZO a stronger signal, which is represented by a line 28 emanating
from the
gain mediurti_ Within the gain medium 16, energy from the pump beam 24 is
coupled to
the signal 26 via stimulated Raman scattering as is well laaown in the art.
As discussed above, the extent of amplification depends on the relation
between
the polarization states of the pump beam and the optical signal, The optical
signal also
comprises electromagnetic waves having as electric field and a magnetic field.
If the
electric field of the optical signal is directed parallel to the electric
field of the pump
beam, the ampli~6cation provided by the gain medium 16 wi71 be ma.~amized.
Conversely,
if the electric fields are perpendicular to each other, a minimum in gain
results_ When the
electric fields are not fully parallel or perpendicular, but contai~o both
parallel and
perpendicular components, the gain will have a value somewhere between the
minimum
and maximum depending on the magnitude of the parallel and perpendicular
components_
Accordingly, as the relative orientation of the electric fields in the pump
beam and the
optical signal vary, the gain will vary. If, however, the pump beam renrtains
entirely
unpolarized, containing no predominant linear polarized component, the gain
wdl not
fluctuate. Thus, by passing the light emitted by the ligtrt source 12 through
the
depolarizes 14, the variations in the amount that the optical signal 26 is
amplified can be
reduced_
In another configuration, the pump beam itself can be amplified by another
pump
beam using an additional gain medium. In this case, using depolarized Light
source to
pump this additional gain medium will reduce the fluctuation of the power of
the pump
beam caused by polarization dependent gain fluctuations.
FIGURES 2-t3 depict preferred embodiments of the optical fiber Raman amplifier
10 of the present invention in which the depolarizes 14 comprises one or more
birefringent optical tl'bers. The one or more birefringent optical fibers are
configured to
at least partially depolari2e light from the light source 12.
Referring now to Figure 2~ the light source 12 advantageously comprises a
semiconductor laser 29 which is coupled through a fiber connector 30 to a
birefringcnt
-6-
CA 02317668 2000-09-06
optical fiber 32. In this embodiment, the birefringent optical fiber 32
functions as part of
the light source 12 and does not function as the depolarixer 14 and,
therefore, is
hereinafter referred to as the non-depolariimg birefringent fiber. This non-
depolarizing
birefringent optical Hber 32 has a fiber Bragg grating 34 inserted therein.
The fiber
Bragg grating 34 comprises a diffracting reflector, which when employed in
association
with the semiconductor laser 29, transmits a wavelength band of light output
the laser.
The non-depolarizing birefi-ingent optical fiber 32 is connected to another
birefringent
optical fiber 36 that serves as the depolarizer x4 aid, accordingly is denoted
the
depolarizing birefringent optical fiber. This depolarizsng birefringent
optical fiber 36,
along with an input optical fiber 38 for carrying the optical signal, are
attached to an
optical coupler 40 that leads to the gain medium 16, namely, an optical fiber
Raman gain
nosdium, which produces gain through stimulated Raman scattering. Preferably,
the
optical fiber Raman gain medium 16 comprises quad and morn preferably, ion-
doped
quartz.
The non-depolari~'!~g and depol 'arcing birefringent optical fibers 32, 36 are
coupled together at a point 42, a close-up of which is depicted in ~GURES 2B
and 2C.
As shown in ~GURES 2B and 2C, a longitudinal axis, z, runs down the length of
the
non depolarizing and depolarising birefringent optical fibers 3Z., 36.
Mutually
perpendicular x (hori~ntal) and y (vertical) axes extend through and are
perpendicular
to the z sags.
The non-depolarizing and depolari2ing birefringent optical fiber 32, 36 each
have
a central core and a c]adding. As is conventional, the core has a refractive
index that is
higher than that of the cladding. Stress imparting layers (not shown) are
disposed in the
cladding, the core sandvsriched therebetareen. As a result of this sandwich
structure, the
2s refractive index of the core is diffr.~rent for light linearly polarized in
the x direction and
tight linearly polarizzd in the y direction, that is, for electromagnetic
radiation having an
electric field parallel to the x axis and electromagnetic radiation having an
electric Geld
parallel to the y axis, respectively. Consequently, linearly polarized light
having a
polari.cation parallel to the horicontal direction travels through the
birefringcnt optical
fiber 32, 36 at a different velocity than light having a polarization parallel
to the vertical
direction. In accordance with convention, and as used herein, one of these
axes, the x
CA 02317668 2000-09-06
axis or the y axis, is referred to as the fast axis, and the other a~ is
referred to the slow
axis. Light having an electric field aligned with the fast axis, propagates
along the length
of the core at a higher velocity than light having an electric field aligned
with the slow
axis. Lice the x and y axes, the fast and slow axes are perpendicular. Also as
used
herein, the term principal axes conrespond$ to the fast and slow axes.
In this embodiment of the invention, the non-depolarizing birefringcnt optical
fiber 32 is oriented such that ono of the principal axes of this fiber matches
the
polarization of the light emitted by the semiconductor laser 29. For example,
the non
depolarizing bircfringent optical fiber 32 may be rotated about its length,
the z axis, such
that its fast axis is aligned and parallel with the electric field of the
electromagnetic
radiation from the semiconductor laser 29 that is transmixted through the non-
depolarising birefringent fiber.
Also, in accordance with the present invention, the depolarizing birefi-ingent
optical fiber 36 is oriented such that the principal axes of the non-
depolarizing
1~ birefringent fiber 32 are not aligned with the principal axes of the
depolarizing
birefringent fiber. An exemplary arrangement of the non-depolarizing and
depolarizing
bircfringent optical fibers 32, 36 is shown in FIGURES 2B and 2C where the non-
depolarizing birefringent optical fiber has a principal axis, e.g., a fast
axis, represented by
a first arrow 44 while the depolarizing birel:ringent optical fiber has a
principal axis, also
a fast axis, represented by a second arrow 46. The fast axis of the
depolarising optical
fiber 36 is rotated about the length oI the fiber, or the z axis, by a non-
zero angle A with
respect to the fast axis of the non-depolarizing optical fiber 32. As shown in
fiIGURE
2C, the angle 8 preferably equals 45°.
rn operation, the semiconductor laser 29 emits a light beam comprising
substantially linearly polarized Light that is coupled into the non-
depolarizing birefi~ingent
optical ;Ober 32 by the fiber connector 30. As discussed above, one of the
principal axes,
the fast or slow axis, of the non-dapolatiziug birelringent optical fiber 32
is parallel to the
electric field of the pump beam. This arrangement maintains the polarization
of the
pump beam as it is transmitted through the non-depolarizing birefiingent
optical fiber 32.
The light within the non~depolarizing birefringent optical fiber 32 passes
through the
fiber Bragg defractive grating 34, which provides a resonator external to the
CA 02317668 2000-09-06
semiconductor laser 29, thereby stab~izing the wavelength of the pump beam and
narrowing its bandwidth.
AIso as desrnbed above, the principal axes of the depolarizing bzrefriungent
optical fiber 36 are nonparallcl to the principal axes of the non depolarizing
birefringent
optical fiber 32. Accordingly, the electric field of the pump beam that is
transmitted
through the non-depolarizing birefringent optical ~ber 32 is nonparallel to
both the fast
and slow axes of the depolari~ng birefringent optical fiber 36. For purposes
of
understanding, the electric field fnr electromagnetic radiation passing
through a
birefringent fiber can be separated into two components, one parallel to the
fast axu and
1.0 one parallel to the slow axes, the vector sum of these two components
being equal to the
electric field. Similarly, light comprising the light source can be separated
into two
components, linearly polarized waves polarized in a direction parallel to the
fast aaas and
linearly polari2cd waves polarized parallel to the slow cads. 'The two sets of
waves are
transmitted through the depolarizing birefringent optical fiber 36, but at
different
x5 velocities. Thus, after passing through the dcpolarir~ing birefringent
optical fiber 36 and
upon reaching the optical coupler 40 and the optical fiber Roman gain medium
16, one of
the sets of waves, the one polarized parallel to the slow axes, experiences
phase delay
with respect to the one polarized parallel to the fast axis.
The phase delay translates into optical path difference between the two sets
of
20 waves. The amount of optical path difference depends on the disparity in
velocity as
well as the length of the dcpolari~ng birefringcnt optical fiber 36. The
longer the optical
path difference, the less correlation in phase between the light polarized in
a direction
parallel to the fast axis and light polarized parallel to the slow axis_ For
sufficiently long
lengths of Faber 36, the optical path difference will be as much as or longer
than the
25 coherence length of the light from the semiconductor laser 29, in which
case, coherence
between the two sets of waves will be Iost. No longer being coherent, the
relative phase
difference between the two sets of waves will vary rapidly and randomly_
Unpolarized light can be synthesized from two incoherent orthogonal linearly
polarized waves of equal amplitude. Since the light polarized in a direction
parallel to
30 the fast axis and the light polarized parallel to the slow cads are
incoherent, orthogonal
linearly polarized light, together they produce unpolarizcd light. This
conclusion arises
_g_
CA 02317668 2000-09-06
because the two sets of waves, which have orthogonal electric fields and a
relative phase
difference that varies rapidly and randomly, combine to form a wave having an
electric
field whose orientation varies raadornly. Light with a randomly varying
electric field
does not have a fixed polarization. 'Thus, light having rapidly varyiag
polarization states,
i.e., unpolarized light, is produced.
'fhe at least partly depolarized pump beam is directed to the optical coupler
40,
which also receives the optical signal transmitted through the input optical
fiber 38. The
propagation of the optical signal through the input optical fiber 38 and to
the optical
coupler 40 is represented by a first arrow 48 shown iun PIGURE 2.A.- 'The two
bums, the
pump beam and the optical signal, arc combined or multiplexed in the optical
coupler 40
and fed into the optical fiber Raman gain medium 16, which transfers energy
from the
pump beam to the optical signal via stimulated Raman scattering. The optical
signal
exits the optical fiber Raman gain medium 16 as an amplified signal indicated
by a second
arrow 50 shown in FIGURE 2A Since the pump beam is at least partly depolarized
upon passing through the dcpolarizi~g birefringcnt optical fiber 36, the
fluctuations in the
amplification provided by the optical ~ber Raman gain medium 16 arc
minirrrized.
Another embodiment of the present invention comprises a LYOT type
depolarizer having two birefringent optical fibers, one fiber having a length
two times or
more as long as the other fiber, i.e_, with respective lengths set by the
ratio of 1:2 or 2:1.
These two optical Ethers 32, 36 are fused together so that the principal axes
thereof are
inclined at an angle 8 of 45° with respect tv each other. The extent
that the depolarizing
birefringent optical fiber 36 is rotated about the z aus determines the amount
of light that
is polarized parallel to the Fast axis and the amount of light that is
polarized parallel to
the slow axis. When B equals 45°, as depicted in FIGURE ZC, the
magnitude of the
electric fields Lor the waves propagating parallel to the East and slow axis
are the same;
thus, the intensities of the two waves arc equal. As discussed above,
unpolarized light
can be synthesized from two incoherent orthogonal linearly polarized waves of
equal
amplitude. Since the magnitudes of the two incoherent orthogonal linearly
polari:ced
wavca are equivalent, substantially unpolarized light can be produced.
For other values of 8 not equal to 45°, the magnitudes of the electric
fields for
the waves propagating parallel to the fast and slow axis are not the same as
for the
_10_
CA 02317668 2000-09-06
configuration shown in FIG. 2B. For the purposes of understandhtg, the
combination of
the fast and slow waves can be separated into a sum of two parts. The first
part
comprises equal magnitude orthogonal incoherent waves haying electric fields
parallel to
the fast and slow axis, the combination of which produces uapvlarired light.
The second
part comprises the remainder, a component from the larger of the two waves,
which has
an electric field parallel either to the fast or slow axis. This part is
linearly polarized.
Thus, a portion of the light wdl be unpolarized and a portion of the light
wtll be linearly
polarized. The pump beam will not be completely depolarized_
A ratio of the intensities of the polarized component to the sum of the
intensities
of the polarized and unpolarizcd components is known in the art as the degree
of
polarization (DOP). The DOP is generally expressed in percentage_ Changing the
angle
between the principal axes of the non-depolarising and depolarizing
birefringent optical
fiber 32, 36 changes the DOP. For example, if the angle 8 is changed from
45°, on
condition that the depola 'ruing birefringent optical her has the same length,
the degree
of polarization (DOP) of the pump beam becomes larger. Accordingly, the angle
between the principal axes of the non-depolarizing and depolarizing
birefringent optical
fiber 32, 36, in part, controls the DOP.
FIGURES 3A-3C depict other preferred embodiments of the invention wherein
the semiconductor laser 29 is joined to the depolarizing birefringent fiber 36
through the
fiber connector 30. This depolarizing birefringent her 36 is directly attached
with the
optical coupler 40, which receives the optical input fiber 3~ and is connected
to the
optical fiber Raman gain medium x6. This depolari~ng birefiingent fiber 36 is
also
oriented such that its principal axes are not aligned with the electric field
of the beam
output by the semiconductor laser 29. For example, FIGURES 38 and 3C show
tight
emitted by the semiconductor laser 29 that is polarized in the vertical
direction as
indicated by a first arrow 52. However, one of the principal axes of the
depolari~ng
birefringent optical hbcr 36 (represented by a second arrow 54) is rotated
about the z
axis by a non-zero angle 8 with respect to the vertical direction- As shown in
FIGURE
3C, the angle 8 preferably equals 45° such that equal amounts of light
polarized parallel
to the fast and slow axes propagate through the depolari~ng birefringent
optical fiber 36.
_11_
CA 02317668 2000-09-06
In another embodiment of the present imrention depicted in FIGURE 4, the fight
source 12 additionally comprises a polarization controller 56 inserted between
the non-
depolarizng and depolarizing birefringent optical fibers 32, 36. Sinu7ar to
the Raman
amplifiers 10 descnbed with reference to FIGURES 2A 2C, the semiconductor
laser 29
is coupled to one end of the non-depolarising birefringent optical fiber 32
through the
fiber connector 30, the non-depolarirsng birefringent optical fiber having a
fiber Bragg
grating 34 inserted therein. The other end of the non-depolarizing
birefringenl optical
fiber 32, however, is joined to the polarization controller 56, which is
connected to the
depolarising birefringent optical fiber 36. The depolarizing birefringent
optical fiber 36
leads to the optical coupler 40, which is connected to the Raman ampliEer gain
medium
16. The input optical fiber 38 is also attached to the optical coupler 40 as
descn'bed
above.
The light emitted by the semiconductor laser 29 after passing through the non-
depolarizing bire&ingent optical fiber 32 reaches the polarization controller
56. The
polarization controher 56 provides the light, which is directed into the
depolarizing
birefringent optical :fiber 36, with a preferred state of polarization. Thus,
rather than
rotating the orientation of the depolariziag birefringent optical fiber 36
about the z axis,
the polarization is rotated about the z axis. In the embodiments depicted in
FIGURES
2A 2C, as well as those depicted in FIGURES 3A 3C, the depolarizing
bircFringent
optical fiber 36 is rotated to misalign the principal axis of the depolarizing
fiber and the
electric field of the pump br;am. In contrast, in the embodiment shown in
FIGURE 4, the
electric field of the light emitted by the laser 29 is rotated with respect to
the principal
axes of the depolarizing birefringent optical frber 36 using the polarization
controller 56.
In either case, the extent of rotation determines the amount of light
polarized
parallel to the fast and the slow axes of the depolarizing birefringent
optical fiber 36 or
alternatively, the amount of light coupled into fast and slow modes supported
by the
optical Ober. The depolarizing birefringent optical fiber 36 supports two
independent
pole 'rvation modes, a fast mode and a slow mode; that is, the fiber transmits
light
polaritzed parallel to the fast axis and light polarized parallel to the slow
axis. The
linearly polarized pump beam can be divided into light of two orthogonal
linearly
-12-
CA 02317668 2000-09-06
polarized states, a first polaJization state corresponding to light coupled
into the fast
mode and a second polarization state corresponding to light coupled into the
slow modes
The amount of light in the first linearly polarized state aad the second
linearly
polarized state is determined by the orientation v:f the electric field of the
light with
respect to the fast and slow axis of the depolarizing birefringent optical
fiber 36. If the
light is linearly polarized in the direction of the fast ass, all the tight
w~71 be coupled into
the fast mode and no light will be coupled into the slow anode. Ily however,
the light has
an electric field directed at an angle of 45° with respect to both the
Cast and the slow
axes, then half the light will be couplod into the fast mode and half wfil be
coupled into
7.0 the slow mode. Sizrnlarly, for other linearly polarized states, unequal
portions of the light
will be coupled into the fast and slow modes of the depolarizing bire~ringent
optical ~ber.
Thus, by varying the polarization state of the light emitted by the laser 29,
and in
particular, by rotating the electric field of linearly polarized laser output
about the z aaas,
the portion of the light coupled into the fast and slow modes can be
controlled.
15 Preferably, equal portions of the light arc distnbuted to the fast and slow
modes of the
depolarizing birefringcnt optical fiber. Thus, the polarization controller
preferably is
adjusted to provide linearly polarized light having an electric field directed
at an angle of
45° with respect to both the fast and Lhe slow aces. With use of the
polarization
controller 56, the non-depolarizing and depolari2ing birefringent optical
fibers 32, 36
2U need not be fixed irt a specific orientation about the z axis to achieve
this distribution that
optimizes depolarization of the laser light.
)~GURES 5-8 depict other embodiments of the present invention that include an
optical distributor 58 connecting the non-depolaci~ing birefringent optical
fiber 32 to first
aad second depolarizing bircfringcnt optical fibers 36a, 36b. As is the Raman
amplifiers
25 described above with reference to FIGURES 2-4, the semiconductor laser 29
is coupled
to the non-depolarizing birefringent optical fiber 32 through the fiber
connector 30- The
non-depvlariring birefringent optical ~ber 32 leads to the optical distnbutvr
58, which
may comprise a wavelength division muhiplex (WDI~ coupler or a polarization
demultiplexer- Preferably, however, the optical distributor 58 preserves the
polari~.ation
30 of the beam passing therethrough. 7"he optical distnbutor 5~ is connected
to one end of
the first and second depolarizing birefringcnt optical fibers 36a, 36b, which
are
-7 3-
CA 02317668 2000-09-06
terminated at another end by a beam combiner 60. A single-mode optical fiber
62
extends from the beam eombiner 60 and leads to the optical coupler 40. As
descn'bed
above, the optical coupler 40 receives the input optical fiber 38 and is
connected to the
optical fiber Raman gain medium 16.
In one embodiment, the light beam from the semiconductor laser 29 is guided
through the non-depolarizing birefiingent optical fiber 32 to the optical
distn'butor 58,
rwhich directs equal fractions of the beam into the first and second
depolarizing
birefringent optical fibers 36a, 36b. In this embodiment, the optical
distributor 58 directs
into the 5rst depolariza~ng birefringent optical fiber priman'ly only light
that is linearly
polarized parallel to the fast axis of the first depolarizing birefringent
fiber 36a. Similarly,
the optical distr~utor 58 directs into the second depolarizing birefringent
optical fiber
36b primarily only light that is linearly polarized parallel to the slow ass
of the second
depolarizing birefringent fiber. Accordingly, the optical distributor 58
couples one
portion, preferably half of the beam into the fast mode of the first
depolarizing
birefringent optical fiber 36a and another equal portion, preferably the other
had into the
slow mvdc of the second depolarizing birefringent optical ~ber 36b. The light
in the fast
mode propagates at a higher velocity than the Iight propagating the slow mode,
thereby
imparting phase delay as the light propagates in the C~rst and second
depolariadng
bircfringent optical fibers 36a, 36b. As described above, this phase delay
translates unto
optical path difference. In this embodiment, the first and second depolar~ng
birefringent optical fibers 36a, 36b each have appro~mately equal lengths.
This length is
chosen to produce an optical path difference that is su~ciently large to
reduce the
coherence between the two portions (i.e., halves) of the beam and to thereby
at least
partially depolarize the beam. Alternatively, the &rst and second depvlarizi~
birefringent optical fibers 36a, 36b can have different lengths. In this case,
the optical
path di~~crence will be caused both by the disparity in the refractiYe index
and the
propagation velocities for the fast and slow polarization modes in the two
depolarizing
birefringent optical fibers and by the unequal lengths of the two depolarizing
bireftingent
optical fibers. Again, the lengths can be chosen such that the optical path
difference is
sufficient to reduce the coherence between the two portions (i.e., halves) of
the pump
beam and to produce a depolariTing effect-
-14-
CA 02317668 2000-09-06
The two poztions of the beam in the first and second depolarizing birefrungcnt
optical fibers 36a, 36b, respectively, are combined in the beam combiner 60.
Preferably,
the beam combiner 60 comprises a polarization preserving beam combiner and the
beams
transmitted through the first and second birefa-ingent optical fibers 36a, 36b
are linearly
polarized perpendicular to each other when the pump beam is output from the
beam
combiner_
In another configuration, the optical distrbutor 58 directs equal portions of
the
beam from the laser 29 into the first and second birefring~nt optical fbcrs
36a, 36b
without restricting the polarization of the light. Thus, light is coupled into
both the fast
~0 and slow modes of the first depolarizing birefrutgent optical fiber 36a and
into both the
fast and slow rnodcs of the second depolarizing biref=irigent optical fiber
36b. The first
and second depol3rizhtg bireliingent optical fibers 36a, 36b, however, have
different
lengths. The difference in length of the two depolarizing bire~ringent optical
fibers 36a,
36b is large enough tv produce sufficient optical path difference to reduce
the coherence
between the light in the two fibers and to at least partially depola 'rvx the
pump beam.
The light in the first and second depolarirang birefringent optical fibers
36a, 36b is
combined in the beam cornbiner 60, and this pump beam is directed to the
optical fiber
Ratnan gain medium 16 after being transmitted Through the single mode optical
fiber 62
and coupled with the optical signal in the f.~ber optic coupler 40_
Alternatively, equal portions of the beam from the laser 29 are coupled into
the
fast mode of the first birefringent optical fiber 36a as well as the fast mode
of the second
bircfringent optical fiber 36b or ixtto the slow mode of the 5rst and second
birefringent
optical fibers 36a, 36b. Additionally, the $rst and second birefiingent fibers
36a, 36b
have different lengths so as to introduce an optical path difference greater
than the
coherence length between the light exiting the two fibers. As iri the other
cor~gurations,
the two beams are brought together in the beam combiner 60, and are directed
to the
optical fiber Raman gain medium ~6 after being transmitted through the single
mode
optical fiber 62 and combined with the optical signal in the fiber optic
coupler 40-
FIGURES 6-8 differ in that in FIGURE 6, the Sber Bragg grating 34 is inserted
in the non-d~;polarizing birefiingent optical Eber 32, in FIGURE 7, the
polarisation
controller 56 is inserted in the non-depolarising birefringent optical fiber,
and in
-15-
CA 02317668 2000-09-06
FIGURE 8, both the fiber Bragg grating and the polarization controller are
inserted in
the non-depolarizing birefringent . optical fiber. ,As discussed above, by
providing the
non-depolarizing birefringent optical fiber 32 with a fiber Bragg grating 34,
an external
resonator is formed for the semiconductor laser 29. The fiber Bragg grating 34
reflects
light fix'om the semiconductor laser 29 and narrows and stabilizes the
wavelength
distnbution of the laser output beam. Also as discussed above, the
polarization
controller 56 adjusts the polarization of the beam input to the depolarizer I4
so as to
optimize depolarization.
As shown in FIGURES 9-11, a Raman fiber amplifier 10 may comprise a
plurality of semiconductor lasers 29 cash emitting a light beam of a same or
di~ereat
wavelength. In one configuration illustrated in FIGURES 9A and 913, a separate
depolarizer 14 is associated with each individual laser 29, with this
plurality of
depola~rizers being optically connected to a multi-wavelength optical coupler
64. Each of
the depolarizers 14 receives light emitted from one of the semiconductor
lasers 29 and
produces at least partially depolarized light. The resultant plurality of
partly depolari:ced
beams of light arc combined into a single pump beam within the mufti-
wavelength optical
coupler 64. A separate fiber Bragg grating 34 can be inserted between each
semiconductor laser 29 and the respective depolarizer 14 to tailor the
wavelength
distribution of the light output by the semiconductor lasers as shown in
FIGURE 9B.
The same methods far producing and depolarizing light beam$ and for amplifying
the
signal as described above may be employed for a plurality of wavelengths. For
example,
as shown in FIGURE 10, ouch laser 29 in the plurality of semiconductor lasers
is coupled
to one of the fiber connectors 30, which is connected to respective non-
depolarizing
birefringent optical fibers 32. Each of the non-depolarizing birefiingent
optical fibers 32
has the fiber Bragg grating 34 connected thereto, which is joined to one
depolarizing
birefringent optical fiber 36. Each depolari-rsng biro&ingent optical fiber 36
is linked to
the mufti-wavelength optical coupler 64, which bas an optical fiber 66
exteading
therefrom. In general, an optical coupler such as the mufti-wavelength optical
coupler
64 shown in FIGURES 9A 9B, 10, and 11A-91B comprises one or more input lines
3U connected to one or more output lines. The number of input and output lines
depends
on the application. In FIGURES 9~ 9B, 10, and 11A I1B, the number of output
lines is
-16-
CA 02317668 2000-09-06
less than the number of input lines_ More specifically, in FIGURE 10, three
input lines
are coupled to the single optical fiber 66. This optical fiber 66 leads to the
other optical
coupler 40 that receives the input optical fiber 38. The optical fiber Raman
gain medium
X6 is attached to this optical coupler 40 as well.
finch laser 29 crnits a beam in a di~ercnt wawelcngth band. These beams, which
are at least partly depolarized upon passing through the separate depola~
birefringent optical fibers 36, arc combined in the multi-wavelength optical
coupler 64.
The combined beam is transmitted through the optical fiber 66 to the other
optical
coupler 40 and sent on to the optical fiber Raman gain medium 16 along with
the optical
signal also received by the optical coupler. Yn this manner, a plurality of
beams having
same or different wavelengths can be at least partially depolarized and
combined to form
a pump beam for pumping the optical fiber Raman gain medium 16. Simt7arly, in
any of
the embodiments discussed above, a plurality of semiconductor lasers 29 can be
employed to generate a beam comprising light in one or more wavelength bands,
which
x5 is subsequently depolarized at least partially_
FIGURE 1 XA and 11B depict an alternative arrangement wherein the multi-
wavelength optical coupler 64 precedes the depolarizes 14. In particular, the
lasers 29
are connected to non-depolarizing optical fibers 32 that run to the mufti-
wavelength
optical coupler 64. As illustrated in FIGURE 11B, fiber Bragg gratings 34 can
be
inserted between two sections of the non-depot 'anzing optical fibers 32 to
control and
stabilize the wavelength light emitted by the sem5conductor lasers 29. As in
the
embodiment shown in 1~'lCiLIRFS 9A, 9B and 70, the optical fiber 66 extends
from the
mufti-wavelength optical coupler 64, however, here the optical fiber leads to
the
depolarizes 14.
Thus, separate light beams having same or different wavelengths are generated
by
the plurality of lasers 29. These beams arc guided through the non-
depolarizing optical
fibers 32 and to the mufti-wavelength optical coupler 64 where they ate
combined and
output into the optical fiber 66. The combined beam travels through the
optical fiber 66
to the depolarizes J.4 where the muati-wavelength beam is at least partially
depolarized.
After depolarization, the pump beam proceeds to the gain medium 16 as
described above.
In this manner, a light beam comprising a plurality of same or different laser
wavelengths
-17-
CA 02317668 2000-09-06
can be at least partially depolarized and employed to pump the optical fiber
Ramaa gain
16 medium in the Kaman amplifies' 10. The use of a single depolarizes 1.4 as
shown is
FIGURES 11A and 11B simplifies the Kaman amplifier 10 as compared to the
embodiments depicted in P'IGURF,S 9A, 9B and 10, which include a plurality of
depolarizers. Depolarization, however, may not be as complete unless the
polarization
of each of the sercaieonductor lasers is aligned, e.g., with individual
polarization
transformers.
In accordance with the present invention, the length of the depolarixing
biref~ring~nt optical fiber 36 can be adjusted to alter the degree of
polarization (DOP).
The value of DOP depends on the coherence length of the pump beam and the
optical
path difference between the tight coupled into the fast and slow modes of the
depolarizinng birefiiitgent optical fiber 36. Tlie optical path difference is
determined in
part by the length of the depolarizing birefringent optical fiber 36.
Accordingly, DOP
depends on the length of the depolarizing birefringcnt optical fiber 36. In
particular, the
polarized component decreases with increasing length of the depolarizing
birefringent
optical fiber 36 as shown in lrIGURE 12, which plots the relationship between
the DOP
and the length of the depolarizing birefringent optical fiber. Values for DOP
were
measured at the end of the depolarizing birefringent optical fiber 36
connected to the
optical coupler 40. This plot confirms that the DOP can be controlled by
adjusting the
length of the depolarizing birefringent optical fiber or fibers. It will be
appreciated that
any decrease in the polarization of the beam prior to catering the gain medium
is
advantageous. I-lowever, using the depolarization principles of the present
invention, the
degree of polarization (DOP) of the pump beam is advantageously decreased to
at least
about 40% or less. More prcCerably, the DOP is decreased below approximately
20% .
It has been round that the DOP of the pump beam can be reduced to less than
about 10%
in some embodiments of the invention.
As described above, varying the DOP of the pump beam can control fluctuations
in the optical fiber Kaman gain. The level oC fluctuations in gain is
characterized by the
polarization dependence of the optical fiber Kaman gain (PDG), which is
determined by
measuring the difference between the maximum and minimum value of gain while
changing the state of polarization of the signal being amplified_ Measurwnents
of PDG
-18-
CA 02317668 2000-09-06
quantifies polarization dependent loss of the optical amplifier 10. FIGURE 13
plots the
PDG as the DOP of the pump beam is reduced using a preferred embodiment
described
above. The plot shows that the PDG decreases as the degree of polarization
decreases,
the PDG becoming closer to a value of polarization dependent loss, which in
this case is
equals 0.12d13. Thus, optical pumping of an optical fiber Kaman gain medium 16
with
laser light that has been at least par~ially depolari~.ed light reduces the
fluctuations in the
optical fiber Kaman gain.
Accordingly, employing the depolarizes 14 tin the fiber optical Kaman
amplifier
enables the polarization dependent gain fluctuations to be reduced. Stable
gain is
10 possble while using a single scrniconductor laser 29 to pump the optical
fiber Kaman
gain medium 16. The laser output need not be combined with light from a second
source.
Tbc complexity of the Kaman amplifier 10 is thus reduced as less semiconductor
laser
devices are required to optically pump the optical fiber Kaman gain medium 16.
As
Illustrated in FIGURES 2-11, this Kaman amplifier 10 can operate with or
without the
inclusion of the fiber Bragg grating 34. Irlowever, optical pumping with light
having a
narrow wavelength distribution is advantageously provided by employing the
$ber l3ragg
grating 34.
Although a plurality of Kaman amplifiers 10 having diuff~rent schemes for
depolarising the pump beam are shown in FIGURES 2-19., other depolarizers 14,
such as
other LYOT type depolarizcrs as well as Cornu type depolarixers can be
employed in
accordance with the invention to produce an at least partly depolarized pump
beam.
,A,ccordingly, the depolarizes 14 may comprise birefxingent components other
than
birefringent i~'bcr such as birefringcnt crystal. Nevertheless, fiber
dopolari~.ers hke the
LYOT bbcr depolarizes are preferred for integration into a fiber optic
communication
system 2. Additionally, other components within the optical amplifier 20 may
comprise
optical fiber, optical integratc,d waveguide devices, or both. For example,
any of the
optical couplers (optical coupler 40, optical distributor Sg, beam combines
60, multi-
wavelength optical coupler 64) may be fiber or integrated optic waveguide
devices or
combinations thereof.
Furthermore, as described above, the sZmiconductor laser light sources 29
output
substantially linearly polarized light, which can be at least partially
depolarized so as to
-19-
CA 02317668 2000-09-06
avoid variation in gain provide by the amplifier 10. The usefulness of the
depolarizes 14,
howevex, is not so limited, that is, the methods descr~bc herein can be
employed for light
sources that output non-linearly polarised light. For example, cnrcularly or
elliptically
polarized light can be at Least partially depolarized, e.g., by coupling this
light into a
birefringent optical fiber, so as to minimize fluctuations in amplification
provided by the
Raman gain medium 16.
The present invention may be embodied in other specific forms without
departing
from the essential characteristics as descn'bed herein. The embodiments
descnbed above
arc to be considered in all respects as illustrative only,and not restrictive
inn any manaer_
As is also stated above, it should be noted that the use of particular t~loBY
~~
desenbing certain features or aspects of the invention should not be taken to
imply that the
terlnmology is being re-defined herein to be restricted to inelu~ng any speck
characteristics
of the features or aspects of the invention with which that ternunology is
associated. 1'he
scope of any invention is, therefore, 'indicated by the following claims
rather than the
foregoing description. Any and all changes wliich come within the meaning and
range of
equivalency of the claims are to be considered in their scope.
_20_
