Note: Descriptions are shown in the official language in which they were submitted.
1N0 93/22855
PCT/G B93/00863
1
Optical clock recovery
FIELD OF THE INVENTION
The present invention relates to the recovery of a
clock signal from a data stream, and in particular to an
a11-optical system for carrying out such a function. The
invention also encompasses a novel mode-locked laser.
In this specification the term "optical" is intended
to refer to that part of the electromagnetic spectrum which
is generally known as the visible region together with
those parts of the infra-red and ultraviolet regions at
each end of the visible region which are capable for
example of being transmitted by dielectric optical
waveguides such as optical fibres.
BACKGROUND ART
Clock or timing recovery circuits have the purpose of
extracting a timing wave at the symbol rate from a
conditioned signal pulse stream. Once recovered, such a
timing wave might then be used, for example, to clock
subsequent signal processing stages for the pulse stream.
The present invention is particularly concerned with the
case where the original data stream is in the form of a
modulated optical signal, as might be found, for example,
in an optical telecommunications system. Hitherto, in
order to recover a clock signal from such an optical data
stream the optical signal has been converted from the
optical to the electronic domain using e.g. a
photodetector, and the electronic signal subsequently
filtered to derive the timing wave. Since the bandwidth of
the electronic circuitry is inevitably far smaller than
that of the optical circuits, this clock recovery stage
acts as a bottle-neck, limiting the performance of the
system of which it forms a part.
WO 93/22855 PCT/GB93/0086'i-
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SUMMARY OF THE INVENTION
According to a first aspect of the present invention,
there is provided a system for recovery of a clock from an
optically encoded signal comprising:
a mode-locked laser;
a modulator in the optical path of the optical cavity
of the mode-locked laser;
means for applying an optically encoded input signal to the
modulator; and
means for outputting an optical pulse stream from the
mode-locked laser;
the modulator in response to the optically encoded
signal modulating the phase and/or amplitude of light in
the optical path the laser cavity thereby locking the phase
and frequency of the output pulse stream to the timing wave
of the optically encoded signal.
The present invention provides an a11-optical clock
recovery system, suitable for use with an RZ pulse stream.
A laser is mode-locked via a modulator driven by the
optical stream of data. This results in the generation in
the laser of a pulse stream corresponding to the timing
wave of the optical signal applied to the modulator. This
a11-optical system can operate at data rates as high as 100
GHz, far above the rates attainable with conventional
electronic clock recovery circuits. Moreover the resulting
pulse stream can be employed directly in further optical
processing applications, without requiring an additional
electro-optical conversion stage. It is suitable for use
therefore with optical multiplexing, regeneration and
memory access circuits.
Preferably the output pulse stream is used to clock a
subsequent optical processing stage for the optically
encoded signal.
Preferably the modulator is a non-linear optical
modulator (NOM) connected in common with an optical
transmission path for the optically encoded signal and with
the laser cavity.
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3
Preferably, the laser cavity includes a passive signal
shaping element.
The inclusion of a passive shaping element in the
cavity makes the output pulse stream more robust, and less
susceptible to variations in the modulating data.
Preferably the modulator is arranged as a cross-phase
modulator (XPM).
Preferably the mode-locked laser is a fibre laser.
The use of a fibre laser is particularly advantageous in
facilitating integration of the system with optical fibre
based transmission systems. Preferably the active medium
of the mode-locked laser serves as part of the optical
transmission path for the optically encoded signal, thereby
amplifying the signal.
The configuration adopted for this preferred aspect of
the invention, using a fibre laser with a part of the laser
cavity serving also as a transmission path for a modulating
signal, offers significant advantages over conventional
mode-locked lasers and is not limited in application to the
system for clock recovery of the first aspect of the
invention.
A clock recovery circuit in accordance with the first
aspect of the invention can advantageously form the basis
of an a11-optical signal regenerator, capable of operating
at high data rates.
According to a second aspect of the present invention,
there is provided a regenerator for an optical signal
comprising, in combination, a clock recovery system in
accordance with the first aspect of the present invention,
and a modulator circuit connected to the output of the
clock recovery system and to a source of a data carrying
signal and arranged to produce a modulated signal
substantially aligned to the timing of the clock signal.
Preferably he modulator circuit comprises:
a first optical signal path carrying the optical
signal;
WO 93/22855 ~ ~ ~ ~ ~~ ~ ~ PCT/GB93/00863
4
a second optical signal path carrying the clock signal
output from the clock recovery system; and
a non-linear optical medium connected in common to the
first and second optical signal paths, the clock signal
pumping the medium thereby cross-phase modulating the
optical signal and substantially aligning the timing of the
optical signal to the clock signal.
Preferably, the non-linear optical medium in the
modulator circuit is a fibre modulator. Preferably, the
fibre modulator is connected to the first and second
optical signal paths in a loop mirror configuration.
The use of a loop mirror configuration in a clock
recovery circuit, or in a regenerator stage following a
clock recovery circuit, is particularly advantageous in
that it provides a circuit with a high degree of immunity
to the effects of vibration or other physical disturbance.
In the loop mirror interference between signals travelling
around the loop in opposite directions converts the phase
shifts into amplitude modulation.
According to a third aspect of the present invention,
there is provided a mode-locked laser system comprising:
a fibre laser;
means for pumping the fibre laser;
a fibre modulator connected in the path of the optical
cavity of the fibre laser and connected in common with an
optical transmission path for a driving signal; and
means for outputting an optical pulse stream from the
laser;
the driving signal modulating the phase and/or
amplitude of light in the optical path of the laser cavity,
thereby mode-locking the laser.
According to a fourth aspect of the present invention,
there is provided a method of recovering a clock from an
optically encoded signal, including the step of driving a
modulator in a mode-locked laser with the optically encoded
signal thereby generating in the laser a pulse train locked
"~O 93/22855 ~ ~' '~~ ~ PCT/GB93/00863
in phase and frequency to the timing wave of the optically
encoded signal.
DESCRIPTION OF THE DRAWINGS
5 Embodiments of a system for optical clock recovery in
accordance with the present invention will now be described
in detail, by way of example only, with reference to the
accompanying drawings, in which:-
Figure 1 is a diagram illustrating a clock recovery
circuit;
Figures 2A and 2B show the time domain picture and
corresponding power spectrum for a periodic train of
impulses and a random train of impulses respectively;
Figures 3A and 38 show two alternative examples of
a11-optical clock recovery circuits;
Figure 4 is a graph illustrating the evolution of the
laser from a DC field input for a periodic pump pulse train
and for a random data pulse train;
Figure 5 is a delay characteristic for a fibre
modulator;
Figure 6 is a diagram showing schematically a
wavelength-selective coupler;
Figure 7 is a diagram showing an alternative linear
laser cavity circuit;
Figure 8 is a diagram showing a recovery circuit
implemented as a semiconductor device;
Figure 9 is an a11-optical regenerator circuit;
Figure 10 is a frequency-time plot illustrating the
shift in timing effected by the circuit of Figure 11; and
Figure lI is an alternative a11-optical regenerator
circuit.
DETAILED DESCRIPTION OF EXAMPLES
Figure 1 shows the basic configuration of one example
of a system embodying the present invention. An optical
signal encoded with data in the form of an RZ pulse stream
is carried through the system in a transmission fibre 1.
WO 93/22855 '~ '~ ~' ~ PCT/GB9310086'~-
~ ,.w
6
A non-linear optical modulator (NOM) 2 is connected in the
path of the signal in the transmission fibre. The NOM 2 is
also connected in the optical path of the laser cavity 3 of
a mode-locked laser 4. The laser generates a pulse train
locked to the timing waveform of the input data. The pulse
train is output via an output coupler 5.
Figure 3A shows in detail a first example of-a clock
recovery circuit. In this example, the mode-locked laser
is a fibre laser 6, incorporating a fibre modulator 8 in
the optical cavity. The fibre modulator 8 is connected in
common with the optical cavity and the transmission system
associated with the circuit. It comprises a single-mode
optical fibre, which, in the present example, has a length
of 8.8 km. A suitable fibre is available commercially
under the trade name SMF/DS CPC3 from Corning Inc., a US
corporation of Corning, New York 14831. This is a
dispersion-shifted fibre designed to operate in the 1550 nm
region. It has a mode field diameter of 8.1 microns, a
cladding diameter of 125 microns and a coating outside
diameter of 250 microns. The effective group index of
refraction is 1.476 at 1550 nm. The dispersion
characteristics are shown in Figure 5. They are such that
the dispersion minimum falls between the wavelengths of the
data and of the laser cavity.
The length needed for the fibre modulator is
determined by the required phase shift. In general, the
phase shift need not be as big as ~r. The phase shift is
proportional to the product of the peak power and the
interaction length, so that there is a trade-off between
these two. The selected length of 8.8 km makes it possible
for the modulator to function at peak powers less than 10
mW, e.g. 5 mW. If the power is increased to 100 mW, then
the required length comes down by a corresponding factor of
100. In general, for passive mode-locking of the laser
cavity by the data stream to be effective, a phase shift of
0.3 radians in the fibre modulator is sufficient.
r
--CVO 93/22855 ~ ~ ~ PCT/GB93/00863
7
Wavelength-selective couplers WDM1 and WDM2 couple the
ends of the fibre modulator 8 to the transmission system
and to the rest of the laser cavity.
Each of the couplers WDM1, WDM2 is a bi-directional
device incorporating an interference f filter f ormed as an
evaporation-deposited stacked dielectric. One wavelength
passes straight through the filter while another wavelength
is reflected. Figure 6 is a diagram showing schematically
the input and output channels. Channel 1 is a channel in
a wavelength range from 1.535 to 1.541 microns. Channel 2
is a channel at 1.555 to 1.561 microns. Channel 3 is an
output channel common to both wavelength ranges. An
appropriate device having these properties is available
commercially from the company JDS FITEL as WDM coupler
model number WD1515Y-A1. A similarly constructed
wavelength-selective coupler WDM3 is used to couple the
laser diode pump to the fibre laser 6. This has Channel 1
transmitting an input at 1.468 to 1.496 microns and Channel
2 transmitting at 1.525 to 1.580 microns. This WDM coupler
is available commercially as WD141F-M2 also from JDS FITEL.
In operation, different wavelengths corresponding to
the different channels of the couplers WDM1 and WDM2 are
chosen for the laser and for the data. The two couplers
(WDM1, WDM2) then allow the data stream to pass straight
through the clock recovery circuit via the common fibre,
while permitting the laser wavelength, which is defined by
the spectral filter F, to be coupled around the simple ring
laser configuration. The filter F in this example is an
interference filter. The peak wavelength and transmission
peak width of the filter can be varied by altering the
angle of the filter element with respect to the incident
beam. The bandwidth of the filter F may be used to control
the laser pulse width.
The fibre laser 6 is based on an erbium-doped fibre
(operating range 1.52-1.6 um) pumped by a high-power laser
diode (LD). Both signal and laser wavelengths in this
example lie within the erbium fibre gain window as in
~, ~,,',
WO 93/22855 c~~:w; ~ PCT/GB93/00863
8
currently proposed a11-optical non-linear fibre loop
(de)multiplexing schemes (Nelson, B.P., Blow, K.J.,
Constantine, P.D., Doran, N.J., Lucek, J.K., Marshall,
I.W., and Smith, K.: "A11-optical Gbit/s switching using
nonlinear optical loop mirror", Electron. Lett., 27, p. 704
( 199l) ) . The erbium f fibre laser employs an AL203-GeOz-Si02
host with an erbium doping level of 200-300 ppm, a core
diameter of 5.4 ~cm and an index difference of 0.01. A
suitable erbium fibre laser is described in Smith, K.,
Greer, E.J., Wyatt, R., Wheatley, P., Doran, N.J., and
Lawrence, M.: "Totally Integrated Erbium Fibre Soliton
Laser Pumped by a Laser Diode", Electron. Lett., 1991, 27,
p.244.
The laser is pumped by a laser diode 7. This may be
a GRINSCH InGaAsP device as described in the above cited
paper by Smith et al.
The circuit includes a fibre stretcher FS which is
used to facilitate fine adjustment of the cavity length and
to ensure that the cavity round-trip time is matched to an
exact multiple of the data rate. The fibre stretcher may
comprise a pair of cylindrical drums around which part of
the fibre forming the optical cavity is wound. The drums
may then be driven apart, for example by a piezo-electric
element, to stretch the fibres. Alternatively, the fibre
may be wound round a single expansible cylinder of piezo-
electric material. The cylinder then expands in response
to an applied control signal in order to stretch the fibre.
The fibre stretcher may be used in a feedback loop to
stabilize the mode-locked laser. Part of the signal in the
laser loop is split and fed to a phase-sensitive detector
which compares the phase of the signal with a fixed
reference signal. The resulting error signal is fed back
to the piezo-electric element of the fibre stretcher so as
to correct any variations. Since both the fibre modulator
and the circuit as a whole are temperature sensitive, the
stability of this circuit may be further optimized by
placing the entire circuit within a sealed thermally-
> ~. ~;
-CVO 93/22855 PCf/GB93/00863
9
insulated housing. Thermostatic control of the temperature
of the circuit environment may also be provided where
necessary.
The cavity incorporates an intra-cavity ffibre isolator
I which ensures that the cavity functions unidirectionally
and allows the full laser power to be accessed in a single
stable pulse train via the output coupler 5. This coupler
is a fused fibre device available commercially from BT & D
Technologies, model number SMC0202-155-OC. The fibre
isolator I is available commercially from BT & D
Technologies with model number OIC-1100-1550.
A further example of a clock recovery circuit is shown
in Figure 3B which is similar to the first circuit but
utilises a two-wavelength non-linear loop mirror to act as
an amplitude modulator for the laser cavity. In this case,
the shared fibre (again between WDM1 and WDM2) forms part
of the non-linear loop and hence the phase modulation is
converted to amplitude modulation.
The circuit configurations described above, as well as
being particularly suitable for clock recovery, can also be
used more generally for applications requiring a mode
locked laser. In place of the input data stream a
dedicated driving signal may be generated and input to the
fibre modulator. The frequency of the driving signal is
chosen in accordance with the characteristics of the laser
cavity and the desired output pulse stream.
Although the circuit configuration and components
described above are preferred for their ease of integration
with current and proposed fibre transmission systems, other
components and configurations are possible as alternatives.
As an alternative to an erbium-doped fibre laser, a
praesodynium doped fibre laser may be used. Such a laser
operates in the region of 1.3 microns. Accordingly, a
circuit using such a laser~may be advantageous where it is
desired to translate pulses from the 1.5 micron
transmission window to the 1.3 micron region. To this end,
such a circuit might be used in combination with an all-
WO 93/22855 w~ '~, ~ PCT/GB93/00863
optical regenerator as described further below. One
possible modification to the Figure 3A circuit is the
addition of a Fabry-Perot filter FP, shown in dashed lines.
This provides a narrow-band filter tuned to stop the laser
5 jumping in wavelength.
As an alternative to the use of wavelength-selective
devices for the couplers, the data signal and the pump
signal may be distinguished by different polarisation
states in which case polarisation splitters PDM are used in
10 place of the wavelength-selective devices. An appropriate
fibre modulator could be formed using a birefringent fibre.
This is a fibre including stress centres to give an
anisotropic refractive index. If light polarised parallel
to one of the fibre axes is launched along that axis, then
it maintains its polarisation state. Such fibres typically
introduce a delay of 1 ps/m and so over a long length this
would result in separation of the data and the laser pulse
stream. Accordingly, where long fibre lengths are used,
the overall length may be constructed from two shorter
lengths coupled end to end, with a relative shift in the
orientation of the fibres so as to cancel out the effects
of the delay. For shorter lengths, e.g. 10 metres or less,
this is unnecessary and a single length of birefringent
fibre may be used to form the fibre modulator.
The mode-locked laser need not be a ring laser as in
the circuits previously described, but may alternatively
use a linear laser cavity as shown in Figure 7. Such a
cavity would typically be formed between two termination
mirrors M1,M2 or between a termination mirror and a
ref lective output coupler/ f filter . Such a ref lective f filter
may be based on a fibre grating produced by optical writing
into a photo-sensitive fibre. This grating would then
combine the functions of the output coupler FC and the
filter F described in the above circuit. In the Figure, P
is the pump, C1 and C2 fibre couplers and Er the erbium
laser.
n ~'~~~~~ .
The modulator need not be a fibre modulator of the
type described above, and in principle any suitable non-
linear material may be used. Particularly appropriate may
be a capillary waveguide filled with an organic non-linear
liquid such as CS2. Such a device gives a non-linearity as
much as 100 times greater than a fibre modulator, although
at the cost of a greater response time, typically of the
order of 1 ps.
Further alternatives for the modulator are provided by
semiconductor doped glasses, semiconductor device
waveguides and semiconductor laser amplifiers operating at
their transparency point (i.e. with gain = 1). These are
found to be particularly fast and have a response time less
than 1 ps.
As may be seen in Figure 8, the entire clock recovery
circuit may be implemented in an integrated semiconductor
device including a saturable absorber 9 which is pumped by
the optical signal, and an associated semiconductor laser
gain medium 10. The gain medium is formed in a channel in
a semiconductor substrate 11. A Bragg filter 12 would then
be formed in the semiconductor structure to tune the device
to a required wavelength. An appropriate structure is
shown schematically in Figure 8.
Irrespective of the particular components or
configuration chosen, the method utilises the incoming data
stream to drive or "pump" a mode-locked laser, which in
turn generates short, picosecond duration pulse trains at
the base rate (or an exact multiple) of the data. The data
serves to modulate either the amplitude or phase of the
light in the laser cavity. Providing this modulation takes
place with a period (T) equal to (or an integer multiple
of) the laser round-trip time, mode-locking of the laser
follows (Siegman, A.E.: Chapter 7 of "Lasers", University
Science Books, Mill Valley, California, (1986) and
references therein). The ensuing stream of pulses (at a
repetition rate 1/T) can be accessed via the output
coupler.
w:
WO 93/22855 . (~~~ PCT/GB93/00863_
"~,~
12
The mode-locking laser can conveniently be analysed as
follows. For a linear laser cavity of length, L, the
adjacent axial modes axe spaced by (c/2nL) where c is the
velocity of light in free space and n is the refractive
index of the cavity medium. (For a ring laser the axial
mode spacing is c/nL.) These modes oscillate independently
with randomly varying phases and amplitudes and
consequently the output of the laser will vary in an
uncontrolled manner. In the case of mode-locking, a
modulator (either phase or amplitude) is placed within the
laser cavity and driven at the axial mode frequency
separation. A periodic modulation (period, T) gives rise
to a set of frequency sidebands separated by 1/T. This is
illustrated in Figure 2A where we show a period train of
impulses in time (specifically a periodic array of delta
functions, y(t) = Ed(t-nT)) and the corresponding frequency
power spectrum. If the 1/T frequencies coincide with the
axial laser modes, then these modes are forced to have a
definite phase relationship with each other. In this case,
the output of the mode-locked laser has the following
characteristics:
(i) The pulse repetition frequency is c/2nL for a
linear cavity (c/nL for a ring).
(ii) For N locked oscillating modes of equal
amplitude the width of the pulses is given by 1/Du, where
t1v is oscillation linewidth of the laser.
(iii) The peak power is N times the average laser
power without mode-locking.
In practice, for the circuits using fibre lasers we
employ "harmonic mode-locking". The idea behind this is
essentially as described above, but now the driving
frequency is tuned to a high (Nth) harmonic of the cavity
frequency i.e. 1/T - N x (c/2nL). The laser modes
therefore become organised into N more or less independent
sets of coupled modes. This method,is commonly applied to
fibre lasers since long overall cavity lengths (typically
tens of metres) are required to obtain sufficient gain for
.. ,WO 93/22855 ~ ~ ~ ~~~ ~ PCT/GB93/00863
13
laser action and, therefore, the associated fundamental
frequencies are only -MHz.
The modulating signal can be considered to be a random
train of impulses such as those depicted in Figure 2B. The
data sequence is not completely random since the pulse
arrival times are defined by T. In this case, the pulse
train can be represented by y(t)=Ea~d(t-nT) where a~ is
either 1 or 0. The corresponding power spectrum can be
calculated (Kanefsky, M.: Chapter 2 of "Communication
Techniques for Digital and Analog Signals", Harper & Row
(1985)) and is also shown in Figure 2B. It is similar to
that of the periodic impulse sequence in Figure 2A but with
the addition of a white noise component. As we can see,
providing that we ensure that the axial mode-spacing
coincides with the 1/T data rate, the periodic modulation
can serve to couple the cavity modes and thereby mode-lock
the laser. We also note that by tuning the laser length
such that the 1/T data period coincides with sub-multiples
of the axial mode spacing, we can force the laser to
oscillate at integer multiples of the data rate.
The basic mode-locking mechanism for the systems
described here is that of XPM. This effect occurs because
the effective refractive index of a wave depends not only
on the intensity of that wave but also on the intensity of
other copropagating waves. We imagine that the data
stream, at a wavelength ~S, modifies the refractive index
periodically for the second (laser) wavelength at ~~. We
can easily estimate the magnitude of this effect. If we
assume that the group delays of the two wavelengths are
closely matched, then the phase change, 0~, imposed by the
pump wavelength upon the "weak" signal can be calculated
from the following:
4~r n~PL
3 5 e~
A ~r
WO 93/22855 ~ PCT/GB93/00863
14
where nz is the non-linear (Kerry coefficient (=3.2x102°
mZ/W) , Aeff iS the effective fibre core area, ~ is the
wavelength, P is the peak power of the optical "pump"pulse,
and L is the length of the shared fibre link. As an
example, taking an effective area of 50 um2, L - 1 km, and
a wavelength of 1.55 um, we can calculate a peak power, P
600 mW for ~~ = n. In terms of a non-linear loop mirror
modulator, this gives rise to a 100% amplitude modulation.
In general, however, this strength of modulation is well in
excess (by approximately an order of magnitude) of that
required for good, stable mode-locking (Siegman, A.E.:
Chapter 7 of "Lasers", University Science Books, Mill
Valley, California (1986) and references therein). We can
therefore infer that peak powers -60 mW should be
sufficient. The above equation applies to a pump and
signal with the same linear polarization state. In the
real world, the peak power estimate may be increased by a
factor - 1.5 by the evolution of the relative pump and
signal polarizations. These peak powers can readily be
achieved by incorporation of fibre amplifiers within the
proposed optical circuit. If the link length were to
increase to, say, 5 km then soliton pulses in a11-optical
systems may perform the modulation directly (e.g. for 25 ps
duration pulses in fibre with a dispersion constant D of
around +1 ps/nm.km fibre, the path average power for the
fundamental soliton, P~--5 mW) .
An important characteristic of the clock recovery
circuit is the time required for the circuit to respond to
the incoming data stream - the "lock-up" time. In the case
of a mode-locked laser, this will be determined by the
number of round-trips required to establish the mode-locked
train, together with the round-trip time of the cavity. We
can estimate the number of round-trips required by
employing the transient analysis of active mode-locking
described by Kuizenga and Siegma-n. According to their
analysis, the number of round-trips, NSS, required to form
~~ 'CVO 93/2285S I ~ ,~ ~ ~ PCT/GB93/00863
a pulse width within -20% of the final steady state value
is given by
NSS - ~8 Go ~}-t/2 (Ofa/fm]
5
where Go is the steady state gain coefficient, ~ is the
depth of modulation, fife is the cavity bandwidth and fm is
the modulation frequency. Taking typical values for our
laser conf iguration of Go - 1. 5 , Gm - 2 ( Of a - 10o GHz ( -1
10 nm) and fm - 2 GHz we estimate NSS to be -10. Therefore,
for a cavity length of 1 km, the lock-up time would appear
to be -50 his. By employing shorter cavity lengths, say
-100 m, this time is reduced to a few us. These estimates
have been verified by some simple numerical computer
15 simulations. We consider the idealised case in which the
laser is initially oscillating in only a single axial mode
at the time the amplitude modulator is first turned on.
The cavity also contains a simple spectral filter which
serves to limit the final pulse duration. The results are
plotted in Figure 4 where we show the evolution of the
pulse width (arbitrary units) vs. round-trip number for
both a truly periodic pumping sequence (full curve) and a
"random" sequence (dashed curve). The full curve shows
that the pulse width is close to its final value within -10
round-trips which is in good agreement with the predictions
of Kuizenga and Siegman. In the case of the dashed curve
we use the same laser parameters as for the full curve, but
on every cavity round-trip we use a random number generator
to select whether the modulator is "on" (1) or "off" (0).
We can see that a "1" serves to compress the mode-locked
pulse, while a "0" leads to a pulse broadening due to the
sole action of the filter. According to these simple
computer simulations, the effect of using a random data
sequence mode-lock on the laser is two-fold: Firstly,
there is a mechanism for laser pulse width variations, and
secondly, the resultant average pulse width generated is
somewhat broader than the truly periodic modulation case.
WO 93/22855 ~ ~Y~ ~ ,~~~ PCT/GB93/00863
16
It appears, however, that the circuit lock-up time with a
"random" pump is comparable to that of the simple mode-
locked laser. It is worth noting that the actual number of
cavity round-trips required to form the mode-locked pulse
is relatively small (only -10's). As a consequence, in
terms of the lock-up time, advantages are gained by using
short cavity lengths. For fibre lasers, the use of highly
doped fibres facilitates cavity lengths -10's of cm with
associated nanosecond lock-up times. In the case of an
Erbium laser, for example, doping levels might be increased
to around 4000 ppm, an order of magnitude greater than the
200-300 ppm doping levels conventionally adopted. To
accomplish this we would also require highly non-linear
shared links and fibre modulators. Potential candidates
for these links are semiconductor doped fibres or
semiconductor laser amplifiers. A suitable amplifier is
described in the paper by C.T. Hultgren and E.P. Ippen:
"Ultrafast refractive index dynamics in AlGaAs diode laser
amplifiers", Appl. Phys. Lett. 59(6), 5.8.91. Such an
amplifier would combine the functions of the fibre
modulator and the laser in the circuits of Figure 3.
A further feature of the circuits is that changes in
the frequency of the incoming data signal can be tracked by
adjustment of the laser cavity length. The circuits
depicted in Figure 3, as noted above, incorporate fibre
stretchers from which it is reasonable to obtain length
changes, DL - 10 cm. We would therefore expect a
corresponding change in the axial mode-spacing, Of (= c
OL/nL2), of -1.5 kHz for a 100 m long cavity.
Another consideration is the response of the circuit
to a temporal fitter on the incoming signal. In terms of
a long-distance soliton system, this so called Cordon-Haus
fitter (Cordon, J.P., and Haus, H.A.: "Random walk of
coherently amplified solitons in optical fibre
transmission", Opt. Lett., il, p. 665, (1986)) is the
result of amplified spontaneous emission from the fibre
amplifiers. Since the switching window of the a11-optical
W0 93/22855 ~ ~ ~ ~ r~ ~ ~ PCT/GB93/00863
17
modulator is determined by both the data pulse width and
the group delay difference between the signal and laser
wavelength, the pulse walk-through to some extent cancels
out the effects of the timing fitter. As discussed further
below, this effect may be exploited to enable the circuit
to function as an optical regenerator.
As discussed above in relation to Figure 4, the shape
of the pulses output from the clock recovery circuit shows
some dependence on the modulating data stream. In use, the
fibre modulator tends to sharpen the pulses in the laser
cavity, while the filter tends to broaden them. If the
data stream contains a stream of zeros then during that
time the modulator is not effective to sharpen the pulses
and so the width of the pulses tends to increase. In a
modification to the Figure 3A circuits shown in dashed
lines, a passive shaping element PSE is added to the laser
cavity. This is an element which tends to sharpen the
pulses, even in the absence of a modulating signal. The
fibre modulator itself may provide the required passive
shaping if it has non-linearities chosen to be soliton
supporting at the optical powers at which the circuit
operates. For the fibre modulator employed in the circuit
of Figure 3A, this is the case when the power in the laser
cavity is greater than approximately 100 mW. If these non-
linearities are further increased then the circuit becomes
passively mode-locked, i.e. it generates output pulses even
in the absence of a modulating data stream, as opposed to
being passively mode-locked as in the unmodified circuits
of Figure 3. As an alternative to using the non-
linearities of the fibre modulator, the passive shaping
element may be an additional component formed, for example,
from a non-linear semiconductor waveguide.
A clock recovery circuit in accordance with the
present invention may form the basis of an a11 optical
regenerator. Such a circuit is shown in Figure 9. The
first stage of the circuit, referenced A in the diagram, is
the clock recovery circuit of Figure 3a. The following
J WO 93/22855 PCT/GB93/00863
18
stage, referenced B, uses a configuration similar to that
adopted for the circuit of Figure 3b and like that circuit
uses a nonlinear loop mirror fibre modulator LM. This
modulator uses, in the present example, 6.5 km of the same
type of dispersion shifted fibre as the clock recovery
circuit. As in the clock recovery circuit, the loop mirror
fibre modulator is used for cross phase modulation of two
signals. The first signal is the data carrying -signal
output from the clock recovery stage. This signal is first
amplified in an erbium doped fibre amplifier EDFA before
being input to the loop mirror modulator LM via a
wavelength selective coupler WDM38. The other signal is
the clock output from stage A. This passes first through
an isolator and a fibre stretcher and is then input to the
loop mirror modulator via a 50:50 fibre coupler FC and the
two wavelength selective couplers WDM3H, WDM4H. Stage H
incorporates polarisation controllers PC.
In this circuit stage B modulates the clock pulse
train using the data stream in the non-linear loop mirror,
so that the clock pulses become the regenerated pulse
stream. Typically, the clock and the data carrying signal
are at different wavelengths, 1.54 and 1.56 microns in the
present example, and so this form of regeneration is
particularly advantageous where translation of the signal
between two wavelengths is required.
In a second example of a regeneration circuit, the
clock recovery stage is again constructed as shown in stage
A of Figure 9. Stage B however is formed as shown in
Figure 11. A modulator NOM, which may again be a fibre
loop modulator, or which may alternatively be a simple
length of ffibre, is configured so that cross phase
modulation of the data carrying signal by the clock signal
peturbs the frequency of the data, in the manner described
below, so that subsequent dispersion in the transmission
fibre retimes the data. Neglecting the peturbation itself,
the retimed data is at the same frequency as the input data
and so this circuit is appropriate where a "straight
~'O 93/22855 PCT/GB93/00863.
,~ ~ ~k ~ ,~ ;
19
through" operation, without translation between two
wavelength ranges, is required.
As already discussed above in relation to clock
recovery, the interaction of the two signals in the fibre
modulator results in a phase shift 6~ in the data carrying
signal, and a corresponding frequency shift dv. The
magnitude of the frequency shift is proportional to the
rate of change of phase. The frequency shifts imposed on
the signal are then converted into appropriate changes in
timing by dispersion in the transmission fibre following
the regeneration circuit. If a signal pulse arrives ahead
of the centre of the time slot, then the pulse receives a
net negative frequency shift. Provided that the
transmission fibre is anomalously dispersive at the signal
wavelength, i.e. a positive group delay dispersion, the
down-shifted pulse is slowed down and hence moves towards
the centre of the time slot. Conversely, a pulse arriving
late in the time window acquires a net positive frequency
shift and is speeded up. The frequency shift is shown in
Figure 10, with the time shifted data shown in dashed
lines. The magnitude of the frequency shift is readily
controlled by adjustment of the clock pulse power. For a
typical phase response time of 20 ps and a peak phase shif t
of n peak frequency shifts around +/-25 GHz are obtained.
Random frequency shifts in a soliton carrying transmission
system arising from the Gordon-Haus effect are usually
considerably smaller, typically of the order of a few MHz
acquired over e.g. 5000 km of fibre path. The retiming
provided by the optical regenerator is therefore more than
adequate to compensate for such effects.
Other circuit configurations and different components
may be used to realise the a11 optical regenerator. In
particular, the various alternatives to the use of a loop
fibre modulator discussed above in relation to the clock
recovery circuit may also be adopted for stage B of a
regenerator circuit.
WO 93/22855 ~ ~~~PCT/GB93/00863
As a further alternative approach to a11-optical
signal regeneration, stage A, that is the clock recovery
circuit, may itself be used to produce directly the
frequency shift required to retime the data. In normal
5 operation, the clock pulses generated in the optical cavity
will in turn cross-phase-modulate the data stream,
producing a phase and frequency shift. In the power
regimes previously discussed this effect is generally
negligible, but if the laser power is increased to be as
10 great as the power of the data carrying signal, then the
magnitude of the frequency shift experienced by the data as
it passes through the fibre modulator is suf f iciently great
to produce the required retiming when the signal
subsequently undergoes dispersion in the transmission
15 fibre. There is then no need for an additional stage such
as stage B to effect signal regeneration. In this
configuration the output pulse stream is the retimed data,
and it is not necessary to output the pulse stream from the
laser cavity.
