Dark pulse generation

An optical pulse generator comprises a resonant source of optical radiation, a modulator, and a source of modulating signals, wherein the modulator is operative to mode lock the resonant source and phase modulate the optical radiation in accordance with the modulating signals in such a manner as to produce dark pulses in the optical radiation.

FIELD OF THE INVENTION 
This invention relates to an optical pulse generator for generating dark 
pulses such as dark soliton pulses. 
BACKGROUND 
An optical pulse is usually considered to comprise a burst of optical 
carrier radiation with a given modulation envelope shape. When the pulse 
has a particular initial envelope shape e.g. U(t)-N sech(t), where N is an 
integer, the pulse can be transmitted as a soliton in an optical fibre. 
For such particular envelope shapes, the wavelength dispersion produced in 
the pulse by the fibre, or so-called chirp, is counterbalanced by the 
fibre's non-linear dependence of refractive index on amplitude, which 
produces a self phase modulation (SPM by which the phase of the pulse is 
modulated by its own intensity. This counterbalance results in a self 
maintaining pulse or soliton, which tends to maintain its envelope shape 
with time as it is transmitted along the fibre. The non dispersive nature 
of solitons makes them attractive for data transmission through optical 
fibres over long distances. 
A pulse having the characteristics just described is known as a bright 
pulse. It is also possible to generate so called dark pulses such as dark 
solitons, which occur when an essentially continuous burst of optical 
radiation contains temporal gaps or regions of reduced intensity 
radiation. Such gaps are known as dark pulses. It can be shown that for 
the particular case of solitons, dark solitons may have a general envelope 
shape given by U(t)-N tanh(t), where N is an integer. For a fuller 
discussion, reference is directed to Nonlinear propagation effect in 
optical fibres: numerical studies--K. J. Blow & N. J. Doran Chapter 4, 
Optical Solitons--Theory and Experiment, edited by J. R. Taylor, Cambrigde 
University Press 1992. 
As used herein, the term dark pulse includes both a black pulse in which 
the intensity drops to zero a grey pulse in which the intensity drops only 
partially towards zero. 
Dark solitons have been produce experimentally for example as described on 
pages 394-396 of "Optical Solitons-Theory Experiment" Supra. In this 
arrangement, pulses from a dye laser have their frequency components 
spatially dispersed by means of a grating and then individually weighted 
by means of a mask. The resultant weighted amplitude components are then 
recombined by another grating. The pulse is accordingly imparted with a 
desired temporal profile according to a fourier transform of the desired 
pulse shape. Using this technique, dark pulses closely resembling the 
expected black and grey solitons have been generated. 
However, a problem with this prior arrangement is that the fourier 
transform performed by mask and grating is not readily controlable. 
The use of a modulator, responsive to input modulating signals, to produce 
bright solitons is described in D. M. Pataca et al: "Actively Mode-locked 
Pr.sup.3+ -doped fluoride fibre laser" Electronics Letters 9th Jun. 1994, 
Vol. 30, No. 12, pp. 964-5. In this arrangement, the praseodymium 
(Pr.sup.3+)-doped fibre is included in a resonant cavity pumped by a 
Nd:YAG pump laser. The cavity is defined by a semi-reflective mirror at 
one end of the fibre and a filly reflective mirror at the other. The 
cavity also includes a electro-optical modulator. In use, the modulator is 
driven by a sinusoidal waveform which produces positive and negative 
sinusoidal variations in the refractive index of the modulator, so as to 
phase modulate light resonating in the cavity. If the period of the 
modulation is selected to correspond to the transit time for light 
resonating in the cavity, the cavity is said to be mode-locked. The 
sinusoidal phase modulation produced by the modulator causes positive and 
negative going chirp for successive half cycles of the modulation 
frequency. When the resulting chirp is negative going, it compensates for 
the dispersive characteristics of the Pr.sup.3+ fibre so that bright 
solitons are produced during successive negative half cycles of the 
modulating waveform. For the other half cycles, the positive going chirp 
that is produced, adds to the dispersion produced by the fibre and as a 
result, broad unstable pulses are produced. 
Reference is also directed to E. J. Green and K. Smith, Electronics 
Letters, Vol. 28, no. 18, 27 Aug. 1992, pp 1741-1743 in which another mode 
locked laser configuration is described, which may produce bright 
solitons. 
Dark solitons have been produced using an electro-optical modulator, but 
without using a mode locking technique, as described by W. Zhao et al, 
Optics Letters, Vol. 5, no. 8, 15 Apr. 1990, pp 405-407. 
SUMMARY OF THE INVENTION 
The present invention provides an improved way of producing dark pulses. 
Broadly stated, the present invention provides an optical pulse generator 
comprising the source of optical radiation, a modulator, and a source of 
modulating signals, the modulator being operative to phase modulate the 
optical radiation in accordance with the modulating signals in such a 
manner as to produce dark pulses in the optical radiation. 
The radiation source may comprise a laser that is mode-locked by He 
modulator. More particularly, the optical pulse generator according to the 
invention may comprise an optical cavity, an optically dispersive medium 
in the cavity, means for producing optical resonance within the cavity, 
and a modulator for cyclically phase modulating optical signals in the 
cavity for mode locking the resonance to produce pulses of a given 
periodicity, wherein the dispersive characteristics of the cavity and the 
phase modulating characteristics of the modulator are selected to produce 
dark output pulses. 
The optically dispersive medium within the cavity may comprise an optical 
waveguide, for example an optical fibre doped to provide a particular 
wavelength dispersion characteristic. 
Whilst the generator according to the invention generates dark pulses, it 
can also be configured to produce bright pulses and, to this end the 
cavity may also include dispersive means having a predetermined wavelength 
dispersion characteristic so that radiation resonant in the cavity is 
subject to wavelength dispersion as a function of the dispersion 
characteristics of both the waveguide and the dispersive means. 
The dispersive means may comprise grating with graticules that have a 
spatial frequency which varies along the length thereof. 
The modulator may comprise an optical modulator, for example an optical 
fibre connected in the cavity, with an optical modularing source for 
directing optical modulating pulses into the fibre to modulate its optical 
characteristics. In this way, phase modulation, which produces mode 
locking, can be achieved.

DESCRIPTION OF THE EMBODIMENTS 
Referring now to FIG. 1, this shows a prior art phase locked Pr.sup.3+ 
-doped fluoride fibre laser as proposed by D. M. Pataca et al, Electronic 
letters Jun. 1994, Vol. 30, No. 12 P.964 supra. The laser consists of a 
cavity between a fully reflective mirror M1 and a partially reflective 
mirror M2. 
Optical excitation is provided by a mode locked Nd: YAG laser 1 operating 
in the long wavelength wing of the Pr.sup.3+ ion absorption (0.32 dB/m 
absorption @1.064 .mu.m). The pumping energy is coupled through a 
wavelength division multiplexer WDM1 and a mode matching silica fibre 2 
into a Pr.sup.3+ -doped fibre 3. Because the upper state life time of the 
Pr.sup.3+ ion is long (.about.100 .mu.s) compared to the pumping period 
(.about.10 ms), the excitation is essentially continuous. 
Light leaving the Pr.sup.3+ fibre is collimated by a lens 4 and directed 
to a 0.25 mm thick glass runing etalon 5, and thence to a modulator 6. In 
addition, the cavity includes a set of mechanical polarisation control 
disks 7. 
The modulator 6 is operative to achieve FM mode locking by phase modulating 
the laser emission of the Pr.sup.3+ fibre within the cavity. The 
modulator 6 was operated with a periodicity corresponding to the transit 
time within the cavity between the mirrors (M1, M2) and as a result, the 
laser produced output pluses, through the mirror M2, at a rate 
corresponding to the frequency of the modulator or sub-multiples thereof. 
The modulator 6 comprises a bulk lithium niobate electro-optic phase 
modulator, the peak phase shift of which is a few tenths of 2 radian, and 
the frequency of which (.about.420 MHz) was thus tuned to a harmonic of 
the cavity mode spacing (.about.10 MHz). 
Thus, the laser produced bright pulse, mode locked at the modulation 
frequency of the modulator 6, at the lasing wavelength of 1.3 .mu.m 
associated with the Pr.sup.3+ fibre 3. Further details of the 
experimental results are given in DM. Pataca et al, supra. The bright 
output pulses consisted of a succession of short bright pulses, 
interleaved with broad unstable pulses, when the modulator was driven by a 
sinusoidal electrical signal. The reason for this will now be explained 
with reference to FIG. 2. 
In FIG. 2a, the drive signal to the phase modulator 6 comprises a 
conventional sinusoidal electrical waveform This produces a corresponding 
sinusoidal phase modulation of the light .O slashed.(t) in the cavity of 
the laser which, as shown in FIG. 2b, in region 1, produces a positive 
going rate of change of frequency or chirp in the FM mode locked pulses. 
This resultant positive going chirp combines with the positive dispersion 
of the Pr.sup.3+ fibre 3 so as to produce a pulse spreading effect which 
results in a broad, relatively unstable bright output pulse, shown in FIG. 
2c. 
In contrast, for the negative going half cycle of the phase modulation 
shown in region II, for FIG. 2a, a negative going chirp is produced which 
combines with the positive dispersion of the Pr.sup.3+ fibre 3 so as to 
produce a relatively short duration compressed pulse, 2s shown in FIG. 2c 
In accordance with the invention, it has been appreciated that the effect 
shown in region 1 of FIG. 2 can be modified in order to achieve dark 
output pulses and an embodiment of pulse generator in accordance with the 
invention will now be described with reference to FIG. 3. 
In FIG. 3, a Pr.sup.3+ doped fibre laser is shown, which instead of being 
mode locked by a electro-optical modulator as in FIG. 1, is FM mode locked 
by an optical modulator. As in FIG. 1, the Pr.sup.3+ fibre 3 is coupled 
in the cavity between fully reflective mirror M1 and a partially 
reflective output coupler M2 The fibre was 10m long and pumped by a Nd:YAG 
laser 1 operating at 1.64 .mu.m through a wavelength division multiplexer 
WDM1. The end mirror M1 was a 100% reflector butted to the Pr.sup.3+ 
doped fibre 3. The output coupler M2 comprises either a .about.92% 
reflecting mirror or an optically written fibre grating as will be 
described in more detail hereinafter. 
In the embodiment of FIG. 3, the FM phase modulation is achieved in the 
silica fibre 2 To this end, modulating pulses from a 1.56 .mu.m DFB laser 
7 are coupled trough a wavelength division multiplexer WDM 2 into the 
silica fibre 2. 
The modulating pulses produce changes in the refractive index of the silica 
fibre 2 via non linear (Kerr) effect; sometimes known as cross phase 
modulation (XPM), and by selecting the pulse repetition rate in accordance 
with the resonant frequency of the cavity, mode locking can be achieved. 
In practice, the pulses from the laser 7 have a duration of 35ps and are 
amplified in an erbium-doped fibre amplifier (not shown) to a maximum mean 
power of 30 mW. The pulse propagate along the of 500 m of the silica fibre 
2 between the wavelength division multiplexers WDM 2 and WDM 3, and then 
exit from the cavity. The modulating pulses produce changes in the 
refractive index of the silica fibre so as to produce a positive going 
phase modulation of the Pr.sup.3+ laser light that resonates in the 
cavity between the mirrors M1, M2. As a result, dark pulses are produced 
in the output of the laser, as will now be explained with reference to 
FIG. 4. 
In FIG. 41, the phase shift .O slashed.(t) produced by the modulating 
pulses in the silica fibre 2, is shown. It is to be noted that each 
successive modulation pulse produces the same positive phase window. The 
modulating fibre was chosen to have a dispersion zero at 1.44 .mu.m such 
that the group delays at 1.3 .mu.m (the resonant wavelength of the 
Pr.sup.3+) laser and at 1.565 .mu.m (the wavelength of the modulating 
pulses from DFB laser 7) were reasonably matched. The total dispersion for 
the laser cavity was estimated at 14 ps/m resulting from a 4 ps/nm and 10 
ps/nm contributions from the Pr.sup.3+ fibre and the modulator fibre 3 a 
the modulator fibre 2 respectively. The resulting positive going chirp 
produced by the phase modulation shown in FIG. 4a, combines with the 
positive dispersion of the cavity to produce dark pulses as shown in FIG. 
4b. 
FIG. 7a illustrates an example of output dark pulses produced when 
modulating pulses from the laser 7 are applied to fibre 2 with a 
repetition frequency of .about.700 MHz The combination of the positive 
frequency shifts produced by the modulating pulses from laser 7, and the 
normal dispersion of the cavity tends to "push" light of the modulation 
time slots into the unmodulated regions, giving rise to a broad, 
essentially continuous wave output, separated by dark optical pulses. 
The apparatus shown in FIG. 3 additionally can be reconfigured to produce 
bright. This can be achieved by changing the cavity dispersion by 
replacing the mirror M2 by a so-called "chirped" grating. Such a grating 
may be produced as described in R. Kashyap et al: Novel method of 
producing all fibre photoinduced chirped gratings", Electronics letters, 
Vol 30, No. 12, pp. 996-998, 1994. Briefly, different gratings are 
produced by establishing standing wave in a photosensitive fibre to 
produce a grating pattern. The pattern is repeated in the fibre at 
different levels of applied stress so that, when the stress is released, 
of different spacings are recorded in the same fibre. A schematic 
illustration of a fibre including two such gratings is shown in FIG. 5, 
with its output characteristics being shown in FIG. 6. Referring to FIG. 
5, the grating includes two spaced grating portions A, B, which exhibit 
peak reflectivities at .lambda..sub.1 and .lambda..sub.2 respectively. It 
can be seen that the spacing between the regions A, B will introduce a 
chirp into the pulses that resonate in the cavity, which by appropriate 
selection of the graticule spacings for regions A, B, result in a negative 
going dispersion, in order to compensate for the positive dispersion of 
the Pr.sup.3+ fibre 3. 
FIG. 7b and c show the laser output for the device of FIG. 3, where firstly 
the semi-reflecting mirror M2 is used (FIG. 7b) and when the mirror is 
replaced by the chirped (FIG. 7c). In both cases, modulating pulses from 
laser 7 were fed into the fibre 2 with a repetition rate of .about.2.8 
GHz. In absence of the grating as shown in FIG. 7b, dark pulses were 
produced and the results were qualitatively similar to those shown in FIG. 
7a. However, when the grating was introduced, a net positive group delay 
was produced within the fibre, capable of supporting bright solitons and 
as a result, a stream of narrow .about.50 ps bright optical pulses were 
produced. 
Many modifications and variations of the described examples are possible. 
For example, it may be possible to replace the phase modulator by an 
amplitude modulator and achieve FM mode locking, and generate the dark 
pulses. In all of the described examples, the dark pulses may be solitons. 
As used herein, the term optical radiation includes visible light and 
non-visible radiation such as ultraviolet and infra-red radiation.