Generation of pulses of electromagnetic radiation by use of the induced modulational instability

Pulses of electromagnetic radiation having predetermined spacing between pulses can be generated by coupling amplitude modulated carrier radiation into an appropriate nonlinear transmission medium that has anomalous dispersion in some spectral region. The amplitude modulation is to be such that the amplitude has peaks with the predetermined spacing, and the carrier wavelength is to be in the anomalous dispersion region of the medium. Interaction between the medium and the radiation then results in contraction of the width of the amplitude peaks, which can result in formation of narrow pulses. A preferred transmission medium is monomode fiberguide, e.g., silica-based single mode optical fiber. In addition to the method for producing a sequence of pulses, a communication system using the inventive method is disclosed. Furthermore, a communication system comprising Raman amplification of the signal is disclosed.

FIELD OF THE INVENTION 
This invention pertains to the generation of pulses of electromagnetic 
radiation, particularly in the optical and infrared spectral region, and 
to fiberguide communication systems using pulses generated by the 
inventive method. 
BACKGROUND OF THE INVENTION 
Pulses of electromagnetic radiation, typically in the visible or infrared 
part of the electromagnetic spectrum, find many uses in science and 
technology. For instance, almost all existing or planned optical 
communication systems are of the digital type and thus employ pulses of 
electromagnetic radiation. Other applications of such pulses are, inter 
alia, in optical radar, optical ranging, optoacoustic spectroscopy, and 
reaction rate studies. 
The prior art knows many techniques for forming optical pulses, such as 
rotating chopper discs, pulsed lasers, diodes, or flash lamps. However, 
most older prior art techniques cannot conveniently and inexpensively 
produce a train of very short pulses, with pulse width of the order of one 
nanosecond or less. 
Recently some methods have been developed that are capable of producing 
exceedingly short pulses, in the picosecond, and even femtosecond, range. 
These include pulse compression methods, (see, e.g., C. V. Shank et al, 
Applied Physics Letters, Vol. 40(9), pp. 761-763) and the solution laser 
method (L. F. Mollenauer et al, Optics Letters, Vol. 9, pp. 13-15). Such 
ultrashort pulses are of great scientific interest, since they permit 
previously unattainable time resolution in a number of scientific 
experiments. In addition to their scientific usefulness, such short pulses 
potentially may be useful in very high bit rate optical communication 
systems. 
Such communication systems, which, it is believed, may be able to operate 
at bit rates of hundreds of gigabits/second, and even as high as a 
terabit/second, are based on the use of shape-preserving optical pulses, 
also referred to as optical solitons. See, for instance, U.S. Pat. No. 
4,368,543, issued Jan. 11, 1983 to A. Hasegawa; U.S. Pat. No. 4,406,516, 
issued Sept. 27, 1983 to A. Hasegawa; and Y. Kodama and A. Hasegawa, 
Optics Letters, Vol. 8(6), pp. 342-344 (1983). The above patents are 
co-assigned with this. 
Such systems, in order to approach the high data transmission rate of which 
they are capable, require means that can produce narrow optical pulses at 
a very high rate. This application discloses a relatively simple and 
inexpensive method for producing such pulses. 
As will be discussed below in detail, the inventive method utilizes the 
modulational instability of continuous wave (cw) radiation in an 
appropriate optical medium. This instability has previously been used to 
produce tunable coherent infrared and far infrared electromagnetic 
radiation. See, U.S. Pat. No. 4,255,017, issued Mar. 10, 1981, to A. 
Hasegawa, co-assigned with this, and A. Hasegawa and W. F. Brinkman, IEEE 
Journal of Quantum Electronics, Vol. QE-16(7), pp. 694-697. The prior art 
method comprises injection of unmodulated cw radiation, the carrier, into 
single mode optical fiber, the carrier wavelength chosen to lie within the 
regime of anomalous dispersion of the fiber core material. Due to the 
combined effect of the anomalous dispersion and the nonlinear Kerr effect, 
side bands of the carrier are produced; in other words, amplitude 
modulation of the injected unmodulated carrier wave results. Rectification 
of the modulated carrier yields an output signal of a frequency 
proportional to the square root of the power in the carrier wave. 
GLOSSARY OF TERMS 
By "optical" radiation (and its equivalents) I mean radiation of a 
wavelength used, or potentially useful, in communication systems using 
fiberguide as transmission medium. Typically optical radiation is in the 
visible and infrared spectral region, from about 0.4 .mu.m to about 100 
.mu.m wavelength. 
"Radiation of nominal wavelength .lambda..sub.o " is intended to mean 
"radiation having a finite (but narrow) spectral width, the spectrum 
approximately centered on .lambda..sub.o ". 
The spacing between two pulses (or amplitude peaks) is the spacing between 
corresponding parts of the two pulses (or amplitude peaks) (e.g., 
peak-to-peak spacing) in vacuum, and can be expressed in terms of a 
length, e.g., .lambda..sub.M, or in terms of a time, e.g., .tau..sub.M, 
where .tau..sub.M =.lambda..sub.M /c, c being the speed of light in 
vacuum. 
Amplitude modulation is the process by which the amplitude of a carrier 
wave (herein radiation of wavelength .lambda..sub.o) is varied in 
accordance with some property (typically the amplitude) of a quantity, the 
modulating signal. The modulating signal can be, but need not be, 
sinusoidal, and can have, but need not have, substantially constant 
wavelength. Amplitude modulation of cw carrier radiation of substantially 
constant amplitude produces amplitude peaks in the carrier radiation. 
The width of a radiation amplitude peak in a carrier herein is the width, 
in vacuum, at the half-maximum, i.e., halfway between minimum amplitude 
and maximum amplitude. Similarly, the width of a pulse is the width at 
half-maximum. Pulse width and width of a radiation amplitude peak can also 
be expressed in terms of length or in terms of time, and the two modes of 
expressing the above quantities (and of pulse spacing and amplitude peak 
spacing) are used interchangeably herein. 
The modulation depth of an amplitude modulated carrier wave herein is one 
half of the difference between maximum and minimum carrier amplitude, 
divided by the average carrier amplitude. 
A fiberguide is a dielectric filament having a refractive index profile 
such as to longitudinally guide electromagnetic radiation of the design 
wavelength .lambda..sub.o. A monomode fiberguide is a fiberguide which can 
carry only one mode (the fundamental mode) of radiation at the design 
wavelength. 
Transmission media useful for the practice of the invention have a region 
of anomalous dispersion, a nonlinear index of refraction, and are 
substantially transparent for radiation of wavelength .lambda..sub.o. A 
medium is "substantially transparent" if .vertline.q.vertline..sup.2 
&gt;.GAMMA.. For definitions of these terms, see the material following 
equation 1 below. 
A transmission medium, having an (group velocity) index of refraction N 
(defined as c/u.sub.g, where u.sub.g is the group speed) at the wavelength 
.lambda., has anomalous dispersion at .lambda. if 
.differential.N/.differential..omega.&lt;0 at .lambda. with .omega. the 
radial frequency of the radiation of wavelength .lambda.. A fiberguide has 
a spectral region of anomalous dispersion if 
.differential.N/.differential..omega.&lt;0 over a finite range of 
wavelengths. 
Silica-based fiberguide herein has a core and optically active cladding 
that everywhere comprises more than 50%, by weight, of SiO.sub.2. 
It is to be understood that all wavelengths herein refer to wavelength in 
vacuum. 
SUMMARY OF THE INVENTION 
I have invented a method for generating a sequence of pulses in 
electromagnetic radiation of nominal wavelength .lambda..sub.o, typically 
a train of optical pulses. The pulse spacing can be essentially constant, 
or it can be varied, typically in accordance with some pre-established 
scheme. The method utilizes the modulational instability which can occur 
in any nonlinear transmission medium having an anomalous dispersion 
regime, combined with an (external) amplitude modulation of the carrier 
(of nominal wavelength .lambda..sub.o). 
The inventive method for producing a sequence of pulses of electromagnetic 
radiation, the sequence of pulses comprising at least two adjacent pulses 
with predetermined spacing therebetween, comprises coupling radiation of 
nominal wavelength .lambda..sub.o into an appropriate transmission medium, 
the medium having anomalous dispersion in a spectral region that comprises 
.lambda..sub.o, the radiation propagating through the medium from the 
coupling location to an output location. The amplitude of the radiation 
coupled into the medium comprises a sequence of amplitude peaks, the 
spacing between two adjacent peaks being substantially equal to the 
predetermined spacing between the two pulses formed from the two peaks. 
The length of the propagation path in the transmission medium is such that 
the width of a pulse is at least 10% less than the width of the amplitude 
peak from which the pulse was formed. It is to be noted that amplitude 
peaks, after propagation through the transmission medium and undergoing 
contraction, are referred to herein as pulses. 
Pulse production occurs through interaction of the electromagnetic 
radiation with the transmission medium. The nonlinearity of the medium 
produces self-steepening of the amplitude peaks, and the spacing between 
the (externally produced) amplitude peaks determines the spacing between 
the pulses formed from the peaks. 
The invention has broad applicability, and can be practiced with any 
medium, e.g., a vapor, having the requisite anomalous dispersion and low 
absorption, as will be appreciated by those skilled in the art. However, 
the discussion herein will primarily be in terms of the preferred 
transmission medium, namely, monomode fiberguide, typically silica-based 
monomode fiberguide. This is merely to facilitate exposition and is not 
intended to limit the scope of the invention. 
The inventive method can advantageously be applied in a high data rate 
communication system, e.g., a fiberguide system that uses soliton pulses. 
Such a system typically comprises, in addition to means for generating a 
sequence of radiation pulses by the inventive method, means for modifying 
the generated sequence of pulses in accordance with a predetermined scheme 
(e.g., by removal of pulses from the pulse train), means for coupling the 
modified pulse train into monomode fiberguide, and means for detecting 
radiation pulses after their transmittal through the fiber. A soliton 
communication system using Raman amplification is also disclosed.

DETAILED DESCRIPTION 
The theory that predicts the existence and properties of the instability of 
unmodulated cw electromagnetic radiation propagating through a single mode 
fiberguide has been outlined in U.S. Pat. No. 4,255,017 (the '017 patent), 
incorporated herein by reference, and will not be repeated here in any 
detail. 
As has been shown before (e.g., the '017 patent) an essentially constant 
amplitude cw carrier wave can exhibit instability due to small random 
perturbations in amplitude of wavelength greater than a critical 
wavelength .lambda..sub.c. For a carrier of the form given by Equation 1 
of the '017 patent, .lambda..sub.c is, in the dimensionless units of the 
patent, equal to .pi.[10.sup.4.5 (.pi.n.sub.2).sup.1/2 .PHI.].sup.-1, with 
all the symbols as defined in the patent. This instability leads, as I 
have taught previously, to the development of side bands of the carrier 
wave. 
I have discovered that the same instability can be utilized to form a 
sequence of pulses having a predetermined spacing between adjacent pulses. 
More generally, the instability can be used to produce a narrowing of the 
amplitude maxima of amplitude modulated cw electromagnetic radiation of 
appropriate wavelength. 
The method comprises coupling amplitude modulated cw radiation into an 
appropriate transmission medium, e.g., into a single mode fiberguide, of 
predetermined length. A necessary condition for the existence of the 
instability, and therefore for the practice of the invention, is the 
existence of a spectral region in which the medium has anomalous 
dispersion. The carrier wavelength .lambda..sub.o is then selected to be 
within the anomalous dispersion regime. 
For a given fiber, radiation intensity and wavelength, modulation frequency 
and depth, the degree of narrowing typically is a function of the fiber 
length, increasing with increasing fiber length. However, as will be 
further discussed below, fully formed pulses can split into doublets, and 
perhaps even higher multiplets, in a fiber that is longer than the length 
required for development of a fully formed pulse. 
An exemplary embodiment of the inventive method is schematically depicted 
in FIG. 1. Unmodulated cw radiation 11, of wavelength .lambda..sub.o, is 
produced by radiation generator 10, and is amplitude modulated by means of 
amplitude modulator 12. The wavelength of the modulating signal 13 is 
.lambda..sub.M. Although 13 is shown as a sinusoidal signal of constant 
wavelength and amplitude, such is not necessarily the case, and other, 
more complex, waveforms could, in principle, also be used to modulate the 
carrier. Amplitude modulated cw radiation 14 is then coupled into single 
mode optical fiberguide 15 and, after interaction with the fiber, emerges 
at the output end of the fiber as output radiation 16, with spacing 
between pulses equal to .lambda..sub.M. The output radiation may also 
contain radiation that is not contracted into the pulses (or amplitude 
maxima), as indicated schematically in 16. This radiation will be referred 
to as the background, and will be further discussed below. For ease of 
representation, 16 does not show the individual waveforms of the carrier 
radiation, and is not drawn to the same scales as 14. 
Any suitable means for effecting the amplitude modulation of the carrier 
wave is considered to be within the scope of the invention. For instance, 
the means can be a waveguide electrooptic modulator of a type discussed by 
R. C. Alferness in IEEE Transactions on Microwave Theory and Techniques, 
Vol. MTT-30(8), pp. 1121-1137, (1982). Means for generating radiation 10, 
means for coupling radiation into the amplitude modulator or into the 
fiberguide, attenuators, and other necessary components are well known to 
those skilled in the art and need no discussion here. For instance, a 
convenient source for the cw radiation might be a solid state laser 
emitting in the 1.5 .mu.m wavelength range, e.g., a InGaAsP laser. 
Alternative to the above scheme which shows separate means for generating 
and for amplitude modulating the carrier wave, it is of course possible to 
combine these two functions in a single means, e.g., an amplitude 
modulated laser, and this and other obvious variations are intended to be 
within the scope of the invention. 
Exemplary fiberguide useful in the practice of the inventive method is 
standard low loss silica-based single mode optical fiber of the type 
familiar to those skilled in the art. SiO.sub.2 has a region of anomalous 
dispersion for wavelengths greater than about 1.3 .mu.m, extending to 
wavelengths that are not of interest herein, due to strong radiation 
absorption at these wavelengths. Since doping of SiO.sub.2 typically 
causes a shift of the lower limit of the anomalous dispersion region 
(.lambda..sub.d herein), in silica-based fiberguide .lambda..sub.d 
typically is larger than 1.3 .mu.m. High purity silica has very low 
absorption at about 1.55 .mu.m, and wavelength between about 1.3 .mu.m and 
about 1.7 .mu.m are therefore of considerable interest for communication 
purposes. If the wavelengths are also in the region of anomalous 
dispersion of silica-based fiberguide, e.g., fiberguide with germania 
doped core, they can be advantageously used to practice the invention. 
I will now describe a computer simulation of pulse formation by the induced 
modulational instability. It is well established that the nonlinear 
Schrodinger equation describes accurately the space-time evolution of the 
complex envelope amplitude .PHI.(x, t) of electromagnetic radiation of 
wavelength .lambda..sub.o in single mode fiberguide. The equation can be 
written as follows: 
##EQU1## 
where .xi.=10.sup.-9 x/.lambda., 
.tau.=10.sup.4.5 (-.lambda.k").sup.-1/2 (t-x/u.sub.g) 
q=10.sup.4.5 (.pi.n.sub.2).sup.1/2 .PHI., and 
.GAMMA.=10.sup.9 .lambda..gamma.. 
.lambda. is the wavelength of the carrier in free space (usually 
.lambda..sub.o herein), x is the distance of transmission in the 
fiberguide, u.sub.g is the group speed, t is the time, n.sub.2 is the Kerr 
coefficient of the fiberguide material (about 1.2.multidot.10.sup.-22 
m.sup.2 /V.sup.2 for SiO.sub.2), .PHI. is defined above, .gamma. is the 
loss rate of the radiation in the fiber, 
k'=.differential.k/.differential..omega., k"=.differential..sup.2 
k/.differential..omega..sup.2, and i is the imaginary unit. 
Equation 1 is solved numerically in a periodic boundary condition, with 
period .tau.=48, with the initial condition 
##EQU2## 
where A.sub.M is the depth of modulation (0&lt;A.sub.M .ltoreq.1), and 
.tau..sub.M is .lambda..sub.M /c. The assumed value of .lambda. 
corresponds to a loss rate of 0.3 dB/km, and .lambda. is 1.55 .mu.m. The 
fiber is assumed to have a core cross-section of 20 .mu.m.sup.2, the group 
dispersion, defined as (.lambda..sub.o k"), is 8.52.multidot.10.sup.-17 
seconds for .lambda..sub.d of 1.50 .mu.m. Then .vertline.q.vertline.=1 
corresponds to a power of 105 mW, .tau.=1 to 2.7 ps, and .xi.=1 to 1.55 
km. 
FIG. 2 shows the radiation envelope .vertline.q.vertline. (modulation 
period .tau..sub.M =32.4 ps) at .xi.=0, and FIGS. 3 and 4 the computed 
envelope at .xi.=1.29 (2.0 km), and .xi.=1.74 (2.6 km), respectively. As 
can be seen, the amplitude maxima of the input radiation contract in width 
and increase in amplitude. The process can lead to formation of pulses 
whose amplitude approaches six times the initial average amplitude (=1). 
For longer propagation distances, however, the peaked structure can 
deform, splitting into two (or more) peaks, as is shown by FIG. 4. This 
and other calculations show that a pulse sequence, with pulse width 
.tau..sub.o .ltoreq.1, can be produced, the repetition period given by the 
modulation length .lambda..sub.M, independent of the initial depth of 
modulation A.sub.M. The calculations also show that the length of fiber 
needed to form fully developed pulses varies, inter alia, as a function of 
A.sub.M and .tau..sub.M. 
This is illustrated by FIGS. 5 and 6, which show the computed envelope 
structure at 2.5 km for .tau..sub.M of 21.5 ps, and at 10 km for 
.tau..sub.M of 64.8 ps, respectively. All other parameters are as given 
above. Table 1 shows further calculated results, namely, the distance 
.xi..sub.M at which the single peak structure is substantially fully 
developed, as well as the maximum .vertline.q.sub.M .vertline. and minimum 
.vertline.q.sub.m .vertline. radiation amplitude at that distance, for 
various values of modulation depth A.sub.M and modulation period 
.tau..sub.M. 
TABLE 1 
______________________________________ 
.tau..sub.M = 8 
12 24 
______________________________________ 
A.sub.M = 0.2 
.xi..sub.M 
= 2.90 3.55 
6.45 
.vertline.q.sub.M .vertline. 
= 2.5 2.4 2.4 
.vertline.q.sub.m .vertline. 
= 0.6 0.7 0.7 
A.sub.M = 0.5 
.xi..sub.M 
= 1.61 1.94 
3.23 
.vertline.q.sub.M .vertline. 
= 3.4 3.9 3.8 
.vertline.q.sub.m .vertline. 
= 0.5 0.7 0.8 
A.sub.M = 0.8 
.xi..sub.M 
= 1.08 1.29 
2.26 
.vertline.q.sub.M .vertline. 
= 4.4 5.7 4.7 
.vertline.q.sub.m .vertline. 
= 0.5 0.8 1.2 
______________________________________ 
The above values can, inter alia, be used to determine the results at 
different power levels, since Equation 1 is invariant if q'=.alpha.q is 
substituted, so long as the new variables .tau.'=.tau./.alpha., 
.xi.'=.xi./.alpha..sup.2, and .GAMMA.'=.alpha..sup.2 .GAMMA. are used. If, 
for instance, the average power is increased by a factor of four, and the 
modulation period halved, the distance at which pulses are fully developed 
is quartered, in a fiber having quadrupled loss. I have also found that, 
generally, the pulse formation distance and the pulse width increase in 
proportion to the square root of the group dispersion at the carrier 
wavelength. 
As can be seen from FIGS. 3-6, the radiation is typically not completely 
collapsed into pulses, but a cw background remains. If desired, this 
background radiation can be removed, for instance, by intensity 
discriminating means such as were disclosed by R. H. Stolen et al in 
Optics Letters, Vol. 7(10), pp. 512-514 (1982). 
I have also found that, in order to avoid stimulated Brillouin 
backscattering in the fiberguide, it is advantageous to use input cw 
radiation having finite, although still relatively narrow, bandwidth. For 
instance, in silica-based fiber, the growth rate of the modulational 
instability exceeds that of the stimulated Brillouin scattering if the 
carrier bandwidth is greater than about 4 GHz. 
As will be understood from the above discussion, the inventive method does 
not necessarily have to be used to form fully developed pulses. A 
parameter, e.g., the fiber length, can be chosen such that substantial 
narrowing of the amplitude peaks, typically by at least about 10%, 
probably at least about 50%, results. The modulation depth can be anywhere 
between 0 and 1, but preferably is greater than 0.5. The average amplitude 
of the radiation coupled into the fiberguide is to be such that at that 
point .vertline.q.vertline..sup.2 &gt;.GAMMA., where the bar indicates the 
average value, and all other symbols were defined before. The minimum 
spacing between adjacent amplitude peaks is to be greater than about 
10.sup.9 .pi..lambda..sub.o [10.sup.4.5 (.pi.n.sub.2).sup.1/2 
.PHI.].sup.-1. 
The inventive method can advantageously be used in a communication system, 
e.g., a system using monomode fiberguide as transmission medium and 
soliton pulses as information carrier. Such systems may use silica-based 
fiberguide, and in that case preferably use a carrier wavelength 
.lambda..sub.o at or near 1.55 .mu.m, but typically between about 1.3 and 
1.7 .mu.m. For the parameter values used in the examples herein, the pulse 
spacing .lambda..sub.M may typically be between about 1 mm 
(3.multidot.10.sup.-11 sec) and about 10 cm (3.multidot.10.sup.-9 sec). 
The pulse width typically is substantially less than the pulse spacing, 
preferably no more than about 0.25.tau..sub.M, due to the possibility of 
interaction between adjacent soliton pulses. 
A communication system according to the invention typically comprises, in 
addition to the inventive means for generating a sequence of pulses, 
described above, means for selectively modifying at least some of the 
pulses of the sequence of pulses, modification being in accordance with a 
predetermined scheme, whereby information can be impressed upon the pulse 
sequence. Such modification can be by any appropriate means, e.g., by 
means of a waveguide electrooptic modulator (see R. C. Alferness, op. 
cit.). However, modification is not necessarily by removal or attenuation 
of pulses, and the scope of the invention is intended to encompass all 
suitable modification means. 
The modified pulse sequence is then coupled into the fiberguide by known 
means, transmitted therethrough, and detected by any appropriate means, at 
a location different from the input location. 
A system according to the invention can be a soliton system or a nonsoliton 
system. In the former case, the transmission channel fiberguide has 
anomalous dispersion at .lambda..sub.o. Furthermore, a soliton system 
advantageously comprises means for transforming the pulses generated by 
means of the inventive method into soliton pulses. Such means typically 
are attenuating means. In a nonsoliton system the transmission channel 
fiberguide is chosen to have normal dispersion at .lambda..sub.o, or, 
advantageously, to have the wavelength of zero group dispersion at 
.lambda..sub.o. 
An exemplary communication system according to the invention is 
schematically depicted in FIG. 7. Reference numerals 11-16 refer to 
previously discussed pulse generating means. Pulse train 16 is modified in 
optical switch 70 in response to signal 71, modified pulse train 72 is 
coupled into optical fiber 73 and, after transmission therethrough, 
detected by detector 74, whose output 75 is available for processing by 
means not shown. 
A system according to the invention may also comprise means for removing 
the background radiation, and means for amplifying the pulses in the 
fiber, in addition to such well-known components as coupling means, 
attenuators, switches, taps, and the like. An all-fiber system in which 
pulse formation, optical switching, and transmission all takes place in 
optical fiberguide is in principle possible and is considered to be within 
the scope of the invention. Furthermore, in principle the inventive method 
can be used to form a modulated pulse stream, i.e., a sequence of pulses 
carrying information, thereby obviating the need for separate 
pulse-modifying means. The modulation can be achieved by using an 
appropriately amplitude-modulated carrier, e.g., a carrier in which 
predetermined amplitude maxima are absent or shifted. In such an 
embodiment of the invention, the pulse-forming fiber could be directly 
joined to the pulse-transmitting fiber, or both functions could be 
achieved in the same fiber. 
A particularly advantageous method for amplifying soliton pulses is by 
means of the stimulated Raman process, described, for instance, by A. 
Hasegawa in Optics Letters, Vol. 8(12), pp. 650-652 (1983), incorporated 
herein by reference. Raman amplification requires injection of cw 
(unmodulated) pump radiation into the fiberguide at an intermediate point 
along the fiber, and can lead, by means of a transfer of energy from the 
pump radiation to the pulses, to amplification and reshaping of the 
pulses. This is shown schematically in FIG. 8, where modified pulse train 
72 is coupled into fiber 73, and radiation transmitted through the fiber 
is detected by detector 74. At one or, as shown, several, intermediate 
locations pump radiation 82, produced by pump sources 81, is injected into 
the fiberguide by means not shown, but known to those skilled in the art. 
See, for instance, U.S. Pat. No. 4,054,366, issued Oct. 18, 1977 to M. K. 
Barnoski et al. 
The pump wavelength in a Raman amplifier is chosen to be different from the 
signal wavelength .lambda..sub.o. In silica based fiberguide, for 
instance, the pump wavelength is advantageously chosen to be less than 
.lambda..sub.o, typically by about 0.05-0.2 .mu.m, preferably by about 0.1 
.mu.m. For a discussion of the theory of stimulated Raman scattering see, 
for instance, Y. R. Shen et al, Physical Review, Vol. 137, No. 6A, pp. 
A1787-A1805 (1965). Although it is possible to inject pump radiation so as 
to propagate in only one direction in the fiber, it is advantageous to 
inject it substantially symmetrically, i.e., such that pump radiation is 
propagating in both fiber directions, since this gives the possibility of 
amplifying signal pulses propagating in either direction. In a soliton 
system, it is advantageous to space the pump radiation injection points 
such that the pulses reach an injection point before they have ceased to 
be solitons, and to choose the system parameters such that amplification 
does maintain the pulses as fundamental solitons. I have found that 
advantageous distances between adjacent injection points are less than 
2.pi. in units of .xi. (see above). I have also found that it is 
advantageous to use pump radiation of finite bandwidth, in SiO.sub.2 -base 
fiber typically at least about 20 GHz, with a reasonable upper bandwidth 
limit in such fiber being about 200 GHz. The pump power typically can be 
in the range 3-100 mW, and in low loss fiber (e.g., 0.2 dB/km) a typical 
distance between injection points may be up to about 50 km, but may be 
considerably less for low pump power injection.