Apparatus and method for improving signal-to-noise ratio in wavelength division multiplexing soliton transmission systems

For use in a soliton optical pulse transmission system, an apparatus for, and method of, increasing a signal-to-noise ratio of the system. The apparatus includes: (1) a component for receiving a soliton and an accompanying background noise from the system and increasing a power density of the soliton and the accompanying background noise and (2) a saturable absorber, having a predetermined recombination rate, for receiving and absorbing a portion of the soliton and the accompanying background noise, the predetermined recombination rate causing the saturable absorber to absorb a lesser portion of the soliton than of the accompanying background noise, the component having increased the power density of the soliton and the accompanying background noise to increase a sensitivity of the saturable absorber, the apparatus thereby increasing the signal-to-noise ratio of the system.

TECHNICAL FIELD OF THE INVENTION 
The present invention is directed, in general, to optical communications 
systems and, more specifically, to an apparatus and method for improving 
signal-to-noise ratio in wavelength division multiplexing ("WDM") soliton 
transmission systems. 
BACKGROUND OF THE INVENTION 
Given the rapid generation, dissemination and absorption of information in 
today's society, high speed communication of lightwave signals over 
optical fiber links provides, more than ever, a focal point of intense 
interest for scientists and engineers alike. 
Often, optical fiber transmission links include both electronic and optical 
components. Since optical signals often require purely optical processing, 
it is often appropriate to generate optical signals from, and eventually 
transform optical signals back into, electronic signals. 
Conventionally, an optical transmitter, comprising a drive circuit and a 
light source (such as a solid state laser), converts an electronic signal 
into an optical signal (most often consisting of a series of individual 
pulses). The optical signal is caused to propagate through an optical 
fiber having a cladding surrounding a core. The material of choice for 
both the cladding and core is pure silica, doped with impurities to yield 
different respective indices of refraction, with the objective being to 
contain the optical signal within the core by a phenomenon called "total 
internal reflection." 
The optical fiber may contain splices along its length; the splices are 
designed to be as transparent as possible to the optical signal. The 
optical fiber may also feed into beam splitters or couplers that 
respectively divide, or combine other optical signals into, the optical 
signal. Because the optical fiber distorts or attenuates the optical 
signal as it travels through the optical fiber, interspersed electronic or 
optical regenerative repeaters or amplifiers may be required to restore or 
increase the optical signal's intensity periodically. Finally, an optical 
receiver, comprising a photodetector (such as a solid state photodiode), 
an amplifier and a signal conditioner, transforms the optical signal back 
into electronic form for use with electronic circuitry. 
As mentioned above, repeaters or amplifiers may be required to restore an 
optical signal that has degraded by virtue of having travelled through an 
optical fiber. Often, optical signals must be restored after travelling a 
distance of 100 km or so. The chief culprit responsible for degrading 
optical signals is chromatic dispersion, brought about by physical 
interaction of the optical signal with the material constituting the 
optical fiber. Dispersion causes the frequency components of a given 
optical pulse to spread, redistributing the energy in the pulse past the 
point at which the information carried in the pulse becomes unrecoverable. 
Although optical fibers may be approximated as linear waveguides, it has 
been found that they do exhibit certain nonlinearities in their response 
to optical signals. Among other effects, the nonlinearities may modulate 
the frequency of the carrier wave underlying a given optical pulse 
(so-called "self-phase modulation"). Such modulation of the carrier wave 
is referred to as "chirping." Depending upon carrier wave frequency and 
the shape and power of the pulse, chirping most often causes the pulse to 
disperse more rapidly. However, if conditions are right, chirping may 
actually concentrate the pulse to a minimum width before assisting in its 
dispersion. 
If the pulse is of the right shape and power and the underlying carrier 
wave is of the right frequency, concentration of the pulse brought about 
by chirping may be made exactly to counteract spreading of the pulse 
brought about by dispersion, resulting in a net balanced response by the 
optical fiber to the pulse. The optical fiber maintains the pulse in its 
original shape and thereby transmits the pulse nondispersively over 
distances far greater than 100 km. In fact, transoceanic distances are 
theoretically attainable without regeneration. 
A pulse meeting such requirements is called a "soliton." Although solitons 
do not solve the problem of dispersion in optical fibers, they do offer 
dramatic increases over ordinary optical pulses in data rate and the 
maximum distance they can travel before requiring regeneration. 
One objective in designing optical communication systems is maximizing the 
data rate therethrough. In addition to simply increasing the rate of a 
single channel, multiple channels may be created over a single optical 
fiber. So-called "wavelength division multiplexing" ("WDM") allows at 
least two separate channels of optical signals of different wavelength 
(frequency) to be carried on a single optical fiber. 
It is natural to combine the concepts of solitons and WDM to produce a 
soliton transmission system with multiple channels and therefore carry a 
great amount of information over long distances. However, it has been 
found that ultra-long soliton transmission systems operating at data rates 
of 2.5 gigabits per second (Gbps) and above develop a noise background or 
"pedestal" that is broadband in spectrum and roughly continuous in time by 
virtue of cross-coupling between the channels. The noise background 
degrades the signal-to-noise ratio of the system, and therefore ultimately 
limits the information capacity of the system as a whole. What is needed 
in the art is an apparatus and method for improving the signal-to-noise 
ratio in WDM soliton transmission systems. However, the apparatus and 
method must be able to operate with solitons, which only contain about 1 
picojoule (pJ) of energy. 
SUMMARY OF THE INVENTION 
To address the above-discussed deficiencies of the prior art, the present 
invention provides, for use in a soliton optical pulse transmission 
system, an apparatus for, and method of, increasing a signal-to-noise 
ratio of the system. 
The apparatus includes: (1) a component for receiving a soliton and an 
accompanying background noise from the system and increasing the power 
density of the soliton and the accompanying background noise and (2) a 
saturable absorber, having a predetermined recombination rate, for 
receiving and absorbing a portion of the soliton and the accompanying 
background noise, the predetermined recombination rate causing the 
saturable absorber to absorb a lesser portion of the soliton than of the 
accompanying background noise, the component having increased the power 
density of the soliton and the accompanying background noise to increase a 
sensitivity of the saturable absorber. In this manner, the apparatus 
thereby increases the signal-to-noise ratio of the system. 
The present invention therefore allows a power-density-increasing component 
(such as an amplifier or an optical focussing element) and a saturable 
absorber to cooperate to reduce the background noise inherent in soliton 
transmission systems. The saturable absorber is "tuned" to the wavelength 
of the soliton to minimize its attenuation. In a manner to be described, 
increasing the power density of the soliton results in increased 
saturability within the saturable absorber. 
In one embodiment of the present invention, the component is a first 
optical component for spatially concentrating the soliton and the 
accompanying background noise, the apparatus further comprising a second 
optical component for receiving and spatially restoring a remaining 
portion of the soliton and the accompanying background noise, the second 
optical component returning the spatially-restored soliton and the 
accompanying background noise to the system. In this embodiment, the power 
density is increased by focussing the soliton into a smaller area, rather 
than by increasing its overall power. 
In a more specific embodiment, the first and second optical components are 
refractive elements composed of a material having an index of refraction 
higher than that of a core of an optical fiber of the system. 
Alternatively, diffractive components may be employed to focus the soliton 
and its accompanying noise. 
In one embodiment of the present invention, the apparatus is generally 
spherical, the first and second optical components being generally 
hemispherical and disposed on opposite sides of the saturable absorber. 
The spherical shape is ideally compact, allowing the apparatus to be 
placed within a hole or slot in a silicon substrate. 
In one embodiment of the present invention, the component is an amplifier 
for increasing the power of the soliton and the accompanying background 
noise. Again, the present invention increases the power density of the 
soliton and the accompanying background noise to increase the sensitivity 
of the saturable absorber. The amplifier, while not necessary to the 
present invention, increases the power density by increasing the overall 
power. 
In one embodiment of the present invention, the saturable absorber is 
composed of a material selected from the group consisting of: (1) gallium 
arsenide (GaAs), (2) indium gallium arsenide (InGaAs), (3) gallium 
aluminum arsenide (GaAlAs), (4) indium gallium aluminum arsenide 
(InGaAlAs) and (5) indium phosphide (InP). Those of ordinary skill in the 
art will recognize that other semiconducting materials may be employed to 
advantage in the saturable absorber, as long as the recombination rate of 
the photocarriers therein is appropriate or an electroabsorptive effect 
may be advantageously created therein. 
In one embodiment of the present invention, the system is a WDM system, the 
apparatus further comprising a WDM filter for separating the plurality of 
channels carried thereon into separate optical paths, each of the separate 
optical paths having one of the apparatus for increasing the 
signal-to-noise ratio associated therewith, the WDM filter recombining the 
plurality of separate optical paths. The present invention finds 
advantageous use in a WDM system, wherein cross-coupling produces 
continuous, broadband noise. However, soliton transmission systems, in 
general, would benefit from the present invention. 
In one embodiment of the present invention, the apparatus further comprises 
conductive layers disposed about the saturable absorber for generating an 
electric field proximate the saturable absorber, the electric field 
stimulating an electroabsorptive effect in the saturable absorber to 
increase the absorbing of the portion of the spatially-concentrated 
soliton and accompanying background noise. In a manner to be described, 
the electroabsorptive effect provides additional absorption of the 
background noise. However, the apparatus may be completely passive, and 
therefore not field-driven. 
The foregoing has outlined, rather broadly, preferred and alternative 
features of the present invention so that those skilled in the art may 
better understand the detailed description of the invention that follows. 
Additional features of the invention will be described hereinafter that 
form the subject of the claims of the invention. Those skilled in the art 
should appreciate that they can readily use the disclosed conception and 
specific embodiment as a basis for designing or modifying other structures 
for carrying out the same purposes of the present invention. Those skilled 
in the art should also realize that such equivalent constructions do not 
depart from the spirit and scope of the invention in its broadest form.

DETAILED DESCRIPTION 
Referring initially to FIG. 1, illustrated is a diagram of a soliton 100 
and an accompanying continuous, broadband background noise 110 before 
absorption by the apparatus of the present invention. The soliton 100 
appears as a pulse or spike. The noise 110 appears as a roughly continuous 
pedestal upon which the soliton 100 sits. As previously described, in 
soliton transmission systems, and particularly in WDM soliton transmission 
systems, photocarrier cross-coupling in the optical fiber creates 
broadband, continuous noise 110 that degrades the signal-to-noise ratio, 
and ultimately the range and information-carrying capacity, of the system. 
Turning now to FIG. 2, illustrated is a diagram of the soliton 100 and 
accompanying continuous, broadband background noise 110 after absorption 
by the apparatus of the present invention. The primary object of the 
present invention is to increase the signal-to-noise ratio of a soliton 
transmission system. This may be done by disproportionately increasing the 
signal level, disproportionately decreasing the noise 110 level, or both. 
The present invention advantageously employs a saturable absorber to 
decrease the noise 110 level disproportionately. The soliton 100 has a 
higher peak power than the noise 110. The saturable absorber is designed 
to have a photocarrier lifetime (or recombination rate) about equal to the 
pulsewidth of the soliton 100. Interaction among the soliton 100, the 
noise 110 and the photocarriers causes energy in the soliton 100 and the 
noise 110 to be dissipated into the orbits of the photocarriers. The 
intensity dependence of the dissipation is such that relatively less of 
the energy (or peak power) of the soliton 100 is dissipated. The noise 110 
is attenuated more than the soliton 100, because the noise 110 has a lower 
peak power. Indeed, FIG. 2 shows that, while the soliton 100 level is 
decreased, the noise 110 level is disproportionately decreased, resulting 
in an overall improvement in signal-to-noise ratio. 
Turning now to FIG. 3, illustrated is a WDM soliton optical pulse 
transmission system employing an apparatus, constructed according to the 
present invention, for increasing a signal-to-noise ratio of the system. 
The system, generally designated 300, comprises an optical transmitter 
310, that itself comprises a drive circuit 311 and a light source 312 
(such as a solid state laser). The optical transmitter 310 converts an 
electronic signal into an optical signal (most often consisting of a 
series of individual pulses and, more specifically, solitons). 
The optical signal is caused to propagate through an optical fiber 320 
having a cladding surrounding a core (not separately shown in FIG. 3). The 
optical fiber 320 may contain splices 330 along its length. The splices 
330 are designed to be as transparent as possible to the optical signal. 
The optical fiber may also feed into beam splitters or couplers 340 that 
respectively divide, or combine other optical signals into, the optical 
signal. 
Because the optical fiber distorts or attenuates the optical signal as it 
travels through the optical fiber 320, interspersed electronic or optical 
regenerative repeaters or amplifiers 350 may be required to restore or 
increase the optical signal's intensity periodically. 
The present invention is shown in FIG. 3 as an apparatus 360, interposed 
into the system 100, for increasing the signal-to-noise ratio of the 
system 300. Finally, an optical receiver 370, comprising a photodetector 
371 (such as a solid state photodiode), an amplifier 372 and a signal 
conditioner 373, transforms the optical signal back into electronic form 
for use with electronic circuitry (not shown). 
Turning now to FIG. 4, illustrated is a cross-sectional view of an 
embodiment of the apparatus 360 of FIG. 3 employing a 
spatially-concentrating first optical component 420 and a 
spatially-restoring second optical component 430. Shown on either side of 
the apparatus 360 is the optical fiber 320 of FIG. 3. As previously 
described, the optical fiber has a core 410 and a cladding 415. The 
soliton 100 and noise 110, upon which the present invention operates, 
reside within the core 410. 
In the embodiment of the present invention illustrated in FIG. 4, the 
apparatus 360 comprises the first and second optical components 420, 430 
with a saturable absorber 440 disposed therebetween. The first and second 
optical components 420, 430 are illustrated as being in the form of 
hemispheres and therefore act as convex lenses, converging any light 
incident thereon. Preferably, the first and second optical components 420, 
430 are composed of a transparent material having an index of refraction 
higher than that of the core 410 of the optical fiber 320 (such as 
silicon, which has an index of refraction n of about 3.5). If the first 
and second optical components 420, 430 are composed of silicon and the 
wavelength .gamma. of the soliton 100 is 1550 nm, the first optical 
component 420 will spatially concentrate the soliton 100 onto a spot size 
having a radius of about 0.44 micron. If the radius of the core 410 is 
about 9 microns, the soliton received therefrom is increased in power 
density and reduced in area, resulting in a gain in intensity of about 
418. After the saturable absorber 440 acts on the soliton 100 and noise 
110, the second optical component collimates the light diverging therefrom 
to restore the soliton to its original area. 
The saturable absorber 440 is shown as a thin layer of material interposed 
between the first and second optical components 420, 430. The saturable 
absorber 440 is preferably composed of a material, such as p-doped InGaAs, 
that is capable of entering into saturation to absorb energy from light. 
The thin layer should at least be translucent, and is most preferably 
transparent, to the soliton 100. 
Whether by spatially-concentrating the soliton 100 with the first optical 
component 420 or by increasing power density through some other means, the 
soliton 100 and the noise 110 are preferably increased in power density to 
increase a sensitivity of the saturable absorber 440 to the energy in the 
soliton 100 and the noise 110. With respect to the soliton 100, if it is 
assumed that the soliton contains 1 pJ of energy (10.sup.-12 J), the 
number of photons within the soliton 100 is about 8.times.10.sup.6. Given 
a 0.44 micron spot size, the focussed intensity of the photons is about 
5.times.10.sup.15 photons per cm.sup.2. Absorption by saturable absorber 
440 of the photons in the soliton 100 is about 1%, resulting in an excited 
carrier density of about 5.times.10.sup.13 photocarriers per cm.sup.2. 
This is about 100 times more than the excitation saturation density of 
InGaAs, thereby driving it into saturation. 
As described above, WDM soliton transmission systems carry multiple 
channels on a single optical fiber 320. In such systems, interaction among 
the photocarriers in the saturable absorber causes the separate channels 
to cross-couple, degrading system performance. One solution is to separate 
the channels and pass the solitons therein through separate saturable 
absorbers. 
Accordingly, turning now to FIG. 5, illustrated is a schematic diagram of 
an alternative embodiment of the apparatus of FIG. 3 employing a WDM 
filter and separate apparatus for each channel. The apparatus illustrated 
is designed for a 7 channel WDM soliton transmission system. Accordingly, 
there are 7 separate saturable absorbers, designated SA.sub.1 through 
SA.sub.7. 
Solitons 100 and the accompanying background noise 110 enter the apparatus 
via a lefthand portion (as shown) of the optical fiber 320. An optional 
amplifier 510 increases the power of the solitons 100 and the accompanying 
background noise 110. A WDM filter 520 separates the solitons 100 
corresponding to each of the 7 channels, placing each of the channels on a 
separate optical fiber 321, 322, 323, 324, 325, 326, 327. The separate 
channels therefore may be treated individually in each of the 
corresponding saturable absorbers SA.sub.1 through SA.sub.7. 
Following processing in the saturable absorbers SA.sub.1 through SA.sub.7, 
the separate channels are recombined in a WDM coupler 530. After 
combination, the channels may again be amplified in an optional amplifier 
540 and delivered to the righthand portion (as shown) of the optical fiber 
320. Again, the advantage in dividing the channels for processing through 
separate saturable absorbers is that interaction between the channels 
during the absorption process is not possible. Therefore, it is not 
necessary to address cross-coupling with, for example, sliding-guiding 
filters. 
Turning now to FIG. 6, illustrated is a schematic diagram of a further 
alternative embodiment of the apparatus of FIG. 3 employing an amplifier 
and a butt-coupled saturable absorber. In this further alternative 
embodiment, the power density of the incoming solitons 100 is increased by 
increasing the power of each soliton (instead of decreasing the area over 
which the soliton is spread). 
Accordingly, solitons 100 and the accompanying background noise 110 again 
enter the apparatus via a lefthand portion (as shown) of the optical fiber 
320. An amplifier 610 increases the power of the solitons 100 and the 
accompanying background noise 110. A WDM filter 620 separates the solitons 
100 corresponding to each of the 7 channels, placing each of the channels 
on the separate optical fibers 321, 322, 323, 324, 325, 326, 327. 
Following processing in a saturable absorber 625, the separate channels are 
recombined in a WDM coupler 630. After combination, the channels may again 
be amplified in an optional amplifier 640 and delivered to the righthand 
portion (as shown) of the optical fiber 320. 
In FIG. 6, the saturable absorber 625 is shown as a single layer of 
saturable absorber material. Instead of providing separate substrates, 
each with a layer of saturable absorbing material associated therewith (as 
with the saturable absorbers SA.sub.1 through SA.sub.7 of FIG. 5), a 
single silicon substrate may be provided with a broad layer of saturable 
absorbing material. 
Turning now to FIG. 7, illustrated is a schematic diagram of a further 
alternative embodiment of the apparatus of FIG. 3 employing an optional 
amplifier 610, a WDM filter 620, butt-coupled saturable absorbers 700, a 
reflector 710 and a circulator 720. Recognizing that the structure of FIG. 
5 is symmetric about the saturable absorbers SA.sub.1 through SA.sub.7, a 
reflector 710 may be employed to reflect the solitons back through the 
first optical component 420, thereby additionally employing the first 
optical component 420 in the role formerly occupied by the second optical 
component 430 of FIG. 4. 
Turning now to FIG. 8, illustrated is a schematic diagram of still a 
further alternative embodiment of the apparatus of FIG. 3 employing a 
passive InP saturable absorber. Solitons 100 and the accompanying 
background noise 110 again enter the apparatus via a lefthand portion (as 
shown) of the optical fiber 320. An amplifier 610 increases the power of 
the solitons 100 and the accompanying background noise 110. A WDM filter 
620 separates the solitons 100 corresponding to each of the 7 channels, 
placing each of the channels on the separate optical fibers 321, 322, 323, 
324, 325, 326, 327. 
Following processing in separate, passive InP saturable absorbers 625, the 
separate channels are recombined in a WDM coupler 630. After combination, 
the channels may again be amplified in an optional amplifier 640 and 
delivered to the righthand portion (as shown) of the optical fiber 320. 
One alternative method of making a saturable absorber is to employ a 
field-screening electroabsorptive saturable absorber. A field-screening 
saturable absorber has several advantages over other types of saturable 
absorbers. First, a field-screening saturable absorber can be made more 
sensitive, thereby requiring less energy to achieve the necessary 
saturation. Second, the recovery time of the saturation can be controlled 
through changes in electrical parameters, such as resistance, capacitance 
or material resistivities. Finally, the absorption strength can be 
controlled by electrical parameters, such as the voltage or current level 
of a control signal applied to the field-screening saturable absorber. 
Turning now to FIG. 9, illustrated is a highly schematic diagram of a 
field-screening saturable absorber constituting yet another embodiment of 
the apparatus of FIG. 3. The saturable absorber 900 takes the form of an 
electrical diode having three distinct regions. Region 910 is a p-doped 
semiconductor contact layer. Region 930 is an n-doped semiconductor 
contact layer. Both regions 910 and 930 are advantageously chosen to be 
transparent. An interposed electroabsorptive region 920 is a layer of 
material chosen so that its optical absorption depends on the electric 
field across it, an effect known as electroabsorption. Advantageously, it 
may be chosen to be a semiconductor layer with low or no doping. The 
saturable absorber 900 then constitutes a convenient structure and means 
for applying and changing the electric field across the electroabsorptive 
region 920, for example by reverse-biasing the saturable absorber 900. 
Electroabsorptive effects are well-known in semiconductor materials. One 
such effect is the Franz-Keldysh effect, seen in direct-gap bulk 
semiconductor materials, such as GaAs, InP and InGaAs. The 
electroabsorptive region 920 could be composed of such materials, or other 
semiconductor direct gap materials well known to those skilled in the art. 
Another such electroabsorptive effect is the quantum confined Stark 
effect, seen in quantum well materials, and the electroabsorptive region 
920 could be composed of such quantum well materials. Quantum well 
materials can be made from alternating thin layers of at least two 
different semiconductor materials, such as GaAs and GaAlAs, or InGaAs and 
InGaAlAs, or other such materials known to those skilled in the art. Yet 
another special case of quantum well or multiple thin layered 
semiconductor electroabsorptive materials are those displaying 
electroabsorption due to the Wannier-Stark effect, and such materials 
could also be used to make the electroabsorptive region 920 as is well 
known to those skilled in the art. 
In operation of the saturable absorber 900 of FIG. 9 an input light beam 
940, composed most preferably of solitons, is shone on an entrance surface 
970 of the saturable absorber 900. In FIG. 9, the light beam is shown 
impinging on a surface of the p-doped region 910, but the input light beam 
940 may be shone on any convenient surface of the saturable absorber 900 
that allows the input light beam 940 to reach the electroabsorptive region 
920. Initially, some of the input light beam 940 is absorbed by the 
material of the electroabsorptive region 920. This absorption generates 
photocarriers in the material of the electroabsorptive region 920. These 
photocarriers then move under the action of the electric field in the 
electroabsorptive region 920. The electric field initially present in the 
electroabsorptive region 920 may be set by a biasing voltage supply 960. 
The movement of the photocarriers changes the electric field in the 
electroabsorptive region 920, and this change in electric field changes 
the absorption of the material constituting the electroabsorptive region 
920. Typically (but not necessarily) the movement of the carriers will 
lead to a reduction of the electric field. In the cases of any of the 
three electroabsorption mechanisms: the Franz-Keldysh effect, the quantum 
confined Stark effect or the Wannier-Stark effect, choosing the operating 
wavelength to be somewhat longer than the spectral position of the zero 
field band gap of the semiconductor material leads to a reduction of 
absorption as the electric field is reduced, hence leading to a saturating 
absorption as desired. This saturating absorption can be observed on a 
transmitted output beam 950. It is understood that mirrors may also be 
used with the saturable absorber 900 so that the transmitted output beam 
950 may actually emerge from the entrance surface 970 if desired. 
A resistor 980 and any capacitance associated with the saturable absorber 
900 can control the speed of recovery of the saturable absorber. It is 
understood that the resistor 980 could be partly or wholly composed of 
resistance internal to the regions 910 and 930. It is also understood that 
the device could also operate without any biasing voltage supply 960, with 
the field in this "self-biased" case being set by the built-in field of 
the saturable absorber 900. 
The physics of the recovery speed of such a device is described in 
"High-Speed Absorption Recovery in Quantum Well Diodes by Diffusive 
Electrical Conduction" by G. Livescu, et al., Applied Physics Letters, 
Vol. 54, No. 8, 20 Feb. 1989, pp. 748-750. This article describes how to 
design the resistivity of the regions 910 and/or 930 to achieve a desired 
speed of recovery of the absorption in such saturable absorber 900. 
The time taken for the absorption to change after the initial absorption of 
optical energy in the electroabsorptive region 920 depends on the time 
taken for the photocarriers to transport to the electrodes, and, in the 
case of quantum well structures, the time taken for the photocarriers to 
be emitted from the quantum wells. The physics of these processes is well 
known. At high electrical fields (such as 10.sup.4 V/cm) in many 
semiconductor materials, the electron and hole velocities (the electrons 
and holes being the kinds of photocarriers created in such semiconductors) 
are typically of the order of 10.sup.7 cm/s, corresponding to about 10 
picoseconds (ps) to move about 1 micron of distance. Such length scales 
(e.g., 1 micron) and fields (e.g., 10.sup.4 V/cm-10.sup.5 V/cm, 
corresponding to 1-10 V/micron), are typical in such electroabsorptive 
saturable absorbers 900, with the electroabsorptive region 920 being 
typically of a total thickness in the range 0-1 micron to 5 microns 
(though it is understood that thicknesses outside this range are within 
the scope of the present invention). The emission time of photocarriers 
from quantum wells can be adjusted over a broad range by the design of the 
quantum wells and the choice of operating field. This area has been 
investigated and discussed in the article "Simultaneous Measurement of 
Electrons and Hole Sweep-Out from Quantum Wells and Modeling of 
Photoinduced Field Screening Dynamics" by J. A. Cavailes, et al., IEEE 
Journal of Quantum Electronics, Vol. 28, No. 10, October 1992, pp. 
2486-2497. 
It is known, for example, that by using low barriers in quantum wells, the 
total time for carrier emission and transport through a quantum well 
region can be of the order of 4 ps, as discussed in the article "Fast 
Escape of Photocreated Carriers Out of Shallow Quantum Wells" by J. 
Feldman, et al., Applied Physics Letters, Vol. 59, No. 1, 1 July 1991, pp. 
66-68. Hence, it is possible to make the change in absorption due to 
field-screening electroabsorption occur much faster than the 50 ps time 
scale typical of pulse lengths in soliton transmission systems, while 
controlling the recovery time of the change of absorption to be of the 
order of 50 ps through the choice of the resistor 980. These properties 
are desirable for the use of a saturable absorber in reducing noise in a 
soliton transmission system. 
One particularly advantageous way of using the concept of a field-screening 
electroabsorptive saturable absorber is shown in FIG. 10. In this case, a 
current source 1000 is used. As those of ordinary skill in the art 
understand, a current source is an electrical supply that delivers 
substantially the same current over a usable range of output voltages. The 
combination of the current source 1000 and a capacitance 1010 ensures that 
the average current passing through the modulator under the desired 
operating conditions is the desired value I.sub.s, while still allowing 
transient currents, associated with the rapid recovery of the electrical 
voltage over the saturable absorber 900 as the absorption recovers, to 
flow as required. In this case the capacitor serves as an AC low impedance 
to pass the transient currents without substantially affecting the average 
DC current I.sub.s. It is understood that, in practice, the capacitance 
1010 may be capacitance that is intrinsic to the physical structure used 
to make the saturable absorber 900, or may be stray capacitance associated 
with wiring, or may be capacitance intrinsic to the actual structure used 
to make the current source 1000. 
It is well known, and described in the article "The Quantum Well 
Self-Electrooptic Effect Device: Optoelectronic Bistability and 
Oscillation, and Self-Linearized Modulation" by D. A. B. Miller, et al., 
IEEE Journal of Quantum Electronics, Vol. QE-21, No. 9, September 1985, 
pp. 1462-1476, that driving an electroabsorption-modulating diode (the 
saturable absorber 900) with a current source in a region where absorption 
increases with increasing reverse bias can lead to a useful operating mode 
referred to as "self-linearized modulation." In this mode, over some 
useful range of absorption and voltage, the voltage over the diode 
automatically adjusts so that the photocurrent generated by the diode is 
(on average at least) equal to the average drive current I.sub.s. Since it 
is typically the case in many such diode structures that one electron of 
photocurrent is generated for each photon absorbed in the 
electroabsorptive region 920, the number of photons absorbed per second on 
the average (and hence the average absorbed power) is controlled by the 
current source. An important point about this automatic control is that 
the same average power is absorbed independent of the precise wavelength 
of the light, and independent of device operating temperature, at least 
over usable operating ranges of wavelengths and temperatures. Hence, in 
operation of the saturable absorber 900, the average fractional absorption 
of the saturable absorber can be automatically set, independent of the 
precise wavelength of the light or the precise operating temperature. 
Hence the need for precise temperature stabilization and any need to have 
different devices of different device control parameters for operation at 
different wavelengths is avoided. 
Although the present invention has been described in detail, those skilled 
in the art should understand that they can make various changes, 
substitutions and alterations herein without departing from the spirit and 
scope of the invention in its broadest form.