Saturated semiconductor laser amplifier for compensation of optical fibre dispersion

The span of an optical dispersion-limited fiber for propagating optical pulses is improved by simultaneously chirping and amplifying the stream by means of a saturated semiconductor laser amplifier. The chirp causes a compression of the pulses as they propagate through an initial portion of the fiber, whereby the span of the fiber is increased.

TECHNICAL FIELD 
This invention relates to fiber-optic communication systems for digital 
transmission of information and to methods of operating such systems. 
BACKGROUND OF THE INVENTION 
In fiber-optic communication systems, digital data are transmitted from 
sending stations to receiving stations by means of optical pulses 
propagating in optical fibers. Each of the fibers thus carries 
(propagates) a digital data stream (sequence) of optical pulses typically 
formed by pulse code modulation, or other kind of pulse modulation, of a 
continuous optical wave called the "carrier". This carrier typically is a 
monochromatic continuous-wave optical beam of radiation, as supplied by a 
laser oscillator source of light having a wavelength in the near 
infra-red, for example, a wavelength of about 1.5 .mu.m (1.5 micrometers). 
During each predetermined time interval ("slot") the amplitude of the 
carrier wave is modulated in accordance with one bit of digital 
information, so that during each such time slot the carrier wave contains 
a pulse of standard height or no pulse at all, in order to represent a 
binary digital "1" or "0", respectively, for example. The thus 
pulse-modulated optical beam propagates through fiber segments (fiber 
transmission lines), and each of such fiber segments extends from a 
sending station to a receiving station. At the receiving station, 
typically the fiber segment terminates in a signal regenerator device to 
restore the signal--i.e., to reduce noise and to restore the amplitude of 
the pulses--to a standard level for further propagation in fiber or for 
other uses. 
As the technology progresses, the desired data rates and hence pulse 
repetition rates increase in order to increase the data handling capacity 
of the system without concomitantly increasing the required number of 
relatively costly fiber transmission lines. As these pulse repetition 
rates are increased to values of 8 GHz (8 gigahertz), corresponding to 
data rates of 8 Gb/s (8 gigabits/second), or more, a major problem arises 
because of the phenomenon of optical dispersion by the transmission 
medium, to wit, the fiber material, particularly in case of a carrier 
wavelength of about 1.5 .mu.m. Dispersion is the inherent property of any 
transmission medium that different optical frequencies (different 
wavelengths) propagate through the medium at different velocities. Thus, 
since even a purely monochromatic (single optical frequency) beam of 
continuous-wave optical radiation is not purely monochromatic after it is 
modulated, e.g. pulsed, the phenomenon of dispersion of any pulsed optical 
beam occurs during propagation through fiber. As a result of this 
dispersion in fiber material, when a stream of digital optical pulses is 
introduced into one end (input end) of an optical fiber segment, it 
emerges from the other end (output end) as a stream of optical output 
pulses that are degraded, i.e., are spread out ("smeared") in time and 
space. As the length of the fiber segment is increased, the effects of 
dispersion accumulate, whereby the smearing of the pulses becomes more 
severe. Thus, as the length of a fiber segment is increased beyond a 
threshold, the output pulses become unrecognizable (unrestorable) as such. 
That is, it becomes impossible to determine the transmitted 
information--i.e., to decide at the output end of the fiber which of the 
time slots had been designated for carrying a pulse and hence are 
supposedly representing binary digital "1" or which of the time slots are 
not carrying a pulse and hence are supposedly representing binary digital 
"0". 
As is known from Fourier transform theory, the product of the half-width of 
the optical Fourier spectrum of an optical pulse and the width of the 
pulse (measured in units of time) is equal to unity or more, depending 
upon the pulse shape (profile). Thus for a given pulse shape, as the pulse 
repetition rate and hence the data rate increases, the width of each pulse 
must decrease, and hence the half-width of optical Fourier spectrum must 
increase. Consequently, the spread of optical frequency Fourier components 
increases as the data rate increases, whereby the spread of optical 
propagation velocities due to the fiber material dispersion property 
increases, and hence for a given fiber span (length) the degradation of 
signal becomes more severe. Even if the pulse shape is such that its 
optical spectrum is Fourier transform limited, and thus minimizes the 
effects of dispersion and hence maximizes the fiber span, the degradation 
of signal by dispersion in fiber can still be a major problem. 
At the same time the fiber disperses a propagating signal it also absorbs 
light by scattering or other phenomena whereby signal level is undesirably 
reduced. Thus, although the dispersion problem could be alleviated by 
using a carrier having a wavelength that undergoes less dispersion, such 
as a carrier having a wavelength of about 1.3 .mu.m, the absorption 
phenomena would then impose an even more serious limitation on fiber span. 
In a paper "Use of Chirp Pulses to Improve the Pulse Transmission 
Characteristics in a Dielectric Optical Waveguide" by T. Suzuki, published 
in Electronics and Communications in Japan, Vol. 59-C, No. 3, pp. 117-125 
(1976), it was proposed that a chirped pulse technique be used to modulate 
the optical frequency of the pulse stream by passing it through a lithium 
niobate crystal whose refractive index was being frequency-modulated 
(equivalent to time-dependent phase-modulation) by means of an applied 
external a.c. electric field of frequency in sychronism with the pulse 
repetition rate, whereby the optical frequency of the carrier wave varied 
monotonically across the pulse from its leading (front) edge to it 
trailing (rear) edge; thus the pulse was chirped. Consequently, the pulse 
was compressed by dispersion during its transmission through an initial 
portion of optical fiber: the fiber dispersion slows the propagation of 
the front portion of the chirped pulse and speeds the propagation of the 
rear portion thereof. In this way, the chirping of the pulses served to 
compensate the effects of dispersion, and the total length of fiber 
segment that was capable of propagating the pulse in a recognizable shape 
(form) was increased; that is, chirping the pulses increased the span. 
However, this technique has several shortcomings including a significant 
lowering of signal level (modulation loss) while propagating through the 
electro-optic crystal, such as lithium niobate, as well as the requirement 
of relatively large applied electric fields in the crystal and hence the 
undesirable consumption of unduly large amounts of energy required to 
produce a significant chirp and hence a significant increase in the fiber 
span--i.e., an increase of at least about 50% (a factor of at least about 
1.5). As pulse repetition rates increase, and hence as the frequency of 
electric fields applied to the electro-optic crystal increase, these 
shortcoming become increasingly severe, so that this technique is 
impractical for pulse repetition rates equal to or greater than about 1 
GHz. Another technique for chirping and hence compressing optical pulses 
involves the chirping, by self-phase modulation in fiber itself, of 
intense optical pulses while propagating through the fiber, as described 
in U.S. Pat. No. 4,588,957, issued on May 13, 1986 to Balant et al. 
entitled "Optical Pulse Compression Apparatus and Method". However, for 
pulse repetition rates of 10 GHz and higher, that requires an energy per 
pulse of more than 10 picojoule per pulse, which is undesirably high. 
Therefore it would be desirable to have a technique for increasing the span 
of fiber capable of propagating optical pulses having a carrier wavelength 
in the near infra-red and a pulse repetition rate of about 8 GHz or more 
that avoids the shortcomings of prior art. 
SUMMARY OF THE INVENTION 
This invention is based upon our discovery that a saturated semiconductor 
laser amplifier--i.e., a semiconductor laser amplifier operated in a 
condition of gain saturation--not only can increase the amplitude of an 
optical beam propagating through the amplifier but also can significantly 
phase-modulate, and hence frequency-modulate, the optical beam. In 
particular, we have discovered that a saturated semiconductor laser 
amplifier can impose a significant chirp upon optical pulses having a 
pulse repetition rate in the range of 8 to 16 GHz, and having an optical 
carrier wave of wavelength in the near infra-red, whereby the chirped 
pulses are significantly compressed after introduction into and 
propagating through an initial portion of an optical fiber segment, so 
that degradation of the pulses because of dispersion does not commence 
until after such initial portion of the fiber segment has been traversed 
by the propagating pulses. Thus, the chirp in the pulse compensates the 
dispersion in the fiber so as to compress the pulse as it propagates 
through the initial portion of the fiber and thereby enhances the span. 
Obviously, if the length of this initial portion is a significant fraction 
of the (entire) length of the segment, then and only then will the span be 
significantly increased. 
In a specific embodiment, nearly Fourier-transform-limited optical pulses 
that have monochromatic carrier wavelengths of about 1.5 .mu.m are chirped 
by passage through a saturated semiconductor laser amplifier 
advantageously having the configuration of a channeled substrate buried 
heterostructure. In this way, for pulses having a repetition rate of about 
16 GHz, the chirping of the pulses by the laser amplifier serves to 
increase the fiber span from about 15 km (15 kilometers) to about 70 km, 
i.e., by a factor of more than 4. At the same time, the laser amplifier 
produces an increase in pulse height (amplitude) and hence also in signal 
level, which thereby compensates any optical scattering and absorption or 
other signal loss during propagation through the fiber. 
The use of a saturated semiconductor laser amplifier to chirp the pulses 
can thus increase the fiber span to a value which is significantly greater 
than it would have been in the absence of the chirping. Accordingly, in a 
fiber optic communication system for digital data transmission, a 
saturated semiconductor laser amplifier can also advantageously be used in 
a regenerator for the two-fold purpose of increasing the amplitude of 
optical pulses (modulation gain) and of compensating for fiber material 
dispersion. 
In another aspect, the invention involves a method of operating a 
fiber-optic communication system including the step of operating a 
semiconductor laser amplifier in a condition of gain saturation while an 
optical pulse stream having a pulse repetition rate of about 8 GHz or more 
passes through and is amplified by the laser amplifier, emerges from the 
laser amplifier, and enters into and propagates through an optical fiber 
segment which exhibits significant optical dispersion. Advantageously, the 
laser amplifier has a structure such that during its operation the 
amplifier imposes a phase modulation on the pulses passing therethrough 
whereby the phase modulation results in a significant chirp of the pulses 
such that a pulse compression occurs after propagation of the pulses 
through an initial portion of the fiber segment, the initial portion 
having a length which is a significant fraction of the fiber span, whereby 
a significant increase of the fiber span is enabled, even for pulses 
having energies as small as 0.1 pJ (picojoule) per pulse.

DETAILED DESCRIPTION 
As shown in the FIGURE, a modulated laser device 11 is optically coupled to 
a saturated semiconductor laser amplifier 13. The modulated laser device 
11 generates and delivers an output stream of optical pulses through a 
fiber segment 12 (or other optical coupler) to the saturated semiconductor 
laser amplifier 13. The laser device 11 is modulated by an input voltage 
V.sub.in whereby the output stream is a sequence of optical pulses on an 
optical carrier wave. The output stream as delivered to the fiber segment 
12, has a symmetric Fourier spectrum 16 centered at a frequency f.sub.o, 
the carrier frequency. 
The semiconductor laser amplifier 13 is coupled to and receives the output 
stream of optical pulses from the fiber segment 12 and simultaneously 
amplifies and chirps the stream of optical pulses. The laser amplifier 13 
is coupled to an optical fiber segment 14 and thus generates and delivers 
a resulting stream of chirped optical pulses to the optical fiber segment 
14 in response to the stream from the fiber segment 12. The fiber segment 
14, but not the segment 12, has such a long length that it exhibits 
significant optical dispersion. Emerging from the laser amplifier 13, 
owing to the chirping the stream has a Fourier spectrum 17 which is not 
symmetric and is centered at a frequency f.sub.01 which is ordinarily 
lower than f.sub.0. As this stream propagates through an initial 
(left-hand) portion of the fiber segment 14 the pulses are compressed; as 
the stream propagates through an intermediate portion of the fiber segment 
14 the pulses expand to their original width; and as the stream propagates 
through a final (right-hand) portion of the fiber segment 14 the pulses 
expand further but not above the limit of recognizability, the fiber 
segment 14 having a suitable length which is no greater than the threshold 
length of unrecognizability, as discussed. 
The output end (right-hand end) of the fiber 14 is coupled to a 
photodetector 25, such as a standard avalanche photo-diode or a standard 
PIN photo-diode. The resulting electrical output signal produced by the 
photodetector 25 is electrically coupled to a comparator 26 which is 
arranged to produce a binary digital electrical output signal, e.g., a 
"high" level to represent the presence of a pulse during a given time slot 
and a "low" level to represent the absence of such pulse. The output end 
of the comparator 26 is electrically coupled either to another modulated 
laser device 31 or to a utilization device 27, or to both. Thus the 
modulated laser 31 or the utilization device 27, or both, receives the 
binary digital electrical output from the comparator 26. The modulated 
laser 31 has its output end optically coupled to an input end of a fiber 
segment 32, or of any standard optical coupler, for optically coupling the 
modulated laser 31 to another saturated semiconductor laser amplifier 33. 
When the modulated laser 31 receives the output signal from the comparator 
26, this laser 31 produces an output stream of pulses which is received by 
the fiber segment 32 and delivered by it without significant modification 
to the saturated semiconductor laser amplifier 33. 
The output end of the saturated laser amplifier 33 is optically coupled to 
an input end of yet another fiber segment 34. Thus, the stream received by 
the segment 32 is amplified and chirped by the saturated semiconductor 
laser amplifier 33, in much the same way as the laser amplifier 13, to 
produce a stream of chirped pulses that enter into and propagate through 
the fiber segment 34 in much the same way as described above for the 
stream of pulses propagating through the fiber segment 14. 
The photodetector 25, comparator 20, modulated laser 31, segment 32, and 
saturated semiconductor laser amplifier 33 together thus form a 
regenerator 30 which restores the pulses to their original amplitude and 
shape, i.e., to the same Fourier spectrum as that which they had when they 
emerged from the laser amplifier 13. 
Similarly, another photodector 35, comparator 36, modulate laser 41, 
segment 42, and saturated laser 43 together form another regenerator whose 
output is delivered to yet another fiber segment 44 or to another 
utilization device 37, or to both. The semiconductor laser amplifiers 13, 
33, and 43 advantageously have substantially identical structures, 
typically channeled substrate buried heterostructures. 
The chirp thus imposed by these laser amplifiers is nearly linear over a 
central part of each pulse and downshifts the frequency by shifting the 
phase by a significant fraction of 2.pi., typically a phase shift of about 
.pi.. The net gain produced by each of these saturated laser amplifiers 
can be in the range of about 10 to 15 dB (decibels) or more, taking into 
account insertion losses. 
Typically the modulated lasers 11, 31, and 41 are substantially identical 
mode locked semiconductor lasers that are internally or externally 
modulated by electrical signals (V.sub.in, for example), each having a 
structure suitable for supplying at least nearly Fourier transform limited 
pulses of an optical carrier having a wavelength of about 1.5 .mu.m. 
The fiber segments 12, 14, 32, 34, 42, and 44 are typically standard 
commercial grade silica fibers. The lengths of fiber segments 12, 32, and 
42 are advantageously all equal to less than about 10 km, whereas the 
lengths of fiber segments 14, 34, and 44 are all equal to about 70 km, for 
the case where the optical carrier wavelength (=c/f.sub.o) is about 1.5 
.mu.m and the pulse repetition rate is about 16 GHz, according to actual 
successful tests. For the case where the pulse repetition rate is about 8 
GHz, the lengths of the fiber segments 14, 34, and 44 can be as much as 
about 220 km, according to extrapolation (fixed product of bit rate and 
fiber length) from the tests at 16 GHz. The chirping of the pulses by the 
laser amplifier downshifts the central frequency f.sub.o to a slightly 
lower frequency f.sub.01, typically a shift of about 40 GHz, equivalent to 
a wavelength increase of about 0.0003 .mu.m. 
Although the invention has been described in detail in terms of a specific 
embodiment, various modifications can be made without departing from the 
scope of the invention.