Optical communications system using frequency shift keying

Frequency shift keying optical communications devices using a cleaved coupled cavity laser are described. The cleaved coupled cavity laser comprises first and second laser diode sections which are mutually optically coupled to each other and means for adjusting the refractive index of the first and second sections relative to each other. The cleaved coupled cavity laser is part of a light source which further comprises means for selecting at least one desired output frequency from a group of at least two output frequencies.

TECHNICAL FIELD 
This invention relates generally to optical communications systems and 
particularly to such systems and components thereof using optical 
frequency shift keying. 
BACKGROUND OF THE INVENTION 
Many techniques, such as, for example, bandwidth compression, for 
increasing the information handling capabilities of communications systems 
exist. However, the desire for communications systems capable of 
transmitting even greater quantities of information in a given time period 
has almost inevitably led to the development of such systems capable of 
operating at ever higher frequencies. Communications systems using 
electromagnetic radiation were initially developed for operation at very 
low frequencies, less than 10 MHz, and the possibility of using 
electromagnetic radiation in the visible or near visible region has always 
been of interest because of the very high data rate transmission, relative 
to low frequency systems, possible in this short wavelength, high 
frequency region. 
The lack of a suitable radiation source, which had long hindered 
developments in this area, was solved, at least in principle, with the 
invention of the laser, and the light source presently contemplated for 
most such systems is a semiconductor laser. Several transmission media are 
possible for use in communications systems operating in the visible or 
near visible region, but after the development of low loss glass 
transmission lines, commonly referred to as optical fibers, such optical 
fibers become the preferred transmission media. The optical fiber 
typically comprises a silica based glass having a high refractive index 
core surrounded by a low refractive index clad. The optical communications 
systems presently contemplated have a light source and photodetector 
optically coupled to each other by the optical fiber. 
For transmission over extended distances, for example, more than 20 km, the 
optical signal is regenerated at one or more intermediate points by a 
device commonly referred to as a repeater. The repeater unit detects the 
incoming optical pulse and reshapes it into the desired electrical shape 
which is then applied to a laser. The repeater thus enables the system to 
operate over larger distances than are possible with a single fiber 
segment. 
All optical fiber systems presently in commercial use are based on the 
encoding of the information by amplitude modulation (AM) and direct 
detection of the transmitted optical energy, i.e., they are two-level, 
one-channel systems. In other words, information is transmitted as an 
optical pulse is either transmitted or not transmitted within 
predetermined time intervals. However, more sophisticated schemes of 
encoding the transmitted information afford possibilities of either or 
both higher data transmission rates or longer repeater spacings than are 
possible with amplitude modulation. Multi-level and/or multi-channel 
systems should significantly increase the information transmission 
capacity of optical fiber communications systems. For example, optical 
frequency modulation (FM) might improve either the data transmission rate 
or permit the repeater spacings to be increased. 
Although there has been interest recently in the modulation and 
demodulation of coherent laser radiation, the development of FM optical 
communications systems has been relatively slow. This is due to several 
factors including the absence of a laser that might be easily tuned 
through a suitable frequency range and the stringent requirements imposed 
upon the system by heterodyne detection. For example, the frequency shift 
keying (FSK) system described by Saito et al in IEEE Journal of Quantum 
Electronics, QE-17, pp. 935-941, June 1981, was a two-level, 
single-channel system using a continuously tuned laser and heterodyne 
detection. The frequency tuning rate is extremely small, approximately 100 
MHz/mA, and limited to a tuning range of less than approximately 1 GHz. 
However, the stringent requirements imposed on the system by heterodyne 
detection could be considerably relaxed if there were a laser easily 
tunable over a wide frequency width and which had a very narrow frequency 
output. 
SUMMARY OF THE INVENTION 
We have found that a device comprising a light source may be used for 
multi-level optical frequency shift keying when the light source comprises 
at least one cleaved coupled cavity semiconductor laser that may be 
optically coupled to an optical fiber, and means for varying the 
refractive index of at least one section of the laser relative to the 
other section to select one of at least two desired output frequencies. 
The system may further comprise a wavelength dispersive photodetector 
module which, together with the laser, comprises a repeater unit. The 
system may further comprise an optical fiber which optically couples the 
photodetector and light source. In one embodiment, a single laser is used 
and a four-level, two-channel frequency shift keying system is obtained. 
Unlike prior art two-level frequency shift keying systems, the frequency 
shift of the cleaved coupled cavity laser is so large and the output so 
narrow in frequency that a direct detection scheme may be employed instead 
of heterodyne detection. This is desirable because it eliminates the use 
of an ultra-stable local oscillator.

For reasons of clarity, the elements of the invention are not drawn to 
scale in the FIGURES. 
DETAILED DESCRIPTION 
A frequency shift keying optical communications system according to our 
invention is schematically depicted in FIG. 1. Depicted are a cleaved 
coupled cavity laser which is the light source 100, an optical fiber 300, 
and a wavelength dispersive photodetector module 500. The light source 
100, which is tunable to at least two frequencies, and photodetector 
module 500 are optically coupled to each other by means of the optical 
fiber 300. The optical fiber may comprise any of the conventional and 
well-known silica-based optical fibers. 
A cross-sectional view of the cleaved coupled cavity laser of FIG. 1 
through the active layers is shown in FIG. 2. Section 3 comprises a first 
layer 31, a first cladding layer 32, active layer 33, second cladding 
layer 34, and layer 35. Section 5 comprises first layer 51, first cladding 
layer 52, active layer 53, second cladding layer 54, and layer 55. 
Further, there is a built-in p-n junction in the active layer in each 
section. The active layer, i.e., the region in which electron-hole 
recombination occurs, has a first bandgap energy and a first refractive 
index. The active layers have a width through which current is injected 
that is less than 20 .mu.m to prevent multifilament lasing, i.e., the 
active layers have a width less than 20 .mu.m. The cladding layers have a 
second bandgap energy which is greater than the first bandgap energy and a 
second refractive index which is less than the first refractive index. The 
cladding layers thus provide both carrier and optical confinement for 
these index guided sections. Both sections 3 and 5 are disposed on 
substrate 15. Sections 3 and 5 are electrically contacted by electrodes 7 
and 9, respectively, and there is further a circuit element, such as 
variable resistance 73, which permits the current through section 3 to be 
adjusted relative to the current of section 5. Variable resistance 73 thus 
provides means for adjusting the refractive index of the first and second 
sections relative to each other. The substrate 15 forms a common 
electrical contact to sections 3 and 5. The electrical contacts to 
sections 3 and 5 may also be used to adjust the current through the two 
sections. Although the active layers are depicted as being closer to the 
substrate than to the top surface, they may also be positioned the latter 
way, i.e., closer to the top surface. All the layers are epitaxially grown 
and are approximately lattice matched to each other. A variable resistance 
may also be connected to section 5. Further, laser heterostructures other 
than the one described may also be used and other means for adjusting the 
refractive index of the first and second sections relative to each other 
may also be used. Other index guided structures may also be used. 
Additionally, the layer structures in sections 3 and 5 may differ. 
The cavities are mutually optically coupled to each other through cleaved 
mirror surfaces and the active stripes are aligned with respect to each 
other, i.e., the stripes are positioned essentially co-linearly with 
respect to each other, and separated by a relatively small distance, 
typically less than 10 .mu.m, but greater than the carrier tunneling 
distance, thus providing electrical isolation between the sections. This, 
i.e., the carrier tunneling distance, is approximately 200 Angstroms for 
electrons in InGaAsP. 
The sections should be electrically isolated from each other, i.e., they 
should be capable of being electrically biased relative to each other. 
This permits the carrier concentrations in the sections to be separately 
controllable. 
The composition of the layers and substrate is not critical, although 
layers and substrate should be at least approximately lattice matched to 
each other, and may be selected from Group III-V and Group II-VI compound 
semiconductors. These compositions, for example, InGaAsP, permit the 
wavelength of the emitted radiation to be in those areas, 1.35 and 1.55 
.mu.m, presently of greatest interest for optical communications. 
An exemplary method for forming devices comprising coupled cavities from a 
unitary structure according to this invention will be briefly described. A 
standard semiconductor laser wafer having a plurality of active stripes 
has, on one surface, a plurality of gold pads which are formed by, for 
example, electroplating. The precise structures of the active stripes and 
adjacent layers are not critical and they may be buried heterostructures, 
buried crescent heterostructures, or yet other types. The desired layers 
may be grown on the substrate by growth techniques, such as molecular beam 
or liquid phase epitaxy, that are well known to those skilled in the art. 
However, the isolated thick gold pads should preferably be approximately 
the same size as the standard diode and are electroplated onto the wafer 
side having the epitaxial layers, if the diode is CW bonded epitaxial 
layer (epilayer) side down, or onto the substrate side, if the diode is 
bonded epilayer side up. Other deposition techniques, such as evaporation, 
may also be used. Metals other than gold may be used if they adhere to the 
semiconductor and may be plastically deformed. Standard and well-known 
cleaving procedures are now applied. At one position, the bars separate 
from each other as these positions are not contacted by the gold pads. 
However, at the other positions, the presence of the gold pad holds the 
adjacent cleaved bars together. The lengths are not critical and can be 
selected as desired. The individual pairs of diodes, which are still held 
together by the gold pads, are now separated from each other by sawing or 
scribing. The resulting structure of a single pair of laser diode sections 
has two precisely self-aligned and extremely closely optically coupled 
Fabry-Perot cavities. The mirrors of the cavities are formed by the 
cleaved surfaces. 
A typical separation of the coupled cavities is approximately 1 .mu.m. If a 
larger separation is desired, it can be easily obtained by, for example, 
moving the two Fabry-Perot diodes with respect to each other using the 
gold pad as the hinge. The precise separation is not critical but it 
should be greater than the carrier tunneling distance and less than 
approximately 10 .mu.m. Of course, the last movement should place the 
mirror faces parallel or approximately parallel to each other so that the 
active stripes are aligned with respect to each other. The mirror faces 
need not be precisely parallel to each other as the angular distribution 
of the emitted radiation is sufficiently broad to optically couple the 
cavities. However, the faces should not contact each other, as electrical 
isolation between the sections is desired. Further, the Fabry-Perot diodes 
may be slightly twisted with respect to each other with the active stripes 
forming the twist axis. This twist has a transverse mode filtering effect, 
i.e., certain transverse modes can be suppressed. 
To complete the fabrication of the device, the two Fabry-Perot diodes 
hinged together by the gold pad are bonded, using, e.g., indium, 
simultaneously epilayer side down on a heat sink, such as gold plated 
copper. Standard CW bonding procedures may be employed. Separate 
electrical connections are made to each diode on the substrate side. Of 
course, if the gold pads are on the substrate side, the Fabry-Perot diodes 
are bonded simultaneously substrate side down, respectively, and the 
separate electrical connections are made on the epilayer side. 
Use of the gold bonding pads is not essential. For example, the laser diode 
sections may be formed by cleaving a standard wafer without bonding pads 
and then positioning the diodes with respect to each other on the heat 
sink and bonding. However, the use of the gold pads facilitates relatively 
accurate positioning of the active stripes with respect to each other. 
The wavelength dispersive photodetector module comprises at least one 
photodetector and a wavelength dispersive element. The module is thus 
capable of separate detection of the individual wavelengths. The 
wavelength dispersive element may be, for example, a grating, 
interferometer, etc., which diverts the separate wavelengths into 
different individual photodetectors, within the module, in which the 
optical pulses are absorbed and corresponding output voltage or current 
pulses produced at the different individual detectors responsible for 
detecting the different wavelengths. 
The cleaved coupled cavity laser is operated as follows in a frequency 
shift keying communications system. One of the diode sections of the 
cleaved coupled cavity laser operates as a laser and the second diode 
section operates under lasing threshold as a frequency modulator. A change 
in the current applied to the modulator diode produces a change in the 
carrier density which, in turn, produces a corresponding change in the 
effective refractive index of cavity. The change in the effective 
refractive index of the cavity results in a slight shift of the 
Fabry-Perot modes of the modulator with respect to those of the laser 
diode. Such a slight shift results, because of the optical coupling 
between the two cavities, in a large shift, typically 15 Angstroms, of the 
enforced mode of the coupled cavity to the adjacent Fabry-Perot mode of 
the laser diode section. FIG. 3 shows various spectra obtained with 
different current levels applied to the modulator diode section. The laser 
was an InGaAsP buried crescent laser such as those described in 
Electronics Letters, 18, pp. 95-96, 1982. The wavelength is plotted 
horizontally and the output intensity is plotted vertically. The current 
through the modulator section is indicated on each spectrum. As is 
evident, a frequency shift of 150 Angstroms can be obtained. As is also 
evident, a frequency tuning rate of 10 Angstroms/mA can also be obtained. 
Such a large tuning range and tuning rate permit fabrication of a 
multi-channel, multilevel frequency shift keying transmission system. As 
the current through the modulator section is varied, the frequency of the 
emitted radiation is also varied. Thus, the means for varying the current 
through the modulator section comprises means for selecting a desired 
output frequency from a group of at least two output frequencies. Such 
frequency shifting can be achieved in less than 1 nanosecond implying that 
bit rates greater than 1 Gigabit/second can be achieved. It should also be 
noted that the cleaved coupled cavity laser operates with a single 
longitudinal mode even under high speed modulation. That is, the ratio of 
the most intense longitudinal mode relative to suppressed longitudinal 
modes is at least 50 when the laser is CW operated. 
The operation of a two-channel, four-level frequency shift keying system 
will be better understood by reference to a specific example. Electrical 
pulses from two channels, A and B, either alone or multiplexed (A+B), may 
be applied to the modulator diode. The current pulses for channels A and B 
are of different magnitude. The laser diode has a dc current applied. 
Because the output lasing wavelength is a function of the current applied 
to the modulator section of the cleaved coupled cavity laser, the three 
different current levels that result from multiplexing channels A and B 
will yield output lasing modes at three different wavelengths. The fourth 
wavelength is obtained when neither channel A nor B has a pulse. The TABLE 
shows the coding and decoding scheme of a four-level two-channel optical 
frequency shift keying system according to our invention. 
TABLE 
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CHANNEL 
A B .LAMBDA. 
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CODING 
0 0 .lambda..sub.0 
1 0 .lambda..sub.1 
0 1 .lambda..sub.2 
1 1 .lambda..sub.3 
DECODING 
A = .lambda..sub.1 + .lambda..sub.3 
B = .lambda..sub.2 + .lambda..sub.3 
______________________________________ 
FIG. 4 depicts a repeater unit useful in the optical frequency shift keying 
communications system of this invention. The repeater unit comprises a 
wavelength dispersive element 90, which receives the optical pulse from 
the previous laser, signal regenerating means 120 and a cleaved coupled 
cavity laser. The wavelength dispersive element 90, for example, a 
grating, directs the separate wavelengths, .lambda..sub.1, .lambda..sub.2, 
.lambda..sub.3, and .lambda..sub.4 into the individual photodetectors 
D.sub.1, D.sub.2, D.sub.3, and D.sub.4, respectively. Means 120 receives 
electrical signals from a plurality of individual photodetectors D.sub.1, 
D.sub.2, D.sub.3, and D.sub.4 and regenerates the electrical current 
pulses in a manner well known to those skilled in the art. These pulses 
are then applied to the cleaved coupled cavity laser to produce output at 
the desired wavelength. 
It will be apparent to those skilled in the art that, for example, an 
eight-level, three-channel frequency shift keying system can also be 
constructed in a similar manner. Other modifications are contemplated. For 
example, the laser and photodetector may be optically coupled through air 
or free space.