Laser diode phase modulation technique

Apparatus, and a corresponding method, for phase-modulating an optical carrier signal. An input data signal is passed through a derivative filter to obtain a time-differentiated data signal, for application directly to a laser diode, which functions inherently as a frequency modulator. The carrier signal is frequency-modulated by the time derivative of the input signal, and this is equivalent to being phase-modulated by the input signal.

BACKGROUND OF THE INVENTION 
This invention relates generally to optical communication systems and, more 
particularly, to techniques for phase modulation of an optical carrier 
signal. The established method for phase modulation of optical signals in 
a phase-shift-keying (PSK) modulation system is to use an electro-optical 
(EO) modulator with a polarizer. The EO modulator method employs a 
property of modulation theory: that a double sideband suppressed carrier 
(DSB-SC) signal, obtained by ampliponent, modulation by .+-.1 with a 
supressed carrier component is equivalent to shifting the carrier 
.+-..pi.2 in phase. Thus, phase modulation can be obtained by 
amplitude-modulating the carrier and ensuring that the carrier frequency 
component is suppressed. 
Another known method of phase modulation of laser diodes avoids the use of 
an EP modulator and involves injection locking with a second laser diode. 
This was described in Electronics Letters, 18, 5, pp. 210-22, 1982. 
However, this method requires two laser diodes and is limited to 
modulation bandwidths less than half of the injection locking bandwidth. 
Moreover, it has complexities associated with maintaining injection 
locking of the two laser diodes. 
The use of PSK for modulation of optical signals has approximately a 6 dB 
(decibels) advantage over FSK (frequency shift keying), but the use of an 
EO modulator imposes a penalty of approximately 3-4 dB, which practically 
negates this advantage. Further, the use of an EO modulator poses 
additional problems of reduced reliability, and increased bulk and weight, 
all of which are of concern in many applications. 
It will be appreciated from the foregoing that there is still a significant 
need for an alternative approach to direct phase modulation of laser 
diodes used in optical communication systems. The present invention is 
directed to this end. 
SUMMARY OF THE INVENTION 
The present invention resides in apparatus, and a related method, for 
direct phase modulation of a laser diode, without the use of an 
electro-optical modulator. Briefly, and in general terms, the method of 
the present invention comprises the steps of inputting a time-varying data 
signal to be used as a modulating signal, differentiating the data signal 
with respect to time, generating an optical carrier signal in a laser 
diode, and modulating the optical carrier signal with the 
time-differentiated data signal, whereby the laser diode operates to 
frequency-modulate the time-differentiated data signal, which is 
equivalent to phase modulation of the data signal. 
The method also includes the step of equalizing the differentiated data 
signal prior to the modulating step, to compensate for nonlinearities in 
operation of the laser diode as a frequency modulator. 
In terms of novel apparatus, the invention comprises a laser diode, for 
generating an optical carrier signal, a derivative filter connected to 
receive a data signal as input, and constructed to produce an output 
signal proportional to the time derivative of the input data signal, and 
means for coupling the derivative filter output signal to the laser diode, 
wherein the laser diode functions as a frequency modulator and produces a 
modulated optical carrier signal that is modulated in phase in accordance 
with variations in the input data signal. 
The phase modulation apparatus also comprises an equalizing filter 
connected between the derivative filter and the laser diode, to compensate 
for nonlinearities in operation of the laser diode as a frequency 
modulator. 
It will be appreciated from the foregoing that the present invention 
represents a significant advance in the field of optical communication 
systems. In particular, the invention provides a more efficient technique 
for direct phase modulation of an optical signal. Other aspects and 
advantages of the invention will become apparent from the following more 
detailed description, taken in conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
As shown in the drawings for purposes of illustration, the present 
invention is concerned with techniques for direct phase modulation of 
laser diodes. Phase modulation of laser diodes can be usefully employed in 
optical communications systems. In the past, phase modulation has been 
effected by means of electro-optical modulators, or by relatively complex 
techniques employing two injection locked laser diodes. 
In accordance with the invention, a laser diode is directly modulated in 
phase by a data signal. The invention may be considered the combination of 
two properties from different fields. One is a known property of laser 
diodes: that they will inherently operate as frequency modulators. This is 
a result of two physical effects observable in laser diodes, which cause 
their operating frequencies to change with changes in injection current. 
One is a thermal effect at low modulation frequencies. This causes changes 
in frequency of several gigahertz per milliampere of injection current 
change (GHz/mA), at modulation frequencies below a few megahertz (MHz). 
The thermal effect is due to heating of the device by electrical resistive 
losses, which changes the refractive index of material inside the laser 
cavity. This, in turn, changes the effective cavity length and therefore 
its lasing frequency. The other physical effect causing changes in 
frequency is a carrier density effect at high modulation frequencies. This 
causes changes in frequency of several hundred MHz/mA over a modulation 
range from a few MHz to a multiple GHz. The carrier density effect is due 
to variations in the refractive index of material inside the laser diode, 
with variations in the number of charge carriers. 
The other property employed in the invention is a known relationship 
between frequency modulation and phase modulation. Both are forms of angle 
modulation, and, since the frequency is basically the time differential of 
the angle, they differ only in the presence or absence of a time integral 
or a time differential term. In simple mathematical terms, frequency 
modulation may be expressed as: 
EQU c(t)=A cos[.omega..sub.c t+.DELTA..sub..omega. .intg.m(t)dt+.PSI.], 
and a phase modulated carrier has the form: 
EQU c(t)=A cos[.omega..sub.c t+.DELTA.m(t)+.PSI.], 
where 
A =the carrier amplitude, 
.omega..sub.c =carrier frequency, 
m(t) =signal modulation, 
.DELTA..sub..omega. =frequency deviation coeff. (rps/volt), 
.DELTA. =phase modulation index (radians/volt), 
.PSI. =carrier phase. 
From the above expressions, it might be deduced that, if the time 
derivative of m(t), referred to as m'(t), were input to an FM modulator, 
the inherent integration of the FM modulator would cancel an initial 
"pre-biasing" of the input signal by time differentiation. This property 
has been known for some time. See, for example, a text by Carlson, 
Communication Systems, p. 223 (1975), in which it is stated that 
integrating and differentiating networks can be used to convert a phase 
modulator to a frequency modulator, and vice versa. 
The optical phase modulator of the invention has only three components: a 
laser diode, indicated by reference numeral 10, an equalizing filter 12 
and a derivative filter 14. As discussed above, the laser diode is 
inherently a frequency modulator, and may be illustrated as comprising a 
laser light source 10.1 and a frequency modulator 10.2. Because of the 
thermal effect and the optical carrier effect described above, the 
frequency modulation effected by the laser diode 10 is not perfectly 
linear over a range of modulation frequencies of interest. Greatly 
improved linearity can be obtained by means of the equalizing filter 12, 
which is of simple passive resistance and capacitance (RC) design. The 
derivative filter 14 provides a time derivative dm/dt of the input signal 
m(t), and this is applied to the laser diode 10 through the equalizing 
filter 12. As a result, frequency modulation of the differentiated input 
signal produces an output signal in which the carrier signal is phase 
modulated by the input signal m(t). 
The remainder of FIG. 1 is of interest only for its depiction of a complete 
communications system employing the invention. The resulting PSK signals 
may be transmitted though a sending telescope 20 and a receiving telescope 
22, or through an optical fiber 24, depending on application requirements. 
At a receiving end of the system, a PSK modulated carrier signal is input 
to a detector 26, together with an optical signal from a local-oscillator 
laser 28. The detector produces a corresponding electrical signal 
resulting from optical demodulation of the incoming optical carrier 
signal. This electrical PSK signal is amplified in preamplifier 30, and 
then passed to a PSK demodulator 32, which decodes the data in the PSK 
signal. 
The derivative filter 14 may take any convenient form. When the input 
signals are digital PSK modulating signals, the filter 14 may conveniently 
take the form of a finite impulse response (FIR) filter like the one shown 
in FIG. 4. However, the invention is not limited to a particular 
implementation of the derivative filter, nor to a phase modulator that 
uses a phase shift keying (PSK) data modulation scheme. 
To gain a better understanding of manner in which the invention operates in 
a PSK modulation scheme, the modulation and demodulation steps were 
simulated in a computer simulation system, as shown in simplified form in 
FIG. 2. Pulse code modulation (PCM) pulses were generated, as indicated in 
block 40, and input to a low-pass filter 42, the purpose of which is to 
give the pulses a rise and fall time of about 0.1 T, where T is the time 
between data bits. The data bits consist of .+-.1 pulses, and the filter 
42 is a fourth-order filter of the Butterworth type, chosen such that BT 
=5, where B is the 3 dB bandwidth of the filter. FIG. 3a shows the input 
PCM pulses in the time domain after bandlimiting by the lowpass filter 42, 
and FIG. 3b shows the magnitude spectrum of the same signal. 
Next the signals from the filter 42 are input to the derivative filter, 
here indicated as 14'. The derivative filter in the simulation was 
implemented as a digital optimal equiripple filter, designed using the 
Parks-McClellan algorithm, and taking the form of a 40-tap FIR structure 
as shown in FIG. 4. The time waveform after passing the bandlimited data 
pulses through the derivative filter 14' is shown in FIG. 5a. Consistent 
with the notion of time differentiation, transitions between signal levels 
of the original data are represented in FIG. 5a by spikes in the output of 
the derivative filter 14'. The magnitude spectrum of the derivative filter 
output signals is shown in FIG. 5b. 
A second lowpass filter 44 is placed after the derivative filter, in the 
simulation, to simulate the effects of a real bandwidth-limited frequency 
modulator. The waveform after filter 44 is shown in FIG. 6a, for a filter 
bandwidth such that BT =2. It can be seen from FIG. 6a that the derivative 
spikes of FIG. 5a have been broadened and flattened by the presence of the 
filter 44. The corresponding magnitude spectrum is shown again, in FIG. 
6b. 
The bandlimited derivative filter output is finally transmitted to an ideal 
frequency modulator 46, and the resultant output is shown in FIG. 7a. 
Because of the simulation technique employed, the output is shown at 
baseband rather than at carrier frequency. Thus, the phase variations show 
again as basically binary transitions between .+-.1. The corresponding 
magnitude spectrum is shown in FIG. 7b. 
The frequency modulator output is transmitted to a PSK demodulator 48 which 
recovers the original data signals again, as shown in FIG. 8a, with the 
corresponding magnitude spectrum shown in FIG. 8b. 
From the foregoing description, it will be appreciated that the present 
invention represents a significant advance in the field of optical phase 
modulation techniques. In brief, the invention achieves direct optical 
phase modulation of a laser diode by means of a relatively inexpensive 
electrical filter, thereby avoiding the problems inherent in the use of a 
bulky, expensive, and less reliable electro-optical modulator. It will 
also be appreciated that, although an embodiment of the invention has been 
described in detail for purposes of illustration, various modifications 
may be made without departing from the spirit and scope of the invention. 
Accordingly, the invention is not to be limited except as by the appended 
claims.