Semiconductor diode laser system

A semiconductor diode laser system with microwave mode locking. The system includes a semiconductor laser diode and an external reflector positioned to receive radiation that emits from the diode and reflects the same back into the diode, the microwave mode locking serving to modulate the radiation and the external reflector being properly positioned to return the radiation to the diode at a return time equal to the period of the drive signal or a submultiple thereof.

The present invention relates to semiconductor laser systems. 
Picosecond optical pulse sources have been playing an increasingly 
important role in the study of ultra-fast processes and have potential 
application to high speed electronics. Interest in such techniques has 
recently been stimulated by the generation of picosecond pulses at high 
repition rate, but conventional sources have remained large and cumbersome 
laboratory systems. The present invention, however, as hereinafter 
disclosed, presents a compact picosecond pulse generator using a cw 
semiconductor diode laser. Such compact laser pulse generator has a number 
of practical uses. Among such uses are optical communications, ultra-fast 
computers, testing of optical fibers, producing one-hundred percent 
modulation of a laser at GHz repetition rates, testing optical switches 
and gates, testing fast detectors and optical communication channels, 
sample and hold circuitry, plasma diagnostics, optical sampling of fast 
phenomena, picosecond spectroscopy, investigation of material properties, 
fast electronics, and so forth. 
Accordingly, a principal object is to provide a small-size, economical 
laser whose output pulses are very short. 
Another object is to provide such a laser with laser pulses as short as 
twenty picoseconds at a repetition rate of three billion per second. 
Still another object is to provide a semiconductor laser with the foregoing 
characteristics and one that provides continuous wave (cw) pulses. 
A further object is to provide the pulse generator to a pulse code 
modulated communication system. 
A further object is to provide a source of well-separated pulses that can 
be modulated at a relatively convenient rate and multiplexed to provide a 
high rate optical communication channel. 
A further object is to provide a source for an optical radar system for 
high resolution distance measurements. 
These and still further objects are addressed hereinafter. 
The foregoing objects are achieved in a compact, mode-locked semiconductor 
laser system that includes a semiconductor laser diode and an external 
reflector, the external reflector being positioned to receive optical 
radiation and to reflect the same back into the diode. Means is provided 
to pump the diode to create therein inverted electron population of the 
electronic energy levels of the semiconductor to provide, by radiative 
recombination, said optical radiation therein. Microwave signal drive 
means is connected to the diode to modulate the optical radiation. The 
external reflector is appropriately positioned relative to the diode so as 
to form an optical resonator whose optical length serves to provide a 
return time of the reflected optical radiation that is related to the 
period of the drive signal of the microwave signal drive in a way that 
said return time about equals the period of the microwave drive signal, 
or, preferably, a submultiple of said period.

Turning now to FIG. 1, the circuitry labeled 101 includes a compact laser 
100 consisting of a laser diode 102 with an optical resonator or resonant 
cavity that has an external member, namely, a mirror or other reflector 
103. (In later figures the optical resonator or resonant cavity disclosed 
has additional external members.) The external reflector 103 in an actual 
device is a five centimeter mirror which is precisely positioned relative 
to the diode 102 to receive optical radiation that emits from the diode 
and to reflect the same back into the diode at a very precise return time, 
as noted below. Pumping of the diode 102 in FIG. 1 to provide the 
necessary temperature inversion is provided by a dc bias 104 through a 
bias tee 106. A further signal to the diode 102 is derived from a 
microwave signal drive 105 which is also connected to the diode 102 
through the bias tee 106. As is evident in the explanation below, the 
microwave signal drive and the bias are combined in the bias tee 106 to 
supply a composite signal to the diode 102 to modulate the optical 
radiation labeled 1 that emits therefrom. The output radiation 1 (also 
labeled h.upsilon. in FIG. 1) is focused by the lens or lenses 107 and 
delivered to analyzing circuit 108 for analysis, as later discussed. For 
present purposes, the external reflector 103 is appropriately positioned 
relative to the diode 102 so that the return time of the optical radiation 
labeled 1A in FIG. 1 about equals a submultiple of the period of the drive 
signal of the microwave signal from the drive 105 to provide mode-locking; 
in a special case the return time of the optical radiation 1A can equal 
the period of the drive signal. 
An actual device of the type described in the previous paragraph has been 
built and tested. It generated twenty picosecond pulses in a laser system 
that is about the size of a facial tissue box and included the elements 
100, 104, 105 and 106. Such short pulses have previously been observed 
only with large, very expensive and high-powered lasers. A laser pulse of 
twenty picoseconds in air is about six millimeters long. About three 
billion pulses per second were generated on a cw basis, a rate convenient 
for communication uses. Some details of the lasting portion of FIG. 1 are 
now taken up. 
The actual system made and tested, as above noted, generates twenty 
picosecond optical pulses at microwave repetition rate from a GaAlAs 
double heterostructure diode operating cw at room temperature. The diode 
is operated in an optical resonator that includes an external member or 
members and is actively modulated at 3 GHz. The pulses are measured by 
auto-correlation using SHG in LiIO.sub.3. They are the shortest pulses 
ever reported for a cw laser diode. 
If the laser diode optical gain is to be modulated so as to produce mode 
locking, the relatively long relaxation time of the population inversion 
of the diode 102 requires operation of the diode with the external 
reflector 103 to give a resonant cavity roundtrip time of the order of a 
nanosecond. There follows a short discussion of the analyzing circuit or 
detection system 108. 
The detection system 108 includes a beam splitter 109 which separates the 
pulse-train incident upon it into the two replicas which are reflected by 
corner reflectors 110 and 111. One of the corner reflectors is movable so 
that a variable delay can be introduced between the two pulse train 
replicas. The beam labeled 1' is a recombination of the two beams 
reflected by the corner reflectors 110 and 111, but the beam labeled 1" is 
just a portion of the beam reflected by the corner reflector 111. 
The horizontal beam is reflected by a mirror 112 into a LiIO.sub.3 crystal 
113 which doubles the frequency of the incident radiation. The doubled 
radiation is detected in a photomultiplier 114. The photomultiplier output 
is fed to a lock-in amplifier 115. One of the optical beams is chopped by 
a chopper 116; the chopping rate is fed also into the lock-in amplifier 
115, resulting in increased sensitivity of detection of the 
photomultiplier output. 
The vertical recombined beam is split in a beam splitter 117. Part of the 
beam is detected in a fast photodiode 118 whose output is displayed by a 
microwave spectrum analyzer 121. The spectrum analyzer 121 shows beats of 
noise at a frequency corresponding to the inverse roundtrip time, i.e., 3 
GHz in the present example, as explained below. These beats are useful in 
monitoring the correct modulation frequency and also indicate the success 
of modelocking by a radical change in the spectrum when modelocking is 
achieved. The other part of the beam passing through the beam splitter 117 
continues through a scanning Fabry Perot 119 to a detector 120 which 
monitors the optical spectrum of the laser diode and provides an output 
signal to an oscilloscope 122. 
In the mode-locked, semiconductor laser system shown in FIG. 1, the laser 
diode 102 is uncoated on both output surfaces and is placed at the center 
of curvature of the external aluminum mirror 103. The mirror 103 is 
spherical and has, as above noted, a radius of five centimeters. The 
composite resonator or resonant cavity consists of the cavity formed by 
the external mirror 103 and the outermost diode cleavage plane plus an 
internal Fabry-Perot type resonator defined by the two faces of the laser 
diode 102. The isolated laser diode has a threshold current of 190 mA; the 
output wavelength is centered at .about.810 nm and is divided between 
several longitudinal modes 3.17 .ANG. apart. The composite resonator 
reduces the threshold to 145 mA and the emission line width to no more 
than 0.5 .ANG., as measured by a grating spectrometer. There are, however, 
multiple external resonator modes present as evidenced by a 3 GHz=c/2L 
beat signal on the microwave spectrum analyzer 121. The dc drive current 
of the laser is modulated with several milliwatts of microwave power at 
the observed 3 GHz beat frequency. The effect of tuning the modulation 
frequency may be observed by the spectrum analyzer 121. 
Microwave modulation in the actual system previously described was applied 
to the laser 100 through the bias tee 106, as above noted. Impedance 
matching was achieved by a shunt capacitance wired in cascade with a 
50.OMEGA. microstrip line which was terminated by the diode 102. The 3 dB 
matching bandwidth was 150 MHz. No more than 6 mW of microwave drive was 
used in this experiment. Intensity correlation of the beam 1 was made by 
SHG in the LiIO.sub.3 crystal 113. The fundamental (infrared) beam 
incident on the LiIO.sub.3 crystal 113 is composed of two beams of 
relative intensities 4:5. The weaker beam is chopped when correlation 
measurements are made. 
An intensity correlation curve was made which showed pulses of 23 ps FWHM, 
assuming a Gaussian pulse shape. The unmodulated laser output had a 
correlation peak about twice as wide and with a much lower contrast ratio. 
It was found that dc drive currents between 150 and 200 mA did not change 
the pulse width significantly; 200 mA were never exceeded when modulation 
was on, for fear of destroying the laser diode 102. Aligning the laser 
diode 102 with the external mirror 103 is critical in obtaining short 
pulses. 
As is noted above, the optical resonator or resonant cavity of the laser 
100 includes members within and without the laser diode 102. FIGS. 2-4, 
6A, 6B, 7, 8A, 8B and 9 discussed below show variations of the resonator 
of FIG. 1 and FIG. 5 shows a variation of the laser diode 102. In the 
discussion below of the further figures, an attempt is made to apply the 
same or similar labels to elements that perform the same or similar 
functions to the elements in FIG. 1. 
The compact laser shown at 100A in FIG. 2 includes one or more lenses 20A 
and 20B interposed between the laser diode 102 and a plane (or curved) 
mirror or other external reflecting surface 103A. Th optical resonator of 
the compact laser 100A includes the lenses 20A and 20B as well as the air 
space between the laser diode and the reflecting surface 103A, plus the 
propagating distance of the optical radiation within the laser diode 102. 
In the laser labeled 100B in FIG. 3, the optical resonator further includes 
an optical fiber 22 terminating in a reflecting surface which may be the 
surface 103A or a cleavage plane at the end of the optical fiber 22. 
Coupling optics 21 serve to couple optical radiation to and from the fiber 
22. 
The laser marked 100C in FIG. 4 includes as part of its optical resonator 
an optical waveguide 23 which has at its far end a reflecting surface 103B 
to return optical radiation to the laser diode 102 in FIG. 4. 
The compact lasers 100D, 100E, 100F, 100G, 100H, 100I and 100J in FIGS. 6A, 
6B, 7, 8A, 8B, 8C and 9, respectively, are further modifications of the 
laser 100. The laser 100D has a lens or series of lenses 20A and a prism 
24 within the optical cavity thereof; the laser 100E has a grating 103C 
that forms its external reflecting surface; other bandwidth controlling 
elements such as bandpass filter, can be employed; the laser 100F includes 
an external reflector 103D in addition to the external reflector 103; the 
laser 100G includes two laser diodes 102' and 102" plus the reflecting 
surface 103; the laser 100 H also has two laser diodes 102' and 102" 
disposed respectively at opposite ends of an optical waveguide, again 
marked 23 in FIG. 8B; the laser 100I has laser diodes 102' and 102", 
optical fiber 22A with coupling optics 21A and 21B; and in the laser 100J 
the laser diode 102 is located between lenses 20A and 20B so that its 
optical radiation 1A' and 1A" is focused thereby upon the reflecting 
surface 103A and a partially transparent mirror 103D. 
Whereas the laser diode 102 discussed above is a single-section diode, the 
laser diode labeled 102A in FIG. 5 is a two-section diode composed of 
sections 40A and 40B separated by a SiO.sub.2 insulating strip 43 (etched 
or scribed interfaces may also be used to separate the two sections). The 
laser diode 102A may be used in the configuration above discussed. The two 
sections 40A and 40B can be energized and modulated in an almost 
independent manner by currents I.sub.A and I.sub.B, respectively. The 
laser diode 102A consists of an Au-Zn metallizations 41A and 41B and an 
Au-Sn metallization 42 which serve as electrodes. A GaInAsP active region 
44 is formed between an InP(n) substrate 45 and an InP(p) cap 46. Light 
emits from the active region 44. The operation is now taken up. 
Previously it is explained that the threshold for lasing of a laser diode 
such as the laser diode 102 differs from a condition in which the diode is 
employed within a resonator having parts external to the diode and a 
condition in which the resonator is wholely within the diode. The 
discussion here on the matter of threshold is in the context of the former 
situation, that is, in the context of a laser like that shown in FIG. 1, 
for example. 
In the above context, the section 40B, say, can be drawn below threshold so 
as to act as a saturable absorber. In that situation, if the section 40A 
is drawn above threshold and modulated by a microwave signal as previously 
discussed, the saturable-absorber action of the section 40B will lead to 
pulse shortening. Alternatively, the section 40B, below threshold, can be 
driven by a microwave signal to achieve loss modulation independent of the 
dc gain produced in the section 40A. The separation of the modulation 
function from the gain excitation gives additional degrees of freedom in 
optimizing the mode-locking operation. Both sections can be operated above 
threshold. The same principles can be realized by two separate diodes 
placed in the same resonator; see the diodes 102' and 102" in FIGS. 8A-8C. 
Further modifications of the invention herein disclosed will occur to 
persons skilled in the art and all such modifications are deemed to be 
within the spirit and scope of the invention as defined by the appended 
claims.