Method and an apparatus for the synchronous mode locking of the longitudinal laser modes of a semiconductor diode laser

A method and an apparatus are described by producing optical radiation pulses with a duration of the order of picoseconds (10.sup.-12 s) and less using external mode locking of the longitudinal laser modes of a semiconductor laser element operating in an external resonator. In the method of the invention the gain modulation, needed for synchronous mode locking, of the active semiconductor laser element is effected by modulation of the electrical pumping current by way of a high-speed optoelectronic switch, that for its part is controlled by a mode locked master laser. The present method combines the advantages of synchronous optical pumping, as used in commercial mode locked dye laser systems, with the simple type of electrical pumping of commercially available semiconductor laser diodes, more especially double heterostructure laser diodes. It respresents a simple and economic alternative to synchronously pumped dye laser systems and considerably extends the spectral range within which radiation pulse may be produced with a duration of the order of picoseconds.

BACKGROUND OF THE INVENTION 
The invention relates to a method and apparatus for the synchronous mode 
locking of longitudinal laser modes in a semiconductor diode laser. 
Optical light pulses with a duration on the order of picoseconds and 
fractions thereof may be generated by locking the phases of oscillation 
modes excited in a laser resonator, i.e. by so-called "mode locking". The 
minimum pulse width then possible decreases with an increase in the gain 
band width of the laser-active medium. Consequently laser media with a 
large gain band width, as for example solutions of organic dyes, F center 
crystals and semiconductors are more especially suitable for producing 
pulses of optical radiation of extremely short duration. 
A commonly-used form of mode locking is synchronous optical pumping or 
excitation of a laser by a mode-locked second laser with a lesser band 
width, see for example the paper of W. H. Glenn et al. in Appl. Phys. 
Lett. 12, 54, 1968. To cause synchronous mode locking the optical length 
of the laser producing the pumping pulses has to be equal to the optical 
length of the laser resonator of the pumped laser or to an integral 
multiple thereof. Synchronously locked dye laser systems on these lines 
are commercially available and make it feasible to produce picosecond 
light pulses in the visible and near infrared range as far as 
approximately 0.9 .mu.m. 
When compared with dye lasers etc. semiconductor lasers may be seen to be 
characterized by such features as compactness (typical dimensions being 
200 .mu.m by 200 .mu.m by 100 .mu.m) and more especially by the simple 
method of excitation using an electric current and by the small power 
requirement (typically being some ten to some hundred mW). Furthermore, 
semiconductor lasers may cover the full spectral range between about 0.7 
.mu.m and 30 .mu.m. Semiconductor lasers for the spectral range of 0.7 to 
1.6 .mu.m have reached an extraordinarily advanced stage of technical 
perfection and at room temperature may be operated continuously for 
periods far in excess of 100,000 hours. 
A number of different methods have been developed for mode locking 
semiconductor lasers, more especially: 
(a) Passive mode locking (E. P. Ippen et al., Appl. Phys. Lett. 37, 267 
(1980); J. P. van der Ziel et al., Appl. Phys. Lett. 39, 525 (1981)); 
(b) Active-passive mode locking (J. P. van der Ziel et al., Appl. Phys. 
Lett. 39, 867 (1981)); 
(c) Active mode locking by gain modulation (J. P. van der Ziel et al., 
Journal Appl. Phys. 52, 4435 (1981); J. C. AuYeung et al., Appl. Phys. 
Lett 40, 112 (1982)); 
(d) Synchronous mode locking with optical excitation (R. S. Putman et al., 
Appl. Phys. Lett. 40, 660 (1982)). 
For methods (a), (b) and (d) it is not possible to use commercially 
available semiconductor diode lasers. Method (a) and (b) necessitate an 
elaborate preparation of one laser end (involving proton or ion 
bombardment for producing an internal saturable absorber). Generally 
method (d) requires cooling of the semiconductor laser diode to low 
temperatures. 
In method (c) mode locking is produced by gain modulation, that for its 
part results from a modulation of the feed current of the semiconductor 
diode. The modulation of the current takes place purely electronically 
using radio frequency or pulse generators. As is the case with the methods 
(a) and (b) as well, in method (c) simultaneous generation of synchronized 
mode locked radiation pulse trains is not possible at different emission 
wavelengths. Not one of the above-noted methods is compatible with 
commercial synchronously pumped dye laser systems. 
Furthermore the publication of E. O. Goebel et al. in Appl. Phys. Lett. 42 
(1), Jan. 1, 1983, pages 25 to 27 refers to the use of a high-speed 
optoelectronic GaAs switch to modulate the gain of a semiconductor laser. 
The switch is controlled by the radiation pulses of a mode locked dye 
laser. However, the emission of the semiconductor laser is not mode 
locked. 
SHORT SUMMARY OF THE PRESENT INVENTION 
One object of the present invention is to devise a simple method and a 
simple apparatus for the synchronous mode locking of the longitudinal 
laser modes of a semiconductor laser, with which the laser radiation 
pulses may be generated in a wide wavelength range and with a short 
duration, more especially on the order of picoseconds and less. 
In order to effect this and other aims, in the invention a semiconductor 
laser diode, that is located in an external optical resonator, is supplied 
with current from an optoelectronic pulse generator, that comprises an 
optoelectronic switch controlled by the radiation pulses from a master 
laser. 
In the present method the semiconductor laser is controlled by electrical 
driving pulses with a high time stability, this constituting a basic 
requirement for optimum mode locking of the semiconductor diode laser 
synchronously excited by such pulses. 
The semiconductor laser is operated in an external optical resonator, one 
crystal end face of the semiconductor laser diode can be used as the exit 
mirror if desired. The reflectivity of the second crystal end face, that 
is placed in the optical path of the laser radiation, is preferably 
decreased, as for example by producing dielectric layers thereon by vapor 
coating. This A R coating may be undertaken on regular commercial 
semiconductor laser diodes at litte expense. 
The synchronous mode locking is produced by matching the length of the 
external resonator with the length of the master laser controlling the 
optoelectronic switch, this being in keeping with the principle of Glenn 
disclosed in the above-mentioned publication. 
In consequence of the synchronous gain modulation of the semiconductor 
laser diode being by electrical excitation, it is possible (unlike the 
case of optical excitation) to use commercially available semiconductor 
laser diodes. Apart from the A R coating of one cyrstal end face, that has 
been necessary in all methods of mode locking proposed so far, no further 
modification of the laser diode is needed, in contrast to former methods 
for active-passive and passive mode locking in accordance with the 
publications noted supra. 
As compared with synchronously optically pumped mode locked dye laser 
systems, the novel laser systems disclosed herein are more economical by 
several orders of magnitude. In principle, the invention makes possible 
the adaptation of existing synchronously pumped mode locked dye laser 
systems, since the gas lasers employed for synchronous optical pumping may 
be used without any modification for driving the optoelectronic switch of 
the laser system in accordance with the present invention. By such a 
subsequent adaptation the available spectral range of picosecond laser 
systems, that supply an uninterrupted pulse train, may be considerably 
extended. 
In addition to the switch element referred to in the paper by Goebel et 
al., the optoelectronic switch may be of another type, as for example a 
conventional photodetector with a suitably fast response characteristic 
and a sufficiently high dynamic range, and for example avalanche 
photodiodes and PIN photodiodes. 
The optoelectronic semiconductor switch may with advantage be fabricated so 
as to be integrated with the respective semiconductor laser, as for 
example on a common insulating substrate so that the present method may be 
practiced with an extremely compact component as may be produced by 
presently available semiconductor technology.

DESCRIPTION OF A PREFERRED EMBODIMENT 
The apparatus shown in FIG. 1 comprises, as its main parts, a mode locked 
master laser 10, a semiconductor laser 1,2, an excitation or pump power 
supply 14 for the semiconductor laser and a high-speed optoelectronic 
switch 16, that is connected between the pump power supply 14 and a 
semiconductor laser diode 18 of the semiconductor laser 12. 
As shown, the master laser 10 is preferably a mode locked gas ion laser, 
such as an Ar.sup.+ or a Kr.sup.+ laser for example, that may comprise a 
laser gas vessel 20 and a mode lock prism 22, that is located in an 
optical resonator, that is delimited by a 100% reflecting mirror 24 and a 
partially transmissive mirror 26. The laser 10 may be of conventional 
construction so that the pump energy supply is omitted for the sake of 
simplicity. 
In lieu of a gas laser it is possible to utilize another type of laser as a 
master laser 10, as for example a mode locked solid state laser, such as 
an Nd:YAG or ruby laser, or a mode locked dye laser. 
The semiconductor laser diode 18 may be a commercially available BH 
GaAs/GaAlAs semiconductor diode (BH=buried heterostructure). 
The optoelectronic switch 16 comprises a photoconductor element 28, that 
consists of chromium doped gallium arsenide. In place of such a GaAs:Cr 
photoconductor it is also possible to employ avalanche or PIN photodiodes 
with a sufficiently fast response characteristic. 
As shown, the optoelectronic switch 16 is connected between the inner 
conductor and the outer conductor of two radio frequency or transmission 
lines, such as coaxial cables 30 and 32, respectively, of which the one 
cable 30 connects the photoconductor 28 with the excitation energy source 
14 and the other cable 32 connects the photoconductor 28 through a 
series-connected matching resistor 34 with the laser diode 18. The 
resistor 34 matches the typically relatively low resistance of the laser 
diode 18 to the characteristic impedance of the radio frequency line 32, 
but may be omitted. The semiconductor laser diode would then essentially 
form a short circuit of the radio frequency line. 
The semiconductor laser 12 comprises an external optical resonator, that at 
one end is defined by a mirror 36 whose reflectivity is as high as 
possible and at the other end by a crystal end face 38 of the 
monocrystalline semiconductor member of the laser diode 18, said end face 
38 reflecting only partially. The other crystal end face, that is opposite 
to the crystal end face 38 functioning as an exit mirror, is provided with 
an antireflection (AR) layer 40. The optical cavity or resonator of the 
semiconductor laser 12 includes frequency selective means or optical band 
filter means 42, as a thin film interference filter and/or a Fabry-Perot 
etalon. The optical resonator of the semiconductor laser may be defined 
alternatively by a pair of external mirrors with different reflectivities, 
it then being preferred for the two crystal end faces of the semiconductor 
member of the laser diode to have an additonal A R coating for producing a 
suitable modification of its reflection properties. In the case of the use 
of a resonator with only one external mirror the crystal end face, 
functioning as a second resonator mirror, may be modified and more 
specially enhanced in its reflectivity by an additional dielectric coating 
in order to modify the power and duration of the output pulses. 
The length of the resonator of the semiconductor laser 12 is able to be 
adjusted for matching with respect to the optical length of the resonator 
of the master laser 10 by displacing the reflector 36. The reflector 36 is 
for this purpose supported by an adjustment device 44, that comprises a 
coarse adjustment device 46 and a fine adjustment device 48. The coarse 
adjustment device 46 may, as illustrated, comprise a carriage that is 
adjusted by a lead screw, the fine adjustment device 48 being mounted on 
the carriage. The fine adjustment device 48 may be a conventional 
piezoelectric transducer, on which the reflector 36 is mounted. 
OPERATION 
For operation the first step is to match the length of the resonator 36-38 
of the semiconductor laser 12 to the length of the resonator 22-24 of the 
master laser 10. The optical length of the resonator of the master laser 
10 may be equal to, or an integral multiple of, the length of the 
resonator of the semiconductor laser. The master laser is operated, as 
known, with mode locking and supplies an optical pulse train 50 of short 
optical pulses, whose duration may for example be of the order of 100 
picoseconds and less. 
Such pulses control the optoelectronic switch 16, that in response to each 
pulse of the pulse train 50 shortcircuits the end, remote from the laser 
diode 18, of the radio frequency line 32. Thus, short current pulses, that 
are very accurately timed, and are produced by the discharge of the 
capacitance of the radio frequency line 30 are superimposed on the 
relatively low constant current normally flowing through the semiconductor 
laser diode. These current pulses modulate the gain of the laser diode, 
that consequently, cooperating with the matched optical resonator 36-38 
supplies very short, mode locked optical output pulses 52, the duration 
whereof is of the order picoseconds and less. 
The wavelength of the output radiation of the semiconductor laser 12 may be 
adjusted in a conventional manner using the band filter 42. 
Furthermore the present invention provides a simple way of mode locking two 
or more semiconductor lasers in external resonators synchronously in 
parallel. For this purpose the pulse train of the master laser 10 may be 
split up by beam splitters 60a, 60b, 60c etc. into two or more component 
beams 62a, 62b 62c as is illustrated in FIG. 2 for the case of three beam 
splitters and three component beams. Each of the component beams controls 
a corresponding optoelectronic switch 16a, 16b or 16c, respectively, that 
for its part then controls a semiconductor laser, not shown in FIG. 2, 
corresponding to the semiconductor laser 12 in FIG. 1. Such parallel 
operation of a number of semiconductor lasers is more especially made 
possible by the fact that the power, that is needed for controlling the 
optoelectronic switches and thus the semiconductor lasers, is extremely 
low. 
The master laser may furthermore be a pulsed mode locked laser, which then 
supplies corresponding bursts. The synchronously mode locked semiconductor 
laser or lasers then supply corresponding bursts. This latter feature is 
more particularly significant when using passively mode locked Nd:YAG or 
ruby lasers and flash lamp pumped dye lasers. 
It will be appreciated that the invention provides a method for producing 
optical radiation pulses with a duration in the order of picoseconds 
(10.sup.-12.sbsp.s) and less by synchronous mode locking of the 
longitudinal laser modes of a semiconductor laser element operating in an 
external resonator. In the method of the invention the gain modulation, 
necessary for synchronous mode locking, of the active semiconductor laser 
element is effected by modulation of the electric pumping current, the 
modulation of the pumping current in turn being effected by a high-speed 
optoelectronic switch, that for its part is controlled by a mode locked 
laser. The present method combines the advantages of synchronous optical 
pumping, as employed in commercial mode locked dye laser systems, with the 
simple type of electrical pumping of commercially available semiconductor 
laser diodes, more especially double heterostructure laser diodes. It 
represents a simple and economic alternative to synchronously pumped dye 
laser systems and substantially increases the spectral range, in which 
radiation pulses may be produced with a duration of the order of 
picoseconds. 
In a practical embodiment of the invention, the master laser 10 was a 
commercially available actively mode locked Ar.sup.+ ion laser of the 
company Spectra Physics, Model 171. The photoconducting element 28 
comprised a member measuring 1 mm by 5 mm by 0.3 mm of commercially 
available, semi-insulating, Cr-doped gallium arsenide substrate material 
with a typical resistivity of 10.sup.8 ohm.cm, as available from Wacker 
Chemitronic, Burghausen, Western Germany. The parts of the rf. lines in 
contact with the body of the photoconductor 38 are formed by a stripline. 
The width of the stripline was 250 .mu.m, the thickness of the 
photoconductor member being matched to this. 
The gap between the ends of the stripline on the side facing the master 
laser 10 of the photoconductor 28 was 25 .mu.m wide. The stripline had a 
characteristic impedance of 50 ohms. 
The length of the rf. line 30 is not critical. In practice a 1 meter piece 
of commercial coaxial cable with a characteristic impedance of 50 ohms was 
used. The rf. line 32 has to be as short as possible. In the embodiment it 
was a 5 centimeter length of 50 ohm coaxial cable. The voltage of the 
power supply 14 typically amounts to 25 volts maximum. The resistance of 
the matching resistor 34 depends on the resistance of the optoelectronic 
switch in the illuminated condition and may be between 1 and 50 ohms. In 
the present case a matching resistance 34 of 50 ohms was employed. 
The laser diode 38 was a diode of the type HL-3400 of The Hitachi Company, 
Japan. The AR coating of the end face 40 was produced in a known manner 
(see for example G. Eisenstein, L. W. Stulz, Applied Optics 23, 161 
(1984)) in the form of a dielectric quarter wavelength coating. The 
wavelength of the radiation from the master laser amounted to 514.5 nm. 
The length of the resonator of the master laser and of the external 
resonator of the laser diode amounted each to approximately 180 cm. The 
laser light emitted from the exit face 40 was collimated with a microscope 
objective with a focal length of 0.25 cm. For checking the bandwidth and 
tuning the laser diode output radiation use was made of a narrow-band 
interference filter (bandwidth 5 nm), in combination with an additional 80 
.mu.m thick etalon (R=30%). 
The laser diode supplied radiation pulses with a wavelength of 841 nm and a 
duration of 30 ps FWHM with a repetition rate of 80.32 MHz and a mean 
output power of typically 250 microwatts.