Single longitudinal mode laser without seeding

The use of feedback circuit in a laser device with a frequency selective element such as an etalon allows for the production of a substantially single longitudinal mode output indefinitely. The feedback circuitry is adapted to adjust the length of the laser cavity so that a longitudinal mode of the laser device is near transmission peak of the frequency selective element.

BACKGROUND OF THE INVENTION 
The present invention relates to laser devices that produce an output that 
is substantially a single longitudinal mode. In some applications, it is 
desired that the output of a laser device be substantially a single 
longitudinal mode. One benefit of producing a single longitudinal mode 
output is the reproducibility of the pulse outputs. Reproducibility of 
pulse outputs is desirable for some measurements or scientific 
experiments. As discussed below, more than one longitudinal mode in an 
output will modulate the output. This modulation may be random and 
unpredictable. 
Additionally, the laser device may be used in a system with non-linear 
crystals. Non-linear crystals are used in frequency doublers, and 
parametric oscillators or amplifiers. The conversion efficiencies of 
non-linear crystals may depend upon the pulse-shape of the laser device 
output. For that reason, it is desired that the laser device output be 
substantially a single longitudinal mode so that the pulse shape is 
predictable. 
One difficulty maintaining a laser device in a single longitudinal mode is 
that the cavity length and gain characteristics of the laser device drift 
with temperature changes. 
It is desired to have a laser device that can maintain its output in a 
single longitudinal mode even when the cavity length and gain 
characteristics drift with temperature changes. 
SUMMARY OF THE INVENTION 
An advantage of the present invention is the use of feed-back circuitry to 
adjust the length of the laser cavity so that substantially only a single 
longitudinal mode is output from the laser device. The laser device of the 
present invention does not require any seeding from another laser. 
The present invention uses a frequency selective element, such as an 
etalon, in the laser cavity to narrow the frequency range of the round 
trip gain characteristics of the laser device. The frequency selective 
element has a transmissive peak at a certain frequency or frequencies. 
Varying the length of the cavity shifts frequencies of the longitudinal 
modes of the laser device. In this way, the feed-back circuit can maintain 
the frequency of the single longitudinal mode near a transmissive peak of 
the frequency selective element. 
Maintaining the frequency of a longitudinal mode near the transmissive peak 
of the frequency selective element is necessary because the cavity length 
and transmissive characteristics of the frequency selective element can 
drift with temperature. In many cases, it is impractical to maintain the 
temperature of the laser device to the precision required to prevent the 
effects of the temperature drift. 
Producing a substantially single longitudinal mode output of a laser device 
requires that one longitudinal mode be amplified much more than 
neighboring longitudinal modes. Even relatively small amounts of 
additional longitudinal mode radiation in the output of the laser device 
can cause modulation or "beating" in the output. 
The feedback circuitry in one embodiment of the present invention minimizes 
the time between the start of a pump pulse and the detection of 
"pre-lasing" radiation. The "pre-lasing" radiation shows up as a series 
of spikes and is often called "relaxation oscillation". The closer the 
frequency of the longitudinal mode is to the transmissive peak of the 
frequency selective element the shorter it will take for the "pre-lasing" 
radiation to appear. The "pre-lasing" radiation is detected soon after the 
gain of the gain material overcomes the cavity losses. The timing of the 
"crossover" point is dependent on the cavity losses. The intracavity 
losses are minimum when the lasing mode coincides in frequency with the 
frequency selective elements resonance. 
The present invention can also use a Q-switch in the laser cavity. The 
Q-switch can be adjusted so that when the Q-switch is "off" a certain 
amount of transmission is allowed. This can be done by detuning a 
quarter-wave plate from its fully "closed" position so that some radiation 
passes through the Q-switch when it is "off". The detuning of the 
quarter-wave plate can be adjusted so that "pre-lasing" radiation is 
detected near the end of the pump pulse so that more of the pump pulse 
energy is available to the Q-switched output pulse. 
Another part of the present invention comprises keeping "pre-lasing" 
radiation many round trips within the laser cavity before it is output in 
a Q-switched pulse. The build up of the "pre-lasing" radiation to 
detectable levels once the gain becomes greater than the cavity loses is 
relatively slow since the gain is only slightly greater than the 
intracavity losses. It can take thousands of round trips in order to build 
up the "pre-lasing" radiation to detectable levels. The large number of 
round trips within the cavity greatly increases the frequency selectivity 
of the laser device. The detected "pre-lasing" radiation is for that 
reason substantially a single longitudinal mode. Once the pre-lasing 
radiation is detected, the Q-switch switch is turned on and the 
amplification of the laser device radiation is greatly increased. The 
"pre-lasing" radiation is quickly amplified into a Q-switched giant pulse 
which maintains the substantially single longitudinal mode character of 
the "pre-lasing" radiation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 is a graph that shows a computer simulation of a Q-switched pulse 
where a main longitudinal mode is modulated by a neighboring longitudinal 
mode. The modulation is a waveform having a frequency equal to the 
frequency difference between the main and neighboring longitudinal modes. 
Ideally, the neighboring longitudinal modes are completely suppressed. In 
practice, this is not possible but the modulation depth can be kept below 
some predetermined amount. As seen in FIG. 1., the modulation depth is 
defined as M/A. The modulation depth is related to the intensity of the 
main and side longitudinal modes by the equation 
##EQU1## 
wherein the output intensity of the main longitudinal mode is I.sub.m and 
the output intensity of the neighboring mode is I.sub.n. For this reason, 
in order to keep the modulation depth below 4%, it is necessary that 
##EQU2## 
FIG. 2 is a schematic view of the laser device 20 of the present invention. 
The laser device 20 is designed to produce substantially single 
longitudinal mode radiation. In a preferred embodiment, the laser device 
will make use of second and fourth harmonic generation in nonlinear 
crystals (not shown) to generate nanosecond pulses at 266 nm, at a 
repetition rate of 300 Hz. Pulse-to-pulse energy stability in the 
harmonics is desired since one of the foreseen applications of the machine 
will be in micro-electronics and micromachining. Due to the unpredictable 
effects of the modulation by side longitudinal modes, the modulation depth 
of the Q-switched output of the laser device 20 must be minimized. 
The laser device 20 defines laser cavity between the rear mirror 22 and 
output mirror 24. The laser device will support longitudinal modes at 
frequencies such that the cavity length is equal to an integer number of 
half-wavelengths. The cavity length in a preferred embodiment is 20 cm so 
that the free spectral range of the resonator, or the frequency separation 
between longitudinal modes, is about 0.75 Ghz. 
Arranged in the laser cavity is the gain material 26. The gain material 26 
is preferably made of Nd:YAG. Nd:YAG is a well known laser material with a 
peak absorption frequency of 808 nm. Other gain materials can be used in 
the present invention, such as Nd:YLF, Nd:YVO.sub.4, Nd:Doped Glass, or 
any other lasant material. The gain material 26 is pumped by stacks 28 of 
laser diode bars positioned along opposite sides of the gain material 26. 
The gain material 26 has Brewster angled ends to define the preferred 
polarization of the laser beam. 
FIG. 3 is a graph that shows a computer simulation of the Lorenzian gain 
profile of the gain material for a single pass amplification. The 
frequency scale along the bottom shows the frequency difference from the 
center frequency of the gain material. 
Looking again at FIG. 2, a frequency selective element 30 such as a 
Fabry-Perot etalon is placed in the laser cavity. The etalon is a piece of 
glass or fused-silica with parallel faces having a reflective coating 
placed on the faces. In a preferred embodiment, the etalon comprises a 5 
mm thick slab of fused-silica with a 40% reflective coating on the faces. 
The etalon is available from CVI located in Albuquerque, N. Mex. 
FIG. 4 is a graph of a computer simulation showing the transmissive peaks 
100, 102 and 104 of an etalon. The etalon has transmissive peaks, such as 
transmissive peaks 100,102 and 104, of near 100% transmission at 
frequencies such that the etalon optical length is equal to an integer 
number of half-wavelengths. 
The etalon can be placed on a temperature controlled mount (not shown) to 
get some control of the frequency of a transmissive peak of the etalon 
using the 2.7 Ghz/.degree.C. tuning coefficient for fused silica etalons. 
In this manner, the transmissive peak 102 of the etalon can be placed near 
the center frequency (shown in FIG. 3) of the gain material 26, shown in 
FIG. 2. 
The frequency selective element may alternately be a resonant reflector. 
The resonant reflector is comprised of two slabs of a high refractive 
index material such as YAG separated by a spacer. One of the slabs has 
parallel faces. The spacer also has parallel faces so that three faces of 
the two slabs are parallel to each other. The fourth face of the slabs has 
an anti-reflective coating placed thereon. The fourth face need not be 
parallel to the other three faces. The resonant reflector has reflectivity 
peaks at close to 51% reflectivity. This resonant reflector could be used 
as an output mirror of the laser device. In this manner, the "pre-lasing" 
radiation having frequencies close to a reflectivity peak of the resonant 
reflector would be maintained within the laser cavity longer. A large 
number of round trips within the cavity will sharpen this frequency 
selective effect. 
FIG. 5 is a graph of a computer simulation showing the round trip 
amplification in the laser device of the present invention. This graph 
shows the effects of the Lorenzian lineshape of the gain material, the 
etalon transmissive peaks and shows the positions of different 
longitudinal modes. 
The preferred embodiment of the present invention has "pre-lasing" 
radiation staying within the laser cavity for a large number of round 
trips so as to reduce the modulation depth of the Q-switched output. This 
effect can be seen in FIGS. 6-7. FIG. 6 is a graph of a computer 
simulation of the amplification in the laser device after 10 round trips. 
Note that the amplification of the neighboring longitudinal modes 108 and 
110 are within an order of magnitude of the amplification of the main 
longitudinal mode 106. FIG. 7 is a graph of a computer simulation showing 
the amplification in the laser device after 288 round trips. In this case, 
the amplification of the neighboring longitudinal modes 114 and 116 is 
10.sup.-4 times the amplification of the main longitudinal mode 112. This 
means that the modulation depth of a Q-switched pulse will be about 4% if 
the Q-switched pulse maintains this amplification ratio. As discussed 
below, in a preferred embodiment, the "pre-lasing" radiation makes over 
1000 round trips within the cavity. 
Looking again at FIG. 2, one way of causing the "pre-lasing" radiation to 
make many trips within the laser cavity involves the Q-switch 32. The 
Q-switch 32 comprises a quarter-wave plate 34 and a Pockels cell 36. In 
the typical Q-switch operation, transmission through the Q-switch is very 
low when the Q-switch is off, and high when the Q-switch is switched on so 
that an output Q-switched pulse quickly uses the energy stored in the gain 
material. FIG. 8 is a graph of a computer simulation showing Q-switched 
pulse evolution without "pre-lasing". Before time A, the Q-switch is off 
so that the gain in the laser device is smaller than the threshold gain 
set by the laser devices losses. The gain material can be pumped with a 
pump pulse in this period without losing energy due to lasing. At time A, 
the Q-switch is turned on, dropping the threshold gain. From time A to 
time B (at about 60 round trips), the actual gain is much greater than the 
threshold gain so that lasing occurs very rapidly. Within about 60 round 
trips the energy within the gain material is used up so that the actual 
gain drops below the threshold gain at time B. For the system shown in 
FIG. 8, a single longitudinal mode output cannot be maintained since there 
is not enough passes through the etalon to adequately suppress the 
neighboring longitudinal modes. 
Looking again at FIG. 2, a small detuning in the quarter-wave plate 34 from 
its fully closed orientation can increase the transmission through the 
Q-switch in its "off" state from a nominally zero amount to an amount 
proportional to sin .sup.2 (2.PSI.) where .PSI. is the quarter-wave plate 
detuning angle. As discussed below, the quarter-wave plate can be detuned 
until "pre-lasing" radiation is detected near the end of the pump period. 
FIG. 9 is a graph of a computer simulation showing Q-switched pulse 
evolution in the presence of "pre-lasing". At time A, the gain in the 
laser device becomes greater than the threshold gain of the system and 
"pre-lasing" begins. From time A to time B, the actual gain increases 
linearly as the gain material is pumped. The difference between the actual 
gain and the threshold gain is small, however, so that it takes many 
cavity round-trips for the optical intensity in the laser cavity to build 
up to a detectable intensity. Because of the over a thousand round-trips 
in the laser cavity, the "pre-lasing" radiation is overwhelmingly one 
single longitudinal mode. At time B, the "pre-lasing radiation" is 
detected and the Q-switch is turned on. The "pre-lasing" radiation is 
quickly amplified into a Q-switched output pulse that maintains the 
substantially single longitudinal mode character of the "pre-lasing" 
pulse. The Q-switched output pulse quickly uses up the pump pulse energy 
stored in the gain material so that at time C the actual gain is below the 
threshold gain. 
Looking again at FIG. 2, a polarizer 38 deflects some cavity radiation to a 
sensor such as photo-diode 40. When the photo-diode 40 detects a certain 
level of "pre-lasing" radiation the Q-switch 32 is turned on. 
A problem with maintaining the Q-switched output as substantially a single 
longitudinal mode is temperature drift in the laser device. The 
frequencies of the longitudinal modes drift with temperature due to the 
thermal expansion of the laser cavity. A figure for this thermal drift may 
be around 7 Ghz/.degree.C. for the frequencies and temperatures of 
interest. The transmission peaks of the etalon and the peak of the gain 
material also drift with temperature changes. For some applications, 
maintaining the desired temperature stability with an oven would be 
impractical. 
FIG. 10 is a graph of the amplification of the laser device after 288 round 
trips showing the effects of temperature drift. This figure shows a 
frequency shift, .DELTA.f, of half the free spectral range or longitudinal 
mode separation. This frequency shift could be the result of a combination 
of any of the temperature drifts discussed above. The result of this 
temperature drift is that the intensity of longitudinal modes 120 and 122 
are equal and the laser device is far from a single longitudinal mode 
output. Of course, smaller frequency shifts will also increase the 
modulation depth of the Q-switched output. It is impractical to control 
the temperature accurately enough to maintain the output in a 
substantially single longitudinal mode output over long periods of time. 
Looking again at FIG. 2, the laser device 20 is maintained in a 
substantially single longitudinal mode by the feedback circuitry 42 and 
piezoelectric actuator 44. The piezo-electric actuator 44 is attached to 
the rear mirror 22. A change of the voltage input to the piezo-electric 
actuator 44 adjusts the cavity length by a certain small amount. Changing 
the cavity length shifts the frequencies of the longitudinal modes. The 
feedback circuitry 42 is adapted to control the piezoelectric actuator 44 
so as to adjust the length of the laser cavity so that a longitudinal mode 
of the laser device is near the frequency of a transmission peak of the 
frequency selective element 30. One way of maintaining a longitudinal mode 
near the frequency of a transmission peak of the frequency selective 
element 30 is to minimize the period of time between the beginning of a 
pump pulse to the gain material 26 and the detection of the "pre-lasing" 
radiation in the laser cavity. 
FIG. 11 is a graph of a computer simulation showing the gain evolution at a 
center and at an offset frequency. If the pump pulse to the gain material 
is rectangular, the gain within the laser device will grow at a linear 
rate during the pump pulse. The slope of the gain will be slightly higher 
at the center frequency rather than at the offset frequency. More 
importantly the losses will be greater at the offset frequency than at the 
center frequency. The center frequency is at the frequency of a 
transmissive peak of the frequency selective element. The "pre-lasing" 
radiation is detected soon after the gain over takes the losses. The 
detection time of the "pre-lasing" radiation is the shortest at the 
frequency of the transmissive peak of the frequency selective element. As 
discussed above, adjusting the quarter-wave plate also adjusts the level 
of the losses so that the "crossover" point can be set near the end of the 
pump pulse. In this manner, the Q-switched output pulse can use as much of 
the pump energy as possible. 
FIG. 12 is a graph of a computer simulation showing the "pre-lasing" 
radiation Build-up-Time versus rear mirror displacement. At Zero 
displacement, one longitudinal mode is at the frequency of a transmissive 
peak of the etalon. At .lambda./4 displacement, two longitudinal modes are 
equally transmitted through the etalon as was shown in FIG. 10. 
FIG. 13 is a block diagram showing an embodiment of the feedback circuit 
42' of the present invention. In one embodiment, the feedback circuit 42' 
causes the cavity length to alternate on different pump pulses between 
different lengths and the feedback circuitry 42' attempts to minimize the 
difference between the build-up-times at the two lengths. The time to 
voltage converter 130 converts the build-up-time into a voltage. This 
voltage is sent to a sample-and-hold circuit 132. When two voltages are 
stored in the sample-and-hold circuitry 132 corresponding to two different 
dithered cavity lengths, these voltages are sent to a difference amplifier 
134. This difference is the error signal used by the feedback circuit. 
Looking again at FIG. 12, three sets of dithered rear mirror displacements 
A,B, and C are shown. For set A, position I has a greater build up time 
than position II. For set B, position I has an equal build up time as 
position II. For set C, position I has a lesser build up time than 
position II. FIG. 14 is a graph of a voltage or current error signal 
created by the feedback circuitry for sets A, B, and C of FIG. 12. The 
feedback circuitry will work to set the error signal as close to zero as 
possible. 
Looking again at FIG. 13, the error signal is an indication of the desired 
change in position of the piezoelectric actuator 44'. This error signal is 
sent to an integrator 136 to get a voltage indication, V.sub.int, of a 
desired position of the piezoelectric actuator 44'. This voltage, 
V.sub.int,is added to the alternating positive and negative dither voltage 
V.sub.dl. The high voltage bridge amplifier 150 produces a voltage, 
V.sub.hv, which is a multiple of the input voltage. This voltage V.sub.hv 
is used to drive the piezoelectric actuator 44'. 
Due to the feedback circuit 42', the length of the cavity will change so 
that a longitudinal modes will track the peak transmissive frequency of 
the etalon. If because of a temperature shift, the peak transmissive 
frequency of the etalon increases or decreases, the length of the cavity 
will shift so a longitudinal mode is near the peak transmissive frequency 
of the etalon. If because of a temperature shift, the cavity length 
increases or decreases, the length of the cavity will shift back due to 
the feedback circuitry. The dithering caused by the feedback circuitry 42' 
means that a longitudinal mode is not exactly at a peak transmissive 
frequency of the etalon. Due to the large number of round-trips of the 
"pre-lasing" radiation in the laser cavity, a substantially single 
longitudinal mode output is produced. This single longitudinal mode output 
can be maintained indefinitely due to the feedback circuit. 
The feedback circuit discussed above will work in a similar manner if 
instead of placing an etalon in the laser cavity a resonant reflector is 
used as an output mirror. At a reflectivity peak of the resonant 
reflector, the build-up-time of the "pre-lasing" radiation will be at a 
minimum because more of the "prelasing" radiation will be reflected back 
into the laser cavity. In order to minimize the build-up-time, the 
feedback circuit will adjust the cavity length so that a longitudinal mode 
of the laser cavity is close to the reflectivity peak of the resonant 
reflector. 
FIGS. 15-18 show a preferred embodiment of the feedback circuit. FIG. 15 is 
a schematic showing time-to-voltage converter 130'. The reset time for a 
preferred embodiment is about 250 .mu.secs. FIG. 16 is a schematic showing 
timing circuitry 138' and Sample-and-Hold circuit 132'. FIG. 17 is a 
schematic showing difference amplifier 134', Integrator 136' and adder 
139', timing circuitry 138' and dither circuitry 140'. FIG. 18 is a 
schematic showing the High Voltage Bridge Amplifier 150'. 
The feedback circuit could be designed in a different manner than that 
described above. For example, the feedback circuit could be implemented in 
a "digital" rather than "analog" fashion. Additionally, the feedback 
circuit could include a computer which uses a computer program to control 
the level of feedback. 
Looking again at FIG. 2, pin hole 46 is placed in the cavity to ensure the 
output is a TEM.sub.00 transverse mode rather than some higher order 
transverse mode. Cylindrical lens 48 and spherical lens 50 may also be 
placed in the laser cavity to shape the mode volume in the cavity and 
maximize energy extraction. 
In a preferred embodiment, the piezoelectric actuator 44 is available from 
the DDO Corporation, Electro-Ceramic division located in Salt Lake City, 
Utah; the Pockels cell 36 is available from Inrad Incorporated located in 
North Vale, N.J.; and the gain material 26 is available from Litton 
Airtron located in Charlotte, N.C. 
Various details of the implementation and method are merely illustrative of 
the invention. It is to be understood that various changes in such details 
may be within the scope of the invention, which is to be limited only by 
the appended claims.