Pulsed solid state ring laser injection locking stabilizer

An injection seeded, single frequency ring laser source is presented wherein stabilization and single frequency control is accomplished by measuring the intensity or power of the portion of the high power laser beam generated in the ring slave laser and which is directed to return to the seed laser. When the intensity of the return beam falls below a preset threshold level, the in-phase operation of the laser system has been established and high energy single frequency pulses are generated and emitted.

FIELD OF THE INVENTION 
This invention relates to an injection-seeded stabilized laser source for 
use in interferometric or heterodyne detection systems wherein the ring 
slave oscillator provides a powerful output pulse of optical energy at a 
single frequency suitable for laser radar applications. 
BACKGROUND OF THE INVENTION 
Doppler laser radar systems operating in heterodyne detection mode are used 
for remote measurements of atmospheric winds. In order to be used within a 
laser radar system, the laser must provide a powerful pulse of optical 
energy at a very narrow bandwidth (single longitudinal and transverse 
mode). Such a narrow line is achieved utilizing two laser components in an 
injection-seeding master oscillator/power-amplifier system. Generally, in 
such a system the master oscillator or seed laser is a low power highly 
stable, continuous wave laser while the power amplifier or slave laser is 
a high power pulsed laser operating at a single frequency established by 
the seed laser. 
Utilizing a ring resonator configuration for the slave oscillator provides 
important advantages such as: a traveling wave (eliminating "spatial hole 
burning"); a long resonator within a compact frame; and direct and simple 
laser seeding. Active control of the ring resonator length is essential 
for single frequency operation of the laser. U.S. Pat. No. 5,099,486 
presents a ring laser resonator invention with a type of resonant path 
length control which, although independent of the invention presented 
herein, may be utilized in combination with the present invention to 
improve path length control. 
It is an object of this invention to present a method and apparatus for 
providing a high power, high repetition rate, single frequency laser 
source. 
It is another object of this invention to present a new method and 
apparatus for providing injection-seeding feedback control for ring slave 
lasers for use in such laser sources. 
It is another object of the present invention to provide an active 
injection seeding stabilizer for ring slave lasers for use in such laser 
sources. 
It is a further objective of this invention to provide an actively 
stabilized injection seeded ring laser system. 
SUMMARY OF THE INVENTION 
This invention presents a novel laser source with a technique for 
stabilization of an injection seeded ring laser that enhances its use in 
interferometric or heterodyne detection modes. The laser emits single 
frequency pulses of high power with high repetition rate capability. 
The fundamental embodiment of the invention includes a stable single 
frequency. continuous wave master oscillator coupled to a higher power 
ring slave oscillator through an optical isolator. The slave oscillator is 
activated to generate single mode pulses of energy at the master 
oscillator single frequency which is controllable within the slave 
oscillator ring resonant cavity by one or more adjustable corner mirrors. 
The single frequency operation of the slave laser is achieved via active 
longitudinal mode selection performed by a feedback resonator length 
control. The slave laser within the ring resonant cavity of the slave 
oscillator normally allows bi-directional flow of the laser beam around 
the ring resonant cavity. However, when tuned correctly, the injection 
seeding of the single frequency master oscillator forces the slave laser 
to mainly lase unidirectionally in the direction of the seed beam. 
Tuning the slave laser ring resonant cavity is accomplished by adjusting 
the total pathlength of the ring cavity to equal an integral multiple of 
the wavelength emitted by the master oscillator. This ring resonator 
length control is enable by monitoring the return beam output (in the 
opposite direction from the master oscillator beam path) from the slave 
oscillator back to the optical isolator located between the master and 
slave oscillators. This return beam is diverted by the optical isolator to 
be monitored by a detector. The measured pulse power and build-up time is 
then fed to a controller which controls the position of a controllable 
mounted corner mirror which adjusts the ring resonator pathlength. Once 
resonant injection-seeding is achieved by the active control of the corner 
mirror, the pulse energy in the return beam substantially decreases. 
The change in intensity between the return beam with the resonant cavity 
out-of-phase with the master oscillator signal and in-phase with that 
signal is significant. The out-of-phase return signal intensity is 
generally several times greater than when in-phase. This allows for the 
controller to be set to simply detect a drop in intensity to below some 
preselected threshold value as indicative that proper phasing has 
occurred. At this point the controller control can be paused, and 
reactivated later if the resonant condition is lost. 
If the resonant condition is later lost, it may be re-acquired by 
programming the controller to search again for resonance by any of a 
myriad search techniques known in the art, e.g., using some form of 
sawtooth scan. An alternate and complimentary means of regaining resonance 
is to utilize the method taught by Achareker et al in U.S. Pat. No. 
5,099,486 by hunting for time difference minimization between the time of 
turn-on for the Q-switch of the slave oscillator and the time of 
occurrence of the pulse output from the slave laser in the forward 
direction for rough determination of the mirror adjustment for resonance, 
and then to transfer to the technique taught herein for fine adjustment 
and control.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
This invention presents a novel technique and apparatus of adjusting a 
slave laser to emit high energy pulses at a single frequency set by a 
reference seed laser. The invention comprises a seed laser resonator 
serving as a master oscillator; a ring laser resonator serving as a slave 
laser connected to said seed laser, and having a closed path for its laser 
beam; means for changing said closed pathlength of said ring laser 
resonator; means, serving as part of said ring laser resonator, for 
switching into and out of resonance said ring laser resonator whereby said 
high energy output pulses are created; means for detecting a return laser 
beam emitted from said slave laser back toward said seed laser resonator; 
and means for comparing the intensity of said return laser beam with a 
predetermined threshold level and for causing said means for changing to 
adjust said closed pathlength of said ring laser resonator by a 
predetermined amount before a next single output laser pulse is created in 
a manner preset to be responsive to said compared intensity and said 
threshold level. 
The ring laser resonator further comprises a plurality of mirrors for 
guiding a laser beam along said closed path, a slave laser rod of a 
preselected material for generating the high energy pulses and a Q-switch 
positioned in said closed path. Measurement of the return beam intensity 
is performed by a programmable controller connected to said means for 
detecting and programmed to output a control signal to said means for 
changing said closed pathlength. 
When the controller determines that the measured return beam intensity is 
below the preselected threshold level, the slave laser is in phase with 
the seed laser, and single frequency pulses are being generated and 
emitted by the slave laser. Further adjusting of the mirror means which 
changes the pathlength of the closed path is not necessary unless the 
controller detects that the laser has shifted from this condition as 
indicated by the measured intensity slipping higher than the threshold 
level. 
FIG. 1 shows the preferred embodiment used for this invention. Shown are a 
seed laser resonator 1, used as the master oscillator, a beam steerer 
assembly 3, a beam shaper 5, a half wave length wave plate 7, a Brewster 
polarizer 9, and an optical isolator 11. Optical isolator 11 contains a 
45.degree. Faraday rotator 13 sandwiched between an input Brewster window 
polarizer 12 and an output Brewster window polarizer 14 that have their 
principal planes at 45.degree.. These components are aligned collinear 
with the lazing axis of seed laser 1 and their output enters into the ring 
slave oscillator 15. Optical isolator 11 serves to transmit the forward 
propagating seed laser light with very little attenuation while laser 
radiation emitted from the ring slave laser in a backward direction along 
path 28 is at 90.degree. to Brewster polarizer 12 and is largely 
deflected. 
The slave laser portion of the embodiment comprises the ring resonant 
cavity containing a flashlamp pumped Holmium laser head 23, corner mirrors 
25 and 26, a Q-switch 22, an aperture 24, and a piezoelectric transducer 
with mounted mirror 21. Partially reflective mirror 26 allows transmission 
of the pulse output from the slave oscillator 15 along paths 27 and 28. 
Mirror 21 also serves as a corner mirror for the closed path and is 
adjustable under control of the piezoelectric transducer to make 
pathlength changes in the closed path. 
When laser head 23 is fired by Q-switch 22, it normally emits laser 
radiation in both directions. The linearly polarized radiation directed 
back towards seed laser 1 along path 28, after passing through output 
Brewster window polarizer 14 and Faraday rotator 13, is reflected from 
Brewster window polarizer 12 to a detector 17. Detector 17 measures the 
intensity of the return beam and feeds this information to a controller 
19. The controller then controls adjustment of corner mirror 21. 
The master oscillator beam deflected by Brewster polarizer 9 can be used as 
a local oscillator beam for use in interferometric combination with output 
beam 27 after said output beam has interacted with a target medium located 
outside of the laser system. 
FIG. 2 shows a plot of the power (or intensity) envelope for a laser pulse 
generated through slave laser 23 as received by detector 17 in an in-phase 
and an out-of-phase condition. The figure is a plot of the detector output 
power on the ordinate and time on the abscissa. A power threshold level 
has been indicated parallel to the abscissa by the dotted line identified 
as 34. 
During operation, master oscillator 1 emits a low power single frequency 
signal. Its signal is directed into laser head 23 where it aids in 
establishing a limited frequency band for generation of a higher power 
laser pulse by the laser head 23 under control of Q-switch 22. Not shown, 
but part of laser head 23, is a flashlamp (or other source of excitation 
energy) and a laser rod for generating the desired high intensity pulse. 
Such elements are well known in the art. 
Laser beam energy created by laser head 23 is emitted in both directions 
around the closed path. If the path is not balanced in-phase with the 
wavelength emitted by master oscillator 1, the intensity or power of the 
counter rotating laser beams are similar in level and are represented by 
curve 30 in FIG. 2. However, when the pathlength of the closed path loop 
is adjusted to be in-phase with the wavelength emitted by master 
oscillator 1, i.e., made to equal an integral number of the wavelength 
emitted by master oscillator 1, the return beam intensity drops 
considerably as compared with the beam moving forward (in a counter 
clockwise direction around the close path). The power and beam shape of 
such a return beam is shown as curve 32 in FIG. 2. 
The difference in power levels between the return laser beam when 
out-of-phase with master oscillator 1 and when in-phase with master 
oscillator 1 is substantial and may be more than ten times in magnitude. 
By setting a threshold level 34 as shown in FIG. 2, it is relatively easy 
to detect when the system goes from an out-of-phase, unseeded operations 
to an in-phase, seeded operation by the dramatic change in the intensity 
of the return laser beam. 
Therefore, in operating this system, as pulses are triggered to emit from 
laser head 23, detector 17 measures the intensity of the return beam and 
feeds this information to controller 19. While the intensity of the return 
beam is greater than the preselected threshold, controller 19 causes the 
corner mirror 21 to readjust itself after each pulse seeking a state where 
the intensity of the return beam drops below threshold level 34. The 
system can be programmed by one of any of several methods known in the art 
for searching for the in-phase condition. Once the in-phase condition is 
found, controller 19 continues to monitor detector 17 while it stops 
issuing adjustment commands between pulses to mirror 21. The system should 
now remain stable in an in-phase condition for generating and emitting 
high energy single frequency laser pulses along path 27. 
Temperature and other environmental conditions may cause the pathlength of 
the closed path to shift out of an in-phase condition. This will be 
detected by an increase in the return beam intensity to a point above the 
threshold level. At this time detector 17, controller 19 and adjustable 
mirror 21 must re-engage their search procedures to again reestablish the 
in-phase condition. 
A second preferred embodiment of the present invention incorporates further 
advantages offered by material covered in U.S. Pat. No. 5,099,486 by 
Achareker et al, which is incorporated herein by reference. In the 
Achareker et al patent a method and apparatus is presented where control 
of the frequency of pulses emitted by a resonant ring slave laser is 
forced to a single frequency defined by a master oscillator laser, also 
through the control of the pathlength of the ring laser resonant path. In 
this patent, Achareker et al monitored the time difference between turn-on 
of the Q-switch within the ring laser resonant cavity and the time of 
occurrence of the output pulse from the slave laser. By effecting changes 
to the ring laser resonant path through an adjustable corner mirror the 
time difference measured between the turn-on of the Q-switch and the 
output pulse from the slave laser could be minimized. When so minimized 
the frequency of the output pulse from the slave laser is driven to a 
single frequency as defined by the master oscillator. 
FIG. 3 shows the diagram of FIG. 1 modified to incorporate the additional 
capability offered in the Achareker et al patent. In addition to the 
components already described, a beam splitter 40 is shown intersecting the 
path of output pulse 27, a detector 42 is shown receiving part of the 
output beam, and the Q-switch driver 44 is shown connected to Q-switch 22 
and to controller 19. 
The advantage of this combination is that for initial rough location of the 
occurrence of the in-phase condition, the method of Achareker et al 
provides a more rapid procedure. Following identification of which 
direction to adjust mirror 21 to approach the in-phase condition by the 
Achareker et al method, completion of the search and mirror adjustment is 
more accurately and continually accomplished utilizing the specifics of 
the invention presented herein. 
In FIG. 3, rough localization of a resonant in-phase condition for the 
closed cavity operation of the combined system is as follows. Part of 
output pulse 27 is deflected by beam splitter 40 to detector 42. Detector 
42 feeds its signal to controller 19. Controller 19 is connected to 
monitor firing of Q-switch 22 through Q-switch driver 44. Therefore, 
controller 19 is capable of monitoring the time difference (.DELTA.t) 
between firing of Q-switch 22 and the output of the resultant pulse 
through beam splitter 40. Following each pulse cycle and the resultant 
measurement of the time difference (.DELTA.t) mirror 21 is adjusted 
slightly. Thereby, over a period of two or more measurement cycles of 
times .DELTA.t, the direction for mirror 21 adjustment in locating the 
in-phase condition of the closed cavity can be identified. 
Continued measurement and control utilizing time difference measurements 
could continue, but preferably control would be turned over to the 
invention as presented herein. That control would then allow for more 
precise fine tuning for positioning mirror 21 to optimize the closed path 
to an in-phase condition and to deactivate the mirror 21 while the 
in-phase condition exists. If the novel method presented herein detects 
that the in-phase condition has been lost, then reacquisition may be 
implemented using either or both techniques once again. 
The technique described in this invention is applicable to various types of 
solid state lasers including Thulium, Holmium:YLF (Tm,Ho:YLF), Thulium:YAG 
(Th:YAG), and Neodimium:YAG (Nd:YAG). The technique is also applicable to 
diode laser pumped lasers as well as flashlamp pumped lasers. 
While this invention has been described with reference to its presently 
preferred embodiment its scope is not limited thereto. Rather such scope 
is only limited insofar as defined by the following set of claims and 
includes all equivalents thereof.