CA2068348A1 - Single frequency, long tail solid state laser interferometer system - Google Patents

Abstract

Attorney's Docket APD 90-01 SINGLE FREQUENCY, LONG TAIL SOLID STATE LASER INTERFEROMETER
SYSTEM

Abstract A single frequency solid state laser has been designed for generation of a high power Q-switch pulse followed by a quasi-CW low power tail by properly controlling the laser resonator transmission via an intracavity Q-switch. The generation of a stable single frequency pulse is accomplished by a primary feedback control system containing a central timing and prelase control unit. Generation and smoothing control of a quasi-CW pulse tail is accomplished through a secondary feedback control system.

APD\PATAPP\APD9001 4/25/91

Description

Attorn-y ' ~ Dock-t APD 90-01 8ING~B FREQUE:NCY, LONG TAIL 80LID 8TAT~1 I~A8~Z:R I~ITlSR~l~OJI~3T~
8Y8T~21 Background of the I~vention The development of laser sources to emit large pulses of energy followed by a long tail of low energy quasi CW signal are known in the prior art. Several investigators have developed laser sources with this characteristic, but none seem to have enlarged on its use in interferometric circuit devices designed for practical applications where efficient detection is required, and frequency and intensity stabilities of the local oscillator portion of the pulse are critical. Such a practical application is the measurement of atmospheric conditions such a wind turbulence at variable distances ahead of a flying aircraft.
Stephen Marcus and Theodore Quist in U.S. Patent 4,447,149 have developed a laser apparatus utilizing a Q-switched laser unit to generate laser pulse signals with a low intensity trailing tail. The low intensity tail is utilized as a local oscillator signal that is combined with the target return from the emitted pulse signal.
Their device is utilized as a pulsed laser radar system and, in a general sense, embodies the minimum elements which theoretically would form a target sensing, single source created target and reference beam system. Their disclosure Attorn~y'a Dock~t APD 90-01 seems directed to the USQ of gas lasers only, and tends only to conjecture the control necessary Or a few critical parameters for effectively making such a system viable for practical use. No insight or discussion is provided on creating a usable system with solid state lasers.
Robert Harney in U.S. Patent 4,298,280 has presented an infrared radar system comprised of an infrared laser to provide a succession of transmitted pulses shaped to have a high intensity spiXe followed by a relatively low intensity quasi-CW portion. However, the lower intensity tail is not used to create the reference beam. A local oscillator laser is used for that purpose. The high intensity spike is used to obtain range information while the following lower intensity portion i5 used for providing the stable frequency part for Doppler analysis.
I. Goldstein and A. Chabot have published the article ~Characteristics of a Traveling-Wave Ruby Single-Mode Laser as a Laser Radar Transmitter~ in the Journal of Quantum Electronics, Vol. QE-2, No. 9, September 1966, pp. 519-523.
Their published work describes a solid state ruby laser set up to emit spike shaped pulse followed by a low level CW
portion of much longer duration. The CW portion was planned to serve as a reference 3ignal to perform optical heterodyning with the target echo of the spike pulse.

, . - : - . :.

. . . . - - , ,:
:

Attorney'~ Dock~t APD 90-01 ~3--Goldstein and Chabots paper described their studies Or this laser device with regard to matters such as laser coherence, frequency shift during the pulse and the method for achieving the single mode. They did find problem~ with frequency stability. They also observed the oscillating nature of the CW portion, but felt it not important for their purposes.
U.S. Patent 4,690,551 by Edwards et al presents a laser radar utilizing pulse-tone waveforms, but utilizes a separate CW laser to generate local oscillator reference signal for multiplexing with the target return signal.
~.S. Patent 3,856,402 by Low et al presents a clear air turbulence detector utilizing a gaseous C02 master oscillator laser for emitting a signal shaped into pulses to a target area, and then beats their return echoes against a second portion of the CW C02 signal.
Notwithstanding the material presented in the prior art, there remains a major need to develop a compact device for air turbulQnce measurement from a moving aircraft.
An ob~ect of this invention is to present a solid state laser syQtem operated in a single frequency mode to provide both a high energy target pulse for reflection from an atmospheric disturbance, and a longer duration low energy quasi CW signal coherent with the high energy pulse for :' - ~, Attorn~y'- Dock-t A~D 90-01 heterodyning the two at the detection stage ~or accurate interferometric and Doppler analysis.
A further object of thi~ invention i~ to present a solid state laser system comprising beam splitters and detectors positioned to monitor and control the creation o~ a single frequency high energy pulse and a smooth low energy tail of the same frequency for use in interferometric and Doppler detection and analysis of air turbulence.
Another object of thi8 invention is to present a laser system as mentioned above which will occupy mir.imum spacs and operate from an aircraft in flight.
Field of the Invention This invention relates to a laser source for use in an interferometric system whereby the laser source provides a powerful pulse of optical energy to transit one leg of the interferometer including reflection off a target and return along the same path as the outward beam, and a low level quaRi CW tail off the same pulse to transit a second reference path of the interferometer and then recombined with th~ returning pulse for analysis.
Summary of the I~vçntion This invention presents a specialized laser source and an air tur~ulence detection system designed to occupy small volume and to allow the detection and measurement of air f~ttorn-y' Dock~t AP~ 90-~1 turbulence at several distances ahead of an aircra~t while in flight. It comprises a solid state laser source, preferably a Holmium laser, configured with feedback control to.initiate and form a Q-switched pulse of high energy for emission to a target atmosphere, and ~to form a lower energy quasi-CW
reference signal for heterodyning with the returning echo pulse.
To form a high energy pulse to be focused ahead of an aircraft into a turbulent medium a solid state laser rod is activated by a flashlamp to establish the lasing action within a laser cavity. A Q-switch in the form of an acousto-optic modulator (AOM) under the control of an RF
signal intercepts the lasing beam of light within the cavity.
A portion of the incident laser beam is deflected by the AOM to a detector in a primary feedback circuit for the control of the applied RF power. The intensity of the deflected beam is approximately proportional to the applied RF power.
Ths RF power is applied to the AOM as soon as the laser flash lamp pump turns on. The detector then outputs a current signal level proportional to the intensity of the deflected beam outside the laser resonator. When, during the prelase phase, the intensity within the resonator reaches a preselected level, the RF power to the AOM is partially Attorn-y ' ~ Dock-t A~D 90-01 turned off thereby causing an intense Q-switched pulse to pass through the AOM for focusing to a turbulent medium.
Shortly after the high energy pulse has been generated the RF power to the AOM can be further reduced to allow the pulse energy to decay into a long guasi CW tail. It is a portion of this low energy tail which is used as a local oscillator to combine with the reflected pulse returning to the instrument for analysis of the pulse signal by interferometric techniques as well as Doppler analysis.
However, the intensity of the quasi-CW pulse tail is not constant but rather is strongly amplitude modulated. To overcome problems with this modulation, these fluctuatisns are controlled by using a secondary feedback circuit to continue a finer level of RF power control of the Q-switch AOM during the quasi-CW tail period following the pulse.
A second AO~ placed in tha laser beam path down3tream from the Q-switch AOM is triggsred on between the passage of the high energy pulse and the beginning of the quasi-CW tail to da~lect a portion of this tail through a second detector into the secondary feedback circuit. This secondary feedback circuit is programmed to smooth fluctuations detected in the tail signal by adjusting the RF signal controlling the Q-switch AOM to adjust tha intensity of the laser tail passing through it.

?

Attorn-y ' ~ Dock~t APD 90-01 By these means a more stable quasi-CW tail is available for heterodyning with the returning echo pulse. The analysis of the content of the returned echo pulse i~ thereby more effectively accomplished to yield data on the velocity structure, reflectivity intensity, and phase information pertaining to air turbulencs located in the path of the laser pulse.
Brief Description o~ e ~rawings Figure 1 shows a single feedback control loop for generation of a single frequency pulse.
Figure 2 is a gain plot showing mode selection accomplished with etalon.
Figure 3 is a series of plots relating the ~F power, the pump power, the Q-switch AOM transmission, the gain medium, and the Log of the output laser power to time.
Figure 4 shows this laser system coRfiguration with the primary feedback loop for generation of the pulse and the secondary feedback loop for generation of a constant, stable quas$-CW tail.
Figure 5 shows a representative plot of the RF power applied to the Q-switch AOM throughout the entire cycle to accomplish the objectives of the invention.

Attorn-y'- Dock-t A~D 90-01 ~e~cri~tion of the Pre~erred E~bodi~ent In a broad per~pective, this inv-ntion presents over the prior art a device and method for generating a high peak Q-switched laser pulse to be transmitted to a distant reflecting medium; the high peak power pulse being thereafter followed by a low power quasi-CW tail. Returning echoes from the high peak power pulse are heterodyned with the low power guasi-CW tail for analysis of information contained in the reflected pulse.
An important aspect of this invention is the ability to timely trigger the Q-switch to release a pulse at a single frequency. The invention accomplishes this through detection of a preselected threshold during prelase phase of the laser output en~rgy to identify a time to trigger the Q-switch for emission o~ the high energy pulse.
A sQcond aspect of the invention comprises the capability to sQnse the low energy quasi-CW tail energy level and, through a feedback circuit, to control an acousto-optic modulator ~witch to effect stabiliiation of the amplitude of Z0 this tail.
A laser resonator is shown in Figure 1 with a feedback loop showing detection of a diverted beam 46 from the laser cavity by a detector 14. The configuration shown in . .
. ~ . . -. .~ ., , , . ~ . .

Attorr~-y'- Dock-t APD 90-01 _g_ Figure 1 is the ~eedback control loop ~or generation o~
single frequency pulses.
The laser cavity comprises a reflecting mir~or 6, a laser rod 2, a flashlamp 4, an acousto-optic modulator serving aq a Q-switch 8, ~n etalon 16, and a semi-transparent mirror 18 at the front end o~ the cavity. When the cavity is lasing light traverses along a laser axis 44 with some portion being output to a remote target. Except for possible simple on-off switching purposes, Q-switch 8 is recognized as not essential for the normal operation of a general purpose laser cavity, and would normally not be included except for the purposes of this invention.
An aperture 20 is placed in the cavity to limit laser oscillation to the lowest order transverse mode. The aperture 20 may be combined with a lens to compensate for thermal gain medium lensing and to increase efficiency.
The laser or gain medium 2 utilized in the invention has been a cr, Th, Ho.YSGG laser operating at 2.1~. The flashlamp 4 serving as a pump source has been a Xe flashlamp.
The etalon, or multi-mirror resonator 16 is configured to have few (3-10) resonant modes occur within that part of the gain medium spectrum above a preselected threshold as shown in Figure 2. In one testsd resonator a 4 mirror .

Att~rn-y'- Dock-t ~PD 90-~1 resonator with power reflectivities of 99.5~, 3.5~, 3.5%, and 80~ was employed.
The Q-switch 8 serves to control the capabili~y oS the laser cavity to generate laser pulses and to control fine adjustments to the amplit~de of laser pulses emitted from the laser cavity. The Q-switch 8 employed has been a sio2 acousto-optic modulator operated by a RF power signal which allowed flexible temporal control by the feedback loop.
The feedback control loop comprises detector 14 which connects to a control unit 12 and then to a Q-switch driver 10 which provides the RF power signal for control of Q-switch 8. The control unit 12 monitors and controls the timing functions and generates a control signal to modulate the RF
power signal gsnerated by Q-switch driver 10. The modulated RF power signal then controls the laser beam amplitude allowed to exit the laser cavity through the switching function of Q-switch 8 as mentioned above. Although the electronic configuration within control unit 12 may be uni~ue in its specific design to the present inventors embodiment, it is recognized that such designs are varied and within the common knowledge availa~le from the prior art.
During the prelase condition of laser cavity 1, Q-switch 8 is in an off state of operation, i.e., receiving an RF
signal from Q-switch driver 10. In this off state Q-switch 20683~8 Attorn~y'- Dock-t ~PD 90-01 8 causes the light beam to be deflected 46 towards detector 14 and prevents the laser cavity ~rom generating a strong laser signal or pulse.
Single frequency operation of the laser is achieved via the combination of transverse mode control by aperture 20, longitudinal mode selection by etalon 16, and monitoring the prelase stage through feedbac~ detector 14.
Prelasing is the technique employed here to discriminate against all but one frequency. This technigue allows the mode with the largest gain (least loss) to build-up in intensity faster than other (highar loss) frequencies. When Q-switch 8 is activated for transmission of the laser beam along axis 44 to produce the high power laser pulse the dominant frequency will cause depletion of the gain medium before any other competing freguencies can participate, thus producing a predominately single frequency output.
The details of control through the prelase phase and generation of the high peak power pulse are exhibited as functions of time in Figure 3a through e. Figure ~a shows a plot with ti~e of the pump power provided by flashlamp 4.
The RF power Rignal to Q-switch 8 is shown in Figure 3b. The transmission through Q-switch 8 i~ shown in Figure 3c.
As shown in Figure 3a the laser is activated by pump source 4 through a large intensity flash of energy at the ~.

: :

Attorn-y'- Dock-t A~D 90-01 early portion o~ the operation. Control of Q-switch 8, shown in Figure 3b, has the RF ~ignal high to a time T~ at which time the signal i9 dropped in intensity to a seco~d preset level for a further time interval to T2. During the time di~ference from Tl to T2 the high pea~ energy pulse will be generator and will output the laser.
Transmission capability through the acousto-optic modulator Q-switch 8 as shown in Figure 3c is low up to time Tl and then becomes higher between times Tl and T2 and is essentially open in this example after time T2. This time plot represents the capability o~ the Q-switch transmission and not the intensity of the light travelling through and from said Q-switch.
The gain medium intensity as a function of time is shown in Figure 3d. Figure 3e then shows the log of the output power along path 44 from the laser throughout the entire proce~.
Looking at Figure 3e, we note that up to time Tl we are in a prelase phase 70, and between T~ and T2 the high peak ZO power Q-switch pulse is emitted. Following the time T2 the low power guasi-CW tail 74 occur~. The quasi-C~ tail is produced when the Q-switch is only partially opened at time Tl to form Q-switch pulse 72, then further opened at time T2 during the downward decay of pulse 72.

, .

"~
Attor~-y~oDock-t APD 90-C1 Figure 2 allows U8 a vision at understanding how singlefrequency operation of the laser is actually accomplished.
Shown is the mode spectrum for the laser with its gain plotted as a function of the frequency being transmitted within the gain medium.~ By setting a threshold, one can isolate a range for frequency bQtween a high value F~ and a low value FL as shown. It is to be noted that the frequency gain spectrum envelop changes as time proceeds during pumping by pump source 4.
From Figure 2, we see that under the gain spectrum envelop we would have many frequency modes 92 if etalon 16 were not installed. With etalon 16 in place mode selection has occurred and just a few modes 90 are actually available under the ~nvelop.
To achieve a single mode to be in existence at the time the high peaked power pulse 72 is generated, it is necessary to monitor through detector 14 in the primary feedback loop the prelase power output as shown in Figure 3e. By experience and experimentation one is able to determine a threshold level occurring during the prelase phase which will occur at some time To from which it is known that a certain preselected time differencs T~ minus To is a correct time delay befors partially opening Q-switch 8. Opening of the Q-switch at this preselected time Tl serves to sort out a .

.

~ttorn-y'- Dock-t APD 90-01 single frequency mode to be transmitted as the high peak power Q-switch pulse 72, which also continues to later comprise the low power quasi-CW tail 74.
Had the Q-switch not been partially turned on at time T~, Figure 3e also shows that the prelase condition would have continued as an oscillating low energy level beam of light 76 within the laser cavity l. Detection of the prelase threshold level 78 at To occurs on the first oscillation in the prelase signal 76. The freguency generated by this technigue is not always the same frequency for every generated high energy laser pulse. However, when utilizing the instrument in an interferometric mode this creates no problem in its use.
In theory, the log of the output power exhibited in Figure 3e as a function of time would occur with this apparatus and method, particularly noting that the low power quasi-CW tail 74 appears more or lesc stable for the re~aining portion of time exhibited. Realistically, the low power quasi-CW tail 74 tends to oscillate in a noisy ~anner, ther-by making it difficult to work with for analysis when heterodyned with a returning echo of the high peak power pulse 72. Figure 4 shows a plot of the log of the output power as a function of time with the high peak power pulse 50 followed by an unsmoothed, oscillating, low energy quasi-CW

. : ~ . . .

, , , . . . . ` . ~ . i .

, . . . , - - . ~ .

~ttorrl-y'- Dock-t A~D 90-01 tail 5Z. In order to ~mooth and control oscillations in guasi-CW tail 52, a second reedback circuit i5 utilized as shown in Figure 4.
Figure 4 comprise~ the laser cavity 1 shown in Figure 1, ies feedback detector 14 and associated feedback loop to Q-switch 8, and in addition, show~ the use of a second acousto-optic modulator 38 deflecting the quasi-Cw tail to a detector .32 and back through a pulse tail and feedback control 30.
In this embodiment feedback detector 14 feeds its signal to a central timing and control unit 24 which outputs a control signal to Q-switch driver 10 as described in the embodiment of Figure 1. During generation of the high energy Q-switched pulse as described above this primary feedback loop i~ functional.
The central timing and control unit 24 directs the timing operation of the entire laser system by outputting a la~er trigger 26 which feed~ laser rlashlamp 4 thereby starting a pulse generation cycle of the laser system, a control s1gnal 28 to an AOM driver 39 for switching operation of AOM 38 via a RF signal output from AO~ driver 39, and a timing control signal to pul~e shaping feedback control 30.
The secondary feedback loop is now utilized to control the oscillations in the low power quasi-cW tail 52. This feedback loop comprises the second acousto-optic modulator .: . . . . .
.. ~ .. , , . . ................... , . -., :.. : . ,; ~ ~
. . ~ , . - :

Attorn--y' a Dock-t APD 90-01 38, AOM driver 39, reflector 36, beam splitter 34, the ~econd feedback detector 32, and pulse shaping feedback control unit 30 which is then connected to control Q-switch driver 10 and Q-switch 8. This feedback circuit i8 configured to create a smoothed pulse tail 70 as shown in Figure 4 as a function of time . Acousto-optic modulator 38 is not triggered to deflect beam 40 to mirror 36 until after the high peak power Q-switch pulse 60 is emitted from the system as the target pulse. Pulse 60, of course, would travel to a target which would cause a portion of the pulse to be reflected back and received at so~e later time at the system.
once Q-switch pulse 60 has been transmitted, central timing and control unit 24 commands the tail to be diverted by AOM driver 39 and AOM 38 to mirror 36 and beam splitter 34, where it i8 again reflected along a "local oscillator"
path 42 to be heterodyned with the returning echo o~ the Q-switch pulse at some later time. A portion of the tail is transmitted through beam splitter 34 and is received by the second detector 32.
Detettor 32 monitors the fluctuations of the pulse tail.
The detector output is fed to pulse shaping feedback control unit 30 which, also under timing control from central timing and control unit 24, generates a voltage signal with an amplitude directly related to the square root of the pulse ' ;

' .' ' 206834~

Attornoy'~ Dock-e APD 90-01 tail intensity. This signal i9 then output ~rom pulse shaping feedbacX control unit 30 to modulate the RF signal generated and ampli~ied by Q-awitch driver 10. Thia results in a Q-switch 8 deflection efficiency that is proportional to S the pulse tail intensity.
The design of pulse shaping ~eedback control unit 30 takes into account the time delays associated with Q-switch 8 and the resonant energy buildup time in laser cavity 1.
The pulse shaping feedback control unit 30 estimates the pulse tail intensity at a lead time equal to the total resonant response time of laser cavity 1 by measuring the present amplitude of detector 32 output current and its temporal derivative.
In summary the voltage output of pulse shaping feedback control unit 30 is given by:
v(t) ~ K~ [id(t + b)]m and the Q-switch deflection efficiency (i.e., inversa of tran~mission) is egual to N(t) - R2P~(t) ~ v2(t) = R~2i~(t + b) where t is time, id is detector 32 output current, b is the resonant response time of laser cavity 1, R~ and R2 are constants, and P~(t) is applied RF signal power.

.: ~ ' '.,.

Attorn-y'~ Dock-t APD 90-01 Flgur- S how- a plot a~ a tunction of ti~ ot th- RF
control Jignal to acou~to-optic nodulator 8 Th- portion of the nvolop during la~ing op r~tion 100 fro- ~he ti~e tla hlu~p 4 w~- activat-d to the ti~ that acou~to-optic ~ ~odulator 8 w - open d to start tran- iJ-ion of th Q-switch :: :
; pulsQ~ i9 repre~ented~by the ti~ l-ngth ~t~ ~t2 rQpr-~Qnts tSe tim diffQr-nc-~ betw -n T~ and T2 as shown in prior figur~s which i~ tSe p~riod during which th Q-switch pulse ls~cr-~t-d Aft r tim ~T2 th- RF~slgnal 1~ allow~d to slowly 0 ~ d cay ~03~ to so~e~pr d~t-r~in d ti~- ~T3 at which ti~ a lgn~l tro~c ntral ti~ing and~control unit 24 to acousto-optic ~odulator 38~ itted to activate acou~to-optic ~odulator 38 ~At ti-e-~after T3~ nd durlng an int rv l ~t~
. ~ ~
~; pul Q ~haping t ~ k control UDit 30 tunetions to inilize o clllatlon~ in tSe t~il n rgy ~
Th purpo-- ot slowly~dbc~ring RF ~ignal 103 is to r-~ov ~o~o o~ th initial int-n~ity fluctuations tSat would ooclr in th qua~l-CW pul~- tall At tire T3 pul- sSaping ~nd t e~ control 30 will set in and apply a ~ignal whose ~a plltud- 10~ a~tun tion of and deter~ined by d tector 32 ~ o tput curr-nt Thls slgnal whlch controls acou~to-optic -~ ~odulator 8 ~-rv g to control the intensity of the bea~
~ltt-d through acousto-optic ~odulator 8 in a ~annor that th intensity of the quasi-cW puls- tail is smooth ~ ~ .
:

'. ~: . . : -Attorn-y'- Dc~ck-t A~D 90-01 Smoothing Or the quasi-CW pulsQ tail racilitates more effectiv- and useful analysis o~ the returned signal infor~ation when the cho o~ the Q-~witch pulsQ rQturns and is heterodyned with the guasi-CW pulse tail signal.
While this invention hai~ been described with re~erence 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 thereo~.

, . ~ ;: -: -

Claims (29)

1. A single frequency laser system which comprises a laser source resonator cavity;
a means for controlling said laser source resonator cavity for generating a single frequency laser beam signal shaped as a high energy pulse signal immediately followed by a lower energy quasi-CW tail signal;
a means for directing said high energy pulse signal along a first path towards a preselected external reflective substance which constitutes a target medium for reflecting a portion of said high energy pulse signal back upon itself;
a first means for switching positioned in said first path to intercept and direct said lower energy quasi-CW tail signal along a second path not interacting with said target medium; and a means for receiving said reflected portion of said high energy pulse signal and combining it with said lower energy quasi-CW tail signal on said second path to form a heterodyned signal.

2. The single frequency laser system according to Claim 1 wherein the laser source resonator cavity comprises:
gain medium;
flashlamp placed proximate to said gain medium whereby said gain medium is induced to laze when said flashlamp is excited thereby creating a laser beam along the optical axis of said gain medium;
a concave mirror positioned at one end of said cavity and on said optical axis whereby said cone-ave mirror will intercept and reflect said laser beam back along said optical axis;
a partially reflecting planar mirror positioned at an opposite end of said cavity and on said optical axis whereby said planar mirror will intercept and reflect part of said laser beam back along said optical axis and allow part of said laser beam to pass through;
an aperture positioned on said optical axis whereby a single transverse mode of said laser beam is allowed to propagate; and an etalon positioned on said optical axis whereby a few frequency modes are allowed to propagate.

3. The single frequency laser system according to Claim 2 wherein the laser source resonator cavity further comprises:
a lens in combination with said aperture to enhance allowance of said single transverse mode selection.

4. The single frequency laser system according to Claim 3 further comprising:
a means for analysing and displaying said heterodyned signal.

5. The single frequency laser system according to Claim 2 wherein the means for controlling comprises:
a second means for switching, positioned in the laser beam path within said laser source resonator cavity, to controllably switch the laser beam between a resonant laser beam path which exits to said first and second paths and a detector beam path positioned off-axis to said resonant laser beam path;
a first detector positioned in said detector beam path to intercept the laser beam and measure its intensity during a prelase portion of said laser beam signal; and a central timing and control means connected with an input from said first detector, and connected to output a control signal to said second switching means.

6. The single frequency laser system according to Claim 5 wherein the central timing and control means comprises:
a first electronic circuit programmed to detect a time of occurrence, T0, of a preselected intensity threshold level during the prelase portion of said laser beam, then at a preselected time difference following T0, occurring at T1, said electronic circuit causes a control signal to be emitted by said central timing and control means to activate said second switching means to open a preselected amount thereby allowing said laser beam to propagate along said resonant laser beam path of said laser resonator cavity until a third preselected time, T2, whereby said high energy pulse signal portion of said single frequency laser beam signal is formed, and thereafter causing said second switching means to open fully thereby allowing the remainder of said laser beam to propagate along said resonant laser beam path whereby said lower energy quasi-CW tail signal is formed.

7. The single frequency laser system according to Claim 6 further comprising:
a means for analysing and displaying said heterodyned signal.

8. The single frequency laser system according to Claim 1 wherein the means for controlling comprises:
a second means for switching, positioned in the laser beam path within said laser source resonator cavity, to controllably switch the laser beam between a resonant laser beam path which exits to said first and second paths and a detector beam path positioned off-axis to said resonant laser beam path;
a first detector positioned in said detector beam path to intercept the laser beam and measure its intensity during a prelase portion of said laser beam signal; and a central timing and control means connected with an input from said first detector, and connected to output a control signal to said second switching means.

9. The single frequency laser system according to Claim 8 wherein the central timing and control means comprises:
a first electronic circuit programmed to detect a time of occurrence, T0, of a preselected intensity threshold level during the prelase portion of said laser beam, then at a preselected time difference following T0, occurring at T1, said electronic circuit causes a control signal to be emitted by said central timing and control means to activate said second switching means to open a preselected amount thereby allowing said laser beam to propagate along said resonant laser beam path of said laser resonator cavity until a third preselected time, T2, whereby said high energy pulse signal portion of said single frequency laser beam signal is formed, and thereafter causing said second switching means to open fully thereby allowing the remainder of said laser beam to propagate along said resonant laser beam path whereby said lower energy quasi-CW tail signal is formed.

10. The single frequency laser system according to Claim 9 further comprising:
a means for analysing and displaying said heterodyned signal.

11. The single frequency laser system according to Claim 1 wherein the means for controlling comprises:
a second means for switching, positioned in the laser beam path within said laser source resonator cavity, to controllably switch the laser beam between a resonant laser beam path which exits to said first and second paths and a detector beam path positioned off-axis to said resonant laser beam path;
a first detector positioned in said detector beam path to intercept the laser beam and measure its intensity during a prelase portion of said laser beam signal;
a central timing and control means connected with an input from said first detector and an output to an amplifying means; and an amplifying means connected to receive a control signal from said central timing and control means and to output the amplified control signal to said second switching means.

12. The single frequency laser system according to Claim 11 wherein the central timing and control means comprising:
a first electronic circuit programmed to detect a time of occurrence, T0, of a preselected intensity threshold level during the prelase portion of said laser beam, then at a preselected time difference following T0, occurring at T1, said electronic circuit causes a control signal to be emitted by said central timing and control means to activate said second switching means to open a preselected amount thereby allowing said laser beam to propagate along said resonant laser beam path of said laser resonator cavity until a third preselected time, T2, whereby said high energy pulse signal portion of said single frequency laser beam signal is formed, and thereafter causing said second switching means to open fully thereby allowing the remainder of said laser beam to propagate along said resonant laser beam path whereby said lower energy quasi-CW tail signal is formed.

13. The single frequency laser system according to Claim 1 which further comprises:
a means for monitoring and modifying only said lower energy quasi-CW tail signal.

14. The single frequency laser system according to Claim 13 wherein the means for monitoring is a second feedback circuit which comprises:
a second detector positioned in said second path to measure the intensity of said lower energy quasi-CW tail signal; and a pulse shaping feedback control means connected with inputs from said and detector and said controlling means, an output control to said first switching means whereby said lower energy quasi-CW tail signal is switched to said second path, and an output control connected to said second switching means.

15. The single frequency laser system according to Claim 14 wherein the pulse shaping feedback control comprises:
a second electronic circuit programmed to activate said first switching means to direct said lower energy quasi-CW
tail signal along said second path upon receiving a timing control signal at time T2 from said controlling means, then measure oscillations in the intensity of said lower energy quasi-CW tail signal, and to output a smoothing control signal to said first switching means.

16. The single frequency laser system according to Claim 15 further comprising:
a means for analysing and displaying said heterodyned signal.

17. The single frequency laser system according to Claim 6 which further comprises:
a means for monitoring and modifying only said lower energy quasi-CW tail signal.

18. The single frequency laser system according to Claim 17 wherein the means for monitoring is a second feedback circuit which comprises:
a second detector positioned in said second path to measure the intensity of said lower energy quasi-CW tail signal; and a pulse shaping feedback control means connected with inputs from said second detector and said controlling means, an output control to said first switching means whereby said lower energy quasi-CW tail signal is switched to said second path, and an output control connected to said second switching means.

19. The single frequency laser system according to Claim 18 wherein the pulse shaping feedback control comprises:
a second electronic circuit programmed to activate said first switching means to direct said lower energy quasi-CW tail signal along said second path upon receiving a timing control signal at time T2 from said controlling means, then measure oscillations in the intensity of said lower energy quasi-CW tail signal, and to output a smoothing control signal to said first switching means.

20. The single frequency laser system according to Claim 19 further comprising:
a means for analysing and displaying said heterodyned signal.

21. The single frequency laser system according to Claim 9 which further comprises:
a means for monitoring and modifying only said lower energy quasi-CW tail signal.

22. The single frequency laser system according to Claim 21 wherein the means for monitoring is a second feedback circuit which comprises:
a second detector positioned in said second path to measure the intensity of said lower energy quasi-CW tail signal; and a pulse shaping feedback control means connected with inputs from said second detector and said controlling means, an output control to said first switching means whereby said lower energy quasi-CW tail signal is switched to said second path, and an output control connected to said second switching means.

23. The single frequency laser system according to Claim 22 wherein the pulse shaping feedback control comprises:
a second electronic circuit programmed to activate said first switching means to direct said lower energy quasi-CW tail signal along said second path upon receiving a timing control signal at time T2 from said controlling means, then measure oscillations in the intensity of said lower energy quasi-CW tail signal, and to output a smoothing control signal to said first switching means.

24. The single frequency laser system according to Claim 23 further comprising:
a means for analysing and displaying said heterodyned signal.

25. The single frequency laser system according to Claim 5 wherein the first and second means for switching comprise:
an acousto-optic switch known as a Q-switch.

26. The single frequency laser system according to Claim 8 wherein the first and second means for switching comprise:

an acousto-optic switch known as a Q-switch.

27. A method for a single frequency laser system which comprises the following steps:
generating a laser beam in a laser source resonator cavity;
controlling said laser source resonator cavity for generating a single frequency laser beam signal which forms a high energy pulse signal immediately followed by a lower energy quasi-CW tail signal;
directing said high energy pulse signal portion along a first path towards a preselected external reflective substance which constitutes a target medium for reflecting a portion of said high energy pulse signal back upon itself;
switching said lower energy quasi-CW tail signal along a second path not interacting with said target medium;
monitoring oscillations in the intensity of said lower energy quasi-CW tail signal;
shaping said lower energy quasi-CW tail signal whereby said undesired oscillations in said lower energy quasi-CW
tail signal are removed by smoothing; and combining said reflected high energy pulse signal with said smoothed lower energy quasi-CW tail signal and heterodyning said two signals.

28. A method for a single frequency laser system which comprises the following steps:

generating a laser beam in a laser source resonator cavity containing a mode selector and etalon;
blocking said laser beam from exiting said laser source cavity with a switching means during the prelase period of said laser beam;
detecting a time of occurrence, T0, of a preselected intensity threshold level during the prelase period of said laser beam;
opening said switching means a predetermined amount at a predetermined time T2, determined from knowledge of T0 to allow a high energy pulse signal portion of said single frequency laser beam signal to be formed and to exit said source laser cavity;
opening said switching means completely at a predetermined time T2, determined by knowledge of T1, whereby a lower energy quasi-CW tail signal is formed and emitted;
directing said high energy pulse signal portion along a first path towards a preselected external reflective substance which constitutes a target medium for reflecting a portion of said high energy pulse signal back upon itself; and switching said lower energy quasi-CW tail signal along a second path not interacting with said target medium.

29. A method for a single frequency laser system which comprises the following steps:
generating a laser beam in a laser source resonator cavity containing a mode selector and etalon;
blocking said laser beam from exiting said laser source cavity with a switching means during the prelase period of said laser beam;
detecting a time of occurrence, T0, of a preselected intensity threshold level during the prelase period of said laser beam;
opening said switching means a predetermined amount at a predetermined time T, determined from knowledge of T0 to allow a high energy pulse signal portion of said single frequency laser beam signal to be formed and to exit said source laser cavity;
opening said switching means completely at a predetermined time T2, determined by knowledge of T1, whereby a lower energy quasi-CW tail signal is formed and emitted;
directing said high energy pulse signal portion along a first path towards a preselected external reflective substance which constitutes a target medium for reflecting a portion of said high energy pulse signal back upon itself;
switching said lower energy quasi-CW tail signal along a second path not interacting with said target medium;
monitoring oscillations in the intensity of said lower energy quasi-CW tail signal;
shaping said lower energy quasi-CW tail signal whereby said undesired oscillations in said lower energy quasi-CW tail signal are removed by smoothing; and combining said reflected high energy pulse signal with said smoothed lower energy quasi-CW tail signal and heterodyning said two signals.