CA1180797A - Injection frequency locked tea laser - Google Patents
Injection frequency locked tea laserInfo
- Publication number
- CA1180797A CA1180797A CA000374194A CA374194A CA1180797A CA 1180797 A CA1180797 A CA 1180797A CA 000374194 A CA000374194 A CA 000374194A CA 374194 A CA374194 A CA 374194A CA 1180797 A CA1180797 A CA 1180797A
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- CA
- Canada
- Prior art keywords
- frequency
- cavity
- optical
- signal
- resonant cavity
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
- 238000002347 injection Methods 0.000 title abstract description 4
- 239000007924 injection Substances 0.000 title abstract description 4
- 241001122767 Theaceae Species 0.000 title 1
- 230000003287 optical effect Effects 0.000 claims abstract description 66
- 230000004044 response Effects 0.000 claims abstract description 21
- 230000010287 polarization Effects 0.000 claims description 11
- 230000000087 stabilizing effect Effects 0.000 claims description 11
- 230000008859 change Effects 0.000 claims description 5
- 238000001514 detection method Methods 0.000 abstract description 9
- 230000001427 coherent effect Effects 0.000 abstract description 6
- 230000006641 stabilisation Effects 0.000 description 16
- 238000011105 stabilization Methods 0.000 description 16
- 229910002092 carbon dioxide Inorganic materials 0.000 description 10
- 239000007789 gas Substances 0.000 description 10
- CURLTUGMZLYLDI-UHFFFAOYSA-N Carbon dioxide Chemical compound O=C=O CURLTUGMZLYLDI-UHFFFAOYSA-N 0.000 description 6
- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 description 4
- 230000010355 oscillation Effects 0.000 description 4
- 230000005855 radiation Effects 0.000 description 4
- 238000001228 spectrum Methods 0.000 description 4
- 238000010521 absorption reaction Methods 0.000 description 3
- 230000001276 controlling effect Effects 0.000 description 3
- 238000010586 diagram Methods 0.000 description 3
- 230000005684 electric field Effects 0.000 description 3
- 229910052734 helium Inorganic materials 0.000 description 3
- 239000001307 helium Substances 0.000 description 3
- SWQJXJOGLNCZEY-UHFFFAOYSA-N helium atom Chemical compound [He] SWQJXJOGLNCZEY-UHFFFAOYSA-N 0.000 description 3
- 230000007246 mechanism Effects 0.000 description 3
- 239000000203 mixture Substances 0.000 description 3
- 230000003134 recirculating effect Effects 0.000 description 3
- 238000010408 sweeping Methods 0.000 description 3
- 239000003990 capacitor Substances 0.000 description 2
- 230000000875 corresponding effect Effects 0.000 description 2
- 229910052757 nitrogen Inorganic materials 0.000 description 2
- 230000007704 transition Effects 0.000 description 2
- QGZKDVFQNNGYKY-UHFFFAOYSA-N Ammonia Chemical class N QGZKDVFQNNGYKY-UHFFFAOYSA-N 0.000 description 1
- 229910000661 Mercury cadmium telluride Inorganic materials 0.000 description 1
- 239000006096 absorbing agent Substances 0.000 description 1
- 230000009471 action Effects 0.000 description 1
- 230000003466 anti-cipated effect Effects 0.000 description 1
- 230000005540 biological transmission Effects 0.000 description 1
- MCMSPRNYOJJPIZ-UHFFFAOYSA-N cadmium;mercury;tellurium Chemical compound [Cd]=[Te]=[Hg] MCMSPRNYOJJPIZ-UHFFFAOYSA-N 0.000 description 1
- 239000001569 carbon dioxide Substances 0.000 description 1
- 230000000779 depleting effect Effects 0.000 description 1
- 238000000605 extraction Methods 0.000 description 1
- 238000003384 imaging method Methods 0.000 description 1
- 238000005259 measurement Methods 0.000 description 1
- 238000000329 molecular dynamics simulation Methods 0.000 description 1
- 239000003381 stabilizer Substances 0.000 description 1
- 230000002123 temporal effect Effects 0.000 description 1
- 230000001960 triggered effect Effects 0.000 description 1
Classifications
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
- H01S3/10084—Frequency control by seeding
- H01S3/10092—Coherent seed, e.g. injection locking
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
- H01S3/13—Stabilisation of laser output parameters, e.g. frequency or amplitude
- H01S3/139—Stabilisation of laser output parameters, e.g. frequency or amplitude by controlling the mutual position or the reflecting properties of the reflectors of the cavity, e.g. by controlling the cavity length
- H01S3/1394—Stabilisation of laser output parameters, e.g. frequency or amplitude by controlling the mutual position or the reflecting properties of the reflectors of the cavity, e.g. by controlling the cavity length by using an active reference, e.g. second laser, klystron or other standard frequency source
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01W—METEOROLOGY
- G01W1/00—Meteorology
- G01W2001/003—Clear air turbulence detection or forecasting, e.g. for aircrafts
Landscapes
- Physics & Mathematics (AREA)
- Electromagnetism (AREA)
- Engineering & Computer Science (AREA)
- Plasma & Fusion (AREA)
- Optics & Photonics (AREA)
- Lasers (AREA)
- Optical Radar Systems And Details Thereof (AREA)
Abstract
INJECTION FREQUENCY LOCKED TEA LASER
Abstract A very high power stable frequency TEA laser receiver-transmitter combination for applications requiring coherent detection. The transmitter includes a conventional CW laser tube positioned in a resonant cavity also containing a high power TEA laser. The high power output pulses are injection locked to the CW laser for frequency stability. The frequency of the laser pulses is tuned by changing the optical length of the resonant cavity in response to the frequency of a portion of the output signal. Electrical to optical power efficiency is improved as a result of the high percentage of power extracted from the cavity.
Abstract A very high power stable frequency TEA laser receiver-transmitter combination for applications requiring coherent detection. The transmitter includes a conventional CW laser tube positioned in a resonant cavity also containing a high power TEA laser. The high power output pulses are injection locked to the CW laser for frequency stability. The frequency of the laser pulses is tuned by changing the optical length of the resonant cavity in response to the frequency of a portion of the output signal. Electrical to optical power efficiency is improved as a result of the high percentage of power extracted from the cavity.
Description
)7 9 ~
Background of the Invention There is a serious need Eor a general purpose very high power transmitter-receiver combination for clear air turbu-lence detection, pointing and tracking, imaging, wind shear and trailing vortex measurements and similar applications.
Upyrading of the present master oscillator power amplifier unit can be carried further, but will eventually be limited by serious complexities and holding off self-oscillation in the power amplifier section. Also, additional gain achieved by use of saturable absorbers or isolators can be expected to desrade the output diffraction limited performance. For applications requiring coherent detection, the high power pulse available from a TEA laser would be desirable, but ltS
use is precludea by the uncertainty in the lasing frequency, making it difficult to design the appropriate local oscilla-tor. This frequency instability is characteristic of the TEA laser.
~,~6 ~ ~ 80~
Summary of the Invention In accordance with this invention, there is provided an optical resonant cavity, a high power pulsed laser cell disposed within the resonant cavity, means for injecting a low power CW signal into the resonant cavity and means for stabili~-ing the frequency within the resonant cavity.
Further in accordance with this invention, there is provided an optical resonant cavity, a pulsed laser cell and a CW laser cell disposed within the resonant cavity, and means for stabilizing the resonance frequency of the resonant cavity in response to a predetermined frequency of the CW laser cell.
Preferably, the stabilizing means comprise means for deriving an output beam from the resonant cavity due to the CW laser cell, means for varying the opticai length of the resonant cavity and means for controlling the optical length varying means in response to a frequency of the output beam.
This invention further provides for an optical resonant cavity, a pulsed laser cell and a CW laser cell disposed lon-gitudinally within the resonant cavity, means for transmitting a signal from the resonant cavity, means for receiving a return signal reflected by a target, a laser for producing a local oscillator signal, a detector and means for projecting the local oscillator and received signaIs on the detector.
- Moreover, the projecting means may comprise mean~ for pro-viding the same polarization of the local oscillator and received signal. Preferably, there are also provided means for stabilizing the re~onant frequency of the optical cavity in response to a predetermined frequency of the CW laser cell.
There may also be provided means ~or offsetting the frequency of the local oscillator signal with respect to the frequency ~ ` ` ~
)79~
o* the transmitted signal. The offsetting means may comprise means for varying the optical length of the local oscillator laser in response to an output signal of the detector.
This invention also provides for an optical resonant cavity, a pulsed laser cell and a CW laser cell of a predeter-mined frequency, means for aligning signals from said pulsed and continuous wave laser cells, means for transmitting a signal from the resonant cavity and means for adjusting the resonant frequency of the optical cavity in response to the transmitted signal. Additionally, there are means for receiv-ing a portion of the transmitted signal re1ected by a target, means for providing a local oscillator signal and means for projecting the local oscillator and received signals on a detector. The means for adjusting the resonant frequency may also comprise means for dithering the optical cavity at a fixed frequency and means for varying the optical length of the optical cavity in response to frequency components of the transmitted signal for optimi~ing harmonics of the dithering frequency from the transmitted signal: alternatively, they may comprise tunable filter means connected to a portion of the transmitted signal, sweeping means for tuning said filter means through a range of frequencies about the resonant frequency, and means for varying the optical length of the resonant cavity - in response to the output signal of the filter means.
~3.~ J~
In accordance with the present invention, there is provided in combinatlon: an optical resonank cavity; means for producing an output beam from said cavity comprising a high power pulsed laser cell disposed within said resonant cavity;
and means for controlling the frequency of said output beam comprising: means, disposed within the cavity, for producing a low power continuous wave signal having a predetermined frequency; and, a feedback loop responsive to the frequency of said low power continuous wave signal for automatically tuning the resonant frequency of said cavity as a function of the frequency of said continuous wave signal.
In accordance with another aspect of the invention, there is provided in combination: an optical resonant cavity having a tunable resonant frequency; means for producing an output beam from said cavity, said output beam producing means comprising a pulsed laser cell disposed within said optical cavity for producing a pulsed beam of energy; a continuous wave cell disposed within said optical cavity for producing a continuous beam of energy, having a predetermined freque~ncy, adapted to pass through said pulsed laser cell; and a feedback circuit responsive to the predetermined frequency of the continuous beam of energy for automatically tuning the resonant frequency of the optical cavity to stabilize the frequency of the pulsed beam.
In accordance with a further aspect of the invention, there is provided in combination: an optical resonant cavity having a tunable resonant frequency; means for producing an output beam of energy having a first predetermined frequency and polarization from said cavity comprising a pulsed laser cell disposed within said resonant cavity for producing a beam pulse;
means for stabilizing, at the first predetermined frequency, the - 3a -frequency of said beam pulse, said stabilizing means comprising:
a continuous wave laser cell disposed within said resonant cavity for injecting a continuous wave beam, having a frequency substantially equal to the first predetermined frequency, into said pulsed laser cell; and, a feedback circuit for automatically turning the resonant frequency of said optical cavity in response to the frequency of the continuous wave beam;
means for transmitting said output beam from said resonant cavity; means for receiving a return beam reflected by a target;
a local oscillator laser for generating a reference beam having a second predetermined frequency and polarization; and means for mixing said return beam with said reference beam comprising a detector and means for projecting said reference beam and return beam on said detector.
In accordance with a further aspect of the invention, there is provided in combination: an optical resonant cavity having a nominal resonant frequency; means for producing an output signal having a predetermined frequency from said cavity, said output signal producing means comprising: a pulsed laser cell disposed within the cavity for generating signal pulses;
and, a continuous wave laser cell disposed within said resonant cavity for generating continuous wave signals having the predetermined frequency, said signal pulses and continuous wave signals being aligned, one wi-th the other; means for transmitt-ing said output signal from said resonant cavity; and a feedback circuit, responsive to the continuous wave signal components of the output signal and non-responsive to the pulsed signal components of the output signal, for automatically and continuously maintaining the resonance of said optical cavity at said nominal resonant frequency.
In accordance with a further aspect of the invention, - 3b -~ ~ ~3 ~
there .is pro~ided laser apparatus comprising: an optical resonant cavity; a pulsed laser cell disposed within said cavity; a continuous wave laser cell disposed within said resonant cavity; and means for automatically stabilizing the resonance frequency of the resonant cavity in response to a predetermined frequency of the resonant cavity in response automatically to a predetermined frequency of the continuous wave laser cell.
In accordance with a further aspect of the invention, there is provided laser apparatus comprising: an optical resonant cavity; a pulsed laser cell disposed within the resonant cavity for producing a pulse of energy having a frequency related to the length of the resonant cavity; a continuous wave laser cell disposed within said cavity for producing continuous wave energy having a frequency related to the length of the resonant cavity, said continuous wave energy passing through the pulsed laser cell; means automatically responsive to a portion of the continuous wave energy for generating a control signal in accordance with the frequency of the produced continuous wave energy; and means directly response to said control signal for automatically controlling the length of the cavity.
- 3c -~ ~ ` ~
~ ~ 8 ~
Brief Description of the Drawings These and other objects and advantages of the invention will be better understood from the following detailed descrip-tion used in conjunction with the drawings in which like referenced numbers refer to like parts of items and in which:
FIGURE 1 is a block diagram of one embodiment of the frequency locked TEA laser showing a frequency stabilization scheme;
FIGURE 2 is a plot of a typical pulse from the frequency locked TEA laser of the present invention;
FIGURE 3 is a block diagram showing one embodiment of the stabilized frequency locked TEA laser of the present invention used in a coherent detection system; and FIGURE 4 is a block diagram showing another embodiment of the stabili~.ed frequency locked TEA laser of the present invention used in a coherent detection system.
:
~ 1 8 ~
Description of the Preferred Embodiment Referring now to Figure 1 there is shown an embodiment of a laser system oE the present invention. The injection-locked pulsed TEA laser 10 consists o reflecting mirror 20, partially transmitting mirror 50, CW laser cell 30 and the TEA laser cell 40. Reflecting mirror 20 is mounted on a piezoelectric transducer 25 whose function will become appar-ent as the description of the invention progresses. CW laser cell 30 can be one of several conventional continuous wave laser cells, for instance, the one used is a low pressure, approximately 20 Torr total pressure, C02~N2-He longitudinal dis~harge tube with Brewster angle windows. TEA laser cell 40 is a transverse electric atmospheric pressure pulse laser.
- The pulse generator 39 for TEA laser cell 40 consists, for instance, of a capacitor charged by a power supply. When a spark gap is triggered, the capacitor discharges throuyh a combination of preionizing and main electrodes within a TEA
laser cell. The preionizing electrodes are designed in a conventional manner to provide a preionizing discharge prior to the main discharge and the main electrodes are of the Rogowski configuration. The gases between the electrodes can be a conventional mix of nitrogen, carbon dioxide and helium.
The total pressure can be atmospheric for convenience although - other pressures are possible. A glow discharge is established between the electrodes and the usual molecular dynamics of a C2 laser takes place. The nitrogen is excited to its first vibrational level and collides with C02 molecules transferring energy to themO Laser action originates from transitions in the C02 molecules~ Helium is present to thermalize the gas mix so that more energy can be extracted at the desired lasing ~ :3 sn~
frequency. When the optical gain has risen high enough to exceed the optical losses, a laser pulse builds up in the TEA
laser cavity formed by mirrors 20 and 50 and a portion of this radiation transmits through the output mirror 50 of frequency locked TEA laser 10. It is possible to flow gas ~ransverse to the electric field and also transverse to the optical beam in order to replenish the gas between the electrodes for the next laser pulse if such an operation is needed.
A characteristic of TE~ lasers is to generate a high power pulse of ~hort duration in the order of 50 nanoseconds.
However, the frequency of oscillation is not stable. Several emission lines can oscilla-te simultaneously and the frequency will chirp depending on the exact spacing of the TEA laser mirrors. A laser of this type is not readily usable in a coherent detection system since the lasing frequency varies from pulse to pulse making it difficult to obkain a corre-sponding local oscillator reference signal.
It was discovered that by inserting a CW laser cell along with a TEA laser cell in an optical cavity, the output fre~uency of the TEA laser cell could be stabilized~ This is achieved by adjusting the spacing of mirrors 20 and 50 such that they are resonant for a specific line of the CO2 lasing transitions. For instance, by adjusting mirrors 20 and 50 to - resonate for the P-20 line of the C2 only the P-20 line will oscillate and CW laser 30 will emit a low power continuous laser beam on the P-20 line. While this laser is oscillating CW, TEA cell 40 is pulsed. The gain of a TEA cell rises very rapidly in all its emission lines. However, since there is a recirculating power density at the P-20 line, only the P-20 line receives significant power. Thus, the output power of the unlt rises very rapidly in the P-20 line. The presence of helium in the discharge thermalizes the other lines and allows them to relax into the P-20 line and add more energy to the P-20 oscillations. TEA laser cell 40 then, is effec-tively frequency-locked by the presence of a recirculating power density at the P-20 line from CW laser cell 30. The power continues to rise until the gain in TEA cell 40 begins to saturate and then it levels off and decays at the expense of the gain in TEA cell 40. A major portion of the stored energy in TEA cell 40 is extracted at the P-20 line. No energy is extracted at any other line. The output power continues to fall until it reaches the initial low CW level existing before TEA cell 40 was pulsed. The energy of a TEA
laser pulse is the same as its-frequency-locked equivalent, however, the frequency-locked pulse has less power but longer duration in the order of several microseconds.
This change in pulse duration is brought about by the contribution of an existing power level, from the CW laser cell 30, in the resonant cavity pr1or to the pulse buildup.
This is a significant factor in the simultaneous mechanisms responsible for the rate of change of both power and gain within the cavity. This pre-existing power level effectively reduces the peak instantaneous power, otherwise obtainable, - by depleting gain at the outset. This lower instantaneous power contributes to extracting laser energy for a longer time. The net result is that the pulse from the frequency-locked TEA laser cell 40 is stretched as compared to the pulse from an uncompensated TEA laser~ Figure 2 shows a plot of power as a function of time for a typical pulse of the injection frequency-lccked TEA laser of the present invention.
07~7 The power level is shown in relative units ince the actual power level depends on several parameters while the temporal characteristic of the pulse remains the same.
The pul9e repetition rate is primarily determinea by the time required to change the gas mixture between pulses or if the cavity is a sealed unit by the time needed for the gas to stabilize. Fresh gas can be pumped in one side of the electrodes until it fills the volume where the discharge will oc~ur again. The extracted gas can be recirculated, cooled and reused~ Alternatively, it can be discharged to the atmosphere since the device works slightly above atmo-spheric pressure. A combination of recirculating and fresh gas can also be used~ Trigger control 90 determines the - pulse repetition rate by sending a trigger signal to pulse generator 3~.
Referring back to Figure 1, a beam is extracted from the laser cavity through partial transmitting mirror 50.
Part o~ this output beam is reflected by beamsplitter 60 toward detector 70. The output of detector 70, which can be a pyroelectric type, is utilized by stabilization control 80 to stabilize the frequency of frequency-locked TEA laser 10 to a selected line, the P-20 line, for instance.
This tuning operation is done with a CW laser cell 30 - operating in its normal mode and with the TEA laser cell 40 turned off~ Resonant cavity forming mirror 20 is mounted on a pieæoelectric ~ransducer 25 so as to enable tuning of the resonant frequency of the cavity by adjusting the position of mirror 20. Stabilization control 80 applies an AC signal to piezoelectric transducer 25 so as to dither mirror 20.
Piezoelectric transducer 25 is initially positioned so as to o~
be wi~hin the selected P-20 line oscillation range. With an A-C dither signai at 1 KHz, a 2 KHz dither will be super-imposed on the output signal, since the P-20 line spectrum is a bell shaped curve and sweeping back and forth results in a frequency doubling. When the dithering or scanning occurs at the peak of the power spectrum of the P-20 line, the output `
signal at mirror 50 has only the fundamental 2 K~z component superimposed on it. If the mirror spacing is such that the dithering occurs away from the peaX for the line selec~ed, then the output signal will have superimposed a signal con-sisting of the fundamental 2 KHz frequency and many other harmonics depending on how far the resonant frequency of the cavity is from the peak of the line spectrum. Thus, stabilization control 80 generates an error signal which is applied to piezoelectric transducer 25 in response to the presence of harmonics of the 2 KHz dither signal to change the spacing between the two mirrors 20 and 50 so as to elim-inate these harmonics. When this is achieved, stabilization co~trol 80 turns off the dithering signal and keeps mirror 20 in position for optimum resonance in the selected line spectrum. The detailed circuitry of this Eeedback loop is not shown since the implementation of this type of feeaback loop is well known to those skilled in the artO
Figure 3 shows the injection-locked TEA laser system 10 used in a coherent detection system. The output beam, which is linear1y polarized, is transmitted through par-tially transmitting mirror 50 and beamsplitter 100, which is positiQned at tne brewster angle. Quarter wave-length plate 110 changes the linearly polarized output beam to circular polarization. Additionally, a telescope 120 ls )7 ~ ~
used to enlarge the output beam. Target 130 reflects some of the output beam, but the reflected signal is circularly polarized opposite than the output beam. The reflected signal is received back through the quarter wavelength plate 110 where it becomes linearly polarized orthogonal to that of the output beam of frequency-locked TEA laser system 10 and it then reflects from the brewster angle beamsplitters 100 and 140 on to a mixing detector 160 through lens 150.
Local oscillator laser 200 is arranged with its polarization orthogonal to that of frequency-locked TEA laser system 10 so that the polarizations of local oscillator beam and the return signal beam are identical. These signals are directed onto lens 150 by beamsplitter 140. Lens 150 projects these two signals on detector 160 from where the h-eterodyne signal can be directed to a signal processor/display 170. Detector 160 can be a mercury cadmium telluride photovoltaic detector.
It effectively mixes the recei~ed signal and the local oscil-lator signal to produce an output signal proportional to their frequency diference representing the Doppler shift.
Processor/display 170 aIso receives a timing signal from timing and stabilization control 180 concurrently with a trigger signaI to pulse generator 39. The timing signal can be used ~o start the horizontal sweep on an A scope in order to display range information, while the Doppler shift signal can be processed to control the vertical displacementO
Figure 3 also shows a mechanism for setting and stabi~
lizing the frequency of local oscillator laser 200. One of the mirrors forming the resonant ca~tity of local oscillator laser 200 is connected to a piezoelectric transducer 210.
The frequency of laser 200 can be stabilized by timing and stabilization control 180 utilizing the same scheme as described in the stabilization of the frequency-locked TEA
laser system 10 of Figure 1. This time both the CW and TEA
laser are turned of, and only a portion of the local oscil-lator signal is transmitted to detector 190 by beamsplitter 140 and mirror 195. Again timing and stabilization control 180 applies a 1 KH~ dither to piezoelectric transducer 210 resulting in a superimposed 2 KHz component on the local oscillator laser 200 output signal. Timing and s~abilization control 180 then g~nerates an error signal until harmonics of the 2 KHz dither are removed signifying that the local oscil-lator laser 200 has been stabilized.
Next, TEA Laser 10 is stabilized against stabilized local oscillator laser 200, using the com~ination of two beams on the heterodyne detector 160. The first of these two beams is the portion of the local oscillator beam that transmits through beamsplitter 140, lens 150 and onto detector 160.
This beam is linearly polarized orthogonally to that of TEA
laser system 10. The other beam is derived TEA laser system 10 when it is operating CW between pulses. This beam is directed to telescope 120 through quarter-wave plate 110, where small portion of the beam is backscattered and directed again through quarter-wave plate 110 so that its linear polarization - is the same as that of the local oscillator signal. This back-scattered signal is then reflected by beamsplitters 100 and 140 through lens 150 and onto detector 160. Since polarization is maintained correctly, the two signals are heterodyned by mixing detector 160. A portion of this heterodyne signal i5 directed to the timing and stabilization control 180 where an error signal is derived and applied to piezoelectric transducer ) 7 ~ ~
25, thus stabilizing TEA laser system 10. Timing and stabili-zation control 180 can also be used to offset the frequency of the local oscillator laser rom the signal frequency of the frequency-locked l'EA laser, by utilizing part of the signal of heterodyne detector 160 to apply a control signal to piezo-electric transducer 210 to offset the resonant frequency of the local oscillator laser cavity. In this offset approach, the CW laser 30 is turned on so that its beam heterodynes with the local oscillator beam on detector 160.
Figure 4 shows an alternate embodiment of the frequency stabilization mechanism. The frequency-locked TEA laser system 10 is the same as the one described in connection with Figure 1.
The frequency of local oscillator laser 200 is stabilized while the frequency-locked TEA laser 10 is turned off. A portion of the local oscillator laser signal is reflected from beamsplitter 140 and directed by mirror 300 through a stark cell 310 and onto a detector 320. The stark cell is driven by an audio-oscillator with a transverse electric field from the timing and stabilization control 330. The stark cell is filled with low pressure deuterated ammonia (NH2D) which has a very narrow absorption line range corresponding to the P-20 line of CO2.
The frequency of this absorption line is swept by the electrlc field for the audio-osciilator. When the electric field of the audio-oscillator is adjusted to exactly tune the absorption lin~ of the stark cell 310 to the P-20 line of the C02, there is a marked dip in the transmission of thP stark cell, As the audio-oscillator sweeps around this voltage, the stark cell txansmission is ~requency doubled. The output of detector 320 is directed to the timing and stabilization control 330 where it is compared with the phase and frequency o the audio-1 ~81n7~J~
oscillator. An error signal can then be derived to adjust the piezoelectric transducer 210 on local oscillator laser 200 in order to move its frequency to that required by the stark cell for zero error signal. This will allow the local oscillator laser to be precisely tuned on the P-20 emission line of C02.
Next, TEA laser 10 is stabilized against the stark cell stabilized local oscillator laser 200, using the signal from de~ector 160, as described in the stabilization of laser system 10 of Figure 3. Any predetermined ofPset between frequency-locked TEA oscillator 10 and local oscillator laser 200 can then be achieved. A predetection amplifier can be introduced, if needed, in the return signal path prior to the beam entering beamsplitter 140. As mentioned above, the detailed circuitry for stabilizing local oscillator laser 200 and laser system 10 is not shown, since the implementation of the described feed-back loops are well known to those sXilled in the art. For instance, it can be implemented by the Lock-in Stabilizer unit - manufactured by Lansing as Part No. 80-214. It provides the sweeping high voltage necessary for stark cell 310 and the dithering voltage for the piezoelectric transducers 25 and 210.
It has an input for the detector signal.
As discussed earlier, the output pulse duration of the frequency-locked TEA oscillator is approximately 2 microseconds.
The signal to noise (S/N) ratio is proportional to the power of the pulse and inversely proportional to the signal bandwidth.
For an optimally designed system, the signal bandwidth is pro-portional to the inverse of the duration of the pulse. Thus, the S/N ratio is proportional to the energy of a pulse which indicates that frequency locking the TEA la~er pulse does not degrade the S/N ratio of the ~ystem since the pulse maintains the same energy.
D 3L ~ 3 r~
Another important consideration i9 the system's uncer-tainty in speed of detected targets. It is due to the frequency shift caused by a moving target, which is related to the wavelength of the laser beam, and its resolution obtain-able from the laser pulse; For an optimum system, the best resolution that can be achieved is due to the bandwidth of the laser, which is the inverse of the duration of the pulse.
Using the ten micron wavelength of the P-20 line of the C02 and a pulse duration of at least 2 microseconds give a velocity resolution of at least approximately 8 feet per second which is much more than adequate for most applications. It was found that after the main pulse has been transmitted, the frequency holds steady for ten to twenty microseconds and then runs very rapidly. Within a ~ew milliseconds, the oscillations return to their initial injection-locXed frequency.
Another feature is that since the frequency returns to the injection-locked value within a few milliseconds, a maximum repetition rate of a few hundred hertz is possible if the gas can be made to lase that quickly. Also, it is important to realize that the output energy scales directly with volume.
The pulse width, however, does not scale with volume and can be maintained in the several microsecond range. Thus, the anticipated output power for a system with a one joule of ene gy is approximately 250 kilowatts. Scaling up to larger units has the potential of yielding several megawatts of output power.
For these larger units, there is a possibility that bacX-scattered radiation onto the heterodyne detector may interfere with detection or saturate the detector. This problem can be solved by the addition of a mechanical chopper disc in front 7 ~ '~
of the detector in the region where the local oscillator and the signal beams are both focussed. The mechanical chopper disc can be used as the primary timing for the entire system.
For instance, with a chopper disc closed, the system is turned on and the pulse leaves the instrument just before the chopper disc opens and allows radiation to be received by the detector.
This feature may or may not be required. It may be suf~icient merely to electronically gate the detector as in the present CAT systems.
An operational characteristic of the system is the low CW radiation emerging from the instrument. In some instances of targets at long range, the primary return may be received simultaneously with a CW return from a much more highly - reflecting close target. Frequency discrimination normally allows resolution of these targets. If that is not possible, then the CW discharge tube can be turned off or reduced in level to bring the gain down below losses in a transmitter so that the transmitter actually turns off during the time when detection is desired. The CW laser could then be turned up
Background of the Invention There is a serious need Eor a general purpose very high power transmitter-receiver combination for clear air turbu-lence detection, pointing and tracking, imaging, wind shear and trailing vortex measurements and similar applications.
Upyrading of the present master oscillator power amplifier unit can be carried further, but will eventually be limited by serious complexities and holding off self-oscillation in the power amplifier section. Also, additional gain achieved by use of saturable absorbers or isolators can be expected to desrade the output diffraction limited performance. For applications requiring coherent detection, the high power pulse available from a TEA laser would be desirable, but ltS
use is precludea by the uncertainty in the lasing frequency, making it difficult to design the appropriate local oscilla-tor. This frequency instability is characteristic of the TEA laser.
~,~6 ~ ~ 80~
Summary of the Invention In accordance with this invention, there is provided an optical resonant cavity, a high power pulsed laser cell disposed within the resonant cavity, means for injecting a low power CW signal into the resonant cavity and means for stabili~-ing the frequency within the resonant cavity.
Further in accordance with this invention, there is provided an optical resonant cavity, a pulsed laser cell and a CW laser cell disposed within the resonant cavity, and means for stabilizing the resonance frequency of the resonant cavity in response to a predetermined frequency of the CW laser cell.
Preferably, the stabilizing means comprise means for deriving an output beam from the resonant cavity due to the CW laser cell, means for varying the opticai length of the resonant cavity and means for controlling the optical length varying means in response to a frequency of the output beam.
This invention further provides for an optical resonant cavity, a pulsed laser cell and a CW laser cell disposed lon-gitudinally within the resonant cavity, means for transmitting a signal from the resonant cavity, means for receiving a return signal reflected by a target, a laser for producing a local oscillator signal, a detector and means for projecting the local oscillator and received signaIs on the detector.
- Moreover, the projecting means may comprise mean~ for pro-viding the same polarization of the local oscillator and received signal. Preferably, there are also provided means for stabilizing the re~onant frequency of the optical cavity in response to a predetermined frequency of the CW laser cell.
There may also be provided means ~or offsetting the frequency of the local oscillator signal with respect to the frequency ~ ` ` ~
)79~
o* the transmitted signal. The offsetting means may comprise means for varying the optical length of the local oscillator laser in response to an output signal of the detector.
This invention also provides for an optical resonant cavity, a pulsed laser cell and a CW laser cell of a predeter-mined frequency, means for aligning signals from said pulsed and continuous wave laser cells, means for transmitting a signal from the resonant cavity and means for adjusting the resonant frequency of the optical cavity in response to the transmitted signal. Additionally, there are means for receiv-ing a portion of the transmitted signal re1ected by a target, means for providing a local oscillator signal and means for projecting the local oscillator and received signals on a detector. The means for adjusting the resonant frequency may also comprise means for dithering the optical cavity at a fixed frequency and means for varying the optical length of the optical cavity in response to frequency components of the transmitted signal for optimi~ing harmonics of the dithering frequency from the transmitted signal: alternatively, they may comprise tunable filter means connected to a portion of the transmitted signal, sweeping means for tuning said filter means through a range of frequencies about the resonant frequency, and means for varying the optical length of the resonant cavity - in response to the output signal of the filter means.
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In accordance with the present invention, there is provided in combinatlon: an optical resonank cavity; means for producing an output beam from said cavity comprising a high power pulsed laser cell disposed within said resonant cavity;
and means for controlling the frequency of said output beam comprising: means, disposed within the cavity, for producing a low power continuous wave signal having a predetermined frequency; and, a feedback loop responsive to the frequency of said low power continuous wave signal for automatically tuning the resonant frequency of said cavity as a function of the frequency of said continuous wave signal.
In accordance with another aspect of the invention, there is provided in combination: an optical resonant cavity having a tunable resonant frequency; means for producing an output beam from said cavity, said output beam producing means comprising a pulsed laser cell disposed within said optical cavity for producing a pulsed beam of energy; a continuous wave cell disposed within said optical cavity for producing a continuous beam of energy, having a predetermined freque~ncy, adapted to pass through said pulsed laser cell; and a feedback circuit responsive to the predetermined frequency of the continuous beam of energy for automatically tuning the resonant frequency of the optical cavity to stabilize the frequency of the pulsed beam.
In accordance with a further aspect of the invention, there is provided in combination: an optical resonant cavity having a tunable resonant frequency; means for producing an output beam of energy having a first predetermined frequency and polarization from said cavity comprising a pulsed laser cell disposed within said resonant cavity for producing a beam pulse;
means for stabilizing, at the first predetermined frequency, the - 3a -frequency of said beam pulse, said stabilizing means comprising:
a continuous wave laser cell disposed within said resonant cavity for injecting a continuous wave beam, having a frequency substantially equal to the first predetermined frequency, into said pulsed laser cell; and, a feedback circuit for automatically turning the resonant frequency of said optical cavity in response to the frequency of the continuous wave beam;
means for transmitting said output beam from said resonant cavity; means for receiving a return beam reflected by a target;
a local oscillator laser for generating a reference beam having a second predetermined frequency and polarization; and means for mixing said return beam with said reference beam comprising a detector and means for projecting said reference beam and return beam on said detector.
In accordance with a further aspect of the invention, there is provided in combination: an optical resonant cavity having a nominal resonant frequency; means for producing an output signal having a predetermined frequency from said cavity, said output signal producing means comprising: a pulsed laser cell disposed within the cavity for generating signal pulses;
and, a continuous wave laser cell disposed within said resonant cavity for generating continuous wave signals having the predetermined frequency, said signal pulses and continuous wave signals being aligned, one wi-th the other; means for transmitt-ing said output signal from said resonant cavity; and a feedback circuit, responsive to the continuous wave signal components of the output signal and non-responsive to the pulsed signal components of the output signal, for automatically and continuously maintaining the resonance of said optical cavity at said nominal resonant frequency.
In accordance with a further aspect of the invention, - 3b -~ ~ ~3 ~
there .is pro~ided laser apparatus comprising: an optical resonant cavity; a pulsed laser cell disposed within said cavity; a continuous wave laser cell disposed within said resonant cavity; and means for automatically stabilizing the resonance frequency of the resonant cavity in response to a predetermined frequency of the resonant cavity in response automatically to a predetermined frequency of the continuous wave laser cell.
In accordance with a further aspect of the invention, there is provided laser apparatus comprising: an optical resonant cavity; a pulsed laser cell disposed within the resonant cavity for producing a pulse of energy having a frequency related to the length of the resonant cavity; a continuous wave laser cell disposed within said cavity for producing continuous wave energy having a frequency related to the length of the resonant cavity, said continuous wave energy passing through the pulsed laser cell; means automatically responsive to a portion of the continuous wave energy for generating a control signal in accordance with the frequency of the produced continuous wave energy; and means directly response to said control signal for automatically controlling the length of the cavity.
- 3c -~ ~ ` ~
~ ~ 8 ~
Brief Description of the Drawings These and other objects and advantages of the invention will be better understood from the following detailed descrip-tion used in conjunction with the drawings in which like referenced numbers refer to like parts of items and in which:
FIGURE 1 is a block diagram of one embodiment of the frequency locked TEA laser showing a frequency stabilization scheme;
FIGURE 2 is a plot of a typical pulse from the frequency locked TEA laser of the present invention;
FIGURE 3 is a block diagram showing one embodiment of the stabilized frequency locked TEA laser of the present invention used in a coherent detection system; and FIGURE 4 is a block diagram showing another embodiment of the stabili~.ed frequency locked TEA laser of the present invention used in a coherent detection system.
:
~ 1 8 ~
Description of the Preferred Embodiment Referring now to Figure 1 there is shown an embodiment of a laser system oE the present invention. The injection-locked pulsed TEA laser 10 consists o reflecting mirror 20, partially transmitting mirror 50, CW laser cell 30 and the TEA laser cell 40. Reflecting mirror 20 is mounted on a piezoelectric transducer 25 whose function will become appar-ent as the description of the invention progresses. CW laser cell 30 can be one of several conventional continuous wave laser cells, for instance, the one used is a low pressure, approximately 20 Torr total pressure, C02~N2-He longitudinal dis~harge tube with Brewster angle windows. TEA laser cell 40 is a transverse electric atmospheric pressure pulse laser.
- The pulse generator 39 for TEA laser cell 40 consists, for instance, of a capacitor charged by a power supply. When a spark gap is triggered, the capacitor discharges throuyh a combination of preionizing and main electrodes within a TEA
laser cell. The preionizing electrodes are designed in a conventional manner to provide a preionizing discharge prior to the main discharge and the main electrodes are of the Rogowski configuration. The gases between the electrodes can be a conventional mix of nitrogen, carbon dioxide and helium.
The total pressure can be atmospheric for convenience although - other pressures are possible. A glow discharge is established between the electrodes and the usual molecular dynamics of a C2 laser takes place. The nitrogen is excited to its first vibrational level and collides with C02 molecules transferring energy to themO Laser action originates from transitions in the C02 molecules~ Helium is present to thermalize the gas mix so that more energy can be extracted at the desired lasing ~ :3 sn~
frequency. When the optical gain has risen high enough to exceed the optical losses, a laser pulse builds up in the TEA
laser cavity formed by mirrors 20 and 50 and a portion of this radiation transmits through the output mirror 50 of frequency locked TEA laser 10. It is possible to flow gas ~ransverse to the electric field and also transverse to the optical beam in order to replenish the gas between the electrodes for the next laser pulse if such an operation is needed.
A characteristic of TE~ lasers is to generate a high power pulse of ~hort duration in the order of 50 nanoseconds.
However, the frequency of oscillation is not stable. Several emission lines can oscilla-te simultaneously and the frequency will chirp depending on the exact spacing of the TEA laser mirrors. A laser of this type is not readily usable in a coherent detection system since the lasing frequency varies from pulse to pulse making it difficult to obkain a corre-sponding local oscillator reference signal.
It was discovered that by inserting a CW laser cell along with a TEA laser cell in an optical cavity, the output fre~uency of the TEA laser cell could be stabilized~ This is achieved by adjusting the spacing of mirrors 20 and 50 such that they are resonant for a specific line of the CO2 lasing transitions. For instance, by adjusting mirrors 20 and 50 to - resonate for the P-20 line of the C2 only the P-20 line will oscillate and CW laser 30 will emit a low power continuous laser beam on the P-20 line. While this laser is oscillating CW, TEA cell 40 is pulsed. The gain of a TEA cell rises very rapidly in all its emission lines. However, since there is a recirculating power density at the P-20 line, only the P-20 line receives significant power. Thus, the output power of the unlt rises very rapidly in the P-20 line. The presence of helium in the discharge thermalizes the other lines and allows them to relax into the P-20 line and add more energy to the P-20 oscillations. TEA laser cell 40 then, is effec-tively frequency-locked by the presence of a recirculating power density at the P-20 line from CW laser cell 30. The power continues to rise until the gain in TEA cell 40 begins to saturate and then it levels off and decays at the expense of the gain in TEA cell 40. A major portion of the stored energy in TEA cell 40 is extracted at the P-20 line. No energy is extracted at any other line. The output power continues to fall until it reaches the initial low CW level existing before TEA cell 40 was pulsed. The energy of a TEA
laser pulse is the same as its-frequency-locked equivalent, however, the frequency-locked pulse has less power but longer duration in the order of several microseconds.
This change in pulse duration is brought about by the contribution of an existing power level, from the CW laser cell 30, in the resonant cavity pr1or to the pulse buildup.
This is a significant factor in the simultaneous mechanisms responsible for the rate of change of both power and gain within the cavity. This pre-existing power level effectively reduces the peak instantaneous power, otherwise obtainable, - by depleting gain at the outset. This lower instantaneous power contributes to extracting laser energy for a longer time. The net result is that the pulse from the frequency-locked TEA laser cell 40 is stretched as compared to the pulse from an uncompensated TEA laser~ Figure 2 shows a plot of power as a function of time for a typical pulse of the injection frequency-lccked TEA laser of the present invention.
07~7 The power level is shown in relative units ince the actual power level depends on several parameters while the temporal characteristic of the pulse remains the same.
The pul9e repetition rate is primarily determinea by the time required to change the gas mixture between pulses or if the cavity is a sealed unit by the time needed for the gas to stabilize. Fresh gas can be pumped in one side of the electrodes until it fills the volume where the discharge will oc~ur again. The extracted gas can be recirculated, cooled and reused~ Alternatively, it can be discharged to the atmosphere since the device works slightly above atmo-spheric pressure. A combination of recirculating and fresh gas can also be used~ Trigger control 90 determines the - pulse repetition rate by sending a trigger signal to pulse generator 3~.
Referring back to Figure 1, a beam is extracted from the laser cavity through partial transmitting mirror 50.
Part o~ this output beam is reflected by beamsplitter 60 toward detector 70. The output of detector 70, which can be a pyroelectric type, is utilized by stabilization control 80 to stabilize the frequency of frequency-locked TEA laser 10 to a selected line, the P-20 line, for instance.
This tuning operation is done with a CW laser cell 30 - operating in its normal mode and with the TEA laser cell 40 turned off~ Resonant cavity forming mirror 20 is mounted on a pieæoelectric ~ransducer 25 so as to enable tuning of the resonant frequency of the cavity by adjusting the position of mirror 20. Stabilization control 80 applies an AC signal to piezoelectric transducer 25 so as to dither mirror 20.
Piezoelectric transducer 25 is initially positioned so as to o~
be wi~hin the selected P-20 line oscillation range. With an A-C dither signai at 1 KHz, a 2 KHz dither will be super-imposed on the output signal, since the P-20 line spectrum is a bell shaped curve and sweeping back and forth results in a frequency doubling. When the dithering or scanning occurs at the peak of the power spectrum of the P-20 line, the output `
signal at mirror 50 has only the fundamental 2 K~z component superimposed on it. If the mirror spacing is such that the dithering occurs away from the peaX for the line selec~ed, then the output signal will have superimposed a signal con-sisting of the fundamental 2 KHz frequency and many other harmonics depending on how far the resonant frequency of the cavity is from the peak of the line spectrum. Thus, stabilization control 80 generates an error signal which is applied to piezoelectric transducer 25 in response to the presence of harmonics of the 2 KHz dither signal to change the spacing between the two mirrors 20 and 50 so as to elim-inate these harmonics. When this is achieved, stabilization co~trol 80 turns off the dithering signal and keeps mirror 20 in position for optimum resonance in the selected line spectrum. The detailed circuitry of this Eeedback loop is not shown since the implementation of this type of feeaback loop is well known to those skilled in the artO
Figure 3 shows the injection-locked TEA laser system 10 used in a coherent detection system. The output beam, which is linear1y polarized, is transmitted through par-tially transmitting mirror 50 and beamsplitter 100, which is positiQned at tne brewster angle. Quarter wave-length plate 110 changes the linearly polarized output beam to circular polarization. Additionally, a telescope 120 ls )7 ~ ~
used to enlarge the output beam. Target 130 reflects some of the output beam, but the reflected signal is circularly polarized opposite than the output beam. The reflected signal is received back through the quarter wavelength plate 110 where it becomes linearly polarized orthogonal to that of the output beam of frequency-locked TEA laser system 10 and it then reflects from the brewster angle beamsplitters 100 and 140 on to a mixing detector 160 through lens 150.
Local oscillator laser 200 is arranged with its polarization orthogonal to that of frequency-locked TEA laser system 10 so that the polarizations of local oscillator beam and the return signal beam are identical. These signals are directed onto lens 150 by beamsplitter 140. Lens 150 projects these two signals on detector 160 from where the h-eterodyne signal can be directed to a signal processor/display 170. Detector 160 can be a mercury cadmium telluride photovoltaic detector.
It effectively mixes the recei~ed signal and the local oscil-lator signal to produce an output signal proportional to their frequency diference representing the Doppler shift.
Processor/display 170 aIso receives a timing signal from timing and stabilization control 180 concurrently with a trigger signaI to pulse generator 39. The timing signal can be used ~o start the horizontal sweep on an A scope in order to display range information, while the Doppler shift signal can be processed to control the vertical displacementO
Figure 3 also shows a mechanism for setting and stabi~
lizing the frequency of local oscillator laser 200. One of the mirrors forming the resonant ca~tity of local oscillator laser 200 is connected to a piezoelectric transducer 210.
The frequency of laser 200 can be stabilized by timing and stabilization control 180 utilizing the same scheme as described in the stabilization of the frequency-locked TEA
laser system 10 of Figure 1. This time both the CW and TEA
laser are turned of, and only a portion of the local oscil-lator signal is transmitted to detector 190 by beamsplitter 140 and mirror 195. Again timing and stabilization control 180 applies a 1 KH~ dither to piezoelectric transducer 210 resulting in a superimposed 2 KHz component on the local oscillator laser 200 output signal. Timing and s~abilization control 180 then g~nerates an error signal until harmonics of the 2 KHz dither are removed signifying that the local oscil-lator laser 200 has been stabilized.
Next, TEA Laser 10 is stabilized against stabilized local oscillator laser 200, using the com~ination of two beams on the heterodyne detector 160. The first of these two beams is the portion of the local oscillator beam that transmits through beamsplitter 140, lens 150 and onto detector 160.
This beam is linearly polarized orthogonally to that of TEA
laser system 10. The other beam is derived TEA laser system 10 when it is operating CW between pulses. This beam is directed to telescope 120 through quarter-wave plate 110, where small portion of the beam is backscattered and directed again through quarter-wave plate 110 so that its linear polarization - is the same as that of the local oscillator signal. This back-scattered signal is then reflected by beamsplitters 100 and 140 through lens 150 and onto detector 160. Since polarization is maintained correctly, the two signals are heterodyned by mixing detector 160. A portion of this heterodyne signal i5 directed to the timing and stabilization control 180 where an error signal is derived and applied to piezoelectric transducer ) 7 ~ ~
25, thus stabilizing TEA laser system 10. Timing and stabili-zation control 180 can also be used to offset the frequency of the local oscillator laser rom the signal frequency of the frequency-locked l'EA laser, by utilizing part of the signal of heterodyne detector 160 to apply a control signal to piezo-electric transducer 210 to offset the resonant frequency of the local oscillator laser cavity. In this offset approach, the CW laser 30 is turned on so that its beam heterodynes with the local oscillator beam on detector 160.
Figure 4 shows an alternate embodiment of the frequency stabilization mechanism. The frequency-locked TEA laser system 10 is the same as the one described in connection with Figure 1.
The frequency of local oscillator laser 200 is stabilized while the frequency-locked TEA laser 10 is turned off. A portion of the local oscillator laser signal is reflected from beamsplitter 140 and directed by mirror 300 through a stark cell 310 and onto a detector 320. The stark cell is driven by an audio-oscillator with a transverse electric field from the timing and stabilization control 330. The stark cell is filled with low pressure deuterated ammonia (NH2D) which has a very narrow absorption line range corresponding to the P-20 line of CO2.
The frequency of this absorption line is swept by the electrlc field for the audio-osciilator. When the electric field of the audio-oscillator is adjusted to exactly tune the absorption lin~ of the stark cell 310 to the P-20 line of the C02, there is a marked dip in the transmission of thP stark cell, As the audio-oscillator sweeps around this voltage, the stark cell txansmission is ~requency doubled. The output of detector 320 is directed to the timing and stabilization control 330 where it is compared with the phase and frequency o the audio-1 ~81n7~J~
oscillator. An error signal can then be derived to adjust the piezoelectric transducer 210 on local oscillator laser 200 in order to move its frequency to that required by the stark cell for zero error signal. This will allow the local oscillator laser to be precisely tuned on the P-20 emission line of C02.
Next, TEA laser 10 is stabilized against the stark cell stabilized local oscillator laser 200, using the signal from de~ector 160, as described in the stabilization of laser system 10 of Figure 3. Any predetermined ofPset between frequency-locked TEA oscillator 10 and local oscillator laser 200 can then be achieved. A predetection amplifier can be introduced, if needed, in the return signal path prior to the beam entering beamsplitter 140. As mentioned above, the detailed circuitry for stabilizing local oscillator laser 200 and laser system 10 is not shown, since the implementation of the described feed-back loops are well known to those sXilled in the art. For instance, it can be implemented by the Lock-in Stabilizer unit - manufactured by Lansing as Part No. 80-214. It provides the sweeping high voltage necessary for stark cell 310 and the dithering voltage for the piezoelectric transducers 25 and 210.
It has an input for the detector signal.
As discussed earlier, the output pulse duration of the frequency-locked TEA oscillator is approximately 2 microseconds.
The signal to noise (S/N) ratio is proportional to the power of the pulse and inversely proportional to the signal bandwidth.
For an optimally designed system, the signal bandwidth is pro-portional to the inverse of the duration of the pulse. Thus, the S/N ratio is proportional to the energy of a pulse which indicates that frequency locking the TEA la~er pulse does not degrade the S/N ratio of the ~ystem since the pulse maintains the same energy.
D 3L ~ 3 r~
Another important consideration i9 the system's uncer-tainty in speed of detected targets. It is due to the frequency shift caused by a moving target, which is related to the wavelength of the laser beam, and its resolution obtain-able from the laser pulse; For an optimum system, the best resolution that can be achieved is due to the bandwidth of the laser, which is the inverse of the duration of the pulse.
Using the ten micron wavelength of the P-20 line of the C02 and a pulse duration of at least 2 microseconds give a velocity resolution of at least approximately 8 feet per second which is much more than adequate for most applications. It was found that after the main pulse has been transmitted, the frequency holds steady for ten to twenty microseconds and then runs very rapidly. Within a ~ew milliseconds, the oscillations return to their initial injection-locXed frequency.
Another feature is that since the frequency returns to the injection-locked value within a few milliseconds, a maximum repetition rate of a few hundred hertz is possible if the gas can be made to lase that quickly. Also, it is important to realize that the output energy scales directly with volume.
The pulse width, however, does not scale with volume and can be maintained in the several microsecond range. Thus, the anticipated output power for a system with a one joule of ene gy is approximately 250 kilowatts. Scaling up to larger units has the potential of yielding several megawatts of output power.
For these larger units, there is a possibility that bacX-scattered radiation onto the heterodyne detector may interfere with detection or saturate the detector. This problem can be solved by the addition of a mechanical chopper disc in front 7 ~ '~
of the detector in the region where the local oscillator and the signal beams are both focussed. The mechanical chopper disc can be used as the primary timing for the entire system.
For instance, with a chopper disc closed, the system is turned on and the pulse leaves the instrument just before the chopper disc opens and allows radiation to be received by the detector.
This feature may or may not be required. It may be suf~icient merely to electronically gate the detector as in the present CAT systems.
An operational characteristic of the system is the low CW radiation emerging from the instrument. In some instances of targets at long range, the primary return may be received simultaneously with a CW return from a much more highly - reflecting close target. Frequency discrimination normally allows resolution of these targets. If that is not possible, then the CW discharge tube can be turned off or reduced in level to bring the gain down below losses in a transmitter so that the transmitter actually turns off during the time when detection is desired. The CW laser could then be turned up
2~ and stabilization re-established before the next pulse is emitted. It has also been found that the beam is essentially diffraction limited which is a desira~le feature for a C02 laser radar.
- Additionally, this injection-locked unit shows potential of being packaged in a much smaller volume than the original CAT instrument~. The electrical to optical efficiency is much higher for the injection-locked unit primarily because a major-ity of the energy is extracted in contrast to the CAT unit which uses long amplifiers with very little energy extraction.
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Other modiEications to the described embodiments will be apparent to those skilled in tha axt without departing from the spirit and scope o this invention~ Accordingly, it is intended that this invention be no~ limited except as defined by the appended claims.
- Additionally, this injection-locked unit shows potential of being packaged in a much smaller volume than the original CAT instrument~. The electrical to optical efficiency is much higher for the injection-locked unit primarily because a major-ity of the energy is extracted in contrast to the CAT unit which uses long amplifiers with very little energy extraction.
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Other modiEications to the described embodiments will be apparent to those skilled in tha axt without departing from the spirit and scope o this invention~ Accordingly, it is intended that this invention be no~ limited except as defined by the appended claims.
Claims (15)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. In combination:
an optical resonant cavity;
means for producing an output beam from said cavity comprising a high power pulsed laser cell disposed within said resonant cavity; and means for controlling the frequency of said output beam comprising: means, disposed within the cavity, for producing a low power continuous wave signal having a predetermined frequency; and, a feedback loop responsive to the frequency of said low power continuous wave signal for automatically tuning the resonant frequency of said cavity as a function of the frequency of said continuous wave signal.
an optical resonant cavity;
means for producing an output beam from said cavity comprising a high power pulsed laser cell disposed within said resonant cavity; and means for controlling the frequency of said output beam comprising: means, disposed within the cavity, for producing a low power continuous wave signal having a predetermined frequency; and, a feedback loop responsive to the frequency of said low power continuous wave signal for automatically tuning the resonant frequency of said cavity as a function of the frequency of said continuous wave signal.
2. In combination:
an optical resonant cavity having a tunable resonant frequency;
means for producing an output beam from said cavity, said output beam producing means comprising a pulsed laser cell disposed within said optical cavity for producing a pulsed beam of energy;
a continuous wave cell disposed within said optical cavity for producing a continuous beam of energy, having a predetermined frequency, adapted to pass through said pulsed laser cell; and a feedback circuit responsive to the predetermined frequency of the continuous beam of energy for automatically tuning the resonant frequency of the optical cavity to stabilize the frequency of the pulsed beam.
an optical resonant cavity having a tunable resonant frequency;
means for producing an output beam from said cavity, said output beam producing means comprising a pulsed laser cell disposed within said optical cavity for producing a pulsed beam of energy;
a continuous wave cell disposed within said optical cavity for producing a continuous beam of energy, having a predetermined frequency, adapted to pass through said pulsed laser cell; and a feedback circuit responsive to the predetermined frequency of the continuous beam of energy for automatically tuning the resonant frequency of the optical cavity to stabilize the frequency of the pulsed beam.
3. The combination of claim 2 wherein said optical resonant cavity has a predetermined optical length and said feedback circuit further comprises:
means for extracting a portion of the output beam from said resonant cavity;
means for detecting the frequency of said extracted portion of the output beam and for generating an electrical signal in response to variations in said frequency;
means for varying the optical length of said optical resonant cavity; and means responsive to said electrical signal for controlling said optical length varying means to change the optical length of said optical resonant cavity to substantially eliminate the variations of said output frequency.
means for extracting a portion of the output beam from said resonant cavity;
means for detecting the frequency of said extracted portion of the output beam and for generating an electrical signal in response to variations in said frequency;
means for varying the optical length of said optical resonant cavity; and means responsive to said electrical signal for controlling said optical length varying means to change the optical length of said optical resonant cavity to substantially eliminate the variations of said output frequency.
4. In combination:
an optical resonant cavity having a tunable resonant frequency;
means for producing an output beam of energy having a first predetermined frequency and polarization from said cavity comprising a pulsed laser cell disposed within said resonant cavity for producing a beam pulse;
means for stabilizing, at the first predetermined frequency, the frequency of said beam pulse, said stabilizing means comprising: a continuous wave laser cell disposed within said resonant cavity for injecting a continuous wave beam, having a frequency substantially equal to the first predetermined frequency, into said pulsed laser cell; and, a feedback circuit for automatically tuning the resonant frequency of said optical cavity in response to the frequency of the continuous wave beam;
means for transmitting said output beam from said resonant cavity;
means for receiving a return beam reflected by a target;
a local oscillator laser for generating a reference beam having a second predetermined frequency and polarization;
and means for mixing said return beam with said reference beam comprising a detector and means for projecting said refer-ence beam and return beam on said detector.
an optical resonant cavity having a tunable resonant frequency;
means for producing an output beam of energy having a first predetermined frequency and polarization from said cavity comprising a pulsed laser cell disposed within said resonant cavity for producing a beam pulse;
means for stabilizing, at the first predetermined frequency, the frequency of said beam pulse, said stabilizing means comprising: a continuous wave laser cell disposed within said resonant cavity for injecting a continuous wave beam, having a frequency substantially equal to the first predetermined frequency, into said pulsed laser cell; and, a feedback circuit for automatically tuning the resonant frequency of said optical cavity in response to the frequency of the continuous wave beam;
means for transmitting said output beam from said resonant cavity;
means for receiving a return beam reflected by a target;
a local oscillator laser for generating a reference beam having a second predetermined frequency and polarization;
and means for mixing said return beam with said reference beam comprising a detector and means for projecting said refer-ence beam and return beam on said detector.
5. The combination of claim 4 wherein said projecting means further comprises:
means for aligning the polarization of said reference and received beams.
means for aligning the polarization of said reference and received beams.
6. The combination of claim 5 further comprising:
means for off-setting the frequency of said reference beam with respect to the frequency of said output beam.
means for off-setting the frequency of said reference beam with respect to the frequency of said output beam.
7. The combination of claim 6 wherein the local oscillator laser has a predetermined optical length and said off-setting means comprises:
means for varying the optical length of said local oscillator laser in response to an output signal of said detector.
means for varying the optical length of said local oscillator laser in response to an output signal of said detector.
8. In combination:
an optical resonant cavity having a nominal resonant frequency;
means for producing an output signal having a predetermined frequency from said cavity, said output signal producing means comprising: a pulsed laser cell disposed within the cavity for generating signal pulses; and, a continuous wave laser cell disposed within said resonant cavity for generating continuous wave signals having the predetermined frequency, said signal pulses and continuous wave signals being aligned, one with the other;
means for transmitting said output signal from said resonant cavity; and a feedback circuit, responsive to the continuous wave signal components of the output signal and non-responsive to the pulsed signal components of the output signal, for automatically and continuously maintaining the resonance of said optical cavity at said nominal resonant frequency.
an optical resonant cavity having a nominal resonant frequency;
means for producing an output signal having a predetermined frequency from said cavity, said output signal producing means comprising: a pulsed laser cell disposed within the cavity for generating signal pulses; and, a continuous wave laser cell disposed within said resonant cavity for generating continuous wave signals having the predetermined frequency, said signal pulses and continuous wave signals being aligned, one with the other;
means for transmitting said output signal from said resonant cavity; and a feedback circuit, responsive to the continuous wave signal components of the output signal and non-responsive to the pulsed signal components of the output signal, for automatically and continuously maintaining the resonance of said optical cavity at said nominal resonant frequency.
9. The combination of claim 8 further comprising:
means for receiving a portion of said transmitted signal reflected by a target;
means for providing local oscillator signal;
means for projecting said local oscillator and received signals on a detector.
means for receiving a portion of said transmitted signal reflected by a target;
means for providing local oscillator signal;
means for projecting said local oscillator and received signals on a detector.
10. The combination of claim 9 wherein said optical resonant cavity has a nominal optical length and wherein said feedback circuit further comprises:
means for deriving a portion of said output signal;
means for automatically generating an electrical control signal, said electrical control signal having an amplitude that changes in accordance with changes in the frequency of the continuous wave signal components of said output signal portion; and means for varying the optical length of said resonant cavity from said nominal optical length in response to the amplitude changes of said control signal.
means for deriving a portion of said output signal;
means for automatically generating an electrical control signal, said electrical control signal having an amplitude that changes in accordance with changes in the frequency of the continuous wave signal components of said output signal portion; and means for varying the optical length of said resonant cavity from said nominal optical length in response to the amplitude changes of said control signal.
11. The combination of claim 10 further comprising:
means for off-setting the frequency of said local oscillator signal from the frequency of said output signal.
means for off-setting the frequency of said local oscillator signal from the frequency of said output signal.
12. The combination of claim 9 wherein said optical resonant cavity has a nominal optical length and wherein said feedback circuit further comprises:
means for dithering, at a fixed frequency, the optical length of said optical cavity about said nominal optical length, said dithering producing harmonics of said fixed dithering frequency on said output signal when the resonant frequency of said optical cavity differs from said nominal resonant frequency;
means for deriving a portion of said output signal;
means for detecting the presence of said harmonics on the derived portion of said output signal and for generating a feedback signal in response thereto; and means, responsive to said feedback signal, for varying the nominal optical length of said optical cavity to substantially eliminate the harmonics of said fixed dithering frequency on said output signal.
means for dithering, at a fixed frequency, the optical length of said optical cavity about said nominal optical length, said dithering producing harmonics of said fixed dithering frequency on said output signal when the resonant frequency of said optical cavity differs from said nominal resonant frequency;
means for deriving a portion of said output signal;
means for detecting the presence of said harmonics on the derived portion of said output signal and for generating a feedback signal in response thereto; and means, responsive to said feedback signal, for varying the nominal optical length of said optical cavity to substantially eliminate the harmonics of said fixed dithering frequency on said output signal.
13. The combination of claim 9 wherein said optical resonant cavity has a predetermined optical length and wherein said feedback circuit further comprises:
tunable filter means connected to a portion of said transmitted output signal, said filter means generating an output signal in accordance with the frequency of the continuous wave signal components of said transmitted output signal;
means for tuning said filter means through a range of frequencies about said nominal resonant frequency; and means for varying the optical length of said optical resonant cavity in response to the output signal of said filter means.
tunable filter means connected to a portion of said transmitted output signal, said filter means generating an output signal in accordance with the frequency of the continuous wave signal components of said transmitted output signal;
means for tuning said filter means through a range of frequencies about said nominal resonant frequency; and means for varying the optical length of said optical resonant cavity in response to the output signal of said filter means.
14. Laser apparatus comprising:
an optical resonant cavity;
a pulsed laser cell disposed within said cavity;
a continuous wave laser cell disposed within said resonant cavity; and means for automatically stabilizing the resonance frequency of the resonant cavity in response to a predetermined frequency of the resonant cavity in response automatically to a predetermined frequency of the continuous wave laser cell.
an optical resonant cavity;
a pulsed laser cell disposed within said cavity;
a continuous wave laser cell disposed within said resonant cavity; and means for automatically stabilizing the resonance frequency of the resonant cavity in response to a predetermined frequency of the resonant cavity in response automatically to a predetermined frequency of the continuous wave laser cell.
15. Laser apparatus comprising:
an optical resonant cavity;
a pulsed laser cell disposed within the resonant cavity for producing a pulse of energy having a frequency related to the length of the resonant cavity;
a continuous wave laser cell disposed within said cavity for producing continuous wave energy having a frequency related to the length of the resonant cavity, said continuous wave energy passing through the pulsed laser cell;
means automatically responsive to a portion of the continuous wave energy for generating a control signal in accordance with the frequency of the produced continuous wave energy; and means directly responsive to said control signal for automatically controlling the length of the cavity.
an optical resonant cavity;
a pulsed laser cell disposed within the resonant cavity for producing a pulse of energy having a frequency related to the length of the resonant cavity;
a continuous wave laser cell disposed within said cavity for producing continuous wave energy having a frequency related to the length of the resonant cavity, said continuous wave energy passing through the pulsed laser cell;
means automatically responsive to a portion of the continuous wave energy for generating a control signal in accordance with the frequency of the produced continuous wave energy; and means directly responsive to said control signal for automatically controlling the length of the cavity.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US14656280A | 1980-05-05 | 1980-05-05 | |
| US146,562 | 1988-01-21 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1180797A true CA1180797A (en) | 1985-01-08 |
Family
ID=22517954
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000374194A Expired CA1180797A (en) | 1980-05-05 | 1981-03-30 | Injection frequency locked tea laser |
Country Status (6)
| Country | Link |
|---|---|
| JP (1) | JPS571280A (en) |
| CA (1) | CA1180797A (en) |
| DE (1) | DE3117717A1 (en) |
| FR (1) | FR2481848B1 (en) |
| GB (1) | GB2093630B (en) |
| IT (1) | IT1170918B (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5014277A (en) * | 1989-04-21 | 1991-05-07 | Driel Henry M Van | Laser mode-coupling via a pulsed modulator |
Families Citing this family (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FR2544561B1 (en) * | 1983-04-13 | 1985-06-14 | Comp Generale Electricite | GAS LASER DEVICE CAPABLE OF EMITTING RADIATION PULSES HAVING A SINGLE FREQUENCY |
| JPS60129086A (en) * | 1983-12-16 | 1985-07-10 | 株式会社ナカ技術研究所 | Drying apparatus |
| ITMI20031675A1 (en) * | 2003-08-29 | 2005-02-28 | Cesi Ct Elettrotecnico Sperimen Tale Italiano | LASER OSCILLATOR PULSED IN SINGLE LONGITUDINAL MODE |
| US9601895B2 (en) | 2013-10-18 | 2017-03-21 | Bae Systems Information And Electronic Systems Integration Inc. | Ultra fast semiconductor laser |
Family Cites Families (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3471803A (en) * | 1967-04-28 | 1969-10-07 | Hughes Aircraft Co | Laser having a stabilized output spectrum |
| US3646469A (en) * | 1970-03-20 | 1972-02-29 | United Aircraft Corp | Travelling wave regenerative laser amplifier |
| US3740664A (en) * | 1972-02-22 | 1973-06-19 | United Aircraft Corp | Hybrid frequency stable laser system |
-
1981
- 1981-03-30 CA CA000374194A patent/CA1180797A/en not_active Expired
- 1981-04-10 GB GB8111336A patent/GB2093630B/en not_active Expired
- 1981-04-23 JP JP6190081A patent/JPS571280A/en active Pending
- 1981-04-24 IT IT48343/81A patent/IT1170918B/en active
- 1981-05-04 FR FR8108772A patent/FR2481848B1/en not_active Expired
- 1981-05-05 DE DE19813117717 patent/DE3117717A1/en not_active Ceased
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5014277A (en) * | 1989-04-21 | 1991-05-07 | Driel Henry M Van | Laser mode-coupling via a pulsed modulator |
Also Published As
| Publication number | Publication date |
|---|---|
| IT8148343A0 (en) | 1981-04-24 |
| GB2093630B (en) | 1984-02-01 |
| GB2093630A (en) | 1982-09-02 |
| FR2481848A1 (en) | 1981-11-06 |
| IT1170918B (en) | 1987-06-03 |
| FR2481848B1 (en) | 1985-09-13 |
| DE3117717A1 (en) | 1982-03-04 |
| JPS571280A (en) | 1982-01-06 |
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