Efficient generation and heterodyne detectionof 4.75-pam light with second-harmonic generation
David A. Russell and Reinhard Ebert
Second-harmonic generation energy conversion efficiencies of C0 2 laser radiation as high as 56% havebeen measured in 2-cm-long AgGaSe2 crystals with 30-ns pulses. The value of the nonlinear coefficientd36, which best fits the measured data, is (32 ± 4) x 10-12 m/V. Continuous-wave second-harmonicgeneration in tellurium generated up to 0.27 mW with an efficiency of 6.8(10)-5. These pulsed andcontinuous beams were combined to demonstrate heterodyne detection at 4.75 Elm, which increased thesignal strength of weak signals by at least a factor of 25 over direct detection. Simultaneous heterodynedetection of 5- and 10-[um light is demonstrated by use of both the pump and second-harmonic beamsfrom the cw laser as independent local oscillators.
As the use of passive thermal-imaging systems in the3-5-pim atmospheric window increases, an activelaser source in this window is also desirable forpurposes such as range finding, target designation, orcommunication. No adequate laser source, that is,one that is efficient and portable, has existed at atransmission peak within this wavelength range.However, because the CO2 laser serves as a good lasersource for the 8-12-jim window, one might considerusing second-harmonic generation to convert the CO2laser's radiation into the 3-5-jim region. When the9P14 line of a CO2 laser, with its wavelength of 9.50jim, is frequency doubled, its second harmonic has awavelength of 4.75 jlm, which coincides with a trans-mission peak in the 3-5-jm window.
Until now, second-harmonic generation of CO2laser radiation has only been moderately efficient.When silver gallium selenide (AgGaSe2) has beenused, energy conversion efficiencies up to 14% havebeen reported., 23 This paper reports measured ex-ternal energy conversion efficiencies up to 56%.
Detection of the second-harmonic signal has alsobeen improved. Detection in the 3-5-jim region hasbeen dependent on direct detection. With the im-
The authors are with Forschungsgesellschaft fur AngewandteNaturwissenschaften, Forschungsinstitut fur Optik, Schloss Kress-bach, W-7400 Tfibingen 1, Germany.
provements in cw second-harmonic generation (SHG)reported here, a local oscillator at 4.75 m is nowpossible; this permits heterodyne detection, whichhas the advantages of increased sensitivity and phase-information detection. Heterodyne detection at 4.75jim is reported, with the signal strength being in-creased at least 25 times in comparison with directdetection when weak signals must be detected.Additionally, simultaneous two-color heterodyne de-tection, which is simultaneous heterodyne detectionat two wavelengths with one detector, is demon-strated by detecting both the pump and the second-harmonic signals and then filtering the electricalsignal to separate the widely spaced intermediatefrequencies of the two wavelengths.
Procedure for Measuring the Second-HarmonicEfficiency
In this paper, the second-harmonic conversion effi-ciency is defined as the ratio of the energy in thesecond-harmonic pulse after it leaves the crystal tothe energy in the pump pulse before it enters thecrystal. Conversion efficiency is reported becausethe absolute energy of the second-harmonic pulsedepends on the absolute energy of the pump beam.However, given a peak intensity, an intensity distribu-tion, and an effective crystal length, and assumingoptimum alignment, only one efficiency will be gener-ated. Therefore the conversion efficiency was mea-sured as a function of the peak intensity of theincident pump pulse.
Because conversion efficiency is related to the peak
intensity of the pump beam, higher intensities aredesirable to produce higher efficiencies. However,conversion efficiency in AgGaSe2 is currently limitedby the damage threshold of the crystal and its coat-ings3'4 and appears to be dependent on the surfaceroughness of the particular crystal.5 Therefore thedamage threshold of our AgGaSe2 crystals was mea-sured in terms of the energy density per pulse todetermine the highest possible intensity without in-ducing damage. By use of a 30-ns rectangular CO2laser pulse, visible plasmas were produced with asingle shot at energy densities of 1.8-2.0 J/cm 2 , anddamage was observed on the surface of the crystalafter several hundred pulses at energy densitiesexceeding 1.0 J/cm2 . This difference of a factor of 2between single-shot plasma damage and damage aftermany pulses is consistent with previous damagemeasurements. 6
To increase the incident intensity and thereforeincrease the conversion efficiency without exceedingthe damage threshold, the temporal intensity distribu-tion of the transversely excited atmosphere (TEA)C02 laser pulse was altered external to the laser witha pulse-shaping system. The high-power spike effi-ciently generates second-harmonic energy, whereasthe low-power tail produces little second-harmonicenergy. However, the tail, containing approxi-mately 50% of the energy of the pulse, contributeslargely to the crystal damage, because the damagethreshold is proportional to the energy density.Selecting only the peak power from the spike greatlyreduces the energy density, so that the crystal is notso easily damaged, but does not drastically reducethe amount of second-harmonic energy generated.Therefore a temporally narrower pulse can be focusedtighter, producing a higher intensity and a higherefficiency without exceeding the damage threshold.
The optical layout for the efficiency measurementsis shown in Fig. 1. The TEA C02 laser, whichoperates with a single longitudinal mode on the 9P14line (X = 9.50 jlm), typically emitted 50-60-mJ pulsesat a repetition rate of 2-10 Hz, with a nearly Gauss-ian spatial intensity distribution. The linearly polar-ized TEA pulse passed through the pulse-shapingsystem, which selected a rectangular 30-ns pulsefrom the peak power of the TEA pulse. The pulseshaper consisted of an optically triggered spark gap,an electro-optic modulator, and an analyzer, whichtogether transmitted a 2-mJ pulse, with polarizationthat was rotated 900. Figure 2 shows the temporal
Fig. 1. Schematic diagram of the second-harmonic generationexperiment.
SZscd
ED
Time (50 ns/div)
Fig. 2. Temporal intensity profile of the spike of the TEA laserpulse (with electrical noise), shown at the top, and the 30-ns shapedpulse (bottom).
profile of the laser pulse before and after the pulseshaper. The shaped pulse was indeed rectangular,with a rise time and a fall time faster than the 1-nsresolution of the measurement equipment. Thebeam then passed through a series of attenuators,which permitted the intensity at the crystal to bechanged without changing the spot size. The beamwas focused to produce a beam waist at the AgGaSe2crystal with a 1/e2 intensity radius of 0.50 0.03mm. With this large spot size, the wave fronts canbe assumed to be planar, and walkoff is negligible.When this configuration was used, peak pump beamintensities up to 17 MW/cm2 were attainable.Throughout this paper, peak intensity is defined as2E/rrr 2t, where E is the energy in the pulse, r is theradius of the Gaussian spot, and t is the pulseduration.
To determine the efficiency of the second-harmonicgeneration, we measured the energy of the pumppulse with a pyroelectric energy detector, which wasinserted just before the crystal. Following the gen-eration of the second-harmonic beam in the crystal,the pump beam was blocked with three filters: a5-mm CaF2 window, a 5-mm MgF2 window, and a1-mm infrared glass N6 window. No detectable pumppower passed through these filters, and the measuredtransmission of these filters at the second-harmonicwavelength was 29%. The energy of the second-harmonic pulse was then measured by focusing thebeam onto a more sensitive pyroelectric energy detec-tor and accounting for the transmission of the filters.Within their range of overlapping sensitivity, the twodetectors agreed in their energy measurements for agiven beam. Additionally, the temporal profile ofthe entire TEA pulse and the rectangular pulse weremonitored with mercury cadmium telluride detec-tors, which sampled the beam before the pulse-shaping system and after the crystal, respectively.
To attain higher intensities, and therefore higherconversion efficiencies, we completed a second seriesof measurements with multiple longitudinal modesrunning in the laser. The total energy in the unat-tenuated 30-ns pulse increased to a typical level of 4mJ and remained constant from pulse to pulse,although the distribution of the energy among themodes changed from pulse to pulse. The energydistribution varied from being predominantly in one
mode, producing a nearly rectangular temporal inten-sity profile, to being equally divided between twomodes, producing deep sinusoidal modulations in thetemporal profile. In all cases, additional weak longi-tudinal modes were present. In addition, the spotsize at the crystal was reduced to 0.36 0.03 mm,and again walkoff and divergence were negligible.This configuration allowed intensities up to 65MW/cm2 to be obtained.
Three AgGaSe2 crystals, grown by Cleveland Crys-tals, Inc. (Cleveland, Ohio), were used in the measure-ments. All three crystals were antireflection coatedfor X = 9.50 jim on the input side and for X = 4.75 jimfor the exit side to increase the conversion efficiency.The crystals were cut for phase matching at X = 9.50jim, although one crystal was cut for 49.94', thephase matching angle predicted by the Sellmeiercoefficients from Bhar,7 and the other two crystalswere cut for 48.48°, the phase-matching angle pre-dicted by the Sellmeier coefficients from Kildal andMikkelsen.4 The maximum second-harmonic signalwas reached when the two 48.48° crystals wereinserted at near normal incidence. The maximumsignal for the crystal cut to 49.94° was obtained onlywhen the crystal was obviously tilted by approxi-mately 4°. This indicates that the refractive indicesderived from Kildal and Mikkelsen's Sellmeier coeffi-cients are more accurate at X = 9.50 jim than Bhar'scoefficients.
To accurately model the laboratory conditions andproperly predict the conversion coefficiencies, weneeded to measure the reflection coefficients for bothsides of the crystals and the loss coefficient for thebulk crystal. Because of the two different coatingswith unknown reflectivities, conventional transmis-sion measurements cannot determine the necessaryinformation, such as the amount of the pump beamentering the crystal. Instead the reflection coeffi-cients and the loss coefficient were measured at thepump wavelength with a Fabry-Perot-type measure-ment technique. The spacing between the ends ofthe crystals was changed by a small increase in thetemperature, temporarily producing an interferomet-ric variation in the coherent transmission or reflection.This variation was sufficient to separate the reflectionlosses of each surface from the bulk material absorp-tion or scattering losses. The results, summarizedin Table 1, indicate that the antireflection coatings onthe input sides have very low reflectivities (less than1%) at the pump wavelength and that the loss coeffi-cients compare favorably with previous measure-
ments.3 5 The slightly higher loss coefficient of the6-mm crystal may be because of the fact that thecrystal already had significant surface damage, whichmay have been coupled with the loss coefficient.
Second-Harmonic Efficiency Results
The results of the experimentally obtained conver-sion efficiencies for AgGaSe2 are presented below andare much better than the results of previous research-ers.123 The data for single longitudinal mode opera-tion are presented first. From these data, an experi-mentally determined value of the nonlinear coefficientd36 for AgGaSe2 is extracted. After that, the multiple-mode operation data are presented, showing efficien-cies up to 56%. In all cases, the experimental dataare compared with results from computer models,which predict the conversion efficiency for a singlelongitudinal mode pump beam.
The single longitudinal mode conversion efficiencydata for the 16.4-mm AgGaSe2 crystal are plottedagainst the incident-peak pump beam intensity inFig. 3. Each plotted point represents an averageover many individual pulses, and every measureddata point is shown in the figure. The scatter of datapoints is caused by a variation in performance fromday to day as well as from measurement to measure-ment. At a given intensity, the higher points weremeasured at near optimal performance, whereas thelower points were measured under poorer conditions,most likely because of the varying spatial-intensityprofile of the beam or optical misalignments, such asphase mismatching, which may have reduced theamount of generated or detected second-harmonicpower. Typical error bars representing the uncer-tainty in the measurements are shown for one datapoint. The vertical bar represents the pulse-to-pulsevariation in pump or second-harmonic power, and thehorizontal bar represents the uncertainty in measur-ing the spot size and the pulse-to-pulse fluctuation inpump power.
..
on
00
MPI0
C6)2
0~
.,-x
8 - _ __0 a 6 9 i12 5 ±6
Peak Pump Beam Intensity (MW/cm2)
Fig. 3. Theoretical and experimental SHG external energy conver-sion efficiencies for the 16.4-mm-long AgGaSe 2 crystal duringsingle longitudinal mode operation. The theoretical curve as-sumes that d36 = 34 x 10-12 m/V.
The solid curve represents the theoretical perfor-mance prediction from a computer model, whichassumes planar wave front interaction with negligiblewalkoff but includes transmission losses and pumpdepletion. The pump beam's spatially Gaussian andtemporally rectangular intensity profile is modeled inthe computer program. Additionally, the transmis-sion at X = 4.75 jim is assumed to be the same as themeasured transmission at X = 9.50 jim. Because ofthe large disparity in the reported values of theAgGaSe 2 nonlinear coefficient d36 (Refs. 1, 3, 4, and8-10) a correct value could not be found in theliterature. Instead, the nonlinear coefficient wasvaried until the curve coincided with the highest datapoints. For the 16.4-mm crystal, a value of d36 =
34(10)-12 m/V provides the best fit to the measureddata. The contour of the theoretical curve matchesthe optimal measurement points over the range of thepump beam intensity.
The maximum efficiency of 21.6% was reached at apump beam intensity of 17.0 MW/cm2. Under theseconditions, 430 jiJ of second-harmonic energy weregenerated with 2.0 mJ of pump energy focused ontothe 16.4-mm crystal. Although this was the highestattainable intensity with this configuration, the en-ergy density was only 0.51 J/cm2 , which is approxi-mately half the damage threshold.
The single longitudinal mode experimental andtheoretical efficiency data for the 20.8-mm crystal areshown in Fig. 4. For a given intensity, the efficiencyis higher than for the 16-mm crystal, as expectedfrom a longer crystal. The maximum measuredefficiency of 17.9% was reached at an intensity ofapproximately 12 MW/cm 2 . Unfortunately difficul-ties with the laser prevented measurements at higherintensities with the same optical arrangement.
Two theoretical curves are shown for the 20.8-mmcrystal, using two nonlinear coefficients. The solidcurve passes through the center of the data points forall intensities and was produced with a coefficient of
24
*C)
1U
00
8
0)
20
16
12
B
4
06 9
Peak Pump Beam Intensity (MW/cm2)
Fig. 4. Theoretical and experimental SHG external energy conver-sion efficiencies for the 20.8-mm-long AgGaSe 2 crystal duringsingle longitudinal mode operation. The solid theoretical curveuses d36 = 28 x 10-12 m/V and the dashed curve uses d36 = 32 x10-12 m/V.
28(10)-12 m/V. The dashed curve passes throughthe best point and was produced with a coefficient of32(10)-12 m/V. This crystal has a large dip in itsincoherent transmission at 4.75 jim. If the losscoefficient or the reflectivities at 4.75 m are worsethan assumed, these errors will artificially cause thenonlinear coefficient to appear to be lower. Withoutaccurate transmission data at 4.75 jim, these un-knowns will be coupled into the nonlinear coefficientto increase its uncertainty.
From these measurements and from similar mea-surements of the 6-mm crystal, we determine thevalue of d36 for AgGaSe2 to be (32 ± 4) x 10-12 m/V.This agrees well with four of the six values reportedin the literature. 48"0 The two most recently re-ported measurements3 9 are significantly higher, al-though no uncertainties are reported for those mea-surements.
Multiple longitudinal modes increase the SHGenergy conversion efficiency compared with a singlelongitudinal mode." When all other conditions areunchanged, two longitudinal modes, each containinghalf the energy of the single mode, generate 1.5 timesmore second-harmonic energy than a single longitudi-nal mode. The increase is caused by the temporalsinusoidal modulation, which creates intensity peaksthat are twice as high as the intensity of a singlelongitudinal mode. These higher peaks generatemore second-harmonic energy than is lost in thetroughs of the modulation, so the energy in thesecond-harmonic pulse increases.
The measurements of multiple longitudinal modeSHG energy conversion efficiency for the 16.4-mmand the 20.8-mm crystals are shown in Figs. 5 and 6,respectively. The theoretical curve in both cases isthe single longitudinal mode prediction, assumingthat d36 = 32(10)-12 m/V. Because the laser variedbetween one longitudinal mode and two nearly equalmodes, the energy conversion efficiency also variedfor a given peak pump beam intensity. The mea-
60
2..0
00
0)
s0
40
30
20
10)
0
I I _ I
3 20 30 40 50
Peak Pump Beam Intensity (MW/cm2)
Fig. 5. Theoretical and experimental SHG external energy conver-sion efficiencies for the 16.4-mm-longAgGaSe2 crystal. The pumplaser varied between a single mode and two longitudinal modeswith equal energy. The theoretical curve assumes d36 = 32 x10-12 m/V.
Fig. 6. Theoretical and experimental SHG external energy conver-sion efficiencies for the 20.8-mm-longAgGaSe 2 crystal. The condi-tions are the same as in Fig. 5.
sured data is represented by boxes in the figures,where each box contains at least 50 individual datapoints. The width of the box represents the pulse-to-pulse variation in the pump beam energy. Theheight of the box represents the variation in thesecond-harmonic energy as the pump beam variedfrom one to two modes. When the laser emittedessentially one mode, the efficiency was at the bottomof the box, which corresponds quite well with thetheoretical prediction, up until the point wheredamage started to occur (intensity = 30 MW/cm 2).Beyond that point, the surface of the crystal incurreddamage and the measured efficiency fell away fromthe theoretical prediction, as expected. The top ofthe measurement boxes corresponds to efficiencymeasurements, when the laser emitted two longitudi-nal modes of near equal power. These points are afactor of 1.5 higher than the theoretical curves,exactly as expected from the theory. The middle ofthe box represents an uneven split of the energybetween the two modes. The highest measured effi-ciency was 56% with the 20.8-mm AgGaSe2 crystal.At this point, 1.4 mJ of = 4.75 jirm light weregenerated from two equal modes at a peak pumpbeam intensity of 44 MW/cm2 . Because the theoreti-cal curve continues upward and because divergenceand walkoff are negligible, the damage thresholdcontinues to be the limiting factor in the conversionefficiency.
Second-harmonic conversion efficiencies in AgGaSe2from this paper and from other researchers arecompared in Table 2. This paper shows a conversionefficiency that is almost four times better than inpreviously reported research. Further improve-ments in the SHG conversion efficiency will requireadditional changes to the pump pulse shape, thegrowth of longer crystals, or improvements in thedamage threshold of the crystals and their coatings.
Heterodyne Detection
Efficient generation of laser light at 4.75 m is onestep in developing an efficient range finder or laser
Table 2. Comparison of Energy Conversion Efficiencies
Iseler et al.b 16 10.6 - 7 1.4 mJEckardt et al.c 21 10.25 12 14d
Single-mode 16.4 9.50 17.0 21.6 14.4 kWpeak
Multimode 20.8 9.50 46 56 93 kWpeak
aRef 1.bRef 2.CRef. 3.dCalculated internal energy efficiency.
communication system in the 3-5 jim window.Another step improves the sensitivity of detection.Detection is ultimately limited by the sensitivity ofthe detector hardware, which is beyond the scope ofthis work. However, heterodyne detection uses alocal oscillator to increase the sensitivity of anydetector hardware beyond its sensitivity in directdetection. The optical principles needed to increasethe sensitivity of a given detector were successfullydemonstrated by developing a heterodyne detectionsystem operating at 4.75 jim.
The TEA laser pulse and an AgGaSe2 crystalefficiently generate a 4.75-jim pulse with high peakpower, which can be transmitted through the atmo-sphere. However, several requirements must be sat-isfied to create a suitable local oscillator. The inter-mediate frequency needs to be low enough to detect,so an appropriate local oscillator frequency can onlybe generated by also doubling the frequency of the9P14 CO2 line. The local oscillator must also main-tain a constant power, so that the amplitude of theheterodyne signal is independent of the arrival timeof the signal to be detected. Because cw lasers emitsubstantially less power than the peak power of apulsed laser, adequate local oscillator second-har-monic power is achievable only if relatively highefficiency at low intensities is possible. This re-quires that the nonlinear material used to generatethe local oscillator has a very large nonlinear coeffi-cient.
Tellurium has the largest second-order nonlinearcoefficient of the known materials-in it frequencydoubling of 10-jm radiation is possible-and tellu-rium is approximately 20 times more efficient thanAgGaSe2. Unfortunately tellurium suffers from two-photon absorption at higher intensities, which limitsits conversion efficiency to approximately 6% (Ref.12). The only known report of cw second-harmonicgeneration in tellurium' 3 claimed an efficiency of"less than 10-6" with a 5-W unfocused beam generat-ing "about 0.5 iLW" in a 7-mm crystal. For compari-son, we directed 3 W of unfocused 9.56-pm light intoan 8.3-mm Te crystal and produced 0.3 jiW of second-
harmonic power-approximately the same results aspreviously reported.
To increase the efficiency of the second-harmonicgeneration, we increased the intensity of the pumpbeam by tightly focusing the beam onto the crystal,but the pump beam had to be chopped at a duty cycleof 0.1% to avoid damaging the Te crystal. Althoughthe tight focusing restricted the effective length of thecrystal because of walkoff between the pump and thesecond-harmonic beams, the 4-W pump beam with anintensity of approximately 1.3 MW/cm2 generated0.27 mW of second-harmonic power. This efficiencyof 6.8(1O)-5 is more than a factor of 60 better than thepreviously reported unfocused efficiency, and almost3 orders of magnitude more second-harmonic powerwas produced. This amount of cw power proved tobe enough to serve as a local oscillator for heterodynedetection.
The combination of the good efficiency of AgGaSe2at high intensities and adequate generation of cwpower in Te permitted the 4.75-jim heterodyne sys-tem in Fig. 7 to be assembled. The TEA laser beamwas focused onto the AgGaSe2 crystal to generate thetransmitted signal. To simplify the setup, the entireTEA laser pulse (not the rectangular pulse) was usedin this demonstration. The entire experiment wascompleted in the laboratory, so the attenuation thatwas due to transmission was simulated by firstattenuating the TEA pump beam and further attenu-ating the second-harmonic beam until it was barelyvisible on the MCT detector in direct detection. Thecw CO2 laser beam was focused onto the Te crystal togenerate the local oscillator, and then it was com-bined with the TEA beam at a beam splitter to reachthe detector. Filters blocked the pump beams fromreaching the detector.
The results of successful heterodyne detection at4.75-jim are shown in Fig. 8, an oscillograph with twosignals from the MCT detector. The upper trace isthe heterodyne signal, resulting from mixing thesecond-harmonic from the TEA pulse and the second-harmonic from the cw laser. The dc level of theheterodyne signal corresponds to the power of thelocal oscillator. The amplitude of the modulation isproportional to the field strength of the TEA pulse,and the frequency of the modulation is the differencein the frequencies of the two second-harmonic beams.The lower trace is the second-harmonic of the TEApulse in direct detection, when the local oscillatorbeam was simply blocked. A slight temporal offsetbetween the curves is observed in Fig. 8, because thecurves were recorded from two temporally distinct
Fig. 7. Schematic diagram of the heterodyne detection experi-ment.
s - 2S;0?00,1'U~~l' i'v,.l
Time (125 ns/div)
Fig. 8. Heterodyne (top) and direct (bottom) detection of a =
4.75-pLm pulse.
pulses. The amplitude of the modulation correspond-ing to the peak of the TEA pulse is seven times largerthan the height of the peak in direct detection.However, the tail of the TEA pulse, which is lost inthe noise in direct detection, is clearly visible inheterodyne detection, and is approximately 25 timeslarger than the electronic noise. Although neitherlaser was frequency stabilized, the relative stabilitybetween the lasers, as determined by watching theintermediate frequency, was sufficient to permit theheterodyne signal to be observed for several minuteswithout adjustment.
Furthermore, simultaneous heterodyne detectionof 5-jim and 10-jim light was demonstrated for thefirst time, using only one detector. When the filtersblocking the pump beams were removed, the 4.75-and the 9.50-jm beams from the TEA laser hit thedetector. The local oscillator needed slight realign-ment for both of its beams to reach the detector,because walkoff separated the two beams in thecrystal. CaF2 filters were added to attenuate thepump beams to match the second-harmonic ampli-tudes. The result of this two-color heterodyne detec-tor is shown in Fig. 9. In addition to the heterodynesignal and the TEA signal in direct detection, as inFig. 8, Fig. 9 also shows the local oscillator signal.The TEA pulse and the local oscillator each containlight at 4.75 jim and 9.50 m, although that light isimpossible to see in direct detection. However, thetwo colors are obvious in the heterodyne signal,where two modulations are visible. The lower fre-quency modulation (24 MHz) is the intermediatefrequency of the pump beams (W 9 .5 0 = °signal - lo)The higher frequency modulation (48 MHz) is theintermediate frequency of the second harmonic.Because the second-harmonic frequencies are doublethe pump frequencies, the second-harmonic interme-diate frequency is exactly twice the intermediatefrequency of the pump beam (A(04.75 = 2)signal -2wl = 2Aw9.50). Thus the combined heterodyne sig-nal is the summation of two independent heterodynesignals, one for each wavelength. Filtering of the
Fig. 9. Simultaneous two-color heterodyne detection. The het-erodyne signal (top) has two frequency components: 48 MHz forX = 4.75 pm and 24 MHz for X = 9.50 pAm. Also shown is the localoscillator signal (middle) and the peak of the TEA laser pulse(bottom), both in direct detection and both containing two colors.
electrical signal from the detector could easily sepa-rate the two intermediate frequencies and, therefore,distinguish between the two wavelengths, althoughonly one detector was used.
Thus with only the equipment required for 10-pLmheterodyne detection plus two nonlinear crystals,simultaneous heterodyne detection in both the 3-5-and the 8-12-pm bands is possible. Many possibili-ties can now be imagined in which one could takeadvantage of simultaneously detecting two wave-lengths and also obtaining phase information fromboth wavelengths. As an example, range finding inboth of the thermal infrared atmospheric windows isnow possible with only one set of equipment.
Conclusion
Modification of the shape of a TEA CO2 laser pulsehas permitted a substantial increase in the conver-sion efficiency and the generated power of second-harmonic generation (SHG) with AgGaSe2. Contin-uous-wave second-harmonic generation in telluriumhas created almost 3 orders of magnitude more powerthan previously reported. These two improvements
combined to increase the sensitivity of a detectorsystem by permitting heterodyne detection at 4.75pLm. When the pump beams are also used, only oneset of equipment is necessary for simultaneous two-color heterodyne detection in both the 3-5 pm and8-12 gum atmospheric windows.
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