Automated rejection of parasitic frequency sidebands in heterodynedetection LIDARapplicationsCarlos Esproles, David M. Tratt, and Robert T. Menzies Citation: Review of Scientific Instruments 60, 78 (1989); doi: 10.1063/1.1140581 View online: http://dx.doi.org/10.1063/1.1140581 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/60/1?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Interfacial water in the vicinity of a positively charged interface studied by steady-state and time-resolvedheterodyne-detected vibrational sum frequency generation J. Chem. Phys. 141, 18C527 (2014); 10.1063/1.4897265 Communication: Quantitative estimate of the water surface pH using heterodyne-detected electronic sumfrequency generation J. Chem. Phys. 137, 151101 (2012); 10.1063/1.4758805 Ultrafast vibrational dynamics of water at a charged interface revealed by two-dimensional heterodyne-detectedvibrational sum frequency generation J. Chem. Phys. 137, 094706 (2012); 10.1063/1.4747828 Direct evidence for orientational flip-flop of water molecules at charged interfaces: A heterodyne-detectedvibrational sum frequency generation study J. Chem. Phys. 130, 204704 (2009); 10.1063/1.3135147 Heterodyne-detected electronic sum frequency generation: “Up” versus “down” alignment of interfacial molecules J. Chem. Phys. 129, 101102 (2008); 10.1063/1.2981179

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Automated rejection of parasitic frequency sidebands in heterodyne­detection llDAR applications

Carlos Esproles

Ball Systems Engineering Division, 260 South Los Robles Avenue, Pasadena, California 91101

David M. Tratt and RobertT. Menzies

Jet Propulsion Laboratory, California Institute o/Technology, 4800 Oak Grove Drive, Pasadena, California 91109

(Received 8 August 1988; accepted for publication 25 September 1988)

The authors describe an electronic technique for detecting the sporadic onset of multiaxial mode behavior of a normally single-mode TEA C02 1aser and demonstrate how this information can be used to facilitate rejection of unsuitable pulses in a heterodyne-detection LIDAR context.

INTRODUCTION

The increasing importance of LIDAR techniques for re­mote-sensing applications has precipitated a demand for sys­tems that possess a high degree of operational autonomy. Several such instruments currently in the proposal, con­struction, or operating stages incorporate heterodyne detec­tion for maximal sensitivity and range capability. I While this approach has found particular favor within the CO2 LI-

DAR community,1-7 its successful application entails the use of a stable, single-frequency laser transmitter offset locked by some arbitrary amount from the local oscillator.

LASER TRIGGER (FROM SPARK GAP)

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OSCILLOSCOPE © (VISUAL MONITOR)

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In the case of pulsed transverse excitation at atmospher­ic pressure (TEA) CO2 transmitters, the free-running out­put characteristically consists of several axial modes, so that some way of inducing the laser to emit a single mode (single frequency) must be employed. This requirement, in general, presents no problem, as numerous methods have evolved

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(FIG. 4)

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FIG. L Block schematic of the multimode discriminator.

78 Rev. Sci.lnstrum. 60 (1), January 1989 0034-6748/89/010078-04$01.30 @ 1988 American institute of Physics 78

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FIG. 2. Typical time sequences of multimode (upper frame), and single­mode (lower frame) laser operation, showing the initial noise burst, fol­lowed by the arrival of the optical pulse about 1 f.ls later. The lower trace associated with each pulse image shows the corresponding output of the bandpass filter. Note the earlier arrival time of the single-mode pulse rela­tive to that of the multimode output. Horizontal scale: 200 liS div - I.

over the past few years that address this very situation.8 Al­though these techniques typically offer excellent reliability, there will nevertheless be a finite failure rate, causing occa­sional reversion to multimode operation. Such pulses exhibit an overall bandwidth considerably in excess of the hetero­dyne receiver acceptance window, and thus do not contrib­ute to the detected signal. In the case of a data system that individually logs each shot (see, for example, Ref. 5) this need not represent a problem, since each data record can be assessed in software for its suitability prior to further pro­cessing. However, the ground-based atmospheric back­scatter LIDAR at JPL 4 records data with a hardware aver­ager, so that it was judged desirable to have at hand some means by which multimode pulses could be identified in real time and rejected by the data acquisition system, with a COll­

sequent improvement in overall signal-to-noise ratio. This paper describes the constructional principles, un­

deriying concepts, and performance characteristics of one such device that accomplishes these tasks.

79 Rev. ScI. Instrum., Vol. 50, NO.1, January 1989

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! FIG. 3. As ill Fig. 2, but displaying the appropriaic rf switch output in place oflhat from the bandpass filter. The horizontal bar between the frames de­notes the transmission window of the rf switch.

I. PRINCIPLE OF OPERATION

The most obvious diagnostic denoting the transition be­tween single-mode and multimode operation is the appear­ance in the pulse temporal profile of a strong modulation component at the interaxial mode frequency separation of the laser cavity (noting. however, that such modulation will only be observable when, as here, the pulse length exceeds the cavity round-trip period). The modus operandi of our technique relies on the isolation of this frequency component and its subsequent use for trigger-delivery decision-making purposes.

The input signal for this diagnostic is obtained by sam­pling a small portion of the laser output, in our case by means of a low-reflectivity beam splitter, and directing it onto a room-temperature HgCdTe photoresistor (Boston Elec­tronics model ROO4; rise time 1 ns). Following amplifica­tion, part of the detector output is split to a storage oscillo­scope (for ease ofvisuai monitoring) while the remainder is sent to the multimode discrimination circuit for further analysis. This signal is first sent through a bandpass filter to eliminate the pulse envelope (Fig. 1). The 3-dB bandwidth of this filter was specified at 10 MHz and was centered at 63

Heterodyne-detection llOAR 79

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FIG. 4. As in Fig. 2, but displaying the appropriate rf detector output in place of that from the bandpass filter.

MHz <the interaxial mode spacing for our 2.4-m cavity; giv­en by c12L, where L is the cavity mirror separation).

The broadband noise burst due to the TEA discharge precedes the optical output (Fig. 2) and is capable of gener­ating a spurious multimode flag within the discriminator unit regardless of the actual status of the laser pulse. It was therefore found necessary to blank the input to the discri­minator for the initial interval immediately subsequent to the firing of the TEA laser. In addition, modulation occa­sionally appearing in the pulse tail (probably due to higher order transverse modes at frequencies outside the mode se­lection zone) was also sufficient to cause pulse rejection. Thus, the input to the discrimination circuitry was gated via a rf switch (Daico l00C1282) timed so as to transmit only the time interval containing predominantly the gain­switched spike (recognizing that this gate width must be sufficient to account for the arrival-time difference between a single- and a multimode pulse,8 as illustrated by Fig. 2). The output of the rf switch under both single- and multi­mode conditions is shown in Fig. 3. Note that the extraneous modulation contributions identified above, and clearly ap­parent in Fig. 2, have been completely eliminated.

A rf detector (slew-rate-l V II'-s) integrates the 63-MHz signal output by the rfswitch (Fig. 4), which is then further amplified and fed into a programmable comparator.

80 Rev. Scl.lnstrum., Vol. 60, No.1. January 1989

The comparator outputs a negative transistor-transistor logic convention (TTL) pulse if the input signal magnitude exceeds the preset threshold value (decided empirically by considering what level of sideband contamination may be tolerated). Finally, this TTL pulse is AND gated with a posi­tive TTL pulse derived from the laser discharge (and de­layed so as to be synchronous with the comparator output) to generate the trigger for the data system. Hence, if a multi­mode pulse was detected, then data collection will be inhibit­ed.

II. OPERATIONAL NOTES

The elapsed time between the arrival of the laser pulse and the transmission of the data system trigger is approxi­mately 2 fts using the circuit design described here. For our LIDAR system this presented no problem, since this period corresponds to a region of space that is inaccessible for rea­sons of transceiver geometry.9 If it became necessary to re­trieve data from within this instrumental "dead zone," then a data-acquisition unit with pretrigger retention capability must be used.

In order to provide the operator with a rapid visual as­sessment of the system response, a peak detector is also in­corporated that displays the rf detector output on a digital voltmeter and also a light-emitting diode (LED) bar graph. The bar graph option can be especially useful in the case of systems operating at pulse repetition frequencies (PRFs) in excess of 5 Hz, such as that currently being assembled at JPL in preparation for the NASA GLOBE (Global Backscatter Experiment) mission in 1989.6 To facilitate maximal unat­tended operation, the system includes an audio alarm trig­gered by the negative TTL comparator ouiput, releasing the system operator from unnecessary in situ monitoring tasks.

III, DISCUSSION

A simple technique, implemented using primarily com­mercial circuit modules, has been devised to discriminate between single-mode (single-frequency) and multimode output conditions of a mode-controlled pulsed laser. The technique was successfully demonstrated with an existing coherent atmospheric LIDAR facility utilizing an injection­seeded, single-mode TEA CO2 laser. Subject only to the pulse length proviso mentioned previously, there exists, in principle, no limitation as to the potential range of possible applications to which the technique is suited.

ACKNOWLEDGMENTS

The research described in this paper was carried out by the Jet Propulsion Laboratory, California Institute of Tech­nology, under contract with the National Aeronautics and Space Administration (NASA) while David Tratt held a NASA-National Research Council Resident Research As­sociateship.

'D. K. Killinger, N. Menyuk, andW. E. DeFeo, App!. Opt. 22, 682 (1983). 21. W. Bilbro, C. DiMarzio, D. Fitzjarrald, S. Johnson, and W. Jones, App\. Opt. 25, 3952 (1986).

3R. M. Hardesty, App\. Opt. 23, 2545 (1984). 4R. T. Menzies, M. J. Kavaya, P. H. Flamant, and D. A. Haner, App\. Opt. 23,2510 (1984).

Heterodyne-detectlon LlDAR 80

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5W. B. Grant, J. S. Margolis, A. M, Brothers, and D, M. TraIt, App!. Opt. 26,3033 (1987),

6 LA WS Instrument Panel Report, Earth Observing System (National Aero­nautics and Space Administration, Washington, DC, 1987), VoL lIg.

7 A. A, Woodfield and J. M. Vaughn, Int. J. Aviat. Safety 1, 207 (1983).

81 Rev. Scl.lnatrum., Vol. 60, No.1, January 1989

~D. M. Trat!, A. K. Kar, and R. G. Harrison, Prog. Quantum Electron. 10, 229 (1985).

9G, M. Ancellet, M. J. Kavaya, R. T. Menzies, and A. M. Brothers, App/. Opt. 25, 2886 (1986),

Heterodyne-detection llDAR 81

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