Upgraded 1:56 μm lidar at IMK–IFU with0:28 J=pulse

Thomas TricklKarlsruher Institut für Technologie, Institut für Meteorologie und Klimaforschung (IMK-IFU),

Kreuzeckbahnstrasse 19, D-82467 Garmisch-Partenkirchen, Germany(thomas.trickl@kit.edu)

Received 20 January 2010; revised 31 May 2010; accepted 1 June 2010;posted 9 June 2010 (Doc. ID 123054); published 24 June 2010

The 1:56 μm eye-safe aerosol lidar at the Institut für Meteorologie und Klimaforschung was upgraded,which resulted in a doubled laser pulse energy of 0:28J and an improved performance of the detectionelectronics. The high pulse energy, achieved by stimulated Raman backscattering of 0:9J from a Nd:YAGlaser in deuterium (33% conversion efficiency when neglecting the residual optical losses), was mainlyachieved by improving the beam quality of the pump beam. The Stokes pulses showed a subnanosecondpeak with some underlying 10ns component. Because of the substantial increase in pulse energy, thisreliable and simple method considerably gains in value. Examples are presented, demonstrating anoperating range between 0.4 and more than 10km for quasi-horizontal measurements. © 2010 OpticalSociety of AmericaOCIS codes: 280.0280, 280.1100, 280.3640.

1. Introduction

There has been ongoing interest in eye-safe IRaerosollidar systems during the past one and a half decades[1–7]. Despite the fact that a suitable calibration withthe Rayleigh return is highly demanding at wave-lengths substantially greater than 1 μm, these sys-tems are attractive because, being background free,they offer the advantage of high sensitivity for parti-cles. Eye safety becomes an important issue whenautomatic operation or fast angular scanning of alidar is intended.

Most frequently, these lidar systems have been op-erated in the atmospheric window around 1:55 μm,where laser radiation can be generated by stimulatedRaman shifting the emission of an Nd:YAG laser inmethane or deuterium. Various approaches to reachsuitable pulse energies by this method have beenfollowed. For two Raman-shifted laser source pulsesused in eye-safe IR lidar systems, energies of morethan 100mJ have been reported [8,9]. Up to 390mJof Stokes output in methane, corresponding to a con-

version efficiency of 43%, has been achieved by Spulerand Mayor by using a folded beam path inside theRamancell (with five passes) to extend the interactionrange [9]. This design avoids focusing and the relatedsoot formation. The problems with soot formation infocused geometries were circumvented by Carnuthand Trickl by using deuterium as the Raman gas[8,10]. After realizing a very low forward Stokes emis-sion, they were, however, able to produce 140mJpulses at 1:560 μm by backward scattering 0:9 J froma narrowband Nd:YAG laser operated at 10Hz.Hooper et al. reached 250mJ per pulse with 35% effi-ciency by using an optimized dual-deuterium-celldesign, which, however, required a careful focal-pointalignment [11].

In addition, tunable radiation in this wavelengthregion can be produced by optical parametric oscilla-tors (OPOs). High pulse energies of radiation atabout 1:56 μm have been reported by Harrell et al.[1] and Richter et al. [4] for OPO systems pumpedat 1:064 μm, 170mJ (at 10Hz repetition rate), and125mJ (at 20Hz), respectively. The bandwidths ofthese OPOs were as high as 1:26nm [1] and 0:60nm[4]. A similar average power, but with lower pulse en-ergy (27mJ), was achieved by Roy and Mathieu by

0003-6935/10/193732-09$15.00/0© 2010 Optical Society of America

3732 APPLIED OPTICS / Vol. 49, No. 19 / 1 July 2010

using a 100Hz pump laser [12]. All these OPOs ex-hibit large beam divergences of several milliradians.Optical parametric oscillators require an additionalwavelength control and, therefore, are more complexin operation. This effort pays off in the case of trace-gas measurements in the IR spectral region, also in-cluding the generation of narrowband radiation.

In this paper, substantial upgrading of the1:560 μmbackscatter lidar system described by Carnuth andTrickl [8] is reported. This lidar systemhad been usedparallel to our extensively used (e.g., [13–20]) mobilethree-wavelength backscatter lidar during two fieldcampaigns [21,22]. The main result of the upgradingis a substantial increase in pulse energy (Section 2).The important details of the Raman conversion areanalyzed. The requirements for eye-safe operationare discussed in Section 3. As to signal processing,the problems with the preamplifier and with introdu-cing a faster (40MHz) moderate-cost 12 bit transientdigitizer were solved (Section 4). The modified lidarwas successfully applied during a field campaign in1997. Examples from this campaign, demonstratingthe highly satisfactory overall performance of thelidar, are given in Section 5.

2. Laser System

Almost the same setup was used for the generation ofthe 1:56 μm radiation as that described in Ref. [8]; seeFig. 1. An Nd:YAG laser with intracavity etalon,more than 0:9J of narrowband output pulse energy,and 10Hz repetition rate was used (Quanta-Ray,GCR 3). The beam profile is specified by the manu-facturer as “70% Gaussian”; this number refers to anintensity deficit on and next to the beam axis. Asmall (<10%) diffraction ring exists next to the edge.The laser beam was focused into a Raman shifter1:7m long with an f ¼ 0:9m lens. As described inRef. [8], the narrowband operation (specified band-width:<0:2 cm−1) increases the stimulated backwardRaman scattering by a factor of 4, to 140mJ in the

configuration shown in Fig. 1. The tilt angle ofthe intracavity etalon was chosen to maximize theStokes emission, not the 1:064 μm output, which re-sulted in a slightly reduced pump pulse energy. Thepowerful backscattered 1:56 μm radiation was thentransmitted through a dichroic beam splitter (Laser-optik GmbH, R > 99:99% at 1:064 μm; T ¼ 95:53% at1:56 μm; in an earlier, custom-made version, a loss ofless than 1% had been achieved at 1:56 μm), spec-trally discriminated by a Pellin–Broca prism andthen directed by a beam-steering mirror into the at-mosphere through a pair of iris apertures alignedparallel to the telescope axis. The 1:56 μm energy wasmonitored during the data acquisition by a photo-diode that received a tiny fraction of the emission,coupled out by a Brewster plate, through a fiberbundle. This signal, at the same time, served as thetrigger source.

In the initial setup, an optical diode, formed by aGlan polarizer and a Faraday rotator, was placed be-tween the laser and themirrorM2 (seeFig. 1), in orderto avoid potential damage to the Nd:YAG laser by sti-mulated Brillouin backscattering. This possibilitywas carefully examined, but no Brillouin backscatter-ingwas found.As a consequence, the optical diode andtheFresnel rhomb (used for orientation of the linearlypolarized radiation) could be removed. Thediffractionstructure superimposed on the transmitted 1:064 μmbeam (despite antireflection coatings on all opticalsurfaces, as claimed by the manufacturers) disap-peared, and the near-Gaussian pump beampropagat-ed to the Raman cell unchanged. Quite surprisingly,the output of the Raman shifter immediately rose toabout 220mJ, indicating a high importance of thebeam quality on conversion efficiency. A positive re-sult of the improved pump beam quality was alsothe end of occasionally damaging the dichroic beamsplitter.

Fig. 1. Layout of the transmitter section of the 1:56 μm lidar: M1–M4, high-reflecting mirrors; P, Glan polarizer; FR, Faraday rotator;FRb, Fresnel rhomb; PB, Pellin–Broca prism; DB, dichroic beam splitter; IA, iris aperture; F, filter; FG, Cajon VCR fitting with filtergasket; GC, gas circulator; V, valve; and B, beam stop.

1 July 2010 / Vol. 49, No. 19 / APPLIED OPTICS 3733

The orientation of the Pellin–Broca prism had tobe carefully reset for negligible loss (Brewster angle).At the end, 280mJ were measured behind the prismfor a pump pulse energy inside the Raman cellof 895mJ (a �5% uncertainty of the absolute energymeasurement was derived from an intercompar-ison of four Scientech powermeters [8] that alsodetermined the recalibration factor used). The pulse-to-pulse fluctuations were 3% (standard deviation).The 1:56 μm beam profile was visualized with a fre-quency doubler disk (Technisches Büro für Laserser-vice). As before, it was very smooth and did notexhibit the ring structure seen on the pump beam.The beam divergence approximately matches thatof the pump laser (0:5mrad).

The deuterium pressure range for optimum conver-sion to 1:56 μm, 1.5 –2.0 kPa, remained unchanged.For the studies described here, we operated theRaman shifter with 2.0 kPa, where the peak of theconversion efficiency was estimated.

The Raman shifter is equipped with a gas circula-tor installed because of the reported fluctuations ofthe forward emission of the cell during the earlyphase. For clean gas, operating the circulator im-proved the output by just 5%. A very high cleanlinessof the deuterium fill is required for optimum perfor-mance. Apart from occasional problems with thequality of the delivered gas, the main source of gascontamination was ablation of material from the wallof the stainless steel cell during laser beam align-ment. In these cases, gas cleaning could be achievedby sucking the deuterium fill through the filters.However, the maximum output could not alwaysbe regained after circulating the gas.

The gas quality could best be judged by watchingthe forward anti-Stokes emission. This emissionstrongly depended on the purity of the Raman gas.Under optimum conditions the anti-Stokes rings(caused by four-wavemixing) even overlapped to formwhite light clearly visible as a strong white beam dur-ing nighttime. Although the anti-Stokes cones couldbe clearly seen, their pulse energy was rather lowand was not determined.

The pulse shape of the 1:56 μm radiation was mea-sured with a fast photodiode (Electro-Optics Technol-ogy,Model EOT-3000, 175ps rise and fall times) and a500MHz digital oscilloscope (Tektronix). In order toavoid light reflections from optical holders and othersurfaces, the detector was mounted several metersoutside the lidar trailer. The results are shown inFig. 2. Almost half of the energy is concentrated ina spike 1ns long (i.e., about 0:5ns after correctingfor the response time of the oscilloscope). This beha-vior is in agreement with the known [23] pulse com-pression for Raman backscattering. However, also arather long component exists. The position of the in-tensity maximum of the broad feature is shifted tolonger times with growing output energy.

The forward emission was analyzed by using a sec-ond Pellin–Broca prism for separating it from thetransmitted pump beam. A large powermeter was ro-

tated in the plane defined by the deflected pumpbeam and the cell. Any 1:56 μm emission must havebeen below 1mJ.

The complete absence of forward Stokes radiationcould mean that the conversion is either eaten up bythe strong backward emission or destroyed by elec-tric breakdown in the cell. As a matter of fact, therehad been some indication that breakdown could beinvolved. Thirty to 40mJ forward Stokes outputhad been obtained with an f ¼ 1:1m focusing lens[10]. This emission had strongly fluctuated in pulseenergy, suggesting thermal problems inside the focalvolume. These fluctuations had been much more se-vere with methane as the Raman gas [10].

There is also the possibility of gain depletion byStokes–anti-Stokes coupling. Bloembergen and Shenpointed out that both the Stokes and the anti-Stokesemission are suppressed in the case of exact momen-tum matching of the photons [24]. The first focusedexperimental verification of this effect was publishedby Duncan et al. [25]. Perrone et al. found that forincreasingly tight focusing of UV radiation from a xe-non chloride laser with f<0:4m lenses, the forwardStokes emission is diminished, whereas the conver-sion efficiency for the first rotational Stokes compo-nent rose to as much as 35% [26]. This was ascribedto the increasing divergence angle of the pump beamexceeding the phase-matching angles of both theStokes and anti-Stokes emissions. However, theStokes emission was not completely suppressed.

The dispersion of hydrogen and deuterium in thenear IR is much less pronounced than that in thevisible and UV regions. As a result, one would, inprinciple, expect narrower cone angles for the Ramanemission and the possibility of reaching the gain-depletion regime with longer focal lengths as inthe setup described here. From the tables on the re-fractive index kindly provided by Perrone (Ref. [23]

Fig. 2. Temporal shape of the 1:56 μm emission for different le-vels of output pulse energy, measured with a digital oscilloscopewith 500 MHz analog bandwidth; the length of the 1:064 μm pumppulse is 10ns. The 1:56 μm pulse energy of 220mJ corresponds tothe full pump intensity and includes the optical losses for the pre-liminary adjustment.

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in Ref. [26]) the following expressions were derived,by least-squares fitting, for D2 and H2, respectively:

Deuterium (101.325 kPa, 273:15K):

ðn − 1Þ × 106 ¼ 134:2778þ 9:75037 × 105 × λ−2

þ 1:15615 × 1010 × λ−4;λ in nm:

ð1Þ

Hydrogen (101.325 kPa, 273:15K, added for conve-nience):

ðn − 1Þ × 106 ¼ 136:1915þ 1:00046 × 106 × λ−2

þ 1:21766 × 1010 × λ−4;λ in nm:

ð2Þ

The asymptotic values agree with measuredvalues for very long wavelengths obtained from theInternet (from several sources, e.g., [27]), but exhibita much lower uncertainty of about 0.02 (standard de-viation from numerical fit), slightly higher than the aposteriori standard deviations for the individual datapoints. In order to obtain the refractive index for ele-vated pressures, we, for simplicity, assumed n − 1 tobe proportional to the gas density.

The two equations for the diagram for thewave vec-tors~k under phase-matched conditions were solved toyield thephase-matchinganglesϑS andϑaS for Stokesand anti-Stokes scattering, respectively:

cosϑS ¼ ð2kPÞ2 þ k2S − k2aS2ð2kPÞkS

; ð3Þ

cosϑaS ¼ ð2kPÞ2 − k2S þ k2aS2ð2kPÞkaS

; ð4Þ

where subscripts P, S, and aS denote “pump,”“Stokes,” and “anti-Stokes,” respectively. Testing thisapproach with published results for both H2 and D2[25,26] yielded reasonable agreement,with the excep-tion of the 41 atmD2 case in Ref. [25] where a Stokesangle of 5:6mrad instead of 7mrad is obtained fromEqs. (1) and (3). For 2.0 kPa of deuterium at 20 °C, weobtainϑS ¼ 5:59mrad andϑaS ¼ 4:05mrad. The conehalf-angle for f ¼ 0:9m and a beam diameter of8:0mm is 4:44mrad and almost covers both these an-gles. Thus, one cannot exclude an influence of gainsuppression due to Stokes–anti-Stokes coupling.

Figure 3 shows pulse energies obtained for dif-ferent positions in the system and the conversion ef-ficiency as a function of the pump pulse energy insidethe Raman shifter. The transmitted pump pulse en-ergy was corrected for the small loss in the cell out-put window. The backward Stokes radiation is givenas measured behind the Pellin–Broca prism and, inaddition, corrected for the measured losses in the cellinput window (almost negligible) and, in particular,in the not optimized replacement dichroic beamsplitter. The conversion efficiencies were calculated

from the corrected pulse energies and, thus, repre-sent the maximum achievable values (33:8ð�0:5Þ%at EP ¼ 450mJ, 32:7ð�0:5Þ% at full pump pulseenergy).

From Fig. 3 it becomes obvious that there is a sub-stantial energy loss in the pump beam around thethreshold for Raman conversion. Because, duringtests outside the Raman cell, spark formation in airwas observed in a wide range of pulse energies, it isreasonable to assume that optical breakdown causesthis reduction in pulse energy also in D2. Here, it isimportant to mention that, due to the not compen-sated thermal lensing at low and moderate energies,the effective focal length significantly exceeds 0:9m.The nonlinear behavior of the curves in Fig. 3 is, tosome extent, explained by the change in thermal len-sing: below 0:3 J exclusively due to the growing flash-lamp power in the laser oscillator, above 0:3 J due tothe growing flashlamp power in the amplifier rod.When approaching the full output pulse energy, the1:064 μmbeam becomes fully collimated and the focallength reaches the nominal value of 0:9m. Quite ob-viously, the relative losses due to overload in the focalpoint diminish as the backward Raman emissionincreases.

It is important to recall in this context the existenceof spatially stable pronounced forward anti-Stokesemission in the visible spectral range. This suggeststhat the intensity-induced losses discussed above donotmuch influence the presence of this emission. Alsoin the case of methane as the medium [10], there hadbeen some indication that the anti-Stokes radiationwas much less affected, but we had observed strongpulse-to-pulse fluctuations of the cone shape due tosome obviously higher thermal loading than in deu-terium. In summary, the observations indicate thatthe strong forward–backward asymmetry of the

Fig. 3. Energy budgets for different GCR 3 pump pulse energies:the pump pulse energy inside the Raman shifter is taken (Ein).“Without losses” means that corrections for the losses in the di-chroic beam splitter and the cell window were made.

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Stokes gain for the narrowband laser should be thedominating factor in explaining the absence offorward Stokes emission for f ¼ 0:9m. By contrast,for shifting our narrowband krypton fluoride lasersystem in hydrogen and deuterium a rather high for-ward Stokes emission is observed [28,29].

3. Eye-Safety Considerations

The strongly enhanced level of output pulse energynecessitates reviewing eye-safety conditions. At thetime of our first publication [8], the threshold pulseenergy for eye safety at a repetition rate of 10Hzwas 32 Jm−2, with local enhancement by two decadesat 1:54 μm in the military regulation STANAG 3606.Meanwhile, the AmericanNational Standard for SafeUse of Lasers ANSI Z136.1-2000 (American NationalStandard Institute, New York, 2000) prescribes asingle-pulse eye-safety threshold of 104 Jm−2 in theentire wavelength range between 1:5 μm and 1:8 μm[30]. However, an excessive exposure of 10 s mustbe assumed, and the single-pulse value must be di-videdby thenumber of pulses in10 s. Thepulse lengthis not regarded, and, thus, no influence of the complexpulse shape depicted in Fig. 2 must be estimated. Forthe upgraded lidar with a beam diameter of 10mmand a repetition rate of 10Hz, an eye-safety thresholdof just 7:9mJ is obtained. For this much lower limit, abeamexpansion to adiameter of62mmis required forachieving full eye safety also for 0:3 J pulses. Eye-safeoperation is mandatory in rapidly scanning systems[6,31–33] and in the case of a fully automatic mea-surement series.

4. Lidar Receiver and Data-Acquisition System

The receiver consists of a Newtonian telescope with a38 cm diameter and a focal length of the primarymirror of 2m (Intercon Spacetec), an eyepiece with9mm focal length with a built-in aperture, and a2:0mm diameter InGaAs photodiode (EG&G, ModelC30642E). The receiver is mounted underneath thelaser table, which can be tilted to elevation angles upto 45°. The image of the primary mirror (exit pupil) is1:7mm in diameter, and, thus, it fully fits onto thedetector. Unfortunately, the aperture determinesthe onset of the near-field cutoff at about 0:4km. Be-cause of a change in capacitance at the edge of thediode, the secondary mirror of the telescope mustbe carefully aligned in order to avoid a distorted fre-quency response. An edge filter (custom made byLaseroptik GmbH) almost completely blocks the so-lar background.

The photodiode is mounted inside a shielded boxalso containing a preamplifier. The supply voltageis taken from a ground-free large rechargeable 12Vbattery in another shielded box, connected to thedetector box by a shielded cable. The signal is trans-ferred to the instrument rack via a doubly shieldedcable 6m long (Suhner, G03332). The inner shield isconnected at both ends, the outer shield only on oneside. In this way, electromagnetic interference by thelaser could be completely avoided when using an

appropriate data-acquisition system (see the lastparagraph of this section).

The residual problems with the time response ofthe detector-preamplifier unit [8] could be solvedby using a new custom-made preamplifier yieldingan overall bandwidth set to about 10MHz (RomanskiSensors). A linear voltage response for signals up to1:1V was verified, given a saturation voltage of1:65V. An overall range resolution of 8:2m (FWHM)is estimated from the lidar returns from a rock, mea-sured with the transient digitizer (see below). Someresidual component (<0:25% of the peak signal) withslow decay is left after the end of themain pulse. Thiscomponent is very likely caused by the insufficientcompensation of the differing detector capacitancenear the edge of the photodiode (a problem thatcan be even more severe for some other photodiodetypes). However, no larger detector (3mm) was cho-sen because of some fear concerning the strong in-crease in noise.

A moderate-cost solution was looked for in the caseof the transient digitizer. A two-channel PC card witha 40MHz sample rate and 12 bits for the digitizationwas selected (FAST ComTec, Model TR40). This cardallowed one to read out all 8192 channels between twotrigger pulses for repetition ratesup to38Hz (ona486PC), sufficient for the 10Hz repetition rate of the la-ser. After replacing the input operational amplifiersby faster amplifiers (Comlinear, CLC 409) and afterremoving the feedback capacitors, optimum linearitywas achieved. This was concluded from testing theTR40 card with a precision sine-wave generator to-gether with our two other 12 bit transient digitizersystems (both with a 20MHz sample rate) purchasedduring the same year (1995) from DSP Technologyand Licel for our ozone lidar systems [34–36]. Therewas no detectable difference in linearity betweenthese three digitizers, the TR40 output exhibitingsome constant sensitivity to the trigger pulse. How-ever, the TR40 noise level was about six times higherthan for the Licel system and four times higher thanfor the DSP Technology system, most likely due to op-erating the TR40 inside the lidar PC. However, due tothe enhanced sampling rate, this can, in part, be com-pensated for by smoothing. A slight constant offset inthe presence of the trigger pulse was found. As a con-sequence, the trigger pulse length was extended toseveral hundred microseconds.

During the lidar operation, a broad (single-oscillation) background feature with varying ampli-tude of the order of 50 μV was found. The parasiticvoltage with an overall length of about 0:4ms (0:2msafter the trigger pulse) was ascribed to the magneticfield caused by the current pulse for the laser flash-lamps (U ∼ −dI=dt). Its occurrence was very likelythe result of an insufficiently defined zero voltageof the card. This interference is absent when usingthe digital oscilloscope of the lidar system (Tektronix,TDS 420). After testing several solutions, the inter-ference was fully removed by magnetically shielding

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the PC inside an iron pocket as well as the coaxialcable inside grounded iron pipes.

5. Examples of Measurements

The availability of a longer wavelength in addition tothe three standard wavelengths of our older mobilethree-wavelength aerosol lidar, operated at 355, 532,and 1064nm, has been an attractive option for the in-vestigation of aerosol.With this widewavelength cov-erage, the influence of the aerosol composition on thewavelength dependence can bemuchbetter seen thanfor the most commonly used emission of an Nd : YAGlaser and its harmonics. We obtained very nice exam-ples during our 1993 coastal studies within theMAPTIP (Marine Aerosol Properties and ThermalImager Performance) field campaign in The Nether-lands [37]. Here, the wind-depending changesbetween continental and marine aerosol could be ob-served. In addition, the sea spray emission into theatmosphere in the structured surf zone, reachingtop heights of 400m, was visualized [22]. In Fig. 4we show again the case with the most nonlinearwavelength dependence during MAPTIP, obtainedduring a period of moderate fog. We see a transitionfrom typical aerosol backscattering to Rayleigh back-scattering with a growing wavelength, presumably toa rather small size of the fogdroplets. Theatmospherewas already rather transparent at 1:560 μmwith a vi-sual range of far beyond 10km; see Fig. 7 in Ref. [8].

After upgrading the 1560nm lidar, the systemwas,again, used in a coastal study on the German islandof Sylt in August 1997. The lidar was placed on adune (“Bunker Hill,” 3km north of Hörnum, the vil-lage next to the south cape), about 25mabove the seasurface, and mostly carried out almost horizontalmeasurements over the sea (fixed azimuth: 270°,

roughly perpendicular to the coast) relevant forthe characterization of the atmosphere in the fieldof view of the thermal imagers of the partner insti-tute. Occasional slant-path measurements weremade for mapping the marine boundary layer. Thethree-wavelength lidar was not operated duringthe Sylt campaign.

The laser output was conservatively set to115ð�5ÞmJ in view of the gradually warming coolingwater (extracted from and returned to a barrel) andin order to avoid potential degradation of compo-nents for the highest pulse energies (220mJ) duringthe extended time period of the field campaign. Evenfor this reduced pulse energy, the signal level wasvery high and could reach even more than 500mVin the presence of surf-induced aerosol. For a homo-geneous situation with a 1:56 μm visual range ofabout 260km, the near-field signal was about 30mV.

The IR solution of the Klett algorithm yields ex-tinction does not explicitly contain the backscatter-to-extinction ratio BP [38], at least for constant BP,an assumption dictated during this campaign bythe absence of additional information from otherchannels. The near-field lidar return was calibratedin extinction, because extinction coefficients could beretrieved under occasionally occurring homogeneousconditions. For the time period around a calibrationthe extinction data were adjusted by comparing thenear-field signal levels. Because this requires assum-ing a BP value not changing during this period, someuncertainty arises, but there was no other choice.

The observational period (11–21August ) was char-acterized by exceptional conditions: low to moderatewind and air temperatures up to 35 °C due to a high-pressure zone to the east, causing a persistent advec-tion from lower latitudes, includingAfrica. Because ofmostly low wind speeds, no similarly spectacular ob-servations of sea sprayweremade as duringMAPTIP,

Fig. 4. Wavelength dependence of the extinction coefficientsretrieved from horizontal lidar measurements in homogeneouslydistributed moderate coastal fog (Katwijk, the Netherlands,3 November 1993).

Fig. 5. Slant-path measurement on 14 August 1997 at an eleva-tion angle of 20°, showing the marine boundary layer up to about2:84kmðr ¼ 8:3kmÞ and the onset of cirrus clouds at 6:94kmðr ¼20:3kmÞ and beyond. The data were smoothed with a seven-channel gliding average. The times are given in Central EuropeanTime (CET ¼ UTCþ 1h).

1 July 2010 / Vol. 49, No. 19 / APPLIED OPTICS 3737

when sinusoidal patterns created by different surfzones were observed out to long distances [22].

As during MAPTIP, no direct lidar return from thesea surface was found when lowering the laser beamto hit the water next to the horizon. Also, no signalfrom the significant exhaust of the engines of the re-mote aeroplanes participating in the campaign wasachieved after systematically searching, even afterthe plume had expanded after some time. Whatwas seen on a number of days were almost ubiqui-tous low-level aerosol plumes, which, in part, werecaused by the rather dense coastal ship traffic.

In Fig. 5 a slant-path measurement through theboundary layer is shown. The layer top is observedat 2:84km. In order to reduce the noise, the datawere smoothed with a seven-channel gliding arith-metic mean, corresponding to a vertical resolutionof about 13:4m [39,40]. The signal-to-noise ratio isacceptable for distances up to 10km. However, someaerosol structure is visible also in the free tropo-sphere (distances beyond 9km), as well as a weak re-

turn from the edge of a horizontally confined cloudlayer at distances beyond 20:3km.

Figure 6 displays three examples of measurementsduring a period with a strongly variable aerosol dis-tribution. The upper edge of the boundary layer isnot sharp. The height corresponding to the half-dropnear 3km is just 0:3km, a low value even for aboundary-layer height in the early morning. Theaerosol content in the layer above the boundary layeris negligible. This also reflects the perfectly constantbackground level of the lidar signal.

A particularly spectacular demonstration of thespatial variability of the aerosol distribution some-times observed above the sea surface is given inFig. 7. The three measurements at small elevationangles were carried out within just 7 min. A minorchange in angle resulted in major changes in the dis-tribution, with a presumable ship plume that is mostclearly seen at an elevation of 0:5 °. This plume couldbe visually discerned.

A few measurements were made at the Institut fürMeteorologie und Klimaforschung (IMK-IFU) withthe full output achieved shortly before the end ofthe project. Because of the late period in that year,very clear conditions prevailed, reducing the signalto very low values. Under presumable Rayleighconditions, a peak signal level of about 8mV was ob-tained for the highest pulse energies available(0:28 J). For averaging over 5000 laser shots in onetest case (20° elevation), a range of more than 6kmcould be achieved.

6. Conclusions

In this paper a surprisingly significant increase inefficiency in the backward Raman scattering of1:064 μm in deuterium is described. The pulse ener-gies achieved exceed those so far published for deu-terium, with a simpler approach and almost equalconversion efficiency. Unless a narrow emissionbandwidth is needed, the method is advantageous,as it does not require sophisticated alignment orgas circulation. The beam divergence approximatelycorresponds to that of the pump laser, and a smoothbeam profile was obtained from a near-Gaussianpump beam with residual ring structure. Eye safetyunder the rather restrictive current eye-safety regu-lations can be achieved by a beam expansion to about62mm. In addition to the laser upgrading, the perfor-mance of the detection electronics and the data-acquisition system could be strongly improved.

Although measurements to distances exceeding10km were routinely achieved in the presence ofaerosol, the signal-to-noise ratio could be greatly im-proved by enhancing the preamplifier gain and byusing the full energy of the Raman-shifted laser. Inthe near-field channel the minimum distance (fulloverlap) reduced to less than 0:2km by using an eye-piece without aperture, such as an aspheric lens. Theoperation of the lidar with better near-field perfor-mance and with a higher signal will require a near-field/far-field separation, as is done for all trace-gas

Fig. 6. Two horizontal measurements on 12 August 1997, demon-strating the high variability of the near-coastal aerosol on thatevening. The third trace shows the extinction coefficient for aslant-path measurement during that period, indicating a softupper boundary (center: 0:3km) of the low boundary layer.

Fig. 7. Quasi-horizontal measurements over the sea surface re-vealing a substantial complexity of the surface layers. The two ar-rowsmark distances corresponding to the same altitude in the 0:5°and 1:2° profiles (68m).

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lidar systems of IMK-IFU [29,34–36,41]. It is impor-tant to note that even under very clear (Rayleigh-type) conditions as they sometimes occur in ourAlpine valley, a reasonable background signal couldbe found up to several kilometers. This suggests thata calibration of the backscatter signal or the systemconstant is possible, in particular after improving thesignal-to-noise ratio (also by using a higher perfor-mance digitizer; see Section 4).

The use of 1:56 μm as an additional wavelength inan aerosol lidar based on an Nd:YAG laser and itsharmonics is seen as a valuable extension. The trans-mitter layout can be further simplified and reducedin size (e.g., by removing the gas circulator) and,therefore, could fit into conventional transmitters.

The author thanks W. Carnuth and A. Kohnle forthe excellent cooperation over many years, D. Haaks(Aerolaser) for his great help with the preliminarypreamplifier design and the shielding, and M. Mückfor his assistance in the final optimization of the sys-tem. He also thanks M. R. Perrone for her input re-garding the Raman gain suppression and R.Schwall and W. Wilhelm (FAST ComTec) for their as-sistance in improving and programming the TR40transient digitizer. This work was funded by the Ger-man Bundesministrerium für Verteidigung undercontract T/R320/Q0004/Q0300 and the Forschungsin-stitut für Optik (Tübingen, Germany; meanwhile:Fraunhofer-FOM, Ettingen, Germany) under con-tract FGAN 4501 8220 as well as by the GermanBun-desministerium für Bildung und Forschung withintheGermanAerosolLidarNetwork (contractAF109).

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