6 wind profile measurements by coherent doppler …3 2μmlaser crystals rare-earth ion, er, tm, ho...

1 Introduction

Space-borne lidar with eye-safe laser isexpected to be able to measure directly alti-tude profiles of many kinds of atmosphericparameters such as aerosols, clouds, somekinds of molecules, and water vapers, whichare not observable by passive sensors. One ofthe desired meteorological parameters to beimplemented is the global wind profile, lidarobservations of which may be a promisingmethod. A space-borne Doppler lidar is con-sidered to be the observation system capableof such global measurement of wind profilesin the troposphere and urgently needed [1].Development of a space-borne Doppler lidarusing an all-solid-state eye-safe laser hasrecently begun and it has not been applied tospace. Aiming at full time operation satellitesfor wind profiling, it is necessary to developthe basic technology of Doppler lidar and asystem for a demonstration mission of tech-nology and availability. With the intention ofperforming the demonstration of the space-borne Doppler lidar, we are making research

on coherent Doppler lidar (CDL) having 2μmsolid-state lasers for the observation of windprofiles in the troposphere. Using coherentlidars, we may be possible to measure aerosolsand CO2 profiles, which are related to theevaluation of the global change models.

2 Eye-safe solid laser andDoppler lidar

Eye-safety is necessary for space-bornelidars because of laser beam direction to theearth. Lasers emitting in 1.5μm or longerwavelength, where the output level in eye-safety is high, are suitable for space-borneinstruments. We have been engaged in theresearch and development of 2μm solid-statelaser as candidate laser for space-borne exper-iments. On the point of view of space deploy-ment, all-solid-state system pumped by LaserDiodes (LD) is preferable. We are investigat-ing all-solid-state laser of high efficiency andhigh output energy, which are necessary forspace-borne Doppler lidar in the future.

R&D of coherent Doppler lidars using

MIZUTANI Kohei et al. 167

6 Wind Profile Measurements byCoherent Doppler Lidar

MIZUTANI Kohei, ITABE Toshikazu, ISHII Shoken, SASANO Masahiko, AOKI Tetsuo, ASAI Kazuhiro, and SATO Atsushi

Global wind profiling by a space-borne Doppler lidar with an eye-safe laser is expectedto bring big progress in numerical weather prediction and the studies on global climatemodeling. CRL has been conducting studies on the lasers for the space-borne coherentDoppler lidar to observe wind and aerosol profiles and developing the algorithm for the windprofiling through ground-based and airborne wind observations by a coherent Doppler lidar.We are also making studies on systems for a demonstration of the global wind profiling bythe space-borne coherent Doppler lidar.

Keywords Coherent, Doppler lidar, Eye-safe laser, Space-borne, Wind profile

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2μm solid-state laser started in 1990’. It iscompact and efficient; it is thus suitable formobile, air-borne and space-borne system.Coherent detection is performed by coherentDoppler lidar, which determines the frequencyof a beat signal with a local laser to deduce anDoppler shift of return signal with a highdegree of accuracy. The objects reflectinglaser light are aerosols and clouds in the tropo-sphere. As shown in Fig.1, coherent Dopplerlidar uses a injection locking pulse laser whichis seeded and wavelength controlled by a mas-ter laser. The laser light backscattered andDoppler shifted by aerosols is mixed with thelocal laser light (sometime the same as themaster laser) and combined in a detector. Lowfrequency beat signals are amplified with anIF amplifier, A/D converted, and recorded asdigital signals. Doppler frequency is derivedand converted to a wind velocity after analysisin frequency domain and subtraction of theoffset component. At a wavelength of 2μm, afrequency shift of 1 MHz corresponds to 1 m/sof wind velocity along the line of sight. Wecan also derived aerosol distributions fromintensity profiles of signals.

3 2μm laser crystals

Rare-earth ion, Er, Tm, Ho etc., dopedcrystals are used for 1.5-2μm laser. We maderesearch on Tm:YAG lasers in cw and pulseoscillation modes[2][3]. As high efficiency and

powerful output energy is needed for space-based application, we are investigating basiclasing properties of new 2μm laser crystals torealize higher performance. Tm and Ho dopedcrystals are promising because of its possibili-ty of powerful output power. Table 1 showslasing properties of the materials which weremeasured in a laser resonator configuration.

Rods of host materials, LuLiF and GdVO,seem to be efficient. Threshold energy of YLF,which was well studied in the past, is low andit lases easily.

4 Ground-based and airborneobservations

We are developing an airborne Dopplerlidar with 2μm laser to observe wind profilesdownward from a jet plane. Figure 2 showsthe jet plane equipped with the system (WindTracer: CLR) at the upper picture and theuncovered pod holding it at the lower picture.We will investigate the algorithms required toextract the Doppler shift, compensate for air-plane attitude and velocity, and measure windprofiles through airborne experiment. Thetransceiver of the Doppler lidar is placed in apod attached to the bottom of the jet planebody and is controlled from the inside of thejet plane. A silicon wedge is rotated to scan alaser beam downward along a cone with anadir angle of 20 degrees. The Doppler lidarfeatures a Tm:YAG laser of an output wave-length of 2.01μmand output energy of 6mJ ata repetition rate of 100 Hz.

The lidar can be removed from the podand be placed on the ground to transmit laserlight upward for wind measurements of the

Journal of the National Institute of Information and Communications Technology Vol.51 Nos.1/2 2004

Schematic of Coherent Doppler lidarFig.1

Lasing properties of 2 µm laser crys-tals

Table 1

troposphere. The photograph in Fig.3 showsthe experimental system. The same scanner isused to select the beam direction and theobservation cone has a zenith angle of 20degrees in this case. Figure 4 shows the alti-tude change of spectrum intensities which are

derived from FFT analysis of each range dataof one shot. We can recognize gradual changeof wind with altitude, and see the spectralpeak around 0 m/s by the scattering inside ofthe system under the altitude of about 600 m.The lower limit of the measurement is actuallydetermined by this altitude (about 600 m).Wind vector profiles are measured in a fewminutes, after repeating rotation of the scan-ner, stop and measurement (for about 1000shots), rotation of the scanner…. Figure 5shows the results of wind velocity and direc-tion observed simultaneously by the Dopplerlidar, VHF radar and radio sonde at Wakkanaiin September, 2002. Wind profiles from thesethree instruments agree well until the altitude


Airborne coherent Doppler lidarFig.2

Ground-based coherent Doppler lidarFig.3

Spectrum intensities derived from one shot dataFig.4

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of 9 km, which was the observation limit ofthe Doppler lidar in that time. The agreementof the Doppler lidar and radio sonde is espe-cially good. These results show that we canmeasure the accurate wind profiles with the

Doppler lidar. In Fig.6, time series of windprofiles near Kiyokawa of Yamagata prefec-ture from 15:00 to 24:00 in August 6, 2003 areshown and these data are used for the analysisof local wind.


Wind velocity and direction observed by coherent Doppler lidar(CDL), VHF radar(VR), andradio sonde (Sonde)

Fig.5

Time series of wind profiles observed near Kiyokawa on August 8, 2003Fig.6


Layout of JEM/CDLFig.7

Doppler wind lidar data quality profiles simulated for JEM/CDL (upper figures) and reducedenergy model (lower figures)

Fig.8

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5 Research on space-borneDoppler lidar

5.1 Space-borne Doppler lidarPrior to deployment of operational satel-

lite, it will be necessary to demonstrate that aDoppler lidar system is practical and useful inoperational measurement of global winds. Wehave studied a coherent Doppler lidar system(CDL) capable of deployment on board theexposed facilities (EF) of the Japanese Experi-ment Module (JEM) of the International SpaceStation (ISS) as a candidate demonstrationsystem. In a sub-group meeting of the formerEarth Observation Committee (Chief Examin-er: Prof. T. Iwasaki, Tohoku University) deal-ing with coherent Doppler lidar, a report enti-tled “Science plan on the wind measurementsby the ISS (International Space Station)/JEM-borne coherent Doppler lidar” was proposed[1]. In this report, the required levels of accu-racy for the horizontal wind vector observedby the JEM/CDL were 2 m/s to 3 m/s for ahorizontal resolution of 100 km in the lowertroposphere.

A wind velocity along the line of sight ismeasured by heterodyne detection ofbackscattered light from aerosols in the atmos-phere. The horizontal velocity of wind isobtained by combining line-of-sight velocitiesin two diagonal directions (to the front of andbehind of a satellite). Thus it is necessary toemploy a mechanism that may be used tomeasure wind velocities in two directions.Then, we are considering the use of two fixed40-cm telescopes. We are also investigatingTm,Ho:YLF laser (λ:2.06μm) as a candidatefully solid-state laser of 2-Joule output powerand 10 Hz repeatability, and are making a trialmanufacture of a sub-scale laser as describedin the following section. Figure 7 is aschematic layout of the JEM/CDL. Usingpulse laser light at a rate of 10 Hz, 70 pulsescorrespond to a horizontal distance of 100 km.We expect that the averaging of measurementsmade at these pulses will result in error levelsfor wind velocity that are within the accept-able range specified in the Science Plan. It

should be noted that the predicted errorswould depend heavily on the assumed profileof aerosol.

The exposed facility’s standard payload islimited to 500 kg in weight and 3 kW of elec-tric power [4]. The present model weighs470 kg and needs a power supply of 1489 W,both of which values are below the limits. Theentire power supply for the JEM is, however,only 5.4 kW. Thus reduced consumption ofpower may be important. The lasers consumethe most of the power, as well as generate heatthat must be dissipated by JEM’s liquid cool-ing system of fluorinert. In this regard, we areconducting further investigation of 2μm lasercrystals of high efficiency and a trial manufac-ture of a sub-scale laser, as highly efficientlasers will contribute to reductions not only inpower consumption but also in exhaust heat.The present JEM/CDL model is designed torelease the heat of cooling apparatuses oflaser-rod and LD to a liquid coolant. If the liq-uid coolant system is replaced by radiationcooling system, the reduction in power may beas much as 500 W. Radiation cooling will beavailable for a demonstration mission by aFree Flyer Unit, and eventual deployment in afuture operational satellite (in addition to pos-sible JEM/CDL applications in part).

Results of observation simulation on windmeasurement quality distributions are shownin Fig.8 ( in cooperation with Dr.G.D.Emmittat Simpson Weather Associates). Simulationswere performed in the cases of backgroundaerosol model and enhanced aerosol model.According to the simulation, chance withaccuracy of 1m/s or better in JEM/CDL modelis about 80% in the enhanced aerosol modeland is between 30% and 60% in the back-ground aerosol model, where existences ofclouds increase the chance of observation withhigh quality. The lower two figures in Fig.8show the results of simulation with 500 mJlaser output instead of 2 J. The simulationindicates that the probability of observationswith accuracy of 2 m/s or better is not smalleven in the case of a laser output of a quarterof 2 J. Thus, we can reduce the resource by


employment of radiation cooling and reduc-tion of laser output power in a satellite, whichcan not prepare sufficient resource.

5.2 Sub-scale laser experimentThe most important component for the

space-borne coherent Doppler lidar is a spce-borne 2μm laser. We are developing a sub-


Layout of the sub-scale laserFig.9

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scale laser with 500 mJ output, in which con-duction cooling of Tm,Ho:YLF laser rods isused to be applicable to the space model. Fig-ure 9 shows the layout of this laser. Unidirec-tional operation is performed in the slaveoscillator, the laser rod of which is side-pumped. The injection seeding enables thelaser to oscillate in the single longitudinalmode needed for the Doppler lidar [5]. Thepre-amp is a 4-pass amp with a end pump con-figuration. The post-amp was designed as a 4-pass amp with a side pump configuration, butit is actually used in 2-pass because of suffi-cient amplification in 2-pass [6]. The sub-scalelaser is installed in an optical table and theevaluation of it is now proceeding (Fig.10).The pre-amp does not work properly in suffi-cient output at 10 Hz because of a thermalproblem in it. Then we are now operate thewhole system at 1 Hz and in the output pulseenergy of about 400 mJ. The post-amp ispumped at 10 Hz even in this case. Thus, theamplification of the post-amp to 500 mJ at

10 Hz, is mostly proved. The problem of iso-lation between pre-amp and post-amp, whichlimits the maximum output energy, will be set-tled soon by adjustments of the suitable parts.

We have good prospects to get capabilityof generating 500 mJ pulse at 10Hz. Consider-ing applications to space-borne model, meas-urements with accuracy of acceptable levelmay be possible in fairly wide regions by theinstrument of this level of output pulse energy.Thus, a small lidar model of a free flyer with alaser of 500 mJ at 10 Hz may be another pos-sibility for the demonstration mission, consid-ering the present status of the satellite sched-ule. A smaller model with a laser of 200 mJmay be considerable to demonstrate only thetechnology feasibility by a coherent Dopplerlidar which can measure backscattering fromthe ground, planetary boundary layer, andclouds. If we adopt more efficient laser rodlike Tm,Ho:LuLiF, realization of higher effi-ciency in generating laser pulse and reductionof power consumption may be possible.


The sub-scale laserFig.10

6 Conclusion

Space-borne Doppler lidar may permitobservation of vertical profiles of winds on aglobal scale with an accuracy of 1 to 2 m/s.This is expected to be a promising means tosolve the problem of the shortage of the accu-racy and distribution in the current wind data.As Doppler lidar provides direct observationsof winds, it may be used for verification andcalibration of other types of observed data.Since a Doppler lidar has never been used inspace, we must demonstrate its feasibility inspace. Moreover, as there are vibration-rota-tion lines of CO2 and H2O in the wavelengthrange of 2μm solid lasers, coherent lidar mayused as a DIAL for these absorption lines inthe future. The measurement of CO2 with1 ppm accuracy is especially needed. Weshould investigate further the possibility of

such measurements by space-borne coherentlidar.

So far we have studied models that couldbe deployed on JEM’s exposed facilities andthat would meet the requirements described inthe Science Plan. We have good prospects of500 mJ output at 10 Hz in the conductioncooling sub-scale laser which could be a smallmodel of space-borne laser for JEM/CDL. Weneed to continue the work on improving thesystem’s efficiency, reducing its weight, andestablishing the fundamental technologiesinvolved. Another possibility, e.g. a free flyer,for a demonstration mission besides ofJEM/CDL is also valuable to be considered.Development of an algorithm for applicationof the lidar system will also be necessary,using an air-borne lidar system for windobservations.


References1 Ed. T.Iwasaki, "Science Plan on the Wind Measurements by ISS/JEM-borne Coherent Doppler Lidar",

ESTO(Earth Science& Technology Organization),1999. (in Japanese)

2 A.Sato, K.Asai, T.Itabe, “Double-pass-pumped Tm:YAG laser with a simple cavity configuration”, Applied

Optics, Vol.27, No.27, pp.6395-6400, 1998.

3 A.Sato, K.Sousumi, K.Asai, T.Itabe “Characteristics for normal mode and Q-switched operations of flash-

lamp-pumped Cr,Tm(,Ho):YAG laser”, Review of Laser Engineering, Vol.25, No.1, pp.45-49, 1997. (in

Japanese)

4 "Manual of JEM Exposed Facility", NASDA JBX-96154A, 1998. (in Japanese)

5 H.Fukuoka, M.Kadoya, K.Asaba, K,Asai, K.Mizutani, T.Itabe “Injection Seeded Hm,Ho:YLF Laser”, SPIE

Vol.4153, pp.455-462, 2000.

6 M.W.Phillips, S.W.Henderson, M.Poling , R.M.Huffaker, “Coherent LIDAR development for Doppler wind

measurement from the International Space Sation” SPIE Vol.4153, pp.376-384, 2000.

176 Journal of the National Institute of Information and Communications Technology Vol.51 Nos.1/2 2004

MIZUTANI Kohei, Ph. D.

Leadar, Lidar Group, AppliedResearch and Standards Department

Laser Remote Sensing

ITABE Toshikazu, Dr. Sci.

Executive Director, Basic andAdvanced Research Department


ISHII Shoken, Ph. D.

Researcher, Lidar Group, AppliedResearch and Standards Department


SASANO Masahiko, Ph. D.

Researcher, Remote Sensing ResearchGroup, Environment and EnergyDepartment, National MaritimeReseach Institute


AOKI Tetsuo, Ph. D.

Leadar, Network and Information Sys-tems Group, Research Support Divi-sion, General Affairs Department

Optical Remote Sensing

ASAI Kazuhiro, Ph. D.

Professor, Tohoku Institute of Technol-ogy


SATO Atsushi, Ph. D.

Lecturer, Tohoku Institute of Technolo-gy

Laser Physics

6 wind profile measurements by coherent doppler …3 2μmlaser crystals rare-earth ion, er, tm, ho...

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