Preliminary study on ground based coherent differential absorption LIDAR for vertical profiling of water vapor density using 1.

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1 8 th Coheret Laser Radar Coferece Prelmary study o groud based coheret dfferetal absorpto LIDAR for vertcal proflg of water vapor desty usg.53 µm wavelegth Masaharu IMAKI, Ryota KOJIMA, Takayuk YANAGISAWA, ad Shumpe KAMEYAMA Mtsubsh Electrc Corporato, Iformato Techology R&D Ceter, 5-- Ofua, Kamakura, Kaagawa, Japa, Lead Author e-mal address: Imak.Masaharu@dc.MtsubshElectrc.co.jp Abstract: We performed prelmary study o a groud based coheret dfferetal absorpto ldar (DIAL for vertcal proflg of water vapor desty usg.53m laser wavelegth. We assumed the atmospherc codto usg the fast atmospherc sgature code (FASCODE, ad estmated the measuremet accuracy usg coheret ldar equato. Smulato results show a 0 % accuracy of water vapor desty from a alttude of 0. km to 4.5 km wth 0 mutes average. Keywords: Water vapor, Dfferetal absorpto, Coheret. Itroducto Water vapor the atmosphere affects growg cumulombus whch causes localzed heavy rafall, therefore the measuremet of water vapor desty profle s requred for the early predcto of ths rafall. Heght profle of water vapor measuremet usg coheret dfferetal absorpto ldar (DIAL techque was reported Ref. ad. The cotuous heght profle measuremet was demostrated, however, the measuremet accuracy was flueced by solar backgroud lght. O the other had, coheret DIAL techque ca overcome ths ssue ad was reported Ref. 3 ad 4. The good agreemet result o water vapor desty comparso wth that of radosode soudgs was show. Sce late 990s, we have bee developg the wd sesg coheret Doppler LIDARs wth.5 µm wavelegth. The LIDAR wth.5 µm wavelegth has advatages such as ts compactess ad relablty by usg telecom products. Here, we cosder to apply ths techology to DIAL, ad perform prelmary study o a groud based coheret DIAL for vertcal proflg of water vapor desty usg.53 µm laser wavelegth. I the followgs, we dscuss the cocept, cofgurato, ad smulato results of the ldar... SYSTEM CONFIGURATION The system cofgurato s show Fg.. A CW laser lghts wth.5 µm wavelegth from two laser dodes are locked to the ON ad OFF wavelegth for water vapor. Ether ON or OFF wavelegth lghts are selected by a optcal swtch. Selected laser lght s modulated wth a pulse waveform, s amplfed, ad s trasmtted to the atmosphere. A part of the CW lght s tapped ad used for a local sgal lght for heterodye detecto. The receved lghts are coverted to electrcal sgals by a photodetector. The receved sgal s aalog-to-dgtal coverted ad spectral aalyss ad accumulato s performed the sgal processor. A coheret ldar has a advatage daytme measuremet compared wth coheret ldar because the fluece of backgroud lght s greatly low. I addto, the ldar ca smultaeously measure wd speed ad water vapor desty. A dfferetal absorpto optcal depth DAOD ad water vapor desty are derved from a rato of the ormalzed measured testes of the two wavelegths, ad are wrtte by CLRC 06, Jue 6 July

2 8 th Coheret Laser Radar Coferece Laser dode (λ o Laser dode (λ off Optcal couplers Optcal Swtch Drver Detector Pulse modulator A/D Coverter Polarzato beam spltter Optcal Amplfer Sgal Processer Wave plate Telescope Fgure. Schematc dagram of coheret dfferetal absorpto ldar. Optcal fber Electrc sgal cable DAOD z ( ( z+ P ( off Po = l, ( Po + Poff z DAPD z ( = z ( ko koff, ( where P off (z s receved power of OFF wavelegth electrc doma ad P o (z s receved power of ON wavelegth electrc doma at rage z, z s rage resoluto of the ldar, k o ad k off are absorpto coeffcet of ON wavelegth ad OFF wavelegth, respectvely. If the absorpto coeffcet s costat, the radom relatve error of the water vapor desty / s expressed by SNR + SNRoff SNRo ( z + SNRoff ( z DAPD o + ( z = + +, (3 SNRo ( z =, (4 PShot + ( N PS z N where P shot s shot ose power electrc doma ad N s umber of laser shot. The frst term a deomator s the sgal-to-ose rato (SNR deped o the shot ose, ad the secod term a deomator s the SNR deped o the speckle ose. 3. WAVELENGTH SELECTION The ma requremets for ON wavelegth selecto.5 µm wavelegth rego are ( strog le testy ad hgh weghtg fucto ear the groud the rego of the bad of optcal amplfer, ad ( low temperature depedece of absorpto. Fgure shows the le testy of water vapor absorpto les for 700 m - 00 m rage (upper ad 50 m m rage (bottom. Although a absorpto coeffcet of.5 µm wavelegth rego s lower tha oe of 0.8 µm wavelegth rego whch has bee used wdely the past lterature, t s sutable for log rage measuremet. I the bad of CLRC 06, Jue 6 July

3 8th Coheret Laser Radar Coferece IMAKI, Masaharu optcal amplfer, the strog le testy s shorter wavelegth, ad the value s (cm/molecule at m wavelegth. Le testy (cm/molecule Eye-safe wavelegth.e-9 Bad of optcal amplfer.e-0.e-.e-.e-3.e-4.e-5.e Wavelegth (m E-3 Le testy (cm/molecule Bad of optcal amplfer.e-4.e Wavelegth (m Fgure. Le testy of water vapor for m rage (upper ad m rage (bottom. Temperature depedece of the le testy s expressed by exp h c ν h c E Q(T96 k T, S T (T = S (T96 exp Q(T k T T0 exp h c ν k T 96 (5 where S(T96 s le testy at temperature of 96 K, Q(T s the total teral partto sum, h s Plak costats, c s speed of lght, k s Boltzma costat, E s the lower-state eergy of the trasto, ad ν s the waveumber of the spectral le trasto vacuum. Fgure 3 shows the temperature depedece of the water vapor le testy. The le color of red, orage, yellow, gree, ad blue s the le testy.5 µm wavelegth rego. The whte color le s the le testy 0.8 µm wavelegth rego whch s used for the typcal coheret DIAL system []. The mmum temperature depedece s m the.5 µm wavelegth rego, ad we selected ths wavelegth as the ON wavelegth. CLRC 06, Jue 6 July 3

4 8 th Coheret Laser Radar Coferece Fgure 3. Temperature depedece of the water vapor le testy. 4. CALCULATION RESULTS OF RANDOM RELATIVE ERROR IN WATER VAPOR DENSITY MEASUREMENT We calculated the radom relatve error of the water vapor desty by usg eq. 3. The system parameters are descrbed as follows. O ad OFF wavelegth are m ad m, respectvely. The output laser eergy s.6 mj, pulse wdth s 650 s whch correspod to about 00 m rage resoluto, ad repetto frequecy s 5 khz. The recever dameter s 50 mm, ad system effcecy s -4 db. The umber of laser shot s 30,000 for each wavelegth, whch correspod to the 0 mutes observato tme. About the atmospherc model, U.S. stadard atmosphere 976 s used, ad the heght profle model of aerosol backscatter coeffcet from Ref. 5 ad water vapor desty from Ref. 6 are used. These are show Fg. 4 (a ad (b. Calculato results of the receved power electrcal doma ad relatve error of water vapor for 00 m, 300 m, ad 500 m rage resoluto are show Fg. 4 (c ad (d. Smulato results show a 0 % accuracy of water vapor desty from a alttude of 0.km to 4.5km wth 0 mutes averaged. These results dcate that coheret DIAL has the potetal of the water vapor desty measuremet for weather forecast of localzed heavy rafall. CLRC 06, Jue 6 July 4

5 8 th Coheret Laser Radar Coferece 6 5 λo_00m λoff_00m Rage resoluto 00m 300m 500m 4 Heght(km 3 (a (b (c (d 0.E-0.E-08.E Backscatter coeffcet Mxg rato (g/kg (/m/sr Recved power (dbm 0 00 Relatve error (% Fgure 4. Heght profle model of aerosol backscatter coeffcet (a, water vapor mxg rato (b, the receved power for λo ad λoff at 00 m rage resoluto (c, ad the relatve error of water vapor for 00 m, 300 m, ad 500 m rage resoluto (d. 5. REFERENCES [] A. R. Nehrr, et al., Joural of Atmospherc ad Oceac Techology 8 ( [] S. M. Spuler, et al., Atmospherc Measuremet Techques 8 ( [3] M. J. Kavaya, et al., Appl. Opt. 8 ( [4] R. M. Hardesty, Appl. Opt. 3 ( [5] J. D. Sphre, et al., Appl. Opt., 36, ( [6] G. P. Aderso, et al., AFGL-TR-86-00, (986. CLRC 06, Jue 6 July 5