Studies on candidate approaches for satellite-ground laser communications

Transcription

1 Proc. Internatonal Conference on Space Optcal Systems and Applcatons (ICSOS) 22, 5-3, Ajacco, Corsca, France, October 9-2 (22) Studes on canddate approaches for satellte-ground laser communcatons Yoshhsa Takayama, Yoshsada Koyama, Hdek Takenaka, Hroo Kunmor and Moro Toyoshma atonal Insttute of nformaton and communcatons Technology Tokyo, Japan Abstract Actvtes to perform satellte-ground laser communcatons and approaches to smply the optcal ground staton are ntroduced. We show a small optcal transponder for a small satellte, the analyss of cloud dstrbuton to estmate the possblty of satellte-ground lnks for the ste dversty scheme, and the estmaton of performance of a smplfed optcal ground staton. As an unconventonal dea, we descrbe an approach to employ the negatve refractve behavor for passve beam couplng. Keywords- small optcal transponder, SOTA, ste dversty, optcal ground staton, OGS, negatve refractve behavor. I. ITRODUCTIO Recently the successful demonstratons of the nter-satellte communcatons and the satellte-ground communcatons attract attentons as one of the promsng technologes to provde broadband and long dstance communcatons [-6]. In the feld of satellte applcatons, the use of small satelltes seems to be one of the nterestng topcs. The relatvely short development perod of a small satellte enables us to have deas and plans to utlze sophstcated technologes such as a hgh resoluton magng, wdeband measurement, and so on. The survey of the earth observaton satelltes reports that the data transmsson s less than Mbps f the mass of a satellte s less than kg n most cases [7]. Thus, one of our challenges s to provde a small optcal communcaton termnal for a small satellte, where the data rate s more than Mbps and the mass of the satellte s about 5kg. The optcal communcaton termnal s named as the Small Optcal Transponder (SOTA) [8]. When we look at an optcal ground staton (OGS) for satellte-ground laser communcatons, selecton of the locatons of OGSs s mportant to mprove the avalablty of the satellte-ground lnk wth the ste dversty scheme. Therefore we have estmated the cloud dstrbuton over the Japan area to fnd a set of stes that mutually shows small correlaton n the cloud dstrbuton tendences [9]. Concernng the functons such as gmbals, fast steerng mrrors and sensors n the optcal system of OGS, one of our actvtes s to look for a way to smplfy the OGS s structure. The study to use an optcal fber of a large core dameter to couple the receved beam s an example, where the degree of the arrval angle of the sgnal beam has been studed to estmate the couplng effcency of the receved sgnal beam wthout the fast steerng mrror. In other actvtes, we have proposed an unconventonal method to couple the receved beam nto a small detector or an optcal fber of a small core dameter, where an optcal devce of perodcal structure has been used to obtan the negatve refractve behavor. In ths work, we ntroduce our actvtes concernng the small optcal transponder to be mounted on a satellte, the ste selecton for optcal ground statons, and the estmaton of potental of a smplfed OGS. Besdes, an dea to mprove the couplng effcency of lght s descrbed as an unconventonal approach. II. OPTICAL COMMUICATIO EQUIPMET OBOAD A. Small Optcal Transponder (SOTA) The optcal part of SOTA s shown n Fg.. The total mass ncludng the optcal and the electrcal parts are 6.2kg. The dmensons of the optcal part ncludng the fork mount gmbals are (W)78mm, (D)7mm, and (H)278mm. SOTA s equpped wth three wavelengths laser sources of.8μm,.98μm and.5μm, respectvely. They are labeled as Tx for.98μm, Tx2 and Tx3 for.8μm and Tx4 for.5μm. Wth those wavelengths, the conventonal laser communcatons and a basc experment for the quantum key dstrbuton are planned [8]. The dameter of the acquston and the trackng senor s 23mm and the dameter of the man telescope for Tx4 s 5mm. For the downlnk, SOTA selects one of the two data rates of Mbps and Mbps. The average transmsson powers of the downlnks are 24.3dBm for Tx, and 6.dBm for Tx4, respectvely. Due to the lmtaton of the power consumpton, Tx and Tx4 should be turned on exclusvely. SOTA chooses ether Tx or Tx4 for the downlnk to perform the conventonal laser communcaton demonstratons, whle, n the basc experment for the quantum key dstrbuton, Tx2, Tx3 and Tx4 are turned on smultaneously. The downlnk lasers of Tx and Tx4 are modulated wth a pseudo nose code or mage data of camera nstalled n the satellte. Besdes, the codng technologes such as the Reed-Solomon and LDGM can be supermposed for error correcton. Copyrght (c) ICSOS 22. All Rghts Reserved.

2 Proc. Internatonal Conference on Space Optcal Systems and Applcatons (ICSOS) 22, 5-3, Ajacco, Corsca, France, October 9-2 (22) coeffcent wth aha s almost, whch mples that the cloud dstrbuton of the cty s ndependent to the one of Tokyo. Therefore f we select one cty to combne wth Tokyo n order to avod the cloud blockages, aha would be an effectve locaton. Fgure. Optcal part of SOTA protoflght model III. OPTICAL GROUD STATIO A. Selecton of locaton When the satellte-ground laser communcatons are carred out, the blockage of the optcal paths by the clouds s a problem to be avoded. For the avodance of the cloud blockages, we are lookng at the ste dversty scheme where the plural optcal ground statons mutually connected through the terrestral communcaton networks are used. The laser communcaton lnk could be establshed f at least one of the optcal ground statons under the clear sky can access to the satellte. To select the canddate locatons for the optcal ground statons, the cloud dstrbuton s calculated by usng mages taken by a meteorologcal satellte. The sample mage s gven n Fg. 2, where the northern part of Japan s found n the mddle of the pcture. The detecton wavelength ranges between.3μm and.3μm. For our study, such mages are prepared for one year, from June n 27 to May 3 n 28. We select stes n Japan area and compute each mage to extract the pxel values at all the selected ponts. Although the atmospherc characterstcs such as the transparency are beyond the consderatons due to the approach based on the pxel values of the mages, the evaluaton gves us good nsght to fnd the combnaton effect of the multple ground statons. The numercal correlatons of the sequentally extracted pxel values at Tokyo wth the ones at the other locatons are computed by the Pearson product-moment correlaton coeffcent gven as Correlaton coeffcent Fgure 2. Sample mage used for calculaton. Tokyo.8.6 Sapporo.4.2 Fukuoka aha Poston number Fgure 3. Correlaton coeffcent on cloud dstrbuton. r X X Y Y () 2 X X Y Y where s the total number of the processed mages, X means the pxel value at Tokyo n the -th mage, Y means the pxel value at a certan selected locaton n the -th mage, and are the average of X and Y, respectvely. The correlaton coeffcent s gven n Fg. 3, where the selected ctes are arranged along the abscssa mostly from the north of Japan to the south. In the fgure, four canddate ctes are shown for reference. Those locatons are ndcated n Fg. 4. In Fg. 3, the correlaton coeffcent labelled Tokyo s as the self-correlaton. Here we fnd that the correlaton 2 Fgure 4. Canddate stes for OGSs B. Smplfed optcal ground staton In most cases, laser communcaton termnals are equpped wth the coarse pontng functon, the fne pontng functon and the pont-ahead functon as well as the transmsson and recepton functons of the sgnal beam. The termnals acqure and track the beam from the counter termnal by controllng the coarse pontng and the fne pontng functons, and the transmsson drecton of the sgnal beam s modfed by the pont-ahead functon. Copyrght (c) ICSOS 22. All Rghts Reserved.

3 Proc. Internatonal Conference on Space Optcal Systems and Applcatons (ICSOS) 22, 5-3, Ajacco, Corsca, France, October 9-2 (22) Here we consder a smplfed optcal termnal shown n Fg. 5 for OGSs, where the coarse pontng functon of the optcal antenna s equpped wth the sgnal recever and the beacon transmtter. The optcal axs of the telescope s parallel to the drecton of the beacon transmsson. When we conduct the satellte-ground laser communcatons wth such a smplfed OGS, the performance s drectly nfluenced by the atmospherc turbulences, the lack of pont-ahead angle compensaton, and so on. Those nfluences appear even f the optcal termnal on a satellte s fully functonal to transmt the sgnal beam toward the OGS n the deal drecton, because the transmsson drecton of the optcal termnal onboard s determned by the arrval angle of lght from the OGS. Among the atmospherc nfluences, one of the domnant phenomena s the beam wander. Fg. 6 shows that the beacon beam from the OGS collected at the satellte s movng on the recevng plane of the optcal termnal, where s a varance of the dstance between the deal center of the beacon llumnaton area wth no atmospherc nfluences and the center of the llumnaton area affected by atmosphere. The angle varaton caused by the beam wander s obtaned as /, where L s the dstance between the OGS and the satellte []. For computaton, we assume that the orbt of the satellte s crcular wth the alttude 6km and the OGS locates wthn the orbt plane. In ths case, the requred pont-ahead angle for OGS s about 2μrad at the elevaton angle deg and ncreases up to 5μrad at the elevaton angle 9deg. Besdes, the angular fluctuaton due to the beam wander s added to the pont-ahead angle. By usng the Hufnagel-Valley(H-V) model, the error angle caused by the beam wander and the pont-ahead s obtaned as shown n Fg. 7. Snce the satellte s assumed to transmt the beam toward the deal drecton, the angular error n the beacon beam from the OGS drectly affects the transmsson drecton of the sgnal beam of the satellte, and that s observed as the arrval angle error n the OGS. The arrval angle error causes the spread of the exposed area of the receved beam on the OGS recevng plane. The spread of the exposed area s proportonal to the focal length of the OGS recevng optcs. Here we assume that the rato of the OGS telescope radus to the focal length s., and the results are shown n Fg. 8. Here the case wth the pont-ahead compensaton and the case wthout the compensaton are plotted. The use of optcal fbers provdes easer handng of the optcal setup than an optcal setup where lenses and mrrors are arranged. Therefore, f we use an optcal fber to couple the lght that the OGS receves from the satellte, the core area s requred to be larger than the exposed area on the OGS recevng plane. A dotted lne n Fg. 8 s drawn at 3μm of the spot wander range, for example, that ntersects the plots at.7m and.4m on the transmsson radus, respectvely. Therefore, f a fber wth the core radus of 3μm s used n ths OGS, the effectve radus of the telescope s less than 7cm n the case wthout the pont-ahead compensaton and 4cm n the case of the pont-ahead compensaton. Besdes, the numercal aperture of the fber should be taken nto consderatons to estmate the acceptable data rate. Accordng to the geometrcal optcs, the acceptable bandwdth of the fber s computed and shown n Fg.9, where the numercal aperture of the fber s assumed as.2, and the fber s the step ndex type wth the refractve ndex of the core s.49 []. The acceptable bandwdth s a functon of the length of the fber as shown n Fg. 9. Error angle ( rad) Spot wander rage ( m) Fgure 5. Smplfed optcal ground staton Fgure 6. Beam wander Transmsson radus (m) Elevaton angle (deg) Fgure 7. Beam wander angle Beam wander and pont-ahead angle Beam wander wth pont-ahead compensaton Transmsson radus (m) Fgure 8. Spot wander range Copyrght (c) ICSOS 22. All Rghts Reserved.

4 Proc. Internatonal Conference on Space Optcal Systems and Applcatons (ICSOS) 22, 5-3, Ajacco, Corsca, France, October 9-2 (22) Acceptable bandwdth (GHz) Length of fber (m) Fgure 9. Acceptable bandwdth of a fber. j A b f A b j f lens negatve refractve behavor Fgure. Propagaton of lght caused by the negatve refractve behavor. C. Unconventonal approach for couplng of receved beam It s known that a slab of a negatve refractve ndex medum placed behnd a focal pont of a lens causes the rays extng from the focal pont A f to gather agan at the pont A b as shown n Fg. [2]. For example, the ray denoted by the bold arrow labeled j f goes nto the slab from ar, refracts toward a negatve drecton n the angular sense, and fnally exts as the ray labeled j b.when the boundares of the slab are mutually parallel, the arrows j f and j b pont n the same drecton. Therefore, an optcal detector acceptng lght at the poston A f can be replaced by the poston at A b to receve the lght wthn the same numercal aperture. Here we note that the lne connectng the pont A f and A b s perpendcular to the boundares of the slab. Ths lne of thought led us to consder an approach shown n Fg., where the focal ponts of the lght receved by a telescope spread to the range W f, but the range s reduced to W b behnd the negatve refractve medum slab. Here, the propagaton drecton of each ray n front of and behnd the medum remans the same f the boundares n each case are mutually parallel. Therefore, all the rays can be coupled f an optcal detector or a fber wth a dameter larger than W b s put behnd the medum [3]. For a materal of the negatve refractve behavor, we look at a photonc crystal of two-dmensonal lattce structure as shown n Fg. 2. The rectangular rods wth the cross secton sze.d by.4d are arranged n a square where d s the dstance of the neghborng rods. We assume d s equal to.5λ, λ s the wavelength of the lght, and the refractve ndces of the rods n r and the background n b are 3.35 and.55, respectvely. In Fg. 3, the materal showng the negatve refractve behavor has curved boundares and the rays come out of three ponts A, B, and C wthn the wdth W f at x=. The dstances between the neghborng ponts are normalzed to. The boundares b and b 2 are segments of crcles wth the radus 3 and 28, respectvely. The range W f s reduced to W b at x=9 wth the reducton rato of 37.5%. In the sample calculaton, the recreated focuses behnd the crystal seems to become blurred but the propagaton drectons of the rays are almost the same as the ones n front of the crystal. ormalzed dstance y W f Fgure. Spatal shft of focal ponts Fgure 2. Photonc crystal of latce strucutre - W f.d b b 2 C B A n b.4d crystal Fgure 3. Reducton of the range that all the beams passes through IV. COCLUSIOS We have ntroduced our actvtes concernng satellteground laser communcatons. As the equpment on a satellte, the small optcal transponder SOTA s ntroduced frst. As the actvtes on the optcal ground statons, the cloud dstrbuton analyss for the ste selecton of optcal ground statons has been carred out. The calculaton has found a combnaton of stes wth almost no correlaton even wthn Japan area. For the estmaton of potental of a smplfed OGS, the use of large core dameter s studed. Besdes, an dea to utlze the negatve refractve behavor has been descrbed to gather the lght wthout expandng the propagaton angles. d n r d W b W b 2 ormalzed dstance x References Copyrght (c) ICSOS 22. All Rghts Reserved.

5 Proc. Internatonal Conference on Space Optcal Systems and Applcatons (ICSOS) 22, 5-3, Ajacco, Corsca, France, October 9-2 (22) [] T. Tokeresen, G. Oppenhaeuser, In-orbt test result of an operatonal optcal ntersatellte lnk between ARTEMIS and SPOT4, SILEX, Proc. SPIE 4635, 22, pp.-5. [2] T. Jono, Y. Takayama, K. Shratama, I. Mase, B. Demelenne, Z. Sodnk, A. Brd, M. Toyoshma, H. Kunmor, D. Gggenbach,. Perlot, M. Knapek, K. Ara, Overvew of the nter-orbt and the orbt-to-ground lasercom demonstraton by OICETS, Proc. SPIE, 6457, 27, pp [3] M. Toyoshma, H. Takenaka, C. Schaefer,. Myashta, Y. Shoj, Y. Takayama, Y. Koyama, H. Kunmor, S. Yamakawa, E. Okamoto, Results from phase-4 Krar optcal communcaton demonstraton experments wth the ICT optcal ground staton (KODE), AIAA ICSSC3.4.2, 29. [4]. Perlot, M. Knapek, D. Gggenbach, J. Horwath, M. Brechtelsbauer, Y. Takayama, T. Jono, Results of the Optcal Downlnk Experment KIODO from OICETS Satellte to Optcal Ground Staton Oberpfaffenhofen (OGS-OP), Proc. SPIE, 6457, 27, pp [5] B. Smutny, R. Lange, H. Kämpfner, D. Dallmann, M. Gregory, G. Mühlnkel, Hgh Data Rate Optcal Inter-Satellte Lnks, Proc. ICSOS,, ICSOS29-2, 29, pp. -5. [6] K. E. Wlson, J. M. Kovalk, A. Bswas, M. W. Wrght, W. T. Roberts, Y. Takayama, S. Yamakawa, Prelmnary results of the OCTL to OICETS optcal lnk experment (OTOOLE), Proc. SPIE 7587, 2, pp [7] M. Toyoshma, H. Takenaka, Y. Shoj, Y. Takayama, Y. Koyama, M. Akoka, Small Optcal Transponder for Small Satelltes, Proc. CSDSP 2, OWC-9, 2, pp [8] Y. Takayama, M. Toyoshma, Y. Koyama, H. Takenaka, M. Akoka, K. Shratama, I. Mase, O. Kawamoto, Current development status of Small Optcal TrAnsponder (SOTA) for satellte-ground laser communcatons, Proc. SPIE 8246, 22, pp [9] Y. Takayama, M. Toyoshma, Y. Koyama, H. Takenaka, M. Akoka, K. Shratama, I. Mase, O. Kawamoto, Development of small optcal transponder for satellte-ground laser communcaton demonstratons, 7th Ka and BroadBand Communcatons Conference, 2, pp. -6. [] L. C. Andrews, R. L. Phllps, R. J. Sasela, R. R. Parent, Strehl rato and scntllaton theory for uplnk Gaussan-beam waves: beam wander effects, Opt. Eng. 45, 7, 26, pp [] G. Keser, Optcal fber communcatons, 3rd edton, McGraw-Hll Hgher Educaton, Sngapore, 2, Chap. 3. [2] J. B. Pendry, egatve refracton makes a perfect lens, Phys. Rev. Lett. 85, 8, 2, pp [3] Y. Takayama, W. Klaus, K. Shnohara, Passve approach to expand the feld of vew of recevers for free-space laser communcatons, Opt. Eng. 44, 5, 25, pp Copyrght (c) ICSOS 22. All Rghts Reserved.