Performance Evaluation of Next Generation Free

IEICE TRANS. ELECTRON., VOL.E90–C, NO.2 FEBRUARY 2007

381

PAPER

Special Section on Evolution of Microwave and Millimeter-Wave Photonics Technology

Performance Evaluation of Next Generation Free-Space Optical Communication System Kamugisha KAZAURA†a) , Member, Kazunori OMAE† , Student Member, Toshiji SUZUKI† , Mitsuji MATSUMOTO† , Edward MUTAFUNGWA†† , Members, Tadaaki MURAKAMI††† , Nonmember, Koichi TAKAHASHI††† , Hideki MATSUMOTO††† , Kazuhiko WAKAMORI††† , and Yoshinori ARIMOTO†††† , Members

SUMMARY Free-space optical communication systems can provide high-speed, improved capacity, cost effective and easy to deploy wireless networks. Experimental investigation on the next generation free-space optical (FSO) communication system utilizing seamless connection of freespace and optical fiber links is presented. A compact antenna which utilizes a miniature fine positioning mirror (FPM) for high-speed beam control and steering is described. The effect of atmospheric turbulence on the beam angle-of-arrival (AOA) fluctuations is shown. The FPM is able to mitigate the power fluctuations at the fiber coupling port caused by this beam angle-of-arrival fluctuations. Experimental results of the FSO system capable of offering stable performance in terms of measured bit-error-rate (BER) showing error free transmission at 2.5 Gbps over extended period of time and improved fiber received power are presented. Also presented are performance results showing stable operation when increasing the FSO communication system data rate from 2.5 Gbps to 10 Gbps as well as WDM experiments. key words: free-space optical (FSO) communication, fine positioning mirror (FPM), scintillation, angle-of-arrival (AOA) fluctuations, atmospheric turbulence

1.

Introduction

The increase demand of wireless links which are easier, faster and less expensive to deploy has renewed interest in the use of free-space optics in digital transmission of signal in the atmosphere [1]–[6]. Recently, free-space optical (FSO) communication systems have become low cost, simple and easy to install, and are therefore increasingly deployed to offer highspeed, broader bandwidth communication links. Optical wireless communication systems (just like microwave and millimetre-wave wireless communication systems) can easily provide high-speed communications without the difficulty and cost of deploying high-capacity optical fiber cables [7], [8]. Manuscript received June 5, 2006. Manuscript revised September 28, 2006. † The authors are with the Global Information and Telecommunication Institute (GITI), Waseda University, Honjo-shi, 367-0035 Japan. †† The author is with the Helsinki University of Technology, Finland. ††† The authors are with the Advanced Info Communication Promotion Community, Tokyo, 164-8512 Japan. †††† The author is with the National Institute of Information and Communication Technology (NICT), Koganei-shi, 184-8795 Japan. a) E-mail: [email protected] DOI: 10.1093/ietele/e90–c.2.381

Currently, widely deployed FSO systems use 0.8 µm wavelength band and communication systems which are able to provide up to 1.5 Gbps are in practical use. However, the use of 0.8 µm wavelength optical devices makes the FSO system incompatible with most of the current high capacity optical fiber systems. Therefore in order to overcome such technical barriers, devices and components developed for long-haul optical fiber communication are effectively utilized to achieve high-speed, improved capacity FSO communications system [9], [10]. In optical fiber communication using 1.55 µm band, wavelength division multiplexing (WDM) technology is possible with eye safe limits thus making it a suitable operating wavelength for FSO data links [11]. Because optical fiber communication devices and components are generally designed for operation with single mode fiber (SMF), when compared to traditional optical wireless system devices there is little or no difference in their design. In reference [9] by applying multimode fiber a simplified optical system technique for coupling a free-space optical beam into a fiber is presented. Unfortunately, multimode fibers are not widely deployed in optical fiber communication networks thus the availability of such devices is limited. In this paper we present an experimental investigation of a next generation FSO utilizing seamless connection of a free-space optical beam to an SMF. The viability of the communication system using an optical antenna with a miniature fine positioning mirror (FPM) for high-speed beam tracking and AOA fluctuation compensation is evaluated. This system was deployed in Tokyo for communication experiments over a 1 km distance. Results of the experiment show that by using an antenna with a high-speed beam tracking and control unit, the system is able to reduce the intensity fluctuations of the received optical beam coupled to the SMF. Furthermore, we verified that a stable connection was achieved and an effective seamless free-space and fiber FSO system was realized. Section 2 gives an outline of seamless connection of free-space and fiber system and Sect. 3 describes the developed optical antenna used in our experimental performance evaluation of next generation FSO communication system. In Sect. 4 the experimental results as well analysis are discussed and in Sect. 5 the paper is concluded.

c 2007 The Institute of Electronics, Information and Communication Engineers Copyright


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2.

Seamless Connection of Free-Space and Fiber System

In traditional FSO systems a fiber transceiver converts the electrical signal to optical signal. The electrical signal is amplified by a laser driver which provides enough current to drive the laser diode. Modulated light from the laser diode is directed through space to the receiver, which focuses the beam onto an Si APD. The APD converts the optical signal to an electrical signal. After noise filtering and reshaping the electrical signal is converted at the fiber transceiver back to an optical signal. This process is depicted in Fig. 1(a). FSO communications systems which operate in this fashion can transmit data rates up to 1.5 Gbps. They can not operate above this data rate because of the power and bandwidth limitation of the optical devices [12]. To overcome the above-mentioned limitations, technology originally designed for long haul fiber optics transmission operating in 1.31 µm or 1.55 µm as transmitting wavelengths is utilized. Unfortunately, the 1.31 µm wavelength is a poor choice for FSO transmission because of the high atmospheric absorption near the 1.31 µm from water vapour. There is relatively less absorption for FSO transmission at the 1.5 µm wavelength. This wavelength is appropriate for seamless connection of free-space and fiber system. In seamless connection of free-space and fiber systems an optical beam is emitted directly from a fiber (SMF) termination to free-space using an optical antenna. At the receiver, the transmitted optical beam is focused directly to a fiber and then sent down the fiber for detection. This is shown in Fig. 1(b). However, depending on the deployment environment, the optical signal’s transmitted power may not be sufficient for free-space transmission. Therefore the signal power is boosted by an erbium doped fiber amplifier (EDFA) and the resulting high-powered optical signal can be transferred from the SMF termination to free-space as shown in Fig. 1(b). In this method the need to convert the optical signal from electrical to optical formats or vice

Fig. 1 FSO system using (a) O/E and E/O conversion and (b) seamless connection of FSO beam and single mode fiber.

versa for transmitting or receiving through space is eliminated. Furthermore, this system is protocol transparent, the need for reconfiguration of the transmitting antenna is eliminated even when the nature of the transmitted signal changes due to varying bit-rate, signal format (analogue or digital) or wavelength channel [13]. Since fiber and free-space optical transmission links carry the same optical signal, the scheme can utilize mature technologies and optical components developed for high bit-rate fiber transmission. Essentially, by omitting the propagation path turbulence factor in the atmosphere, a free-space transmission channel can be considered to be approximately equivalent to that of an SMF. Previously, FSO systems utilizing a freespace optical beam coupled to an SMF for communication between orbiting satellites have been proposed in [14]– [16]. By seamlessly interfacing fiber and free-space channels a hybrid optical transmission scheme that enables a fiber transmitted optical signal to be emitted directly into free-space is achieved as described previously. In the reverse direction the free-space transmitted optical signal is focused onto the fiber by using beam size converters, such as lenses described in reference [13]. However, the optical beam transmitted through the atmosphere requires a large aperture lens because of the huge beam diameter at the receiver. It is difficult to focus the optical signal into an SMF which has a core diameter of about 10 µm. Furthermore, the beam experiences atmospheric turbulence as it propagates through the atmosphere, as well as vibrations of the device at the installation site and beam distortion occurrence. The consequence of these effects is the fluctuation of the beam angle-of-arrival (AOA) which in turn leads to significant variation in the power of the light focused into the SMF. It is therefore difficult to maintain an error free communications link due to the received signal power occasionally dropping below the receiver sensitivity. As a result of these implementation difficulties, until recently, FSO systems have been restricted to research publication and theoretical verification experiments [13], [16], [17]. In practical FSO communication systems various techniques have been developed or are applied to mitigate atmospheric effects such as scintillation or beam wander. These techniques include adaptive optics (AO), use of large receive apertures, diversity techniques and fast tracking antennas [18]. Adaptive optics techniques, originally developed for atmospheric compensation in astronomical sites, restores the distorted wave-front to its original state before it was destroyed by atmospheric turbulence. Although AO have shown limited success, they require bulky and computationintensive systems to achieve wave-front sensing and correction. Alternatively, the use of large receive apertures for atmospheric turbulence mitigation requires the antenna telescope to be equally large. The use of diversity techniques increases the likelihood that the detected signal will be read correctly by propagating the optical wave-front in at least two distinct ways. Diversity can occur in the form of spatial diversity (requiring multiple transmitters and/or receivers),

KAZAURA et al.: PERFORMANCE EVALUATION

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Fig. 2

Fig. 3

Experimental FSO communication system setup.

temporal diversity (requiring signal to be transmitted twice, separated by a time delay) and wavelength diversity (requiring the transmission of data on at least two distinct wavelengths). Even though diversity techniques are promising, they do require significant electronic overhead in the retiming and synchronization process [19]. It is also possible to improve the reliability of FSO system by utilizing coding schemes used in RF and wired communication systems. Although coding provides an additional layer of information security, studies have shown that even the best Forward Error Correction (FEC) codes cannot negate the effect of atmospheric turbulence alone. To compensate for atmospheric turbulence induced signal fading, the antenna used in our experiment utilizes a miniature fine positioning mirror to control and steer the received beam to the SMF connection port. The major incentive of incorporation of an FPM is the active prevention of long term data loss by compensating or at least mitigating atmospheric turbulence induced wave-front phase distortion. Compared to the techniques outlined above, the merit of the FPM employed in our antenna is that it manages to improve the FSO system performance with less complexity and minimum electronics overhead while maintaining the compact size of the antenna. The basic configuration of our experimental setup for seamless free-space and optical fiber transmission investigation is shown in Fig. 2. Because the light is fiber coupled at both ends, the EDFA receive electronics and other measurement and data collection devices can be conveniently placed inside the building. The fibers are run to the respective rooftops and then coupled directly to the transceiver as depicted in Fig. 2. The experimental hardware setup including data collection and other measurement devices are placed in the experiment room as shown in Fig. 3. The collected data include weather data (visibility, temperature, precipitation and fog), bit error rate (BER) and the optical received power. Also placed in the experimental room is a PC for remote antenna monitoring and control. The challenge in seamless connection of free-space and SMF systems is not only to design an effective beam track-

Experimental hardware setup.

ing and antenna alignment technique, but also an efficient mechanism for focusing the light into the fiber at the receiver. An active tracking is required to control and steer the received light to the SMF. This is described in more detail in the next section. 3.

Optical Antenna Description

The narrow transmission beam of a free-space optical signal makes alignment of FSO communication terminals difficult compared to the wider beam RF systems. FSO systems are faced with the challenge of designing effective pointing and tracking mechanism that must keep the receiver aligned with the transmitter. In this experiment a compact optical antenna specifically designed to address this challenge is used. The internal structure of the optical antenna is shown in Fig. 4. The antenna uses a Cassegrain type telescope configuration designed using 3 free form surface (FFS) optics consisting of primary mirror, secondary mirror and a collimating mirror [20]. A 1.55 µm wavelength beam is used for data reception and transmission offering full-duplex (simultaneous bidirectional) data at gigabit-per-second rates. A 0.98 µm beacon is used for antenna alignment and tracking purpose. The beacon light is emitted from four output windows placed on the four corners in front of the antenna. The antenna utilizes an automatic active tracking mechanism with a CCD camera for rough tracking or initial alignment and a quadrant detector (QD) for accurate tracking. Coupling of the transmitted optical beam directly to the SMF is accomplished by a miniature fine positioning mirror. The FPM, which has a vital function in fine tracking process, is placed at the fiber radiation pupil position vicinity. The information of the arrival beam fluctuations is provided by the QD in order to always lead the horizontal optical axis to the fiber connection port thus achieving a feedback control setup. The FPM antenna tracking speed is selected to be able to mitigate the effects of random atmospheric turbulences on the received beam and steer most of the received optical signal to the SMF. By combining two tracking methods as outlined above a more effective beam tracking and control is achieved. The


384

Fig. 5 Beam intensity fluctuation (top) caused by scintillation effects and (below) 1550 nm communication beam intensity variations as a result of AOA fluctuation. Fig. 4

Optical antenna internal structure.

CCD beam position detector and digital signal processing (DSP) performs actual beam position and analysis and feed the information back to the system for counter measures. The QD used for fine tracking consists of four separate detector elements arranged in a matrix. Part of the 0.98 µm beacon beam is directed to the QD by a beam splitter. The four elements of the QD collect the beam separately, and when the signal output for all the detectors is the same then the spot is located exactly at the middle of the detector array. If the beam spot moves, the amount of beam collected by each different detector will be different, resulting in variation of the level of the output signal. By analyzing and comparing these four output signals the direction of the spot movement on the detector array is determined and a corrective control signal is sent to drive the FPM mirror actuator to control and steer the received 1.55 µm data signal to the SMF. 4.

Experimental Results and Analysis

In order to establish the optimum antenna’s FPM tracking speed, we first determine, approximately, the magnitude of the atmospheric turbulence in our deployment environment. This is done by examining the relation and correlation between intensity fluctuation caused by the influence of atmospheric turbulence and the 1.55 µm beam intensity variations as a result of AOA fluctuations. By doing so we can quantify, to some extent, the magnitude of atmospheric turbulence experienced by the propagating optical beam. Figure 5 top shows the measured beam intensity fluctuation in terms of detected voltage level as a result of scintillation effect and Fig. 5 below shows the 1.55 µm communication beam intensity variation because of AOA fluctuations caused by atmospheric turbulence. It is rather difficult, if not impossible, to measure the beam wave-front phase changes during strong intensity scintillations in the receiver aperture [21]. Thus the AOA fluctuations are measured in

Fig. 6 Power spectra showing the relationship between intensity fluctuation caused by scintillation and 1550 nm communication beam variations caused by AOA fluctuations.

terms of the detected electrical signal level (in Volts) when the antenna tracking was set to OFF. The measured data is recorded after every 5 minutes. The 5 minutes periods are split into blocks of 3 seconds wherein the sampling rate is 10 kHz. By producing the power spectra of the time series data shown in Fig. 5 we can obtain the frequency characteristics of the data and therefore are able to establish the similarity in the characteristics of the beam intensity fluctuation as a result of scintillation and the beam AOA fluctuations. Figure 6 depicts the frequency characteristics of the beam intensity fluctuations and AOA fluctuations. From the figure it can be observed that the AOA fluctuations of the 1.55 µm data beam for frequency above 100 Hz are closely correlated to scintillation variations observed on the 0.8 µm scintillation measurement beam. It should be noted that the intensity variation as a result of scintillation is measured by an antenna installed on the same site thus having almost the same propagation path as the antenna under test. Therefore, it can be correctly assumed that the atmospheric turbulence


385 Table 1

Selected specifications of equipment used in the experiment.

Parameter Specification

Unit

Value

Data rate Gbit/s 2.5 E/O (directly mod. DFB laser) OOK/NRZ Test data pattern 223 − 1 PRBS Boost EDFA output mW 100 Receiver optical filter 0.5 dB BW nm ± 11 Antenna aperture mm 40 Rec. sensitivity (BER = 10−12 ) dBm −30 E/O - Electrical/Optical, OOK/NRZ - On Off Keying with non return to zero, PRBS - pseudo random bit sequence

Fig. 7 Fiber received power—(top) antenna tracking set to OFF and (bottom) antenna tracking set to 1 kHz.

will influence the AOA fluctuation of the 1.55 µm communication beam in a similar way to the scintillation effect observed on the 0.8 µm wavelength beam. From the above the FPM tracking speed which will enable the mirror to steer and focus to a great extent the received optical beam to the SMF can be closely approximated. From the experimental results and observation of beam AOA fluctuations frequency characteristics (obtained by producing the power spectra as shown in Fig. 6) the selected FPM antenna tracking speed value should be set above 500 Hz. In our case we set the tracking speed to 1 kHz which is currently considered to be the fastest tracking system for any available FSO communication system. The improvement in the fiber received power is depicted in Fig. 7. The figure shows the result for accurate tracking by controlling the beam AOA fluctuations. Figure 7 top shows the fiber received power when the antenna FPM tracking is set to OFF while Fig. 7 bottom shows fiber received power when the antenna FPM tracking speed is set to 1 kHz. It is observed that the high-speed tracking capability of the FPM manages to control and steer most of the received light to the SMF. The intensity fluctuations observed when the antenna FPM tracking is set to OFF are remarkably suppressed and improvement in the fiber received power is realized as depicted in Fig. 7 bottom. To evaluate the communication quality of the system a 2.5 Gbps bit error rate tester (BERT) pattern generator is used to directly modulate (on-off keying) a single frequency distributed feedback (DFB) laser at 1.55 µm wavelength, with a 2−23 − 1 pseudo random bit sequence (PRBS) pattern length. The data encoded optical signal, is amplified by a 100 mW EDFA. At the receiver the received optical beam is focused by the FPM to the SMF. The optical circulator is used to isolate the transmitted and the received signals. At the reception side, the received signal is equally split into two arms by a passive 3 dB coupler. Half of the received signal enters the O/E converter and the data and clock signals extracted for BER measurements and the other half of the received signal is used to monitor

Fig. 8

Bit error rate and fiber received power characteristics.

the received optical power (this setup is depicted in Fig. 2). The 3 dB output is coupled directly to an optical power meter with a 100 msec averaging time and the optical power meter data is averaged and logged every 30 secs. Some of the primary specifications of the FSO communication devices used in the experiment are listed in Table 1. The regenerated data and clock signals derived from the 2.5 Gbps optical signal, were applied to the 2.5 Gbps error detector for link performance monitoring. The BER characteristics when the antenna FPM tracking speed is set to 1 kHz is shown in Fig. 8. For a 24 hour period, error free transmission is achieved which confirms the stability and good performance of the system. For this system the minimum back-to-back error-free received power is −30 dBm. Even though the antenna tracking speed is enough to compensate for atmospheric turbulence effects, occasional burst errors occur as shown in Fig. 8. These burst errors are attributed to perhaps the non-linearity in the tracking system or the tracking dynamic range might be insufficient in situations of strong atmospheric turbulence. Another likely cause of the burst errors is phase scintillation due to atmospheric turbulence of the incoming signal beam resulting in the decrease of the received power. These occasional burst errors hardly have any influence on the overall performance of the FSO system and the system can be used for reliable stable communication. Consecutively by using a digital communication analyzer we evaluated the data handling capability of the FSO communication system using the eye pattern technique. The essential key wave shape parameters including period, rise/fall time, clock to data jitter, overshoot, ringing, noise and signal to noise ratio are observed to be within accept-


386

The total bandwidth of this FSO communication system can be increased considerably as more channels can be activated in the more complex WDM or dense wavelength division multiplexing (DWDM) schemes. 5.

Fig. 9 Eye pattern with Mask showing a standard STM16/OC48 test for 2.5 Gbps transmission.

Conclusion

A free-space optical communication system using specially designed compact antenna for easy, cost effective means of constructing a robust and reliable high-speed link for next generation FSO system was investigated. The FSO communication system offered seamless connection of free space and fiber system. The transceiver incorporates a FPM for high-speed beam tracking and control function, therefore, having the capability to mitigate the effects of atmospheric turbulence on the transmitted optical beam. The antenna FPM tracking speed is 1 kHz which is currently considered the fastest tracking speed for a FSO system. The FSO communication system performance was verified and error free transmission over an extended period of time was demonstrated. The system performance expressed in terms of BER performance was also evaluated and showed to be consistently above acceptable levels. Stable performance after increasing the system bandwidth using WDM technology was also attained. Acknowledgments

Fig. 10

10 Gbps transmission test eye pattern.

This work is supported by a grant from the National Institute of Information and Communications Technology (NICT) of Japan. References

Fig. 11

WDM received signal spectrum.

able tolerance. This is depicted in Fig. 9 for a 2.5 Gbps single channel transmission and in Fig. 10 which depicts eye pattern for a single channel 1.5 µm data link operating at 10 Gbps. Alternatively, by using a relatively straightforward method of increasing the bandwidth by employing WDM technology, four 2.5 Gbps individual channels with an output power of 100 mW per wavelength can be realised. The four 2.5 Gbps channels were combined for a total wireless throughput of 10 Gbps. By employing this technique stable communication was accomplished without any fluctuation or interferences between wavelengths as shown in Fig. 11.

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Kamugisha Kazaura received B.Eng. degree in Electronics and Communications Engineering from the University of Bath in UK in 1995. He worked for Tanzania Telecommunications Company Ltd. (TTCL) between 1996 and 1999. He later joined Waseda University’s Graduate School of Global Information and Telecommunication Studies where he received his M.Sc. in Information and Telecommunication Engineering in 2002. Currently he is working on his PhD degree. His research interest includes fixed and mobile high-speed wireless communications networks, broadband wireless networks and free-space optical communication systems.

Kazunori Omae received the B.Sc. degree in Liberal Arts Industry and Technology from the University of the Air in 2004 and the M.Sc. degree in Information and Telecommunication Engineering from Waseda University, Graduate School of Global Information and Telecommunication Studies in 2006. He is currently employed with Cable and Wireless IDC Inc and his research interests include free-space optical communication systems, millimeter wave wireless communication systems.

Toshiji Suzuki received the M.Sc. in Electronic and Electrical Engineering from Sophia University Graduate School of Science and Engineering in 1972. From 2002 he joined the Global Information and Telecommunication Institute of Waseda University as a visiting researcher after a success career with Canon Electronics. His research interests include free-space optical communication systems, infrared and visible light communication.

Mitsuji Matsumoto Since joining NTT labs in 1970, Dr. Matsumoto has been engaged in research and CCITT standardization activities in the field of protocol architecture and terminal design for facsimile, telematics and multimedia services and systems. He joined the Global Information and Telecommunication Institute of Waseda University, Tokyo, Japan as professor in 1996. Currently he is vice director of GITI. In 2000–2004 study period he became Vice Chairman of ITU-T SG16 (Multimedia) and since 2004 he is the vice president of Infrared Data Association.

Edward Mutafungwa received the B.Eng. degree in electronic systems engineering and the M.Sc. degree in telecommunications and information systems from the University of Essex, Colchester, U.K., in 1996 and 1997, respectively, and the Dr.Sc.Tech. degree in communications engineering from the Helsinki University of Technology (HUT), Espoo, Finland, in 2004. Since 1997, he has been lecturing and researching in various projects at the Communications Laboratory of HUT and has consulted for various companies. His research interests lie within the general fields of optical networking, network design, broadband wireless communications, intelligent transport systems, and intelligent computing.

Tadaaki Murakami received the M.Sc., from the University of Electro-Communication in Tokyo in 1999. He is currently working for Koito Industry Ltd. which he joined in 1999. From 2005 he joined the Global Information and Telecommunication Institute of Waseda University, Tokyo, Japan as a visiting researcher. He is engaged in research and development of freespace optical communications equipment and applied technology.


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Koichi Takahashi received the M.Sc. from Muroran Institute of Technology, Department of Engineering Applied Solid-State physics in 1984. He currently works for Olympus Corporation and in 2004 he joined the Global Information and Telecommunication Institute of Waseda University as a visiting researcher. His research interest include development of microlithography lens with focus on optical system as well as research and development of a free curved surface optical system, optical communication related to free space optical communication.

Hideki Matsumoto received the M.Sc. from Niigata University, Graduate School of Science and Engineering in 1982. In 2004 he joined the Global Information and Telecommunication Institute of Waseda University as a visiting researcher. He is engaged in research of optical wireless LAN system, spread spectrum communication technology, as well as research and development of optical transmission device.

Kazuhiko Wakamori received the M.Sc. from Shizuoka University, Graduate School of Information Engineering. From 1979 he joined Hamamatsu Photonics K.K. He has been a visiting researcher at the Global Information and Telecommunication Institute of Waseda University from 2004. His research interest include development of optical communication for a highspeed optical device and research and development of free-space optical communication.

Yoshinori Arimoto received the M.Sc. in Physics from Osaka University, Osaka, Japan in 1979. From 1979 he joined the Radio Research Laboratory (Communication Research Laboratory), Ministry of Posts and Telecommunication. During 1983–1985 he was with the Telecommunications Satellite Corporation of Japan (TSCJ). From 1990 to 1993 he worked at the Advanced Telecommunications Research Institute (ATR) Optical and Radio Communications Research Laboratory. In 1999 to 2001 he was with the National Space Development Agency of Japan. His research interest include communication related to space infrastructure, such as freespace laser communication.