Shiro Yamakawa

Long-term statistics of laser beam propagation in an optical ground-to-geostationary satellite communications link

A ground-to-space laser communications experiment was conducted to verify the optical interfaces between a laser communications terminal in an optical ground station and an optical payload onboard a geostationary satellite 38 000 km away. The end-to-end optical characteristics such as intensity, sensitivity, wavelength, polarization, and the modulation scheme of optical signals as well as acquisition sequences of the terminals were tested under fairly good atmospheric conditions. The downlinks bit error rate was on the order of 10/sup -10/ in spite of atmospheric turbulence. Atmospheric turbulence-induced signal fading increased the uplink bit error rate, the best value of which was 2.5 /spl times/10/sup -5/ because the turbulent layer near the earth surface affects the uplink signal more than it does the downlink one. The far-field optical antenna patterns were measured through the ground-to-satellite laser links. The long-term statistics of the optical signal data is in good agreement with the calculated joint probability density function due to atmospheric turbulence and pointing jitter error effects, which means the stationary stochastic process can be applied to not only the static link analysis but also the dynamic link performance of the optical communications link. The equivalent broadened optical beam pattern should be used for the fading analysis even though the atmospheric coherence length is larger than the antenna diameter or the optical beam diameter of the transmitter. From these results, a more accurate dynamic link design of the optical communications link can be performed that would be useful for system designers, especially for designers of commercial systems.

Latest results and trade-off of high-power optical fiber amplifiers for optical interorbit communications

Optical inter-orbit communications (Optical IOC) technology is necessary for the space activities of the next century, because of the wide bandwidth of the optical region, and the light weight and small size of Optical IOC equipment. Moreover, transmitter subsystems using optical fiber amplifiers are strong candidates for Optical IOCs high power transmitter. In this paper, R&D activities on optical fiber amplifiers in NASDA are described. Several high power optical fiber amplifiers which have been developed very recently are also discussed. At the end of this paper, we consider a trade off results, for example Which optical fiber amplifiers are best for Optical IOC?

Trade-off between IM-DD and coherent system in high-data-rate optical interorbit links

Trade-off between Intensity Modulation Direct Detection (IM- DD) and coherent system oriented to the inter-orbit lightwave communication link is performed. Improvement in sensitivity of IM-DD system adopting high power optical fiber booster/pre- amplifiers is discussed preciously. In this paper, IM-DD system with booster/pre-amplifier and five kinds of coherent system (ASK-heterodyne, FSK-heterodyne, PSK-heterodyne, DPSK- heterodyne, and PSK-homodyne) are compared for high-data rate communication. Numerical analysis shows the superior characteristics of coherent communication system in low BER transmission under severe ASE noise.

Coherent lightwave receivers with a laser diode local oscillator for interorbit optical communication

Coherent lightwave receivers are experimentally demonstrated with a laser diode as local light source. The laser diode local oscillator is available for not only reducing weight and power consumption of a coherent receiver, but expanding the wavelength tuning range of receiver. In our study, a narrow line-width (<50 kHz) DFB laser diode is introduced into coherent receivers as a local oscillator light source. And two kinds of heterodyne coherent lightwave receiver (ASK-envelope detection and PSK-synchronous detection) are designed and evaluated. In the experiment, the signal light was generated in a system consist of a DFB laser diode operated at the wavelength of 1.55 μm, an electro-optic extra-cavity wideband modulator and an Erbium doped fiber amplifier (EDFA). After attenuating power, the signal light was introduced into the receiver. An optical attenuator was inserted between the EDFA and the receiver instead of space loss in actual communication link. Coherent lightwave detection using a laser-diode local light source was successfully performed at data rate of 2.5 Gbps. Sensitivity under 174 photons/bit at Bit Error Rate (BER) of 10-9 is experimentally achieved in the PSK coherent system. The PSK coherent receiver also shows good wavelength tracking characteristics for the variation of signal beam wavelength. It is demonstrated that the combination of tunable local optical sources and an intermediate frequency-phase lock loop (PLL) is effective to compensate Doppler effect, which is indispensable in inter-orbit communication link. In addition, a trade-off between the intensity modulation direct detection (IM-DD) system and coherent system oriented to the inter-orbit optical communication link is discussed based on the experimental results.

Analysis of noises generated in high-power-operated erbium-doped fiber amplifiers and their influences on IM-DD/coherent OIC system performance

The noise characteristics of high-power operated erbium- doped fiber amplifiers are evaluated experimentally and theoretically. Optical high-power post-amplifiers are indispensable for the transmitter in high-bit rate optical inter-orbit communication (OIC) links applications in which repeaters is not applicable. Thus, the effects of optical amplification noise on communication performance must be analyzed to establish reliable optical links. The dominant noise component and the influence on receiver sensitivity are discussed for various kinds of OIC links in our study. In addition, intensity modulation direct detection (IM-DD) scheme utilizing optical amplifiers are evaluated for OIC link applications. Experimental demonstrations of coherent detection are also performed with optical fiber amplifiers. The performance of IM-DD and coherent optical communication system are discussed in case of operation under the heavy amplified spontaneous emission (ASE) noise environment induced by the optical amplifiers. The requirements for optical band-pass filters to reduce ASE noise are clarified for IM-DD scheme and the superior feature of coherent systems are shown in aspects of tolerance for bandwidth of optical filter. High-sensitive receiving operation under 50 photons/bit at bit error rate of 10-9 is achieved in the system with a 500 mW optical booster-amplifier assuming GEO-GEO links (approximately 40,000 km).

Small-size two-axis mechanical devices for FPM and PAM using piezoelectronic actuators

We have developed two kinds of small size and light weight mechanical devices which consist of 2-axis piezoelectronic driving actuators and elastic hinges for fine pointing mechanism (FPM) and point ahead mechanism (PAM) optical inter- orbit communication (optical IOC) equipment. These are Beam Trapping Mechanism (BTM) and Beam Scanning Mechanism (BSM) respectively. High sensitivity receive system using single- mode optical fiber should be needed to increase communication data rate, which are a receiver of heterodyne detection in optical fiber and a receiver using optical pre-amplifier (for example, Er doped fiber amplifier). Therefore, both improvement of precision of FPM and control of the end of optical fiber are needed. In order to realize these requirements, we have designed, developed and tested two kinds of mechanical devices. At first, results of BTM, which consists of an optical fiber, four piezoelectric actuators, elastic hinges and a housing which mounts them, are described. The end facet of optical fiber supported by elastic hinges is controlled by piezoelectronic actuators in order to trap spatial received beam into optical fiber with high efficiency and high speed. The functional test results of BTM show a moving range of 140 (mu) rad, a resolution of less than 0.2 (mu) rad and natural frequency of 390 Hz. These values show BTM has enough performance for low-loss received beam trapping into optical fiber core. Furthermore, small-sized and lightweight BTM was realized using piezoelectronic actuators. We designed a BSM which has function of piezoelectronic driving 2-axis FPM in order to improve FPM. BSM have mirror of which size is 15 * 12 mm2. The mechanical concept of BSM is as same as BTM. In BSM, mirror is supported by elastic hinges, replace with optical fiber. Function test results show scanning angle range of more than 2.8 mrad at azimuth direction, 2.0 mrad at elevation direction, scanning resolution of 1.0 (mu) rad and natural frequency is 1.1 kHz. Furthermore, BSM is smaller and lighter compared with electro- magnetic devices.