**2. Transition of the Free Space Optical Communication systems**

The various concepts and architectures of Free Space Optical Communication (FSO) systems are illustrated in Figure 1. Figure 1(a) shows a conventional FSO system [Infrared][ Free-Space]. Since it uses as a transmission media the difference wavelength as an optical fiber, it includes optical and electrical conversion between an optical fiber side and the free-space side. Furthermore, it is necessary to prepare a different interface which performs individual modulation and demodulation, coding, etc. for every communications service. Figure 1(b) shows the concept of a NG-FSO system [Takahashi 2008][Matsumoto 2008][Kazaura 2007]. In seamless connection of free-space and fiber systems an optical beam is emitted directly from a single mode fiber (SMF) termination to free-space using a new concept FSO antenna. Loss of the optical signal power caused by space transmission can be compensated using a fiber amplifier using the same wavelength band of 1550 nm as an optical fiber network [Khaleghi 1996][ Luo 1998]. In this method the need to convert the optical signal from electrical to optical formats or vice versa for transmitting or receiving through space is eliminated. Figure 1(c) shows the concept of a Radio on Free Space Optical communication (RoFSO) system for realizing application in radio service combining with Radio on Fiber (RoF) technology [Al-Raweshidy 2002][ Hai 2006]. Main advantage and goal of RoFSO systems is they can be used to quickly and effectively provide heterogeneous wireless service [Komaki 2003][Tsukamoto 2006] for example WiFi (IEEE 802.11) ,WiMAX (IEEE 802.16), cellular based 3G signals etc, simultaneously.

However, the challenge in NG-FSO system design is making seamless connection of free space propagated beam to the SMF. The optical signal which has been propagated through free-space is reduced by the optical elements like lens etc. in the antenna and focused to the core of SMF. The optical beam transmitted through the free-space is influenced by various weather conditions, such as attenuation by rain, fog, snow etc., and by atmospheric absorption. Furthermore, the beam experiences atmospheric turbulence as it propagates through free-space, 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 AOA which in turn leads to significant variation in the power of the light focused into the SMF. Although AOA is not a problem for conventional FSO system thanks to a large area of the photo detector, it is a problem for NG-FSO because of little margin between the arrival beam and SMF. It is therefore difficult to maintain a stable link performance.

**Figure 1.** The concept of various FSO systems, (a) conventional, (b) NG-FSO and (c) RoFSO

202 Optical Communication

FSO (RoFSO) system.

communication systems will be used not only for the space communications [Kaliski 1999][Borocom 2005][Koyama 2004] but also the terrestrial long-distance photonic network. In this paper, we discuss demand specifications of optical antennas considering phenomena such as the scintillation which occurs by atmospheric turbulence. And then we explain optics design and the design results to satisfy demand specifications. We also mention the fine tracking mechanism using the fine pointing mirror (FPM) and the feedback of the signal's incident angle detection by the quadrant detector (QD) for the antenna. The influences of fluctuation of laser beam angle-of-arrival (AOA) are reduced effectively. We mention the following research studies of the two optical wireless communication systems such as next generation free-space optical communication (NG-FSO) system and radio on

**2. Transition of the Free Space Optical Communication systems** 

802.16), cellular based 3G signals etc, simultaneously.

The various concepts and architectures of Free Space Optical Communication (FSO) systems are illustrated in Figure 1. Figure 1(a) shows a conventional FSO system [Infrared][ Free-Space]. Since it uses as a transmission media the difference wavelength as an optical fiber, it includes optical and electrical conversion between an optical fiber side and the free-space side. Furthermore, it is necessary to prepare a different interface which performs individual modulation and demodulation, coding, etc. for every communications service. Figure 1(b) shows the concept of a NG-FSO system [Takahashi 2008][Matsumoto 2008][Kazaura 2007]. In seamless connection of free-space and fiber systems an optical beam is emitted directly from a single mode fiber (SMF) termination to free-space using a new concept FSO antenna. Loss of the optical signal power caused by space transmission can be compensated using a fiber amplifier using the same wavelength band of 1550 nm as an optical fiber network [Khaleghi 1996][ Luo 1998]. In this method the need to convert the optical signal from electrical to optical formats or vice versa for transmitting or receiving through space is eliminated. Figure 1(c) shows the concept of a Radio on Free Space Optical communication (RoFSO) system for realizing application in radio service combining with Radio on Fiber (RoF) technology [Al-Raweshidy 2002][ Hai 2006]. Main advantage and goal of RoFSO systems is they can be used to quickly and effectively provide heterogeneous wireless service [Komaki 2003][Tsukamoto 2006] for example WiFi (IEEE 802.11) ,WiMAX (IEEE

However, the challenge in NG-FSO system design is making seamless connection of free space propagated beam to the SMF. The optical signal which has been propagated through free-space is reduced by the optical elements like lens etc. in the antenna and focused to the core of SMF. The optical beam transmitted through the free-space is influenced by various weather conditions, such as attenuation by rain, fog, snow etc., and by atmospheric absorption. Furthermore, the beam experiences atmospheric turbulence as it propagates through free-space, 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 AOA which in turn leads to significant variation in the power of the light focused into the SMF.
