Virtual single-mode connector enables free-space optical communications
Free-space optical (FSO) communications can provide flexible, easy-to-install, and license-free line-of-sight wireless-communications links. The high speed and large bandwidth offered by light-wave communication technology make them very attractive as means to meet future demand for broadband Internet access and high-definition television broadcasting services. However, the bit rate of existing FSO links using near-IR lasers in the 780–850nm range is still limited to 1–2.5Gb/s. This is due to both the upper limit to laser power usually adopted to maintain eye-safe light levels and the lack of existing high-speed optical devices required to build multi-gigabit optical terminals.
Longer-wavelength spectral regimes, such as the 1.5μm band, have maximum permissible exposures that are less critical than that for the 780–850nm range. They are also attractive because of the wide variety of existing optical devices suitable for multi-gigabit operation. Particularly, erbium-doped fiber amplifiers (EDFAs), which can be engineered for operation at longer wavelengths, are among the most important elements in high-power transmitters and sensitive receivers. To use EDFAs as high-speed optical devices, we must efficiently couple a stable free-space optical beam to a single-mode fiber (SMF)—characterized by a mode-field diameter of approximately 10μm—because almost all high-speed fiber-optic components are connected by SMFs.
In space communications, FSO links are considered the ultimate media to establish high bit-rate data links between satellites. They use diffraction-limited laser beams with large-aperture optics (telescopes) and require accurate pointing and tracking technology. Using these space laser-communication technologies and terrestrial fiber-optic components, we designed a new, compact FSO terminal that provides seamless connectivity with terrestrial fiber-optic networks1 (see Figure 1). It can be used for link distances of up to 1km. A refractive telescope with an effective aperture of 2.4cm is used as an optical antenna to collect the incoming laser beam and convert it into a thin, collimated beam with an internal diameter of 2mm. A fast-steering mirror (FSM) is placed at the telescope's exit pupil to stabilize the angle-of-arrival (AOA) fluctuations of the free-space laser beam. (The latter are caused by vibrations and/or thermal deformations of the terminal support structures and by atmospheric turbulence in the propagation path.)
The stabilized beam is focused into the SMF at the fiber coupler. A tracking sensor using a silicon-quadrant photo detector is integrated into the fiber coupler to detect AOA fluctuations and alignment errors. Based on the horizontal and vertical error signals, an analog proportional-integral-differential servo controller with a bandwidth of >5kHz drives the FSM. After SMF coupling, an optical circulator is used to separate incoming and transmitting optical signals. A near-IR beacon is used for bidirectional tracking. We selected wavelengths of 972 and 982nm, both within the EDFA's pump-laser band, for operational tests. The transmitting signal and beacon laser light are multiplexed by a wave-division-multiplexing (WDM) coupler and then transmitted to the opposite terminal using the same optical path as the signal light, i.e., through the fiber coupler, the FSM, and the optical antenna. The overall optical-signal attenuation from the optical aperture to the SMF connector is approximately 2.0dB. The terminal size is 12×12×20cm3, and its total weight is less than 1kg (see Figure 2 for an internal view). The electrical power required for terminal operation including FSM servo and beacon-laser driver is less than 1W.
We conducted several demonstration experiments using the new FSO terminal. In December 2007, an optical code-division multiple-access signal, requiring a wide transmission bandwidth, was successfully transmitted over 380m through glass windows.2 In the three weeks from 25 August to 12 September 2008, we performed 1.28Tb/s WDM signal transmission (40Gb/s through 32 channels) between two buildings separated by 210m.3 In summer daylight conditions (strong sunshine), we achieved more than 99.9% availability for a link quality (bit error rate) of better than 10−9 for more than six hours, without any forward-error corrections. Our experiments indicate that the new FSO terminal has an operational quality enabling Tb/s-class link capacity, as well as link stability and reliability similar to that of an SMF in free space. We plan to use the new terminal to seamlessly connect fiber-optic and wireless networks.
Yoshinori Arimoto is a senior research engineer. His work focuses on laser communications in space.
