Abstract:
To show the possibility to provide a high-speed laser communication to the mobile terminals on aircrafts, ships, trains and cars, small and light-weight laser terminals are developed for a high-altitude stratospheric platform test flight. The laser terminal onboard consists of an optical antenna, two-axes gimbals, fine-tracking system and EDFA-based optical receiver and is capable of analogue signal transmission compatible to digital TV signal at UHF band. Implemented technologies as well as a preliminary result of the test flight up to a 4-km altitude are reported.

Figure 1: Internal layout of optical antenna module

Figure 2: Configuration of onboard optical receiver

Introduction
As the high-speed and broadband communication services provided by fiber optic network penetrates into our daily life, we will need a media to connect the mobile terminal with the fiber optic network and to offer multi-gigabit communications. One of the ways to answer such a need will be a point-to-point free-space laser communication because it will not be difficult to provide a same kind of link-speed, such as 10Gbps or more, once the acquisition and tracking of narrow laser beam is stably established.
At NICT, we have been studying the possibility to provide a high-speed laser communication to the mobile terminals on aircrafts, ships, trains and cars, by applying the existing technology in the area of space laser communication, including a precise tracking and acquisition system and optical transmitter/receiver technology, and we recently developed a prototype laser communication terminal for a test flight vehicle of the high-altitude stratospheric platform. A digital TV broadcasting signal at UHF band was to be transmitted to the receiver at the test-flight airship via an optical analog (feeder) link.
The development program was started in May, 2003 and the first flight was successfully performed on 22nd November, 2004. However, we could only demonstrate a bidirectional acquisition/tracking using a 980-nm/970-nm laser beacon and could not perform a fine tracking and optical signal transmission, due to the flight time was limited to a few hours. This paper describes the configuration of the laser communication terminals and key technology used for the demonstration experiment.

Figure 3: Internal layout of miniature fast steering mirror

Figure 4: Pull-in performance of fine tracking system (in elevation angle)

Figure 5: Principle of integrated fiber coupler

Figure 6: Beacon tracking experiment using a test flight airship. (a) Before beacon tracking. (b) After acquisition and tracking

System configuration
The system design was performed considering following two conditions; (1) As the maximum altitude of the test flight airship was designed to be up to 4.0 km, the maximum distance of optical feeder link is only a few km’s. (2) As the test flight vehicle is unmanned it assumes a clear visibility for a pilot at ground control center.
We used same optical antenna modules for both airship receiver and ground transmitter. The aperture the primary optical antenna is 4.0cm through which a 1.550-nm laser signal will be transmitted/received and a 980/970-nm beacon signal will be received. Four dedicated beacon transmitting collimators with a smaller (5mm) aperture are prepared at the perimeters of the primary antenna. An off-the-shelf two-axis gimbal assembly was used to point the optical system including the primary antenna and the four beacon collimators. The optical antenna was specially designed based on a three mirror telescope concept to have a wide imaging FOV in the acquisition sensor (CCD) port as well as a diffraction limited performance at the fine tracking and feeder link signal port. The mirrors are made with plastic (ZEONEX) and shaped by a numerical controlled machining. Internal layout of the antenna module is shown in Figure 1. The weight of the optical antenna module is 2.4 kg. Total wave front accuracy of the antenna module was measured to be less than 1/10 waves at 1550-nm wavelength.

Figure 7: Vehicle flight path and antenna pointing error

We used two EDFAs for both transmitter and receiver. The 1550-nm optical signal generated by a DFB laser is then intensity modulated by a Lithium Niobate (LN) external modulator and amplified by a transmitting EDFA to 100mW. Another EDFA was used at the receiver front to amplify and stabilize the receiving optical signal intensity. A DSP-based automatic level control (ALC) system was implemented to stabilize the amplitude of the signal component in the photo current from a InGaAs photo-detector. The configuration of the onboard optical receiver is shown in Figure 2.

Key technologies
Two key technologies to reduce the size and weight of optical terminals are implemented inside the optical antenna module. One is a fast steering mirror (FSM). It is very important to have a high performance fine-tracking system for a robust feeder link transmission because there are several sources of the jitters in the laser beam direction not only caused by vehicle basement and other mechanical vibration but also by the atmospheric turbulence. As the frequency spectrum of this jitter extends to a few 100Hz and higher frequency amplitude is less than 5 micro-radians, a wideband fine tracking system was used with a commercially developed miniature fast steering mirror (shown in Figure 3). The aperture of the mirror is 2.3mm x 2.8mm and the steering bandwidth is extended to 10 kHz. A closed loop servo bandwidth, combined with an analog PID controller, is 2 kHz. A typical pull-in performance of the fine tracking system is shown in Figure 4 for elevation angle. A large overshoot is appeared in the elevation tracking error, but, it settled within 1 ms.
Another key technology is a fiber coupler and a fine tracking sensor. It is difficult to focus and guide the incoming laser beam, which is captured by an optical antenna, into a single mode fiber (SMF). Near perfect alignment between the focused beam profile (always Airy profile) and the single-mode transmissible condition (near Gaussian profile) will be required to satisfy good receiving efficiency. A newly designed fiber coupler with tracking error sensing optics is introduced. Figure 5 shows the principle of its integration. If there is no difference in the refractive index between the grass ferrule and the clad of the single mode fiber, the light which is focused in front of the SMF face but not coupled to the transmissible mode will diverge from the focal point near the fiber face through the clad of the SMF and go straight in the glass ferrule maintaining the tracking error information, in this case, offset of the focal point from the center of the SMF face. There is no penalty in coupling efficiency for the signal beam at 1550-nm wavelength because there is one focusing lens in front of the SMF face. For the tracking error sensing, beacon laser wavelength at 970/980nm was used. All the glass surface including focusing lens, SMF face and surface of the glass ferrule are anti-reflective (AR) coated at each operating wavelength.

Result of flight experiment
Figure 6 shows the typical images of acquisition and tracking. These images were taken by the acquisition sensor (CCD camera) at the ground optical terminal. The left image shows that the ground terminal is tracking to the airship in a pre-acquisition phase and the right image is an example of beacon tracking. The FOV of the CCD camera was 1.8 degrees in diagonal and the beam divergence of the laser beacon is 2.0 degrees for uplink and 0.5 degrees for downlink. The CCD camera is already saturated in this case. We turned the fine tracking system and the feeder link transmission in this experiment, but, there was no indication of the fine tracking and signal transmission.
Figure 7 shows an example of the flight data analysis. The figure shows a 2km x 2km square area centered by the ground control center. Blue dots correspond to the vehicle flight path and the red dots show the antenna pointing direction which is calculated from the vehicle GPS position and attitude and angle of the two-axis gimbals. The variation of the red dots from the center of this figure correspond the error of initial acquisition direction. The error was within 0.5 degrees which is more than enough to start an initial acquisition.