A popular data transmission method for large-capacity communications is propagating a laser beam in free space. The high directivity of the coherent light reduces the propagation loss and provides strong irradiance at the receiving plane. The size of the optical antennas, or telescopes, that are used can therefore be much smaller than those used for propagating radio frequencies. In addition, the laser propagation technique provides a high data rate without frequency coordination, making installation of equipment simpler than for other methods.
An optical link between the transmitter and the receiver must be maintained during data communications. However, using a sharp beam sometimes causes difficulties. This is especially the case when the relative positions of the communicating terminals change dramatically. Precise control over the direction of beam emission from the transmitter is required, and light arriving at the receiver must be accurately tracked and acquired.1–3
The optical communication equipment we have designed has two functions for beam tracking so that we can achieve such angular accuracy. The first is a coarse pointing function that controls the direction of the optical antenna so that it remains in contact with the accompanying terminal. A second, fine pointing, function is contained within an inner optical setup and is used to remove the residual angular errors.
We conducted a demonstration of our laser communication technique using a moving car and a fixed ground station (as illustrated in Figure 1). We obtained a data rate of 40Gb/s at a wavelength of 1.5μm, over a distance of several hundred meters. We tested two different coarse pointing functions in our experiments. All of the optics in our moving terminal system, including the telescope and the fine pointing function, are installed within a ball-like container that also includes the coarse tracking function (see Figure 2). This terminal has been improved from an earlier version.3–5 The previous system was designed so that the terminal was suspended for installation, but our design enables the terminal to be installed upside down and increases its flexibility.
Figure 1. Setup for the demonstration. The optical terminal on the car transmits data to the optical terminal on the ground with a data rate of 40Gb/s at a wavelength of 1.5μm.
Figure 2. Optical terminal mounted on the car. All the optics, including the telescope and the fine pointing function, are installed within the ball-like container.
Figure 3 shows the second coarse pointing function we used in our tests as the fixed terminal. This version has a pair of mirrors place in front of a fixed optical telescope. The fine pointing function is also contained inside the terminal. The laser beam is emitted from the moving terminal and tracked to the fixed terminal simultaneously. We obtained video imagery of our demonstration using four cameras that shows even when the car turned sharply, the optical link between the two stations was maintained. This allowed us to investigate how the coarse and fine pointing functions operated. The optical terminal on the car moved through 30° at a maximum angular velocity of 1.8°/s during the test.
Figure 3. Optical terminal used as the fixed ground station. The coarse pointing function consists of a pair of mirrors placed in front of a fixed optical telescope. The fine pointing function is also contained within the terminal.
We have successfully demonstrated that our data transmission equipment can be used when installed on a moving vehicle. We now plan to use the optical terminal (see Figure 2) on an aircraft to transmit images, taken during a flight, to a ground station. This technique could potentially be useful in delivering large volumes of synthetic aperture radar or high-resolution imaging data acquired from aircraft or satellites.
Our demonstration was performed using equipment of the Japanese Ministry of Internal Affairs and Communications, which also funded an earlier version of the terminal.
Yoshihisa Takayama, Hideki Takenaka, Yoshisada Koyama, Hiroo Kunimori, Morio Toyoshima, Kohei Mizutani
National Institute of Information and Communications Technology
Motoaki Shimizu, Toshiaki Yamashita
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