Mitsubishi Electric Corp. developed a prototype of an optical receiver for use in laser communication terminals (LCTs). The company claimed that the prototype is the first to integrate space optical communication using laser beams and a function to detect the direction of received beams in the 1.5-μm band, a general purpose band used for terrestrial optical fiber communications and other applications.
High-resolution satellite imagery can be used to assess damage caused by disasters, but since such images are transmitted via radio waves it has been difficult to transmit high-resolution images in real time due to limitations in data capacity and the size of satellite antennas, an announcement from Mitsubishi Electric said. As a result, large-capacity, high-speed space optical communications that do not require optical fiber are required to support fast and accurate damage assessments following disasters. However, space optical communications use very narrow laser beams, about one-thousandth of that of radio waves. It has been a challenge to determine how to precisely align laser beams with satellites traveling at high speed.
The optical receiver integrates functions to detect both four phase changes of laser light and beam direction to enable space optical communication with 10× the speed, capacity, and distance of radio-wave communication. Since the wavelength is much shorter, smaller antennas can be used in compact communication units that can be installed in locations difficult for optical fiber, such as between buildings. Installations also are possible in areas where normal infrastructure is not available, such as disaster zones, developing countries, or remote areas, thereby expanding the use of wireless communications in a variety of situations.
Mitsubishi’s optical receiver prototype for laser communication terminal. Courtesy of Mitsubishi Electric.
The photoelectric converter of the system, which receives laser light and converts it into electrical signals, is divided into four segments, and the direction of received laser beams is detected by comparing the output signal intensity of each segmented element. The dedicated sensor for detecting beam directions in conventional systems is no longer necessary. The small optical receiver integrates functions for space optical communication and laser angle-of-arrival detection in a photodetector.
The prototype detects four phase changes of laser light and beam direction, using an optical circuit on a small 5-cm × 5-cm glass substrate with two photodetectors mounted to a single printed circuit board. The optical circuit detects 0 °F, 90 °F, 180 °F, and 270 °F, in contrast to conventional two-phase detection that detects 0 °F and 180 °F. The beam direction function eliminates the need for a dedicated sensor. The entire system is contained in a 10-cm3 lightweight module.
The coherent detection method enables communication even with weaker laser beams compared to the conventional method of detecting intensity changes due to beams turning on and off, thereby enabling communication over longer distances using the same laser-beam intensity. Additionally, the coherent method is less influenced by sunlight and other background light for more stable communications.
The photodetectors’ beam-direction detection function additionally eliminates the need for a dedicated sensor. Furthermore, the optical circuit is contained on a small 5-cm × 5-cm glass substrate, mounting two photodetectors on a single printed circuit board. The single-module configuration realizes a lightweight optical receiver measuring just 10 cm3, less than one-fourth the size of Mitsubishi Electric's previous model.