Nonlinear optics for free-space laser communications

Free-space laser transmission represents a promising option for communication, for example, in optical intersatellite links (OISLs). However, precisely locating the communicating parties and tracking the signal beams continue to be major challenges. The narrow emission angle of lasers demands micro-radian pointing precision. Without adequate beam pointing and target-tracking precision, a link can easily be lost due to such factors as orbit uncertainty and satellite jitter.¹

Traditional optical communication systems employ mechanically steered mirrors to point and track laser beams. The standard approach has been to use a corner mirror for beam return and a two-stage (coarse-fine) dual-detector concept. This approach, however, is complex, involves moving parts, and limits the efficiency with which the signal is fed to the terminal's fiber.

A different, ‘all-optical’ approach to beam control uses nonlinear optical materials to locate a distant, moving counterpart and to point the communication signal at the detected recipient.^2,3 This concept allows automatic coupling of emitters and receivers (e.g., optical fibers or telecommunication satellite antennas) using a single optical element. It also eliminates the need for ultraprecise mechanical steering. The system provides fully automated, continuous, direct- and return-beam tracking between communicating parties.

This has become possible with so-called double phase conjugation (DPC), shown in Figure 1. DPC has been extensively studied both theoretically and experimentally in materials with photorefractive nonlinearity.⁴ But telecommunication applications require fast, millisecond-scale response time of a nonlinear material sensitive in the 1550nm range. Liquid crystals, with their extraordinarily high thermal and orientational nonlinearities (see Figure 2), were proposed for this application at the Canadian Space Agency (CSA) and have been the only materials known so far to satisfy the relevant requirements.⁵

CSA is now studying the feasibility of this novel technique for both intersatellite and satellite-to-ground optical communications. The OISL scenario is illustrated schematically in Figure 3. Each satellite is equipped with identical optical terminals. First, the data-modulated optical signal to be transmitted is merged with the beacon beam, resulting in a summation beam (s) in the fiber interface. Then, both beams are sent into the nematic liquid crystals, where the DPC-induced hologram directs the signals toward the respective communication counterparts. Beacon beams form gratings at each end of the link. Since each grating is rewritten in real time by every beacon pulse, it tracks the relative positions of the two satellites, automatically linking them and precisely pointing the signal beam to the targeted receiver.

Unlike classical optomechanical beam steering, this all-optical method does not merely redirect the beam but redistributes the angular directivity pattern into a much narrower angular spread, pointing the beam precisely to remote recipients (see Figure 4). The current CSA system requires only a few megawatts of optical power and shows a millisecond-range response time.

To date, nonlinear optical tracking has been successfully demonstrated in the laboratory. Nonlinear beam control and fine tracking can be efficiently used in a variety of applications, including high-speed free-space optical links between fast-moving parties (ground and marine vehicles, aircraft, satellites, and so on), optical-cable-cross precise beam addressing, and precision aiming at other types of targets.