Nonlinear optics for free-space laser communications

Alexander Dudelzak
2007 SPIE Newsroom  
Nonlinear optical materials provide precision beam pointing and tracking in 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 targettracking precision, a
more » ... cking 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 Figure 1. In double phase conjugation (DPC), two incoherent beams intersect in a nonlinear optical material, where stimulated scattering of both beams induces a dynamic holographic grating. The hologram conjugates the wavefronts of the incident beams, resulting in phaseconjugated (PC) beams redirected precisely toward each other's source. Thus, the two sources are automatically linked, regardless of their relative angular positions Figure 2. Holographic grating recording is based on a nematic liquid crystal (NLC) cell sensitized at 1.5µm using an absorbing indium-tin oxide (ITO) coating. The E 1 and E 2 beams record a hologram in the cell. The solid purple curve schematically shows the resulting thermal grating diffused by the ITO coating into the NLC. CSA is now studying the feasibility of this novel technique for both intersatellite and satellite-to-ground optical communi-Continued on next page
doi:10.1117/2.1200702.0545 fatcat:sqztmkkpfbeozf5a4z3kilglcy