Two-Photon Absorption Long-Wavelength Optical Beam Tracking

�A new optical beam tracking approach for free-space optical communication links using two-photon absorption (TPA) in a high-bandgap detector material was investigated. TPA is used to track a long-wavelength transmit laser while direct absorption on the same sensor simultaneously tracks a shorter-wavelength beacon. The TPA responsivity was measured for silicon using a PIN photodiode at a laser beacon wavelength of �550 nm. As expected, the responsivity shows a linear dependence with incident power level. The responsivity slope is 5E-�2 A/W/W/cm 2 . Also, optical beam spots from the �550-nm laser beacon were characterized on commercial charge-coupled device (CCD) and complementary metal-oxide semiconductor (CMOS) imagers with as little as �30 nW of optical power. This new tracker technology offers an innovative solution to reduce system complexity, improve transmit/ receive isolation, improve optical efficiency, improve signal-to-noise ratio (SNR), and reduce cost for a free-space optical communications transceiver.