A new optical beam tracking approach for free-space optical communication links using two-photon absorption (TPA) in a high-bandgap detector material was demonstrated. This tracking scheme is part of the canonical architecture described in the preceding article. 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 1,550 nm. As expected, the responsivity shows a linear dependence with incident power level. The responsivity slope is 4.5 × 10–7 A/W2. Also, optical beam spots from the 1,550-nm laser beacon were characterized on commercial charge-coupled device (CCD) and complementary metal-oxide semiconductor (CMOS) imagers with as little as 13.7 μW of optical power (see figure). 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 free-space optical communications transceivers.

Two-Photon Absorption generated signal levels caused by a 1,550 nm laser focused spot on a silicon CMOS focal plane array detector at various power levels. Note that the spot is distinguishable even with incident power levels in the 10’s of microwatts.

This work was done by Gerardo G. Ortiz and William H. Farr of Caltech for NASA’s Jet Propulsion Laboratory. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Physical Sciences category. NPO-46063

This Brief includes a Technical Support Package (TSP).
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Two-Photon-Absorption Scheme for Optical Beam Tracking

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This article first appeared in the January, 2011 issue of Photonics Tech Briefs Magazine.

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