Photoacoustic communications: delivering audible signals via absorption of light by atmospheric H2O.

We describe a means of communication in which a user with no external receiver hears an audible audio message directed only at him/her. A laser transmits the message, which is encoded upon a modulated laser beam and sent directly to the receiver's ear via the photoacoustic effect. A 1.9 µm thulium laser matched to an atmospheric water vapor absorption line is chosen to maximize sound pressure while maintaining eye-safe power densities. We examine the photoacoustic transfer function describing this generation of audible sound and the important operational parameters, such as laser spot size, and their impact on a working system.

Signal-to-noise ratio in squeezed-light laser radar.

The formalism for computing the signal-to-noise ratio (SNR) for laser radar is reviewed and applied to the tasks of target detection, direction finding, and phase-change estimation with squeezed light. The SNR for heterodyne detection of coherent light using a squeezed local oscillator is lower than that obtained using a coherent local oscillator. This is true for target detection, for phase estimation, and for direction finding with a split detector. Squeezing the local oscillator also lowers SNR in balanced homodyne and heterodyne detection of coherent light. Loss places an upper bound on the improvement that squeezing can bring to direct-detection SNR.

Optical detection of terahertz using nonlinear parametric upconversion.

We extend our work to perform sensitive, room-temperature optical detection of terahertz (THz) by using nonlinear parametric upconversion. THz radiation at 700 GHz is mixed with pump light at 1,550 nm in a bulk GaAs crystal to generate an idler wave at 1,555.6 nm. The idler is separated, coupled into optical fiber, and detected using a gated Geiger-mode avalanche photodiode. The resulting THz detector has a power sensitivity of 4.5 pW/Hz and a timing resolution of 1 ns.

Optical detection of terahertz radiation by using nonlinear parametric upconversion.

We describe and demonstrate sensitive room-temperature detection of terahertz (THz) radiation by nonlinearly upconverting terahertz to the near-infrared regime, relying on telecommications components. THz radiation at 700 GHz is mixed with pump light at 1550 nm in a bulk GaAs crystal to generate an idler wave at 1555.6 nm, which is separated and detected by using a commercial p-i-n diode. The THz detector operates at room temperature and has an intrinsic THz-to-optical photon conversion efficiency of 0.001%.

Squeezing the local oscillator does not improve signal-to-noise ratio in heterodyne laser radar.

The signal-to-noise ratio for heterodyne laser radar with a coherent target-return beam and a squeezed local-oscillator beam is lower than that obtained using a coherent local oscillator, regardless of the method employed to combine the beams at the detector.