Scalable modulator for frequency shift keying in free space optical communications.

Frequency shift keyed (FSK) modulation formats are well-suited to deep space links and other high loss links. FSK's advantage comes from its use of bandwidth expansion. I.e., FSK counteracts power losses in the link by using an optical bandwidth that is greater than the data rate, just as pulse position modulation (PPM) does. Unlike PPM, increasing FSK's bandwidth expansion does not require increased bandwidth in electronic components. We present an FSK modulator whose component count rises logarithmically with the bandwidth expansion. We tested it with four-fold bandwidth expansion at 5 and 20 Gbit/s. When paired with a pre-amplified receiver, the required received power was about 4 and 5 dB from the theoretical best for such receivers. We also tested the FSK transmitter with a photon counting receiver.

Laser vibrometry from a moving ground vehicle.

We investigated the fundamental limits to the performance of a laser vibrometer that is mounted on a moving ground vehicle. The noise floor of a moving laser vibrometer consists of speckle noise, shot noise, and platform vibrations. We showed that speckle noise can be reduced by increasing the laser spot size and that the noise floor is dominated by shot noise at high frequencies (typically greater than a few kilohertz for our system). We built a five-channel, vehicle-mounted, 1.55 µm wavelength laser vibrometer to measure its noise floor at 10 m horizontal range while driving on dirt roads. The measured noise floor agreed with our theoretical estimates. We showed that, by subtracting the response of an accelerometer and an optical reference channel, we could reduce the excess noise (in units of micrometers per second per Hz(1/2)) from vehicle vibrations by a factor of up to 33, to obtain nearly speckle-and-shot-noise-limited performance from 0.3 to 47 kHz.

Photon-counting 1.55 microm optical communications with pulse-position modulation and a multimode upconversion single-photon receiver.

We report single-photon frequency upconversion of multimode light from 1.55 to 0.532microm and demonstrate an end-to-end optical link that emulates photon-starved communications and atmospheric propagation. Using 64-ary pulse-position modulation and a half-rate serially concatenated turbo code we achieve a decoded efficiency of 0.3 detected photons/bit.

Contributed Review: Advanced three-dimensional laser radar imaging with the airborne optical systems testbed.

We describe the development of a state-of-the-art airborne 3D laser radar system capable of area coverage rates in excess of 150 km2/h. The ladar testbed can operate day and night and uses a low-power, eye-safe, and covert near-IR laser transmitter and a large-pixel-format photon-counting detector array coupled with a precision, fast-scanning, beam pointing mechanism. Mission areas include wide-area down-looking high-resolution open terrain and foliage-penetrating imaging, side-looking and up-looking laser ranging and tracking, and sensor fusion with electro-optical infrared cameras and hyperspectral payloads. We describe the airborne optical systems testbed ladar system, present recently collected 3D data, and discuss testbed configurations that can support advanced imaging applications.

Two-photon coincident-frequency entanglement via extended phase matching.

We demonstrate a new class of frequency-entangled states generated via spontaneous parametric down-conversion under extended phase-matching conditions. Biphoton entanglement with coincident signal and idler frequencies is observed over a broad bandwidth in periodically poled KTiOPO4. We demonstrate high visibility in Hong-Ou-Mandel interferometric measurements under pulsed pumping without spectral filtering, which indicates excellent frequency indistinguishability between the down-converted photons. The coincident-frequency entanglement source is useful for quantum information processing and quantum measurement applications.

Efficient single-photon counting at 1.55 microm by means of frequency upconversion.

We demonstrate efficient single-photon detection at 1.55 microm by means of sum-frequency mixing with a strong pump at 1.064 microm in periodically poled lithium niobate followed by photon counting in the visible region. This scheme offers significant advantages over existing InGaAs photon counters: continuous-wave operation, higher detection efficiency, higher counting rates, and no afterpulsing. We achieved single-photon upconversion efficiency of 90% at 21.6 W of circulating power in a resonant pump cavity with a 400-mW Nd:YAG laser. We observed high background counts at strong circulating pump powers due to efficient upconversion of pump-induced fluorescence photons.

Efficient generation of tunable photon pairs at 0.8 and 1.6 microm.

We demonstrate efficient generation of collinearly propagating, highly nondegenerate photon pairs in a periodically poled lithium niobate cw parametric downconverter with an inferred pair generation rate of 1.4x10(7)/s/mW of pump power. Detection of an 800-nm signal photon triggers a thermoelectrically cooled 20%-efficient InGaAs avalanche photodiode for the detection of the 1600-nm conjugate idler photon. Using single-mode fibers as spatial mode filters, we obtain a signal-conditioned idler-detection probability of ~3.1%.

Three-dimensional imaging laser radar with a photon-counting avalanche photodiode array and microchip laser.

We have developed a threedimensional imaging laser radar featuring 3-cm range resolution and single-photon sensitivity. This prototype direct-detection laser radar employs compact, all-solid-state technology for the laser and detector array. The source is a Nd:YAG microchip laser that is diode pumped, passively Q-switched, and frequency doubled. The detector is a gated, passively quenched, two-dimensional array of silicon avalanche photodiodes operating in Geigermode. After describing the system in detail, we present a three-dimensional image, derive performance characteristics, and discuss our plans for future imaging three-dimensional laser radars.