Sumanth Kaushik

NLOS UV communication for distributed sensor systems

Atmospheric scattering of ultraviolet light is examined as a mechanism for short-range, non-line-of-sight (NLOS) communication between nodes in energy-constrained distributed sensor networks. The physics of scattering is discussed and modeled, and progress in the development of solid state sources and detectors is briefly summarized. The performance of a representative NLOS UV communication system is analyzed by means of a simulation model and compared to conventional RF systems in terms of covertness and transceiver power. A test bed for evaluating NLOS UV communication hardware and modulation schemes is described.

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%.

Amplification in Ni-like Nb at 204.2 Å pumped by a table-top laser

We identified for the first time the 3d94d1S − 3d94p1P line in Ni-like Nb at 204.2 Å that was predicted to show gain. When pumped with a train of pulses containing less than 1 J per pulse, significant emission was recorded at 204.2 Å following the second and the third pulses. We measured the small signal gain coefficient per Joule of incident laser energy to be 1.49±0.42 cm−1 J−1 for this laser transition, which is higher by several orders of magnitude than that reported for other collisional laser systems in this wavelength range.

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.

Terahertz Interferometer That Senses Vibrations Behind Barriers

A terahertz sensor that measures vibrations behind optically opaque barriers (cardboard, plastic, wool, cotton) is described and demonstrated. Using interferometric techniques, submicrometer displacements can be resolved. Measured spectral response agrees with commercial vibrometer

MIT/LL development of broadband linear frequency chirp for high-resolution ladar

The development of a high-resolution laser radar (ladar) exhibiting sub-mm resolution would have a great impact on standoff identification applications. It would provide biometric identification capabilities such as three-dimensional facial recognition, interrogation of skin pore patterns and skin texture, and iris recognition. The most significant technical challenge to developing such a ladar is to produce the appropriate optical waveform with high fideltiy. One implementation of such a system requires a 1.5-THz linear frequency sweep in 75 &mgr;s. Previous demonstrations of imaging with such waveforms achieved a 1 THz sweep in > 100 ms, and required additional corrections to compensate for sweep nonlinearity. The generation of high fidelity, temporally short frequency-swept waveforms is of considerable interest to the DoD community. We are developing a technique that utilizes a novel method to generate a 1 THz sweep in 50 &mgr;s from a mode-locked laser. As a proof-of-principle demonstration of this technique we have successfully generated a 20 GHz sweep in 1 µs with a fidelity sufficient to produce better than -20 dB sidelobes for a range measurement without using any additional corrections. This method is scalable to produce the entire 1 THz sweep in 50 &mgr;s.

Ultrasensitive, Room Temperature Detection of THz Radiation Using Nonlinear Parametric Conversion

We demonstrate ultrasensitive, room temperature optical detection of terahertz by using nonlinear parametric upconversion. Terahertz radiation is mixed with pump light at 1550 nm in quasi-phase-matched GaAs crystal to generate an optical sideband or idler wave that is coupled into optical fiber and detected using a Geiger-mode APD. The resulting terahertz detector has a noise equivalent power of 78 fW/Hz1/2 with a timing resolution of 1 ns.

Observation of gain in a recombining H-like boron plasma

We have observed gain at 26.2 nm in a laser-produced, recombining boron plasma. We conservatively estimate the gain for the B v 3d−2p transition to be 3.7 ± 0.2 cm−1, yielding a gain–length product of 3.0. The highest value of gain was measured 400 μm from the solid target surface and was obtained with the use of a steel blade. The pump-laser power was 1.1 J in 60 ps.

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.

A semiempirical line shape model of GaAs MQW structures

We have developed a semiempirical model to describe the line shape in the vicinity of the exciton absorption lines in GaAs multiple quantum well (MQW) structures. This model is based on a conventional line-broadening analysis similar in spirit to that used in atomic and plasma physics. The absorption spectrum is determined through the line positions, oscillator strengths, homogeneous and inhomogeneous broadening, and continuum shifts. Values for these parameters are derived from a combination of theoretical models, some existing in the literature and other developed by the authors, and experimental data where theory is lacking. Our results are in good agreement with the absorption measurements and the nonlinear coefficients available in the literature. This model shows improvements in the nonlinearity at low temperature and small inhomogeneous linewidth, which is as expected. >