William S. Rabinovich

Large-aperture multiple quantum well modulating retroreflector for free-space optical data transfer on unmanned aerial vehicles

We describe progress in the development of a multiple quan- tum well modulating retroreflector, including a description of recent dem- onstrations of an infrared data link between a small rotary-wing un- manned airborne vehicle and a ground-based laser interrogator using the device designed and fabricated at the Naval Research Laboratory (NRL). Modulating retroreflector systems couple an optical retroreflector, such as a corner cube, and an electro-optic shutter to allow two-way optical communications using a laser, telescope, and pointer-tracker on only one platform. The NRL modulating retroreflector uses a semiconductor-based multiple quantum well shutter capable of modula- tion rates greater than 10 Mbps, depending on link characteristics. The technology enables the use of near-infrared frequencies, which is well known to provide covert communications immune to frequency allocation problems. This specific device has the added advantage of being com- pact, lightweight, covert, and requires very low paper. Up to an order of magnitude in onboard power can be saved using a small array of these devices instead of the radio frequency equivalent. In the described dem- onstration, a Mbps optical link to an unmanned aerial vehicle in flight at a range of 100 to 200 feet is shown. Near real-time compressed video was also demonstrated at the Mbps level and is described.

Thermo-optic tuning and switching in SOI waveguide Fabry-Perot microcavities

Compact silicon-on-insulator (SOI) waveguide thermo-optically tunable Fabry-Perot microcavities with silicon/air Bragg mirrors are demonstrated. Quality factors of Q=4,584 are measured with finesse F=82. Tuning is achieved by flowing current directly through the silicon cavity resulting in efficient thermo-optic tuning over 2 nm for less than 50 mW applied electrical power. The high-Q cavities enable fast switching (1.9 mus rise time) at low drive power (<10 mW). By overdriving the device, rise times of 640 ns are obtained. Various device improvements are discussed.

Persistent photoconductivity in n-type GaN

Persistent photoconductivity has been observed in n-type GaN:Si. The effect is seen at room temperature in both nonoptimally grown films as well as in device quality channel layers. The relaxation dynamics are found to agree with a stretched exponential model of recovery. A comparison between different samples, based upon stretched exponential parameters, Hall measurements, and photoluminescence data is made. The data suggest that the cause of persistent photoconductivity is the same among the different samples and that there is a transition in the relaxation dynamics between room temperature and 130 °C.

Free-space optical communications link at 1550 nm using multiple-quantum-well modulating retroreflectors in a marine environment

A 1550-nm eye-safe, free-space optical communications link is demonstrated at rates up to 5 Mbits/s over a distance of 2 km in the Chesapeake Bay, using quantum-well-based modulating retroreflectors. Tests are conducted under various atmospheric conditions over a time period of about a year. The experimental and theoretical link budgets are compared and statistical measurements of the effects of scintillation are collected.

Integrated waveguide Fabry-Perot microcavities with silicon/air Bragg mirrors.

We demonstrate in-plane microfabricated Fabry-Perot cavities with cryogenically etched silicon/air distributed Bragg reflector (DBR) mirrors and integrated silicon-on-insulator rib waveguides. Several DBR configurations and cavity lengths were measured. Various devices exhibit Q=26963, FWHM=0.060 nm, finesse F=489, free spectral range FSR=81.7 nm, and DBR mirror reflectance R=99.4%. Thermo-optic tuning over 6.7 nm is also demonstrated.

Spatial hole burning effects in distributed feedback lasers

The effects of spatial hole burning in a steady-state distributed feedback (DFB) laser are examined by numerically solving the coupled mode equations that describe the system. An approximate solution for the gain above threshold is derived and compared to the exact solution. It is shown that the self-induced grating that arises due to spatial hole burning significantly reduces the mode discrimination of index-coupled DFB lasers. This makes it difficult for these lasers to maintain single-longitudinal-mode behavior above threshold. However, it is found in addition that bulk-modulated (gain-coupled) DFB lasers do not lose their mode selectivity above threshold, indicating that these lasers may be better choices for narrow-linewidth operation. >

High-Current Photodetectors as Efficient, Linear, and High-Power RF Output Stages

Recent progress in high-current photodiodes now makes it possible to efficiently generate over 26 dBm of RF power directly from the output of a photodiode. This paper describes two photodetector designs which demonstrate excellent large- and small-signal behavior. Maximum small-signal compression currents have increased to over 700 mA at 300 MHz. Output RF power amplifier stage efficiencies of over 45% (class AB operation) and 35% (class A) have been achieved from 0.3 to 6 GHz with RF power outputs over 26 dBm. The linearity figure of merit (LFOM) is also shown to be greater than 50, leading to the possibility of implementing very high linearity RF power amplifiers in the future.

A cat's eye multiple quantum-well modulating retro-reflector

A new kind of modulating retro-reflector using cats eye optics and a multiple quantum-well electro-absorption modulator array is described. The device exhibits retro-reflection over a 30/spl deg/ field of view and can support data rates of up to 50 Mb/s using 1-mm pixels. The use of the device in free-space optical communication is discussed.

Design and analysis of a diffraction-limited cat's-eye retroreflector

We describe a design for a modified, cats-eye retroreflector. The design is catadioptric, containing a single concave mirror and several lenses. This retroreflector design exhibits a unique combination of performance characteristics. It is diffraction limited over a large field angle while operating with a large aperture and numerical aperture. There is little vignetting of the optical beam, even at large field angles, providing good light return at all angles of incidence. It brings the light beam to a focus and allows access to the light near the focal plane, thus decoupling the size of the aperture from the size of devices used with the retroreflector. This final feature makes the design appealing for use with spatial light modulators, or other optical or electro-optical components.

Intrinsic multiple quantum well spatial light modulators

Large improvements are reported in the sensitivity of optically addressed multiple quantum well spatial light modulators. In prior work with these materials the quantum well region has been made semi‐insulating. It is shown that this is unnecessary and in fact detrimental to performance. By placing layers containing high trap concentrations at the ends of the structure and leaving the active quantum well layers intrinsic the speed of the device at a given illumination is improved by more than four times, diffraction efficiency is enhanced and spatial resolution is almost the same.