Scott R. Henion

Power penalty for optical signals due to dispersion slope in WDM filter cascades

We calculate the receiver power penalty due to the accumulated dispersion slope of optical filter cascades in wavelength-division-multiplexed optical networks. For flat-top fiber Bragg grating and thin-film filters, dispersive rather than amplitude filter effects dominate the cascade power penalty, which increases with dispersion slope and signal frequency detuning from the cascade zero-dispersion point. The dispersion slope of these filters varies as the inverse cube of their bandwidth, using increased penalties for narrow filters in systems with small channel spacing, For a 60-GHz-wide Bragg grating filter with a 1.3-ns/nm/sup 2/ dispersion slope, a cascade of 26 filters can be tolerated at 10 Gbit/s with /spl plusmn/5 GHz allowed detuning. Optically preamplified receivers have significantly larger dispersion penalties than thermal-noise-limited receivers.

Architecture overview and data summary of a 5.4 km free-space laser communication experiment

MIT Lincoln Laboratory designed and built two free-space laser communications terminals, and successfully demonstrated error-free communication between two ground sites separated by 5.4 km in September, 2008. The primary goal of this work was to emulate a low elevation angle air-to-ground link capable of supporting standard OTU1 (2.667 Gb/s) data formatting with standard client interfaces. Mitigation of turbulence-induced scintillation effects was accomplished through the use of multiple small-aperture receivers and novel encoding and interleaver hardware. Data from both the field and laboratory experiments were used to assess link performance as a function of system parameters such as transmitted power, degree of spatial diversity, and interleaver span, with and without forward error correction. This work was sponsored by the Department of Defense, RRCO DDR&E, under Air Force Contract FA8721-05-C-0002. Opinions, interpretations, conclusions and recommendations are those of the authors and are not necessarily endorsed by the United States Government.

Diode-pumped composite-cavity electrooptically tuned microchip laser

A diode-pumped composite-cavity electrooptically tuned microchip laser has been constructed that tunes up to 30 GHz. The laser incorporates a LiTaO/sub 3/ electrooptic tuning element with acoustic damping to provide linear voltage-to-frequency conversion with a tuning sensitivity of approximately=14 MHz/V at modulation frequencies from DC to >1.3 GHz.<<ETX>>

Air-to-ground lasercom system demonstration

This article presents an overview of the air to ground lasercom demonstration performed under the Free-Space Optical Communications Airborne Link (FOCAL) program. Techniques used to mitigate fading, demonstrated in 2008 and reported earlier, are reviewed as are the basic equipment approach, also reported earlier. We overview the new results for tracking, fiber coupling, channel measurements and communications performance for the air-to-ground link. This work was sponsored by the Department of Defense, RRCO DDR&E, under Air Force Contract FA8721-05-C-0002. Opinions, interpretations, conclusions and recommendations are those of the authors and are not necessarily endorsed by the United States Government.

Electrooptic phased array transmitter

An optically controlled transmit phased array was designed and a four-element linear array was built using integrated optics for beamforming and fiber optics for flexible remoting of the antenna. Electrooptic phase shifters remote from the antenna allowed simple electronic control of beam pointing. This phased array transmitter operated at 10 GHz with an effective isotropic radiated power (EIRP) of 14 dBW. The radiated power corresponded to approximately 0.4 W per element. The transmitted beam was electronically steered to any angle between -30/spl deg/ and 30/spl deg/ in less than 5 /spl mu/s. The measured antenna patterns agreed well with theory.

Differential Phase-Shift Keying in Spatial Diversity Transmitters for Fade Mitigation

We investigate the use of differential phase-shift keying (DPSK) in multiwavelength spatial diversity transmitters for mitigation of atmospheric fading. By selecting the transmitter wavelengths to coincide with the transmission peaks of the delay-line interferometer in the receiver, the only modification required to a standard DPSK receiver is a wider optical filter. We examine the wavelength separation required to minimize penalties from beating between the wavelengths and find that a separation of twice the data rate is sufficient for a four-wavelength system with narrow filtering. We also demonstrate a reduction in scintillation loss for a four-wavelength DPSK system in a fading channel.

Dispersion-induced power penalty in fiber-Bragg-grating WDM filter cascades

In wavelength-division-multiplexed optical networks signals pass through cascades of multiplexing/demultiplexing filters. We present experimental measurements and theoretical modeling of transmission power penalty at 10 Gbps due to dispersion slope of fiber-Bragg-grating and thin-film cascades.

Performance and qualification of a multi-rate DPSK modem

Recently, we demonstrated a multi-rate DPSK modem with high-sensitivity over a wide dynamic range, which can significantly benefit performance and cost of NASA’s Laser Communication Relay Demonstration. This increased flexibility, combined with the need to verify robust operation under challenging free-space environmental conditions, results in a large number of operational states which must be accurately and thoroughly tested. To support this, we developed test and diagnostic capabilities that can be easily reconfigured to assess modem performance across a wide range of data rates and operational modes. These capabilities include internal self-test modes in which test waveforms can be directed from the transmitter into the receiver to determine modem communications performance. We used these self-test capabilities to demonstrate robust performance in realistic environments during thermal-vacuum, shock/vibration, and EMI/EMC testing.

Adaptive wideband optical nulling for an antenna system

Nulling is a technique to reduce unwanted interference by selecting specifically against some characteristic of the interference. In the case of an antenna with several subapertures or beams, spatial discrimination against the interference is implemented through appropriate complex weighting of the antenna outputs prior to combining to make the net antenna sensitivity orders of magnitude lower in the direction of the interference. This paper describes how optics has been used to achieve a wide- bandwidth (2 GHz) null centered at 6 GHz for the case of two antenna outputs. The broadband null-depth is approximately 40 dB over the 5 to 7 GHz RF band. Such a deep null over this wide band could not be achieved using microwave technology alone.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Differential Phase-Shift Keying in Multi-Wavelength Spatial Diversity Links

We investigate the use of differential phase-shift keying in multi-wavelength spatial diversity transmitters that mitigate atmospheric fading.