Thomas M. Shay

High-sensitivity direct detection optical communication system that operates in sunlight

In freespace laser communication systems, optical background noise rejection is a very important issue. We have designed and built a building to building direct detection optical communication system that uses a Faraday Anomalous Dispersion Optical Filter (FADOF) in the receiver. The FADOF is a narrow bandpass optical filter, which can provide a background noise rejection of 105 approximately 106, while transmitting the signal with up to 80% efficiency. The FADOF also has a signal bandwidth that is variable between 0.5 GHz and 5 GHz, a field-of-view that is flat over +/- 20 degree(s). FADOFs offer new capabilities to freespace laser communication by effectively reducing the solar background radiation that reaches the photodetector. Using the FADOF receiver, we have demonstrated that 27nW of received signal power gives a bit error rate of 10-6 (limited by the photoreceiver electronic noise) independent of solar noise up to 0.15Watt. We also repeated these measurements under the same operation conditions after replacing the FADOF with an interference filter. The experiments showed three orders higher background noise rejection capability for the FADOF receiver.

400 Hz frequency stability of a GaAlAs laser frequency locked to the Rb (D2) line

A commercial GaAlAs injection laser is frequency locked to the Rb (D2) transition. A relative frequency stability of 400 Hz is measured for a 24-s averaging time. The frequency stability of this device with only temperature control is 4 kHz. This is a factor of 20 improvement in the state of the art for temperature control stability for semiconductor lasers.

Theoretical model for a background noise limited laser-excited optical filter for doubled Nd lasers

A simple theoretical model for the calculation of the dependence of filter quantum efficiency versus laser pump power in an atomic Rb vapor laser-excited optical filter is reported. Calculations for Rb filter transitions that can be used to detect the practical and important frequency-doubled Nd lasers are presented. The results of these calculations show the filters quantum efficiency versus the laser pump power. The required laser pump powers required range from 2.4 to 60 mW/cm/sup 2/ of filter aperture. >

Wide dynamic range Doppler-shift compensation for space-borne optical communications

We model and demonstrate a novel concept for the Doppler-shift compensation of optical transmitters. Optical frequency offset locking is demonstrated using a simple Fabry-Perot laser diode transmitter and a frequency-lock loop. Our system simultaneously demonstrates a continuous tracking range of 21 GHz and a tracking speed of 44 THz/s. The transmitter frequency is adjusted over the entire 21-GHz range by merely adjusting the frequency of a microwave oscillator.

Doppler shift compensation for spaceborne optical communication using a Faraday anomalous dispersion optical filter (FADOF)

Optical frequency offset-locking is demonstrated for application to Doppler shift compensation for a spaceborne optical communication system using a laser diode transmitter and a narrow band-pass filter with a small tuning range at the receiver. The system performs closed loop computer control of a laser diode output frequency and allows tuning of this frequency with respect to a reference. A tuning range of 21 GHz and a tuning speed of 44 THz/sec is demonstrated while maintaining a transmitting stability of <80 MHz. The theory modeling the system is shown to have agreement with the experimental results to within 5%.

A potassium Faraday anomalous dispersion optical filter

The characteristics of a potassium Faraday anomalous dispersion optical filter operating on the blue and near infrared transitions are calculated. The results show that the filter can be designed to provide high transmission, very narrow pass bandwidth, and low equivalent noise bandwidth. The Faraday anomalous dispersion optical filter (FADOF) provides a narrow pass bandwidth (about GHz) optical filter for laser communications, remote sensing, and lidar. The general theoretical model for the FADOF has been established in our previous paper. In this paper, we have identified the optimum operational conditions for a potassium FADOF operating on the blue and infrared transitions. The signal transmission, bandwidth, and equivalent noise bandwidth (ENBW) are also calculated.

Stark anomalous dispersion optical filter for doubled Nd:YLF lasers

The Stark anomalous dispersion optical filter is a wide-frequency-tunable ultra-narrow bandwidth optical filter. The first theoretical investigation of this filter, which matched wavelength with doubled Nd:YLF lasers for deepspace laser communications, is reported. The results show that the filter may provide about 86% transmission, 1.6 GHz bandwidth, 3 GHz noise equivalent bandwidth, and wide frequency tuning range.

Incorporation of a FADOF to an ESPI system

Electronic Speckle Pattern Interferometry (ESPI) has been used for many years in nondestructive testing applications in laboratories. Field applications of ESPI systems have been limited by the need to restrict the amount of light, sunlight and other sources, during operation. Interference filters and other techniques have been tried to increase the applicability of ESPI systems in daylight environments. Each of these attempts have been moderately successful. The FADOF (Faraday Anomalous Dispersion Optical Filter) is a revolutionary filter that improves throughput, field-of-view, and the signal-to-noise ratio of the laser signal returning from the test object. This paper describes the basics of a FADOF and how the filter can be incorporated into an ESPI system.

Simple low-loss technique for frequency-locking lasers

We report the results of a novel method for frequency stabilizing laser diodes based on the resonant Faraday effects. A Faraday cell with crossed Brewster windows comprises the intracavity frequency selective element. In this arrangement a laser pull-in range of 4A was measured and the laser operated at a single frequency with a linewidth less than 6 Mhz.

Simple technique for measuring the injection current-induced frequency deviation of semiconductor lasers

A simple but reliable method for measuring the injection-current induced frequency deviation of semiconductor lasers is presented. This technique can be used for modulation frequencies of the order of several hundred megahertz.(AIP)