Scott H. Bloom

Ultranarrow line filtering using a Cs Faraday filter at 852 nm

To achieve quantum-noise-limited performance, background-limited laser receivers require narrow-band optical filters. We measured and modeled the ultranarrow-band transmission spectrum of a Cs Faraday filter at 852 nm. The transmission spectrum consisted of passbands on either side of the 6 (2)S((1/2))-6 (2)P(3/2) hyperfine doublet lines, making a total of four. The passbands may be simple peaks or highly modulated, depending on the operating parameters. We observed peaked passbands of near-unity transmission with a 0.6-GHz bandwidth and modulated bands with features as sharp as 100 MHz. Excellent agreement with our calculations at 852 nm allows us to predict confidently a 0.7-GHz transmission band for Cs at 455 nm.

Long-range, noncoherent laser Doppler velocimeter

An experimental demonstration of a long-range, noncoherent laser Doppler velocimeter (LDV) is presented. The LDV detects incoming Doppler-shifted signal photons by using the sharp spectral absorption features in atomic or molecular vapors. The edge of the absorption feature is used to convert changes in frequency to large changes in transmission. Preliminary measurements of wind velocity using seeded aerosols showed that the LDV results agreed with mechanical anemometer measurements to within the accuracy of the LDV measurements. With optimization the LDV will provide accurate range-resolved and vibration-tolerant wind-speed measurements at large distances.

Helicopter plume detection by using an ultranarrow-band noncoherent laser Doppler velocimeter

An experimental demonstration of helicopter plume detection by using a range-resolved, narrow-bandwidth, noncoherent laser Doppler velocimeter is presented. The laser Doppler velocimeter detects incoming Doppler-shifted signal photons by using the sharp spectral transmission features in a Faraday magneto-optic atomic line filter.

Effect of aperture averaging on a 570-Mbps 42-km horizontal path optical link

Optical communications offer high data rate satellite to ground communications in a small, low mass, and low power consumption package. However, turbulence-induced scintillation degrades the link performance as the zenith angle increases. To investigate the effect of atmospheric turbulence on the optical link at high zenith angles, we performed a 570 Mbps optical communications link across a 42 km horizontal path, and have measured the effects of aperture averaging on the irradiance variance. The variance clearly showed a dependence on the aperture size, decreasing with increasing aperture size. These results were used to calculate the log-amplitude variance and the atmospheric structure constant, Cn2, across the link. The bit error rates across the link were also measured. The results show that the link performance was dominated by burst errors with error rates that ranged from 10-6 to 10-2, increasing with decreasing aperture size.

Status of BMDO/IST lasercom advanced technology demonstration

Laser communications between satellites, high flying aircraft (such as JSTARS), and the ground offer the potential to transfer extremely high amounts of information faster and with a much smaller package than is possible using current radio frequency and microwave technologies. This can be especially important in downlinking time sensitive satellite reconnaissance information because the satellite stays within range of a ground station or aircraft for only a few minutes. A capability to downlink from a satellite to an aircraft can provide all weather performance, and multiple data transfers for every satellite orbit. Over the last few years, SDIO (now BMDO) has funded a number of technology efforts through the US Army Space and Strategic Defense Command reducing the risks associated with laser communications. This paper describes one of these efforts which is now being carried forward to an Advanced Technology Demonstration at ThermoTrex Corporation. The program will lead to the demonstration of high data rate communications of 270 MBPS (Mega Bits Per Second) to 1.08 GBPS (Giga Bits Per Second) between high altitude aircraft and possibly between a satellite and the ground. The Laser Communications Terminals incorporate Atomic Line Filter technology for background light rejection during acquisition, reactionless Roto-Lok offset cable drive gimbals for fast slewing and high accuracy pointing, and direct digital modulation of semiconductor diode lasers detected with low noise avalanche photodiodes. We present designs and preliminary performance results for both a simplified terminal appropriate for a near term satellite-to-ground data transfer experiment, and a full capability terminal appropriate for ground, aircraft, or satellite operations.

Status of SDIO/IS&T lasercom testbed program

Laser satellite networking is a key element of effective communications operations to support both strategic and tactical missions. Lasercom offers a number of important advantages over conventional RF satellite communications. The shorter wavelength available using lasers provides higher data rates at less power and smaller apertures, both resulting in lower weight requirements. On the other hand, lasercom entails more difficulty in acquisition and tracking because of the narrow beams used. Technology problems to be overcome before intersatellite laser communications can reliably outperform RF communications include acquisition in the presence of significant background light from the earth, tracking to resolutions of a few microradians, high speed modulation of semiconductor lasers with close to one watt of power in a diffraction limited beam, high bandwidth low noise detector response, and demonstrated long term performance. We have developed critical technologies to solve some of these problems, and demonstrated them in a laboratory testbed which also supports development and testing of network protocols and algorithms. Our hardware provides new capability in background light rejection by using innovative atomic line filter technology, improved tracking accuracy by using innovative zero backlash Roto-Lok drive gimbal telescopes, and increased communications bandwidth by incorporating multi-link networking protocols.

Results of 150-km, 1-Gbps lasercom validation experiment using aircraft motion simulator

Laser communications between high flying aircraft such as high altitude unmanned aerial vehicles and between the aircraft and the ground (weather permitting) offers the potential to transfer extremely high amounts of information faster and with a smaller package than is currently possible with a radio frequency and microwave technologies. Over the last few years, BMDO has funded a number of technology efforts through the U.S. Army Space and Strategic Defense Command reducing the risks associated with laser communications. One of these efforts, at ThermoTrex Corporation in San Diego, California, is now being carried forward towards an advanced technology demonstration. The program will lead to the demonstration of high data rate communications of 274 Mbit/s to 1.2 Gbit/s between high altitude aircraft and between a satellite and the ground. To identify and further reduce the risks associated with operating the lasercom system on a high flying aircraft, a demonstration of a long range link in the presence of simulated aircraft motion was performed. Link acquisition utilizes an interface to an inertial navigation unit for initial pointing and atomic line filter technology for background light rejection. In addition, data was taken on the effects of atmospheric turbulence on the intensity of received laser light across the 150 km path. We present the development of the tracking system and results of the experiment performed at Mauna Loa, Hawaii and Haleakala, Maui in May and September 1995.

High-elevation terrestrial validation of Ballistic Missile Defense Organization (BMDO) lasercom system at 1.1 Gbit/s

Laser communications between high flying aircraft such as high altitude Unmanned Aerial Vehicles and between the aircraft and the ground (weather permitting) offers the potential to transfer extremely high amounts of information faster and with a much smaller package than is possible using current radio frequency and microwave technologies. This can be especially important in transferring time sensitive reconnaissance information because the value of the data can deteriorate rapidly with time. A capability to downlink from a satellite to an aircraft can provide all weather performance, and multiple data transfers for every satellite orbit. Over the last few years, BMDO has funded a number of technology efforts through the US Army Space and Strategic Defense Command reducing the risks associated with laser communications. One of these efforts, at ThermoTrex Corporation in San Diego, California, is now being carried forward towards an Advanced Technology Demonstration. The program will lead to the demonstration of high data rate communication of 270 MBPS (Mega Bits Per Second) to 1.2 GBPS (Giga Bits Per Second) between high altitude aircraft and between a satellite and the ground. The Laser Communications Terminals incorporate Atomic Line Filter technology for background light rejection during acquisition, reactionless Roto-Lok offset cable drive gimbals for fast slewing and high accuracy pointing, and direct digital modulation of semiconductor diode lasers detected with low noise avalanche photodiodes. We present results of a 42 km, 1.2 GBPS laser communications demonstration performed at NASA/JPL Table Mountain facility in Wrightwood, CA. Also presented are designs for a space qualifiable satellite terminal presently under construction.

Fast atomic line filter/field ionization detector

An experimental demonstration of a fast atomic line filter/field ionization detector (FALF/FID) is presented. The FALF/FID detects incoming signal photons by resonant absorption in an atomic vapor cell containing a strong electric field. Excited atoms are electric-field ionized after further excitation to a Stark-shifted Rydberg level by a pump laser tuned to a resonance in the ionization spectrum, providing an observed enhancement in the ionization rate of ten times over the continuum ionization threshold. Preliminary measurements of time response (<10 nsec) and quantum efficiency (>25%) indicate that with optimization the FALF/FID will provide high quantum efficiency, fast time response, and narrow-linewidth detection.

Proposed near-term, 1-Gbps space laser communications demonstration system

The Laser Communications Demonstration System (LCDS) Phase A/B was initiated by NASA-Headquarters through JPL to show that improvements in both technology and multi- discipline system engineering expertise since the initiation of previous programs enable the practical demonstration of space laser communication terminals that exhibit the anticipated benefits relative to rf communications. This paper presents an overview of the Laser Communication Demonstration System as developed by the Ball team.