George F. Lutes

Applications of ultra-stable fiber optic distribution systems

Present and future applications of fiber-optic frequency distribution systems are discussed and it is noted that for applications requiring distribution stability greater than one part in 10/sup 15/ at 1000 s averaging times, a stabilized fiber optic link is the only choice presently available. A stabilized fiber optic system that can improve distribution stability by more than 100 times is also described. It uses a cable delay compensator.<<ETX>>

Reference Frequency Distribution Over Optical Fibers: a Progress Report

The results were recently reported of an experiment in which a 100-MHz reference frequency was transmitted over a 14-km long fiber-optic link. A differential stability of 1.5 X for 1000 seconds averaging times was measured for this link. However, there were several factors that limited the stability of the link. A phase change due to bending the cable resulted in a sensitivity o microphonics and a degradation to the Allan variance. Also, variations of phase delay due to temperature changes caused degradation of the Allan variance as well. This paper considers fundamental and practical stability limits on reference frequency distribution over fiber-optic links. It also reports on recent progress in reducing the sources of instability, including the u s e of optical isolators to greatly reduce sensitivity to cable bending.

Optical calibration phase locked loop for the Shuttle Radar Topography Mission

The Shuttle Radar Topography Mission (SRTM) is an interferometric synthetic aperture radar system that flew on the space shuttle in February 2000, SRTM has an inboard antenna in the shuttle cargo bay and an outboard antenna at the end of a 60-m mast, extending from the cargo bay. In order to meet the elevation mapping accuracy requirement, the relative phase delay between the radar signals received via the outboard channel, compared with the inboard channel has to be known to within 80 at 5.3 GHz. This paper describes the design solutions and constraints, the devices, the analysis, and validation used to implement an optical calibration loop for SRTM. The calibration method involves injecting a tone into one panel of the inboard antenna, and sending an optical copy of the tone via a fiber-optic cable to be injected into the outboard antenna. A portion of the optical signal is reflected off an outboard partial mirror and travels back via the fiber to the inboard calibration system. There, it is converted back into a radio frequency tone and its phase is compared with the phase of the original tone. As the temperature of the mast fiber changes, a phase error is detected in the phase comparator. This error is used to control a custom designed optical phase shifter connected in series with the mast fiber. This phase-locked-loop guarantees that the phase of the calibration tone at the outboard stages within 1/spl deg/ relative to the phase of the calibration tone at the inboard antenna.

Status of frequency and timing reference signal transmission by fiber optics

The authors report recent progress in high-stability fiber-optic distribution of frequency and timing reference signals. They outline state-of-the-art performance at 100 MHz, 1 GHz, and 8.4 GHz for these systems. A lower cost, lower performance distribution system for the user who does not need full H-maser stability is described. Future fiber-optic system developments and their potential impact on systems which use high stability frequency reference distribution are discussed. System hardware and cost-performance tradeoffs are considered. A cost-performance tradeoff is presented, along with a cost savings suggestion to use a single fiber-optic transmitter to transmit a signal to several users simultaneously.<<ETX>>

Distribution of ultra-stable reference frequency signals over fiber optic cable (for radiotelescopes)

Radiotelescope systems, which operate primarily at microwave frequencies, are used for radio and radar astronomy, very large baseline interferometry (VLBI), geodynamic measurements, and spacecraft navigation. Experimenters have struggled for years to overcome the deficiencies of metallic coaxial cables and waveguides which have limited the stability and accuracy of measurements made with radiotelescope systems. Advances in fiber optic technology are on the verge of eliminating transmission lines as the major source of error in these systems. A description is given of high-stability fiber optic links which are used to distribute reference frequencies over distances as far as 29 km. Reference signals generated by hydrogen masers are distributed over these links and maintain a stability of 1 part in 10/sup 15/ for 1000-s averaging times.<<ETX>>

Optical isolator system for fiber-optic uses.

A low loss optical isolator suitable for fiber-optic uses has been assembled from commercial components. The isolator exhibits reverse isolation of >70 dB, with a forward loss of <1.3 dB. This system provides an effective approach for reducing instabilities encountered in the output signal of semiconductor lasers in certain applications of fiber-optic systems. The paper presents a phenomenological explanation for the superior performance of the isolator system.

The fractional frequency stability of a 34-m-diameter beam-waveguide antenna

Advances in fiber-optic technology have made it possible to develop a new technique to isolate and measure the frequency stability of a large beam-waveguide (BWG) antenna. Through the use of the technique described in the paper, at both 46.5/spl deg/ and 37/spl deg/ elevation angles, under good weather conditions, the degradation of the the fractional frequency stability of signals passing through the antenna path was measured to be between 1.3 and 2.2/spl times/10/sup -15/ for sampling intervals of 1024 s. These stability values apply to the portion of the antenna that includes the main reflector, subreflector, tripod legs, and six BWG mirrors. These test results are believed to be the first successful fractional frequency stability measurements made on the microwave optics portion of a large antenna to a level of 1 or 2 parts in 10/sup 15/. >

Impact of semiconductor laser frequency deviations on fiber optic frequency reference distribution systems

An analysis is presented of the effect of the linewidth of a single-longitudinal-mode semiconductor laser on the frequency stability of a frequency reference transmitted over single-mode optical fiber. The interaction of the random laser frequency deviations with the dispersion of the optical fiber is considered to determine theoretically the effect on the Allan deviation of the transmitted frequency reference. It is shown that the magnitude of this effect may determine the limit on the ultimate stability possible for frequency reference transmission on optical fiber, although it is not a serious limitation to present system performance.<<ETX>>

A new test of relativity using hydrogen maser standards

A novel test of relativity that was performed using advanced time and frequency technology of the Deep Space Network is discussed. The instrumentation, procedure. and data analyses are described. By directly measuring the propagation-delay variation between two masers, limits on a possible anisotropy in the one-way velocity of light have been obtained.<<ETX>>