Richard B. Warrington

Subpicometer length measurement using heterodyne laser interferometry and all-digital rf phase meters

We present an all-digital phase meter for precision length measurements using heterodyne laser interferometry. Our phase meter has a phase sensitivity of 3 μrad/√Hz at signal frequencies of 1 Hz and above. We test the performance of our phase meter in an optical heterodyne interferometric configuration, using an active Sagnac interferometer test bed that is flexible and low noise. We demonstrate more than 70 dB of laser frequency noise suppression to achieve an optical phase sensitivity of 5 μrad/√Hz and a corresponding displacement sensitivity of 0.5 pm/√Hz at signal frequencies above 10 Hz. In addition, we demonstrate the ability of our phase meter to follow full fringe signals accurately at 100 Hz and to track large signal excursions in excess of 10(5) fringes without cycle slipping. Finally, we demonstrate a cyclic error of ≤1 pm/√Hz, above 10 Hz.

Long-term operation and performance of cryogenic sapphire oscillators

Cryogenic sapphire oscillators (CSO) developed at the University of Western Australia (UWA) have now been in operation around the world continuously for many years. Such oscillators, due to their excellent spectral purity are essential for interrogating atomic frequency standards at the limit of quantum projection noise; otherwise aliasing effects will dominate the frequency stability due to the periodic sampling between successive interrogations of the atomic transition. Other applications, which have attracted attention in recent years, include tests on fundamental principles of physics, such as tests of Lorentz invariance. This paper reports on the long-term operation and performance of such oscillators. We compare the long-term drift of some different CSOs. The drift rates turn out to be linear over many years and in the same direction. However, the magnitude seems to vary by more than one order of magnitude between the oscillators, ranging from 1014 per day to a few parts in 1013 per day

All-Digital Radio-Frequency Signal Distribution Via Optical Fibers

We present a radio-frequency (RF) signal distribution system via optical fibers. We utilize an all-digital platform that encompasses a phase-locked loop, numerically-controlled oscillator, and fiber phase noise cancellation system. Our system achieves a fractional frequency transfer stability of 4 × 10-13 at 1 s and 6 × 10-17 at one day for the distribution of RF signals over 70 km of optical fiber. We demonstrate that this performance can be achieved with standard crystal oscillators. Our system is scalable, configurable, and flexible, allowing distribution of signals at different frequencies while maintaining over two orders of magnitude of the fiber phase noise suppression.

High-Precision Optical-Fiber Transfer Of A Radio-Frequency Reference For Remote Environmental And Industrial Sensing

We show long-distance precise synchronization of a radio-frequency signal over an optical-fiber network without actively stabilizing the optical path length, yielding long-term stability comparable to a hydrogen maser for remote sensing applications.

Long-distance fiber-optical transfer of a radio-frequency control signal for radio-astronomy and sensing applications

Passive phase-conjugation enables precise fiber-optical transfer of radio-frequency reference signals over long distances, yielding high long-term stability for radio-astronomy and remote-sensing applications without active path-length stabilization.

Optical cavity enhanced real-time absorption spectroscopy of CO2 using laser amplitude modulation

We present a spectrometer based on the cavity enhanced amplitude modulated laser absorption spectroscopy (CEAMLAS) technique for measuring molecular gas absorption. This CEAMLAS spectrometer accurately measured a CO2 absorption line at 1572.992 nm with effectively 100% measurement duty cycle. It achieved an absorption sensitivity of 5.2 × 10−9 Hz−1∕2 using a linear Fabry-Perot cavity with a modest finesse of ≈1000. We also used the spectrometer to perform preliminary measurements of the 13C/12C isotopic ratio in CO2, yielding an isotopic signature δ13C of −83±9‰ for our CO2 sample.

Precision length measurement using an all-digital phasemeter for heterodyne laser interferometry

We report on an all-digital radio-frequency phasemeter for application in precision length measurements using heterodyne laser interferometry [1]. Our phasemeter has a phase sensitivity of 200 nrad/√Hz at signal frequencies of 0.2 Hz and above. We demonstrate the use of our phasemeter in an optical heterodyne interferometric configuration, using an active Sagnac interferometer test-bed. In our low-noise Sagnac test-bed, we demonstrate more than 70 dB of laser frequency noise suppression to achieve an optical phase sensitivity of 5 μrad/√Hz, and a corresponding displacement sensitivity of 0.5 pm/√Hz at signal frequencies above 10 Hz. In addition, our phasemeter has the ability to follow full fringe signals accurately at 100 Hz and to track large signal excursions in excess of 105 fringes without cycle slipping. Finally, we measure a cyclic error of ≤ 1 pm/√Hz.