Michael J. Wouters

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

Frequency Measurement Capability of a Fiber-Based Frequency Comb at 633 nm

A fiber-based frequency comb has been developed to measure the frequency of a 633-nm iodine-stabilized laser at the National Metrology Institute of Japan (NMIJ). The measured frequency was consistent with the previous results measured with a Ti:sapphire-based comb. The NMIJ comb was shipped to Australia for the validation of the measurement capability of the National Measurement Institute, Australia (NMIA) and NMIJ combs using a common microwave reference and a common optical frequency. Consequently, the frequency consistency of the two combs was approximately 8 times10-17. Furthermore, we demonstrate that an absolute mode number of the comb can easily and clearly be determined by using the two combs.

Density of a single crystal natural silicon sphere

The density of a single-crystal natural silicon sphere identified as ldquoAvo#3rdquo has been redetermined at the National Measurement Institute, Australia (NMIA), as part of an international collaboration to determine the value of the Avogadro constant. A systematic review of the measurement system found that a significant bias was introduced in the measurement of the volume of the sphere, which is calculated from the interferometric measurements of the mean diameter. It was found that the dynamic response of the laser that is used to generate the frequency steps for the phase-shifting interferometry introduced a bias to the returned phase value. We determined the magnitude of the bias and then applied corrections to obtain the final measured value. The density reported here is based on some new and some recalculated measurements taken between April 2000 and May 2006 during the ongoing development of the measurement system.

Frequency comparison of two fiber-based frequency combs at 633 nm

Two fiber-based frequency combs were compared at 633 nm. An offset laser was locked to a frequency comb, and the locked laser was measured with another comb. Consequently, the frequency uncertainty of the combs was less than 3 times 10-16 at 10 000 s averaging, which is sufficiently lower than the uncertainty of UTC.

Long-distance telecom-fiber transfer of a radio-frequency reference for radio astronomy

Very-long-baseline interferometry (VLBI) for high-resolution astronomical imaging requires phase-stable frequency references at widely separated radio-telescope antennas. For the first time to our knowledge, we have disseminated a suitable radio-frequency (RF) reference for VLBI over a “real-world” telecom optical-fiber link between radio telescopes that are >100  km apart, by means of an innovative phase-conjugation technique. Bidirectional optical amplification is used in parallel with live traffic, and phase perturbations in the effective optical-fiber path length are compensated. This RF-over-fiber approach obviates the need for separate hydrogen masers at each antenna, offering significant advantages for radio-astronomy facilities such as the Square Kilometer Array.

An optical fiber-based system for high-stability distribution of reference radio-frequencies

We present a novel optical fiber-based radio-frequency distribution system that incorporates low-cost commercially available components. It has a fractional frequency stability of 7×10⁻¹⁷ (averaged over 10⁴ s) for distribution of an 80-MHz signal.

Time-transfer over optical fibre using pseudo-random noise ranging

The National Measurement Institute, Australia has been developing methods for time and frequency transfer over optical fibre with an eye towards applications in radio-astronomy. We report here on a digital time-transfer system that uses pseudo-random noise-ranging and is implemented on a Field Programmable Gate Array. The system has demonstrated a time deviation of a few ps on a 100 km link.

Development and applications of a traceable time-transfer system

The National Measurement Institute, Australia (NMIA) has been developing GPS Common View (GPSCV) time-transfer systems since the late 1990s. These provide traceability to UTC(AUS) for laboratory frequency references and network time. We describe our system, including advice on the use of low-cost receivers for GPSCV, summarise our 15 years of providing services based on it and conclude with a summary of current and future developments.

An optical fibre-based frequency dissemination network for Australia

An optical-fibre network is being planned for distribution of optical and microwave reference frequencies in Australia. This network would connect a number of research facilities and the National Measurement Institute (NMI), allowing long-range comparison of frequency standards to high precision and providing reference signals for a wide range of applications. First results for transfer in the laboratory achieve a fractional frequency stability of 7×10−17 at an averaging time of 104 s for the distribution of an 80 MHz radio frequency signal. We present details of the technique used to cancel noise introduced by the fibre, describe the all-digital radio-frequency phasemeter used to characterise performance, and outline plans for the further development of the network.