We present the preliminary design and experimental results of a 1550 nm solid-state beam pointing system based on an optical phased array (OPA) architecture. OPAs manipulate the distribution of optical power in the far-field by controlling the phase of individual emitters in an array. This allows OPAs to steer the beam in the far field without any mechanical components (e.g., steering mirrors). The beam-steering system presented here uses waveguide electro-optic modulators to actuate the phase of each element in a 7-emitter OPA, enabling kHz bandwidth steering with sub-milliradian pointing precision. The control system used to stabilize and control the phase of each emitter in the OPA exploits a technique called digitally enhanced heterodyne interferometry, allowing the phase of each emitter to be measured simultaneously at a single photodetector, dramatically simplifying the optical system. All digital signal processing is performed using a field-programmable gate-array. Applications of this technology include free-space link acquisition and tracking for satellite-to-satellite laser communications and light detection and ranging (LiDAR).
A new type of sodium guidestar laser based on semiconductor laser technology is being developed by the astronomy, space, and laser communication communities in Australia and the United States, in partnership with laser manufacturer Arete Associates. Funding has been secured from the Australian Research Council and the Australian National University, with support from academic (UNSW) and industry partners (AAO, GMTO, EOS, Lockheed Martin). The consortium aims to develop a full scale prototype of the Semiconductor Guidestar Laser. The laser, to be delivered in 2019, will be initially installed on the EOS Satellite and Debris Tracking Station 1.8m telescope at Mount Stromlo Observatory where it will be thoroughly tested, on sky and in real operation conditions. This will be the first time that a Laser Guide Star is created in Australian skies. We present the project motivation and objectives, laser development and test plans, and the preliminary test results obtained to date.
The Gravity Recovery and Climate Experiment (GRACE) has produced a wealth of data on Earth gravity, hydrology, glaciology and climate research. To continue that data after the imminent end of the GRACE mission, a follow-on mission is planned to be launched in 2017, as a joint USGerman project with a smaller Australian contribution. The satellites will be essentially rebuilt as they were for GRACE using microwave ranging as the primary instrument for measuring changes of the intersatellite distance. In addition and in contrast to the original GRACE mission, a Laser Ranging Interferometer (LRI, previously also called ‘Laser Ranging Instrument’) will be included as a technology demonstrator, which will operate together with the microwave ranging and supply a complimentary set of ranging data with lower noise, and new data on the relative alignment between the spacecraft. The LRI aims for a noise level of 80 nm/√Hz over a distance of up to 270km and will be the first intersatellite laser ranging interferometer. It shares many technologies with LISA-like gravitational wave observatories. This paper describes the optical architecture including the mechanisms to handle pointing jitter, the main noise sources and their mitigation, and initial laboratory breadboard experiments at AEI Hannover.
We use digitally enhanced heterodyne interferometry to measure the stability of optical fiber laser frequency references. Suppression of laser frequency noise by over four orders of magnitude is achieved using post processing time delay interferometry. This approach avoids dynamic range and bandwidth issues that can occur in feedback stabilization systems. Thus long fiber lengths may be used resulting in better frequency discrimination, a reduction in spatially uncorrelated noise sources and an increase in interferometer sensitivity. We achieve an optical stability of 30 Hz/√Hz for quasi-static frequencies as low as 20 mHz.
Laser frequency fluctuations limit the ultimate resolution in interferometric fiber sensors. In this work, we demonstrate
an interferometric sensor insensitive to the effects of frequency change in an interrogating laser. A system is
characterized showing a minimum of 4.5 orders of magnitude frequency change reduction, and a demonstrated
broadband improvement of up to 1.5 orders of magnitude for signals between 100 mHz and 1 Hz. Using this technique a
resolution of less than a nanostrain/rtHz was achieved for a broad range of frequencies.
The resolution of fiber optic interferometry sensors is often limited by frequency noise in the laser. For this reason, prestabilization
techniques have been used to reduce laser frequency fluctuations and improve signal resolution. However,
for multi-element systems this becomes cumbersome and difficult to implement. In this paper, we demonstrate the use of
digitally-enhanced interferometry for the interrogation of a multi-element sensing system. Over 50 dB of cross-talk
rejection was found, with displacement resolutions of ~ 100 pm. Furthermore, using this technique, sub-frequency noise
displacement resolution was obtained without the need for high performance sensors.
Steady developments in cost and reliability in fiber optic sensors have seen an increase of their deployment in numerous
monitoring and detection applications. In high-end applications, greater resolution is required, especially in systems
where the environment is quiet, but the signal is weak. In order to meet these requirements the most dominant noise
source, laser frequency noise, must be reduced. In this paper we present a quasi-static strain sensing referenced to a
molecular frequency reference. A DFB CW diode laser is locked to a fiber Fabry-Perot sensor, transferring the detected
signals onto the laser frequency and suppressing laser frequency noise. The laser frequency is then read off using an
H<sup>13</sup>C<sup>14</sup>N absorption line. Phase modulation spectroscopy is used to both lock the laser to the sensor and read off the
signals detected by the sensor. The technique is capable of resolving signals below 1 nanostrain from 20 mHz, reaching a
white noise floor of 10 picostrain at several Hz.
We give an overview of the design and planned operation of the metrological Scanning Probe Microscope (mSPM)
currently under development at the National Measurement Institute Australia (NMIA) and highlight the metrological
principles guiding the design of the instrument. The mSPM facility is being established as part of the nanometrology
program at NMIA and will provide the link in the traceability chain between dimensional measurements made at the
nanometer scale and the realization of the SI meter at NMIA. The instrument will provide a measurement volume of
100 μm × 100 μm × 25 μm with a target uncertainty of 1 nm for the position measurement.
A fiber accelerometer array is presented with an unprecedented breakthrough combination of high acceleration resolution
after 100 km of fiber, in a bandwidth down to the infrasonic, with high multiplexing density and low crosstalk. The
demonstrated resolution is better than 60 ng/√Hz for all channels down to 10 Hz, even after the 100 km length of fiber.
Moreover, the system can accommodate 80 channels per fiber in wavelength division multiplexed operation with better
than -64 dB crosstalk. The dynamic range is 120 dB in a 300 Hz bandwidth.
Point-to-point laser metrology systems can be used to stabilize large structures at the nanometer levels required for
precision optical systems. Existing sensors are large and intrusive, however, with optical heads that consist of several
optical elements and require multiple optical fiber connections. The use of point-to-point laser metrology has therefore
been limited to applications where only a few gauges are needed and there is sufficient space to accommodate them.
Range-Gated Metrology is a signal processing technique that preserves nanometer-level or better performance while
enabling: (1) a greatly simplified optical head - a single fiber optic collimator - that can be made very compact, and (2) a
single optical fiber connection that is readily multiplexed. This combination of features means that it will be
straightforward and cost-effective to embed tens or hundreds of compact metrology gauges to stabilize a large structure.
In this paper we describe the concept behind Range-Gated Metrology, demonstrate the performance in a laboratory
environment, and give examples of how such a sensor system might be deployed.
We present the latest results from our multiplexed fiber optic Fabry-Perot acoustic sensor array using modulated lasers. It
offers the possibility of a robust, reliable and easy to deploy system, meeting the demands of geophysical survey.
The Laser-Interferometer-Space-Antenna (LISA) is a space-based interferometer with arm lengths of 5*10 9 m. Its design goal is to measure gravitational waves with a strain sensitivity of 10-23 at 10 mHz. Unlike in earth-based interferometers the arm lengths can differ by up to 2% or 108 m. For that reason frequency noise in the λ ~ 1 μm laser will not cancel in the direct interference signal. A laser locked to a ULE reference cavity in a 1°μK/square root Hz environment will have about 10 Hz/square root Hz frequency noise. The LISA sensitivity goal requires for the laser noise of less than 10-5 Hz/square root Hz, about a factor 10<sup>-6</sup> below what has been achieved (1). Cancellation of laser frequency noise can be achieved by time-delayed-interferometry (TDI) (2,3). We describe a laboratory test of TDI with an unequal arm interferometer. The intent is to ascertain the performance limitations and proof-of-concept for 6 orders of magnitude frequency noise suppression.
The optical paths on the LISA bench must have a length instability of less than 10~pm/square root Hz over time scales of 1s to 1000s. A small rigid interferometer has been constructed to measure the optical path length changes using various bonding techniques. The interferometer was constructed entirely from ultra-low expansion (ULE) glass by optically contacting ULE beamsplitters to a ULE bench. Preliminary results taken with the interferometer operating in air indicate optical path length fluctuations of approximately 100 pm/ sqaure root Hz or less for frequencies between 1 mHz and 1 Hz.