We have investigated the effect of the frequency shift associated with an acousto-optic (AO) interaction on the spectral quality of acousto-optically tuned external cavity semiconductor lasers. A ring-cavity was built and the wavelength controlled by a frequency compensating acousto-optic tunable filter pair, which allowed user control over the frequency shift of the interacting light. Laser spectra were recorded for a range of frequency shifts, and a 30 kHz driving frequency gave the optimal spectral response where the output spectral shape did not vary as a function of operating wavelength and resulted in <0.1 nm linewidths.
We report on the development of a single frequency laser operating at a wavelength of 2.1 μm. This laser uses a holmium-doped distributed Bragg reflector (DBR) structure pumped at 1.15 μm. These lasers have applications in precision measurement, coherent laser radar and beam combination. Initial characterisation of the laser is included.
A holmium non-linear polarisation rotation (NLPR) all-fibre mode-locked laser was characterised. The radiofrequency (RF) spectrum of the pulsed train output was used as an automation metric. Non-ideal pulsing behaviour was corrected by changing the intracavity polarisation state using actuators in an electronic polarisation controller. The optimum cavity produced mode-locked pulses with a central wavelength of 2.061 μm and an 8.7 nm optical full-width-at-half-maximum (FWHM).
Gravitational wave detectors rely on the interference of light at the asymmetric port of a Michelson interferometer. The required light sources for these detectors are high power (200 W) with low intensity and frequency noise . These requirements are currently met with the amplification of a monolithic single-frequency Nd:YAG non-planar-ringoscillator (NPRO) . To improve the sensitivity of next generation detectors, cryogenic silicon test masses have been proposed to reduce the thermal coating noise. Laser sources near 2000 nm appear promising as they lie in the silicon transmission window and have reduced absorption in the amorphous silica coatings which reduces the heat-load on the cryogenically cooled mirrors . High power thulium-doped fibre amplifiers provide broad emission in this wavelength range and have previously been demonstrated with high power and narrow linewidths . Suitable thulium NPRO sources are not commercially available and an alternative seed source is required to determine the noise characteristics of these amplifiers.
The performance of mid-infrared fiber lasers operating on the 3.5 μm transition in erbium has improved significantly since the first demonstration that dual wavelength pumping allowed efficient operation. In this contribution, we will discuss the progress of fiber lasers that operate on this transition with an emphasis on advances towards short pulse generation and wavelength agility. Mode-locked operation using saturable absorption is a robust means of achieving ultra-short pulse operation in the near infrared but achieving this in the mid-infrared has been elusive. We will also describe our characterization of the mid-infrared performance of graphene, a material which has been very successfully applied to mode-locked pulse generation in the near infrared.
We report on a numerical model and supporting experiments to show that a high peak power, pulse
burst, Na guide-star waveform, suitable for use with adaptive optics systems requiring dynamic
refocusing to avoid guide star elongation, is capable of producing a return comparable to
conventional guide star laser of comparable output power. The predictions from our numerical
model using coherent pumping by short, high peak power pulses, or
so-called π-pulse pumping,
indicate that very bright fluorescence returns can be achieved in this regime. This is supported by
experimental results where fluorescence is observed in alkali atoms (cesium) using variable input
power and pulse lengths. The model is used to predict very bright Na guide stars, using short pulses
to excite most of the Na atoms available, followed by sufficient time to let them decay.
We demonstrate for the first time the practical feasibility of a new sodium guide star laser with a
pulsed burst output of sufficient energy at 589nm to be useful for current applications and readily
scalable to meet future requirements. We describe complete experimental design verification results
of the pulse burst laser concept, optimized to eliminate guide-star elongation issues and to meet all
requirements for Multi Conjugate Adaptive Optics (MCAO) for future extremely large ground-based
telescopes (ELTs). It makes use of sum frequency generation (SFG) of two, Q-switched, injection
mode-locked, wavelength stabilized Nd:YAG lasers, producing a
macro-micro, pulse-burst output
which is optimized in power and bandwidth to maximize the fluorescence from the high altitude
We describe a new, improved approach for sodium guide-star lasers for the correction of atmospheric aberrations in
telescopes, which satisfies all current requirements for advanced pulse burst waveforms. It makes use of sum frequency
generation (SFG) of two pulsed, Q-switched, injection mode-locked Nd:YAG lasers, resulting in a macro-micro pulse-burst
output, optimized in power and bandwidth to maximize the fluorescence from the high altitude sodium layer. The
approach is robust and power scalable and satisfies the requirements for Multi Conjugate Adaptive Optics (MCAO) for
current and future telescopes, including extremely large ground telescopes (ELTs). It is also adaptable for advanced
design options. Here we describe the approach in detail, the results from critical design verification experiments, the
current status and plans for further work required to demonstrate a complete sodium guide-star laser.
Long-period fiber Bragg gratings (LPG) where the grating period is much longer than the wavelength of light have many unique characteristics and find uses in gain-flattening filters and mode converters. This paper describes the characteristics of the initial LPGs fabricated at the University of Adelaide using an infrared CO2 laser. The optical system implemented promotes uniform irradiation of the full circumference of the fiber, avoiding many of the non-uniformities, associated with a single sided system. Some initial gratings have been made using this method, which typically show an attenuation of 10dB within a wavelength range (FWHM) of 8 nm. Work is now focused on improving these devices through an understanding of the writing process and its effect on the transmitted spectrum.
The Australian Consortium for Gravitational Astronomy has built a High Optical Power Test Facility north of Perth, Western Australia. Current experiments in collaboration with LIGO are testing thermal lensing compensation, and suspension control on an 80m baseline suspended optical cavity. Future experiments will test radiation pressure instabilities and optical spring in a high power optical cavity with ~200kW circulating power. Once issues of operation and control have been resolved, the facility will go on to assess the noise performance of the high optical power technology through operation of an advanced interferometer with sapphire tests masses, and high performance suspension and isolation systems. The facility combines research and development undertaken by all consortium members, which latest results are presented.
Pulsed THz (T-ray) spectroscopy is sensitive, non-invasive tool for studying materials from physics to biology, but transmission measurements of liquid samples, especially water, have been limited by noise. This paper shows that the accuracy of T-ray material parameter measurements of liquid samples can be greatly increased, especially for highly-absorbing liquids, by using a rapid modulation of the liquid in the T-ray beam path, coupled with a novel implementation of mean and amplitude detection to T-ray spectroscopy. The experiments are supported by calculations quantifying the sources of uncertainty. Liquid transmission T-ray studies are valuable for understanding solvation dynamics of salts, exploring long-range structure in mixtures and probing biomolecules in suspension.
This paper describes a study conducted into the limit on spectral resolution due to the dynamic range of a T-ray spectrometer. The pulsed nature of terahertz time-domain spectroscopy (THz-TDS) sets a fundamental limit on its spectral resolution. The spectral resolution of THz-TDS can be improved by increasing the duration of the temporal measurement, but is limited by the dynamic range of the system in the time-domain. This paper presents calculations and experimental results relating the temporal dynamic range of a THz-TDS system to its spectral resolution. We discuss three typical pulsed terahertz sources in terms of their dynamic range and hence achievable spectral resolution.
In this article photonic implementations of two oversampled analog-to-digital converter architectures are discussed. The first, and simplest design is that of pulse-code-modulation. Simulations and an experiment are developed employing a Multiple-Quantum-Well p-i-n diode comparator. Agreement between simulation and experiment is demonstrated and the design is subsequently extended to a higher performance first order unipolar sigma-delta architecture. Results of the signal-to-noise ratio as a function of input amplitude are presented for an oversampling ratio of 100.
Pulsed THz (T-ray) spectroscopy is an increasingly wide-spread tool for studying materials from physics to biology. Liquid transmission T-ray studies are valuable for understanding solvation dynamics of salts, exploring long-range structure in mixtures and probing biomolecules in suspension. In this paper the uncertainty in parameter estimation of liquid samples is shown to be dependent on the thickness change of the sample, and on the noise of the T-ray spectrometer. For many important liquids, such as water, with high THz absorption, we show that measurement uncertainty can be greatly decreased using a dynamically-modulated liquid sample, using differential T-ray time-domain spectroscopy (DTDS). Preliminary experiments support these calculations.
The Australian Consortium for Interferometric Gravitational wave Astronomy (ACIGA) is carrying out research on the detection of gravitational waves using laser interferometry. Here we discuss progress on each of the major sub systems: data analysis, lasers and optics, isolation suspension and thermal noise, and configurations, and report on the development of a high optical power test facility in Gingin, Western Australia.
A new bioaffinity sensor based on pulsed terahertz (THz) spectroscopy is able to sensitively detect the presence of ultra-thin bound biomolecular layers. The protein avidin and lipid biotin are noted for their very high binding affinity, and the ease for which they can be attached to residues with importance in many biosensing applications. We demonstrate the sensitivity of the pulsed THz spectrometer to thin avidin layers and to avidin amplified with micron-sized agarose beads. The experimental results can be simply modelled by considering transmission of the THz radiation at the thin film interfaces. We detect less than 10.3 ng/cm2 avidin, giving the THz system a detection capability of sub-thin solid films better than ellipsometry and reflectometry techniques.
The study of enzymatic protein molecules using terahertz time-domain spectroscopy (THz-TDS) has the potential to reveal molecular activity in real time without the use of labelling. Molecular hydration, or bound water, is a critical parameter in enzyme activity and THz-TDS measurements. For the first time we experimentally measure the terahertz-frequency response of nano-sized particles of protein and their level of molecular hydration. These measurements are valuable in understanding the terahertz response of biological systems and in studying the interaction between bound water and proteins.
Characterizing the optical and dielectric properties of thin films in the GHz to THz range is critical for the development of new technologies in integrated circuitry, photonics systems and micro-opto-electro-mechanical systems (MOEMS). Terahertz differential time-domain spectroscopy (DTDS) is a new technique that uses pulsed terahertz (THz) radiation to detect phase changes of less than 0.6 femtoseconds (fs) and absorption changes corresponding to several molecular monolayers. This paper shows how DTDS can be combined with double modulation in the pump-probe system to improve sensitivity by an order of magnitude. The technique is experimentally verified using 1 μm thick samples of silicon dioxide on silicon.
T-ray systems o er an exciting range of capabilities for chemical and biological diagnostics using the emerging technology of terahertz pulse imaging.1,2 We report results from the rst Australian T-ray program and discuss how MOEMS techniques can be applied to decrease the system size.3 A small portable T-Ray system will cost less and is needed, for example, in endoscopic applications.4
The rapidly developing field of terahertz (T-ray) imagin promises to provide a non-invasive method of identifying the composition of various objects. Biomedical diagnostics, semiconductor device diagnostics, trace gas analysis and moisture analysis for agriculture are among the growing number of important T-ray applications. We present results using an electro-optical sampling method and discuss how this can be simplified by sign a micro antenna array realized by MEMS technology. The challenges and advantages of both approaches are compared.
We evaluate the efficiency of a new optoelectronic quenched avalanche sensor, with the potential of enabling high resolution imagin through turbid media with femtosecond lasers. Our target application is for imaging cancer in the human breast.
CCD holography is to be used to measure the phase conjugation fidelity of an SBS experiment at TRW. The diagnostic has been shown capable of phase measurements with an RSS accuracy of 1/80 (lambda) . The technique and experiments designed to quantify its accuracy are described. The application of this technique to the SBS experiment and the peculiarities of doing the experiment in the IR are also discussed.