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Molecular and Optical Physics Laboratory

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Projects

The Molecular and Optical Physics Laboratory (MOPL) provides an excellent setting for laser-based research in optical or molecular physics, with various projects available to interest research students. Previous Honours and PhD graduates are now employed in the photonics industry and in R&D institutions (universities, CSIRO, DSTO, …) in Australia, Europe and the USA.
MOPL activities range from fundamental high-resolution studies of molecular spectroscopy and energy transfer, through development of tunable laser and nonlinear-optical devices, to laser-based diagnostic sensing applications in the environment (e.g., atmospheric processes), industry (e.g., control of combustion, smelting, etc.), biomedicine (e.g., clinical breath analysis, coherent Raman spectroscopic imaging) and forensics (e.g., trace detection of explosives, post-blast residues and pathogens). Specific topics suitable for new research student projects include:

Much of MOPL's ongoing work is slanted towards possible commercial uptake of new laser-based instruments and sensing techniques that have been developed. Several items of our laser-based intellectual property have been through the preliminary stages of commercialisation.

Cavity Ringdown Spectroscopy

In cavity ringdown (CRD) spectroscopy, tunable coherent light (e.g., from a laser) interacts efficiently with gas-phase molecules in a highly reflective resonant optical cavity. Tuned coherent radiation bounces back and forth in the cavity, travelling many kilometres. Weak absorption spectra (e.g., at low molecular concentration) are then measurable with high sensitivity and photometric precision. The rate at which light 'leaks' out of the cavity is readily converted to an absorption spectrum. The conventional form of CRD spectroscopy employs pulsed coherent light, but new ways to use continuous-wave (cw) sources have been developed. Research on CRD spectroscopy within MOPL has led to compact CRD spectrometer designs based on a rapidly-swept optical cavity, miniature tunable diode lasers and other optical telecommunications components. A key feature of the MOPL innovations is optical-heterodyne detection, which allows the optical transmitter and receiver to be located in a single module that is linked to one or more CRD cavities by optical fibre. This results in powerful analytical chemistry instruments for diagnostic sensing applications in industry, medicine, agriculture, defence, forensics, and the environment. It is a bit like having a laser-based sniffer dog! This is a promising way to perform medical breath tests, yielding non-invasive ways to diagnose a variety of diseases - blood, liver, lung, pancreas, stomach, etc. (e.g., 13CO2 or NH3 breath tests for Helicobacter pylori, associated with peptic ulcers). There are prospects of CRD-based spectroscopic sensing of industrial process gases in/from smelters, furnaces, etc. (e.g., CO/CO2 ratios to monitor and control combustion efficiency). In the laboratory, CRD spectroscopy can be used to investigate gas-phase chemical reactions (e.g., detecting reaction intermediates) or molecular dynamics (e.g., collision-induced processes in gases and molecular beams).

 Elements of MOPL research
Cavity Ringdown Spectrometer Florian Englich, Brian Orr & Yabai He seem pleased with a laboratory prototype of their new cavity ringdown (CRD) spectrometer, as depicted in an article entitled 'Sniffing out disease' in the September 2003 issue of Macquarie University News. The rapidly swept CRD sample cell is in front of the multi-wavelength optical-heterodyne-detected transmitter/receiver unit that is coupled to it by optical fibre. This unit is being compacted down for field-based sensing in hospitals, etc.

Optical Parametric Oscillators

The MOPL group is a leader in high-performance optical parametric oscillator (OPO) systems. OPOs are nonlinear-optical (NLO) sources of tunable coherent radiation, useful for spectroscopic sensing applications in industry, the environment and science. Ongoing MOPL research is evolving novel wavelength control schemes for pulsed tunable OPOs. Three types of OPO are depicted: (a) a free-running OPO (no active wavelength control); (b) OPO with intracavity tuning element (T), e.g., grating, filter or étalon; (c) an injection-seeded OPO. The active medium has NLO susceptibility c(2) and is either birefringently phase-matched or quasi-phase-matched. Input (pump P, seed) & output (signal S, idler I) light waves, with optical frequency wj & wave vector kj (j = P, S, I), are represented by arrows The OPO resonator contains at least 2 mirrors (M1,2).Schemes (a) & (b) are conventional, but the MOPL approach favours scheme (c), where narrowband seed radiation (e.g., from one or more external tunable diode lasers) controls the OPO output wavelength(s). MOPL's research focuses on multi-wavelength OPOs for atmospheric LIDAR, ultra-narrowband tunable OPOs in basic science, and greatly simplified tuning schemes for pulsed OPOs.

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