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Natural gas leaks are a serious concern not only because of methane emission but also an explosive threat. We report designing of a portable, miniaturized, laser-based methane tracker for rapid detection of natural gas leaks.

Cavity-enhanced spectroscopy is a sensitive optical absorption technique but one where the practical applications have been limited to studying small wavelength ranges. This Letter shows that wideband operation can be achieved by combining techniques usually reserved for the communications community with that of cavity-enhanced spectroscopy, producing a multiplexed real-time cavity-enhanced spectrometer. We use multiple collinear laser sources operating asynchronously and simultaneously while being detected on a single photodetector. This is synonymous with radio frequency (RF) cellular systems in which signals are detected on a single antenna but decoded uniquely. Here, we demonstrate results with spectra of methyl salicylate and show parts-per-billion per root hertz sensitivity measured in real-time.

The application of quantum optics to spatial measurements with continuous-wave laser beams is discussed. In particular, we summarize the work on measuring small displacements and tilt. We present the required theoretical multi-mode model and discuss how it applies to split detectors and spatial homodyne detectors. We show how spatial measurements can be improved using squeezed light. Finally, we present a demonstration of improved spatial homodyne measurements using squeezed light in a higher order spatial mode.

We present new results from a variant of cavity enhanced spectroscopy that allows large spectral bandwidths to be analysed in real time, and demonstrate new methods to apply the technique to real-time monitoring of species with broad absorbances.

A system is described that employs a diode-pumped Nd:YAG continuous-wave laser source servolocked to a three-mirror optical cavity and an analog detection circuit that extracts the ring-down rate from the exponentially decaying ring-down waveform. This scheme improves on traditional cavity ring-down spectroscopy setups by increasing signal acquisition rates to tens of kilohertz and reducing measurement noise sources. For example, an absorption spectrum of a weak CO2 transition at 1064 nm is obtained in less than 10 s at a spectral resolution of 75 kHz employing a cavity with an empty-cavity ring-down decay lifetime of 2.8 μs and a total roundtrip path length of 42 cm. The analog detection system enables laser frequency scan rates greater than 500 MHz/s. The long-term sensitivity of this system is 8.8×10−12 cm−1 Hz−1/2 and the short-term sensitivity is 1.0×10−12 cm−1 Hz−1/2.

We present unified theoretical expressions for laser intensity noise in the presence of injection locking and feedback. We discuss optimum control strategies for different configurations and frequency regions. We illustrate the various effects with experimental results from Nd : YAG non-planar ring oscillator lasers.

We present design and performance details of three photodetector circuits that have been developed in the authors laboratory over the past eight years. These detectors have been optimized to meet the unique demands of experiments such as high power, high sensitivity interferometry, nonlinear optics, and laser noise measurements. The circuits are: a low-noise dc coupled (dc 20 MHz) general purpose detector, a low-noise broadband (15–1100 MHz) detector capable of detecting 10 mW of light, and a high-power large dynamic range detector (30 kHz–60 MHz) capable of detecting up to 100 mW of light. We present bandwidth dynamic range and noise performance details for all three designs. In addition, we present detailed circuit schematics along with design and construction guidelines to enable assembly and use of these designs.

Polarization-selective optical heterodyne detection is shown to enhance the practical sensitivity of cavity ring-down spectroscopy. Initial experiments demonstrate a signal-to-noise ratio above 31 dB. Minor improvements should yield shot-noise-limited operation.

A fully quantum-mechanical model of feedback to the pump source of a four-level laser is developed with a view to predicting the achievable intensity noise reduction. For solid-state lasers, the model shows that quantum noise sources due to laser dynamics have a significant impact on the optimization of the feedback loop, Experimental results obtained with a diode pumped neodymium:yttrium aluminum garnet (Nd:YAG) laser are found to be in good agreement with the theoretical model. The ultimate limit to the noise suppression comes from the quantum noise due to the measurement processes in the feedback loop. A scheme to overcome this limit using a squeezed vacuum is theoretically demonstrated to be a highly efficient method of generating bright intensity squeezed light, particularly in the kHz regime where bright squeezing is otherwise difficult to obtain. [S1050-2947(97)08111-0].

It is commonly known that we are made up of approximately 60% water, but if fact we are 100% MOLECULES! Sci-Fi shows invent simple instruments to diagnose humans in real-time. Is it science fiction or fact?

Recent advances in digital signal processing have made it possible to make real-time hypertemporal spectroscopic measurements over several wavelength bands simultaneously. This talk will outline the progress made in this area of research and present a view of future application for this technology.

We experimentally investigate the polarisation dependence of the locking bandwidth and transfer of amplitude modulation within an injection locked laser system. We find that the measured locking bandwidth of our injection locked laser varies by a factor of 2 as a function of polarisation. The transfer function for amplitude modulation on the master laser is strongly polarisation dependent and does not reflect a simple reduction in effective master laser power. As an explanation of the effects observed, we introduce a ''polarisation resonance'' condition for injection locking a laser cavity.

We present a new variant of the Cavity Ringdown Spectroscopy (CRDS) that is able to scan across more than 1400 nm of spectral bandwidth around 6000 nm, acquiring and analysing more than 150,000 spectral datapoints in less than four seconds.

We develop an extended Kalman filter to estimate the ring-down time of a multimode Fabry-Perot optical cavity with a pulse train input. First the extended Kalman filter is applied to data generated from a simulator to show that the estimate of the filter converges to the expected value of the ring-down time. It is also shown that the multimode filter improves the estimation performance, compared to a simplified filter designed from a single-mode model. Then the filter is applied to a set of experimentally obtained output intensity data.

We present experimental results that illustrate how laser intensity noise near the quantum-noise limit is transferred in an injection-locked cw Nd:(yttrium aluminum garnet) nonplanar ring-oscillator laser. We show that these results are in extremely good agreement with our quantum-mechanical model describing the injection locking process [T. C. Ralph, C. C. Harb, and H.-A. Bachor, Phys. Rev. A]. Three regions in the intensity-noise spectrum are identified and we show that different minimum noise levels exist in these regions. Finally, we show that the injection-locked laser can generate and preserve nonclassical states. \textcopyright{} 1996 The American Physical Society.

We present a new variant of the Cavity Ringdown Spectroscopy (CRDS) that is able to scan across more than 1400 nm of spectral bandwidth, acquiring and analysing more than 150,000 spectral datapoints in less than four seconds.

Real lasers show intrinsic noise well above the standard quantum noise limit. We review the properties of lasers and of techniques to suppress this noise. Experimental results of electro-optic feedback techniques and of squeezing in a second harmonic generator are presented. Finally we show that the optimum light source could well be a cascade of different cavities, each one performing a specific task in the noise suppression.

This paper presents offline extended Kalman filter (EKF) estimation results for the decay time-constant for an experimental Fabry-Perot optical cavity for cavity ring-down spectroscopy (CRDS). The decay time for an optical cavity is defined as the amount of time it takes for the light intensity inside the cavity to decay to 1/e times its original intensity in the absence of a sustained light source. An estimation of the decay time for a cavity depends upon the absorbing species inside the cavity along with other miscellaneous losses due to the elements used to construct the cavity. Once the miscellaneous losses are computed experimentally, the decay time due to the absorbing species can be directly computed. The decay rate, as a function of wavelength, can be used to identify trace gas elements such as chemicals and their compounds. For estimation purposes, the cavity is modeled as a low pass filter with unity DC gain and the experimentally obtained light intensity at the output of the cavity is used as measurement. During the process of recording the intensity data, the cavity's resonant frequency is held in sync with the input laser frequency via a proportional-integral (PI) controller. Finally, the estimation results for the decay time of the cavity using the low pass filter model presented in this paper, are compared with the estimation results using a quadrature model for the cavity from a previous work. The estimation results are also compared on filter execution times.

This Letter presents offline estimation results for the decay-time constant for an experimental Fabry-Perot optical cavity for cavity ring-down spectroscopy (CRDS). The cavity dynamics are modeled in terms of a low pass filter (LPF) with unity DC gain. This model is used by an extended Kalman filter (EKF) along with the recorded light intensity at the output of the cavity in order to estimate the decay-time constant. The estimation results using the LPF cavity model are compared to those obtained using the quadrature model for the cavity presented in previous work by Kallapur et al. The estimation process derived using the LPF model comprises two states as opposed to three states in the quadrature model. When considering the EKF, this means propagating two states and a (2×2) covariance matrix using the LPF model, as opposed to propagating three states and a (3×3) covariance matrix using the quadrature model. This gives the former model a computational advantage over the latter and leads to faster execution times for the corresponding EKF. It is shown in this Letter that the LPF model for the cavity with two filter states is computationally more efficient, converges faster, and is hence a more suitable method than the three-state quadrature model presented in previous work for real-time estimation of the decay-time constant for the cavity.

One of the main challenges in remote Fourier transform infrared (FT-IR) spectroscopy is the collection of a reliable background spectrum. Although suggested as a method to address the problem in prior literature, super clip apodization (SCA) has had little reported success for wide spectral features. SCA is a technique that involves the manipulation of different parts of the interferogram to calculate an absorbance spectrum from a single interferogram. A new method called complementary super clip apodization (CSCA) is developed here and is successfully used in conjunction with SCA in an iterative optimization algorithm. The umbrella term of super clip mathematics is also defined to encompass spectral calculation using SCA, CSCA or both in combination. The validity of super clip mathematics is demonstrated in an experimental study of gas-phase nitromethane. In an effort to mimic errors present in standoff detection, uniformly distributed noise and/or wavenumber shifting is added to the interferometric sample data to test the robustness of the algorithm. It will be shown that the implementation of SCA and CSCA in combination is more successful for concentration assessment than using SCA or CSCA alone.© (2012) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

This paper presents results from a pulsed-laser cavity ring-down spectrometer with novel field programable gate array real-time data collection. We show both theoretically and experimentally that the data extraction can be achieved from a single cavity ringdown event, and that the absorbance can be determined without the need to fit the ringdown time explicitly. This methodology could potentially provide data acquisition rate up to 1 MHz, with the accuracy and precision comparable to nonlinear least squares fitting algorithms.

We investigate the intensity noise properties of a continuous-wave diode pumped Nd:YAG ring-laser system and present results for an active feedback loop that suppresses the relaxation oscillation noise. This system reduces the intensity noise to within 6.1 dB of the quantum noise equivalent level (which is at 1.5/spl times/10/sup -8///spl radic/Hz for 1.5 mA) for frequencies between 10 kHz to 300 kHz and to less than 1/spl times/10/sup -7///spl radic/Hz for frequencies between 300 Hz and 10 kHz. The technical properties of the optimized feedback system are presented. The theoretical limits of performance for the system are discussed and it is shown that the performance is within 3.1 dB of these limits. We also present data from an optical beat experiment demonstrating that the intensity control system does not introduce any new features into the frequency noise spectrum. >

This paper presents an application of the discrete-time extended Kalman filter (EKF) for offline estimation of the ring-down time constant of an experimental Fabry-Perot optical cavity for the purpose of cavity ring-down spectroscopy (CRDS). The ring-down time corresponds to the time taken by the light inside an optical cavity to decay to 1/e of its initial intensity. The estimation of ring-down time (or decay time) for a cavity is a direct indication of the absorbing species contained in it and can be used to detect improvised explosive devices and concealed explosives. The data for the estimation process is obtained in the form of light intensity at the output of the optical cavity from a CRDS experimental setup. During the process of data collection, the resonant frequency of the optical cavity is held in lock with the input laser frequency with the aid of a proportional-integral (PI) controller in order to allow for maximum constructive interference of light within the cavity.

A consortium of researchers at University of New South Wales (UNSW@ADFA), and Loyola University New Orleans (LU NO), together with Australian government security agencies (e.g., Australian Federal Police), are working to develop highly sensitive laser-based forensic sensing strategies applicable to characteristic substances that pose chemical, biological and explosives (CBE) threats. We aim to optimise the potential of these strategies as high-throughput screening tools to detect prohibited and potentially hazardous substances such as those associated with explosives, narcotics and bio-agents.

Phase error compensation is an important consideration in Fourier transform spectroscopy, particularly when obtaining background and sample information from one interferogram. A phase angle optimization algorithm is discussed to address this issue.

This paper considers the application of a discrete-time extended Kalman filter (EKF) to a problem of estimating the ring-down time constant of a Fabry-Perot optical cavity for the purpose of cavity ring-down spectroscopy (CRDS). The ring-down time corresponds to the time taken by the light inside an optical cavity to decay to 1/e of its initial intensity. The online estimation of ring-down time (or decay time) for a cavity is a direct indication of the absorbing species contained in it and can be used to detect improvised explosive devices and concealed explosives. Two cases are considered for EKF design depending on whether the resonant frequency of the optical cavity is perfectly locked to the input laser frequency, or not. In the case of a perfect lock, a three-state EKF is designed to estimate the magnitude and phase quadrature components of the cavity and the ring-down time. In the other case where there is not a perfect lock between the two frequencies, leading to non-zero detuning, a four-state EKF is designed to estimate the detuning parameter in addition to the magnitude and phase quadrature components of the cavity and the ring-down time. The detuning parameter gives an indication of the deviation of laser frequency from the resonant frequency of the cavity, which can then be used by a controller to maintain detuning at zero.

We experimentally demonstrate the generation of optical squeezing at multiple longitudinal modes and transverse Hermite-Gauss modes of an optical parametric amplifier. We present measurements of approximately 3 dB squeezing at baseband, 1.7 GHz, 3.4 GHz and 5.1 GHz which correspond to the first, second and third resonances of the amplifier. We show that both the magnitude and the bandwidth of the squeezing at the higher longitudinal modes is greater than can be observed at baseband. The squeezing observed is the highest frequency squeezing reported to date.

We combine experimental improvements to present an in principle demonstration of the use of multiple co propagating modes for quantum protocols.

Cavity ringdown spectra of ammonia at 10 parts in 10(9) by volume (ppbv) and higher concentrations were recorded by use of a 16-mW continuous-wave quantum-casacde distributed-feedback laser at 8.5 mum whose wavelength was continuously temperature tuned over 15 nm. A sensitivity (noise-equivalent absorbance) of 3.4x10(-9) cm(-1) Hz(-1/2) was achieved for ammonia in nitrogen at standard temperature and pressure, which corresponds to a detection limit of 0.25 ppbv.

The latest results on the experimental generation of the position and momentum (x-p) entanglement for bright optical beams are presented. The measurements of the quantum correlations in the TEM10 mode with two optical parametric amplifiers are demonstrated. TEM10 quadrature entanglement is also demonstrated. Two position squeezed beams on a 50:50 beam splitter are used to demonstrate full x-p entanglement.

In this experiment we use periodically poled potassium titanyl phosphate (PPKTP) as the nonlinear crystal in an optical parametric oscillator (OPO) for the generation of squeezed light on the D1 line of rubidium which lies at 795 nm. Although OPOs have long been used as sources of squeezed light, the shorter wavelengths required for atomic interactions have always been a challenge due to a limited choice of nonlinear crystals. PPKTP is a new material that has recently been shown to be highly effective over a very wide range of optical frequencies (Akamatsu, 2006).

The spatial properties of laser beams can be used to encode, transfer and detect quantum information into high order modes with high efficiency. We demonstrate the use of such states, including spatial squeezing and entanglement.

A quantum mechanical model is formulated to describe the coupling between pump intensity noise and laser frequency noise in a solid-state laser. The model allows us to investigate the limiting effects of closed-loop stabilization schemes that utilize this coupling. Two schemes are considered: active control of the quadrature phase noise of the laser and active control of the amplitude noise of the laser. We show that the noise of the laser in the actively stabilized quadrature is ultimately limited by the vacuum noise introduced by the feedback beamsplitter in both schemes. In the case of active control of the quadrature phase noise, the noise is also limited by the intensity noise floor of the detection scheme. We also show that some sources of noise in the passively stabilized quadrature can be suppressed and that it is possible to achieve simultaneous quadrature amplitude and phase stabilization of a solid-state laser. However, the quantum mechanically driven noise in the passively stabilized quadrature cannot be suppressed. While this poses the ultimate limit to the noise in the passively stabilized quadrature, we show that it is experimentally feasible to observe squeezing directly generated by a solid-state laser using this technique. © 2007 The American Physical Society.

We exploit second order (χ(2)) nonlinear optical phase matching for the selection of individual high order transverse modes. The ratio between the generated components can be adjusted continuously via changes in the phase-matching condition.

We report on the generation of more than 5 dB of vacuum squeezed light at the Rubidium D1 line (795 nm) using periodically poled KTiOPO$_{4}$ (PPKTP) in an optical parametric oscillator. We demonstrate squeezing at low sideband frequencies, making this source of non-classical light compatible with bandwidth limited atom optics experiments. When PPKTP is operated as a parametric amplifier, we show a noise reduction of 4 dB stably locked within the 150 kHz-500 kHz frequency range. This matches the bandwidth of Electromagnetically Induced Transparency (EIT) in Rubidium hot vapour cells under the condition of large information delay., 8 pages, 3 figures. Accepted for Journal of Physics B

We demonstrate quantum correlations in the transverse plane of continuous wave light beams by producing -4.0 dB, -2.6 dB and -1.5 dB of squeezing in the TEM00, TEM10 and TEM20 Hermite- Gauss modes with an optical parametric amplifier, respectively. This has potential applications in quantum information networking, enabling parallel quantum information processing. We describe the setup for the generation of squeezing and analyze the effects of various experimental issues such as mode overlap between pump and seed and nonlinear losses., Comment: 7 pages, 4 figures

The multimode operation of an optical parametric oscillator (OPO) operating below threshold is calculated. We predict that squeezing can be generated in a comb that is limited only by the phase matching bandwidth of the OPO. Effects of technical noise on the squeezing spectrum are investigated. It is shown that maximal squeezing can be obtained at high frequency even in the presence of seed laser noise and cavity length fluctuations. Furthermore the spectrum obtained by detuning the laser frequency off OPO cavity resonance is calculated.

We demonstrate a scheme based on homodyne detection with a TEM/sub 10/ local oscillator for optimum small displacement measurements beyond the quantum noise limit. It is demonstrated to be directly extendable to optimum tilt measurement.

Multimode operation of an optical parametric oscillator operating below threshold is calculated. We show that maximal squeezing can be obtained at high frequency even in the presence of seed laser noise and cavity length fluctuations.

We report an experimental demonstration of optimal measurements of small displacement and tilt of a Gaussian beam - two conjugate variables - involving a homodyne detection with a TEM10 local oscillator. We verify that the standard split detection is only 64% efficient. We also show a displacement measurement beyond the quantum noise limit, using a squeezed vacuum TEM10 mode within the input beam., 9 pages, 8 figures

We demonstrate close to optimum control of the intensity noise of a laser by combining the techniques of laser injection locking and negative electronic feedback. The greatly increased stability of the electronic feedback loop from the output of the slave laser to its pump source that can be obtained by injection locking the slave laser is used to strongly suppress the intensity noise of this laser. We simultaneously use the injection locked output to negatively feed back to an amplitude modulator in the master laser beam and, with both control loops running, we can demonstrate intensity noise suppression of the composite system to within 1 dB of the theoretical limit at 5 kHz.

Optical cavity enhancement is a highly desirable process to make sensitive direct-absorption spectroscopic measurements of unknown substances, such as explosives, illicit material, or other species of interest. This paper reports advancements in the development of real-time cavity ringdown spectroscopy over a wide-bandwidth, with the aim to make headspace measurements of molecules at trace levels. We report results of two pulsed quantum cascade systems operating between (1200 to 1320)cm(-1) and (1316 to 1613)cm(-1) that measure the headspace of nitromethane, acetonitrile, acetone, and nitroglycerin, where the spectra are obtained in less than four seconds and contain at least 150,000 spectral wavelength datapoints.

We demonstrate a novel intensity noise suppression configuration which combines laser injection locking and electronic feedback. We use two feedback loops which together suppress the intensity noise of the injection locked laser to 4 dB above the quantum noise limit.

Nous realisons une experience de mesure optimale de petits deplacements d'un faisceau gaussien TEM 00 , basee sur une detection homodyne employant un oscillateur local TEM 10 . Nous montrons une amelioration de 56% du signal detecte par rapport a une detection a deux zones. Ce nouveau dispositif permet egalement de mesurer de facon optimale de petites valeurs de tilt, la quantite conjuguee du deplacement. Enfin, nous montrons que la compression du mode TEM 10 du faisceau incident permet une mesure de deplacement au dela de la limite quantique standard.

Intensity stable laser sources are requires for both applied and fundamental research fields, such as telecommunications or gravitational wave antenna research. The aim of this collaboration between the Australian National University and Laser Zentrum Hannover is to examine techniques for producing stable light sources that are operating at their fundamental limit as set by the quantum properties of the light.

Experimental investigations illustrating the intensity noise properties of injection-locked solid-state lasers were performed. The measured results are predicted by a quantum mechanical model. The theory describes the intensity noise transfer to the slave laser’s output for large modulation signals as well as for signals at the quantum noise level. Different frequency regions in the intensity noise spectrum are identified and it is shown that there exist different minimum noise levels in these regions. Under appropriate conditions, injection-locked lasers can generate non-classical light.

In this paper we report on the development of a Fourier-transform based signal processing method for laser-locked Continuous Wave Cavity Ringdown Spectroscopy (CWCRDS). Rather than analysing single ringdowns, as is the norm in traditional methods, we amplitude modulate the incident light, and analyse the entire waveform output of the optical cavity; our method has more in common with Cavity Attenuated Phase Shift Spectroscopy than with traditional data analysis methods. We have compared our method to Levenburg-Marquardt non linear least squares fitting, and have found that, for signals with a noise level typical of that from a locked CWCRDS instrument, our method has a comparable accuracy and comparable or higher precision. Moreover, the analysis time is approximately 500 times faster (normalised to the same number of time domain points). Our method allows us to analyse any number of periods of the ringdown waveform at once: this allows the method to be optimised for speed and precision for a given spectrometer.

Second harmonic generation (SHG) can be used as a technique for controlling the spatial mode structure of optical beams. We demonstrate experimentally the generation of higher-order spatial modes, and the possibility to use nonlinear phase matching as a predictable and robust technique for the conversion of transverse electric modes of the second harmonic output. The details of this effect are well described by our wave propagation models, which include mode dependent phase shifts. This is, to our knowledge, the first detailed study of spatial mode conversion in SHG. We discuss potential applications of this effect.