Charles C. Harb

A laser-locked cavity ring-down spectrometer employing an analog detection scheme

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.

Cavity ringdown spectroscopy using mid-infrared quantum-cascade lasers.

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.

Suppression of the intensity noise in a diode-pumped neodymium:YAG nonplanar ring laser

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. >

Optical heterodyne detection in cavity ring-down spectroscopy

Abstract 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.

Photodetector designs for low-noise, broadband, and high-power applications

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.

Squeezed light for bandwidth-limited atom optics experiments at the rubidium D1 line

We report on the generation of more than 5 dB of vacuum squeezed light at the rubidium D1 line (795 nm) using periodically poled KTiOPO4 (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.

Entangling the Spatial Properties of Laser Beams

Position and momentum were the first pair of conjugate observables explicitly used to illustrate the intricacy of quantum mechanics. We have extended position and momentum entanglement to bright optical beams. Applications in optical metrology and interferometry require the continuous measurement of laser beams, with the accuracy fundamentally limited by the uncertainty principle. Techniques based on spatial entanglement of the beams could overcome this limit, and high-quality entanglement is required. We report a value of 0.51 for inseparability and 0.62 for the Einstein-Podolsky-Rosen criterion, both normalized to a classical limit of 1. These results are a conclusive optical demonstration of macroscopic position and momentum quantum entanglement and also confirm that the resources for spatial multimode protocols are available.

Intensity-noise dependence of Nd:YAG lasers on their diode-laser pump source

Typically, the intensity noise of solid-state lasers is dominated by a resonant relaxation oscillation, RRO, at intermediate frequencies (kilohertz to megahertz) and by pump-source noise at frequencies below the RRO. The RRO is driven by vacuum fluctuations as well as by pump-source fluctuations and is therefore present irrespective of the pump-source noise level. However, the intensity noise at frequencies below the RRO can be substantially lowered by use of a low-noise pump source. This behavior is experimentally studied for diode-pumped Nd:YAG ring lasers. An experimental comparison is made between pumping with a single-element diode laser (SEDL) or with a diode-laser array (DLA). We find good agreement with theory for the SEDL but not for the DLA because the DLAs output intensity noise is spatially variant. We also show that pump-source frequency noise has only a minor effect on the intensity noise of the Nd:YAG laser. The requirements for low-noise operation of solid-state lasers are discussed.

Frequency-switched heterodyne cavity ringdown spectroscopy

When the frequency of light coupled into a cavity is suddenly shifted, the radiation emanating from the input port of the previously excited cavity can beat with the reflection of the frequency-shifted input on the surface of a photodetector. When the beat frequency is stable, the time decay of the resulting optical heterodyne signal can be used to measure intracavity absorption spectra with near quantum-limited sensitivity.

Pulsed quantum cascade laser-based CRDS substance detection: real-time detection of TNT

This paper presents experimental results from a pulsed quantum cascade laser based cavity ringdown spectrometer used as a high-throughput detection system. The results were obtained from an optical cavity with 99.8% input and output coupling mirrors that was rapidly swept (0.2s to 7s sweep times) between 1582.25 cm(-1) (6.3201μm) and 1697.00 cm(-1) (5.8928μm). The spectrometer was able to monitor gas species over the pressure range 585 torr to 1μtorr, and the analysis involves a new digital data processing system that optimises the processing speed and minimises the data storage requirements. In this approach we show that is it not necessary to make direct measurements of the ringdown time of the cavity to obtain the system dynamics. Furthermore, we show that correct data processing is crucial for the ultimate implementation of a wideband IR spectrometer that covers a range similar to that of commercial Fourier transform infrared instruments.