Clinton B. Carlisle

Quantum noise-limited FM spectroscopy with a lead-salt diode laser.

This paper reports the demonstration of quantum-noise-limited sensitivity in frequency modulation spectroscopy (FMS) with a lead-salt diode laser operating in the 4.8-microm region. Experimentally determined sensitivities of 10(-7) in a 1-Hz bandwidth have been demonstrated for detection of CO and CO(2) in a single-pass absorption cell. The high sensitivity obtained with our FMS apparatus can be attributed to the use of improved lead-salt diode lasers, a new optical fringe reduction method, and careful attention to optical processing of the diode laser beam.

Signal-to-noise ratio enhancement in frequency-modulation spectrometers by digital signal processing.

A signal-to-noise ratio enhancement of an order of magnitude was observed when digital signal-processing algorithms were applied to two diode-laser frequency-modulation spectrometers in the near-infrared and midinfrared spectral ranges. These algorithms include digital bandpass filters, a Wiener filter, a matched filter, and a least-squares fit. Digital signal processing has a practical advantage over other noise suppression techniques because it is easy to implement and to adapt to all experiment configurations without any physical modifications or additions to the spectrometer.

Design of an open path near-infrared diode laser sensor: application to oxygen, water, and carbon dioxide vapor detection.

An open path diode laser sensor was constructed with near-infrared diode lasers and two-tone frequency-modulation spectroscopy. The sensor incorporates several novel features (such as digital signal-processing algorithms, a computerized line-locking routine, and discontinuous wavelength scanning) that are important in a field instrument. The sensor was used to monitor oxygen, water, and carbon dioxide in the near-infrared spectral range. For oxygen, an absorbance detection sensitivity of 2 × 10(-6) in a 10-Hz bandwidth was demonstrated with a GaALAs laser at 760.56 nm. The stability of the sensor was 0.1% over a period of 10 h when an absorbance of 6 × 10(-3) was monitored.

CO 2 laser-based differential absorption lidar system forrange-resolved and long-range detection of chemical vapor plumes

A dual CO(2) laser-based differential absorption lidar (DIAL) system has been constructed and demonstrated for range-resolved mapping of chemical vapor plumes. The system acquires high range resolution through the use of plasma-shutter pulse clippers that extinguish the nitrogen tail of the CO(2)-laser output. Aprogrammable servomotor-driven scanner allows full hemispherical coverage of the interrogated field. A high-speed direct-detection receiver subsystem is used to gather, process, and display vapor-concentration data in near real time. Data demonstrating range-resolved detection of low concentrations of chemical plumes from ranges of 1 to 2 km are presented. In the column-content detection mode, trace levels of secondary vapors from various organophosphate liquids were monitored. Detection of an SF(6) vapor plume released 16 km from the DIAL system is also adduced.

Tunable-diode-laser frequency-modulation spectroscopy using balanced homodyne detection

Quantum-limited frequency-modulation spectroscopy (FMS) has been performed with tunable diode lasers at 5.4 and 1.3 microm by employing a dual-beam balanced homodyne detection scheme to suppress optical fringes, residual amplitude modulation, and excess laser noise by 15-20 dB. Performing FMS at the two wavelengths involved different lasers (lead salt and InGaAsP, respectively), optics, and detectors, which emphasizes the versatility and applicability of the technique. System sensitivities of 1 x 10(-7) and 2 x 10(-7) were obtained for the respective systems.

Tunable diode laser frequency modulation spectroscopy through an optical fiber : high-sensitivity detection of water vapor

Frequency modulation spectroscopy through a single‐mode optical fiber has been demonstrated for monitoring H2O vapor at 7665 cm−1. Using a 2.5 mW distributed feedback laser and 100 m of optical fiber, a minimum detectable absorption of 5×10−7 was achieved. To obtain this sensitivity limit, very large spurious signals and technical noise were suppressed using a dual‐channel detection scheme.

Measurement of 12 CO 2 : 13 CO 2 ratios for medical diagnostics with 1.6-μm distributed-feedback semiconductor diode lasers

We have observed some of the absorption lines from the molecules (12)CO(2) and (13)CO(2)in the 1.6-µm spectral region with the use of specially fabricated single-mode InGaAsP distributed-feedback semiconductor diode lasers. Using a 23.6-m-long multipass absorption cell in combination with radio-frequency modulation and detection techniques, we measured the (12)CO(2):(13)CO(2) isotopic ratio of two specific lines at 6253.73 and 6253.90 cm(-1) with sufficient precision for diagnostic medical tests that analyze CO(2) on human breath.

High-sensitivity FM spectroscopy with a lead-salt diode laser

We report the demonstration of near-quantum-noise-limited sensitivity with a lead-salt diode laser using FM spectroscopy and high-speed sweep integration. Using both single-tone and two-tone techniques, we demonstrated an absorption sensitivity of ~2 x 10(-7) in a 2.44-Hz bandwidth from a Doppler-broadened NO line at 1856.5 cm(-1). The sensitivity in our experiments was limited by laser excess noise, which was about four times the expected quantum-noise level.

EXPLOSIVES DETECTION WITH A FREQUENCY MODULATION SPECTROMETER

An explosives detection instrument was designed and tested at SRI International. The instrument uses frequency modulation spectroscopy with midinfrared lead-salt diode lasers to perform high-sensitivity detection of characteristic nitrogen-containing decomposition products of explosives. Ultimately, the instrument should be capable of detecting and identifying subpicogram levels of plastic explosives, which would be suitable for screening passengers at airports. Using the laboratory breadboard instrument and two different explosive vapor generators, we demonstrated a lower limit of detection of 5-10 pg for cyclotrimethylene trinitramine and linearity of the signal over an order of magnitude.

Kalman filtering of tunable diode laser spectrometer absorbance measurements

A recursive Kalman time-series filter was applied to absorbance measurements obtained with a tunable diode laser spectrometer. The spectrometer uses frequency modulation spectroscopy and a nearinfrared diode laser operating at 1.604 µm to monitor the CO(2)-vapor concentration in a 30-cm absorption cell. The Kalman filter enhanced the signal-to-noise ratio of the spectrometer by an order of magnitude when an absorbance of 6 × 10(-5) was monitored.