Haris Riris

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.

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.

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.

Measurement of the strengths of 1 ← 0 and 3 ← 0 transitions of HI using frequency modulation spectroscopy

Abstract The linestrengths for five P -branch lines in the fundamental and seven R -branch lines, including three high- J transitions, in the second overtone band of HI have been measured using frequency modulation spectroscopy. From these data, we have obtained new values for the rotationless dipole-moment matrix elements and Herman-Wallis coefficients for these bands. Combining this new information with previously measured intensity data, we have deduced a refined dipole moment function for HI.

Near-Infrared InGaAs/InP Distributed-Feedback Lasers for Spectroscopic Applications

Near-infrared diode-laser-based systems using laser-absorption molecular spectroscopy can sensitively monitor atmospheric gases, pollutants, and toxic gases. They can also monitor trace gases on the human breath for medical diagnostics. The detection levels are equal to or less than parts per million. Sarnoff/SRI has made and tested room-temperature InGaAsP/InP DFB lasers operating at 1.39, 1.6, and 1.65 micrometers . All of these devices had output powers of 10 mW or more. The current-tuning rates varied from -580 to -1240 MHz/mA. The temperature tuning rate was about 0.1 nm/K for all devices. Continuous tuning ranges were 7 nm for the 1.39 micrometers lasers and 5 nm for the 1.6 and 1.65 micrometers lasers. We observed H2O at 1.39 micrometers , CO and CO2 at 1.6 micrometers , and CH4 at 1.65 micrometers . We monitored the ratio of 13CO2 to 12CO2 on human breath samples as the initial step towards clinical trials for medical diagnostics.

Tunable single-frequency III-V semiconductor diode lasers with wavelengths from 0.76 to 2.7 μm

We have fabricated single-frequency diode lasers from a number of III-V semiconducting compounds. These diode lasers were specifically designed for laser absorption spectroscopy. Their emission wavelengths span the internal of 0.76 to 2.7 micrometers . Water vapor, CO, CO2, NH3, CH4 HF, and O2 have been detected using them. After a brief review of their physical structure and principles of operation, we present representative output characteristics of these lasers, along with a discussion of several important applications.

Signal processing strategies in tunable diode laser spectrometers

Abstract Digital signal processing algorithms were applied to diode laser frequency modulation spectrometers in the near- and mid-infrared spectral ranges. These algorithms include digital bandpass filters, a Wiener filter, a matched filter, and a Kalman filter. Digital signal processing is easy to implement and adapt to all experimental configurations without any physical modifications or additions to the spectrometer.

Single-beam diode-laser technique for optical path-length measurements

A simple single-beam technique employing radio-frequency modulation of a tunable diode laser with homodyne demodulation is demonstrated as a means of measuring optical path lengths. This technique offers a straightforward method for determining path lengths traversed through optical multipass cells or performing optical range-finding over short (i.e., tens of meters) standoff distances. The radio-frequency phase-sensitive nature of the technique permits narrow-band detection and high signal-to-noise ratios, even when range-finding measurements are made with range resolutions of «1 m. This compares favorably with traditional short-pulse, wide-bandwidth optical range finders.