Quantitative chemical identification of four gases in remote infrared (9-11 mum) differential absorption lidar experiments

High Repetition Rate, Tunable Mid-Infrared BaGa4Se7 Optical Parametric Oscillator Pumped by a 1 μm Nd:YAG Laser

A tunable and compact mid-infrared optical parametric oscillator (OPO) based on BaGa4Se7 (BGSe) crystal with a repetition rate up to 250 Hz was demonstrated. A high energy and more approachable side-pumped Q-switched Nd:YAG laser was employed as the pump for the BGSe OPO. Due to the pump double-pass single-resonant oscillator (DP-SRO) configuration, the maximum average power of 250 mW and the maximum pulse energy of 1.28 mJ at 4.06 μm was achieved with the repetition rate of 250 Hz and 100 Hz, respectively. The tunable mid-infrared output from 3.42–4.73 μm was obtained. The influence of repetition rate and cavity length was studied and the thermal effect was analyzed.

Long-wave infrared multi-wavelength optical source for standoff detection of chemical warfare agents

We have designed and built a wavelength-tunable optical source for standoff detection of gaseous chemicals by differential absorption spectrometry in the long-wave infrared. It is based on a nanosecond 2 µm single-frequency optical parametric oscillator, whose idler wave is amplified in large aperture Rb:PPKTP crystals. The signal and idler waves are mixed in ZnGeP₂ crystals to produce single-frequency tunable radiation in the 7.5-10.5 µm range. The source was integrated into a direct detection lidar to measure sarin and sulfur mustard inside a closed chamber, in an integrated path configuration with a noncooperative target.

Long-wave infrared multi-wavelength IPDA lidar for standoff detection of chemical warfare agents: theoretical study

We discuss and evaluate the expected performance of a tunable multi-wavelength integrated-path differential absorption lidar operating in the long-wave infrared between 7.5 and 11 µm, for standoff measurement of chemical agents. Interference issues with natural gas compounds throughout the entire 7.5-11 µm band are first discussed. Then, the study focuses on four interest species, three warfare agents, and a simulant. A performance model is derived and exploited to assess the expectable measurement precision of the lidar for these four species in the integrated-path mode within a 2 min alert time and seventeen emitted wavelengths. Measurement precisions better than the targeted sensitivity levels look reachable at the kilometer range with laser power below 100 mW. Performance optimization strategies are discussed, either by adjusting the pulse energy/pulse repetition rate for a given laser power and lidar range or by reducing the wavelength sequence in an optimal way. Finally the system's receiving operating characteristic curves are derived to describe the expected detection performance in terms of probability of false alarm rate and probability of detection.

High-energy, tunable, long-wave mid-infrared optical parametric oscillator based on BaGa4Se7 crystal

A high-energy, tunable, long-wave mid-infrared optical parametric oscillator (OPO) based on the BaGa₄Se₇ crystal was demonstrated in this Letter with 1064 nm laser pumping. The mid-infrared OPO was designed as a double-pass single resonant oscillator (DP-SRO) to reduce the threshold and improve the outputs. Further optimization on the cavity length was theoretically and experimentally studied. With a short cavity length of 30 mm, the output energy of over 1 mJ/pulse at 11 µm was obtained with the pump energy of 39.5 mJ/pulse. In addition, a wide tuning range of 8-14 µm was experimentally achieved by rotating the BaGa₄Se₇ crystal.

In-fiber high-speed recognition of incoherent-light broadband energy spectrum patterns

A fiber-optic system is proposed and experimentally demonstrated for real-time, on-the-fly identification of an incoherent-light energy spectrum pattern based on dispersion-induced time-spectrum convolution. In the proposed system, the incoming frequency-spectrum patterns to be identified are modulated by a time-mapped version of the target intensity profile. Following propagation through a suitable fiber-optic dispersive medium, the measured output temporal waveform provides a correlation of the incoming spectra with the programmed target pattern. This enables direct, real-time detection of the matching energy spectra, without any further numerical post-processing. We experimentally demonstrate successful recognition of a target infrared spectral pattern, extending over a bandwidth of 1.5 THz with a resolution of ∼12  GHz, with sub-megahertz update rates. A path for further performance improvements is also suggested.

Minimum description length approach for unsupervised spectral unmixing of multiple interfering gas species

We address an original statistical method for unsupervised identification and concentration estimation of spectrally interfering gas components of unknown nature and number. We show that such spectral unmixing can be efficiently achieved using information criteria derived from the Minimum Description Length (MDL) principle, outperforming standard information criteria such as AICc or BIC. In the context of spectroscopic applications, we also show that the most efficient MDL technique implemented shows good robustness to experimental artifacts.

Differential absorption lidar CO2 laser system for remote sensing of TATP related gases

A CW tunable 10.6 microm CO(2) laser differential absorption lidar (DIAL) system has been developed, for the first time to our knowledge, for the remote sensing of triacetone triperoxide (TATP) gas vapors, which have strong absorption lines at several wavelengths, including 3.3, 8.3, and 10.6 microm. The DIAL laser beam was transmitted through an enclosed absorption cell containing TATP or SF(6), and backscattered returns were measured from a retroreflector array target at ranges of 5-100 m. DIAL sensitivity for the detection of TATP was about 0.5 ng/microl [52 parts in 10(6)(ppm)] for a 0.3 m path.

Multiwavelength discrimination and measurements of a two-gas mixture by use of a broadly tunable mid-infrared semiconductor laser

Spectroscopic detection of gases can be achieved by measuring a few species-specific absorption lines, requiring very accurate wavelength control. Alternatively, it can be achieved by using many wavelengths spread over a wide range; each wavelength need not be optimal spectroscopically, but all collectively form a unique fingerprint for the species of interest. Statistical regression can be used to quantify their concentrations. An experimental evaluation of this concept involved using a 3.1 microm broadly tunable Sb-based mid-IR laser to discriminate and measure mixtures of acetylene and water vapor with absorption spectral overlaps. As many as 30 wavelengths from approximately 3200 to approximately 3280 cm-1 were used to measure 5 x 5 combinations of the two-gas concentration. Statistical analysis of the results validates the concept. Each gas concentration was consistently and reliably measured without any problem of interference from the other. In addition, the method was sufficiently sensitivite to detect unusual discrepancies by use of statistical analysis. Optimization of the system's detection capability and its receiver-operating characteristics is demonstrated. The results suggest that the statistical multiwavelength broadband approach to detection of gas mixture can be a highly effective alternative to species-specific single-line spectroscopy.

Tunable 7 12-microm optical parametric oscillator using a Cr,Er:YSGG laser to pump CdSe and ZnGeP(2) crystals

A CdSe optical parametric oscillator (OPO) pumped by a 2.79-mum , Cr, Er:YSGG laser yielded a 59% signal-plus-idler slope efficiency (eta), a total idler output of 1.2-2.4mJ between 8.5 and 12.3 mum , and an idler beam that was 2.2-2.5 times the diffraction limit. A ZnGeP(2) OPO operated with a lower threshold, eta = 29% , and a forward idler output of 0.7-2.4 mJ from 6.9 to 9.9 microm . The signal and idler bandwidths were typically 4 cm(-1) for each OPO.