Alfred Y. Cho

Cavity ringdown spectroscopic detection of nitric oxide with a continuous-wave quantum-cascade laser

A spectroscopic gas sensor for nitric oxide (NO) detection based on a cavity ringdown technique was designed and evaluated. A cw quantum-cascade distributed-feedback laser operating at 5.2 mum was used as a tunable single-frequency light source. Both laser-frequency tuning and abrupt interruptions of the laser radiation were performed through manipulation of the laser current. A single ringdown event sensitivity to absorption of 2.2 x 10(-8) cm(-1) was achieved. Measurements of parts per billion (ppb) NO concentrations in N(2) with a 0.7-ppb standard error for a data collection time of 8 s have been performed. Future improvements are discussed that would allow quantification of NO in human breath.

Raman injection laser.

Stimulated Raman scattering is a nonlinear optical process that, in a broad variety of materials, enables the generation of optical gain at a frequency that is shifted from that of the incident radiation by an amount corresponding to the frequency of an internal oscillation of the material. This effect is the basis for a broad class of tunable sources known as Raman lasers. In general, these sources have only small gain (∼ 10-9 cm W-1) and therefore require external pumping with powerful lasers, which limits their applications. Here we report the realization of a semiconductor injection Raman laser designed to circumvent these limitations. The physics underlying our device differs in a fundamental way from existing Raman lasers: it is based on triply resonant stimulated Raman scattering between quantum-confined states within the active region of a quantum cascade laser that serves as an internal optical pump—the device is driven electrically and no external laser pump is required. This leads to an enhancement of orders of magnitude in the Raman gain, high conversion efficiency and low threshold. Our lasers combine the advantages of nonlinear optical devices and of semiconductor injection lasers, and could lead to a new class of compact and wavelength-agile mid-and far-infrared light sources.

Trace-gas detection in ambient air with a thermoelectrically cooled, pulsed quantum-cascade distributed feedback laser

A pulsed quantum-cascade distributed feedback laser operating at near room temperature was used for sensitive high-resolution IR absorption spectroscopy of ambient air at a wavelength of approximately 8 microm. Near-transform-limited laser pulses were obtained owing to short (approximately 5-ns) current pulse excitation and optimized electrical coupling. Fast and slow computer-controlled frequency scanning techniques were implemented and characterized. Fast computer-controlled laser wavelength switching was used to acquire second-derivative absorption spectra. The minimum detectable absorption was found to be 3 x 10(-4) with 10(5) laser pulses (20-kHz repetition rate), and 1.7 x 10(-4) for 5 x 10(5) pulses, based on the standard deviation of the linear regression analysis.

Effective utilization of quantum-cascade distributed-feedback lasers in absorption spectroscopy

A variable duty cycle quasi-cw frequency scanning technique was applied to reduce thermal effects resulting from the high heat dissipation of type I quantum-cascade lasers. This technique was combined with a 100-m path-length multipass cell and a zero-air background-subtraction technique to enhance detection sensitivity to a parts-in-10(9) (ppb) concentration level for spectroscopic trace-gas detection of CH4, N2O, H2O, and C2H5OH in ambient air at 7.9 micrometers. A new technique for analysis of dense high resolution absorption spectra was applied to detection of ethanol in ambient air, yielding a 125-ppb detection limit.

Thermoelectrically cooled quantum-cascade-laser-based sensor for the continuous monitoring of ambient atmospheric carbon monoxide

We report the first application of a thermoelectrically cooled, distributed-feedback quantum-cascade laser for continuous spectroscopic monitoring of CO in ambient air at a wavelength of 4.6 microm. A noise-equivalent detection limit of 12 parts per billion was demonstrated experimentally with a 102-cm optical pathlength and a 2.5-min data acquisition time at a 10-kHz pulsed-laser repetition rate. This sensitivity corresponds to a standard error in fractional absorbance of 3 x 10(-5).

Transportable automated ammonia sensor based on a pulsed thermoelectrically cooled quantum-cascade distributed feedback laser

A compact ammonia sensor based on a 10-microm single-frequency, thermoelectrically cooled, pulsed quantum-cascade laser with an embedded distributed feedback structure has been developed. To measure NH3 concentrations, we scanned the laser over two absorption lines of its fundamental v2 band. A sensitivity of better than 0.3 parts per million was achieved with just a 1-m optical path length. The sensor is computer controlled and automated to monitor NH3 concentrations continuously for extended periods of time and to store data in the computer memory.

Single frequency quantumcascade-DFB laser based spectrometer for trace gas detection

Performance characteristics of a single frequency mid-infrared quantum cascade distributed feedback (QC-DFB) laser will be reported. Sensitive spectroscopic trace gas detection of CH4, N2O, H2O and C2H5OH in the 7.9 μm spectral region was demonstrated using a 100-m Herriott multipass cell and a rapid “zero-air” background subtraction technique.