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The high-power continuous-wave laser action of a quantum cascade laser was described. A modulation-doped funnel injector featuring a three-well vertical-transition active region was used to achieve gain optimization and decreased heating effects. In addition, InP was utilized as the waveguide cladding material to enhance lateral conductance and thermal dissipation. Results showed that the lasers exhibited high performance as proven by maximum gain coefficient and injection efficiency as well as reduced carrier scattering.

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 [micro] m 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.sup.-8] [cm.sup.-1] was achieved. Measurements of parts per billion (ppb) NO concentrations in [N.sub.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. OCIS codes: 140.5960, 280.3420, 300.6320, 170.4580.

Distributed-feedback quantum-cascade (QC) lasers are expected to form the heart of the next-generation mid-IR laser absorption spectrometers, especially as they are applied to measurements of trace gases in a variety of environments. The incorporation of room-temperature-operable, single-mode QC lasers should result in highly compact and rugged sensors for real-world applications. We report preliminary results on the performance of a laser absorption spectrometer that uses a QC laser operating at room temperature in a quasi-cw mode in conjunction with balanced ratiometric detection. We have demonstrated sensitivities for [N.sub.2]O [10 parts in [10.sup.6] volume-mixing ratio for a 1-m path (ppmv-m)] and NO [520 parts in [10.sup.9] volume-mixing ratio for a l-m path (ppbv-m)] at 5.4 [micro]m. System improvements are described that are expected to result in a 2 orders of magnitude increase in sensitivity. [C] 2001 Optical Society of America OCIS codes: 010.1120, 120.1740, 140.3070, 300.1030, 300.6260, 140.5960.

A tunable quantum-cascade (QC) laser has been flown on NASA's ER-2 high-altitude aircraft to produce the first atmospheric gas measurements with this newly invented device, an important milestone in the QC laser's future planetary, industrial, and commercial applications. Using a cryogenically cooled QC laser during a series of 20 aircraft flights beginning in September 1999 and extending through March 2000, we took measurements of methane ([CH.sub.4]) and nitrous oxide ([N.sub.2]0) gas up to ~20 km in the stratosphere over North America, Scandinavia, and Russia. The QC laser operating near an 8-[micro]m wavelength was produced by the groups of Capasso and Cho of Bell Laboratories, Lucent Technologies, where QC lasers were invented in 1994. Compared with its companion lead salt diode lasers that were also flown on these flights, the single-mode QC laser cooled to 82 K and produced higher output power (10 mW), narrower laser linewidth (17 MHz), increased measurement precision (a factor of 3), and better spectral stability (~0.1 [cm.sup.-1] K). The sensitivity of the QC laser channel was estimated to correspond to a minimum-detectable mixing ratio for methane of approximately 2 parts per billion by volume. [C] 2001 Optical Society of America OCIS codes: 010.0010, 120.0120, 140.0140, 300.0300.

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 ~8 (m. Near-transform-limited laser pulses were obtained owing to short (~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.sup.-4] with [10.sub.5] laser pulses (20-kHz repetition rate), and 1.7 x [10.sup.-4] for 5 x [10.sup.5] pulses, based on the standard deviation of the linear egression analysis. [C] 2000 Optical Society of America OCIS codes: 140.5960, 280.3420, 300.6320, 010.1280.

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.sup.9] (ppb) concentration level for spectroscopic trace-gas detection of [CH.sub.4], [N.sub.2]O, [H.sub.2]O, and [C.sub.2][H.sub.5]OH in ambient air at 7.9 [micro]m. 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. [C] 2000 Optical Society of America OCIS codes: 140.5960, 280.3420, 300.6320, 010.1280.