Clifford Frez

High-power laterally coupled distributed-feedback GaSb-based diode lasers at 2 μm wavelength

We report on GaSb-based laterally coupled distributed-feedback (DFB) diode lasers designed to operate at wavelengths near 2.05 μm. Second-order Bragg gratings were etched alongside narrow ridge waveguides to enable single-mode DFB operation in 2-mm-long laser diodes. At a heat-sink temperature of 10 °C, the lasers emit more than 40 mW continuous-wave in a single longitudinal mode, while increasing the current beyond 300 mA results in multimode operation due to spectral shifting of the laser gain with respect to the peak grating reflectivity. At −10 °C, we observe DFB operation at higher current, with single-facet emission exceeding 80 mW.

Laser generation of gas bubbles: Photoacoustic and photothermal effects recorded in transient grating experiments

Absorption of high power laser radiation by colloidal suspensions or solutions containing photoreactive chemicals can result in bubble production. Here, transient grating experiments are reported where picosecond and nanosecond lasers are used to initiate photoinduced processes that lead to bubble formation. Irradiation of colloidal Pt suspensions is found to produce water vapor bubbles that condense back to liquid on a nanosecond time scale. Laser irradiation of Pt suspensions supersaturated with CO(2) liberates dissolved gas to produce bubbles at the sites of the colloidal particles. Laser induced chemical reactions that produce bubbles are found in suspensions of particulate C in water, and in the sensitized decarboxylation of oxalic acid. Theory based on linear acoustics as well as the Rayleigh-Plesset equation is given for description of the bubble motion.

Photoacoustic transients produced by laser generated, ultrahigh thermal gradients

Irradiation of an absorbing surface in contact with a transparent fluid by a pulsed laser can result in generation of enormous thermal gradients. Here it is shown that the effect of such thermal gradients on photoacoustic waves is the production of fast transients that accompany the leading edge of the wave. Calculations show that the transients can be accounted for by an additional source term in the wave equation for pressure. Experiments are reported showing the predicted transients on waves from absorbing layers submerged in transparent fluids irradiated with 10 ns laser pulses.

Portable 4.6 Micrometers Laser Absorption Spectrometer for Carbon Monoxide Monitoring and Fire Detection

The air quality aboard manned spacecraft must be continuously monitored to ensure crew safety and identify equipment malfunctions. In particular, accurate real-time monitoring of carbon monoxide (CO) levels helps to prevent chronic exposure and can also provide early detection of combustion-related hazards. For long-duration missions, environmental monitoring grows in importance, but the mass and volume of monitoring instruments must be minimized. Furthermore, environmental analysis beyond low-Earth orbit must be performed in-situ, as sample return becomes impractical. Due to their small size, low power draw, and performance reliability, semiconductor-laser-based absorption spectrometers are viable candidates for this purpose. To reduce instrument form factor and complexity, the emission wavelength of the laser source should coincide with strong fundamental absorption lines of the target gases, which occur in the 3 to 5 micrometers wavelength range for most combustion products of interest, thereby reducing the absorption path length required for low-level concentration measurements. To address the needs of current and future NASA missions, we have developed a prototype absorption spectrometer using a semiconductor quantum cascade laser source operating near 4.6 micrometers that can be used to detect low concentrations of CO with a compact single-pass absorption cell. In this study, we present the design of the prototype instrument and report on measurements of CO emissions from the combustion of a variety of aerospace plastics.

Transient gratings generated by particulate suspensions : The uniformly irradiated sphere and the point source

Expressions for the time dependence of the state variables in a transient grating experiment carried out on suspensions of particles can be determined by integration over space of the solutions for the temperature and photoacoustic pressure for a single particle. The method relies on independent computation of the thermal and acoustic modes of wave motion which are combined to give the temperature, pressure, and density in the grating as a function of time. Calculations are given for the uniformly irradiated droplet and the point source, the latter including the effects of a temperature-dependent thermal expansion coefficient. Transient grating experiments are reported in colloidal Pt that show features described in the calculation.

Low power consumption lasers for next generation miniature optical spectrometers for major constituent and trace gas analysis

The air quality of any manned spacecraft needs to be continuously monitored in order to safeguard the health of the crew. Air quality monitoring grows in importance as mission duration increases. Due to the small size, low power draw, and performance reliability, semiconductor laser-based instruments are viable candidates for this purpose. The minimum instrument size requires lasers with emission wavelength coinciding with the absorption of the fundamental frequency of the target gases which are mostly in the 3.0-5.0 micrometers wavelength range. In this paper we report on our progress developing high wall plug efficiency type-I quantum-well GaSb-based diode lasers operating at room temperatures in the spectral region near 3.0-3.5 micrometers and quantum cascade (QC) lasers in the 4.0-5.0 micrometers range. These lasers will enable the development of miniature, low-power laser spectrometers for environmental monitoring of the spacecraft.

Generation of photoacoustic transients from optically induced thermal gradients

Irradiation of an absorbing surface in contact with a transparent fluid with a pulsed laser can result in the generation of extremely large thermal gradients. For example, when a laser with a pulse width of 10 ns and a fluence of 1 J/cm2 irradiates a region with an absorption of 1 cm1 having the thermal properties of liquid water, a thermal gradient on the order of 105 K/m at the interface is produced. Here, it is shown that the effect of such thermal gradients on photoacoustic waves from an infinite half space and from a uniformly irradiated sphere is the production of fast transients on the leading edges of the waves. The character of the transients is determined from an additional source term in the wave equation for pressure that obtains when heat conduction is taken into account. Experiments are reported showing the predicted transients on photoacoustic waves from absorbing layers in contact with transparent fluids irradiated with 10 ns laser pulses.