William J. Kessler

Ultrasensitive dual-beam absorption and gain spectroscopy: applications for near-infrared and visible diode laser sensors

A dual-beam detection strategy with automatic balancing is described for ultrasensitive spectroscopy. Absorbances of 2 × 10(-7) Hz(-½) in free-space configurations and 5 × 10(-6) Hz(-½) in fiber-coupled configurations are demonstrated. With the dual-beam technique, atmospherically broadened absorption transitions may be resolved with InGaAsP, AlGaAs, and AlGaInP single-longitudinal-mode diode lasers. Applications to trace measurements of NO(2), O(2), and H(2)O are described by the use of simple, inexpensive laser and detector systems. Small signal gain measurements on optically pumped I(2) with a sensitivity of 10(-5) are also reported.

Diode laser-based air mass flux sensor for subsonic aeropropulsion inlets

An optical air mass flux sensor based on a compact, room-temperature diode laser in a fiber-coupled delivery system has been tested on a full-scale gas turbine engine. The sensor is based on simultaneous measurements of O(2) density and Doppler-shifted velocity along a line of sight across the inlet duct. Extensive tests spanning engine power levels from idle to full afterburner demonstrate accuracy and precision of the order of 1-2% of full scale in density, velocity, and mass flux. The precision-limited velocity at atmospheric pressure was as low as 40 cm/s. Multiple data-reduction procedures are quantitatively compared to suggest optimal strategies for flight sensor packages.

Observations of gain on the I(P1∕22→P3∕22) transition by energy transfer from O2(aΔg1) generated by a microwave discharge in a subsonic-flow reactor

The excitation of I(P1∕22) in the reactions of discharge-generated O2(aΔg1) and O(P3) with I2 has been investigated in a microwave discharge-flow reactor at 1.5Torr and ∼350K using a suite of optical absorption and emission diagnostics to detect O2(aΔg1), O, I(P1∕22), and I(P3∕22) with high sensitivity. For O2(aΔg1) yields of 0.20–0.25 generated by the microwave discharge, positive gain on the I(P1∕22→P3∕22) transition at 1.315μm was observed when O concentrations were reduced by reaction with added NO2. The results imply quenching mechanisms for I(P1∕22) which are much faster than direct collisional deactivation by O.

Diode laser-based sensors for chemical oxygen iodine lasers

We describe several diode laser-based instruments that can detect important species in chemical oxygen iodine lasers (COIL). Species detected include: water vapor, atomic iodine, and ground state oxygen. The sensors allow non-intrusive, real-time measurements from which one can determine small signal gain and the singlet delta oxygen yield. The water vapor concentrations can also be continuously monitored. The sensitivities of the sensors are sufficient for all the conditions found in typical COIL devices. The room temperature diode lasers are miniature and fiber coupled. Data for all three species are presented.

Simultaneous water vapor concentration and temperature measurements using 1.31-micron diode lasers

This paper reports the development of a compact, inexpensive sensor for simultaneous water vapor concentration and temperature measurements suitable for aeropropulsion exhaust applications. High sensitivity is achieved with an electronically balanced dual detector strategy that circumvents requirements for custom-fabricated lasers operating at specific wavelengths or high-frequency modulation techniques. Using widely available, broadly tunable InGaAsP diode lasers near 1.31 jxm, simultaneous measurements are demonstrated in a fiber-coupled, wavelength multiplexed configuration with a limiting density sensitivity of 1015 cm~3 in a 50-cm path and an rms standard deviation of 42 K over a range from 300 to 1300 K. Initial results suggest the possibility of extending this temperature range to 1900 K and above using other line pairs. (1) where Iv is the monochromatic laser intensity at frequency v, mea- sured after propagating a pathlength t through a medium with an absorbing species number density N. The strength of the absorp- tion is determined by the temperature-dependent line strength S(T), and the line shape function g(v — v()). The line shape function de- scribes the temperature- and pressure-dependent broadening mech- anism of the fundamental line strength. The temperature dependence of the line strength arises from the Boltzmann population statistics governing the internal energy level population distribution of the absorbing species. The single-mode distributed feedback (DFB) diode lasers used in this work are sufficiently narrow in frequency that they may be con- sidered essentially monochromatic with respect to the absorption line shape. The laser frequency may be tuned over a range that en- compasses the entire line shape function so that the resultant trans- mission can be integrated to remove the pressure and temperature dependence of the line-broadening mechanisms. The recorded ab- sorbance, then, is proportional only to the temperature-dependent line strength and the absorbing species number density. It is usu- ally possible to select an absorbing ground state whose line strength is relatively constant over some target temperature range so that the absorbance is a direct measurement of species number density. Separate temperature measurements may be used to correct for tem- perature variations, if necessary. Alternatively, two absorption transitions may be probed (using one or two lasers, depending on the target transition separation and the laser tuning range). The ratio of the integrated absorbance of each transition is a pure function of temperature,2

Fabrication of silicon-on-insulator adiabatic tapers for low loss optical interconnection of photonic devices

Steps are described for fabricating, preparing, and assembling pigtailed optical mode converters being developed for low loss coupling of optical fibers to high index contrast waveguide devices and arrays. The mode converters comprise adiabatic waveguide tapers fabricated from silicon-on-insulator (SOI) wafers, utilizing the silicon device layer as a waveguide core and the buried oxide as the underlying clad. Polished facets comprise the input and output ends of the tapers. The mode shape at the input typically matches that of an SMF-28 fiber, while the output ends can be sized to match various waveguide device mode shapes, typically ranging from 1 to 5 microns with aspect ratios as high as 5:1. Semiconductor planar processing techniques are employed to form the tapers upon commercial SOI wafers. An additional oxide layer is deposited upon the tapers to provide a symmetric clad around the silicon. Once fabricated, the wafers are diced into chips containing rows of tapers. The input and output facets are then lapped and polished, using a precision end point process, after which an anti reflective (AR) coating is applied. Following AR coating the chips are aligned and bonded to either single fibers or V-groove fiber arrays, creating the final pigtailed mode converter device. The insertion loss for completed mode converters ranges from 0.5 to 1 dB depending upon output facet size and asymmetry.

Velocity field imaging in supersonic reacting flows near atmospheric pressure

A combined experimental and analytical effort was conducted to demonstrate the applicability of OH Doppler-shifted fluorescence imaging of velocity distributions in supersonic combustion gases near and above atmospheric pressure. The experiments were conducted in the underexpanded exhaust flow from a 6.8-atm, 2400-K, H 2 -O 2 -N 2 burner exhausting into the atmosphere. The effects of pulse-to-pulse variations in the dye laser band shape and inplane variations in temperature and pressure were examined in detail. A modification was developed to increase the single-pulse laser bandwidth, thereby increasing the intra-image velocity dynamic range as well as reducing the sensitivity of the velocity measurement to the gas property variations

Rotational level-dependent collisional broadening and line shift of the A2σ+−X2∏ (1, 0) band of OH in hydrogen-air combustion gases

Abstract Measurements of the collisional broadening and line shift of the (1, 0) band of the A2σ+−X2∏ system of OH are reported in atmospheric pressure hydrogen-air combustion gases. The measurements were made using a single-mode, narrow linewidth, frequency-doubled ring dye laser operating near 283 nm. The OH was generated in the combustion gases of a flat flame H2-air burner. Collisional broadening parameters for equilibrium mixtures of H2, O2, H2O, and N2 were obtained spanning a range of fuel/air equivalence ratios from 0.6 to 1.6 and temperatures from 1500 to 2050 K. Measurements were obtained spanning rotational quantum numbers from J″ = 4.5 to 16.5. The collision induced frequency shift was determined to be 0.1 that of the collisional broadening.

Collisional broadening of absorption lines in water vapor and atomic iodine relevant to COIL diagnostics

We present results for collisional broadening for selected absorption lines in water vapor and atomic iodine relative to diode laser- based diagnostics for chemical oxygen iodine lasers. For water vapor we measured broadening of the 1.3925 µm line by numerous gases including oxygen, water vapor, nitrogen, and helium. Preliminary measurements were also completed on the (3,4) hyperfine line in atomic iodine at 1.3152 µm.

Accurate prediction of collapse temperature using optical coherence tomography-based freeze-drying microscopy

The objective of this study was to assess the feasibility of developing and applying a laboratory tool that can provide three-dimensional product structural information during freeze-drying and which can accurately characterize the collapse temperature (Tc ) of pharmaceutical formulations designed for freeze-drying. A single-vial freeze dryer coupled with optical coherence tomography freeze-drying microscopy (OCT-FDM) was developed to investigate the structure and Tc of formulations in pharmaceutically relevant products containers (i.e., freeze-drying in vials). OCT-FDM was used to measure the Tc and eutectic melt of three formulations in freeze-drying vials. The Tc as measured by OCT-FDM was found to be predictive of freeze-drying with a batch of vials in a conventional laboratory freeze dryer. The freeze-drying cycles developed using OCT-FDM data, as compared with traditional light transmission freeze-drying microscopy (LT-FDM), resulted in a significant reduction in primary drying time, which could result in a substantial reduction of manufacturing costs while maintaining product quality. OCT-FDM provides quantitative data to justify freeze-drying at temperatures higher than the Tc measured by LT-FDM and provides a reliable upper limit to setting a product temperature in primary drying.