Accurate Measurement of the Optical Constants n and k for a Series of 57 Inorganic and Organic Liquids for Optical Modeling and Detection.

For optical modeling and other purposes, we have created a library of 57 liquids for which we have measured the complex optical constants n and k. These liquids vary in their nature, ranging in properties that include chemical structure, optical band strength, volatility, and viscosity. By obtaining the optical constants, one can model most optical phenomena in media and at interfaces including reflection, refraction, and dispersion. Based on the works of others, we have developed improved protocols using multiple path lengths to determine the optical constants n/k for dozens of liquids, including inorganic, organic, and organophosphorus compounds. Detailed descriptions of the measurement and data reduction protocols are discussed; agreement of the derived optical constant n and k values with literature values are presented. We also present results using the n/k values as applied to an optical modeling scenario whereby the derived data are presented and tested for models of 1 µm and 100 µm layers for dimethyl methylphosphonate (DMMP) on both metal (aluminum) and dielectric (soda lime glass) substrates to show substantial differences between the reflected signal from highly reflective substrates and less-reflective substrates.

Characterization of a swept external cavity quantum cascade laser for rapid broadband spectroscopy and sensing.

The performance of a rapidly swept external cavity quantum cascade laser (ECQCL) system combined with an open-path Herriott cell was evaluated for time-resolved measurements of chemical species with broad and narrow absorption spectra. A spectral window spanning 1278 - 1390 cm(-1) was acquired at a 200 Hz acquisition rate, corresponding to a tuning rate of 2x10(4) cm(-1)/s, with a spectral resolution of 0.2 cm(-1). The capability of the ECQCL to measure < 100 ppbv changes in nitrous oxide (N(2)O) and 1,1,1,2-tetrafluoroethane (F134A) concentrations on millisecond timescales was demonstrated in simulated plume studies with releases near the open-path Herriott cell. Absorbance spectra measured using the ECQCL system exhibited noise-equivalent absorption coefficients of 5x10(-9) cm(-1)Hz(-1/2). For a spectrum acquisition time of 5 ms, noise-equivalent concentrations (NEC) for N(2)O and F134A were measured to be 70 and 16 ppbv respectively, which improved to sub-ppbv levels with averaging to 100 s. Noise equivalent column densities of 0.64 and 0.25 ppmv × m in 1 sec are estimated for N(2)O and F134A.

Precision control of multiple quantum cascade lasers for calibration systems.

We present a precision, 1-A, digitally interfaced current controller for quantum cascade lasers, with demonstrated temperature coefficients for continuous and 40-kHz full-depth square-wave modulated operation, of 1-2 ppm/ °C and 15 ppm/ °C, respectively. High precision digital to analog converters (DACs) together with an ultra-precision voltage reference produce highly stable, precision voltages, which are selected by a multiplexer (MUX) chip to set output currents via a linear current regulator. The controller is operated in conjunction with a power multiplexing unit, allowing one of three lasers to be driven by the controller, while ensuring protection of controller and all lasers during operation, standby, and switching. Simple ASCII commands sent over a USB connection to a microprocessor located in the current controller operate both the controller (via the DACs and MUX chip) and the power multiplexer.

Real-time trace gas sensing of fluorocarbons using a swept-wavelength external cavity quantum cascade laser.

We present results demonstrating real-time sensing of four different fluorocarbons at low part-per billion (ppb) concentrations using an external cavity quantum cascade laser (ECQCL) designed for infrared vibrational spectroscopy of molecules with broad absorption features. The ECQCL was repeatedly swept at 20 Hz over its full tuning range of 1145-1265 cm(-1) providing a scan rate of 3535 cm(-1) s(-1), and a detailed characterization of the ECQCL scan stability and repeatability is presented. The ECQCL was combined with a 100 meter path length multi-pass cell for direct absorption spectroscopy. A portable sensor system is described, which was deployed on a mobile automotive platform to provide spatially-resolved detection of fluorocarbons in outdoor experiments. Noise-equivalent detection limits of 800-1000 parts-per-trillion (ppt) are demonstrated for 1 s integration times.

Note: Switch-mode hybrid current controllers for quantum cascade lasers.

Chemical sensing in the mid-infrared has been revolutionized by the advent of quantum cascade lasers. Their higher current and voltage demands compared to laser diodes however, negatively impact size, weight, and power footprint for fieldable systems, particularly in hot environments. This Note presents a switch-mode/linear hybrid controller, furnishing drive currents up to 2 A at compliance voltages up to 15 V, with output noise near levels determined by shot and Johnson contributions from within the linear regulator, from frequencies below 100 Hz to the switching frequency. Current modulation from near 0 to 1.8 A is also demonstrated at frequencies up to a kilohertz.

Infrared near-field spectroscopy of trace explosives using an external cavity quantum cascade laser.

Utilizing a broadly-tunable external cavity quantum cascade laser for scattering-type scanning near-field optical microscopy (s-SNOM), we measure infrared spectra of particles of explosives by probing characteristic nitro-group resonances in the 7.1-7.9 µm wavelength range. Measurements are presented with spectral resolution of 0.25 cm(-1), spatial resolution of 25 nm, sensitivity better than 100 attomoles, and at a rapid acquisition time of 90 s per spectrum. We demonstrate high reproducibility of the acquired s-SNOM spectra with very high signal-to-noise ratios and relative noise of <0.02 in self-homodyne detection.

Intracavity sensing via compliance voltage in an external cavity quantum cascade laser.

We demonstrate a technique for gas phase spectroscopy and sensing by detecting changes in compliance voltage of an external cavity quantum cascade laser due to intracavity absorption. The technique is characterized and used to measure the absorption spectrum of water vapor and Freon-134a.

Low-noise high-performance current controllers for quantum cascade lasers.

Quantum cascade lasers have ushered in a new era of enhanced capability for chemical sensing. The higher current and voltage demands of these devices over their laser diode counterparts have also ushered in the demand for more capable drive electronics. The current-sensitivity and high frequency response of these devices have continued the desire for low noise, stability, and agility enjoyed by the laser diode community for many years. This article addresses the issue of maintaining these characteristics at the currents and voltages required, and presents example performance of current controllers developed by the author at Pacific Northwest National Laboratory, achieving output currents up to 2 A and compliance voltages of 15 V, with noise levels close to the Johnson noise of the internal resistors, typically a few nA/√Hz. Rapid full-depth current modulation up to 100 kHz is also demonstrated.

Single-mode, narrow-linewidth external cavity quantum cascade laser through optical feedback from a partial-reflector.

An external-cavity (EC) quantum cascade (QC) laser using optical feedback from a partial-reflector is reported. With this configuration, the otherwise multi-mode emission of a Fabry-Perot QC laser was made single-mode with optical output powers exceeding 40 mW. A mode-hop free tuning range of 2.46 cm(-1) was achieved by synchronously tuning the EC length and QC laser current. The linewidth of the partial-reflector EC-QC laser was measured for integration times from 100 µs to 4 seconds, and compared to a distributed feedback QC laser. Linewidths as small as 480 kHz were recorded for the EC-QC laser.

Stabilization, injection and control of quantum cascade lasers, and their application to chemical sensing in the infrared.

Quantum cascade lasers (QCLs) are a relatively new type of semiconductor laser operating in the mid- to long-wave infrared. These monopolar multilayered quantum well structures can be fabricated to operate anywhere between 3.5 and 20 microm, which includes the molecular fingerprint region of the infrared. This makes them an ideal choice for infrared chemical sensing, a topic of great interest at present. Frequency stabilization and injection locking increase the utility of QCLs. We present results of locking QCLs to optical cavities, achieving relative linewidths down to 5.6 Hz. We report injection locking of one distributed feedback grating QCL with light from a similar QCL, demonstrating capture ranges of up to +/-500 MHz, and suppression of amplitude modulation by up to 49 dB. We also present various cavity-enhanced chemical sensors employing the frequency stabilization techniques developed, including the resonant sideband technique known as NICE-OHMS. Sensitivities of 9.7 x 10(-11) cm(-1) Hz(-1/2) have been achieved in pure nitrous oxide.

Frequency stabilization of quantum-cascade lasers by use of optical cavities.

We report a heterodyne beat with a linewidth of 5.6+/-0.6 Hz between two cavity-stabilized quantum-cascade lasers operating at 8.5 microm . We also present a technique for measuring this beat that avoids the need for extreme isolation of the optical cavities from the environment, that of employing a third servo loop with low bandwidth to force one cavity to track the slow drifts and low-frequency fluctuations of the other. Although it is not fully independent, this technique greatly facilitates heterodyne beat measurements for evaluating the performance of cavity-locked lasers above the bandwidth of the third loop.