Clinton J. Smith

Signal-to-noise ratio in chirped laser dispersion spectroscopy

Quantitative studies and experimental validation of noise sources occurring in chirped laser dispersion spectroscopy (CLaDS) are reported. Their impact on the signal-to-noise ratio (SNR) achievable with the CLaDS sensing method is analyzed through a noise model supported by experimental results. In particular the model shows that the SNR is optimal for a given value of the laser chirp rate. The experimental studies are conducted with a quantum cascade laser operating at 5.2 µm for the detection of nitric oxide. Optical fringing has been found to be a significant non-random source of noise and an effective reduction method that can improve the SNR is also discussed.

Sensing nitrous oxide with QCL-coupled silicon-on-sapphire ring resonators.

We report the initial evaluation of a mid-infrared QCL-coupled silicon-on-sapphire ring resonator gas sensor. The device probes the N(2)O 2241.79 cm(-1) optical transition (R23 line) in the ν(3) vibrational band. N(2)O concentration is deduced using a non-linear least squares fit, based on coupled-mode theory, of the change in ring resonator Q due to gas absorption losses in the evanescent portion of the waveguide optical mode. These early experiments demonstrated response to 5000 ppmv N(2)O.

Next generation infrared sensor instrumentation: remote sensing and sensor networks using the openPHOTONS repository

We describe our novel instrumentation architectures for infrared laser spectrometers. Compact, power efficient, low noise modules allow for optimized implementation of cell phone sized sensors using VCSELs, diode, and quantum cascade laser sources. These sensors can consume as little as 0.3W with full laser temperature (<0.001K/Hz1/2) and current control (<2ppm/Hz1/2 noise), photodiode preamplification (<2pA/Hz1/2 noise floor, 1MΩ transimpedance), and digital lock-in amplification with 3 independent channels. We have implemented sensors based on laser absorption spectroscopy, photoacoustic spectroscopy, and Faraday rotation spectroscopy using the openPHOTONS systems, with performance rivaling standalone laboratory measurement instrumentation. Additionally, as openPHOTONS is an open source software repository, this instrumentation can be quickly adapted to new optical configurations and applications. Such modules allow the development of flexible sensors, whether implementing closed path spectrometers, open path perimeter monitoring, or remote backscatter based sensors. This work is also the enabling technology for wireless sensor networks (WSN) of precision sensors, a desirable sensing paradigm for long term, wide area, precision, temporally and spatially resolved studies. This approach can complement existing remote sensing and mapping technologies including satellite observations and sparse networks of flux towers.

Real-time calibration of laser absorption spectrometer using spectral correlation performed with an in-line gas cell

A real-time drift correction and calibration method using spectral correlation based on a revolving in-line gas cell for laser-based spectroscopic trace-gas measurements has been developed and evaluated experimentally. This technique is relatively simple to implement in laser spectroscopy systems and assures long-term stability of trace-gas measurements by minimizing the effects of external sources of drift in real-time. Spectroscopic sensitivity sufficient for environmental monitoring and effective drift suppression has been achieved for long-term measurements of CO₂ with a quantum cascade laser based spectrometer.

Remote mid-infrared sensing using chirped laser dispersion spectroscopy

A new spectroscopic technique for remote molecular detection is presented. Chirped Laser Dispersion Spectroscopy (CLaDS) uses a two-color dynamic interferometric heterodyne detection to measure optical dispersion caused by molecular transitions. The dispersion sensing is based on measurement of instantaneous frequency of an optical heterodyne beatnote which provides high immunity to optical power fluctuations. Thus CLaDS is well suited to long distance remote sensing and open-path monitoring. In this work we present CLaDS experimental setup for remote sensing of nitric oxide using 5.2 μm quantum cascade laser. System performance as well as advantages and limitations are discussed.

High frequency polarization switching VCSEL clock using subwavelength quarter-wave plate

We demonstrated an external cavity vertical-cavity-surface-emitting-laser (VCSEL) clock using a subwavelength quarter-wave plate and achieved a polarization self-switching frequency as high as 7.2 GHz with an oscillation frequency FWHM of 6 MHz.

Cost-effective manufacturing of compact TDLAS sensors for hazardous area applications

Tunable Diode Laser Absorption Spectroscopy (TDLAS) is finding ever increasing utility for industrial process measurement and control. The technique’s sensitivity and selectivity benefit continuous concentration measurements of specific gas components in complex gas mixtures which are often laden with liquids or solid particulates. Tradeoff options among optical path length, absorption linestrength, linewidth, cross-interferences, and sampling methodology enable sensor designers to optimize detection for specific applications. Emerging applications are demanding increasing numbers of distributed miniaturized sensors at diminishing costs. In these applications, the TDLAS specificity is a key attribute, and its high sensitivity enables novel sampling package designs with short optical path lengths. This paper describes a miniature hermetically-sealed backscatter TDLAS transceiver package designed for high-volume production at acceptable cost. Occupying a volume less than 1in3 and weighing less than 0.06 lb, the transceiver is a key component of TDLAS sensors intended for in-situ measurements of potentially explosive gas mixtures.

In-line reference cell for real-time calibration of laser absorption spectrometers

We discuss a novel method of tunable diode laser sensor calibration using an in-line reference gas cell. The technique employs wavelength modulation spectroscopy at higher harmonics to simultaneously probe the sample and reference spectra.

A rotational sample/reference cell for high-accuracy realtime spectroscopic trace-gas sensing

We demonstrate real-time drift correction of a quantum cascade laser based direct absorption trace-gas sensor using a rotating in-line reference cell. High accuracy and performance sufficient for long-term environmental monitoring has been demonstrated.

Electric Current Tuning the Self-Oscillation Frequency of EC-VCSELs

We demonstrate a new way to electrically tune the self-oscillation frequency of an external cavity VCSEL by changing the electrical current injected into the VCSEL, without using any mechanical moving parts. We found that for a self-oscillation frequency from 500 MHz to 4 GHz, the tuning range is up to 15% and the tuning rate is up to 800 MHz/mA. Our experiments and simulation show that the injection current tuning of self-oscillation frequency is due to a change in the VCSEL birefringence induced by changing the injection current.