Mark F. Witinski

Single-mode 2.65 µm InGaAsSb/AlInGaAsSb laterally coupled distributed-feedback diode lasers for atmospheric gas detection

We demonstrate index-coupled distributed-feedback diode lasers at 2.65 µm that are capable of tuning across strong absorption lines of HDO and other isotopologues of H2O. The lasers employ InGaAsSb/AlInGaAsSb multi-quantum-well structures grown by molecular beam epitaxy on GaSb, and single-mode emission is generated using laterally coupled second-order Bragg gratings etched alongside narrow ridge waveguides. We verify near-critical coupling of the gratings by analyzing the modal characteristics of lasers of different length. With an emission facet anti-reflection coating, 2-mm-long lasers exhibit a typical current threshold of 150 mA at 20 °C and are capable of emitting more than 25 mW in a single longitudinal mode, which is significantly higher than the output power reported for loss-coupled distributed-feedback lasers operating at similar wavelengths.

High tuning stability of sampled grating quantum cascade lasers.

Predictable tuning behavior and stable laser operation are both crucial for laser spectroscopy measurements. We report a sampled grating quantum cascade laser (QCL) with high spectral tuning stability over the entire tuning range. We have determined the minimum loss margin required to suppress undesired lasing modes in order to ensure predictable tuning behavior. We have quantified power fluctuations and drift of our devices by measuring the Allan deviation. To demonstrate the feasibility of sampled grating QCLs for high-precision molecular spectroscopy, we have built a simple transmission spectroscopy setup. Our results prove that sampled grating QCLs are suitable light sources for highly sensitive spectroscopy measurements.

Pump-enhanced difference-frequency generation at 3.3 μ m

The demonstration of continuous wave intracavity difference-frequency generation in the mid-infrared (mid-IR) is presented. A cavity for pump laser enhancement is constructed around a periodically poled lithium niobate crystal, and the cavity length is locked to the frequency of the pump laser using the Pound-Drever-Hall technique, producing a gain of 12 in the resultant idler power compared to the single-pass case. A widely tunable single-mode 3.3 microm idler beam with a power of nearly 10 mW is available for direct absorption spectroscopy. The pump-enhancement method demonstrated here should be readily scalable to produce hundreds of milliwatts of mid-IR light by using higher power signal and pump lasers.

Calibration and Quality Assurance of an Airborne Turbulence Probe in an Aeronautical Wind Tunnel

AbstractThe Best Aircraft Turbulence (BAT) probe is used by multiple research groups worldwide. To promote an accurate interpretation of the data obtained from the probe’s unusual nine-port design, a detailed understanding of the BAT probe’s function along with a characterization and minimization of its systematic anomalies is necessary. This paper describes recent tests to enhance understanding of the probe’s behavior. The tests completed in the Wright Brothers Wind Tunnel at the Massachusetts Institute of Technology (MIT) built on earlier findings at Purdue University. Overall the true-vertical wind relative to the probe was found to have a systematic anomaly of about 10%–15%, an acceptable value borne out by considerable field experience and further reducible by modeling and removing. However, significant departure from theoretical behavior was found, making detailed generalization to other BAT probes still inadvisable. Based on these discoveries, recommendations are made for further experiments to exp...

Two-tone frequency-modulation spectroscopy in off-axis cavity

As opposed to a conventional optical resonator, an off-axis-aligned cavity is able to transmit without distortion radiation modulated at a frequency even far above the cavity bandpass. This allows us to implement a simple spectroscopic technique that combines the cavity path-length enhancement of integrated cavity output spectroscopy (ICOS) and the noise reduction associated with radio-frequency modulation (FM). An FM-ICOS spectrometer is demonstrated for the first time using a two-tone modulation technique. The performance is compared to the traditional ICOS by examining the acetylene absorption at 1543.77 nm. A signal-to-noise ratio improvement by a factor 3.5 is found with our proof-of-concept setup. Larger improvements are expected in a more optimized setup.

Nitrous oxide quartz-enhanced photoacoustic detection employing a broadband distributed-feedback quantum cascade laser array

We present a gas sensing system based on quartz-enhanced photoacoustic spectroscopy (QEPAS) employing a monolithic distributed-feedback quantum cascade laser (QCL) array operated in a pulsed mode as a light source. The array consists of 32 quantum cascade lasers emitting in a spectral range from 1190 cm−1 to 1340 cm−1. The optoacoustic detection module was composed of a custom quartz tuning fork with a prong spacing of 1 mm, coupled with two micro-resonator tubes to enhance the signal-to-noise ratio. The QEPAS sensor was validated by detecting the absorption of the P- and R-branches of nitrous oxide. The measurements were performed by switching the array QCLs in sequence while tuning their operating temperature to retrieve the fine structure of the two N2O branches. A sensor calibration was performed, demonstrating a linear responsivity for N2O:N2 concentrations from 1000 down to 200 parts-per-million. With a 10 s lock-in integration time, a detection sensitivity of less than 60 parts-per-billion was achieved permitting the monitoring of nitrous oxide at global atmospheric levels.

Monolithic Quantum Cascade Laser Arrays for Broadband Portable Infrared Spectroscopy

This presentation presents the spectroscopic concepts and results enabled by arrays of Distributed Feedback (DFB) QCLs, with each element at a slightly different wavelength than its neighbor. In optical systems, such as standoff detectors and in situ gas analyzers, this increases analyte sensitivity and selectivity by broadening spectral source coverage and by allowing for extremely fast all-electronic wavelength tuning with no moving parts. The QCL array is also an increasingly essential solution to the power scaling of QCLs. This talk will present the QCL array concept and our packaging systems before moving to discuss molecular spectroscopy results for a) multigas sensing, b) standoff explosives detection, and c) power scaling for directed energy.

Monolithic Packaged QCL Arrays for Portable High Performance Spectroscopy

Summary form only given. This presentation presents the spectroscopic concepts and results enabled by arrays of Distributed Feedback (DFB) QCLs, with each element at a slightly different wavelength than its neighbor. In optical systems, such as standoff detectors and in situ gas analyzers, this increases analyte sensitivity and selectivity by broadening spectral source coverage and by allowing for extremely fast all-electronic wavelength tuning with no moving parts. The QCL array is also an increasingly essential solution to the power scaling of QCLs. This talk will present the QCL array concept and our packaging systems before moving to discuss molecular spectroscopy results for a) multigas sensing, b) standoff explosives detection, and c) power scaling for directed energy.The data show how monolithic and all-electronic tuning enables next-generation spectroscopes that are not only more robust and miniature than those that utilize external cavity-tuned lasers, but that are inherently more stable in terms the shot-to-shot amplitude and wavelength parameters. This enhanced stability increases signal to noise for a given configuration (pathlength, averaging time, concentration, etc...). Some discussion of how to maximize the benefits of high speed, highly reproducible tuning is presented, including detector, preamplifier, and digitization considerations for both backscattered and closed path configurations. Time permitting, preliminary results on monolithic beam combining of QCLs for both power scaling and for improved spectroscopic integration will be discussed.

Recent advances in QCLs, high finesse optical cavities and robotic instrumentation: Addressing climate change with new experimental strategies

Te key irrefutable evidence for irreversible change in the Earths climate structure lies in the high Arctic. Specifcally, the observed loss of permanent foating ice in the Arctic Ocean, which has decreased from 16 × 103 km3 in 1979 to 3 × 103 km3 in 2012, represents an onset of climate change that was not predicted to occur at anything like the observed rate - even as late as the 2007 IPCC report. Te rapid loss of permanent ice is still not quantitatively understood in terms of the mechanisms controlling the fow of thermal energy into the system, yet both the rate of loss and the increasing rate of that loss place the time scale for the loss of all permanent foating ice to be within this decade. Te remarkable consistency of the quantitative loss in ice volume, as thermal energy fowed into the ice structure, is demonstrated by both the absolute volume as a function of time and the steepening curvature in the time dependence of volumetric loss as shown in Figure 1. Tis increasing curvature is a clear refection of the feedbacks that are accelerating further loss as the Arctic Ocean foating ice recedes.

Testing Space-based Infrared Sensors for Systematic Errors

Recent developments in compact, monochromatic, high-power infrared light sources allow the implementation of analogs of laboratory measurement tests on-orbit for infrared sensors. This paper presents experimental results to demonstrate this concept.