Andreas Hangauer

High frequency modulation capabilities and quasi single-sideband emission from a quantum cascade laser.

Both intensity- (IM) and frequency-modulation (FM) behavior of a directly modulated quantum cascade laser (QCL) are measured from 300 Hz to 1.7 GHz. Quantitative measurements of tuning coefficients has been performed and the transition from thermal- to electronic-tuning is clearly observed. A very specific FM behavior of QCLs has been identified which allows for optical quasi single sideband (SSB) modulation through current injection and has not been observed in directly modulated semiconductor lasers before. This predestines QCLs in applications where SSB is required, such as telecommunication or high speed spectroscopy. The experimental procedure and theoretical modeling for data extraction is discussed.

Gain Compression and Linewidth Enhancement Factor in Mid-IR Quantum Cascade Lasers

We have observed and quantified the adiabatic and transient chirp in a directly modulated quantum cascade laser (QCL). Those wavelength tuning effects are well-characterized in diode lasers, and the rate equation model that successfully describe the diode laser behavior also provides an excellent fit for the QCL data. In this study, we have extracted the linewidth enhancement factor (αH) from the transient chirp and the gain compression factor from the adiabatic chirp. We postulate that the extraction of the αH from the transient chirp is valid for the QCL case, despite additional tuning effects in QCLs (e.g., voltage tuning) that are negligible in diode lasers. Also in the QCL the adiabatic chirp coefficient strongly increases with laser output power but still stays an order of magnitude below typical values known from diode lasers. We hypotesize possibility of the adiabatic chirp to be connected to the χ(3) nonlinearity (responsible for four-wave mixing that was recently attributed to the FM self-locking in QCLs), but the exact origin remains to be experimentally confirmed in future work.

Wavelength modulated multiheterodyne spectroscopy using Fabry-Pérot quantum cascade lasers.

Multiheterodyne spectroscopy implemented with semiconductor Fabry-Pérot lasers is a method for broadband (> 20 cm-1), high spectral resolution (~1 MHz) and high time resolution (< 1 µs/spectrum) spectroscopy with no moving parts utilizing off-the-shelf laser sources. The laser stabilization approach demonstrated here enables continuous frequency tuning (at 12.5 Hz repetition rate) while allowing for multiheterodyne wavelength modulation spectroscopy (WMS). Spectroscopic detection of N2O around 1185 cm-1 is experimentally realized, which shows a direct absorption sensitivity limit of ~1.5⨯10-3/√Hz fractional absorption per mode. This can be lowered using WMS down to 5⨯10-4/√Hz per mode, limited by optical fringes. This approaches the range of sensitivities of standard single-mode laser based spectrometers, which demonstrates that the multiheterodyne method is well-suited for chemical sensing of spectrally broadened absorption features or for multi-species measurements.

Gas sensing fiber network with simultaneous multi-node detection using range-resolved chirped laser dispersion spectroscopy

We present a laser-spectroscopic method for continuous and simultaneous interrogation of multiple passive gas sensor nodes in a fiber based sensor network. Enabled by chirped laser dispersion spectroscopy, methane leak detection is demonstrated as an example application.

Chirped laser dispersion spectroscopy with directly modulated quantum cascade laser

A feasibility study of chirped laser dispersion spectroscopy (CLaDS) with utilizing direct modulation of a quantum cascade laser instead of external modulators is presented. Optimization of laser parameters enables nearly single- and dual-sideband CLaDS operation.

High-speed modulation characteristic of a quantum cascade laser

A high-frequency (100kHz-1GHz) modulation of the quantum cascade laser amplitude and phase is presented. Plasma-effect tuning with coefficients in 0.5-1.7MHz/mA range is observed depending on bias current. Carrier dynamics effects appear at frequencies >100MHz.

FPGA-based chirped laser dispersion spectrometer

We present a FPGA based fast data acquisition system which enables continuous, real time chirped laser dispersion spectroscopic chemical sensing. System performance is evaluated using atmospheric methane detection sensing as an example application.

Remote Sensing of Atmospheric Methane with Simultaneous Ranging Using Chirped Laser Dispersion Spectroscopy

We present a new sensing technology that allows for simultaneous sensing and ranging using chirped laser dispersion spectroscopy (CLaDS). In conjunction with previous works demonstrating the effectiveness of CLaDS for remote sensing, this new configuration yields spectroscopic and ranging information from a single measurement, and is implemented for continuous, multi-path detection of atmospheric methane. 1. INTRODUCTION In remote sensing of atmospheric trace gases, knowledge of the path length is crucial to accurate extraction of the species concentration. However, in most case, this optical path length must be known beforehand or measured separately. Currently, the standard technique for atmospheric ranging and sensing is lidar (light radar), where pulsed light probes the target environment. The timing of the returned backscatter is used to extract the path length, and the received intensity is analyzed to determine the path-integrated concentration. We present an alternative, continuous wave (CW), approach based on the fundamentals of chirped laser dispersion spectroscopy (CLaDS). A proof-of-concept study has shown that both species concentration and path length can be extracted from a single, CW measurement. Development of a system that combines this CW sensing and ranging feature, with a CLaDS system that monitors atmospheric methane will be presented.

Fundamental limits in chirped laser dispersion spectroscopy

We present performance analysis of chirped laser dispersion spectroscopy (CLaDS) under shot noise limited conditions. A comparison to direct laser absorption spectroscopy (DLAS) is also provided.

Noise properties in multi-heterodyne spectrometers based on quantum- and interband-cascade lasers

The noise properties of two multi-heterodyne spectrometers based on multi-mode semiconductor lasers (QCLs and ICLs) are investigated to determine their detection performance limitations. Both laser technologies provide well-defined multi-heterodyne beat-note structures suitable for spectroscopic measurements.