We develop a spectroscopy platform for industrial applications based on semiconductor quantum cascade laser (QCL)
frequency combs. The platform’s key features will be an unmatched combination of bandwidth of 100 cm<sup>-1</sup>, resolution of
100 kHz, speed of ten to hundreds of μs as well as size and robustness, opening doors to beforehand unreachable
markets. The sensor can be built extremely compact and robust since the laser source is an all-electrically pumped
semiconductor optical frequency comb and no mechanical elements are required. However, the parallel acquisition of
dual-comb spectrometers comes at the price of enormous data-rates. For system scalability, robustness and optical
simplicity we use free-running QCL combs. Therefore no complicated optical locking mechanisms are required. To
reach high signal-to-noise ratios, we develop an algorithm, which is based on combination of coherent and non-coherent
averaging. This algorithm is specifically optimized for free-running and small footprint, therefore high-repetition rate,
comb sources. As a consequence, our system generates data-rates of up to 3.2 GB/sec. These data-rates need to be
reduced by several orders of magnitude in real-time in order to be useful for spectral fitting algorithms.
We present the development of a data-treatment solution, which reaches a single-channel throughput of 22% using a
standard laptop-computer. Using a state-of-the art desktop computer, the throughput is increased to 43%. This is
combined with a data-acquisition board to a stand-alone data processing unit, allowing real-time industrial process
observation and continuous averaging to achieve highest signal fidelity.
Room temperature, continuous wave (CW) operation of distributed feedback (DFB) quantum cascade lasers with widely
spaced operation frequencies is reported. The relatively small temperature tuning range of a single device, smaller or
equal to approximately 1 % of the wavelength, usually limits their efficiency for spectroscopic investigations. By using a
bound-to-continuum active region to create a broad gain spectrum and monolithic integration of different DFB gratings,
we achieved high-performance devices with single-mode emission between 7.7 and 8.3 &mgr;m at a temperature of +30 °C.
This frequency span corresponds to 8 % of the center frequency. The maximum CW operation temperature achieved was
63 °C at the gain center and as much as 35 °C and 45 °C, respectively, at the limits of the explored wavelength range.
We report a realization of single-frequency quantum-cascade lasers in continuous-wave mode on thermo-electrical cooler at frequencies ~ 1830 cm<sup>-1</sup> (~ 5.46 μm) and ~ 1900 cm<sup>-1</sup> (~ 5.26 μm). The active region of the lasers is based on the broad gain bound-to-continuum concept. We report a 1.5 mm-long, 18 μm-wide quantum-cascade laser exhibiting single-mode emission over the entire investigated temperature and current ranges with a side-mode suppression ratio > 25 dB. Output powers up to 54 mW at -30°C and 1.2 mW at +27°C are demonstrated. A tuning range of 12.8 cm<sup>-1</sup> (0.7%) can be obtained between 1823.1 cm<sup>-1</sup> and 1835.9 cm<sup>-1</sup>. A different device, 1.5 mm-long, 12 μm-wide, is reported in the range 1892.8 cm<sup>-1</sup> to 1905.5 cm<sup>-1</sup>, exhibiting output power of 59 mW at -30°C and 0.8 mW at +20°C. The objective of this development is to obtain a room-temperature continuous-wave quantum-cascade laser at 1900cm<sup>-1</sup>, important for NO (nitric oxide) measurements. We demonstrate also Fabry-Pérot continuous-wave operation of quantum-cascade lasers grown by metal organic vapour phase epitaxy up to -5°C without the need of buried heterostructure processing.
The quantum cascade laser is an unipolar semiconductor laser source emitting in the mid-infrared range between 3.5 and 25 μm. During the past ten years after their invention, this technology has
reached the level of maturity required for commercialization, and QC
lasers have thus become very attractive for a large number of
applications, including gas sensing, pollution detection, atmospheric chemistry, detection of compounds, non-invasive medical
diagnostics, free-space optical data transmission or even LIDAR. Most common requirements are single-mode operation on thermoelectric cooler, high power and/or continuous-wave. Nowadays several high-power single-mode QC lasers are available at Alpes Lasers in the range from 4.3 to 16.5 μm, with a side-mode suppression ratio larger than 30 dB. We present here a specific high-average power Fabry-Perot quantum cascade laser and a distributed-feedback quantum cascade laser operating near 8 μm.
Continuous wave (CW) operation of quantum cascade lasers is reported up to a temperature of 312 K. The junction down mounted devices were designed as buried heterostructure lasers with high-reflection coatings on both facets. This resulted in CW operation at an emission wavelength of 9.1μm with an optical power ranging from 17 mW at 293 K to 3 mW at 312 K. A distributed feedback type device was fabricated and tested as well. It showed CW singlemode operation up to 260 K. These results demonstrate the potential of quantum cascade lasers as CW mid-infrared light sources for high-resolution spectroscopy and free space telecommunication systems.
Physics and applications of recent quantum cascade laser active region designs are discussed. Specifically, the use of bound-to-continuum and two-phonon resonance active regions for high temperature, high duty cycle operation is reviewed. Threshold current densities as low as 3kA/cm<SUP>2</SUP> at T=300K, operation with a peak power of 90mW at 425K, and single mode, high power operation up to temperatures above 330K at (lambda) approximately equals 16micrometers are demonstrated. QC lasers able to operate at high duty cycles (50%) on a Peltier cooler were used in a demonstration of a 300MHz free space optical link between two buildings separated by 350m.