Tunable Microcavity-Stabilized Quantum Cascade Laser for Mid-IR High-Resolution Spectroscopy and Sensing

Quantum cascade lasers are unipolar semiconductor lasers based on intersubband transitions in heterostructures. These lasers, which have demonstrated continuous wave operation at room temperature in the mid-infrared spectral range, are well suited for the realization of compact, ultra-sensitive, trace-gas sensors based on absorption spectroscopy. Up to now, only distributed feedback (DFB) single-mode devices have been used for such applications. DFB quantum cascade lasers have proven to be effective for gas sensing, but their relatively narrow tuning range, smaller or equal to about 1% of the wavelength, makes them not very versatile and limits their usefulness for spectroscopic investigations. In this thesis we developed broadly tunable external cavity quantum cascade lasers. The main advantage of these sources compared with DFBs is their broader tuning range, which is limited only by the spectral bandwidth of the gain element. We particularly studied broad gain bandwidth active regions based on bound-to-continuum designs. With that kind of active region, we have demonstrated a tuning range equal to 15% of the center wavelength at ~ 10 m, which was three times broader than the best values reported in the literature at that time, as well as good performance in pulsed mode at room temperature. Using a strain-compensated bound-to-continuum design emitting near 5.2 m, we have demonstrated for the first time continuous-wave operation of an external cavity quantum cascade laser on a thermoelectric cooler. The tuning range was comparable to that of pulsed devices, but with a much better side-mode suppression ratio and a much narrower linewidth. This continuous-wave device has successfully been applied to the spectroscopy of nitric oxide in collaboration with Prof. Tittel's Laser Science Group at Rice University. High resolution absorption spectra of that gas could be acquired over a large wavelength range. We also studied heterogeneous cascade structures, that is quantum cascade structures in which the various stages emit at several different wavelengths, as a way to increase further the tuning range of external cavity quantum cascade lasers. A pulsed laser tunable from 8.2 to 10.4 m, that is over 24% of the center wavelength has been realized using an active region composed of two substacks of bound-to-continuum designs. We conclude on the feasibility of room temperature, continuous-wave, broadly-tunable external cavity quantum cascade lasers in the mid-infrared spectral range. These devices will be useful for the analysis of multi-component gases, for high resolution spectroscopic investigations in fundamental science, and for the detection of heavy molecules with broad absorption features which DFB quantum cascade lasers cannot scan entirely.

Stable operation is one of the most important requirements for a laser source for high-precision applications. Many efforts have been made to improve the stability of lasers by employing various techniques, e.g., electrical and/or optical injection and phase locking. However, these techniques normally involve complex experimental facilities. Therefore, an easy implementation of the stability evaluation of a laser is still challenging, especially for lasers emitting in the terahertz (THz) frequency range because the broadband photodetectors and mature locking techniques are limited. In this work, we propose a simple method, i.e., relative phase locking, to quickly evaluate the stability of THz lasers without a need of a THz local oscillator. The THz laser system consists of a THz quantum cascade laser (QCL) frequency comb and a single-mode QCL. Using the single-mode laser as a fast detector, heterodyne signals resulting from the beating between the single-mode laser and the comb laser are obtained. One of the heterodyne beating signals is selected and sent to a phase-locked loop (PLL) for implementing the relative phase locking. Two kinds of locks are performed by feeding the output error signal of the PLL, either to the comb laser or to the single-mode laser. By analyzing the current change and the corresponding frequency change of the PLL-controlled QCL in each phase-locking condition, we, in principle, are able to experimentally compare the stability of the emission frequency of the single-mode QCL (fs) and the carrier envelope offset frequency (fCEO) of the QCL comb. The experimental results reveal that the QCL comb with the repetition frequency injection locked demonstrates much higher stability than the single-mode laser. The work provides a simple heterodyne scheme for understanding the stability of THz lasers, which paves the way for the further locking of the lasers and their high-precision applications in the THz frequency range.

Mid-infrared lasers, emitting in the spectral region of 3–12 μm that contains strong characteristic vibrational transitions of many important molecules, are highly desirable for spectroscopy sensing applications. High-efficiency quantum cascade lasers have been demonstrated with up to watt-level output power in the mid-infrared region. However, the wide wavelength tuning that is critical for spectroscopy applications still largely relies on incorporating external gratings, which have stability issues. Here, we demonstrate a monolithic, broadly tunable quantum cascade laser source emitting between 6.1 and 9.2 μm through an on-chip integration of a sampled grating distributed feedback tunable laser array and a beam combiner. High peak power up to 65 mW has been obtained through a balanced high-gain active region design, efficient waveguide layout, and the development of a broadband antireflection coating. Nearly fundamental transverse-mode operation is achieved for all emission wavelengths with a pointing stability better than 1.6 mrad (0.1°). The demonstrated laser source opens new opportunities for mid-infrared spectroscopy.

After more than 20 years of development, quantum cascade lasers (QCLs) have become the leading laser source emitting in the mid-infrared spectrum region between 3 and 12μm, where CBRN and explosives have strong absorption signatures. The Center for Quantum Devices at Northwestern University currently holds the record for both the highest room temperature continuous wave output power of 5.1 W and the most efficient QCL of 53% wall plug efficiency operating at cryogenic temperature. This book chapter presents our latest development results on mid-infrared QCLs with wavelength tuning and high power beam steering capabilities, which significantly enhances QCL’s suitability for remote spectroscopy and sensing applications.

In this paper, we review the recent progress in broadly tunable single-mode mid-infrared quantum cascade lasers (QCLs). With a brief introduction on various applications of broadly tunable single-mode mid-infrared QCLs, we first categorize these lasers based on their configurations and tuning schemes. Then recent progress in each scheme is investigated, together with detailed discussions. Furthermore, the pros and cons of different approaches are compared, and the opportunities and challenges for future developments from our point of view are listed. Last but not least, potential new directions and prospects for obtaining broadly tunable single-mode characteristics are also provided. It is foreseeable that, with the development of high performance broadly tunable single-mode mid-infrared QCLs, more applications could be exploited in the near future with new phenomena to be discovered.

We have recently demonstrated a continuous wave molecular laser emission based on ammonia optically pumped by a quantum cascade laser. We report here new results obtained with an optimized laser prototype pumped by a tunable external cavity quantum cascade laser. New laser lines with high output power are demonstrated at frequencies between 1 and 5.5 THz.

The differential absorption lidar (DIAL) can help us to obtain the vertical distribution of the atmospheric CO2 concentration, which is important to the study of carbon sources and carbon sinks. We design a seeder injected pulsed laser system, working as the laser source of the CO2 DIAL. Unlike the other CO2 DIALs, our laser source is the result of difference frequency of two lasers at the wavelengths of 1064 nm and 634 nm, respectively. It should be pointed out that the high frequency (wavelength) accuracy and stability of the emission laser, especially the on-line one, are greatly required in the CO2 DIAL system. However, the mechanical properties of the dye laser (634 nm) and the application of laser difference frequency technique make the wavelength drift constantly; besides, the extremely unstable energy of the pulsed laser increases the difficulty in identifying and stabilizing the on-line wavelength. Hence, a fast and efficient frequency (wavelength) stabilization method is needed to achieve a stable emission wavelength. Aiming at the research gap of the high precision requirements of on-line laser for this kind of pulsed DIAL, we propose a frequency stabilization method based on matching algorithm. The basic idea is to utilize the saturable absorption of CO2 molecule, by measuring the differential residual-intensity after the laser passing through dual absorption cells to calculate the optical depth (OD) and obtain the so-called pseudo CO2 absorption spectrum, which can be used to identify the on-line laser accurately. Finally, based on the matching algorithm of one-dimensional image, treating the OD as the gray value in the image, we implement the OD matching as a most important part in the process of frequency stabilization, and determine the exact position of the real-time output laser in the measured pseudo absorption spectrum. Thus, when some errors happen to the monitored ODs, by continuously adjusting the wavelength of the laser, the proposed method can fulfill the wavelength adjustment and accomplish the continuous frequency stabilization for on-line laser. Experimental results show that the frequency stabilization algorithm based on OD matching can satisfy the requirements for pulsed on-line laser frequency stabilization, and the sum of squares of deviation method is the optimal one in this application, with a stabilization accuracy of 0.3 pm. Besides, the proposed method can also be introduced in other laser frequency stabilization.

Infrared active stand-off detection techniques often employ high power tunable quantum cascade lasers (QCLs) for target illumination. Due to the distances involved, any fluctuation of the laser beam direction and/or beam profile is amplified at the sample position. If not accounted for, this leads to diminished performance (both sensitivity and selectivity) of the detection technique as a direct result of uncertainties in laser irradiance at each imaged pixel of the sample. This is especially true for detection approaches which illuminate a relatively small footprint at the target since the laser beam profile spatial fluctuations are often comparable to the (focused) laser spot size. Also, there is often a necessary trade-off between high output QCL power and beam quality. Therefore, precise characterization of the laser beam profile and direction as a function of laser properties (tuning wavelength, current and operating mode: pulsed or CW) is imperative. We present detailed measurements of beam profiles, beam wander and power fluctuations and their reproducibility as function of laser wavelength and stand-off distance for a commercially available tunable quantum cascade laser. We present strategies for improving beam quality by compensating for fluctuations using a motorized mirror and a pair of motorized lenses. We also investigate QCL mode hops and how they affect laser beam properties at the sample. Detailed mode-hop stability maps were measured.

A rugged, mid-infrared (IR) CO laser absorption diagnostic has been developed to monitor the amount of CO produced by a high-pressure CH4-O2 combustor test rig operating at supercritical CO2 conditions (30 MPa and 1150°C). The laser system operates at the fundamental absorption band, ν″ = 0, R(12), of CO near 4.5 μm. The mid-IR diagnostic was constructed from a tunable quantum cascade laser (QCL), an absorption cell with two window ports for monitoring CO exhaust concentration, and two IR photodetectors. Temperature and pressure sensors were mounted near the absorption cell to monitor exhaust flow conditions, and the operational wavelength of the laser was determined by a calibration process using a known mixture of CO and N2. Environmental conditions at the remote outdoor test facility posed significant difficulties in the data acquisition process for the IR diagnostic. Fluctuating environmental temperatures proved to be problematic when operating cryogenic photodetectors and stabilizing a QCL designed to operate with an internal temperature of −15°C. Improvements to the IR system included elimination of problematic stagnation regions via a new absorption cell design and an increase in the CO detection limit. During steady state conditions, the mid-IR diagnostic measured the CO concentration to within ± 80.6 ppm. The IR diagnostic was shown to have superior CO detection response time and the ability to resolve features not detected by other CO detector counterparts.

A potential diagnostic application of molecular rotational absorption lines at high redshift is to test the invariance of physical constants. This can be done by comparing the observed redshifted frequency of a molecular absorption line with redshifted lines from other types of transitions such as the 21cm hyperfine transition or electronic resonance transitions. In order to set stringent limits, it is necessary to achieve the greatest possible frequency resolution. This makes radio lines well suited for this purpose.

Supercontinuum sources are all-fiber pulsed laser-driven systems that provide high power spectral densities within ultra-broadband spectral ranges. The tailored process of generating broadband, bright, and spectrally flat supercontinua-through a complex interplay of linear and non-linear processes-has been recently pushed further towards longer wavelengths and has evolved enough to enter the field of mid-infrared (mid-IR) spectroscopy. In this work, we review the current state and perspectives of this technology that offers laser-like emission properties and instantaneous broadband spectral coverage comparable to thermal emitters. We aim to go beyond a literature review. Thus, we first discuss the basic principles of supercontinuum sources and then provide an experimental part focusing on the quantification and analysis of intrinsic emission properties such as typical power spectral densities, brightness levels, spectral stability, and beam quality (to the best of the authors' knowledge, the M2 factor for a mid-IR supercontinuum source is characterized for the first time). On this basis, we identify key competitive advantages of these alternative emitters for mid-IR spectroscopy over state-of-the-art technologies such as thermal sources or quantum cascade lasers. The specific features of supercontinuum radiation open up prospects of improving well-established techniques in mid-IR spectroscopy and trigger developments of novel analytical methods and instrumentation. The review concludes with a structured summary of recent advances and applications in various routine mid-IR spectroscopy scenarios that have benefited from the use of supercontinuum sources.

Frequency tuning and stability of Nd:YVO 4 in a dual coupled cavity

We present the burn-in behavior and power stability of multiple quantum cascade lasers (QCLs) that were measured to investigate their long-term performance. For these experiments, the current to the QCL was cycled every ten minutes, and the output power was monitored over time for durations as long as two months. A small increase in power for a given injection current is observed for almost all of the QCLs tested during the burn-in period. Although the amount and duration of the burn-in varied among the devices tested, we observed that QCLs that operated with a lower threshold current exhibited a smaller burn-in change from initial conditions for the first ten hours of operation. This correlation, however, disappeared at longer operation times. The effect of packaging the QCLs is also investigated to determine its impact on performance and reliability. The power stability is measured for the packaged QCLs along with changes in the operational conditions. Although the temperature of the QCL is kept constant with a thermistor and thermoelectric cooler (TEC) inside the package, the case temperature is varied to monitor its correlation with any changes in power or frequency. Power changes and small frequency shifts are observed under these conditions. One possible explanation for these changes is the influence of optical feedback from the anti-reflection (AR) coated window in the package. The data from all of these experiments is presented.

Spectroscopy at midand far-infrared wavelengths is important for environmental sensing (trace analysis, pollutants), medical diagnostics (blood analysis, breath control), and is a promising candidate for future high-security applications (airport security, identification of hazardous chemicals).1 Sensing chemical components in gases and liquids requires powerful coherent light sources with narrow spectral bandwidth and small divergence.2, 3 In the past decade, this demand has promoted quantum cascade lasers (QCLs), which enhance the performance and shrink down the dimensions of today’s midand farinfrared sensor systems.4 In contrast to conventional semiconductor lasers, QCLs can be designed to hit the target wavelength of a spectroscopic application. The coupling of semiconductor lasers results in an increased output power,5 stable single-mode operation, and wavelength tunability.6, 7 When integrated in a spectroscopic measurement system, monolithic coupling schemes can maximize the sensitivity, selectivity, and applicability of the sensor. However, a homogeneous laser beam is essential to avoid complicated optics and high losses of light. Therefore, the light inside the laser cavity has to be understood and controlled. Figure 1 shows a sketch of a Y-coupled QCL with corresponding dimensions. Two laser waveguides (branches) spaced by 60μm merge into a single trunk towards a single facet. The Yshaped QCLs have a waveguide width of 10μm and imply a bending radius of 8.3mm. The lasers were fabricated from a gallium arsenide/aluminum gallium arsenide heterostructure by lithography, etching, sputtering, and evaporation techniques.8 The devices were operated with 100ns current pulses at 78K. Their emission wavelength peaks at 10.5μm. To analyze the output beams on both sides of the laser, both the single and double facets were imaged with a midinfrared micro-bolometer camera. The image of the single facet Figure 1. Sketch of the Y-shaped quantum cascade laser (QCL) with corresponding dimensions. Near-field images illustrate the intensity distributions emitted from (left) the single facet and (right) the double facet.

We report operation of tunable external cavity quantum cascade lasers with emission wavelength controlled by an acousto-optic modulator (AOM). A long-wave infrared quantum cascade laser wavelength tuned from ∼8.5 μm to ∼9.8 μm when the AOM frequency was changed from ∼41MHz to ∼49 MHz. The laser delivered over 350 mW of average power at the center of the tuning curve in a linewidth of ∼4.7 cm−1. Measured wavelength switching time between any two wavelengths within the tuning range of the QCL was less than 1 μs. Spectral measurements of infrared absorption features of Freon demonstrated a capability of obtaining complete spectral data in less than 20 μs.