Characterization and control of tunable quantum cascade laser beam parameters for stand-off spectroscopy

Techniques which apply tunable quantum cascade lasers (QCLs) for target illumination suffer from fluctuations of the laser beam direction. This manuscript describes a method to stabilize the beam direction by using an active feedback loop. This approach corrects and stabilizes the laser pointing direction using the signal from a 4-element photo sensor as input to control an active 2 dimensional Galvo mirror system. Results are presented for measurements using known perturbations as well as actual mode hops intrinsic to external cavity QCL during wavelength tuning.

Summary form only given. Birefringent filters are routinely used as a wavelength tuning element in bulk optics, broadly tunable lasers such Ti:Sapphire [1] and dye lasers. A detailed description of birefringent filter operation can be found in ref. [2]. In this paper, we report the design, fabrication and characterisation of an integrated electro-optically tunable birefringent filter for wavelength and polarisation tuning of quantum cascade lasers (QCLs) [3].Our generic design for an integrated tunable birefringent filter (ITBF) for QCLs is shown in Fig. 1(a). It consists of 2 quarter waveplates with a differential phase shift (DPS) section between them. The integrated equivalents of waveplates are polarization mode convertors (PMC) [4] that converts the TM (Transverse Magnetic) polarised light from the gain section into circularly polarised light. In a QCL there is only gain for TM modes because of the intersubband transition selection rules. The DPS section adjusts the relative phase of the TM and TE (Transverse Electric) components of the circularly polarized light from the first PMC (PMC1 in figure 1(a)); this is because the birefringence of DPS section can be adjusted by injection current. The relative phase of the TE and TM components at the input into the second PMC (PMC 2 in figure 1(a)) determines the output polarization. In a QCL, only TM light will experience gain and the wavelengths that are rotated back to TM polarization are determined by the current in the DPS section. Thus we have a QCL that has its operating wavelength selected by an electro-optic tunable birefringent filter. For wavelength tuning the gain section was operated pulsed (100 ns pulse width at 60 kHz repetition rate) above threshold, at a current density of 6.8 kA/cm, and wavelength tuning was recorded (shown in Fig. 1(b)) for various values of DC injection current density into the DPS up to 24nm at 0.41kA/cm2. In our polarization control experiments at 0 kA/cm2 a maximum throughput polarisation angle of 37O was recorded and this angle increases up to 82O with an applied DC current of 0.97 kA/cm2 into the DPS and no further change in polarization angle recorded above 0.97 kA/cm2 current density (shown in Fig. 1(c)).In summary we have presented results on a QCL that includes an integrated birefringent filter. The results show that the output wavelength and polarisation can be electro-optically tuned by injecting a DC current into a differential phase shift section.

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.

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.

Whether you're new to lasers or you've been working with them for some time, you may be wondering what all the fuss is over laser beam profiling. Why worry about the quality of the laser beam that you've just put into production? Or, if you think your process is humming along nicely, why fix what isn't broken? You might think that laser beam quality has more than likely been addressed at the research and development stage of the laser you have, or even at the manufacturing or integration stages of the system that you've received. This article shows the importance of laser beam profiling and how the user can benefit from it.

We experimentally demonstrate speckle noise reduction and beam wander mitigation by using a rotating diamond/KBr pellet and a multimode fiber (MMF). As the diamond/KBr diffuser is rotated, the reflected speckle images that are captured by an infrared camera are temporally averaged. We demonstrate 78% speckle noise reduction by averaging 25 frames, which is within 80% of the theoretical contrast reduction. Large beam position fluctuations are also significantly suppressed by adding the MMF. This combination of beam wander mitigation and speckle reduction offers significant benefits for emerging optical technologies that use quantum cascade lasers as illumination sources.

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.

We have been investigating the influence of atmospheric turbulence on the propagation of a picosecond laser pulse. The figure of merit of presented results is the time of propagation, its absolute delay and jitter. Phase wavefront deformation or beam profile changes were not studied. The correlation of the atmospheric turbulence with the propagation delay fluctuation was measured. The research was motivated by the needs of highly precise laser ranging of ground, air, and space objects; and highly precise and accurate time transfer ground-to-space and ground-to-ground by means of picosecond optical laser pulse. Firstly for comparison, lets briefly summarize the effects of a turbulent atmosphere to continuous laser beam. The total effect of atmospheric turbulences on a continuous laser beam propagation is a highly complex subject. Atmospheric turbulences can be defined as random spatial variations in the refraction index of the atmosphere resulting in a distortion of the spatial phase fronts of the propagating signal. Spatial phase front distortion induces the variable path of light energy and thus all effects described later on. Variations of the refraction index are caused by the turbulent motion of the atmosphere due to the variations in temperature and gradients in the water vapour. Following (Degnan, 1993), the optically turbulent atmosphere produces three effects on low power laser beams: 1) beam wander, 2) beam spread and 3) scintillations. Severe optical turbulence can result in a total beam breakup. Beam wander refers to the random translation of the spatial centroid of the beam and is generally caused by the larger turbulent eddies through which the beam passes. In astronomical community it is usually referred as seeing. Beam spread is a short term growth in the effective divergence of the beam produced by smaller eddies in the beam path. The two effects are often discussed together in terms of a “long term” and “short term” beam spread. The “long term” beam spread includes the effects of beam wander, whereas the “short term” beam spread does not. For more details, see (Degnan, 1993). Maximum turbulence occurs at mid-day in the desert (low moisture) under clear weather conditions. For the usual laser wavelength of 532 nm one can expect 2.4-4.6 cm for the coherence length at zenith angles of 0° and 70° respectively. At the tripled Nd:YAG wavelength (355 nm) the corresponding values are 3.1 and 1.6 cm (Degnan, 1993). Turbulence induced beam spreading will only have a significant impact on beam divergence (and hence signal level) if the coherence length is on the order of, or smaller than, the original effective beam waist radius. Since a typical 150 μrad beam implies an effective waist radius of 2.26 mm, the effect of beam spread on signal level for such systems is relatively small, i.e. a few percent.

Many medical studies have investigated the safety threshold of ocular media at various wavelengths. Understanding and determining the damage threshold of laser exposure to the retina is important in order to set safety guidelines. A phenomenon known as thermal lensing where a media's index of refraction changes as the laser energy is absorbed causes the beam profile and subsequently the position of the focal point of the beam to change as the temperature changes within the media. These changes in ocular media such as vitreous and aqueous humor, the cornea, crystalline lens, and retina all result in a slight change in the beam profile making the prediction of the focal point and laser spot size at the retina difficult to accurately predict. This paper describes how the effects of thermal lensing in the eye can be investigated to aid in determining the damage threshold for pulse durations of a few nanoseconds and continuous wave, CW, laser irradiation delivered to the retina and the relationship of how the damage is related to multiple pulse exposures. We present a first-order modeling effort for the thermal lensing effect in the eye along with initial estimates of impact on damage thresholds as predicted through computational biophysics thermal damage models.

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.

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.

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.

The quantum cascade laser (QCL) is an important laser source in the mid-infrared and terahertz frequency range. The past twenty years have witnessed its tremendous development in power, wall plug efficiency, frequency coverage and tunability, beam quality, as well as various applications based on QCL technology. Nowadays, QCLs can deliver high continuous wave power output up to 5.1 W at room temperature, and cover a wide frequency range from 3 to 300 μm by simply varying the material components. Broadband heterogeneous QCLs with a broad spectral range from 3 to 12 μm, wavelength agile QCLs based on monolithic sampled grating design, and on-chip beam QCL combiner are being developed for the next generation tunable mid-infrared source for spectroscopy and sensing. Terahertz sources based on nonlinear generation in QCLs further extend the accessible wavelength into the terahertz range. Room temperature continuous wave operation, high terahertz power up to 1.9 mW, and wide frequency tunability form 1 to 5 THz makes this type of device suitable for many applications in terahertz spectroscopy, imaging, and communication.

We describe development of spectrally tunable micro-engineered filters operating in the longwave infrared (LWIR) region from 8 to 12 μm based on using the guided-mode resonance (GMR) effect. The device structure consists of a subwavelength dielectric grating on top of a homogeneous waveguide using high index dielectric transparent materials, i.e., germanium (Ge) with a refractive index of 4.0 and zinc selenide (ZnSe) substrate with refractive index of 2.4. The filters are designed to reflect the incident broadband light at one (or more) narrow spectral band while fully transmitting the rest of the light. The tuning of the reflection wavelength is achieved by changing the angle of incidence of light by mechanically tilting the filter. Filters based on one-dimensional gratings are polarization dependent and those based on two-dimensional gratings are polarization independent at normal incidence of light. Both 1-D and 2-D gratings were designed and characterized. Anti-reflection coatings were applied on substrate interfaces to improve transmission over the entire spectral region. We carried out transmission measurements of these filters using an automated tunable room temperature quantum cascade laser (QCL) system operating from 7.3 to 12.9 μm, and a modified Fourier Transform Spectrometer with a custom designed chamber for normal incidence of light on the sample. We present detailed characterization experiments and compare the theoretical and experimental results.

ArF-dry microlithography is currently switching from pre-production to mass-production and the target node is shifted from 90 nm to 65 nm. ArF-wet or F₂ laser lithography will then be an important player for the next generation node below 45 nm. Therefore, high throughput and high-resolution exposure tools for VUV lithography require VUV light sources (ArF and F₂ lasers) with high power and narrow bandwidth. In this paper, we describe the beam quality of the new- type injection lock (MOPO, master oscillator power oscillator) ArF laser system we developed and compare it with the beam quality of a master oscillator power amplifier (MOPA) ArF laser system. A high power and narrow bandwidth ArF laser can be achieved with twin laser chambers in a MOPA or an injection lock laser configuration. Compared to the MOPA system, the injection lock laser system has an excellent performance (e.g. high efficiency, long pulse duration and narrow spectrum). On the other hand, the injection lock system has some disadvantages in beam quality showing high spatial coherence, broadband emission and having a beam profile with a hole. These technical issues have been solved, however, with the following two new breakthrough-technologies: (1) a new-type injection lock system having low spatial coherence and a beam profile with no hole and (2) the optimization of the injection seed energy and discharge timing between the twin chambers for low broadband emission. The spatial coherence, the broadband spectrum and the beam profile of the new-type injection lock system were measured with a Young's interferometer, a wide range spectrometer with etalons and a 2-dimensional beam profiler, respectively. The new-type injection lock ArF laser system had a lower spatial coherence than a conventional injection lock system, a very low broadband emission level thus preventing deterioration of exposure tools resolution, and a beam profile with no hole. Moreover, we reconfirmed that the new-type injection lock system has the same excellent performance as the conventional injection lock system.