Rf-modulation of mid-infrared distributed feedback quantum cascade lasers

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.

We present the design and the performance of midinfrared distributed feedback (DFB) quantum cascade lasers (QCLs) made of strain balanced Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> As/Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</sub> In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-y</sub> As material on InP substrates for sensing CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O, and NO in the middle wavelength range of mid-IR from 4 to 6 μm. We present the performances of our DFB QCLs at three different aspects: high power, low threshold power consumption, and wide wavelength coverage. We demonstrated a continuous wave (CW) DFB QCL with an output power of 220mW at 20°C, a CW DFB QCL with low threshold voltage of 8V and low-divergent far-field angles of 27.5° × 32.9° by reducing the number of quantum cascaded stages, a CW DFB QCL with a low threshold power consumption of 0.7W by reducing doping density in the active core, and DFB QCLs wide wavelength coverage of 325 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , 16.9% of center wavelength at 5.2 μm, within one wafer by changing the grating period. 12 DFB QCL chips have being aged for 3000 h under 25°C and a constant quasi-CW current of 0.37 A. No decrease in power was observed.

Two-segment distributed feedback (DFB) GaAs/AlGaAs bound-to-continuum quantum cascade lasers emitting at ∼10.8 µm were fabricated and investigated. Reversible switching between two different DFB modes by current injection control yielding sidemode suppression ratios up to >23 dB is reported. Two different devices were fabricated for which the distance between the different DFB modes is 1.5 and 2.5 cm−1, respectively, and quasi-continuous tuning over a range of >9 cm−1 (105 nm) between 120 and 250 K was observed.

Because of an intrinsically low linewidth-enhancement factor, the quantum cascade laser (QCL) is especially favorable for patterning with a recently proposed 2-D photonic crystal (PC) lattice that substantially increases the device area over which optical coherence can be maintained. In this work, we use an original time-domain Fourier-transform (TDFT) algorithm to theoretically investigate the beam quality and spectral purity of gain-guided PC distributed-feedback (DFB) quantum cascade lasers. The conventional 1-D DFB laser and also the angled-grating DFB (/spl alpha/-DFB) laser are special cases of the PCDFB geometry. By searching the parameter space consisting of tilt angle, coupling coefficients, stripe width, and cavity length, we have theoretically optimized the PCDFB gratings for QCL gain regions. At a wavelength of 4.6 /spl mu/m, the simulations project single-mode emission from stripes as wide as 1.2 mm, and etendues of no more than three times the diffraction limit for 2-mm stripes. We also examine the tolerances required for single-mode and high-brightness operation. Comparisons are made to analogous simulations of a-DFB QCL lasers.

Complex-coupled edge-emitting photonic crystal distributed feedback quantum cascade lasers at λ ∼ 7.6 μm

We report on the development of λ ~ 4.7 μm continuous-wave (CW) operation of surface grating distributed feedback (DFB) quantum cascade lasers (QCLs). A high wall plug efficiency (WPE) of 7% is obtained at 15°C from a single facet producing over 0.85 W of CW output power. The threshold current density of DFB QCL is as low as 1.19 kA/cm at 15°C and 2.29 kA/cm at 90°C in CW mode. Single-mode operation with side mode suppression ratio above 25 dB is observed in the working temperature of 15°-95°C.

Continuous tuning of a room-temperature-operated distributed feedback quantum cascade laser in the range of 0.6 cm−1 (20 GHz) was achieved using 1.3 μm telecom diode laser as a pumping source. We demonstrate wavelength modulation of the quantum cascade laser at frequencies up to 300 MHz.

Nitrogen oxides (NOx), including nitric oxide (NO) and nitrogen dioxide (NO2) play important roles in determining the photochemistry of the ambient atmosphere, controlling the production of tropospheric ozone, affecting the concentration levels of the hydroxyl radical, and forming acid precipitation. A sensor system capable of simultaneous measurements of NO and NO2 by using a commercial 76 m astigmatic multi-pass gas cell (MPGC) was developed in order to enable fastresponse NOx detection. A continuous wave (CW), distributed-feedback (DFB) quantum cascade laser (QCL) and a CW external-cavity (EC) QCL were employed for targeting a NO absorption doublet at 1900.075 cm-1 and a NO2 absorption line at 1630.33 cm-1, respectively. Both laser beams were combined and transmitted through the MPGC in an identical optical path and subsequently detected by a single mid-infrared detector. A frequency modulation multiplexing scheme was implemented by modulating the DFB-QCL and EC-QCL at different frequencies and demodulating the detector signal with two Labview software based lock-in amplifiers to extract the corresponding second-harmonic (2f) components. Continuous monitoring of NO and NO2 concentration levels was achieved by locking the laser frequencies to the selected absorption lines utilizing a reference cell filled with high concentrations of NO and NO2. The experimental results indicate minor performance degradation associated with frequency modulation multiplexing and no cross talk between the two multiplexed detection channels. The performance of the reported sensor system was evaluated for real time, sensitive and precise detection of NO and NO2 simultaneously.

A sensor system capable of simultaneous measurements of NO and NO2 was developed using a wavelength modulation-division multiplexing (WMDM) scheme and multi-pass absorption spectroscopy. A continuous wave (CW), distributed-feedback (DFB) quantum cascade laser (QCL) and a CW external-cavity (EC) QCL were employed for targeting a NO absorption doublet at 1900.075 cm−1 and a NO2 absorption line at 1630.33 cm−1, respectively. Simultaneous detection was realized by modulating both QCLs independently at different frequencies and demodulating the detector signals with LabView-programmed lock-in amplifiers. The sensor operated at a reduced pressure of 40 Torr and a data sampling rate of 1 Hz. An Allan–Werle deviation analysis indicated that the minimum detection limits of NO and NO2 can reach sub-ppbv concentration levels with averaging times of 100 and 200 s, respectively.

We demonstrate surface-grating distributed feedback quantum cascade lasers (QCLs) with a watt-level power output at 4.75 μm. A device with a 5 mm cavity length exhibits an output power of 1.1 W in room-temperature cw operation. Single-mode operation with a side mode suppression ratio of 30 dB is obtained in the working temperature of 15–105 °C. A double-lobed far field with negligible beam steering is observed. The significance of this demonstration lies in its simplicity and readiness to be applied to standard QCL wafers with the promise of high-power performances.

We demonstrate phase locking of a 4.7 THz distributed feedback quantum cascade laser (QCL) to a stable laboratory synthesizer. The QCL operates at 4747.9 GHz with 1 mW output power, mounted on the first stage of a pulse-tube cryocooler with mechanical dampers. Despite residual mechanical vibrations, the QCL radiation is successfully coupled out through a series of cold and warm mirrors and focused onto a room-temperature superlattice diode (SLD). The SLD is pumped by a 182.6 GHz signal from a diode frequency multiplier chain and generates the 26th harmonic at 4748.2 GHz for harmonic mixing as a phase reference. This produces a 327 MHz intermediate frequency (IF) that is amplified, filtered, and fed to a balanced diode mixer phase detector. The phase-locked loop employs a loop filter to generate correction signals for QCL current adjustment, reducing the unlocked linewidth from approximately 10 MHz to 1 Hz (limited by resolution bandwidth of spectrum analyzer). In addition of providing absolute frequency reference and ability of introducing fine frequency steps, the system demonstrates remarkable robustness, maintaining phase lock for several hours and even under low signal-to-noise conditions such as 1 dB of signal over noise at 3 MHz resolution bandwidth.This robustness comes from tailored PLL design, incorporating a diode ring mixer as phase detector and a linear phase narrow band filter, and proves its suitability for practical heterodyne array receivers targeting the [OI] line at 4.7 THz.

Optical feedback cavity-enhanced absorption spectroscopy (OF CEAS) has been demonstrated with a thermoelectrically cooled continuous wave distributed feedback quantum cascade laser (QCL) operating at wavelengths around 7.84 μm. The QCL is coupled to an optical cavity which creates an absorption pathlength greater than 1000 m. The experimental design allows optical feedback of infra-red light, resonant within the cavity, to the QCL, which initiates self-locking at each TEM00 cavity mode frequency excited. The QCL linewidth is narrowed to below the mode linewidth, greatly increasing the efficiency of injection of light into the cavity. At the frequency of each longitudinal cavity mode, the absorption coefficient of an intracavity sample is obtained from the transmission at the mode maximum, measured with a thermoelectrically cooled detector: spectral line profiles of CH4 and N2O in ambient air were recorded simultaneously and with a resolution of 0.01386 cm−1. A minimum detectable absorption coefficient of 5.5×10−8 cm−1 was demonstrated after an averaging time of 1 s for this completely thermoelectrically cooled system. The bandwidth-normalised limit for a single cavity mode is 5.6×10−9 cm−1 Hz−1/2 (1σ).

We report on an application of a pulsed distributed feedback quantum cascade laser (QCL) for long open path readout system of multiple passive temperature, pressure, humidity or other sensors. Unlike the usually used active sensors, containing radio or optical data transmitters, the presented system does not contains any active transmitters at the sensors side. The QCL and the receiver are displaced on one side of the path. On the opposite path side, a modified Michelson interferometer, based on corner cube retroreflectors, is placed. One retroreflector is fixed and another one is attached to the appropriate sensor, which moves it when the measured parameter is changed. The useful travel range of the movable retroreflector is less than lambda/2 (4.8 mum) to obtain a monotonic transfer function between the measured parameters and the received signals. This system measures distances in the micrometer and sub-micrometer range but displaced up to a few kilometers from the transmitter/receiver station. A relatively long laser pulse of 150-200 ns is used to scan the interferometers and to read out the information. During the pulse, the QCL frequency is swept within a 40 GHz range. This frequency sweeping allows decreasing significantly the influence of the atmospheric turbulence over the accuracy of the data transfer. When the swept radiation illuminates the interferometer, oscillations in the received signal occur due to the interference between the different parts of the beam. The period of the oscillations and the oscillation phase depend on the distance between retroreflectors and the changes in this distance, respectively. The main advantage of the system proposed is that a single transmitter-receiver pair allows reading out multiple sets of completely passive sensors offering high level of data protection due to the low beam divergency

Semiconductor quantum cascade lasers (QCLs) that emit mid-infrared light in the wavelength range 4-9 μm are unipolar and the laser emission is due to intersubband transitions in a repeated stack of multiple quantum wells. The thermal management of these devices is a challenge. The overheating of the active region (referred to as “core” throughout this paper) in these lasers decreases the optical power and ultimately results in laser failure. In this paper, a detailed finite element-based numerical modeling of the thermal behavior of these devices as well as the measurements performed to validate the models are presented. The studies include the effect of submount material, mounting schemes such as epi-side down or epi-side up mounting, and, finally, the effect of core geometry on the thermal impedance. Various core designs, such as the split core, are analyzed. Various experiments were conducted to correlate the results with the numerical modeling by measuring the thermal impedance between the laser diode's core and the bottom of the substrate as well as the temperature change within a pulse of a distributed feedback (DFB) QCL which emits in a single longitudinal mode of narrow linewidth. The temperature of the active core of a DFB QCL can be determined by measuring the lasing frequency, which changes with the temperature of the active core. By measuring the lasing frequency as a function of time within a pump current pulse, the temperature change in the active core region can be determined and thereby thermal conductance of the laser structure. In the conclusion, various recommendations for efficient thermal performance of these QCLs are provided.

A non-invasive CO2 gas temperature sensing technique at or close to the room temperature range based on quantum cascade laser absorption spectroscopy is presented. The method probes thermally populated ground state and hot-band rotational-vibrational transitions of CO2 in the frequency range from 2349 to 2351 cm−1 from which the gas temperature is obtained from Boltzmann statistics. Transmission spectra are recorded by injection-current driven frequency-scans, the so-called inter pulse mode, of a pulsed distributed feedback quantum cascade laser. The statistical uncertainty (1σ) in temperature for single frequency scans with time resolution of 10 ms is 4 K and can be further reduced down to ∼50 mK by long-time averaging of about 1 min. The technique is evaluated with particular emphasis on implementation, data acquisition, data analysis and potential improvements.