Widely tunable, monolithically integrated mid-infrared semiconductor laser operates at room temperature.
MANIJEH RAZEGHI, WENJIA ZHOU, DONGHAI WU, RYAN MCCLINTOCK AND STEVEN SLIVKEN, NORTHWESTERN UNIVERSITY
The atoms in a molecule can bend, stretch and rotate with respect to one another, and these excitations are largely optically active. Most molecules, from simple to moderately complex, have a characteristic absorption spectrum in the 3- to 14-µm wavelength range that can be uniquely identified and quantified in real time. Infrared spectroscopy has been used to study these absorption features and develop different molecular “fingerprints.” The benefits of this optical technique, as opposed to chemical sensors or chromatography, are that the detection mechanism requires minimal sample pretreatment and is very fast. While high-performance infrared detectors have existed for a long time, the main challenge in achieving the full potential of tunable laser spectroscopy lies in the performance limitation of the tunable mid-infrared (IR) laser sources.
Before the invention of quantum cascade lasers (QCLs), few mid-IR sources were available. Among them were optical parametric oscillators that convert an input laser wave into two output waves of lower frequency by means of nonlinear optical interaction. Although these oscillators can achieve high output power, they are generally bulky, require large external power supplies and cooling systems, and are easily subject to misalignment or damage.
There are several kinds of interband semiconductor lasers that emit at infrared wavelengths. Among them are lead salt lasers, HgCdTe lasers and Sb-based interband lasers. Since they all are categorized as semiconductor lasers, they are inherently compact. However, interband lasers suffer from performance degradation as the operating temperature and wavelength increases. This is largely due to a rapid increase of the nonradiative recombination rate as a function of temperature.
In contrast, QCLs are well-established mid-IR laser sources, based on intersubband transitions in a semiconductor heterostructure, rather than a combination of electron-hole pairs across the semiconductor bandgap. A basic intersubband transition region incorporates mini band gaps that form an injector region that effectively collects the electrons and injects them into the active region of the next stage. This cascading scheme also gives the name to the quantum cascade laser. QCLs, based on the GaInAs/AlInAs material system, feature very high performance over a broad range of wavelengths between 3 and 14 µm.
For intersubband transitions, the bandgap design becomes independent of interband material bandgap, which is a characteristic property of the compound semiconductor. The emission wavelength can be tailored by changing only the relative thickness of multiple quantum wells and barriers without changing the material composition. In a broadband heterogeneous QCL (HQCL), multiple stacks of discrete wavelength quantum cascade (QC)
stages are incorporated within the same quantum cascade laser waveguide core. The summation of the effective gain of the QC stages features a broad gain over a large wavelength range. This allows selection of the laser emitting wavelength over a wide range using an appropriate tunable feedback mechanism.
Strain-balanced Al0.63In0.37As/Ga0.35In0.65As/Ga0.47In0.53As QCLs were shown to have good performance across the wavelength range 5.2 to 11 µm, and they can be incorporated with a single process step into a HQCL active region. When designing a broadband laser core, it is critical to use wavefunction engineering to avoid any cross-absorption loss. Photons emitted by the optical transition of a long wavelength QC stage are often cross-absorbed by optical transitions between the upper laser level and the next excited state of other QC stages, due to large oscillator strength between them and a large electron population in the upper laser level at high electric fields. The absorption decreases with detuning of the photon energy from the resonant absorption energy. As a result, the energy separation between upper laser level and the next excited state must be reduced well below 124 meV (approximately 10 µm). This may lessen the overall efficiency of a shorter wavelength emitter by decreasing the injection efficiency into the upper laser level and increasing the thermal escape of electrons from the upper laser level, but is necessary in a HQCL to minimize excess loss at longer wavelengths. In our design, a compromise value of 55 to 60 meV was chosen for a balance in the efficiency and the range of operation in the broadband HQCL, (Figure 1a).
Figure 1. Conduction band diagram and relevant wavefunctions for one emitting stage of a quantum cascade laser based on the Al0.63In0.37As/Ga0.35As/Ga0.47In0.53As material system (a). Simulated gain curve of a five-core heterogeneous quantum cascade laser (QCL) (b). Courtesy of Northwestern University.
Notably, all the cores must operate at the same current density simultaneously, so that maximum gain has to be reached simultaneously for all cores. The modal intensity profile within the waveguide and the modal confinement factors of the QC stages at different photon energies have to be balanced. The net gain curve of the HQCL is calculated by the sum of individual QC stage gain, weighted by the respective modal confinement factors. The total modal gain based on simulation, at a current density of approximately 4 kA/cm2, is shown in Figure 1b. Wavefunction engineering ensures that the strong losses corresponding to resonant absorption from upper laser levels to higher states (near 60 meV) does not overlap with the HQCL emission energies of interest. The gain is designed to be very flat throughout the wavelength range.
The broadband structure was grown by gas source molecular beam epitaxy on an InP substrate. The threshold current of the HQCLs are much higher than single core devices due to reduced modal confinement factors of each QC stage with respect to a single core device. The devices are doped higher than single core QCLs to ensure that all wavelengths can reach the threshold current before their maximum current at resonant field. The wafer was characterized using a distributed feedback (DFB) laser array. Single mode emission between 5.8 and 9.9 µm was obtained with a maximum power above 300 mW, and most DFB lasers have power output above 100 mW. A relatively flat threshold current between 5.8 and 9.0 µm indicates a flat gain.
Monolithic widely tunable QCL source
A widely tunable laser requires not only a broadband gain medium but also a robust tuning mechanism that allows selecting any wavelength at will within that wide wavelength range. External cavity QCLs with a wide tuning range are commercially available and have been used for many practical applications. However, the peripheral optical components make it sensitive to mechanical shocks and vibrations. Furthermore, the tuning speed is limited by the movement of large grating objects. Monolithic and electrical tuning is possible with DFB QCLs, which have been demonstrated to cover a few tens of cm-1. However, because each element in the array can only tune over such a small range of a few cm-1, the size of the array could be prohibitively large in order to cover a range of hundreds of cm-1. Fortunately, the DFB lasers can be replaced by sampled grating DFB (SGDFB) lasers, which have been demonstrated to extend the tunability of a QCL by one order of magnitude.
Multichannel wavelength-selectable sampled grating distributed feedback (SGDFB) laser arrays can be monolithically integrated with an optical beam combiner to realize even wider tuning on a single chip. The benefit of the beam combiner is that all wavelengths come out of a single output aperture, along with a compact size and fast electric tuning.
The integrated device consists of an eight-laser SGDFB array and a beam combiner section (Figure 2). The primary emitting wavenumber of lasers is spaced by tens of cm-1 by controlling the grating period. The device was fabricated using standard micro fabrication technologies. During operation, one laser is selected and tuned with injected continuous wave (CW) current to the two SGDFB laser sections. The injected CW currents locally change the section temperature, which leads to the change of effective refractive index. A wavelength range of 6.2 to 9.1 µm from a single emitting aperture was obtained by integrating the 8-laser SGDFB laser array with an on-chip beam combiner1. This is achieved in a device smaller than a penny.
Figure 2. Schematic of the wavelength tunable QCL source with a monolithically integrated sampled grating distributed feedback (SGDFB) laser array and beam combiner. Courtesy of Northwestern University.
In order to make the tunable laser source usable in the field, a self-contained tunable laser system was designed and built (Figure 3), weighing about 3 kilograms and measuring 127 mm × 203 mm × 184 mm. It works off of one 48-VDC power supply and contains all of the electronics and control algorithms necessary to drive the individual lasers within the array and coordinate the driving of the laser array and produce the desired wavelength. The system has both a local touchscreen user interface and several remote interfaces (GPIB, USB, RS232 and Ethernet). The lasers in the array are beam-combined into a single aperture and the emission comes out as single collimated output from the front of the system.
Figure 3. Tunable laser system with the additional power supply. Courtesy of Northwestern University.
The system was calibrated using custom written automatic calibration software to perform nearly 43,000 scans of the laser array and record the spectrum with a Fourier transform infrared (FTIR) spectrometer for different lasers and DC current combinations. For each of the lasers in the array, a colormap is made showing the peak wavelength vs. SGDFB Section A and Section B currents. This is then used to generate optimized paths through the data and create a table of precalibrated scan paths. The software is then able to interpolate along these paths with a resolution of better than 0.1 nm.
In order to use the system for chemical spectroscopy, a wavelength region of interest is selected and each discrete scan-state is downloaded to a dedicated RAM chip on the control board. An on-board or off-board time-base is then used to step through the states stored in the RAM. The upper limit on the laser stabilization time limits the scan rate to about 1000 Hz. In order to demonstrate the scanning capability of the system, a 100-mm single-pass gas cell was filled with natural gas composed primarily of methane at 1 atmosphere. The transmitted light was collected with a cryogenic mercury cadmium telluride (MCT) detector and digitized by a high-speed lock-in amplifier. Figure 4 shows a comparison of the reference spectrum (gray, obtained with a FTIR spectrometer) to that measured with the tunable laser system (red points). Generally, the tunable laser system can accurately measure the spectrum of natural gas. In particular, the inset of Figure 4 shows excellent agreement with some of the fine spectral features.
Figure 4. Comparison of the spectrum measured with the tunable laser system (red dots) to the expected spectrum measured with the Fourier transform infrared (FTIR) spectrometer. The inset shows a zoomed-in region showing the excellent agreement. Courtesy of Northwestern University.
This technology is at once extremely sophisticated engineering, yet it’s also practical and reproducible. Any wavelength in the laser’s range can be accessed on demand at room temperature, which is ideal for sensing applications. The compact system, which drives the individual lasers within the array with the conditions necessary to produce any accessible wavelength, is able to do fast wavelength scanning without being limited by the driving hardware electronics. This broadly tunable QCL source with no moving parts will open new opportunities for mid-IR spectroscopy and chemical sensing.
Meet the authors
Manijeh Razeghi is the Walter P. Murphy Professor and director of the Center for Quantum Devices (CQD) at Northwestern University, Evanston, Ill.; email: [email protected]. Wenjia Zhou is a doctoral student, Donghai Wu is a postdoctoral researcher, Ryan McClintock is a research associate professor, and Steven Slivken is a research associate professor, all at CQD.
Acknowledgment
This research was partially supported by the Department of Homeland Security (DHS), Science and Technology Directorate. This material represents the position of the authors and not necessarily that of DHS.
Reference
1. W. Zhou and N. Bandyopadhyay, et al. (2016). Monolithically, widely tunable quantum cascade lasers based on a heterogeneous active region design. Sci Rep, 6, article no. 25213.