Monolithically, widely tunable quantum cascade lasers based on a heterogeneous active region design

Quantum cascade lasers (QCLs) have become important laser sources for accessing the mid-infrared (mid-IR) spectral range, achieving watt-level continuous wave operation in a compact package at room temperature. However, up to now, wavelength tuning, which is desirable for most applications, has relied on external cavity feedback or exhibited a limited monolithic tuning range. Here we demonstrate a widely tunable QCL source over the 6.2 to 9.1 μm wavelength range with a single emitting aperture by integrating an eight-laser sampled grating distributed feedback laser array with an on-chip beam combiner. The laser gain medium is based on a five-core heterogeneous QCL wafer. A compact tunable laser system was built to drive the individual lasers within the array and produce any desired wavelength within the available spectral range. A rapid, broadband spectral measurement (520 cm−1) of methane using the tunable laser source shows excellent agreement to a measurement made using a standard low-speed infrared spectrometer. This monolithic, widely tunable laser technology is compact, with no moving parts, and will open new opportunities for MIR spectroscopy and chemical sensing.

across the 6-10 μ m wavelength range 16 . By varying only the relative layer thicknesses all emitters can be incorporated into a heterogeneous QCL core in a single growth run. When operated at the resonant field, current densities of all QC stages are nominally ~10 kA/cm 2 , which is accomplished by adjusting the doping concentration and upper laser level electron lifetime. As shown in Fig. 1b, the gain curve of the heterogeneous QCL is predicted to be broad and flat, which allows selection of the laser emitting wavelength over a wide range with an appropriate feedback mechanism. In order to characterize the gain bandwidth and flatness of the designed laser, the grown wafer is characterized with a DFB laser array in the same way as in ref. 7. Electroluminescence (EL) of the heterogeneous QCL wafer is shown in Fig. 1c. The EL spectrum has a full width at half maximum (FWHM) of 1050 cm −1 with a center near 1420 cm −1 . As shown in Fig. 1d, single mode DFB emission between 5.8 to 9.9 μ m is obtained from a twenty-one DFB laser array of 3 mm long with both facets AR-coated with 1300 nm Y 2 O 3 . The residual reflectivity is below 7% across the 6-10 μ m range with a minimum of 1.1% at 8 μ m. The current threshold is relatively flat between 5.8 and 9.0 μ m and increases rapidly beyond 9.0 μ m. The maximum power output is about 300 mW per facet at a wavelength of 7.7 μ m, and most DFB lasers have power output above 100 mW.
Monolithically tunable QCL source with a single output. The main limitation of the demonstrated DFB laser array is that it has multiple output apertures and limited tuning for each discrete single mode. To fill in the spectral gaps and realize a single output aperture, the twenty-one DFB QCLs are converted to an eight SGDFB QCL array and a beam combiner (see Methods section). Figure 2a shows the schematic of the integrated device. Two electric isolation channels are defined. One isolates the two SGDFB sections, and the other isolates the beam combiner section from the SGDFB laser array. Each SGDFB laser is 5.5 mm long with a ridge width of 10 μ m. The general SGDFB laser source is comprised of a series of short gratings (with grating period and number of periods defined as Λ g and N g , respectively) periodically sampled on the two sections with two different sampling periods Z 1 and Z 2 . For this demonstration, both sections of every SGDFB laser are sampled with N g = 20 for 16 times, and Λ g ranges from 0.97 to 1.40 μ m from laser #1 to #8. As only a short region is patterned, the 750 nm grating layer is completely etched to provide a maximum coupling strength. The sampling periods Z 1 = 161.8 μ m and Z 2 = 171.5 μ m are used for the front and back sections, respectively. The ridge waveguide is dry-etched by an inductively coupled plasma (ICP) and the cross section is shown in Fig. 2b. The plasma etching produces vertical sidewalls and precise shape for curved waveguides. The separation between each laser is as narrow as 100 μ m, which is chosen to enable direct wire bonding. The eight lasers are routed to a single output using a three-stage tree array Y-junction and S-bend waveguides. Given the combiner section length of 3 mm, the bending radius of the S-bend waveguide is 1800 μ m. A critical consideration for the beam combiner design is the preservation of the fundamental transverse mode. Higher order mode operation has been reported for Y-junction QCLs 17 . As a solution, we replaced the last Y-junction with a two-in-one funnel combiner 15 . As shown in Fig. 2c, the funnel combiner is designed to partially filter out high order modes that may be present at the input waveguide. Mode filtering improves as the relative angle between the input and output waveguide decreases, though there is some tradeoff in power transmission. For this demonstration, the insertion angle is set at 3.5° with a funnel combiner length and width of 82 μ m and 100 μ m, respectively.
In order to use the beam combiner as a single pass amplifier, after cleaving, the front facet is AR-coated with 1300 nm Y 2 O 3 . Tuning spectrum measurements were carried out on a thermo-electrically cooled (TEC) stage at 15 °C using a Fourier transform infrared spectrometer (FTIR) at a resolution of 0.125 cm −1 (see Methods section). One laser at a time in the SGDFB laser array was selectively biased, and the spectrum of the beam coming out of the single aperture beam combiner was measured. Synchronized pulse currents with 100 ns pulse width and 5% duty cycle (500 kHz repetition rate) were first applied, with separate drivers, to both sections of the selected SGDFB laser to reach threshold (~21 V). The voltage was kept the same on both sections and was further increased until single mode emission could be maintained (~24 V, I = 1.35I th ). Because the beam combiner section has the same gain material as the laser sections, a third synchronized pulsed current was applied to the beam combiner section for power amplification. For the current demonstration, single mode emission can be maintained, with high side mode suppression ratios (SMSR), for a combiner voltage of ~19 V. Wavelength tuning is realized by applying additional independent continuous wave (CW) currents to the two SGDFB laser sections. The injected CW currents locally change the section temperature, which leads to a change of the effective refractive index. The tuning of SGDFB lasers is the combination of steplike Vernier tuning, which depends on the CW current difference between the two sections, and the continuous tuning of each emission wavelength, achieved by changing the current in the two sections proportionally. Based on the SGDFB grating design, the estimated Vernier tuning step size is 9.7 cm −1 and 9.2 cm −1 with front and back section tuning, respectively. Figure 3a shows the combined Vernier tuning of 520 cm −1 between 6.2 and 9.1 μ m of the beam coming out of the single aperture beam combiner. The tuning of each laser is 64, 52, 60, 52, 58, 55, 65 and 45 cm −1 for lasers #1 through #8, respectively. The power of each single mode emission line is measured with a thermopile placed in front of the beam combiner facet. With the beam combiner amplification, an optical output power of several milliwatts has been achieved. The highest peak power of 5 mW is from laser #6, where the emission wavelength is close to a maximum in the gain curve. The relatively low power of the SGDFB laser compared to the DFB laser array is due to a significantly higher threshold (I th,SGDFB = 2.2I th,DFB ) and the coupling losses in the beam combiner region. Potentially, the power performance can be improved with better thermal management or stronger grating coupling, which will decrease the threshold current. Also, the output power is limited by the low beam combiner current due to multi-mode lasing at high current conditions. Reducing facet reflectivity, using multi-layer AR-coating or angled facet amplifier 18 , can increase the multi-mode lasing threshold. With future development, there is substantial room for power improvement by reducing the facet reflectivity. Even more, the power output across the entire tuning range can be uniformed by adjusting the beam combiner current.
As mentioned above, the Vernier tuning mechanism is not continuous, with a step of 9.2-9.7 cm −1 corresponding to the frequency spacing of the supermodes formed by the sampled gratings. Continuous tuning, however, can help fill in these gaps. Figure 3b shows the side mode suppression ratio (SMSR) of all the compiled single mode spectra measured from the beam combiner facet selected from automatic continuous tuning measurement by changing the front and back section CW currents. Most spectra have a side mode suppression ratio (SMSR) above 20dB, which suggests these wavelengths are suitable for spectroscopy applications. However, full wavelength coverage between 6.2 and 9.1 μ m is not achieved with these lasers, because the continuous tuning of a single mode is, at this time, limited to a maximum of 5.8 cm −1 . Based on the temperature-dependent spectral measurement of a DFB laser, which is processed from the same wafer, by increasing the laser stage temperature, a tuning coefficient of 0.1 cm −1 /K is estimated. This means that a ~60-degree increase in temperature makes the laser unable to reach threshold. With future development, the continuous tuning gaps can be bridged by decreasing the supermode space by increasing the sampling period length Z 1 and Z 2 .
Broadband absorption spectroscopy of methane. In order to automatically scan across all available wavelengths for chemical spectroscopy, a self-contained tunable laser system was designed and built as shown in Fig. 4a. This system works off of a single 48 V DC power supply and contains all of the electronics necessary to drive the individual lasers within the array and coordinate the driving of the laser array to produce a desired wavelength (see Methods section). The laser beam comes out as a single collimated output from the front of the system. The system was calibrated using custom automatic calibration software, which records the laser output spectrum with a FTIR as a function of tuning current combinations. For each of the lasers in the array, a colormap is made showing the peak wavelength as function of front and back section current set points. This is then used to generate optimized paths through the data and create a table of pre-calibrated scan paths, as shown in Fig. 4b. 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 wavelength scanning, a wavelength region of interest is selected and a scan is generated by specifying start, stop, and step conditions. In order to support high speed scanning, each discrete scan-state is downloaded to a dedicated random access memory (RAM) chip on the control board. A crystal oscillator and a phase-locked loop and frequency divider within the control field programmable gate array (FPGA) are then used to step through the states stored in the RAM. The electronic hardware supports a scan rate of up to 32 kHz. Figure 4c shows the setup for the absorption spectroscopy measurement, which consists of the tunable laser system, the gas cell, and a cryogenic mercury cadmium telluride (MCT) detector (see Methods section). The gas cell is filled with methane at 1 atmosphere. In order to measure the broadband absorption spectroscopy of methane, the tunable laser system was scanned from 6.2 μ m to 9.1 μ m with a step size of 0.1 nm at a scan rate of 512 Hz, which is limited by the maximum data-sampling rate of our lock-in amplifier. Because the tunable device has a single output and a lens installed, we can scan over all eight lasers in the array without needing realignment. The gas cell was first scanned with a vacuum FTIR in order to obtain a reference spectrum. Figure 5 shows a comparison of the reference spectrum (grey) to that measured with the tunable laser system (red points). In general, the tunable laser system is able to accurately measure the spectrum of methane. In particular, the inset of Fig. 5 shows excellent agreement with some of the fine spectral features.

Discussion
In conclusion, we demonstrated a tunable, broadband QCL source that emits at wavelengths from 6.2 μ m to 9.1 μ m (520 cm −1 ) using an eight-laser SGDFB array integrated with an on-chip beam combiner. A compact tunable laser system was built to drive the individual lasers within the array with the conditions necessary to produce any accessible wavelength. The system is able to do fast wavelength scanning without being limited by the driving hardware electronics. The spectral measurement of methane from 6.2 μ m to 9.1 μ m shows excellent agreement with FTIR measurement. This broadly tunable QCL sources with no moving parts will open new opportunities for mid-IR spectroscopy and chemical sensing.

Methods
Growth and fabrication. The QCL structure presented in this work is based on the Al 0.63 In 0.37 As/ Ga 0.35 In 0.65 As/Ga 0.47 In 0.53 As material system grown by gas-source molecular beam epitaxy on a semi-insulating InP substrate. The layer sequence and the waveguide doping are as follows: 3-μ m InP buffer layer (Si, ~2 × 10 16 cm −3 ), five-core heterogeneous laser core, 100-nm InP spacer layer (Si, ~2 × 10 16  Device Testing. All measurements were performed at room temperature. Spectral measurements were performed with a Bruker Fourier transform infrared (FTIR) spectrometer at a resolution of 0.125 cm −1 . The lasers operate in pulsed mode (100 ns pulse width, 500 kHz repetition rate), with the addition of DC biases to different sections to tune the laser frequencies. Three independent drivers were used to inject the pulse/DC currents for spectral and power measurements. Output mid-IR power in pulsed mode was measured using a calibrated thermopile detector for the average power and the peak power was calculated from the measured average power and the known duty cycle.
Tunable laser system. Within the system, the laser array is mounted on a thermo-electrically cooled (TEC) stage held at a stable heatsink temperature of 15 °C. This stage is mounted on invar-alloy posts for maximum laser pointing stability. The lasers within the array share a common cathode which is grounded to the heatsink. The system has three separate positive polarity laser drivers, each with a variable DC current and pulsed resistively loaded AC voltage. High-efficiency buck-converter power supplies (Texas Instruments TPS54360) with either current or voltage feedback being mixed with a digital to analog converter (DAC) signal are used to dynamically control the DC current and pulsed voltage of each driver. Two 1 × 8 multiplexers consisting of p-FETs (Vishay Si7309DN) select the laser array pair to be driven. The pulsed voltage is applied to the selected laser pair and the beam combiner via n-FETs (Vishay SiS892ADN). All of the FETs are driven by an embedded field programmable gate array (FPGA) via isolated gate drivers (Analog Devices ADuM4223). A separate FPGA controls the DAC settings and monitors the power supplies. The system uses an ARM cortex A9 based embedded single board computer (Toradex Colibri T30) as the main controller.
Spectroscopy measurement setup. The photoconductive detector is biased with 30 mA of DC current though a bias tee, and the output is connected to a high-speed lock-in amplifier (Stanford Research Systems SR488). A 500 kHz synchronization pulse locked to the laser pulsing is used as the reference input to the lock-in amplifier. An external measurement trigger output from the laser system is used to trigger the lock-in amplifier to record a data point into the lock-in's internal storage. The scanning and download of data after a scan are controlled remotely via a custom LabVIEW program. The tunable laser system can scan between a wavelength range with a designated step size, first with an open beam and then with the methane filled gas cell in the beam path. After correcting for the reflectivity of the windows, these two scans were then divided by each other to reveal the transmission spectrum of the methane.