Demonstration of High-Power and Stable Single-Mode in a Quantum Cascade Laser Using Buried Sampled Grating
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High-power, low-threshold stable single-mode operation buried distributed feedback quantum cascade laser by incorporating sampled grating emitting at λ ~ 4.87 μm is demonstrated. The high continuous wave (CW) output power of 948 mW and 649 mW for a 6-mm and 4-mm cavity length is obtained at 20 °C, respectively, which benefits from the optimized optical field distribution of sampled grating. The single-mode yields of the devices are obviously enhanced by controlling cleaved positions of the two end facets precisely. As a result, stable single-mode emission and mode tuning linearly without any mode hopping of devices are obtained under the different heat sink temperatures or high injection currents.
KeywordsQuantum cascade laser Distributed feedback Sampled grating
Full width at half maximum
Molecular beam epitaxy
Metal organic vapor phase epitaxy
Plasma-enhanced chemical vapor deposition
Quantum cascade laser
Quantum cascade lasers (QCLs) have turned out to be one of the most promising mid-infrared light sources and attracted much attention for the applications of remote sensing, high-resolution spectroscopy, and industrial process monitoring after its first demonstration due to its highlight features such as large wavelength covering range, compact size, and high output power [1, 2, 3, 4]. As for those applications, single-mode emission and high output power are usually desired, which can be achieved by a distributed feedback (DFB) QCL. The buried grating approach has been adopted widely for a smaller waveguide loss, lower threshold current density, and higher single-mode yields compared with the surface grating [5, 6]. Up to now, a series of significant breakthroughs based on a buried grating approach have been made in improving the performance of DFB QCLs of single-mode stability and output power [7, 8], but an over-coupled feedback mechanism of buried grating hinders the output power from enhancing further. The typical value of continuous wave (CW) output power of buried uniform grating DFB QCLs emitting around 4.6–5 μm is less than 300 mW at room temperature [5, 9]. Theoretically, the coupling coefficient of buried grating can be improved by optimizing the grating depth and the duty cycle. However, the distributed feedback performance levels are very sensitive to the profile of etching of grating in the semiconductor layer close to the active area. Any tiny variation of the etching depth and the duty cycle would strongly influence the grating coupling coefficient [10, 11]. Moreover, it is also difficult to improve the grating coupling by controlling the grating depth and the duty cycle precisely based on a low-cost holographic lithography technique and wet chemical etching. Generally, the conventional DFB QCLs oscillate at two frequencies slightly shifted from the Bragg frequency, which can lase depending on the optical loss influenced by the facet random phase [12, 13, 14].
In this work, we propose the use of buried sampled grating with a small sampling duty cycle for optimizing the coupling coefficient and improving the optical field distribution. The prominent advantage of this method is it is able to increase the cavity length of device for enough optical gain while maintaining a desirable grating coupling strength. To improve the single-mode yields and ultimate performance, cleaved position of the two end facets is precisely controlled to avoid the effect of the end facet random phase. On the one hand, this approach retains the advantages of small waveguide loss for a low threshold current density and is compatible with buried heterostructure processing. Furthermore, the sampled grating is fabricated only through conventional holographic exposure combined with the optical photolithography, which leads to improved flexibility, repeatability, and cost-effectiveness. As a result, low threshold and high-output power single-mode DFB QCLs emitting at λ ∼ 4.87 μm are achieved simultaneously in the buried sampled grating structure. The threshold current density of these DFB-QCLs is as low as 1.05 kA/cm2 and the single facet produced 948 mW of CW output power for the device with a 6-mm cavity length at 20 °C.
The QCL structure was grown on an n-InP (Si, 2 × 1017 cm−3) substrate by solid-source molecular beam epitaxy (MBE). The active core consisted of 40 stages of strain-compensated In0.67Ga0.33As/In0.37Al0.63As quantum wells and barriers providing the electron transition channel to produce photon, which was surrounded by the upper and lower InGaAs confinement layers. The grating was defined on the upper InGaAs confinement layer using a double-beam holographic lithography technique combined with conventional optical lithography. Then the upper waveguide layer was grown by metal organic vapor phase epitaxy (MOVPE). After that, the wafer was processed into a double-channel ridge waveguide laser with an average core width of about 10 μm filling with semi-insulating InP:Fe for efficient heat removal. A 450-nm-thick SiO2 layer was then deposited by plasma-enhanced chemical vapor deposition (PECVD) for insulation, and electrical contact was provided by a Ti/Au layer deposited by electron beam evaporation. An additional 5-μm-thick gold layer was electroplated for improving heat dissipation. After being thinned down to about 140 μm, a Ge/Au/Ni/Au metal contact layer was deposited on the substrate side. Then the waveguides were cleaved into 4-mm- and 6-mm-long bars, and the high reflectivity (HR) coating consisting of Al2O3/Ti/Au/Al2O3 (200/10/100/120 nm) was deposited on one of the facets by electron beam evaporation, leaving an uncoated facet for the measurement of edging emitting power. Lastly, the lasers were mounted with the epilayer side-down on a diamond heat sink with an indium solder, which were subsequently soldered on copper heat sinks for effective heat dissipation.
Results and Discussion
The spectra of devices were tested by a Fourier transform infrared spectrometer with a resolution of 0.25 cm−1. The lasers were then mounted on a holder containing a thermistor combined with a thermoelectric cooler to monitor and adjust the sub-mount temperature. The emitted optical power was measured with a calibrated thermopile detector placed in front of the laser facet without any correction.
In conclusion, low-threshold, high-output power stable single-mode emission sampling grating DFB QCLs have been achieved. The maximum CW output power and threshold current density are 0.948 W (0.649 W) and 1.05 kA/cm2 (1.59 kA/cm2) for a 6-mm (4 mm) cavity. A major improvement in distribution of the optical field is realized by introducing the small sampled duty cycle to reduce the coupling strength. A single lobe far-field profile is also observed. So for buried distributed feedback quantum cascade lasers, incorporating sampled grating is a simple and effective method to achieve the devices with high-output power, low-threshold, stable single-mode emission and high single-mode yields.
The authors would like to thank Ping Liang and Ying Hu for their help in device processing.
This work was supported by the National Key Research and Development Program (Grant No. 2016YFB0402303, 2018YFA0209103), the National Natural Science Foundation of China (NSFC) (Grant Nos. 61774150, 61790583, 61574136, 61627822, and 61774146) and the Key projects of Chinese Academy of Sciences ( Grant No. QYZDJ-SSW-JSC027), the Instrument training project of Beijing science and Technology Commission (Grant No. Z181100009518002).
Availability of Data and Materials
All data are fully available without restriction.
FMC designed the device structure, fabricated the devices, calculated the theoretical model, performed the testing, and wrote the paper. JCZ and FQL provided the concept, polished the paper, and supervised the project. DBW and ZHG improved the theoretical model. SQZ and SML improved the design. LJW and JQL completed the MOCVD growth. NZ modulated the active region structure and completed the MBE growth. ZGW supervised the project. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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