High Efficiency, Low Power-Consumption DFB Quantum Cascade Lasers Without Lateral Regrowth
© The Author(s). 2017
Received: 16 March 2017
Accepted: 7 April 2017
Published: 19 April 2017
Very low power-consumption distributed feedback (DFB) quantum cascade lasers (QCLs) at the wavelength around 4.9 μm were fabricated by conventional process without lateral regrowth of InP:Fe or using sidewall grating. Benefitted from the optimized materials and low waveguide loss, very low threshold current density of 0.5 kA/cm2 was obtained for a device with cavity length of 2 mm. Combined with the partial-high-reflection coating, the 1-mm-long DFB QCL achieved low power-consumption continuous wave (CW) operation up to 105 °C. The CW threshold power-consumptions were 0.72 and 0.78 W at 15 and 25 °C, respectively. The maximum CW output power was over 110 mW at 15 °C and still more than 35 mW at 105 °C. At 15 °C, wall-plug efficiency of 5.5% and slope efficiency of 1.8 W/A were deduced, which were very high for low power-consumption DFB QCLs.
KeywordsQuantum cascade laser Distributed feedback Low power-consumption High efficiency
In recent years, quantum cascade lasers (QCLs) have proven to be very efficient coherent light sources in the mid-infrared region with a wide variety of applications, such as trace gas sensing, high-resolution spectroscopy, and free space communication [1–3]. Since the invention of QCLs , distributed feedback (DFB) QCLs have also attracted much attention for their single longitudinal mode operation and compact module [5–7]. In portable applications, the input electrical power consumption of the DFB QCL is required to be as low as possible, so that the total volume and power consumption of the system can be minimized. Moreover, in order to obtain stable and narrow line-width single-mode emission, continuous wave (CW) operation of DFB QCL above ambient temperature is desired. Several works on the demonstration of room temperature, low power-consumption DFB QCLs have been reported, in which narrow ridge and short cavity device structure are the common approach to lower the power consumption, with the typical ridge width of 2–5 μm [8–10]. However, the narrow ridge width will increase the waveguide loss due to the lateral optical absorption and scattering, leading to a high threshold current density (J th) and preventing the high temperature CW operation. To reduce waveguide loss of narrow ridge devices, the buried heterostructure (BH) configuration is necessary, which involves a complex fabrication process. Meanwhile, the BH structure reduces the optical confinement factor, which is also against to achieve low J th operation. In , Briggs et al. used sidewall grating as an alternative to the BH structure, but the loss is still high for the narrow ridge width (4 μm). As the result, the J th was high (1.91 kA/cm2 at 20 °C). In short, narrow ridge structure can lower the threshold current and power consumption at the expenses of J th, slope efficiency, and wall-plug efficiency (WPE), which is not helpful to further reduce power consumption. The complex process and high cost are also unfavorable to the fabrication and application of DFB QCLs.
In a previous report, we demonstrated the very low threshold operation of QCL in CW mode with the J th of 0.56 kA/cm2 at 20 °C by optimizing growth parameter of the epitaxial materials, wide ridge width was used to lower the waveguide loss and no lateral regrowth of InP:Fe was necessary . The very low J th is encouraging for developing low power-consumption DFB QCLs without a very narrow ridge.
In this work, an active region based on two-phonon resonance design was modified and optimized. DFB QCLs with ridge width of 9 μm were fabricated by a conventional process without regrowth of InP:Fe or using sidewall grating. At 20 °C, the DFB QCLs with high-reflection (HR) coating and different cavity lengths (1, 1.5, and 2 mm) can all achieve CW single-mode operation with low J th (0.84, 0.52, and 0.5 kA/cm2). In order to reduce the high mirror loss of short cavity devices, partial-high-reflection (PHR) coating consisting of Al2O3/Ge was designed and used . After PHR coating on the front facet, the CW J th of the 1-mm-long device was reduced to 0.64 and 0.69 kA/cm2 at 15 and 25 °C corresponding to the power consumptions of 0.72 and 0.78 W, respectively. Single-mode emission was maintained from 15 to 105 °C, and the lasing wavenumber at 150 mA was tuned from 2045.5 to 2031.2 cm−1; no mode hopping was observed. The maximum CW output power was over 110 mW at 15 °C and still more than 35 mW at 105 °C. Wall-plug efficiency (WPE) of 5.5% and slope efficiency of 1.8 W/A were deduced at 15 °C, which were very high for low power-consumption DFB QCLs.
The QCL active region was based on the two-phonon resonant design, consisting of strain compensated In0.67Ga0.33As/In0.37Al0.63As superlattice, which was similar to that in ref. . In order to obtain the lasing wavelength around 4.9 μm, the active region structure of ref.  was modified. The thicknesses of In0.67Ga0.33As quantum well layers were increased by 8%. According to our theoretical simulation on the modified active region structure, the dipole matrix element and the phonon scattering lifetime between the upper laser level and the lower laser level were 1.60 nm and 2.55 ps, respectively. The simulation results indicated that high performance can be maintained compared to ref. . Moreover, the drop of the transition levels in the modified structure lead to lower electric field intensity, meaning the decrease of operating voltage.
To lower the mirror loss for short cavity DFB QCL, a partial-high-reflection (PHR) coating consisting of Al2O3/Ge was deposited on the front facet of the device by electron beam evaporation. The design of Al2O3/Ge coating was based on the concept of optical admittance . The reflectivity (R) of the Al2O3/Ge coating for the wavelength of 4.9 μm as a function of the thicknesses of the two layers is plotted in Fig. 1b. As shown, the reflectivity of the Al2O3/Ge coating can be varied from 3.85 × 10−4 to more than 0.9 by changing the thicknesses of the two layers. Here we chose the thickness of Al2O3 and Ge to be 220 and 240 nm, corresponding to R ~ 0.5, as marked by the blue star. It should be pointed out that since the waveguide loss of a specific device cannot be obtained accurately and the coating layer thicknesses cannot be controlled precisely by electron beam evaporation, the choice of R ~ 0.5 was not optimal, only with the purpose to lower the J th.
Results and Discussion
The lasers were mounted on a thermoelectrically cooled holder with a thermistor for monitoring and adjusting the heat sink temperature. The optical power and spectra from front facets of the lasers were measured with a calibrated thermopile detector placed directly in front of the laser facets and a Fourier transform infrared (FTIR) spectrometer, respectively.
Low Threshold Continuous Wave Operation
High Temperature Low Power-Consumption Performance
After the CW measurement above, an Al2O3/Ge coating consisting of 220-nm-thick Al2O3 and 240-nm-thick Ge was deposited on the front facet of the 1-mm-long DFB QCL. The 1-mm-long device with both HR and PHR coatings realized very high CW performance, including high temperature, low power-consumption, high output power, stable single-mode emission, and high slope efficiency.
As shown, the performance of the 1-mm-long DFB QCL was improved significantly by the PHR coating. We expect to further reduce the power consumption of our DFB QCL by shortening the cavity length and using higher reflectivity PHR coating.
High Wall-Plug Efficiency
Very low power-consumption DFB QCLs were fabricated by conventional process without lateral regrowth or using sidewall grating. The low J th room temperature CW operation was demonstrated, which can be attributed to the optimized materials and low waveguide loss. The 1-mm-long PHR and HR-coated DFB QCL achieved very high CW performance, including high operating temperature (up to 105 °C), low threshold power-consumption (0.72 and 0.78 W at 15 and 25 °C), stable single-mode emission (SMSR >20 dB and mode-hopping free), high WPE of 5.5% and high slope efficiency of 1.8 W/A at 15 °C. The maximum CW output power was over 110 mW at 15 °C and still more than 35 mW at 105 °C. Compared with the previous reports on low power-consumption DFB QCLs, the fabrication process was simple and low cost. Meanwhile, the high temperature CW operation with low power-consumption allowed for more compact laser package, which is significant to develop portable laser system and for applications in harsh thermal environments.
Fourier transform infrared
- I th :
- J th :
Threshold current density
Molecular beam epitaxy
Metal organic vapor phase epitaxy
- PIV :
Quantum cascade laser
- R :
- η s :
The authors acknowledge the contributions of Ping Liang and Ying Hu in the device fabrication.
This work was supported by the National Basic Research Program of China (Grant Nos. 2013CB632803/01), National Key Research and Development Program (Grant No. 2016YFB0402303), National Natural Science Foundation of China (Grant Nos. 61435014, 61627822, 61574136, 61404131), Key Projects of Chinese Academy of Sciences (Grant No. ZDRW-XH-2016-4), and Beijing Natural Science Foundation (Grant No. 4162060, 4172060).
ZWJ designed the device structure, fabricated the device, performed the testing, and wrote the paper. LJW and FQL provided the concept, polished the paper, and supervised the project. JCZ and JQL improved the design. YHZ and SQZ performed the testing. DBW and XFJ 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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