Open Access
Add to BibSonomy
Abstract
In this article, we report a high power quantum cascade laser (QCL) at λ$\sim$7.4 µm with a broad tuning range. By carefully designing and optimizing the active region and waveguide structure, a continuous-wave (CW) output power up to 1.36 W and 0.5 W is achieved at 293 K and 373 K which shows the excellent temperature stability. A high wall-plug efficiency (WPE) of 8% and 13.6% in CW and pulsed mode at 293 K are demonstrated. The laser shows a characteristic temperature T0 of 224 K and T1 of 381 K over a temperature range from 283 K to 373 K. In addition, a far field of pure zero order transverse mode and a fairly wide external cavity (EC) tuning range (280 cm-1) from 6.54 µm to 8 µm are achieved in pulsed operation. In addition, an EC single mode output power of 226 mW is obtained under CW operation at 293K.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Quantum cascade lasers (QCLs), as the ideal light source in mid-wave infrared (MIR) and long-wave infrared (LWIR), have experienced a significant progress since the first demonstration in 1994 [1]. The LWIR (7-12 $\mu m$) region is of great importance in many fields like chemical sensing and free space communication. For commercial applications such as trace gas sensing, macromolecules and explosives detection, high power and wide tuning range of the QCLs means a lower detection limit and the possibility of simultaneous detection of multiple absorption peaks and components, which are very necessary for quantitative and precise measurement. [2–4] Therefore, high performance QCLs with both broad tuning range and high power are of great value.
Lots of high performance QCLs in LWIR region have been reported in recent years. [5–9] Taking the 7-8 $\mu m$ region as an example which contains important absorption peaks of gas like CH4 and explosives like TNT. QCLs with a 7.3-7.8 $\mu m$ lasing range using strained active region were reported in 2010 [10], but the power didn’t reach watt-level. Richard Maulini et al. optimized LWIR voltage defect of ∼100 meV and achieved a maximum CW output power of 1.38 W and maximum wall-plug efficiency (WPE) of 10.0% at 7.1 $\mu m$ [11]. Based on slightly-diagonal bound to continuum design, Huan Wang et al [12]. achieved a watt-level CW output power with a WPE of 9.08% at 7.7 $\mu m$. Quanyong Lu et al. reported a strong-coupled strain-balanced design which realized a broad electroluminescence (EL) of 380 cm-1 and pulsed wall-plug efficiency of 10% around 7.8$\mu m$ [13]. By using a high differential gain active region, W. Zhou et al [14]. obtained a CW output power of 2.0 W at 8 $\mu m$ and a high WPE of 20.4% in pulsed mode operation and 12.8% in CW operation. Besides, Botez et al. demonstrated a high performance QCL with a pretty high internal efficiency by using step-taper resonant extraction (STA-RE type) active region design [15–17], but the epitaxial process of this type of QCLs was pretty complicated.
However, it’s hard for those high power QCLs to obtain a wide tuning range which is limited by their active region design. The typical bound to bound (BTB) QCLs have narrow linewidth (full width at half maximum, FWHM, ϒ∼20 meV) of EL spectrum [18]. Meanwhile, the bound to continuum (BTC) designed QCLs are sensitive to the electric field, the EL spectrum linewidth decreases steeply with increasing voltage [19] which is owing to the increased spacing between energy levels in the continuum. To get a broad tuning range, a five-stack laser design with a 432 cm-1 tuning range was demonstrated by Andreas Hugi et al. [20], still, the peak power reached 1 W in EC operation. However, this kind of QCLs may have an inhomogeneous spectral behavior. Dual-upper-state (DAU) design, which can broaden the gain spectrum range and tuning range in principle. For instance, a broad EC tuning range of 22% (320 cm-1) of the center wavelength was reported at 6.8 $\mu m$ by Fujita et al. [18,21], and they also reported a wide EL spectrum linewidth (330 cm−1) around 8.4 $\mu m$ [19]. Tatsuo Dougakiuchi et al. [22] reported a tuning range from 9.5 to 11.4 $\mu m$ (176 cm−1, 18% of the center wavelength around 10 $\mu m$). These QCLs are all based on DAU design, but none of those DAU design QCLs can exceed 1W CW operation at room temperature. However, high power and broad tuning range are equally important in commercial applications.
Here, we report a high power QCLs (λ∼7.4 $\mu m$) with a broad tuning range by carefully designing the energy level of the dual-upper-state and waveguide structure. A CW output power up to 1.36 W and 0.5 W at 293 K and 373 K was reported, still, a relatively broad electroluminescence spectrum (42 meV at 7.0 V) and wide EC tuning range (280 cm-1) was achieved.
2. Active region and waveguide design
The conduction band diagram of the active region under an electric field of 48 kV/cm was shown in Fig. 1. The broad tuning range and high power QCLs reported here was based on DAU design. The active region was based on a strain compensation Al0.58In0.42As / In0.6Ga0.4As structure.
Fig. 1. Conduction band diagram and wave functions of relevant energy levels of the active region. An electric field of 48 kV/cm was applied to align the structure. The structures are based on In0.6Ga0.4As/In0.42Al0.58As material system, layer thicknesses, starting from the injection barrier were as follows: 38/24/27//61/9/49/11/45/12/37 15/32/16/31/19/29/22/29/25/28/29/26 where doped layers (Si, 2.5 E17 cm−3) are underlined. The barriers layers are in bold, and wells layers are in roman.
The transition taken place between the two upper states numbered by 4,3 and the lower miniband numbered by 2 which contributed to a broad-gain spectrum. In order to obtain better laser performance, we design a strong-coupling two upper level with energy splitting of 2 ħΩ=20 meV, which can effectively improve the carrier tunneling rate into the upper levels [23] and help to improve the injection efficiency. The transition matrix element of the transitions from the two upper states to lower miniband are slightly different such as Z32 = 2.0 nm and Z42 = 1.5 nm (from two upper states to the same lower state in miniband 2) to avoid strong mode competition. A relatively high energy gap ΔEC5∼190 meV could effectively suppress the leakage current and ΔE54∼60 meV helps to a high performance even at high temperature. The structure was grown on the InP substrate (Si∼2E17 cm-3), with a 5 $\mu m$ thick InP lower waveguide, the 35-period active regions was sandwiched between two 300 nm thick In0.53Ga0.47As to improve the optical confinement. Then, a 3.5 µm InP cladding layer (Si∼3E16 cm-3) was grown, followed by 0.15 µm graded doped (Si∼1-3E17 cm-3) and 0.85 µm highly doped (Si∼5E18 cm-3) InP layer. A relatively thicker layer of waveguide was designed to obtain a lower waveguide loss and higher confinement. The metal organic chemical vapor deposition (MOCVD) was used for upper InP layers after the growth of the active region and two InGaAs layer by molecular beam epitaxial (MBE).
3. Fabrication and characterization of the device
Considering the dissipation of the heat and also the far field divergence angle, the complete wafer was processed into device with a ridge width of 7.1 $\mu m$. Laser bars of 4 mm length with HR facet coating were cleaved and down mounted on diamond heatsink for testing. The EL spectrum was measured under a pulsed operation (100 kHz,1 $\mu s$) at different voltage at room temperature. As showing in Fig. 2(a), there was a strong H2O absorption peak around the half maximum of the spectrum. After fixing the water absorption by testing the background water absorption, the result was showed in Fig. 2(b). The inserted graph in Fig. 2(b). showed the FWHM linewidth of the device under different voltage. The linewidth was about 42 meV at 7 V which was much wider than the typical linewidth of the reported BTC QCLs. [24]
Fig. 2. EL spectrum of the strip device at different voltage at 298 K (a)with water absorption (b)after removing water absorption, Inset in (b): the FWHM linewidth at different voltage.
The CW and pulsed power-current-voltage (P-I-V) curves, along with the WPE results of the 7.1 $\mu m$ wide, 4 mm long cavity, HR-coated device at different temperature were shown in Fig. 3(a) and Fig. 3(b). A maximum CW optical output power of 1.36 W was obtained at 293 K, and the CW power still reached 500 mW at 373 K which is much higher (more than twice) than other reported QCLs based on DAU design [18,19,22]. We attributed the performance improvement to the optimized designing of the two upper energy level and the waveguide structure. The maximum CW WPE of 8% was measured at 293 K. The maximum WPE and the maximum peak power in pulsed mode was 13.6% and 2.3 W at 293 K respectively. The performance of this device was comparable to those high performance QCLs in 7-8 $\mu m$[10–13]. Meanwhile, the high power under high temperature and the high temperature stability enriched its practical application scenarios.
Fig. 3. (a) CW P-I-V characteristics of the 7.1 $\mu m$ wide,4 mm long, HR-coated with a diamond sub-mount at different temperature. (b) pulsed P-I-V characteristics of the 7.1 $\mu m$ wide,4 mm long, HR-coated with a diamond sub-mount at different temperature under 20 kHz,0.5 $\mu s$. Right axis: solid line: output power; dashed line: wall-plug efficiency. Inset in (b): spectrum at 303 K,1.25 Ith.
We also analyzed the characteristic temperature of the device based on the CW and pulsed P-I-V test results. Derived by the exponential function:
Where T refers to the temperature of the heatsink, T0 and T1 refers to the characteristic temperature coefficient. As shown in Fig. 4(a) and Fig. 4(b), the characteristic temperature T0 and T1 under CW test condition were 168 K and 240 K, respectively. Meanwhile, the characteristic temperature of T0 and T1 under pulsed test condition were 224 K and 384 K, which also demonstrated the high temperature stability of the device.Fig. 4. Threshold current density, characteristic temperature, and slope efficiency of the device under (a)CW operation. (b) pulsed operation with duty cycle of 1%.
4. EC tuning performance
The far-field image of the laser at driving current slightly exceeded the threshold current was shown in Fig. 5(a) which consisted of a pure zero order transverse mode. The picture was taken with a pyroelectric camera placed 25 cm away from the laser, with lens (Lightpath,390037IR1) collimation. In order to verify the broadband characteristics of the laser, we constructed the external cavity system of Littrow configuration. As shown in Fig. 5(b), the 4 mm long cavity device emitting on both sides was coated with AR (Al2O3/Ge) at the front facet for higher feedback, and the reflectivity of the front facet is less than 1%. The feedback beam was provided by the first-order diffraction of the grating (Newport, 33095FL02-142R), and by changing the reflection angle of the grating, we obtained the spectrum and peak output power under different blazed grating angles as shown in the Fig. 5(c). The results showed a broad tuning range from 6.54 $\mu m$ (1529.15 cm-1) to 8.0 $\mu m$ (1248.741 cm-1), $\mathrm{\Delta} \lambda $ = 1.48 $\mu m$ (280 cm-1, which gives a pretty wide tuning performance $\frac{{\mathrm{\Delta} \lambda }}{\lambda } \approx 20.7$%), which was benefit from the DAU active region design. The maximum peak output power of the 4 mm external cavity tunable laser was 252mW at center wavelength of 7.47 $\mu m$ (1338.58cm−1), under the duty cycle of 2% (10 kHz,2 $\mu s$) and current of 650 mA, and the peak power can be higher with higher injection current.
Fig. 5. (a) Far field picture of the QCL measured in pulsed mode at room temperature under a current of 540 mA with a duty cycle of 4% (20 kHz 2 $\mu s$) The horizontal and vertical beam sizes were 1.99 mm and 1.97 mm, respectively (b) Schematic illustration of EC-QCL for QCLs emitting on both sides, with AR coat (Al2O3/Ge) (c) Measurements of the EC-tuning spectrum and peak power of the QCLs at 293 K with a duty cycle of 2% (10 kHz, 2 $\mu s$) at 650 mA.
We also tested the EC tuning performance under CW operation. Considering the high output power of the QCLs, we changed the AR coat from Al2O3/Ge (600nm/70nm) to ZrO2 (350nm) for the reason that Al2O3/Ge AR coat may be damaged at very high output power (over 1 W) while ZrO2 can withstand it. However, due to the difficulty of thick coating, the reflectivity of ZrO2 coat (350nm) is 16% which is much higher than 1% of Al2O3/Ge coat and may leads to lower feedback. A HR coated back facet was used for lower threshold and higher output power. Therefore, we changed the EC configuration and tested the output power by using a beam splitter with a reflectance of 30% around 7.4 $\mu m$ as shown in Fig. 6(a). We chose a relatively high transmittance of the beam splitter for a better feedback, still, a relatively high output power. The pulsed and CW tuning performance and output power of the reflected beam are also shown in Fig. 6. The reduced tuning range of both pulsed and CW operation are due to the high reflectivity of the AR coat, but as shown in Fig. 6(b) and (c), the tuning performance under CW operation and pulsed operation are pretty close (215 cm-1 and 193 cm-1) which indicates the gain of the laser is strong enough. Also, the EC single mode output power of the reflected beam can reach 226 mW under CW operation at 293K with an injection current of 1.4 A with only 30% reflectance of the beam splitter around 7.4 $\mu m$. The output power is directly measured from the output beam without parameter correction which can be larger if the reflectance is optimized.
Fig. 6. (a) Schematic illustration of EC-QCL with both facets coated, emitting on single side, with AR coat (ZrO2), HR coat (Al2O3/Au/ Al2O3), the reflectance of the beam splitter around 7.4 $\mu m$ is about 30%. (b) Measurements of the EC-tuning spectrum from 1246 cm-1 to 1461 cm-1 (215 cm-1) under pulsed operation of the QCLs at 293 K with a duty cycle of 2% (10 kHz, 2 $\mu s$) at 600 mA. (c) EC-tuning spectrum from 1250 cm-1 to 1443 cm-1 (193 cm-1) and output power under CW operation of the QCLs at 293 K at 1.4A.
5. Conclusion
In summary, we demonstrated a high performance quantum cascade laser at λ$\sim$7.4 $\mu m$ with a broad tuning range. By carefully designing and optimizing the active region and waveguide structure, a broad EL linewidth (42 meV at 7 V) and a CW output power up to 1.36 W and 0.5 W was achieved at 293 K and 373 K respectively. A high wall-plug efficiency of 8% and 13.6% in CW and pulsed mode at 293 K were achieved for a 4 mm long, 7.1 $\mu m$ narrow ridge laser mounted epi-side down on diamond heatsink. A far field of pure zero order transverse mode and a broad EC tuning range from 6.54 $\mu m$ to 8 $\mu m$ (280 cm-1, $\frac{{\mathrm{\eta} \lambda }}{\lambda } \approx 20.7$%) were reported. In addition, an EC single mode output power of 226 mW is obtained under CW operation at 293 K. The LWIR QCLs with both high power and broad tuning range are of great value for broadband detection applications.
Funding
National Key Research and Development Program of China (2018YFA0209103); National Natural Science Foundation of China (61790583, 61991430, 62174158); Youth Innovation Promotion Association of the Chinese Academy of Sciences (2021107).
Acknowledgments
The authors would like to thank Ping Liang and Ying Hu for their help in device processing.
Disclosures
The authors declare no conflicts of interest.
Data Availability
The data that support the findings of this study are available from the corresponding authors upon reasonable request.
References
1. J. Faist, F. Capasso, D. L. Sivco, C. Sirtori, A. L. Hutchinson, and A. Y. Cho, “Quantum cascade laser,” Science 264(5158), 553–556 (1994). [CrossRef]
2. O. Hertzberg, A. Bauer, A. Kuderle, M. A. Pleitez, and W. Mantele, “Depth-selective photothermal IR spectroscopy of skin: potential application for non-invasive glucose measurement,” Analyst 142(3), 495–502 (2017). [CrossRef]
3. Y. Fu, H. Liu, and J. Xie, “100-m standoff detection of a QCL-induced photo-vibrational signal on explosives using a laser vibrometer,” Opt. Lasers Eng. 107, 241–246 (2018). [CrossRef]
4. Y. Yoon, C. J. Breshike, C. A. Kendziora, R. Furstenberg, A. R. McGill, R. A. Crocombe, L. T. Profeta, and A. K. Azad, “Control of quantum cascade laser sources in stand-off detection of trace explosives,” Proc. SPIE 10983, 109830G (2019). [CrossRef]
5. A. Lyakh, R. Maulini, A. Tsekoun, R. Go, and C. K. N. Patel, “Multiwatt long wavelength quantum cascade lasers based on high strain composition with 70% injection efficiency,” Opt. Express 20(22), 24272–24279 (2012). [CrossRef]
6. T. Fei, S.Q. Zhai, J.C. Zhang, N. Zhuo, J.Q. Liu, L.J. Wang, S.M. Liu, Z. W. Jia, K. Li, Y. Q. Sun, K. Guo, F. Q. Liu, and Z. G. Wang, “High power λ ∼ 8.5 µm quantum cascade laser grown by MOCVD operating continuous-wave up to 408 K,” J. Semicond. 42(11), 112301 (2021). [CrossRef]
7. M. Troccoli, A. Lyakh, J. Fan, X. Wang, R. Maulini, A. G. Tsekoun, R. Go, and C. K. N. Patel, “Long-wave IR quantum cascade lasers for emission in the λ = 8-12µm spectral region,” Opt. Mater. Express 3(9), 1546 (2013). [CrossRef]
8. X. Feng, C. Caneau, H. P. Leblanc, D. P. Caffey, L. C. Hughes, T. Day, and Z. Chung-en, “Watt-level room temperature continuous-wave operation of quantum cascade lasers with λ >10 µm,” IEEE J. Sel. Top. Quantum Electron. 19(4), 1200407 (2013). [CrossRef]
9. B. Schwarz, C. A. Wang, L. Missaggia, T. S. Mansuripur, P. Chevalier, M. K. Connors, D. McNulty, J. Cederberg, G. Strasser, and F. Capasso, “Watt-level continuous-wave emission from a bifunctional quantum cascade laser/detector,” ACS Photonics 4(5), 1225–1231 (2017). [CrossRef]
10. R. P. Leavitt, John L. Bradshaw, Kevin M. Lascola, Gregory P. Meissner, Frankie Micalizzi, Frederick J. Towner, and John T. Pham, “High-performance quantum cascade lasers in the 7.3- to 7.8-µm wavelength band using strained active regions,” Opt. Eng. 49(11), 111109 (2010). [CrossRef]
11. R. Maulini, A. Lyakh, A. Tsekoun, and C. K. N. Patel, “λ∼7.1 µm quantum cascade lasers with 19% wall-plug efficiency at room temperature,” Opt. Express 19(18), 17203–17211 (2011). [CrossRef]
12. H. Wang, J.C. Zhang, F.M. Cheng, N. Zhuo, S.Q. Zhai, J.Q. Liu, L.J. Wang, S.M. Liu, F.Q. Liu, and Z.G. Wang, “Watt-level, high wall plug efficiency, continuous-wave room temperature quantum cascade laser emitting at 7.7 µm,” Opt. Express 28(26), 40155–40163 (2020). [CrossRef]
13. Quanyong Lu, Dong hai Wu, Saumya Sengupta, Steven Slivken, and Manijeh Razeghi, “Room temperature continuous wave, monolithic tunable THz sources based on highly efficient mid-infrared quantum cascade lasers,” Sci. Rep. 6(1), 23595 (2016). [CrossRef]
14. W. Zhou, Q. Y. Lu, D. H. Wu, S. Slivken, and M. Razeghi, “High-power, continuous-wave, phase-locked quantum cascade laser arrays emitting at 8 µm,” Opt. Express 27(11), 15776–15785 (2019). [CrossRef]
15. D. Botez, C. C. Chang, and L. J. Mawst, “Temperature sensitivity of the electro-optical characteristics for mid-infrared (λ = 3–16 µm)- emitting quantum cascade lasers,” J. Phys. D: Appl. Phys. 49(4), 043001 (2016). [CrossRef]
16. J. D. Kirch, C. C. Chang, C. Boyle, L. J. Mawst, D. Lindberg, T. Earles, and D. Botez, “86% internal differential efficiency from 8 to 9µm-emitting, step-taper active-region quantum cascade lasers,” Opt. Express 24(21), 24483–24494 (2016). [CrossRef]
17. D. Botez, J. D. Kirch, C. Boyle, K. M. Oresick, C. Sigler, H. Kim, B. B. Knipfer, J. H. Ryu, D. Lindberg, T. Earles, L. J. Mawst, and Y. V. Flores, “High-efficiency, high-power mid-infrared quantum cascade lasers,” Opt. Mater. Express 8(5), 1378 (2018). [CrossRef]
18. K. Fujita, S. Furuta, A. Sugiyama, T. Ochiai, A. Ito, T. Dougakiuchi, T. Edamura, and M. Yamanishi, “High-performance quantum cascade lasers with wide electroluminescence (600cm-1), operating in continuous-wave above 100°C,” Appl. Phys. Lett. 98(23), 231102 (2011). [CrossRef]
19. K. Fujita, T. Edamura, S. Furuta, and M. Yamanishi, “High-performance, homogeneous broadgain quantum cascade lasers based on dual-upper-state design,” Appl. Phys. Lett. 96(24), 241107 (2010). [CrossRef]
20. A. Hugi, R. Terazzi, Y. Bonetti, A. Wittmann, M. Fischer, M. Beck, J. Faist, and E. Gini, “External cavity quantum cascade laser tunable from 7.6 to 11.4 m,” Appl. Phys. Lett. 95(6), 061103 (2009). [CrossRef]
21. T. Dougakiuchi, K. Fujita, N. Akikusa, A. Sugiyama, T. Edamura, and M. Yamanishi, “Broadband tuning of external cavity dual-upperstate quantum-cascade lasers in continuous wave operation,” Appl. Phys. Express 4(10), 102101 (2011). [CrossRef]
22. T. Dougakiuchi, K. Fujita, A. Sugiyama, A. Ito, N. Akikusa, and T. Edamura, “Broadband tuning of continuous wave quantum cascade lasers in long wavelength (> 10µm) range,” Opt. Express 22(17), 19930–19935 (2014). [CrossRef]
23. Jacob B. Khurgin, Yamac Dikmelik, Peter Q. Liu, et al., “Role of interface roughness in the transport and lasing characteristics of quantum cascade lasers,” Appl. Phys. Lett. 94(9), 091101 (2009). [CrossRef]
24. R. Maulini, M. Beck, J. Faist, and E. Gini, “Broadband tuning of external cavity bound-to-continuum quantum-cascade lasers,” Appl. Phys. Lett. 84(10), 1659–1661 (2004). [CrossRef]
