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Abstract
In this work, we report high power continuous wave room-temperature operation single mode quantum cascade lasers in the mid-infrared spectral range from 3.8 to 8.3 µm. Single mode robustness and dynamic range are enhanced by optimizing the distributed feedback grating coupling design and the facet coatings. High power single mode operation is secured by circumventing the over-coupling issue and spatial hole burning effect. Maximum single-facet continuous-wave output power of 5.1 W and wall plug efficiency of 16.6% is achieved at room temperature. Single mode operation with a side mode suppression ratio of 30 dB and single-lobed far field with negligible beam steering is observed. The significantly increased power for single mode emission will boost the QCL applications in long-range free-space communication and remote sensing of hazardous chemicals.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The mid-infrared spectral region (3-35 µm) is important for chemical sensing, analysis and identifying and studying chemical substances [1]. Plenty of gas- and liquid-phase chemicals have characteristic absorption features in mid-infrared region that can be used for sensing and identification. Many devices have been developed to access the mid-IR range; among them, quantum cascade lasers (QCLs) emerged as a coherent light sources in 1990s. Owing to its unique intersubband transition and cascade features, the emitting wavelength of a QCL can be widely tailored from 3 to 25 µm as well as deep into the THz range [2]. The recent rapid development in power, efficiency, and spectral tuning, is making QCLs the leading light source in mid-IR and THz ranges [3,4].
In many QCL application scenarios, like gas sensing and analysis, medium power (< 100 mW) is sufficient owing to the rather sensitive absorption features in the mid-IR range [5]. However, under certain circumstances, like stand-off detection of explosives and dangerous chemicals, long-range free-space communications, and infrared countermeasure, high power output with narrow linewidth is highly advantageous [6,7]. To satisfy these application requirements, significant effort has been consistently devoted to developing high power high efficiency QCLs in the past two decades [8–11]. Up to now, the most powerful QCL from a single chip in continuous wave (CW) operation can emit to 5.1 W with 21% wall plug efficiency at room temperature, which was demonstrated in Northwestern University [12]. Since Fabry–Pérot (FP) cavity has been utilized in these high power demonstrations, the device usually emits a broad spectrum, which can be potentially converted into frequency comb operation for gas detection application when dispersion cancellation technique is applied [13]. Nevertheless, due to the sporadic absorption feature in the atmosphere, part of the laser spectrum could be significantly attenuated in a long-range propagation application. As a result, a certain mode selection mechanism should be applied to narrow the spectrum and target the high transmission lines in the atmosphere to avoid the dramatic attenuation for the outdoor applications. Currently, distributed feedback (DFB) grating [14], distributed Bragg reflector (DBR) [15], and multi-section sampled grating DFB [16] have been used in QCLs to achieve narrow linewidth emission. Previously, high power up to 2.4 W at room temperature CW operation was demonstrated from a surface-plasmon coupled DFB QCL design [17]. Besides the high efficiency active region design, the key to achieve high power single mode operation is proper coupling strength with sufficient modal loss discrepancy between the lasing mode and other modes.
Here we report an anti-reflection (AR)-AR facilitated master oscillator power amplifier (MOPA) QCL design, and demonstrate single mode, high power output up to 5 W in room temperature continuous wave operation spanning a wavelength range of 3.8 to 8.3 µm.
2. Device modelling
In this work, the master oscillator (MO) section consists of a short DFB section with a strong coupling coefficient for single mode operation and the power amplifier section consists of a long section of uncorrugated active waveguide for power amplification [18]. The AR-AR coatings are deposited to front and back facets to enhance the modal loss difference. Unlike the surface-plasmon coupled DFB design which relies on high-reflection (HR) coating on the back facet to strengthen the DFB feedback for single mode operation, the HR coating for the strongly coupled MO sections could undermine the single mode operation, as shown in Fig. 1. A simulation based on coupled wave theory is performed [19].We have assumed a back and front facet reflectivity of 10% and 2%, respectively, for an AR-AR coated device, 100% and 2% for a HR-AR coated device, a coupling coefficient of 55 cm-1, and zero relative phases at the ending mirror positions. Compared with the HR-AR coating scheme, the AR-AR coating scheme not only increases the modal loss discrimination between the two band-edge modes and other modes from 0.5 cm-1 to 1.3 cm-1, but also increases the loss discrimination between the two band edge modes from -0.12 to 0.45 cm-1. The negative value means the lower-frequency mode have lower modal loss. The increased loss discrimination is attributed to the back-facet radiation from the AR coated device [20]. The small modal loss discrimination of 0.12 cm-1 for the HR-AR coating scheme could lead to dual mode operation.
Fig. 1. (a) Calculated modal loss for different longitudinal modes as a function of frequency detuning for the HR-AR (dark blue) and AR-AR (red) coating conditions. (b) Modal loss for the two band edge modes as a function of different back facet mirror phase positions for the HR-AR coated (dark blue) and AR-AR (red) coated devices. (c) and (d) Longitudinal mode profile of the optical mode field with the lowest modal loss in the HR-AR and AR-AR coated device, respectively.
The relative phase at the ending mirror position with certain reflectivity can change modal loss difference dramatically. Since the front facet is coated with AR coating with negligible reflection, the front mirror phase position doesn’t post a significant influence on the modal losses of the two band-edge modes. Thus, we simulated the modal loss difference as a function of back facet mirror phase relative to the middle of grating tooth while the relative phase for the front facet is fixed at 0. Obviously, the loss difference for the HR-AR coated device is mainly confined within ±0.12 cm-1 for nearly all the mirror phase positions, while the mirror loss greater than 0.12 cm-1 for the AR-AR coated device corresponds to 70% of the mirror phase positions. This will greatly improve the probability of single mode operation for AR-AR coated devices. In addition, the HR coating on the back facet also leads to over-coupling inside the MO section, while the AR-AR coatings on both facets exhibits lower overall coupling and improves the power ratio between the PA and MO section from 8.7 to 9.5, as seen in Figs. 1(c) and 1(d).
3. MOPA QCLs at λ∼4.9 µm
The QCL structure presented in this work was grown by gas-source molecular beam epitaxy (GSMBE) on an n-InP substrate using a strain-balanced technique. The active region design is identical to that in Ref. [12], containing a stage number of 40. A 300-nm InGaAs grating layer is grown on top of the active region for buried grating fabrication. The grating is patterned into the 300-nm InGaAs layer using e-beam lithography and inductively coupled plasma dry etching. The coupling coefficient is estimated to κ∼ 55 cm-1 at λ∼4.9 µm, which results in a coupling strength of κLDFB∼5.5, sufficient for single mode operation. LDFB=1 mm is the grating cavity length. After the grating fabrication, a 3-µm-thick n+ InP cladding layer (graded Si doping, ∼ 0.2-2.0 ×1017 cm−3) is grown by metalorganic chemical vapor deposition (MOCVD) followed by a 1-µm-thick InP (Si, ∼ 1×1019 cm−3) cap layer. The sample is processed into buried ridge waveguide with a ridge width of 5.6 µm.
Testing was done by mounting the laser on a micro-impingement cooler held at room temperature. For CW measurement, the optical power was measured with a calibrated thermopile detector placed directly in front of the laser facet. The peak power in pulsed mode operation (500 ns pulse width at 2% duty cycle) was calculated using the same calibrated thermopile detector where the measured average power is divided by the duty cycle. The lasing spectra measurement was performed on a Bruker Fourier transform infrared (FTIR) spectrometer with a liquid nitrogen cooled Mercury-Cadmium-Telluride (MCT) photodetector in rapid scan mode at a resolution of 0.125 cm-1. The lasing far fields were obtained by placing the laser on a rotational stage and monitoring the power received by a remote (∼50 cm) MCT detector with an attached 1 mm wide aperture at a step size of 1° [21].
Two 8-mm long MOPA devices with HR-AR and AR-AR coatings are tested and compared with a 5-mm long, 8.8-µm wide uncoated FP device. For the HR-AR device, the back facet is coated with Y2O3/Au (400 nm/100 nm) and the front facet is coated with Y2O3 (580 nm) for HR and AR coatings, respectively. The Y2O3 film is deposited with ion beam sputtering deposition system, the Au film is deposited with e-beam evaporation system. For the AR-AR device, the back facet is coated with 400-nm Y2O3 and the front facet is coated with 580-nm Y2O3. The single Y2O3 layer with 400 nm and 580 nm on the facets will reduce the facet reflectivity down to ∼10 and 2%, respectively. The AR coating on the back facet also acts as a passivation layer preventing the short-circuit issue induced by indium which is used as the solder material during the epi-down bonding process.
The devices were epilayer-down bonded to diamond submounts for efficient heat removal. Figure 2 shows the pulsed-mode power-current-density-voltage (P-J-V) and wall plug efficiency (WPE) characteristics at room temperature. Surprisingly, the AR-AR coated device exhibits improved power and efficiency over the HR-AR coated device. The AR-AR coated device emits over 8 W with a threshold current density (Jth) of 1.85 kA/cm2, slope efficiency (ηs) 5.1 W/A, and WPE of 21.7%, respectively. These are higher than the HR-AR coated device with a maximum power over 7 W, Jth=1.80 kA/cm2, ηs=4.3 W/A, and WPE = 19.0%, and even comparable to the 5-mm long uncoated FP device with maximum power over 9 W, Jth=1.90 kA/cm2, ηs=5.3 W/A and WPE = 22.8%. This was enabled by a moderate coupling strength and avoiding over-coupling effects. An HR coating can be crucial for a weakly coupled surface grating and can enhance the coupling strength for securing single mode operation [17]. However, for a strongly coupled buried DFB, the HR coating can lead to significant spatial hole burning (SHB) effects [22]. This will in turn affect the outcoupling efficiency and reduce the output power and slope efficiency. The AR-AR coated device exhibits comparable slope efficiency and wall plug efficiency to the FP reference device, indicating that the over coupling and SHB effects are largely suppressed.
Fig. 2. Pulsed mode P-J-V characterization (a) and WPE characterization (b) of the 8-mm long HR-AR coated MOPA device, AR-AR coated MOPA device, and 5-mm long uncoated FP device at λ∼4.9 µm.
In continuous wave operation, as shown in Fig. 3(a), the AR-AR coated device emits to 5.1 W with a Jth = 1.92 kA/cm2, ηs = 4.2 W/A, and WPE = 16.6%, respectively, while the HR-AR coated device emits to 4.9 W with Jth=1.86 kA/cm2, ηs=3.7 W/A, and WPE = 14.5%. A scanning electron microscopic image of the MOPA device front facet before AR coating is provided in Fig. 3(b). A double-step wet etching and regrowth processing has been used to obtain a narrow ridge [23]. What is more interesting is the spectral behavior change between the two devices. Figure 3(c) shows the lasing spectra of the HR-AR coated device in CW operation at different currents with a step of 0.2 A at room temperature. The device primarily emits in dual modes with limited single-mode operation near the lasing threshold. This is due to the reduction of the loss difference among the two DFB modes induced by the HR coating. On the contrary, the AR-AR coated device emits predominantly single mode with a side mode suppression ratio (SMSR) of 30 dB and an electrical tuning rate of -3.6 cm-1/A, as shown in Fig. 3(d). The improved single mode dynamic range and robustness is attributed to the enhanced loss difference induced by AR coatings on both facets. Multiple lasers from different laser bars with HR-AR and AR-AR coating conditions were tested and repeatable dual mode and single mode behaviors were observed. This solidifies the above findings on modal loss differences with different coating conditions.
Fig. 3. (a) Continuous wave power-current-voltage (P-I-V) and WPE characterizations of the HR-AR coated MOPA device and AR-AR coated MOPA device at λ∼4.9 µm. (b) Scanning electron microscopic image of the device front facet before AR coating. The dashed square indicates the position of the active region. The white bar represents 5 µm. Continuous wave lasing spectra for HR-AR coated MOPA device (b) and AR-AR coated MOPA device (c) at different currents with a step of 0.1 A.
The beam quality of the AR-AR coated laser was further investigated by monitoring the far-field distribution of the emitted light along the slow axis (denoted as ϕ, perpendicular to the growth direction). Figure 4(a) shows the far-field profiles of the laser in room temperature CW operation at different currents with a step of 0.1 A. The output beam shows a stable single-lobed far field pattern with negligible beam steering. The full width at half maximum (FWHM) of the far field varies from 29° to 32° as current increases. A two dimensional far-field characterization of the device along slow and fast axes (denoted as θ, parallel to the growth direction) at a CW current of 1.6 A is performed, as shown in Fig. 4(b). The device emits a near Gaussian beam with a divergence angle of 31° and 41° in the slow and fast axes, respectively.
Fig. 4. (a) Continuous wave far-field profiles for the AR-AR coated device at λ∼4.9 µm at different currents with a step of 0.1 A in the slow axis. (b) Two-dimensional far field scan at a continuous wave current of 1.6 A.
4. MOPA QCLs at λ∼3.8 µm and 8.3 µm
Unlike the optical phased array which has a limited bandwidth for a fixed multimode interferometer structure [23], the MOPA is basically a broadband single mode architecture and can be readily applied to other wavelengths. Another two QCL structures at λ∼3.8 and 8.3 µm with similar active region designs to Refs. [24] and [23] were grown by gas-source MBE on n-InP substrates. The stage numbers of the active region are 30 and 45 for devices at λ∼3.8 and 8.3 µm, respectively. The waveguide and active region are identical to that in the references except that a 300-nm InGaAs layer is grown on top of the active region for grating fabrication.
The two samples are processed into buried ridge waveguides with ridge widths of 7.0 and 7.8 µm, respectively. A double-step wet etching and regrowth processing procedure is used in the device fabrication. Considering the relatively lower threshold power density for the sample at λ∼8.3 µm, the PA waveguide width is tapered to 16 µm at the ending facet to enhance the output power. The InP cladding layer thickness for sample at λ∼8.3 µm is increased to 4 µm to reduce the optical loss. The back facets for the λ∼3.8 and 8.3 µm devices are coated with 340-nm and 650-nm thick Y2O3 coatings, respectively, while the Y2O3 coating thicknesses on the front facets are 400 nm and 980 nm, respectively, to target 10% and 2% for the two facets. Figure 5 shows the power efficiency, spectrum, and the far field characteristics for device at λ∼3.8 µm. The AR-AR coated device emits to 1.6 W (3.6 W) with Jth = 2.55 kA/cm2 (2.05 kA/cm2), ηs=1.38 W/A (2.14 W/A), and WPE of 4.1% (8.5%), for CW (pulsed mode) operation. Like the MOPA device at λ∼4.9 µm, the AR-AR coating scheme improves the single mode operation robustness at λ∼3.8 µm, as shown in Fig. 5(b). The electrical tuning rate was measured to be -3.9 cm-1/A for the device. Single lobed far field distribution is observed with a FWHM varying from 25° to 29° as the current increases.
Fig. 5. (a) Continuous wave P-I-V and WPE characterizations of the AR-AR coated MOPA device at λ∼3.8 µm. (b) Continuous wave lasing spectra for AR-AR coated MOPA device at different currents with a step of 0.1 A. (c) Continuous wave far-field profiles at different currents with a step of 0.1 A in the slow axis.
The testing result for the MOPA device at λ∼8.3 µm is summarized in Fig. 6. The AR-AR coated device emits to 3.4 W (5.2 W) with Jth = 1.06 kA/cm2 (1.02 kA/cm2), ηs=2.26 W/A (2.41 W/A), and WPE of 13% (15.4%), for CW (pulsed mode) operation. Similarly, the AR-AR coated device exhibits improved single mode operation at λ∼8.3 µm, as shown in Fig. 6(b), with an electrical tuning rate of -2.15 cm-1/A. Single lobed far field is observed with the FWHM varying from 23° to 25° as the current increases. The reduced electrical tuning rate compared with the devices with shorter wavelengths is attributed to the lower electrical power (operating voltage) injected into the device at λ∼8.3 µm which leads to less heat generated.
Fig. 6. (a) Continuous wave P-I-V and WPE characterizations of the AR-AR coated MOPA device at λ∼8.3 µm. (b) Continuous wave lasing spectra for AR-AR coated MOPA device at different currents with a step of 0.1 A. (c) Continuous wave far-field profiles at different currents with a step of 0.1 A in the slow axis.
5. Conclusion
To summarize, we demonstrated high power single mode operation of MOPA QCLs from λ∼3.8 to 8.3 µm at room temperature in continuous wave operation. High power up to 1.6 W, 5.1 W, and 3.4 W with maximum WPE of 4.1%, 16.6%, and 13% were obtained for λ∼3.8, 4.9, and 8.3 µm, respectively. Both the power and efficiency are significantly improved over previous for QCL devices at λ∼3.8 and 8.3 µm. The single mode robustness is improved with AR-AR coating on the front and back facets. Single mode operation with a SMSR about 30 dB and a single-lobed symmetric far field with negligible beam steering are observed. The demonstrated high-power single mode QCLs across the mid-IR spectral range are highly desired for long-range free-space communication and remote sensing of hazardous chemicals applications.
Acknowledgements
The authors would like to acknowledge the interest and encouragement of Dr. Dan Green, Kevin Leonard, Myron Paoli, K. K. Law, Jason Auxier, Paul Mac, Rich Spinosa from NAVY, and Mark Rosker from DARPA.
Disclosures
The authors declare no conflicts of interest.
References
1. F. K. Tittel, D. Richter, and A. Fried, “Mid-infrared laser applications in spectroscopy,” in Solid-State Mid-Infrared Laser Sources, I. T. Sorokina and K. L. Vodopyanov, eds. (Springer, 2003).
2. M. S. Vitiello, G. Scalari, B. Williams, and P. D. Natale, “Quantum cascade lasers: 20 years of challenges,” Opt. Express 23(4), 5167 (2015). [CrossRef]
3. M. Razeghi, Q. Y. Lu, N. Bandyopadhyay, W. Zhou, D. Heydari, Y. Bai, and S. Slivken, “Quantum cascade lasers: from tool to product,” Opt. Express 23(7), 8462–8475 (2015). [CrossRef]
4. J. Faist, Quantum Cascade Lasers (OUP Oxford, 2013).
5. R. F. Curl, F. Capasso, C. Gmachl, A. A. Kosterev, B. McManus, R. Lewicki, M. Pusharsky, G. Wysocki, and F. K. Tittel, “Quantum cascade lasers in chemical physics,” Chem. Phys. Lett. 487(1-3), 1–18 (2010). [CrossRef]
6. X. Chen, L. Cheng, D. Guo, Y. Kostov, and F.-S. Choa, “Quantum cascade laser based standoff photoacoustic chemical detection,” Opt. Express 19(21), 20251–20257 (2011). [CrossRef]
7. S. M. Johnson, E. Dial, and M. Razeghi, “High-speed free-space optical communications based on quantum cascade lasers and type-II superlattice detectors,” Proc. SPIE 11288, 1128814 (2020). [CrossRef]
8. M. Razeghi, W. J. Zhou, S. Slivken, Q. Y. Lu, D. H. Wu, and R. McClintock, “Recent progress of quantum cascade laser research from 3 to 12 µm at the Center for Quantum Devices [Invited],” Appl. Opt. 56(31), H30–H44 (2017). [CrossRef]
9. 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]
10. Y. Bai, S. Slivken, S. Kuboya, S. R. Darvish, and M. Razeghi, “Quantum cascade lasers that emitmore light than heat,” Nat. Photonics 4(2), 99–102 (2010). [CrossRef]
11. F. Xie, C. Caneau, H. P. Leblanc, D. P. Caffey, L. C. Hughes, T. Day, and C. E. Zah, “Watt-Level Room Temperature Continuous-Wave Operation of Quantum Cascade Lasers With lambda > 10 µm,” IEEE J. Sel. Top. Quantum Electron. 19(4), 1200407 (2013). [CrossRef]
12. Y. Bai, N. Bandyopadhyay, S. Tsao, S. Slivken, and M. Razeghi, “Room temperature quantum cascade lasers with 27% wall plug efficiency,” Appl. Phys. Lett. 98(18), 181102 (2011). [CrossRef]
13. Q. Y. Lu, S. Manna, D. H. Wu, S. Slivken, and M. Razeghi, “Shortwave quantum cascade laser frequency comb for multi-heterodyne spectroscopy,” Appl. Phys. Lett. 112(14), 141104 (2018). [CrossRef]
14. J. Faist, C. Gmachl, F. Capasso, C. Sirtori, D. L. Sivco, J. N. Baillaergeion, and A. Y. Cho, “Distributed feedback quantum cascade lasers,” Appl. Phys. Lett. 70(20), 2670–2672 (1997). [CrossRef]
15. F. Xie, C. G. Caneau, H. P. LeBlanc, M. Ho, J. Wang, S. Chaparala, L. C. Hughes, and C. Zah, “High power and high temperature continuous-wave operation of distributed Bragg reflector quantum cascade lasers,” Appl. Phys. Lett. 104(7), 071109 (2014). [CrossRef]
16. S. Slivken, N. Bandyopadhyay, S. Tsao, S. Nida, Y. Bai, Q. Y. Lu, and M. Razeghi, “Sampled grating, distributed feedback quantum cascade lasers with broad tunability and continuous operation at room temperature,” Appl. Phys. Lett. 100(26), 261112 (2012). [CrossRef]
17. Q. Y. Lu, Y. Bai, N. Bandyopadhyay, S. Slivken, and M. Razeghi, “2.4 W room temperature continuous wave operation of distributed feedback quantum cascade lasers,” Appl. Phys. Lett. 98(18), 181106 (2011). [CrossRef]
18. M. Troccoli, C. Gmachl, F. Capasso, D. L. Sivco, and A. Y. Cho, “Mid-infrared ((∼ 7.4 (m) quantum cascade laser amplifier for high power single-mode emission and improved beam quality,” Appl. Phys. Lett. 80(22), 4103–4105 (2002). [CrossRef]
19. R. Kazarinov and C. Henry, “Second-order distributed feedback lasers with mode selection provided by first-order radiation losses,” IEEE J. Quantum Electron. 21(2), 144–150 (1985). [CrossRef]
20. W. Streifer, D. R. Scifres, and R. D. Burnham, “Coupled wave analysis of DFB and DBR lasers,” IEEE J. Quantum Electron. 13(4), 134–141 (1977). [CrossRef]
21. Q. Y. Lu, Y. Bai, N. Bandyopadhyay, S. Slivken, and M. Razeghi, “Room-temperature continuous wave operation of distributed feedback quantum cascade lasers with watt-level power output,” Appl. Phys. Lett. 97(23), 231119 (2010). [CrossRef]
22. Q. Y. Lu, F. H. Wang, D. H. Wu, S. Slivken, and M. Razeghi, “Room temperature terahertz semiconductor frequency comb,” Nat. Commun. 10(1), 2403 (2019). [CrossRef]
23. W. J. 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]
24. N. Bandyopadhyay, Y. Bai, B. Gokden, A. Myzaferi, S. Tsao, S. Slivken, and M. Razeghi, “Watt level performance of quantum cascade lasers in room temperature continuous wave operation at lambda similar to 3.76 (m,” Appl. Phys. Lett. 97(13), 131117 (2010). [CrossRef]
