50 mW cw in a Single Spectral Mode - Northwestern...
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Corrugated-Sidewall Interband Cascade Lasers Emitting > 50 mW cw in a Single Spectral Mode Mijin Kim a ¸ Chul Soo Kim b ¸ Charles D. Merritt b , William W. Bewley b , Chadwick L. Canedy b , Igor Vurgaftman* b , Joshua Abell b , and Jerry R. Meyer b a Sotera Defense Solutions, MacLean, VA, USA 22102 b Code 5613, Naval Research Laboratory, Washington, DC, USA 20375 BIOGRAPHY Jerry R. Meyer completed his Ph.D. in Physics at Brown University in 1977. Since then he has carried out basic and applied research at the Naval Research Laboratory in Washington DC, where he is the Navy Senior Scientist for Quantum Electronics (ST) and Acting Head of the Quantum Optoelectronics Section. His investigations have focused on semiconductor optoelectronic materials and devices, such as the development of new classes of semiconductor lasers and detectors for the infrared. He is a Fellow of OSA, APS, IEEE, IOP, and SPIE. He has co-authored over 340 refereed journal articles, 14 book chapters, 27 patents (4 licensed), and over 130 invited conference presentations. [jerry.meyer@nrl;.navt.mil; (202) 767-3276] TECHNICAL ABSTRACT Mid-IR interband cascade lasers (ICLs) incorporating “carrier rebalanced” designs [1], epitaxial-side-down mounting, and other improvements have recently displayed pulsed threshold current densities as low as 134 A/cm 2 at 300 K and maximum cw operating temperatures as high as 119 o C. However, the application of these devices to trace gas sensing based on laser spectroscopy will also require operation in a single spectral mode, with output in a narrow line whose wavelength can be tuned, e.g., by varying the injection current and/or thermoelectric cooler temperature. While the powers required by some detection systems are quite modest ( 1 mW), remote sensing will generally require somewhat higher single-mode laser powers. In previous work, we reported the etching of a corrugated pattern into the sidewalls of narrow ICL ridges to serve the dual purpose of suppressing higher-order lateral lasing modes, by increasing their loss, and also forming a 4 th -order distributed feedback (DFB) grating to select a single longitudinal mode [2]. Devices employing this approach emitted up to 12 mW cw in a single spectral mode at room temperature and 45 mW at -20 o C with a tuning range of 11 nm. Here we discuss the unexpected spectral performance of a series of ICL narrow ridges with corrugated sidewalls having a pitch not specifically designed to match a given DFB resonance. Light-current (L-I) and far-field characterizations confirmed that the ridges having corrugated sidewalls could generate more power in a good than sister devices having straight sidewalls, since the ridges could be wider before higher-order lateral modes became dominant. For example, the corrugated-sidewall devices reported here lased in a single lateral mode despite having a ridge width of 13.2 m. The narrow ridges were fabricated by optical lithography and Cl-based inductively coupled plasma dry etching. The etch proceeded to the GaSb separate-confinement layer below the active region, and was followed by a wet phosphoric-acid-based clean-up etch. The optical lithography mask patterned a lateral grating with period 2 m into both sides of the ridges. A 200-nm-thick conformal Si 3 N 4 layer was next deposited by plasma-enhanced chemical vapor deposition, followed by sputtering of 100 nm of SiO 2 to fill occasional pinholes in the Si 3 N 4 . Windows were opened by contact lithography, followed by e-beam evaporation of the Ag/Ti/Pt/Au contact metallization. Gold was electroplated on top of the structure to a thickness of 5 m, cavities were cleaved to a length of 4 mm, high-reflection (HR) and anti- reflection (AR) coatings were deposited on the facets, and mounting was either epitaxial-side-up or epitaxial-side down. The emission spectra were obtained with a Bomem DA-8 FTIR. Figure 1 illustrates the emission spectra from an epi-up device when operated at T = 25 ºC just below (55 mA, black) and just above (65 mA, red) the lasing threshold. Most of the ridges display single-mode emission with narrow linewidth at some temperatures and currents, although that behavior was more robust for some than others. Figures 2-4 summarize the spectral and L-I characteristics for an epi-down device whose output remains single-mode over an extended range of operating conditions. Figure 2 indicates that the wavelength of the single-mode output shifts by 2 nm
Transcript of 50 mW cw in a Single Spectral Mode - Northwestern...
Corrugated-Sidewall Interband Cascade Lasers Emitting > 50 mW cw in a Single Spectral Mode
Mijin Kima¸ Chul Soo Kimb¸ Charles D. Merrittb, William W. Bewleyb, Chadwick L. Canedyb,
Igor Vurgaftman*b, Joshua Abellb, and Jerry R. Meyerb aSotera Defense Solutions, MacLean, VA, USA 22102
bCode 5613, Naval Research Laboratory, Washington, DC, USA 20375
BIOGRAPHY Jerry R. Meyer completed his Ph.D. in Physics at Brown University in 1977. Since then he has carried out basic and applied research at the Naval Research Laboratory in Washington DC, where he is the Navy Senior Scientist for Quantum Electronics (ST) and Acting Head of the Quantum Optoelectronics Section. His investigations have focused on semiconductor optoelectronic materials and devices, such as the development of new classes of semiconductor lasers and detectors for the infrared. He is a Fellow of OSA, APS, IEEE, IOP, and SPIE. He has co-authored over 340 refereed journal articles, 14 book chapters, 27 patents (4 licensed), and over 130 invited conference presentations. [jerry.meyer@nrl;.navt.mil; (202) 767-3276]
TECHNICAL ABSTRACT Mid-IR interband cascade lasers (ICLs) incorporating “carrier rebalanced” designs [1], epitaxial-side-down
mounting, and other improvements have recently displayed pulsed threshold current densities as low as 134 A/cm2 at 300 K and maximum cw operating temperatures as high as 119 oC. However, the application of these devices to trace gas sensing based on laser spectroscopy will also require operation in a single spectral mode, with output in a narrow line whose wavelength can be tuned, e.g., by varying the injection current and/or thermoelectric cooler temperature. While the powers required by some detection systems are quite modest (� 1 mW), remote sensing will generally require somewhat higher single-mode laser powers. In previous work, we reported the etching of a corrugated pattern into the sidewalls of narrow ICL ridges to serve the dual purpose of suppressing higher-order lateral lasing modes, by increasing their loss, and also forming a 4th-order distributed feedback (DFB) grating to select a single longitudinal mode [2]. Devices employing this approach emitted up to 12 mW cw in a single spectral mode at room temperature and 45 mW at -20oC with a tuning range of 11 nm.
Here we discuss the unexpected spectral performance of a series of ICL narrow ridges with corrugated sidewalls having a pitch not specifically designed to match a given DFB resonance. Light-current (L-I) and far-field characterizations confirmed that the ridges having corrugated sidewalls could generate more power in a good than sister devices having straight sidewalls, since the ridges could be wider before higher-order lateral modes became dominant. For example, the corrugated-sidewall devices reported here lased in a single lateral mode despite having a ridge width of 13.2 �m. The narrow ridges were fabricated by optical lithography and Cl-based inductively coupled plasma dry etching. The etch proceeded to the GaSb separate-confinement layer below the active region, and was followed by a wet phosphoric-acid-based clean-up etch. The optical lithography mask patterned a lateral grating with period 2 �m into both sides of the ridges. A 200-nm-thick conformal Si3N4 layer was next deposited by plasma-enhanced chemical vapor deposition, followed by sputtering of 100 nm of SiO2 to fill occasional pinholes in the Si3N4. Windows were opened by contact lithography, followed by e-beam evaporation of the Ag/Ti/Pt/Au contact metallization. Gold was electroplated on top of the structure to a thickness of 5 �m, cavities were cleaved to a length of 4 mm, high-reflection (HR) and anti-reflection (AR) coatings were deposited on the facets, and mounting was either epitaxial-side-up or epitaxial-side down. The emission spectra were obtained with a Bomem DA-8 FTIR.
Figure 1 illustrates the emission spectra from an epi-up device when operated at T = 25 ºC just below (55 mA, black) and just above (65 mA, red) the lasing threshold. Most of the ridges display single-mode emission with narrow linewidth at some temperatures and currents, although that behavior was more robust for some than others. Figures 2-4 summarize the spectral and L-I characteristics for an epi-down device whose output remains single-mode over an extended range of operating conditions. Figure 2 indicates that the wavelength of the single-mode output shifts by 2 nm
when the current is varied from 150 mA to 400 mA at the fixed temperature of 25 ºC. The measured spectral linewidth of � 0.15 nm is limited by the spectrometer resolution. The sidemode suppression ratio of � 17 is somewhat lower than for earlier devices [2], presumably because the grating was not designed or optimized for DFB operation. Figure 3 illustrates the spectral shift with temperature when the current is held fixed at 250 mA. The temperature range of 20-35 ºC is seen to provide a wavelength tuning range of 6 nm. Figure 4 plots the L-I characteristics for the same device at 10, 25, and 40 ºC, showing only the current ranges over which the output is single-mode at each temperatures. Note that the single-mode cw output is up to 55 mW at 25 ºC and > 70 mW at 10 ºC. The ambient-temperature result is by far the highest reported single-mode cw power to be generated by an interband lasers emitting beyond 3 �m. A second epi-down device performs similarly to the results shown in Figs. 2-4, while two epi-up devices show similar spectral properties with somewhat lower maximum cw output powers.
3.590 3.595 3.600 3.605 3.610
I = 55 mA
13.2 �m x 4 mmHR/AR, Epi-UpT = 25 °C
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W avelength (�m) Figure 1. Cw output spectra for an epitaxial-side-up Figure 2. Cw output spectra for a corrugated-sidewall ridge with corrugated sidewalls at currents just epi-down ridge at a series of currents and a fixed below (55 mA) and just above (65 mA) threshold. temperature of 25 ºC.
3.594 3.596 3.598 3.600 3.602 3.6040.0
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25 oC T = 20 oC
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Epi-DownI = 250 mA
0 100 200 300 400 5000
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T = 10 °C13.2 �m x 4 mm
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)
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Figure 3. Cw output spectra for the same epi-down device as in Fig. 2, at a fixed current of 250 mA and a series of temperatures.
Figure 4. Cw L-I characteristics for the same device as in Figs. 2 and 3, over the range of currents for which single-mode output is maintained at each temperature.
[1] I. Vurgaftman, W. W. Bewley, C. L. Canedy, C. S. Kim, M. Kim, C. D. Merritt, J. Abell, J. R. Lindle, and J. R. Meyer, Nature Commun. 2, 585 (2011).
[2] C. S. Kim, M. Kim, J. R. Lindle, W. W. Bewley, C. L. Canedy, J. Abell, I. Vurgaftman, and J. R. Meyer, Appl. Phys. Lett. 95, 231103 (2009). Keywords: Interband cascade lasers, mid-infrared diode lasers, distributed-feedback lasers, chemical sensing.
