Sep 11, 2014 - with continuous-wave output power above. 15 mW at room temperature. R. Liang, T. Hosoda, L. Shterengas, A. Stein, M. Lu,. G. Kipshidze and ...
Distributed feedback 3.27 µm diode lasers with continuous-wave output power above 15 mW at room temperature R. Liang, T. Hosoda, L. Shterengas, A. Stein, M. Lu, G. Kipshidze and G. Belenky GaSb-based type-I quantum well laterally coupled distributed feedback diode lasers emitting in the methane absorption band near 3.27 µm were designed and fabricated. The first-order index grating with a period of 480 nm was defined by e-beam lithography and etched on both sides of 6 µm-wide shallow ridge waveguide. Coated 2 mmlong devices demonstrated stable continuous-wave single-frequency operation in a wide temperature range with an output power of 15 mW at +17°C and 40 mW at −20°C. The Bragg wavelength temperature tuning rate was ∼0.27 nm/K.
3.27 µm (379 meV) was apparent in the measured gain spectra. The mode frequency was enforced by the index grating with a period of 480 nm etched on both sides of the narrow ridge waveguide (Fig. 2). The measured lateral near-field pattern (inset to Fig. 2) confirmed that extension of the lateral grating by ∼10 µm from the ridge sidewalls was adequate to couple most of the optical field. The coupling coefficient κ was estimated to be ∼8 cm−1 following [7]. The overlap of the fundamental mode with a grating region of ∼0.15% was estimated based on the measured lateral near-field distribution and the calculated vertical near-field distribution calibrated by the measured fast axis far-field pattern.
Ÿ = 480 nm
Introduction: Tunable diode laser absorption spectroscopy (TDLAS) is an effective approach for methane detection and monitoring. Methane sensing finds applications in industries ranging from oil production to planetary science. Robust and efficient single spectral mode 3.27 µm diode lasers operating near room temperature (RT) in the continuouswave (CW) regime are required for methane TDLAS. Both distributed feedback (DFB) and external cavity single-frequency diode lasers operating in CW near RT near the methane absorption band have been reported, see for instance [1–4]. The available CW output power was limited to several milliwatts so far although higher values are often desirable for improved system sensitivity. In this Letter, we report on the design and development of 3.27 µm emitting DFB lasers based on the efficiency of the diode laser heterostructure similar to the one reported in [5]. The devices demonstrate single-frequency operation in a wide temperature range with a CW output power level well above 15 mW. The lasers operated in the multimode CW regime below −30°C and above +20°C because of gain detuning from ∼3.27 µm DFB line. Fabrication: Three quantum well (QW) laser heterostructures were grown by solid-source molecular beam epitaxy on Te-doped GaSb substrates. Narrow ridge waveguide laterally coupled DFB lasers were fabricated by inductively coupled plasma (ICP) reactive ion etching (RIE) in the Oxford Plasmalab System 100. The etching was performed using ∼300 nm-thick Si3N4 masks and under the conditions given in Table 1.
Table 1: ICP RIE parameters SiCl4/N2/Ar (sccm) 10/5/20
Pressure (mTorr) 2
RIE (W) 60
ICP (W) 250
DC bias (V) −210
Etching rate (nm/min) ∼800
0
modal gain, cm–1
–10
–20
168 mA 150 mA 120 mA
–30 30 mA
–40
60 mA
90 mA
L = 1 mm, uncoated, 4 MHz/25 ns, 17C –50 0.35
0.36
0.37 0.38 0.39 photon energy, eV
0.40
0.41
Fig. 1 Current dependence of modal gain spectra for 1 mm-long, uncoated DFB diode lasers Inset: Lasing spectra corresponding to operation at Fabry‐Perot cavity mode and at grating defined Bragg wavelength
Narrow (∼6 µm-wide) ridge lasers emitted in fundamental mode below threshold and above threshold. Modal gain spectra were measured by the Hakki‐Paoli technique [6] with no spatial filtering applied thanks to the stable single spatial mode operation. The parameters of compressively strained GaInAsSb QWs yielded the modal optical gain peak near 3.33 µm (372 meV) at 17°C (Fig. 1). The grating related distortion near
~6 mm
–10
–5
0
5
CFN 5.0 kV 8.0 mm × 4.50 k SE(U,LA0) 3/7/2014
10 mm 10.0 mm
Fig. 2 SEM image of as-cleaved mirror of fully processed DFB diode laser Upper inset: Cross-section view of etched gratings Lower inset: Lateral near-field profiles measured for currents in range from 200 to 500 mA at RT
The uncoated 1 mm-long devices were indium soldered epi-up for pulsed characterisation (200 ns/100–500 kHz). The AR/HR (∼5 and ∼95%) coated 2 mm-long devices were soldered epi-down onto polished gold-plated copper blocks for characterisation in the CW regime. Experimental results and discussion: Fig. 3 shows the light–current– voltage characteristics measured in the CW regime for the AR/HR coated 2 mm-long devices. Single-frequency output power reached above 10 and 40 mW at +20 and −20°C, respectively. The insets to Fig. 3 show the representative laser spectrum and lateral far-field pattern. The devices demonstrated stable single-frequency operation for the detuning of 10 mW Inset: Temperature dependence of laser spectra measured at CW current of 300 mA
Conclusion: RT operated 3.27 µm single-frequency mode diode lasers with an output power level of above 15 mW have been designed and fabricated using GaSb-based type-I diode laser heterostructures. The laterally coupled DFB lasers were fabricated using e-beam lithography and silicon-chloride-based ICP RIE. The 10 µm-wide first-order index grating with a period 480 nm was formed on both sides of the ∼6 µm-wide ridge waveguide. Anti-/high-reflection coated 2 mm-long devices demonstrated stable DFB operation in the temperature range from −20 to +20°C, with a Bragg wavelength temperature tuning rate of 0.27 nm/K. Acknowledgments: The authors thank C. Frez for guidance with ICP RIE and e-beam resistive mask preparation and S. Forouhar for fruitful discussion. This work was supported by the US Army Research Office, grant W911NF1420070 and by the Air Force Office of Scientific Research, grant FA95501110136. The research was carried out in part at the Center for Functional Nanomaterials, Brookhaven National Laboratory, which is supported by the US Department of Energy, Office of Basic Energy Sciences, under contract no. DE-AC02-98CH10886.
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ELECTRONICS LETTERS 11th September 2014 Vol. 50 No. 19 pp. 1378–1380
