53% Wall-plug Efficiency 975 nm Distributed Feedback Broad Area Laser

M. Kanskar, Y. He, Jason Cai, C. Galstad, S. H. Macomber, E. Stiers, D. Botez and L.J. Mawst

100 µm-stripe, 2 mm long, DFB diode lasers with narrow spectral widths of 2.3 Å FWHM were achieved to a CW power of 5 W. The center wavelength was locked at the Bragg condition and shifts at a rate of 0.065 nm K^-1. 1 mm-long DFB laser showed a record high 53% wall-plug efficiency for grating-stabilized semiconductor lasers at 25°C heatsink temperature and 2 W CW output power.

Introduction: Multimode, Fabry-Perot, semiconductor diode lasers emitting near 975 nm wavelength are of interest for pumping the upper transition states of Yb, Er and Yb/Er co-doped solid state and fiber lasers. In particular, the 0.975 mm multimode diode lasers are very attractive pump sources for co-doped, dual-clad fiber lasers and amplifiers, especially in pulsed mode operation. At this pump wavelength, the quantum defect is minimal and the absorption cross-section (2.5 dB m^-1) is much higher relative to transition states near the 0.92 mm band (~0.7 dB m^-1) which allows for the same gain to be achieved with one-third the gain fiber length [1]. The use of shorter gain fiber helps to mitigate deleterious nonlinear effects such as the Stimulated Raman Scattering (SRS) and the Stimulated Brillouin Scattering (SBS). However, the absorption bandwidth at 0.975 mm is narrow (< 9 nm FWHM). So, either expensive thermal stabilization measures or costly and sensitive external wavelength-locking mechanisms have to be employed making 0.975 mm diode lasers less attractive as pump sources for these applications. Monolithically wavelength-stabilized [2] and emission-bandwidth narrowed, high-power 0.975 mm semiconductor diode laser pumps provide a unique solution that is cost-effective, robust, and simple to deploy. We report on the integration of a Bragg grating in a semiconductor laser cavity forming a low-loss, weak distributed feedback (DFB) device, which results in record high 53% wall-plug efficiency at 2 W CW operation and 25°C heatsink temperature from a 100µm-stripe DFB diode laser.

Laser Diode Structure: The laser structure is schematically shown in Figure 1. The DFB lasers were made with a two-step epitaxial growth process. The first growth comprised epitaxy of 0.6-µm thick N-cladding and 1.1-µm thick broad-waveguide core with 3.21 and 3.39 index of refraction, respectively, terminated with 0.2-µm thick InGaP layer of 3.21 index of refraction, which subsequently serves as the bottom portion of the grating layer. 100-nm deep second-order gratings were fabricated in this layer using holographically exposed photoresist patterning and subsequent transfer of these grating patterns onto the underlying cladding layer using reactive ion etching (RIE) technique. The second order grating has a pitch of Λ= λ/n_eff = 0.290µm, where λ = 0.975 mm is the vacuum wavelength and n_eff = 3.36 is the effective index of the fundamental mode in the waveguide. The remaining 0.6-µm thick P-cladding layer comprised a 0.3-µm thick (n = 3.32) layer and a 0.3-µm thick (n = 3.23) layer. Finally a 0.15-µm thick p⁺-cap GaAs layer was then grown to complete the laser structure. This structure provides a narrow transverse and lateral beam-widths, θ_⊥, θ_//, of approximately 38° and 7° FWHM respectively. The active region consists of an 85 Å InGaAs quantum well. The primary and the secondary crystal growth were performed by low-pressure metal organic vapor phase epitaxy (LP-MOVPE) in a multi-wafer reactor. Laser diodes were fabricated with 100-µm wide aperture and they were coated with conventional dielectric films to achieve approximately 4% and 95% front and back facet reflectivity, respectively. The diodes were mounted p-side down on copper-tungsten heatsink using gold-tin solder. CW L-I-V measurements were performed at 25°C heatsink temperature.

Method: The location of the grating with respect to the fundamental optical mode, etch-depth of the grating and the index of refraction of the overgrown cladding material were judiciously chosen to keep κL ~ 1, where κ is the coupling constant and L is the grating length. In order to extract high optical power a 100 µm stripe was electrically pumped. As a result, the device operates multi(lateral)-spatial mode at a single longitudinal-cavity mode at low powers, and at a couple of longitudinal-cavity modes at high powers [3]. Additionally, effort was made to fabricate smooth grating surface and regrowth conditions were optimized for low interface scattering loss. Both 1 and 2 mm cavity-length devices were fabricated. The 1 mm long DFB laser had 0.9 W/A slope efficiency and 53% peak wall-plug efficiency at 2 W CW operation and 25°C heatsink temperature as shown in Figure 2. The threshold current for this laser was only 230 mA. The 2 mm cavity-length device was driven up to 8 A producing CW output power of 5.5 W at 25°C heatsink temperature. The CW spectral width was measured to be 1.8 Å FWHM at the peak efficiency and broadened to 2.3 Å FWHM at 5 W of CW operation as shown in the inset to Figure 3. The high quality of regrowth is evident from the low operating voltage achieved in these devices as shown in Figure 2. The turn-on voltage was measured to be 1.32 V and the series resistance was 44 and 22 mΩ for the 1 and the 2 mm long devices respectively. Temperature dependence of the spectrum peak was measured to be approximately 0.65 Å K^-1. Output wavelengths remained locked at the Bragg condition from 10 to 50°C heatsink temperature at 2 A drive condition, providing over 40°C locking range as shown in Figure 4.

Conclusion: We have demonstrated record high 53% wall-plug efficiency, 100 µm-stripe DFB laser with 1 mm cavity length emitting near λ=0.975 mm wavelength. This record high efficiency was achieved due to implementation of low loss, high quality grating with good overgrown cladding layer and the choice of κL ~ 1. In CW operation, spectral width of 2.3 Å FWHM was achieved up to power levels of 5 W and heatsink temperature of 25°C from a 2 mm long device. Furthermore, wavelength-locking range of greater than 40°C was also observed.

M. Kanskar, Y. He, Jason Cai, C. Galstad, S. H. Macomber, E. Stiers (Alfalight, Inc. 1832 Wright St. Madison, WI, 53704, United States)
E-mail: mkanskar@alfalight.com

D. Botez and L.J. Mawst (Department of Electrical and Computer Engineering, University of Wisconsin at Madison, 1415 Engineering Dr., Madison, WI, 53706, United States)

References:

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2. EARLES, T., MAWST, L.J., and BOTEZ, D.: '1.1 W continuous-wave, narrow spectral width (< 1Å) emission from broad-stripe, distributed-feedback diode lasers (l=0. 0.897mm)', Appl. Phys. Lett., 1998, 73, (15), pp. 2072-2074
3. CHANG, C.H., EARLES, T, and BOTEZ, D.: 'High CW power narrow-spectral width (< 1.5 Å) 980nm broad-stripe distributed-feedback diode lasers', Electronics Lett. , 2000, 36, (11) , pp.954-955.