808 nm broad area DFB laser for solid-state laser pumping application

Y. He, H. An, J. Cai, C. Galstad, S. Macomber and M. Kanskar

Wavelength stabilised 808 nm broad area distributed feedback (DFB) diode lasers with high continuous-wave (CW) output have been realised. The centre wavelength was locked at the Bragg condition with narrow spectral widths of 0.3 nm FWHM and shifted at a rate of 0.062 nm/°C. The 2 mm-long laser showed a record high 57% wall-plug efficiency and more than 4W CW output power at 25°C heat- sink temperature. It is an ideal source for pumping solid-state lasers such as Nd-doped YAG and YVO₄.

Introduction: Multimode, Fabry-Pérot, semiconductor diode lasers emitting near 808 nm wavelength have been extensively used for pumping Nd in solid-state gain media for high power solid-state lasers and microchip lasers [1]. However, the absorption bandwidth of Nd at 808 nm is narrow (< 5 nm FWHM in YAG and about 12 nm in YVO₄) [2]. In comparison, the spectrum from typical pump diode lasers is 1–2 nm wide and shifts with temperature at approximately 0.3 nm/°C. As a result, wavelength stabilisation against temperature variation is necessary, either by an active thermal management method (such as a thermal-electric cooler (TEC) or chillers) or by external wavelength locking mechanisms (such as volume Bragg gratings). Both techniques have been used to maintain the optimum pumping efficiency over useful operating temperatures. However, besides adding cost, TECs are power-hungry and inefficient, making them unsuitable for many applications requiring a battery as the power supply. The external wavelength-locking approach is expensive and sensitive to packaging conditions and has tight alignment tolerances that make it less robust. In applications, where 808 nm pumped Nd gain media is frequency doubled using nonlinear crystals to generate green for display applications, total efficiency, compactness of the system as well as a wide operating temperature range become indispensable. Monolithically wavelength stabilised [3] and emission bandwidth narrowed, high power 808 nm semiconductor diode laser pumps provide a unique solution that is cost-effective, compact, robust and simple to deploy. We report on the integration of a Bragg grating in a semiconductor laser Fabry-Pérot cavity forming a low-loss, weak distributed feedback (DFB) device, which results in record high 57% wall-plug efficiency at CW operation and 25°C heatsink temperature from a 2 mm cavity length, 100 mm stripe, 808 nm DFB diode laser.

Laser diode structure: Fig. 1 shows the refractive index profile of the semiconductor laser structure. The DFB lasers were made with a two-step epitaxial growth process. The first growth comprised epitaxy of 0.8 mm-thick n-cladding and a 1.3 µm-thick broad-waveguide core with 3.230 and 3.303 index of refraction, respectively, terminated with a thin grating layer. Second-order gratings were fabricated on this layer using holographically exposed photoresist patterning and sub- sequent transfer of these grating patterns onto the underlying grating layer using reactive ion etching (RIE) technique. The second-order grating has a pitch of Λ = λ / n _eff = 244 nm, where l λ = 808 nm is the vacuum wavelength and n _eff = 3.31 is the effective index of the fundamental mode in the waveguide. The remaining 0.8 mm-thick p-cladding layer and the final 0.15 µm-thick p⁺-cap GaAs layer were then grown on top of the etched grating layer. This structure provides a narrow transverse beamwidth, &theta _⊥, of approximately 38.68° FWHM. The active region consists of a 5 nm compressively-strained InGaAsP quantum well. The primary growth and the regrowth were performed by low-pressure metal organic vapour phase epitaxy (LP-MOVPE). Laser diodes were fabricated with a 100 µm wide aperture for electrical injection 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 heatsinks using gold-tin solder. CW L-I-V measurements were performed at 25°C heatsink temperature.

Method: The laser was designed to achieve a high quality of the regrown p-cladding layer. Unlike the 976 nm DFB laser [4], lower refractive index (higher bandgap) material is necessary to provide wave-guiding and poses a growth challenge. Efforts were made to optimise the regrowth conditions for low refractive index material. As a result, low optical loss was demonstrated in the laser, leading to very high efficiency of operation. The location of the grating with respect to the fundamental optical mode, the etch-depth of the grating and the index of refraction of the overgrown cladding material were judiciously chosen to be kept at κL ∼ 1, where κ is the grating-coupling coefficient and L is the grating length. 100 µm stripe and 2 mm-long devices were fabricated with more than 1 W/A slope efficiency and 57% wall-plug efficiency at 3 W CW operation and 25°C heatsink temperature as shown in Fig. 2. The threshold current for this laser was only 450 mA (225 A/cm²). The CW spectral width was measured to be 0.3 nm at FWHM at the peak efficiency and remained so even when exceeding 4 W of CW operation. The high quality of regrowth is evident from the low operating voltage achieved in these devices, as shown in Fig. 2. The turn-on voltage and series resistance were measured to be 1.57 V and 55 milliohms, respectively. Fig. 3 shows the emission spectra of an 808 nm wavelength stabilised diode laser as heatsink temperature varies. Temperature dependence of the spectrum was measured to be approximately 0.062 nm/°C as long as the output wavelength is locked at the Bragg condition. The temperature range of effective locking is limited by detuning between the optical gain peak wavelength and the Bragg wavelength, which is a function of the junction temperature at a given operating current. Fig. 4 maps the locked output (100% of the power in the Bragg emission bandwidth) over heatsink temperature and operating current. The 808 nm wavelength stabilised diode remains locked at the Bragg condition at 2.5 W (about 3 A operating current) from 15 to 45°C heatsink temperature, providing over 30°C of locking range.

Conclusion: We have demonstrated a record high 57% wall-plug efficiency, 100 µm stripe DFB laser with 2 mm cavity length emitting near an important wavelength of 808 nm. This record high efficiency was achieved owing to implementation of low-loss grating, fabricated by a high quality regrown cladding layer and choice of κL ∼ 1. In CW operation, spectral width of 0.3 nm at FWHM was achieved at power levels . 4 W and heatsink temperature of 25°C. Furthermore, a wavelength-locking range greater than 30°C was also observed, providing unique solution to temperature-independently pumped Nd-doped solid-state lasers.

Y. He, H. An, J. Cai, C. Galstad, S. Macomber and M. Kanskar (Alfalight, Inc., 1832 Wright St., Madison, Wisconsin 53704, USA)
E-mail: yhe@alfalight.com

References

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