APPLIED PHYSICS LETTERS 96, 131110 共2010兲
975 nm high power diode lasers with high efficiency and narrow vertical far field enabled by low index quantum barriers P. Crump,a兲 A. Pietrzak, F. Bugge, H. Wenzel, G. Erbert, and G. Tränkle Ferdinand-Braun-Institut, Leibniz-Institut für Höchstfrequenztechnik, Gustav-Kirchoff-strasse 4, 12489 Berlin, Germany
共Received 4 January 2010; accepted 12 March 2010; published online 31 March 2010兲 For optimal coupled power into fiber, high power diode lasers should operate efficiently with smallest possible vertical far field emission angle. Although waveguide and cladding layers can be designed to achieve small angles, the refractive index profile of the active region itself restricts the minimum achievable value. We show that the use of low index quantum barrier layers leads to substantially reduced far field angles, while sustaining high power conversion efficiency. 90 m stripe lasers that use such designs have narrow vertical far field angles of 30° 共95% power content兲, power conversion efficiency of 58% and operate reliably at 10 W output. © 2010 American Institute of Physics. 关doi:10.1063/1.3378809兴 Diode lasers provide the optical energy for most high performance laser systems. For industrial applications, the maximum reliable power must be coupled into an optical system with a restricted aperture, for example a 100 m core optical fiber. Therefore, designs choices for high power, high power conversion efficiency, c, and narrow far field angle must be balanced. The vertical angle with 95% power content, ⌰95%, must typically be optimized, to eliminate stray light. 980 nm GaAs-based 100 m stripe broad area lasers, for example, operate with c ⬃ 60%, ⌰95% ⬃ 45° – 60°, and reliable continuous wave 共CW兲 power PCW ⬃ 10 W.1,2 Design optimization for reduced ⌰95% remains under active study, with ⌰95% = 15° – 20° recently reported.3,4 Such small ⌰95% values are achieved in part by using thick optical waveguides 共ⱖ8.6 m兲. It is challenging to sustain high efficiency for thick waveguides, due to their resistance and the increased volume for carrier recombination.3,4 A central challenge in the optimization of the vertical far field is the influence of the active region. The quantum wells and barriers have high refractive indices, leading to substantial optical waveguiding. One option is to use special cladding structures, which guide light away from the active region.5 Photonic band crystal designs are also promising.6 As an alternative, we investigate here using the barrier layers themselves to reduce the vertical far field. To illustrate our approach, we select a GaAs-based laser design operating at 976 nm, with an active region comprising of three 9 nm thick In0.15Ga0.85As quantum wells, separated by 8 nm thick GaAsxP1−x barriers and enclosed in 5 nm GaAsxP1−x spacer layers. A 4.8 m thick Al0.20Ga0.80As asymmetric waveguide with 1.7 m thick p-side 共waveguide refractive index, nw = 3.39兲 and Al0.85Ga0.15As cladding layers were used. In Fig. 1, the vertical refractive index and calculated near field profiles are illustrated for two barrier compositions 共calculations follow the techniques of Ref. 7兲. The near field is concentrated near the active region for conventional barrier and spacer layers 共GaAs0.80P0.20, barrier and spacer refractive index nbs = 3.42⬎ nw兲. When an alternative a兲
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low-index barrier 共GaAs0.55P0.45 , nbs = 3.31⬍ nw兲 is used, the near field spreads out. Typically high power diode lasers do not use such barriers as they have a higher energy band gap than the waveguide, which can reduce the internal quantum efficiency, i.8,9 However, the potential reduction in far field justifies a detailed investigation. As a first step, the far field angle was calculated as a function of the composition of barrier and spacer layers 共waveguide and cladding layers remain unchanged throughout兲, as shown in Fig. 2. Low refractive index barriers 共nbs ⬍ nw兲 reduce the overall far field, to below ⌰95% = 30°. At a barrier composition of GaAs0.10P0.90 共nbs = 3.16, 3.2% tensile strain兲, the calculated far field angle ⌰95% = 19.5° is equivalent to that of the same waveguide with no active region. If the waveguide thickness is instead increased for fixed GaAs0.80P0.20 barriers, ⌰95% remains ⬎33°, even for thicknesses ⬎8 m. However, at high levels of crystal strain, misfit dislocations will form in the semiconductor.4,10 For a single layer of a given strain, the maximum allowable thickness for stable material can be calculated, termed the critical thickness, tc.10 Strain compensation by adjacent layers can inhibit defect formation, as can optimized growth conditions.4 We calculate tc for GaAsxP1−x following the approach of Ref. 10 共double kink limit兲. We find that GaAs0.10P0.90 has tc
FIG. 1. 共Color online兲 Vertical refractive index and calculated mode profile vs vertical position for designs with 共a兲 standard GaAs0.80P0.20 and 共b兲 lowindex GaAs0.55P0.45 quantum barriers. In 共b兲, the mode profile for GaAs0.80P0.20 is reproduced 共dashed line兲 for comparison.
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FIG. 2. 共Color online兲 Calculated 共line兲 and measured 共points兲 vertical far field angle as a function of composition of GaAsxP1−x barrier layers. Inset: Measured far field profiles for x = 0.55 共dashed兲 and 0.80 共solid兲.
= 5 nm, so could not be used 共we use 8 nm barriers兲. However, intermediate levels should improve far field. To test this projection, experimental structures were grown using low pressure metal-organic vapor-phase epitaxy on GaAs substrates, with fixed waveguide 共4.8 m thick Al0.20Ga0.80As兲 and varied barrier and spacer composition. GaAs0.55P0.45 共nbs = 3.31, tc = 12 nm兲, GaAs0.66P0.44 共nbs = 3.34, tc = 17 nm兲, and GaAs0.80P0.20 共nbs = 3.42, tc = 33 nm兲 were compared. The material was processed into 100 m stripe broad area lasers and assessed unmounted and uncoated under pulsed 共1 s兲 conditions. ⌰95% values were measured for devices with 1 mm cavity length, shown in Fig. 2. Example measured far field profiles can be seen in the inset. Good agreement with simulation is seen, with ⌰95% = 27° for GaAs0.55P0.45. Next, the electro-optical performance was assessed. Following standard practice,3,4 the light-current characteristics were measured as a function of resonator length 共100 m stripe width兲 and the performance parameters extracted, as shown in Fig. 3. Plotting the inverse external quantum efficiency, 1 / d, as a function of cavity length, L, allows us to extract i, and optical loss, ␣i. There is no indication of differences between the designs, with i = 0.93 and ␣i
FIG. 3. 共Color online兲 Derivation of optical parameters from cavity length dependent measurements. 共a兲 Optical loss and internal quantum efficiency, 共b兲 transparency current and modal gain.
Appl. Phys. Lett. 96, 131110 共2010兲
FIG. 4. 25 ° C CW characteristics for 3 mm cavity, 90 m stripe broad-area diode lasers with GaAs0.66P0.44 barriers: 共a兲 Optical power 共left axis兲 and conversion efficiency 共right axis兲 vs current, 共b兲 optical power vs time for two samples.
= 1.1 cm−1 in all cases. In contrast, in Ref. 4, where low index barriers were not used, a 7 m thick waveguide was needed to achieve, ⌰95% ⬃ 30°, and i was ⬍60%. Second, plotting the logarithm of the threshold current density, Jth, as a function of 1 / L provided values for modal gain, ⌫g0, and transparency current density, Jtrans. Lower ⌫g0 is expected for designs with wider far field: the calculated confinement factors, ⌫, are ⌫ = 2.0%, 2.5% and 2.7% for GaAs0.55P0.45, GaAs0.66P0.34, and GaAs0.80P0.20, respectively. The extracted ⌫g0 values are 23, 27, and 29 cm−1, respectively, scaling as expected. A reasonable Jtrans of 210 A / cm2 was derived for GaAs0.80P0.20 and GaAs0.66P0.34 designs. However, the GaAs0.55P0.45 design exhibited Jtrans = 250 A / cm2, potentially indicating the presence of additional nonradiative defect centers. Cathodoluminescence measurements confirmed this assessment:4 GaAs0.55P0.45 designs showed dark spot defects but GaAs0.80P0.20 and GaAs0.66P0.34 did not. For GaAs0.55P0.45, tc is close to the barrier thickness and local thickness and compositional variations may have caused defect formation. To assess CW performance and reliability, material using the GaAs0.66P0.34 design was processed into lasers with 90 m stripe width and 3 mm cavity length. After cleaving, the facets were passivated4 then coated to yield 98% and 0.5% front and rear reflectivities, respectively. Devices were mounted junction side down with hard solder onto passively cooled copper heatsinks, with copper-diamond carriers used for stress matching. CW test results are shown in Fig. 4. A peak c of 58% was observed, and c remains above 55% to PCW ⬎ 10 W. Figure 4 also shows constant power endurance measurements for two samples. No degradation was seen after 1700 h at PCW = 7 W and 25 ° C. PCW was then raised to 10 W, and no failures have occurred to date after a further 450 h. In conclusion, low-index quantum-barriers significantly reduced vertical far field angle, below what is possible by changing the waveguide thickness. The resulting 90 m stripe broad area lasers achieved ⌰95% = 30°, 10 W reliable power and c = 58%, comparable to designs with significantly wider far fields.1,2 This work was supported by the German Federal Ministry of Education and Research 共BMBF兲 under Contract No. 13N9725.
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