10 W continuous-wave monolithically integrated master-oscillator power-ampliﬁer H. Wenzel, K. Paschke, O. Brox, F. Bugge, J. Fricke, A. Ginolas, A. Knauer, P. Ressel and G. Erbert A semiconductor-based master-oscillator power-ampliﬁer operating at 977 nm is demonstrated to emit more than 10 W continuous wave in a nearly diffraction limited beam with a narrow spectral bandwidth. The device consists of a distributed Bragg reﬂector laser and a ﬂared ampliﬁer monolithically integrated on a single chip. Introduction: Compact lasers emitting single-frequency, diffraction limited beams at a continuous-wave (CW) optical power of several Watts are required for many applications including frequency conver- sion, free-space communications, and pumping of ﬁbre lasers and ﬁbre ampliﬁers. Conventional narrow stripe or broad area semicon- ductor lasers do not meet these requirements, either due to limited output power or poor beam quality and wavelength stability. In the past, numerous concepts to maintain good beam quality and wave- length stability in the Watt range have been proposed, of which several have been successfully demonstrated. One of the most promising devices is the monolithically integrated master-oscillator power-ampliﬁer (MOPA), where either a distributed Bragg reﬂector (DBR) laser [1, 2] or a distributed feedback (DFB) laser [3] and a ﬂared (or tapered) gain-region ampliﬁer are combined on a single chip. CW optical powers of 2 W at 970 nm [1], 2.2 W at 854 nm [2] and 1.5 W at 980 nm [3] have been achieved. During recent years, no further improvement towards higher output power has been reported. In this Letter, an improved MOPA is demonstrated which emits a CW power of more than 10 W at 977 nm in a nearly diffraction limited beam and narrow spectral bandwidth of 40 pm. Device structure and fabrication procedure: The MOPA investigated has a total length of 8 mm with antireﬂection coated front and rear facets (R f < 0.1%, R r  1%). The PA has a length of 4 mm with a total ﬂare angle of 6  . The current conﬁnement is realised by a shallow He implantation outside of the ﬂared p-electrode. The 4 mm-long MO is a three-section DBR laser consisting of a passive 500 mm-long DBR section, an active gain section and a passive 65 mm-long DBR section towards the PA. The lateral optical and current conﬁnement in the MO is provided by a 2.2 mm-wide dry-etched ridge-waveguide with an effective index step of about 6  10 3 . The maximum reﬂectivities of the DBRs are 70 and 2%, respectively, estimated from the ratio of the optical powers emitted at the rear and front facets of an AR coated DBR laser fabricated from the same wafer. The layer structure grown by two-step metal-organic vapour-phase epitaxy consists of a Al 0.45 Ga 0.55 As=Al 0.70 Ga 0.30 As super large optical cavity (SLOC) with a 3.6 mm broad waveguide core as described in [4] and [5]. The vertical divergence of the edge-emitted power is 21  full-width at half maximum (FWHM). The second-order Bragg gratings of the MO are deﬁned holographically and formed by wet-chemical etching into an InGaP=GaAs=InGaP layer sequence. More details of the fabrica- tion procedure of the gratings can be found in [5] and [6]. For the experimental characterisation in CWoperation, the MOPA is mounted p-side up with AuSn solder on a CuW submount which is then soldered with PbSn on a specially designed conductively cooled package capable of absorbing up to 40 W thermal power. The p-side contact was formed by wire bonding. The input currents to the MO and PA are individually controllable. Results: All experimental results presented in the following have been obtained at a heatsink temperature of 15  C. Fig. 1 shows the dependence of the CW optical power on the input current I PA to the PA for different input currents I MO to the MO. The shape of the characteristics change strongly with the currents. For I PA <2A shown in the inset, optical power depends almost exponentionally on I PA and rises with increasing I MO as is typical for an unsaturated ampliﬁer. 0 5 10 MO current, A 0.10 0.15 0.20 0.25 0.30 power, W PA current, A 15 10 5 0 0 0 1 2 0.1 0.2 Fig. 1 CW optical power against input current I PA to PA at 15  C heatsink temperature Parameter is the input current I MO to the MO Inset: Power dependence for I PA between 0 and 1.5 A For I PA > 2 A and I MO > 0.1 A, the output power becomes in part independent of I MO . However, if I MO and thus the power injected into the PA is not high enough and if I PA exceeds a certain value, the output power depends nonlinearly on I PA , which occurs e.g. for I MO ¼ 0.15 A and I PA > 3.5 A. At very high currents I PA > 11 A, an oscillating behaviour of the output power can be observed which is possibly caused by compound-cavity effects. For 3 A < I PA < 10 A and I MO  0.25 A, the power-current characteristic is almost linear with a slope efﬁciency of 1W= A. For I PA > 10 A, the slope efﬁciency decreases. This is probably because if the optical power is much larger than the saturation power, the output power of the ampliﬁer is a linear function of the unsaturated modal gain which depends itself sublinearly (e.g. logarithmic) on the injected current density. At I PA ¼ 13A, the output power exceeds 10 W. Within the current range investigated, thermal effects are almost negligible. 10 –4 10 –3 10 –2 10 –1 10 0 power spectral density, a.u. 0 0.5 1.0 977.25 977.20 977.15 977.6 977.2 976.8 l, nm b l, nm a Fig. 2 Optical spectrum at power of 10 W and 15  C heatsink temperature a Logarithmic scale b Linear scale Input currents to MO and PA are I MO ¼ 0.2 A and I PA ¼ 13 A, respectively An optical spectrum recorded at 10 W optical power with the optical spectrum analyser (OSA) Advantest Q8347 is depicted in Fig. 2. The logarithmic plot demonstrates single-longitudinal mode emission with a sidemode suppression ratio greater than 30 dB, limited by the dynamic ELECTRONICS LETTERS 1st February 2007 Vol. 43 No. 3