Petersen, P.M.

Thestrup, B.

Online Research Database In Technology

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Downloaded from orbit.dtu.dk on: Oct 25, 2019External cavity setup for beam quality improvement of a broad-stripe segmented diodelaserThestrup, B.; Petersen, P.M.Publication date:2005Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Thestrup, B., & Petersen, P. M. (2005). External cavity setup for beam quality improvement of a broad-stripesegmented diode laser. Abstract from European Conference on Lasers and Electro-Optics - European QuantumElectronics Conference 2005, Munich, Germany.External cavity setup for beam quality improvement  of a broad-stripe segmented diode laser Birgitte Thestrup and Paul Michael Petersen, Optics and Plasma Research Department, Risø National Laboratory, Frederiksborgvej 399, 4000 Roskilde, Denmark.   Abstract  We demonstrate a new setup for selective off-axis feedback of a broad-stripe segmented diode laser leading to significant improvement of the overall spatial coherence of the output beam. Results with a 5 segmented diode are presented.      Introduction Segmented diode lasers and diode laser bars are attractive due to their compactness and simplicity of operation. However, these devices suffer from poor spatial coherence along the slow axis, and this prohibits coupling of the laser beams into, e.g., thin core fibres. The spatial coherence properties of broad single emitters can be improved significantly by introducing feedback to the diode by off-axis self-injection locking1. However, this technique cannot be directly applied to segmented diode lasers as the segments usually are mutually incoherent.  Here, we present an off-axis self-injection locking setup of a segmented diode laser that includes two feedback arms. The purpose of the first feedback arm is to overlap the output beams from the different segments. Thereby, it is possibly to apply off-axis self-injection locking to the individual segments successfully, which is performed in the second arm. The procedure results in an overall improvement of the beam quality that can exceed the beam quality of the individual emitters. Results with a 5 segmented diode are presented.  400 600 800Double feedbackSingle feedback  Intensity[arbitrary units]X [µm] Experimental A schematic of the double feedback setup is shown in Fig. 1.  The laser system consists of a microlens that collimates the beam along the fast axis (y-axis), a cylindrical lens that collimates the beam along the slow axis (x-axis), a diffraction grating placed in the Fourier plane with respect to the slow axis, an output coupler and a mirror. The principle behind the external cavity design is selective off-axis self-injection locking of the individual elements in the diode along the slow axis with the aid of spectral beam combining2.  Results Fig. 2 shows slow axis intensity profiles of the output beam at single and double feedback, measured at beam waist, for a 5W, 500µm wide, 5 segmented 808 nm diode laser operating at 1.6 A. The output beam is focused with a cylindrical lens. Using single feedback from the output coupler, the beam quality can be improved from an M2 value above 25 to M2= 7.  Introducing the double feedback, the beam quality is improved even further to around M2=3. In the figure, this improvement is seen as a more Gaussian-like intensity profile at double feedback. Similar results has been obtained for a drive current of 2.2 A (1.5 times the threshold current).    Summary A new setup for improving the beam quality of a segmented broad-stripe diode laser is presented. The beam quality of a 5 segmented diode laser is improved from an M2 value above 25 to M2= 3 at a drive current of 1.6 A using this setup. Segmented, broad-area diode laserMicro lensCyl. lensOutput couplerOutputbeamFilterMirrorGratingFourier planeIntensity profile of 0th order beamIntensity profile ofoutput beam              Fig. 1 Schematic of the diode laser system  Fig. 2 Intensity profiles of focused output beams at single and double feedback. The curve heights in the two cases are not comparable.    References  1. B. Thestrup, M. Chi, B. Sass, P.M. Petersen, Appl. Phys. Lett. 82 (2003) 680-682 2. V. Daneu, A. Sanchez, T.Y. Fan, H.K. Choi, G.W. Turner, C.C. Cook, Opt. Lett. 25 (2000) 405-407 

External cavity setup for beam quality improvement of a broad-stripe segmented diode laser

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External cavity setup for beam quality improvement of a broad-stripe segmented diode laser

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