1 W tunable near diffraction limited light from a broad area laser diode in an external cavity with a line width of 1.7 MHz

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1 Optics Communications 277 (27) W tunable near diffraction limited light from a broad area laser diode in an external cavity with a line width of 1.7 MHz Andreas Jechow a, *, Volker Raab a, Ralf Menzel a, Michael Cenkier b, Sandra Stry b, Joachim Sacher b a University of Potsdam Institute of Physics, Chair of Photonics, Am Neuen Palais 1, Potsdam, Germany b Sacher Lasertechnik, Hannah Arendt Strasse 3-7, 3537 Marburg, Germany Received 16 October 26; received in revised form 17 April 27; accepted 5 May 27 Abstract A novel unstable external cavity for a broad area laser diode is presented. The cavity is based on a V-shaped setup that improves the slow axis beam quality by coupling the internal modes of a gain guided laser diode. The novelty here is the compact unstable resonator design without lenses in direction of the slow axis. For frequency stabilisation and to narrow the line width of the laser diode emission a diffraction grating in a Littrow configuration is used. With this setup up to 1 W of near diffraction limited light with a beam quality of M and a line width of 1.7 MHz could be achieved. The external cavity laser was tunable over a range of 35 nm (FWHM) around the center wavelength of 976 nm. Ó 27 Elsevier B.V. All rights reserved. PACS: f; Px; 42.6.Da 1. Introduction * Corresponding author. Tel.: address: ajechow@uni-potsdam.de (A. Jechow). URLs: (A. Jechow), (J. Sacher). Compact high power laser sources in the near infrared region are required for various applications such as spectroscopy, nonlinear optics and medical diagnostics. Therefore, narrow line width and mostly tunability of the laser emission is essential. Furthermore particularly the nonlinear applications demand high power densities. Since the introduction of periodically poled crystals and their high conversion efficiencies diode laser sources are considered for frequency conversion into the visible. Especially very compact, cheap and robust pump laser sources that fulfill the requirements of nonlinear applications are of high interest. Broad area laser (BAL) diodes feature long lifetimes combined with compactness and relative low prices by providing high optical output powers up to several Watts [1]. Nevertheless these devices show poor spatial and longitudinal mode properties. Especially the slow axis beam quality is very poor because of the large emitter width which is typically in the region of 1 4 lm in that direction. Furthermore the spectral emission shows a band width of a few nanometers. Thus broad area diodes are limited to certain applications such as pumping of solid state lasers and are not suitable for the applications mentioned above. Several approaches where made to improve both the power and the beam quality of the laser diodes. Progress was reached especially in the field of vertical emitting diodes [2], tapered laser diodes [3] and amplifiers and distributed feedback (DFB) laser diodes [4]. All of these solutions have their advantages but they also have certain drawbacks. Mainly they are much more expensive than broad area diode lasers for the same output power. Especially for custom solutions with low quantities they are usually not adequate. Another way to obtain high output powers, narrow line width and good beam quality out of a standard laser diode 3-418/$ - see front matter Ó 27 Elsevier B.V. All rights reserved. doi:1.116/j.optcom

2 162 A. Jechow et al. / Optics Communications 277 (27) is to build an external cavity that improves the spatial and temporal laser diode emission [5 13]. Several solutions for beam shaping with external cavities have already been proposed [5 7]. Unfortunately, when higher powers were achieved the effort usually was correspondingly high. This includes the use of phase conjugation elements [9] as well as MOPA systems [1,13]. A simpler way to improve the beam quality is to couple the internal modes of a gain guided broad area laser diode by using a V-shaped cavity design [6,7]. Here a new compact and simple unstable setup based on this V-shaped design that provides narrow band width, tunability and high brightness is presented. 2. Experimental setup The unstable external cavity is depicted in Fig. 1. It consists of only four optical components: a gain guided broad area laser diode, an aspherical cylindrical lens with a high numerical aperture (NA =.7) and a focal length of.9 mm, the so called fast axis collimator (FAC), a half wave plate and a holographic diffraction grating. The laser diode has an emitter size of 4 lm in x-direction (slow axis) 1 lm in y-direction (fast axis) and a cavity length of 15 lm. To suppress the internal longitudinal and transversal modes of the diode itself the front facet has to be anti-reflection (AR) coated in the order of R <1 5. Without AR-coating the diode provides about 2 W at an injection current of I = 2.5 A with an M 2 >6 and a line width of 2 nm centered around 976 nm. With the AR-coating applied the diode shows a broad luminescence spectrum of 3 nm (FWHM) width. The gain guiding of the laser diode results in a stripe array structure in the x-direction (slow axis). This structure is essential for the understanding of the operation of the V- shaped cavity. In contrast to an index guided laser diode the active region extends completely along the x-axis without a change in the refractive index. The carriers are aperture (optional) outcoupling branch injected from narrow stripes into the active region which leads to a variation of the carrier density between pumped and unpumped areas. The variation of the carrier density leads to a variation of the refractive index. This electrical confinement is lower than the optical confinement in index guided diode laser and a weak coupling between neighboring modes can be observed. The BAL diode that was used during the experiments had contact stripes with a pitch of d = 2 lm applied onto the active region. The schematic in Fig. 1 shows the setup in plane of the slow axis. The V-shaped cavity is split into a feedback branch and an outcoupling branch. The feedback branch contains a half wave plate and the diffraction grating in a Littrow configuration and the outcoupling branch contains only an optional aperture. Feedback is centered around the angle a FB that is determined by an out of phase oscillation of the contact stripes [6]. It is given by a FB = k/d. With the pitch of the contact stripes d = 2 lm and the wavelength k = 976 nm follows a FB = 48.8 mrad = 2.8. This V-shaped design was already used in [6,7]. However in both cases a slow axis lens in combination with a slit aperture was used to force the skew emission under the angle a FB. In this setup no collimation or focusing lens in direction to the slow axis is needed. Instead an unstable laser resonator is realized. This is possible due to interplay of the high gain, the stripe array structure of the laser diode and the relative short cavity length of 4 mm. The high divergence emission of the diode in direction of the fast axis is collimated by an AR-coated cylindrical lens with a focal length of 9 lm the so called FAC (fast axis collimator). Practically all emitted light is collimated into an almost parallel beam with a width of 1.5 mm and a remaining divergence off less than 1 mrad without vignetting. This beam hits the grating with g = 18 lines/mm which is positioned in 4 mm distance to the diode. The second novelty to the design described in [7] is the rotation of the grating by 9 so that the grooves of the grating are positioned in the plane of epitaxy (Fig. 2). The light emitted by the diode is polarized in the plane of epitaxy (slow axis). To match the high 1st order diffraction efficiency of the grating it is necessary to rotate the "slow-axis" diode FA C /2 symmetry axis grating axis" "fast diode FAC λ/2 grating feedback branch 4 mm Fig. 1. Schematic drawing of the unstable V-shaped cavity in plane of the slow axis. It consists of an AR-coated broad area laser diode, a fast axis collimator (FAC), a half wave plate and a grating in a Littrow configuration on the feedback side. The optional aperture on the outcoupling side can be inserted to suppress the amplified stray emission (ASE). Fig. 2. Schematic drawing of the resonator in plane of the fast axis. Wavelength selection is realized by the combination of FAC and diode without further spectral filters. Due to its small height only a small part of the spectrum is fed back into the active region via the FAC. The half wave plate rotates the polarization of the diode by 9 which is necessary to match the high diffraction efficiency of the grating.

3 A. Jechow et al. / Optics Communications 277 (27) polarization by 9 using an AR-coated k/2 retardation plate. In a Littrow configuration the 1st order diffraction is reflected back into itself and only a small spectral part of the light is focused back into the 1 lm aperture of the active region by the FAC. Thus no further slit apertures have to be used for spectral filtering in this cavity. If the aperture of the semiconductor in slow axis is approximated by a hard stop one would expect an angular intensity distribution proportional to (sin c(a)) 2 = (sin(a)/a) 2. This intensity distribution is fed back by the grating. Because of the divergency of the beam in slow axis and the unstable setup the gain region of the laser diode extracts only the central part of the maximum. This part of the distribution is an almost plane wave so that no further spatial mode filtering is necessary. Therefore the well-known distortions and spherical aberrations of lenses are completely avoided inside the cavity. Since the gain of the laser diode is very high, the loss of the unstable cavity design does not come into play. The coupling between the feedback branch and the outcoupling branch takes place inside the semiconductor chip. The chip constitutes a diffraction grating caused by the stripe array geometry which leads to a gain modulation. The gain modulation in turn leads to a modulation of the refractive index. This coupling is rather weak. Correspondingly the resonator finesse is low and it cannot be expected to obtain line widths well below the grating lens monochromator. 3. Experimental results The output power of the external cavity laser as a function of the injection current behind the optional aperture is depicted in Fig. 3. A maximum output power of 1 W could be obtained at a pumping current of 2.8 A. The laser threshold is reached at.7 A resulting in a slope efficiency of.48 W/A. Beyond an injection current of 2.8 A the output power saturates as thermal roll over sets in. Fig. 4 represents the beam quality measurements along the slow axis. The beam quality was measured using a moving slit technique and the beam radii where determined using the second momentum of the intensity distribution. A hyperbolic fit through the data was used to obtain the M 2 value. This follows completely the ISO11146 standard. At an injection current of 2.8 A and a wavelength of 976 nm yielding 1 mw of optical output power a beam quality of M 2 = 1.3 in slow axis and M 2 = 1.2 in fast axis were determined. Since the slow axis beam quality of the diode without external cavity is around M 2 > 6 this means an improvement by a factor of 44 in that axis was obtained. The fast axis beam quality remains nearly diffraction limited. By changing the Littrow angle of the grating the center wavelength of the laser system can be tuned. Fig. 5 shows the optical output power as a function of the wavelength for three different injection currents. The maximum of the output power is centered around 97 nm and shifts to lower wavelengths as the injection current is raised. This is caused by the band gap filling effect. At an injection current of 2.8 A tuning range of 35 nm (FWHM) from 953 nm to 988 nm could be achieved. The total tuning range was 5 nm from 945 nm to 995 nm. Beam radius [µm] Position [mm] Fig. 4. Beam caustic along the slow axis at an injection current of 2.8 A and an optical output power of 1 mw. The beam radii where measured with a moving slit technique using the second momentum of the intensity resulting in M 2 = 1.3. Power [mw] utput power [mw] A 2.5 A 2. A Injection current [A] Fig. 3. Optical output power of the external cavity laser as a function of injection current. The maximum output power of 1 mw is reached at an injection current of 2.8 A. The slope efficiency is.48 W/A. Fig. 5. Optical output power as a function of the center wavelength for three different injection currents. A maximum tuning range of 35 nm (FWHM) could be achieved.

4 164 A. Jechow et al. / Optics Communications 277 (27) Fig. 6 shows the normalized spectra at different center wavelengths at 2.5 A injection current measured with an optical spectrum analyzer (OSA). Even at the tails of the tuning curve the side mode suppression was well above 3 db. In all cases line widths around 5 pm (FWHM) were obtained, which equals the resolution limit of the used OSA. A single spectrum near the tuning maximum at nm and an injection current of 2.8 A is depicted in Fig. 7.Atan optical output power of 95 mw a side mode suppression of better than 45 db and a band width of 5 pm could be obtained. Using a Fabry Perot Interferometer (FPI) with a free spectral range (FSR) of 8 GHz line widths of Dm < 8 MHz could be measured. This is well below the resolution limit of the OSA and very close to the resolution limit of the FPI. For further investigation of the line width a heterodyne interference analysis was done. In this experiment a reference laser with a known linewidth of Dm REF = 1 MHz (SACHER LASERTECHNIK TEC 3_96_5) was used to interfere with the unstable laser cavity. The beat signal depicted in Fig. 8 was detected using a fast photo diode and an electronic spectrum analyzer (ESA) with a resolution of 4 khz at a measuring time of 1 ms (4 MHz/ms). rel.power [db] Fig. 6. Normalized spectra for different center wavelengths at an injection current of 2.5 A. For all cases a line width around 5 pm (FWHM) was obtained. The side mode suppression was better than 3 db in all cases. norm. Power [db] FWHM 5 pm Fig. 7. Spectrum at an injection current of 2.8 A and an optical output power of 95 mw. Intensity [a.u.] The beat signal is centered around 12 MHz and has a width of Dm BEAT = 2 MHz. By the known line width of Dm REF = 1 MHz p of the reference laser this gives a line width of Dm ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Dm REF þ Dm BEAT ¼ 1:73 MHz for the external cavity laser. The measurement was done at a pumping current of 2 A. Compared to the free running diode with a line width about 2 nm this is an improvement of the brilliance by several magnitudes. 4. Conclusion Based on previous concepts a novel unstable V-shaped cavity is presented. The setup stands out by its simplicity and compactness. It consists of only four optical components or five considering the optional aperture which results in a cavity length of only 4 mm. It was possible to achieve up to 1 W of near diffraction limited light with M for both axis. The slow axis beam quality was improved by a factor of more than 4. Furthermore the line width of the broad area laser diode could be narrowed to 1.73 MHz by maintaining a tuning range of more than 35 nm (FWHM). The slow axis brightness could be improved by the factor of about 2 compared to the free running laser diode. Furthermore the very simple concept can be easily adapted to other wavelengths. With the high brightness and the excellent line width it is possible to use this external cavity laser for nonlinear as well as spectroscopic applications. References Δν FWHM = 2 MHZ Heterodyne frequency [MHz] Fig. 8. Beat signal of the external cavity emission and a reference laser with a line width of 1 MHz using a ESA with a resolution of 4 khz. The resulting line width is around 1.7 MHz. [1] E. Lassila, R. Hernberg, Appl. Opt. 45 (26) [2] M. Miller, M. Grabherr, R. King, R. Jäger, R. Michalzik, K.J. Ebeling, IEEE J. Sel. Top. Quant. Electron. 7 (21) 21. [3] M. Kelemen, J. Weber, G. Kaufel, G. Bihlmann, R. Moritz, M. Mikulla, G. Weimann, Electron. Lett. 41 (25) 111. [4] H. Wenzel, J. Fricke, A. Klehr, A. Knauer, G. Erbert, Phot. Techn. Lett. 18 (26) 737. [5] S. Wolff, H. Fouckhardt, Opt. Express 7 (2) 222. [6] V. Raab, R. Menzel, Opt. Lett. 27 (22) 167.

5 A. Jechow et al. / Optics Communications 277 (27) [7] V. Raab, D. Skoczowsky, R. Menzel, Opt. Lett. 27 (22) [8] A. Wicht, M. Rudolf, P. Huke, R.H. Rinkleff, K. Danzmann, Appl. Phys. B 78 (24) 137. [9] C. Pedersen, R. Hansen, Opt. Express 13 (25) [1] M. Chi, O.B. Jensen, J. Holm, C. Pedersen, P.E. Andersen, G. Erbert, B. Sumpf, P.M. Petersen, Opt. Express 13 (25) [11] M. Duval, G. Fortin, M. Piché, N. McCarthy, Appl. Opt. 44 (25) [12] H. Stoehr, F. Mensing, J. Helmcke, U. Sterr, Opt. Lett. 31 (26) 736. [13] M. Maiwald, S. Schwertfeger, R. Güther, B. Sumpf, K. Paschke, C. Dzionk, G. Erbert, G. Tränkle, Opt. Lett. 31 (26) 82.

High brightness semiconductor lasers M.L. Osowski, W. Hu, R.M. Lammert, T. Liu, Y. Ma, S.W. Oh, C. Panja, P.T. Rudy, T. Stakelon and J.E.

QPC Lasers, Inc. 2007 SPIE Photonics West Paper: Mon Jan 22, 2007, 1:20 pm, LASE Conference 6456, Session 3 High brightness semiconductor lasers M.L. Osowski, W. Hu, R.M. Lammert, T. Liu, Y. Ma, S.W. Oh,