High spatial and temporal coherent output from a broad-stripe laser diode with phase conjugate feedback

Abstract

We report the first demonstration, to our knowledge, of high bright output with low M2 and narrow linewidth from a broad-stripe laser diode (BS-LD) with a phase conjugate feedback. Fig. 1 is the experimental setup. A 1W-class BS-LD (SONY SLD304), whose polarization is parallel to die y-axis in Fig.1, is mounted onto heat sink. The output of BS-LD is collimated and passes through a half wave plate, which rotates the polarization by 90°, so that the polarization of BS-LD lies in an undoped BaTiO3 extra-ordinary plane. One-half of BS-LD beam is out-coupled by a prism mirror placed in the far field, and the other is focused onto the BaTiO3. The BaTiO3 has dimensions of 5mm × 6mm × 7mm, and operates as a self-pumped CAT phase conjugator. The free-running BS-LD had 1.5nm line-width (FWHM) and poor beam quality of Mx2~5 and My2>100 in the directions along the x and y-axes. After the BS-LD switching on, the phase conjugator turned on within 10minutes. After an additional 5-10minutes, the system reached a steady state and the line-width of BS-LD had narrowed to less than 0.3nm (Fig.2). The out-coupled beam, which was focused by a lens with the focal length of 100mm, is shown in Fig.3. Its Mx2 and My2 factors had reduced to less than 20% and 35% compared with them at free-running. And then, out-coupled power still exceeded 50% of the total lasing power of BS-LD. It means that the focused output beam should have 7 times higher brightness than at free-running. Previously beam-clean-up of a high-power-array-laser-diode with phase conjugate feedback has been reported. In previous reports, because the phase conjugator works not as spatial filter but just as self-alignment external feedback mirror, an additional spatial filter was required for picking off one-half of twin-lobe output in far-field. Our technique is very different from previous ones, because it required no element such as the spatial filter for improving beam quality and narrowing the lasing frequency in BS-LD apart from the external phase conjugator. We therefore conclude that the BS-LD with phase-conjugate feedback is an effective source for pumping solid state laser materials at high efficiency.