Page 182 - High Power Laser Handbook
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150   Diode Lasers                          High-Power Diode Laser Arrays     151


                                   Output   Slow axis  Input

                                                               Z = 1, 3, 5, ...
                             Z = 1  Z = 2
                                                               Z = 2, 4, 6, ...



                           Output 6 beams-     Fast axis     Input 2 × 3 beams-
                             pitch 0.9 mm                      pitch 1.8 mm
                                                Z = 1
                                                Z = 2
                                                Z = 3
                                                Z = 4
                                                Z = 5
                                                Z = 6
                      Figure 6.18  Refractive method of interleaving beams from a diode laser
                      stack.


                      the  electro-optical  efficiency  is  greater  than  55  percent  and  where
                      power levels greater than 100 W per diode bar are available. The dif-
                      ferent wavelengths can be multiplexed with dielectric-edged mirror
                      into a common beam. Assuming a 12-bar stack with an internal pitch
                      of 1.8 mm and an average power of 120 W per diode bar, the total
                      power in a 21 × 10 mm beam size will be 2880 W after interleaving,
                      and 5742 W after polarization coupling, assuming a 5 percent cou-
                      pling loss. Multiplexing all seven available wavelengths, the power
                      will reach more than 35 kW CW from a 2-cm  aperture. The full beam
                                                           2
                      parameter product (BPP) can be as low as 80 × 250 mm-mrad for the
                      two axes, or a combined BPP of 330 mm-mrad. This enables fiber cou-
                      pling into an 800-mm core diameter fiber with an NA of 0.22, which
                      has an intrinsic BPP of 352 mm-mrad.
                      6.6.3  Fiber-Coupled High-Power Diode Laser Devices
                      Following the above described example of extreme power, fiber-coupled
                      diode laser stacks have been developed and placed in the market as a
                      replacement for lamp-pumped solid-state lasers. Because the beam
                      quality of lamp-pumped solid-state lasers enabled fiber coupling into
                      a 400- or 600-mm core fiber with an NA of 0.12, the diode laser stacks
                      had to be designed for the same beam parameter product of less than
                      120 mm-mrad (full angle). An example of such a design is shown in
                      Fig. 6.19. The slow-axis beam quality is improved by rearranging
                      the emitter from the horizontal line in the diode bar to a vertical stack
                      of emitters, using two stacks of parallel glass plates similar to the
                      interleaving concept. By choosing the correct number of emitters
                      for the diode bar, the stack’s beam quality can be made uniform in
                      both directions and, therefore, most effectively coupled into a fiber.
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