Anaprism.len - A beam circularizer for diode lasers

Anaprism.len - A beam circularizer for diode lasers

Most diode lasers emit asymmetric beams. The numerical aperture is different in the yz and xz planes, and often the beam has astigmatism, which is a separate issue. One way to make the beam circular is to use a pair of anamorphic prisms, as shown in this example. The prisms used here are available as Melles Griot part number 06GPU001. They work in collimated light (otherwise they would add astigmatism), so in an actual application the prisms must be used in combination with a collimator. Such a system is included as the file diodassy.len, described below.

The prisms have a vertex angle of 29.43333 degrees, a width of 12mm, and a maximum thickness perpendicular to the back face of 8.5mm. For a given entry angle to the first prism, the angle of the second prism is fixed by the requirement that the beam emerge parallel to itself. The displacement of the beam depends on the prism separation. The listing below shows how the system should be set up in OSLO. Surfaces 4 and 6 are expressed in the coordinate system of surface 1 using a return_coordinates (rco) command. The rco command goes on the preceding surface and indicates that the coordinates of the next surface are to be taken according to the dcx, dcy, dcz, tla, tlb, and tlc relative to a base surface (here, surface 1).

In order to prevent a confusing drawing caused by the tilted surfaces, the surfaces themselves are marked not drawable (in the Surface Control spreadsheet). A plan view of the system shows just the ray trajectories. Note that although the drawing makes it look like there are only two surfaces, there are actually 4. The rays are close enough to normal incidence on the other two that the drawing doesn’t show them.

To substitute for the missing prism surfaces, the entire assembly has been placed in a box, using bdi (boundary information) data. OSLO graphics routines can be instructed to put 3D objects on a drawing that are totally unrelated to the optical function of the depicted system. These objects are specified by a list of vertices (vx) and polygon faces (pf), as shown in the listing below. To enter such data yourself, open the lens file in the text editor and use the same scheme. The vertex and face information must be preceded by a bdi command, which gives the number of data items. The final solid-model drawing of the system is as follows:

*LENS DATA
Anamorphic prism assembly
SRF      RADIUS      THICKNESS   APERTURE RADIUS       GLASS SPE   NOTE
  0        --        1.0000e+20    1.0000e+18             AIR

  1        --          6.000000      4.000000 AS          AIR   *

  2        --          5.872557    1.0000e-06            SF11 C *
  3        --            --        1.0000e-06             AIR   *

  4        --          5.872557    1.0000e-06            SF11 C *
  5        --            --        1.0000e-06             AIR   *

  6        --            --          4.000000             AIR   * Prism assy

  7        --         -0.003198      4.179133 S

*TILT/DECENTER DATA
  2     DT    1         DCX       --       DCY       --       DCZ       --
                        TLA   -59.800000   TLB       --       TLC       --
  3     RCO   1
        DT    1         DCX       --       DCY       --       DCZ       --
                        TLA    29.433333   TLB       --       TLC       --
  4     DT    1         DCX       --       DCY    -6.400000   DCZ    16.866459
                        TLA    29.292833   TLB       --       TLC       --
  5     RCO   1
        DT    1         DCX       --       DCY       --       DCZ       --
                        TLA   -29.433333   TLB       --       TLC       --
  6     DT    1         DCX       --       DCY    -6.400000   DCZ    23.000000
                        TLA       --       TLB       --       TLC       --

*SURFACE TAG DATA
  1     LMO EGR  (6 surfaces)
  1     DRW AP
  6     DRW AP

*BOUNDARY DRAWING DATA
SRF 1:
VX NBR         X            Y            Z        COORD SURF
    1        7.000000     7.000000       --             1
    2       -7.000000     7.000000       --             1
    3       -7.000000   -14.000000       --             1
    4        7.000000   -14.000000       --             1
    5        7.000000     7.000000    23.000000         1
    6       -7.000000     7.000000    23.000000         1
    7       -7.000000   -14.000000    23.000000         1
    8        7.000000   -14.000000    23.000000         1
PF NBR        VX1          VX2          VX3          VX4
    1            1            2            3            4
    2            1            5            6            2
    3            5            8            7            6
    4            8            7            3            4
    5            1            4            8            5
    6            2            3            7            6

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