Diodassy.len - Shaping a diode laser beam

Diodassy.len - Shaping a diode laser beam

This file combines a diode-laser collimator (diodcoll.len), a cylindrical lens, and an anamorphic prism assembly (anaprism.len) to create an overall system that converts the light from a hypothetical diode laser having a beam divergence ratio of 3:1 and 10 microns of astigmatism into a collimated circular Gaussian beam having a wavefront quality of better than 0.25l. The diode is assumed to be single mode, and to have a numerical aperture in the xz plane of 0.3, and a numerical aperture in the yz plane of 0.1. The general layout of the system is as shown below. See diodcoll.len for additional information on the collimator and anaprism.len for additional information on the prism assembly.

*LENS DATA
Astigmatic diode/prism assembly
SRF      RADIUS      THICKNESS   APERTURE RADIUS       GLASS SPE   NOTE
  0        --          1.578414    1.0000e-06             AIR

  1   ELEMENT GRP     10.200000      2.500000 A          SF11 C *
  6    Collimator      3.000000      4.500000             AIR     Collimator

  7       ELEMENT      1.000000      4.000000             BK7 C *
  8    Astig corr        --          4.000000             AIR   * Astig corr

  9   ELEMENT GRP     17.745114      4.000000             AIR   *
14    Prism assy        --          4.000000             AIR   * Prism assy

15       ELEMENT      1.000000      4.000000             BK7 C *
16    Out window        --          4.000000             AIR     Out window

17        --            --          2.515146 S

The astigmatism of the source is listed as the general operating condition sasd on the general operating conditions, as shown below. The value is the distance between the apparent source locations in the yz and xz meridians, 0.01 millimeters in the present example.

*OPERATING CONDITIONS: GENERAL
   .
   .
   Source astigmatic dist:    0.010000    Ray aiming mode:          Aplanatic
   Temperature:              20.000000    Pressure:                  1.000000

The numerical aperture of the system is listed as 0.3 on the surface data spreadsheet. This tacitly assumes that the beam is circular. The ellipticity of the beam is indicated in the spot diagram operating conditions, since that is the place where it is important. The spot size in the y-direction is called ssy, and the spot size in the x direction is called ssx. Since the diode aperture is specified in NA, the spot size must be given as ss = th[0]*tan(asin(NA)). Which yields ssy = .159, ssx = .496.

The data below show the results of a spot diagram. Note that since the system is afocal, the spot data appears in angular measure (radians). Note also that the spot is much larger in the x direction than the y direction, as confirmed by the plot.

*SPOT DIAGRAM: MONOCHROMATIC APODIZED
APDIV    11.050000
WAVELENGTH 1
WAV WEIGHTS:
       WW1
    1.000000
GAU   SSY         SSX
    0.100000    0.300000
NUMBER OF RAYS TRACED:
       WV1
        96
PER CENT WEIGHTED RAY TRANSMISSION:     6.532767

*SPOT SIZES
  GEO RMS YA  GEO RMS XA  GEO RMS RA  DIFFR LIMIT    CENTYA      CENTXA
  1.3927e-06  9.4077e-06  9.5102e-06  6.8377e-05      --          --

*WAVEFRONT RS
WAVELENGTH 1
   PKVAL OPD     RMS OPD  STREHL RATIO    RSY         RSX         RSZ
    0.031658    0.007022    0.998711  1.1417e-10      --          --

Spot diagrams only show the intersection points of rays with the image surface, not the ray weights. In the present case, the different ssx and ssy values put different weights on the rays (you can confirm this using the Calculate >> Display spot diagram command and selecting ray weights). The weights affect calculations such as energy distributions, and more particularly Fourier transforms, which are used to compute the intensity distribution in the emergent beam. The plot below shows the point spread function (i.e. the far-field intensity distribution) for the present system. The abscissa is in radians, since the evaluation is in afocal mode.

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