2.2. Optimally illuminating a digital holography setup
For an illustrative SLM-based beam shaping application, we
used a diffractive optical setup in an optical Fourier transform
geometry, as shown in Fig. 2, which consists of a diode-pumped
solid-state laser (Laser Quantum Excel, λ0¼532 nm) with beam
diameter 2w0¼1.5 mm. The horizontally polarized laser beam is
de-magnified (1/1.5 times) to meet the specifications of the fixed
GPC light shaper module which generates a small rectangular
output beam profile with a 4:3 aspect ratio. The beam is magnified
before passing through a rectangular iris which blocks peripheral
light, also allowing direct comparison with a hard-truncated
Gaussian. The resulting rectangular beam is then projected to a
phase-only spatial light modulator (Hamamatsu Photonics,
792 600 pixels, 9.9 mm 7.5 mm active area) to read out holographic phase patterns encoded on the SLM. The modulated beam
is Fourier transformed using a lens (f¼250 mm) and subsequently
imaged to a beam profiler (Gentec-EO, Beamage 3.0).
For the hard-truncated input beam, the phase mask and phase
contrast filter (PCF) are retained but the PCF is slightly displaced to
move the phase shifting region away from the beam path and
disable the phase contrast effect. This ensures that the input beam
encounters the same perturbations along the optical beam path to
the SLM and onwards. During comparison of the GPC-enhanced
and hard-truncated hologram read outs, laser power is kept constant. Hence, any improvement is attributed to the beam shaping
involved prior to the phase modulation at the SLM.