Split-beam Fourier filter

DETAILED DESCRIPTION FIG.
1 shows the elements comprising one embodiment of a single stage split-beam Fourier filter.
The filter is a flat plate of glass (1) with one edge (2) carefully polished perpendicular to the plate surface--this edge splits the beam of a fibre beam expander consisting of a first fibre (3) glued to a lens (4) with transparent glue (5) and a second fibre (6), lens (7) and glue (8).
Anti-reflection coatings (9) on the lenses and plate improve transmission and suppress cavity resonances.
Light passing through the plate experiences a wavelength dependent phase shift compared with the light that does not pass through the plate.
If the phase shift is zero or a multiple of 2.
pi.
, then the beam is unchanged and the transmission is maximum (100%).
If the phase shift is .
pi.
, then the electric field (E field) is inverted in one half of the beam compared to the other, giving an antisymmetric E field distribution in the beam.
The result at the output fibre tip is an E field distribution which is the two dimensional Fourier transform of the beam E field which is also an antisymmetric function.
The overlap of this distribution with the fundamental fibre mode is zero therefore no light will be launched into the fibre if the fibre is single mode.
Since the phase shift is wavelength dependent, the device transmission is wavelength dependent.
A filter of this type was constructed using a fibre beam expander and a 92 .
mu.
m thick optically flat plate of glass which was edge polished and mounted on a positioner allowing insertion into the beam in the direction of the arrow (10) and rotation about this axis (11).
A maximum extinction greater than 35 dB and a transmission loss of 0.
20 dB were observed (using a 1 mm beam diameter).
The wavelength period was 54 nm.
Tuning through more than a complete wavelength period was accomplished by tilting the plate about the arrowed axis (10) with negligible increase in loss.
Polarisation dependence was measured to be less than 0.
1 dB.
FIG.
2 shows the transmission characteristic of a split-beam Fourier filter with various extinctions and centre wavelengths