Page 214 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Contamination and Industrial Systems

Contamination and Industrial Systems 207

Detector
assembly

Path-length
10–1000mm

Sample
Bubble meniscus:
pressure difference
controlling curvature

Polymer membrane
250μm thick with
O-Ring
seal 5mm dia. hole
Air
Air inlet to
adjust curvature Cavity
and clean the
meniscus Heated sapphire
window Optical
Path

Collimated
source assembly
Figure 9.15 Better control of the holey bucket is obtained by supporting the liquid
column primarily on a pressurized air cushion. The meniscus curvature may be adjusted
using feedback control from the differential pressure. Plane menisci are possible, or we
can make use of the optical power of a curved liquid surface for beam collimation.

whole meniscus area. Now the hole can grow to 6 or 10mm diameter without
great difficulty, allowing the use of large-scale sources.
By varying the pressure in the supporting cavity, the meniscus curvature
can be varied from convex (positive optical power) to concave (negative optical
power). With the correct pressure, the meniscus can also be adjusted to be
accurately flat, suitable for collimated beam transmission. The pressure is
of course a function of the water head, ambient temperature, surface tension,
etc., so closed loop control is necessary to have a useful system. The figure shows
a sensitive pressure sensor which can resolve a few microns change in water
head, measuring the pressure difference Dp across the meniscus. As long as
surface tension g is constant, Dp defines the meniscus radius of curvature R
identically:

2g
Dp = (9.1)
R

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