Page 182 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Stability and Tempco Issues

Stability and Tempco Issues 175

package type, batch number if possible, operating temperature, illumination
power and density, light wavelength, polarization state, and optical modes. Any
detraction from equivalent operating conditions can degrade the tracking of the
two receivers’ outputs. In the same way the electrical channels should be care-
fully matched. Large differences in bandwidth, for example between signal and
reference channels, will degrade high-speed tracking. It has also been found
preferable to synchronously detect both signal and reference intensities. Cost
reduction by using an AC-coupled reference path was found to lead to poor per-
formance. The noise contributions of both receiver paths should be understood,
as both will contribute to the overall noise level.
There are also many seemingly trivial things that can go wrong. These
include mechanical instability and movement of the beam-splitter and photo-
diodes, vignetting and variable loss of light from each diode, interference fringes
on the two detectors, differential contamination of the sample and reference
optical paths, and different interfering illuminations. Figure 8.12 shows the
simplest type of division-of-wavefront beamsplitter. This is not ideal with semi-
conductor laser sources due to their significant variation in emitted angular
power distribution, which changes with current drive and temperature. Smaller
variations take place with LEDs too. It is also important to avoid ambient light
falling on PD2. Any power variations here will be mirrored in power variations
in the controlled source.

8.5.2 Beam-splitters
The beam-splitter is also a frequent source of problems (Fig. 8.13). The com-
monest type is the cemented cube, formed using a metal, dielectric or hybrid
metal/dielectric thin film on one prism, which is encapsulated against another
prism. These are robust and easy to mount, but can have significant polariza-
tion sensitivity. Different coating types perform quite differently, so it is worth-
while to study the detailed specifications. Good descriptions of the trade-offs
are given in the Newport and Melles-Griot optical catalogs. The accurately
parallel surfaces increase the risk of disturbing interference fringes. They can
be operated off-normal incidence, but this increases optical aberrations with
nonparallel beams.
A simple microscope slide is sometimes pressed into service as a beam-
splitter, especially with small diameter free-space beams from lasers. The glass
with refractive index n provides a normal-incidence power reflectivity in air of
2
((n - 1)/(n + 1)) at each surface, about 4 percent for common borosilicate glass
slides (n = 1.51). However, the 1mm thickness of the glass plate might be com-
parable to the input beam diameter, meaning that some overlap of the two
reflected beams will occur. With sufficiently coherent sources, this will lead to
interference and large relative power fluctuations at the reference detector from
the reflected light (Fig. 8.14). As the two reflected beams are of similar inten-
sities, the visibility of the interferences seen there can approach 100 percent.
In the transmitted beam interference occurs with a beam that has suffered two

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