Page 190 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Stability and Tempco Issues

Stability and Tempco Issues 183

making sure they come from the same part of the source beam, in the same
polarization state, free from asymmetrical intensity fluctuations caused by
interference fringes in windows and other components and so on, the effective
measurement noise level can be brought down to within a decibel of the shot
noise limit over a wide bandwidth. This constitutes an enormous improvement.
In one stroke a large number of laser sensor systems are turned from offering
marginal performance to delivering real precision.

8.5.4 Split detectors
Another useful application of optical referencing avoids the discrete beam-
splitter altogether, making use of different parts of the source beam as in Fig.
8.12. High resolution position sensors can be constructed by projecting a high
intensity beam onto a split photodetector. Figure 8.19 shows two photodiodes
made in a single substrate. The structure has common diode anodes. An opaque
aperture is used to cast a well-defined spot of light from an infrared LED onto
the junction between the two sensitive surfaces. If the spot is symmetrically
placed, the signal formed from the difference between the two photocurrents
V L - V R is zero. With a displacement right or left we have a positive- or
negative-going signal.
With an optimized choice of the spot-diameter the transfer function can be
adequately linear for simple position sensing applications. A square aperture of
uniform illumination would improve linearity, but even a bare LED placed close
to the photodetector can be used (lens removed by polishing). The sensor gain
or slope of the transfer function then depends on the axial separation Z. Gain
stability is improved through use of the aperture to restrict the angular diver-

Left
Left
-
+A
V -V L
-
R
LED Right +
Source -
Right +A
± X Z -
V -V L
R
-(V L +V R
+
Set-point +Set-point)
X
Summed intensities
control LED current
Figure 8.19 Very simple but high resolution 1- and 2-dimensional position sensors can be fabri-
cated using split (dual or quadrant) photodiodes illuminated by light from an LED, preferably
cleanly apertured. The photocurrent difference gives the position, while the sum can be used to
control the source intensity. Remanent scale factor errors occur if the axial separation Z changes.

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