Page 116 - Photodetection and Measurement - Maximizing Performance in Optical Systems

System Noise and Synchronous Detection

System Noise and Synchronous Detection 109

a lock-in amplifier to detect office fluorescent lighting! The 50/60Hz line voltage
would be applied, suitably attenuated, to the lock-in reference input, the
detected fluorescent light to the input. The dominant modulated light compo-
nent is at 100/120Hz since the light output when driven by AC is independent
of the sign of the current and hence is at double the drive frequency.
As mentioned earlier, the newest lock-in amplifiers digitize the signal at an
early stage and perform much of the processing numerically. Hence they tend
to offer an even wider range of features. The distinction between these beauti-
ful systems and vector voltmeters, spectrum analyzers, and scalar and vector
analyzers is now becoming blurred. There is presumably still a market for
analog instruments at the high-frequency end of laboratory measurements, but
even this niche will be eroded as time goes by.

5.7.1 Phase shifting
The other crucial function block in all lock-in amplifiers is a phase shifter. We
have seen that to obtain maximum output from the multiplier–integrator, it is
necessary to align clock and input signals. There will always be some delay
between the reference clock and the received modulated light. This is generally
not due to time-of-flight effects, but to group delays in the photoreceiver. It is
good practice to design the receiver to have adequate but not excessive band-
width. This means that the square-wave modulated light is detected as a less-
than-square electrical signal. The receiver rise time will be slower than that
needed to accurately reproduce the square input light, and this slowing of the
transitions is equivalent to a time-shift of the fundamental frequency which is
detected in the demodulator. To compensate for this delay, the reference channel
is provided with a wide-range, finely adjustable, and stable phase shifter that
allows accurate alignment of reference and signal.
Where direct electronic modulation of the source is not possible, for example
in some types of slow turn-on lasers and high thermal capacity sources, alter-
native intensity-modulators are needed. All the well-known optical modulator
types may be used for this (e.g., Pöckels cells, acoustooptic modulators, wave-
guide and bulk electrooptic modulators, polarization modulators, liquid-crystal
devices) as can mechanical modulators such as moving mirrors, moving fibers,
and vibrating and rotating “choppers.”
The classic chopper, a rotating disk with drilled holes or machined sectors, is
still very useful and widely used (Fig. 5.18). As long as the beam transmitted
through the sectors is small compared with the sector angular width, almost
square-wave modulation of the light is possible. Where the beam is too large,
the transmitted beam modulation is more trapezoidal. As we only detect the
f mod fundamental, this is of little consequence. A reference signal is provided by
an optocoupler, magnetic Hall-effect device, capacitive sensor, etc. It is possible
with this approach to vary the phase difference between transmitted beam and
reference signal by moving the optocoupler azimuthally around the disk or
equivalently by shifting the main beam. In practice both are usually fixed, with

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