Page 234 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Measurand Modulation

Measurand Modulation 227

configured for multiple small samples. A track around the perimeter of the
50mm diameter paper disk can hold about 50 samples each of 1mL, either all
different or multiple instances of the same sample. Two timing mark systems
should be provided, one to signal the start of a revolution, the others to signal
the angular position around the disk. They can be provided either on the
support arbor or printed onto the paper itself. As the disk spins, each sample
is read out and the area under the absorption signal is digitized in time. Similar
systems have been built based on chemical analyses done in multilayer films in
tape form. With a single sample divided into multiple patches and spun at high
speed, we effectively have a diffuse reflectance modulated by a periodic modu-
lation of sample concentration. Large improvements in LOD are possible with
this approach.

10.7 Remote Sensing: Fiber
Refractometer Revisited
In the modulation techniques described so far, the instrument has been avail-
able, and sample modulation has been straightforward in principle. In the case
of optical fiber sensors, however, the location where we would like to modulate
may be in a remote or inhospitable place. We have already seen the difficulties
of achieving even one-bit of resolution in a fiberoptic reflectometer. Due to
fiber attenuation which can be highly variable from installation to installation
and over time, even detecting the presence or absence of a switched mirror can
be difficult. The signal of interest is, because of attenuation, a tiny fraction of
the swamping intensity due to unterminated fiber ends, connectors, and mate-
rial back-scatter. Modulation of the source, although convenient for intensity
quantification and industrial interference removal, does not improve the S/N of
the true signal. However, if we can perform the modulation at the distal end of
the fiber, that is, of the light incident on the sensor-head, then large improve-
ments are possible.
Figure 10.15 shows a fiber refractometer with a mechanical intensity modu-
lator, formed from a stepper-motor driven chopper wheel. If this is battery-
driven, or powered via a solar cell from ambient light or light guided in the fiber,
and the power consumption can be kept of the order of 1mW or less, then con-
venient and low cost-of-ownership fiber sensors can be constructed. Even micro-
electro-mechanical-systems (MEMS) clockwork motors driven by mechanical
strain energy may be attractive. With a low-reflection chopper wheel, the signal
intensity will change periodically, with the intensity difference being that due
to the sensor head reflection. The received signal will contain the disturbing
reflections in the unterminated reference fiber arm (9mW), other fiber arms and
in-line connectors, with superimposed the small modulated intensity from the
liquid under test (minimum 1.5nW). By reducing the detection bandwidth we
should have no difficulty in detecting the refractometer signal, with an accu-
racy determined by the intensity stability, detector tempco etc. Without the
measurand modulation, detection of small changes in the 1.5nW minimum

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