Page 187 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Stability and Tempco Issues

180 Chapter Eight

Clamped
fiber

Steel
Plastic mandrel
supports

Monitor
photodiode
Glass
waveguide N
Magnet

Figure 8.15 Light can be tapped from a fiber using a small-radius bend.
A magnet and steel mandrel make a convenient clamp, and even allow
the light to be coupled to an embedded photodiode.

fiber with a known radius of curvature and to press it against a glass baseplate.
If a primary-coated fiber is used, light couples out of the coating and into the
glass slide at the point of contact. The glass plate acts as a simple light-guide
coupling to a small photodiode such as a BPW34. The steel mandrel can be held
in place using a small magnet fixed underneath the glass.
Schemes for optical referencing in these ways provide some of the highest
resolution measurement techniques available. The papers by Allen (1995) and
Imasaka (1983) give good examples of what is possible in chemical photometry.

8.5.3 Intensity noise reduction
Dual-detector referencing has another very important use—source noise reduc-
tion. In systems that have high received optical powers, which should easily be
shot-noise limited in detection, the measured noise level is often found to be far
greater than expected from the shot calculation. This is because the source itself
exhibits excess intensity noise. Sometimes this is due to poor regulation of
the power supplies feeding the source. This can usually be seen with a low-
frequency spectrum analyzer. Even with good supplies, laser systems in partic-
ular are prone to excess noise, usually exacerbated by spurious optical feedback
into the laser cavity from lenses, windows, fibers etc. Even focussing the laser
beam with a microscope objective onto a cleaved fiber end-face for coupling
forms an efficient retroreflector. The noise level is often 30 to 60dB worse than
shot noise. Just as we can suppress slow intensity drifts caused by source
temperature fluctuations, so we can use the configurations such as seen in
Fig. 8.10 to reduce the effects of fast excess intensity noise. For example, we
can divide signal by reference in an analog divider IC as previously mentioned
(Fig. 8.16a). Alternatively, if the two signals are adjusted to be nominally equal,
they can be subtracted to achieve the same compensation (Fig. 8.16b). This is
capable of lower noise and higher bandwidth.

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