Biomedical Engineering Reference
In-Depth Information
In order to avoid detection of laser FM noise and frequency drift, the length
difference must be minimized. A practical limit is imposed by fiber han-
dling and dressing procedures. Experience shows that attempting to match
lengths closer than 1 or 2 mm is unproductive and unnecessary. Use of bal-
anced differential detectors minimizes common mode effects caused by
laser amplitude fluctuations.
Referencing the resolution measurements to a 2 kHz signal frequency is
one approach to avoid some of the effects of 1/
f
noise present in the laser
and electronics. The anticipated signal frequencies are much less than 2 kHz.
The simple homodyne detection scheme shown in Figure 4.25 is subject to
1/
f
noise; hence, the sensitivity will be diminished by noise. We will show
that more than adequate sensitivity can be achieved. Some additional areas
where component improvements will benefit system performance are as
follows:
• Source diode
• Stabilized diodes
• Reduction of 1/
f
• Long life
• Integrated optics
• Low cross talk
• Low loss
• High efficiency
• Polarizers
• High extinction ratio
• Availability of single polarization fiber
• Couplers
• Extra low loss
• High birefringence
• Modulators
• Frequency shifter
• High frequency, high efficiency
• Fibers
• Low loss, high birefringence
• Low loss polarizing
4.18.3 Feasibility Analysis
To determine feasibility, we begin with the noise-limiting value of 1 μrad
and degrade it by the difference in 1/
f
noise between 2 kHz and 1 Hz signal
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