Biomedical Engineering Reference
In-Depth Information
FIGURE 6.3
Representative output from ISC mode profile
and loss determination setup.
away so that coupling is not induced), measuring
any radiation scattered out of the waveguide [1].
The use of the fiber probe incorporated in a micro-
scope allows the distance between the fibers and
the waveguide surface to be kept constant, thereby
improving the accuracy of this technique. The use
of relatively high power AlGaAs lasers, which are
now commercially available, makes this technique
quite viable, as more power input results in more
scattered power.
Several other techniques exist for measur-
ing waveguide propagation loss. A generaliza-
tion of the fiber probe measurement is to use a
video camera to image scattered radiation along
the propagation path; then, by employing digital
image processing, the propagation loss can be
calculated [2]. To use this technique with AlGaAs
waveguides, an infrared video camera is often
used. Another possibility to measure waveguide propagation loss is the
use of Fabry-Perot techniques [3]. Here the optical length of the wave-
guide or the wavelength of the source is varied over a small range with
the output intensity being monitored. The optical length can be changed
by slightly heating or cooling the sample, while varying the wavelength
of the laser source over a small range by inducing the appropriate varia-
tions in the power supply current. Radiation losses are measured in bends
in a manner similar to the absorptions loss; however, by calibrating and
subtracting the other two loss mechanisms, the loss due to radiation alone
can be evaluated.
Fiber probe
Detector
Fiber
- Out of plane scattered
light proportional to loss
FIGURE 6.4
Fiber probe loss measure-
ment technique.
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