Optical Diagnostics and Imaging - Applied Optics Fundamentals and Device Applications - page 293

6
Optical Dia gnosticsandImaging
6.1 OpticalCharacterization
A variety of specialized test and characterization techniques are employed
in the evaluation of optical circuits. Correlation of the information obtained
from each characterization provides a complete analysis of the electronic
and optical properties of the optical circuit. These are discussed in the fol-
lowing sections.
Figure 6.1 illustrates the experimental setup most commonly used in end-
fire coupled loss measurements. Direct end-fire coupling is achieved by
focusing laser light into and out of the waveguide with an appropriate lens
system. Systems designed for use with solid-state laser diodes eliminate the
elliptical beam pattern and astigmation common in these types of lasers and
provide micron spot sizes. Mode selective, nondestructive loss measurements
can be performed using the prism coupling technique shown in Figure 6.2.
Here, light is evanescently coupled from the prism into a waveguide mode at
a specific coupling angle determined by the prism geometry and index and
the mode order.
Coupling into multiple closely spaced waveguides on a single chip is also
often required. This is most easily accomplished by butt-coupling optical
fibers to the input waveguide. A chuck is fabricated from a suitable semi-
conductor material by etching grooves in the material to hold the fibers. Due
to precision of the photolithographic techniques used to etch the grooves
and precision of standard optical fiber dimensions, this method provides a
highly accurate means of aligning the fibers and waveguides.
The output image can be analyzed by a variety of methods. The method
illustrated in Figure 6.2 is to sweep the focused image across a detector-slit
combination by use of a scanning mirror. The detector output is displayed
on an oscilloscope, with its sweep synchronized to the mirror scan. This
approach is often modified to remove the sweeping mirror and detector-slit
combination by using a precision stepping motor. The detector output and
stepping motor position can drive an X-Y plotter to produce a plot similar to
the oscilloscope display of the previous setup.
An alternative to these techniques is the use of a two-dimensional (2-D)
self-scanning pyroelectric detector array. This device, when coupled with an
oscilloscope and computer, permits storage and output of mode profile and