Page 146 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Useful Electronic Circuits and Construction Techniques to Get You Going

Useful Electronic Circuits and Construction Techniques to Get You Going 139

an LED source from the receiver, with just a small hole to couple the two func-
tions. All internal surfaces and the dividing wall should be spray-painted matte
black. The lid and box are usually fairly well sealed by the die molding used. If
you find that any light gets in, glue some angle-aluminum around the edge of
the lid/base to shield the join. The use of the lid makes mounting of circuits
under test more convenient, and the internal dividing wall can be butchered
and replaced to suit the circuits. The good suppression of fluorescent room lights
and electrical noise eases measurements of the fundamental noise performance
of the receivers. If you have problems with power-supply-borne interference,
there is room inside for batteries.

6.11 Butchering LEDs
It is often helpful to remove the molded lens from “water-clear” visible LEDs
to allow external optics to be used or just to allow an optical fiber to be placed
closer to the emitting chip. The construction of these LED packages is usually
a metal header with a depression in it, in which the LED is bonded, used as
the substrate contact. A gold wire bond makes the other connection, coupled to
the other lead. The form of the wire bond varies considerably between differ-
ent devices; some are in the form of large loops, while others are almost hori-
zontal from chip to lead. You can easily see the internal construction using a
small loupe.
Most of the lens needs to be ground away with a file or coarse grinding paper.
You can grip the LED between your fingers or in a pair of pliers, but it is prefer-
able to make a simple jig. One approach is to take a small sheet of plastic or
aluminum, 6mm thick and about the size of a PC mouse. Drill a hole to take
the LED in the center of one edge. Try to get it tight, so that the LED pushes
in, with the chip just showing out the other side. Alternatively, make a screwed
clamp to press the LED into an undersized hole. Glue spacers such as two
12-mm washers or scribed and broken pieces of microscope slide glass to the
other edge (Fig. 6.13). The idea is to have a three-point mounting, with the LED
as one point.
With the LED firmly fixed remove most of the lens with a file, then go to fine
waterproof grinding paper (800 to 1000 mesh is good). A piece of float glass
makes a flat support. Grind the three points with a circular or figure-of-eight
motion until the LED surface looks uniformly matte. It is worth doing this with
the LED powered up, just to make sure you haven’t gone through the wire-
bond! Try to get to within 250mm of the bond, checking frequently under the
loupe or stereo microscope. Then clean it all off carefully under a running tap
with tissue paper or cotton-wool sticks.
Polish it on something finer. A tiny dot of 6mm diamond paste on a piece of
white bond paper works well, or a creamy slurry of alumina or cerium oxide
powder on a cotton cloth, or even toothpaste will work. A minute of movement
on the paper will bring the LED surface to a glass-like finish, free from
scratches. Just wash off the paste with water or isopropanol.

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