174 Fiber Optic Essentials
A mask, or “photomask” is a square glass plate with a patterned emul-
sion of metal film on one side. This transfers the pattern to the photoresist.
The mask can be placed in physical contact with the wafer, in close
proximity to the wafer, or the mask can be avoided by using a projection
printing technique, where the UV scanned across the surface is an image.
Projection printers that step the mask image over the wafer surface are
called step-and-repeat systems, and can attain 1 micron resolutions. The
next step is development. The photoresist is dissolved where appropri-
ate. Mesas are etched in the device. The final step is hard-baking. This
hardens the photoresist and improves the adhesion of the photoresist to
the wafer surface.
The above description of photolithography in some ways simplifies the
multi-stage process a laser undergoes in production. In reference  a full
description of the fabrication of a typical VCSEL is described, including
multiple steps of photolithography – on the p-side photolithography is
first used to define the initial mesa diameter, and later to define the
VCSEL emission aperture on the mesa, while gold interconnect pads
are also deposited by a photoresist liftoff process. The n-side also had
Ni/AuGE/Au metal contacts deposited.
Several devices are usually formed on a single wafer. This wafer is
either cleaved, which is split with a sharp instrument along the natural
division in the crystal lattice, or sawed, to separate the individual chips.
Edge-emitting lasers are often cleaved, VCSELs are often sawed .
8.4 Measurement Equipment
In this section, we describe some of the important optical measurement
devices which may be used both in the laboratory and in commercial fiber
installations to characterize an optical fiber link. These tools may be used
when planning the installation of a new link, to determine whether the
desired capacity can be achieved. They are also useful in fault isolation
and troubleshooting of failures in optical links.
8.4.1 FIBER OPTIC POWER METER
The fiber optic power meter measures how much light is coming out of
a fiber optic cable; it can be used to determine the amount of light being
generated by an optical source, or the amount of light being coupled into
an optical receiver. Optical power is usually measured in dBm, or decibels
referenced to 1 mW. These devices measure the average optical power,