Fiber Optic Essentials

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 [6] 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 [6].

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,