Page 224 - Photonics Essentials an introduction with experiments

Optical Fibers and Optical Fiber Amplifiers

218 Advanced Topics

der of 50 dB), passage though several amplifier stages is possible be-
fore signal regeneration is required.
An optical amplifier can also be made out of a semiconductor laser
simply by removing the mirrors on each end. This is achieved by put-
ting on antireflection coatings and tilting the propagation path rela-
tive to the coated laser facets. A semiconductor optical amplifier 100
microns in length can achieve the same gain as an erbium-doped opti-
cal fiber that is several meters long. Furthermore, the SOA is electri-
cally pumped by current, so no additional pump lasers are needed.
The usable gain spectrum at 1550 nm is typically larger—about 50
nm instead of 30 nm. The gain spectrum can be tuned at will by
changing the material composition of the SOA. With all these addi-
tional advantages, semiconductor lasers have not replaced erbium-
doped fiber amplifiers. This is not due to oversight.
There are three important differences between these two kinds of
optical amplifiers (Fig. 9.16). Both of the differences lead to a higher
contribution to the noise of the SOA compared to the Er-doped fiber
amplifier.
The first important factor has to do with coupling loss between the
semiconductor optical amplifier and an optical fiber. The mode diame-
ter of the optical fiber is about 9 microns. The mode diameter of a
semiconductor laser is much smaller, about 1 micron. The mode mis-
match can be appreciated by comparing the ratio of the area of each
mode: ~ 80 to 1. This mismatch in size leads to coupling losses going
from the fiber to the SOA and from the SOA to the fiber. Special mode
adapters are used to reduce the mode–size mismatch , and antireflec-
tion coatings are used to eliminate Fresnel reflection losses between

Figure 9.16. An erbium-doped fiber amplifier (on the left) and a semiconductor optical
amplifier on the right. Both components shown here are made by the same company.
The semiconductor optical amplifier is much more compact. (© JDS Uniphase, repro-
duced by permission.)

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.