The Optical Communications Reference

network in Japan in the early 1990s [11]. This trend

persisted for a while before EDFAs emerged on the

scene and led to a dramatic change in technology trends.

3.1.3 Emergence of fiber

amplifiers and DWDM systems

3.1.3.1 EDFAs

In the late 1980s typical state-of-the-art, repeater-less,

transmission distances were about 40–50 km at the 560

Mb/s transmission rate. Because maximum launched

optical power was below 100 mW, it was difficult to im-

prove system lengths beyond this specification, and the

use of electronic repeate rs became inevitable. At a re-

peater, the so-called 3R-regeneration functions (ream-

plification, retiming, and reshaping) are performed in the

electric domain on the incoming attenuated as well as

distorted (due to dispersion) signals after detection by

a photode-tector and before the revamped signal is fed to

a laser diode drive circuit, from which these cleaned

optical pulses are launched to the next section of the

fiber link. However, these complex functions are ex-

pensive and unit replacement is required when network

capacity is to be upgraded to higher bit transmission rates

because electronic components are bit rate sensitive.

Because these units are required to convert photons to

electrons and back to photons, often at modulation rates

approaching the limits of the then available electronic

switching technology, a bottleneck was encountered in

the late 1980s. What was needed was an optical amplifier

to bypass this electronic bottleneck. In 1986, the research

group at Southampton University in England reported

success in incorporating rare earth trivalent erbium ions

into host silica glass during fiber fabrication [22]. Erbium

is a well-known lasing species characterized by strong

fluorescence at 1550 nm. Subsequently, the same group

demonstrated that excellent noise and gain performance

is feasible in a large part of the 1550-nm window with

erbium-doped standard silica fibers [5].

The concept of optical amplification in fiber is almost

as old as the laser itself. Today, EDFAs seem like an

outstanding breakthrough, but they are really an old

idea. In 1964, Koester and Snitzer [23] demonstrated

a gain of 40 dB at 1.06 mm in a 1-meter-long Nd-doped

fiber side pumped with flash lamps. The motivation at

that time was to find optical sources for communica-

tion, but the impressive development of semiconductor

lasers that took place in subsequent years pushed fiber

lasers to the background. The operation of an EDFA is

very straightforward [24]. The electrons in the 4f shell

of the erbium ions are excited to higher energy states by

absorption of energy from a pump. Absorption bands

most suitable as pumps for obtaining amplification of

1550-nm signals are the 980- and 1480-nm wavelengths

(Fig. 3.1.9a); Fig. 3.1.9b shows a schematic of an EDFA.

When pumped at either of these wavelengths, an

erbium-doped fiber was found to amplify signals over

a band of almost 30–35 nm at the 1550-nm wavelength

region (see Fig. 8.9 in Chapter 8).

Typical pump powers required for operating an EDFA

as an amplifier range from 20 to 100 mW. Absorption of

pump energy by the erbium ions leads to population in-

version of these ions from the ground state (4I

15/2

)to

either 4I

11/2

(980 nm) or 4I

13/2

(1480 nm) excited

states: the 4I

13/2

level effectively acts as a storage of

pump power from which the incoming weak signals may

stimulate emission and experience amplification [13].

Stimulated events are extremely fast, and hence the

amplified signal slavishly follows the amplitude modula-

tion of the input signal . EDFAs are accordingly bit rate

transparent. EDFAs are new tools that system plan ners

now almost routinely use for designing networks. EDFAs

can be incorporated in a fiber link with an insertion loss of

w0.1 dB, and almost full population inversion is

achievable at the 980-nm pump band. Practical EDFAs

with an output power of around 100 mW (20 dBm),

a 30-dB small signal gain, and a noise figure of <5 dB are

now available commercially. In a 10-Gb/s transmission

experiment, a record receiver sensitivity of 102 photons

per bit was attained in a two-stage composite EDFA

having a noise figure of 3.1 dB [25].

Rapid

non-radiative

transitions

980 nm 1480 nm 1520 ~ 1570 nm

EDFA

Weak WDM signals

@ 1550 nm band

123

123

Amplified WDM

signals

Fiber

Fiber

32

≈ 1 s

λ λ λ

λ λ λ

τ

τ

sp

≈ 10 ms

4I

15/2

4I

13/2

4I

11/2

(a)

(b)

Fig. 3.1.9 (a) Energy level diagram of an Er

þ3

ion. (b) Schematic

layout of an EDFA (for details see Chapter 8).

95

Optical fibers for broadband communication CHAPTER 3.1