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PH221 MODERN OPTICS

ERBIUM DOPED FIBRES

AND THEIR APPLICATIONS

ABHISHEK PANCHAL

SC11B157

BTECH PHYSICAL SCIENCES



1. Introduction

1.1 Erbium

Erbium is a chemical element in the lanthanide series, with the symbol Er and atomic

number 68. It is a rare earth element. A trivalent element, pure erbium metal is malleable

(or easily shaped), soft yet stable in air, and does not oxidize as quickly as some other rare-

earth metals. Its salts are rose-colored, and the element has characteristic sharp absorption

spectra bands in visible light, ultraviolet, and near infrared. Otherwise it looks much like the

other rare earths. Its sesquioxide is called erbia. Erbium's properties are to a degree dictated

by the kind and amount of impurities present.

1.1 Optical Amplifier

Optical amplifiers are a key enabling technology for optical communication

networks. Together with wavelength-division multiplexing (WDM) technology, which allows

the transmission of multiple channels over the same fiber, optical amplifiers have made it

possible to transmit many terabits of data over distances from a few hundred kilometres

and up to transoceanic distances, providing the data capacity required for current and

future communication networks.

A basic optical communication link comprises a transmitter and receiver, with anoptical fiber cable connecting them. Signals propagating in optical fiber suffer far less

attenuation than in other mediums, such as copper, there is a limit of about 100km because

after that signal gets too noisy to be detected. Before, it was necessary to electronically

regenerate the optical signals every 80-100 km in order to achieve transmission over long

distances. This meant receiving the optical signal, cleaning and amplifying it electronically,

and then retransmitting it over the next segment of the communication link.

While this is done easily when transmitting a single low capacity optical channel, it

quickly becomes unfeasible when transmitting tens of high capacity WDM channels,

resulting in a highly expensive, power hungry and bulky regenerator station. Furthermore,the regeneration hardware depends on the number of channels, as well as the bit-rate,

protocol, and modulation format of each individual channel, so that any upgrade to the link

would automatically require upgrades to the regenerator stations.

In contrast, an ideal optical amplifier is designed to directly amplifier any input

optical signal, without needing to transform it first to an electronic signal. It can amplify all

WDM channels together, and is generally transparent to the number of channels, their bit-

rate, protocol, and modulation format. Thus, a single optical amplifier can replace all the

multiple components required for an electronic regeneration station. Furthermore, the

transparency of the optical amplifier means that the link can be upgraded without the need

to replace the amplifier



2. Erbium Doped Fibre Amplifiers

2.1 Introduction

Erbium-doped fiber amplifiers are the by far most important fiber amplifiers in the

context of long-range optical fiber communications; they can efficiently amplify light in the

1.5-μm wavelength region, where telecom fibers have their loss minimum.

2.2 Principle:

Erbium Doped Fibre Amplifier consist of optical fibre of which the core has been

doped erbium ions. The electrons of the erbium doped fibre can be excited to higher energy

levels by pumping with shorter wavelength light. The amplification will take place in boththe C band (1530 nm – 1560 nm) as well as the L band (1570 nm – 1560 nm), which makes

erbium an excellent choice for an amplifier. Pump wavelengths of 980 nm or 1480 nm are

used to excite erbium’s quantum levels, as this will not lead to great losses in the optical

fibre. The pump supplies higher energy to electrons in an active medium raising them to a

higher level as shown in figure 3.1



Figure 3.1 Erbium energy levels and stimulated emmisions

2.3 Setup

A typical setup of a simple erbium-doped fiber amplifier (EDFA) is shown in Figure3.2. Its core is the erbium-doped optical fiber, which is typically a single-mode fiber. In the

shown case, the active fiber is “pumped” with light from two laser diodes (bidirectional

pumping), although unidirectional pumping in the forward or backward direction (co-

directional and counter-directional pumping) is also very common. The pump light, which

most often has a wavelength around 980 nm and sometimes around 1450 nm, excites the

erbium ions (Er3+) into the 4I13/2 state (in the case of 980-nm pumping via 4I11/2), from

where they can amplify light in the 1.5-μm wavelength region via stimulated emission back

to the ground-state manifold 4I15/2.

2.3 Erbium Doped Fiber Amplifier

Figure 2.2: Schematic setup of a simple erbium-doped fiber amplifier. Two laser diodes (LDs)

provide the pump power for the erbium-doped fiber. The pump light is injected via dichroic

fiber couplers. Pig-tailed optical isolators reduce the sensitivity of the device to back-

reflections.

Figure 2.2



The setup shown also contains two “pig-tailed” (fiber-coupled) optical isolators. The

isolator at the input prevents light originating from amplified spontaneous emission from

disturbing any previous stages, whereas that at the output suppresses lasing (or possibly

even destruction) if output light is reflected back to the amplifier. Without isolators, fiber

amplifiers can be sensitive to back-reflections.

Apart from optical isolators, various other components can be contained in a

commercial fiber amplifier. For example, there can be fiber couplers and photo detectors for

monitoring optical power levels, pump laser diodes with control electronics and gain-

flattening filters. For particularly compact packages, various passive optical components can

be combined into a photonic integrated circuit (planar lightwave circuit).

Very high signal gains, as used, e.g., for the amplification of ultrashort pulses to high

energies, are usually realized with amplifier chains, consisting of several amplifier stages

with additional optical elements (e.g. isolators, filters, or modulators) in between.

Figure 2.3 Erbium excitation

2.4 Gain Spectrum

The shape of the erbium gain spectrum depends on the absorption and emission

cross sections, which depend on the host glass. Also, the spectral shape of the gain and not

only its magnitude is substantially influenced by the average degree of excitation of the

erbium ions, because these have a quasi-three-level transition. Figure 2 shows data for a

common type of glass, which is some variant of silica with additional dopants e.g. to avoid

clustering of erbium ions. Other glass compositions can lead to substantially different gain

spectra.

2.5 Noise

The principal source of noise in DFAs is Amplified Spontaneous Emission (ASE), which

has a spectrum approximately the same as the gain spectrum of the amplifier. Noise figure

in an ideal DFA is 3 dB, while practical amplifiers can have noise figure as large as 6 –8 dB.

As well as decaying via stimulated emission, electrons in the upper energy level can

also decay by spontaneous emission, which occurs at random, depending upon the glass

structure and inversion level. Photons are emitted spontaneously in all directions, but aproportion of those will be emitted in a direction that falls within the numerical aperture of



the fiber and are thus captured and guided by the fiber. Those photons captured may then

interact with other dopant ions, and are thus amplified by stimulated emission. The initial

spontaneous emission is therefore amplified in the same manner as the signals, hence the

term Amplified Spontaneous Emission. ASE is emitted by the amplifier in both the forward

and reverse directions, but only the forward ASE is a direct concern to system performance

since that noise will co-propagate with the signal to the receiver where it degrades system

performance. Counter-propagating ASE can, however, lead to degradation of the amplifier's

performance since the ASE can deplete the inversion level and thereby reduce the gain of

the amplifier.

2.6 Erbium Gain

Figure 2.3: Gain and absorption (negative gain) of erbium (Er3+) ions in a phosphate glass

for excitation levels from 0 to 100% in steps of 20%.

Figure 3.4

Strong three-level behavior (with transparency reached only for > 50% excitation)

occurs at 1535 nm. In that spectral region, the unpumped fiber exhibits substantial losses,

but the high emission cross section allows for a high gain for strong excitation. At longer

wavelengths (e.g. 1580 nm), a lower excitation level is required for obtaining gain, but the

maximum gain is smaller.

The maximum gain typically occurs in the wavelength region around 1530 –1560 nm,with the 1530-nm peak being most pronounced for high excitation levels, whereas low

excitation levels lead to gain maxima at longer wavelengths. The local excitation level

depends on the emission and absorption cross sections and on the pump and signal

intensity (apart from that of ASE light). The average excitation level over the whole fiber

length, as is relevant for the net gain spectrum, depends on the pump and signal powers,

but also on the fiber length and the erbium concentration. Such parameters (together with

the choice of glass composition) are used to optimize EDFAs for a particular wavelength

region, such as the telecom C or L band.

A good flatness of the gain in a wide wavelength region (→ gain equalization), as

required e.g. for wavelength division multiplexing (see below), can be obtained by using



optimized glass hosts (e.g. telluride or fluoride fibers, or some combination of amplifier

sections with different glasses) or by combination with appropriate optical filters, such as

long-period fiber Bragg gratings.

2.6 Erbium doped Amplifiers in Telecom Systems

EDFAs can serve various functions in systems for optical fiber communications; the most

important applications are the following:

1. The power of a data transmitter may be boosted with a high-power EDFA before

entering a long fiber span, or a device with large losses, such as a fiber-optic splitter.

Such splitters are widely used e.g. in cable-TV systems, where a single transmitter is

used to deliver signals into many fibers.

2. A fiber amplifier may also be used in front of a data receiver, if the arriving signal is

weak. Despite the introduction of amplifier noise, this can improve the signal-to-noise ratio and thus the possible data transmission rate, since the amplifier noise

may be weaker than the input noise of the receiver. It is more common, however, to

use avalanche photodiodes, which have some built-in signal amplification.

3. In-line EDFAs are used between long spans of passive transmission fiber. Using

multiple amplifiers in a long fiber-optic link has the advantage that large

transmission losses can be compensated without (a) letting the optical power drop

to too low levels, which would spoil the signal-to-noise ratio, and (b) without

transmitting excessive optical powers at other locations, which would cause

detrimental nonlinear effects due to the unavoidable fiber nonlinearities. Many of

these in-line EDFAs are operated even under difficult conditions, e.g. on the ocean

floor, where maintenance would be hardly possible.

4. Although data transmitters are normally not based on erbium-doped devices, EDFAs

are often part of equipment for testing transmission hardware. They are also used in

the context of optical signal processing.

These functions can be realized in the telecom C and L bands. Other types of fiber

amplifiers, e.g. based on praseodymium, have been considered for other bands, but none

can compete with erbium-based devices in terms of gain and gain efficiency.



3. Why Erbium Doped Fibres?

A particular attraction of EDFAs is their large gain bandwidth, which is typically tens of nanometers and thus actually more than enough to amplify data channels with the highest

data rates without introducing any effects of gain narrowing. A single EDFA may be used for

simultaneously amplifying many data channels at different wavelengths within the gain

region; this technique is called wavelength division multiplexing. Before such fiber amplifiers

were available, there was no practical method for amplifying all channels e.g. between long

fiber spans of a fiber-optic link: one had to separate all data channels, detect and amplify

them electronically, optically resubmit and again combine them. The introduction of fiber

amplifiers thus brought an enormous reduction in the complexity, along with a

corresponding increase in reliability. Very long lifetimes are possible by using redundant

down-rated pump diodes.

The only competitors to erbium-doped fiber amplifiers in the 1.5-μm region are Raman

amplifiers, which profit from the development of higher power pump lasers. Raman

amplification can also be done in the transmission fiber. Nevertheless, EDFAs remain very

dominant.

3.1 Technical Details

The most common pump wavelength for EDFAs is around 980 nm. Light at this

wavelength pumps erbium ions from their ground-state manifold 4I15/2 to the 4I11/2

manifold, from where there is a quick non-radiative transfer to the upper laser level 4I13/2.

Due to that quick transfer, there is essentially no de-excitation via stimulated emission by

pump light, and very high excitation levels can be achieved. Therefore, this approach makes

it possible to achieve the highest gain efficiency (order of 10 dB/mW) and the lowest noise

figure, although the power efficiency is not ideal due to the significant quantum defect.

A higher power efficiency can be achieved by in-band pumping around 1450 nm.

However, stimulated emission by pump light then limits the achievable excitation level,

hence also the gain per unit length, and the maximum gain occurs at longer wavelengths.

The noise figure will also be higher.

Due to the not very high laser cross sections, the saturation power of an EDFA is

fairly high compared with that of a semiconductor optical amplifier. Therefore, single

symbols in high bit rate data transmissions have a much too low energy to cause any

significant gain saturation. Only over thousands or millions of symbols, the gain adjusts itself

to the average signal power level.

In high-gain amplifiers, amplified spontaneous emission (ASE) is often a factor limiting the

achievable gain. Due to the quasi-three-level nature of the erbium ions, ASE powers can be

different between forward and backward direction, and the maximum ASE can occur at a

wavelength which differs from that of maximum gain.



The noise figure of an EDFA is slightly larger than the theoretical limit of 3 dB for a

high-gain amplifier; this is mainly due to the quasi-three-level nature. Relatively low-noise

performance can be achieved with suitable amplifier design, taking into account particularly

the erbium excitation level near the signal input end, which can be strongly influenced by,

e.g., the pump direction.

Various aspects of erbium-doped amplifiers can be analyzed with suitable fiber

simulation software. The resulting quantitative understanding can be the basis for

optimization of devices in terms of performance and required components.

− Variants and Other Applications of Erbium-doped Fiber Amplifiers

3.2 Ytterbium can also be used

A high gain in a shorter length can be achieved with ytterbium-sensitized fibers (also

called Er:Yb:glass fibers or ytterbium-codoped fibers). In addition to the erbium dopant,these contain some significant concentration of Yb3+ ions (typically much more ytterbium

than erbium). Ytterbium ions may then be excited e.g. with 980-nm pump light (or even at

longer wavelengths such as 1064 nm) and transfer their energy to erbium ions. For a proper

choice of the material composition of the fiber core, this energy transfer can be fairly

efficient. However, the use of pure erbium-doped fibers is more common in the telecom

area, because ytterbium sensitization has no essential advantages here and possibly leads to

a reduced gain bandwidth due to the modified chemical composition.

Erbium-doped double-clad fibers can be used for generating very high output

powers of tens of watts or even more. As the pump absorption efficiency can be weak inthis case, an ytterbium-sensitized core may again be useful.

It is also possible to amplify ultrashort pulses in the 1.5-μm region to relatively high

energies, using EDFAs in the form of amplifier chains. One exploits the relatively high

saturation energy of such amplifiers, particularly when using erbium-doped large mode area

fibers.



4. L-band Erbium-doped fiber laser

4.1 Introduction

Owing to the insufficient channel capacity of the dense-wavelength-division-

multiplexed (DWDM) systems at C-band, the long-wavelength band (L-band) Erbium-doped

fiber amplifier (EDFA) covering a wavelength range from 1570 nm to 1610 nm1 have been

comprehensively investigated to enlarge the transmission capacity. Versatile economic and

efficient schemes of the wavelength-tuneable Erbium-doped fiber lasers (EDFLs) operated at

L-band have also emerged to meet the demand on testing L-band fiber-optic devices.

Typically, the gain media such as the dense erbium-doped fiber, the erbium-

ytterbium codoped double clad fiber2, and the brillouin-erbium fiber3, etc., were employed

to configure the L-band EDFLs in dual resonant cavity4, linear overlapping cavity5, and single

ring cavity6.Different approaches for wavelength-tuning the EDFL with the intra-cavity Fabry –

Perot filters and the fiber Bragg gratings (FBGs) were proposed. In particular, the

wavelength tunability of L-band EDFL via a cavity-loss control was demonstrated by opto-

machanically bending the single-mode fiber in the EDFL cavity9. In this work, we present a

coupling-ratio controlled scheme to demonstrate a full L-band EDFL with a wavelength-

tuning range over 45 nm and a low variation on the output powers of different channels. By

using a highly doped EDF with an optimized length, the ultra-high quantum efficiency and

power conversion rate of the EDFL can be approached under optimized bi-directional

pumping scheme, which shows an improvement on conversion efficiency of more than 10%

than those reported using conventional L-band EDFA configurations.

4.2 Setup

The setup of the coupling-ratio controlled wavelength-tunable EDFL is shown in Fig. 5.1.

It consists of an L-band EDFA with a bi-directionally 980nm/1480nm pumping scheme. The

specific EDF (Fibercore L-band fiber, DF1500L) exhibits a large absorption of 17.5 dB/m at

1531nm. Under an optimized operation, both a forward pumping of 17.5 mW at 980 nm and

a backward pumping of 200 mW at 1480 nm are employed.

Fig. 4.1 A coupling-ratio controlled wavelength tunable L-band EDFL with a tunable-ratio

optical coupler (TROC).

Figure 4.1



This EDFA further takes the advantage on using a reduced length of the specially

designed L-band fiber with high Erbium (Er3+) concentration, which offers an ultra-wide

amplified spontaneous emission spectrum ranged between 1538 nm and 1628 nm (see Fig.

2) with a comparable gain and a suppressed noise power. Versatile pumping schemes have

been investigated to construct a low-noise and high-gain L-band EDFA as the gain medium.

The forward 980nm and backward 1480nm cascaded pumping geometry is selected for

pumping the 45m-long EDF via the 980nm/1550nm and 1480nm/1550nm WDM couplers,

respectively, in which the forward pumping at 980nm is effective for improving the noise

characteristics, and the backward pumping at 1480nm benefits from a better quantum

conversion efficiency and gain coefficient11, 12. With such a simplified EDFA, an extremely

high power conversion efficiency (PCE) of 37% associated with a small-signal gain of 34.8 dB

and a wavelength dependent gain deviation of 6 dB is achieved.

To configure the EDFL, two optical isolators are used to ensure the unidirectional

propagation of the light, preventing the spatial hole burning effect in the EDFA from

bidirectional operation and allowing a stable single-frequency operation simultaneously. In

particular, a 1×2 tuneable-ratio optical coupler (TROC) is inserted into the EDFL ring-cavity,in which the coupling ratio can be detuned from 0.5% to 99.5%. The maximum output

power is obtained at an output ratio of 90%, whereas the longest output wavelength has to

be achieved at a coupling ratio of 10 %.

4.3 Results and discussion

The amplified spontaneous emission of the EDFA has revealed a wide spectral

response covering the C- and L-bands, as shown in Fig. 2. In principle, the wavelength

tuneablility of an EDFL can be greatly affected by several cavity parameters such as the

intra-cavity loss, the output coupling ratio (or reflectivity), and the active fiber length14-19.A deeply saturated

EDFA under high-power feedback injection has shown the capability to offer wide-

band and flat gain20, and the L-band EDFL can be readily available by minimizing the intra-

cavity loss to keep largest gain in the EDFL cavity. The total intra-cavity loss has previously

been understood as the most important factor that affects both the output power and the

wavelength tuning range21. The tuning bandwidth increases significantly as the intra-cavity

loss is reduced, whereas the output power of the EDFL shows an opposite trend. That is, the

wavelength tuning range can be broadened by reducing the output coupling ratio.

Previously, a numerical modeling for the C+L-band EDFL at different intra-cavity losses also

supports this statement11, the numerical results on the wavelength dependent outputpower for an EDFL at different intra-cavity losses has confirmed that the reducing cavity loss

can effectively extend the wavelength tuning rage of the EDFL.

4.4 Conclusion

It is demonstrated that an output-coupling-ratio controlled, full long wavelength

band erbium-doped fiber ring laser by using a bi-directionally dual-wavelength pumped

EDFA in close-loop with an output coupler of tuneable coupling ratio. The L-band EDFL is

wavelength tuneable from 1567 nm to 1612 nm at a maximum quantum efficiency of 42%,

respectively with ultra-high power conversion efficiency of 37%, comparable gain of 34 dB,

and maximum output power of up to 91mW. The minimum wavelength tuning resolution of 0.3 nm is achieved under the maximum wavelength tuning range of up to 45 nm covering



whole L-band, while a low channel power variation of <1.2dB and a stable output with

0.04% power fluctuation is reported.



References

1.

en.wikipedia.org2. www.rp-photonics.com/erbium_doped_fiber_amplifiers.html

3. citeseerx.ist.psu.edu

4. www.lightwavestore.com

5. www.ieee.org

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