edfa chapman

Erbium-doped fibre amplifiers:

the latest revolution in optical communications

by D.A. Chapman

Like many technological advances associated with optical fibre communication, erbium-doped fibre amplifiers have moved

from the laboratory to the field with extraordinary speed. They are capable of low-noise, low-distortion,

high-gain and high-power operation over a broad bandwidth. This paper describes the physical principles behind their

operation and shows how their unique characteristics are set t o bring about a qualitative change in the way in which optical

fibre is used.

1 Background

Despite the remarkable changes in telecommunications already brought about by optical fibre, current uses only scratch the surface of the possibilities and it has long been recognised that the full capabilities of single-mode fibre will only be realised if the signal can be kept optical throughout an end-to-end link.’ The main ‘electrical bottlenecks’ which need to be eliminated are the electrical switching and the electrical regeneration. Erbium-doped fibre amplifiers remove the need for regenerators by providing wideband, low-distortion, optical amplification

derives from the long distances and high data rates achievable from this deceptively simple scheme. Long distances are achievable because optical fibre can now be manufactured with very low attenuation (say 0.3 dB/km, so that it takes 10 km for the power to be halved) and because the attenuation is independent of the signalling rate. The high data rates are achievable not only because the attenuation is independent of signalling rate, but also because systems can be constructed to give very little dispersion. Consequently, virtually the only degradation to a signal while propagating along the fibre is the uniform attenuation, and there is an incentive to develop an optical

with gains of over 20 dB. In the long te rn , they could also contribute to the realisation of practical optical switching, either merely by compensating for loss in switching devices, or by providing an active

amplifier to compensate for the attenuation.

component in the switch element itself.2 Conventionally, optical fibre is used

as shown in Fig. 1. A light source (semiconductor laser diode) is modulated, usually with essentially on- off digital pulsing, the light is coupled into the fibre and propagates to the receiver. At the receiver is a semiconductor photodiode which provides a current output proportional

+ curmnt

t

to the incident optical power. The success of optical fibre communication

I Fig. 1 Conventional arrangement for optical-fibre communications

ELECTRONICS & COMMUNICATION ENGINEERING JOURNAL APRIL 1994 59

similarity between the doped fibre and the transmission medium with which it is being

erbium doped fibre (about 50 m) a o:oznal

input signal wavelength (1550 nm) mEci;i:er

splice

zt pump light source (980 or 1460 nm)

used enables simple joining with low-loss splices

There arevarious refinements on the design shown in Fig. 2. In practice, it is usual to include an isolator at the amplifier output This is an optical ‘diode’, which only allows light to propagate in one direction. Without the isolator, reflections from imperfect splices, couplers, connectors etc could be amplified in

Fig. 2 Erbium-doped fibre amplifier

Erbium-doped fibre amplifiers have been developed to the point that they are now available commercially and the technology is thocght to be sufficiently mature that they will be used in transatlantic optical-fibre communication systerns due for construction in 1995. They have all sorts of possible applications however (they can provide the gain necessary to support soliton transmission3 and they can be used in resonant structures to produce fibre-based laser light sources? to give just two examples) and their full impact on the way that optical fibre is used is yet to become clear.

2 Erbium-doped fibre amplifiers

Erbium-doped fibre amplifiers (EDFAs) operate in the long-wavelength optical-fibre transmission ‘window’ at around 1550 nm. One of their attractions is the simplicity of the design. At their most basic, all they need is a length of doped fibre spliced into the transmission path, a pump source and a means of coupling the light from the pump into the doped fibre (Fig. 2). Energy is transferred, via the erbium dopant, from the pump light to the signal, thereby effecting amplification. The pump source needs to be reasonably powerful - typically 1-100 mW, depending upon the required output signal power - but this is now available from semiconductor laser diodes. Coupling of the pump light into the fibre can be done at very low loss (say under 0.2 dB) with a fused-fibre wavelength division multiplexer. The doped fibre is made from silica glass, and differs from standard telecommunications fibre mainly by the presence of erbium (and possibly other dopants) in the core region. Being based upon standard telecommunications fibre has two advantages: the manufacture uses established processes enabling the production of high-quality fibre at low cost, and the

t metastable state

0 8 eV (1 550 nm)

ground state ~

Fig. 3 Energy levels of erbium ion electrons

at worst causing instability, at best leading to a small increase in noise. Fig. 2 shows the pump light injected so that it propagates in the same direction as the signal along the doped fibre. This is known as co- directionalpurnping. Counterdirectional pumping, with the pump injected at the far end of the doped fibre and propagating in the opposite direction to the signal, is equally possible. The two options have different advantages and disadvantages. For example, counterdirectional pumping might allow higher output signal powers whereas co-directional pumping tends to give better noise performance. There are all sorts of variations in possible configurations - such as combinations of pumping direction and wavelength, and the way in which isolators are used - aimed at optimising the performance for particular applications. Furthermore the length of the doped fibre (typically of the order of tens of metres) and the dopant concentration and distribution needs to be optimised.

These details will not be considered further here (see References 5-8 for examples of some of the variations considered.) The rest of this Section looks at the amplification mechanisms and discusses how they influence the performance characteristics of EDFAs.

Amplification mechanism EDFAs amplify using stimulated emission. (They are

laser devices in the sense that they use light amplification by stimulated emission of radiation, but the term ‘laser’ is usually reserved for a device in which the amplifying medium is contained within a resonant cavity in order to make a light source.)

The electrons associated with the erbium ions can exist in a number of different energy levels, some of which are shown (in a much simplified form) in Fig. 3. The first point to note is the existence of the metastable level at about 0.8 eV (about 1.3 joules) above the ground state. This is about the same as the energy of the photons of 1550 nm wavelength light, so that if a 1550 nm photon interacts with an erbium ion in the metastable state, a quantum- mechanical resonance effect causes the ion to return to the ground state and an extra photon to be emitted at exactly the same wavelength as (and in phase with) the incident wavelength (Fig. 4).

One incident photon has become two photons and we have the basis of optical amplification.The reverse can also happen unfortunately: an incident photon at 1550 nm can be absorbed by exciting an ion from the ground state to the

60 ELECTRONICS & COMMUNICATION ENGINEERING JOURNAL APRIL 1994

metastable state. However, net amplification is achievable if, on average, more photons cause resonant emission rather than absorption. This will happen if incident photons are more likely to encounter ions in the metastable state than in the ground state. The condition in which there are more electrons in the excited, metastable state than in the ground state is referred to as population inversion and is achieved by the process known as pumping. In an EDFA pumping is performed optically by 980 nm or 1480 nm light - photons from the pump excite electrons from the ground state to the metastable state.

There are two routes for the pumping, depending upon the pump wavelength (Fig. 5). If the pump wavelength is around 1480 nm the energy of the pump photons is very similar to the energy of the signal photons and the absorption of a pump photon excites an electron from the ground state directly to the metastable state. (Note that all the energy levels have a finite width and so correspond to a small range of energies. See the Panel on ’Energy level broadening’.) Alternatively, pumping can be at 980 nm, in which case absorption of a pump photon excites the electron to a higher energy state. The higher state has a very short ‘lifetime’ and the electron very rapidly ‘drops’ down to the metastable state, releasing its energy as mechanical vibrations in the fibre. At the atomic level these vibrations take the form of waves which, in some ways, may be viewed as particles (phonons) carrying energy, just as photons are viewed as waves carrying energy. Thus in the nonradiative decays the transition emits one or more phonons, whereas in radiative decays the transition emits photons.

Noise Some photons will return to the ground state

spontaneously, without being stimulated by a signal photon. In doing so they emit a photon unrelated to the signal. This is the only significant source of noise in an EDFA. Photons generated by spontaneous emission can propagate in either direction along the doped fibre. Whichever way they go they may themselves stimulate emission of more photons. The stimulated emission by spontaneously emitted photons causes two problems. Firstly, it amplifies the noise, hence the description amplified spontaneous emission (ASE) noise. Secondly it removes some of the electrons from the metastable level. thereby reducing the degree of population inversion. This second effect means that even though backward- propagating ASE will not contribute directly to the amplifier output noise, it does degrade the EDFA performance.

Theoretical analysis shows that the probability of an excited ion spontaneously emitting a photon is closely related to

Fig. 4

the probability of an incident photon exciting a stimulated transition. Doing the calculations gives the result that, in an ideal amplifier with complete population inversion, the power spectral density of the output noise would be given

Photon multiplication by stimulated emission

by

where h is Planck‘s constant and G is the power gain of the amplifier at frequency v (so hv is the photon energy). The factor 2 comes from the fact that single-mode fibre supports two (degenerate) orthogonal polarisation states. Real amplifiers will always be worse than this, and the noise output is modelled by

P,,(v) = 2 ns,,hv (G- 1)

where nsp is the amplifier excess noise factor. The noise characteristics of EDFAs are usually described by the ‘noise figure’, which, as explained in the discussion of pre- amplifiers below, is equal to 2n,(1010g,,(2n,~) in decibels),

Saturation As with any amplifier, as the magnitude of the output

from an EDFA increases, the amplifier eventually starts to saturate. In an EDFA saturation is caused by the reduced extent of the population inversion (fewer erbium ions in the excited state) leading to a reduction in gain, with characteristics of the form shown in Fig. 6. However, there is an important difference between the saturation in EDFAs and the more familiar saturation in electrical amplifiers, or, indeed, some other optical amplifiers.

The difference is that the saturation is a long-term effect (‘long’, in this context, meaning around 10 ms). Because of the long lifetime of the metastable state (the time constant

- nonradiative

transition

1480 nm

a b

I Fig. 5 (a) a t 1480 nm (b ) at 980 nm

Optical pumping in erbium-doped fibre:

for spontaneous decay from the metastable state is about 14 ms), and the corresponding low probability of individual photons stimulating emission from individual ions, high gain is achieved by having a large ‘reservoir’ of excited erbium ions with which incident photons may interact. If the average signal power is high, the reservoir empties and the


Al) . This is known

Is, electrons will tend to be

1480 nm 1550 nm

more likely to occupy the lower end of the range. This explains the success of pumping at 1480 nm (Fig. U). the higher energy 1480 nm photons excite electrons

from where they may be top end of the ground state, by

(lower energy) 1550 nm photoile. The wavelength range over which stimulated emission, and therefore

- - Stark splimng thermal broadening

Fig. A I

Fig. A2

gain, can be achieved conveniently spans much of the low-loss silica fibre window. Notice also that the broad energy levels involved also mean that the precise pump wavelength is not critical. m e higher level, used by 980 pumping, is narrower, so that the exact wavelength of 980 nm pumps is important to within a few nanometres.)

If the doped fibre is cooled the thermal broadening will be reduced, reducing the overlap within the manifold and giving a level structure more like the middle of Fig. Al. This is known as inhomogeneous broadening. The exact spacing of the levels for any one erbium ion depends upon the electric fields experienced by the individual ion. Because of the amorphous nature of the glass in which the ions are embedded, the electric field, and therefore the level structure, are different for each ion. Thus the inhomogeneously broadened fibre will have levels all the way across the same range as the homogeneously broadened fibre, but different levels will be due to different erbium ions and so will, to some extent, be independent.

gain drops. Short pulses of high power in an otherwise low- power signal, however, have access to the large reservoir of excited ions and experience the same gain as lower power pulses in the same signal.

There are a number of consequences of this slow saturation. Firstly, the amplifier is unaffected by fluctuations in the pump power above about 50 kHz. Secondly, the amplifier can be operated in the saturation region without degrading the extinction ratio of the digital signal, for signalling rates above about 100 kbaud. Similarly, amplifiers will not severely distort analogue signals, even when saturating, provided the signal spectrum is predominantly above 100 kHz.

The reduced population inversion of a saturating amplifier does, however, affect the characteristics in a number of other ways. A useful feature is that it increases the efficiency with which pump energy is converted to signal energy because there is a high probability of all the pump photons interacting with unexcited erbium ions and therefore being ‘used’, rather than passing through the doped fibre unabsorbed. Unfortunately however, the

reduced population inversion also reduces the noise figure. This is because a proportion of the signal photons will pump ground state ions to the metastable state (and be absorbed in the process). In effect, the degree of population inversion contributing to spontaneous decay is then greater than the degree of population inversion usefully contributing to the amplifier gain.

3 Applications

The most obvious application of an optical amplifier is to extend the reach of a point-to-point transmission link. There are three generic configurations (Fig. 7): use as a post-amplifier to boost the signal prior to transmission; as a preamplifier, to amplify the signal immediately before detection at the receiver: or as an in-line amplifier as a nonregenerative repeater (combinations of all three are also possible). The first two have the advantage of locating the amplifier only at a terminal, where power and maintenance access are readily available, but the third allows the possibility of the longest links


(extendible by using more amplifiers). Some insight into the features and benefits of optical amplifiers is gained by considering these three configurations in more detail, so the following subsections look at each in turn.

The postamplifier The EDFA is being used here as a power amplifier

following a lower-powered signal source - paralleling the usual practice for high-powered electrical sources. The amplifier is operated in saturation (see above), where it is most efficient in terms of converting pump power to signal power but has an increased noise figure. Because the noise is attenuated during transmission with the signal however, the increased noise figure is unlikely to be significant. High-power pump sources are needed, and safety considerations will be important because of the high- powered output signal.

The preamplifier The performance limit of conventional optical-fibre

systems using pin photodiodes derives from the noise generated in the receiver, thermal noise in the photodiode load resistor and amplifier noise in the following electrical preamplifier. Any means of increasing the signal power prior to the load resistor and electrical preamplifier will improve the signal-to-noise ratio for a given receiver. A ‘conventional’ approach is to use an avalanche photodiode (APD) to provide gain within the photodiode by the ‘avalanche’ of electrons released by each incoming photon. As a statistical process, however, the avalanche gain itself introduces noise. Coherent detection is an alternative approach, whereby a (relatively) high-power local optical source is mixed with the received signal at the photodiode, to provide true optical amplification. Coherent detection can be very successful, but requires a complex receiver with phase or frequency locking of the local oscillator, and polarisation control (see Reference 9). In addition, both the optical signal source and the local oscillator lasers need to have very narrow linewidths.

Using an EDFA as an optical preamplifier is an alternative which provides better performance than the use of an avalanche photodiode and is much simpler than coherent detection. There is no particular constraint on the source linewidth and there is no need for polarisation control. It is necessary, however, to consider the effect of the noise (see the Panel on T h e effect of ASE on an optical receiver’).

A concept which is commonly used to quantify the effect of amplifier ASE is the noise figure, which is derived by comparing the signal-to-noise ratio at the output of an ideal receiver with and without an EDFA used as a preamplifier. By an ideal receiver is meant one which introduces no noise itself (no thermal or electrical amplifier noise), so that without the EDFA the only noise present is the shot noise (quantum noise) which arises due to the quantum statistical nature of detection in a photodiode. The signal power (current squared) out from the photodiode is given by f = P:p, where P, is the received optical signal power

t

input power, log scale

Fig. 6 EDFA gain characteristic

and Ris the photodiode responsivity (the ratio of the output current to incident optical power). R is given by qe/hv, with q , the quantum efficiency, equal to unity-because we are assuming an ideal receiver and e the charge on an electron.

The shot noise power (current-squared) is given by <i,’> = 2eIB, where B is the bandwidth of the receiver noise

a

Tx -k Rx

, pfeamplHier

b

i G R X E I

in-tine amwier

C

9. 7 The three generic configurations for using an optical amplifier: (a) post-amplifier; (b) preamplifier; (c) in-line amplifier


The effect of ASE on an optical receiver

Spectrum of the received optical signal Spectra of the three output noise components

power spectral density pmportional to mean received optical power (nB,)

power spectral density pmportiinal The AS€ spaclrum has banhrldlh & Fdlomrq lhn phomdiode, low-pass electrical llnerinp is irlcdelled 86 having a band*ndth &, In general E- is much greater than W,

c -I 2 BWI

The analysis of a receiver in the presence of amplified spontaneous emission @SE) noise is qualitatively different from the analysis of the receiver in a conventional system. Whereas all the noise in conventional systems is electrical noise added after the photodiode, ASE originates before the photodiode and results in three noise components on the photodiode output. The first component is the extra shot noise from the increased mean optical power. The other two arise due to the fact that the photodiode output is proportional to the incident optical intensity, which is in turn proportional to the square of the electromagnetic field. This nonlinearity (squarelaw) of the photodiode ‘mixes’ different frequencies to generate two sets of beat frequencies: mixing between the signal and ASE generates ‘signal- spontaneous beat noise’; and mixing

The spectrum 01 the SpOnlaneOUS-SpOntatmws beat noise can be thought 01 as generated by the trequenwdomam mnvdution 01 lhe AS€ -rum with itsslt, togsther with a DC term generated by the mean w a r of me AS€. The DC term pmvldns a fixed o W t which may be negktnd (but generates the extra shot noise, Shown above)

power spectral density

slgnal x ASE (ns) , , pyortional to signal-spontaneous

beat noise

power proportional to nSB-

& frequency 6.- - BOD!

The spectrum of the slgnal-spontaneous beat n o w can be thought of as generaled by frequency domain convolution 01 the sgnal spaarum with the AS€ spectrum

between different frequency of the ASE noise at the receiver input components within the ASE and can be reduced by putting an specbum generates ‘spontaneous optical filter before the photodiode. spontaneous beat noise’. The Optical filtering also reduces the magnitude of the spontaneous magnitude of the extra shot noise, spontaneous beat noise is but not of the signal-spontaneous proportional to the optical bandwidth beat noise.

filter, so the signal-to-noise ratio is power, GP,, converted to a current by a photodiode with responsivity R and having a bandwidth of B.)

The signal-twnoise ratio becomes

Putting an EDFA in front of the receiver increases the optical signal power by the gain G, but also adds noise due to the ASE. Assuming the use of an optical filter before the

component is given by

<z?,,~~~,.> = 4 P n & v ( G - 1)GPJ Paralleling the usual definition of noise figure as the ratio

(This is the ASE noise, ns#v (G - 1) , beating with the signal between the signal-to-noise ratio at the input and the signal-


GZP:P + 4Pn&v (G - 1) GP,B mD =

photodiode, it can be shown that the extra noise GPP 2eB(1+ ZnSp(G - 1))

to-noise ratio at the output of an amplifier, the noise figure of an EDFA is defined as the ratio between the signal-to- noise ratio with and without the EDFA, thus

For large gain, this approximates to Zn,. A perfect amplifier would have n, = 1, giving a noise figure of 2 (3 dB). That is, with an ideal receiver, using a perfect optical amplifier would halve (degrade) the signal-to-noise ratio. Real receivers are very much worse than the ideal receiver because of thermal and other noise, so a real EDFA (which can approach an ideal EDFA to within a factor of 2) can give a substantial increase (improvement) in signal-to-noise ratio.

In-line amplifiers Using in-line amplifiers is effectively going back to a

technique used in the days of analogue transmission, when linear amplifiers were used to maintain the signal power along trunk transmission systems.

In a digital era, when the received wisdom is that regeneration (detecting and recreating the digital pulse stream afresh to prevent the accumulation of noise and distortion) is superior to linear amplification, it might seem that (linear) optical amplification offers limited benefits. However, an optical amplifier is much simpler than a regenerator, and potentially more reliable. Furthermore, as explained above, the propagation of an optical signal in fibre can be virtually noise and distortion-free: noise only appears in the electronics at the receiver. Thus the weakest link of existing systems is at the receiver, and there is scope for accepting some in-line signal degradation (generated by amplifiers) without degrading the overall performance. In practice, if a series of optical amplifiers are concatenated the noise (&E) builds-up (the noise power from successive amplifiers adds) and the receiver noise is soon dominated by the ASE. Nevertheless, the advantage of simplicity remains.

There is furthermore the advantage of transparency. Regenerators have to be designed for a particular system. They are compatible with a given signalling rate and line code. Amplifiers, on the other hand, are almost completely transparent to the conveyed waveform. Thus an installed system using amplifiers may be changed - upgraded - by changing only the terminals, but a regenerated system requires the replacement of all the regenerators. For example, the next proposed transatlantic transmission system, TAT-12, will be installed in 1995 to operate at 5 Gbit/s using 133 amplifiers for the 6000 km link between Rhode Island and Land's End. It might be possible at some future date to upgrade to, say, 10 Gbit/s, merely by changes at the terminals.

The ability to operate EDFAs in saturation without distorting the conveyed signal can be exploited when concatenating amplifiers, as it provides a means of passive automatic gain control. As can be seen from Fig. 8, if the input power to a saturating amplifier changes, then its gain

changes in the right direction to compensate. Saturating operation of amplifiers in a transmission system may be ensured by making the total loss between repeaters less than the small signal gain of the amplifiers.

Distributed amplification The gain in an EDFA is inherently distributed over the

length of the doped fibre - typically tens of metres. It is possible deliberately to distribute the gain over even longer distances by using a lower erbium doping concentration in the fibre. In this way it is possible to arrange for the gain just to compensate for the fibre attenuation, creating a zero attenuation transmission medium. If the pump wavelength of 1480 nm is used, then the silica fibre attenuation of the pump can be kept low (1480 nm being at the edge, but within, the 1550 nm transmission window), so a single pump can pump several kilometres of fibre used in this mode.

The use of distributed amplification to create a zero loss fibre is particularly attractive for use with soliton transmission, where it maintains the pulse amplitude required to generate the nonlinearities necessary for the stability of solitons.

Wavelength division multiplexing The transparency of EDFAs across much of the 1550 nm

fibre transmission window makes them suitable for use with systems employing wavelength division multiplexing (WDM). A number of channels can be simultaneously amplified in a single EDFA, and, because of the slow gain- response time (see the discussion of saturation above), there is negligible crosstalk between channels above about 100 kHz. However, there is a problem with using strings of concatenated amplifiers operated in saturation (the configuration proposed in the discussion of the in-line amplifier above). If an amplifier is homogeneously

input power input power input power drops, gain 2

rises to drops to compensate Compensate

operating point

L rises. gain

Fig. 8 Operation of an EDFA in saturation


Fig. 9 A twin-cored fibre can provide automatic gain control for each channel of a wavelength division multiplexed signal

broadened (see Panel on 'Energy level broadening'), the operating point will be determined by the average power across all channels, which will tend to overamplify the more powerful channels and underamplify the weaker channels. With concatenated amplifiers the effect builds- up along the route and causes initial slight differences in channel powers to be exaggerated. Even if the channels are all launched with identical powers, the variation in amplifier gain across the kequency spectrum, though generally small, is sufficient to create, then emphasise, power differences.

A number of solutions to this particular problem are currently being investigated. Flattening the gain of amplifiers,'" or building filters to equalise the amplifier gain spectrumlL helps, assuming that the channels are launched with identical powers. If a return path is available, feedback to adjust the channel launch powers on an individual basis can be used to equalise the received channel powers. Cooling the amplifiers, so that the broadening becomes inhomogeneous rather than homogeneous (see the Panel on 'Energy level broadening') is an interesting, but complex a p p r ~ a c h . ' ~ The inhomogeneously broadened amplifier saturates to some extent independently at each of the different wavelengths because the different wavelengths are amplified by different levels in the inhomogeneous spectrum. This approach is particularly attractive because it offers the possibility of independent automatic gain control for each channel.

The same benefit is available from another recently proposed scheme,14 without the need for cooling. This uses double-cored, doped fibre as the amplifying medium. Light propagating in double-cored fibre moves back and forth between the two cores with a cycle length that depends upon the light wavelength. Thus different channels are present in the cores at different locations (Fig. 9), and some of the gain for each channel derives from different physical locations in the cores. Saturation at the different locations will adjust to the appropriate channel, again providing the required automatic gain control separately for each channel.

System problems By overcoming the attenuation limits on transmission

distance, fibre amplifiers bring to the fore other limitations. A major problem is that of dispersion. Most of the fibre presently installed in the field (there is a lot of it) is standard single-mode fibre ('standard' rather than

'dispersion shifted'). This has a dispersion minimum around 1300 nm and is highly dispersive at 1550 nm. EDFAs amplify only in the 1550 nm window. Research is in progress to try to find similar amplifier technologies suitable for 1300 nm (see below), but nothing as successful as the EDFA looks likely to emerge in the short term. A number of techniques for overcoming the disDersion limits at

I

1550 nm are being investigated. Methods have included equalisation using lengths of fibre designed to have strong dispersion characteristics in the opposite sense to standard fibre15 and the exotic technique of phase conjugation.16 This lattel: method involves inverting the spectrum of the signal half way along a transmission path, so that the second half of the transmission 'undoes' all the dispersion done in the first half.

Another set of problems brought to light by the use of amplifiers arises from fibre nonlinearities. At the power levels and over the repeater spacings used in the past, it has been sufficient to treat silica fibre as though its optical properties (specifically dielectric constant) were purely linear. With high powers and longer distances, the nonlinear effects can become noticeable. In practice, however, provided the power levels are kept below about +18 dBm, nonlinearities can be kept small such that they are not the limiting factor in system perf~rmance. '~

4 Fibre ampli f iers for operat ion a t 1300 n m

A fibre laser operable at the 1300 nm window is a very desirable goal, because ofthe dispersion zero at 1300 nm in standard singlemode fibre. Praseodymium has the right energy levels for pumping at 1000 nm to give gain at 1300 nm, and amplifiers based on praseodymium doping have been demonstrated.The difficulty is that the lifetime of the metastable state of praseodymium in silica is very short (1 ps), due to nonradiative decays. This means that very high pump powers are required to achieve population inversion. The nonradiative decay occurs because energy can be removed by phonons in the silica glass. To some extent this can be improved by using a different type of glass which supports different phonon energies. Specifically, fluoride glasses have been developed in the past for optical fibre transmission at longer wavelengths (around 2400 nm), and the energies of phonons in fluoride glasses are significantly lower than in silica glasses. Consequently more phonons are needed to carry the energy from a given transition, and so nonradiative decay is less probable than in silica. Amplifiers for the 1300 nm window based upon praseodymium-doped fluoride glass fibres are therefore of current interest and show promise.'8 But since even in fluoride glass the metastable lifetime is only 100 ps (compared to 14 ms for erbium in silica), high pump powers are still required and it is unlikely that praseodymium-doped fluoride fibre amplifiers (PDFFAs) will ever be as successful as EDFAs.


5 Conclusions

Erbium-doped fibre amplifiers provide an almost perfect technical solution to the need for Optical gain in the 1550 nm optical-fibre transmission ’window’. They have been developed to the state where the noise performance approaches theoretical limits to within a few decibels, and the power handling capacity is more than adequate for any currently proposed telecommunications application. They will change the way in which fibre is used in telecommunication networks by effectively removing the limitations due to attenuation. As amplifiers come to be used instead of regenerators, end-to-end links will become transparent to the conveyed signal, opening up the possibility of widespread use of wavelength division multiplexing capable of transporting a wide variety of analogue and/or digital signals.

Further reading

For fiirther reading, a detailed description of the theory of optical-fibre amplifierswill be found in a book edited by P.W. France (Reference 19), but inevitably in a subject moving as fast as this is, books tend to date very quickly and the latest information is only available kom the technical journals and conferences. Papers describing the latest work appear in a number of different physics and engineering journals, but in particular, Electronics Letters (IEE), Photonics Technology Letters (IEEE) and Journal of Lightwave Technology (IEEE) have relevant material in every issue. All the major conferences on optical communication include papers on EDFAs, but in recent years the Optical Society of America has held an annual topical meeting on ‘Optical amplifiers and their applications’; the 1994 meeting will be held in the USA on 3rd-5th August.

References

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TAYLOR J.R: ‘Optical solitons - theory and experiment’. Cambridge Studies in Modem Optics, 10, (Cambridge University Press, 1992) TAKARA, H., KAWANISHI, S., and SARUWATARI, M.: ‘20 GHz, 3.5 ps transform-limited optical pulse generation from a highly stable, tuneable actively modelocked Er-doped fibre laser‘. Cod. on Optical ampliers and their applications, Yokohama, Japan 4th4th July 1993, Technical Digest pp.310-313 (Optical Society ofAmerica, Washington DC) DIGIOVANNI, D.J., WYSOCKI, P.F.. and DAVEY, S.T.: Tailor fiber design to optimise amplier performance,’ Laser Focus World, September 1993,29, (9), pp.95106 MELLIS, J., GARNER, P., MARTIN, J.N., MIDDLEDITCH, D.J., COLLINS, J.V., WHEATLEY, P., and PARKER BE.: ‘Modular erbiumdoped fibre ampliers for optical communications systems’, BT Technol. J., October 1991, 9, (4), pp.12-18

7 W, A, OMAHONY, M.J., and SIDDIQUI, AS.: ‘Analysis of optical gain enhanced erbiumdoped fiber ampliers using optical filters’, Photon. Technol. Lett., July 1993, 5, (7), pp.773-775

8 ZERVAS, M.N., LAMMING., RI., and PAYNE. D.N.: Trade- off and design considerations of the erbiumdoped fibre amplifier’. IEE Colloquium on Optical amplifiers for communications, London, 20th May 1992, Digest No, 1992/124, Paper 7

9 STANLEY, I.W.: ‘A tutorial review of techniques for coherent optical fiber transmission systems’, IEEE Commun. Mag., August 1985,23, (8), pp.37-53

10 DA SILVA, V.L., SILBERBERG, Y., WANG.. J.S.. GOLDSTEIN, E.L., and ANDREJCO. M.: ‘Automatic gain flattening in Er-doped fiber amplifiers’. OFC/IOOC‘93, San Jose, California, 21st-26th February 1993, Technical Digest pp.174-175

11 KASHIWADA, T., NAKAZATO, IC, OHNISHI, M., KANAMORI, H., and NISHIMURA. M.: ‘Spectral gain behaviour of Er-doped fiber with extremely high aluminium concentration’. Conf. on Optical ampliers and their applications, Yokohama, Japan, 4th4th July 1993, Technical Digest pp.104-107 (Optical Society of America, Washington DC)

12 WILLNEQ AE., and HWANG, S.-M.: ‘Passive equalization of non-uniform EDFA gain by optical filtering for megameter transmission of 20 WDM channels through a cascade of EDFAs’. Conf. on Optical amplifiers and their applications, Yokohama, Japan, 4th-6th July 1993, Technical Digest pp.178-181 (Optical Society of America, Washington DC)

13 GOLDSTEIN, E.L., ESKILDSEN, L., DA SILVA, V., ANDREJCO, M., and SILBERBERG, Y.: ‘Suppression of dynamic cross-saturation in multiwavelength lightwave networks with inhomogeoneously broadened fiber amplifiers’. Conf. on Optical amplifiers and their applications, Yokohama, Japan, 4th-6th July 1993, Technical Digest pp.70-73 (Optical Society of America, Washington DC)

14 LAMING, RI., MINELLY, J.D., DONG, L.. and ZERVAS. M.N.: ‘Erbium-doped-fiber amplifier with passive spectral- gain equalisation’. OFC/IOOC’93, San Jose, California. 21st-26th February 1993, Techical Digest pp.175-177

15 PAYNE, D.N., LAMING RI.. RICHARDSON, D.J., and GRUDININ, A: ‘Unleashing the full capacity of the installed fibre base’. 19th European Conference on Optical communication (ECOC’93). Montreaux, 12th-16th September 1993, Proceedings Volume 1, pp.92-94 (available from Swiss Electrotechnical Association (SEV), CH-8034 Zurich)

16 GNAUCK, AH.. JOPSON, RM.. and DEROSIER, RM.: ‘10 Gb/s 360 km transmission over dispersive fiber using midsystem spectral inversion’, Photon. Technol. Lett., June 1993,5, (6), pp.663-666

17 GOSSET, N., and DUGAN, J.M.: ‘Stretching span distance’. J. Lightwave Technol., November 1993,. 10, (12). pp.5658

18 WHITLEY, T.J., WYATT, R.. SZEBESTA, D.. and DAVEY, S.T.: Towards a practical 1.3 pm optical fibre amplifier’, ET Technol.J.,April1993.11, (2) pp.115-127

19 FRANCE, P.W.: ‘Optical fibre lasers and amplifiers’ (Blackie and Son, Glasgow and London, and CRC Press, USA and Canada, 1991)

0 IEE 1994 Received 14th December 1993

The author is with the Faculty of Technology, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK.

ELEC’TRONICS & COMMUNICATION ENGINEERING JOURNAL APRIL 1994 67

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LETTER TO THE EDITOR - Dear Sir - M.B. Sandler’s article in the December 1993 issue of Electron. & Commun. Eng. J. was both excellent and fascinating. He indicates that the basic concept of pulse width modulation (PWM), which he describes, is not particularly new: however, readers may be interested to hear of its full antiquity. In the late nineteen sixties, at least two semiconductor manufacturers offered ICs using the PWM technique. Mullard produced devices for a wide range of applications. The most advanced device, TDA2600, appeared in the early seventies. This IC produced the scanning current required for vertical deflection of the 30AX range of colour picture tubes. The circuit was conceived by Brian Attwood at Mullard Central Applications Laboratory at Mitcham. He, together with Brian Simpson, brought the circuit to fruition.

Philips used theTDA2600 in their G11 chassis. Kenneth Smith carried out the field circuit development at Philips Croydon Works. Naturally, equipment with high-gain VHF ampliliers had problems of interference from the pulse switching edges, but careful attention to earth paths solved these. Also, novel circuitry gave novel mechanisms of non- interlace, but these were discovered and remedied. One and a half million receivers were made. The specification called for a life of 10 years and 20000 hours with a failure

rate below 0.2%/1000 hours. These figures were easily achieved - a few receivers are known to be in present regular use - not least that in the shop of the writer’s father!

The technique fell into disuse for field scanning in TV receivers owing to advances in the technology of heat extraction from IC power transistors. Thus the advantage of high-efficiency bottomed output devices lost its appeal. ICs with conventional Class B output stages were simpler and cheaper.

The author’s memory is grateful for help received from J.C. Warren and J. Douglas.

R. TALKS (F) Croydon

15th February 1994

Brief letters, preferably typed, on technical and professional matters are welcomed. They should be sent to the Staff Editor, Electronics & Communication Engineering Journal, IEE, Michael Faraday House, Six Hills Way, Stevenage, Herts SG1 2AY. The Editor reserves the right to edit letters which are published. The Editor’s decision on which letters are published is final.

fi8 ELECTRONICS & COMMUNICATION ENGINEERING JOURNAL APRIL 1994

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