GB2116391A - Single-mode optical fibre telecommunication apparatus - Google Patents

Abstract

Pulses of electromagnetic energy of appropriate peak power can form soliton pulses in monomode fibre (12). The width of such pulses changes during propagation, and can decrease for certain ranges of peak power, e.g. between about Po and 9/4Po, where Po is the "balanced" peak power. Using this property, repeaterless transmission of soliton pulses is possible if the pulses are amplified in non-electronic amplifiers (13) (meaning amplifiers in which the optical pulses keep the form of optical pulses throughout the amplification process) by appropriate amounts at appropriate intervals. Exemplary amplifiers are glass laser, Raman laser, semiconductor laser medium. and cw injection. <IMAGE>

Description

SPECIFICATION Single-mode optical fibre telecommunication apparatus This application relates to apparatus for transmission of electromagnetic pulse signals by means of monomode fibre.

Impressive progress has recently been made in the field of optical telecommunications.

Systems are now being installed that permit transmission of data at a rate of many megabits/sec over distances of several kilometres between repeaters. However, since the economics of systems, such as, for instance, intercontinentai submarine cable systems, are strongly affected by data rate and repeater spacing, work directed towards improvement in such system parameters continues.

Although currently available fibre can transmit signals with relatively low loss and low dispersion, and although further improvement in these respects can reasonably be anticipated, fibre telecommunication links require, and will probably continue to require, regeneration of the signal at so-called "repeaters" at points intermediate between the sending or input end and the receiving or output end of the fiber communication channel. "Input end" and "output end" refer, of course, to a single transmission, and can be reversed for a subsequent transmission.

Repeaters typically carry out two functions, namely raising the power level of the signal pulse, and reshaping the pulse. In addition, repeaters frequently also retime the pulse. Raising of the power level is required because of the attenuation suffered by the signal in any real fibre. Reshaping is required because, owing to dispersive effects in the fibre, pulses typically spread. Retiming is found to be often necessary to maintain proper pulse spacing.

Repeaters in fibre telecommunication systems typically comprise means for detecting the signal, e.g., a photodiode, means for operating on the output of the photo detector, e.g., amplifying and reshaping the electrical output signal of the detector, and a source for optical radiation, modulated typically by the amplified and reshaped output signal of the detector, as well as means for again coupling the output of the optical source into the fibre. Repeaters of the type described are not only being used now but are being considered also for future fibre telecommunication systems. See, for instance, P. E.

Radley, and A. W. Horsley, Proceedings of the International Conference on Submaring Telecommunication Systems, London, February 1980, pp.173-176.

Conventional repeaters are typically complex devices containing a significant number of components. For instance, a typical optical regenerator contains around 50 transistors (ibid., page 174). This "electronic" complexity, particularly in high bit-rate systems, as well as reliability problems encountered with laser sources, is making repeater costs a major cost item for the fibre telecommunication systems that are currently under consideration.

The conventional response to these facts has, inter alia, been an effort to improve fibre quality, with the results that now repeater spacing of about 50 km appear feasible. Nevertheless, difficulties associated with the use of repeaters are sufficiently severe to make consideration of alternative solutions important, and this application pertains to such an alternative solution. We will next discuss some fibre characteristics relevant to the invention.

Pulses of electromagnetic energy transmitted through optical fibre experience attenuation and dispersion, with the latter resulting in a broadening of the pulse in the time domain. If such broadening is sufficiently severe, adjacent pulses can overlap, resulting in loss of signal detectability. In monomode fibre, (i.e., fibre in which only the fundamental mode of the signal can propagate at the operating wavelength of the system) the two principal dispersion mechanisms are material dispersion and waveguide dispersion.

A material of index of refraction n exhibits material dispersion at the wavelength A if

at that wavelength. Physically, this implies that the phase velocity of a plane wave travelling in such a medium varies nonlinearly with wavelength, and consequently a light pulse will broaden as it travels through such medium.

Waveguide dispersion typically also is wavelength dependent. We will refer herein to the combined material and waveguide dispersion as "chromatic" dispersion. As an example, typical of magnitudes of chromatic dispersion effects in a typical monomode fibre, a 10 ps pulse of carrier wavelength 1.5 ym doubles its width after about 650 metres.

If in a medium

throughout a certain wavelength regime, then the medium is said to be normally dispersive in that regime. On the other hand, a wavelength regime throughout which

constitutes a so-called anomalous dispersion regime. In silica, for instance, a regime of normal dispersion extends from short wavelengths to about 1.27 ,um, and an anomalous dispersion regime from about 1.27 ,um to longer wavelengths. Separating the two regimes is a wavelength at which

i.e., at which material dispersion is zero to first order. This wavelength depends on the composition of the medium. The wavelength at which chromatic dispersion vanishes to first order similarly is composition dependent and, in addition, depends on such fibre parameters as diameter and doping profile.It can, for instance, be as high as about 1.5 ,um in appropriately designed monomode silica-based fibers.

A natural choice of carrier wavelength in a high data rate fibre telecommunication system is the wavelength of first-order zero chromatic dispersion in the fibre. However, even at this wavelength there is pulse spreading due to higher order terms in the dispersion. See, for instance, F.

P. Kapron, Electronics Letters, Vol. 13, pp. 96- 97 (1977).

Recently, it has been proposed to use the nonlinear change of dielectric constant (Kerr effect) of a monomode fibre to compensate for the effect of chromatic dispersion, i.e. to utilize "solitons".

A solition pulse occurs when the broadening effect due to chromatic dispersion is balanced by a contraction due to the nonlinear dependence of the index of refraction on electric field. The existence of solitons in monomode fibre and the possibility of their stationary transmission was predicted by A. Hasegawa and F. Tappert, Applíed Physics Letters" Vol. 23(3), pp. 142-144, (1973). That paper dealt with lossless monomode fibres, and taught the existence of a minimum pulse peak power, dependent, inter alia, on fibre parameters, pulse width and carrier wavelength, above which solitons can exist.These predictions of Hasegawa and Tappert have been verified by demonstrating dispersionless transmission of a 7 ps pulse with a peak power of about 1 Watt at 1.45 m through monomode fibre for a distance of about 700 metres. See, L. F. Mollenauer et al, Physical Review Letters, Vol. 45(13), pp.1095 1098, (1980). Mollenauer et al also verified the predicted by Hasegawa and Tappert that soliton pulses of peak power in excess of the so-called "balanced" peak power P0 undergo pulse narrowing.

Recently, A. Hasegawa and Y. Kodama have proposed the use of soliton pulses in high data rate monomode fibre telecommunication systems. See Proceedings of the IEEE, Vol. 69(9), September 1981, pp. 1145-11 50. That paper contained an extensive discussion of the properties of solitons in ideal fibre and of the effects of higher order dispersion and of loss on solitons, as well as design examples and criteria.

The proposed telecommunication systems utilize the self-confinement effect to achieve high rates of data transmission. They do not, however, address the question of pulse regeneration, and the difficulties inherent in conventional approaches of regeneration that were alluded to above.

Optical fibre telecommunication apparatus according to the present invention comprises a single-mode optical fibre capable of transmitting soliton pulses and one or more non-electronic amplifiers each located at an intermediate location along the fibre of amplifying soliton pulses propagating in the fibre, a non-electronic amplifier being defined as an amplifier adapted to amplify optical pulse signals wherein the signals keep the form of optical pulses throughout the amplification process.

An example of a suitable amplifier is a glass laser, i.e. a glass medium, typically a fibre, doped with an appropriate ion species (that is, ions having energy levels separated by an energy substantially equal to hc/Ro, where h is Planck's constant and c is the speed of light in vacuum), and pumped with electromagnetic radiation adapted to producing a population inversion in the energy levels. Another exemplary amplifying means is a Raman amplifier, e.g. a glass medium, typically a fibre, in which A0 is within a "Stokes" wavelength band of a pump radiation (see, for instance, Optical Fiber Telecommunications, S. E.

Miller and A. G. Chynoweth, editors, Academic Press, (1979), pp. 127-132). Still another exemplary amplifying means provides injection of a continuous wave (cw) of wavelength essentially equal to A0, in phase with the soliton, and of amplitude substantially lower than the pulse amplitude, whereby, through nonlinear interaction between pulse and cw, a pulse amplitude increase can result. And still another exemplary amplifying means is a semiconductor laser operated as an amplifying medium.

The above exemplary amplifiers are examples of non-electronic amplifiers. A feature which they all share is that they permit preservation of the phase of the pulse.

It will be appreciated that a soliton pulse does not attain its final (i.e., asvmptotic) shape and pulse height at the moment of "amplification", i.e., when energy is transferred to the pulse (see, Hasegawa and Kodama, op. cit.), but rather, the pulse typically undergoes a change of pulse width and amplitude while it propagates through the fibre after having undergone the "amplification", to attain its final shape and amplitude after propagation for a distance of the order of LNL, to be defined below.

It is advantageous to choose the initial pulse power and pulse width as well as the amplifier spacing and amplification factors such that the above-referred-to changes are a pulse narrowing and an amplitude increase.

Some embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which: Fig. 1 schematically shows an exemplary telecommunication system embodying the invention; Fig. 2 shows a computed pulse shapes for a pair of soliton pulses absent amplification, and for a pair of linear pulses; Fig. 3 shows computed pulse shapes for a pair of periodically amplified soliton pulses; Fig. 4 shows computed pulse shapes for a pair of soliton pulses with periodic cw injection, and for the same pair absent the cw injection; and Figs. 5 and 6 show computed pulse shapes for a pair of periodically amplified soliton pulses after propagation through about 1000 km and about 6000 km of fibre, respectively.

As was shown by Hasegawa and Kodama (op.

cit., page 1147), the balanced peak electric field jO of a soliton pulse, i.e. the peak electric field in the signal which causes the pulse to retain its shape indefinitely in lossless ideal fibre, is

In this experssion, A is the free space carrier wavelength, n is the index of refraction of the fibre, ct)O is the carrier angular frequency, to is the half-pulse width, and n2 is the nonlinear index of refraction of the fibre. Equation 1 can be used to define the balanced peak power PO.

In this expression, vg is the group velocity c/n, where c is the speed of light, E0 is the dielectric constant of vacuum, S is the cross-sectional area of the fibre, and n is the index of refraction of the fibre.

As was shown by Hasegawa and Kodama, a one-soliton pulse can exist for peak power between one-fourth andrnine-fourths PO. A soliton having peak power between P0 and ninefourths P0 will undergo pulse narrowing during transmission, and a soliton having peak power between one-fourth P0 and P0 will undergo pulse broadening. These authors also teach requirements to be observed in the production of a soliton pulse, as well as conditions to be observed in the design of a monomode fibre soliton transmission system, and these requirements and conditions will not be repeated here.Multiple soliton pulses have peak powers in excess of nine-fourths PO. Although their use in the instant invention is possible, they are not preferred.

It is convenient to distinguish between pulse regeneration and pulse amplification. By "regeneration" we mean a process in which at least the pulse amplitude is increased and the pulse width is decreased. Regeneration is typically carried out in an identifiable piece of apparatus, usually referred to as a repeater, and typically involves a change in the nature of the signalcarrying entity, from photons to e.g. electrons, and back to photons.

On the other hand, by "amplification" we mean a process by which only the pulse amplitude is substantially changed, but no means are provided in the amplifying means for changing the pulse width or shape. The change in amplitude contemplated herein is an increase. At least part of the amplification process is typically carried out in an identifiable piece of apparatus, to be referred to herein as an amplifier.

We do not exclude the possibility of practising the invention by providing, in addition to the amplifiers, additional non-electronic pulse shaping means, e.g. sections of fibre comprising material having a large Kerr coefficient.

Since fibre loss is the only factor which contributes to the deterioration of a soliton pulse through pulse spreading, while at the same time neither the loss nor chromatic dispersion, including higher order dispersion, substantially changes the fundamental shape of the soliton pulse, it is possible to obtain a pulse-width preserving channel by providing means for nonelectronic amplification of a soliton pulse, without need for extrinsic pulse shaping means.

That is to say non-electronic amplification of the soliton pulse takes place of typically more complicated pulse regeneration hitherto required.

Fig. 1 schematically depicts a generalized fibre telecommunication system embodying the invention. Pulses of electromagnetic radiation, emitted by pulse generating means 10, are coupled by coupling means 11 into monomode fibre 12. Pulse generation is controlled by means of input signal 1 5. Since any real fibre causes attenuation of pulses transmitted therethrough, pulses arriving at non-electronic amplifier 1 3 are lower in amplitude than they were when they were coupled into the input end of the fibre. After amplification in amplifier 13, pulses continue their transit through the fibre, being periodically reamplified at further amplifiers 13, until the pulses reach the end of the transmission channel at its output end and are detected by detecting means 14. Reshaping of the pulse typically takes place inherently during transmission.Signal 1 6 is derived from the detecting means and contains essentially the information that has been carried by signal 15.

A requirement for the existence of soliton pulses is that the carrier wavelength is in the region of anomalous dispersion of the fibre. By "carrier wavelength" we mean the centre wavelength of the pulse spectrum. For silicabased fibre, this condition implies that the carrier wavelength has to be above about 1.27,us.

Advantageous wavelengths for operation of a system according to the invention are near 1.5 ym, since silica-based fibre typically has a loss minimum in this wavelength region, with fibre loss being potentially as low as 0.2 db/km.

Any possible source of coherent electromagnetic radiation of the appropriate wavelength and intensity can be used. For instance, such a source could be an appropriate semiconductor laser, or a gas laser. Means for coupling the pulsed radiation into the fibre are also well known to those skilled in the art and will not be discussed here. Similarly, means for detecting the signal pulses are well known to those in the art and do not require discussion.

Exemplary amplifying means are doped glass lasers, Raman lasers, semiconductor laser media, and amplifiers employing continuous wave (cw) injection, a process that will be discussed in greater detail below.

Fig. 2 shows in time sequence the computed development of the pulse shape of two pulses, originally having pulse width of approximately 14 ps each, spaced about 57 ps apart, as they are propagating through silica-based monomode fibre, the fibre having core cross-sectional area of 20 yam2. The pulses have a carrier wavelength of 1.5 pm, and the fibre is assumed to have 0.2 db/km loss at that wavelength. The pulses are assumed to have an input amplitude of 1.26.106 volt/metre, and the balanced peak power P0 for the assumed conditions is 105 mW. The curves 20 are the computed pulse shaped for solitons, i.e.

the appropriate nonlinear index of refraction was used in the calculation (n2=1 .2x 10-22 (m/V)2). As can be seen from Fig. 2, in the absence of amplification the soliton pulses broaden sufficiently that after 22.5 km the pulses have substantially merged. Curve 21 is the computed pulse shape for two linear pulses of the same initial amplitude and width as the pulses of curve 20. By "linear" we mean that the nonlinear coefficient of the index of refraction was assumed to be zero. As can be seen, after about 7.5 km the linear pulses have undergone drastic change.

Fig. 3 shows the computed pulse shape for two periodically amplified solitons. Fibre properties were assumed to be the same as in Fig. 2, and similarly, the same initial pulse shape and amplitude was used. Application by 1.9 db was assumed to take place after 9.4 km, 18.8 km and 28.2 km. As can be seen, under these conditions, the soliton pulses substantially retain their shape and other attributes.

Fig. 4 shows the computed pulse shapes for two solitons, initially idential to those assumed in Figs. 2 and 3, propagating through fibre having the same properties as assumed previously, for the case of periodic cw injection. The cw is assumed to have identical wavelength as the carrier wavelength of the pulses, to be in phase with the soliton, and to have an amplitude 11% of the initial soliton peak amplitude. Injection is assumed to occur at 9.4 km, 18.8 km, and 28.2 km. As can be seen from Fig. 4, under the assumed conditions, the soliton pulses also substantially retain their shape and other attributes.

Figs. 5 and 6 show a computed pair of soliton pulses after 1080 km and 5940 km, respectively, with amplification by about 1.3 db every 6.75 km.

The assumed fibre properties are those used in Fig. 2, the input peak power is 11.2 mW, the full pulse width at half maximum is about 42 ps, the pulse spacing is about 1 70 ps, and the carrier wavelength 1.5 pm. Curve 50 in Figs. 5 and 6 represents the input pulse, curve 51 of Fig. 5 the solitons after transmission through more than 1000 km of fibre, and curve 60 of Fig. 6 the same soliton pair after transmission through about 6000 km of fibre. As can be seen, pulse shapes are remarkably well preserved under the assumed conditions. The change of pulse spacing observable in Fig. 6 is due to the interaction between the two solitons.

As was stated by Hasegawa and Kodama (op.

cit. page 1147), a pulse having an initial peak power other than the balanced peak power will, during propagation, undergo a change in pulse width and amplitude. For instance, an input pulse whose envelope is given by aq0sech(qj), with 1/2 < a < 3/2, will yield asymptotically a onesolition pulse (in the absence of attenuation) of envelope shape given by a,q,sech(a,q,z).

a,=(1+2a), and a=1 +, with IaI < < 1/2. Thus, for a=3/2, the maximum possible amplitude for a one-soliton pulse, ago=2, the peak power of the asympototic solition is about 4 times the peak power of the input pulse, and the asymptotic width is reduced to about one-half of the width of the initial pulse. Similarly, if a soliton pulse is amplified by a factor a (a > 1) then, absent fibre loss, the resulting asymptotic soliton increases in amplitude by about (2a-1) times the original soliton pulse and decreases in width by about (2a--1)-' times the original pulse width.

It is to be noted that, before the pulse shape settles down to its asymptotic one soliton, its shape oscillates and the pulse loses some energy.

For silica-based fibre, the oscillation period LNL in metres is approximately given by

where P(W) is the peak power of the pulse in watt, A(,um) is the carrier wavelength in pm, and S is the core cross section of the fibre core in (pom)2.

An exemplary means for amplifying the soliton pulse employs injection into the fibre of cw of essentially the same wavelength and substantially the same phase as the soliton carrier. Such injection can result in a soliton pulse of narrower width and larger amplitude. The portions of the cw which are not utilized for the amplification of the pulse can be eliminated through further injection of cw at appropriately located later amplification points, resulting in destructive interference of the cw.

Analysis shows that the solition amplitude increases by about X times the cw amplitude, and the width decreases by the same amount, if wavelength and phase equality between carrier and cw exists. Thus, if cw of amplitude E0 is injected into a fibre whenever the soliton amplitude has decreased, due to attenuation, by about 7rEo, then the original solition structure can be recovered.

Unwanted accumulation of unused cw can be avoided if injection points are spaced so as to result in destructive interference of successively injected cw, while at the same time resulting always in constructive interference with the solition carrier. That is possible since the phase of the soliton shifts continuously during propagation, whereas cw maintains a constant phase. Hasegawa and Kodama (op. cit.) have given expression for determining the soliton phase as a function of propagation distance, and appropriate cw injection points can be determined by the use of these or equivalent expressions. If the initial phase of the soliton is To then cw injection is advantageously arranged after a propagation distance T such that the phase

where m is a positive integer.

As an example of the amplification by repeated injection of cw, using the parameters recited in the description of Fig. 2 above, it is possible to maintain a soliton pulse substantially unchanged through injection of cw of amplitude 1.8 1 105V/m every 9.4 km, if the original soliton has a width of about 14 ps, yielding a balanced peak power of about 105 mW.

Claims (6)

Claims

1 Optical fibre telecommunication apparatus comprising a single-mode optical fibre capable of transmitting soliton pulses and one or more nonelectronic amplifiers each located at an intermediate location along the fibre for amplifying soliton pulses propagating in the fibre, a non-electronic amplifier being defined as an amplifier adapted to amplify optical pulse signals wherein the signals keep the form of optical pulses throughout the amplification process.

2. Apparatus as claimed in claim 1 wherein the or each amplifier comprises a glass laser amplifier.

3. Apparatus as claimed in claim 1 wherein the or each amplifier comprises a Raman amplifier.

4. Apparatus as claimed in claim 1 wherein the or each amplifier comprises means for injecting continuous wave electromagnetic radiation, substantially in phase with the carrier wave of the pulses, into the fibre.

5. Apparatus as claimed in claim 1 wherein the or each amplifier comprises a semiconductor laser amplifier.

6. Optical fibre telecommunication apparatus substantially as herein described with reference to the accompanying drawings.