WO2002079822A2

WO2002079822A2 - Method and apparatus for providing dispersion in dispersion compensation modules

Info

Publication number: WO2002079822A2
Application number: PCT/GB2002/001552
Authority: WO
Inventors: Nicholas John Doran; Liam Gleeson; Timothy Ellingham
Original assignee: Marconi Uk Intellectual Property Ltd
Priority date: 2001-03-30
Filing date: 2002-03-28
Publication date: 2002-10-10
Also published as: WO2002079822A3; GB0108084D0; AU2002244848A1

Abstract

The invention relates to the field of chromatic-dispersion compensation. A circulator (30) is connected in-line with an optical fibre transmission line (50) to permit a double passage through a length of dispersion-compensation fibre (40). Light propagating in the transmission line (50) enters the circulator at a first port (10), is coupled to the dispersion-compensation fibre (40) at a second port, propagates along dispersion-compensation fibre (40) to a reflector (60), is reflected back along dispersion-compensation fibre (40) to the second port and is coupled back into the transmission line (50) at a third port (20). The compensation provided is thus double that of a single-pass arrangement. Furthermore the dispersion-compensation fibre (40) is optically pumped by Raman pumping thereby providing Raman amplification of the light. In one arrangement the reflector (40) is substantially transmissive at the pump wavelength and pumping means (e.g.a laser) optically pumps the fibre (40) through the reflector.

Description

METHOD OF. AND APPARATUS FOR, PROVIDING DISPERSION IN

DISPERSION COMPENSATION MODULES

This invention relates to dispersion compensation modules for compensating for

chromatic dispersion in optical communication systems. More especially the invention

concerns a method of, and apparatus for, providing the dispersion in such a module.

Optical fibres form an increasingly important part of the world's telecommunications

infrastructure. Information is transmitted along such fibres using optical radiation in the

form of pulses of light. Such a pulse contains light of many frequencies (wavelengths).

Unfortunately, light of one frequency does not, in general, travel along an optical fibre at

the same speed as light of a different frequency; rather the fibre exhibits chromatic

dispersion. Consequently, the information-carrying pulses of optical radiation can

become distorted by propagation along long lengths of optical fibre.

Optical fibres that are used as long-distance transmission lines generally exhibit positive

(often termed "anomalous") dispersion at wavelengths near their operating wavelength

(typically 1500 nm): light of a longer wavelength ("redder" light) travels slower than

light of a shorter wavelength ("bluer" light). To compensate for transmission line

chromatic dispersion, lengths of fibre having negative (often termed "normal") dispersion are introduced into the transmission line at regular intervals along its length;

in such "normal" optical fibre, light of a shorter wavelength travels slower than light of a longer wavelength. The net dispersion experienced by light transmitted along the

transmission line may thereby be controlled.

Relatively short lengths of fibre with high negative dispersion may thus be used to

compensate for transmission line dispersion. The dispersion compensating fibre (DCF)

is usually provided as a dispersion compensation module, containing sufficient DCF to

compensate for the dispersion of a given length (optical path length) of transmission

fibre. The module is inserted in the line of the transmission fibre, so that light passes

directly from the transmission fibre, straight through the DCF and back into the

transmission fibre.

Dispersion compensating fibre is expensive to produce, particularly in long lengths (the

cost of a DCF module scales very approximately with the amount of compensation

offered) and thus the amount of compensation that can be provided is usually limited by

cost considerations. Known DCF typically can be used to compensate for up to 100 km

of large-effective-area fibre (LEAF).

An object of the invention is to provide dispersion compensation for longer lengths of

transmission fibre than the lengths typically compensated by prior art dispersion-

compensation devices and to provide that compensation at little extra cost. A further

object of the invention is to provide additional functionality to the compensation method

and apparatus. According to the invention there is provided A dispersive optical device for compensating chromatic dispersion in a dispersive optical-fibre transmission line, the

device comprising: (i) a circulator having a first port, a second port and a third port, the

circulator being arranged to couple light entering the first port to the second port and

light entering the second port to the third port, the first and third ports being connectable

to the transmission line; (ii) an optical fibre, for provision of compensating dispersion,

connected to the second port; and (iii) a reflector arranged to reflect, at least partially,

light that has passed through the optical fibre from the second port back through the

optical fibre to the second port; characterised by pumping means operable to optically

pump the fibre by Raman pumping.

The invention thus enables a doubling of the dispersion offered by each dispersion-

compensation module without the need for additional compensating fibre and

additionally provide amplification of the light. That not only reduces cost but allows

greater dispersion than is readily possible with prior art fibre modules.

Use of a circulator substantially prevents back reflection along the input path.

Circulators are, in themselves, well-known devices and can be embodied in a wide

variety of designs (see, for example, Harry J.R. Dutton "Understanding Optical

Communications" Prentice Hall PTR, New Jersey (1998), pp 253-257 for a discussion

of circulators including a discussion of operation of an example of a circulator). The

key principle in relation to the invention is that light entering the circulator at the first

port exits at the second port (to the optical fibre) and light entering the circulator at the second port (from the optical fibre) exits at the third port.

In one arrangement the pumping means preferably comprises a fibre coupler arranged in

line with the optical fibre and a pump amplifier connected to the coupler.

Advantageously with such an arrangement the reflector is highly reflective at the pump

wavelength and the signal wavelength. Raman pumping at a wavelength reflected by the

reflector provides efficient gain: the double pass by pump light significantly increases

efficiency, particularly when gain in the optical fibre is large, because the signals make

two passes through a region of high pump power.

Alternatively the reflector is substantially transmissive at the pump wavelength and the

pumping means optically pumps the fibre through the reflector. By introducing the

pump light into the fibre through the reflector to provide optical gain, this affects signals

as they propagate in each direction in the optical fibre. The reflector may have a short-

wavelength cut-off in its reflectivity characteristic. Radiation at wavelengths below the

cut-off wavelength may thus be coupled into the optical fibre through the reflector.

Preferably the device is arranged to provide substantially chirp-free pulses at the

reflector. Such an arrangement provides the benefit of using the second pass through

the optical fibre after reflection to pre-compensate for dispersion in a next stretch of

transmission line connected to the third port of the circulator.

The idea of such pre-compensation is considered inventive in its own right and according to a second aspect of the present invention there is provided a dispersive

optical device for compensating chromatic dispersion in a dispersive optical-fibre

transmission line, the device comprising: (i) a circulator having a first port, a second

port and a third port, the circulator being arranged to couple light entering the first port

to the second port and light entering the second port to the third port, the first and third

ports being connectable to the transmission line; (ii) an optical fibre, for provision of

compensating dispersion, connected to the second port; and (iii) a reflector arranged to

reflect, at least partially, light that has passed through the optical fibre from the second

port back through the optical fibre to the second port; characterised by being arranged to

provide substantially chirp-free pulses at the reflector.

In either aspect of the invention the optical fibre preferably has a negative ("normal")

dispersion at an operating wavelength of the device; optical fibre transmission lines

usually have "anomalous" dispersion at their operating wavelength. The optical fibre

provides the same dispersion to light propagating in either direction. The dispersion of

the optical fibre is preferably between -40 ps km^"1 and -200 ps km^"1, although in some

applications it may be as much as -300 ps km^"1 or -400 ps km^"1. The optical fibre is

preferably between 1 km and 20 km in length. More preferably, the fibre has a length of

between 5 km and 15 km.

The reflectivity of the reflector may differ from unity (i.e. be less than) by an amount

sufficient to provide a tap at the end of the fibre, by means of which propagating light

pulses may be monitored. Advantageously the device further comprises means to monitor the tapped pulses.

The reflector may have a band-pass reflectivity profile (i.e. a wavelength 'gap¹). Such an

arrangement allows channels to be dropped and/or added at the reflection point; that has

the particular advantages that the reflection point is suitable for signal detection and that

the channel adding/dropping takes place at the mid-point of the dispersion

compensation.

The reflector may have a variable reflectivity; for example, it may be a fibre-loop

mirror. Fibre-loop mirrors are, in themselves, well-known (see, for example, G.P.

Agrawal, "Non-linear Fibre Optics", 2^nd Edition, Academic Press Ltd., London(1995),

pp 121-125). The reflectivity of the fibre-loop mirror depends on the splitting ratio of a

fibre coupler that couples an input part of the optical fibre and an output part of the

optical fibre to the two ends of the loop itself. The splitting ratio itself is dependent on

the power of the incident signal pulses.

The reflector may be a fibre Bragg grating.

Preferably the reflector is formed directly on the end-face of the optical fibre; losses may

thereby be reduced because there is then no need for a splice at the exit of the optical

fibre. Preferably, the reflector is a dielectric stack.

Alternatively, the reflector is formed on the end-face of a length of standard optical fibre, which is spliced to the optical fibre for provision of compensating dispersion. In

that case, there would be a loss of approximately twice the splice loss but there would

be no need to alter irreversibly the dispersion-compensating fibre, so it could be re-used

in other applications.

According to a third aspect of the invention there is provided a method of compensating

chromatic-dispersion in a dispersive optical-fibre transmission line, comprising:

inputting light into a circulator that couples the light into an optical fibre providing

dispersion compensation; propagating the light along the fibre to a reflector; at least

partially reflecting the light at the reflector; propagating the reflected light back along

the fibre to the circulator; and outputting the reflected light from the circulator;

characterised by optically pumping the optical fibre by Raman pumping.

Preferably the method further comprises adding and/or dropping channels in a wave-

division multiplexing system through the reflector.

Preferably the input light is input from the transmission line and the output light is

output to the transmission line.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, of which: Figure 1 is a schematic diagram of a dispersion compensation module according to the

invention;

Figure 2 is a schematic diagram of a dispersion compensation module according to the

invention incorporating a fibre loop mirror;

Figure 3 is a schematic diagram of a dispersion compensation module incorporating an

end-pumping scheme;

Figure 4 is a schematic diagram of a dispersion compensation module according to the

invention incorporating a Raman pumping scheme; and

Figure 5 is a schematic diagram of a dispersion compensation module according to the

invention incorporating a band-pass mirror.

Each of the illustrated embodiments comprises signal input 10, signal output 20,

circulator 30 and a length of dispersion-compensating optical fibre 40. The dispersion

compensation module is inserted into a length of optical-fibre transmission line 50. The

optical fibre transmission line includes (not shown) a pre-amplifier connected upstream

of the circulator and a booster amplifier connected downstream of the circulator.

Substantially all signal light (at around 1550 nm) propagating in the transmission line 50 is directed into the optical fibre 40 by the circulator 30. In the first illustrated embodiment (Figure 1), a reflector in the form of a mirror 60 is provided at the end of

the optical fibre; the reflectivity of the mirror is substantially unity for all the signal

wavelengths. The mirror is directly formed as a dielectric stack on the end of the

dispersion compensating fibre in order to reduce losses by eliminating the need for a

splice. The additional loss in the device, compared with a prior art module, is that of the

circulator (< 1 dB) and the loss of the mirror.

Alternatively, if mirror 60 has a reflectivity of less than unity, light transmitted through

the mirror may be monitored as a tap. In DM (Dispersion Managed) soliton systems

that tap will be close to the point where chirp-free pulses are expected.

In the embodiment of Figure 2, the reflector is a fibre loop mirror 70, which can provide

variable reflectance.

In the embodiment of Figure 3, fibre 40 is pumped through a partially reflecting mirror

80. Mirror 80 has a long-wavelength cut-off in its reflectance, which allow coupling of

pump light into the fibre, whereas signal light is substantially completely reflected.

Signal light makes a double pass through the pumped region.

In Figure 4, fibre 40 is Raman-pumped through a fibre coupler 42, two ports of which

are connected to fibre 40 and a third port of which is connected to an optical amplifier

44 that amplifies light at the Raman pump wavelength. Mirror 46 is highly reflective at both the signal and the pump wavelengths (it has near unity reflectivity from 1400 nm to

1650 nm). In Raman pumping, pump photons of a high frequency (short wavelength)

excite electrons in atoms of the fibre to higher energy levels, from which they decay to

give signal photons of the lower signal frequency (longer wavelength) plus phonon

energy. Thus pump photons are converted into signal photons, providing gain to the

signal. The pump light is of five equally spaced wavelengths between 1421 nm and

1495 nm, which results in a flat gain spectrum at signal wavelengths between 1530 nm

and 1565 nm (C band).

In wave-division multiplexing (WDM) communications systems, signal pulses are

transmitted in wavelength channels each having a different carrier wavelength. In the

embodiment of Figure 5, channels can be dropped and added at the reflection point by

means of a mirror 90 with a band-pass reflectivity spectrum. The reflectivity of the

reflector 90 is low for frequencies in a band (of 50 GHz width) within the operating

bandwidth and high for all other frequencies in that bandwidth (which extends from

1530 nm to 1605 nm). Reflector 90 is a fibre Bragg grating, which is a length of optical

fibre having a substantially periodic or chirped variation in refractive index, which

causes only some frequencies to be reflected. Channels added at the reflector 90 are

conveniently at or near the mid-point of the dispersion compensation.

Claims

CLAIMS:

1. A dispersive optical device for compensating chromatic dispersion in a

dispersive optical-fibre transmission line, the device comprising: (i) a circulator having

a first port, a second port and a third port, the circulator being arranged to couple light

entering the first port to the second port and light entering the second port to the third

port, the first and third ports being connectable to the transmission line; (ii) an optical

fibre, for provision of compensating dispersion, connected to the second port; and (iii) a

reflector arranged to reflect, at least partially, light that has passed through the optical

fibre from the second port back through the optical fibre to the second port;

characterised by pumping means operable to optically pump the fibre by Raman

pumping.

2. A device as claimed in claim 1, in which the pumping means comprises a fibre

coupler arranged in line with the optical fibre and a pump amplifier connected to the

coupler.

3. A device as claimed in claim 2, in which the reflector is highly reflective at the

pump wavelength and the signal wavelength.

4. A device as claimed in claim 1, in which the reflector is substantially

transmissive at the pump wavelength and the pumping means optically pumps the fibre

through the reflector.

5. A device as claimed in claim 4, in which the reflector has a short-wavelength

cut-off in its reflectivity characteristic.

6. A device as claimed in any preceding claim which is arranged to provide

substantially chirp-free pulses at the reflector.

7. A dispersive optical device for compensating chromatic dispersion in a

fibre from the second port back through the optical fibre to the second port;

characterised by being arranged to provide substantially chirp-free pulses at the reflector.

8. A device as claimed in any preceding claim, in which the optical fibre has a

negative ("normal") dispersion at an operating wavelength of the device.

9. A device as claimed in claim 8, in which the dispersion of the optical fibre is

between -40 ps km^"1 and -200 ps km^"1 or between -300 ps km^"1 and —400 ps km -1

10. A device as claimed in claim 8, in which the dispersion of the optical fibre is

between 1 km and 20 km in length.

11. A device as claimed in any preceding claim, in which the reflectivity of the

reflector differs from unity by an amount sufficient to provide a tap at the end of the

fibre, by means of which propagating light pulses may be monitored.

12. A device as claimed in claim 11, further comprising means to monitor the tapped

pulses.

13. A device as claimed in any preceding claim, in which the reflector has a band¬

pass reflectivity profile.

14. A device as claimed in any preceding claim, in which the reflector is a fibre-loop

mirror.

15. A device as claimed in any of claims 1 to 13, in which the reflector is a fibre

Bragg grating.

16. A device as claimed in any of claims 1 to 13, in which the reflector is formed

directly on the end-face of the optical fibre.

17. A device as claimed in claim 16, in which the reflector is a dielectric stack.

18. A device as claimed in any of claims 1 to 13, in which the reflector is formed on

the end-face of a length of standard optical fibre, which is spliced to the optical fibre for

provision of compensating dispersion.

19. A method of compensating chromatic-dispersion in a dispersive optical-fibre

transmission line, comprising: inputting light into a circulator that couples the light into

an optical fibre providing dispersion compensation; propagating the light along the fibre

to a reflector; at least partially reflecting the light at the reflector; propagating the

reflected light back along the fibre to the circulator; and outputting the reflected light

from the circulator; characterised by optically pumping the optical fibre by Raman

pumping.

20. A method according to claim 19, in which channels in a wave-division

multiplexing system are added and/or dropped through the reflector.

21. A method according to claim 19 or claim 20, in which the input light is input

from the transmission line and the output light is output to the transmission line.