GB2379327A

GB2379327A - Amplifier

Info

Publication number: GB2379327A
Application number: GB0120882A
Authority: GB
Inventors: Keith John Dowling; Hayden Scott Fews
Original assignee: Marconi Caswell Ltd; Marconi Optical Components Ltd; Bookham Technology PLC
Current assignee: Marconi Caswell Ltd; Marconi Optical Components Ltd; Lumentum Technology UK Ltd
Priority date: 2001-08-30
Filing date: 2001-08-30
Publication date: 2003-03-05
Also published as: GB0120882D0

Abstract

An optical amplifier comprises a multi-mode pump source 5, single-mode input 1 and output fibres 3, and a circulator 2 having at least three ports (11,12,13). The input fibre is connected to a first port, the pump source is fed to a second port and the output fibre connected to a third port, wherein the pump source 5 is connected to the second port via a gain fibre 4. The gain fibre 4 is provided with a reflector 6 to reflect a light signal back towards the optical circulator 3. The gain fibre 4 may be a double clad fibre. Preferably, the reflector 6 is a Bragg grating which has a gain flattening function. A pigtail fibre 3 may be provided between the double clad fibre 4 and the circulator 2.

Description

Amplifier The invention relates to a fibre optical amplifier with exemplary use in an optical fibre communication system Optical fibre amplifiers are standard components in optical fibre communication systems and generally comprise lengths of appropriately doped single-mode fibre connected in series in the communication route. The lengths of doped fibre are pumped with light from laser diodes through appropriate coupling arrangements. The wavelength of the pump laser being different to that of the communication optical signal wavelength.

In one known approach, known as core pumping, both the signal wavelength and the pump light co-propagate in the same core of a doped single-mode fibre. This achieves maximum absorption of the pump light by the doping ions. However, the pump light must be provided in a single-mode optical fibre, and power that can be obtained from known single-mode laser diodes is limited. To achieve high powers any such system requires a number of pump diodes, thereby increasing the cost and complexity of the system In another known approach, known as cladding pumping, multi-mode pump laser diodes can be used which achieve much higher power output. In such arrangements, rare-earth-doped double-clad fibres are used that have a single-mode guide for the optical signal surrounded by a multi-mode pump guide. Although such an amplifier has a lower pump absorption per unit length than the core-pumped approach, this configuration enables the amplifier to be pumped using high power, low cost multimode pump sources whilst still having a single-mode communication optical input and output.

However, this arrangement suffers from the problem of coupling a multi-mode pump source and a single-mode communication input and output into the same double-clad fibre. It is physically impossible to focus the multi-mode pump light into a singlemode optical fibre without high losses. The known solution to this problem generally involves spatially separating the multi-mode pump port from the single-mode fibre

port and modifying the double-clad fibre in such a way that permits the pump light and optical signal to be co-launched. An example of such an approach is disclosed in US5999673.

Although the known approaches achieve reasonable pump coupling efficiences, the coupling means are quite complex to manufacture. The present invention seeks to provide an amplifier having an alternative coupling arrangement that is simpler to produce.

According to the invention there is provided an optical amplifier comprising a multimode pump source, single-mode input and output fibres, an optical circulator and a multiple clad gain fibre, wherein the input signal fibre being connected to a first port of the optical circulator, the gain fibre is connected to a second port and the output signal fibre is connected to a third port, the pump source being coupled into the end of the gain fibre remote from the second port, which end of the gain fibre is provided with a reflector to reflect a light signal back towards the optical circulator and to transmit the pump wavelength, which is coupled into the gain fibre.

The arrangement of the invention advantageously overcomes the problem of multiplexing the pump source and optical signal into the gain fibre. The design also advantageously halves the length of gain fibre required as the signals pass twice through the gain medium. The pump signal is minimally affected by the reflector.

Preferably the further comprises a gain flattening function substantially in the middle of the amplifier thereby minimising noise figure and maximising gain bandwidth.

Preferably a fibre pigtail is provided between the double-clad gain fibre and the optical circulator to prevent unabsorbed pump light reaching the circulator. preferably the incoming light is polarised and the gain fibre and the fibre pigtail are polarisation maintaining and a polarisation rotator is provided such that counter propagating signals in the gain fibre are orthogonally polarise. Preferably, the reflector is a fibre Bragg grating. Preferably, the gain fibre is a double clad fibre.

An exemplary embodiment of the invention will now be described in greater detail with reference to the drawings in which :-

Fig. I shows a schematic diagram of an optical amplifier according the invention Fig. 2 shows a schematic flow diagram of a circulator Fig. 1 shows a schematic diagram of an optical amplifier comprising a single-mode input fibre I connected to a first port of a three port circulator 2. The second port of the circulator 2 is connected to a single-mode fibre pigtail 3, which in turn is spliced to a gain fibre 4 at B. The end of the gain fibre 4 remote from the single-mode fibre pigtail 3 is connected to a multimode pump laser 5 at A. The gain fibre 4 is provided with a fibre Bragg grating, or other reflector, 6 at the end adjacent to the connection to the multi-mode pump laser 5. A single-mode output fibre 7 is connected to the third port of the circulator 2.

The gain fibre 4 will typically be a double-clad fibre, such as described in US5566196. These fibres use a single-mode guide for the optical signal surrounded by a multi-mode pump guide. The single-mode core is doped with rare-earth ions to provide the desired gain. The cladding surrounding the core is itself surrounded by a low index polymer, or glass, second cladding which allows it to become a guiding structure. In use, pump light is launched into the fibre into the undoped cladding, where it propagates in multi-mode fashion interacting with the doped core as it travels along the fibre.

Fig. 2 shows a schematic flow diagram of a circulator. The circulator 2 is a passive multi-fibre junction in which an incoming signal is routed to another fibre. In use, a signal incident at port 11 is routed typically using a Faraday rotator and optically active rotator in series to port 12. Similarly, a signal incident a port 12 is routed to port 13. In general, this degree of funct lity is sufficient but in some designs it is possible to route signals from port 13 to port 11. Four port circulator designs are also known.

With reference to Figure I and 2, in use, an optical signal at wavelength à is introduced into the amplifier at port I I via the fibre 1 and is routed to port 12 by the

circulator 2. The optical signal is then passed down the fibre pigtail 3 to the gain fibre 4, at B The pump laser signal at wavelength Ap is introduced at the other end of the gain fibre 4 at A. In order that the optical signal exits the fibre at the same end from which it was launched, the gain fibre 4 is provided with a reflector for the signal

wavelength c. The amplified optical signal c'then enters the circulator at port 12 and is routed to port 13 the output of the amplifier. Reflection at port 12 being kept to a minimum to avoid the optical path between the port 12 and the reflector 6 acting as a laser cavity.

The reflector 6 may be any reflector and may also be reflectivity profiled so as to flatten the gain curve by strengthening, or weakening, the reflectivity with wavelength.

The pump laser 5 input at wavelength Â. p propagates along the cladding surrounding the doped inner core of the double-clad gain fibre. Concurrently, the optical signal at wavelength ? propagates along the doped inner core of the gain fibre. The multimode pump laser energy excites to a higher level the energy state of the dopant in the inner core. This elevated energy state cannot be sustained indefinitely and drops back down to its equilibrium state by releasing photonic energy in sympathy with the coexisting photonic wavelength Xc. Thus the communication optical signal), c is amplified as % c'. An advantage of the preferred embodiment is that the length of double-clad gain fibre required is significantly reduced.

For clarity of explanation the optical signal and wavelength have been described in the singular. The invention has greater application for a plurality of optical signals on a plurality of wavelengths, wherein the reflector 6 reflects at the plurality of wavelengths. Such a situation exists where the invention is used within a wavelength divisional multiplex (WDM) or dense wavelength divisional multiplex (DWDM) optical communications system.

The circulator can be a polarisation sensitive component and some circulators polarise the light signal to facilitate the routing. This can advantageously be used in a further embodiment to reduce crosstalk in the gain medium. Whilst the invention is described

using a three port circulator, in practice, a circulator with more than three ports may be used.

In this further embodiment, the fibre pigtail 3 and the gain fibre 4 are both polarisation maintaining fibres. A polarisation changer is provided adjacent to the reflector 6, so that the polarisation of reflected light is changed, e. g. from TE mode to TM mode. As the different polarisations will not interact, cross talk will be minimal.

The invention has been described in an exemplary communication system application.

However, the amplifier means can be used in any other optical system, for example, medical applications in surgical and diagnostic procedures, or industrial cutting tools.

Claims

Claims 1. An optical amplifier comprising a multi-mode pump source, single-mode input and output fibres, an optical circulator and a multiple clad gain fibre, wherein the input signal fibre being connected to a first port of the optical circulator, the gain fibre is connected to a second port and the output signal fibre is connected to a third port, the pump source being coupled into the end of the gain fibre remote from the second port, which end of the gain fibre is provided with a reflector to reflect a light signal back towards the optical circulator and to transmit the pump wavelength, which is coupled into the gain fibre.
2. An optical amplifier according to Claim 1, wherein the gain fibre is a doubleclad fibre
3. An optical amplifier according to Claim 2, wherein the reflector further comprises a gain flattening function substantially in the middle of the amplifier thereby minimising noise figure and maximising gain bandwidth.
4. An optical amplifier according to any one of Claims 1 to 3, wherein a fibre pigtail is provided between the double-clad gain fibre and the optical circulator to prevent unabsorbed light reaching the circulator.
5. An optical amplifier according to any one of Claims 1 to 4, wherein the incoming light is polarised and the gain fibre and the fibre pigtail are polarisation maintaining and a polarisation rotator is provided such that counter propagating optical signals in the gain fibre are orthogonally polarise.
6. An optical amplifier according to any one of Claims 1 to 5, wherein the reflector is a fibre Bragg grating.