GB2253514A

GB2253514A - Optical amplifiers

Info

Publication number: GB2253514A
Application number: GB9104711A
Authority: GB
Inventors: Michael John Shearme; Hwa Yaw Tam
Original assignee: GEC Marconi Ltd; Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1991-03-06
Filing date: 1991-03-06
Publication date: 1992-09-09
Also published as: GB9104711D0

Abstract

An optical fibre pre-amplifier and amplifier comprises first and second optical fibres respectively, each fibre including a doped length, at least one optical pump source for counter-pumping the second optical fibre, and coupling means whereby a proportion of the output from the optical pump source is used to counterpump the first optical fibre. Alternatively, an optical fibre pre-amplifier and amplifier comprises first and second optical fibres respectively, each fibre including a doped length, a first optical pump source for co-pumping the second optical fibre and a coupling means whereby a proportion of the output from the first pump source counterpumps the first optical fibre and a second optical pump source for counter-pumping the second optical fibre. Fig 2 and (Figs 3, 4, 6) illustrate alternative embodiments. Fibre (38) (Fig 4) is pumped by the residual power of pump source (20). Filter (62) (Figs 6, 7) stops out-of-band ASE, for example. <IMAGE>

Description

OPTICAL AMPLIFIERS This application relates to doped fibre optical amplifiers.

Current systems employ silica fibres which have been doped with erbium and other trace elements such as germanium and aluminium. The fibre is pumped using an optical source at a frequency which causes some of the dopant to be excited into a higher energy state. This higher state can decay by stimulated emission in response to an input signal, thus causing amplification of the input signal.

In order to obtain high output powers from these amplifiers it is common to pump the fibre from both directions, although systems can use only one pump if required (co- or counter-pumped).

Particularly in the case of bidirectional pumping it is difficult to achieve high gain whilst at the same time achieving maximum output power. The solution to this has been to provide a second amplifier in front of the power amplifier, which in the past has been achieved with a separate pre-amplifier.

The object of this invention is to produce a system which is cheaper than such a double amplifier system.

According to the present invention there is provided an optical fibre pre-amplifier and amplifier comprising first and second optical fibres respectively, each fibre including a doped length, at least one optical pump source for counter-pumping the second optical fibre, and coupling means whereby a proportion of the output from the optical pump source is used to counterpump the first optical fibre.

There is further provided an optical fibre pre-amplifier and amplifier comprising first and second optical fibres respectively, each fibre including a doped length, a first optical pump source for co-pumping the second optical fibre and a coupling means whereby a proportion of the output from the first pump source counterpumps the first optical fibre and a second optical pump source for counter-pumping the second optical fibre.

The present invention will now be described, by way of example, with reference to the accompanying drawings in which Figure 1 shows a prior art optical amplifier using bidirectional pumping; Figures 2 to 4 show various embodiments of the present invention using bidirectional pumping; Figure 5 shows an enlarged detail of a wavelength division multiplexer of Figure 4; Figure 6 shows a further embodiment using a single counter-pumping source; Figure 7 shows the response of the filter used in the embodiment of Figure 6.

In Figure 1 an amplifier 1 includes an optical fibre 2, having undoped lengths 4, 6 and a doped length 8 therebetween. The lengths 4, 6 of the fibre 2 are connected to respective wavelength division multiplexers (WDMs) 10, 12 at ports 14, 16. Optical pump sources 18, 20 are connected to each WDM 10, 12 at respective ports 22, 24 by optical fibres 26, 28.

The output from the pump source 18 is coupled to the fibres 2 by the WDM 10, so as to travel from length 4 to length 6.

Similarly the output from the pump source 20 is coupled to the fibre 2 by the WDM 12, so as to travel from length 6 to length 4.

As a result of this arrangement the fibre 2 is subjected to bidirectional pumping to increase the output power from the amplifier.

An input signal 30 is applied to the WDM 10 at a port 32 whereby it is coupled to the fibre 2.

The dopant in the doped length 8 is excited into a higher energy state by the pumping of the fibre and decays from this higher energy state by stimulated emission, being stimulated by the input signal 30 causing amplification thereof to produce an output signal 34.

In Figure 2 is shown a first embodiment of the present invention which includes an amplifier as shown in Figure 1, the equivalent components having the same identification numbers as in that figure. Additionally, there is a pre-amplifier 36 which comprises a optical fibre 38 including a similarly doped length 40.

The output end 42 of the fibre 38 is connected to a WDM 44 at a port 46. A length of fibre 48 connects the input port 32 of WDM 10 to the output port 50 of the WDM 44.

A proportioncc % of the output of the pump source 18 is tapped off from the fibre 26 by use of a coupler 52 the remainder of the output p % being directed to the WDM 10 as before. The- proportion ct % is routed by an optical fibre 54 to the WDM 44 at a port 56. An isolator 58 connected to the fibre 38 prevents any excess output from the pump source 18 reaching the source of the input signal and overloading it. This isolator 58 may not be necessary in all cases.

A variation of this embodiment is shown in Figure 3 where the WDMS 10, 44 are combined into a single four-part WDM 60. All the other parts of the system are similar to those in Figure 2 and have the same reference numbers.

Typical performance figures for the embodiments of Figures 2 and 3 give a pre-amplifier gain of 13dB with an output power of around 15dim. The required gain is 20dB. The pump powers are 30-40mW. If 5mW is tapped off to power the pre-amplifier 36 the output power is reduced by less than 0.5dB but the necessary extra gain of 7dB would be achieved easily.

A further embodiment is shown in Figures 4 and 5 which is similar to the embodiment shown in Figure 2 and equivalent ports are given the same reference numbers. The WDM 44 of Figure 2 is omitted and the end 42 of the fibre 38 is connected to the port 32 of the WDM 10.

As a result, rather than the output of the pump source 8 being shared between the WDMs 10, 44 the fibre 38 is pumped by the residual power of the pump source 20. If the pump sources 18, 20 have different optical frequenciesXP1 and > P2 then a wavelength division multiplexer 10 could be used which will combine the co-pump 18 outputA P1 with the signal5, but let the residual power of the counterpump 20 pass to the pre-amplifier 36 as shown in Figure 5.

A final embodiment is shown in Figures 6 and 7. The residual counter pump power is used to pump the pre-amplifier 36 as in Figure 4. Between the pre-amplifier 36 and the amplifier 1 is a filter arrangement 62 the response of which is shown in Figure 7.

The filter arrangement 62 passes the pump frequency\ and the signal frequency Xs but stops the unwanted signals such as the out-of-band Amplified Spontaneous Emission (ASE).

The result of using this configuration is that the unwanted signals are not passed to the power amplifier 1 and pump energy is not wasted in it. Accordingly, the amplifier output is improved, firstly by not wasting power in amplifying unwanted signals and secondly by improving the quality of the signal and obviating the need to place a filter after the power amplifier 1 which causes a power loss.

While the embodiment shows a counter pumped system using a single pumped source, the approach can be used with any optical amplifier having at least one counterpumping source and may be extended to multiple lengths of optical fibre.

Claims

1. An optical fibre pre-amplifier and amplifier comprising first and second optical fibres respectively, each fibre including a doped length, at least one optical pump source for counter-pumping the second optical fibre, and coupling means whereby a proportion of the output from the optical pump source is used to counterpump the first optical fibre.

2. An optical fibre pre-amplifier and amplifier comprising first and second optical fibres respectively, each fibre including a doped length, a first optical pump source for co-pumping the second optical fibre and a coupling means whereby a proportion of the output from the first pump source counterpumps the first optical fibre and a second optical pump source for counter-pumping the second optical fibre.

3. An optical fibre pre-amplifier and amplifier as claimed in Claim 2 wherein the coupling means is a wavelength division multiplexer.

4. An optical fibre pre-amplifier and amplifier as claimed in Claim 1 wherein the coupling means is an optical filter.

5. An optical fibre pre-amplifier and amplifier substantially as hereinbefore described with reference to and as illustrated in Figure 2, or Figure 3, or Figures 4 and 5 or Figure 6 and 7 of the accompanying drawings.