WO2000050943A1

WO2000050943A1 - High isolation couplers

Info

Publication number: WO2000050943A1
Application number: PCT/IE2000/000023
Authority: WO
Inventors: Patrick William Burke; Satoru Moriya; Hillary Patricia Cronin; Ooto Kazuhiro
Original assignee: Sumicem Opto-Electronics (Ireland) Ltd.
Priority date: 1999-02-22
Filing date: 2000-02-16
Publication date: 2000-08-31
Also published as: AU2569200A; IES990137A2

Abstract

The invention relates to wavelength couplers (11) with high isolation and in particular it is directed and describes devices that include a fused taper wavelength division multiplexer (DWM) which incorporates wavelength selective optical reflection elements such as fibre Bragg gratings. The incorporation of wavelength filtering means (12, 13) located on at least one of the output ports (7, 8) on the coupler allow the filtering of light of particular wavelengths through the output ports (7, 8).

Description

High Isolation Couplers

Field of Invention

The present invention relates to couplers with high isolation and in particular is directed to devices that include a fused taper wavelength division multiplexer (WDM) and w avelength selective optical reflection element such as a fibre Bragg grating.

Background to the Invention

Fibre optic couplers are known for a variety of uses including the splitting of light into two or more component wavelengths or splitting of power as required. T\ pically the type of coupler used for the splitting of wavelengths is known as a WDM coupler. They may be manufactured using a process called Fused Biconical Taper (FBT) technology. A FBT fibre coupler consists of two or more optical fibres whose optical cladding has been fused together. The structure is tapered by elongation while it is hot until appropriate coupling properties are achieved that allows light to transfer between the fibres.

As detailed in US 5,388,173, US4,474,427, US5, 104,209 and US 5367,588 gratings are already known. Gratings in fibre may be achieved by photo inducing an index change in the glass of the fibre by exposing the glass to UN light and thus changing the refractive index. Photosensitivity is the fibre characteristics that make the w riting of the fibre grating possible. A fibre bragg grating is a periodic modulation of the index of refraction in the core of the fibre. When light travelling down the core of the fibre comes in contact with the bragg grating a selected band of wavelengths is reflected back in the opposite direction. Bragg gratings are short period gratings \\ orking in reflection, other grating types available are long period gratings or intermodal coupling gratings. Conventional methods of forming such gratings on fibre optic are described in US 5042 897, US 5061 032 and US 5367 588. Figure 1 shows a bi-directional WDM coupler 1 according to the prior art. It consists of two fibres 2. 3 whose optical cladding has been fused together to form a coupling region 4. The WDM coupler has two input ports 5, 6 and two output ports 7, 8. Light entering an input port 5 at two different wavelengths λ_{l 5} λ₂ is separated into the two components λ, and λ₂. In an ideal WDM coupler the wavelength separation is complete, i.e. that 100% of λ_ exits the coupler through output port 7 and 100% of λ₂ exits through output port 8. In reality this degree of separation is not possible and some percentage of λ, will also exit through output port 8 and some percentage of λ₂ through port 7. Wavelength isolation, or crosstalk, is a measure of how well the different wavelengths are separated at the two output ports, is expressed in dB, and using the example of Figure 1 can be defined as:

isolation λ = -lOlog (λ,(port 8)/ λι(port 7))

where

λ,(port 8) is the amount of light at wavelength λ_ passing though port 8, and λι(port 7) is the amount of light at wavelength λ_ passing through port 7.

Using the example of the 1480/ 1550 nm coupler above, the isolation of the 1480 nm wavelengths port is the measure of 1550 light that is outputted through that port, conversely the isolation of 1550 port is the measure of the 1480 light outputted.

The majority of transmission over fibre optic lines operates at 1550nm wavelength. 1550nm is the most popular wavelength due to the existence of an optical amplifier which has the capability to amplify signals at this wavelength. This optical amplifier is an Erbium Doped Fibre Amplifier (EDFA). The EDFA is one of the most critical components in a long haul fibre optic network that amplifies the transmission signal by use of a 980 or 1480nm pump laser. To amplify the optical signal the pump wavelength (980 or 1480nm) is introduced to excite a rare element known as Erbium that is doped into a piece of fibre. When a transmission signal in the 1550nm window passes through the same fibre, the excited ions collide with the 1550nm photons, thus boosting the signal as it exits the amplifier.

WDM couplers are commonly used in amplifiers such as that described above to combine pump and signal wavelengths and thus allow amplification to occur.

Essentially the WDM facilitates the addition of the 980 or 1480 wavelengths which excites the Erbium. For example a 1480/1550 WDM splits or combines 1480 and 1550 wavelengths. Two of the critical parameters for WDM couplers are to have high isolation while keeping insertion loss low. High isolation is essential to ensure maximum separation of the two wavelengths, while low insertion loss is a critical parameter for any optical component or system.

Using Fused Biconical Taper technology alone the isolation requirements for some EDFA\s are very difficult to achieve. The coupling ratio of WDM couplers today are designed based on sine wave wavelength dependency, which causes poor isolation with wider wavelength range. This can be seen especially in the example of a 1480/1550 WDM due to narrow wavelength separation. For example if typical isolation values of a 1480/1550 WDM are considered, isolation typically achievable at 1550nm would be 23dB, whereas at 1570nm, isolation can by typically approximately lOdB. With a 980/1550 WDM, isolation typically achievable at 1550nm would 25dB, whereas at 1570nm, isolation can by typically approximately 20dB. The fundamental difference between these devices is the wavelength separation of the two wavelengths, the WDM 1480/1550 has the narrower wavelength separation, and thus has poorer isolation across the wider wavelength range (1550-1570nm). Using the currently available WDM couplers acceptable isolation figures are currently not achievable, and as a result amplifier manufacturers have to utilise bulk optic WDM systems in order to utilise the coupler within amplification systems.

There are several known methods to achieve high isolation WDM couplers. One such method is to cascade several WDM couplers together, the disadvantages of this method are that they introduce higher insertion loss due to using several WDM's plus the package size is large. As mentioned previously another method used is bulk optics technology, which can achieve high isolation but components are expensive and show higher insertion loss. In addition with the bulk optic technology, several components need to be combined in order to achieve high isolation.

The limitations of known fused taper WDM couplers in today's market is that they do not always achieve satisfactory levels of high isolation across the required wavelength range, without the addition of other physical components such as isolators or filters. The limitations of the technologies described so far are cost, size and complexity. There are no known one piece components available that have the necessary specifications.

Object of Invention

It is an object of the present invention to provide a WDM coupler which provides an additional improvement in isolation. It is a further object of the present invention to provide an all fibre WDM with isolation values not previously achievable with an all fibre component.

Summary of Invention

Accordingly the present invention provides a fibre optic coupler comprising: at least one input port, at least two output ports, and wavelength filtering means located substantially at, or on, at least one of said at least two output ports, the wavelength filtering means predefined to allow light of particular wavelengths to pass through said at least one of said at least two output ports and to reflect light of all other wavelengths.

The coupler is preferably an all fibre optic component. The wavelength filtering means is preferably an integral part of the all fibre optic coupler. Where two or more wavelength filtering means are used on two or more output ports, the choice of which wavelength filtering means is used at which port is made based on the desired wavelength to be emitted through said port

The wavelength filtering means is preferably a fibre reflection grating

The fibre grating is preferably a Bragg fibre grating

The grating is either written, or etched, directly on said at least one of said at least two output ports, or written on a piece of fibre which is subsequently spliced to said at least one of said at least two output ports

The coupler is preferably for use with optical amplifiers to combine pump and signal wavelengths, and more preferably for use with the type of optical amplifiers known as Erbium Doped Fibre Amplifier (EDFA)

The coupler may also be used for the splitting or combining of signals in optical transmission systems

The coupler is preferably used in conjunction with inputted light of wavelengths 1480 and 1550nm where it is desired to separate the light at output ports into the component wavelengths

Alternatively the coupler may be used in combination with inputted light of u avelengths of 1310 and 1550 nm where it is desired to separate the light at output ports into the component wavelengths

The coupler may be used in conjunction with any other wavelength combination w here high isolation is required

The coupler preferably achieves isolation values in excess of 20dB over the range 1530 to 1560 nm Brief Description of the Drawings

Figure 1 shows a known WDM coupler, Figure 2 shows a WDM coupler according to the present invention,

Detailed Description of the Invention

As detailed above in the section "Background to the Invention" Figure 1 shows a known WDM coupler 1. Figure 2 shows a WDM coupler 11 according to one embodiment of the present invention. The same reference numerals are used for similar parts. The coupler comprises two fibres 2, 3, the optical cladding of which has been stripped back and fused together to form a coupling region 4. The coupler has two input ports 5, 6 and two output ports 7, 8. In this embodiment two gratings 12, 13 are written on the output ports 7, 8 respectively, but depending on the application or requirement for the coupler the number of gratings can be one or more, i.e. both ports may have gratings written to them or either one of them may have. The choice of which grating is written to which port is made on the basis of which wavelengths of light are required to pass through this port and which wavelengths are required for reflection. A high isolation WDM coupler is thus achieved by writing a particular fibre grating on the required output port. The grating can be placed inside or outside the WDM coupler package, and can be written on the output port, or on a fibre spliced to the output port, in order to reflect the unwanted wavelength at that output port. Gratings can be achieved on standard off the shelf single mode fibre or photosensitive fibre.

Referring to Figure 2 and using the example of a WDM coupler operating with incoming light at input port 5 of a mixture of 1480/1550 nm, which requires separation to the component wavelengths at the output ports 7, 8. The grating is written on output port 7 to allow light of wavelength 1480 nm to pass through and to reflect light of wavelengths 1550 nm. Conversely the choice of grating 13 made at output port 8 is such to reflect light wavelength 1480 nm and to allow light of wavelength 1550 nm through. Using the isolation formula highlighted previously this elimination of the mixture of wavelengths passing through each output port improves the isolation of the wavelengths.

To explain further with an example of a high isolation 1480/1550WDM grating coupler, on the 1550nm output port the 148ϋnm power range will be reflected back. Conversely on the 1480nm port the 1550nm wavelength range power will be reflected. As such a WDM coupler is achieved that provides high isolation across specified wavelength ranges and as gratings produces very low power loss their combination with WDM's has minimal effect on the WDM coupler loss.

Although described with reference to a 1480/ 1550 nm WDM coupler it will be appreciated by those skilled in the art that the coupler of the present invention can be modified to utilise any wavelength combination, such as 980/ 1550 nm or 1310/ 1550 nm. It will also be apparent to those skilled in the art that although the invention has been described with reference to a "2 X 2" coupler, i.e. 2 input ports and 2 output ports, it will be equally applicable to any "n X n" ( where n is a whole number) coupling device. It will also be apparent to those skilled in the art that depending on the application, any number of input ports may be terminated so as to make them redundant.

The combination of the fused taper device & fibre grating allows dramatic improvement in the isolation value normally achievable with fused taper WDM couplers. By incorporating the grating as an integral fibre component the size of the complete package is reduced and the complexity of the high isolation WDM system is minimised.

Typical isolation figures achievable using the device described in Fig 2 are greater than 2ϋdB.

Example of manufacture of WDM coupler of the present invention

A fused taper WDM coupler is manufactured using standard FBT manufacturing techniques, such as that described in US 4 392 712. A Bragg fibre grating is etched or written on an equivalent fibre, in accordance with known methodologies, which is subsequently attached to the fused taper WDM coupler. This may be achieved by a technique such as fusion splicing or other standard techniques.

Alternatively the fibre grating is etched on the required output port of the fused taper WDM coupler.

Example of Isolation Results obtained using the WDM coupler of the present invention

The following table (Table 1) illustrates typical isolation results over a wavelength range 1530 to 1560 nm. The tests were done using standard optical test procedures and equipment.

Table 1

As can be seen from the table isolation results with an improvement of approximately 20dB are achieved across the entire tested range.

Advantages of the present invention

By using the device of the present invention it is possible to achieve a high isolation coupler in a one piece component without using additional physical components such as filters or isolators. The words "comprises/comprising" and the words "having/including" when used herein with reference to the present invention are used to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

Claims

1. A fibre optic coupler 11 comprising: a) at least one input port 5, 6, b) at least two output ports 7, 8, and c) wavelength filtering means 12, 13 located substantially at, or on, at least one of said at least two output ports, the wavelength filtering means predefined to allow light of particular wavelengths to pass through said at least one of said at least two output ports and to reflect light of all other wavelengths.

2. The coupler as claimed in claim 1 whereby the coupler is an all fibre optic WDM coupler, and the wavelength filtering means is an integral part of the coupler.

3. The coupler as claimed in any preceding claim whereby where two or more wavelength filtering means are used on two or more output ports, the choice of which wavelength filtering means is used at which port is made based on the desired wavelength to be emitted through said port.

4. The coupler as claimed in any preceding claim whereby the wavelength filtering means is a fibre reflection grating, preferably a Bragg fibre grating, either written, or etched, directly on said at least one of said at least two output ports, or written on a piece of fibre which is subsequently spliced to said at least one of said at least two output ports.

The coupler as claimed in any preceding claim wherein the light of particular wavelengths is selected from one of the following wavelength types: a) light of wavelength of 1310 and 1550 nm, or b) light of wavelength of 1480 and 1550 nm.

6. The couplers claimed in any preceding claim wherein isolation values in excess 20dB are achieved over the range 1530 to 1560nm.

7. Use of the coupler as claimed in any preceding claim with optical amplifiers.

8. Use of the coupler as claimed in any preceding claim with optical amplifiers of the type known as Erbium Doped Fibre Amplifiers (EDFA).