GB2072876A - Bidirectional coupler for communication over a single fibre - Google Patents

Abstract

An assembly for bidirectional signal transmission over a single fibre is disclosed together with its method of manufacture. A hot coating (12) of dichroic material is applied to a surface of a glass substrate (20). The substrate is processed into dichroic wafers having larger dimensions than the cross section of the fibre with which it is employed. The dichroic wafer is positioned over the end face of a polished fibre beamsplitter (24) and secured thereto by a thin layer (30) of optical grade epoxy. A second polished fibre beamsplitter (34) may then be secured in a similar manner to the opposite side of the wafer. the coated wafer forms an acute angle of about 25 DEG with a plan perpendicular to the axis of each beamsplitter half. A bidirectional coupler is thereby formed. <IMAGE>

Description

SPECIFICATION Bidirectional coupler for communication over a single fibre This invention relates to bidirectional signal transmission over a single fibre utilizing wavelength duplexing.

There are many instances where duplex communication is desirable over a single optical fibre. Dichroic filters (multi-layer dielectric coatings) have typically been used in conjunction with the fibres to distinguish between two wavelengths.

A "cold" dichroic coating has been directly applied to an end face of an optical fibre. Such application is often characterized by poor adhesion of the coating. A "hot" coating would be generally preferred over a "cold" coating because of the latter's increased adhesion and durability.

Hitherto a "hot" coating method has been impractical, however, due to the different thermal expansion coefficients between the fibre, the epoxy holding the fibre to its substrate, and the substrate. Thus it is found that as the coated end face is cooled, the multi-layer dielectric dichroic coating usually becomes detached or the adhesion becomes so weakened that it very readily becomes detached at a later time. The survival rate is small which results in both wasted time and material. "Cold" coatings typically have poor adhesion characteristics and are not as durable as "hot" coatings. Both "hot" and "cold" dichroic coatings have been examined.

Although a "cold" multi-layer dielectric dichroic coating can be applied to the fibre end face, the adhesion and durability is poor. Accordingly the dichroic surfaces often do not survive the alignment process.

It is an object of the invention to provide an optical fibre with a dichroic filter such that the disadvantages inherent with prior art methods are avoided.

According to the present invention there is provided a method for producing a bidirectional coupler for duplex communication over a single optical fibre, which method includes the steps of applying a "hot" dichroic coating upon a major surface of a glass substrate; of processing said coated substrate into at least one dichroic wafer; and attaching said wafer to an optical fibre by means of an adhesive.

The invention also resides in an assembly for bidirectional signal transmission over a single optical fibre, which assembly includes a glass wafer having a major surface with a dichroic coating thereon which coating has been applied by a "hot" method, and an optical fibre attached to said surface of said wafer by a thin layer of adhesive.

The dichroic filter provided by the invention is a device which is extremely thin, e.g., less than about 50-75 micrometers. A glass substrate, typically of fused silica is provided. A mutli-layer dielectric dichroic coating (hot) is applied to the glass substrate. The dichroic filter is ground and polished to reduce its thickness to a minimum practical value. Its dichroic surface is protected during thb grinding and polishing operation.

The reduced thickness coated substrate is then diced into either square or rectangular wafers on a diamond saw. The wafer size is variable, but it must-be larger than the fibre to which it is ultimately applied. A nominal cross section of two millimeters by two millimeters can be employed. A dichroic wafer is positioned over a polished fibre beamsplitter half. A thin layer (less than 25 micrometers) of optical epoxy is applied to the fibre beamsplitter half. The dichroic wafer is then directly attached by means of the epoxy to the half. It is important that the epoxy layer be quite thin such that axial displacement is small.

Otherwise coupling loss will increase. This coupler half is cured according to the cure properties of the epoxy.

A second uncoated fibre beamsplitter half may be micromanipulated with epoxy on its surface and brought into contact with the first dichroic beamsplitter half. The two parts are positioned for maximum signal throughput. The completed device is cured and removed from the assembly station. Transmission and reflectivity measurements are performed to characterize the dichroic coupler.

There follows a description of the manufacture of a bidirectional coupler embodying the invention in a preferred form. The description refers to the accompanying drawings in which: Figure 1 is a perspective view of a glass substrate having a multi-layer dielectric dichroic coating thereon; Figure 2 is a sectional elevational view of a dichroic wafer secured to a fibre beamsplitter half; Figure 3 is similar to Figure 2 and further includes a second uncoated fibre beamsplitter half secured to the opposite side of the wafer.

Additionally, a third port tapoff fibre or light pipe is bonded to the coupler using optical grade epoxy.

Referring to the drawings, a glass substrate 10 is provided having a thickness of about 125 to 1 50 micrometers. The substrate may be fused silica. A multi-layer dielectric dichroic coating 1 2 is applied to one of the major faces of the glass substrate 10. The substrate may be in the form of a rectangular slab as shown in Figure 1 or may alternatively take other forms. The coating 12 of the multi-layer dielectric dichroic material is applied by a "hot" process to enhance adhesion and durability. A dichroic filter 1 4 is thereby formed.

After deposition of the coating, the dichroic filter 14 is ground and polished to reduce its thickness to a minimum practical value. The dichroic surface 1 2 is protected during the grinding and polishing step. This protection is achieved by applying a soluble adhesive 1 6 thereto which will not harm the dichroic coating.

The filter 14 is placed upon a grinding and polishing apparatus 1 8 as shown in Figure 1. The dichroic filter 14 is then diced into a plurality of square or rectangular wafers by a diamond saw.

The wafer size is not critical, but it must have larger dimensions than the cross section of the fibre with which it is employed. A representative cross section of two millimeters by two millimeters can be utilized.

Referring now to Figure 2, the dichroic wafer 20 is positioned over a polished beamsplitter half 22. The fibre beamsplitter half 22 includes a portion including a bare optical fibre 24 and another portion including the fibre with a jacket of fibre cladding material 26. Both portions are contained within a glass substrate 28. A thin layer 30 of optical grade epoxy is applied to the end face of the fibre beamsplitter half. The layer should be less than 25 micrometers in thickness to minimize coupler transmission loss due to axial separation of the fibres. The dichroic wafer is then directly attached to the fibre beamsplitter half by the layer 30 of epoxy. The plane of the wafer forms an acute angle 8 with a plane perpendicular to the axis of the beamsplitter half. In the embodiment shown in Figure 2 this angle 0 is about 250.This incident angle is critical for optimum optical performance of the fibre dichroic coupler. An acute angle of 250 is selected such that the polarization effects, i.e., Brewster's angle, are not approached (incident acute angle plus finite half angle of the fibre). Brewster's angle for the fibre core/epoxy interface is about 430. The fibre acceptance half angle is about 80; thus, a portion of the reflected beam will have an angle of about 350. This is below the polarization angle and will allow good performance of the dichroic coating.

A second uncoated fibre beamsplitter half 32 is micropositioned with respect to the first fibre beamsplitter half 22 to maximize signal throughput. It is secured to the opposite side of the dichroic wafer 20 by means of a thin layer 30 of optical grade epoxy. The fibre 34 within the second half has substantially the same diameter as that in the first half 1 8. The coupler shown in Figure 3 is thereby obtained wherein the fibres are aligned. Additionally, either a tapoff fibre or light pipe 36 is aligned and epoxied to the substrate 28 near the wafer 20. Maximum tapoff efficiency is accordingly achieved as reflected light is collected for processing.

It has been found that the bidirectional coupler produced according to the invention is sufficiently durable for practical use in duplex communication.

The polished dichroic wafer 1 6 shown in the figures is approximately between less than 50 and 75 micrometers in thickness with a cross section of two millimeters by two millimeters. It will be appreciated, however, that alternative dimensions are also possible. It has also been found that, for chemical vapor deposition type fibres, when using an optical epoxy as the adhesive medium, coupling efficiency is quite acceptable provided the axial separation of the fibres 24, 34 is within two fibres diameters.

Once the coupler is assembled, it is cured and removed from the assembly station. Transmission and reflectivity measurements are performed to characterize the dichroic coupler.

Claims (21)

1. A method for producing a bidirectional coupler for duplex communication over a single optical fibre, which method includes the steps of applying a "hot" dichroic coating upon a major surface of a glass substrate; of processing said coated substrate into at least one dichroic wafer; and attaching said wafer to an optical fibre by means of an adhesive.

2. A method as claimed in claim 1 wherein said wafer is formed to be less than 75,um in thickness.

3. A method as claimed in claim 1 or 2, wherein said substrate is diced into a plurality of dichroic wafers.

4. A method as claimed in claim 1,2, or 3, including the additional step of providing a second optical fibre and attaching said second optical fibre to a side of said wafer opposite from where the optical fibre is attached, both fibres being secured to said wafer by means of a thin layer of optical grade epoxy.

5. A method as claimed in any preceding claim wherein said glass substrate is fused silica-SiO2.

6. A method as claimed in any preceding claim which method includes the step of polishing and grinding said substrate after applying the dichroic coating.

7. A method as claimed in any preceding claim which method includes the step of attaching said wafer to said optical fibre at such an angle with respect to a plane perpendicular to the axis of said fibre that Brewster angle polarization effects are not approached.

8. A method as claimed in claim 7, wherein said angle is about 250.

9. A method as claimed in any preceding claim which method includes the step of bonding a tapoff fibre to said coupler near said wafer to collect reflected light for processing.

10. A method for producing a bidirectional coupler for duplex communication over a single optical fibre, which method is substantially as hereinbefore described with reference to the accompanying drawings.

11. A bidirectional coupler made by the method claimed in any preceding claim.

1 2. An assembly for bidirectional signal transmission over a single optical fibre, which assembly includes a glass wafer having a major surface with a dichroic coating thereon which coating has been applied by a "hot" method, and an optical fibre attached to said surface of said wafer by a thin layer of adhesive.

13. An assembly as claimed in claim 12 including a second optical fibre attached to a surface of said wafer opposite said coated surface by a thin layer of adhesive.

14. An assembly as claimed in claim 12 or claim 13 wherein said wafer is less than 75m thick.

15. An assembly as claimed in claim 13 or 14 wherein said fibres are separated by a distance within two fibre diameters.

1 6. An assembly as claimed in claim 12, 13, 14 or 15, wherein said adhesive is optical grade epoxy.

17. An assembly as claimed in any claim of claims 12 to 1 6 wherein said wafer is attached to said optical fibre at such an angle with respect to a plan perpendicular to the axis of said fibre that Brewster angle polarization effects are not approached.

18. An assembly as claimed in claim 17 wherein said angle is about 250.

19. An assembly as claimed in any claim of claims 1 2 to 1 7 wherein said first optical fibre is embedded within a glass substrate.

20. An assembly as claimed in claim 1 9 wherein a tapoff fibre is bonded to said glass substrate near said wafer such that tapoff efficiency is maximized.

21. An assembly as claimed in claim 13, or any subsequent claim appended thereto, wherein both fibres are embedded within a glass substrate.