Optical fibre directional coupler

• US 4,474,431 A
• Filed: 12/05/1983
• Issued: 10/02/1984
• Est. Priority Date: 12/20/1978
• Status: Expired due to Fees

• Current Assignee: STC PLC (f/k/a Standard Telephones and Cables)
• Original Assignee: International Standard Electric Corporation (ITT, Inc.)
• Inventors: Bricheno, Terry
• Primary Examiner(s): Sikes, William L.
• Assistant Examiner(s): Gonzalez, Frank
• Application Number:
  US06/558,259
• Time in Patent Office: 302 Days
• Field of Search: 350/96.15, 350/96.16, 350/96.21, 356/73.1
• US Class Current: 385/24
• CPC Class Codes: G02B 6/2821 using lateral coupling between contiguous fibres to split or combine optical signals
• CPC Class Codes: G02B 6/2835 formed or shaped by thermal treatment, e.g. couplers
• CPC Class Codes: G02B 6/4246 Bidirectionally operating package structures
• CPC Class Codes: G02B 6/441 built up from sub-bundles G02B6/4429, G02B6/4439, G02B6/4479 take precedence

Abstract

An optical fibre directional coupler has first and second optical fibres clamped together by a piece of low index heat-shrunk tubing 12. The first fibre consists solely of a core 10 and cladding 11 while the second has a core 13 a primary cladding 14 whose cross-sectional area is larger than that of the core, and a secondary cladding 15. The outermost layer of each fibre is absent over a portion of its length within the tubing 12 so that the core 10 of the first is in intimate contact with the primary cladding 14 of the second. The second fibre may have a mode stripper 45 to strip modes guided by the primary/secondary cladding interface. Fibre break detection apparatus and a single-fibre two-way transmission system are described which use such directional couplers.

Claims

1. An optical fibre directional coupler comprising:

a first optical fibre having oppositely located first and second ports, an optical core of a predetermined refractive index and an optical cladding surrounding said core and having a lower refractive index than said core;

a second optical fibre having oppositely located third and fourth ports, a core of a predetermined refractive index, a primary optical cladding surrounding said core and having an index of refraction lower than said core and a cross-sectional area larger than the cross-sectional area of the core, and a secondary optical cladding surrounding said primary cladding and having a refractive index lower than said primary cladding, said secondary cladding having a refractive index less than said core of said first optical fibre, said first and second optical fibres being positioned adjacent each other to form a coupling region wherein the optical cladding of the first fibre and the secondary cladding of the second fibre have been removed, said first and third ports being positioned on the same side of the coupling region positioned in use that said third port receives light and transmits the core modes thereof through said core of said second fibre to said fourth port and into an additional fibre positioned adjacent thereto, said fourth port receives a greater proportion of cladding modes than core modes from the additional fibre, and the cladding modes from the fourth port are conveyed through the coupling region predominantly to said first port for detection thereat; and

means for holding said first and second optical fibres in intimate contact in said coupling region.

2. The optical fibre directional coupler as claimed in claim 1 wherein the diameter of the core of the first fibre is larger than that of the primary cladding of the second fibre.

3. The optical fibre directional coupler as claimed in claim 1 wherein the cladding of the first fibre and the secondary cladding of the second fibre are both made of plastic and the core of the first fibre and the core and primary cladding of the second fibre are made of glass.

4. The optical fibre directional coupler as claimed in claim 3 wherein said glass is selected from the group consisting of pure silica and doped silica.

5. The optical fibre directional coupler as claimed in claim 1 further comprising a cladding modes mode-stripper positioned on said second optical fibre before the coupling region.

6. The optical fibre directional coupler as claimed in claim 5 wherein said mode stripper is provided by a region of optically absorbing dielectric material replacing a portion of the secondary cladding of the second fibre.

7. A single optical fibre two-way transmission system comprising:

first and second directional couplers;

first and second light sources;

first and second photodetectors; and

a single optical fibre transmission link,

said first and second directional couplers each including

a first optical fibre having oppositely located first and second ports, an optical core of a predetermined refractive index and an optical cladding surrounding said core and having a lower refractive index than said core,

a second optical fibre having oppositely located third and fourth ports, a core of a predetermined refractive index, a primary optical cladding surrounding said core and having an index of refraction lower than said core and a cross-sectional area larger than the cross-sectional area of the core, and a secondary optical cladding surrounding said primary cladding and having a refractive index lower than said primary cladding, said secondary cladding having a refractive index less than said core of said first optical fibre, said first and second optical fibres being positioned adjacent each other to form a coupling region wherein the optical cladding of the first fibre and the secondary cladding of the second fibre have been removed,

means for holding said first and second optical fibres in intimate contact in said coupling region,

said first light source being coupled to said third port of said second optical fibre of said first coupler,

said first photodetector being coupled to said first port of said first optical fibre of said first coupler,

said second light source being coupled to said fourth port of said second fibre of said second coupler,

said second photodetector being coupled to said third port of said first fibre of said second coupler,

said single optical fibre transmission link connected to said fourth port and said third port respectively of said first and second directional couplers, and

said first and third ports of each coupler being positioned on the same side of the respective coupling regions, said ports being so positioned in use that said third port and said fourth port respectively of said first and second couplers receive light and transmit the core modes thereof through said cores of said second fibres to said fourth and third ports respectively of said first and second couplers and into said single optical fibre transmission link, said fourth and third ports respectively of said first and second couplers receive a greater proportion of cladding modes than core modes from said link, and the cladding modes from said fourth and third ports respectively of said first and second couplers are conveyed predominantly to said first and second ports respectively of said first and second couplers through the coupling regions for detection thereat.

8. A single optical fibre two-way transmission system as claimed in claim 7 wherein the core diameter of the single optical fibre transmission link matches the diameter of the primary cladding of the second fibres of both directional couplers.