Optical fibre directional coupler

Claim 2

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.

Claim 3

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.

Claim 4

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.

Claim 5

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.

Claim 6

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.

Claim 7

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.

Claim 8

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.

Claim 9

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.

Claim 10

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.