GB2147715A - Optical coupler - Google Patents

Abstract

A wavelength division multiplexing or demultiplexing optical coupler includes optical element (18) having a convex spherical surface (22) on one end and a diffraction grating (26) on a portion of its other end. The remaining portion of its other end receives a multiple fibre array (12) for transmitting and receiving the light to be multiplexed or demultiplexed. <IMAGE>

Description

SPECIFICATION Optical coupler This invention relates to optical wavelength division multiplexing or demultiplexing couplers and, more particularly, to optical couplers of the diffraction grating type.

Diffraction grating couplers used as optical multiplexing or demultiplexing devices take light from the input fibre or fibres, respectively, and couple it back into output fibres or fibre, respectively. These couplers utilise a diffraction grating, that is, an angularly dispersive device, that diffracts away incident collimated light at an angle dependent upon the incidence angle and the wavelength of the incident light. In this way, light can be separated by wavelength and coupled as desired.

There are several types of diffraction grating couplers. One common type uses a concave diffraction grating and another common type uses a radially graded refractive index (hereafter GRIN) lens with a plane diffraction grating. The concave diffraction grating type device has the advantage of not requiring any light collimating and/or refocusing optics. Its disadvantages are that extremely tight control must be exercised in forming the spherical concave surface and also in forming the grating configuration. Compounding the latter requirements is the fact that the ruling tool used to form the grating must swing through an arc as it traverses the spherical surface. In addition, concave grating type devices have a low diffraction efficiency and can suffer from image astigmatism.

The disadvantage of the GRIN lens type device is that it includes an additional optical device to collimate and focus the light.

An object of the present invention is to overcome the disadvantages of the two types of diffraction grating couplers noted above.

According to the invention in its broadest aspect, there is provided an optical coupler comprising an elongated optical component formed of light-transmitting material, one end of the component having a convex surface and the other end having a generally planar surface, one portion of said planar surface having a diffraction grating formed thereon and the remaining portion being adapted to receive a multiple fibre array.

The shape of the convex surface is such that the path of the light from the convex surface to any fibre in the array is equal to approximately one focal length.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a perspective view of a diffraction grating coupler in accordance with this invention; Figure 2 is a schematic illustration of a diffraction grating coupler in accordance with this invention; and Figure 3 is a schematic illustration of another diffraction grating in accordance with this invention.

In Figure 1 there is illustrated a diffraction grating coupler 10 and a multiple fibre array 12. In this embodiment the coupler 10 is functioning as a multiplexer and, thus, the multiple fibre array includes a plurality of fibres (in this embodiment, eight such fibres) 14a, b, c, d, e, f, g and h, each connected to a light source (not shown), for example, a laser or a light-emitting diode. It should be understood that.any number of fibres can be included in the array. Each light source provides light at a different wavelength. The light from each fibre is the incident light and is combined by the coupler 10 and coupled into an output or link fibre 16 connected into a fibre optics system.It should be understood that if the coupler 10 is functioning as a demultiplexer, the incident light would include all of the wavelengths and would be transmitted along the fibre 16 to the coupler which would distribute each wavelength to its appropriate fibre 14a to 14h. In this mode, each of the fibres 14a to 14h inclusive would be connected to a suitable light detector, for example, an avalanche photodiode or a PIN diode.

In Figures 1 and 2, the coupler 10 can be seen to include an elongated optical component 18 formed of a good light-transmitting material. In this embodiment the light-transmitting material is glass and is preferably, pure fused silica. Any one of various materials can be used and it should have a generally uniform index of refraction. As shown in Figure 1, the optical component 18 has a generally rectangular cross-section, but other shapes are also usable.

As also shown in Figure 1, the component 18 actually comprises two blocks of pure fused silica 18a and 18b having generally planar mating surfaces in abutting relationship. The blocks 18a and 18b are fixed together to form a unitary component. Conveniently the blocks 18a and 18b are fixed together by a good optical grade epoxy 20, i.e. an epoxy that transmits light. The use of two blocks of silica is preferred in some instances since it facilitates the making of coupler 10 as will be clear from later descriptions of the invention. It should be understood that a single block of material could be used as illustrated in Figure 2 or 3.

One end of the elongated optical component 18 is formed with a convex surface 22 coated with a light-reflecting material. Gold or silver are preferred materials coated on the surface 22 and this surface can be formed by any conventional technique. The preferred configuration for surface 22 is spherical where the radius of curvature is such that light emitted from the fibre array into the component 18 travels a path to the surface 22 having a length equal to about one focal length. In this way, the light is collimated by the spherical surface 22.

In some cases it may be desirabie to have the configuration of surface 22 an aspherical surface, that is, a surface defined by more than one equation in order to minimise aberration. Thus, the term spherical as used herein should be construed to include an aspherical surface.

The other end of the optical component is formed with a generally planar surface 24, one portion of which is formed with a diffraction grating 26. It should be understood that the diffraction grating comprises a large number of grooves as shown in Figure 2 of the drawing. It should also be understood that the size of the grooves is greatly exaggerated in Figure 2 for the sake of clarity. The diffraction grating 26 is also coated with a light-reflecting material such as gold or silver. The remaining portion of the planar surface 24 is that portion to which the multiple fibre array 12 is secured, This can also be accomplished by the use of a suitable optical grade epoxy.

The diffraction grating 26 can be formed on the planar end surface 24 by a conventional ruling tool, usually a diamond blade. The diffraction grating can also be formed on a wedge which is fixed to the planar surface of block 18b with an optical epoxy 20, as shown in Figure 1. It is preferred, however, to replicate the grating 26 on the end face 24. Replication can be accomplished by coating the one portion of the planar surface 24 with a suitable optical grade resin and pressing a master die having the diffraction grating pattern on its contact surface into the resin while it is still soft enough to form. Thereafter, the resin is cured and coated with the reflecting material in accord with conventional techniques. The use of two blocks 18a and 18b is preferred when the diffraction grating 26 is replicated because handling of the material is facilitated.

Various resins can be used and should have an index of refraction when cured, approximately equal to that of the light-transmitting material.

Suitable resins are made by Bausch and Lomb, Microscopy and Image Analysis Division, located in Rochester, New York.

With reference to Figure 2, it can be seen that incident light travelling down an input fibre is trans mitted through the optical component 10 until it strikes the mirrored spherical surface 22 where it is collimated and reflected through the component to the diffraction grating 26. When the collimated light strikes the diffraction grating 26 it is diffracted back to the mirrored spherical surface 22 where it is reflected and focused into an output fibre.

As shown in Figure 2, the convex spherical surface 22 is centered with respect to the optical axis A of the component 18. With the surface 22 so centered the planar surface 24 forms an angle T with a line perpendicular to the optical axis. Angle T is approximately equal to one-half the grating in cidence angle required for efficient grating opera tion. In some embodiments, as shown in Figure 3, it may be desirable to have the planar surface per pendicular to the optical axis of the component.

Such a perpendicular surface is shown at 24a in the component 18a. In these embodiments the convex spherical surface 22a should be off-centre so that it, in effect, inclines the surface 26 to an ef fective angle equal to one-half of the incidence an gle required for efficient grating operation.

The outer surfaces of the component 18, exclud ing surfaces 22 and 24, may have a ground glass finish to decrease internal scattering from the con vex surface 22 and from the diffraction grating 26.

The ground glass finished surfaces may be black ened or otherwise treated to enhance their lighttrapping ability.

Claims (12)

1. An optical coupler comprising an elongated optical component formed of light-transmitting material, one end of the component having a convex surface and the other end having a generally planar surface, one portion of said planar surface having a diffraction grating formed thereon and the remaining portion being adapted to receive a multiple fibre array.

2. An optical coupler in accordance with claim 1, wherein the convex surface and the one portion of the planar surface are coated with a light-reflecting material.

3. An optical coupler in accordance with claim 1, wherein the optical component is formed of a material having a generally uniform index of refraction.

4. An optical coupler in accordance with claim 1, wherein the optical component is formed of glass.

5. An optical coupler in accordance with claim 1, wherein the other surfaces of the optical component have a ground glass finish and are coated with materials that enhance their light-trapping ability.

6. An optical coupler in accordance with claim 1, wherein the optical component is a single member.

7. An optical coupler in. accordance with claim 1, wherein the optical component comprises two blocks of material joined together by an epoxy.

8. An optical coupler in accordance with claim 1, wherein the generally planar surface forms an angle with the optical axis of the component and the spherical surface is centered with respect to the axis.

9. An optical coupler in accordance with claim 1, wherein the generally planar surface is perpendicular to the optical axis of the component and wherein the spherical surface is off-centre with respect to the axis.

10. An optical coupler in accordance with claim 1, wherein the one portion of the planar surface in cludes a resin coating in which the diffraction grat ing is replicated.

11. An optical coupler in accordance with claim 1, wherein the path of light emitted from a multiple fibre array to the convex surface is approximately equal to the focal length.

12. An optical coupler substantially as described with reference to the accompanying draw ings.