GB2170923A - Optical waveguide structures and their fabrication - Google Patents

Abstract

An optical waveguide structure 4 is formed utilizing a material having a first state in which it does not undergo plastic flow and is homogeneous, dielectric and isotropic, and is essentially transparent to electromagnetic radiation at a wavelength that is within the optical spectrum, and also having a second state, the material being convertible from one of its first and second states to the other state by irradiation with energy at a predetermined wavelength. A layer of the material 6 in its unconverted state is deposited on a support member 2 having a refractive index less than that of the material in its first state. Lateral boundaries of a body to be formed on the support member 2 are defined by means of a mask 8 having areas 10 that are substantially transparent to energy at the predetermined wavelength and areas that are substantially opaque to energy at the predetermined wavelength. The layer of material is irradiated through the mask with energy at the predetermined wavelength and material in the second state is removed from the substrate without removing material in the first state that is between the lateral boundaries. <IMAGE>

Description

SPECIFICATION Optical waveguide structures and the fabrication thereof This invention relates to optical waveguide structures and the fabrication thereof.

Background of the Invention The art of fabrication of optical fibers is quite highly developed, and it is possihle to manufacture fibers of virtually indefinite length and with essentially uniform properties throughout that length. Moreover, a good deal of effort has been devoted to techniques for splicing two fibers together, end to end. Splicing devices that receive the ends of fibers to be spliced and secure them in accurately aligned fashion are commercially available.

However, fabrication of a device for joining one fiber to two other fibers, i. e. a beam combiner or beam splitter, is much more involved. One known method of forming such a coupler involves twisting together the fibers to be joined, fusing the fibers along at least a part of the twisted portion and then drawing the fibers in the fused condition so as to merge the fibers together. This method of fabrication of a coupler, known as the fused biconical taper technique, is time consuming and involves highly skilled labor. Accordingly, couplers manufactured by this technique are expensive.

Summary of the Invention According to a first aspect of the present invention there is provided a method of forming an optical waveguide structure utilizing a material having a first state in which it does not undergo plastic flow and is homogeneous, dielectric and isotropic, and is essentially transparent to electromagnetic radiation at a wavelength within the optical spectrum, and also having a second state, said material being convertible from one of said states to the other state by irradiation with energy at a predetermined wavelength, said method comprising the following steps, steps (a) and (b) not necessarily being in the order stated, (a) depositing a layer of said material in its said one state on a support member having a refractive index less than that of said material in its first state, (b) defining lateral boundaries of a body to be formed on said support member by means of a mask having areas that are substantially transparent to energy at said predetermined wavelength and areas that are substantially opaque to energy at said predetermined wavelength, (c) irradiating said layer of material through said mask with energy at said predetermined wavelength, whereby following such irradiation the material between said lateral boundaries is in said first state and material outside said boundaries is in said second state, and (d) removing the material in said second state from the substrate without removing material in said first state that is between said lateral boundaries.

According to a second aspect of the invention there is provided an optical waveguide structure comprising a support member and a body of material secured to said support member, said material being in a state in which it has a refractive index greater than that of the support member and does not undergo plastic flow and is homogeneous, dielectric and isotropic and is essentially transparent to electromagnetic radiation at a wavelength within the optical spectrum, the material also having a second state and being convertible from one of said states to the other state by irradiation with energy at a predetermined wavelength.

Brief Description of the Drawings For a better understanding of the invention, and to show how the same may be carried into effect, reference now will be made, by way of example, to the accompanying drawing in which: FIG. 1 is a light ray diagram illustrating total internal reflection, FIG. 2 is a perspective view of an optical coupler comprising a waveguide structure fabricated in accordance with the present invention, FIG. 3 is an enlarged partial sectional view taken on the line Ill-Ill of FIG. 2, FIG. 4 is a view similar to FIG. 3 illustrating one of the steps during manufacture of the waveguide structure, FIG. 5 illustrates a variation on the method illustrated by reference to FIG. 4, FIG. 6 is an enlarged sectional view of the waveguide structure of FIG. 5, and shows the end of an optical fiber waveguide, and FIG. 7 illustrates two modifications of the method described with reference to FIGS. 2-4.

Detailed Description As used herein, the term "optical waveguide structure" relates to a structure that comprises an elongate body of dielectric material which confines light launched into the body within the lateral boundaries of the body. Thus, an optical fiber is itself an optical waveguide structure.

The mechanism whereby an optical waveguide confines a light beam is known as total internal reflection. If a light beam (FIG. 1) propagating in a body A of refractivc index n1 is incident on an interface with a body B of refractive index n2 (less than n1) at a small angle a (measured between the incident light beam and the normal to the interface), the light beam enters the body B and is refracted away from the normal to the interface, the angle of refraction B between the refracted beam r and the normal within the body B being related to the angle of incidence a and the values of n1 and n2 by sina/sinfi = n2/nl When the value of a increases, the value of ss also increases until, at a value a, known as the critical angle and given by sina, = n2/n1, the value of ss reaches 90 degrees. When the value of a is greater than the critical angle i.e.

sin a is greater than nun1, the light beam is not refracted at the interface but undergoes total internal reflection, the angle of reflection ,' being equal to the angle of incidence. It will be readily understood that total internal reflection cannot take place unless n1 is greater than n2.

The waveguide structure illustrated in FIGS.

2 and 3 comprises a body 4 of UV cured adhesive material formed within predetermined lateral boundaries on a substrate 2 of quartz.

The adhesive material in the cured state is homogeneous and isotropic and has a refractive index of 1.56, whereas the refractive index of quartz is 1.55. Thus, light that enters the waveguide by way of one end and is incident on a wall of the waveguide at an angle greater than the critical angle will undergo total internal reflection and will therefore be confined within the lateral boundaries of the body 4. It will be seen from FIG. 2 that the body 4, in plan, is generally Y-shaped, and it will be seen from FIG. 3 that the body 4 is rectangular in cross-section. The body 4 is optically coupled at the distal ends of the three limbs of the Y to three optical fibers 12, 14 and 16.The waveguide thus forms a coupler, whereby light entering the body 4 from the fiber 12 is split into two beams directed into the fibers 14 and 16 respectively, or light entering from the fibers 14 and 16 is combined into a single beam directed into the fiber 12.

The body 4 is formed on the substrate 2 by depositing a layer 6 (FIG. 4) of uncured adhesive material on the substrate 2, placing over the layer 6 a mask 8 that defines an aperture 10 corresponding in configuration to the desired configuration of the waveguide and then exposing the layer 6 to a collimated beam of ultraviolet light. Material of the layer 6 that lies beneath the aperture 10 is exposed to the light and is cured, whereas other material of the layer 6 is shielded from the light by the mask 8 and is not cured. The mask is then removed, and the uncured material is washed from the substrate. If desired, a layer of protective cladding material may then be deposited over the body 4. It will be appreciated that the cladding material would have to have a refractive index less than that of the material of the body 4.

The optical fibers 12. 14 and 16 that are optically coupled by the waveguide are adhesively bonded to the substrate 2. Preferably, this is is done before deposit of the layer 6, so that the mask 8 can be used to ensure that the fibers are properly positioned on the substrate and thereafter the mask can be aligned with reference to the fibers and thereby insure that the body 4 will be properly positioned relative to the fibers, and also in order to avoid the need for use of indexmatching material to displace air between the cores of the fibers and the ends of the body 4, but the fibers could be secured to the substrate 2 after formation of the body 4.

An alternative way of forming the body 4 is illustrated in FIG. 5. In the case of FIG. 5, the substrate 2 is transparent to ultraviolet light in the range of wavelengths used to cure the adhesive material, and the mask 8' is formed by a layer of metal or other UV-opaque material that is deposited on the substrate. The mask 8' is initially deposited as a uniform layer over the surface of the substrate and is then selectively etched to define the mask aperture 10', or is deposited by a technique allowing definition of the aperture 10' at the time of deposition, for example by vapor deposition using a mask having features complementary to those of the mask 8'. After the mask 8' has been formed, the layer 6 of uncured adhesive material is deposited on the upper surface of the substrate/mask composite, and the adhesive material is exposed to ultraviolet light from beneath.As before, the adhesive material at locations corresponding to the aperture 10' is cured and the material at other locations remains uncured, and the uncured material is then removed, possibly together with the mask material 8'.

Commercially available optical fiber comprises a core 18 (FIG. 6) of high index transparent material, a cladding layer 19 of lower index material and a protective covering (not shown) of synthetic plastic material, which may be opaque. The external diameter of the core typically ranges from nine m for single mode transmission to about 62 m for multimode transmission, the external diameter of the cladding being the same, typically 125 m, regardless of the diameter of the core. In coupling the fiber to the end of the waveguide, the outer protective covering of the fiber is removed and the exposed cladding layer is placed in contact with the substrate. It will be apparent from FIG. 6 that this might be satisfactory so far as launching light from the fiber into the waveguide is concerned, in that the entire area of the end face of the core 18 is optically coupled to the body 4, but that since a large proportion of the area of the end face of the body 4 is not optically coupled to the core 18 a substantial proportion of any optical energy directed towards the core 18 from the body 4 will not enter the core of the fiber.

This problem is alleviated by the arrangement shown in FIG. 7(a). The waveguide shown in FIG. 7(a) is formed by depositing a layer 20 of low index material on the substrate prior to depositing the adhesive material 6. The layer 20 is coextensive with the layer 6 with respect to longitudinal directions of the body 4', but need not be coextensive with respect to lateral directions. The thickness of the layer 20 is substantially equal to the difference between the inner and outer radii of the cladding layer 19 of the fiber. The layer 6 is formed to a thickness equal to the diameter of the core of the fiber. Accordingly, the proportion of the area of the end face of the body 4 that is not optically coupled with the core of the fiber is substantially reduced with respect to the arrangement shown in FIG. 5.The proportion may be reduced still further, by appropriate selection of the thicknesses of the layers 20 and 6 relative to the dimensions of the core and cladding of the fiber, as shown in FIG.

7(b), and this would reduce still further the loss suffered in introducing light into the fiber from the body 4', but this would be at the expense of loss in introducing light from the fiber into the body 4'.

As shown in FIG. 7(b), an additional layer 22 of low index cladding material may be deposited over the body 4'.

Suitable adhesive materials for the body 4 are Norland Optical Adhesives type 61 and type 65. Type 65, when cured, is a soft, rubbery material, whereas type 61 has greater dimensional stability and meets quite rigorous specifications with respect to transparency and its stability under change in temperature.

These adhesives can be cured using ultraviolet light of wavelength in the range from 350 to 380 nm. Either type can be readily removed from the substrate 2 when in the uncured state using carbon tetrafluoride (Freon).

It will be appreciated that the invention is not restricted to the particular wavelength structures that have been described and illustrated, and that variations may be made therein without departing from the scope of the invention as defined in the appended claims, and equivalents thereof. For example, it is not necessary that the material of the layer 6 be curable by exposure to ultraviolet light. Other methods for producing on the sub strate a body of homogeneous, isotropic, die lectric, transparent material within desired lateral boundaries will readily present them selves to persons skilled in the art. For example, the body may be formed from a transparent plastic material that is degradable by exposure to ultraviolet light, e.g. polymethyl methacrylate, by securing a plate of the plastic material to the substrate, masking those portions of the plate that are to consti tute the body 4 or 4', and exposing the rest of the plate to ultraviolet light. The exposed material may then be removed without remov ing the unexposed material. In addition, it is not necessary that the substrate 2 be made of quartz, even in the case of the method described with reference to FIG. 5, because appropriate glasses are available.

Claims (15)

1. A method of forming an optical waveguide structure utilizing a material having a first state in which it does not undergo plastic flow and is homogeneous, dielectric and isotropic, and is essentially transparent to electromagnetic radiation at a wavelength that is within the optical spectrum, and also having a second state, said material being convertible from one of said states to the other state by irradiation with energy at a predetermined wavelength, said method comprising the following steps, steps (a) and (b) not necessarily being performed in the order stated:: (a) depositing a layer of said material in its said one state on a support member having a refractive index less than that of said material in its first state, (b) defining lateral boundaries of a body to formed on said support member by means of a mask having areas that are substantially transparent to energy at said predetermined wavelength and areas that are substantially opaque to energy at said predetermined wavelength, (c) irradiating said layer of material through said mask with energy at said predetermined wavelength, whereby following such irradiation the material between said lateral boundaries is in said first state and material outside said lateral boundaries is in said second state, and (d) removing the material in said second state from the substrate without removing material in said first state that is between said lateral boundaries.

2. A method according to claim 1, wherein step (a) is performed before step (b) and the mask is positioned so that said layer lies between the mask and the support member.

3. A method according tc claim 1, wherein step (b) is performed before step (a) and is accomplished by forming said mask an the substrate, step (a) being performed by depositing said layer of material over the mask and exposed portions of said one face of the support member.

4. A method according to claim 1, wherein the waveguide structure is formed on a substrate member having a main face and said method comprises, prior to steps (a) and (b), forming said support member as a layer on said main face.

5. A method according to claim 4, wherein said body is elongate and is terminated in at least one longitudinal direction by an end face that is spaced from the periphery of the substrate member, and the support member is formed so that it extends no farther, in said one longitudinal direction of said body, than said end face.

6. A method according to claim 1, wherein said one state of said material is the second state thereof, the material in said second state being plastically flowable and being soluble in a solvent in which the material in said first state is not soluble, step (d) being performed by rinsing the structure with said solvent.

7. A method according to claim 1, wherein said one state of the material is the first state thereof, the areas of the mask that are substantially transparent to energy at said predetermined wavelength being areas outside the lateral boundaries of the waveguide, whereby the material of such areas is degraded to its second state in step (c).

8. An optical waveguide structure comprising a support member and a body of material secured to said support member, said material being in a state in which it has a refractive index greater than that of the support member and does not undergo plastic flow and is homogeneous, dielectric and isotropic and is essentially transparent to electromagnetic radiation at a wavelength within the optical spectrum, the material also having a second state and being convertible from one of said states to the other state by irradiation with energy at a predetermined wavelength.

9. A waveguide structure according to claim 8, wherein said material is in said one state.

10. A waveguide structure according to claim 8, wherein said material is in said other state.

11. A waveguide structure according to claim 8, formed on a substrate member having a main face, said support member being constituted by a layer on said main face.

12. A waveguide structure according to claim 11, wherein said body is elongate and is terminated in at least one longitudinal direction by an end face that is spaced from the periphery of the substrate member, said support member extending no farther, in said one longitudinal direction of the body, than said end face.

13. In combination, a waveguide structure according to claim 12 and an optical fiber comprising an inner core and an outer cladding layer, said optical fiber having an end face and being secured to the substrate member of the waveguide structure in end-to-end alignment with said body, with the end face of the fiber and the end face of said body in mutually confronting relationship, the layer constituting said support member being at least as thick as said outer cladding layer.

14. A method of forming an optical wave guide structure substantially as hereinbefore described.

15. An optical waveguide structure substantially as hereinbefore described with reference to the accompanying drawings.