WO1986001304A1

WO1986001304A1 - Wavelength division multiplexer and demultiplexer

Info

Publication number: WO1986001304A1
Application number: PCT/GB1985/000362
Authority: WO
Inventors: John W. Y. Lit; Daniel W. Hewak
Original assignee: Canadian Industrial Innovation Centre/Waterloo; Idea Corporation; Asquith, Anthony
Priority date: 1984-08-17
Filing date: 1985-08-14
Publication date: 1986-02-27
Also published as: JPS62500053A; EP0222765A1; AU4679085A; KR860700295A; CA1262580A

Abstract

A thin-film waveguide lens and a wavelength division multiplexer/demultiplexer (1) embodying such lens. The thin film waveguide lens comprises a thin-film waveguide with end planes essentially normal to the lens axis. A plano-convex overlay layer integral with the waveguide extends the length of the waveguide along the axis, its profile being selected so as to produce a graded effective refractive index in the lens in order to collimate focussed rays and focus collimated rays entering at one end plane for substantially collimated or focussed arrival at the other end plane. The multiplexer/demultiplexer (1) comprises the above thin-film waveguide lens, with one of the end planes constituting an entrance/exit plane (A A') for receiving optical fibres (F1, F2, F3) in an abutting connection, and the other end plane (B B') bearing a diffraction/reflection grating (5). In accordance with the preferred embodiments, the shape of the plano-convex overlay layer is such as to provide an effective index of refraction profile in the waveguide according to the formula n = n0sech(gr), where n is the effective refractive index at the distance r from the axis of the waveguide, n0 is the effective refractive index at the waveguide axis, and g is a constant equal to pi/2f, where is the focal length of the lens, being essentially the distance between end planes.

Description

Wavelength Division Multiplexer and Demultiplexer

This invention relates to fiber optic communications systems, and particularly to a wavelength division multiplexer/demultiplexer comprising a novel thin-film waveguide lens.

In a fiber optic communication system, each fiber typically carries waves of a particular wavelength. Existing multiplexing technology allows hundreds and thousands of signals to be sent on this wavelength through a single fiber through time multiplexing. However, when the upper limit is reached, a separate fiber has to be installed in order to satisfy heavier information traffic demands. Instead of increasing the number of fibers, an additional wavelength may be used in one fiber. Wavelength division multiplexers and demultiplexers are the devices which allow this to be achieved. At the transmission end, signals carried by different wavelengths are fed into one single fiber by means of multiplexers. At the receiver end, the signals are descrambled into groups, each belonging to a separate wavelength, by a demultiplexer.

The wavelengths currently used commonly in optical communications are in the 800 n and 1300 nm regions. A third potential region is the 1500 nm one. Usual commer- cial demultiplexers separate only two very different wave¬ lengths, one in each of the regions. However, as the technology of light sources and laser light sources in particular improves, it will become possible to use many more different wavelengths to increase the signal-carrying capacity of each optical fibre. Multiplexers and demulti¬ plexers are needed to effectively handle such increased numbers of wavelengths.

An accompanying technology is integrated optics. Among the growing number of devices being developed for integrated optics, the optical waveguide lens remains the most basic. An optical waveguide lens performs a variety of important functions, including focusing and collimating, Fourier transformation, imaging, spatial filtering, and the integration of guided-optical beams.

There are many important design criteria for an optical waveguide lens. The position of the focal plane, focal spot size, angular field of view, along with the intensity profile of the first diffraction spot, the energy in the sidelobes and the throughput losses must all be considered. Equally important, the fabrication techniques should be^' simple, inexpensive and compatible with present technology.

Presently-marketed wavelength division multiplexers and demultiplexers nearly always use thin-film filters. When more than two wavelengths, .say n, are used, the use of thin-film filters is clumsy, because n-1 filters must be used in a cascaded structure. Such devices are expensive to fabricate, and suffer from signal attenuation.

Grating devices can be used as an alternative to thin-film filters. The use of a grating to separate wavelengths is of course well known in spectroscopy, and is also known in optical signal multiplexing and demultiplexing from Canadian patent no. 1,089,932, described below. Another method of separating wavelengths is in the use of prisms, which however are not commercially practical.

All grating devices require means to collimate the light rays for arrival at the grating, and means to focus the rays subsequently. A thin-film optical waveguide lens can be used as the means for effecting this collimating and focussing.

Three types of thin-film optical waveguide lens have been proposed and demonstrated for use with guided optical beams: mode index lenses such as thin-film Luneberg lenses, geodesic waveguide lenses, and Fresnel diffraction lenses. Each of these lenses utilizes a localized change in the optical waveguide: an index gradient, a spherical depression, and a grating respectively, to alter the wave- front curvature and obtain the desired focusing effect. This localized change in the waveguide structure contributes to losses by introducing scattering and mode conversion at the lens edge.

One specific type of multiplexer/demultiplexer employing a thin-film lens is that described in Canadian patent no. 1,089,932 granted to Northern Telecom Limited on November 18, 1980, which employs a collimating lens to collimate the light from an optical fibre, a diffraction/ reflection grating, a focusing lens, and an array of light detectors'and sources. ;

In three-dimensional optics, the graded [refractive] index, or "GRIN" rod lens has become important in many applications. Among the different GRIN rod lens refractive index distributions, it is generally known that the profile n = n₀sech(gr), where g is the effective refractive index at the waveguide axis, g is a constant

(equal to τr./2f, where f is the focal length), and r is the. radial distance from the axis of the waveguide, focusses all meridional rays exactly.

It is an object of the present invention to provide a thin-film waveguide lens for use in a wavelength division multiplexer/demultiplexer to overcome some of the problems and offer advantages over the wavelength division multiplexers and demultiplexers in the prior art.

Thus in accordance with one aspect of the present invention there is provided a thin-film waveguide lens for use in a multiplexer/demultiplexer, comprising a thin-film waveguide having a longitudinal axis and having end planes essentially normal to the axis at each end of the waveguide. A plano-convex overlay layer integral with the waveguide extends the length of the waveguide along the axis, the profile of the overlay layer being selected so as to produce a graded effective refractive index in the lens in order to collimate focussed rays and focus collimated rays entering at one end plane for substantially collimated or focussed arrival at the other end plane.

In accordance with another aspect of the invention, there is provided a multiplexer/demultiplexer comprising the above thin-film waveguide lens, in which one of the end planes constitutes an entrance/exit plane for receiving optical fibres in an abutting connection, and the other end plane bears a diffraction/reflection grating. Focussed rays entering the lens at the entrance/exit plane are substantially collimated for arrival at the diffraction/reflection grating, and rays reflecting from the diffraction/reflection grating are substantially focussed on returning to the entrance/exit plane, the locations for abuttment of the optical fibres being matched to the spot locations of the focussed rays along the entrance/exit plane.

In accordance with the preferred embodiments of the above aspects of the invention, the shape of the plano-convex overlay layer is such as to provide an effective index of refraction profile in the waveguide according to the formula n = nøsech(gr), where n is the effective refrac¬ tive index at the distance r from the axis of the waveguide, ng is the effective refractive index at the waveguide axis, and g is a constant (equal to T(/ 2f , where f is the focal length). Further features of the invention will be described or will become apparent in the course of the following detailed description.

In order that the invention may be more clearly understood, the preferred embodiment thereof will now be described in detail by way of example, with reference to the accompanying drawings, in which:

Fig.1 is a top view of the preferred embodiment of the multiplexer/demultiplexer, depicting in representational form the case of two wavelengths;

Fig.2 is a view of the entrance plane A-A¹ of the lens of the multiplexer/demultiplexer; and

Fig.3 is a cross-section at the end plane B-B¹ of the lens.

Referring to the drawings, there is illustrated the multiplexer/demultiplexer 1 of the preferred embodiment. For convenience, the device will be treated as a demulti¬ plexer in this description, although as will become apparent, the device can operate in either role, depending simply on the direction of the signals.

An input fiber F1 feeds signals carried by different wavelengths into the device at an entrance/exit plane A- A¹. Fibers F2 and F3 are representative output fibers, each carrying demultiplexed or wavelength divided signals of a different wavelength. In practice, there of course may be more than two wavelengths, and there will thus be more than the two representative output fibers F2 and F3. Due to the current limitations of light sources, it is not likely that more than about ten output fibers would be involved at present, but as laser sources in particular improve, there could be many more output fibers corres- ponding to different transmission wavelengths.

The device consists of a uniform flat substrate 2, on which is a uniform thin-film waveguide 3, on which is an

5 overlay layer 4 with a carefully selected relief profile. The overlay layer 4 may of the same material (a three- layer case, including the overlying medium, usually air), or of a different material (a four-layer case) from the- material of the waveguide portion. The optical fibers F1 ,

10 F2 and F3 are butt-joined to the uniform thin-film waveguide portion 3 of the entrance/exit plane A-A¹ of the device.

In the case of an overlay layer 4 of the same material as 1.5 the thin-film waveguide portion 3, then instead of viewing the overlay layer and the waveguide 3 as being separate, they may be properly viewed as being one and the same. For convenience of description, however, they will be referred to as separate elements throughout this specification. A 20 dotted line is used in Figs. 2 and 3 to indicate the plane of division between these elements, whether real or notional.

At the end plane B-B¹ of the device remote from the 5 entrance/exit plane A-A¹ is an integral diffraction/ reflection grating 5. The grating 5 is scribed or embossed or otherwise prepared on the end plane B-B', and a suitable thin film is deposited on the grating 5 to turn it into a reflection grating with high efficiency and low 0 losses.

The profile of the overlay layer 4 is selected so as to produce a graded effective refractive index in the lens in order to collimate the rays for arrival at the 5 diffraction/reflection grating 5 and so as to focus the wavelength-divided light rays leaving the grating at spots along the entrance/exit plane A-A', corresponding to the locations of the output fibres F2, F3, etc.. This results in a plano-convex shape for the overlay layer 4, aligned along the longitudinal axis of the thin-film waveguide 3, as illustrated in the accompanying drawings, and extending from the entrance/exit plane A-A¹ all the way back to the end plane B-B' with the grating 5. The grating 5 is of course slightly angled from the longitudinal axis so that the diffracted/reflected signals are directed slightly away from the input fibre F1 , towards representative output fibres F2 and F3.

It is known that a GRIN rod lens with an index gradient according to the formula n = ngs chCgr) focusses all meridional rays exactly. In the preferred embodiment of the present invention this index distribution is adapted to the two dimensions of the thin-film waveguide 3, with the plane of the thin-film waveguide 3 coincident with a meridional plane of the GRIN rod lens, the result being a thin-film lens which focusses all incident rays parallel to its axis. While other suitable profiles may be developed, it has been determined by the inventors that this particular profile is quite suitable as a profile for the overlay layer 4 in the present invention.

The position of each output fiber is of course dependent on the wavelength to be picked up by that fiber. The separation between fibers thus depends on the separation between wavelengths. A certain physical separation, as for example at least one fiber diameter between centers, is desireable to avoid undesireable crosstalk in the event of inaccuracies in signal focusing, which are to a certain extent unavoidable. Ideally each spot is focussed essentially entirely within the circumference of the corresponding fibre connection point, so that there is essentially no crosstalk, and the focussing properties of the present invention are such that this should be essentially achievable. To improve signal focusing, the entrance/exit plane A-A' is angled slightly from the plane normal to the axis of the thin-film waveguide 3, as can be seen in exaggerated fashion from Fig.1, based on calculations from ray-trace data, to provide a flat-plane approximation of the optimum focal plane for the different focal points of the various wavelengths. Thus both the end plane B-B¹ with the grating 5 and the entrance/exit plane A-A¹ are preferably slightly angled. The location of the optimum focal plane is naturally dependent on the angle selected for the diffrac¬ tion grating 5.

By ray-tracing through the refractive index distribution, it can be shown that diffraction-limited focusing can be obtained for many focusing and collimating requirements. From the ray-trace data, the sizes and positions of the focal spots are determined. This permits a proper choice of 'the output fiber diameters and the position of the focal plane.

To fabricate the index distribution given by the above- mentioned equation in a planar waveguide, the concept of an effective index is used. Although the bulk refractive index in a waveguide is constant, changing the thin-film thickness alters the velocity of a guided optical wave. The result is that the propogating phase front has a velocity that is equivalent to bulk propogation in a material having the appropriate effective index. In a waveguide constructed with the appropriate bulk materials and with the properly shaped profile or appropriate overlay layer 4, the required effective distribution can be obtained. This method is most effective with single- mode waveguides. However, since telecommunications are presently using single-mode fibers, and since integrated optics is best suited for single-mode techniques, this is not a serious restriction. To determine whether there exist practical materials for the realization of this lens, the dispersion curves for propogation in multilayer films were examined. For both the three-layer and four-layer cases, there exist a mini- mum and maximum waveguide thickness corresponding to cut¬ off of the TE_Q and TE-. modes, respectively. Within this thickness range there is an allowable effective index distribution. It was found that for the three-layer case with a glass substrate a variety of focal lengths could be constructed with most waveguide materials. The only res¬ triction occurs with short focal lengths when the bulk waveguide refractive index approaches that of the sub¬ strate 2. Similar construction flexibility is found for the four-layer case, and with different substrate materials.

The exact waveguide profile required to obtain the required effective index distribution is obtained in tabu¬ lated form by. solving the eigenvalue equations, which yield the dispersion curves, for the local thickness, given the required local effective index, the wavelength to be focused and the bulk refractive indices of the materials to be used. Such steps are considered to be routine and within the ordinary knowledge of those knowledgeable in the field.

This design enables all the wavelengths, which may be close together, to be separated with only oηe lens element and grating of integral construction. The integral construction of the device is particularly advantageous since there are no discontinuities in the path of the signal, the same element being used for both collimating and focusing. The lens structure uses the entire thin-film waveguide to obtain the collimating and focussing effects, by virtue of the effective index gradient which extends from the entrance/exit plane A-A¹ to the diffraction/ reflection grating 5. This structure, in contrast to existing devices such as those in the above-mentioned Canadian patent no. 1,089,932, eliminates two dielectric boundaries, between planar waveguide and lens, and between lens and planar waveguide. Such prior art devices used isolated thickness varying regions. This resulted in a localized lens element bounded by a planar waveguide. Eliminating these boundaries provides the potential to minimize mode-conversion and scattering losses.

It will be readily appreciated from the above description that the device can operate either as a multiplexer or as a demultiplexer with equal facility, depending merely on the direction of the signals. When operating as a multiplexer, the signals from the fibres F2, F3, etc. are combined at the diffraction/reflection grating 5, producing an output signal which is focussed at the fibre F1.

The lens can be manufactured using existing techniques used for Luneberg lens construction, using a wide variety of materials from low loss optical glasses to active materials such as LiNbθ3.

Two methods could conceivably be used to fabricate such a lens. First, sputtering or evaporation could be used with appropriate masks. Alternatively, a lens with a suitable profile could be embossed on a dip-coated deformable gel film on a substrate. Subsequent heat treatment would transform the molded gel into an inorganic hard oxide material. Each of these techniques requires computer-aided design and a high precision mask or mold. However, once the mask or mold is made, it can be used to fabricate many lenses.

It will be extremely important to keep the fabrication process within tolerances, since any error in the waveguide thickness will increase aberrations and result in a corresponding error in the lens focal length. However, small shifts in the focal plane position may be tolerated since the optimum focal plane can be reached through grinding and polishing of the waveguide edge.

The principal technologies involved are listed belov/ with a brief description of their applications to the fabrication of the different parts of the device:

1. Fiber cutting - the ends of each fiber must be creaved with flat surfaces perpendicular to the fiber axis.

2. Polishing - the two ends of the integrated thin- film lens (surfaces AA¹ and BB¹ in Figs. 2 and 3) and also the fiber ends must be polished.

3. Butt-joiήing - fibres have to be very accurately butt-joined to the end of the integrated thin-film waveguide 3 by a suitable method, possibly by using a suitable epoxy or by fusion.

4. Thin-film deposition - the film with an appro- priate thickness must be deposited on the flat surface of a substrate. Also, the end surface BB¹ (Fig. 3) must be provided with an appropriate film which is used to fabricate a grating 5. A dipping and baking technique is proposed.

5. Embossing - this technique will be used to shape the curved surface with a pre-determined relief profile on top of the integrated thin-film waveguide 3. It will also be used to produce the grating 5.

6. Conventional thin-film deposition - a thick film has to be deposited on the grating 5 to turn it into a reflection grating with very high efficiency and very low losses. A sputtering or a high-vacuum evaporation system may be used.

This new optical waveguide lens has been analyzed and its fabrication and focusing properties have been theoretically examined. It has been found to have reasonable fabrication tolerances compatible with existing shadow masking sputtering techniques and a new embossing technique. Considerable design flexibility allows its construction with a variety of materials. Ray tracing reveals that diffraction limited focusing should be possible. Also, low f-numbers can be obtained, making the design promising for use in miniaturized integrated optic circuits. The embossing technique will likely be suitable for mass production, resulting in lower costs when compared with other existing methods.

It should be noted that although in Figs. 2 and 3 the plane of division between the thin-film waveguide portion 3 and the overlay layer 4 is shown as being such that the overlay layer has zero thickness at the edges of the waveguide, the plane could be positioned so that the overlay layer does have a definite thickness at the waveguide edge. This of course is only relevent in the four-layer case, since in the three-layer case the division is notional rather than real.

It will be appreciated that the above description relates to the preferred embodiment by way of example only. Many variations on the invention will be obvious to those knowledgeable in the field, and such obvious variations are within the scope of the invention as described and claimed, whether or not expressly described.

Claims

Claims :

1. A thin-film waveguide lens for use in a multiplexer/demultiplexer, comprising: a thin-film waveguide having a longitudinal axis; end planes essentially normal to said axis at each end of said waveguide; a plano-convex overlay layer integral with said waveguide and extending the length of said waveguide along said axis, the profile of said overlay layer being selected so as to produce a graded effective refractive index in the lens in order to collimate focussed rays and focus collimated rays entering at one said end plane for substantially collimated or focussed arrival at the other said end plane.

2. A lens as recited in claim 1 , in which the shape of said plano-convex overlay layer is such as to provide an effective index of refraction profile in said waveguide according to the formula n = ngsech (gr) , where n is the effective refractive index at the distance r from said axis, Ώ. is the effective refractive index at said axis, and g is a constant equal to tf/2f, where f is the focal length of said lens, being the distance between said end planes.

3. A lens as recited in claim 1, in which the material of said overlay layer is the same as the material of said waveguide.

4. A lens as recited in claim 2, in which the material of said overlay layer is the same as the material of said waveguide.

5. A multiplexer/demultiplexer comprising a thin- film waveguide lens, said lens comprising: a thin-film waveguide having a longitudinal axis; end planes essentially normal to said axis at each end of said waveguide; a plano-convex overlay layer integral with said waveguide and extending the length of said waveguide along said axis, the profile of said overlay layer being selected for the length of said lens so as to produce a graded effective refractive index in the lens in order to collimate focussed rays and focus collimated rays entering at one said end plane for substantially collimated or focussed arrival at the other said end plane; one said end plane constituting an entrance/exit plane for receiving optical fibres in an abutting connection; the other said end plane bearing a diffraction/ reflection grating; whereby focussed rays entering the lens at said entrance/ exit plane are substantially collimated for arrival at said diffraction/relection grating, and whereby rays reflecting from said diffraction/reflection grating are ■ substantially focussed on returning to said entrance/exit plane, the locations for abuttment of said optical fibres being matched to the spot locations of the focussed rays along the entrance/exit plane.

6. A multiplexer/demultiplexer as recited in claim 5, in which the shape of said plano-convex overlay layer is such as to provide an effective index of refraction profile in said waveguide according to the formula n = TΪ_Q sech (gr), where n is the effective refractive index at the distance r from said axis, ng is the effective refractive index at said axis, and g is a constant equal to /2f, where f is the focal length of said lens, being the distance between said entrance/exit plane and said diffraction/reflection grating.

7. A multiplexer/demultiplexer as recited in claim 5, in which the material of said overlay layer is the same as the material of said waveguide.

8. A multiplexer/demultiplexer as recited in claim 6, in which the material of said overlay layer is the same as the material of said waveguide.

9. A multiplexer/demultiplexer as recited in claim

5, in which said end plane constituting an entrance/exit plane is angled slightly from the plane normal to said waveguide axis, so as to provide a flat-plane approxi¬ mation of the optimum focal plane for the spot locations of the focussed rays along the entrance/exit plane.

10. A multiplexer/demultiplexer as recited in claim

6, in which said end plane constituting an entrance/exit plane is angled slightly from the plane normal to said waveguide axis, so as to provide a flat-plane approxi¬ mation of the optimum focal plane for the spot locations of the focussed rays along the entrance/exit plane.

11. A multiplexer/demultiplexer as recited in claim

7, in which said end plane constituting an entrance/exit plane is angled slightly from the plane normal to said waveguide axis, so as to provide a flat-plane approxi¬ mation of the optimum focal plane for the spot locations of the focussed rays along the entrance/exit plane.

12. A multiplexer/demultiplexer as recited in claim

8, in which said end plane constituting an entrance/exit plane is angled slightly from the plane normal to said waveguide axis, so as to provide a flat-plane approxi¬ mation of the optimum focal plane for the spot locations of the focussed rays along the entrance/exit plane.