Patent Application
20060093298
The present application claims priority of Italian Patent Application No. RM2004A000544 filed Nov. 4, 2005, entitled INTEGRATED OPTICAL WAVEGUIDE AND PROCESS FOR ITS FABRICATION, which is incorporated herein in its entirety by this reference.
The present invention relates to the field of optical waveguides and more particularly to an integrated optical waveguide and process for its fabrication.
In the field of telecommunications and data transmission, the current tendency to use optical signals instead of traditional electrical signals is well known. Transmission of optical signals takes place by means of optical waveguides and generation and processing of the optical signals takes place by means of optical devices, such as laser sources, amplifiers, modulators, and the like. Many of these devices can be produced in the form of integrated planar optical circuits using manufacturing techniques typical of integrated planar semiconductor electronic circuits. According to one of said techniques the waveguides are formed, together with other optical components, on a silicon substrate or on a dielectric substrate. First, a layer of silicon dioxide with a relatively low refractive index (e.g. 1.458) is deposited on the substrate, intended to make up the lower cladding of the cores of the waveguides; then another layer of silicon dioxide with a relatively high refractive index (nc>1,46) is deposited on the lower cladding; the cores of the waveguides are obtained from this layer by means of selective anisotropic etching using normal photolithographic techniques; finally, a further layer of silicon dioxide is deposited, usually with the same refractive index as the lower cladding, in order to cover both the sides and top of the cores. With this technique it is possible to obtain waveguides with a substantially square cross-section.
One important aspect in designing optical systems is the coupling of different devices both inside and outside the same integrated optical circuit, for example coupling between an integrated waveguide and an optic fiber. The ends of the waveguides to be coupled together may have very different cross-sections. For example, the input waveguide of a device to be interfaced with a laser source may have one end with a square section with sides of 5 μm while the laser source emits a luminous power from a circular or elliptical cross-section with axes between 0.5 μm and 2 μm in a solid angle with openings between 20 and 40 degrees on both axes, or the output waveguide of a device to be interfaced with an optic fiber may have one end with a square cross-section with sides of 5 μm and the input end of the optic fiber may have a circular cross-section with a 9 μm diameter. In these conditions, the efficiency of the coupling is generally very low.
In order to obtain more efficient couplings, various techniques are known: some of these require interposition of optical systems between the waveguides to be coupled, others envisage modifications to the terminal segments of either one or the other or both the waveguides to be coupled, gradually increasing or reducing the cross-section adiabatically, i.e. substantially without loss and maintaining the single-mode transmission characteristics of the guide. These techniques, however, are rather complex and, since they require an increase in the cross-sections of the input and output ends of the waveguides, they are not suitable to be used in integrated optical circuits with multiple input/output ports.
Another approach to increase the efficiency of the coupling between waveguides having different cross-sections comprises increasing the refractive index of the core of the waveguide with the smaller cross-section. In this way, the effective area of the end of the waveguide with the smaller cross-section is increased but immunity to noise of the entire waveguide is reduced.
The present invention provides a waveguide and a process for its fabrication that permits high waveguide coupling efficiency without foregoing the most convenient index contrast for most of its length.
The integrated optical waveguide of the present invention is defined by a core and cladding and includes a terminal segment having an input/output end. The core has a refractive index in the terminal segment that varies gradually in a longitudinal direction and increases towards the input/output end.
In a preferred process for fabricating an integrated optical waveguide on a substrate, a layer of a material for the lower cladding of the waveguide is deposited on the substrate. Material for the waveguide core is deposited on the lower cladding material. The waveguide core material is selectively removed to form the waveguide core and define an input/output end of the waveguide. Upper cladding material is deposited to cover the top and sides of the waveguide core such that the operation to deposit a layer for the core includes the successive deposition of multiple layers of a material with a pre-established refractive index, with deposition of each layer followed by implantation of dopant impurities (P+) in a pre-established area of the respective layer so as to modify the pre-established refractive index. The method includes a high-temperature treatment to diffuse the dopant impurities. The reciprocal position of the pre-established areas of the various layers and the implantation dosages and energy are selected such that after the high-temperature treatment, the refractive index of the core in a segment of the waveguide that terminates in an input/output end, varies gradually in a longitudinal direction and increases towards the input/output end of the waveguide.
The invention will become more apparent from the following detailed description of an embodiment thereof as illustrated in the accompanying drawings, in which the various figures represent a terminal segment of a waveguide according to the invention in various steps of the fabrication process, and more particularly:
a to 12a illustrate lateral cross-sections of a preferred embodiment of the present invention; and
b to 12b illustrate longitudinal cross-sections of the invention shown in
With reference to the drawings, in particular
A multi-layer 13 (
The mask 14 is then removed and a second layer 13.2 of silicon dioxide is deposited (
The mask 18 is then removed, and a third layer 13.3 of silicon dioxide is deposited (
Once the deposition and implantation operations have been terminated, high-temperature treatment (annealing) is carried out, during which the implanted impurities spread inside the multi-layer 13, creating a region 16 where, as illustrated in
A photoresist mask 22 is then formed on the multi-layer 13 (
As is clear from the above description and drawing figures, the refractive index of the terminal segment of the waveguide core gradually increases longitudinally from the value of the longest part of the waveguide, which is constant if the layers that make up the multi-layer 13 have the same refractive index as in the embodiment described, to a higher value near the end of the guide itself; therefore, the end 24 of the waveguide has an effective area greater than it would have had without the above-described treatment. It should be noted that in this embodiment the refractive index of the terminal segment also varies transversally. In particular, it decreases gradually from the center towards the lower cladding layer and towards the upper cladding layer.
In this way, a waveguide is obtained whose core has the most suitable refractive index for the transmission characteristics desired for most of its length and a higher refractive index at its input/output ends; in this way, coupling with another waveguide is more efficient.
Furthermore, the terminal segment has all the advantages of the waveguides whose refractive index gradually decreases towards the perimeter, such as good luminous energy confinement and good noise immunity.
It is understood that although only one exemplary embodiment of the invention has been illustrated and described, numerous modifications are possible without departing from the scope and spirit of the invention. For example, the multi-layer for the waveguide cores can be made up of more than three layers, each of which can be subjected to selective implantation with appropriate elements, dosages and energy in order to obtain the desired profile for the refractive index of the terminal segments of the waveguide; the material of the layers can be different from silicon dioxide provided that its refractive index can be modified through implantation; moreover, the openings of the implantation masks adjacent to the respective main openings can be more or less than three, or even totally absent: in this latter case, the gradual profile of the refractive index of the waveguide terminal segments is determined only by the reciprocal dimensions of the main areas and by the parameters of the respective implantation operations.
While there have been described above the principles of the present invention in conjunction with specific memory architectures and methods of operation, it is to be clearly understood that the foregoing description is made only by way of example and not as a limitation to the scope of the invention. Particularly, it is recognized that the teachings of the foregoing disclosure will suggest other modifications to those persons skilled in the relevant art. Such modifications may involve other features which are already known per se and which may be used instead of or in addition to features already described herein. Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure herein also includes any novel feature or any novel combination of features disclosed either explicitly or implicitly or any generalization or modification thereof which would be apparent to persons skilled in the relevant art, whether or not such relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as confronted by the present invention. The applicants hereby reserve the right to formulate new claims to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom.
| Number | Date | Country | Kind |
|---|---|---|---|
| RM2004A000544 | Nov 2004 | IT | national |
