Birefringent Optical Waveguide Structure Having a Strip Waveguide Buried in a Cladding Layer and the Method of Manufacturing

The present invention is directed to a birefringent optical waveguide structure which is buried in a cladding layer of a second glass and to a method for manufacturing the waveguide structure.

Passive optical components that split the signal into orthogonally polarized parts are needed for coherent optical heterodyne reception independently of the polarization of the incoming signal. Such components can be executed with the means of integrated optics when adequately birefringent, integrated optical waveguides or structures of such waveguides can be manufactured with which the division of arbitrarily polarized light into two mutually orthogonally polarized components can be carried out.

It is an object of the present invention to provide a birefringent optical waveguide structure comprising at least one strip waveguide buried in a cladding layer of glass, which can be manufactured with adequately high birefringence so that the division for arbitrarily polarized light into mutual orthogonal components is attainable or enabled.

To accomplish this object, the invention is directed to a birefringent optical waveguide structure comprising at least one strip waveguide of a first glass or material being buried in a cladding layer of a second glass or material and at least one additional layer being provided on the cladding layer, said first glass having a refractive index higher than the refractive index of the second glass and the additional layer applying a mechanical stress to each strip waveguide with a component of the stress extending perpendicular to said strip waveguide. Preferably, the additional layer is a glass layer and is applied, preferably, on both surfaces or sides of the cladding layer. The glass of the additional layer has a different coefficient of thermal expansion and a different elasto-optical coefficient in comparison to the coefficient of thermal expansion and elasto-optical coefficient of the first and second glasses.

The method of producing such a birefringent optical waveguide structure comprises providing a substrate having a cladding glass layer of a second glass with a strip waveguide of the first glass being embedded in the cladding layer and at least one additional layer on the cladding layer to form a structure, then heating the structure to an elevated temperature and subsequently rapidly cooling the structure from the elevated temperature to a relatively low temperature to create thermal stresses between the cladding layer and the additional layer. The step of providing the substrate containing the cladding glass layer with the waveguide structure buried therein can be obtained by providing the cladding glasslayer on the substrate and embedding the waveguide in the cladding glass by field-induced ion exchange, by diffusion or by ion implantation. It is also possible to produce the buried strip waveguide structure in the cladding glass by providing a layer of the cladding glass, providing a layer of the first glass forming the strip waveguide, processing, such as be etching, the layer of the first glass to form the waveguide structure, then coating this structure with a layer of the second glass forming the cladding glass. Preferably, the step of providing the structure includes providing a substrate, providing an additional layer directly on the substrate, applying the cladding layer, forming the waveguide structure, either directly in the cladding layer or on the cladding layer and covering it with a second cladding glass layer, then applying an outer or second additional layer. The applying of the glass layers, for both the additional layer and cladding glass, as well as the layer for forming the structure, can be depositing from a vapor phase or by depositing by sputtering, it also can be formed by depositing in accordance to a CVD method.

Known structures of integrated optics, such as, for example, branchings, optical directional couplers, cascades of optical directional couplers, waveguide crossings and combinations of these elements can be manufactured with the waveguide of the invention and with the method of the invention. The invention is based on the fact that an adequate birefringence in glass can be produced by mechanical stresses which arise, for example, when glasses connected to one another and having different coefficients of thermal expansion and elasto-optical coefficients, particularly glasses that have different compositions, are cooled down in common from a high temperature.

Other advantages and features of the invention will be readily apparent from the following description of the preferred embodiments, the drawings and claims.

FIG. 1 is a plan view of a waveguide structure in the form of a cascade optical directional coupler on a substrate; and

FIG. 2 is a cross sectional view taken along the lines II--II of FIG. 1.

The principles of the present invention are particularly useful when incorporated in the waveguide structure of FIG. 1, which comprises a cascade of optical directional couplers RK1, RK2 and RK3 with the coupler RK1 being a traditional 3dB coupler for both intrinsic modes conducted in, respectively, one of two waveguides LW1 and LW2. The couplers RK2 and RK3 are polarization-sensitive couplers that enable the complete separation of two intrinsic modes supplied in the respective waveguides LW1 and LW2 and orthogonal relative to one another. Thus, one intrinsic mode is conducted to the photodiode PO2 and PO3 in the waveguide LW1 or, respectively, LW2, whereas the intrinsic mode orthogonal thereto and separated therefrom in the directional coupler RK2 will be conducted by the light waveguide LW21 to a photodiode pp1 and the mode separated by the directional coupler RK3 will be conducted in the light waveguide LW32 to a photodiode PO4.

The structure of FIG. 1 is suitable for an optical diversity receiver, whereby the elliptically polarized, optical signal FS conducted to the receiver and, for eaxmple, a transmission fiber, is coupled into one of the waveguides LW1 or LW2 and a linearly polarized optical signal LO or a local oscillator of the receiver is coupled into the other waveguide LW2 or, respectively, LW1.

The optical directional couplers RK1, RK2 and RK3 are formed in a knownmanner in that the two waveguides LW1 and LW2, and LW1 and LW21, or LW2 and LW32 are conducted in such a close proximity next to one another in a respective coupling path, such as L1, L2 or L3, that the overall coupling of light between them occurs. However, these waveguides are conducted at such a great distance from one another side-by-side outside of the coupling path L1, L2 and L3 that no such overcoupling will occur.

As already mentioned, the directional coupler RK2 and RK3 differ from the directional coupler RK1 in that they are polarization sensitive and separate the two supplied intrinsic modes polarized orthogonally relative to one another from one another. To that end, the two waveguides LW1 and LW21 for the coupler RK2 and the waveguides LW2 and LW32 for the coupler RK3 are composed of highly birefringent strip waveguides in the region of these directional couplers RK2 and RK3. For example, they are highly birefringent at least in their coupling path L2 or, respectively, L3.

An example for the manufacture of such a birefringent strip waveguide is set forth with reference to FIGS. 1 and 2. As illustrated in FIG. 2, a glass layer GSl that has a different coefficient of thermal expansion and a different elasto-optical coefficient from the jacket glass of the waveguide which has a predetermined index of refraction is deposited on a substrate Sub in the region B of FIG. 1. The directional couplers RK2 and RK3, but not the directional coupler RK1, are situated in this region B. A glass layer GS2, which is a cladding layer of cladding or jacket glass, is deposited on this first-mentioned layer GS1. To form the strip waveguide, such as LW32, LW2, LW1 and LW21, a glass layer having a refractive index that is higher than the refractive index of the jacket glass of the layer GS2 is placed on the layer GS2. This newly applied glass layer will have the same coefficient of thermal expansion and the same elasto-optical coefficient as the jacket glass insofar as possible.

This glass layer is then etched off with the assistance of known etching and masking techniques, so that only the waveguides LW1, LW2, LW21 and LW32 remain on the jacket glass layer GS2 as elevated strips. The jacket glass layer GS2 and the waveguides LW1, LW2, LW21 and LW32 situated thereon are then covered with a second cladding layer GS3 of the jacket or cladding glass. An additional glass layer GS4 that, again, has a coefficient of thermal expansion and elasto-optical coefficient different from the jacket glass is deposited on the cladding layer GS3. This will form a unit or waveguide structure. After the application of all the layers, the structure of the layers are heated to a high temperature and are then quickly cooled in common to a relatively low temperature. Due to the different coefficients of thermal expansion, mechanical stresses having at least a component Sp extending perpendicular to the waveguides LW1, LW2, LW21 and LW32 will be formed.

Instead of manufacturing the waveguides by depositing a glass layer on a jacket or cladding glass layer, etching the layer to form the waveguide strips and then coating with a second layer of the cladding glass, the waveguides can be formed by providing one or more layers of the cladding glass and then forming or burying the waveguides in this cladding glass by a field-induced ion exchange or by a diffusion process or an ion implantation. All of these processes are known techniques.

The depositing of the glass layers can occur from a vapor chase, for example, by a CVD method or can also occur by means of sputtering the glass layers.

In a specific exemplary embodiment, the substrate Sub is composed of silicon. The additional layers GS1 and GS4 are composed of an SiO.sub.2 that is doped with Al.sub.2 O.sub.3. The cladding or jacket layers GS2 and GS3 is composed of SiO.sub.2 and the waveguides LW1, LW2, LW21 and LW32 are composed of SiO.sub.2 that has been doped with either GeO.sub.2 or TiO.sub.2 . These glass layers are heated to 700.degree. C. and are then quickly cooled to room temperature. A mechanical stress having a component Sp perpendicular to the waveguides LW1, LW2, LW21 and LW32 will occur and a relative difference of .alpha..beta./.beta. in the propagation constant having a value of about 10.sup.-4 will occur for the intrinsic modes conducted in the waveguides and polarized orthogonally relative to one another.

Although various minor modifications may be suggested by those versed in the art, it should be understood that we wish to embody within the scope of the patent granted hereon all such modifications as reasonalby and properly come within the scope of our contribution to the art.