Optical waveguide device

Abstract: An optical waveguide device comprises a low refractive index layer (8) having a refractive index n b , a core layer (6-1, 6-2) having a refractive index n c (where n c > n b ) and a cladding layer (9) having a refractive index n c1 (where n c1 < n c ) formed on a substrate (7). Thermal expansion co­ efficients of the low refractive index layer (8), the core layer (6-1, 6-2) and the cladding layer (9), respectively, are made substantially equal to each other at respective boun­ daries.

Description: BACKGROUND OF THE INVENTION The present invention relates to an optical waveguide device which is less or scarcely dependent on the plane of polarization of the input signal. In optical fiber communication, wavelength multiplex transmission is very important from the view point of achieving economy and flexibility of the optical communication system. Furthermore, optical coupling/branching devices are indispensable for light wavelength multiplex transmission. In recent years there has been a trend towards studying and developing an optical coupling/decoupling waveguide structure in a one-chip monolithic structure thereby causing a reduction in manufacturing cost. A typical example of such an optical device, is disclosed in N. Takato et al's article entitled "Low-loss Directional Coupler Using High-silica Embedded Channel Waveguides" contained in "OEC '86 Technical Digest", A3-3 (July, 1986), pp. 22 and 23. In this device, optical branching characteristics are obtained by making use of the fact that the coupling of two optical waveguides is dependent on wavelength. More specifically, light signals of wavelengths λ₁ and λ₂ inputted in to a first port propagate through a coupling region, reciprocating periodically between two cores, wherein the light signal of wavelength λ₁ leaves via through a third port with the light signal of wavelength λ₂ leaving via a fourth port. The substrate of the optical device is usually, a Si-substrate or a SiO₂-substrate. A low refractive index layer (having a refractive index n b ) is formed of a SiO₂-film. The refractive index n c of the core has to be greater than the index n b of the low refractive index layer. For this reason, the SiO₂-film is ordinarily doped with refractive index controlling dopant such as TiO₂, P₂O₅, GeO₂ or the like. A cladding layer is formed of a material having the same refractive index as that (n b ) of the buffer layer (low refractive index layer) and is usually formed from SiO₂. In the structure of the optical waveguide device mentioned above, thermal expansion coefficients of the core, the low refractive index layer and the cladding layer differ from one another. Fig. 11A of the accompanying drawings shows the relationship between doping content of and refractive index for various dopants, and Fig. 11B shows the relationship between doping content and thermal expansion for various dopants. As will be seen in Figs. 11A and 11B, when the dopant TiO₂, GeO₂, P₂O₅ etc. is contained in SiO₂, the thermal expansion coefficient of the latter assumes correspondingly different values. Further, as the content of dopant increases, the magnitude of change in the thermal expansion coefficient becomes greater. As a consequence, thermal stress is produced between the core and the cladding, between the core and the low refractive index layer and additionally between the low refractive index layer and the substrate. The thermal stress in turn brings about anisotropy in the refractive indices, resulting in the occurrence of shift of wavelength which exerts adverse influence on the coupling efficiency between E 11X -mode and E 11Y -mode. The wavelength shift causes a degradation in isolation in the optical waveguide coupling/branching device, leading to an increase in the cross-talk between the light signals of different wavelengths. An optical waveguide device having the features included in the first part of claim 1 is known from JP-A-61-284704. To reduce the difference in the thermal expansion coefficients between the low-refractive index layer and the core layer, the difference in doping between the layers is made very small. As a result, however, the difference in the refractive indices is also very small (about 1 %) which impairs the light guiding capability of the core layer. SUMMARY OF THE INVENTION An object of the present invention is to provide an optical waveguide coupling/branching device as well as other optical waveguide devices which are substantially immune from the drawbacks mentioned above. This object is met by the optical waveguide device defined in claim 1. As described previously, the core layer is in contact with the low refractive index layer and the cladding layer. Accordingly, by selecting the material composition such that the respective thermal expansion coefficients at the boundaries between the core layer and the other layers are substantially equal to each other, thermal stresses due to differences in the thermal expansion coefficient can be alleviated. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1A to 4B are sectional views illustrating distributions of thermal expansion coefficients and refractive indexes in optical waveguide devices according to exemplary embodiments of the invention, respectively, in the direction thicknesswise of the device; Fig. 5 shows an optical waveguide branching device to which the invention can be applied; Fig. 6 shows an optical waveguide star coupler device to which the invention can be applied; Fig. 7 shows an optical waveguide switch device to which the invention can be applied; Fig. 8 is a view showing a method of fabricating an optical waveguide device according to an embodiment of the invention; Fig. 9 is a plan view of an optical module to which an embodiment of the invention is applied; Figs. 10A and 10B are views showing a structure of the optical waveguide coupling/branching device according to an embodiment of the invention; Fig. 11A is a view showing relations between contents of dopants used for realizing the optical waveguide device according to the invention and refractive indexes thereof; and Fig. 11B is a view showing the relationship between dopant content and the thermal expansion for various materials in the optical waveguide device according to the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Figs. 10A and 10B show an optical waveguide coupling/branching device according to a first embodiment of the present invention. Referring to the figures, reference numeral 7 denotes a SiO₂-substrate of 5 cm (2 in) wide and 0.45 mm in thickness. A SiO₂-film of 4 µm in thickness doped with B₂O₃ and P₂O₅ is formed on the SiO₂-substrate 7 to serve as a low reference index layer 8. In that case, the contents of dopants B₂O₃ and P₂O₅ increases progressively with distance from the substrate. More specifically, in the plane over which the SiO₂-film or low refractive index layer 8 lies in contract with the SiO₂-substrate 7, the layer 8 contains no dopants. As the distance of the low refractive index layer 8 from the substrate is increased, the contents of the dopants B₂O₃ and P₂O₅ are increased so that the thermal expansion coefficient of the low refractive index layer 8 is increased continuously (or substantially continuously or stepwise), as will be seen in Fig. 1A which shows the distribution of the thermal expansion coefficient in a section Y - Y of the device as shown in Fig. 10B. The pure SiO₂-film portion as well as the SiO₂-film doped with B₂O₃ and P₂O₅ mentioned above can be formed by a CVD (chemical vapour deposition) method, a sputtering method, a flame deposition method or the like. By continuously changing the thermal expansion coefficient in this manner, thermal stress due to a difference between the thermal expansion coefficients of the substrate 7 and the low refractive index layer 8 can be alleviated. On the other hand, core layers 6-1 and 6-2 are so formed that the thermal expansion coefficient thereof is substantially equal to that of the low refractive index layer 8 at the boundaries between the layers 6-1 and 6-2 and the layer 8, whereby thermal stress is prevented from being produced at the boundaries. Further, a cladding layer 9 is also formed to have the thermal expansion coefficient equal to that of the core layers 6-1 and 6-2 at the boundaries. Accordingly, no thermal stress can take place at the boundaries between the cladding layer 9 and the core layers 6-1 and 6-2. It should be noted that distribution of the refractive indices is realized such that the refractive index n c of the core layers 6-1 and 6-2 is larger than that of the low refractive index layer 8 and the cladding layer 9. Next, a numerical example of the optical waveguide device will be described. The core layers 6-1 and 6-2 are each constituted by a phosphosilicate glass film (having a thickness of about 8 µm) formed of SiO₂ doped with about 3 mol % of P₂O₅. The cladding layer 9 is constituted by a phosphoborosilicate glass film (having a thickness of about 18 µm) formed of SiO₂ doped with about 3 mol % of B₂O₃ and about 3 mol % of P₂O₅. The low refractive index layer 8 is also formed from the phosphoborosilicate glass film mentioned above, whose composition in however so selected that no dopants are present in SiO₂ in the plane over which the low refractive index film 8 contacts with the SiO₂-substrate 7 and contents of dopants P₂O₅ and B₂O₃ are increased by substantially equal amount in mol %, as the thickness of the low refractive index layer 8 is increased (i.e. the distance of the layer 8 from the substrate 7 is increased), until the contents of the dopants P₂O₅ and B₂O₃ in SiO₂ have attained about 3 mol %, respectively, at or in the vicinity of the boundary to the core layers 6-1 and 6-2. In this manner, the characteristics illustrated in Figs. 1A and 1B can be realized, the reasons for which will be readily understood by referring to Figs. 11A and 11B. Fig. 2A to 2B illustrate a second embodiment of the optical waveguide device according to the invention, in which the thermal expansion coefficient of the cladding layer 9 is decreased, as the thickness thereof is increased. The core layers, the low refractive index layer and the substrate are identical with those of the first embodiment shown in Figs. 1A and 1B. More specifically, the cladding layer 9 is formed from a phosphoborosilicate glass film which is realized by doping SiO₂ with B₂O₃ and P₂O₅ to have a profile whereby about 3 mol % of B₂O₃ and P₂O₅, respectively, are contained in the cladding layer 9 at the boundary with the core layers 6-1 and 6-2, wherein the contents of both dopants are decreased progressively by equal amounts, as the thickness of the cladding layer 9 is increased, until the SiO₂-film of the layer 9 finally contains no dopants. The thickness of the cladding layer 9 is about 18 µm. Figs. 3A and 3B show a third embodiment of the optical waveguide device according to the invention, in which the content of the dopant B₂O₃ in the cladding layer 9 is selected to be higher than that of P₂O₅ so that the refractive index of the cladding layer 9 is lower than that of the low refractive index layer 8. More specifically, the cladding layer 9 is realized by doping SiO₂ with 4 mol % of B₂O₃ and 2 mol % of P₂O₅, respectively, at the region in contact with the core layers 6-1 and 6-2 to form a glass film, wherein the contents of the dopants B₂O₃ and P₂O₅ are decreased, as the thickness of the cladding layer 9 is increased, while maintaining constant the content ratio of both dopants. Figs. 4A and 4B show a fourth embodiment of the optical waveguide device according to the invention, in which the content of the dopant P₂O₅ in the cladding layer 9 is selected to be higher than that of B₂O₃ so that the cladding layer 9 has a higher refractive index than the low refractive index layer 8. As will be appreciated from the above description of the embodiments of the invention, the present invention teaches that the occurrence of thermal stress is minimized by making the thermal expansion coefficients at the boundaries between the core layers, the low refractive index layer and the cladding layer at least approximately equal to each other, to thereby reduce the shift of the center wavelength. It should however be understood that the present invention is not restricted to the illustrated embodiments but numerous modifications will readily occur to those skilled in the art without departing from the scope of the invention. By way of example, in addition to the combination of dopants P₂O₅ and B₂O₃ in SiO₂, many other combinations of the dopants shown in Figs. 11A and 11B such as, for example, combinations of Al₂O₃ and B₂O₃, GeO₂ and B₂O₃, TiO₂ and P₂O₅, TiO₂ and P₂O₅ and B₂O₃ and others can be used. Further, the present invention can equally be applied to an optical branching device such as shown in Fig. 5, an optical star coupler such as shown in Fig. 6, an optical switch constituted by providing electrodes 16-1 and 16-2 on a cladding layer of a coupling region of a directional coupler as shown in Fig. 7, an optical modulator and other devices in addition to the optical waveguide coupling/branching device described hereinbefore. According to the teachings of the present invention, these optical waveguide devices can be made with a minimum shift of the center wavelength brought about by variation in temperature, whereby optical waveguide devices of high performance (exemplified by a reduction of cross-talk due to high degree of inter-wavelength isolation and reduction in the transmission loss) can be obtained. Besides, by virtue of the matched thermal expansion coefficient at the inter-layer boundary, generation of cracks and breakage of the glass film upon formation thereof can positively be prevented. Additionally, deviation in the length of the coupling region due to a bend of the substrate can also be excluded. Next, a method of fabricating an optical waveguide device according to an embodiment will be described. Referring to Fig. 8 which shows a manufacturing process, a low refractive index layer 8 is deposited on a substrate (which may be a SiO₂-substrate or other semiconductor substrate such as Si- or GaAs-substrate) by a low-temperature CVD method at a reaction temperature of 390°C. More specifically, a layer containing dopants B₂O₃ and P₂O₅ is formed on the SiO₂-substrate over a whole width thereof by blowing thereon monosilane gas, diborane gas and phosphine gas together with O₂-gas and N₂-gas. In that case, the flow rates of the diborane and phosphine gases, respectively, are controlled by gas flow controllers so as to be increased, as deposition of the film proceeds, in accordance with a predetermined control program. Subsequently, a core layer 6 is deposited on the low refractive index layer 8 thus formed. Deposition of this core layer 6 on the layer 8 is performed such that the thermal expansion coefficients of these layers 6 and 8 are at least approximately equal to each other at the boundary therebetween and that the core layer 6 has a higher refractive index than that of the low refractive layer 8. To this end, the diborane gas is not fed but only the monosilane and phosphine gases and O₂- and N₂-gases are blown to thereby form the film through deposition. After deposition of the film, anneal processing is conducted about one hour at a temperature of about 1200°C in the O₂-atmosphere. Next, a WSi x -film (where x ≈ 0.4) is deposited in thickness of about 1 µm by sputtering. Thereafter, a photoresist film is formed over the WSi x -film in a thickness of about 1.5 µm by application, being followed by patterning of the photoresist film by using a photomask. The sample undergone the patterning is then dry-etched by using a NF₃-gas to thereby carry out the patterning of the WSi x -film. Subsequently, the sample is dry-etched by using a mixture gas of C₂F₆ and CHF₃ for thereby patterning the core layer 6 to form core patterns 6-1 and 6-2 of the directional coupler. After the photoresist and the WSi x -film have been removed by etching, soot of glass to form a cladding layer 9 is deposited through a flame deposition process by using a flame hydrolysis burner. More specifically, soot is produced by using an oxyhydrogen burner of a coaxial four-tube structure, wherein vapours of SiCl₄, POCl₃ and BBr₃ are fed to the center tube of the oxyhydrogen burner with Ar-gas being used as a carrier gas to thereby bring about a flame hydrolysis reaction in the burner flame. The soot mentioned above consists of glass soot of SiO₂, P₂O₅ and B₂O₃. After deposition of the glass soot, vitrification of the soot is realized by heating about one hour at a temperature of about 1200°C in the atmosphere of He. It should be mentioned that the present invention is not restricted to the abovementioned method. By way of example, deposition of the glass film can also be realized for example by sputtering, vapour deposition, flame deposition, plasma CVD method instead of the CVD method described above. In the foregoing, individual embodiments of the optical waveguide devices have been described separately. However, it goes without saying that the invention can equally be applied to combinations of optical devices such as, for example, combinations of the optical waveguide coupling/branching devices shown in Figs. 1A and 1B with the optical waveguide branching device shown in Fig. 5, combinations of the former with the optical switch shown in Fig. 7 or other combinations. Additionally, the present invention can be applied to an optical module composed of a combination of optical devices in accordance with the invention such as an optical waveguide coupling/branching device, an optical switch and/or an optical waveguide branching device with a semiconductor laser, a light receiving element, as is shown in Fig. 9. More specifically, Fig. 9 shows an embodiment of the optical module resulting from combination of an optical waveguide coupling/branching device, a semiconductor laser 18 and an opto-electronic light receiving element 17. A light signal of wavelength λ₁ is injected into to the optical module 19 and is received by the light receiving element 17. On the other hand, a light signal of wavelength λ₂ emitted by the semiconductor laser 18 leaves a port 1 of the optical module through the optical waveguide coupling/branching device. Thus, a bidirectional transmission module with two wavelengths λ₁ and λ₂ can be realized. In Fig. 9, reference numeral 20 designates an electric circuit which serves for amplifying and identifiing of the electric signal resulting from the conversion by the light receiving element 17 and for driving the light receiving element. Reference numeral 21 denotes a transmitting circuit including a semiconductor laser drive circuit, a comparator circuit, a monitor circuit and others. In the device according to the present invention, the substrate is positively protected against a bend or warp. Accordingly, mounting the active elements mentioned above will bring about substantially no degradation in the characteristics (such as deviation of the optical axis and others). As will now be appreciated from the foregoing description, the matching in the thermal expansion coefficients at the boundaries between the core layer, clad layer and the low refractive index layer can be realized satisfactorily according to the teachings of the invention, whereby deviation (shift) of center wavelength in the optical waveguide branching characteristics, cracking of the glass film and other undesirable phenomena due to occurrence of thermal stress can be positively suppressed. Thus, there is provided the optical device of improved performance and enhanced reliability.

Claims: An optical waveguide device, comprising a substrate (7), a low-refractive index layer (8) disposed on the substrate (7) and having a refractive index n b , a core layer (6) disposed on the low-refractive index layer (8) and having a refractive index n c , with n c > n b , and a cladding layer (9) disposed on the core layer (6) and having a refractive index n c1 , with n c1 < n c , characterised in that the dopant concentrations within the layers are such that the resulting thermal expansion coefficients of the low-refractive index layer (8), the core layer (6) and the cladding layer (9) are substantially equal at the boundaries between the layers. The device of claim 1, wherein in a transverse direction going from the substrate towards the core the thermal expansion coefficient of the low-refractive index layer (8) increases substantially continuously with distance from the substrate. The device of claim 1, wherein in a transverse direction going away from the core the thermal expansion coefficient of the cladding layer (9) decreases substantially continuously with increasing distance from the core. The device of any of claims 1 to 3, wherein the substrate (7) is formed of SiO₂-glass. The device of any of claims 1 to 4, including a pair of core layers (6-1, 6-2) of rectangular cross-section, disposed on the low-refractive index layer in parallel side-by-side relationship so as to be distributively coupled to each other. The device of any of claims 1 to 4, wherein at least one end of the core layer (6) is connected to at least two optical branch waveguides (11...14).

Inventors: IMOTO KATSUYUKI, IMOTO, KATSUYUKI
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1988
Main classification: G02B6
IPC codes: G02F1, G02B6