Waveguide path type polarization independent optical wavelength tunable filter

[0002] 1. Field of the Invention

[0003] The present invention relates to a waveguide path type optical device used in an optical wavelength division multiplex transmission system, and more particular to a waveguide path type polarization independent optical wavelength tunable filter used in a wavelength tunable multiplexer-demultiplexer for multiplexing and demultiplexing any signal optical wavelength.

[0004] 2. Description of the Related Art

[0005] There is proposed, as a conventional filter, an optical wavelength filter wherein a channel type optical waveguide path is provided on a substrate, a surface wave exciting electrode is provided on the substrate through a buffer layer in the vicinity of a middle portion of the input terminal and output terminal of the channel type optical waveguide path, and an element for separating two linearly polarized lights orthogonal to each other is provided in the vicinity of said middle portion and the output terminal (Japanese Patent Application Laid-Open No. 4-51114).

[0006] There is also proposed an optical wavelength filter wherein a channel type optical waveguide path is provided on a substrate exhibiting an acousto-optic effect, and a surface wave exciting electrode, a phase shifter electrode and a polarized light separation element are provided on the channel type optical waveguide path (Japanese Patent Application Laid-Open No. 4-159516).

[0007] Recently, however, as optical communication systems have been put to practical use, demand rises for a high-capacity, multifunctional, advanced communication system. Also, the addition of new functions of such as generating a higher speed optical signal, multiplexing wavelengths within a single light transmission path, and switching over and changing light transmission paths are required.

[0008] In these circumstances, as optical fiber amplifiers have been, in particular, rapidly put to practical use, an optical wavelength division multiplex transmission (WDM) system is being developed actively.

[0009] The WDM transmission cannot be realized without a wavelength tunable multiplexer-demultiplexer for multiplexing and demultiplexing any signal optical wavelength. A wide range of wavelength tunable widths, high speed operation and the like are required of an optical wavelength tunable filter used in the wavelength tunable multiplexer-demultiplexer.

[0010] As for optical wavelength tunable filters, TE (transverse-electric)-TM (transverse-magnetic) mode conversion type filters using the AO (acousto-optic) effect capable of easily tuning a selected wavelength by changing the frequency of a surface acoustic wave are being developed extensively.

[0011] FIG. 1 is a view for showing the structure of a conventional TE-TM mode conversion type waveguide path type polarization independent optical wavelength tunable filter.

[0012] The wavelength tunable filter shown therein is obtained by forming a titanium (Ti) diffusion optical waveguide paths 1a, 1band anisotropic optical waveguide paths 2a, 2b, 2cand 2dare formed on the surface of a substrate 4aof an X-cut lithium niobate (LiNbO₃).

[0013] Portions having slightly higher refraction indexes than the substrate 4abecome the Ti diffusion optical waveguide paths 1a, 1band the anisotropic optical waveguide paths 2a, 2b, 2cand 2d. The Ti diffusion optical waveguide paths 1aand 1bare formed by thermally diffusing Ti to the substrate 4a, whereas the anisotropic optical waveguide paths 2a, 2b, 2cand 2dare formed by thermally diffusing Ti to the substrate 4aand then conducting ion (proton) exchange processing.

[0014] A surface acoustic wave (SAW) excitation interdigital transducer 5aand surface acoustic wave absorbers 6aand 6bare provided right over the Ti diffusion optical waveguide paths 1aand 1b.

[0015] The polarization beam splitter 11aconsists of the Ti diffusion optical waveguide paths 1a, 1band the anisotropic optical waveguide paths 2a, 2b. The polarization beam splitter 11bconsists of the Ti diffusion optical waveguide paths 1a, 1band the anisotropic optical waveguide paths 2c, 2d. The TE-TM mode converter 12aconsists of the Ti diffusion optical waveguide paths 1a, 1band the surface acoustic wave (SAW) exciting interdigital transducer 5a.

[0016] Now, the operational principle of this wavelength tunable filter will be described.

[0017] First, description will be given to the operations of the TE-TM mode converter 12aof the polarization beam splitter, taking the polarization beam splitter 11aas an example.

[0018] FIG. 2 is a view for explaining the operation of the polarization beam splitter 11a.

[0019] In FIG. 2, a TE polarization component 15a, which is an extraordinary ray, and a TM polarization component 15b, which is an ordinary ray, of a light 14aincident on the Ti diffusion optical waveguide path 1aare separately introduced to the anisotropic optical waveguide path 2aand the Ti diffusion optical waveguide path 1a, respectively at a polarized light separation basic structural part 10a.

[0020] Further, the TE polarization component 15ais multiplexed to the Ti diffusion optical waveguide path 1bat a polarized light separation basic structural part 10b. Both of the polarization components 15aand 15bof the incident light 14aare, therefore, polarization-separation outputted to the Ti diffusion optical waveguide paths 1band 1a, respectively.

[0021] Likewise, as regards a light 14dincident on the Ti diffusion optical waveguide path 1b, a TE polarization component 15gand a TM polarization component 15hof the incident light 14dare separation-polarization outputted to the Ti diffusion optical waveguide paths 1aand 1b, respectively.

[0022] It is noted that the TE mode refers to the component of the wave-guided light 14awhich electric field is parallel to the substrate, whereas the TM mode refers to the component of the guided wave light 15 which electric field is perpendicular to the substrate.

[0023] Next, the operation of the TE-TM mode converter 12awill be described. FIG. 3 is a view for explaining the operation of the TE-TM mode converter.

[0024] In FIG. 3, the surface acoustic wave excited by applying an RF signal 13afrom an oscillation circuit to the interdigital transducer 5aacts as a periodic refractive index grating for the wave-guided light 15aand 15b.

[0025] In this case, it is assumed that the guided wave 15ahas only a TE polarization component and the guided wave 15bhas only a TM polarization component.

[0026] By using the X cut lithium niobate substrate and setting the direction of optical wave transmission to Y direction, the Ti diffusion optical waveguide paths 1aand 1bdiffer in the effective refractive index of both the TE and TM modes.

[0027] If the following phase matching conditional equation (1), where the refractive index grating period formed by the surface acoustic wave is Λ and the effective refractive indexes of the TE mode and TM mode are NTE and NTM, respectively, is satisfied, a wavelength λ is subjected to TE-TM mode conversion by interaction with the refractive index grating.

[0028] λ=|NTE−NTM|Λ  (1)

[0029] The refractive index grating period Λ is inversely proportional to the frequency f of the RF signal 13a. Due to this, it is possible to change easily wavelength λ to be subjected to TE-TM mode conversion by changing the frequency of the RF signal 13a. Thus, by appropriately setting the frequency of the RF signal 13a, any wavelength can be subjected to TE-TM mode conversion.

[0030] In case of the mode converter 12a, therefore, the TE polarization wave-guided light 15a, only if it is the wave-guided light of a wavelength satisfying the phase matching condition with respect to the refractive index grating period Λ, is TE-TM mode converted to a TM polarization wave-guided light 15con the Ti diffusion optical waveguide path 1b.

[0031] On the other hand, on the Ti diffusion optical waveguide path 2a, the TM polarization guided wave 15b, only if it is a guided wave of a wavelength satisfying the phase matching condition with respect to the refractive index grating period Λ, is TE-TM mode converted to a TE polarization wave-guided light 15d.

[0032] Based on the operations of the above-stated polarization beam splitters 11a, 11band the mode converter 12a, the operational principle of the wavelength tunable filer will be described with reference to FIG. 1.

[0033] In FIG. 1, the light 14aapplied from an input port 3ais separated into the TE polarization component 15aand the TM polarization component 15bintroduced to the Ti diffusion optical waveguide paths 1band 1a, respectively, by the polarization beam splitter 11a.

[0034] Only the wavelength which satisfies the phase matching condition by interaction with the refractive index grating Λ, of the TE polarization component 15aincident on the Ti diffusion optical waveguide path 1bis converted from the TE polarization component 15ato a TM polarization component 15cby the TE-TM mode converter 12a. Thereafter, the resultant TM polarization component 15cis introduced to the Ti diffusion optical waveguide path 1bby the polarization beam splitter 11band is outputted as the TM polarization component of the light 14cfrom a port 3d.

[0035] The TE polarization component 15awhich has not been subjected to mode conversion is introduced to the Ti diffusion optical waveguide path 1aby the polarization beam splitter 11band outputted as the TE polarization component of the light 14bfrom a port 2c.

[0036] Meanwhile, only the wavelength which satisfies the phase matching condition by interaction with the refractive index grating Λ, of the TM polarization component 15bincident on the Ti diffusion optical waveguide path 1ais converted from the TM polarization component 15bto a TE polarization component 15dby the TE-TM mode converter 12aas in the same manner as stated above. Thereafter, the resultant TE polarization component 15dis introduced to the Ti diffusion optical waveguide path 1bby the polarization beam splitter 11band outputted as the TE polarization component of the light 14cfrom the port 3d.

[0037] The TM polarization component 15bwhich has not been subjected to mode conversion is introduced to the Ti diffusion optical waveguide path 2aby the polarization beam splitter 11band outputted as the light 14bfrom the port 3c. By appropriately setting the frequency of the RF signal 13a, therefore, it is possible to select only a desired wavelength as an outgoing light 14cfrom the port 3dno matter how the input light 14ais polarized. It is also possible to output only non-selected light as the outgoing light 14bfrom the port 3c.

[0038] FIG. 4A is an explanatory view showing an example of light intensity attenuated wavelength characteristics (to be referred to as “filter characteristics” hereinafter) of the output light 14cfrom the port 3dof the conventional waveguide path type polarization independent optical wavelength tunable filter stated above. FIG. 4B is an explanatory view showing an example of filter characteristics of the output light 14bfrom the port 3cthereof.

[0039] The problem with the conventional filter is, however, that the attenuation of the non-selected light intensity in respect of the light intensity of the selected central wavelength does not exceed a certain level throughout wavelength bands in the filter characteristics of the selected light 14c. The reason for the problem will be described with reference to FIG. 5 as shown below.

[0040] In FIG. 5, the TM polarization component 15cof the selected light, which has been subjected to mode conversion as stated above, is introduced to the Ti diffusion waveguide path 1b. At the same time, at the polarized light separation basic structural part 10a, the TE polarization component 15eof the non-selected light with polarized light separation quantity according to the control accuracy of, for example, an actual production process is incident on the Ti diffusion optical waveguide path 1b.

[0041] Likewise, on the Ti diffusion optical waveguide path 1a, the TE polarization component 15dof the selected light which has been subjected to mode conversion passes through the anisotropic optical waveguide path 2band introduced to the Ti diffusion optical waveguide path 1b. At the same time, at the polarized light separation basic structural part 10d, the TM polarization component 15fof the non-selected light with polarized light separation quantity according to the control accuracy of, for example, an actual production process, is incident on the Ti diffusion optical waveguide path 1b.

[0042] As a result, the TE polarization component 15eof the non-selected light incident on the Ti diffusion optical waveguide path 1band the TM polarization component 15fof the non-selected light become noise components for the selected light, thereby deteriorating attenuation characteristics. The noise components are resulted from the polarized light separation characteristics of the polarization beam splitter according to the control accuracy of, for example, an actual production process. For that reason, it is difficult to improve filter characteristics.

[0043] It is, therefore, an object of the present invention to provide a waveguide path type polarization independent optical wavelength tunable filter with a high tolerance during manufacture, capable of easily improving filter characteristics.

[0044] A waveguide path type polarization independent optical wavelength tunable filter according to the present invention comprises a dielectric substrate; two polarization beam splitters provided on the dielectric substrate; and a TE-TM mode converter arranged between the two polarization beam splitters on the dielectric substrate. The two polarization beam splitters include a first optical waveguide path formed on a surface of the dielectric substrate and an anisotropic optical waveguide path. At least one of the polarization beam splitters is formed on the first optical waveguide path, and includes a first thin film formed on the first optical waveguide path and having a refractive index lower than that of the first optical waveguide path; a second thin film formed on the first thin film and having a refractive index higher than that of the first optical waveguide path; and a metal film formed on the anisotropic optical waveguide path of at least one of the polarization beam splitters. The TE-TM mode converter includes a second optical waveguide path continuous to the first optical waveguide path of the polarization beam splitters formed on the surface of the substrate; and an interdigital transducer, arranged right on the second optical waveguide path, for exciting a surface acoustic wave.

[0045] According to the waveguide path type polarization independent optical wavelength tunable filter of the present invention, a layered film, consisting of the first thin film having a refractive index lower than that of the optical waveguide path and the second thin film having a refractive index higher than that of the optical waveguide path, is formed right on the optical waveguide path of the polarization beam splitter. Due to this, the TE polarization component of the non-selected light on the optical waveguide path is transferred to the layered film and removed from the interior of the optical waveguide paths.

[0046] Meanwhile, by forming a metal film right on the anisotropic optical waveguide path, the TM polarization component of the non-selected light on the anisotropic optical waveguide path is absorbed by the metal film and removed from the interior of the anisotropic optical waveguide path.

[0047] As described above, according to the waveguide path type polarization independent optical wavelength tunable filter of the present invention, a layered film, consisting of the first thin film having a refractive index lower than that of the optical waveguide path and the second thin film having a refractive index higher than that of the optical waveguide path, is formed right on the optical waveguide path of the polarization beam splitter, and a metal film is formed right on the anisotropic optical waveguide path.

[0048] Due to this, the layered film of the first and second thin films allows the non-selected light components on the optical waveguide path to be transferred to the layered film, to thereby remove the components from the interior of the optical waveguide path. In addition, the non-selected light components on the anisotropic waveguide path can be absorbed by the metal film to thereby remove the components from the interior of the anisotropic waveguide path.

[0049] Accordingly, it is possible to easily remove noise resulting from the non-selected light components in the polarization beam splitters and to thereby improve filter characteristics without the need to ensure strict accuracy for manufacturing the polarization beam splitter parts.

[0050] FIG. 1 shows a structure of a conventional TE-TM mode conversion, waveguide path type polarization independent optical wavelength tunable filter;

[0051] FIG. 2 is an explanatory view for explaining an operation of a polarization beam splitter of the optical wavelength tunable filter shown in FIG. 1;

[0052] FIG. 3 is an explanatory view for explaining an operation of a TE-TM mode converter of the optical wavelength tunable filter shown in FIG. 1;

[0053] FIGS. 4A and 4B are views showing examples of filter characteristics of the optical wavelength tunable filter shown in FIG 1;

[0054] FIG. 5 is an explanatory view for explaining occurrence of noise due to non-selected components in a polarization beam splitter of the optical wavelength tunable filter shown in FIG 1;

[0055] FIG. 6 is a view showing a waveguide path type polarization independent optical waveguide path tunable filter according to one embodiment of the present invention;

[0056] FIG. 7 is an explanatory view for explaining the operation of the optical wavelength tunable filter shown in FIG. 6;

[0057] FIG. 8 is a view showing an example of coupling characteristics of a TE polarization component to an Si film;

[0058] FIGS. 9A and 9B show examples of filter characteristics of the optical wavelength tunable filter shown in FIG. 6; and

[0059] FIG. 10 shows a waveguide path type polarization independent optical wavelength tunable filter according to another embodiment of the present invention.

[0060] The waveguide path type polarization independent optical wavelength tunable filter in embodiments according to the present invention will be described hereinafter.

[0061] FIG. 6 shows a waveguide path type polarization independent optical wavelength tunable filter in this embodiment. Description will be given while the constituent elements of the filter in this embodiment common to those of the conventional filter are denoted by the same reference numerals.

[0062] The waveguide path type polarization independent optical tunable filter in the embodiment shown in FIG. 6 consists of two polarization beam splitters 11 (11a, 11b) including Ti diffusion optical waveguide paths 1 (1a, 1b) formed on a dielectric substrate 4aand anisotropic waveguide paths 2 (2ato 2d), and a mode converter 12aincluding the Ti diffusion optical waveguide path 1, a surface acoustic wave interdigital transducer 5aand an acoustic wave absorber 6a.

[0063] With the structure shown in FIG. 6, a layered film consisting of an SiO₂film 8aand Si film 9ais formed right on the Ti diffusion optical waveguide path 1 of the polarization beam splitter 11b. A TE polarization component of a non-selected light on the Ti diffusion optical waveguide path 1 is thereby transferred to the layered film 8a, 9aand removed from the Ti diffusion optical waveguide path 1. Metal layers 7a, 7bare formed right on the anisotropic optical waveguide paths 2c, 2dof the polarization beam splitter 11b. A TM polarization component of a non-selected light on the anisotropic optical waveguide paths 2c, 2dis transferred to the metal films 7a, 7band removed from the anisotropic optical waveguide paths 2cand 2d.

[0064] The waveguide path type polarization independent optical wavelength tunable filter in this embodiment will be described in detail in due order.

[0065] The waveguide path type polarization independent optical wavelength tunable filter in this embodiment is an X-cut Y-propagation LiNbO₃waveguide path type optical wavelength tunable filter by using X-cut LiNbO₃for the dielectric substrate 4aand setting a transmission axis to a Y axis.

[0066] Now, description will be given to a method for manufacturing the X-cut Y-propagation LiNbO₃waveguide path type optical wavelength tunable filter.

[0067] In FIG. 6, a Ti film is deposited on the LiNbO₃substrate 4a. After the resultant layer is patterned into an optical waveguide path pattern, it is subjected to thermal diffusion for a few hours to thereby form the Ti diffusion optical waveguide paths 1a, 1band the anisotropic optical waveguide paths 2a, 2b, 2cand 2d. Thereafter, masking is effected so that ions can be exchanged only on the anisotropic optical waveguide paths 2a, 2b, 2cand 2dand then proton-exchange is effected for a few hours to thereby form the anisotropic optical waveguide paths 2a, 2b, 2cand 2d.

[0068] Next, a surface acoustic wave exciting interdigital transducer 5ais formed out of a metal film above the Ti diffusion optical waveguide paths 1a, 1b. Acoustic wave absorbers 6a, 6bare formed in front of and back of the surface acoustic wave exciting interdigital transducer 5a. Thereafter, a layered film consisting of an SiO₂film 8aand Si film 9ais formed on the Ti diffusion optical waveguide path 1awithin the polarization beam splitter 11band metal films 7aand 7bare formed on the anisotropic optical waveguide paths 2cand 2d.

[0069] The polarization beam splitter 11aconsists of the Ti diffusion waveguide paths 1a, 1band the anisotropic waveguide paths 2a, 2b. The mode converter 12aconsists of the Ti diffusion optical waveguide paths 1a, 2b, the surface acoustic exciting interdigital transducer 5a, and the acoustic wave absorbers 6a, 6b. The polarization beam splitter 11bconsists of the Ti diffusion waveguide paths 1a, 1b, the anisotropic optical waveguide paths 2c, 2d, the metal films 7a, 7b, the SiO₂film 8aand the Si film 9a.

[0070] Next, description will be given to an example of the operation of the waveguide path type polarization independent optical waveguide path tunable filter shown in FIG. 6, with reference to FIG. 7.

[0071] In FIG. 6, the light 14aincident on the input port 3ais separated into a TE polarization component 15aand a TM polarization component 15bintroduced to the Ti diffusion optical waveguide paths 1band 1a, respectively, by the polarization beam splitter 11a.

[0072] Only the wavelength which satisfies the phase matching condition by interaction with the refractive index grating Λ, of the TE polarization component 15aincident on the Ti diffusion optical waveguide path 1bis converted from the TE polarization component 15ato the TM polarization component 15cby the TE-TM mode converter 12a. Thereafter, the resultant TM polarization component 15cis introduced to the Ti diffusion optical waveguide path 1bby the polarization beam splitter 11band outputted as the TM polarization component of the light 14cfrom the port 3d.

[0073] In addition, the TE polarization component 15awhich has not been subjected to mode conversion is introduced to the Ti diffusion optical waveguide path 1aby the polarization beam splitter 11band outputted as the TE polarization component of the light 14bfrom the port 3c.

[0074] At this moment, at the polarized light separation basic structural part 10a, the TE polarization component 15eof the non-selected light with polarized light separation quantity according to the control accuracy of, for example, an actual production process is also incident on the Ti diffusion optical waveguide path 1b. As shown in FIG. 7, while being transmitted to the Ti diffusion optical waveguide path 1b, the TE polarization component 15eof the non-selected light is coupled to the Si film 9aby setting the appropriate thickness of the SiO₂film 8aand that of the Si film 9aas shown in FIG. 8, and removed from the Ti diffusion optical waveguide path 1b.

[0075] Meanwhile, only the wavelength which satisfies the phase matching condition by interaction with the refractive index grating Λ, of the TM polarization component 15bincident on the Ti diffusion optical waveguide path 2ais converted from the TM polarization component 15bto the TE polarization component 15dby the TE-TM mode converter 12ain the same manner as stated above. Thereafter, the resultant TE polarization component 15dis introduced to the Ti diffusion optical waveguide path 1bby the polarization beam splitter 11band outputted as the TE polarization component of the light 14cfrom the port 3d.

[0076] The TM polarization component 15bwhich has not been subjected to mode conversion is introduced to the Ti diffusion optical waveguide path 1aby the polarization beam splitter 11b, and is outputted as the light 14bfrom the port 3c.

[0077] At this moment, at the polarized light separation basic structural part 10d, the TM polarization component 15fof the non-selected light with polarized light separation quantity according to the control accuracy of, for example, an actual production process is incident on the Ti diffusion optical waveguide path 1b. AS shown in FIG. 7, while being transmitted to the anisotropic optical waveguide path 2d, the TM polarization component 15fof the non-selected light is removed from the anisotropic optical waveguide path 2dby the metal film 7b.

[0078] The TE polarization component 15eof the non-selected light and TM polarization component 15fof the non-selected light, which become noise components for the selected light, incident on the Ti diffusion optical waveguide path 1bcan be removed as stated above. Owing to this, it is possible to improve attenuation characteristics.

[0079] Next, description will be given to the result of manufacturing the waveguide path type polarization independent optical wavelength tunable filter shown in FIG. 6 and measuring the film characteristics.

[0080] First, X-cut LiNbO₃was used as the dielectric substrate 4aand the transmission axis was set as the Y axis. The Ti diffusion optical waveguide paths 1a, 1bwere formed to be 100 nm in Ti film thickness, 6 μm in Ti strip width at a diffusion temperature of 1050° C. for 8 hours.

[0081] The anisotropic optical waveguide paths 2a, 2b, 2cand 2dwere formed to be 100 nm in Ti thickness, 6 μm in Ti strip width at a diffusion temperature of 1050° C. for 8 hours. Thereafter, the formed waveguide paths were subjected to proton-exchange at 250° C. for 3 minutes in a benzoic acid atmosphere and then to thermal treatment at 360° C. for 80 minutes in an oxygen atmosphere.

[0082] The interdigital transducer 5afor surface acoustic wave excitation was provided by forming a Cr film of a thickness of 50 nm and an Al film of a thickness of 200 nm by means of sputtering and patterning the resultant films. The metal films 7aand 7bwere provided only right on the anisotropic optical waveguide paths 2cand 2dby forming an Al film of a thickness of 200 nm by means of spattering, and by patterning the resultant film to be 15 μm in width and 8 mm in length.

[0083] The SiO₂film 8aand Si film 9awere provided by forming a film of a thickness of 200 nm and that of a thickness of 90 nm by means of sputtering, respectively and by patterning the resultant films to be 200 μm in width and 10 nm in length so as to be arranged only right on the Ti diffusion optical waveguide paths 1a, 1bwithin the polarization beam splitter 11b.

[0084] The acoustic wave absorbers 6a, 6bwere formed by coating and thermosetting silicone adhesive. The interaction length of the TE-TM mode converter 12awas set at 30 nm.

[0085] The characteristics of the waveguide path type polarization independent optical wavelength tunable filter with the above-stated structure will be described.

[0086] In FIG. 6, if the RF signal 13awas inputted with a frequency of 171.58 MH_zat power of 30 mW, the filter characteristics of the selected light, i.e., the output light 14cfrom the port 3dwas, for example, as shown in FIG. 9A. The filter characteristics was improved to −25.6 dB at a wavelength spaced from the transmission central wavelength by 10 nm, and to −31.3 dB at a wavelength spaced from the transmission central wavelength by 20 nm.

[0087] The filter characteristics of the non-selected light, i.e., the output light 14bfrom the port 3cwas, for example, shown in FIG. 9B. The ratio (that is, through cross stroke) of the light intensity of a selection central wavelength at the selected light port to that at the non-selected light port was improved to 32.5 dB.

[0088] As stated so far, according to the waveguide path type polarization independent optical wavelength tunable filter in this embodiment, non-selected light components which become noise can be easily removed by the metal films 7a, 7band SiO₂/Si films 8a, 9ain the polarization beam splitter 11b. As a result, filter characteristics can be improved without the need to maintain high accuracy for manufacturing the polarization beam splitter part.

[0089] Although the above-stated embodiment has illustrated a case where the metal films 7aand 7bare made of Al, it is possible to obtain equivalent advantage by forming the metal films 7a, 7bout of Au, Ti, Cr, Pt or the like. Besides, if the silicon (Si) film thickness is 90±5 nm, the same advantage can be obtained, as well.

[0090] The above-stated embodiment has illustrated a case where the polarization beam splitter 11bis provided with the SiO₂film 8a, Si film 9aand metal films 7a, 7b. If the polarization beam splitter 11ais similarly provided with the SiO₂film 8b, Si film 9band metal films 7c, 7das shown in FIG. 10, greater advantage can be obtained.

[0091] Since the constitutions and operations of the SiO₂film 8b, Si film 9band metal films 7c, 7dare the same as those of the SiO₂film 8a, Si film 9aand metal films 7a, 7b, no description will be given herein.