JP2848144B2 - Tunable optical filter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a tunable optical filter having a function of electrically tuning and branching an optical signal of a desired frequency, and has a small structure, a wide tuning range, and a fast response time. The present invention relates to an optical filter.

[0002]

2. Description of the Related Art Optical frequency division multiplexing (FDM) transmission for wavelength multiplexing 50 or more optical signals in one wavelength region is expected as a future optical communication system. In this FDM transmission, it is essential to develop an optical filter having a function of splitting an optical signal of a desired frequency with low loss. As this optical filter, a Mach-Zehnder optical filter is well known. However, in this optical filter, the frequency interval between the _two frequencies f ₁ and f ₂ output to the output terminal is shifted due to a slight deviation of the structural parameters (length, thickness, width, relative refractive index difference, etc. of the waveguide). There was a problem that would.

For this purpose, conventionally, as shown in FIG.
A thin-film heater (Cr heater) 83 is formed in the vicinity of one of the waveguides 81 and 82 having two different lengths, a voltage is applied to the thin-film heater 83, and the voltage is adjusted to adjust the applied voltage. A method of causing the expansion and contraction of the wave path ΔL / 2 and thereby splitting an optical signal having a desired frequency is being studied. In FIG. 8, reference numeral 84 denotes a directional coupler,
5 and 86 are input ports, and 87 and 88 are output ports.

[0004]

The optical filter shown in FIG. 8 has the following problems.

(1) Even if a voltage is applied to the heater 83 and the applied voltage is adjusted, it is difficult to widen the tuning range of the frequencies f ₁ and f ₂ . This is because the optical filter has a silica-based waveguide structure,
This is because the refractive index of the waveguide hardly changes even if the temperature is changed in the range of 10 to 100 ° C. by the heater heating (refractive index change amount: 10 ⁻⁵ / ° C.). Therefore, in this configuration, the tuning of the frequency depends on the adjustment of the waveguide length rather than the adjustment of the refractive index. However, there is a problem in that the extension and contraction of the waveguide length brings a deflection characteristic to the demultiplexing characteristic.

(2) Since the optical filter is formed of a silica-based waveguide structure, the relative refractive index difference between the core and the clad cannot be made large (the current maximum relative refractive index is not more than 1).
%). Therefore, the size of the optical filter cannot be reduced. The larger the area, the slower the heat response characteristics and the slower the tuning response time.

(3) Since the relative refractive index difference cannot be made large, the radius of curvature R of the waveguides 81 and 82 becomes large, and as a result, it is difficult to reduce the propagation loss.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a tunable optical filter having a small structure, a wide tuning range, and a fast response time, which solves the above-mentioned disadvantages of the prior art.

[0009]

In order to achieve the above object, the present invention provides a first cladding layer having a first refractive index on a substrate, and a first refractive index layer on the first cladding layer. A core having a second refractive index larger than the refractive index is formed, and the upper part thereof is covered with a second cladding layer having another refractive index to form a waveguide, and glass is used for the first cladding layer and the core. , The second cladding layer is made of a polymer material, and a heater for heating the waveguide is provided.

Alternatively, a first cladding layer having a first refractive index is provided on a substrate, and a core having a second refractive index larger than the first refractive index is formed on the first cladding layer. A waveguide is formed by covering the upper part with an intermediate layer having a third refractive index smaller than the second refractive index and a second clad layer, and forming a waveguide using the first clad layer, the core, and the intermediate layer, Second
The cladding layer may be formed using a polymer material, and a heater for heating the waveguide may be provided.

The refractive indices of the two waveguides may be different at least in one part.

The heater may be provided near one of the two waveguides or both cores, and may be connected to a voltage application terminal.

[0013] Silicon may be used as the polymer material of the second cladding layer.

The thickness of the intermediate layer may be in the range of 0.01 μm to 8 μm.

A heat blocking hole may be provided in the substrate between the two waveguides.

[0016] A heat blocking slit may be provided on the substrate between the two waveguides.

[0017]

According to the above construction, since the polymer material is used for the second cladding layer, the refractive index of the polymer material can be largely changed by applying a voltage to the heater. As a result, the center frequency can be widened. Can be controlled.

Further, since the polymer material has a lower refractive index than glass, the relative refractive index difference can be made large, and as a result, a small-sized optical filter can be realized.

Further, since the second cladding layer made of a polymer material can be easily formed by coating, the production process cost can be reduced.

In addition, when the size is reduced, the response of the center frequency due to the heating of the heater is improved, and tuning can be performed with low power consumption.

Further, by reducing the size, the propagation loss of the waveguide can be reduced.

Further, since the present invention is a method of tuning the center frequency using a change in the refractive index of the waveguide caused by heating the heater, the polarization dependency is smaller than that of a conventional method in which the waveguide length is changed.

[0023]

FIG. 1 shows an embodiment of a waveguide type tunable optical filter according to the present invention. FIG. 1B is a front view, and FIG.
Is a top view, but shows a cross-sectional view taken along the line AA in FIG. In this configuration, a layer 2 of a first clad (for example, SiO ₂ ) having a refractive index of n _c1 is formed on a substrate 1 (for example, Si), and a refractive index of n is formed on the first clad layer 2.
_w (n _w > n _c1 ) and a substantially rectangular core 3 (3-1,3-
2, 3-3 and 3-4) are formed using, for example, SiOxNyHz. Further, on the first cladding layer 2, a thin-film heater 7 (formed of, for example, a Cr or Ti film) and a power supply line 8 (8-1 and 8-2) are also patterned. Then, the cores 3-1 to 3-4, the heater 7,
Refractive index on the surface of feeder lines 8-1 and 8-2 is n _c2 (n _c2
<N _w ) of the second cladding layer 4 (polymer material such as silicone). The cores 3-1 to 3-4 are the first
An input side and an output side directional coupler 5 are provided on the cladding layer 2.
Two waveguides 6 having different lengths between (5-1 and 5-2) (first waveguide 6-1 and second waveguide 6-2)
The Mach-Zehnder type optical filter connected by is patterned. Voltage application terminals 9-1 and 9-2 are provided between the power supply lines 8-1 and 8-2 so that a voltage can be applied.
The feature of this configuration is that the first cladding layer 2 and the cores 3-1 to 3-1
3-4 is that glass is used, and the second cladding layer 4 is made of a polymer material. That is, since a polymer material is used for the second cladding layer 4, when a voltage is applied to the heater 7 to change the temperature near the first waveguide 6-1, the refraction of the second cladding layer 4 near the first waveguide 6-1 is changed. The center frequencies f ₁ , f output from the output side directional coupler 5-2.
₂ can be tuned. Where f ₁ , f
₂ , the equivalent refractive index n ₁ on the first waveguide 6-1 side (the equivalent refractive index depends on the relative refractive index difference between the core and the clad), the waveguide length l ₁ , the second waveguide The relationship between the equivalent refractive index n ₂ on the 6-2 side and the waveguide length l ₂ is shown by the result of analysis by the present inventors,

[0024]

(Equation 1)

Where C is the speed of light in a vacuum m = 0, 1, 2, 3,...

As can be seen from equations (1) and (2), n
_It can be seen that by changing ₁ , f ₁ and f ₂ can be controlled. Next, FIG. 2 shows an embodiment of how much n ₁ can be changed depending on the temperature in the above configuration. FIG. 2 shows the temperature dependence of the refractive index of quartz glass and silicone (made by Shin-Etsu Silicone, model name OF8). The refractive index of silicone can be changed by about ± 0.6% in the range of 20 ° C. to 80 ° C. That is, f ₁ and f ₂ can be tuned to the same extent. It is apparent from the temperature dependence of the refractive index of quartz glass that it is difficult to tune f ₁ and f ₂ in temperature control when the second cladding layer 4 is glass.

FIG. 3 shows a waveguide type tunable optical filter according to a second embodiment of the present invention. 1 in that the second cladding layer 4 is formed on the surfaces of the cores 3-1 to 3-4, the heater 7, and the power supply lines 8-1 and 8-2 formed on the first cladding layer 2. Before coating on the intermediate layer 1
0 (for example, SiO ₂ ). This intermediate layer 10 is made of glass and has a thickness of 0.01 μm to 8 μm,
Chosen refractive index _{n i (n i <n w} ). The intermediate layer 10 is used as a protective film for the cores 3-1 to 3-4, and light propagating in the cores 3-1 to 3-4 causes non-uniformity of the interface between the core and the second cladding layer 4. Scattering can be suppressed. In addition, in terms of long-term reliability, an effect of suppressing the OH group from the second cladding layer 4 from diffusing into the cores 3-1 to 3-4 and entering the cores 3-1 to 3-4 can be obtained. If the thickness of the intermediate layer 10 is too large, it is difficult to change n ₁ even when the temperature is changed by the heater 7. This intermediate layer 10 has cores 3-1 to
After patterning 3-4, for example, plasma CVD
It can be formed by a low pressure CVD method or the like. In FIGS. 1 and 3, it is preferable that the heater 7 be juxtaposed with the first waveguide 6-1 (interval: 20 μm or more) in order to improve the response of the refractive index change. It is preferable that the length is as long as possible.

FIG. 4 shows a third embodiment of the waveguide tunable optical filter according to the present invention. This is a configuration in which heaters are juxtaposed near the cores of the first and second waveguides 6-1 and 6-2 as in 7-1 and 7-2. In this case, between the voltage application terminals 9-1 and 9-2, 9-3
And 9-4 are applied independently of each other.
Note that, in FIG. 1, a glass having a high refractive index such as borosilicate glass or a glass having a low refractive index such as quartz glass may be used for the substrate 1 other than Si. The first cladding 2
Also, as in the embodiment, at least one kind of P, B, etc. may be added as a refractive index control additive other than SiO ₂ .

FIG. 5 shows a fourth embodiment of the waveguide tunable optical filter according to the present invention. This is the first
In this configuration, a slit 11 on the heat blocking surface is provided between the waveguide 6-1 and the second waveguide 6-2. Holes 12-1
12-5 is provided. By providing the heat-blocking slit 11 and the holes 12-1 to 12-5, the temperature near the first waveguide 6-1 can be controlled by the heater 7, and the second waveguide 6-2 can be controlled. The influence of the temperature control on the side can be reduced. Note that at least one hole may be provided instead of the slit 11. Conversely, slits may be used instead of the holes 12-2 to 12-4. The slit 11 and the holes 12-1 to 12-1
A liquid, a refrigerant, etc. may be put in 2-5 to shut off heat. It is more convenient to make these liquids, refrigerants and the like circulate to reduce the raised temperature.

FIG. 6 shows a fifth embodiment of the waveguide tunable optical filter according to the present invention. This is a configuration in which the heater 7 is formed on the second clad layer 4 instead of being formed on the first clad layer 2. In this embodiment, the heater 7 is disposed at a position deviated from directly above the first waveguide 6-1. However, the heater 7 may be disposed immediately above the first waveguide 6-1. In this embodiment, since the substrate 1 is made of quartz or Vycor glass having a low refractive index, the first cladding layer 2 is omitted.

The waveguide type tunable optical filter of the present invention is not limited to the above embodiment. First, equations (1) and (2)
As can be seen from the above, n ₁ and n ₂ may be made different from the beginning. For example, the refractive index can be easily changed by changing the amount of N in the composition SiOxNyHz of the first waveguide 6-1 and the second waveguide 6-2. As a method of changing the refractive index, the cores 3-1 to 3-4 are the first.
After patterning on the cladding layer 2, one or both of the first and second waveguides 6-1 and 6-2 are
For example, laser light (laser light of Ar, CO ₂ , N _2, etc.)
And the refractive index can be changed by changing the irradiation light amount, irradiation time and the like. An example is shown in FIG.

As the polymer material of the second clad 4, other than silicone, a fluorine polymer material (for example, polyvinylidene fluoride), polymethyl methacrylate, or the like can be used.

The cores 3-1 to 3-4 have a refractive index n _w of 1.4.
Materials with values in the range of 65 to 1.60 can be used. For example, in addition to SiOxNyHz, P, Ge, T
i, Zn, Al, Na, at least one inclusive SiO ₂ system or SiOxNyH the refractive index control additive K such
A z-based material can be used. The SiOxNyHz film used for the core can be formed by a plasma CVD method. However, since the warpage (that is, the generation of stress) of the substrate generated during the film formation is extremely small, an optical filter having little polarization dependence is used. There is also a feature that it can be realized. Also, a plurality of optical filters of the present invention are connected in series,
Alternatively, they may be used as optical filters combined in parallel. For example, in addition to splitting _two frequencies f ₁ and f ₂ , the present invention can also be applied to an optical filter that splits four waves of f ₁ , f ₂ , f ₃ , and f ₄ , and further more. it can. Further, a ring resonator is added in juxtaposition with the first or second waveguide, and the present invention can be applied to an optical filter realizing higher isolation characteristics. Further, the tunable optical filter of the present invention uses a material having a high refractive index for the core and a material having a low refractive index for the second cladding layer, so that the relative refractive index difference can be further increased. It has the feature that the size can be significantly reduced, resulting in lower loss.

[0034]

As described above, the tunable optical filter of the present invention has the following effects.

(1) Since a polymer material is used for the second cladding layer, the refractive index of the polymer material can be greatly changed by applying a voltage to the heater. As a result,
The center frequency can be controlled over a wide range.

(2) The high refractive index of the polymer material is lower than that of glass, and a high refractive index can be realized by increasing the content of N since SiOxNyHz is used for the core. An optical waveguide having a large relative refractive index difference between the core and the clad can be configured. for that reason,
A small-sized optical filter can be realized. If the size is reduced, the temperature control by the heater becomes easy, and a tunable optical filter having a high response speed can be configured.

(3) Since the second cladding layer made of a polymer material can be easily formed by coating, the manufacturing process cost can be reduced.

(4) SiOxNyHz used for the core
Since the film generates less stress at the time of film formation, an optical filter with little polarization dependence can be realized.

[Brief description of the drawings]

FIG. 1 is a front view and a top sectional view of a tunable optical filter according to an embodiment of the present invention.

FIG. 2 is a characteristic diagram for comparing the temperature dependence of the refractive index of silicone with that of quartz glass in the present invention.

FIG. 3 is a front view and a top cross-sectional view of a tunable optical filter showing another embodiment of the present invention.

FIG. 4 is a front view and a top cross-sectional view of a tunable optical filter showing another embodiment of the present invention.

FIG. 5 is a front view and a top cross-sectional view of a tunable optical filter showing another embodiment of the present invention.

FIG. 6 is a front view and a top cross-sectional view of a tunable optical filter showing another embodiment of the present invention.

FIG. 7 is a characteristic diagram showing the temperature dependence of the refractive index when a core is irradiated with an Ar gas laser beam in the present invention.

FIG. 8 is a schematic diagram of a Mach-Zehnder type optical filter showing a conventional example.

[Explanation of symbols]

 Reference Signs List 1 substrate 2 first cladding layer 3 core 4 second cladding layer 7 heater

Claims (8)

(57) [Claims] 1. A two-directional coupler having a different length between two directional couplers.
In a Mach-Zehnder type optical filter connected by two waveguides, a first cladding layer having a first refractive index is provided on a substrate, and a second refractive index larger than the first refractive index is provided on the first cladding layer. Is formed, and the upper part thereof is covered with a second cladding layer having a different refractive index to form a waveguide. Glass is used for the first cladding layer and the core, and a polymer is used for the second cladding layer. A tunable optical filter comprising a material and a heater for heating the waveguide. 2. A Mach-Zehnder optical filter in which two directional couplers are connected by two waveguides of different lengths, wherein a first cladding layer having a first refractive index is provided on a substrate, A core having a second refractive index larger than the first refractive index is formed on one clad layer, and an upper part of the core and a second layer having a third refractive index smaller than the second refractive index are formed on the clad layer.
Forming a waveguide by covering with a cladding layer, a first cladding layer,
A tunable optical filter comprising a core and an intermediate layer made of glass, a second clad layer made of a polymer material, and a heater for heating the waveguide. 3. The optical filter according to claim 1, wherein at least one portion of the two waveguides has a different refractive index. 4. The heater according to claim 1, wherein the heater is provided near one of the two waveguides or near both cores, and is connected to a voltage application terminal.
The tunable optical filter according to any of the above. 5. The tunable optical filter according to claim 1, wherein silicon is used as a polymer material of the second cladding layer. 6. The tunable optical filter according to claim 2, wherein the thickness of the intermediate layer is in a range of 0.01 μm to 8 μm. 7. The tunable optical filter according to claim 1, wherein a heat blocking hole is provided in the substrate between the two waveguides. 8. The tunable optical filter according to claim 1, wherein a slit for heat insulation is provided on the substrate between the two waveguides.