JPH0667129A - Optical wavelength filter - Google Patents

Abstract

PURPOSE:To provide the optical wavelength filter with which light of a specific wavelength among incident lights is selectable without depending on the kinds of the polarization of the incident lights and in which an electrooptical effect is utilized. CONSTITUTION:A Mach-Zehnder interferometer 33 consists of a first Y branch 33a, second Y branch 33b, first waveguide 33c and second waveguide 33d is formed on an LiNbO3Z plate 31. The second waveguide 33d is so formed as to generate an optical path length difference with the first waveguide 33c by adjusting its refractive index. The first waveguide 33c is provided with electrodes 39a, 39b for selective wavelength tuning and the second waveguide 33d is provided with electrodes 41a, 41b for adjusting the optical path length. The ratio between the change in refractive index with a TM wave and the change in refractive index in a TE wave generated by the electrooptical effect in the first waveguide 33c and the ratios between the change in the optical path length with the TM wave and the change in the optical path length in the TE wave generated by the electrooptical effect in the first waveguide 33c and the second waveguide 33d are set at the same ratios.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is an optical wavelength filter capable of selecting light having a specific wavelength of incident light regardless of the type of polarization of the incident light (hereinafter may be simply referred to as "element"). It is about.

[0002]

2. Description of the Related Art As a conventional element of this type, one using a Mach-Zehnder interferometer is disclosed in, for example, Japanese Patent Laid-Open No. 3-672.
No. 04 publication. FIG. 7A is a plan view schematically showing the structure of this element, and FIG. 7B is a sectional view taken along the line I-I of FIG. 7A of this element.

This conventional device includes a silicon substrate 11 and
The SiO ₂ -based clad layer 13 formed on the silicon substrate 11, the first waveguide 15 and the second waveguide 17 formed in the clad layer 13, and the waveguides 15 and 1
It was provided with directional couplers 19a and 19b for coupling 7 and a heater 21 provided on the first waveguide 15. Both the first waveguide 15 and the second waveguide 17 were made of a silica glass material. Further, the waveguide length of the first waveguide 15 is L, and the waveguide length of the second waveguide 17 is L + ΔL. In FIG. 7, 23
a and 23b are input ports, and 25a and 25b
Each of b is an output port.

In this device, when two signal lights f ₁ and f ₂ separated by an optical frequency interval Δf are incident from the input port 23b, if ΔL is a predetermined value, these signal lights f ₁ and f ₂
f ₂ could be taken out from separate output ports. That is, the multiplexer / demultiplexer operation was possible. However, in practice, the optical path length difference (n
L: where n is the refractive index of the waveguide. ) Had to be adjusted with high precision. Therefore, in this conventional element, the thermo-optical effect is generated in the first waveguide 15 by the heater 21 to adjust the optical path length difference.

When the thermo-optic effect is used, the change in the refractive index of the waveguide has no polarization dependence. Therefore, in this conventional device, if the structure of the waveguide is devised so as not to have polarization dependence, incident light It was possible to construct an optical wavelength filter that can select a specific wavelength regardless of the type of polarized light.

[0006]

However, when the optical path length is adjusted by the thermo-optic effect, the time required for this is m.
Since it is on the order of sec, there is a limit in obtaining an optical wavelength filter that operates at high speed. Also, to achieve high-speed operation,
It is possible to utilize the electro-optic effect, but heretofore this kind of element has not existed.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a Mach-Zehnder type optical wavelength filter using the electro-optical effect.

[0008]

To achieve this object, according to the present invention, an optical wavelength filter comprising a Mach-Zehnder interferometer having a first waveguide and a second waveguide is provided. The two waveguides are provided on a substrate having an electro-optical effect, and the ratio of the change in the refractive index to the first polarized light and the change in the refractive index to the second polarized light caused by the electro-optical effect in the first waveguide described above, The above-mentioned first waveguide and the second waveguide between the optical path length difference with respect to the first polarization and the ratio of the optical path length difference with respect to the second polarization described above are set to be the same. To do.

Here, the same ratio between the changes in the refractive index and the difference between the optical path lengths is not limited to the same case, but also includes an approximate state considered to be the same within the scope of the object of the present invention. .

As the substrate having the electro-optical effect, various substrates such as a LiNbO ₃ substrate and a semiconductor substrate can be used.

[0011]

According to the structure of the present invention, it is possible to obtain an optical wavelength filter exhibiting a light transmittance spectrum having at least one transmission wavelength peak common to the first polarized light and the second polarized light.

Further, since the selective wavelength adjustment and the optical path length adjustment can be carried out by utilizing the electro-optical effect, an operating frequency of about 1 GHz can be expected, so that a higher speed operation can be expected as compared with the conventional element using the thermo-optical effect.

Further, in the case of a configuration in which a plurality of optical wavelength filters in which the length of the second waveguide is set to a value different from each other by a value defined by a geometric series of 2 are connected in series, each has a peak wavelength λ _0. However, a device in which a plurality of wavelength filters having different light transmittance spectra are connected in series can be configured.
Therefore, it is possible to obtain a bandpass filter which has polarization independence and has good wavelength selectivity (good selection characteristic of wavelength λ ₀ ) even for wideband incident light.

Further, in the case where each of the first and second waveguides is constituted by a diffusion waveguide and the difference in refractive index between the substrate of the second waveguide and the substrate of the first waveguide is made larger than that. , The length of the second waveguide remains the same as the length of the first waveguide, and an optical path length difference is given between the first and second waveguides. If the lengths of the first and second waveguides can be made the same, device design,
Advantages can be expected in device fabrication, device dimensions, and the like.

[0015]

Embodiments of the optical wavelength filter of the present invention will be described below with reference to the drawings. However, the drawings used in the description merely schematically show the dimensions, shapes, and arrangement relationships of the respective constituents to the extent that the present invention can be understood.

1. First Embodiment FIG. 1 is a plan view showing the configuration of the optical wavelength filter of the first embodiment. In FIG. 1, reference numeral 31 is a LiNbO ₃ substrate as a substrate having an electro-optical effect, which is the Z plate in this case. The substrate 31 has a first Y branch 33a,
A Mach, which is composed of the second Y branch 33b and the first waveguide 33c and the second waveguide 33d provided between these Y branches.
A Zener interferometer 33, an input (output) port 35, and an output (input) port 37 are provided. These Y branches 3
3a, 33b, waveguides 33c, 33b, input / output port 3
5 and 37 each have a waveguide structure in which Ti (titanium) is diffused in the LiNbO ₃ substrate 31. The lengths of the first waveguide 33c and the second waveguide 33d are both L _{i in} the case of the first embodiment. However, regarding the second waveguide 33d, the diffusion amount of Ti when manufacturing the second waveguide 33d is different from the diffusion amount of Ti of other waveguide structure portions (specifically, on the first waveguide formation planned region). By controlling the film thickness of the Ti film formed as the diffusion source), a refractive index different from that of the other waveguide portions is shown, and the optical path length (=) is provided between the first and second waveguides 33c and 33d.
(Refractive index x length of waveguide) difference is generated.

Further, in this optical wavelength filter, tuning electrodes 39a and 39b for adjusting the selection wavelength by generating an electric field in the Z direction in the first waveguide 33c, and the electrode 39a are on the first waveguide 33c. The electrodes 39b are provided along the side of the first waveguide 33c. Further, in order to generate an electric field in the Y direction in the second waveguide 33d to adjust the optical path length of this waveguide, the optical path length adjusting electrodes 41a and 41b are provided on both sides of the first waveguide 33d along the waveguide. Is provided. This optical path length adjusting electrode 4
By controlling the voltage applied to 1a and 41b, it is possible to adjust the ratio between the optical path length difference for the first polarized light and the optical path length difference for the second polarized light between the first waveguide 33c and the second waveguide 33d ( Details will be described later.). Each of these electrodes 39a, 39b, 41a, 41b is formed by forming a metal thin film suitable for electrode formation on the LiNbO ₃ substrate 31 on which the Ti waveguide is formed by a known film forming technique, and forming the thin film by the lithography technique. It can be formed by patterning.

Next, let us consider a case where, for example, a TM wave as the first polarized light and a TE wave as the second polarized light are incident on the optical wavelength filter of the first embodiment. In this case,
The transmission wavelength λ ₀ ^M of the TM wave in this optical wavelength filter is shown by the following equation (1) as is well known, and the transmission wavelength λ ₀ ^E of the TE wave is shown by the following equation (2) as well known. However, in these formulas, Derutaderutaenu _e is a second waveguide 33d for extraordinary light (extraordinary light because it uses Z-plate of LiNbO ₃ as in this case the substrate is a TM wave or first polarization.) It Shows the difference in the refractive index from the other waveguide structure portion (the first waveguide 33c in this case), and δΔn _o is the second waveguide 33d and the other waveguide for ordinary light (TE wave in this case, the second polarization). The refractive index difference from the waveguide structure portion (in this case, the first waveguide 33c) is shown. Also, Δn _e , Δn
_o Each shows the change in refractive index for each of extraordinary light and ordinary light due to the concentration of Ti diffused in the LiNbO ₃ substrate. Further, m and m ′ are orders of interference. This m, m '
Is an integer and is in the wavelength region of λ ₀ ^E and λ ₀ ^M (for example, 1.
3 μm band or 1.55 μm band), δΔn _e ,
Determined by δΔn _o and L _i .

Λ ₀ ^M = 2πδΔn _e L _i / 2mπ (1) λ ₀ ^E = 2πδΔn _o L _i / 2m′π (2) As can be understood from the formulas (1) and (2). , Λ ₀ ^M = λ
_{At 0} ^E , both the first polarized light and the second polarized light are transmitted. Then, in order to be λ ₀ ^M = λ ₀ ^E , (1)
From equation (2), it can be seen that the condition of equation (3) below is necessary.

ΔΔn _e / m = δΔn _o / m '(3) Here, when a voltage is applied to the tuning electrodes 39a and 39b of the optical wavelength filter of the first embodiment to adjust the selected wavelength. , ΔΔn _e changes by δn _e, and δΔ
n _o changes by δn _o. And, in order to satisfy λ ₀ ^M = λ ₀ ^E even after such a change occurs,
As is clear from the equation (3), the following equation (4) needs to be established.

[0021] is a _{(δΔn e + δn e) /} m = (δΔn o + δn o) / m '··· (4) In other _{words, δn e / m = δn o} / m' ··· (5).

Therefore, the tuning electrodes 39a, 3
In order to satisfy λ ₀ ^M = λ ₀ ^E even after the adjustment by 9b, after all, from the expressions (3) and (5), the following (6)
Expression conditions are required.

[0023] In _{δΔn e / δΔn o = m /} m '= δn e / δn o ··· (6) This equation (6), δn _e / δn _o were tuning electrodes 39a, the voltage 39b is applied At this time, the refractive index change δn _e with respect to the first polarized light and the refractive index change δ with respect to the second polarized light caused by the electro-optic effect in the first waveguide.
The ratio with respect to n _o (in other words, the ratio of the magnitude of the change in the refractive index for each polarized light caused by the electro-optic effect). Further, as described above, δΔn _e is the refractive index difference between the second waveguide 33d and the first waveguide 33c for the first polarized light, and δΔn _o is the second waveguide 33d for the second polarized light.
And the first waveguide 33c. Since the dimensions of the first waveguide and the second waveguide are both L _i , δΔn _e is the difference in optical path length between the first waveguide and the second waveguide with respect to the first polarized light, and δΔn _o. Is the optical path length difference for the second polarized light between the first waveguide and the second waveguide, and as a result, δΔn _e / δΔn _o is the optical path length difference for each polarization between the first and second waveguides. That is the ratio.

Therefore, from the above, if the following formula (7) is satisfied in the optical wavelength filter of the first embodiment, this optical wavelength filter can transmit light of wavelength λ ₀ without polarization dependence. .

[0025] _{δΔn e / δΔn o = δn e} / δn o ··· (7) FIG optical transmittance spectrum with respect to the first polarization in the optical wavelength filter of the first embodiment constructed as described above (TM wave) Two
The light transmittance spectrum for the second polarized light (TE wave) is shown in FIG. 2A, and FIG. In addition,
In these figures, the horizontal axis represents wavelength and the vertical axis represents transmittance (the same applies to FIGS. 4 and 5 below). FSR (spacing between a plurality of transmission wavelength peaks) and FWHM (spectrum half-width) in each characteristic are δΔn _e / δΔn _o
Therefore, the ratio is different for each polarized light, that is, for each TM wave and TE wave. However, according to the optical wavelength filter of the first embodiment, regarding the light in the band inside the transmission wavelength peak including the selected wavelength λ ₀ and adjacent to this (the band indicated by S ₁ in FIG. 2A), Light of wavelength λ ₀ can be selectively transmitted regardless of the polarization of incident light.

2. Second Example In the optical wavelength filter of the first example, regarding the light in the band inside the transmission wavelength peak including the selected wavelength λ ₀ and adjacent to this, the light of the wavelength λ ₀ is irrespective of the polarization type of the incident light. Although it was possible to selectively transmit the light, it is desirable that it is compatible with a wider band in practical use. This second embodiment is such an example.

Therefore, in the optical wavelength filter of the second embodiment, as shown in FIG. 3, the optical wavelength filter of the first embodiment has the lengths of the first and second waveguides 33c and 33d of L. _i individual light wavelength filter 51, same L _i / 2 individual light wavelength filter 53, same L _i / 4 individual light wavelength filter 5
5 are sequentially connected in series to form an optical filter. That is, the lengths of the first and second waveguides 33c and 33d are different from each other so as to indicate a geometric series of 2 (L _i , L _i / 2, L
_i / 4. ...) except that a plurality of optical wavelength filters having the same configuration as the first embodiment are prepared and are connected in series.
In addition, in the example of FIG. 3, the three-stage configuration is adopted, but it is two depending on the design.
Of course, the number of steps may be four or more.

Each individual optical wavelength filter 51 for the first polarized light (TM wave) in the optical wavelength filter of the second embodiment,
The light transmittance spectra at 53 and 55 and the light transmittance spectrum as a whole are set to the first and second waveguides 33c and 33.
FIG. 4 (A) about 51 intended length L _i of d, for 53 ones of the L _i / 2 in FIG. 4 (B), the same L _i /
No. 4 55 is shown in FIG. 4 (C), and the whole 55 is shown in FIG. 4 (D). Also, similar light transmittance spectra for the second polarized light (TE wave) in the optical wavelength filter of the second embodiment are shown in FIGS.

As is apparent from FIGS. 4 and 5, according to the optical wavelength filter of the second embodiment, the light of wavelength λ ₀ is selected irrespective of the kind of polarization of the incident light as in the first embodiment. It can be seen that the light can be transmitted as desired and also functions as a bandpass filter for light in a wider band than in the case of the first embodiment.

3. Description of Design Example Next, a procedure for actually designing the optical wavelength filter of the first embodiment (the same applies to the optical wavelength filters of each stage of the second embodiment) will be described.

FIG. 6 is a diagram used for the explanation, and LiN
When Ti is diffused in the Z plate 31 of bO ₃ , the substrate 31
3 is a diagram showing how the refractive index of each of the first polarized light (TM wave) and the second polarized light (TE wave) changes depending on the Ti concentration. However, the wavelength of both TM wave and TE wave is 1.15 μm. In FIG.
The characteristic indicated by I is for the TM wave, and the characteristic indicated by II is for the TE wave. It can be seen that for both TM waves and TE waves, the substrate refractive index changes similarly regardless of the polarization up to a Ti concentration of about 0.6 mol%, and then changes differently.

Therefore, in the production of the optical wavelength filter of the embodiment, the waveguide structure portion other than the second waveguide 33c, that is, the input port 35, the first Y branch 33a, the first waveguide 33c, and the second waveguide 33c shown in FIG. The Y branch 33b and the output port are formed by diffusing Ti into the substrate 41 so that the refractive index difference from the substrate 41 is Δn in FIG. In addition, the second waveguide 33d
Is formed so that the difference in refractive index between it and the substrate is larger than Δn. Therefore, the second waveguide 3 for the TM wave
3d and the refractive index difference δΔn _e between the first waveguide 33c and T
Refractive index difference Derutaderutaenu _o between the second waveguide 33d and the first waveguide 33c for the E wave is as shown in FIG. 6, respectively.

In this embodiment, the substrate 31 is L
Since iNbO ₃ is used, δn is
_{Since e} / δn _o = α≈3, and it is necessary to satisfy the above equation (7) for the purpose of the present invention, after all,
It is necessary to set δΔn _e / δΔn _o ≈α. However, this is generally not the case. This is because the polarization dependence of the electro-optic effect and the polarization dependence of the refractive index change due to Ti diffusion are different. Therefore, δΔn _e / δΔn _o that holds only by the Ti concentration is set as β. Further, when a voltage is applied to the optical path length adjusting electrodes 41a and 41b (see FIG. 1), only δΔn _o can be changed by the electro-optical effect γ ₂₂ , so this value is set to ε. Then, δΔn _e / δΔn _o
≈α can be electrically adjusted by the electrodes 41a and 41b as follows: δΔn _e / (δΔn _o + ε) = α (8) When modifying the equation (8), divided by _{δΔn e = α (δΔn o +} ε) further Arufaderutaderutaenu both sides of this equation _o, deforming the _{δΔn e / αδΔn o = 1 +} αε / αδΔn o Furthermore this equation, ε / δΔn _o = (δΔn _e / αδΔn _o ) −1 (9). Then, as described above, δΔn _e / δΔn
_{Since o} = β, δΔn _e / (δΔn _o
In order to set + ε) = α, ε = δΔn _o [(β / α) -1] (10) is set by the optical path length adjusting electrodes 41a and 41b.
Should be adjusted.

In the above description, the electrodes 41a, 41
b was used only for adjusting the birefringence of the second waveguide, and was not used for tuning the selected wavelength. However, the electrodes 39a, 39b, 41a, 41b may be used respectively for tuning the selected wavelength. In that case,
Based on δΔn _e / δΔn _o = β, δn _e / (δn
_The refractive index change ε that occurs in the electrodes 41a and 41b may be adjusted so that _o + ε) = β holds. That is, δn _e /
By transforming _{(δn o + ε) = β} , δn e = βδn o + βε next, further divided by Betaderutaenu _o this of both sides, the _{δn e / βδn o = 1 +} ε / δn o ··· (11). Here, since δn _e / δn _o = α is set in advance, in the end, ε = δn _o [(α / β) −1] (12) is generated in the electrodes 41 a and 41 b. The refractive index change ε may be adjusted.

In the case of using LiNbO _3, from its nature, α / β ≒ 1.5, δΔn o about 10 ^-3, .DELTA.n _o is about 10 ^-4. Therefore, these values are (10)
As can be seen by substituting into the equation (12), ε given by the equation (12) becomes smaller than ε given by the equation (10). This is the wavelength selection for the electrodes 39a, 39b, 4
Electrodes 39a, 39 are more effective when using 1a and 41b.
This means that the voltage applied to the electrodes 41a and 41b in order to adjust the ratio of the optical path length differences can be reduced compared to the case where only b is used.

Further, in the above description, the first waveguide 33c
Although the second waveguide 33d and the second waveguide 33d have the same length, the present invention can be applied even when the lengths of the two waveguides are different. The length of both waveguides 33c and 33d is, for example, ΔL.
If it is different, the voltage may be applied to the electrodes 41a and 41b so that (δΔn _e L _i + n _e ΔL) / (δ Δn _o L _i + n _o ΔL) = α. When the lengths of the first and second waveguides 33c and 33d are different, it is not always necessary to make the Ti concentration of the second waveguide 33d different from the Ti concentration of the first waveguide 33c.
This is because by making the lengths of the two waveguides different from each other, it is possible to secure a structure that already causes a difference in optical path length.

In addition, in the configuration of the Mach-Zehnder interferometer 33 described in the embodiment, the first Y branch 33a and the second Y branch 33a.
Even if directional couplers are used instead of the branches 33b, the same effect as that of the embodiment can be expected.

[0038]

As is apparent from the above description, according to the present invention, in a Mach-Zehnder interferometer having a first waveguide and a second waveguide, these waveguides are formed on a substrate having an electro-optical effect. Since the ratio of the change in the refractive index of each polarization caused by the electro-optic effect in the first waveguide and the ratio of the difference in the optical path length of each polarization between the first and second waveguides are set to be the same, An optical wavelength filter having an optical transmittance spectrum having at least one transmission wavelength peak common to each of the first polarized light and the second polarized light (for example, a peak of wavelength λ ₀ ) is obtained. Therefore, it is possible to obtain an optical wavelength filter using the electro-optical effect and having polarization independence.

Since the electro-optic effect is used,
Higher speed operation can be expected than the conventional device that used the thermo-optic effect.

Further, in the case of a configuration in which a plurality of optical wavelength filters in which the length of the second waveguide is set to a value defined by a geometric series of 2 and different from each other are connected in series, there is polarization independence. In addition, a bandpass filter having good wavelength selectivity (good selection characteristic of wavelength λ ₀ ) even for high-bandwidth incident light can be obtained.

Further, the first and second waveguides are respectively constituted by diffusion waveguides, the lengths of both the waveguides are the same, and the difference in the refractive index between the two waveguides is different from that of the substrate. In the case of a structure that gives a difference in length, advantages can be expected in element design, element production, element size, and the like.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of an optical wavelength filter according to a first embodiment.

FIG. 2A and FIG. 2B are diagrams showing optical transmittance spectra for TM wave and TE wave respectively in the optical wavelength filter of the first embodiment.

FIG. 3 is a diagram for explaining an optical wavelength filter according to a second embodiment.

FIG. 4A to FIG. 4D are diagrams showing the light transmittance spectrum of each individual light wavelength filter and the entire light transmittance spectrum with respect to the TM wave in the light wavelength filter of the second embodiment. .

5A to 5D are diagrams showing a light transmittance spectrum of each individual light wavelength filter and a whole light transmittance spectrum with respect to a TE wave in the light wavelength filter of the second embodiment. .

FIG. 6 is a diagram for explaining an example, and is a diagram for explaining a design example of the optical wavelength filter of the example.

FIG. 7 is a diagram for explaining a conventional optical wavelength filter.

[Explanation of symbols]

31: Substrate having electro-optic effect (in this case LiNbO
₃ substrates) 33: Mach-Zehnder interferometer 33a: first Y branch 33b: second Y branch 33c: first waveguide 33d: second waveguide 35: input port 37: output port 39a, 39b: for selective wavelength tuning Electrodes 41a, 41b: Optical path length adjusting electrodes 51, 53, 55: Individual optical wavelength filters

Claims (4)

[Claims] 1. An optical wavelength filter comprising a Mach-Zehnder interferometer having a first waveguide and a second waveguide, wherein the first and second waveguides are provided on a substrate having an electro-optical effect. Caused by electro-optic effect in the waveguide,
The ratio of the change in the refractive index to the first polarized light and the change in the refractive index to the second polarized light, the optical path length difference between the first waveguide and the second waveguide for the first polarized light, and the second polarized light. The optical wavelength filter is characterized in that the ratio of the optical path length differences with respect to is set to be the same. 2. The optical wavelength filter according to claim 1, wherein the first waveguide is provided with a tuning electrode for adjusting a selective wavelength of the optical wavelength filter, and the second waveguide is provided with an optical path length adjustment. An optical wavelength filter, which is provided with electrodes for use. 3. The optical wavelength filter according to claim 1, wherein the length of the second waveguide is set to a value defined by a geometric series of 2 and different from each other. An optical wavelength filter characterized in that are connected in series. 4. The optical wavelength filter according to claim 1, wherein the first and second waveguides are diffusion waveguides, respectively, and a difference in refractive index between the second waveguide and the substrate is An optical wavelength filter, wherein the refractive index difference between the first waveguide and the substrate is made larger.