JPH05323248A - Tunable optical filter - Google Patents

Abstract

PURPOSE:To sufficiently suppress the side lobe in a turnable optical filter of TE-TM mode conversion type using acoustooptical effect. CONSTITUTION:An optical waveguide 2 and an electrode 3 exciting surface acoustic wave traveling on the same line as an optical signal propagating in the optical waveguide 2 are formed on a dielectric substrate 1. In the turnable optical filter providing a polarizer 15 for incident light coupling only the polarization in the specified direction among the incident optical signal to be inputted to the optical waveguide 2 and a polarizer 16 for transmitting light separating the transmitted light from the optical waveguide 2 according to the polarization component, the electrode 3 is formed to be the curved shape which has the radius of curvature so that the coupling coefficients in the propagating direction in the interactive region of the optical signal propagating in the optical waveguide 2 and surface acoustic wave are distributed following the prescribed function.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a tunable optical multiplexer / demultiplexer for multiplexing and demultiplexing an optical signal of an arbitrary wavelength (frequency) in an optical wavelength multiplex transmission system or an optical frequency multiplex transmission system. The present invention relates to a tunable optical filter used in. In particular, TE-using the acousto-optic (AO) effect
The present invention relates to a tunable optical filter that can easily achieve phase matching in a TM mode conversion type optical filter by changing the frequency of a surface acoustic wave.

[0002]

2. Description of the Related Art FIG. 7 shows a TE-TM based on an acousto-optic effect.
It is a figure which shows the structure of the conventional tunable optical filter using mode conversion.

In the figure, the dielectric substrate LiNbO ₃
On the surface of the substrate 11, an optical waveguide 12 in which titanium (Ti) is thermally diffused so that the propagation direction is perpendicular to the optical axis (z axis).
A linear electrode 13 for exciting a surface acoustic wave traveling on the same straight line as the optical signal propagating through the optical waveguide 12 and a sound absorbing material 14 for absorbing the surface acoustic wave are formed. Further, before and after the optical waveguide 12, an incident light polarizer 15 that couples only the polarized light in a specific direction of the incident light to the optical waveguide 12, and the optical waveguide 1
The tunable optical filter is configured by arranging the outgoing light polarizer 16 for separating the outgoing light of 2 according to the polarization component.

When the optical signal propagating through the optical waveguide 12 and the surface acoustic wave excited by the linear electrode 13 travel on the same straight line, the principal axis of the index ellipsoid is changed by the diffraction grating by the surface acoustic wave. As a result, a diagonal element is generated in the permittivity tensor, and only the optical signal having a wavelength satisfying the phase matching conditions of the surface acoustic wave and TE mode and TM mode is selectively subjected to TE-TM mode conversion.

By utilizing the mode conversion function by the acousto-optic effect, the polarization direction of the incident light is preliminarily aligned by the incident light polarizer 15, and the outgoing light polarizer 16 is used for TE.
A bandpass filter or a bandstop filter can be constructed by selecting an optical signal of either polarization mode or TM mode. This prior art is described in the literature (David A. Smith, Jane E. Baran, J.
ohn J. Johonson and Kwok-Wai Cheng, "Integrated-Op
tic Acoustically-Tunable Filter for WDMNetworks ",
IEEE JOURNAL OF SELECTED AREAS IN COMUNICATIONS,
vol.8, No.6, pp1151-1159, 1990).

Here, FIG. 8 shows propagation directions of an optical signal and a surface acoustic wave in a conventional tunable optical filter using TE-TM mode conversion by the acousto-optic effect. TE mode,
The TM mode field distributions E ₃ and H ₃ are defined as A _E (x ₁ ) and A _M (x ₁ ) respectively indicating changes in the amplitude in the propagation direction, and F _E (x ₂ ) in the field distribution in the propagation direction and the vertical direction. , x ₃ ), F _M (x ₂ , x ₃ ), the angular frequency of the optical signal is ω, and their propagation constants are β _E ,
β _M

[0007]

[Equation 1]

It can be expressed as Further, the permittivity in vacuum is ε ₀ , the elastic optical constant is p ^E _kjmn , the electro-optical constant is r ^s _kjm , the strain caused by the displacement of the surface acoustic wave is S _mn, and the electric field due to the surface acoustic wave is E ^{If you say a} _m ,
The displacement δε _il of the dielectric constant tensor due to surface acoustic waves is

[0009]

[Equation 2]

[0010] Here, the strain S and the electric field E ^{a due} to the surface acoustic wave are proportional to the square root of the surface acoustic wave power.
The TE mode and TM mode equivalent refractive indices are N _TE and N _TM , the light wavelength is λ, the surface acoustic wave wavelength is Λ when the light wavelength λ is 1.55 μm, and the wavelength λ is 1.55 μm. If the amount of phase mismatch of is Δk, the phase matching condition is

[0011]

[Equation 3]

[0012] At this time, the coupling mode equation of the TE mode and the TM mode has the coupling coefficient between the modes as C _ME ,
C _EM

[0013]

[Equation 4]

[0014] The relationship between the coupling coefficients C _ME and C _EM between modes and the TE-TM mode conversion is

[0015]

[Equation 5]

There is a relationship of Here, since the strain S and the electric field E ^{a due} to the surface acoustic wave are proportional to the square root of the surface acoustic wave power, using the relations of the equations (3) and (7), the propagation direction of the coupling coefficient between the modes is The distribution also has a relationship proportional to the square root of the surface acoustic wave power.

[0017]

By the way, the conventional linear electrode 13 has a linear shape, and the surface acoustic wave power distribution in the propagation direction in the interaction area of the surface acoustic wave and light is substantially constant. Therefore, the coupling coefficient becomes almost constant in the interaction region and can be regarded as a constant. The TE-TM mode conversion efficiency (transmission characteristic) η at this time is given by the following formula:

[0018]

[Equation 6]

It becomes Here, the transmission characteristic of the filter obtained by substituting Δk in equation (8) into equation (8) is shown in FIG. In Fig. 9, the horizontal axis represents the central wavelength of light 1.
It is the deviation (nm) from 55 μm, and the vertical axis is the filter transmittance (dB). The interaction distance was 20 mm.

The tunable optical filter using the TE-TM mode conversion by the acousto-optic effect has the advantages that the driving power is smaller and the tuning range is wider than other tunable optical filters.

However, in the conventional tunable optical filter, since the linear electrode 13 for exciting the surface acoustic wave has a linear shape, it exists on both sides of the main transmission region (main lobe) including the central wavelength, as shown in FIG. The difference with the sub-transmissive region (side lobe) is about 9 dB. In other words, the maximum value of the side lobe is not negligible compared to the main lobe, which increases crosstalk between optical signals of different wavelengths and is a factor that significantly limits the wavelength arrangement of optical signals in wavelength division multiplexing communication. Was becoming.

The present invention is a TE-TM based on the acousto-optic effect.
An object of the present invention is to provide a tunable optical filter that can sufficiently suppress side lobes in the tunable optical filter using mode conversion.

[0023]

According to a first aspect of the present invention, an optical waveguide and an electrode for exciting a surface acoustic wave traveling on the same straight line as an optical signal propagating in the optical waveguide are provided on a dielectric substrate. An incident light polarizer that couples only the polarized light in a specific direction among the optical signals that are formed and incident on the optical waveguide, and an outgoing light polarizer that separates the outgoing light of the optical waveguide according to the polarization component. In the provided tunable optical filter, the electrode has a curved shape having a radius of curvature that distributes a coupling coefficient in a propagation direction in an interaction region of an optical signal propagating through an optical waveguide and a surface acoustic wave with a predetermined function. And

According to a second aspect of the present invention, in the tunable optical filter according to the first aspect, there is provided a surface acoustic wave waveguide for guiding the surface acoustic wave in an interaction region between the optical signal and the surface acoustic wave. The present invention is characterized in that the curved electrode and the surface acoustic wave waveguide are used to distribute the coupling coefficient in the propagation direction in the interaction region with a predetermined function.

[0025]

According to the first aspect of the present invention, the electrode for exciting the surface acoustic wave is formed into a curved shape having a predetermined radius of curvature so that T in the interaction area between the surface acoustic wave and the optical signal is increased.
An appropriate distribution is given to the coupling coefficient of E-TM mode conversion.

According to a second aspect of the present invention, an electrode for exciting a surface acoustic wave has a curved shape having a predetermined radius of curvature, and a surface acoustic wave waveguide for guiding the converged surface acoustic wave is provided. By including the same, an appropriate distribution is similarly given to the coupling coefficient of TE-TM mode conversion in the interaction region between the surface acoustic wave and the optical signal.

Here, as the coupling coefficient proportional to the square root of the surface acoustic wave power, the optical waveguide is provided so as to give a distribution such as a Gaussian distribution or a Hamming function that is small at both ends of the interaction region and large at the center. When the surface acoustic wave power on the road is functionally distributed, it is possible to realize a tunable optical filter having excellent transmission characteristics in which side lobes are significantly suppressed.

For example, in the tunable optical filter in which the Hamming function shown in FIG. 2 is given as the distribution of the coupling coefficient in the interaction region, the coupling equations (simultaneous differential equations) shown in the equations (5) and (6) are used. According to the numerical analysis, the transmission characteristic as shown in FIG. 3 can be realized. From this analysis result, it can be seen that the filter transmittance of the side lobe with respect to the main lobe is reduced by about −30 dB, and good filter characteristics are realized.

In order to make the coupling coefficient have such a function distribution, the electrode for exciting the surface acoustic wave is curved, the convergence of the surface acoustic wave is enhanced, and the surface acoustic wave power distribution is changed.

[0030]

FIG. 1 is a diagram showing the configuration of an embodiment of a tunable optical filter of the invention described in claim 1. In FIG.

In the figure, L of Ycut which is a dielectric substrate
On the surface of the iNbO ₃ substrate 1, the propagation direction is the optical axis (z axis).
An optical waveguide 2 in which titanium (Ti) is thermally diffused so as to be perpendicular to, a curved electrode 3 that excites a surface acoustic wave traveling on the same straight line as the light propagating in the optical waveguide 2, and a surface acoustic wave is absorbed. The sound absorbing material 4 is formed. Further, before and after the optical waveguide 2, an incident light polarizer 15 that couples only the polarized light in a specific direction of the incident light to the optical waveguide 2, and an outgoing light that separates the outgoing light of the optical waveguide 2 according to the polarization component. The tunable optical filter is configured by arranging the polarizer 16 for use.

In this embodiment, the length of the interaction area (distance from the electrode 3 to the sound absorbing material 4) is set to 20 mm, and the curved electrode 3
The opening width was set to 920 μm, and the radius of curvature was set to 10 mm so that the surface acoustic wave was focused on the center of the interaction region.

FIG. 4 shows the power distribution of the surface acoustic wave excited by the conventional linear electrode 13 and the curved electrode 3 of this embodiment in each interaction region on the optical waveguides 12, 2. .. The solid line indicates the surface acoustic wave power distribution by the curved electrode 3, and the broken line indicates the linear electrode 1 having a linear shape.
3 shows the distribution of surface acoustic wave power according to No. 3.

The coupling coefficient is distributed in proportion to the square root of the surface acoustic wave power distribution shown here, and a transmission characteristic as shown in FIG. 5 can be obtained. In FIG. 5, the horizontal axis represents the deviation (nm) from the central wavelength of light of 1.55 μm, and the vertical axis represents the filter transmittance (dB). The solid line shows the transmission characteristic of the curved electrode 3, and the broken line shows the conventional transmission characteristic of the linear electrode 13 (FIG. 9). As shown here, it is understood that the side lobe according to the present embodiment is suppressed by about 8 dB as compared with the conventional configuration. That is, by controlling the power distribution of the surface acoustic wave using the curved electrode 3 of the present embodiment, side lobes can be suppressed and a tunable optical filter with low crosstalk can be realized.

FIG. 6 is a diagram showing the construction of an embodiment of the tunable optical filter of the invention described in claim 2. In FIG. In this embodiment, the LiNbO ₃ substrate 1, the optical waveguide 2, the curved electrode 3, the sound absorbing material 4, the incident light polarizer 15 and the outgoing light polarizer 16 are the same as those of the embodiment shown in FIG. The feature of this embodiment is that the surface acoustic wave waveguide 7 for confining the surface acoustic waves excited from the curved electrode 3 is provided. The surface acoustic wave waveguide 7 is formed of, for example, S as a buffer layer.
It can be formed by depositing gold on the io ₂ layer. It can also be formed by thermally depositing high-concentration titanium outside the surface acoustic wave waveguide region.

By guiding the surface acoustic wave converged by using the surface acoustic wave waveguide 7, the power distribution in the propagation direction of the surface acoustic wave coupled to the optical signal propagating in the optical waveguide 2 is increased. , The distribution of the coupling coefficient in the interaction region can be finely controlled. That is, the surface acoustic wave excited from the curved electrode 3 is converged and coupled to the surface acoustic wave waveguide 7. At this time, the distribution of the coupling coefficient can be set to a predetermined distribution by adjusting the radius of curvature of the curved electrode 3 and the shape of the surface acoustic wave waveguide 7 by trial and error.

[0037]

As described above, according to the present invention, the electrode for exciting the surface acoustic wave is curved with a predetermined radius of curvature, so that the coupling coefficient is easily distributed and the side lobe in the filter transmission characteristic is suppressed. be able to. Further, by combining the curved electrode and the surface acoustic wave waveguide that converges the surface acoustic wave, the side lobe suppressing effect can be enhanced. Therefore, crosstalk can be significantly reduced in the tunable optical filter.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of an embodiment of a tunable optical filter of the invention described in claim 1.

FIG. 2 is a diagram showing an example in which a Hamming function is a distribution of coupling coefficients in an interaction region.

FIG. 3 is a diagram showing filter transmission characteristics with respect to the coupling coefficient distribution of FIG.

FIG. 4 is a diagram showing a power distribution of surface acoustic waves excited by a conventional linear electrode and a curved electrode of the present embodiment.

5 is a diagram showing filter transmission characteristics with respect to the surface acoustic wave power distribution of FIG.

FIG. 6 is a diagram showing a configuration of an embodiment of a tunable optical filter of the invention described in claim 2.

FIG. 7 is a diagram showing a configuration of a conventional tunable optical filter using TE-TM mode conversion by an acousto-optic effect.

FIG. 8 is a diagram showing propagation directions of an optical signal and a surface acoustic wave in a conventional tunable optical filter.

FIG. 9 is a diagram showing filter transmission characteristics of a conventional tunable optical filter.

[Explanation of symbols]

1 LiNbO ₃ substrate 2 optical waveguide 3 curved electrode 4 sound absorbing material 7 surface acoustic wave waveguide 11 LiNbO ₃ substrate 12 optical waveguide 13 linear electrode 14 sound absorbing material 15 incident light polarizer 16 outgoing light polarizer

Claims (2)

[Claims] 1. A dielectric substrate is provided with an optical waveguide and an electrode for exciting a surface acoustic wave propagating on the same straight line as an optical signal propagating through the optical waveguide, and the optical signal is incident on the optical waveguide. A tunable optical filter including an incident light polarizer that couples only polarized light in a specific direction, and an outgoing light polarizer that separates outgoing light of the optical waveguide according to a polarization component, wherein the electrode is A tunable optical filter having a curved shape having a radius of curvature for distributing a coupling coefficient in a propagation direction in an interaction region of an optical signal propagating through the optical waveguide and the surface acoustic wave with a predetermined function. 2. The tunable optical filter according to claim 1, further comprising a surface acoustic wave waveguide that guides the surface acoustic wave in an interaction region between the optical signal and the surface acoustic wave, and a curved electrode. A tunable optical filter having a structure in which a coupling coefficient in a propagation direction in the interaction region is distributed by a predetermined function by the surface acoustic wave waveguide.