JP2001056454A - Fabry-perot type optical waveguide filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the propagating mode of an optical waveguide from transferring to a cut-off area and multi-mode area when high positive and negative electric fields are applied as well with respect to a Fabry-Perot type optical waveguide filter capable of being used as the wavelength variable filter selecting an optical signal of one optical wavelength from wavelength multipleed optical signals. SOLUTION: In the Fabry-Perot type optical waveguide filter wherein mirrors 3 having high reflectance are provided in both end surfaces of the optical waveguide 2 formed in an electrooptical crystal substrate 1 and also electrodes 4 are arranged in both sides of the optical waveguide, optical waveguide forming parameters dx1, 2.dy1 are set in the area within a single waveguide mode and moreover in the area apart from a cut-off area and multi-mode area.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Fabry-Perot type optical waveguide filter which can be used as a wavelength tunable filter for selecting an optical signal having an arbitrary wavelength from a wavelength multiplexed optical signal.

[Summary of the Invention] With the progress of high-definition and digitalization of broadcasting, introduction of a wavelength division multiplexing optical transmission device for transmitting a high-definition digital signal between studios in a broadcasting station has been promoted. Since the transmission capacity of this wavelength division multiplexing optical transmission device is proportional to the number of wavelength division multiplexing, high density of wavelength division multiplexing is a decisive factor for large capacity.

On the receiving side of the transmission apparatus, a wavelength tunable optical filter capable of selecting any one wavelength from such a high-density wavelength-division multiplexed optical signal is required. This has a narrow band transmission characteristic. In addition, a Fabry-Perot type optical waveguide filter that can change the center wavelength of one transmission band to the center wavelength of another transmission band by using the electro-optic effect is considered promising (Ref. 1: Saito et al., “Fabry-Perot Ti : LiNbO
₃ Optical Waveguide Filter Characteristics, ”Proceedings of the 98th IEICE General Conference, C-3-179, pp. 345).

The substrate material constituting this Fabry-Perot type optical waveguide filter has a large change in the refractive index due to the application of an electric field, and is conventionally applied to an optical waveguide device, and is manufactured using lithium niobate (for which the manufacturing method and reliability have been established. LiNbO ₃ ) is suitable.

However, when a Fabry-Perot type optical waveguide filter is formed using a lithium niobate substrate, the range of an applied electric field for changing the center wavelength is about ten times that of a conventional optical waveguide device such as an optical modulator. It is necessary to assume until. At this time, the problem is that the propagation mode of the optical waveguide shifts to the cutoff region, and the transmission loss at the center wavelength increases.

SUMMARY OF THE INVENTION It is an object of the present invention to suppress an increase in transmission loss at the center wavelength when a high electric field is applied by reviewing parameters relating to an optical waveguide shape from values used in a conventional optical waveguide device. More particularly, the present invention relates to a Fabry-Perot graded three-dimensional optical waveguide filter formed by thermally diffusing, for example, metal titanium (Ti) to a lithium niobate substrate.

[0007]

2. Description of the Related Art FIG. 4 shows a configuration example of a conventional Fabry-Perot type optical waveguide filter. In FIG. 4, a Fabry-Perot type optical waveguide filter comprises a mirror 3 having a high reflectivity deposited on both end faces of an optical waveguide 2 formed on a lithium niobate substrate 1 which is an electro-optic crystal substrate, and both sides of the optical waveguide 2. The electrode 4 is disposed on the optical waveguide 2 so that a predetermined electric field can be applied to the optical waveguide 2.

Here, the optical waveguide 2 is a graded three-dimensional optical waveguide formed by thermally diffusing, for example, metallic titanium (Ti). This graded three-dimensional optical waveguide is described in detail in Reference 2: Nishihara et al., "Optical Integrated Circuit" (Ohm Co., Ltd., 2.4 three-dimensional optical waveguide, pp. 31-40). An outline will be described.

[0009] As shown in FIG. 5, in the graded-type three-dimensional optical waveguide, the refractive index of the x direction, for example, Gaussian distribution in the y-direction both refractive index n _fx of the optical waveguide than the refractive index n _s of the periphery of the substrate Will also be higher. Thereby, the light incident on the optical waveguide is confined in the optical waveguide and propagates as a waveguide mode.

[0010] In FIG. 5, _{d x} is x-direction (depth direction) of the diffusion depth, _{d y} represents the diffusion depth in the y direction (the width direction). Then, _{d x} and 2 · _{d y} is a parameter representing the shape of the optical waveguide, is 2 _{· d} y / d _x, referred to as the aspect ratio.

Aspect ratio and normalized frequency V_dI(V
_dI= K₀・ D_x・ (N_fX ²-N_S ²)^1/2) With
The relationship is as shown in FIG. In the figure, the shaded area is simply
Represents a region where one mode propagates. And on the left side of the shaded area
The region indicates the cut-off region where light cannot be guided,
Area is a multi-mode area where higher order modes can propagate.
Represents.

That no light is guided in the cutoff region;
Unnecessary higher-order modes propagate in the multi-mode region, and a plurality of transmission peaks appear. Therefore, when configuring a Fabry-Perot optical waveguide filter, these regions are not used and a single-mode region in a shaded portion is used. Will use.

Here, conventionally, as described in the above-mentioned reference 2, as a parameter representing the shape of the optical waveguide, the radiation loss of the waveguide mode due to branching or bending of the optical waveguide is reduced as shown in FIG. as shown by the two-dot B, it had set the parameters d _x2 and 2 · d _y2 close to the cut-off region.

FIG. 7 is a transmission characteristic diagram of a Fabry-Perot type optical waveguide filter manufactured using optical waveguide shape parameters _dx2 and 2 · _dy2 . FIG. 7 shows transmission characteristics when the electric field applied to the optical waveguide is -3 V / μm, 0 V / μm, and 3 V / μm.

As shown in FIG. 7, in the Fabry-Perot optical waveguide filter, the electric field applied to the optical waveguide is 0 V / μm.
Switching from m to −3 V / μm switches the transmission band to the shorter wavelength side, and switching from 0 V / μm to 3 V / μm switches the transmission band to the longer wavelength side. By sweeping the center wavelength in this way, a desired wavelength can be selected.

[0016]

However, as shown in FIG. 7, when a negative electric field is applied to the optical waveguide, the refractive index _nfx of the optical waveguide decreases due to the electro-optic effect, and the center wavelength shifts to the shorter wavelength side. However, there is a problem that the insertion loss at the center wavelength increases at the same time. Hereinafter, this phenomenon will be described with reference to FIG.

[0017] In FIG 8, the range of B'- point B '+ point represents a mode change range of the conventional optical waveguide device fabricated using the optical waveguide shape parameter d _x2 and 2 · d _y2. The range from the point B− to the point B + indicates the mode change range of the Fabry-Perot optical waveguide filter manufactured using the optical waveguide shape parameters _dx2 and 2 · _dy2 .

In a conventional optical waveguide device such as an optical modulator, the magnitude of an applied electric field is as small as 1 V / μm or less, and the amount of change in normalized frequency is small. Therefore, as the refractive index _nfx of the optical waveguide decreases, the normalized frequency decreases. However, as shown in FIG. 8, the transmission region moves from point B to point B'- in the single mode region, and the propagation mode Does not move to the cutoff region.

However, the Fabry-Perot type optical waveguide fill
In the case of a conventional optical waveguide device,
Since it is assumed to be about 10 times the chair, the refraction of the optical waveguide
Rate n _fxIs significantly smaller than before.
Then, the normalized frequency is greatly reduced, and FIG.
Move from point B to point B- in the cutoff area as shown
Happens to happen. In this case, the propagation mode is a single mode
The transition from the load region to the cutoff region
The insertion loss at the center wavelength increases.
is there.

In view of the above circumstances, the present invention provides a Fabry-Perot optical waveguide using an optical waveguide shape parameter such that the propagation mode of the optical waveguide does not shift to a cutoff region or a multimode region even when a positive or negative high electric field is applied. It is intended to provide a filter.

[0021]

In order to achieve the above object, the present invention provides a graded three-dimensional optical waveguide formed on an electro-optic crystal substrate, at both ends of which a high reflectivity mirror is provided. In a Fabry-Perot type optical waveguide filter having electrodes disposed on both sides, parameters for determining the shape of the optical waveguide are cut to such an extent that the parameters exist in a single waveguide mode region even when a high electric field is applied for sweeping center wavelength. It is characterized in that it is set in a region distant from the off region and the multi-mode region.

According to the present invention, in the Fabry-Perot type optical waveguide filter, attention is paid to the fact that there is no branch or bend in the optical waveguide, and the optical waveguide shape parameter is not set in the vicinity of the cutoff region as in the prior art. , A region separated from the cut-off region or the multi-mode region.

As a result, even when a positive or negative high electric field is applied, the propagation mode of the optical waveguide is prevented from shifting to the cut-off region or the multi-mode region, and the optical filter generated when a high voltage is applied to the optical waveguide. The phenomenon of increase in transmission loss at the center wavelength is suppressed.

[0024]

FIG. 1 shows the configuration of an embodiment of a Fabry-Perot optical waveguide filter according to the present invention. In addition. 1, the same components as those of the conventional example shown in FIG. 4 are denoted by the same reference numerals and names. Hereinafter, a description will be given mainly of the portion according to the present embodiment.

In FIG. 1, the Fabry-Perot type optical waveguide filter of the present embodiment has an optical waveguide shape parameter of
Instead of the conventional d _x2 and 2 · d _y2 , d _x1 and 2 · d
_y1 .

This optical waveguide shape parameter d _x1 , 2 ·
_dy1 is set within a single guided mode region and away from the cutoff region and the multimode region. In other words, in the present embodiment, the optical waveguide shape parameters d _x1 and 2 · _{dy 1} that do not shift to the cutoff region or the multimode region when a positive or negative high electric field is applied are selected. Even if the optical waveguide shape parameter is not set in the vicinity of the cutoff region, the optical waveguide of the Fabry-Perot type optical waveguide filter has no branching or bending, so that the radiation loss of the waveguide mode can be kept small.

Next, the operation of the present embodiment will be described with reference to FIGS. FIG. 2 illustrates the propagation mode of the optical waveguide shape parameters d _x1 and 2 · _{dy 1} of the present embodiment. FIG. 3 is a transmission characteristic diagram of the Fabry-Perot type optical waveguide filter of the present embodiment.

In FIG. 2, an area 21 surrounded by a curve P and a curve Q indicated by oblique lines is almost at the center of the single mode area.
Are the optical waveguide shape parameters d _x1 , 2 · d of the embodiment.
_The normalized frequency V _dI of the optical waveguide when _y1 is used,
It shows the range that the aspect ratio (2d _y / d _x ) can take.
In other words, the region 21 has the optical waveguide shape parameter d _x
This is the range that the propagation mode can take when ₁ , 2 · _dy1 is used. The area 21 will be discussed below.

The boundary between the cut-off region and the single mode region and the boundary between the single mode and the multimode of the heat diffusion type optical waveguide are expressed by the following expressions (1) and (2), respectively.

2 · d _y1 / d _x = {π / (2 · b _I )} ^1/2 / V _dI (1) 2 · d _y1 / d _x = 3 · {π / (2 · b _I) )｝ ^1/2 / V _dI (2)

Where b _I is the normalized waveguide refractive index, and the normalized frequency V _dI is expressed by the following equation (3) using the refractive index of the waveguide and the refractive index of the substrate.

V _dI = k ₀ · d _x · (n _fX ² -n _S ² ) ^1/2 (3)

[0031] When applying a positive maximum electric field necessary to sweep the refractive index changes from _{n fx} to _{n fx + Δn fx} +. At this time, if the aspect ratio is kept constant, the operating point shifts to the right and moves from point A to point A +. In order to prevent the transition from the single mode region to the multimode region, the operating point at the maximum electric field must be on the left side of the boundary between the single mode and the multimode. That is, the operating point A when no electric field is applied is at the boundary (= curve P).

[Number 3] _{2 · d y / d x =} 3 · {π / (2 · b I)} 1/2 / {V dI 2 + k 0 2 d x 2 · Δn fX + · (Δn fX + -2n fX)} ^1/2 ... (4) Must be to the left. The above equation (4) represents the curve P.

[0032] Similarly, upon application of a negative electric field maximum, the refractive index from _{n fx n fx -Δn fx -} changes to. At this time, if the aspect ratio is kept constant, the operating point shifts to the left and moves from point A to point A-. In order to prevent the transition from the single mode region to the multimode region, the operating point at the maximum electric field must be on the right side of the boundary between the single mode and the multimode. That is, the operating point A when no electric field is applied is at the boundary (= curve Q).

Equation 4] _{2 · d y / d x =} {π / (2 · b I)} 1/2 / {V dI 2 + k 0 2 d x 2 · Δn fX- · (Δn fX- + 2n fX)} 1 ^{/ 2} Must be to the right of (5). The above equation (5) represents the curve Q.

Therefore, in order to eliminate the increase in the insertion loss at the center wavelength during the sweep, it is sufficient to design the operating point A when no electric field is applied in the area 21 between the curves P and Q.

In this embodiment, the shape parameter of the optical waveguide is selected from the region 21 far from both regions so as not to shift to the cut-off region when a negative electric field is applied and to shift to the multi-mode region when a positive electric field is applied.

As a result, for example, point A in area 21 is
When a negative electric field is applied, it moves to the point A-, but does not shift to the cut-off region but remains in a single region. Point A moves to point A + when a positive electric field is applied, but does not move to the multi-mode region.

Therefore, even when the transmission band is switched by applying an electric field, that is, even when the transmission band is swept, the transmission loss at the center wavelength can be kept substantially constant. FIG. 3 shows that the width of the optical waveguide is 7 μm to 14 μm, which is an example of a conventional optical waveguide device.
4 shows the transmission characteristics when enlarged. When the width of the optical waveguide is increased, the normalized frequency can be increased while maintaining the aspect ratio.

As shown in FIG. 3, even when the applied electric field is swept positively and negatively, the transmission loss at the center wavelength is almost constant.

[0038]

As described above, according to the present invention,
Since the shape parameter of the optical waveguide is set in a region away from the cutoff region and the multimode region, the waveguide mode can be prevented from shifting to the cutoff region and the multimode region even when a high positive and negative electric field is applied.

Therefore, the value of the transmission loss at the center wavelength during the sweep is maintained substantially constant.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a Fabry-Perot optical waveguide filter according to an embodiment of the present invention.

FIG. 2 shows an optical waveguide shape parameter d _x1 ,
It is explanatory drawing of the propagation mode of 2 * _dy1 .

FIG. 3 is a transmission characteristic diagram of the Fabry-Perot type optical waveguide filter of the present embodiment.

FIG. 4 is a configuration diagram of a conventional Fabry-Perot type optical waveguide filter.

FIG. 5 is a refractive index distribution diagram of a graded three-dimensional optical waveguide.

FIG. 6 is a relationship diagram between an aspect ratio and a normalized frequency V _dI .

FIG. 7 shows a conventional optical waveguide shape parameter d _x2 , 2 · d
It is a transmission characteristic figure of the Fabry-Perot type optical waveguide filter manufactured using _y2 .

FIG. 8 shows a conventional optical waveguide shape parameter d _x2 , 2 · d
It is explanatory drawing of the propagation mode of _y2 .

[Explanation of symbols]

REFERENCE SIGNS LIST 1 electro-optic crystal substrate (lithium niobate substrate) 2 optical waveguide (titanium thermal diffusion optical waveguide) 3 mirror with high reflectivity 4 electrode 21 determination of propagation mode when using optical waveguide shape parameters d _x1 , 2 · _{dy 1} Area to gain

Claims (1)

[Claims] 1. A Fabry-Perot optical waveguide filter in which high-reflectance mirrors are provided on both end faces of a graded three-dimensional optical waveguide formed on an electro-optic crystal substrate, and electrodes are arranged on both sides of the optical waveguide. The parameters that determine the shape of the optical waveguide are set in the cut-off region and the region away from the multi-mode region to the extent that they exist in the single waveguide mode region even during the application of a high electric field for center wavelength sweeping. A Fabry-Perot type optical waveguide filter characterized by the above-mentioned.