JPH07174928A - Optical wavelength filter - Google Patents

Abstract

PURPOSE:To obtain an optical wavelength filter capable of ensuring a larger number of selective wavelength channels than a conventional filters, and reducible in size. CONSTITUTION:This optical wavelength filter has a directional coupler 13 with two or more waveguides 13a, 13b different from each other in refractive index or equiv. refractive index, gratings 19 fitted to the waveguides 13a, 13b and causing Bragg reflection at many wavelengths and electrodes 15a, 15b for controlling the difference in refractive index or equiv. refractive index between the waveguides 13a, 13b and the refractive index of each of the waveguides.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an element for selecting light having a specific wavelength from a plurality of wavelength-multiplexed optical signals.

[0002]

2. Description of the Related Art As a conventional example of an element (optical wavelength filter) for separating an optical signal having a specific wavelength λ ₀ from wavelength-multiplexed optical signals, for example, reference I (IEEE Communicative) is used.
ation Magazine, October 1989, p.
53-63). This document I
The optical wavelength filter disclosed in, is a: Fabry-Perot type,
There are four types: b: Mach-Zehnder type, c: mode conversion type, and d: Bragg reflection type. Considering that the transmission wavelength λ ₀ of the optical wavelength filter is changed from the design reference wavelength λ to λ + Δλ, the following expressions (1) are established for the wavelength change amount Δλ in the types a, b and d, and the type c Then, the following equation (2) is established for the wavelength variation Δλ. Where λ ₀ = λ
It can be expressed as + Δλ.

Δλ / λ = Δn / n (1) Δλ / λ = Δn / δn (2) where Δn is obtained by electrically changing the refractive index of the waveguide provided in the optical wavelength filter. Change in refractive index, n
Is the refractive index of the waveguide provided in the optical wavelength filter, and δn is the refractive index difference between modes, for example, the refractive index difference between the TM and TE modes.

Generally, the design reference wavelength λ is a constant which is uniquely determined from the shape, size, forming material, etc. of the optical wavelength filter constituent element. However, among the c-type ones, which utilize the acousto-optic effect (AO effect), the period of the grating for converting the light mode can be electrically changed, so that the design reference wavelength λ should be variably controlled. You can Therefore, the refractive index change amount Δ that is electrically variably controlled
Although n has an upper limit, the variable range (tuning width) of the transmission wavelength λ ₀ is the widest in the type c.

The _full width at half maximum Δλ _FWHM of the light transmittance peak in each of the a, b, c and d filters can be expressed by the following equations (3), (4), (5) and (6). In the formula, L represents the electrode length of the optical wavelength filter, and R represents the reflectance of the input / output end face of the optical wavelength filter.

Δλ _FWHM / λ = {λ / (2 · L · n)} · {λ / (π · R ^1/2 )} (3) Δλ _FWHM / λ = λ / (L · n)… ( 4) Δλ _FWHM / λ = λ / (L · δn) (5) Δλ _FWHM / λ = λ / (2 · L · n) (6) Since normally δn << n, (3) to (3) As can be understood from the equation (6), the _full width at half maximum Δλ _{FWHM in} the types a, b and d becomes very narrow, but the _full width at half maximum Δλ _{FWHM in} the type c becomes very wide.

Here, if the transmission bandwidth per channel of the optical wavelength filter is expressed by the half width Δλ _FWHM , the number of channels C
H can be expressed by the following expression (7) for the type a, the following expression (8) for the types b and c, and the following expression (9) for the type d. Δn _max in the formula represents the maximum Δn in the changeable range.

CH = {(2 · L · Δn _max ) / λ} · {(π · R ^1/2 ) / (1-R)} (7) CH = (L · Δn _max ) / λ ( 8) CH = (2 · L · Δn _max ) / λ (9) However, since the type a is subject to the limitation of FSR (Free Spectral Range), CH = π · R ^1/2 /
(1-R).

Therefore, if Δn _max ≈0.01, then R ≈0.9 for the type a and several tens of channels (potentially 80 channels if the FSR is ignored) due to the limitation of the FSR, and L for the type b. Approx. 1 cm for 80 channels, c type for L = 1 mm for 8 channels and d type for L =
There are 8 channels for 500 μm.

[0010]

However, in the conventional optical wavelength filters of the types a, b, and d described above, even if the transmission band width Δλ _FWHM per channel can be narrowed, the tuning width (transmission wavelength λ ₀ Since the variable range) cannot be widened, the number of channels (= tuning width / transmission bandwidth per channel) cannot be increased. Further, in the c type optical wavelength filter, the tuning width can be widened, but the number of channels cannot be increased because the transmission bandwidth Δλ _FWHM cannot be narrowed.

Considering increasing the number of channels,
In the a, b, and d types, if the element length L is increased, the transmission bandwidth Δ
Although λ _FWHM can be narrowed and therefore the number of channels can be increased, if the transmission bandwidth Δλ _FWHM is too narrow, the optical wavelength filter becomes unwieldy and impractical. In the type c, the number of channels cannot be increased unless the element length L is extremely long (for example, L = 1 m).

[0012]

Therefore, the optical wavelength filter of the present invention includes a directional coupler having two or more waveguides having different refractive indexes or equivalent refractive indexes, and each of the two or more waveguides. The multi-wavelength Bragg-reflecting grating is provided, and an electrode for controlling the refractive index difference between these two or more waveguides or the equivalent refractive index difference and the refractive index of each waveguide.

[0013]

The operation of the present invention will be described by taking the case of two waveguides (referred to as first and second waveguides) as an example.

Since the refractive index or equivalent refractive index of the first and second waveguides forming the directional coupler are different, the wavelength dispersion is different in each waveguide. Therefore, the propagation constants of the waveguides match only at a specific wavelength λ _i . Then, the light input from the outside to the first waveguide (or the second waveguide) moves to the second waveguide (or the first waveguide) only at this wavelength λ _i . Here, the refractive index control electrode has a function of controlling the wavelength λ _i . However,
Since the refractive index difference between the first and second waveguides is different, the wavelength tuning width can be made wider than in the case where they are not.

Next, the operation of the grating will be described. Without the grating, the light of the above wavelength λ _i would be output as the transmission wavelength with the half width that it had. However, since the present invention has the grating that Bragg-reflects at multiple wavelengths, the light of the wavelength λ _i that has moved from the first waveguide (or the second waveguide) to the second waveguide (or the first waveguide). Is reflected by the above-mentioned grating. Here, if the wavelength pitch in the Bragg reflection of multiple wavelengths and the half width at each reflection are set as predetermined, the light of the above wavelength λ _i is reflected at a certain wavelength in the grating and has a smaller half width than in the case where there is no grating. It becomes the light of.

[0016]

Embodiments of the optical wavelength filter of the present invention will be described below with reference to the drawings. It should be noted that the drawings used in the description merely schematically show the dimensions, shapes, and arrangement relationships of the respective constituent components to the extent that the present invention can be understood. Further, in each of the drawings used for description, the same components are denoted by the same reference numerals.

1. First Embodiment FIG. 1 is a plan view for explaining an optical wavelength filter 10 according to a first embodiment of the present invention. This optical wavelength filter 10
A substrate 11 having an electro-optical effect and first and second waveguides 13 a and 13 b formed side by side on the substrate 11 and having different refractive indexes to form a directional coupler 13. Waveguides 13a and 13b, refractive index control electrodes 15a and 15b, and input / output ports 17a and 17b
And a predetermined grating 19. In addition,
In FIG. 1, reference numeral 21 is a waveguide 13a, 13b for facilitating the connection between the waveguide 13a, 13b and an optical fiber or the like.
Is a part for expanding the interval (waveguide interval expanding part) and is not an essential part.

Here, as the substrate 11, a substrate having an electro-optical effect, for example, a compound semiconductor substrate or LiN is used.
A bO ₃ substrate can be used. Further, the two waveguides 13a and 13b can have a known waveguide structure. For example, various types of waveguide structures such as a buried type, a ridge type, and a loaded type (for example, the document “Optical integrated circuit”, Ohmsha (Showa 60), p.196) are used to design the type of substrate to be used and the optical wavelength filter. It is better to respond. In addition, the refractive index control electrodes 15a and 15b are used for the refractive index difference between the waveguides 13a and 13b or the equivalent refractive index difference and the waveguides 13a and 13b.
This is for controlling the refractive index of 13b, and is provided on each of the waveguides 13a and 13b here. The other poles of these electrodes 15a and 15b are provided on the back surface of the substrate 11, for example. The grating 19 is provided in each of the waveguides 13a and 13b and performs Bragg reflection at multiple wavelengths. This grating 1
9 is composed of a sampled grating here. Further, here, the grating 19 is formed across both the waveguides 13a and 13b. Further, the gratings in both the waveguides 13a and 13b have the same period. However, the grating 19 is not limited to the sampled grating and may be any other suitable one, for example, a super-periodic grating may be adopted. Further, the grating 19 need only be provided in both the waveguides 13a and 13b, and may not be continuous in the substrate portion between the both waveguides.

Next, the operation of each part of the optical wavelength filter 10 of the first embodiment and the operation of the optical wavelength filter 10 will be described. This description will be given with reference to FIGS.

An optical signal λ in which light of wavelengths λ _{1 to} λ _N , for example, is multiplexed into one of the input / output ports (input / output port 17a in the example of FIG. 1) of the optical wavelength filter
input. In this optical wavelength filter 10, since the refractive indexes of the first and second waveguides 13a and 13b are different, the wavelength dispersion in each waveguide is different. Therefore, the propagation constants of the waveguides match only at a specific wavelength λ _i . Therefore, from the input / output port 17a to the first waveguide 13a
The light input to the second waveguide 13 is only at this wavelength λ _i.
Move to b. This state is shown in FIG. Of course, the light of the wavelength λ _i moves to the second waveguide 13b with a half value width due to the characteristic of the directional coupler 13 in the first place (details will be described later). In FIG. 2A, (I) shows the propagation constant of the first waveguide, (II) shows the propagation constant of the second waveguide, and (III) shows the transition rate. Further, here, the refractive index control electrode 1
Reference numerals 5a and 15b indicate the function of controlling the wavelength λ _i .
The wavelength tuning width Δλ at this time is on the order of Δλ = Λdn. However, dn is the refractive index control electrode 15
is a change in the electro-optical refractive index due to a and 15b, and Λ is represented by the following equation. In the following equation, n ₁ is the refractive index of the first waveguide 13a, n ₂ is the refractive index of the second waveguide, λ ₀ is the center value of the wavelength range used, and λ = λ ₀ means that the value at the wavelength λ ₀ is adopted.

[0021]

[Equation 1]

As is clear from the formula of the wavelength tuning width Δλ = Λdn, if Λ is sufficiently large, the electro-optical refractive index change dn caused by the refractive index control electrodes 15a and 15b.
Even if is small, broadband wavelength tuning can be obtained.

Next, the operation of the grating 19 will be described. Since the grating 19 is designed to perform Bragg reflection at a plurality of wavelengths, its wavelength-reflectance characteristic is as shown in FIG. 2 (B). That is, the light of each of a plurality of specific wavelengths is reflected. Wavelength tuning rate Δλ _B / λ _{0 in} this grating 19
Is Δλ _B / λ ₀ = dn / n. Where n is the first
It is the refractive index of the waveguide and the second waveguide (when λ = λ ₀ , the refractive index of the first waveguide is n ₁ and the refractive index of the second waveguide is n _2).
Is n ₁ = n ₂ = n). The light λ _i transferred from the first waveguide 13a to the second waveguide 13b is reflected by the grating 19
Reflected at. However, here, of the light in the wavelength region of wavelength λ _i having a certain half-width, light that matches a plurality of specific wavelengths set in the grating 19 is reflected. Light reflected in this way (λ _i = λ
₀ ) is output to the outside as a transmission wavelength from the input / output port 17b (see FIG. 2C). Here, the half-value width Δλ of each spectrum of the Bragg reflection generated in the grating 19
_BW / λ ₀ is of the order of Δλ _BW / λ ₀ = λ ₀ / 2nNl. However, Nl is the total length of the grating.
Here, N is the number of grid portions G (see FIG. 1) (the number of divisions, 6 in the illustrated example), and l (see FIG. 1) is the length of the lattice portion G. On the other hand, the FWHM Δλ _b of the directional coupler 13 itself when light is transferred from the first waveguide (or the second waveguide) to the second waveguide (or the first waveguide).
/ Λ ₀ is on the order of Δλ _b / λ ₀ = Λ / L. However, L is the length of each of the first and second waveguides 13a and 13b. Here, the number of channels CH in the optical wavelength filter is given by tuning width / half-width. Further, in the present invention, as described above, the tuning width Δλ can be widened by increasing Λ. However, the FWHM Δλ _{b of the} directional coupler 13 in the first place
Since / λ ₀ is on the order of Δλ _b / λ ₀ = Λ / L as described above, if Λ is increased, the half width Δλ _b / λ ₀ is also increased, so that the number of channels cannot be increased. However, in the present invention, the grating 19
Therefore, even if the full width at half maximum Δλ _b / λ ₀ of the directional coupler 13 is widened by increasing Λ in order to widen the tuning width Δλ, the half width can be narrowed by this grating. More specifically, if the grating 19 is not provided, the number of channels is Δλ.
/ Δλ _b = dnL / λ ₀ order (above Δλ = Λ
It can be obtained from the expression dn and the expression Δλ _b / λ ₀ = Λ / L. ). Further, the number of channels in the case of only the grating 19 is Δλ _b / Δλ _BW = (2Nl / λ ₀ ) dn order (the wavelength tuning rate Δλ _B / λ ₀ =
The expression dn / n and Δλ _BW / λ ₀ = λ ₀ / 2nNl
It is obtained from the formula. ). However, in the present invention, the tuning width is Δλ and the half width is the grating 19
Since the value of Δλ _BW / λ ₀ at, the number of channels is Δλ / Δ
λ _BW = (Λ / λ ₀ ² ) 2nNldn. From this formula,
It can be seen that by making Λ / λ ₀ sufficiently large, the number of channels can be dramatically increased as compared with the conventional optical wavelength filters a to d.

The pitches of a plurality of wavelengths of Bragg reflection in the grating 19 are Δλ _B / λ ₀ = λ ₀ /
[2 (L / N) n] and the half width Δλ in the directional coupler
_Since it is necessary to match _b / λ ₀ , λ ₀ / [2 (L /
N) n] = Λ / L, and thus Λ = λ ₀ N / 2n. Therefore, this Λ is Δλ / Δλ _BW = (Λ /
λ ₀ ² ) Substituted for 2nNldn, the number of channels Δ
λ / Δλ _BW is (N ² l / λ ₀ ) dn.

2. Second Embodiment An example (second embodiment) will be described in which the configuration of the first embodiment is further provided with a grating for even mode and odd mode coupling (hereinafter, also referred to as “even / odd mode coupling grating”). To do. 3A and 3B are plan views for explaining the optical wavelength filter 30 of the second embodiment. In this case, the first and second waveguides 13a and 13b
One of them has a grating 31 for even / odd mode combination.
An example in which is provided is shown. In this embodiment, the even / odd mode coupling grating 31 is constructed by periodically varying the width of the waveguide. The details are shown in FIG. The period Λ of this grating 31
(See FIG. 3B) is obtained by the formula described in the first embodiment. Of course, the configuration of the even / odd mode coupling grating 31 may be any suitable configuration.

In the optical wavelength filter 30 of the second embodiment, the light input to the input / output port 17a excites the even mode in which the amplitude of the light is concentrated in the first waveguide 13a. Then, only the light of the wavelength λ ₀ corresponding to the period of the even / odd mode coupling grating 31 is converted into the odd mode in which the amplitude of the light is concentrated in the second waveguide 13b. That is, with the light of wavelength λ ₀ , the propagation constants of the first and second waveguides are as if they matched. The conversion characteristic in the second embodiment is the same as the characteristic described in the first embodiment with reference to FIG. Since the subsequent operation of the grating 19 is the same as that of the first embodiment, the light that has been converted into the odd mode and has a specific wavelength set in the grating 19 is reflected and entered. It is output to the output port 17b.

3. Third Embodiment An example (third embodiment) in which extension waveguides that are not coupled to each other are connected to the respective waveguides forming the directional coupler will be described. FIG. 4 is a plan view for explaining the optical wavelength filter 40 of the third embodiment.

The optical wavelength filter 40 of the third embodiment is
The extension waveguide 4 which is not coupled to the first and second waveguides 13a and 13b of the optical wavelength filter of the first embodiment.
1a and 41b (meaning that there is no coupling between the waveguides 41a and 41b) are connected via a branch 43, and further, each of the extended waveguides 41a and 41b is provided with a grating 19 for Bragg reflection at multiple wavelengths. Configured. However, here, the first and second waveguides 13a,
An example is shown in which the refractive indices of 13b are changed by changing the width of 13b. Further, here, an example is shown in which an electrode for controlling the refractive index difference between the two waveguides 13a and 13b is provided on one of the waveguides 13a. In addition, the extension waveguides 41a, 41
Electrodes 45a and 45b for controlling the refractive index of these waveguides are provided on the respective b.

In the optical wavelength filter 40 of the third embodiment, light other than the wavelength λ ₀ shown in FIG. 2A is input to the input / output port 17a, the first waveguide 13a and the first waveguide. The light of the specific wavelength shown in FIG. 2 (B), which propagates through the connected extension waveguide 41a, is reflected by the grating 19 and returns to the input / output port 17a again. On the other hand, the light having the wavelength λ ₀ is equally divided into the first and second waveguides 13a and 13b, and the extension waveguide 41
Light of a specific wavelength shown in FIG. 2B propagated to a and 41b, respectively, is reflected by the grating 19 and returned to the directional coupler 13 part, and is output from the input / output waveguide 17b. It The input / output waveguide 17
Of the light input from a, the light reflected by the grating 19 in the extension waveguide 41a is the input / output waveguide 1
Although not theoretically returning to 7a again, it may occur due to a manufacturing error of the element. In that case, this can be compensated by known phase adjusting means.

In the case of the structure of the third embodiment in which the extended waveguide is provided and a predetermined grating is provided therein, the same effect as that of the first embodiment can be obtained. Further, although the element length is lengthened by providing the extended waveguide, there is also a special advantage that the coupling coefficient of light between the both waveguides 13a and 13b does not need to be small.

4. Fourth Embodiment FIG. 5 is a plan view for explaining the fourth embodiment of the present invention. The optical wavelength filter 50 according to the fourth embodiment corresponds to the configuration obtained by combining the second embodiment and the third embodiment described above. In the optical wavelength filter 50 of the fourth embodiment, for example, the light input from the first waveguide 13a is sent to the second waveguide 13b only at a specific wavelength λ ₀ corresponding to the grating 31 for even / odd mode coupling. Transition. The transferred light is reflected by the grating 19 and output to the input / output waveguide 17b.

5. Fifth Embodiment In the first to fourth embodiments described above, the desired optical wavelength filter is obtained by using the directional coupler and the grating. However, in the optical wavelength filters of the first and second embodiments of the above embodiments, the coupling coefficient of the two waveguides 13a and 13b forming the directional coupler 13 must be sufficiently reduced (ideal. In some cases, the number of reflection peaks due to the grating may be two unless it is set to 0. As a result, the output wavelength may be two.
Since there are many cases where the coupling coefficient cannot be made sufficiently small due to variations in the manufacturing conditions of the optical wavelength filter, it is preferable to take measures against it. This fifth embodiment is such an example. First, the structure of the fifth embodiment will be described with reference to FIG. Here, FIG. 6 is a plan view of the optical wavelength filter of the fifth embodiment.

The optical wavelength filter 60 of the fifth embodiment is
The structure of the first embodiment has the same structure as the first and second waveguides 13a and 13a.
This is an example in which an electrode (hereinafter, referred to as “coupling coefficient control electrode”) for controlling the coupling coefficient of light between b and b is further provided. 1
In the part between the waveguide 13a and the second waveguide 13b of
It is provided along these waveguides 13a and 13b.
The other pole of the coupling coefficient control electrode 61 is provided on the back surface of the substrate 11, for example. In FIG. 6, 63a, 63b, 6
Reference numeral 3c is an electrode for adjusting the phase between two eigenmodes (two eigenmodes, symmetrical and antisymmetric, which will be described later). With these electrodes 63a to 63c, the phase of the optical signal is adjusted so that the power is concentrated at the end portion of the waveguide (for example, the first waveguide 13a) to which the input light is input, on the side of the input / output port 17a. it can.

The optical wavelength filter 60 of the fifth embodiment operates as follows. However, here, the wavelengths λ _{1 to} λ _N
The optical signal λ in which the light of
Consider the case of input from the optical wavelength filter 60 to the optical wavelength filter 60. Light having a wavelength other than the specific wavelength λ ₀ in the optical signal λ excites only one eigenmode because the eigenmode field distribution depends on one of the waveguides 13a and 13b. In addition, of such light having a wavelength other than the wavelength λ ₀ , light having a wavelength that is not reflected by the grating 19a is discarded from the end of the second waveguide 13a. In addition, the grating 1 of the light having a wavelength other than the wavelength λ _{0 is}
The light reflected by 9a returns to the input / output port 17a side.

On the other hand, the light of wavelength λ ₀ in the optical signal λ is the first
Of the symmetric and antisymmetric eigenmodes of the second waveguide 13a and the second waveguide 13b. The two eigenmodes are reflected by the grating 19. The reflection characteristic of one of the modes is as shown in FIG. 7 (A), and the reflection characteristic of the other mode is as shown in FIG. 7 (B). That is, the position of the reflection peak is displaced. In addition, in FIGS. 7A and 7B, n ₊ and n ₋ are
It is the refractive index felt by the symmetric and antisymmetric eigenmodes. As described above, the reason why the position of the emission peak is displaced is that the refractive indexes n ₊ and n felt by the two eigenmodes are different because the propagation constants of the first waveguide 13a and the second waveguide 13b are different.
_- due to the fact that different. Since the difference in the propagation constant between the two eigenmodes can be controlled by adjusting the coupling coefficient between the waveguides 13a and 13b, the coupling coefficient control electrode 61 provided in the fifth embodiment can be provided with an appropriate electric signal (voltage or the like). )
Is applied to adjust the coupling coefficient to control the propagation constant difference. This makes it possible to match one of the many reflection peaks of each of the two eigenmodes (see P in FIGS. 7A and 7B).

Here, the state in which the light of wavelength λ ₀ is transferred from the first waveguide 13a to the second waveguide 13b is the same as that described in the first embodiment, so that FIG. As shown, it has a certain half-value width. However, also in the fifth embodiment, as in the first embodiment, the grating 19
Is provided, the half-value width can be narrowed. Moreover,
In the fifth embodiment, since the coupling coefficient controlling electrode 61 is further provided, even if the coupling coefficient of light between the two waveguides is different due to manufacturing variations or the like, this can be adjusted by the electrode 61. 13b Each grating 19
The reflection peaks at can be matched. As a result,
In the optical wavelength filter 60 of the fifth embodiment, FIG.
Of the light in the wavelength band shown in FIG. 7, the light having the peak wavelength shown in FIGS.
Will be output.

In the fifth embodiment, an example in which the coupling coefficient control electrode is provided in the optical wavelength filter of the first embodiment has been described, but this idea is based on each of the optical wavelength filters of the second to fourth embodiments. It can also be applied to the optical wavelength filter of the sixth embodiment described later. In that case, a coupling coefficient control electrode may be provided between the first waveguide 13a and the second waveguide 13b, for example, in the second to fourth embodiments and the sixth embodiment.

6. Sixth Embodiment Next, a structural example (sixth embodiment) for further improving the crosstalk characteristics in the optical wavelength filters of the above-described first to fifth embodiments will be described. Here, an example in which this structural example is applied to the optical wavelength filter of the second embodiment will be described. First, the structure of the sixth embodiment will be described with reference to FIG. Here, FIG. 8 is a plan view of the optical wavelength filter of the sixth embodiment.

The optical wavelength filter 70 of the sixth embodiment is characterized in that the periods of the gratings 119a and 119b provided in the extended waveguides 41a and 41b are different from each other. However, in the sixth embodiment, the electrodes for controlling the refractive index between the waveguides and the refractive index of each waveguide are the electrode 15a provided on the first waveguide 13a and the extended waveguide 41.
a and 41b, and electrodes 15x and 15y provided in the portions where the gratings are formed. Furthermore, in the sixth embodiment, the electrodes 15 of the extended waveguides 41a and 41b are
Each grating 119 is provided on the part other than that where x and 15y are provided.
Phase control electrodes 71a and 71b for adjusting the phases of the lights returning to the directional coupler 13 after being reflected by a and 119b are provided in the extension waveguides 41a and 41b.

The wavelength filter 70 of the sixth embodiment operates as follows. However, here, consider a case where an optical signal λ in which lights of wavelengths λ _{1 to} λ _N are multiplexed is input to the optical wavelength filter 70 from the input / output port 17a.

First, in the portion of the directional coupler 13, light is emitted from the first waveguide 13 only at wavelengths near the specific wavelength λ _0.
a to the second waveguide 13b (see FIG. 9A), and such light is extended to the extended waveguides 41a and 41b.
Is divided into two equal parts. At other wavelengths, the input light travels from the input / output port 17a to the first waveguide 13a to the extension waveguide 41a and is discarded from the end of the extension waveguide 41a. In addition, the grating 119 of the light of other wavelengths referred to here
The light reflected by a returns to the input / output port 17a.

Here, the reflectance characteristic of the filter composed of the grating 119a provided in the extension waveguide 41a has a plurality of peaks as shown in FIG. 9B. In addition, the reflectance characteristic of the filter including the grating 119b provided in the extension waveguide 41b is shown in FIG.
It has a plurality of peaks as shown in (C). However, the reflectance characteristics of the gratings 119a and 119b are different because the periods of the gratings are different. In such a configuration, the grating 119
Light P (see FIGS. 9B and 9C) having a wavelength corresponding to the portion where the reflection peaks of a and 119b overlap each other is reflected equally to the first waveguide 13a and the second waveguide 13b of the directional coupler 13. However, the light of wavelengths other than that is guided by the waveguides 13a and 13a.
Only one side of b is reflected. Due to the property of the directional coupler, the light reflected back to only one of the waveguides has a loss of 6 dB. As a result, the total characteristics of the two filters composed of the gratings 119a and 119b are as shown in FIG. 9 (D). That is, the reflectance of the portion where the reflection peaks of the gratings 119a and 119b overlap is higher than that of the other portions. Therefore, in the filter 70 of the sixth embodiment,
Since the filter characteristic of the directional coupler 13 is affected by the filter characteristic shown in FIG. 9D, the overall filter characteristic is mainly the light P of a specific wavelength as shown in FIG. It becomes transparent. Further, here, the filter characteristic of the directional coupler 13 has a crosstalk portion X (dB) as shown in FIG. The crosstalk X is usually 10 dB. However, in the case of the optical wavelength filter 70 according to the sixth embodiment, the gratings 119a and 11g with respect to the crosstalk X are generated.
Since the effect of the filter characteristic of FIG. 9D due to 9b is exerted, the crosstalk that was normally 10 dB only with the directional coupler 13 can be reduced to 16 dB.

The phase control electrodes (also referred to as phase adjustment electrodes) 71a, 71b are the gratings 119a,
This is for adjusting the phase of the light reflected from the 119b and returning to the waveguides 13a and 13b so that they are aligned at the wavelength λ ₀ . If the light reflected by the gratings 119a and 119b and returned to the directional coupler 13 does not have the same phase, a loss occurs at the wavelength λ ₀ due to the property of the directional coupler, so that it is appropriate for the phase control electrodes 71a and 71b. This is prevented by applying an electrical signal (eg voltage).

In the sixth embodiment, an example in which the idea of the invention of the sixth embodiment is applied to the optical wavelength filter of the second embodiment has been described, but the idea is the first, the third to the fifth. It can be applied to each optical wavelength filter of the embodiment.

[0045]

As is apparent from the above description, according to the optical wavelength filter of the present invention, the wavelength tuning width can be made wider than the conventional one because it has the predetermined directional coupler and the refractive index control electrode. Further, since the predetermined grating is provided, the full width at half maximum of the transmission wavelength can be narrowed. Since the number of channels is defined by the wavelength tuning width / half-value width, the number of channels can be increased in the present invention as compared with the conventional case. In addition, the number of channels can be increased with the length of the directional coupler or the element length such that an extension waveguide is added thereto. Therefore, it is possible to provide an optical wavelength filter that can have a large number of selected wavelength channels and can be downsized. For example, in each of the optical wavelength filters a to d described in the prior art, the number of channels is about the number of channels≈ (element length × dn) / λ ₀ , whereas in the present invention, the number of channels≈ (Λ × because it is 2n × device length × dn) / λ ₀ ^2, Taking a large ratio of lambda / lambda ₀ the number of channels can be made lambda / lambda ₀ times the conventional. Here, Λ is given by the above equation, and λ ₀ is the center value of the wavelength range used.

Further, in the structure in which the electrodes for adjusting the coupling coefficient of light between the waveguides forming the directional coupler are provided,
For example, even when the coupling coefficient of light between the waveguides deviates from a desired value due to manufacturing variations, this can be corrected, so that the effect of the present invention can be secured.

Further, in the structure in which the grating period is different, the crosstalk characteristic can be improved.

[Brief description of drawings]

FIG. 1 is a diagram for explaining a first embodiment and is a plan view for explaining a configuration.

2A to 2C are diagrams for explaining the first embodiment, and are diagrams for explaining the operation and the operation.

FIG. 3A and FIG. 3B are diagrams for explaining a second embodiment.

FIG. 4 is a plan view for explaining a third embodiment.

FIG. 5 is a plan view for explaining a fourth embodiment.

FIG. 6 is a diagram for explaining the fifth embodiment and is a plan view for explaining the configuration.

7A to 7C are diagrams for explaining the fifth embodiment and are diagrams for explaining the operation and the operation.

FIG. 8 is a diagram for explaining the sixth embodiment and is a plan view for explaining the configuration.

9A to 9D are diagrams for explaining the sixth embodiment, and are diagrams for explaining the operation and the operation.

FIG. 10 is a diagram for explaining the sixth embodiment and is a diagram following FIG. 9 for explaining the action and operation.

[Explanation of symbols]

10: Optical wavelength filter of the first embodiment 11: Substrate (having electro-optical effect) 13a: First waveguide 13b: Second waveguide 13: Directional coupler 15a, 15b, 15x, 15y: Refraction Rate control electrodes 17a, 17b: input / output ports 19: Bragg reflection at multiple wavelengths 21: waveguide spacing expansion portion 30: optical wavelength filter of the second embodiment 31: even / odd mode coupling grating 40: third Optical wavelength filters 41a and 41b of the embodiment: Extended waveguides 43: Branches 45a and 45b: Refractive index control electrodes 50: Optical wavelength filter of the fourth embodiment 60: Optical wavelength filter of the fifth embodiment 61: Coupling coefficient control Electrodes 63a to 63c: phase adjustment electrode 70: optical wavelength filter 71a, 71b of the sixth embodiment: phase control electrode (also referred to as phase adjustment electrode) 11 a, 119b: grating period are different from each other

Claims (7)

[Claims] 1. A directional coupler having two or more waveguides different in refractive index or equivalent refractive index, a multi-wavelength Bragg-reflecting grating provided in each of the two or more waveguides, An optical wavelength filter comprising: an electrode for controlling a refractive index difference or equivalent refractive index difference between two or more waveguides and a refractive index of each waveguide. 2. The optical wavelength filter according to claim 1, wherein extension waveguides that are not coupled to each other are connected to the respective waveguides that form the directional coupler, and An optical wavelength filter characterized by being provided in each of these extension waveguides instead of being provided in the sex coupler part. 3. The optical wavelength filter according to claim 1, wherein the two or more waveguides forming the directional coupler are provided with gratings for even mode and odd mode coupling. Optical wavelength filter characterized by. 4. The optical wavelength filter according to claim 1, for controlling a light coupling coefficient between the two or more waveguides forming the directional coupler. An optical wavelength filter characterized by further comprising the electrode of. 5. The optical wavelength filter according to claim 1 or 2, wherein the gratings have different periods. 6. The optical wavelength filter according to claim 4, further comprising a phase adjustment electrode for adjusting a phase between eigenmodes generated in the directional coupler. . 7. The optical wavelength filter according to claim 5, further comprising a phase adjustment electrode for adjusting the phase of lights returning to the directional coupler after being reflected by each grating. Optical wavelength filter characterized by.