JPH0635007A - Variable wavelength optical filter - Google Patents

Abstract

PURPOSE:To narrow down the transmission band width of each channel of the variable wavelength optical filter and also widen the tuning width. CONSTITUTION:A filter part 10 is constituted by connecting Mach-Zehnder type interferometers 14 in stages, the electrode length Le of each of the interferometers 14 is made so different as to generate a geometric sequence whose common ratio is 2, and the structural optical length Lo between arms 24 and 26 of each of the interferometers 14 is made so different as to generate a geometric sequence whose common ratio is 2 in the increasing order of the electrode length Le. Then the Le/Lo of the interferometer 14 belonging to the filter part 10 is made constant. A filter part 12 is constituted similarly to the filter part 10 and the Le/Lo=alpha of the interferometers 34 of the filter part 12 is made different from the Le/Lo=beta of the filter part 10.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical filter capable of variably controlling a light transmission wavelength.

[0002]

2. Description of the Related Art Conventionally, as an optical filter for separating an optical signal of a specific wavelength λ ₀ from wavelength multiplexed optical signals, for example, Reference 1: IEEE Communication Magazine.
(IEE Communication Magazine)
October 1989, p53-63. The optical filter disclosed in Document 1 is classified into four types: a: Fabry-Perot type, b: Mach-Zehnder type, c: mode conversion type, and d: Bragg reflection type. Considering that the transmission wavelength λ ₀ of the optical filter is changed from the design reference wavelength λ to λ + Δλ, the following equation (1) is established for the wavelength variation Δλ in the types a, b, and d, and the wavelength is changed in the type c. The following expression (2) is established for the change amount Δλ. Where λ ₀ = λ
It can be expressed as + Δλ. Δλ / λ = Δn / n (1) Δλ / λ = Δn / δn (2) In the formula, Δn is the refractive index change amount obtained by electrically changing the refractive index of the waveguide included in the optical filter, n is the refractive index of the waveguide provided in the optical 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 by the shape, size, forming material, etc. of the optical filter constituent element. However, among the c-type, the acousto-optic effect (A
In the case of utilizing the O effect), the period of the grating for converting the light mode can be electrically changed, so that the design reference wavelength λ can be variably controlled. Therefore, the refractive index change amount Δn that is electrically variably controlled
Although there is an upper limit in the range, 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), respectively. In the equation, L represents the electrode length of the optical filter, and R represents the reflectance of the input / output end face of the optical 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, the formulas (3) to (6) are given. As can be understood from the above, the _full width at half maximum Δλ _{FWHM in} the types a, b and d is very narrow, but the _full width at half maximum Δλ _{FWHM in} the type c is very wide.

Here, if the transmission bandwidth per channel of the optical filter is expressed by the half width Δλ _FWHM , the number of channels CH is expressed by the following equation (7) for the type a, and the following equation (8) for the types b and c:
Further, in the type of d, it can be expressed as the following equation (9). Δn in the formula
_max 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.

[0007]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
In the b and d type optical filters, although the transmission bandwidth Δλ _FWHM per channel can be narrowed, the tuning width (variable range of the transmission wavelength λ ₀ ) cannot be widened, so the number of channels (=
(Tuning width / transmission bandwidth per channel) cannot be increased. Further, in the c type optical 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 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).

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned conventional problems, and an optical wavelength optical filter and an optical wavelength filter device capable of narrowing the transmission bandwidth per channel within a practical range and widening the tuning width. To provide.

[0010]

In order to achieve this object, the variable wavelength optical filter of the present invention comprises a plurality of filter sections connected in multiple stages. Each filter unit is formed by connecting a plurality of Mach-Zehnder interferometers in multiple stages, and each interferometer is composed of two 1 × 2Y branch outputs and 2 × 1Y branch inputs coupled through a connection waveguide. An arm, an electrode provided on one of the two arms to variably control the refractive index of the arm, and an optical path length difference provided on the other of the two arms to provide a structural optical path length difference between the two arms. And a generator.

The electrode lengths L _e of the interferometers belonging to the same filter section are made different so as to form a geometric progression of a common ratio 2, and the structural optical path lengths of the interferometers belonging to the same filter section are set. The difference L _o is made different so as to form a geometric progression with a common ratio of 2 in ascending order of the electrode length L _e . Further, the ratio represented by (electrode length L _e ) / (structural optical path length difference L _o ) of each interferometer belonging to the same filter unit is made equal, and the ratio is made different for each filter unit.

[0012]

According to such a configuration, (1) the electrode lengths L _e of the interferometers belonging to the same filter section are made different so as to form a geometric progression of a common ratio of 2, and (2) the same filter is used. The structural optical path length difference L _o of each interferometer belonging to the section is made different so as to form a geometric progression of a common ratio 2 in the ascending order of the electrode length L _e .
(3) (electrode length L of each interferometer belonging to the same filter unit
_e ) / (structural optical path length difference L _o ), and (4) the ratio (electrode length L _e ) / (structural optical path length difference L _o ), for each filter section. Make them different.

Therefore, the light transmission characteristics of each filter section are such that a plurality of light transmission bands a appear and the peak positions of these light transmission bands a are separated by a constant wavelength interval b. Moreover, the size of the wavelength interval b of each filter portion is different. Since the light transmission band A of the wavelength tunable optical filter of the present invention is a wavelength range in which the transmission bands a of the respective filter parts coincide with each other, a plurality of light transmission bands A appear and the peak position of these light transmission bands A is a constant wavelength. Space B apart. Since the wavelength interval B is the least common multiple of the wavelength interval b of each filter unit, the wavelength interval B can be widened.

Further, when the equivalent refractive index difference between the one and the other waveguides of the interferometer is changed via the electrode, the peak position of the light transmission band a of the filter portion moves. Therefore, the peak position of the light transmission band A is moved by appropriately and appropriately changing the equivalent refractive index difference between the one and the other waveguides for each filter unit, and thus the transmission wavelength of the wavelength tunable optical filter of the present invention is changed. Can be changed.

Further, since a filter portion having a wide width of the light transmission band a, especially a wide half value width and a narrow filter portion can be formed, even if the peak position of the light transmission band a of each filter portion is slightly deviated from the wavelength of the light to be separated, this The decrease in the output intensity of the light output from the tunable optical filter of the invention can be reduced.

[0016]

Embodiments of the present invention will be described below with reference to the drawings. It should be noted that the drawings are merely schematic representations so that the present invention can be understood, and therefore the present invention is not limited to the illustrated examples.

FIG. 1 is a plan view schematically showing the structure of an embodiment of the present invention. The tunable wavelength optical filter of this embodiment includes two stages of filter units 10 and 12, and the output of the preceding stage filter unit 10 is connected to the input of the next stage filter unit 12. (Configuration of Filter Unit 10) The filter unit 10 includes a plurality of stages of Mach-Zehnder interferometers 14
4 is connected in multiple stages. Each interferometer 14 includes an arm 24 configured by coupling one output of the 1 × 2Y branch 16 and one input of the 2 × 1Y branch 18 via a connection waveguide 20, and
An arm 26 configured by coupling the other output of the 1 × 2Y branch 16 and the other input of the 2 × 1Y branch 18 via a connection waveguide 22, and the arm 26 provided on one arm 24.
Electrode 28 for variably controlling the refractive index of 4 and the other arm 2
6 and an optical path length difference generation unit 30 that provides a structural optical path length difference between the arms 24 and 26.

Further, each interferometer 1 belonging to the filter section 10
4 of the electrode length L _e, with varied so as to form a geometric progression of common ratio 2, the interferometer 1 belonging to the filter unit 10
The structural optical path length difference L _o of No. 4 is made different so as to form a geometric progression with a common ratio of 2 in ascending order of the electrode length L _e . Then, (electrode length L _e ) / of each interferometer 14 belonging to the filter unit 10
The ratio α represented by (structural optical path length difference L _o ) is made equal.

In this embodiment, each interferometer 14 of the filter section 10 is provided on the substrate 32, and the output of the interferometer 14 is connected to the input of the interferometer 14 at the next stage of the interferometer 14 in order. Then, the interferometers 14 at the respective stages are connected to make a multi-stage connection.

The Y branches 16 and 18 of each interferometer 14 are symmetrical Y
The branching and connecting waveguides 20 and 22 are straight waveguides having the same length. The arm 24 is one output branch 1 of the Y branch 16.
6a, the connection waveguide 20, and one input branch 18a of the Y branch 18 are sequentially connected, and the arm 26 is formed by the Y branch 16a.
The other output branch 16b, the connection waveguide 22, and the Y branch 1
The other input branch 18b of 8 is sequentially connected. The electrode 28 is connected to a part or the whole of the arm 24, for example, the connection waveguide 20.
, And the refractive index of the connection waveguide 20 is electrically variably controlled via the electrode 28. For example, an electric field is applied to the connection waveguide 20 to control the refractive index by the electro-optic effect, or by injection of carriers into the connection waveguide 20 and the plasma effect.

Further, the optical path length difference generating part 30 is provided in a part or the whole of the arm 26, for example, in the connection waveguide 22, and the shape of the waveguide of the arm 26 part (hereinafter, arm 26 part I) in which the optical path length difference generating part 30 is provided. , The structural equivalent refractive index difference is provided in advance between the arm 26 portion I and the arm 24 by arbitrarily setting the forming material and other design conditions. The equivalent refractive index of the arm 26 where the optical path length difference generator 30 is not provided is equal to the equivalent refractive index of the arm 24. For example, the waveguide width of the arm 26 part I and the arm 24
By changing the waveguide width of the arm 2 or the arm 2
The equivalent refractive index difference is provided by forming the 6 part I and the arm 24 from forming materials having different refractive indexes.

Between the light path length from the start point P1 of the arm 24 to the end point Q1 (refer to FIG. 1 for the points P1 and Q1) and the light path length from the start point P1 of the arm 26 to the end point Q1. As a method of structurally providing a difference in optical path length, the equivalent refractive index is made different, and, for example, the geometric length from the start point P1 of the arm 24 to the end point Q1 and the geometric length from the start point P1 of the arm 26 to the end point Q1 are set. May be different. In this case, the arm 26 portion I, for example, the connection waveguide 22 is used as a curved waveguide to form the arm 24,
It suffices to provide a geometrical length difference between 26.

The filter unit 10 is composed of a total of j interfering devices 14, and the electrode length L _e of each interfering device 14 is represented by L _e101 , L _e102 , ..., L _{e10j in} ascending order of electrode length. If so, the electrode length L _e101 of the shortest interferometer 14 = l
_e10 , the electrode length of the second shortest interferometer 14 L _e102 = 2 · l
_e10 , ..., The electrode length L _{e10j of} the j-th shortest interferometer 14 =
2 ^j-1 · l _e10 . Here, the electrode length is the length of the electrode 28 along the arm 24.

Further, the arms 24, 26 of each interferometer 14 are
Between the structural optical path lengths L _o in the order of increasing electrode length L _e
The light path length difference L _o101 = l _o10 , the light path length difference of the interferometer 14 having the second shortest electrode length is expressed as _o101 , L _o102 , ..., L _o10j. L _o102 = 2 ・
l _o10, ......, and the optical path length difference ^{_{L o10j = 2 j-1 ·}} l o10 short interferometer 14 to j-th electrode length. And each interferometer 1
4 (electrode length L _e ) / (structural optical path length difference L _o ) = α. For example, L _e101 / L _o101 = L _e102 / L _o102 =
...... = L _e10j / L _o10j = _α≈1 . (Configuration of Filter Unit 12) The filter unit 12 includes a plurality of stages of Mach-Zehnder type interferometers 34.
4 is connected in multiple stages. Each interferometer 34 includes an arm 42 configured by coupling one output of the 1 × 2Y branch 36 and one input of the 2 × 1Y branch 38 via a connection waveguide 40,
An arm 46 configured by coupling the other output of the 1 × 2Y branch 36 and the other input of the 2 × 1Y branch 38 via a connection waveguide 44, and the arm 4 provided on the one arm 42.
The electrode 48 for variably controlling the refractive index of 2 and the other arm 4
6 and an optical path length difference generating section 50 that provides a structural optical path length difference between the arms 42 and 46.

Further, each interferometer 3 belonging to the filter section 12
4 of the electrode length L _e, with varied so as to form a geometric progression of common ratio 2, the interferometer 3 belonging to the filter unit 12
The structural optical path length difference L _o of No. 4 is made different so as to form a geometric progression with a common ratio of 2 in ascending order of the electrode length L _e . Then, (electrode length L _e ) / of each interferometer 34 belonging to the filter unit 12
The ratio β represented by (structural optical path length difference L _o ) is made equal, and the ratio β is made different from the ratio α for the interferometer 14 of the other filter unit 10 (α ≠ β).

In this embodiment, each interferometer 34 of the filter section 12 is provided on the substrate 52, and the output of the interferometer 34 is connected to the input of the interferometer 34 of the next stage of the interferometer 34 in order. Then, the interferometers 34 at each stage are connected to make a multi-stage connection.

The Y branches 36 and 38 of each interferometer 34 are symmetrical Y
The branching and connecting waveguides 40 and 44 are straight waveguides having the same length. The arm 42 is one output branch 3 of the Y branch 36.
6a, the connection waveguide 40, and one input branch 38a of the Y branch 38 are sequentially connected, and the arm 46 is connected to the Y branch 36.
The other output branch 36b, the connection waveguide 44, and the Y branch 3
The other input branch 38b of 8 is sequentially connected. The electrode 48 is connected to a part or the whole of the arm 42, for example, the connection waveguide 40.
, And the refractive index of the connection waveguide 40 is electrically variably controlled via the electrode 48.

Further, the optical path length difference generating part 50 is provided in a part or the whole of the arm 46, for example, in the connection waveguide 44, and the arm 46 part provided with the optical path length difference generating part 50 (hereinafter, arm 46 part).
A structural equivalent refractive index difference is previously provided between the arm 46 portion II and the arm 42 by arbitrarily and appropriately defining the waveguide shape, forming material, and other design conditions of II). The equivalent refractive index of the arm 46 where the optical path length difference generating unit 50 is not provided is made equal to the equivalent refractive index of the arm 42.

Similar to the optical path length difference generating section 28, the optical path length difference generating section 50 can be variously modified. For example, the geometric length from the start point P2 of the arm 42 to the end point Q2 may be different from the geometric length from the start point P2 of the arm 46 to the end point Q2 (refer to FIG. 1 for the points P2 and Q2). ).

The filter unit 12 is composed of a total of k interfering devices 34, and the electrode length L _e of each interfering device 34 is represented as L _e121 , L _e122 , ..., L _{e12k in the} ascending order of electrode length. For example, the electrode length L _e121 = l _{e12 of} the shortest interferometer 34,
The electrode length L _{e122 of the} second shortest interferometer 34 = 2 · l _e12 ,
The electrode length of the k-th shortest interferometer 34 L _e12k = 2 ^k-1
_Let it be l _e12 . Here, the electrode length is the length of the electrode 48 along the arm 42.

Further, the arms 24, 26 of each interferometer 34 are
Between the structural optical path lengths L _o in the order of increasing electrode length L _e
o121, L _o122, ......, if represents the _L o12k, the optical path length difference between the 1st short interferometer 34 of the electrode length L _o121 = l _o12, the optical path length difference between the shorter interferometer 34 to the second electrode length L _o122 = 2
l _o12, ......, and the optical path length difference ^{_{L o12k = 2 k-1 ·}} l o12 short interferometer 34 to the k-th electrode length. And each interferometer 3
4 (electrode length L _e ) / (structural optical path length difference L _o ) = β, and the ratio β is different from the ratio α of the filter unit 10 for the interferometer 14. For example, L _e121 / L _o121 = L
_{Let e122} / _Lo122 = ... = _Le12k / _Lo12k = _β≈4 .

Β / α = (2 ^j- _{1l 10} ) / (2 ^k- _1 )
L _o12 ), the electrode lengths of both filter parts 10 and 12 can be made equal, and therefore, the tunable wavelength optical filter of this embodiment can be constructed by using the substrates ₃₂ and _{52 of the} same substrate size. Like in this case,
Preferably, l _o10 = l _o12 · 2 ^h (h = 1, 2, 3, ...
...) is good.

The first-stage interferometer 34 included in the filter unit 12
Is connected to the output of the interferometer 14 at the final stage of the filter unit 10 to form the tunable wavelength optical filter of this embodiment.

Next, the operation of the variable wavelength optical filter of this embodiment will be briefly described. FIG. 2 is a diagram for explaining the principle of operation of the variable wavelength optical filter of this embodiment. Here, assuming that k = j and h = 2 (in this case, α≈1, β≈4), the light transmission characteristics of the filter unit 10 alone are shown in FIG.
FIG. 2A shows the light transmission characteristics of the filter unit 12 alone.
FIG. 2C shows the light transmission characteristics obtained by combining the light transmission characteristics of the filter units 10 and 12 with each other, that is, the light transmission characteristics of the variable wavelength optical filter of this embodiment. In these figures, the vertical axis represents the light output intensity and the horizontal axis represents the wavelength of light.

The light transmission characteristics of the filter section 10 alone are such that a plurality of light transmission bands a ₁₀ appear and the peak positions of these bands a ₁₀ are separated by a constant wavelength interval b ₁₀ (see FIG. 2A). )). Similarly, in the light transmission characteristics of the filter unit 12 alone, a plurality of light transmission bands a ₁₂ appear and these band a
_The characteristic is such that the ₁₂ peak positions are separated by a constant wavelength interval b ₁₂ (FIG. 2 (B)). Since α ≠ β, the wavelength interval b ₁₀ ≠ b ₁₂ . Further, since the optical path length difference L _o of the interferometer 14 included in the filter unit 10 is set to be longer than the optical path length difference L _{o of the} interferometer 34 included in the filter unit 12, the transmission band a ₁₀ of the filter unit 10 is larger. It is narrower than the transmission band a _{12 of 12} .

Therefore, the transmission band A of the variable wavelength optical filter of this embodiment is a range where the transmission bands a ₁₀ and a ₁₂ coincide with each other, so that the transmission band A can be narrowed. Moreover, since the wavelength interval b ₁₀ ≠ b ₁₂ , the wavelength interval B at the peak position of the transmission band A is the least common multiple of the wavelength intervals b ₁₀ and b ₁₂ . Therefore, a variable wavelength filter having a light transmission characteristic in which the bandwidth of the transmission band A is narrow and the wavelength interval B is wide can be configured. By setting β / α = 2 ^g (g = 1, 2, 3, ...), the least common multiple of the wavelength intervals b ₁₀ and b ₁₂ can be set to b ₁₂ .

The peak position of the transmission band a ₁₀ of the filter section 10 moves when the equivalent refractive index of the arm 24 is electrically changed via the electrode 28. Similarly, the filter unit 12
The peak position of the transmission band a ₁₂ of is moved when the equivalent refractive index of the arm 42 is electrically changed via the electrode 48. Thus, the peak position of the transmission band A of the variable wavelength optical filter of this embodiment can be moved by arbitrarily and appropriately moving the peak positions of the transmission bands a ₁₀ and / or a ₁₂ .

Preferably, when the equivalent refractive indexes of both arms 24 and 42 are not electrically changed, the peak position of the transmission band a ₁₀ of the filter section 10 and the filter section at a desired wavelength λ ₀ which is a design reference. The filter units 10 and 12 are designed so that the peak positions of the transmission band a ₁₂ of ₁₂ coincide with each other. At this time, when a desired wavelength among the wavelengths represented by λ ₀ ± h · b ₁₀ (h is an integer) is set as the transmission wavelength of the variable wavelength optical filter of this embodiment (when rough adjustment of the transmission wavelength is performed) 2A, the equivalent refractive index of the arm 42 of the filter unit 12 is changed electrically while the equivalent refractive index of the arm 24 of the filter unit 10 is not changed electrically as shown by the solid line in FIG. Then, the peak position of the transmission band a ₁₂ of the filter unit 12 is made to coincide with the peak position of the transmission band a ₁₀ of the filter unit 10 as indicated by the dotted line in FIGS. 2B and 2C. When a desired wavelength among the wavelengths represented by (λ ₀ ± h · b ₁₀ ) + γ is set as the transmission wavelength of the variable wavelength optical filter of this embodiment (when fine adjustment of the transmission wavelength is performed). 2A, the equivalent refractive index of the arm 42 of the filter unit 10 is electrically changed to move the peak position of the transmission band a ₁₀ by γ, and then the transmission band a _{10 as} shown by the dashed line in FIG. Is moved by γ, the equivalent refractive index of the arm 42 of the filter unit 12 is electrically changed to transmit the light through the filter unit 12 as shown by a dotted line in FIGS. 2B and 2C. The peak position of the band a ₁₂ is matched with the peak position of the transmission band a ₁₀ of the filter unit 10.

In this way, when the peak positions of the transmission bands a ₁₀ and a ₁₂ are made to coincide with each other, if the bandwidths of the transmission bands a ₁₀ and a ₁₂ are both narrow, the light output from the variable wavelength optical filter of this embodiment is In order to sufficiently increase the intensity of P, the deviation of the peak positions of the transmission bands a ₁₀ and a ₁₂ must be reduced. However, in this embodiment, since the bandwidth of the transmission band a ₁₀ is narrow and the bandwidth of the transmission band a ₁₂ is wide,
Even if the peak positions of these bands a ₁₀ and a ₁₂ are slightly deviated, the intensity of the light output from the tunable wavelength optical filter of this embodiment can be made sufficiently large, and therefore the adjustment accuracy of these peak positions can be made gentle. You can

When the difference in optical path length is given in advance between the arms 24 and 26 of the filter section 10 by structurally providing the difference in optical path length, the equivalent index of refraction of the arm 24 in the filter section 10 is electrically changed. when electrically varied from the equivalent refractive index n + [Delta] n in the state of changing to a maximum of the equivalent refractive index n in the absence, if the peak position of the transmission band a ₁₀ is changed by [Delta] [lambda] _T10, [Delta] [lambda] _T10 is next It can be expressed as in Expression (10). It is preferable to make Δλ _T10 = b ₁₀ . Δλ _T10 / λ ₀ = α · (Δn / δn) (10) However, δn represents the equivalent refractive index difference structurally given by the optical path length difference generating unit 30 between the arms 24 and 26 of the filter unit 10. . Further, the refractive index of the portions of the filter unit 10 excluding the optical path length difference generating unit 30 of the branches 16 and 18 and the arms 24 and 26 is equal to the refractive index n in a state where it is not electrically changed.

In this case, the transmission band a _{10 of the} filter unit ₁₀
The full width at half maximum Δλ _W10 can be expressed by the following equation (11). Δλ _W10 / λ = λ / (2 · L _omax10 · _δn ) (11) where L _omax10 represents the maximum optical path length difference, and L _omax10 = 2
^{It is j-1} · ₁₀ .

Similarly, the arm 42 of the filter unit 12 is
When the equivalent refractive index n is changed from the equivalent refractive index n in the state where it is not electrically changed to the equivalent refractive index n + Δn in the state where it is changed to the maximum electrically, the peak position of the transmission band a ₁₂ is Δλ. If only _T12 changes, Δλ _T12 can be expressed by the following equation (12). Preferably Δλ _T12 = b
₁₂ is good. Δλ _T12 / λ ₀ = β · (Δn / δn) (12) where δn ₁₂ is an equivalent refractive index difference structurally provided by the optical path length difference generating unit 50 between the arms 42 and 46 of the filter unit 10. Represent Further, the refractive index of the portions of the filter unit 12 excluding the optical path length difference generating unit 50 of the branches 36 and 38 and the arms 42 and 46 is equal to the refractive index n ₁₂ in a state where it is not electrically changed.

In this case, the transmission band a _{12 of the} filter unit ₁₂
The full width at half maximum Δλ _W12 can be expressed by the following equation (13). Δλ _W12 / λ = λ / (2 · L _omax12 · _δn ) (13) where L _omax12 represents the maximum optical path length difference, and L _omax12 = 2
^k-1 · l _o12 .

When the geometrical lengths of the arms 24 and 26 of the filter section 10 are made different to give a difference in optical path length, the arm 24 of the filter section 10 is made.
Δ when the equivalent refractive index of is changed from n to n + Δn
λ _T10 can be expressed by the following equation (14). Δλ _T10 / λ ₀ = α · (Δn / n) (14) However, the branches 16 and 18 of the filter unit 10 and the arm 2
The refractive indices of 4 and 26 are equal to the refractive index n in a state where they are not electrically changed.

In this case, the transmission band a _{10 of the} filter unit ₁₀
The full width at half maximum Δλ _W10 can be expressed by the following equation (15). Δλ _W10 / λ = λ / (2 · L _omax10 · n) (15) Similarly, the geometric lengths of the arms 42 and 46 of the filter unit 12 are made different, and the optical path length is changed. When the difference is given, Δλ when the equivalent refractive index of the arm 42 of the filter unit 12 is changed from n to n + Δn
_T12 can be expressed by the following equation (16). Δλ _T12 / λ ₀ = β · (Δn / n) (16) However, the branches 36 and 38 of the filter unit 12 and the arm 4
The refractive indices of 2 and 46 are equal to the refractive index n in a state where they are not electrically changed.

In this case, the transmission band a _{12 of the} filter unit ₁₂
The full width at half maximum Δλ _W12 can be expressed by the following equation (17). Half-width of _{Δλ W12 / λ = λ / (} 2 · L omax12 · n) ... (17) transmission band A of the variable wavelength optical filter of this embodiment, the least common multiple of the wavelength interval b _10, b ₁₂ and b ₁₂ if,
It is equal to Δλ _W10 / λ ₀ expressed by the equation (11) or the equation (15), and the tuning width (the maximum movement width of the peak position of the transmission band A) is expressed by the equation (12) or the equation (16). Δλ
_{Equal to T12} / λ ₀ , and the number of channels is Δλ _T12 / Δ
λ _W10 = β · (2 · Δn · _Lomax10 ) / λ. According to this embodiment, the number of channels can be increased by β times that of the conventional one. When the maximum value of the electrode length of the filter unit 12 is made equal to L _{omax10 of the} filter unit 10, β = L
It becomes _omax10 / L _omax12 .

The present invention is not limited to the above-mentioned embodiments, and therefore, the shape, size, arrangement position, numerical conditions and the like of each component can be arbitrarily changed.

[0048]

As is apparent from the above description, according to the tunable wavelength optical filter of the present invention, it is possible to form a filter portion having a wide light transmission band width, particularly a half value width, and a narrow filter portion. Even if the peak position of the light transmission band a is slightly deviated from the wavelength of the light to be separated, the decrease in the output intensity of the light output from the wavelength tunable optical filter of the present invention can be suppressed. Therefore, the adjustment accuracy of the peak position of the transmission band a can be moderated to facilitate the tuning of the filter.

Further, the transmission band A of the variable wavelength optical filter of the present invention is a range in which the transmission bands a of the respective filter parts coincide with each other, so that the transmission band A can be narrowed. Moreover, since the wavelength interval B between the peak positions of the transmission band A of the variable wavelength optical filter of the present invention is the least common multiple of the wavelength interval b between the peak positions of the transmission band a of each filter section, the wavelength interval B can be widened. .

Therefore, according to the present invention, it is possible to provide a variable wavelength optical filter having a narrow transmission band per channel, a wide tuning width, and easy tuning.

[Brief description of drawings]

FIG. 1 is a plan view schematically showing the configuration of an embodiment of the present invention.

2A to 2C are diagrams for explaining the principle operation of the embodiment.

[Explanation of symbols]

 10, 12: Filter unit 14, 34: Mach-Zehnder type interferometer 16, 32: 1 × 2Y branch 18, 38: 2 × 1Y branch 20, 22, 40, 44: Connection waveguide 24, 26, 42, 46 : Arm 28, 40: Electrode 30, 50: Optical path length difference generator

Claims (1)

[Claims] 1. A plurality of filter sections connected in multiple stages, each filter section being formed by connecting a plurality of Mach-Zehnder interferors in multiple stages, each interferometer comprising a 1 × 2Y branch output and a 2 × 1Y branch. Two arms formed by coupling branch inputs through a connection waveguide, an electrode provided on one of the two arms to variably control the refractive index of the arm, and provided on the other of the two arms An optical path length difference generating section that gives a structural optical path length difference between these two arms is formed, and the electrode lengths L _e of the interferometers belonging to the same filter section form a geometric progression of a common ratio of 2. And the structural optical path length difference L of each interferometer belonging to the same filter unit.
The _o, electrode length ascending order of L _e varied so as to form a geometric progression of Oyakehi 2, each interferometer that belong to the same filter unit (electrode length L _e) /
A variable wavelength optical filter characterized in that a ratio represented by (structural optical path length difference L _o ) is made equal and the ratio is made different for each filter section.