JP2731307B2 - Wavelength selection element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical wavelength filter element for selectively separating light according to a wavelength, an optical amplifier for selectively amplifying light according to a wavelength, or a light to a wavelength. The present invention relates to a wavelength selection element suitable for forming an optical oscillator that selectively oscillates according to the wavelength.

[0002]

2. Description of the Related Art Conventionally, as a filter element for separating an optical signal of a specific wavelength λ ₀ from wavelength-multiplexed optical signals, for example, reference I: IEEE Common
ation Magazine, October 1989 p5
3 to 63 are disclosed. The filter elements disclosed in Document I are a: Fabry-Perot type, b: Mach-Zehnder type, c: mode conversion type, and d: Bragg reflection type.
Classified into species. Considering changing the transmission wavelength λ _{0 of the} filter from the design reference wavelength λ to λ + Δλ, a,
For the types b and d, the equation (1) in Table 1 holds for the wavelength change Δλ, and for the type c, the equation (2) for Table 1 holds for the wavelength change Δλ. Note that λ ₀ = λ + Δλ can be expressed.

In the equation, Δn is the amount of refractive index that can be electrically changed with respect to the waveguide provided in the filter element, n
Represents a refractive index of a waveguide provided in the filter element, and δn represents a refractive index difference between modes, for example, a refractive index difference between TM and TE modes.

Generally, the design reference wavelength λ is uniquely determined from the shape, dimensions, forming material and the like of the filter element components, and is a constant. However, among the c-types that use 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 is variably controlled. Can be. Therefore, the refractive index change Δ that is electrically variably controlled.
Although there is an upper limit for n, the variable range (tuning width) of the transmission wavelength λ ₀ is the largest for the type of c.

The half-width Δλ _FWHM of the light transmittance peak in each of the filters a, b, c and d is calculated by the equations (3), (4), (5) and (6) in Table 1, respectively. ) Expression. L in the formula is the element length (electrode length) of the filter element,
R represents the reflectance of the input / output end face of the filter element.

[0006] Since normally δn << n, the expression (3)
As can be understood from the equation (6), the half width Δλ _{FWHM in} the types a, b and d is very narrow, but the half width Δλ _{FWHM in} the type c is very wide.

Here, _assuming that the transmission bandwidth (half width) per channel of the filter element is Δλ _FWHM , the number of channels CH is given by equation (7) in Table 1 for the type a, and the number of channels CH in the types b and c. Equation (8) in Table 1 and the type of d can be expressed as in Equation (9) in Table 1. Δn _max in the equation represents the maximum Δn in a changeable range. However, in the case of type a, FSR (FreeS
Therefore, CH = π · R ^1/2 / (1−R) when the element is used alone because of the limitation of the spectral range.

[0008] Therefore, if Δn _max ≒ 0.01, Rａ0.9 in the case of type a becomes several tens of channels (potentially 80 channels if FSR is ignored) due to the restriction of FSR, and L ≒ 0.9 in the case of type b. 80 channels for ≒ 1 cm, 8 channels for c ≒ 1 mm and L =
Eight channels are set at 500 μm.

[0009]

The above-mentioned conventional a,
In filter elements of types b and d, even if the transmission bandwidth Δλ _FWHM per channel can be reduced, the tuning width (variable range of the transmission wavelength λ ₀ ) cannot be increased, so the number of channels (= tuning width / per channel) Transmission bandwidth)
Cannot be increased. In the case of the c-type filter element, the tuning width can be widened, but the transmission bandwidth Δλ _FWHM cannot be narrowed, so that the number of channels cannot be increased.

When considering the number of channels,
In the types a, b, and d, if the element length L is increased, the transmission bandwidth Δ
Although λ _FWHM can be narrowed and thus the number of channels can be increased, if the transmission bandwidth Δλ _FWHM is too narrow, the filter element becomes difficult to handle and becomes impractical. In the case of type c, the element length L
Is extremely long (for example, L = 1 m), the number of channels cannot be increased.

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

[0012]

In order to achieve this object, a wavelength selecting element according to the present invention is provided on a plurality of waveguides forming a directional coupler and on a waveguide forming an optical resonator. A light reflecting portion, and an electrode provided on the waveguide for variably controlling the equivalent refractive index of the waveguide, wherein the equivalent refractive indices of the plurality of waveguides are different without being electrically changed. It has a configuration.

In practicing the present invention, a semiconductor laser structure having a waveguide as an active layer may be provided.
In this case, the wavelength selecting element of the present invention can constitute an optical oscillator that selectively oscillates light according to the wavelength or an amplifier that amplifies light according to the wavelength.

In practicing the present invention, a semiconductor laser structure using a waveguide as an active layer may not be provided. In this case, with the wavelength selection element of the present invention,
An optical wavelength filter that separates light according to wavelength can be configured.

[0015]

When the inventor of the present application examined the transmission band I of light passing through the wavelength selection element of the present invention by numerical analysis, it was found that two transmission bands a and b having different properties were superimposed on each other. It turns out that I should consider it.

The peak position of the transmission band a has the property of moving when the equivalent refractive index difference between the waveguides is electrically changed. The wavelength range in which the peak position of the transmission band a can be moved is wide, but the half width of the transmission band a is wide. On the other hand, the peak position of the transmission band b has the property of moving when the equivalent refractive index of the waveguide is electrically changed. Transmission band b
Is narrow, but the wavelength range in which the transmission band b can be moved is narrow. For example, when the directional coupler is composed of two waveguides c and d, when the equivalent refractive index difference between these waveguides c and d is changed, the peak position of the transmission band a moves, and When the equivalent refractive index is changed, the peak position of the transmission band b moves. The transmission band I is a wavelength region where the transmission bands a and b coincide with each other. When the peak position of the transmission bands a and / or b is moved, the peak position of the transmission band I moves.

These transmission bands a and b appear with or without a semiconductor laser structure having a waveguide as an active layer. When the semiconductor laser structure is not provided, the transmission band I
Of light can be selectively separated. When a semiconductor laser structure is provided, light in the transmission band I can be selectively amplified and output, or light in the transmission band I can be selectively oscillated.

[0018]

Embodiments of the present invention will be described below with reference to the drawings. It should be noted that the drawings are only schematically shown to the extent that the present invention can be understood, and thus the present invention is not limited to the illustrated examples.

FIG. 1 is a plan view schematically showing the structure of the first embodiment of the present invention. The wavelength selection element of the first embodiment is an optical wavelength filter. The wavelength selection element of this embodiment includes a plurality of, for example, two waveguides 10, 1 constituting a directional coupler.
6 and a light reflecting portion 1 provided in a waveguide to constitute an optical resonator
2 and 14 and electrodes 18 and 20 for variably controlling the equivalent refractive index of the waveguide. Waveguide (first waveguide) 1
The equivalent refractive indices of 0 and the waveguide (second waveguide) 16 are different when these equivalent refractive indices are not electrically changed.

In this embodiment, for example, the widths of the first waveguide 10 and the second waveguide 16 are made different, or the materials of the first waveguide 10 and the second waveguide 16 are made different. Or the state where the equivalent refractive index of the first waveguide 10 and the second waveguide 16 is not electrically changed by adding a refractive index control substance to the first waveguide 10 and the second waveguide 16. The waveguides 10 and 16 have different equivalent refractive indices.

The waveguides 10 and 16 are arranged close to each other so as to cause light interaction, and are provided on the substrate 22. The waveguides 10 and 16 form a directional coupler. The first waveguide 10 is provided from one substrate end surface 22a to the other substrate end surface 22b, and the second waveguide 16 is located at a position X not extending from one substrate end surface 22a to the other substrate end surface 22b.
It is provided so as to extend to

The light reflecting portions 12 and 14 are high reflection films provided on the substrate end faces 22a and 22b.
The light reflecting portions 12 and 1 are provided on one end surface of the
4 is provided, and the light reflecting portion 12 is provided on one end face of the second waveguide 16. The first waveguide 10, the second waveguide 16, and the light reflectors 12 and 14 form an optical resonator. An input port 24 is provided at an end of the first waveguide 10 on the side where the light reflecting section 14 is provided, and an output port 26 is provided at an end of the second waveguide 16 on the side where the light reflecting section 12 is provided.

When light enters the input port 24 provided in the first waveguide 10, light having a wavelength in a transmission band I, which will be described later, exits from the output port 26 provided in the second waveguide 10, and light having a wavelength other than that. Light is applied to the substrate 2 from the end of the second waveguide 16 at the position X.
Emitted or absorbed during zero.

The substrate 22 is a ferroelectric crystal substrate or a compound semiconductor substrate. The waveguide structure and the electrode structure can have any suitable structures according to the substrate material. In the illustrated example, the electrodes 18 and 20 are provided on the first waveguide 10 and the second waveguide 16.

Next, the operation characteristics of the wavelength selection element of the first embodiment will be described. Wavelength λ output from output port 26
The light intensity (| output |) can be expressed as in Table 2 (10) using a matrix expression relating to the light transmission characteristics of the directional coupler. Where r is the reflectance of the light reflecting portion 12, 14, L ₁ is the length of the portion first waveguide 10 and second waveguide 16 for coupling to produce the interaction of light (see FIG. 1), L ₂ Is the resonator length (see FIG. 1), n is the refractive index of the substrate 22, and Δn is the waveguide 10 when the equivalent refractive index of the first waveguide 10 and the second waveguide 16 is not electrically changed. , And K represent the coupling coefficient between the first waveguide 10 and the second waveguide 16.

From the analysis based on the equation (10), the wavelength band (transmission band I) of the light passing through the wavelength selection element of this embodiment may be considered by superimposing two transmission bands a and b having different properties. You can see that.

FIGS. 2A to 2C are views for explaining the light transmission characteristics of the wavelength selection element of the first embodiment. FIG.
(A), (B) and (C) show the distribution of the transmission band a, transmission band b and transmission band I of the wavelength selection element, respectively. In these figures, the vertical axis represents the light intensity and the horizontal axis represents the wavelength. .

In equation (10), δ ₁ is used to simplify the equation.
Consider << K. In this embodiment, if δ ₁ is large, it becomes difficult for light having a wavelength other than the wavelength of the transmission band I described later to migrate from the first waveguide 10 to the second waveguide 16. Then δ ₁ <<
By setting K, light having a wavelength other than the transmission band I is easily transferred from the first waveguide 10 to the second waveguide 16. When the expression (29) in Table 7 is satisfied, the denominator in the expression (10) becomes a minimum with respect to 2 · k · L.
Appears. By substituting equation (29) into equation (10) and transforming, | output | of equation (30) in Table 7 is obtained. (3
From equation (0), a transmission band a as shown in FIG. 2A is obtained. The peak position of the transmission band a has the property of moving when the equivalent refractive index difference between the first waveguide 10 and the second waveguide 16 is electrically changed.

When δ ₁ << K, r in equation (10)
Is closer to 1, | output | approaches 0 when Cos (2 · k · L ₂ ) ≠ 1, and a large output is obtained in the transmission band b when Cos (2 · k · L ₂ ) = 1. The peak position of the transmission band b has the property of moving when the equivalent refractive index of the first waveguide 10 is electrically changed, and a plurality of peak positions of the transmission band b having such properties appear. The peak of the transmission band b is 2 · k · L ₂ = 2 · from Cos (2 · k · L ₂ ) = 1.
It occurs at a wavelength λ that satisfies m · π (where m represents a positive integer). The plurality of transmission bands b have a constant wavelength interval Δλ.
Appears in _FSR . The transmission band I of the first embodiment is a wavelength region where the transmission bands a and b coincide as shown in FIG. In this embodiment, there are a plurality of transmission bands a, and the plurality of transmission bands a are separated at a constant wavelength interval.
The separation interval of the transmission band a is the separation interval Δλ of the transmission band b.
_Wider than _FSR .

When the equivalent refractive indices of the first waveguide 10 and the second waveguide 16 are not electrically changed, the peak of the transmission band a occurs at a wavelength λ _O satisfying C ₂ = 1. C
_{When 2} = 1, Table 3 (11) holds, and therefore the wavelength λ _O
Can be expressed as in Table 3 (12) from Equation (11).

When the equivalent refractive index difference between the first waveguide 10 and the second waveguide 16 is electrically changed from Δn to Δn + δΔn, the wavelength at which the peak of the transmission band a occurs is λ.
_O changes to λ _O + Δλ _OT, and the peak position of the transmission band a moves by Δλ _OT . The shift amount Δλ _OT of the peak position of the transmission band a can be expressed as in Table 3 (13). The expression (14) is obtained from the expression (13).

The peak position of the transmission band b can be moved by electrically changing the equivalent refractive index of the first waveguide 10. However, the peak position of the transmission band b can be moved without moving the peak position of the transmission band a. In order to move only the peak position, the equivalent refractive index of the first waveguide 10 and the equivalent refractive index of the second waveguide 16 are made equal while the equivalent refractive index of the second waveguide 16 is made equal. The rate is changed electrically. Transmission band b
Can be considered in the same manner as in the case of moving the transmission band of the Fabry-Perot resonator type optical wavelength filter, and both the equivalent refractive indices of the first waveguide 10 and the second waveguide 16 are changed by δn. And the peak position of the transmission band b is Δ
Move by λ _FT . Movement amount Δ of peak position of transmission band b
λ _FT can be expressed as shown in Table 3 (15). Usually, δn ≒ δ
Since Δn and n> Δn, Δλ _OT > Δλ _FT , and therefore the wavelength range in which the peak position of the transmission band a can be shifted is wider than the wavelength range in which the peak position of the transmission band b can be shifted. .

Although not necessarily limited to this,
It is preferable that the peak position of the transmission band b and the peak position of the transmission band a occur at the same wavelength λ _O in a state where the equivalent refractive index of the first waveguide 10 and the second waveguide 16 is not electrically changed.

When the peak positions of the transmission bands a and / or b are moved, the peak position of the transmission band I is moved, so that the wavelength (transmission wavelength) of the light transmitted through the wavelength selection element changes. In order to widen the variable range (tuning width) of the light transmission wavelength, the shift amount Δλ _FT of the peak position of the transmission band b when Δn is changed to the maximum is equal to or larger than Δλ _FSR so that the shift amount Δλ _FT Can be varied from zero to at least Δλ _FSR . Table 3 (16)
By configuring the wavelength selection element so as to satisfy the expression, the moving amount Δλ _FT when δn is changed to the maximum is Δλ
Can be equal to _FSR . Here, δn _max represents δn when the maximum change is made.

Move the moving amount Δλ _FT from zero to at least Δλ _FSR
By being able to change
The peak positions of the transmission bands a and b can be matched over the entire range of the variable range Δλ _OTmax of the peak position of the transmission band a, so that the variable range of the transmission wavelength of light is δΔn _max / Δn (= Δλ _OTmax ). It can be. Here, δΔn _max represents Δn when the maximum change is made.

Since the half width of the transmission band b is smaller than the half width of the transmission band a, the half width Δλ _W of the transmission band I is equal to the half width of the transmission band b. Table 3 (17) shows the half width Δλ _W
It can be expressed like an expression. Therefore, the number of channels (=
Δλ _OTmax / Δλ _W ) can be expressed as shown in Table 3 (18).
It can be seen that the number of channels in this embodiment is n / Δn times the number of channels of a normal Fabry-Perot optical wavelength filter.

When the peaks of the transmission bands a and b coincide with each other, as shown in FIG.
Contains a plurality of transmission bands b, and therefore, in transmission band I, a peak of transmission band I (main peak MP) and other peaks (sub-peaks SP) appear. Considering the main peak MP as the wavelength of the light to be separated, it is preferable to make the ratio of the light intensity of the main peak MP and the light intensity of the sub-peak SP sufficiently large. Therefore, the light intensity X of the sub-peak SP separated from the main peak MP by Δλ _FSR is considered. By variation .DELTA..delta ₁ minute [delta] ₁ in the vicinity of S ₂ = 0 for considering the light intensity X a (10) to expand, further developed (10) [delta] ₁ << K and r ≒ 1 and approximation in equation I do. Then, Table 4 (1
9) is obtained. Here, using the expression (20) in Table 4, (1
By transforming equation (9), equations (21) and (22) in Table 4 are obtained. Therefore, for example, X = 0.032 (−15d)
B) and Δn / n = 0.01, from equation (21), r
= 0.999, and r = 0.99 and Δn / n =
If 0.02, then X = 0.025 from equation (22). By setting the magnitudes of r and Δn / n to any suitable values, the sub-peak SP can be sufficiently reduced for practical use.

Next, the coupling coefficient K will be considered. δ ₁ << K
In this case, it is sufficient to satisfy the expression (23) in Table 4 (where α is a constant satisfying α >> 1). Therefore, equation (24) is obtained from equations (16) and (23). If M = (element length) / (coupling length), then from equation (24), M = α ·
By determining the coupling coefficient K such that (λ _O / Δλ _OTmax ) is satisfied, δ ₁ << K can be satisfied.

In this embodiment, the first waveguide 10 and the second waveguide 16 are not provided with the function of amplifying light. However, in this embodiment, a Fabry-Perot type semiconductor laser structure using the first waveguide 10 as an active layer is used. Is provided, and if the light amplifying action is given to the first waveguide 10, the wavelength selection element of this embodiment can be used as an optical amplifier or a variable wavelength optical oscillator. When used as an optical amplifier, light of a specific wavelength can be amplified and selectively output, and the loss of light in the waveguide can be compensated to prevent characteristic deterioration due to the loss of light. When used as a variable wavelength optical oscillator, light of a specific wavelength can be selectively oscillated.

Further, in the above-described embodiment, the first waveguide 1
Although the 0 and second waveguides 16 are provided in parallel in a plane, the first waveguide 10 and the second waveguide 16 may be provided three-dimensionally. When setting up three-dimensionally,
The first waveguide 10, the intermediate layer, and the second waveguide 16 may be sequentially stacked, and these waveguides 10 and 16 may be coupled via the intermediate layer so as to cause light interaction.

FIG. 3 is a plan view schematically showing the structure of the second embodiment of the present invention. The components corresponding to the components of the first embodiment are denoted by the same reference numerals. In the following description, points different from the first embodiment will be described, and detailed description of the same points as the first embodiment will be omitted.

The wavelength selecting element of the second embodiment is a light reflecting section 2
8, 30, 32 are provided. The light reflecting portions 28 to 32 are Bragg reflectors or diffraction gratings. The light reflecting portions 28 and 30 are provided at one and the other end of the first waveguide 10, and the light reflecting portion 32 is Provided at one end. Although the light reflecting portions 28 to 32 are provided on the upper surface of the waveguide in the illustrated example, the light reflecting portions 28 to 32 may be provided on the bottom surface or the side surface of the waveguide.

In the wavelength selecting element of the second embodiment, a Bragg reflection type semiconductor laser structure having the entire first waveguide 10 as an active layer is provided, or
If a distributed feedback type semiconductor laser structure using these waveguide portions as active layers is provided in each of the waveguide portions provided with 8, 30, and 32, the wavelength selection element of this embodiment can be used as an optical resonator. Alternatively, it can be used as an optical amplifier.

FIG. 4 is a plan view schematically showing the structure of the third embodiment of the present invention. The components corresponding to the components of the first embodiment are denoted by the same reference numerals. In the following description, points different from the first embodiment will be described, and detailed description of the same points as the first embodiment will be omitted.

In the first embodiment, as shown in FIG. 1, the second waveguide 16 is moved from one substrate end face 22a to the other substrate end face 2a.
The light reflecting portion 12 is provided at one end of the second waveguide 16 provided up to the position X which does not reach 2b, but in the third embodiment, as shown in FIG. 4, one end and the other end of the second waveguide 16 are provided. The second waveguide 16 is provided at the center of the substrate 22 so that the portion does not contact the one and the other substrate end surfaces 22a and 22b. Therefore, in the third embodiment, the light reflection portion 12 is not provided on one and the other end surfaces of the second waveguide 16, and light having a wavelength other than the transmission band I is emitted from the one and the other end portions of the second waveguide 16. From each of them is radiated or absorbed into the substrate. Symbols Y and X in FIG. 3 indicate one and the other end face positions of the second waveguide 16.

The input port 24 is provided at the end of the first waveguide 10 on the side where the light reflecting portion 14 is provided, and the output port 26 is provided at the end of the first waveguide 10 on the side where the light reflecting portion 12 is provided. .
The first waveguide 10 and the light reflecting portions 12 and 14 constitute an optical resonator.

The | output | of the wavelength selection element of the third embodiment can be expressed as shown in Table 5 (25). The operation characteristic of the third embodiment obtained from the equation (25) is the same as that of the first embodiment, but the relational expression obtained with respect to the light intensity X in the third embodiment is the equation (19) of the first embodiment. , (21) and (22), L ₁ → L _1/2 .

FIG. 5 is a plan view schematically showing the configuration of the fourth embodiment of the present invention. The components corresponding to the components of the third embodiment are denoted by the same reference numerals. In the following description, points different from the third embodiment will be described, and detailed description of the same points as the third embodiment will be omitted.

The wavelength selecting element of the fourth embodiment is a light reflecting section 34.
And 36. The light reflecting portions 34 and 36 are Bragg reflectors or diffraction gratings, and the light reflecting portions 34 and 36
Are provided at one end and the other end of the first waveguide 10. The rest is the same as the third embodiment.

FIG. 6 is a plan view schematically showing the configuration of the fifth embodiment of the present invention. The components corresponding to the components of the third embodiment are denoted by the same reference numerals. In the following description, points different from the third embodiment will be described, and detailed description of the same points as the third embodiment will be omitted.

The wavelength selecting element of the fifth embodiment is the second waveguide 1
6 has waveguides 38 and 40 provided at one end and the other end. For example, the waveguides 38 and 40 are linear waveguides having a waveguide width equal to the waveguide width of the first waveguide 10.
A waveguide 38 coupled to one end of the second waveguide 16;
The waveguide 38 is provided on the substrate 22 so as to extend from one end of the second waveguide 16 to a position not reaching the one substrate end face 22a. Similarly, the waveguide 40 is coupled to the other end of the second waveguide 16, and the waveguide 40 is extended from the other end of the second waveguide 16 to a position not reaching the other substrate end face 22b. 22. Electrodes 48 and 50 are provided on the waveguides 38 and 40, respectively. Even when the equivalent refractive index of the first waveguide 10 is electrically changed or not changed, the equivalent refractive index of the first waveguide 10 is made equal to the equivalent refractive index of the waveguides 38 and 40. , Electrically control the equivalent refractive index of the waveguides 38 and 40 via the electrodes 48 and 50.

In the first to fourth embodiments, by setting δ ₁ << K, light having a wavelength other than the transmission band I can be transmitted to the first waveguide 10.
From the first waveguide 16 to the second waveguide 16.
However, in the fifth embodiment, the waveguides 38 and 40 having the same equivalent refractive index as the first waveguide 10 are provided in the second waveguide 16, so that light having a wavelength other than the transmission band I is not transmitted even if δ ₁ << K. The transition from one waveguide 10 to the second waveguide 16 is facilitated.

FIG. 7 is a plan view schematically showing the structure of the sixth embodiment of the present invention. The components corresponding to the components of the first embodiment are denoted by the same reference numerals. In the following description, points different from the first embodiment will be described, and detailed description of the same points as the first embodiment will be omitted.

The wavelength selecting element of the sixth embodiment is the second waveguide 1
6 has a waveguide 42 provided at the other end. For example, the waveguide 42 is a straight waveguide having a waveguide width equal to the waveguide width of the first waveguide 10. The waveguide 42 is connected to the second waveguide 1
6, and the waveguide 42 is provided on the substrate 22 so as to extend from the other end of the second waveguide 16 to a position not reaching the other substrate end face 22b. Waveguide 42
An electrode 52 is provided thereon. Even when the equivalent refractive index of the first waveguide 10 is electrically changed or not changed, the guiding is performed so that the equivalent refractive index of the first waveguide 10 and the equivalent refractive index of the waveguide 42 become equal. The equivalent refractive index of the waveguide 42 is
2 is electrically controlled.

In the sixth embodiment, the waveguide 42 having the same equivalent refractive index as the first waveguide 10 is provided in the second waveguide 16.
Even if δ ₁ << K is not satisfied, light having a wavelength other than the transmission band I easily shifts from the first waveguide 10 to the second waveguide 16.

FIG. 8 is a plan view schematically showing the structure of the seventh embodiment of the present invention. The components corresponding to the components of the fifth embodiment are denoted by the same reference numerals. In the following description, points different from the fifth embodiment will be described, and detailed description of the same points as the fifth embodiment will be omitted.

The wavelength selecting element of the seventh embodiment has a low refractive index portion 4.
4 is provided. The low-refractive-index portion 44 is a portion having a lower refractive index than the substrate 22. For example, a groove is formed in the substrate 22, and air filled in the groove is used as the low-refractive-index portion 44. May be used as the low refractive index portion 44. The low refractive index portion 44 is provided between the first waveguide 10 and the second waveguide 16 so as to extend from the position X to the position Y.

Transmission band I emitted from output port 26
The reflectance r of the light reflecting portions 12 and 14 required to sufficiently reduce the amount of light having wavelengths other than the above can be reduced. In order to sufficiently reduce the amount of light having a wavelength other than the transmission band I emitted from the output port 26, it is preferable that r 第一 1 in the above-described first to sixth embodiments, but the seventh embodiment Then r ≒
Even if it is not 1, the amount of light having a wavelength other than the transmission band I emitted from the output port 26 can be reduced.

The | output | of the wavelength selection element of the seventh embodiment can be expressed as shown in Table 6 (26). In particular, Cos ² (L _c
When K) = Sin ² (L _c · K) = １ ／, | output | can be expressed as shown in Table 6 (27). Here L _c represents the length of the waveguide 38, 40 (see FIG. 8).

In equation (26), Cos (2 · k · L ₁
+ 2 · δ ₁ · L ₁ ) = − 1, the transmission band a is obtained. The peak of the transmission band b is Sin (δ ₁ · L ₁ ) = 1.
That is, a wavelength λ satisfying δ ₁ · L ₁ = (π / 2) · m
Occurs in Therefore, also in the seventh embodiment, equation (14) is established in the same manner as in the first embodiment.

FIG. 9 is a plan view schematically showing the configuration of the eighth embodiment of the present invention. The components corresponding to the components of the first embodiment are denoted by the same reference numerals. In the following description, points different from the first embodiment will be described, and detailed description of the same points as the first embodiment will be omitted.

In the eighth embodiment, the first waveguide 10 is provided between the substrate end faces 22a and 22b so as to extend from one substrate end face 22a to a position X which does not reach the other substrate end face 22b. Further, the second waveguide 16 is provided between the substrate end surfaces 22a and 22b so as to extend from the other substrate end surface 22b to a position Y not reaching the one substrate end surface 22a. Input port 2
4 to the other end of the second waveguide 16 and the output port 26
Is provided at one end of the first waveguide 10. First waveguide 1
The equivalent refractive indices of the zero and second waveguides 16 are made different to provide a propagation constant difference Δβ between the waveguides 10 and 16.

Further, a light reflecting portion 12 is provided at one end of the first waveguide 10, and a light reflecting portion 14 is provided at the other end of the second waveguide 16. The first waveguide 10, the second waveguide 16, and the light reflecting portions 12, 14 constitute an optical resonator. The first waveguide 10 and the second waveguide 16 are so close as to cause light interaction between the positions X and Y, and the first waveguide portion and / or the second waveguide between the positions X and Y. A grating 46 for phase matching is provided in the portion. Then, the electrode 18 is provided on the first waveguide 10 and the grating 46. The grating 46 matches the phase of light guided through the first waveguide 10 with the phase of light guided through the second waveguide 16, and achieves light transfer between the waveguides 10 and 16. is there.

The wavelength selecting element of the eighth embodiment transmits light having a wavelength λ at a transmittance represented by the following equation (28). (28)
Where Δβ is the propagation constant difference between the first waveguide 10 and the second waveguide 16 due to the equivalent refractive index difference between these waveguides,
Λ represents the period of the grating 46.

As can be understood from equation (28), the wavelength selection element of the eighth embodiment transmits only light having a wavelength satisfying δ ₁ = 0. The envelope representing the transmission band a in FIG.
The denominator of the expression representing the transmittance of the expression (28) is obtained under the condition that the change in kL is minimized, and (1-r) ² · B ² /
(1−r · B ² ) ² . In the other embodiment described above, a plurality of transmission bands a appear periodically, but in the eighth embodiment, the number of transmission bands a is one and does not appear periodically.

FIG. 10 is a plan view schematically showing the configuration of the ninth embodiment of the present invention. The components corresponding to the components of the fifth embodiment are denoted by the same reference numerals.
In the following description, points different from the fifth embodiment will be described, and detailed description of the same points as the fifth embodiment will be omitted.

In the ninth embodiment, the waveguide 38 coupled to the second waveguide 16 is provided from the position Y to one of the substrate end faces 22a, and the first waveguide 10 is located at a position not extending from the substrate end face 22b to the substrate end face 22a. Provide up to Z. The light reflecting portion 12 is provided at the end of the waveguide 38 on the substrate end surface 22a side, and the light reflecting portion 14 is provided at the substrate end surface 22b of the first waveguide 10.
On the side end. The light reflecting portion 12 is not provided at the end of the first waveguide 10 on the substrate end surface 22a side. First waveguide 10,
The second waveguide 16, the waveguide 38, and the light reflection parts 12, 14 constitute an optical resonator. The output port 26 is provided at the end of the waveguide 38 on the substrate end surface 22a side.

FIG. 11 is a plan view schematically showing the configuration of the tenth embodiment of the present invention. The components corresponding to the components of the seventh embodiment are denoted by the same reference numerals.
In the following description, points different from the seventh embodiment will be described, and detailed description of the same points as the seventh embodiment will be omitted.

In the tenth embodiment, the waveguide 38 coupled to the second waveguide 16 is provided from the position Y to one of the substrate end faces 22a, and the first waveguide 10 is positioned not to reach the substrate end face 22b from the substrate end face 22b. Provide up to Z. The light reflecting portion 12 is provided at the end of the waveguide 38 on the substrate end surface 22a side, and the light reflecting portion 14 is provided at the substrate end surface 22b of the first waveguide 10.
On the side end. The light reflecting portion 12 is not provided at the end of the first waveguide 10 on the substrate end surface 22a side. First waveguide 10,
The second waveguide 16, the waveguide 38, and the light reflection parts 12, 14 constitute an optical resonator. The output port 26 is provided at the end of the waveguide 38 on the substrate end surface 22a side.

The present invention is not limited only to the above-described embodiment, and accordingly, the configuration, shape, arrangement position, number of arrangements, and other conditions of each component can be arbitrarily and suitably changed.

For example, the first waveguide portion and the second waveguide portion in a region where the first and second waveguides are close to each other so as to cause light interaction may be a straight waveguide or a curved waveguide. Further, the number of waveguides constituting the directional coupler can be two or more.

[0072]

[Table 1]

[0073]

[Table 2]

[0074]

[Table 3]

[0075]

[Table 4]

[0076]

[Table 5]

[0077]

[Table 6]

[0078]

[Table 7]

[0079]

As is apparent from the above description, according to the wavelength selecting element of the present invention, the transmission type of the wavelength selecting element of the present invention is obtained by superposing two types of transmission bands a and b having different properties. It was found that the band I should be considered.

The peak position of the transmission band a has the property of moving when the equivalent refractive index difference between the first and second waveguides is electrically changed. The wavelength range in which the peak position of the transmission band a can be moved is wide, but the half width of the transmission band a is wide. On the other hand, the peak position of the transmission band b has the property of moving when the equivalent refractive index of the first waveguide is electrically changed. Although the half width of the transmission band b is narrow, the wavelength range in which the transmission band b can be moved is narrow. The transmission band I is a wavelength region where these transmission bands a and b coincide with each other. When the peak position of the transmission bands a and / or b is moved, the transmission band I
The peak position moves. Since the peak position of the transmission band I becomes the transmission wavelength of the wavelength selection element of the present invention, and the range in which the peak position of the transmission band a can be varied is wide, the transmission wavelength of the wavelength selection element of the present invention can be changed over a wide range. . Further, the half width of the transmission band I is equal to the half width of the transmission band b and accordingly becomes narrow. The transmission wavelength of the wavelength selection element of the present invention can be changed over a wide range,
Moreover, since the half bandwidth of the transmission band I is narrow, the number of channels can be increased.

[Brief description of the drawings]

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

FIGS. 2A to 2C are diagrams for explaining the light transmission characteristics of the first embodiment.

FIG. 3 is a plan view schematically showing a configuration of a second embodiment of the present invention.

FIG. 4 is a plan view schematically showing a configuration of a third embodiment of the present invention.

FIG. 5 is a plan view schematically showing a configuration of a fourth embodiment of the present invention.

FIG. 6 is a plan view schematically showing a configuration of a fifth embodiment of the present invention.

FIG. 7 is a plan view schematically showing a configuration of a sixth embodiment of the present invention.

FIG. 8 is a plan view schematically showing a configuration of a seventh embodiment of the present invention.

FIG. 9 is a plan view schematically showing a configuration of an eighth embodiment of the present invention.

FIG. 10 is a plan view schematically showing a configuration of a ninth embodiment of the present invention.

FIG. 11 is a plan view schematically showing a configuration of a tenth embodiment of the present invention.

[Explanation of symbols]

 10, 16: Waveguide 12, 14: Light reflecting portion 18, 20: Electrode

Continuation of the front page (56) References JP-A-3-10228 (JP, A) JP-A-64-25587 (JP, A) JP-A-61-42189 (JP, A) JP-A-6-130237 (JP, A) , A) APPL. PHYS. LETT. , VOL. 49 NO. 1 PP. 10-12 (July 1986) K. KOROKY ET. AL. , "INTEGRATE D-OPTIC, NARROW-LIN EWIDTH LASER"

Claims (2)

(57) [Claims] 1. A plurality of waveguides forming a directional coupler, a light reflecting portion provided on the waveguide for forming an optical resonator, and an equivalent refraction of the waveguide provided on the waveguide. An electrode for variably controlling the indices, wherein the equivalent refractive indices of the plurality of waveguides are different without being electrically changed. 2. The wavelength selection device according to claim 1, further comprising a semiconductor laser structure using said waveguide as an active layer.