Optical filter and wavelength locker device

Abstract

To provide an optical filter which offers both temperature independence of an operating wavelength and high integration of optical circuitry, and to provide a wavelength locker device.SOLUTION: An optical filter disclosed herein comprises a core portion covered with a cladding and configured to extend in one direction to enable propagation of light in multiple waveguide modes, and satellite portions covered with the cladding and configured to extend in the one direction apart from the core portion and to have a higher effective refractive index than that of the cladding, the satellite portions being smaller than a minimum size of an optical material capable of propagating light in a single mode.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to optical filters and wavelength locker devices. [Background technology]

[0002] It is known that the refractive index of the optical material constituting an optical device changes with temperature. For example, in an optical filter, which is one example of an optical device, the refractive index changes due to a change in temperature of the optical circuit, which may result in a change in the operating wavelength of the optical filter.

[0003] For example, a known configuration is to provide a temperature control mechanism using a Peltier element or the like next to an optical filter to control the temperature of the optical circuit at a constant level. However, such a configuration requires costs for the introduction of the temperature control mechanism, and requires a lot of effort and power consumption to strictly control the temperature using the temperature control mechanism.

[0004] Under these circumstances, there is a demand for an optical filter with an extremely low temperature dependency of the operating wavelength.For example, Patent Document 1 discloses an optical filter that includes two waveguides having a region where the first and second cores are close to each other and optically coupled, and that reduces the temperature dependency of the center wavelength by providing a periodic structure for forming periodic perturbations in the refractive index in the optically coupled region. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2006-330104 A Summary of the Invention [Problem to be solved by the invention]

[0006] In the technology disclosed in Patent Document 1, two independent cores are optically coupled via a cladding. The intensity of the evanescent light that seeps from the core into the cladding attenuates exponentially the farther it is from the core. Therefore, in the technology disclosed in Patent Document 1, the optical coupling strength between the two cores is relatively weak.

[0007] If the coupling strength is weak, the size of the periodic structure for forming the periodic perturbation of the refractive index becomes large, so that in the technology disclosed in Patent Document 1, the device length of the optical filter becomes relatively long, making it difficult to achieve high integration.

[0008] An object of the present disclosure is to provide an optical filter and a wavelength locker device that can achieve both temperature independence of the operating wavelength and high integration of the optical circuit. [Means for solving the problem]

[0009] The optical filter of the present disclosure comprises a core portion covered by a cladding portion, extending in one direction, and capable of propagating light in a plurality of waveguide modes, and a satellite portion covered by the cladding portion, extending away from the core portion in the one direction, having an effective refractive index higher than that of the cladding portion, and being smaller than the minimum size required for an optical material capable of propagating light in a single mode.

[0010] The optical filter of the present disclosure comprises a core portion covered by a cladding portion, extending in one direction, and having a plurality of waveguide modes, and a satellite portion covered by the cladding portion, extending away from the core portion in the one direction, having an effective refractive index higher than that of the cladding portion, and having no waveguide modes.

[0011] The wavelength locker device of the present disclosure comprises a first optical power monitor that measures the optical intensity of at least a portion of the laser output from a laser light source, a second optical power monitor that measures the optical intensity of at least a portion of the laser output that has passed through the above-mentioned optical filter, and a temperature controller that adjusts the temperature of the laser light source so that the difference between the output value of the first optical power monitor and the output value of the second optical power monitor becomes zero. Effect of the Invention

[0012] According to the present disclosure, it is possible to achieve both temperature independence of the operating wavelength and high integration of the optical circuit. [Brief description of the drawings]

[0013] [Figure 1] FIG. 1 is a schematic diagram showing an example of the structure of an optical filter according to a first embodiment; [Diagram 2] Partially enlarged view to explain the periodic structure of the satellite part [Diagram 3] FIG. 1 is a diagram showing characteristics of an optical filter according to a first embodiment; [Figure 4] Diagram to explain Grating-Assisted Coupler [Diagram 5] Diagram to explain Grating-Assisted Coupler [Figure 6] FIG. 1 is a diagram for explaining an example of an optical filter according to a first embodiment; [Figure 7] FIG. 1 is a diagram for explaining the range in which the temperature independence of the optical filter is achieved when the width w of the core portion and the distance wg between the core portion and the satellite portion are changed. [Figure 8] FIG. 1 is a diagram for explaining selectable elements of each optical filter structure for achieving temperature independence. [Figure 9] FIG. 1 is a diagram showing a first modified example of an optical filter utilizing the EO effect. [Figure 10] FIG. 1 shows a second modified example of an optical filter utilizing the EO effect. [Figure 11] FIG. 13 shows a third modified example of an optical filter utilizing the TO effect. [Figure 12] FIG. 13 is a diagram showing a fourth modified example of an optical filter utilizing a mechanical effect. [Figure 13] FIG. 5 is a diagram showing a fifth modified example of an optical filter utilizing an optical nonlinear effect. [Figure 14] FIG. 13 is a diagram showing an eighth modified example of an optical filter that allows refractive index adjustment by ion implantation. [Figure 15] FIG. 13 is a diagram for explaining the structure of an optical filter according to a second embodiment; [Figure 16] FIG. 13 is a diagram showing characteristics of an optical filter according to a second embodiment; [Figure 17] FIG. 13 is a diagram for explaining the structure of an optical filter according to a third embodiment; [Figure 18] A top view of an example of a Mach-Zehnder interferometer in which the incoming light is split into two arms and then recombined. [Figure 19] FIG. 13 is a diagram for explaining the principle by which an optical filter according to a third embodiment behaves as an AZMI; [Figure 20] FIG. 1 is a diagram showing an example of the configuration of a wavelength locker device using an optical filter according to the present disclosure; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. Elements common to multiple drawings will be given the same reference numerals, and detailed descriptions of those elements will not be repeated. However, descriptions that are more detailed than necessary, such as detailed descriptions of already well-known matters and duplicate descriptions of substantially identical configurations, may be omitted.

[0015] <Summary> The present disclosure provides an optical filter capable of achieving both temperature independence of the operating wavelength and high integration of the optical circuit, and a wavelength locker device using the optical filter. The optical filter of the present disclosure can be applied to both a single-peak filter having a characteristic of transmitting or removing a single wavelength, and a periodic filter (periodic filter) having a periodic transmission characteristic.

[0016] In the first embodiment, an example in which the optical filter of the present disclosure is applied to a single-peaked filter, particularly a notch filter that blocks light of a specific wavelength, is described. In the second embodiment, an example in which the optical filter of the present disclosure is applied to a single-peaked filter, particularly a bandpass filter that transmits light of a specific wavelength, is described. In the third embodiment, an example in which the optical filter of the present disclosure is applied to a periodic filter having a property that transmittance varies periodically with respect to wavelength is described.

[0017] Moreover, a wavelength locker device can be provided using the optical filter of the present disclosure. In the fourth embodiment, an example of a wavelength locker device using the optical filter of the present disclosure will be described.

[0018] <First embodiment> An example in which the optical filter of the present disclosure is applied to a notch filter, which is an example of a single-peak filter, will be described below.

[0019] FIG. 1 is a schematic diagram showing an example of the structure of an optical filter 10 according to a first embodiment of the present disclosure. FIG. 1A is a schematic diagram of the optical filter 10 viewed from above. FIG. 1B is a schematic cross-sectional diagram taken along line AA in FIG. 1A. In FIG. 1A and FIG. 1B, the shape and size of each component are shown diagrammatically and are not accurate. The optical filter 10 shown in FIG. 1A and FIG. 1B is assumed to be installed on a substrate.

[0020] In FIG. 1, the vertically upward direction is indicated as the y direction, the longitudinal direction of the optical filter 10 is indicated as the z direction, and the width direction of the optical filter 10 is indicated as the x direction.

[0021] The optical filter 10 includes an input portion 11, a tapered portion 12, a core portion 13, a tapered portion 14, an output portion 15, a cladding portion 16, and a satellite portion 17. Although a channel-type waveguide structure is illustrated in Fig. 1B, the present invention is not limited to this, and the optical filter 10 may have, for example, a rib-type structure.

[0022] Light is incident on the incident portion 11 from the outside. The light incident on the incident portion 11 is incident on the core portion 13 via the tapered portion 12. The tapered portion 12 has a tapered shape that widens from the connection portion with the incident portion 11 toward the connection portion with the core portion 13. The core portion 13 extends along one direction. In the present disclosure, the one direction is the longitudinal direction of the optical filter 10 (the z direction shown in FIG. 1 ).

[0023] Tapered portion 14 is connected to the terminal end of core portion 13. Tapered portion 14 has a tapered shape that narrows from the connection portion with core portion 13 toward the connection portion with emission portion 15.

[0024] The incident portion 11, the tapered portion 12, the core portion 13, the tapered portion 14, and the exit portion 15 are covered with the cladding portion 16. The incident portion 11, the tapered portion 12, the core portion 13, the tapered portion 14, and the exit portion 15 are formed of the same material. The material forming the incident portion 11, the tapered portion 12, the core portion 13, the tapered portion 14, and the exit portion 15 has a higher refractive index than the material forming the cladding portion 16. In the optical filter 10, the incident portion 11, the tapered portion 12, the core portion 13, the tapered portion 14, the exit portion 15, and the cladding portion 16 form an optical waveguide.

[0025] Examples of materials for the input portion 11, the tapered portion 12, the core portion 13, the tapered portion 14, and the output portion 15 include high refractive index materials such as silicon (Si), silicon nitride (SiN), III-V semiconductors (GaAs, InP, InGaAs, InGaAsP), and ferroelectrics (LiNbO3, BBO, KTP). Examples of materials for the cladding portion 16 include silicon dioxide (SiO2), indium phosphide (InP), and air.

[0026] The input portion 11, the output portion 15 and the cladding portion 16 form a single mode waveguide. The core portion 13 and the cladding portion 16 form a multimode waveguide. The width and height of the input portion 11 and the output portion 15 are set to a width and height that can confine only the light of the zeroth mode (fundamental mode). The width and height of the core portion 13 are set to a width and height that can confine light of a plurality of waveguide modes and can achieve temperature independence of the filter characteristics of the optical filter 10, in other words, temperature independence of the operating wavelength of the multimode waveguide, as described later.

[0027] In the following embodiments, the multimode waveguide formed by the core 13 and the cladding 16 will be described as a waveguide capable of propagating (confining) the zeroth mode and the first mode. However, the present disclosure is not limited to this, and the multimode waveguide formed by the core and the cladding may be capable of confining (propagating) light of second or higher modes in addition to light of the fundamental mode and the first mode.

[0028] Satellite portions 17 are arranged on both sides of core portion 13 in the width direction (x direction shown in FIG. 1) and substantially parallel to core portion 13. Satellite portions 17 are covered with cladding portion 16. Core portion 13 and satellite portion 17 are arranged apart from each other with cladding portion 16 interposed therebetween.

[0029] The width and height of the satellite portion 17 are set smaller than the width and height that can confine the fundamental mode light of the light incident on the optical filter 10. This allows the satellite portion 17 to have a structure that does not guide light. In other words, the satellite portion 17 does not have a guided mode (eigenmode).

[0030] In the optical filter 10, the satellite portion 17 is configured not to have a waveguide mode for the following reason. The satellite portion 17 reflects the light component that oozes out from the core portion 13 to the cladding portion 16 when light propagates through the multimode waveguide, and has a role of adjusting the effective refractive index of the multimode waveguide. If the satellite portion 17 had a waveguide function, energy transfer would occur between the satellite portion 17 and the core portion 13, which is not desirable.

[0031] The satellite portion 17 is arranged such that its structure changes at a constant period. In the example shown in FIG. 1A, the satellite portion 17 has a zigzag structure in which a region 171 relatively close to the core portion 13 and a region 172 relatively far from the core portion 13 in the width direction (x direction in FIG. 1A) of the optical filter 10 are arranged in a zigzag along the longitudinal direction. In the following description, the period of the periodic structure of the satellite portion 17 (the total length in the longitudinal direction of the region 171 relatively close to the core portion 13 and the region 172 relatively far from the core portion 13) is denoted as Λ.

[0032] FIG. 2 is a partially enlarged view for explaining the periodic structure of the satellite portion 17. As shown in FIG. 2, the distances from the core portion 13 to the satellite portions 17 arranged on both sides in the width direction of the core portion 13 are different in the first half and the second half of one period along the longitudinal direction.

[0033] In the example shown in FIG. 2, there are two types of distances d1 and d2 as the distances from the core portion 13 to the satellite portion 17. In the example shown in FIG. 2, the distance from the core portion 13 to one satellite portion 17 (region 171) is indicated by d1, and the distance from the core portion 13 to the other satellite portion 17 (region 172) is indicated by d2. In the example shown in FIG. 2, d1 < d2, but the present disclosure is not limited to this.

[0034] Such a periodic structure allows satellite 17 to change the guided mode of light propagating through the multimode waveguide formed by core 13 and cladding 16. The principle by which satellite 17 changes the guided mode of light propagating through the multimode waveguide will be described later.

[0035] 2, satellite portion 17 has regions 171 close to core portion 13 and regions 172 far from core portion 13 arranged alternately along the longitudinal direction, but the present disclosure is not limited to this. For example, satellite portion 17 of the present disclosure may have a serpentine shape such that the distance from the core portion changes along the longitudinal direction.

[0036] The period Λ is expressed by the following equation (1).

number

[0037] The propagation constant is determined by the effective refractive index of each guided mode. If the effective refractive index in the 0th mode is n0 and the effective refractive index in the 1st mode is n1, the propagation constants β0 and β1 are expressed by equation (2).

number

[0038] As shown in formula (2), the propagation constant is determined by the wavelength of light incident on the optical filter 10. Therefore, the wavelength of light handled by the optical filter 10 needs to be set in advance, and the size of the periodic structure of the satellite portion 17 in the optical filter 10 is determined based on the wavelength set in advance. In addition, the size (width and height) and length of the core portion 13 (i.e., the length of the multimode waveguide formed by the core portion 13 and the cladding portion 16) are set to predetermined values, and the method of setting these values ​​will be described later.

[0039] The behavior of the optical filter 10 having such a structure when a specific light is incident on it will be described below. The light input to the incident portion 11 is transferred in the tapered portion 12 to the zeroth mode of the multimode waveguide formed by the core portion 13 and the cladding portion 16.

[0040] Due to the periodic structure of the satellite portions 17, the light of the target wavelength is modulated as it travels inside the core portion 13, and is shifted to the first mode just before the end portion of the core portion 13. The tapered portion 14 is configured to radiate (scatter) the light in the first mode, and no light is output to the output portion 15. This causes the optical filter 10 to operate as a notch filter that removes only the target wavelength, as shown in Fig. 3. Fig. 3 is a diagram showing the characteristics of the optical filter 10 of the first embodiment.

[0041] <Principle> The principle by which light propagating in a multimode waveguide formed by the core 13 and the cladding 16 is shifted from the zeroth-order mode to the first-order mode due to the periodic structure of the satellite 17 will be described.

[0042] 4 and 5 are diagrams for explaining a Grating-Assisted Coupler, which is a known technique. As shown in FIG. 4A, A , β BConsider two waveguides, each represented by . The graphs shown under each waveguide in Fig. 4A show examples of the light intensity distribution in each waveguide. When these waveguides are brought close to each other as shown in Fig. 4B, coupling occurs due to the overlap of electric fields. The strength of this coupling is κ. This coupling causes the respective modes to mix as shown in Fig. 4B, and two new eigenmodes β0 and β1 are generated. The graphs in Fig. 4B show examples of the light intensity distribution in each of the two eigenmodes generated by the coupling.

[0043] Here, the propagation constant β A and β B If the difference between them is large, e.g., if the difference is larger than the coupling strength κ, or if the coupling between the two waveguides is weak, the propagation constant of each eigenmode will be smaller than that of the original mode β A or β B becomes closer to.

[0044] Let ε be the difference between the propagation constants of the two original waveguides divided by 2 (ε = (β B -β A ) / 2). When ε=0, i.e., when the two waveguides have the same structure and their propagation constants are equal, the two new eigenmodes are completely symmetric or antisymmetric, as shown in FIG. 4C. On the other hand, when ε≠0, i.e., when the two waveguides have different structures and their propagation constants are different, the two new eigenmodes have similar distributions to the respective waveguides, as shown in FIG. 4D. FIG. 4E is a graph showing the relationship between the propagation constants β0 and β1 of the new eigenmodes and the difference ε in the propagation constants of the original waveguides. β0 corresponds to the 0th mode (fundamental mode) in a multimode waveguide, and β1 corresponds to the 1st mode.

[0045] As shown in Fig. 5A, when ε≠0, let us assume that only the mode with β0 is excited among the two new eigenmodes. Fig. 5A is a top view of two waveguides close to each other, as shown in Fig. 4B, etc. In Fig. 5A, a relatively narrow waveguide has a propagation constant β A and the relatively wide waveguide has a propagation constant β BIn this case, the light corresponding to the β0 mode shown in FIG. 4D propagates directly to the right.

[0046] Here, as shown in Fig. 5B, it is assumed that one of the waveguides is given a change in structure at a predetermined period along the longitudinal direction. Specifically, for example, a concave-convex structure is formed along the longitudinal direction. The period Λ of the concave-convex structure (the total length of the concave structure and the convex structure) is set based on the difference between the propagation constants β0 and β1, as shown in the above formula (1).

[0047] With this structure, the light propagating through the waveguide can be modulated by the periodic structure, and as it travels through the waveguide, it can transition from one mode (β0) to another mode (β1). This technology is called Grating Assisted Coupler (Reference: D. Marcuse, J. Lightwave Technol. LT-5 (2), 268-273 (1987). D. Marcuse, J. Lightwave Technol. LT-5 (2), 268-273 (1987)).

[0048] In the Grating Assisted Coupler, the period L for mode transition crit is expressed by the following equation (3). L crit =π / δ (3) δ is a value indicating the degree of modulation of light by the periodic structure of the waveguide.

[0049] As shown in FIG. 5C, the length of the waveguide is L crit By setting the angle β0 to β1, the mode of light incident at β0 can be shifted to β1 at the time of emission.

[0050] The optical filter 10 described in the first embodiment has a length L crit(an integer multiple of). As a result, in the optical filter 10 of the first embodiment, by arranging satellite portions 17 having a periodic structure on both sides in the width direction of the multimode waveguide constituted by the core portion 13 and the cladding portion 16, a grating assisted coupler is constituted, and light of the zeroth mode incident on the multimode waveguide can be shifted to the first mode when it is output. By providing a tapered portion 14 that radiates light of the first mode behind such a waveguide structure, as described above, the optical filter 10 can operate as a notch filter that removes only a specific wavelength.

[0051] <Temperature Dependence> The optical filter 10 having the structure described above can match the changes in refractive index in multiple waveguide modes for light of a specific wavelength λ, even if the temperature of each material constituting the waveguide changes due to light propagation, etc. In other words, the optical filter 10 can achieve temperature independence of the operating wavelength.

[0052] In order to achieve temperature independence at the wavelength λ0 in the optical filter 10, strictly speaking, the following formula (4) must be satisfied.

number

[0053] n eq0 (λ,T) is the effective refractive index n0 in the zeroth mode as a function of wavelength λ and temperature T. eq1 (λ,T) is the effective refractive index n1 in the first mode as a function of wavelength λ and temperature T.

[0054] The wavelength incident on the optical filter 10 is λ p In this case, the wavelength λ of the optical filter 10 is p Temperature dependence in ∂λ p / ∂T can be expressed by the following equation (5).

number

[0055] Here, the desirable temperature dependence of the optical filter 10 is, in practice, 10 -4 [μm / K], and more preferably 3×10 -6 It can be expressed as [μm / K]. 10 -4 In this case, the following equation (6) can be derived from equation (5).

number

[0056] In other words, if the difference in temperature coefficient between the zeroth mode and the first mode is sufficiently small with respect to the term obtained by dividing the difference in effective refractive index between the zeroth mode and the first mode by the wavelength, it can be said that the temperature dependence of the optical filter 10 is achieved.

[0057] In the optical filter 10, the effective refractive index n eff can be expressed by the following equation (7).

number

[0058] Here, the ratio W3 of the light intensity in the satellite 17 is determined by the distance between the core 13 and the satellite 17. Specifically, as the distance between the core 13 and the satellite 17 decreases, W3 increases, and the overall effective refractive index n eff On the other hand, when the distance between the core 13 and the satellite 17 increases, W3 decreases, and the effective refractive index n effIn this way, by adjusting the distance between the core portion 13 and the satellite portion 17, the effective refractive index of the entire optical filter 10 in a certain waveguide mode can be changed.

[0059] <Example> An example of an optical filter having the structure described in the first embodiment and how the example can achieve temperature independence will be described. Fig. 6 is a diagram for explaining an example of the optical filter 10 of the first embodiment.

[0060] Fig. 6A is a schematic top view of the waveguide structure of the optical filter 10 as an embodiment. Fig. 6B is a schematic cross-sectional view of the waveguide structure of the optical filter 10 as an embodiment, showing a cross section of line BB in Fig. 6A. In the embodiment shown in Fig. 6A, the periodic structure of the satellite portion 17 is formed into a meandering structure. In the embodiment shown in Fig. 6B, the core portion 13 and the satellite portion 17 are formed into a rib structure. In the embodiment shown in Fig. 6, the core portion 13 and the satellite portion 17 are formed of silicon (Si), and the cladding portion 16 is formed of silicon dioxide (SiO2).

[0061] In the embodiment shown in FIGS. 6A and 6B, the period L of the satellite portion 17 corresponds to the period Λ of FIG. period The length L of the multimode waveguide formed by the core portion 13 and the cladding portion 16 is 5.81 μm. device The width of the core portion 13 is 0.9 μm, and the width of the satellite portion 17 is 0.2 μm. The distance from the core portion 13 to one of the satellite portions 17 is 0.38 μm.

[0062] Fig. 6C is a graph showing the relationship between the wavelength of light incident on the optical filter 10 shown in Fig. 6A and Fig. 6B and the spectrum of the optical filter 10. As shown in Fig. 6C, it is known that when the temperature of the optical filter 10 is changed, the direction of the spectrum shift differs depending on the wavelength. In the example shown in Fig. 6C, when the temperature of the substrate is changed in the range from 10°C to 40°C, the wavelength of the spectral dip does not change near 1.54 μm, so it is understood that temperature independence is achieved near the wavelength of 1.54 μm.

[0063] FIG. 6D is a graph showing the relationship between the temperature dependence of wavelength (∂λ / ∂T) and wavelength. According to FIG. 6D, at a wavelength of 1.538 μm, the temperature dependence is 3 pm / K (=3×10 -6 It can be seen that a desired temperature independence is achieved at the wavelength.

[0064] In the optical filter 10 of the first embodiment, the conditions necessary to achieve temperature independence are not limited to the example shown in Fig. 6. The size (width and height) of each structure of the optical filter 10 is not limited to the size shown in Fig. 6A and Fig. 6B, and may be set based on the wavelength to be used.

[0065] 6 is merely one embodiment, and in reality, there are countless options for the size of each structure of the optical filter 10 to achieve temperature independence in this disclosure. g FIG. 1 is a diagram for explaining the range in which the temperature independence of the optical filter 10 is achieved when the distance w between the core 13 and the satellite 17 is changed. g means the average distance between the core portion 13 and the satellite portions 17 arranged in a zigzag structure.

[0066] 7A and 7C are examples of cross sections of the optical filter 10. In the example shown in Fig. 7A, the core portion 13 and the satellite portion 17 are formed in a rib structure, with the rib height being 220 nm and the slab portion having a height of 110 nm. In the example shown in Fig. 7A, the width of the satellite portion 17 is fixed at 0.2 μm. On the other hand, in the example shown in Fig. 7C, the width of the satellite portion 17 is fixed at 0.3 μm.

[0067] FIG. 7B shows the relationship between the width w of the core portion 13 and the distance w between the core portion 13 and the satellite portion 17 when the width of the satellite portion 17 is 0.2 μm. g 7D shows the relationship between the width w of the core 13 and the distance w between the core 13 and the satellite 17 when the width of the satellite 17 is 0.3 μm and the difference (∂n1 / ∂T-∂n0 / ∂T) between the temperature coefficients of the zeroth mode and the first mode. g and the difference (∂n1 / ∂T-∂n0 / ∂T) between the temperature coefficients of the zeroth and first modes. Figures 7B and 7D show that the whiter the region, the closer (∂n1 / ∂T-∂n0 / ∂T) is to 0. The w and w corresponding to the dashed line region are g By selecting , sufficient temperature independence of the optical filter 10 can be achieved.

[0068] In FIG. 7, the width w of the core portion 13 and the distance w between the core portion 13 and the satellite portion 17 are g In addition to the above, the width w of the satellite portion 17 is selected. p The height h of the core portion 13 and the satellite portion 17, the wavelength λ of the light, etc. may be selectable.

[0069] Fig. 8 is a diagram for explaining elements that can be selected to achieve temperature independence among the structures of the optical filter 10. Fig. 8 shows a cross section of the optical filter 10 including the core section 13 and the satellite section 17. Fig. 8A shows an example in which the core section 13 and the satellite section 17 have a channel-type structure as shown in Fig. 1B.

[0070] (First example) In a first example, in the channel structure shown in Fig. 8A, the wavelength of light incident on the optical filter 10 is set to about 1.55 µm. In this case, each selectable element is given by the following equation (8). w=0.673+Δw w g =0.25+Δw g w p =0.20+Δw p h=0.22+Δh λ=1.55+Δλ (8) The unit of the value is μm. Δw, Δw g , Δw p , Δh, and Δλ indicate the deviation from the central value in each element.

[0071] Δw, Δw g , Δw p , Δh, and Δλ satisfy the relationship expressed by equation (9).

number

[0072] It is desirable that the coefficients α, β, γ, δ1, and δ2 in the formula (9) take values ​​shown in the following formula (10), for example. α=0.211 β=-0.196 γ=-1.20 δ1=2.19 δ2=39.3 (10)

[0073] Using equations (8) to (10), Δw, Δw g , Δw p , we can see what value Δh should be set to.

[0074] Specifically, in the first example, Δw, Δw g , Δw p When Δh and Δλ are all 0, that is, from equation (8), w = 0.673, wg =0.25, w p = 0.20, h = 0.22, and λ = 1.55, the optical filter 10 can achieve temperature independence.

[0075] Next, the width w of the satellite part 17 p Consider the case where Δw is set to 0.21. In this case, from equation (8), p = 0.01. In this case, for example, Δw g If =Δh=Δλ=0, then from equation (9), Δw=αΔw p It is.

[0076] Therefore, w = 0.673 + (0.211 × 0.01), w g =0.25, w p = 0.21, h = 0.22, and λ = 1.55, the optical filter 10 can achieve temperature independence.

[0077] In the first example, Δw, Δw g , Δw p As a result of setting Δh and Δλ, w, w p , w g It is desirable that the values ​​of h and λ fall within the numerical range shown in the following equation (11). 0.5≦w≦0.9 0.1≦w p ≦0.5 0.1≦w g ≦0.8 0.18≦h≦0.26 1.5≦λ≦1.6 (11)

[0078] The numerical values ​​included in the formulas (8) to (11) are calculated by a simulation to achieve temperature independence of the optical filter 10. In the example shown in FIG. 8A, the cross-sectional shapes of the core 13 and the satellite 17 are almost rectangular, but in the simulation, the above numerical values ​​are calculated assuming that the cross-sectional shapes of the core 13 and the satellite 17 are rectangular. In the actual manufacturing process, the cross-sectional shapes of the core 13 and the satellite 17 tend to be trapezoids with the bottom side longer than the top side, in which case the side surfaces of the core 13 and the satellite 17 are tapered. If the cross-sectional shape becomes a trapezoid as a result of actual manufacturing, it is possible to deal with it by performing a simulation using parameters for a rectangle with the same cross-sectional area.

[0079] (Second example) In a second example, in the channel structure shown in Fig. 8A, the wavelength of light incident on the optical filter 10 is set to about 1.31 µm. In this case, each selectable element is given by the following equation (12). w=0.541+Δw w g =0.25+Δw g w p =0.20+Δw p h=0.22+Δh λ=1.31+Δλ (12) The numerical values ​​are in μm.

[0080] Δw, Δw g , Δw p , Δh, and Δλ satisfy the relationship expressed by equation (9).

[0081] It is desirable that the coefficients α, β, γ, δ1, and δ2 in the formula (9) take values ​​shown in the following formula (13), for example. α=0.185 β=-0.294 γ=-627 δ1=2.12 δ2=-17.2 (13)

[0082] Using equations (9), (12), and (13), Δw, Δw g , Δw p , we can see what value Δh should be set to.

[0083] In the second example, w, w p , w g It is desirable that the values ​​of h and λ fall within the numerical range shown in the following equation (14). 0.3≦w≦0.8 0.1≦w p ≦0.5 0.1≦w g ≦0.8 0.18≦h≦0.26 1.26≦λ≦1.36 (14)

[0084] Next, FIG. 8B shows an example in which the core portion 13 and the satellite portion 17 have a rib-type structure. In this case, the height h of the slab structure can be selected as an optional element. r More will be added.

[0085] (Third example) In a third example, in the rib structure shown in Fig. 8B, the wavelength of light incident on the optical filter 10 is set to about 1.55 µm. In this case, each selectable element is given by the following equation (15). w=0.862+Δw w g =0.48+Δw g w p =0.20+Δw p h=0.22+Δh h r =0.11+Δh r λ=1.55+Δλ (15) The unit of the value is μm. Δh r indicates the deviation in height of the slab structure from the central value.

[0086] Δw, Δw g , Δw p , Δh, Δh r , Δλ satisfies the relationship expressed by equation (16).

number

[0087] It is desirable for the coefficients α, β, γ, δ1, δ2, and ε in equation (16) to take values ​​shown in equation (17) below, for example. α=0.368 β=-0.198 γ=-2.98 δ1=3.56 δ2=98.6 ε=3.78 (17)

[0088] Using equations (15) to (17), Δw, Δw g , Δw p , Δh, Δh r You can see what value you should set it to.

[0089] In the first example, w, w p , w g , h, h r It is desirable that the values ​​of and λ are within the numerical range shown in the following formula (18). 0.6≦w≦1.1 0.1≦w p ≦0.5 0.1≦w g ≦0.8 0.18≦h≦0.26 0.08≦h r ≦0.14 1.5≦λ≦1.6 (18)

[0090] (Example 4) In a fourth example, in the channel structure shown in Fig. 8B, the wavelength of light incident on the optical filter 10 is set to about 1.31 µm. In this case, each selectable element is given by the following equation (19). w=0.716+Δw w g =0.48+Δw g w p =0.20+Δw p h=0.22+Δh h r =0.11+Δh r λ=1.31+Δλ (19) The numerical values ​​are in μm.

[0091] Δw, Δw g , Δw p , Δh, Δh r , Δλ satisfies the relationship expressed by equation (16), as in the third example.

[0092] It is desirable that the coefficients α, β, γ, δ1, and δ2 in the formula (16) take values ​​shown in the following formula (20), for example. α=0.527 β=-0.173 γ=-2.13 δ1=0.328 δ2=-34.1 ε=3.25 (20)

[0093] Using equations (16), (19), and (20), Δw, Δw g , Δw p , Δh, Δh r You can see what value you should set it to.

[0094] In the second example, w, w p , w g , h, h rIt is desirable that the values ​​of and λ are within the numerical range shown in the following formula (21). 0.5≦w≦1.0 0.1≦w p ≦0.5 0.1≦w g ≦0.8 0.18≦h≦0.26 0.08≦h r ≦0.14 1.26≦λ≦1.36 (21)

[0095] The size of each component of the optical filter 10 of the present disclosure is not limited to the first to fourth examples described in relation to FIG. 8, but may be set appropriately within a range that can achieve temperature independence based on the target wavelength.

[0096] <Action and effect> The operation and effects of the optical filter 10 described in the first embodiment of the present disclosure will be described.

[0097] The optical filter 10 has a structure in which a core portion 13 extending along the longitudinal direction and satellite portions 17 extending along the longitudinal direction on both sides of the core portion 13 are covered with a cladding portion 16. The satellite portions 17 are arranged at periodically different positions along the longitudinal direction so that the distance between the core portion 13 and the satellite portions 17 changes periodically with a period set based on the propagation constants of two waveguide modes (zeroth mode and first mode) in which the multimode waveguide propagates. With this configuration, the satellite portions 17 have a cross section smaller than the cross section capable of propagating light in a single mode.

[0098] With this configuration, the effective refractive index of the multimode waveguide composed of the core 13 and cladding 16 changes periodically due to the periodic structure of the satellite 17. This allows the optical filter 10 to cause mode transition of light incident on the multimode waveguide as it travels through the waveguide. By appropriately adjusting the length of the multimode waveguide, for example, light incident in the zeroth mode can be transitioned to light in the first mode when it is output.

[0099] By arranging the tapered section 14 that radiates first-order mode light behind such a multimode waveguide structure, the optical filter 10 operates as a single-peak filter (notch filter) that removes first-order mode light.

[0100] In the optical filter 10, since light of two guided modes propagates in one core 13, the coupling between the two guided modes can be made relatively strong. This allows the device length to be shorter than that of a conventional waveguide structure using a Grating-Assisted Coupler. This allows high integration to be achieved.

[0101] In addition, in optical filter 10 having the above structure, by adjusting parameters such as the cross-sectional size (width and height) of core 13, the size of satellite 17, and the distance between core 13 and satellite 17, it is possible to make the difference in the temperature dependence (temperature coefficient) of the multi-mode waveguide for the zeroth mode and the first mode approximately zero. This makes it possible to achieve temperature independence of the optical filter for a specific wavelength.

[0102] (Modification of the first embodiment) A modification of the first embodiment will now be described.

[0103] In the first embodiment, the satellites 17 arranged on both sides of the core 13 have a periodic structure along the longitudinal direction. This structure causes the refractive index of the multimode waveguide formed by the core 13 and the cladding 16 to change periodically, which allows the multimode waveguide to change the mode of light incident on it to another mode when it is emitted. This allows the optical filter 10 to function as a notch filter that removes light of a specific wavelength.

[0104] In the present invention, the satellite portion does not necessarily have a periodic structure in the longitudinal direction. The optical filter of the present invention has a multimode waveguide and a satellite structure, and the periodic change in the refractive index of the multimode waveguide may be realized by using, for example, the EO effect, the TO effect, the mechanical effect, or the nonlinear effect.

[0105] (First Modification) The electro-optical (EO) effect is an effect in which the refractive index of a material changes when a voltage is applied to the material. The following is an example of a case in which the EO effect is applied to the optical filter of the present disclosure.

[0106] Fig. 9 is a diagram showing a first modified example of an optical filter utilizing the EO effect. Fig. 9A is a schematic top view. Fig. 9B is a cross-sectional view taken along line CC in Fig. 9A. Fig. 9C is a cross-sectional view taken along line DD in Fig. 9A. As shown in Fig. 9A, an optical filter 10A of the first modified example is similar to the first embodiment in that satellite portions 17A are arranged along the longitudinal direction of a core portion 13A. However, the satellite portions 17A are arranged linearly and do not have a zigzag structure.

[0107] The optical filter 10A has an n-type semiconductor structure 18A and a p-type semiconductor structure 19A. The n-type semiconductor structure 18A and the p-type semiconductor structure 19A each have a comb-like shape, and are arranged so that the comb-like shapes interlock with each other and join with each other. The n-type semiconductor structure 18A and the p-type semiconductor structure 19A are also arranged so as to contact the core portion 13A and the satellite portion 17A with each other. This causes the position of the pn junction to change periodically along the longitudinal direction, as shown in Figures 9B and 9C. The period during which the position of the pn junction changes is set to the period shown in the above formula (1).

[0108] Electrodes are connected to the n-type semiconductor structure 18A and the p-type semiconductor structure 19A, respectively. When a forward bias voltage or a reverse bias voltage is applied to the n-type semiconductor structure 18A and the p-type semiconductor structure 19A via the electrodes, the width of the depletion layer in which no electrons or holes exist changes, and the carrier density at the pn junction can be controlled. This makes it possible to change the refractive index of the multimode waveguide formed by the core portion 13A and the cladding portion 16A.

[0109] (Second Modification) Fig. 10 is a diagram showing a second modified example of an optical filter utilizing the EO effect, Fig. 10A is a schematic top view, and Fig. 10B is a cross-sectional view taken along line EE in Fig. 10A.

[0110] In the optical filter 10B of the second modification, a quantum well layer 41B is formed in the core portion 13B and the satellite portion 17B. RF electrodes 42B are arranged along the longitudinal direction on both outer sides of the satellite portion 17B as viewed from the core portion 13B. An AC power supply is connected to each RF electrode 42B. The quantum well layer 41B is formed on a substrate such as GaAs.

[0111] When an AC voltage is applied to each of the RF electrodes 42B, a standing electric field distribution is generated between the RF electrodes 42B. The distribution of two-dimensional electron gas (2DEG) in the quantum well layer 41B is modulated by the electric potential formed by the electric field. This makes it possible to periodically change the refractive index of the multi-mode waveguide formed by the core portion 13B and the cladding portion 16B. In the second modification, it is possible to control the period for changing the refractive index by the frequency of the AC voltage applied to the RF electrodes 42B.

[0112] (Third Modification) The optical filter 10C of the third modification utilizes the TO effect. FIG. 11 is a diagram showing a fourth modification of the optical filter utilizing the TO effect. FIG. 11A is a schematic top view. FIG. 11B is a cross-sectional view taken along the line FF in FIG. 11A. Specifically, thin-film heaters 43C are periodically arranged on the substrate on both outer sides of the satellite portion 17C arranged along the longitudinal direction of the core portion 13C or in the cladding portion 16C. This allows a temperature distribution according to the amount of heat of the thin-film heater 43C to be generated on the multi-mode waveguide, thereby periodically changing the refractive index of the multi-mode waveguide.

[0113] (Fourth Modification) Fig. 12 is a diagram showing a fourth modified example of the optical filter utilizing a mechanical effect, Fig. 12A is a schematic top view, Fig. 12B is a cross-sectional view taken along line GG in Fig. 12A.

[0114] In the optical filter 10D of the fourth modification, the core 13D, cladding 16D, and satellite 17D are fixed to a substrate, and a movable beam 45D is supported by a substrate 44D via a spring. The movable beam 45D can be moved toward or away from the multi-mode waveguide by an actuator (not shown). The refractive index of the multi-mode waveguide can be changed depending on the position to which the movable beam 45D is moved.

[0115] (Fifth Modification) 13 is a diagram showing a fifth modified example of an optical filter utilizing an optical nonlinear effect. In an optical filter 10E of the fifth modified example, two light waves of a control light CL having a wavelength different from that of the signal light SL are input to an input portion 11E in a waveguide mode different from the waveguide mode in which the signal light SL propagates. The propagation constants of the two control light CL1, CL2 are set to be slightly different from each other. Due to the difference in the propagation constants, the two control light CL1, CL2 interfere with each other, causing a periodic change in intensity of the control light CL propagating through the multimode waveguide.

[0116] This produces an optical nonlinear effect called the optical Kerr effect, which makes it possible to periodically change the refractive index of the multimode waveguide formed by the core portion 13E and the cladding portion 16E in accordance with the intensity of light in the multimode waveguide.

[0117] In the first to fifth modified examples, examples have been described in which the optical filters 10A, 10B, 10C, 10D, and 10E periodically change the refractive index of the multi-mode waveguide by the EO effect, the TO effect, the mechanical effect, the optical nonlinear effect, etc. In such modified examples, the amount of change in the refractive index can be controlled by controlling parameters according to the configuration.

[0118] In this case, the optical filters 10A, 10B, 10C, 10D, and 10E can be used as modulators. The modulators realized by the present disclosure have the characteristics of the optical filters of the present disclosure, and therefore have a specific wavelength filter shape and can achieve temperature independence.

[0119] The modulation method of the modulator can be a first modulation method that controls the amount of change in the refractive index, and a second modulation method that controls the period of change in the refractive index. In the first modulation method, the transmittance can be changed by control without changing the center wavelength of the filter. In the second modulation method, the center wavelength of the filter is changed by control.

[0120] In the optical filters 10A, 10B, 10C, 10D, and 10E of the first to fifth modified examples, the satellite portion does not have a periodic structure because the refractive index is periodically changed by the EO effect, the TO effect, the mechanical effect, the optical nonlinear effect, etc. In this case, the satellite portion is provided to achieve temperature independence of the optical filters 10A, 10B, 10C, 10D, and 10E by adjusting the size of the satellite portion and the distance from the core portion.

[0121] (Sixth Modification) In the optical filter 10 described in the first embodiment above, as shown in FIG. 1A, etc., the two satellite portions 17 are arranged on either side of the core portion 13 so that the distance between the satellite portions 17 is approximately constant even if they have different periods (mirror-antisymmetric).

[0122] When the symmetry is different between the two modes (zeroth mode and first mode) used in the multimode waveguide formed by the core portion 13 and the cladding portion 16, such a structure can produce a periodic refractive index change in the multimode waveguide.

[0123] On the other hand, when the two modes have the same symmetry, the satellites on both sides of the core may be arranged in a mirror symmetrical manner.

[0124] (Seventh Modification) In the optical filter 10 described in the first embodiment above, as shown in FIG. 1A etc., the position (distance) of the satellite portion 17 relative to the core portion 13 is periodically changed, thereby causing a periodic change in the refractive index of the multi-mode waveguide.

[0125] As a modification of the seventh modification, instead of periodically changing the position of the satellites, an optical filter may be formed in which the side surface of the core has an uneven shape when viewed from above. In such an optical filter of the seventh modification, the distance between the core and the satellites changes periodically.

[0126] (Eighth Modification) It is known that when a waveguide structure is made of silicon, the refractive index can be adjusted during use by injecting ions into the waveguide during fabrication and then heating the portion to a high temperature for annealing (see JJ Ackert, et al., Optics Express 19(13), 11969 (2011)). This technology may be applied to the optical filter 10 of the first embodiment, in which ions are injected into the satellite portion 17 during fabrication and the satellite portion is heated to a high temperature during use, allowing the effective refractive index of the entire optical filter 10 to be adjusted.

[0127] FIG. 14 is a diagram showing an eighth modified example of an optical filter that allows refractive index adjustment by ion implantation. In the optical filter 10F of the eighth modified example, the satellite portion 17F provided on both sides of the core portion 13F is periodically provided with ion implantation regions 46F. By heating the satellite portion 17F to a high temperature during calibration before use, the refractive index of the satellite portion 17F, and therefore the effective refractive index of the entire optical filter 10F, can be arbitrarily adjusted. With this configuration, the effective refractive index of the optical filter 10F can be adjusted after the optical filter 10F is manufactured. Therefore, the manufacturing precision of the optical filter 10F can be relaxed, and the manufacturing cost and design cost of the optical filter 10F can be significantly reduced.

[0128] In addition, the ion-implanted region 46F may absorb light. However, in the optical filter 10F, the satellite portion 17F does not guide light. This prevents the light from being absorbed by the ion-implanted region 46F provided in the satellite portion 17F, resulting in a decrease in the intensity of the light propagated by the optical filter 10F.

[0129] (Ninth Variation) In the first embodiment described above, the terminal end of satellite portion 17 is formed parallel to the longitudinal direction as shown in Fig. 1A. The optical filter of the present disclosure is not limited to this, and the optical filter of the present disclosure may have a satellite portion formed by bending or widening the terminal end in the width direction in the opposite direction to core portion 13.

[0130] This makes it possible to prevent light from being scattered at the ends of the satellite portions 17.

[0131] <Second embodiment> In the second embodiment described below, an optical filter 20 will be described as a bandpass filter that extracts only light of a specific wavelength. The optical filter 20 has a different structure of a terminal portion from the optical filter 10 of the first embodiment described above.

[0132] In the following description, the same components as those in the first embodiment are denoted by the same reference numerals, and the description thereof may be omitted.

[0133] Fig. 15 is a diagram for explaining the structure of an optical filter 20 according to the second embodiment. As shown in Fig. 15, the optical filter 20 includes an input portion 11, a tapered portion 12, a core portion 13, a cladding portion 16, a satellite portion 17, a tapered portion 21, waveguides 22 and 23, and an output portion 24.

[0134] As shown in FIG. 15, the optical filter 20 has the same structure from the input portion 11 to the terminal end of the core portion 13 as the optical filter 10 of the first embodiment.

[0135] The tapered portion 21 connects the end of the core portion 13 and the waveguide 22. The waveguide 22 is formed to be wider than the core portion 13. Therefore, the tapered portion 21 has a tapered shape that widens from the end of the core portion 13 to the start end of the waveguide 22. In FIG. 15, the length of the tapered portion 21 is shown to be relatively short compared to other configurations, but in reality, the tapered portion 21 may be formed long, for example, to prevent interference with the end of the satellite portion 17.

[0136] The waveguides 22 and 23 are arranged parallel to each other in the longitudinal direction and close to each other. As a result, the waveguides 22 and 23 form a directional coupler. The propagation constant of the first mode of the waveguide 22 and the propagation constant of the zeroth mode of the waveguide 23 are set to the same value. The length L of the waveguides 22 and 23 is dc is set to the perfect bond length.

[0137] The waveguide 22 is formed to be wider than the core portion 13, and constitutes a multimode waveguide together with the surrounding cladding portion 16. The waveguide 22 has a zeroth mode and a first mode.

[0138] On the other hand, the waveguide 23 is formed to be narrower than the waveguide 22, and is a single mode waveguide having only the first mode.

[0139] At this time, the first mode of waveguide 22 and the zeroth mode of waveguide 23 are coupled, and the first mode light propagating through waveguide 22 can be extracted to waveguide 23. The zeroth mode light propagating through waveguide 22 is not coupled to waveguide 23, and is radiated (scattered) from, for example, the terminal end of waveguide 22.

[0140] As a result, only the light in the first mode is emitted from the emission portion 24. Fig. 16 is a diagram showing the characteristics of the optical filter 20 of the second embodiment. As shown in Fig. 16, the optical filter 20 acts as a bandpass filter that extracts only specific light.

[0141] In the optical filter 20 of the second embodiment, the ends of the waveguides 22 and 23 may be bent or widened to suppress scattering of light. Also, a structure equivalent to the tapered section 14 and the emission section 15 of the first embodiment may be added to the end of the waveguide 23 to extract light in the zeroth mode from the waveguide 23.

[0142] In the example shown in FIG. 15, the waveguide 23 is provided only on one side of the waveguide 22, but a waveguide similar to the waveguide 23 may be provided on the other side as well.

[0143] For example, when the width of waveguide 22 is 1.19 μm, the width of waveguide 23 is 0.43 μm, and the distance between waveguide 22 and waveguide 23 is 0.25 μm, the perfect coupling length at which the total energy of the light in the first mode of waveguide 22 can be extracted from waveguide 23 is 13.8 μm.

[0144] In the example shown in FIG. 15, both of the waveguides 22 and 23 have a structure in which the width does not change along the longitudinal direction, but for example, either of the waveguides 22 and 23 may be formed in a tapered shape.

[0145] In the optical filter 20 of the second embodiment, as in the optical filter 10 of the first embodiment, the temperature independence of the optical filter 20 can be achieved by adjusting the size of the core portion 13, the size of the satellite portion 17, and the distance between the core portion 13 and the satellite portion 17, etc.

[0146] <Third embodiment> In the third embodiment, an optical filter 30 having a structure different from those in the first and second embodiments will be described.

[0147] 17 is a schematic top view illustrating the structure of an optical filter 30 according to the third embodiment. As shown in FIG. 17, the optical filter 30 has a first input portion 31, a second input portion 32, a Y branch 33, a core portion 34, a satellite portion 35, a Y branch 36, a first output portion 37, a second output portion 38, and a cladding portion 39.

[0148] The first incident portion 31, the second incident portion 32, the Y branch 33, the core portion 34, the satellite portion 35, the Y branch 36, the first exit portion 37, and the second exit portion 38 are covered by a cladding portion 39. The first incident portion 31, the second incident portion 32, the first exit portion 37, and the second exit portion 38, together with the cladding portion 39, form a single-mode waveguide. The core portion 34, together with the cladding portion 39, form a multi-mode waveguide.

[0149] In this way, the optical filter 30 has two inputs and two outputs with a multi-mode waveguide between them, and acts as a periodic filter whose transmittance varies periodically with respect to wavelength.

[0150] The principle by which the optical filter 30 according to the third embodiment functions as a periodic filter will be described below.

[0151] <Principle> Generally, an interferometer known as a Mach-Zehnder interferometer is known, which splits light from a single light source into two parallel beams and measures the phase difference between the parallel beams.

[0152] Fig. 18 is a schematic top view showing an example of a Mach-Zehnder interferometer in which incident light is split into two arms and then merged. In the Mach-Zehnder interferometer 50 shown in Fig. 18, the second arm 52 is formed longer than the first arm 51. In the example shown in Fig. 18, the second arm 52 has an area formed to be thicker than the first arm 51. The Mach-Zehnder interferometer 50 having such an asymmetric structure is sometimes called an asymmetric Mach-Zehnder interferometer (AZMI).

[0153] When the difference in length between first arm 51 and second arm 52 is ΔL and the length of the region of second arm 52 that is thicker than first arm 51 is L', the temperature dependence of the Mth mode wavelength is expressed by the following equation (22).

number

[0154] In such a conventional AMZI, the first arm 51 and the second arm 52 are arranged physically separated from each other, so that a partial temperature change may have a different effect on the first arm 51 and the second arm 52. In other words, it is difficult for the conventional AMZI to achieve temperature independence with respect to local temperature changes.

[0155] Here, the optical filter 30 according to the third embodiment shown in Fig. 17 has two arms close to each other and has the core 34 behave as a one-core multimode waveguide, thereby achieving performance equivalent to that of the AZMI with two arms close to each other. In the optical filter 30 according to the third embodiment, the two arms are not physically separated, so that even if a local temperature change occurs in a part of the core 34, a temperature difference between the two arms is unlikely to occur. This makes it possible to provide an optical filter 30 that is resistant to local temperature changes.

[0156] FIG. 19 is a diagram for explaining the principle of the optical filter 30 according to the third embodiment behaving as an AZMI. FIG. 19A shows a schematic cross-sectional view of the optical filter 30 in a plane perpendicular to the longitudinal direction. FIG. 19B shows a schematic mode shape when light is incident only from the first incident part 31. FIG. 19C shows an example of the mode shape when light is incident only from the first incident part 31, and the shapes of the symmetric mode and the antisymmetric mode. FIG. 19D shows a schematic mode shape when light is incident only from the second incident part 32. FIG. 19E shows an example of the mode shape when light is incident only from the second incident part 32, and the shapes of the symmetric mode and the antisymmetric mode.

[0157] Fig. 19A shows the core 34 and the satellite 35 in cross section. In Fig. 19A, the graphs shown under the core 34 and the satellite 35 show examples of light intensity distribution. As shown in Fig. 19A, by arranging the satellite 35 on both sides of the core 34, it is possible to utilize a lower-order mode and a higher-order mode in one core.

[0158] When light is incident only from the first incident part 31, the mode shape at the Y branch 33 becomes the shape shown in Fig. 19B. This shape is close to the shape obtained by adding a symmetric (S) mode and an antisymmetric (AS) mode in phase, as shown in Fig. 19C.

[0159] On the other hand, when light is incident only from the second incident portion 32, the mode shape at the Y branch 33 becomes the shape shown in Fig. 19D. This shape is close to the shape obtained by adding a symmetric (S) mode and an antisymmetric (AS) mode in antiphase, as shown in Fig. 19E.

[0160] With this configuration, when light is incident on the optical filter 30, it behaves as follows. For example, when light is incident only from the first incident portion 31, both S mode and AS mode light are propagated by the core portion 34. At this time, the phase difference between the S mode and AS mode of the light that reaches the Y branch 36 at the end portion of the core portion 34 changes depending on the wavelength of the incident light.

[0161] The length of the core portion 34 is L core Then, the phase of the S mode at the Y branch 36 is β S ×L core , the phase of the AS mode is β AS ×L core Here, β S is the propagation constant of the S mode, β AS is the propagation constant of the AS mode.

[0162] L core By setting the length of the Y branch 36 using the following equation (23), the S mode and the AS mode can be made in phase with each other in the Y branch 36. L core =2π m / (β S -β AS ) (twenty three) Here, m is an integer.

[0163] In the Y branch 36, when the S mode and the AS mode are in phase, the light proceeds to the first exit portion 37 and is emitted. In addition, in the Y branch 36, when the S mode and the AS mode are in opposite phase, the light proceeds to the second exit portion 38 and is emitted. Therefore, the exit portion from which the light is emitted changes depending on the wavelength of the incident light. This realizes a periodic wavelength filter.

[0164] In the optical filter 30 according to the third embodiment, when the effective refractive indices in the S mode and the AS mode are n0 and n1, respectively, the temperature independence of the optical filter 30 can be achieved by adjusting the size of each component and the distance between the components so that equations (5) and (6) are satisfied, as in the optical filter 10 according to the first embodiment.

[0165] In the optical filter according to the third embodiment, the phase difference between two modes can be controlled by the EO effect, the TO effect, the mechanical effect, or the optical nonlinear effect. The configurations of the modified examples using the EO effect, the TO effect, the mechanical effect, or the optical nonlinear effect are almost the same as the first to fifth modified examples described in the first embodiment, and therefore the description thereof will be omitted.

[0166] <Fourth embodiment> A wavelength locker device can be manufactured using the optical filters according to the first to third embodiments of the present disclosure. FIG. 20 is a diagram showing an example of the configuration of a wavelength locker device 100 using the optical filters according to the present disclosure. The wavelength locker device 100 has a first optical power monitor 110, a second optical power monitor 120, a beam splitter 130, an optical input port 140, and an optical filter 10 according to the present disclosure. Note that FIG. 20 shows an example in which the optical filter 10 according to the first embodiment is adopted as an example of the optical filter according to the present disclosure, but the wavelength locker device according to the present disclosure can also adopt the optical filters 10A to 10F shown in the modified examples of the first embodiment, the optical filter 20 according to the second embodiment, and the optical filter 30 according to the third embodiment.

[0167] A portion of the laser output from the laser source 200 is input to the wavelength locker device 100 via the optical input port 140 by the beam splitter 210. A portion of the input laser output is input to the first optical power monitor 110 by the beam splitter 130, and the remainder is input to the second optical power monitor 120 via the optical filter 10. The first optical power monitor 110 and the second optical power monitor 120 each measure the intensity of the input light, and output the measurement results to the temperature controller 220 as a control signal S1 and a control signal S2, respectively.

[0168] The temperature controller 220 associated with the laser light source 200 automatically adjusts the set temperature based on the control signals S1 and S2 so as to make the difference between the output values ​​of the first and second optical power monitors 110 and 120 zero. This makes it possible to prevent changes in the wavelength of the laser light source 200 due to temperature changes and stabilize (lock) the wavelength of the light output by the laser light source 200.

[0169] In the fourth embodiment, a wavelength locker device has been described as an application example of the optical filter of the present disclosure, but the optical filter of the present disclosure can be applied to various other optical devices.

[0170] Although the preferred embodiments have been described in detail above, the present disclosure is not limited to such specific embodiments, and various modifications and variations are possible within the scope of the description set forth in the claims. [Industrial Applicability]

[0171] The present disclosure is useful in optical filters and rocker devices. [Explanation of symbols]

[0172] 10, 10A, 10B, 10C, 10D, 10E, 10F, 20, 30 Optical filters 11,11E Entrance part 12 Tapered section 13, 13A, 13B, 13C, 13D, 13E, 13F Core part 14 Tapered section 15. Emitter 16, 16A, 16B, 16C, 16D, 16E, 16F Clad section 17, 17A, 17B, 17C, 17D, 17E, 17F Satellite Section 171,172 areas 18A n-type semiconductor structure 19A p-type semiconductor structure 41B Quantum well layer 42B Electrode 43C Thin Film Heater 44D Board 45D movable beam 46F Ion implantation area 21 Tapered section 22,23 Waveguide 24 Emission section 31 1st entrance part 32 2nd entrance part 33 Y-junction 34 Core 35 Satellite Division 36 Y-junction 37 First exit section 38 Second exit section 39 Clad section 100 Wavelength Locker Device 110 First Optical Power Monitor 120 2nd Optical Power Monitor 130 Beam splitter 140 Optical Input Port SL signal light CL, CL1, CL2 control light

Claims

1. a core portion covered by a cladding portion, extending in one direction, and capable of propagating light in a plurality of waveguide modes; a satellite portion that is covered by the cladding portion, extends in the one direction away from the core portion, has an effective refractive index higher than that of the cladding portion, and is smaller than the minimum size of an optical material capable of propagating light in a single mode; An optical filter comprising:

2. a multi-mode waveguide formed by the core portion and the clad portion, which transitions light in a first guided mode to a second guided mode; 2. The optical filter of claim 1.

3. an emission section that does not emit light of the second waveguide mode shifted by the multi-mode waveguide, and emits light of another waveguide mode; 3. The optical filter of claim 2.

4. an emission section that emits light of the second waveguide mode shifted by the multi-mode waveguide and does not emit light of other waveguide modes; 3. The optical filter of claim 2.

5. the satellite portion has a periodic structure that periodically changes the effective refractive index of the multi-mode waveguide along the one direction; An optical filter according to any one of claims 2 to 4.

6. The periodic structure is a structure in which the distance between the core portion and the satellite portion is changed periodically.

6. The optical filter of claim 5.

7. The period Λ of the periodic structure along the one direction is expressed by Equation (1):

7. The optical filter of claim 6. [0010] β ０ is the propagation constant in the first guided mode, β １ is the propagation constant for the second guided mode.

8. The effective refractive index n of the first guided mode of the multimode waveguide ０ Temperature coefficient ∂n ０ / ∂T and the effective refractive index n １ Temperature coefficient ∂n １ / ∂T has the relationship of formula (2). An optical filter according to at least one of claims 2 to 7. [0025] T is the temperature and λ is the wavelength of the light.

9. The core and satellite have a channel-type waveguide structure, and the width of the core and the width of the satellite are ｐ , the distance between the core and the satellite ｇ , the height h of the core portion and the satellite portion is 1.5≦λ≦1.6 0.5≦w≦0.9 0.1≦w ｐ ≦0.5 0.1≦w ｇ ≦0.8 0.18≦h≦0.26 is set to 9. The optical filter of claim 8.

10. The core and satellite have a channel-type waveguide structure, and the width of the core and the width of the satellite are ｐ , the distance between the core and the satellite ｇ , the height h of the core portion and the satellite portion is 1.26≦λ≦1.36 0.3≦w≦0.8 0.1≦w ｐ ≦0.5 0.1≦w ｇ ≦0.8 0.18≦h≦0.26 is set to 9. The optical filter of claim 8.

11. The core and satellite have a rib-type waveguide structure, and the width of the core and the width of the satellite are ｐ , the distance between the core and the satellite ｇ , the height h of the core and the satellite, and the height h of the slab structure ｒ teeth, 1.5≦λ≦1.6 0.6≦w≦1.1 0.1≦w ｐ ≦0.5 0.1≦w ｇ ≦0.8 0.18≦h≦0.26 0.08≦h ｒ ≦0.14 is set to 9. The optical filter of claim 8.

12. The core and satellite have a rib-type waveguide structure, and the width of the core and the width of the satellite are ｐ , the distance between the core and the satellite ｇ , the height h of the core and the satellite, and the height h of the slab structure ｒ teeth, 1.26≦λ≦1.36 0.5≦w≦1.0 0.1≦w ｐ ≦0.5 0.1≦w ｇ ≦0.8 0.18≦h≦0.26 0.08≦h ｒ ≦0.14 is set to 9. The optical filter of claim 8.

13. the satellite portion changes a phase difference between two guided modes in a multi-mode waveguide formed by the core portion and the cladding portion between a start end and a end end of the multi-mode waveguide; 2. The optical filter of claim 1.

14. A first incident portion and a second incident portion connected to the starting end portion; a first exit portion and a second exit portion connected to the terminal end portion; The optical filter of claim 13 further comprising:

15. The core section and the satellite section constitute an asymmetric Mach-Zehnder interferometer (AZMI).

15. An optical filter according to claim 13 or 14.

16. The effective refractive index n in one guided mode of the multimode waveguide ０ Temperature coefficient ∂n ０ / ∂T and the effective refractive index n １ Temperature coefficient ∂n １ / ∂T has the relationship of formula (3). An optical filter according to at least one of claims 13 to 15. [0030] T is the temperature and λ is the wavelength of the light.

17. a core portion covered with a cladding portion, extending in one direction, and having a plurality of waveguide modes; a satellite portion that is covered by the cladding portion, extends in the one direction away from the core portion, has an effective refractive index higher than that of the cladding portion, and does not have a waveguide mode; An optical filter comprising:

18. a first optical power monitor configured to measure the optical intensity of at least a portion of a laser output from a laser light source; a second optical power monitor configured to measure the optical intensity of at least a portion of the laser output that has passed through the optical filter according to any one of claims 1 to 17; a temperature controller that adjusts the temperature of the laser light source so that a difference between an output value of the first optical power monitor and an output value of the second optical power monitor becomes zero; A wavelength locker device comprising:

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