Optical filter and wavelength locker device

Abstract

An optical filter (10) according to the present disclosure comprises: a core part (13) that is covered with a cladding part (16), extends in one direction (z), and can propagate light in a plurality of waveguide modes; and a satellite part (17) that is covered with the cladding part (16), extends in the one direction (z) while separated from the core part (13), has a higher effective refractive index than the cladding part (16), and is smaller than the minimum possible size of an optical material capable of propagating light in a single mode.

Description

Optical filters and wavelength lockers

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

[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 an example of an optical device, the refractive index changes with temperature change of the optical circuit, which can cause the operating wavelength of the optical filter to change.

[0003] For example, a known configuration is to install a temperature control mechanism using a Peltier element next to the optical filter to maintain a constant temperature in the optical circuit. However, this configuration requires costly installation of the temperature control mechanism, and requires a great deal 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 whose operating wavelength has very low temperature dependence. For example, Patent Document 1 discloses an optical filter that includes two waveguides having a region where first and second cores are optically coupled close to each other, and that has a periodic structure for forming periodic perturbations in the refractive index in the optically coupled region, thereby reducing the temperature dependence of the center wavelength.

[0005] Japanese Patent Application Laid-Open No. 2006-330104

[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 exponentially decays with increasing distance 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, and therefore, 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.

[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 satellite portions 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 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 satellite portions 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 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.

[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.

[0013] FIG. 1 is a schematic diagram showing an example of the structure of an optical filter according to a first embodiment, where FIG. 1A is a schematic diagram of the optical filter as viewed from above, and FIG. 1B is a schematic cross-sectional diagram taken along line A-A in FIG. 1A. FIG. 2 is a partially enlarged view illustrating the periodic structure of a satellite portion. FIG. 3 is a diagram illustrating the characteristics of the optical filter according to the first embodiment. FIGS. 4A to 4E are diagrams illustrating a grating-assisted coupler. FIGS. 5A to 5C are diagrams illustrating a grating-assisted coupler. FIG. 6 is a diagram illustrating an example of the optical filter according to the first embodiment, where FIG. 6A is a schematic top view of a waveguide structure of the optical filter according to the example, FIG. 6B is a schematic cross-sectional diagram of the waveguide structure of the optical filter according to the example, FIG. 6C is a graph showing the relationship between the wavelength of light incident on the optical filter shown in FIGS. 6A and 6B and the spectrum of the optical filter, and FIG. 6D is a graph showing the relationship between the temperature dependence of wavelength (∂λ / ∂T) and wavelength. FIG. 7 shows the relationship between the width w of the core and the distance w between the core and the satellite. ｇ 7A and 7C show examples of the cross section of an optical filter, and FIG. 7B shows the relationship between the width w of the core and the distance w between the core and the satellite when the width of the satellite is 0.2 μm. ｇ and the difference between the temperature coefficients of the zeroth and first modes (∂n １ / ∂T-∂n ０ 7D shows the relationship between the width w of the core and the distance w between the core and the satellite when the width of the satellite is 0.3 μm. ｇ and the difference between the temperature coefficients of the zeroth and first modes (∂n １ / ∂T-∂n ０ / ∂T). FIG. 8 is a diagram for explaining elements that can be selected to achieve temperature independence among the various structures of an optical filter. FIG. 8A shows a cross section of an optical filter in which the core and satellite sections have a channel-type structure as shown in FIG. 1B, and FIG. 8B shows a cross section of an optical filter in which the core and satellite sections have a rib-type structure. 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 section taken along line C-C in FIG. 9A, and FIG. 9C is a cross section taken along line D-D in FIG. 9A. 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 section taken along line E-E in FIG. 10A. FIG. 11 is a diagram showing a third modified example of an optical filter utilizing the TO effect. FIG. 11A is a schematic top view, and FIG. 11B is a cross section taken along line F-F in FIG. 11A. FIG. 12 is a diagram showing a fourth modified example of an optical filter utilizing a mechanical effect, where FIG. 12A is a schematic top view, and FIG. 12B is a cross-sectional view taken along line G-G in FIG. 12A . FIG. 13 is a diagram showing a fifth modified example of an optical filter utilizing an optical nonlinear effect. FIG. 14 is a diagram showing an eighth modified example of an optical filter that enables refractive index adjustment by ion implantation. FIG. 15 is a diagram illustrating the structure of an optical filter according to a second embodiment. FIG. 16 is a diagram illustrating the characteristics of the optical filter according to the second embodiment. FIG. 17 is a diagram illustrating the structure of an optical filter according to a third embodiment. FIG. 18 is a schematic top view showing an example of a Mach-Zehnder interferometer that splits incident light into two arms and then combines them. FIGS. 19A to 19E are diagrams illustrating the principle by which the optical filter according to the third embodiment behaves as an AZMI. FIG. 20 is a diagram illustrating an example of the configuration of a wavelength locker device using an optical filter according to the present disclosure.

[0014] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. Elements common to multiple drawings will be assigned the same reference numerals, and detailed descriptions of those elements will be omitted. However, descriptions that are more detailed than necessary, such as detailed descriptions of well-known matters or redundant descriptions of substantially identical configurations, may be omitted.

[0015] The present disclosure provides an optical filter that can achieve 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 unimodal filter that transmits or removes a single wavelength, and a periodic filter (periodic filter) that has periodic transmission characteristics.

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

[0017] Furthermore, 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 Hereinafter, an example in which an optical filter according to the present disclosure is applied to a notch filter, which is an example of a single-peak filter, will be described.

[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 as viewed from above. FIG. 1B is a schematic cross-sectional view taken along line A-A in FIG. 1A. In FIGS. 1A and 1B, the shape and size of each component are shown schematically and are not accurate. The optical filter 10 shown in FIGS. 1A and 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 from the outside onto the incident portion 11. The light incident from the incident portion 11 is incident onto 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 towards 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] The tapered portion 14 is connected to the terminal end of the core portion 13. The tapered portion 14 has a tapered shape that narrows from the connection portion with the core portion 13 toward the connection portion with the 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 by 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 materials forming the incident portion 11, the tapered portion 12, the core portion 13, the tapered portion 14, and the exit portion 15 have 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 incident portion 11, the tapered portion 12, the core portion 13, the tapered portion 14, and the emission portion 15 include silicon (Si), silicon nitride (SiN), III-V group semiconductors (GaAs, InP, InGaAs, InGaAsP), and ferroelectrics (LiNbO ３ Examples of materials for the cladding portion 16 include high refractive index materials such as silicon dioxide (SiO ２ ), indium phosphide (InP), air, etc.

[0026] The incident 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 multi-mode waveguide. The width and height of the incident portion 11 and the output portion 15 are set to a width and height that can confine only light in 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 in multiple waveguide modes and that can achieve temperature independence of the filter characteristics of the optical filter 10, as will be described later, in other words, temperature independence of the operating wavelength of the multi-mode waveguide.

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

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

[0029] The width and height of the satellite section 17 are set smaller than those that can confine the fundamental mode light incident on the optical filter 10. This allows the satellite section 17 to have a structure that does not guide light. In other words, the satellite section 17 does not have a waveguide 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 has the role of reflecting the light component that leaks from the core portion 13 into the cladding portion 16 when light is propagated through the multimode waveguide, thereby adjusting the effective refractive index of the multimode waveguide. If the satellite portion 17 had a waveguide function, energy would be transferred between the satellite portion 17 and the core portion 13, which would be undesirable.

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

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

[0033] 2, there are two distances d1 and d2 from the core unit 13 to the satellite unit 17. In the example shown in Fig. 2, the distance from the core unit 13 to one satellite unit 17 (region 171) is shown as d1, and the distance from the core unit 13 to the other satellite unit 17 (region 172) is shown as 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 the satellite portions 17 to change the guided mode of light propagating through the multimode waveguide formed by the core portion 13 and the cladding portion 16. The principle by which the satellite portions 17 change the guided mode of light propagating through the multimode waveguide will be described later.

[0035] 2, satellite portion 17 has regions 171 that are close to core portion 13 and regions 172 that are far from core portion 13, which are alternately arranged 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). However, β ０ is the propagation constant of the multimode waveguide in the zeroth mode, β １ is the propagation constant of the multimode waveguide in the first mode, κ is the coupling strength between the core 13 and the satellite 17, and ε is the β ０ and β １ This is the difference between the propagation constants (difference in propagation constants) divided by 2.

[0037] The propagation constant is determined by the effective refractive index for each guided mode. Let the effective refractive index for the zeroth mode be n ０ , the effective refractive index in the first mode is n １ Then, the propagation constant β ０ and β １ is expressed by equation (2). λ is the wavelength of light.

[0038] As shown in equation (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 section 17 in the optical filter 10 is determined based on the preset wavelength. Furthermore, the size (width and height) and length of the core section 13 (i.e., the length of the multi-mode waveguide formed by the core section 13 and the cladding section 16) are set to predetermined values, and the method for 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 multi-mode waveguide formed by the core portion 13 and the cladding portion 16.

[0040] Due to the periodic structure of the satellite section 17, light of the target wavelength is modulated as it travels inside the core section 13, and is shifted to the first-order mode just before the end of the core section 13. The tapered section 14 is configured to radiate (scatter) light in the first-order mode, so that no light emerges at the output section 15. As a result, the optical filter 10 operates as a notch filter that removes only the target wavelength, as shown in Figure 3. Figure 3 is a diagram showing the characteristics of the optical filter 10 of the first embodiment.

[0041] <Principle> The principle by which propagating light in a multimode waveguide formed by the core 13 and 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 Ｂ Consider 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 the electric fields. The strength of this coupling is denoted as κ. This coupling causes the respective modes to mix, as shown in Fig. 4B, and two new eigenmodes β ０ and β １ The graph in FIG. 4B shows an example of the light intensity distribution in each of the two eigenmodes resulting from the coupling.

[0043] Here, the propagation constant β Ａ and β Ｂ If the difference between them is large, for example, 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 It becomes closer to.

[0044] The difference between the propagation constants of the two original waveguides divided by 2 is defined as ε (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 new two eigenmodes become perfectly symmetric or antisymmetric modes, as shown in Figure 4C. On the other hand, when ε ≠ 0, i.e., when the two waveguides have different structures and their propagation constants are different, the new two eigenmodes have similar distributions to each other, as shown in Figure 4D. Figure 4E shows the propagation constant β of the new eigenmode. ０ , β １ 10 is a graph showing the relationship between β and the difference ε in the propagation constant of the original waveguide. ０ corresponds to the zeroth mode (fundamental mode) in the multimode waveguide, and β １ corresponds to the first mode.

[0045] As shown in FIG. 5A, when ε≠0, of the two new eigenmodes, β ０ 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 Ｂ In this case, the β ０ The light corresponding to the mode propagates to the right.

[0046] Here, as shown in Figure 5B, it is assumed that one of the waveguides has a structure that is changed at a predetermined period along the longitudinal direction. Specifically, for example, a concave-convex structure is formed along the longitudinal direction. The period of the concave-convex structure (the total length of the concave and convex structures) Λ is calculated by multiplying the propagation constant β by the above-mentioned formula (1). ０ and β １ It is set based on the difference between

[0047] This structure modulates the light propagating through the waveguide by the periodic structure, and as it travels through the waveguide, one mode (β ０ ) to the other mode (β １This technology is called Grating Assisted Coupler (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 is ｃｒｉｔ is expressed by the following formula (3): ｃｒｉｔ =π / δ (3) where 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 β ０ The incident light mode is β １ can be transferred to.

[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 multi-mode waveguide constituted by the core portion 13 and the cladding portion 16, a grating-assisted coupler is formed, and light in the zeroth mode incident on the multi-mode waveguide can be transitioned to the first mode when it is output. By providing a tapered portion 14 that radiates first-order mode light after such a waveguide structure, the optical filter 10 can operate as a notch filter that removes only a specific wavelength, as described above.

[0051] <Regarding 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 the optical filter 10, the wavelength λ ０ In order to achieve temperature independence in the temperature range, strictly speaking, the following formula (4) must be satisfied:

[0053] n ｅｑ０ (λ, T) is the effective refractive index n ０ as a function of wavelength λ and temperature T. ｅｑ１ (λ, T) is the effective refractive index n １ 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 ｐ Temperature dependence of ∂λ ｐ / ∂T can be expressed by the following equation (5). ∂n ０ / ∂T and ∂n １ / ∂T are called the temperature coefficients in the zeroth and first modes, respectively.

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

[0056] In other words, if the difference in the 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 dependency 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). n ｃｏｒｅ is the refractive index of the core portion 13, n ｃｌａｄ is the refractive index of the cladding portion 16, n ｓ is the refractive index of the satellite portion 17. １ , W２ , W ３ is a value indicating the proportion of the optical intensity in each of the core 13, cladding 16, and satellite 17 to the entire multimode waveguide.

[0058] Here, the ratio W of the light intensity in the satellite part 17 ３ is determined by the distance between the core 13 and the satellite 17. Specifically, when the distance between the core 13 and the satellite 17 becomes smaller, W ３ becomes larger, and the overall effective refractive index n ｅｆｆ On the other hand, when the distance between the core 13 and the satellite 17 increases, W ３ becomes smaller, and the overall effective refractive index n ｅｆｆ In this way, by adjusting the distance between the core 13 and the satellite 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 example. FIG. 6B is a schematic cross-sectional view of the waveguide structure of the optical filter 10 as an example, showing a cross section along line B-B in FIG. 6A. In the example shown in FIG. 6A, the periodic structure of the satellite portion 17 is formed into a meandering structure. In the example shown in FIG. 6B, the core portion 13 and the satellite portion 17 are formed into a rib structure. In the example shown in FIG. 6, the core portion 13 and the satellite portion 17 are made of silicon (Si), and the cladding portion 16 is made of silicon dioxide (SiO ２ ) are formed respectively.

[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 13 and the clad 16 is 5.81 μm. ｄｅｖｉｃｅThe width of the core 13 is 0.9 μm, and the width of the satellite 17 is 0.2 μm. The distance from the core 13 to one of the satellites 17 is 0.38 μm.

[0062] 6A and 6B , the wavelength of the light incident on the optical filter 10 and the spectrum of the optical filter 10. As shown in FIG. 6C, 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 substrate temperature is changed in the range from 10° C. to 40° C., the wavelength of the spectral dip near 1.54 μm does not change, indicating 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 of wavelength is 3 pm / K (=3×10 －６ It can be seen that a desired temperature independence is achieved at the wavelength.

[0064] It should be noted that the conditions necessary to achieve temperature independence in the optical filter 10 of the first embodiment 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 sizes shown in Fig. 6A and Fig. 6B, and may be set based on the wavelength to be used.

[0065] 6 is merely one example, 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 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 height being 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 13 and the distance w between the core 13 and the satellite 17 when the width of the satellite 17 is 0.2 μm. ｇ and the difference between the temperature coefficients of the zeroth and first modes (∂n １ / ∂T-∂n ０ 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. ｇ and the difference between the temperature coefficients of the zeroth and first modes (∂n １ / ∂T-∂n ０ 7B and 7D show the relationship between the (∂n １ / ∂T-∂n ０ / ∂T) is close to 0, and the w and w corresponding to the dashed line area 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 also 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] 8A and 8B are diagrams illustrating selectable elements of the optical filter 10 for achieving temperature independence. A cross section of the optical filter 10 including the core 13 and the satellite 17 is shown in Fig. 8A. An example in which the core 13 and the satellite 17 have a channel-type structure as shown in Fig. 1B is shown.

[0070] (First Example) In the first example, in the channel structure shown in Fig. 8A, the wavelength of light incident on the optical filter 10 is set to around 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 numerical values ​​is μm. Δw, Δw ｇ , Δw ｐ , Δh, and Δλ indicate deviations from the central value in each element.

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

[0072] Coefficients α, β, γ, and δ in equation (9) １ and δ ２ It is desirable that the values ​​of α and β be, for example, values ​​shown in the following formula (10): α=0.211, β=-0.196, γ=-1.20, and δ ... １ = 2.19 δ ２ = 39.3 (10)

[0073] Using equations (8) to (10), Δw and Δw are determined depending on how much the wavelength to be filtered by the optical filter 10 deviates from 1.55 μm (Δλ). ｇ , Δw ｐ , Δh can be set to.

[0074] Specifically, in the first example, Δw, Δw ｇ , Δw ｐ , Δh, and Δλ are all 0, that is, from equation (8), w = 0.673, w ｇ = 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 portion 17 ｐLet us 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 we set = Δh = Δλ = 0, then from equation (9), Δw = αΔw ｐ 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 w, h, and λ are within the range of values ​​shown in the following formula (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 equations (8) to (11) were 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 approximately rectangular, but the above numerical values ​​were also calculated in the simulation assuming that the cross-sectional shapes of the core 13 and the satellite 17 are rectangular. In an actual manufacturing process, the cross-sectional shapes of the core 13 and the satellite 17 tend to be trapezoidal, with the bottom side longer than the top side. In this case, the side surfaces of the core 13 and the satellite 17 are tapered. If the cross-sectional shape becomes trapezoidal as a result of actual manufacturing, it can be addressed 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 wｇ = 0.25 + Δw ｇ w ｐ = 0.20 + Δw ｐ h=0.22+Δh λ=1.31+Δλ (12) The unit of the numerical values ​​is μm.

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

[0081] Coefficients α, β, γ, and δ in equation (9) １ and δ ２ It is desirable that the values ​​of α, β, γ, and δ are expressed by the following formula (13): α=0.185, β=-0.294, γ=-627, and δ=-1. １ = 2.12 δ ２ = -17.2 (13)

[0082] Using equations (9), (12), and (13), Δw and Δw can be calculated depending on how much the wavelength to be filtered by the optical filter 10 deviates from 1.31 μm (Δλ). ｇ , Δw ｐ , Δh can be set to.

[0083] In the second example, w, w ｐ , w ｇ It is desirable that the values ​​of w, h, and λ are within the numerical ranges shown in the following formula (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 numerical value is μm. Δh ｒ indicates the deviation of the height of the slab structure from the central value.

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

[0087] Coefficients α, β, γ, and δ in equation (16) １ , δ ２ , and ε preferably take the values ​​shown in the following equation (17): α=0.368 β=−0.198 γ=−2.98 δ １ = 3.56 δ ２ =98.6 ε=3.78 (17)

[0088] Using equations (15) to (17), Δw and Δw are determined depending on how much the wavelength to be filtered by the optical filter 10 deviates from 1.55 μm (Δλ). 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 w and λ are within the range of values ​​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≦hｒ ≦0.14 1.5≦λ≦1.6 (18)

[0090] (Fourth Example) 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 around 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 unit of the numerical value is μm.

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

[0092] Coefficients α, β, γ, and δ in equation (16) １ and δ ２ It is desirable that the values ​​of α=0.527, β=-0.173, γ=-2.13, and δ=0.527, β=-0.173, and γ=-2.13 are given by the following equation (20). １ = 0.328 δ ２ =-34.1 ε=3.25 (20)

[0093] Using equations (16), (19), and (20), Δw and Δw can be calculated depending on how much the wavelength to be filtered by the optical filter 10 deviates from 1.31 μm (Δλ). 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 ｒ It is desirable that the values ​​of w and λ are within the numerical ranges 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] <Functions and Effects> Functions 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 13 extending along the longitudinal direction and satellites 17 extending along the longitudinal direction on both sides of the core 13 are covered by a cladding 16. The satellites 17 are arranged at periodically different positions along the longitudinal direction so that the distance between the core 13 and the satellites 17 changes periodically with a period set based on the propagation constants of two waveguide modes (zeroth-order mode and first-order mode) in which light propagates through the multi-mode waveguide. In addition to this configuration, the satellites 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 multi-mode waveguide formed by 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 multi-mode waveguide as it travels through the waveguide. By appropriately adjusting the length of the multi-mode waveguide, for example, light incident in zero-order mode can be transitioned to light in first-order mode upon emission.

[0099] By arranging the tapered section 14 that emits 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, light of two waveguide modes propagates within one core 13, and therefore the coupling between the two waveguide modes can be relatively strong. This allows the device length to be shorter than that of a conventional waveguide structure using a grating-assisted coupler, thereby enabling high integration.

[0101] Furthermore, in the optical filter 10 having the above structure, by adjusting parameters such as the cross-sectional size (width and height) of the core 13, the size of the satellite 17, and the distance between the core 13 and the 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 almost zero, thereby achieving 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 along the longitudinal direction have a periodic structure. This structure causes the refractive index of the multimode waveguide formed by the core 13 and the cladding 16 to change periodically, thereby enabling the multimode waveguide to change the mode of incident light 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 multi-mode waveguide and a satellite structure, and the periodic change in the refractive index of the multi-mode waveguide may be realized by using, for example, the EO effect, the TO effect, a mechanical effect, or a nonlinear effect.

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

[0106] 9A and 9B are diagrams 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 and join with each other. The n-type semiconductor structure 18A and the p-type semiconductor structure 19A are also arranged so that they contact the core portion 13A and the satellite portion 17A. This causes the position of the pn junction to change periodically along the longitudinal direction, as shown in Figures 9B and 9C. The period for which the position of the pn junction changes is set to the period shown in equation (1) above.

[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 a depletion layer where no electrons or holes exist changes, thereby controlling the carrier density at the pn junction. This allows the refractive index of the multi-mode waveguide formed by the core portion 13A and the cladding portion 16A to be changed.

[0109] (Second Modification) Figure 10 shows a second modification of the optical filter that utilizes the EO effect. Figure 10A is a schematic top view. Figure 10B is a cross-sectional view taken along line EE in Figure 10A.

[0110] In the optical filter 10B of the second modification, a quantum well layer 41B is formed in the core 13B and the satellite 17B. RF electrodes 42B are arranged along the longitudinal direction of the satellite 17B on both outer sides thereof as viewed from the core 13B. An AC power supply is connected to each RF electrode 42B. The quantum well layer 41B is formed on a substrate made of, for example, 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 electric potential created by the electric field modulates the distribution of two-dimensional electron gas (2DEG) in the quantum well layer 41B. This allows the refractive index of the multi-mode waveguide formed by the core portion 13B and the cladding portion 16B to be periodically changed. In the second modification, the period for changing the refractive index can be controlled by the frequency of the AC voltage applied to the RF electrodes 42B.

[0112] (Third Modification) An optical filter 10C of the third modification utilizes the TO effect. FIG. 11 shows a third 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 F-F in FIG. 11A. Specifically, thin-film heaters 43C are periodically arranged on the substrate on both outer sides of satellite portions 17C arranged along the longitudinal direction of a core portion 13C or in a cladding portion 16C. This creates a temperature distribution in the multi-mode waveguide that corresponds to the amount of heat generated by the thin-film heaters 43C, thereby periodically changing the refractive index of the multi-mode waveguide.

[0113] (Fourth Modification) Figure 12 shows a fourth modification of the optical filter that utilizes a mechanical effect. Figure 12A is a schematic top view. Figure 12B is a cross-sectional view taken along line GG in Figure 12A.

[0114] In the optical filter 10D of the fourth modification, a core 13D, a cladding 16D, and a satellite 17D are fixed to a substrate, and a movable beam 45D is supported by a substrate 44D via springs. 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] 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 control light CL having a different wavelength from that of signal light SL are input to an input portion 11E in a waveguide mode different from the waveguide mode in which 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 propagation constants, the two control light CL1, CL2 interfere with each other, causing a periodic change in the intensity of the control light CL propagating through the multi-mode waveguide.

[0116] This produces an optical nonlinear effect called the optical Kerr effect, which allows the refractive index of the multimode waveguide formed by the core portion 13E and the cladding portion 16E to be changed periodically according to the intensity of the light within 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, TO effect, mechanical effect, 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 same characteristics as 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 classified into a first modulation method that controls the amount of refractive index change and a second modulation method that controls the refractive index change period. In the first modulation method, the transmittance can be changed 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 modifications, the satellite portions do not have a periodic structure because the refractive index is periodically changed by the EO effect, TO effect, mechanical effect, optical nonlinear effect, etc. In this case, the satellite portions are provided to achieve temperature independence of the optical filters 10A, 10B, 10C, 10D, and 10E by adjusting their sizes and distances from the core portions.

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

[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 cause a periodic refractive index change in the multimode waveguide.

[0123] On the other hand, if 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, 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 positions of the satellite portions, an optical filter may be formed in which the side surfaces of the core portion have an uneven shape when viewed from above. In such an optical filter of the seventh modification, the distance between the core portion and the satellite portion changes periodically.

[0126] (Eighth Modification) When a waveguide structure is made of silicon, it is known that the refractive index can be adjusted during use by implanting ions into the waveguide during fabrication and then annealing the portion at a high temperature during use (see J.J. Ackert, et al., Optics Express 19(13), 11969 (2011)). This technique may be applied to the optical filter 10 of the first embodiment, where ions are implanted into the satellite portion 17 during fabrication and the satellite portion is heated to a high temperature during use, thereby enabling the effective refractive index of the entire optical filter 10 to be adjusted.

[0127] FIG. 14 shows an eighth modified example of an optical filter that enables refractive index adjustment by ion implantation. In the eighth modified example, satellite regions 17F, which are provided on both sides of a core region 13F, have periodic ion implantation regions 46F. By heating the satellite regions 17F to a high temperature during calibration before use, the refractive index of the satellite regions 17F, and therefore the effective refractive index of the entire optical filter 10F, can be adjusted as desired. This configuration allows the effective refractive index of the optical filter 10F to be adjusted after its manufacture. This allows for a reduction in manufacturing precision for the optical filter 10F, significantly reducing the manufacturing and design costs of the optical filter 10F.

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

[0129] 1A , in the first embodiment described above, the terminal ends of the satellite portions 17 are formed parallel to the longitudinal direction. However, the optical filter of the present disclosure is not limited to this, and the optical filter of the present disclosure may have satellite portions formed by bending or widening the terminal ends in the width direction opposite to the core portion 13.

[0130] This prevents light from scattering 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 the 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 will be denoted by the same reference numerals and the description thereof will be omitted.

[0133] 15 is a diagram illustrating 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 as the optical filter 10 of the first embodiment from the input portion 11 to the terminal end of the core portion 13 .

[0135] The tapered portion 21 connects the end portion 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 portion of the core portion 13 to the start end of the waveguide 22. In Figure 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 longer to prevent interference with the end portion of the satellite portion 17, for example.

[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 full bond length.

[0137] The waveguide 22 is formed to be wider than the core portion 13, and forms 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-order mode.

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

[0140] As a result, only 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 according to the second embodiment. As shown in Fig. 16, the optical filter 20 functions as a bandpass filter that extracts only specific light.

[0141] In the optical filter 20 of the second embodiment, the scattering of light may be suppressed by bending or widening the ends of the waveguides 22 and 23. Furthermore, structures equivalent to the tapered section 14 and the exit section 15 of the first embodiment may be added to the end of the waveguide 23 so that light in the zeroth mode can be extracted 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 also be provided on the other side.

[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 waveguides 22 and 23 is 0.25 μm, the perfect coupling length at which all the 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, the waveguides 22 and 23 are both structured so that 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] As described above, the optical filter 30 has two input ports and two output ports with a multi-mode waveguide between them, and functions as a periodic filter whose transmittance varies periodically with 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 called a Mach-Zehnder interferometer is known, which splits light from one light source into two parallel beams and measures the phase difference between the parallel beams.

[0152] Fig. 18 is a top view schematic diagram showing an example of a Mach-Zehnder interferometer in which incident light is split into two arms and then combined. 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 a region formed to be thicker than the first arm 51. A 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 the first arm 51 and the second arm 52 is ΔL and the length of the region of the second arm 52 that is thicker than the first arm 51 is L′, the temperature dependence of the M-th mode wavelength is expressed by the following equation (22).

[0154] In such a conventional AMZI, the first arm 51 and the second arm 52 are physically separated from each other, so that a partial temperature change may have a different effect on the first arm 51 than on 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] 17 shows an optical filter 30 according to a third embodiment. The two arms are arranged close to each other, and the core 34 behaves as a single-core multimode waveguide, thereby achieving performance equivalent to that of an AZMI with two arms arranged 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 by which the optical filter 30 according to the third embodiment behaves 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 view of the mode shape when light is incident only from the first incident portion 31. FIG. 19C shows an example of the mode shape when light is incident only from the first incident portion 31, as well as examples of the shapes of the symmetric mode and the antisymmetric mode. FIG. 19D shows a schematic view of the mode shape when light is incident only from the second incident portion 32. FIG. 19E shows an example of the mode shape when light is incident only from the second incident portion 32, as well as examples of 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 below 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 within one core.

[0158] When light is incident only from the first incident portion 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, the behavior is 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 of the core portion 34 changes depending on the wavelength of the incident light.

[0161] The length of the core portion 34 is Lｃｏｒｅ Then, the phase of the S mode in 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 L 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. ｃｏｒｅ = 2π m / (β Ｓ -β ＡＳ ) (23) where m is an integer.

[0163] In the Y branch 36, when the S mode and the AS mode are in phase, the light travels to and is emitted from the first exit portion 37. In addition, in the Y branch 36, when the S mode and the AS mode are out of phase, the light travels to and is emitted from the second exit portion 38. 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, the effective refractive index in each of the S mode and the AS mode is n ０ , n １ As in the optical filter 10 of the first embodiment, by adjusting the size of each component and the distance between the components so that equations (5) and (6) are satisfied, the optical filter 30 can be made temperature independent.

[0165] In the optical filter according to the third embodiment, the phase difference between two modes can be controlled by the EO effect, TO effect, mechanical effect, or optical nonlinear effect. The configurations of the modified examples using the EO effect, TO effect, mechanical effect, or optical nonlinear effect are almost the same as the first to fifth modified examples described in the first embodiment, and therefore will not be described here.

[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 an optical filter according to the present disclosure. The wavelength locker device 100 includes 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 used as an example of the optical filter according to the present disclosure, but the wavelength locker device according to the present disclosure can also employ the optical filters 10A to 10F shown in the modifications 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 light 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 control signals S1 and 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 prevents changes in the wavelength of the laser light source 200 due to temperature changes, and makes it possible to stabilize (lock) the wavelength of the light output by the laser light source 200.

[0169] Although the fourth embodiment has been described with reference to a wavelength locker device as an example of application of the optical filter of the present disclosure, 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.

[0171] The disclosures of the specification, drawings and abstract contained in Japanese Patent Application No. 2023-182082, filed on October 23, 2023, are incorporated herein by reference in their entirety.

[0172] The present disclosure is useful in optical filters and rocker devices.

[0173] 10, 10A, 10B, 10C, 10D, 10E, 10F, 20, 30 Optical filter 11, 11E Incident portion 12 Tapered portion 13, 13A, 13B, 13C, 13D, 13E, 13F Core portion 14 Tapered portion 15 Emitting portion 16, 16A, 16B, 16C, 16D, 16E, 16F Cladding portion 17, 17A, 17B, 17C, 17D, 17E, 17F Satellite portion 171, 172 Region 18A n-type semiconductor structure 19A p-type semiconductor structure 41B Quantum well layer 42B Electrode 43C Thin film heater 44D Substrate 45D Movable beam 46F Ion implantation region 21 Tapered portion 22, 23 Waveguide 24 Output section 31 First input section 32 Second input section 33 Y branch 34 Core section 35 Satellite section 36 Y branch 37 First output section 38 Second output section 39 Cladding section 100 Wavelength locker device 110 First optical power monitor 120 Second optical power monitor 130 Beam splitter 140 Optical input port SL Signal light CL, CL1, CL2 Control light

Claims

1. An optical filter comprising: 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.

2. The optical filter according to claim 1, wherein a multi-mode waveguide formed by said core portion and said cladding portion transfers light of a first guided mode to a second guided mode.

3. The optical filter according to claim 2, further comprising an output section that does not output light of the second waveguide mode shifted by the multi-mode waveguide, but outputs light of another waveguide mode.

4. The optical filter according to claim 2, further comprising an output section that outputs light of the second waveguide mode shifted by the multi-mode waveguide and does not output light of other waveguide modes.

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

6. The optical filter according to claim 5, wherein the periodic structure is a structure in which the distance between the core portion and the satellite portion changes periodically.

7. The optical filter according to claim 6, wherein the period Λ of the periodic structure along the one direction is expressed by Equation (1). 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 8. The optical filter according to claim 2, wherein / ∂T satisfies the relationship of formula (2). 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 w of the core and the width w 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, and 0.1≦w ｐ ≦0.5 0.1≦w ｇ 9. The optical filter according to claim 8, wherein h is set to ≦0.8 0.18≦h≦0.

26.

10. The core and satellite sections have a channel-type waveguide structure, and the width w of the core section and the width w of the satellite section 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 9. The optical filter according to claim 8, wherein h is set to ≦0.8 0.18≦h≦0.

26.

11. The core and satellite have a rib-type waveguide structure, and the width w of the core and the width w 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 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 The optical filter of claim 8 , wherein the ΔE is set to ≦0.

14.

12. The core and satellite sections have a rib-type waveguide structure, and the width w of the core section and the width w of the satellite section 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 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 The optical filter of claim 8 , wherein the ΔE is set to ≦0.

14.

13. The optical filter according to claim 1, wherein the satellite portion changes a phase difference between two guided modes in a multimode waveguide formed by the core portion and the cladding portion between the start end and the end end of the multimode waveguide.

14. The optical filter according to claim 13, further comprising: first and second input portions connected to said starting end portion; and first and second output portions connected to said terminal end portion.

15. The optical filter according to claim 13 or 14, wherein the core portion and the satellite portion form an asymmetric Mach-Zehnder interferometer (AZMI).

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 16. The optical filter according to claim 13, wherein / ∂T satisfies the relationship of formula (3). T is the temperature and λ is the wavelength of the light.

17. An optical filter comprising: 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.

18. A wavelength locker device comprising: a first optical power monitor that measures the optical intensity of at least a portion of a 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 an optical filter described in any one of claims 1 to 17; 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.

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