Wavelength filter

Abstract

This wavelength filter comprises: an input waveguide into which input light is input; a first ring resonator having a first coupler and a second coupler; an intermediate waveguide connected to the second coupler; a second ring resonator having a third coupler and a fourth coupler; and an output waveguide connected to the fourth coupler. The first ring resonator and the second ring resonator are configured to have a specific wavelength region, which is a wavelength region in which, as the wavelength increases, the transmittance and the half-value width of a transmission peak of the first ring resonator exhibit wavelength dependence in which the transmittance and the half-value width monotonically increase or decrease, while the transmittance and the half-value width of a transmission peak of the second ring resonator exhibit wavelength dependence in which the transmittance and the half-value width monotonically change in a direction opposite to that of the first ring resonator.

Description

Wavelength filter

[0001] This disclosure relates to wavelength filters.

[0002] Conventionally, wavelength filters (vernier filters) are known that can expand the FSR (Free Spectrum Range) compared to a ring resonator consisting of a single ring waveguide by connecting two ring resonators with different FSRs (Free Spectrum Ranges) in series, through the vernier effect (see, for example, Patent Document 1).

[0003] Japanese Patent Publication No. 2006-278769

[0004] In the wavelength filters described above, the coupling coefficient of the coupler in each ring resonator changes depending on the wavelength of light input to the wavelength filter, and may also change due to manufacturing variations in the coupler. Furthermore, the transmission characteristics of each ring resonator (for example, the transmittance of light (hereinafter simply referred to as "transmittance") and the full width at half maximum (FWHM) of the transmission peak (hereinafter simply referred to as "FWHM")) change depending on the coupling coefficient of the coupler constituting each ring resonator. Generally, the transmission characteristics of a ring resonator alone tend to show a large FWHM in the wavelength range with high transmittance and a small FWHM in the wavelength range with low transmittance, but for a wavelength filter, it is preferable to have high transmittance and a small FWHM. The transmission characteristics of a vernier filter are determined by the superposition of the transmission characteristics of two ring resonators. For this reason, for example, when a vernier filter is constructed by combining two ring resonators that exhibit similar transmission characteristics, the transmission characteristics of the vernier filter tend to show a large FWHM in the wavelength range with high transmittance and a small transmittance in the wavelength range with a small FWHM, as a result of the superposition of the transmission characteristics of each ring resonator. As a result, the range in which a suitable transmittance and suitable full width at half maximum can be achieved simultaneously as a vernier filter (i.e., the wavelength tuning range in which it operates properly as a wavelength filter) may become narrower.

[0005] Therefore, one aspect of this disclosure aims to provide a wavelength filter that can suitably secure a wavelength tunable range.

[0006] This disclosure includes the following wavelength filters [1] to

[13] .

[0007] [1] A first ring resonator comprising: an input waveguide into which input light is input; a first coupler connected to the input waveguide and having one upstream port into which the input light is input; a second coupler having one upstream port connected to one downstream port of the first coupler and one downstream port connected to another upstream port of the first coupler; an intermediate waveguide connected to another downstream port of the second coupler; a third coupler connected to the intermediate waveguide and having one upstream port into which light from the other downstream port of the second coupler is input; a fourth coupler having one upstream port connected to one downstream port of the third coupler and one downstream port connected to another upstream port of the third coupler; and an output waveguide connected to another downstream port of the fourth coupler. A wavelength filter in which the first ring resonator and the second ring resonator are configured to have a specific wavelength range in which the transmittance and full width at half maximum of the transmission peak of the first ring resonator exhibit a wavelength dependence in which the transmittance and full width at half maximum of the transmission peak of the second ring resonator exhibit a wavelength dependence in which the transmittance and full width at half maximum of the transmission peak of the second ring resonator exhibit a wavelength dependence in which the transmission and full width at half maximum of the transmission peak change monotonically in the opposite direction to that of the first ring resonator.

[0008] The wavelength filter described in [1] above has a configuration in which an input waveguide, a first ring resonator, an intermediate waveguide, a second ring resonator, and an output waveguide are connected in series. It is configured to have a specific wavelength range in which the transmittance and full width at half maximum of the first ring resonator exhibit a wavelength dependence in which they monotonically increase or decrease as the wavelength of the incident light increases, while the transmittance and full width at half maximum of the second ring resonator exhibit a wavelength dependence in which they monotonically change in the opposite direction to that of the first ring resonator. In other words, in the specific wavelength range, the first ring resonator and the second ring resonator are configured to change their transmission characteristics in accordance with the change in wavelength so that they cancel out (reduce) each other's wavelength dependence. By providing such a specific wavelength range in the above wavelength filter, it is possible to suitably secure a wavelength range in which both transmittance and full width at half maximum are balanced (i.e., a wavelength tuning range in which the wavelength filter operates suitably) from within that specific wavelength range.

[0009] [2] The wavelength filter of [1], wherein the first to fourth couplers are directional couplers, and the parameters of each of the first to fourth couplers are adjusted so that the first ring resonator and the second ring resonator have the specific wavelength range.

[0010] According to the configuration described in [2] above, the first to fourth couplers can be made smaller compared to the case where the first to fourth couplers are made of MZI couplers, thus enabling miniaturization of the wavelength filter. In addition, compared to the case where the first to fourth couplers are made of MMI couplers, the loss of transmitted light in the first to fourth couplers can be suppressed.

[0011] [3] The wavelength filter of [1], wherein the first to fourth couplers are MMI couplers, and the parameters of each of the first to fourth couplers are adjusted so that the first ring resonator and the second ring resonator have the specific wavelength range.

[0012] According to the configuration described in [3] above, compared to the case where the first to fourth couplers are configured as directional couplers or MZI couplers, the characteristic variations due to manufacturing errors of the first to fourth couplers (e.g., variations in the coupling coefficient) can be reduced, thereby suppressing product-to-product variations in the transmission characteristics of the wavelength filter. Furthermore, in the case of MMI couplers, if a tapered or bent type is adopted, the light can be split at a splitting ratio other than 1:1 while suppressing the loss of transmitted light.

[0013] [4] The wavelength filter of [1], wherein the first to fourth couplers are MZI couplers, and the parameters of each of the first to fourth couplers are adjusted so that the first ring resonator and the second ring resonator have the specific wavelength range.

[0014] According to the configuration described in [4] above, the optical branching ratio in the first to fourth couplers can be easily adjusted by an electrical signal.

[0015] [5] The first ring resonator and the second ring resonator have a first connection configuration or a second connection configuration, the first connection configuration is such that the first downstream port of the first coupler is a cross port to the first upstream port of the first coupler, the other downstream port of the second coupler is a cross port to the first upstream port of the second coupler, the first downstream port of the third coupler is a bar port to the first upstream port of the third coupler, and the other downstream port of the fourth coupler is a bar port to the first upstream port of the fourth coupler, the second connection configuration is such that the first downstream port of the first coupler is a bar port to the first upstream port of the first coupler, the other downstream port of the second coupler is a bar port to the first upstream port of the second coupler, and the first downstream port of the third coupler is a cross port to the first upstream port of the third coupler, A wavelength filter of any of [1] to [4], wherein the other downstream port of the fourth coupler is a cross port to the one upstream port of the fourth coupler.

[0016] According to the configuration in [5] above, by employing the same type of coupler (a coupler with the same wavelength dependence of coupling coefficients) for the first to fourth couplers, a wavelength filter can be configured such that the second ring resonator has opposing transmission characteristics with respect to the first ring resonator. Therefore, while obtaining the same effect as the wavelength filter in [1] above, design resources can be reduced by limiting the type of coupler constituting the first to fourth couplers to just one type.

[0017] [6] The wavelength filter of [5], wherein the first connection configuration further comprises a radiation waveguide connected to another downstream port of the third coupler and discarding non-resonant peak light other than resonant peak light passing through the first ring resonator and the second ring resonator, and a radiation waveguide connected to another upstream port of the fourth coupler and discarding the non-resonant peak light, and the second connection configuration further comprises a radiation waveguide connected to another downstream port of the first coupler and discarding the non-resonant peak light, and a radiation waveguide connected to another upstream port of the second coupler and discarding the non-resonant peak light.

[0018] According to the configuration described in [6] above, in both the first and second connection configurations, light other than the resonant peak light (non-resonant peak light) can be appropriately discarded via the radiating waveguide inside the ring-shaped waveguide of the first or second ring resonator. This makes it possible to suppress the generation of stray light due to non-resonant peak light.

[0019] [7] A first ring resonator comprising: an input waveguide into which input light is input; a first coupler connected to the input waveguide and having one upstream port into which the input light is input; a second coupler having one upstream port connected to one downstream port of the first coupler and one downstream port connected to another upstream port of the first coupler; an intermediate waveguide connected to another downstream port of the second coupler; a third coupler connected to the intermediate waveguide and having one upstream port into which light from the other downstream port of the second coupler is input; a fourth coupler having one upstream port connected to one downstream port of the third coupler and one downstream port connected to another upstream port of the third coupler; and an output waveguide connected to another downstream port of the fourth coupler. A wavelength filter comprising a first ring resonator and a second ring resonator, wherein the first and second ring resonators are configured to have at least one of a first wavelength range in which the coupling coefficients of the first and second couplers increase monotonically as the wavelength increases, while the coupling coefficients of the third and fourth couplers decrease monotonically as the wavelength increases, and a second wavelength range in which the coupling coefficients of the first and second couplers decrease monotonically as the wavelength increases, while the coupling coefficients of the third and fourth couplers increase monotonically.

[0020] According to the configuration of [7] above, by setting the wavelength dependence of the coupling coefficients of the first to fourth couplers as described above, a configuration in which the first ring resonator and the second ring resonator have a specific wavelength range (a wavelength range corresponding to at least one of the first wavelength range and the second wavelength range) can be reliably and easily obtained.

[0021] [8] The wavelength filter of [7], wherein the first ring resonator and the second ring resonator have both the first wavelength range and the second wavelength range.

[0022] According to the configuration described in [8] above, the wavelength dependence of the coupling coefficient of the first to fourth couplers can be suitably suppressed.

[0023] [9] The wavelength filter of [7], wherein the first ring resonator and the second ring resonator have only one of the first wavelength range and the second wavelength range.

[0024] According to the configuration of [9] above, the design of the wavelength filter can be facilitated as compared with the case where the wavelength filter is configured to have both the first wavelength range and the second wavelength range.

[0025]

[10] A wavelength filter according to any one of [1] to [9], which is mounted on a wavelength tunable laser and functions as an external resonator that controls the wavelength tunable range of the laser light emitted from the wavelength tunable laser.

[0026] According to the configuration of

[10] above, a wavelength tunable laser having a wide wavelength tunable range can be provided.

[0027]

[11] The wavelength filter according to

[10] , wherein the wavelength tunable range is a range of ±2% or more of the center wavelength from the center wavelength of the wavelength tunable range.

[0028] According to the configuration of

[11] above, the wavelength tunable range of the wavelength tunable laser can be suitably widened.

[0029]

[12] The wavelength filter according to

[10] , wherein the wavelength tunable range is a range of ±5% or more of the center wavelength from the center wavelength of the wavelength tunable range.

[0030] According to the configuration of

[12] above, the effect of the configuration of

[11] can be further improved.

[0031]

[13] The wavelength filter according to

[10] , wherein the wavelength tunable range is a range of ±10% or more of the center wavelength from the center wavelength of the wavelength tunable range.

[0032] According to the configuration of

[13] above, the effect of the configuration of

[12] can be further improved.

[0033] According to one aspect of the present disclosure, it is possible to provide a wavelength filter that can suitably secure a wavelength tunable range.

[0034] FIG. 1 is a diagram showing a configuration example of a wavelength tunable laser including a wavelength filter according to an embodiment. FIG. 2 is a diagram showing a configuration example of a directional coupler. FIG. 3 is a diagram showing an example of the wavelength dependence of the coupling coefficient of each coupler. FIG. 4 is a diagram showing an example of the transmission characteristics of the wavelength filter. FIG. 5 is a diagram showing the transmission characteristics of the wavelength filter according to a comparative example. FIG. 6 is a diagram showing a configuration example of an MMI coupler. FIG. 7 is a diagram showing a modified example of the wavelength dependence of the coupling coefficient of each coupler. FIG. 8 is a diagram showing a first modified example of the wavelength filter. FIG. 9 is a diagram showing a second modified example of the wavelength filter. FIG. 10 is a diagram showing a modified example of the wavelength tunable laser.

[0035] Hereinafter, an embodiment of the present disclosure will be described in detail with reference to the drawings. In the following description, the same or corresponding elements are denoted by the same reference numerals, and redundant descriptions are omitted.

[0036] Referring to FIG. 1, a wavelength tunable laser 1 including a wavelength filter 10 according to this embodiment will be described. As shown in FIG. 1, the wavelength tunable laser 1 includes a semiconductor optical amplifier 2 and an optical integrated circuit 3. The optical integrated circuit 3 includes a substrate 4 and various optical elements including an optical waveguide provided on the substrate 4. The substrate 4 is, for example, a rectangular plate-shaped silicon substrate. In the wavelength tunable laser 1, a wavelength filter 10 and a mirror portion 70 are mounted on one main surface (the front surface in FIG. 1) of the substrate 4 constituting the optical integrated circuit 3. In the present disclosure, the side where the semiconductor optical amplifier 2 is located with respect to the wavelength filter 10 is defined as "upstream", and the side where the wavelength filter 10 is located with respect to the semiconductor optical amplifier 2 is defined as "downstream".

[0037] The semiconductor optical amplifier 2 is constituted by, for example, a quantum cascade laser (QCL). The semiconductor optical amplifier 2 emits input light L from one end face 2a to the optical integrated circuit 3. ｉｎ The wavelength filter 10 outputs, as output light L, the resonant peak light among the input light L incident from the upstream side of the wavelength filter 10. ｉｎ The resonant peak light is light having a wavelength (resonant peak wavelength) that passes through (transmits through) the wavelength filter 10 and is emitted from the downstream side of the wavelength filter 10. ｏｕｔ

[0038] Output light L ｏｕｔ is reflected by the mirror unit 70 and returned to the wavelength filter 10 as the return light L ｒ The return light L returned to the wavelength filter 10 passes through the wavelength filter 10 from the downstream side to the upstream side. When the return light L ｒ enters the semiconductor optical amplifier 2 (end face 2a), light of the resonance peak wavelength is amplified inside the semiconductor optical amplifier 2. A part of the light amplified inside the semiconductor optical amplifier 2 is emitted again from the end face 2a to the optical integrated circuit 3, and similarly its return light L ｒ is returned to the semiconductor optical amplifier 2, and the light of the resonance peak wavelength is further amplified inside the semiconductor optical amplifier 2. A part of the light amplified inside the semiconductor optical amplifier 2 in this way is emitted to the outside from the other end face 2b of the semiconductor optical amplifier 2 as the laser light L.

[0039] The wavelength filter 10 includes an input waveguide 20, a first ring resonator 30, an intermediate waveguide 60, a second ring resonator 40, and an output waveguide 50. The wavelength filter 10 is provided on the substrate 4. The wavelength filter 10 outputs the resonance peak light among the input light L ｉｎ incident on the input waveguide 20 as the output light L ｏｕｔ from the output waveguide 50. Light other than the resonance peak light (non-resonance peak light) is discarded to each terminator (terminators 36, 38, 46, 48) via each radiation waveguide (radiation waveguides 35, 37, 45, 47) described later. In the present disclosure, since the upstream and downstream are defined as described above, in each coupler 31, 32, 41, 42 included in the wavelength filter 10, the side where the resonance peak light among the light input from the input waveguide 20 is input is the "upstream", and the side where the resonance peak light is output is the "downstream".

[0040] ​​The first ring resonator 30 includes a first coupler 31, a second coupler 32, and two waveguides (first waveguide 33 and second waveguide 34) that connect the first coupler 31 and the second coupler 32 in a ring shape. The first coupler 31 and the second coupler 32 are configured as directional couplers having 4 (2 x 2) ports, for example. For example, the first coupler 31 and the second coupler 32 are configured as identical couplers exhibiting the same wavelength dependence.

[0041] The second ring resonator 40 includes a third coupler 41, a fourth coupler 42, and two waveguides (third waveguide 43 and fourth waveguide 44) that connect the third coupler 41 and the fourth coupler 42 in a ring shape. The third coupler 41 and the fourth coupler 42 are configured as directional couplers having 4 (2 x 2) ports, for example. For example, the third coupler 41 and the fourth coupler 42 are configured as identical couplers exhibiting the same wavelength dependence.

[0042] Figure 2 schematically shows several configuration examples of directional couplers. Figure 2(a) shows an example of a linear directional coupler. In a linear directional coupler, two parallel waveguides are in close proximity inside the coupler (central part). In a directional coupler, the coupling coefficient, which will be described later, is adjusted by parameters such as the length of this proximity section (coupling length Lc), the distance between the two waveguides (gap G), and the width B of each waveguide.

[0043] Figure 2(b) shows an example of an adiabatic directional coupler. In an adiabatic directional coupler, the width B of each waveguide is configured to vary along the waveguide. In the example in Figure 2(b), one waveguide (upper side in the figure) is configured so that the width B gradually increases from upstream (left side in the figure) to downstream (right side in the figure). On the other hand, the other waveguide (lower side in the figure) is configured so that the width B gradually decreases from upstream to downstream. In the case of such an adiabatic directional coupler, the coupling coefficient is adjusted by the same parameters (coupling length Lc, gap G, width B) as described above for the linear type. However, in an adiabatic directional coupler, the width B of each waveguide, among the parameters of the linear type described above, can be defined not as a fixed value but as a variable B(x) that corresponds to the waveguide position x. Thus, the parameters that affect the coupling coefficient of a directional coupler can differ depending on the type of directional coupler.

[0044] Figure 2(c) shows an example of a bent directional coupler. The bent directional coupler differs from the linear type shown in Figure 2(a) mainly in that the adjacent portion of the two waveguides, which extend parallel to each other in the linear type, is curved. Such a bent directional coupler may include parameters that affect the coupling coefficient, such as the degree of bending of the adjacent portion, in addition to the linear type parameters (coupling length Lc, gap G, width B) mentioned above.

[0045] The first coupler 31 has an upstream port 31a (one upstream port), an upstream port 31b (another upstream port), a downstream port 31c (one downstream port), and a downstream port 31d (another downstream port). The downstream port 31c is the cross port of the upstream port 31a (i.e., the bar port of the upstream port 31b). The downstream port 31d is the bar port of the upstream port 31a (i.e., the cross port of the upstream port 31b). The input waveguide 20 is connected to the upstream port 31a of the first coupler 31.

[0046] The second coupler 32 has an upstream port 32a (one upstream port), an upstream port 32b (another upstream port), a downstream port 32c (another downstream port), and a downstream port 32d (one downstream port). The downstream port 32c is a cross port of the upstream port 32a (i.e., a bar port of the upstream port 32b). The downstream port 32d is a bar port of the upstream port 32a (i.e., a cross port of the upstream port 32b).

[0047] The first waveguide 33 is a waveguide connecting the downstream port 31c and the upstream port 32a. The second waveguide 34 is a waveguide connecting the downstream port 32d and the upstream port 31b. The first waveguide 33 and the second waveguide 34 are configured in a ring shape via the first coupler 31 and the second coupler 32. The first waveguide 33 and the second waveguide 34 may be formed from the same material as the substrate (e.g., silicon), or from a different material than the substrate (e.g., silicon nitride, germanium, etc.) provided on the substrate. For example, the first waveguide 33 and the second waveguide 34 may be configured as rib waveguides formed by processing the surface of the substrate into a convex shape. The same applies to the other waveguides.

[0048] The downstream port 31d of the first coupler 31 is connected to the terminator 36 via the radiating waveguide 35. The upstream port 32b of the second coupler 32 is connected to the terminator 38 via the radiating waveguide 37. The terminator 36 prevents light incident from the downstream port 31d of the first coupler 31 into the radiating waveguide 35 from returning to the downstream port 31d due to reflection, etc. The terminator 38 prevents light incident from the upstream port 32b of the second coupler 32 into the radiating waveguide 37 from returning to the upstream port 32b due to reflection, etc.

[0049] The first ring resonator 30 further includes a phase shifter 39 for filter control, which is arranged along the first waveguide 33. The phase shifter 39 is, for example, a thermal phase shifter that generates heat when power is supplied. The phase shifter 39 is composed of, for example, a thin-film heater. By generating heat, the phase shifter 39 changes (increases) the refractive index of the portion of the ring-shaped waveguides 33 and 34 constituting the first ring resonator 30 that is along the phase shifter 39 (in the example of Figure 1, the portion along a part of the first waveguide 33), thereby shifting the phase of light circulating around the ring-shaped waveguides 33 and 34. By controlling the power supplied to the phase shifter 39, the amount of phase adjustment (phase shift amount) of the first ring resonator 30 can be controlled.

[0050] The second ring resonator 40 is connected to the first ring resonator 30 via an intermediate waveguide 60. The intermediate waveguide 60 is a waveguide that connects the downstream port 32c of the second coupler 32 to the upstream port 41a of the third coupler 41.

[0051] The third coupler 41 has an upstream port 41a (one upstream port), an upstream port 41b (another upstream port), a downstream port 41c (one downstream port), and a downstream port 41d (another downstream port). The downstream port 41c is a cross port of the upstream port 41a (i.e., a bar port of the upstream port 41b). The downstream port 41d is a bar port of the upstream port 41a (i.e., a cross port of the upstream port 41b).

[0052] The fourth coupler 42 has an upstream port 42a (one upstream port), an upstream port 42b (another upstream port), a downstream port 42c (another downstream port), and a downstream port 42d (one downstream port). The downstream port 42c is the cross port of the upstream port 42a (i.e., the bar port of the upstream port 42b). The downstream port 42d is the bar port of the upstream port 42a (i.e., the cross port of the upstream port 42b). The output waveguide 50 is connected to the downstream port 42c of the fourth coupler 42.

[0053] The third waveguide 43 is a waveguide connecting the downstream port 41c and the upstream port 42a. The fourth waveguide 44 is a waveguide connecting the downstream port 42d and the upstream port 41b. The third waveguide 43 and the fourth waveguide 44 are configured in a ring shape via the third coupler 41 and the fourth coupler 42.

[0054] The downstream port 41d of the third coupler 41 is connected to the terminator 46 via the radiating waveguide 45. The upstream port 42b of the fourth coupler 42 is connected to the terminator 48 via the radiating waveguide 47. The terminator 46 prevents light incident from the downstream port 41d of the third coupler 41 into the radiating waveguide 45 from returning to the downstream port 41d due to reflection, etc. The terminator 48 prevents light incident from the upstream port 42b of the fourth coupler 42 into the radiating waveguide 47 from returning to the upstream port 42b due to reflection, etc.

[0055] The second ring resonator 40 further includes a phase shifter 49 for filter control, which is positioned along the third waveguide 43. The phase shifter 49 is, for example, a thermal phase shifter that generates heat when power is supplied. The phase shifter 49 is composed of, for example, a thin-film heater. By generating heat, the phase shifter 49 changes (increases) the refractive index of the portion of the ring-shaped waveguides 43 and 44 constituting the second ring resonator 40 that is along the phase shifter 49 (in the example in Figure 1, the portion along a part of the third waveguide 43), thereby shifting the phase of light circulating around the ring-shaped waveguides 43 and 44. By controlling the power supplied to the phase shifter 49, the amount of phase adjustment (phase shift amount) of the second ring resonator 40 can be controlled.

[0056] The end of the output waveguide 50 opposite to the side connected to the downstream port 42c is optically connected to the mirror section 70 (in this embodiment, the upstream port of the 1x2 coupler that constitutes the mirror section 70). The mirror section 70 receives the output light L emitted from the output waveguide 50. ｏｕｔ It is configured to return at least a portion of it to the output waveguide 50. In the tunable laser 1, the mirror section 70 has a reflectivity of 100% and the output light L ｏｕｔ Return light L ｒIt is configured to return the output waveguide 50. The mirror section 70 is configured as a loop mirror in which the two ports on the output side (downstream side) of a 1x2 coupler are connected by a loop-shaped waveguide. However, the configuration of the mirror section 70 is not limited to the above configuration. For example, the output light L ｏｕｔ Return light L ｒ The mirror portion, which has the function of reflecting light, may be made of a highly reflective film provided on the side surface of the substrate 4.

[0057] A phase shifter 80 for longitudinal mode control is positioned near the output waveguide 50, along the output waveguide 50. However, the position of the phase shifter 80 is not limited to along the output waveguide 50. For example, the phase shifter 80 may be positioned along the input waveguide 20 or the intermediate waveguide 60.

[0058] The phase shifter 80 is a thermal phase shifter similar to, for example, phase shifters 39 and 49. By generating heat, the phase shifter 80 changes the refractive index of the portion of the output waveguide 50 along the phase shifter 80, thereby shifting the longitudinal modes of the resonator formed by the semiconductor optical amplifier 2 and the optical integrated circuit 3 (wavelength filter 10 and mirror section 70). As a result, the output light L passing through the output waveguide 50 ｏｕｔ The wavelength can be changed by the phase shifter 80. ｏｕｔ It is possible to fine-tune the wavelength.

[0059] The first ring resonator 30 and the second ring resonator 40 have different circumference lengths. That is, the first ring resonator 30 and the second ring resonator 40 have different FSRs. By being composed of these two ring resonators 30 and 40, the wavelength filter 10 has an expanded FSR due to the Vernier effect compared to when it is composed of a single ring resonator (either the first ring resonator 30 or the second ring resonator 40).

[0060] Using Figure 3, an example of the wavelength dependence of the coupling coefficients of each coupler constituting the wavelength filter 10 will be explained. In general, couplers have the characteristic that their coupling coefficients change depending on the wavelength of the incident light (wavelength dependence). In this embodiment, the coupling coefficient is the ratio of the light incident on the upstream port of the coupler that is branched to the cross port relative to the upstream port. For example, if the coupling coefficient of a coupler at a certain wavelength is κ, the coupler branches the light of that wavelength input to the upstream port to the cross port and the bar port relative to the upstream port in a ratio of "κ:1-κ".

[0061] The solid line C1 in Figure 3 shows the wavelength dependence of the coupling coefficients of the first coupler 31 and the second coupler 32 of the first ring resonator 30. The dashed line C2 in Figure 3 shows the wavelength dependence of the coupling coefficients of the third coupler 41 and the fourth coupler 42 of the second ring resonator 40. In the graph in Figure 3, the horizontal axis represents wavelength (nm), and the vertical axis represents the coupling coefficient.

[0062] As shown by line C1, each of the first coupler 31 and the second coupler 32 is configured such that the coupling coefficient increases monotonically as the wavelength increases, by adjusting the parameters of the directional coupler described above. On the other hand, as shown by line C2, each of the third coupler 41 and the fourth coupler 42 is configured such that the coupling coefficient decreases monotonically as the wavelength increases, by adjusting the parameters of the directional coupler described above. In other words, in this embodiment, the wavelength dependence of the coupling coefficients of the couplers constituting the first ring resonator 30 (first coupler 31 and second coupler 32) has characteristics opposite to those of the wavelength dependence of the coupling coefficients of the couplers constituting the second ring resonator 40 (third coupler 41 and fourth coupler 42).

[0063] In the example shown in Figure 3, the first coupler 31 and the second coupler 32 each have a wavelength dependence (linear C1) in which the coupling coefficient increases monotonically and linearly in response to changes in wavelength, but they may also have a wavelength dependence in which the coupling coefficient increases monotonically and nonlinearly (for example, quadratic) in response to changes in wavelength. Similarly, the third coupler 41 and the fourth coupler 42 each have a wavelength dependence (linear C2) in which the coupling coefficient decreases monotonically and linearly in response to changes in wavelength, but they may also have a wavelength dependence in which the coupling coefficient decreases monotonically and nonlinearly (for example, quadratic) in response to changes in wavelength. For example, by configuring couplers 31, 32, 41, and 42 as linear directional couplers (see Figure 2(a)), the coupling coefficients of couplers 31, 32, 41, and 42 can be given a linear wavelength dependence (for example, linear C1 and C2). Furthermore, by configuring couplers 31, 32, 41, and 42 as bendable directional couplers (see Figure 2(c)), the coupling coefficients of couplers 31, 32, 41, and 42 can be given a nonlinear wavelength dependence (for example, curves C10 and C20 in Figure 7, which will be described later).

[0064] An example of the transmission characteristics of the wavelength filter 10 will be explained using Figure 4. Since the first ring resonator 30 is composed of couplers 31 and 32 having the wavelength dependence of the coupling coefficient as described above, the transmission characteristics of the first ring resonator 30 (e.g., transmittance and full width at half maximum) have a wavelength dependence that changes according to the wavelength of the incident light. Similarly, the transmission characteristics of the second ring resonator 40 also have a wavelength dependence corresponding to the wavelength dependence of the coupling coefficient of couplers 41 and 42.

[0065] Figure 4(a) shows the transmission characteristics T1 (solid line) of the first ring resonator 30 and T2 (dashed line) of the second ring resonator 40. Figure 4(b) shows the transmission characteristics T of the wavelength filter 10 (i.e., the transmission characteristics as a vernier filter combining the first ring resonator 30 and the second ring resonator 40). In each graph in Figure 4, the horizontal axis represents wavelength (nm) and the vertical axis represents transmittance (dB).

[0066] The transmission characteristics T1 and T2 each have periodically occurring peaks in transmittance. The wavelengths of each peak in the transmission characteristics T1 and T2 change with the circumference length of each ring resonator (first ring resonator 30 and second ring resonator 40), and correspond to the wavelengths when the circumference length is an integer multiple of the wavelength of light passing through each ring resonator 30 and 40. In this embodiment, if a phase shifter 39 is provided, the transmission characteristic T1 can be adjusted by the phase shifter 39. Similarly, if a phase shifter 49 is provided, as in this embodiment, the transmission characteristic T2 can be adjusted by the phase shifter 49.

[0067] The transmission characteristics T1 and T2 vary depending on the coupling coefficient of the couplers included in each ring resonator. Generally, in wavelength ranges where the coupling coefficient of the couplers constituting the ring resonator is relatively large, the transmittance and full width at half maximum of each peak tend to be relatively large, and in wavelength ranges where the coupling coefficient of the couplers is relatively small, the transmittance and full width at half maximum of each peak tend to be relatively small. For this reason, the first ring resonator 30 and the second ring resonator 40 have the wavelength dependence of their transmission characteristics as described below.

[0068] The coupling coefficient (linear curve C1) of the first coupler 31 and the second coupler 32 constituting the first ring resonator 30 is adjusted so that the coupling coefficient increases monotonically as the wavelength increases from the short wavelength side to the long wavelength side. As a result, the first ring resonator 30 is configured to exhibit a transmission characteristic T1 in which the transmittance of each peak (i.e., the transmittance at the peak of each peak) and the full width at half maximum H1 increase monotonically as the wavelength increases from the short wavelength side to the long wavelength side. In other words, in this embodiment, the transmission characteristic T1 of the first ring resonator 30 changes in one direction (in the direction in which the transmittance of each peak and the full width at half maximum H1 increase) as the wavelength increases in a predetermined wavelength range (in this embodiment, the entire wavelength range). In this embodiment, the transmittance of each peak of the first ring resonator 30 is adjusted so that it changes monotonically (increases in this example) in the region in the wavelength tuning range of the tunable laser 1 where the transmittance of each peak is -3 dB or more.

[0069] The coupling coefficient (linear curve C2) of the third coupler 41 and the fourth coupler 42 constituting the second ring resonator 40 is adjusted so that the coupling coefficient decreases monotonically as the wavelength increases from the short wavelength side to the long wavelength side. As a result, the second ring resonator 40 is configured to exhibit a transmission characteristic T2 in which the transmittance and full width at half maximum H2 of each peak decrease monotonically as the wavelength increases from the short wavelength side to the long wavelength side. In other words, in this embodiment, the transmission characteristic T2 of the second ring resonator 40 changes in one direction (in the direction in which the transmittance and full width at half maximum H2 of each peak decrease) as the wavelength increases in a predetermined wavelength range (the entire wavelength range). In this embodiment, the transmittance of each peak of the second ring resonator 40 is adjusted so that it changes monotonically (decreases in this example) in the region where the transmittance of each peak is -3 dB or more in the wavelength tuning range of the tunable laser 1.

[0070] As described above, in this embodiment, the first ring resonator 30 and the second ring resonator 40 have transmission characteristics T1 and T2 that have wavelength dependences in opposite directions. In other words, the first ring resonator 30 and the second ring resonator 40 are configured to have a wavelength range (specific wavelength range) in which their respective transmission characteristics T1 and T2 change monotonically in opposite directions as the wavelength increases. In this embodiment, the entire wavelength range is the specific wavelength range.

[0071] Here, we will explain the transmission of light in the wavelength filter 10. In the wavelength filter 10, the input light L incident from the input waveguide 20 to the first ring resonator 30 (upstream port 31a of the first coupler 31) ｉｎ Of these, the light that passes through the first ring resonator 30 with a transmittance corresponding to the transmission characteristic T1 is the intermediate light L ｍｉｄ The intermediate light L is then emitted into the intermediate waveguide 60. ｍｉｄ Of these, the light that passes through the second ring resonator 40 with a transmittance corresponding to the transmission characteristic T2 is the output light L ｏｕｔThe light is then emitted to the output waveguide 50. Therefore, as shown in Figure 4(b), the transmission characteristic T of the wavelength filter 10 is the sum of the transmission characteristic T1 of the first ring resonator 30 and the transmission characteristic T2 of the second ring resonator 40. The returned light L from the mirror section 70 ｒ This transmits through the wavelength filter 10 in the opposite direction to that described above.

[0072] Figure 5 is a graph showing the transmission characteristics of a wavelength filter according to the comparative example (hereinafter simply referred to as the "comparative example"). The comparative example differs from the example (wavelength filter 10) in that the couplers 41 and 42 constituting the second ring resonator 40 are of the same type as the couplers 31 and 32 constituting the first ring resonator 30, exhibiting the same wavelength dependence (straight line C1 in Figure 3).

[0073] Figure 5(a) shows the transmission characteristics T1 of the first ring resonator 30 and T2c of the second ring resonator 40 according to the comparative example. As described above, in the comparative example, the coupling coefficients of the couplers 41 and 42 constituting the second ring resonator 40 have the same wavelength dependence as couplers 31 and 32 (straight line C1 in Figure 3). Therefore, the transmission characteristics T2c of the second ring resonator 40 have the same wavelength dependence as the transmission characteristics T1 of the first ring resonator 30. That is, both the transmission characteristics T1 and T2c are configured such that the transmittance and full width at half maximum H1 and H2c of each peak increase monotonically as the wavelength increases from the short wavelength side to the long wavelength side. In other words, in the comparative example, the first ring resonator 30 and the second ring resonator 40 are configured to have only wavelength ranges in which their respective transmission characteristics T1 and T2c change monotonically in the same direction as the wavelength increases.

[0074] As shown in Figure 5(b), the transmission characteristic Tc of the comparative example is the superposition of the transmission characteristics T1 and T2c of the two ring resonators constituting the comparative example. As shown in Figure 5(a), in the comparative example, the transmission characteristics T1 and T2c have a wavelength dependence that changes monotonically in the same direction. Therefore, the transmission characteristic Tc will show the same tendency as the transmission characteristics T1 and T2c (a tendency in which the wavelength dependencies of each are reinforced). In other words, the transmission characteristic Tc of the comparative example has the same characteristics as the individual transmission characteristics T1 and T2c (i.e., a wavelength dependence that increases monotonically from the short wavelength side to the long wavelength side) as a result of superimposing the transmission characteristics T1 and T2c of each ring resonator 30 and 40, which have similar wavelength dependencies.

[0075] As a result, in the comparative example, the transmittance of each periodically appearing peak Pc is not constant, but is configured to change in one direction according to the wavelength (monotonically increasing from the short wavelength side to the long wavelength side). Furthermore, in the comparative example, the full width at half maximum of each peak Pc is also not constant, but is configured to change in one direction according to the wavelength (monotonically increasing from the short wavelength side to the long wavelength side).

[0076] Generally, in wavelength filters, it is preferable to have high transmittance and low full width at half maximum (FMAX). However, in the comparative example, although a vernier effect that increases FSR can be obtained by combining two ring resonators 30 and 40, similar to the example (wavelength filter 10), it is difficult to obtain desirable filter characteristics from the above viewpoint. Specifically, in the comparative example, the transmittance shows suitable characteristics in the wavelength range where the wavelength is relatively large, while the FMAX shows suitable characteristics in the wavelength range where the wavelength is relatively small. As a result, the wavelength range in which suitable characteristics with a good balance of both transmittance and FMAX are shown becomes limited (for example, the range between the second peak Pc from the left and the third peak Pc from the left in Figure 5(b)). In other words, in the comparative example, the range in which suitable transmittance and suitable FMAX can be achieved simultaneously (i.e., the wavelength tuning range in which the wavelength filter operates appropriately) becomes narrow.

[0077] In contrast, the wavelength filter 10 has a specific wavelength range (in this embodiment, the entire wavelength range) in which the transmission characteristics T1 and T2 of the two ring resonators 30 and 40 change monotonically in opposite directions. In other words, in the wavelength filter 10, the transmission characteristic T1 of the first ring resonator 30 and the transmission characteristic T2 of the second ring resonator 40 are configured to cancel each other out (reduce each other's characteristics) in the specific wavelength range (the entire wavelength range). As a result, as shown in Figure 4(b), the wavelength dependence of the transmission characteristic T of the wavelength filter 10 is effectively reduced. That is, in the wavelength filter 10, in the specific wavelength range, the transmittance and full width at half maximum (i.e., transmission characteristic T) of the periodic peak P have reduced wavelength dependence compared to the transmission characteristic Tc of the comparative example (see Figure 5(b)).

[0078] As described above, the wavelength filter 10 has a configuration in which the input waveguide 20, the first ring resonator 30, the intermediate waveguide 60, the second ring resonator 40, and the output waveguide 50 are connected in series. It is configured to have a specific wavelength range in which, as the wavelength of the incident light increases, the transmittance and the full width at half maximum of the transmission peak of the first ring resonator 30 monotonically increase or decrease, while the transmittance and the full width at half maximum of the transmission peak of the second ring resonator 40 monotonically change in the opposite direction to that of the first ring resonator 30. In other words, in the specific wavelength range, the transmission characteristics of the first ring resonator 30 and the second ring resonator 40 change in accordance with the change in wavelength so that they cancel out (reduce) each other's wavelength dependence. By providing such a specific wavelength range in the wavelength filter 10, it is possible to suitably secure a wavelength range within that specific wavelength range in which both transmittance and full width at half maximum are balanced (i.e., a wavelength tuning range in which the wavelength filter operates suitably).

[0079] Furthermore, the wavelength filter 10, when mounted on the tunable laser 1, functions as an external resonator that controls the tunable range of the laser light L emitted from the tunable laser 1. In the wavelength filter 10, the transmittance at the periodic peak P of the transmission characteristics is preferably -3 dB or higher. It is even more preferable that there are four or more consecutive peaks P having a transmittance of -3 dB or higher. In addition, the difference between the transmittance at peak P and the transmittance at the second largest peak adjacent to peak P (SMSR: Side Mode Suppression Ratio) is preferably 1 dB or higher.

[0080] In the wavelength filter 10, as described above, the first ring resonator 30 and the second ring resonator 40 are configured to cancel out (reduce) the wavelength dependence of each other's transmission characteristics. By implementing the wavelength filter 10 configured in this way, the tunable laser 1 can be made into a tunable laser 1 with a wide wavelength tuning range. For example, in the tunable laser 1, by configuring the wavelength filter 10 (first ring resonator 30 and second ring resonator 40) to have a specific wavelength range as described above, the tunable range of the tunable laser 1 may be set to a range of ±2% or more (preferably ±5% or more, more preferably ±10% or more) from the center wavelength of the tunable range. This allows the tunable range of the tunable laser 1 to be suitably widened.

[0081] Furthermore, in the wavelength filter 10, as described above, the transmittance of each peak in the first ring resonator 30 and the second ring resonator 40 is adjusted to monotonically increase or decrease in the region where the transmittance of each peak is -3 dB or more within the wavelength tuning range of the tunable laser 1. With such first ring resonators 30 and second ring resonators 40, the range of variation in the transmission characteristics T1 and T2 due to changes in wavelength (i.e., the range of variation in the transmittance of each peak) can be kept within a relatively small range, thus effectively suppressing the wavelength dependence of the transmission characteristics T1 and T2. As a result, the wavelength dependence of the transmission characteristic T of the wavelength filter 10, which is configured as a vernier filter combining the first ring resonator 30 and the second ring resonator 40, can be reduced.

[0082] In the wavelength filter 10, each coupler 31, 32, 41, and 42 is a directional coupler, and the first ring resonator 30 and the second ring resonator 40 are configured to have a specific wavelength range by adjusting the parameters of each directional coupler 31, 32, 41, and 42. As shown in Figure 3, in this embodiment, each coupler 31 and 32 is configured to have a wavelength dependence (linear C1) in which the coupling coefficient increases monotonically as the wavelength increases, by adjusting the parameters of the directional couplers of each coupler 31 and 32. On the other hand, each coupler 41 and 42 is configured to have a wavelength dependence (linear C2) in which the coupling coefficient decreases monotonically as the wavelength increases, by adjusting the parameters of the directional couplers of each coupler 41 and 42. As a result, the transmission characteristics T1 of the first ring resonator 30 and the transmission characteristics T2 of the second ring resonator 40 are configured to have a specific wavelength range (the entire wavelength range in this embodiment) with opposite wavelength dependencies.

[0083] In this embodiment, each coupler 31, 32, 41, 42 is configured as a directional coupler, and the parameters of the directional couplers constituting each coupler 31, 32, 41, 42 are adjusted. Compared to the case where each coupler 31, 32, 41, 42 is configured as an MZI coupler, each coupler 31, 32, 41, 42 can be miniaturized, thus enabling miniaturization of the wavelength filter 10. Furthermore, compared to the case where each coupler 31, 32, 41, 42 is configured as an MMI coupler (see Figure 6, described later), the loss of transmitted light in each coupler 31, 32, 41, 42 can be suppressed. Moreover, when each coupler 31, 32, 41, 42 is configured as an adiabatic or bent directional coupler (see Figures 2(b) and (c)), the wavelength dependence of the coupling coefficient of each coupler 31, 32, 41, 42 can be reduced compared to the case where a linear directional coupler (see Figure 2(a)) is used.

[0084] However, the couplers 31, 32, 41, and 42 are not limited to directional couplers and may be composed of couplers other than directional couplers. For example, each coupler 31, 32, 41, and 42 may be an MMI (Multi-Mode Interference) coupler as shown in Figure 6. In this case, the parameters of each MMI coupler of each coupler 31, 32, 41, and 42 may be adjusted so that the first ring resonator 30 and the second ring resonator 40 have a specific wavelength range.

[0085] Figure 6(a) shows an example of a rectangular MMI coupler. The rectangular MMI coupler has a waveguide (hereinafter simply referred to as the "coupler") formed in a rectangular shape when viewed from the thickness direction of the substrate 4 inside the coupler (central part). In an MMI coupler, the coupling coefficient is adjusted by parameters such as the length of the coupler (coupler length Lc), the gap G between the two waveguides at the end face of the coupler, and the width B of the coupler.

[0086] Figure 6(b) shows an example of a tapered MMI coupler. In a tapered MMI coupler, the width B of the coupling section is configured to vary along the longitudinal direction of the coupling section. In the example in Figure 6(b), the width B of the coupling section is smallest inside the coupler (central part) and gradually increases towards the upstream and downstream sides. In the case of such a tapered MMI coupler, the coupling coefficient is adjusted by the same parameters (coupling length Lc, gap G, width B) as described above for the rectangular type. However, in a tapered MMI coupler, the width B of the coupling section, among the parameters of the rectangular type described above, can be defined not as a fixed value but as a variable B(x) depending on the waveguide position x. Thus, the parameters that affect the coupling coefficient of an MMI coupler can differ depending on the type of MMI coupler.

[0087] Figure 6(c) shows an example of a bent MMI coupler. The bent MMI coupler differs from the rectangular type mainly in that it has a curved, arched coupling section. Such a bent MMI coupler may include parameters that affect the coupling coefficient, such as the degree of bending of the coupling section, in addition to the rectangular type parameters (coupling length Lc, gap G, width B) mentioned above.

[0088] Each coupler 31, 32, 41, and 42 is composed of an MMI coupler, and the parameters of the MMI couplers constituting each coupler 31, 32, 41, and 42 are adjusted. Compared to the case where each coupler 31, 32, 41, and 42 is composed of a directional coupler or an MZI coupler, the characteristic variation (e.g., variation in coupling coefficient) due to manufacturing errors of each coupler 31, 32, 41, and 42 can be reduced, thereby suppressing product-to-product variations in the transmission characteristics of the wavelength filter 10. Furthermore, if a tapered or bent type, as shown in Figures 6(b) and (c), is adopted for the MMI coupler, the light can be split at a splitting ratio other than 1:1 while suppressing the loss of transmitted light.

[0089] Furthermore, each coupler 31, 32, 41, and 42 may be an MZI (Mach-Zehnder Interferometer) coupler. In this case, the parameters of each MZI coupler of each coupler 31, 32, 41, and 42 may be adjusted so that the first ring resonator 30 and the second ring resonator 40 have a specific wavelength range. As an example, an MZI coupler has a configuration in which two directional couplers are arranged in series, and a thermal phase shifter is placed along one of the two waveguides connecting the two directional couplers. In such a case, the parameters of the MZI coupler may include the parameters of each directional coupler included in the MZI coupler and the amount of shift by the thermal phase shifter (the amount of change in refractive index, which will be described later). In such an MZI coupler, the refractive index of a part of the waveguide constituting the MZI coupler can be changed by controlling the power supplied to the thermal phase shifter.

[0090] In a configuration where each coupler 31, 32, 41, and 42 is composed of an MZI coupler, and the parameters of the MZI couplers constituting each coupler 31, 32, 41, and 42 are adjusted, the optical branching ratio in each coupler 31, 32, 41, and 42 can be easily adjusted by an electrical signal (for example, an electrical signal supplied to the thermal phase shifter described above).

[0091] As shown in Figure 3, in the wavelength filter 10, the first ring resonator 30 and the second ring resonator 40 are configured to have a first wavelength range in which the coupling coefficient (linear line C1) of the first coupler 31 and the second coupler 32 increases monotonically as the wavelength increases, while the coupling coefficient (linear line C2) of the third coupler 41 and the fourth coupler 42 decreases monotonically. With the above configuration, by setting the wavelength dependence of the coupling coefficients of each coupler 31, 32, 41, and 42 as described above, a configuration in which the first ring resonator 30 and the second ring resonator 40 have the above-mentioned specific wavelength range can be reliably and easily obtained.

[0092] In the wavelength filter 10, contrary to the example shown in Figure 3, the first ring resonator 30 and the second ring resonator 40 may be configured to have a second wavelength range in which the coupling coefficients of the first coupler 31 and the second coupler 32 decrease monotonically as the wavelength increases, while the coupling coefficients of the third coupler 41 and the fourth coupler 42 increase monotonically. For example, the first coupler 31 and the second coupler 32 may be configured to have a wavelength dependence of the coupling coefficient shown by the straight line C2 in Figure 3, while the third coupler 41 and the fourth coupler 42 may be configured to have a wavelength dependence of the coupling coefficient shown by the straight line C1 in Figure 3. In this case, the relationship between the transmission characteristics of the first ring resonator 30 and the transmission characteristics of the second ring resonator 40 will be the opposite of the relationship shown in Figure 4(a). In other words, the transmission characteristics of the first ring resonator 30 will be similar to the transmission characteristics T2 shown in Figure 4(a), and the transmission characteristics of the second ring resonator 40 will be similar to the transmission characteristics T1 shown in Figure 4(a).

[0093] Even if the transmission characteristics of the first ring resonator 30 and the second ring resonator 40 are swapped in this way, the transmission characteristics of the superimposed wavelength filter 10 will be the same as those of the embodiment described above (transmission characteristics T shown in Figure 4(b)). Therefore, even when the first ring resonator 30 and the second ring resonator 40 are configured to have a second wavelength range, the same effects as in the embodiment described above (i.e., when the first ring resonator 30 and the second ring resonator 40 are configured to have a first wavelength range) can be obtained.

[0094] Furthermore, as shown in the example in Figure 3, configuring each coupler 31, 32, 41, and 42 to have a wavelength dependence in which the coupling coefficient increases or decreases monotonically across the entire wavelength range is easier than configuring each coupler 31, 32, 41, and 42 to have both a first wavelength range and a second wavelength range (for example, setting each coupler 31, 32, 41, and 42 to have a complex wavelength dependence (curves C10, C20) as shown in Figure 7 later). Therefore, when configuring the wavelength filter 10 (first ring resonator 30 and second ring resonator 40) to have only one of the first wavelength range or the second wavelength range, the design of the wavelength filter 10 can be made easier compared to when the wavelength filter 10 is configured to have both the first and second wavelength ranges.

[0095] Furthermore, the first ring resonator 30 and the second ring resonator 40 may be configured to have both the first wavelength range and the second wavelength range described above. For example, as shown in Figure 7, the coupling coefficients of the first coupler 31 and the second coupler 32 constituting the first ring resonator 30 may be configured to have the wavelength dependence shown in curve C10. On the other hand, the coupling coefficients of the third coupler 41 and the fourth coupler 42 constituting the second ring resonator 40 may be configured to have the wavelength dependence shown in curve C20.

[0096] Curve C10 is an upward-convex curve and has a monotonically increasing region (wavelength range smaller than the wavelength corresponding to the peak of curve C10) where the coupling coefficient monotonically increases as the wavelength increases, and a monotonically decreasing region (wavelength range larger than the wavelength corresponding to the peak of curve C10) where the coupling coefficient monotonically decreases. Curve C20 is an downward-convex curve and has a monotonically decreasing region (wavelength range smaller than the wavelength corresponding to the peak of curve C20) where the coupling coefficient monotonically decreases as the wavelength increases, and a monotonically increasing region (wavelength range larger than the wavelength corresponding to the peak of curve C10) where the coupling coefficient monotonically increases. Furthermore, the monotonically increasing region of curve C10 overlaps with the monotonically decreasing region of curve C20, and the monotonically decreasing region of curve C10 overlaps with the monotonically increasing region of curve C20.

[0097] As described above, when the wavelength dependence of the coupling coefficients of each coupler 31, 32, 41, and 42 is set, the wavelength range where the monotonically increasing region of curve C10 and the monotonically decreasing region of curve C20 overlap corresponds to the first wavelength range described above, and the wavelength range where the monotonically decreasing region of curve C10 and the monotonically increasing region of curve C20 overlap becomes the second wavelength range described above.

[0098] Thus, when the first ring resonator 30 and the second ring resonator 40 are configured to have both a first wavelength range and a second wavelength range, as shown in Figure 7, the range of variation in the coupling coefficient of each coupler 31, 32, 41, 42 (i.e., the range by which the coupling coefficient changes with wavelength, and the width in the vertical axis direction of curves C10, C20) can be made smaller compared to the linear wavelength dependence (straight lines C1, C2 in Figure 3). This allows for the effective suppression of the wavelength dependence of the coupling coefficient of each coupler 31, 32, 41, 42. As a result, the wavelength dependence of the transmission characteristics (transmittance and full width at half maximum) of each ring resonator 30, 40 composed of couplers 31, 32, 41, 42 can be reduced, and consequently, the wavelength dependence of the transmission characteristics of the wavelength filter 10 can be reduced.

[0099] In the example shown in Figure 7, the wavelength dependence of the coupling coefficients of couplers 31 and 32 is an upward-convex curve (curve C10), and the wavelength dependence of the coupling coefficients of couplers 41 and 42 is a downward-convex curve (curve C20). However, the wavelength dependence of the coupling coefficients of couplers 31 and 32 may be a downward-convex curve (for example, a curve similar to curve C20), and the wavelength dependence of the coupling coefficients of couplers 41 and 42 may be an upward-convex curve (for example, a curve similar to curve C10).

[0100] (First Modified Example of Wavelength Filter) Figure 8 shows an example of the configuration of the wavelength filter 10A according to the first modified example. In the wavelength filter 10, the couplers 31, 32, 41, and 42 had the connection configuration shown in Figure 1. More specifically, in the connection configuration of the wavelength filter 10, the downstream port 31c of the first coupler 31 (the port connected to the waveguide 33) was a cross port to the upstream port 31a of the first coupler 31 (the port connected to the input waveguide 20), the downstream port 32c of the second coupler 32 (the port connected to the intermediate waveguide 60) was a cross port to the upstream port 32a of the second coupler 32 (the port connected to the waveguide 33), the downstream port 41c of the third coupler 41 (the port connected to the waveguide 43) was a cross port to the upstream port 41a of the third coupler 41 (the port connected to the intermediate waveguide 60), and the downstream port 42c of the fourth coupler 42 (the port connected to the output waveguide 50) was a cross port to the upstream port 42a of the fourth coupler 42 (the port connected to the waveguide 43).

[0101] In contrast, as shown in Figure 8, the connection configuration of the wavelength filter 10A (first connection configuration) differs from the connection configuration of the wavelength filter 10 shown in Figure 1 in that the downstream port 41c of the third coupler 41 (the port connected to the waveguide 43) is a bar port to the upstream port 41a of the third coupler 41 (the port connected to the intermediate waveguide 60), and the downstream port 42c of the fourth coupler 42 (the port connected to the output waveguide 50) is a bar port to the upstream port 42a of the fourth coupler 42 (the port connected to the waveguide 43). In other words, in the wavelength filter 10A, the downstream port 41d of the third coupler 41 is a cross port to the upstream port 41a, and the downstream port 42d of the fourth coupler 42 is a cross port to the upstream port 42a.

[0102] According to the first connection configuration of the wavelength filter 10A, by employing the same type of coupler (a coupler with the same wavelength dependence of coupling coefficients) for each coupler 31, 32, 41, and 42, the wavelength filter 10A can be configured such that the second ring resonator 40 has opposing transmission characteristics with respect to the first ring resonator 30. Therefore, while obtaining the same effects as the wavelength filter 10, design resources can be reduced by limiting the type of coupler constituting each coupler 31, 32, 41, and 42 to one type. Furthermore, with the wavelength filter 10A, even if the wavelength dependence of the coupling coefficients of each coupler 31, 32, 41, and 42 changes due to heat, manufacturing errors, etc., a specific wavelength range always exists, making it more resistant to temperature changes and manufacturing errors compared to the wavelength filter 10.

[0103] The above-mentioned effects of the wavelength filter 10A will be explained in more detail. As described above, when the same type of coupler is used for each coupler 31, 32, 41, and 42, each coupler 31, 32, 41, and 42 has a common coupling coefficient κ for a given wavelength. Here, the first connection configuration of the wavelength filter 10A shown in Figure 8 is equivalent to the connection configuration of the wavelength filter 10 shown in Figure 1, where the coupling coefficient of couplers 31 and 32 is set to "κ" and the coupling coefficient of couplers 41 and 42 is set to "1-κ". Therefore, in the wavelength filter 10A, when the coupling coefficient "κ" of each coupler 31, 32, 41, and 42 increases in accordance with the change in wavelength, in a connection configuration equivalent to the wavelength filter 10A and similar to Figure 1, the coupling coefficient "κ" of couplers 31 and 32 constituting the first ring resonator 30 increases, while the coupling coefficient "1-κ" of couplers 41 and 42 constituting the second ring resonator 40 decreases monotonically. This allows the first ring resonator 30 and the second ring resonator 40 to have wavelength dependencies in opposite directions. Therefore, with the wavelength filter 10A, similar to the wavelength filter 10, the first ring resonator 30 and the second ring resonator 40 can be configured to have a wavelength range (specific wavelength range) in which their transmission characteristics T1 and T2 change monotonically in opposite directions as the wavelength increases.

[0104] Furthermore, in the wavelength filter 10A, the second ring resonator 40 has a radiating waveguide 45 connected to the downstream port 41d of the third coupler 41 (a port different from the one connected to the waveguide 43) and which discards non-resonant peak light other than the resonant peak light passing through the wavelength filter 10A, and a radiating waveguide 47 connected to the upstream port 42b of the fourth coupler 42 (a port different from the one connected to the waveguide 43) and which discards non-resonant peak light. With the above configuration, light other than the resonant peak light (non-resonant peak light) can be appropriately discarded within the ring-shaped waveguides 43 and 44 of the second ring resonator 40 via the radiating waveguides 45 and 47. This makes it possible to suppress the generation of stray light due to non-resonant peak light. Note that instead of terminators 46 and 48, grating couplers or flip-up mirrors may be provided at the ends of the radiating waveguides 45 and 47.

[0105] (Second Modification of Wavelength Filter) Figure 9 shows an example of the configuration of the wavelength filter 10B according to the second modification. As shown in Figure 9, the connection configuration of the wavelength filter 10B (second connection configuration) differs from the connection configuration of the wavelength filter 10 shown in Figure 1 in that the downstream port 31c of the first coupler 31 (the port connected to the waveguide 33) is a bar port to the upstream port 31a of the first coupler 31 (the port connected to the input waveguide 20), and the downstream port 32c of the second coupler 32 (the port connected to the intermediate waveguide 60) is a bar port to the upstream port 32a of the second coupler 32 (the port connected to the waveguide 33). In other words, in the wavelength filter 10B, the downstream port 31d of the first coupler 31 is a cross port to the upstream port 31a, and the downstream port 32d of the second coupler 32 is a cross port to the upstream port 32a.

[0106] The second connection configuration of the wavelength filter 10B also allows the wavelength filter 10B to be configured such that the second ring resonator 40 has opposing transmission characteristics to the first ring resonator 30, by employing the same type of coupler (a coupler with the same wavelength dependence of coupling coefficients) for each coupler 31, 32, 41, and 42, for the same reasons as the first connection configuration of the wavelength filter 10A. Therefore, the same effect as the wavelength filter 10A can be obtained with the wavelength filter 10B.

[0107] Furthermore, in the wavelength filter 10B, the first ring resonator 30 has a radiating waveguide 35 connected to the downstream port 31d of the first coupler 31 (a port different from the side connected to the waveguide 33) and which discards non-resonant peak light other than resonant peak light passing through the wavelength filter 10B, and a radiating waveguide 37 connected to the upstream port 32b of the second coupler 32 (a port different from the side connected to the waveguide 33) and which discards non-resonant peak light. With the above configuration, light other than resonant peak light (non-resonant peak light) can be appropriately discarded within the ring-shaped waveguides 33 and 34 of the first ring resonator 30 via the radiating waveguides 35 and 37. This makes it possible to suppress the generation of stray light due to non-resonant peak light. Note that instead of terminators 36 and 38, grating couplers or flip-up mirrors may be provided at the ends of the radiating waveguides 35 and 37.

[0108] Although several embodiments of this disclosure have been described above, this disclosure is not limited to the configurations shown in each of the embodiments described above. The materials and shapes of each configuration are not limited to the specific materials and shapes described above, but a variety of other materials and shapes can be used. In addition, some of the configurations included in each of the embodiments described above may be omitted or modified as appropriate, or can be combined arbitrarily. For example, the number of ring resonators constituting the wavelength filter 10 is not limited to two, but may be three or more. Also, the wavelength dependence of the coupling coefficient of each coupler is not limited to changing linearly or curved as shown in Figures 3 and 7, but may change in a broken line or step, for example.

[0109] Furthermore, the form of the tunable laser on which the wavelength filter 10 is implemented is not limited to the form of tunable laser 1. For example, the wavelength filter 10 may be implemented in a tunable laser 1A according to the modified example shown in Figure 10. The tunable laser 1A differs from tunable laser 1 in that it is configured to output laser light L to other circuits on the optical integrated circuit 3 rather than from the semiconductor optical amplifier 2 (end face 2b). As an example, the tunable laser 1A has a mirror section 70A composed of a 2x2 coupler and a loop-shaped waveguide instead of a mirror section 70. Also, the tunable laser 1A has a waveguide 90 connected to an upstream port other than the upstream port to which the output waveguide 50 in the mirror section 70A is connected. In the tunable laser 1A, the output light L incident on the mirror section 70A ｏｕｔ A portion of this is output to other circuits on the optical integrated circuit 3 via the waveguide 90.

[0110] Furthermore, in the above embodiment, an example was given in which each coupler 31, 32 constituting the first ring resonator 30 is composed of the same type of coupler exhibiting the same wavelength dependence. However, the wavelength dependence of the coupling coefficients of the two couplers 31, 32 may be configured to exhibit different characteristics (tendencies), or they may be configured to exhibit the same characteristics but to different degrees. As an example of the former, one of the couplers 31, 32 may be configured to exhibit a wavelength dependence in which the coupling coefficient increases as the wavelength increases, while the other coupler 31, 32 may be configured to exhibit a wavelength dependence in which the coupling coefficient decreases as the wavelength increases. As an example of the latter, the couplers 31, 32 may be configured to exhibit a wavelength dependence in which the coupling coefficient changes (increases or decreases) in the same direction as the wavelength increases, while the amount of change in the coupling coefficient (magnitude of the slope) with respect to the change in wavelength may be different between the couplers 31, 32. The same applies to each coupler 41, 42 constituting the second ring resonator 40. In other words, the wavelength dependence of the coupling coefficients of couplers 31, 32, 41, and 42 only needs to be adjusted so that the wavelength dependence of the respective transmission characteristics of ring resonators 30 and 40 is ultimately opposite to that of the other couplers, and two couplers included in the same ring resonator do not necessarily have to be of the same type that exhibit the same wavelength dependence.

[0111] 1, 1A...Tunable laser, 10, 10A, 10B...Wavelength filter, 20...Input waveguide, 30...First ring resonator, 31...First coupler, 31a...Upstream port (one upstream port), 31b...Upstream port (other upstream port), 31c...Downstream port (one downstream port), 31d...Downstream port (other downstream port), 32...Second coupler, 32a...Upstream port (one upstream port), 32b...Upstream port (other upstream port), 32c...Downstream port (other downstream port), 32d...Downstream port (one downstream port), 35, 37, 4 5, 47... Radiation waveguide, 40... Second ring resonator, 41... Third coupler, 41a... Upstream port (first upstream port), 41b... Upstream port (other upstream port), 41c... Downstream port (first downstream port), 41d... Downstream port (other downstream port), 42... Fourth coupler, 42a... Upstream port (first upstream port), 42b... Upstream port (other upstream port), 42c... Downstream port (other downstream port), 42d... Downstream port (first downstream port), 50... Output waveguide, 60... Intermediate waveguide, H1, H2... Half-width, L... Laser light, L ｉｎ ...Input light.

Claims

1. A first ring resonator comprising: an input waveguide into which input light is input; a first coupler connected to the input waveguide and having one upstream port into which the input light is input; a second coupler having one upstream port connected to one downstream port of the first coupler and one downstream port connected to another upstream port of the first coupler; an intermediate waveguide connected to another downstream port of the second coupler; a third coupler connected to the intermediate waveguide and having one upstream port into which light from the other downstream port of the second coupler is input; a fourth coupler having one upstream port connected to one downstream port of the third coupler and one downstream port connected to another upstream port of the third coupler; and an output waveguide connected to another downstream port of the fourth coupler. A wavelength filter in which the first ring resonator and the second ring resonator are configured to have a specific wavelength range in which the transmittance and full width at half maximum of the transmission peak of the first ring resonator exhibit a wavelength dependence in which the transmittance and full width at half maximum of the transmission peak of the second ring resonator exhibit a wavelength dependence in which the transmittance and full width at half maximum of the transmission peak of the second ring resonator exhibit a wavelength dependence in which the transmission and full width at half maximum of the transmission peak change monotonically in the opposite direction to that of the first ring resonator.

2. The wavelength filter according to claim 1, wherein the first to fourth couplers are directional couplers, and the first ring resonator and the second ring resonator are configured to have the specific wavelength range by adjusting the parameters of each of the first to fourth couplers.

3. The wavelength filter according to claim 1, wherein the first to fourth couplers are MMI couplers, and the first ring resonator and the second ring resonator are configured to have the specific wavelength range by adjusting the parameters of each of the first to fourth couplers' MMI couplers.

4. The wavelength filter according to claim 1, wherein the first to fourth couplers are MZI couplers, and the first ring resonator and the second ring resonator are configured to have the specific wavelength range by adjusting the parameters of each of the first to fourth couplers' MZI couplers.

5. The first ring resonator and the second ring resonator have a first connection configuration or a second connection configuration, wherein the first connection configuration is such that the first downstream port of the first coupler is a cross port to the first upstream port of the first coupler, the other downstream port of the second coupler is a cross port to the first upstream port of the second coupler, the first downstream port of the third coupler is a bar port to the first upstream port of the third coupler, and the other downstream port of the fourth coupler is a bar port to the first upstream port of the fourth coupler, and the second connection configuration is such that the first downstream port of the first coupler is a bar port to the first upstream port of the first coupler, the other downstream port of the second coupler is a bar port to the first upstream port of the second coupler, and the first downstream port of the third coupler is a cross port to the first upstream port of the third coupler, The wavelength filter according to any one of claims 1 to 4, wherein the other downstream port of the fourth coupler is a cross port to the one upstream port of the fourth coupler.

6. The wavelength filter according to claim 5, wherein in the first connection configuration, the second ring resonator further comprises a radiating waveguide connected to another downstream port of the third coupler and discarding non-resonant peak light other than resonant peak light passing through the first ring resonator and the second ring resonator, and a radiating waveguide connected to another upstream port of the fourth coupler and discarding the non-resonant peak light, and in the second connection configuration, the first ring resonator further comprises a radiating waveguide connected to another downstream port of the first coupler and discarding the non-resonant peak light, and a radiating waveguide connected to another upstream port of the second coupler and discarding the non-resonant peak light.

7. A first ring resonator comprising: an input waveguide into which input light is input; a first coupler connected to the input waveguide and having one upstream port into which the input light is input; a second coupler having one upstream port connected to one downstream port of the first coupler and one downstream port connected to another upstream port of the first coupler; an intermediate waveguide connected to another downstream port of the second coupler; a third coupler connected to the intermediate waveguide and having one upstream port into which light from the other downstream port of the second coupler is input; a fourth coupler having one upstream port connected to one downstream port of the third coupler and one downstream port connected to another upstream port of the third coupler; and an output waveguide connected to another downstream port of the fourth coupler. A wavelength filter comprising a first ring resonator and a second ring resonator, wherein the first and second ring resonators are configured to have at least one of a first wavelength range in which the coupling coefficients of the first and second couplers increase monotonically as the wavelength increases, while the coupling coefficients of the third and fourth couplers decrease monotonically as the wavelength increases, and a second wavelength range in which the coupling coefficients of the first and second couplers decrease monotonically as the wavelength increases, while the coupling coefficients of the third and fourth couplers increase monotonically.

8. The wavelength filter according to claim 7, wherein the first ring resonator and the second ring resonator have both the first wavelength range and the second wavelength range.

9. The wavelength filter according to claim 7, wherein the first ring resonator and the second ring resonator have only one of the first wavelength range and the second wavelength range.

10. A wavelength filter according to any one of claims 1 to 9, which is mounted on a tunable laser and functions as an external resonator that controls the wavelength tuning range of the laser light emitted from the tunable laser.

11. The wavelength filter according to claim 10, wherein the wavelength tunable range is a range of ±2% or more of the center wavelength from the center wavelength of the wavelength tunable range.

12. The wavelength filter according to claim 10, wherein the wavelength tunable range is a range of ±5% or more of the center wavelength from the center wavelength of the wavelength tunable range.

13. The wavelength filter according to claim 10, wherein the wavelength tunable range is a range of ±10% or more of the center wavelength from the center wavelength of the wavelength tunable range.

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