Optical filter, optical filter control system, and optical filter control method

Abstract

The MZI filter (1) optically connects the input-side optical multiplexer / demultiplexer (3) and the output-side optical multiplexer / demultiplexer (5), respectively, and each of the first asymmetric arm (4a) and second asymmetric arm (4b), which have different optical path lengths, is provided with an adjustment waveguide portion in the waveguide constituting the asymmetric arm, in which the nonlinearity of the change in effective refractive index with respect to temperature changes is relatively larger than in other parts, and at least a part of at least one of the adjustment waveguide portions is provided with a temperature-dependent adjuster (7a, 7b) that changes the temperature dependence of the effective refractive index of at least that part.

Description

[Technical Field]

[0001] The present invention relates to an optical filter, particularly an optical filter including an MZI (Mach-Zehnder interferometer) type optical filter, which has a waveguide structure that splits incident light into multiple waveguides having different optical path lengths and outputs the emitted light from each waveguide, an optical filter control system, and an optical filter control method. [Background technology]

[0002] One method for increasing the capacity of optical communication systems is the digital coherent communication method. This method transmits signals not only on the intensity of light but also on its phase. Since the phase information of light is extracted using the phenomenon of optical interference, both the light source of the transmitter that sends the signal and the light source of the local emission that becomes the interference light at the receiver that receives the signal must have their wavelengths precisely controlled. One type of light source used for these purposes is the single-mode laser. Single-mode lasers oscillate at a single wavelength, but their oscillation wavelength and optical output intensity can change due to manufacturing errors and ambient temperature. Therefore, a wavelength monitor and optical intensity monitor are essential within a light source module for digital coherent communication that incorporates a single-mode laser. In particular, precise control of the oscillation wavelength to less than 0.1 nm is required.

[0003] For example, Patent Document 1 discloses the structure of a one-chip wavelength monitor using an MZI-type optical filter for controlling the oscillation wavelength of a laser. Here, an MZI-type optical filter is an optical filter that has a waveguide structure that constitutes a Mach-Zehnder interferometer (MZI), in which incident light is split and incident to two waveguides with different optical path lengths, and the emitted light from each waveguide is combined and output. These two waveguides are sometimes called "asymmetric arms". Hereafter, the MZI-type optical filter will be simply referred to as an "MZI filter". Patent Document 1 describes how using Si waveguides and SiN waveguides, which have mutually different temperature dependences on the effective refractive index, in the asymmetric arms of an MZI filter reduces the temperature dependence of the MZI filter. Therefore, a wavelength monitor using this MZI filter can be expected to operate stably regardless of ambient temperature. Hereinafter, this stable operation, which is not affected by ambient temperature, may be referred to as "temperature-independent operation." [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] International Publication No. 2024 / 218927 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] When manufacturing MZI filters, individual differences in manufacturing, particularly in the shape, structure, and material properties of each waveguide constituting the asymmetric arm of the MZI filter, can result in the design characteristics not being achieved regarding temperature dependence, and some units may exhibit operating characteristics that deviate from the temperature-independent operation expected in the design.

[0006] The disclosed technology was developed to solve the problems described above, and aims to provide an optical filter, an optical filter control system, and an optical filter control method that can adjust the temperature dependence even after manufacturing. [Means for solving the problem]

[0007] According to one aspect of an optical filter according to an embodiment of the present disclosure, the optical filter includes: a first optical multiplexer / demultiplexer provided on a light incident side; a second optical multiplexer / demultiplexer provided on a light exit side; and a first waveguide and a second waveguide that have different optical path lengths from each other and optically connect the first optical multiplexer / demultiplexer and the second optical multiplexer, respectively. The first waveguide consists of a first waveguide portion and a first adjustment waveguide portion, and the second waveguide consists of a second waveguide portion and a second adjustment waveguide portion. The first adjustment waveguide portion and the 2nd second adjustment waveguide portion have relatively larger non-linearity of change in effective refractive index with respect to temperature change compared with the first waveguide portion and the second waveguide portion. The optical filter is characterized by comprising a temperature dependency adjuster that is provided on at least one of the first adjustment waveguide portion and the second adjustment waveguide portion, for changing the temperature dependency of the effective refractive index in the waveguide at the provided position. Effects of the Invention

[0008] In the optical filter according to the technology of the present disclosure, each asymmetric arm is provided with a waveguide portion for temperature dependency adjustment, and a temperature dependency adjuster for changing the temperature dependency of the effective refractive index is provided in the waveguide portion for temperature dependency adjustment. Therefore, the temperature dependency of the optical filter can be adjusted even after the manufacture of the optical filter. Accordingly, even for an optical filter product that cannot achieve the performance expected in design in terms of low dependency on environmental temperature due to individual variations caused in manufacturing of the optical filter, the performance can be improved by adjustment after manufacture. Brief Description of the Drawings

[0009] [Figure 1] Fig. 1 is a schematic diagram illustrating the configuration of a waveguide structure of an MZI filter, an optical filter control system including the MZI filter, and a wavelength monitor element including the MZI filter according to Embodiment 1. [Figure 2]Figure 2 is a graph showing the change in the second derivative of the effective refractive index with respect to temperature, d2neff3 / dT2, as a function of the width of a silicon waveguide element with a height of 220 nm and an effective refractive index of neff3, used in the waveguide portion for temperature-dependent adjustment. [Figure 3] Figure 3A is a graph showing the wavelength-dependent change in the transmittance of the MZI filter with respect to ambient temperature before adjustment with the temperature-dependent regulator. Figure 3B is a graph showing the wavelength-dependent change in the transmittance of the MZI filter with respect to ambient temperature after adjustment with the temperature-dependent regulator. [Figure 4] Figure 4 is a flowchart showing the optical filter control method according to Embodiment 1. [Figure 5] Figure 5A is a hardware configuration diagram of the optical filter control system. Figure 5B is a hardware configuration diagram of the optical filter control system using a dedicated processing circuit. [Figure 6] Figure 6 shows the waveguide structure of the MZI filter in Embodiment 2. [Modes for carrying out the invention]

[0010] Hereinafter, in order to explain this disclosure in more detail, the forms for implementing this disclosure will be described with reference to the attached drawings. In the following, identical elements will be denoted by the same reference numerals and their descriptions will be omitted.

[0011] Embodiment 1. In this embodiment, we will explain using an MZI filter as an example, which comprises two waveguides forming a pair of asymmetric arms, with the input and output sides of each waveguide coupled by optical multiplexers.

[0012] (Waveguide structure) Figure 1 shows a schematic diagram illustrating the waveguide structure of the MZI filter 1, the optical filter control system 100 including the MZI filter 1, and the configuration of the wavelength monitor element 200 including the MZI filter in this embodiment.

[0013] The following description will use an MZI filter 1, which has a first asymmetric arm 4a and a second asymmetric arm 4b corresponding to a first waveguide and a second waveguide with mutually different optical path lengths, respectively, and is constructed using a planar waveguide type optical monitor made of a silicon photonics chip, as a specific example.

[0014] The MZI filter 1 comprises an input-side photomultiplexer / demultiplexer 3, a first asymmetric arm 4a, a second asymmetric arm 4b, a phase adjuster 6 and temperature-dependent adjusters 7a and 7b, and an output-side photomultiplexer / demultiplexer 5. The first asymmetric arm 4a and the second asymmetric arm 4b correspond to the first waveguide and the second waveguide in the claims, respectively. The phase adjuster 6 and the temperature-dependent adjusters 7a and 7b are devices for changing the temperature dependence of the effective refractive index in the waveguide at the locations where they are provided.

[0015] The first and third ports of the input-side optical multiplexer / demultiplexer 3 of the MZI filter 1 may be optically connected to a first optical coupler 2a and a second optical coupler 2b, respectively, for coupling the input light to the input-side optical multiplexer / demultiplexer 3. Any optical coupler can be used in this case, but examples include grating couplers, edge couplers, and elephant couplers. In the input-side optical multiplexer / demultiplexer 3, the first port is connected to the second port via a silicon waveguide which is the first optical waveguide 3a, and the third port is connected to the fourth port via a silicon waveguide which is the second optical waveguide 3b. In the output-side optical multiplexer / demultiplexer 5, the first port is connected to the second port via a silicon waveguide which is the first optical waveguide 5a, and the third port is connected to the fourth port via a silicon waveguide which is the second optical waveguide 5b.

[0016] When the MZI filter 1 is used in the wavelength monitoring element 200, the second and fourth ports of the output-side photomultiplexer / demultiplexer 5 of the MZI filter 1 are optically connected to the photodetectors 9a and 9b, respectively. The first and third ports of the input-side photomultiplexer / demultiplexer 3 are optically connected to the light source device whose wavelength is to be monitored via the first optical coupler 2a and the second optical coupler 2b, respectively, as needed.

[0017] The MZI filter 1 is a planar waveguide type optical monitor made of a silicon photonics chip formed by integrating, for example, a first optical coupler 2a, a second optical coupler 2b, an input-side optical multiplexer / demultiplexer 3, a first asymmetric arm 4a, a second asymmetric arm 4b, an output-side optical multiplexer / demultiplexer 5, a first photodetector 9a, and a second photodetector 9b on a planar surface 201a of a silicon (Si) substrate 201A.

[0018] The input node of the first asymmetric arm 4a is optically connected to the second port of the input-side optical multiplexer / demultiplexer 3, and the output node of the first asymmetric arm 4a is optically connected to the first port of the output-side optical multiplexer / demultiplexer 5. The input node of the second asymmetric arm 4b is optically connected to the fourth port of the input-side optical multiplexer / demultiplexer 3, and the output node of the second asymmetric arm 4b is optically connected to the third port of the output-side optical multiplexer / demultiplexer 5.

[0019] The first asymmetric arm 4a is composed of silicon nitride waveguides 41a1 and 41a2, a silicon waveguide 41a3, a temperature-dependent adjustment waveguide 42a, and a temperature-dependent regulator 7a provided in the temperature-dependent adjustment waveguide 42a. Although the silicon waveguide 41a3 is shown only between the silicon nitride waveguides 41a1 and 41a2 in Figure 1, connecting waveguides may be used to optically connect each silicon nitride waveguide 41a1, 41a2, the temperature-dependent adjustment waveguide 42a, and each photomultiplexer / demultiplexer 3 and 5. If connecting waveguides are used, they may be waveguides of the same material, composition and structure as the silicon waveguide 41a3. It is also possible to construct the connecting waveguides from a different waveguide material than the silicon waveguide 41a3. Of the first asymmetric arm 4a, the portion other than the temperature-dependent adjustment waveguide 42a is the first waveguide portion in the claims. The temperature-dependent adjustment waveguide 42a is also the first adjustment waveguide portion in the claims. Furthermore, the waveguide portion composed of silicon nitride waveguides 41a1 and 41a2 and silicon waveguide 41a3 may hereafter be referred to as the "first asymmetric portion."

[0020] The second asymmetric arm 4b is composed of silicon nitride waveguides 41b4 and 41b5, silicon waveguides 41b1, 41b3 and 41b2, a phase adjuster 6 provided in silicon waveguide 41b1, a temperature-dependent adjustment waveguide 42b, and a temperature-dependent adjuster 7b provided in temperature-dependent adjustment waveguide 42b. Similar to the first asymmetric arm 4a, connecting waveguides may be used between each waveguide of the second asymmetric arm 4b and between each waveguide and each optical multiplexer / demultiplexer. If connecting waveguides are used, they may be waveguides made of a material and structure having the same optical properties as silicon waveguides 41a3 and silicon waveguides 41b1, 41b3, and 41b2. Of the second asymmetric arm 4b, the portion other than the temperature-dependent adjustment waveguide 42b is the second waveguide portion in the claims. The temperature-dependent adjustment waveguide 42b is also the second adjustment waveguide portion in the claims. Furthermore, the waveguide portion composed of silicon nitride waveguides 41b4 and 41b5 and silicon waveguides 41b1, 41b3, and 41b2 may hereafter be referred to as the "second asymmetric portion."

[0021] As will be described later, silicon nitride waveguides 41b4 and 41b5 are elements provided with the aim of reducing the difference in the degree of propagation loss at the connection points between the two asymmetrical parts by ensuring that the number of connection points between waveguides with different characteristics is the same in the first asymmetrical part and the second asymmetrical part.

[0022] When connecting waveguides are used in the first and second asymmetric arms 4a and 4b, all such connecting waveguides shall be made of materials and structures with common optical properties, and shall be designed so that the sum of the physical lengths of the connecting waveguides in each asymmetric arm is equal.

[0023] Each silicon nitride waveguide 41a1, 41a2 and each silicon waveguide 41b1, 41b2 correspond to the first and second waveguide elements of the claims, respectively. Under this correspondence, each silicon nitride waveguide 41b4, 41b5 corresponds to the third waveguide element of the claims. The relationship between each silicon nitride waveguide 41a1, 41a2 and each silicon waveguide 41b1, 41b2 and the first and second asymmetric arms 4a, 4b may be reversed from that shown in Figure 1 above. In that case, each silicon nitride waveguide 41b4, 41b5 will be provided on the first asymmetric arm 4a, and in this case, each silicon nitride waveguide 41b4, 41b5 will correspond to the fourth waveguide element of the claims.

[0024] (Asymmetrical part) The first asymmetrical portion is formed in a folded shape, for example, a U-shape. However, it is not limited to a folded shape or a U-shape.

[0025] The following explanation will use the case where a U-shape as shown in Figure 1 is selected as the folded shape as an example. In the first asymmetrical section, in an example configured in a U-shape, silicon nitride waveguides 41a1 and 41a2 are located on the respective arms of the U-shape and are arranged opposite each other. The physical lengths of silicon nitride waveguide 41a1 and silicon nitride waveguide 41a2 are designed to be the same. One end of silicon nitride waveguide 41a1 is optically connected to the second port of the input-side optical multiplexer / demultiplexer 3 via a connecting silicon waveguide as needed. In one example, the waveguide structure of the first asymmetrical portion is symmetrical. In this specification and in the claims, the terms "same" or "equal" include the concepts of "approximately the same" and "approximately equal" in terms of manufacturing tolerances and errors.

[0026] The other end of the silicon nitride waveguide 41a2 is optically connected to the temperature-dependent adjustment waveguide 42a via a connecting silicon waveguide as needed, and the temperature-dependent adjustment waveguide 42a is optically connected to the first port of the output-side optical multiplexer / demultiplexer 5 via a connecting silicon waveguide as needed. The silicon waveguide 41a3 is optically connected between the other end of the silicon nitride waveguide 41a1 and one end of the silicon nitride waveguide 41a2, and is located at the folded portion of the first asymmetrical section, i.e., the bottom of the U-shape.

[0027] In one example of a U-shaped configuration, in the second asymmetrical portion, silicon waveguides 41b1 and 41b2 are located in the respective arms of the U-shape and are positioned opposite each other. The physical length of silicon waveguide 41b1 and the physical length of silicon waveguide 41b2 are the same in design. Silicon nitride waveguides 41b4 and 41b5 are located in the respective arms of the U-shape and are positioned opposite each other. The physical length of silicon nitride waveguide 41b4 and the physical length of silicon nitride waveguide 41b5 are the same in design.

[0028] One end of the silicon waveguide 41b1 is optically connected to the fourth port of the input-side optical multiplexer / demultiplexer 3. Furthermore, since the silicon waveguide 41b1 and the silicon waveguide that is the second optical waveguide 3b of the input-side optical multiplexer / demultiplexer 3 are formed continuously if they are made of the same material and have the same structure, one end of the silicon waveguide 41b1 and the fourth port of the input-side optical multiplexer / demultiplexer 3 do not physically exist. In one example, the waveguide structure of the second asymmetrical portion is symmetrical except for the presence or absence of a temperature controller. Even when a connecting waveguide is interposed between one end of the silicon waveguide 41b1 and the fourth port of the input-side optical multiplexer / demultiplexer 3, these three waveguide elements are formed continuously if they are made of the same material and have the same structure, so there is no physical presence between one end of the connecting waveguide and the fourth port of the input-side optical multiplexer / demultiplexer 3.

[0029] The other end of the silicon waveguide 41b2 is optically connected to the temperature-dependent adjustment waveguide 42b, and the temperature-dependent adjustment waveguide 42b is optically connected to the third port of the output-side optical multiplexer / demultiplexer 5 via a connecting silicon waveguide.

[0030] The first asymmetrical portion of the first asymmetrical arm 4a and the second asymmetrical portion of the second asymmetrical arm 4b are classified into a common portion A, which is common to both asymmetrical portions, and a non-common portion B, which is not common to both asymmetrical portions, as shown in Figure 1.

[0031] In the first asymmetrical portion, the silicon nitride waveguide 41a1 and the silicon nitride waveguide 41a2 are divided into a common portion A and a non-common portion B. In the silicon nitride waveguide 41a1, as shown in Figure 1, the common portion A is the part that is optically connected to the silicon waveguide 41a3 and the part that is optically connected to the second port of the input-side optical multiplexer / demultiplexer 3, while the non-common portion B is the remaining part of the silicon nitride waveguide 41a1, i.e., the central portion. In silicon nitride waveguide 41a2, the common portion A and non-common portion B are similar to those in silicon nitride waveguide 41a1, with the common portion A being at both ends and the non-common portion B being in the middle.

[0032] In the second asymmetrical portion, as shown in Figure 1, silicon nitride waveguides 41b4, 41b5, and 41b3 form a common portion A with respect to the first asymmetrical portion. In other words, the physical length of the silicon nitride waveguide 41b4 in the second asymmetric portion is designed to be the same as the physical length of a portion of the silicon nitride waveguide 41a1 in the first asymmetric portion, the physical length of the silicon nitride waveguide 41b5 in the second asymmetric portion is designed to be the same as the physical length of a portion of the silicon nitride waveguide 41a2 in the first asymmetric portion, and the physical length of the silicon waveguide 41b3 in the second asymmetric portion is designed to be the same as the physical length of the silicon waveguide 41a3 in the first asymmetric portion.

[0033] Effective refractive index n in silicon nitride waveguides 41a1, 41a2, 41b4, 41b5 eff2 The temperature dependence of the effective refractive index n in silicon waveguides 41a3, 41b1, 41b3, and 41b2 is given by n eff1 This differs from its temperature dependence.

[0034] The physical length of the non-common portion B in silicon nitride waveguide 41a1 and the physical length of the non-common portion B in silicon nitride waveguide 41a2 are designed to be (L+ΔL) / 2, respectively. Therefore, the non-common portion B in the first asymmetric portion is composed of a silicon nitride waveguide, and its design physical length is (L + ΔL).

[0035] Silicon waveguides 41a3 and 41b3 are included in common section A. They have equal physical length and are both L1. The sum of the physical lengths of the silicon nitride waveguides 41b4 and 41b5 included in the common section A of the second asymmetrical section is, in design terms, equal to the sum of the physical lengths of the portions of the silicon nitride waveguides 41a1 and 41a2 included in the common section A of the first asymmetrical section, and both are L'. Furthermore, as will be described later, silicon nitride waveguides 41b4 and 41b5 are elements intended to match propagation losses, so their combined physical length may be shorter than the combined physical length of silicon nitride waveguides 41a1 and 41a2.

[0036] The physical length of the non-common portion B in silicon waveguide 41b1 and the physical length of the non-common portion B in silicon waveguide 41b2 are both L / 2. Therefore, the non-common portion B in the second asymmetric portion is composed of silicon waveguides and has a physical length of L. The physical length of the non-common portion B in the first asymmetric portion is designed to be ΔL longer than the physical length of the non-common portion B in the second asymmetric portion.

[0037] In this embodiment, the first asymmetrical portion is composed of silicon nitride waveguides 41a1, 41a2 and silicon waveguide 41a3, and the second asymmetrical portion is composed of silicon waveguides 41b1 to 41b3 and silicon nitride waveguides 41b4 and 41b5. Therefore, the first asymmetrical portion and the second asymmetrical portion have the same number of connection points between waveguides with different characteristics. That is, in the first asymmetrical portion, silicon nitride waveguides 41a1 and 41a2 have a total of 4 connection points with silicon waveguides, and in the second asymmetrical portion, silicon nitride waveguides 41b4 and 41b5 have a total of 4 connection points with silicon waveguides. Thus, the difference in propagation loss between the first asymmetrical portion and the second asymmetrical portion can be minimized. Furthermore, by forming each asymmetrical portion into a U-shape and symmetrically arranging silicon nitride waveguides 41a1 and 41a2, and silicon nitride waveguides 41b4 and 41b5, respectively, at each leg of the U-shape, there is the advantage of easier length adjustment during manufacturing.

[0038] (modified version) Note that silicon waveguides 41b1 and 41b2 do not necessarily have to be waveguides of the same material and structure as silicon waveguide 41b3. If silicon waveguides 41b1 and 41b2 and silicon waveguide 41b3 do not have the same material and structure, it is conceivable to provide a pair of silicon waveguides on the first asymmetric arm 4a that are of the same material and structure as silicon waveguides 41b1 and 41b2, but have a total physical length shorter than the combined physical length of silicon waveguides 41b1 and 41b2, for example, on the input side of silicon nitride waveguide 41a1 and on the output side of silicon nitride waveguide 41a2. In this modified example, the pair of silicon waveguides corresponds to the fourth waveguide element of the claims.

[0039] (Temperature dependent adjustment waveguide) The temperature-dependent adjustment waveguide 42a provided on the first asymmetric arm 4a and the temperature-dependent adjustment waveguide 42b provided on the second asymmetric arm 4b are waveguides provided to compensate for the ambient temperature dependence of the MZI filter 1 after the MZI filter 1 has been manufactured. Both the temperature-dependent adjustment waveguide 42a and the temperature-dependent adjustment waveguide 42b have a physical length of L3 and an effective refractive index of n. eff3 It consists of waveguides having a common structure and materials. Effective refractive index n of temperature-dependent adjustment waveguides 42a and 42b eff3 The temperature dependence of is relatively more nonlinear than that of the waveguides constituting the first and second asymmetric portions, and therefore the effective refractive index n eff3 The second derivative of the temperature is relatively larger than that of the other parts of the first asymmetric arm 4a. Details regarding the specific configuration of the temperature-dependent adjustment waveguides 42a and 42b will be described later. Furthermore, in this specification and the claims, "effective refractive index" is a function of temperature and does not refer to any fixed value.

[0040] (others) In this embodiment, each silicon nitride waveguide is divided into two sections, but it is also possible to divide them into three or more sections. On the other hand, if the transition loss occurs at the waveguide connection interface, a smaller number of divisions is preferable, and silicon nitride waveguides 41a1, 41a2, silicon nitride waveguides 41b4, and 41b5 may each be composed of individual waveguides. When dividing each silicon nitride waveguide, it is preferable to divide it by the same number of divisions between the two asymmetric arms 4a and 4b in order to make the transmission loss between the two asymmetric arms 4a and 4b and the optical effect at the connection interface the same for both asymmetric arms 4a and 4b. Furthermore, the connection order of the waveguides in each asymmetric arm 4a, 4b may differ from the example described with reference to Figure 1. As a result, an arrangement can be adopted in which each element of the first and second asymmetric portions is distributed on both sides of the temperature-dependent adjustment waveguides 42a, 42b.

[0041] As the input-side optical multiplexer / demultiplexer 3 and the output-side optical multiplexer / demultiplexer 5, for example, a directional coupler having a first port to a fourth port and having an optical propagation path from the first port to the second port and an optical propagation path from the third port to the fourth port, or a multi-mode interferometer (MMI) can be used. The output-side photomultiplexer / demultiplexer 5 may be an asymmetric photomultiplexer / demultiplexer with a power splitting ratio other than 0.5.

[0042] The phase adjuster 6 and temperature-dependent adjusters 7a and 7b only need to be able to change the phase state of the light output from the MZI filter 1, and are not limited to heaters; other heating and cooling means may be used. Alternatively, they may be phase changers such as pin-type photocurrent injection type refractive index adjusters or photocurrent extraction type refractive index adjusters, which utilize photocurrent injection or extraction via a pn junction, or quantum confinement Stark effect or Pockels effect by voltage application. A method in which the waveguide is constructed of a photo-induced refractive index changing material and the temperature dependence of the effective refractive index is changed by ultraviolet irradiation can also be employed.

[0043] While a silicon substrate was used as an example for the substrate 201A constituting the silicon photonics chip above, other substrates such as indium phosphide (InP), a compound semiconductor, or a glass substrate can also be used.

[0044] Furthermore, the MZI filter 1 of this embodiment may be constructed using an optical circuit built in free space not integrated on the wafer, with optical fibers, multiplexers / demultiplexers, and couplers.

[0045] (Optical filter control system) The phase adjuster 6 and temperature-dependent adjusters 7a and 7b are controlled via the control unit 10 and act on at least a portion of the silicon waveguide 41b1 and the temperature-dependent adjusting waveguides 42a and 42b, respectively, to change the temperature dependence of the effective refractive index of each optical guide. In this embodiment, means for heating at least a portion of each of the silicon waveguide 41b1 and temperature-dependent adjusting waveguides 42a and 42b are employed as phase adjusters and temperature-dependent adjusters. Any heating means can be used for this heating; specifically, a heater can be used. By heating each of these phase adjusters and temperature-dependent adjusters, the temperature of at least a portion of each of the silicon waveguides 41b1 and temperature-dependent adjusting waveguides 42a and 42b changes, thereby changing the effective refractive index of each waveguide portion. When the MZI filter 1 is configured as a planar waveguide type device using a silicon photonics chip, the phase adjuster 6 and temperature-dependent adjusters 7a and 7b are, for example, located on a portion of the silicon photonics chip and formed on an insulating film (not shown) formed on the surface of the silicon photonics chip, and adjust the temperature by passing a current in response to control by the control unit 10. The phase adjuster 6 may be installed in the waveguide of the first asymmetrical portion to control its temperature. The MZI filter 1 and control unit 10 in this embodiment constitute an optical filter control system 100.

[0046] (Wavelength monitoring element) The MZI filter 1 of this embodiment, together with the photodetectors 9a and 9b, can constitute a wavelength monitor element 200 in a wavelength rocker for wavelength control of laser light from a light source device such as a semiconductor laser. In this configuration, the control unit 10 of the optical filter control system 100 may also serve as the control unit for the wavelength monitor element 200. When the control unit 10 also serves as the control unit for the wavelength monitor element 200, the outputs of the photodetectors 9a and 9b are input to the control unit 10.

[0047] The light receivers 9a and 9b are, for example, waveguide-type light receivers or surface-incident-type light receivers. As an example, a photodiode that is a SiGe (silicon germanium) light receiver can be used.

[0048] Design of Asymmetric Portion In the first and second asymmetric portions, the common portions A have equal designed optical path lengths as described above, so no phase difference occurs between light passing through each common portion A of the first and second asymmetric portions. On the other hand, in each non-common portion B, as described above, there are differences between the first and second asymmetric portions in the effective refractive index, material, and physical length of the waveguide.

[0049] When the MZI filter 1 of the present embodiment is used in a wavelength monitoring element 200 of a wavelength locker for a semiconductor laser or the like, it is required that the phase difference between the first asymmetric arm 4a and the second asymmetric arm 4b with respect to a target wavelength λ is not affected by changes in environmental temperature. Particularly, in a semiconductor laser light source, since temperature control is performed for oscillation wavelength control, the wavelength monitoring element disposed near the semiconductor laser device can also be subjected to fluctuations in environmental temperature due to such influence. Although it is possible to perform constant-temperature control on the MZI filter 1 using a heating device or the like, this consumes energy.

[0050] Between the first and second asymmetric arms of the MZI filter 1 of the present embodiment, when none of the phase adjusters and temperature dependence adjusters are activated, due to the difference in effective refractive index and physical length between the non-common portions B, n eff2 (L+ΔL)-n eff1 L ··· (Formula a) the optical path length difference as described above occurs. To perform wavelength locking using the MZI filter 1, this optical path length difference needs to have a fixed phase difference θ with respect to a certain target wavelength λ. Therefore, n eff2 (L+ΔL)-n eff1 L=(λ / 2π)θ=mλ ··· (Formula b) the above relationship needs to hold. Here, "m" is a coefficient that relates the wavelength λ to the optical path length difference of the non-common portion B, and is a variable proportional to the phase difference θ.

[0051] For the relationship in equation b to be independent of ambient temperature, it is sufficient that the temperature derivative of equation b is always zero. That is, if we represent temperature as T, (dn eff2 / dT)(L+ΔL)-(dn eff1 / dT)L =mdλ / dT=0 (formula c) It would be ideal if this always held true. L, (dn eff2 (dT) and (dn eff1 Since the phase difference (dT) is known from the material and structure of the corresponding waveguide, an appropriate ΔL can be calculated from this equation c to obtain the desired phase difference θ with respect to the target wavelength λ, i.e., the desired m value. Then, by designing the first and second asymmetrical portions based on this ΔL, a temperature-independent MZI filter 1 can be realized. In the MZI filter 1 designed in this way, the value of m can be adjusted by changing the temperature dependence of the effective refractive index of a part of the waveguide of the second asymmetrical portion or the first asymmetrical portion using the phase adjuster 6.

[0052] (Isolator) It is preferable to provide an isolator 8 in the gap between the phase adjuster 6 and the temperature-dependent adjusters 7a and 7b at their respective installation locations, in order to thermally or electrically isolate each of the phase adjusters and temperature-dependent adjusters. As the isolator 8, an air gap may be used to suppress thermal crosstalk. If the MZI filter 1 is constructed from a silicon photonics chip, for example, the layer structure of the substrate 201A constituting the silicon photonics chip can be etched to suppress thermal crosstalk. If it is desired to suppress electrical crosstalk between the phase adjuster 6 and the temperature-dependent adjusters 7a and 7b, then, for example, metal isolation can be employed.

[0053] (Details of the waveguide for temperature-dependent adjustment) As described above, the MZI filter 1 is designed so that the phase difference generated in the first and second asymmetric portions is independent of ambient temperature for an incident light of a certain target wavelength λ. However, during the manufacturing of the MZI filter 1, variations occur in the material, structure, and physical length of each waveguide constituting the first asymmetric arm 4a and the second asymmetric arm 4b, respectively. As a result, products with performance and characteristics that differ from the design may be produced, and individual differences may occur in the independence from ambient temperature. However, as described above, the first and second asymmetric portions are designed on the premise that each waveguide constituting them has characteristics as per the design values, and it is difficult, or virtually impossible, to adjust for the dependence on ambient temperature after manufacturing. Therefore, in this embodiment, a temperature-dependent adjustment waveguide 42a having a temperature-dependent regulator 7a is provided on the first asymmetric arm 4a, and a temperature-dependent adjustment waveguide 42b having a temperature-dependent regulator 7b is provided on the second asymmetric arm 4b, making it possible to adjust the environmental temperature dependence due to individual differences in the MZI filter after manufacturing.

[0054] In this embodiment, the effective refractive index n of each silicon waveguide in the first and second asymmetrical portions. eff1 and the effective refractive index n of silicon nitride waveguides eff2 The relationship between the effective refractive index and the temperature change is approximately linear in the practical temperature range; that is, the temperature derivative of the effective refractive index is approximately constant with respect to temperature. Therefore, the effective refractive index n of each silicon waveguide in the first and second asymmetrical portions is approximately constant with respect to temperature. eff1 and the effective refractive index n of each silicon nitride waveguide eff2 and the second derivative d with respect to temperature 2 n eff1 / dT 2 d 2 n eff2 / dT 2 In the practical temperature range, these values ​​are all small enough to be approximated as 0. In contrast, the temperature-dependent adjustment waveguides 42a and 42b have an effective refractive index n eff3The relationship between the effective refractive index and the temperature change, i.e., the temperature dependence of the effective refractive index, is such that waveguide elements with stronger nonlinearity are used in the first and second asymmetrical portions compared to silicon waveguides and silicon nitride waveguides in the practical temperature range. As a result, in the temperature-dependent adjustment waveguides 42a and 42b, the effective refractive index n is eff3 The second derivative of d with respect to temperature 2 n eff3 / dT 2 is, n eff1 and n eff2 It is relatively large compared to the second derivative with respect to each temperature.

[0055] Waveguide elements with relatively high nonlinearity in the temperature dependence of the effective refractive index, which can be used in the temperature-dependent waveguides 42a and 42b, can be waveguide elements of various materials or structures. When the MZI filter 1 is configured as a planar waveguide type using a silicon photonics chip, an example of a waveguide structure that can be used as the temperature-dependent adjustment waveguides 42a and 42b is shown below. Figure 2 shows the effective refractive index n with respect to the change in the width of the silicon waveguide when the waveguide element is composed of a silicon waveguide with a height of 220 nm. eff3 The second derivative of d with respect to temperature 2 n eff3 / dT 2 This shows the change. Generally, silicon waveguides in silicon photonics chips use a single-mode waveguide with a rectangular waveguide cross-section perpendicular to the waveguide axis, with a height of 220 nm and a width of 500 nm. However, if the width is narrowed to about 250 nm to 350 nm while maintaining the height, the change in the ratio of light distribution between the core and cladding becomes large, resulting in an effective refractive index n eff3 Figure 2 shows that the second derivative with respect to temperature is up to approximately 7 times larger than that of the 500 nm width. As an example, the temperature-dependent adjustment waveguides 42a and 42b in this embodiment can employ a silicon waveguide structure with a height of approximately 500 nm and a width of approximately 250 to 350 nm.

[0056] In this embodiment, silicon-based materials were used for each waveguide structure, but waveguides made of InP, GaAs, LN, BTO, PLZT, or glass-based materials may also be used.

[0057] The input-side optical multiplexer / demultiplexer 3 and the output-side optical multiplexer / demultiplexer 5 may be, for example, directional couplers. However, any device capable of splitting light, such as an MMI (multimode interference coupler), may be used.

[0058] For the photodetectors 9a and 9b, as mentioned above, germanium-based photodiodes can be used, but InP-based and GaAs-based photodetectors may also be used. Furthermore, either waveguide-type or surface-incident photodetectors may be used.

[0059] In this embodiment, the phase adjuster 6 and temperature-dependent adjusters 7a and 7b are shown exemplarily as being powered by heaters, but are not limited to this. As means for adjusting the effective refractive index of each waveguide in each phase adjuster and temperature-dependent adjuster, it is sufficient to change the refractive index of either or both the core and cladding of the waveguide at the mounting location of the phase adjuster and temperature-dependent adjuster. Various known adjustment methods or adjustment forms can be employed, such as other known heating and cooling methods, pressure application, current injection or extraction via a pn junction, quantum confinement Stark effect by voltage application, or the Pockels effect.

[0060] For the isolator 8, an air gap, for example, can be used if the purpose is to suppress thermal crosstalk. If the MZI filter 1 is constructed from a silicon photonics chip, thermal crosstalk can be suppressed by etching the layer structure of the silicon photonics chip. If electrical crosstalk between phase adjusters and temperature-dependent adjusters needs to be suppressed, for example, metal isolation can be employed.

[0061] (Operating principle) Next, the operating principle of the MZI filter 1 of this embodiment will be explained. The optical path lengths la and lb of the first asymmetric arm 4a and the second asymmetric arm 4b at a certain ambient temperature are given by equations (1a) and (1b), respectively. TIFF0007927214000001.tif21166 TIFF0007927214000002.tif27166 Here, the temperature difference ΔTa represents the temperature increase due to heating applied by the temperature-dependent regulator 7a to the physical length La range of the temperature-dependent regulating waveguide 42a. Here, "temperature increase due to heating" can be rephrased as the temperature difference with respect to the temperature of the unheated portion other than the phase regulator and temperature-dependent regulator mounting portion in each of the first asymmetric arm 4a and the second asymmetric arm 4b. The same applies to other temperature differences below. The temperature differences ΔTb and ΔTc are the temperature differences applied to the silicon waveguide 41b1 and the temperature-dependent adjustment waveguide 42b within the ranges of physical lengths Lb and Lc, respectively, by the phase adjuster 6 and the temperature-dependent adjuster 7b. L1 is the physical length of silicon waveguides 41a3 and 41b5, respectively. L' is the physical length of the portion of silicon nitride waveguides 41a1 and 41a2, and silicon nitride waveguides 41b4 and 41b5 that is included in the common section A.

[0062] In this embodiment, the difference in optical path length between the first asymmetric arm 4a and the second asymmetric arm 4b of the MZI filter 1 is expressed as the difference la-lb between equations (1a) and (1b), as shown in equation (2) below. TIFF0007927214000003.tif27166

[0063] In equation (2), the first and second terms on the left side are common terms with equation b above, and correspond to the optical path length difference caused by the non-common portion B in the first and second asymmetric portions. In this embodiment as well, without adjustment by each phase adjuster and temperature-dependent adjuster, both terms satisfy equation c above, and the optical path length difference is designed not to have temperature dependence. The fourth and fifth terms on the left side of equation (2) correspond to the heating control of the physical length La and Lc portions of the temperature-dependent adjustment waveguides 42a and 42b, respectively, by the temperature-dependent regulators 7a and 7b. The third term on the left side of equation (2) corresponds to the heating control by the phase adjuster 6 for the physical length Lb portion of the silicon waveguide 41b1 in the second asymmetric portion. By adjusting the effective refractive index of the silicon waveguide 41b1 by the phase adjuster 6 corresponding to this third term, the magnitude of the optical path length difference generated between the first and second asymmetric portions can be adjusted, thereby adjusting the phase difference between the two asymmetric portions with respect to a certain target wavelength λ. The physical lengths Lb, La, and Lc of the heated portion may be equal to the total length of the target silicon waveguide 41b1, temperature-dependent adjustment waveguides 42a and 42b, respectively.

[0064] Next, differentiating both sides of equation (2) with respect to temperature yields equation (3a). TIFF0007927214000004.tif35166

[0065] The correspondence between the first to fifth terms on the left-hand side of equation (3a) and the waveguides, phase adjusters, and temperature-dependent adjusters of the first and second asymmetric arms 4a and 4b is the same as the correspondence described for the first to fifth terms on the left-hand side of equation (2) above. The first and second terms on the left side of equation (3a) are the same as those in equation c above. In this embodiment, both asymmetric arms are designed such that, without adjustment by the respective phase adjusters and temperature-dependent adjusters, the first and second terms on the left side become zero, and the difference in optical path length between the two asymmetric parts is not temperature-dependent. The third term on the left side of equation (3a) corresponds to the heating of the silicon waveguide 41b1 by the phase adjuster 6. The temperature dependence of the effective refractive index of the silicon waveguide 41b1 is approximately linear, and the second derivative of the effective refractive index with respect to temperature in the third term on the left side is negligible, or rather, can be approximated to zero, compared to the second derivatives of the effective refractive index with respect to temperature in the fourth and fifth terms on the left side of equation (3a) relating to the temperature-dependent adjusting waveguide, where the temperature dependence of the effective refractive index is nonlinear. Therefore, equation (3a) relating to the ambient temperature dependence of the optical path length difference between the two asymmetric arms can be approximated by zeroing the third term on the left side and expressed as equation (3b) below. TIFF0007927214000005.tif31166

[0066] (action) In this embodiment, the MZI filter 1 is designed such that the calculated values ​​of the first and second terms on the left side of equation (3b), which correspond to the non-common part B, are zero. However, due to individual manufacturing variations in the MZI filter 1, for example, the physical lengths corresponding to L and ΔL may deviate from the design, or variations in the waveguide material may affect the effective refractive index n. eff1 , n eff2 If the characteristics deviate from the design values, the temperature dependence of the non-common part B does not cancel each other out, and the calculated values ​​of the first and second terms on the left side of equation (3b) do not become zero. In other words, products may be produced in which temperature dependence remains in the optical path length difference between the first and second asymmetrical arms. For such products, if the first and second asymmetrical arms are composed only of the first and second asymmetrical parts, post-manufacturing adjustment becomes impossible. For example, wavelength filters used as wavelength monitors for light source devices in optical communication systems require extremely precise filter fabrication accuracy on the order of 10 MHz / K relative to ambient temperature. For example, in a configuration disclosed in Patent Document 1, where a semiconductor laser and a wavelength filter are mounted on the same temperature controller, the temperature controller can compensate for ambient temperature tolerance. However, if operation without a temperature controller is required for further power saving, the tolerance for individual manufacturing differences in a wavelength filter without a waveguide for temperature-dependent adjustment becomes extremely small in order to obtain the required low temperature dependence. Therefore, it is necessary to manufacture the filter with extreme precision so that the first and second terms on the left side of equation (3b) are zero, as designed.

[0067] In contrast, in this embodiment, temperature-dependent adjusting waveguides 42a and 42b are provided on the first and second asymmetric arms 4a and 4b, respectively, and temperature-dependent adjusting waveguides 42a and 42b are provided on each of the first and second asymmetric arms 4a and 4b. The sum of the fourth and fifth terms on the left side of equation (3b), which corresponds to the temperature dependence occurring in the first and second asymmetric portions, is adjusted individually for the MZI filter product after manufacturing by setting appropriate ΔTa and ΔTc for the temperature-dependent adjusting devices 7a and 7b, thereby making the left side of equation (3b) zero or close to zero, and suppressing the temperature dependence in the optical path length difference between the first and second asymmetric arms of the product.

[0068] Factors that may cause temperature dependence in the optical path length difference between the two asymmetric arms due to manufacturing variations in the MZI filter include deviations from the design value in the non-common section B mentioned above, as well as manufacturing deviations from the design value in the silicon waveguide including the other common section A. Taking this into consideration, the optical path length difference outside of the non-common section B is expressed as a function of temperature "l err If we represent this as (T), then equations (2) and (3b) can be expressed as equations (2') and (3b) below, respectively. TIFF0007927214000006.tif26166 TIFF0007927214000007.tif39166

[0069] As shown in equation (3b'), l errEven considering (T), in the MZI filter 1 of this embodiment, similar to equation (3b), the temperature dependence of the MZI filter can be suppressed by setting the left side of equation (3b') to zero or close to zero by appropriately setting the temperature-dependent regulators 7a and 7b in the temperature-dependent adjustment waveguides 42a and 42b.

[0070] The direction in which the sum of the first and second terms on the left side of equation (3b), or the first, second, and fifth terms on the left side of equation (3b'), shifts due to individual differences can vary depending on the manufacturing conditions of the MZI filter. However, for the third and fourth terms on the left side of each of these equations, ΔT a ΔT c Even if each term is limited to positive values, since the signs of the third and fourth terms on the left-hand side are opposite, compensation is possible regardless of the positive or negative sign of the deviation.

[0071] Furthermore, each waveguide of the MZI filter 1 has wavelength dependence, and for example, the values ​​of the first and second terms on the left side of equations (3b) and (3b') may change depending on whether the wavelength is 1550 nm or 1600 nm. In this case, ΔT depends on the wavelength range used. a and ΔT c By adjusting this setting, it is possible to ensure operation with sufficiently low temperature dependence across all wavelength ranges.

[0072] Figures 3A and 3B are graphs showing the relationship between the wavelength spectrum of the transmittance of an MZI filter fabricated according to this embodiment and ambient temperature for three different ambient temperatures (80°C, 40°C, and 0°C). Figure 3A shows the ΔT before adjustment by temperature-dependent regulators 7a and 7b. a =ΔT c This graph shows the wavelength spectra of the transmittance of MZI filter 1 at 0°C for the three ambient temperatures mentioned above. In Figure 3A, the wavelength spectra for the three ambient temperatures are shifted relative to each other in the wavelength direction, indicating that the wavelength spectrum of transmittance is temperature-dependent. On the other hand, Figure 3B shows that the MZI filter 1 of this embodiment used in Figure 3A is modified by temperature-dependent regulators 7a and 7b to control ΔTa =32℃, ΔT c This graph shows the wavelength spectrum of the transmittance of MZI filter 1 after adjustment to set it to =0°C, for the three ambient temperatures mentioned above. The wavelength spectra for each ambient temperature all overlap almost completely, indicating that the temperature dependence of the transmittance wavelength spectrum was suppressed by controlling the temperature-dependent regulators 7a and 7b to constant values.

[0073] Furthermore, since the first and second temperature-dependent adjustment waveguides have the same structure and physical length in terms of design, for example, when designing the optical circuit of this embodiment by modifying a conventional MZI filter that has only the first and second asymmetrical portions in the first and second asymmetrical arms, the temperature dependence between the two asymmetrical arms cancels out for the first and second temperature-dependent adjustment waveguides, and there is no need to consider the characteristics of the temperature-dependent adjustment waveguides. Therefore, no design modification is required for the optical circuit design so that the first and second asymmetrical portions satisfy the relationship in equation c.

[0074] However, the first and second temperature-dependent adjustment waveguides are not limited to having the same structure and physical length in terms of design. For example, if there are challenges such as wanting to reduce the number of heaters, minimize waveguide losses, or reduce the device area, the structure of the first and second temperature-dependent adjustment waveguides may be asymmetric as needed. In such cases, as an example, a structure can be designed in advance such that the sum of the first and second terms on the left side of equations (3b) and (3b') is less than zero, taking individual differences into account, and ΔT b One possible design is one in which the left-hand side of each equation can be reduced to zero simply by heating. In this example, it is necessary to design the wavelength dependence of equation (2) and the temperature dependence of equations (3b) and (3b') after considering the characteristics of the first and second temperature-dependent adjusting waveguides.

[0075] This embodiment eliminates the need for constant temperature control to maintain a constant temperature of the filter tip in order to maintain wavelength characteristics, even though the MZI filter uses a silicon waveguide which has a high temperature dependence. This reduces the power consumed by the temperature controller for constant temperature control, and furthermore, enables operation without a temperature controller. By simply setting the first and second asymmetrical sections on the first and second asymmetrical arms, respectively, the temperature dependence of the MZI filter can be canceled out in terms of design, and operation without a temperature controller, i.e., operation with zero energy consumption, can be expected. However, in reality, with this optical circuit alone, variations in the manufacturing of MZI may result in products that do not achieve a sufficiently low temperature dependence for practical use. As long as such products are anticipated, constant temperature control will be required for the silicon photonics chip on which the MZI filter is formed. In contrast, this embodiment further provides temperature-dependent adjustment waveguides 42a and 42b, and individually controls their temperatures using temperature-dependent regulators 7a and 7b, respectively. This makes it possible to compensate for and reduce the temperature dependence of the MZI filter 1 that occurs outside the design due to individual manufacturing differences after the MZI filter is manufactured. Therefore, even if temperature dependence is expected to occur due to individual manufacturing differences, constant temperature control of the filter chip is not required, and the power used for the temperature regulator for constant temperature control can be eliminated or at least reduced.

[0076] (Optical filter control method) Next, the optical filter control method according to this embodiment will be described. Figure 4 is a flowchart showing an optical filter control method according to Embodiment 1, illustrating the process of adjusting the temperature dependence of the MZI filter 1 by controlling a temperature-dependent regulator.

[0077] First, the optical circuit of the MZI filter 1 of this embodiment, which is the subject of temperature-dependent adjustment, is adjusted to the yth ambient temperature (step ST1). Here, m is an integer greater than or equal to 2, and y is an integer such that 1 ≤ y ≤ m. Ambient temperature can be controlled by any method, such as by controlling a temperature controller (not shown) attached to the wavelength monitor element 200 incorporating the MZI filter 1, or by using equipment such as a constant temperature room (not shown). However, the optical circuit of the MZI filter 1 as a whole must be adjusted to the ambient temperature.

[0078] Next, with the optical circuit of the MZI filter 1 adjusted to the above ambient temperature, laser light of the xth wavelength is incident from a laser light source (not shown) to the first and third ports of the input-side optical multiplexer / demultiplexer 3 of the MZI filter 1, and the light transmitted through the MZI filter is incident on the first and second photodetectors 9a and 9b, which are optically connected to the second and fourth ports of the output-side optical multiplexer / demultiplexer 5, respectively (step ST2). Here, let n be an integer greater than or equal to 2, and x be an integer such that 1 ≤ x ≤ n. Laser light of multiple wavelengths may be laser light generated by multiple single-wavelength lasers, each emitting light at a different wavelength, or it may be laser light obtained by wavelength scanning using a tunable laser, such as a semiconductor laser. Furthermore, the laser light may be incident on the first and third ports of the input-side optical multiplexer / demultiplexer 3 via the first optical coupler 2a and the second optical coupler 2b, which are optically connected to each of the ports. Furthermore, when a tunable laser light source such as a semiconductor laser is used as the laser light source, the wavelength of the incident laser light may be monitored separately. In this case, the wavelength value of the separately monitored laser light source may be input to the control unit 10.

[0079] The laser light incident on the first photodetector 9a is photoelectrically converted by the first photodetector 9a and outputs a photocurrent indicating the first monitor value to the control unit 10. The laser light incident on the second photodetector 9b is photoelectrically converted by the second photodetector 9b, and outputs a photocurrent indicating the second monitor value to the control unit 10.

[0080] The control unit 10 converts the first monitor value obtained by photocurrent into a voltage, and further converts the analog to digital to obtain the first monitor value obtained by digital information. The control unit 10 converts the second monitor value obtained by photocurrent into a voltage, and further converts the analog to digital to obtain a second monitor value obtained by digital information. The conversion from photocurrent to voltage in the control unit 10 may be performed by other devices other than the control unit 10, which may or may not be included in the optical filter control system 100. In short, the control unit 10 only needs to obtain a first monitor value in digital information from the output of the first light receiver 9a, and a second monitor value in digital information from the output of the second light receiver 9b.

[0081] Next, the control unit 10, having obtained the first and second monitor values, calculates the wavelength monitor value, which is the difference between the first and second monitor values, and the optical power monitor value, which is the sum of the first and second monitor values. Furthermore, it calculates the wavelength monitor value by dividing the wavelength monitor value by the optical power monitor value (step ST3). Here, the optical power monitor value is proportional to the total power value of the laser light emitted from the first and third ports of the input-side optical multiplexer / demultiplexer 3, and the wavelength monitor value changes according to the wavelength and light intensity of the incident laser light. Therefore, by dividing the wavelength monitor value by the optical power monitor value, the wavelength monitor value is normalized with respect to the light intensity of the laser light. In this embodiment, it is sufficient to measure the wavelength spectrum of the light transmitted through the MZI filter 1, so it is also possible to use the wavelength monitor value without dividing it by the optical power monitor value.

[0082] Next, the control unit 10 records the wavelength monitor value in an internal or external storage medium of the control unit 10, associating it with the first ambient temperature and the first wavelength (step ST4).

[0083] Steps ST1 to ST4 are repeated for mutually different second to nth wavelengths and mutually different second to mth ambient temperatures.

[0084] Furthermore, when a semiconductor laser is used as the laser light source, the wavelength of the laser light incident on the MZI filter 1 can change continuously. In this case as well, in step ST4, the optical power monitor value is recorded in relation to some discrete wavelength value, such as the wavelength value of the laser light source monitored separately or the laser wavelength known as a function of time. In effect, this is no different from performing steps ST3 to ST4 while discretely changing the wavelength of the laser light source from the 1st to the nth wavelength.

[0085] Furthermore, the order in which steps ST1 to ST4 are performed is arbitrary, as long as steps ST1 to ST4 are performed for multiple sets of mutually different ambient temperatures.

[0086] Steps ST1 to ST4 are repeated for multiple ambient temperatures and multiple wavelengths to obtain the transmission wavelength spectrum of the MZI filter 1 for each ambient temperature from the recorded wavelength monitor value data (step ST5).

[0087] Step ST6 determines whether there is a change in the transmission wavelength spectrum of the MZI filter 1 for each ambient temperature obtained in step ST5, i.e., whether there is an ambient temperature dependence of the transmission wavelength spectrum. This determination can be made, for example, by quantifying the magnitude of the deviation in the acquired wavelength spectrum due to changes in ambient temperature using an arbitrary scale, and comparing the quantified magnitude of the deviation with a predetermined reference value.

[0088] For example, as illustrated in Figure 3B, if there is an ambient temperature dependence in which the wavelength spectrum of the transmitted light from the MZI filter 1 changes with changes in ambient temperature, the temperature-dependent regulators 7a and 7b are controlled according to the magnitude of the change in the wavelength spectrum to adjust the ambient temperature dependence to a small degree (step ST7). Various procedures can be employed for this adjustment, such as a procedure that adjusts based on data stored in advance as a table or function regarding the relationship between the degree of ambient temperature dependence of the wavelength spectrum and the adjustment amounts of temperature-dependent regulators 7a and 7b, or a trial-and-error procedure that repeats step ST7 and steps ST1 to ST6. Once the adjustments are complete, proceed to step ST8.

[0089] In step ST6, if it is determined that there is no change in the wavelength spectrum in response to ambient temperature, or that the degree of such change is smaller than the reference value, then the MZI filter 1 being evaluated can be said to have been manufactured largely as designed. In this case, no adjustment for ambient temperature dependence is necessary, so proceed to step ST8.

[0090] When the MZI filter 1 of this embodiment is used in a wavelength monitor element 200 for wavelength locking of a laser light source, for example, the phase adjuster 6 is adjusted so that the absolute value of the slope of the wavelength spectrum shown by the wavelength monitor value is maximized at the target wavelength λ to be locked (step ST8).

[0091] During step ST7, so-called crosstalk may occur, where the heat or signal from one phase tuner and temperature-dependent tuner affects the waveguide portion corresponding to another phase tuner or other temperature-dependent tuner. If crosstalk is a concern, after step ST7 is completed, return to step ST1, re-check the temperature dependence of the MZI filter 1, and perform steps ST2 to ST7 sequentially to readjust the phase tuner and temperature-dependent tuner. This feedback control is repeated until readjustment is no longer necessary.

[0092] (Hardware configuration) The following describes the hardware configuration that realizes the functions of the control unit 10 of the optical filter control system 100. The functions of the control unit 10 are realized by a processing circuit. Specifically, the control unit 10 includes a processing circuit for executing steps ST3 to ST7 of the temperature-dependent adjustment method shown in Figure 4. The processing circuit may be dedicated hardware, or it may be an arithmetic unit that executes a program.

[0093] Figure 5A is a block diagram showing the hardware configuration for executing the software that implements the functions of the control unit 10 of the optical filter control system 100. Figure 5B is also a block diagram showing the hardware configuration for implementing the functions of the control unit 10 of the optical filter control system. If the above processing circuit is a circuit comprising the arithmetic unit 10a and the storage unit 10b shown in Figure 5A, for example, the storage unit 10b stores a program that commands the execution of steps ST3 to ST7 of the temperature-dependent adjustment method shown in Figure 4. The arithmetic unit 10a loads and executes the program stored in the storage unit 10b, thereby realizing the functions of the control unit 10.

[0094] If the above processing circuit is a dedicated hardware processing circuit 10c as shown in Figure 5B, the processing circuit 10c may be, for example, a single circuit, a composite circuit, a programmed processor, a parallel programmed processor, an ASIC (Application Specific Integrated Circuit), an FPGA (Field-Programmable Gate Array), or a combination thereof. Furthermore, the functions of the control unit 10 may be implemented by separate processing circuits, or these functions may be implemented together in a single processing circuit.

[0095] In this embodiment, an isolator 8 is placed between the phase adjuster 6 and the temperature-dependent adjusters 7a and 7b to suppress crosstalk, thereby reducing the need for readjustment and facilitating control. If the effects of crosstalk are not considered, step ST9 can be omitted.

[0096] The procedure for determining the presence and magnitude of temperature dependence of the wavelength spectrum of transmitted light from MZI filter 1 is not limited to the procedure of steps ST1 to ST6 in the above flowchart; any other procedure can be used.

[0097] This embodiment relates to an MZI-type optical filter equipped with two asymmetrical arms having different optical path lengths. However, for interference-type optical filters with three or more asymmetrical arms having mutually different optical path lengths, where the input-side multiplexer / demultiplexer distributes incident light and the output-side multiplexer / demultiplexer combines the waves before outputting them to the receiver, it is possible to adjust the ambient temperature dependence of the optical filter due to individual differences that occur during manufacturing after the optical filter has been produced, by providing an ambient temperature-dependent adjustment waveguide with a temperature-dependent adjuster for each asymmetrical arm, similar to this embodiment.

[0098] Embodiment 2. Figure 6 shows the optical circuit configuration of the MZI filter 1' according to Embodiment 2. Elements whose position on the optical circuit is the same as in Embodiment 1 are given the same reference numerals as in Embodiment 1. As shown in Figure 6, the MZI filter 1' is configured in the optical circuit configuration of Embodiment 1 above, in which the first asymmetric portion is made of a single silicon nitride waveguide 41a1 with a physical length L + ΔL, and the second asymmetric portion is made of a single silicon waveguide 41b1 with a physical length L. In this case, the entire individual silicon nitride waveguide 41a1 with physical length L + ΔL in the first asymmetrical portion and the silicon waveguide 41b1 with physical length L in the second asymmetrical portion each constitute a non-common portion B. The input-side optical multiplexer / demultiplexer 3 and the silicon nitride waveguide 41a1 and silicon waveguide 41b1, as well as the silicon nitride waveguide 41a1 and silicon waveguide 41b1 and the respective temperature-dependent adjustment waveguides 42a and 42b, are all optically connected by silicon waveguides, and these parts can also be considered as a common part A. The distinguishing feature of Embodiment 2 is that the second asymmetrical portion in Embodiment 1 is formed entirely from a single silicon waveguide. Therefore, as a modification, the silicon nitride waveguide 41a1 may be divided into two via the silicon waveguide 41a3, similar to Embodiment 1. In that case, a portion corresponding to the silicon waveguide 41b3 is defined in the second asymmetrical portion as well, and both are included in the common portion A. The arrangement of other optical circuit elements in the first asymmetric arm 4a and the second asymmetric arm 4b, as well as the phase adjuster and temperature-dependent adjuster, is the same as in Embodiment 1.

[0099] In Embodiment 2, the difference in optical path length between the first and second asymmetrical arms can still be expressed by (2b) and (2b'), and therefore, by satisfying equation (3b) or equation (3b') between L and ΔL, an MZI filter 1 can be realized in which the difference in optical path length between the two asymmetrical arms does not depend on temperature for the target wavelength λ.

[0100] Furthermore, the relationship between the first asymmetric arm 4a and the second asymmetric arm 4b is such that in the non-common portion B, the physical length of the optical waveguides and the effective refractive index are different, and this is not limited to a planar waveguide type optical monitor using a silicon photonics chip, but may also be a planar waveguide type optical monitor using a compound semiconductor indium phosphide substrate or a glass substrate.

[0101] The operating principle and function of the temperature-dependent adjustment waveguide of the MZI filter 1' in Embodiment 2 are the same as those in Embodiment 1.

[0102] In Embodiment 2, the waveguide materials for waveguides 41a1 and 41b1 can be selected from any material and structure, as long as the relationship is satisfied that the rate of change of the effective refractive index with respect to temperature changes is relatively small for waveguide 41a1 with a longer physical length and relatively large for waveguide 41a1 with a shorter physical length.

[0103] Embodiment 3. The MZI filter 1'' (not shown) according to Embodiment 3 is configured such that, in the optical circuit configuration of the MZI filter 1'' of Embodiment 2 shown in Figure 6, the first asymmetrical portion is composed of a single waveguide 41a1 with a physical length L + ΔL, and the second asymmetrical portion is composed of a single waveguide 41b1 with a physical length L. Waveguides 41a1 and 41b1 are made of the same material and structure. Elements whose position on the optical circuit is the same as in Embodiment 1 are given the same reference numerals as in Embodiment 1. In this case, the entire waveguide 41a1 with physical length L+ΔL in the first asymmetrical portion and the entire waveguide 41b1 with physical length L in the second asymmetrical portion each constitute a non-common portion B. Waveguides 41a1 and 41b1 may both be made of silicon waveguides, but waveguides of other materials can also be used, as long as the optical path lengths of the first asymmetrical portion and the second asymmetrical portion are different.

[0104] The operating principle and function of the temperature-dependent adjustment waveguide of the MZI filter 1'' in Embodiment 3 are the same as those in Embodiments 1 and 2.

[0105] (Common matters) In each of the above embodiments, the phase adjuster 6 may be omitted as necessary, and either the temperature-dependent adjuster 7a or 7b may be omitted as necessary. [Industrial applicability]

[0106] The optical filter and optical filter control system disclosed herein can be used, for example, as an optical filter in a wavelength monitor element for monitoring the wavelength of a light source, and as a means for controlling said optical filter, in optical communication systems that require precise control of the wavelength of a light source. In addition, it can be used as an optical filter for precisely monitoring the wavelength of light in various fields that require precise monitoring of optical wavelengths, such as the oscillation control of laser light. [Explanation of Symbols]

[0107] 1 MZI filter, 2a,2b Optical coupler, 3 Input side photomultiplexer / demultiplexer, 4a First asymmetric arm, 4b Second asymmetric arm, 5 Output side photomultiplexer / demultiplexer, 6 Phase adjuster, 7a,7b Temperature-dependent adjuster, 8 Isolator, 9a First photodetector, 9b Second photodetector, 10 Control unit, 10a Calculation unit, 10b Memory unit, 10c Processing circuit, 41a1,41a2 Silicon nitride waveguide, 41a3 Silicon waveguide, 41b1~41b3 Silicon waveguide, 41b4,41b5 Silicon nitride waveguide, 42a,42b Waveguides for temperature-dependent adjustment, 100 Optical filter control system, 200 Wavelength monitor element, 201a Substrate surface, 201A Substrate.

Claims

1. A first photomultiplexer / demultiplexer is provided on the light incident side, A second photomultiplier / demultiplier is provided on the light emission side, The system comprises a first waveguide and a second waveguide, each having a different optical path length, which optically connect the first optical multiplexer / demultiplexer and the second optical multiplexer / demultiplexer, respectively. The first waveguide consists of a first waveguide portion and a first regulating waveguide portion. The aforementioned second waveguide consists of a second waveguide portion and a second regulating waveguide portion. The first and second adjustment waveguide sections exhibit relatively greater nonlinearity in the change of effective refractive index with respect to temperature changes compared to the first and second waveguide sections. The device includes a temperature-dependent regulator provided in at least one of the first or second adjustment waveguide portion, for changing the temperature dependence of the effective refractive index in the waveguide at the location where the regulator is provided. Light filter.

2. The invention is characterized by comprising a phase adjuster provided in at least one of the first waveguide portion or the second waveguide portion, for changing the temperature dependence of the effective refractive index in the waveguide portion at the location where the adjuster is provided. The optical filter according to claim 1.

3. At least a portion of the first waveguide section is composed of a first waveguide element. At least a portion of the second waveguide section is composed of a second waveguide element having a different effective refractive index and physical length from the first waveguide element. The optical path lengths of the first waveguide portion and the second waveguide portion are different, characterized in that The optical filter according to claim 1 or claim 2.

4. The first waveguide element and the second waveguide element are characterized in that the waveguide element with a relatively larger rate of change in the effective refractive index with respect to temperature changes has a relatively shorter physical length. The optical filter according to claim 3.

5. The optical filter according to claim 3, comprising at least one of the following: a third waveguide element provided in the second waveguide portion, having the same effective refractive index as the first waveguide element and a physical length shorter than the first waveguide element; or a fourth waveguide element provided in the first waveguide portion, having the same effective refractive index as the second waveguide element and a physical length shorter than the second waveguide element.

6. The first waveguide element is a silicon nitride waveguide, and the second waveguide element is a silicon waveguide, characterized in that The optical filter according to claim 3.

7. At least one of the first waveguide element or the second waveguide element is composed of two or more elements. If the first waveguide element is composed of two or more elements, each element constituting the first waveguide element is arranged at two or more locations in the first waveguide portion. If the second waveguide element is composed of two or more elements, each element constituting the second waveguide element is arranged at two or more locations in the second waveguide portion. The optical filter according to claim 3.

8. The optical filter according to claim 1 or 2, wherein, when a plurality of phase adjusters or temperature-dependent adjusters are provided together, the optical filter is further provided with an isolator that thermally or electrically isolates the plurality of phase adjusters or temperature-dependent adjusters from each other.

9. A silicon photonics chip configured on a silicon substrate, The optical filter according to claim 1 or claim 2.

10. It includes a heater for heating the waveguide at the location where a phase adjuster or temperature-dependent adjuster is installed. The optical filter according to claim 1 or claim 2.

11. The optical filter according to claim 1 or claim 2, Two photodetectors, each optically connected to the second photomultiplexer / demultiplexer, The optical filter includes a temperature-dependent regulator and a control unit electrically connected to the two light receivers, A light filter control system equipped with, The control unit, Based on the magnitude of the photocurrent of the laser light incident on the first photomultiplexer / demultiplexer and received by the two photodetectors, the amount of light transmitted through the optical filter is calculated. Based on the amount of transmitted light associated with multiple ambient temperatures and multiple wavelengths of the laser light, the presence or absence of temperature dependence of the transmitted wavelength spectrum of the optical filter is determined. If it is determined that there is a temperature dependence, a signal is output to the temperature dependence regulator provided in the optical filter to operate the temperature dependence regulator according to the degree of the temperature dependence. Optical filter control system.

12. A method for controlling an optical filter using the optical filter control system described in claim 11, The control unit, A step of calculating the amount of light transmitted through the optical filter based on the magnitude of the photocurrent of the laser light incident on the first photomultiplexer and received by the two photodetectors, respectively; A step of determining whether or not the transmitted wavelength spectrum of the optical filter has a temperature dependence based on the amount of transmitted light associated with a plurality of ambient temperatures and a plurality of wavelengths of the laser light, If it is determined that there is a temperature dependence, the step of outputting a signal to the temperature-dependent regulator provided in the optical filter to operate the temperature-dependent regulator according to the degree of the temperature dependence, A method for controlling optical filters, comprising:

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