Optical Monitor Device

Abstract

The purpose of the present invention is to provide, in a small size and at low cost, an optical monitor device whereby the power of an optical signal incident on a multicore optical fiber can be accurately measured irrespective of the polarization state of the optical signal.　The optical monitor device pertaining to the present disclosure comprises a first branching part (33A) for branching incident light incident from a prescribed incidence region into two directions including a first direction and a second direction, a first light-receiving element (5A) for receiving the branched light branched in the second direction by the first branching part (33A) and detecting the intensity of the light branched by the first branching part for each incidence position in the incidence region, a second branching part (33B) for branching the branched light branched in the first direction by the first branching part (33A) into two directions including the first direction and a third direction perpendicular to both the first direction and the second direction, and a second light-receiving element (5B) for receiving the branched light branched in the third direction by the second branching part and detecting the intensity of the light branched by the second branching part (33B) for each incidence position in the incidence region.

Description

[Technical field]

[0001] The present disclosure relates to an optical monitoring device, and more particularly to an optical monitoring device for detecting the intensity of light in an optical transmission device or the like and feeding back the detection result to other components. [Background technology]

[0002] In recent years, with the increase in Internet traffic, there is a strong demand for increasing communication capacity in communication systems. To achieve this, communication systems using optical fibers are used in the access networks between communication offices and users' homes, and in the core networks connecting communication offices. In optical fiber communication, the detection of the optical intensity propagating through the optical fiber is often used to control communication and check the soundness of the equipment. For example, in access networks, test light is propagated through the optical fiber, and the loss and soundness of the optical fiber, as well as the target and connection of the core wires, are confirmed by detecting the optical intensity. In addition, WDM (Wavelength Division Multiplex) transmission used in core networks requires monitoring of the optical intensity for feedback control.

[0003] In optical power monitoring of access networks, a technology is used that splits light at a fixed splitting ratio using two parallel waveguides (see, for example, Patent Document 1), which makes it possible to measure the optical signal power and propagation loss in the access network.

[0004] In optical intensity monitoring in WMD transmission, a technique is used that simultaneously monitors the intensity of optical signals in multiple optical fibers by combining optical fibers arranged one-dimensionally with a dielectric multilayer film (see, for example, Patent Document 2).

[0005] However, the conventional optical monitor device still has the following problems.

[0006] As optical communications become more widespread and the number of fiber cores in optical facilities / cables increases, firstly, in the case of optical monitoring devices that use an optical coupler for each optical fiber core, the cost and size increase in proportion to the number of cores.Even in the case of optical monitoring devices in which optical fibers and optical intensity sensors are arranged in a one-dimensional array, there is a limit to the array arrangement of optical fibers, and if the number of optical fiber cores increases, the cost and size increase in proportion to the number of cores.

[0007] There is also a technology that uses Fresnel reflection as a spatial optical system for constructing such an optical monitor device. However, since Fresnel reflection has different reflectances for the incident p-polarized and s-polarized light, the branching ratio of the light branched to the optical sensor side changes depending on the polarization state of the incident light, making it difficult to perform accurate measurements. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Patent No. 3450104 [Patent Document 2] Patent Publication No. 2004-219523 Summary of the Invention [Problem to be solved by the invention]

[0009] The present disclosure has been made in consideration of these points, and aims to realize a small, low-cost optical monitor device that can accurately measure the power of an optical signal, regardless of the polarization state of the incident optical signal, for optical fibers with a large number of cores, such as tens of cores. [Means for solving the problem]

[0010] In order to achieve the above object, the optical monitor device of the present disclosure comprises: An optical monitor device for detecting the intensity of light propagating through a plurality of optical fibers, comprising: a first branching unit that branches incident light incident through a defined incident region into two, that is, light in a first direction and light in a second direction; a first light receiving element that receives the branched light branched in the second direction by the first branching section and detects the intensity of the light branched by the first branching section for each incident position in the incident region; a second branching unit that branches the branched light branched in the first direction by the first branching unit into two directions, a third direction perpendicular to both the first direction and the second direction, and the first direction; a second light receiving element that receives the branched light branched in the third direction by the second branching unit and detects an intensity of the light branched by the second branching unit for each incident position in the incident region; Equipped with. Effect of the Invention

[0011] According to the present disclosure, it is possible to realize a small, low-cost optical monitor device that can accurately measure the power of an optical signal, regardless of the polarization state of the incident optical signal, for optical fibers with a large number of cores, such as tens of cores. [Brief description of the drawings]

[0012] [Figure 1] 1 is a diagram illustrating the relationship between the angle of incidence and reflection of s-polarized light and p-polarized light. [Diagram 2] FIG. 1 illustrates an optical monitor device according to the present disclosure. [Diagram 3] FIG. 1 illustrates an optical monitor device according to the present disclosure. [Figure 4] FIG. 1 illustrates an optical monitor device according to the present disclosure. [Diagram 5] 4 is a diagram for explaining the state of light at first and second refractive index interfaces. FIG. [Figure 6] 13 is a diagram for explaining the state of light at third and fourth refractive index interfaces. FIG. [Figure 7] 4 is a diagram for explaining the state of light at first and second refractive index interfaces. FIG. [Figure 8]13 is a diagram for explaining the state of light at third and fourth refractive index interfaces. FIG. [Figure 9] 13 is a diagram for explaining the state of light at third and fourth refractive index interfaces. FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Hereinafter, the embodiments of the present disclosure will be described in detail with reference to the drawings. Note that the present disclosure is not limited to the embodiments shown below. These implementation examples are merely illustrative, and the present disclosure can be implemented in various forms with various modifications and improvements based on the knowledge of those skilled in the art. Note that components with the same reference numerals in this specification and drawings are assumed to indicate the same components.

[0014] In order to solve the above problems, the present disclosure provides an optical monitor device that can be realized by the configuration exemplified in FIG. The optical monitor device of the present disclosure comprises: a spatial optical system 30 that splits and emits a part of the incident light 41 in a specific direction (second direction), a remaining part in another specific direction (third direction), and a majority of the remaining in yet another specific direction (first direction) at a specific splitting ratio; An incident-side optical fiber 11 that propagates a plurality of light beams arranged in a two-dimensional array so as to input the light beams to a spatial optical system 30; an output optical fiber 12 for propagating a plurality of light beams arranged to receive a majority of the output light beam 42 from the spatial optical system 30; a first light receiving element 5A arranged to receive a first portion of the emitted light 43A from the spatial optical system 30; A second light receiving element 5B arranged to receive a second portion of the emitted light 43B; an incident-side optical lens 21 that is disposed between the spatial optical system 30 and the incident-side optical fiber 11 and converts the light incident on the spatial optical system 30 into parallel light; an output optical lens 22 that is disposed between the spatial optical system 30 and the output optical fiber 12 and efficiently couples the output light from the spatial optical system 30 to the output optical fiber 12; has.

[0015] As shown in FIG. 2, the spatial optical system 30 includes: a first member 30A connected to the incident side optical lens 21 and having a uniform refractive index; a first monolayer film 33A in contact with the first member 30A and having a uniform refractive index different from that of the first member 30A; a second member 30B in contact with the first monolayer film 33A and having the same refractive index as the first member 30A; a second monolayer film 33B in contact with the second member 30B and having the same refractive index as the first monolayer film 33A; a third member 30C connected to the second single layer film 33B and the output side optical lens 22 and having the same refractive index as the first member 30A; It may be composed of:

[0016] Here, the refractive index of the first monolayer film 33A and the refractive index of the second monolayer film 33B are arbitrary. For example, the refractive index of the first monolayer film 33A and the refractive index of the second monolayer film 33B are lower than the refractive indexes of the first member 30A, the second member 30B, and the third member 30C. In addition, the refractive index of the first monolayer film 33A and the refractive index of the second monolayer film 33B may be the same or different.

[0017] The first single-layer film 33A functions as a first branching section that branches the incident light into two, a first direction and a second direction. The second single-layer film 33B functions as a second branching section that branches the branched light, which has been branched in the first direction by the first single-layer film 33A, into two, a third direction perpendicular to both the first direction and the second direction, and the first direction.

[0018] The spatial optical system 30 includes: the first refractive index interface 31A and the second refractive index interface 31B are parallel to each other and are provided at a specific angle with the optical axis of the incident light 41; and the third refractive index interface 31C and the fourth refractive index interface 31D are parallel to each other and are provided at the specific angle with the optical axis of the incident light 41 and with normals perpendicular to the normals of the first refractive index interface 31A and the second refractive index interface 31B, The first direction in which most of the emitted light 42 is emitted is the direction in which the light is transmitted through the first to fourth refractive index interfaces (31A, 31B, 31C, 31D), the second direction in which the first portion of the emitted light 43A is emitted is the direction in which the first portion of the emitted light 43A is reflected at the first refractive index interface 31A; The third direction in which the second portion of emitted light 43B is emitted is the direction in which it is reflected at the third refractive index interface 31C.

[0019] In addition, the spatial optical system 30 includes: The optical fiber 10 may have a first refractive index interface 31A and a second refractive index interface 31B that are parallel to each other and are provided at a specific angle with respect to the optical axis of the incident light 41, and a third refractive index interface 31C and a fourth refractive index interface 31D that are located closer to the output side optical fiber 12 than the first refractive index interface 31A and the second refractive index interface 31B and correspond to a surface obtained by rotating the first refractive index interface 31A and the second refractive index interface 31B by 90 degrees in the circumferential direction around the optical axis of the incident light 41.

[0020] In FIG. 2, the boundary surface between the first component 30A and the first monolayer film 33A is the first refractive index interface 31A, the boundary surface between the second component 30B and the first monolayer film 33A is the second refractive index interface 31B, the boundary surface between the second component 30B and the second monolayer film 33B is the third refractive index interface 31C, and the boundary surface between the third component 30C and the second monolayer film 33B is the fourth refractive index interface 31D.

[0021] FIG. 2 and FIGS. 3 to 9 described below show an example in which the first direction is the x-axis direction, the second direction is the z-axis direction, and the third direction is the y-axis direction, but these directions can be any directions depending on the optical design of the spatial optical system 30.

[0022] In order to facilitate understanding of the spatial optical system 30 shown in Fig. 2, Fig. 4 shows only one of the two-dimensionally arranged incident-side optical fiber 11 and outgoing-side optical fiber 12. In the following description, the light transmitted through the first single-layer film 33A in the x-axis direction is referred to as most of the outgoing light 42A, and the light transmitted through the second single-layer film 33B in the x-axis direction is referred to as most of the outgoing light 42B. Also, the optical axis of the incident light 41 is in the x-axis direction.

[0023] Fig. 5 shows a cross section (xz plane) including the incident light 41 and the first part of the emitted light 43A of the first monolayer film 33A shown in Fig. 4. Fig. 6 shows a cross section (xy plane) including the most part of the emitted light 42A and the second part of the emitted light 43B of the second monolayer film 33B shown in Fig. 4.

[0024] When the refractive indexes of the member 30A and the single layer film 33A are different, as shown in FIG. 5, a part of the incident light 41 is reflected at the first refractive index interface 31A to become the outgoing light 43A, and the rest is refracted at the first refractive index interface 31A. When the members 30A and 30B have the same refractive index, The light refracted at the surface 31A is refracted again at the second refractive index interface 31B to become most of the outgoing light 42A, which becomes parallel to the incident light 41. Similarly, in Fig. 6, when the members 30B and 30C have the same refractive index, most of the outgoing light 42A and most of the outgoing light 42B become parallel. Note that, although not shown in Figs. 5 and 6 and Figs. 7 to 9 to be described later for ease of understanding, a part of the incident light 41 is also reflected at the second refractive index interface 31B to become outgoing light 43A, and a part of the most of the outgoing light 42A is also reflected at the fourth refractive index interface 31D to become outgoing light 43B.

[0025] 5 is a plane including the incident light 41 incident on the first monolayer film 33A along the first direction (x direction) and the first part of the outgoing light 43A reflected by the first monolayer film 33A in the second direction (z direction), and thus serves as the incident plane of the first monolayer film 33A. FIG. 6 is a plane including the majority of the outgoing light 42A incident on the second monolayer film 33B along the first direction (x direction) and the second part of the outgoing light 43B reflected by the second monolayer film 33B in the third direction (y direction), and thus serves as the incident plane of the second monolayer film 33B. As described above, by providing the first monolayer film 33A and the second monolayer film 33B so that the third direction is perpendicular to the first direction and the second direction, the incident plane of the first monolayer film 33A and the incident plane of the second monolayer film 33B can be made perpendicular to each other.

[0026] 4, as described above, the incident plane of the first single-layer film 33A and the incident plane of the second single-layer film 33B are perpendicular to each other, so that the p-polarized light in the first single-layer film 33A becomes the s-polarized light in the second single-layer film 33B, and the s-polarized light in the first single-layer film 33A becomes the p-polarized light in the second single-layer film 33B. Since the first single-layer film 33A and the second single-layer film 33B have the same angle with the optical axis and the same refractive index, the branching ratio of the p-polarized light in the first single-layer film 33A and the branching ratio of the s-polarized light in the second single-layer film 33B are equal, and the branching ratio of the s-polarized light in the first single-layer film 33A and the branching ratio of the p-polarized light in the second single-layer film 33B are also equal.

[0027] The intensity of p-polarized light in the intensity I of the incident light 41 is I p , the intensity of s-polarized light is I s The splitting ratio of p-polarized light and s-polarized light in the first monolayer film 33A is K p , K s Then, the optical power entering the first light receiving element 5A is expressed by Equation 1, and the optical power entering the second light receiving element 5B is expressed by Equation 2. (Number 1) K p I p +K s I s (Formula 1) (Number 2) K s (1-K p)I p +K p (1-K s )I S (Formula 2)

[0028] From the above, the total value of the optical power entering the first light receiving element 5A and the second light receiving element 5B is given by Equation 3. (Number 3) (K s +K p -K s K p )(I p +I s )=(K s +K p -K s K p )I (Equation 3)

[0029] Branching ratio K s , K p depends only on the refractive index and the incident angle of the spatial optical system 30, the ratio of the sum of the optical powers entering the two light receiving elements 5A and 5B to the optical power of the incident light 41 is constant regardless of the polarization state.

[0030] 2 and 4, the light from the incident optical fiber 11 is converted into parallel light by the incident optical lens 21, preventing loss due to diffusion. Furthermore, most of the emitted light 42 is guided to the output optical lens 22 by the spatial optical system 30. The output optical lens 22 collects most of the emitted light 42 that has passed through the spatial optical system 30, and couples it to the output optical fiber 12. In this way, most of the emitted light 42 emitted from the incident optical fiber 11 can be guided to the output optical fiber 12 with little loss.

[0031] FIG. 7 shows a case where incident light 41 including light of multiple wavelengths is incident on the first single layer film 33A. The incident light 41 travels in different directions in the single layer film 33A depending on the wavelength. Therefore, the incident position on the refractive index interface 31B varies depending on the wavelength. On the other hand, when the members 30A and 30B have the same refractive index, the light incident on the member 30B from the refractive index interface 33B travels in the same direction as the incident light 41 due to refraction between the single layer film 33A and the member 30B. Hereinafter, of the light (most of the exit light 42A) incident on the member 30B from the refractive index interface 33B, the light with a long wavelength is referred to as most of the exit light 42A-L, and the light with a short wavelength is referred to as most of the exit light 42A-S. In FIG. 7, the light with a long wavelength has a smaller refraction angle, but this is not limited to the above. In this case, the optical axes of the incident light 41 and most of the outgoing light 42A-L are shifted by Z1 in the z direction, and the optical axes of the incident light 41 and most of the outgoing light 42A-S are shifted by Z2 in the z direction (Z1 <Z2)だけずれる。

[0032] Fig. 8 shows how most of the emitted light 42A-L shown in Fig. 7 is transmitted and reflected by the single-layer film 33B. As described above, when the refractive indexes of the member 30B and the single-layer film 33B are different and the refractive indexes of the members 30B and 30C are the same, most of the emitted light 42A-L and most of the emitted light 42B-L are parallel, and their optical axes are shifted by Y1 in the y direction. Note that the part of the emitted light 42B-L refers to light with a long wavelength among the majority of the emitted light 42B.

[0033] Fig. 9 shows how most of the emitted light 42A-S shown in Fig. 7 is transmitted and reflected by the single-layer film 33B. As described above, when the refractive indexes of the member 30B and the single-layer film 33B are different and the refractive indexes of the members 30B and 30C are the same, most of the emitted light 42A-S and most of the emitted light 42B-S are parallel, and their optical axes are shifted by Y2 in the y direction. Note that the partial emitted light 42B-S refers to light with a short wavelength among the majority of the emitted light 42B.

[0034] As a result, even if the optical axes of the output optical fibers 12 at the incident end faces are arranged in parallel, the transmitted light can be coupled to the output optical fibers 12 regardless of the wavelength.

[0035] However, as shown in FIG. 7 to FIG. 9, the optical axis of the incident light 41 and the majority of the emitted light 42B are misaligned. Therefore, in the present disclosure, the position and lens diameter of the output optical lens 22 are determined according to the wavelength range of the incident light 41. Moreover, the greater the thickness of the single-layer film 33A or 33B, the greater the misalignment of the optical axis of the incident light 41 and the majority of the emitted light 42B. Therefore, the position of the output optical lens 22 is determined according to the thickness of the single-layer film 33A and 33B. Moreover, by setting the diameter of the output optical lens 22 to a value or more determined according to the wavelength width of the incident light 41 and the thickness of the single-layer film 33A and 33B, the optical loss can be reduced. On the other hand, if the diameter of the output optical lens 22 is greater than the installation interval of the input fiber, it will collide with the adjacent lens, so the diameter of the output optical lens 22 needs to be less than the installation interval of the input fiber.

[0036] On the other hand, some of the emitted light 43A and 43B branched by the spatial optical system 30 are guided to the light receiving element 5A or 5B arranged in a different direction from the majority of the emitted light 42. In this way, the intensity of a part of the light propagating from the incident side optical fiber 11 to the output side optical fiber 12 can be measured.

[0037] The ratio between the sum of the optical power measured by the two light receiving elements 5A and 5B and the intensity of the emitted light 42 is constant and known in advance. For example, if the ratio is 1:N, and the sum of the optical intensities measured by the light receiving elements 5A and 5B is L [mW], it can be known that the optical intensity incident from the incident side optical fiber 11 is (N+1)×L [mW], and the optical intensity propagated to the exit side optical fiber 12 is N×L [mW].

[0038] As described above, in the optical monitoring devices illustrated in Figures 2 and 4, incident light is branched by Fresnel reflection at the refractive index interface. However, since Fresnel reflection does not depend on wavelength but on refractive index, light can be branched over a wide wavelength range.

[0039] (First embodiment) FIG. 3 shows an embodiment of the present disclosure. In FIG. 3, only one of the two-dimensionally arranged incident side optical fiber 11 and the output side optical fiber 12 is shown for ease of understanding. The members 30A, 30B, and 30C can be made of, for example, quartz glass. For the first single-layer film 33A and the second single-layer film 33B, a spacer 34 of a predetermined thickness is placed between each member, and an air layer can be used by leaving a gap. The incident side optical lens 21 and the output side optical lens 22 can be realized by a collimator having a GRIN (GRaded INdex) fiber built in a rectangular ferrule 13 or 14 used in optical connectors, etc. The optical fibers 11 and 12 are also built in the rectangular ferrule 13 or 14, respectively, and the optical axes of the incident side optical fiber 11, the incident side optical lens 21, the output side optical fiber 12, and the output side optical lens 22 can be aligned using a guide pin 15 and a guide hole, as in the optical connector. The light receiving elements 5A and 5B can be realized by a commercially available optical sensor element or optical image sensor. By filling the connecting portions other than the single layer films 33A and 33B with a refractive index matching material, it is possible to suppress unnecessary Fresnel reflection.

[0040] Regarding Fresnel reflection when light is incident on air from quartz glass, the relationship between the angle of incidence and the reflectance for each of s-polarized light and p-polarized light is shown in Figure 1. As shown in Figure 1, there is an angle of incidence at which the reflectance of p-polarized light is zero. At this time, the branching ratio of p-polarized light branched by single-layer films 33A and 33B is also zero, so if first single-layer film 33A and second single-layer film 33B are arranged so that the angle of incidence is this angle, the total optical power entering first light-receiving element 5A and second light-receiving element 5B described above will be K s Therefore, the relationship between the sum of the light intensities measured by the light receiving elements 5A and 5B and the incident light power becomes easier. For example, the first single-layer film 33A and the second single-layer film 33B may be provided so that the incident angle is 30 degrees or less.

[0041] 2 to 4, the input optical fiber 11 and output optical fiber 12 are arranged two-dimensionally, and the two-dimensionally arranged light beam is branched by the spatial optical system 30. This has the effect of making it possible to reduce the size compared to using an optical monitor device for each individual core or an optical monitor device in which optical fibers are arranged one-dimensionally. In addition, since the number of components is small, it is easy to reduce costs. In addition, the power of the optical signal can be accurately monitored regardless of the polarization state of the incident optical signal.

[0042] 2 to 4 show an example in which the incident side optical fibers 11, the exit side optical fibers 12, the incident side optical lenses 21, and the exit side optical lenses 22 are arranged in a two-dimensional array of 3 × 3, but the combination may be any number equal to or greater than 2 × 2. Furthermore, the intervals in the two-dimensional array of the incident side optical fibers 11 and the exit side optical fibers 12 may be the same or different.

[0043] The above are examples of the embodiment, but the present invention is not limited to these. For example, the spatial optical system 30 is not limited to a cubic shape, and may be any shape, such as a rectangular parallelepiped. The light receiving element 5 may be disposed at any position where the light branched by the spatial optical system 30 can be received. For example, the light receiving element 5 may be embedded inside the spatial optical system 30.

[0044] The optical monitoring device of the present disclosure can be used to monitor any light transmitted in an optical transmission system. For example, the optical monitoring device of the present disclosure can be mounted in any device used in an optical transmission system, such as a transmitting device, a receiving device, or a repeater, and the measurement result of the light receiving element 5 can be used for feedback or feedforward to any component inside or outside the device. Also, the optical monitoring device of the present disclosure can be inserted in the middle of a transmission line in an optical transmission system to measure the intensity and propagation loss of an optical signal in the transmission line. [Industrial Applicability]

[0045] The present disclosure can be applied to the information and communications industry. [Explanation of symbols]

[0046] 5A, 5B: Light receiving element 11: Input optical fiber 12: Output optical fiber 13: Input ferrule 14: Output ferrule 15: Guide pin 21: Input optical lens 22: Output optical lens 31: Refractive index interface 33A, 33B: Single layer membrane 34: Spacer 41: Incident light 42: Most of the light emitted 43A, 43B: Part of the emitted light 30: Spatial optical system 30A, 30B, 30C: Materials

Claims

1. An optical monitor device for detecting the intensity of light propagating through a plurality of optical fibers, comprising: a first branching unit that branches incident light incident through a defined incident region into two directions, a first direction and a second direction; a first light receiving element that receives the branched light branched in the second direction by the first branching portion and detects an intensity of the light branched by the first branching portion for each incident position in the incident region; a second branching unit that branches the branched light, which is branched in the first direction by the first branching unit, into two directions, that is, into a third direction perpendicular to both the first direction and the second direction, and into the first direction; a second light receiving element that receives the branched light branched in the third direction by the second branching unit and detects an intensity of the light branched by the second branching unit for each incident position in the incident region; Equipped with the first branch and the second branch have equal refractive indices and angles of incidence; by summing the light intensities of the first light receiving element and the second light receiving element, the intensity of the light propagating through the plurality of optical fibers is detected. Optical monitor device.

2. an optical component having the first branching portion and the second branching portion; a plurality of incident side optical fibers arranged in a two-dimensional array so as to make light incident on the incident region of the optical component; a plurality of output side optical fibers arranged in a two-dimensional array so as to receive each output light from the optical component in the first direction; an incident-side optical lens disposed between the optical component and the incident-side optical fiber, and converting each light beam incident on the optical component into a parallel beam; an output optical lens disposed between the optical component and the output optical fiber, for coupling each output light from the optical component to the output optical fiber; 2. The optical monitor device according to claim 1, further comprising:

3. The optical component is a first member connected to the incident side optical lens and having a uniform refractive index; a first monolayer film in contact with the first member and having a uniform refractive index lower than the refractive index of the first member; a second member in contact with the first monolayer film and having the same refractive index as the first member; a second monolayer film in contact with the second member and having a uniform refractive index lower than the refractive indexes of the first member and the second member; a third member connected to the second single-layer film and the output side optical lens and having the same refractive index as the first member; the first monolayer film functions as the first branch portion, The second monolayer film functions as the second branch portion.

3. The optical monitor device according to claim 2.

4. a first interface between the first member and the first monolayer film and a second interface between the second member and the first monolayer film have a specific angle with an optical axis of the incident light, a third boundary surface between the second member and the second monolayer film and a fourth boundary surface between the third member and the second monolayer film have a normal line perpendicular to a normal line of the first boundary surface and a normal line of the second boundary surface, the first direction is a direction transmitted from the first boundary surface through the fourth boundary surface, the second direction is a direction reflected at the first boundary surface, The third direction is a direction reflected at the third boundary surface.

4. The optical monitor device according to claim 3.

5. a first interface between the first member and the first monolayer film and a second interface between the second member and the first monolayer film have a specific angle with an optical axis of the incident light, a third boundary surface between the second member and the second monolayer film and a fourth boundary surface between the third member and the second monolayer film correspond to surfaces obtained by rotating the first boundary surface and the second boundary surface by 90 degrees in a circumferential direction around an optical axis of the incident light, the first direction is a direction transmitted from the first boundary surface through the fourth boundary surface, the second direction is a direction reflected at the first boundary surface, The third direction is a direction reflected at the third boundary surface.

4. The optical monitor device according to claim 3.

6. The specific angle is an angle at which the reflectance of p-polarized light of the incident light is zero.

6. An optical monitor device according to claim 4 or 5.

7. The first monolayer film and the second monolayer film are air layers.

7. An optical monitor device according to claim 3, wherein the optical monitor device is a semiconductor laser.

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