Optical multiplexer used in optical detection devices

Abstract

To provide an optical multiplexer that outputs a plurality of interference light beams in which the phase difference between signal light and reference light has been changed.SOLUTION: The optical multiplexer used in optical detection devices comprises: a first input port which signal light enters; a second input port which reference light of the same wavelength as the signal light enters; an optical multiplexing coupler for multiplexing signal light and reference light and generating a plurality of interference light beams; and a plurality of output ports for outputting the plurality of interference light beams. Each of the plurality of output ports has an optical path difference which is the difference between the shortest distance to the first input port and the shortest distance to the second input port in the inside of the core of the optical multiplexing coupler. The optical path difference is constituted to be different between the plurality of output ports.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The technology disclosed in this specification relates to an optical multiplexer used in a photodetector device. [Background technology]

[0002] Various optical devices require a technique for estimating the phase state of light propagating through an optical waveguide. For example, in a Mach-Zehnder optical modulator, light propagating through an optical waveguide is branched into two optical waveguides, and different phases are imparted to the light propagating through one or both optical waveguides, thereby estimating the phase state from the difference in the intensity of the combined light. Furthermore, Patent Documents 1 and 2 disclose techniques for estimating the phase state of light propagating through each of multiple optical waveguides in an optical phased array (OPA) having multiple optical waveguides in order to calibrate manufacturing variations in each of the multiple optical waveguides and characteristic variations in phase shifters. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2023-079493 [Patent Document 2] Special Publication No. 2023-508155 Summary of the Invention [Problem to be solved by the invention]

[0004] In an optical device having a plurality of optical waveguides, in order to estimate the phase state of each of the plurality of optical waveguides with high accuracy, it is desirable to acquire a large amount of data in which the phase difference between signal light propagating through the optical waveguides and reference light whose phase state is known or does not fluctuate is changed. This specification provides an optical multiplexer used in an optical detection device, which generates a plurality of interference lights in which the phase difference between signal light and reference light is changed. [Means for solving the problem]

[0005] The optical multiplexer used in the optical detection device disclosed in this specification may include a first input port into which signal light is input, a second input port into which reference light having the same wavelength as the signal light is input, an optical multiplexing coupler that multiplexes the signal light and the reference light to generate multiple interference lights, and multiple output ports that output the multiple interference lights. Each of the multiple output ports may have an optical path difference within the core of the optical multiplexing coupler that is the difference between the shortest distance between the first input port and the shortest distance between the second input port and the output port. The optical path difference may be configured to be different among the multiple output ports.

[0006] The optical multiplexer described above includes an optical multiplexing coupler with an adjusted optical path difference, and therefore, by multiplexing the signal light and the reference light, it is possible to generate a plurality of interference lights with different phase differences between the signal light and the reference light. [Brief explanation of the drawings]

[0007] [Figure 1] FIG. 1 is a diagram schematically illustrating a configuration of a photodetector device. [Figure 2] FIG. 10 is a diagram showing sine curves fitted to a plurality of interference signals measured in a photodetector device. [Figure 3] FIG. 10 is a diagram schematically illustrating an example of a modified optical multiplexer. [Figure 4] FIG. 10 is a diagram schematically illustrating an example of a modified optical multiplexer. DETAILED DESCRIPTION OF THE INVENTION

[0008] The optical detection device disclosed in this specification will be described below with reference to the drawings. Note that in some figures, for the purpose of clarifying the illustration, only some of the components that are repeatedly arranged are labeled with reference numerals. The optical detection device disclosed in this specification is not particularly limited, but may be configured, for example, to be mounted on an optical phased array and to detect propagating light propagating through each of a plurality of optical waveguides that constitute the optical phased array.

[0009] As shown in FIG. 1, the optical detection device 1 includes an optical multiplexer 10, a photodetector 20, and an estimation unit 30.

[0010] The optical multiplexer 10 has a first input port 12, a second input port 14, an optical multiplexing coupler 16, and multiple output ports 18. These components that make up the optical multiplexer 10 are composed of a cladding layer (e.g., a silicon oxide layer) stacked on a semiconductor substrate, and a core layer (e.g., a silicon layer) that is covered by the cladding layer and has a refractive index higher than that of the cladding layer. FIG. 1 shows the shape of the core layer in a direction perpendicular to the main surface of the semiconductor substrate, i.e., when the semiconductor substrate is viewed in a plan view. Hereinafter, when describing the shape of the optical multiplexer 10, the shape when the semiconductor substrate is viewed in a plan view will be described.

[0011] The first input port 12 is configured with a single-mode optical waveguide and is an input port into which the signal light S, the phase of which is to be measured, is input. In this example, the initial phase of the signal light S is set to φ s For example, when the optical multiplexer 10 is connected to the rear stage of an optical phased array, the first input port 12 may receive propagating light that has propagated through an optical waveguide to be measured among the multiple optical waveguides that constitute the optical phased array.

[0012] The second input port 14 is configured with a single-mode optical waveguide, and is an input port into which the reference light R having the same wavelength as the signal light is input. In this example, the initial phase of the reference light R is set to φ r For example, when the optical multiplexer 10 is connected to the rear stage of the optical phased array, branched light obtained by branching input light to be input to the optical phased array before input to the optical phased array may be input to the second input port 14 as the reference light R, or light obtained by multiplexing propagated light that has propagated through one or more of the optical waveguides that constitute the optical phased array other than the optical waveguide to be measured may be input as the reference light R.

[0013] Each of the plurality of output ports 18 is formed of a single-mode optical waveguide, and as will be described later, is an output port from which a corresponding interference light among the plurality of interference lights generated by the optical multiplexing coupler 16 is output. In this example, six output ports 18 are illustrated. The plurality of output ports 18 is formed of N ports (N is an even number), and preferably six or more ports.

[0014] The optical multiplexing coupler 16 is composed of a multimode optical waveguide, and multiplexes the signal light S and the reference light R to generate a plurality of interference lights having different initial phases, and outputs each of the generated interference lights to a corresponding output port 18. The optical multiplexing coupler 16 is composed of a flat core layer, and has an input end face 15 to which the first input port 12 and the second input port 14 are connected, and an output end face 17 to which the plurality of output ports 18 are connected.

[0015] The input end face 15 of the optical multiplexing coupler 16 is a circular arc with a radius A. The output end face 17 of the optical multiplexing coupler 16 is a circular arc with a radius B that passes through the center point of the arc that constitutes the input end face 15 and is smaller than radius A. The bisector of the input end face 15 and the bisector of the output end face 17 are coincident, and this bisector is shown in the figure as center line 16C. The first input port 12 and the second input port 14 are connected to the input end face 15 in a symmetrical positional relationship with respect to the center line 16C of the optical multiplexing coupler 16. The multiple output ports 18 are connected to the output end face 17 in a symmetrical positional relationship with respect to the center line 16C of the optical multiplexing coupler 16.

[0016] The optical axis of the first input port 12 is perpendicular to the input end face 15. The position of the optical axis of the first input port 12 on the input end face 15 is designated 12a. The optical axis of the second input port 14 is perpendicular to the input end face 15. The position of the optical axis of the second input port 14 on the input end face 15 is designated 14a. The optical axis of each of the multiple output ports 18 is perpendicular to the output end face 17. The position of the optical axis of any output port 18 (the first output port is shown as an example in Figure 1) on the output end face 17 is designated 18a. The shortest distance between positions 12a and 14a is designated d, the shortest distance between position 18a and center line 16C is designated x, and the distance between positions 12a and 18a (and between positions 14a and 18a) measured in a direction parallel to center line 16C is designated A'. The angle between the optical axis of the second input port 14 (and the optical axis of the first input port 12) and the center line 16C is α, and the angle between the optical axis of any output port 18 and the center line 16C is β. The distances d, x, and A' are expressed by the following equations.

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[0017] Within the core of the optical multiplexing coupler 16, the shortest distance between the position 12a and the position 18a, that is, the optical path length L1 from the first input port 12 to any output port 18, can be calculated as follows.

number

[0018] Within the core of the optical multiplexing coupler 16, the shortest distance between the position 14a and the position 18a, that is, the optical path length L2 from the second input port 14 to any output port 18, can be calculated as follows.

number

[0019] Therefore, the optical path difference, which is the difference between the optical path length L1 and the optical path length L2, is expressed by the following formula.

[0020]

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[0021] If the wavelengths of the signal light S and the reference light R are λ, the phase difference Δθ between the signal light S and the reference light R input to any output port 18, which occurs due to the optical path difference, is expressed by the following formula.

[0022]

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[0023] In this way, the optical multiplexer 10 can adjust the phase difference Δθ between the signal light S and the reference light R input to any output port 18 by adjusting the effective refractive index n of the optical multiplexing coupler 16 and various design dimensions A', d, and x. The optical multiplexer 10 has a structure that allows easy adjustment of various design dimensions, yet can multiplex the signal light S and the reference light R to generate multiple interference lights having different initial phases. The phase difference Δθ between the signal light S and the reference light R at each of the six output ports 18, which is caused by the optical path difference, may be designed as follows. [Table 1]

[0024] The photodetector 20 has a plurality of balanced photodetectors (BPDs) 22. Each of the plurality of balanced photodetectors 22 is connected to a corresponding two of the plurality of output ports 18, and the interference light output from each of the two output ports 18 is differentially amplified and photoelectrically converted to generate a plurality of interference signals ΔP outThe multiple interference signals are input to the estimation unit 30. The phase difference of the interference light output from each of the two output ports 18 connected to any balanced photodetector 22 is π. In the example shown in Table 1, balanced photodetectors 22 are connected to the output ports 18 numbered 1 and 4, balanced photodetectors 22 are connected to the output ports 18 numbered 2 and 5, and balanced photodetectors 22 are connected to the output ports 18 numbered 3 and 6. In this way, by detecting the interference light with a phase difference of π using the balanced photodetector 22, an interference signal ΔP from which common mode noise (DC component) has been removed is obtained. out is obtained.

[0025] The estimation unit 30 has an input / output port 32, a CPU 34, a ROM 36, and a RAM 38. The input / output port 32 is connected to the photodetector 20 and receives a plurality of interference signals from the photodetector 20. The CPU 34 is connected to the input / output port 32, the ROM 36, and the RAM 38 and executes processing for estimating the phase state of the signal light S based on the plurality of interference signals. The ROM 36 stores various programs for estimating the phase state. The RAM 38 temporarily stores data required when the CPU 34 executes the various programs.

[0026] When the number of output ports 18 of the optical multiplexer 10 is generalized to N (N is an even number), the output P of the interference light from each of the output ports 18 is out is expressed by the following formula. s is the electric field strength of the signal light S, and E r is the electric field intensity of the reference light R, and Δφ is the initial phase φ of the signal light S at the input end face 15 of the optical multiplexing coupler 16. s and the initial phase φ of the reference beam R r is the difference between the signal light S and the reference light R, and Δθ is the phase difference between the signal light S and the reference light R caused by the optical path difference inside the core of the optical multiplexing coupler 16.

number

[0027] The output P of the interference light shown in the first half of Equation 8 out1 ~P out(N / 2) is the output of the interference light output from one of the two output ports 18 connected to each of the plurality of balanced photodetectors 22, and the output P of the interference light shown in the latter half of Equation 8 is out(N / 2+1) ~P out(N) is the output of the interference light output from the other output port 18 of the combination of two output ports 18 connected to each of the plurality of balanced photodetectors 22. For example, the output P out1 and output P out(N / 2+1) corresponds to the combination of two interference lights input to one balanced photodetector 22, and the output P out(N / 2) and output P out(N) corresponds to the combination of two interference lights input to one balanced photodetector 22. Therefore, the interference signal ΔP output from the photodetector 20 is out is expressed by the following formula:

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[0028] FIG. 2 shows the interference signal ΔP obtained by the photodetector 20 for each phase difference Δθ. out The estimation unit 30 calculates the phase difference Δθ and the interference signal ΔP out The data is fitted to a sine wave, and the phase difference Δφ, which is the phase difference of the signal light S relative to the reference light R, and the amplitude 4E s E r And it can be asked.

[0029] In this way, the photodetection device 1 of this embodiment can acquire data on interference light having different phase differences in a single measurement. The number of data pieces depends on the number N of output ports 18 of the optical multiplexer 10. When the number N of output ports 18 of the optical multiplexer 10 is six or more (i.e., three or more combinations of two output ports 18), the phase differences of the interference signals can be comprehensively formed within the range of ±π. As a result, the photodetection device 1 of this embodiment can estimate the phase state (i.e., phase and amplitude) of the signal light with high accuracy and high efficiency.

[0030] In the photodetection device 1 of this embodiment, the electric field strength of the signal light S propagating to each of the multiple output ports 18 is approximately equal among the multiple output ports 18, and the electric field strength of the reference light R propagating to each of the multiple output ports 18 is also equal among the multiple output ports 18. This makes it possible to ignore differences in the electric field strength of the input light S and the reference light R among the multiple output ports 18, so that the phase state (i.e., phase and amplitude) of the signal light S can be estimated with higher accuracy.

[0031] The first input port 12 and the second input port 14 may have a tapered shape in which the width changes monotonically toward the input end face 15 of the optical multiplexing coupler 16, i.e., along the light propagation direction. For example, the first input port 12 and the second input port 14 may have an inverse tapered shape in which the width decreases monotonically toward the input end face 15 of the optical multiplexing coupler 16. In this case, the numerical aperture of each of the first input port 12 and the second input port 14 increases, and the divergence angle of light inside the core of the optical multiplexing coupler 16 increases. This allows the optical multiplexer 10 to be miniaturized.

[0032] 3 shows a modified optical multiplexer 100. In this optical multiplexer 100, the radius of the arc of the input end face 15 of the optical multiplexing coupler 16 is the same as the radius of the arc of the output end face 17. In this example, the amount of light coupled to the multiple output ports 18 is more uniform.

[0033] 4 shows a modified optical multiplexer 102. In this optical multiplexer 102, both the input end face 15 and the output end face 17 of the optical multiplexing coupler 16 are flat. The input end face 15 and the output end face 17 of the optical multiplexing coupler 16 disclosed in this specification are not limited to being arc-shaped, and other shapes can be adopted.

[0034] The photodetector 1 disclosed in this specification is applicable to various optical devices. The photodetector 1 disclosed in this specification is particularly useful when applied to a large-scale system such as a LiDAR system, although it is not particularly limited thereto.

[0035] The features of the technology disclosed in this specification are summarized below. Note that the technical elements described below are independent technical elements that exhibit technical usefulness either alone or in various combinations.

[0036] (First feature) An optical multiplexer, a first input port to which a signal light is input; a second input port to which a reference light having the same wavelength as the signal light is input; an optical multiplexing coupler that multiplexes the signal light and the reference light to generate a plurality of interference lights; a plurality of output ports for outputting the plurality of interference lights, each of the plurality of output ports has an optical path difference within the optical multiplexing coupler that is the difference between the shortest distance between the first input port and the output port and the shortest distance between the second input port and the output port; An optical multiplexer configured such that the optical path difference of each of the plurality of output ports differs among the plurality of output ports.

[0037] (Second feature) the optical multiplexing coupler has an input end face to which the first input port and the second input port are connected, and an output end face to which the plurality of output ports are connected, the input end surface is a first arc having a first radius; 2. The optical multiplexer according to Feature 1, wherein the output end face is a second arc that passes through the center point of the first arc and has a second radius that is smaller than the first radius.

[0038] (Third feature) the optical multiplexing coupler has an input end face to which the first input port and the second input port are connected, and an output end face to which the plurality of output ports are connected, the input end surface is a first arc having a first radius; 2. The optical multiplexer according to claim 1, wherein the output end face is a second arc having the first radius.

[0039] (Fourth feature) The number of the plurality of output ports is N (N is an even number equal to or greater than 6), 4. The optical multiplexer according to any one of Features 1 to 3, wherein the phase difference between the two interference lights output from the two output ports included in each of the N / 2 combinations of the plurality of output ports is π.

[0040] Although specific examples of the present invention have been described in detail above, these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and variations of the specific examples exemplified above. Furthermore, the technical elements described in this specification or drawings exhibit technical utility alone or in various combinations, and are not limited to the combinations described in the claims at the time of filing. Furthermore, the technology exemplified in this specification or drawings can achieve multiple objectives simultaneously, and achieving one of these objectives itself has technical utility. [Explanation of symbols]

[0041] 1: detection device, 10: optical multiplexer, 12: first input port, 14: second input port, 15: input end face, 16: optical multiplexing coupler, 17: output end face, 18: output port, 20: photodetector, 22: balanced photodetector, 30: estimation unit

Claims

1. An optical multiplexer used in an optical detection device, a first input port to which the signal light is input; a second input port to which a reference light having the same wavelength as the signal light is input; an optical multiplexing coupler that multiplexes the signal light and the reference light to generate a plurality of interference lights; a plurality of output ports for outputting the plurality of interference lights, each of the plurality of output ports has an optical path difference within a core of the optical multiplexing coupler that is a difference between a shortest distance between the output port and the first input port and a shortest distance between the output port and the second input port; An optical multiplexer configured such that the optical path differences differ among the plurality of output ports.

2. the optical multiplexing coupler has an input end face to which the first input port and the second input port are connected, and an output end face to which the plurality of output ports are connected, the input end surface is a first arc having a first radius; 2. The optical multiplexer according to claim 1, wherein the output end face is a second arc that passes through the center point of the first arc and has a second radius that is smaller than the first radius.

3. the optical multiplexing coupler has an input end face to which the first input port and the second input port are connected, and an output end face to which the plurality of output ports are connected, the input end surface is a first arc having a first radius; 2. The optical multiplexer according to claim 1, wherein said output end face is a second arc having said first radius.

4. the number of output ports is N (N is an even number); 4. The optical multiplexer according to claim 1, wherein the phase difference between two interference lights output from two output ports included in each of the N / 2 combinations of the plurality of output ports is π.

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