Optical devices

Abstract

To provide an optical device which can reduce the variation in speckle by a simple structure.SOLUTION: The optical device comprises a light source 10, a separation optical system 11, an irradiation optical system 12, a light reception optical system 13, a multiplex optical system 14, a multi-mode waveguide 15, a mode separation optical system 16, light detectors 17, and an adder 18. The multi-mode waveguide 15 allows multiplexed light from the multiplex optical system 14 to propagate in a plurality of modes. The mode separation optical system 16 outputs light in the plurality of modes propagating through the multi-mode waveguide 15 separately. The light detector 17 receives light in each of the modes having been separated by the mode separation optical system 16, and outputs each of these as a received light signal (electrical signal). The adder 18 adds up the received light signals from the light detectors 17 and outputs the resulting signal.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an optical device that receives light reflected from an object and converts it into a received signal. [Background technology]

[0002] When a highly coherent light such as a laser beam is irradiated onto an object and the reflected light is received, a phenomenon known as speckle occurs in which the intensity of the received signal varies randomly.Speckle occurs when the object has uneven surfaces and the reflected light interferes with each other due to these uneven surfaces.

[0003] When measuring vibration information of an object by irradiating the object with light and measuring the reflected light, variations in the intensity of the received light signal due to speckles (hereinafter referred to as speckle variations) can make the measurement unstable. Therefore, there is a demand for technology to reduce speckle variations.

[0004] Patent Document 1 describes a method for reducing speckle variations by receiving reflected light using multiple light-receiving optical systems and corresponding multiple light-receiving elements, and adding up the light-receiving signals from each light-receiving element. This method takes advantage of the fact that the speckle variations in the light-receiving signals obtained from each light-receiving element are independent, and the probability of each light-receiving signal decreasing simultaneously is low. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 03 / 089955 Summary of the Invention [Problem to be solved by the invention]

[0006] However, the method of Patent Document 1 requires multiple light-receiving optical systems, which makes the optical systems complex and increases the size of the device. Also, when the position of the target changes, as in LiDAR, it is necessary to align the observation positions of each light-receiving optical system, which is difficult.

[0007] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an optical device that can reduce speckle variations with a simple configuration. [Means for solving the problem]

[0008] The present invention provides a waveguide that propagates reflected light from an object in a plurality of modes, a mode separation unit that separates the light propagating through the waveguide into each mode, a plurality of photodetectors that receive the light separated into each mode by the mode separation unit and output a photodetection signal, and an adder that adds up the photodetection signals from each photodetector. The waveguide and mode separation section are realized by multiple waveguides corresponding to each mode and one diffraction grating antenna. The optical device is characterized by the above. The present invention also provides an optical device comprising: a waveguide that propagates reflected light from an object in a plurality of modes; a mode separation section that separates the light propagating through the waveguide into modes; a plurality of photodetectors that receive the light separated into modes by the mode separation section and output received light signals; and an adder that adds up the received light signals from each photodetector, wherein the waveguide propagates modes with small overlap in their distributions; the plurality of photodetectors are arranged adjacent to the output end of the waveguide and are arranged corresponding to each mode at positions where overlap with other modes is small; and the mode separation section is realized by the arrangement of the plurality of photodetectors.

[0009] The present invention also provides an optical device comprising: a waveguide that propagates reflected light from an object in a plurality of modes; a mode separation unit that separates the light propagating through the waveguide into modes; a phase shifter that controls the phase difference between each mode of the light separated into modes by the mode separation unit; a photodetector that receives light from the phase shifter and outputs a received light signal; and a control device that adjusts the amount of phase shift of the phase shifter so that the output of the received light signal from the photodetector is maximized. [Effects of the Invention]

[0010] According to the present invention, speckle variations can be reduced with a simple configuration. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a diagram showing the configuration of an optical device according to a first embodiment. [Figure 2] FIG. 2 is a diagram showing a diffraction grating antenna 20. [Figure 3]FIG. 10 is a diagram showing an example of mode separation by the arrangement of a photodetector 17. [Figure 4] A diagram showing the sum of exponential distributions. [Figure 5] A diagram showing the light intensity of each mode. [Figure 6] A diagram showing a scatter plot of light intensity between each mode. [Figure 7] FIG. 10 is a diagram showing the configuration of an optical device according to a second embodiment. [Figure 8] FIG. 10 is a diagram showing a modified example of the optical device of the second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0013] (First embodiment) Fig. 1 is a diagram showing the configuration of an optical device according to the first embodiment. As shown in Fig. 1, the optical device according to the first embodiment includes a light source 10, a separation optical system 11, an irradiation optical system 12, a light-receiving optical system 13, a multiplexing optical system 14, a multi-mode waveguide 15, a mode separation optical system 16, a photodetector 17, and an adder 18.

[0014] The light source 10 is a light source that emits coherent light, such as a laser diode. The wavelength is, for example, in the infrared band. The light source may be continuous light or pulsed light.

[0015] The separation optical system 11 is an optical system that separates the light from the light source 110 into measurement light and reference light, for example, by a beam splitter.

[0016] The irradiation optical system 12 is an optical system that irradiates the object 100 with the measurement light separated by the separation optical system 11. For example, the measurement light is converted into parallel light by a lens and then irradiated onto the object 100.

[0017] The light receiving optical system 13 is an optical system that receives light reflected from the object 100. For example, the light reflected from the object 100 is collected and received by a lens.

[0018] The combining optical system 14 is an optical system that combines the reflected light from the light receiving optical system 13 and the reference light from the separation optical system 11 and outputs the combined light. For example, the reflected light and the reference light are combined by a beam splitter.

[0019] The multimode waveguide 15 is a waveguide that propagates the multiplexed light from the multiplexing optical system 14 in multiple modes. The number of modes is arbitrary as long as it is two or more. The structure of the multimode waveguide 15 is arbitrary as long as it can propagate multiple modes, and a multimode fiber or a silicon waveguide can be used. For example, in the case of a waveguide with a circular core, the LP 01 , LP 11a , LP 11b , LP 21a , LP 21b , LP 02 It is sufficient that two or more of the above modes can be propagated.

[0020] The mode separation optical system 16 is an optical system that separates and outputs each of the multiple modes of light propagating through the multimode waveguide 15. The mode separation is performed using, for example, a mode demultiplexer. It is not necessary to separate and output all of the multiple modes propagating through the multimode waveguide 15, but it is sufficient to separate and output two or more modes. It is preferable to separate three or more modes in order to efficiently reduce speckle variations. For example, 01 , LP 11a , LP 11b The above three modes are output separately.

[0021] The separated modes do not necessarily have to be orthogonal to each other, but it is preferable to separate them so that they are orthogonal to each other, as this makes it possible to reduce speckle variations more efficiently.

[0022] It is also preferable to average the mode distribution using a mode scrambler or the like to make the light output equal in each mode. This is because speckle variations can be reduced more efficiently. In this case, it is not necessary for the output to be strictly equal in each mode; for example, the ratio of the minimum output to the maximum output in each mode may be 0.4 to 1.

[0023] The photodetector 17 is a device, such as a photodiode, that receives the light of each mode separated by the mode separation optical system 16 and outputs it as a received light signal (electrical signal).

[0024] The adder 18 is a device that adds the light receiving signals from the photodetectors 17 and outputs the result.

[0025] In the first embodiment, the measurement light and reference light are multiplexed upstream of the multi-mode waveguide 15, but they may be multiplexed at any position upstream of the photodetector 17. However, to simplify the configuration of the multiplexing optical system 14, it is preferable to multiplex the measurement light and reference light before mode separation by the mode separation optical system 16. More preferably, the measurement light and reference light are multiplexed upstream of the multi-mode waveguide 15 as in the first embodiment. This is because, as described above, it is preferable to make the light output uniform in each mode, and therefore it is preferable to couple the reference light uniformly into each mode. In other words, if the measurement light and reference light are multiplexed before light is propagated in each mode by the multi-mode waveguide 15, the reference light will be coupled uniformly into each mode.

[0026] Next, the reason why speckle variations are reduced in the optical device of the first embodiment will be described.

[0027] As shown in Figure 4, when mode separation is not performed, the detected light intensity varies due to speckle and has an exponential distribution. In the graph, the horizontal axis represents the normalized detected light intensity, and the vertical axis represents frequency. Therefore, there is a high probability that the detected light intensity will be low, and there is a high probability that the value will be below the threshold and will not be detected.

[0028] In the first embodiment, the modes are separated, detected, and then added together, so the distribution of detected light intensity is the sum of multiple exponential distributions. If the exponential distributions are similar to each other (highly correlated), the sum of the distribution is also an exponential distribution, and variation due to speckle is not reduced. On the other hand, if the exponential distributions are independent of each other (uncorrelated), the sum of the distribution is a gamma distribution (see Figure 4).

[0029] Generally, if each mode is orthogonal, the variation in the detected light amount in each mode will also be independent, and each exponential distribution will be independent of each other. Therefore, in the first embodiment, the detected light amount is the sum of mutually independent exponential distributions, resulting in a gamma distribution. As a result, the variation in the detected light amount is reduced. Furthermore, the distribution peak shifts to the right (in the direction in which the light amount increases), and the proportion of light amounts exceeding the threshold increases, enabling stable light detection.

[0030] As described above, the optical device of the first embodiment can efficiently reduce speckle variations with a simple configuration and obtain a stable light receiving signal. Furthermore, since the first embodiment receives light in multimode, it can obtain a larger amount of light than the general method of receiving light in single mode.

[0031] Next, we will explain the experimental results that show that the speckle variation in the detected light amount in each mode is independent. Random pattern speckles were generated 10,000 times in a simulation, and the coupled light amount between the electric field distribution of the speckles and the electric field distribution of each mode was calculated, and the correlation coefficient was calculated for the data of each mode. The modes were LP 01 , LP 11a , LP 11b The three were:

[0032] Figure 5 is a graph showing the number of trials and the light intensity of each mode, and Figure 6 is a scatter plot of the light intensity between each mode. As shown in Figure 6, it can be seen that there is almost no correlation between each mode. The calculation result of the correlation coefficient is LP 01 and LP 11a is -0.01, LP 01 and LP11b is 0.01, LP 11a and LP 11b From the above, it was confirmed that the speckle variations in the detected light amount in each mode were independent of each other.

[0033] (Second embodiment) In the second embodiment, the configuration of the optical device of the first embodiment is modified as follows. As shown in Fig. 7, the configuration up to the mode separation optical system 16 is the same as in the first embodiment, but the configuration of the stages subsequent to the mode separation optical system 16 is different. That is, instead of the multiple photodetectors 17 and adder 18 in the optical device of the first embodiment, the second embodiment has multiple phase shifters 21, a multiplexer 22, a control device 23, and a photodetector 24.

[0034] The phase shifter 21 is a device that controls the phase difference between modes of light separated into each mode by the mode separation optical system 6. Specifically, a phase shifter is provided for each mode other than the reference mode, and the phase shift amount of each phase shifter is controlled. For example, a fiber-type phase shifter or the like is used as the phase shifter.

[0035] The multiplexer 22 is a device that multiplexes the light from the mode separation optical system 16, the phase difference between each mode of which has been controlled by the phase shifter 21.

[0036] The photodetector 24 is a device that receives the light from the multiplexer 22 and outputs it as a received light signal (electrical signal).

[0037] The control device 23 measures the intensity of the received light signal output from the photodetector 24 and controls the phase shift amount of each phase shifter 21 so that the intensity is maximized. By controlling the phase difference and combining in this way, it is possible to obtain the same result as when signals from multiple photodetectors 17 are added as in the first embodiment.

[0038] In the second embodiment, the reference light is multiplexed before being input to the multi-mode waveguide as in the first embodiment, but multiplexing may be performed anywhere prior to the photodetector 24. For example, the light of each mode and the reference light may be multiplexed by the multiplexer 22, which can further simplify the device configuration. In this case, it is preferable to control the phase difference between the light of each mode and the reference light by the phase shifter 21 (see FIG. 8).

[0039] The optical device of the second embodiment can achieve the same effects as the first embodiment. That is, it is possible to efficiently reduce speckle variations with a simple configuration and obtain a stable light receiving signal. Furthermore, since light is received in multiple modes, a greater amount of light can be obtained compared to the general method of receiving light in a single mode. Furthermore, while the first embodiment required a photodetector 17 for each mode, the second embodiment has the advantage of requiring only one photodetector 24.

[0040] (Modification of the mode separation method) A modification of the mode separation method in the first and second embodiments will be described.

[0041] (Variation 1) A diffraction grating antenna 20 may be used instead of the multimode waveguide 15 and the mode separation optical system 16 (see Figure 2). By using the diffraction grating antenna 20, it is possible to receive only a specific propagation mode and propagate it to the corresponding waveguide, thereby achieving the functions of the multimode waveguide 15 and the mode separation optical system 16 in one. The diffraction grating antenna 20 is an element in which a diffraction grating is built into a waveguide. The combined light from the combining optical system 14 is guided from outside the waveguide into the waveguide via the diffraction grating, but depending on the structure of the diffraction grating, only a specific mode can be guided directly to the waveguide. Therefore, by using multiple waveguides corresponding to each mode and one diffraction grating antenna 20, it is possible to propagate light in each mode and separate each mode at the same time. In Figure 2, the LP 01 , LP 11a , LP 11bThe example shows a single diffraction grating antenna 20 that can receive the three modes and propagate them through three waveguides corresponding to the respective modes. 01 , LP 11a , LP 11b The three modes are directly separated.

[0042] (Variation 2) Mode separation can also be achieved by arranging multiple photodetectors 17. Each mode has a different spatial distribution of light intensity in a cross section perpendicular to the central axis of the multimode waveguide 15. Therefore, mode separation can be achieved by arranging multiple photodetectors 17 as follows. Modes with small overlapping distributions are selected and propagated in the multimode waveguide 15. Photodetectors 17 corresponding to each mode are arranged adjacent to the output end of the multimode waveguide 15 at positions where overlap with other modes is small. Arrangement in this manner allows each mode to be separated and detected (see FIG. 3). Small overlap in distributions means, for example, that the area of ​​overlapping light intensity relative to the area over which the light intensity is distributed is 60% or less, preferably 50% or less.

[0043] For example, as shown in Figure 3(b), LP 11a The distribution of LP 11b The distribution is rotated by 90° around the central axis of the multi-mode waveguide 15, and there is little spatial overlap. Therefore, four photodetectors 17 are arranged in a 2 × 2 matrix near the output end of the multi-mode waveguide 15 (see FIG. 3(a)), and a pair of photodetectors 17 is used as the LP 11a Another pair of photodetectors 17 is placed at the position where the output of LP 11b By placing it at a position where the output of LP is large (see Fig. 3(c)), 11a and LP 11b can be separated and detected. [Industrial Applicability]

[0044] The present invention can be used in LiDAR and the like. [Explanation of symbols]

[0045] 10:Light source 11: Separation optical system 12: Irradiation optical system 13: Light receiving optical system 14: Combined optical system 15: Multimode waveguide 16: Mode separation optical system 17, 24: Photodetector 18: Adder 21: Phaser 22: Multiplexer 23: Control device

Claims

1. a waveguide that propagates reflected light from an object in multiple modes; a mode separation unit that separates light propagating through the waveguide into individual modes; a plurality of photodetectors that receive the light separated into modes by the mode separation unit and output received light signals; an adder that adds together the light receiving signals from the photodetectors; and An optical device characterized in that the waveguide and the mode separation section are realized by a plurality of waveguides respectively corresponding to the respective modes and one diffraction grating antenna.

2. a waveguide that propagates reflected light from an object in multiple modes; a mode separation unit that separates light propagating through the waveguide into individual modes; a plurality of photodetectors that receive the light separated into modes by the mode separation unit and output received light signals; an adder that adds together the light receiving signals from the photodetectors; and the waveguide propagates modes whose distributions have little overlap; The plurality of photodetectors are arranged adjacent to the output end of the waveguide, and are arranged corresponding to the respective modes at positions where overlap with other modes is small, An optical device, characterized in that the mode separation section is realized by an arrangement of a plurality of the photodetectors.

3. a waveguide that propagates reflected light from an object in multiple modes; a mode separation unit that separates light propagating through the waveguide into individual modes; a phase shifter that controls a phase difference between each mode of the light separated into modes by the mode separation unit; a photodetector that receives light from the phase shifter and outputs a light reception signal; a control device that adjusts the phase shift amount of the phase shifter so that the output of the light receiving signal from the photodetector is maximized; An optical device comprising:

4. 4. The optical device according to claim 3, wherein the waveguide and the mode separation section are realized by a plurality of waveguides corresponding to the respective modes and one diffraction grating antenna.

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