Grating period consistency measuring method based on asymmetric interference

Abstract

The invention provides a grating period consistency measuring method based on asymmetric interference, which comprises the following steps of: measuring the mass center offset of a light spot based on a shearing differential asymmetric interference measuring system; setting a grating parameter range, initializing a particle swarm position and a particle speed, each particle representing a group of candidate grating parameters; by taking the light spot centroid offset as input and the grating period as output, calculating optical response to obtain a plurality of particles corresponding to different grating periods; the fitness of each particle is calculated, the historical optimum of each particle and the global optimum of all particles in the group are updated, and the speed and the position of the particles are iteratively updated; and when a preset maximum number of iterations is reached, outputting a global optimal grating period consistency parameter. According to the invention, the shearing differential asymmetric interference is applied to the grating period consistency measurement, the asymmetric interference is combined with the grating period inversion algorithm, and the method has the advantages of high precision, no damage, rapidness and low cost.

Description

Technical Field

[0001] The present invention relates to the technical field of high-precision optical measurement, and particularly to a method for measuring the grating period consistency based on asymmetric interference. Background Technique

[0002] With the continuous progress of grating manufacturing, new high-precision gratings are increasingly widely used. Fiber Bragg gratings are widely used in fields with extremely demanding measurement requirements such as aerospace, ships, and medical applications. The fiber Bragg grating sensor mainly because the central wavelength of its reflected wave is related to the period of the grating. When the external temperature or stress changes, it will cause the period of the grating to change, affecting the central wavelength of the grating reflected wave. By demodulation, external measured information such as temperature and strain can be obtained. The grating period consistency is at the nanometer level and is highly correlated with the measured information. Therefore, high-precision measurement methods are required for measuring the grating period.

[0003] Among the currently commonly used grating period measurement devices, the atomic force microscope is a probe-based measurement method, and its measurement accuracy can reach the nanometer level, but the requirements for probe preparation are high and the equipment cost is large. Scanning electron microscopes and transmission electron microscopes are a type of measurement method based on transmitted electron beams, and their measurement accuracy can also reach the nanometer level. Samples often need to be damaged to make thinner slices; at the same time, both of these devices are based on a point-by-point scanning measurement method during measurement, and the measurement process is slow.

[0004] Therefore, the current measurement methods using devices such as atomic force microscopes, scanning electron microscopes, and transmission electron microscopes cannot take into account the fast, low-cost, non-destructive high-precision measurement of grating structure parameters. Therefore, to meet the requirements of various high-precision optical applications, there is an urgent need for a high-precision grating period consistency detection means that can simultaneously meet the requirements of high speed, low cost, and no need to damage the sample. Summary of the Invention

[0005] Aiming at the deficiencies of the existing technology, the present invention proposes a method for measuring the grating period consistency based on asymmetric interference to solve the technical problem that there is a lack of a high-precision grating period consistency detection means in the existing technology that can simultaneously meet the requirements of high speed, low cost, and no need to damage the sample.

[0006] The technical solution adopted by the present invention is as follows:

[0007] In the first aspect, a method for measuring the grating period consistency based on asymmetric interference is provided, including:

[0008] Measuring the centroid offset of the light spot based on a shear differential asymmetric interference measurement system;

[0009] Set the range of grating parameters, initialize the positions and velocities of the particle swarm, and each particle represents a set of candidate grating parameters;

[0010] Taking the centroid offset of the light spot as the input and the grating period as the output, calculate the optical response to obtain multiple particles corresponding to different grating periods;

[0011] Calculate the fitness for each particle, update the personal historical best of each particle and the global best of all particles in the population, and iteratively update the velocity and position of the particles;

[0012] When the preset maximum number of iterations is reached, output the global optimal grating period consistency parameter.

[0013] Further, the grating parameters include period, duty cycle, and depth.

[0014] Further, when taking the centroid offset of the light spot as the input and the grating period as the output to calculate the optical response, the relationship between the centroid offset of the light spot and the grating period is as follows:

[0015]

[0016] In the above formula, E x (g,D) represents the centroid offset of the light spot, g is the beam translation distance, z is the beam waist distance, K is the linear coefficient between the grating period and the diffraction phase, D is the grating period, k is the wave vector, w 0 is the beam waist radius, and w(z) is the light spot radius at z.

[0017] Further, when calculating the fitness for each particle, the fitness function is where is the measured value, is the theoretical value.

[0018] Further, when updating the velocity and position of the particles, the velocity update formula is as follows:

[0019] v t+1 = w·v t + c 1 r 1 (pBest - x t ) + c 2 r 2 (gBest - x t )

[0020] In the above formula, v represents the velocity, w is the inertia weight, c1, c2 are learning factors, r1, r2 are random numbers, pBest is the personal historical best of each particle, gBest is the global best of the population, and x t represents the position of the particle;

[0021] The position update formula is as follows:

[0022] x t+1 = x t + v t+1

[0023] In the above formula, x t represents the position of the particle, and v represents the velocity.

[0024] Furthermore, when the fitness is lower than the set threshold and reaches the preset maximum number of iterations; the threshold is the grating period consistency error < 1×10 -4 nm.

[0025] In a second aspect, a shear differential asymmetric interference measurement system is provided for the grating period consistency measurement method based on asymmetric interference described in the first aspect, including a laser and a beam splitter;

[0026] The laser emits randomly polarized light, and the randomly polarized light is divided into first laser light and second laser light after entering the beam splitter; on the optical path of the first laser light, a laser, a collimating coupler, a first birefringent crystal, a first polarizer, a beam splitter, and a second birefringent crystal are arranged in sequence; on the optical path of the first laser light, a compensating plate, a quarter-wave plate, a half-wave plate, a second polarizer, and a camera lens are arranged in sequence; the included angle between the optical path of the first laser light and the optical path of the second laser light is 90 degrees.

[0027] Furthermore, the laser is a helium-neon laser, the beam splitter is a 50 / 50 beam splitter, the first birefringent crystal and the second birefringent crystal are both calcite, and the first polarizer and the second polarizer are both Glan-Taylor polarizing prisms.

[0028] Furthermore, the shear differential asymmetric interference measurement system is placed in a sealed enclosure, and the measurement system reaches a thermal equilibrium state in the sealed enclosure through a water-cooling method.

[0029] Furthermore, the first polarizer, the second polarizer, the quarter-wave plate, and the half-wave plate for adjusting the system compensation phase are remotely driven by a motor.

[0030] From the above technical solutions, the beneficial technical effects of the present invention are as follows:

[0031] 1. By adopting the measurement method of this embodiment, shear differential asymmetric interference is applied to the measurement of grating period consistency, and the combination of asymmetric interference and the grating period inversion algorithm has the measurement advantages of high precision, non-destructive, fast, and low cost.

[0032] 2. It has broken through the current measurement accuracy of traditional interference gratings in terms of accuracy, and the measurement level of the consistency of the grating period reaches below the nanometer level. It has high application prospects in the fields of semiconductor manufacturing, optical communication devices, lasers and spectrometers, and quality control of micro-nano processing. Brief Description of the Drawings

[0033] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following will briefly introduce the drawings required for use in the description of the specific embodiments or the prior art. In all the drawings, similar elements or parts are generally identified by similar reference numerals. In the drawings, the elements or parts do not necessarily draw according to the actual scale.

[0034] Figure 1 Schematic diagram of the shear differential asymmetric interference measurement system according to the embodiment of the present invention;

[0035] Figure 2 Schematic diagram of the conversion process of physical quantities in the grating measurement according to the embodiment of the present invention;

[0036] Figure 3 Graph of the relationship between the pointer offset and the period according to the embodiment of the present invention;

[0037] Figure 4 Schematic diagram of the main process of the grating period consistency measurement method of asymmetric interference according to the embodiment of the present invention. Detailed Embodiments

[0038] The following will describe in detail the embodiments of the technical solutions of the present invention in conjunction with the drawings. The following embodiments are only used to more clearly illustrate the technical solutions of the present invention, so they are only examples and cannot be used to limit the protection scope of the present invention.

[0039] It should be noted that unless otherwise specified, the technical terms or scientific terms used in this application should be the ordinary meanings understood by those skilled in the art to which the present invention belongs.

[0040] Embodiment

[0041] This embodiment provides a grating period consistency measurement method based on asymmetric interference. The shear differential asymmetric interference measurement system used in this measurement method is as Figure 1 shown, including a laser and a beam splitter. The laser emits randomly polarized light. After the randomly polarized light enters the beam splitter, it is divided into the first laser and the second laser. On the optical path of the first laser, a laser, a collimating coupler, a first birefringent crystal, a first polarizer, a beam splitter, and a second birefringent crystal are arranged in sequence. On the optical path of the first laser, a compensator, a quarter-wave plate, a half-wave plate, a second polarizer, and a camera lens are arranged in sequence; the included angle between the optical path of the first laser and the optical path of the second laser is 90 degrees.

[0042] The working process of the above-mentioned shear differential asymmetric interferometric measurement system is as follows:

[0043] Random polarized light is emitted from a laser, and the laser light is introduced into the measurement system through a single-mode fiber and collimated into a Gaussian beam by a collimating coupler. In a specific embodiment, the wavelength of the random polarized light is 600 - 800 nm, and the laser is a helium-neon laser JDSU, 1145, 22.5 mW.

[0044] The first birefringent crystal splits the two polarization components of the Gaussian beam into two parallel beams separated by 2 mm in the x-z plane; in a specific embodiment, the first birefringent crystal is calcite.

[0045] The first polarizer is used as a pre-selection to prepare the beam into a pre-selected state; in a specific embodiment, the first polarizer is a Glan-Taylor polarizing prism.

[0046] The 50 / 50 beam splitter at a 45° angle to the beam does not affect the propagation direction of the beam, and the beams of different polarization components are affected equally in terms of position and phase.

[0047] The second birefringent crystal splits the two polarization components of the beam into two parallel beams separated by 0.5 mm in the x-z plane; in a specific embodiment, the second birefringent crystal is calcite.

[0048] The grating to be measured is placed behind the second birefringent crystal in the first laser light path, and the position of the grating to be measured is set to rotate by an angle θ around the z-axis, so that the reflected light forms an angle of 2θ with the original beam in the x-y plane. The two reflected beams are combined again by the second birefringent crystal and then enter the beam splitter.

[0049] The beam is split after reflection and enters the second laser light path. It first passes through a phase compensation device composed of a compensator, a quarter-wave plate, and a half-wave plate, and then passes through a second polarizer at 45° to achieve post-selection; in a specific embodiment, the second polarizer is a Glan-Taylor polarizing prism.

[0050] Finally, a CCD lens of a camera is used to detect the intensity distribution shift of the light spot, which is the centroid shift of the light spot.

[0051] To avoid the influence of instrument heating, air flow, and vibration, the entire measurement system is placed in a sealed enclosure during the experiment, and instrument devices such as light sources, computers, and motor controllers that are prone to vibration are placed far away from the measurement system. In addition, to avoid interference from the human body temperature to the measurement system, the first polarizer, the second polarizer, the quarter-wave plate, and the half-wave plate for adjusting the compensation phase of the system are driven by a motor, and their angular resolution is 0.01 degrees, enabling remote control of the rotation of the wave plates.

[0052] After the installation is completed and the measurement system is preliminarily debugged to the working range, since the detector of the measurement system will heat up and is located near the measurement optical path, and considering that the measurement system does not have an environmental airtight or vacuum environment in the experiment, the measurement system reaches a thermal equilibrium state in the sealed cover through a water-cooling method to stabilize the air flow.

[0053] The measurement grating is installed on a six-axis nano-displacement stage (PI, H-824); in some embodiments, the translational accuracy of the displacement stage is 100 nm, and the angular rotation resolution is better than 1.74 μrad (10 - 4 μrad); the attitude of the displacement stage does not need to be adjusted significantly after being adjusted to the initial state.

[0054] Combined with the above measurement system, the grating period consistency measurement method based on asymmetric interference in this embodiment has a measurement principle that uses a lateral shear optical path to convert grating parameters into optical path difference information, and then uses the weak value amplification effect caused by asymmetric interference to achieve high-precision measurement of the optical path difference, thereby achieving high-precision measurement of the grating period consistency. The physical conversion process of the measurement process is as Figure 2 shown. Under specific optical path parameters, the relationship between the measured pointer offset and the period is as Figure 3 shown. When the angle between the two beams of light is very small, the weak value amplification effect can be used to achieve higher-sensitivity detection.

[0055] According to the above measurement principle, the grating period consistency measurement method based on asymmetric interference in this embodiment includes the following steps:

[0056] S1. Based on the shear differential asymmetric interference measurement system, measure the centroid offset of the light spot

[0057] The centroid offset of the light spot corresponds to the pointer offset described in the above measurement principle. The method for obtaining the centroid offset of the light spot can be found in the measurement process described above. In a specific implementation, for example, when measuring, the incident light is at the Littrow angle (i.e., the incident angle is equal to the reflection angle), and the wavelength is 632.8 nm.

[0058] S2. Set the grating parameter range, initialize the particle swarm position and particle velocity, and each particle represents a set of candidate grating parameters

[0059] The grating parameters include period, duty cycle, and depth. For example, set the grating period between 1000.01 and 1000.02 nm, the duty cycle to 0.5, and the depth to 800 nm;

[0060] Initialize the particle swarm position, randomly generate particle positions, and each particle represents a set of candidate grating parameters; initialize the particle velocity, randomly generate particle velocities, and the velocity determines the direction and distance of the particle's next movement.

[0061] In this embodiment, since it is mainly for the measurement of grating period consistency, the particle swarm algorithm only needs to perform single-parameter iteration; in this embodiment, only the grating period D is iterated, so the position of the particle is a one-dimensional variable, directly representing the candidate grating period; in a specific implementation, the search range is between 1000.01 and 1000.02 nm and is continuously adjusted during iteration.

[0062] S3. Taking the spot centroid offset as the input and the grating period as the output, calculate the optical response to obtain multiple particles corresponding to different grating periods

[0063] Combining the weak measurement theory and the rigorous coupled-wave theory, at a specific grating period, the relationship between the spot centroid offset and the grating period is as follows:

[0064]

[0065] In the above formula, E x (g, D) represents the spot centroid offset, g is the beam translation distance, z is the beam waist distance, K is the linear coefficient between the grating period and the diffraction phase, and its value range is generally between 30 and 50, D is the grating period, k is the wave vector, w 0 is the beam waist radius, and w(z) is the spot radius at z.

[0066] According to the above relationship, taking the spot centroid offset as the input and the grating period as the output, calculate the optical response to obtain the grating period; each particle corresponds to 1 grating period. In this embodiment, the value of the grating period corresponding to the spot centroid position, so the measured spot centroid offset corresponds to the grating period consistency.

[0067] S4. Calculate the fitness for each particle

[0068] For the spot centroid offset obtained by experimental measurement, the theoretical value can also be calculated through a numerical model. The theory is the target data of the experimental measurement result and is used to construct the fitness function. In this embodiment, the numerical model for calculating the theoretical value is not limited and is implemented in any achievable manner of the prior art. For example: using the RCWA numerical model and centroid calibration to calculate the theoretical value. Centroid calibration refers to the one-to-one correspondence between the centroid and the phase under the preset grating parameters.

[0069] Because it is an iteration for a single parameter of the grating period, the fitness function is where is the measured value, is the theoretical value; the smaller the fitness, the closer the parameter of the particle is to the true value.

[0070] S5. Update the personal historical best of each particle and the global best of all particles in the population

[0071] Find the position with the best fitness among the historical positions of each particle as the personal historical best (pBest) of each particle. Find the position with the best fitness among the historical positions of all particles in the population as the global best (gBest). The update rule is as follows:

[0072] If the fitness of the current particle is better than its own personal historical best, then update pBest;

[0073] If the fitness of the current particle is better than the global best of the population, then update gBest.

[0074] S6. Iteratively update the velocity and position of the particles

[0075] The velocity update formula is as follows:

[0076] v t+1 = w·v t + c 1 r 1 (pBest - x t ) + c 2 r 2 (gBest - x t )

[0077] In the above formula, v represents the velocity, w is the inertia weight, c1 and c2 are learning factors, r1 and r2 are random numbers, pBest is the personal historical best of each particle, gBest is the global best of the population, and x t represents the position of the particle.

[0078] The position update formula:

[0079] x t+1 = x t + v t+1

[0080] In the above formula, x t represents the position of the particle, and v represents the velocity.

[0081] S7. Reach the preset maximum number of iterations and output the global optimal grating period consistency parameter

[0082] When reaching the preset maximum number of iterations and the fitness is lower than the set threshold, in a specific implementation, the set threshold is that the grating period consistency error < 1×10 -4 nm and the fitness change tends to be stable; output the global optimal grating period consistency parameter.

[0083] By adopting the measurement method of this embodiment, the shear differential asymmetric interference is applied to the measurement of grating period consistency. The combination of asymmetric interference and the grating period inversion algorithm has the advantages of high precision, non-destruction, rapidity and low cost. In terms of precision, it breaks through the measurement precision of the current traditional interference grating, and the measurement level of grating period consistency reaches below the nanometer level. It has high application prospects in the fields of semiconductor manufacturing, optical communication devices, lasers and spectrometers, and micro-nano processing quality control, etc.

[0084] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features. And these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention, and they should all be covered within the scope of the claims and the description of the present invention.

Claims

1. A method for measuring grating period consistency based on asymmetric interference, characterized in that: include: Based on the shear differential asymmetric interferometry system, the centroid offset of the light spot is measured; Set the grating parameter range, initialize the particle group position and particle velocity, and each particle represents a set of candidate grating parameters; Taking the center-of-mass offset of the light spot as input and the grating period as output, the optical response is calculated to obtain multiple particles corresponding to different grating periods; Calculate the fitness of each particle, update the historical optimum of each particle and the global optimum of all particles in the group, and iteratively update the speed and position of the particle; When the preset maximum number of iterations is reached, the global optimal grating period consistency parameters are output.

2. The method for measuring grating period consistency based on asymmetric interference according to claim 1, characterized in that: The grating parameters include period, duty cycle and depth.

3. The method for measuring grating period consistency based on asymmetric interference according to claim 1, characterized in that: When the optical response is calculated with the center offset of the light spot as input and the grating period as output, the relationship between the center offset of the light spot and the grating period is as follows: In the above formula, E x (g,D) represents the centroid offset of the light spot, g is the beam translation distance, z is the beam waist distance, K is the linear coefficient between the grating period and the diffraction phase, D is the grating period, k is the wave vector, w0 is the beam waist radius, and w(z) is the spot radius at z.

4. The method for measuring grating period consistency based on asymmetric interference according to claim 1, characterized in that: When calculating the fitness of each particle, the fitness function is in is the measured value, is the theoretical value.

5. The method for measuring grating period consistency based on asymmetric interference according to claim 1, characterized in that: When updating the velocity and position of a particle, the velocity update formula is as follows: v t+1 =w·v t +c1r1(pBest-x t )+c2r2(gBest-x t ) In the above formula, v represents speed, w is inertia weight, c1, c2 are learning factors, r1, r2 are random numbers, pBest is the best historical result of each particle, gBest is the global best result of the group, and x t represents the position of the particle; The position update formula is as follows: x t+1 =x t +v t+1 In the above formula, x t represents the position of the particle and v represents its velocity.

6. The method for measuring grating period consistency based on asymmetric interference according to claim 1, characterized in that: When the fitness is lower than the set threshold, the preset maximum number of iterations is reached; the threshold is the grating period consistency error <1×10 - 4 nm.

7. A shear differential asymmetric interferometry system, characterized in that: A method for measuring grating period consistency based on asymmetric interference as described in any one of claims 1 to 6, comprising a laser and a spectroscope; The laser emits randomly polarized light, which is divided into a first laser and a second laser after entering a beam splitter; a laser, a collimating coupler, a first birefringent crystal, a first polarizer, a beam splitter, and a second birefringent crystal are arranged in sequence on the optical path of the first laser; a compensation plate, a 1 / 4 wave plate, a 1 / 2 wave plate, a second polarizer, and a camera lens are arranged in sequence on the optical path of the first laser; an angle between the optical path of the first laser and the optical path of the second laser is 90 degrees.

8. The shear differential asymmetric interferometry system according to claim 7, characterized in that: The laser is a helium-neon laser, the beam splitter is a 50 / 50 beam splitter, the first birefringent crystal and the second birefringent crystal are both calcite, and the first polarizer and the second polarizer are both Glan-Taylor polarizing prisms.

9. The shear differential asymmetric interferometry system according to claim 7, characterized in that: The shear differential asymmetric interference measurement system is placed in a sealed cover, and water cooling is used to allow the measurement system to reach a thermal equilibrium state in the sealed cover.

10. The shear differential asymmetric interferometry system according to claim 7, characterized in that: The first polarizer, the second polarizer, the 1 / 4 wave plate and the 1 / 2 wave plate used for adjusting the system compensation phase are remotely driven by a motor.

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