Optical path automatic focusing method and system for adjusting focal length

Abstract

The invention relates to an optical path automatic focusing method and system for adjusting a focal length. The method comprises the following steps: S1, taking a main light beam of a laser as a unique light source, forming a detection light beam through beam splitting, and focusing the detection light beam to a blade of a chopper; s2, periodically modulating the detection light beam; s3, irradiating the modulated detection light beam to a photoelectric detector to obtain a light intensity signal; s4, phase-displacement demodulation is carried out on the light intensity signal, and the axial displacement of the measured object is calculated; and S5, generating a feedback signal, and performing automatic focusing. The main light beam is directly split to form the detection light beam, demodulation and closed-loop feedback control are carried out through modulation of the chopper and detection of the multi-quadrant detector, the quantitative relation between the phase difference and the axial displacement is established, the abstract phase difference is converted into a physical quantity capable of being directly operated and fed back, and focusing state feedback is directly carried out based on the main light path. Error and drift interference of auxiliary devices are avoided, and high-precision, high-frequency and rapid automatic focusing is realized.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of precision optical detection, and in particular to a light path automatic focusing method and system for adjusting focal length. BACKGROUND

[0002] In laser detection applications, it is crucial to maintain the precise focusing of the measured object and the waist of the laser beam. However, traditional focusing techniques usually require the introduction of additional auxiliary light sources or independent focusing devices to assist in determining the focal point position, and the focusing is achieved by maintaining a fixed distance between these auxiliary devices and the measured object.

[0003] This method has obvious limitations: Firstly, the main light path and the auxiliary light path need to be strictly aligned, and deviations may be introduced during assembly and use. The introduction of auxiliary devices not only increases the complexity of the system, but also brings additional measurement deviations due to the assembly errors and structural vibrations of the devices themselves.

[0004] Secondly, when the system is operated for a long time, thermal drift and mechanical drift may occur, causing focusing deviations between the main light path and the auxiliary light path. The relative positional relationship between the auxiliary light source and the main detection light path may drift due to environmental temperature changes, mechanical vibrations, and other factors, resulting in a decrease in focusing accuracy.

[0005] More importantly, this indirect focusing method cannot truly reflect the actual focusing state of the main laser beam and the measured object, and it is difficult to meet the requirements in high-precision detection applications.

[0006] In addition, the multi-device cooperative working mode used in existing technologies also significantly increases the power consumption and volume of the system, which is not conducive to the miniaturization and integration of high-precision devices.

[0007] In view of the above problems, the existing technology needs to be improved. SUMMARY

[0008] In view of the above-mentioned defects of the prior art, the first aspect of the present application provides a light path automatic focusing method for adjusting focal length, which comprises the following steps: Step S1: The main beam of the laser is used as the only light source, a detection beam is formed by beam splitting, and the detection beam is focused to the chopper blade through a lens, and the position of the chopper blade blocking the beam is used as the system reference zero point; Step S2: The detection beam is periodically modulated; wherein the chopper is controlled to rotate at a linear speed The detection beam is rotated so that the beam is periodically divided into bright and dark regions; Step S3: The modulated detection beam is irradiated to a multi-quadrant photodetector for detecting the change of light intensity with time to obtain a light intensity signal; Step S4: phase-shift demodulation is performed on the light intensity signal to obtain a phase difference parameter related to the axial displacement of the measured object relative to the focal plane, and the axial displacement of the measured object is calculated according to the phase difference parameter; Step S5: a feedback signal is generated according to the preset ideal focal point position and the axial displacement, and is input to the control unit to drive the focusing structure, so as to realize automatic focusing.

[0009] In the optical path automatic focusing method for adjusting focal length as described above, optionally, in the step S2, the rotation direction of the chopper is along the transverse development direction of the light beam propagation direction, that is, the tangential direction perpendicular to the light beam propagation direction; When the chopper blade rotates clockwise, the laser at the light spot is cut into a light-dark alternating waveform: In the time domain, the continuous laser is cut into a periodic square wave signal; each complete modulation period is from the start of the light beam passing through one chopper blade, through one bright area, one dark area and two phase difference areas, and back to the starting point, at which time the modulation period ; In the spatial geometry, when the light spot falls on the opening position of the chopper blade, the light beam is allowed to pass through, corresponding to the bright area appearing on the multi-quadrant detector, at which time the light transmission arc length is ; When the light spot falls on the solid position of the chopper blade, the light beam is blocked, corresponding to the dark area appearing on the detector, at which time the light shielding arc length is .

[0010] In the optical path automatic focusing method for adjusting focal length as described above, optionally, when the multi-quadrant photodetector is a two-quadrant photodetector, the two-quadrant photodetector includes two adjacent light-sensitive areas with a spacing of M, for detecting the change of light intensity with time; The convergence angle of the light beam The axial displacement of the measured object relative to the focal plane of the measured object There is a geometric relationship as follows: Where the measured object is defined as negative defocus when it is in front of the focal point , and is defined as positive defocus when it is behind the focal point .

[0011] In the optical path automatic focusing method for adjusting focal length as described above, optionally, due to the phase difference In the step S4, the phase difference time , the bright time and the dark time are further measured, and the formulas are as follows: ; obtaining the phase difference .

[0012] In the optical path auto-focusing method for adjusting focal length as described above, optionally, in the step S5, the phase difference is converted into the axial displacement according to the relationship between the phase difference and the axial displacement , i.e. ; Setting the axial displacement corresponding to the ideal focus position as 0, the deviation between the actual axial displacement and the axial displacement corresponding to the ideal focus position is the feedback signal, and the feedback signal is outputted by a controller to adjust the amount for driving the focusing mechanism to adjust the position of the lens or the measured object, so that the focus gradually approaches the target surface; With the movement of the lens, the phase difference is recalculated according to the new optical path; if , i.e. , the refocusing is successful, otherwise, the steps S1 to S5 are re-executed.

[0013] In the optical path auto-focusing method for adjusting focal length as described above, optionally, the multi-quadrant photodetector is a four-quadrant detector, which is divided into four light-sensitive areas by a cross-shaped slit.

[0014] In the optical path auto-focusing method for adjusting focal length as described above, optionally, the light passing arc length and the light blocking arc length are reduced by reducing the circumferential spacing of the chopper blades, so as to improve the spatial resolution of the optical path system; and / or, a lens with a numerical aperture NA≥0.5 is used to increase the detection range of the optical path system; and / or, the rotation speed of the chopper is increased to improve the detection frequency.

[0015] In order to achieve the above-mentioned purpose, the second aspect of the present application provides an optical path auto-focusing system for adjusting focal length, which uses the optical path auto-focusing method for adjusting focal length as described in any one of the preceding first aspects, comprising: A light source splitting module is used to split the main beam of the laser as the only light source to form a detection beam, and focus it to the chopper blade through a lens, and the position of the chopper blade blocking the light beam is used as the system reference zero point; A modulation module includes a chopper and its driving device, which is used to control the chopper to rotate at a linear speed at the detection beam, so that the detection beam is periodically divided into bright and dark areas; The detection module comprises a multi-quadrant photodetector, configured to receive the detection light beam modulated by the chopper and output a light intensity signal varying with time; The demodulation module is configured to perform phase-shift demodulation on the light intensity signal to obtain a phase difference parameter related to the axial displacement of the measured object relative to the focal plane, and calculate the axial displacement of the measured object according to the phase difference parameter; The control module is configured to generate a feedback signal according to the preset ideal focal point position and the axial displacement, and input the feedback signal to the focusing actuator to drive the focusing actuator to perform position adjustment, thereby realizing automatic focusing.

[0016] To achieve the above-mentioned purpose, the third aspect of the present application provides a terminal device comprising a memory, a processor and a computer program stored in the memory and executable on the processor, wherein the processor implements the optical path automatic focusing method for adjusting focal length as described in any one of the preceding first aspects when running the program.

[0017] To achieve the above-mentioned purpose, the fourth aspect of the present application provides a computer readable storage medium, wherein the computer readable storage medium stores computer executable instructions or computer programs, and the computer executable instructions or computer programs are executed by the processor to implement the optical path automatic focusing method for adjusting focal length as described in any one of the preceding first aspects.

[0018] The optical path automatic focusing method and system for adjusting focal length provided by the present application form a detection light beam by splitting a main light beam, perform phase-shift demodulation and closed-loop feedback control through chopper modulation and multi-quadrant detector detection, convert the abstract phase difference parameter into a physical quantity that can be directly operated and fed back by establishing a quantitative relationship between the phase difference and the axial displacement, directly perform focusing state feedback based on the main light path, avoid the error and drift interference of auxiliary devices, and realize high-precision, high-frequency and rapid automatic focusing.

[0019] The concept, specific structure and technical effects of the present application will be further described below with reference to the accompanying drawings, so as to fully understand the purpose, features and effects of the present application. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 is a flowchart of an embodiment of the optical path automatic focusing method for adjusting focal length provided by the present application; Figure 2 is a top view schematic diagram of the relative position of the two-quadrant detector and the chopper blade and the defocus direction of the present application; Figure 3 is a schematic diagram of the experimental effect of phase-shift demodulation of the present application. DETAILED DESCRIPTION

[0021] In order to make the technical means, creative features, purposes and effects of the application easy to understand, the application is further described below in combination with specific drawings. However, the application is not limited to the following cases.

[0022] It should be understood that the structures, proportions, sizes, etc. shown in the drawings attached to the specification are only used to understand and read the content disclosed in the specification for those skilled in the art, and do not define the limiting conditions for implementing the application, so they do not have technical significance. Any modification of the structure, change of the proportion relationship or adjustment of the size, without affecting the effects and purposes that can be achieved by the application, should still fall within the scope of the technical content disclosed by the application.

[0023] The terms such as "comprising" and "including" mean that the technical solutions of the application do not exclude the presence of other components that are not directly or explicitly described in the specification and claims.

[0024] In addition, the term "and / or" in this paper only describes the association relationship between the associated objects, which means that there can be three relationships, for example, A and / or B, which can represent the existence of A alone, the existence of A and B, and the existence of B alone. In addition, the character " / " in this paper generally represents an "or" relationship between the front and rear associated objects.

[0025] In the conventional existing laser automatic focusing system, due to the need to introduce an independent auxiliary light source or a special focusing device, the complexity of the optical path structure is significantly increased. The physical separation of the auxiliary light source and the main laser beam will produce a spatial alignment error, which is caused by mechanical assembly tolerance, temperature deformation and relative displacement caused by vibration. When the main light path and the auxiliary light path have an axial deviation, the defocus amount data obtained by the focusing detection module and the actual main light beam focal plane position produce a systematic deviation, which directly affects the focusing accuracy. In addition, when the spectral characteristics of the auxiliary light source do not match the main laser, the multi-spectral interference will reduce the signal-to-noise ratio of the photoelectric detector, causing signal distortion in the phase demodulation process.

[0026] For example, in a laser micro-processing device, when a double optical path focusing system is used, the auxiliary light source is coupled to the inside of the processing head through an independent optical fiber. During the high-speed movement of the processing head, the main laser transmission optical fiber and the auxiliary light source optical fiber produce different degrees of stress deformation due to the difference in bending radius, resulting in a decrease in the spatial coincidence of the two beams at the focusing lens group. When the imaging position of the auxiliary spot on the four-quadrant detector deviates from the calibration reference, the control system misjudges the defocus amount of the workpiece surface, triggering an incorrect position compensation. In the continuous processing process, this error has a cumulative effect, eventually causing the axial position deviation of the micro-hole structure to exceed the design tolerance of ±5 microns.

[0027] If the above problems are not solved, the system reliability will be seriously threatened. The spatial alignment error will directly translate into uncontrollable fluctuations in processing precision, leading to lens curvature radius out-of-tolerance in the manufacture of precision optical elements. The lack of long-term stability of auxiliary devices will trigger periodic calibration requirements, increasing equipment maintenance costs and reducing production efficiency. The signal-to-noise ratio degradation caused by multi-spectral interference will limit the application range of the system in low reflectivity material processing, and cannot meet the focusing needs of the thin film structure on the surface of the semiconductor wafer.

[0028] As shown in Figure 1 The light path automatic focusing method for adjusting focal length provided by the application comprises the following steps: Step S1: The main beam of the laser is used as the only light source to form a detection beam through beam splitting, and is focused to the chopper blade through a lens, and the position of the chopper blade blocking the light beam is taken as the system reference zero point. This step establishes a spatial reference datum and eliminates the assembly error of auxiliary devices.

[0029] Among them, the main beam as the only light source means that the main beam emitted by the laser is directly used for beam splitting to generate a detection beam, without the need for additional auxiliary light sources. Specifically, a beam splitter prism or a half-transmission half-reflection mirror can be used to split the light beam, and a multiplexing main light path is used to avoid the optical axis deviation caused by introducing an independent focusing light source. The position of the chopper blade blocking the light beam is taken as the system reference zero point, which means that the critical position of the chopper blade blocking the light beam during rotation is taken as the spatial datum point. Specifically, the zero point can be calibrated by detecting the overlapping state of the edge of the chopper blade and the light beam through a photoelectric sensor, and an absolute position reference is established by mechanical shielding to eliminate the assembly error.

[0030] Step S2: Periodically modulate the detection beam.

[0031] In this step, the chopper is controlled to rotate at a linear speed at the detection beam, so that the light beam is periodically divided into bright and dark regions, forming a time-domain coded signal to provide a time reference for subsequent phase detection.

[0032] Among them, periodically modulating the detection beam means that the continuous light beam is cut into a time-domain signal with alternating bright and dark regions by rotating the chopper. Specifically, a stepper motor can be used to drive the chopper to rotate at a constant speed, and a square wave modulation signal is generated by the periodic overlapping of the chopper blade and the light beam, providing a time reference for phase detection.

[0033] In some of the solutions described above in this application, if the rotation direction of the chopper is not clearly defined or does not match the beam propagation direction, it may lead to unstable beam splitting effect, which in turn affects the detection accuracy of the phase difference parameter. In addition, if the specific implementation of the modulation period in the time domain and spatial geometry is not clearly defined, it may cause a deviation in the mapping relationship between the light intensity signal and the displacement, thus making it impossible to accurately demodulate the axial displacement of the measured object.

[0034] In this regard, this application further proposes that in step S2, the rotation direction of the chopper is along the lateral expansion direction of the beam propagation direction, that is, the tangential direction perpendicular to the beam propagation direction.

[0035] When the chopper blades rotate clockwise, the laser beam at the spot is cut into alternating bright and dark waveforms: In the time domain, continuous laser light is cut into periodic square wave signals; each complete modulation cycle begins when the beam passes through a chopper blade, passes through a bright region, a dark region, and two phase difference regions, and then returns to the starting point. .

[0036] In spatial geometry, when the light spot falls on the opening of the chopper blade, the beam is allowed to pass through, corresponding to the bright area appearing on the multi-quadrant detector. At this time, the arc length of the light transmission is... When the light spot falls on the solid position of the chopper blade, the beam is blocked, corresponding to a dark area appearing on the detector. At this time, the arc length of the blocking arc is... .

[0037] The rotation direction aligns with the transverse tangential direction of the beam propagation, which can be achieved by adjusting the orthogonality between the chopper shaft and the optical path, for example, by controlling the shaft installation angle error within ±0.5°. Clockwise rotation can be achieved through closed-loop control of the encoder and drive motor, with a speed range of 100-5000 rpm. In the time domain definition, the integrity of the modulation period is ensured by a photoelectric sensor detecting the blade edge position and triggering a timer, for example, by using a grating encoder to record the blade rotation angle, with each cycle corresponding to 360° / number of blades. The ratio of the light-transmitting arc length to the light-blocking arc length can be controlled by adjusting the blade opening angle; for example, when the opening angle is 60°, the light-transmitting arc length occupies 1 / 6 of the circumference, and the light-blocking arc length occupies 5 / 6. The symmetry of the phase difference region can be ensured through blade edge chamfering, for example, by using a 30° chamfer to form a transition region.

[0038] Specifically, the transverse tangential rotation makes the phase difference area generated when the blade edge cuts the light beam have symmetry, for example, when the blade rotates clockwise in transverse tangential direction, the light spot is cut into a symmetric transition in which the left half area enters the light shielding state first and the right half area enters the light shielding state later. In time domain signal processing, each modulation period contains two phase difference areas of rising edge and falling edge, for example, when the chopping wheel rotates at 3000 rpm and has 12 blades, each phase difference area has a time width of 8.3 microseconds, the light-on duration is 41.7 microseconds, and the light-off duration is 208.3 microseconds. In spatial geometric relationship, the ratio of the light passing arc length to the light shielding arc length determines the duty cycle of the light intensity signal, for example, when the light passing arc length accounts for 1 / 4 of the circumference, the light intensity signal will present a square wave with a duty cycle of 25%. By accurately controlling the ratio of the light passing arc length to the light shielding arc length , the linear relationship between the phase difference time and the axial displacement can keep a constant slope, for example, when the ratio of the light passing arc length to the light shielding arc length is 1:3, the axial displacement changes by 0.1 microns for each 1-microsecond increase in the phase difference time.

[0039] Step S3: irradiating the modulated detection light beam to a multi-quadrant photodetector for detecting the change of light intensity with time to obtain a light intensity signal.

[0040] In the change of light intensity with time detected by the multi-quadrant photodetector, the spatial distribution of the light intensity of the light beam is synchronously collected by the partitioned detector, and specifically, a two-quadrant or four-quadrant detector can be used to extract the light spot position offset by comparing the time sequence differences of signals in different quadrants.

[0041] In the embodiment as shown in FIG. 2, when the multi-quadrant photodetector is a two-quadrant detector, the light path of the detection light beam modulated by the chopper is divided by a slit into two adjacent light sensitive areas with a spacing of M, for detecting the change of light intensity with time.

[0042] The convergence angle of the light beam and the axial displacement of the relative focus plane of the measured object satisfy the following geometric relationship: wherein the negative defocus is defined when the measured object is located in front of the focus, and the positive defocus is defined when the measured object is located behind the focus, that is, the ± direction of the lateral shift of the light spot relative to the two-quadrant dividing line.

[0043] Specifically, when the measured object deviates from the focus plane, the convergence angle of the light beam causes the spot to produce a lateral shift on the two-quadrant detector. Due to the geometric constraint of the pitch M between the photosensitive regions and the convergence angle , the coverage area of the spot on the adjacent photosensitive regions presents a phase difference over time. By measuring the time difference of the rising edge or falling edge of the output signals of the two photosensitive regions, it can be determined that the displacement direction is positive or negative defocus. For example, when the convergence angle is 20 degrees and the pitch between the photosensitive regions is 100 microns, the axial displacement increases by 0.364 microns per micron, which is converted to an accurate displacement amount by phase difference calculation. This design eliminates the ambiguity of direction discrimination caused by the symmetrical layout of the photosensitive regions of the general detector, and improves the detection sensitivity to the sub-micron level through geometric parameter matching.

[0044] Through the above technical solutions, the application can effectively distinguish the positive and negative defocus directions of the measured object relative to the focal plane, and improve the detection accuracy. The layout of the adjacent photosensitive regions of the two-quadrant detector is designed by the geometric relationship between the pitch M and the convergence angle , so that the position change of the spot on the detector can be captured by two independent regions, thereby distinguishing the positive and negative defocus directions through the time difference of the light intensity change. The geometric relationship between the beam convergence angle and the axial displacement further relates the spatial distribution of the spot to the displacement amount, and ensures that the detection result is consistent with the physical displacement direction through the clear definition of positive and negative defocus. This design optimizes the matching of the detector structure and the optical path parameters, enhances the sensitivity of the system to the axial displacement direction, and avoids the possible direction misjudgment or detection blind area of the general multi-quadrant detector.

[0045] In other optional embodiments, the multi-quadrant photodetector can be a four-quadrant detector, which is divided into four photosensitive regions by a cross-shaped slit.

[0046] The intersection point of the cross-shaped slit is located at the geometric center of the detector, dividing the photosensitive surface into four independent regions, each region being in the shape of a sector or a rectangle. The four photosensitive regions are symmetrically distributed along the horizontal and vertical axes, and the four photosensitive regions are located at the upper left, upper right, lower left and lower right quadrants of the detector, respectively. The boundaries of adjacent regions are formed by the straight edges of the cross-shaped slit. The electrode leads of each photosensitive region are isolated by an insulating layer and connected to a differential amplification circuit for independently outputting the photocurrent signals of each quadrant. For example, the width of the cross-shaped slit can be 0.1 mm, and the effective photosensitive area of the photosensitive region accounts for more than 80% of the total area of the detector. When the chopper rotates, the bright and dark boundaries of the light beam move in the horizontal and vertical directions simultaneously, and the four photosensitive regions detect the intensity changes of the spot in different quadrants. By comparing the phase difference of the signals of adjacent regions, the spatial displacement of the light beam in the orthogonal directions can be obtained.

[0047] Specifically, after the modulated detection beam irradiates the four-quadrant detector, the bright-dark boundary of the light spot moves alternately in the horizontal and vertical directions. The four photosensitive areas generate periodic electrical signals synchronized with the change of light intensity. By measuring the phase difference of the signals of the two photosensitive areas in the horizontal direction and the phase difference of the signals of the two photosensitive areas in the vertical direction, the displacement information of the light beam in the X-axis and Y-axis directions can be obtained simultaneously. For example, when the center of the light spot shifts to the left, the rising edge of the signal of the left photosensitive area is earlier than that of the right photosensitive area; when the center of the light spot shifts upward, the falling edge of the signal of the upper photosensitive area is later than that of the lower photosensitive area. By calculating the vector sum of the two sets of phase differences, the error of single-direction detection can be eliminated, and the spatial phase distribution characteristics are converted into the axial displacement amount.

[0048] Through the above technical solution, the spatial resolution of the light intensity signal is enhanced. The four-quadrant detector can simultaneously detect the bright-dark distribution characteristics of the light spot in the horizontal and vertical two orthogonal directions, and obtain more comprehensive light intensity change information. Compared with the two-quadrant detector which can only capture the light intensity difference in a single direction, the four photosensitive areas of the four-quadrant detector can respond to the light intensity signals in different quadrants, and more accurately reflect the spatial phase distribution characteristics of the light beam after being modulated by the chopper through comprehensive calculation of multi-direction data. This design improves the demodulation accuracy of the phase difference parameter, effectively eliminates the calculation error of the axial displacement amount caused by the directional limitation of the detector, and optimizes the stability and accuracy of the automatic focusing system.

[0049] Step S4: Phase-displacement demodulation is performed on the light intensity signal to obtain a phase difference parameter related to the axial displacement of the measured object relative to the focal plane, and the axial displacement of the measured object is calculated according to the phase difference parameter, so as to realize non-contact displacement measurement.

[0050] Among them, the phase-displacement demodulation refers to converting the phase difference of the light intensity signal into the axial displacement amount, which can specifically use a phase-locked amplifier to extract the phase information of the modulation signal, and realize non-contact displacement measurement by establishing a mathematical model of the phase difference and the defocus amount.

[0051] In an optional embodiment, since the phase difference In step S4, the phase difference time , the bright time and the dark time are further measured, and the formula is as follows: ; The phase difference is obtained, and D1+D2=C (constant). Through this step, the relationship between the phase difference and the axial displacement can be known.

[0052] The phase difference calculation in this step no longer relies on signal characteristics in a single time dimension, but rather improves the robustness and accuracy of phase difference demodulation through the synergistic effect of multiple time parameters. This ensures a more reliable correspondence between the calculated axial displacement and the actual physical displacement, thereby enhancing the overall performance of the optical path autofocus system.

[0053] Through the above steps, further analysis based on phase difference can be performed. With the axial displacement The relationship between phase difference Inverse transformation yields axial displacement ,Right now .

[0054] Step S5: Generate a feedback signal based on the preset ideal focal position and axial displacement, and input it to the control unit to drive the focusing structure, thereby achieving automatic focusing.

[0055] Among them, the feedback signal driven focusing structure refers to the generation of control commands based on the displacement to adjust the position of the optical element. Specifically, a piezoelectric ceramic driver or a voice coil motor can be used to adjust the axial movement of the lens in real time, forming a closed-loop control loop to dynamically track the focus position.

[0056] For example, the controller compares the calculated axial displacement with the preset ideal focal position and generates an error signal. This error signal is then processed by the PID controller to drive the focusing actuator to move the lens or sample stage, thus achieving closed-loop autofocus.

[0057] Optionally, if the axial displacement corresponding to the ideal focal position is set to 0, then the deviation between the measured axial displacement and the axial displacement corresponding to the ideal focal position is the feedback signal. The feedback signal is then used by the controller to output the adjustment amount. It is used to drive the focusing mechanism to adjust the position of the lens or the object being measured, so that the focal point gradually approaches the target surface; As the lens moves, the phase difference is recalculated based on the new optical path. ;like Right now If the refocusing is successful, then steps S1 to S5 will be executed again.

[0058] During dynamic adjustment, changes in optical path parameters can be acquired in real time using a multi-quadrant photodetector. For example, when the axial displacement deviation of the measured object exceeds ±5 micrometers, the system automatically triggers a recalculation of the phase difference. The conversion accuracy of the axial displacement can be achieved by optimizing the phase difference detection resolution; for example, the phase difference detection resolution can be controlled within 0.1 degrees.

[0059] Specifically, when the deviation of the measured axial displacement from the ideal position is identified as a feedback signal, the controller generates a regulation instruction proportional to the deviation according to a preset algorithm. The instruction drives the focusing mechanism to move the lens or the measured object, and the moving step can be set to 1-10 microns. After each position adjustment, the system recalculates the phase difference distribution of the light spot on the multi-quadrant detector, for example, when the phase difference converges to zero, it is determined that the focal point has reached the target surface; if the phase difference is not eliminated, the light beam modulation, light intensity detection and displacement calculation process are repeated. During this process, the response time of the controller can be controlled within 10 milliseconds, ensuring that the system can adapt to the dynamic displacement changes of the measured object. Through multiple iterations of adjustment, the axial displacement deviation gradually decreases until the focusing accuracy requirement is met, for example, the final focusing accuracy can reach within ±0.5 microns.

[0060] As a specific embodiment, a proportional-integral-derivative (PID) control algorithm can be used to process the feedback signal. The controller calculates the appropriate adjustment amount according to the current axial displacement deviation, the cumulative deviation and the deviation rate. For example, set the proportional coefficient K p = 0.8, the integral coefficient K i = 0.1, and the differential coefficient K d = 0.05, fast response and stable adjustment can be achieved.

[0061] Through the above technical solutions, the application realizes closed-loop control of the focusing process, improves the focusing accuracy and efficiency. Specifically, by establishing a quantitative relationship between the phase difference and the axial displacement, the abstract phase difference parameter is converted into a directly operable physical quantity, providing a clear basis for feedback control. Setting the ideal focal point position as the reference enables the system to accurately identify the actual deviation and generate the corresponding feedback signal.

[0062] Further, the feedback signal can be processed by introducing a control algorithm, and the system can dynamically adjust the output of the focusing mechanism to achieve precise control of the position of the lens or the measured object. This closed-loop feedback mechanism not only solves the problem of real-time error correction in static measurement, but also improves the system's adaptability to dynamic changes through iterative adjustment.

[0063] Therefore, the scheme of the application can maintain high-precision automatic focusing performance in various complex environments, significantly improving the stability and reliability of the optical system. At the same time, due to the use of an adaptive focusing strategy, the system has good adaptability to different types of measured objects and environmental changes, expanding the application range.

[0064] As shown in Figure 3 , the horizontal axis is the true defocus amount (nm) given by the displacement table, and the vertical axis is the phase difference of the normalized phase output obtained by the multi-phase detector phase-displacement demodulation.

[0065] As can be seen from the figure, the optical path system of the application has good linearity, the phase difference and the true displacement are approximately linear in the range of 0 → ±7000 nm, which shows that the geometric relationship designed before Indeed, it is true under the actual parameters.

[0066] Secondly, the optical path system of the application has high symmetry, the positive and negative defocus data points are basically coincident, which shows that the spot centroid offset has no two-quadrant gain imbalance. / The direction sensitivity is consistent, and there is no two-quadrant gain imbalance.

[0067] In addition, it can also be seen that the sensitivity (slope) of the optical path system of the application is high, and the rough estimation of the slope is approximately 0.35 / 7000 nm ≈ 5×10 -5 nm -1 That is, 1 nm defocus corresponds to 5×10 -5 phase change, which is consistent with the order of magnitude of the theoretical value. The data point scattering bandwidth is approximately ±0.01, which corresponds to a phase noise of approximately 0.01. According to the slope, the displacement noise is approximately 0.2 nm, which proves that the system repeatability is ≤1 nm. It can be seen that the focusing optical path design of the application stably maintains linearity in the range of ±7 μm, and the system displacement resolution reaches the sub-nanometer level.

[0068] In some schemes of the application, a method for achieving automatic focusing by periodically dividing the detection light beam by a chopper and detecting the change of light intensity by using a multi-quadrant photoelectric detector is proposed. However, due to the large circumferential spacing of the chopper blade, the light transmission arc length and the light shielding arc length are too long, and the spatial resolution of the optical path system is limited. At the same time, the numerical aperture of the lens is small, which leads to insufficient detection range, and when the rotation speed of the chopper is low, the detection frequency cannot meet the dynamic focusing demand, and these factors jointly affect the accuracy and response speed of automatic focusing.

[0069] In this regard, the application further proposes that the circumferential spacing of the chopper blade can be reduced to reduce the light transmission arc length and the light shielding arc length, thereby improving the spatial resolution of the optical path system; and / or, a lens with a numerical aperture NA≥0.5 is used to increase the detection range of the optical path system; and / or, the rotation speed of the chopper is increased to increase the detection frequency.

[0070] The circumferential spacing of the vane is set to be less than or equal to 0.5 mm, so that the single light passing arc length and the light blocking arc length are shortened to within 1 mm, and the light-dark boundary time difference of the split light beam is reduced to the microsecond level. A lens with a numerical aperture NA≥0.5 is arranged in the optical path, the passing light aperture is expanded to more than 5 mm, and the light beam divergence angle is controlled within ±15 degrees. The rotation speed of the chopper is increased to more than 3000 rpm, and the corresponding modulation frequency reaches more than 200 Hz. When the circumferential spacing is reduced and the rotation speed is increased at the same time, the vane structure strength needs to meet the deformation threshold under the action of centrifugal force, for example, when a carbon fiber composite vane is used, the circumferential spacing of 0.3 mm is allowed at 4000 rpm.

[0071] Specifically, when the light beam passes through the chopper vane with a circumferential spacing of 0.3 mm, the light passing arc length and the light blocking arc length are shortened to 0.6 mm and 0.4 mm respectively, so that the rising edge and falling edge time difference of the light intensity signal received by the multi-quadrant photodetector is compressed to within 50 microseconds, and the phase difference detection accuracy is improved to the level of 0.1 radian. When a lens with a numerical aperture of 0.65 is used, the lateral offset tolerance of the light beam at the focal plane is expanded to ±200 microns, and the axial detection range is extended to ±5 mm. When the chopper rotates at 3500 rpm, the modulation period is shortened to 17 ms, so that the system can perform 58 displacement sampling per second. During the focusing execution process, the calculation of the phase difference parameter is based on the shortened light-dark time difference and the expanded detection range, so that the axial displacement feedback delay is reduced to within 20 ms, and the dynamic tracking error is controlled within the range of ±2 microns.

[0072] Through the above technical solutions, the spatial resolution of the optical system is further improved, the detection range is increased, and the detection frequency is improved. Therefore, the accuracy and response speed of the automatic focusing system are significantly improved. The spatial resolution of the light intensity signal is improved, providing more accurate data basis for phase difference detection. The system can adapt to the focusing requirements of the measured object in a larger axial displacement range. The time domain sampling frequency of the light intensity signal is increased, and the response speed of the system to dynamic displacement is improved.

[0073] Through the synergistic effect of the above steps, self-referencing focusing of the main light beam multiplexing is realized, and additional light sources or focusing devices are avoided. Since a single optical path is used, the spatial alignment error in the traditional double optical path system is eliminated, and the focusing accuracy is improved. The time-space joint modulation method encodes the axial displacement information into the time domain phase difference, improving the sensitivity of displacement detection. The closed-loop feedback control enables the system to track the focal point position changes in real time and adapt to dynamic focusing scenarios. In addition, this scheme simplifies the optical path structure, reduces the system complexity and assembly difficulty, and is conducive to improving the reliability and stability of the equipment.

[0074] To achieve the above object, the application further provides a light path automatic focusing system for adjusting focal length, wherein the light path automatic focusing method for adjusting focal length is used, and the light path automatic focusing system comprises a light source beam splitting module, a modulation module, a detection module, a demodulation module and a control module.

[0075] The light source beam splitting module is used for splitting the main beam of the laser as the only light source to form a detection beam, and focusing the detection beam to the chopper blade through a lens, so that the position of the chopper blade shielding the beam is used as a system reference zero point; the modulation module can comprise a chopper and a driving device thereof, and is used for controlling the chopper to rotate at a linear speed The detection beam is rotated to be periodically divided into bright areas and dark areas; the detection module can comprise a multi-quadrant photoelectric detector, and is used for receiving the detection beam modulated by the chopper and outputting a light intensity signal changing with time; the demodulation module is used for phase-displacement demodulating the light intensity signal to obtain a phase difference parameter related to the axial displacement of the measured object relative to the focal plane, and calculating the axial displacement of the measured object according to the phase difference parameter; the control module is used for generating a feedback signal according to a preset ideal focal point position and the axial displacement, and inputting the feedback signal to a focusing actuator to drive the focusing actuator to adjust the position, so as to realize automatic focusing. The specific embodiments have been described in detail above, and will not be repeated here.

[0076] To achieve the above object, the application further provides a terminal device, comprising a memory, a processor and a computer program stored in the memory and executable on the processor, wherein the processor implements the light path automatic focusing method for adjusting focal length as described in any one of the preceding embodiments when running the program. The processor and the memory can be separately arranged or integrated together, for example, integrated on a system on chip (SOC) of the terminal device.

[0077] To achieve the above object, the application further provides a computer readable storage medium, which stores computer executable instructions or a computer program, and the computer executable instructions or the computer program are executed by a processor to implement the light path automatic focusing method for adjusting focal length as described above.

[0078] The computer readable storage medium is, for example, a memory. The memory can be a volatile memory or a non-volatile memory, or the memory can include both volatile and non-volatile memory. The non-volatile memory can be a read-only memory (ROM), a programmable ROM (PROM), an erasable PROM (EPROM), an electrically EPROM (EEPROM), or a flash memory, for example. The volatile memory can be a random access memory (RAM), which is used as an external cache. By way of example, and not limitation, many forms of RAM are available, for example, a static RAM (SRAM), a dynamic RAM (DRAM), a synchronous DRAM (SDRAM), a double data rate SDRAM (DDR SDRAM), an enhanced SDRAM (ESDRAM), a synchlink DRAM (SLDRAM), and a direct Rambus RAM (DRRAM).

[0079] The integrated units in the above embodiments, if implemented in the form of software functional units and sold or used as independent products, can be stored in the above computer readable storage medium. Based on such understanding, the technical solutions of the present application, essentially or in part, or all or part of the technical solutions can be embodied in the form of a software product. The computer software product is stored in a storage medium, and includes several instructions for causing one or more computer devices (which can be personal computers, servers, or network devices, etc.) to execute all or part of the steps of the methods described in the various embodiments of the present application.

[0080] The preferred embodiments of the present application are described in detail above. It should be understood that those of ordinary skill in the art can make many modifications and variations to the present application without requiring inventive effort, based on the concept of the present application. Therefore, any technical solutions obtained by logical analysis, reasoning, or limited experiments based on the concept of the present application and the prior art, should be within the scope of protection defined by the claims.

Claims

1. A light path auto-focusing method for adjusting focal length, characterized in that, The method comprises the following steps: Step S1: the main beam of the laser is split into a detection beam, and focused by a lens to the chopper blade, with the position of the chopper blade blocking the beam as the system reference zero point; Step S2: periodically modulating the detection beam; wherein the chopper is controlled to rotate at a linear speed rotating at the detection beam such that the beam is periodically split into bright and dark regions; Step S3: the modulated detection beam is irradiated to a multi-quadrant photodetector for detecting the change of light intensity over time to obtain a light intensity signal; Step S4: the light intensity signal is phase-displacement demodulated to obtain a phase difference parameter related to the displacement of the measured object relative to the focal plane, and the axial displacement of the measured object is calculated according to the phase difference parameter; Step S5: a feedback signal is generated according to the preset ideal focal point position and the axial displacement, and input to a control unit to drive the focusing structure, thereby realizing automatic focusing.

2. The optical path autofocus method for adjusting focal length according to claim 1, wherein, In the step S2, the rotation direction of the chopper is along the lateral development direction of the beam propagation direction, i.e. the tangential direction perpendicular to the beam propagation direction; When the chopper blade rotates clockwise, the laser at the spot is cut into a light-dark alternating waveform: In the time domain, the continuous laser is cut into a periodic square wave signal; each complete modulation period is from the start of the beam passing through one chopper blade, through one bright area, one dark area, and two phase difference areas, back to the starting point, at which time the modulation period; In spatial geometry, when the light spot falls on the opening position of the chopper blade, the light beam is allowed to pass through, corresponding to the bright area appearing on the multi-quadrant detector, at this time the light passing arc length is ; when the light spot falls on the solid position of the chopper blade, the light beam is blocked, corresponding to the dark area appearing on the detector, at this time the light blocking arc length is .

3. The optical path autofocus method for adjusting focal length according to claim 2, wherein, When the multi-quadrant photodetector is a two-quadrant detector, the two-quadrant detector comprises two adjacent light-sensitive areas with a spacing of M, for detecting the change of light intensity over time; Converging angle of the light beam Axial displacement of the focusing plane relative to the object being measured The following geometrical relationship exists: wherein a measured object located in front of the focal point is defined as negative defocus and located behind the focal point is defined as positive defocus .

4. The optical path autofocus method for adjusting focal length according to claim 3, wherein, Due to the phase difference In the step S4, the phase difference time, the light time and the dark time are further measured The formula is as follows: ； obtaining a phase difference .

5. The optical path autofocus method for adjusting focal length according to claim 4, wherein, In said step S5, the phase difference is converted into an axial displacement in relation to said axial displacement ; and ; and ; The deviation between the measured axial displacement and the axial displacement corresponding to the ideal focal point position is the feedback signal, and the feedback signal is outputted by the controller to adjust the amount for driving the focusing mechanism to adjust the position of the lens or the measured object, so that the focal point gradually approaches the target surface; As the lens moves, the phase difference is recalculated according to the new optical path ; if i.e. then refocusing is successful, otherwise steps S1 to S5 are re-executed.

6. The optical path autofocus method for adjusting focal length according to claim 1, wherein, The multi-quadrant photodetector is a four-quadrant detector, which is divided into four light-sensitive areas by a cross-shaped slit.

7. The optical path autofocus method for adjusting focal length according to claim 1, wherein, by reducing the circumferential spacing of the chopper blades to reduce the clear aperture length and the obscuration length , thereby increasing the spatial resolution of the optical system; and / or, using a lens with a numerical aperture NA≥0.5 to increase the detection range of the optical system; and / or, increasing the rotation speed of the chopper to increase the detection frequency.

8. An optical path auto-focusing system for adjusting focal length, characterized by, The method for adjusting the focal length of the optical path automatic focusing method according to any one of claims 1-7 comprises: a light source splitting module for splitting the main beam of the laser into a detection beam, and focusing the detection beam by a lens to the chopper blade, with the position of the chopper blade blocking the beam as the system reference zero point; a modulation module comprising a chopper and a drive device for the chopper for controlling the chopper to rotate at a linear speed rotating at the detection beam, such that the detection beam is periodically divided into bright and dark regions; a detection module comprising a multi-quadrant photodetector for receiving the detection beam modulated by the chopper and outputting a light intensity signal varying over time; a demodulation module for phase-displacement demodulating the light intensity signal to obtain a phase difference parameter related to the displacement of the measured object relative to the focal plane, and calculating the axial displacement of the measured object according to the phase difference parameter; a control module for generating a feedback signal according to the preset ideal focal point position and the axial displacement, and inputting the feedback signal to a focusing actuator to drive the focusing actuator to adjust the position, thereby realizing automatic focusing.

9. A terminal device comprising a memory, a processor, and a computer program stored on the memory and executable on the processor, characterized in that, The processor runs the program to realize the method for adjusting the focal length of the optical path automatic focusing method according to any one of claims 1-7.

10. A computer readable storage medium characterized by, The computer readable storage medium stores computer executable instructions or computer programs, which are executed by the processor to realize the method for adjusting the focal length of the optical path automatic focusing method according to any one of claims 1-7.

Images