CN102494623A - Non-contact measuring method and device for center distance of optical surface in lens - Google Patents

Abstract

The invention relates to a non-contact measurement method and a measurement device for the center distance of an optical surface in a lens. In the invention, the light emitted by a broadband light source is passed through an interference structure to generate an interference signal, and the interference signal is sampled and the image is reconstructed to obtain the center of the lens. Two-dimensional tomographic image of the optical surface, using a thin parallel beam as the scanning beam in the sample arm, changing the starting position of the coherent tomography by moving the fiber collimator, and then obtaining the characteristic images of each optical surface at different depths in the lens , and finally the distance between the centers of the two optical surfaces is obtained according to the position of the optical surface in its characteristic image and the moving distance of the imaging start position. The invention has the advantages of non-contact and non-destructive measurement, high measurement accuracy and simple data processing, and can be flexibly applied to the fields of optical processing, optical assembly and calibration, optical detection and the like.

Description

Non-contact measuring method and measuring device for distance between centers of optical surfaces in lens

technical field

The invention belongs to the field of optical measurement, and in particular relates to a non-contact measuring method and a measuring device for the distance between the centers of optical surfaces in a lens.

Background technique

The calibration of the optical system, especially the calibration of the objective lens, directly affects the imaging quality and performance of optical instruments, and is a very critical process. The calibration process of the objective lens mainly has three requirements: 1) correct the eccentric error of each surface; 2) ensure the air gap; 3) ensure that the mirror surface is not deformed under the premise of firm installation. The measurement and control of the air gap is one of the key processes in the production of objective lenses. If the air gap cannot be strictly controlled, it will cause spherical aberration, chromatic aberration and affect the focal length, magnification, etc., and even seriously affect the imaging quality of the objective lens. The measurement and control of the air gap is one of the key processes in the production of objective lenses.

At present, there are many measurement methods for optical surface spacing, which are mainly divided into two categories: contact measurement and non-contact measurement. The contact method is still widely used in China for measurement. There are usually two contact measurement methods: one is to measure the distance between the upper vertex of the previous lens and the upper vertex of the latter lens, and then subtract the lens thickness; the second is to measure the distance from the vertex of the spherical surface to the section of the lens holder. Contact measuring instruments mainly include: dial indicator, dial indicator or grating micrometer, etc. The main disadvantage of contact measurement is that it is easy to scratch the lens surface. In order to avoid scratches, a layer of protective paper is usually added between the measuring head and the surface to be measured, so the measurement accuracy is low. Moreover, contact measurement takes a long time, and it is impossible to realize real-time guidance in the process of optical processing and assembly. For some surfaces coated with special coatings, contact measurement is strictly prohibited, so non-contact measurement must be used.

In the Chinese patent application number "01133730.3", a new non-contact optical system air gap measurement method is proposed. The main optical detection system is composed of a Fizeau interferometer, so that the focus of the standard lens of the interferometer is focused on the apex of the measured lens. Through a photoelectric imaging converter, the wavefront of the vertex of the measured lens mirror is reversed and positioned by self-collimating interference, indicating that the grating is linked with the standard lens of the interferometer, and the movement of the standard lens is read through a reading system composed of a grating sensor and a digital display. Thus, the air separation value of the optical system is obtained.

Compared with contact measurement, this measurement method has the following advantages: it realizes non-contact non-destructive measurement, which greatly improves the measurement accuracy; it can be used for the installation and inspection of special lenses such as coatings; the reading is convenient and simple. This measurement method realizes non-contact measurement, but it still has many defects: the focus positioning of the standard lens is realized by adjusting the bending degree of the interference fringes, and the adjustment process is cumbersome, the workload is large, and a large human error is introduced .

Optical coherence tomography (OCT) is a newly developed tomographic technology that combines low-coherence interferometer and confocal scanning microscopy, and uses heterodyne detection technology to obtain internal information of samples. It is a high-resolution non-destructive real-time imaging tool with broad prospects in many fields, and the spatial resolution is maintained at the μm level.

Contents of the invention

The object of the present invention is to aim at the deficiencies in the prior art, to provide a non-contact measuring method and measuring device for the distance between the centers of the optical surfaces in the lens, which can measure the distance between the centers of the optical surfaces in the lens in a non-contact manner, and has the function of measuring It has the advantages of high precision, simple data processing, real-time guidance and flexible use.

The technical solution provided by the present invention is: a non-contact measurement method for the distance between the centers of the optical surfaces in the lens, the steps are as follows:

Step 10, performing optical coherence tomography on the first optical surface of the lens to be tested;

Wherein, step 00 is also included before step 10, making the scanning light beam of the sample arm pass through the optical axis of the lens to be measured; the criterion for making the scanning light beam of the sample arm pass through the optical axis of the lens to be measured is that after covering the reflected light of the reference arm, the photodetector The detected light intensity is maximum.

Step 20, changing the starting position of coherence tomography, and performing optical coherence tomography on the next optical surface of the lens to be tested;

Wherein, step 20 includes:

Step 21, moving the starting position of the coherence tomography by Δz to obtain an optical coherence tomography image;

Step 22, observe whether there is a characteristic image of the next optical surface in the optical coherence tomography image obtained in step 21, if not, continue to perform step 21, if not, perform step 23;

Step 23, record the total amount of movement of the starting position of the coherence tomography as x _i , i∈{1, 2, ..., N-1}, N is the total number of optical surfaces related to the distance between the optical surfaces to be measured, That is, there are N-1 distances between adjacent optical surfaces.

Step 30, if the characteristic images of all optical surfaces related to the distance between the optical surfaces of the lens to be tested have been obtained, perform step 40, otherwise perform step 20;

Step 40, performing data processing on the obtained characteristic image to obtain the required optical surface spacing value;

Wherein, step 40 includes:

Step 41, determine the center position of the optical surface in the obtained characteristic image, record the pixel point position of the jth optical surface in its characteristic image as z _j , j∈{1, 2, ..., N};

i∈{1，2，...，N-1}，x_i为对应相干层析成像起始位置移动量，n_i为间隔的材料折射率，d为层析图像单个像素点对应光程值；Step 42, calculate the distance between two adjacent optical surfaces, and record the distance between the i-th adjacent optical surfaces as l _i , the calculation formula is

i ∈ {1, 2, ..., N-1}, x _i is the movement amount of the starting position of the corresponding coherence tomography, n _i is the refractive index of the interval material, and d is the optical path corresponding to a single pixel of the tomographic image value;

In step 43 , according to the distance between two adjacent optical surfaces obtained in step 42 , the required distance between two optical surfaces is obtained.

Wherein, the scanning light beam in step 00 is a thin parallel light beam.

Wherein, the method for changing the starting position of the coherent tomography in step 20 is to move the fiber collimator in the sample arm or the fiber collimator in the optical delay line, or move the fiber collimator in the sample arm and the fiber collimator in the optical delay line at the same time. Fiber collimator.

Wherein, the characteristic image described in step 30 is a two-dimensional cross-sectional tomographic image obtained by optical coherence tomography.

Wherein, the Δz in step 21 is smaller than the axial imaging range of the coherent tomography system.

Wherein, the movement of the starting position of the coherent tomography is to move toward the sample position along the incident direction of light.

The invention also provides a non-contact measuring device for the distance between the centers of the optical surfaces in the lens, the measuring device includes a broadband light source, a fiber coupler, two polarization controllers, a phase modulator, an optical delay line, a sample scanning device, and a photoelectric detector device, low noise preamplifier, data acquisition card, computer;

The measurement device adopts a fiber-optic Michelson interference structure. After the light emitted from the broadband light source is split by the fiber coupler, it enters the reference arm and the sample arm respectively; the path entering the reference arm passes through a polarization controller, a phase modulator and an optical delay in sequence. The optical delay line adopts a fast scanning delay line in the spectral domain. The optical delay line is composed of a fiber collimator, a diffraction grating, a Fourier lens, a scanning galvanometer, and a mirror; the way into the sample arm passes through another polarization controller and Sample scanning device, the sample scanning device includes fiber optic collimator, high-precision guide rail, motorized micro-displacement platform and the sample to be tested. The two reflected lights interfere when the fiber coupler converges. After the interference signal is detected by the photodetector, the The noise preamplifier is amplified, filtered, and then connected to the data acquisition card and computer.

The present invention has the advantage over prior art that:

1. The non-contact measurement method for the center distance of the optical surface in the lens according to the present invention is non-destructive to the sample and can be flexibly applied to the field of installation and detection;

2. The present invention utilizes optical coherence tomography technology, so the measurement accuracy is high, up to micron level;

3. The present invention uses thin parallel light beams to scan, and moves the optical fiber collimator by high-precision guide rails, thereby changing the range of tomographic imaging, and the measurement range reaches tens of millimeters; the data processing is simple.

Description of drawings

Fig. 1 is a work flow chart of the present invention;

Fig. 2 is the structural representation of detection device of the present invention;

3 is a schematic diagram of the structure of an optical delay line used in an embodiment of the present invention;

4 is a schematic structural diagram of a sample scanning device used in an embodiment of the present invention;

Fig. 5 is a schematic diagram of data processing;

Figure 6 Schematic diagram of the measurement results of the distance between the objective lens and the sample.

Detailed ways

The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments.

As shown in Figure 2, the device of the present invention includes a broadband light source 1, a fiber coupler 2, polarization controllers 3 and 4, a phase modulator 5, an optical delay line 6, a sample scanning device 7, a photodetector 8, and a low-noise front Amplifier 9, data acquisition card 10, computer 11.

The measurement device adopts a fiber-optic Michelson interference structure. The light emitted from the broadband light source 1 is split by the fiber coupler 2 and enters the reference arm and the sample arm respectively. All the way into the reference arm passes through the polarization controller 3, the phase modulator 5 and the optical delay line 6 in sequence. Straightener 12, diffraction grating 13, Fourier lens 14, scanning galvanometer 15, reflecting mirror 16 are made up, as shown in Figure 3; All the way that enters sample arm passes polarization controller 4 and sample scanning device 7, sample scanning device 7 It includes an optical fiber collimator 18, a high-precision guide rail 17, an electric micro-displacement platform 19 and a sample to be tested 20, as shown in FIG. 4 . The two reflected lights interfere when the fiber coupler 2 converges, and the interference signal is detected by the photodetector 8 , amplified and filtered by the low-noise preamplifier 9 , and then connected to the data acquisition card 10 and the computer 11 .

The principle of the present invention is: due to the low coherence of the broadband light source 1, when the reference arm optical path is fixed, only when the optical path difference between the reference arm reflected light and the sample arm backreflected light is smaller than the coherence length of the broadband light source 1, can interference occur. That is, only the retroreflected light at a specific depth of the sample can interfere, and the intensity of the interference light is maximum when the optical path difference between the two arms is zero, and decreases rapidly with the increase of the optical path difference. When the optical delay line 6 scans the optical path, it interferes with the retroreflected light at different depths of the sample, and the interference signal envelope represents the refractive index information in the depth direction of the sample. For an optical surface with a sudden change in refractive index, the center of the envelope represents the position information of the optical surface, so the distance between the centers of adjacent envelopes represents the distance between adjacent optical surfaces, and the optical path change of the optical delay line 6 is strictly controllable, so The distance value can be obtained by obtaining the optical path difference corresponding to the center of the adjacent envelope and dividing it by the refractive index of the material between the adjacent optical surfaces.

The current signal generated by the photodetector 8 is first sent to the low-noise preamplifier 9 for amplification, voltage conversion, and filtering to filter out the DC component. The special transformation extracts the envelope information of the interference signal, that is, the amplitude information, and then converts the amplitude signal into the gray value of the image. The computer obtains the characteristic image of the corresponding depth range of the sample according to the corresponding algorithm. This algorithm has been maturely applied in the prior art. Here, No details are given here.

From the above, the axial gray envelope of the feature image is the interference signal envelope. For an optical surface with a sudden change in refractive index, the extreme gray points represent the position information of the optical surface, so the distance between the extreme gray points represents the distance between the corresponding optical surfaces, and the optical path change of the optical delay line 6 is strictly controllable Yes, the imaging depth range can be precisely calibrated, so the actual distance between the corresponding optical surface and the imaging starting position can be obtained by obtaining the position of the gray extreme point in the characteristic image.

The optical path scanning range of the optical delay line 6 is generally 1-3mm, and the optical path difference between the optical surfaces of a lens can reach hundreds of millimeters. In the present invention, interference signals of optical surfaces in different depth ranges are obtained by changing the optical path matching conditions of the reference arm and the sample arm. When the reference arm remains unchanged, the fiber collimator 18 is moved through the high-precision guide rail 17 in the sample arm, and the sample depth range where interference may occur also moves accordingly; when the sample arm remains unchanged, the fiber collimator 12 in the moving reference arm also has The same effect; or the fiber collimator 12 in the reference arm and the moving fiber collimator 18 in the sample arm can also be moved simultaneously. In this embodiment, the first solution is adopted, that is, the reference arm remains unchanged, and the fiber collimator 18 is moved through the high-precision guide rail 17 in the sample arm. Finally, the distance between adjacent optical surfaces is calculated in combination with the moving distance of the imaging start position and the positions of adjacent optical surfaces in the respective feature images. In addition to extending the penetration depth, a thin parallel beam is used for scanning in the sample arm.

其相干长度约为7.97μm；光纤耦合器2选用2×2宽带耦合器，中心波长在1310nm，带宽100nm，分光比50∶50；高精度导轨17读数精度1μm，用于光纤准直器18的上下精确移动，光纤准直器18整形的细平行光束直径为500μm。In this embodiment, the broadband light source 1 is a near-infrared broadband light source with a central wavelength of 1310nm and a full width at half maximum of 95nm. According to the formula

Its coherence length is about 7.97 μm; the fiber coupler 2 is a 2×2 broadband coupler with a center wavelength of 1310 nm, a bandwidth of 100 nm, and a splitting ratio of 50:50; Moving up and down precisely, the diameter of the thin parallel beam shaped by the fiber collimator 18 is 500 μm.

In this embodiment, the polarization controller 3 is used to match the polarization state of the light reflected by the reference arm and the light reflected back by the sample, thereby improving the signal-to-noise ratio of the system. The interference signal carrier frequency technology is adopted, and the modulation frequency of the phase modulator 5 in the reference arm is set at 500kHz, so that the signal and low-frequency noise can be effectively separated in the frequency domain. In-circuit filtering and subsequent software digital filtering are implemented using a low-noise preamplifier. The device scans the sample in the depth direction (z direction) by RSOD, and selects the sample scanning mode for lateral scanning, which is realized by using a motorized micro-displacement platform 17 .

Use a signal generator to generate a triangular wave signal with a frequency of 500 Hz and a peak-to-peak value of 2 V to drive the scanning galvanometer 15, and the signal generator simultaneously generates a gate trigger matrix signal with a fixed delay and an adjustable duty cycle with the triangular wave signal for triggering data acquisition The card 10 performs data collection, and the sampling frequency is 5M/s. In this implementation, the gate trigger matrix signal is set to be delayed by 20% relative to the triangular wave signal, 2048 points are sampled in a single axial scan, the axial imaging range of the system is 1.074mm (in air), the image resolution is 800*406, and a single pixel corresponds to The distance d in the air is 2.645 μm.

The measurement steps are further described below (see Figure 1).

Step 00, first make the scanning beam of the sample arm pass through the optical axis of the lens to be tested. The specific method is to turn on the light source, cover the reflected light of the reference arm, and slightly move the position of the lens to be tested. When the light intensity detected by the photodetector 8 is the largest, it is considered to be scanning The light beam has passed the optical axis of the lens under test.

Step 10, performing optical coherence tomography on the first optical surface of the lens to be tested.

Execute the software to control the signal generator to generate a triangular wave signal, drive the optical delay line 6 to start optical path scanning, and at the same time trigger the signal to trigger the data acquisition card 10 to start data acquisition, the electric micro-displacement platform 19 performs lateral reciprocating motion, and the computer 11 performs image reconstruction and display , to obtain a 2D cross-sectional tomographic image of the first optical surface.

Step 20, changing the starting position of coherence tomography, and performing optical coherence tomography on the next optical surface of the lens to be tested.

Moving the optical fiber collimator 18 in the sample arm to the sample position along the incident direction of the light by a distance Δz, the starting position of the coherent tomography is also moved in the same direction, so that tomography can be performed at a deeper position in the sample. The moving distance Δz of the fiber collimator 18 can be read by the high-precision guide rail 17. Δz is smaller than the axial imaging range of the system (in air), and observe whether the next optical surface is included in the image. If not, continue to move the fiber collimator 18Δz. If not, execute In the next step, at the same time, record the total amount of movement of the fiber collimator 18 as x _i . i∈{1, 2, ..., N-1}, N is the total number of optical surfaces related to the distance between optical surfaces to be measured, that is, there are N-1 distances between adjacent optical surfaces.

Step 30, if the characteristic images of all the optical surfaces related to the distance between the optical surfaces to be measured have been obtained, execute step 40, otherwise execute step 20.

Step 40, data processing is performed on the obtained feature image to obtain the required distance between optical surfaces.

Step 40 includes:

Step 41, determine the center position of the optical surface in the characteristic image, record the pixel point position of the jth optical surface in its characteristic image as z _j , j∈{1, 2, . . . , N}.

Along the depth direction of the image, the gray value projection statistics are done. In this embodiment, the image size is 800*406. At each depth position (406 places in total), the sum of the gray value of all pixels (800) is counted. Value point locations represent optical surface locations.

i∈{1，2，...，N-1}，x_i为对应相干层析成像起始位置移动量，n_i为间隔的材料折射率，d为层析图像单个像素点对应光程值。Step 42, calculate the distance between two adjacent optical surfaces, and record the distance between the i-th adjacent optical surfaces as l _i , the calculation formula is

i ∈ {1, 2, ..., N-1}, x _i is the movement amount of the starting position of the corresponding coherence tomography, n _i is the refractive index of the interval material, and d is the optical path corresponding to a single pixel of the tomographic image value.

Fig. 5 is a schematic diagram of data processing.

Fig. 6 is a schematic diagram of the measurement results of an objective lens. The objective lens has 5 optical surfaces in total, and the characteristic image of each optical surface is as shown in the figure. According to the above-mentioned data processing method, the distance values of 4 adjacent optical surfaces are calculated as: 1 ₁ =6.021 mm, l ₂ =4.066 mm, l ₃ =10.750 mm, l ₄ =5.929 mm.

In step 43 , according to the distance between two adjacent optical surfaces obtained in step 42 , the required distance between two optical surfaces is obtained.

The distance between the optical surfaces required for the objective lens in Figure 6 is the air gap, that is, the distance d between the third optical surface and the fourth optical surface, and the total thickness of the objective lens, that is, the distance L between the first optical surface and the fifth optical surface. From the distance values of the four adjacent optical surfaces obtained in step 42, it can be obtained: d=l ₃ =10.750 mm, L=l ₁ +l ₂ +l ₃ +l ₄ =26.766 mm.

Various modifications can be made to the above contents by those skilled in the art without departing from the spirit and scope of the present invention defined by the claims. Therefore, the scope of the present invention is not limited to the above description, but is determined by the scope of the claims.

Claims (10)

1. A non-contact measuring method of optical surface center distance in lens, it is characterized in that, comprising: Step 10, performing optical coherence tomography on the first optical surface of the lens to be tested; Step 20, changing the starting position of coherence tomography, and performing optical coherence tomography on the next optical surface of the lens to be tested; Step 30, if the characteristic images of all optical surfaces related to the distance between the optical surfaces of the lens to be tested have been obtained, perform step 40, otherwise perform step 20; Step 40, data processing is performed on the obtained feature image to obtain the required distance between optical surfaces. 2. the non-contact measuring method of optical surface center distance in a kind of lens according to claim 1, is characterized in that, also comprises step 00 before step 10, makes sample arm scanning light beam pass through the optical axis of lens to be measured; The criterion for making the scanning beam of the sample arm pass through the optical axis of the lens to be tested is that the light intensity detected by the photodetector is the largest after covering the reflected light of the reference arm. 3. The non-contact method for measuring the distance between the centers of optical surfaces in a lens according to claim 1, wherein step 20 comprises: Step 21, moving the starting position of the coherence tomography by Δz to obtain an optical coherence tomography image; Step 22, observe whether there is a characteristic image of the next optical surface in the optical coherence tomography image obtained in step 21, if not, continue to perform step 21, if not, perform step 23; Step 23, record the total amount of movement of the starting position of the coherence tomography as x _i , i∈{1, 2, ..., N-1}, N is the total number of optical surfaces related to the distance between the optical surfaces to be measured, That is, there are N-1 distances between adjacent optical surfaces. 4. The non-contact method for measuring the distance between centers of optical surfaces in a lens according to claim 1, wherein step 40 comprises: Step 41, determine the center position of the optical surface in the obtained characteristic image, record the pixel point position of the jth optical surface in its characteristic image as z _j , j∈{1, 2, ..., N}; Step 42, calculate the distance between two adjacent optical surfaces, and record the distance between the i-th adjacent optical surfaces as l _i , the calculation formula is i ∈ {1, 2, ..., N-1}, x _i is the movement amount of the starting position of the corresponding coherence tomography, n _i is the refractive index of the interval material, and d is the optical path corresponding to a single pixel of the tomographic image value; In step 43 , according to the distance between two adjacent optical surfaces obtained in step 42 , the required distance between two optical surfaces is obtained. 5 . The non-contact method for measuring the distance between centers of optical surfaces in a lens according to claim 2 , wherein the scanning beam is a thin parallel beam. 6 . 6. The non-contact measuring method of the distance between the centers of optical surfaces in a lens according to claim 1, wherein the method for changing the starting position of coherent tomography in step 20 is to move the fiber collimator in the sample arm Or move the fiber collimator in the optical delay line, or move the fiber collimator in the sample arm and the fiber collimator in the optical delay line at the same time. 7. The non-contact method for measuring the distance between centers of optical surfaces in a lens according to claim 1, wherein the characteristic image in step 30 is a two-dimensional cross-sectional tomographic image obtained by optical coherence tomography . 8 . The non-contact method for measuring the distance between centers of optical surfaces in a lens according to claim 3 , wherein the Δz in step 21 is smaller than the axial imaging range of the coherent tomography system. 9 . The non-contact method for measuring the distance between centers of optical surfaces in a lens according to claim 6 , wherein the movement of the starting position of the coherent tomography is to move toward the sample position along the incident direction of the light. 10. A non-contact measuring device for the distance between the centers of optical surfaces in a lens, characterized in that the measuring device comprises a broadband light source (1), a fiber coupler (2), two polarization controllers (3, 4), a phase Modulator (5), optical delay line (6), sample scanning device (7), photodetector (8), low-noise preamplifier (9), data acquisition card (10), computer (11); The measuring device adopts a fiber-optic Michelson interference structure. After the light emitted from the broadband light source (1) is split by the fiber coupler (2), it enters the reference arm and the sample arm respectively; all the way into the reference arm passes through the polarization controller ( 3), a phase modulator (5) and an optical delay line (6), the optical delay line (6) adopts a fast scanning delay line in the spectral domain, and the optical delay line (6) is composed of a fiber collimator (12), a diffraction grating (13 ), a Fourier lens (14), a scanning galvanometer (15), and a mirror (16); the path entering the sample arm passes through another polarization controller (4) and a sample scanning device (7), and the sample scanning device ( 7) It includes a fiber optic collimator (18), a high-precision guide rail (17), a motorized micro-displacement platform (19) and a sample to be tested (20); the two reflected lights interfere when the fiber coupler (2) converges, and the interference After the signal is detected by the photoelectric detector (8), it is amplified and filtered in the low-noise preamplifier (9), and then connected to the data acquisition card (10) and the computer (11).

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