CN116147889A

CN116147889A - Optical centering method and device based on optical fiber point diffraction interference

Info

Publication number: CN116147889A
Application number: CN202211405295.1A
Authority: CN
Inventors: 陈凌峰; 陈伟雄; 张旭升; 赵子默; 耿敦好
Original assignee: Beijing Institute of Technology BIT
Current assignee: Beijing Institute of Technology BIT
Priority date: 2022-11-10
Filing date: 2022-11-10
Publication date: 2023-05-23

Abstract

The invention discloses an optical centering method and device based on optical fiber point diffraction interference, and belongs to the technical field of optical precision measurement. The two optical fiber point light sources axially interfere, the intersection point of the connecting line of the two optical fiber point light sources and the image plane of the camera is positioned at the position of the maximum optical path difference, and the coordinate of the intersection point can be obtained by processing the interference pattern on the image plane. The camera is adjusted to enable the center of the image plane to coincide with the maximum value point of the optical path difference, and a virtual reference axis can be determined through the connecting line. Then, a lens is arranged between the two point light sources according to the assembly sequence, so that the extreme point of the optical path difference always coincides with the center of the image plane of the camera, and the lens group can be centered. The sensitivity to the centering errors of the lens group can be improved by adding the auxiliary lens. The centering method has the advantages of simple principle and structure, high centering precision and no need of rotating the lens element through a precise rotating shaft.

Description

Optical centering method and device based on optical fiber point diffraction interference

Technical Field

The invention belongs to the technical field of optical precision measurement, and relates to an optical centering method and device based on optical fiber point diffraction interference.

Background

The centering error in the lens assembly and adjustment process is an important index for evaluating the assembly quality of the lens, and the centering error can increase advanced aberration, damage the coaxiality of the lens group and seriously influence the image quality of an optical system. Therefore, research on the centering method of the lens group is of great importance to improve the optical performance. The existing optical centering method mainly comprises a collimation method, an interference method and an image polarization method. The collimation method is classified into a stationary method and a rotation method. The static collimation method needs higher mechanical precision, the stability of the reference shaft is not high, no compensation measures are adopted, the error probability is high, and the centering precision is not high. The rotary auto-collimation method is a centering method which is currently used in a large number, such as the opticontric centering (decentration measurement) system manufactured by trioctics corporation of germany. The method mainly comprises a transmission type and a reflection type. The collimated light beam is converged on the CCD after passing through or being reflected by the lens, and if the lens is eccentric, a circular path is drawn on the CCD by the focused image or the reflected spherical center image converged by the lens. But this type of approach requires precise rotation of the lens element. The transmission method fails when the principal point of the image side of the lens to be centered is located on the rotation axis, while the reflection method requires focusing the objective lens to the spherical center position of the lens to be centered, and a focusing error is introduced. Image polarization methods place a lens to be centered between two linear polarizers and then a focused or divergent polarized light beam passes through a half wave plate. The magnitude of decentration produced by the lens is determined by the changes in the pattern and color of the polarized image. The method is suitable for centering plastic lenses, and has low light intensity of polarized images when the glass lenses are centered, so that the measurement effect is affected. Interferometry is also widely used for optical centering due to its high measurement accuracy, and is largely classified into reflection type and transmission type interferometry. In the reflection type method, the centering position of the center interferometry is the center of the lens, and when the reflected light of the front optical surface and the rear optical surface of the lens to be centered interferes with reference light, the centering error can be determined by analyzing interference fringes; edge interferometry utilizes optical surface edge reflected light interferometry to measure centering errors. The cyclic optical path interferometry is a transmission type centering method which separately irradiates light beams from both surfaces of a lens to be centered and then calculates decentration of the lens from a difference between the center of the generated interference fringe and the center of a spot of the light beam on an image plane, but this method centers only a single lens and it requires an additional device to provide a reference. In general, the interference method has higher centering precision, is applicable to spherical and aspherical lenses, but the current centering device has more complex optical path structure, uses more optical devices, increases the error generation probability, is difficult to machine and calibrate, and has strict requirements on the reflectivity of the surface to be measured by a reflection method, so that the method has certain limitation.

Disclosure of Invention

The purpose of the invention is that: because the auto-collimation centering method commonly used at present needs to rotate the lens, and the requirement on rotation precision is higher; the existing interference type centering method has more optical devices and complex optical path structure. Therefore, we propose a new type of transmission interference centering method that uses dual fiber point diffraction interference to create a virtual reference axis in space, locate the reference axis by interference imaging, and center the lens group with this as the reference.

First, the principle of optical fiber point diffraction interference optical centering is introduced. As shown in fig. 1, two point light sources P in space _a 、P _b The emitted spherical waves interfere axially. N is any point on the surface xoy, C is P _a 、P _b Intersection of the line of (c) with the plane xoy. From the geometrical relationship, P _a 、P _b The optical path difference to any point C on xoy is larger than the optical path difference to N, and C is the extreme point of the optical path difference on xoy and is also the geometric center of the interference ring, and the position of the interference ring can be determined by processing the interference pattern. Thus P _a 、P _b A reference axis can be determined. As shown in FIG. 2, if at P _a 、P _b A lens group P is arranged between _a ' is P _a Is a pixel of the image. When the optical axes of all lenses are fully coincident with the reference axis, line P _a ′P _b Still pass through C.

As shown in FIG. 3, if one of the lenses is decentered or tilted, P _a Image point P of (2) _a ' will deviate from the reference axis h, then P _a ′P _b Intersecting xoy at point I, i.e., the optical path difference extreme point is shifted to point I. Let P be _a ' and P _b The axial distance of the optical path difference maximum value point is l, and the offset of the optical path difference maximum value point can be obtained according to the similarity relationCI is

If the position and angle of the eccentric or tilted lens are adjusted so that the optical path difference extreme point I returns to point C again, the optical axis of the lens coincides with the reference axis. Thus, when assembling a lens group, centering of the lens group can be achieved by sequentially adjusting each lens so that point I always coincides with point C.

The method is to amplify the displacement of the image point of the lens group from the reference axis, and the amplification rate represents the sensitivity of the optical path to the decentration or inclination of the lens, and the sensitivity depends on the offset of the extreme point I of the optical path difference on the plane xoy. From the geometrical relationship in FIG. 3, it can be seen that when the axial distance l is smaller or CP is smaller _b The larger the offset CI of the point I on the image plane is, the larger. Although it is possible to directly adjust P _a 、P _b To reduce the axial distance l, but this will change the position of the reference axis. If the axial distance CP is extended by the moving surface xoy _b The interference fringes on xoy will quickly become sparse, which will reduce the phase range and the amount of phase information, reducing the positioning accuracy of point I. In order to reduce the axial distance l, an auxiliary mirror can be added before the mirror group to be centered, and the image point P can be changed by axially moving the auxiliary mirror _a ' position, as shown in fig. 4. This not only reduces the axial distance l, but also ensures a sufficient number of interference fringes on xoy, thus further increasing the sensitivity of the optical path to decentration or tilting of the lens.

The acquired interferogram is processed, phase information is acquired, and the extraction precision of the coordinates of the extreme points of the optical path difference can be improved to a sub-pixel level through polynomial fitting.

The invention is realized by the following technical scheme:

1. the optical centering method based on the optical fiber point diffraction interference comprises a laser, a one-to-two optical fiber beam splitter, optical fibers (a, b), an auxiliary mirror, a thin film spectroscope (BS), piezoelectric ceramics (PZT) and a controller, and a lens-free CMOS camera.

According to the determinationThe heart principle builds a set of devices. As shown in fig. 5, P _a 、P _b Is two fiber point light sources formed by coupling and splitting the laser through optical fibers, P _a After passing through the lens group to be centered and the BS, the lens group to be centered is connected with P _b Interference and imaging on a camera, and processing the interference pattern by a computer. Wherein the point light source P _b The optical fiber is wrapped around the PZT and stretched as the PZT expands radially to introduce a phase shift. When the lens group is assembled, each lens is installed and adjusted in sequence, so that the extreme point of the optical path difference is always positioned at the center of the image plane of the camera, and the centering of the lens group can be realized.

The experiment uses a single longitudinal mode solid laser with the wavelength of 532nm as a light source; selecting a single mode fiber with a core diameter of 3 mu m and a numerical aperture of 0.11; the thickness of the film spectroscope is 2 mu m; the pixel size of the CMOS sensor of the camera is 5.86 mu m, the pixel size is 1920 multiplied by 1200, and the sampling range is large; the introduction of additional aberrations can be avoided by lens-free imaging. The scheme comprises the following steps:

the first step: constructing a light path: two fiber point light sources formed by coupling and splitting of laser through optical fibers, and fiber point light source P _a And P _b Interference occurs after passing through the BS, and interference fringes are formed on an image surface of the camera;

and a second step of: determining a reference axis: sending the interference pattern on the acquisition camera to a computer for processing, and adjusting P _a 、P _b The relative position of the optical path difference extreme point and the camera image surface enables the optical path difference extreme point to be positioned at the center of the camera image surface;

and a third step of: centering the lens group: according to the assembly sequence of the lens group, each lens is fixed between two optical fiber point light sources respectively, the positions and the directions of the lenses are adjusted, so that the extreme points of the optical path difference are always positioned at the center of the image plane of the camera, and the optical axes of the extreme points are coincident with the reference axis at the moment, thus the centering of the lens group can be completed.

The beneficial effects of the invention are as follows:

1. the method has the advantages of generating approximately ideal spherical waves and having high measurement accuracy by an interferometry;

2. a virtual reference axis is established, a lens is not required to be rotated, and the mechanical precision requirement and cost are reduced;

3. the failure condition of the transmission type auto-collimation centering method does not exist, and the method is always sensitive to centering errors. As shown in fig. 6, the transmission type auto-collimation centering method measures the offset of the focus F' after the parallel light passes through the lens after rotation. If the lens image Fang Zhudian J' is on the reference axis, the focal point cannot describe a circular path on the image plane when the lens is rotated, and the method fails. The centering method of the present invention reflects the decentration of the lens by shifting the image point. As shown in fig. 7, even if the image Fang Zhudian J 'of the lens is located on the reference axis, the image point P' must deviate from the reference axis once the optical axis of the lens does not coincide with the reference axis. The centering method in the invention does not have the failure condition of the transmission type auto-collimation centering method.

Drawings

FIG. 1 is a schematic diagram of determining a reference axis based on fiber point diffraction interference.

Fig. 2 is a position of an optical path difference extreme point when the optical axes of all lenses coincide with a reference axis.

FIG. 3 is a pixel P _a The position of the optical path difference extreme point on the image plane when 'off the optical axis'.

Fig. 4 is a diagram of increasing the offset of the auxiliary mirror change point I on the image plane.

Fig. 5 is a schematic diagram of a fiber point diffraction interference centering scheme.

Fig. 6 is the position of the focal point F 'of the parallel beam when the image Fang Zhudian J' is on the reference axis.

Fig. 7 is the position of the image point P 'when the image Fang Zhudian J' is on the reference axis.

Fig. 8 is an optical path diagram for determining a reference axis.

Fig. 9 is an interferogram and phase profile after reference axis determination.

Fig. 10 is an optical path diagram of lens group centering.

Fig. 11 is an interferogram and phase profile after lens group centering.

Fig. 12 is an optical design parameters of the auxiliary mirror and the mirror group to be centered.

Fig. 13 is an decentration measurement process of the lens 1.

Fig. 14 is an decentration measurement process of the lens 2.

Fig. 15 is an interferogram and phase profile during an eccentricity measurement.

Fig. 16 is a tilt measurement process of the lens 1.

Fig. 17 is a tilt measurement process of the lens 2.

Fig. 18 is an interferogram and phase profile during tilt measurement.

Detailed Description

Specific embodiments of the present invention will be described in further detail below with reference to examples and drawings.

1. Determining a reference axis

The position of the reference axis is first determined. The optical path is arranged as shown in FIG. 8 such that P _a 、P _b Axial interference is formed and interference fringes are received at the camera. Collecting and processing interference patterns in real time, calculating coordinates of extreme points of optical path difference, and adjusting P _a 、P _b And the position relative to the camera is that the extreme point of the optical path difference is positioned at the center C of the image plane xoy of the camera. Fig. 9 shows the interferograms and phase distribution after reference axis determination.

2. Lens group centering

Before centering, in order to ensure that the centering device is sensitive to the eccentricity of the lens, an auxiliary mirror is added in front of the lens group to enable the image point P _a ' as close to P as possible in the axial direction _b . The offset of the extreme point of the optical path difference is remarkable when the lens is eccentric by moving the auxiliary mirror, and the phase extraction precision is high. As shown in FIG. 10, in the order of assembly of the lens groups, the lens groups are assembled in the following order _a And P _b The optical axis of the lens is approximately adjusted to be near the reference axis, and then the position and the direction of the lens are adjusted, so that the extreme point of the optical path difference is always positioned at the point C, and the optical axis of the lens is coincident with the reference axis. Since the current optical path configuration enables the lens 2 to have a higher sensitivity, the centering of the lens 2 does not require the configuration of an auxiliary mirror. Fig. 11 shows the interferograms and phase distribution after reference axis determination.

To verify the method, a lens group including 2 lenses was centered, and the offset of the extreme point of the optical path difference on the image plane of the camera after the lenses were decentered and tilted, respectively, was measured. The optical design parameters of the auxiliary mirror and the set of mirrors to be centered are shown in fig. 12.

3. Eccentricity measurement

Fig. 13 and 14 show the decentration measurement process of the lens 1 and the lens 2, respectively. After each lens is centered in sequence, the lens is translated in the y-direction, and the interference fringes move accordingly on the camera image plane. The amount of translation was monitored by a digital dial gauge. The interferograms are then acquired and processed at each translational position, and fig. 15 is a partial interferogram and phase distribution during the eccentricity measurement. Table 1 lists the measurement results of the optical path difference extreme point offset CI after each lens was translated from 0 to 60 μm.

Table 1 eccentricity measurement results

4. Tilt measurement

Fig. 16 and 17 show tilt measurement processes of the lens 1 and the lens 2, respectively. Similar to the decentration measurement, the lens group is re-centered according to the order of installation of the lens group. Each lens is then tilted separately so that its optical axis is tilted to the reference axis, and the interference fringes are then correspondingly moved across the camera image plane. The tilt angle alpha is monitored by means of an autocollimator. The interferograms are then acquired and processed at each tilt position, and FIG. 18 is a partial interferogram and phase distribution during tilt measurements. Table 2 lists the measurement results of the optical path difference extreme point offset CI after each lens is tilted from 0 to about 300 ".

Table 2 tilt measurement results

The invention provides a novel optical centering method and device based on optical fiber point diffraction interference. It determines a virtual reference axis based on the line connecting the two point light sources. When the optical axis of the lens is not coincident with the reference axis, the extreme point of the optical path difference on the camera is offset, so that the lens group can be centered conveniently and rapidly. Compared with an auto-collimation type centering instrument, the method does not need a precise rotary table to rotate the lens, and has the advantage of high positioning accuracy of interference. The optical path is flexible in configuration, can adapt to the centering of different lens groups, and has higher sensitivity. In addition, the method can also measure wave aberration after the lens group is completely centered if necessary, and quantitatively test and feed back the assembly adjustment of the lens group.

The above description of the embodiments of the invention has been presented in connection with the drawings but these descriptions should not be construed as limiting the scope of the invention, which is defined by the appended claims, and any changes based on the claims are intended to be covered by the invention.

Claims

1. The optical centering method based on optical fiber point diffraction interference is characterized by that two optical fiber point light sources are axially interfered, the intersection point of their connecting line and image plane is formed in the position where the optical path difference is maximum, the coordinate of said intersection point can be obtained by processing interference pattern, so that said connecting line can define a virtual reference axis, then the lens is mounted between two point light sources according to the assembling sequence, and the position of the optical path difference extreme point is remained unchanged, so that the lens group can be centered.

2. The optical centering device based on optical fiber point diffraction interference is characterized by comprising a laser, a one-to-two optical fiber beam splitter, optical fibers (a and b), an auxiliary mirror, a thin film spectroscope (BS), piezoelectric ceramics (PZT) and a controller, wherein the optical path is arranged as follows:

the laser is coupled by optical fibers, and two optical fiber point light sources P are formed after beam splitting _a And P _b Interference occurs through BS and forms interference fringes on the camera, point light source P _b The optical fiber is wound on the piezoelectric ceramic phase shifter, and when the phase shifter expands radially, the optical fiber is stretched to introduce phase shift, and the interference pattern on the image plane of the camera is collected and sent to a computer for processing.

3. The optical centering detailed scheme based on the optical fiber point diffraction interference is characterized by comprising the following steps of:

the first step: determining a reference axis: adjusting P _a 、P _b The relative position of the optical path difference extreme point and the camera is used for collecting and processing the coordinates of the optical path difference extreme point, so that the optical path difference extreme point is positioned at the center of an image plane of the camera;

and a second step of: centering the lens group: according to the assembly sequence of the lens group, each lens is respectively fixed between two optical fiber point light sources, the positions and the directions of the lenses are adjusted, so that the extreme points of the optical path difference are always at the center of the image plane of the camera, and the optical axes of the extreme points and the reference axis are proved to coincide, so that the centering of the lens group can be completed.