Oblique incidence phase shift interferometer and rectangular prism large surface measurement methodTechnical Field
The invention relates to the field of optical and metal surface shape precision measurement, in particular to an oblique incidence phase shift interferometer and a rectangular prism large surface measurement method.
background
with the development of optical cold processing technology, the matched surface shape measurement also depends on the automatic measurement of digital wave surface mainly by phase-shift interferometer.
the prism is an optically cold-worked product, which is the largest amount except for the lens, and is widely used in various optical systems. At present, microprisms are also different military projections and are gradually used in a mobile phone camera system as a steering device as shown in fig. 1, the microprisms are large in batch and high in quality requirement, and optical cold processing enterprises feel great pressure. As a plane optical element, the surface shape precision of each surface of a right-angle prism is the most important index, wherein the right-angle surface can be checked by using a traditional interferometer, the large surface is interfered by total internal reflection light, and at present, only a template contact type measuring method can be adopted, so that a large amount of labor is consumed, and the surface smoothness is reduced. As shown in fig. 2, the cause of such interference formation is expressed: the two right-angle surfaces of the right-angle prism perform total internal reflection on the light rays for two times, the reflected light intensity (90%) of the right-angle prism is far greater than that (4%) of the measured large surface, and the two are parallel and cannot be separated.
When the requirement on the processing precision of the early right-angle prism is low, an enterprise measures the large surface of the large right-angle prism by adopting an interferometer visual observation method, and the principle is that the angle error of the right-angle prism is large, and the total internal reflection light and the large surface vertical reflection light have an error angle as shown in figure 3, so that a part of large surface interference fringes can be seen by a method of continuously shielding and probing. The method needs experienced inspectors to estimate and read through scattered stripes, is low in efficiency and poor in accuracy, and cannot be applied to mass production of mobile phone microprisms. And after the right-angle prism machining precision is improved to a certain degree, the area which can be observed by shielding and probing is smaller and smaller until the area is completely overlapped, at the moment, the surface shape of a large surface cannot be seen by a conventional interferometer, and digital measurement such as phase-shift interference is impossible.
Oblique incidence measurement of phase-shifting interferometers is generally used to measure elongated pieces of material beyond the interferometer's aperture, in which collimated light from the interferometer is reflected twice from the surface to be measured of the lapping surface flat, thus resulting in a W test result for the lapping surface flatL(x, y) direct acquisition of wavefront W rather than phase-shifting interferometer0(x, y), but need to be scaled by a scaling factor related to, for example, the incident light angle θ:
for general tests, accurate measurement of the angle of incidence is cumbersome and is also unfriendly to the measurer when viewed visually, since the aperture of the screen displaying the interferogram does not coincide with the actual aperture and is also scaled.
Disclosure of Invention
The purpose of the invention is as follows: the invention aims to provide a method for measuring a large-area right-angle prism by avoiding the interference of total internal reflection of the right-angle prism and an oblique incidence phase-shift interferometer.
the technical scheme is as follows: an oblique incidence phase shift interferometer comprises an interferometer, a transmission flat crystal, a reflection flat crystal and a right-angle prism test objective table; the transmission flat crystal is vertical to the emergent wavefront of the interferometer; the right-angle prism is placed on the right-angle prism test object stage, and the right-angle prism can rotate relative to the right-angle prism test object stage; the transmission flat crystal and the reflection flat crystal are separated on two sides of the right-angle prism; the emergent wavefront of the interferometer vertically passes through the transmission flat crystal and obliquely enters the large surface of the right-angle prism, and then the wavefront is reflected to the reflection flat crystal; the reflecting flat crystal vertically reflects the wave front and returns the reflected wave front along the original optical path, and forms interference fringes with the reflected wave front of the transmitting flat crystal to be received by the interferometer.
Specifically, the oblique incidence measurement of the outgoing wavefront of the interferometer adopts a 60-degree large-surface incidence angle designed by a fixed structure, so that the proportionality coefficient in the formula 1 can be realized
The method ensures that the phase-shift interference result can be directly used, and the vertical double-fold compressed display of the interference image is adopted on the software interface, so that the shape and the interference image of the displayed visual measured piece are completely the same as those under the normal incidence condition, and the observation habit of a measurer is kept unchanged. In addition, the interference cavity integrated structure adopted by the invention also ensures the accurate measurement and correction of the incident angle and the proportionality coefficient before use. Therefore, the included angle between the interferometer and the large surface of the right-angle prism is 60 degrees.
Specifically, the right-angle prism test object stage comprises a fixed two-dimensional adjusting frame, and a hole is formed in the two-dimensional adjusting frame; the right-angle prism test objective table also comprises a rectangular hole connecting piece, and the rectangular hole connecting piece is connected in the opening of the two-dimensional adjusting frame and can rotate; the right-angle prism matching jig is fixed in the rectangular hole connecting piece, and the large surface of the right-angle prism is aligned to the light path.
Specifically, the interferometer, the transmission flat crystal, the reflection flat crystal and the right-angle prism test object stage are all fixed on the integrated bottom plate; the large plane of the two-dimensional adjusting frame, the transmission flat crystal and the reflection flat crystal are vertical to the integrated bottom plate; the emergent wavefront of the interferometer is parallel to the integrated bottom plate.
specifically, the transmission flat crystal and the reflection flat crystal are symmetrically arranged on two sides of the right-angle prism.
The invention also discloses a method for measuring the large surface of the right-angle prism by using the oblique incidence phase-shift interferometer, which comprises the following steps:
1) switching the interferometer to a point-to-point mode, and firstly, seeing that a reflection light spot of a transmission flat crystal is positioned at the center of the cross reticle;
2) placing the tested right-angle prism on a matched jig of a right-angle prism test objective table, wherein the emergent wave of the interferometer is obliquely emitted into the large surface of the right-angle prism, and the reflected light spot cluster of the right-angle prism can be seen on a monitoring screen; the micro-rotation rectangular hole connecting piece outer frame is provided with a pair of light spot clusters which symmetrically move outwards and a group of independent light spot clusters which are fixed;
3) Adjusting a two-dimensional adjusting frame, and adjusting original immobile independent light spot clusters to be between cross partitions so that central light spots of the light spot clusters coincide with reflection light spots of reflection flat crystals;
4) Slightly rotating the outer frame of the matched jig to enable the symmetrical light spot clusters to leave the center and reach the edge of the view field;
5) Switching the interferometer to a test state, and adjusting the inclination of the two-dimensional adjusting frame to enable the number of interference fringes to meet the phase shift measurement requirement;
6) Visual fringe interpretation or phase shift interferometry is performed.
specifically, the emergent wavefront of the interferometer in the step 2) is obliquely emitted into the large surface of the right-angle prism at an angle of 60 degrees.
The principle of the method for measuring the large surface of the right-angle prism is as follows: the method comprises the following steps of carrying out 3D deflection on light rays obliquely incident into a measured right-angle prism at an angle of 60 degrees in the emergent wavefront of an interferometer, firstly, obliquely incident collimated light emitted by the interferometer onto the large surface of the prism at an angle of 60 degrees, returning a light original path through a high-reflectivity reference reflector (namely, a reflective flat crystal), and further realizing double-beam interference with reflected light of a standard transmissive flat crystal; and secondly, deflecting the edge line of the right-angle prism and the incident light plane by a small angle, so that the total internal reflection light of the right-angle prism deviates from the incident plane and is separated from the large-plane reflection light. In the process, the light clusters related to the total internal reflection of the right-angle prism are separated into the incident plane and the incident plane, and finally the light is blocked by the aperture light in the interferometer and cannot enter the CCD to form interference light.
The above-mentioned right angle prism inside light 3D deflection that relates to, its principle is: after the interference light beam cluster which is incident to the measured right-angle prism by the interferometer is obliquely incident and deflected by the ridge line, most energy of the interference light beam cluster is deflected out of an incident surface and does not enter an image acquisition system of the interferometer. As shown in fig. 4 (definition of reflection surface) and fig. 5, the right-angle reflection surfaces of the right-angle prism are a and B, the large surfaces of the right-angle prism are C (outer reflection surface) and D (inner reflection surface), and the reference mirror surface is R. Then the effective ray cluster (one order of magnitude or more of the intensity of the light reflected from the large surface, which may interfere with the measurement interferogram) is shown in table 1.
TABLE 1 construction of light clusters
note: the table lists only the resulting clusters of light during one-way transmission, which actually need to go back and forth once each time the interferometer is returned, thus doubling the complexity of the clusters.
When the rotation angle of the edge line of the measured right-angle prism relative to the incident plane exceeds the field angle of the interferometer, the reflection direction of the large surface to the light is unchanged, such as a light beam 1; whereas the primary reflections related to the right-angled surfaces all deviate from the incident and exit planes, e.g. beams 2, 3; even when reflected by the mirror RF back into the interferometer again, it is blocked by the internal aperture stop.
The light emitted from the right-angle prism for the second time is reflected once by the large-surface inner surface (D), so the emitting direction of the light is independent of the right-angle error and the rotation of the ridge line, and the light becomes the source of background interference light. However, the secondary reflected light becomes "tolerable" interference light for several reasons:
1) the edge difference of the right-angle prism will cause the symmetrical separation of the forward interference light (CR-BA-D-AB) and the backward interference light (AB-D-BA-RC) from the large-area reflected light as shown in FIG. 6. It has an angle phi with the large surface reflection light (CRC)2＝asin(n·sin(φ1+4α)) (2)
In the formula, phi1Is the angle of incidence, phi2Is the exit angle, n is the refractive index of the prism material, and α is the edge difference. The relationship between the error of the exit angle and the edge difference when the incident angle is 60 ° is shown in fig. 7. It can be seen that the higher the refractive index, the more significant the exit angle error. It can be calculated that when the edge difference is larger than 1', the emergent light can be shifted out of the receivable range of the interferometer, which is why the rectangular prism with low processing precision can be directly measured on the common interferometer, but the processing precision cannot be improved.
2) The interference light entering the oblique incidence interferometer is the light beams after multiple multi-surface reflection (for example, the light beams 4, 5, 6 and 7 undergo transmission/reflection of 10 surfaces and transmission inside a prism twice in one turn, and the elements of the light beams 8 and 9 are higher by 16), the light intensity is smaller than that of the large-surface reflection light, and the light beams have superposition of multi-surface wave front errors, and a multi-beam interference fringe formed by the light beams and the large-surface reflection light is a fine fringe, and the influence on phase shift interferometry is about 0.02-0.03 lambda. The measurement accuracy can be regarded as a small error compared with the 0.1 lambda of the large-area measurement accuracy.
has the advantages that: the oblique incidence phase shift interferometer and the method for measuring the large surface of the right-angle prism can accurately measure the large surface precision of the right-angle prism, the interferometer can be used for large surface measurement, other auxiliary tools are not needed, and the smooth finish of the large surface cannot be influenced; the method for measuring the large surface of the right-angle prism has the advantages of not contacting the surface of a measured piece, having good stability of interference fringes, reducing the reflection interference of the right-angle surface to an acceptable degree and the like, and is suitable for the rapid detection of the large-scale large surfaces of the right-angle prism strips (roots) and the microprisms.
drawings
FIG. 1 is a right angle prism of the present invention;
FIG. 2 is a total internal reflection ray diagram of a right angle prism;
FIG. 3 is a graph of total internal reflection ray shift due to right angle error of a right angle prism;
FIG. 4 is a reflection surface definition of a ray cluster;
FIG. 5 is a view of the part of the ray cluster orientation under the parallel ridges;
FIG. 6 is a perspective view of a portion of the light cluster under the parallel ridges;
FIG. 7 is a graph of the effect of edge differences on the orientation of a light cluster;
FIG. 8 is a graph of edge difference versus exit angle error;
FIG. 9 is a schematic diagram of a tilted-incidence phase-shifting interferometer configuration;
FIG. 10 is a schematic perspective view of a rectangular prism test stage of an oblique incidence phase-shifting interferometer;
FIG. 11 is a bottom view of a rectangular prism test stage of an oblique incidence phase-shifting interferometer;
Fig. 12 is a schematic view of a rectangular hole connecting member.
Detailed Description
the invention is further described below with reference to the figures and examples.
With reference to fig. 1 to 7, a method for measuring a rectangular prism by oblique incidence, the measuring method comprising the following processes:
1) switching the interferometer 1 to a point-to-point mode, and firstly, seeing that a reflection light spot of the transmission flat crystal 2 is positioned at the center of the cross reticle;
2) placing the tested right-angle prism 5 on a matched jig 7 of a right-angle prism test object stage 4, and enabling collimated light of the interferometer 1 to be incident on the large surface of the right-angle prism 5 at an angle of 60 degrees, so that a reflected light spot cluster of the right-angle prism 5 can be seen on a monitoring screen; the micro-rotating rectangular hole connecting piece 42 has an outer frame, a pair of light spot clusters symmetrically move outwards, and a group of independent light spot clusters are fixed;
3) adjusting a two-dimensional adjusting frame 41, adjusting the original immobile independent light spot cluster to be between the cross partitions, and enabling the central light spot to be superposed with the reflection light spot of the reflection flat crystal 3;
4) Slightly rotating the outer frame of the matched jig 7 to enable the symmetrical light spot clusters to leave the center and reach the edge of the view field;
5) Switching the interferometer 1 to a test state, and adjusting the inclination of the two-dimensional adjusting frame 41 to make the number of interference fringes meet the phase shift measurement requirement;
6) Performing visual fringe interpretation or phase shift interferometry;
7) The interference image display area of the monitor or the computer display compresses the image by half in a single direction, so that the proportion of the right-angle prism in the displayed image is consistent with the actual proportion, and observation and judgment in the measuring process are facilitated.
an oblique incidence phase shift interferometer comprises an interferometer 1, a transmission flat crystal 2, a reflection flat crystal 3 and a right-angle prism test objective table 4; the transmission flat crystal 2 is vertical to the emergent wavefront of the interferometer; the right-angle prism 5 is placed on the right-angle prism test objective table 4 and can rotate relative to the right-angle prism test objective table 4; the transmission flat crystal 2 and the reflection flat crystal 3 are separated from two sides of the right-angle prism 5; the emergent wave front of the interferometer 1 vertically passes through the transmission flat crystal 2, is obliquely incident on the large surface of the right-angle prism 5, and then is reflected to the reflection flat crystal 3; the reflecting plate 3 reflects the wavefront vertically and returns the reflected wavefront along the original optical path, and forms interference fringes with the reflected wavefront transmitted through the plate 2 to be received by the interferometer 1.
FIG. 9 is a schematic diagram of a tilted incidence phase-shifting interferometer according to the present invention. Collimated light emitted by the phase-shift interferometer is partially reflected on the working surface of the transmission flat crystal 2 to become a reference wavefront; other light wave fronts are incident to the large surface of the tested right-angle prism 5 at 60 degrees, are reflected to reflect the flat crystal 3, then return along the original optical path, and enter the interferometer to become the tested wave fronts. The reference wave front and the measured wave front form interference fringes, and a phase shift interference method can also be adopted for digital measurement. The configuration shown in fig. 8 is inverted, with the piece to be measured placed above, and is suitable for measuring larger size rectangular prism strips; the micro-prism can also be turned over to form a bottom-mounted micro-prism product which is suitable for measuring the micro-prism product after cutting processing.
therefore, in order to enable collimated light emitted by the interferometer to enter the large surface of the right-angle prism 5 to be measured at 60 degrees, the included angle between the interferometer 1 and the large surface of the right-angle prism 5 is 60 degrees.
As shown in fig. 10 to 12, the rectangular prism test stage 4 includes a fixed two-dimensional adjusting frame 41, and a circular hole is formed in the two-dimensional adjusting frame 41, and the size of the circular hole should be more than twice of the aperture of the interferometer and the installation allowance of the supporting jig 7 is reserved; because of the 60 ° oblique incidence, the actual effective measurement area of the interferometer 1 is an ellipse whose major axis is twice the aperture of the interferometer. The rectangular prism test object stage 4 further comprises a circular rectangular hole connecting piece 42, the purpose of the connecting piece is to allow the tested piece (the rectangular prism 5) to rotate slightly to obtain a necessary ridge included angle theta, and the rectangular hole connecting piece 42 is connected in the opening of the two-dimensional adjusting frame 41 and can rotate; the jig 7 matched with the right-angle prism 5 is fixed in the rectangular hole connecting piece 42, and the large surface of the right-angle prism 5 is aligned with the light path.
in interferometry, the source of vibration is wide, but only that part of the cavity length of the interference cavity is affected to cause the interference fringes to dither. Thus, the optical parts constituting the interference cavity are reinforced in rigidity, and the influence of vibration can be suppressed or even eliminated. After the reflection flat crystal 3 is separated from the interferometer host, the reflection flat crystal has smaller mass and volume and is easier to be rigidly and directly fixed; in addition, after the bottom plate of the interference cavity does not bear the interferometer, the smaller area is not easy to generate warping vibration. The bottom plate is fixed on the whole frame of the equipment through flexible connection, and the transmission of high-frequency vibration is also cut off. The interferometer 1, the transmission flat crystal 1, the reflection flat crystal 3 and the right-angle prism test objective table 4 are all fixed on an integrated bottom plate 6; the large plane of the two-dimensional adjusting frame 41, the transmission flat crystal 2 and the reflection flat crystal 3 are vertical to the integrated bottom plate 6; the interferometer emergent wavefront is parallel to the integrated base plate 6. The transmission flat crystal 1 and the reflection flat crystal 3 are symmetrically arranged on two sides of the right-angle prism 5 and are separated from the interferometer 1. The jig two-dimensional adjusting frame 41 of the measured part is also fixed on the same integrated bottom plate 6 through two right-angle side blocks 43 to form a rigid structure.
fig. 10 and 11 are schematic diagrams of a device under test tool of an oblique incidence phase-shifting interferometer according to the present invention. It consists of two parts: the circular rectangular hole connecting piece 42 can rotate slightly in the two-dimensional adjusting frame 41, and has the function of screwing the edge line of the measured right-angle prism 5 out of the incident plane. The inner frame is a replaceable matched jig 7 manufactured according to the size of the measured right-angle prism, and a V-shaped jig is manufactured under the condition that the interferometer 1 is arranged on the inner frame and is used for placing the right-angle edge of the right-angle prism 5 on the jig, and the large surface of the right-angle prism is upward; and under the condition that the interferometer is arranged below, a three-point supporting type jig is manufactured and used for directly placing the large surface of the right-angle prism to enable the large surface to be downwards aligned with the light path.