CN104019762B - High-precision long-range surface shape detector for optical surface - Google Patents

Abstract

The invention discloses a high-precision long-range optical surface profile detector, which comprises a first optical head, a reference mirror and a second optical head, the first optical head is used to scan the optical element to be tested, and the reference mirror is fixedly arranged On the side wall of the first optical head, the second optical head projects a reference beam to the reference mirror, and detects the reference beam reflected by the reference mirror, the first optical head and the second optical head Heads come in different grades of precision. In the present invention, the first optical head and the second optical head are adopted, the first optical head scans and measures the optical element to be tested, and the second optical head performs scanning motion error detection of the first optical head. The two optical heads are based on different detection requirements and Level to set the accuracy, measurement range and beam width, so that the optical components to be tested can be measured more accurately, and it is not easily disturbed by the external environment.

Description

A high-precision long-range optical surface shape detector

technical field

The invention relates to an optical surface shape detector, in particular to the structure of a high-precision, long-range optical surface shape detector.

Background technique

In the fields of scientific research, information technology, aerospace, national defense industry, astronomical observation, etc., especially in the field of synchrotron radiation optical engineering, optical components with extremely high surface accuracy (on the order of 1 nanometer and 10 nanorads) are required. The processing technology of such high-precision optical components largely depends on high-precision surface shape detection technology.

The long-range surface shape detectors commonly used at present are all based on the f-θ optical system for angle measurement, even if the thin beam enters the Fourier transform (FT) lens and uses a line array or area array detector to detect the focal spot pattern on the focal plane of the lens The position of , the position information reflects the angle information of the incident beam, the relationship between the two is:

Δd=f*Δθ,

Where Δd is the movement distance of the pattern position on the detector, f is the focal length of the lens, and Δθ is the angle change of the incident beam.

Before the light beam enters the FT lens, it first translates and scans the surface of the optical element to be tested in a straight line and is reflected by the surface before entering the FT lens. Therefore, the angle change Δθ of the reflected beam is twice the angle change Δα of the sample scanning point during the scanning process: Δd= 2f*Δα is In this way, the angles of different positions on the surface to be measured are obtained to restore its surface shape.

Long-range surface shape detectors are roughly divided into two categories:

One type is a long-range profiler (Long Trace Profiler, hereinafter referred to as LTP) based on a thin laser beam f-θ system. LTP changes the above thin beams into two coherent thin beams split by one laser beam, then an interference pattern can be obtained on the detector instead of the focal spot pattern, which helps to improve the measurement resolution and suppress multiple Some additional interference effects from optical surfaces. LTP is further divided into LTP II and pp-LTP.

The other is nanometer optical metrology (NOM) based on f-θ autocollimator, that is, the above thin beam is generated by the autocollimator through the light-limiting aperture and returned to the autocollimator for detection. The use of white light LED autocollimator can better eliminate the pointing error of the light source, and can also eliminate additional interference. However, due to the limited intensity of the light source, the limited aperture must be opened relatively large to obtain a better signal-to-noise ratio, resulting in a decrease in the spatial resolution of the instrument.

Due to the high-precision long-distance (usually up to 1m) scanning detection, the long-distance surface shape detector is a relatively large-scale experimental testing instrument, including all its accessories, which covers an area of tens of square meters.

Internationally, as early as 1975, the research work of non-contact surface shape detection was carried out. At that time, the measurement method of focusing the laser on the surface to be measured was adopted, and the measurement accuracy was low, and it was not suitable for optical surfaces with large curvature. element. In 1982, Von Bieren proposed a pen-beam interferometer based on the wavefront interference method, which greatly improved the measurement accuracy and scope of application. However, the optical paths of the two beams of coherent light are not equal, which is greatly affected by the unstable factors of the laser and the environment. In 1989, Peter Z. Takacs of BNL in the United States and Shinan Qian of NSRL in Hefei proposed a pen-beam interferometer based on an equal optical path spectroscopic unit and named it LTP, which realized high-precision measurement of optical surfaces. LTP can easily adjust the distance between the two beams to change the spatial period of the interference fringes on the CCD, but since the scanning head uses a light pen to directly scan the structure, the surface shape measurement accuracy is greatly affected by the accuracy of the guide rail; for this reason, the instrument uses a high-precision but mechanical Air bearing guide rail with complex structure and high cost. In 1992, S.C.Irick and W.R.Mckinny of LBL proposed LTP II, which used the reference mirror compensation technology to correct the measurement error caused by the instability of the laser beam pointing in the light pen interferometer; at the same time, it also partially corrected the uneven air temperature and air flow on the measurement accuracy In addition, the improvement of the optical head structure has suppressed most of the scanning motion errors.

The optical structure of LTP II is shown in Figure 1. The laser light source 1 is transformed into two beams with a half-wavelength difference through the phase plate 2, and then divided into two beams by the beam splitter 3. One beam is the measuring beam, which is projected to the test beam. The surface of the optical element 4 is reflected to the FT lens 7 to convert the angle information of the measuring beam into the position information of the focal spot on the area array detector 8; The plane reflector 6 is reflected and returned to the Duff prism 5, and then the reference beam angle information is converted into the focal spot position information on the area array detector 8 by the FT lens 7. The function of the Duff prism 5 is to convert the directivity error of the light source 1 and The scanning motion errors are synthesized and measured together through the same reference optical path.

In 1995, S.N. Qian and others continued to develop LTP, and proposed ppLTP (pentaprism Long TraceProfiler-pentaprism long-range profiler), using a flexible and compact pentagonal prism to scan instead of the overall scan of the optical head of the light pen interferometer, which also makes most of the scanning motion error is suppressed; and the directivity of the laser beam in the interferometer is improved by using the laser fiber collimation technology. Laboratories such as BNL in the United States and Elettra in Italy have established ppLTP.

The optical structure of ppLTP is shown in Figure 2. The laser light source p1 is transformed into two beams with a half-wavelength difference through the phase plate p2, and then divided into two beams by the beam splitter p3. The two reflections are projected onto the surface of the optical element p6 to be tested, and then returned to the pentaprism p5, and then the angle information of the measuring beam is converted into the position information of the focal spot on the area array detector p8 by the FT lens p7. The function of the pentaprism p5 is to make the output The beam and the incident beam maintain a fixed angle, and are not affected by the pitch error of the scanning motion of the pentaprism p5 itself, so ppLTP does not set a scanning motion reference optical path; the other beam is the light source directivity reference beam, which is projected to the plane mirror p4 and then reflected to the FT lens p7 converts the reference beam angle information into the focal spot position information on the area array detector p8.

The NOM device was established by BESSY-II in 2004. The scanning mechanism is the same as that of ppLTP, which uses the insensitivity of the pentaprism to the rotation error. The system structure includes: pentaprism, autocollimator and light bar. The use of white light LED autocollimator can better eliminate the pointing error of the light source, and can also eliminate additional interference. However, due to the limited intensity of the light source, the limited aperture must be opened relatively large to obtain a better signal-to-noise ratio, resulting in a decrease in the spatial resolution of the instrument. The optical structure of NOM is shown in Figure 3. The optical head N100 is an autocollimator fixed on the support of the optical table. The collimated light beam generated is deflected by the pentaprism N5, and then passes through the light-limiting aperture N6 and is projected to the optical element to be tested. The N300 is reflected back and passes through the light-limiting aperture N6, the pentaprism N5 to the autocollimator N100 to detect the angle change. The pentaprism N5 and the adjustable light-limiting aperture N6 constitute the scanning movement part of the N200; the autocollimator N100 contains a white LED light source N1, which is limited by the light-limiting slit N2 as a light source, and then collimated by the lens N4 after passing through the beam-splitting prism N3. It is emitted as a parallel beam; the return beam is converged by the lens N4 and reflected by the beam splitting prism N3, and the focal spot is located on the area array detector N7; the angle change information of the optical element to be tested is reflected by detecting the position change of the focal spot. The function of the pentaprism N5 is to maintain a fixed angle between the outgoing beam and the incident beam, and is not affected by the pitch error of the scanning motion of the pentaprism N5 itself, so NOM does not set a reference optical path for scanning motion; the light source of the autocollimator N100 is stably limited , so NOM does not set the light source directivity reference beam.

The performance of existing long-range surface shape detectors is limited by the following aspects:

1. Due to the large lateral displacement of the optical path on the non-ideal optical components inside the instrument, the measurement accuracy is reduced;

The optical components used inside the instrument are always not ideal (there are aberrations, surface errors, uneven refractive index, etc.), resulting in different additional errors for different positions of the optical components and different angles of incident light, so the thin beam When a large lateral displacement occurs on the optical element, the measurement accuracy of the instrument will be reduced.

There are two reasons for the lateral displacement of the optical path: one is that the beam angle changes so that the light beam with a long optical path sweeps on the optical element, and the other is that the optical path connecting the fixed element and the scanning moving element is not parallel to the direction of the scanning movement, causing the light beam to move on the optical element. Lateral displacement on. Both factors have a strong influence on ppTLP and NOM. For LTP II, in order to separate the reference beam from the measurement beam on the detector, it is required to tilt the reference beam, causing the second, more serious lateral displacement.

2. It is difficult to calibrate the instrument due to the large change in the length of the optical path;

The above problems can theoretically be alleviated by using standard angle generating equipment for calibration and calibration. However, the calibration data must obtain the calibration data of each incident angle at each lateral position of the optical element of the instrument. Since the lateral position at the same incident angle is determined by the optical path length Therefore, when the optical path length is inclined and changes greatly, it is necessary to calibrate the calibration data of different incident angles (two-dimensional angles in space) under different optical path lengths. This is a three-dimensional calibration, which is difficult to achieve because the calibration amount is too large. In addition, when the measurement application after calibration is completed, it is necessary to accurately provide the change of the optical path length in real time to utilize the calibration data, which is not easy to realize. The existing surface shape testing instruments all have the problem of difficult calibration.

3. The angle range of the optical surface suitable for detection is small;

For the existing ppLTP and NOM, due to the large optical path length, when measuring the optical surface with a large angle range, the lateral displacement of the measuring beam is large and causes a large error, so it is not suitable for surface shape measurement with a large angle range .

4. The reference measurement accuracy of scanning motion error and light source pointing error is low.

Existing LTPs use the same optical head to complete optical surface measurement and reference measurement, but the measurement characteristics of the two are quite different. Optical surface measurement requires a large measurement range, while reference measurement requires only a small range of measurements; optical surface measurement requires high spatial resolution with a narrow beam, reference measurement does not have this requirement. An optical head with a large measurement range and high spatial resolution must be sacrificed for measurement accuracy. The existing LTP reference measurement introduces the same optical head to measure the optical surface, resulting in the reduction of the reference measurement accuracy of the LTP scanning motion error and the light source directivity error.

Contents of the invention

In view of the problems existing in the prior art, the object of the present invention is to provide a high-precision long-distance optical surface shape detector with high detection accuracy and strong anti-interference ability.

To achieve the above object, the present invention provides the following technical solutions:

A high-precision long-range optical surface shape detector, including a first optical head, a reference mirror and a second optical head, the first optical head is used to scan the optical element to be tested, and the reference mirror is fixedly arranged on the second optical head On the side wall of an optical head, the second optical head projects a reference beam to the reference mirror, and detects the reference beam reflected by the reference mirror, the accuracy level of the first optical head and the second optical head different.

Further, the optical element to be tested is arranged on an optical platform, the first optical head is arranged above the optical platform, and the second optical head is fixedly arranged.

Further, the optical element to be tested is arranged horizontally, the first optical head is arranged close to the optical element to be tested, and performs a scanning movement in a horizontal direction, and the reference mirror is arranged vertically.

Further, the first optical head is an f-θ system of a thin laser beam, including a laser, a coupling lens, an optical fiber, a collimating lens, a phase plate, a beam splitter, a plane mirror, a Fourier transform lens and an area array detector The laser projects a beam to the beam splitter through a coupling lens, an optical fiber, a collimating lens and a phase plate in sequence, and the beam splitter projects a part of the beam to the plane mirror, and then passes through the The reflection of the beam splitter passes through the Fourier transform lens to the area detector, and the other part projects to the light source element to be measured, and then passes through the reflection of the beam splitter and passes through the Fourier transformation lens. Transform the lens to the area array detector.

Further, the plane mirror is arranged obliquely relative to the light beam projected by the beam splitter.

Further, the first optical head is an autocollimator.

Further, the second optical head is an autocollimator.

Further, the second optical head is an f-θ system of thin laser beams as described above.

Further, the reference mirror is a plane mirror.

Compared with the prior art, the present invention adopts the first optical head and the second optical head in the present invention, the first optical head scans and measures the optical element to be tested, and the second optical head performs scanning motion error detection of the first optical head, The accuracy, measurement range and beam width of the two optical heads are set according to different detection requirements and levels, so that the optical components to be tested can be measured more accurately and are not easily disturbed by the external environment.

Description of drawings

Below in conjunction with accompanying drawing, the present invention is described in further detail:

Figure 1 is a schematic diagram of the existing LTP II optical structure;

FIG. 2 is a schematic diagram of an existing ppLTP optical structure;

FIG. 3 is a schematic diagram of an existing NOM optical structure;

Fig. 4 is a schematic structural diagram of the high-precision long-distance optical surface shape detector of the present invention.

detailed description

Typical embodiments embodying the features and advantages of the present invention will be described in detail in the following description. It should be understood that the present invention is capable of various changes in different embodiments without departing from the scope of the present invention, and that the description and drawings therein are illustrative in nature and not limiting. this invention.

As shown in FIG. 4 , the high-precision long-distance optical surface profile detector of the present invention includes a first optical head 100 , a reference mirror 300 and a second optical head 200 . Wherein, the first optical head 100 is used to scan the optical element 400 to be tested, the reference mirror 300 is fixedly arranged on the side wall of the first optical head 100, the second optical head 200 projects a reference beam to the reference mirror 300, and detects the reference mirror 300 For the reflected reference beam 600 , the accuracy levels of the first optical head 100 and the second optical head 200 are different.

In the present invention, the optical element 400 to be tested is arranged on an optical platform (not shown in the figure), the first optical head 100 is arranged above the optical platform, and the second optical head 200 is fixedly arranged. The optical element 400 to be tested is arranged horizontally, the first optical head 100 is arranged close to the optical element 400 to be tested, and performs a scanning movement along the horizontal direction, and the reference mirror 300 is arranged vertically.

In the present invention, the first optical head 100 can be an autocollimator, or an f-θ system of a thin laser beam. As shown in FIG. 4 , in this embodiment, the first optical head 100 is an f-θ system of a thin laser beam. Specifically, the first optical head 100 includes a laser 101, a coupling lens 102, an optical fiber 103, a collimating lens 104, a phase plate 105, a beam splitter 106, a plane mirror 107, a Fourier transform lens 108, and an area array detector 109 , laser 101 , coupling lens 102 , optical fiber 103 , collimator lens 104 , phase plate 105 , beam splitter 106 , plane mirror 107 , Fourier transform lens 108 and area array detector 109 are all placed in housing 110 . The laser 101 projects the light beam 500 to the beam splitter 106 through the coupling lens 102, the optical fiber 103, the collimating lens 104 and the phase plate 105 in sequence, and the beam splitter 106 projects a part of the light beam 500 to the plane mirror 107, and then passes through the beam splitter 106 Reflected by the Fourier transform lens 108 to the area array detector 109, the other part is projected to the light source element 400 to be measured, then reflected by the beam splitter 106 and passed through the Fourier transform lens 108 to the area array detector 109 . The plane mirror 107 is arranged obliquely relative to the beam 500 projected by the beam splitter 106 .

In the present invention, the second optical head 200 can be an autocollimator, or an f-θ system of thin laser beams as described above. In this embodiment, as shown in FIG. 4 , the second optical head is an autocollimator. In this embodiment, the reference mirror 300 is a plane mirror.

In this embodiment, the first optical head 100 is an f-theta measurement optical head of a thin laser beam with a built-in light source directivity reference optical path. The light source of the head 100, through the phase plate 105, becomes a light beam with a half-wavelength difference between the two halves, and then is divided into two beams by the beam splitter 106, and one beam is a measuring beam, which is projected onto the surface of the optical element 400 to be tested and then reflected to the Fourier Transformation (FT) lens 108 converts the measurement beam angle information into focal spot position information on the area array detector 109; the other beam is the light source directivity reference beam, which is projected onto the plane reflector 107 fixed inside the first optical head 100 through Reflected to the Fourier transform (FT) lens 108 to convert the angle information of the reference beam into the position information of the focal spot on the area array detector 108 . The second optical head 200 adopts a high-precision and small-range wide-beam autocollimator, which is vertically projected to the reference mirror 300 fixed on the first optical head 100 along the scanning motion direction, and the reflected light returns to the autocollimator parameter first optical head 100 scan motion error.

Compared with prior art, the beneficial effect of the present invention is:

1. The lateral displacement of the optical path on the non-ideal optical components inside the instrument is greatly reduced, thereby improving the measurement accuracy. Specifically include the following aspects:

1) Low lateral displacement of the measuring beam. Compared with the existing ppLTP and NOM, the measurement optical path length of the detector of the invention is very short, which greatly reduces the lateral displacement caused by the change of the measurement beam angle during the scanning process. In addition, the measurement optical path length of the detector of the present invention is almost fixed (the change is only the height change of the mirror to be measured), which almost eliminates the lateral displacement of the inclined measurement beam during the scanning process of the optical path length change.

2) Low lateral displacement of the source pointing reference beam. The light source directional reference beam is completely confined inside the first optical head 100, which is also a very short fixed-length optical path, and the lateral displacement of the optical path has little effect on the accuracy of the instrument so that it can be completely ignored.

3) Low lateral displacement of the scanning motion reference beam. Since the second optical head 200 is dedicated to complete the reference measurement of the scanning motion, the reference beam is designed to be non-inclined, that is, completely parallel to the direction of the scanning motion. Compared with the inclined reference beam design of LTP II, the lateral direction of the scanning motion reference beam Displacement is almost completely eliminated.

2. The instrument is easy to calibrate and calibrate;

All optical path lengths in the first optical head 100 of the present invention are basically fixed, which is easy to calibrate. Although the optical path length of the second optical head 200 changes, there is almost no lateral displacement due to the non-inclined light beam, so it is not necessary to consider the influence of the optical path length change during calibration, which is easy to calibrate.

3. It can be applied to optical surface detection in a wide range of angles;

In the present invention, the measuring optical path is very short, and the lateral displacement caused by the same angle change of the measuring point is very small, so it can be used to detect optical surfaces with a wide range of angles.

4. The reference measurement accuracy of scanning motion error and light source directivity error is high.

In the present invention, the light source directivity reference optical path is completely enclosed inside the first optical head 100, the optical path length is short and fixed, there is no lateral displacement, and it is not easily affected by environmental instability, so the measurement accuracy of light source directivity is greatly improved. The non-tilted scanning motion reference beam of the second optical head 200 of the present invention basically completely eliminates the lateral displacement. At the same time, the beam uses a wide beam, which is not easily affected by environmental instability and lateral displacement, so the scanning motion measurement accuracy is greatly improved.

The technical solution of the present invention has been disclosed by the preferred embodiments as above. Those skilled in the art should realize that changes and modifications made without departing from the scope and spirit of the present invention disclosed by the appended claims of the present invention are within the protection scope of the claims of the present invention.

Claims (8)

1. a kind of high accuracy long-range Optical Surface detector is it is characterised in that include the first optical head, reference mirror and second Optical head, described first optical head is used for scanning optical element to be measured, and described reference mirror is fixedly installed on described first optical head Side wall on, described second optical head projects reference beam to described reference mirror, and detects the reference light of described reference mirror reflection Bundle, described first optical head is different from the accuracy class of described second optical head, and described first optical head is the f- θ of narrow laser beam System, saturating including laser instrument, coupled lens, optical fiber, collimation lens, phase board, beam splitter, plane mirror, Fourier transformation Mirror and planar array detector, described laser instrument passes sequentially through coupled lens, optical fiber, collimation lens and phase board and throws to described beam splitter Irradiating light beam, a described light beam part is projected to described plane mirror by described beam splitter, then passes through described beam splitter again Reflect and pass through described Fourier transform lens to described planar array detector, another part is projected to described optical element to be measured, Then again by the transmission of described beam splitter and by described Fourier transform lens to described planar array detector, described second light Learn head vertically to throw to the described reference mirror being fixed on described first optical head along scanning motion direction.

2. high accuracy long-range Optical Surface detector as claimed in claim 1 is it is characterised in that described treat photometry unit Part is arranged on an optical table, and described first optical head is arranged above described optical table, and described second optical head is fixed Setting.

3. high accuracy long-range Optical Surface detector as claimed in claim 2 is it is characterised in that described treat photometry unit Part is horizontally disposed with, and described first optical head is arranged close to described optical element to be measured, and is scanned in the horizontal direction moving, institute State reference mirror to be vertically arranged.

4. high accuracy long-range Optical Surface detector as claimed in claim 1 is it is characterised in that described plane mirror The light beam of relatively described beam splitter projection is obliquely installed.

5. described high accuracy long-range Optical Surface detector as arbitrary in claim 1-3 is it is characterised in that described second Optical head is an autocollimator.

6. described high accuracy long-range Optical Surface detector as arbitrary in claim 1-3 is it is characterised in that described second Optical head is the f- θ system of described narrow laser beam.

7. described high accuracy long-range Optical Surface detector as arbitrary in claim 1-3 is it is characterised in that described reference Mirror is plane mirror.

8. a kind of high accuracy long-range Optical Surface detector is it is characterised in that include the first optical head, reference mirror and second Optical head, described first optical head is used for scanning optical element to be measured, and described reference mirror is fixedly installed on described first optical head Side wall on, described second optical head projects reference beam to described reference mirror, and detects the reference light of described reference mirror reflection Bundle, described first optical head is different from the accuracy class of described second optical head, and described first optical head is an autocollimator, institute State the second optical head vertically to throw to the described reference mirror being fixed on described first optical head along scanning motion direction.