Multiple Reflection Ultra-Fast High-Precision Interferometer and Surface Shape Measurement Method

Abstract

The present invention discloses a multiple-reflection ultrafast high-precision interferometer and a surface shape measurement method. The interferometer includes a short-pulse laser source, an optical energy adjustment device, a linear polarizer, a half-wave plate, a polarization beam splitter prism, a first quarter-wave polarization plate, a second quarter-wave polarization plate, a first reference standard mirror, a second reference standard mirror, a standard mirror, a polarization device, a framing camera, and a controller. The framing camera and the short-pulse laser are both connected to the controller. The short-pulse laser emitted by the short-pulse laser source is emitted from the light source and then sequentially passes through the optical energy adjustment device, the linear polarizer, and the half-wave plate and then enters the polarization beam splitter prism, and is decomposed into two beams of light to enter the measurement branch and the reference branch. The present invention increases the detection sensitivity by using the method of multiple reflections and oscillations of the short-pulse laser in the equal optical path cavity, and uses a framing camera with a delay synchronization shutter to achieve time gating of the multiple-reflection interference pulse signal, so as to realize the surface shape detection of the optical mirror at the picometer level.

Description

Technical Field

[0001] The present invention belongs to the technical field of interferometers, and relates to a multi-reflection ultrafast high-precision interferometer and a surface shape measurement method. Background Art

[0002] Ultra-high-precision optical mirror detection equipment can be used to manufacture ultra-high-precision objectives for extreme ultraviolet (EUV) lithography machines, and high-precision soft and hard X-ray synchrotron radiation optical elements. Current solutions include methods such as Fizeau interferometers in the optical band, point diffraction interferometry, Shack-Hartmann sensors, etc. However, their detection accuracy is limited by photoelectric noise, coherent noise, air disturbances, vibration interference, etc. Even under extremely harsh environmental conditions, their detection repeatability can only reach the sub-nanometer level. The detection accuracy of the current highest-precision commercial interferometer only reaches an RMS of 0.1 nm. Summary of the Invention

[0003] The purpose of the present invention is to provide a multi-reflection ultrafast high-precision interferometer and a surface shape measurement method to solve the technical problems of low detection accuracy, coherent noise interference, and mechanical vibration interference existing in the prior art.

[0004] To achieve the above purpose, the present invention adopts the following technical solutions to solve:

[0005] On the one hand, the present invention provides a multi-reflection ultrafast high-precision interferometer, including a short-pulse laser source, an optical energy adjustment device, a linear polarizer, a half-wave plate, a polarization beam splitter prism, a first quarter-wave polarization plate, a second quarter-wave polarization plate, a first reference standard mirror, a second reference standard mirror, a standard mirror, a polarization device, a framing camera, and a controller. The framing camera and the short-pulse laser are both connected to the controller. The first reference standard mirror, the second reference standard mirror, the measured mirror, and the standard mirror are all plane mirrors, or the first reference standard mirror and the second reference standard mirror are both plane mirrors, and the measured mirror and the standard mirror are both spherical mirrors; where:

[0006] The short-pulse laser emitted by the short-pulse laser source enters the polarization beam splitter prism after passing through the optical energy adjustment device, the linear polarizer, and the half-wave plate in sequence after being emitted from the light source, and is decomposed into two beams of light with polarization directions parallel and perpendicular to the main section of the polarization beam splitter prism respectively. The beam parallel to the main section of the polarization beam splitter prism enters the measurement branch, and the beam perpendicular to the main section of the polarization beam splitter prism enters the reference branch;

[0007] The light wave entering the measurement branch becomes a circularly polarized light after passing through the first quarter-wave plate, enters the oscillating optical cavity formed by the reference mirror and the measured mirror to be measured after passing through the reference mirror, and the light wave is reflected multiple times in the oscillating optical cavity. Each time it returns from the measured mirror and passes through the reference mirror, a measured echo signal is formed, thus obtaining a series of measured echo signals; the measured echo signal becomes a linearly polarized light after passing through the first quarter-wave plate, and its polarization direction rotates 90 degrees compared with when the light beam first passes through the first quarter-wave plate. This linearly polarized light enters the polarization beam splitter prism and is reflected;

[0008] The light wave entering the reference branch becomes a circularly polarized light after passing through the second quarter-wave plate, enters the oscillating optical cavity formed by the first reference mirror and the second reference mirror after passing through the first reference mirror, and the light wave is reflected multiple times in the oscillating optical cavity. Each reflection on the surface of the second reference mirror forms a reference light pulse signal respectively, thus obtaining a series of reference light pulse signals; the reference light pulse signal becomes a linearly polarized light after passing through the second quarter-wave plate, and its polarization direction rotates 90 degrees compared with when the light beam first passes through the second quarter-wave plate, and then passes through the polarization beam splitter prism;

[0009] The optical path from the standard surface of the first reference mirror to the splitting surface of the polarization beam splitter prism is equal to the optical path from the standard surface of the reference mirror to the splitting surface of the polarization beam splitter prism; the optical path of the optical cavity formed by the first reference mirror and the second reference mirror is equal to or an integer multiple ratio of the optical path of the optical cavity formed by the reference mirror and the measured mirror to be measured;

[0010] The measured echo pulse signal and the reference light pulse signal at the same moment are combined after passing through the polarization beam splitter prism, and then form multiple interference images after passing through the polarization device composed of multiple polarizers with different polarization directions and are acquired by the frame camera.

[0011] Further, the light energy adjustment device includes at least two attenuation sheets, and the transmittance of one of the attenuation sheets is variable.

[0012] Further, a first collimating mirror is provided between the short-pulse laser source and the light energy adjustment device.

[0013] Further, the first collimating mirror includes two optical lenses and a small hole between them.

[0014] Further, a second collimating mirror is provided between the polarization beam splitter prism and the polarization device.

[0015] Further, the second collimating mirror includes two optical lenses and a small hole between them.

[0016] Further, reflective films are coated on the standard surfaces of the first reference mirror and the reference mirror.

[0017] On the other hand, the present invention also provides a method for measuring the surface shape by multiple reflections. This method is based on the multiple-reflection ultrafast high-precision interferometer of the present invention. In the multiple-reflection ultrafast high-precision interferometer, the first reference standard mirror, the second reference standard mirror, the measured mirror, and the standard mirror are all plane mirrors. The specific steps are as follows:

[0018] Step 1: Set the delay time and the shutter time width of the framing camera. The controller receives the pulse signal sent by the short-pulse laser source in real time as the trigger signal, and controls the framing camera to obtain the interference fringes according to the delay time and the shutter time width. Fix the position of the standard mirror, and adjust the optical axis direction and position of the first reference standard mirror so that the optical path from the standard surface of the first reference standard mirror to the beam-splitting surface of the polarization beam splitter PBS is equal to the optical path from the standard surface of the standard mirror to the beam-splitting surface of the PBS until the interference fringes obtained by the framing camera are zero fringes;

[0019] Among them, the delay time = zero reflection optical path / light speed; among them, the zero reflection optical path is the total optical path from the short-pulse laser to the framing camera when the light beam does not enter the oscillating optical cavity, and the shutter time width is not less than the laser pulse width and less than the laser pulse interval;

[0020] Step 2: Reset the delay time. The delay time = (zero reflection optical path + number of reflections * reflection optical path) / light speed, where the reflection optical path = cavity interval * 2 * air refractive index; the number of reflections is selected as 1;

[0021] Adjust the cavity interval between the measured mirror and the standard mirror to be equal to the cavity interval between the first reference standard mirror and the second reference standard mirror so that the modulation degree of the interference fringes of the framing camera is the highest; then adjust the angle between the second reference standard mirror and the measured mirror so that the interference fringes obtained by the framing camera are zero fringes;

[0022] Step 3: Reset the delay time. The delay time = (zero reflection optical path + number of reflections * reflection optical path) / light speed, where the reflection optical path = cavity interval * 2 * air refractive index; the number of reflections is selected as a natural number, generally 5 to 100 times;

[0023] Step 4: Adjust the light energy adjustment device and the rotation angle of the 1 / 2 wave plate to make the modulation degree of the interference pattern formed on the focal plane of the framing camera the largest;

[0024] Step 5: When adjusting the cavity interval between the measured mirror and the standard mirror or the cavity interval between the first reference standard mirror and the second reference standard mirror, make the modulation degree of the interference pattern the largest;

[0025] Step 6: The controller controls the framing camera to collect the interference image, and performs data processing on the interference image to obtain the interference wavefront;

[0026] Step 7: The controller removes the influence of the reference mirror on the wavefront data from the interference wavefront obtained in Step 6 to obtain the surface shape of the measured mirror.

[0027] Step 8: Gradually increase the number of reflections to obtain a new delay time, and return to Step 3 until the required measurement accuracy is achieved.

[0028] Further, before the said Step 1, place the multiple-reflection ultrafast high-precision interferometer under vacuum, low pressure or stable constant-temperature laminar air conditions.

[0029] On the other hand, the present invention also provides a surface shape measurement method using multiple reflections, which is based on the multiple-reflection ultrafast high-precision interferometer of the present invention. In the multiple-reflection ultrafast high-precision interferometer, both the first reference standard mirror and the second reference standard mirror are plane mirrors, and both the measured mirror and the standard mirror are spherical mirrors. The method specifically includes the following steps:

[0030] Step 1: Set the delay time and the shutter time width of the frame camera. The controller receives the pulse signal sent by the short-pulse laser source in real time as the trigger signal, and controls the frame camera to obtain interference fringes according to the delay time and the shutter time width. Fix the position of the standard mirror, and adjust the optical axis direction and position of the first reference standard mirror so that the optical path from the standard surface of the first reference standard mirror to the splitting surface of the polarization beam splitter is equal to the optical path from the standard surface of the standard mirror to the splitting surface of the polarization beam splitter until the interference fringes obtained by the frame camera are zero fringes.

[0031] Wherein, the delay time = zero reflection optical path / light speed; wherein, the zero reflection optical path is the total optical path from the short-pulse laser to the frame camera when the light beam does not enter the oscillation optical cavity, and the shutter time width is not less than the laser pulse width and less than the laser pulse interval.

[0032] Step 2: Reset the delay time, and the delay time = (zero reflection optical path + number of reflections * reflection optical path) / light speed, wherein the reflection optical path = cavity interval * 2 * air refractive index; the number of reflections is selected as 1.

[0033] Adjust the angle and position of the measured mirror so that the interference fringes obtained by the frame camera are zero fringes; then adjust the cavity interval between the first reference standard mirror and the second reference standard mirror to be equal to the cavity interval between the measured mirror and the standard mirror so that the modulation degree of the interference fringes of the frame camera is the highest; then adjust the angle between the second reference standard mirror and the measured mirror so that the interference fringes obtained by the frame camera are zero fringes.

[0034] Step 3: Reset the delay time, and the delay time = (zero reflection optical path + number of reflections * reflection optical path) / light speed, wherein the reflection optical path = cavity interval * 2 * air refractive index; the number of reflections is selected as a natural number, generally 5 - 100 times.

[0035] Step 4: Adjust the light energy adjustment device and the rotation angle of the half-wave plate to maximize the modulation degree of the interference pattern formed on the focal plane of the framing camera.

[0036] Step 5: Adjust the cavity interval between the first reference standard mirror and the second reference standard mirror to maximize the modulation degree of the interference pattern.

[0037] Step 6: The controller controls the framing camera to collect the interference image, and performs data processing on the interference image to obtain the interference wavefront.

[0038] Step 7: The controller removes the influence of the standard mirror on the wavefront data from the interference wavefront obtained in Step 6 to obtain the surface shape of the measured mirror.

[0039] Step 8: Gradually increase the number of reflections to obtain a new delay time, and return to Step 3 until the required measurement accuracy is achieved.

[0040] Furthermore, before the said Step 1, place the multiple-reflection ultrafast high-precision interferometer under vacuum, low pressure or stable constant-temperature laminar air conditions.

[0041] Compared with the prior art, the present invention has the following technical effects:

[0042] 1. The measurement accuracy is improved. The present invention uses a short-pulse laser with a pulse width from fs to us. At the same time, the framing camera and the methods of optical path compensation and integral time gating are used to realize the separation of multiple-reflection signals and adjustable number of reflections, making multiple-reflection interference measurement possible and greatly improving the measurement accuracy. At the same time, by using a polarization beam splitter prism, a half-wave plate, and a quarter-wave polarization plate, the laser polarization states match each other at different stages. By rotating the angle of the half-wave plate, the signal modulation degree can be adjusted. Finally, four phase-shifted interference patterns are simultaneously formed on the focal plane of the framing camera through pixelated polarization devices, with high energy utilization rate and a wide applicable range of the reflectivity of the measured mirror. In addition, instant interference is realized, making the system insensitive to overall vibration and greatly improving the measurement accuracy.

[0043] 2. The signal-to-noise ratio of multiple-reflection signals is ensured. In the present invention, a first reference standard mirror and a standard mirror are added to the two arms of the traditional Twyman-Green interferometer, and the standard surfaces of the first reference standard mirror and the standard mirror are coated with films to reduce the multiple-reflection attenuation amount and ensure the signal-to-noise ratio of multiple-reflection signals.

[0044] 3. The accuracy improves with the number of reflections. Inside the interference cavity, the wavefront of the light wave is affected by the surface shape of the measured mirror and increases linearly with the reflection order. Except for the standard surface and the synchronous increase in the air disturbance error, other interference detection errors such as photon noise, electronics noise, the influence of the wavefront of the pre-collimation system, the polarization ratio of the polarization optical device, and the alignment angle are all independent of the number of reflections. As the number of reflections increases, the influence of other interference factors on the measurement result is relatively reduced by multiples. This is manifested as a multiple reduction in the measurement repeatability level. Compared with the 0.1 - 0.5 nm level of typical instruments, the present invention can achieve a repeatability accuracy better than 0.05 nm under the condition of 20 reflections.

[0045] 4. Coherent noise is avoided. Due to the time-gating effect formed under the shutter modulation in the framing camera, the duty cycle can reach 1:10 - 1:1000. The time points at which the stray light or unwanted light formed by the laser source passing through different channels reach the framing camera are different from those of the effective optical pulse signal. Thus, its main part will be shielded outside the effective detection range and cannot form a response, avoiding the interference of non-examined surfaces and scattered stray light on the detection result. In particular, it avoids the interference of multiple reflections in the reference mirror and the interference formed by multiple reflections on the surface of components in the interferometer, significantly improving the quality of the interference pattern, and there is no need to use methods such as ground glass to destroy the spatial coherence to solve the coherent noise problem.

[0046] 5. The present invention can measure the surface shapes of the front and back surfaces of a thin parallel plate respectively (adjustment and selection can be carried out by relying on optical path compensation). By using time gating to avoid mutual interference, and after the multiple reflections increase the time misalignment amount, compared with the thickness limitation (>0.5 mm) of parallel plates by other types of interferometers, it can be reduced to more than 5 times.

[0047] 6. Vibration resistance. Since the pulse width of the pulsed laser used in the detection is extremely small (the typical value is 100 ps - 10 ps), the mechanical vibration level within this period is much less than the pm level.

[0048] 7. The requirements for the wavelength stability and coherence length of the laser light source are low. Due to the use of near-equal optical path interference conditions (that is, all the interference signals entering the framing camera have almost equal optical paths traveled, and they are all divided from the same optical pulse, so their coherence does not need to be very high), the requirement for the laser wavelength stability is significantly relaxed to 1 / 100 - 1 / 10000 compared with the usual Fizeau type and Twyman - Green type interferometers. Also due to the near-equal optical path interference, within the allowable range of the adjustment ability, the laser coherence length greater than the millimeter level is sufficient, greatly relaxing the restrictions on the laser.

[0049] In summary, the present invention proposes a method of increasing the detection sensitivity by using a short-pulse laser to reflect and oscillate multiple times in an interferometric equal-optical-path cavity. At the same time, a framing camera with a delay-synchronized shutter is used to achieve time gating of the multiple-reflection interference pulse signal, thereby realizing the detection of the optical mirror surface shape at the picometer level. Description of the Drawings

[0050] Figure 1 FIG. is a schematic structural diagram of a multiple-reflection ultrafast high-precision interferometer of the present invention. Among them, the first reference standard mirror, the second reference standard mirror, the standard mirror, and the measured mirror are all plane mirrors.

[0051] Figure 2 FIG. is a schematic structural diagram of a multiple-reflection ultrafast high-precision interferometer of the present invention. Among them, the first reference standard mirror and the second reference standard mirror are both plane mirrors, and the standard mirror and the measured mirror are both spherical mirrors.

[0052] The present invention will be further explained below in conjunction with the drawings and specific embodiments. Specific Embodiments

[0053] As Figure 1 shown, the multiple-reflection ultrafast high-precision interferometer of the present invention includes a short-pulse laser source, an optical energy adjustment device, a linear polarizer, a half-wave plate, a polarization beam splitter prism, a first quarter-wave polarization plate, a second quarter-wave polarization plate, a first reference standard mirror, a second reference standard mirror, a standard mirror, a polarization device, a framing camera, and a controller. Among them:

[0054] The short-pulse laser emitted by the short-pulse laser source passes through the optical energy adjustment device, the linear polarizer, and the half-wave plate in sequence after being emitted from the light source, and then enters the polarization beam splitter prism, where it is decomposed into two beams of light with polarization directions parallel and perpendicular to the main section of the polarization beam splitter prism respectively. The beam parallel to the main section of the polarization beam splitter prism enters the measurement branch, and the beam perpendicular to the main section of the polarization beam splitter prism enters the reference branch (which can be exchanged according to the characteristics of the polarization beam splitter prism);

[0055] The light wave entering the measurement branch becomes a circularly polarized light after passing through the first quarter-wave polarization plate, passes through the standard mirror, and then enters the oscillating optical cavity formed by the standard mirror and the measured reflecting mirror. The light wave reflects multiple times in the oscillating optical cavity, and each time it returns from the measured reflecting mirror and passes through the standard mirror, a measured echo signal is formed, thereby obtaining a series of measured echo signals (that is, each laser pulse emitted by the short-pulse laser source excites a corresponding series of measured echo signals); the measured echo signal becomes a linearly polarized light after passing through the first quarter-wave polarization plate, and its polarization direction rotates 90 degrees compared with when the light beam first passes through the first quarter-wave polarization plate. This linearly polarized light enters the polarization beam splitter prism and is reflected.

[0056] The light wave entering the reference branch becomes a circularly polarized light after passing through the second 1 / 4 polarization wave plate, enters the optical oscillation cavity formed by the first reference standard mirror and the second reference standard mirror after passing through the first reference standard mirror. The light wave is reflected multiple times in the optical oscillation cavity, and each reflection on the surface of the second standard mirror forms a reference light pulse signal respectively, so as to obtain a series of reference light pulse signals. The reference light pulse signal becomes a linearly polarized light after passing through the second 1 / 4 polarization wave plate, and its polarization direction rotates 90 degrees compared with when the light beam first passes through the second 1 / 4 polarization wave plate, and then passes through the polarization beam splitter prism.

[0057] The optical path from the standard surface of the first reference standard mirror to the splitting surface of the polarization beam splitter prism (PBS) is equal to the optical path from the standard surface of the standard mirror to the splitting surface of the polarization beam splitter prism, so as to ensure the co-phase of the echoes of the measurement branch and the reference branch.

[0058] The optical path of the optical cavity formed by the first reference standard mirror and the second reference standard mirror is equal to or an integer multiple ratio of the optical path of the optical cavity formed by the standard mirror and the measured reflecting mirror (when the optical paths are equal, the number of reflections of the two arms is equal, and when it is an integer multiple ratio, the number of reflections of the two arms is different).

[0059] The measured echo pulse signal and the reference light pulse signal at the same moment are combined after passing through the polarization beam splitter prism, and then form multiple interference images after passing through the polarization device composed of multiple polarizers with different polarization directions, which are acquired by the framing camera, so as to obtain a series of interference images.

[0060] In the above technical solution, the 1 / 2 wave plate is used to correct the energy and the energy contrast of the two polarized lights;

[0061] Preferably, a first collimating mirror is provided between the short-pulse laser source and the optical energy adjusting device, which is used to solve the illumination uniformity and spatial coherence. Specifically, the first collimating mirror includes two optical lenses and a small hole between them. The optical lenses are used to collimate into parallel light, and the small hole is used for spatial filtering.

[0062] Preferably, a second collimating mirror is provided between the polarization beam splitter prism and the polarization device, which is used to match the detector size of the framing camera or locally magnify the interference image. Specifically, the second collimating mirror includes two optical lenses and a small hole between them. The optical lenses are used to image the interference fringe image, and the small hole is used for spatial filtering and reducing the influence of stray light.

[0063] Preferably, reflective films (reflectivity is 30% - 99.95%) are coated on the standard surfaces of the first reference standard mirror and the standard mirror, which reduces the attenuation amount of multiple reflections and ensures the signal-to-noise ratio of the multiple reflection signals.

[0064] Specifically, the first reference standard mirror and the second reference standard mirror can both be plane mirrors or spherical mirrors, and the standard mirror and the mirror under test can be plane mirrors or spherical mirrors. Among them, when the first reference standard mirror and the standard mirror are spherical mirrors, the spherical mirror is a Fizeau-type standard transmission spherical mirror.

[0065] Specifically, the light energy adjustment device includes at least two attenuation sheets, and the transmittance of one of the attenuation sheets is variable for adjusting the laser transmittance.

[0066] The external synchronization lines of the framing camera and the short-pulse laser are both connected to the controller, and the synchronization controller controls the synchronization of the short-pulse laser and the framing camera according to the preset delay time.

[0067] The present invention provides a method for measuring the surface shape of multiple reflections. This method uses the above-mentioned multiple-reflection ultrafast high-precision interferometer for measurement. The following takes the case where the mirror under test, the standard mirror, the first reference standard mirror, and the second reference standard mirror are all plane mirrors as an example for description, and specifically includes the following steps:

[0068] Step 1: Set the delay time and the shutter time width of the framing camera. The controller receives the pulse signal sent by the short-pulse laser source in real time as the trigger signal, and controls the framing camera to obtain the interference fringes according to the delay time and the shutter time width. Fix the position of the standard mirror, and adjust the optical axis direction and position of the first reference standard mirror so that the optical path from the standard surface of the first reference standard mirror to the splitting surface of the polarization beam splitter is equal to the optical path from the standard surface of the standard mirror to the splitting surface of the polarization beam splitter prism until the interference fringes obtained by the framing camera are zero fringes.

[0069] Among them, the delay time = zero reflection optical path / light speed; among them, the zero reflection optical path is the total optical path from the short-pulse laser to the framing camera when the light beam does not enter the oscillation optical cavity, and the shutter time width is not less than the laser pulse width and less than the laser pulse interval.

[0070] Step 2: Reset the delay time. The delay time = (zero reflection optical path + number of reflections * reflection optical path) / light speed, where the reflection optical path = cavity interval * 2 * air refractive index; the number of reflections is selected as 1.

[0071] Adjust the cavity interval between the mirror under test and the standard mirror to be equal to the cavity interval between the first reference standard mirror and the second reference standard mirror to make the modulation degree of the interference fringes of the framing camera the highest; then adjust the angle between the second reference standard mirror and the mirror under test so that the interference fringes obtained by the framing camera are zero fringes.

[0072] Step 3: Reset the delay time. The delay time = (zero reflection optical path + number of reflections * reflection optical path) / light speed, where the reflection optical path = cavity interval * 2 * air refractive index; the number of reflections is selected as a natural number, generally 5 to 100 times.

[0073] Step 4: Adjust the optical energy adjustment device and the rotation angle of the half-wave plate to maximize the modulation degree of the interference pattern formed on the focal plane of the framing camera.

[0074] Step 5: Adjust the cavity interval between the measured mirror and the reference mirror or the cavity interval between the first reference standard mirror and the second reference standard mirror to maximize the modulation degree of the interference pattern.

[0075] Step 6: The controller controls the framing camera to collect the interference image, and performs data processing on the interference image to obtain the interference wavefront.

[0076] Step 7: The controller removes the influence of the reference mirror on the wavefront data from the interference wavefront obtained in Step 6 to obtain the surface shape of the measured mirror.

[0077] Step 8: Gradually increase the number of reflections to obtain a new delay time, and return to Step 3 until the required measurement accuracy is achieved.

[0078] Preferably, before Step 1, the optical system part of the present invention is placed under vacuum, low pressure or stable constant-temperature laminar air conditions. This is because the optical path difference caused by the error of the reference mirror and air disturbance does not decrease with the change of the number of reflections. The error of the reference mirror can be solved by precise calibration, while the air disturbance changes with time. Placing the optical system part of the present invention under vacuum, low pressure or stable constant-temperature laminar air conditions can effectively avoid the error caused by air disturbance.

[0079] On the other hand, when both the measured mirror and the reference mirror are spherical mirrors (the reference mirror is preferably a Fizeau-type standard transmission spherical mirror), and both the first reference standard mirror and the second reference standard mirror are plane mirrors, in this case, the focal points of the measured surface and the reference mirror coincide, and the optical path cannot be adjusted after it is determined. Therefore, when using the method of the present invention described above, Step 2 needs to be replaced with: adjusting the angle and position (six degrees of freedom) of the measured mirror to make the interference fringes obtained by the framing camera zero fringes; then adjusting the cavity interval between the first reference standard mirror and the second reference standard mirror to be equal to the cavity interval between the measured mirror and the reference mirror to make the modulation degree of the interference fringes of the framing camera the highest; then adjusting the angle between the second reference standard mirror and the measured mirror to make the interference fringes obtained by the framing camera zero fringes; and Step 5 needs to be replaced with: adjusting the cavity interval between the first reference standard mirror and the second reference standard mirror to maximize the modulation degree of the interference pattern.

Claims

1. A multiple - reflection ultrafast high - precision interferometer, characterized in that, it includes a short - pulse laser source, an optical energy adjustment device, a linear polarizer, a half - wave plate, a polarization beam splitter prism, a first quarter - wave polarizing plate, a second quarter - wave polarizing plate, a first reference mirror, a second reference mirror, a standard mirror, a polarization device, a framing camera and a controller. The framing camera and the short - pulse laser are both connected to the controller. The first reference mirror, the second reference mirror, the measured mirror and the standard mirror are all plane mirrors. Or, the first reference mirror and the second reference mirror are both plane mirrors, and the measured mirror and the standard mirror are both spherical mirrors. Among them: The short - pulse laser emitted by the short - pulse laser source, after being emitted from the light source, successively passes through the optical energy adjustment device, the linear polarizer and the half - wave plate, and then enters the polarization beam splitter prism, and is decomposed into two beams of light with polarization directions parallel and perpendicular to the main section of the polarization beam splitter prism respectively. The beam parallel to the main section of the polarization beam splitter prism enters the measurement branch, and the beam perpendicular to the main section of the polarization beam splitter prism enters the reference branch; The light wave entering the measurement branch becomes a circularly polarized light after passing through the first quarter - wave polarizing plate, passes through the standard mirror and then enters the oscillating optical cavity composed of the standard mirror and the measured mirror. The light wave is reflected multiple times in the oscillating optical cavity. Each time it returns from the measured mirror and passes through the standard mirror, a measured echo signal is formed, so as to obtain a series of measured echo signals. The measured echo signal becomes a linearly polarized light after passing through the first quarter - wave polarizing plate, and its polarization direction rotates 90 degrees compared with when the light beam first passes through the first quarter - wave polarizing plate. This linearly polarized light enters the polarization beam splitter prism and is reflected; The light wave entering the reference branch becomes a circularly polarized light after passing through the second quarter - wave polarizing plate, passes through the first reference mirror and then enters the oscillating optical cavity composed of the first reference mirror and the second reference mirror. The light wave is reflected multiple times in the oscillating optical cavity. Each reflection on the surface of the second reference mirror forms a reference light pulse signal respectively, so as to obtain a series of reference light pulse signals. The reference light pulse signal becomes a linearly polarized light after passing through the second quarter - wave polarizing plate, and its polarization direction rotates 90 degrees compared with when the light beam first passes through the second quarter - wave polarizing plate, and then passes through the polarization beam splitter prism; The optical path from the standard surface of the first reference mirror to the splitting surface of the polarization beam splitter prism is equal to the optical path from the standard surface of the standard mirror to the splitting surface of the polarization beam splitter prism; The optical path of the optical cavity composed of the first reference mirror and the second reference mirror is equal to or an integer - multiple ratio of the optical path of the optical cavity composed of the standard mirror and the measured mirror; The measured echo pulse signal and the reference light pulse signal at the same moment are combined after passing through the polarization beam splitter prism, and then form multiple interference images after passing through the polarization device composed of multiple polarizers with different polarization directions, and are acquired by the framing camera.

2. The multiple - reflection ultrafast high - precision interferometer according to claim 1, characterized in that, the optical energy adjustment device includes at least two attenuation sheets, and the transmittance of one of the attenuation sheets is variable.

3. The multiple - reflection ultrafast high - precision interferometer according to claim 1, characterized in that, a first collimating mirror is provided between the short - pulse laser source and the optical energy adjustment device.

4. The multi-reflection ultrafast high-precision interferometer according to claim 3, characterized in that the first collimating mirror comprises two optical lenses and a small hole therebetween.

5. The multi-reflection ultrafast high-precision interferometer according to claim 1, characterized in that a second collimating mirror is provided between the polarization beam splitter prism and the polarization device.

6. The multi-reflection ultrafast high-precision interferometer according to claim 5, characterized in that the second collimating mirror comprises two optical lenses and a small hole therebetween.

7. The multi-reflection ultrafast high-precision interferometer according to claim 1, characterized in that reflective films are coated on the standard surfaces of the first reference standard mirror and the standard mirror.

8. A method for measuring surface shape by multi-reflection, characterized in that the method is based on the multi-reflection ultrafast high-precision interferometer according to any one of claims 1 to 7. In the multi-reflection ultrafast high-precision interferometer, the first reference standard mirror, the second reference standard mirror, the measured mirror and the standard mirror are all plane mirrors, and specifically includes the following steps: Step 1: Set the delay time and the shutter time width of the framing camera. The controller receives the pulse signal sent by the short-pulse laser source in real time as the trigger signal, and controls the framing camera to obtain interference fringes according to the delay time and the shutter time width; fix the position of the standard mirror, and adjust the optical axis direction and position of the first reference standard mirror so that the optical path from the standard surface of the first reference standard mirror to the splitting surface of the polarization beam splitter prism is equal to the optical path from the standard surface of the standard mirror to the splitting surface of the polarization beam splitter prism until the interference fringes obtained by the framing camera are zero fringes; wherein, the delay time = zero reflection optical path / light speed; wherein, the zero reflection optical path is the total optical path from the short-pulse laser to the framing camera when the light beam does not enter the oscillation optical cavity, and the shutter time width is not less than the laser pulse width and less than the laser pulse interval; Step 2: Reset the delay time, and the delay time = (zero reflection optical path + number of reflections * reflection optical path) / light speed, wherein, the reflection optical path = cavity interval * 2 * air refractive index; the number of reflections is selected as 1; Adjust the cavity interval between the measured mirror and the standard mirror to be equal to the cavity interval between the first reference standard mirror and the second reference standard mirror so that the modulation degree of the interference fringes of the framing camera is the highest; then adjust the angle between the second reference standard mirror and the measured mirror so that the interference fringes obtained by the framing camera are zero fringes; Step 3: Reset the delay time, and the delay time = (zero reflection optical path + number of reflections * reflection optical path) / light speed, wherein, the reflection optical path = cavity interval * 2 * air refractive index; the number of reflections is selected as a natural number, generally 5 to 100 times; Step 4: Adjust the optical energy adjusting device and the rotation angle of the 1 / 2 wave plate to make the modulation degree of the interference pattern formed on the focal plane of the framing camera the largest; Step 5: When adjusting the cavity interval between the measured mirror and the standard mirror or the cavity interval between the first reference standard mirror and the second reference standard mirror to make the modulation degree of the interference pattern the largest; Step 6: The controller controls the framing camera to collect and obtain the interference image, and performs data processing on the interference image to obtain the interference wavefront; Step 7: The controller removes the influence of the standard mirror on the wavefront data from the interference wavefront obtained in Step 6 to obtain the surface shape of the measured mirror. Step 8: Gradually increase the number of reflections to obtain a new delay time, and return to Step 3 until the required measurement accuracy is achieved.

9. The method for measuring the surface shape by multiple reflections according to claim 8, characterized in that, before the said Step 1, the multiple-reflection ultrafast high-precision interferometer is placed under vacuum, low pressure or stable constant-temperature laminar air conditions.

10. A method for measuring the surface shape by multiple reflections, characterized in that, this method is based on the multiple-reflection ultrafast high-precision interferometer described in any one of claims 1 to 7. In the multiple-reflection ultrafast high-precision interferometer, both the first reference standard mirror and the second reference standard mirror are plane mirrors, and both the measured mirror and the standard mirror are spherical mirrors. The specific steps are as follows: Step 1: Set the delay time and the shutter time width of the framing camera. The controller receives the pulse signal sent by the short-pulse laser source in real time as the trigger signal, and controls the framing camera to obtain interference fringes according to the delay time and the shutter time width; fix the position of the standard mirror, adjust the optical axis direction and position of the first reference standard mirror so that the optical path from the standard surface of the first reference standard mirror to the splitting surface of the polarization beam splitter is equal to the optical path from the standard surface of the standard mirror to the splitting surface of the polarization beam splitter until the interference fringes obtained by the framing camera are zero fringes; wherein, the delay time = zero reflection optical path / light speed; wherein, the zero reflection optical path is the total optical path from the short-pulse laser to the framing camera when the light beam does not enter the oscillating optical cavity, and the shutter time width is not less than the laser pulse width and less than the laser pulse interval; Step 2: Reset the delay time, the delay time = (zero reflection optical path + number of reflections * reflection optical path) / light speed, wherein, the reflection optical path = cavity interval * 2 * air refractive index; the number of reflections is selected as 1; Adjust the angle and position of the measured mirror so that the interference fringes obtained by the framing camera are zero fringes; then adjust the cavity interval between the first reference standard mirror and the second reference standard mirror to be equal to the cavity interval between the measured mirror and the standard mirror so that the modulation degree of the interference fringes of the framing camera is the highest; then adjust the angle between the second reference standard mirror and the measured mirror so that the interference fringes obtained by the framing camera are zero fringes; Step 3: Reset the delay time, the delay time = (zero reflection optical path + number of reflections * reflection optical path) / light speed, wherein, the reflection optical path = cavity interval * 2 * air refractive index; the number of reflections is selected as a natural number, generally 5 to 100 times; Step 4: Adjust the optical energy adjustment device and the rotation angle of the half-wave plate to make the modulation degree of the interference pattern formed on the focal plane of the framing camera the highest; Step 5: Adjust the cavity interval between the first reference standard mirror and the second reference standard mirror to make the modulation degree of the interference pattern the highest; Step 6: The controller controls the framing camera to collect and obtain the interference image, and performs data processing on the interference image to obtain the interference wavefront; Step 7: The controller removes the influence of the standard mirror on the wavefront data from the interference wavefront obtained in Step 6 to obtain the surface shape of the measured mirror; Step 8: Gradually increase the number of reflections to obtain a new delay time, and return to Step 3 until the required measurement accuracy is achieved.

11. The method for measuring the surface shape by multiple reflections according to claim 10, characterized in that, the standard mirror is a Fizeau-type standard transmission spherical mirror.

12. The multi-reflection surface shape measurement method according to claim 10, characterized in that, before the step 1, the multi-reflection ultrafast high-precision interferometer is placed under the conditions of vacuum, low pressure or stable constant-temperature laminar air.

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