A multi-surface topography measurement method based on coherent and controllable laser

Abstract

The present invention discloses a multi-surface topography testing method based on coherent controllable laser. A coherent controllable laser light source is composed of a broadband white noise source, a filter, an adjustable RF attenuator, an RF amplifier, a DC drive power supply, a bias module, and a semiconductor laser. Aiming at the multi-surface interference characteristics of parallel flat optical elements, short coherent light with a high sidelobe suppression ratio is obtained by modulating the semiconductor laser with broadband white noise current to avoid the crosstalk of multi-surface interference fringes. The adjustable RF attenuator is used to adjust the intensity of the white noise signal, realizing the controllable coherence length from self-coherence to short coherence, gradually shortening the existence interval of the interference fringes, quickly locating the zero optical path difference position of the interference measurement, and completing the measurement of the topography of the multi-surface interference element. Compared with the existing methods, the method proposed by the present invention increases the positioning speed of the zero optical path difference position while improving the measurement accuracy, device integration degree, and simplicity of the multi-surface topography testing, and is more suitable for industrialization.

Description

Technical Field

[0001] The present invention belongs to the technical field of interferometer light sources, and particularly relates to a multi-surface topography testing method based on coherent controllable laser light. Background Art

[0002] With the rapid development of high-precision and advanced technology fields such as astronomical observation, lithography machines, and laser nuclear fusion, multi-surface interference elements such as large-aperture and high-precision parallel flat plates, silicon wafers, and ICF target balls are increasingly widely used. The surface shape deviation of these multi-surface interference elements has an important impact on the application effect and also poses new requirements for their topography detection. When the topography of multi-surface interference elements is measured by interfering with equal-phase coherent light sources such as He-Ne lasers, the reflected light from the front and back surfaces both interferes, and there are multiple sets of interference fringes in the interference fringe pattern, resulting in the inability to correctly calculate the multi-surface topography information.

[0003] Common multi-surface topography testing methods include: ① Using a wavelength-tunable laser for wavelength-scanning interference measurement, which can separate multi-surface interference information to calculate the multi-surface topography information. However, this method is limited by the wavelength tuning range of the laser and can only measure the topography of multi-surface interference elements with a thickness of millimeters, and cannot meet the measurement requirements of elements such as silicon wafers; ② White-light scanning interferometry, which can achieve high-precision topography detection of thin-layer multi-surface interference elements at the 0.1 nm level. However, its light source brightness is low, the field of view is small, and it is difficult to locate interference fringes, and it is only applicable to microscopic topography detection; ③ Short-coherence interferometry, which uses a light source with a short coherence length to avoid multi-surface interference fringe crosstalk and can accurately measure the surface topography of multi-surface interference elements. Zhang Wenxi et al. from the Chinese Academy of Sciences used a current-modulated semiconductor laser with a frequency of 700 MHz to obtain a short-coherence light source with a coherence length of 80 μm; Chu Fenghong et al. from Shanghai University of Electric Power, by studying the influence of laser slope efficiency, bias current, radio frequency signal frequency, and amplitude on the coherence length of LD, used a current-modulated semiconductor laser with 950 MHz and an amplitude of 19 dBm to obtain a short-coherence light source with a coherence length of 90 μm, improving the interference fringe contrast. However, when the coherence length of the short-coherence light source is as low as hundreds or even dozens of micrometers, the measured object can only generate interference fringes within a very short distance. At the same time, limited by the adjustment step and precision error of the optical path displacement device, it is difficult to locate the fringes.

[0004] Aiming at the problem of difficult positioning of interference fringes in short-coherence interferometric measurement, a laser light source with controllable coherence length is used to expand the existence range of interference fringes to find the interference fringes, and then gradually shorten the coherence length to narrow the search range of interference fringes, so as to achieve rapid and accurate positioning of the zero optical path difference position. In addition, the existing short-coherence light source schemes have problems of many interference side lobes and low side lobe suppression ratio, which affect the search for the main peak of the interference envelope. The broadband white noise current modulation semiconductor laser has a high side lobe suppression ratio, which can reduce the influence of interference side lobes on the positioning of the zero optical path difference position and interferometric measurement. The present invention proposes a multi-surface topography measurement method based on coherence-controllable laser, which can adapt to different-structured interferometers to complete the interferometric measurement of the surface topography of multi-surface interference elements, and adopts an adjustable radio frequency attenuator and a broadband white noise source to solve the two key problems of difficult positioning of the zero optical path difference position and interference side lobe suppression. Summary of the Invention

[0005] The purpose of the present invention is to provide a multi-surface topography measurement method based on coherence-controllable laser, which is simple to operate, has a high degree of device integration, and can quickly locate the zero optical path difference position.

[0006] The technical solution of the present invention is as follows: a multi-surface topography measurement method based on coherence-controllable laser, and the steps are as follows:

[0007] Step 1: Connect the coherence-controllable laser light source and the interferometer through a fiber optic connector, and then proceed to Step 2.

[0008] Among them, the coherence-controllable laser light source includes a broadband white noise source, a first radio frequency connection line, a filter, a second radio frequency connection line, an adjustable radio frequency attenuator, a third radio frequency connection line, a radio frequency amplifier, a fourth radio frequency connection line, a bias module, a fifth radio frequency connection line, a semiconductor laser, and a polarization-maintaining fiber connected in sequence; it also includes a DC drive power supply connected to the bias module through a wire; the polarization-maintaining fiber is connected to the interferometer through a fiber optic connector.

[0009] Step 2: Place the multi-surface interference element in the test optical path inside the interferometer, control the cavity length of the interferometer so that the optical path of the test optical path is 1-3 cm longer than that of the reference optical path, and then proceed to Step 3.

[0010] Step 3: Turn on the DC drive power supply, the semiconductor laser outputs long-coherence laser, observe the polarization camera in the interferometer to receive the multi-surface interference fringes, and adjust the pitching and tilting postures of the multi-surface interference element to make the corresponding two groups of interference fringes clearly displayed, and then proceed to Step 4.

[0011] Step 4: First, turn on the broadband white noise source, then turn on the RF amplifier. Gradually reduce the attenuation of the adjustable RF attenuator, observe the presence of interference fringes in the grayscale image received by the polarization camera in the interferometer, and during this process, move the multi-surface interference element back and forth in the direction of decreasing the optical path of the test optical path of the interferometer 12, so that interference fringes exist in the image received by the polarization camera. Continue to reduce the attenuation of the adjustable RF attenuator. The coherence length of the coherent controllable laser source shortens as the attenuation of the adjustable RF attenuator decreases. When the coherence length of the coherent controllable laser source is lower than the thickness of the multi-surface interference element, the light source behaves as a short coherence light source. At this time, the crosstalk of multiple groups of interference fringes on the multi-surface interference element disappears, and proceed to Step 5.

[0012] Step 5: Repeat Step 4 until the attenuation of the adjustable RF attenuator reaches the minimum. Further adjust the front and back positions of the multi-surface interference element so that the interference fringes exist and have the best contrast in the image received by the polarization camera, that is, the front surface of the multi-surface interference element reaches the zero optical path difference position of the interferometer. Collect the current interference fringe pattern through the polarization camera in the interferometer, and calculate the surface topography of the front surface of the multi-surface interference element through the four-step phase-shifting algorithm, and proceed to Step 6.

[0013] Step 6: Continue to move the multi-surface interference element in the direction of decreasing the optical path of the test optical path of the interferometer. When the contrast of the interference fringes in the image received by the polarization camera is the best again, the back surface of the multi-surface interference element reaches the zero optical path difference position of the interferometer. Collect the current interference fringe pattern through the polarization camera in the interferometer, and calculate the surface topography of the back surface of the multi-surface interference element through the four-step phase-shifting algorithm.

[0014] Compared with the prior art, the significant advantages of the present invention are as follows:

[0015] (1) The coherence length of the coherent controllable laser source is controllable. By controlling the shortening of the coherence length, gradually reduce the existence interval of interference fringes, narrow the search range of the zero optical path difference position of the interferometer, and finally control the existence interval of interference fringes within hundreds of micrometers, position the front and back surfaces of the multi-surface interference element to be measured at the zero optical path difference position of the interferometer, and avoid the influence of multi-surface interference fringe crosstalk on the measurement result.

[0016] (2) The coherent controllable laser source uses direct current modulation, without expensive phase modulation devices such as electro-optic phase modulators, with low device cost and good robustness.

[0017] (3) The coherent controllable laser source uses broadband white noise signal current modulation, with high output stability, few interference envelope side lobes, high side lobe suppression ratio, and high measurement accuracy.

[0018] (4) The coherent controllable laser light source uses an FP cavity multimode semiconductor laser coupled with a polarization-maintaining optical fiber for output. It has high output power and good spatial coherence, and can be used for the transient detection of wavefront aberration of optical elements with low reflectivity and large aperture.

[0019] (5) The testing process is simple to operate and convenient to adjust. Brief Description of the Drawings

[0020] Figure 1 FIG. is a schematic diagram of the multi-surface topography testing structure based on the coherent controllable laser of the present invention.

[0021] Figure 2 FIG. is a comparison diagram of the broadened spectra under different intensity broadband white noise current modulations of the coherent controllable laser light source.

[0022] Figure 3 FIG. is a schematic diagram of the multi-surface interference fringe positioning and measurement principle of the coherent controllable laser light source.

[0023] Figure 4 FIG. is a graph of the interference fringes and their contrast near the zero optical path difference position when the coherence length of the coherent controllable laser light source is the shortest.

[0024] Figure 5 FIG. is a graph of the scanned interference fringes and their contrast for broadband white noise and single-point noise current modulations, where FIG. (a) shows the scanned interference fringes for the two current modulations, and FIG. (b) shows the scanned contrast curves for the two current modulations.

[0025] Figure 6 FIG. is a schematic diagram of the optical path structure of the present invention applied to a Twyman-Green dynamic interferometer.

[0026] Figure 7 FIG. is an effect diagram of the multi-surface topography testing of the present invention applied to a Twyman-Green dynamic interferometer, where FIG. (a) shows four interference phase-shifted images collected by the interferometer, and FIG. (b) shows the surface topography of the measured part obtained by calculation.

[0027] Figure 8 FIG. is a schematic diagram of the optical path structure of the present invention applied to a Fizeau dynamic interferometer.

[0028] Figure 9 FIG. is an effect diagram of the multi-surface topography testing of the present invention applied to a Fizeau dynamic interferometer, where FIG. (a) shows the multi-surface interference pattern of a sapphire parallel plate, FIG. (b) shows the interference fringes of the front surface, FIG. (c) shows the interference fringes of the rear surface, FIG. (d) shows the topography of the front surface, and FIG. (e) shows the topography of the rear surface. Detailed Embodiments

[0029] The present invention will be further described in detail below with reference to the drawings.

[0030] Combined with Figure 1, a multi-surface topography testing method based on coherent controllable laser, comprising the following steps:

[0031] Step 1: Connect the coherent controllable laser light source and the interferometer 12 through the fiber optic connector 11, and proceed to Step 2.

[0032] Among them, the coherent controllable laser light source includes a broadband white noise source 1, a first RF connecting line 2-1, a filter 3, a second RF connecting line 2-2, an adjustable RF attenuator 4, a third RF connecting line 2-3, an RF amplifier 5, a fourth RF connecting line 2-4, a bias module 6, a fifth RF connecting line 2-5, a semiconductor laser 9, and a polarization maintaining fiber 10 connected in sequence; it also includes a DC drive power supply 8 connected to the bias module 6 through a wire 7; the polarization maintaining fiber 10 is connected to the interferometer 12 through the fiber optic connector 11.

[0033] Further, the broadband white noise source 1 is connected to the filter 3 through the first RF connecting line 2-1, and then connected to the input end of the RF amplifier 5 through the second RF connecting line 2-2, the adjustable RF attenuator 4, and the third RF connecting line 2-3. The output end of the RF amplifier 5 is connected to the AC input end of the bias module 6 through the fourth RF connecting line 2-4. The DC drive power supply 8 is connected to the DC input end of the bias module 6 through the wire 7. The DC signal and the AC signal are superimposed by the bias module 6 and then input into the semiconductor laser 9 through the fifth RF connecting line 2-5. The semiconductor laser 9 is connected to the fiber optic connector 11 through the polarization maintaining fiber 10.

[0034] Step 2: Place the multi-surface interference element (multiplesurfaceinterference) 13 in the test optical path inside the interferometer 12, control the cavity length of the interferometer so that the optical path of the test optical path is 1-3 cm longer than the reference optical path, and proceed to Step 3.

[0035] Step 3: Turn on the DC drive power supply 8, the semiconductor laser 9 outputs long coherent laser, observe the polarization camera in the interferometer 12 to receive the multi-surface interference fringes, adjust the pitch and tilt postures of the multi-surface interference element 13 to make the corresponding two sets of interference fringes clear, and 4-10 fringes are received in the field of view of the polarization camera, and proceed to Step 4.

[0036] Further, the current setting of the DC drive power supply 8 is lower than the current at the center of the linear region of the current-power curve of the semiconductor laser 9 and higher than the threshold current of the semiconductor laser 9. Too high a bias current will reduce the spectral broadening width, and too low a bias current will result in low output optical power.

[0037] Step 4: After turning on the broadband white noise source 1, turn on the radio frequency amplifier 5. After the broadband white noise signal is filtered by the filter 3, it enters the adjustable radio frequency attenuator 4 for attenuation. The attenuated white noise signal enters the radio frequency amplifier 5 for amplification, and then the bias current generated by the DC drive power supply 8 and the attenuated white noise signal enter the bias module 6 through the DC input terminal and the AC input terminal of the bias module 6 respectively for superposition. The superimposed broadband white noise modulation signal is input into the semiconductor laser 9 through the fifth radio frequency connection line 2-5. The semiconductor laser 9 is modulated by the broadband white noise current, and the output spectrum is broadened and output through the polarization-maintaining optical fiber 10 and the optical fiber connector 11.

[0038] Combined with Figure 2 , when the attenuation of the adjustable radio frequency attenuator 4 is decreased, the intensity of the broadband white noise signal entering the semiconductor laser 9 increases accordingly, and the output optical spectrum linewidth is gradually broadened. When the attenuation of the attenuator is the smallest, the broadband white noise intensity is the largest and the output spectrum linewidth is the widest.

[0039] Combined with Figure 3 , gradually decrease the attenuation of the adjustable radio frequency attenuator 4 to shorten the coherence length of the light source, and observe the existence of the interference fringes in the grayscale image received by the polarization camera of the interferometer 12. When the interference fringes disappear, move the multi-surface interference element 13 back and forth in the direction of decreasing the optical path of the test optical path until the interference fringes are received again in the polarization camera. Continue to decrease the attenuation of the adjustable radio frequency attenuator 4 and go to Step 5.

[0040] Furthermore, the coherence length of the coherence controllable laser light source shortens as the attenuation of the adjustable radio frequency attenuator 4 decreases. When the coherence length of the coherence controllable laser light source is lower than the thickness of the multi-surface interference element 13, the light source behaves as a short coherence light source. At this time, the crosstalk of multiple groups of interference fringes on the multi-surface interference element 13 disappears, and only the interference fringes are generated on the front surface within the coherence length range.

[0041] Step 5: Repeat Step 4 until the attenuation of the adjustable radio frequency attenuator 4 reaches the minimum. Further adjust the front and back positions of the multi-surface interference element 13 to make the interference fringes exist and have the best contrast in the image received by the polarization camera, that is, the front surface of the multi-surface interference element 13 reaches the zero optical path difference position of the interferometer 12. Collect the current interference fringe pattern through the polarization camera in the interferometer 12, and calculate the surface topography of the front surface of the multi-surface interference element 13 through the four-step phase-shifting algorithm, and go to Step 6.

[0042] Combined with Figure 4, when the attenuation of the adjustable radio frequency attenuator 4 is minimized, the coherence length of the coherent controllable laser light source is the shortest. Only near the zero optical path difference position of the multi-surface interference element 13 in the interferometer 12, there are interference fringes, and the contrast of the interference fringes rapidly weakens as the distance from the zero optical path position increases. When the multi-surface interference element 13 is moved and it is observed that the interference fringes suddenly become sharp and the contrast is the highest, the front surface of the multi-surface interference element 13 is at the zero optical path difference position, that is, the peak position of the interference wave packet. Scan the fringes near the zero optical path difference position and draw the interference wave packet curve. The shortest coherence length of the coherent controllable laser light source is calculated to be 381 nm. Then, for the multi-surface interference element 13 with a thickness less than 191 nm, multi-surface interference fringe crosstalk can be avoided, and the multi-surface topography can be successfully measured.

[0043] Combined with Figure 5 , when the spectral shape of the short coherence light source is not an ideal Gaussian line shape, there will be interference side lobes outside the coherence length. There are interference fringes at both the main peak of the interference envelope where the zero optical path difference position is located and the positions of the interference side lobes. The existence of the interference side lobes interferes with the judgment of the zero optical path difference position, and when at the positions of other reflecting surfaces, it may also generate crosstalk of miscellaneous fringes and affect the interference measurement result. Move the measured part 13 before and after the zero optical path difference position and record the contrast of the interference fringes at different optical path difference positions. The interference side lobe suppression ratio of the short coherence light source based on broadband white noise current modulation is 0.73. Replace the short coherence light source of the semiconductor laser modulated by a single-point frequency noise signal current and test the change of the interference fringe contrast with the optical path difference under the same conditions. By comparison, it can be seen that the side lobe suppression effect of the present invention using a semiconductor laser modulated by broadband white noise current is significantly improved.

[0044] Step 6: Continue to move the multi-surface interference element 13 in the direction of decreasing the optical path of the test optical path of the interferometer 12. When the contrast of the interference fringes in the image received by the polarization camera is the best again, the rear surface of the multi-surface interference element 13 reaches the zero optical path difference position of the interferometer 12. Collect the current interference fringe pattern through the polarization camera in the interferometer 12, and solve and obtain the surface topography of the rear surface of the multi-surface interference element 13 through the four-step phase-shifting algorithm.

[0045] The models and parameters of the above functional modules are preferably as follows:

[0046] The model of the broadband white noise source is: QM-NO1200, the frequency bandwidth range is 0.1 - 2000 MHz, and the broadband white noise power is -10 dBm. The model of the filter is: QM-LPF20040S, and the passband frequency range is 0 - 200 MHz. The model of the adjustable RF attenuator is: QM-AT0960375S, the operating frequency is 9k - 6 GHz, the attenuation step is 0.25 dB, and the attenuation range is 0.25 - 31.75 dB. The model of the RF amplifier is: QM-PA011200T, the amplification frequency bandwidth range is 1 - 1200 MHz, and the gain is 35 dB. The DC drive power supply uses a self-developed constant current source, and the output DC current range is 0 - 240 mA. The model of the bias module is: Mini-Circuits, ADCH-80A, and the operating frequency is 10 MHz–10 GHz. The semiconductor laser uses an F-P semiconductor laser, the central wavelength is 638 nm, the original line width is 0.1 nm, and the output power is 30 mW.

[0047] Example 1

[0048] Combined with Figure 6 and Figure 7 , the multi-surface topography testing method based on coherent controllable laser of the present invention is applied to the optical path structure schematic diagram of the Twyman-Green dynamic interferometer 12-1.

[0049] Step 1: Connect the coherent controllable laser light source and the Twyman-Green dynamic interferometer 12-1 through the fiber optic connector 11.

[0050] Step 2: Place the multi-surface interference element 13 in the test optical path of the Twyman-Green dynamic interferometer 12-1, and control the cavity length so that the optical path of the test optical path is 1 - 3 cm longer than the reference optical path.

[0051] Step 3: Turn on the DC drive power supply 8, the semiconductor laser 9 outputs long coherent laser light, observe the multi-surface interference fringes received by the polarization camera of the Twyman-Green dynamic interferometer 12-1, and adjust the pitch and tilt of the multi-surface interference element 13 so that the two groups of interference fringes are clearly displayed, and there are 4 - 10 fringes in the field of view.

[0052] Step 4: After turning on the broadband white noise source 1, turn on the RF amplifier 5. The coherent controllable laser light source undergoes spectral broadening. The broadened light enters the Twyman-Green dynamic interferometer 12-1 through the polarization-maintaining flange 14. After being collimated by the collimating mirror 15, it is incident on the polarization beam splitter 16 and split into P light and S light. The S light is used as a reference beam and is reflected by the polarization beam splitter 16 to the reference standard 18, and then reflected back to the polarization beam splitter 16. During this process, after passing through the first λ / 4 wave plate 17-1 twice back and forth, the polarization direction rotates by 90° and becomes P light, and then passes through the polarization beam splitter 16 and is incident on the imaging optical path; the P light is used as a test beam and passes through the polarization beam splitter 16 and is incident on the multi-surface interference element 13, and then is reflected back to the polarization beam splitter 16. Similarly, during this process, after the P light passes through the second λ / 4 wave plate 17-2 twice, it becomes S light and is reflected by the polarization beam splitter 16 to the imaging optical path; in the imaging optical path, the orthogonal polarization beams carrying the surface topography information of the multi-surface interference element 13 are changed into circularly polarized lights with opposite rotation directions by the third λ / 4 wave plate 17-3, and then the polarization camera 19 performs polarization phase shifting to form 4 interference fringe patterns with equal phase difference intervals.

[0053] Gradually reduce the attenuation of the adjustable RF attenuator 4 to shorten the coherence length of the light source, and observe the presence of the interference fringes in the gray-scale image received by the polarization camera of the interferometer 12. When the interference fringes disappear, move the multi-surface interference element 13 back and forth in the direction of decreasing the optical path of the test optical path until the interference fringes are received again in the polarization camera, and then continue to reduce the attenuation of the adjustable RF attenuator 4.

[0054] Step 5: Repeat Step 4 until the attenuation of the adjustable RF attenuator 4 reaches the minimum. Further adjust the front and back positions of the multi-surface interference element 13 so that the interference fringes in the image received by the polarization camera exist and have the best contrast, that is, the front surface of the multi-surface interference element 13 reaches the zero optical path difference position of the Twyman-Green dynamic interferometer 12-1. Collect the current interference fringe pattern through the polarization camera in the Twyman-Green dynamic interferometer 12-1, and calculate the surface topography of the front surface of the multi-surface interference element 13 through the four-step phase-shifting algorithm.

[0055] Step 6: Continue to move the multi-surface interference element 13 in the direction of decreasing the optical path of the test optical path of the Twyman-Green dynamic interferometer 12-1. When the contrast of the interference fringes in the image received by the polarization camera is the best again, the back surface of the multi-surface interference element 13 reaches the zero optical path difference position of the Twyman-Green dynamic interferometer 12-1. Collect the current interference fringe pattern through the polarization camera in the Twyman-Green dynamic interferometer 12-1, and calculate the surface topography of the back surface of the multi-surface interference element 13 through the four-step phase-shifting algorithm.

[0056] Example 2

[0057] Combined with Figure 8 and Figure 9, the multi-surface topography measurement method based on coherent controllable laser of the present invention is applied to the schematic optical path structure of the Fizeau dynamic interferometer 12-2.

[0058] Step 1: Connect the coherent controllable laser light source and the Fizeau dynamic interferometer 12-2 through the fiber optic connector 11.

[0059] Step 2: Place the multi-surface interference element 13 in the test optical path of the Fizeau dynamic interferometer 12-2, and control the cavity length of the interferometer so that the optical path of the test optical path is 1-3 cm longer than that of the reference optical path.

[0060] Step 3: Turn on the DC drive power supply 8, the semiconductor laser 9 outputs long coherent laser light, observe the multi-surface interference fringes received by the polarization camera of the Fizeau dynamic interferometer 12-2, and adjust the pitch and tilt of the multi-surface interference element 13 to make the two groups of interference fringes clear, and there are 4-10 fringes in the field of view.

[0061] Step 4: Turn on the broadband white noise source 1 and then turn on the RF amplifier 5. The coherent controllable laser light source undergoes spectral broadening. The broadened light enters the Fizeau dynamic interferometer 12-2 through the polarization-maintaining flange 20, enters the polarization fiber beam splitter 22 through the first polarization-maintaining fiber 21-1 and is divided into P light and S light. The P light and S light respectively pass through the second polarization-maintaining fiber 21-2, the fiber optic tunable RF attenuator 23, the third polarization-maintaining fiber 21-3, the fourth polarization-maintaining fiber 21-4, the fiber optic delay line 24, and the polarization-maintaining fiber 21-5 and enter the polarization fiber combiner 25. The combined short coherent light passes through the fifth polarization-maintaining fiber 21-6 and the fiber optic output head 26 and is incident on the negative lens 27. After being expanded by the negative lens 27, it enters the beam splitting prism 28 and is collimated by the positive lens 29. It is reflected at the standard part 30 and the multi-surface interference element 13. The reflected light is converged by the positive lens 29 and then reflected by the beam splitting prism 28 into the imaging optical path. After being collimated by the collimating mirror 31, the orthogonal polarization light beam carrying the surface topography information of the multi-surface interference element 13 is changed into circularly polarized light with opposite rotation directions by the λ / 4 wave plate 32, and then four interference fringe images with equal phase difference intervals are formed by polarization phase shifting by the polarization camera 33.

[0062] Gradually reduce the attenuation amount of the tunable RF attenuator 4 to shorten the coherence length of the light source, and observe the presence of the interference fringes in the gray-scale image received by the polarization camera of the interferometer 12. When the interference fringes disappear, adjust the delay amount of the fiber optic delay line 24 (equivalent to moving the multi-surface interference element 13 forward and backward) in the direction of decreasing the optical path of the test optical path until the interference fringes are received again by the polarization camera, and continue to reduce the attenuation amount of the tunable RF attenuator 4.

[0063] Step 5: Repeat Step 4 until the attenuation of the adjustable RF attenuator 4 reaches the minimum. Further adjust the delay of the optical fiber delay line 24 or the front and back positions of the multi-surface interference element 13 to make the interference fringes exist and have the best contrast in the image received by the polarization camera, that is, the front surface of the multi-surface interference element 13 reaches the zero optical path difference position of the Fizeau dynamic interferometer 12-2. Collect the current interference fringe pattern through the polarization camera in the Fizeau dynamic interferometer 12-2, and calculate the surface topography of the front surface of the multi-surface interference element 13 through the four-step phase-shifting algorithm.

[0064] Step 6: Continue to adjust the delay of the optical fiber delay line 24 in the direction of reducing the optical path of the test optical path of the Fizeau dynamic interferometer 12-2 or move the front and back positions of the multi-surface interference element 13. When the contrast of the interference fringes in the image received by the polarization camera is the best again, the back surface of the multi-surface interference element 13 reaches the zero optical path difference position of the Fizeau dynamic interferometer 12-2. Collect the current interference fringe pattern through the polarization camera in the Fizeau dynamic interferometer 12-2, and calculate the surface topography of the back surface of the multi-surface interference element 13 through the four-step phase-shifting algorithm.

[0065] In summary, the present invention can be applied to interferometers with different structures to measure the topography of multi-surface interference elements, avoid crosstalk of multi-surface interference fringes, improve the measurement accuracy, device integration and simplicity of multi-surface topography measurement while quickly positioning the zero optical path difference position of the interferometer, and is more suitable for industrialization.

Claims

1. A multi-surface topography testing method based on coherent controllable laser, characterized in that, The steps are as follows: Step 1: Connect the coherent controllable laser light source and the interferometer (12) through the fiber optic connector (11), and then go to Step 2; Among them, the coherent controllable laser light source includes a broadband white noise source (1), a first RF connecting line (2-1), a filter (3), a second RF connecting line (2-2), an adjustable RF attenuator (4), a third RF connecting line (2-3), an RF amplifier (5), a fourth RF connecting line (2-4), a bias module (6), a fifth RF connecting line (2-5), a semiconductor laser (9), and a polarization-maintaining fiber (10) connected in sequence; it also includes a DC drive power supply (8) connected to the bias module (6) through a wire (7); the polarization-maintaining fiber (10) is connected to the interferometer (12) through the fiber optic connector (11); Step 2: Place the multi-surface interference element (13) in the test optical path inside the interferometer (12), control the cavity length of the interferometer to make the optical path of the test optical path 1-3 cm longer than that of the reference optical path, and then go to Step 3; Step 3: Turn on the DC drive power supply (8), the semiconductor laser (9) outputs long coherent laser light, observe the multi-surface interference fringes received by the polarization camera in the interferometer (12), and adjust the pitching and tilting postures of the multi-surface interference element (13) to make the corresponding two sets of interference fringes clear, and then go to Step 4; Step 4: First turn on the broadband white noise source (1), then turn on the RF amplifier (5), gradually reduce the attenuation of the adjustable RF attenuator (4), observe the presence of the interference fringes in the grayscale image received by the polarization camera in the interferometer (12), and move the multi-surface interference element (13) back and forth in the direction of reducing the optical path of the test optical path of the interferometer (12) during this process to make the interference fringes exist in the image received by the polarization camera, and continue to reduce the attenuation of the adjustable RF attenuator (4). The coherence length of the coherent controllable laser light source shortens as the attenuation of the adjustable RF attenuator (4) decreases. When the coherence length of the coherent controllable laser light source is lower than the thickness of the multi-surface interference element (13), the light source behaves as a short coherence light source. At this time, the crosstalk of multiple sets of interference fringes on the multi-surface interference element (13) disappears, and then go to Step 5; Step 5: Repeat Step 4 until the attenuation of the adjustable RF attenuator (4) reaches the minimum, and further adjust the front and back positions of the multi-surface interference element (13) to make the interference fringes exist and have the best contrast in the image received by the polarization camera, that is, the front surface of the multi-surface interference element (13) reaches the zero optical path difference position of the interferometer (12). Collect the current interference fringe pattern through the polarization camera in the interferometer (12), and calculate the surface topography of the front surface of the multi-surface interference element (13) through the four-step phase-shifting algorithm, and then go to Step 6; Step 6: Continue to move the multi-surface interference element (13) in the direction of decreasing the optical path of the test optical path of the interferometer (12). When the contrast of the interference fringes in the image received by the polarization camera is optimal again, the rear surface of the multi-surface interference element (13) reaches the zero optical path difference position of the interferometer (12). Collect the current interference fringe pattern through the polarization camera in the interferometer (12), and calculate the surface topography of the rear surface of the multi-surface interference element (13) through the four-step phase-shifting algorithm.

2. The multi-surface topography testing method based on coherent controllable laser according to claim 1, characterized in that: In the coherent controllable laser light source, the broadband white noise source (1) is connected to the filter (3) through the first RF connection line (2-1), and then through the second RF connection line (2-2), the adjustable RF attenuator (4), and the third RF connection line (2-3) to the input end of the RF amplifier (5). The output end of the RF amplifier (5) is connected to the AC input end of the bias module (6) through the fourth RF connection line (2-4). The DC drive power supply (8) is connected to the DC input end of the bias module (6) through the wire (7). The DC signal and the AC signal are superimposed by the bias module (6) and then input into the semiconductor laser (9) through the fifth RF connection line (2-5). The semiconductor laser (9) is connected to the fiber optic connector (11) through the polarization-maintaining fiber (10); The broadband white noise source (1) generates a broadband white noise signal, which enters the filter (3) through the first RF connection line (2-1) for filtering. The filtered white noise signal enters the adjustable RF attenuator (4) through the second RF connection line (2-2) for signal attenuation. The attenuated white noise signal enters the RF amplifier (5) through the third RF connection line (2-3) for signal power amplification. The amplified white noise signal enters the AC input end of the bias module (6) through the fourth RF connection line (2-4). The DC drive power supply (8) generates a bias current, which enters the DC input end of the bias module (6) through the wire (7). The bias current and the white noise signal are coupled by the bias module (6) and then enter the semiconductor laser (9) through the fifth RF connection line (2-5). The output optical spectrum of the semiconductor laser (9) is broadened and output through the polarization-maintaining fiber (10) connected to the fiber optic connector (11).

3. The multi-surface topography testing method based on coherent controllable laser according to claim 2, wherein: The semiconductor laser (9) in the coherent controllable laser light source is an FP cavity semiconductor laser, which outputs multiple longitudinal modes under a DC power supply. Under the action of the broadband white noise current, the output spectral intensity and phase are modulated by the noise current to produce spectral broadening, and the broadening effect is positively correlated with the intensity of the broadband white noise.

4. The multi-surface topography testing method based on coherent controllable laser according to claim 3, characterized in that: The broadband white noise source (1) in the coherent controllable laser light source directly generates broadband white noise through the RF circuit, or collects the amplified spontaneous emission ASE optical signal through a photodetector and converts it into an electrical signal to obtain a Gaussian white noise signal.

5. The multi-surface topography testing method based on coherent controllable laser according to claim 3, wherein: The adjustable RF attenuator (4) in the coherent controllable laser light source controls the attenuation amount through a single-chip microcomputer. The smaller the attenuation amount, the higher the intensity of the broadband white noise signal entering the RF amplifier (5), the wider the output spectral broadening of the semiconductor laser (9), and the shorter the coherence length of the coherent controllable laser light source.

6. The multi-surface topography testing method based on coherent controllable laser according to claim 3, characterized in that: The DC drive power supply (8) in the coherent controllable laser light source is set with a current lower than the current at the center of the linear region of the current-power curve of the semiconductor laser (9) and higher than the threshold current of the semiconductor laser (9). If the bias current is too high, the spectral broadening width will be reduced; if it is too low, the output optical power will be low.

7. The multi-surface topography testing method based on coherent controllable laser according to claim 3, characterized in that: The bias module (6) in the coherent controllable laser light source uses a Bias Tee circuit to superimpose a DC signal and a broadband white noise signal and apply them to the semiconductor laser (9). It blocks AC on the DC signal terminal and blocks DC on the AC signal terminal to avoid signal crosstalk and burning out the device.

8. The multi-surface topography testing method based on coherent controllable laser according to claim 1, characterized in that: The interferometer (12) uses a Twyman-Green type or Fizeau type dynamic interferometer, and the multi-surface interference element (13) is a parallel plate type element.

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