CN105890878B - Measure the measurement apparatus and method of speculum damage threshold in real time using femtosecond laser - Google Patents

Abstract

The invention discloses a measuring device and method for real-time measuring the damage threshold of a reflector by using a femtosecond laser. In the present invention, after splitting the incident laser beams, one beam is used as the main laser beam and the other beam is frequency-multiplied as the detection light; the main laser beam is incident on the surface of the mirror sample, and if it is damaged, plasma will be generated on the surface; a part of the detection light carries plasma information , the part carrying the plasma information interferes with the part not carrying the plasma information to generate interference fringes, the delay of the detection light is changed by delaying the optical path, and the interference pattern at different times after the main laser passes is detected, and the main laser is incident on the mirror sample The evolution process of the plasma generated later is inverted to obtain the damage of the reflector sample; the invention monitors the damage of the femtosecond laser to the 0° reflector in real time under the conditions of different energies and different power densities, and can be used for the damage threshold in the field of optical coatings The measurement of the femtosecond mirror has guiding significance for the improvement of the method of improving the damage threshold of the femtosecond mirror.

Description

Measurement device and method for real-time measurement of mirror damage threshold using femtosecond laser

technical field

The invention belongs to the field of optical measurement, and in particular relates to a measuring device and method for real-time measuring the damage threshold of a mirror by using a femtosecond laser.

Background technique

The interaction between femtosecond laser and matter is one of the recent research hotspots, especially in the ignition of inertial confinement fusion (ICF) and laser accelerated particles. The first ruby laser was invented by American Maiman in 1960, and it was tuned in 1962. The emergence of Q technology enables people to obtain laser pulses of the nanosecond (ns) level. In 1963, the invention of laser mode-locking technology made the pulse width enter the picosecond (ps) level. Since then, the development of laser technology has been relatively flat until 1985. D. Strickland and G. Mouro proposed Chirped-pulse Amplification (CPA) to make the generation of high-power small desktop lasers a reality, but the problem that followed was that the transmission of high-power lasers required a damage threshold Higher optical components, especially mirrors used for femtosecond lasers, are therefore particularly important for research on the damage threshold of mirrors. Unlike general long-pulse laser (such as picosecond, nanosecond, etc.) damage, femtosecond laser The action time is short, and the instantaneous damage is different from the thermal diffusion process of the mirror damage caused by the long pulse, which is mainly caused by the electric field effect of the laser. As a probe, the femtosecond probe light can detect the instantaneous damage of the incident laser to the mirror and generate surface plasma. After the probe light passes through, it can carry the plasma expansion process and density distribution information. The coating layer of the mirror is improved, so the real-time measurement of the damage threshold of the mirror is of great significance for the transmission of femtosecond laser and the development of laser technology.

The femtosecond laser is used as a probe to detect the plasma density, which is mainly based on the interference principle of light. In the plasma density detection, because the refractive index of the region where the plasma exists is different from the general optical path, the small optical path difference introduced is reflected in the interference fringes. . Mitsuo Takeda used the Fourier transform method to extract the phase information in the interferogram in the article "Fourier-transform method of fringe-patternanalysis for computer-based topography and interferometry", and the intensity I(x,y) distribution of the interference fringe:

A(x,y) represents the background of the fringe, B(x,y) is the amplitude of the light intensity change of the fringe in the interference area, with Respectively represent the space carrier frequency,

φ(x,y) is the phase distribution of the measured wavefront; the intensity distribution of the interference fringes is Fourier transformed:

The distribution expression of i(x,y) interference fringes in the frequency domain, a(f _x ,f _y ) is the fundamental frequency component in the frequency domain, and For the high-frequency term containing the phase, one of the two is filtered out and moved to the fundamental frequency position, and the inverse Fourier transform is performed to obtain:

C(x,y)=b(x,y)exp(iφ(x,y))=FFT ^-1 (b′(f _x ,f _y ))

C(x, y) is an intermediate transformation item containing phase information, φ _m represents the phase change introduced by the probe light passing through the plasma region, then

The phase information obtained by the Fourier transform method is combined with the Abbe inverse transform to obtain the density distribution of the plasma area, and the damage can be judged by the collected interferogram at the moment of damage.

In the article "Dynamics of Relativistic Laser-Plasma Interaction on Solid Targets", Y.Ping et al. found that when the laser interacts with the solid target, the damage occurs due to the movement of the plasma surface, and the scattering spectrum has a certain shift, and the damage degree is different. The offset is also different.

In the case of high-power laser output, it is more common to use a half-wave plate to change the output energy. After the half-wave plate, a polarization beam splitter is added to ensure the degree of polarization, so that a certain polarization component of the output light is completely transmitted or reflected. This combination can It is more convenient to change the output energy when the polarization is guaranteed.

Contents of the invention

Based on the current ultra-fast detection process and according to the movement of the incident spectrum in the laser acceleration experiment, the present invention proposes a measurement device and method for real-time monitoring of the damage of femtosecond lasers to general reflectors under different energies and different power densities. , can be used to measure the damage threshold in the field of optical coatings, combined with interferometry, analyze the physical process of damage from the obtained interference image, which has guiding significance for improving the method of improving the damage threshold of femtosecond mirrors.

An object of the present invention is to propose a measurement device for real-time measurement of the damage threshold of a mirror by using a femtosecond laser.

The measurement device of the present invention for measuring the damage threshold of a reflector in real time using a femtosecond laser comprises: a collimation device, a first beam splitter, an energy tuner, a main laser light path adjustment device, a first lens, a second lens, a frequency doubling crystal, The third lens, the detection light adjustment optical path device, the delay optical path device, the second beam splitter, the rectangular prism, the 0° reflector and the receiving CCD; wherein the linearly polarized laser is collimated by the collimation device and passes through the first beam splitter , a beam of light reaches the energy tuner as the main laser, and maintains the original polarization direction after passing through the energy tuner, while the energy changes; the optical path device is adjusted through the main laser to ensure that the position of the main laser spot on the surface of the mirror sample remains unchanged; The first lens placed on the first translation stage is focused to adjust the spot size of the main laser incident on the mirror sample; the main laser is incident on the surface of the mirror sample at an angle of 0°; if the main laser damages the mirror sample, then Plasma is generated on the surface of the mirror sample; after passing through the first beam splitter, another beam of light is focused on the frequency doubling crystal by the second lens, and the emitted frequency doubling light is collimated by the third lens to form parallel light as a detection The light is incident on the detection light adjustment optical path device; after the detection light is adjusted by the detection light adjustment optical path device, the delay of the detection light is adjusted by the delay optical path device; after exiting from the delay optical path device, the detection light is incident on the reflector at an angle of 90° Sample; the beam diameter of the probe light is larger than the width of the plasma region generated by the mirror sample, and part of the probe light after passing through the mirror sample carries plasma information, and the other part does not carry plasma information; the probe light passes through the second beam splitter Divided into two beams, one beam is reflected by a right-angle prism, and the part carrying plasma information and the part not carrying plasma information in this beam are turned over; the other beam returns to the original path through the 0° reflector, and the two beams combine again. After combining the beams, the part carrying the plasma information and the part not carrying the plasma information interfere to generate interference fringes; after passing through the first filter, the receiving CCD receives the interference patterns at different times; the inversion obtains the damage degree of the mirror sample.

The collimation device adopts a pair of first and second total reflection mirrors parallel to each other to realize optical path collimation.

The energy tuner includes a rotating glass plate and two polarizing mirrors. The rotating wave plate can change the polarization direction of the main laser. The degree of polarization can be changed while changing the energy; the energy of the main laser can be automatically adjusted to realize the measurement of the damage degree of the sample under the same pulse width, the same spot and different laser energies.

The main laser adjusting optical path device includes a pair of first and second partial light reflectors parallel to each other. By adjusting the first and second partial light reflectors, the position of the incident light spot of the main laser on the sample surface of the reflector remains unchanged. An energy meter is set behind the first partial light reflector. After the main laser passes through the first partial light reflector, a part of the light passes through and enters the energy meter for real-time energy monitoring. After the second part of the light reflector, an optical filter and a return light monitoring CCD are arranged. After the main laser passes through the second part of the light reflector, part of the light passes through the second light filter and enters the return light monitoring CCD to receive the return light signal. When the main laser is incident on the reflector sample and the reflector sample is not damaged, there will be a fundamental frequency return light at the backscattered light monitoring position, which cannot be detected on the return light monitoring CCD after passing through the second filter; if the reflector sample When the main laser is incident, plasma is generated instantly on the surface of the mirror sample, and the backscattered light is mixed with light of other frequencies. After passing through the second filter, bright spots can be detected on the return light monitoring CCD. The second optical filter adopts a fundamental frequency optical filter.

The first lens is placed on the first translation stage, and the first translation stage moves one-dimensionally along the direction perpendicular to the mirror sample surface to adjust the distance between the first lens and the mirror sample surface, thereby adjusting the main laser incident to the reflector The size of the light spot on the mirror sample, so the size of the light spot can be changed arbitrarily.

The main laser is incident on the surface of the mirror sample at an angle of 0°, and the mirror sample is a 0° mirror.

The incident linearly polarized laser is divided into two beams after passing through the first beam splitter, one beam is used as the main laser beam, and the other beam is used as the probe light after frequency doubling, which avoids the interference of the main laser fundamental frequency light, and the measured interferogram The background is low.

The frequency doubling crystal adopts one of barium metaborate BBO crystal, potassium dihydrogen phosphate KDP and lithium triborate LBO, and the frequency doubling light emitted from the frequency doubling crystal is used as the detection light.

The detection light adjusting optical path device includes fourth and fifth total reflection mirrors perpendicular to each other, and the detection light is collimated by the fourth and fifth total reflection mirrors and then incident on the surface of the mirror sample at an angle of 90°.

The delay optical path device includes sixth and seventh total reflection mirrors perpendicular to each other, and the sixth and seventh total reflection mirrors perpendicular to each other are fixed on the second translation platform, and the translation platform is moved one-dimensionally along the direction parallel to the optical path, thereby changing The time delay of the detection light detects the evolution process of the plasma generated after the main laser is incident on the surface of the mirror sample within a certain time scale, and the different reactions after femtoseconds, picoseconds and nanoseconds, the main laser is incident on the mirror When the probe light passes behind the sample surface at different times, interference patterns at different times can be obtained in the receiving CCD.

Another object of the present invention is to provide a method for measuring the damage threshold of a reflector in real time by using a femtosecond laser.

The measuring method utilizing femtosecond laser of the present invention to measure mirror damage threshold in real time comprises the following steps:

1) The linearly polarized laser is collimated by the collimation device, passes through the first beam splitter, and a beam of light is used as the main laser to the energy tuner. After passing through the energy tuner, the original polarization direction is maintained, and the energy is changed at the same time;

2) Adjust the optical path device through the main laser to ensure that the position of the light spot of the main laser on the surface of the mirror sample remains unchanged;

3) adjusting the spot size of the main laser beam incident on the mirror sample by focusing the first lens placed on the first translation stage;

4) The main laser is incident on the surface of the mirror sample at an angle of 0°;

5) If the main laser causes damage to the mirror sample, plasma will be generated on the surface of the mirror sample;

6) The other beam of light passing through the first beam splitter is focused by the second lens on the frequency doubling crystal, and the emitted frequency doubling light is collimated by the third lens to form parallel light, which is incident to the detection light adjusting optical path device as the detection light ; After the detection light is adjusted by the detection light adjusting optical path device, the time delay of the detection light is adjusted by the delay optical path device;

7) After exiting from the delay optical path device, the probe light is incident on the mirror sample at an angle of 90°; the beam diameter of the probe light is larger than the width of the plasma region generated by the mirror sample, and a part of the probe light after passing through the mirror sample carries Plasma information, the other part does not carry plasma information;

8) The probe light is divided into two beams by the second beam splitter, one beam is reflected by a rectangular prism, and the part carrying plasma information and the part not carrying plasma information in this beam are folded; the other beam is passed by a 0° reflector Returning to the original path, the two beams merge again, and the part carrying the plasma information and the part not carrying the plasma information interfere to generate interference fringes;

9) Obtain the interference pattern received by the receiving CCD after passing through the first optical filter;

10) The plasma distribution on the surface of the mirror sample is obtained by inversion of interference fringes;

11) Change the delay of the probe light by delaying the optical path, detect the interference pattern at different times after the main laser is incident on the surface of the mirror sample, and obtain the evolution process of the plasma generated after the main laser is incident on the mirror sample, and invert it to obtain Damage to mirror samples.

Wherein, in step 5), it is judged whether the main laser causes damage to the reflector sample, which specifically includes: the main laser adjusting optical path device includes first and second partial light reflections parallel to each other; an optical filter is arranged behind the second partial light reflector And the return light monitoring CCD, the return light reflected by the surface of the reflector sample passes through the second part of the light reflector, and then enters the return light monitoring CCD through the second filter to receive the return light signal. When the main laser is incident on the reflector On the sample, when the reflector sample is not damaged, there will be a strong fundamental frequency return light at the backscattered light monitoring position, which cannot be detected on the return light monitoring CCD after passing through the filter; if the reflector sample is damaged, after the main laser incident Plasma is generated instantly on the surface of the mirror sample, and the backscattered light is mixed with other frequency light, and after passing through the band-stop filter, bright spots are detected on the return light monitoring CCD.

In step 10), the interference fringe inversion is obtained to obtain the damage degree of the mirror sample, which specifically includes the following steps:

i. Determine the phase difference change

in, Indicates the phase difference introduced by the probe light passing through the plasma region, λ represents the wavelength of the probe light, ∈ is the refractive index of the plasma region, s represents the integration path, and I(x) is the change of the optical path difference obtained after integration;

ii. Obtain the optical path difference I(x) according to the integral of the refractive index change value on the path:

Wherein, I(x) represents the optical path difference obtained after integration, ∈(r) represents the refractive index along the integration radius, r is the radius when integrating along the axial direction, and R is the integration radius of the plasma boundary region;

iii. Perform Abbe inverse transformation on the optical path difference I(x) to obtain the relationship between the phase difference and the refractive index of the plasma, and obtain the density of the plasma from the refractive index of the plasma, and then obtain the damage degree of the mirror sample, so The plasma distribution on the surface of the mirror sample can be obtained by using the change of the optical path difference.

If curved interference fringes can be detected in the interference pattern, it means that the mirror surface of the mirror sample is damaged. If there are only parallel fringes, it means that there is no damage. The measured plasma density distribution can directly show the appearance of the damaged surface. The density of the formed plasma is related to the expansion time of the formed plasma after the main laser is incident on the mirror sample surface.

Advantages of the present invention:

In the present invention, after splitting the incident laser light, one beam is used as the main laser beam and the other beam is frequency-multiplied as the detection light; the main laser beam is incident on the surface of the mirror sample, and if it is damaged, plasma will be generated on the surface; a part of the detection light carries plasma Information, the part carrying the plasma information interferes with the part not carrying the plasma information to generate interference fringes, the delay of the detection light is changed by delaying the optical path, and the interference patterns at different times after the main laser is incident on the surface of the mirror sample are detected, and the main The evolution process of the plasma generated after the laser is incident on the reflector sample is reversed to obtain the damage of the reflector sample; the present invention monitors the damage of the femtosecond laser to the 0° reflector in real time under the conditions of different energies and different power densities, and can be used It is used in the measurement of damage threshold in the field of optical coatings, combined with interferometry, and the physical process of damage is analyzed from the obtained interference image, which has guiding significance for improving the method of improving the damage threshold of femtosecond mirrors.

Description of drawings

Fig. 1 is the schematic diagram of an embodiment of the measuring device utilizing femtosecond laser real-time measurement mirror damage threshold of the present invention;

Fig. 2 is a partially enlarged schematic diagram of the detection light divided into two beams and interfering in the measuring device for real-time measuring the damage threshold of the mirror by using femtosecond laser according to the present invention.

detailed description

The present invention will be further elaborated below through specific embodiments in conjunction with the accompanying drawings.

As shown in Figure 1, the measuring device utilizing femtosecond laser to measure the damage threshold of reflector in real time in this embodiment includes: first and second total reflection mirrors 1 and 2 parallel to each other, first beam splitter 3, rotating slide 4 An energy tuner composed of the first and second polarizing reflectors 6 and 7, the first and second partial light reflectors 8 and 9 parallel to each other, the first lens 10, the second lens 11, the frequency doubling crystal 12, The third lens 13, the fourth and fifth total reflection mirrors 14 and 15 perpendicular to each other, the sixth and seventh total reflection mirrors 16 and 17 perpendicular to each other, the second beam splitter 20, the rectangular prism 21, the 0° reflection mirror 22 And receiving CCD29; Wherein, linearly polarized laser light is collimated through the collimation device that the first and second total reflection mirrors 1 and 2 that are parallel to each other form, through the first beam splitter 3, the reflected 90% energy is used as main laser light, Reach the rotating glass slide 5 after being reflected by the third total reflection mirror 4, change the polarization direction, keep the original polarization direction after the first and second polarizing mirrors 6 and 7 parallel to each other, and the energy changes simultaneously; The main laser adjustment optical path device composed of the first and second partial light reflectors 8 and 9 ensures that the position of the spot of the main laser beam after the first lens 10 is focused on the surface of the reflector sample remains unchanged; the first lens focus 10 is installed on the first On the translation stage, adjust the spot size of the main laser incident on the reflector sample; the main laser is incident on the surface of the reflector sample at an angle of 0°; 5% of the leaked light from the first part of the light reflector 8 is reflected by the beam splitter 24 and then enters the second part. Pulse width measurement is carried out in the order correlator 25; 50% of the light transmitted by the beam splitter 24 enters the energy meter 26 for real-time energy monitoring; the second part of the mirror 9 has 5% light leakage, when the mirror sample 19 When there is damage, the second optical filter 27 and the return light monitoring CCD28 are installed behind the second part of the reflector 9 to receive the return light signal; 10% of the light transmitted through the first beam splitter 3 is focused through the second lens 11 to On the frequency doubling crystal 12, the outgoing frequency doubling light is collimated by the third lens 13 and is used as the detection light to be parallel incident to the detection light adjusting optical path device formed by the mutually perpendicular fourth and fifth total reflection mirrors 14 and 15; After the probe light adjusts the optical path device to be collimated, the time delay of the probe light is adjusted by the delay optical path device composed of the mutually perpendicular sixth and seventh total reflection mirrors 16 and 17 fixed on the second translation platform; , the probe light is incident on the mirror sample 19 at an angle of 90°, that is, the probe light passes along the surface of the mirror sample; as shown in Figure 2, the beam diameter of the probe light is greater than the width of the plasma region generated by the mirror sample, and the probe light One part carries plasma information, and the other part does not carry plasma information. The black dots in FIG. Covering the centerline of the rectangular prism 21, the part carrying the plasma information and the part not carrying the plasma information in this bundle are turned over; Combine the beams again, after the beams combine, the part that carries the plasma information and the part that does not carry The plasma information part interferes to generate interference fringes. After combining the beams, both the upper half and the lower half can generate interference. The receiving CCD can only receive a part of the interference fringes; after passing through the first filter 23, the receiving CCD 29 receives different The interference pattern at time; the damage degree of the mirror sample is obtained by inversion.

Finally, it should be noted that the purpose of the disclosed embodiments is to help further understand the present invention, but those skilled in the art can understand that various replacements and modifications can be made without departing from the spirit and scope of the present invention and the appended claims. It is possible. Therefore, the present invention should not be limited to the content disclosed in the embodiments, and the protection scope of the present invention is subject to the scope defined in the claims.

Claims (10)

1. A measuring device for measuring a damage threshold of a mirror in real time using a femtosecond laser, the measuring device comprising: the device comprises a collimating device, a first beam splitting sheet, an energy tuner, a main laser adjusting light path device, a first lens, a second lens, a frequency doubling crystal, a third lens, a detection light adjusting light path device, a time delay light path device, a second beam splitting sheet, a right-angle prism, a 0-degree reflector and a receiving CCD (charge coupled device); the linear polarization laser is collimated by the collimating device, passes through the first beam splitting sheet, reaches the energy tuner as a main laser, keeps the original polarization direction after passing through the energy tuner, and simultaneously changes the energy; the position of the light spot of the main laser on the surface of the reflector sample is ensured to be unchanged by the main laser light path adjusting device; focusing by a first lens arranged on a first translation platform, and adjusting the size of a light spot of the main laser incident on a reflector sample; the main laser is incident to the surface of the reflector sample at an angle of 0 degrees; if the main laser damages the reflector sample, plasma is generated on the surface of the reflector sample; another beam of light after passing through the first beam splitting sheet is focused on a frequency doubling crystal through a second lens, and the emergent frequency doubling light is collimated through a third lens to form parallel light which is used as detection light and is incident to a detection light adjusting light path device; the detection light is adjusted by the detection light adjusting light path device, and then the time delay of the detection light is adjusted by the time delay light path device; after the light is emitted from the time-delay light path device, the detection light is incident to the reflector sample at an angle of 90 degrees; the beam diameter of the detection light is larger than the width of a plasma area generated by the reflector sample, one part of the detection light passing through the reflector sample carries plasma information, and the other part of the detection light does not carry the plasma information; the detection light is divided into two beams by the second beam splitter, one beam is reflected by the right-angle prism, and the part carrying the plasma information in the beam is folded with the part not carrying the plasma information; the other beam returns through the original path of the 0-degree reflector, the two beams are combined again, and the combined beam carries the plasma information part to interfere with the part which does not carry the plasma information part to generate interference fringes; interference patterns at different moments are obtained by receiving the interference patterns by the receiving CCD after passing through the first optical filter; and (5) obtaining the damage degree of the reflector sample through inversion.

2. A measuring device according to claim 1, wherein said collimating means employs a pair of first and second total reflecting mirrors parallel to each other.

3. The measuring device according to claim 1, wherein the energy tuner comprises a rotating glass plate and two polarizing mirrors, the rotating wave plate changes the polarization direction of the main laser, and after passing through the two mutually parallel polarizing mirrors, the main laser is in the original polarization direction, so that the polarization degree of the main laser is ensured, and the energy is changed at the same time.

4. A measuring apparatus according to claim 1, wherein said primary laser light conditioning optical path means comprises a pair of first and second partially reflective mirrors which are parallel to each other.

5. The measuring device according to claim 4, wherein an energy meter is arranged behind the first part of the light reflecting mirror, and after the main laser passes through the first part of the light reflecting mirror, a part of light penetrates into the energy meter to perform real-time energy monitoring; and after the main laser passes through the second light reflector, part of the light passes through the second light filter and enters the return light monitoring CCD to receive return light signals.

6. The measurement apparatus of claim 1 wherein the first lens is placed on a first translation stage that moves in one dimension in a direction perpendicular to the sample surface of the mirror.

7. The measuring apparatus according to claim 1, wherein said probe light adjusting optical path means includes fourth and fifth total reflection mirrors which are perpendicular to each other.

8. The measuring apparatus according to claim 1, wherein said delayed optical path means comprises sixth and seventh total reflection mirrors which are perpendicular to each other, and the sixth and seventh total reflection mirrors which are perpendicular to each other are fixed to a second translation stage which moves one-dimensionally in a direction parallel to the optical path.

9. A measuring method for measuring a damage threshold of a reflector in real time by using femtosecond laser is characterized by comprising the following steps:

1) the linear polarization laser is collimated by a collimating device, passes through a first beam splitting sheet, a beam of light is taken as main laser to an energy tuner, and after passing through the energy tuner, the original polarization direction is kept, and meanwhile, the energy is changed;

2) the position of the light spot of the main laser on the surface of the reflector sample is ensured to be unchanged by adjusting the light path device through the main laser;

3) focusing through a first lens arranged on a first translation platform, and adjusting the size of a light spot of main laser incident on a reflector sample;

4) the main laser is incident to the surface of the reflector sample at an angle of 0 degrees;

5) if the main laser damages the reflector sample, plasma is generated on the surface of the reflector sample;

6) the other beam of light passing through the first beam splitting sheet is focused on a frequency doubling crystal through a second lens, and the emergent frequency doubling light is collimated through a third lens to form parallel light which is used as detection light and is incident to a detection light adjusting light path device; the detection light is adjusted by the detection light adjusting light path device, and then the time delay of the detection light is adjusted by the time delay light path device;

7) after the light is emitted from the time-delay light path device, the detection light is incident to the reflector sample at an angle of 90 degrees; the beam diameter of the detection light is larger than the width of a plasma area generated by the reflector sample, one part of the detection light passing through the reflector sample carries plasma information, and the other part of the detection light does not carry the plasma information;

8) the detection light is divided into two beams by the second beam splitter, one beam is reflected by the right-angle prism, and the part carrying the plasma information in the beam is folded with the part not carrying the plasma information; the other beam returns through the original path of the 0-degree reflector, the two beams are combined again, and the combined beam carries the plasma information part to interfere with the part which does not carry the plasma information part to generate interference fringes;

9) receiving the interference pattern by a receiving CCD after passing through a first optical filter;

10) the interference fringe inversion obtains the plasma distribution condition of the surface of the reflector sample;

11) and changing the delay of the detection light through the delay light path, detecting interference patterns at different moments after the main laser is incident on the surface of the reflector sample, obtaining the evolution process of plasma generated after the main laser is incident on the reflector sample, and obtaining the damage condition of the reflector sample through inversion.

10. The measurement method according to claim 9, wherein in step 10), the plasma distribution of the mirror sample is obtained by interference fringe inversion, and the method specifically comprises the following steps:

i. determining phase difference changes

Wherein,the phase difference of the detection light introduced through the plasma region is represented, lambda represents the detection light wavelength, epsilon is the refractive index of the plasma region, s represents an integral path, and I (x) is the optical path difference change obtained after integration;

obtaining an optical path difference i (x) from the integration of the refractive index change value over the path:

<mrow> <mi>I</mi> <mrow> <mo>(</mo> <mi>x</mi> <mo>)</mo> </mrow> <mo>=</mo> <mn>2</mn> <msubsup> <mo>&amp;Integral;</mo> <mi>x</mi> <mi>R</mi> </msubsup> <mo>&amp;lsqb;</mo> <mo>&amp;Element;</mo> <mrow> <mo>(</mo> <mi>r</mi> <mo>)</mo> </mrow> <mi>r</mi> <mo>/</mo> <msup> <mrow> <mo>(</mo> <msup> <mi>r</mi> <mn>2</mn> </msup> <mo>-</mo> <msup> <mi>x</mi> <mn>2</mn> </msup> <mo>)</mo> </mrow> <mrow> <mn>1</mn> <mo>/</mo> <mn>2</mn> </mrow> </msup> <mo>&amp;rsqb;</mo> <mi>d</mi> <mi>r</mi> </mrow>

wherein, I (x) represents the optical path difference obtained after integration, epsilon (R) represents the refractive index along the integral radius, R is the radius when integrating along the axial direction, and R is the integral radius of the plasma boundary region;

and iii, performing Abbe inverse transformation on the optical path difference I (x) to obtain the relation between the phase difference and the refractive index of the plasma, and obtaining the plasma distribution condition according to the refractive index of the plasma.