CN112857752A - Absolute measurement system and method for angle-resolved scattering of optical element - Google Patents
Absolute measurement system and method for angle-resolved scattering of optical element Download PDFInfo
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Abstract
The invention discloses an absolute measurement system and method for angular resolution scattering of an optical element, which are based on a cavity ring-down technology, simultaneously measure a cavity ring-down signal transmitted by an output cavity mirror and an angular resolution scattering cavity ring-down signal of the optical element to obtain the ratio of the amplitudes of the two cavity ring-down signals, obtain the absolute value of the angular resolution scattering of the optical element by calibrating the transmittance of the output cavity mirror, and obtain the angular resolution scattering distribution of the optical element by rotating the position of a scattering measurement photoelectric detector along the angular direction. The invention aims to provide an absolute measurement system and method for angular resolution scattering of an optical element, which are based on the cavity ring-down technology to improve the sensitivity of scattering measurement and eliminate the influence of stray light, and utilize the transmittance of a high-reflection cavity mirror with extremely low degree to calibrate the angular resolution scattering, thereby improving the absolute measurement precision of weak scattering.
Description
Technical Field
The invention relates to the technical field of optical characteristic testing of optical elements, in particular to an absolute measurement system and method for angular-resolved scattering of an optical element.
Background
The scattering property of the optical element is an important optical property parameter, and the scattering loss not only reduces the light energy transmittance of the optical system, but also affects the imaging quality of the optical system. Meanwhile, the angle-resolved scattering distribution also contains the information of the surface quality of the optical element, and the surface processing characteristics of the optical element can be inverted by measuring the angle-resolved scattering distribution characteristics of the optical element, so that the measurement data can be provided for the processing of the high-performance optical element. Therefore, it is particularly important to accurately measure the angular resolved scattering properties of the optical element.
The measurement of angle-resolved scattering of optical elements typically employs a spectrophotometric method, i.e., the magnitude of scattering is determined by measuring the ratio of the intensity of scattered light to the intensity of incident light over a range of solid angles (determined by the light collection area of the photodetector and the distance between the detector and the point at which the optical element is being measured) directly at an angle using the photodetector. For high performance optical elements with good surface quality, the scattering intensity is low, while the output power of the laser light source used for scattering measurement of the optical element is generally in the order of 10mW, and the intensity of the scattered light measured for angle-resolved scattering measurement is generally in the order of nW or less, so the photodetector used for measuring angle-resolved scattering requires not only extremely high detection sensitivity but also an extremely large dynamic range (at least 10 orders of magnitude). Meanwhile, angle-resolved scatterometry typically uses standard optical element samples known to scatter close to 100% to calibrate the scattered signal intensity, and this method of calibrating very low scatter values with high scatter values necessarily results in higher absolute measurement errors. In addition, in order to suppress or eliminate the influence of stray light generated by a laser light source and optical elements used in the scatterometry system on scatterometry, the stray light is generally eliminated by using a long beam shaping optical path, a complicated beam filtering, a diaphragm, and the like, which complicates the scatterometry optical system, makes alignment adjustment difficult, and is costly.
Disclosure of Invention
The invention aims to provide an absolute measurement system and method for angular resolution scattering of an optical element, which are based on the cavity ring-down technology to improve the sensitivity of scattering measurement and eliminate the influence of stray light, and utilize the transmittance of a high-reflection cavity mirror with extremely low degree to calibrate the angular resolution scattering, thereby improving the absolute measurement precision of weak scattering.
The invention is realized by the following technical scheme:
an absolute measurement system for angle-resolved scattering of an optical element is characterized by comprising a detection laser light source, a ring-down cavity, a first cavity ring-down signal detection unit, a sample bearing table, a second cavity ring-down signal detection unit, a function generator, a data acquisition card and a computer;
the detection laser light source is used for inputting a detection laser beam into the ring-down cavity;
the function generator is used for periodically modulating the intensity of the output of the detection laser light source;
the ring-down cavity is used for carrying out intra-cavity multiple reflection accumulation on the detection laser beam so as to obtain a cavity ring-down signal on the falling edge of the periodic modulation of the intensity of the detection laser light source;
the sample bearing table is placed in the ring-down cavity and used for placing an optical element to be measured;
the first cavity ring-down signal detection unit is used for acquiring a first cavity ring-down signal; wherein the first cavity ring-down signal is the cavity ring-down signal transmitted from an output cavity mirror of the ring-down cavity;
the second cavity ring-down signal detection unit is used for acquiring a second cavity ring-down signal, and the second cavity ring-down signal is the cavity ring-down signal scattered from the ring-down cavity by the optical element to be detected;
the data acquisition card is used for acquiring the first cavity ring-down signal and the second cavity ring-down signal;
the computer is used for fitting the amplitude I of the first cavity ring-down signal acquired by the data acquisition card according to a single exponential decay function00And the amplitude I of the second cavity ring-down signal01And obtaining an angle-resolved scattering absolute value of the optical element to be measured according to the following formula;
S＝I01T0/I00M；
wherein S is the absolute value of angle-resolved scattering of the optical element to be measured, T0The transmittance of the output cavity mirror; and M is the ratio of the amplification factors of the photoelectric detector of the second cavity ring-down signal detection unit and the photoelectric detector of the first cavity ring-down signal detection unit.
Preferably, the ring-down cavity comprises a first plano-concave mirror and a second plano-concave mirror;
the detection laser beam is transmitted by the first plano-concave reflecting cavity mirror and then is input into the ring-down cavity, and is reflected and accumulated for multiple times back and forth in the ring-down cavity formed by the first plano-concave reflecting cavity mirror and the second plano-concave reflecting cavity mirror, and a cavity ring-down signal is obtained on the detection laser light source intensity periodic modulation falling edge; the cavity ring-down signal is transmitted to the first cavity ring-down signal detection unit for detection after being transmitted by the second concave reflection cavity mirror, and the cavity ring-down signal is transmitted to the second cavity ring-down signal detection unit for detection after being scattered by the optical element to be detected.
Preferably, the ring-down cavity comprises a first plano-concave reflecting cavity mirror, a planar coupling cavity mirror and a second plano-concave reflecting cavity mirror;
the detection laser beam is transmitted by the plane coupling cavity mirror and then is input into the ring-down cavity, and is reflected and accumulated for many times back and forth in the ring-down cavity formed by the first plano-concave reflecting cavity mirror, the plane coupling cavity mirror and the second plano-concave reflecting cavity mirror, and a cavity ring-down signal is obtained on the periodic modulation falling edge of the detection laser light source intensity; the cavity ring-down signal is transmitted to the first cavity ring-down signal detection unit for detection after being transmitted by the second concave reflection cavity mirror, and the cavity ring-down signal is transmitted to the second cavity ring-down signal detection unit for detection after being scattered by the optical element to be detected.
Preferably, when the optical element to be detected is a high-reflection optical element, the detection laser beam output by the detection laser light source is reflected by the optical element to be detected and then transmitted to the second plano-concave reflecting cavity mirror.
Preferably, when the optical element to be detected is a transmissive optical element or an uncoated optical element, the detection laser beam output by the detection laser light source is transmitted by the optical element to be detected and then transmitted to the second plano-concave mirror.
Preferably, the first cavity ring-down signal detection unit includes a focusing lens and a first photodetector, and when the first cavity ring-down signal is transmitted from the output cavity mirror, the first cavity ring-down signal is focused to the first photodetector through the focusing lens.
Preferably, the single exponential decay function is:
Ij(t)＝I0je-t/τ+Aj；
wherein j is 0 or 1, and j is 0, which is the first cavity ring-down signal; when j is 1, the second cavity ring-down signal is obtained; i is0jFor signal amplitude and τ ring-down time, AjIs a dc bias.
A method of using an absolute measurement system for angular resolved scattering of an optical element as described above, comprising the steps of:
s1: acquiring a first cavity ring-down signal and a second cavity ring-down signal;
s2: fitting the amplitude I of the first cavity ring-down signal according to a single exponential decay function00And the amplitude I of the second cavity ring-down signal01；
S3: obtaining an angle-resolved scattering absolute value of the optical element to be measured according to the following formula:
S＝I01T0/I00M；
wherein S is the absolute value of angle-resolved scattering of the optical element to be measured, T0The transmittance of the output cavity mirror; and M is the ratio of the amplification factors of the photoelectric detector of the second cavity ring-down signal detection unit and the photoelectric detector of the first cavity ring-down signal detection unit.
Preferably, the single exponential decay function fitting is performed by selecting the first cavity ring-down signal detected by the photodetector of the first cavity ring-down signal detection unit and the second cavity ring-down signal detected by the photodetector of the second cavity ring-down signal detection unit after 0.1 μ s.
Preferably, the method further includes step S4, in which step S4 is used to scan the photodetector position of the second cavity ring-down signal detection unit along the angular direction, and steps S1 to S3 are repeated to obtain the angle-resolved scattering distribution of the optical element under test.
Compared with the prior art, the invention has the following advantages and beneficial effects:
1. the high-sensitivity and high-precision absolute measurement of the angular resolution scattering of the optical element is realized based on the cavity ring-down technology;
2. the requirement of scattering measurement on the sensitivity of the photoelectric detector is reduced by utilizing the optical amplification effect of the optical cavity resonant cavity;
3. the cavity ring-down method is utilized to inhibit the stray light;
4. the scattering value is calibrated by using the very low transmittance (usually lower than 0.01%) of the high-reflection second plano-concave mirror, so that the complexity and the cost of the angle-resolved scattering measurement system are greatly reduced, and the measurement precision is improved.
Drawings
The accompanying drawings, which are included to provide a further understanding of the embodiments of the invention and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the invention and together with the description serve to explain the principles of the invention. In the drawings:
FIG. 1 is a schematic diagram of the overall structure of angular resolution scattering measurement when the initial optical resonant cavity is a V-shaped cavity and the optical element to be measured is a high-transmittance optical element or an uncoated optical element according to the present invention;
FIG. 2 is a schematic diagram of the overall structure of the angular resolution scattering measurement when the initial optical resonant cavity is a V-shaped cavity and the measured optical element is a high-reflectivity optical element according to the present invention;
FIG. 3 is a schematic diagram of the overall structure of the angular resolved scattering measurement when the initial optical resonant cavity is a straight cavity and the optical element to be measured is a high-transmittance optical element or an uncoated optical element according to the present invention;
FIG. 4 is a schematic diagram of the overall structure of the angular resolved scatterometry measurement with a straight cavity as the initial optical resonator and a highly reflective optical element as the measured optical element according to the present invention;
FIG. 5 is a graph of the simultaneous measurement recording of the cavity ring-down signal transmitted through the second plano-concave mirror and the angularly resolved scattered cavity ring-down signal of the present invention;
reference numbers and corresponding part names in the drawings:
1. detecting a laser light source; 2. a planar coupling cavity mirror; 3. a first plano-concave reflective cavity mirror; 4. a second plano-concave mirror; 5. a focusing lens; 6. a first photodetector; 7. a sample carrier; 8. an optical element to be measured; 9. a second photodetector; 10. a function generator; 11. a data acquisition card; 12. and (4) a computer.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is further described in detail below with reference to examples and accompanying drawings, and the exemplary embodiments and descriptions thereof are only used for explaining the present invention and are not meant to limit the present invention.
Examples
An absolute measurement system for angle-resolved scattering of an optical element comprises a detection laser light source 1, a ring-down cavity, a first cavity ring-down signal detection unit, a sample bearing table 7, a second cavity ring-down signal detection unit, a function generator 10, a data acquisition card 11 and a computer 12;
the detection laser light source 1 is used for inputting a detection laser beam into the ring-down cavity;
a function generator 10 for periodically modulating the intensity of the output of the detection laser light source 1;
the ring-down cavity is used for carrying out multiple reflection accumulation in the cavity on the detection laser beam so as to obtain a cavity ring-down signal on the falling edge of the detection laser light source 1 with the intensity periodically modulated;
the sample bearing table 7 is arranged in the ring-down cavity and used for placing an optical element 8 to be measured;
the first cavity ring-down signal detection unit comprises a focusing lens 5 and a first photoelectric detector 6 and is used for acquiring a first cavity ring-down signal; the first cavity ring-down signal is a cavity ring-down signal transmitted from an output cavity mirror of the ring-down cavity; when the first cavity ring-down signal is transmitted out of the output cavity mirror, the first cavity ring-down signal is focused to a first photoelectric detector 6 through a focusing lens 5 for detection;
the second cavity ring-down signal detection unit is a second photoelectric detector 9 and is used for acquiring a second cavity ring-down signal, and the second cavity ring-down signal is a cavity ring-down signal scattered from the ring-down cavity by the optical element 8 to be detected;
the data acquisition card 11 is used for acquiring a first cavity ring-down signal detected by the first photoelectric detector 6 and a second cavity ring-down signal detected by the second photoelectric detector 9;
a computer 12 for fitting the amplitude I of the first cavity ring-down signal acquired by the data acquisition card 11 according to a single exponential decay function00And amplitude I of the second cavity ring-down signal01And obtaining an angle-resolved scattering absolute value of the optical element 8 to be measured according to the following formula;
S＝I01T0/I00M；
wherein S is the absolute value of the angle-resolved scattering of the optical element 8 to be measured, T0The transmittance of the output cavity mirror; m is the ratio of the magnification of the second photodetector 9 to the first photodetector 6, where, in this embodiment, the single exponential decay function is:
Ij(t)＝I0je-t/τ+Aj；
wherein, j is 0 or 1, and j is 0, which is the first cavity ring-down signal; when j is 1, the signal is a second cavity ring-down signal; i is0jFor signal amplitude, τ is ring down time, AjIs a dc bias.
In the scheme, the light amplification effect of the optical resonant cavity is skillfully utilized to amplify the detection laser beam output by the detection laser 1, so that the intensity of scattered light is increased, the photoelectric detector can detect the scattered light, and the requirement of scattering measurement on the sensitivity of the photoelectric detector is reduced; meanwhile, stray light is restrained by adopting a cavity ring-down method, and compared with the prior art, the stray light is eliminated by adopting a long light beam shaping light path, a complex light beam filtering mode, a diaphragm mode and the like, so that the complexity of the system can be effectively reduced.
Further, in the scheme, the ring-down cavity can be a straight cavity or a V-shaped cavity; the optical element 8 to be measured can be a high-reflection optical element, a transmission optical element or an uncoated optical element; according to the type of ring-down cavity and the type of the optical element 8 to be measured, the following four cases can be classified, as shown in fig. 1 to 4:
in fig. 1, the ring-down cavity is a V-shaped cavity, and the optical element 8 to be measured is a transmissive optical element or an uncoated optical element. The V-shaped cavity comprises a first plano-concave reflecting cavity mirror 3, a plane coupling cavity mirror 2 and a second plano-concave reflecting cavity mirror 4, and a sample bearing platform 7 is arranged between the first plano-concave reflecting cavity mirror 3 and the plane coupling cavity mirror 2. When the detection laser beam is transmitted to the V-shaped cavity through the plane coupling cavity mirror 2, the detection laser beam is reflected and accumulated in the V-shaped cavity for many times back and forth, and a cavity ring-down signal is obtained on the falling edge of the intensity periodic modulation of the detection laser light source 1, and on one hand, the cavity ring-down signal is transmitted through the second concave reflection cavity mirror 4, then leaves the V-shaped cavity and is detected by the first photoelectric detector 6; on the other hand, the light is scattered by the optical element 8 to be measured and then leaves the V-shaped cavity, and is collected by the second photoelectric detector 9.
In fig. 2, the ring-down cavity is a V-shaped cavity, and the optical element 8 to be measured is a high-reflection optical element. The V-shaped cavity comprises a first plano-concave reflecting cavity mirror 3, a plane coupling cavity mirror 2 and a second plano-concave reflecting cavity mirror 4, and a sample bearing platform 7 is arranged on the opposite side of the first plano-concave reflecting cavity mirror 3 and the plane coupling cavity mirror 2. When the detection laser beam is transmitted to the V-shaped cavity through the plane coupling cavity mirror 2, the detection laser beam is reflected and accumulated in the V-shaped cavity for many times back and forth, and a cavity ring-down signal is obtained on the falling edge of the intensity periodic modulation of the detection laser light source 1, and on one hand, the cavity ring-down signal is transmitted through the second concave reflection cavity mirror 4, then leaves the V-shaped cavity and is detected by the first photoelectric detector 6; on the other hand, the light is scattered by the optical element 8 to be measured and then leaves the V-shaped cavity, and is collected by the second photoelectric detector 9.
In fig. 3, the ring-down cavity is a straight cavity, and the optical element 8 to be measured is a transmissive optical element or an uncoated optical element. Wherein, the straight cavity comprises a first plano-concave reflecting cavity mirror 3 and a second plano-concave reflecting cavity mirror 4, and the sample bearing platform 7 is arranged between the first plano-concave reflecting cavity mirror 3 and the second plano-concave reflecting cavity mirror 4. When the detection laser beam is transmitted to the straight cavity through the first plano-concave reflecting cavity mirror 3, the detection laser beam is reflected and accumulated repeatedly in the straight cavity, and a cavity ring-down signal is obtained at the falling edge of the intensity periodic modulation of the detection laser light source 1, and because the optical element 8 to be detected is a transmission optical element or an uncoated optical element, the cavity ring-down signal on one hand leaves the ring-down cavity through the optical element 8 to be detected and the second plano-concave reflecting cavity mirror 4 and is detected by the first photoelectric detector 6; on the other hand, the light is scattered by the optical element 8 to be detected and then leaves the ring-down cavity to be detected by the second photoelectric detector 9.
In fig. 4, the ring-down cavity is a straight cavity, and the optical element 8 to be measured is a high-reflection optical element. Wherein, the straight cavity comprises a first plano-concave reflecting cavity mirror 3 and a second plano-concave reflecting cavity mirror 4, and the sample bearing platform 7 is arranged at an included angle with the first plano-concave reflecting cavity mirror 3 and the second plano-concave reflecting cavity mirror 4. When the detection laser beam is transmitted to the straight cavity through the first plano-concave reflecting cavity mirror 3, the detection laser beam is reflected and accumulated for multiple times back and forth in the straight cavity, and a cavity ring-down signal is obtained at the falling edge of the intensity periodic modulation of the detection laser light source 1, and because the optical element 8 to be detected is a high-reflection optical element, the cavity ring-down signal is transmitted to the second plano-concave reflecting cavity mirror 4 after being reflected by the optical element 8 to be detected, leaves the ring-down cavity and is detected by the first photoelectric detector 6; on the other hand, the light is scattered by the optical element 8 to be detected and leaves the ring-down cavity, and is detected by the second photodetector 9.
A method of using an absolute measurement system for angle-resolved scattering of an optical element as described above, comprising the steps of:
s1: acquiring a first cavity ring-down signal and a second cavity ring-down signal;
s2: fitting the amplitude I of the first cavity ring-down signal according to the single exponential decay function00And amplitude I of the second cavity ring-down signal01；
S3: the absolute value of the angle-resolved scattering of the optical element 8 to be measured is obtained as follows:
S＝I01T0/I00M；
wherein S is the absolute value of the angle-resolved scattering of the optical element 8 to be measured, T0The transmittance of the second plano-concave mirror 4; m is the ratio of the magnification of the second photodetector 9 to the first photodetector 6.
Further, in the scheme, in order to eliminate the influence of stray light on angle-resolved scatterometry, the amplitude I of the cavity ring-down signal is fitted by using a single exponential decay function00And amplitude I of angle-resolved cavity ring-down signal01Then the first cavity attenuation detected by the first photodetector 6 before 0.1 mus may be removedAnd fitting the oscillation signal and a second cavity ring-down signal detected by the second photoelectric detector 9 by using a single exponential decay function.
It should be noted that, to ensure accuracy, the ratio M of the amplification factors of the second photodetector 9 and the first photodetector 6 should be determined by measuring the same detected light signal with both photodetectors; transmittance T of second plano-concave reflecting cavity mirror 40The method can be determined by a variable-angle cavity ring-down method or a spectrophotometry method, and the scheme is not further elaborated because the method is not an improvement point of the scheme.
Further, in the scheme, in order to realize angle-resolved scatterometry of the optical element 8 to be measured at different positions, and obtain angle-resolved scatterometry distribution of the optical element 8 to be measured, the optical element 8 to be measured may be mounted on a two-dimensional translation stage to move its lateral (or longitudinal) position, and the second photodetector 9 may also be rotated to obtain angle-resolved scatterometry cavity ring-down signals at different angles, thereby obtaining absolute scatterometry values at different angles, and further obtaining angle-resolved scatterometry distribution of the optical element 8 to be measured at different positions.
The technical solution of the present solution is further explained below with reference to the measurement system of fig. 1 and 2:
the detection laser light source 1 adopts a continuous detection laser, and the function generator 10 is adopted to modulate the intensity output of the detection laser and inject laser into the stable optical resonant cavity. The stable initial optical resonant cavity is formed by a plane coupling cavity mirror 2 and two identical plano-concave high reflection cavity mirrors (a first plano-concave reflection cavity mirror 3 and a second plano-concave reflection cavity mirror 4). An optical element 8 to be measured is inserted into the initial optical resonant cavity, the incident angle is the using angle of the optical element 8 to be measured, and the optical element 8 to be measured is arranged on the optical element sample bearing platform 7. If the optical element 8 to be tested is a transmission optical element or an uncoated optical element, the stable test optical resonant cavity is formed without moving the position of the first plano-concave reflecting cavity mirror 3, as shown in fig. 1. If the optical element 8 to be tested is a highly reflective optical element, the position of the first plano-concave mirror 3 is correspondingly shifted to form a stable test optical resonator, as shown in FIG. 2. In the measuring system shown in fig. 1 and 2, the second plano-concave reflectionThe transmitted light of the cavity mirror 4 is focused by the focusing lens 5 into the first photodetector 6 for detection, and the angle-resolved scattering loss signal of the optical element 8 to be detected is detected by the second photodetector 9. The detection laser beam is injected into the optical resonant cavity through the plane coupling cavity mirror 2 and oscillates in the optical resonant cavity. At the falling edge of the square wave, the laser is turned off, producing a ring down signal. Simultaneously recording the cavity ring-down signals detected by the first photodetector 6 and the second photodetector 9, as shown in fig. 5, and recording the recorded cavity ring-down signals according to a single exponential decay function Ij(t)＝I0je-t/τ+AjFitting to obtain the amplitude I of the first cavity ring-down signal00And amplitude I of the second cavity ring-down signal01And from this the angle-resolved scattering S ═ I of the optical element 8 to be measured is obtained01T0/I00And M. While the angle-resolved scatter distribution of the optical element under test is obtained by scanning the position of the second photodetector 9 in the angular direction. In order to eliminate the influence of stray light on angle-resolved scatterometry, the cavity ring-down signal in the time range before 0.1 μ s is removed when the single exponential decay function is fitted to the measured cavity ring-down signal.
The above-mentioned embodiments are intended to illustrate the objects, technical solutions and advantages of the present invention in further detail, and it should be understood that the above-mentioned embodiments are merely exemplary embodiments of the present invention, and are not intended to limit the scope of the present invention, and any modifications, equivalent substitutions, improvements and the like made within the spirit and principle of the present invention should be included in the scope of the present invention.
Claims (10)
1. An absolute measurement system for angle-resolved scattering of an optical element is characterized by comprising a detection laser light source (1), a ring-down cavity, a first cavity ring-down signal detection unit, a sample bearing table (7), a second cavity ring-down signal detection unit, a function generator (10), a data acquisition card (11) and a computer (12);
the detection laser light source (1) is used for inputting a detection laser beam into the ring-down cavity;
the function generator (10) is used for carrying out intensity periodic modulation on the output of the detection laser light source (1);
the ring-down cavity is used for carrying out intra-cavity multiple reflection accumulation on the detection laser beam so as to acquire a cavity ring-down signal on the falling edge of the detection laser light source (1) with periodically modulated intensity;
the sample bearing table (7) is placed in the ring-down cavity and used for placing an optical element (8) to be measured;
the first cavity ring-down signal detection unit is used for acquiring a first cavity ring-down signal; wherein the first cavity ring-down signal is the cavity ring-down signal transmitted from an output cavity mirror of the ring-down cavity;
the second cavity ring-down signal detection unit is used for acquiring a second cavity ring-down signal, and the second cavity ring-down signal is the cavity ring-down signal scattered from the ring-down cavity by the optical element to be detected (8);
the data acquisition card (11) is used for acquiring the first cavity ring-down signal and the second cavity ring-down signal;
the computer (12) is used for fitting the amplitude I of the first cavity ring-down signal acquired by the data acquisition card (11) according to a single exponential decay function00And the amplitude I of the second cavity ring-down signal01And obtaining an angle-resolved scattering absolute value of the optical element (8) to be measured according to the following formula;
S＝I01T0/I00M；
wherein S is the absolute value of the angle-resolved scattering of the optical element (8) to be measured, T0The transmittance of the output cavity mirror; and M is the ratio of the amplification factors of the photoelectric detector of the second cavity ring-down signal detection unit and the photoelectric detector of the first cavity ring-down signal detection unit.
2. An absolute measurement system for angle-resolved scattering of optical elements according to claim 1, wherein the ring-down cavity comprises a first plano-concave mirror (3) and a second plano-concave mirror (4);
the detection laser beam is transmitted by the first plano-concave reflecting cavity mirror (3) and then is input into the ring-down cavity, and is reflected and accumulated for multiple times back and forth in the ring-down cavity formed by the first plano-concave reflecting cavity mirror (3) and the second plano-concave reflecting cavity mirror (4), and a cavity ring-down signal is obtained on the falling edge of the intensity periodic modulation of the detection laser light source (1); the cavity ring-down signal is transmitted to the first cavity ring-down signal detection unit for detection after being transmitted by the second concave reflection cavity mirror (4), and the cavity ring-down signal is transmitted to the second cavity ring-down signal detection unit for detection after being scattered by the optical element (8) to be detected.
3. An absolute measurement system for angular resolved scattering of an optical element according to claim 1, wherein the ring-down cavity comprises a first plano-concave mirror (3), a planar coupling mirror (2) and a second plano-concave mirror (4);
the detection laser beam is transmitted by the plane coupling cavity mirror (2) and then is input into the ring-down cavity, and is reflected and accumulated for many times back and forth in the ring-down cavity formed by the first plano-concave reflecting cavity mirror (3), the plane coupling cavity mirror (2) and the second plano-concave reflecting cavity mirror (4), and a cavity ring-down signal is obtained on the intensity periodic modulation falling edge of the detection laser light source (1); the cavity ring-down signal is transmitted to the first cavity ring-down signal detection unit for detection after being transmitted by the second concave reflection cavity mirror (4), and the cavity ring-down signal is transmitted to the second cavity ring-down signal detection unit for detection after being scattered by the optical element (8) to be detected.
4. An absolute measurement system for angle-resolved scattering of optical elements according to claim 2 or 3, wherein when the optical element (8) to be measured is a high-reflection optical element, the detection laser beam outputted from the detection laser source (1) is reflected by the optical element (8) to be measured and then transmitted to the second plano-concave reflection cavity mirror (4).
5. An absolute measurement system for angle-resolved scattering of optical elements according to claim 2 or 3, wherein when the optical element (8) to be measured is a transmissive optical element or an uncoated optical element, the detection laser beam outputted from the detection laser light source (1) is transmitted through the optical element (8) to be measured and then transmitted to the second plano-concave reflective cavity mirror (4).
6. An absolute measurement system for angle-resolved scattering of optical elements according to claim 1, wherein the first cavity ring-down signal detection unit comprises a focusing lens (5) and a first photodetector (6), and when the first cavity ring-down signal is transmitted from the output cavity mirror, the first cavity ring-down signal is focused to the first photodetector (6) through the focusing lens (5).
7. An absolute measurement system of angular resolved scattering of an optical element according to claim 6, wherein the single exponential decay function is:
Ij(t)＝I0je-t/τ+Aj；
wherein j is 0 or 1, and j is 0, which is the first cavity ring-down signal; when j is 1, the second cavity ring-down signal is obtained; i is0jFor signal amplitude, τ is ring down time, AjIs a dc bias.
8. A method of using an absolute measurement system of angular resolved scattering of an optical element according to any of claims 1-7, comprising the steps of:
s1: acquiring a first cavity ring-down signal and a second cavity ring-down signal;
s2: fitting the amplitude I of the first cavity ring-down signal according to a single exponential decay function00And the amplitude I of the second cavity ring-down signal01；
S3: the absolute value of the angular resolution scattering of the optical element (8) to be measured is obtained according to the following formula:
S＝I01T0/I00M；
wherein S is the angle of the optical element (8) to be measuredResolving the absolute value of scattering, T0The transmittance of the output cavity mirror; and M is the ratio of the amplification factors of the photoelectric detector of the second cavity ring-down signal detection unit and the photoelectric detector of the first cavity ring-down signal detection unit.
9. The method of claim 8, wherein the fitting of the single exponential decay function is performed using the first cavity ring-down signal detected by the photodetector of the first cavity ring-down signal detection unit and the second cavity ring-down signal detected by the photodetector of the second cavity ring-down signal detection unit after 0.1 μ s.
10. A method for absolute measurement of angular resolved scattering of an optical element according to claim 8 or 9, further comprising step S4, wherein the step S4 is used to scan the photodetector position of the second cavity ring-down signal detection unit in an angular direction, and the steps S1-S3 are repeated to obtain the angular resolved scattering distribution of the optical element under test (8).
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