Coaxial multi-wavelength transverse shear wave front measurement system and measurement method

Abstract

The application discloses a coaxial multi-wavelength transverse shearing wavefront measurement system. The system comprises a gradient separation unit, a wavefront measurement unit and a recovery unit. The signal wavefront to be measured firstly enters a beam splitting unit composed of a diffraction lens to realize gradient separation of different wavelength wavefronts, and then passes through the wavefront measurement unit to realize four-wave transverse shearing interference and obtain a shearing interference graph. In the recovery unit, a computer realizes beam separation in the interference graph spectrum according to the different wavefront gradients through Fourier transform, and a subsequent recovery algorithm obtains gradient introduction according to the spectral position of each wavelength, and then recovers the original wavefront. The coaxial multi-wavelength transverse shearing wavefront measurement system disclosed by the application solves the problem that the existing coaxial multi-wavelength beam is difficult to measure without beam splitting, realizes single measurement of the multi-wavelength beam, has the advantages of simple structure, convenient measurement and the like, and can also measure large-diameter, single-shot high-energy laser.

Description

Technical Field

[0001] This invention belongs to the field of optical technology, and relates to shearing interferometry, and particularly to a coaxial multi-wavelength transverse shearing wavefront measurement system and method. Background Technology

[0002] In optical frequency doubling technology, the fundamental frequency light is phase-matched through a frequency doubling crystal to generate frequency-doubled light. This beam has a very small walk-off angle and is a coaxial multi-wavelength beam. A common method for measuring this beam is to separate the beam using a wedge lens and then use wavefront detection technology for measurement.

[0003] A diffractive lens is a device that uses surface structure design to create a diffraction effect, thereby focusing a light beam. Unlike optical lenses, the focal length of a diffractive lens is specific to a particular wavelength of light. The focusing distance of light beams of different wavelengths through a diffractive lens is inversely proportional to the wavelength, thus creating gradient separation.

[0004] The basic principle of shearing interferometry is to split the wavefront to be measured into two or more branches using a wavefront splitter, and to create a certain degree of misalignment among these branches, thereby causing interference between the wavefront to be measured and itself. It is mainly divided into transverse shearing and radial shearing. Transverse shearing interferometry is known for its high sensitivity and high precision, and has advantages such as simple structure, common optical path, and easy optical system construction. A modified Hartmann grating is commonly used as the splitter. This measurement method is currently applied in the single-wavelength domain.

[0005] To address the challenges of measuring coaxial multi-wavelength beams and overcome the drawbacks of beam splitting, this invention proposes a coaxial multi-wavelength transverse shear wavefront measurement system. By using diffractive lenses to separate the gradients of wavefronts of different wavelengths, and employing an algorithm to reconstruct the original wavefronts for each wavelength, the system solves the problem of measuring coaxial multi-wavelength beams. Summary of the Invention

[0006] To overcome the above problems, this invention proposes a coaxial multi-wavelength transverse shear wavefront measurement system. Using this system, coaxial single measurement of multi-wavelength beams is realized. The measurement principle is simple, the device is easy to build, and the measurement results are accurate and reliable.

[0007] The technical solution of the present invention is as follows:

[0008] The coaxial multi-wavelength transverse shear wavefront measurement system is characterized by including a gradient separation unit, a wavefront measurement unit, and a recovery unit.

[0009] The gradient separation unit is used to focus incident light of different wavelengths onto different focal points, so that the focused beam has different gradients, which facilitates the subsequent wavefront measurement unit to extract the required interference information.

[0010] The wavefront measurement unit is used to generate and acquire transverse shearing interferograms, and then process them to obtain the wavefront information to be measured.

[0011] The restoration unit is used to restore multi-wavefront information from the acquired interferogram.

[0012] The gradient separation unit consists of a diffractive lens, which has different focal lengths for different wavelengths of light. The focal length is inversely proportional to the wavelength, so that each beam has a different gradient.

[0013] The wavefront measurement unit consists of an improved Hartmann grating and a CCD imaging system.

[0014] The restoration unit is a computer.

[0015] The present invention also provides a method for measuring coaxial multi-wavelength transverse shear wavefront, comprising the following steps:

[0016] Step ① Focus incident light of different wavelengths onto different focal points to make the focused beams have different gradients, and collect an interferogram;

[0017] Step ② Perform Fourier transform on the interferogram to obtain the spectrum;

[0018] Step ③ The intensity information can be obtained by the inverse Fourier transform of the center 0th order frequency S0;

[0019] Step 4: After plane wave calibration, the phase information of each wavelength has fixed positions at +1 order in the X and Y directions of the spectrum. Due to different gradients, the phase of the wavefront under test is separated at the +1 order of the spectrum. Let a be the distance between this position and the center of the spectrum. After filtering the +1 order of the wavefront under test of a certain wavelength in the X and Y directions, shift it a distance a to the center of the spectrum, and then perform an inverse Fourier transform to obtain the imaginary angles. This will give us the differential wavefront phase in the X and Y directions.

[0020] Step ⑤: Perform a Fourier transform on the differential wavefront phases in both directions based on the shearing amount (calculated from the grating period and the grating CCD spacing).

[0021] After least squares integration, the phase of the wavefront to be measured is obtained by inverse Fourier transform.

[0022] Compared with the prior art, the advantages of this invention are:

[0023] (1) A coaxial multi-wavelength transverse shear wavefront measurement system can be used to perform a single coaxial measurement of multi-wavelength beams without beam splitting. This eliminates the need for a beam splitting system during measurement, thus expanding the applicability of multi-wavelength beams. It can handle various coaxial multi-wavelength beams, including optical frequency doubling systems and optical frequency mixing systems.

[0024] (2) When the wavefront measurement unit can simultaneously acquire interferograms of all wavelengths, the system can also process off-axis wavefronts.

[0025] (3) The transverse shearing interferometry technique upon which this invention is based is typically used for single-wavelength wavefront measurements, but multiple measurements are required for multi-wavelength scenarios. This invention extends this measurement technique to the single-time multi-wavelength domain and performs multi-wavelength corrections in the algorithm. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the coaxial multi-wavelength transverse shear wavefront measurement system of the present invention.

[0027] Figure 2 This is an interferogram spectrum diagram using a dual-wavelength example in this invention.

[0028] Figure 3 This is a flowchart of the restoration unit in this invention.

[0029] In the diagram, Ⅰ represents the gradient separation unit, Ⅱ the wavefront measurement unit, and Ⅲ the restoration unit; 1 is the diffraction lens, 2 is the improved Hartmann grating, 3 is the CCD imaging system, and 4 is the computer. S0 represents the spectral center point (order 0), S x1 S x2 S represents two +1 level points in the X direction. y1 S y2 These represent two +1 level points in the Y direction. The remaining points, representing frequency usage, are not shown. Detailed Implementation

[0030] The present invention will be further described below with reference to the accompanying drawings and specific embodiments. It should be noted that... Figure 1 The examples included are used to explain the present invention, but should not be used to limit the scope of protection of the present invention.

[0031] The purpose of this invention is to provide a coaxial multi-wavelength transverse shear wavefront measurement system, which aims to address the needs of coaxial multi-wavelength wavefront measurement and solve the problem of beam splitting for multi-wavelength light measurement.

[0032] See Figure 1 , Figure 1 This is a schematic diagram of a coaxial multi-wavelength transverse shear wavefront measurement system. The wavefront of the signal to be measured enters the beam splitter unit. Since the focal length of light of different wavelengths is inversely proportional to the wavelength, the diffraction lens 1 achieves gradient separation of wavefronts of different wavelengths. The multi-wavelength converged light passes through the Hartmann grating 2, generating four-wave transverse shear interference. The transverse shear interference patterns of each signal are superimposed on the imaging CCD 3 for reception, and finally analyzed and processed by the computer 4 to obtain the information of the original wavefront to be measured.

[0033] See Figure 2 , Figure 2This is a spectrum diagram of an interferogram using a dual-wavelength example. After performing a Fourier transform on the interferogram, the intensity information can be obtained from the inverse transform of the center 0th-order frequency S0. Due to the different gradients, the phase information of each wavelength is separated at the +1 order of the spectrum. Where S... x1 S y1 For the shorter wavelength test light, S is the +1 order in the X and Y directions. x2 S y2 This represents the +1 order of the longer wavelength light being measured in the X and Y directions. These orders are relatively well separated and do not interfere with each other.

[0034] See Figure 3 The method for restoring the four-wave transverse shearing interferometry is as follows: Fourier transform the interferogram to obtain the spectrum; after identifying the spectrum points, obtain the separated S... x1 S y1 S x2 S y2 The +1 order spectral centers for different wavelengths are determined. After plane wave calibration, for each wavelength of light, there are fixed +1 order positions in the X and Y directions of the spectrum. Let 'a' be the distance between this position and the spectral center. After filtering the +1 order in the X and Y directions of the wavefront to be measured for a certain wavelength, shifting it a distance towards the spectral center and then performing an inverse Fourier transform yields the differential wavefronts in the X and Y directions. This shifting method eliminates the gradient introduced by the diffraction lens, ensuring accurate reconstruction. Then, based on the shearing amount (calculated from the grating period and the grating CCD spacing), the wavefront to be measured for that wavelength can be integrated to obtain the final wavefront.

[0035] In summary, this invention, based on the principle of transverse shearing interferometry, achieves gradient separation through diffraction lenses, enabling coaxial multi-wavelength wavefront measurement. This eliminates the need for a beam splitter system, expanding the applicability of multi-wavelength beams. Furthermore, it extends transverse shearing interferometry to the field of multi-wavelength measurement and provides a correction method. It can handle various coaxial multi-wavelength beams, including optical frequency doubling systems and optical frequency mixing systems. When the wavefront measurement unit can simultaneously acquire interferograms of all wavelengths, the system can also handle off-axis wavefronts.

[0036] The above description is only used to illustrate the technical solution of the present invention. It should be noted that, for those skilled in the art, modifications can be made to the above technical solution or equivalent substitutions can be made to some of the features without departing from the principle of the present invention. These modifications and substitutions should also be considered within the scope of protection of the present invention.

Claims

1. A coaxial multi-wavelength transverse shear wavefront measurement system, characterized in that, It includes a gradient separation unit (Ⅰ), a wavefront measurement unit (Ⅱ), and a restoration unit (Ⅲ); The gradient separation unit (Ⅰ) is used to focus incident light of different wavelengths onto different focal points, so that the focused beam has different gradients, so as to facilitate the subsequent wavefront measurement unit to extract the required interference information. The wavefront measurement unit (Ⅱ) is used to generate and acquire transverse shearing interferograms, and then process them to obtain the wavefront information to be measured. The restoration unit (Ⅲ) is used to restore multi-wavefront information from the acquired interferogram; The gradient separation unit (Ⅰ) is a diffraction lens (1). This diffraction lens has different focal lengths for different wavelengths of light, and the focal length is inversely proportional to the wavelength, so that each beam has a different gradient. The wavefront measurement unit (Ⅱ) consists of an improved Hartmann grating (2) and a CCD imaging system (3). The multi-wavelength converged light passes through the improved Hartmann grating (2) and generates four-wave transverse shearing interference. The transverse shearing interference patterns of each signal are superimposed on the CCD imaging system (3) for reception. The interference pattern is subjected to Fourier transform to obtain the spectrum. The spectrum is obtained from the center 0th order frequency. The intensity information is obtained by inverse Fourier transform of S0. The phase information of each wavelength is calibrated by plane wave and has +1 fixed positions in the X and Y directions in the spectrum. Let the distance between this position and the center of the spectrum be a. After filtering the +1 order of a certain wavelength wavefront to be measured in the X and Y directions, shift it a distance a to the center of the spectrum and then perform an inverse Fourier transform to obtain the imaginary angle. The differential wavefront phases in the X and Y directions can be obtained. The differential wavefront phases in the two directions are then subjected to Fourier transform according to the shearing amount. After integration by least squares method, the inverse Fourier transform is used to obtain the phase of the wavefront to be measured.

2. The coaxial multi-wavelength transverse shear wavefront measurement system according to claim 1, characterized in that: The restoration unit (Ⅲ) is a computer (4).

3. A coaxial multi-wavelength transverse shear wavefront measurement method, characterized in that, Includes the following steps: Step ① The incident light of different wavelengths is focused at different focal points by diffraction lenses so that the focused beam has different gradients. The focused light of different wavelengths passes through the modified Hartmann grating to generate four-wave transverse shearing interference. The transverse shearing interference patterns of each signal are superimposed on the CCD imaging system for reception. Step ② Perform Fourier transform on the interferogram to obtain the spectrum; Step ③ The intensity information can be obtained by the inverse Fourier transform of the center 0th order frequency S0; Step 4: After plane wave calibration, the phase information of each wavelength has fixed positions in the X and Y directions at +1 level in the spectrum. Due to different gradients, the phase of the wavefront to be measured is separated in the +1 level of the spectrum. Let a be the distance between this position and the center of the spectrum. After filtering the +1 level of the wavefront to be measured in the X and Y directions, shift it a distance a to the center of the spectrum and then perform the imaginary angle after inverse Fourier transform to obtain the differential wavefront phase in the X and Y directions. Step 5: Perform Fourier transform on the differential wavefront phases in the two directions based on the shearing amount, integrate them using the least squares method, and then perform an inverse Fourier transform to obtain the wavefront phase to be measured. The shearing amount is calculated from the grating period and the grating CCD spacing.

Images