CN107036711B - On-line measurement device and method for multi-wavelength compound light field - Google Patents

Abstract

An on-line measurement device and method for multi-wavelength composite light field, the device includes a three-step random phase plate and a CCD detector fixed together by a connecting sleeve, the three-step random phase plate surface and the CCD detector target surface In parallel, the output end of the CCD detector is connected to the input end of the computer, and the computer has corresponding data recording acquisition and processing software for recording light spots and data processing. By recording a diffraction spot, the present invention can quickly and real-time obtain the amplitude and phase of light fields with three different wavelengths: fundamental frequency, double frequency, and triple frequency, and provide reliable corresponding parameters for the operation of high-power laser drivers. The technical difficulty of not being able to simultaneously measure the composite light field. Especially suitable for wavefront measurement of pulsed beams in high power laser drives. The invention has the characteristics of simple structure, small volume, low environmental requirements, fast convergence speed and high measurement precision.

Description

On-line measurement device and method for multi-wavelength compound light field

technical field

The invention relates to a high-power laser driver, in particular to an on-line measurement device and method for multi-wavelength composite light field.

Background technique

In the high-power laser drive system, the quality of the beam directly determines the success or failure of the physical experiment, so it is necessary to monitor and measure the beam quality. However, due to the large and complex system of high-power lasers and thousands of optical components, the measurement of the wavefront of the beam faces many difficult operational difficulties. Compared with other devices, large-scale high-power laser drivers have two prominent features: first, the measurement space is limited, and larger measuring instruments are difficult to use; second, the measured beam is a pulsed beam, which is difficult to measure by general methods. Limited by the above two points, many instruments and methods for wavefront detection are limited in the use of high-power laser drive, such as common interferometers, shearing interferometers, Hartmann sensors, etc.

Coherent Diffractive Imaging (CDI) uses the diffraction theory and the convolution theorem to record the light intensity information on one or more diffraction planes under the condition of satisfying the Nyquist sampling theorem, and then iterates through the computer repeatedly. Satisfy different constraints, make the calculated wave function approach the real distribution successively, and finally obtain the phase distribution that meets the accuracy requirements. Due to its simple structure and theoretical precision reaching the diffraction limit, it has attracted more and more attention and applications in the fields of X-ray, electron beam, fluorescence, and visible laser imaging. In 2010, the phase recovery method based on phase modulation Algorithm (Coherent Modulation Imaging, referred to as CMI) was proposed. This method only needs one diffraction spot and can still guarantee a fast convergence speed. It has a simple structure and low requirements for environmental stability. Wavefront measurement and diagnostics of beams.

The application of coherent diffraction imaging theory to the measurement of wavefront and element surface shape in high-power laser drivers has related patented technologies and practical applications. Since the theory of coherent diffraction imaging needs to meet the condition that the light to be measured is coherent light, so before However, the light field that finally enters the target chamber in the high-power laser driver is a compound light field composed of fundamental frequency light, frequency doubled light and triple frequency light. We need to analyze these three The light field is measured simultaneously. To this end, we propose a device and method for on-line measurement of multi-wavelength composite optical field wavefronts.

Contents of the invention

The present invention provides an on-line measurement device and method for multi-wavelength composite light field. By recording a diffraction spot, the amplitude and phase of three different wavelength light fields of fundamental frequency, multiplied frequency and tripled frequency can be obtained quickly and in real time. Provide reliable corresponding parameters for the operation of high-power laser drivers, and solve the current technical problem that the composite light field cannot be measured simultaneously. Especially suitable for wavefront measurement of pulsed beams in high power laser drives. The invention has the characteristics of simple structure, small volume, low environmental requirements, fast convergence speed and high measurement precision.

Technical solution of the present invention

A multi-wavelength composite light field online measurement device is characterized in that the device includes a three-step random phase plate and a CCD detector and is fixed together by a connecting sleeve, and the three-step random phase plate surface and the CCD detector target surface Parallel, the distance between the three-step random phase plate surface and the target surface of the CCD detector is L, the phase distribution of the three-step random phase plate is P _m , the output of the CCD detector is connected to the computer The input terminal is connected, and the computer has corresponding data recording acquisition and processing software, which is used for recording light spots and data processing.

The three-step random phase plate has strong scattering ability and high transmittance for different light fields. By performing depth differential etching on the quartz glass, the optical phase retardation of each wavelength There are three components, and they are randomly distributed. If the three phase delays of the fundamental frequency light are [a b c], then the corresponding phase delays of the frequency multiplier and triple frequency light are [2a 2b 2c], [3a 3b 3c] ].

Utilize above-mentioned on-line measuring device to the measurement method of multi-wavelength composite light field of fundamental frequency, double frequency and triple frequency, comprise the following steps:

1) Select the corresponding beam reducer according to the size of the actual optical wave field to be measured so that the light spot to be recorded can be completely received by the CCD detector;

2) Place the on-line measurement device in the optical path to be measured and make the three-step random phase plate perpendicular to the light beam, the incident light wave is a spherical wave and the focal plane is called the incident plane, which is located at the Before the three-step random phase plate surface, determine the distance as T. If the light wave to be measured is pulsed light, an external trigger signal should be added to the CCD detector;

3) After the compound light field to be measured is scattered by the three-step random phase plate, a diffraction spot is detected by the CCD detector and sent to the computer for storage. The computer uses the software written to perform the following iterative calculations, including the following steps:

①In the first iteration, we initially guess that the distribution of the three beams of light at the focal plane is φ _1,m ,

②Let n=n+1, the nth iteration process is described as follows:

According to the diffraction theory, the light field φ _n,m at the focal plane S1 is propagated to the phase plate surface to obtain the incident light field distribution on the three-step random phase plate surface as ψ _n,m ;

Wherein, the subscript m represents the light field of different wavelengths: m=1 represents the fundamental frequency light, m=2 represents the double frequency light, m=3 represents the triple frequency light, and the subscript n represents the nth iteration, such as φ _{3, 1} represents the light field complex amplitude distribution of the fundamental frequency light in the third iteration;

③The outgoing light field behind the phase plate is W _n,m =ψ _n,m *P _m ;

④ After the phase plate, the outgoing light field propagates to the CCD target surface to obtain the diffraction spot as

in, Indicates the process of light field W _n,m traveling distance L, Represents the Fourier transform and calculates the error E _n of the nth iteration:

Wherein, I is the recorded spot intensity, and ∑u represents the summation of the rows and columns of the matrix;

⑤ Use the recorded actual spot I to correct the calculated wavefront amplitude of each light and keep the phase unchanged, that is

Different from single-wavelength update, here we multiply the corresponding coefficient according to the energy ratio of each light in the iterative process when updating the amplitude;

⑥To inversely propagate the updated wavefront in ⑤ to the three-step random phase plate surface, we use Represents the reverse propagation distance L along the incident direction of light, and eliminates the phase plate phase modulation to obtain the corrected phase plate front illumination light ψ′ _n,m ;

⑦ Backpropagation to the focal plane According to the feature that the focal spot is usually concentrated in the central area on the focal plane, the focal plane limitation is introduced, and the wavefront distribution of the three beams of light at the focal plane is obtained after correction: φ _n+1,m = S _R(n,m) φ′ _{n, m} +γ(1-S _R(n,m) )(φ′ _n,m -φ _n,m ) is used as the initial value of the next iteration, where S _R(n,m) is a diameter that gradually increases with the number of iterations The enlarged hole function will no longer increase after reaching the maximum value, and the updated weight value γ is between [0,1];

⑧Return to step ①, when the diffraction spot plane error If it is less than 10-3, the iterative process ends.

Technical effects and advantages of the present invention

By recording a diffraction spot, the present invention can quickly and real-time obtain the amplitude and phase of light fields with three different wavelengths: fundamental frequency, double frequency, and triple frequency, and provide reliable corresponding parameters for the operation of high-power laser drivers. The technical difficulty of not being able to simultaneously measure the composite light field. Especially suitable for wavefront measurement of pulsed beams in high power laser drives.

The invention has the characteristics of simple structure, small volume, low environmental requirements, fast convergence speed and high measurement precision.

Description of drawings

Fig. 1 is a schematic diagram of the multi-wavelength compound light field online measurement device of the present invention

Figure 2 is a schematic diagram of the iterative plane of the multi-wavelength compound light field online measurement method

In the figure: 1-three-step random phase plate, 2-CCD detector, 3-fixed connecting sleeve, 4-computer, S1-incidence surface, S2-phase plate surface, S3-CCD detector target surface

Fig. 3 is a flow chart of the present invention utilizing the diffraction transmission theory to realize wavefront reconstruction through computer iterative operations

Detailed ways

Please refer to Fig. 1 and Fig. 2 first, it can be seen from the figures that the multi-wavelength compound light field online measuring device of the present invention includes a three-step random phase plate 1 and a CCD detector 2 and is fixed together by a connecting sleeve 3, the three The step random phase plate surface S2 is parallel to the CCD detector target surface S3, the distance between the three-step random phase plate surface S2 and the CCD detector target surface S3 is L, and the phase of the three-step random phase plate 1 The distribution is P _m , the output end of the CCD detector 2 is connected to the input end of the computer 4, and the computer 4 has corresponding data recording acquisition and processing software for recording light spots and data processing.

The three-step random phase plate 1 has strong scattering ability and high transmittance for different light fields, and the optical phase retardation for each wavelength is achieved by performing differential etching on the quartz glass. There are three components and they are randomly distributed. If the three phase delays of the fundamental frequency light are [a b c], then the corresponding phase delays of the doubled and tripled light are [2a 2b 2c], [3a 3b 3c].

Utilize above-mentioned on-line measuring device to the measurement method of multi-wavelength composite light field of fundamental frequency, double frequency and triple frequency, comprise the following steps:

1) Select a corresponding beam reducer according to the size of the actual optical wave field to be measured so that the light spot to be recorded can be completely received by the CCD detector 2;

2) Place the online measurement device in the optical path to be measured and make the three-step random phase plate surface S2 perpendicular to the light beam, the incident light wave is a spherical wave and the focal plane is called the incident surface S1, and the incident surface S1 is located at Before the three-step random phase plate surface S2, determine the distance as T, if the light wave to be measured is pulsed light, an external trigger signal should be added to the CCD detector 2;

3) After the compound light field to be measured is scattered by the three-step random phase plate, a diffraction spot is detected by the CCD detector 2 and sent to the computer 4 for storage. The computer uses the software written to perform the following iterative calculations, including the following steps (see Fig. 3):

①In the first iteration, we initially guess that the distribution of the three beams of light at the focal plane is φ _1,m ,

②Let n=n+1, the nth iteration process is described as follows:

According to the diffraction theory, the light field φ _n,m at the focal plane S1 is propagated to the phase plate surface to obtain the incident light field distribution on the three-step random phase plate surface S2 as ψ _n,m ;

Wherein, the subscript m represents the light field of different wavelengths: m=1 represents the fundamental frequency light, m=2 represents the double frequency light, m=3 represents the triple frequency light, and the subscript n represents the nth iteration, such as φ _{3, 1} represents the light field complex amplitude distribution of the fundamental frequency light in the third iteration;

③The outgoing light field behind the phase plate surface S2 is W _n,m =ψ _n,m *P _m ;

④ After the phase plate, the outgoing light field propagates to the CCD target surface S3 to obtain the diffraction spot as

in, Indicates the process of light field W _n,m traveling distance L, Represents the Fourier transform and calculates the error E _n of the nth iteration:

Wherein, I is the recorded spot intensity, and ∑u represents the summation of the rows and columns of the matrix;

⑤ Use the recorded actual spot I to correct the calculated wavefront amplitude of each light and keep the phase unchanged, that is

Different from single-wavelength update, here we update the amplitude according to

The energy ratio of each light in the iteration process is multiplied by the corresponding coefficient;

⑥ Backpropagating the updated wavefront in ⑤ to the three-step random phase plate surface S2, we use Represents the reverse propagation distance L along the incident direction of light, and eliminates the phase plate phase modulation to obtain the corrected phase plate front illumination light ψ′ _n,m ;

⑦ Backpropagation to the focal plane According to the feature that the focal spot is usually concentrated in the central area on the focal plane, the focal plane limitation is introduced, and the wavefront distribution of the three beams of light at the focal plane is obtained after correction: φ _n+1,m = S _R(n,m) φ′ _{n, m} +γ(1-S _R(n,m) )(φ′ _n,m -φ _n,m ) is used as the initial value of the next iteration, where S _R(n,m) is a diameter that gradually increases with the number of iterations The enlarged hole function will no longer increase after reaching the maximum value, and the updated weight value γ is between [0,1];

⑧Return to step ①, when the diffraction spot plane error If it is less than 10 ^-3 , end the iterative process.

The following are the parameters of one embodiment of the invention:

The measurement light field includes a compound light field of fundamental frequency light, double frequency light and triple frequency light. The phase delay of the three-step random phase plate 1 to the fundamental frequency light is [0, π/2, π], the phase delay to the double frequency light is [0, π, 2π], and the phase delay to the triple frequency light The quantity is [0,3π/2,3π]. The distance between the phase plate 1 and the CCD detector 2 is 70mm, the resolution of the CCD detector 2 is 4008*2672, and the smallest unit is 9um. Record a diffraction spot, and calculate the propagation process through diffraction theory. Specific steps include:

The initial guess is that the distribution of the three beams of light at the focal plane is φ _1,m , and the subscript m represents the light field of different wavelengths (m=1 represents the fundamental frequency light, m=2 represents the double frequency light, m=3 represents the triple frequency light ), the subscript n indicates the nth iteration:

In the first iteration, we initially guess that the distribution of the three beams of light at the focal plane is φ _1,m , and the nth iteration process is described as follows:

According to the diffraction theory, the optical field φ _n,m at the focal plane S1 is propagated to the phase plate surface, and the optical field distribution on the three-step random phase plate surface S2 is ψ _n,m .

The phase distribution of the three-step random phase plate is P _m , and the phase distribution is known information through measurement in advance, so the outgoing light field behind the phase plate surface S2 can be expressed as W _n,m =ψ _n,m *P _m .

we use Indicates the process of light field W _n,m traveling distance L, Indicates Fourier transform, after the phase plate, the outgoing light field propagates to the CCD target surface to obtain a diffraction spot in L is the distance from the phase plate surface S2 to the CCD target surface S3. During this step, we simultaneously calculate the error E _n of this iteration, Among them, I is the recorded spot intensity, and Σu represents the summation of the rows and columns of the matrix.

Use the recorded actual spot I to correct the calculated wavefront amplitude of each light and keep the phase constant, that is,

Different from single-wavelength update, here we multiply the corresponding coefficient according to the energy ratio of each light in the iterative process when updating the amplitude.

To backpropagate the updated wavefront in 4) to the three-step random phase plate surface S2, we use Represents the reverse propagation distance L along the incident direction of the light, and eliminates the phase plate phase modulation to obtain the corrected phase plate front illumination light ψ′ _n,m .

Backpropagation to the focal plane According to the characteristics that the focal spot is usually concentrated in the central area on the focal plane, the focal plane limitation is introduced, and the wavefront distribution of the three beams of light at the focal plane is obtained after correction

φ _n+1,m ＝SR _(n,m) φ′ _n,m +γ(1- _SR(n,m) )(φ′ _n,m -φ _n,m )

And as the initial value of the next iteration, where S _R(n,m) is a hole function whose diameter gradually increases with the number of iterations, and will not increase after reaching the maximum value, and the update weight value γ is between [0,1] .

Repeat the above steps until the diffraction spot plane error is sufficiently small, and the iterative process ends.

Experiments show that, by recording a diffraction spot, the present invention can quickly and real-time obtain the amplitude and phase of three different wavelength light fields of fundamental frequency, double frequency and triple frequency, and provide reliable corresponding parameters for the operation of high-power laser drivers. Solve the current technical problem that it is impossible to measure the composite light field at the same time. Especially suitable for wavefront measurement of pulsed beams in high power laser drives. The invention has the characteristics of simple structure, small volume, low environmental requirements, fast convergence speed and high measurement precision.

Claims (2)

1. A multi-wavelength composite optical field online measuring device is characterized in that the device includes three steps random phase plate (1) and CCD detector (2) and is fixed together by connecting sleeve (3), and the three The step random phase plate surface (S2) is parallel to the CCD detector target surface (S3), and the distance between the three-step random phase plate surface (S2) and the CCD detector target surface (S3) is L, and the described The phase distribution of the three-step random phase plate (1) is P _m , the output end of the CCD detector (2) is connected with the input end of the computer (4), and the computer (4) has corresponding data record acquisition and Processing software, used to record light spots and data processing; the three-step random phase plate (1) has strong scattering ability and high transmittance to different light fields, and the depth difference There are three components to the optical phase delay of each wavelength, and they are randomly distributed. If the three phase delays of the fundamental frequency light are [abc], then the corresponding phases of the frequency-doubled and triple-frequency light The delay amounts are [2a 2b 2c], [3a 3b 3c], respectively. 2. Utilize the on-line measuring device described in claim 1 to the measuring method of fundamental frequency, double frequency and triple frequency multi-wavelength composite light field, it is characterized in that the method comprises the following steps: 1) Select a corresponding beam reducer according to the size of the actual optical wavefield to be measured so that the light spot to be recorded can be completely received by the CCD detector (2); 2) Place the online measurement device in the optical path to be measured and make the three-step random phase plate surface (S2) perpendicular to the light beam, the incident light wave is a spherical wave and the focal plane is called the incident surface (S1), the The incident surface (S1) is located in front of the three-step random phase plate (S2), and the distance is determined to be T. If the light wave to be measured is pulsed light, an external trigger signal should be added to the CCD detector (2); 3) After the compound light field to be measured is scattered by the three-step random phase plate, a diffraction spot is detected by the CCD detector (2) and sent to the computer (4) for storage. The computer uses the software written to perform the following iterative calculations, including the following step: ①In the first iteration, the initial guess distribution of the three beams of light at the focal plane is φ _1,m , ②Let n=n+1, the nth iteration process is described as follows: According to the diffraction theory, the light field φ _n,m at the focal plane S1 is propagated to the phase plate surface to obtain the incident light field distribution on the three-step random phase plate surface S2 as ψ _n,m ; Among them, the subscript m represents the light field of different wavelengths: m=1 represents the fundamental frequency light, m=2 represents the double frequency light, m=3 represents the triple frequency light, the subscript n represents the nth iteration, φ _3,1 Represents the light field complex amplitude distribution of the fundamental frequency light in the third iteration process; ③The outgoing light field behind the phase plate surface S2 is W _n,m =ψ _n,m *P _m ; ④ After the phase plate, the outgoing light field propagates to the CCD target surface to obtain the diffraction spot as in, Indicates the process of light field W _n,m traveling distance L, Represents the Fourier transform and calculates the error E _n of the nth iteration: Wherein, I is the recorded spot intensity, and ∑u represents the summation of the rows and columns of the matrix; ⑤ Use the recorded actual spot I to correct the calculated wavefront amplitude of each light and keep the phase unchanged, that is Different from single-wavelength update, when the amplitude is updated, the corresponding coefficient is multiplied according to the energy ratio of each light in the iterative process; ⑥ Backpropagate the updated wavefront in ⑤ to the three-step random phase plate surface S2, using Represents the reverse propagation distance L along the incident direction of light, and eliminates the phase plate phase modulation to obtain the corrected phase plate front illumination light ψ′ _n,m ; ⑦ Backpropagation to the focal plane According to the characteristics that the focal spot is usually concentrated in the central area on the focal plane, the focal plane limitation is introduced, and the wavefront distribution of the three beams of light at the focal plane is obtained after correction: φ _n+1,m ＝SR _(n,m) φ′ _n,m +γ(1- _SR(n,m) )(φ′ _n,m -φ _n,m ) and as the next iteration Initial value, where S _R(n,m) is a hole function whose diameter gradually increases with the number of iterations, and does not increase after reaching the maximum value, and the update weight value γ is between [0,1]; ⑧Return to step ①, when the diffraction spot plane error If it is less than 10 ^-3 , end the iterative process.