Space-time light field dynamic reconstruction method based on monochromatic numerical mode decomposition

Abstract

The invention discloses a space-time light field dynamic reconstruction method based on monochromatic numerical mode decomposition, and belongs to the field of space-time mode-locked pulse fiber lasers and multimode nonlinear fiber optics, and the method comprises the following steps: S1, building an analysis platform which comprises a space-time mode-locked laser, a folded mirror, a spectral filter, a processing device and an acquisition device, the processing device adjusts the state of the spectral filter, and the acquisition device points to the folding mirror; s2, calibrating the analysis platform; s3, the space-time mode-locked laser generates an incident beam, after spectral filtering, the acquisition device continuously acquires spatial light intensity corresponding to each spectral component, and spectrum-spatial intensity information is obtained; and S4, converting the spectrum-space intensity information into space-time structure information. By adopting the method, the output mode of the space-time mode-locked pulse is monitored in real time, the mode selective excitation in the space-time mode-locked laser cavity is fed back and controlled, and a solution is provided for a high-performance space-time mode-locked fiber laser with a controllable output mode.

Description

Technical Field

[0001] The invention relates to the field of spatiotemporal mode-locked pulsed fiber lasers and multimode nonlinear fiber optics, and in particular to a spatiotemporal light field dynamic reconstruction method based on monochromatic numerical mode decomposition. Background Art

[0002] Spatiotemporally mode-locked fiber lasers have a larger mode area and additional spatial degrees of freedom, and are expected to achieve higher-performance ultrafast laser output, but they also face the problem of uncontrollable spatial mode distribution and output beam quality. For multimode nonlinear processes, the challenge associated with the measurement system is how to effectively obtain all the information that fully characterizes broadband and highly multimode beams. An effective measurement system needs to minimize measurement time, experimental cost and complexity, and must be conducive to accurate comparison of theoretical analysis with experimental results in order to optimize and control laser output performance. Currently, the commonly used single-mode fiber scanning or sampling method can easily perform qualitative analysis, but cannot be directly compared with theory. Spatial mode multiplexing passive devices based on photon lanterns or multi-channel beam control can only achieve physical decomposition of a small number of modes and are not suitable for multi-mode light fields. The full-field measurement method using holographic wavelength scanning can perform mode decomposition based on wavelength. Although the device is relatively simple, it takes a long time and cannot achieve dynamic reconstruction of spatiotemporal light fields. Summary of the invention

[0003] The purpose of the present invention is to provide a method for dynamic reconstruction of a spatiotemporal light field based on monochromatic numerical mode decomposition. A quasi-4-f system is formed by a programmable digital micromirror device, a cylindrical lens and a transmission diffraction grating to realize an adjustable optical filter. The spectral bands are opened in sequence to realize wavelength scanning and the corresponding spatial distribution is recorded by a high-speed camera, so that spatial-spectral intensity information can be obtained. The collected spectral-spatial distribution data is subjected to mode decomposition by a non-iterative algorithm, and the amplitude and phase of the light field can be restored in real time. The spectral-spatial distribution is then subjected to a frequency domain to time domain Fourier transform to obtain the spatiotemporal structure information, and the reconstruction time can be shortened to the order of 1 ms. The understanding of the mode selection of three-dimensional spatiotemporal mode-locked pulses and the regulation mechanism of their spatiotemporal characteristics can be deepened, and an effective solution is provided for the design optimization and mode control of adaptively controlled spatiotemporal mode-locked fiber lasers.

[0004] To achieve the above object, the present invention provides a method for dynamic reconstruction of space-time light field based on monochromatic numerical pattern decomposition, comprising the following steps: S1: Establishing an analysis platform, including a spatiotemporal mode-locked laser, a folding mirror, a spectrum filter, a processing device and a collection device connected in sequence, wherein the processing device adjusts the working state of the spectrum filter, and the collection device points to the folding mirror; S2: Calibrate the analytical platform using a spontaneous emission fluorescence light source and a spectrometer; S3: The spatiotemporal mode-locked laser generates an incident light beam, which is processed by the transmission surface of the folding mirror and the spectral filter to obtain a spectral filter and return along the original optical path. The spectral filter is finally output from the refractive surface of the folding mirror. The acquisition device continuously collects the spectral filtered light field, records the spatial light intensity corresponding to each spectral component, and obtains the spectral-spatial intensity information; S4: Perform non-iterative numerical calculation on the spectral-spatial intensity information to restore the amplitude and phase of the light field, and then perform Fourier transform on it to obtain the spatiotemporal structure information.

[0005] Preferably, in step S1, a scanning galvanometer is arranged behind the optical path of the spatiotemporal mode-locked laser, which can improve the refresh rate of the spectral filter.

[0006] Preferably, in step S1, the spectral filter includes a transmission diffraction grating, a cylindrical mirror and a digital micromirror device arranged in sequence along the optical path, and the processing device is connected to the digital micromirror device to adjust the switching state of different positions of the digital micromirror device to achieve adjustable wavelength scanning spectral filtering.

[0007] Preferably, in step S2, the calibration process is as follows: use a spontaneous radiation fluorescence light source to output spontaneous radiation fluorescence input into a folding mirror, and incident the spontaneous radiation fluorescence on a digital micromirror device with all pixels turned on, and use a spectrometer to measure its fluorescence spectrum; then turn off some pixel units on the digital micromirror device, and incident the spontaneous radiation fluorescence on the digital micromirror device with closed pixel units, and use a spectrometer to measure its output spectrum, and by comparing the attenuated spectral component intervals on the two output spectra with the pixel intervals of the corresponding closed pixel units on the digital micromirror device, the center wavelength and the wavelength interval corresponding to each unit pixel can be located, and the functional parameters of the analysis platform can be calibrated and calibrated.

[0008] Preferably, the specific process in step S3 is as follows: the bandwidth of the spectral filter is set to 70nm, the spectral resolution is set to 0.04nm, the refresh rate is set to 32kHz, the spectral filter will select a specific spectral band to constitute spectral filtering, the processing device is set to a computer, and the acquisition device is set to a high-speed camera or a photodiode array. The spatial light intensity distribution diagram corresponding to each spectral component is recorded with a high-speed camera or a photodiode array to finally obtain spectral-spatial intensity information.

[0009] Preferably, in step S4, the process of converting the spectral-spatial intensity information into the spatiotemporal structure information is as follows: a non-iterative algorithm is used to perform pattern decomposition on the collected spectral-spatial intensity information, and the nonlinear pattern is converted into a cumbersome linear part and a simple nonlinear part. A matrix solution is performed using a pseudo-inverse matrix and the fiber intrinsic mode light intensity distribution to restore the amplitude and phase of the light field, and then the spectral-spatial distribution is Fourier transformed from the frequency domain to the time domain to obtain the spatiotemporal structure information.

[0010] Therefore, the present invention adopts the above-mentioned spatiotemporal light field dynamic reconstruction method based on monochromatic numerical pattern decomposition, which has the following advantages: (1) In the present invention, the spectral filter device composed of a programmable digital micromirror device and a transmission diffraction grating has the advantages of a wide spectral range and real-time scanning. Compared with other tunable optical filter solutions, it can obtain a higher degree of freedom and has the advantages of quick operation, simple structure and programmability.

[0011] (2) In the present invention, a monochrome numerical pattern decomposition technique is used, and the pattern decomposition time is less than 1 ms. The total reconstruction time is determined by the time of scanning the spectral band of the digital micromirror device. The refresh rate of the existing digital micromirror device determines that the reconstruction time is in the order of tens of ms. The refresh rate can be increased to tens of MHz by using a scanning galvanometer, and the total reconstruction time can be reduced to less than 1 ms. This pattern-resolved spatiotemporal measurement system is simple, efficient, low-cost, and can dynamically reconstruct spatiotemporal pattern information.

[0012] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 This is a structural diagram of an analysis platform in a method for dynamic reconstruction of a spatiotemporal light field based on monochromatic numerical pattern decomposition according to the present invention; Figure numerals: 1. folding mirror; 2. transmission diffraction grating; 3. cylindrical mirror; 4. digital micromirror device; 5. processing device; 6. collection device; 7. time-space mode-locked laser. DETAILED DESCRIPTION

[0014] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. The components of the embodiments of the present invention described and shown in the drawings here can be arranged and designed in various different configurations. The specific model specifications need to be selected and determined according to the actual specifications of the device, and the specific selection calculation method adopts the existing technology in the field, so it will not be described in detail.

[0015] Example like Figure 1 As shown, the present invention provides a method for dynamic reconstruction of a spatiotemporal light field based on monochromatic numerical pattern decomposition, comprising the following steps: S1: Establish an analysis platform, including a space-time mode-locked laser 7, a folding mirror 1, a spectral filter, a processing device 5 and a collection device 6 connected in sequence, the processing device 5 adjusts the working state of the spectral filter, and the collection device points to the folding mirror 1. The spectral filter includes a transmission diffraction grating 2, a cylindrical mirror 3 and a digital micromirror device 4 arranged in sequence along the optical path. The processing device 5 is connected to the digital micromirror device 4 to adjust the switching state of the digital micromirror device 4 at different positions; a scanning galvanometer can be additionally arranged after the optical path of the space-time mode-locked laser 7 to improve the refresh rate of the spectral filter and thereby speed up the later reconstruction time.

[0016] S2: Use the spontaneous emission fluorescence light source and spectrometer to calibrate the analysis platform. The calibration process is as follows: A spontaneous fluorescence light source is used to output spontaneous fluorescence, which is input to the transmission surface of the folding mirror 1, and finally incident on the digital micromirror device 4 with all pixels turned on, forming a spectrum filter return, which is output through the refractive surface of the folding mirror 1, and its output spectrum is measured using a spectrometer; some pixel units in the digital micromirror device 4 are turned off. In this embodiment, two vertical pixel units are turned off, and then the spontaneous fluorescence is incident on the digital micromirror device 4 with the pixel units turned off, and its new output spectrum is measured using a spectrometer. By comparing the interval of the two spectral components attenuated on the two output spectra with the pixel interval of the corresponding closed pixel units on the digital micromirror device 4, the central wavelength and the wavelength interval corresponding to each unit pixel can be located, so as to calibrate and calibrate the functional parameters such as the working wavelength and spectral selection resolution of the analysis platform; S3: The spatiotemporal mode-locked laser 7 generates an incident light beam, which is processed by the transmission surface of the folding mirror 1 and the spectral filter to obtain a spectral filter and return along the original optical path. The spectral filter is finally output from the refractive surface of the folding mirror 1; The bandwidth of the spectral filter is set to 70nm, the spectral resolution is set to 0.04nm, the processing device 5 is set to a computer, the acquisition device 6 is set to a high-speed camera or a photodiode array, a wide wavelength tuning range, and a refresh rate is set to 32kHz to achieve high-speed wavelength scanning. A specific spectral band is selected from the refractive surface output of the folding mirror 1 to form spectral filtering, and the spatial light intensity corresponding to each spectral component is recorded by the acquisition device 6 to obtain spectral-spatial intensity information; S4: The collected spectral-spatial intensity information is decomposed using a non-iterative algorithm, and the nonlinear mode is converted into a linear part and a simple nonlinear part. The pseudo-inverse matrix and the fiber intrinsic mode light intensity distribution are used to solve the matrix, and then the amplitude and phase of the light field are restored. The spectral-spatial distribution is transformed from the frequency domain to the time domain by Fourier transform to obtain the spatiotemporal structure information. The calculation process is as follows: The lateral distribution of the electric field in the optical fiber corresponding to the wavelength-scanned spectral band can be expressed as a linear combination of characteristic modes. : ; ; In the above formula, Representing coordinates The electric field at are the complex coefficients representing the k-th eigenmode, represents the amplitude of the kth eigenmode, represents the relative phase of the k-th eigenmode.

[0017] The experiment uses a high-speed camera or photoelectric array diode to measure the spatial intensity distribution corresponding to the wavelength scan. The formula is as follows: ; In the above formula, Representing coordinates The spatial intensity distribution at the fiber output. Mode decomposition is to use the intensity distribution at the fiber output to determine the coefficients , for support The optical fiber with characteristic modes needs to be determined Amplitude and Phase (assuming ), a total of If the spectral intensity distribution is captured by a high-speed camera or a photodiode array, the resulting image can be used to recover the coefficients .

[0018] For example: Consider a The input image consists of pixels, then it can be written as The system of equations is as follows: ; In the above formula, represents the spatial intensity distribution of the mth point, and the subscript and Indicates the kth and jth order eigenmodes, ranging from 1 to N , represents the conjugate of the jth eigenmode complex coefficient.

[0019] Combined with the pseudo-Moore-Penrose inverse matrix calculation of the eigenmode of the optical fiber at a given wavelength, the complex coefficients of the characteristic mode, namely the amplitude and phase parts, are calculated, and then the obtained spectral-spatial distribution is transformed from the frequency domain to the time domain by Fourier transform to obtain the spatiotemporal structure information.

[0020] Therefore, the present invention adopts a method for dynamic reconstruction of spatiotemporal light fields based on monochromatic numerical mode decomposition, wherein the accuracy of the spectral filter based on the programmable digital micromirror device can be controlled in real time to realize the spatial mode intensity information acquisition of wavelength scanning, and reconstruct the three-dimensional light field with the help of the monochromatic numerical mode decomposition method, which provides a new method for the full-field measurement of the spatiotemporal light field. Compared with other light field measurement systems, this system has the advantages of less time consumption, simple structure and programmability, and can further promote the adaptive control and optimization of high-energy spatiotemporal mode-locked fiber lasers with customizable output characteristics.

[0021] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solution of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solution to deviate from the spirit and scope of the technical solution of the present invention.

Claims

1. A method for dynamic reconstruction of spatiotemporal light fields based on monochromatic numerical pattern decomposition, characterized in that: The following steps are involved: S1: Establishing an analysis platform, including a spatiotemporal mode-locked laser, a folding mirror, a spectrum filter, a processing device and a collection device connected in sequence, wherein the processing device adjusts the working state of the spectrum filter, and the collection device points to the folding mirror; S2: Calibrate the analytical platform using a spontaneous emission fluorescence light source and a spectrometer; S3: The spatiotemporal mode-locked laser generates an incident light beam, which is processed by the transmission surface of the folding mirror and the spectral filter to obtain a spectral filter and return along the original optical path. The spectral filter is finally output from the refractive surface of the folding mirror. The acquisition device continuously collects the spectral filtered light field, records the spatial light intensity corresponding to each spectral component, and obtains the spectral-spatial intensity information; S4: Perform non-iterative numerical calculation on the spectral-spatial intensity information to restore the amplitude and phase of the light field, and then perform Fourier transform on it to obtain the spatiotemporal structure information.

2. The method for dynamic reconstruction of spatiotemporal light field based on monochromatic numerical pattern decomposition according to claim 1, characterized in that: In the step S1, a scanning galvanometer is arranged behind the optical path of the spatiotemporal mode-locked laser, which can increase the refresh rate of the spectral filter.

3. The method for dynamic reconstruction of spatiotemporal light field based on monochromatic numerical pattern decomposition according to claim 1, characterized in that: In step S1, the spectral filter includes a transmissive diffraction grating, a cylindrical mirror and a digital micromirror device arranged in sequence along the optical path. The processing device is connected to the digital micromirror device to adjust the switching state of different positions of the digital micromirror device to achieve adjustable wavelength scanning spectral filtering.

4. The method for dynamic reconstruction of spatiotemporal light field based on monochromatic numerical pattern decomposition according to claim 3, characterized in that: In step S2, the calibration process is as follows: Use a spontaneous fluorescence light source to output spontaneous fluorescence and input it into a folding mirror, so that the spontaneous fluorescence is incident on a digital micromirror device with all pixels turned on, and use a spectrometer to measure its fluorescence spectrum; Then, some pixel units on the digital micromirror device are turned off, and the spontaneous radiation fluorescence is incident on the digital micromirror device with turned-off pixel units. The output spectrum is measured using a spectrometer. By comparing the intervals of the attenuated spectral components on the two output spectra with the pixel intervals of the corresponding turned-off pixel units on the digital micromirror device, the central wavelength and the wavelength interval corresponding to each unit pixel can be located, and the functional parameters of the analysis platform can be calibrated and calibrated.

5. The method for dynamic reconstruction of spatiotemporal light field based on monochromatic numerical pattern decomposition according to claim 4, characterized in that: The specific process in step S3 is as follows: The bandwidth of the spectral filter is set to 70nm, the spectral resolution is set to 0.04nm, and the refresh rate is set to 32kHz. The spectral filter will select a specific spectral band to constitute spectral filtering. The processing device is set to a computer, and the acquisition device is set to a high-speed camera or a photodiode array. The high-speed camera or photodiode array is used to record the spatial light intensity distribution diagram corresponding to each spectral component, and finally the spectral-spatial intensity information is obtained.

6. The method for dynamic reconstruction of spatiotemporal light field based on monochromatic numerical pattern decomposition according to claim 5, characterized in that: In step S4, the process of converting the spectral-spatial intensity information into the spatiotemporal structure information is as follows: A non-iterative algorithm is used to decompose the collected spectral-spatial intensity information, converting the nonlinear pattern into complicated linear parts and simple nonlinear parts. The pseudo-inverse matrix and the fiber intrinsic mode light intensity distribution are used to solve the matrix, and then the amplitude and phase of the light field are restored. The spectral-spatial distribution is then transformed from the frequency domain to the time domain by Fourier transform to obtain the spatiotemporal structure information.

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