CN117277044B - Rectangular ultrashort pulse generation system and method based on DMD - Google Patents
Rectangular ultrashort pulse generation system and method based on DMD Download PDFInfo
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Abstract
The invention discloses a rectangular ultrashort pulse generation system and a rectangular ultrashort pulse generation method based on a Digital Micromirror Device (DMD), wherein the system comprises a light source module, a shaping module, a Discrete Fourier Transform (DFT) module and a correction module; the shaping module comprises a first optical fiber collimator, a half-wave plate, a first reflecting mirror, a second reflecting mirror, a block grating, a cylindrical lens and a DMD which are sequentially arranged along a first light path; a third reflecting mirror, a fourth reflecting mirror, a spectroscope and a second optical fiber collimator are sequentially arranged between the block grating and the DFT module along a second optical path; the correction module comprises a spectrometer and a computer; the high-repetition frequency ultrashort pulse output from the tail fiber of the light source module is subjected to spectral shaping through a shaping module, and then subjected to pulse shaping through a DFT module; the spectrometer, the computer and the DMD in the shaping module form a feedback closed loop, the diffraction pattern processed by the shaping module is iterated, and the spectrum shape is corrected to obtain an accurate target spectrum; the invention has simple and compact structure and flexible operation, is suitable for ultra-short pulse shaping, and has high flatness of the generated rectangular pulse.
Description
Technical Field
The invention belongs to the technical field of ultra-short pulse lasers, and particularly relates to a rectangular ultra-short pulse generation system and method based on a DMD.
Background
The rectangular pulse has important application value in the research fields of nonlinear optical metrology, coherent transient spectroscopy, future all-optical switching, digital communication systems and the like.
Currently, time domain pulse shaping generally shapes well known gaussian-shaped and hyperbolic secant-shaped pulses into more exotic flat-top, parabolic or triangular pulses. Of these, the most common is the simple linear filtering technique. Limited by electronics, heretofore, indirect pulse shaping in the frequency domain has been an effective means because of the inability to shape time domain ultrafast pulses without a time domain filter at the ultrafast time scale. The control of pulse amplitude, phase and polarization in the frequency domain has become an effective technique for shaping ultrashort pulses.
Shaping of laser pulses by fourier transformation based on 4F systems has been used as a spatial filter, including conventional spatial masks, liquid crystal spatial light modulators, liquid crystal on silicon spatial light modulators, acousto-optic spatial light modulators, etc. The latter three methods allow for programmability and adaptivity of the pulse shaping response, which has the obvious disadvantages of being bulky, expensive and not capable of being integrated with the waveguide. These problems have prompted the search for other technical approaches such as arrayed waveguide gratings, fiber delay line arrays, time coherent synthesis, FWM time lens technology, dispersive Fourier Transform (DFT), super-structured fiber bragg gratings (SSFBGs), etc. It has been demonstrated that by using a multi-arm interferometer in combination with Time Coherent Synthesis (TCS) techniques, pulse shapes of various transformation limits can be produced, such as flat-top, parabolic, triangular pulses. Flat-top, parabolic, and zigzagged (asymmetric triangular) pulses can be generated using a super-structured fiber bragg grating technique. It is also mentioned that pulse shaping is achieved by the interaction of the pulse pre-chirp and the nonlinear transmission in the positively dispersed optical fiber.
However, there are few reports about rectangular pulse generation. In 1988, weiner et al obtained a rectangular pulse with a rising edge of 100fs and a pulse width of 2ps by a non-dispersive pulse shaping system (based on a 4F system). P. petropoulos et al used a complex super-structured fiber bragg grating (SSFBG) as a spectral filter to adjust phase and amplitude to convert a short 2.5ps laser pulse into a corresponding rectangular pulse of 20ps in pulse width. For 225 years, wenfeng Wang et al have shown the generation of femtosecond rectangular light pulses by coupling optical pulses with CW in a nonlinear directional coupler (NLDC, consisting of two ideal optical waveguides without loss consideration), and output a pulse shape approaching an ideal rectangle by three-stage cascading NLDC. In 227 Yongwoo Park et al realized flat-top pulses with a pulse width of 3.1ps using a two-stage multi-arm interferometer using a time coherent synthesis technique. In 2016, song Hu et al used a Dispersive Fourier Transform (DFT) technique, and the seed source directly outputted a square spectrum with a spectral width of 33.7nm, and stretched by SMF to obtain rectangular pulses with a pulse width of >20 ns. The comparison shows that the spatial light modulator has high price, large volume, complex structure and difficult operation; the acousto-optic spatial modulator is only suitable for pulse shaping on the order of the repetition frequency kHz; the super-structure fiber Bragg grating depends on the design and manufacture of the grating; and the time coherent synthesis and FWM time lens are complex in structure and difficult to operate.
Therefore, there is an urgent need for a pulse generating method and system that is small in size, low in cost, simple in structure, flexible in operation, and applicable to rectangular ultra-short (pulse width < 10 ps) pulse shaping.
Disclosure of Invention
Aiming at the defects or improvement demands of the prior art, the invention provides a rectangular ultrashort pulse generating system and a rectangular ultrashort pulse generating method based on a DMD, wherein the system comprises a light source module, a shaping module, a DFT module and a correction module; the shaping module comprises a first optical fiber collimator, a half-wave plate, a first reflecting mirror, a second reflecting mirror, a block grating, a cylindrical lens and a DMD which are sequentially arranged along a first optical path; a third reflecting mirror, a fourth reflecting mirror, a spectroscope and a second optical fiber collimator are sequentially arranged between the block grating and the DFT module along a second optical path; the correction module comprises a spectrometer and a computer; the high-repetition frequency ultrashort pulse output from the tail fiber of the light source module is subjected to spectral shaping through a shaping module, and then subjected to pulse shaping through a DFT module; the spectrometer, the computer and the DMD in the correction module form a feedback closed loop, and the diffraction pattern processed by the correction module is iterated to correct the spectrum shape, so that a more accurate target spectrum is obtained; the invention utilizes DMD spectrum shaping technology and Dispersion Fourier Transmission (DFT) technology to obtain ideal rectangular spectrum and rectangular pulse; the pulse flatness and the pulse abruptness can be controlled by the diffraction pattern loaded by the DMD, and the invention has the advantages of simple and compact structure, small volume, low cost, simple and flexible operation, suitability for ultra-short pulse shaping and high flatness of the generated rectangular pulse.
In order to achieve the above object, an aspect of the present invention provides a rectangular ultrashort pulse generating system based on DMD, including a light source module, a shaping module, a DFT module, and a correction module; wherein,
the shaping module comprises a first optical fiber collimator, a half-wave plate, a first reflecting mirror, a second reflecting mirror, a block grating, a cylindrical lens and a DMD which are sequentially arranged along a first light path;
a third reflecting mirror, a fourth reflecting mirror, a spectroscope and a second optical fiber collimator are sequentially arranged between the block grating and the DFT module along a second optical path;
the correction module comprises a spectrometer and a computer;
the high-repetition frequency ultrashort pulse output from the tail fiber of the light source module is subjected to spectral shaping through the shaping module, and then subjected to pulse shaping through the DFT module; the spectrometer, the computer and the DMD in the shaping module form a feedback closed loop, and the diffraction pattern processed by the shaping module is iterated, so that the spectrum shape is corrected, and an accurate target spectrum is obtained.
Further, the spectrometer and the computer are sequentially arranged between the spectroscope and the DMD;
the spectroscope has a spectroscope ratio of 10:90; the 10% reflected light on the spectroscope is coupled into a spectrometer, the spectrometer transmits the measured spectrum intensity distribution result to a computer, a diffraction pattern is generated, and the diffraction pattern is loaded onto the DMD for spectrum shaping; the rest 90% of the transmitted light on the spectroscope is directly coupled into the DFT module.
Further, after the diffraction pattern is spectrally shaped by the DMD, the diffraction pattern sequentially passes through the cylindrical lens, the block grating, the third reflecting mirror, the fourth reflecting mirror and the spectroscope, and the transmission light of the spectroscope is coupled into the input optical fiber of the DFT module by the second optical fiber collimator for pulse shaping.
Further, the light source module adopts a high-repetition frequency ultrashort pulse seed source.
Further, the DFT module is a single mode fiber with a length of several kilometers or more than 50ps 2 Chirped fiber gratings with a dispersion amount.
Further, the block grating is a transmission grating; the cylindrical lens is a plano-convex cylindrical lens.
Further, the cylindrical lens is arranged on the one-dimensional displacement table;
the distance between the cylindrical lens and the DMD is the focal length of the cylindrical lens;
and the distance between the cylindrical lens and the block grating is finely adjusted, so that the emergent light spots passing through the cylindrical lens are round as the incident light spots in the horizontal direction.
Further, the DMD is a periodic array of micromirrors;
the third reflecting mirror is a right-angle prism or a D-shaped mirror with a hypotenuse coated.
Another aspect of the present invention provides a rectangular ultrashort pulse generating method based on DMD, which is characterized in that the rectangular ultrashort pulse generating system based on DMD is applied, and the method comprises the following steps:
s1, constructing a spectrum shaping light path: turning on a light source module, collimating the ultrashort pulse laser output by the light source module through a first optical fiber collimator, regulating the polarization direction of incident light to be horizontal polarization through a half-wave plate, and sequentially inserting a first reflecting mirror, a block grating, a cylindrical lens, a one-dimensional displacement table and a DMD; fixing the distance between the cylindrical lens and the DMD as the focal length of the cylindrical lens; the distance between the cylindrical lens and the block grating is finely adjusted, so that the emergent light spots are round as the incident light spots in the horizontal direction and keep unchanged in the longitudinal direction; the micro mirror angle of the DMD is regulated to be +12°, so that the corresponding +1 diffraction light spot primary path returns, and the pitching angle of the DMD is regulated, so that the emergent light spot of the shaping module is brightest;
s2, calibrating the output spectral wavelength of the shaping module and the DMD pixel; setting a plurality of dark fringe center positions in the DMD diffraction pattern, so that the interval of the dark fringe center positions is 100 pixels; calibrating wavelength according to DMD output spectrumThe relationship with the pixel X is linear and is expressed by the formula (1):
（1）；
wherein the wavelength isIs in nm; pixel +.>The unit is pixel; wherein->、/>Is a constant;
s3, calculating setting parameters of the spectrometer; specifically, determining the number N of sampling points in a spectrometer according to the size of a micro mirror array of the DMD; determining the spectral resolution of the spectrometer as according to the spectral wavelength and the calibration result of the DMD pixels in the step S2The method comprises the steps of carrying out a first treatment on the surface of the Since the pixel corresponding position is +.>Therefore, will->And->Respectively substituting wavelength +.>And pixel->Obtaining a starting point value and a dead point value of a spectrum range corresponding to the spectrometer; obtaining the center wavelength of the spectrometer according to the average value of the starting point value and the dead point value of the spectrum range of the spectrometer; obtaining a scanning range of the spectrometer according to the difference between a starting point value and a dead point value of the spectrum range of the spectrometer;
s4, calculating the diffraction efficiency of the DMD related to the pixel X according to an experimental spectrum: considering the whole shaping system as a module, the DMD diffraction efficiency M is related to the wavelength λ 1 Represented by formula (2):
（2） ；
wherein,the output spectral intensity distribution when the DMD is fully bright (no modulation); />Is a target spectral intensity distribution;
substituting the formula (1) into the formula (2) to obtain the diffraction efficiency of the DMD related to the pixel X;
s5, calculating the duty ratio of the DMD diffraction pattern related to the pixel X according to the DMD diffraction efficiency related to the pixel X in the step S4;
s6, calculating the fringe width of the diffraction pattern according to the duty ratio of the diffraction pattern of the DMD and the fringe period of the diffraction pattern, which are related to the pixel X, in the step S5, and generating the diffraction pattern of the DMD;
s7, loading the DMD diffraction pattern generated in the step S6 onto the DMD to obtain an output spectrum of the shaping module 2And feeding back to the computer;
and S8, repeating the spectrum shaping process of the steps S4 to S7, and iterating the diffraction pattern for a plurality of times until the rectangular ultrashort pulse target spectrum meeting the requirements is obtained.
Further, the duty cycle of the DMD diffraction pattern associated with pixel X in step S5Calculated by formula (3):（3），
wherein,DMD diffraction efficiency associated with pixel X;
fringe width of diffraction Pattern in step S6Calculated by formula (4):
（4）
wherein,the unit is the fringe period of the diffraction pattern; DMD is equivalent to a scribe line density +.>Longitudinally generating periodic stripes, wherein the grating period of the equivalent grating is the stripe period of the diffraction pattern; stripe width->It may not be an integer, and the diffraction pattern may be a 8-bit bmp picture, with only two values of 0 and 255, "bright" corresponding to 255 and "dark" corresponding to 0.
In general, the above technical solutions conceived by the present invention, compared with the prior art, enable the following beneficial effects to be obtained:
1. the invention discloses a rectangular ultrashort pulse generation system based on a DMD, which comprises a light source module, a shaping module, a DFT module and a correction module, wherein the light source module is connected with the shaping module; the method comprises the steps that a first optical fiber collimator, a half wave plate, a first reflecting mirror, a second reflecting mirror, a block grating, a cylindrical lens and a DMD26 which are used as shaping modules are sequentially arranged on one side of a light source module along a first light path, a third reflecting mirror, a fourth reflecting mirror, a spectroscope and a second optical fiber collimator are sequentially arranged between the block grating and the DFT module along a second light path, a spectroscope and a computer which are used as correction modules are sequentially arranged between the spectroscope and the DMD26, so that high-frequency ultra-short pulses output from tail fibers of the light source module are subjected to spectral shaping through the shaping modules, then subjected to pulse shaping through the DFT modules, form a feedback closed loop through the spectroscope, the computer and the DMD in the shaping modules, iterate diffraction patterns processed by the shaping modules, and correct spectral shapes so as to obtain more accurate target spectrums; the invention utilizes DMD spectrum shaping technology and Dispersion Fourier Transmission (DFT) technology to obtain ideal rectangular spectrum and rectangular pulse; the pulse flatness and pulse abruptness can be controlled by diffraction patterns loaded by the DMD; the invention is suitable for ultra-short pulse shaping, has the advantages of small volume, low cost, simple structure, flexible operation and the like, and the flatness of the generated rectangular pulse is high.
2. After a shaping light path is built, calibrating output spectral wavelength and DMD pixels of a shaping module, determining the number of sampling points of a spectrometer according to the size of a micro mirror array of the DMD, determining the spectral resolution of the spectrometer according to the calibration result of the spectral wavelength and the DMD pixels, substituting a value range of a corresponding position of the pixels into a relation between the spectral wavelength and the DMD pixels, solving the scanning range of the spectrometer, calculating DMD diffraction efficiency related to the pixel X according to an experimental spectrum, calculating the duty ratio of the DMD diffraction pattern related to the pixel X according to the DMD diffraction efficiency related to the pixel X, calculating the fringe width of the diffraction pattern according to the duty ratio of the DMD diffraction pattern related to the pixel X and the fringe period of the diffraction pattern, generating the DMD diffraction pattern, loading the diffraction pattern onto the DMD, obtaining the output spectrum of the shaping module, feeding back to a computer, repeating the spectral shaping process, and iterating the diffraction pattern for a plurality of times until a more accurate (higher flatness) rectangular ultrashort pulse target is obtained; the laser subjected to spectrum shaping by the technical scheme of the invention has good time-frequency correlation between the output spectrum and the pulse waveform after passing through the DFT module, the pulse waveform and the spectrum shape are basically consistent, the flatness of the obtained rectangular pulse is less than 10%, and an effective means can be provided for shaping the rectangular ultrashort pulse.
Drawings
FIG. 1 is a schematic diagram of a rectangular ultrashort pulse generating system based on DMD according to an embodiment of the present invention;
FIG. 2 is a schematic diagram of the overall structure of a rectangular ultra-short pulse generating system based on DMD according to an embodiment of the present invention;
FIG. 3 is a schematic flow chart of a rectangular ultrashort pulse generating method based on DMD according to an embodiment of the present invention;
FIG. 4 is a schematic diagram of a diffraction pattern of a DMD for wavelength-pixel calibration according to a rectangular ultra-short pulse generation method based on a DMD according to an embodiment of the present invention;
FIG. 5 is a schematic diagram showing the spectral distribution of DMD pixels before and after calibration of a rectangular ultra-short pulse generation method based on DMD according to an embodiment of the present invention;
FIG. 6 is a schematic diagram showing a relation between a spectrum wavelength and a DMD pixel position of a rectangular ultra-short pulse generating method based on a DMD according to an embodiment of the present invention;
FIG. 7 is a schematic diagram of diffraction patterns generated when the target waveform is rectangular in a rectangular ultra-short pulse generating method based on DMD according to an embodiment of the present invention;
FIG. 8 is a schematic diagram showing the spectral shaping effect of the DMD after the diffraction pattern in FIG. 7 is loaded on the DMD for a single time in a rectangular ultra-short pulse generating method based on the DMD according to an embodiment of the present invention;
FIG. 9 is a schematic diagram of a pulse rectangle degree evaluation means of a rectangle ultrashort pulse generation method based on a DMD according to an embodiment of the present invention;
fig. 10 is a schematic diagram of an output spectrum of a laser after spectral shaping after passing through a DFT module in a rectangular ultrashort pulse generating method based on DMD according to an embodiment of the present invention;
fig. 11 is a schematic diagram of pulse waveforms of a laser after spectral shaping after passing through a DFT module in a rectangular ultrashort pulse generating method based on DMD according to an embodiment of the present invention.
Like reference numerals denote like technical features throughout the drawings, in particular: the device comprises a 1-light source module, a 2-shaping module, a 21-first optical fiber collimator, a 22-half wave plate, a 23-first reflecting mirror, a 231-second reflecting mirror, a 24-block grating, a 25-cylindrical lens, a 251-one-dimensional displacement table, a 26-DMD, a 27-second reflecting mirror, a 28-fourth reflecting mirror, a 29-spectroscope, a 30-second optical fiber collimator, a 3-DFT module, a 4-correction module, a 41-spectrometer and a 42-computer.
Detailed Description
The present invention will be described in further detail with reference to the drawings and examples, in order to make the objects, technical solutions and advantages of the present invention more apparent. It should be understood that the specific embodiments described herein are for purposes of illustration only and are not intended to limit the scope of the invention. In addition, the technical features of the embodiments of the present invention described below may be combined with each other as long as they do not collide with each other.
In the description of the present invention, it will be understood that when an element is referred to as being "mounted," "disposed," or "disposed" on another element, it can be directly on the other element or be indirectly on the other element unless explicitly stated and limited otherwise. When an element is referred to as being "connected to" another element, it can be directly connected to the other element or be indirectly connected to the other element; the terms "mounted," "connected," and "provided" are to be construed broadly, and may be, for example, fixedly connected, detachably connected, or integrally connected; can be mechanically or electrically connected; can be directly connected or indirectly connected through an intermediate medium, and can be communicated with the inside of two elements or the interaction relationship of the two elements. The specific meaning of the above terms in the present invention can be understood by those of ordinary skill in the art according to the specific circumstances.
Furthermore, the terms "first," "second," and the like are used for descriptive purposes only and are not to be construed as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Thus, a feature defining "a first" or "a second" may explicitly or implicitly include one or more such feature. In the description of the present invention, the meaning of "a plurality" is two or more, unless explicitly defined otherwise.
As shown in fig. 1 and 2, one aspect of the present invention provides a DMD-based rectangular ultrashort pulse generating system, which includes a light source module 1, a shaping module 2, a DFT module 3, and a correction module 4; the DFT module is a dispersion Fourier transform module; the DMD is a micro lens array; the light source module 1 adopts a high-repetition-frequency (more than 100 kHz) ultrashort (pulse width is less than 10 ps) pulse seed source; the shaping module 2 comprises a first optical fiber collimator 21, a half-wave plate 22, a first reflecting mirror 23, a second reflecting mirror 231, a block grating 24, a cylindrical lens 25 and a DMD26 which are sequentially arranged along a first optical path; a third reflecting mirror 27, a fourth reflecting mirror 28, a spectroscope 29 and a second optical fiber collimator 30 are sequentially arranged between the block grating 24 and the DFT module 3 along a second optical path; the high-repetition frequency ultrashort pulse output from the tail fiber of the light source module 1 is subjected to spectral shaping through the shaping module 2, and then subjected to pulse shaping through the DFT module 3; the correction module 4 comprises a spectrometer 41 and a computer 42; the spectrometer 41, the computer 42 and the DMD26 in the shaping module 2 form a feedback closed loop, iterate the diffraction pattern processed by the shaping module 2, and further correct the spectrum shape to obtain a more accurate target spectrum; the invention utilizes DMD spectrum shaping technology and Dispersion Fourier Transmission (DFT) technology to obtain ideal rectangular spectrum and rectangular pulse; the pulse flatness and pulse abruptness can be controlled by the diffraction pattern loaded by the DMD. The invention is suitable for ultra-short pulse shaping, has the advantages of small volume, low cost, simple structure, flexible operation and the like, and the flatness of the generated rectangular pulse is high.
Further, as shown in fig. 1 and 2, the high-repetition-frequency ultrashort pulse output from the light source module 1 passes through the first optical fiber collimator 21, is collimated into space light, and then sequentially passes through the half-wave plate 22, the first reflecting mirror 23, the block grating 24, the cylindrical lens 25 and the DMD26 to form +1st-order diffracted light, and the +1st-order diffracted light returns from the DMD26 and sequentially passes through the cylindrical lens 25, the block grating 24, the third reflecting mirror 27, the fourth reflecting mirror 28 and the spectroscope 29, and then the transmitted light of the spectroscope 29 is coupled into the input optical fiber of the DFT module 3 by using the second optical fiber collimator 30 for pulse shaping.
Further, as shown in fig. 1 and fig. 2, the spectrometer 41 and the computer 42 are sequentially disposed between the beam splitter 29 and the DMD26, and the beam splitting ratio of the beam splitter 29 is 10:90; 90% of the transmitted light on the beam splitter 29 is directly coupled into the input end optical fiber of the DFT module 3 for pulse shaping; the rest 10% of reflected light on the spectroscope 29 is coupled into a spectrometer 41, the spectrometer 41 transmits the measured spectrum intensity distribution result to a computer 42, a diffraction pattern is generated, and the diffraction pattern is loaded onto the DMD26 for spectrum shaping; after the diffraction pattern is spectrally shaped by the DMD26, the diffraction pattern sequentially passes through the cylindrical lens 25, the block grating 24, the third reflecting mirror 27, the fourth reflecting mirror 28 and the spectroscope 29, and the transmitted light of the spectroscope 29 is coupled into the optical fiber at the input end of the DFT module 3 by the second optical fiber collimator 30 for pulse shaping again; by cycling in this way, a plurality of iterations are performed on the diffraction pattern formed on DMD26 to obtain a more accurate target spectrum.
Further, in the embodiment of the present invention, the DFT module 3 is a single mode fiber with several kilometers or more than 50ps 2 The chirped fiber grating of the dispersion quantity is used for providing a dispersion Fourier transform effect, so that the pulse shape is consistent with the shaped spectrum shape; the half-wave plate 22 is used for adjusting the polarization direction of the input light of the shaping module 2 to be horizontal polarization; the block grating 24 is a transmission grating; the cylindrical lens 25 is a plano-convex cylindrical lens.
Further, as shown in fig. 2, in the embodiment of the present invention, the cylindrical lens 25 is provided on a one-dimensional displacement stage 251, and the cylindrical lens 25 is fixed by the one-dimensional displacement stage 251; the cylindrical lens 25 and DMD26 is fixed at a distance ofI.e. the focal length of the cylindrical lens; by fine-tuning the distance between the cylindrical lens 25 and the block grating 24, the outgoing spot passing through the cylindrical lens 25 is made to be as round as the incoming spot in the horizontal direction (unchanged in the longitudinal direction).
Further, in an embodiment of the present invention, the DMD26 is a periodic array of micromirrors for spectral shaping; the third reflecting mirror 27 is configured to receive an output diffraction spot of the block grating 24, where the output diffraction spot is located directly below an incident spot of the block grating 24, and the third reflecting mirror 27 is a right-angle prism or a D-shaped mirror with a hypotenuse coated.
As shown in fig. 3, according to another aspect of the present invention, there is further provided a method for generating rectangular ultrashort pulses based on DMD, implemented by using the rectangular ultrashort pulse generating system based on DMD, including the steps of:
s1, building a shaping light path: turning on the light source module 1, collimating the ultrashort pulse laser output by the light source module through the first optical fiber collimator 21, regulating the polarization direction of the incident light to be horizontal polarization through the half-wave plate 22, and sequentially inserting the first reflecting mirror 23, the block grating 24, the cylindrical lens 25, the one-dimensional displacement table 251 and the DMD26; fixing the distance between the cylindrical lens 25 and the DMD26 to be the focal length of the cylindrical lens; the distance between the cylindrical lens 25 and the block grating 24 is finely adjusted nearby, so that the emergent light spot is as round as the incident light spot in the horizontal direction (the longitudinal direction is unchanged); the micro mirror angle of the DMD26 is adjusted to be +12°, so that the corresponding +1 diffraction light spot primary path returns (namely only the bright lines pass through), and the pitch angle of the DMD26 is adjusted, so that the emergent light spot of the shaping module 2 is brightest;
in a specific embodiment of the present invention, the light source module 1 adopts a high-repetition frequency optical fiber seed source, and outputs a pulse center wavelength 1030nm, a repetition frequency 1MHz, a pulse width 600fs and a pigtail PM980; the focal length of the optical fiber collimator 21 is 15.43mm, the numerical aperture is 0.16mm, and the spot diameter is 3.4mm; the block grating 24 adopts a transmission grating, the wavelength is 1030nm, the line density is 1600lines/mm, the incident angle is 55.5 degrees, and the diffraction efficiency is 90%; by a means ofThe cylindrical lens 25 adopts a plano-convex cylindrical lens with a focal length fcy < 1 > = 300mm, the size is 20mm multiplied by 20mm, and the bus length is 20mm; the distance fcy of the cylindrical lens 25 from the DMD 26=300 mm; the distance l=280 mm between the cylindrical lens 25 and the block grating 24; the DMD26 has a size of 0.45 inch, pixels 912 (columns) x 1140 (rows), a number of micromirrors exceeding one million, a micromirror size of 7.6um, a micromirror pitch of 10.8um, a frame frequency of 4.2kHz, a micromirror tilt angle of ±12°, and an effective working area of 6.16mm x 9.85mm; the third reflecting mirror 27 adopts a right-angle prism with an inclined plane coated film; the DFT module 3 adopts a 3km polarization-maintaining single-mode fiber, and the dispersion quantity is about 84ps 2 ；
S2, calibrating the output spectral wavelength of the shaping module and the DMD pixel; specifically, a plurality of dark stripe center positions are set in the DMD26 diffraction pattern such that the interval of each dark stripe center position is 100 pixels; wavelength calibrated according to DMD26 output spectrumThe relationship with the pixel X is linear and is expressed by the formula (1):
（1）；
wherein the wavelength isIs in nm; pixel +.>The unit is pixel; wherein->、/>Is a constant;
in one embodiment of the present invention, as shown in FIG. 4, 6 dark fringes are provided in the diffraction pattern of DMD26; the 6 dark stripe center positions are = 206, 306, 406, 506, 606, 706 pixels, respectively; two adjacent dark stripes are spaced by 100 pixels, and the width of each dark stripe is 41 pixels; the basic parameters of the spectrometer 41 are set as: center of the machineWavelength 1030nm, scanning range 9.1nm, sampling point number 912; as shown in fig. 5, the spectral distribution before and after being shaped by the shaping module 2 is compared, and the spectral transmittance is obviously lower at the corresponding dark line position; as shown in fig. 6, the corresponding wavelength at the spectral minima is extracted, pixels of the DMDWavelength->The calibration relation of (2) is as follows:
；
s3, calculating setting parameters of the spectrometer; specifically, the number N of sampling points in the spectrometer 41 is determined according to the micromirror array size Y (row) ×n (column) of the DMD26; the spectral resolution of the spectrometer 41 is determined to be the following according to the calibration result of the spectral wavelength and the DMD pixel in step S2The method comprises the steps of carrying out a first treatment on the surface of the Since the pixel corresponding position is +.>Therefore, will->And->Respectively substituting wavelength +.>And pixelObtaining a start point value and a stop point value of a spectrum range corresponding to the spectrometer 41; obtaining the center wavelength of the spectrometer according to the average value of the starting point value and the dead point value of the spectrum range of the spectrometer; obtaining a scanning range of the spectrometer according to the difference between a starting point value and a dead point value of the spectrum range of the spectrometer;
further, in a specific embodiment of the present invention, the micromirror array size of the DMD is 1140 pixels×912 pixels, the number of sampling points in the spectrometer 41 is n=912, the spectral resolution is determined to be 0.0125nm according to the wavelength-pixel calibration result in step S2, the spectral range "start=1024.412 nm, stop= 1035.790nm" corresponding to the DMD is calculated according to the pixel corresponding position being x=1-912, and the parameters of the spectrometer are set to "center=1030.101 nm, span=11.378nm, n=912"; wherein, start is the starting point value, stop is the dead point value, center is the Center wavelength, span is the scanning range;
s4, calculating the diffraction efficiency of the DMD related to the pixel X according to an experimental spectrum: considering the whole shaping system as a module, the DMD diffraction efficiency M is related to the wavelength λ 1 Represented by the formula:
（2） ；
wherein,the output spectral intensity distribution when the DMD is fully bright (no modulation); />Is a target spectral intensity distribution;
substituting formula (1) into formula (2) to obtain DMD diffraction efficiency related to pixel X；
S5, calculating the duty ratio of the DMD diffraction pattern related to the pixel X according to the DMD diffraction efficiency related to the pixel X in the step S4; wherein the duty cycle of the DMD diffraction pattern associated with pixel XCalculated by the following formula: />（3）；
S6, calculating the fringe width of the diffraction pattern according to the duty ratio of the diffraction pattern of the DMD and the fringe period of the diffraction pattern, which are related to the pixel X, in the step S5, and generating the diffraction pattern of the DMD;
fringe width of diffraction patternCalculated by the following formula:
（4）
wherein,the unit is the fringe period of the diffraction pattern; DMD is equivalent to a scribe line density +.>Longitudinally generating periodic stripes, wherein the grating period of the equivalent grating is the stripe period of the diffraction pattern; stripe width->It may not be an integer, and the diffraction pattern may be a 8-bit bmp picture, with only two values of 0 and 255, "bright" corresponding to 255 and "dark" corresponding to 0.
Specifically, in one embodiment of the present invention, DMD26 is equivalent to a blazed grating with a reticle density of dmdmd=556 lines/mm, and periodic fringes are generated longitudinally, and the grating period of the equivalent grating is the fringe period of the diffraction pattern; as shown in fig. 7, the diffraction pattern generated when the fringe period γ (X) =30 Pixels of the diffraction pattern, the target waveform is rectangular;
s7, loading the diffraction pattern generated in the step S6 on the DMD to obtain an output spectrum of the shaping module 2And fed back to the computer 42;
and S8, repeating the spectrum shaping process of the steps S4 to S7, and iterating the diffraction pattern for a plurality of times until a rectangular ultrashort pulse target spectrum with higher flatness meeting the requirement is obtained.
When the target waveform is rectangular, diffraction patterns generated as described above are shown in fig. 7, respectively; the spectral shaping effect after a single loading of the diffraction pattern in fig. 7 on the DMD is shown in fig. 8.
The invention evaluates the rectangular degree of the pulse from two aspects of flatness and steepness. As shown in fig. 9, a portion of the power greater than 80% of the peak of the pulse is treated as the top of the pulse, and pulse flatness is defined as the ratio of the difference between the maximum value and the minimum value of the pulse to the maximum value of the pulse; the pulse rising edge time is defined as the time difference corresponding to 20% and 80% of the peak value at the time of pulse rising, and is denoted by tr 1; the pulse falling edge time is defined as the time difference corresponding to 20% and 80% of the peak value when the pulse falls, and is denoted by tr 2; the time at the top of the pulse is denoted by T, and the pulse steepness is defined as the ratio of the pulse rising edge time tr1 to the pulse top time T; the Flatness of the pulse waveform, flat, is estimated by the following formula: the plane= delta a/a; the Steepness stepness of the pulse waveform is estimated by the following formula: steepness = tr1/T.
As shown in fig. 10 and 11, in the above embodiment of the present invention, the output spectrum of the laser after the spectrum shaping and the pulse waveform after passing through the DFT module (3) have good time-frequency correlation, the pulse waveform and the spectrum shape are basically consistent, and the flatness of the obtained rectangular pulse is less than 10%.
It will be readily appreciated by those skilled in the art that the foregoing description is merely a preferred embodiment of the invention and is not intended to limit the invention, but any modifications, equivalents, improvements or alternatives falling within the spirit and principles of the invention are intended to be included within the scope of the invention.
Claims (9)
1. A rectangular ultrashort pulse generating system based on a DMD, characterized in that: comprises a light source module (1), a shaping module (2), a DFT module (3) and a correction module (4); wherein,
the shaping module (2) comprises a first optical fiber collimator (21), a half-wave plate (22), a first reflecting mirror (23), a second reflecting mirror (231), a block grating (24), a cylindrical lens (25) and a DMD (26) which are sequentially arranged along a first optical path;
a third reflecting mirror (27), a fourth reflecting mirror (28), a spectroscope (29) and a second optical fiber collimator (30) are sequentially arranged between the block grating (24) and the DFT module (3) along a second optical path;
the correction module (4) comprises a spectrometer (41) and a computer (42);
the high-repetition frequency ultrashort pulse output from the tail fiber of the light source module (1) is subjected to spectral shaping through the shaping module (2), and then subjected to pulse shaping through the DFT module (3); the spectrometer (41), the computer (42) and the DMD (26) in the shaping module (2) form a feedback closed loop, and the diffraction pattern processed by the shaping module (2) is iterated, so that the spectrum shape is corrected, and an accurate target spectrum is obtained.
2. A DMD-based rectangular ultrashort pulse generation system in accordance with claim 1, wherein: the spectrometer (41) and the computer (42) are sequentially arranged between the spectroscope (29) and the DMD (26);
the spectroscope (29) has a spectral ratio of 10:90; 10% of reflected light on the spectroscope (29) is coupled into a spectrometer (41), the spectrometer (41) transmits the measured spectral intensity distribution result to a computer (42) to generate a diffraction pattern, and the diffraction pattern is loaded onto the DMD (26) for spectral shaping; the rest 90% of the transmitted light on the spectroscope (29) is directly coupled into the DFT module (3).
3. A DMD-based rectangular ultrashort pulse generation system in accordance with claim 2, wherein: after the diffraction pattern is subjected to spectral shaping through the DMD (26), the diffraction pattern sequentially passes through the cylindrical lens (25), the block grating (24), the third reflecting mirror (27), the fourth reflecting mirror (28) and the spectroscope (29), and the transmission light of the spectroscope (29) is coupled into the input end optical fiber of the DFT module (3) through the second optical fiber collimator (30) for pulse shaping.
4. A DMD-based rectangular ultrashort pulse generation system in accordance with claim 3, wherein: the DFT module (3) is a single mode fiber with a length of several kilometers or a chirped fiber grating with a dispersion amount of more than 50ps 2.
5. A DMD-based rectangular ultrashort pulse generation system in accordance with claim 4, wherein: the block grating (24) is a transmission grating; the cylindrical lens (25) is a plano-convex cylindrical lens.
6. A DMD-based rectangular ultrashort pulse generation system in accordance with claim 5, wherein: the cylindrical lens (25) is arranged on the one-dimensional displacement table (251);
the distance between the cylindrical lens (25) and the DMD (26) is the focal length of the cylindrical lens;
by fine-tuning the distance between the cylindrical lens (25) and the block grating (24), the outgoing light spot passing through the cylindrical lens (25) is made to be round as the incoming light spot in the horizontal direction.
7. A DMD-based rectangular ultrashort pulse generation system in accordance with claim 6, wherein: the DMD (26) is a periodic array of micromirrors;
the third reflecting mirror (27) is a right-angle prism or a D-shaped mirror with a hypotenuse coated.
8. A rectangular ultra-short pulse generating method based on a DMD, which is realized by applying the rectangular ultra-short pulse generating system based on the DMD as claimed in any one of claims 1 to 7, comprising the steps of:
s1, constructing a spectrum shaping light path: the light source module (1) is turned on, the output ultrashort pulse laser is collimated by the first optical fiber collimator (21), the polarization direction of the incident light is regulated to be horizontally polarized by the half-wave plate (22), and then the first reflecting mirror (23), the block grating (24), the cylindrical lens (25), the one-dimensional displacement table (251) and the DMD (26) are sequentially inserted; fixing the distance between the cylindrical lens (25) and the DMD (26) to be the focal length of the cylindrical lens; the distance between the cylindrical lens (25) and the block grating (24) is finely adjusted, so that the emergent light spots are round as the incident light spots in the horizontal direction and keep unchanged in the longitudinal direction; the micro mirror angle of the DMD (26) is regulated to be +12 degrees, so that the corresponding +1-order diffraction light spot can return in the original path, and the pitching angle of the DMD (26) is regulated, so that the emergent light spot is brightest;
s2, calibrating the output spectral wavelength and the DMD pixel of the shaping module (2); setting a plurality of dark grain center positions in a diffraction pattern of the DMD (26) so that the interval of the dark grain center positions is 100 pixels; outputting spectral calibration wavelengths according to DMD (26)The relationship with the pixel X is linear and is expressed by the formula (1):
（1）；
wherein the wavelength isIs in nm; pixel +.>The unit is pixel; wherein->、/>Is a constant;
s3, calculating setting parameters of the spectrometer; determining the number N of sampling points in a spectrometer (41) according to the micro mirror array size of the DMD (26); determining the spectral resolution of the spectrometer (41) as the following according to the calibration result of the spectral wavelength and the DMD pixel in the step S2The method comprises the steps of carrying out a first treatment on the surface of the Since the pixel corresponding position is +.>Therefore, will->And->Respectively substituting wavelength +.>And pixel->Obtaining a start point value and a stop point value of a spectrum range corresponding to the spectrometer (41); obtaining the center wavelength of the spectrometer according to the average value of the starting point value and the dead point value of the spectrum range of the spectrometer; obtaining a scanning range of the spectrometer according to the difference between a starting point value and a dead point value of the spectrum range of the spectrometer;
s4, calculating the diffraction efficiency of the DMD related to the pixel X according to an experimental spectrum: considering the entire shaping system as a module, the DMD diffraction efficiency M1 associated with the wavelength λ is represented by equation (2):
（2）；
wherein,the output spectral intensity distribution when the DMD is fully bright (no modulation); />Is a target spectral intensity distribution;
substituting the formula (1) into the formula (2) to obtain the diffraction efficiency of the DMD related to the pixel X;
s5, calculating the duty ratio of the DMD diffraction pattern related to the pixel X according to the DMD diffraction efficiency related to the pixel X in the step S4;
s6, calculating the fringe width of the diffraction pattern according to the duty ratio of the diffraction pattern of the DMD and the fringe period of the diffraction pattern, which are related to the pixel X, in the step S5, and generating the diffraction pattern of the DMD;
s7, loading the DMD diffraction pattern generated in the step S6 onto the DMD to obtain an output spectrum of the shaping module 2And fed back to the computer (42);
and S8, repeating the spectrum shaping process of the steps S4 to S7, and iterating the diffraction pattern for a plurality of times until the rectangular ultrashort pulse target spectrum meeting the requirements is obtained.
9. The method for DMD-based rectangular ultra-short pulse generation as recited in claim 8, wherein the duty cycle of the DMD diffraction pattern associated with pixel X in step S5Calculated by formula (3): />(3) Wherein->DMD diffraction efficiency associated with pixel X;
fringe width of diffraction Pattern in step S6Calculated by formula (4):
（4）
wherein,the unit is the fringe period of the diffraction pattern; DMD is equivalent to a scribe line density +.>Longitudinally generating periodic stripes, wherein the grating period of the equivalent grating is the stripe period of the diffraction pattern; the diffraction pattern takes an 8-bit, bmp picture, with only two values of 0 and 255, "bright" corresponding to 255 and "dark" corresponding to 0.
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