CN102313605A - Method and device for measuring self-referenced spectral interference femtosecond laser pulse in real time - Google Patents
Method and device for measuring self-referenced spectral interference femtosecond laser pulse in real time Download PDFInfo
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Abstract
The invention relates to a method and a device for measuring a self-referenced spectral interference femtosecond laser pulse in real time. In the method, firstly, laser is divided into three beams and two beams of laser are focused and irradiated on a non-linear transparent optimal medium to generate two beams of first-order self-diffraction light; any one beam of self-diffraction light is used as a reference laser pulse, the reference laser pulse is collinear with a third beam of laser pulse in space and is regulated to be coincident with the third beam of laser pulse, and a laser interference spectrum can be obtained by regulating delay time; and the interference spectrum is measured, so that a laser spectrum and a spectral phase can be obtained by inverse calculation through a self-referenced spectral coherence method, and thus, the width and the shape of the laser pulse can be measured. In the invention, an SRSI (self-referenced spectral interferometry) method and an FROG (frequency-resolved optical gating) method and the advantages thereof are combined; and the method and the device have the characteristics of high speed, simpleness and convenience and are suitable for single shot measurement, real-time measurement and monitoring of the widths and the shapes of femtosecond laser pulses with different pulse widths and different wavelengths.
Description
Technical Field
The invention relates to femtosecond laser pulses, in particular to a real-time measuring method and a real-time measuring device for self-reference spectrum interference femtosecond laser pulses (shapes and widths). The method can be applied to a femtosecond optical system with the pulse width of 10-300fs in the spectral range of 200-3000 nm. The invention is not only suitable for the laser pulse with megahertz repetition frequency, but also suitable for the measurement of the pulse width and the pulse shape of the single femtosecond laser pulse.
Background
With the development of the femtosecond laser technology, the femtosecond laser pulse is more and more widely applied to various social fields such as scientific research, processing, biology, medical treatment, national defense, communication and the like. In the laboratory, the femtosecond laser pulse is used as a tool in a plurality of important advanced scientific basic researches and application basic researches such as astronomical physics, X-ray laser, laser electron and proton acceleration, attosecond laser pulse generation and the like, femtosecond chemistry, femtosecond nonlinear optical microscopic imaging and the like. Femtosecond laser processing enables finer and smoother surface shapes to be obtained in industrial and medical applications relative to nanosecond and picosecond lasers. Femtosecond laser pulses have also been recently used to perform ophthalmic lens cutting surgery. In all of these important basic scientific studies and applications, femtosecond laser pulse width is an important optical parameter, and its measurement or detection is essential in many experiments. Therefore, a simple, convenient and effective method and device for measuring and monitoring the laser pulse width are very important for promoting the development and application of the femtosecond laser technology field. With the development of femtosecond laser technology, femtosecond laser pulse width measurement technology is also continuously developed. The most commonly used method at present is an autocorrelation method [ see document 1: trebino, Frequency-Resolved Optical grading: the Measurement of Ultrashort laser pulses (Kluwer Academic Publishers) (2000), frequency-resolved grating (FROG) method [ see document 2: r.t. trenbino, k.w.delong, d.n.fitinghoff, j.n.sweetser, m.a.krumbugel, b.a.richman, and d.j.kane, "Measuring ultra laser pulses in the time-frequency domain using frequency-resolved optical gating," rev.sci.instrument.68 (9), 3277-3295(1997) ] and spectral phase coherent direct electric field reconstruction (spectral phase interaction for direct electric-field reconstruction, short SPIDER) methods [ see document 3: iaconis and I.A.Walmsley, "Spectral phase interaction for direct electric-field communication of ultrashort optical pulses," Opti.Lett.23 (10), 792- & 794(1998) ". The principle and the structure of the autocorrelation method are simple, but the phase information of the femtosecond laser pulse cannot be obtained. The FROG and SPIDER methods can obtain the pulse phase. However, the FROG and SPIDER methods typically require a longer time to reconstruct the pulse. In the SPIDER method, a nonlinear optical crystal is typically required to convert and generate the measurement signal. This limits the application of these methods over a wide spectral range because each measurement instrument can only be adapted to a specific spectral range due to the phase matching conditions of the nonlinear optical crystal.
Recently, a cross-polarized wave (XPW) based method [ see document 4: jullien, l.canova, o.albert, d.boschetto, l.antonuci, y.h.cha, j.p.rousseau, p.chaudet, g.cheriaux, j.ethepare, s.kortet, n.minkovski, and s.m.saliel, "Spectral branched and pulse duration reduction cross-polar generation: self-referenced spectroscopic interferometry (SRSI) method using the fluorescence of the spectroscopic phase, "application. phys. b 87(4), 595-601(2007) ] as reference light [ see document 5: t.oksenhendler, s.coudreau, n.forget, v.crozatier, s.grabielle, r.herzog, o.gobert, and d.kaplan, "Self-compensated spectral interferometry," appl.phys.b 99(1), 7-12 (2010.) ] is used to measure the laser pulses. In this method, the spectrum and the spectral phase of the measurement laser can be obtained quickly by only 3 simple iterative calculations. This is by far the simplest and most convenient way to make a pulse width single shot measurement. However, this method requires an optical polarizing element. Since the polarizing optical element is also only effective for a specific laser wavelength and has a certain spectral bandwidth, this limits the application of the method and apparatus to a specific spectral range. The chromatic dispersion of the polarizing optical element also makes it limited for short pulse measurements below 10 fs.
Disclosure of Invention
The present invention aims to propose a method based on the self-diffraction effect [ see document 6: a real-time measurement method of self-reference spectrum interference femtosecond laser pulse of J.Liu, K.Okamura, Y.Kida, and T.Kobayashi, "Temporal correlation enhancement of femto-connected pulse side a self-diffusion process in a bulk Kerr medium", Opt.express 18(21), 22245-22254(2010) ], provides a real-time measurement device of femtosecond laser pulse shape. The invention combines the two methods of SRSI and FROG and the advantages thereof, has the characteristics of high speed, simplicity and convenience, and is suitable for single-shot measurement and real-time measurement and monitoring of the pulse width and the pulse shape of the femtosecond laser with different pulse widths and different wavelengths.
The technical solution of the invention is as follows:
a method for real-time measurement of femtosecond laser pulses, characterized in that the method comprises the following steps:
firstly, a first-order self-diffraction laser beam is generated by utilizing a third-order nonlinear optical effect:
the laser beam to be measured is divided into three beams of laser by a beam splitting system, wherein one beam of laser energy is 5% of the total pulse energy for measurement, and the other two beams of laser energy are equal and focused on a piece of thinner (usually 100um for quartz glass) Kerr transparent nonlinear dielectric material at a certain non-collinear included angle (usually about 2 °) to generate a first-order self-diffraction beam according to the expression [ see document 6 ]:
wherein: omegasd1、ω1And ω-1The first-order self-diffraction light and the two incident laser angular frequencies are respectively. Δ kz(ωsd1，ω1，ω-1) For phase mismatch during self-diffraction, L is the nonlinear dielectric material thickness.
According to expression (1), since the self-diffracted laser light generated has a smoother and wider laser spectrum than the incident laser light pulse to be measured due to the third-order nonlinear effect, we used this self-diffracted laser light beam as a reference beam for self-reference spectral interference (SRSI) femtosecond laser pulse measurement [ see document 5 ].
In the SRSI measurement, we first adjust the laser light to be measured and any one of the first-order self-diffracted light (hereinafter referred to as the reference light) to be collinear and spatially coincident into the high-precision spectrometer.
Firstly, blocking light to be measured and reference light respectively, measuring the independent spectrum of the reference light or laser to be measured respectively by using a high-precision spectrometer, enabling the intensity of the reference light to be stronger than that of the laser to be measured (better stronger by about 3 times), and storing a data file.
And then, adjusting the time delay between the reference light and the laser pulse to be detected, wherein the time delay tau is proper, and the spectral interference fringes of the two beams of light can be obviously seen on a spectrometer. Increasing the delay can increase the spectral fringe density and thus the accuracy of the spectral and spectral phase measurements, but this requires a higher accuracy spectrometer. Here, we typically adjust the delay τ so that the spectral fringe spacing is around 2 nm. Meanwhile, two laser beams are optimized to be overlapped in space to obtain spectral interference fringes D (omega, tau) with the maximum modulation depth, and a data file is stored. The measured spectral interference fringe D (ω, τ) can be expressed as [ see document 5 ]:
D(ω，τ)＝|Eref(ω)+E(ω)eiωτ|2
＝|Eref(ω)|2+|E(ω)|2+f(ω)eiωτ+f*(ω)e-iωτ (2)
where ω is the laser angular frequency, S0(ω)＝|Eref(ω)|2+|E(ω)|2Is the sum of the spectra of the laser to be measured and the first-order self-diffraction light signal;is the spectral interference term of the two lasers.
Since we assume that the reference spectral phase is known, the laser spectrum and spectral phase to be measured can be obtained by fourier transform and iterative computation procedure as in fig. 1. Wherein S is0(τ), f (τ) are each S0(ω), f (ω) Fourier transforms the values in the time domain. As can be seen from fig. 1, the acquisition of the laser spectrum and the spectral phase requires the following steps:
1. transforming the measured interference spectrum signal D (omega, tau) to a time domain by utilizing Fourier;
2. by applying a suitable window function (e.g. a super-Gaussian function) to S0(τ), f (τ) is extracted from the time domain signals obtained in step 1;
3. s obtained in step 20(tau), f (tau) are respectively inverse Fourier transformed to frequency domain to respectively obtain S0(ω)，f(ω)；
4. Using S obtained in step 30(omega), f (omega), the following formula is adopted to determine the spectral amplitude | E (omega) | of the laser to be measured and the spectral amplitude | E of the reference lightref(ω) | is directly expressed linearly [ see document 5]：
And
thereby preliminarily obtaining the spectrum | E (omega) & gtof the laser to be measured2And spectrum | E of reference lightref(ω)|2；
5. By performing a dephasing operation (argf (ω)) on f (ω), the preliminary spectral phase of the laser to be measured is iteratively calculated using the following formula:
wherein,
and
the spectral phases of the laser light to be measured and the reference light, respectively (initially assumed to be 0). And C is a spectral phase constant introduced by a dispersive optical element such as a beam splitter and can be directly obtained by calculation.
6. Fourier transform is carried out on the preliminarily obtained laser spectrum and the spectrum phaseTransforming to obtain the pulse shape | E (t) of laser pulse to be measured2And pulse width, E (t) being the value of the fourier transform of E (ω).
7. Since the reference light phase is not always absolutely equal to 0, the above steps need to be further iteratively calculated according to the following iterative method to optimize the laser spectrum and the spectral phase output. The iterative method is as follows:
(i) obtaining the time domain electric field shape of the reference light E (t) xE (t) y according to the time domain expression of the formula (1) by using the pulse shape signal obtained in the step 62And performing inverse Fourier transform on the spectrum to obtain a reference light spectrum | Eref(ω)|2And the spectral phase
(ii) The reference light spectrum phase substitution
Namely, a new laser spectrum phase to be measured can be obtained, and the pulse shape | E (t) of the laser pulse to be measured is obtained by carrying out Fourier transform on the laser spectrum phase to be measured2And a pulse width;
(iii) and (i) repeating the steps (i) and (ii), so that the calibrated laser spectrum to be detected and the spectrum phase are obtained, and further the calibrated laser spectrum, the laser pulse shape and the pulse width are obtained.
The invention relates to a real-time measuring device of femtosecond laser pulse, which mainly comprises:
(a) a beam splitting system: the laser to be measured is divided into three beams, two beams are used for generating self-diffraction light, and the other weaker beam is used as the light to be measured.
(b) Self-diffraction process system: two laser beams are focused on a third-order nonlinear medium to generate a system of self-diffraction light.
(c) A bundle combining system: the generated self-diffraction light and the laser to be measured are collimated and superposed in space.
(d) The spectral measurement system comprises: high precision spectrometers are used to measure laser spectra and interference spectra.
The structure of the real-time measuring device of the femtosecond laser pulse is as follows: the laser beam detection device comprises a first 45-degree total reflection mirror, a first beam splitting sheet and a third 45-degree total reflection mirror in sequence along the direction of a laser beam to be detected, wherein a fourth 45-degree total reflection mirror is arranged in the direction of reflected light of the third 45-degree total reflection mirror, a first concave reflection mirror is arranged in the direction of the fourth 45-degree total reflection mirror, a third-order nonlinear dielectric material and a light barrier are arranged in sequence in the direction of reflected light of the first concave reflection mirror, the third-order nonlinear dielectric material is arranged on the focus of the first concave reflection mirror, a second concave reflection mirror is arranged in the direction of a first-order self-diffraction beam of the third-order nonlinear dielectric material, a third beam splitting sheet is arranged in the direction of the second concave reflection mirror, and the first-order self-diffraction beam is called as a reference beam;
the laser beam to be measured is divided into a transmission beam and a reflection beam by a first beam splitter, a second beam splitter and a first beam delayer are arranged in sequence in the direction of the reflection beam, a fifth 45-degree total reflection mirror is arranged in the direction of the output beam of the first beam delayer, and the reflection light of the fifth 45-degree total reflection mirror enters the first concave reflecting mirror;
and a second light beam delayer is arranged in the direction of a reflected light beam of the second beam splitting sheet, a second 45-degree total reflection mirror is arranged in the direction of an output light beam of the second light beam delayer, a reflected light beam passing through the second 45-degree total reflection mirror is still called as a light beam to be detected, the light beam to be detected is reflected by a sixth 45-degree total reflection mirror after penetrating through the third beam splitting sheet, and a pinhole diaphragm and a high-precision spectrometer are arranged in the direction of a reflected light of the sixth 45-degree total reflection mirror. And the high-precision spectrometer is used for measuring the laser spectrum and the interference spectrum.
The invention has the following remarkable characteristics:
(a) according to the invention, the obtained interference laser spectrum is calculated by program software written by a computer by utilizing linear calculation formulas (3), (4) and (5) of an SRSI method (see document 5), and the information of the laser spectrum, the spectrum phase, the pulse shape and the pulse time domain phase of the laser pulse to be measured can be obtained only by carrying out about three times of iterative calculation. Compared with the prior art, the invention obviously improves the spectrum and the pulse width adaptive range of femtosecond laser pulse measurement, improves the calculation speed and can monitor the laser pulse shape in real time.
(b) The invention adopts the self-reference spectrum method to measure the laser pulse, has high response speed and can be used for single-shot measurement and real-time monitoring of the pulse width of the femtosecond laser pulse.
(c) The invention adopts the first-order self-diffraction light generated by the self-diffraction effect as the reference light, thereby being capable of operating in the 200-3000nm broadband range and the 10-300fs pulse width range.
(d) The invention can also be used for the SD-FROG method to measure the laser pulse, thereby expanding the application range.
Drawings
FIG. 1 is a flow chart of a method for calculating femtosecond laser pulse shape, laser spectrum and spectral phase.
FIG. 2 is a block diagram of a system structure of a real-time measuring device for self-reference spectral interference femtosecond laser pulses according to the invention.
FIG. 3 is a schematic diagram of an optical path configuration of an example of the apparatus for real-time measurement of self-referenced spectral interference femtosecond laser pulses according to the present invention.
FIG. 4 is a graph showing the experimental results of the device of the present invention measuring the laser pulse with the center wavelength of 800nm and 55 fs.
FIG. 5 is a graph showing the experimental results of measuring a laser pulse with a central wavelength of 8fs at 400nm by using the example apparatus.
Detailed Description
The present invention will be further described with reference to the following examples and drawings, but the scope of the present invention should not be limited thereto.
Referring to fig. 1, the real-time measurement method of self-reference spectral interference femtosecond laser pulse of the present invention includes the following steps:
1) and generating a first-order self-diffraction laser beam by utilizing a third-order nonlinear optical effect:
the laser beam 1 to be measured is divided into three beams of laser by the beam splitting system, wherein one beam of laser energy is 5% of the total pulse energy for measurement and is called as a beam to be measured, the other two beams of laser energy are equal and are focused on the transparent nonlinear medium 12 at a certain non-collinear included angle of 2 degrees to generate a first-order self-diffraction beam which is called as a reference beam, and the light intensity I of the reference beamsd1(ωsd1) Comprises the following steps:
wherein: omegasd1、ω1And ω-1Angular frequencies, Δ k, of the reference beam and of the two incident lasers, respectivelyz(ωsd1，ω1，ω-1) For the amount of phase mismatch in the self-diffraction process, L is the thickness of the nonlinear medium 12,
2) adjusting the light beam to be measured and the reference light beam to be collinear and enter the high-precision spectrometer (19) in a spatial coincidence mode to obtain a spectral interference fringe D (omega, tau) with the maximum modulation depth:
D(ω，τ)＝|Eref(ω)+E(ω)eiωτ|2
＝|Eref(ω)|2+|E(ω)|2+f(ω)eiωτ+f*(ω)e-iωτ
wherein: omega is the laser angular frequency, S0(ω)＝|Eref(ω)|2+|E(ω)|2Is the sum of the reference beam signal spectrum and the measured beam signal spectrum;
is an interference term of the reference beam signal spectrum and the measured beam signal spectrum;
3) and carrying out data processing on the spectral interference fringe D (omega, tau) according to the following steps to obtain a laser spectrum and a spectral phase:
carrying out Fourier transformation on the spectral interference fringes D (omega, tau) to time domain signals, and respectively extracting S from the time domain signals0(τ) and f (τ);
② for the S0(tau), f (tau) are respectively inverse Fourier transformed to respectively obtain frequency domains S0(ω) and frequency domain f (ω);
utilizing the S0(ω), f (ω) and the spectral amplitude | E of the reference light is obtained by the following formularef(ω)|：
And laser spectrum amplitude | E (omega)' to be measured
Preliminary spectrum | E (ω) & gtnon of laser to be measured2And spectrum | E of reference lightref(ω)|2；
Fourthly, performing dephasing operation on the f (omega) to calculate argf (omega), and calculating the initial spectrum phase of the laser to be measured by using the following formula:
wherein,and
the initial reference light spectrum phase is set to be 0, and C is a spectrum phase constant introduced by a dispersive optical element such as a beam splitting sheet (16);
performing Fourier transform on the preliminarily obtained laser spectrum and spectrum phase to obtain pulse shape | E (t) of laser pulse to be measured2And pulse width, E (t) being the value of the fourier transform of E (ω);
sixthly, because the phase of the reference light is not necessarily absolutely equal to 0, further iterative computation is needed according to the following iterative method, and the laser spectrum and the spectrum phase output are optimized, wherein the iterative method comprises the following steps:
(i) using the pulse shape signal | E (t) obtained in step 5 to count charge2Obtaining the time-domain electric field shape of the reference light E (t) | E (t) & lty & gt) as Y ray according to the time-domain expression of the formula (1)2Performing inverse Fourier transform on the electric field to obtain a reference light spectrum and a reference light spectrum phase
(ii) The spectral phase of the reference light is adjusted
Substituting into a formula:
(iii) and repeating the iteration steps i and ii for about three times to obtain the calibrated laser spectrum to be measured and the calibrated spectrum phase, and further obtain the calibrated laser pulse shape and pulse width.
Referring to fig. 2 and 3, fig. 2 is a block diagram of a system structure of a real-time measuring device for self-referencing spectral interference femtosecond laser pulses according to the present invention. FIG. 3 is a schematic diagram of an optical path configuration of an example of the apparatus for real-time measurement of self-referenced spectral interference femtosecond laser pulses according to the present invention. As can be seen from the figure, the real-time measuring device for the self-reference spectrum interference femtosecond laser pulse comprises the following components: the laser beam detection device comprises a first 45-degree total reflection mirror 2, a first beam splitting sheet 3 and a third 45-degree total reflection mirror 8 which are sequentially arranged along the direction of a laser beam 1 to be detected, wherein a fourth 45-degree total reflection mirror 9 is arranged in the direction of reflected light of the third 45-degree total reflection mirror 8, a first concave reflection mirror 11 is arranged in the direction of the fourth 45-degree total reflection mirror 9, a third-order nonlinear dielectric material 12 and a light barrier 14 are sequentially arranged in the direction of reflected light of the first concave reflection mirror 11, the third-order nonlinear dielectric material 12 is arranged at the focus of the first concave reflection mirror 11, a second concave reflection mirror 15 is arranged in the direction of a first-order self-diffraction beam 13 of the third-order nonlinear dielectric material 12, a third beam splitting sheet 16 is arranged in the direction of the second concave reflection mirror 15, and the first-order self-diffraction beam 13 is called as a reference beam;
the laser beam 1 to be measured is divided into a transmission beam and a reflection beam by a first beam splitter 3, a second beam splitter 4 and a first beam delayer 5 are arranged in the direction of the reflection beam in sequence, a fifth 45-degree total reflection mirror 10 is arranged in the direction of the output beam of the first beam delayer 5, and the reflection light of the fifth 45-degree total reflection mirror 10 enters a first concave reflecting mirror 11;
the second beam delayer 6 is arranged in the direction of the reflected beam of the second beam splitting plate 4, the second 45-degree total reflection mirror 7 is arranged in the direction of the output beam of the second beam delayer 6, the reflected beam passing through the second 45-degree total reflection mirror 7 is still called as a beam to be measured, the beam to be measured is reflected by the sixth 45-degree total reflection mirror 17 after passing through the third beam splitting plate 16, and the aperture diaphragm (18) and the high-precision spectrometer (19) are arranged in the direction of the reflected light of the sixth 45-degree total reflection mirror (17).
The nonlinear medium 12 is an optical medium of any transparent material.
In this example, pulse width measurements were made on commercial femtosecond laser pulses using an apparatus as shown in fig. 3. In a specific implementation light path, the repetition frequency of the laser beam 1 to be measured is 1kHz, and the central wavelength is 800 nm. After passing through the first and second beam splitters 3 and 4 having a thickness of 1mm, the two split lasers are focused on the fused silica glass 12 having a thickness of 0.1mm by the first silvered concave mirror 11 having an R of-700 mm. One of the first-order self-diffracted lights 13 is collimated by the second silvered concave mirror with the thickness of-1000 mm, and then enters the third beam splitting sheet 16 with the thickness of 1mm to be spatially superposed with the other laser beam to be measured. The two beams of coincident laser enter a high-precision spectrometer 19 through a total reflection mirror 17 and a small aperture diaphragm 18 to measure laser spectrums and interference laser spectrums. Fig. 4(a) shows an interference spectrum (thin solid line) measured when the time delay between the laser to be measured and the reference laser is 1.25ps, a thick solid line in the middle shows a laser spectrum of first-order self-diffracted light (reference light), and a lower dotted line shows a spectrum of the laser to be measured. By using the measured interference spectrum data and the calculation method and flow shown in fig. 1 in the invention content, the laser spectrum and the spectrum phase of the pulse to be measured can be calculated. The point and the hollow circle line in fig. 4(b) are the laser spectrum to be measured by the spectrometer, the thick solid line is the laser spectrum obtained by the SD-SRSI method, and the short line is the laser spectrum phase obtained by the SD-SRSI method.
A laser pulse having a central wavelength of 400nm to 7.5fs compressed by this method is applied to a hollow fiber using an experimental apparatus similar to that shown in fig. 3 [ see document 7: the pulse width measurement experiments were performed J.Liu, K.Okamura, Y.Kida, T.Teramoto, andT.Kobayashi, "Clean sub-8-fs pulses at 400nm generated by a hold fiber compressor for an ultra violet ultra fast pump-probe spot," Opt.express 18(20), 20645-20650(2010) ]. Fig. 5(a) shows (thin solid line) an interference spectrum measured when the time delay between the laser light to be measured and the reference laser light is 400fs, the middle thick solid line shows a spectrum of first-order self-diffracted light (reference light), and the lower dotted line shows a spectrum of the laser light to be measured. All spectra were measured using a USB4000 spectrometer from oceanooptics; by using the measured interference spectrum data and the calculation method and flow shown in fig. 1 in the invention content, the laser spectrum and the spectrum phase of the pulse to be measured can be calculated. FIG. 5(b) shows the laser spectrum obtained by the SD-SRSI method in the thick solid line, the laser spectrum obtained by the SD-SRSI method in the short line, and the pulse width of 7.5fs obtained by Fourier transform.
Claims (3)
1. A real-time measurement method of self-reference spectrum interference femtosecond laser pulse is characterized by comprising the following steps:
1) and generating a first-order self-diffraction laser beam by utilizing a third-order nonlinear optical effect:
the laser beam (1) to be measured is divided into three beams of laser by a beam splitting system, wherein one beam of laser energy is 5% of the total pulse energy for measurement and is called as a beam to be measured, and the other two beams of laser energy are equal and are focused on a transparent nonlinear medium (12) at a certain non-collinear included angle of 2 degrees to generate a first-order self-diffraction beam which is called as a reference beamLight beam, light intensity I of the reference beamsd1(ωsd1) Comprises the following steps:
wherein: omegasd1、ω1And ω-1Angular frequencies, Δ k, of the reference beam and of the two incident lasers, respectivelyz(ωsd1，ω1，ω-1) L is the thickness of the nonlinear medium (12) for the amount of phase mismatch in the self-diffraction process,
2) adjusting the light beam to be measured and the reference light beam to be collinear and enter the high-precision spectrometer (19) in a spatial coincidence mode to obtain a spectral interference fringe D (omega, tau) with the maximum modulation depth:
D(ω，τ)＝|Eref(ω)+E(ω)eiωτ|2
＝|Eref(ω)|2+|E(ω)|2+f(ω)eiωτ+f*(ω)e-iωτ
wherein: omega is the laser angular frequency, S0(ω)＝|Eref(ω)|2+|E(ω)|2Is the sum of the reference beam signal spectrum and the measured beam signal spectrum;
is an interference term of the reference beam signal spectrum and the measured beam signal spectrum;
3) and carrying out data processing on the spectral interference fringe D (omega, tau) according to the following steps to obtain a laser spectrum and a spectral phase:
carrying out Fourier transformation on the spectral interference fringes D (omega, tau) to time domain signals, and respectively extracting S from the time domain signals0(τ) and f (τ);
② for the S0(tau), f (tau) are respectively inverse Fourier transformed to respectively obtain frequency domains S0(ω) and frequency domain f (ω);
utilizing the S0(ω), f (ω), the following formula was used to solveSpectral amplitude of reference light | Eref(ω)|：
And laser spectrum amplitude | E (omega)' to be measured
Preliminary spectrum | E (ω) & gtnon of laser to be measured2And spectrum | E of reference lightref(ω)|2；
Fourthly, performing dephasing operation on the f (omega) to calculate argf (omega), and calculating the initial spectrum phase of the laser to be measured by using the following formula:
wherein,
and
the initial reference light spectrum phase is set to be 0, and C is a spectrum phase constant introduced by a dispersive optical element such as a beam splitting sheet (16);
fifthly, theFourier transform is carried out on the preliminarily obtained laser spectrum and the spectrum phase to obtain the pulse shape | E (t) of the laser pulse to be measured2And pulse width, E (t) being the value of the fourier transform of E (ω);
sixthly, because the phase of the reference light is not necessarily absolutely equal to 0, further iterative computation is needed according to the following iterative method, and the laser spectrum and the spectrum phase output are optimized, wherein the iterative method comprises the following steps:
(i) using the pulse shape signal | E (t) obtained in step 5 to count charge2Obtaining the time-domain electric field shape of the reference light E (t) | E (t) & lty & gt) as Y ray according to the time-domain expression of the formula (1)2Performing inverse Fourier transform on the electric field to obtain a reference light spectrum and a reference light spectrum phase
(ii) The spectral phase of the reference light is adjusted
Substituting into a formula:
(iii) and repeating the iteration steps i and ii for about three times to obtain the calibrated laser spectrum to be measured and the calibrated spectrum phase, and further obtain the calibrated laser pulse shape and pulse width.
2. A real-time measuring apparatus of femtosecond laser pulses for implementing the real-time measuring method of self-reference spectral interference femtosecond laser pulses according to claim 1, characterized by comprising: a first 45-degree total reflection mirror (2), a first beam splitting sheet (3) and a third 45-degree total reflection mirror (8) are arranged along the direction of a laser beam (1) to be measured in sequence, a fourth 45-degree total reflection mirror (9) is arranged in the reflected light direction of the third 45-degree total reflection mirror (8), a first concave reflector (11) is arranged in the direction of the fourth 45-degree total reflector (9), a third-order nonlinear dielectric material (12) and a light barrier (14) are sequentially arranged in the direction of the reflected light of the first concave reflector (11), the third-order nonlinear dielectric material (12) is arranged on the focus of the first concave reflecting mirror (11), a second concave reflector (15) is arranged in the direction of the first-order self-diffraction light beam (13) of the third-order nonlinear dielectric material (12), a third beam splitter (16) is arranged in the direction of the second concave reflector (15), and the first-order self-diffraction light beam (13) is called a reference light beam;
the laser beam (1) to be measured is divided into a transmission beam and a reflection beam by a first beam splitter (3), a second beam splitter (4) and a first beam delayer (5) are arranged in sequence in the direction of the reflection beam, a fifth 45-degree total reflection mirror (10) is arranged in the direction of the output beam of the first beam delayer (5), and the reflection light of the fifth 45-degree total reflection mirror (10) enters a first concave reflecting mirror (11);
a second light beam delayer (6) is arranged in the direction of a reflected light beam of the second beam splitting sheet (4), a second 45-degree total reflection mirror (7) is arranged in the direction of an output light beam of the second light beam delayer (6), the reflected light beam passing through the second 45-degree total reflection mirror (7) is still called as a light beam to be measured, the light beam to be measured is reflected by a sixth 45-degree total reflection mirror (17) after penetrating through the third beam splitting sheet (16), and a pinhole diaphragm (18) and a high-precision spectrometer (19) are arranged in the direction of the reflected light of the sixth 45-degree total reflection mirror (17).
3. A real-time measuring device of femtosecond laser pulses according to claim 2, wherein the nonlinear medium (12) is an optical medium of any transparent material.
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