CN100432643C - Femtosecond laser camera - Google Patents

Abstract

The invention relates to a device for measuring various characteristics of a femtosecond pulse laser. The technical problem to be solved is that the device should be able to accurately measure the width (amplitude) of the femtosecond laser pulse, quickly measure the initial phase and its distribution, and intuitively detect the chirp state, carrier center frequency and spectral bandwidth of the femtosecond laser pulse, and be able to A waveform image of a femtosecond laser pulse is plotted. The technical solution is: femtosecond laser camera, along the laser pulse direction of the laser input port of the camera, a mode converter, a reflection mirror on one side and a spectral phase modulator are arranged in sequence, and a focusing lens is set, which is reflected by the mode converter The direction of the laser pulse is provided with a Michelson interferometer with a time delay shutter, and the focusing lens, frequency doubling crystal, spatial filter aperture, gain selection switch, spectrometer and video camera are arranged in sequence, and reflected by the gain selection switch The direction of the laser pulse is set by the interferometric autocorrelation receiver SHG, which is also equipped with a computer.

Description

femtosecond laser camera

technical field

The invention relates to a device for measuring ultrashort pulse laser, in particular to a device for femtosecond pulse laser capable of accurately measuring pulse width, quickly measuring phase, chirp and other characteristics.

Background technique

At present, ultrashort pulse lasers generally refer to lasers whose pulse width is less than picosecond (ps, 10 ^-12 seconds) or less. Since the advent of lasers in the 1960s, the initial ultrashort pulse laser was a few nanoseconds (ns, 10 ^-9 seconds) width of light generated by a free-oscillating laser with a Q-switched switch. In the 1990s, due to the invention of the Kerr Lens mode-locking and soliton mode-locking technology, and solid-state self-mode-locked lasers using new broadband laser media (such as titanium-doped sapphire Ti:Al ₂ O ₃ ), have been able to directly generate several femtoseconds (fs, 10 ^-15 seconds) width of light. Femtosecond pulsed lasers have been widely used in the research of various ultrafast phenomena and physical behaviors under strong fields due to their extremely high time-resolution characteristics and extremely high peak energy. Such as the relaxation process of molecules, the metabolism of biological cells, the kinetics of photochemical reactions, and laser nuclear fusion. However, the current ultrashort pulse laser measurement technology is relatively lagging behind. So far, due to the limitation of the time limit resolution of electronic detection technology (generally about 1ps), there is no instrument that can directly measure femtosecond pulsed laser. In order to adapt to the measurement of narrower and narrower ultrashort pulse lasers, people can only still use coherent detection technology. The basic idea of this technique is to use the measurement results satisfying the conditions of optical coherence (or correlation) to indirectly calculate the required information to be measured through mathematical methods. With the rapid development of ultrashort pulse laser technology, accurate measurement of femtosecond pulses has become a very urgent need. Especially when the pulse width is very narrow (less than 10fs), it is not only necessary to measure the pulse width accurately, but also to measure information such as pulse phase and chirp characteristics at the same time. However, the traditional ultra-short pulse laser measurement devices (such as two-photon fluorescence method, streak camera, etc.) that can only measure the pulse width are no longer applicable, and only the coherent method has been used until now. On the other hand, in the research field of femtosecond lasers, in order to obtain the narrowest pulse, it is necessary to compensate the chirp as much as possible; at the same time, in order to quickly and directionally compensate the chirp, compress the pulse and improve the pulse shape, it is also necessary to be able to Measuring device for fast measurement of pulse phase and chirp characteristics. Therefore, the development and development of femtosecond pulse laser measurement technology and the complete understanding of femtosecond pulse width, phase and chirp information are very important in the field of ultrafast technology research. After long-term exploration, many coherent detection methods have been proposed successively. Among them, the two most internationally recognized standard femtosecond pulsed laser measurement methods are: frequency-resolved optical switching method (FROG) and spectral phase coherent direct electric field reconstruction method (SPIDER).

Frequency-resolved optical switching (FROG) is a coherent detection method proposed in the 1990s that can indirectly obtain information such as the pulse width and phase to be measured. Its basic principle is: divide the measured light into two beams, one beam is used as the probe light, and the other beam is introduced into the time delay τ as the switch light, and then the two beams are converged into a nonlinear medium to interact with each other. Second-order or third-order nonlinear effect, receiving the sum-frequency light of the interaction between the probe light and the switch light as the signal light, and then passing through the spectrometer and using the CCD camera to measure the image of its light intensity distribution (in fact, it is a time- and frequency-related image) Relevant two-dimensional function), the measured pulse width and phase information can be obtained by using a computer and using iterative operations. In the FROG method, there are many different design schemes according to different optical switch functions. Among them, based on the traditional second-harmonic-frequency-resolved optical switch method (SHG-FROG), it is easy to adjust and has high sensitivity, so it is widely used. Specifically as shown in Figure 9: the measured femtosecond pulsed laser beam _G1 is divided into two beams by the beam splitter _B1 : one beam passes through the Michelson interferometer (including right-angle mirrors _M1 , _M2 , half mirror B ₁ , the fixed right-angle mirror M ₁ in the mirror M ₃ ), as the probe light; the other beam passes through the movable right-angle mirror M ₂ in the Michelson interferometer, as the switch light (to introduce a time delay τ) ; and make the two beams of light incident parallel to the lens L1 and focus into the nonlinear medium S ₁ (BBO or KDP) to generate interaction, after the spectrum is expanded by the spectrometer T1, it is received by the CCD, so as to obtain the light intensity information after the interaction, Also known as the "trace spectrum" of SHG-FROG. It is the distribution of the frequency spectrum of a series of different delay times and intensity autocorrelation signals along the delay time axis, expressed mathematically as:

${\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; FROG</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&omega;</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; \&tau;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>{<span\; class="notranslate">\; |</span>\underset{<span\; class="notranslate">\; -</span><span\; class="notranslate">\; \&infin;</span>}{\overset{<span\; class="notranslate">\; \&infin;</span>}{<span\; class="notranslate">\; \&Integral;</span>}}{\mathrm{<span\; class="notranslate">\; E.</span>}}_{\mathrm{<span\; class="notranslate">\; sig</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; \&tau;</span>}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; exp</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; j\&omega;t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; |</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; =</span>{<span\; class="notranslate">\; |</span>{\mathrm{<span\; class="notranslate">\; E.</span>}}_{\mathrm{<span\; class="notranslate">\; sig</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&omega;</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; \&tau;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; |</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}$

in: ${\mathrm{E.}}_{\mathrm{sig}}(t,\mathrm{\&tau;})=\widetilde{\mathrm{E.}}\left(t\right)\widetilde{\mathrm{E.}}(t-\mathrm{\&tau;})$ is the envelope expression of signal light.

$\mathrm{E.}\left(t\right)=\widetilde{\mathrm{E.}}\left(t\right)\exp [-j\left({\mathrm{\&omega;}}_{c}t\right)-\stackrel{\&Right\; Arrow;}{K}r]$ is the electric field expression of the femtosecond pulsed laser to be measured.

为波矢，

为待测飞秒脉冲的时域复包络。ω _c is the center carrier frequency,

for wave vector,

is the time-domain complex envelope of the femtosecond pulse to be measured.

In fact, I _FROG (ω,τ) is the square of the magnitude of the Fourier transform of E _sig (t,τ). Therefore, the SHG-FROG trace (pictured image) data is passed through the computer, and an iterative algorithm (normalized Gerchberg-Saxton algorithm) is used to obtain a unique solution, that is, the pulse of the femtosecond laser pulse to be measured can be uniquely determined. width and phase information. Its advantage is that the result is accurate, but its disadvantage is that it is slow and not suitable for real-time detection.

The spectral phase coherent direct electric field reconstruction method (SPIDER) or the self-reference spectral phase coherent direct electric field reconstruction method is a method proposed in the 1990s based on the spectral coherent method that can quickly calculate the phase of the pulse to be measured. The Frenchman C.Froehly first proposed the concept of spectral coherence in the 1970s. Its basic principle is to pass two beams of light satisfying the coherent condition through a nonlinear medium, and use a spectrometer (which is used to convert time-domain optical signals into frequency domain optical signal, that is, Fourier transform) to record interference fringes (the frequency interval of fringes is inversely proportional to the delay time τ of two beams of coherent light, which is 2π/τ), and if the interference fringes are inversely transformed by Fourier, the two beams The spectral phase difference of light. If the spectral phase of one beam of reference light is known, the spectral phase of the other beam can be obtained. However, in fact, the spectral phase of the reference light is also unknown, so the self-reference spectral coherence method is proposed, that is, the light pulse to be measured is copied into a pair of mirror image pulses with a relative delay time τ through the Michelson interferometer, but due to The spectral phase difference between them is zero, and the spectral phase of the pulse cannot be obtained yet. Until the 1990s, a self-referencing spectral coherence method (ie, SPIDER method) using "spectral clipping" solved the above problems. The core of the SPIDER method is to use pulse stretching technology to try to add a known small frequency shift Ω to the reference light, and make the frequency shift not affect the phase of the measured light pulse, and then use the above spectral coherence method to calculate The phase of the light pulse being measured. Specifically as shown in Figure 10: the measured laser pulse _G2 is divided into two paths by the beam splitter _B4 , one of which is reflected to the Michelson interferometer (including the fixed right-angle mirror _M4 and the movable right-angle mirror M ₅ , the semi-reflective half-lens B ₂ , B ₃ ) is divided into two beams of exactly the same pulsed light with a relative time delay τ, and the two beams of pulsed light are combined and sent to the focusing mirror L2; the other pulsed light is sent by the beam splitter B ₄ is transmitted into the nonlinear medium as the spectral phase modulator J and is stretched. Since the parameters such as the dispersion coefficient of the medium material of the spectral phase modulator (also known as the stretcher) are known, it can be realized that the reference light can be given as described above. Add a known small frequency shift; the linear positive chirped pulse light that is broadened and endowed with a frequency shift, after being reflected by the total reflection mirrors M ₆ and M ₇ , enters the focusing mirror L ₂ in parallel with the previous light pulse pair , and enter the nonlinear crystal _S2 for frequency conversion, and then received by the spectrometer T2; due to the time delay τ and the existence of chirp, the two beams of pulsed light sum at different frequencies, and produce a frequency difference in the frequency domain, It is called the spectral shear amount (expressed in Ω); the spectrometer records the interference spectrum image (also known as the SPIDER trace) of the pulse pair with the spectral shear amount Ω and sends it to the computer for simple calculation to obtain the phase value. The mathematical expression of the SPIDER trace is:

$\mathrm{<span\; class="notranslate">\; D.</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&omega;</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; \&tau;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>{<span\; class="notranslate">\; |</span>\stackrel{<span\; class="notranslate">\; ~</span>}{\mathrm{<span\; class="notranslate">\; E.</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; C</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; \&Omega;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; |</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{<span\; class="notranslate">\; |</span>\stackrel{<span\; class="notranslate">\; ~</span>}{\mathrm{<span\; class="notranslate">\; E.</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; C</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; |</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; |</span>\stackrel{<span\; class="notranslate">\; ~</span>}{\mathrm{<span\; class="notranslate">\; E.</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; C</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; \&Omega;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; |</span><span\; class="notranslate">\; |</span>\stackrel{<span\; class="notranslate">\; ~</span>}{\mathrm{<span\; class="notranslate">\; E.</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; C</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; |</span><span\; class="notranslate">\; \&times;</span>\mathrm{<span\; class="notranslate">\; COS</span>}<span\; class="notranslate">\; [</span>\mathrm{<span\; class="notranslate">\; \&theta;</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; C</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; \&omega;\&tau;</span>}<span\; class="notranslate">\; ]</span>$

τ为相对时间延迟，

为展宽器的二阶色散，

与

为两束光脉冲的频域表达式。in

τ is the relative time delay,

is the second-order dispersion of the stretcher,

and

is the frequency domain expression of the two light pulses.

Since the SPIDER method does not have high requirements on the energy of the optical pulse, it can be detected directly from the output of the laser oscillation stage (no optical power amplification stage), and its processing operation for reconstructing the pulse phase does not require an iterative algorithm, is simple and fast, and can be used as a real-time A tool for detecting phase information of light pulses, but this method cannot directly give information such as pulse width.

Contents of the invention

The technical problem to be solved by the present invention is to overcome the deficiencies of the above-mentioned background technology, and provide a femtosecond laser pulse detection device, which should be able to accurately measure the width (amplitude) of the femtosecond laser pulse, quickly measure the initial phase and its distribution, and be intuitive Detect the chirp state, carrier center frequency and spectral bandwidth of the femtosecond laser pulse, and draw the waveform image of the femtosecond laser pulse.

The present invention adopts following technical scheme:

Femtosecond laser camera, the camera has a laser input port, along the direction of the laser pulse of the port, a mode converter, a reflection mirror on one side and a spectral phase modulator are arranged in sequence, and the reflection mirror on one side transmits the other side A mirror for transmitting laser pulses is set between the direction of the laser pulses and the focusing lens, so that the laser pulses first pass through the mode converter, one side is transmitted through the other side of the mirror, and then sent to the spectral phase modulator for frequency clipping, and then along the original The path returns to one side, transmits the other mirror and reflects to the focusing lens; a Michelson interferometer with a time-delay shutter is set in the direction of the laser pulse reflected by the mode converter, and the laser pulse output by the Michelson interferometer Set the focusing lens, frequency doubling crystal BBO, spatial filter aperture, gain selection switch and spectrometer in sequence, and set the interferometric autocorrelation receiver SHG in the direction of the laser pulse reflected by the gain selection switch. In addition, it is equipped with a computer and a camera Machine CCD, so that the laser pulse reflected by the other routing mode converter first passes through the Michelson interferometer, and is output from the Michelson interferometer, and then input in parallel with the aforementioned laser pulse reflected by one side and the other mirror After being combined with the focusing lens, it enters the frequency doubling crystal BBO for sum frequency; the laser pulse after the sum frequency is divided into two beams of laser pulses after passing through the spatial filter aperture and the gain selection switch, and one beam of laser pulses passes through the gain selection switch and enters the spectrometer. The camera CCD takes pictures and sends them to the computer to process the image data, and the other laser pulse is reflected by the gain selection switch to the interference autocorrelation receiver SHG, and then outputs the signal to the computer for processing.

The mode converter is provided with a circular optical mirror that can rotate around the center of the circle, wherein half of the circle is made into a total reflection mirror, and the other half of the circle is made into a half mirror, which is driven by a precision motor and controlled by a computer. Control so that the detection state of the femtosecond laser camera is in full information mode, instant phase mode, chirp state mode or comparison mode.

The described gain selection switch is provided with a circular optical mirror that can rotate around the center of the circle, wherein 1/3 of the circle is made into a total reflection mirror, 1/3 of the circle is made into a half mirror, and 1/3 of the circle is Full lens, the mirror is driven by a precision motor and controlled by a computer.

The time-delay shutter includes a precision screw screw, a stepper motor, a precision displacement sensor and related circuits arranged in an oil-filled inner cavity, and is connected to a computer for DN control, and also includes a movable right-angle reflector in the Michelson interferometer mirror.

The spatial filtering aperture is an aperture whose central hole area can be continuously adjusted, and is fixed by an optical multi-dimensional adjustment frame.

The spectral phase modulator includes a grating forming a certain angle with each other, a right-angle mirror and a movable mirror.

The detection device provided by the invention has multiple detection functions:

1. When the mode converter is in "full information mode", it has the detection function of SHG-FROG method and SHG method. At this time, the measured laser pulse is reflected by the total reflection mirror area of the optical mirror in the mode converter. The gain selection switch can choose two states: In the full transmission state, the pulse width (amplitude), phase, carrier center frequency and spectral bandwidth information of the measured femtosecond laser pulse can be measured by the SHG-FROG method; the gain selection switch selects half In the anti-semi-transmission state, the traditional SHG method can be used to measure the pulse width (amplitude), carrier center frequency and spectral bandwidth information of the measured femtosecond laser pulse.

2. When the mode converter is in "chirp state mode", it has a chirp state detection function. At this time, the measured laser pulse is reflected by the total reflection mirror area of the optical mirror in the mode converter. The gain selection switch can select the total reflection state, and at the same time, the interferometric autocorrelation receiver needs to be enabled, which can completely and accurately measure the size and sign of the chirp contained in the femtosecond laser pulse, and can achieve the effect of quantitative analysis.

3. When the mode converter is in "instant phase mode", it has the SPIDER method detection function. At this time, the measured laser pulse is incident on the semi-reflective half-lens area of the optical mirror in the mode converter, and the gain selection switch selects the full transmission state, which can easily and quickly obtain the phase information of the measured laser pulse, which can be used as a real-time measurement tool.

4. When the mode converter is in "comparison mode", it has the function of detecting and comparing the same measured laser pulse by SHG-FROG method and SPIDER method. At this time, the measured laser pulse is incident on the mode converter that rotates at a certain rate, so that the measured laser pulse is regularly reciprocated by the mode converter for total reflection and semi-transmission, and the gain selection switch is in a state of total transmission, and can be obtained Measurement results of two different methods (FROG method and SPIDER method). Therefore, the measurement results can be compared and their accuracy can be judged; the average value of the results obtained by the two measurement methods can also be used as the final measurement result, so that the measurement data is closer to the real situation. Therefore, the invention can be widely used in the field of ultrashort pulse laser technology and application, especially the development, production and debugging of ultrashort pulse laser.

Description of drawings

Fig. 1 is a schematic structural view of the present invention.

Fig. 2 is a front view structural diagram of the mode converter in the present invention.

Fig. 3 is a right view structural diagram of the mode converter.

Fig. 4 is a schematic structural diagram of a time-delay shutter in the present invention.

Fig. 5 is a front view structural diagram of the gain selection switch in the present invention.

Fig. 6 is a right view structural diagram of the gain selection switch.

Fig. 7 is a schematic diagram of the state of the present invention when it is in the "chirp state mode".

Fig. 8 is a schematic diagram of the state of the present invention when it is in "full information mode".

FIG. 9 is a schematic diagram of an existing second harmonic frequency-resolved optical switching method.

Fig. 10 is a schematic diagram of the existing spectral phase coherent direct electric field reconstruction method.

Detailed ways

As shown in Figure 1, the femtosecond laser camera has a laser pulse input port R (generally a hollow aluminum tube with a black-plated inner wall, which is used as the introduction path of the measured laser pulse, and is integrated with the mode converter MO), Along the input direction of the measured laser pulse at the input port, a mode converter MO is arranged sequentially, one side transmits the other mirror BF and the spectral phase modulator (stretcher) GT, and one side transmits the laser reflected by the other mirror BF Between the pulse direction and the focusing lens L3 is a mirror M ₁₂ that transmits the laser pulses to the focusing lens L3. Set Michelson interferometer in the direction of the laser pulse reflected by the mode converter [including right-angle mirror M ₈ , a movable right-angle mirror M ₉ arranged at right angles to the mirror (a part of the time-delay shutter YC) and a beam splitter mirror (also known as half mirror) B ₆ , B ₅ ], and in the direction of the laser pulse output by the Michelson interferometer, the focusing lens L3, the frequency doubling crystal BBO, the spatial filter aperture GQ, and the gain selection switch are sequentially set MG and spectrometer GPY, and an interference autocorrelation receiver SHG is set in the direction of the laser pulse reflected by the gain selection switch, and is equipped with a computer DN and a camera CCD. The arrow in the figure is the running direction of the measured laser pulse, and the position of the arrow is the running route of the measured laser pulse.

In the mode converter shown in Fig. 2 and Fig. 3, the circular optical mirror can rotate around the central axis, form a 45-degree angle with the centerline of the hollow tube at the input port and be fixed by a functional bracket. The half circle 2 of the circular optical mirror is made into a total reflection mirror surface, and the other half circle 1 is made into a semi-reflective half-lens surface, and the mirror can be driven by a stepping motor 3 to rotate and controlled by a computer. When it is necessary to measure the pulse width (amplitude), phase, carrier center frequency, and spectral bandwidth of the pulse, the detection status of the femtosecond laser camera should be in "full information mode", and the mode converter is a total reflection mirror at this time; when it is necessary to measure the pulse When the chirp feature is used, the detection state of the femtosecond laser camera should be in the "chirp state mode", and the mode converter is also a total reflection mirror at this time; when the pulse phase needs to be measured, the detection state of the femtosecond laser camera should be in the "instant phase Mode", at this time the mode converter is a semi-reflective and semi-transmissive mirror; when two methods (FROG method and SPIDER method) need to be used to measure the same pulse to be measured, the detection status of the femtosecond laser camera should be in "comparison mode", At this time, the mode converter is controlled by a computer to make it rotate at a certain rate, and alternately change to a total reflection mirror and a semi-reflective mirror. The selection of the above-mentioned modes is determined by the operator according to the needs.

Described spectral phase modulator GT (also known as stretcher) has multiple forms, such as nonlinear medium (i.e. dispersion medium), Fabry-Perot etalon, dielectric film mirror of special coating and the present invention adopts The grating + right-angle mirror are relatively mature technologies and can be directly adopted. The grating+right-angle mirror structure of the present invention includes grating G (1200 lines/mm) forming a certain angle with each other, right-angle mirror M ₁₀ , and a total reflection mirror M that can be moved to change the distance relative to the grating G ₁₁ , wherein the distance between the grating G and the rectangular mirror M ₁₀ is also adjustable. The light entering the phase modulator GT is the light to be measured transmitted by the mode converter MO as described above. The light is firstly reflected by the transmission surface of the other transmission mirror BF on one side, incident on the grating G in the spectral phase modulator GT and reflected to the right-angle mirror _M10 , reflected by _M10 to the grating G, and then by the grating G reflects to the total reflection mirror M ₁₁ , returns to the grating G by the original path from the total reflection mirror M ₁₁ , and _{is reflected by the grating G to the right-angle mirror M 10 by} the original path again, and then returns to the The grating G, and enters the optical path of the phase modulator GT according to the original, is reflected to the reflective surface of one reflective mirror BF and the other reflective mirror BF. Therefore, the measured laser pulse is reflected four times by the grating in the phase modulator GT, then the dispersion amount provided by the phase modulator GT, the spectral clipping amount Ω, and The appropriate range value of the relative time delay τ of the image light of the Michelson interferometer with a time delay shutter is stored in the computer for later use.

The Michelson interferometer with a time-delay shutter includes a right-angle mirror M ₈ , a movable right-angle mirror M ₉ arranged at right angles to the mirror, and beam splitters (also known as half mirrors) B ₆ , B ₅ , wherein the movable right-angle mirror _M9 is an integral part of the time-delay shutter YC, and the time-delay shutter (shown in Figure 4) includes a precision screw screw 15, a stepping motor 14, The precision displacement sensor 11 and related circuits are connected with the computer DN for control, so that the stepper motor rotates according to the measurement needs and drives the screw and the connected movable right-angle mirror M9 to move a corresponding distance, and the CCD shutter only shoots once, and repeats in turn This is done to obtain the desired image of each correlated detection signal with a time delay of τ.

The interferometric autocorrelation receiver SHG mainly includes a photodetector and an A/D converter. The interferometric autocorrelation receiver SHG, after adding the Michelson interferometer part and the computer in the present invention, has the same structure as the existing Chinese patent (ZL200420110194.2), which can completely and accurately measure the femtosecond The size and sign of the chirp contained in the laser pulse can achieve the effect of quantitative analysis, and can completely reproduce the complete waveform of the "frequency-resolved autocorrelation interference second harmonic envelope", and the test results expressed through the image, the image Intuitive and easy to understand.

The gain selection switch MG is mainly to expand the detection function of the femtosecond laser camera. In addition to the convenient and intuitive detection of femtosecond laser pulse chirp characteristics as mentioned above, the "total reflection mirror state" of the gain selection switch MG "With the described autocorrelation receiver SHG, plus the Michelson interferometer part and the computer in the present invention, a traditional ultrashort laser pulse detection device of the second harmonic autocorrelation (SHG) method is formed. On the other hand, it splits the measured signal light according to needs, or performs total reflection and total transmission to reduce the energy loss of the measured signal light. As a relatively simple structure, the gain selection switch MG can be a disk-shaped optical plane mirror that can rotate around the central axis, and the optical mirror surface is 45 degrees to the centerline direction of the introduction channel tube of the input port of the femtosecond laser camera. corner placement and the ability to fine-tune or lock in that direction. The circular optical plane mirror is divided into three equal parts on average, one third of which is a total reflection mirror 22, one third is a semi-transmissive mirror 21, and one third is empty (the total transmission surface 23). The gain selection switch is driven by a stepper motor 26 and controlled by a computer to meet the needs of the measurement.

When the femtosecond laser camera is in the "full information mode" state (that is, the state shown in Figure 8), the measured laser pulse passes through the mode converter MO, and is totally reflected by MO to the Michelson interference with time-delayed shutter. In the instrument, it is divided into two beams by the beam splitter B6 in the interferometer, one of which is reflected by the beam splitter B6 and enters the fixed right-angle mirror M8 of the Michelson interferometer. Beam mirror B5 and transmitted to the focusing lens L3; The component part of the time delay shutter YC mentioned above, and can move the corresponding distance along the precision oil-filled screw according to the measurement needs, thus forming a light with a relative time delay τ with the above-mentioned detection beam, which is used as the switch light (Obviously with the above-mentioned probe light just having relative time delay τ without any other image pulse of difference), reflected by M9 to the beam splitter B5 and then reflected (making it incident parallel to the probe light) to the focusing lens L3, after the focusing lens is incident into the frequency-doubling crystal BBO (or KDP) for coherent sum-frequency conversion, the unnecessary stray light is filtered out by the spatial filter aperture GQ, and the obtained sum-frequency signal light is incident to the gain selection switch MG [according to the measurement needs, the MG can select two states of total transmission and semi-reflection and semi-transmission (as described in Figure 8, the signal light semi-reflected by the MG is represented by a dashed arrow)]; when the MG is When fully transmitted, the signal beam directly enters the spectrometer GPY, and the two-dimensional spectrogram of intensity versus frequency and time is recorded by a CCD camera. After the data is processed by the computer for many iterations, the pulse width (amplitude), phase, spectral bandwidth and other information of the measured laser pulse can be obtained; when the gain selection switch MG is semi-reflective and semi-transmissive, the obtained sum frequency signal beam will be Divided into two routes: one route transmits the MG to the above-mentioned spectrometer GPY and performs the above-mentioned detection process, and the other routes the MG to reflect to the said total reflection mirror M ₁₃ and reflect to the said interference mirror through M ₁₃ After the autocorrelation receiver SHG, the signal is also output to the computer for processing, and the chirp information contained in the femtosecond laser pulse is given.

When the femtosecond laser camera is in the "instant phase mode", the light to be measured is in a semi-reflective and semi-transmissive state after being incident on the mode converter MO, and the time gain selection switch MG is in a fully transmissive state. At this time, the state of the femtosecond laser camera is basically the same as that in FIG. 1, except that there is no signal light between the time gain selection switch MG and the total reflection mirror _M13 .

When the femtosecond laser camera is in the "chirp state mode" (state shown in Figure 7), the light to be measured is in a total reflection state after it is incident on the mode converter MO, and the time gain selection switch is also in a total reflection state, and the interference is turned on Autocorrelation receiver SHG.

When the femtosecond laser camera is in the "comparison mode", the mode converter MO rotates at a certain speed, so that the measured light is alternately in the semi-transparent and total reflection state, and the time gain selection switch is in the total transmission state.

The various optical mirrors mentioned above should meet the spectral bandwidth requirements for measurement, and use quartz materials with small absorption losses. Among them, the nonlinear frequency doubling crystal (BBO, or KDP, etc.) has a thickness of about 100 μm, and one side transmits the other side of the mirror. In the wide enough wavelength range of the carrier center frequency band of the measured light, it is as consistent and sufficient as possible. High transmittance and reflectivity. It is worth pointing out that in order to make the measurement results of the instrument accurate, the precision of each component must have higher requirements, and the basic conditions of optical self-coherence detection should be met as much as possible. For example, the flatness of the reflective mirror should be better than 1/50 wavelength, and the delay system ensures stability without shaking. It adopts oil-filled precision screw, delay shutter and spatial filter aperture, and the corresponding optical mirror is installed in a lockable, coarse-tuning and fine-tuning, three-dimensional multi-dimensional On the precision adjustment frame, it is easy to install and adjust. The important optical path needs to be protected from dust and light, so as to improve the signal-to-noise ratio (S/N), detection accuracy and sensitivity of the measurement system.

It should be pointed out that in order to ensure accurate detection, the entire device should be installed on a shock-proof optical platform. During the measurement, it should be ensured that the whole machine is airtight and light-proof, and it is also very necessary to avoid the generation of vibration sources in the surrounding environment as much as possible.

What needs to be explained is: except for the gain selection switch MG, mode converter, and time-delay shutter, all other components and materials can be purchased; the required computer software can also be compiled by ordinary programmers.

Several detection processes of the present invention are:

1. In the "full information mode", the measured femtosecond laser pulse A enters from the input port R, and then passes through the mode converter MO (in a state of total reflection) and the Michelson interferometer with a time-delayed shutter. One beam of light from the right-angle mirror _M8 is used as the probe light E(t), and the other beam is introduced into a relative time delay τ by the movable right-angle mirror _M9 as the switching light signal E(t-τ), and the two beams of light are passed through The focusing lens L3 converges into the frequency doubling crystal (BBO) to generate the sum frequency light Esig(t, τ), which is the signal light required for detection. According to the principle of nonlinear optics, it should satisfy the relationship: Esig(t, τ )∝E(t)(t-τ). Since other lights in different directions are also generated in the nonlinear medium, the unnecessary stray light can be filtered out to the greatest extent by using the spatial filter aperture GQ, and the signal light Esig(t, τ) is directly incident on the spectrometer GPY (At this time, the gain selection switch should be in the full transmission state), and the measured image (SHG-FROG trace) is recorded by the camera CCD, and the image and related data are input to the computer for DN processing, and the iterative operation program is used , the amplitude (pulse width), phase, spectral bandwidth and other information of the measured femtosecond laser pulse can be obtained, and the waveform image can be drawn. Apparently, the detection process is the second harmonic frequency-resolved optical switching method (SHG-FROG method) described in FIG. 9 .

2. In the "chirp state mode" (at this time, the mode converter is a total reflection mirror, and the gain selection switch is in the total reflection state), the measured laser pulse is incident on the mode converter and is totally reflected to the shutter with time delay The beam splitter in the Michelson interferometer is divided into two beams, one of which is reflected by the fixed right-angle mirror of the Michelson interferometer with a time-delay shutter as "fixed light", and the other beam is attached The reflected light of the movable right-angle mirror of the Michelson interferometer with a time-delayed shutter (activate its precision slow-moving gear) is used as "movable light", and the minimum step size is selected under computer control to make the "movable light" Scan the above "fixed light" and make the stepper motor further collect one data per revolution. Therefore, the second harmonic autocorrelation plane curve graph with resolvable interference fringes can be obtained, and the relevant data can be processed by a computer to obtain the intuitive chirp size and positive and negative information contained in the femtosecond laser pulse, which can be achieved as in the Chinese patent (ZL200420110194. 2) The effect.

3. In the "instant phase mode", the measured femtosecond laser pulse A enters from the input port R, and passes through the mode converter MO in turn. At this time, the mode converter is in a semi-reflective and semi-transmissive state, so that the measured laser pulse is divided into transmitted light and Reflected light: The transmitted light passes through the transmission surface of the other mirror, enters the spectral phase modulator GT (broadening device) and is modulated by giving a linear positive chirp, and then returns along the original path to one side and transmits the other side reflective surface of the mirror, and reflect to the total reflection mirror M ₁₂ , and then reflect to the focusing lens L3. On the other hand, the measured laser pulse reflected by the mode converter passes through the Michelson interferometer with time delay shutter, focusing lens, frequency doubling crystal, spatial filter aperture, gain selection switch (the gain selection switch is all transmission state) and a spectrometer. The measured laser pulse passes through the Michelson interferometer with a time-delayed shutter to generate a mirrored pulse beam with a relative delay time τ: E(t) and E(t-τ), and transmits the mirror through the other mirror with the aforementioned The reflected beam E′(t) with a linear positive chirp is incident parallel to the focusing lens and enters the frequency doubling crystal BBO to generate a coherent sum frequency. Due to the time delay τ and the existence of the chirp, the image pulse beam is endowed with a linear The positively chirped light beam sums frequency at different frequencies, thus generating a frequency difference, which is called the spectral clipping amount, expressed in Ω. The beam after the sum frequency passes through the spatial filter aperture and the gain selection switch (at this time the gain selection switch is in a fully transmissive state), enters the spectrometer and is recorded by the CCD camera as a spectrally coherent interference fringe image, and its mathematical expression is:

$\mathrm{<span\; class="notranslate">\; D.</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&omega;</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; \&tau;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>{<span\; class="notranslate">\; |</span>\stackrel{<span\; class="notranslate">\; ~</span>}{\mathrm{<span\; class="notranslate">\; E.</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; C</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; \&Omega;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; |</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{<span\; class="notranslate">\; |</span>\stackrel{<span\; class="notranslate">\; ~</span>}{\mathrm{<span\; class="notranslate">\; E.</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; C</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; |</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; |</span>\stackrel{<span\; class="notranslate">\; ~</span>}{\mathrm{<span\; class="notranslate">\; E.</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; C</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; \&Omega;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; |</span><span\; class="notranslate">\; |</span>\stackrel{<span\; class="notranslate">\; ~</span>}{\mathrm{<span\; class="notranslate">\; E.</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; C</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; |</span><span\; class="notranslate">\; \&times;</span>\mathrm{<span\; class="notranslate">\; COS</span>}<span\; class="notranslate">\; [</span>\mathrm{<span\; class="notranslate">\; \&theta;</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; C</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; \&omega;\&tau;</span>}<span\; class="notranslate">\; ]</span>$

及

是输入脉冲对的频域表达式；Ω是光谱剪裁量，且

τ是延迟时间，是光谱相位调制器的二阶色散；ω_c和ω_c-Ω分别为经光谱剪裁后两脉冲的中心频率。Wherein, D (ω, τ) is the expression of the interference fringe image of taking;

and

is the frequency domain expression of the input pulse pair; Ω is the spectral clipping amount, and

τ is the delay time, is the second-order dispersion of the spectral phase modulator; ω _c and ω _c -Ω are the center frequencies of the two pulses after spectral clipping, respectively.

The phase information of the femtosecond laser pulse can be obtained by inputting the above-mentioned spectrally coherent interference fringe image (or SPIDER trace data) into the computer and processing it with the inversion algorithm of SPIDER. Due to the simple and fast operation, the real-time phase information of the measured laser pulse can be obtained.

Obviously, the detection process is the spectral phase coherent direct electric field reconstruction method (SPIDER method) described in FIG. 10 .

4. In the "comparison mode", the measured femtosecond laser pulse A enters from the input port R, at this time the mode converter operates at a certain rate (according to the SHG-FROG method corresponding to the "full information mode" and "instant phase mode" and the SPIDER method to measure the time required for each) rotation, so that the measured laser pulse is regularly reciprocated by the total reflection and semi-transmission of the optical mirror (at this time the gain selection switch is in a state of total transmission), then two different methods can be used ( FROG method and SPIDER method) measure the same measured femtosecond laser pulse, compare the measurement results, and give the average value, thereby further improving the measurement accuracy.

Initial state setting and adjustment:

(1) During the initial measurement, adjust the correct incident angle of the measured laser pulse into the input port R, so that the corresponding indicator light of the observation display window of the femtosecond laser camera is on. Make the mode converter in "full information mode" (total reflection state) through the computer, enable the semi-reflection and semi-transmission state with the gain selection switch, enable the interferometric autocorrelation receiver SHG, and turn on the Michelson interferometer with time-delay shutter The fast-moving block moves back and forth once in the whole process, and the measured laser pulse passes through the optical path of the "full information mode" as mentioned above, and the maximum value of the autocorrelation signal of the measured laser pulse can be observed on the observation display screen of the femtosecond laser camera, thereby The optimal delay time τ value required for SHG-FROG measurement can be determined and stored in the computer, and it will be executed by the time delay shutter when it is to be measured. At the same time, the center carrier frequency and spectral bandwidth of the measured laser pulse can be measured through the spectrometer and photodetector, and these parameters are stored in the computer for later use.

的变化情况，从而可以确定SPIDER法测量所需要的最佳延迟时刻τ及Ω的取值范围，存入计算机待测量时由时间延迟快门执行。(2) It is also necessary to put the mode converter in the "instant phase mode" (in a semi-reflective and semi-transmissive state) through the computer, enable the gain selection switch to enable the semi-reflective and semi-transmissive state, and enable the interferometric autocorrelation receiver SHG at the same time, and the measured laser pulse passes through The optical path of the "instant phase mode" as mentioned above, and the fast moving block of the Michelson interferometer with a time-delayed shutter is turned on to move back and forth in the whole process, and the movable total reflection mirror M in the spectral phase modulator (stretcher) is adjusted at the same time ₁₁ the distance from the grating (the value of its second-order dispersion can be changed), and the measured laser pulse coherent light intensity can be observed on the observation screen of the femtosecond laser camera for different delay times τ and

Therefore, the value range of the optimal delay time τ and Ω required for SPIDER method measurement can be determined, and stored in the computer to be measured by the time-delay shutter.

Claims (5)

1, femtosecond laser camera, it is characterized in that this camera has a laser input port R, along the laser pulse direction of this port is provided with mode converter MO, one side transmits the other side mirror BF and spectral phase modulator GT successively, and A mirror M ₁₂ for transmitting laser pulses is set between the laser pulse direction reflected by the mirror BF on one side and the focusing lens L3, so that the laser pulses of this path first pass through the mode converter MO, one side is transmitted and the other side is reflected The mirror BF is sent to the spectral phase modulator GT for frequency clipping, and then returns along the original path to one side, transmits the other mirror BF and reflects to the focusing lens L3; and sets a time delay in the direction of the laser pulse reflected by the mode converter MO The Michelson interferometer of the shutter, and the focusing lens L3, the frequency doubling crystal BBO, the spatial filter aperture GQ, the gain selection switch MG and the spectrometer GPY are set in sequence in the direction of the laser pulse output by the Michelson interferometer, and in the direction of the gain selection switch MG The direction of the reflected laser pulse is set to the interferometric autocorrelation receiver SHG, which is also equipped with a computer DN and a camera CCD, so that the laser pulse reflected by another routing mode converter MO first passes through the Michelson interferometer, and is transmitted from the Michelson After the output of the interferometer, together with the aforementioned laser pulse reflected by the mirror BF on the other side, it is input to the focusing lens L3 in parallel, and then enters the frequency doubling crystal BBO for sum frequency; the laser pulse passing through the sum frequency passes through the spatial filter aperture After GQ and gain selection switch MG, it is divided into two beams of laser pulses. One beam of laser pulses passes through the gain selection switch and enters the spectrometer GPY. It is captured by a CCD camera and sent to the computer DN to process image data. After the selector switch is reflected to the interference autocorrelation receiver SHG, the output signal is given to the computer DN for processing; a circular optical mirror that can rotate around the center of the circle is arranged in the described mode converter MO, wherein half of the circle is made into a total reflection mirror (2 ), the other half of the circle is made into a semi-reflective half mirror (1), which is driven by a precision motor (3) and controlled by a computer, so that the detection status of the femtosecond laser camera is in full information mode, instant phase mode, chirp status mode or compare mode. 2. The femtosecond laser camera according to claim 1, characterized in that said gain selection switch MG is provided with a circular optical mirror that can rotate around the center of the circle, wherein 1/3 of the circle is made into a total reflection mirror ( 22), 1/3 circle is made semi-reflective mirror (21), and 1/3 circle is full lens (23), and this mirror is driven by precision motor and is controlled by computer. 3. The femtosecond laser camera according to claim 1, characterized in that the time-delayed shutter comprises a precision screw (15), a stepping motor (14), The precision displacement sensor (11) and related circuits are connected to the computer for DN control, and the movable right-angle mirror _M9 in the Michelson interferometer is also included. 4. The femtosecond laser camera according to claim 1, 2 or 3, characterized in that the spatial filter aperture GQ is an aperture whose central aperture area can be continuously adjusted, and is fixed by an optical multi-dimensional adjustment frame. 5. The femtosecond laser camera according to claim 4, characterized in that the spectral phase modulator GT comprises a grating G forming a certain angle with each other, a right-angle mirror M ₁₀ and a movable mirror M ₁₁ .

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