CN112595425B - Ultrashort laser pulse measuring method and measuring system - Google Patents
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Abstract
The invention belongs to the field of ultrafast laser pulse measurement, and particularly discloses an ultrashort laser pulse measurement method and a measurement system, wherein the method comprises the following steps: measuring a second harmonic frequency spectrum generated by the ultrashort laser pulse to be measured through the nonlinear crystal by adopting a device for measuring the second harmonic process generated by the nonlinear crystal by utilizing the nonlinear crystal, wherein an included angle between an optical axis of the nonlinear crystal and a light propagation direction is changed by rotating the nonlinear crystal, the spectrum width of the second harmonic frequency spectrum generated by the nonlinear crystal under each included angle is monitored, and an included angle corresponding to the spectrum width for analyzing all information of the ultrashort laser pulse to be measured is determined; and acquiring an ultra-short laser pulse trace graph under the included angle and extracting pulse information of the ultra-short laser pulse to be detected from the trace graph. The invention monitors and acquires the trace graph which can be used for analyzing all information of the ultrashort laser pulse to be detected by rotating the phase matching angle of the nonlinear crystal, thereby avoiding the problem that the existing measuring method has strict requirements on the nonlinear crystal.
Description
Technical Field
The invention belongs to the technical field of ultrafast laser pulse measurement, and particularly relates to an ultrashort laser pulse measurement method and an ultrashort laser pulse measurement system.
Background
Nowadays, femtosecond ultrashort laser pulses have strong applicability in high-field physics, such as the exploration of the kinetic process of molecular atoms and the generation of attosecond pulses. Accurate measurement of ultrashort laser pulses is particularly important.
At present, there are many mature ultrashort pulse measurement methods, but these measurement methods have some disadvantages. At first, people propose an autocorrelation measurement method, which can only obtain the time domain intensity information of the pulse and cannot obtain the phase information of the time domain; then, improvement is carried out on the basis of autocorrelation measurement, and a spectrometer part, namely (SHG-FROG), is added behind autocorrelation, so that time domain phase information of pulses can be obtained through a reconstruction method, but the measurement method puts certain requirements on the thickness of a nonlinear crystal and needs to use a very thin crystal to meet the problem of phase matching bandwidth; later, the crystal angle rotation-integration (angle-dependent) method can meet the bandwidth requirement of thick nonlinear crystal measurement by rotating the nonlinear crystal angle to match different wavelengths within the exposure time of the spectrometer, but the device is complex to operate; then, the method goes to a Spectral Phase interference method (SPIDER) of Electric Field Direct Reconstruction, and the device is relatively complex and needs three beams of light; as far as the Dispersion-Scan device is concerned, this device and the SHG-FROG device are relatively simple, but require very thin nonlinear crystals to generate the second harmonic signal to meet the phase matching bandwidth. For thin nonlinear crystals, processing is very difficult.
Disclosure of Invention
The invention provides an ultrashort laser pulse measuring method and a measuring system, which are used for solving the technical problem that the existing ultrashort laser pulse measuring method has high requirements on nonlinear crystal processing.
The technical scheme for solving the technical problems is as follows: an ultrashort laser pulse measurement method, comprising:
measuring a second harmonic frequency spectrum generated by the ultrashort laser pulse to be measured through a nonlinear crystal by adopting a device for measuring the second harmonic process generated by the nonlinear crystal by utilizing the nonlinear crystal, wherein the included angle between the optical axis of the nonlinear crystal and the light propagation direction is changed by rotating the nonlinear crystal, the spectrum width of the second harmonic frequency spectrum generated by the nonlinear crystal under each included angle is monitored, and the included angle corresponding to the spectrum width for analyzing all information of the ultrashort laser pulse to be measured is determined;
and acquiring an ultra-short laser pulse trace graph under the included angle, and extracting pulse information of the ultra-short laser pulse to be measured from the trace graph to finish measurement of the ultra-short laser pulse to be measured.
The invention has the beneficial effects that: the invention provides a method for measuring ultrashort laser pulse by using nonlinear crystal with thickness of 10-300um based on a device for measuring ultrashort laser pulse by using the process of generating second harmonic by using nonlinear crystal. The method has the advantages that the trace graph which can be used for analyzing all information of the ultrashort laser pulse to be detected is monitored and collected by rotating the phase matching angle of the nonlinear crystal, the problem that the phase matching bandwidth of the relatively thick nonlinear crystal is narrow under the phase matching angle is solved, the method is flexible and simple, the cost is low, the material processing is relatively easy, and the problem that harsh requirements are required to be provided for the nonlinear crystal for measuring all information of the ultrashort laser pulse is solved ingeniously.
On the basis of the technical scheme, the invention can be further improved as follows.
Further, the extracting the pulse information of the ultrashort laser pulse to be detected from the trace diagram specifically includes:
and performing spectrum correction on the trace graph, and reconstructing the corrected trace graph by adopting a reconstruction algorithm to obtain frequency domain phase information of the ultrashort laser pulse to be measured so as to complete measurement of the ultrashort laser pulse to be measured.
Further, the nonlinear crystal has a thickness greater than 10 microns.
Further, the included angle corresponding to the spectrum width for analyzing all information of the ultrashort laser pulse to be detected is the monitored included angle corresponding to the widest frequency spectrum of the second harmonic generated by the nonlinear crystal, and the phase matching wavelength of the nonlinear crystal corresponding to the included angle is out of the frequency spectrum range of the ultrashort laser pulse to be detected.
The invention also provides an ultrashort laser pulse measuring system, comprising:
the device is used for measuring the ultrashort laser pulse by utilizing the process of generating the second harmonic wave by the nonlinear crystal and is used for measuring the second harmonic frequency spectrum generated by the ultrashort laser pulse to be measured through the nonlinear crystal;
the angle adjusting module is used for rotating the nonlinear crystal to change an included angle between an optical axis of the nonlinear crystal and a light propagation direction, monitoring the spectrum width of a second harmonic generated by the nonlinear crystal under each included angle, determining the included angle corresponding to the spectrum width for analyzing all information of the ultrashort laser pulse to be detected, and collecting an ultrashort laser pulse trace diagram under the included angle;
and the data processor is used for extracting the pulse information of the ultrashort laser pulse to be detected from the trace graph.
The invention has the beneficial effects that: the invention provides a system for measuring ultrashort laser pulses by using nonlinear crystals with the thickness of 10-300um based on a device for measuring ultrashort laser pulses by using a process of generating second harmonic by using the nonlinear crystals. The phase matching angle of the nonlinear crystal is rotated through the angle adjusting module, so that a trace graph which can be used for analyzing all information of the ultrashort laser pulse to be detected is monitored and collected, the problem that the phase matching bandwidth of the relatively thick nonlinear crystal is narrow under the phase matching angle is avoided, the cost is low, the material processing is relatively easy, and the problem that harsh requirements are required to be provided for the nonlinear crystal for measuring all information of the ultrashort laser pulse is ingeniously solved.
Further, the data processor is specifically configured to: and performing spectrum correction on the trace graph, and reconstructing the corrected trace graph by adopting a reconstruction algorithm to obtain frequency domain phase information of the ultrashort laser pulse to be measured so as to complete measurement of the ultrashort laser pulse to be measured.
Further, the nonlinear crystal has a thickness greater than 10 microns.
Further, the included angle corresponding to the spectrum width for analyzing all information of the ultrashort laser pulse to be detected is the monitored included angle corresponding to the widest frequency spectrum of the second harmonic generated by the nonlinear crystal, and the phase matching wavelength of the nonlinear crystal corresponding to the included angle is out of the frequency spectrum range of the ultrashort laser pulse to be detected.
Drawings
Fig. 1 is a flow chart of an ultra-short laser pulse measurement method according to an embodiment of the present invention;
FIG. 2 is a schematic diagram of an ultra-short laser pulse measurement system according to an embodiment of the present invention;
FIG. 3 shows the second harmonic spectrum simulation results of the 8fs 730nm Gaussian ultrashort laser pulse with different BBO thicknesses provided by the embodiment of the present invention;
FIG. 4 is a schematic diagram comparing a trace diagram and a reconstruction result diagram of an ultrashort laser pulse 8.3fs with a given pulse intensity and phase in a 40 ° 100um BBO according to an embodiment of the present invention;
fig. 5 is a schematic diagram comparing a trace graph and a reconstruction result graph of an ultrashort laser pulse to be measured by using a BBO of 150um at 37.6 ° according to an embodiment of the present invention;
FIG. 6 is a graph comparing the results of reconstructions with 37.6 deg. 150um BBO and 35 deg. 5um BBO, respectively, provided by an embodiment of the present invention.
The same reference numbers will be used throughout the drawings to refer to the same or like elements or structures, wherein:
the system comprises an ultrashort laser pulse to be detected 1, a beam splitter 2, planar reflectors 3, 5 and 6, fused quartz 4, a displacement table 7, a cylindrical mirror 8, a nonlinear crystal BBO9, a vertical slit 10, ultraviolet concave reflectors 11, 13 and 16, a horizontal slit 12, an isosceles prism 14, an ultraviolet planar reflector 15, an ultraviolet camera 17 and a data processing module 18.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. In addition, the technical features involved in the embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.
Example one
An ultrashort laser pulse measuring method, as shown in fig. 1, includes:
measuring a second harmonic frequency spectrum generated by the ultrashort laser pulse to be measured through the nonlinear crystal by adopting a device for measuring the second harmonic process generated by the nonlinear crystal by utilizing the nonlinear crystal, wherein the included angle between the optical axis of the nonlinear crystal and the light propagation direction is changed by rotating the nonlinear crystal, the spectrum width of the second harmonic frequency spectrum generated by the nonlinear crystal under each included angle is monitored, and the included angle corresponding to the spectrum width for analyzing all information of the ultrashort laser pulse to be measured is determined;
and acquiring an ultra-short laser pulse trace graph under the included angle, and extracting pulse information of the ultra-short laser pulse to be measured from the trace graph to finish measurement of the ultra-short laser pulse to be measured.
It should be noted that an included angle between the optical axis of the nonlinear crystal and the light propagation direction is a phase matching angle.
The embodiment is based on a device for measuring ultrashort laser pulses by utilizing a process of generating second harmonic waves by using a nonlinear crystal, and provides a method for measuring ultrashort laser pulses by using the nonlinear crystal with the thickness of 10-300 um. The method is flexible and simple, has low cost, and skillfully solves the problem that the prior method needs to provide harsh requirements for the nonlinear crystal for measuring all information of the ultrashort laser pulse.
When an ultrashort laser pulse to be detected passes through a thick nonlinear crystal with a phase matching wavelength near the center of the frequency spectrum of the pulse to be detected, the conversion efficiency of frequency components near the phase matching wavelength is far higher than that of other frequency components of the pulse to be detected, so that the frequency spectrum width observed on a detector is narrowed. However, when the phase matching wavelength of the nonlinear crystal is outside the spectrum range of the pulse to be measured or near the boundary of the spectrum range, the conversion efficiency of each frequency component of the pulse to be measured is comparable, so that the complete second harmonic signal spectrum of the pulse to be measured can be measured at the phase matching angle, and can be applied to the measurement of the short pulse.
Preferably, the extracting the pulse information of the ultrashort laser pulse to be detected from the trace graph specifically includes: and performing spectrum correction on the trace graph, and reconstructing the corrected trace graph by adopting a reconstruction algorithm to obtain frequency domain phase information of the ultrashort laser pulse to be measured so as to complete measurement of the ultrashort laser pulse to be measured.
Preferably, the thickness of the nonlinear crystal is greater than 10 μm.
The conversion efficiency of the nonlinear crystal is related to the thickness of the crystal and the phase matching angle, the thicker the crystal, the higher the conversion efficiency of the second harmonic, and the more the phase matching angle deviates from the frequency spectrum of the pulse to be measured, the lower the conversion efficiency of the crystal. Therefore, when the nonlinear crystal with the thickness of less than 10 microns is matched with the phase of the pulse to be measured, the intensity of the generated second harmonic is lower. If the angle is changed, the intensity of the corresponding second harmonic is lower.
Preferably, the included angle corresponding to the spectrum width for analyzing all information of the ultrashort laser pulse to be detected is the included angle corresponding to the widest monitored second harmonic spectrum generated by the nonlinear crystal, and the phase matching wavelength of the nonlinear crystal corresponding to the included angle is outside the spectrum range of the ultrashort laser pulse to be detected.
The degree that the phase matching wavelength of the nonlinear crystal deviates from the central wavelength of the ultrashort laser pulse to be detected needs to be large, the acquired trace pattern is complete, and the PCGPA (or dispersion scanning reconstruction algorithm) reconstruction result is more accurate.
In order to make the method of the present invention clearer, the following steps are optionally adopted when the method is implemented:
(1) the spectrum (for example, 600-.
(2) The ultrashort laser pulse to be measured is subjected to a device (such as SHG-FROG, Dispersion-Scan) for measuring ultrashort laser pulse by utilizing the process of generating second harmonic by using the nonlinear crystal, a second harmonic signal is generated, and the frequency spectrum of the second harmonic is obtained, wherein the thickness of the nonlinear crystal is 10-300 um.
(3) The change of the second harmonic signal frequency spectrum is observed through the included angle between the nonlinear crystal optical axis and the light propagation direction in the rotation measuring device, namely the phase matching angle. As the phase matching angle changes, the corresponding phase matching wavelength gradually deviates from the central wavelength of the second harmonic, and the spectrum of the second harmonic signal is gradually widened, and when the second harmonic signal reaches a certain angle (37.6 degrees through experiments), the spectrum is widest. When the spectrum width is equivalent to the spectrum range calculated in step (1) (300-450nm), the trace map can be collected.
(4) And (3) performing spectrum correction on the acquired trace diagram, then reconstructing by using PCGPA (or dispersion scanning reconstruction), and reconstructing the frequency spectrum phase of the pulse when the error is small, thereby obtaining all information of the ultrashort laser pulse (8.6fs) to be detected.
It should be noted that the spectrometer used in step (1) and the spectrometer used in the apparatus in step (2) may be of the same type or different types, and the method is not limited and does not substantially differ the technical problem to be solved by the present invention. It should be noted that the device in step (2) has a high frequency response to the frequency in the spectrum range of the second harmonic signal of the ultra-short laser pulse to improve the measurement accuracy.
Example two
An ultrashort laser pulse measurement system, comprising:
the device is used for measuring the ultrashort laser pulse by utilizing the process of generating the second harmonic wave by the nonlinear crystal and is used for measuring the second harmonic frequency spectrum generated by the ultrashort laser pulse to be measured through the nonlinear crystal;
the angle adjusting module is used for rotating the nonlinear crystal to change an included angle between an optical axis of the nonlinear crystal and a light propagation direction, monitoring the spectrum width of a second harmonic generated by the nonlinear crystal under each included angle, determining the included angle corresponding to the spectrum width for analyzing all information of the ultrashort laser pulse to be detected, and collecting an ultrashort laser pulse trace diagram under the included angle;
and the data processor is used for extracting the pulse information of the ultrashort laser pulse to be detected from the trace graph.
Preferably, the data processor is specifically configured to: and performing spectrum correction on the trace graph, and reconstructing the corrected trace graph by adopting a reconstruction algorithm to obtain frequency domain phase information of the ultrashort laser pulse to be measured so as to complete measurement of the ultrashort laser pulse to be measured.
Preferably, the thickness of the nonlinear crystal is greater than 10 μm.
Preferably, the included angle corresponding to the spectrum width for analyzing all information of the ultrashort laser pulse to be detected is the included angle corresponding to the widest monitored second harmonic spectrum generated by the nonlinear crystal, and the phase matching wavelength of the nonlinear crystal corresponding to the included angle is outside the spectrum range of the ultrashort laser pulse to be detected.
The related technical description of the above system technical solution is the same as the first embodiment, and is not repeated herein.
In order to make the measuring system of the invention clearer, the following example is given:
as shown in fig. 2, the system comprises 2 beam splitters, 3, 5 and 6 plane mirrors, 4 fused quartz, 7 displacement stages, 8 cylindrical mirrors, 9 nonlinear crystals BBO, 10 vertical slits, 11, 13 and 16 ultraviolet concave mirrors, 12 horizontal slits, 14 isosceles prisms, 15 ultraviolet plane mirrors, 17 ultraviolet cameras and 18 data processing modules.
The device shown in fig. 2 is a single-shot SHG-FROG device, the ultrashort laser to be measured is divided into two beams by a beam splitter 2, the two beams of light are focused on BBO9 by a cylindrical mirror 8, time-space coincidence of the two beams of light on BBO is ensured by adjusting a manual displacement table 7, a second harmonic signal is generated, the two beams of light are imaged on a horizontal slit by an ultraviolet concave reflector 11, a vertical slit 10 is used for blocking the ultrashort laser pulse to be measured and the second harmonic signal generated by the ultrashort laser pulse to be measured and the two beams of light, the two beams of light are collimated by an ultraviolet concave reflector 13, the light is split on an isosceles prism 14, and the light is incident to a CCD ultraviolet camera 17 through an ultraviolet concave reflector 16. And changing the phase matching angle of the BBO to enable a relatively complete second harmonic spectrum signal to be seen on the CCD ultraviolet camera 17, performing spectrum correction after collection, and reconstructing the frequency domain phase of the pulse to be detected by using PCGPA.
In the ultrashort laser pulse measuring device, the nonlinear crystal with the thickness of 10-300um is used, so that the problem of phase matching bandwidth of the relatively thick nonlinear crystal is avoided, the device is simple, and the material processing is relatively easy.
Specifically, the device shown in fig. 2 is used to measure the ultrashort laser pulse to be measured, as follows:
as shown in FIG. 2, an infrared laser pulse with a center wavelength of 800nm (750nm-850nm)25fs generated by a Titanium sapphire (Titanium sapphire) laser is incident into a hollow fiber filled with 0.6atm Ar and with a length of 1m to perform spectral broadening, the wavelength range of an emergent pulse is 600-900nm, and then dispersion compensation is performed through a chirped mirror and fused silica, then the light beam enters a single-shot SHG-FROG device and is divided into two beams by a beam splitter, focusing on a 29.2-degree 150-micron BBO crystal through a cylindrical mirror, imaging on a CCD camera through a self-made prism spectrometer, observing the width of the frequency spectrum range of a second harmonic signal through CCD software, and observing complete frequency spectrum width on a CCD camera when the phase matching angle is 37.6 degrees, collecting a trace graph, performing spectrum correction, and then reconstructing by using PCGPA to obtain all information of the ultrashort laser pulse to be detected.
As shown in fig. 3, the gaussian ultrashort laser pulse with 8fs 730nm undergoes the second harmonic spectrum simulation results with different BBO thicknesses, two different phase matching angles of 32 ° and 40 ° are selected, the phase matching wavelengths of 32 ° and 40 ° are 732nm and 600nm, respectively, and the gaussian curve in the figure is the spectrum self-convolution of the 8fs pulse, i.e. the ideal second harmonic spectrum. It can be seen that the phase matching angle is at 32 °, the width of the second harmonic spectrum is gradually reduced as the BBO thickness increases, while the phase matching is at 40 °, the second harmonic spectrum width is not substantially changed as the BBO thickness increases, which proves that: the angle of the nonlinear crystal is adjusted to enable the phase matching wavelength of the crystal to be out of the spectrum range of the pulse to be measured, so that the spectrum width of the second harmonic signal can be completely measured.
As shown in fig. 4, the trace plot and the reconstructed result plot of the ultrashort laser pulse 8.3fs in 40 ° 100um BBO at a given pulse intensity and phase were simulated and calculated using the apparatus of fig. 2. The upper two graphs in fig. 4 are the reconstructed trace graph and the calculated trace graph, respectively, the solid lines in the lower two graphs are the pulse intensity and the spectrum intensity and the phase used for calculation, and the dotted lines are the reconstructed pulse intensity and the spectrum intensity and the phase. In fig. 4 is seen: and the reconstructed pulse intensity and the spectral intensity, the phase and the input pulse information are well matched when the phase matching wavelength (600nm) is outside the spectral range (600-900nm) of the pulse to be measured.
As shown in fig. 5, the trace pattern and the reconstructed result pattern of the ultrashort laser pulse to be measured are measured by using the apparatus of fig. 2 and using 37.6 ° 150um BBO. In fig. 5, the upper two graphs are a reconstructed trace graph and a measured and spectrally corrected trace graph, respectively, and the black line in the lower two graphs is the reconstructed pulse intensity and the spectral intensity, the gray line is the phase of the reconstructed pulse and the spectrum, and the half-height width of the reconstructed pulse is 8.6 fs.
As shown in FIG. 6, using the apparatus FIG. 2, a comparative plot of the results was reconstructed with 37.6 deg. 150um BBO and 35 deg. 5um BBO, respectively. The black line in the graph is the measured spectral intensity of the pulse to be measured, the solid gray line is the intensity and phase of the pulse and spectrum reconstructed by the 35 ° 5um BBO, and the dashed black line is the intensity and phase of the pulse and spectrum reconstructed by the 37.6 ° 150um BBO. The phase matching bandwidth of 35-degree 5um BBO covers the spectrum width of the whole ultrashort laser pulse to be detected, and the reconstruction result can be used as a standard. It can be seen that the two measurements agree very well, again proving the correctness of the invention.
The results show that the method can accurately, simply and conveniently measure the unknown optical ultrashort laser pulse. Compared with the existing method, the method has specific advantages and has a wide application prospect in the field of ultrafast measurement.
It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.
Claims (4)
1. An ultrashort laser pulse measurement method, comprising:
measuring a second harmonic frequency spectrum generated by the ultrashort laser pulse to be measured through a nonlinear crystal by adopting a device for measuring the second harmonic process generated by the nonlinear crystal by utilizing the nonlinear crystal, wherein the included angle between the optical axis of the nonlinear crystal and the light propagation direction is changed by rotating the nonlinear crystal, the spectrum width of the second harmonic frequency spectrum generated by the nonlinear crystal under each included angle is monitored, and the included angle corresponding to the spectrum width for analyzing all information of the ultrashort laser pulse to be measured is determined; the thickness of the nonlinear crystal is greater than 10 microns; the included angle corresponding to the spectral width for analyzing all information of the ultrashort laser pulse to be detected is the included angle corresponding to the widest monitored second harmonic frequency spectrum generated by the nonlinear crystal, and the phase matching wavelength of the nonlinear crystal corresponding to the included angle is out of the spectral range of the ultrashort laser pulse to be detected;
and acquiring an ultra-short laser pulse trace graph under the included angle, and extracting pulse information of the ultra-short laser pulse to be measured from the trace graph to finish measurement of the ultra-short laser pulse to be measured.
2. The method of claim 1, wherein the extracting the pulse information of the ultrashort laser pulse to be measured from the trace map specifically comprises:
and performing spectrum correction on the trace graph, and reconstructing the corrected trace graph by adopting a reconstruction algorithm to obtain frequency domain phase information of the ultrashort laser pulse to be measured so as to complete measurement of the ultrashort laser pulse to be measured.
3. An ultrashort laser pulse measurement system, comprising:
the device is used for measuring the ultrashort laser pulse by utilizing the process of generating the second harmonic wave by the nonlinear crystal and is used for measuring the second harmonic frequency spectrum generated by the ultrashort laser pulse to be measured through the nonlinear crystal;
the angle adjusting module is used for rotating the nonlinear crystal to change an included angle between an optical axis of the nonlinear crystal and a light propagation direction, monitoring the spectrum width of a second harmonic generated by the nonlinear crystal under each included angle, determining the included angle corresponding to the spectrum width for analyzing all information of the ultrashort laser pulse to be detected, and collecting an ultrashort laser pulse trace diagram under the included angle; the thickness of the nonlinear crystal is greater than 10 microns; the included angle corresponding to the spectral width for analyzing all information of the ultrashort laser pulse to be detected is the included angle corresponding to the widest monitored second harmonic frequency spectrum generated by the nonlinear crystal, and the phase matching wavelength of the nonlinear crystal corresponding to the included angle is out of the spectral range of the ultrashort laser pulse to be detected;
and the data processor is used for extracting the pulse information of the ultrashort laser pulse to be detected from the trace graph.
4. The ultrashort laser pulse measurement system of claim 3, wherein the data processor is specifically configured to: and performing spectrum correction on the trace graph, and reconstructing the corrected trace graph by adopting a reconstruction algorithm to obtain frequency domain phase information of the ultrashort laser pulse to be measured so as to complete measurement of the ultrashort laser pulse to be measured.
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