CN211373846U - Ultraviolet femtosecond seed laser pulse width measuring device of free electron laser - Google Patents
Ultraviolet femtosecond seed laser pulse width measuring device of free electron laser Download PDFInfo
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Abstract
The utility model provides an ultraviolet femtosecond seed laser pulse width measuring device of free electron laser, which comprises an oscillator for outputting probe light, an amplifier for outputting the light to be measured, a frequency tripler and a measuring light path, wherein the oscillator, the amplifier, the frequency tripler and the measuring light path are sequentially connected; the beam combining mirror is positioned at the intersection point of the light paths of the light to be measured and the probe light, the wavelength of the probe light is 800nm, the wavelength of the light to be measured is 266nm, the thickness of the sum frequency crystal is at most 0.1mm, the cutting angle is 44.3 degrees, the thickness of the frequency doubling crystal is at least 0.5mm, and the cutting angle is 29.2 degrees. The laser pulse width measuring device of the utility model has high time resolution and high measuring accuracy; through collinear structural design, the space of an optical platform is saved, and the device can be used as an online diagnosis device of ultraviolet femtosecond laser pulse width for real-time monitoring.
Description
Technical Field
The utility model relates to a measuring device of ultraviolet femtosecond seed laser pulse width in a free electron laser device.
Background
The application of femtosecond laser in scientific research, biology, medical treatment, processing, communication, national defense and other fields is deeply developed. One important application is that the ultraviolet femtosecond laser is used as an external seed of a free electron laser device and is injected into an undulator together with an electron beam, when the resonance relation is satisfied, the seed laser interacts with the electron beam and is continuously amplified until the seed laser is output in a saturated mode, so that the radiation of the FEL inherits the characteristics of the seed laser, namely, the FEL has the characteristics of good coherence and stability, extremely high power (GW), narrow spectrum, narrow width (dozens to hundreds of femtoseconds) and the like. The pulse width and the pulse shape of the femtosecond laser are important optical parameters of laser pulse, and are the source of an external seed type free electron laser device (FEL); for a free electron laser device (FEL) of a specific mode, the pulse width of the FEL can be directly calculated from the pulse width of the seed laser, so that real-time diagnosis of the uv femtosecond laser is very necessary in experiments of the FEL.
In the field of free electron laser, no literature or patent is found through research on how to accurately measure the pulse width of the ultraviolet femtosecond seed laser.
In the field of optics, the femtosecond laser pulse width measuring instrument discussed in the current utility model CN 203824652U employs infrared femtosecond pulse width measurement suitable for a center wavelength of 800 nm. The single picosecond laser pulse width measuring device disclosed in utility model patent CN 101699233 a also measures the pulse width of picosecond magnitude by optical autocorrelation method, adopts KDP frequency doubling crystal to produce signal light, and is only suitable for infrared wave band about 800 nm.
In 2010, the university of east china, the resident, describes a cross-correlation measurement method of ultraviolet pulses (see [ the resident; research on ultraviolet femtosecond laser pulse width measurement [ D ], university of east china, 2010 ]), which can be used to measure a pulse width of 266 nm. However, the thickness of the BBO crystal used is 0.2mm, and due to the influence of the group velocity mismatch effect, the thickness of the crystal can significantly reduce the accuracy of the measurement result, so that the cross-correlation curve obtained by the measurement has obvious distortion, and there is no discussion about how to obtain an accurate measurement result through the distribution of the cross-correlation curve; in addition, due to the adoption of a non-collinear measurement method, under the condition that the incident angle of 2 beams of incident light is very small (less than 1 degree), the emergent light needs to be transmitted for a longer distance (more than X meters) to separate out the signal light, so that the occupied space of an optical platform is larger; further, this structure is not convenient for finding the position of the correlated signal light for a probe light having a single pulse energy of about 10 nJ.
In 1988, d.c. edelstein et al, when studying the method of generating uv Femtosecond pulses by BBO crystals, set forth a cross-correlation measurement method of measuring pulse width with a wavelength of about 210nm and a pulse width of less than 100fs (see [ d.c. edelstein, e.s.wachman, l.k.cheng et al, "femtocell ultra pulse generation in β -BBO", appl.phys.lett 52(26),1988 ]), which uses sum-frequency BBO crystals with a cut angle of 69 °, limited by manufacturing conditions, and a minimum thickness of 0.085mm for BBO crystals, and separates out cross-correlation signal light of 210nm by using a uv monochromator, but it only gives a pulse width calculation method in the case where the light to be measured is hyperbolic secant distribution.
In 2013, by ying Yang et al (see [ ding Yang, Feng Yang, jin ya Zhang et al, "Pulse broad of deep ultra violet ferromagnetic from second coherent formation in KBe2BO3 crystal", optics communications 288,2013) ], a non-collinear UV femtosecond Pulse width measuring device was designed, the wavelength of light to be measured was 210nm, the angle to the probe light was 2 °, the thickness of BBO difference frequency crystal was 0.1mm, and the cut was 76 °. The influence of the included angle on the accuracy of the measurement result is not researched; the pulse width of the probe light is 249fs in the measurement process, so that the time resolution of the measurement system is limited; the pulse energy of the probe light is 400 muj, the energy of the light pulse to be detected is 9 muj, and no discussion is given on how to efficiently search the position of the cross-correlation signal light under the nJ condition.
SUMMERY OF THE UTILITY MODEL
An object of the utility model is to provide a free electron laser device's ultraviolet seed laser pulse width measuring device to realize the online real-time pulse width diagnosis of ultraviolet femto second seed laser, improve measuring result's time resolution and accuracy.
In order to achieve the above object, the utility model provides a free electron laser's ultraviolet femto second seed laser pulse width measuring device, it is installed on an optical platform, including oscillator, amplifier, tripler and the measurement light path that links to each other in proper order, the oscillator sets up the input light into output probe light sum amplifier, and the amplifier sets up the input light into the oscillator and carries out energy amplification, the tripler sets up the light conversion output of amplifier output for the light that awaits measuring, the measurement light path includes the first, second and third ultraviolet plane high-reflection mirror, beam combiner, changeable beta-BBO sum frequency crystal and beta-BBO frequency doubling crystal, dispersion prism, band pass filter and photomultiplier that arrange along the trend of the light path of awaiting measuring light in proper order, and this photomultiplier is connected with an oscilloscope; the beam combining mirror is located at the intersection point of the light path of the to-be-measured light and the light path of the probe light, the included angle between the beam combining mirror and the light path of the to-be-measured light is 45 degrees, the included angle between the beam combining mirror and the light path of the to-be-measured light is 0 degree, the central wavelength of the probe light is 800nm, the bandwidth is 10nm, the central wavelength of the to-be-measured light is 266nm, the bandwidth is 1.6nm, the thickness of the beta-BBO sum frequency crystal is at most 0.1mm, the cutting angle theta is 44.3 degrees, the thickness of the beta-BBO frequency doubling crystal is at least 0.5mm, and the cutting angle theta is 29.2 degrees.
The first ultraviolet plane high-reflection mirror and the second ultraviolet plane high-reflection mirror are both arranged on the same high-precision electric displacement table.
The stroke of the electric displacement table is larger than 200mm, the repetition precision is +/-0.05 mu m, the precision is +/-1.7 mu m, the bidirectional repetition precision is +/-0.25 mu m, and the minimum displacement increment of scanning is 20 nm.
The electric displacement platform comprises a guide rail arranged on the optical platform, a platform arranged on the guide rail through a screw nut pair and a servo motor used for driving the screw nut pair, and the first ultraviolet plane high-reflection mirror and the second ultraviolet plane high-reflection mirror are arranged on the platform.
The center wavelength of the band-pass filter is 400nm, and the bandwidth is 20 nm.
The polarization state of probe light sets up to be perpendicular with the polarization state who treats the photometry, probe light is e light, treat that the photometry is o light.
The oscillator is a titanium gem laser oscillator, the bandwidth of the probe light is 30nm, the pulse width is 50fs +/-10 fs, the average power is 0.7W, the frequency is 79.33MHz, and the single pulse energy is 8.8 nJ.
The frequency tripler with be equipped with first diaphragm and second diaphragm between the high reflection mirror of first ultraviolet plane, be equipped with third diaphragm and fourth diaphragm between the high reflection mirror of first ultraviolet plane and the high reflection mirror of second ultraviolet plane, be equipped with the fifth diaphragm between dispersion prism and the band-pass filter.
Two or any plurality of ultraviolet plane high-reflection mirrors can be arranged between the oscillator and the beam combining mirror.
The utility model discloses an ultraviolet seed laser pulse width measuring device of free electron laser adopts changeable BBO sum frequency crystal and frequency doubling crystal, and the thickness 1 of sum frequency crystal is 0.05mm, and the crystal cut angle is 44.3 to make 800 nm's probe light and 266 nm's ultraviolet ray collineatly pass through sum frequency crystal and then produce about 400nm difference frequency signal after closing the bundle on the beam combining mirror, improve measurement system's time resolution and accuracy, and through this kind of collineation setting, saved spatial position, be convenient for measurement system miniaturization; the thickness of the frequency doubling crystal for frequency doubling of the probe light is at least 0.5mm, the generated 400nm frequency doubling light can efficiently and directly observe the position of the 400nm light after passing through the prism, the signal light can be conveniently and efficiently detected in the actual measurement process, and the position of the photomultiplier can be adjusted according to the position of the signal light. Furthermore, the utility model discloses an ultraviolet seed laser pulse width measuring device's of free electron laser the pulse width of probe light is about 50fs to at probe light low energy, under the condition, adopt high sensitivity's photomultiplier as the detector, with improvement measurement system's sensitivity.
Drawings
Fig. 1 is a schematic structural diagram of the ultraviolet femtosecond seed laser pulse width measuring device of the free electron laser of the present invention.
Fig. 2(a) -2(b) are the utility model discloses a probe light detects the map with the thick synchro-regulation process who treats the photometry among the ultraviolet femto second seed laser pulse width measuring device of free electron laser, wherein, fig. 2(a) shows that 266nm treats photometry advanced 800nm probe light 4.72ns, fig. 2(b) shows that 266nm treats photometry and 800nm probe light and realizes thick synchronization, and synchronous error is about 200ps scope.
Fig. 3 is a graph of the results of 3 repeated measurements of the distribution of the intensity of the difference frequency signal light with the delay time of the uv femtosecond seed laser pulse width measuring apparatus of the free electron laser according to the present invention.
Fig. 4 is a distribution diagram of a typical cross-correlation signal measured by the uv femtosecond seed laser pulse width measuring device of the present invention.
Fig. 5 is a schematic diagram of the measurement result of the free electron laser of the uv femtosecond seed laser pulse width measurement device with an excessive photomultiplier gain index in the measurement process.
Detailed Description
The present invention will be further described with reference to the following specific embodiments. It should be understood that the following examples are illustrative of the present invention only and are not intended to limit the scope of the present invention.
Fig. 1 shows an apparatus for measuring a pulse width of a free electron laser according to an embodiment of the present invention, which is installed on an optical platform 100 and includes an oscillator 1, an amplifier 2, a frequency tripler 3 and a measuring optical path 4 connected in sequence.
Wherein the oscillator 1 is a titanium sapphire laser oscillator having 2 functions, and is configured to output probe light having a center wavelength of 800nm, a bandwidth of 30nm, a pulse width of 50fs ± 10fs, an average power of about 0.7W, a frequency of 79.33MHz, and a single pulse energy of 8.8nJ and input light to an amplifier; the amplifier 2 is arranged to amplify the energy of the input light of the oscillator 1, and outputs light with the wavelength of about 800nm at the frequency of 1-1000Hz, the single pulse power of the amplifier is 30GW, the single pulse energy is 3mJ, the bandwidth is 10nm, and the pulse width is 100 fs; the frequency tripler 3 is set to convert the light output by the amplifier into light to be measured, the central wavelength of the light to be measured is 266nm, the light to be measured is ultraviolet femtosecond seed laser with the pulse width to be measured, the single pulse power of the light to be measured is larger than 2GW, the frequency is 1Hz-1000Hz, the single pulse energy is 300uJ, the bandwidth is 1.6nm, and the pulse width is hundred femtosecond magnitude. In general, the parameters of the light to be detected output by the frequency tripler 3 are changed by adjusting the parameters of the amplifier 2 without adjusting the parameters of the oscillator 1.
The measurement optical path 4 comprises a first ultraviolet plane high-reflection mirror M1, a second ultraviolet plane high-reflection mirror M2, a third ultraviolet plane high-reflection mirror M3, a beam combining mirror M4, a switchable beta-BBO sum frequency crystal C1 and a beta-BBO frequency doubling crystal C2, a dispersion prism P1, a band-pass filter T6 and a photomultiplier tube D6 which are sequentially arranged along the direction of the optical path of light to be measured, wherein the photomultiplier tube D6 is connected with an oscilloscope (not shown). The trigger signal for triggering the sampling of the oscilloscope and the probe light output by the laser oscillator have a fixed time difference, and the time difference between the trigger signal and the probe light is ensured by a timing system.
The area photometry is in incident angle on first, second and third ultraviolet plane high reflection mirror M1, M2, M3 is 45, beam combining mirror M4 is located treat the light path of photometry with the nodical of the light path of probe light, and with the contained angle of the light path of treating photometry is 45, with the contained angle of the light path of probe light is 0, and it sets up to reflect the probe light of treating photometry and seeing through the oscillator output.
In addition, in other embodiments, several additional uv plane high-reflection mirrors may be added according to the available space of the optical platform 100 and the requirements for adjusting the optical path and collimating the optical path, for example, two or any more uv plane high-reflection mirrors may be further disposed between the oscillator 1 and the beam combiner to ensure that the beam combiner M4 is located at the intersection of the optical path of the light to be measured and the optical path of the probe light.
Therefore, the triple frequency multiplier 3 outputs light to be measured, and the light passes through the ultraviolet high-reflection mirrors M1, M2, M3 and the beam combining mirror M4 and then irradiates the center of the sum frequency crystal C1; when the probe light output by the oscillator 1 passes through the beta-BBO frequency doubling crystal C2 to generate frequency doubling signal light with the wavelength of 400nm when the probe light is switched to the beta-BBO frequency doubling crystal C2, the probe light passes through the dispersion prism P1 to separate the light with the wavelengths of 800nm, 400nm, 266nm and 3 on the space, and the difference frequency signal passes through the band-pass filter T6 to adjust the position of the photomultiplier tube D6 according to the position of the frequency doubling signal light; when the voltage is switched to beta-BBO sum frequency crystal C1, probe light output by the oscillator 1 is transmitted by a beam combiner M4, then is superposed with light to be measured in time and transverse positions, and is irradiated on the sum frequency crystal C1, meanwhile, after passing through the sum frequency crystal C1, difference frequency signal light with the wavelength of about 400nm is generated, after passing through a dispersion prism P1, light with the wavelengths of 800nm, 400nm, 266nm and 3 is separated in space, after passing through a band-pass filter T6, noise light with other wave bands is filtered, only light with the wavelength of 400nm is allowed to pass, the influence of the probe light, the light to be measured and ambient light on measurement accuracy is reduced, and the influence is irradiated on a photomultiplier D6, the photomultiplier D6 converts detected light intensity into corresponding voltage signals, and an oscilloscope is used for collecting the intensity of the difference frequency signal light.
In order to improve the time resolution and accuracy of the measuring system, the thickness of the beta-BBO and frequency crystal C1 is at most 0.1mm, the cutting angle theta is 44.3 degrees, under the condition of the angle, when probe light with the wavelength of about 800nm and light to be measured with the wavelength of 266nm pass through the crystal, difference frequency signal light with the wavelength of 400nm can be generated, the influence of group velocity mismatch on cross-correlation signal broadening is reduced by the beta-BBO and frequency crystal with the thickness of less than or equal to 0.1mm, the time resolution of the measuring device is improved, and the measured time resolution is enabled to be less than 30 fs; the thickness of beta-BBO frequency doubling crystal C2 is at least 0.5mm, and the cutting angle theta is 29.2 degrees, so that when probe light of 800nm passes through beta-BBO frequency doubling crystal C2 under the angle, 400nm frequency doubling signal light can be generated. The switchable beta-BBO sum frequency crystal C1 and the beta-BBO frequency doubling crystal C2 are both arranged on a 4-dimensional direction fine-adjustable optical adjusting frame, and the adjustable directions are rotation in a horizontal plane, a vertical plane and rotation directions in a horizontal plane. In the measuring process, the phase matching angle of the beta-BBO sum frequency crystal C1 or the beta-BBO frequency doubling crystal C2 is finely adjusted by a knob on an optical adjusting frame, and the difference frequency signal light or the frequency doubling signal light with the strongest intensity is obtained. The center wavelength of the band-pass filter T6 is 400nm, and the bandwidth is 20nm, so that the influence of probe light, light to be detected and ambient light on the measurement accuracy is reduced.
In addition, the polarization state of the probe light (o light) needs to be perpendicular to that of the light to be detected (e light), and only when the perpendicular condition is met, the probe light and the BBO crystal generate difference frequency signal light of 400nm, and before measurement, the polarization states of 2 kinds of light are checked to ensure that one is horizontally polarized and the other is vertically polarized. In this embodiment, the probe light is o light, and the light to be detected is e light. The polarization state of the generated difference frequency signal light of 400nm is the same as that of the probe light of 800 nm.
The first ultraviolet plane high-reflection mirror M1 and the second ultraviolet plane high-reflection mirror M2 are both installed on the same high-precision electric displacement table 5, the electric displacement table 5 is controlled to move towards the direction close to or far away from the frequency tripler 3 to realize the time coincidence of the light to be detected and the probe light, so that the light to be detected and the probe light can generate difference frequency signal light after passing through a sum frequency crystal C1, moreover, a distribution curve between the intensity of the difference frequency signal light and the position of a high-precision delay line can be obtained through the scanning of the high-precision electric displacement table 5, and after the full width at half maximum of the curve is calculated, the pulse width of the ultraviolet seed laser to be detected can be obtained through simple conversion. The stroke of the electric displacement table 5 is larger than 200mm, the repetition precision is +/-0.05 mu m, the precision is +/-1.7 mu m, and the bidirectional repetition precision is +/-0.25 mu m, so that the measurement resolution is better than 30fs, difference frequency signals are conveniently searched, remote automatic scanning can be realized, the minimum displacement increment of scanning is 1 mu m, and light to be measured with the wavelength of 266nm and the pulse width of 100fs to dozens of ps can be measured.
The electric displacement table 5 comprises a guide rail mounted on the optical platform 100, a platform mounted on the guide rail through a screw-nut pair, and a servo motor for driving the screw-nut pair, and the first and second ultraviolet plane high-reflection mirrors M1 and M2 are mounted on the platform, so that the screw-nut pair is driven by the servo motor, and after a screw in the screw-nut pair rotates, the nut matched with the screw can make linear motion, and further the first and second ultraviolet plane high-reflection mirrors M1 and M2 on the platform are driven to move towards the direction close to or far away from the frequency tripler 3.
A first diaphragm I1 and a second diaphragm I2 are arranged between the frequency tripler 3 and the first ultraviolet plane high-reflection mirror M1 and are used for positioning the transverse position of incident light to be detected. And a third diaphragm I3 and a fourth diaphragm I4 are arranged between the second ultraviolet plane high-reflection mirror M2 and the third ultraviolet plane high-reflection mirror M3 and are used for positioning the transverse position of the emergent light to be detected. And a fifth diaphragm I5 is arranged between the dispersion prism P1 and the band-pass filter T6 and is used for filtering stray light.
The ultraviolet femtosecond seed laser pulse width measuring method based on the ultraviolet femtosecond seed laser pulse width measuring device of the free electron laser comprises the following steps:
step S1: the method comprises the steps that an oscillator 1, an amplifier 2 and a frequency tripler 3 which are connected in sequence are installed, the oscillator 1 is used for outputting probe light and input light of the amplifier, the frequency tripler 3 is used for outputting light to be detected, a first ultraviolet plane high-reflection mirror M1, a second ultraviolet plane high-reflection mirror M2 and a third ultraviolet plane high-reflection mirror M3 are installed in sequence along the direction of a light path of the light to be detected, a light intensity detector is arranged at the intersection point of the light to be detected and the probe light, so that the optical path of the probe light and the light to be detected is preliminarily measured, and the positions of the first ultraviolet plane high-reflection mirror M1 and the second ultraviolet plane high-reflection mirror M2 are adjusted according to the measurement result to realize rough;
the wavelength of the probe light is 800nm, the wavelength of the light to be measured is 266nm, the preliminary measurement is realized by connecting an input signal of the light intensity detector to an oscilloscope and using the optical path difference between the probe light and the light to be measured, the optical path difference between the probe light and the light to be measured of the preliminary measurement is shown in fig. 2(a), and the optical path difference between the probe light and the light to be measured of 266nm in fig. 2(a) exceeds the optical path difference between the probe light and the light to be measured of 800nm by 4.72ns, namely the optical path difference between the probe light and the light to be measured is 1.416.
The first ultraviolet plane high-reflection mirror M1 and the second ultraviolet plane high-reflection mirror M2 are both arranged on the same high-precision electric displacement table 5, the positions of the first ultraviolet plane high-reflection mirror M1 and the second ultraviolet plane high-reflection mirror M2 are adjusted by adjusting the electric displacement table 5, and the optical path difference of 2 kinds of light is adjusted to about 200ps as shown in figure 2 (b). In FIG. 2(b), there are only 2 peaks, the first large peak is 266nm, the large peak includes a small peak 800nm inside the same as the second one, and since the large peak and the small peak are overlapped, i.e., roughly synchronized, it appears that there is only one large peak, and therefore, it is estimated that the optical path difference at this time is adjusted to about 200 ps.
Step S2: the ultraviolet femtosecond seed laser pulse width measuring device of the free electron laser is constructed, namely, a beam combining mirror M4, a beta-BBO frequency doubling crystal C2 with the thickness of at least 0.5mm, a dispersion prism P1, a band-pass filter T6 and a photomultiplier tube D6 are sequentially arranged along the direction of the light path of light to be measured, the photomultiplier tube D6 is connected with an oscilloscope, frequency doubling signal light with the wavelength of 400nm is generated at the moment, the position of a light spot of the frequency doubling signal light is directly observed, and the photomultiplier tube D6 is adjusted to the position. In order to improve the search efficiency of the difference frequency light, the sum frequency crystal needs to be switched to a beta-BBO frequency doubling crystal C2, the repetition frequency of the generated frequency doubling signal light is very high (79.33MHz), and the position of the frequency doubling signal light is convenient to directly observe, so that the positioning of 400nm light is realized before formal measurement. The thickness of beta-BBO frequency doubling crystal C2 is at least 0.5mm, and the larger the thickness, the higher the efficiency and the stronger the signal.
Step S3: switching a beta-BBO frequency doubling crystal C2 into a beta-BBO frequency doubling crystal C1, finding out difference frequency signal light in a +/-30 mm scanning range by adopting a high-precision electric displacement table 5, then scanning by adopting the electric displacement table 5 and acquiring the intensity of the difference frequency signal light by adopting an oscilloscope, and further obtaining a distribution diagram of the intensity of the difference frequency signal light along with delay time (namely an intensity distribution curve of the difference frequency signal light);
the delay time can be obtained by dividing the displacement (i.e., delay distance) of the electric displacement table 5 during scanning by the speed of light. A fifth diaphragm I5 is arranged between the dispersion prism P1 and the band-pass filter T6, 800nm probe light and 266nm light to be detected can generate cross-correlation signals with the wavelength of 400nm after passing through a beta-BBO sum frequency crystal, 3 wavelengths of light enter the prism at a certain angle, the fifth diaphragm I5 filters stray light with the wavelength of 800nm and 266nm after the light is split by the prism, so that difference frequency signal light with the wavelength of 400nm enters a high-sensitivity photomultiplier D6 after passing through a diaphragm 5, and the photomultiplier D6 converts the intensity of the detected signal light into a voltage signal, and therefore the intensity of the difference frequency signal light can be acquired through an oscilloscope.
In order to verify the repeatability of the pulse width measurement system, 3 repeated scans are performed before formal measurement, and the distribution graph of the intensity of the difference frequency signal light obtained by the scanning result along with the delay time is shown in fig. 3, which shows that the system has high repeatability. A typical profile of the intensity of the resulting difference frequency signal light with delay time is shown in fig. 4.
Wherein the difference frequency signal light IcsThe strength of (A) is:
wherein, I1(t) intensity of probe light having a wavelength of 800nm, I2(t- τ) is the intensity of light to be measured at 266 nm.
In addition, when the gain of the photomultiplier tube D6 is too large, the cross-correlation signal may have a flat-top distribution, and when the gain is too large, the distribution of the intensity of the frequency signal light with the delay time is shown in fig. 5. In addition, the signal value is 0 when the signal on the oscilloscope exceeds the display range.
Step S4: a typical intensity profile of the difference frequency signal light is shown in fig. 4, and the difference frequency obtained from the profile after obtaining the profile is shownFull width at half maximum σ of signal lightCsValue, and thus the pulse width σ of the light to be measured2。
Pulse width sigma of light to be measured2The calculation formula of (2) is as follows:
Wherein the full width at half maximum σ of the intensity distribution curve of the difference frequency signal lightcsContains the broadening amount caused by the group velocity mismatch, β -BBO and the thickness of C1 crystal have significant influence on the group velocity mismatch, and in order to eliminate the influence, the full width at half maximum sigma of the intensity distribution curve of the difference frequency signal light is requiredcsFitting is performed to improve the accuracy of the measurement, and the specific fitting method is as follows:
for the fact that the time domain distribution of the light to be measured is Gaussian distribution, the influence of broadening caused by β -BBO and the thickness of a frequency crystal C1 is considered, and the accurate pulse width value sigma of the light to be measured can be obtained according to the following formula fittingUV：
If the time domain distribution of the light to be measured is hyperbolic secant distribution, considering the influence of the broadening amount caused by β -BBO and the thickness of the frequency crystal C1, and fitting according to the following formula, the accurate pulse width value sigma of the light to be measured can be obtainedUV：
Wherein the content of the first and second substances,
The group velocity mismatch value
The specific calculation method is as follows:
wherein v isogIs the group velocity of o light, vegIs the group velocity of e light, nogIs the group refractive index of o light, negThe refractive index of e light, c the speed of light, c 3 × 108m/s。
Group refractive index n of o light and e lightog，negThe calculation method of (2) is as follows:
wherein λ is the wavelength of light, and has a unit of μm, no(λ) is the refractive index of o light with a wavelength λ, ne(λ) is the refractive index of e-light with wavelength λ.
Step S5: according to the pulse width sigma of the light to be measured2The pulse width of the free electron laser light pulse is obtained.
For a specific mode of free electron laser pulse, based on the measured pulse width of the light to be detected (i.e. the UV femtosecond seed laser), the pulse width T of the free electron laser pulseFELComprises the following steps:
wherein n is the number of harmonic series, TseedIs the pulse width of the light to be measured (i.e., the uv seed femtosecond laser).
What has been described above is only the preferred embodiment of the present invention, not for limiting the scope of the present invention, but various changes can be made to the above-mentioned embodiment of the present invention. All the simple and equivalent changes and modifications made according to the claims and the content of the specification of the present invention fall within the scope of the claims of the present invention. The present invention is not described in detail in the conventional technical content.
Claims (9)
1. The ultraviolet femtosecond seed laser pulse width measuring device of the free electron laser is arranged on an optical platform (100), and is characterized by comprising an oscillator (1), an amplifier (2), a frequency tripler (3) and a measuring light path (4) which are sequentially connected, wherein the oscillator (1) is used for outputting probe light and input light of the amplifier, the amplifier (2) is used for amplifying energy of the input light of the oscillator (1), the frequency tripler (3) is used for converting the light output by the amplifier into light to be measured, and the measuring light path (4) comprises a first ultraviolet plane high-reflection mirror (M1, M2, M3), a beam combining mirror (M4), a switchable beta-BBO sum frequency crystal (C1), a beta-BBO crystal (C2), a dispersion prism (P1) and a second ultraviolet plane high-reflection mirror (M1, M2, M3), a switchable double frequency crystal (C4) and a beta-BBO crystal (C2) which are, The band-pass filter (T6) and the photomultiplier (D6), the photomultiplier (D6) is connected with an oscilloscope; the beam combining mirror (M4) is located at the intersection point of the light path of the light to be detected and the light path of the probe light, the included angle between the light path of the light to be detected and the included angle between the light path of the probe light is 45 degrees, the included angle between the light path of the probe light is 0 degree, the central wavelength of the probe light is 800nm, the bandwidth is 10nm, the central wavelength of the light to be detected is 266nm, the bandwidth is 1.6nm, the thickness of the beta-BBO sum frequency crystal (C1) is at most 0.1mm, the cutting angle theta is 44.3 degrees, the thickness of the beta-BBO frequency doubling crystal (C2) is at least 0.5mm, and the cutting angle theta is 29.2 degrees.
2. The UV femtosecond seed laser pulse width measurement device of free electron laser according to claim 1, wherein the first and second UV plane high-reflection mirrors (M1, M2) are both mounted on the same high-precision electric displacement table (5).
3. Uv femtosecond seed laser pulse width measurement device for free electron laser according to claim 2, characterized in that the stroke of the motorized displacement stage (5) is larger than 200mm, the repetition accuracy is ± 0.05 μm, the accuracy is ± 1.7 μm, the bidirectional repetition accuracy is ± 0.25 μm, and the minimum displacement increment of the scan is 20 nm.
4. The uv femtosecond seed laser pulse width measurement device of a free electron laser according to claim 2, wherein the motorized displacement stage (5) comprises a guide rail mounted on the optical stage (100), a stage mounted on the guide rail through a screw-nut pair, and a servo motor for driving the screw-nut pair, the first and second uv plane high-reflection mirrors (M1, M2) being mounted on the stage.
5. The uv femtosecond seed laser pulse width measurement device of free electron laser according to claim 1, wherein the band-pass filter (T6) has a central wavelength of 400nm and a bandwidth of 20 nm.
6. The uv femtosecond seed laser pulse width measurement apparatus for free electron laser according to claim 1, wherein the polarization state of the probe light is set to be perpendicular to the polarization state of the light to be detected, the probe light is e-light, and the light to be detected is o-light.
7. The UV femtosecond seed laser pulse width measurement apparatus of a free electron laser according to claim 1, wherein the oscillator (1) is a Titania laser oscillator, the bandwidth of the probe light is 30nm, the pulse width is 50fs ± 10fs, the average power is 0.7W, the frequency is 79.33MHz, and the single pulse energy is 8.8 nJ.
8. The device for measuring the pulse width of the free electron laser in the femtosecond ultraviolet seed laser as claimed in claim 1, wherein a first diaphragm (I1) and a second diaphragm (I2) are disposed between the frequency tripler (3) and the first ultraviolet plane high-reflection mirror (M1), a third diaphragm (I3) and a fourth diaphragm (I4) are disposed between the second ultraviolet plane high-reflection mirror (M2) and the third ultraviolet plane high-reflection mirror (M3), and a fifth diaphragm (I5) is disposed between the dispersion prism (P1) and the band-pass filter (T6).
9. The uv femtosecond seed laser pulse width measurement device of free electron laser according to claim 1, characterized in that two or any multiple uv plane high-reflection mirrors are further provided between the oscillator (1) and the beam combining mirror.
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