XFROG-based high-precision mid-infrared ultra-short pulse electric field measuring device

Abstract

The application discloses a high-precision mid-infrared ultra-short pulse electric field measuring device based on XFROG and relates to the technical field of ultra-short pulse electric field measurement. The XFROG system and the frequency scheme can improve the power of the sum frequency pump light, can effectively perform frequency conversion on weak mid-infrared pulses, can convert more of the mid-infrared pulses to the visible-near infrared waveband, and can complete detection. Compared with the FROG scheme, the mid-infrared pulse that can be detected by the XFROG scheme can be weaker, and when strong mid-infrared pulses are detected, the XFROG scheme also has the advantages of improving the power of the pump light and improving the conversion efficiency. The more the pulse light converted to the visible-near infrared waveband, the closer the pulse information obtained by the XFROG scheme to the original pulse information.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of ultra-short pulse electric field measurement, and specifically relates to a high-precision mid-infrared ultra-short pulse electric field measurement device based on XFROG. BACKGROUND

[0002] Since the first laser was invented in 1960, laser technology has made great progress, and the pursuit of laser technology is to obtain shorter pulse width and higher intensity. Ultra-short pulses have important applications in industrial processing, medical treatment, ultra-fast phenomenon detection and the like, but the extremely short pulse width is far beyond the limit of traditional devices such as oscilloscopes.

[0003] In order to measure the pulse width of an ultra-short pulse, people have gradually proposed methods such as autocorrelation, frequency-resolved optical gating (FROG) and self-referencing spectral phase interferometry for direct electric field reconstruction (SPIDER).

[0004] A typical autocorrelator is shown in FIG. 1, which has a structure similar to that of a Michelson interferometer. First, the measured pulse light is divided into two beams, a controllable time delay is added to one of the two beams, and then the two beams are focused onto a frequency doubling crystal to obtain a frequency doubling signal. Then, the frequency doubling signal is received by a photomultiplier tube or a photodiode with an amplifier. Essentially, the autocorrelator changes the overlap of the two pulses by using time delay, and then the pulse width is inversely deduced by the intensity change of the frequency doubling signal. Figure 1 At present, FROG is a core technology for measuring ultra-short pulses, and different measurement methods and algorithms are derived therefrom. A typical FROG optical path is shown in FIG. 2. The basic principle of FROG is to use nonlinear effects as optical gating switches, measure the spectrum of the nonlinear signal under different time delays, and then use computer reconstruction algorithms to process the information, so as to finally obtain all the information of the input pulse, including the pulse waveform, amplitude and phase. The existing FROG is mainly used for near-infrared pulses, and there are few reports on mid-infrared frequency doubling. Moreover, the frequency doubling process cannot be used to detect weak mid-infrared signals.

[0005] Figure 2 Based on this, the present application provides a solution. SUMMARY

[0006] The present application aims to at least solve one of the technical problems existing in the prior art;

[0007] The present application aims to at least solve one of the technical problems existing in the prior art; ​

[0008] To this end, the application provides a high-precision mid-infrared ultra-short pulse electric field measuring device based on XFROG, comprising:

[0009] The fiber coupling head two FC1 is used for outputting the pulse light with a center wavelength of 1040 nm of the fiber laser to a spatial light path, and then the pulse light is divided into two beams through a polarization beam splitter PBS, the power of the two beams of pulse light is changed by adjusting a wave plate group before the PBS, one of the two beams is combined with continuous light with a center wavelength of 1560 nm through a dichroic mirror DM, the polarization of the two beams of light is adjusted by using a wave plate two HWP, and the lens L in front of the dichroic mirror DM is moved to focus the two beams of light into a PPLN crystal to generate pulse light with a center wavelength of 3120 nm;

[0010] The 3120 nm mid-infrared pulse changes the propagation direction through a mirror M, and then the first mid-infrared lens three L3 is used to avoid excessive divergence, the 3120 nm mid-infrared pulse is combined with the other 1040 nm pulse light divided by the PBS through a dichroic mirror one DM1, the optical path lengths of the two beams of light are adjusted to be equal, the lens two L2 in front of the dichroic mirror one DM1 is moved to focus the two beams of light into a crystal PPLN1, the temperature of the crystal PPLN1 is adjusted to the best matching temperature, and pulse light with a center wavelength of 780 nm is generated, after the pump light is filtered out, the pulse light generated by the frequency mixing is coupled into a fiber coupling head three FC2 through a spatial light path to connect a spectrometer, and the spectral data of the pulse light is analyzed;

[0011] The specific way of adjusting the crystal temperature to the best matching temperature is:

[0012] First, the best matching temperature range recommended when the crystal is designed is obtained, and the recommended range is marked;

[0013] Then, starting from the lowest temperature value in the recommended range, the real-time temperature is set, then the real-time temperature is determined in the form of increasing the set temperature value, and then the output power corresponding to each real-time temperature is obtained, and the real-time temperature with the maximum output power is marked as the best matching temperature; In this process, quasi-phase matching is involved, and the quasi-phase matching is specifically:

[0014] ω1-ω2=ω3

[0015] k1-k2=k3

[0016] The two formulas represent the energy and momentum conservation conditions required in the difference frequency process, and in some cases, the phase matching cannot be satisfied. At this time, quasi-phase matching can be used, that is, a periodic polarization is introduced into the crystal to introduce additional momentum, so that the nonlinear process that cannot be phase matched is realized, and the second formula becomes:

[0017] k1-k2+Δk=k3

[0018] Using the refractive index-wavelength curve of the LN crystal, the period of the crystal is designed to introduce additional momentum, and since the refractive index is also related to temperature, after the design is completed, the best matching temperature needs to be calculated for temperature control;

[0019] When using quasi-phase matching, since the planned period of the crystal does not strictly match the designed period, the crystal temperature needs to be adjusted to the actual best matching temperature during actual use, at which time the detected difference frequency light power is the highest.

[0020] Then the mirror group electric displacement table is controlled, the time delay of the 1040nm pulse light is adjusted, the spectral data corresponding to the time delay is recorded, the measured data is processed by writing a computer pulse iterative algorithm to realize the reconstruction of the waveform; when the error between the calculated intensity distribution and the measured result is lower than a preset error value.

[0021] Compared with the prior art, the beneficial effects of the present application are:

[0022] The existing autocorrelation scheme uses a frequency doubling process, and the frequency doubled signal is connected to the CCD for detection, only the approximate width of the pulse can be obtained, and the CCD in the 1.5 to 2.5 μm band is not as mature as the visible and near-infrared, and the direct connection of the pulse light to the CCD is also easy to cause damage to the CCD. The XFROG system can reconstruct the mid-infrared pulse waveform by using the collected spectral data, and the visible-near-infrared spectrometer is mature and easy to obtain;

[0023] The existing FROG scheme using frequency doubling technology can only convert the mid-infrared pulse (3 to 5 μm) to 1.5 to 2.5 μm, and the detection wavelength range of the spectrometer is still very high. The XFROG system uses a sum frequency scheme to convert the mid-infrared pulse to the visible-near-infrared band, which can be detected by the mature visible-near-infrared spectrometer;

[0024] Nonlinear optics is based on strong light, the existing FROG system needs to divide the pulse into two beams when detecting weak mid-infrared pulse, from the power point of view, at least one of the light power is less than or equal to half of the original, the frequency conversion efficiency will be very low, so the minimum detection power of the spectrometer has very high limit, in principle, the FROG system cannot detect particularly weak mid-infrared pulse. The sum frequency scheme of the XFROG system can increase the power of the sum frequency pump light, which can effectively convert weak mid-infrared pulse to visible-near infrared band, and complete the detection. Compared with the FROG scheme, we can detect weaker mid-infrared pulses, and when detecting strong mid-infrared pulses, we can also increase the pump light power and improve the conversion efficiency. The more pulse light converted to visible-near infrared, the closer the pulse information we get to the original pulse. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 It is a schematic diagram of a typical autocorrelator;

[0026] Figure 2 It is a schematic diagram of a typical FROG optical path;

[0027] Figure 3 It is a schematic diagram of the optical path of the application;

[0028] Figure 4 It is a flowchart of the pulse iteration algorithm of the application. DETAILED DESCRIPTION

[0029] The technical solutions of the application will be described clearly and completely in combination with the embodiments. Obviously, the described embodiments are only part of the embodiments of the application, not all. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the application.

[0030] 3-5 μm mid-infrared spectral region is the window of space air, which has strong penetration in dense fog, smoke and other environments; mid-infrared spectrum contains a large number of atomic and molecular absorption peaks, which also corresponds to many material electronic transition energy levels and quantum confined semiconductor band transition energy levels, which are all related to the characterization of ultra-short pulses;

[0031] XFROG is the full name of cross-correlation frequency-resolved optical gating, cross-correlation refers to measuring the pulse width of mid-infrared pulse light with another pulse light, rather than dividing one pulse light into two pulse lights for measurement as described in the FROG principle;

[0032] Now a common scheme to generate mid-infrared pulse is to use two beams to difference frequency, but the nonlinear process conversion efficiency is not high, the generated mid-infrared pulse is weak; if you want to use FROG technology to measure the pulse waveform of the mid-infrared pulse, the first problem is the lack of suitable frequency doubling crystal, secondly, the frequency doubling conversion efficiency is not high, the requirement for the detector is higher, and the wavelength of the mid-infrared pulse light is in 3 to 5 μm, the wavelength after frequency doubling is in 1.5 to 2.5 μm, the detector in this band is also relatively scarce; the FROG based on second harmonic SHG cannot retrieve the weak mid-infrared pulse, and the frequency mixing process can efficiently convert the signal light through strong pump light and detect through near-infrared band detector.

[0033] As Figures 3-4 , in order to realize the measurement of weak mid-infrared pulse signal, a measurement device based on XFROG is proposed, which specifically comprises:

[0034] The fiber coupling head two FC1, and the wave plate three QWP1, the wave plate four HWP1, and the polarization beam splitter PBS arranged in sequence on the side of the fiber coupling head two FC1 along the light path two;

[0035] The fiber coupling head two FC1 is used to output 1040nm pulse light, the wave plate three QWP1 is a quarter wave plate, and the wave plate four HWP1 is a half wave plate;

[0036] The wave plate five HWP2, the mirror one M1, and the lens one L1 are arranged in sequence on the side of the polarization beam splitter PBS along the light path three, and then combined with the dichroic mirror DM; the wave plate five HWP2 is a half wave plate;

[0037] It also includes a fiber coupling head one FC, and a wave plate one QWP, a wave plate two HWP, a lens L, a dichroic mirror DM, and a crystal PPLN arranged in sequence on the side of the fiber coupling head one FC along the light path one; the light path one and the light path three are combined through the dichroic mirror DM and the crystal PPLN, and the combined light path is sequentially provided with a filter Filter, a mirror M, a lens three L3, a mirror M2, a lens two L2, and a dichroic mirror one DM1;

[0038] The wave plate one QWP is a quarter wave plate, and the wave plate two HWP is a half wave plate;

[0039] The mirror group electric displacement table, the lens L4 are arranged in sequence on the other side of the polarization beam splitter PBS along the light path four, and then combined with the dichroic mirror one DM1; forming a combined light, and sequentially provided with a crystal one PPLN1, a filter one Filter1, and a fiber coupling head three FC2 along the light path of the combined light;

[0040] The mirror group electric displacement table includes two symmetrically arranged mirrors;

[0041] The fiber coupling head three FC2 is used for outputting 780nm pulsed light.

[0042] In specific operation, the fiber laser center wavelength of 1040nm pulsed light is output to the spatial light path by means of the fiber coupling head two FC1, is divided into two beams through the polarization beam splitter PBS, and we can change the power of the two pulsed light beams by adjusting the wave plate group before PBS, that is, the wave plate three QWP1 and the wave plate four HWP1. One of the two pulsed light beams is combined with the center wavelength of 1560nm continuous light, the 1560nm continuous light is output by the fiber coupling head one FC, and the dichroic mirror DM is used for combining the two beams. The polarization of the two beams is adjusted by the wave plate two HWP, and the lens L in front of the dichroic mirror DM is moved to focus the two beams into the PPLN crystal for difference frequency. The crystal temperature is adjusted to the best matching temperature to generate pulsed light with a center wavelength of 3120nm.

[0043] The specific way of adjusting the crystal temperature to the best matching temperature is as follows:

[0044] First, the best matching temperature range recommended when the crystal is designed is obtained, which is marked as the recommended range.

[0045] Then, starting from the lowest temperature value in the recommended range, set it as the real-time temperature. Then determine a real-time temperature in the form of increasing the set temperature value. Then, an output power corresponding to each real-time temperature is obtained. The real-time temperature with the maximum output power is marked as the best matching temperature. In this process, quasi-phase matching is involved, which is specifically as follows:

[0046] ω1-ω2=ω3

[0047] k1-k2=k3

[0048] The two formulas represent the energy and momentum conservation conditions that need to be met in the difference frequency process. In some cases, our phase matching cannot be satisfied. At this time, quasi-phase matching can be used, that is, a periodic polarization is introduced into the crystal to introduce additional momentum, so that the nonlinear process that cannot be phase matched is realized. The second formula becomes:

[0049] k1-k2+Δk=k3

[0050] The refractive index-wavelength curve of the LN crystal is used to design the crystal period to introduce additional momentum. Since the refractive index is also related to the temperature, after the design is completed, the best matching temperature needs to be calculated for temperature control.

[0051] When quasi-phase matching is used, since the planned period of the crystal does not strictly match the designed period, the crystal temperature needs to be adjusted to the actual best matching temperature in actual use, at which the detected difference frequency light power is the highest.

[0052] After filtering out the two pump beams and the frequency-doubled light of the two pump beams, the 3120 nm mid-infrared pulse changes the propagation direction through the mirror M, and is coupled into the fiber coupling head three FC2 through a spatial light path to couple into the spectrometer after avoiding excessive divergence through the first mid-infrared lens three L3, and being combined with another 1040 nm pulse light separated by the PBS through the dichroic mirror one DM1, adjusting the optical path of the two paths to be equal, and moving the lens two L2 in front of the dichroic mirror one DM1 to focus the two paths into the crystal PPLN1, adjusting the temperature of the crystal PPLN1 to the optimal matching temperature, generating a pulse light with a center wavelength of 780 nm, and after filtering out the pump light, the pulse light generated by the frequency mixing can be coupled into the fiber coupling head three FC2 through a spatial light path to couple into the spectrometer, and the spectral data is analyzed;

[0053] Then the electric displacement table is controlled to adjust the time delay of the 1040 nm pulse light, and the spectral data corresponding to the time delay is recorded, and the measured data is processed through the computer pulse iterative algorithm to realize the reconstruction of the waveform, and the specific mode is as follows:

[0054] The signal obtained by frequency mixing of the two beams can be understood as a multiplication operation:

[0055] E sig (t,τ)=E1(t)×E2(τ)

[0056] Where τ represents the delay time;

[0057] Then the mirror group electric displacement table is controlled to adjust the time delay of the 1040 nm pulse light, and the spectral data corresponding to the time delay is recorded, and the obtained spectral data is placed together, and the strongest place of the frequency-mixed wavelength light intensity obtained by the nonlinear process is defined as the point with zero time delay, and the trace diagram of this experiment is obtained. By analyzing the trace diagram, whether the entire pulse is measured can be observed;

[0058] At this time, we can use the pulse iterative algorithm to reconstruct the intensity and phase of the pulse. The principle of the retrieval method is to randomly generate an initial solution, then calculate the corresponding FROG diagram in the forward direction, and compare it with the experimental results to calculate the error size, then fine-tune the solution to find a solution with reduced error. Specifically, the amplitude and phase of the pulse are guessed by using the computer, so that the trace diagram obtained by the guessed solution and the experimental measurement results reach good consistency.

[0059] When the error between the calculated intensity distribution and the measured results is lower than an acceptable degree, it is considered that the pulse reconstruction is completed; the mid-infrared pulse measurement scheme based on XFROG can effectively detect the mid-infrared pulse waveform generated by the difference frequency process.

[0060] The above examples are only used to illustrate the technical method of the present application but not limit the present application. Although the present application is described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical method of the present application can be modified or equivalently replaced without departing from the spirit and scope of the technical method of the present application.

Claims

1. A high-precision mid-infrared ultrashort pulse electric field measurement device based on XFROG, characterized in that, include: The fiber coupler FC1 is used to output pulsed light with a center wavelength of 1040nm from the fiber laser into the spatial optical path. Then, it is split into two beams by the polarizing beam splitter PBS. The power of the two pulsed light beams is changed by adjusting the waveplate group in front of the PBS. One of the beams is combined with the continuous light with a center wavelength of 1560nm through the dichroic mirror DM. The polarization of the two beams is adjusted by the waveplate HIWP. The lenses L of the two beams in front of the dichroic mirror DM are moved to focus them into the PPLN crystal for difference frequency. The crystal temperature is adjusted to the optimal matching temperature to generate pulsed light with a center wavelength of 3120nm. After the 3120nm mid-infrared pulse changes its propagation direction by the reflector M, it is prevented from excessive divergence by the first mid-infrared lens L3. It is then combined with another 1040nm pulse beam separated from the PBS by the dichroic mirror DM1. The two optical paths are adjusted to be equal. The lenses L2 of the two optical paths in front of the dichroic mirror DM1 are moved to focus the light into the crystal PPLN1. The temperature of the crystal PPLN1 is adjusted to the optimal matching temperature to generate a pulse light with a center wavelength of 780nm. After filtering out the pump light, the pulse light generated by the sum-frequency is coupled into the optical fiber coupler FC2 through the spatial optical path to the spectrometer for analysis of its spectral data. Next, the electric displacement stage of the reflector group is operated to adjust the time delay of the 1040nm pulse light, and the spectral data corresponding to the time delay is recorded. The measured data is processed by a pulse iteration algorithm written in the computer to realize waveform reconstruction. When the error between the calculated intensity distribution and the measured result is lower than a preset error value, the waveform reconstruction is considered complete.

2. The high-precision mid-infrared ultrashort pulse electric field measurement device based on XFROG according to claim 1, characterized in that, The waveplate group includes waveplate three QWP1 and waveplate four HWP1.

3. The high-precision mid-infrared ultrashort pulse electric field measurement device based on XFROG according to claim 1, characterized in that, 1560nm continuous light is output from the fiber optic coupler FC.

4. The high-precision mid-infrared ultrashort pulse electric field measurement device based on XFROG according to claim 1, characterized in that, The 3120nm mid-infrared pulse is generated by filtering out two pump beams and the frequency-doubled light of the two pump beams from the 3120nm pulse light.

5. The high-precision mid-infrared ultrashort pulse electric field measurement device based on XFROG according to claim 1, characterized in that, The crystal is a periodically polarized lithium niobate crystal; The specific method for adjusting the crystal temperature to the optimal matching temperature is as follows: First, obtain the optimal matching temperature range recommended during crystal design and mark it as the recommended range; Then, starting from the lowest temperature value in the recommended range, set it as the real-time temperature; then determine a real-time temperature by increasing the set temperature value. For each real-time temperature generated, obtain the output power corresponding to each real-time temperature, and mark the real-time temperature with the highest output power as the optimal matching temperature.

6. The high-precision mid-infrared ultrashort pulse electric field measurement device based on XFROG according to claim 3, characterized in that, The fiber optic coupler FC1 generates two pulsed beams through the following structure, which includes: Fiber optic coupler FC1, and waveplate 3 QWP1, waveplate 4 HWP1, and polarizing beam splitter PBS are arranged sequentially along one side of fiber optic coupler FC1. The polarizing beam splitter PBS splits the beam into optical path 3 and optical path 4. Fiber optic coupler FC1 is used to output 1040nm pulsed light, waveplate 3 QWP1 is a quarter-wave plate, and waveplate 4 HWP1 is a half-wave plate.

7. The high-precision mid-infrared ultrashort pulse electric field measurement device based on XFROG according to claim 6, characterized in that, The generation of 3120nm pulsed light is achieved through the following structure, specifically including: Along the optical path three, a waveplate 5 HWP2, a mirror 1 M1, and a lens 1 L1 are arranged sequentially on one side of the polarizing beam splitter PBS, and then merge with the dichroic mirror DM; the waveplate 5 HWP2 is a half-waveplate.

8. A high-precision mid-infrared ultrashort pulse electric field measurement device based on XFROG according to claim 6, characterized in that, The specific structure of the fiber optic coupler-FC outputting 1560nm continuous light includes: Fiber optic coupler FC, and waveplate QWP, waveplate HWP, lens L, dichroic mirror DM, and crystal PPLN arranged sequentially along one side of fiber optic coupler FC; optical paths 1 and 3 are combined via dichroic mirror DM and crystal PPLN, and a filter Filter, mirror M, lens L3, mirror M2, lens L2, and dichroic mirror DM1 are arranged sequentially on the combined optical path; Waveplate 1 (QWP) is a quarter-wave plate, and waveplate 2 (HWP) is a half-wave plate.

9. A high-precision mid-infrared ultrashort pulse electric field measurement device based on XFROG according to claim 8, characterized in that, On the other side of the polarizing beam splitter PBS, along the optical path, there are a reflector group, an electric displacement stage, and a lens L4, which then merge with the dichroic mirror DM1 to form a combined beam. Along the optical path of the combined beam, there are a crystal PPLN1, a filter Filter1, and an optical fiber coupler FC2.

10. A high-precision mid-infrared ultrashort pulse electric field measurement device based on XFROG according to claim 9, characterized in that, The electric displacement stage for the reflector assembly includes two symmetrically arranged reflectors; The fiber optic coupler FC2 is used to output 780nm pulsed light.

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