High-precision intermediate infrared ultrashort pulse electric field measuring device based on XFROG

Abstract

The invention discloses a high-precision mid-infrared ultrashort pulse electric field measuring device based on an XFROG, and relates to the technical field of ultrashort pulse electric field measurement, the sum frequency scheme of an XFROG system can improve the power of sum frequency pump light, and can effectively perform frequency conversion on weak mid-infrared pulses and convert more weak mid-infrared pulses to a visible-near infrared band to complete detection. Compared with an FROG scheme, the method has the advantages that the detected intermediate infrared pulse can be weaker, the pump light power can be improved and the conversion efficiency can be improved when the strong intermediate infrared pulse is detected, and the more the visible-near infrared pulse light is converted, the more the obtained pulse information is closer to the pulse information.

Description

Technical Field

[0001] The invention belongs to the technical field, and in particular is a high-precision mid-infrared ultrashort pulse electric field measurement device based on XFROG. Background Art

[0002] Since the advent of the first laser in 1960, laser technology has achieved a huge leap forward, among which pulsed lasers with shorter time width and higher intensity are the goals that laser technology is constantly pursuing. Ultrashort pulses have important applications in industrial processing, medical treatment, and ultrafast phenomenon detection, but their extremely short pulse width far exceeds the limits of traditional equipment such as oscilloscopes.

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

[0004] Autocorrelation methods such as Figure 1 The schematic diagram of a typical autocorrelator is shown in Figure 1. Its structure is similar to that of a Michelson interferometer. First, the measured pulse light is divided into two beams, and a controllable time delay is added to one of the beams. Then, the two beams are combined and focused on a frequency-doubling crystal to obtain a frequency-doubling signal. Then, a photomultiplier tube or a photodiode with an amplifier is used to receive the frequency-doubling signal. In essence, it uses the time delay to change the overlap of the two pulses, so as to infer the pulse width through the intensity change of the frequency-doubling signal.

[0005] Currently, FROG is a core technology for measuring ultrashort pulses, and different measurement methods and algorithms have been derived from it. A typical FROG optical path is as follows: Figure 2 As shown in the figure, the basic principle of FROG is to use nonlinear effects as optical gate switches, measure the spectrum of nonlinear signals under different delays, and then use computer reconstruction algorithms to process the information, and finally obtain all the information of the input pulse, including pulse waveform, amplitude and phase. The existing FROG is mainly aimed at near-infrared pulses, and there are few reports on mid-infrared frequency doubling. In addition, the frequency doubling process cannot detect weak mid-infrared signals.

[0006] Based on this, the present application provides a solution. Summary of the invention

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

[0008] To this end, the present invention proposes a high-precision mid-infrared ultrashort pulse electric field measurement device based on XFROG, comprising:

[0009] Fiber coupling head FC1 is used to output the pulse light with a central wavelength of 1040nm from the fiber laser to the spatial optical path, and then split it into two beams through the polarization beam splitter PBS. The power of the two pulse light beams is changed by adjusting the wave plate group in front of the PBS. One of the beams is combined with the continuous light with a central wavelength of 1560nm through the dichroic mirror DM. The polarization of the two beams is adjusted by using the wave plate HWP. The lens L of the two beams in front of the dichroic mirror DM is moved to focus them into the PPLN crystal for difference frequency. The crystal temperature is adjusted to the best matching temperature to generate pulse light with a central wavelength of 3120nm.

[0010] After the 3120nm mid-infrared pulse changes its propagation direction through the reflector M, it passes through the first mid-infrared lens L3 to avoid excessive divergence, and is combined with another 1040nm pulse light separated from the PBS through the dichroic mirror DM1, and the two optical paths are adjusted to be equal. The lens L2 of the two lights in front of the dichroic mirror DM1 is moved to focus them into the crystal PPLN1, and the temperature of the crystal PPLN1 is adjusted to the best matching temperature to generate a pulse light with a central wavelength of 780nm. After filtering out the pump light, the pulse light generated by the sum frequency is coupled into the fiber coupling head FC2 through the spatial optical path to connect to the spectrometer, and its spectral data is analyzed;

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

[0012] First, obtain the best matching temperature range recommended during crystal design and mark it as the recommended range;

[0013] 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 manner of increasing the set temperature value, and obtain the output power corresponding to each real-time temperature each time a real-time temperature is generated, and mark the real-time temperature with the largest output power as the best matching temperature; this process involves quasi-phase matching, which is specifically:

[0014] ω1-ω2=ω3

[0015] k1-k2=k3

[0016] These two equations represent the energy and momentum conservation conditions that need to be satisfied in the frequency difference process. In some cases, our phase matching cannot be satisfied. In this case, quasi-phase matching can be used, that is, periodic polarization is introduced inside the crystal, and additional momentum is introduced to realize the nonlinear process that was originally unable to phase match. The second equation above becomes:

[0017] k1-k2+Δk=k3

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

[0019] When using quasi-phase matching, since the planned period of the crystal is not strictly consistent with the designed period, the crystal temperature needs to be adjusted to the actual optimal matching temperature in actual use, at which time the detected difference frequency optical power is the highest.

[0020] Then, the electric translation stage of the reflector group is operated to adjust the delay of the 1040nm pulse light, and the spectral data corresponding to the delay is recorded. The waveform is reconstructed by processing the measured data through the pulse iteration algorithm written in the computer; 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 present invention has the following beneficial effects:

[0022] The existing autocorrelation scheme uses a frequency doubling process, and the frequency doubling signal is connected to the CCD for detection, which can only obtain the approximate width of the pulse. The mid-infrared pulse (3 to 5μm) is 1.5 to 2.5μm after frequency doubling. The CCD in this band is not as mature as the visible and near-infrared, and the direct connection of pulse light to the CCD is also likely to cause damage to the CCD. However, the XFROG system can iteratively reconstruct the waveform of the mid-infrared pulse light using the collected spectral data, and the spectrometer in the visible-near-infrared band is mature and easy to obtain;

[0023] The frequency doubling technology used in the existing FROG solution can only convert mid-infrared pulses (3 to 5 μm) to 1.5 to 2.5 μm, and the detection wavelength range of the spectrometer is still very high. The sum frequency solution used in this XFROG system can convert mid-infrared pulses to the visible-near infrared band, which can be detected using the current 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 pulses. From the perspective of power, the power of at least one beam of light is less than or equal to half of the original one, and the frequency doubling conversion efficiency will be very low. Therefore, there is an extremely high limit on the minimum detection power of the spectrometer. In principle, the FROG system cannot detect particularly weak mid-infrared pulses. The sum frequency scheme of this XFROG system can increase the power of the sum frequency pump light, and can effectively perform frequency conversion on weak mid-infrared pulses, converting more of them into the visible-near infrared band to complete detection. Compared with the FROG scheme, the mid-infrared pulses we can detect can be weaker, 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 the visible-near infrared, the closer the pulse information we get is to the pulse itself. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0026] Figure 2 This is a typical FROG optical path diagram;

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

[0028] Figure 4 It is a flow chart of the pulse iteration algorithm of the present invention. DETAILED DESCRIPTION

[0029] The technical solution of the present invention will be clearly and completely described below in conjunction with the embodiments. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0030] The 3-5μm mid-infrared spectrum is a window to the air in space, and has strong penetrating power in environments such as dense fog and smog. The mid-infrared spectrum contains a large number of atomic and molecular absorption peaks, and also corresponds to the electronic transition energy levels of many materials and the interband transition energy levels of quantum confined semiconductors. These applications are inseparable from the characterization of ultrashort pulses.

[0031] The full name of XFROG is cross-correlation frequency-resolved optical switching. Cross-correlation refers to measuring the pulse width of a mid-infrared pulse light with another pulse light, rather than measuring by splitting a pulse light into two pulse light beams as described in the FROG principle.

[0032] A common scheme for generating mid-infrared pulses now is to use two beams of light for difference frequency, but the conversion efficiency of this nonlinear process is not high, and the generated mid-infrared pulses are weak. If one wants to use FROG technology to measure the pulse waveform of the mid-infrared pulse, the first problem faced is the lack of suitable frequency doubling crystals, and the second is the low frequency doubling conversion efficiency, which places high demands on the detector. In addition, the wavelength of mid-infrared pulse light is between 3 and 5μm, and the wavelength after frequency doubling is between 1.5 and 2.5μm, and detectors in this band are also relatively scarce. Usually, FROG based on second harmonic SHG cannot retrieve weaker mid-infrared pulses, while the sum frequency process can efficiently convert signal light through strong pump light and detect it through near-infrared band detectors.

[0033] like Figure 3-Figure 4 In order to realize the measurement of weak mid-infrared pulse signals, we proposed a measurement device based on XFROG, which specifically includes:

[0034] The second optical fiber coupling head FC1, and the third wave plate QWP1, the fourth wave plate HWP1, and the polarization beam splitter PBS are sequentially arranged on one side of the second optical fiber coupling head FC1 along the second optical path, and the polarization beam splitter PBS splits the light beam into the third optical path and the fourth optical path;

[0035] Fiber coupling head FC1 is used to output 1040nm pulse light, wave plate QWP1 is a quarter wave plate, and wave plate HWP1 is a half wave plate;

[0036] On one side of the polarization beam splitter PBS, wave plate five HWP2, reflector one M1, lens one L1 are sequentially arranged along the optical path three, and then merged with the dichroic mirror DM; wave plate five HWP2 is a half wave plate;

[0037] It also includes a fiber coupling head FC, and a wave plate QWP, a wave plate HWP, a lens L, a dichroic mirror DM, and a crystal PPLN which are sequentially arranged on one side of the fiber coupling head FC along the optical path 1; the optical path 1 and the optical path 3 are combined via the dichroic mirror DM and the crystal PPLN, and the optical path after the combination is sequentially provided with a filter Filter, a reflector M, a lens L3, a reflector M2, a lens L2, and a dichroic mirror DM1;

[0038] Wave plate one QWP is a quarter wave plate, wave plate two HWP is a half wave plate;

[0039] On the other side of the polarization beam splitter PBS, a reflector group electric translation stage and a lens L4 are sequentially arranged along the optical path 4, and then merged with a dichroic mirror DM1 to form a combined light beam. A crystal PPLN1, a filter Filter1 and a fiber coupling head FC2 are sequentially arranged along the optical path of the combined light beam.

[0040] The reflector group electric translation stage includes two symmetrically arranged reflectors;

[0041] The fiber-coupled head FC2 is used to output 780nm pulse light.

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

[0043] The specific method of adjusting the crystal temperature to the best matching temperature is:

[0044] First, obtain the best matching temperature range recommended during crystal design and mark it 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 manner of increasing the set temperature value, and obtain the output power corresponding to each real-time temperature each time a real-time temperature is generated, and mark the real-time temperature with the largest output power as the best matching temperature; this process involves quasi-phase matching, which is specifically:

[0046] ω1-ω2=ω3

[0047] k1-k2=k3

[0048] These two equations represent the energy and momentum conservation conditions that need to be satisfied in the frequency difference process. In some cases, our phase matching cannot be satisfied. In this case, quasi-phase matching can be used, that is, periodic polarization is introduced inside the crystal, and additional momentum is introduced to realize the nonlinear process that was originally unable to phase match. The second equation above becomes:

[0049] k1-k2+Δk=k3

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

[0051] When using quasi-phase matching, since the planned period of the crystal is not strictly consistent with the designed period, the crystal temperature needs to be adjusted to the actual optimal matching temperature in actual use, at which time the detected difference frequency optical power is the highest.

[0052] After filtering out the two pump lights and the doubled frequency lights of the two pump lights, the 3120nm mid-infrared pulse changes its propagation direction through the reflector M, and then uses the first mid-infrared lens L3 to avoid excessive divergence, and combines it with another 1040nm pulse light separated from the PBS through the dichroic mirror DM1, adjusts the two optical paths to be equal, moves the lens L2 of the two lights in front of the dichroic mirror DM1 to focus them into the crystal PPLN1, adjusts the temperature of the crystal PPLN1 to the best matching temperature, and generates a pulse light with a central wavelength of 780nm. After filtering out the pump light, the pulse light generated by the sum frequency can be coupled into the optical fiber coupling head FC2 through the spatial optical path to connect to the spectrometer, and its spectral data is analyzed;

[0053] Then, the electric translation stage is operated to adjust the time delay of the 1040nm pulse light, and the spectral data corresponding to the time delay is recorded. The waveform is reconstructed by programming a pulse iteration algorithm on the computer to process the measured data. The specific method is as follows:

[0054] The signal obtained by adding two beams of light 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, we manipulated the electric translation stage of the reflector group to adjust the time delay of the 1040nm pulse light, recorded the spectral data corresponding to the time delay, put the obtained spectral data together, and defined the strongest point of the sum frequency wavelength light intensity obtained by the nonlinear process as the point where the time delay is zero, and obtained the trace diagram of this experiment. By analyzing the trace diagram, we can observe whether we have measured the entire pulse;

[0058] At this point, we can use the pulse iteration algorithm to reconstruct the intensity and phase of the pulse. The principle of the retrieval method is to randomly generate an initial solution, then forward calculate the corresponding FROG diagram, compare it with the experimental results, calculate the error size, and then fine-tune this solution to find a solution with reduced error. Specifically, the computer is used to guess the amplitude and phase of the pulse, so that the trajectory diagram obtained by the guessed solution is consistent with the experimental measurement results.

[0059] When the error between the calculated intensity distribution and the measured result is below an acceptable level, the pulse reconstruction can be considered complete; this XFROG-based mid-infrared pulse measurement scheme can effectively detect the mid-infrared pulse waveform generated by the difference frequency process.

[0060] The above embodiments are only used to illustrate the technical method of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical method of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical method of the present invention.

Claims

1. A high-precision mid-infrared ultrashort pulse electric field measurement device based on XFROG, characterized in that: include: Fiber coupling head FC1 is used to output the pulse light with a central wavelength of 1040nm from the fiber laser to the spatial optical path, and then split it into two beams through the polarization beam splitter PBS. The power of the two pulse light beams is changed by adjusting the wave plate group in front of the PBS. One of the beams is combined with the continuous light with a central wavelength of 1560nm through the dichroic mirror DM. The polarization of the two beams is adjusted by using the wave plate HWP. The lens L of the two beams in front of the dichroic mirror DM is moved to focus them into the PPLN crystal for difference frequency. The crystal temperature is adjusted to the best matching temperature to generate pulse light with a central wavelength of 3120nm. After the 3120nm mid-infrared pulse changes its propagation direction through the reflector M, it passes through the first mid-infrared lens L3 to avoid excessive divergence, and is combined with another 1040nm pulse light separated from the PBS through the dichroic mirror DM1, and the two optical paths are adjusted to be equal. The lens L2 of the two lights in front of the dichroic mirror DM1 is moved to focus them into the crystal PPLN1, and the temperature of the crystal PPLN1 is adjusted to the best matching temperature to generate a pulse light with a central wavelength of 780nm. After filtering out the pump light, the pulse light generated by the sum frequency is coupled into the fiber coupling head FC2 through the spatial optical path to connect to the spectrometer, and its spectral data is analyzed; Then, the electric translation stage of the reflector group is operated to adjust the delay of the 1040nm pulse light, and the spectral data corresponding to the delay is recorded. The waveform is reconstructed by processing the measured data through the pulse iteration algorithm written in the computer; when the error between the calculated intensity distribution and the measured result is lower than a preset error value.

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

3. The high-precision mid-infrared ultrashort pulse electric field measurement device based on XFROG according to claim 1, characterized in that: The 1560nm continuous light is output by the fiber coupling head 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 lights and the doubled frequency lights of the two pump lights 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 poled lithium niobate crystal; The specific method of adjusting the crystal temperature to the best matching temperature is: First, obtain the best matching temperature range recommended during crystal design and mark it as the recommended range; Then start from the lowest temperature value in the recommended range and set it as the real-time temperature; then determine a real-time temperature in the manner of increasing the set temperature value, and each time a real-time temperature is generated, obtain the output power corresponding to each real-time temperature, and mark the real-time temperature with the largest output power as the best 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 coupling head FC1 generates two pulsed lights, which is specifically realized by the following structure, which includes: A second optical fiber coupling head FC1, and a third wave plate QWP1, a fourth wave plate HWP1, and a polarization beam splitter PBS are sequentially arranged on one side of the second optical fiber coupling head FC1 along the second optical path. The polarization beam splitter PBS splits the light beam into the third optical path and the fourth optical path. The fiber coupling head FC1 is used to output 1040nm pulse light, the wave plate QWP1 is a quarter wave plate, and the wave plate 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 by the following structure, specifically including: On one side of the polarization beam splitter PBS, a wave plate five HWP2, a reflector one M1, and a lens one L1 are sequentially arranged along the optical path three, and then merged with the dichroic mirror DM; the wave plate five HWP2 is a half wave plate.

8. The 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-coupled head FC outputting 1560nm continuous light includes: A fiber coupling head FC, and wave plate QWP, wave plate HWP, lens L, dichroic mirror DM, and crystal PPLN are sequentially arranged on one side of the fiber coupling head FC along the optical path 1; the optical path 1 and the optical path 3 are combined via the dichroic mirror DM and the crystal PPLN, and the optical path after the combination is sequentially provided with a filter Filter, a reflector M, a lens L3, a reflector M2, a lens L2, and a dichroic mirror DM1; Wave plate one QWP is a quarter wave plate, and wave plate two HWP is a half wave plate.

9. The 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 polarization beam splitter PBS, a reflector group electric translation stage and a lens L4 are sequentially arranged along the optical path 4, and then merged with a dichroic mirror DM1 to form a combined beam. A crystal PPLN1, a filter Filter1 and a fiber coupling head FC2 are sequentially arranged along the optical path of the combined beam.

10. The high-precision mid-infrared ultrashort pulse electric field measurement device based on XFROG according to claim 9, characterized in that: The reflector group electric translation stage includes two symmetrically arranged reflectors; The fiber-coupled head FC2 is used to output 780nm pulse light.

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