CN114200215B - Real-time autocorrelator based on radio frequency spectrum conversion and waveform measuring method - Google Patents
Real-time autocorrelator based on radio frequency spectrum conversion and waveform measuring method Download PDFInfo
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Abstract
The invention discloses a real-time autocorrelator based on radio frequency spectrum conversion and a waveform measuring method, and belongs to the field of time domain pulse waveform measurement. The method comprises the following steps: s1, respectively outputting pulse laser and continuous laser as a signal to be detected and pump light, and coupling the signal to be detected and the pump light to form a beam of light; s2, performing cross phase modulation on the coupled light beam to load radio frequency information of the signal to be measured on the spectrum of the pump light to obtain a radio frequency spectrum of the signal to be measured; s3, performing dispersion stretching on the radio frequency spectrum of the signal to be measured to form a radio frequency spectrum mapped to a time domain; and S4, performing photoelectric conversion on the radio frequency spectrum mapped to the time domain to obtain a radio frequency signal, and performing inverse Fourier transform on the radio frequency signal to obtain a pulse autocorrelation waveform which changes in real time. In summary, the present invention can improve the measurement rate of the real-time autocorrelation waveform and improve the observation window of the autocorrelation waveform.
Description
Technical Field
The invention belongs to the field of time domain pulse waveform measurement, and particularly relates to a real-time autocorrelator based on radio frequency spectrum conversion and a waveform measurement method.
Background
The autocorrelator as an instrument for representing the time domain autocorrelation function has the advantages of high resolution, high sensitivity, convenient use and the like, and can measure the ultra-narrow pulse width of ps and fs magnitude.
The most commonly used autocorrelator is based on the michelson interference principle, a pulse to be measured is divided into two paths by a light splitting prism, one path of signal is scanned by a one-dimensional directional reflector, so that time delay is brought, then two beams of light with time delay difference pass through a nonlinear crystal to generate an autocorrelation signal, the time measurement of the pulse is converted into intensity measurement, and an autocorrelation image is obtained. The interferometric autocorrelator is divided into multiple measurements and single measurement according to different measurement modes. The multiple measurements, in which one of the laser beams after splitting is scanned back and forth over the mirrors of a stepper motor, result in an autocorrelation waveform representing a time delay, are limited by the rate of mechanical movement, with refresh times typically on the order of s (Kikuchi, K. "high sensitive interferometric auto-correlator using Si avalanche photodiode as two-photon absorber." Electronics Letters 34.1 (1998): 123-125.). The single measurement method avoids errors caused by a stepping motor in multiple measurement processes, and improves the measurement speed to Hz level (Raghuramaiah, M., et al. "A second-order auto-resolver for single-shot measurement of interferometric laser pulse duration." Sadhana 26.6 (2001): 603-611.). The interferometry is simple to operate, but the measurement rate is limited by the mechanical scanning rate, and pulse signals can be observed in real time in the Hz magnitude. Spectral coherent electric field reconstruction (SPIDER) the measured autocorrelation waveform is obtained by mixing two pulses of a beam with a chirped pulse in a Nonlinear crystal, the chirped pulses being coherent in a spectrometer, followed by Fourier transformation of the curve (Tawfik, walid. "precision measurement of an ultra laser using a spectral phase interference for direct-field reception." Journal of Nonlinear Optical Physics & Materials 24.04 (2015): 0041550.). The coherent electric field reconstruction method has high sensitivity and more accurate measurement, but has a complex structure and complicated operation, and the measurement rate is limited by the measurement rate of the spectral measurement device, usually several Hz.
Besides the measurement method of the autocorrelation waveform, the autocorrelation waveform can be obtained by performing inverse Fourier transform on a radio frequency spectrum, the measurement precision of the autocorrelation waveform is effectively improved due to the large radio frequency measurement bandwidth, and the measurement resolution of the radio frequency spectrum determines the observation window of the autocorrelation waveform. The traditional radio frequency measurement method is based on a spectrometer (ESA) and a Photodetector (PD), but is limited by the detection bandwidth of the spectrometer and the detector, the measurement range of a radio frequency spectrum does not exceed 100GHz, and the observation window of an autocorrelation waveform is limited.
In addition to the electrical measurement method, the optical measurement method loads the intensity of the radio frequency information to be measured on the phase of the pump light by means of a nonlinear medium, breaks through the limitation of the electrical bandwidth, and the measurement bandwidth can reach above THz (Dorrer, christoph, and d.n. maywar. "" RF spectrum analysis of optical signal using nonlinear optics. "" Journal of bright wave technology 22.1 (2004): 266.), but the measurement rate is still limited by the spectral measurement device. The rate of measurement of the autocorrelation waveform is limited by the speed-limited radio frequency spectrum measurement system, which can only reach hundreds of Hz.
Disclosure of Invention
Aiming at the defects and improvement requirements of the prior art, the invention provides a real-time autocorrelation instrument based on radio frequency spectrum conversion and a waveform measuring method, and aims to improve the measuring rate of real-time autocorrelation waveforms and improve the observation window of the autocorrelation waveforms.
To achieve the above object, according to an aspect of the present invention, there is provided a real-time autocorrelation waveform measuring method based on radio frequency spectrum conversion, comprising the steps of:
s1, respectively outputting pulse laser and continuous laser as a signal to be detected and pump light, and coupling the signal to be detected and the pump light to form a beam of light;
s2, performing cross phase modulation on the coupled light beam, and loading radio frequency information of the signal to be detected onto the spectrum of the pump light to obtain a radio frequency spectrum of the signal to be detected;
s3, performing dispersion stretching on the radio frequency spectrum of the signal to be measured to form a radio frequency spectrum mapped to a time domain;
and S4, performing photoelectric conversion on the radio frequency spectrum mapped to the time domain to obtain a radio frequency signal, and performing inverse Fourier transform on the radio frequency signal to obtain a pulse autocorrelation waveform which changes in real time.
Further, in step S1, before coupling the signal to be measured and the pump light into a beam of light, the method further includes adjusting the polarization states of the signal to be measured and the pump light to keep the polarization states of the signal to be measured and the pump light consistent.
Further, in step S1, before coupling the signal to be measured and the pump light into a beam of light, power amplification is performed on the output signal to be measured and the output pump light.
Further, in step S3, before performing dispersion stretching on the radio frequency spectrum of the signal to be measured, filtering the radio frequency spectrum of the signal to be measured is further included.
According to another aspect of the present invention, there is provided a real-time autocorrelation apparatus based on radio frequency spectrum conversion, comprising:
the pulse light source is used for outputting pulse laser as a signal to be detected;
a continuous pumping light source for outputting continuous pumping light;
the optical coupler is used for coupling the signal to be detected and the pump light to synthesize a beam of light;
the nonlinear medium is used for performing cross phase modulation on the coupled beam combination light to load the radio frequency information of the signal to be measured on the spectrum of the pump light to obtain the radio frequency spectrum of the signal to be measured;
the large dispersion unit is used for performing dispersion stretching on the radio frequency spectrum of the signal to be measured to form a radio frequency spectrum mapped to a time domain;
the photoelectric detector is used for performing photoelectric conversion on the radio frequency spectrum of the time domain to obtain a radio frequency signal;
and the high-speed digital-to-analog converter is used for recording the radio-frequency signals mapped to the time domain and performing inverse Fourier transform on the radio-frequency signals to obtain the pulse autocorrelation waveform which changes in real time.
Further, the large dispersion unit is a chirped bragg grating or consists of a dispersion compensation fiber and a large effective area fiber.
Further, the nonlinear medium is a highly nonlinear optical fiber or a nonlinear waveguide.
Further, still include: the polarization controller comprises a first polarization controller and a second polarization controller, wherein the first polarization controller is positioned between the pulse light source and the optical coupler, and the second polarization controller is positioned between the continuous pumping light source and the optical coupler.
Further, the method also comprises the following steps: the first amplifier is positioned between the pulse light source and the optical coupler, and the second amplifier is positioned between the continuous pumping light source and the optical coupler.
Further, still include: a first filter positioned between the pulsed light source and the optical coupler; and/or a second filter located between the nonlinear medium and the large dispersion unit.
Generally, by the above technical solution conceived by the present invention, the following beneficial effects can be obtained:
(1) The invention relates to a real-time autocorrelation instrument and a method based on radio frequency spectrum conversion, which are characterized in that a radio frequency spectrum of a signal to be measured is obtained by performing cross phase modulation on the pulse signal to be measured loaded on a continuous optical spectrum, the signal to be measured is mapped to a time domain through dispersion stretching, and a real-time pulse autocorrelation waveform is obtained after inverse Fourier transform, namely the real-time autocorrelation waveform is obtained through rapid radio frequency spectrum measurement, and the measurement rate of the obtained autocorrelation waveform is consistent with the pulse repetition frequency of the signal to be measured. The measurement rate of the autocorrelation waveform is improved by setting the high pulse repetition frequency of the signal to be measured; the characteristic of a large observation window of an autocorrelation waveform can be realized by setting a high measurement bandwidth of the photoelectric detector and the high-speed digital-to-analog converter.
(2) Preferably, the original pump light and the signal to be detected can be filtered out by filtering and denoising the radio frequency spectrum of the signal to be detected, only the pure radio frequency spectrum of the signal to be detected is left, and the quality of the obtained autocorrelation waveform is improved.
(3) Preferably, the subsequent nonlinear modulation can work in an optimal state by adjusting and adjusting the polarization states of the signal to be measured and the pump light to keep the polarization states of the signal to be measured and the pump light consistent.
In summary, the real-time autocorrelation instrument and method based on radio frequency spectrum conversion of the present invention improve the measurement rate of the autocorrelation waveform on the basis of improving the observation window of the autocorrelation waveform; and the traditional autocorrelation device is simplified, and the function of recording the pulse width change in real time is realized, so that the short plate of the prior art in the aspect of measuring the speed is made up, and the measuring speed of the autocorrelation waveform is improved.
Drawings
Fig. 1 is a schematic structural diagram of a real-time autocorrelator according to the present invention.
Fig. 2 is a time domain diagram of 6 periods of the signal pulse to be measured in embodiment 1.
Fig. 3 is a dual soliton with a time interval of 600ps in example 1.
Fig. 4 is a theoretical autocorrelation waveform obtained by directly performing autocorrelation operation on the time domain spectrum of the dual solitons in embodiment 1.
Fig. 5 shows the coupled pump light and the filtered signal to be measured in example 1.
Fig. 6 is a frequency domain radio frequency spectrum after cross-phase modulation in example 1.
Fig. 7 is an autocorrelation waveform converted from a frequency domain radio spectrum in embodiment 1.
Fig. 8 is a time domain radio frequency spectrum obtained after intermediate frequency time mapping in embodiment 1.
FIG. 9 is a time domain radio frequency spectrum and a detail view of a single frame in embodiment 1
Fig. 10 shows the converted dual soliton autocorrelation waveform and single frame resolution in example 1.
FIG. 11 is a graph of the observation window characterization in example 1, which can be as high as 600ps.
FIG. 12 is a resolution characterization chart of example 1, which can be as high as 300fs.
Fig. 13 is a time domain diagram of a single-frame three soliton with unequal spacing in embodiment 2.
Fig. 14 is an autocorrelation waveform obtained by directly performing autocorrelation operation on the three solitons in embodiment 2.
Fig. 15 is an autocorrelation waveform of a three-soliton in radio frequency spectrum inversion measured by the system in example 2.
The same reference numbers will be used throughout the drawings to refer to the same or like elements or structures, wherein:
the device comprises a pulse light source 1, a first filter 2, a first amplifier 3, a first polarization controller 4, a continuous pumping light source 5, a second amplifier 6, a second polarization controller 7, an optical coupler 8, a nonlinear medium 9, a second filter 10, a large dispersion unit 11, a photoelectric detector 12 and a high-speed digital-to-analog converter 13.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and do not limit the invention. In addition, the technical features involved in the embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.
In the present invention, the terms "first", "second", and the like in the description and the drawings are used for distinguishing similar objects, and are not necessarily used for describing a particular order or sequence.
As shown in fig. 1, the present invention provides a radio frequency spectrum-based autocorrelation waveform measuring apparatus, which obtains a real-time autocorrelation waveform by rapidly measuring a radio frequency spectrum of a pulse source with a narrow pulse width, a high repetition frequency, and a rapid change, and has a continuous characterization capability, and the obtained autocorrelation apparatus has characteristics of a high frame rate, a high resolution, and a large measurement window. The method comprises the following steps: the device comprises a pulse light source 1, a continuous pumping light source 5, an optical coupler 8, a nonlinear medium 9, a large dispersion unit 11, a photoelectric detector 12 and a high-speed digital-to-analog converter 13.
The pulse light source 1 is used for outputting pulse laser as a signal to be detected of the system to be characterized; specifically, the optical frequency comb is used in this embodiment.
A continuous pumping light source 5 for pumping light for performing a nonlinear process.
And the optical coupler 8 is used for coupling the signal to be measured and the pump light to synthesize a beam of light, so that the beam of light enters the nonlinear medium 9 together.
And the nonlinear medium 9 is used for performing cross phase modulation, and loading the radio frequency information of the signal to be measured on the spectrum of the pump light to obtain the radio frequency spectrum of the signal to be measured. The measurement bandwidth of the radio frequency spectrum is related to the nonlinear coefficient of the nonlinear medium, and the larger the nonlinear coefficient is, the larger the measurement bandwidth of the radio frequency spectrum is. The nonlinear medium may be a highly nonlinear optical fiber or a nonlinear waveguide, and in this embodiment, the nonlinear medium is a nonlinear waveguide.
A large dispersion unit 11, configured to perform dispersion stretching (frequency-time mapping) on a radio frequency spectrum of a signal to be measured to form a radio frequency spectrum mapped to a time domain; specifically, the large dispersion unit may be a chirped bragg grating or a large dispersion unit composed of a dispersion compensation fiber DCF and a large effective area fiber LEAF.
And the photoelectric detector 12 is used for photoelectrically converting the radio frequency spectrum of the time domain to obtain a radio frequency signal.
And the high-speed digital-to-analog converter 13 is used for recording the radio-frequency signals mapped to the time domain in real time and performing inverse Fourier transform on the radio-frequency signals to obtain the pulse autocorrelation waveform which changes in real time. In particular, the high speed digital-to-analog converter 13 may be a real time oscilloscope.
Preferably, a first filter 2 may be further designed between the pulsed light source and the optical coupler for filtering the signal to be measured and filtering out an effective spectral width.
Preferably, a first amplifier 3 and a second amplifier 6 can be respectively designed between the pulse light source and the optical coupler and between the continuous pumping light source and the optical coupler, and are respectively used for amplifying the power of the signal to be detected and the pumping light, so that the power requirement of the subsequent nonlinear process on the required power is met; specifically, the first amplifier 3 and the second amplifier 6 may be erbium-doped fiber amplifiers, semiconductor optical amplifiers, or raman amplifiers. Meanwhile, the first filter can also be used for filtering out an effective amplification range, namely filtering out a spectrum width in the amplification range of the amplifier.
Preferably, a first polarization controller 4 and a second polarization controller 7 may be respectively disposed between the pulse light source and the optical coupler and between the continuous pumping light source and the optical coupler, and are respectively used to ensure that the polarization states of the signal to be measured and the pumping light are consistent, so as to obtain a better nonlinear state; .
Preferably, a second filter 10 may be further disposed between the nonlinear medium and the large dispersion unit, for filtering out the original pump light and the signal to be measured, and only leaving the radio frequency spectrum of the signal to be measured;
the main devices are all optical fiber devices.
During working, the pulse light source 1 sends out a pulse laser signal with the pulse width of fs-ps magnitude as a signal to be measured, an effective measurement range is filtered out through the first filter 2, and power amplification is carried out through the first amplifier 3.
The continuous pumping light source 5 emits continuous light, which is used as pumping light of the system and is amplified in power by the second amplifier 6.
For a nonlinear process, when the polarization directions of the two beams of light are consistent, the efficiency is highest, so that the first polarization controller 4 and the second polarization controller 7 are respectively polarization controllers for adjusting the polarization states of the signal to be measured and the pump light, and the polarization states of the signal to be measured and the pump light are kept consistent.
The detection light and the pumping light passing through the polarization controller are coupled into a nonlinear medium 9 with a larger nonlinear coefficient through an optical coupler 8 to generate cross phase modulation (XPM), and the intensity information of the signal to be measured is loaded on the phase of the pumping light 5, so that the radio frequency spectrum of the signal to be measured is obtained on the spectrum.
After selective filtering by a second filter 10, filtering out original pump light and a signal to be detected, only leaving the loaded radio frequency spectrum, performing dispersion stretching by a large dispersion unit 11 to form a radio frequency spectrum which is mapped to a time domain, and further obtaining radio frequency information in the time domain;
the radio frequency spectrum mapped to the time domain is converted into an electric signal through the photoelectric detector 12, and then is collected and displayed through the high-speed digital-to-analog converter 13, so that ultra-fast radio frequency spectrum measurement is realized; finally, through inverse Fourier transform, a real-time autocorrelation waveform can be obtained, and the measurement rate of the real-time autocorrelation waveform corresponds to the pulse repetition frequency of the signal to be measured, namely the higher the pulse repetition frequency to be measured is, the faster the measurement rate is.
The invention maps the pulse of the signal to be measured loaded on the continuous optical spectrum to the time domain through the large dispersion optical fiber, realizes the ultra-fast radio frequency spectrum measurement by utilizing the high-speed digital-to-analog converter, and then obtains the autocorrelation waveform of the pulse which changes in real time through inverse Fourier transform. The measured autocorrelation waveform has the characteristic of a real-time measurement, large observation window. The cross phase modulation occurs in the nonlinear waveguide, and the higher loading efficiency can be realized by adjusting the optical power, polarization and relative wavelength of the signal to be measured and the pump light. Based on the principle that the loading efficiency is higher and the bandwidth is larger, the finally obtained radio frequency spectrum measurement bandwidth can reach the THz magnitude, the measurement rate depends on the repetition frequency of the signal to be measured, the higher the repetition frequency of the signal to be measured is, the higher the measurement rate of the obtained radio frequency spectrum is, and the MHz magnitude can be reached.
The invention obtains the real-time autocorrelation instrument through the conversion relation between the radio frequency spectrum and the autocorrelation waveform, the resolution ratio of the real-time autocorrelation instrument mainly depends on the measurement bandwidth of the radio frequency spectrum, the observation window of the autocorrelation waveform depends on the resolution ratio of the measurement of the radio frequency spectrum, and the real-time autocorrelation instrument is mainly related to the measurement bandwidth of a photoelectric detector and a high-speed digital-to-analog converter. Namely, the wider the loaded radio frequency spectrum bandwidth is, the higher the resolution of the obtained autocorrelation waveform is; the larger the measurement bandwidth of the photoelectric detector and the digital-to-analog converter is, the higher the measurement precision of the radio frequency spectrum is, and the larger the corresponding measured autocorrelation waveform observation window is. The characteristics of a large observation window of the autocorrelation instrument can be realized by setting high measurement bandwidths of the photoelectric detector and the high-speed digital-to-analog converter; and obtaining a high radio frequency spectrum measurement rate by setting a high pulse repetition frequency to be measured, namely obtaining a high measurement rate of the autocorrelator. The real-time autocorrelation waveform measurement based on radio frequency spectrum conversion can make up for a short board with limited measurement speed of the traditional autocorrelation instrument, and provides a new method for measuring the pulse width of an ultrafast pulse laser.
The specific real-time autocorrelator measurement system specifically comprises the following steps:
1) The pulse light source 1 is an optical fiber mode-locked femtosecond laser source, the center wavelength of the laser source is 1580nm, and the electric field expression is assumed to be E comb (t), intensity can be written as I comb (t)＝|E comb (t)| 2 According to the definition of the radio spectrum and of the autocorrelation, its radio spectrum S comb (omega) and autocorrelation waveform I auto (t) the expression can be written as:
I auto (t)＝I comb (t)★I comb (t) (2)
wherein the content of the first and second substances,
representing a fourier transform symbol, ≧ represents an autocorrelation symbol.
2) The mathematical expression of the pump light of the continuous pumping light source 5 is E (t) =Aexp(-iω pump t) where ω is pump Representing the wavelength of the continuous light, set at 1550nm, a represents the intensity of the continuous light. The signal light pulse to be measured and the pump continuous light are coupled into the nonlinear medium to generate cross phase modulation, and the intensity information of the pulse light can be loaded on the phase of the pump light:
E o (t)＝exp(-iω pump t)exp(-imI comb (t)) (3)
neglecting transmission losses, m being a parameter related to the non-linear medium, equal to 4 π Ln 2 /λ，n 2 The nonlinear medium parameter, L is the length of the nonlinear medium, and lambda is the wavelength of the pulse light of the signal to be measured, when the phase shift satisfies the small signal approximation, i.e. | mI comb (t) | < 1, and the electric field after loading can be written as follows:
E XPM (t)＝(1+mI comb (t))·exp(-iω pump t) (4)
and the intensity spectrum at this time can be written as:
showing that the radio frequency information is loaded onto the spectrum of the pump light.
3) Then, it passes through a large dispersion unit 10, which consists of DCF and LEAF, since they have opposite second-order and third-order dispersion coefficients, so that with proper length matching, the second-order dispersion required for the experiment and negligible third-order dispersion can be obtained. After the dispersion stretching of the large dispersion unit, the third-order dispersion can be eliminated and pure second-order dispersion can be obtained, and the frequency domain expression of the combined dispersion unit can be written as follows:
where Φ represents the combined second-order dispersion amount, and according to fourier transform, a time-domain response function can be obtained:
4) After dispersive transmission, the output result is equal to E XPM And the product in the H-frequency domain, the convolution in the time domain, thus yielding:
according to the large dispersion condition Φ > τ 2 Therefore, the quadratic term is omitted, formula (8) is obtained, the square is obtained, time domain intensity information is obtained, the photoelectric detector converts the optical signal into an electric signal, and the high-speed digital-to-analog converter performs real-time acquisition and observation:
wherein, t pump The time point corresponding to the pump light is represented, and the frequency-time mapping relation is as follows: Δ t/Φ = Δ ω. It can be seen from the formula that the radio frequency spectrum obtained by cross-phase modulation is mapped to the time domain through the large dispersion unit, and the obtained radio frequency spectrum is still the radio frequency spectrum of the signal to be measured. This is also a way to get a fast measurement of the pulses. After filtering, original pumping light and pulse to be measured are filtered, and only loaded radio frequency spectrum S is left comb . According to the frequency-time mapping relation, making Δ t/Φ = Δ ω, performing inverse fourier transform on the Δ t/Φ = Δ ω, and obtaining:
the formula shows that the inverse Fourier transform of the radio frequency spectrum is the autocorrelation spectrum of the measured signal, the pulse width is consistent with the pulse width of the autocorrelation waveform which is directly measured, and only the difference in the intensity is a proportionality coefficient, which can be solved through normalization. In conjunction with the above derivation, fast radio frequency spectrum measurement can result in fast autocorrelation waveform measurement, which will be described as an application example.
Example 1
In order to verify that the scheme has the characteristics of high speed, large observation window and high resolution for measurement of the autocorrelation waveform, the scheme implements a real-time autocorrelation waveform measurement system with 50MHz frame rate, 300fs resolution and 600ps observation window, compared with the traditional autocorrelation instrument, the measurement speed is increased by 6 orders of magnitude, the observation window is increased by one order of magnitude, and the resolution is mainly limited by the measurement bandwidth of radio frequency spectrum. In this case, a nonlinear waveguide with a nonlinear coefficient of 1w/m is used so that the 6dB bandwidth of the radio spectrum of the pulse signal is 1THz. In this case, the repetition frequency of the set signal to be measured is 50MHz, the pulse width is 250fs, and in order to verify the measurement window of the scheme, a double soliton with a time interval of 600ps is set. Secondly, in the process of cross phase modulation, in order to avoid spectrum overlapping, the central wavelength of the signal to be measured is set at 1580nm and the wavelength of the pump light is set at 1550nm. According to the frequency-time mapping relation, under the condition that the frames are not overlapped, the dispersion amount is set to be a little larger as possible, and then the larger radio frequency spectrum resolution can be obtained, and the setting is 0.9ns/nm in the case. In addition, based on actual measurement conditions, the detection bandwidth of the photoelectric detector is set at 60GHz, and the radio frequency spectrum detection bandwidth determines the measurement resolution of the autocorrelator.
As shown in fig. 2, a dual soliton pulse of a mode-locked laser with 6 cycles is given, the repetition frequency is 50MHz, the time interval is 20ns, the pulse width is 250fs, the repetition frequency determines the measurement rate of the autocorrelation spectrum, when the repetition frequency is changed, the change of the measurement rate can be realized, and the higher the repetition frequency is, the faster the measurement rate is.
As shown in fig. 3, a partial enlarged view of the dual solitons is given, the dual solitons with an interval of 600ps can be seen, and the signal to be measured can be changed by setting the number of the solitons and the soliton interval;
as shown in fig. 4, given the autocorrelation waveform obtained by directly performing autocorrelation operation on the time domain spectrum, a double soliton with an interval of 600ps can be seen, and for the autocorrelation process, the generated sideband pulse intensity is twice lower than the central pulse intensity, which also conforms to the definition of autocorrelation.
As shown in fig. 5, the coupled signal to be measured and the pump light are shown, the signal to be measured is filtered out to have a spectral width of 20nm and a center wavelength of 1580nm, and in order to prevent the overlapping of the subsequent cross-phase modulation spectra, the signal to be measured is set at 1550nm as a continuous light of the pump light.
As shown in fig. 6, the radio frequency spectrum loaded to both sides of the pump spectrum after cross-phase modulation, and the pump light and the signal pulse to be measured before being filtered are shown. The loaded radiofrequency spectrum envelope can be seen to be wavy, which is the result of double soliton interference. The number of solitons and the soliton spacing are changed, and the obtained radio frequency spectrum envelope is also changed. And then filtering to remove the original pump light and the pulse to be measured, and only leaving a radio frequency spectrum which takes 1550nm as a center and has a spectrum width of 34 nm.
As shown in fig. 7, the autocorrelation spectrum obtained by directly performing inverse fourier transform on the filtered radio frequency spectrum is given, and the shape of the double solitons with an interval of 600ps can be seen, and is consistent with the autocorrelation calculation spectrum given in fig. 4.
As shown in fig. 8, the radio frequency spectrum obtained in the time domain after the frequency-time mapping is given is consistent with the time domain spectrum of the original probe signal, the repetition frequency is 50MHz, which is 6 cycles, and the shape of the time domain spectrum also maintains the shape of the radio frequency spectrum in the frequency domain.
As shown in fig. 9, a single-period radio frequency spectrum in the time domain is given, and since a detection bandwidth of 60GHz is set, an oscillation envelope shown in the figure is a large envelope of dual soliton interference in the spectrum, and is limited by a filtering bandwidth of the second filter, a signal-to-noise ratio of the oscillation envelope is poor, and if there is no limitation of the detection bandwidth, a mapped time-domain waveform is also consistent with a radio frequency spectrum display in the frequency domain.
As shown in fig. 10, an autocorrelation waveform is given, a single frame signal is subjected to inverse fourier transform, and the reconstructed autocorrelation waveform is consistent with an autocorrelation waveform obtained by radio frequency spectrum conversion in a frequency domain, and has a time interval of 600ps. According to the autocorrelation definition, for a gaussian pulse, the pulse width needs to be multiplied by a coefficient of 0.707, and through pulse width conversion of a gaussian function, the resolution of a single pulse is 300fs.
As shown in fig. 11, a measurement window of an autocorrelation waveform is shown, and the soliton spacing of the dual solitons is changed by taking 100ps as a step, assuming that the normalized autocorrelation spectrum intensity is in the magnitude of volts (V), the minimum magnitude that can be observed by an oscilloscope is millivolts (mV), and as a definition, when the soliton spacing is 600ps, the sideband of the autocorrelation is just reduced to 1mV, and the reduction of the sideband is mainly limited by the detection bandwidth of the photodetector.
As shown in FIG. 12, the resolution change curves at 100ps,200ps,300ps,400ps,500ps and 600ps are shown, and it can be seen that, in the range of 600ps, the resolution oscillates around 300fs, which indicates that the resolution of the present invention is not affected by the observation window and remains around 300fs.
Example 2
In example 1, the observation window, resolution and rate of the real-time autocorrelation system are verified, and in order to verify the performance of the system, in this case, the three solitons and the four solitons with different distances are measured by using the same pulse width, wavelength setting, dispersion amount and detector detection bandwidth as those in example 1.
As shown in fig. 13, a time domain spectrum of three solitons with a distance of 150ps and 250ps within 2ns is given, the pulse width of each soliton is kept at 250fs, and the intensities of the three pulses are kept consistent.
As shown in fig. 14, an autocorrelation waveform obtained by directly performing autocorrelation operation on a time-domain pulse is given, and since the autocorrelation waveform is a three-soliton, the height of the obtained pulse is one third of the height of the main peak according to the definition of autocorrelation. The time intervals are 150ps,250ps,400ps, consistent with the soliton time domain spacing.
As shown in fig. 15, which shows the autocorrelation waveform obtained by inverse fourier transform of the radio frequency spectrum measured using the apparatus of example 1, the same soliton spacing can be observed in comparison with fig. 14, demonstrating the accuracy of the measurement. Since the autocorrelation waveform is related to the envelope of the soliton radio frequency spectrum, the radio frequency spectrum mapped to the time domain is limited by the bandwidth of the photoelectric converter and the oscilloscope in the measurement process, the envelope part of high frequency is suppressed, and the heights of sidebands of the measured autocorrelation waveform are not equal.
Example 3
The embodiment provides a real-time autocorrelation waveform measuring method based on radio frequency spectrum conversion, which mainly comprises the following steps:
s1, outputting pulse laser as a signal to be detected of a system and continuous laser as pump light;
preferably, the step S1 further includes filtering and denoising the output pulse laser;
s2, coupling the signal to be detected and the pump light to form a beam of light;
preferably, before coupling the signal to be measured and the pump light into one beam of light in step S2, power amplification is further performed on the output signal to be measured and the output pump light.
Preferably, in step S2, before coupling the signal to be measured and the pump light into one beam of light, the method further includes adjusting the polarization states of the signal to be measured and the pump light to keep the polarization states of the signal to be measured and the pump light consistent.
S3, performing cross phase modulation on the beam combination light to load the radio frequency information of the signal to be measured on the spectrum of the pump light to obtain the radio frequency spectrum of the signal to be measured;
s4, performing dispersion stretching (frequency-time mapping) on the radio frequency spectrum of the signal to be detected to form a radio frequency spectrum mapped to a time domain;
preferably, before performing dispersion stretching on the radio frequency spectrum of the signal to be measured, the method further includes filtering the radio frequency spectrum of the signal to be measured, filtering out original pump light and the signal to be measured, and only leaving the pure radio frequency spectrum of the signal to be measured.
Step S5, performing photoelectric conversion on the time domain radio frequency spectrum obtained in the step S4 to obtain a radio frequency signal;
and S6, recording the radio frequency signal mapped to the time domain, and performing inverse Fourier transform on the radio frequency signal to obtain a pulse autocorrelation waveform which changes in real time.
It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.
Claims (10)
1. A real-time autocorrelation waveform measuring method based on radio frequency spectrum conversion is characterized by comprising the following steps:
s1, respectively outputting pulse laser and continuous laser as a signal to be detected and pump light, and coupling the signal to be detected and the pump light to form a beam of light;
s2, performing cross phase modulation on the coupled light beam, and loading radio frequency information of the signal to be detected onto the spectrum of the pump light to obtain a radio frequency spectrum of the signal to be detected;
s3, performing dispersion stretching on the radio frequency spectrum of the signal to be measured to form a radio frequency spectrum mapped to a time domain;
s4, performing photoelectric conversion on the radio frequency spectrum mapped to the time domain to obtain a radio frequency signal, and performing inverse Fourier transform on the radio frequency signal to obtain a pulse autocorrelation waveform which changes in real time; and the measurement rate of the autocorrelation waveform is consistent with the pulse repetition frequency of the signal to be measured.
2. The method according to claim 1, wherein before coupling the signal to be measured and the pump light into a beam of light in step S1, the method further comprises adjusting the polarization states of the signal to be measured and the pump light to keep the polarization states of the signal to be measured and the pump light consistent.
3. The real-time autocorrelation waveform measuring method based on radio frequency spectrum conversion as claimed in claim 1, wherein before coupling the signal to be measured and the pump light into a beam of light in step S1, further comprising power amplifying the output signal to be measured and the pump light.
4. The method according to claim 3, further comprising filtering the RF spectrum of the signal to be measured before the step S3 of performing dispersion stretching on the RF spectrum of the signal to be measured.
5. A real-time autocorrelator based on radio frequency spectrum conversion, comprising:
the pulse light source (1) is used for outputting pulse laser as a signal to be detected;
a continuous pumping light source (5) for outputting continuous pumping light;
the optical coupler (8) is used for coupling the signal to be detected and the pump light to synthesize a beam of light;
the nonlinear medium (9) is used for performing cross phase modulation on the coupled beam combination light to load the radio frequency information of the signal to be measured on the spectrum of the pump light to obtain the radio frequency spectrum of the signal to be measured;
the large dispersion unit (11) is used for performing dispersion stretching on the radio frequency spectrum of the signal to be measured to form a radio frequency spectrum mapped to a time domain;
the photoelectric detector (12) is used for carrying out photoelectric conversion on the radio frequency spectrum of the time domain to obtain a radio frequency signal;
and the high-speed digital-to-analog converter (13) is used for recording the radio frequency signals mapped to the time domain and carrying out inverse Fourier transform on the radio frequency signals to obtain the pulse autocorrelation waveform which changes in real time.
6. The real-time autocorrelation instrument based on radio frequency spectrum conversion according to claim 5 wherein said large dispersion unit is a chirped Bragg grating or consists of a dispersion compensation fiber and a large effective area fiber.
7. The real-time autocorrelation instrument based on radio frequency spectrum conversion of claim 6 wherein said nonlinear medium is a highly nonlinear optical fiber or a nonlinear waveguide.
8. The real-time autocorrelation apparatus based on radio frequency spectrum conversion of claim 7 further comprising: a first polarization controller (4) and a second polarization controller (7), the first polarization controller (4) being located between the pulsed light source (1) and the optical coupler (8), the second polarization controller (7) being located between the continuous pumping light source (5) and the optical coupler (8).
9. The real-time autocorrelation apparatus based on radio frequency spectrum conversion of claim 6 further comprising: a first amplifier (3) and a second amplifier (6), the first amplifier (3) is positioned between the pulsed light source (1) and the optical coupler (8), and the second amplifier (6) is positioned between the continuous pumping light source (5) and the optical coupler (8).
10. The real-time autocorrelation apparatus based on radio frequency spectrum conversion as claimed in any one of claims 6 to 9 further comprising: a first filter (2) located between the pulsed light source (1) and the optical coupler (8); and/or a second filter (10) located between the nonlinear medium (9) and the large dispersion unit (11).
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