Real-time measurement method and system for polarization state of vector ultrafast optical signal
阅读说明：本技术 一种矢量超快光信号偏振态实时测量方法与系统 (Real-time measurement method and system for polarization state of vector ultrafast optical signal ) 是由 杨中民 林巍 文晓晓 韦小明 于 2020-05-30 设计创作，主要内容包括：本发明公开了一种矢量超快光信号偏振态实时测量方法与系统。所述方法为：待测矢量超快光信号通过色散元件进行时间拉伸时频变换,然后经过光放大器补偿色散元件引入的光功率衰减,得到放大光信号,将其经偏振分束元件分为两路偏振方向彼此正交的光信号,其中一路光信号通过偏振控制元件将光偏振方向旋转90度,另一路光信号通过光延迟线保证两路光信号的光程足够接近,然后利用光耦合器将两路光信号合束并经过偏振相关元件得到单一偏振方向的光信号；通过高速信号采集部件实时记录处理后光信号的实时强度信息,并通过傅里叶算法得到待测矢量超快光信号的偏振态斯托克斯参数。本发明能够实现高刷新率的矢量光信号偏振态分析,实时测量偏振态的斯托克斯参数。(The invention discloses a method and a system for measuring the polarization state of a vector ultrafast optical signal in real time. The method comprises the following steps: the vector ultrafast optical signal to be measured is subjected to time stretching time-frequency conversion through a dispersion element, then is subjected to optical power attenuation introduced by an optical amplifier compensation dispersion element to obtain an amplified optical signal, the amplified optical signal is divided into two paths of optical signals with mutually orthogonal polarization directions through a polarization beam splitting element, wherein one path of optical signal rotates the light polarization direction by 90 degrees through a polarization control element, the other path of optical signal ensures that the optical paths of the two paths of optical signals are close enough through an optical delay line, and then the two paths of optical signals are combined by an optical coupler and pass through a polarization related element to obtain an optical signal with a single polarization direction; and recording the real-time intensity information of the processed optical signal in real time through a high-speed signal acquisition component, and obtaining the polarization state Stokes parameter of the ultrafast optical signal of the vector to be detected through a Fourier algorithm. The invention can realize the polarization state analysis of the vector optical signal with high refresh rate and measure the Stokes parameters of the polarization state in real time.)
1. A real-time measurement system for the polarization state of a vector ultrafast optical signal is characterized by comprising a dispersion element, an optical amplifier, a polarization beam splitting element, a polarization control element, an optical delay line, an optical coupler, a polarization correlation element and a high-speed signal acquisition component;
the dispersion element is used for performing time stretching on an input vector ultrafast optical signal to be measured to obtain a time-frequency conversion optical signal and outputting the time-frequency conversion optical signal to the optical amplifier; the optical amplifier is used for amplifying the optical signal with attenuated power, compensating the optical power attenuation introduced by the dispersion element, obtaining an amplified optical signal and outputting the amplified optical signal to the polarization beam splitting element; the polarization beam splitting element is used for separating two polarization components in the vector optical signal, forming two paths of optical signals with polarization directions orthogonal to each other and respectively outputting the two paths of optical signals to the optical delay line and the polarization control element; the polarization control element is used for rotating the polarization direction of one path of optical signal so as to realize coherent superposition with the other path of optical signal; the optical delay line is used for adjusting the optical path difference of the two paths of optical signal transmission; the optical coupler is connected with the polarization control element and the optical delay line, and is used for re-combining and re-beam two optical signals and outputting the two optical signals to the polarization related element; the polarization-dependent element is used for selecting the optical signal in the specific polarization direction, obtaining the optical signal in the single polarization direction and outputting the optical signal to the high-speed signal acquisition component; the high-speed signal acquisition component is used for converting an input optical signal in a single polarization direction into an electric signal, then carrying out real-time data acquisition and digitization, and finally completing polarization state analysis to realize real-time measurement of the polarization state of the vector ultrafast optical signal.
2. The system of claim 1, wherein the dispersion element has a dispersion DTSThe pulse width tau of the vector ultrafast signal to be detected meets the Fraunhofer far-field diffraction condition DTS＞＞τ2。
3. The system of claim 1, wherein the optical amplifier comprises a rare-earth doped fiber, a solid optical amplifier, or a semiconductor optical amplifier.
4. The system according to claim 1, wherein the polarization direction of the polarization control element is perpendicular to the polarization direction of the light signal, and the introduced phase difference is equal to an odd multiple of pi; the length of the optical delay line ensures that the optical path difference Delta l of the two paths satisfies the relational expression Delta l between the wavelength Lambda and the wavelength resolution Delta Lambda2/Δλ。
5. The system of claim 1, wherein the optical coupler comprises a spatial coupler or an optical fiber coupler.
6. The system of claim 1, wherein the high speed signal acquisition component comprises a photodetector and a high speed analog-to-digital converter for converting the received ultrafast optical signal into a digital signal.
7. A method for measuring the polarization state of a vector ultrafast optical signal in real time is characterized by comprising the following steps:
s1, carrying out time stretching time-frequency conversion on the vector ultrafast optical signal to be detected through a dispersion element, and then, compensating optical power attenuation introduced by the dispersion element through an optical amplifier to obtain an amplified optical signal;
s2, dividing the amplified optical signal obtained in the step S1 into two paths of optical signals with polarization directions orthogonal to each other through a polarization beam splitting element, wherein one path of optical signal rotates the polarization direction of light by 90 degrees through a polarization control element, the other path of optical signal ensures that the optical paths of the two paths of optical signals are close enough through an optical delay line, and then the two paths of optical signals are combined by an optical coupler and pass through a polarization related element to obtain an optical signal with a single polarization direction;
and S3, recording the real-time intensity information of the processed optical signal in real time through the high-speed signal acquisition component, and obtaining the polarization state Stokes parameter of the ultrafast optical signal of the vector to be detected through a Fourier algorithm.
8. The method according to claim 7, wherein in step S1, the optical signal (u) is assumed to be polarization-locked by the vector solitons1,u2)TAny component of (a) satisfies:
wherein u'j(t) is the normalized time domain optical signal after time stretching time-frequency transformation,for the optical signal u to be measuredj(t) corresponding frequency domain expression, DTSτ is the pulse width of the ultrafast optical signal, i represents an imaginary unit, and t and ω represent time and angular frequency, respectively;
in order to meet the requirements of subsequent signal detection and acquisition, the optical amplifier performs power amplification on the vector optical signal processed by the dispersion element:
wherein, (u ″)1(t),u″2(t)) is ultrafast optical signal after passing through optical amplifier, and the power is P after respectively corresponding to the amplification1,P2Thus, the pulse energy ratio between the polarization components before and after amplification is unchanged:
where the symbol |) represents the 2-norm of the function.
9. The method as claimed in claim 7, wherein in step S2, the polarization beam splitter splits the vector light signal into two components u and v with polarization directions perpendicular to each other,
the optical signal v is transformed by the polarization control element as follows:
wherein R is a unitary matrix representing a transformation matrix of the polarization control element with respect to the original optical signal,representing conjugate transposition, wherein theta is a rotation angle of the element main shaft direction relative to an original coordinate system, and M is a Jones matrix of the polarization control element;
without loss of generality, assume that 2 θ ═ pi/2 yields an expression for the two optical signal components u 'and v' when entering the optical coupler:
wherein, t0Δ l/c, where Δ l is the effective optical path difference and c is the speed of light;
setting the polarization main axis direction of the polarization related element to be consistent with the original optical signal coordinate system, and obtaining the optical signal u passing through the polarization related element after being combinedc(t) expression:
10. the method for real-time measurement of the polarization state of the vector ultrafast optical signal as claimed in claim 7, wherein in step S3, the signal is detected, collected and digitized by the high-speed signal collecting component, and the extraction of the polarization state information is completed by fast fourier transform; the time domain intensity signal is as follows:
Ic＝|uc(t)|2；
the time domain intensity signal IcEquivalently written as:
wherein, the symbol
where superscript denotes the corresponding fourier image function, superscript denotes the conjugate of the function, c.c denotes the complex conjugate of the preceding term, simplifying the above to the following form:
Ic＝S0+S1+S-1；
according to the formula, the signal I is collectedcFourier image function ofSide lobe s of its two measurements1(t-t0) And s-1(t+t0) The relative amplitude and phase information of the polarization component of the vector optical signal are included, and then the Stokes parameter p of the polarization state is calculated2And p3：
p2＝Real(Ic*Rect(t-t0))；
p3＝Imag(Ic*Rect(t-t0))；
Where symbol denotes convolution, Real () and Imag () denote the Real and imaginary parts of the fetch function, respectively, and Rect () is a rectangular function.
Technical Field
The invention relates to the field of ultrafast signal measurement, in particular to a method and a system for measuring the polarization state of a vector ultrafast optical signal in real time.
Background
The ultrafast optical signal propagates in the birefringent medium, and evolves vector characteristics due to the cross phase modulation effect to form vector solitons. Thus, in birefringent media, typically lasers and amplifiers constructed as single mode, polarization maintaining fibers, the pulses are usually present as vector solitons with either regular or irregular polarization evolution. The polarization state stability is one of important parameter indexes for ensuring a laser system, and the analysis of the polarization state of the vector ultrafast optical signal is particularly critical.
At present, the polarization state analysis and measurement method of the vector ultrafast optical signal mainly comprises the following steps: 1) the commercial polarization measuring instrument is not generally designed for vector ultrafast optical signals with large bandwidth, and has low refresh rate and narrow response bandwidth due to limited mechanical rotation speed and applicable spectral bandwidth of the front glass; 2) liu et al, utilizing this technique, analyzed the frequency domain information of the vector ultrafast optical signal (Liu M.dynamic mapping of a polarization rotation vector source in a fiber laser. optics Letters,2017,42(2):330-333), but could not analyze the polarization state evolution of the optical signal due to the lack of relative phase data.
The invention provides a method and a system for measuring the polarization state of a vector ultrafast optical signal in real time in order to realize the real-time measurement of the polarization state of the optical signal. The method combines a time stretching time-frequency transformation technology and a phase inversion technology of coherent superimposed optical signals, solves the problems of low refresh rate and narrow response bandwidth of the traditional polarization state analyzer, can improve the refresh rate to be more than MHz magnitude, and simultaneously ensures that the response bandwidth is more than 100 nm. The method has wide application prospect in the field of ultrafast measurement, particularly in the aspect of real-time measurement of the frequency domain and polarization state information of the vector optical signal.
Disclosure of Invention
The invention aims to realize the real-time measurement of the polarization state of a vector ultrafast optical signal and break through the problems of low refreshing speed and narrow response bandwidth of the traditional polarization state analyzer.
The purpose of the invention is realized by at least one of the following technical solutions.
A real-time measurement system for the polarization state of a vector ultrafast optical signal comprises a dispersion element, an optical amplifier, a polarization beam splitting element, a polarization control element, an optical delay line, an optical coupler, a polarization related element and a high-speed signal acquisition component;
the dispersion element is used for performing time stretching on an input vector ultrafast optical signal to be measured to obtain a time-frequency conversion optical signal and outputting the time-frequency conversion optical signal to the optical amplifier; the optical amplifier is used for amplifying the optical signal with attenuated power, compensating the optical power attenuation introduced by the dispersion element, obtaining an amplified optical signal and outputting the amplified optical signal to the polarization beam splitting element; the polarization beam splitting element is used for separating two polarization components in the vector optical signal, forming two paths of optical signals with polarization directions orthogonal to each other and respectively outputting the two paths of optical signals to the optical delay line and the polarization control element; the polarization control element is used for rotating the polarization direction of one path of optical signal so as to realize coherent superposition with the other path of optical signal; the optical delay line is used for adjusting the optical path difference of the two paths of optical signal transmission; the optical coupler is connected with the polarization control element and the optical delay line, and is used for re-combining and re-beam two optical signals and outputting the two optical signals to the polarization related element; the polarization-dependent element is used for selecting the optical signal in the specific polarization direction, obtaining the optical signal in the single polarization direction and outputting the optical signal to the high-speed signal acquisition component; the high-speed signal acquisition component is used for converting an input optical signal in a single polarization direction into an electric signal, then carrying out real-time data acquisition and digitization, and finally completing polarization state analysis to realize real-time measurement of the polarization state of the vector ultrafast optical signal.
Further, the dispersion amount D of the dispersion elementTSThe pulse width tau of the vector ultrafast signal to be detected meets the Fraunhofer far-field diffraction condition DTS＞＞τ2。
Further, the optical amplifier comprises a rare earth doped optical fiber, a solid optical amplifier or a semiconductor optical amplifier.
Further, the polarization main axis direction of the polarization control element and the polarization direction of the optical signalThe phase is vertical, and the introduced phase difference is equal to odd times of pi; the length of the optical delay line ensures that the optical path difference Delta l of the two paths satisfies the relational expression Delta l between the wavelength Lambda and the wavelength resolution Delta Lambda2/Δλ。
Further, the optical coupler includes a space type coupler or a fiber type coupler.
Further, the high-speed signal acquisition component comprises a photoelectric detector and a high-speed analog-to-digital converter, and is used for converting the received ultrafast optical signal into a digital signal.
A real-time measurement method for the polarization state of a vector ultrafast optical signal comprises the following steps:
s1, carrying out time stretching time-frequency conversion on the vector ultrafast optical signal to be detected through a dispersion element, and then, compensating optical power attenuation introduced by the dispersion element through an optical amplifier to obtain an amplified optical signal;
s2, dividing the amplified optical signal obtained in the step S1 into two paths of optical signals with polarization directions orthogonal to each other through a polarization beam splitting element, wherein one path of optical signal rotates the polarization direction of light by 90 degrees through a polarization control element, the other path of optical signal ensures that the optical paths of the two paths of optical signals are close enough through an optical delay line, and then the two paths of optical signals are combined by an optical coupler and pass through a polarization related element to obtain an optical signal with a single polarization direction;
and S3, recording the real-time intensity information of the processed optical signal in real time through the high-speed signal acquisition component, and obtaining the polarization state Stokes parameter of the ultrafast optical signal of the vector to be detected through a Fourier algorithm.
Further, in step S1, the optical signal (u) is assumed to be polarization-locked with the vector solitons1,u2)TAny component of (a) satisfies:
wherein u isj' (t) is the normalized time domain optical signal after time stretching time-frequency transformation,for the optical signal u to be measuredj(t) corresponding frequency domain expression, DTSτ is the pulse width of the ultrafast optical signal, i represents an imaginary unit, and t and ω represent time and angular frequency, respectively;
in order to meet the requirements of subsequent signal detection and acquisition, the optical amplifier performs power amplification on the vector optical signal processed by the dispersion element:
wherein, (u ″)1(t),u″2(t)) is ultrafast optical signal after passing through optical amplifier, and the power is P after respectively corresponding to the amplification1,P2Thus, the pulse energy ratio between the polarization components before and after amplification is unchanged:
where the symbol |) represents the 2-norm of the function.
Further, in step S2, the polarization beam splitting element splits the vector optical signal into two components u and v whose polarization directions are perpendicular to each other,
the optical signal v is transformed by the polarization control element as follows:
wherein R is a unitary matrix representing a transformation matrix of the polarization control element with respect to the original optical signal,
without loss of generality, assume that 2 θ ═ pi/2 yields an expression for the two optical signal components u 'and v' when entering the optical coupler:
wherein, t0Δ l/c, where Δ l is the effective optical path difference and c is the speed of light;
setting the polarization main axis direction of the polarization related element to be consistent with the original optical signal coordinate system, and obtaining the optical signal u passing through the polarization related element after being combinedc(t) expression:
further, in step S3, the high-speed signal acquisition component detects, acquires, and digitizes the polarization state information, and extracts the polarization state information through fast fourier transform; the time domain intensity signal is as follows:
Ic＝|uc(t)|2；
the time domain intensity signal IcEquivalently written as:
wherein, the symbol
where superscript denotes the corresponding fourier image function, superscript denotes the conjugate of the function, c.c denotes the complex conjugate of the preceding term, simplifying the above to the following form:
Ic＝S0+S1+S-1；
according to the formula, the signal I is collectedcFourier image function ofSide lobe s of its two measurements1(t-t0) And s-1(t+t0) The relative amplitude and phase information of the polarization component of the vector optical signal are included, and then the Stokes parameter p of the polarization state is calculated2And p3：
p2＝Real(Ic*Rect(t-t0))；
p3＝Imag(Ic*Rect(t-t0))；
Where symbol denotes convolution, Real () and Imag () denote the Real and imaginary parts of the fetch function, respectively, and Rect () is a rectangular function.
Compared with the traditional polarization state analysis means, the polarization state analysis method has the advantages that:
the invention simultaneously utilizes the dispersion Fourier transform technology and the phase inversion technology of coherent superposition optical signals, solves the problem of low refresh rate of the traditional method, and can improve the refresh rate to be more than MHz-GHz magnitude. In the field of ultrafast measurement, a fast and effective technical means is provided for the analysis of vector optical signals; meanwhile, the response and feedback speed of the polarization control system can be potentially improved by applying the measurement method. The invention includes, but is not limited to, applications in the fields of ultrafast measurement, ultrafast imaging, and the like.
Description of the drawings:
FIG. 1 is a schematic structural diagram of a system for real-time measurement of polarization state of a vector ultrafast optical signal according to an embodiment of the present invention;
FIG. 2 is a schematic diagram of a spectrum of a signal to be measured after coherent superposition of optical signal components in two polarization directions in an embodiment of the present invention;
FIG. 3 is a schematic representation of the real part of a Fourier transformed and filtered complex function of the spectrum of FIG. 2 in an embodiment of the present invention;
fig. 4 is a schematic diagram of the position change of the polarization state of the ultrafast optical signal on the stokes parameter plane at different times, which is obtained by the measurement system in the embodiment of the present invention.
The specific implementation mode is as follows:
in order to make the objects, technical solutions and advantages of the present invention more apparent, embodiments of the present invention are described in detail below with reference to the accompanying drawings.