CN113098595B - Method, system and device for measuring group delay of few-mode optical fiber differential mode - Google Patents
Method, system and device for measuring group delay of few-mode optical fiber differential mode Download PDFInfo
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Abstract
The invention discloses a method, a system and a device for measuring group delay of a few-mode optical fiber differential mode, and belongs to the field of optical communication sensing and measurement. The invention uses the frequency modulation continuous optical signal of the chirp signal with monotonous frequency increment, the difference frequency component is directly obtained by carrying out Fourier transform on the electrical signal after photoelectric conversion, the complexity is lower, and the measuring speed is faster. Meanwhile, when the time delay of a plurality of differential mode groups is measured, the traditional microwave interference scheme needs to carry out two-time fast Fourier transform to analyze a frequency spectrum peak value, but the system can realize the measurement of the time delay of the plurality of differential mode groups only by one-time fast Fourier transform, thereby realizing the simplification of a processing algorithm. The system converts the traditional amplitude-frequency response measurement into the difference frequency item detection generated by the intermode interference by utilizing the intermode interference of the frequency modulation continuous optical signals with different modes, thereby reducing the requirement of a receiver on the measurement bandwidth of the measurement system and simultaneously ensuring higher precision, thereby reducing the overall cost of the measurement system.
Description
Technical Field
The invention belongs to the technical field of optical communication sensing and measurement, and particularly relates to a method, a system and a device for measuring group delay of a few-mode optical fiber differential mode.
Background
The mode division multiplexing technology in few-mode optical fiber is one of space division multiplexing technologies, and is one of the most competitive capacity expansion schemes at present. However, due to the mode coupling, mode-dependent loss, differential mode group delay and other factors existing in the few-mode fiber, the signals in different mode channels have differences when reaching the receiving end and interaction exists between the modes, which further degrades the signal receiving quality and limits the system transmission distance. Therefore, in order to facilitate optimization of few-mode optical fiber theoretical models, preparation processes and the like, the few-mode optical fiber parameter measurement work is taken as a theoretical basis of subsequent work, and the importance of the few-mode optical fiber parameter measurement work is also self-evident.
The existing differential mode group delay measurement technology of few-mode optical fiber can be divided into two types, namely a time domain scheme and a frequency domain scheme, according to the category of analysis quantity, wherein the time domain scheme mainly depends on analyzing the difference of time domain pulse response of a basic mode optical signal and a high-order mode optical signal, such as time delay difference between arrival pulses, so as to obtain differential mode group delay through analysis, but in order to accurately analyze the time domain pulse response, the sampling rate of a system receiver is required to be higher; the frequency domain scheme is mainly based on the microwave interference principle, namely, the differential mode group delay is obtained by backstepping through analyzing the amplitude-frequency response characteristic of signal light after being transmitted by a few-mode optical fiber.
In 2018, Varun Kelkar et al put forward a method for measuring multimode fiber Differential Mode Group Delay (DMGD) by using a Fourier domain mode-locked laser (FDML), wherein the method realizes the Measurement of the multimode fiber differential mode group delay by using a self-built fast frequency-swept laser based on a frequency modulation continuous wave technology. However, the swept-frequency laser has a complicated structure and expensive components, and has modulation nonlinearity, which reduces the measurement accuracy. In addition, an additional photodetector is needed in the scheme, so that the low-frequency noise of the swept-frequency laser is suppressed.
Disclosure of Invention
Aiming at the defects and improvement requirements of the prior art, the invention provides a method, a system and a device for measuring the differential mode group delay of a few-mode optical fiber, and aims to realize the differential mode group delay measurement in the few-mode optical fiber with low cost, low complexity and high precision by utilizing the frequency modulation continuous wave technology.
To achieve the above object, according to a first aspect of the present invention, there is provided a method for measuring group delay in a few-mode fiber differential mode, the method comprising the steps of:
s1, generating a frequency modulation continuous optical signal of a chirp signal which is periodic and has monotonous frequency increment in the period;
s2, splitting the frequency-modulated continuous optical signal into two paths of coherent optical signals with different modes, respectively serving as a basic mode optical signal and a high-order mode optical signal, and then performing mode division multiplexing to obtain a few-mode frequency-modulated continuous optical signal;
s3, carrying out mode decomposition multiplexing on the few-mode frequency modulation continuous optical signal transmitted by the few-mode optical fiber to be detected to obtain a basic mode optical signal and a high-order mode optical signal which are subjected to mode decomposition multiplexing, and then combining to generate an inter-mode interference optical signal;
s4, performing photoelectric conversion on the inter-mode interference optical signals to obtain a plurality of difference frequency components;
s5, demodulating a corresponding differential mode group delay value for each difference frequency component, wherein the formula is as follows:
wherein the content of the first and second substances,
representing a difference frequency component, TcRepresenting the period of the chirp signal, L representing the length of the few-mode fiber to be measured, fstopRepresenting the stop frequency, f, of the chirp signalstartIndicating the start frequency, tau, of the chirp signalDMGDRepresenting a differential mode group delay value.
Has the advantages that: the invention adopts mode division multiplexing to excite a high-order mode in a few-mode optical fiber; by adopting mode decomposition multiplexing, noise items which are introduced by mode crosstalk and are irrelevant to measurement in the few-mode optical fiber are reduced.
Preferably, in step S1, the frequency-modulated continuous optical signal of the chirp signal having periodicity and monotonous frequency increment within the period is generated by:
(1) generating a linearly polarized narrow linewidth optical carrier and a chirped electrical signal having low phase noise characteristics;
(2) and (3) loading the chirp electrical signal to a linear polarization state narrow linewidth optical carrier through photoelectric intensity modulation to obtain a frequency modulation continuous optical signal.
Preferably, before the chirped electrical signal is loaded to the linearly polarized narrow linewidth optical carrier, the polarization state of the optical carrier is matched with the polarization state of the device for realizing the photoelectric intensity modulation, so as to realize the maximum signal modulation efficiency.
Has the advantages that: the invention matches the polarization state of the optical carrier with the polarization state of the device for realizing the photoelectric intensity modulation to realize the maximum signal modulation efficiency, thereby reducing the signal-to-noise ratio.
Preferably, in step S2, after the beam splitting and before the mode division multiplexing, the following processing is performed on the two coherent optical signals with different modes:
respectively matching the polarization states of the two paths of optical signals with the polarization states of devices for realizing the mode division multiplexing;
and performing delay matching on the two paths of optical signals after polarization state matching.
Has the advantages that: the invention adopts polarization state matching to reduce the inter-mode crosstalk when different modes are excited; and time delay matching is adopted to reduce time delay errors introduced by the non-to-be-detected few-mode optical fiber.
To achieve the above object, according to a second aspect of the present invention, there is provided a few-mode fiber differential mode group delay measurement system, comprising:
the frequency modulation continuous optical signal generating module is used for generating a frequency modulation continuous optical signal of a chirp signal which is periodic and has monotonous frequency increment in the period;
the module of the mode division multiplexing, is used for splitting the frequency modulation continuous optical signal into two routes of coherent different mode optical signals, regard as the optical signal of fundamental mode and optical signal of high-order mode separately, and then carry on the mode division multiplexing, get the few-mode frequency modulation continuous optical signal;
the few-mode optical fiber to be detected is used for transmitting few-mode frequency modulation continuous optical signals;
the module of mode decomposition multiplexing, is used for carrying on the mode decomposition multiplexing through the few-mode frequency modulation continuous optical signal that the few-mode optical fiber to be measured transmits, basic mode optical signal and high-order mode optical signal through the demultiplexing of the mode division;
the photoelectric conversion module is used for combining the fundamental mode optical signal and the high-order mode optical signal subjected to the mode division demultiplexing to generate an intermode interference optical signal, and performing photoelectric conversion on the intermode interference optical signal to obtain a plurality of difference frequency components;
the differential mode group delay demodulation module is used for demodulating a corresponding differential mode group delay value for each difference frequency component, and the formula is as follows:
wherein the content of the first and second substances,
representing a difference frequency component, TcRepresenting the period of the chirp signal, L representing the length of the few-mode fiber to be measured, fstopRepresenting the stop frequency, f, of the chirp signalstartIndicating the start frequency, tau, of the chirp signalDMGDRepresenting a differential mode group delay value.
Preferably, the frequency modulated continuous optical signal generating module includes:
the narrow-linewidth light source generating unit is used for generating a linear polarization state narrow-linewidth optical carrier with low phase noise characteristic and transmitting the linear polarization state narrow-linewidth optical carrier to the photoelectric intensity modulating unit;
the chirp electric signal source is used for generating a chirp signal which is periodic and has monotonous frequency increment in the period;
and the photoelectric intensity modulation unit is used for modulating and loading the chirp electrical signal to the optical carrier with the narrow linewidth in the warp polarization state through photoelectric intensity to obtain a frequency modulation continuous optical signal.
Has the advantages that: the invention adopts the external modulator to realize frequency modulation continuous waves, and because the external modulator is adopted, taking the Mach-Zehnder modulator as an example, when the external modulator is biased at a power linear point and the modulation depth is small enough, the output signal has higher modulation linearity, thereby avoiding the nonlinear effect of wavelength modulation brought by a fast frequency-sweeping laser, further improving the measurement precision, and reducing the cost and the system complexity.
Preferably, the frequency modulated continuous optical signal generating module further comprises:
and the first polarization control unit is used for matching the polarization state of the optical carrier with the polarization state of a device for realizing photoelectric intensity modulation before the chirp electric signal is loaded to the linear polarization state narrow linewidth optical carrier so as to realize the maximum signal modulation efficiency.
Preferably, the module for mode division multiplexing includes:
the first optical coupling unit is used for generating two paths of optical signals which are modulated by the photoelectric intensity modulation unit, namely a fundamental mode optical channel and a high-order mode optical channel, and the two paths of optical signals are used as two independent measurement signals and are respectively incident to the second polarization control unit and the third polarization control unit;
the second polarization control unit is used for matching the polarization state of the incident optical signal with the polarization state of a device for realizing the mode division multiplexing;
the third polarization control unit is used for matching the polarization state of the incident optical signal with the polarization state of a device for realizing the mode division multiplexing;
the optical delay adjusting unit is used for performing delay matching on the two paths of optical signals after polarization state matching and transmitting the optical signals to the mode division multiplexing unit;
and the mode division multiplexing unit is used for selectively exciting the fundamental mode light and the high-order mode light in the few-mode optical fiber to be tested.
Has the advantages that:
(1) the invention uses the time delay calibration module to realize the mode selective multiplexing and the optical link time delay matching of different modes in the demultiplexer, because the ideal to-be-measured value is only introduced for the few-mode optical fiber to be measured, the inter-mode time delay introduced by the mode multiplexing and demultiplexing device belongs to the error, and in order to reduce the error, the time delay calibration module is added in one path.
(2) Compared with the traditional method for exciting the space mode by the bias optical fiber, the mode selective photon lantern has good mode isolation, and the excited space mode in the few-mode optical fiber can be similar to two groups of space modes to be detected, so that the signal transmitted by the few-mode optical fiber is less influenced by unexpected mode crosstalk. The receiving end processes the same. Thereby achieving noise suppression.
To achieve the above object, according to a third aspect of the present invention, there is provided a few-mode fiber differential mode group delay measuring apparatus, comprising:
a transmitting end, the transmitting end comprising: the system comprises a frequency modulation continuous optical signal generating module and a mode division multiplexing module;
the frequency modulation continuous optical signal generating module is used for generating a frequency modulation continuous optical signal of a chirp signal which is periodic and has monotonous frequency increment in the period;
the module of the mode division multiplexing, is used for splitting the frequency modulation continuous optical signal into two routes of coherent different mode optical signals, regard as the optical signal of fundamental mode and optical signal of high-order mode separately, and then carry on the mode division multiplexing, get the few-mode frequency modulation continuous optical signal;
a receiving end, comprising: the device comprises a mode decomposition multiplexing module, a photoelectric conversion module and a differential mode group delay demodulation module;
the module decomposition multiplexing module is used for carrying out module decomposition multiplexing on the few-mode frequency modulation continuous optical signal at the receiving end to obtain a basic mode optical signal and a high-order mode optical signal which are subjected to module decomposition demultiplexing;
the photoelectric conversion module is used for combining the fundamental mode optical signal and the high-order mode optical signal subjected to the mode division demultiplexing to generate an intermode interference optical signal, and performing photoelectric conversion on the intermode interference optical signal to obtain a difference frequency component;
the differential mode group delay demodulation module is used for demodulating a corresponding differential mode group delay value for each difference frequency component, and the formula is as follows:
wherein the content of the first and second substances,
representing a difference frequency component, TcRepresenting the period of the chirp signal, L representing the length of the few-mode fiber to be measured, fstopRepresenting the stop frequency, f, of the chirp signalstartIndicating the start frequency, tau, of the chirp signalDMGDRepresenting a differential mode group delay value;
when the measuring device works, the optical fiber to be detected is connected between the mode division multiplexing module and the mode decomposition multiplexing module, so that optical signal transmission is carried out.
Preferably, the frequency modulated continuous optical signal generating module includes:
the narrow-linewidth light source generating unit is used for generating a linear polarization state narrow-linewidth optical carrier with low phase noise characteristic and transmitting the linear polarization state narrow-linewidth optical carrier to the photoelectric intensity modulating unit;
the chirp electric signal source is used for generating a chirp signal which is periodic and has monotonous frequency increment in the period;
and the photoelectric intensity modulation unit is used for loading the chirp electrical signal to a linear polarization state narrow linewidth optical carrier through photoelectric intensity modulation to obtain a frequency modulation continuous optical signal of the chirp signal with monotonous frequency increment.
Generally, by the above technical solution conceived by the present invention, the following beneficial effects can be obtained:
(1) compared with the scheme of measuring the multimode fiber differential mode group delay by the existing Fourier domain mode-locked laser, the method uses the frequency modulation continuous optical signal of a sinusoidal signal generated by a complex device, the differential frequency component is the frequency-frequency analysis of multiple Fourier transform on the electrical signal to extract the peak frequency point, and simultaneously needs a plurality of photoelectric conversion units to carry out noise suppression. Meanwhile, when the time delay of a plurality of differential mode groups is measured, the traditional microwave interference scheme needs to carry out two-time fast Fourier transform to analyze a frequency spectrum peak value, but the system can realize the measurement of the time delay of the plurality of differential mode groups only by one-time fast Fourier transform, thereby realizing the simplification of a processing algorithm.
(2) The invention provides a few-mode optical fiber differential mode group delay measurement and measurement system, which is characterized in that through a frequency modulation continuous wave technology, when a frequency modulation continuous wave takes a linear chirp signal as an example, a few-mode optical fiber to be measured is input and different space modes are simultaneously excited, and after the frequency modulation continuous wave is transmitted through the few-mode optical fiber, the signal frequencies of different space modes at the same moment can generate difference due to the differential mode group delay effect among the space modes. At the receiver, the optical signals of different spatial modes are combined, and the signals produce inter-mode interference at the photodetector, and thus a differential mode group delay-dependent low frequency beat signal. By detecting the low-frequency beat frequency signal, the differential mode group delay value can be reversely deduced, so that other expensive high-bandwidth devices and detection equipment are omitted.
(3) Compared with the traditional differential mode group delay measurement scheme based on time domain impulse response, the system converts the minimum differential mode group delay from the time domain to the frequency domain by utilizing the intermode interference of the frequency modulation continuous optical signals of different modes, thereby reducing the sampling rate of a receiver of the measurement system and the requirements of the transmitted time domain pulse waveform;
(4) compared with the traditional differential mode group delay measurement scheme based on microwave interference, the system converts the traditional amplitude-frequency response measurement into the difference frequency item detection generated by the inter-mode interference by utilizing the inter-mode interference of the frequency modulation continuous optical signals of different modes, thereby reducing the requirement of a receiver on the measurement bandwidth of the measurement system, and simultaneously ensuring higher precision, thereby reducing the overall cost of the measurement system.
Drawings
Fig. 1 is a schematic structural diagram of a few-mode fiber differential mode group delay measurement system provided in the present invention;
fig. 2 is a structural diagram of a system for measuring group delay in a few-mode fiber differential mode according to an embodiment of the present invention;
fig. 3 is a schematic diagram of a difference frequency principle of the chirp signal provided by the present invention for realizing time delay detection;
fig. 4 is a spectrum diagram of a signal at a receiving end according to the present invention.
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 are not intended to 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.
The invention provides a few-mode optical fiber differential mode group delay measuring method, which comprises the following steps:
step S1, generating a frequency modulation continuous optical signal of a chirp signal which is periodic and has monotonous frequency increment in the period.
Preferably, in step S1, the frequency-modulated continuous optical signal of the chirp signal having periodicity and monotonous frequency increment within the period is generated by:
(1) generating a linearly polarized narrow linewidth optical carrier and a chirped electrical signal having low phase noise characteristics;
(2) and (3) loading the chirp electrical signal to a linear polarization state narrow linewidth optical carrier through photoelectric intensity modulation to obtain a frequency modulation continuous optical signal.
Preferably, before the chirped electrical signal is loaded to the linearly polarized narrow linewidth optical carrier, the polarization state of the optical carrier is matched with the polarization state of the device for realizing the photoelectric intensity modulation, so as to realize the maximum signal modulation efficiency.
And S2, splitting the frequency modulation continuous optical signal into two paths of coherent optical signals with different modes, respectively serving as a basic mode optical signal and a high-order mode optical signal, and then performing mode division multiplexing to obtain a few-mode frequency modulation continuous optical signal.
Preferably, in step S2, after the beam splitting and before the mode division multiplexing, the following processing is performed on the two coherent optical signals with different modes:
respectively matching the polarization states of the two paths of optical signals with the polarization states of devices for realizing the mode division multiplexing;
and performing delay matching on the two paths of optical signals after polarization state matching.
And S3, carrying out mode decomposition multiplexing on the few-mode frequency modulation continuous optical signal transmitted by the few-mode optical fiber to be detected to obtain a basic mode optical signal and a high-order mode optical signal which are subjected to mode decomposition and demultiplexing, and then combining to generate an inter-mode interference optical signal.
And S4, performing photoelectric conversion on the inter-mode interference optical signals to obtain a plurality of difference frequency components.
S5, demodulating a corresponding differential mode group delay value for each difference frequency component, wherein the formula is as follows:
wherein the content of the first and second substances,
representing a difference frequency component, TcRepresenting the period of the chirp signal, L representing the length of the few-mode fiber to be measured, fstopRepresenting the stop frequency, f, of the chirp signalstartIndicating the start frequency, tau, of the chirp signalDMGDRepresenting a differential mode group delay value.
In order to implement the above method, the present invention further provides a system for measuring group delay in a few-mode fiber differential mode, where the system is shown in fig. 1 and includes:
and the frequency modulation continuous optical signal generating module is used for generating a frequency modulation continuous optical signal of a chirp signal which is periodic and has monotonous frequency increment in the period.
And the mode division multiplexing module is used for splitting the frequency modulation continuous optical signal into two paths of coherent optical signals with different modes, respectively serving as a basic mode optical signal and a high-order mode optical signal, and then performing mode division multiplexing to obtain a few-mode frequency modulation continuous optical signal.
And selectively exciting the two optical signals into basic mode light and high-order mode light in the few-mode optical fiber to be tested by using the mode division multiplexing unit, and coupling and outputting the basic mode light and the high-order mode light, namely completing the time delay calibration and the mode division multiplexing of the two-mode channel.
The few-mode optical fiber to be tested is used for transmitting few-mode frequency modulation continuous optical signals.
The few-mode optical fiber to be tested is used as a device to be tested and is a main factor for introducing differential mode group delay.
And the mode decomposition multiplexing module is used for carrying out mode decomposition multiplexing on the few-mode frequency modulation continuous optical signal transmitted by the few-mode optical fiber to be detected to obtain a basic mode optical signal and a high-order mode optical signal which are subjected to mode division demultiplexing.
And the photoelectric conversion module is used for combining the fundamental mode optical signal and the high-order mode optical signal subjected to the mode division demultiplexing to generate an intermode interference optical signal, and performing photoelectric conversion on the intermode interference optical signal to obtain a plurality of difference frequency components.
The photoelectric conversion unit is used for realizing the inter-mode signal interference between the two modes through square law detection, and thus, the difference frequency detection is realized.
The differential mode group delay demodulation module is used for demodulating a corresponding differential mode group delay value for each difference frequency component, and the formula is as follows:
wherein the content of the first and second substances,
representing a difference frequency component, TcRepresenting the period of the chirp signal, L representing the length of the few-mode fiber to be measured, fstopRepresenting the stop frequency, f, of the chirp signalstartIndicating the start frequency, tau, of the chirp signalDMGDRepresenting a differential mode group delay value.
Preferably, the frequency modulated continuous optical signal generating module includes:
the narrow-linewidth light source generating unit is used for generating a linear polarization state narrow-linewidth optical carrier with low phase noise characteristic and transmitting the linear polarization state narrow-linewidth optical carrier to the photoelectric intensity modulating unit;
the chirp electric signal source is used for generating a chirp signal which is periodic and has monotonous frequency increment in the period;
and the photoelectric intensity modulation unit is used for modulating and loading the chirp electrical signal to the optical carrier with the narrow linewidth in the warp polarization state through photoelectric intensity to obtain a frequency modulation continuous optical signal.
Preferably, the frequency modulated continuous optical signal generating module further comprises:
and the first polarization control unit is used for matching the polarization state of the optical carrier with the polarization state of a device for realizing photoelectric intensity modulation before the chirp electric signal is loaded to the linear polarization state narrow linewidth optical carrier so as to realize the maximum signal modulation efficiency.
Preferably, the module for mode division multiplexing includes:
the first optical coupling unit is used for generating two paths of optical signals which are modulated by the photoelectric intensity modulation unit, namely a fundamental mode optical channel and a high-order mode optical channel, and the two paths of optical signals are used as two independent measurement signals and are respectively incident to the second polarization control unit and the third polarization control unit;
the second polarization control unit is used for matching the polarization state of the incident optical signal with the polarization state of a device for realizing the mode division multiplexing;
the third polarization control unit is used for matching the polarization state of the incident optical signal with the polarization state of a device for realizing the mode division multiplexing;
the optical delay adjusting unit is used for performing delay matching on the two paths of optical signals after polarization state matching and transmitting the optical signals to the mode division multiplexing unit;
and the mode division multiplexing unit is used for selectively exciting the fundamental mode light and the high-order mode light in the few-mode optical fiber to be tested.
To further illustrate the system of the present invention, a few-mode fiber differential mode group delay measurement system with low bandwidth receiver requirement is described in detail in conjunction with the following embodiments:
the system structure of the embodiment of the invention is shown in fig. 2.
Further, the measurement of the group delay of the few-mode fiber differential mode by using the frequency modulation continuous optical signal is discussed by combining formula derivation and a theoretical model.
In an example of the invention, a frequency modulated continuous wave light source generating module is shown in fig. 2, the module comprising: a narrow linewidth laser, a first polarization controller, a photoelectric intensity modulator (taking a Mach-Zehnder modulator as an example), and an arbitrary waveform generator. After the polarization state of the narrow-linewidth laser output by the laser is matched with the polarization state of the main axis of the modulator crystal through the polarization controller, the narrow-linewidth laser enters the Mach-Zehnder modulator to load an electric signal, wherein the electric signal is a periodic chirp signal, and can be expressed as linear chirp by taking the example of the linear chirp as
Wherein the content of the first and second substances,v (t) is the loaded chirp electrical signal, A1Is the amplitude of the electrical signal and,
is the initial phase, T, of the electrical signalcIs the period of the chirp signal, fstopIs the chirp maximum frequency, i.e. the stop frequency, fstartThe lowest frequency of the chirp, i.e., the starting frequency. If VπIs the half-wave voltage of the Mach-Zehnder modulator, when the bias voltage of the Mach-Zehnder modulator is
When the modulator is biased at a power linear point and works in a push-pull mode, after an electric signal is loaded, the output light field and the output light intensity of the modulator can be expressed as
Wherein for the sake of analysis it is assumed that the modulator is an ideal modulator, E0For narrow line width light source amplitude, omegacIs the optical carrier frequency, phicIs the optical carrier phase. Further, for the convenience of subsequent analysis, for the light intensity signal, according to the bezier expansion, the light intensity signal can be expressed as
When in use
When the signal modulation depth is small, the third order and above components in the light intensity signal can be ignored, and further the simplification can be continued to
Further, as shown in fig. 2, the module for calibrating delay and performing modulo division multiplexing includes: the device comprises a first optical coupler, a second polarization controller, a third polarization controller, a tunable optical delay line and a mode division multiplexer. The modulator outputs an optical signal and splits the optical signal into two paths of optical signals after passing through the first optical coupler, and if the first optical coupler is an ideal 1: 1 coupler, the two signal light fields output can be respectively expressed as
One signal passes through the adjustable optical delay line and is used for matching signal delay caused by two single-mode fiber links when the system does not access the few-mode fiber to be measured so as to reduce measurement errors. Then the two paths of signals enter two single-mode input ends of the mode division multiplexer and are excited into different modes in the few-mode optical fiber.
Further, as shown in fig. 2, the module for mode division multiplexing and signal receiving includes: the module decomposition multiplexer, the second optical coupler, the photoelectric detector and the oscilloscope. And after two paths of signals are excited into different modes in the few-mode optical fiber by the mode division multiplexer and transmitted, under the condition of not considering residual delay introduced by a single-mode optical fiber link part, due to mode difference, namely differential mode group delay, the two signals experience different delays when arriving at the mode decomposition multiplexer, and the signals after being transmitted by the few-mode optical fiber can be respectively expressed as
The optical field of the output signal of the two signals after being combined by the second optical coupler can be expressed as
Further, when the optical signal enters the photodetector, the obtained photocurrent signal can be expressed as
Wherein R is the responsivity of the photodetector, Iinter(t,τ1,τ2) Is shown below
Iinter(t,τ1,τ2) I.e. the term representing the inter-modal interference, cos (ω), generated at the receiving end after the transmission of the two mode signalsc(τ1-τ2) Means a phase difference introduced by the optical carrier. Further, I after simplificationout(t) substituting to obtain the final expression of the received photocurrent signal as
The first three terms are DC component and chirp signal component of the signal, the fourth term is the difference frequency and frequency combination term of two chirp signals, if it is, the third term is the DC component and chirp signal component of the signal, the fourth term is the difference frequency and frequency combination term of two chirp signals
When the temperature of the water is higher than the set temperature,
the Taylor expansion is carried out on the root number in the difference frequency combination term and the second-order term is ignored, and then the received signal is simplified into
Further expanding the trigonometric function to obtain
Wherein
Where Δ f (t, τ)1,τ2) In order to obtain the frequency-synthesizing term,
the difference frequency term is a term whose frequency does not change with time and the sum frequency term Δ f (t, τ)1,τ2)≥2fstartWhen the bandwidth of the photodetector is small, particularly when the bandwidth is much smaller than fstartIn time, the frequency combination term is filtered out, and after the direct current component is removed, the received photocurrent signal can be further simplified into
As can be seen from the above formula, when the low-bandwidth DC blocking receiver is adopted, only the difference frequency terms of the two paths of signals are reserved in the finally received signal, so that the difference frequency terms can be measured by the receiver with smaller bandwidth
And calculating the differential mode group delay according to the definition formula and the known parameters
L is the length of the optical fiber to be measured. The signal frequency time diagram for realizing the differential mode group delay through the difference frequency detection is shown in fig. 3, compared with the traditional microwave interference scheme that the differential mode group delay can be measured only by scanning the amplitude-frequency response of a few-mode optical fiber in the whole frequency spectrum range, the differential mode group delay detection method can be used for measuring the differential mode group delayThe differential mode group delay measurement with relatively high precision can be realized only by the difference frequency component with lower frequency, and the equal-amplitude response analysis equipment of a vector network analyzer is not needed, so that the cost of the measurement system is reduced. Further, by analogy, when a plurality of differential time delay amounts exist, the invention can obtain a plurality of difference frequency values by measuring at a receiver, and only one fast Fourier transform algorithm is needed to obtain the plurality of difference frequency values by measuring. In the microwave interference scheme, when a plurality of differential time delay quantities exist, firstly, a fast Fourier transform algorithm is needed to measure amplitude-frequency response, and then, a second fast Fourier transform algorithm is needed to analyze frequency components of an amplitude-frequency response curve. Therefore, the invention can simultaneously realize the reduction of the algorithm complexity under specific conditions.
The difference frequency measurement diagram of the differential mode group delay in the few-mode optical fiber realized by the invention is shown in fig. 4. The analysis shows that the difference frequency signal has higher signal-to-noise ratio and can be well distinguished from noise, and meanwhile, the bandwidth requirement of the difference frequency signal is not high.
In general, the method for detecting the group delay of the differential mode by the difference frequency through the intermode interference between chirp signals of different modes, which is adopted by the invention, is better improved in the aspects of the bandwidth requirement of a receiver and the algorithm complexity compared with the traditional scheme.
According to the content, the few-mode optical fiber differential mode group delay measurement system required by the low-bandwidth receiver can be constructed, and after the transmission measurement of the two-module few-mode optical fiber based on 10 kilometers and the photoelectric detector with the 2GHz bandwidth, the differential mode group delay measurement with the precision of 0.002ps/m can be realized, namely the differential mode group delay measurement with higher precision.
Correspondingly, the invention also provides a device for measuring the group delay of the few-mode optical fiber differential mode, which comprises the following components:
a transmitting end, the transmitting end comprising: the system comprises a frequency modulation continuous optical signal generating module and a mode division multiplexing module;
the frequency modulation continuous optical signal generating module is used for generating a frequency modulation continuous optical signal of a chirp signal which is periodic and has monotonous frequency increment in the period;
the module of the mode division multiplexing, is used for splitting the frequency modulation continuous optical signal into two routes of coherent different mode optical signals, regard as the optical signal of fundamental mode and optical signal of high-order mode separately, and then carry on the mode division multiplexing, get the few-mode frequency modulation continuous optical signal;
a receiving end, comprising: the device comprises a mode decomposition multiplexing module, a photoelectric conversion module and a differential mode group delay demodulation module;
the module decomposition multiplexing module is used for carrying out module decomposition multiplexing on the few-mode frequency modulation continuous optical signal at the receiving end to obtain a basic mode optical signal and a high-order mode optical signal which are subjected to module decomposition demultiplexing;
the photoelectric conversion module is used for combining the fundamental mode optical signal and the high-order mode optical signal subjected to the mode division demultiplexing to generate an intermode interference optical signal, and performing photoelectric conversion on the intermode interference optical signal to obtain a difference frequency component;
the differential mode group delay demodulation module is used for demodulating a corresponding differential mode group delay value for each difference frequency component, and the formula is as follows:
wherein the content of the first and second substances,
representing a difference frequency component, TcRepresenting the period of the chirp signal, L representing the length of the few-mode fiber to be measured, fstopRepresenting the stop frequency, f, of the chirp signalstartIndicating the start frequency, tau, of the chirp signalDMGDRepresenting a differential mode group delay value;
when the measuring device works, the optical fiber to be detected is connected between the mode division multiplexing module and the mode decomposition multiplexing module, so that optical signal transmission is carried out.
Preferably, the frequency modulated continuous optical signal generating module includes:
the narrow-linewidth light source generating unit is used for generating a linear polarization state narrow-linewidth optical carrier with low phase noise characteristic and transmitting the linear polarization state narrow-linewidth optical carrier to the photoelectric intensity modulating unit;
the chirp electric signal source is used for generating a chirp signal which is periodic and has monotonous frequency increment in the period;
and the photoelectric intensity modulation unit is used for loading the chirp electrical signal to a linear polarization state narrow linewidth optical carrier through photoelectric intensity modulation to obtain a frequency modulation continuous optical signal of the chirp signal with monotonous frequency increment.
The device also comprises a display device for displaying the measurement result.
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 few-mode fiber differential mode group delay measurement method is characterized by comprising the following steps:
s1, generating a frequency modulation continuous optical signal of a chirp signal which is periodic and has monotonous frequency increment in the period;
s2, splitting the frequency-modulated continuous optical signal into two paths of coherent optical signals with different modes, respectively serving as a basic mode optical signal and a high-order mode optical signal, and then performing mode division multiplexing to obtain a few-mode frequency-modulated continuous optical signal;
s3, carrying out mode decomposition multiplexing on the few-mode frequency modulation continuous optical signal transmitted by the few-mode optical fiber to be detected to obtain a basic mode optical signal and a high-order mode optical signal which are subjected to mode decomposition multiplexing, and then combining to generate an inter-mode interference optical signal;
s4, performing photoelectric conversion on the inter-mode interference optical signals to obtain a plurality of difference frequency components;
s5, demodulating a corresponding differential mode group delay value for each difference frequency component, wherein the formula is as follows:
wherein the content of the first and second substances,
representing a difference frequency component, TcRepresenting the period of the chirp signal, L representing the length of the few-mode fiber to be measured, fstopRepresenting the stop frequency, f, of the chirp signalstartIndicating the start frequency, tau, of the chirp signalDMGDRepresenting a differential mode group delay value.
2. The method according to claim 1, wherein in step S1, the frequency modulated continuous optical signal of the chirp signal having periodicity and monotone frequency increment within the period is generated by:
(1) generating a linearly polarized narrow linewidth optical carrier and a chirped electrical signal having low phase noise characteristics;
(2) and (3) loading the chirp electrical signal to a linear polarization state narrow linewidth optical carrier through photoelectric intensity modulation to obtain a frequency modulation continuous optical signal.
3. The method of claim 2, wherein the polarization state of the optical carrier is matched to the device polarization state that achieves the electro-optical intensity modulation to achieve maximum signal modulation efficiency before the chirped electrical signal is applied to the linearly polarized narrow linewidth optical carrier.
4. The method according to claim 1, wherein in step S2, before the mode division multiplexing after the beam splitting, the following processing is performed on the two coherent optical signals with different modes:
respectively matching the polarization states of the two paths of optical signals with the polarization states of devices for realizing the mode division multiplexing;
and performing delay matching on the two paths of optical signals after polarization state matching.
5. A few-mode fiber differential mode group delay measurement system, comprising:
the frequency modulation continuous optical signal generating module is used for generating a frequency modulation continuous optical signal of a chirp signal which is periodic and has monotonous frequency increment in the period;
the module of the mode division multiplexing, is used for splitting the frequency modulation continuous optical signal into two routes of coherent different mode optical signals, regard as the optical signal of fundamental mode and optical signal of high-order mode separately, and then carry on the mode division multiplexing, get the few-mode frequency modulation continuous optical signal;
the few-mode optical fiber to be detected is used for transmitting few-mode frequency modulation continuous optical signals;
the module of mode decomposition multiplexing, is used for carrying on the mode decomposition multiplexing through the few-mode frequency modulation continuous optical signal that the few-mode optical fiber to be measured transmits, basic mode optical signal and high-order mode optical signal through the demultiplexing of the mode division;
the photoelectric conversion module is used for combining the fundamental mode optical signal and the high-order mode optical signal subjected to the mode division demultiplexing to generate an intermode interference optical signal, and performing photoelectric conversion on the intermode interference optical signal to obtain a plurality of difference frequency components;
the differential mode group delay demodulation module is used for demodulating a corresponding differential mode group delay value for each difference frequency component, and the formula is as follows:
wherein the content of the first and second substances,
representing a difference frequency component, TcRepresenting the period of the chirp signal, L representing the length of the few-mode fiber to be measured, fstopRepresenting the stop frequency, f, of the chirp signalstartIndicating the start frequency, tau, of the chirp signalDMGDRepresenting a differential mode group delay value.
6. The system of claim 5, wherein the frequency modulated continuous optical signal generating module comprises:
the narrow-linewidth light source generating unit is used for generating a linear polarization state narrow-linewidth optical carrier with low phase noise characteristic and transmitting the linear polarization state narrow-linewidth optical carrier to the photoelectric intensity modulating unit;
the chirp electric signal source is used for generating a chirp signal which is periodic and has monotonous frequency increment in the period;
and the photoelectric intensity modulation unit is used for modulating and loading the chirp electrical signal to the optical carrier with the narrow linewidth in the warp polarization state through photoelectric intensity to obtain a frequency modulation continuous optical signal.
7. The system of claim 6, wherein the frequency modulated continuous optical signal generating module further comprises:
and the first polarization control unit is used for matching the polarization state of the optical carrier with the polarization state of a device for realizing photoelectric intensity modulation before the chirp electric signal is loaded to the linear polarization state narrow linewidth optical carrier so as to realize the maximum signal modulation efficiency.
8. The system of claim 6 or 7, wherein the modulo division multiplexing module comprises:
the first optical coupling unit is used for generating two paths of optical signals which are modulated by the photoelectric intensity modulation unit, namely a fundamental mode optical channel and a high-order mode optical channel, and the two paths of optical signals are used as two independent measurement signals and are respectively incident to the second polarization control unit and the third polarization control unit;
the second polarization control unit is used for matching the polarization state of the incident optical signal with the polarization state of a device for realizing the mode division multiplexing;
the third polarization control unit is used for matching the polarization state of the incident optical signal with the polarization state of a device for realizing the mode division multiplexing;
the optical delay adjusting unit is used for performing delay matching on the two paths of optical signals after polarization state matching and transmitting the optical signals to the mode division multiplexing unit;
and the mode division multiplexing unit is used for selectively exciting the fundamental mode light and the high-order mode light in the few-mode optical fiber to be tested.
9. A few-mode fiber differential mode group delay measuring device is characterized by comprising:
a transmitting end, the transmitting end comprising: the system comprises a frequency modulation continuous optical signal generating module and a mode division multiplexing module;
the frequency modulation continuous optical signal generating module is used for generating a frequency modulation continuous optical signal of a chirp signal which is periodic and has monotonous frequency increment in the period;
the module of the mode division multiplexing, is used for splitting the frequency modulation continuous optical signal into two routes of coherent different mode optical signals, regard as the optical signal of fundamental mode and optical signal of high-order mode separately, and then carry on the mode division multiplexing, get the few-mode frequency modulation continuous optical signal;
a receiving end, comprising: the device comprises a mode decomposition multiplexing module, a photoelectric conversion module and a differential mode group delay demodulation module;
the module decomposition multiplexing module is used for carrying out module decomposition multiplexing on the few-mode frequency modulation continuous optical signal at the receiving end to obtain a basic mode optical signal and a high-order mode optical signal which are subjected to module decomposition demultiplexing;
the photoelectric conversion module is used for combining the fundamental mode optical signal and the high-order mode optical signal subjected to the mode division demultiplexing to generate an intermode interference optical signal, and performing photoelectric conversion on the intermode interference optical signal to obtain a difference frequency component;
the differential mode group delay demodulation module is used for demodulating a corresponding differential mode group delay value for each difference frequency component, and the formula is as follows:
wherein the content of the first and second substances,
representing a difference frequency component, TcRepresenting the period of the chirp signal, L representing the length of the few-mode fiber to be measured, fstopRepresenting the stop frequency, f, of the chirp signalstartIndicating the start frequency, tau, of the chirp signalDMGDRepresenting a differential mode group delay value;
when the measuring device works, the few-mode optical fiber to be measured is connected between the mode division multiplexing module and the mode decomposition multiplexing module, so that optical signal transmission is carried out.
10. The apparatus of claim 9, wherein the frequency modulated continuous optical signal generating module comprises:
the narrow-linewidth light source generating unit is used for generating a linear polarization state narrow-linewidth optical carrier with low phase noise characteristic and transmitting the linear polarization state narrow-linewidth optical carrier to the photoelectric intensity modulating unit;
the chirp electric signal source is used for generating a chirp signal which is periodic and has monotonous frequency increment in the period;
and the photoelectric intensity modulation unit is used for loading the chirp electrical signal to a linear polarization state narrow linewidth optical carrier through photoelectric intensity modulation to obtain a frequency modulation continuous optical signal of the chirp signal with monotonous frequency increment.
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