CN110048781B - Method and device for identifying optical signal modulation format - Google Patents
Method and device for identifying optical signal modulation format Download PDFInfo
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Abstract
The invention discloses a method and a device for identifying an optical signal modulation format, wherein the method comprises the following steps: extracting a portion of the light from the optical signal; carrying out time delay self-coherent processing on the first optical signal to obtain a first optical signal; converting the first optical signal into an electric signal and sampling; carrying out distribution statistical analysis on the sampling result, and drawing a frequency histogram of interference light intensity distribution; and extracting and analyzing the characteristic parameters of the frequency histogram of the interference light intensity distribution, and distinguishing the modulation format on the optical signal according to the characteristic parameters. The invention obtains the light intensity change of the modulation signal after the delayed self-coherence by processing the extracted optical signal by the delayed self-coherence interferometer, and identifies the modulation format according to the characteristics of different changes of the delayed self-coherence intensity of the optical signal of different modulation formats.
Description
Technical Field
The invention relates to the field of optical fiber communication, in particular to a method and a device for identifying an optical signal modulation format.
Background
In recent years, coherent optical communication technology and digital signal processing technology have promoted the capacity and transmission distance of optical fiber communication systems to be continuously increased; accordingly, optical network construction will become more dynamic, complex, transparent, flexible, and reconfigurable. In order to better meet the requirements of users and fully utilize fiber link resources, dynamic optical networks such as flexible optical networks and software defined optical networks are produced.
In a dynamic optical network, due to different service requirements of users and different fiber service carrying capacities at different places, a situation that optical signals of different services exist simultaneously occurs in the same fiber. The optical signals of different services have different modulation formats and different requirements on signal quality, and the digital signal processing algorithms adopted in the subsequent data recovery are different; therefore, in both the test and maintenance of the optical fiber link, the modulation format of the optical signal needs to be known in advance when the signal is recovered at the receiving end.
The existing method for identifying the modulation format of the optical signal mainly comprises the following three steps:
(1) a modulation format identification method based on a coherent receiver and digital signal processing. The method uses a balanced receiver to carry out coherent detection to obtain IQ components of signals, compensates damage (such as chromatic dispersion, polarization mode dispersion and the like) in the optical signal transmission process through a digital signal processing algorithm, then recovers a constellation diagram of the signals, and then can identify the modulation format of the signals according to the characteristics of the constellation diagram, or can obtain a frequency histogram first and then identify the modulation format according to the frequency histogram. The method adopts a set of complete coherent receiving system, and at least four high-speed optical detectors, sampling circuits and corresponding digital signal processing modules are needed for a dual-polarization system, so that the realization difficulty is high, and the cost is high; in addition, the digital signal processing algorithm generally needs to know the transmission distance of the optical signal in the optical fiber in advance to effectively compensate for the signal damage, and in the elastic optical network and the software defined optical network, the transmission distance of the signal light cannot be known in advance in many cases.
(2) A method based on asynchronous sampling. The method needs to sample the signal twice in a code element period, then interval the signal for several code element periods, sample the signal twice in the same code element period, and so on, and continuously sample; and the time interval between two samplings in the code element period needs to be kept fixed, thus high requirements are put on the sampling control, and the system implementation cost is still high.
(3) A modulation format identification method based on a spectrogram. The problem of this method is that it is impossible to identify signals with close spectral characteristics, such as QPSK signal and 16QAM signal with 10GBaud/s or more, and their spectral patterns are not significantly different after transmission through optical fiber with 200km or more, so the method has limited applicability.
In view of the above, there is an urgent need to improve the existing signal modulation format recognition scheme to reduce the implementation difficulty and cost, and have a wide application range.
Disclosure of Invention
The technical problem to be solved by the invention is that the signal modulation format identification scheme has the problems of high implementation difficulty, high cost and certain application limitation.
In order to solve the above technical problem, the technical solution adopted by the present invention is to provide an optical signal modulation format identification method, including the following steps:
step S10, extracting partial light from the optical signal, and performing time-delay self-coherent processing on the partial light to obtain a first optical signal;
step S20, converting the first optical signal into an electric signal and sampling;
step S30, performing distribution statistical analysis on the sampling result, and drawing a frequency histogram of the interference light intensity distribution;
step S40, extracting and analyzing characteristic parameters of the frequency histogram of the interference light intensity distribution, and distinguishing the modulation format on the optical signal according to the characteristic parameters.
In the above method, the step S30 includes the steps of:
carrying out statistical analysis on the sampling result output by the sampling circuit;
equally dividing the sampling result range into a plurality of sections according to the maximum value and the minimum value of the sampling result;
and counting the frequency of the sampling result falling on each segment, thereby obtaining a frequency histogram of the strength distribution of the delayed coherent signal.
In the above method, in step S10, 5% of the light is extracted from the optical signal using a 95:5 power coupler.
In the above method, in step S10, a delayed self-coherent interferometer is used to perform delayed self-coherent processing on the extracted partial light;
the delay difference between the two arms of the delay self-coherent interferometer is larger than the code element period of the optical signal and smaller than the light source coherence time.
In the method, the time-delay self-coherent interferometer is realized by adopting an optical device based on an optical fiber coupler and separation or based on silicon-based integration.
In the method, the delay difference between the two arms of the time-delay self-coherent interferometer is a fixed value or a tunable value.
In the above method, the modulation format includes: NRZ-OOK, RZ-OOK, QPSK, and 16 QAM.
In the method, before sampling, an electric signal converted from an optical signal is amplified by an amplifying circuit, so that a voltage value is matched with an operating voltage range of an analog-to-digital converter for sampling.
The invention also provides a device for identifying the modulation format of the optical signal, which comprises:
an optical power coupler for extracting a portion of the light from the optical signal;
the time delay self-coherent interferometer is used for carrying out time delay self-coherent interference processing on the extracted partial light to obtain a first light signal;
a photodetector for converting the first optical signal into an electrical signal;
the sampling circuit is used for sampling the electric signal to obtain a sampling result;
the signal intensity distribution statistical module is used for carrying out distribution statistical analysis on the sampling result to obtain a signal intensity distribution statistical result;
the frequency histogram feature extraction and analysis module: the frequency histogram is used for drawing a frequency histogram of interference light intensity according to the signal intensity distribution statistical result, and characteristic parameters related to signal modulation format identification in the frequency histogram are extracted;
and the modulation format type identification module is used for identifying the modulation format of the optical signal according to the characteristic parameters.
In the above apparatus, the minimum bandwidth of the photodetector, amplifying circuit and sampling circuit is greater than half of the baud rate of the signal.
Compared with the prior art, the method has the advantages that the extracted optical signals are subjected to the processing of the time-delay self-coherent interferometer, and three indexes which reflect the influence of modulation signals and noise, the line width of a light source and the phase difference caused by the time-delay self-coherent interferometer on the light intensity are additionally introduced; because the phase difference caused by the time-delay self-coherent interferometer is fixed, and the light intensity change caused by the light source phase noise is slower than the light intensity change of the modulation signal and the noise, the light detector actually detects the light intensity change of the modulation signal after the time-delay self-coherent, and according to the characteristic that the light signals of different modulation formats have different time-delay self-coherent intensity changes, the modulation format of the signal can be identified through the subsequent algorithm processing; the optical detector and the sampling circuit are fewer, the requirement on delay control is not high, the bandwidth of the high-speed detector and the sampling circuit is only more than half of the baud rate of the signal, complex digital signal processing such as dispersion compensation and carrier recovery is not needed to be carried out on the sampled signal, and the system has the advantages of being simple and low in implementation cost.
Drawings
Fig. 1 is a flowchart of a method for identifying a modulation format of an optical signal according to the present invention;
FIG. 2 is a block diagram of an implementation of the present invention when a time-delay self-coherent interferometer employs fiber-based couplers and split optics;
fig. 3 is a block diagram of an optical signal modulation format recognition apparatus according to the present invention.
Detailed Description
The invention discloses a method and a device for identifying an optical signal modulation format based on delayed self-coherent direct detection, which can be used for identifying the optical signal modulation format of a single-span and ultra-long-span optical fiber communication system and a multi-span and long-distance optical fiber communication system. The invention is described in detail below with reference to the drawings and the detailed description.
As shown in fig. 1, the method for identifying a modulation format of an optical signal provided by the present invention includes the following steps:
step S10, extracting a part of light from the signal light; and carrying out time delay self-coherent processing on the first optical signal to obtain a first optical signal.
A95: 5 power coupler can be adopted to extract 5% of light from the signal light, a delay self-coherent interferometer is adopted to carry out delay self-coherent processing on the extracted 5% of light, the delay difference between two arms of the delay self-coherent interferometer is larger than the code element period of the optical signal and smaller than the light source coherence time, and therefore, the delay difference slightly smaller than the light source coherence time can be selected under the condition that the optical signal speed is not known in advance, and the normal work of the system is guaranteed.
Step S20 is to convert the first optical signal into an electrical signal and perform sampling.
In the invention, the optical detector is used for photoelectric conversion, before sampling, because the electric signal output by the optical detector is a photocurrent signal, the photocurrent signal needs to be converted into a voltage signal by an amplifying circuit, the voltage signal needs to be amplified, the voltage value is matched with the working voltage range of the analog-to-digital converter so as to be convenient for sampling, the analog signal is converted into a digital signal, and the sampling result reflects the change of the interference light intensity.
Step S30, performing distribution statistical analysis on the sampling result, and drawing a frequency histogram of the interference light intensity distribution.
And step S40, extracting and analyzing the characteristic parameters of the frequency histogram of the interference light intensity distribution, distinguishing the modulation format NRZ-OOK, RZ-OOK, QPSK or 16QAM on the optical signal according to the characteristic parameters of the frequency histogram, and outputting the modulation format.
Wherein: under the conditions of small dispersion and low noise, the judgment can be carried out according to the number and the relative size of the maximum values of the frequency histogram: including "0" maxima (very close to 0, but not necessarily exactly equal to 0), OOK signals have 2 maxima, QPSK signals have 3 maxima, 16QAM signals have 9 maxima;
the differentiation of NRZ-OOK and RZ-OOK can be judged according to the ratio of 0 maximum to nonzero maximum, the ratio of NRZ-OOK is minimum, and the ratios of 0 maximum to nonzero maximum of 2/3RZ-OOK, 1/2RZ-OOK and 1/3RZ-OOK are increased in sequence.
Under large dispersion, high noise conditions, the frequency histograms of all signals collapse to only one "0" maximum, but the frequency histograms of different modulation format signals are differently clustered.
The aggregation degree of 16QAM, QPSK, NRZ-OOK, 2/3RZ-OOK, 1/2RZ-OOK and 1/3RZ-OOK frequency histograms increases in sequence.
Therefore, the modulation format identification can be performed according to the bandwidth of the frequency histogram, and in order to ensure the accuracy of the modulation format identification, three bandwidths of 3dB, 10dB and 20dB can be selected for judgment.
In the invention, the time-delay self-coherent interferometer is realized by adopting an optical device based on an optical fiber coupler and separation or based on silicon-based integration; the delay difference between the two arms of the delay self-coherent interferometer can be set as a fixed value and also can be a tuning value, and by utilizing the tunable delay difference, a plurality of groups of data can be collected to distinguish nonlinear noise and spontaneous radiation noise, so that the distinguishing accuracy of modulation formats can be improved; the value range of the delay difference between two arms of the delay self-coherent interferometer is required to be between the code element period of an optical signal and the coherent time of an optical source, no matter the delay difference is a fixed value or a tunable value.
In addition, after one time of delay adjustment is completed, the current delay difference needs to be fixed for a period of time to ensure that the system completes subsequent data acquisition.
When a time-delayed self-coherent interferometer employs fiber coupler-based and split optics, as shown in fig. 2, the optical field of the optical signal entering the first 3dB coupler is assumed to be represented as:
wherein, S (t) is the light field change caused by modulation signals and noise;
representing slow phase change of factors such as light source line width and the like; j is a unit imaginary number; t is a time independent variable; after passing through the first 3dB coupler, the optical fields of the two optical signals are respectively expressed as:
wherein E is1(t) is the optical field of the upper arm optical signal, E2(t) is the optical field of the lower arm optical signal,
and
respectively representing the phase change caused by the first 3dB coupler on the upper branch and the lower branch; after passing through the symbol delay module, the optical fields of the two optical signals after reaching the second 3dB coupler are respectively expressed as:
wherein tau represents the delay difference caused by the symbol delay module,
and
representing the phase change caused by other parts of the link where the upper arm and the lower arm are located; then the combined light intensity I reaching the light detector0(t) can be expressed as:
wherein I (t) is the change of the incident light intensity with time, and I (t-tau) is the change of the signal light intensity with time after the time delay tau; s (t-tau) is the light field envelope after a delay of tau;
(t- τ) is the intrinsic phase change of the optical field after the time delay τ;
compared to direct detection, direct detection based on delayed autocorrelation introduces an additional term, namely
Wherein the first parenthesized term reflects the effect of modulation signal and noise on light intensity; the second parenthetical term represents the effect of the light source line width on the light intensity; the third parenthetical term represents the effect on light intensity of the phase difference caused by the two 3dB couplers in the time-delayed self-coherent interferometer.
In general, the phase difference caused by the 3dB coupler is fixed, and the light intensity change caused by the phase noise of the light source is slower than that of the modulation signal and the noise, so that the light intensity change after the modulation signal is delayed from being coherent is actually detected. The optical signals with different modulation formats have different delay autocorrelation intensity changes, so that the modulation formats of the signals can be identified through algorithm processing according to the change characteristics of the delay autocorrelation intensity.
In the present invention, step S30 specifically includes:
carrying out statistical analysis on the sampling result output by the sampling circuit;
equally dividing the sampling result range into a plurality of sections according to the maximum value and the minimum value of the sampling result;
and counting the frequency of the sampling result falling on each segment, thereby obtaining a frequency histogram of the strength distribution of the delayed coherent signal.
It should be noted that the minimum bandwidth (3dB) of the optical detector, the amplifying circuit and the sampling circuit employed in the present invention should be greater than half of the baud rate of the signal.
As shown in fig. 3, the apparatus for identifying a modulation format of an optical signal according to the present invention includes:
an optical power coupler 10 for extracting a portion of the light from the optical signal;
a delayed self-coherent interferometer 20, configured to perform delayed self-coherent interference processing on the extracted partial light to obtain a first optical signal;
a photodetector 30 for converting the first optical signal into an electrical signal;
the sampling circuit 50 is used for sampling the electric signals to obtain sampling results and converting the analog signals into digital signals;
the signal intensity distribution statistical module 60 is configured to perform distribution statistical analysis on the sampling result, that is, divide the sampling result into a plurality of segments according to the size range of the digital signal, and count the frequency of the sampling value on each segment to obtain a signal intensity distribution statistical result;
a frequency histogram feature extraction and analysis module 70, configured to draw a frequency histogram of interference light intensity according to the signal intensity distribution statistical result, and extract feature parameters related to signal modulation format identification in the frequency histogram;
and a modulation format type identification module 80, configured to determine an optical signal modulation format according to the characteristic parameters extracted from the frequency histogram, and output the result.
In the present invention, the optical fiber sampling circuit further includes an amplifying circuit 40, since the electrical signal output by the optical detector 30 is a photocurrent signal, the amplifying circuit is configured to convert the photocurrent signal into a voltage signal, and amplify the voltage signal, so that the voltage value matches with the working voltage range of the sampling circuit.
The present invention is not limited to the above-mentioned preferred embodiments, and any structural changes made under the teaching of the present invention shall fall within the scope of the present invention, which is similar or similar to the technical solutions of the present invention.
Claims (8)
1. An optical signal modulation format identification method is characterized by comprising the following steps:
step S10, extracting partial light from the optical signal, and performing time-delay self-coherent processing on the partial light to obtain a first optical signal;
step S20, converting the first optical signal into an electric signal and sampling;
step S30, performing distribution statistical analysis on the sampling result, and drawing a frequency histogram of the interference light intensity distribution;
step S40, extracting and analyzing characteristic parameters of a frequency histogram of interference light intensity distribution, and distinguishing modulation formats on optical signals according to the characteristic parameters;
in step S10, a delayed self-coherent interferometer is used to perform delayed self-coherent processing on the extracted partial light; the delay time difference between two arms of the delay self-coherent interferometer is greater than the code element period of the optical signal and less than the light source coherence time;
in step S40, the modulation format includes: NRZ-OOK, RZ-OOK, QPSK, and 16 QAM;
the distinguishing of the modulation formats on the optical signal according to the characteristic parameters specifically includes: under the conditions of small dispersion and low noise, judging according to the number and relative size of maximum values of the frequency histogram; the distinction between NRZ-OOK and RZ-OOK is judged according to the ratio of zero maximum to non-zero maximum; and under the conditions of large dispersion and high noise, carrying out modulation format identification according to the bandwidth of the frequency histogram.
2. The method according to claim 1, wherein step S30 comprises the steps of:
carrying out statistical analysis on the sampling result output by the sampling circuit;
equally dividing the sampling result range into a plurality of sections according to the maximum value and the minimum value of the sampling result;
and counting the frequency of the sampling result falling on each segment, thereby obtaining a frequency histogram of the strength distribution of the delayed coherent signal.
3. The method of claim 1, wherein in step S10, a 95:5 power coupler is used to extract 5% of the light from the optical signal.
4. The method of claim 1, wherein the time-delayed self-coherent interferometer is implemented using fiber coupler and splitter-based or silicon-based integrated optics.
5. The method of claim 1, wherein the delay difference between the two arms of the time-delayed self-coherent interferometer is a fixed value or a tunable value.
6. The method of claim 1, wherein in step S20, before sampling, the electrical signal converted from the optical signal is amplified by an amplifying circuit to match the voltage value with the operating voltage range of the analog-to-digital converter performing sampling.
7. An apparatus for identifying a modulation format of an optical signal, comprising:
an optical power coupler for extracting a portion of the light from the optical signal;
the time delay self-coherent interferometer is used for carrying out time delay self-coherent interference processing on the extracted partial light to obtain a first light signal, and the time delay difference between two arms of the time delay self-coherent interferometer is greater than the code element period of the light signal and less than the light source coherence time;
a photodetector for converting the first optical signal into an electrical signal;
the sampling circuit is used for sampling the electric signal to obtain a sampling result;
the signal intensity distribution statistical module is used for carrying out distribution statistical analysis on the sampling result to obtain a signal intensity distribution statistical result;
the frequency histogram feature extraction and analysis module is used for drawing a frequency histogram of interference light intensity according to the signal intensity distribution statistical result and extracting feature parameters related to signal modulation format identification in the frequency histogram;
a modulation format type identification module, configured to identify a modulation format of the optical signal according to the characteristic parameter, where the modulation format includes: NRZ-OOK, RZ-OOK, QPSK, and 16 QAM;
the identifying, according to the characteristic parameter, a modulation format of the optical signal specifically includes: under the conditions of small dispersion and low noise, the modulation formats are distinguished according to the number of maximum values and relative size of the frequency histogram; the distinction between NRZ-OOK and RZ-OOK is judged according to the ratio of zero maximum to non-zero maximum; and under the conditions of large dispersion and high noise, carrying out modulation format identification according to the bandwidth of the frequency histogram.
8. The apparatus of claim 7, wherein a minimum bandwidth of the photodetector, amplifying circuit, and sampling circuit is greater than half of a signal baud rate.
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