CN103399418A - Method and device for compensating nonlinearity of electro-absorption modulator - Google Patents
Method and device for compensating nonlinearity of electro-absorption modulator Download PDFInfo
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Abstract
The invention provides a method and a device for compensating nonlinearity of an electro-absorption modulator. A system comprises a laser, an electro-absorption modulator, a spectral filter, a preprocessing module, and an optical receiver, wherein the laser is used for generating an optical signal, the electro-absorption modulator connected with the laser is used for modulating the optical signal to obtain multi-sideband spectrum of the optical signal, the spectral filter connected with the electro-absorption modulator is used for controlling the amplitude of 0-order spectral sideband of the optical signal according to the multi-sideband spectrum so as to obtain the compensated optical signal, the processing module connected with the spectral filter is used for preprocessing the compensated optical signal, and the optical receiver connected with the preprocessing module is used for converting the preprocessed optical signal into an electrical signal and outputting the electrical signal. The method and the device have the advantages that the 0-order spectral sideband of the optical signal is processed, thus the nonlinearity of the modulator is compensated only by spectral band-stop filtering and limitation of the microwave photonic system is avoided.
Description
Technical Field
The invention relates to the field of microwave electronic application, in particular to a method and a device for compensating nonlinearity of an electroabsorption modulator.
Background
Microwave photonics is the field of photonics means to handle microwave millimeter wave signals. Due to the characteristics of high frequency, large bandwidth and the like, the method can break through the technical bottleneck of conventional electronic means, and is applied to the fields of radio-over-fiber systems, optical true delay systems, microwave photonic filters, photonic generation of radio-frequency arbitrary waveforms and the like. Electro-absorption modulators (EAMs) are one of the most common electro-optic modulators, and operate at frequencies up to the millimeter-wave range. The modulator is widely applied to microwave photon systems due to the characteristics of high working frequency, small driving voltage, low power consumption, small size, easy integration with semiconductor devices and the like. However, these systems have many inherent problems to be solved, including system dynamic range degradation due to nonlinear distortion inherent in electro-absorption modulators, and the like. To increase the dynamic range of the system, techniques to compensate for the non-linearity of the electro-absorption modulator must be implemented.
Current methods of compensating for electro-absorption modulator nonlinearities can be classified into 2 categories:
1. a circuit predistortion method. According to the method, a special pre-distortion circuit is designed to perform waveform pre-distortion on an input signal, so that the signal still keeps high fidelity after the nonlinear distortion of an electro-absorption modulator. This is a commonly used technical approach in the field of electronics. However, the circuit structure is signal dependent, and the operating frequency range of the circuit usually does not exceed 1GHz, and is only up to a specific frequency band of 3.1GHz to 4.8 GHz. The application requirements for the microwave frequency band and even the millimeter wave frequency band cannot be met. In addition, the carrier-to-interference ratio can be improved by only about 15dB to 20 dB.
2. A method of photonics. The method has 3 specific implementation means at present.
1) Means for dual wavelength input. An EAM is a wavelength dependent device with different wavelengths of operating light, having different modulation curves, i.e., having different non-linear characteristics. By utilizing the nonlinear difference between the two wavelengths, a set of dual-wavelength input optical path system is designed, and the two modulated signals are mutually superposed to enable nonlinear components contained in the two modulated signals to be mutually inhibited, so that the function of compensating nonlinearity is realized. The method can improve the distortion-free dynamic range of the system by 8 dB. The approach additionally adds an input light source, so that the system becomes more complex and the energy consumption is increased correspondingly. And because the nonlinear difference between the dual wavelengths is not arbitrary, the control depends on the control of the wavelength and the control of the bias voltage of the modulator, the control is complex, and the realization effect is not ideal.
2) A means of dual polarization electro-absorption modulator. An EAM is a polarization dependent device in which the modulation curves of the input light for the two polarization states are also different. Thus, before entering the EAM, the polarizer is placed and adjusted in polarization direction to distribute the power ratio of the two polarized light entering the EAM. And the output end of the EAM is provided with and adjusts the polarization analyzing direction of the polarization analyzer, so that the two polarized lights are mutually superposed, the nonlinear components are mutually inhibited, and the function of compensating nonlinearity is realized. The distortion-free dynamic range of the system can be improved by 9.5dB by using the method. Although the method only adds polarization and polarization analysis functions, the nonlinear adjustment depends on the control of the polarization direction and the control of the bias voltage of the modulator, and the control is complicated. The integration of the EAM with other semiconductor devices is not facilitated by the introduction of polarization control to the modulator.
3) Means for parallel modulation by dual modulators. Two modulation optical paths are designed by utilizing the nonlinear difference between the two EAMs, so that two parallel modulation signals are mutually superposed, the nonlinear components are mutually inhibited, and the function of compensating nonlinearity is realized. The method additionally adds key photoelectric devices such as a light source and a modulator, the light path structure is complex and large, and the original light path structure is damaged.
Disclosure of Invention
The object of the present invention is to solve at least one of the technical drawbacks mentioned above.
To this end, it is an object of the invention to propose a device for compensating for the non-linearity of an electro-absorption modulator.
It is another object of the present invention to provide a method for compensating for electro-absorption modulator nonlinearity.
To achieve the above object, an embodiment of an aspect of the present invention provides an apparatus for compensating nonlinearity of an electro-absorption modulator, including: a laser for generating an optical signal; the electric absorption modulator is connected with the laser and is used for modulating the optical signal to obtain a multi-sideband spectrum of the optical signal, wherein the multi-sideband spectrum comprises a 0-order spectral sideband, a 1-order spectral sideband and a 2-order spectral sideband; the spectral filter is connected with the electric absorption modulator and is used for controlling the amplitude of a 0-order spectral sideband of the optical signal according to the multi-sideband spectrum of the optical signal so as to obtain a compensated optical signal; the preprocessing module is connected with the spectral filter and used for preprocessing the compensated optical signal; and the optical receiver is connected with the preprocessing module and used for converting the preprocessed optical signal into an electric signal and outputting the electric signal.
According to the device provided by the embodiment of the invention, the 0-order spectral sideband of the optical signal is processed, so that the nonlinearity compensation of the modulator is realized only in a spectral band-stop filtering mode, and the device is not limited by a microwave photonic system structure.
In one embodiment of the invention, the pre-processing comprises amplification, filtering and transmission of the optical signal.
In one embodiment of the present invention, the spectral filter controls the amplitude of the 0 th order spectral sideband of the optical signal by band-stop filtering the multi-sideband spectrum of the optical signal to obtain a compensated optical signal.
In one embodiment of the invention, the spectral filter controls the attenuation power of the 0 th order spectral sideband of the optical signal by the stop-band rejection ratio of the spectral filter to control the amplitude of the 0 th order spectral sideband of the optical signal.
In one embodiment of the invention, the spectral filter controls the attenuated power of the 0 th order spectral sideband of the optical signal, and the amplitude of the third order intermodulation (IMD 3) frequency component in the electrical signal detected by the receiver is represented by the equation,
wherein,
for the responsivity of the photodetector, E1Is a constant term, alpha is 0 order spectral sideband attenuation power unit is dB, En,p(Vb,V0) Is the amplitude of the optical frequency component, n is the optical frequency component in the corresponding n-order spectral sideband, VbIs the bias voltage of an electro-absorption modulator, V0For input to electro-absorption modulatorsThe amplitude of the electrical signal is,
for the optical frequency component with respect to the central optical carrier ω0The phase of (c).
To achieve the above object, another aspect of the embodiments of the present invention provides a method for compensating nonlinearity of an electro-absorption modulator, including:
modulating laser light emitted by a laser to obtain a multi-sideband spectrum of the optical signal, wherein the multi-sideband spectrum comprises a 0-order spectral sideband, a 1-order spectral sideband and a 2-order spectral sideband; performing amplitude control on a 0-order spectral sideband of the optical signal according to a multi-sideband spectrum of the optical signal to obtain a compensated optical signal; preprocessing the compensated optical signal; and converting the preprocessed compensation optical signal into an electrical signal and outputting the electrical signal.
According to the method provided by the embodiment of the invention, the 0-order spectral sideband of the optical signal is processed, so that the nonlinear compensation of the modulator is realized only in a spectral band-stop filtering mode, and the method is not limited by the structure of a microwave photonic system.
In one embodiment of the invention, the pre-processing comprises amplification, filtering and transmission of the optical signal.
In one embodiment of the invention, the compensated optical signal is obtained by band-stop filtering a multi-sideband spectrum of the optical signal and controlling the amplitude of the 0 th order spectral sideband of the optical signal.
In one embodiment of the invention, the attenuation power of the 0 th order spectral sideband of the optical signal is controlled by the stopband rejection ratio of the bandstop filter to control the amplitude of the 0 th order spectral sideband of the optical signal.
In one embodiment of the invention, the attenuated power of the 0 th order spectral sidebands of the optical signal is controlled, and the amplitude of the third order intermodulation (IMD 3) frequency component in the resulting electrical signal detected by the receiver is represented by the equation,
wherein,for the responsivity of the photodetector, E1Is a constant term, alpha is 0 order spectral sideband attenuation power unit is dB, En,p(Vb,V0) Is the amplitude of the optical frequency component, n is the optical frequency component in the corresponding n-order spectral sideband, VbIs the bias voltage of an electro-absorption modulator, V0To input the amplitude of the electrical signal to the electroabsorption modulator,
for the optical frequency component with respect to the central optical carrier ω0The phase of (c).
Additional aspects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.
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The foregoing and/or additional aspects and advantages of the present invention will become apparent and readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which:
FIG. 1 is a block diagram of an apparatus for compensating for electro-absorption modulator nonlinearity according to one embodiment of the present invention;
FIG. 2 is a schematic diagram of an embodiment of a spectral filter that is programmable, in accordance with one embodiment of the present invention;
FIGS. 3 (a) and 3 (b) are graphs of stopband rejection ratio versus fundamental power and IMD3 output power, respectively, in accordance with one embodiment of the present invention;
fig. 4 is a graph of carrier-to-interference ratio as a function of stopband rejection ratio according to one embodiment of the present invention;
FIG. 5 is a schematic diagram of an apparatus for compensating for electro-absorption modulator nonlinearity in accordance with one embodiment of the present invention;
FIG. 6 is a graph of the output spectrum of an optical transmit module and an ideal power response curve of a spectral filter according to one embodiment of the present invention;
FIG. 7 (a) and FIG. 7 (b) are graphs of the frequency spectrum of the output electrical signal without and with compensation processing, respectively, for the optical signal according to an embodiment of the present invention;
FIG. 8 is a graph of dynamic range boosting performance according to one embodiment of the present invention; and
FIG. 9 is a flow diagram of a method of compensating for electro-absorption modulator nonlinearity in accordance with one embodiment of the present invention.
Detailed Description
Reference will now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like or similar reference numerals refer to the same or similar elements or elements having the same or similar function throughout. The embodiments described below with reference to the drawings are illustrative only and should not be construed as limiting the invention.
In the description of the present invention, it is to be understood that the terms "first", "second" and the like are used for descriptive purposes only and are not to be construed as indicating or implying relative importance or implying any number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of the present invention, "a plurality" means two or more unless specifically defined otherwise.
Fig. 1 is a block diagram of an apparatus for compensating for non-linearity of an electro-absorption modulator according to an embodiment of the present invention. As shown in fig. 1, the apparatus for compensating nonlinearity of an electro-absorption modulator according to an embodiment of the present invention includes a laser 100, an electro-absorption modulator 200, a spectral filter 300, a pre-processing module 400, and an optical receiver 500.
Wherein the laser 100 is used to generate an optical signal.
Electro-absorption modulator 200 the electro-absorption modulator is coupled to the laser for modulating the optical signal to obtain a multi-sideband spectrum of the optical signal, wherein the multi-sideband spectrum includes a 0 th order spectral sideband, a 1 st order spectral sideband, and a 2 nd order spectral sideband. The spectral filter 300 is connected to the electro-absorption modulator, and is configured to perform amplitude control on a 0 th-order spectral sideband of the optical signal according to a multi-sideband spectrum of the optical signal to obtain a compensated optical signal.
The present invention uses the spectral variation of the two-tone signal input and output as an analysis of the non-linearity characteristics of the electro-absorption modulator and an evaluation of the non-linearity compensation performance.
Suppose the input microwave signal is of amplitude V0Angular frequency of omega1And Ω2The two-tone signal of (a), i.e. the input signal, is:
Assume that the input laser is E0exp(jω0t) in which E0As amplitude of the light field, omega0Is the laser angular frequency. Assuming an attenuation of the 0 th order sideband power of the modulator output spectrum by α dB, the 5 spectral sidebands contained by the electro-absorption modulator output are:
Where n represents an nth-order spectral sideband, and n =0 (term 2 in the formula (2)) represents a 0 th-order spectral sideband. E1Is a constant term, ω0+pΩ1+(n-p)Ω2For all optical frequency components in the nth order spectral sidebands, En,p(Vb,V0) Is the amplitude of the optical frequency component, which is the electroabsorption modulator bias voltage VbSum signal amplitude V0As a function of (a) or (b),for the optical frequency component with respect to the central optical carrier ω0And has a phase ofn,p(Vb,V0)=E-n,-p(Vb,V0)，
After the optical signal is subjected to square-law detection by a photoelectric detector, the output electric signal comprises countless multiple frequencies which can be p omega1+qΩ2Indicating that where all frequencies of p + q = s constitute the s-order spectral band, the output signal theoretically contains an infinite number of spectral bands. The frequency bands (e.g. s and s +1 order frequency bands) across the octave can be directly filtered by the band pass filter, and what the present invention intends to suppress is third order intermodulation distortion in the signal band, so that the 1 order frequency band (s = 1) where the fundamental signal is located is the frequency band of interest of the present invention. Assuming that the photodetector responsivity is
So that the 1 st order spectral band of the output microwave signal is
Wherein,
For the fundamental frequency (i.e. frequency omega)1And Ω2) And IMD3 frequency (i.e., 2 Ω)2-Ω1And 2 Ω1-Ω2) Taking n =0 and n =1 as examples, the frequencies correspond to the frequency Ω2And 2 Ω2-Ω1The amplitudes are obtained as follows:
From the above formula, it can be seen that V is suitable forbThe value of α can be such that there is a value of α in the above formula such thatI1(= 0), and I0Not equal to 0, thereby achieving IMD3 suppression while preserving fundamental frequencies.
In one embodiment of the present invention, the laser 100 and the electroabsorption modulator 200 are typically integrated onto one chip.
The center wavelength of the stop band of the spectral filter corresponds to the wavelength output by the laser 100, and the stop band rejection ratio (corresponding to the above-mentioned 0 th order spectral sideband power attenuation factor α) may be fixed, or a filter with a programmable rejection ratio control may be used. When the band-stop filter with fixed stop-band rejection ratio is adopted, the specific stop-band rejection ratio is designed according to requirements.
In one embodiment of the present invention, any device having an optical band-stop filtering function can be used for the nonlinear compensation apparatus according to the present invention, such as a band-stop filter of a Fiber Bragg Grating (FBG).
In one embodiment of the invention, the spectral filter controls the amplitude of the 0 th order spectral sidebands of the optical signal by band-stop filtering the multi-sideband spectrum of the optical signal to obtain a compensated optical signal. The spectral filter controls the attenuation power of the 0 th order spectral sideband of the optical signal by the stop-band rejection ratio of the spectral filter to control the amplitude of the 0 th order spectral sideband of the optical signal.
The preprocessing module 400 is connected to the spectral filter and is configured to preprocess the compensated optical signal. The pre-processing includes any sub-function of the microwave optical subsystem and combinations of sub-functions including, but not limited to, amplification, filtering, and transmission of optical signals.
The optical receiver 500 is connected to the preprocessing module, and is configured to convert the preprocessed optical signal into an electrical signal and output the electrical signal.
Fig. 2 is a schematic diagram of an embodiment in which the spectral filter is a programmable spectral filter according to an embodiment of the present invention. As shown in fig. 2, the programmable spectral filter comprises a spectrum-to-space mapped spectrum demultiplexer 201, a multi-way parallel optical modulator 202, and a space-to-spectrum mapped spectrum multiplexer 203. The spectrum demultiplexer 201 and the spectrum multiplexer 203 may use a grating and a lens to form a spatial light path type frequency-space multiplexing demultiplexing module, the optical modulator 202 may use a liquid crystal spatial optical modulator, and spatial light paths are formed between the spectrum demultiplexer 201 and the optical modulator 202 and between the optical modulator 202 and the spectrum multiplexer 203. The spectrum demultiplexer 201 and the spectrum multiplexer 203 may also use an optical fiber type wavelength division multiplexing demultiplexing device such as an arrayed waveguide grating, the optical modulator 202 correspondingly uses a modulator array, and the spectrum demultiplexer 201 and the optical modulator 202 and the spectrum multiplexer 203 are connected by a multiplex optical fiber. The spectrum demultiplexer 201 branches different wavelengths to different positions on the optical modulator 202, the optical modulator 202 is controlled by a control signal, independent optical amplitude modulation is performed on the optical waves at different positions, different power attenuation can be achieved on the optical waves with different wavelengths, and therefore the filtering function of a signal spectrum is achieved.
In one embodiment of the present invention, the band-stop filter can be programmed to obtain the optimum power attenuation required for a particular electro-absorption modulator by adjusting the stopband rejection ratio α. Fig. 3 (a) and 3 (b) are graphs of stopband rejection ratio versus fundamental power and IMD3 output power, respectively, according to one embodiment of the present invention. As shown in fig. 3 (a) and 3 (b), as a is changed, the fundamental frequency and IMD3 output power are also changed. The modulator bias point at this time was set to 2V and the microwave input power was 10 dBm. As can be seen from fig. 3 (a) and 3 (b), the fundamental frequency power shows a certain fading trend with the increase of α, but the change is not drastic. IMD3, however, exhibits a tendency to decrease first and then increase, and a rapid power fade will occur near a certain value of α, causing IMD3 to rapidly drop to 0 at that value. Fig. 4 is a graph of carrier-to-interference ratio as a function of stopband rejection ratio according to one embodiment of the invention. As shown in fig. 4, the corresponding carrier-to-interference ratio varies with a. When the IMD3 power is 0, the corresponding carrier-to-interference ratio reaches a maximum value, and the corresponding stop-band rejection ratio α is the optimal 0-order spectral sideband (0-OSB) power attenuation value. In this embodiment, the carrier-to-interference ratio reaches a maximum and IMD3 is completely suppressed when the power attenuation of 0-OSB is about 21.2 dB.
FIG. 5 is a schematic diagram of an apparatus for compensating for electro-absorption modulator nonlinearity in accordance with one embodiment of the present invention. As shown in fig. 5, the input microwave signal is modulated onto an optical carrier by an electro-absorption modulator 102, the output spectrum of the electro-absorption modulator 102 contains 5 spectral sidebands,
the spectral filter 104 performs amplitude control on the 0 th order spectral sideband of the optical signal according to the multi-sideband spectrum of the optical signal to obtain a compensated optical signal, the pre-processing module 105 pre-processes the compensated optical signal, and finally, the optical receiver 106 converts the pre-processed optical signal into an electrical signal and outputs the electrical signal.
In one embodiment of the invention, the integrated module 103 uses Apogee Photonics' 40Gb optical transmitter integrated module (LIM 400), which contains a laser 101 and an electro-absorption modulator 102, and is also called EML. The bias voltage of the electro-absorption modulator 102 can be adjusted between 0-5V. When the laser driving current is set to 70mA, the output wavelength is 1546.95nm, and the corresponding output optical power is 6dBm when the modulator bias point is 0V. FIG. 6 is a graph of the output spectrum of an optical transmit module and an ideal power response curve of a spectral filter according to one embodiment of the present invention. When the input signal is a two-tone signal with frequencies of 19.46GHz and 19.54GHz, the output spectrum of the optical transmitting module 103 is shown by a solid line in fig. 6, and the ideal power response curve corresponding to the spectral filter 104 is shown by a dashed line in fig. 6. The 0 th order spectral sideband in fig. 6 is the spectral sideband to be processed by the present invention, and the stop-band rejection ratio α of the filter is the power attenuation factor of the 0 th order spectral sideband.
Fig. 7 (a) and 7 (b) are frequency spectrums of output electrical signals without and with compensation processing on optical signals, respectively, according to an embodiment of the present invention. As shown in fig. 7 (a) and 7 (b), when the microwave input power is 5dBm, the corresponding spectrum change before and after the nonlinearity is compensated. The carrier-to-interference ratio is improved from 33dB before the suppression to 69dB after the suppression, and is improved by 36 dB.
FIG. 8 is a graph of dynamic range boosting performance according to one embodiment of the present invention. As shown in FIG. 8, the noise floor measured by the system of the present example was-146 dBm/Hz using Agilent E4446A. Thus, the dynamic range of the system is from
Is lifted to
The dynamic range of the system is improved by 14.7 dB.
According to the device provided by the embodiment of the invention, the 0-order spectral sideband of the optical signal is processed, so that the nonlinearity compensation of the modulator is realized only in a spectral band-stop filtering mode, and the device is not limited by a microwave photonic system structure.
FIG. 9 is a flow diagram of a method of compensating for electro-absorption modulator nonlinearity in accordance with one embodiment of the present invention. As shown in fig. 9, a method of compensating for electro-absorption modulator nonlinearity according to an embodiment of the present invention includes the steps of:
step S101, modulating laser light emitted by a laser to obtain a multi-sideband spectrum of an optical signal, wherein the multi-sideband spectrum includes a 0-order spectral sideband, a 1-order spectral sideband, and a 2-order spectral sideband.
And S102, performing amplitude control on a 0-order spectral sideband of the optical signal according to the multi-sideband spectrum of the optical signal to obtain a compensated optical signal.
Step S103, preprocessing the compensated optical signal. The pre-processing includes any sub-function of the microwave optical subsystem and combinations of sub-functions including, but not limited to, amplification, filtering, and transmission of optical signals.
And step S104, converting the preprocessed compensation optical signal into an electric signal and outputting the electric signal.
In one embodiment of the invention, the compensated optical signal is obtained by band-stop filtering a multi-sideband spectrum of the optical signal and controlling the amplitude of the 0 th order spectral sideband of the optical signal. The stopband rejection ratio through the bandstop filtering controls the attenuation power of the 0 th order spectral sideband of the optical signal to control the amplitude of the 0 th order spectral sideband of the optical signal. The attenuated power of the 0 th order spectral sidebands of the optical signal is controlled and the amplitude of the third order intermodulation (IMD 3) frequency component in the resulting electrical signal, which is ultimately detected by the receiver, is expressed by the equation,
wherein,
for the responsivity of the photodetector, E1Is a constant term, alpha is 0 order spectral sideband attenuation power unit is dB, En,p(Vb,V0) Is the amplitude of the optical frequency component, n is the optical frequency component in the corresponding n-order spectral sideband, VbIs the bias voltage of an electro-absorption modulator, V0To input the amplitude of the electrical signal to the electroabsorption modulator,
for the optical frequency component with respect to the central optical carrier ω0The phase of (c).
The present invention uses the spectral variation of the two-tone signal input and output as an analysis of the non-linearity characteristics of the electro-absorption modulator and an evaluation of the non-linearity compensation performance.
Assuming input microwave signal as amplitudeIs a V0Angular frequency of omega1And Ω2The two-tone signal of (a), i.e. the input signal, is:
Assume that the input laser is E0exp(jω0t) in which E0As amplitude of the light field, omega0Is the laser angular frequency. Assuming an attenuation of the 0 th order sideband power of the modulator output spectrum by α dB, the 5 spectral sidebands contained by the electro-absorption modulator output are:
Where n represents an nth-order spectral sideband, and n =0 (term 2 in the formula (2)) represents a 0 th-order spectral sideband. E1Is a constant term, ω0+pΩ1+(n-p)Ω2For all optical frequency components in the nth order spectral sidebands, En,p(Vb,V0) Is the amplitude of the optical frequency component, which is the electroabsorption modulator bias voltage VbSum signal amplitude V0As a function of (a) or (b),
for the optical frequency component with respect to the central optical carrier ω0And has a phase ofn,p(Vb,V0)=E-n,-p(Vb,V0)，
After the optical signal is subjected to square-law detection by a photoelectric detector, the output electric signal comprises countless multiple frequencies which can be p omega1+qΩ2Indicating that where all frequencies of p + q = s constitute the s-order spectral band, the output signal theoretically contains an infinite number of spectral bands. The frequency bands (e.g. s and s +1 order frequency bands) across the octave can be directly filtered by the band pass filter, and what the present invention intends to suppress is third order intermodulation distortion in the signal band, so that the 1 order frequency band (s = 1) where the fundamental signal is located is the frequency band of interest of the present invention. Assuming that the photodetector responsivity isSo that the 1 st order spectral band of the output microwave signal is
Wherein,
-formula (4)
For the fundamental frequency (i.e. frequency omega)1And Ω2) And IMD3 frequency (i.e., 2 Ω)2-Ω1And 2 Ω1-Ω2) Taking n =0 and n =1 as examples, the frequencies correspond to the frequency Ω2And 2 Ω2-Ω1The amplitudes are obtained as follows:
From the above formula, it can be seen that V is suitable forbThere may be a value of α in the above formula such that I1(= 0), and I0Not equal to 0, thereby achieving IMD3 suppression while preserving fundamental frequencies.
Fig. 3 (a) and 3 (b) are graphs of stopband rejection ratio versus fundamental power and IMD3 output power, respectively, according to one embodiment of the present invention. As shown in fig. 3 (a) and 3 (b), as a is changed, the fundamental frequency and the output power of IMD3 are also changed. The modulator bias point at this time was set to 2V and the microwave input power was 10 dBm. As can be seen from fig. 3 (a) and 3 (b), the fundamental frequency power shows a certain fading trend with the increase of α, but the change is not drastic. IMD3, however, exhibits a tendency to decrease first and then increase, and a rapid power fade will occur near a certain value of α, causing IMD3 to rapidly drop to 0 at that value. Fig. 4 is a graph of carrier-to-interference ratio as a function of stopband rejection ratio according to one embodiment of the invention. As shown in fig. 4, the corresponding carrier-to-interference ratio varies with a. When the IMD3 power is 0, the corresponding carrier-to-interference ratio reaches a maximum value, and the corresponding stop-band rejection ratio α is the optimal 0-order spectral sideband (0-OSB) power attenuation value. In this embodiment, the carrier-to-interference ratio reaches a maximum and IMD3 is completely suppressed when the power attenuation of 0-OSB is about 21.2 dB.
According to the method provided by the embodiment of the invention, the 0-order spectral sideband of the optical signal is processed, so that the nonlinear compensation of the modulator is realized only in a spectral band-stop filtering mode, and the method is not limited by the structure of a microwave photonic system.
It should be understood that the specific description of the method embodiment of the present invention is the same as the operation and processing of the various modules and units of the apparatus embodiment, and thus will not be described in detail.
Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention, and that variations, modifications, substitutions and alterations can be made in the above embodiments by those of ordinary skill in the art without departing from the principle and spirit of the present invention.
Claims (10)
1. An apparatus for compensating for non-linearity of an electro-absorption modulator, comprising:
a laser for generating an optical signal;
an electro-absorption modulator connected to the laser for modulating the optical signal to obtain a multi-sideband spectrum of the optical signal, wherein the multi-sideband spectrum includes a 0-order spectral sideband, a 1-order spectral sideband and a 2-order spectral sideband;
the spectral filter is connected with the electric absorption modulator and is used for carrying out amplitude control on a 0-order spectral sideband of the optical signal according to a multi-sideband spectrum of the optical signal so as to obtain a compensated optical signal;
the preprocessing module is connected with the spectral filter and used for preprocessing the compensated optical signal; and
and the optical receiver is connected with the preprocessing module and used for converting the preprocessed optical signal into an electric signal and outputting the electric signal.
2. An apparatus for compensating for electro-absorption modulator nonlinearity as in claim 1, wherein said pre-processing comprises amplification, filtering and transmission of said optical signal.
3. The apparatus for compensating for non-linearity of an electro-absorption modulator of claim 1 wherein the spectral filter controls the amplitude of the 0 th order spectral sidebands of the optical signal by band-stop filtering a multi-sideband spectrum of the optical signal to obtain a compensated optical signal.
4. The apparatus according to claim 3, wherein the spectral filter controls the attenuation power of the 0 th order spectral sideband of the optical signal by the stop-band rejection ratio of the spectral filter to control the amplitude of the 0 th order spectral sideband of the optical signal.
5. The apparatus for compensating for electro-absorption modulator nonlinearity of claim 4, wherein said spectral filter controls the attenuation power of the 0 th order spectral sideband of said optical signal, and the amplitude of the third order intermodulation frequency components in the electrical signal detected by said receiver is represented by the following equation,
wherein,
for the responsivity of the photodetector, E1Is a constant term, alpha is 0 order spectral sideband attenuation power unit is dB, En,p(Vb,V0) Is the amplitude of the optical frequency component, n is the optical frequency component in the corresponding n-order spectral sideband, VbIs the bias voltage of an electro-absorption modulator, V0To input the amplitude of the electrical signal to the electroabsorption modulator,
for the optical frequency component with respect to the central optical carrier ω0The phase of (c).
6. A method of compensating for electro-absorption modulator nonlinearity comprising the steps of:
modulating laser light emitted by a laser to obtain a multi-sideband spectrum of the optical signal, wherein the multi-sideband spectrum comprises a 0-order spectral sideband, a 1-order spectral sideband and a 2-order spectral sideband;
performing amplitude control on a 0-order spectral sideband of the optical signal according to a multi-sideband spectrum of the optical signal to obtain a compensated optical signal;
preprocessing the compensated optical signal; and
and converting the preprocessed compensation optical signal into an electrical signal and outputting the electrical signal.
7. A method of compensating for electro-absorption modulator nonlinearity as in claim 6, wherein said pre-processing comprises amplification, filtering and transmission of said optical signal.
8. The method of compensating for electrical absorption modulator nonlinearity of claim 6, wherein said compensated optical signal is obtained by band-stop filtering a multi-sideband spectrum of said optical signal and controlling the amplitude of the 0 th order spectral sideband of said optical signal.
9. The method of compensating for electro-absorption modulator nonlinearity of claim 8, wherein the attenuation power of the 0 th order spectral sideband of the optical signal is controlled by the stopband rejection ratio of the bandstop filter to control the amplitude of the 0 th order spectral sideband of the optical signal.
10. The method of compensating for electro-absorption modulator nonlinearity of claim 9, wherein the attenuated power of the 0-order spectral sidebands of the optical signal is controlled, and the amplitude of the third-order intermodulation frequency components in the electrical signal detected by the receiver is represented by the equation,
wherein,for the responsivity of the photodetector, E1Is a constant term, alpha is 0 order spectral sideband attenuation power unit is dB, En,p(Vb,V0) Is the amplitude of the optical frequency component, n is the optical frequency component in the corresponding n-order spectral sideband, VbIs the bias voltage of an electro-absorption modulator, V0To input the amplitude of the electrical signal to the electroabsorption modulator,
for the optical frequency component with respect to the central optical carrier ω0The phase of (c).
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