CN108692816B - Fast spectrum measurement method and device based on image frequency suppression - Google Patents
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Abstract
The invention discloses a fast spectrum measurement method based on image frequency suppression, which utilizes a 90-degree optical mixer to mix frequency of an optical signal to be measured and an optical local oscillation signal to generate four paths of differential signals; using two balanced photoelectric detectors to carry out balanced photoelectric detection on the four paths of differential signals; converting the output I, Q two paths of analog signals into digital signals, performing phase shift of +90 degrees/90 degrees on the Q paths of digital signals in a digital domain, and adding the Q paths of digital signals and the I paths of digital signals to obtain frequency spectrums on the left side/right side of the center frequency of the optical signal to be measured in the frequency band corresponding to the optical local oscillation signal; changing the frequency of the optical local oscillator signal and repeating the steps; and finally, combining the obtained series of optical signal frequency spectrums to be detected into a broadband frequency spectrum of the optical signal to be detected. The invention also discloses a rapid spectrum measuring device based on image frequency suppression. The invention can greatly improve the measuring speed on the premise of ensuring the measuring resolution.
Description
Technical Field
The invention relates to a spectral measurement method, in particular to a fast spectral measurement method and a fast spectral measurement device based on image frequency suppression, and belongs to the technical field of microwave photonics.
Background
With the rapid development of photonic technology, broadband services, increasing service quality requirements, and exponentially increasing access devices are emerging in recent years, so that an optical fiber broadband network becomes a key development content. The development and application of Dense Wavelength Division Multiplexing (DWDM) and other high-speed modulation techniques are becoming necessary trends. However, this has brought with it that the bandwidth (or granularity of spectral multiplexing) of a single channel of an optical information system is getting smaller, for example: the channel interval of an ultra-dense wavelength division multiplexing passive optical network (UDWDM-PON) which is one of the next-generation optical access network standards is in GHz level; optical frequency division orthogonal multiplexing (OFDM) system subcarrier bandwidths are typically on the order of hundreds of MHz; microwave photon systems require that wireless channels with intervals of tens of MHz can be distinguished, the resolution of the traditional spectrum instrument is far greater than the precision, and sub-picometers (1 pm-10) are urgently needed-12m) and even femtocells (1fm 10-15m) high-performance optical parameter detection instrument equipment such as a spectrometer with magnitude resolution.
After the ultra-wide wavelength range Tuned Laser (TLS) technology is mature, a coherent optical spectrum analysis technology (COSA) is developed, a series of coherent spectrums between signal light to be measured and local oscillator light are obtained by wavelength scanning of a scanning laser as the local oscillator light by utilizing the interference principle in wave optics, and then the spectrum of the signal light to be measured is obtained by a signal processing means based on known local oscillator light parameters. Coherent spectrometers with resolutions up to 5MHz (0.04pm) have been developed by French APEX.
Fig. 1 is a schematic structural diagram of a typical spectral measurement device based on a swept-frequency light source and coherent photoelectric detection, which mainly includes a main control unit, a swept-frequency laser, an optical coupler, a balanced optical detector, and a signal acquisition and processing module. The working principle is as follows: at any moment, the local oscillator optical signal (output signal of the frequency sweeping light source) and the optical signal to be measured interact in the optical coupler; then, coherent reception is carried out by using a balanced photoelectric detector, the analog electric signal is converted into a digital signal by using a signal acquisition and processing module, and a narrow-band electric signal carrying optical signal information to be detected at the instantaneous frequency of a local oscillator signal is output; at the next moment, the output frequency of the local oscillation optical signal is changed, and the process is repeated; and finally, the main control unit processes data by using an amplitude extraction algorithm by taking the output of the swept-frequency light source as a reference.
The spectrum measurement method based on the swept-frequency light source and the coherent photoelectric detection utilizes the low-frequency photoelectric detector to extract the components of the signal to be measured at two sides of the swept-frequency sideband, and takes a proper integration range to integrate the power spectrum as the amplitude value of the signal to be measured at the frequency of the swept-frequency sideband. Although the measurement scheme has measurement accuracy in the order of MHz, the resolution of the scheme depends on the frequency sweep interval of the frequency sweep light source and the bandwidth of the low-pass filter, the higher the resolution is, the smaller the frequency interval of adjacent scanning points is, the more the number of frequency sweep points is needed in the same measurement range, and the longer the measurement time is. The contradiction between resolution and measurement range and measurement speed is the biggest obstacle to the practical use of the technology, and no effective solution is reported at present.
Disclosure of Invention
The invention aims to overcome the defect of too low measurement speed of the existing spectral measurement technology based on a sweep frequency light source and coherent photoelectric detection, and provides a fast spectral measurement method and a fast spectral measurement device based on image frequency suppression, which can greatly improve the measurement speed on the premise of ensuring the measurement resolution.
The invention specifically adopts the following technical scheme to solve the technical problems:
a fast spectrum measurement method based on image frequency suppression utilizes a 90-degree optical mixer to mix frequency of an optical signal to be measured and an optical local oscillation signal to generate four-way differential signals; using two balanced photoelectric detectors to carry out balanced photoelectric detection on the four paths of differential signals; converting the output I, Q two paths of analog signals into digital signals, performing phase shift of +90 degrees/90 degrees on the Q paths of digital signals in a digital domain, and adding the Q paths of digital signals and the I paths of digital signals to obtain frequency spectrums on the left side/right side of the center frequency of the optical signal to be measured in the frequency band corresponding to the optical local oscillation signal; changing the frequency of the optical local oscillator signal and repeating the steps; and finally, combining the obtained series of optical signal frequency spectrums to be detected into a broadband frequency spectrum of the optical signal to be detected.
Preferably, the I, Q output paths are converted into digital signals, and the Q paths are added to the I paths of digital signals after +90 °/-90 ° phase shift in the digital domain, and implemented by using a digital processor DSP.
Preferably, the optical local oscillator signal is a carrier-suppressed optical single sideband signal.
Further preferably, the carrier suppressed optical single sideband signal is obtained by the following method: the method comprises the steps of dividing a microwave local oscillation signal into two paths of orthogonal signals with equal power by using a 90-degree microwave directional coupler, modulating the two paths of orthogonal signals on a narrow-linewidth optical carrier by using a double-parallel Mach-Zehnder double-arm modulator, and enabling an output signal to be an optical single-side-band signal with carrier suppression by adjusting bias voltage of the double-parallel Mach-Zehnder double-arm modulator.
Further preferably, the optical local oscillator signal is a carrier-suppressed +1 order optical single-sideband signal or a carrier-suppressed +2 order optical single-sideband signal.
The following technical scheme can be obtained according to the same invention concept:
a fast spectral measurement device based on image frequency suppression, comprising:
the local oscillation light generating unit is used for generating an optical local oscillation signal;
the 90-degree optical mixer is used for mixing the optical signal to be detected and the optical local oscillation signal to generate four paths of differential signals;
the two balanced photoelectric detectors are used for carrying out balanced photoelectric detection on the four paths of differential signals;
the digital signal processing unit is used for converting I, Q paths of analog signals obtained by balanced photoelectric detection into digital signals, performing phase shift of +90 degrees/90 degrees on Q paths of digital signals in a digital domain, and adding the Q paths of digital signals and I paths of digital signals to obtain frequency spectrums on the left side/right side of the center frequency of the optical signal to be detected in the frequency band corresponding to the optical local oscillation signal; and the main control unit is used for controlling the frequency change of the optical local oscillation signal and combining the obtained series of optical signal frequency spectrums to be detected into a broadband frequency spectrum of the optical signal to be detected.
Preferably, the digital signal processing unit is a digital processor DSP.
Preferably, the optical local oscillation signal generated by the local oscillation light generation unit is a carrier-suppressed optical single sideband signal.
Further preferably, the local oscillation light generation unit includes: the microwave scanning device comprises a microwave frequency sweeping source, a narrow linewidth laser, a 90-degree microwave directional coupler and a double-parallel Mach-Zehnder double-arm modulator; the input end of the 90-degree microwave directional coupler is connected with the output end of the microwave frequency sweeping source, two output ends of the 90-degree microwave directional coupler are respectively connected with two microwave input ports of the double-parallel Mach-Zehnder double-arm modulator, and the optical input port of the double-parallel Mach-Zehnder double-arm modulator is connected with the output end of the narrow-linewidth laser.
Further preferably, the optical local oscillator signal is a carrier-suppressed +1 order optical single-sideband signal or a carrier-suppressed +2 order optical single-sideband signal.
Compared with the prior art, the technical scheme of the invention and the further improvement or preferred technical scheme thereof have the following beneficial effects:
on the basis of the existing spectrum measurement method based on the sweep frequency light source and coherent photoelectric detection, the invention utilizes the image frequency suppression technology of photoelectric combination to extract the single-side beat frequency signal of the local oscillation light, and because the signals at the two sides of the sweep frequency local oscillation light are not mixed, the spectrum of the optical signal to be measured can be restored by using Digital Signal Processing (DSP). Mature electric analog-digital converter (ADC) and DSP technologies can meet the requirements of a system dynamic range, meanwhile, the frequency spectrum resolution and precision are determined by the performance of a frequency sweeping light source and the performance of a DSP, the frequency sweeping interval is determined by the bandwidth of the DSP, the number of scanning points can be greatly reduced on the premise of ensuring the measurement resolution, and the measurement speed is improved.
Drawings
FIG. 1 is a schematic structural diagram of a conventional spectral measurement device based on a swept-frequency light source and coherent photoelectric detection;
FIG. 2 is a schematic structural diagram of a spectral measuring device according to the present invention;
FIG. 3 is a schematic view of the structural principle of a preferred embodiment of the spectrum measuring apparatus of the present invention
FIG. 4 is a schematic diagram of the frequency spectrum of points A-E in the device shown in FIG. 3;
fig. 5 is a schematic block diagram of an optical single sideband modulator in a preferred embodiment.
Detailed Description
Aiming at the defects of the prior art, the invention adopts the solution that on the basis of the existing spectrum measurement method based on a sweep frequency light source and coherent photoelectric detection, a single-side beat frequency signal of local oscillator light is extracted by utilizing a photoelectric combined image frequency suppression scheme, and because signals on two sides of the sweep frequency local oscillator light are not mixed, the frequency spectrum of the optical signal to be measured can be restored by using Digital Signal Processing (DSP). Mature electric analog-digital converter (ADC) and DSP technologies can meet the requirements of a system dynamic range, meanwhile, the frequency spectrum resolution and precision are determined by the performance of a frequency sweeping light source and the performance of a DSP, the frequency sweeping interval is determined by the bandwidth of the DSP, the number of scanning points can be greatly reduced, and the measurement speed can be greatly improved on the premise of ensuring the measurement resolution.
The spectrum measuring method specifically comprises the following steps: mixing the optical signal to be detected and the optical local oscillation signal by using a 90-degree optical mixer to generate four paths of differential signals; using two balanced photoelectric detectors to carry out balanced photoelectric detection on the four paths of differential signals; converting the output I, Q two paths of analog signals into digital signals, performing phase shift of +90 degrees/90 degrees on the Q paths of digital signals in a digital domain, and adding the Q paths of digital signals and the I paths of digital signals to obtain frequency spectrums on the left side/right side of the center frequency of the optical signal to be measured in the frequency band corresponding to the optical local oscillation signal; changing the frequency of the optical local oscillator signal and repeating the steps; and finally, combining the obtained series of optical signal frequency spectrums to be detected into a broadband frequency spectrum of the optical signal to be detected.
As shown in fig. 2, the fast spectrum measuring device based on image frequency suppression of the present invention includes:
the local oscillation light generating unit is used for generating an optical local oscillation signal;
the 90-degree optical mixer is used for mixing the optical signal to be detected and the optical local oscillation signal to generate four paths of differential signals;
the two balanced photoelectric detectors are used for carrying out balanced photoelectric detection on the four paths of differential signals;
the digital signal processing unit is used for converting I, Q paths of analog signals obtained by balanced photoelectric detection into digital signals, performing phase shift of +90 degrees/90 degrees on Q paths of digital signals in a digital domain, and adding the Q paths of digital signals and I paths of digital signals to obtain frequency spectrums on the left side/right side of the center frequency of the optical signal to be detected in the frequency band corresponding to the optical local oscillation signal; and the main control unit is used for controlling the frequency change of the optical local oscillation signal and combining the obtained series of optical signal frequency spectrums to be detected into a broadband frequency spectrum of the optical signal to be detected.
The optical local oscillation signal LO output by the local oscillation light generating unit and the optical signal S to be detected are subjected to coherent frequency mixing in a 90-degree optical mixer to generate four paths of differential signals I +, I-, Q + and Q-, and the I + and the I-are input into the balanced photoelectric detector 1 to obtain an in-phase component I; the Q + and Q-are input to the balanced photodetector 2 to obtain the quadrature component Q. The output of the two balanced photoelectric detectors is connected with the input of the signal acquisition and processing module, the analog signals I and Q are converted into digital signals through an analog-to-digital conversion unit in the signal processing module, the digital signals are input into the main control unit after being processed, and the main control unit processes and displays data.
The optical local oscillator signal is preferably a carrier-suppressed optical single sideband signal, and a carrier-suppressed +1 order optical single sideband signal, a +2 order optical single sideband signal or a higher order single sideband signal can be used.
The digital signal processing unit can adopt MCU, DSP, etc., preferably adopts DSP to realize.
To facilitate understanding of the public, the following preferred embodiment will describe the technical solution of the present invention in more detail.
The structure of the spectrum measuring apparatus in this embodiment is shown in fig. 3, wherein the local oscillator light generating unit is configured to generate a carrier-suppressed optical single-sideband signal, and includes a microwave frequency sweeping source, a narrow-linewidth light source, and a single-sideband modulator, where the single-sideband modulator modulates a microwave local oscillator signal output by the microwave frequency sweeping source onto an optical carrier output by the narrow-linewidth light source to generate the carrier-suppressed optical single-sideband signal. The digital signal processing unit adopts DSP, and the main control unit adopts a computer.
In the actual working process of the device, an optical single-sideband signal of a suppressed carrier generated by a local oscillator light generating unit is used as a local oscillator signal, the optical signals to be measured on the left side and the right side of a central frequency in a frequency band corresponding to the local oscillator light are assumed to be L and R respectively, and the optical signals to be measured and the local oscillator signal are mixed by using a 90-degree optical mixer to generate four-path differential signals; then, two balanced photoelectric detectors are used for carrying out balanced photoelectric detection on the four paths of differential signals, L, R signals with the same phase are obtained in the path I, and L signals with-90-degree phase shift and R signals with + 90-degree phase shift are obtained in the path Q; then, I, Q two paths of signals are input into the real-time oscilloscope, analog signals are converted into digital signals, then the Q path of signals are subjected to-90-degree phase shift through the DSP to obtain reversed-phase L signals and same-phase R signals, then the reversed-phase L signals and the same-phase R signals are added with the I path of signals, and finally only the R signals on the right side of the center frequency in the frequency band corresponding to the local oscillation light are output; then, changing the output frequency of the microwave frequency sweeping source, and repeating the measuring process to obtain a series of R signals in different frequency bands; finally, the series of R signals are combined into a broadband spectrum of the light signal to be measured.
In fig. 3, a is a schematic diagram of mixing an optical single-sideband modulation signal of a suppressed carrier and an optical signal to be measured in a 90 ° optical mixer; b is the output signal of the I route; c is Q output signal; d is a schematic process diagram of processing the I, Q two-path signals by the signal processing module; and E is the spectrum of the light signal to be measured on the right side of the local oscillator light obtained by processing. The frequency spectrum of signals a-E is shown in fig. 4.
The basic structure of the optical single-sideband modulator adopted in the embodiment is shown in fig. 5, and the optical single-sideband modulator consists of a 90-degree microwave directional coupler and a double-parallel Mach-Zehnder double-arm modulator; the input end of the 90-degree microwave directional coupler is connected with the output end of the power-divided microwave frequency sweeping source, the two output ends of the 90-degree microwave directional coupler are respectively connected with two microwave input ports of a double-parallel Mach-Zehnder double-arm modulator (MZM), and the optical input port of the double-parallel Mach-Zehnder double-arm modulator is connected with the output end of the narrow-linewidth light source. The input microwave signal is divided into two paths of orthogonal signals with equal power through a 90-degree microwave directional coupler and respectively transmitted to two microwave input ports of a double-parallel Mach-Zehnder double-arm modulator. And given proper direct current bias, the modulator is used for modulating two paths of orthogonal microwave signals input from the microwave input port on an optical carrier input from the optical input port to generate an optical single sideband signal for suppressing the carrier.
The working principle of the spectral measuring device is briefly described as follows:
if two paths of microwave signals output by the 90-degree directional coupler are S respectivelye1＝Vcos(ωet) and Se2＝Vsin(ωet) the optical carrier signal is So＝Voexp(iωot)，
The phase difference of the two arms of the MZM1 and the MZM2 respectively,
is the phase difference between MZM1 and MZM2, the optical single sideband signal that can be output is:
wherein, β is the modulation factor,
regulating
And
i.e., three DC biases, to
m00, i.e. an angular frequency of ωLO＝ω0+ωeIs used to modulate the signal. Taking the optical sideband as a local oscillation signal, the local oscillation signal may be represented as:
wherein A isLOAnd ωLORespectively, the amplitude and angular frequency of the local oscillator signal (assuming a phase of 0). To simplify the principle analysis, we consider the signal under test as ωLOSignals having a center frequency and including left and right side frequency spectrums are denoted as
Wherein A issIs the amplitude, omega, of the optical signal to be measuredL(t) and ωR(t) represents ωLOSpectral components on the left and right sides. After the optical signal to be detected and the local oscillator signal are subjected to coherent mixing in the 90-degree optical mixer, four paths of differential output signals are as follows:
E1,2∝Es±ELO,E3,4∝Es±jELO
the output ac signal passing through the two balanced photodetectors is:
II(t)＝R|AsALO(ωLO)|{cos[ωL(t)]+cos[ωR(t)]}
IQ(t)＝R|AsALO(ωLO)|{sin[ωL(t)]-sin[ωR(t)]}
where R is the response coefficient of the balanced photodetector. I isI(t) and IQ(t) acquiring and converting into digital signals by a real-time oscilloscope, storing the digital signals in a computer, and processing the digital signals to obtain IQ(t) phase-shifted by-90 DEG and with II(t) adding to obtain the optical signal to be measured as
I＝R|AsALO(ωLO)|{cos[ωL(t)]+cos[ωR(t)]}
+R|AsALO(ωLO)|{sin[ωL(t)-π/2]-sin[ωR(t)-π/2]}
＝R|AsALO(ωLO)|{cos[ωL(t)]+cos[ωR(t)]}
+R|AsALO(ωLO)|{-cos[ωL(t)]+cos[ωR(t)]}
＝2R|AsALO(ωLO)|cos[ωR(t)]
From the above formula, ω isLOThe left-hand spectra cancel each other out due to the opposite phase, and ω isLOThe spectra on the right side are mutually enhanced due to the same phase. Therefore, the final output signal will come entirely from ωLOThe spectrum on the right. Similarly, as will IQ(t) phase-shifting by +90 DEG and with II(t) add, the final output signal is then derived entirely from ωLOThe left-hand spectrum. In actual measurement, the master control computer controls the microwave frequency sweeping source to sweep omegaeTherefore, the frequency of the local oscillator signal is scanned, and finally, the complete optical signal frequency spectrum to be detected can be obtained.
In this embodiment, the optical single-side band frequency-sweeping signal is obtained by modulating the microwave frequency-sweeping signal onto an optical carrier, and thus has a very high resolution (the light source generally employs a narrow linewidth laser with a linewidth of 300Hz, and thus the resolution of the measuring apparatus is about 300 Hz); meanwhile, the 90-degree optical mixer and the balanced photoelectric detector are adopted in the device for coherent detection, so that system noise can be suppressed, and measurement errors introduced by high-order sidebands in optical single-sideband signals can be eliminated.
When the apparatus is used to measure the spectrum of an optical signal, it is preferable to perform calibration of the measuring apparatus in advance, and specifically, the following calibration method can be used: removing an optical signal to be measured in the measuring device, connecting the output of an optical single-sideband modulator with the local oscillation input end of a 90-degree optical mixer, connecting a known double-sideband modulated optical signal with the signal input end of the 90-degree optical mixer, keeping the other devices unchanged, controlling a microwave frequency sweeping source by a computer to carry out frequency scanning, processing and recording an optical signal frequency spectrum, and comparing the frequency spectrum with the actual frequency spectrum of the known optical signal to obtain the frequency spectrum response of the system so as to correct the measuring result; during actual measurement, the spectrum of the measured optical signal to be measured is corrected by adopting the frequency spectrum response of the system obtained by calibration, and system errors are eliminated, so that the accurate spectrum of the optical signal to be measured is obtained.
Claims (10)
1. A fast spectrum measurement method based on image frequency suppression is characterized in that a 90-degree optical mixer is used for mixing an optical signal to be measured and an optical local oscillation signal to generate four paths of differential signals; using two balanced photoelectric detectors to carry out balanced photoelectric detection on the four paths of differential signals; converting I, Q two paths of analog signals output by a balanced photoelectric detector into digital signals, performing phase shift of +90 degrees/90 degrees on Q paths of digital signals in a digital domain, and adding the Q paths of digital signals and I paths of digital signals to obtain frequency spectrums of the optical signals to be measured on the left side/right side of the central frequency in a frequency band corresponding to the optical local oscillation signals; changing the frequency of the optical local oscillator signal and repeating the steps; and finally, combining the obtained series of optical signal frequency spectrums to be detected into a broadband frequency spectrum of the optical signal to be detected.
2. The method of claim 1, wherein the converting of the I, Q output two analog signals into digital signals and the adding of the Q digital signals after +90 °/-90 ° phase shifting in the digital domain and the I digital signals are implemented using a digital processor.
3. The method of claim 1, wherein the optical local oscillator signal is a carrier-suppressed optical single sideband signal.
4. The method of claim 3, wherein the carrier suppressed optical single sideband signal is obtained by: the method comprises the steps of dividing a microwave local oscillation signal into two paths of orthogonal signals with equal power by using a 90-degree microwave directional coupler, modulating the two paths of orthogonal signals on a narrow-linewidth optical carrier by using a double-parallel Mach-Zehnder double-arm modulator, and enabling an output signal to be an optical single-side-band signal with carrier suppression by adjusting bias voltage of the double-parallel Mach-Zehnder double-arm modulator.
5. The method according to claim 3, wherein the optical local oscillator signal is a carrier-suppressed +1 order optical single sideband signal or a carrier-suppressed +2 order optical single sideband signal.
6. A fast spectral measurement device based on image frequency suppression, comprising:
the local oscillation light generating unit is used for generating an optical local oscillation signal;
the 90-degree optical mixer is used for mixing the optical signal to be detected and the optical local oscillation signal to generate four paths of differential signals;
the two balanced photoelectric detectors are used for carrying out balanced photoelectric detection on the four paths of differential signals;
the digital signal processing unit is used for converting I, Q paths of analog signals obtained by balanced photoelectric detection into digital signals, performing phase shift of +90 degrees/90 degrees on Q paths of digital signals in a digital domain, and adding the Q paths of digital signals and I paths of digital signals to obtain frequency spectrums on the left side/right side of the center frequency of the optical signal to be detected in the frequency band corresponding to the optical local oscillation signal;
and the main control unit is used for controlling the frequency change of the optical local oscillation signal and combining the obtained series of optical signal frequency spectrums to be detected into a broadband frequency spectrum of the optical signal to be detected.
7. The apparatus of claim 6, wherein the digital signal processing unit is a digital processor.
8. The apparatus of claim 6, wherein the optical local oscillator signal generated by the local oscillator light generation unit is a carrier suppressed optical single sideband signal.
9. The apparatus of claim 8, wherein the local oscillator light generation unit comprises: the microwave scanning device comprises a microwave frequency sweeping source, a narrow linewidth laser, a 90-degree microwave directional coupler and a double-parallel Mach-Zehnder double-arm modulator; the input end of the 90-degree microwave directional coupler is connected with the output end of the microwave frequency sweeping source, two output ends of the 90-degree microwave directional coupler are respectively connected with two microwave input ports of the double-parallel Mach-Zehnder double-arm modulator, and the optical input port of the double-parallel Mach-Zehnder double-arm modulator is connected with the output end of the narrow-linewidth laser.
10. The apparatus of claim 8, wherein the optical local oscillator signal is a carrier-suppressed +1 order optical single sideband signal or a carrier-suppressed +2 order optical single sideband signal.
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