CN103763022B - A high spatial resolution optical frequency domain reflectometer system based on high-order sideband swept frequency modulation - Google Patents

Abstract

The invention discloses a high-spatial-resolution optical frequency domain reflectometer system based on high-order sideband frequency sweep modulation. The system comprises a frequency sweeping light source part, a test light path part, a receiver and a signal processing part, wherein the frequency sweeping light source part uses a narrow linewidth laser as an original light source, and emergent light generates a frequency sweeping sideband optical signal through external modulation. In the external modulation process, the frequency sweeping radio frequency signal is amplified by a high-power radio frequency amplifier, high voltage is loaded to an electro-optic modulator with lower half-wave voltage to generate multi-order sidebands, the high-order optical sidebands of broadband frequency sweeping are obtained by filtering of a narrow-band optical filter, the high-order sidebands are led into an optical path system as frequency sweeping carrier light sources, backscattered and reflected optical signals are collected, and optical frequency domain reflection analysis is realized through local coherent detection and signal processing. The frequency sweep range can be expanded by utilizing high-order sideband frequency sweep, so that the optical frequency domain reflectometer can achieve higher spatial resolution.

Description

A High Spatial Resolution Optical Frequency Domain Reflectometer Based on High-Order Sideband Sweep Frequency Modulation system

technical field

The invention relates to the technical fields of electro-optical modulation, distributed optical fiber sensing, optical reflectometer, etc., and in particular relates to a high spatial resolution optical frequency domain reflectometer system based on high-order sideband frequency sweep modulation.

Background technique

Optical fiber communication has developed rapidly since the 1970s due to its characteristics of transmission frequency bandwidth and low loss. my country started the construction of large-scale commercial optical fiber communication systems in the early 1990s. Optical fiber sensing technology is developed rapidly along with the development of optical fiber communication technology. It uses light wave as carrier and optical fiber as medium to perceive and transmit external measured signals. The light wave as the carrier of the measured signal and the optical fiber as the light wave propagation medium have a series of unique advantages that are difficult to compare with other carriers and media. Light waves are not afraid of electromagnetic interference, are easily received by various photodetection devices, can be easily converted into photoelectricity or electro-optic, and are easy to match with highly developed modern electronic devices and computers. The optical fiber is small in size and light in weight, easy to lay and transport, rich in material sources, non-radiative, and difficult to eavesdrop. Large communication capacity and long transmission distance; the potential bandwidth of an optical fiber can reach 20THz.

At present, the optical fiber communication system has become an optical cable transmission network carrying huge information capacity. In order to ensure safety and smoothness, it is necessary to have instruments that can accurately measure the transmission characteristics of optical fibers. Optical reflectometer is a very important means of fiber link status monitoring and maintenance. Currently, Optical Time Domain Reflectometer (OTDR) is widely used. OTDR uses backscattered light to measure the propagation characteristics of optical fibers. The light source emits a beam of light pulses into the fiber, where backscattering occurs in the fiber. Some parameters in the optical fiber to be tested will be modulated onto the pulsed light during the scattering process. Therefore, using OTDR technology, the system can detect the distribution of optical fiber parameters by measuring the variation of backscattered light intensity with time, so as to determine the distribution of optical fiber parameters. The length of and the parameter distribution information of each place. OTDR detects by analyzing the time difference and optical path difference of backscattered light. The improvement of its spatial resolution requires shortening the pulse width of the light source and increasing the bandwidth of the receiver, and shortening the pulse width of the light source means that the signal energy is reduced, and the system noise is proportional to the receiver bandwidth, so increasing the receiver bandwidth means The system dynamic range and signal-to-noise ratio decrease, so there is a contradiction between the resolution of the OTDR system and the signal-to-noise ratio, dynamic range, and measurement time, and its spatial resolution can only reach the order of meters. In order to solve this problem, other time-domain reflectometry methods are also being studied continuously. Such as correlation detection of pseudo-random detection signal, complementary Gray code detection, etc.

In order to improve performance such as spatial resolution and measurement sensitivity, the backscattering method is applied to the optical frequency domain, and an OFDR (Optical Frequency Domain Reflectometer) has been developed. The high spatial resolution of the OFDR system will make it more widely used in many fields that have high requirements for measurement accuracy [Opt. Express 19,19790-19796 (2011)]. The single-sideband modulation of the narrow-linewidth laser is performed by frequency-sweeping RF signals to obtain a linearly-sweeping optical signal, which is used in the corresponding OFDR system, and long-distance and high-spatial-resolution line detection has been realized [J.Lightwave Technol. 6, 3287-3294 (2008)]. However, the complex modulation process and poor stability of SSB modulation limit the corresponding performance. On the other hand, the sweeping range of RF sweeping signals is limited by electronic components, generally only in the order of GHz [J. Lightwave Technol. 30, 1015-1024 (2012)]. To obtain OFDR with high spatial resolution, it is necessary to increase the scanning range. Therefore, it is necessary to develop novel broadband swept source technology for achieving high spatial resolution OFDR.

Contents of the invention

The purpose of the present invention is that in the OFDR system, in order to overcome the problems of non-linear frequency sweep that may be generated by using the internal modulation of the light source, the external modulation of the light source can be used to realize the frequency sweep. The sidebands are swept. Limited by the electronic bottleneck of electronic components, the scanning frequency range of radio frequency signals is limited. In order to expand the scanning range and improve the spatial resolution, the present invention utilizes the principle of high-voltage modulation of low-half-wave voltage electro-optic modulators to generate multi-order sidebands, providing A high spatial resolution optical frequency domain reflectometer system based on high order sideband swept frequency modulation.

In order to achieve the above object, the present invention adopts the following technical solutions:

A high spatial resolution optical frequency domain reflectometer system based on high-order sideband frequency sweep modulation, including a frequency sweep light source part, a test optical path part, a receiver and a signal processing part; it is characterized in that: the frequency sweep light source part includes Laser, low half-wave voltage electro-optic modulator, radio frequency signal generator, circulator, fiber Bragg grating, erbium-doped fiber amplifier and optical filter; the narrow linewidth laser emitted by the laser is loaded according to the radio frequency signal sent by the radio frequency signal generator After the bias voltage is connected to the electro-optic modulator for modulation, a broadband optical comb is formed; the obtained optical comb is introduced into a circulator connected with a fiber Bragg grating, and the reflected light wave is amplified by an erbium-doped fiber amplifier and injected into an adjustable Optical filter; the filtered high-order sweep sideband is introduced into the test optical path part, interferes with the local light and is finally received by the receiver, and processed by the signal processing part.

It is further characterized in that: the low half-wave voltage electro-optic modulator is a Mach-Zehnder type electro-optic modulator or a phase-type electro-optic modulator, the half-wave voltage of the electro-optic modulator is not greater than 4 volts, and the drive of the radio frequency signal of the radio frequency signal generator The power is not lower than 27dBm.

The optical filter is a narrow-band filter based on a fiber Bragg grating or an optical filter based on a diffraction grating.

The reflectivity of the fiber Bragg grating is greater than 99%, its shape is Flat-top, and its extinction ratio is not less than 40dB.

The equivalent scanning range of the sweeping light source used above is _ΔF , the minimum modulation frequency of the RF signal generated by the RF signal generator used is flow, the scanning frequency rate of the RF signal used is γ, and the scanning time of the RF signal used is τ , the central wavelength and bandwidth of the optical filter used are adjustable; the theoretical limit of spatial resolution is determined by Δl=c/2nΔF, the frequency sweep range of the radio frequency signal Δf _RF =γτ, and the frequency sweep range of the modulated Nth order sideband For Δf _N =Nγτ, Δf _N < f _low .

Technical effect of the present invention:

1. The electro-optic modulator with low half-wave voltage can generate an optical frequency comb with multiple sidebands when using high-voltage modulation. For high-order optical sidebands, it can obtain a magnified optical frequency sweep range.

2. By using fiber Bragg grating and tunable optical filter, a certain high-order sideband can be filtered out from the modulated optical frequency comb and other sidebands and noise can be suppressed at the same time.

3. The optical frequency domain reflectometer system that uses high-order sideband sweep light instead of the general first-order sideband sweep light as the sweep light source can obtain a spatial resolution that is doubled.

Description of drawings

Figure 1 is a schematic diagram of the basic structure of the system of the present invention;

The figure includes: Optical frequency: fiber frequency; Time: time; FL: fiber laser; PC: polarization controller; MZ-modulator: Mach-Zehnder modulator; RF synthesizer: radio frequency signal generator; Triggersource: trigger signal; CIR: Circulator; FBG: Fiber Bragg Grating; Bias: Bias voltage; EDFA: Erbium-doped fiber amplifier; BPF: Optical filter; FUT: Fiber under test; BPD: Balanced photodetector; Polarization diversity: Polarization diversity; OC: 3dB Optocoupler; ADC: analog-to-digital conversion module; Computer: computer.

Fig. 2 is a schematic diagram of an optical frequency comb with multi-order sidebands modulated by a Mach-Zehnder modulator with a lower half-wave voltage in the present invention;

In the figure, Power: signal strength; Optical Comb: optical comb; Wavelength: wavelength.

Fig. 3 is a schematic diagram of the tenth-order sideband spectrum filtered from the modulated optical frequency comb by using a fiber Bragg grating and a tunable optical filter in the present invention;

In the figure, Power: signal strength; Carrier: carrier; +10th order sideband: tenth order sideband; Wavelength: wavelength.

Fig. 4 is a schematic diagram of the method for measuring the experimental value of the spatial resolution of the present invention;

In the figure, Reflectivity: reflection coefficient; Distance: distance; FWHM: Fresnel reflection peak width.

detailed description

Embodiment one:

Figure 1 shows a basic schematic diagram of the system structure described in this embodiment. The laser FL is connected to the Mach-Zehnder modulator MZ-modulator with a lower half-wave voltage through the polarization controller PC, and is modulated according to the RF signal of the RF signal generator RFsynthesizer. When the applied bias voltage Bias is higher, it can generate multi-order sidebands, forming a broadband optical comb. The obtained optical comb is introduced into the circulator CIR connected with the fiber Bragg grating FBG, and the reflected light wave is amplified by the erbium-doped fiber amplifier EDFA and then injected into the tunable optical filter BPF. Through the designed and matched fiber Bragg grating FBG and tunable filter BPF, the desired sideband can be filtered out at a higher rejection ratio. The filtered high-order sweep sideband is introduced into the optical system, and the single-mode fiber is used as the fiber under test. The light reflected from the FUT of the fiber under test interferes with the local light and is finally transmitted by the 8-bit ADC of the 8-bit analog-to-digital conversion module. Cooperate with computer for detection.

Fig. 2 shows a schematic diagram of an optical frequency comb with multi-order sidebands modulated by a Mach-Zehnder modulator with a lower half-wave voltage in this embodiment. The highest one in the figure is the carrier, and the optical frequency comb has more than 20 subcarriers, indicating that at least the tenth order sideband can be filtered out.

Fig. 3 shows a schematic diagram of the tenth-order sideband spectrum filtered out from the modulated optical frequency comb by using a fiber Bragg grating and a tunable optical filter in this embodiment. The tenth order sideband is more than 20dB higher than the other order sidebands.

Fig. 4 shows a schematic diagram of the measurement method of the experimental value of the spatial resolution in this embodiment. The method shown is to measure the full width at half maximum of the reflected Fresnel peak at the end of the fiber under test, and a larger width represents a lower spatial resolution. The figure shows the Fresnel reflection peak obtained by using the tenth-order sideband to scan when the equivalent frequency sweep range of the RF signal is 800MHz. The full width at half maximum of the peak is used as the experimental value of the spatial resolution, which is 1.5cm.

Claims (5)

1. a High-spatial-resolutoptical optical frequency domain reflectometer system based on the modulation of high-order sideband frequency sweep, including swept light source portion Point, optical system for testing part, receiver and signal processing；It is characterized in that: described swept light source part includes laser instrument, low Half-wave voltage electrooptic modulator, radio-frequency signal generator, circulator, Fiber Bragg Grating FBG, erbium-doped fiber amplifier and light filter Ripple device；Radiofrequency signal that the narrow-linewidth laser that described laser instrument sends sends according to radio-frequency signal generator also loads bias voltage After be connected to described low half-wave voltage electrooptic modulator and be modulated, form a broadband light comb；The light comb obtained is imported even Having connect the circulator of Fiber Bragg Grating FBG, the light wave reflected injects tunable optical filter after erbium-doped fiber amplifier amplifies Ripple device；The high-order frequency sweep sideband leached is imported optical system for testing part, interferes with local light and finally received by receiver, With signal processing part divisional processing.

High-spatial-resolutoptical optical frequency domain reflectometer system based on the modulation of high-order sideband frequency sweep the most according to claim 1, It is characterized in that: described low half-wave voltage electrooptic modulator is that Mach increases Dare type electrooptic modulator or phase type Electro-optical Modulation Device, the half-wave voltage of electrooptic modulator is not more than 4 volts, and the driving power of the radiofrequency signal of radio-frequency signal generator is not less than 27dBm。

High spatial resolution optical frequency domain reflectometer system based on the modulation of high-order sideband frequency sweep the most according to claim 1 and 2 System, it is characterised in that: described optical filter is the light filtering of narrow band filter based on Fiber Bragg Grating FBG or diffraction grating Device.

High spatial resolution optical frequency domain reflectometer system based on the modulation of high-order sideband frequency sweep the most according to claim 1 and 2 System, it is characterised in that: described Fiber Bragg Grating FBG reflectance is more than 99%, and it is shaped as Flat-top type, and its extinction ratio is the least In 40dB.

High spatial resolution optical frequency domain reflectometer system based on the modulation of high-order sideband frequency sweep the most according to claim 1 and 2 System, it is characterised in that: described swept light source equivalence sweep limits is Δ F, and what the radio-frequency signal generator of employing produced penetrates Frequently the lowest modulation frequency of signal is f_low, the radiofrequency signal rate of scanning speed of employing is γ, during the radiofrequency signal frequency sweep of employing Between be τ, centre wavelength and the bandwidth of the optical filter of employing are adjustable；Spatial resolution theoretical boundary is by Δ l=c/2n Δ F certainly Fixed, radiofrequency signal swept frequency range Δ f_RF=γ τ, the swept frequency range of the N rank sideband modulated is Δ f_N=N γ τ, Δ f_N< f_low。