CN103401606A - Coherent optical time-domain reflectometer based on detection frequency coding - Google Patents

Abstract

The invention discloses a coherent optical time-domain reflectometer based on detection frequency coding, which comprises a laser, a first coupler, a frequency coder, a radio-frequency driver, a circulator, an optical interface, an optical filter, a second coupler, a photoelectric detector, a radio-frequency amplifier, a data acquisition module, a signal processing module and a display module. Laser light emitted by the laser passes through the frequency coder for frequency coding to obtain pulses of detection frequency pulse light and filling light which is complementary to the time sequence of the detection frequency pulse light. The frequency of the pulse of the detection frequency pulse light is different from the frequency of the pulse of the filling light. After the respective backward Raleigh scattering signals of the detection frequency pulse light and the filling light in an optical fiber line are fed back, the detection frequency pulse light and the filling light enter the optical filter through the third port of the circulator, the filtered detection light signal is coherent with local oscillator light at the second coupler, then coherent medium-frequency signals are output by the photoelectric detector and finally the medium-frequency signals are acquired and processed to obtain a time-domain curve which reflects the characteristics of the optical fiber line.

Description

A Coherent Optical Time Domain Reflectometer Based on Detection Frequency Coding

technical field

The invention belongs to the communication field, and in particular relates to a coherent optical time domain reflectometer used for optical fiber communication line characterization, event identification and fault location.

Background technique

Coherent optical time-domain reflectometry is usually used to monitor multi-relay ultra-long-distance optical fiber communication lines such as transoceanic submarine optical cables. Coherent optical time domain reflectometry uses the principle of lidar to locate the scattering and/or reflection point by injecting a detection light pulse into the optical fiber and recording the return time of the Rayleigh scattered light and/or reflected light of the light pulse in the optical fiber. The pulse width of the optical pulse corresponds to the spatial resolution of the measurement, and the two are proportional to each other. For example, the detection optical pulse width of 1 microsecond corresponds to the spatial resolution of 100 meters, and the pulse period is slightly longer than the round-trip time of the detection optical signal in the line. At the same time, it uses the coherent detection method to transfer the power of the detection signal light to the heterodyne intermediate frequency signal by heterodyning the detection light and the local oscillator light, so that most of the out-of-band noise can be suppressed by narrow-band filtering the intermediate frequency signal. This increases the dynamic range of the measurement. For multi-relay ultra-long-distance optical fiber communication lines, the longer the line distance, the larger the pulse period, and to obtain high spatial resolution, the pulse width should be as small as possible, so the duty cycle of the detection optical pulse is extremely small, for example, For the monitoring of a 10,000-kilometer transoceanic submarine optical cable, the pulse period is slightly longer than 100 milliseconds, and the pulse width is usually in the range of 1 microsecond to 100 microseconds. Since the lines mostly use erbium-doped fiber amplifiers as relay amplifiers, low duty-cycle optical pulses will cause transient effects in erbium-doped fiber amplifiers, and the transient effects will form optical surges when accumulated in each erbium-doped fiber amplifier, thereby The light pulse is seriously deformed, and even the erbium-doped fiber amplifier is damaged.

For optical surge suppression, frequency shift keying technology is usually used. It introduces a filling light pulse that is complementary to the detection light pulse in terms of timing. The detection light pulse and the filling light pulse correspond to their respective lasers and pulse modulators, and then use a coupler or a wavelength division multiplexer to combine the two into a quasi-continuous Light, this kind of quasi-continuous light can well suppress the light surge. In addition, Evangelides Stephen and others proposed a coherent optical time domain reflectometer scheme based on frequency pulse sweep, and applied for a world patent, the patent number is: US20040794174, and the patent name is "COTDR ARRANGEMENT WITH SWEPT FREQUENCY PULSE GENERATOR FOR AN OPTICAL TRANSMISSION SYSTEM ". The frequency of the frequency pulse involved in the method jumps with time but the laser power is continuous. Continuous laser light enters the erbium-doped fiber amplifier in the fiber line to suppress the transient effect and avoid optical surges. However, the frequency pulse is a frequency sweep Yes, in order to stabilize the coherent intermediate frequency signal generated by the probe light and the local oscillator light, the frequency of the local oscillator light should be changed accordingly. In addition, the frequency sweep does not change the continuity and linewidth of the laser. The Brillouin of the continuous light The threshold is low, which limits the peak power of the frequency pulse and thus limits the dynamic range of the measurement.

Contents of the invention

Aiming at the deficiencies of the prior art, the present invention proposes a coherent optical time-domain reflectometer based on detection frequency coding, which uses the same light source to obtain detection light, filling light and local oscillator light with constant frequency, and the detection light frequency is coded in time sequence , thus improving the measurement speed and dynamic range.

A coherent optical time domain reflectometer based on detection frequency coding proposed by the present invention is improved in that the coherent optical time domain reflectometer includes: a laser 1, a first coupler 2, a frequency encoder 3, and a radio frequency driver 4. Circulator 5, optical interface 6, optical filter 7, second coupler 8, photodetector 9, radio frequency amplifier 10, analog-to-digital conversion module 11, signal processing module 12 and display module 13;

The laser light emitted by the laser 1 is divided into two paths by the first coupler 2, one path is input into the frequency encoder 3, and the other path is directly input into the second coupler 8;

The radio frequency driver 4 generates a radio frequency signal whose frequency changes with time sequence to drive the frequency encoder 3, and the frequency encoder 3 modulates the incident light to generate pulsed light coded by the detection frequency and a filling optical frequency pulse complementary to it in time sequence; The detection frequency pulsed light and the filling light pulse are input into the first port of the circulator 5, output from the second port of the circulator 5, and injected into the optical fiber communication line under test through the optical interface 6;

The back Rayleigh scattering signals of the detection frequency pulse light and the filling light frequency pulse in the optical fiber under test are returned, and input to the optical filter 7 through the third port of the circulator 5;

The detection optical signal filtered out by the optical filter 7 enters the second coupler 8 and is mixed with the local oscillator light, and the intermediate frequency signal generated by the mixing of the two is received by the photodetector 9 and outputs a corresponding intermediate frequency signal;

After the intermediate frequency signal is amplified by the intermediate frequency amplifier 10, it is collected by the analog-to-digital conversion module 11, and the collected data is transmitted to the signal processing module 12 for data processing, and is displayed by the display module 13 to represent optical fiber communication. Optical time domain reflectance curve of line attenuation information.

Wherein, the coherent optical time domain reflectometer includes a polarization scrambler 14 arranged between the frequency encoder 3 and the circulator 5 for suppressing polarization noise.

Wherein, the coherent optical time domain reflectometer includes an erbium-doped fiber amplifier 15 arranged between the polarization scrambler 14 and the circulator 5 to increase the power of the pulsed light at the detection frequency, thereby increasing the dynamic range of the measurement.

Wherein, the frequency encoder 3 is an electro-optic phase modulator.

Alternatively, the frequency encoder 3 is an electro-optical intensity modulator.

Wherein, the radio frequency driver 4 is an arbitrary waveform generator, which is used to generate a radio frequency signal whose frequency changes with time sequence.

Wherein, the optical filter 7 is a fiber grating.

Wherein, the photodetector 9 is a balanced photodetector for improving detection sensitivity.

Wherein, the analog-to-digital conversion module 11 is a data acquisition card for converting the analog intermediate frequency signal into a digital signal.

Wherein, the signal processing module 12 includes an FPGA chip for processing digital signals.

Compared with the prior art, the beneficial effects of the present invention are:

The detection frequency pulse obtained by the frequency encoding method does not destroy the continuity of the laser power. Compared with the frequency shift keying technology, it does not need to separately configure the laser and the acousto-optic modulator for filling the light. Compared with the frequency pulse scanning In the way of frequency, the present invention changes the single-frequency pulse into multi-frequency, so that the Brillouin threshold of continuous light can be effectively improved, so that the device can obtain a larger measurement dynamic range. In addition, the coherent optical time domain reflectometer involved in the present invention The optical frequency of the local oscillator is constant, thereby reducing the difficulty of frequency modulation and control of the detection light, and facilitating the acquisition and processing of signals. Moreover, the present invention uses an optical filter to filter out the detection signal light, thereby effectively suppressing the detection of other optical frequencies. Interference can further improve the signal-to-noise ratio of the measurement.

Description of drawings

FIG. 1 is a schematic structural diagram of a coherent optical time domain reflectometer based on detection frequency coding proposed by the present invention.

Fig. 2 is a schematic diagram of the detection light pulse sequence of the frequency pulse encoding proposed by the present invention.

Figure 3 is the output power spectrum of the phase modulator driven by a single frequency.

Fig. 4 is the optical time domain reflectance curve corresponding to the detection frequency of each code.

FIG. 5 is a detection curve obtained by a coherent optical time domain reflectometer based on detection frequency coding proposed by the present invention.

Detailed ways

The specific implementation manners of the present invention will be further described in detail below in conjunction with the accompanying drawings.

This embodiment provides a coherent optical time domain reflectometer based on detection frequency coding, which includes:

Laser 1, configured to provide probe light, fill light and single-frequency local oscillator light;

The first coupler 2 is used for light splitting;

The frequency encoder 3 is an electro-optic phase modulator or an electro-optic intensity modulator, which is used to modulate the single-frequency continuous laser and obtain frequency pulse output;

RF driver 4, select RF generator, such as Agilent's E8257D, the RF range is from 200kHz to 26.5GHz, it provides frequency coded RF signal to drive the phase modulator;

The circulator 5 provides respective channels for the sending and receiving of light;

Optical interface 6, used for optical path access;

The optical filter 7 is selected from a fiber grating, which is used to filter out miscellaneous frequency light and improve the optical signal-to-noise ratio;

The second coupler 8 is used for coupling of two paths of light;

The photodetector 9 is a balanced photodetector for photoelectric reception;

Radio frequency amplifier 10, used for the amplification of radio frequency signal;

Analog-to-digital conversion module 11 selects a data acquisition card for converting radio frequency electrical signals into digital signals;

The signal processing module 12 mainly includes an FPGA chip for computing and processing digital signals;

Display module 13, including a display screen, etc., for displaying measurement results;

Polarizer 14, used to randomly change the polarization state of light;

Erbium-doped fiber amplifier 15, used to boost optical power;

Specifically, after the laser 1 is connected to the first coupler 2, the frequency encoder 3, the circulator 5 and the optical interface 6 in sequence, it is connected to the optical fiber communication line; the first coupler 2 is connected to the second coupler 8, and the circulator 5 is also connected to the second coupler 8 through the optical filter 7. The second coupler 8 is then connected to the photodetector 9 , the radio frequency amplifier 10 , the analog-to-digital conversion module 11 , the signal processing module 12 and the display module 13 in sequence.

Preferably, in this embodiment, in order to suppress polarization noise, a polarization scrambler 14 is added between the frequency encoder 3 and the circulator 5; An erbium-doped fiber amplifier 15 is added between the devices 5, thereby improving the dynamic range of the measurement.

As shown in Figure 1, the laser light emitted by the laser 1 is divided into two paths by the first coupler 2, one path is input into the frequency encoder 3, and the other path is directly input into the second coupler 8;

The radio frequency driver 4 _generates radio frequency signals _{whose frequency changes with time sequence as shown in FIG.} frequency, and the rest are detection frequencies. In Fig. 2, the duration of each detection frequency is 1 microsecond, and they are frequency pulses existing in time series. The RF driver 4 drives the frequency encoder 3, and the frequency encoder 3 uses a phase modulator, and its output power spectrum under single-frequency drive is shown in Figure 3. The interval of each optical frequency in Fig. 3 is equal to the radio frequency frequency that radio frequency driver 4 outputs, like this, the radio frequency driving frequency coder 3 of sequence as shown in Fig. 2 modulates the laser light of laser device 1 to send all the way, will obtain power spectrum and change with time sequence The spectrum, so as to realize the encoding of the detected light frequency.

Corresponding to the radio frequency modulation spectrum in Fig. 2, the frequency encoder 3 generates the detection frequency pulse light and the filling light pulse complementary to it in time sequence, and then, the detection pulse light and the filling light pulse are connected to the first port of the circulator 5, from The second port output of the circulator 5 is injected into the optical fiber communication line of multi-relay amplification through the optical interface 6;

The back Rayleigh scattering signal of the probe frequency pulse light and the filling light pulse in the optical fiber under test is returned, and connected to the optical filter 7 through the third port of the circulator 5;

The detection optical signal filtered out by the optical filter 7 enters the second coupler 8 and mixes with the local oscillator light (that is, another laser beam split from the first coupler 2), and the intermediate frequency signal generated by the mixing of the two is received by the photodetector 9 And output corresponding intermediate frequency electrical signals, the intermediate frequencies corresponding to each detection frequency are Δf1, Δf2, Δf3, . . . , Δfn, as shown in FIG. 4 .

The intermediate frequency signal is amplified by the intermediate frequency amplifier 10 so that the analog-to-digital conversion module 11 performs high-precision analog-to-digital conversion. The analog-to-digital conversion module 11 can select a data acquisition card, and the collected data is transmitted to the signal processing module 12 for data processing. 12 Perform digital band-pass filtering on the digital signal to extract each detection frequency separately, then perform digital down-conversion and digital low-pass filtering on each detection frequency to finally obtain the power of the detection signal, and then superimpose the results of multiple measurements to obtain Reduce the random noise of the measurement. In this way, the optical time domain reflection curves corresponding to each detection frequency pulse are shown in FIG. The display module 13 displays the optical time domain reflectance curve representing attenuation information of the optical fiber communication line, as shown in FIG. 5 .

Compared with similar commercial equipment MW90010A, the present invention can obtain the probe light pulse and the filling light pulse from the same laser light source, without using the filling light source and the pulse modulator separately to obtain the filling light pulse, and, because the frequency of the probe light It is coded in time sequence, and each detection frequency corresponds to a detection curve. The result of superposition and averaging of multiple detection curves can reduce the fading noise of the detection curve more quickly and improve the dynamic range of the measurement.

Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to the above embodiments, those of ordinary skill in the art should understand that: the present invention can still be Any modification or equivalent replacement that does not depart from the spirit and scope of the present invention shall be covered by the scope of the claims of the present invention.

Claims (10)

1. Coherent optical time domain reflectometer based on look-in frequency coding, it is characterized in that, described Coherent optical time domain reflectometer comprises: laser (1), the first coupler (2), frequency coding device (3), radio driver (4), circulator (5), optical interface (6), optical filter (7), the second coupler (8), photodetector (9), radio frequency amplifier (10), analog-to-digital conversion module (11), signal processing module (12) and display module (13);

The laser that described laser (1) sends is divided into two-way by described the first coupler (2), a road input described frequency coding device (3), and described the second coupler (8) is directly inputted on another road;

Described radio driver (4) produces the described frequency coding device of radio frequency signals drive (3) that frequency changes with sequential, and described frequency coding device (3) modulating the incident light produces the pulsed light of look-in frequency coding and the filling light pulse of complementation on sequential with it; The first port of described look-in frequency pulsed light and the described filling light pulse described circulator of input (5), and export from the second port of described circulator (5), through described optical interface (6), be injected into the tested optical fiber communication line;

Described look-in frequency pulsed light and the dorsad Rayleigh scattering signal of described filling light pulse in tested optical fiber return, through the 3rd port input described optical filter (7) of described circulator (5);

The detection light signal that described optical filter (7) leaches enters described the second coupler (8) to be mixed with local oscillator light, and the two mixes the intermediate-freuqncy signal that produces and is received and export corresponding intermediate-freuqncy signal by described photodetector (9);

Described intermediate-freuqncy signal is gathered by described analog-to-digital conversion module (11) after described intermediate frequency amplifier (10) amplifies, the transfer of data that collects carries out the data processing for described signal processing module (12), and shows by described display module (13) the optical time domain reflection curve that characterizes the optical fiber telecommunications line decay characteristics.

2. Coherent optical time domain reflectometer as claimed in claim 1, is characterized in that, described Coherent optical time domain reflectometer comprises the scrambler (14) that is arranged between described frequency coding device (3) and circulator (5), be used to suppressing polarization noise.

3. Coherent optical time domain reflectometer as claimed in claim 2, it is characterized in that, described Coherent optical time domain reflectometer comprises the erbium-doped fiber amplifier (15) that is arranged between described scrambler (14) and circulator (5), for promoting the power of look-in frequency pulsed light, and then improve the dynamic range of measuring.

4. Coherent optical time domain reflectometer as claimed in claim 1, is characterized in that, described frequency coding device (3) is selected electro-optic phase modulator.

5. Coherent optical time domain reflectometer as claimed in claim 1, is characterized in that, described frequency coding device (3) is selected the electric light intensity modulator.

6. Coherent optical time domain reflectometer as claimed in claim 1, is characterized in that, described radio driver (4) is selected AWG (Arbitrary Waveform Generator), for generation of the radiofrequency signal of frequency with the sequential variation.

7. Coherent optical time domain reflectometer as claimed in claim 1, is characterized in that, described optical filter (7) is selected fiber grating.

8. Coherent optical time domain reflectometer as claimed in claim 1, is characterized in that, described photodetector (9) is selected the balance photodetector, be used to promoting detectivity.

9. Coherent optical time domain reflectometer as claimed in claim 1, is characterized in that, described analog-to-digital conversion module (11) is selected data collecting card, for the intermediate-freuqncy signal by simulation, converts digital signal to.

10. Coherent optical time domain reflectometer as claimed in claim 1, is characterized in that, described signal processing module (12) comprises fpga chip, for the treatment of digital signal.

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