WO2012079480A1

WO2012079480A1 - Dynamic frequency deviation correction method and coherence optical time-domain reflectometer system

Info

Publication number: WO2012079480A1
Application number: PCT/CN2011/083566
Authority: WO
Inventors: 冯志勇; 王顺; 张旭苹; 张益昕
Original assignee: 华为技术有限公司
Priority date: 2010-12-14
Filing date: 2011-12-06
Publication date: 2012-06-21
Also published as: CN102170307A; CN102170307B

Abstract

A dynamic frequency deviation correction method and a coherence optical time-domain reflectometer system are disclosed. The method comprises: splitting a laser beam output by a laser, to generate probe light and control light; inputting the control light into an MZI for interference, and performing photoelectric conversion on the interfered control light to obtain a first electric signal; controlling, according to the first electric signal, an optical path difference between optical signals transmitted on two arms of the MZI, so that the optical path difference is a quarter of the wavelength of the laser beam; calculating, according to the first electric signal, a deviation degree of an actual center frequency of the laser beam relative to a standard value of the center frequency of the laser beam; and performing corresponding compensation according to the deviation degree. In the present invention, the laser beam output by the laser is split into the probe light and control light, and the deviation degree of the actual center frequency of the laser beam relative to the standard value of the center frequency of the laser beam is calculated according to the control light; corresponding correction control is performed, according to the deviation degree, on the laser beam output by the laser, so as to improve optical coherent detection performance.

Description

Dynamic frequency offset correction method and coherent optical time domain reflectometer system The present application claims to be submitted to the Chinese Patent Office on December 14, 2010, application number 201010601965.8, entitled "Dynamic frequency offset correction method and coherent optical time domain reflectometer" The priority of the Chinese Patent Application, the entire disclosure of which is hereby incorporated by reference. Technical field

The present invention relates to the field of optical fiber communication technologies, and in particular, to a dynamic frequency offset correction method and a coherent optical time domain reflectometer system. Background technique

With the development of optical fiber communication technology, fiber opticization of the network has always been one of the main trends in network development. The submarine cable is also an important part of network fiber optics. Real-time monitoring of long-distance seabed cables has also become an important part of network maintenance.

In the detection of submarine cables, a coherence optical time-domain reflectometer (COTDR) is used for testing. In the COTDR system, lasers are required to detect ultra-long-range laser reflection signals. The frequency drift during the measurement will affect the receiver reception frequency. In order not to lose the signal, it is necessary to enlarge the receiving pass band of the receiver, thereby causing an inrush of noise power, thereby causing a decrease in the signal-to-noise ratio of the receiving system. Therefore, the detection performance of COTDR will be subject to the frequency stability of the light source. In the conventional scheme, a laser with extremely high frequency stability is often used as a light source, which greatly increases the cost of a communication system with COTDR.

In the prior art solution, the frequency of the measuring laser is monitored from the center frequency, and if a deviation occurs, the light source is adjusted. However, the prior art has the following disadvantages: since the COTDR requires a high line width of the light source, the adjustment of the source frequency affects the line width of the light source, and therefore, the detection effect is affected. Summary of the invention

The invention provides a dynamic frequency offset correction method and a coherent optical time domain reflectometer system, which are used for improving the coherent light detection performance, reducing the requirement of the line width of the light source, and improving the effect of correcting the frequency offset.

Embodiments of the present invention provide a method for dynamic frequency offset correction, which is applied to a coherent optical time domain reflectometry system. Eliminate the effects of laser frequency drift, including:

Performing spectroscopic processing on the laser outputted by the laser to generate probe light and control light;

The control light is input into the Mach-Zehnder interferometer MZ I to perform interference, and the interference control light is obtained;

Performing photoelectric conversion on the interference control light to obtain a first electrical signal;

Controlling, according to the first electrical signal, an optical path difference of an optical signal transmitted on both arms of the MZ I such that the optical path difference is one quarter of a wavelength of the laser;

Determining, according to the first electrical signal, a degree of deviation of a true center frequency of the laser light from a standard value of a center frequency of the laser;

Corresponding compensation is performed according to the degree of deviation such that the actual center frequency of the baseband signal obtained after receiving the optical signal transmitted from the downlink converges to a standard value of the center frequency of the baseband signal.

Embodiments of the present invention also provide a coherent optical time domain reflectometer system, including: a laser, a first beam splitter, a Mach Zehnder interferometer MZI, a photodetector, a first controller, a second controller, a modulator, and Modulation source

The laser for generating a laser;

The first beam splitter is configured to split the laser light to obtain detection light and control light; and the MZI is configured to interfere with the input control light to obtain interference control light; the photoelectric detection And performing photoelectric conversion on the interference control light to obtain a first electrical signal;

The first controller is configured to generate a control signal according to the first electrical signal to control the MZI such that an optical path difference of an optical signal transmitted on both arms of the MZ I is the laser One quarter of the wavelength;

The modulator is configured to modulate the probe light to generate a probe light pulse;

The modulation source is configured to generate a modulation control signal to control the modulator to modulate the probe light;

The second controller is configured to calculate a deviation degree of an actual center frequency of the laser light from a standard value of a center frequency of the laser light according to the first electrical signal, and generate a control based on the deviation degree a signal to control the modulation source to output an output frequency of the modulation control signal such that an actual center frequency of the probe light pulse generated by the modulator modulation converges to a center frequency of the laser The standard value.

The embodiment of the invention further provides a coherent optical time domain reflectometer system, the system comprising: a laser, a first beam splitter, a Mach Zede interferometer MZI, a photodetector, a first controller, a second controller, Modulator, modulation source, coherent receiver, mixer and oscillator;

The laser for generating a laser;

The coherent receiver is configured to receive an optical signal transmitted from the downlink, and perform coherent processing on the optical signal transmitted from the downlink to obtain a second electrical signal;

The oscillator is configured to generate an electrical local oscillator signal;

The mixer is configured to mix the electrical local oscillator signal and the second electrical signal to obtain a baseband signal;

The second controller is configured to calculate a deviation degree of an actual center frequency of the laser light from a standard value of a center frequency of the laser light according to the first electrical signal, and generate a control based on the deviation degree The signal is controlled to control a frequency at which the oscillator tunes its output of the electrical local oscillator signal such that an actual center frequency of the baseband signal obtained by the mixer converges to a standard value of a center frequency of the laser.

The invention divides the laser outputted by the laser into the probe light and the control light, and calculates the deviation degree of the actual center frequency of the laser light from the standard value of the center frequency of the laser according to the control light; according to the degree of deviation, corresponding compensation is performed to eliminate the laser The effect of the drift of the center frequency of the laser improves the optical coherence detection performance and can reduce the linewidth requirement of the light source. DRAWINGS

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, a brief description of the drawings used in the embodiments or the prior art description will be briefly described below. Obviously, the drawings in the following description It is a certain embodiment of the present invention, and other drawings can be obtained from those skilled in the art without any creative work.

1a is a flowchart of a method for dynamic frequency offset correction according to an embodiment of the present invention;

FIG. 1b is a schematic diagram of a coherent optical time domain reflectometer system according to an embodiment of the present invention; FIG.

1 c is a schematic diagram of a coherent optical time domain reflectometer system according to an embodiment of the present invention;

FIG. 1d is a schematic diagram of a coherent optical time domain reflectometer system according to an embodiment of the invention. detailed description

The technical solutions in the embodiments of the present invention are clearly and completely described in the following with reference to the accompanying drawings in the embodiments of the present invention. It is a partial embodiment of the invention, and not all of the embodiments. All other embodiments obtained by a person of ordinary skill in the art based on the embodiments of the present invention without creative efforts are within the scope of the present invention.

The embodiment of the invention detects the frequency offset of the laser and corrects the corresponding frequency offset, thereby eliminating the influence of the frequency drift of the laser output by the laser, thereby reducing the bandwidth requirement of the communication system and improving The performance of a coherent optical time domain reflectometer system.

FIG. 1 is a flowchart of a method for dynamic frequency offset correction according to an embodiment of the present invention, including:

Step 11. Perform beam splitting on the laser output from the laser to generate probe light and control light.

In this embodiment, the laser is split by a beam splitter such as a coupler to generate a probe light, and the power thereof is much larger than the power of the control light. Alternatively, the ratio of the probe light to the control light may be as shown in the figure. Shown 99:1.

Step 12: input the control light into the Mach-Zehnder interferometer MZI to perform interference, and obtain the control light after the interference;

Step 13. Perform photoelectric conversion on the interference control light to obtain a first electrical signal. Step 14. Control light of the optical signal transmitted on the two arms of the MZI according to the first electrical signal. a step such that the optical path difference is one quarter of a wavelength of the laser;

In the present embodiment, the control light is interfered by using the Mach Zed interferometer MZI to obtain the interference control light, and then the interference control light is photoelectrically converted to obtain a first electric signal. Since the power level of the first electrical signal can reflect the optical path difference of the two arms in the MZI (that is, the optical path difference of the optical signals transmitted on the two arms), the optical path of the MZI arms can be adjusted according to the first electrical signal. The difference is that the optical path difference is one quarter of the laser, thereby correcting the influence of external factors such as temperature and vibration on the MZI detection. MZI can adjust the optical path difference of the two arms of the MZI by adjusting the flexibility of the piezoelectric ceramic. When the optical path difference of the two arms of the MZI is exactly one quarter of the laser wavelength, the output of the MZI is in the linear output region, and the output intensity is proportional to the degree of drift of the center frequency of the laser output from the laser.

Step 15. Calculate, according to the first electrical signal, a degree of deviation of an actual center frequency of the laser light from a standard value of a center frequency of the laser.

Step 16. Perform corresponding compensation according to the degree of deviation, so that the actual center frequency of the baseband signal obtained after receiving the optical signal transmitted from the downlink converges to a standard value of the center frequency of the baseband signal.

After obtaining the deviation of the actual center frequency of the laser from the standard value of the laser center frequency, the following two ways may be adopted to transmit the baseband signal obtained by receiving the optical signal of the coherent optical time domain reflectometer system from the downlink. The actual center frequency converges to the standard value of the center frequency of the baseband signal to eliminate the effects of the center frequency drift of the laser output from the laser.

Method 1: coherently processing the probe light and the optical signal from the downlink received by the coherent optical time domain reflectometer system, and performing photoelectric conversion on the optical signal obtained by the coherent processing to obtain a second electrical signal; Deviating, controlling, for mixing with the second electrical signal to obtain a frequency of the electrical local oscillator signal of the baseband signal, such that an actual center frequency of the baseband signal converges to a center frequency of the baseband signal standard value.

Manner 2: when modulating the probe light to generate a probe light pulse, controlling an output frequency of the modulation source for performing the modulation according to the degree of deviation, such that a center frequency of the probe light pulse converges in the The standard value of the center frequency of the laser.

In this embodiment, the probe light may be divided by a beam splitter, and a part of the divided probe light is output to a coherent receiver, and the optical signal transmitted from the downlink is coherently processed by the coherent receiver to obtain a coherent processing. a second electrical signal; mixing the electrical local oscillator signal with the second electrical signal to obtain a baseband signal; and then controlling, according to the degree of deviation, for mixing with the second electrical signal The frequency of the electrical local oscillator signal to the baseband signal is such that the actual center frequency of the baseband signal converges to a standard value of the center frequency of the baseband signal.

The following describes the method for correcting the dynamic frequency offset in combination with a specific application scenario. The coherent optical time domain reflectometer system of this embodiment may be a system as shown in FIG. 1b, including: laser 1, coupler A2, photodetector 7, a first controller 8, a second controller 10, a modulation source 11, a modulator 12, and an MZI5; wherein, the MZI5 includes a coupler B3, a piezoelectric ceramic 9, and a coupler C6.

The method of dynamic frequency offset correction will be described in detail below in conjunction with the coherent optical time domain reflectometer system. The optical power outputted by the laser 1 is divided into two outputs through the coupler A2, one of which outputs most of the power as the probe light for detection; the other has a smaller output power and is used as the control light for control.

Control light input MZI5 coupler B3 splits into two paths, two paths of light arrive through different paths

The MZI5 coupler C6 is coherently processed, and the intensity of the output light will reflect the difference in optical path between the two arms. After the MZI5 interferes with the input control light, the obtained control light is input to the photodetector 7, and the photodetector 7 photoelectrically converts it to obtain a first electrical signal for the controller to generate a control signal.

The photodetector 7 transmits the first electrical signal to the first controller 8. The first controller 8 makes a control decision, generates a control signal to control the MZI5, controls the expansion and contraction of the piezoelectric ceramic 9 in the MZI5 to adjust the optical path difference of the two arms of the MZI5, so that the optical path difference is four points of the laser wavelength. One, to correct the influence of external factors such as temperature and vibration on MZI5, so that the output of MZI5 is always in the linear output area. The feedback control of the first controller 8 to the MZI5 can correct the influence of external factors on the MZI5 to some extent.

Further, the following steps are performed to further eliminate the drift of the center frequency of the laser output from the laser.

Inputting the first electrical signal outputted by the photodetector 7 to the second controller 10; calculating, by the second controller 10, the actual center frequency of the laser light relative to the center frequency of the laser light according to the first electrical signal The degree of deviation of the standard value is determined by the second controller 10 according to the degree of deviation to compensate for the drift of the laser center frequency. The second controller 10 makes a control decision according to the degree of deviation. When the probe light is modulated to generate a probe light pulse, the output frequency of the modulation source 11 for performing modulation control is controlled according to the degree of deviation, so that the The actual center frequency of the probe light pulse converges to a standard value of the center frequency of the laser.

After the modulation source 11 is adjusted, the probe light enters the modulator 12 to receive modulation, and a probe light pulse is generated. The center frequency of the probe light pulse is corrected, and the monitored light is entered through the uplink 13. The cable is such that the center frequency of the probe light pulse of the input cable is always near the standard value. Since the center frequency of the probe light pulse generated in the embodiment of the present invention has been corrected, the probe light pulse reflects and/or scatters the optical signal in the optical fiber, and the downlink transmits back to the coherent optical time domain reflectometer system. The actual center frequency of the received baseband signal can converge to the standard value of the center frequency of the baseband signal, thereby eliminating the influence of the center frequency drift of the laser.

In another application scenario, the coherent optical time domain reflectometer system can be designed as a system as shown in FIG. 1c, including: laser 1, coupler A2, photodetector 7, first controller 8, second controller 10, a modulation source 11, a modulator 12, and an MZI5; wherein, the MZI5 includes a coupler B3, a piezoelectric ceramic 9, and a coupler C6.

Based on this, the system further includes: a coherent receiver 15, a mixer 17, an oscillator 16, and a baseband filter 18.

The method of dynamic frequency offset correction will be described in detail below in conjunction with the coherent optical time domain reflectometer system.

The optical power outputted by the laser 1 is divided into two outputs through the coupler A2, one of which outputs most of the power as the probe light for detection; the other has a smaller output power and is used as the control light for control.

Control light input MZI5 coupler B3 is divided into two paths, two paths of light arrive through different paths

The photodetector 7 transmits the first electrical signal to the first controller 8. The first controller 8 makes a control decision, generates a control signal to control the MZI5, controls the expansion and contraction of the piezoelectric ceramic 9 in the MZI5 to adjust the optical path difference of the arms of the MZI5, and makes the optical path difference of the arms of the MZI5 the laser. One quarter of the wavelength.

In the system, the modulation source 11 generates a modulation control signal to control the modulator 12 to modulate the probe light; the modulator 12 modulates the probe light to generate a probe light pulse.

The coherent receiver 15 receives the optical signal transmitted from the downlink, and performs coherent processing on the optical signal transmitted from the downlink to obtain a second electrical signal.

The oscillator 16 is used to generate an electrical local oscillator signal.

A mixer 17 is configured to mix the electrical local oscillator signal with the second electrical signal to obtain a baseband signal.

The method for correcting the dynamic frequency offset in this embodiment is as follows:

Inputting the first electrical signal output by the photodetector 7 into the second controller 10; by the second controller 10 calculating, according to the first electrical signal, a degree of deviation of a true center frequency of the laser with respect to a standard value of a center frequency of the laser; and generating a control signal to control the oscillator 16 to be tuned based on the degree of deviation The frequency of the electrical local oscillation signal outputted by the mixer 17 causes the actual center frequency of the baseband signal obtained by the mixer 17 to converge to a standard value of the center frequency of the laser, thereby realizing correction of dynamic frequency offset .

In this embodiment, the probe light may be divided by a coupler, and a part of the divided probe light is output to the coherent receiver. At this time, the coherent optical time domain reflectometer system is shown in FIG. D, the probe light is divided into two parts, one part is input to the modulator 12 for modulation, and the other part is input to the coherent receiver 15 for coherent processing with the optical signal transmitted from the downlink to generate a second electric signal, and the second controller 10 The electrical local oscillator signal generated after the tuning is mixed with the second electrical signal, and the actual center frequency of the obtained baseband signal converges to a standard value of the center frequency of the baseband signal, thereby realizing the correction of the dynamic frequency offset.

In this embodiment, the laser outputted by the laser is divided into the probe light and the control light, and the degree of deviation of the actual center frequency of the laser light from the standard value of the center frequency of the laser is calculated according to the control light; corresponding compensation control is performed according to the degree of deviation, Eliminate the effects of the laser's center frequency drift, improve optical coherent detection performance, and reduce the linewidth requirements of the source.

An embodiment of the present invention provides a coherent optical time domain reflectometer system, and a schematic structural diagram of the system can be seen in FIG. Including: laser 1, first beam splitter 2 (can be the coupler A in the figure),

MZI5, photodetector 7, first controller 5, second controller 10, modulator 12 and modulation source 11.

The laser 1 is used to generate a laser;

The first beam splitter 2 is configured to split the laser light to obtain the probe light and the control light; the MZI5 is configured to interfere with the input control light to obtain interference control light; a detector 7 for photoelectrically converting the interference control light to obtain a first electrical signal;

The first controller 8 is configured to generate a control signal according to the first electrical signal, to

The MZI5 is controlled such that the optical path difference of the optical signals transmitted on the two arms of the MZI5 is one quarter of the wavelength of the laser;

The modulator 12 is configured to modulate the probe light to generate a probe light pulse; the modulation source 11 is configured to generate a modulation control signal to control the modulator 12 to modulate the probe light; The second controller 10 is configured to calculate, according to the first electrical signal, a degree of deviation of a true center frequency of the laser light from a standard value of a center frequency of the laser, and generate the The control signal controls the modulation source 11 to output an output frequency of the modulation control signal such that the actual center frequency of the generated probe light pulse modulated by the modulator 12 converges to a standard value of a center frequency of the laser.

Furthermore, the system of this embodiment further includes: a coherent receiver 15 for receiving an optical signal transmitted from the downlink and performing coherent processing on the optical signal transmitted from the downlink.

Based on the coherent optical time domain reflectometer system shown in Fig. 1c, the probe light can be divided by a beam splitter, and a part of the divided probe light is output to the coherent receiver 15. Further, the system needs to be further improved. The structure of the improved coherent optical time domain reflectometer system can be seen in FIG. 1D. At this time, the system further includes: a second optical splitter 4 (may be a diagram Id) The coupler D) shown is for separating a portion of the light from the probe light and inputting it to the coherent receiver 15.

The interaction mechanism and function between the modules of the coherent optical time domain reflectometer system in this embodiment can be referred to the description of the corresponding embodiments in FIG. 1 to FIG. 1d, and details are not described herein again.

An embodiment of the present invention provides a coherent optical time domain reflectometer system, and a schematic structural diagram of the system can be seen in FIG. Including: laser 1, first beam splitter 2 (may be coupler A in the figure), MZI5, photodetector 7, first controller 8, second controller 10, modulator 12, modulation source 11, coherent Receiver 15, mixer 17 and oscillator 16.

The laser 1 is used to generate a laser;

The first controller 8 is configured to generate a control signal according to the first electrical signal, to The MZI5 is controlled such that the optical path difference of the optical signals transmitted on the two arms of the crucible 5 is one quarter of the wavelength of the laser;

The modulator 12 is configured to modulate the probe light to generate a probe light pulse; the modulation source 11 is configured to generate a modulation control signal to control the modulator 12 to modulate the probe light;

The coherent receiver 15 is configured to receive an optical signal transmitted from the downlink, and perform coherent processing on the optical signal transmitted from the downlink to obtain a second electrical signal;

The oscillator 16 is configured to generate an electrical local oscillator signal;

The mixer 17 is configured to mix the electrical local oscillator signal and the second electrical signal to obtain a baseband signal;

The second controller 10 is configured to calculate, according to the first electrical signal, a degree of deviation of a true center frequency of the laser light from a standard value of a center frequency of the laser, and generate the Controlling a signal to control a frequency at which said oscillator 16 tunes its output of said electrical local oscillator signal such that said actual center frequency of said baseband signal obtained by said mixer 17 converges to a standard value of a center frequency of said laser .

In this embodiment, the probe light may be divided by a beam splitter, and a part of the divided probe light is output to the coherent receiver 15. The system further includes: a second beam splitter 4 (may be a coupler D in the figure) And for extracting a part of the light from the probe light and inputting it into the coherent receiver 15 for coherence with the optical signal transmitted from the downlink.

For the interaction mechanism and function between the modules of the device for the dynamic frequency offset correction of the optical fiber communication system in this embodiment, reference may be made to the description of the corresponding embodiments in FIG. 1 to FIG. 1d, and details are not described herein again.

It should be noted that the above embodiments are only for explaining the technical solutions of the present invention, and are not intended to be limiting; although the present invention has been described in detail with reference to the foregoing embodiments, it will be understood by those skilled in the art that: The technical solutions described in the foregoing embodiments are modified, or some of the technical features are equivalently replaced. The modifications and substitutions do not depart from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

Claim

A dynamic frequency offset correction method for eliminating the effects of frequency drift of a laser in a coherent optical time domain reflectometer system, comprising:

Performing spectroscopic processing on the laser light output by the laser to generate probe light and control light; inputting the control light into the Mach-Zehnder interferometer MZI to perform interference, and obtaining control light after interference;

Controlling, according to the first electrical signal, an optical path difference of an optical signal transmitted on both arms of the MZI such that the optical path difference is one quarter of a wavelength of the laser;

2. The method according to claim 1, wherein the compensation is performed according to the degree of deviation, such that an actual center frequency of the baseband signal obtained after receiving the optical signal transmitted from the downlink converges to the The standard value of the center frequency of the baseband signal, including:

Performing coherent processing on the optical signal from the downlink received by the coherent optical time domain reflectometer system, and photoelectrically converting the optical signal obtained by the coherent processing to obtain a second electrical signal; And controlling a frequency for mixing the second electrical signal to obtain an electrical local oscillator signal of the baseband signal such that an actual center frequency of the baseband signal converges to a standard value of a center frequency of the baseband signal.

The method according to claim 1, wherein the compensation is performed according to the degree of deviation, so that the actual center frequency of the baseband signal obtained after receiving the optical signal transmitted from the downlink is located at the baseband The standard value of the center frequency of the signal, including:

When the probe light is modulated to generate a probe light pulse, an output frequency of the modulation source for performing the modulation is controlled according to the degree of deviation such that a center frequency of the probe light pulse converges to a center frequency of the laser light The standard value.

4. The method according to claim 1, wherein the power of the probe light is greater than the Control the power of light.

5. A coherent optical time domain reflectometer system, comprising: a laser, a first beam splitter, a Mach Zehnder interferometer MZI, a photodetector, a first controller, a second controller, a modulator, and a modulation Source

The laser for generating a laser;

The first beam splitter is configured to split the laser light to obtain the probe light and the control light; and the MZI is configured to interfere with the input control light to obtain interference control light; the photoelectric detection And performing photoelectric conversion on the interference control light to obtain a first electrical signal;

The first controller is configured to generate a control signal according to the first electrical signal to control the MZI such that an optical path difference of an optical signal transmitted on two arms of the MZI is a wavelength of the laser One quarter of;

The second controller is configured to calculate a deviation degree of an actual center frequency of the laser light from a standard value of a center frequency of the laser light according to the first electrical signal, and generate a control based on the deviation degree And a signal for controlling the modulation source to output an output frequency of the modulation control signal such that an actual center frequency of the probe light pulse generated by the modulator modulation converges to a standard value of a center frequency of the laser.

6. The coherent optical time domain reflectometer system of claim 5, wherein the system further comprises:

A coherent receiver is configured to receive an optical signal transmitted from the downlink and perform coherent processing on the optical signal transmitted from the downlink.

7. The coherent optical time domain reflectometer system of claim 5, wherein the system further comprises:

A second beam splitter for separating a portion of the light from the probe light and inputting it to the coherent receiver.

8. A coherent optical time domain reflectometer system, the system comprising: a laser, a first beam splitter, a Mach Zede interferometer MZI, a photodetector, a first controller, a second controller, Modulator, modulation source, coherent receiver, mixer and oscillator;

The laser for generating a laser;

The oscillator is configured to generate an electrical local oscillator signal;

9. The coherent optical time domain reflectometer system of claim 8, wherein the system further comprises:

A second beam splitter is configured to split a portion of the light from the probe light and input it into the coherent receiver for coherence with the optical signal transmitted from the downlink.