OTDR system

Abstract

The invention discloses an OTDR (Optical Time Domain Reflectometer) system, which comprises a signal generator; a circulator; a variable optical attenuator which attenuates the received backward light and outputs an attenuated optical signal; the single-photon detector is used for converting the received attenuated optical signal into an electric signal; the photon counter is used for counting the received electric signals; and a controller that restores the backward light incident on the variable optical attenuator on the basis of the count signal and the attenuation amount of the variable optical attenuator. According to the OTDR system, the variable optical attenuator is used for attenuating the backward light, and the attenuated optical signal enters the single-photon detector, so that the saturation of the single-photon detector is avoided, the anti-saturation capability of the single-photon detector is improved, and the technical problem that the anti-saturation capability of the single-photon detector is poor is solved. Moreover, because the single photon detector is accurate in detection, the counting signal output by the photon counter can be ensured to be accurate, and the controller can also be ensured to accurately restore the back light emitted into the variable optical attenuator.

Description

Technical Field

[0001] The invention belongs to the technical field of optical fiber detection, and in particular relates to an OTDR system. Background Art

[0002] Optical Time-Domain Reflectometry (OTDR) is a precision photoelectric instrument made by using the backscattering produced by Rayleigh scattering and Fresnel reflection when light is transmitted in an optical fiber. It is widely used in the maintenance and repair of optical cable lines. It can measure joint attenuation, optical fiber transmission attenuation, optical fiber length and fault location.

[0003] At present, there are two main types of OTDRs used for optical fiber fault location monitoring: single-photon detection OTDR for single-photon level weak light detection and ordinary OTDR for non-single-photon level light detection. Avalanche photodiode (APD) is a common photosensitive detection element in the field of laser communication. It has high detection sensitivity and is an indispensable component of OTDR.

[0004] Among them, the avalanche photodiode of the single-photon detection OTDR works in the gated Geiger mode, while the avalanche photodiode of the ordinary OTDR works in the linear mode; the avalanche photodiode working in the linear mode does not need to consider the after-pulse effect and can work in the continuous signal acquisition state, which has the advantage of fast measurement time. However, since the avalanche photodiode working in the linear mode has a low gain and cannot detect weak optical signals, the measurement accuracy and measurement distance of the ordinary OTDR are limited.

[0005] Although single-photon detection OTDR can detect extremely weak optical signals that are smaller than thermal noise, it can achieve higher measurement accuracy, longer measurement distance and larger dynamic range than ordinary OTDR, which can make up for the shortcomings of ordinary OTDR. However, in the single-photon detection OTDR detection process, due to the order of magnitude difference in the intensity of Rayleigh scattering and Fresnel reflection, the reflected light exceeds the intensity range of the single-photon detector, which can easily lead to distortion or saturation of the single-photon detector. Summary of the invention

[0006] The invention provides an OTDR system, which solves the technical problem that the single-photon detector has poor anti-saturation capability.

[0007] In order to achieve the above technical objectives, the present invention adopts the following technical solutions:

[0008] An OTDR system, comprising:

[0009] A signal generator, which is used to generate a pulse light signal;

[0010] A circulator, whose input end is connected to the output end of the signal generator and whose common end is connected to the optical fiber;

[0011] a variable optical attenuator, which is used to receive the backlight outputted from the output end of the circulator, attenuate the received backlight, and output an attenuated optical signal;

[0012] a single-photon detector, which is used to receive the attenuated optical signal output by the variable optical attenuator, convert the received attenuated optical signal into an electrical signal, and output it;

[0013] A photon counter, which is used to receive the electrical signal output by the single-photon detector and count the received electrical signal;

[0014] The controller is used for receiving the counting signal output by the photon counter and restoring the backlight incident on the variable optical attenuator according to the counting signal and the attenuation amount of the variable optical attenuator.

[0015] In some embodiments of the present application, the variable optical attenuator is a passive variable optical attenuator;

[0016] The controller is also used to control the attenuation of the variable optical attenuator.

[0017] In some embodiments of the present application, the controller controls the attenuation of the variable optical attenuator by controlling the driving voltage of the variable optical attenuator.

[0018] In some embodiments of the present application, the controller is further used to: establish a relationship curve between attenuation and counting signal; find the linear region of the relationship curve; adjust the current attenuation of the variable optical attenuator so that the current attenuation is in the linear region, and then re-receive the counting signal output by the photon counter, and restore the backlight incident on the variable optical attenuator based on the received counting signal and the current attenuation.

[0019] In some embodiments of the present application, establishing a relationship curve between attenuation and counting signal specifically includes: controlling the attenuation of the variable optical attenuator to gradually change from a first preset value to a second preset value according to a set step size, and recording the counting signal output by the photon counter received at each attenuation amount; and fitting the relationship curve between the attenuation and counting signal.

[0020] In some embodiments of the present application, the variable optical attenuator is an active variable optical attenuator, which is also used to detect the optical power of the received backlight. When it is detected that the optical power of the backlight exceeds a set threshold, the backlight is attenuated and an attenuated optical signal is output, so that the optical power of the attenuated optical signal does not exceed the set threshold; and the attenuation amount is fed back to the controller.

[0021] In some embodiments of the present application, the variable optical attenuator includes a beam splitter, a detection unit, an attenuation unit, and a control unit;

[0022] The beam splitter is used to receive the backlight outputted from the output end of the circulator and split the received backlight into two beams, wherein one optical signal is transmitted to the attenuation unit and the other optical signal is transmitted to the detection unit;

[0023] The detection unit is used to detect the optical power of the received optical signal and transmit the detected optical power to the control unit;

[0024] The control unit is used to determine whether the received optical power exceeds a set threshold; if it exceeds the set threshold, generate an attenuation amount, and send the generated attenuation amount to the attenuation unit and the controller;

[0025] The attenuation unit is used to receive the attenuation amount sent by the control unit and the optical signal sent by the beam emitter, attenuate the optical signal according to the attenuation amount, output the attenuated optical signal, and send it to the single-photon detector.

[0026] In some embodiments of the present application, the splitting ratio of the beam splitter is any value between 9:1 and 99:1.

[0027] In some embodiments of the present application, the signal generator includes:

[0028] A pulse generator for generating a pulse electrical signal;

[0029] The laser is used for receiving the pulse electric signal sent by the pulse generator and generating a pulse optical signal.

[0030] In some embodiments of the present application, the OTDR system further includes a clock; the clock generates a clock signal and sends it to the photon counter, the single photon detector, and the signal generator.

[0031] Compared with the prior art, the advantages and positive effects of the present invention are as follows: the OTDR system of the present invention attenuates the incident backlight by designing a variable optical attenuator, and the attenuated optical signal is then incident to the single-photon detector, thereby avoiding the saturation of the single-photon detector, ensuring the accurate detection of the single-photon detector, improving the anti-saturation capability of the single-photon detector, and solving the technical problem of the poor anti-saturation capability of the single-photon detector. Moreover, since the single-photon detector detects accurately, the counting signal output by the photon counter can be guaranteed to be accurate, and thus the controller can be guaranteed to accurately restore the backlight incident to the variable optical attenuator.

[0032] Other features and advantages of the present invention will become more apparent after reading the detailed description of the embodiments of the present invention in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other accompanying drawings can be obtained based on these accompanying drawings without paying creative work.

[0034] Figure 1 It is a principle block diagram of an embodiment of the OTDR system proposed by the present invention;

[0035] Figure 2 is a functional block diagram of an embodiment of a variable optical attenuator;

[0036] Figure 3 is the signal curve graph of the received backlight;

[0037] Figure 4 It is a graph of the relationship between incident photons and photon counts. DETAILED DESCRIPTION

[0038] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0039] In the description of the present application, it should be understood that the terms "center", "up", "down", "front", "back", "top", "bottom", "inside", "outside", etc., indicating orientations or positional relationships, are based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as a limitation on the present application.

[0040] In the description of this application, it should be understood that the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of the feature. In the description of this application, unless otherwise specified, "plurality" means two or more.

[0041] In the description of this application, it should be noted that, unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in this application can be understood according to specific circumstances.

[0042] The OTDR system of this embodiment includes a signal generator, a circulator, a variable optical attenuator (VOA), a single photon detector, a photon counter, a controller, etc. Figure 1 shown.

[0043] A signal generator is used to generate a pulse light signal.

[0044] The circulator has an input end connected to the output end of the signal generator, a common end connected to the optical fiber, and an output end connected to the variable optical attenuator.

[0045] The variable optical attenuator is used for receiving the backlight outputted from the output end of the circulator, attenuating the received backlight, and outputting an attenuated optical signal.

[0046] The single-photon detector is used to receive the attenuated optical signal output by the variable optical attenuator, convert the received attenuated optical signal into an electrical signal, and output it.

[0047] The photon counter is used to receive the electrical signal output by the single-photon detector, count the received electrical signal, and output a counting signal.

[0048] The controller is used for receiving the counting signal output by the photon counter and restoring the backlight incident on the variable optical attenuator according to the counting signal and the attenuation amount of the variable optical attenuator.

[0049] The signal generator sends out a pulse light signal, which is incident on the optical fiber through a circulator. When there is a connector or a breakpoint in the optical fiber, backlight will be generated. The backlight is incident on the variable optical attenuator through the circulator. The variable optical attenuator attenuates the incident light signal, outputs the attenuated light signal, and sends it to the single-photon detector. The single-photon detector converts the light signal into an electrical signal and sends it to the photon counter. The photon counter generates a counting signal and sends the counting signal to the controller. The controller restores the backlight incident on the variable optical attenuator according to the counting signal and the attenuation amount of the variable optical attenuator.

[0050] Suppose a single-photon detector can receive 1 to 100 photons, but the backlight has 1000 photons. When the number of photons incident on the single-photon detector is 100, 200, 300 or more, the single-photon detector outputs the same signal, which is called the saturation of the single-photon detector. Figure 4 As shown in the curve, when the single-photon detector is not saturated, the more incident photons, the greater the photon count. As the number of photons incident on the single-photon detector increases, the single-photon detector gradually reaches saturation, that is, it outputs the same electrical signal, causing the photon counter to also output the same photon count. In order to avoid this saturation, a variable optical attenuator is added in front of the single-photon detector. The variable optical attenuator attenuates the optical signal, reduces the optical power of the optical signal, avoids the saturation of the single-photon detector, and enables the single-photon detector to accurately detect the optical signal.

[0051] The OTDR system of this embodiment is designed with a variable optical attenuator, which attenuates the incident backlight, and then the attenuated optical signal is injected into the single-photon detector, thereby avoiding the saturation of the single-photon detector, ensuring the accurate detection of the single-photon detector, improving the anti-saturation capability of the single-photon detector, and solving the technical problem of the poor anti-saturation capability of the single-photon detector. Moreover, since the single-photon detector detects accurately, the counting signal output by the photon counter can be guaranteed to be accurate, and thus the controller can be guaranteed to accurately restore the backlight injected into the variable optical attenuator.

[0052] In some embodiments of the present application, the variable optical attenuator is a passive variable optical attenuator. The controller is also used to control the attenuation of the variable optical attenuator.

[0053] The variable optical attenuator is a passive variable optical attenuator that cannot control the attenuation by itself. The attenuation is controlled by a controller. The variable optical attenuator attenuates the optical signal according to the attenuation.

[0054] The variable optical attenuator receives the backlight outputted from the output end of the circulator, attenuates the backlight according to the attenuation amount, and outputs an attenuated optical signal, which is simple and convenient.

[0055] In some embodiments of the present application, the controller controls the attenuation of the variable optical attenuator by controlling the driving voltage of the variable optical attenuator, which is convenient for control.

[0056] The driving voltage of the variable optical attenuator is proportional to the attenuation. The greater the driving voltage, the greater the attenuation. The unit of attenuation is dB. The greater the attenuation, the smaller the photon count.

[0057] In some embodiments of the present application, the controller is also used to: establish a relationship curve between attenuation (or driving voltage) and counting signal; find the linear area of ​​the relationship curve; adjust the current attenuation of the variable optical attenuator so that the current attenuation is located in the linear area of ​​the relationship curve, and then re-receive the counting signal output by the photon counter, and restore the backlight incident on the variable optical attenuator based on the received counting signal and the current attenuation.

[0058] In the linear region of the relationship curve, the attenuation is proportional to the counting signal, which can ensure that the single-photon detector is not saturated and that the single-photon detector detects accurately.

[0059] Each time the optical signal is detected, the controller first establishes a relationship curve between the attenuation and the counting signal, then finds the linear area of ​​the relationship curve, adjusts the current attenuation of the variable optical attenuator to be in the linear area of ​​the relationship curve, and then receives the counting signal again. Finally, based on the received counting signal and the current attenuation, the backlight incident on the variable optical attenuator is restored to ensure that the single-photon detector is not saturated, avoid inaccurate detection due to the saturation of the single-photon detector, and improve the anti-saturation ability of the single-photon detector.

[0060] In some embodiments of the present application, a relationship curve between attenuation and counting signal is established, specifically including: controlling the attenuation of the variable optical attenuator to gradually change from a first preset value to a second preset value according to a set step size, and recording the counting signal output by the photon counter received at each attenuation amount; fitting the relationship curve between the attenuation and counting signal.

[0061] Through the above steps, the relationship curve between the attenuation amount and the counting signal can be simply and accurately established.

[0062] Assume that the first preset value is the minimum attenuation (such as 0), and the second preset value is the maximum attenuation, and the attenuation of the variable optical attenuator is controlled to gradually increase from the first preset value to the second preset value according to a set step size (such as 10 dB).

[0063] Therefore, each time the OTDR system detects an optical signal, the controller first gradually changes the driving voltage of the variable optical attenuator to change the attenuation, so that the attenuation gradually changes from the first preset value to the second preset value according to the set step length, and records the counting signals corresponding to each attenuation, and then fits the relationship curve between the attenuation and the counting signal. Find the linear area of ​​the relationship curve, adjust the current attenuation of the variable optical attenuator, so that the current attenuation is in the linear area of ​​the relationship curve, and then re-receive the counting signal output by the photon counter, and restore the backlight incident on the variable optical attenuator according to the received counting signal and the current attenuation.

[0064] In some other embodiments of the present application, the variable optical attenuator is an active variable optical attenuator. The variable optical attenuator is also used to detect the optical power of the received backlight, and when it is detected that the optical power of the backlight exceeds a set threshold, the backlight is attenuated, an attenuated optical signal is output, so that the optical power of the attenuated optical signal does not exceed the set threshold; and the attenuation amount is fed back to the controller.

[0065] When an optical signal with an optical power exceeding a set threshold is incident on a single-photon detector, the single-photon detector will be saturated; when an optical signal with an optical power not exceeding the set threshold is incident on a single-photon detector, the single-photon detector will not be saturated.

[0066] Therefore, the active variable optical attenuator detects the optical power of the received backlight. When it is detected that the optical power of the backlight does not exceed the set threshold, the backlight is not attenuated (or the attenuation is 0), and then the backlight is emitted into the single-photon detector. When the active variable optical attenuator detects that the optical power of the backlight exceeds the set threshold, the backlight is attenuated and an attenuated optical signal is output. The active variable optical attenuator controls the attenuation by itself so that the optical power of the generated attenuated optical signal does not exceed the set threshold, thus avoiding saturation of the single-photon detector, saving time and improving efficiency.

[0067] In some embodiments of the present application, the variable optical attenuator includes a beam splitter, a detection unit, an attenuation unit, a control unit, etc. Figure 2 shown.

[0068] The beam splitter is used to receive the backlight outputted from the output end of the circulator and split the received backlight into two beams, wherein one beam of light signal is transmitted to the attenuation unit, and the other beam of light signal is transmitted to the detection unit.

[0069] The detection unit is used to detect the optical power of the received optical signal and transmit the detected optical power to the control unit.

[0070] The control unit is used to determine whether the received optical power exceeds a set threshold; if it exceeds the set threshold, an attenuation amount is generated and the generated attenuation amount is sent to the attenuation unit and the controller.

[0071] The attenuation unit is used to receive the attenuation amount sent by the control unit and the optical signal sent by the beam emitter, attenuate the optical signal according to the attenuation amount, output the attenuated optical signal, and send it to the single-photon detector.

[0072] The attenuation unit attenuates the optical signal according to the attenuation amount and outputs the attenuated optical signal, so that the optical power of the attenuated optical signal does not exceed the set threshold value, thereby avoiding saturation of the single-photon detector.

[0073] The variable optical attenuator is designed to be a beam splitter, a detection unit, a control unit and an attenuation unit; wherein the beam splitter is used to split the optical signal, the detection unit is used to detect the optical power, the control unit is used to judge whether the optical power exceeds the set threshold and generate the attenuation; the attenuation unit is used to attenuate the optical signal; each part performs its respective functions to realize the detection of the optical power and the attenuation of the optical signal, the structure is simple, and it is easy to build and realize.

[0074] In some embodiments of the present application, the splitting ratio of the beam splitter is any value between 9:1 and 99:1.

[0075] The ratio of the optical power transmitted by the beam splitter to the attenuation unit to the optical power transmitted to the detection unit is any value within a range of 9:1 to 99:1.

[0076] In some embodiments of the present application, the splitting ratio of the beam splitter is 9:1, that is, the ratio of the optical power transmitted to the attenuation unit to the optical power transmitted to the detection unit is 9:1.

[0077] In some other embodiments of the present application, the splitting ratio of the beam splitter is 99:1, that is, the ratio of the optical power transmitted to the attenuation unit to the optical power transmitted to the detection unit is 99:1.

[0078] By designing the above splitting ratio range, it is possible to ensure that the detection unit has sufficient light intensity to accurately detect the optical power, while avoiding affecting the light intensity transmitted to the attenuation unit.

[0079] In some embodiments of the present application, in order to facilitate the generation of a stable pulsed light signal, the signal generator includes a pulse generator and a laser.

[0080] A pulse generator is used to generate a pulse electrical signal.

[0081] The laser is used to receive the pulse electrical signal sent by the pulse generator and generate a pulse optical signal according to the received pulse electrical signal.

[0082] In some embodiments of the present application, the OTDR system further includes a clock. The controller controls the operation of the clock. The clock generates a clock signal and sends it to the photon counter, the single photon detector, and the signal generator to achieve signal synchronization and ensure the smooth operation of the OTDR system.

[0083] The operating principle of the OTDR system is described in detail below with reference to the accompanying drawings.

[0084] The pulse light signal emitted by the laser passes through the optical fiber to be tested, and its back-returned light is input into the variable optical attenuator. The variable optical attenuator attenuates the light signal to below the detector saturation and feeds back the attenuation amount to the controller. After the photon counter obtains the count at a certain detection position, it calculates and restores the original signal with the current attenuation amount. For example, when there is no reflection signal, the OTDR does not attenuate, and when there is a strong reflection signal, it attenuates 100 times. Then, the attenuation multiple is multiplied by the obtained count signal to restore the original signal.

[0085] See also Figure 3 As shown, the laser emits a modulated pulse light signal, which passes through a circulator and is incident on the optical fiber. When there is a connector or a breakpoint in the optical fiber, a backscattered signal will be generated. The backscattered signal passes through the circulator and will no longer be incident on the laser, but will be incident on the variable optical attenuator.

[0086] Variable optical attenuators can be either passive or active.

[0087] For passive variable optical attenuators (i.e. variable optical attenuators without communication), such as Figure 1 As shown, the controller can change the attenuation by changing the driving voltage of the passive variable optical attenuator, and establish a relationship curve between the attenuation and the photon count. When the attenuation weakens, that is, when the incident photons increase, the photon counter counts gradually saturate, and the current attenuation is recorded. During OTDR detection, the single photon detector is always kept below saturation to obtain accurate data.

[0088] For active variable optical attenuators (i.e. variable optical attenuators with communication), such as Figure 2 As shown, after the incident light enters the beam splitter, a portion of the optical signal enters the detection unit. When the optical power of the optical signal exceeds the set threshold, the control unit will automatically control the attenuation unit to attenuate and feed back the attenuation amount to the controller.

[0089] For a passive variable optical attenuator (attenuation controlled by voltage), the controller controls the attenuation by controlling the change in the driving voltage. Backlight enters the single-photon detector, which outputs an electrical signal, which is input into the photon counter for counting. In this way, the relationship between voltage (equivalent to attenuation) and count is obtained. When the voltage (attenuation) and count no longer show a linear relationship, the detector is considered to be saturated. The controller adjusts the voltage (attenuation) to the linear region, and multiplies the attenuation multiple by the obtained count to restore the signal. By controlling the attenuation in the linear region, back-reflected light and scattered light can be restored, such as Figure 3 As shown, different amounts of attenuation can be independently performed at different connector locations to improve the dynamic range of the OTDR.

[0090] For active variable optical attenuators, such as Figure 2As shown, a beam splitter, a detection unit and a control unit are added to the passive variable optical attenuator. After the back-light signal is input into the variable optical attenuator, it first passes through the beam splitter, and a part of the light is injected into the detection unit. The detection unit detects the optical power. The setting threshold of the control unit can be set. When the control unit determines that the input optical power exceeds the set threshold (single-photon detector saturation value), the control unit will automatically control the attenuation amount so that the optical power of the optical signal is attenuated below the set threshold, and transmit the attenuation amount to the controller. The controller combines the obtained counting signal curve with the attenuation amount to restore the entire OTDR curve. This method improves the detector's anti-saturation ability, saves time, and improves efficiency.

[0091] The OTDR system of this embodiment uses a variable optical attenuator with feedback, which can attenuate in real time according to the input optical signal; and the variable optical attenuator can communicate with the controller and feed back the attenuation amount to the controller; the variable optical attenuator can improve the anti-saturation capability of the OTDR and improve the dynamic range.

[0092] The OTDR system of this embodiment adds a variable optical attenuator to the link, avoids the saturation of the single-photon detector, can achieve real-time attenuation, reduce detection time, and improve efficiency; can realize the feedback of attenuation, effectively avoid the saturation of the single-photon detector. The controller can calculate the restored signal based on the feedback attenuation to improve the dynamic range of the OTDR. The OTDR system of this embodiment improves the anti-saturation ability of the single-photon detector, feeds back the attenuation amount to the controller, and effectively solves the problem of saturation of the single-photon detector under strong reflection signals.

[0093] The OTDR system of this embodiment sets a variable optical attenuator before the single-photon detector, attenuates the optical signal before injecting it into the single-photon detector, avoids saturation of the single-photon detector, and improves the anti-saturation ability of the single-photon detector; moreover, it has the function of automatically adjusting the attenuation amount and has feedback and communication functions.

[0094] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. An OTDR system, Features: include: A signal generator, which is used to generate a pulse light signal; A circulator, whose input end is connected to the output end of the signal generator and whose common end is connected to the optical fiber; a variable optical attenuator, which is used to receive the backlight outputted from the output end of the circulator, attenuate the received backlight, and output an attenuated optical signal; a single-photon detector, which is used to receive the attenuated optical signal output by the variable optical attenuator, convert the received attenuated optical signal into an electrical signal, and output it; A photon counter, which is used to receive the electrical signal output by the single-photon detector and count the received electrical signal; The controller is used for receiving the counting signal output by the photon counter and restoring the backlight incident on the variable optical attenuator according to the counting signal and the attenuation amount of the variable optical attenuator.

2. The OTDR system according to claim 1, Features: The variable optical attenuator is a passive variable optical attenuator; The controller is also used to control the attenuation of the variable optical attenuator.

3. The OTDR system according to claim 2, Features: The controller controls the attenuation of the variable optical attenuator by controlling the driving voltage of the variable optical attenuator.

4. The OTDR system according to claim 2, Features: The controller is further used to: establish a relationship curve between attenuation and counting signal; find the linear region of the relationship curve; adjust the current attenuation of the variable optical attenuator so that the current attenuation is in the linear region, and then re-receive the counting signal output by the photon counter, and restore the backlight incident on the variable optical attenuator according to the received counting signal and the current attenuation.

5. The OTDR system according to claim 4, Features: The establishing of the relationship curve between the attenuation and the counting signal specifically includes: controlling the attenuation of the variable optical attenuator to gradually change from a first preset value to a second preset value according to a set step length, and recording the counting signal output by the photon counter received at each attenuation; and fitting the relationship curve between the attenuation and the counting signal.

6. The OTDR system according to claim 1, Features: The variable optical attenuator is an active variable optical attenuator, and is also used to detect the optical power of the received backlight. When it is detected that the optical power of the backlight exceeds a set threshold, the backlight is attenuated and an attenuated optical signal is output, so that the optical power of the attenuated optical signal does not exceed the set threshold; and the attenuation amount is fed back to the controller.

7. The OTDR system according to claim 6, Features: The variable optical attenuator includes a beam splitter, a detection unit, an attenuation unit, and a control unit; The beam splitter is used to receive the backlight outputted from the output end of the circulator and split the received backlight into two beams, wherein one optical signal is transmitted to the attenuation unit and the other optical signal is transmitted to the detection unit; The detection unit is used to detect the optical power of the received optical signal and transmit the detected optical power to the control unit; The control unit is used to determine whether the received optical power exceeds a set threshold; If the set threshold is exceeded, an attenuation amount is generated, and the generated attenuation amount is sent to the attenuation unit and the controller; The attenuation unit is used to receive the attenuation amount sent by the control unit and the optical signal sent by the beam emitter, attenuate the optical signal according to the attenuation amount, output the attenuated optical signal, and send it to the single-photon detector.

8. The OTDR system according to claim 7, Features: The splitting ratio of the beam splitter is any value between 9:1 and 99:

1.

9. The OTDR system according to any one of claims 1 to 8, Features: The signal generator comprises: A pulse generator for generating a pulse electrical signal; The laser is used for receiving the pulse electric signal sent by the pulse generator and generating a pulse optical signal.

10. The OTDR system according to any one of claims 1 to 8, Features: The OTDR system also includes a clock; the clock generates a clock signal and sends it to the photon counter, the single photon detector and the signal generator.

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