Optical time domain reflectometer with large dynamic range and method for testing optical fibers

Abstract

The application discloses a large dynamic range optical time domain reflectometer and a fiber testing method, which comprises a continuous light source; an optical amplifier for gain amplification of the continuous laser; an optical attenuator for attenuation of the optical power of the continuous laser; a time domain filter for modulating the continuous laser into a periodic pulsed light signal; a circulator for outputting the periodic pulsed light signal to a measured optical fiber and receiving a backscattering signal returned from the measured optical fiber; an optical signal detection unit for converting the backscattering signal into an electric signal; a signal processing unit for extracting the relationship between the backscattering power and the position of the optical fiber from the electric signal; and a control unit for dynamically adjusting the gain of the optical amplifier, setting the attenuation value of the optical attenuator and adjusting the modulation pulse width of the time domain filter. The application can realize large dynamic range backscattering signal detection and is compatible with conventional quartz optical fibers and hollow core optical fibers.

Description

Technical Field

[0001] This invention belongs to the field of hollow fiber testing, specifically relating to a large dynamic range optical time domain reflectometer and fiber testing method. Background Technology

[0002] Optical Time Domain Reflectometer (OTDR) is a conventional fiber optic testing technology that can characterize the attenuation characteristics of optical fibers in a distributed manner and reflect some event points on the optical fiber link. It has important applications in the production, construction, operation and maintenance of optical fibers and cables.

[0003] Anti-resonant hollow fiber consists of a specific arrangement of capillary structures that generate an anti-resonance effect and suppress light leakage, confining light within an air-medium core for transmission. Because the guiding medium is air, hollow fiber exhibits significantly better performance in terms of attenuation, dispersion, time delay, and nonlinearity than traditional silica fiber, and has seen rapid development in recent years, with an attenuation coefficient reaching 0.05 dB / km.

[0004] However, testing hollow-core fiber presents significant challenges for OTDRs. The backscattering coefficient of air (down to -110 dB / m) is 30-40 dB lower than that of quartz (approximately -72 dB / m), while the dynamic range of conventional commercial OTDRs is only about 40-50 dB (corresponding to a pulse width of 20 µs). Therefore, conventional commercial OTDRs struggle to test the attenuation of hollow-core fiber. Specifically, the only technical means for testing hollow-core fiber with conventional commercial OTDRs is to use a larger pulse width; however, this sacrifices test resolution and dead zones. Even with a 1 µs pulse width, the dynamic range is still insufficient for testing long hollow-core fiber links, and due to the large dead zone and low resolution, it is impossible to accurately characterize the event conditions of the fiber link.

[0005] Patent application CN118801977A proposes using an optical amplifier to amplify optical pulse signals and employing a dispersion compensation component to compensate for the dispersion of the amplified optical signal, resulting in a chirp-free optical signal. This scheme uses a dispersion compensation component to compress the pulse and increase its power. However, the dispersion compensation scheme mentioned in this application typically targets fs or ps-level pulse width compression, while OTDRs operate at ns or µs-level pulse widths, making the pulse width compression effect negligible. Furthermore, dispersion compensation components usually cause significant signal loss, thus limiting the output optical pulse power. In addition, this scheme does not consider the noise problem of the optical amplifier during pulsed light amplification; although it can increase the output optical pulse power, the dynamic range is not significantly improved. Moreover, due to the nonlinear effects of optical fibers, this scheme is difficult to integrate with conventional optical fibers and hollow-core optical fibers. Summary of the Invention

[0006] In view of the above-mentioned defects or improvement needs of the existing technology, the present invention proposes a large dynamic range optical time domain reflectometer and optical fiber testing method.

[0007] To achieve the above objectives, according to one aspect of the present invention, a large dynamic range optical time-domain reflectometer is provided, comprising:

[0008] A continuous light source, used to output continuous laser light;

[0009] Optical amplifier, used to amplify the gain of continuous laser light;

[0010] An optical attenuator is used to attenuate the optical power of a continuous laser beam after gain adjustment.

[0011] A time-domain filter is used to modulate continuous laser light into periodic pulsed light signals;

[0012] A circulator is used to output periodic pulsed optical signals to the optical fiber under test and to receive backscattered signals returned from the optical fiber under test.

[0013] An optical signal detection unit is used to convert backscattered signals into electrical signals;

[0014] The signal processing unit is used to extract the relationship between backscattered power and fiber position from the electrical signal;

[0015] The control unit is used to dynamically adjust the gain of the optical amplifier, set the attenuation value of the optical attenuator, and adjust the modulation pulse width of the time-domain filter based on the relationship between backscatter power and fiber position, as well as the power range of the linear operating range of the optical signal detection unit.

[0016] According to the above scheme, the optical amplifier is a stage 1 amplifier; or

[0017] The optical amplifier consists of at least two cascaded amplifier stages, with a broadband filter between each stage.

[0018] According to the above scheme, the continuous light source, optical amplifier, optical attenuator, and time-domain filter are connected sequentially; or

[0019] The continuous light source, optical amplifier, time-domain filter, and optical attenuator are connected in sequence.

[0020] According to the above scheme, the output power of the continuous light source is -30dBm to 13dBm; the gain adjustment range of the optical amplifier is 10dB to 70dB, and the maximum output power of the optical amplifier is 43dBm; the sensitivity range of the optical signal detection unit is -90dBm to -100dBm.

[0021] According to another aspect of the present invention, a fiber optic testing method using the aforementioned large dynamic range optical time domain reflectometer is provided, comprising the following steps:

[0022] Based on the known and estimated parameters of the optical fiber under test, the gain of the optical amplifier, the attenuation value of the attenuator, and the modulation pulse width of the time-domain filter are initially set.

[0023] Real-time acquisition of the relationship between backscattered power and fiber position;

[0024] Based on the relationship between backscattered power and fiber position, and the power range of the linear operating range of the optical signal detection unit, the gain of the optical amplifier is dynamically adjusted, the attenuation value of the optical attenuator is set, and the modulation pulse width of the time-domain filter is adjusted.

[0025] According to the above method, the known parameters of the fiber under test include the type of fiber under test and the backscattering coefficient, and the estimated parameters include the length range of the fiber under test, the number of fiber fusion splices and the number of connection points.

[0026] The principles for initially setting the gain of the optical amplifier, the attenuation value of the attenuator, and the modulation pulse width of the time-domain filter using the above method are as follows:

[0027] Estimate the total loss of the fiber under test based on the known and estimated parameters.

[0028] Based on the initially set gain of the optical amplifier, the attenuation value of the attenuator, and the modulation pulse width of the time-domain filter, the known system loss and parameters of the continuous light source, as well as the backscattering coefficient and estimated total loss of the fiber under test, the estimated backscattering signal power at the start and end of the fiber under test is calculated, so that the estimated backscattering signal power at the start and end of the fiber under test is within the power range that can be linearly detected by the optical signal detection unit.

[0029] According to the above method, during the initial setup, first determine the magnitude of the nonlinear effect of the fiber under test based on the type of fiber under test, and set the combined output power of the optical amplifier and attenuator to a level that does not produce nonlinear effects. At the same time, based on the backscattering coefficient of the fiber under test and the estimated total loss, set the modulation pulse width of the time domain filter to 1.5-3 times the minimum pulse width that meets the principle, so as to achieve a higher test resolution and signal-to-noise ratio.

[0030] According to the above method, during dynamic adjustment, first gradually decrease the attenuation value of the optical attenuator, and then gradually increase the gain of the optical amplifier, thereby increasing the combined output power of the optical attenuator and the optical amplifier, until the backscattered power of different fiber positions acquired in real time is within the linear working range of the optical signal detection unit, and when the combined output power of the optical attenuator and the optical amplifier continues to increase, the detectable fiber end position does not change, and the adjustment is stopped.

[0031] If adjusting the combined output power of the optical attenuator and optical amplifier to produce a nonlinear effect in the fiber under test, or adjusting it to the maximum output power, still fails to ensure that the backscattered power at different fiber positions acquired in real time is within the linear operating range of the optical signal detection unit, then the modulation pulse width of the time-domain filter should be increased.

[0032] Following the above method, when the fiber under test is a single-mode quartz fiber, the initial settings are as follows:

[0033] The combined output power of the optical attenuator and optical amplifier is set to a level that does not produce fiber nonlinear effects. Based on the measured backscattering power level, the modulation pulse width of the time-domain filter is set to 1.5-3 times the minimum pulse width that meets the principle.

[0034] To set the combined output power of the optical attenuator and optical amplifier to a level that does not produce fiber nonlinear effects, first set the optical attenuation value to 0-10dB, and then adjust the gain of the optical amplifier.

[0035] Using the above method, when the fiber under test is a hollow fiber, in the initial settings, first set the modulation pulse width of the time domain filter to a smaller pulse width, and set the combined output power of the optical attenuator and optical amplifier to a smaller value that conforms to the principle. The smaller value is 2~5dB more than the minimum value.

[0036] Using the method described above, a smaller pulse width is less than or equal to 100 ns.

[0037] When setting the combined output power of the optical attenuator and optical amplifier to a smaller value that meets the principle, as described above, first set the optical attenuation value to 0~10dB, and then adjust the gain of the optical amplifier.

[0038] In summary, compared with the prior art, the above-described technical solutions conceived by this invention can achieve the following beneficial effects:

[0039] By employing a two-stage optical power adjustment method involving an optical amplifier and an optical attenuator, the gain of the optical amplifier, the attenuation value of the optical attenuator, and the modulation pulse width of the time-domain filter are dynamically adjusted based on the real-time acquired backscattered signal. This ensures that the predicted backscattered signal power at both the start and end of the fiber under test lies within the linearly detectable power range of the optical signal detection unit, thereby accurately detecting the OTDR test curve of the fiber under test. Using this invention, the upper limit of the detectable backscattered signal power of the fiber under test is determined by the saturation power of the optical signal detection unit, while the lower limit is determined by the sensitivity limit of the optical signal detection unit. This achieves backscattered signal detection with a large dynamic range, compatible with both conventional silica fiber and hollow-core fiber. Attached Figure Description

[0040] Figure 1This is a schematic diagram of a device structure provided in an embodiment of the present invention.

[0041] Figure 2 This is a schematic diagram of the multi-stage optical amplifier structure provided in an embodiment of the present invention.

[0042] Figure 3 This is a schematic diagram of the connection between a device and the fiber optic link under test provided in an embodiment of the present invention.

[0043] In the diagram: 1-circulator; 101-first port; 102-second port; 103-third port; 2-fiber under test; 3-optical signal detection unit; 4-coupled point. Detailed Implementation

[0044] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0045] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0046] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this invention, "a plurality of" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0047] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components, unless otherwise explicitly limited. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0048] In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can mean that the first feature is in direct contact with the second feature, or that the first feature is in indirect contact with the second feature through an intermediate medium. Furthermore, "above," "over," and "on top" of the second feature can mean that the first feature is directly above or diagonally above the second feature, or simply that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature can mean that the first feature is directly below or diagonally below the second feature, or simply that the first feature is at a lower horizontal level than the second feature.

[0049] This invention aims to address two main issues: First, existing commercial OTDRs are difficult to use for testing and characterizing hollow-core optical fibers, especially long links composed of multiple hollow-core optical fibers. The blind zone of large pulse width tests is too large, the resolution is too low, and it is impossible to accurately characterize the link attenuation and events of the optical fiber. Second, it addresses the problems existing in current optical amplifier solutions, including pulse amplification, control and noise filtering, as well as the incompatibility with conventional silica optical fibers and hollow-core optical fibers.

[0050] To achieve the above objectives, according to one aspect of the present invention, this embodiment provides a large dynamic range optical time-domain reflectometer, such as... Figure 1 and Figure 3 As shown, the device includes a continuous light source, an optical amplifier, an optical attenuator, a time-domain filter, and a first port 101 of a circulator 1 arranged in sequence. The second port 102 of the circulator 1 is connected to the starting end of the optical fiber 2 under test, and a coupling point 4 is provided between the second port 102 of the circulator 1 and the starting end of the optical fiber 2 under test. The third port 103 of the circulator 1 is connected to an optical signal detection unit 3. The output end of the optical signal detection unit 3 is connected to a signal storage and processing unit, and the signal storage and processing unit is connected to a control unit. The control unit is connected to the adjustment ends of the optical amplifier, the optical attenuator, and the time-domain filter, respectively.

[0051] A continuous light source is used to output continuous laser light, and the output power of the continuous light source is P0 (dBm). In some embodiments, the output power range of the continuous light source is -30dBm to 13dBm.

[0052] An optical amplifier is used to amplify the gain of continuous laser light. The gain of the optical amplifier is G (dB), and the optical power at the output of the optical amplifier is: P1 = P0 + G (dBm) (1). In some embodiments, such as Figure 2 As shown, the optical amplifier is composed of two cascaded amplifier stages, with a broadband filter between the two stages. The number of stages in the optical amplifier can be adjusted according to the required gain G. In some embodiments, the optical amplifier may also consist of a single-stage amplifier. In some embodiments, the total gain adjustment range of the optical amplifier is 10dB to 70dB, and the total maximum output power of the optical amplifier is 43dBm.

[0053] An optical attenuator is used to attenuate the optical power of a continuous laser beam after gain adjustment. If the attenuation of the optical attenuator is set to A (dB), then the optical power at the output of the optical attenuator is: P2 = P0 + GA (dBm) (2).

[0054] A time-domain filter is used to modulate a continuous laser beam after optical power attenuation into a periodic pulsed optical signal with a period of T (ns) and a frequency of F (Hz). The insertion loss of the time-domain filter is IL1 (dB), so the peak power of the output pulsed optical signal is: P3 = P0 + GA - IL1 (dBm) (3).

[0055] The circulator is used to output periodic pulsed optical signals to the fiber under test and to receive backscattered signals returned from the fiber under test. The insertion loss from the first port 101 to the second port 102 of the circulator 1 is denoted as IL2 (dB), the insertion loss from the second port 102 to the third port 103 is denoted as IL3 (dB), and the loss at the coupling point 4 is denoted as C (dB). Then the optical power entering the starting end of the fiber under test is: Pin0 = P3 - IL2 - C = P0 + GA - IL1 - IL2 - C (dBm) (4).

[0056] Given the group refractive index n of the fiber under test and the speed of light c (m / s), a pulse with a period of T (ns) corresponds to the reception of a reverse signal from a fiber of length D: D = c * T / 2n (m)

[0057] The backscattering coefficient of the fiber under test is B (dB / m), therefore the backscattered signal power at the starting end of the fiber is:

[0058] Pb0= P3-IL2-2C-IL3+B+10log10(D)

[0059] = P0+GA-IL1-IL2-2C-IL3+B+10log10(D) (dBm) (5).

[0060] Assuming the total loss of the fiber optic link under test is S (dB), then the backscattered signal power at the end of the fiber optic link (reflected from the fiber end face, not the end) is:

[0061] PbL= P3-IL2-2C-IL3+B+10log10(D)-2S

[0062] = P0+GA-IL1-IL2-2C-IL3+B+10log10(D)-2S (dBm) (6)

[0063] The optical signal detection unit is used to convert backscattered signals into electrical signals.

[0064] The signal storage and processing unit is used to extract and store the relationship between backscattered power and fiber position from the electrical signal. In some embodiments, the signal storage and processing unit is also connected to a display unit for displaying the OTDR test curve.

[0065] The control unit is used to dynamically adjust the gain of the optical amplifier, set the attenuation value of the optical attenuator, and adjust the modulation pulse width of the time-domain filter based on the relationship between backscattering power and fiber position, as well as the power range of the linear operating range of the optical signal detection unit.

[0066] To accurately detect the backscattered signal of the optical fiber, Pb0 and PbL need to be within the power range that the detector can linearly detect. As can be seen from equations (5) and (6), this can be achieved by adjusting the gain G of the optical amplifier, the attenuation setting A of the optical attenuator, and the pulse width of the periodic signal. Since IL1, IL2, IL3, and the coupling loss are essentially fixed values ​​and relatively small, the backscattered power received by the optical signal detection unit mainly depends on the backscattering coefficient B of the fiber under test and the total loss S of the entire link of the fiber under test.

[0067] In this embodiment, the continuous light source, optical amplifier, optical attenuator, and time-domain filter are connected in sequence; in some embodiments, the continuous light source, optical amplifier, time-domain filter, and optical attenuator may also be connected in sequence. That is, the order of the optical attenuator and the time-domain filter can be interchanged.

[0068] According to another aspect of the present invention, this embodiment also provides a fiber optic testing method using the aforementioned large dynamic range optical time domain reflectometer, comprising the following steps:

[0069] S1. Based on the known and estimated parameters of the optical fiber under test, the gain of the optical amplifier, the attenuation value of the attenuator, and the modulation pulse width of the time-domain filter are initially set.

[0070] Known parameters of the fiber under test include, but are not limited to, the type of fiber and the backscattering coefficient. Estimated parameters include, but are not limited to, the length range of the fiber under test and the number of fiber fusion splices or connection points. In some embodiments, some of the above-mentioned estimated parameters may be known, or a range may be estimated based on actual conditions or experience.

[0071] The principles for initially setting the gain of the optical amplifier, the attenuation value of the attenuator, and the modulation pulse width of the time-domain filter are as follows:

[0072] Estimate the total loss of the fiber under test based on the known parameters of the fiber under test.

[0073] Based on the initially set gain of the optical amplifier, the attenuation value of the attenuator, and the modulation pulse width of the time-domain filter, the known system loss and parameters of the continuous light source, as well as the backscattering coefficient and estimated total loss of the fiber under test, the estimated backscattering signal power at the start and end of the fiber under test is calculated, so that the estimated backscattering signal power at the start and end of the fiber under test is within the power range that can be linearly detected by the optical signal detection unit.

[0074] In the initial setup, first determine the magnitude of the nonlinear effect of the fiber under test based on its type. Set the combined output power of the optical amplifier and attenuator to a level insufficient to produce significant nonlinear effects. Simultaneously, based on the backscattering coefficient and estimated total loss of the fiber under test, set the modulation pulse width of the time-domain filter to a reasonable and smaller pulse width to achieve higher test resolution and signal-to-noise ratio. This smaller pulse width is typically 1.5-3 times the minimum pulse width.

[0075] When the fiber under test is a single-mode quartz fiber, the initial setup involves setting the combined output power of the optical attenuator and optical amplifier to a level that does not produce nonlinear effects, prioritizing a smaller optical attenuation value (e.g., 0-10 dB), and then adjusting the gain of the optical amplifier. Based on the measured backscattered power level, the modulation pulse width of the time-domain filter is set to a smaller value that conforms to the principle, such as 1.5-3 times the minimum pulse width.

[0076] When the fiber under test is hollow fiber, in the initial settings, since its nonlinearity level is at least 3 orders of magnitude lower than that of quartz single-mode fiber, the modulation pulse width of the time domain filter can be set to a smaller pulse width, such as no more than 100 ns. The combined output power of the optical attenuator and optical amplifier can be set to a smaller value that meets the principle (such as 2~5 dB more than the minimum value). Among them, a smaller optical attenuation value (such as 0-10 dB) should be set first, and then the gain of the optical amplifier should be adjusted.

[0077] S2. Real-time acquisition of the relationship between backscattered power and fiber position.

[0078] S3. Based on the relationship between backscattering power and fiber position, and the power range of the linear operating range of the optical signal detection unit, dynamically adjust the gain of the optical amplifier, the attenuation value of the optical attenuator, and the modulation pulse width of the time-domain filter.

[0079] During dynamic adjustment, the attenuation value of the optical attenuator is gradually reduced first, and then the gain of the optical amplifier is gradually increased, thereby increasing the combined output power of the optical attenuator and the optical amplifier. This continues until the backscattered power at different fiber positions acquired in real time is within the linear operating range of the optical signal detection unit, and the detectable fiber end position remains unchanged when the combined output power of the optical attenuator and the optical amplifier is further increased. At this point, the adjustment is stopped.

[0080] If adjusting the combined output power of the optical attenuator and optical amplifier to produce a nonlinear effect in the fiber under test, or adjusting it to the maximum output power, still fails to ensure that the backscattered power at different fiber positions acquired in real time is within the linear operating range of the optical signal detection unit, then the modulation pulse width of the time-domain filter should be increased.

[0081] The invention is further illustrated by specific examples below.

[0082] Example 1:

[0083] S1. Based on the known parameters of the fiber under test and the OTDR, make preliminary settings for the gain of the optical amplifier, the attenuation value of the attenuator, and the modulation pulse width of the time-domain filter.

[0084] For the fiber under test, the test link is a 200km hollow-core fiber link composed of 50 reels of 4km fiber, with 49 fiber fusion splices in the link. Given that the group refractive index n of the hollow-core fiber is approximately 1, the backscattering coefficient B is approximately -100dB / m, the estimated average attenuation coefficient of the hollow-core fiber is approximately 0.08dB / km (1550nm), and the average loss of each fusion splice is approximately 0.04dB, then the estimated total loss S of this 200km fiber link is 17.96dB.

[0085] For the OTDR, the initial luminous power P0 of the light source is 0 dBm, the insertion loss IL1 of the time-domain filter is 2 dB, the frequency F of the periodic signal generated by the time-domain filter is 500 Hz, the insertion loss IL2 from port 1 to port 2 of the circulator is 0.5 dB, the insertion loss from port 2 to port 3 is 0.5 dB, the coupling loss C between the output port of the OTDR device and the fiber optic link under test is 0.8 dB (including a 0.3 dB loss from a single-mode connector and a 0.5 dB loss from a single-mode fiber to a hollow fiber connector), the detector sensitivity is -90 dBm, and the saturation power is -20 dBm.

[0086] The modulation pulse width of the time-domain filter is initially set to 50 ns. Then, according to formulas (5) and (6), substituting the known parameters of the fiber under test and the OTDR, the estimated backscattered power at the initial and final ends of the fiber under test is obtained as follows:

[0087] Pb0 = GA - 95.85 (dBm)

[0088] PbL = GA - 131.77 (dBm)

[0089] Pb0 and PbL should be within the linear range of the signal detection unit, that is, Pb0 should be less than -20dBm and PbL should be greater than -90dBm.

[0090] If the attenuation value of the optical attenuator is initially set to 1dB, then the gain G of the optical amplifier will range from 41.77dB to 76.85dB. Since the actual maximum output power of the optical amplifier is generally limited to no more than 43dBm, for a light source with an optical power of 0dBm, the actual gain of the optical amplifier is usually no more than 43dB. Therefore, the gain of the optical amplifier can be initially set to 43dB.

[0091] S2. Real-time acquisition of the relationship between backscattered power and fiber position.

[0092] S3. Based on the relationship between backscattering power and fiber position, and the power range of the linear operating range of the optical signal detection unit, dynamically adjust the gain of the optical amplifier, the attenuation value of the optical attenuator, and the modulation pulse width of the time-domain filter.

[0093] If the actual total loss of the optical fiber is greater, or the backscattering coefficient is lower, the backscattered power at the end of the fiber will be lower than -90dBm. At this point, the optical attenuator cannot be further reduced, and the optical amplifier is already at maximum power. Therefore, the pulse width of the time-domain filter is increased, for example, to 100ns, and the backscattered power at different locations on the fiber is collected again. When the backscattered power at different locations on the fiber is within the linear range of the detector, the pulse width is stopped, and the final OTDR test curve is output.

[0094] Example 2:

[0095] S1. Based on the known parameters of the fiber under test and the OTDR, make preliminary settings for the gain of the optical amplifier, the attenuation value of the attenuator, and the modulation pulse width of the time-domain filter.

[0096] For the fiber under test, the test link is a 2km G.652 single-mode fiber link on a single reel, with 0 fiber fusion splices. Given that the group refractive index n of the G.652 single-mode fiber is approximately 1.467, the backscattering coefficient is approximately -72dB / m, and the estimated average attenuation coefficient is approximately 0.2dB / km (1550nm), the estimated total loss of this 2km fiber link is 0.4dB.

[0097] For an OTDR, the initial luminous power P0 of the light source is 0 dBm, the insertion loss IL1 of the time-domain filter is 2 dB, and the frequency F of the periodic signal generated by the time-domain filter is 500 Hz. The insertion loss IL2 from port 1 to port 2 of the circulator is 0.5 dB, the insertion loss from port 2 to port 3 is 0.5 dB, the coupling loss C between the OTDR output port and the fiber optic link under test is 0.3 dB, the detector sensitivity is -90 dBm, and the saturation power is -20 dBm.

[0098] The modulation pulse width of the time-domain filter is initially set to 50 ns. Then, according to formulas (5) and (6), substituting the known parameters of the fiber under test and the OTDR, the estimated backscattered power at the initial and final ends of the fiber under test is obtained as follows:

[0099] Pb0 = GA - 58.6 (dBm)

[0100] PbL = GA - 59.4 (dBm)

[0101] Pb0 and PbL should be within the linear range of the signal detection unit, that is, Pb0 should be less than -20dBm and PbL should be greater than -90dBm.

[0102] If the initial attenuation value of the optical attenuator is set to 1dB, then the allowable range of the optical amplifier gain G is 0dB to 39.6dB. Therefore, the optical amplifier gain can be set to the minimum value of 10dB. At this point, the optical power injected into the fiber is 6.2dBm, and no nonlinear effect will occur.

[0103] S2. Real-time acquisition of the relationship between backscattered power and fiber position.

[0104] S3. Based on the relationship between backscattering power and fiber position, and the power range of the linear operating range of the optical signal detection unit, dynamically adjust the gain of the optical amplifier, the attenuation value of the optical attenuator, and the modulation pulse width of the time-domain filter.

[0105] In this embodiment, the backscattered power at different locations on the optical fiber that are actually collected is all within the linear range of the detector, and the final OTDR test curve can be directly output.

[0106] Example 3:

[0107] S1. Based on the known parameters of the fiber under test and the OTDR, make preliminary settings for the gain of the optical amplifier, the attenuation value of the attenuator, and the modulation pulse width of the time-domain filter.

[0108] For the fiber under test, the test link is a 200km G.654 single-mode fiber link composed of 100 reels of 2km fiber, with 99 fiber fusion splices in the link. Given that the group refractive index n of the G.654 single-mode fiber is approximately 1.467 and the backscattering coefficient is approximately -72dB / m, the estimated average attenuation coefficient of the tested G.654 single-mode fiber is 0.165dB / km (1550nm), and the average loss of each fusion splice is 0.02dB, then the estimated total loss of this 200km fiber link is 34.98dB.

[0109] For an OTDR, given that the initial luminous power P0 of the light source is 0 dBm, the insertion loss IL1 of the time-domain filter is 2 dB, and the frequency F of the periodic signal generated by the time-domain filter is 500 Hz; the insertion loss IL2 from port 1 to port 2 of the circulator is 0.5 dB, the insertion loss from port 2 to port 3 is 0.5 dB, the coupling loss C between the OTDR output port and the fiber optic link under test is 0.3 dB, the detector sensitivity is -90 dBm, and the saturation power is -20 dBm, then Pb0 should be less than -20 dBm.

[0110] The modulation pulse width of the time-domain filter is initially set to 5000 ns. Then, according to formulas (5) and (6), substituting the known parameters of the fiber under test and the OTDR, the estimated backscattered power at the initial and final ends of the fiber under test is obtained as follows:

[0111] Pb0 = GA - 38.6 (dBm)

[0112] PbL = GA - 108.56 (dBm)

[0113] Pb0 and PbL should be within the linear range of the signal detection unit, that is, Pb0 should be less than -20dBm and PbL should be greater than -90dBm.

[0114] If the initial attenuation value of the optical attenuator is set to 1dB, then the allowable range of the optical amplifier gain G is 19.56dB to 19.6dB. Therefore, the gain of the optical amplifier can be set to the minimum value of 19.6dB. At this time, the optical power entering the single-mode fiber injection end is about 16.6dBm, which is a power level that is not prone to nonlinearity for most G.654 single-mode fibers.

[0115] S2. Real-time acquisition of the relationship between backscattered power and fiber position.

[0116] S3. Based on the relationship between backscattering power and fiber position, and the power range of the linear operating range of the optical signal detection unit, dynamically adjust the gain of the optical amplifier, the attenuation value of the optical attenuator, and the modulation pulse width of the time-domain filter.

[0117] If the actual total loss of the optical fiber is greater, the backscattered power at the fiber end will be lower than -90dBm. At this point, the optical attenuator cannot be further reduced, and increasing the optical amplifier gain may introduce nonlinear effects. Therefore, the pulse width of the time-domain filter should be increased, for example, to 10000ns, and then the backscattered power at different locations on the fiber should continue to be collected. When the backscattered power at different locations on the fiber is within the linear range of the detector, stop increasing the pulse width and output the final OTDR test curve.

[0118] This invention significantly expands the dynamic range of an OTDR through a two-stage optical power adjustment method using an optical amplifier and an optical attenuator. Pulse output and pulse width adjustment are achieved by controlling a time-domain filter. The software automatically adjusts the power of the emitted optical pulse based on the detected backscattered light power level in the fiber under test. When testing shorter conventional single-mode silica fibers, the gain of the optical amplifier can be reduced while the loss of the optical attenuator is increased to avoid nonlinear effects. When testing longer conventional single-mode silica fibers, the gain of the optical amplifier is appropriately increased, and the pulse width is also increased to improve the dynamic range. When testing fibers with low backscattering coefficients, such as hollow-core fibers, although the backscattering coefficient is 30dB to 40dB lower than that of conventional single-mode silica fibers, the very low nonlinear effects of hollow-core fibers allow this scheme to achieve longer-distance testing with shorter optical pulses by increasing the gain of the optical amplifier and reducing the loss of the optical attenuator. Therefore, the test curve resolution of this OTDR is higher than that of conventional OTDRs, and the dead zone is smaller.

[0119] It should be understood that the sequence number of each step in the above embodiments does not imply the order of execution. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of this application.

[0120] It should be noted that, depending on the implementation needs, the various steps / components described in this application can be broken down into more steps / components, or two or more steps / components or parts of the operation of steps / components can be combined into new steps / components to achieve the purpose of this invention.

[0121] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A large dynamic range optical time-domain reflectometer, characterized in that: include: A continuous light source, used to output continuous laser light; Optical amplifier, used to amplify the gain of continuous laser light; An optical attenuator is used to attenuate the optical power of a continuous laser beam after gain adjustment. A time-domain filter is used to modulate continuous laser light into periodic pulsed light signals; A circulator is used to output periodic pulsed optical signals to the optical fiber under test and to receive backscattered signals returned from the optical fiber under test. An optical signal detection unit is used to convert backscattered signals into electrical signals; The signal processing unit is used to extract the relationship between backscattered power and fiber position from the electrical signal; The control unit is used to dynamically adjust the gain of the optical amplifier, set the attenuation value of the optical attenuator, and adjust the modulation pulse width of the time domain filter based on the relationship between backscatter power and fiber position, as well as the power range of the linear operating range of the optical signal detection unit. Based on the known and estimated parameters of the optical fiber under test, the gain of the optical amplifier, the attenuation value of the attenuator, and the modulation pulse width of the time-domain filter are initially set. The known parameters of the fiber under test include the type of fiber and the backscattering coefficient, while the estimated parameters include the length range of the fiber, the number of fiber fusion splices, and the number of connection points. The principles for initially setting the gain of the optical amplifier, the attenuation value of the attenuator, and the modulation pulse width of the time-domain filter are as follows: Estimate the total loss of the fiber under test based on the known and estimated parameters. Based on the initially set gain of the optical amplifier, the attenuation value of the attenuator, and the modulation pulse width of the time-domain filter, the known system loss and parameters of the continuous light source, as well as the backscattering coefficient and estimated total loss of the fiber under test, the estimated backscattering signal power at the start and end of the fiber under test is calculated, so that the estimated backscattering signal power at the start and end of the fiber under test is within the power range that can be linearly detected by the optical signal detection unit. In the initial setup, first determine the magnitude of the nonlinear effect of the fiber under test based on its type, and set the combined output power of the optical amplifier and attenuator to a level that does not produce nonlinear effects. At the same time, based on the backscattering coefficient and estimated total loss of the fiber under test, set the modulation pulse width of the time-domain filter to 1.5-3 times the minimum pulse width that meets the principle, so as to achieve higher test resolution and signal-to-noise ratio.

2. The large dynamic range optical time-domain reflectometer according to claim 1, characterized in that: The optical amplifier is a stage 1 amplifier; or The optical amplifier consists of at least two cascaded amplifier stages, with a broadband filter between each stage.

3. The large dynamic range optical time-domain reflectometer according to claim 1, characterized in that: A continuous light source, an optical amplifier, an optical attenuator, and a time-domain filter are connected in sequence; or The continuous light source, optical amplifier, time-domain filter, and optical attenuator are connected in sequence.

4. The large dynamic range optical time-domain reflectometer according to claim 1, characterized in that: The output power range of the continuous light source is -30dBm to 13dBm; the gain adjustment range of the optical amplifier is 10dB to 70dB, and the maximum output power of the optical amplifier is 43dBm; the sensitivity range of the optical signal detection unit is -90dBm to -100dBm.

5. A fiber optic testing method using a large dynamic range optical time-domain reflectometer as described in any one of claims 1 to 4, characterized in that: Includes the following steps: Based on the known and estimated parameters of the optical fiber under test, the gain of the optical amplifier, the attenuation value of the attenuator, and the modulation pulse width of the time-domain filter are initially set. Real-time acquisition of the relationship between backscattered power and fiber position; Based on the relationship between backscattered power and fiber position, and the power range of the linear operating range of the optical signal detection unit, the gain of the optical amplifier is dynamically adjusted, the attenuation value of the optical attenuator is set, and the modulation pulse width of the time-domain filter is adjusted. The known parameters of the fiber under test include the type of fiber and the backscattering coefficient, while the estimated parameters include the length range of the fiber, the number of fiber fusion splices, and the number of connection points. The principles for initially setting the gain of the optical amplifier, the attenuation value of the attenuator, and the modulation pulse width of the time-domain filter are as follows: Estimate the total loss of the fiber under test based on the known and estimated parameters. Based on the initially set gain of the optical amplifier, the attenuation value of the attenuator, and the modulation pulse width of the time-domain filter, the known system loss and parameters of the continuous light source, as well as the backscattering coefficient and estimated total loss of the fiber under test, the estimated backscattering signal power at the start and end of the fiber under test is calculated, so that the estimated backscattering signal power at the start and end of the fiber under test is within the power range that can be linearly detected by the optical signal detection unit. In the initial setup, first determine the magnitude of the nonlinear effect of the fiber under test based on its type, and set the combined output power of the optical amplifier and attenuator to a level that does not produce nonlinear effects. At the same time, based on the backscattering coefficient and estimated total loss of the fiber under test, set the modulation pulse width of the time-domain filter to 1.5-3 times the minimum pulse width that meets the principle, so as to achieve higher test resolution and signal-to-noise ratio.

6. The optical fiber testing method according to claim 5, characterized in that: During dynamic adjustment, the attenuation value of the optical attenuator is gradually reduced first, and then the gain of the optical amplifier is gradually increased, thereby increasing the combined output power of the optical attenuator and the optical amplifier until the backscattered power of different fiber positions acquired in real time is within the linear working range of the optical signal detection unit, and the detectable fiber end position does not change when the combined output power of the optical attenuator and the optical amplifier is further increased, at which point the adjustment is stopped. If adjusting the combined output power of the optical attenuator and optical amplifier to produce a nonlinear effect in the fiber under test, or adjusting it to the maximum output power, still fails to ensure that the backscattered power at different fiber positions acquired in real time is within the linear operating range of the optical signal detection unit, then the modulation pulse width of the time-domain filter should be increased.

7. The optical fiber testing method according to claim 5, characterized in that: When the fiber under test is a single-mode quartz fiber, the initial settings are as follows: The combined output power of the optical attenuator and optical amplifier is set to a level that does not produce fiber nonlinear effects. Based on the backscattering coefficient of the fiber under test and the estimated total loss, the modulation pulse width of the time-domain filter is set to 1.5-3 times the minimum pulse width that meets the principle.

8. The optical fiber testing method according to claim 7, characterized in that: When setting the combined output power of the optical attenuator and optical amplifier to a level that does not produce fiber nonlinear effects, first set the optical attenuation value to 0-10dB, and then adjust the gain of the optical amplifier.

9. The optical fiber testing method according to claim 5, characterized in that: When the fiber under test is hollow fiber, in the initial settings, first set the modulation pulse width of the time domain filter to a smaller pulse width, and set the combined output power of the optical attenuator and optical amplifier to a smaller value that meets the principle. The smaller value is 2~5dB more than the minimum value.

10. The optical fiber testing method according to claim 9, characterized in that: Smaller pulse widths are less than or equal to 100 ns.

11. The optical fiber testing method according to claim 9, characterized in that: When setting the combined output power of the optical attenuator and optical amplifier to a smaller value that meets the principle, first set the optical attenuation value to 0~10dB, and then adjust the gain of the optical amplifier.

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