Optical time-domain reflection measurement of hollow-core optical fibers

Abstract

A method for performing optical time domain reflectance measurement (OTDR) on a test optical fiber (FUT) is described, the optical fiber comprising a portion of hollow core optical fiber (HCF) and a portion of solid core optical fiber (SCF). The method comprises the steps of determining the position of a transition between the SCF and the HCF within the FUT, setting a time delay using the determined position of the transition in a switch control circuit, and generating an OTDR trace graph using light detected by a photodetector, wherein the time delay is used to control an optical switch to block backscattered light within the SCF from reaching the photodetector (e.g., by absorbing, attenuating, or bypassing the backscattered light).

Description

Background Technology

[0001] Optical Time Domain Reflectometer (OTDR) is a method used for fiber optic network testing, troubleshooting, and the manufacturing of fibers and cables. OTDRs are primarily used to characterize fibers by measuring the amount of light reflected from different points within the fiber. Optical Time Domain Reflectometer is conveniently implemented using a device known as an Optical Time Domain Reflectometer (also called an OTDR). OTDRs are widely used by telecommunications companies, network operators, and technicians involved in the installation, maintenance, and repair of fiber optic networks. They play a crucial role in ensuring the performance and reliability of these networks by providing detailed information on fiber characteristics and detecting potential issues.

[0002] The basic principle of an OTDR involves sending short pulses of light into an optical fiber and analyzing the reflected signals. By measuring the time it takes for the light to travel and the intensity of the reflected signals, an OTDR can determine the distance to various events or anomalies within the optical fiber. The OTDR generates a graphical representation called an OTDR trace or trace graph, which displays the length of the optical fiber and provides information on optical loss, connection points, connectors, and any fiber breakage or bending. The trace graph assists technicians in identifying and locating faults or problems within an optical fiber network.

[0003] The embodiments described below are not limited to embodiments that solve some or all of the disadvantages of known methods for fault location determination and OTDR.

[0004] Hereinafter, a brief overview of the present disclosure is presented to provide the reader with a basic understanding. This overview does not represent the principal or essential features of the claimed subject matter, nor is it intended to limit the scope of the claimed subject matter. Its sole purpose is to briefly present selected concepts disclosed herein as a prelude to the more detailed description that follows.

[0005] A method for performing optical time domain reflectance measurement (OTDR) on a test optical fiber (FUT) is described, the optical fiber comprising a portion of hollow core optical fiber (HCF) and a portion of solid core optical fiber (SCF). The method comprises the steps of determining the position of a transition between the SCF and the HCF in the FUT, setting a time delay using the determined position of the transition in a switch control circuit, and generating an OTDR trace graph using light detected by a photodetector, wherein the time delay is used to control an optical switch to block backscattered light within the SCF from reaching the photodetector (e.g., by absorbing, attenuating, or bypassing the backscattered light).

[0006] Many of the incidental features are more easily understood as they are better understood by referring to the detailed description below, which is considered in combination with the attached drawings. Brief explanation of the drawing

[0007] This description will be better understood by reading the detailed description that follows in reference to the attached drawings. FIGS. 1 to 4 illustrate cross-sectional views of respective examples of hollow core optical fibers. FIG. 5 illustrates an exemplary OTDR trace graph generated using the device of FIG. 6. Figure 6 is a schematic diagram of an OTDR device. FIG. 7 is a schematic diagram of a first example of an improved OTDR device. FIG. 8 is a schematic diagram of a second example of an improved OTDR device. FIG. 9 illustrates a first exemplary method of performing OTDR on a FUT comprising a portion of a hollow core optical fiber and a portion of a solid core optical fiber. FIG. 10 illustrates an exemplary OTDR trace graph generated using the method of FIG. 9. FIG. 11 illustrates a second exemplary method of performing OTDR on a FUT comprising a portion of a hollow core optical fiber and a portion of a solid core optical fiber. FIG. 12 illustrates a third exemplary method of performing OTDR on a FUT comprising a portion of a hollow core optical fiber and a portion of a solid core optical fiber. Similar reference numerals are used to indicate similar parts in the attached drawings. Specific details for implementing the invention

[0008] The detailed description provided below in combination with the attached drawings is intended to describe the example and is not intended to represent the only form in which the example is configured or utilized. This description describes the function of the example and the sequence of operations for configuring and operating the example. However, the same or equivalent functions and sequences may be achieved by other examples. As described, an OTDR may be used to characterize an optical fiber. The OTDR functions by firing an optical probe pulse into the optical fiber (also referred to as the Fiber Under Test (FUT)), and the width of the probe pulse determines the spatial resolution of the OTDR results. Pulse widths of several nanoseconds (e.g., 2 ns) to several microseconds (e.g., 3 μs) may be used. As the probe pulse propagates along the FUT, Rayleigh scattering generates a signal that propagates in the opposite direction to the incident probe pulse. Rayleigh backscattering is caused by the interaction between the incident light and refractive index variations within the FUT (e.g., the core of the FUT). The backscattering signal is subsequently directed to a photodetector (PD). By determining the speed of light within the FUT, the time delay at which a signal is detected can be related to the position within the FUT where the signal is generated. Subsequently, an OTDR trace can be constructed by plotting the power of the backscattered light (detected by the PD) against the position along the optical fiber.

[0009] Although OTDRs are widely used to identify fiber breakage, defects, or other anomalies within optical networks, conventional OTDR technology is not suitable for analyzing network links containing hollow core fibers (HCFs). In standard optical fibers (e.g., single-mode fibers (SMFs) or multimode fibers (MMFs)), there is a solid core (typically made of glass) through which light travels, and the guidance of the light depends on a cladding material surrounding the core that has a lower refractive index than the core material. In HCFs, light does not travel within a solid core but instead travels within a hollow core that can be filled with air, and a different guiding mechanism is used. Two types of HCFs using different guiding mechanisms are hollow core photonic bandgap fibers (PBGFs) and antiresonant hollow core fibers (ARFs), which will be described in more detail below with reference to FIGS. 1 through 4.

[0010] FIG. 1 illustrates a cross-sectional view of an exemplary hollow core PBGF (100). The optical fiber (100) comprises a central tube (102) having a hollow core (104) and a cladding comprising a plurality of uniformly arranged smaller tubular hollow structures (106, 108). The longitudinal axes of the central tube (102) and the smaller tubular structures (106, 108) are substantially parallel.

[0011] FIGS. 2 through 4 illustrate cross-sectional views of three different examples of antiresonant hollow core fibers (ARFs). Light is guided within these optical fibers by an antiresonant optical effect. Each of the optical fibers (200, 300, 400) comprises a tubular outer cladding (or sheath) (202), a structured inner cladding comprising a plurality of tubular cladding capillaries (204, 304, 404), and a hollow core (208). The outer cladding (202) generally has a glass thickness much greater than that of the cladding capillaries (204, 304, 404). In the first example illustrated in FIG. 2, the structured inner cladding comprises five capillaries (204) of the same cross-sectional size and shape, arranged in a single ring within the outer cladding (202) such that the longitudinal axis of each cladding capillary (204) and the longitudinal axis of the outer cladding (202) are substantially parallel. Each cladding capillary (204) contacts (e.g., bonded) the inner surface of the outer cladding (202) at an azimuth position (108), thereby causing the cladding capillaries (204) to be uniformly spaced around the inner circumference of the outer cladding (202) and also spaced from one another by a gap (210) (i.e., so that neighboring capillaries do not come into contact). In some ARF designs, the cladding capillaries (204) may be positioned to come into contact with each other (i.e., not spaced apart as in FIG. 2), but the optical performance of the optical fiber can be improved by spacing them apart to eliminate such contact. The gap (210) eliminates nodes that tend to cause undesirable resonance resulting in high loss at the contact point between adjacent tubes. Thus, an optical fiber having spaced-apart cladding capillaries can be referred to as a "nodeless anti-resonant hollow core optical fiber."

[0012] A cladding capillary (204) arranged annularly along the inner circumference of a tubular outer cladding (202) creates a central space, cavity, or void within the optical fiber, the longitudinal axis of which is also parallel to the longitudinal axis of the outer cladding (202) and the cladding capillary (204), which is the hollow core (206) of the optical fiber. The hollow core (206) is defined by an inwardly facing portion of the outer surface of the cladding capillary (204). This is the core boundary, and the material of the capillary wall constituting this boundary (e.g., glass or polymer) provides the required anti-resonant optical induction effect or mechanism. The cladding capillary (204) has a thickness (t) at the core boundary, which defines the wavelength at which anti-resonant optical induction occurs in the ARF.

[0013] In the second example illustrated in FIG. 3, each primary cladding capillary (204) has a smaller secondary capillary (304) superimposed therein, which is bonded to the inner surface of the primary cladding capillary (204) at the same azimuth position (208) as the bonding point between the primary cladding capillary (204) and the outer cladding (202) in this example. These additional smaller capillaries (304) can reduce optical loss. This type of ARF design having secondary capillaries may be referred to as a "nested antiresonant nodeless fiber (NANF)."

[0014] The third example illustrated in FIG. 4 has two smaller cladding capillaries (304, 404) superimposed inside each cladding capillary (204). As in the example illustrated in FIG. 3, each of the smaller capillaries (304, 404) is bonded to the inner surface of the immediately adjacent larger capillary at the same azimuth position as the bonding point between the primary cladding capillary (204) and the outer cladding (202). In this example, the smaller capillary (404) may be referred to as the secondary cladding capillary, and the smallest cladding capillary (304) may be referred to as the tertiary cladding capillary. The tertiary cladding capillary (304) is bonded to the inner surface of the secondary cladding capillary (404), and the secondary cladding capillary (404) is bonded to the inner surface of the primary cladding capillary (204). This type of ARF design having a secondary cladding capillary and a tertiary cladding capillary may be referred to as a "double-nested antiresonant nodeless fiber (DNANF)." In another additional example (not shown in the drawing), there may be different configurations of the cladding capillaries. For example, there may be a smaller additional capillary within the tertiary capillary (304) to provide an additional level of overlap, and / or there may be multiple secondary cladding capillaries within each primary cladding capillary, each secondary cladding capillary being bonded to the inner surface of the primary cladding capillary at a different azimuth position, and / or each primary cladding capillary may have an internal structure (e.g., one or more partition walls).

[0015] All examples illustrated in FIGS. 2 through 4 include five primary cladding capillaries (204) and thus have five layers of rotational symmetry. Additionally, all capillaries have a circular cross-section. In other examples, there may be a different number of capillaries surrounding the core (e.g., 4, 6, 7, 8, 9, or 10) and / or the capillaries may not have a circular cross-section. Furthermore, in the examples of FIGS. 2 through 4, all primary cladding capillaries (204) have the same size and shape, whereas in other examples, the primary capillaries within the outer cladding (202) may not all have the same size and / or shape.

[0016] When HCF is used in optical networks, it is typically used in combination with single-mode optical fiber (SMF), so that the length of the SMF is connected to the length of the HCF. It is precisely because of this combination of the length of the SMF (or other forms of SCF) and the length of the HCF that the conventional use of OTDRs to detect faults or defects within the HCF is limited. HCF has a much lower backscattering level than SMF. SMF exhibits backscattering of about -70 to -60 dB / m for incident light on the core due to Rayleigh scattering from density fluctuations within the glass. In contrast, the level of backscattering, particularly in HCF, DNANF, and NANF, is about -40 dB lower than that of SMF. (D) The dominant mechanism of backscattering in NANF is Rayleigh backscattering from gas molecules inside the hollow core. This is about -100 dB / m at atmospheric pressure.

[0017] Even if a transition region is introduced between the SCF and the HCF (e.g., by extending the mode of the SMF so that its output field better matches the output field of the HCF and cutting the SCF and HCF at an angle), to reduce losses (and reflections) in the transition region, the difference in backscattering levels between the SMF and the HCF is significantly increased, as shown in Fig. 5, thereby substantially reducing the available dynamic range of the OTDR in the portion of the OTDR trace graph corresponding to the length of the HCF. Consequently, it is much more difficult to identify failures or defects in the HCF over relatively longer distances (e.g., over distances of several kilometers or more), particularly failures or defects that introduce a small amount of backscattering (which still need to be identified and resolved as they can impair the operation of the optical network).

[0018] FIG. 5 shows an exemplary OTDR trace graph (500) for a FUT (600) that can be generated using an OTDR device (610) as illustrated in FIG. 6. The FUT (600) includes a length of SMF (602) connected to a length of HCF (604) through an SMF-HCF coupler (606). The OTDR device (610) includes a light source (612), a circulator (614), and a PD (616). The OTDR device (610) may include other components not illustrated in FIG. 6. A light pulse emitted from the light source (612) is connected to a first port of the circulator (614) and output to the FUT (600) connected to a second port of the circulator (614). This pulse is reflected from the FUT (as described above), and the reflected light enters the circulator (614) at the second port and is output to the PD (616) connected to the third port of the circulator (614). A trace graph (500) is generated using the output signal from the PD (616).

[0019] The horizontal axis of the OTDR trace graph (500) indicates the distance along the FUT, and the vertical axis indicates the backscattering power (absolute or relative). The lengths of the SMF and HCF are shown on the trace graph (arrows (502, 504) respectively). The trace graph (500) shows peaks (506) caused by reflection at the SMF-HCF interface, as well as events (e.g., peaks or lows) (508) caused by imperfections in the HCF. The full dynamic range of the OTDR is shown (arrow 510), which can be seen to be much larger than the available dynamic range of the OTDR device for the HCF (arrow 512); for example, the full dynamic range (510) is more than twice the available dynamic range (512) for the HCF portion of the FUT.

[0020] In this invention, a method and apparatus for performing OTDR on an HCF with an improved dynamic range (e.g., a much higher dynamic range compared to that shown in FIG. 5) are described. By using the method and apparatus described herein, at least a significant proportion of backscattered light from the SMF and SMF-HCF coupler is blocked from reaching the PD. This has the effect of making the entire dynamic range of the OTDR system available for measuring the HCF. This improves the characterization of the HCF that can be performed using the OTDR and enables the detection of smaller faults and defects compared to conventional methods and apparatus (e.g., as shown in FIG. 6). The blocking of light can be achieved by absorbing, attenuating, or diverting the light.

[0021] FIGS. 7 and 8 show two different examples of an improved OTDR device (700, 800) suitable for use with an HCF. Both examples of the improved OTDR device (700, 800) include a light source (612), a circulator (614), a PD (616), and a time-gating circuit (702, 802). The OTDR device (700, 800) may include other components not shown in FIGS. 7 and 8. The time-gating circuit (702, 802) is configured to block all or a significant proportion of backscattered light from the SMF section of the FUT from reaching the PD (616). Blocking the light can be achieved by absorbing, bypassing, or significantly attenuating (e.g., attenuating by more than 50-60 dB) the backscattered light from the SMF section. The PD (616) is connected to the output of the time-gating circuit (702, 802) and may be a non-photon-counting PD.

[0022] The time-gating circuit (702) illustrated in the OTDR device (700) of FIG. 7 includes an optical tap coupler (704), a second PD (706), an optical switch (708), a driving circuit (712) for the optical switch (708), and a switch control circuit (710) including an adjustable delay and pulse width element (714). The combination of the tap coupler (704) and the second PD (706) is used to detect the time when a pulse is emitted by the light source (612) and transmitted to the FUT (600). The tap coupler (704) can tap, for example, 10% of the optical power emitted by the light source (612). Alternatively, a different percentage of light (e.g., 1%, 5%, or 15%) may be tapped by the tap coupler (704). The second PD (706) detects a pulse from the tapped light, which is provided as an input to the switch control circuit (710). The light source and the PD may be an integral part of a commercial OTDR device.

[0023] The switch control circuit (710) outputs a control signal to trigger the optical switch (708) so that the optical switch (708) opens at a time after a pulse is detected by the PD (706), and the time delay between the detection of the pulse by the PD (706) and the opening of the switch (708) is set by an adjustable delay element (714). When the switch (708) is closed (e.g., before receiving a trigger control signal from the switch control circuit (710), light received through the third port of the circulator (614) is blocked by the switch (i.e., attenuated, absorbed, or deflected by the switch (708)), so at least a significant proportion of the light received through the third port of the circulator (614) is not incident on (i.e. does not reach) the PD (616) and is therefore not included in any OTDR trace graph generated. Once the switch (708) is opened (as a result of receiving a trigger control signal from the switch control circuit (710), the light received through the third port of the circulator (614) is not blocked (so it reaches the PD (616)) and is therefore used to generate an OTDR trace graph. The adjustable delay element (714) is set to provide a signal such as a pulse having a specific width and delay, which has the effect of blocking (i.e., attenuating, absorbing, or deflecting) the backscattered light from the SMF by the switch (708). Thus, the adjustable delay element (714) prevents the switch (708) from opening until the light reaching the switch (708) includes backscattered light from the HCF. The combination of the switch control circuit (710) and the switch (708) selectively allows some of the backscattered light within the FUT (600) to reach the PD (616). As described above, this blocks backscattered light within the SMF from reaching the PD (616) and allows backscattered light within the HCF to reach the PD (616).

[0024] The time-gating circuit (802) illustrated in the OTDR device (800) of FIG. 8 includes an optical switch (708), a driving circuit (712) for the optical switch (708), a driving circuit (816) for a light source (612), and a switch control circuit (810) comprising an adjustable delay element (714). The switch control circuit (810) outputs a first control signal to the light source to trigger the light source (612) to output a pulse (e.g., using the driving circuit (816) for the light source), and outputs a second control signal to the optical switch (708) to trigger the switch (708) (e.g., using the driving circuit (712) for the optical switch). The delay between the output first control signal and the output second control signal is set by the adjustable delay element (714). As described above, when the switch (708) is closed (e.g., before receiving the second control signal from the switch control circuit (710)), light received through the third port of the circulator (614) is blocked by the switch (708) (i.e., attenuated, absorbed, or deflected) and does not reach the PD (616) and is therefore not included in any OTDR trace graph generated. Once the switch (708) is opened (as a result of receiving the second control signal from the switch control circuit (710)), light received through the third port of the circulator (614) reaches the PD (616) (e.g., without any significant attenuation by the switch (708)) and is therefore used to generate an OTDR trace graph. As in the configuration of FIG. 7, in the configuration of FIG. 8, the adjustable delay element (714) is set to provide a delay that has the effect of preventing any backscattered light from the SMF from reaching the PD (616) (i.e., not being incident on or detected by the PD (616)) and instead blocking (i.e., attenuated, absorbed, or deflected) it by the switch (708).Therefore, the adjustable delay element (714) prevents the switch (708) from opening until the light reaching the switch (708) includes backscattered light from the HCF.

[0025] The optical switch (708) used in the improved OTDR device (700, 800) described herein may have an extinction ratio that is at least twice the target dynamic range (1010) of the OTDR device (as shown in FIG. 10) (e.g., to ensure that backscattered light from the SMF is absorbed, significantly attenuated, or deflected when the switch is closed). For example, this target dynamic range may be 30 dB or more. The optical switch (708) may also have a switching time that is similar to or faster than the rise and fall times of the pulse emitted by the light source (612) (e.g., rise and fall times of several nanoseconds). By having a switch with a fast switching time, more backscattered light from the HCF can be captured by the PD. The optical switch (708) may take any form capable of performing the aforementioned blocking (e.g., by absorbing, attenuating, or deflecting a significant proportion of the input light when the switch is closed and allowing the light to pass when the switch is open), and may be, for example, an acousto-optic modulator, a variable attenuator, an electro-optic modulator, an intensity or phase modulator, a Mach-Zehnder interferometer, a semiconductor optical amplifier, a modulated optical fiber amplifier, a microelectromechanical switch, etc.

[0026] Although not illustrated in FIGS. 7 and 8, the OTDR device (700, 800) may further include one or two optical amplifiers, one positioned between the light source (612) and the circulator (614), and the other positioned between the circulator (614) and the optical switch (708) (these positions are indicated by arrow 720 in FIGS. 7 and 8). These two optical amplifiers increase both the power of the pulse injected into the FUT and the power of the backscattered light from the FUT. This improves the signal-to-noise ratio (SNR) and, consequently, the dynamic range of the OTDR device, thereby enabling longer testing of the FUT as well as reducing the overall test time.

[0027] Although not illustrated in FIGS. 7 and 8, the OTDR device (700, 800) may include an additional SMF length in front of the circulator (614) to create a fixed signal delay so that the detection and time-gating circuit operates before the SMF backscatter reaches the PD. This additional SMF length may be located between the tap coupler (704) and port 1 of the circulator (614).

[0028] In a variation of the configuration shown in FIGS. 7 and 8, the circulator (614) can be replaced with an alternative optical device that achieves the same optical function, namely, light emitted by the light source is connected to the FUT, and light reflected within the FUT is connected to the input port of the optical switch. For example, the circulator (614) can be replaced with a 4-port coupler.

[0029] FIG. 9 illustrates a first exemplary method of performing OTDR on a FUT comprising a portion of an HCF and a portion of an SCF such as an SMF (e.g., FUT (600)). This method may be implemented using the aforementioned OTDR device (700, 800) illustrated in FIG. 7 or FIG. 8. This method includes the step (block 902) of determining the position of a transition portion between an SCF and an HCF within the FUT. Using the determined position (from block 902), a time delay of an adjustable delay element (714) is set (block 904), and then an OTDR trace graph is generated by using this delay circuit to control an optical switch (708) and selectively blocking (e.g., absorption, significant attenuation, or bypass) a portion of reflected light from reaching the PD (616) (block 906). If the FUT includes the length of the SMF closest to the OTDR device and the length of the HCF connected to the SMF, the portion of the reflected light that is blocked (e.g., absorbed, significantly attenuated, or bypassed) is the initial portion of the pulse of light reflected by the length of the SMF within the FUT. As a result of blocking the reflected light (i.e., at least a significant proportion of the reflected light) by the optical switch, the OTDR trace graph is generated from the light reflected within the HCF (Block 906). This has the effect that the entire dynamic range of the OTDR device (700, 800) is available for measuring the HCF. Even if a small amount of light is allowed to pass through when the switch is closed, this does not significantly affect the dynamic range available for measuring the HCF.

[0030] The method of FIG. 9, and the device illustrated in FIG. 7 and FIG. 8, can not only block (e.g., absorb, attenuate, or bypass) backscattered light from the SMF so that it is not used to generate an OTDR trace graph, but can also be used to block light from reflections caused by any other components within the FUT (or optical train containing the FUT) prior to the HCF, such as switches, connectors, amplifiers, and other optical devices.

[0031] Although the method of FIG. 9 describes generating an OTDR trace graph, it can be understood that this method may output data derived from the OTDR trace graph without outputting the OTDR trace graph. The OTDR trace graph is a measurement of backscattered power, and this method may further include processing this data and outputting data that identifies events (e.g., peaks or troughs) within the link. These events can be identified by analyzing the OTDR trace graph to identify changes in backscattered power. The output event data can identify the position of the identified event within the FUT and / or the characteristics of the identified event (e.g., the characteristics of the event may be determined based on the characteristics of the change in the OTDR trace graph, e.g., whether it is a peak or a trough and / or the magnitude of the change in backscattered power). FIG. 10 illustrates an exemplary OTDR trace graph (1000) for a FUT (600) that can be generated using the method of FIG. 9 and the device of FIG. 7 or 8. The horizontal axis of the OTDR trace graph (1000) indicates the distance along the FUT, and the vertical axis indicates the backscattering power. A time-gate section (1002) of the trace graph corresponding to the length of the SMF (or the total optical path length prior to the HCF) is shown, along with the length of the HCF (1004). The trace graph (1000) displays events (e.g., peaks and troughs) (1008) resulting from the loss and changes in the backscattering coefficient in the HCF. The full dynamic range of the OTDR is indicated by the arrow 1010, which can be seen to be the same as (i.e., equal to) the available dynamic range for the HCF.

[0032] The position of the transition between the SCF and the HCF can be determined by one of several different methods (in block 902). In the first example, this can be determined based on the measurement of the actual physical length of different parts of the FUT using a device such as a tape measure or a ruler. In an additional example, this can be determined using an initial OTDR trace graph as shown in FIGS. 11 and 12. If the position of the transition is determined using an initial OTDR trace graph, this determination can be performed by a delay control module (730) that receives and analyzes the initial OTDR trace graph. The delay control module (730) then determines and sets a time delay (in blocks 904 and 1204). The delay control module (730) may include a processor and memory configured to store instructions executed by the processor to implement the steps of the method of FIGS. 11 and 12, or alternatively, the delay control module (730) may include fixed-function hardware that implements these steps.

[0033] FIG. 11 illustrates a second exemplary method of performing OTDR on a FUT comprising a portion of an HCF and a portion of an SCF such as an SMF (e.g., FUT (600)). This method may be implemented using the aforementioned OTDR device (700, 800) illustrated in FIG. 7 or FIG. 8. This method is a variation of the one illustrated in FIG. 9, which generates an initial OTDR trace graph without blocking any light reaching the photodetector (Block 1102), and then determines the position of the transition between the SCF and HCF within the FUT by analyzing the initial OTDR trace graph to detect the position of the transition between the SCF and HCF within the FUT (Block 1104) (Block 902).

[0034] While generating the initial OTDR trace graph (Block 1102), the optical switch (708) remains open and the driving circuit (712) does not output a control signal. On the other hand, while generating the second OTDR trace graph (Block 906), the optical switch (708) is initially closed and opens only in response to a control signal generated by the driving circuit (712) using an adjustable delay element (714) and output to the optical switch (708). This means that the initial OTDR trace graph (generated in Block 1102) is similar to that shown in FIG. 5, and that any suitable technique can be used for analysis (in Block 1104) to detect the position of the reflection peak (506). For example, the analysis (in Block 1104) can detect the position with maximum backscattering power on the initial trace graph, which can be used to set a time delay (in Block 904) along with the speed of light in the solid-core optical fiber (which can be obtained from a lookup table).

[0035] In the method of FIG. 11, an initial OTDR trace graph can be generated using a first pulse emitted by a light source (612) (block 1102), and a second OTDR trace graph can be generated using a second pulse emitted by a light source (612) (block 906).

[0036] FIG. 12 illustrates a third exemplary method for performing OTDR on a FUT comprising a portion of the HCF and a portion of the SCF, such as an SMF (e.g., FUT (600)). The method illustrated in FIG. 12 is a variation of that illustrated in FIG. 11 and described above; however, the initial OTDR trace graph is generated using an initial time delay (Block 1202), and this time delay is subsequently updated based on the position of the transition portion being determined (Block 1104) (Block 1204). The method of FIG. 12 may be particularly suitable for use when there are other optical components having higher reflection than the HCF-SMF interface prior to the HCF. These higher reflection interfaces or components may interfere with or hinder the determination of the position of the SMF-HCF interface when the method of FIG. 11 is used.

[0037] In one example, the initial delay (used in block 1202) can be determined and set using the estimated length of the SMF or an approximate measurement of the length of the SMF, and then the updated delay (set in block 1204) provides a more accurate value that reflects the detected position of the transition (determined in block 1104).

[0038] In the described example, the FUT (600) includes a portion of the SMF (602) and a portion of the HCF (604). In another example, the FUT may include a plurality of portions in which the HCF and SMF are intersected (e.g., where one or more of the portions of the SMF include an erbium-doped fiber amplifier (EDFA)). In this example, the optical switch (708) may be controlled to selectively block light reflected within any portion of the SMF (e.g., by absorption, attenuation, or by bypassing), as well as the connection (e.g., splice or coupler) between the portion of the HCF and the adjacent portion of the SMF. The method of FIGS. 9, 11, and 12 may be implemented by a computing device connected to or integrated within the described OTDR device (700, 800). A computing device may include a processor and memory arranged to store device-executable instructions that, when executed by the processor, cause the processor to execute some or all of the steps of any of the methods in FIG. 9, 11 and 12.

[0039] As described, to generate an OTDR trace graph that provides a high dynamic range for the HCF, all or part of the backscattered optical power within the SMF is blocked so that it does not enter the PD. This is achieved by using a time-gating circuit. By including an adjustable delay element, the OTDR device can be used for different lengths of the SMF, and by using the method described herein, the position of the transition between the SMF (or other SCF) and the HCF is detected and subsequently used to set the adjustable delay element, so there is no need to know the length of the SMF in advance.

[0040] By using the method described above, it is possible to perform rapid, high-resolution tests to identify failures and defects in the HCF when installed in an optical network. Additional aspects of the invention are described in the following claims:

[0041] Claim A: A method for performing optical time domain reflectometry (OTDR) on a fiber under test (FUT), wherein the FUT comprises a portion of a hollow core fiber (HCF) and a portion of a solid core fiber (SCF), and the method comprises the steps of determining the position of a transition portion between the SCF and the HCF (902), setting a time delay using the determined position of the transition portion in a switch control circuit (904, 1204), and generating an OTDR trace graph using light detected by a photodetector (906), wherein the time delay is used to control an optical switch to block backscattered light within the SCF from reaching the photodetector.

[0042] Claim B: In the method according to Claim A, the optical switch blocks backscattered light within the SCF from reaching the photodetector by attenuating, absorbing, or deflecting backscattered light within the SCF.

[0043] Claim C: In the method according to Claim A or Claim B, the step of determining the position of the transition portion between SCF and HCF includes the step of generating an initial OTDR trace graph without blocking any light reaching the photodetector (1102), and the step (1104) of detecting the position of the transition portion between SCF and HCF by analyzing the initial OTDR trace graph.

[0044] Claim D: In the method according to Claim A or Claim B, the step of determining the position of the transition portion between SCF and HCF includes the step of generating an initial OTDR trace graph using an initial time delay to block a portion of backscattered light from FUT from reaching the photodetector (1202), and the step of analyzing the initial OTDR trace graph to detect the position of the transition portion between SCF and HCF (1104), and the step of setting the time delay using the position of the transition portion determined in the switch control circuit includes the step of updating the time delay using the position of the transition portion determined in the switch control circuit (1204).

[0045] Claim E: In the method according to Claim C or Claim D, the step of detecting the position of the transition portion between SCF and HCF by analyzing the initial OTDR trace graph includes the step of detecting the position of the maximum value in the initial OTDR trace graph.

[0046] Claim F: In the method according to Claim A or Claim B, the step of determining the position of the transition portion between the SCF and the HCF includes the step of measuring the distance between the starting point of the SCF and the transition portion between the SCF and the HCF.

[0047] Claim G: A method according to any one of the preceding claims further comprises the step of analyzing an OTDR trace graph to identify one or more events, and the step of outputting data identifying the events identified in the OTDR trace graph.

[0048] Clause H: An optical time domain reflectometry (OTDR) device (700, 800) for analyzing a hollow core fiber (HCF), the device comprising a light source (612), a time-gating circuit (702, 802) comprising an adjustable time delay element (714) and an optical switch (708), an optical device (614) configured to connect light output by the light source to a test optical fiber (FUT) and to connect light reflected within the FUT to the time-gating circuit (702, 802) - the FUT comprises a portion of a solid core fiber (SCF) and a portion of an HCF - and a photodetector connected to the output of the time-gating circuit, wherein the time-gating circuit is configured to block backscattered light within the FUT from reaching the photodetector for a time period after the light source has generated a pulse of light, and the time period is to the adjustable time delay element It is set by.

[0049] Clause I: The optical switch is configured to block backscattered light within the FUT from reaching the photodetector during the time period after the light source has generated a pulse of light by attenuating, absorbing, or deflecting the light.

[0050] Clause J: In the OTDR device according to Clause H or I, the optical device is an optical circulator.

[0051] Claim K: In an OTDR device according to any one of claims H to J, the time-gating circuit further comprises an optical tap coupler (704) and a second optical detector (706) configured to detect the time at which a pulse of light is emitted by a light source, and a switch control circuit (710) configured to introduce a delay between the detection of the pulse of light and the output of a control signal for opening an optical switch using an adjustable delay element.

[0052] Claim L: In an OTDR device according to any one of claims H to J, the time-gating circuit further comprises a driving circuit (806) configured to output a control signal to a light source that triggers the output of a light pulse, and a switch control circuit (810) configured to introduce a delay between the triggering of the light pulse and the output of the control signal to open the optical switch using an adjustable delay element. Claim M: In an OTDR device according to any one of claims H to J, the device further comprises a delay control module (730) configured to analyze an initial OTDR trace graph to detect the position of a transition portion between the SCF portion and the HCF portion in the FUT and to adjust an adjustable delay element.

[0053] Claim N: A computer-executable instruction that, when executed by a computing device, causes the computing device to perform a method according to any one of claims A through G.

[0054] Clause O: A computer-readable medium configured to store computer-executable instructions according to Clause N.

[0055] Any range or device value described herein may be extended or changed without loss of intended effect, as will be obvious to those skilled in the art.

[0056] Embodiments of the present disclosure include or utilize a special-purpose or general-purpose computer system, including computer hardware such as, for example, a processor system and system memory, as described in more detail below. Embodiments within the scope of the present disclosure also include physical and other computer-readable media for transmitting or storing computer-executable instructions and / or data structures. Such computer-readable media may be any available media accessible by a general-purpose or special-purpose computer system. A computer-readable medium storing computer-executable instructions and / or data structures is a computer storage medium. A computer-readable medium transmitting computer-executable instructions and / or data structures is a transmission medium. Accordingly, embodiments of the present disclosure may include at least two distinctly different types of computer-readable media: computer storage media and transmission media.

[0057] A computer storage medium is a physical storage medium that stores computer-executable instructions and / or data structures. A physical storage medium includes computer hardware such as random access memory (RAM), read-only memory (ROM), electrically erasable programmable ROM (EEPROM), solid state drives (SSDs), flash memory, phase-change memory (PCM), optical disk storage, magnetic disk storage, or other magnetic storage devices, or any other hardware storage device that stores program code in the form of computer-executable instructions or data structures that can be accessed and executed by a general-purpose or special-purpose computer system to implement the disclosed functions.

[0058] A transmission medium includes networks and / or data links that transmit program code in the form of computer-executable instructions or data structures accessible by a general-purpose or special-purpose computer system. A “network” is defined as a data link that enables the transmission of electronic data between a computer system and other electronic devices. When information is transmitted or provided to a computer system via a network or other communication connection (wired, wireless, or a combination thereof), the computer system may regard this connection as a transmission medium. The scope of a computer-readable medium includes combinations thereof.

[0059] Means of program code in the form of computer-executable instructions or data structures can be automatically transferred from a transmission medium to a computer storage medium (or vice versa) upon reaching various computer system components. For example, computer-executable instructions or data structures received via a network or data link can be buffered in RAM within a network interface module and then finally transferred to computer system RAM and / or a low-volatility computer storage medium of the computer system. Accordingly, the computer storage medium can also be included in the computer system component utilizing the transmission medium.

[0060] Computer-executable instructions include instructions and data that, for example, when executed in a processor system, cause a general-purpose computer system, a special-purpose computer system, or a special-purpose processing unit to perform a function or a group of functions. In an embodiment, computer-executable instructions include binary, intermediate format instructions (e.g., assembly language), or source code. In an embodiment, a processor system includes one or more central processing units (CPUs), one or more graphics processing units (GPUs), one or more neural processing units (NPUs), etc.

[0061] Although the subject matter has been described in terms specific to structural features and / or methodological practices, it should be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or practices described. Rather, the specific features and practices described above are disclosed as exemplary forms for implementing the claims. It will be understood that the aforementioned advantages and benefits may relate to one or several embodiments. The embodiments are not limited to solving some or all of the problems mentioned or having some or all of the advantages and benefits mentioned. It will also be understood that mentioning 'one' item refers to one or more of these items.

[0062] The operation of the method described herein may be performed in any suitable order, or, where appropriate, simultaneously. Additionally, individual blocks may be omitted in any method without departing from the scope of the subject matter described herein. Any aspect of any of the above-described embodiments may be combined with any other aspect of the described embodiments to form additional embodiments without losing the effect sought.

[0063] The term 'comprising' is used to mean that the method or device includes the method blocks or components identified herein, but such blocks or components do not constitute an exclusive list and the method or device may include additional blocks or components.

[0064] It will be understood that the foregoing description is given merely as an example and that various modifications may be made by those skilled in the art. The specification, examples, and data provide a complete description of the structure and use of exemplary embodiments. Although various embodiments have been described with some degree of specificity or by reference to one or more individual embodiments, those skilled in the art may make numerous changes to the disclosed embodiments without departing from the scope of this specification.

Claims

Claim 1 A method for performing optical time domain reflectometry (OTDR) on a fiber under test (FUT), wherein the FUT comprises a portion of a hollow core fiber (HCF) and a portion of a solid core fiber (SCF), and the method comprises the steps of determining the position of a transition portion between the SCF and the HCF, setting a time delay using the position of the transition portion determined in a switch control circuit, and generating an OTDR trace graph using light detected by a photodetector, wherein the time delay is used to control an optical switch to block backscattered light within the SCF from reaching the photodetector. Claim 2 A method according to claim 1, wherein the optical switch blocks the backscattered light within the SCF from reaching the photodetector by attenuating, absorbing, or deflecting the backscattered light within the SCF. Claim 3 A method according to claim 1 or 2, wherein the step of determining the position of the transition portion between the SCF and the HCF comprises: generating an initial OTDR trace graph without blocking any light reaching the photodetector; and analyzing the initial OTDR trace graph to detect the position of the transition portion between the SCF and the HCF. Claim 4 A method according to claim 1 or 2, wherein the step of determining the position of the transition portion between the SCF and the HCF comprises: a step of generating an initial OTDR trace graph using an initial time delay to block a portion of backscattered light from the FUT from reaching the photodetector; and a step of analyzing the initial OTDR trace graph to detect the position of the transition portion between the SCF and the HCF; and the step of setting the time delay using the position of the transition portion determined by the switch control circuit comprises a step of updating the time delay using the position of the transition portion determined by the switch control circuit. Claim 5 A method according to claim 3 or 4, wherein the step of detecting the position of the transition portion between the SCF and the HCF by analyzing the initial OTDR trace graph includes the step of detecting the position of the maximum value in the initial OTDR trace graph. Claim 6 A method according to claim 1 or 2, wherein the step of determining the position of the transition portion between the SCF and the HCF comprises the step of measuring the distance between the starting point of the SCF and the transition portion between the SCF and the HCF. Claim 7 A method according to any one of claims 1 to 6, further comprising the step of analyzing an OTDR trace graph to identify one or more events, and the step of outputting data identifying the identified events in the OTDR trace graph. Claim 8 An optical time domain reflectometry (OTDR) device for analyzing a hollow core fiber (HCF), the device comprising: a light source; a time-gating circuit including an adjustable time delay element and an optical switch; an optical device configured to connect light output by the light source to a test optical fiber (FUT) and to connect light reflected within the FUT to the time-gating circuit - the FUT comprises a portion of a solid core fiber (SCF) and a portion of an HCF - and a photodetector connected to the output of the time-gating circuit, wherein the time-gating circuit is configured to block backscattered light within the FUT from reaching the photodetector for a time period after the light source generates a pulse of light, and the time period is set by the adjustable time delay element. Claim 9 An OTDR device according to claim 8, wherein the optical switch is configured to block backscattered light within the FUT from reaching the photodetector during the time period after the light source has generated a pulse of light by attenuating, absorbing, or deflecting the light. Claim 10 In claim 8 or 9, the optical device is an OTDR device that is an optical circulator. Claim 11 An OTDR device according to any one of claims 8 to 10, wherein the time-gating circuit further comprises an optical tap coupler and a second optical detector configured to detect the time at which the pulse of light is emitted by the light source, and a switch control circuit configured to introduce a delay between the detection of the pulse of light and the output of the control signal for opening the optical switch using the adjustable delay element. Claim 12 An OTDR device according to any one of claims 8 to 10, wherein the time-gating circuit further comprises a driving circuit configured to output a control signal to the light source that triggers the output of the light pulse, and a switch control circuit configured to introduce a delay between the triggering of the light pulse and the output of the control signal for opening the optical switch using the adjustable delay element. Claim 13 An OTDR device according to any one of claims 8 to 12, further comprising a delay control module configured to analyze an initial OTDR trace graph to detect the position of a transition portion between a portion of the SCF and a portion of the HCF within the FUT and to adjust the adjustable delay element. Claim 14 A computer-executable instruction that, when executed by a computing device, causes the computing device to perform the method of any one of claims 1 to 7. Claim 15 A computer-readable medium configured to store computer-executable instructions pursuant to paragraph 14.

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