Optical time domain reflectometry for hollow core fibers

Abstract

A method of performing optical time domain reflectometry (OTDR) on a fiber under test (FUT) that comprises a portion of hollow core fiber (HCF) and a portion of solid core fiber (SCF) is described. The method comprises determining a position of a transition between the SCF and the HCF in the FUT; setting a time delay in switch control circuitry using the determined position of the transition; 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 light backscattered within the SCF from reaching the photodetector (e.g. by absorbing, attenuating or diverting the backscattered light).

Description

OPTICAL TIME DOMAIN REFLECTOMETRY FOR HOLLOW CORE FIBERSBACKGROUND

[0001] Optical Time Domain Reflectometry (OTDR) is a method used in fiber optic network testing, troubleshooting and optical fiber and cable manufacturing. OTDR is primarily used to characterize the optical fiber by measuring the amount of light reflected back from different points along the fiber. Optical Time Domain Reflectometry is conveniently implemented using an apparatus known as an Optical Time Domain Reflectometer (also OTDR). OTDRs (the apparatus) 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 about the fiber's characteristics and detecting potential issues.

[0002] The basic principle of an OTDR involves sending a short pulse of light into the fiber and analyzing the reflected signals. By measuring the time it takes for the light to travel and the strength of the reflected signal, the OTDR can determine the distance to various events or anomalies in the fiber. The OTDR generates a graphical representation called an OTDR trace or trace graph, which displays the fiber's length and provides information about the optical loss, splice points, connectors, and any fiber breaks or bends. The trace graph helps technicians identify and locate faults or issues in the fiber optic network.

[0003] The embodiments described below are not limited to implementations which solve any or all of the disadvantages of know n methods of fault location and OTDR. SUMMARY

[0004] The following presents a simplified summary of the disclosure in order to provide a basic understanding to the reader. This summary is not intended to identify key features or essential features of the claimed subject matter nor is it intended to be used to limit the scope of the claimed subject matter. Its sole purpose is to present a selection of concepts disclosed herein in a simplified form as a prelude to the more detailed description that is presented later.

[0005] A method of performing optical time domain reflectometry (OTDR) on a fiber under test (FUT) that comprises a portion of hollow core fiber (HCF) and a portion of Solid Core Fiber (SCF) is described. The method comprises determining a position of a transition between the SCF and the HCF in the FUT; setting a time delay in switch control circuitryusing the determined position of the transition; 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 light backscattered within the SCF from reaching the photodetector (e.g. by absorbing, attenuating or diverting the backscattered light).

[0006] Many of the attendant features will be more readily appreciated as the same becomes better understood by reference to the following detailed description considered in connection with the accompanying drawings.DESCRIPTION OF THE DRAWINGS

[0007] The present description will be better understood from the following detailed description read in light of the accompanying drawings, wherein:FIGs. 1-4 show transverse cross-sectional views of different examples of hollow core fiber;FIG. 5 shows an example OTDR trace graph generated using the apparatus of FIG. 6;FIG. 6 is a schematic diagram of OTDR apparatus;FIG. 7 is a schematic diagram of a first example of an improved OTDR apparatus;FIG. 8 is a schematic diagram of a second example of an improved OTDR apparatus;FIG. 9 shows a first example method of performing OTDR on a FUT that comprises a portion of hollow core fiber and a portion of solid core fiber;FIG. 10 shows an example OTDR trace graph generated using the method of FIG. 9; FIG. 11 shows a second example method of performing OTDR on a FUT that comprises a portion of hollow core fiber and a portion of solid core fiber; and FIG. 12 shows a third example method of performing OTDR on a FUT that comprises a portion of hollow core fiber and a portion of solid core fiber.Like reference numerals are used to designate like parts in the accompanying drawings.DETAILED DESCRIPTION

[0008] The detailed description provided below in connection with the appended drawings is intended as a description of the present examples and is not intended to represent the only forms in which the present examples are constructed or utilized. The description sets forth the functions of the examples and the sequence of operations for constructing and operating the examples. However, the same or equivalent functions and sequences may be accomplished by different examples.

[0009] As described above, OTDR may be used to characterize optical fibers. OTDR works by launching an optical probe pulse into a fiber (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 a few nanoseconds (e.g. 2ns) to a few microseconds (e.g. 3ps) may be used. As the probe pulse propagates along the FUT, Rayleigh scattering gives rise to signals propagating in the opposite direction to the incident probe pulse. Rayleigh backscattering is caused by the interaction of the incident light with refractive index fluctuations in the FUT (e g. in the core of the FUT). The backscattered signals are then redirected into a photodetector (PD). The time delay at which a signal is detected can be related to the position in the FUT where it was generated by knowing the speed of light in the FUT. Then, an OTDR trace can be constructed by plotting the power of the backscattered light (as detected by the PD) vs. position along the fiber.

[0010] OTDR is widely used to identify fiber breaks, flaws or other anomalies in the fiber in optical networks; however, conventional OTDR techniques are not suitable for analyzing network links that include hollow core fiber (HCF). In standard optical fiber (e.g. single mode fiber, SMF, or multimode fiber. MMF). there is a solid core (typically made of glass) in which the light travels and the guidance of the light relies upon the cladding material that surrounds the core having a lower refractive index than the core material. In HCF the light does not travel in a solid core, but instead the light travels in a hollow core which may be filled with air and different guidance mechanisms are used. Two types of HCF that use different guidance mechanisms are hollow' core photonic bandgap fibers (PBGFs) and antiresonant hollow^ core fibers (ARFs) and these are described in more detail below- with reference to FIGs. 1-4.

[0011] FIG. 1 shows a transverse cross-sectional view of an example hollow core PBGF 100. The fiber 100 comprises a central tube 102 with a hollow core 104 and a cladding comprising a plurality of smaller tubular hollow structures 106, 108 arranged uniformly. The longitudinal axes of the central tube 102 and the smaller tubular structures 106, 108 are substantially parallel.

[0012] FIGs. 2-4 show transverse cross-sectional views of three different examples of antiresonant hollow core fibers (ARFs). Light is guided in these fibers by an antiresonant optical effect. Each of the fibers 200, 300, 400 comprises a tubular outer cladding (or jacket) 202, a structured, inner, cladding comprising a plurality of tubular cladding capillaries 204. 304, 404 and a hollow core 206. The outer cladding 202 has a glass thickness that is typically much larger than the cladding capillaries 204, 304, 404. In thefirst example, shown in FIG. 2, the structured, inner, cladding comprises five capillaries 204 of the same cross-sectional size and shape, which are arranged inside the outer cladding 202 in a single ring so that the longitudinal axes of each cladding capillary 204 and of the outer cladding 202 are substantially parallel. Each cladding capillary 204 is in contact with (e.g. bonded to) the inner surface of the outer cladding 202 at an azimuthal location 108, such that the cladding capillaries 204 are evenly spaced around the inner circumference of the outer cladding 202. and are also spaced apart from each other by gaps 210 (i.e. such that there is no contact between neighbouring capillaries). In some designs of ARF, the cladding capillaries 204 may be positioned in contact with each other (in other words, not spaced apart as in FIG. 2), but spacing to eliminate this contact can improve the fiber’s optical performance. The gaps 210 removes nodes that arise at the contact points between adjacent tubes and which tend to cause undesirable resonances that result in high losses. Accordingly, fibers with spaced-apart cladding capillaries may be referred to as “nodeless antiresonant hollow core fibers”.

[0013] The arrangement of the cladding capillaries 204 in a ring around the inside of the tubular outer cladding 202 creates a central space, cavity, or void within the fiber, also with its longitudinal axis parallel to those of the outer cladding 202 and the cladding capillaries 204, which is the fiber’s hollow core 206. The hollow core 206 is bounded by the inwardly facing parts of the outer surfaces of the cladding capillaries 204. This is the core boundary, and the material (glass or polymer, for example) of the capillary walls that make up this boundary provides the required antiresonance optical guidance effect or mechanism. The cladding capillaries 204 have a thickness, t, at the core boundary which defines the wavelength for which antiresonant optical guiding occurs in the ARF.

[0014] In the second example, shown in FIG. 3, each primary cladding capillary 204 has a secondary, smaller capillary 304 nested inside it, bonded to the inner surface of the primary’ cladding capillary 204, in this example at the same azimuthal location 208 as the point of bonding between the primary7cladding capillary7204 and the outer cladding 202. These additional smaller capillaries 304 can reduce the optical loss. ARF designs of this type, with secondary capillaries, may be referred to as “nested antiresonant nodeless fibers” (NANFs).

[0015] The third example, show n in FIG. 4, has two smaller cladding capillaries 304, 404 nested inside each cladding capillary7204. As with the example shown in FIG. 3, each of the smaller capillaries 304, 404 is bonded to the inner surface of the immediately7larger capillary at the same azimuthal location as the point of bonding between the primarycladding 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. ARF designs of this type, with secondary’ and tertiary cladding capillaries may be referred to as ‘"double-nested antiresonant nodeless fibers” (DNANFs). In yet further examples (not shown in the drawings) there may a different configuration of cladding capillaries. For example, there may be smaller further capillaries, within the tertiary' capillary' 304 to provide further levels of nesting and / or there may be a plurality of 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 azimuthal location and / or each primary' cladding capillary may have an internal structure (e.g. one or more dividing walls).

[0016] All of the examples shown in FIGs. 2-4 comprise five primary cladding capillaries 104 and hence have five-fold rotational symmetry. Furthermore, all the capillaries are circular in cross-section. In other examples, there may be a different number of capillaries surrounding the core (e.g. four, six, seven, eight, nine or ten) and / or the capillaries may not be of circular cross-section. Additionally, whilst in the examples of FIGs. 2-4, all the primary’ cladding capillaries 204 are of the same size and shape, in other examples, the primary capillaries within the outer cladding 202 may not all be the same size and / or shape.

[0017] Where HCF is used in an optical network, the HCF is usually used in combination with single mode fiber (SMF), such that a length of SMF is connected to a length of HCF. It is this combination of a length of SMF (or other form of SCF) coupled to a length of HCF that limits the use of OTDR in the conventional manner to look for faults or flaws in the HCF. HCFs have much lower backscattering levels than SMF. SMF shows a backscattering of approximately -70 to -60 dB / m of the incident light on its core due to Rayleigh scattering from density fluctuations in the glass. In contrast, the level of backscattering in HCFs and DNANF and NANF in particular, is approximately' -40 dB below SMF. The dominant mechanism of backscattering in (D)NANF is Rayleigh backscattering from the gas molecules inside the hollow core. This is about -100 dB / m for air atmospheric pressures.

[0018] Even if a transition region is introduced between the SCF and the HCF to reduce losses (and reflections) at the transition (e.g. by expanding the mode of the SMF so that its output field is better matched to that of the HCF and cleaving the SCF and HCF at an angle) the large difference in the backscattering levels between SMF and HCF significantly reduces the usable dynamic range of the OTDR on the part of the OTDR trace graph that corresponds to the length of HCF, as shown in FIG. 5. This makes it much more difficult to identify faults or flaws in the HCF at relatively longer distances (e.g. over distances of a few kilometers or longer), particularly faults or flaws that introduce small amounts of backscattering (but which may still impair the operation of the optical netw ork and so need to be identified and addressed).

[0019] FIG. 5 shows an example OTDR trace graph 500 for a FUT 600 that may be generated using OTDR apparatus 610, as shown in FIG. 6. The FUT 600 comprises a length of SMF 602 coupled to a length of HCF 604 via a SMF to HCF coupler 606. The OTDR apparatus 610 comprises a light source 612, circulator 614 and PD 616. It will be appreciated that the OTDR apparatus 610 may comprise other elements not shown in FIG.6. The optical pulse emitted from the light source 612 is coupled into a first port of the circulator 614 and output to the FUT 600 that is connected to a second port of the circulator 614. The pulse is reflected back 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 that is connected to the third port of the circulator 614. The trace graph 500 is generated using the output signal from the PD 616.

[0020] The horizontal axis of the OTDR trace graph 500 show s the distance along the FUT and the vertical axis show s the (absolute or relative) backscattered power.The length of the SMF and HCF are shown on the trace graph (arrows 502. 504 respectively). The trace graph 500 shows a peak 506 caused by reflections at the SMF-HCF interface as well as events (e.g. peaks or troughs) 508 due to imperfections in the HCF. The overall dynamic range of the OTDR is show n (arrow 510) and it can be seen that this is much larger than the usable dynamic range of the OTDR apparatus for HCF (arrow 512), e.g. the overall dynamic range 510 is more than twice that of the usable dynamic range 512 for the HCF portion of the FUT.

[0021] Described herein are methods and apparatus for performing OTDR on HCF with improved dynamic range (e g. a much higher dynamic range compared to that shown in FIG. 5). Using the methods and apparatus described herein, at least a significant proportion of the backscattered light from the SMF and the SMF to HCF coupler is blockedfrom reaching the PD. This has the effect that the whole dynamic range of the OTDR system is available to measure the HCF. This improves the characterization of the HCF that can be performed using OTDR and enables smaller faults and flaws to be detected compared to conventional methods and apparatus (e.g. as shown in FIG. 6). The blocking of the light may be achieved by absorbing, attenuating or diverting the light.

[0022] FIGs. 7 and 8 show two different examples of improved OTDR apparatus 700, 800 that are adapted for use with HCF. Both examples of improved OTDR apparatus 700, 800 comprise a light source 612, circulator 614, PD 616 and time-gating circuit 702, 802. It will be appreciated that the OTDR apparatus 700, 800 may comprise other elements not shown in FIGs. 7 and 8. The time-gating circuit 702, 802 is configured to block all or a significant proportion of the backscattered light from the SMF section of the FUT from reaching the PD 616. The blocking of the light may be achieved by absorbing, diverting or significantly attenuating (e.g. attenuating by 50-60dB or more) the backscattered light from the SMF section. The PD 616 is coupled to an output of the time-gating circuit 702, 802 and may be a non-photon-counting PD.

[0023] The time-gating circuit 702 shown in the OTDR apparatus 700 in FIG. 7 comprises an optical tap coupler 704, a second PD 706, an optical switch 708 and switch control circuitry' 710 that includes drive circuitry' 712 for the optical switch 708 and an adjustable delay and pulse width element 714. The combination of the tap coupler 704 and second PD 706 are used to detect when a pulse is emitted by the light source 612 and sent into the FUT 600. The tap coupler 704 may, for example, tap 10% of the optical powder emitted by the light source 612. Alternatively, a different proportion of the light may be tapped by the tap coupler 704 (e.g. 1% or 5% or 15%). The second PD 706 detects the pulse in the tapped light and this is provided as an input to the switch control circuitry 710. The light source and PD can be integral parts of a commercial OTDR device.

[0024] The switch control circuitry 710 outputs a control signal to the optical switch 708 to trigger the switch 708 to open at a time after the 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 the adjustable delay element 714. When the switch 708 is closed (e.g. before it receives the trigger control signal from the switch control circuitry 710), light that is received via the third port of the circulator 614 is blocked by the switch (i.e. attenuated, absorbed or deflected by the switch 708) such that at least a significant proportion of the light that is received via the third port of the circulator 614 is not incident upon (i.e. does not reach) the PD 616 and hence is not included in any OTDR trace graph that is generated.Once the switch 708 is opened (as a consequence of receiving the trigger control signal from the switch control circuitry 710), light that is received via the third port of the circulator 614 is not blocked (and so does reach the PD 616) and hence is used to generate an OTDR trace graph. The adjustable delay element 714 is set to provide a signal such as a pulse with a certain width and delay which has the effect that the backscattered light from the SMF is blocked (i.e. either attenuated, absorbed or deflected) by the switch 708. The adjustable delay element 714 therefore prevents the switch 708 from opening until the light that reaches the switch 708 comprises backscattered light from the HCF. The combination of the switch control circuitry 710 and the switch 708 selectively allows portions of the backscattered light within the FUT 600 to reach the PD 616. As described above, it blocks light backscattered within the SMF from reaching the PD 616 and allows light backscattered within the HCF to reach the PD 616.

[0025] The time-gating circuit 802 shown in the OTDR apparatus 800 in FIG. 8 comprises an optical switch 708 and switch control circuitry 810 that includes drive circuitry 712 for the optical switch 708, drive circuitry 816 for the light source 612 and an adjustable delay element 714. The switch control circuitry 810 outputs a first control signal to light source to trigger the light source 612 to output a pulse (e g. using the drive circuitry 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 drive circuitry 712 for the optical switch). The delay between the first control signal being output and the second control signal being output is set by the adjustable delay element 714. As described above, when the swatch 708 is closed (e.g. before it receives the second control signal from the switch control circuitry 710), light that is received via the third port of the circulator 614 is blocked (i.e. attenuated, absorbed or deflected) by the switch 708 such that it does not reach the PD 616 and hence is not included in any OTDR trace graph that is generated. Once the switch 708 is opened (as a consequence of receiving the second control signal from the sw itch control circuitry 710), light that is received via the third port of the circulator 614 does reach the PD 616 (e.g. without any significant attenuation by the switch 708) and hence is used to generate an OTDR trace graph. As in the arrangement of FIG. 7, in the arrangement of FIG. 8, the adjustable delay element 714 is set to provide a delay which has the effect that any backscattered light from the SMF does not reach the PD 616 (i.e. is not incident upon and detected by the PD 616) but is instead blocked (i.e. attenuated, absorbed or deflected) by the switch 708. The adjustable delay element 714 therefore prevents the switch 708 fromopening until the light that reaches the switch 708 comprises backscattered light from the HCF.

[0026] The optical switch 708 that is used in the improved OTDR apparatus 700, 800 described herein may have an extinction ratio that is at least twice a target dynamic range 1010 (as shown in FIG. 10) of the OTDR apparatus (e.g. to ensure that backscattered light from the SMF is absorbed, significantly attenuated or deflected when the switch is closed). In an example, this target dynamic range may be greater than or equal to 30dB. The optical switch 708 may also have a switching time that is comparable to, or faster than, the rise and fall time of the pulse emitted by the light source 612 (e.g. a rise and fall time of the order of a few nanoseconds). By having a switch with a fast switching time, it enables more of the backscattered light from the HCF to be captured by the PD. The optical switch 708 may take any form that can perform the blocking described above (e.g. by absorbing, attenuating or deflecting a significant proportion of the input light when in the switch is closed and allowing the light to pass through when the switch is open) and may, for example, be an acousto-optic modulator, variable attenuator, electro-optic modulator, intensity or phase modulator. Mach-Zehnder interferometer, semiconductor optical amplifier, modulated fiber amplifier, microelectromechanical switch, etc.

[0027] Although not shown in FIGs. 7 and 8, the OTDR apparatus 700, 800 may additionally comprise 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 (with these positions indicated by arrows 720 in FIGs. 7 and 8). These two optical amplifiers increase both the power of the pulse that is injected into the FUT and the pow er of the backscattered light from the FUT. This improves the signal-to-noise ratio (SNR) and hence the dynamic range of the OTDR apparatus which can reduce the overall test time as w ell as enabling testing of longer FUT.

[0028] Although not shown in FIGs. 7 and 8, the OTDR apparatus 700, 800 may comprise an additional SMF length before the circulator 614 to create a fixed signal delay to make sure that the detection and time-gating circuitry operate before the SMF backscattering reaches the PD. Such additional SMF length can be located between the tap coupler 704, and port 1 of the circulator, 720.

[0029] In a variation of the arrangements shown in FIGs. 7 and 8, the circulator 614 may be replaced an alternative optical device which achieves the same optical functions - i.e. light emitted by the light source is coupled into the FUT and light reflected within theFUT is coupled into an input port of the optical switch. For example, the circulator 614 may be replaced with a four-port coupler.

[0030] FIG. 9 shows a first example method of performing OTDR on a FUT that comprises a portion of HCF and a portion of SCF, such as SMF (e.g. FUT 600). The method may be implemented using the OTDR apparatus 700, 800 shown in FIG. 7 or 8 and described above. The method comprises determining a position of a transition within the FUT between the SCF and the HCF (block 902). Using the determined position (from block 902), the time delay of an adjustable delay element 714 is set (block 904) and then an OTDR trace graph is generated using this delay circuit to control an optical switch 708 and selectively block (e.g. absorb, significantly attenuate or divert) a portion of the reflected light from reaching a PD 616 (block 906). Where the FUT comprises a length of SMF closest to the OTDR apparatus and a length of HCF coupled to the SMF, the portion of reflected light that is blocked (e.g. absorbed, significantly attenuated or diverted) is the initial portion of a pulse of light that is reflected by the length of SMF within the FUT. As a consequence of the blocking of the reflected light (i.e. at least a significant proportion of the reflected light) by the optical switch, the OTDR trace graph is generated (in block 906) from light that is reflected within the HCF. This has the effect that the whole dynamic range of the OTDR apparatus 700, 800 is available to measure the HCF. Where the switch allow s a small proportion of the light to pass through when in the closed state, this does not significantly impact the dynamic range that is available to measure the HCF.

[0031] The method of FIG. 9, and apparatus shown in FIGs. 7 and 8, may be used not only to block (e.g. absorb, attenuate or divert) the backscattered light from the SMF so that it is not used to generate the OTDR trace graph but also to block light from reflections caused by any other component in the FUT (or optical train including the FUT) before the HCF, such as switches, connectors, amplifiers and other optical devices.

[0032] Whilst the method of FIG. 9 describes generation of an OTDR trace graph, it will be appreciated that the method may not output the OTDR trace graph but may output data derived from the OTDR trace graph. The OTDR trace graph is a measure of the backscattered power and the method may further comprise processing this data and outputting data identifying events in the link (e g. peaks or troughs). These events may be identified by analysing the OTDR trace graphs to identify changes in the backscattered pow er. The event data that is output may identify' the position of the identified events in the FUT and / or the nature of the identified events (e.g. where the nature of the event may bedetermined based on the nature of the change in the OTDR trace graph, e g. whether it is a peak or a trough and / or based on the magnitude of the change in backscattered power).

[0033] FIG. 10 shows an example OTDR trace graph 1000 for a FUT 600 that may be generated using the method of FIG. 9 and apparatus of FIG. 7 or 8. The horizontal axis of the OTDR trace graph 1000 shows the distance along the FUT and the vertical axis shows the backscattered power. The time-gated section of the trace graph 1002 which corresponds to the length of the SMF (or total optical path length before HCF) is shown, along with the length of the HCF 1004. The trace graph 1000 shows events (e.g. peaks and troughs) 1008 due to loss and backscattering coefficients changes in the HCF. The overall dynamic range of the OTDR is shown (arrow 1010) and it can be seen that this is equal to (i.e. the same as) the usable dynamic range for HCF.

[0034] The position of the transition between the SCF and the HCF may be determined (in block 902) in one of a number of different ways. In a first example, it may be determined based on a measurement of the actual physical length of the different portions of the FUT using a device such as a tape measure or ruler. In further examples, it may be determined using an initial OTDR trace graph, as shown in FIGs. 11 and 12. Where the position of the transition is determined using an initial OTDR trace graph, this determination may be performed by delay control module 730 that receives and analyses the initial OTDR trace graph. The delay control module 730 then determines and sets the time delay (in block 904, 1204). The delay control module 730 may comprise a processor and memory arranged to store instructions that are executed by the processor in order to implement the steps of the methods of FIGs. 11 and 12 or alternatively, the delay control module 730 may comprise fixed function hardware that implements these steps.

[0035] FIG. 11 shows a second example method of performing OTDR on a FUT that comprises a portion of HCF and a portion of SCF, such as SMF (e.g. FUT 600). The method may be implemented using the OTDR apparatus 700, 800 shown in FIG. 7 or 8 and described above. The method is a variation of that shown in FIG. 9 in w hich the position of the transition within the FUT between the SCF and the HCF is determined (in block 902) by generating an initial OTDR trace graph without blocking any light from reaching the photodetector (block 1102) and then analyzing the initial OTDR trace graph to detect the position of the transition w ithin the FUT betw een the SCF and the HCF (block 1104).

[0036] During the generation of the initial OTDR trace graph (in block 1102) the optical switch 708 remains open and the drive circuitry 712 does not output a control signal. In contrast, during generation of the second OTDR trace graph (in block 906), the opticalswitch 708 is initially closed and only opens in response to a control signal that is generated by the drive circuitry 712 using the adjustable delay element 714 and output to the optical switch 708. This means that the initial OTDR trace graph (generated in block 1102) resembles that shown in FIG. 5 and the analysis (in block 1104) may use any suitable technique to detect the position of the reflection peak 506. In an example, the analysis (in block 1104) may detect the position on the initial trace graph with the maximum backscattered power and this, along with the speed of light in the solid core fiber (which may be obtained from a look-up table) may be used to set the time delay (in block 904).

[0037] In the method of FIG. 11, the initial OTDR trace graph may be generated (in block 1102) using a first pulse emitted by the light source 612 and the second OTDR trace graph may be generated (in block 906) using a second pulse emitted by the light source 612.

[0038] FIG. 12 shows a third example method of performing OTDR on a FUT that comprises a portion of HCF and a portion of SCF, such as SMF (e.g. FUT 600). The method shown in FIG. 12 is a variation of that shown in FIG. 11 and described above; ho ever, the initial OTDR trace graph is generated using an initial time delay (in block 1202) and then this time delay is updated (in block 1204) based on the position of the transition that is determined (in block 1104). The method of FIG. 12 may be particularly suited for use where there are other optical components before the HCF that have a higher reflection than the HCF-SMF interface. These higher reflection interfaces or components may prevent or interfere with the determination of the position of the SMF -HCF interface if the method of FIG. 1 1 is used.

[0039] In an example, the initial delay (as used in block 1202) may be determined and set using an estimated length of the SMF or using an approximate measurement of the length of the SMF and then the updated delay (as set in block 1204) provides a more accurate value which reflects the detected position of the transition (as determined in block 1104).

[0040] In the examples described above, the FUT 600 comprises a portion of SMF 602 and a portion of HCF 604. In other examples, the FUT may comprise multiple interleaved portions of HCF and SMF (e.g. where one or more of the portions of SMF comprise an erbium doped fiber amplifier, EDFA). In such examples, the optical switch 708 may be controlled to selectively block (e.g. by absorbing, attenuating or diverting) light that is reflected within any of the portions of SMF as well as the joins (e.g. splices or couplers) between a portion of HCF and an adjacent portion of SMF.

[0041] The methods of FIGs. 9, 11 and 12 may be implemented by a computing device that is connected to, or integrated within, the OTDR apparatus 700, 800 described above. The computing device may comprise a processor and a 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 the method of any of FIGs. 9, 11 and 12.

[0042] As described above, in order to generate an OTDR trace graph that provides a high dynamic range for the HCF, all or part of the light power that is backscattered within the SMF is blocked so that it is not incident upon the PD. This is achieved through the use of the time-gating circuit. By including an adjustable delay element, the OTDR apparatus is usable for different lengths of SMF and by using the methods described herein it is not necessary to know the length of the SMF in advance as the position of the transition between the SMF (or other SCF) and the HCF is detected and then used to set the adjustable delay element.

[0043] By using the methods described above, it is possible to perform fast, high resolution tests on HCF when installed in optical networks to identify faults and flaws.

[0044] Further aspects of the invention are set out in the following clauses:

[0045] Clause A: A method of performing optical time domain reflectometry, OTDR, on a fiber under test, FUT, that comprises a portion of hollow core fiber, HCF, and a portion of solid core fiber, SCF, the method comprising: determining a position of a transition between the SCF and the HCF (902); setting a time delay in switch control circuitry using the determined position of the transition (904, 1204); and generating an OTDR trace graph using light detected by a photodetector, wherein the time delay is used control an optical switch to block light backscattered within the SCF from reaching the photodetector (906).

[0046] Clause B: The method according to clause A, wherein the optical switch blocks light backscattered within the SCF from reaching the photodetector by attenuating, absorbing or deflecting the light backscattered within the SCF.

[0047] Clause C: The method according to clause A or B, wherein determining the position of the transition between the SCF and the HCF comprises: generating an initial OTDR trace graph without blocking any light from reaching the photodetector (1 102); and analyzing the initial OTDR trace graph to detect the position of the transition between the SCF and the HCF (1104).

[0048] Clause D: The method according to clause A or B, wherein determining the position of the transition between the SCF and the HCF comprises: generating an initialOTDR trace graph using an initial time delay to block a portion of backscattered light from the FUT from reaching the photodetector (1202); and analyzing the initial OTDR trace graph to detect the position of the transition between the SCF and the HCF (1104), and wherein setting the time delay in the switch control circuitry using the determined position of the transition comprises: updating the time delay in the switch control circuitry using the determined position of the transition (1204).

[0049] Clause E: The method according to clause C or D, wherein analyzing the initial OTDR trace graph to detect the position of the transition between the SCF and the HCF comprises: detecting a position of a maximum value in the initial OTDR trace graph.

[0050] Clause F: The method according to clause A or B, wherein determining the position of the transition between the SCF and the HCF comprises: measuring a distance between a start of the SCF and the transition between the SCF and the HCF.

[0051] Clause G: The method according to any of the preceding claims, further comprising: analyzing the OTDR trace graph to identify one or more events; and outputting data identifying events identified in the OTDR trace graph.

[0052] Clause H: An optical time domain reflectometry. OTDR, apparatus (700. 800) for analyzing hollow core fiber, HCF, the apparatus 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 couple light output by the light source into a fiber under test, FUT, and to couple light reflected within the FUT into the time-gating circuit (702, 802), wherein the FUT comprises a portion of solid core fiber, SCF, and a portion of HCF; and a photodetector (616) coupled to an output of the timegating circuit, wherein the time-gating circuit (702, 802) is configured to block light backscattered within the FUT from reaching the photodetector during a period of time after generation of a pulse of light by the light source, the period of time set by the adjustable time delay element.

[0053] Clause I: The OTDR apparatus according to clause H, wherein the optical switch is configured to block light backscattered within the FUT from reaching the photodetector during the period of time after generation of the pulse of light by the light source by attenuating, absorbing or deflecting the light.

[0054] Clause J: The OTDR apparatus according to clause H or I, wherein the optical device is an optical circulator.

[0055] Clause K: The OTDR apparatus according to any of clauses H-J, wherein the time-gating circuit additionally comprises: an optical tap coupler (704) and a secondphotodetector (706) configured to detect when the pulse of light is emitted by the light source; and switch control circuitry (710) configured to use the adjustable delay element to introduce a delay between detection of the pulse of light and output of a control signal to open the optical switch.

[0056] Clause L: The OTDR apparatus according to any of clauses H-J, wherein the time-gating circuit additionally comprises: drive circuitry (816) configured to output a control signal to the light source to trigger the output of a pulse of light; and switch control circuitry (810) configured to use the adjustable delay element to introduce a delay between triggering of the pulse of light and output of a control signal to open the optical switch.

[0057] Clause M: The OTDR apparatus according to any of clauses H-L, further comprising: a delay control module (730) configured to analyse an initial OTDR trace graph to detect a position of a transition betw een the portion of SCF and the portion of HCF in the FUT and to adjust the adjustable delay element.

[0058] Clause N: Computer executable instructions that when executed by a computing device, cause the computing device to perform the method of any of clauses A-G.

[0059] Clause O: A computer readable medium configured to store the computer executable instructions according to clause N.

[0060] Any range or device value given herein may be extended or altered without losing the effect sought, as will be apparent to the skilled person.

[0061] Embodiments of the disclosure comprise or utilize a special-purpose or general- purpose computer system that includes computer hardware, such as, for example, a processor system and system memory, as discussed in greater detail below . Embodiments within the scope of the present disclosure also include physical and other computer-readable media for carrying or storing computer-executable instructions and / or data structures. Such computer-readable media can be any available media accessible by a general-purpose or special-purpose computer system. Computer-readable media that store computer-executable instructions and / or data structures are computer storage media. Computer-readable media that carry computer-executable instructions and / or data structures are transmission media. Thus, embodiments of the disclosure can comprise at least two distinctly different kinds of computer-readable media: computer storage media and transmission media.

[0062] Computer storage media are physical storage media that store computerexecutable instructions and / or data structures. Physical storage media include 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(s) which store program code in the form of computer-executable instructions or data structures, which can be accessed and executed by a general-purpose or special-purpose computer system to implement the disclosed functionality.

[0063] Transmission media include a network and / or data links that carry program code in the form of computer-executable instructions or data structures that are accessible by a general-purpose or special-purpose computer system. A "netw ork" is defined as a data link that enables the transport of electronic data between computer systems and other electronic devices. When information is transferred or provided over a netw ork or another communications connection (either hardwired, wireless, or a combination thereof) to a computer system, the computer system may view the connection as transmission media. The scope of computer-readable media includes combinations thereof.

[0064] Upon reaching various computer system components, program code in the form of computer-executable instructions or data structures can be transferred automatically from transmission media to computer storage media (or vice versa). For example, computerexecutable instructions or data structures received over a network or data link can be buffered in RAM w ithin a netw ork interface module and eventually transferred to computer system RAM and / or less volatile computer storage media at a computer system. Thus, computer storage media can be included in computer system components that also utilize transmission media.

[0065] Computer-executable instructions comprise, for example, instructions and data which when executed at a processor system, cause a general-purpose computer system, a special-purpose computer system, or a special-purpose processing device to perform a function or group of functions. In embodiments, computer-executable instructions comprise binaries, intermediate format instructions (e.g., assembly language), or source code. In embodiments, a processor system comprises one or more central processing units (CPUs), one or more graphics processing units (GPUs), one or more neural processing units (NPUs), and the like.

[0066] Although the subject matter has been described in language specific to structural features and / or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims.

[0067] It will be understood that the benefits and advantages described above may relate to one embodiment or may relate to several embodiments. The embodiments are not limited to those that solve any or all of the stated problems or those that have any or all of the stated benefits and advantages. It will further be understood that reference to 'an' item refers to one or more of those items.

[0068] The operations of the methods described herein may be carried out in any suitable order, or simultaneously where appropriate. Additionally, individual blocks may be deleted from any of the methods without departing from the scope of the subject matter described herein. Aspects of any of the examples described above may be combined with aspects of any of the other examples described to form further examples without losing the effect sought.

[0069] The term 'comprising' is used herein to mean including the method blocks or elements identified, but that such blocks or elements do not comprise an exclusive list and a method or apparatus may contain additional blocks or elements.

[0070] It will be understood that the above description is given by way of example only and that various modifications may be made by those skilled in the art. The above specification, examples and data provide a complete description of the structure and use of exemplary embodiments. Although various embodiments have been described above with a certain degree of particularity, or with reference to one or more individual embodiments, those skilled in the art could make numerous alterations to the disclosed embodiments without departing from the scope of this specification.

Claims

CLAIMSWhat is claimed is:

1. A method of performing optical time domain reflectometry, OTDR, on a fiber under test, FUT, that comprises a portion of hollow core fiber, HCF, and a portion of solid core fiber, SCF, the method comprising: determining a position of a transition between the SCF and the HCF; setting a time delay in switch control circuitry using the determined position of the transition; and generating an OTDR trace graph using light detected by a photodetector, wherein the time delay is used control an optical switch to block light backscattered within the SCF from reaching the photodetector.

2. The method according to claim 1 , wherein the optical switch blocks light backscattered within the SCF from reaching the photodetector by attenuating, absorbing or deflecting the light backscattered within the SCF.

3. The method according to claim 1 or 2, wherein determining the position of the transition between the SCF and the HCF comprises: generating an initial OTDR trace graph without blocking any light from reaching the photodetector; and analyzing the initial OTDR trace graph to detect the position of the transition between the SCF and the HCF.

4. The method according to claim 1 or 2, wherein determining the position of the transition between the SCF and the HCF comprises: 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 analyzing the initial OTDR trace graph to detect the position of the transition between the SCF and the HCF, and wherein setting the time delay in the switch control circuitry using the determined position of the transition comprises: updating the time delay in the switch control circuitry using the determined position of the transition.

5. The method according to claim 3 or 4, wherein analyzing the initial OTDR trace graph to detect the position of the transition between the SCF and the HCF comprises: detecting a position of a maximum value in the initial OTDR trace graph.

6. The method according to claim 1 or 2, wherein determining the position of the transition between the SCF and the HCF comprises:measuring a distance between a start of the SCF and the transition between the SCF and the HCF.

7. The method according to any of the preceding claims, further comprising: analyzing the OTDR trace graph to identify one or more events; and outputting data identifying events identified in the OTDR trace graph.

8. An optical time domain reflectometry, OTDR, apparatus for analyzing hollow core fiber, HCF, the apparatus comprising: a light source; a time-gating circuit comprising an adjustable time delay element and an optical switch; an optical device configured to couple light output by the light source into a fiber under test, FUT, and to couple light reflected within the FUT into the time-gating circuit, wherein the FUT comprises a portion of solid core fiber, SCF, and a portion of HCF; and a photodetector coupled to an output of the time-gating circuit, wherein the time-gating circuit is configured to block light backscattered within the FUT from reaching the photodetector during a period of time after generation of a pulse of light by the light source, the period of time set by the adjustable time delay element.

9. The OTDR apparatus according to claim 8, wherein the optical switch is configured to block light backscattered within the FUT from reaching the photodetector during the period of time after generation of the pulse of light by the light source by attenuating, absorbing or deflecting the light.

10. The OTDR apparatus according to claim 8 or 9, wherein the optical device is an optical circulator.

11. The OTDR apparatus according to any of claims 8-10. wherein the timegating circuit additionally comprises: an optical tap coupler and a second photodetector configured to detect when the pulse of light is emitted by the light source; and switch control circuitry configured to use the adjustable delay element to introduce a delay between detection of the pulse of light and output of a control signal to open the optical switch.

12. The OTDR apparatus according to any of claims 8-10, wherein the timegating circuit additionally comprises: drive circuitry configured to output a control signal to the light source to trigger the output of a pulse of light; andswitch control circuitry configured to use the adjustable delay element to introduce a delay between triggering of the pulse of light and output of a control signal to open the optical switch.

13. The OTDR apparatus according to any of claims 8-12, further comprising: a delay control module configured to analy se an initial OTDR trace graph to detect a position of a transition between the portion of SCF and the portion of HCF in the FUT and to adjust the adjustable delay element.

14. Computer executable instructions that when executed by a computing device, cause the computing device to perform the method of any of claims 1-7.

15. A computer readable medium configured to store the computer executable instructions according to claim 14.

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