WO2025151134A1

WO2025151134A1 - Miniature broadband partially coupled directional optical coupler for optical frequency domain reflectometry systems

Info

Publication number: WO2025151134A1
Application number: PCT/US2024/015476
Authority: WO
Inventors: Daniel Kominsky; Tyler GORNEY; Doug HOGAN; Steven Derek ROUNTREE; Stephen Kreger
Original assignee: Luna Innovations Inc
Current assignee: Luna Innovations Inc
Priority date: 2024-01-09
Filing date: 2024-02-13
Publication date: 2025-07-17
Anticipated expiration: 2026-07-09

Abstract

A broadband, partially coupled, directional optical coupler includes an input fiber configured to receive light tuned over a broadband range of frequencies from an optical frequency domain reflectometry (OFDR) instrument, an output fiber configured to return light to the OFDR instrument, and an optical splitter. The optical splitter directs a first portion of the light from the input fiber to a measurement fiber and diverts a second substantial portion of the light from the input fiber to an optical combiner that combines returning light from the measurement fiber with the second substantial portion of light. A non-reciprocal optical element sends a substantial portion of the measurement light or a substantial portion of the combined light to the output fiber. The input fiber, the output fiber, the optical splitter, the optical combiner, and the non-reciprocal optical element are arranged to be substantially colinear in the broadband, partially coupled, directional optical coupler.

Description

MINIATURE BROADBAND PARTIALLY COUPLED DIRECTIONAL OPTICAL COUPLER FOR OPTICAL FREQUENCY DOMAIN REFLECTOMETRY SYSTEMS

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims priority from U.S. provisional patent application serial number 63/618,969, filed January 9, 2024, the contents of which are incorporated herein by reference.

INTRODUCTION

[0002] Optical fibers are used in many applications to sense a variety of parameters like strain, temperature, pressure, position, orientation, fatigue, impending structural failure, etc. by modifying a fiber so that the quantity to be measured modulates the intensity, phase, polarization, wavelength, or transit time of light in the fiber. Optical fibers have many uses in remote sensing. Depending on the application, fiber may be used because of its small size, or because no electrical power is needed at the remote location, or because many sensors can be multiplexed along the length of a fiber by using light wavelength shift for each sensor, or by sensing the time delay as light passes along the fiber through each sensor. Time delay can be determined using an instrument like an optical time domain reflectometer (OTDR), and wavelength shift can be calculated using an instrument implementing optical frequency domain reflectometry (OFDR), or spectrometry.

[0003] In some OFDR systems, a remote module is used that includes multiple fiber coupled passive components spliced together and can be flexibly located at different distances from the actual OFDR instrument. This flexibility of a remote module can reduce or eliminate the sensitivity of OFDR measurements to perturbations in the connecting cable between the OFDR instrument and the remote module. A limiting factor for remote modules is the size of various fiber loops required to connect the remote module optical components to each other while not violating constraints on the allowable or minimum radius of curvature for each fiber loop. That factor limits how small remote modules can be manufactured.

[0004] For purposes of this application, such a remote module is referred to as a broadband, partially coupled, directional optical coupler. [0005] It would be advantageous to reduce the size of a broadband, partially coupled, directional optical coupler by avoiding the physical constraints of fiber loops, fiber optic connectors, and fiber optic splices.

SUMMARY

[0006] In certain example embodiments, a broadband, partially coupled, directional optical coupler includes an input fiber configured to receive light tuned over a range of frequencies from an optical frequency domain reflectometry (OFDR) instrument and an output fiber configured to return light to the OFDR instrument. An optical splitter is configured to direct a first portion of the light from the input fiber to a measurement fiber and divert a second substantial portion of the light from the input fiber to an optical combiner configured to combine returning light from the measurement fiber with the second substantial portion of light. A nonreciprocal optical element is configured to send a substantial portion of the measurement light or a substantial portion of the combined light to the output fiber. The input fiber, the output fiber, the optical splitter, the optical combiner, and the non-rcciprocal optical clement arc arranged to be substantially colinear in the broadband, partially coupled, directional optical coupler.

[0007] In certain example embodiments, the broadband, partially coupled, directional optical coupler is a single integrated component.

[0008] In certain example embodiments, the broadband, partially coupled, directional optical coupler is 1 cm or less in diameter by 5 cm or less in length, thereby substantially reducing its size.

[0009] In certain example embodiments, the optical splitter, the optical combiner, and the non-reciprocal optical element comprise a 3-port optical device with a first port coupled to the input fiber, a second port coupled to the measurement fiber, and a third port coupled to the output fiber.

[0010] Certain example embodiments include a first collimator for the first port, a second collimator for the second port, and a third collimator for the third port.

[0011] In certain example embodiments, the 3-port optical device includes a partial optical reflector configured to reflect the second substantial portion of light to the third port and transmit the first portion of the light to the second port. The partial optical reflector may, for example, include a partially reflecting mirror having a reflectivity in a range of 42%-35%. [0012] Alternatively, the partial optical reflector includes a beam splitter (BS) having a first face oriented towards the first port and the third port, a second face having a substantially reflective surface, and a third face through which light from the first port is coupled to the second port and reflected from the measurement fiber received at the second port is coupled to the third port. A fourth side of the BS may have an absorbing or non-reflective coating to suppress unwanted light paths.

[0013] In other alternative example embodiments, the partial optical reflector includes a first beam splitter (BS) arranged between the first port and the non-reciprocal optical element and a second BS arranged between the third port and the non-reciprocal optical element. The first BS is arranged to direct the first portion of the light from the input fiber to the non- reciprocal optical element and to divert the second substantial portion of the light from the input fiber to the second BS. The second BS is arranged to receive the returning light from the measurement fiber from the second port through the non-reciprocal optical element, combine the returning light with the second substantial portion of the light from the first BS, and direct the combined light towards the third port. A coating on a face of the second BS may be used to prevent or reduce reflection from the face. The first BS and the second BS may be fiber optic splitters, and the second substantial portion of light from the first BS is coupled to the second port of the second BS through optical fibers which are joined by a micro turnaround device.

[0014] In certain example embodiments, the optical splitter includes a 1x2 optical splitter coupled at one end to the second port and coupled to a first leg at the other end to the measurement fiber and to a second leg at the other end to an optical reflector.

[0015] Certain example embodiments further include a micro-turnaround device, an optical isolator, a 3x3 optical splitter, and a fiber loop. The 3x3 optical splitter includes three instrument side legs on an OFDR instrument side and three measurement side legs on a measurement side. The 3x3 optical splitter is arranged to receive at a first instrument side leg of the 3x3 optical splitter OFDR instrument light from the input fiber, direct the first portion of light from a first measurement side leg to the measurement fiber, and direct the second substantial portion of light and the third substantial portion of light from a second measurement side leg to the fiber loop. The fiber loop includes the micro turnaround to connect the second measurement side leg and a third measurement side leg together, and the optical isolator to restrict light to propagate around the fiber loop in a direction from the second measurement side leg to the third measurement side leg. The 3x3 optical splitter is arranged to transmit the second substantial portion of the light received from the isolator and the first substantial portion of the light received from the measurement fiber to a second instrument side leg of the 3x3 optical splitter couple to the output fiber. A third instrument side leg may be connected to a termination to prevent reflections of light in the third instrument side leg from returning to the 3x3 optical splitter.

[0016] In certain example embodiments, the optical splitter includes a micro-turnaround device and a first 2x2 optical splitter, the optical combiner includes a second 2x2 optical splitter, and the first 2x2 optical splitter is arranged to receive at a first instrument side leg of the first 2x2 optical splitter light from the input fiber, direct the first portion of light from a first measurement side leg of the first 2x2 optical splitter to the measurement fiber, and direct the second substantial portion of the light from a second output leg of the first 2x2 optical splitter to the microturnaround device. The micro-turnaround device is arranged to receive the second substantial portion of the light from the second output leg of the first 2x2 optical splitter and return a majority of the second substantial portion of the light to a first measurement side leg of the second 2x2 optical splitter. A second instrument side leg of the first 2x2 optical splitter is arranged to transmit returning light from the measurement fiber to a second measurement side leg of the second 2x2 optical splitter where the returning light from the measurement fiber is combined with the second substantial portion of light received at the first measurement side leg of the second 2x2 optical splitter. A first instrument side leg of the second 2x2 splitter is arranged to send the combined light to the output fiber. A second instrument side leg of the second 2x2 splitter may be connected to a termination to prevent light from reflecting back to the second 2x2 splitter.

[0017] In certain example embodiments, a broadband, partially coupled, directional optical coupler comprises an input fiber arranged to receive input light tuned over a broadband range of frequencies from an optical frequency domain reflectometry (OFDR) instrument, an output fiber configured to return light to the OFDR instrument, a first beam splitter (BS), and a second BS. The first BS is arranged to split light from the input fiber into measurement path light and reference path light. The second BS is arranged to direct a substantial portion of the measurement path light from the first BS into a measurement fiber, combine the reference path light from the first BS with light returned from the measurement fiber; and provide a substantial portion of the combined light to the output fiber. A further example embodiment further includes a first prism and a second prism. The first prism is arranged to direct the reference path light to a second prism, and the second prism is arranged to direct the reference path light to the second BS for combining with the light returned from the measurement fiber. The first prism, the second prism, the first BS, and the second BS are, in example embodiments, micro-optical components that are not fiber coupled. Further example embodiment further includes a first collimator coupled between the input fiber and the first BS, a second collimator coupled between the second PS and the measurement fiber, and a third collimator coupled between the output fiber and the second BS. Absorptive or non-reflective material may also be applied to one or more faces of the first BS and to one or more faces of the second BS to suppress unwanted light paths.

[0018] Certain example embodiments include an optical frequency domain reflectometry (OFDR) system comprising an OFDR instrument including a tunable light source and light detectors, and a broadband, partially coupled, directional optical coupler. The broadband, partially coupled, directional optical coupler includes an input fiber configured to receive light tuned over a broadband range of frequencies from an optical frequency domain reflectometry (OFDR) instrument and an output fiber configured to return light to the OFDR instrument. An optical splitter is configured to direct a first portion of the light from the input fiber to a measurement fiber and divert a second substantial portion of the light from the input fiber to an optical combiner configured to combine returning light from the measurement fiber with the second substantial portion of light. A non-reciprocal optical element is configured to send a substantial portion of the measurement light or a substantial portion of the combined light to the output fiber. The input fiber, the output fiber, the optical splitter, the optical combiner, and the non-reciprocal optical element are arranged to be substantially colinear in the broadband, partially coupled, directional optical coupler.

[0019] This Summary is provided to introduce a selection of concepts that are further described below in the Detailed Description. This Summary is intended neither to identify key features or essential features of the claimed subject matter, nor to be used to limit the scope of the claimed subject matter; rather, this Summary is intended to provide an overview of the subject matter described in this document. Accordingly, it will be appreciated that the abovedescribed features are merely examples, and that other features, aspects, and advantages of the subject matter described herein will become apparent from the following Detailed Description, Figures, and Claims.

BRIEF DESCRIPTION OF THE FIGURES

[0020] These and other features and advantages will be better and more completely understood by referring to the following detailed description of example non-limiting illustrative embodiments in conjunction with the drawings. Unless specific dimensions are given in a figure, the figures are not necessarily to scale and are schematic representations.

[0021] Figure 1 shows an example optical frequency domain reflectometry (OFDR) system.

[0022] Figure 2 shows an example broadband, partially coupled, directional optical coupler and light paths therein.

[0023] Figure 3A illustrates a top view of a known broadband, partially coupled, directional optical coupler including example dimensions.

[0024] Figure 3B illustrates a side view of the broadband, partially coupled, directional optical coupler shown in Figure 3A including example dimensions.

[0025] Figure 4 shows an example embodiment of a broadband, partially coupled, directional optical coupler that includes a 3-port optical device with a partial reflector, where a reference light path is within the broadband, partially coupled, directional optical coupler itself which eliminates the need for the input and output fiber splitters shown in Figure 2.

[0026] Figure 5 shows an example embodiment of a broadband, partially coupled, directional optical coupler that includes a 3-port optical device with a non-polarizing beam splitter (NPBS).

[0027] Figure 6 shows an example embodiment of a broadband, partially coupled, directional optical coupler that includes a 3-port optical device with two non-polarizing beam splitters (NPBSs).

[0028] Figure 7 shows an example embodiment of a broadband, partially coupled, directional optical coupler that includes a 3-port optical circulator and a 1x2 optical splitter. [0029] Figure 8 shows an example embodiment of a broadband, partially coupled, directional optical coupler that includes a 3-port optical circulator and a micro-turnaround device to eliminate fiber loops. [0030] Figure 9 shows an example embodiment of a broadband, partially coupled, directional optical coupler that includes a 3x3 optical splitter, an optical isolator, and a microturnaround device.

[0031] Figure 10 shows an example embodiment of a broadband, partially coupled, directional optical coupler that includes first and second 2x2 optical splitters and a microturnaround device.

[0032] Figure 11 A shows an expanded view of an example micro-optic components embodiment of a broadband, partially coupled, directional optical coupler that includes two beam splitters (BSs) with two prisms.

[0033] Figure 1 IB shows an expanded view of another example micro-optic components embodiment of a broadband, partially coupled, directional optical coupler that includes two BSs with two prisms.

[0034] Figure 11C shows a compact view of the example embodiment shown in Figure 1 IB of a broadband, partially coupled, directional optical coupler as assembled that includes two BSs with two prisms.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

[0035] Specific embodiments are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description are not intended to limit the claims to the particular embodiments disclosed, even where only a single embodiment is described with respect to a particular feature. On the contrary, the intention is to cover all modifications, equivalents and alternatives that would be apparent to a person skilled in the art having the benefit of this disclosure. Examples of features provided in the disclosure are intended to be illustrative rather than restrictive unless stated otherwise.

[0036] Terms, such as first, second, and the like, may be used herein to describe various components. Each of these terminologies is not used to define an essence, order or sequence of a corresponding component but used merely to distinguish the corresponding component from other component(s). For example, a "first" component may be referred to as a "second" component, and similarly, the "second" component may be referred to as the "first" component. “Based on” as used herein covers based at least on. As used throughout this application, the word “may” is used in a permissive sense (i.e., meaning having the potential to), rather than the mandatory sense (i.e., meaning must). The singular forms "a", "an", and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises/comprising" and/or "includes/including" when used herein, specify the presence of stated features, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components and/or groups thereof.

[0037] The same name may be used to describe an element included in the embodiments described above and an element having a common function. Once a component or function is described for one embodiment, that description is not repeated for other embodiments where that component or function operates or performs similarly. Unless disclosed to the contrary, a configuration of components disclosed in any embodiment may be applied to other embodiments, and the specific description of the repeated configuration will be omitted.

[0038] Unless otherwise defined, all terms used herein including technical or scientific terms have the same meaning as commonly understood by one of ordinary skill in the art to which an example belongs. Terms, such as those defined in commonly used dictionaries, are to be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and are not to be interpreted in an idealized or overly formal sense unless expressly so defined herein.

[0039] Definitions are provided for some terms used in this application.

[0040] Broadband means operable over a broad range of wavelengths. For example, broadband may be used to define a range of wavelengths sufficient to discern an interference pattern on the spectrum of the resultant signal.

[0041] Partially-coupled means that part of the input light is sent on a measurement path to a system or device under test, and part of the input light is redirected to a reference path.

[0042] A collimator is a device which converts a diverging beam of light into one in which the light is propagating in a beam of substantially constant diameter, and which converts an incoming beam of light with constant diameter into one which is focused to a small point. Collimators may be used to couple light into and out of optical fibers.

[0043] An optical splitter is a component which receives light enters from one or more fibers on one side of the splitter and distributes portions of that light among two or more fibers on the other side. The number of input and output fibers do not have to match. [0044] A 3-port optical device is a device in which there are three distinct pathways by which light can enter and/or exit the device. Examples of 3-port optical devices include optical circulators, optical filters, optical splitters, and optical switches. A 3-port optical circulator is a device in which light which enters on port 1 is directed to port 2, light which enters on port 2 is directed to port 3. One or more non-reciprocal elements are provided in a 3-port optical circulator to direct light in just one direction along a path; light does not propagate in a reverse direction along the path. In other words, if light is subsequently reflected back into port 2, it does not retrace its path back to port 1, but exits through port 3 instead. Example non-reciprocal elements include one or more beam displacers (e.g., birefringent crystal made, for example, of yttrium orthovanadate (YVO4), rutile or barium borate (a-BBO)), one or more nonreciprocal Faraday rotators made, for example, of yttrium-iron-gamet (YIG), or Bi-added thick film crystals, and one or more beam angle turners such as, for example, birefringent wedges like Rochon prisms and Wollaston prisms, to change a beam propagation direction. Non-reciprocal elements provide the directionality in a broadband, partially coupled, directional optical coupler. An example 3-port circulator with non-reciprocal elements is described in U.S. patent 5,909,310. A 3-port circulator can be either polarization independent or polarization maintaining.

[0045] A beam splitter (BS) is an optical element that splits the light which enters the element into two output paths. A common embodiment of a BS is a cube-shaped component in which a portion of light arriving on any of the four optically active faces of the cube propagates straight through the BS, and the remainder of that light is turned by 90° and then propagates out of the adjacent face. Examples of beam splitters include a polarizing beam splitter (PBS) and a non-polarizing beam splitter (NPBS). A PBS is an optical device in which input light is separated into two orthogonal polarization states and the light of each polarization state exits through a different output face. A NPBS is an optical device which separates the input light into two outputs which are comprised of whatever polarization state the input light possessed.

[0046] A micro-turnaround device is an optical element which uses extreme tapering of an optical fiber and air cladding to allow the fiber to bend in far tighter bends than is achievable in any normal fiber without loss of the light.

[0047] Figure 1 shows an example optical frequency domain reflectometry (OFDR) system 10. Light from an OFDR instrument 12 travels out an input fiber 18 to a known broadband, partially coupled, directional optical coupler 20, which sends measurement light over a measurement fiber 22 corresponding to a sensor (sometimes called a sensor or a device under test (DUT)). The sensor 22 alters the measurement light which is returned to the OFDR instrument 12 by the known broadband, partially coupled, directional optical coupler 20 over an output fiber 24 for detecting, processing, and output of one or more output parameters, e.g., strain, temperature, position, etc., associated with the sensor environment.

[0048] The known broadband, partially coupled, directional optical coupler 20 splits the light from a tunable light source 14, e.g., a tunable laser, in the OFDR instrument 12 into two substantial portions including reference light and measurement light. The broadband, partially coupled, directional optical coupler 20 directs the measurement light to the sensor 22, and then combines reflected measurement light from the sensor 22 with the reference light. The reference path has a nominally constant length from the point where the input light is split to the point where it is combined with the measurement light returning from the sensor 22. The combined light contains an interference signal that is detected by optical detector(s) 16 in the OFDR instrument 12 and then processed, e.g., further optical processing, electronic processing, and computational processing, to extract the measurements of interest.

[0049] Figure 2 shows a known broadband, partially coupled, directional optical coupler 20 along with measurement and reference light paths. The input fiber 18 connects to an input optical splitter 26 which splits the light from the OFDR instrument 12 into measurement and reference light as shown. The measurement light is provided to a first port 1 on one side of a 3- port optical circulator 28 which also includes a second port 2 at an opposite side and coupled to the measurement fiber sensor 22. The 3-port optical circulator 28 includes a third port 3 also on the same side as the first port 1. As described above, measurement light from the input splitter 26 enters port 1 and propagates via non-reciprocal optical elements within the 3-port optical circulator 28 to port 2 where it enters the measurement fiber 22. Reflected measurement light returns to port 2 and propagates via non-reciprocal optical elements within the 3-port optical circulator 28 to port 3 and it is combined at the output splitter 32 with the reference light from the input splitter 26. As described above, one or more non-reciprocal optical directional elements direct received light in path a one direction but not in a path in the opposite direction. In this case, non-reciprocal optical directional elements in the 3-port optical circulator 28 direct input light from port 1 to port 2, and direct reflected measurement light received at port 2 to port 3; however, light does not propagate in the reverse path. [0050] The known broadband, partially coupled, directional optical coupler 20 is typically fabricated by fusion splicing together the components shown in Figure 2 and packaging them in a box. As described in the introduction section above, a limiting factor is the size required for the fiber to connect the components to each other without violating constraints on the allowable radius of curvature of the optical fiber. Figure 3A illustrates a top view of a known broadband, partially coupled, directional optical coupler including example dimensions, and Figure 3B illustrates a side view of the broadband, partially coupled, directional optical coupler shown in Figure 3 A including example dimensions. As shown in this example, the fiber must maintain a radius of curvature of not less than 2.5cm. The solid line indicates the fiber path used to connect two components solely in a clockwise fashion or in a counterclockwise fashion. The dotted path illustrates where a fiber path used to connect two components needs to switch between clockwise and counterclockwise directions. This mandates at least a 10cm length and at least a 6 cm width due to the need for a pair of 2.5cm radius fiber bends. Figures 3A and 3B show a minimum height of at least 1cm. Although there are multiple ways to orient the two splitters 26 and 32 and the 3-port optical circulator 28, the loops of fiber to connect them requires some form of fiber direction reversal to be used. The size dimensions will increase further if further devices such as fiber optic connectors or splices need to be accommodated.

[0051] The inventors in this application invented structures and approaches that eliminate the need for the input splitter 26 and the output splitter 32 shown in the known broadband, partially coupled, directional optical coupler 20 of Figure 2 and that achieve a much smaller broadband, partially coupled, directional optical coupler. In some example embodiments, the entire broadband, partially coupled, directional optical coupler can be achieved in a single cylindrical integrated component measuring 1 cm or less in diameter and 5 cm or less in length, e.g., 0.3cm in diameter by 4cm in length or even smaller. The resulting smaller volume and cross-sectional area save space, increase options for installation, and reduce cost compared to known couplers like that in Figure 2. Another advantage is that a simpler and less costly assembly procedure can be used to construct a broadband, partially coupled, directional optical coupler with, e.g., fewer parts, fewer labor hours, and less skilled technicians. The smaller broadband, partially coupled, directional optical couplers in accordance with example embodiments also reduce path length through the interferometer which is advantageous for low coherence length OFDR. [0052] Figure 4 shows an example embodiment of a new broadband, partially coupled, directional optical coupler 41 that includes a 3-port optical device 43 with a partial reflector (PR) 36 located near port 2. Input light from the OFDR instrument 12 is received at port 1 of the 3- port optical device 43 and collimated in a first collimator (C) before being directed by one or more non-reciprocal elements contained in the 3-port optical device 43 to a partial reflector (PR) 36 which reflects a substantial portion of the input light as reference light along a reference path to port 3 of the 3-port optical device 43. Advantageously, this reference path light (dashed line) is contained within the 3-port optical device 43 itself which eliminates the need for the input and output fiber splitters 26 and 32 shown in Figure 2. Another substantial portion of the input light is not reflected by the partial reflector 36 but instead passes through to a second collimator (C) corresponding to port 2 and through port 2 onto measurement fiber 22 coupled to a DUT. The measurement light (dotted line) continues to the sensing fiber (optionally through one or more fiber connectors) and is reflected by the DUT and returned as represented by the larger curved arrow pointing left. When that returning light reaches the partial reflector from port 2 of the 3- port optical device 43 and the second collimator, some smaller amount of light will be reflected back towards the DUT as represented by the smaller curved arrow pointing right, but most will be passed through the partial reflector 36 and be directed by the one or more non-reciprocal elements in the 3-port optical device 43 to a third collimator (C) corresponding to port 3 where the reference path light and the measurement path light are combined to form the combined (interference) signal sent from port 3 via the output fiber to the OFDR instrument 12.

[0053] An example reflectivity of the partial reflector 36 may be determined so that the amount of light on the reference path approximately equals the amount of light in the measurement path. If the partial reflector 36 returns X% of the light along a reciprocal path, then the light which comprises the interference pattern for the combined light includes X% reference light and (1-X)²% measurement light (because the light on the measurement path is transmitted through the partial reflector 36 twice — once on the outward direction and once on the return direction. One example way of setting a balance between the amount of light on the reference path so that it approximately equals the amount of light in the measurement path is when X = (1 — X)², which is a reflectivity of approximately 38%. About half of the returning measurement light reflections may be lost being reflected back to the DUT by the partial reflector 36. Other different reflectivity values may be used. [0054] Multipath reflections in the measurement path may be accounted for in some example embodiments. The partial reflector 36 reflecting a portion of the reflected DUT light back into the DUT can lead to a secondary reflection appearing at twice the delay of all primary reflections. For Rayleigh scatter sensing, secondary reflection can be reduced or minimized. One example technique is to set a maximum strength of the primary reflection from the DUT, which reduces the impact of the secondary light which reflects twice from the DUT. For example, if partial reflector 36 is set to reflect 50% of the light, and the DUT is specified to 1 allow no more than ^^of the light entering it to be reflected, then the secondary light reflection is (0.5) * * 0.5 * * (0.5) = 0.125 parts per million . Reading the equation from left to right, the terms represent a portion of the light making it through the partial reflector 36, a maximum allowable DUT reflection, a portion of the light bouncing off the partial reflector 36, another instance of the maximum allowable reflection off the DUT, and a portion of the light which has been returned twice by the DUT that makes it through the partial reflector 36.

[0055] Another example technique to account for multipath reflections in the measurement path is to reduce or minimize a fiber distance or length L between the partial reflector 36 and an optical fiber connector (represented in Figure 4 as a point labeled LL) leading to the DUT measurement sensor 22 and providing at least that fiber distance or length L beyond that optical connector before the start of the measurement sensor 22. The optical fiber connector is optional because the DUT fiber may be spliced directly to the broadband, partially coupled, directional optical coupler 41 or the output fiber at port 2 could be used as the DUT. This technique keeps the secondary reflection light at a fiber position that does not overlap with the measurement sensor 22. If the distance from the partial reflector 36 to the optical connector is L, that indicates in example embodiments not using the first length L of the measurement fiber on the DUT side of the optical connector as the sensor. Brighter optical connector reflections may require the distance to be larger, e.g., an n*L distance, where n is the number of multipath reflections at or near a Rayleigh scatter signal level, defined as the strength of the desired reflection from the DUT.

[0056] As mentioned above, example embodiments in accordance with Figure 4 may be implemented without such an optical connector. For example, the broadband, partially coupled, directional optical coupler 41 may be spliced to the DUT directly to the fiber from port 2, a useful approach for low coherence length OFDR systems. [0057] Figure 5 shows an example embodiment of a broadband, partially coupled, directional optical coupler 51 that includes a 3-port optical device 53 with a non-polarizing beam splitter (NPBS) near port 2. Other types of beam splitters (BSs) may be used in place of NPBSs. An advantage of this example embodiment as compared to the embodiment in Figure 4 is that multipath reflections are eliminated. The partial reflector in the 3-port optical device 53 in this embodiment is a NPBS 38 which locates the reference path reflection from mirror (40) along a separate optical path, rather than an inline one as is the case with the partial reflector 36 in the 3- poil optical device 53 of Figure 4. A mirror (Mir) 40 (or other light reflector) located on a face of the NPBS 38 reflects the reference light back into the NPBS 40 for coupling back into nonreciprocal elements 34 of the 3-port optical device 53. A light absorber (Abs) 42 located on an opposing face of the NPBS 38 takes the light from the DUT that is reflected by the NPBS 38, and the portion of the light from mirror 40 that is transmitted by the NPBS 38, and captures it to prevent it from propagating back into the DUT, and in this way, prevents multiple reflections which could otherwise create false content in the measurement results. The mirror/reflector 40 and the absorber 42 may be coatings applied to the faces of the NPBS 38. Examples of mirror type coatings include evaporated metals or dielectric stacks. Examples of light absorbing type coatings include nanostructured materials, absorptive glasses. A similar result can be achieved through any other mechanism that prevents the reflection of the light striking the region 42 from propagating back along the incoming path.

[0058] The path length L between the mirror 40 and the optical connector LL in the example embodiment of Figure 5 is very short, which prevents light returning from the sensor from forming unwanted reflection paths with the mirror 40 and avoids reflections from nearby fiber Rayleigh scatter. Some example implementations may benefit by ensuring that reflections from the NPBS face which are normal to the beam paths, i.e., the vertical sides of NPBS 38 as shown in Figure 5, do not generate unwanted reflections, which can create multiple reference paths. This can be accomplished, for example, by applying an antireflective coating on nonreciprocal elements 34 of the 3-port optical device 53 and the collimators C or by angling the NPBS faces slightly so that any light which is reflected from those faces misses the light path required to couple into port 3.

[0059] Figure 6 shows an example embodiment of a broadband, partially coupled, directional optical coupler 61 that includes a 3-port optical device 63 with a first non-polarizing beam splitter (NPBS) 44 near port 1 and a second non-polarizing beam splitter (NPBS) 46 near port 3. Other types of beam splitters (BSs) may be used in place of NPBSs. The prisms in the first and second NPBSs 44, 46 directly couple the reference light from port 1 to port 3 of the 3- port optical device 63 without entering or propagating in the reciprocal elements 34. An advantage of this example embodiment as compared to the example embodiment in Figure 5 is that sensitivities which might result from having the directional element 34 in the reference path are eliminated. Similar to Figure 5, a light absorbing coating may be applied to unused faces of the first NPBS44 or the second NPBS 46 to absorb light from the DUT to prevent it from propagating back into the DUT, and in this way, prevents multiple reflections. Similar to the example in Figure 5, an equivalent function can be achieved through the use of anti-reflection coatings or angled faces, rather than absorbers.

[0060] Figure 7 shows an example embodiment of a broadband, partially coupled, directional optical coupler 71 that includes a 3-port optical circulator 28 and a 1x2 optical splitter 50. The 1x2 splitter may alternatively be implemented using a 2x2 splitter with one leg terminated. Light from port 1 is coupled to port 2 and then from port 2 via a fiber splice to the 1x2 splitter 50 which splits the light into the reference light and the measurement light. The reference light travels from the splitter 50 to a mirror (M) 40 which reflects the reference light back through the splitter 50. The mirror 40 may be for example a Faraday rotation mirror, a simple mirror coating on the end of a fiber, e.g., evaporated gold, or other type of mirror or reflector. The measurement light from the DUT is combined with the reference light in the 1 2 splitter 50 and the combined interference light is coupled to port 2 of the 3-port optical circulator 28 for output at port 3. Although this example embodiment is larger than the example embodiments of Figures 4-6, it allows all of the components of this broadband, partially coupled, directional optical coupler 71 to be in a single line, and like the example embodiments in Figures 4-6, no loops of fiber are required. However, unlike the example embodiments in Figures 4-6 this example embodiment does include a section of fiber between the individual optical elements 28, 50, 40. As a result, the broadband, partially coupled, directional optical coupler 71 can be contained in a long narrow enclosure or container which is advantageous in some applications, e.g., such as when the access to the point of measurement requires passing through a small hole. [0061] The 3-port optical circulator 28 in the broadband, partially coupled, directional optical coupler 71 of Figure 7 may have some non-zero level of light reflection when incident light arrives at port 2 referred to as “return loss.” A portion of that return loss light may then pass through the 1x2 splitter 50 to the mirror 40 again and return to form a second reference reflection to the measurement light from the DUT. The 1x2 splitter 50 functions as an interferometer mixing point. An interferometer measures the difference in the path length between a reflection on the reference arm and a reflection on a measurement arm with no regard to which of those two paths is longer. If the distance to the mirror 40 and to the start of the DUT at LL are the same, then that is the point in the measurement fiber where there is zero path difference. Moving away from LL either towards or away from the splitter 50 steadily results in higher interference pattern beat frequencies.

[0062] Raleigh scatter reflecting from the fiber between the mirror 40 and the 3-port optical circulator 28 may overlap with the measurement light from the sensor path Rayleigh scatter. Keeping the length of the reference light path between the splitter 50 and the mirror 40 shorter than the length L of the measurement light path between the optical connector LL and the splitter 50 avoids issues associated with this extra Rayleigh scatter signal overlapping that of the DUT/sensor because the interference pattern is defined by the difference in the optical length traveled by the light on the two paths. By keeping the fiber length to the mirror 40 very close to the length L to the optical connector LL, the reflections will be at nearly zero length difference, as opposed to the DUT which would all have larger than zero length.

[0063] Figure 8 shows an example embodiment of a broadband, partially coupled, directional optical coupler 81 that includes a 3-port optical circulator 28 and a micro-turnaround device 52. A first splitter 26, e.g., 2x2 fused tapered coupler, is used to split light from the input path on its way to port 1 of the 3-port optical circulator 28, and the direction of the other split portion of that light is reversed by the micro-turnaround device 52 and combined with light from the DUT sensor exiting port 3 of the 3-port optical circulator 28, in a second splitter 32, e.g., a 2x2 fused tapered coupler. As with the example embodiment shown in Figure 7, even though the example embodiment shown in Figure 8 requires multiple components, all of the components may be contained in a long, thin containing structure which is advantageous in some applications as described above. An example commercially available micro-turnaround device described at https://www.aflglobal.com/Products/Fiber-Optic-Cable/Harsh- Environment/Downhole/MiniBend_for_Downhole_Double-Ended_Systems Optic. aspx has a package size of 2.2mm in diameter by 15mm long. This size allows all fibers to avoid the need for a loop to reverse direction but preserves a fixed length of the fiber paths. It is desirable that the length of the reference arm docs not change because a change in the length of the reference arm results in measurements being reported at the wrong place along the DUT. One example container for the broadband, partially coupled, directional optical coupler 81 includes all of its components in a cylindrical container about 7mm in diameter and 15cm in length.

[0064] Figure 9 shows an example embodiment of a broadband, partially coupled, directional optical coupler 91 that includes a 3x3 optical splitter 56, an optical isolator 54, and a micro-turnaround device 52. This example embodiment reduces component count and can be housed in a very small cross-sectional area container structure. The 3x3 splitter 56 interfaces with the OFDR instrument 12 with a first leg coupled to the input fiber, a second leg coupled to the output fiber, and a third leg being terminated at 58 in such a way as to prevent reflections back towards the splitter 56. The reference path is formed by connecting the micro-turnaround 52 between two of the three output legs of the 3x3 splitter 56. An optical isolator 54 which only allows light to pass through it in one direction is located in the reference path to avoid getting a double reference path which would adversely impact the accuracy of the interference measurements. The isolator 54 may be located on either of the reference path legs between the 3x3 splitter 56 and the micro-turnaround 52, and it may face the direction shown by the bold arrow or in the opposite direction. The reference path in this example embodiment follows a path in transmission (see the arrows in Figure 9), which is less prone to generating unwanted paths through the network than a path in which the light is reflected back along the same path, as shown in Figure 7. One example container for the broadband, partially coupled, directional optical coupler 91 includes all of its components in a cylindrical container about 7mm in diameter and 10cm in length.

[0065] Figure 10 shows an example embodiment of a broadband, partially coupled, directional optical coupler 101 that includes a first 2x2 optical splitter 60, a second 2x2 optical splitter 62, and a micro-turnaround device 52. The second 2x2 optical splitter 62 receives the input light from the OFDR instrument 12 and splits it so that a substantial portion goes to the measurement path and DUT and a substantial portion goes to the micro-turnaround device 52 which directs the reference light to the first 2x2 splitter 60. The reflected measurement light from the DUT passes through the second 2x2 splitter 62 and is combined with the reference path light in the first 2x2 splitter 60, which directs the combined interference light to the output fiber coupled to the OFDR instrument 12. The other leg of that side of the first 2x2 splitter 60 is terminated, c.g., as described above for Figure 9. Alternatively, the first 2x2 splitter 60 may be replaced with a 2x1 splitter.

[0066] The broadband, partially coupled, directional optical coupler 101 may be more power efficient than the broadband, partially coupled, directional optical coupler 91, since the measurement light makes three passes through 50:50 splitters, resulting in 12.5% of light being retained. The broadband, partially coupled, directional optical coupler 91 includes two passes through a 3-way splitter, resulting in the loss of 66% of the light on each pass, with a total of a little less than 11% of light retained. However, the broadband, partially coupled, directional optical coupler 101 includes an additional splitter (two 2x2 splitters 60 and 62) instead of the single 3x3 splitter 56 in Figure 9. Another advantage as compared to the embodiment in Figure 9 is that the reference light for the optical coupler 101 does not pass back through the second 2x2 splitter 62, and therefore, an isolator is not needed as it is in Figure 9.

[0067] Figure 11 A shows an expanded view of an example micro-optic component embodiment of a broadband, partially coupled, directional optical coupler 111 that includes a first BS 44’ and a second BS 46’ and a first prism 64 and a second prism 66. The broadband, partially coupled, directional optical coupler 111 is constructed from micro-optic components and does not require that different components be connected with optical fiber. (The extra space in Figure 11 A between the components is included to make the light paths easier to see). The micro (miniature) optical components (rather than fiber-coupled ones) are assembled together and perform the broadband, partially coupled, directional optical coupler functions of splitting, directing, and combining the two paths.

[0068] For each of the first BS 44’ and second BS 46’ a portion of the light arriving on any of its four optically active faces propagates straight through the BS, and the remaining light is turned by 90° and propagates out of an adjacent face. The input light is collimated at C, and the first BS 44’ splits the light into the reference and measurement paths. The reference path continues forward and is turned 90° by a first prism 64 and directed vertically to the BS 46’. A portion of the reference light which reaches the BS 46’ is turned 90° and passes through a collimator C on its way to the output fiber. The measurement path light split by the first BS 44’ is turned and exits the first BS 44’ vertically to a second prism 66 that rotates the light 180° so that it enters the second BS 46’ which then directs a portion of that light through a collimator C towards the DUT. Measurement light returning from the DUT passes through the second BS 46’ and combines with the reference light with the combined light coupled to the output fiber through collimator C. An absorptive coating on the bottom face of the first BS 44’ may be used to prevent unwanted extra reflections from adding noise into the output signal.

[0069] Advantages of the micro-component embodiment of a broadband, partially coupled, directional optical coupler 111 include greater flexibility in optimizing the optical paths to optimize for different functions. For example, additional optical components, such as polarizers or wave plates can be inserted into the broadband, partially coupled, directional optical coupler 111 to refine the measurements. In some examples, the broadband, partially coupled, directional optical coupler 111 can be fabricated in an enclosure which is approximately 6mm by 9 mm by 3mm.

[0070] Figure 1 IB shows an expanded view of another example micro-optic component embodiment of a broadband, partially coupled, directional optical coupler 112 that includes a first BS 44’ and a second BS 46’ and a first prism 64’ and a second prism 66’ that are shaped differently than the prism in Figure 1 IB. The input light is collimated at C, and the first BS 44’ splits the light into the reference and measurement paths. The reference path light continues forward and is first turned 90° by an angled face of a first trapezoidal shaped prism 64’, turned another 90° by the second angled face of the first prism 64’, and directed vertically down to the BS 46’. A portion of the reference light which reaches the BS 46’ is turned 90° and passes through a collimator C on its way to the output fiber. The measurement path light split by the first BS 44’ is turned 90° and exits the first BS 44’ vertically up to the second BS 46’ that rotates the light 90° which propagates through the center portion of the first prism 64’ and the collimator C towards the DUT. Measurement light returning from the DUT passes through the collimator C, the center portion of the firm prism 64’, and the second BS 46’ and combines with the reference light in the collimator C coupled to the output fiber. An absorptive coating on the bottom face of the first BS 44’ may be used to prevent unwanted extra reflections from adding noise into the output signal.

[0071] Figure 11C shows a compact view of an example embodiment of the example micro-optic component embodiment of the broadband, partially coupled, directional optical coupler 112 of Figure 1 IB as assembled. In this physically compact embodiment, all the micro- optic parts are placed in close proximity, and trapezoidal shape prism 64’ allows relatively simple alignment of the various components.

[0072] In the micro-optic component example embodiments of broadband, partially coupled, directional optical couplers shown in Figures 11A-11C, the optical surfaces between the various micro-optic parts may be anti-reflection (AR) coated to prevent extra pathways for the light, thereby suppressing additional signals that may distort the ultimate interference measurements made by the OFDR instrument. The splitting ratios of the BSs 44’, 46’ may be adjusted through the selection of the materials used to fabricate the BSs to optimize the eventual strength of the interference signal. The reference light passes through two steps of the BSs 44’, 46’, whereas the measurement light passes through three steps of the BSs 44’, 46’ because the measurement light passes through the second BS 46’ twice.

[0073] Further example embodiments of the broadband, partially coupled, directional optical couplers 111 and 112 may add one or more further optical components such as polarizing films, waveplates, or other elements to modify the measurement in ways that are known to one of ordinary skill in the art.

[0074] Each of the example broadband, partially coupled, directional optical couplers described above in conjunction with Figures 4-11 may be used in optical frequency domain reflectometry (OFDR) system like the example OFDR system 10 shown in Figure 1.

[0075] Although various embodiments have been shown and described in detail, the claims are not limited to any particular embodiment or example. None of the above description should be read as implying that any particular element, step, range, or function is essential. All structural and functional equivalents to the elements of the above-described embodiments that are known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed. Features of the embodiments described above may be combined unless clearly technically impossible. Moreover, it is not necessary for a device or method to address each and every problem sought to be solved by the present invention, for it to be encompassed by the invention. No embodiment, feature, element, component, or step in this document is intended to be dedicated to the public.

[0076] All methods described herein can be performed in any suitable order unless otherwise indicated herein. The use of any and all examples, or example language (e.g., “such as”) provided herein, is intended merely to better illuminate the example embodiments and does not pose a limitation on the scope of the claims appended hereto unless otherwise claimed. [0077] The term “about” or “approximately” means an acceptable error for a particular recited value, which depends in part on how the value is measured or determined. In certain embodiments, “about” can mean 1 or more standard deviations. When the antecedent term "about" is applied to a recited range or value it denotes an approximation within the deviation in the range or value known or expected in the art from the measurement’s method. For removal of doubt, it is understood that any range stated herein that does not specifically recite the term “about” before the range or before any value within the stated range inherently includes such term to encompass the approximation within the deviation noted above.

Claims

What is claimed is:

1. A broadband, partially coupled, directional optical coupler, comprising: an input fiber configured to receive light tuned over a broadband range of frequencies from an optical frequency domain reflectometry (OFDR) instrument; an output fiber configured to return light to the OFDR instrument; an optical splitter configured to: direct a first portion of the light from the input fiber to a measurement fiber, and divert a second substantial portion of the light from the input fiber to an optical combiner configured to combine returning light from the measurement fiber with the second substantial portion of light; and a non-reciprocal optical element configured to send a substantial portion of the measurement light or a substantial portion of the combined light to the output fiber, wherein the input fiber, the output fiber, the optical splitter, the optical combiner, and the non-reciprocal optical element are arranged to be substantially colinear in the broadband, partially coupled, directional optical coupler.

2. The broadband, partially coupled, directional optical coupler in claim 1, wherein the broadband, partially coupled, directional optical coupler is a single integrated component.

3. The broadband, partially coupled, directional optical coupler in claim 1, wherein the broadband, partially coupled, directional optical coupler is 1 cm or less in diameter by 5 cm or less in length.

4. The broadband, partially coupled, directional optical coupler in claim 1, wherein the optical splitter, the optical combiner, and the non-reciprocal optical element comprise a 3-port optical device with a first port coupled to the input fiber, a second port coupled to the measurement fiber, and a third port coupled to the output fiber.

5. The broadband, partially coupled, directional optical coupler in claim 4, further comprising a first collimator for the first port, a second collimator for the second port, and a third collimator for the third port.

6. The broadband, partially coupled, directional optical coupler in claim 4, wherein the 3- port optical device includes a partial optical reflector configured to reflect the second substantial portion of light to the third port and transmit the first portion of the light to the second port.

7. The broadband, partially coupled, directional optical coupler in claim 6, wherein the partial optical reflector includes a partially reflecting mirror having a reflectivity in a range of 42%-35%.

8. The broadband, partially coupled, directional optical coupler in claim 6, wherein the partial optical reflector includes a beam splitter (BS) having: a first face oriented towards the first port and the third port, a second face having a substantially reflective surface, and a third face through which light from the first port is coupled to the second port and reflected from the measurement fiber received at the second port is coupled to the third port.

9. The broadband, partially coupled, directional optical coupler in claim 8, further comprising an absorbing or non-reflective coating on a fourth side of the BS to suppress unwanted light paths.

10. The broadband, partially coupled, directional optical coupler in claim 6, wherein: the partial optical reflector includes a first beam splitter (BS) arranged between the first port and the non-reciproc al optical element and a second BS arranged between the third port and the non-reciprocal optical element, the first BS is arranged to direct the first portion of the light from the input fiber to the non-reciprocal optical element and to divert the second substantial portion of the light from the input fiber to the second BS, and the second BS is arranged to receive the returning light from the measurement fiber from the second port through the non-rcciprocal optical element, combine the returning light with the second substantial portion of the light from the first NPBS, and direct the combined light towards the third port.

11. The broadband, partially coupled, directional optical coupler in claim 10, further comprising a coating on or a modification to a face of the first BS and/or the second BS to prevent or reduce reflection from the face or to direct reflection away from the optical paths that leads to the third port.

12. The broadband, partially coupled, directional optical coupler in claim 10, further comprising a micro-turnaround device, wherein: the first BS and the second BS are fiber optic splitters, and the second substantial portion of light from the first BS is coupled to the second port of the second BS through optical fibers which are joined by the micro turnaround device.

13. The broadband, partially coupled, directional optical coupler in claim 1, wherein the optical splitter includes a 1x2 optical splitter coupled at one end to the second port and coupled to a first leg at the other end to the measurement fiber and to a second leg at the other end to an optical reflector.

14. The broadband, partially coupled, directional optical coupler in claim 1, further comprising: a micro-turnaround device; an optical isolator; a 3x3 optical splitter; and a fiber loop, wherein: the 3x3 optical splitter includes three instrument side legs on an OFDR instrument side and three measurement side legs on a measurement side and is arranged to: receive at a first instrument side leg of the 3x3 optical splitter OFDR instrument light from the input fiber, direct the first portion of light from a first measurement side leg to the measurement fiber, and direct the second substantial portion of light and the third substantial portion of light from a second measurement side leg to the fiber loop, wherein the fiber loop includes: the micro turnaround to connect the second measurement side leg and a third measurement side leg together, and the optical isolator to restrict light to propagate around the fiber loop in a direction from the second measurement side leg to the third measurement side leg, and transmit the second substantial portion of the light received from the isolator and the first substantial portion of the light received from the measurement fiber to a second instrument side leg of the 3x3 optical splitter couple to the output fiber.

15. The broadband, partially coupled, directional optical coupler in claim 14, wherein a third instrument side leg is connected to a termination to prevent reflections of light in the third instrument side leg from returning to the 3x3 optical splitter.

16. The broadband, partially coupled, directional optical coupler in claim 1, wherein: the optical splitter includes a micro-turnaround device and a first 2x2 optical splitter; the optical combiner includes a second 2x2 optical splitter; the first 2x2 optical splitter is arranged to: receive at a first instrument side leg of the first 2x2 optical splitter light from the input fiber, direct the first portion of light from a first measurement side leg of the first 2x2 optical splitter to the measurement fiber, and direct the second substantial portion of the light from a second output leg of the first 2x2 optical splitter to the micro-turnaround device; the micro-turnaround device is arranged to receive the second substantial portion of the light from the second output leg of the first 2x2 optical splitter and return a majority of the second substantial portion of the light to a first measurement side leg of the second 2x2 optical splitter; a second instrument side leg of the first 2x2 optical splitter is arranged to transmit returning light from the measurement fiber to a second measurement side leg of the second 2x2 optical splitter where the returning light from the measurement fiber is combined with the second substantial portion of light received at the first measurement side leg of the second 2x2 optical splitter; and a first instrument side leg of the second 2x2 splitter is arranged to send the combined light to the output fiber.

17. The broadband, partially coupled, directional optical coupler of claim 16, wherein a second instrument side leg of the second 2x2 splitter is connected to a termination to prevent light from reflecting back to the second 2x2 splitter.

18. A broadband, partially coupled, directional optical coupler, comprising: an input fiber arranged to receive input light tuned over a broadband range of frequencies from an optical frequency domain rellectometry (OFDR) instrument; an output fiber configured to return light to the OFDR instrument; a first beam splitter (BS); a second BS; wherein: the first BS is arranged to split light from the input fiber into measurement path light and reference path light; the second BS is arranged to: direct a substantial portion of the measurement path light from the first NPBS into a measurement fiber; combine the reference path light from the first BS with light returned from the measurement fiber; and provide a substantial portion of the combined light to the output fiber.

19. The broadband, partially coupled, directional optical coupler in claim 18, further comprising a first prism and a second prism, wherein: the first prism is arranged to direct the reference path light to a second prism, and the second prism is arranged to direct the reference path light to the second BS for combining with the light returned from the measurement fiber.

20. The broadband, partially coupled, directional optical coupler in claim 18, further comprising: a first collimator coupled between the input fiber and the first BS; a second collimator coupled between the second PS and the measurement fiber; and a third collimator coupled between the output fiber and the second BS.

21. The broadband, partially coupled, directional optical coupler in claim 18, further comprising absorptive or non-reflective material applied to one or more faces of the first BS and to one or more faces of the second BS to suppress unwanted light paths.

22. The broadband, partially coupled, directional optical coupler in claim 18, wherein the first prism, the second prism, the first BS, and the second BS are micro-optical components that are not fiber coupled.

23. An optical frequency domain reflectometry (OFDR) system comprising: an OFDR instrument including a tunable light source and light detectors, and a broadband, partially coupled, directional optical coupler including: an input fiber configured to receive light tuned over a broadband range of frequencies from the OFDR instrument; an output fiber configured to return light to the OFDR instrument; an optical splitter configured to: direct a first portion of the light from the input fiber to a measurement fiber, and divert a second substantial portion of the light from the input fiber to an optical combiner configured to combine returning light from the measurement fiber with the second substantial portion of light; and a non-reciproc al optical element configured to send a substantial portion of the measurement light or a substantial portion of the combined light to the output fiber, wherein the input fiber, the output fiber, the optical splitter, the optical combiner, and the non-reciprocal optical element are arranged to be substantially colinear in the broadband, partially coupled, directional optical coupler.