Patent Publication Number: US-2023152568-A1

Title: Directing light into an optical fiber

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 63/034,277, filed Jun. 3, 2020, which is incorporated by reference in its entirety. 
    
    
     FIELD OF THE DISCLOSURE 
     The present disclosure relates to an optical system that can direct light into an optical fiber. 
     BACKGROUND OF THE DISCLOSURE 
     An optical system can use an optical fiber, such as a single-mode fiber. Misalignment of a light beam, with respect to a core of the single-mode fiber, can reduce the coupling efficiency of the beam into the core, and can increase losses in the optical system. 
     SUMMARY OF THE INVENTION 
     In an example, a system can direct light into an optical fiber. The system comprises imaging optics, an actuatable optical element, a processor, and a light source. The imaging optics is configured to form an image of an end of an optical fiber. The actuatable optical element is configured to define an optical path that extends to the actuatable optical element and further extends to the end of the optical fiber. The processor is configured to determine a location in the image of a specified feature in the image. The processor is further configured to cause, based on the location of the specified feature in the image, the actuatable optical element to actuate to align the optical path to a core of the optical fiber. The light source is configured to direct a light beam along the optical path to couple into the core of the optical fiber. 
     In another example, a method is for operating a system to direct light into an optical fiber. The system comprises imaging optics, a processor, and an actuatable optical element. The actuatable optical element defines an optical path, the optical path extending to the actuatable optical element and further extending to the end of the optical fiber. The method comprises: generating, with the imaging optics, an image of an end of the optical fiber; determining, with the processor, a location in the image of a specified feature in the image; causing, with the processor, the actuatable optical element to actuate to align the optical path to a core of the optical fiber based on the location of the specified feature in the image; and directing a light beam along the optical path to couple into the core of the optical fiber. 
     In another example, a computer-readable medium stores instructions that, when executed by a processor of a system for directing light into an optical fiber, can cause the processor to execute operations, such as the method described above or described elsewhere in this description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    shows a schematic drawing of an example of an apparatus that includes a system for directing light into an optical fiber. 
         FIG.  2    shows an end-on view of an example of a light source configured as a fiber bundle, which is suitable for use in the system of  FIG.  1   . 
         FIG.  3    shows a top view of an example of a portion of the system of  FIG.  1   , in which the objective element is configured as an objective mirror. 
         FIG.  4    shows a top view of an example of longitudinal position sensor elements that are suitable for use in the system of  FIG.  1   . 
         FIG.  5    shows a top view of another example of longitudinal position sensor elements that are suitable for use in the system of  FIG.  1   . 
         FIG.  6    shows a top view of another example of longitudinal position sensor elements that are suitable for use in the system of  FIG.  1   . 
         FIG.  7    shows a top view of the longitudinal position adjustor elements in the system of  FIG.  1   . 
         FIG.  8    shows a top view of an example of longitudinal position adjustor elements suitable for use in the system of  FIG.  1   . 
         FIG.  9    shows a flowchart of an example of a method for operating a system to direct light into an optical fiber. 
         FIG.  10    shows a flowchart of an example of another method for operating a system to direct light into an optical fiber. 
     
    
    
     Corresponding reference characters indicate corresponding parts throughout the several views. Elements in the drawings are not necessarily drawn to scale. The configurations shown in the drawings are merely examples and should not be construed as limiting the scope of the invention in any manner. 
     DETAILED DESCRIPTION 
     In an example, a system can direct light into an optical fiber. Imaging optics can form an image of an end of an optical fiber. An actuatable optical element can define an optical path that extends to the actuatable optical element and further extends to the end of the optical fiber. A processor can determine a location in the image of a specified feature in the image. The processor can cause, based on the location of the specified feature in the image, the actuatable optical element to actuate to align the optical path to a core of the optical fiber. A light source can direct a light beam along the optical path to couple into the core of the optical fiber. 
     The system can identify a feature in an image of the end of the optical fiber, then use the location of the feature to actively align an optical path to a core of the optical fiber. With active alignment, the system can improve the robustness of the alignment of the light beam to the core of the optical fiber. Improved robustness of alignment can help compensate for misalignments due to physical misalignment of the optical fiber, such as due to manufacturing tolerances, non-idealities in the mounting of a mechanical holder, and so forth. As a result, the system can achieve a higher coupling efficiency of the beam into the core of the optical fiber compared to an otherwise identical system not utilizing this technique. 
     Further, in various examples, the active alignment can be performed one or more times for each use of a system. Active alignment can be performed before and/or during use of the system. As a specific example, active alignment performed one or more times during operation, can help improve or maintain the alignment while an optical system is in operation. For example, during operation, an optical system can experience movement, temperature change, physical shock or vibration (such as caused by air currents), and/or other environmental or physical change that affects the alignment. The system described below can compensate periodically, in response to a determination of misalignment, or in real time for the environmental or physical change and can help improve or maintain sufficiently high coupling while the optical system is used. The term “sufficiently high” is used here to indicate sufficient coupling to enable the system to function at a performance level (e.g., regarding resolution, accuracy, power consumption, and so forth) for which the system is designed. 
     Further, the system can help achieve sufficiently high coupling efficiency without directly contacting the end of the optical fiber. Because the system uses contactless alignment of the optical path to the fiber, the system can help reduce contamination, physical wear, and the like of the end of the optical fiber. 
     An example of an optical system that can incorporate one or more features from the system shown below is an optical fiber-based strain, temperature, or shape sensing system. As a specific example, an optical system couples light into a multi-core optical fiber to sense, in real time or near real-time, a three-dimensional position in space of an element. As another specific example, an optical system couples light into a multi-core optical fiber to sense, in real time or near real-time, a three-dimensional shape of the optical fiber. 
     Further, an example of an optical system can include a medical or non-medical system. An example of a medical system can include those used for diagnosis or therapy, including surgical systems. In a medical system example, the system described below can be located in an optical path between one or more light sources and one or more cores of a sensing optical fiber, to help establish and/or maintain sufficiently high coupling efficiency (or coupling efficiencies) of light entering the sensing optical fiber over the course of one or more medical procedures. This is but one example of use for the system described in detail below. Other uses are also possible. 
       FIG.  1    shows a schematic drawing of an example of an apparatus  1  that includes a system  100  for directing light into an optical fiber. Because the system  100  can identify a feature in an image of the end of the optical fiber, then use the location of the feature to actively align an optical path to a core of the optical fiber, the system can achieve a relatively robust alignment of a light beam to the core of the optical fiber. 
     A controller  10  can include various optical and electronic components. For medical applications, the controller  10  can be configured as a piece of capital equipment, which can be used and reused for multiple procedures. For applications directed to shape sensing, such as sensing a three-dimensional orientation or shape of an optical fiber, the controller  10  can include a interrogator  12 . The interrogator  12  can direct light into a fiber and analyze light returning from the fiber. The interrogator  12  can use a technique, such as optical frequency domain reflectometry (OFDR), to determine a three-dimensional position of an optical fiber. 
     In some examples, a portion of the equipment can be configured as a replaceable element, which can be used for a part of a procedure or several procedures, or for the entirety of one or several procedures, then discarded. The replaceable element can include a catheter  14 , which can include a sensing optical fiber  16  that extends along at least part of the length of the catheter  14 . In a medical example, the catheter  14  and sensing optical fiber  16  can be maintained or reprocessed in a clean environment, or in a sterile environment if clinically required, prior to use. 
     The system  100  described in detail below can use active alignment to optically connect the optical fiber  16  to the controller  10 . When optically connected, the interrogator  12  can direct light into the sensing optical fiber  16  (through the system  100 ), receive light reflected from locations along a length of the sensing optical fiber  16  (also through the system  100 ), and analyze the reflected light (such as by OFDR) to determine a strain, temperature, or other physical information of the sensing optical fiber  16 . For a shape sensing application, the interrogator  12  is configured to determine a three-dimensional position or shape of the sensing optical fiber  16 . For clarity, the sensing optical fiber  16  will be referred to in the following discussion as the optical fiber  108 . It will be understood that the optical fiber  108  may include the sensing optical fiber  16  or can optionally include a separate portion of fiber coupled to a proximal end of the sensing optical fiber  16 . References below to the optical fiber  108  can include one or both of these cases. 
     The controller  10  can include a fiber connection  18 , such as a multi-core fiber or a plurality of single-mode fibers, that can provide light as input to the system  100 . The plurality of single-mode fibers can also be referred to as a bundle of single-mode fibers or a fiber bundle in this document, although the plurality of single-mode fibers may be bunched in a bundle, disposed in a linear array, and so forth. The system  100  can direct the light provided by the fiber connection  18  through various elements in the system  100  to couple into the sensing optical fiber  16 . The light reflected from locations along the length of the sensing optical fiber  16  can return into the system  100 , can propagate through the various elements in the system  100 , can propagate through the fiber connection  18 , and can be processed by the interrogator  12  in the controller  10 . The system  100  can direct a portion of the light through various elements onto one or more detectors. The detector or detectors can generate one or more control signals. The system  100  can use the one or more control signals to control one or more actuatable elements in the system, to improve the coupling efficiency for light entering the optical fiber  108 . 
     The controller  10  can include an electrical connection  20 , which can provide electrical power to the system  100 . A processor  114 , which can be located in the controller  10  or located in the system  100 , can receive the control signals from the one or more detectors in the system  100  and can drive the one or more actuatable elements in the system  100  to improve the coupling efficiency. 
     During operation, imaging optics in the system  100  can form an image  104  of an end  106  of the optical fiber  108 . An actuatable optical element  110 , such as a pivotable mirror, can define an optical path  112  that extends to the actuatable optical element  110  and further extends to the end  106  of the optical fiber  108  when the optical fiber  108  is present. A processor  114  can determine a location in the image  104  of a specified feature, such as a circumferential edge of the end  106  of the optical fiber  108 , in the image  104 . Although the processor  114  is shown as being located with the system  100 , it will be understood that the processor  114  can alternatively be located with the controller  10 . The processor  114  can cause, based on the location of the specified feature in the image  104 , the actuatable optical element  110  to actuate to align the optical path  112  to a core  116  of the optical fiber  108 . 
     The optical path  112  is a geometrical construct that extends from optical element to optical element between the fiber connection  18  and the optical fiber  108 . Specifically, one end of the optical path is located at the fiber connection  18 , and the other end is located at the optical fiber  108  when the optical fiber  108  is present. The optical path  112  can be bent, translated, rotated, and otherwise aligned by the optical components during operation of the system  100 . During operation of the system  100 , a light beam  120  is directed to propagate along the optical path  112  from optical element to optical element, so that the light beam  120  follows the optical path  112 . It is instructive to clarify that the optical path  112  can be redirected, both when the light beam  120  is present and when the light beam  120  is absent. For configurations in which the optical fiber  108  includes multiple cores, the system  100  can include multiple optical paths  112  that propagate toward respective cores of the optical fiber  108 . 
     A light source  118  can direct the light beam  120  along the optical path  112  to couple into a core  116  of the optical fiber  108 . In some examples, the controller  10  can include one or more light-producing elements, such as light-emitting diodes or laser diodes, and one or more light coupling elements, such as lenses, that can direct light from the light-producing elements into one or more cores of the one or more fibers in the fiber connection  18 . In the configuration of  FIG.  1   , the light source  118  can include a distal end of the fiber connection  18  or a length of fiber that is coupled to a distal end of the fiber connection  18 . For clarity, the fiber in the fiber connection  18  will be referred to in the following discussion as the source optical fiber  138 . It will be understood that the source optical fiber  138  may be the same as the fiber connection  18  or can optionally include a separate portion of fiber coupled to a distal end of the fiber connection  18 . 
     For examples in which the optical fiber  108  includes a single core  116 , the source optical fiber  138  can include a single core  140 . For examples in which the optical fiber  108  includes multiple cores  116 , the source optical fiber  138  can also include multiple cores  140 . The multiple cores  140  can be arranged in a pattern that resembles the pattern of the multiple cores of the optical fiber  108 . As a specific example, the source optical fiber  138  and the optical fiber  108  can each include six cores located in a hexagonal pattern that surrounds the center of the circumferential edge of the fiber. During operation, the system  100  can simultaneously direct light from the multiple cores  140  of the source optical fiber  138  into the multiple cores of the optical fiber  108 . 
     For examples in which the optical fiber  108  includes multiple cores  116 , an alternative to receiving light from multiple cores of a multi-core fiber in the fiber connection  18  is receiving light from the cores of a plurality of single-core fibers, such as fibers in a fiber bundle or a linear array of fibers. In some examples, the optical fiber  108  can be a multi-core optical fiber. The core  116  of the optical fiber  108  can be a first core of a plurality of cores of the multi-core optical fiber. The optical path  112  can be a first optical path of a plurality of optical paths defined by the actuatable optical element  110 . Each optical path of the plurality of optical paths can extend to the actuatable optical element  110  and can further extend to the end  106  of the multi-core optical fiber. The processor  114  can cause the actuatable optical element  110  to actuate to align the optical path  112  to the core  116  by causing the actuatable optical element  110  to actuate to simultaneously align the plurality of optical paths to the plurality of cores of the multi-core optical fiber. The light source  118  can be a first light source of a plurality of light sources. Each light source of the plurality of light sources can direct a corresponding light beam along a corresponding optical path of the plurality of optical paths to couple into a corresponding core of the plurality of cores of the multi-core optical fiber. 
       FIG.  2    shows an end-on view of an example of a light source  118  configured as a fiber bundle  200 , which is suitable for use in the system  100  of  FIG.  1   . The fiber bundle  200  includes a plurality of single-mode fibers  202 ,  204 ,  206 ,  208 ,  210 , and  212 . These single-mode fibers of the plurality include corresponding cores  216 ,  218 ,  220 ,  222 ,  224 , and  226 , respectively. The single-mode fibers of the plurality surround a central fiber  214  having a core  228 . In this example, the single-mode fibers  202 ,  204 ,  206 ,  208 ,  210 , and  212  of the plurality are arranged in a pattern of a regular hexagon surrounding the central fiber  214 . The fiber bundle  200  is suitable for use as a light source  118  for an optical fiber  108  having multiple cores arranged in a similarly shaped hexagonal pattern. The fiber bundle  200  of  FIG.  2    is but one example of a fiber bundle; other arrangements of fibers are also possible. The system  100  can further include magnification optics that can impart a magnification to the plurality of optical paths. The magnification can equal, or substantially equal to within a tolerance, such as 1%, 5%, 10%, or 20%, a ratio of a spacing between adjacent cores of the plurality of cores of the multi-core optical fiber to a spacing between adjacent cores of the plurality of single-core fibers. The magnification optics can include a source objective element  148  (described in detail below), which can collimate light emerging from the light source  118  to form the light beam  120 , and an objective element  122  (also described in detail below), which can focus the light beam  120  to couple into the optical fiber  108 . The ratio of the focal lengths of the source objective element  148  and the objective element  122  can be selected to equal, or substantially equal the ratio of the spacing between adjacent cores of the plurality of cores of the multi-core optical fiber of the optical fiber  108  to a spacing between adjacent cores of the plurality of single-core fibers of the light source  118 . 
     Returning to  FIG.  1   , in some examples, the processor  114  can cause the actuatable optical element  110  to actuate to align the optical path  112  to the core  116  by using at least the following two operations. First, the processor  114  can determine an offset between the location of the specified feature in the image  104  and a predetermined target location in the image  104 . Second, the processor  114  can cause the actuatable optical element  110  to actuate to reduce the offset. The processor  114  can optionally repeat these two operations during operation of the system  100  to help maintain a sufficiently high coupling efficiency into the core  116  during operation. For example, the processor  114  can determine a pixel location (e.g. a set of orthogonal location coordinates, such as x and y) in the image  104  of the specified feature (such as a center of a circumference of the optical fiber  108 ), can compare the determined pixel location to a specified pixel location (e.g., such as a set of values saved in a lookup table or other suitable memory) that corresponds to a well-aligned optical fiber  108 , and cause the actuatable optical element  110  to actuate to move the determined pixel location to coincide with the specified pixel location. 
     In some examples, the specified feature can include part or all of a circumferential edge of the end  106  of the optical fiber  108 . The core  116  can be located at a predetermined core location relative to the circumferential edge of the optical fiber  108 . The processor  114  can cause the actuatable optical element  110  to actuate to align the optical path  112  to the core  116  by causing alignment of the optical path  112  to the predetermined core location. For example, for configurations in which the optical fiber  108  is a single-core fiber, the core  116  can be located at a center of the circumferential edge of the optical fiber  108 . For configurations in which the optical fiber  108  is a multi-core fiber (e.g., a fiber in which a single cladding surrounds multiple cores that are spaced apart from one another), the cores  116  can be located at specified locations with respect to the circumferential edge of the optical fiber  108 . For example, the optical fiber  108  can include four cores, with a center core located at a center of the circumferential edge and three cores located at corners of an equilateral triangle centered about the center core. As another example, the optical fiber  108  can include six cores located in a hexagonal pattern that surrounds the center of the circumferential edge of the optical fiber  108  (e.g., the cores can be located at the corners of a regular hexagon by being spaced apart azimuthally by sixty degrees, about sixty degrees, or sixty degrees to within a tolerance of one degree, two degrees, five degrees, or another suitable value). As yet another example, the optical fiber  108  can include seven cores, with a center core located at a center of the circumferential edge and six cores located at corners of a regular hexagon centered about the center core. Other suitable multi-core configurations can also be used. 
     For configurations in which the optical fiber  108  includes multiple cores, the circumference of the optical fiber  108  can include an optional azimuthal locating feature, such as a partially flattened edge, a notch, a protrusion, or other feature that can mechanically or optically indicate the azimuthal locations of the cores. For example, the optical fiber  108  can include a rod (not a core) that extends along a length of the optical fiber  108 . Such rod can appear as a bright dot (e.g., brighter than an area surrounding the dot) or a dark dot (e.g., darker than an area surrounding the dot) in the image  104  of the end  106  of the optical fiber  108 . In some examples, the azimuthal locating feature can be a mechanical feature of a connecting element. For example, the optical fiber  108  can be held in a ferrule and standard optical connector. When the connector is manufactured, a specific core of the optical fiber  108  can be illuminated, to align the specific core to a key of the standard optical connector. 
     Other specified features can also be used in addition to or instead of the circumferential edge of the end  106  of the optical fiber  108 . For example, the feature can include the appearance of the core  116  of the optical fiber  108  in the image  104 . In some illumination configurations, the core  116  can appear as a dark spot in the image  104 , which can appear darker (e.g., with a lower intensity or brightness) than an area surrounding the core  116 . In other illumination configurations, the core  116  can appear as a bright spot in the image  104 , which can appear brighter (e.g., with a higher intensity or brightness) than an area surrounding the core  116 . Identifying the core  116  directly from bright spots and/or dark spots in the image can also be used with multi-core fibers that have multiple cores. 
     The system  100  can optionally further include an illumination light source  130 . The illumination light source  130  can illuminate the optical fiber  108  with illumination  132 . The illumination  132  can have a wavelength different from a wavelength of the light beam  120 . As a specific example, the wavelength of the light can be 1550 nm, and the wavelength of the illumination  132  can be in the visible spectrum, such as between 400 nm and 700 nm. Other wavelength values can also be used. 
     For configurations that include the illumination light source  130 , at least some of the illumination  132  can reflect or scatter from the optical fiber  108  to form first light. In some examples, illumination  132  that reflects off the end  106  of the optical fiber  108  can produce the first light. In some examples, illumination that enters a side of the optical fiber  108  and exits the end  106  of the optical fiber  108  can form the first light. 
     An objective element  122  can collimate at least some of the first light to form second light. In some examples, such as the configuration of  FIG.  1   , the objective element  122  can include an objective lens. The optical path  112  can extend through the objective lens. As an alternative, the objective element  122  can include an objective mirror.  FIG.  3    shows a top view of an example of a portion of the system  100  of  FIG.  1   , in which the objective element  122  is configured as an objective mirror  302 . The objective mirror  302  can have a cross-section that includes a section of a parabola. Other configurations for the objective element  122  are also possible, including multiple mirrors, multiple lenses, or a combination of at least one mirror and at least one lens. Similarly, the focusing element  126  can include at least one of a focusing lens or a focusing mirror. 
     Returning to  FIG.  1   , a dichroic mirror  124  can direct at least some of the second light away from the optical path  112  to form third light. For example, the dichroic mirror  124  can transmit light in a transmission band that includes 1550 nm. The dichroic mirror  124  can reflect light in a reflection band that includes the wavelength of the illumination  132 , such as in the visible spectrum. This is but one numerical example; other wavelengths and wavelength ranges can also be used. 
     In the configuration of  FIG.  1   , the dichroic mirror  124  is a long pass dichroic mirror, which can transmit relatively long wavelengths (such as those used for performing the shape sensing, optionally in the infrared portion of the electromagnetic spectrum such as 1550 nm), and reflect relatively short wavelengths (such as those used for performing the imaging functions, optionally in the visible portion of the electromagnetic spectrum such as between 400 nm and 700 nm). Alternatively, the dichroic mirror  124  can be a short pass dichroic mirror, which can transmit the relatively short wavelengths (such as those used for performing the imaging functions) and reflect the relatively long wavelengths (such as those used for performing the shape sensing). Replacing the long pass filter with a short pass dichroic mirror would involve swapping the transmitted and reflected arms, so that the optical path  112  would reflect at the dichroic mirror  124  rather than transmit through the dichroic mirror  124  as currently shown in  FIG.  1   . 
     A focusing element  126  can focus the third light to form the image  104  at a focal plane of the focusing element  126 . An imaging array  128  can be located at the focal plane of the focusing element  126  and can sense the image  104 . In some examples, the imaging optics can include the objective element  122 , the dichroic mirror  124 , the focusing element  126 , and the imaging array  128 . The processor  114  can receive from the imaging array  128  an analog and/or a digital signal that corresponds to the image  104 . Other suitable configurations can also be used. 
     In the example of  FIG.  1   , the actuatable optical element  110  is configured as a pivotable mirror. The pivotable mirror can include a single mirror that can pivot in two dimensions, two separated mirrors that can each pivot in a single dimension, multiple mirrors that can each pivot in a single dimension or two dimensions, and other suitable configurations. In the configuration of  FIG.  1   , the pivotable mirror can include a reflective mirror that can pivot about a pivot point, and a linear actuator  136  that can pivot the reflective mirror about the pivot point. Although the pivotable mirror is shown in  FIG.  1    as pivoting in only one dimension, it will be understood that the pivotable mirror can pivot in two orthogonal dimensions, using a pair of linear actuators  136 . The processor  114  can control the linear actuators  136 . The processor  114  can actuate the actuatable optical element  110  to align the optical path  112  to the core  116  of the optical fiber  108  by pivoting the pivotable mirror to steer the optical path  112  based on the location of the specified feature in the image  104 . 
     The optical path  112  can include a fixed portion, extending between the light source  118  and the actuatable optical element  110 . The optical path  112  can include a movable portion, extending between the actuatable optical element  110  and the end  106  of the optical fiber  108 . During operation of the system  100 , the movable portion of the optical path  112  can move in space, while the fixed portion of the optical path  112  may remain stationary. In the configuration of  FIG.  1   , the dichroic mirror  124 , focusing element  126 , and imaging array  128  are located in the fixed portion of the optical path  112 . Other configurations can also be used. 
     In some examples, the actuatable optical element  110  can be located in the optical path  112  to be telecentric. For a telecentric configuration, pivoting the pivotable mirror can produce lateral translation of the optical path  112  at the end  106  of the optical fiber  108  without producing a change in angle of the optical path  112  at the end  106  of the optical fiber  108 . In some examples, locating the pivotable mirror at a rear focal plane (or a back focal plane) of the objective element  122  can produce the telecentric condition. 
     The pivotable mirror of  FIG.  1    is but one example of a suitable actuatable optical element  110 . Other suitable configurations can include a translatable optical element, such as a translatable lens or a translatable mirror. In some examples, the translatable optical element can include the objective element  122 , the full system  100 , and/or the optical fiber  108 . 
     In some examples, the system  100  can include features that allow the system  100  to operate in a separate environment, such as a clean-room environment in an industrial example, or a sterile environment in a medical example involving sterility. For example, in some applications in which a medical procedure is performed, such as when the system  100  can be reusable (e.g., can be capital equipment), and the optical fiber  108  can be replaceable (e.g. can be disposed of after a single-use, or reprocessed and disposed of after multiple uses, or be reprocessed for an indefinite number of uses), the system  100  can optionally include a barrier, such as a window or optical surface. In medical examples, the barrier may meet cleanliness requirements to help provide a clean environment for particular medical procedures not requiring sterility or may meet sterility requirements to help ensure sterility for medical procedures requiring sterility. 
     The window or optical surface can pass the light beam  120  to the optical fiber  108  and can receive light from the optical fiber  108 , without contacting the optical fiber  108 . In some examples, the window or optical surface, can be easily cleaned between uses of the system  100 , to avoid contaminating optical fibers used in subsequent procedures. In some examples, the objective element  122 , such as the objective lens, can form part of a barrier for the system  100 . For example, the objective lens can be plano-convex, with the planar side optionally forming part of the sterile barrier. Other configurations can also be used. As noted above, in some examples, the barrier formed by the system  100  may not be a sterile barrier in that it does not meet sterility requirements. 
     The system  100  can optionally further include a field aligning lens  134  located in the optical path  112  proximate the end  106  of the optical fiber  108 . Such a field aligning lens  134  can improve the coupling efficiency for cases when the optical fiber  108  is positioned away from a central axis of the optical elements of the system  100  (e.g., off-axis performance). The field aligning lens  134  can optionally have a same focal length as the objective element  122 . The field aligning lens  134  can optionally have a diameter (e.g. a clear aperture) than is less than a diameter of the objective element  122 . The field aligning lens  134  can optionally have a numerical aperture (e.g., half the diameter, divided by the focal length) than is less than a numerical aperture of the objective element  122 . The field aligning lens  134  can optionally be formed as a plano-convex lens. The field aligning lens  134  can optionally have a planar side that forms part of a sterile barrier of the system  100 . Because the field aligning lens  134  may be a relatively inexpensive item, the field aligning lens  134  can optionally be configured as a replaceable (e.g., single-use or multi-use) element that can be removed, reprocessed, reused, and/or disposed of. Such a replaceable element can optionally be packaged with, or separately from, the optical fiber  108 . 
     In some examples, the system  100  can optionally monitor an amount of light that is reflected from one or more cores of the optical fiber  108 . For example, in a position-sensing application, the system  100  can couple light into one or more cores of the optical fiber  108 , light can reflect in varying amounts from locations along a length of the optical fiber  108 , and the system  100  can analyze the reflected light, such as by optical frequency domain reflectometry (OFDR) performed by the interrogator  12 , to determine a three-dimensional position of the optical fiber  108 . In some examples, the analysis of the reflected light can include sensing a magnitude or amplitude of the reflected light. Such a sensed magnitude or amplitude can correspond to a coupling efficiency of light entering the optical fiber  108 . The system  100  can actuate the actuatable optical element  110  to raise, maximize, and/or optimize the sensed magnitude or amplitude of the reflected light from the optical fiber  108 . In some examples, the system  100  can use the sensed magnitude or amplitude in concert with the imaging technique described above. For example, the system  100  can use the imaging technique to perform an initial positioning of the optical path  112  near or at the core  116  (e.g., as a coarse alignment procedure), and can use the sensed magnitude or amplitude to more precisely position the optical path  112  with respect to the core  116  (e.g., as a fine alignment procedure). In some examples, the system  100  can use the sensed magnitude or amplitude to position the optical path  112  with respect to the core  116 , without using the imaging technique described above. 
     As explained above, the system  100  can illuminate the optical fiber  108  to capture the image  104  of the end  106  of the optical fiber  108 . For configurations in which the light source  118  includes a source optical fiber  138 , the system  100  can optionally illuminate an end of the source optical fiber  138  and include additional optical elements to superimpose a view of an end  142  of the source optical fiber  138  onto the view of the end  106  of the optical fiber  108  in the image  104 . (As an alternative to illuminating the end  142  of the source optical fiber  138 , or in addition to performing such illumination, the controller  10  can inject illumination into an opposite end of the source optical fiber  138 , which can propagate along the source optical fiber  138  to emerge from the end  142  of the source optical fiber  138 . Because the illumination for imaging can use a different wavelength than the light used for shape sensing, the injection of the illumination can be performed by wavelength division multiplexing at the controller  10 .) Allowing the ends of the two fibers to be viewed simultaneously can provide additional information during the assembly and alignment stages of the system  100 . 
     As explained above, a first illumination light source, such as  130 , can illuminate the optical fiber  108  with first illumination, such as  132 . The first illumination  132  can have a first wavelength different from a wavelength of the light beam  120 . At least some of the first illumination  132  can reflect or scatter from the optical fiber  108  to form first light. An objective element  122 , such as an objective lens or objective mirror, can collimate at least some of the first light to form second light. A second illumination light source  144  can illuminate the source optical fiber  138  with second illumination  146 . The second illumination  146  can have a second wavelength that is different from the first wavelength and different from the wavelength of the light beam  120 . At least some of the second illumination  146  can reflect or scatter from the source optical fiber  138  to form third light. A source objective element  148 , such as a source objective lens or source objective mirror, can collimate at least some of the third light to form fourth light. A dichroic mirror, such as  124 , and a reflector  150 , such as a retroreflector or retroreflecting prism, can superimpose the second light and the fourth light to form fifth light. In the configuration shown in  FIG.  1   , the dichroic mirror  124  can reflect at least some of the fourth light toward the reflector  150 . Alternatively, the dichroic mirror  124  can be oriented to reflect at least some of the second light toward the reflector  150 . A focusing element, such as  126 , can focus the fifth light to form the image  104  at a focal plane of the focusing element  126 . An imaging array, such as  128 , located at the focal plane of the focusing element  126 , can sense the image  104 . Because the ends  106 ,  142  of the fibers can be imaged with light at different wavelengths, the processor  114  can optionally separate the information from the two superimposed views as needed. Forming the superimposed views of the ends  106 ,  142  of the fibers can provide additional information during the assembly and/or alignment stages of the system  100 , and/or during use of the system  100 . For example, because the ends of the fiber can be visible in the image  104 , the image  104  can be used to check the fiber ends for contamination or damage. 
     The optical path  112  can include an optional first pivotable element  152  that can redirect the optical path  112  within an angular range that extends in one dimension or in two dimensions. The first pivotable element  152  can include a mirror on adjustable mount that can controllably pivot about one, two, three, or more axes. Where the pivotable element be pivoted about multiple axes, the axes may intersect or not intersect, or be orthogonal to each other or be rotationally offset by some other angle. The phrase “pivotable element” is intended to include a variety of “tip/tilt elements” that can include, for example, elements such as mirrors, mounted on a tip/tilt stage. A tip/tilt stage can typically pivot about each of two orthogonal and intersecting axes, although other configurations can also be used. In the configuration of  FIG.  1   , the first pivotable element  152  can have a nominal angle of incidence of 45 degrees, or about 45 degrees, so that the optical path  112  can be nominally redirected by 90 degrees, or about 90 degrees. The incidence angle of 45 degrees is but one example of an incident angle; other suitable angles of incidence can also be used. The first pivotable element  152  can provide an additional degree of freedom during the assembly and alignment stages of the system  100 . For example, locating the first pivotable element  152  in the optical path  112  can help relax some placement tolerances on the source optical fiber  138 , and can help compensate for rotations and/or displacements of other optical elements in the optical path  112 . The first pivotable element  152  can be located in the optical path  112  between the light source  118  and the dichroic mirror  124 , between the dichroic mirror  124  and the actuatable optical element  110 , between the actuatable optical element  110  and the optical fiber  108 , or at any other suitable location along the optical path  112 . 
     The optical path  112  can include an optional second pivotable element  154  that can redirect the optical path  112  within an angular range that extends in one dimension or in two dimensions. The second pivotable element  154  can be similar in structure and function to the first pivotable element  152 . The second pivotable element  154  can be located in the optical path  112  between the light source  118  and the dichroic mirror  124 , between the dichroic mirror  124  and the actuatable optical element  110 , between the actuatable optical element  110  and the optical fiber  108 , or at any other suitable location along the optical path  112 . The first pivotable element  152  and the second pivotable element  154  can be located at different locations along the optical path  112  (e.g., can be longitudinally separated along the optical path  112 ). Although the second pivotable element  154  is shown in  FIG.  1    as being adjacent to the first pivotable element  152  with no intervening optical elements between them, the second pivotable element  154  can be located at any suitable location along the optical path  112 , including between beamsplitters, or between a beamsplitter and the actuatable optical element  110 . Using two pivotable elements that are separated along the optical path  112  can be helpful during the assembly and alignment of the optical components in the system  100 . For example, using two longitudinally separated pivotable elements can allow the optical path  112  to be laterally translated (e.g., moved without rotation) to a desired location, or rotated in two dimensions while keeping a fixed spatial location. As a specific example, using two pivotable elements can allow the optical path  112  to pass through a center of a lens, rather than the edge of a lens, to improve the optical performance of the lens. 
     In some examples, it may be beneficial to use one or more pivotable elements to steer the optical path  112  such that the optical fiber  108  and the source optical fiber  138  can be approximately parallel to each other (at least to within a few degrees). Orienting the fibers to be approximately parallel is generally consistent with use of typical physical contact connectors, which typically require insertion of the fibers from opposing sides. The configuration of  FIG.  1    can be modified to achieve this parallelism condition by removing one of the pivotable elements  152  or  154 , adding an additional pivotable element, modifying the dichroic mirror  124  to be a short pass filter rather than a long pass filter, and other geometrical modifications. 
     Tus far, discussion has focused on laterally aligning the optical path  112  to a core  116  of the optical fiber  108 . Specifically, for a coordinate system (x, y, z) at the end  106  of the optical fiber  108 , in which z corresponds to a central axis of the optical path  112 , the discussion above is directed to aligning the optical path  112  in the x- and y-dimensions. For example, capturing an image  104  of the end  106  of the optical fiber  108  can provide x- and y-coordinates of one or more features on the optical fiber  108 , and the system  100  can actively align the optical path  112  in x- and y-dimensions with respect to the feature or features in the image  104 . 
     In some applications, active alignment in x- and y-dimensions may be sufficient to achieve sufficiently high coupling into the optical fiber  108 . These applications can rely on mechanical placement of the end  106  of the optical fiber  108  as being sufficiently accurate to achieve the sufficiently high coupling. For example, the optical fiber  108  may fit into a clamp that can position the end  106  of the optical fiber  108  in a specified plane (in the z-direction) to within a specified tolerance, such that for any z-position within the tolerance, the coupling efficiency is sufficiently high. 
     In other applications, the mechanical placement in the z-direction may not be suitably accurate to achieve the sufficiently high coupling. For these applications, the system  100  can further include one or more longitudinal position sensor elements. The longitudinal position sensor elements can detect a longitudinal separation (e.g., a distance as measured along the optical path  112 ) between a focus of the light beam  120  and the end  106  of the optical fiber  108 . The longitudinal position sensor elements can be located at any suitable location in the fixed portion of the optical path  112 . 
     Similarly, the system  100  can further include one or more longitudinal position adjustor elements. The longitudinal position adjustor element or elements can longitudinally position the focus of the light beam  120  to reduce the longitudinal separation between the focus and the  106  of the optical fiber  108 . The longitudinal position adjustor element or elements can be located at any suitable location in the fixed portion of the optical path  112 . In some examples, the longitudinal position adjustor element or elements can be located in the optical path  112  between the longitudinal position sensor elements and the optical fiber  108 . 
     In the configuration of  FIG.  1   , the longitudinal position sensor elements can include a beamsplitter  156 , such as a dichroic beamsplitter, a 50-50 beamsplitter, or another suitable beam-splitting element. In the configuration of  FIG.  1   , the beamsplitter  156  can be located between the dichroic mirror  124  and the optical fiber  108  along the optical path  112 . Alternatively, the dichroic mirror  124  can be located between the beamsplitter  156  and the optical fiber  108  along the optical path  112 . Other configurations can also be used, including configurations that can swap the functions of the transmitted and reflected paths through the beamsplitter  156 . 
     The beamsplitter  156  can receive light that has been reflected and/or scattered from the end  106  of the optical fiber  108 . The beamsplitter  156  can direct a portion of the reflected light toward a bi-prism  158 , a lens  160 , and a sensor  162 . The lens  160  can focus the light emergent from the bi-prism  158  to form an image  164  at the sensor  162 . The sensor  162  can be coupled to the processor  114 . 
     The bi-prism  158  can impart a wedge angle between opposing halves of the reflected light such that the specified feature, such as the circumferential edge of the optical fiber  108 ) in the image  164  has a corresponding duplicate feature in the image  164 . The processor  114  can further determine, based at least in part on a spacing between the specified feature and the corresponding duplicate feature, the longitudinal separation between the focus and the end  106  of the optical fiber  108 . For these configurations, the spacing can be considered to be a focus error signal. The spacing can be compared against a specified distance, which can be determined in an initial configuration of the system  100 , such as at a factory during initial assembly and alignment of the system  100 . If the spacing is less than the specified distance, the focus can be on one side of the end  106  of the optical fiber  108 , such as outside the optical fiber  108 . If the spacing is greater than the specified distance, the focus can be on the other side of the end  106  of the optical fiber  108 , such as within the optical fiber  108 . 
     The bi-prism configuration of  FIG.  1    is but one example of a configuration for the longitudinal position sensor elements. Other suitable configurations are shown in  FIGS.  4 - 6    and described below. 
       FIG.  4    shows a top view of an example of longitudinal position sensor elements that are suitable for use in the system  100  of  FIG.  1   . The beamsplitter  156  in the optical path  112  directs a light portion  402  of the light beam  120  that has been reflected and/or scattered from the end  106  ( FIG.  1   ) of the optical fiber  108  ( FIG.  1   ) toward a split-field dichroic filter  404 , a lens  406 , and a sensor  410 . A first half  404 A of the split-field dichroic filter  404  can have a first spectral profile (e.g., can pass a first wavelength or a first wavelength band). A second half  404 B of the split-field dichroic filter  404  can have a second spectral profile different from the first spectral profile (e.g., can pass a second wavelength different from the first wavelength or a second wavelength band different from the first wavelength band). The light portion  402  can form an image  408  at the sensor  410 . The sensor  410  can be coupled to the processor  114 . 
     The light portion  402  can have a plurality of wavelengths. The split-field dichroic filter  404  can be configured such that a specified feature in the image  408  has a corresponding duplicate feature in the image  408  at a different wavelength. The processor  114  can further determine, based at least in part on a spacing between the specified feature and the corresponding duplicate feature, the longitudinal separation between the focus and the end  106  of the optical fiber  108 . The spacing can be compared against a specified distance, as explained above. 
       FIG.  5    shows a top view of another example of longitudinal position sensor elements that are suitable for use in the system  100  of  FIG.  1   . The beamsplitter  156  in the optical path  112  directs a light portion  502  of light beam  120  that has been reflected and/or scattered from the end  106  ( FIG.  1   ) of the optical fiber  108  ( FIG.  1   ) toward a liquid crystal on silicon (LCOS) device  504  that has a programmable aperture, a lens  506 , and a sensor  510 . The light portion  502  can form an image  508  at the sensor  510 . The sensor  510  can be coupled to the processor  114 . 
     The LCOS device  504  can use time multiplexing to achieve a similar splitting effect achieved by the elements shown in  FIGS.  1  and  4   . At a first time, a first half  504 A of the aperture of the LCOS device  504  can be reflective, while a second half  504 B of the aperture of the LCOS device  504  can be non-reflective. During the first time, the processor  114  can acquire a first image from the sensor  510 . At a second time after the first time, the first half  504 A of the aperture of the LCOS device  504  can be non-reflective, while the second half  504 B of the aperture of the LCOS device  504  can be reflective. During the second time, the processor  114  can acquire a second image from the sensor  510 . The LCOS device  504  can be configured such that a specified feature in the first image has a corresponding duplicate feature in the second image. The processor  114  can further determine, based at least in part on a spacing between the specified feature and the corresponding duplicate feature, the longitudinal separation between the focus and the end  106  of the optical fiber  108 . The spacing can be compared against a specified distance, as explained above. 
       FIG.  6    shows a top view of another example of longitudinal position sensor elements that are suitable for use in the system  100  of  FIG.  1   . The beamsplitter  156  in the optical path  112  directs a light portion  602  of light beam  120  that has been reflected and/or scattered from the end  106  ( FIG.  1   ) of the optical fiber  108  ( FIG.  1   ) toward a chromatically aberrated lens  606  and a sensor  610 . The light portion  602  can form an image  608  at the sensor  610 . The sensor  610  can be coupled to the processor  114 . 
     The light portion  602  can have a plurality of wavelengths. Because the chromatically aberrated lens  606  includes chromatic aberration, the chromatically aberrated lens  606  can bring light at one wavelength to a first focus at a first focal plane, and can bring light at a second wavelength (the second wavelength different from the first wavelength) to a second focus at a second focal plane that is separated from the first focal plane. Note that in the absence of chromatic aberration, which is typically the case with most well-designed lenses that operate at more than one wavelength, the first and second focal plane are often coincident or are nearly coincident. 
     The chromatically aberrated lens  606  can be configured such that a specified feature in the image  608  has a corresponding duplicate feature in the image  608  at a different wavelength. The processor  114  can further determine, at least in part from a size of the specified feature in the image  608  and a size of the corresponding duplicate feature in the image  608 , the longitudinal separation between the focus and the end  106  of the optical fiber  108 . In addition, or as an alternative, the processor  114  can process the image  608 , such as by perform a two-dimensional Fast Fourier Transform on the image  608 , to evaluate the sharpness of the image  608  at different wavelengths. The sharpness at the different wavelengths can help determine the longitudinal separation between the focus and the end  106  of the optical fiber  108 , and/or can help determine at least a sign of the longitudinal separation (e.g. positive or negative). 
     The configuration of  FIG.  1    uses distinct beamsplitters (such as dichroic mirror  124  and beamsplitter  156 ), distinct focusing elements (such as focusing element  126  and lens  160 ), and distinct sensors (such as imaging array  128  and sensor  162 ) to perform the tasks of imaging (such as using dichroic mirror  124 , focusing element  126 , imaging array  128 ) and of focus sensing (such as using beamsplitter  156 , lens  160 , and sensor  162 ). As an alternative, the tasks and elements can be combined, such as by using a single beamsplitter (such as dichroic mirror  124 ) and moving the longitudinal position sensing elements (such as the bi-prism of  FIG.  1   , the split-field dichroic filter of  FIG.  4   , the LCOS device of  FIG.  5   , or the chromatically aberrated lens of  FIG.  6   ) to be located between the focusing element  126  and the imaging array  128 . Such an alternative can be especially effective if the end  106  of the optical fiber  108  is illuminated with multiple wavelengths, such as three wavelengths that can all be located in a reflection band of the dichroic mirror  124  (or a transmission band if the long pass filter of  FIG.  1    is replaced with a short pass filter). 
       FIG.  7    shows a top view of the longitudinal position adjustor elements in the system  100  of  FIG.  1   . In  FIGS.  1  and  7   , the longitudinal position adjustor element can include a variable focus lens  166 . The variable focus lens  166  can be disposed in the optical path  112 , such as in the fixed portion of the optical path  112 . The processor  114  can further cause the variable focus lens  166  to adjust, based on the longitudinal separation (determined by the longitudinal position sensor elements) between the focus and the end  106  of the optical fiber  108 , a collimation of the light beam  120  to position the focus at the end  106  of the optical fiber  108 . 
       FIG.  8    shows a top view of an example of longitudinal position adjustor elements suitable for use in the system  100  of  FIG.  1   . As an alternative to using the variable focus lens  166  of  FIGS.  1  and  7   , the system  100  can include a linear actuator  802  to longitudinally position the objective element  122  with respect to the end  106  of the optical fiber  108 , to longitudinally position the entire system  100  with respect to the end  106  of the optical fiber  108 , to longitudinally position the optical fiber  108  with respect to the system  100 , or to otherwise controllably vary the separation between the system  100  and the end  106  of the optical fiber  108 . The processor  114  can control the linear actuator  802 , based on the longitudinal separation determined by the longitudinal position sensor elements. The linear actuator  802  can adjust a spacing between the focus and the end  106  of the optical fiber  108 . Other suitable actuators and actuator types can also be used. 
     Any or all of the longitudinal position sensor techniques (such as those that use the bi-prism of  FIG.  1   , the split-field dichroic filter of  FIG.  4   , the LCOS device of  FIG.  5   , the chromatically aberrated lens of  FIG.  6   , or others) can be used with any or all of the longitudinal position adjustor techniques (such as that use the variable focus lens of  FIGS.  1  and  7   , the linear actuator of  FIG.  8   , or others). Further, any or all of the longitudinal position sensor techniques and any or all of the longitudinal position adjustor techniques can be used with any or all of the configurations for the objective element (such as an objective lens or an objective mirror), any or all of the configurations for the optical fiber (such as single-core or multiple-core), any or all of the configurations for the light source (such as a single-core optical fiber, a multi-core optical fiber, a plurality of single-core fibers, or others), and any or all configurations of the pivotable elements (such as including two, omitting one and including just one, omitting both and not including any, or including more than two). 
       FIG.  9    shows a flowchart of an example of a method  900  for operating a system to direct light into an optical fiber. The system can include imaging optics, a processor, and an actuatable optical element. The actuatable optical element can define an optical path. The optical path can extend to the actuatable optical element and can further extend to the end of the optical fiber. The method  900  can be executed on the system  100  of  FIG.  1   , or on any other suitable system. 
     At operation  902 , the system can generate, with the imaging optics, an image of an end of the optical fiber. 
     At operation  904 , the system can determine, with the processor, a location in the image of a specified feature in the image. 
     At operation  906 , the system can cause, with the processor, the actuatable optical element to actuate to align the optical path to a core of the optical fiber based on the location of the specified feature in the image. 
     At operation  908 , the system can direct a light beam along the optical path to couple into the core of the optical fiber. 
     In some examples, the method  900  can optionally further include determining, with the processor, an offset between the location of the specified feature in the image and a predetermined target location in the image. The method  900  can optionally further include causing, with the processor, the actuatable optical element to actuate to reduce the offset. 
     In some examples, the specified feature can be a circumferential edge of the end of the optical fiber. The core can be located at a predetermined core location relative to the circumferential edge of the optical fiber. The method  900  can optionally further include causing, with the processor, the actuatable optical element to actuate to align the optical path to the core by causing alignment of the optical path to the predetermined core location. 
     In some examples, the method  900  can optionally further include illuminating the optical fiber with illumination that has a wavelength different from a wavelength of the light beam. 
     In some examples, the method  900  can optionally further include reflecting or scattering at least some of the illumination from the optical fiber to form first light. The method  900  can optionally further include collimating, with an objective element of the imaging optics, at least some of the first light to form second light. The method  900  can optionally further include directing, with a dichroic mirror of the imaging optics, at least some of the second light away from the optical path to form third light. The method  900  can optionally further include focusing, with a focusing element of the imaging optics, the third light to form the image at a focal plane of the focusing element. The method  900  can optionally further include sensing, with an imaging array located at the focal plane of the focusing element, the image. 
     In some examples, the method  900  can optionally further include detecting, by a longitudinal position sensor, a longitudinal separation between a focus and the end of the optical fiber. The method  900  can optionally further include positioning, with a longitudinal position adjustor, the focus to reduce the longitudinal separation. 
     In some examples, the longitudinal position adjustor can further create a duplicate feature in the image. The method  900  can optionally further include determining, by the processor and based at least in part on a spacing or a size difference between the specified feature and the corresponding duplicate feature, the longitudinal separation between the focus and the end of the optical fiber. 
     In some examples, the longitudinal position adjustor can include a variable focus lens disposed in the optical path. The method  900  can optionally further include causing, by the processor and based on the longitudinal separation between the focus and the end of the optical fiber, the variable focus lens to adjust a collimation of the light beam to position the focus at the end of the optical fiber. 
     In some examples, the longitudinal position adjustor can include an actuatable objective lens that can direct the optical path onto the end of the optical fiber. The method  900  can optionally further include causing, by the processor and based on the longitudinal separation between the focus and the end of the optical fiber, the actuatable objective lens to move to position the focus at the end of the optical fiber. 
       FIG.  10    shows a flowchart of an example of another method  1000  for operating a system to direct light into an optical fiber. The optical fiber can include a core. The system can include imaging optics, a first actuatable optical element, and a second actuatable optical element. The method  1000  can be executed on the system  100  of  FIG.  1   , or on any other suitable system. 
     At operation  1002 , the system can generate, with the imaging optics, a first image of an end of the optical fiber. 
     At operation  1004 , the system can determine, from the first image, a two-dimensional lateral location of the core on the end of the optical fiber. 
     At operation  1006 , the system can cause, based on the two-dimensional lateral location, the first actuatable optical element to actuate to laterally align an optical path to the core. The method  1000  can repeat operations  1002  through  1006  as needed until the system can determine that the optical path is sufficiently aligned to the core. When the system has completed operation  1006 , optical path is considered to be laterally aligned to the core (e.g., aligned in an x-y plane that is orthogonal to the optical path at the end of the optical fiber). 
     At operation  1008 , the system can generate, with the imaging optics, a second image of the end of the optical fiber. 
     At operation  1010 , the system can determine, from the second image, a longitudinal location of the core on the end of the optical fiber. 
     At operation  1012 , the system can cause, based on the longitudinal location, the second actuatable optical element to actuate to bring the optical path to a focus at the end of the optical fiber. The method  1000  can repeat operations  1008  through  1006  as needed until the system can determine that the focus is sufficiently close to the end of the optical fiber. When the system has completed operation  1012 , the focus of the optical path is considered to be longitudinally aligned to the end of the fiber (e.g., aligned in a z-direction that is parallel to the optical path at the end of the optical fiber). 
     In some examples, the first actuatable optical element can include a pivotable mirror. The method  1000  can optionally further include directing a light beam along the optical path to couple into the core of the optical fiber. The method  1000  can optionally further include, repeatedly performing at least the following three operations. First, the system can cause an angular position of the pivotable mirror to dither in two dimensions. Second, the system can sense an amount of light reflected from the core. In some examples, the second operation can be performed by a interrogator of a controller that is coupled to the system. Third, the system can adjust the angular position of the pivotable mirror to increase the amount of light reflected from the core. 
     In some examples, at least one of the controller or a processor included with the system can take measurements at a plurality of angular positions of the pivotable mirror (x, y) and/or a plurality of focus positions (z), measure the amount of light reflected from the core at each of the angular positions and/or focus positions, fit one or more curves to the measured amounts of light, and adjust the angular positions of the pivotable mirror and/or the focus adjustment to correspond to a local maximum of the one or more curves. 
     In some examples, the controller  10  can measure a reflected signal, using OFDR, at each location of the mirror search pattern. Various data processing techniques can be used to evaluate where the best fiber coupling is achieved (e.g., which separation between the focus of the light beam  120  and the end  106  of the optical fiber  108  provides the highest amount of light returning from the optical fiber  108  and therefore provides the highest coupling efficiency into the optical fiber  108 ). 
     In one technique, the controller  10  can sum the reflected amplitude in raw frequency domain data and report the overall reflected amplitude. Such a technique will work, but it is possible to additionally exclude some relatively large background signals. The techniques described below can exclude these relatively large background signals and can therefore increase a signal-to-noise ratio of the coupling efficiency measurement. 
     In another technique, the controller  10  can select a section of the reflected amplitude in the time domain, or optical delay domain, that represents a section of the optical fiber  108  with gratings in the cores, and sum over the selected section. For example, in the time domain, because the horizontal axis corresponds to round-trip propagation time, reflections arising from optical interfaces (such as an interface between glass and air, such as a face of a lens or window) show up as peaks along the horizontal axis. As such, the controller  10  can effectively ignore the peaks arising from these optical interfaces, and analyze data arising from light that is reflected from locations along the length of the optical fiber  108 . The controller  10  can sum the reflected amplitude for data arising from reflection(s) from along the length of the optical fiber  108 , and exclude data arising from reflection(s) from the end  106  of the optical fiber  108  or from other optical surfaces. Selecting which data to use in this manner can increase sensitivity, compared to using all the light that returns to the controller  10 . For example, selecting which data to use in this manner can show a higher signal (e.g., above a noise level) only when light is coupled into the optical fiber  108  and is reflecting from the grating structures along the length of the optical fiber  108 . 
     In still another technique, the controller  10  can select the entire optical fiber  108  region in the time domain in which gratings are present, transform the data into the frequency domain, such as by Fast Fourier Transform, and sum the amplitude only over the spectral region in which the gratings reflect. Processing the amplitude data in this manner can help reduce or eliminate low-level broadband reflected amplitude that can arise from connector reflections and other optical interfaces. 
     A computer-readable medium can store instructions that, when executed by a processor of a system for directing light into an optical fiber, cause the processor to execute operations. The system can include an actuatable optical element that defines an optical path. The optical path can extend to the actuatable optical element and can further extend to the end of the optical fiber. The operations can include at least the following four operations. First, the system can generate, with imaging optics, an image of an end of the optical fiber. Second, the system can determine, from the image, a location in the image of a specified feature in the image. Third, the system can cause the actuatable optical element to actuate to align the optical path to a core of the optical fiber based on the location of the specified feature in the image. Fourth, the system can direct a light beam along the optical path to couple into the core of the optical fiber. 
     Although the various aspects of the present invention have been described with respect to a preferred embodiment, it will be understood that the invention is entitled to full protection within the full scope of the appended claims.