Patent Description:
The technology described in this application relates to optical interrogation system measurements used for fiber optic shape and other sensing applications and to micro optic assemblies and optical interrogation systems that use the micro optic assemblies.

Optical strain sensing is a technology useful for measuring physical deformation of a waveguide caused by, for example, the change in tension, compression, or temperature of an optical fiber. A multi-core optical fiber is composed of several independent waveguides embedded within a single fiber. A continuous measure of strain along the length of a core can be derived by interpreting the optical response of the core using swept wavelength interferometry typically in the form of Optical Frequency Domain Reflectometry (OFDR) measurements. With knowledge of the relative positions of the cores along the length of the fiber, these independent strain signals may be combined to gain a measure of the strain profile applied to the multi-core optical fiber. The strain profile of the fiber refers to the measure of applied bend strain, twist strain, and/or axial strain along the length of the fiber at a high (e.g., less than <NUM> micrometers) sample resolution.

Previous patents have described OFDR-based shape sensing with multi-core optical fibers (e.g., see <CIT> and <CIT>). Some applications for OFDR-based shape sensing fiber require a high degree of confidence in terms of the accuracy and reliability of the shape sensing output. A non-limiting example application is robotic arms used in surgical or other environments.

OFDR systems are typically constructed using discrete optical fiber components such as optical couplers, polarizing beam splitters, polarization controllers, optical connectors, fanout assemblies, etc. OFDR systems become more complex, costly, and require more space as the number of cores increases because another set of discrete optical fiber components must be provided for each additional core. The inventors realized that these problems could be ameliorated if the functions performed by these discrete optical fiber components could be performed using a shared optical assembly. Various example embodiments of shared optical assemblies are described below.

<CIT> discloses an accurate measurement method and apparatus for shape sensing with a multi-core fiber. A change in optical length is detected in ones of the cores in the multi-core fiber up to a point on the multi-core fiber. A location and/or a pointing direction are/is determined at the point on the multi-core fiber based on the detected changes in optical length. The accuracy of the determination is better than <NUM>% of the optical length of the multi-core fiber up to the point on the multi-core fiber. In a preferred example embodiment, the determining includes determining a shape of at least a portion of the multi-core fiber based on the detected changes in optical length.

<CIT> discloses an encoder interferometry system including an encoder scale arranged to receive and diffract a measurement beam. The system further includes one or more optical elements configured and arranged to receive a first diffracted measurement beam and a second diffracted measurement beam from the encoder scale and to redirect the first diffracted measurement beam and the second diffracted measurement beam toward the encoder scale such that the first diffracted measurement beam and the second diffracted measurement beam propagate along non-parallel beam paths having an angular separation α following a second diffraction at the encoder scale. The system further includes a first detector arranged to receive the first diffracted measurement beam and a second detector arranged to receive the second diffracted measurement beam.

The invention relates to an optical system according to claim <NUM>. Further embodiments are claimed by the dependent claims <NUM>-<NUM>.

The following description sets forth specific details, such as particular embodiments for purposes of explanation and not limitation. But it will be appreciated by one skilled in the art that other embodiments may be employed apart from these specific details. In some instances, detailed descriptions of well-known methods, interfaces, circuits, components, and devices are omitted so as not to obscure the description with unnecessary detail. It will be appreciated by those skilled in the art that diagrams herein can represent conceptual views of illustrative circuitry, components, or other functional units.

<FIG> shows an example embodiment of an OFDR-based interrogation system for an example <NUM>-core sensing fiber. Those skilled in the art will appreciate that an OFDR-based interrogation system may be used with a single mode, single core sensing fiber or with a sensing fiber with more or fewer cores than <NUM> cores. Individual blocks shown correspond to various nodes. Those skilled in the art will appreciate that the functions of some of the blocks may be implemented using individual hardware circuits, using software programs and data in conjunction with a suitably programmed digital microprocessor or general purpose computer, and/or using applications specific integrated circuitry (ASIC), and/or using one or more digital signal processors (DSPs). Software program instructions and data may be stored on a non-transitory, computer-readable storage medium, and when the instructions are executed by a computer or other suitable processor control, the computer or processor performs the functions associated with those instructions. The functions of the various illustrated elements may be provided through the use of hardware such as hardware circuitry components and/or hardware circuitry capable of executing software in the form of coded instructions stored on computer-readable medium. Thus, such functions and illustrated functional blocks are to be understood as being either hardware-implemented and/or computer-implemented, and thus, machine-implemented. The term "processor" or "controller" also refers to other hardware capable of performing such functions and/or executing software, such as the example hardware recited above.

Light from a frequency tunable laser <NUM>, controlled in this example by the data processor (also "processor" or "controller") <NUM> rather than by the data acquisition electronics <NUM>, is split with <NUM>/<NUM> coupler between a laser monitor interferometer <NUM> and a measurement interferometer <NUM>. In the laser monitor interferometer <NUM>, the light is split into three paths using a 3x3 coupler. The first path goes to a detector to monitor laser power. The second path passes through a hydrogen cyanide (HCN) gas cell to a detector to provide an absolute wavelength reference. The final path goes through an isolator and another 3x3 coupler to two Faraday rotator mirrors (FRM) with one leg having a known delay difference from the other. The return signals from this interferometer form I/Q signals. With a phase offset of <NUM> degrees, the I/Q signals are converted to quadrature signals and used to measure the change in optical frequency as the laser sweeps.

The light going into the measurement interferometer <NUM> is split using a <NUM>/<NUM> coupler between a reference branch and measurement branch of the interferometer <NUM>. The light in the reference branch is split into six reference signals using cascaded couplers. The light in the measurement branch passes through an isolator and then through a length of erbium-doped fiber. This fiber is pumped with light from a <NUM> pump laser <NUM>, controlled in this example by the processor <NUM> rather than by the data acquisition electronics <NUM>, that couples in through a Wavelength Division Multiplexed (WDM) coupler. This combination of erbium-doped fiber and pump laser <NUM> amplifies the light in the measurement branch of the interferometer. The light passes through another isolator and then through a polarization controller set to flip the light between two orthogonal (or nearly orthogonal) polarization states on subsequent scans. The light is then split with cascading couplers into six measurement channels. The returning light is combined with the six reference paths using 2x2 couplers. These combined signals then pass through polarization beam splitters (PBSs) to two detectors (S and P) for each channel (C, I, J, K, U, V) input to the data acquisition electronics <NUM>, forming a polarization diverse detection scheme. This creates an interferometric measurement of the light reflected from up to six cores of a multicore fiber. The six channels (C, I, J, K, U, V) are connected to each core of a multicore fiber sensor <NUM> using a fanout assembly <NUM> that couples six single core fibers <NUM> to six cores in a multicore cable <NUM> connected by a connector <NUM> to the multicore fiber sensor <NUM>. The controller/data processor <NUM> controls the tunable laser <NUM>, the polarization controller, and the polarization beam splitters, and also drives the pump laser <NUM>. The data processor <NUM> also acquires and processes the data from each of the photodiode detectors provided from the data acquisition electronics <NUM>.

The recorded data is the reflected amplitude as a function of optical frequency for two polarization states, S and P, for each fiber optic core measured. The controller/data processor <NUM> linearizes this recorded data with respect to optical frequency using the data from the laser monitor interferometer <NUM> so that it is represented in equal increments of optical frequency. The linearized data is Fourier transformed into the time domain to represent the amplitude and phase of the reflected light as a function of optical delay along each fiber core. The S and P data from two sequential orthogonal polarization scans are combined to compensate for birefringence in the fiber cores and form a scalar measure of the amplitude and phase of the reflected light from each core. This combined complex signal (amplitude and phase) is compared with interferometric data recorded in a reference scan, and the resulting phase difference/change for each core is the measured signal that is used to compute the current shape of the fiber. The derivatives of the measured phase changes are proportional to the strains in each core. The bends in the x and y directions, the twist, the strain, and the temperature in the fiber may be determined from the derivatives of the measured phase changes. The details of how these parameters are determined is described in other applications and patents assigned to the current assignee of this application such as <CIT> and <CIT> identified in the introduction.

<FIG> highlights in block <NUM> example discrete optical components for one fiber core channel in <FIG>. <FIG> shows in greater detail the example discrete optical components for one fiber core channel highlighted in <FIG>. This block <NUM> from the network uses an optical coupler <NUM> to combine measurement light (M) from fiber <NUM> and reference light (R) from fiber <NUM> from one of the six measurement interferometers <NUM> of <FIG> along with a polarization beam splitter (PBS) <NUM> to split the combined light into two orthogonal linear states detectable by S polarization detecting fiber <NUM> and P polarization detecting fiber <NUM>.

<FIG> shows an example optical assembly that replaces the example discrete optical components shown in <FIG> with a recombination network implemented with bulk optics to form a bulk optic interferometer. The measurement light is provided by a fiber <NUM> that is, in this embodiment, a single mode, single core optical fiber, and is collimated by a collimating lens <NUM>. The reference light is provided by a fiber <NUM> that is, in this embodiment, a single mode, single core optical fiber and is collimated by a collimating lens <NUM>. The collimated measurement and reference light is combined by a <NUM>/<NUM> cube beam splitter <NUM> (which essentially replaces the optical coupler <NUM>) and the combined light is then refracted by a shared polarization beam splitting prism (which replaces the PBS <NUM> in various embodiments) to separate the combined measurement light and reference light into first polarized light and second polarized light that is orthogonal to the first polarized light detected by an S polarization detecting fiber <NUM> that is a single mode, single core, and an orthogonal P polarization detecting fiber <NUM> that is a single mode, single core. Although a <NUM>/<NUM> cube beam splitter is shown, other optical components may be used to perform the combining operation such as a pellicle beam splitter, or a "swiss cheese" splitter where <NUM>% (or close to <NUM>%) reflection areas alternate with <NUM>% transmissive (or close to <NUM>% transmissive) surface areas. One example shared polarization beam splitting prism is a Wallaston prism <NUM> as shown in the Figures, but, other optical components may be used to perform the shared polarization beam splitting operation such as a Nicol prism, a Glan-Taylor prism, a Rochon prism, a Senarmont prism, or a Nomarski prism.

<FIG> shows an example bulk optic interferometer similar to that in <FIG> but with the single mode, single core fibers replaced by multicore fibers including multicore measurement fiber M <NUM>, multicore reference fiber R <NUM>, multicore S polarization detecting fiber <NUM>, and multicore orthogonal P polarization detecting fiber <NUM>. For simplicity, <FIG> and many of the other figures show only two cores for each of the detecting fibers <NUM> and <NUM>, those skilled in the art appreciate that more cores may be used in practice. For example, in cases where the multicore S polarization detecting fiber <NUM>, and multicore orthogonal P polarization detecting fiber <NUM> are parts of a shape sensing system, example embodiments of shape sensing fibers may have three or more cores. Also for simplicity, only one core's light rays are shown for each of the detecting fibers <NUM> and <NUM>, and in practice other cores of these detecting fibers <NUM> and <NUM> will also have light rays. The letter f is the focal length of the collimating lens <NUM>, and 2w is the width of the collimated beam, with w being the radius of the collimated beam. L is the propagation of the collimated beam.

<FIG> gives a detailed view of the geometry of the collimating section of the bulk optic interferometer shown in <FIG>. The multicore measurement fiber <NUM> includes a center core and an offset core separated by a radius rc. The divergence angle θ of the light exiting the center core permits calculation of the beam radius w = f tan θ. The angle φ represents the difference between the propagation angle of the collimated beam from an outer core and the propagation angle (here assumed to be zero) of the collimated beam from the central core and is determined by the equation φ = arctan (rc,f). The beam displacement, Δy , due to the angle displacement of the off-center core is determined using Δy = L rc/f.

<FIG> shows an example bulk optic interferometer with one set of light beams propagating through an example bulk optic interferometer. Both the measurement and reference beams are shown, the prism <NUM> outputs include a measurement + reference s polarization beam and a measurement + reference p polarization beam.

<FIG> shows the same example bulk optic interferometer as in <FIG> but with multiple sets of light beams propagating through an example bulk optic interferometer.

The inventors determined example but non-limiting values for the variables above to demonstrate that the example bulk optic interferometer may be used in place of the optical components shown in block <NUM> of <FIG>. The Rayleigh Range zr is a measure of how far the collimated beam can propagate without substantial divergence, and it is given by: <MAT>. Starting with an example assumption that the light beam remains collimated for L=<NUM> and the Rayleigh range zr is <NUM>, and substituting in this value and a λ of <NUM> × <NUM>-<NUM> results in: <MAT> which reduces to a beam radius w of <NUM> microns. The beam diameter 2w may be rounded up to <NUM> microns with the propagation range of <NUM>. The focal length f of the collimating lens from the multicore fibers is determined using the desired beam radius w and the divergence angle θ of the light exiting the core: w = f tan θ. The divergence angle, θ can be calculated from the numerical aperture NA of the fiber core, which for example is chosen as <NUM> for the NA in air. Recall that beam displacement, Δy , due to the angle displacement of the off-center core is Δy = L rc/f. Setting the beam displacement to be some fraction of the beam diameter, the beam displacement is set to <NUM>% of the beam diameter, or <NUM>% of the beam radius, w: <MAT> Solving for the focal length, f: <MAT> and assuming a <NUM> propagation distance and a <NUM> micron core displacement, results in a focal length of <NUM>: <MAT> and a beam radius of: <MAT> or a beam diameter of D = <NUM>. Using the Lens Makers Equation: <MAT> the radius of the convex lens required to give the desired focal length is estimated as follows: <MAT> <MAT> This convex lens radius R value of <NUM> is a reasonable, non-limiting, and example value for a practical example design of a shared bulk optics interrogator.

<FIG> shows an example bulk optic interferometer with the collimating lenses in contact with the beam splitter/prism. The <NUM>/<NUM> cube beam splitter is shown now as a partial (<NUM>/<NUM>) reflector <NUM>, and the prism <NUM> as a triangle as indicated. Plano-convex collimating lenses <NUM>, <NUM>, and <NUM> are bonded in this example embodiment on their planar surfaces to obtain a monolithic optical assembly as shown that functions in the same way as the optical assembly shown in <FIG>but is more compact.

<FIG> highlights at block <NUM> even more discrete optical components from <FIG> to be consolidated into a shared recombination network implemented with bulk optics. In addition to the six coupler-PBS pairs, block <NUM> includes six coupler-connector pairs plus a fanout assembly that connects to a six core sensing fiber. For reference, one of the channels is labeled including coupler <NUM>, reference fiber <NUM>, measurement fiber <NUM>, coupler <NUM>, and sensing fiber (also "sensor fiber") <NUM>. Those skilled in the art will appreciate that an OFDR-based interrogation system may be used with a single mode, single core sensing fiber that uses just the single labeled channel or with a sensing fiber with any number of cores, such as <NUM> cores or more or fewer cores than <NUM> cores.

<FIG> shows an example shared optical assembly that replaces the example discrete optical components including the fanout portion of the system shown in <FIG> (block <NUM>) with a shared recombination network implemented with bulk optics. This single shared optic assembly is similar to that shown in <FIG> but includes a multicore sensing fiber <NUM> coupling to the assembly via plano-convex collimating lens <NUM> bonded to the splitters/prism and an additional partial (<NUM>/<NUM>) reflector <NUM> so that two partial (<NUM>/<NUM>) reflectors <NUM> and <NUM> are provided. The partial (<NUM>/<NUM>) reflector <NUM> couples the light from the multicore measurement fiber <NUM> to the sensing fiber <NUM>, and the reflected light from the sensing fiber <NUM> is combined with the reference light from reference fiber <NUM> via the partial (<NUM>/<NUM>) reflector <NUM>. The shared polarization beam splitting prism (e.g., a Wallaston prism <NUM> as shown in the Figures or some other suitable prism) separates the combined reflected light and reference light into the s polarized light detected by s detecting fiber <NUM> and second polarized light that is orthogonal to the first polarized light and detected by p detecting fiber <NUM>.

In <FIG>, all of the input and output fibers are multicore optical fibers. However, most fiber optic components in many applications are single core fibers. As a result, the inventors designed a further example embodiment where the focal length of the collimating lenses coupling the light into all of the ports of the assembly other than the sensing fiber port is increased. Increasing the focal length of these collimating lenses moves the image of the cores further apart which means that a bundle of standard single core (e.g., <NUM> micron) fibers can be used in place of the single multicore fiber.

<FIG> shows an example optical assembly similar to that shown in <FIG> that connects to a multicore sensing fiber <NUM> but that uses single core, single mode embodiments of measurement fibers 66a and 66b, single core, single mode embodiments of reference fibers 70a and 70b, single core, single mode embodiments of S detecting fibers 74a and 74b, and single core, single mode embodiments of P detecting fibers 76a and 76b. So even though the sensing fiber <NUM> is a single fiber with multiple cores, it is beneficial for the other input and output fibers to be single core fibers for cost reasons and so that splicing onto standard fiber-optic detectors or sources is available. Given the radius rc and numerical aperture NA of the sensing fiber <NUM>, the spacing and numerical aperture of the single-mode, single core input and output fiber may be calculated using the following equation: <MAT>.

In this non-limiting example, <NUM> microns is selected as the diameter Dio of the single core fiber because this is a standard fiber diameter: <MAT> Solving for the NA of the single core fiber gives a reasonable example number: <MAT> A fiber having a Numerical Aperture of <NUM> is close enough to a commonly-used NA of <NUM> to permit splicing to common fibers without excessive loss. As a result, the multicore fibers in <FIG> are replaced in <FIG> with bundles of <NUM> micron single core fibers and with longer focal length lens collimators <NUM> and <NUM> and de-collimator <NUM>.

One way of implementing each interferometer is to derive the multiple measurement and reference inputs from the power in a single fiber that is split using a 1xN coupler, where N is the number of cores in the multicore sensing fiber <NUM>. But another way described in subsequent example embodiments implements 1xN couplers in the optical assembly using microlens arrays.

The optical assembly in <FIG> removes the need for multicore to single-core conversions, but the multiple connections needed for the reference light and the measurement light requires multiple splices, which are typically undesirable. Moreover, if there are multiple fiber inputs for the reference light, each of these inputs must have a polarization that couples roughly equally into the s and p output polarization ports. Example embodiments address this problem by allowing the light from a single-mode fiber to expand until it is the size of the multi-fiber bundle, and then using a micro lens array to create virtual cores by focusing the light to spots at the appropriate core locations.

<FIG> illustrates an example microlens array <NUM> of two lenses to create two virtual cores so that two single core measurement fibers and two single core reference fibers can each be replaced with one respective single core fiber. The microlens array <NUM> includes a set of nearly overlapping small lenses manufactured onto a single piece of glass or other appropriate substrate. The array allows a single beam to be coupled into multiple cores with minimal losses and is used to create multiple "virtual cores" from a beam emanating from a single core fiber. One of these single core measurement or reference fibers is indicated at <NUM>, and two lenses on the array <NUM> are arranged so that they generate respective light beams as if they came from two "virtual" cores <NUM> and <NUM>. In other words, light coming from two cores is emulated using one core and the two lenses in the array <NUM>.

This example embodiment reduces the number of fibers, and therefore, reduces the size and cost of the fiber bundles at the measurement and reference inputs as shown in <FIG> with the example optical assembly from <FIG> incorporating microlens array <NUM> for the single core measurement fiber <NUM> and microlens array 82R for the single core reference fiber 80R.

In many applications, more than two cores are required. <FIG> shows an example optical assembly like that in <FIG> but for a ten core fiber sensor. Here, a microlens array <NUM> with seven lenses generates light from seven virtual cores from the single core measurement fiber <NUM>, and a microlens array <NUM> with seven lenses generates light from seven virtual cores from the single core reference fiber 80R. The de-collimated outputs from the shared polarization beam splitting prism (e.g., a Wallaston prism <NUM> as shown in the Figures) separate the combined reflected light and reference light into s polarized light and p polarized light that is orthogonal to the s polarized light. The s polarized light is detected by a bundle of seven single core, single mode fibers 74a-<NUM>, and the p polarized light is detected by a bundle of seven single core, single mode fibers 76a-<NUM>.

<FIG> shows a large number of discrete optical components in the example OFDR measurement system of <FIG> indicated at 100A replaced in <FIG> with an example shared optical assembly 100B. The <NUM>/<NUM> coupler and two 1x4 optical couplers in the upper left corner of 100A are replaced by the microlens array <NUM>. The simplification, consolidation, space savings, and cost savings achieved by the example shared optical assembly are significant. Those skilled in the art will appreciate that an OFDR-based interrogation system similar to that shown in <FIG> may be used with a single mode, single core sensing fiber or with a sensing fiber with <NUM> cores, three cores, or some other number or cores that is more or fewer cores than <NUM> cores. For a single mode, single core sensing fiber embodiment, there is no need for the lens arrays, and only one S polarization detecting fiber and only one P polarization detecting fiber are needed.

<FIG> show further example embodiments of a shared optical assembly using bulk optics for remote interferometry. There are advantages to incorporating significant delays in the reference signal, and these significant delays (e.g., tens of nanoseconds) often can only be implemented with lengths of optical fiber. This is why it is useful to provide separate fiber inputs for the measurement and reflection signals. On the other hand, it can also be useful to locate the interferometer close to the multicore fiber sensor. Locating the interferometer close to the multicore fiber sensor eliminates vibration errors in long lead-in fibers to the multicore fiber and can limit the number of connectors required in the system. The optical interferometer assembly shown in <FIG> uses a single input, polarization maintaining (PM) fiber <NUM> to generate both the reference and measurement inputs.

<FIG> shows a front two dimensional view of the optical interferometer assembly with individual elements labeled. <FIG> shows an exploded three dimensional view of the optical interferometer assembly. <FIG> shown a front two dimensional view of the optical interferometer assembly without individual elements labeled but with representative light beams shown and labeled.

In the case where the interferometry is located remotely, it is useful to generate a separate measurement and reference beam for each of the cores in the multicore fiber. Separate beams for each of the cores of the multicore fiber are generated by a microlens array <NUM> and are collimated with a collimating lens <NUM>. Following the micro-lens array into an L-shaped, partially-mirrored prism <NUM>, a partially reflecting (e.g., <NUM>/<NUM> percent) reflecting /transmitting surface <NUM> splits the each of the multiple beams for the multiple cores into a measurement set of beams and a reference set of beams as shown in <FIG>. The measurement light is then redirected down using a <NUM>% mirror <NUM> and passes through a polarization modulating prism <NUM>, e.g., a Faraday prism (a high current crystal) or a Pockels prism (a high voltage crystal), to modulate the polarization of the measurement light. The polarization modulation is controlled by a control signal (provided for example by the controller/data processor <NUM>) actuating electrodes 92a, 92b.

Since the light came in from a polarization maintaining (PM) fiber, the polarization within the optical assembly is deterministic and insures that the axis of the polarization modulating prism <NUM> is aligned to the polarization of the light that is to be modulated. Similarly, the axis of the Wallaston prism <NUM> is chosen to be oriented a <NUM> degrees to the reference beam polarization in order to split the reference beam (which is linearly polarized) into two linearly polarized beams (S and P) with equal power.

Returning to the measurement beam propagation, after the polarization modulation via the activated electrodes 92a, 92b, the measurement beam is reflected off of the <NUM>/<NUM> reflector <NUM> and focused into a core of the sensing fiber <NUM>. Light then reflects off of the Rayleigh scatter within the core or Bragg gratings written into the core of the sensing fiber <NUM>, and this reflected light then exits the core of the sensing fiber <NUM> and retraces the path of the measurement light. The reflected light is re-collimated by the shorter focal length collimating lens <NUM>. When the reflected light encounters the <NUM>/<NUM> reflector <NUM> that directed the measurement light into the core of the sensing fiber <NUM>, half of the reflected light passes through the <NUM>/<NUM> reflector <NUM> to the <NUM>/<NUM> reflector <NUM>. At this <NUM>/<NUM> reflector <NUM>, the light reflected from the core of the sensing fiber <NUM> is combined with the reference light. The combined reference and reflected light of the sensing fiber is then split by the Wallaston prism <NUM> into the two component S and P polarization portions and focused into two separate single cores in the S and P detecting fiber bundles 74a-g and 76a-g.

As shown in <FIG>, the reference light passes from the partially-mirrored prism <NUM> into the prism below where it is combined with the light reflected from the sensing fiber <NUM> cores. The interface between these two prisms is preferably constructed to reduce or minimize reflections. One example way of minimizing these reflections is with an anti-reflective coating <NUM> on the interface surface. Another way would be to use an epoxy that is well-matched in refractive index to the glasses being used for the two prisms that are joined.

Those skilled in the art will appreciate that, although <FIG> and <FIG> show a sensing fiber <NUM> with <NUM> cores, an optical assembly similar to that shown in <FIG> may be used with a single mode, single core sensing fiber or with a sensing fiber with any number of cores, such as more or less cores than <NUM> cores. For a single mode, single core sensing fiber embodiment, there is no need for the lens array <NUM>, and only one S polarization detecting fiber 74a and one P polarization detecting fiber 76a are needed.

Detection of complementary interference signals is desirable for a shared optical assembly in order to form differential pairs of interference signals to cancel out noise and autocorrelation signals. In <FIG>, the beams reflected from the sensor cores are transmitted through the partial reflector <NUM> and combined with the reference beam reflected by the partial reflector <NUM>. This combination of the transmitted sensor light beams and the reflected reference light beams continues to the right and is split into two polarization components by the Wallaston prism <NUM> and coupled into individual fiber cores of fibers 74a-<NUM> and 76a-<NUM> by the de-collimating lens <NUM>.

Another example embodiment that detects complementary interference signals is now described in conjunction with <FIG> and <FIG>. In <FIG>, another complementary combination of the reference and sensor light beams is created by the combination of the transmitted reference light beam (heading down) and the reflected sensor light beam (also heading down). This second, complementary set of combined light beams is also split into its component polarizations by a Wallaston prism <NUM> labeled in <FIG> and coupled into sets of single-mode fibers 110a-<NUM> and 112a-<NUM>. This second, complementary set of combined light beams forms a set of complementary signals complementary to the primary signals captured by the fiber bundles on the right side of the assembly labeled as 74a-<NUM> and 76a and <NUM>. Note that a complementary light beam from an interferometer has a characteristic that the interference is fully constructive on the primary output at 74a-<NUM> and 76a-<NUM>, and the interference is fully destructive on the complementary output captured by the fiber bundles on the bottom side of the assembly at 110a-<NUM> and 112a-<NUM>. Computing the difference between the primary output signal and the complementary output signal provides increased signal-to-noise ratio which means a better output signal.

In some cases, it is also useful to be able to switch between different shape sensing fibers. This is particularly desirable in set-up mechanics where multiple devices are moved or adjusted individually and sequentially. In this case, each device can have an integrated sensor, but a single interrogator channel interrogates each sensor as that mechanical part is moved. Optical switches are expensive, and many switches may be needed if each switch controls a single optical core. Instead of using optical switches, another aspect of the example embodiment in <FIG> and <FIG> (that may be used with or without the differential detection using the multiple sensors aspects just described) allows for inexpensive switching between multicore sensing fibers using a steerable mirror <NUM> to controllably adjust the location of the focused measurement beam through a de-collimating lens <NUM> to match the core locations in two sensing fibers 90a, 90b. The steerable mirror <NUM> may be controlled by a steering control signal (provided for example by the controller/data processor <NUM>) to adjust the direction of the focused measurement beam. For example, by the controller/data processor <NUM> can generate a steering control signal that steers the steerable mirror <NUM> (such as by directing an actuator or other system to steer the mirror portion of the steerable mirror <NUM>) to controllably adjust the location of a focus of the measurement beam between sensing fibers 90a, 90b. The controller/data processor <NUM> adjusts two currents or voltages to adjust the angle of the mirror <NUM>. Only two multicore sensing fibers (also "sensor fibers") 90a, 90b are shown, but more may be added and "switched to" using the steerable mirror <NUM>, e.g., by stacking the multicore sensing fibers in a triangular pattern, a square pattern, or any other compact arrangement. Control over the location of the focal points also means that small changes or errors in the translational position of a multicore sensing fiber can be compensated for by small adjustments in the mirror angle in various embodiments. For example, a control signal from the controller/data processor <NUM> may be configured for making small changes.

Those skilled in the art will appreciate that an optical assembly similar to that shown in <FIG> may be used with a single mode, single core sensing fiber or with a sensing fiber with more or fewer cores than <NUM> cores. A single mode, single core sensing fiber embodiment may be implemented without the lens array <NUM>, and with only one S polarization detecting fiber 74a, one P polarization detecting fiber 76a, only one S polarization detecting fiber 110a, and one P polarization detecting fiber 112a.

The shared optical assemblies described in the example embodiments above have many advantages including reduced size, reduced number of moving parts, simplified operation, reduced cost especially in as the number of cores in the multicore sensing fiber increases over two cores, and increased reliability because of fewer parts that can be isolated in a smaller volume.

The technology described above also has wide and diverse applications. One non-limiting example application is to a fiber optic shape sensing system for a robotic surgical arm in which one or more of the various technical features and/or embodiments described above may be used.

Claim 1:
An optical system comprising:
a measurement fiber (<NUM>) configured to deliver measurement light;
a reference fiber (<NUM>) arranged with the measurement fiber as part of an optical interferometer, the reference fiber (<NUM>) configured to deliver reference light;
a multicore sensing fiber (<NUM>); and
a bulk optics component comprising:
a partial reflector (<NUM>) configured to direct at least some of the measurement light from the measurement fiber (<NUM>) into the multicore sensing fiber (<NUM>) as a measurement beam, the multicore sensing fiber (<NUM>) configured to reflect at least some of the measurement beam from locations along a length of the sensing fiber (<NUM>) to form sensed light;
a beam splitter (<NUM>) to combine the sensed light and the reference light into combined light; and
a polarization beam splitting prism (<NUM>) to separate the combined light into first polarized light and second polarized light that have orthogonal polarization states,
wherein multiple cores of the multicore sensing fiber share the beam splitter and the polarization beam splitting prism.