Optical networks with polarization diversity

Abstract

The present invention relates to an optical network having an optical fiber path 12. The optical fiber path 12 includes a first optical fiber branch 14 and a second optical fiber branch 16 separate from the first optical fiber branch 14, at least one optical switch 18 for switching the input of light to the first optical fiber branch 14 and the input to the second optical fiber branch 16, a polarization beam combiner 20 at the output side of the first and second optical fiber branches 14 and 16, and at least one polarization controller 24, 26, 70 for controlling the polarization in the optical fiber path 12. The optical network further includes an optical power measurement modality 32 for measuring the optical output power of the polarization beam combiner 20. The optical network may be used in an OFDR. Furthermore, OFDR systems, OSS systems, and methods for creating time-multiplexed orthogonal polarization states of light in the optical fiber network 10 are described.

Description

[Technical Field]

[0001] The present invention generally relates to optical networks having polarization diversity. In particular, the present invention relates to optical networks for use in optical frequency domain reflectivity measurement (OFDR). Furthermore, the present invention relates to a system for OFDR, an optical shape sensing system, and a method for creating time-multiplexed orthogonal polarization states of light in an optical fiber network. [Background technology]

[0002] Optical frequency domain reflectance measurements (OFDR) require the cancellation of the birefringence effect of the sensing fiber. This requires interrogation with light having orthogonal polarization states.

[0003] A typical application of OFDR is optical fiber real shape (FORS), also known as optical shape sensing (OSS). FORS is a technique for visualizing the three-dimensional shape of medical devices such as catheters and guidewires. The shape is determined from the analysis of strain measured by a "sensor" optical fiber embedded in the device. The sensor fiber is typically a specially made 4-core optical fiber. The four cores allow for the measurement of bending (2 degrees of freedom), twisting, and temperature along the sensor fiber. One core may be located in the center of the optical fiber. The other three may be spirally wound around the central core. Here, bending of the sensor fiber introduces a linear strain profile in the cross-section of the sensor at the bending point. Twisting shortens or lengthens the outer cores depending on the direction of the twist, but the central core remains unaffected. Temperature and / or tensile strain affect all cores. All cores may have a fiber Bragg grating (FBG) with low refractive index so that the sensor fiber scatters weakly along its entire length.

[0004] The strain in a sensor fiber is discovered by measuring the optical path length and comparing these path lengths to a reference with a known shape, such as a straight line. This known shape is typically measured during the calibration procedure in manufacturing. OFDR is used for dispersion sensing of these path lengths. A wavelength-sensing structure, such as an FBG sensor, is included in the measuring (sensor) arm of the interferometer arm. Scanning the laser wavelength induces a beat frequency in the detector of the measuring interferometer of the interrogator. The beat frequency corresponds to the imbalance in the interferometer, i.e., the difference in optical path length between the measuring arm and the reference arm. The Fourier transform allows the signals from different points on the sensor to be separated. The phase of the beat signal (for all points) is a very sensitive measure of the difference in optical path length, but has the limitation that it is wrapped, i.e., measured modulo 2. The change in the difference in optical path length between points on the sensor fiber when the shape signal is compared to the reference signal is a measure of the accumulated strain between those points.

[0005] A curved optical fiber exhibits birefringence, meaning its refractive index depends on the polarization state (SOP) of the light propagating through the fiber. Consequently, the optical path length measured using OFDR depends on the polarization of the light supplied to the sensor fiber and the shape of the sensor fiber. This is an undesirable effect as it impairs comparison with a reference shape. A solution to this is to use a so-called polarization diversity approach. The sensor is interrogated using two orthogonal polarization states. Orthogonality is maintained throughout the optical path, provided that polarization-dependent loss (PDL) does not play a significant role. Here, the average optical path length is independent of the sensor's input polarization and birefringence.

[0006] Current FORS systems use an electro-optical polarization controller to switch between two polarization states. This type of controller has several advantages. The controller's speed allows for the high frame rate (60 Hz) required for the clinical need to visualize shapes to physicians at video speed. The controller has low polarization mode dispersion (PMD) and therefore has the ability to create orthogonal polarization over the overall wavelength range (e.g., 1530 to 1560 nm) using three control elements, as long as there is low PMD at the controller's input.

[0007] The use of electro-optic polarization controllers may have several drawbacks. The main drawback is that this type of controller can become highly sensitive to a sharp increase in insertion loss after approximately 1000 hours of operation due to aging. Furthermore, the controller's response is highly sensitive to temperature. In practice, this requires (almost) continuous monitoring of polarization to ensure orthogonality of the polarization state. Deviations from orthogonality cannot be directly observed in OFDR data, and therefore, additional measurement means must be present in the system.

[0008] A method for detecting deviations from orthogonality is to use a polarizing beam splitter (PBS) with two photodiodes. First, one state is aligned to the minimum transmission rate of one of the detectors. After switching to the second state, the power of the second detector is a measure of deviation from orthogonality. In the case of orthogonality, the power of the second detector is minimum.

[0009] Piezo-based polarization controllers offer an alternative to electro-optical polarization controllers for creating polarization diversity. However, they suffer from slow response, long-term drift, hysteresis, and inherent PMDs.

[0010] PMD introduces a dependence of polarization on the wavelength of light. Therefore, creating two orthogonal polarization states across the entire wavelength range becomes more complex. Creating orthogonal states requires more degrees of freedom to control polarization. Furthermore, this dependence of polarization on wavelength impairs the observation of deviations from orthogonality using PBS, as it is not possible to create a state with the lowest transmission rate in the photodetector across the entire wavelength range. Therefore, deviations from orthogonality must be observed by different means, such as using a polarization analyzer.

[0011] US2006 / 0164627A1, corresponding to WO2004 / 005973A2, discloses polarization diversity detection without a polarization beam splitter. This document describes an optical fiber measuring device used in an OFDR that performs polarization diversity detection without using a polarization beam splitter. The field vector from one interferometer arm is used as the basis for projecting the field vector from the other interferometer arm. The optical network uses a relatively large number of couplers, thus increasing the system cost.

[0012] US2014 / 0140691A1 discloses a multifunctional optical tool that can be used for embedded fault detection and transceiver source characterization in local optical communication networks. A single device provides swept heterodyne optical spectral analysis (SHOSA) and optical frequency domain reflectivity measurement (OFDR) by utilizing a common interrogation laser light source, common optical components, and common low-bandwidth acquisition hardware. [Overview of the project] [Problems that the invention aims to solve]

[0013] There is still a need for improvement in controlling the orthogonality of polarization states to overcome at least some of the above shortcomings.

[0014] The object of the present invention is to provide an optical network, especially for use in OFDR, which is capable of creating orthogonally polarized states of light, involving the use of durable components resistant to rapid switching over an extended time period.

[0015] A further object of the present invention is to provide an optical network capable of creating substantially orthogonally polarized states of light without the need to detect the state of polarization.

[0016] A further object of the present invention is to provide an optical network capable of creating orthogonally polarized states of light with relaxed requirements for the speed and accuracy of the polarization controller.

[0017] A further object of the present invention is to provide a system for optical frequency domain reflectometry measurement.

[0018] A further object of the present invention is to provide an optical shape sensing system.

[0019] A further object of the present invention is to provide a method for creating time-multiplexed orthogonally polarized states of light in an optical fiber network.

Means for Solving the Problems

[0020] In a first aspect of the present invention, an optical network for use in optical frequency domain reflectometry measurement is provided, the optical network comprising a first optical fiber branch and a second optical fiber branch distinct from the first optical fiber branch, at least one optical switch for switching the input of light to the first optical fiber branch and the input to the second optical fiber branch, a polarization beam combiner on the output side of the first and second optical fiber branches, and at least one polarization controller for controlling the polarization of the optical fiber path, an optical fiber path having the above, an optical power measurement modality for measuring the output power of the polarization beam combiner, It holds.

[0021] The optical network according to the present invention provides a novel polarization diversity method that can be simply called "polarization diversity by switching and coupling". The optical input is switched between a first and a second optical fiber branch via at least one optical switch. That is, the light is alternately input to the first optical fiber branch and the second optical fiber branch. At least one optical switch can switch the input to the first and second optical fiber branches in synchronization with the wavelength scan of the light. For example, the light from the first wavelength scan is input only to the first optical fiber branch, the light from the subsequent second wavelength scan is input only to the second optical fiber branch, the light from the subsequent third wavelength scan is input only to the first optical fiber branch, and so on. The light from the first optical fiber branch and the second optical fiber branch is supplied to a polarization beam combiner (PBC). The PBC may be an optical device having a first input port, a second input port, and one output port. The first optical fiber branch and the second optical fiber branch are connected to the PBC. The first optical fiber branch may be connected to the first input port of the PBC, and the second optical fiber branch may be connected to the second input port of the PBC. The output ports may be connected to a single fiber of an interferometer in an OFDR system, for example. The first input port may provide minimum insertion loss for a first polarization state and maximum insertion loss for a second polarization state orthogonal to the first polarization state. The second input port may provide minimum insertion loss for the second polarization state and maximum insertion loss for the first polarization state. When the PBC is operating ideally, it transmits only the polarization states aligned to each port. In practice, the PBC may have a finite extinction ratio. At the output ports of the PBC, the transmitted polarization states are essentially orthogonal or very close to orthogonal. For example, if the optical output from the first optical fiber branch has a first polarization state aligned to the first input port of the PBC, this polarization state is transmitted by the PBC. If the optical output from the first optical fiber branch has a polarization state orthogonal to the first polarization state, this polarization state is not transmitted by the PBC.If the optical output from the second optical fiber branch has a second polarization state aligned to the second input port of the PBC, this polarization state is transmitted by the PBC. If the optical output from the second optical fiber branch has a polarization state orthogonal to the second polarization state, this polarization state is also transmitted by the PBC. The polarization at the output of the polarizing beam combiner switches between the two polarization states. Due to the optical properties of the polarizing beam combiner, these states have orthogonal polarization (or very close to orthogonal polarization). Therefore, as an advantage, there is no need to detect the polarization state.

[0022] The polarization of light within the optical fiber path is controlled by at least one polarization controller. The polarization controller has the function of setting the polarization state of the light supplied to it. Setting the polarization state may include changing the polarization state from an undefined state to a defined state, or from a first defined state to a second defined state different from the first state. As will be described later, at least one polarization controller may be placed at various locations in the optical fiber path.

[0023] The optical power measurement modality for measuring the output power of a polarized beam combiner in an optical network according to the present invention may be used to control at least one polarization controller to minimize insertion loss in the polarized beam combiner. Measurement of optical power at the output of the polarized beam combiner may be performed using (semi)continuous feedback during manufacturing, maintenance, system startup, periodically, or during system operation. The power measurement modality may be integrated into the system in which the optical network forms part. For example, a signal detector in an OFDR system can be used as a power measurement modality.

[0024] The optical network according to the present invention has several advantages over known optical networks. The only component that operates at high frequency (e.g., 60 cycles per second) is at least one optical switch. It is known for its durability and lasts for about 10 years before wear. 9Optical switches, particularly MEMS-based optical switches, exist that are specified to switch a certain number of times. The optical network according to the present invention does not require a polarization controller that rapidly switches between two polarization states. Therefore, the optical network according to the present invention can be constructed with durable components and is thus resistant to rapid switching over extended time periods.

[0025] The optical network according to the present invention operates over a wide range of wavelengths, requires only very coarse input polarization alignment, does not require means to detect deviations from orthogonality, is resistant to a considerable amount of PMD, does not require components with short lifetimes, and requires only slow polarization control.

[0026] The optical network according to the present invention is particularly advantageous in applications requiring high-speed switching between two orthogonal polarization states. The optical network according to the present invention is particularly advantageous for use in optical frequency domain reflectance measurements, especially in optical shape sensing.

[0027] Preferred embodiments of the present invention are defined in the dependent claims and described herein.

[0028] At least one polarization controller may be located in one of the first and second optical fiber branches. If at least one polarization controller is located in one of the first and second optical fiber branches, the polarization state can be properly aligned with at least one input port of the polarization beam combiner. If the at least one polarization controller includes only one polarization controller, for example located in one of the first and second optical fiber branches, or upstream of at least one optical switch, cost savings can be achieved with fewer controllers.

[0029] Particularly preferred is the case where at least one polarization controller includes a first polarization controller located in the first optical fiber branch and a second polarization controller located in the second optical fiber branch. This arrangement allows the polarization of the light to be well controlled in both the first and second optical fiber branches, and in particular for the minimum insertion loss in the polarization beam combiner, in other words, for the maximum transmission of the corresponding polarization state through the polarization beam combiner. For example, if the first optical fiber branch is connected to the "s" port of the polarization beam combiner and the second optical fiber branch is connected to the "p" port of the polarization beam combiner, the polarization controller of the first optical fiber branch is controlled for "s" polarization and the polarization controller of the second optical fiber branch is controlled for "p" polarization. A further advantage of this embodiment is that both controllers can be optimized and controlled independently of each other.

[0030] Furthermore, at least one polarization controller may include a first polarization controller located in one of the first and second optical fiber branches, and a second polarization controller located upstream of the optical switch.

[0031] In this case, the polarization controller in one of the optical fiber branches cannot be controlled or optimized independently of the polarization controller before the optical switch. In this case, both polarization controllers can be optimized sequentially.

[0032] At least one polarization controller may be a mechanical controller that relies on bending the optical fiber (also known as a "bat ear" or "paddle controller"), an electro-optical polarization controller, or a piezo-based polarization controller. As described above, the optical network according to the present invention requires only low-speed polarization control. Polarization control may be required only on a timescale of external influences on the system, rather than on the frame rate of the system. The polarization controller used in the optical network according to the present invention should have a sufficiently low PMD so that it can control polarization over a desired wavelength range to minimize loss.

[0033] Preferably, at least one optical switch is a MEMS-based optical switch. MEMS-based optical switches are advantageous due to their high durability and fast switching capability.

[0034] Preferably, at least one optical switch includes one bipolar optical switch connected to both the first and second optical fiber branches. In this embodiment, the optical switch has one input port and two output ports, the first of the two output ports connected to the first optical fiber branch and the second of the two output ports connected to the second optical fiber branch.

[0035] Alternatively, at least one optical switch includes a first unipolar optical switch connected to a first optical fiber branch and a second unipolar optical switch connected to a second optical fiber branch. The advantage of this embodiment is that only unipolar optical switches are required, thus reducing costs. In this arrangement, a splitter in the optical fiber path prior to the two unipolar optical switches may be required to split the light to the two unipolar optical switches.

[0036] As already mentioned, when the optical network is configured for use with an OFDR, it is possible and particularly preferable that at least one optical switch be synchronized with the wavelength scanning of light emitted by the light source so that the wavelength scanning is performed alternately in mutually orthogonal polarization states at the output of the PBC.

[0037] More preferably, at least one polarization controller may be configured to set the polarization based on the measured output power of the polarized beam combiner. Thus, the polarization controller may set the polarization state for the maximum output power at the output port of the PBC, i.e., for the minimum insertion loss in the polarized beam combiner. For this purpose, the optical power measurement modality may be configured to control at least one polarization controller.

[0038] In a further aspect of the present invention, a system for measuring reflectance in the optical frequency range is provided, and the system is A light source that emits light, A first type of optical network supplied with light emitted from a light source, It holds.

[0039] Preferably, the light source is configured to trigger at least one optical switch according to a wavelength scan of light.

[0040] In yet another embodiment, an optical shape sensing (OSS) system is provided, comprising the system according to the previous embodiment and an elongated optical fiber connected to an optical network for interrogating the optical fiber. The optical network according to the present invention may be part of the interrogator of the OSS system.

[0041] The elongated optical fiber may be incorporated into a shape-sensing device, such as a guidewire or catheter, or other shape-sensing medical device. The optical output from a polarizing beam combiner may be supplied to the elongated optical fiber of the shape-sensing device.

[0042] According to yet another aspect of the present invention, a method is provided for creating time-multiplexed orthogonal polarization states of light in an optical fiber network, and this method is A step of inputting an optical beam into an optical fiber path, wherein the optical fiber path has a first optical fiber branch and a second optical fiber branch located away from the first optical fiber branch. A step of switching the input of light to the first optical fiber branch and the input of light to the second optical fiber branch, A step of controlling the polarization of light within the optical fiber path, The steps include inputting light from the first and second optical fiber branches into a polarized beam combiner, The steps include measuring the optical output power of a polarized beam combiner, It holds.

[0043] It should be understood that the systems and methods described in the claims have preferred embodiments that are similar to and / or identical to the optical networks described in the claims, particularly as provided in the dependent claims and disclosed herein.

[0044] These and other aspects of the present invention will become apparent from and be explained with reference to the embodiments described below. [Brief explanation of the drawing]

[0045] [Figure 1] This shows the optical layout of an optical network according to one embodiment. [Figure 2] The optical layout of the measurement setup for investigating the effect of finite extinction ratio on polarization is shown. [Figure 3a] This graph shows the dependence of PCB transmission on the angle of incident polarization relative to the maximum transmission axis of the PCB. [Figure 3b] This graph shows the relationship between the polarization state at the PBC exit and the angle between the PBC's maximum transmission axis and the polarization state at the PBC inlet and the angle between the PBC's maximum transmission axis. [Figure 4]This shows the interference experiment setup to verify the orthogonality of the "p" port and "s" port of the PBC. [Figure 5a] The graph shows the results of interferometry to verify the orthogonality of light transmitted by the "p" arm and "s" arm of an optical network, and the results when both arms are aligned for maximum transmission through the PBC. [Figure 5b] The graph shows the results of interferometry used to verify the orthogonality of light transmitted by the "p" arm and "s" arm of an optical network, with the results shown for the case where one arm is aligned for optimal transmission and the other is aligned for minimum transmission. [Figure 6a] The optical layout of an alternative embodiment of the optical network is shown. [Figure 6b] The optical layout of an alternative embodiment of the optical network is shown. [Figure 7a] The optical layout of a further alternative embodiment of the optical network is shown. [Figure 7b] The optical layout of a further alternative embodiment of the optical network is shown. [Figure 8] This shows an optical layout for a further alternative embodiment of the optical network. [Figure 9] This shows an optical layout for a further alternative embodiment of the optical network. [Figure 10] The block diagram of the OFDR system is shown. [Figure 11] A block diagram of the optical shape sensing system is shown. [Modes for carrying out the invention]

[0046] Figure 1 shows a sketch of the optical layout of the optical network 10 according to this disclosure. The optical network 10 provides polarization diversity by switching and coupling (hereinafter referred to as PDSC). The optical network 10 has an optical fiber path 12. The optical fiber path 12 is constructed of one or more optical fibers. The optical fiber path 12 has a first optical fiber branch 14 and a second optical fiber branch 16 separated from the first optical fiber path 14. Each of the first optical fiber branch 14 and the second optical fiber branch 16 may have a single optical fiber. The optical fiber path 12 further has at least one optical switch 18, which here is a single optical switch. The optical switch 18 is a bipolar optical switch, i.e., the switch has an input port IN and two output ports A and B. Output port A is connected to the first optical fiber branch 14, and output port B is connected to the second optical fiber branch 16. The optical switch 18 is configured to switch the optical input to the first optical fiber branch 14 and the optical input to the second optical fiber branch 16. Preferably, the optical switch 18 is a MEMS-based optical fiber switch.

[0047] The optical fiber path 12 further comprises a polarized beam combiner (PBC) 20. The PBC 20 has two input ports, denoted as "s" and "p," and one output port, denoted as "OUT." In each case, the "s" and "p" ports of the PBC 20 provide maximum transmission for polarization states aligned to these ports, and minimum transmission for polarization states not aligned to these ports, particularly those orthogonal to the polarization state of maximum transmission. A first optical fiber branch 14 is connected to the s port, and a second optical fiber branch 16 is connected to the p port of the PBC 20. The output port OUT of the PBC 20 may be connected to a single optical fiber 22, which may be part of an interferometer or connected to an interferometer.

[0048] The optical fiber path 12 further includes at least one polarization controller. In this embodiment, the at least one polarization controller includes a first polarization controller 24 located in the first optical fiber branch 14 and a second polarization controller 26 located in the second optical fiber branch 16. The polarization controllers 24 and 26 may be mechanical controllers (also called butt-ear or paddle controllers), electro-optic controllers, or piezo-based polarization controllers that rely on bending the optical fibers of the respective optical fiber branches 14 and 16. Each polarization controller 24 and 26 has an input port indicated as IN and an output port indicated as OUT. The outputs of both polarization controllers 24 and 26 are supplied to the PBC 20.

[0049] Figure 1 further shows a light source 28. In particular, the light source 28 is a tunable light source. The light source may be a laser, especially a tunable laser. The light source 28 may emit light in a wavelength range, for example, a range with a width of 30 nm. For example, the wavelength range may be 1515 nm to 1545 nm. The light source 28 may form part of the optical network 10 or it may be a separate modality. The light emitted by the light source 28 is supplied to the optical network 10 via a single optical fiber 30 connected to the input port IN of the optical switch 18.

[0050] In the operation of the optical network 10, light from the light source 28 is supplied to the optical switch 18. The optical switch 18 may switch between output ports A and B in synchronization with the wavelength scan of the light source 28, such that after each wavelength scan, the light output from the switch 18 changes from output port A to output port B, or vice versa. Therefore, the light input to branches 14 and 16 may occur alternately after each wavelength scan. In Figure 1, line 29 indicates the light source trigger used to toggle the optical switch 18 in synchronization with the wavelength scan of the light source 28. After each wavelength scan of the light source 28, the light source 28 triggers the switch 18 to change the optical input to the other of optical fiber branches 14 and 16.

[0051] Therefore, the optical input to the PBC20 is switched between the two input ports "s" and "p" of the PBC20. As a result, the polarization at the output of the PBC 20 switches between two polarization states. Due to the optical properties of the PBC 20, these states have orthogonal polarization (or very close to orthogonal polarization).

[0052] The optical network 10 further includes an optical power measurement modality 32 for measuring the optical output power of the PBC 20. One or more output power signals from the optical power measurement modality 32 may be used to control polarization controllers 24 and 26 to set the polarization state at branches 14 and 16 to the "s" and "p" ports of the PBC 20 and optimal alignment, respectively.

[0053] The characteristics of the components of the optical network 10 described above and below will be explained in more detail below.

[0054] A polarized beam combiner (PBC), such as the PBC 20, is an optical device with two input ports labeled "s" and "p" and one output port labeled "OUT". Here, the labels "s" and "p" are used conventionally but do not have clearly defined meanings in optical fiber systems. However, "s" and "p" here refer to mutually orthogonal polarization states. An ideal PBC transmits only polarization states aligned to each input port. That is, the s port transmits s-polarization and absorbs or dissipates p-polarization, and vice versa for the p port. By definition, the p-polarization and s-polarization states are mutually orthogonal.

[0055] In reality, PBCs have a finite extinction ratio. This means that transmission is not zero for misaligned polarization, such as s-polarization supplied to a p-port. The ratio of maximum to minimum transmission is called the extinction ratio ρ and is expressed in a convenient form in dB. Transmission in a PBC with a finite extinction ratio and neglecting all nominal losses is given by Mars's law. T=(1+Γcos(φ in)) / (1+Γ) (1) Here, φ in This represents the angle of incident polarization relative to the maximum transmission axis of the PBC. Angle φ in Γ is measured on a Poincaré sphere (deg pc) and is zero for perfect alignment and 180° for perfect misalignment (where mutually orthogonal polarization states lie on the diameter of the Poincaré sphere and are represented by vectors in opposite directions enclosing an angle of 180°). The "unit loss" Γ has a value between 0 and 1 and is related to the extinction rate ρ by Γ. (1-Γ) / (1+Γ)=10^(-ρ / 10) (2)

[0056] Next, the effect of the finite extinction ratio on the polarization state of light at the output of PBC20 will be explained with reference to Figures 2 to 5. The effect of the finite PBC extinction ratio on the polarization state at the output OUT of PBC20 was tested by two experiments.

[0057] Figure 2 shows a first measurement setup for determining the effect of the finite extinction ratio on polarization at the output port OUT of the PBC20. The measurement setup has a light source, such as a laser, as shown in light source 28 in Figure 1. Light source 28 is connected to a polarization controller 36 via optical fiber 35, and the polarization controller 36 is connected to a splitter 38 via optical fiber 37. The splitter 38 is a 50-50 splitter. One output port of the splitter 38 is connected to a first polarization analyzer 40 via optical fiber 39. The output port of the splitter 38 is connected to one of the input ports ("s" or "p") of the PBC20 via optical fiber 41. The output port OUT of the PBC20 is connected to a second polarization analyzer 42 via optical fiber 43.

[0058] The first experiment investigates the effect of input polarization shift on PBC20. Light from light source 28 is supplied to one of the input ports of PBC20 (port "s" in Figure 2). The polarization at both the input and output of PBC20 is measured using the first and second polarization analyzers 40 and 42.

[0059] The PBC 20 is supplied with a wide range of polarization states. FIG. 3a shows the measured relationship between φ in (as defined above) and the transmission through the PBC 20. Malus' law (Equation 1) provides a good fit of the data, with the fitting resulting in ρ = 24.7 dB.

[0060] φ out If φ is the angle (in deg pc) between the state of polarization (SOP) at the exit or output port OUT of the PBC 20 and the maximum transmission axis of the PBC, then the relationship between φ in and φ out is shown in FIG. 3b. The output SOP appears to be within a few degrees of the maximum transmission axis, regardless of the polarization state entering the PBC 20. The angle φ out increases somewhat for high values of φ in . When φ in approaches 180° (maximum misalignment), a deviation of up to 10° is observed. However, at these high angles, the light experiences very low transmission. Thus, the measured increase in the output angle φ out may be affected by the bias due to the offset or non-linearity of the polarization analyzer 42 at low signal levels. Thus, it may be concluded that a finite extinction ratio does not, or only slightly, affect the output polarization of the PBC 20.

[0061] The second experiment was performed using the measurement setup shown in Figure 4, confirming that the light transmitted through the p-port and s-port is orthogonal. The measurement setup shown in Figure 4 has a light source 28 connected to a splitter 46, in this case a 50-50 splitter, via optical fiber 45. One output port of the splitter 46 is connected to a delay line 48, which is connected to a first optical switch 50 via optical fiber 49. The other output port of the splitter 46 is connected to a further optical switch 52 via optical fiber 51. The output ports of switches 50 and 52 are connected to the input ports IN of polarization controllers 54 and 56 via optical fibers 53 and 55, respectively. The output ports OUT of controllers 54 and 56 are connected to the s-port and p-port of the PCB 20 via fibers 57 and 59, respectively. The PCB 20 is connected to a detector 58. Thus, the PCB 20 is supplied with light from the light source 28, which may be a single scanning laser light source. The delay line 48 provides an intentional difference in arm length to the p-port and s-port of the PCB 20. The output port OUT of the PBC20 is connected to the photodetector 58. This setup takes advantage of the fact that orthogonal polarizations do not interfere with each other, but deviations from orthogonality produce a beat signal in the photodetector 58. The signal D(t) is given by: D(t)=|E p | 2 +|E s | 2 +2|E p ||E s |cos(θ / 2)cos(ωt+φ) (3)

[0062] Here, |E p | 2 and |E s | 2 θ is the power transmitted by the p-port and s-port, respectively, θ is the angle between the polarization state vectors of the light transmitted by the p-port and s-port on the Poincaré sphere, and cos(ωt+φ) is an interference term with a beat frequency ω that is proportional to the difference (delay) in the length of the interferometer arms and the laser scan speed.

[0063] To measure θ, three measurements were performed using the measurement setup shown in Figure 4. During the first measurement, the p-arm was disconnected using the optical fiber switch 52. That is, no light was supplied from the light source 28 to the p-port of the PBC 20. Therefore, the value recorded by the photodetector 58 was equal to the power of the s-arm, and D1(t) = |E| s | 2 Therefore, in the second measurement, the power of the other arm is measured, i.e., the optical switch 50 is opened and the optical switch 52 is closed. Thus, in the second measurement, D2(t) = |E p | 2 However, it is measured. Finally, both switches 50 and 52 are closed so that both arms are connected to measure D3(t) as given in Equation 3 above. Envelope tracking by a bandpass filter BPF and Hilbert transform around ω is used to obtain the magnitude of the beat signal b = Envelope{BPF{D3(t)}}, where θ is obtained using the following: θ = 2 arccos(b / 2|E e ||E o (4)

[0064] Figures 5a and 5b show the measurement results. Figure 5a shows the case where both arms are aligned for maximum transmission through PBC20. The angle θ is measured to be greater than 179.7°. This measurement is limited by noise from light source 28 passing through the bandpass filter. An estimate of the effect of this noise is shown by curve 90 in Figure 5a. This indicates that the measured deviation from orthogonality may be due to the noise.

[0065] The interference measurement setup is also used to verify that the orthogonality of the SOP at the output port OUT of the PBC20 is hardly affected by the polarization alignment at the input ports s and p of the PBC20. This is done by aligning one of the arms in Figure 4 to the minimum transmission and therefore having a strong misalignment, while the other branch is aligned to the maximum transmission. The results of this measurement are shown in Figure 5b. The angle between the s branch and the p branch is determined to be greater than 179°. Again, the measured deviation from orthogonality can be explained by noise. The estimated effect of the noise is also shown by curve 92 in Figure 5b.

[0066] In conclusion, it is observed that the SOPs of light transmitted through the p-port and s-port of the PBC20 are orthogonal for practical purposes, regardless of any misalignment of the SOPs at the PBC input, even in the presence of a finite PBC extinction ratio.

[0067] Another advantage of the PDSC as disclosed here is its operation over a wide range of wavelengths. Most polarization controllers rely on introducing birefringence into optical elements such as optical fibers. As a result, the resulting change in polarization is wavelength-dependent. This effect is even more pronounced when the polarization controller relies on slight modulation of a strong nominal birefringence, such as piezoelectric or thermal-based polarization controllers where polarization is controlled by modulating a large nominal birefringence. In contrast, the PBC 20 relies on loss rather than introducing a difference in optical path length to correct the SOP, and is therefore wavelength-independent. Ultimately, the wavelength range is limited by the support of the individual components. If switch 18 is a MEMS-based optical switch, it accepts a very wide wavelength range of ~400 nm at a nominal wavelength of 1550 nm. The PBC 20 accepts a range of 80 nm at a nominal wavelength of 1550 nm. This is significantly wider than the width required for, for example, optical shape sensing.

[0068] Another beneficial effect of PDSC is that it requires only very coarse input polarization alignment and does not require any means to detect deviations from orthogonality. PDSC creates an essentially orthogonal polarization state as described above. However, to minimize the insertion loss of PBC20, alignment of the input polarization at input ports s and p of PBC20 is required. Preferably, when the optical network 10 is used in an OFDR system, the control signals that can be used to control the polarization controllers 24 and 26 to set the input polarization at branches 14 and 16 for minimum insertion loss by PBC20 can be the magnitude of the OFDR signal already available in the OFDR system. No other means for detecting orthogonality, such as a polarization analyzer, are required.

[0069] The required precision of the input polarization alignment to the input port of the PBC 20 depends on the acceptable level of loss. These losses are given by Mars's law (Equation 1 above). For example, if a loss of 1 dB is acceptable, the alignment must be better than 54°, and precision better than 17° imposes a loss of less than 0.1 dB. In any case, the constraints on alignment precision are orders of magnitude lower than the deviation achieved from orthogonality.

[0070] In the optical network 10, the optimization of the polarization output of the polarization controllers 24 and 26 can be performed using a numerical search method such as the Nelder-Mead algorithm. This has the advantage that it requires minimal (if any) calibration of the polarization controllers 24 and 26. An additional advantage is its strong robustness to environmental changes, such as temperature, and that when the optical network is used in FORS, orthogonality is maintained so that the alignment optimization does not interfere with shape sensing. Only a slight decrease in the signal-to-noise ratio may be observed if transmission is reduced.

[0071] Another advantage of PDSC is its resistance to a considerable amount of polarization mode dispersion (PMD).

[0072] PMD presents a polarization alignment challenge when operating with broadband or scanning light sources such as light source 28. (Center wavelength λ) c A properly aligned system may be misaligned for other wavelengths. PDSC is resistant to several PMDs, either present in the input or introduced by polarization controllers 24, 26. c Assuming proper alignment, PMD introduces misalignment to input ports s and p of PBC 20, and therefore introduces transmission loss to PBC 20. The amount of PMD that can be tolerated depends on how much loss can be tolerated, as with misalignment of controllers 24, 26. For example, consider a system in which the PBC has a center wavelength λ1 = 1545 nm and a final wavelength λ2 = 1555 nm where the maximum transmission occurs. Assuming 1 dB is acceptable, the SOP for λ2 cannot deviate from the SOP for λ1 by more than 17 on the Poincaré sphere. This corresponds to a maximum PMD of 0.012 ps. For an acceptable loss level of 0.1 dB, a PMD of 0.04 ps is still acceptable.

[0073] Another advantage of PDSC is that it does not require components with short lifespans. High-frequency switching (60 cycles / second) between two polarization states can lead to failure of the electro-optic polarization controller. In contrast, in PDSC, the only component that operates at high frequencies is the optical switch 18. MEMS-based optical switches are known for their durability and last ~10 years before wear. 9 It is specified to switch a certain number of times.

[0074] Accelerated lifetime testing was performed on 10 optical fiber MEMS switches (sold by Sercalo Microtechnology Ltd.). The switches were operated at 300 cycles per second, with each cycle being an out-and-back switch. The switches passed 5-10 without any apparent increase in insertion loss, no significant change in switching time, and no missing transitions. 9 We performed more cycles.

[0075] Although not the last, another advantage of PDSC is that it only requires slow polarization control.

[0076] Polarization control in PDSCs is required only on the timescale of external influences on the system, not on the system's frame rate. The main cause of polarization changes is temperature changes. A rough estimate of the maximum drift after startup is 30° per hour. Considering the coarse alignment required to minimize transmission loss, the polarization controllers 24 and 26 must be optimized several times per hour, on the order of tens of degrees. Optimization is typically performed on the order of 10 iterations. In conclusion, the polarization controllers 24 and 26 are expected to operate at an average frequency three orders of magnitude lower than the system's frame rate, unlike conventional polarization diversity methods. Apart from the lifespan advantage, the lower speed requirements also offer significant room for cost reduction, as PDSCs are compatible with a wide range of polarization controller types.

[0077] Further embodiments of the optical network will be described with reference to Figures 6 to 9. Components or parts of the optical network described below that are identical, similar to, or equivalent to the components of the optical network 10 in Figure 1 will be labeled with the same reference numerals as in Figure 1.

[0078] In the following, only the differences between the embodiments shown in Figures 6 to 9 and the embodiment shown in Figure 1 will be described. Unless otherwise indicated, the above description also applies to the following embodiments.

[0079] The optical power measurement modality 32 in Figure 1 is omitted in Figures 6 to 9, but may be present in all embodiments described below.

[0080] Figures 6a and 6b show an optical network 10 in which only the polarization controller 26 is located in the second optical fiber branch 60, and the second polarization controller 70 is located upstream in the optical fiber path 12, i.e., before the optical switch 18. Figure 6b shows an optical network 10 in which the polarization controller 24 is located in the first optical fiber branch 14, and the second polarization controller 70 is located in the optical fiber path 12 before the optical switch 18. In both cases of Figures 6a and 6b, the polarization controllers 26 and 24 become dependent on the polarization controller 70 between the light source 28 and the optical switch 18, respectively. In such cases, the polarization controllers 70 and 26, and 70 and 24, should be optimized sequentially, respectively.

[0081] Figures 7a and 7b show optical networks 10 having only one polarization controller, i.e., one polarization controller 26 in the second optical fiber branch 16, or only one polarization controller 24 in the first optical fiber branch 14. Although control over polarization is reduced in the optical networks 10 shown in Figures 7a and 7b, the cost of these embodiments is reduced compared to Figure 1 because only one polarization controller is used.

[0082] The optical network 10 shown in Figure 8 is similar to the embodiments in Figures 7a and 7b, with only one polarization controller 70 positioned before the optical switch 18. Again, control over polarization is less complete compared to the optical network 10 in Figure 1, but cost reduction is achieved because only one polarization controller is used.

[0083] In the optical network 10 shown in Figure 9, the bipolar optical switch 18 in Figure 1 is replaced by two unipolar switches 18a and 18b. In the embodiment of Figure 9, the splitter 72 is positioned between the light source 28 and the optical switches 18a and 18b, and may be a 50-50 splitter.

[0084] The PDSC-based optical network 10 can be used in any OFDR system. Figure 10 shows a sketch of an OFDR system having a light source L, such as a light source 28, in particular a tunable laser, the optical network ON as taught in this disclosure, and an interferometer I. The light source L may be configured to trigger at least one optical switch 18 or optical switches 18a, 18b for the optical network ON.

[0085] PDSC can be used as a modular hardware component as part of the optical network of a FORS system, thereby making the FORS system more robust, more accurate, and less expensive. Figure 11 shows one embodiment of a FORS system, also referred to as an optical shape sensing system, having a light source L, in particular a tunable laser, an optical network ON as taught in this disclosure, an interferometer I, and a shape sensing device 80, for example, a catheter or guidewire with an optical fiber 82 connected to the interferometer I. The optical network ON as taught in this disclosure may form part of the interrogator of the FORS system.

[0086] Although the present invention has been illustrated and described in detail in the drawings and the foregoing description, such illustrations and descriptions should be considered descriptive or illustrative and not limiting. The present invention is not limited to the disclosed embodiments. Other modifications of the disclosed embodiments can be understood and implemented by those skilled in the art in carrying out the claimed invention, from a consideration of the drawings, disclosure and appended claims.

[0087] In the claims, the words “comprising” do not exclude other components or steps, and the indefinite articles “a” or “an” do not exclude plurality. A single element or other unit may fulfill the function of several items enumerated in the claims. The mere fact that certain means are described in different dependent claims does not imply that combinations of these means cannot be used advantageously.

[0088] No reference numeral in a claim should be construed as limiting the scope.

Claims

1. Optical fiber path, A first optical fiber branch and a second optical fiber branch separate from the first optical fiber branch, At least one optical switch for switching the optical input to the first optical fiber branch and the optical input to the second optical fiber branch, The polarization beam combiners on the output side of the first and second optical fiber branches, and At least one polarization controller for controlling the polarization in the optical fiber path, The optical fiber path having, An optical power measurement modality for measuring the optical output power of the aforementioned polarized beam combiner, An optical network having

2. The optical network according to claim 1, wherein the at least one polarization controller is located in one of the first and second optical fiber branches or upstream of the optical switch.

3. The optical network according to claim 1, wherein the at least one polarization controller comprises a first polarization controller located in the first optical fiber branch and a second polarization controller located in the second optical fiber branch.

4. The optical network according to claim 1, wherein the at least one polarization controller comprises a first polarization controller located in one of the first and second optical fiber branches, and a second polarization controller located upstream of the optical switch.

5. The optical network according to claim 1, wherein the at least one optical switch is a MEMS-based optical switch.

6. The optical network according to claim 1, wherein the at least one optical switch has a bipolar optical switch connected to both the first and second optical fiber branches.

7. The optical network according to claim 1, wherein the at least one optical switch comprises a first unipolar optical switch connected to the first optical fiber branch and a second unipolar optical switch connected to the second optical fiber branch.

8. The optical network according to claim 1, wherein the optical switch is synchronized with the wavelength scanning of light emitted by a light source such that the wavelength scanning is performed alternately at the output of the polarizing beam combiner.

9. The optical network according to claim 1, wherein the at least one polarization controller is configured to set the polarization based on the measured output power of the polarization beam combiner.

10. The optical network according to claim 1, wherein the optical power measurement modality is configured to control the at least one polarization controller.

11. A light source that emits light, An optical network according to any one of claims 1 to 9, supplied with light emitted by the aforementioned light source, A reflectance measurement system in the optical frequency range.

12. The system according to claim 11, wherein the light source is configured to trigger the at least one optical switch by scanning the wavelength of the light.

13. An optical shape sensing system comprising the system according to claim 11 and an elongated optical fiber connected to the optical network for interrogating the optical fiber.

14. In a method for creating time-multiplexed orthogonal polarization states of light in an optical fiber network, A step of inputting an optical beam into an optical fiber path, wherein the optical fiber path has a first optical fiber branch and a second optical fiber branch separate from the first optical fiber branch, A step of switching the input of light to the first optical fiber branch and the input of light to the second optical fiber branch, A step of controlling the polarization of the light in the optical fiber path, The steps include inputting light from the first and second optical fiber branches into a polarizing beam combiner, The steps include measuring the optical output power of the polarized beam combiner, A method of having.

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