WO2014072845A1

WO2014072845A1 - Optical frequency domain reflectometry system with multiple fibers per detection chain

Info

Publication number: WO2014072845A1
Application number: PCT/IB2013/058445
Authority: WO
Inventors: Jeroen Jan Lambertus Horikx; Gert Wim 't Hooft; Milan Jan Henri MARELL; Merel Danielle Leistikow; Jinfeng Huang; Eibert Gerjan VAN PUTTEN
Original assignee: Koninklijke Philips NV
Current assignee: Koninklijke Philips NV
Priority date: 2012-11-09
Filing date: 2013-09-11
Publication date: 2014-05-15
Anticipated expiration: 2015-05-09

Abstract

An optical frequency domain reflectometry (OFDR) system is provided with an optical radiation source (LS) connected to a plurality of measurement paths (MP1, MP2, MP3) and a reference path (RFP) via a first coupling point (CP1). The measurement paths (MP1, MP2, MP3) are arranged for optical connection to respective optical sensing fibers (SF1, SF2, SF3). The measurement paths (MP1, MP2, MP3) have different optical path lengths (L1, L2, L3), so as to allow an optical detection unit (ODU) to essentially uncorruptedly detect optical radiation from parts of the respectiveoptical sensing fibers (SF1, SF2, SF3) that are intended to have a sensory function. The optical detection unit (ODU) is connected to the reference path (RFP) and the measurement paths (MP1, MP2, MP3) via a second coupling point (CP2). Thus, via the common reference path (RFP) for several measurement paths (MP1, MP2, MP3), only one detection chain is required to serve several sensing fibers (SF1, SF2, SF3).

Description

Optical frequency domain reflectometry system with multiple fibers per detection chain

FIELD OF THE INVENTION

The present invention relates to systems for optical analysis, more particularly the present invention relates to an optical frequency domain reflectometry (OFDR) system and a method for obtaining optical frequency domain reflectometry data.

BACKGROUND OF THE INVENTION

In Optical Frequency Domain Reflectometry (OFDR), light from a tunable laser source is coupled into a measurement fiber, or more generally, a device under testing (DUT), and the reflected or backscattered light is made to interfere with light from the same source that has travelled along a reference path yielding information about the fiber, or the DUT.

For measurements on a fiber, in the case that the frequency of the laser source is swept linearly in time, the interference between the light that is coming from a single fixed scattering point on the measurement fiber and the reference light creates a detector signal that has a constant frequency, this frequency being proportional to the difference of the travel time of the light along the measurement path and the reference path. As the propagation velocity of the light and the length of the reference path are known, the position of the scattering point can be computed from the observed frequency.

When multiple scatterers are present in the measurement fiber, the detector signal will be a superposition of different frequencies, each frequency indicative of the position of the respective scatterer. A Fourier transform of the detector signal (a 'scattering profile') can be computed; in graphs of the amplitude and phase of the transformed signal, the amplitude and phase of the different frequencies that are present in the detector signal (which correspond to different scatterer positions) will be shown at their respective positions along the horizontal axis of the graph.

The amplitude and phase of the scattered light can be affected by external influences acting on the fiber. E.g., when the fiber is deformed by external stresses, or when the temperature of the fiber is modified, effects will be seen on the phase and/or amplitude of the scattering profile. From a comparison of the scattering profile of the fiber to the scattering profile of the same fiber in a reference state, information can be obtained about the external influences on the fiber as a function of position along the fiber; i.e. the fiber can be used for distributed sensing.

When multiple sensing fibers are measured simultaneously according to the state of the art of OFDR, a separate interferometer is created for every sensing fiber. Each sensing fiber has its own measurement path (or arm), its own reference (or arm), its own beam combiner and its own detection chain, detector electronics, and one digital acquisition channel per detector. In demanding applications, OFDR systems capable of using polarization diversity are normal. In such polarization diversity systems, a detection chain comprises two detectors attached to a polarizing beam splitter, and a polarization controller for balancing the light from the reference arm on the two detectors.

This means that the cost of building a system will increase significantly with the number of fibers to be measured. Moreover, the settings of the polarization controller required for balancing the amount of light from the reference path or arm on the two detectors attached to the PBS need to be determined separately for each detection channel, increasing the complexity of the alignment procedure with the number of fibers to be measured.

SUMMARY OF THE INVENTION

Following the above, it would be advantageous to provide an improved Optical Frequency Domain Reflectometry (OFDR) system which is suited for handling simultaneous measurement on multiple fibers without the severe increase in cost and complexity of prior art OFDR systems.

In a first aspect, the invention provides an OFDR system comprising:

- an optical radiation source (LS) capable of emitting optical radiation within a certain wavelength band,

- a first coupling point (CP1) arranged for splitting the radiation from the optical radiation source (LS),

- a plurality of measurement paths optically connected to the first coupling point, and wherein the measurement paths are arranged for optical connection to respective optical sensing fibers (SF1, SF2),

- a reference path optically connected to the first coupling point (CP1),

- a second coupling point (CP2) optically connected to the reference path and the plurality of measurement paths, and - an optical detection unit (ODU) optically arranged to detect the combined optical radiation from the reference path and the plurality of measurement paths via the second coupling point (CP2), and to generate an electrical detection signal (E) accordingly,

wherein the plurality of measurement paths have different optical path lengths, so as to allow the optical detection unit (ODU) to essentially uncorruptedly detect optical radiation from parts of the respective optical sensing fibers that are intended to have a sensory function.

In the OFDR system according to the invention, multiple optical sensing fibers share a reference path (or reference arm), and in some embodiments, it is possibly also to share one beam combiner. Consequently, one single optical detection unit can serve more than one sensing fiber. This is obtained by different optical path length for the measurement paths (or measurement arms) to which sensing fibers are connected, and in such a manner that data corruption by overlap in the scattering profile can be prevented or at least reduced to an acceptable degree. This can be done by selection of optical properties of the respective measurement paths so as to ensure that they exhibit optical path lengths which are different to allow the optical detection unit to discriminate the respective reflected radiation parts from the sensing fibers, and/or by proper signal processing on the data provided by the optical detection unit. In some cases a partial overlap can be acceptable, while in other cases the optical lengths of the measurement paths are selected to differ so as to allow non-overlapping optical radiation parts from the relevant parts of radiation from all the connected sensing fibers.

The different optical path lengths result in a relative offset of the frequencies of the detector signal components that result from the respective reflected radiation parts from the sensing fibers. The discrimination occurs because they end up at different positions in the Fourier transform, which separates different frequencies. The underlying reason that a scatterer causes a signal component with a certain frequency is that the travel time of the light that reaches the detector via the scatterer and the travel time of the light that reaches the detector via the reference path are different, so that they correspond to different laser frequencies, as the laser is scanning. The difference in laser frequency, which is the frequency of the detector signal, is proportional to this travel time difference, and can thus be manipulated by changing the optical path lengths of the measurement paths. A typical scan duration is of the order of tens of milliseconds, while preferred changes of the travel time differences or delays of the measurement path lengths are of the order of tens of nanoseconds, for example a difference of travel time difference within 5-100 ns, or within 10-80 ns, or within 20-60 ns. With the OFDR system of the invention, it is possible to lower the price of a measurement system or to simultaneously measure more than one shape sensing fiber using only a single detection system.

Embodiments of the invention for demanding applications of distributed optical sensing using OFDR, e.g. in medical probes or catheters, typically use polarization diversity as this provides the highest precision. However, it should be noted that polarization diversity is not essential to the invention, since the invention also works in embodiments without polarization-diverse measurements.

In one embodiment, the plurality of measurement paths have different optical properties selected so as to provide optical path lengths which differ so as to allow the optical detection unit (ODU) to detect non-overlapping optical radiation from said parts of the respective optical sensing fibers that are intended to have a sensory function. However, in other embodiments, an overlap may be acceptable, which will be explained in the following.

In one embodiment, a separate optical circulator is connected in each of at least two of the plurality of measurement paths between the first coupling point (CP1) and sensing fibers connected to the respective measurement paths.

In one embodiment, each of the plurality of sensing fibers is connected to the second coupling point (CP2) via respective separate optical circulators.

In one embodiment, each of the plurality of measurement paths is connected to the first coupling point (CP1) via respective separate optical circulators.

In one embodiment, at least two sensing fibers are connected to the first coupling point (CP1) via one common optical circulator. Especially, at least two sensing fibers may be connected to the common optical circulator via an optical splitter.

In one embodiment, at least one of the plurality of measurement paths has a delay fiber inserted, so as to influence its optical path length. Especially, a plurality of measurement paths have different delay fibers with providing different delays.

In one embodiment, at least two of the plurality of measurement paths have different physical path lengths serving to provide different optical path lengths. Especially, three or more measurement paths have different physical path lengths serving to provide different optical path lengths.

In one embodiment, at least two of the plurality of measurement paths have different optical properties serving to provide different optical path lengths, e.g. different refraction index values. Especially, three or more measurement paths have different optical properties, e.g. refraction index values, serving to provide different optical path lengths. In one embodiment, at least two of the plurality of measurement paths have different optical path lengths by means of a combination of different physical path lengths and different optical properties.

The OFDR system may comprise a plurality of sensing fibers connected to respective ones of the plurality of measurement paths. In one specific embodiment, the plurality of sensing fibers are arranged within a sensing probe or catheter, so as to allow sensing of a shape of the sensing probe.

The optical radiation source may comprise a laser source arranged for scanning over a wavelength range, e.g. scanning over a small wavelength range within the range 320-2200 nm which is the range over which quartz-based optical fibers could transmit, as a specific example of a scanning wavelength range is 1530-1550 nm. In some

embodiments, the OFDR system comprises a control system arranged to control the scanning wavelength of the laser source.

In embodiments arranged for polarization diverse detection, the detection unit is arranged for polarization-diverse detection of the combined optical radiation from the reference path and the plurality of measurement paths. Especially, the optical detection unit may comprise a polarizing beam splitter (PBS) connected to two detectors (Dl, D2) serving to detect radiation of respective different polarization.

In one embodiment, a polarization controller is arranged between the optical radiation source and the first coupling point.

In preferred embodiments, the OFDR system is based on a transmissive interferometer (a Mach-Zender interferometer).

In the context of the present application, it is to be understood that a scan of the said wavelength band of the radiation source may be considered to include, but not limited to, a substantially continuous variation of the wavelength in an appropriate interval, e.g. within a wavelength of 1530-1550 nm. Under practical conditions, it is further understood that a scan may also be considered as a relatively large number of wavelengths, preferably homogeneously distributed, measured in a fixed order, typically from one end to the other of the interval.

In the context of the present application, it is to be understood that an optical circulator is a non-reversible optical component, where radiation, or light, entering from a first port, exits from a second port and then, upon reentering the optical circulator via the second port, the radiation exits the optical circulator from a third port thereby causing separation between radiation entering the first port and exiting the third port. It would be appreciated by the skilled person in optics that various suitable optical circulators may be applied in the context of the present invention.

In the context of the present application, it is to be understood that the optical path length (OPL) may be considered to be the product of the geometric length and the index of refraction of the medium through which the radiation, or light, is propagating. Optical path length is important because it determines the phase of the light and governs interference and diffraction of light as it propagates.

In the context of the present application, in embodiments utilizing polarization diversity, it is to be understood that the polarization, e.g. the first and second polarization, may be linearly, circularly or elliptically polarized depending on the circumstances and the application of the invention.

In the context of the present application, it is to be understood that by

'coupling point' is understood an optical coupling point of two or more optical paths, e.g. in the form of an optical combiner or optical splitter. Thus, the coupling point serves to combine or split incoming optical radiation from one or more incoming optical paths into one or more outgoing optical paths.

It is envisioned that the present invention may facilitate a broad spectrum of use. The invention is applicable in all fields where distributed sensing using the method of OFDR can be used. Properties that can be measured with this technique are, e.g., strain and temperature. A field of particular interest might be the simultaneous measurement of strain in cores of a helical multi-core fiber, for the purpose of shape-sensing, in particular for medical applications.

The measurement branch of the OFDR system may also find application in other areas of optics, e.g. telecommunication, where such detection and analysis is required.

According to some embodiments, the OFDR system comprises optical sensing fibers connected to respective plurality of measurement paths, the optical sensing fibers being arranged for providing reflections for OFDR along a sensing length (l_s) of the optical sensing fiber. In specific embodiments, there may be more than 2 optical sensing fibers and corresponding measurement paths, e.g. 3, 4, 5, 6, 7, 8, 9 or more optical sensing fibers connected to respective measurement paths. Especially, the optical sensing fibers are arranged within a probe or catheter. In specific embodiments, one or more optical sensing fibers may be placed centrally. In specific embodiments, one or more optical sensing fibers may be placed peripherally, such as being helically arranged. The optical sensing fibers preferably include spatially distributed optical elements, such as Fiber Bragg Gratings or other types of optical elements such as known to the skilled person.

In another specific embodiment, there may be one central optical sensing fiber and one or more, such as three, peripheral optical sensing fibers, such as the one or more peripheral optical sensing fibers being helically arranged. In some embodiments, one optical sensing fiber may have a plurality of optical cores being arranged for e.g. shape-sensing.

The OFDR system may comprise a catheter or probe with the plurality of optical sensing fibers arranged therein and connected to respective measurement paths, and a signal processing system connected to receive the electrical signal generated by the optical detection unit.

Especially, the OFDR system may be or may form part of a medical device or system.

In some embodiments, the OFDR system comprises a plurality of sub systems according to the first aspect, wherein each sub system comprises a plurality of measurement paths connected to a common reference path and an optical detector unit. It is to be understood, that one or more OFDR systems according to the first aspect of the invention, serving to operate a plurality of optical sensing fibers, may be used also together with prior art OFDR systems that serve to operate other optical sensing fiber(s).

According to a second aspect, the invention provides a method for obtaining optical frequency domain reflectometry (OFDR) data, the method comprising:

- providing (SI) an optical radiation source (LS) and emitting (S2) optical radiation within a certain wavelength band, the radiation source being optically connected to a first coupling point (CP1) arranged for splitting the optical radiation from the optical radiation source (LS),

- providing (S3) a reference path (RFP) optically connected to the first coupling point (CP1), - providing (S4) a plurality of measurement paths (17) optically connected to the first coupling point, and arranged for optical connection to respective optical sensing fibers,

- providing (S5) a second coupling point (CP2) optically connected to the reference path and the plurality of measurement paths,

- detecting (S6) a combined optical radiation from the reference path and the plurality of measurement paths via the second coupling point, and

- generating (S7) an electrical signal accordingly.

It is understood that the order in which the method steps are listed is not essential. In a third aspect of the invention, the OFDR system and method of the first and second aspects may be used in a medical application, such as used for medical scanning or medical diagnosis purposes.

In general the various aspects of the invention may be combined and coupled in any way possible within the scope of the invention. These and other aspects, features and/or advantages of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be described, by way of example only, with reference to the drawings, in which

FIG. 1 shows a schematic embodiment of a prior art OFDR system with separate measurement paths and reference paths and detection units for each sensing fiber,

FIG. 2 shows a schematic embodiment of an OFDR system suitable for multiple sensing fibers according to the present invention,

FIG. 3 shows a prior art single sensing fiber polarization diverse OFDR system to explain the principle of polarization diversity,

FIG. 4 shows a schematic embodiment with two sensing fibers connected via separate optical circulators,

FIG. 5 shows a diagram of the scattering profile of a sensing fiber with length l_s, connected to an optical circulator via a connecting fiber with length l_c,

FIG. 6 shows a diagram of the scattering profile for the embodiment of FIG. 4, FIG. 7 shows a schematic embodiment where two sensing fibers are connected via one single optical circulator and an optical splitter,

FIG. 8 shows a diagram of the scattering profile for the embodiment of FIG. 7,

FIG. 9a and 9b show examples of amplitude versus fiber index spectra based on Fourier transforms of detector signals obtained from the embodiment of FIG. 7,

FIG. 10 shows a generalization to N sensing fibers of the embodiment of FIG.

4,

FIG. 11 shows a variant of the embodiment of FIG. 10 for connecting N sensing fibers,

FIG. 12 shows a generalization to N sensing fibers of the embodiment of FIG.

7, and

FIG. 13 shows a flow chart of a method according to the present invention. DESCRIPTION OF EMBODIMENTS

FIG. 1 shows an example of a prior art OFDR system, where two optical sensing fibers SF1, SF2 are connected to a laser source LS via respective measurement paths MP1, MP2 and connected to respective reference paths RFP1, RFP2. Thus, also two detection system with respective detectors Dl, D2 and polarization beam splitters PBS are required, thereby also requiring an analyzing or processing system equipped to analyze the outputs from the total of four detectors Dl, D2. Thus, in systems with many sensing fibers, all elements within the dashed boxes need to be copied, thus resulting in two detector signals for each sensing fiber which needs to be analyzed.

FIG. 2 shows a schematic embodiment of the invention, serving to illustrate the principle of the invention. An optical radiation source in the form of a scanning laser LS, e.g. capable of being scanned over the wavelength range 1530-1550 nm. The laser LS is connected to a first coupling point CPl, e.g. an optical splitter, where the optical radiation is split to a plurality of measurement paths, here three measurement paths MP1, MP2, MP3 are illustrated and connected to respective optical sensing fibers SF1, SF2, SF3. Further, a reference path RFP is connected to receive optical radiation from the first coupling point CPl . The sensing fibers SF1, SF2, SF3 are illustrated here as arranged within a probe or catheter PR, such as an optical shape sensing probe or catheter.

Optical radiation reflected parts from the sensing fibers SF1, SF2, SF3 and measurement paths MP1, MP2, MP3 as well as from reference path RFP are received at a second coupling point CP2, and an optical detection unit ODU is connected to this second coupling point CP2 to receive the combined optical radiation from all measurement paths MP1, MP2, MP3 and the reference path RFP, and to convert them to one or more electrical signals E, e.g. digital electrical signals representing the detected combined optical radiation. The further signal processing equipment required to process the output E from the optical detection unit ODU is not shown, but it is outside the focus of the present invention.

Each measurement path MP1, MP2, MP3 has an optical path length LI, L2, L3 associated thereto, and according to the invention these optical path lengths LI, L2, L3 are selected to be different, so as to allow the optical detection unit ODU to discriminate. The goal of providing different optical path lengths LI, L2, L3 can be achieved in different ways, e.g. introducing optical delays, different physical path lengths, or introducing in general an optical element which serves the to introduce an optical path length difference in one or more of the measurement paths MP1, MP2, MP3. In the following, the principle behind polarization diverse measurement using an OFDR system will be explained in spite of the fact that this is not essential to the invention, however the specific embodiments of the invention described later use this principle.

In OFDR, light from a tunable laser source is coupled into a measurement fiber, and the reflected or backscattered light is made to interfere with light from the same source that has travelled along a reference path. When the frequency of the laser source is swept linearly in time, the interference between the light that is coming from a single fixed scattering point on the measurement fiber and the reference light creates a detector signal that has a constant frequency, this frequency being proportional to the difference of the travel time of the light along the measurement path and the reference path. As the propagation velocity of the light and the length of the reference path are known, the position of the scattering point can be computed from the observed frequency.

When stresses are applied to an optical fiber, e.g. when it is bent, birefringence is induced, which in general will cause a variation of the state of polarization of the light travelling along the fiber. The polarization state of light scattered at different position of the fiber upon arrival at the detector will vary as well. Thus, light reflected from certain parts of the sensing fiber may have a polarization state at the detector which is (nearly) orthogonal to the polarization state of the light that arrives at the detector via the reference path.

Consequently, the strength of the interference signal coming from these certain parts of the sensing fiber will be very low. A known solution to this problem of 'polarization fading' is polarization-diverse detection, usually in the embodiment of a polarizing beam splitter (PBS) with separate detectors for the two polarization states transmitted by the PBS. See, e.g.: B. J. Soller, M. Wolfe, M. E. Froggatt, "Polarization resolved measurement of Rayleigh backscatter in fiber-optic components ", OFC Technical Digest 2005, paper NWD3.

In a birefringent fiber, the refractive index depends on the state of polarization of the light. Consequently, the phases of the Fourier transforms of the detector signals in a polarization-diverse measurement will vary upon modification of the input polarization state of the light that is sent into the measurement fiber. In order to accurately assess the effect of an external influence on the fiber properties two measurements need to be performed; for the second measurement the input polarization state of the light sent to the fiber is made orthogonal to the polarization state used in the first measurement. In this manner, four detector signals are obtained (two detector signals for each of two input polarization states). From the Fourier transforms of these four signals, a single effective scattering profile may be computed that, when compared to the effective scattering profile of the reference state, provides the desired information about the external influences on the fiber as a function of position. See, e.g., patent application US2011/0109898 Al .

Fig. 3 illustrates a prior art OFDR system with one single sensing fiber SF for distributed sensing, and capable of performing the measurements described in the above sections. A tunable laser is connected to an interferometer via a polarization controller pc thus introducing two polarizations PLl, PL2. A splitter distributes the light over the reference path RFP and the measurement path MP of the interferometer. An optical circulator C is used to attach the sensing fiber to the measurement arm. The light from the reference path RFP and the measurement path MP is combined and sent to a polarization-diverse detection system consisting of a polarizing beam splitter PBS and two detectors Dl, D2. For purposes of calibration, a fraction of the laser light is sent to a wavelength reference cell HCN, filled with a gas, e.g. HCN gas, that contains absorption lines with very well-known wavelengths in the range over which the laser LS is scanned. Part of the laser light is also sent to an auxiliary interferometer AUX, which generates a signal that is used to linearize the scan. All detector signals are digitized by a signal acquisition system; the digitized signals are sent to a computer for further processing.

Scan linearization is required to ensure a one-to-one correspondence between scatterer position on the fiber and frequency of the detector signal. Linearization can be done in one of several ways. The signal from the auxiliary interferometer AUX can be used to make the laser frequency depend linearly on time, by means of a feedback loop. Another possibility is to use the signal from the auxiliary interferometer AUX to define the sampling moments of the signal acquisition system. Alternatively, all detector signals can be sampled at a constant rate, but the digitized signal from the auxiliary interferometer AUX is used as input to a resampling algorithm that computes interpolated signals corresponding to a precisely linear scan.

An example of a measurement cycle: the polarization controller pc is used to create a first polarization state PL1 that balances the amount of light from the reference path on the two detectors Dl and D2. The laser LS is then scanned over a wavelength range that is slightly larger than the desired range, and all detector signals are digitized and stored. Then, the polarization is changed to a second state PL2 that is orthogonal to the first state PL1. Again, the laser LS is scanned and the detector signals are digitized and stored. For each scan separately, all data are linearized; the co-linearized wavelength calibration signal is then used to select the sub-ranges of sampled signals that correspond to the desired wavelength interval. The selected signal sub-ranges of Dl and D2 for the first and second polarization state are then Fourier transformed using an implementation of the Discrete Fourier Transform (e.g. a Fast Fourier Transform, FFT), and from these four scattering profiles a single effective scattering profile is computed.

The output of the Discrete Fourier Transform is in the form of discrete bins, which can be referred to by their index number. As is customary in the output data of routines that compute a Discrete Fourier Transform (DFT), the negative frequencies appear in the upper-frequency half of the computed scattering profile.

It can be shown that one unit change of fiber index corresponds to a change of physical path length difference of the interferometer equal to Δ^/ = ^ η - Α7λ ' ¹⁾ where n is the group index in the interferometer, _c is the center of the desired wavelength range of the laser scan and Αλ the desired range. The optical path length difference equals n-Al. For a sensing fiber that is used in reflection, a physical path length difference of Al is equivalent to a change of position along the sensing fiber of Az = Δ//2, so a unit change of fiber index corresponds to a change of position along the sensing fiber equal to

In an example: A_c = 1540 nm, Αλ = 20 nm and « * 1.48, resulting in Δζ ¾ 40 μηι.

As an example, a section of fiber containing Fiber Bragg Gratings that is 2 to 4 meters distant from the point on the sensing fiber for which the length of the measurement arm is equal to the length of the reference arm will then correspond to a fiber index range of approximately 50000-100000.

The time-sampled data used to compute the scattering spectrum may be linearized using a relatively simple, but therefore fast, interpolation algorithm. To keep the interpolation error small the samples need to be close together, resulting in a relatively large number of sampling points. In an example: one million points may be present in the desired wavelength range, and thus in the result of the Discrete Fourier Transform.

Discrete Fourier Transforms (DFTs) of the measured detector signals are used to arrive at the scattering profile of the sensing fiber. Neighboring points of a Fourier transform correspond to points on the sensing fiber that are a distance Az apart, with Az given by Eq.(2). The zero-frequency point of the computed Fourier transform (fiber- index equal to 0) corresponds to the (possibly virtual) point on the sensing fiber for which the lengths of the reference arm and the measurement arm of the interferometer are equal. For an N-point DFT, the maximum frequency (fiber- index N/2) corresponds to a (possibly virtual) point on the sensing fiber that is at a distance L from the zero-frequency point, with L given by

N

L =— Az. (3)

2

The points with fiber indices in the range N/2+1 ... N-l correspond to negative frequencies. In an example with N= 10⁶ and Az = 40 μιη, L will be 20 m.

As already mentioned, although the use of polarization diversity may be preferred in the most demanding applications of distributed sensing using OFDR, this is not essential for the invention to work. The invention also works in applications that do not require polarization-diverse measurements.

FIG. 4 shows an OFDR system embodiment according to the invention that is capable of measuring two sensing fibers SF1, SF2 using a single reference path, a single polarization beam splitter PBS and detectors Dl, D2. In this embodiment, a separate optical circulator CI, C2 is used for each sensing fiber SF1, SF2. The optical path length of the measurement path to which sensing fiber SF1 is attached is made to be different from the length of the measurement path to which sensing fiber SF2 is attached. A delay dl is introduced in the measurement path for sensing fiber SF2 to provide this measurement path difference. The reference path is common to both fibers SF1 , SF2.

As only a single reference path and detection channel is required to operate a plurality of sensing fibers according to the invention, the settings of the polarization controller pc required for balancing the amount of light from the reference path on the two detectors Dl and D2 need to be determined only once for the plurality of sensing fibers, limiting the complexity of the alignment procedure compared to prior art systems.

FIG. 5 illustrates backscattered light, amplitude 'a' and position 'p', from a sensing fiber SF of length l_s which is attached to an optical circulator C of a measurement system via a connecting fiber CF of length l_c. Backscattered light from both the sensing fiber SF and the connecting fiber CF will reach the detectors, and will end up in the computed scattering profile. No backscattered light will reach the detectors from (virtual) positions that lie before the circulator C or after the physical end of the sensing fiber SF. Thus, the use of a circulator C to connect the sensing fiber SF to the measurement system ensures that the backscattered light from the connecting fiber CF and the sensing fiber SF occupies a space of limited extent in the scattering profile.

The zero-frequency point of the scattering profile (i.e. the point with fiber- index equal to 0) corresponds to the (possibly virtual) point on the sensing fiber SF for which the optical lengths of the reference path and the measurement path of the interferometer are equal. FIG. 5 schematically shows the contributions of the connecting fiber CF and the sensing fiber SF to the computed scattering profile for the case that the equal-length point lies at a distance lo to the left of the start of the connecting fiber CF.

The embodiment of FIG. 4 has measurement paths with different optical path lengths for the two sensing fibers SF1 , SF2. Consequently, the distance lo for sensing fiber SF1 is different from the distance lo for sensing fiber SF2. The difference between these distances, expressed in meters (i.e. physical length), is denoted Δ₁₂ in the following. The optical path length difference equals «·Δ₁₂, where n is the refractive index of the fibers. The difference in optical length between the two measurement paths (length of measurement path 2 minus length of measurement path 1) that is required to create this difference is equal to 2«·Δ₁₂. If the refractive indices of the measurement paths and the fibers are all equal, the required difference between the physical lengths of the two measurement paths is equal to 2Δ₁₂. See Eq.(2) for the conversion factor between fiber index and position.

FIG. 6 including schematics (a), (b), and (c) will be described in the following. The location of the start of the connecting fiber for sensing fiber 1 relative to the zero- frequency point for interferometer 1 will be denoted IQ. Thus, the location of the start of the connecting fiber for sensing fiber 2 relative to the zero-frequency point for interferometer 2 will be equal to lo + Δ₁₂. Here, interferometer 1 is the interferometer that consists of the combination of the measurement path to which sensing fiber 1 is attached and the common reference path, and interferometer 2 is the combination of the measurement path to which sensing fiber 2 is attached and the same common reference path.

First, consider the case that both l₀ and to lo + Δ₁₂ are positive, i.e. the reference path is shorter than each of the measurement paths. Labels 1 and 2 are chosen such that Δ₁₂ is positive, cf. FIG. 6 (a).

The magnitude of the shift Δ₁₂ should be chosen in such a manner that the data coming from neither sensing fiber is contaminated. At the very least, this implies that the sensing fiber data do not overlap in the scattering profile, giving rise to the condition

When the shift Δ₁₂ only just satisfies Eq. (4), the data of sensing fiber 1 will have an overlap with the data of connecting fiber 2 in the scattering profile. When the scattered signal of a sensing fiber is much stronger than the Rayleigh scattering of a connecting fiber (e.g. when the sensing fiber contains FBGs) this overlap may be acceptable. However, often the connecting fiber data also contains spurious reflections from connectors, fiber splices etc. that are strong enough to contaminate overlapping sensing fiber data. In this case, or in the case that the signal of the sensing fiber and the connecting fiber are of comparable strength, it would be wise to increase the magnitude of the shift Δ₁₂ ίο a larger value, to prevent any overlap:

^Ai2 > + (5a) The case described by Eq.(5a) is shown in FIG. 6 (a). The magnitude of the shift Δ₁₂ cannot be made arbitrarily large, as the data of sensing fiber 2 would then start to overlap with its negative frequency part (an equivalent way of describing this situation would be: the sampling density becomes insufficient and aliasing starts to occur). Thus, the following condition should also be met:

It may be remarked that L is related to the number of points N in the Discrete

Fourier Transform that is used to compute the scattering profile, cf. Eq.(3). Thus, Eq.(5b) may also be viewed as a lower limit on the number of sampling points in the wavelength scan in order for the invention to work at all. In practice, a larger number of points will likely be used, e.g. in order to be able to apply a simple but fast algorithm for interpolatory

linearization.

Equations 5(a) and 5(b) can be combined into a single equation: hl + hl < ll < ^L - (h +hl + hl) (5C)

It is also possible to place the zero-frequency point for sensing fiber 1 somewhere in its connecting fiber, in which case lo will be negative. To avoid overlap, the following condition should then hold in addition to the condition represented by Eq.(5c):

/₀ > -/_cl / 2 (6)

It will be clear to those skilled in the art that the invention can also be made to work for the case that the reference path is longer than each of the measurement paths, in such a manner that the zero-frequency point lies beyond the end point of both sensing fibers. This case would amount to making l₀ sufficiently negative; the positive and negative frequency parts of the sensing fiber data would then trade places and would appear reversed in position.

More possibilities exist. The shift Δ₁₂ can be made negative in such a manner that for all points on sensing fiber 1 the measurement path is longer than the reference path, while for all points on sensing fiber 2 the measurement path is shorter than the reference path. Only the data for sensing fiber 2 will then appear reversed in position. Depending on the magnitude of Δ₁₂ two situations can be distinguished.

Firstly, when the data for sensing fiber 2 appear to the right of the data for sensing fiber 1 , we have the situation shown in FIG. 6 (b). For this case the following condition must hold in order to avoid overlap (keep in mind that Δι is negative):

²h + hi + hi + hi + hi <

< L + l₀ + l_c2 / 2 (7)

When the zero-frequency point is located in connecting fiber 1 , the condition represented by Eq. (6) must hold in addition.

Secondly, provided lo is sufficiently large, another possibility in the case of negative Δ₁₂ is to place the data for sensing fiber 2 into the empty space above the zero- frequency point, as shown in FIG.6 (c). The immediate vicinity of the zero-frequency point should be avoided, however, as various noise contributions associated with the DC intensity on the detector will be located here. The width of the region to be avoided will be accounted for by an additional parameter <5. The following conditions should then hold (again, keep in mind that Δ₁₂ is negative):

h > hi + S (8a) hi + hl +h +^S <

< ^2/0 + ^min( l , hi ) (8b)

The relations expressed by Eqs.(5-8) are valid for an embodiment in which a separate circulator is used for each sensing fiber, such as is shown in FIG. 4. Other embodiments are possible.

FIG. 7 shows another embodiment in which two sensing fibers SFl , SF2 share an optical circulator C. In this embodiment, an optical splitter '50/50' is used to connect the two sensing fibers SFl , SF2 to a single optical circulator C. The reference path and optical detection unit ODU is still common to both sensing fibers SFl , SF2. A delay dl is introduced in the measurement path of one sensing fiber SF2, while the measurement path of the other sensing fiber SFl does not have such delay.

In the embodiment of FIG. 7, it is not possible to completely avoid overlap in the scattering profile between sensing fiber data and connecting fiber data. For this reason, this embodiment can only be used if the strength of the signal of at least one of the sensing fibers is strong enough to not be corrupted by overlapping Rayleigh scattering data of a connecting fiber. Additionally, the lengths of the connecting fibers between the splitter and the sensing fibers SFl , SF2 must differ by an amount that is sufficient to ensure that the data of the two sensing fibers SFl , SF2 do not overlap in the single scattering profile computed from the detector signals.

FIG. 8 schematically shows the relation between fiber lengths and scattering profile for the case that two sensing fibers SFl , SF2 share an optical circulator C. For clarity, the case is shown when the equal-length point lies at a distance lo to the left of the start of the fiber with length l_c that connects the splitter '50/50' to the circulator C. The fiber labels have been chosen such that connection fiber 2 is longer than connection fiber 1. The signal of sensing fiber SFl needs to be sufficiently strong compared to the (Rayleigh scattering) signal fr m connecting fiber SF2 to not become corrupted by it.

Additionally, no reflection peaks from connectors, fiber splices etc. of sufficient strength to corrupt the data of sensing fiber SFl should be present in the scattering profile in the region occupied by the data from sensing fiber SFl . Preferably, connection fiber 2 should be free from splices and connectors in that region. In the setup shown in FIGS. 7 and 8, it is possible to place the zero-frequency point somewhere in the range between the circulator and the start of sensing fiber SF1 , in which case lo becomes negative. It should be avoided, however, to 'fold back the circulator' into the region occupied by the data of sensing fiber SF1 , as the optical circulator C often corresponds to a strong peak in the scattering profile; this peak would corrupt the data of sensing fiber SF1 (note: the circulator peak is not shown in FIG. 8). This condition is expressed by the following equation:

0 > -( _c + _cl) 2 (10) It will be clear to those skilled in the art that the embodiments of the invention as shown in FIGS. 7 and 8 can also be made to work for the case that the reference path is longer than the measurement path, in such a manner that the zero-frequency point lies beyond the end point of both sensing fibers SF1 , SF2.

FIGS. 9a and 9b show examples of amplitude (logarithmic scale) of the Fourier transforms of the detector signals D l and D2 for a single polarization state for an embodiment of the invention according to FIG. 7. The horizontal axis indicates fiber index. FIG. 9a shows only sensing fiber SF1 attached, while FIG 9b shows sensing fiber SF2 also attached, using 5 m of additional delay fiber. The example measurements are performed on a multi-core sensing fiber containing Fiber Bragg Gratings (FBG), using a setup of FIG. 7. The part of the sensing fiber containing the FBGs corresponds to the region of increased amplitude in the fiber index range of approximately 104000-155000. The FBGs of core 2 of the sensing fiber show up in the fiber index range of 230000-281000, due to the inserted delay fiber. One fiber index corresponds to a distance Az = 40 μιη, cf. Eq.(2).

In the above, the example embodiments are shown with only two sensing fibers connected to respective two measurement paths, but in general the invention can be used for more than two sensing fibers.

FIG. 10 shows a generalized version for measuring N sensing fibers simultaneously, SF1 .. . SFN, and this embodiment is a generalization of the embodiment of FIG. 4. Only the components of the first and last sensing fiber are shown. In FIG. 10, a power splitting ratio of the first coupling point CP1 , i.e. the first splitter, is more general than the nominal 50/50 splitting ratio, and has thus been indicated by the parameter a.

FIG. 1 1 shows an embodiment of the invention for measuring N sensing fibers SF1 , SFN simultaneously which is a variation on the embodiment of FIG. 10. It differs from the embodiment of FIG. 10 in the configuration of the splitters and combiners. In FIG. 10, the first splitter CP1 distinguishes between the single reference path and the common part of all measurement paths, and the separate measurement paths are combined into a common path that is then combined with the single reference path, while in FIG. 11, a single first splitter CP1 distributes the incoming light equally over N measurement paths and the reference path, and the measurement paths and the reference path are combined in a single combining element CP2.

FIG. 12 shows an embodiment of the invention for measuring N sensing fibers SF1, SFN simultaneously which is a variation on the embodiment of FIG. 7. A single optical circulator C is shared by all sensing fibers SF1, SFN. As in the embodiment of FIG. 7, it is not possible to completely avoid overlap in the scattering profile between sensing fiber data and connecting fiber data and for this reason this embodiment can only be used if the strength of the signal of the sensing fibers SF1, SF2 is strong enough to not be corrupted by

overlapping Rayleigh scattering data of multiple connecting fibers.

As is obvious to anyone skilled in the art, combinations of elements of the embodiments of the invention will be possible. An example: it is possible to combine the usage of multiple circulators, as in the embodiments of FIGs. 4, 10, and 11, with the usage of an additional splitter/combiner for attaching multiple fibers to a single optical circulator, as in the embodiments of FIGS. 7 and 12.

It is of interest to compare the signal strengths of the embodiments of this invention, as shown in FIGS. 10, 1 1, and 12 to the signal strength of the embodiment of the state of the art. The relevant signal is created by the interference between light from the reference path and light backscattered by a sensing fiber. The strength of this signal is proportional to the product of the amplitude at the PBS of the light that has travelled along the reference path and the amplitude of the light that comes from the measurement path. Note that the amplitude of the light is proportional to the square root of the power level of the light. The following estimates can be made for the relative signal strength of the various

embodiments (the strength of the backscattered power per unit length of the sensing fiber for unit input power is denoted by η).

State of the art:

Amplitude at PBS via reference path: V{½-(l/N)'½} = ½ (1/N)

Amplitude at PBS via a measurement path: Λ/{½·(1/Ν)·η ·½} = ½-V(r|/N)

Signal strength S proportional to the product of the above two terms: S = Vr|/(4N)

This invention, according to the embodiment of FIG. 10:

Amplitude at PBS via reference path: {(1-α)· ½}

Amplitude at PBS via a measurement path: Λ/{α·(1/Ν)·η ·(1/Ν)·½} = V(½-a-r|)/N Signal strength proportional to the product of the above two terms: S = ½· {α·(1-α)·η}/Ν. The signal strength is maximized by choosing a=½ (i.e. a 50/50 splitter), which yields s=Vy(4N)

This invention, according to the embodiment of FIG. 1 1 :

Amplitude at PBS via reference path: V{ 1/(N+1 1/(N+1)} = 1/(N+1)

Amplitude at PBS via a measurement path: Λ/{ 1/(Ν+1)·η · 1/(N+1)} = Vr|/(N+1)

Signal strength proportional to the product of the above two terms: S = Vr|/(N+1)².

This invention, according to the embodiment of FIG. 12:

Amplitude at PBS via reference path: {(1-α)· ½}

Amplitude at PBS via a measurement path: Λ/{α·(1/Ν)·η ·(1/Ν)· ½} = Λ/(½·α·η)/Ν

Signal strength proportional to the product of the above two terms: S = ½· {α·(1-α)·η}/Ν. The signal strength is maximized by choosing a=½ (i.e. a 50/50 splitter), which yields s=Vy(4N)

An examination of the above results shows that the embodiments according to the invention as shown in FIGS. 10, and 12 give the same strength of the interference signal as the embodiment of the state of the art, when the first splitter is chosen to be a standard 50/50 splitter. The embodiment of the invention as shown in FIG. 1 1 shows a lower signal level; for the case of two fibers sharing a detection chain (N = 2) the signal strength for FIG. 1 1 becomes S = Vr|/9, while for the state of the art, for FIGS. 10 and 12: S = Vr|/8. The relative performance of the embodiment of FIG. 1 1 becomes worse for N>2.

It is a surprise that the signal strengths for the embodiments of the invention of FIGS. 10 and 12 are the same as the signal strength of the state of the art, as light is obviously lost in the combiners that are used to combine the light from all measurement paths into a single common measurement path. The reduction of amplitude of light coming from the measurement paths is compensated for, however, by the increased amplitude of the light from the reference path in the invention as compared to the amplitude of light from the reference path in the state of the art. Because a single common reference path is used in the invention, the light does not have to be divided over several reference paths, as is done in the state of the art embodiment.

FIG. 13 illustrates steps S1-S7 of a method according to the invention for obtaining optical frequency domain reflectometry (OFDR) data. The steps are: - providing (SI) an optical radiation source (LS) and emitting (S2) optical radiation within a certain wavelength band, the radiation source being optically connected to a first coupling point (CP1) arranged for splitting the optical radiation from the optical radiation source (LS),

- generating (S7) an electrical signal accordingly,

wherein the plurality of measurement paths have different optical path lengths, so as to allow the detection of essentially uncorrupted optical radiation from parts of the respective optical sensing fibers that are intended to have a sensory function.

The invention is applicable in all fields where distributed sensing using the method of Optical Frequency Domain Reflectometry can be used. Properties that can be measured with this technique are, e.g., strain and temperature. A field of particular interest might be the simultaneous measurement of strain in cores of a helical multi-core fiber, for the purpose of shape-sensing. The invention can be used to lower the price of a measurement system or to simultaneously measure more than one shape sensing fiber using only a single measurement system. Especially, the invention may be applied in shape sensing for medical applications, where the shape sensing function is used in a medical probe or catheter for spatial location of medical scanning or medical treatment purposes.

To sum up, the invention provides an optical frequency domain reflectometry (OFDR) system with an optical radiation source (LS) connected to a plurality of

measurement paths (MPl, MP2, MP3) and a reference path (RFP) via a first coupling point (CP1). The measurement paths (MPl, MP2, MP3) are arranged for optical connection to respective optical sensing fibers (SFl, SF2, SF3). The measurement paths (MPl, MP2, MP3) have different optical path lengths (LI, L2, L3), so as to allow an optical detection unit (ODU) to essentially uncorruptedly detect optical radiation from parts of the respective optical sensing fibers (SFl, SF2, SF3) that are intended to have a sensory function. The optical detection unit (ODU) is connected to the reference path (RFP) and the measurement paths (MPl, MP2, MP3) via a second coupling point (CP2). Thus, via the common reference path (RFP) for several measurement paths (MP1, MP2, MP3), only one detection chain is required to serve several sensing fibers (SF1, SF2, SF3).

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. In the claims, or the description, the mentioning of "at least one of a first entity, a second entity, and third entity" does not necessarily mean that each of the first entity, the second entity, and third entity are present, hence only the second entity may be present, or alternatively, only the first entity and third entity may be present, and so forth with more entities. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. Any reference signs in the claims should not be construed as limiting the scope.

Claims

CLAIMS:

1. An optical frequency domain reflectometry system comprising:

- a plurality of measurement paths (MP1, MP2, MP3) optically connected to the first coupling point (CP1), and wherein the measurement paths (MP1, MP2, MP3) are arranged for optical connection to respective optical sensing fibers (SFl, SF2, SF3),

- a reference path (RFP) optically connected to the first coupling point (CP1),

- a second coupling point (CP2) optically connected to the reference path (RFP) and the plurality of measurement paths (MP1, MP2, MP3), and

- an optical detection unit (ODU) optically arranged to detect the combined optical radiation from the reference path (RFP) and the plurality of measurement paths (MP1, MP2, MP3) via the second coupling point (CP2), and to generate an electrical detection signal (E)

accordingly,

wherein the plurality of measurement paths (MP1, MP2, MP3) have different optical path lengths (LI, L2, L3), so as to allow the optical detection unit (ODU) to essentially

uncorruptedly detect optical radiation from parts of the respective optical sensing fibers (SFl, SF2, SF3) that are intended to have a sensory function.

2. The optical frequency domain reflectometry system according to claim 1, wherein the plurality of measurement paths (MP1, MP2, MP3) have different optical properties selected to provide optical path lengths (LI, L2, L3) which differ so as to allow the optical detection unit (ODU) to detect non-overlapping optical radiation from said parts of the respective optical sensing fibers (SFl, SF2, SF3) that are intended to have a sensory function.

3. The optical frequency domain reflectometry system according to claim 1, comprising a separate optical circulator (C) connected in each of at least two of the plurality of measurement paths between the first coupling point (CPl) and sensing fibers (SFl, SF2) connected to the respective measurement paths.

4. The optical frequency domain reflectometry system according to claim 1, wherein each of the plurality of sensing fibers (SFl, SFN) are connected to the second coupling point (CP2) via respective separate optical circulators (CI, C2).

5. The optical frequency domain reflectometry system according to claim 1, wherein each of the plurality of measurement paths are connected to the first coupling point (CPl) via respective separate optical circulators (CI, C2).

6. The optical frequency domain reflectometry system according to claim 1, wherein at least two sensing fibers (SFl, SF2) are connected to the first coupling point (CPl) via one common optical circulator (C).

7. The optical frequency domain reflectometry system according to claim 6, wherein the at least two sensing fibers (SFl, SF2) are connected to the common optical circulator (C) via an optical splitter.

8. The optical frequency domain reflectometry system according to claim 1, wherein at least two of the plurality of measurement paths (MPl, MP2, MP3) have different physical path lengths serving to provide different optical path lengths (LI, L2, L3).

9. The optical frequency domain reflectometry system according to claim 1, wherein at least two of the plurality of measurement paths (MPl, MP2, MP3) have different optical properties serving to provide different optical path lengths (LI, L2, L3).

10. The optical frequency domain reflectometry system according to claim 1, wherein at least two of the plurality of measurement paths (MPl, MP2, MP3) have different optical path lengths (LI, L2, L3) by means of a combination of different physical path lengths and different optical properties.

11. The optical frequency domain reflectometry system according to claim 1 , wherein at least one of the plurality of optical sensing fibers (SF1, SF2, SF3) is arranged within a sensing probe (PR), so as to allow sensing of a shape of the sensing probe (PR).

12. The optical frequency domain reflectometry system according to claim 1, wherein the optical detection unit (ODU) is arranged for polarization-diverse detection of the combined optical radiation from the reference path (RFP) and the plurality of measurement paths (MP1, MP2, MP3).

13. The optical frequency domain reflectometry system according to claim 1, comprising a plurality of sub systems, wherein each sub system comprises a plurality of measurement paths connected to a common reference path and an optical detector unit.

14. Use of the optical frequency domain reflectometry system according to claim 1 for a medical application.

15. A method for obtaining optical frequency domain reflectometry data, the method comprising:

- providing (S3) a reference path (RFP) optically connected to the first coupling point (CP1),

- providing (S4) a plurality of measurement paths (17) optically connected to the first coupling point, and arranged for optical connection to respective optical sensing fibers, - providing (S5) a second coupling point (CP2) optically connected to the reference path and the plurality of measurement paths,

- generating (S7) an electrical signal accordingly,