SYSTEM FOR POLARIMETRIC CHARACTERIZATION OF A TARGET

Abstract

A system for polarimetric characterization of a target that includes a liquid light guide (LLG) for propagating light from a light source to the target (S) at least one of a Polarization State Analyzer (PSA) serving to analyze polarization of light having propagated into the LLG and that has been reflected by the target, and a Polarized State Generator (PSG) for modulating the polarization of light injected into the LLG, an optical detector for detecting light backscattered by the target (S) that has been illuminated by the LLG.

Claims

1. A system for polarimetric characterization of a target, comprising: a liquid light guide (LLG) for propagating light from a light source to the target, at least one of: a Polarization State Analyzer (PSA) serving to analyze the polarization of the light having propagated into the LLG and that has been reflected by the target, and a Polarized State Generator (PSG) for modulating the polarization of light injected into the LLG, an optical detector for detecting light backscattered by the target that has been illuminated by the LLG.

2. The system of claim 1, further comprising a detection channel in which light propagates before reaching the optical detector.

3. The system of claim 2, wherein the LLG and the detection channel are distinct and the LLG extends along the detection channel.

4. The system of claim 2, wherein the detection channel extends in a rigid endoscope.

5. The system of claim 2, wherein the LLG and the detection channel extend in a rigid casing.

6. The system of claim 1, further comprising both a PSG through which light is injected into the LLG and a PSA through which light reaches the optical detector.

7. The system of claim 1, wherein the light from the target is detected by the optical detector without having the light reflected by the target propagating through a LLG or a detection channel comprising an optical relay such as a succession of rod lenses.

8. The system of claim 1, further comprising a control system to control the PSA and/or PSG, record signals from the optical detector and compute polarimetric parameters and/or the Mueller matrix of the target and display corresponding information.

9. The system of claim 1, further comprising a bandpass filter or a tri-band filter for narrowing the bandwidth of the light that is reaching the optical detector, the bandwidth is no greater than 30 nm.

10. A method for polarimetric characterization of a target with a system as defined in claim 1, the method comprising: Illuminating the target via the LLG, collecting with the optical detector light reflected by the target thus illuminated, selecting a probe state of at least one of a PSA and PSG and controlling the PSA to analyze at least two different states of polarization of the light reflected by the target and directed to the optical detector and/or controlling the PSG to illuminate the target with at least two different states of polarization and analyzing the light reflected by the target, and computing from the corresponding light intensities measured with the detector at least one polarimetric parameter of the target.

11. The method of claim 10, wherein the collection of light is performed at one or more wavelengths.

12. The method of claim 10, wherein the collection of light is performed through a detection channel distinct from the LLG, the detection channel is an imaging channel comprising a succession of rod lenses in a rigid endoscope, or through a LLG (112) that is distinct from the one that serves to illuminate the target.

13. The method of claim 10, further comprising illuminating the target with polarized light, via a PSG through which light is injected into the LLG, and modulating the polarized state of the light that is injected.

14. The method of claim 13, comprising: illuminating the target with a temporal succession of different polarization probe states, generated by the PSG, the polarized light propagating in the LLG, analyzing the thus illuminated target through the PSA and recording for each generator probe state and analyzer probe state corresponding intensity signals, determining the Mueller matrix M.sub.S of the target under observation based on the recorded intensity signals and knowledge of the polarimetric properties of the system obtained during beforehand calibration thereof.

15. The system of claim 2, wherein the detection channel is an imaging channel comprising a succession of rod lenses or a detection channel made by a LLG different from the one serving to illuminate the target.

16. The system of claim 3, wherein the LLC extends at least on part of its length parallel to the detection channel.

17. The system of claim 5, wherein the casing comprises a tubular body.

18. The system of claim 6, wherein the PSG and/or PSA comprises ferroelectric liquid crystals.

19. The system of claim 9, wherein the bandwidth is no greater than 20 nm.

20. The method of claim 11, wherein the collection of light is performed at three wavelengths using a tri-CCD or tri-CMOS camera.

Description

DETAILED SPECIFICATION

[0061] In the appended drawings:

[0062] FIG. 1 is a partial and schematic view of an example of a Mueller polarimetry endoscopic system made in accordance with the invention,

[0063] FIG. 2 is a schematic cross section of the system,

[0064] FIG. 3 is a partial and schematic axial section of the detection channel,

[0065] FIG. 4 is a schematic view of a first variant embodiment of the invention,

[0066] FIG. 5 is a schematic view of a second variant embodiment of the invention, and

[0067] FIG. 6 is a schematic view of a third variant embodiment of the invention.

[0068] The system 1 shown in FIG. 1 comprises a rigid casing 10 that is configured for introduction into a human or animal body to image a target S of a tissue T.

[0069] The casing 10 may comprise a tubular body that may be 15-35 cm long and is configured for introduction into the human or animal body.

[0070] The system 1 comprises a detection channel that extends inside the casing 10. The detection channel may be formed by a conventional endoscope 11.

[0071] The detection channel may be made in a conventional manner with an optical relay which is preferably made of a succession of rod lenses 17, as shown in FIG. 3. The design of the detection channel may be based on the Hopkins rod lens design, which consists of a succession of lenses characterized by a length longer than their diameter and separation distance.

[0072] The detection channel may comprise an eyepiece near its proximal end and an Objective near its distal end. The objective forms an image of the target which is transmitted by the optical relay system up to the proximal end. The focal plane of the eyepiece may coincide with the image plane of the optical relay, so that the rays come out parallel from the eyepiece and create the image of the target at infinity.

[0073] The structure of the endoscope 11 is for example identical or similar to that of the laparoscope commercialized by the company Karl Storz under reference 26003 AA.

[0074] The endoscope 11 may be of circular or near circular cross section as shown in FIG. 2, of a diameter ranging for example from 2.5 to 10 mm.

[0075] In a variant, the detection channel is directly integrated into the rigid casing 10, without being part of a complete endoscope before introduction into the casing 10.

[0076] The system 1 also comprises an illumination channel that extends parallel to the detection channel inside the tubular body.

[0077] In accordance with the invention, the illumination channel is made of a liquid light guide (LLG) 15. The LLG can be identical or similar to that of a commercial liquid light guide commercialized for example by the company Thorlabs under reference LLG-04H, but the invention is not restricted to any particular kind of LLG.

[0078] The LLG 15 may be of circular or near circular cross section and of a diameter ranging from 2 to 8 mm for example.

[0079] The outside diameter D of the tubular body of the casing 10 is for example ranging from 10 to 20 mm, being preferably no greater than 12 mm.

[0080] The LLG 15 is closed at its ends in any appropriate known manner.

[0081] The system 1 comprises as optical detector a digital camera 20 that receives the light reflected by the surface S illuminated by the LLG 15.

[0082] The system 1 comprises a light source 30 to provide the light that is injected into the LLG 15.

[0083] A PSG 40 is interposed on the path of light between the light source 30 and the LLG 15 and a PSA 41 is interposed on the path of light between the endoscope 11 and the camera 20.

[0084] The PSG 40 and PSA 41 are controlled directly or indirectly by a computer or any other appropriate controller 50. This controller 50 may also be connected to the camera 20 for recording and processing the images thereof The controller 50 enables the synchronization between the switch of the PSG and the PSA and the acquisition of the intensity images by the camera.

[0085] The light source 30 may be a white light source such as a Xenon lamp, LED lamp or halogen lamp, and light may be supplied from this source to the PSG by a fiber bundle 31 or any other appropriate optical system.

[0086] A monochromatic bandpass filter 22 may be interposed on the path of light between the PSA 41 and the camera 20, in which case the camera is monochromatic. One may also use a tri-CDD or tri-CMOS camera, and a tri-band filter.

[0087] The measurements may be made at a given wavelength between 340 and 1000 nm, for example between 450 nm to 700 nm, by selecting the bandpass filter 22.

[0088] In an example using a monochromatic camera 20, the bandpass filter 22 is a filter with a central wavelength of 550 nm and a bandwidth less than 30 nm FWHM. In another example, the filter 22 has a central wavelength of 532 nm with a spectral bandwidth of 10 nm FWMH. In another example, one uses a tri-band filter with a tri-CCD or tri-CMOS camera.

[0089] Light at the output of the PSG 40 is injected into the LLG 15.

[0090] Various optics 43, such as a system of lenses, may be placed before and/or after the PSG, for optimizing the injection of the light into the LLG, thus increasing the efficiency of light transmission.

[0091] The PSG 40 and PSA 41 are known per se and are electrically controlled. The PSG enables to produce four different states of polarization, and the PSA enables to produce four different configurations of analysis, as required by 4×4 Mueller polarimetry.

[0092] The PSG 40 comprises for example in a conventional manner a linear polarizer, a first electrically controllable liquid crystal cell, preferably a ferroelectric liquid crystal, a quarter wave plate and a second electrically controllable liquid crystal cell, preferably a ferroelectric liquid crystal, but other configurations of PSG may be used. The four Stokes vectors corresponding to the four polarization states thus generated are independent and can be arranged in four columns to form a 4×4 modulation matrix denoted W.

[0093] The PSA may comprise optical elements identical to that of the PSG but arranged in the reverse order relative to the direction of light propagation. The Stokes vectors corresponding to the four polarization configurations by the PSA are arranged in four rows to form a 4×4 analysis matrix denoted A.

[0094] For Mueller polarimetry, the PSG temporally modulate the polarization of light illuminating the target under observation by consecutively generating four independent probing polarization states. Each of the four polarization states produced by the PSG after interacting with the sample under observation is analyzed through four consecutive polarization configurations of the PSA. In this way, at least 16 measurements are sequentially performed over a finite interval of time and stacked in a real-valued matrix. This operation can be repeated multiple times in order to acquire the 16-components intensity matrix N times. This enables to improve the signal to noise ratio through an average process.

[0095] For each of the four polarization states produced by the PSG and analyzed through a polarization configuration of the PSA, the intensity measurement is performed for each pixel of the camera in imaging configuration.

[0096] The system is calibrated to account for the polarimetric properties of the optical components of the system and more particularly those of the LLG and of the detection channel.

[0097] Calibration may be performed in conventional manner by placing appropriate optical components after the PSG and before the PSA.

[0098] If one is willing to detail the polarimetric properties of the LLG and of the detection channel, they may be determined by measuring their Mueller matrices in a Triple Step Eigenvalue Calibration Method (T-S ECM) calibration process. Otherwise the system can be directly calibrated in a Single Step Eigenvalue Calibration Method (S-S ECM) calibration process. Both processes are detailed hereunder.

[0099] In such processes, a reflective surface S made of a sandblasted metallic plate is placed in front of the distal end of the imaging channel, and oriented perpendicularly to the longitudinal axis of the imaging channel.

[0100] If one wants to have detailed information about polarimetric properties of the optical components, in particular of the LLG 15 or of the imaging channel of the endoscope 11, one may use the Triple-Step Eigenvalue Calibration Method (T-S ECM).

[0101] In this case, the first step consists in performing a measurement without inserting any optical component in the system 1.

In this case the 16-components intensity matrix B.sub.0 is obtained as:

B.sub.0˜AM.sub.endoM.sub.LLGW  (1)

where M.sub.endo is the 4×4 Mueller matrix of the imaging channel of the endoscope 11 and M.sub.LLG is the Mueller matrix of the LLG 15.

[0102] Several optical components, in particular a polarizer with the transmission axis oriented at 0° with respect to a reference frame of the system (P0°), a polarizer with the transmission axis oriented at 90° (P90°) with respect to the transmission axis of P0° and a waveplate with one of its neutral axis oriented at 30° with respect to the transmission axis of P0°, are consecutively placed in C1.

[0103] In this case one obtains the 16-components intensity matrix B.sub.1i (i=P0°, P90°, L30°) given by:

B.sub.1i˜AM.sub.endoM.sub.LLGM.sub.iW.  (2)

[0104] Then the optical components are consecutively placed in C2 to obtain the 16-components intensity matrix B.sub.2i (i=P0°, P90°, L30°) given by:

B.sub.2i˜AM.sub.endoM.sub.iM.sub.LLGW  (3)

[0105] Then, they are consecutively placed in C3 to obtain the 16-components intensity matrix B.sub.3i (i=P0°, P90°, L30°) given by:

B.sub.3i˜AM.sub.iM.sub.endoM.sub.LLGW.  (4)

[0106] By multiplying the inverse of (1) on the left for (2) and (3), as well as on the right for (3) and (4), the following equations can be obtained:

C.sub.1i=B.sub.0.sup.−1B.sub.1i=W.sup.−1M.sub.iW  (5)

C.sub.2Wi=B.sub.0.sup.−1B.sub.2i=W.sup.−1M.sub.LLG.sup.−1M.sub.iM.sub.LLGW  (6)

C.sub.2Ai=B.sub.2iB.sub.0.sup.−1=AM.sub.endoM.sub.iM.sub.endo.sup.−1A.sup.−1  (7)

C.sub.3i=B.sub.3iB.sub.0.sup.−1=AM.sub.iA.sup.−1  (8).

[0107] The matrices W and A can be obtained from (5) and (8) respectively following the procedure described in the article E. Compain et al “General and self-consistent method for the calibration of polarization modulators, polarimeters, and Mueller-matrix ellipsometers”. Appl. Opt. 38, 3490-3502 (1999) or A. De Martino et al. “General Methods for optimized design and calibration of Mueller polarimeters”, Thin Solid Films 455-456, 112-119 (2004).

[0108] Defining Ŵ=M.sub.LLGW and Â=AM.sub.endo the equations (6) and (7) can be rewritten respectively as:

C.sub.2Wi=W.sup.−1M.sub.LLG.sup.−1M.sub.iM.sub.LLGW=Ŵ.sup.−1M.sub.iŴ  (9)

and:

C.sub.2Ai=AM.sub.endoM.sub.iM.sub.endo.sup.−1A.sup.−1=ÃM.sub.iÃ.sup.−1  (10)

[0109] The matrices Ŵ and  can be respectively obtained from (9) and (10) always following the procedure disclosed in E. Compain et al “General and self-consistent method for the calibration of polarization modulators, polarimeters, and Mueller-matrix ellipsometers”. Appl. Opt. 38, 3490-3502 (1999) or A De Martino et al. “General Methods for optimized design and calibration of Mueller polarimeters”, Thin Solid Films 455-456, 112-119 (2004).

[0110] The matrix M.sub.endo can be obtained from (1) using the formula:

M.sub.endo=A.sup.−1B.sub.0{tilde over (W)}.sup.−1  (11)

[0111] Finally, M.sub.LLG can be obtained from (1) using the formula:

M.sub.LLG=Ã.sup.−1B.sub.0W.sup.−1.  (12)

[0112] In this way, the polarimetric properties of the optical components of the system are characterized.

[0113] In order to measure the Mueller matrix M.sub.S of a target, the target is positioned in front of the system instead of the metallic plate.

[0114] Then, the 16-components intensity matrix B.sub.S is measured, given by:

B.sub.S˜ÃM.sub.SŴ.  (13)

[0115] From (13) it is possible to use the matrices {tilde over (W)} and Ã, previously determined using the T-S ECM, in order to obtain the Mueller matrix M.sub.S of the target by means of:

M.sub.S˜Ã.sup.−1B.sub.S{tilde over (W)}.sup.−1.  (14)

[0116] If a detailed polarimetric characterization of the LLG or of the detection channel is not needed, the Single Step Eigenvalue Calibration Method (S-S EMS) can be used.

[0117] In this case, the first step consists in performing a measurement without inserting any optical component in the system 1.

[0118] In this case the 16-components intensity matrix B.sub.0 in (1) is obtained. Then, the calibration optical elements are consecutively placed in C2 in order to obtain the 16-components intensity matrix B.sub.2i in (3).

[0119] By multiplying on the left the inverse of (1) for (3) it is possible to obtain (6).

[0120] Defining Ŵ=M.sub.LLGW the equation (9) can be obtained from which Ŵ can be derived by using the procedure described in E. Compain et al “General and self-consistent method for the calibration of polarization modulators, polarimeters, and Mueller-matrix ellipsometers”, Appl. Opt. 38, 3490-3502 (1999) or A De Martino et al. “General Methods for optimized design and calibration of Mueller polarimeters”, Thin Solid Films 455-456, 112-119 (2004).

[0121] Then the matrix Ã=AM.sub.endo can be Obtained from (1) as:

AÃ=B.sub.0{tilde over (W)}.sup.−1.  (15)

[0122] Then the 4×4 Mueller matrix M.sub.S of a sample can be derived from (14) by using Ŵ and  determined during calibration.

[0123] Polarimetric parameters, such as tor example the depolarization, retardance and dichroism, can be extracted from the measured Mueller matrix M.sub.S by using for example the Lu-Chipman decomposition, which is widely used in polarimetry and which describes a Mueller matrix as a product of three matrices corresponding respectively to a depolarizer, a retarder and a diattenuator, from which polarimetric properties can be extracted (see S.-Y. Lu and R. A. Chipman “Interpretation of Mueller matrices based on polar decomposition”, J. Opt. Am. A13, 1106(1996))

[0124] The invention is not limited to the above described embodiments.

[0125] In a variant where one desires to measure the 3×Mueller matrix, it is enough for the PSG to produce 3 different probe polarization states and for the PSA to produce 3 different polarization probe configurations.

[0126] Each probe polarization produced by the PSG is analyzed through the three probe polarization configurations of the PSA for a total of 9 intensity measurements.

[0127] In this case the system 1 may be calibrated with the Eigenvalue Calibration Method slightly modified as explained in the article Eigenvalue calibration method for 3×3 Mueller polarimeters from Ji Qi et al, 2362 Vol 44, No. 9/1 May 2019 Optics Letters.

[0128] For Stokes polarimetry, the PSA is left while the PSG is replaced by a polarizer (linear, circular or elliptical0. In a variant, the PSG is left, and the PSA is replaced by a polarizer (linear, circular or elliptical).

[0129] For Orthogonal State Contrast (OSC) polarimetry, the intensity measurement is performed by controlling the PSA in order to produce a polarization configuration that is either parallel (I.sub.parallel) or perpendicular (I.sub.perpendicular) to the polarization state produced by the PSG, or by controlling the PSG in order to produce polarization states that are either parallel (I.sub.parallel) or perpendicular (I.sub.perpendicular) to the polarization. configuration of the PSA.

The intensity of the OSC is obtained by:

[00001]$\begin{array}{cc}{I}_{\mathrm{OSC}}=\frac{I_{\mathrm{parallel}}{I}_{\mathrm{perpendicular}}}{I_{\mathrm{parallel}}+I_{\mathrm{perpendicular}}}& \left(14\right)\end{array}$

This type of measurement enables to characterize only pure depolarizers. OSC can also be performed using circular polarization or elliptical polarization.

[0130] The system 1 may comprise a second LLG 112 to collect the reflected light, instead of the endoscope 11, as illustrated in FIG. 4. The second LLG used to collect the reflected light from the sample is different from the LLG used to illuminate the sample.

[0131] In that example, the camera is replaced with a photodiode detector 20 for rapid single point detection. In that variant, there is no imaging of the target. Such a system is mainly useful to characterize large areas of targets with spatially uniform polarimetric properties.

[0132] The system of FIG. 4 may be used for 4×4 or 3×3 Mueller polarimetry.

[0133] Calibration optical components can be placed at C, in front of the LLG 112, for calibration with S S-ECM.

[0134] In the example of FIG. 5, the difference with FIG. 4 is that there is no PSG but only the PSA 41. Totally depolarized light can be injected in the LEG for Stokes polarimetry.

[0135] Otherwise, for OSC or Stokes polarimetry, a polarizer (not shown), such as a linear, circular or elliptical polarizer, can replace the PSG for injecting polarized light into the LLG.

[0136] For Stokes polarimetry, the PSA will take successively four probe states. For OSC polarimetry, the PSA will take successively two probe states.

[0137] In a variant (not shown), the configuration is the one of FIG. 5 except that the PSA is removed or replaced by a polarizer. The PSG is reintroduced.

[0138] In the embodiment of FIG. 6, reflected light propagates from the target to the PSA 41 without being guided by a LLG or an endoscopic imaging channel with rod lenses as described above or another optical system. The target is illuminated by the LLG 15, which can be fixed on a mount 160 holding its distal tip. The mount may be mobile, being carried for example by a motorized arm, enabling to vary its azimuth angle around the optical axis z of the PSA 41, and/or to vary the distance from the target and/or to vary the incidence angle. Adapted optics (not shown) can be placed at the output of the LLG to obtain different sizes for the beam to illuminate the sample. Such an embodiment can be used for in vivo and ex vivo polarimetry, or for non-medical applications where there is room for moving the LLG around the imaging axis. Acquiring the polarimetric response as a function of the incident angle may serve to generate data for reconstructing a 3D structure of the target, for example. The embodiment of FIG. 6 may be used for 4×4 Mueller polarimetry, 3×3 Mueller polarimetry, Stokes polarimetry or OSC polarimetry.

[0139] In all above described embodiments, the PSG and PSA can be based on components other than ferroelectric liquid crystals, for example photo-elastic modulators (PEMs) or nematic liquid crystals. The PSG and PSA can also be simplified whenever possible to produce less than 4 polarized states, for example if 3×3 Mueller matrix, Stokes polarimetry or OSC is desired. The PSG and PSA should preferably be sufficiently fast to acquire images in 2 seconds maximum and allow a large field of view ideally for in vivo applications.

[0140] The optical detector may also be a Division of Focal Plan polarization camera consisting of a micro-polarizer array as for example commercialized by the company 4D Technology inc. under reference PolarCam. In this case the PSA can be simplified, it no longer requires any polarizer and only 2 retarder configurations are necessary instead of the usual 4 to measure a 4×4 Mueller Matrix.

[0141] The invention may be used for polarimetric characterization of targets other than human or animal tissues. Any application where endoscopes are necessary could be interesting for the invention. This can include industry applications where bulky optical components cannot be used or where remote inspection is required.