OPTICAL COHERENCE TOMOGRAPHY ANALYSIS METHOD AND APPARATUS

Abstract

The present invention relates to an optical coherence tomography analysis method, comprising: Providing a Swept Source Optical Coherence Tomography system (SS-OCT), the SS-OCT system including: a light source, tunable over a spectral band, that generates a coherent light signal; an optical interferometer for dividing the coherent light signal into a reference arm leading to a reference reflector and a sample arm leading to a sample; an optical element to selectively direct a sample light signal exiting the sample arm to a specific portion of the sample, so that for each selection in the optical element a different specific portion of the sample is illuminated; an optical detector for detecting an interference signal generated by a combination of reference and sample returning signals from the reference arm and from the sample arm, reflected by the reference reflector and the sample, respectively; Wherein, for the same selection operated at the optical element level illuminating a specific portion of the sample, the method further comprises: sweeping the light source for a time interval ΔT, so that a wavelength of the coherent light signal, leading to the sample light signal illuminating the specific portion of the sample, changes from a minimum wavelength to a maximum wavelength and wherein the wavelength of the coherent light signal reaches the same value between the minimum wavelength to the maximum wavelength at least twice during the sweeping; detecting the interference signal generated by the sweeping, including the interference signal generated by the sample returning signals of the at least two coherent light signals having the same wavelength; elaborating the detected interference signal generated by the sweeping, including the detected interference signal generated by the sample returning signals of the at least two coherent light signals having the same wavelength, for obtaining an OCT image of the specific portion of the sample.

Claims

1-15. (canceled).

16. An optical coherence tomography analysis method, comprising: Providing a Swept Source Optical Coherence Tomography system (SS-OCT), the SS-OCT system including: a light source, tunable over a spectral band, that generates a coherent light signal; an optical interferometer for dividing the coherent light signal into a reference arm leading to a reference reflector and a sample arm leading to a sample; an optical element to selectively direct a sample light signal exiting the sample arm to a specific portion of the sample, so that for each selection in the optical element a different specific portion of the sample is illuminated; an optical detector for detecting an interference signal generated by a combination of reference and sample returning signals from the reference arm and from the sample arm, reflected by the reference reflector and the sample, respectively; Wherein, for the same selection in the optical element illuminating a specific portion of the sample, the method further comprises: sweeping the light source for a time interval ΔT, so that a wavelength of the coherent light signal leading to the sample light signal illuminating the specific portion of the sample changes from a minimum wavelength to a maximum wavelength and wherein the wavelength of the coherent light signal reaches the same value between the minimum wavelength to the maximum wavelength at least twice during the sweeping; detecting the interference signal generated by the sweeping, including portions of interference signal generated by using the sample returning signals of the at least two coherent light signals having the same wavelength; elaborating the detected interference signal generated by the sweeping, including portions of the detected interference signal generated by using the sample returning signals of the at least two coherent light signals having the same wavelength, for obtaining an OCT image of the specific portion of the sample.

17. The method according to claim 16, wherein sweeping the light source for a time interval ΔT, includes dividing the sweeping in N, where N≥2, sub-sweeping intervals, wherein in each sub-sweeping interval, for a portion thereof, the wavelength of the coherent light signal varies with time substantially identically to the previous sub-sweeping step or varies with time opposite to the previous sub-sweeping step.

18. The method according to claim 16, wherein elaborating the detected interference signal includes excluding a region of the detected interference signal around to the time when the N−1 sub-sweeping interval ends and the N sub-sweeping interval starts.

19. The method according to claim 16, wherein all the sub-sweeping intervals have a substantially identical sub-sweeping duration Δt≤ΔT/2.

20. The method according to claim 16, wherein sweeping the light source for a time interval ΔT includes sweeping the light source for a time interval shorter than 10 μs, preferably shorter than 1 μs.

21. The method according to claim 16, further comprising: dividing the sweeping in N, where N≥2, sub-sweeping intervals; providing the (i−1)-th sub-sweeping interval having a duration Δt.sub.i−1 with the wavelength of the coherent light signal having the following behaviour: λ.sub.t−1(t)=f(t) where f(t) is a monotone function between t.sub.1 and t.sub.2, where t.sub.1 and t.sub.2 ∈Δt.sub.i−1; and providing the i-th sub-sweeping interval having a duration Δt.sub.i with the wavelength of the coherent light signal having the following behaviour: λ.sub.t(t)=−f(t)+C where C is a constant, between t.sub.3 and t.sub.4 where t.sub.3 and t.sub.4 ∈Δt.sub.i.

22. The method according to claim 21, wherein all the sub sweeping intervals have a substantially equal sub sweeping duration Δt and λ.sub.t−1(t)=−λ.sub.t(t)+C where C is a constant for the whole duration of the sub sweeping interval.

23. The method according to claim 16, including: dividing the sweeping in N, where N≥2, sub-sweeping intervals; providing the (i−1)-th sub-sweeping interval having a duration Δt.sub.i−1 with the wavelength of the coherent light signal having the following behaviour: λ.sub.t−1(t)=f(t) where f(t) is a monotone function between t.sub.1 and t.sub.2, where t.sub.1 and t.sub.2∈Δt.sub.i−1; and providing the i-th sub-sweeping interval having a duration Δt.sub.i with the wavelength of the coherent light signal having the following behaviour: λ.sub.t(t)=f(t)+C where C is a constant, between t.sub.3 and t.sub.4 where t.sub.3 and t.sub.4 E Δt.sub.i.

24. The method according to claim 23, wherein all the sub sweeping intervals have equal sub sweeping duration Δt and λ.sub.t−1(t)=λ.sub.t(t)+C where C is a constant for the whole duration of the sub-sweeping interval.

25. The method according to claim 21, wherein f(t) is a substantially linear function.

26. The method according to claim 16, including: dividing the sweeping in N, where N≥2, sub-sweeping intervals all of identical sub-sweeping duration Δt and the wavelength of the coherent light signal is a substantially periodic function with period Δt or 2 Δt.

27. The method according to claim 16, including the step of dividing the sweeping in N sub-sweeping intervals, wherein 2≤N≤15.

28. The method according to claim 16, wherein: the light source (101) has a spectral bandwidth narrower than 40 nm.

29. A Swept Source Optical Coherence Tomography system (SS-OCT), the SS-OCT system including: a. a light source, tunable over a spectral band, that generates a coherent light signal; b. an optical interferometer for dividing the coherent light signal into a reference arm leading to a reference reflector and a sample arm leading to a sample; c. an optical element to selectively direct a sample light signal exiting the sample arm to a specific portion of the sample, so that, for each selection operated at the optical element, a different specific portion of the sample is illuminated; d. an optical detector for detecting an interference signal generated by a combination of reference and sample returning signals from the reference arm and from the sample arm, reflected by the reference reflector and the sample, respectively; e. a processing unit, said processing unit being programmed for, for the same selection in the optical element illuminating a specific portion of the sample: i. defining a sweeping time interval ΔT; ii. changing the coherent light signal leading to the sample light signal illuminating the specific portion of the sample from a minimum wavelength to a maximum wavelength and in the same sweeping modifying the wavelength of the coherent light signal so that it reaches the same value between the minimum wavelength to the maximum wavelength at least twice during the sweeping; iii. elaborating the detected interference signal for obtaining an OCT image of the specific portion of the sample.

30. The SS-OCT system according to claim 29, wherein the light source is a tunable laser source including a liquid crystal tunable element.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0129] The present invention will be better understood with non-limiting reference to the appended drawings, where:

[0130] FIG. 1 represents a behavior of the variation of the wavelength (λ) over time (t) in a light source according to the prior art;

[0131] FIG. 2 is a schematic representation of a SS-OCT system according to the invention;

[0132] FIG. 3A is a detail of the system of FIG. 2;

[0133] FIG. 3B is a detail in enlarged view of FIG. 3A;

[0134] FIG. 4 represents as a solid line a first embodiment of a behavior of the variation of the wavelength (Δλ), expressed in nanometers, over time (t) in a light source of system of FIGS. 2 and 3A-B according to the present invention, the shown dotted line represents the signal of FIG. 1;

[0135] FIG. 5A represents the amplitude (A) in arbitrary units of the resulting interference signal over time (t), expressed in microseconds, when the signal of FIG. 1 is used to illuminate a portion of a sample according to the prior art;

[0136] FIG. 5B represents the amplitude (A) in arbitrary units of the resulting interference signal over time (t), expressed in microseconds, when the signal of FIG. 4 is used to illuminate the same portion of the same sample of FIG. 5A according to the invention;

[0137] FIG. 5C represents the superposition of FIGS. 5A and 5B;

[0138] FIG. 6 represents a second embodiment of a behavior of the variation of the wavelength (Δλ), expressed in nanometers, over time (t) in a light source of system of FIGS. 2 and 3A-B according to the present invention, the shown dotted line represents the signal of FIG. 1 according to prior art;

[0139] FIG. 7A represents the amplitude (A) in arbitrary units of the resulting interference signal over time (t), expressed in microseconds, when the signal of FIG. 1 is used to illuminate a portion of a sample according to the prior art;

[0140] FIG. 7B represents the amplitude (A) in arbitrary units of the resulting interference signal over time (t), expressed in microseconds, when the signal of FIG. 6 is used to illuminate the same portion of the same sample of FIG. 7A according to the invention;

[0141] FIG. 7C represents the superposition of FIGS. 7A and 7B;

[0142] FIG. 8A represents the amplitude (A) in arbitrary units of the resulting interference signal over time (t), expressed in microseconds, when the signal of FIG. 1 is used to illuminate a portion of a sample according to the invention, where two reflections are present;

[0143] FIG. 8B represents the amplitude (A) in arbitrary units of the resulting interference signal over time (t), expressed in microseconds, when the signal of FIG. 6 is used to illuminate the same portion of the same sample of FIG. 8A according to the invention;

[0144] FIG. 8C represents the superposition of FIGS. 8A and 8B;

[0145] FIG. 9A shows the amplitude (A) in arbitrary units of the fast Fourier transform (FFT) over frequency (f) in arbitrary units for the interference signal of FIG. 8A;

[0146] FIG. 9B shows the amplitude (A) in arbitrary units of the fast Fourier transform (FFT) over frequency (f) in arbitrary units for the interference signal of FIG. 8B; and

[0147] FIG. 9C shows the superposition of FIGS. 9A and 9B.

DESCRIPTION OF PREFERRED DETAILED EMBODIMENTS OF THE INVENTION

[0148] In FIG. 2, an optical coherence tomography scanner 100 for SS-OCT is illustrated. The scanner is used to illuminate a sample 110, a typical sample being tissues at the back of the human eye.

[0149] The scanner 100 includes a spatially coherent source of light, 101. This source is preferably a Swept laser Source.

[0150] Further, the scanner includes an interferometer 105, for example including two arms called reference and sample arms, 103, 104 realized with optical fibers.

[0151] Light from source 101, i.e. a coherent light signal, is routed to illuminate the sample 10 via the sample arm 104 of the interferometer 105. Further, the light from source 101 illuminates a reference reflector 106 via the reference arm 103.

[0152] The scanner 100 further includes an optical element 107 positioned between the end of the sample arm 104 and the sample 110. The optical element 107 is able to scan light exiting the arm 104 on the sample 110, so that the beam of light (dashed line 108) sweeps over the area or volume to be imaged. This area or volume of the sample which is imaged at a given time by the optical element is called selected portion of the sample 110.

[0153] The direction of light propagation of the light towards the sample outputted from the sample arm defines a Z direction or depth. A plane perpendicular to it, where the sample 110 lies at least partially, defines a (X, Y) plane.

[0154] Light scattered from the sample 110 is collected, typically into the same sample arm 104 used to route the light for illumination of the selected portion of the sample 110.

[0155] Reference light derived from the same source 101 travels a separate path, involving reference arm 103. The light outputted by the reference arm 103 is reflected by a reflector 108. A reflected light from the reflector is thus travelling backwards in the reference arm 103.

[0156] These two “returning” sample and reference lights back-propagating in the sample and reference arms 103, 104 are collected. Collected sample returning light is combined with collected reference returning light, typically in a fiber coupler 111, to form interference light which is routed to a detector 120, such as a photodiode. The output from the detector 120 is supplied to a processor 130. The results can be stored in the processor.

[0157] The interference causes the intensity of the interfered light to vary across the spectrum. For any scattering point in the sample, there will be a certain difference in the path length between light from the source and reflected from that point, and light from the source traveling the reference path. The interfered light has an intensity that is relatively high or low depending on whether the path length difference is an even or odd number of half-wavelengths, as these path length differences result in constructive or destructive interference, respectively. Thus the intensity of the interfered light varies with wavelength in a way that reveals the path length difference; greater path length difference results in faster variation between constructive and destructive interference across the spectrum.

[0158] The Fourier transform of the interference spectrum reveals the profile of scattering intensities at different path lengths, and therefore scattering as a function of depth in the sample.

[0159] The profile of scattering as a function of depth is called an axial scan (A-scan). A set of A-Scans measured at neighboring locations (various selected portions) in the sample produces a cross-sectional image (tomogram) of the sample 110.

[0160] The range of wavelengths at which the interference is recorded determines the resolution with which one can determine the depth of the scattering centers, and thus the axial resolution of the tomogram.

[0161] A more detailed view of the laser source 101 used in the scanner 100 according to the invention is depicted in FIG. 3A. The laser source, in order to tune the wavelength of the emitted signal, uses a liquid crystal 150 based etalon with a Free Spectral Range of 25 nm and a frequency response of around 10 MHz.

[0162] The laser source 101 includes a cavity 141 delimited by a first and a second mirror 142, 143. The first mirror 142 is a highly reflective mirror, while the second mirror 143 is a partially transparent mirror having a mirror FSR and has the function of output coupler. The output of the etalon 150 is indicated with 146 in the figure.

[0163] The cavity 141 further includes a gain medium or gain chip 144, pumped in a known way, and a collimating lens 145 to focus the light on the etalon 150. Etalon 150 is connected to a voltage generator 160.

[0164] The processor 130 connected to the laser source 101 changes the etalon driving voltage via the voltage generator 160 so that, during an A-scan, the wavelength of the coherent light signal emitted from the laser source 101 changes according to the invention.

[0165] In FIG. 3B, a more detailed view of the etalon 150 is shown in an enlarged view.

[0166] The etalon 150 includes a liquid crystal element 151. The liquid crystal element may include any of: CCN-47, MLC-20180, HNG715600-100 produced by Nematel GmbH (Germany), Merck (USA), Jiangsu Hecheng Display technology (china), respectively.

[0167] The liquid crystal element 151 is doped with a polar addictive, preferably 2, 3-dicyano-4-pentyloxyphenyl 4′-pentyloxybenzoate (DPP), CAS 67042-21-1 produced by UAB Tikslioji Sinteze, Lithuania.

[0168] More information about the used liquid crystal material can be found in “Enhanced nanosecond electro-optic effect in isotropic and nematic phases of dielectrically negative nematics doped by strongly polar additive”, published in Journal of Molecular Physics, December 2017, written by Bingxian Li et al.

[0169] Two opposite sides of the LC element 151 are coated with a high reflectivity dielectric multilayer (reflectivity higher than 95%) 152 and the resulting structure is sandwiched between two electrodes 153 attached to the voltage generator 160.

[0170] Two glass slabs then closes the etalon 150.

[0171] The voltage generator applies a suitable voltage to the electrodes 153 so that the refractive index of the LC element 151 changes. A linear voltage variation implies a linear change in the wavelength of the output 146.

[0172] In FIG. 4, a first preferred embodiment of the sweeping for an A scan which last ΔT is shown, the sweeping duration ΔT is divided is sub intervals of equal duration Δt.

[0173] It is to be understood that the “wavelength” ordinate represents a variation from a minimum wavelength to a maximum wavelength. For practical reasons of representation, the minimum wavelength is represented as if it were the “zero” ordinate, however in reality the minimum wavelength of the coherent light signal emitted by the light source is different from zero. Thus the value shown is always (minimum wavelength)−(maximum wavelength). The same considerations applies to FIG. 1 and FIG. 6.

[0174] In this embodiment, as visible in the figure, in each of these sub intervals of duration Δt, the wavelength of the coherent light output 146 is increased linearly and monotonously for a duration Δt.sub.A. Further, in the same sub sweeping interval, the wavelength is decreased linearly and monotonously for a duration Δt.sub.B where preferably Δt.sub.B<<Δt.sub.A. The resulting wavelength behaviour of the wavelength of the coherent light signal 146 over t is a periodic function in time with period Δt=Δt.sub.A+Δt.sub.B. The wavelength defines substantially, if Δt.sub.B <<Δt.sub.A, a slightly “deformed” sawtooth function of time as represented in FIG. 4. The sawtooth scan can be made or with a very fast reset of the tuneable filter 150 if the electro-optical material is enough fast or using a beam splitter for dividing the light source in two or more portions and an optical delay line(s) to combine said portions in a sawtooth profile.

[0175] In FIG. 4, the prior art tuning of the wavelength is also shown (linear dashed curve equivalent to FIG. 1), where the wavelength linearly increase for the whole duration of the sweeping ΔT.

[0176] A numerical simulation of the signal from the OCT detector 120 of the interference signal obtained in case the signals (prior art and invention) of FIG. 4 is swept over the selected portion of the sample is depicted in FIG. 5A and 5B, in the prior art result in FIG. 5A and the present invention case in FIG. 5B. Further, in FIG. 5C a superposition of the two signals is made (dashed line=prior art, solid curve=present invention).

[0177] In FIG. 5A, prior art case, the interference signal is a sinusoid.

[0178] In FIG. 5B, the interference signal shows a sinusoid and some “noise portions”. It is possible to see from FIG. 5B that the interference signal in the invention presents a plurality of regions where the signal cannot be used. This portions are thus preferably discarded. These regions correspond to the portions Δt.sub.B of the sub sweeping intervals. However, it can also be seen that in the remaining part of the curve (i.e. outside the discarded “noise” portions) the signal is in perfect agreement with the prior art signal, i.e. there is substantially no difference in varying the wavelength continuously from a minimum to a “high” maximum and varying the wavelength from a minimum to a much smaller maximum and repeating this change several times. This can be clearly seen in FIG. 5C where the two signals correspond perfectly outside the “noise” portions.

[0179] It can be shown that, if Δt.sub.B is reduced to a minimum, the resulting portions to be discarded can be reduced as well. The smaller Δt.sub.B is, the smaller the part of the resulting interference signal that needs to be not considered becomes (e.g. the discarded portions become smaller).

[0180] In FIG. 6, a second preferred embodiment of the sweeping for an A scan which last ΔT is shown, the sweeping duration ΔT is divided in sub-intervals of equal duration.

[0181] In each of these sub-intervals of duration Δt, the wavelength is varied linearly and monotonously for the whole duration Δt. However, the variation is alternatively either increasing or decreasing. In a first sub sweeping interval, the wavelength is for example increased linearly and monotonously and in the following sub sweeping interval the wavelength is decreased linearly and monotonously. The slope of the linear curve is the same albeit opposite. In other words, if in the i-th interval the slope of the segment defined by the function wavelength (t) is m, the slope of the curve in the (i+1)-th interval is −m.

[0182] This behaviour of the signal is obtained increasing with a certain speed the voltage applied to the electrodes 153, reaching a maximum, and then decreasing the voltage till the minimum at the same speed of the increase.

[0183] In FIG. 6, the prior art tuning of the wavelength is also shown (linear dashed curve equivalent to FIG. 1), where the wavelength linearly increase for the whole duration of the sweeping ΔT.

[0184] A numerical simulation of the signal from the OCT detector 120 of the interference signal obtained in case the signals (prior art and invention) of FIG. 6 are swept over the selected portion of the sample is depicted in FIG. 7A and 7B. <The prior art results are in FIG. 7A and the present invention case is shown in FIG. 7B. Further, in FIG. 7C, a superposition of the two signals is made (dashed line=prior art, solid curve=present invention).

[0185] In FIG. 7A, prior art case, the interference signal is a sinusoid.

[0186] In FIG. 7B, the interference signal shows a sinusoid and some “noise portions”. It is possible to see from FIG. 7B that the interference signal in the invention presents a plurality of regions where the signal cannot be used. These regions correspond to the boundary between one sub-sweeping interval and the next sub-sweeping interval. They also correspond to the point in which the wavelength changes behavior, from increasing to decreasing. However, it can also be seen that in the remaining part of the curve (i.e. outside the noise portions which should be discarded) the signal is in perfect agreement with the prior art signal, i.e. there is substantially no difference in varying the wavelength continuously from a minimum to a “high” maximum and varying the wavelength from a minimum to maximum and from the maximum to the same minimum, repeating this change several times. This can be clearly seen in FIG. 7C where the two signals correspond perfectly outside the “noise” portions.

[0187] FIG. 8A-8C show the simulations results using the second embodiment sweeping signal of FIG. 6, however in this case two reflections separated by 10 μm are present in the sample.

[0188] A numerical simulation of the signal from the OCT detector 120 of the interference signal obtained in case the signals (prior art and invention) of FIG. 6 are swept over the selected portion of the sample is depicted in FIG. 8A and 8B. The prior art results are shown in FIG. 8A, and the results of the present invention case in FIG. 8B. Further, in FIG. 8C a superposition of the two signals is made (dashed line=prior art, solid curve=present invention).

[0189] In FIG. 8A, prior art case, the interference signal is a superposition of two sinusoids having different frequency. Each frequency represents a different reflection on the sample.

[0190] In FIG. 8B, the interference signal shows also two sinusoids superimposed, and some “noise portions”. It is possible to see from FIG. 8B that the interference signal in the invention presents a plurality of regions where the signal cannot be used. These regions correspond to the boundary between one sub sweeping interval and the next sub sweeping interval. They also correspond to the point in which the wavelength changes behavior, from increasing to decreasing. However, it can also be seen that in the remaining part of the curve (i.e. outside the noise portions which can be considered as discarded portions) the signal is in perfect agreement with the prior art signal, i.e. there is substantially no difference in varying the wavelength continuously from a minimum to a “high” maximum and varying the wavelength from a minimum to maximum and from the maximum to the same minimum, repeating this change several times. This can be clearly seen in FIG. 8C where the two signals correspond perfectly outside the “noise” portions.

[0191] FIG. 9A-9C show the fast Fourier transform (FFT) for the interference signals of FIGS. 8A-8C (respectively) where the two reflections can be clearly distinguished, in the two cases of prior art and present invention. It is possible to see that the two spectral behaviors are very similar with only a small added noise for the present invention case.

EXAMPLES

[0192] The laser can emit light at 1550 nm using InP based gain chip. The emission wavelength change by tuning the intra cavity tunable filter at different transmission wavelength by varying the voltage applied to the electro-optical material (in this case the electro-optical material is a thin liquid Chrystal film inside a Fabry-Perot cavity). The output of the laser is coupled at the input of an interferometer (a 2×2 in fiber coupler). At the other input arm, a fast photodiode (bandwidth around 1 GHz) is coupled and connected with a signal processor. At the end of one of the output arms the reference mirror is fixed and at the other output arm the scanning element based on a collimating lens and a scanning mirror are positioned. The length of the two output arms is preferably balanced for optimum interferometer work.

[0193] The sweeping time is set to be equal to 1 μs and it is divided in N=4 sub sweeping interval, each of 250 ns.

[0194] What is called “prior art” signal is substantially the sweeping of FIG. 1, obtained maintaining the laser source sweeping for 1 μs covering 100 nm.

[0195] The signal as depicted in FIG. 6 is obtained sweeping the laser for 250 ns increasing the output wavelength of 25 nm, then inverting the sweep for other 250 nm returning at the initial wavelength and then the previous two sweeps as described are repeated for a second time. During this 1 μs (4×250 ns), the optical element of the OCT remains fixed on the same measurement point of the sample. Voltage difference values applied to the electrodes vary between 0 and 2 kV which are enough to ensure a laser tunability of at least 20 nm, preferably at least 25 nm.

[0196] The signal of FIG. 4 is obtained sweeping the output wavelength linearly for 225 ns at a slightly higher speed covering 25 nm, then reset in 25 ns and the cycle is repeated four times (see FIG. 4). As in the previous example, during this 1 μs (4×250 ns), the optical element of the OCT remains fixed on the same measurement point of the sample.

[0197] The electrical signal from the photodiode is then amplified and sampled (in the example 10 sample per ns). The resulting 10000 samples are then Fourier transformed using a Cooley-Tukey Fast Fourier Transform (FFT) algorithm.