Method and apparatus for acquiring images by optical coherence tomography

Abstract

Method for acquiring images of a sample by optical coherence tomography comprising the following steps: - acquiring a first interferometric optical signal from a first interferometer optically coupled to a coherent electromagnetic radiation source and a second interferometric optical signal from a second interferometer optically coupled to said source, wherein said first interferometer comprises a pair of interferometric arms optically coupled to each other and configured to receive a radiation coming from said source, wherein an interferometric arm of said first interferometer includes said sample arranged to be illuminated by a radiation coming from said source, wherein said second interferometer comprises a pair of interferometric arms optically coupled to each other and configured to receive a radiation coming from said source; - converting said first interferometric optical signal into a first detection signal and converting said second interferometric optical signal into a second detection signal; - sampling said first and second detection signals; - processing the sampling data of the electrical detection signals to obtain data indicative of a reflectivity profile of the sample along an axis of incidence of the radiation on the sample. In a further aspect thereof, the present invention relates to an apparatus for acquiring images of a sample by optical coherence tomography.

Description

[0001] METHOD AND APPARATUS FOR ACQUIRING IMAGES BY OPTICAL COHERENCE

[0002] TOMOGRAPHY

[0003] DESCRIPTION

[0004] Field of the invention

[0005] The present invention relates to the field of systems for acquiring images by optical coherence tomography. In particular, the present invention relates to a method and to an apparatus for acquiring a sample (for example a biological sample) based on the principle of low optical coherence interferometry.

[0006] Background art

[0007] As is known, optical coherence tomography (OCT), also called phase coherence tomography, is a technology that allows the real time acquisition of high-resolution images (of the order of magnitude of pm) of a sample (for example a biological tissue). The reconstructed information relating to the structure of the sample is obtained from the light radiation back reflected from the different regions of the sample according to their optical properties.

[0008] The most recent optical coherence tomography techniques (Frequency Domain OCT or FD- OCT) substantially consist of analysing the spectral content of an interferometric optical signal indicative of the reflectivity profile of the sample under analysis.

[0009] The optical signal is obtained by illuminating the sample using an electromagnetic radiation having wavelengths belonging to a wide spectral band, chosen based on the optical properties of the sample and on the resolution requirements.

[0010] In general terms, an FD-OCT apparatus includes a coherent electromagnetic radiation source (for example a laser) and an interferometer destined to receive the radiation coming from the previously mentioned source.

[0011] The interferometer divides the incoming light radiation between an optical arm (known as “measurement arm”), at which the sample under analysis is located, and a second optical arm (known as “reference arm”) in which the radiation travels along an optical path of predefined length.

[0012] The interferometer outputs an interferometric optical signal (interferogram) with spectral fringes due to the optical interference between the light radiation output from the measurement arm (substantially the light back reflected and back scattered from the sample illuminated) and the light radiation output from the reference arm. The optical interference signal depends on the phase difference of the optical path in the two arms. Therefore, processing of the interferogram allows the reflectivity to be reconstructed as a function of the position of the reflecting components inside the sample, i.e., of its structure. The interferometric optical signal thus obtained is sent to an optical detector that converts it into a corresponding (electrical) detection signal and an acquisition unit that samples the detection signal thus obtained.

[0013] A signal processing unit processes the sampling data of the detection signal and, based on this analysis, provides a reflectivity profile, i.e., a one-dimensional image (A-Scan image) of the sample in the point of incidence on the transverse plane of the light beam, for different depth values along the axis of incidence of the radiation on the sample.

[0014] By repeating the procedure described above for a series of points of incidence of the light beam on the sample, it is possible to scan the sample. By subsequently recomposing the collection of A-Scan images of the sample thus obtained, it is possible to construct a two-dimensional image in the case of linear scan (B-Scan image) or a three-dimensional image in the case of two- dimensional scan (C-Scan image) of the sample.

[0015] In the field of FD-OCT systems, some solutions of known type (Spectral Domain OCT or SD- OCT) use a broadband light source to provide the light signal to the interferometer. In this case, the detection unit receives the interferometric signal, uses a spectrometer as suitable dispersion means and detects the intensity of each component of the interferometric signal using a linear sensor. The signal acquisition unit synchronously converts the information of each component of the interferometric optical signal into a corresponding component of a detection signal and samples it.

[0016] Other known solutions (Swept Source OCT or SS-OCT) use, instead of a broadband source, a source capable of emitting the radiation according to a series of emission wavelengths. During each emission cycle, the source varies the wavelength of the radiation emitted at high speed, periodically sweeping a predefined wavelength tuning range.

[0017] Systems of this type generally comprise a signal detection and acquisition unit capable of providing a time sequence of sampling data of the detection signal obtained for each emission cycle of the source.

[0018] Both in the systems of SD-OCT and SS-OCT type, the intensity of the detection signal obtained from the interferometric signal is a function of the wavelength of the radiation and is usually coded as a function of the wave number k related thereto. In the ideal case, the intensity of the signal, in the presence of a sample with a single reflective surface, has a sinusoidal trend with respect to the variable k, with periodicity linked to the optical path difference between the arms of the interferometer.

[0019] In the presence of multiple surfaces, the signal is given by the overlapping of sinusoids, and therefore its spectral content directly provides the information of the depths from which the radiation was back reflected in the measurement arm.

[0020] In many systems of FD-OCT type spectral analysis of the intensity of the detection signal (corresponding to the interferogram provided by the interferometer) is carried out by Fourier Transform (FT).

[0021] With Fourier Transform it is relatively easy to obtain a complex function of the single variable z, indicative of the depth along a corresponding axis of incidence of the radiation on the sample. This complex function contains the information relating to the reflectivity of the sample for each depth value z, and hence constitutes the mathematical representation of an A-Scan image of the sample.

[0022] Although spectral analysis of the interferogram by Fourier Transform is highly effective in computational terms, it has some problematic aspects linked to its practical implementation.

[0023] In fact, in real applications the detection signal obtained from the interferometric optical signal is not sampled with a variable that evolves linearly with respect to the wave number k (with k = 2n / A, where A= wavelength of the radiation emitted by the source). Consequently, the trend of the intensity of the signal is not sinusoidal as in the ideal case. In general, it has a chirped waveform, which leads to a dispersion of the spectral content of the relevant sampling data and a consequent degradation of the resolution of the A-Scan images obtained by Fourier Transform.

[0024] In SD-OCT solutions, the waveform distortions in the intensity of the detection signal are generally due to the decoding process of the interferometric optical signal (for example due to a non-linear relationship between the position of the pixels of the acquisition sensor and the wavelength of the incident radiation).

[0025] In SS-OCT solutions, these distortions of the waveform are mainly due to the fact that the source emits the light radiation sequentially varying the wavelength in a non-linear way with respect to the temporal sampling of the acquisition. Moreover, the source varies the emission wavelength in a random and non-repeatable way during each emission cycle of the radiation. The source thus introduces entirely unpredictable instantaneous phase variations at each emission cycle of the radiation and each sampling instant within the same cycle.

[0026] In both SD-OCT and SS-OCT solutions, in addition to the non-linearities due to the emission or to the acquisition of electromagnetic radiation, the optical system introduces unbalanced chromatic dispersions due to the optical components present asymmetrically in the two arms in thickness and in material, i.e., the optical path imbalance of the reference arm with respect to the measurement arm.

[0027] To overcome the loss of resolution due to distortion of the sinusoidal signal caused by the non- l sipneeeadri ptiheost oofdi tohdee sso. urce or of the decoding process of the radiation, in many solutions of the prior art a synchronization signal (often called “k-clock”) is acquired from the source (for example by a suitable additional interferometer) and the sampling data of the detection signal are resampled so that the sampling range thereof depends linearly on the wavelength of interest (identified by a corresponding wave number k) of the light radiation. In other words, the sampling data are resampled so that the interferogram is linearized in space k. In this way, the reflectivity profile of the sample under analysis, along the axis of incidence of the radiation that illuminates it, can be reconstructed with precision and maximum resolution by Fourier Transform after resampling and linearization of the electrical signal, and after optical dispersion compensation.

[0028] Unfortunately, it has been found that resampling of the data of the detection signal (carried out, for example, by suitable interpolation techniques), can lead to a loss of information, above all at the spectral fringes at higher frequencies. This can lead to a reduction in the optical resolution of the image of the sample which occurs above all as the depth inspected increases, along the axis of incidence of the light radiation. Some solutions of the prior art have attempted to overcome this problem using over- sampling techniques of the detection signal. However, this causes a significant increase in the times and computational resources required to provide the A-Scan image of the sample.

[0029] The patents US9506740B2 and EP3239651B1 describe some apparatus of SS-OCT type designed to overcome the drawbacks indicated above. Unfortunately, the solutions proposed are characterized by poor flexibility of use and are relatively difficult and costly to produce on an industrial scale.

[0030] The patent EP10760893B2 describes SS-OCT and SD-OCT systems in which the A-Scan image of a sample is reconstructed from the interferogram without the need to use a Fourier Transform applied subsequently to a resampling of the data.

[0031] Although the solutions of this type currently available in some cases allow the drawbacks described above to be overcome, they still have some problems with regard to the quality of the images provided. In fact, it has been found that at times the images acquired have a resolution that deteriorates as the depth increases (i.e., moving away from the position in which the optical path of the two arms has the same length) and a poor signal-to-noise ratio.

[0032] Moreover, with reference in particular to the patent EP10760893B2, these solutions show a large variability of the resolution values, passing from one emission cycle to another of the source, as they do not consider the instantaneous phase variations relating to the emission and / or to the decoding process of the radiation, but instead use a mean value of the function modelling the phase of the signal, calculated during a calibration session.

[0033] Summary of the invention

[0034] The main aim of the present invention is to provide a method and an apparatus for acquiring images of a sample by optical coherence tomography, which allows the drawbacks of the prior art indicated above to be mitigated or overcome.

[0035] Within this aim, an object of the present invention is to provide a method and an apparatus for acquiring images of a sample by optical coherence tomography that is able to provide images of the sample with relatively high resolution levels, with minimum variabilities, and which are constant along the whole of the depth inspectable with the apparatus.

[0036] A further object of the present invention is to provide a method and an apparatus for acquiring images of a sample by optical coherence tomography that ensure high reliability and flexibility of use.

[0037] A further object of the present invention is to provide a method and an apparatus for acquiring images of a sample by optical coherence tomography that are relatively simple to implement and produce on an industrial scale, at competitive costs with the solutions of the prior art currently available.

[0038] This aim and these objects, together with other objects that will be apparent from the description below and from the accompanying drawings, are achieved, according to the invention, by a method for acquiring images by optical coherence tomography, according to claim 1, and its dependent claims, appended hereto.

[0039] In a further aspect, the present invention relates to an apparatus for acquiring images by optical coherence tomography according to claim 9, and its dependent claims, appended hereto.

[0040] Brief description of the figures

[0041] Features and advantages of the method and apparatus according to the invention will be better understood by referring to the description provided below and to the accompanying figures, provided purely for non-limiting and illustrative purposes wherein:

[0042] Fig. 1 schematically illustrates the apparatus for acquiring images according to the invention; and

[0043] Figs. 2-3 schematically illustrate the method for acquiring images according to the invention; and

[0044] Figs. 4-6 schematically illustrate a measurement method for obtaining data used by the method of acquiring images according to the invention; and

[0045] Fig. 7 schematically illustrates an embodiment of the apparatus for acquiring images according to the invention.

[0046] Detailed description of the invention

[0047] The present invention relates to a method and to an apparatus for acquiring images of a sample 500 by optical coherence tomography (OCT).

[0048] In principle, the sample 500 can be of any type, for example a biological tissue (e.g., the retina of an eye) or any opaque or semi-transparent object.

[0049] The method and the apparatus according to the invention will be described below with particular reference to applications of SS-OCT type, for obvious reasons of brevity of description. This is not in any way meant to limit the scope of the present invention. The method and the apparatus according to the invention can in fact be used effectively in applications of SD-OCT type, as will be discussed in depth below.

[0050] Figs. 1-2 schematically illustrate the main components of the apparatus 1 according to the invention, and the general steps of the method 100 according to the invention, respectively.

[0051] With reference to Fig. 1, the apparatus 1 comprises a coherent electromagnetic radiation source 2, for example a laser.

[0052] The electromagnetic radiation (or light radiation) emitted by the source 2 does not necessarily have a wavelength in the visible range. For example, it could have a wavelength in the infrared range, for example for applications of ophthalmic type.

[0053] In applications of SS-OCT type, the light source 2 is of SS (Swept Source) type. In this case, it is configured to emit a light radiation Lsaccording to a sequence of emission cycles, during each of which the radiation emitted is a very narrow band with a wavelength that is varied very quickly and progressively in a predefined wavelength tuning range.

[0054] The wavelength range swept by the source is a design feature that uniquely determines (together with the central wavelength of the sweep) the achievable theoretical axial resolution of the image obtained from the interferogram, the latter being proportional to the amplitude of the wavelength tuning range and inversely proportional to the square of the central wavelength. Analogously, the bandwidth around the single wavelengths of the radiation emitted is a design feature of the source and uniquely determines the coherence length of the source. For example, when the apparatus 1 is an ophthalmic machine for analysing the retina of the eye, the source 2 can be configured to emit a variable radiation in a wavelength tuning range centred around the value 1000 nm, with amplitude of the order of 100 nm and a cycle repetition frequency of the order of 100 KHz.

[0055] In applications of SD-OCT type, the light source 2 is configured to emit a broadband radiation Ls.

[0056] In general, the source 2 can be produced according to solutions of known type. Therefore, for reasons of brevity it will be described hereunder only with reference to the aspects of interest for the invention.

[0057] The apparatus 1 comprises a first interferometer 3 and a second interferometer 4 optically coupled (according to known modes) to the source 2.

[0058] The first interferometer 3 comprises a pair of interferometric arms 31, 32 optically coupled to each other and configured to receive a portion of the radiation Lsemitted by the source 2.

[0059] The first interferometer 3 comprises a first interferometric arm having a predefined optical length and a second interferometric arm 32 having a variable optical length around the length of the optical path of the measurement arm.

[0060] The first interferometric arm 31 (measurement arm) includes the sample 500 or is optically coupled to the sample. In any case, the sample 500 is arranged to be illuminated by the radiation coming from the source 2.

[0061] The second interferometric arm 32 (reference arm) includes an optical reference 600 (for example a reflective element) arrange to be illuminated by the radiation coming from the source.

[0062] The optical reference 600 defines a reference position for the sample 500, when it is arranged along the sample arm 31. The interferometric arms 31, 32 are thus arranged to form optical paths with an optical path difference (OPD) around zero.

[0063] Preferably, the optical path length of the reference arm 32 can be adjusted to find the point of the measurement arm 31 in which the optical path difference between the two arms of the interferometer is zero, i.e., to position the origin of the reference system of the axial coordinate (z=0) of an A-Scan image of the sample 500 in a position selected by the user within the permitted range (for example +- 20 mm with respect to the origin of the axial coordinate).

[0064] The first interferometer 3 outputs a first interferometric optical signal h (measurement interferogram) when illuminated by the source 2.

[0065] The first signal h is generated by the optical interference between the radiation output from the measurement arm 31 (radiation back reflected and scattered from the illuminated sample 500) and the radiation output from the reference arm 32 (radiation back reflected from the illuminated optical reference 600).

[0066] Preferably, the first interferometer 3 is configured as a Michelson interferometer. The second interferometer 4 comprises a pair of interferometric arms 41, 42 optically coupled to each other and configured to receive a portion of the radiation Lsemitted by the source 2.

[0067] The interferometric arms 41, 42 are arranged to form optical paths with a predefined optical path length 2zc.

[0068] The second interferometer 4 is configured to output a second interferometric optical signal h (reference interferogram).

[0069] The second interferometric signal h is generated by the optical interference between the light radiation output from the interferometric arm 41 and the light radiation output from the interferometric arm 42.

[0070] Preferably, the second interferometer 4 is configured as a Mach-Zehnder interferometer.

[0071] In general, the first and second interferometer 3, 4 can be produced according to known solutions. Therefore, for reasons of brevity they will be described hereunder only with reference to aspects of interest for the invention.

[0072] The apparatus 1 comprises an electronic unit 5 optically coupled (according to known modes) to the first interferometer 3 and to the second interferometer 4 to receive the interferometric signals h, h coming from these.

[0073] The electronic unit 5 (which constitutes a detection, acquisition, and signal processing unit) is advantageously configured to carry out the method 100 according to the present invention.

[0074] With reference to Fig. 2, the method 100 comprises a step 110 of acquiring the first interferometric signal h from the first interferometer 3 and of acquiring the second interferometric signal h from the second interferometer 4. As indicated above, these interferometers are optically coupled to the same coherent electromagnetic radiation source 2. The method 100 comprises a step 120 of converting the first interferometric signal h into a first electrical detection signal he and of converting the second interferometric signal h into a second electrical detection signal he.

[0075] The first detection signal he thus corresponds to the measurement interferogram provided by the first interferometer 3 while the second electrical detection signal he corresponds to the reference interferogram provided by the second interferometer 4.

[0076] The method 100 comprises a step 130 of sampling the first detection signal he and the second detection signal he.

[0077] Sampling of the detection signals he. he is carried out with reference to a suitable sampling variable as a function of the type of light source used.

[0078] In applications of SS-OCT type, sampling of the detection signals he, he is preferably carried out at a series of sampling instants ti (z = 1, 2, ..., N). In applications of SD-OCT type, sampling of the detection signals he, he is preferably carried out at a plurality of sampling positions (groups of pixels) referred to the sensor used.

[0079] The number N of values of the sampling variable considered to sample the electrical detection signals he, he can be selected randomly based on the resources available for decoding and digitalizing the signal.

[0080] The method 100 provides for digitally processing the sampling data of the electrical detection signals he, he to obtain data indicative of a reflectivity profile of the sample 500 along an axis of incidence of the light radiation on said sample. The data thus obtained constitute an A-Scan image of the sample 500.

[0081] With reference again to Fig. 1, the electronic unit 5 preferably comprises an optical detection module 51 optically coupled to the first and second interferometer 3, 4 to receive the interferometric optical signals h, h. The optical detection module can advantageously comprise a first sensor optically coupled to the first interferometer 3 and a second sensor optically coupled to the second interferometer 4.

[0082] The optical detection module 51 is configured to convert the first interferometric signal h into the first detection signal he and to convert the second interferometric signal h into the second detection signal he.

[0083] To carry out the opto-electrical conversion process of the interferometric signals h, h into the respective electrical detection signals he, he, the optical detection module 51 can use optical detection means of known type, the nature of which depends substantially on the type of light source 2 used.

[0084] In applications of SD-OCT type, the optical detection module 51 can include one or more spectrometers and video cameras or high-speed CCD sensors, while in applications of SS-OCT type the optical detection module 51 can advantageously include one or more groups of high- ssppeeeedd pphhoottooddiiooddeess..

[0085] Preferably, the electronic unit 5 comprises a sampling module 52 electrically coupled to the optical detection module 51 to receive the electrical detection signals he, he. The sampling module 52 is configured to sample the first detection signal he and the second detection signal he.

[0086] The sampling module 52 can use known sampling means. Also in this case, the nature of these sampling means depends substantially on the digitalization resources available. In applications of SD-OCT type, the sampling module 52 can be considered as a spectrometer able to provide, in parallel and synchronously, the sampling data of the detection signals he, he depending on the wavelengths of interest. The detection signals he, he are preferably sampled at a plurality of sampling positions (groups of pixels of the video camera or CCD sensor).

[0087] In applications of SS-OCT type, the sampling module 52 can instead comprise a high-speed digitizer (for example with a sampling frequency in the order of GHz) able to provide, in sequence, sampling data of the detection signals he, he as a function of the sampling instant ti. In applications of this type, in order to synchronize each sampling sequence with a corresponding light emission cycle of the source 2 (of SS type), the sampling module 52 can advantageously receive a trigger signal to start to operate. This trigger signal can advantageously be provided directly by the source 2 or by a sensor (for example a Bragg grating) arranged to detect the radiation hsemitted by the source 2 at a predefined wavelength (for example at the start of the sweep).

[0088] From the above, it is in any case evident that the sampling data of the electrical detection signals he, he are formed by vectors of N data, where N is the number of values of the sampling variable considered for analysis of the sample 500, for example the number of sampling instants ti for an application of SS-OCT type, or the number of values of another sampling magnitude for an application of SD-OCT type.

[0089] Preferably, the sampling module 52 includes hardware and / or software resources such as to allow sampling of the detection signals he, he in parallel.

[0090] The method 100 involves digitally processing the sampling data of the electrical detection signals he, he to obtain data indicative of a reflectivity profile of the sample 500 along an axis of incidence of the radiation on the sample. The data thus obtained form an A-Scan image of the sample 500.

[0091] With reference to Fig. 1, the electronic unit 5 advantageously comprises a signal processing module 53 configured to receive and process the sampling data of the electrical detection signals lie, he.

[0092] The signal processing module 53 can be produced according to known solutions, for example solutions of microprocessor or FPGA type. Advantageously, the signal processing module 53 is provided with hardware and / or software resources to allow processing of the sampling data of the electrical detection signals he, he in parallel.

[0093] An essential aspect of the invention consists in that the method 100 comprises carrying out a particular signal processing procedure 200 to process the sampling data of the detection signals he, he. As indicated above, this signal processing procedure is carried out by the processing module of the signals 53.

[0094] The signal processing procedure 200 advantageously includes calculating, in real time (i.e., during analysis of the sample 500), a sort of dictionary of complex functions that describe the instantaneous trend of each signal component derived from the radiation back reflected and scattered at a corresponding value zm of the position in depth of the sample under analysis along an axis of incidence of the radiation on said sample, as a function of the sampling variable of the signals considered.

[0095] The aforesaid dictionary of complex functions allows direct measurement of the reflectivity of the sample under analysis for a corresponding value of the position in depth zm of the sample 500 under analysis along an axis of incidence of the radiation on the aforesaid sample, in the reference system where z=0 is the position along the axis for which the optical path difference between the interferometric arms 31, 32 of the first interferometer 3 is equal to zero. Therefore, this allows an A-Scan image of the sample 500 under analysis to be obtained based directly on the sampling data of the first detection signal he without the need to carry out a Fourier Transform and the relevant necessary resampling or oversampling procedures of the data.

[0096] A particularly important aspect of the invention consists in that the signal processing procedure 200 uses information derived from the second electrical detection signal he (corresponding to the second interferometric optical signal h output by the second interferometer 4) to calculate the previously mentioned dictionary of complex functions. This information is indicative of non-linear phase variations introduced by the source 2 in modulation of the wavelength at each emission cycle (for sources of SS type) and introduced during the decoding and optical detection process of the first interferometric optical signal h (for sources of SD type).

[0097] Therefore, the second interferometric optical signal h does not constitute a simple synchronization signal (k-clock) as occurs in the solutions of the prior art.

[0098] In fact, according to the present invention the second interferometric optical signal h is used as carrier of information useful for reconstructing the non-linear phase dependence with respect to the wave number (k) during the light emission process by the source 2 and during the decoding and optical detection process of the first interferometric optical signal h. In other words, the second detection signal he is used to reconstruct the footprint of the radiation source and of the decoding process with regard to the instantaneous phase variations in the first detection signal he.

[0099] A further particularly important aspect of the invention consists in that the aforesaid dictionary of complex functions is calculated taking into account further information indicative of a phase shift caused by chromatic dispersions due to optical components (for example lenses) arranged along the optical paths inside the first interferometer 3 and to possible asymmetries with regard to the thickness and the materials of these optical components between the interferometric arms 31, 32 of the aforesaid interferometer.

[0100] As will be seen below, this further information is preferably obtained off-line by carrying out a suitable measurement method 300 (Figs. 4-5) during which the interferometers 3, 4 are arranged in a suitable operating configuration (Fig. 6). In this case, both the interferogram of the first interferometer 3 and the interferogram of the second interferometer 4 are used as references to obtain the contribution of chromatic dispersion provided by the optics of the first interferometer 3.

[0101] Fig. 3 illustrates the main steps of the signal processing procedure 200.

[0102] The signal processing procedure 200 will be described hereunder with particular reference to applications of SS-OCT type, for obvious reasons of brevity of description. Nonetheless, it can be effectively used in applications of SD-OCT type.

[0103] As described above, the detection signal he has a frequency distribution that is not linear along the pixels of an array of optical detectors (CCD video cameras) in the case of SD-OCT or along the temporal axis in the case of SS-OCT. The nonlinearity results in a distortion of the signal (chirped signal).

[0104] In general terms, considering a model provided with a single reflective surface in the position of the sample 500, the intensity of the first detection signal he at a generic value of the sampling variable ti (a generic sampling instant in SS-OCT applications or a generic value of another sampling magnitude in SD-OCT applications) can be modelled from the following equation (for the sake of simplicity considering only the cross-interference term of the radiation in the arms of the interferometer, and omitting the self-interference terms and the constant DC term): where: p is an indicative constant parameter proportional to the responsivity of the optical detection module 51 ;

[0105] S(tj) is a parameter indicative of the optical power density of the source 2; zmis a value of the position in depth along the axis of incidence of the radiation on the sample, taken into account in analysis of the sample 500, in the reference system in which z=0 is the position along the axis for which the optical path difference between the interferometric arms 31, 32 of the first interferometer 3 is equal to zero. The term 2zmalso represents the optical path difference between the two interferometric arms; RSmis a parameter indicative of the reflectivity of the sample 500 arranged in the measurement arm 31 of the first interferometer 3 at the depth zm,

[0106] RRis a parameter indicative of the reflectivity of the optical reference 600 arranged in the reference arm 32 of the first interferometer 3;

[0107] G() is a function that describes the (non-linear) relationship between the optical frequency emitted by the source and the value of the sampling variable ti. G(ti) is thus a value indicative of the variations of the non-uniform phase due to the source or to the optical decoding process;

[0108] H() is a function that describes the relationship between the phase shift due to chromatic dispersions and the value of the sampling variable ti. H(tj) is thus a value indicative of the uncompensated chromatic dispersions introduced in the optical paths of the arms of the first interferometer 3.

[0109] Considering the sample 500 as composed of a series of reflecting surfaces located in the positions zm (m=l, 2, M), the first detection signal he at a generic value of the sampling variable ti can be modelled from the following equation: where M is a (random) number of depth values zm along the axis of incidence of the radiation on the sample, considered in analysis of the sample 500.

[0110] As is evident from the equation (2), the detection signal he can be modelled by the superposition of M signal components, each formed by a distorted {chirped) sinusoidal function. The argument G(ti>)2zm+ H (ti) of each sinusoidal function represents the phase of said sinusoidal function of the first detection signal he. The phase depends, in a non-linear way, on the frequency of the radiation emitted by the source or relating to the optical decoding process, and on the chromatic dispersions introduced into the optical paths of the first interferometer 3.

[0111] As mentioned above, the signal processing procedure 200 aims to reconstruct a reflectivity profile (A-Scan image) of the sample 500 under analysis starting directly from sampling data of the first detection signal he (corresponding to the measurement interferogram provided by the first interferometer 3, at which the sample 500 is arranged) and from a dictionary of complex functions / m modelling the signal components (sinusoids) of the first detection signal describing their trend with reference to each single position zm., for a set of positions m = 1, 2, ..., M.

[0112] Based on the aforesaid equations (1) - (2), it is evident that, in order to satisfy this aim, it is necessary to calculate the value G(li) indicative of the non-uniform phase variation and the value H(h) indicative of the phase shift due to the uncompensated chromatic dispersions as a function of the preselected sampling variable ti of the first detection signal he.

[0113] According to the invention, the signal processing procedure 200 comprises a step 201 of acquiring first sampling data Ii[ ] of the first detection signal he and a step 202 of acquiring second sampling data ] of the second detection signal he.

[0114] The first and second sampling data Ii[ ], / ?[ ] comprise, for each sampling value ti a component of the sampling data vector Ii = \h(ti), h(t2), ..., h(tN)} of the first detection signal he and a component of the sampling data vector I 2 = \h(ti), h(t2), ..., h(tN)} of the second detection signal he, respectively.

[0115] As mentioned above, the sampling data Ii[ ], / ?[ ] of the electrical detection signals he, he are constituted by vectors of N components corresponding to the sampling values, where N is the number of values of the sampling variable ti considered for analysis of the sample 500.

[0116] These data are advantageously memorized in a suitable support of the signal processing unit 53 during the sampling process carried out by the sampling module 52. During operation of the apparatus 1, the signal processing unit 53 retrieves the previously mentioned sampling data during the signal processing procedure 200.

[0117] According to the invention, the signal processing procedure 200 comprises a step 203 of calculating third data G[ ] indicative of the non-linear phase variation of the first detection signal he, in particular of each signal component of this detection signal.

[0118] As mentioned above, this phase variation is introduced in both of the electrical detection signals he, he during the radiation emission process by the source 2 and during the decoding and optical detection process of the respective interferometric signals h, h.

[0119] The third data G[ ] indicative of the non-linear phase variation comprise, for each value of the sampling variable ti, a component of the data vector G = {G(ti), G(t2), ..., G(IN)} .

[0120] The third data G[ ] are calculate based on the second sampling data / ?[ ] acquired. The information of the second detection signal he is thus used to reconstruct the footprint of the source and of the optical decoding process on the evolution of the phase variations with respect to the sampling variable of the signal components of the first detection signal he and of the second detection signal he.

[0121] Preferably, the step 203 of calculating the third data G[ ] indicative of the non-linear phase variation comprises the step of calculating seventh data ] indicative of the phase of the second detection signal he.

[0122] The data ] indicative of the phase of the second detection signal he comprise, for each value of the sampling variable ti, a component of the data vector $2 = { ( / >2(ti), < / >2(t2), ..., (faftN)}. The data $?[ ] indicative of the phase of the second detection signal be can be calculated, for example, by carrying out a Hilbert Transform of the second sampling data Z?[ ] and applying a suitable distribution filter of the phase values thus obtained.

[0123] The third data G[ ] indicative of the non-linear phase variation are advantageously calculated based on the data $?[ ] indicative of the phase of the second detection signal he.

[0124] For each value of the sampling variable ti, a component G(ti) of the data vector G can be calculated based on the following relationship: where:

[0125] ( / niti) is a corresponding value indicative of the phase of the second detection signal he in relation to the same value of the sampling variable;

[0126] 2zc is the optical path length difference between the interferometric arms 41, 42 of the second interferometer 4.

[0127] During operation of the apparatus 1, the signal processing unit 53 calculates in real time the third data G[ ] indicative of the non-linear phase variation during the signal processing procedure 200.

[0128] According to the invention, the signal processing procedure 200 comprises a step 204 of acquiring or calculating fourth data d[ ] that describe a phase shift in the first detection signal he. In particular, the fourth data d\ ] are indicative of a complex function that describes a phase shift in the first detection signal he. In practice, the fourth data d\ ] are indicative of a multiplication factor of the signal modelling the phase shift in the first detection signal he.

[0129] This phase shift is due to chromatic dispersions introduced in the first detection signal he by optical components arranged along the optical paths inside the first interferometer 3.

[0130] The fourth data d\ ] comprise, for each value of the sampling variable ti, a component of the vector d = {d(ti), d(t2), ..., d(tx)} of complex values.

[0131] According to preferred embodiments of the invention, the fourth data d[ ] are predefined and are calculated by a specific measurement method 300 (Figs. 4-5) during which the interferometers 3, 4 are arranged in a suitable operating configuration (Fig. 5). This measurement method will be described below.

[0132] According to other possible embodiments of the invention, the fourth data d[ ] can be calculated in real time, for example using a further additional interferometer arranged in a suitable operating configuration. During operation of the apparatus 1, the signal processing unit 53 acquires from a memory support or calculates in real time the fourth data d[ ] during the signal processing procedure 200.

[0133] According to the invention, the signal processing procedure 200 comprises a step 205 of calculating fifth data / T ] including a set of complex functions configured to model the signal components of the first detection signal he describing the trend of these signal components as a function of the sampling variable ti of this detection signal.

[0134] The fifth data / T ] comprise, for each signal component relating to the radiation back reflected and scattered from a single position zm of the sample along the axis of incidence of the radiation, a vector fm= {f(ti), f(tz), of complex values (complex functions).

[0135] Therefore, the fifth data _ / ( ] are formed by an N x M matrix of complex values. This matrix constitutes a sort of dictionary of complex functions modelling the signal components of the first detection signal he and describe their trend as a function of the sampling variable ti.

[0136] In general terms, the fifth data _ / ( ] that describe the signal components of the first detection signal he are calculated based on third data G[ ] indicative of the non-linear phase variation and on the fourth data d[ ] describing a phase shift due to chromatic dispersions.

[0137] The complex function fmthat describes the modelling of the signal obtained from the radiation back reflected and scattered from a position zm of interest, as a function of the sampling variable ti, can be expressed by the following relationship: where:

[0138] - G(ti) is a corresponding value indicative of the non-linear phase variation for the value of the sampling variable ti;

[0139] - d(ti) is a corresponding complex function describing the phase shift due to chromatic dispersions in the first interferometer 3, for the value of the sampling variable ti;

[0140] - Zm is a depth value on the sample 500 (along an axis of incidence of the radiation coming from the source).

[0141] To reduce the computational load of the signal processing module 53, for each value of the sampling variable ti, the value of a generic complex function fm(ti) can be calculated in an approximate manner and / or as update of a corresponding predefined complex function calculated off-line.

[0142] In this case, for each value of the sampling variable ti, the value of the complex function fm(ti) is progressively updated based on a corresponding value G(ti) indicative of the non-linear phase variation (calculated in real time) and on a corresponding value of the complex function d(ti) that describes the phase shift due to chromatic dispersions in the first interferometer 3.

[0143] During operation of the apparatus 1, the signal processing unit 53 calculates in real time the fifth data / T ] indicative of the modelled trend of the components of the first detection signal he as a function of the sampling variable, during the signal processing procedure 200.

[0144] According to the invention, the signal processing procedure 200 comprises a step 206 of calculating sixth data a[ ] indicative of a reflectivity profile of the sample 500 along the axis of incidence of the radiation on the previously mentioned sample.

[0145] The sixth data a[ ] of the reflectivity profile of the sample 500 include a data vector a = {a(l), a(2), a(M)}.

[0146] The data vector a includes M components, where M is the number of depth values zm considered along the axis of incidence of the light radiation on the sample. Each component amof this data vector corresponds to the reflectivity value of the sample at a depth value zm along the axis of incidence of the radiation.

[0147] From the above, it is evident that the sixth data a[ ] constitute an A-Scan image of the sample 500. They are calculated based on first sampling data h[ ] of the first detection signal he and on the fifth data / T ] modelling the trend of the signal components of the first detection signal he as a function of the sampling variable thereof.

[0148] Each component amcorresponding to a predefined depth value zm can be calculated based on the following relationship: where:

[0149] - h(ti) is a sampling value included in the first sampling data h[ ] and corresponding to a generic value of the sampling variable ti;

[0150] - fmiti) is the value of the complex function included in the fifth data _ / ( ] and corresponding to the depth value zm of interest for a generic value of the sampling variable ti;

[0151] - Zm is a depth value on the sample 500 (along an axis of incidence of the radiation coming from the source) of interest.

[0152] During operation of the apparatus 1, the signal processing unit 53 calculates in real time the sixth data a[ ] indicative of the reflectivity profile of the sample 500 during the signal processing procedure 200.

[0153] In this way, the signal processing unit 53 provides a one-dimensional image (A-Scan) of the sample 500 along an axis passing through the point of incidence of the radiation thereon. To obtain a two- or three-dimensional image of the sample 500, the radiation emitted from the source is advantageously shifted (according to a suitable sequence of scans synchronized with emission of the source 2) at a plurality of points of incidence and the method 100 according to the invention must be repeated for each point of incidence of the radiation on the sample.

[0154] It must be noted that to obtain an image of the cross section of the sample 500, perpendicular to the direction of incidence of the radiation, it is sufficient to carry out the processing procedure 200 at a certain depth value zm for all the points of incidence of the radiation coming from the source belonging to the cross section to be viewed. This allows a significant reduction in the computational load of the signal processing unit 53.

[0155] As mentioned above, according to preferred embodiments of the invention, the fourth data d[ ] indicative of a complex function that describes a phase shift of the first detection signal he are calculated by a preliminary measurement method 300.

[0156] In this method the interferometers 3, 4 are arranged according to a particular measurement configuration illustrated in Fig. 6.

[0157] According to this measurement configuration, the first interferometer 3 has a configuration (preferably of Michelson type) similar to that used during analysis of the sample but does not include any sample to be analysed.

[0158] The first interferometric arm 31 (measurement arm) does not include or is optically coupled to a sample 500 but includes a first optical reference 700 (for example a reflective element) arranged in a position, for example, corresponding to that of the sample 500 so as to be illuminated by the radiation coming from the source.

[0159] The second interferometric arm 32 (reference arm) includes the aforementioned second optical reference 600 arranged to be illuminated by the radiation coming from the source.

[0160] The pair of interferometric arms 31, 32 is arranged to have a first predefined optical length difference zr.

[0161] The first interferometer 3 outputs a third interferometric signal h when illuminated by the source 2.

[0162] The third interferometric signal h is generated by the optical interference between the radiation output from the measurement arm 31 (radiation back reflected and scattered from the first illuminated optical reference 700) and the radiation output from the reference arm 32 (radiation back reflected and scattered from the second illuminated optical reference 600).

[0163] The second interferometer 4 has a configuration (preferably of Mach-Zehnder type) identical to that used during analysis of the sample (Fig. 1).

[0164] The interferometric arms 41, 42 are arranged to have a second predefined optical length difference zc.

[0165] The second interferometer 4 outputs a fourth interferometric optical signal h (which can coincide with the previously mentioned second interferometric optical signal h) when illuminated by the source 2.

[0166] The fourth interferometric optical signal U is generated by the optical interference between the radiation output from the interferometric arm 41 and the radiation output from the interferometric arm 42.

[0167] The electronic unit 5, optically coupled to the first interferometer 3 and to the second interferometer 4, is configured to receive the interferometric optical signals h, I4 coming therefrom and carry out the preliminary measurement method 300.

[0168] Fig. 4 illustrates the main steps of the measurement method 300.

[0169] The measurement method 300 includes a step 310 of acquiring the third interferometric signal h output from the first interferometer 3 and a fourth interferometric signal h output from the second interferometer 4.

[0170] The measurement method 300 comprises a step 320 of converting the third interferometric signal h into a third detection signal he and of converting the fourth interferometric signal h into a fourth detection signal he.

[0171] The optical detection modes of the interferometric signals h, h can take place with modes identical to those described above for the interferometric optical signals h, h while carrying out the method 100.

[0172] The measurement method 300 comprises a step 330 of sampling of the third detection signal he and the fourth detection signal he.

[0173] The sampling modes of the electrical detection signals he, he can take place with modes identical to those described above for the electrical detection signals he, he while carrying out the method 100.

[0174] Also in this case, the sampling data of the electrical detection signals he, he are constituted by vectors of N data, wherein N is the number of values of the sampling variable ti considered (N sampling instants in SS-OCT applications or N values of another sampling magnitude in SD- OCT applications).

[0175] The number N of values of the sampling variable ti considered to sample the electrical detection signals he, he advantageously corresponds to the number of values of the sampling variable considered while carrying out the method 100.

[0176] The measurement method 300 involves digitally processing the sampling data of the electrical detection signals he, he so as to obtain the previously mentioned fourth data d\ ]. The measurement method 300 thus comprises a step of carrying out a further signal processing procedure 400 to process the sampling data of the electrical detection signals I3e, I4e. This signal processing procedure is carried out by the signal processing module 53. Fig.5 illustrates the main steps of the signal processing procedure 400. The signal processing procedure 400 comprises a step 401 of acquiring third sampling data I3[ ] of the third detection signal I3eand a step 402 of acquiring fourth sampling data I4[ ] of the fourth detection signal I4e. The third and fourth sampling data I3[ ], I4[ ] are structured in an identical manner to the first and second sampling data I1[ ], I2[ ] acquired during the processing procedure 200. The fourth sampling data I4[ ] are substantially identical to the second sampling data Ι2[ ] acquired during the signal processing procedure 200, described above. The third and fourth sampling data I3[ ], I4[ ] comprise, for each sampling variable value ti, a component of the sampling data vector I3= {I3(t1), I3(t2), ..., I3(tN)} of the third detection signal I3eand a component of the sampling data vector I4= {I4(t1), I4(t2), ..., I4(tN)} of the fourth detection signal I4e, respectively. The signal processing procedure 400 comprises a step 403 of calculating eighth data φ3[ ] indicative of the phase of the third detection signal I3e based on the third sampling data I3[ ] and a step 404 of calculating ninth data φ4[ ] indicative of the phase of the fourth detection signal I4ebased on the fourth sampling data I4[ ]. The ninth data φ4[ ] indicative of the phase of the fourth detection signal I4e are substantially identical to the seventh data φ2[ ] calculated during the signal processing procedure 200, described above. The eighth data φ3[ ] indicative of the phase of the third detection signal I3e comprise, for each value of the sampling variable ti , a component of the data vector φ3= { φ3(t1), φ3(t2), ..., φ3(tN)}. Likewise, the ninth data φ4[ ] indicative of the phase of the fourth detection signal I3e comprise, for each value of the sampling variable ti, a component of the data vector φ4= { φ4(t1), φ4(t2), ..., φ4(tN)}. The data φ3[ ], φ4[ ] indicative of the phase of the third and fourth detection signal I3e, I4e can be calculated by carrying out a Hilbert Transform of the third and fourth sampling data Ι3[ ], Ι and applying a suitable distribution filter of the phase values thus obtained. The signal processing procedure 400 comprises a step 405 of calculating tenth data Η indicative of the phase shift due to unbalanced chromatic dispersions between the two arms, introduced by the optical components of the first interferometer 3. The tenth data H[ ] indicative of the phase shift comprise, for each value of the sampling variable ti, a component of the data vector H = The tenth data H[ ] indicative of the phase shift are calculated based on the eighth data φ3[ ] indicative of the phase of the third detection signal I3eand on the ninth data φ4[ ] indicative of the phase of the fourth detection signal I4e. For each value of the sampling variable ti , the value of the component H(ti) of the data vector indicative of the phase shift can be calculated based on the following relationship: where: is a corresponding value of the data vector indicative of the phase of the third detection signal I3e relating to the value of the sampling variable ti; − φ4(ti) is a corresponding value of the data vector indicative of the phase of the fourth detection signal I4e relating to the value of the sampling variable ti; - 2zris the optical path length difference between the interferometric arms of the first interferometer 3; - 2zc is the optical path length difference between the interferometric arms of the second interferometer 4. From the equation (5), it is evident how the tenth data H[ ] indicative of the phase shift are calculated based on a phase difference normalized at the depth of the sample, between the third detection signal I3e, derived from the interferogram of the first interferometer 3 in which dispersive optical components are present along the path of the light radiation, and the fourth detection signal I4e, derived from the interferogram of the second interferometer 4 in which there are no dispersive optical components present along the path of the light radiation. It must be noted that the tenth data H[ ] are substantially constant in time as they depend substantially on the type and on the layout of the dispersive optical components used in the first interferometer 3. The signal processing procedure 400 comprises a step 406 of calculating the fourth data d[ ] indicative of a complex function that describes the phase shift in the first detection signal I1e(this signal obtained while carrying out the method 100) based on the tenth data H[ ] indicative of the phase shift due to chromatic dispersion in the first interferometer 3. For each value of the sampling variable ti, the component d(ti) of the complex value vector d included in the sampling data d[ ] can be calculated based on the following relationship: where:

[0177] ( / >4(ti) is a corresponding value of the data vector indicative of the phase of the fourth detection signal he relating to the value of the sampling variable tr,

[0178] H(ti) is a corresponding value of the data vector indicative of the phase shift due to chromatic dispersions relating to the value of the sampling variable ti;

[0179] W(ti) is a filter function calculated for the value of the sampling variable ti;

[0180] 2zc is the optical path length difference between the interferometric arms 41, 42 of the second interferometer 4.

[0181] From the equation (7) above it is evident that each component d(ti) of the previously mentioned complex value vector d forms a complex function that describes a phase shift in the first detection signal he (obtained while carrying out the method 100).

[0182] The measurement method 300 can advantageously be repeated if extremely high precision is required with regard to calculation of the tenth data H[ ] indicative of a phase shift caused by chromatic dispersions.

[0183] For each repetition of the measurement method, a different value of zr can advantageously be considered (optical length difference between the interferometric arms 31, 32 of the first interferometer 3).

[0184] The tenth data H[ ] calculated at each repetition cycle can be used to calculate further data / / ’[ ] indicative of the average of the phase shift due to chromatic dispersions in the first interferometer 3.

[0185] These more accurate data can advantageously be used to calculate the fourth indicative data d[ ] modelling the phase shift due to chromatic dispersions in the first detection signal he.

[0186] Example of embodiment of the invention

[0187] The method and the apparatus according to the invention can be used in several types of applications.

[0188] According to some preferred embodiments of the invention, the apparatus 1 is an ophthalmic machine for acquiring images of the retina of the eye. In this case, it can be produced on an industrial scale according to the solution schematically illustrated in Fig. 7.

[0189] The ophthalmic machine 1 comprises a coherent electromagnetic radiation source 2 (for example a laser) of SS (Swept Source) type. Preferably, a fibre coupler 21 is arranged at the output of the light source 2. Such a fibre coupler is configured to divide the light radiation Lsprovided by the source into portions destined for the interferometers 3, 4.

[0190] The ophthalmic machine comprises the first interferometer 3 having a pair of interferometric arms 31, 32.

[0191] The first interferometer 3 comprises an optical attenuator 33 adapted to receive a light radiation coming from the source 2. The optical attenuator 33 is configured to keep the light source directed towards the patient’s eye at a safe level.

[0192] The first interferometer 3 also includes a fibre matrix 34 optically coupled to the optical attenuator 33 and to the interferometric arms 31, 32. At a first input port 34a, the fibre matrix 34 receives the light radiation coming from the source 2 and directs a first portion of this light radiation towards a pass-through port 34b optically coupled to the first interferometric arm 31 and a second portion of this light radiation towards a first output port 34c optically coupled to the second interferometric arm 32.

[0193] The first interferometric arm 31 is destined to be optically coupled with the retina of a patient’s eye, during operation of the apparatus.

[0194] The first interferometric arm 31 comprises a first fibre collimator 311, optically coupled to the pass-through port 34b of the fibre matrix 34, and optical scanning means 312 optically coupled the previously mentioned fibre collimator.

[0195] The optical scanning means 312 comprise a system of movable mirrors electronically controlled by suitable control means 35 (an electronic control circuit board) configured to synchronize the movement of these scanning mirrors with the radiation emission cycles of the source 2.

[0196] The first interferometric arm 31 comprises a system of lenses 314 coupled to the optical scanning means 312. The system of lenses 314 is configured to focus the radiation coming from the source and scanned by the scanning means 312 on the transverse surface of the patient’s retina.

[0197] During operation of the ophthalmic machine, the radiation coming from the source 2 is projected by the fibre collimator 311 towards the scanning means 312 and scanned thereby along the cross section of the retina of the eye upon completion of each acquisition cycle of an A-Scan image of the retina of the eye.

[0198] The radiation back reflected and scattered by the illuminated retina is collected by these scanning means 312 and directed towards the fibre collimator 311 which directs it towards the pass-through port 34b of the fibre matrix. This matrix directs the light radiation coming from the first interferometric arm 31 towards a second output port 34e combining it with a light radiation coming from the second interferometric arm 32.

[0199] The second interferometric arm 32 comprises a second fibre collimator 321, optically coupled to the first output port 34c of the fibre matrix 34.

[0200] The second interferometric arm 32 further comprises an optical diaphragm 322 optically coupled to the second fibre collimator 321. This optical diaphragm can be an adjustable continuously variable diaphragm for filtering purposes.

[0201] The second interferometric arm 32 comprises a reflective element 600 optically coupled to the optical diaphragm 322 and to a third fibre collimator 323.

[0202] The reflective element 600 can advantageously be constituted by a corner cube capable of diverting the incident radiation in a predefined direction regardless of the angle of incidence of said radiation. The reflective element 600 is advantageously arranged to invert the direction of propagation of the incident radiation.

[0203] The second interferometric arm 32 comprises an optical polarization controller 324 optically coupled to the third fibre collimator 323 and to a second input 34d of the fibre matrix 34.

[0204] The optical polarization controller 324 is used to vary the polarization of the radiation, can be of Lefevre type, of fourth wave, or half wave plate type, of mechanical type, and so forth.

[0205] During operation of the ophthalmic machine, the radiation coming from the source 2 is projected by the second fibre collimator 321 towards the reflective element 600. The reflective element 600 inverts the path of the radiation and directs it towards the third fibre collimator 323 which directs the radiation towards the optical polarization controller 324 and the second input port 34d of the fibre matrix. The matrix directs the radiation coming from the second interferometric arm 32 towards the second output port 34e combining it with the radiation coming from the first interferometric arm 31.

[0206] During operation of the ophthalmic machine, at the second output port 34, the first interferometer 3 provides the first interferometric optical signal h obtained from the optical interference of the radiation coming from the interferometric arms 31, 32.

[0207] Preferably, the reflective element 600 can be moved by suitable actuation means along a predefined reference axis. This reflective element thus constitutes a delay line that allows the length difference of the optical paths of the interferometric arms 31, 32 to be modified.

[0208] The optical length of the reference arm 32 can be adjusted to position the reference point of the axial coordinate (z=0) of an A-Scan image of the sample 500 in a position selected by the user in the permitted range (for example +- 20 mm). The ophthalmic machine comprises the second interferometer 4. This interferometer is a Mach- Zehnder interferometer optically coupled to the source 2.

[0209] The second interferometer 4 comprises a pair of interferometric arms (not illustrated) arranged so as to have a predefined optical length difference.

[0210] The second interferometer 4 comprises an input port 43 to receive a radiation coming from the source 2 and an output port 44.

[0211] At this output port 44, during operation of the ophthalmic machine the second interferometer 4 provides the second interferometric optical signal h obtained from the optical interference of the radiation coming from the respective interferometric arms.

[0212] The ophthalmic machine 1 comprises an electronic unit 5 optically coupled to the interferometers 3, 4 described above and configured to carry out the method 100 according to the invention.

[0213] The electronic unit 5 comprises an optical detection module 51 to receive the interferometric optical signals h, h and convert them into corresponding electrical detection signals he, he during operation of the ophthalmic machine.

[0214] The optical detection module 51 includes a first optical detection assembly 51a and a second optical detection assembly 5 lb, which are optically coupled to the output port 34e of the first interferometer 3 and to the output port 44 of the second interferometer 4. Each optical detection assembly 51a, 51b comprises a pair of high-speed photodiodes and a transimpedance amplifier coupled thereto. In this way, each optical detection assembly behaves as a balanced receiver capable of eliminating the common mode noise.

[0215] The optical detection module 51 can, in practice, be constituted by a series of opto-electronic circuits mounted on a corresponding optical detection circuit board.

[0216] The electronic unit 5 comprises a sampling module 52 for sampling the electrical detection signals he, he, during operation of the ophthalmic machine.

[0217] Preferably, during operation of the ophthalmic machine, the sampling module 52 receives a trigger signal coming from the source 2 or from a sensor of the radiation emitted. The trigger signal is destined to synchronize the operation of the sampling module 52 with each radiation emission cycle carried out by the source of SS type.

[0218] The sampling module 52 can, in practice, be constituted by a series of electronic circuits mounted on a corresponding signal acquisition circuit board.

[0219] The electronic unit 5 comprises a signal processing module 53 adapted to process the sampled data of the electrical detection signals he, he, in particular carrying out the signal processing procedures 200, 400. The signal processing module 53 can, in practice, be constituted by a series of electronic circuits mounted on a corresponding signal processing circuit board, preferably integrated with the previously mentioned signal acquisition circuit board.

[0220] The above-mentioned signal acquisition and processing circuit boards are advantageously provided with two signal acquisition and processing channels, to be able to process electrical detection signals he, he in parallel and synchronous mode.

[0221] Advantages of the invention

[0222] The method and the apparatus according to the invention offer numerous advantages with respect to the solutions of the prior art currently available.

[0223] The method and the apparatus according to the invention are able to provide OTC images of a sample with high resolution levels and uniform in depth.

[0224] The method according to the invention uses a sort of dictionary of complex functions that describe the trend of the components relating to the different depths of the electrical detection signal corresponding to the interferometric optical signal output from the interferometer in which the sample under analysis is arranged.

[0225] This dictionary of complex functions is able to consider both undesirable non-linear phase variations introduced by the source or during the decoding and optical detection process, and unbalanced chromatic dispersions introduced by the optical components of the interferometer in which the sample under analysis is arranged.

[0226] The method according to the invention thus allows the image of the sample to be reconstructed directly from the sampled data of the electrical detection signals, without the need for resampling, linearization, or over-sampling operations. The images of the sample thus obtained have relatively high-resolution levels even at depth values of a few millimetres or in the order of one centimetre.

[0227] Moreover, unlike the solutions of the prior art, the resolution of the images is substantially uniform even in the presence of any phenomena of instability and non-repeatability of the emission cycle of the radiation source.

[0228] The method and the apparatus according to the invention have a high flexibility and stability of use. The number N of values of the sampling variable ti to use and the number M of depth values zm of the sample to be considered are at the complete discretion of the user.

[0229] Unlike known solutions of the prior art, in particular those that use a Fourier Transform, the number M of depth values zm of the sample to be considered (and hence the resolution of the images acquired) is not restricted to the number of samples considered or to their specific decimation.

[0230] Naturally, this provides important benefits with regard to management of the computational resources.

[0231] As illustrated, the method according to the invention uses a dictionary of complex functions to produce a direct measurement of the reflectivity of the sample under analysis for corresponding depth values zm of the sample along an axis of incidence of the radiation on the previously mentioned sample.

[0232] This solution is particularly effective if images of a cross section of the sample at a predefined depth zm are required. In this case, the dictionary is reduced to a single complex function as a function of the sampling variable ti and the reflectivity value of each point of the cross section can be obtained as simple scalar product between the electrical detection signal vector and the value vector of the corresponding complex function.

[0233] The method and the apparatus according to the invention are relatively simple to produce on an industrial scale using standardized techniques and procedures and without requiring excessive computational resources. Therefore, they can be produced at competitive costs with respect to the solutions of the prior art currently available.

Claims

CLAIMS1. Method (100) for acquiring images of a sample (500) by optical coherence tomography comprising the following steps: acquiring (110) a first interferometric optical signal (Zy) from a first interferometer (3) optically coupled to a coherent electromagnetic radiation source (2) and a second in- speed pho tetrofdeiroodmeest.ric optical signal (Zz) from a second interferometer (4) optically coupled to said source, wherein said first interferometer (3) comprises a pair of interferometric arms (31, 32) optically coupled to each other and configured to receive a radiation coming from said source, wherein an interferometric arm (31) of said first interferometer includes or is optically coupled to said sample (500) arranged to be illuminated by a radiation from said source, wherein said second interferometer (4) comprises a pair of interferometric arms (41, 42) optically coupled to each other and configured to receive a radiation coming from said source; converting (120) said first interferometric optical signal (Zy) into a first detection signal (he) and converting said second interferometric optical signal (Z2) into a second detec- speed ph tiootnod siigondaels. (he); sampling (130) said first and second detection signals (he, he) ; characterized in that it comprises a step of carrying out a signal processing procedure (200) comprising the following steps: acquiring (201) first sampling data (Z / [ ]) of said first detection signal (he); acquiring (202) second sampling data (h[ ]) of said second detection signal (he); calculating (203) third data (G[ ]) indicative of a non-linear phase variation in said first detection signal (he), wherein said third data (G[ ]) are calculated based on said second sampling data (h[ ]); acquiring or calculating (204) fourth data (d[ ]) indicative of a complex function de- speed ph scortiobdiniogd aes p.hase shift in said first detection signal (he); calculating (205) fifth data (f[ ]) indicative of complex functions describing the behav- speed ph iooutrod oifo sdiegsn.al components of said first detection signal (he), wherein said fifth data (f[ ]) are calculated based on said third data (G[ ]) and said fourth data (d[ ]); calculating (206) sixth data («[ ]) indicative of a reflectivity profile of said sample (500) along an axis of incidence of the radiation on said sample, wherein said sixth data («[ ]) are calculated based on said first sampling data (Zj[ ]) and said fifth data (f[ ]).

2. Method according to claim 1, characterized in that said step (203) of calculating said third data (G[ ]) comprises: - calculating seventh data (φ2[ ]) indicative of the phase of said second detection signal (I2e) based on said second sampling data (I2[ ]); - calculating said third data (G[ ]) based on said seventh data (φ2[ ]).

3. Method according to one of the preceding claims, characterized in that said fourth data (d[ ]) are predefined.

4. Method according to claim 3, characterized in that said fourth data (d[ ]) are calculated through a measurement method (300) comprising the following steps: - acquiring (310) a third interferometric optical signal (I3) from said first interferometer (3) and a fourth interferometric optical signal (I4) from said second interferometer (4), wherein said first interferometer (3) includes a reflective element (700) in a position corresponding to that of said sample (500); wherein the interferometric arms (31, 32) of said first interferometer are arranged to form an optical path having a predefined optical length difference (zr); wherein the interferometric arms (41, 42) of said second interferometer are arranged to form an optical path having a predefined optical length difference (zc); - converting (320) said third interferometric optical signal (I3) into a third detection signal (I3e) and converting said fourth interferometric optical signal (I4) into a fourth detection signal (I4e); - sampling (330) said third and fourth detection signals (I3e, I4e); - carrying out a signal processing procedure (400) comprising the following steps: - acquiring (401) third sampling data (I3[ ]) of said third detection signal (I3e); - acquiring (402) fourth sampling data (I4[ ]) of said fourth detection signal (I4e); - calculating (403) eighth data (φ3[ ]) indicative of the phase of said third detection signal (I3e) based on said third sampling data (I3[ ]); - calculating (404) ninth data (φ4[ ]) indicative of the phase of said fourth detection signal (I4e) based on said fourth sampling data (I4[ ]); - calculating (405) tenth data (Η[ ]) indicative of a phase shift due to unbalanced chro- matic dispersions in said third interferometer (3) based on said eighth data (φ3[ ]) and said ninth data (φ4[ ]); - calculating said fourth data (d[ ]) based on said tenth data (Η[ ]) indicative of a phase shift.

5. Method according to one of the preceding claims, characterized in that said source (2) is of the SS (Swept Source) type.

6. Method according to one of the claims from 1 to 4, characterized in that said source (2) is configured to emit broadband radiation.

7. Method according to one of the preceding claims, characterized in that said first interfer- speed om phetoetrod (3io)d ises c.onfigured as a Michelson interferometer.

8. Method according to one of the preceding claims, characterized in that said second inter- speed fe prohmoteotdeiro (d4e)s i.s configured as a Mach-Zender interferometer.

9. Apparatus (1) for acquiring images of a sample (500) by optical coherence tomography comprising: a coherent electromagnetic radiation source (2); a first interferometer (3) optically coupled to said source (2) and configured to output a first interferometric optical signal (li), wherein said first interferometer comprises a pair of interferometric arms (31, 32) configured to receive a radiation coming from said source, wherein an interferometric arm (31) of said first interferometer includes or is optically coupled to said sample (500) arranged to be illuminated by a radiation from said source; a second interferometer (4) optically coupled to said source and configured to output a second interferometric optical signal (Z2), wherein said second interferometer comprises a pair of interferometric arms (41, 42) configured to receive a radiation coming from said source; characterized in that it comprises an electronic unit (5) configured to carry out a method (100) for acquiring images of said sample (500), according to one of the preceding claims.

10. Apparatus according to claim 9, characterized in that said source (2) is of the SS (Swept Source) type.

11. Apparatus according to claim 9, characterized in that said source (2) is configured to emit broadband radiation.

12. Apparatus according to one of the claims from 9 to 11, characterized in that said first in- speed ter pfheorotomdeiotedre (s3. ) is configured as a Michelson interferometer.

13. Apparatus according to one of the claims from 9 to 12, characterized in that said second interferometer (4) is configured as a Mach-Zehnder interferometer.

14. Apparatus according to one of the claims from 9 to 13, characterized in that it is an oph- speed th pahlmotiocd mioadcehsi.ne (10) for acquiring images of the retina of the eye.

Images