Optical coherence tomography imaging device and method

Abstract

The invention relates to an optical coherence tomography imaging device (100) for imaging a sample (1), which comprises: • a light source (20) for emitting an incident light beam; • a scanning module (30) including an acousto-optic tunable filter (31) configured to spectrally diffract the incident light beam and select a wavelength of the incident light beam, the scanning module (30) providing a diffracted light beam at the selected wavelength, the acousto-optic tunable filter being controlled by an electrical control signal that can be modulated; • a multimode optical fibre (60) arranged to adjust a spatial coherence of the diffracted light beam; • an optical coherence tomography module (90) forming an interference light beam; and • an imaging detector (120) configured to record the interference light beam and to form, on the basis of the interference light beam, an image of the sample.

Description

OPTICAL COHERENCE TOMOGRAPHY IMAGING DEVICE AND METHOD - TECHNICAL FIELD OF THE INVENTION

[0001] The present invention relates generally to an optical coherence tomography imaging device for imaging a sample. It also relates to an optical coherence tomography imaging method for imaging a sample. STATE OF THE ART

[0002] Optical coherence tomography (OCT) is an imaging technique used to measure the two-dimensional or three-dimensional structure of a sample. Specifically, the axial position of the sample's structures is determined using a low-coherence interferometric measurement that measures the time of flight of an optical beam scanning the sample before it is partially reflected or backscattered by a structure of interest.

[0003] Optical coherence tomography (OCT) is distinguished from time-domain OCT, which measures the average of the interference signal over a wide range of wavelengths, thus allowing the measurement of a given depth within a sample. Fourier-domain OCT measures the spectrum of the interference signal, enabling the reconstruction of the sample's axial profile using a Fourier transform.

[0004] Full-field optical coherence tomography (OCT) is an OCT configuration that detects the interferometric signal using a two-dimensional imaging detector optically conjugated to a focal plane located within the imaged sample. This configuration often provides superior transverse spatial resolution while also offering faster image acquisition times.

[0005] For this purpose, full-field optical coherence tomography (FF-OCT) imaging devices are available in the Fourier domain. These devices often use a source module with a scanning module configured to perform a spectral scan of the light source. While functional, these devices do not allow for precise control of the spectral scan. In fact, the spectral scan is constrained by the source scanning module. Consequently, if one wishes to modify the spectral scan performed by the scanning module to optimize image quality or obtain spectroscopic contrast, it is necessary to change the scanning module, which can be expensive and sometimes impossible. Furthermore, such constraints limit the possibilities of optical coherence tomography imaging.PRESENTATION OF THE INVENTION.

[0006] To overcome the aforementioned drawbacks of the prior art, the present invention proposes an optical coherence tomography imaging device for imaging a sample comprising: - a light source to emit an incident light beam exhibiting a spectrum, for example a broad spectrum extending over 120 nm, - a scanning module comprising an acousto-optical tunable filter configured to spectrally diffract the incident light beam and select a wavelength of the incident light beam, the scanning module of the source providing a diffracted light beam at the selected wavelength, the acousto-optical tunable filter being driven by a modulable electrical control signal so as to control a scan of at least a portion of the spectrum of the incident light beam, the selected wavelength being a function of the electrical control signal; - a multimode fiber arranged to adjust the spatial coherence of the diffracted light beam; - an optical coherence tomography module comprising: ii) a reference arm equipped with a reference mirror and an object arm including a support for the sample; ii) a beam splitter arranged to divide the light beam from the multimode optical fiber into a reference light beam in the reference arm and an object light beam in the object arm and / or to recombine the reference light beam and the object light beam into an interference light beam; iii) an imaging detector configured to record the interference light beam and to form, from the interference light beam, an image of the sample on the imaging detector. The imaging device includes a control module that emits the electrical control signal and is configured to modify: - a value of the electrical control signal, and - an application time of a value of the electrical control signal, said electrical control signal not being a radio frequency wave.

[0007] More specifically, an imaging device is proposed in which the scanning module is controllable by a modulating electrical control signal, thus enabling control of the spectral scan. Therefore, thanks to the invention, it is possible to control the desired spectral scan simply and inexpensively.

[0008] In this disclosure, "controllable" means that the spectral sweep performed by the scanning module is not fixed. For example, here, the duration associated with each selected wavelength, as well as the interval between two successive wavelengths, can be modified using the electrical control signal.

[0009] Specifically, changing the value of the electrical control signal allows for modifying its amplitude and thus selecting the desired wavelength of the diffracted light beam. Therefore, such a scanning module can select the wavelengths of the diffracted light beam and the timing of their selection. It is not necessary to scan all wavelengths of the incident light beam's spectrum or a specific spectral band. This makes it possible to select non-contiguous wavelengths, that is, wavelengths that do not follow each other directly.

[0010] Changing the application time of this value allows you to control the duration for which the diffracted light beam at the selected wavelength is emitted. Therefore, it is possible to modulate the formation time of the diffracted light beam at the selected wavelength. Consequently, the spectral scan performed by the scanning module is not necessarily linear.

[0011] Therefore, the spectrum sweep is fully adjustable. This device allows for the simple control or manipulation of various sweep parameters. Thanks to this invention, spectral shaping, also known as spectral modulation, is possible.

[0012] In one embodiment, the electrical control signal is an electrical voltage. Therefore, the electrical control signal is not a radio frequency signal. The electrical control signal is thus an analog signal (here, a voltage or a current). It is therefore easier to implement and allows for inexpensive modulation of the spectral sweep performed by the scanning module. Typically, in this case, the electrical control signal drives the radio frequency antenna of the acousto-optic tunable filter. By "not being a radio frequency wave," we mean, in particular, that the electrical control signal is a control or setpoint signal, whose characteristics (such as the amplitude of the voltage or current) define the parameters of the radio frequency signal that will be generated to drive the transducer of the acousto-optic tunable filter.More precisely, the acousto-optic tunable filter is driven here by voltage (or current) modulation of the electrical control signal, and not by direct radio frequency modulation. Thus, the electrical control signal is physically and functionally distinct from the power radio frequency signal that ensures the acousto-optic interaction within the crystal. This power radio frequency signal typically has a frequency range from 10 MHz to 500 MHz, and more specifically from 40 MHz to 250 MHz (for example, between 94 and 105 MHz).

[0013] In one embodiment, the electrical control signal is configured to drive at least one of the following elements: - a scan step of the scanning module (corresponding here to a difference between two selected wavelengths), - an emission or formation time of the diffracted light beam, by modifying the application time of a value of the electrical control signal.

[0014] Thus, thanks to the invention, the scanning parameters of the scanning module are modifiable, specifically adjustable. It is therefore possible to fully control the spectral scanning performed by the scanning module, unlike the prior art where the spectral scanning parameters are fixed and depend on the scanning module used.

[0015] In one embodiment, the control module can be configured to modify the value of the electrical control signal and the application time of this value of the electrical control signal.

[0016] In one embodiment, the control module is configured to select the wavelength of the diffracted light beam with at least a precision enabling the selection of wavelengths spaced less than 0.2 nm apart, advantageously less than 0.01 nm. Optionally, the control module can be configured to select the wavelength of the diffracted light beam with at least a precision enabling the selection of wavelengths spaced less than 0.2 nm apart.

[0017] In this embodiment, the control module can be configured to adjust the value of the electrical control signal with an accuracy of 1*10' 3in order to select wavelengths from the spectrum of the light source with central wavelengths spaced less than 0.2 nm apart. Thus, it is possible to precisely select any wavelength from the spectrum of the incident light beam.

[0018] For example, at the output of the tunable acousto-optical filter, the filtered light has a spectral width centered around a central tunable wavelength (corresponding to the selected wavelength).

[0019] In one embodiment, the control module is configured to control the scanning of the spectrum of the incident light beam with a spectral scanning step that can be changed according to the value of the selected control signal.

[0020] In one embodiment, the electrical control signal is configured to vary nonlinearly over time. This can be implemented by nonlinearly varying the values ​​of the electrical control signal and / or by modulating the application time of the electrical control signal values. Such a feature allows the power spectral density of the light illuminating the optical coherence tomography device to be varied.

[0021] In one embodiment, the electrical control signal is configured to vary linearly over time for a time period, for example between 5 ms and 15 ms.

[0022] Other advantageous and non-limiting features of the imaging device according to the invention, taken individually or in all technically possible combinations, are as follows: - in one embodiment, the tomography module includes an optical device arranged to collimate the light flux at the output of the multimode fiber into a collimated light beam and direct it towards the beam splitter; - The imaging detector is controlled by a modulating electrical control signal synchronized with the image formed by the imaging device. This electrical control signal allows control of the exposure or activation time of the imaging detector; - the control module is configured to control an activation of the imager detector via a second electrical control signal, said second electrical control signal being modulable; - the control module is configured to change the value of the electrical control signal for at least one time period, said imaging device having a synchronization device configured to synchronize an activation of the imaging detector during the time period, said imaging detector being configured to integrate the interference light beam at each time period and provide an image of the sample from the interference light beam integrated at each time period; - the imaging detector is a two-dimensional (2D) detector or a one-dimensional (1D) detector, for example composed of a line of pixels; - the imaging device is a spectrometer, said imaging device comprising a dispersive element, for example a diffraction grating; - the imaging device includes at least one of the following elements: (i) means for controlling the position of the sample support of the object arm to modify a position of the object arm of the sample according to at least one degree of freedom oriented along an axial direction normal to a surface of the sample; (ii) means for controlling the position of the reference mirror to modify a position of the reference mirror along at least one degree of freedom oriented along an axial direction normal to the reference mirror; and comprises (i) a processing unit coupled to the imaging detector, the control device being configured to control the electrical control signal for n time periods and to change the position of the object arm of the sample and / or the reference mirror at each time period so that each time period is associated with a different position of the sample and / or the reference mirror, each time period being associated with a different position of the object arm, the imaging detector being configured to form an image of the sample at each time period so as to form n images of the sample, n being an integer greater than 1, the processing unit being configured to process the n images of the sample to provide a depth image of the sample; - the control module is configured to modify the value of the electrical control signal for at least one time period, the imaging device comprising a synchronization device configured to synchronize an activation of the imaging detector on values ​​of the electrical control signal within at least one time period so as to record, at each activation, the interference light beam at a given wavelength and to provide an image of the sample at each activation; - the imaging device comprises a processing unit configured to process a set of images of the sample using a spectral processing algorithm in the Fourier domain or in the time domain so as to provide a depth image of the sample from this processing; - the light beam propagating between the source and the tunable acousto-optical filter is propagating in a free field; - the tunable acousto-optical filter is arranged to diffract the incident light beam to a diffraction order of magnitude equal to 1; - the scanning module includes a transmission-operating spatial filter positioned after the acousto-optical tunable filter and configured to select a single diffraction order of the diffracted light beam; - the light source includes at least one of the following light sources: i) a supercontinuum laser; ii) a light-emitting diode; iii) a heat source, such as a heated filament; (iv) an electric arc, or any type of plasma source, including laser-controlled plasma sources; - the multimode fiber has at least one hundred transverse modes and a length between 2 m and 3 m or greater than 10 m; - the imaging device includes a dynamic spatial modulation device arranged to exert pressure on the multimode optical fiber and modulate a phase shift between different transverse modes of the multimode optical fiber; - the optical coherence tomography module is a full-field optical coherence tomography module; - the tomography module includes one of the following interferometers: a Michelson interferometer, a Linnik interferometer, a Mirau interferometer, a Fizeau interferometer, a Mach Zehnder interferometer.

[0023] The invention also relates to an optical coherence tomography imaging method for imaging a sample, said method comprising the following steps: - emission of a beam of light incident by a light source; - spectral filtering by diffraction to select a wavelength of the incident light beam in order to perform a spectral scan of the incident light beam, said filtering step being implemented by a scanning module comprising an acousto-optic tunable filter and forming a diffracted light beam at the selected wavelength; - application of an electrical control signal to drive the tunable acousto-optical filter so as to control a sweep of at least a portion of the spectrum of the incident light beam, said electrical control signal being modulable over time; - adjustment of a spatial coherence of the diffracted light beam by propagation of the diffracted light beam in a multimode optical fiber; - formation of an interference light beam by propagation of the light beam at the output of the optical fiber in an optical coherence tomography module; - capture of the interference light beam by an imaging detector; - formation of an image of the sample from the interference light beam by the imaging detector.

[0024] Other advantageous and non-limiting features of the imaging method according to the invention, taken individually or in all technically possible combinations, are as follows: - The application step includes a sub-step for modifying, by a control module, at least one of the following parameters: i) a value of the electrical control signal; ii) the application time of a value of the electrical control signal; - The imaging method includes: (i) a synchronization step of the imaging detector for at least one time period during which the electrical control signal varies, and ii) a step of integrating the interference light beam for at least one time period to provide an image of the sample from the interference light beam integrated for at least one time period; - the imaging method comprises n iterations of the steps of the imaging method so as to provide n images of the sample, the imaging method comprising a displacement of the reference mirror of the tomography module and / or a displacement of the sample support between each new iteration, the imaging method comprising a step of processing, by a processing unit, the n images to provide a depth image of the sample, n being an integer greater than 1; - the capture step is repeated k times during at least one time period so as to form k images of the sample during the time period, k being an integer greater than 1, and the imaging method includes a step of processing the k images of the sample, by a processing unit, to form a depth image of the sample from the k images of the sample.

[0025] Of course, the different features, variants and embodiments of the invention can be combined with each other in various ways as long as they are not incompatible or mutually exclusive. DETAILED DESCRIPTION OF THE INVENTION

[0026] The description that follows, with regard to the attached drawings, given by way of non-limiting examples, will make it clear what the invention consists of and how it can be carried out.

[0027] Regarding the attached drawings:

[0028] - Figure 1 is a schematic representation of an optical coherence imaging device according to this disclosure;

[0029] - Figure 2 illustrates a first example of a variation of an electrical control signal driving a scanning module of the imaging device illustrated in Figure 1 as well as a variation of an electrical activation signal of an imaging detector of the imaging device illustrated in Figure 1;

[0030] - Figure 3 illustrates a variation in the spectrum of the diffracted light beam over time obtained using the electrical control signal shown in Figure 2;

[0031] - Figure 4 illustrates a second example of a variation of an electrical control signal driving a scanning module of the device illustrated in Figure 1 as well as a variation of an electrical activation signal of an imaging detector of the imaging device illustrated in Figure 1;

[0032] - Figure 5 illustrates a variation of the spectrum of the light beam in wave vector k = — diffracted over time obtained using the electrical signal shown in zo figure 4;

[0033] - Figure 6 illustrates a third example of a variation of an electrical control signal driving a scanning module of the device illustrated in Figure 1;

[0034] - Figure 7 illustrates the variation of the electrical control signal shown in Figure 4, as well as a variation of an electrical activation signal of an imaging detector of the imaging device shown in Figure 1;

[0035] - Figure 8 illustrates a fourth example of variations of an electrical control signal driving a scanning module of the device illustrated in Figure 1 as well as a variation of an electrical activation signal of an imaging detector of the imaging device illustrated in Figure 1;

[0036] - Figure 9 illustrates spectral bands obtained with variations of the electrical control signal illustrated in Figure 4;

[0037] - Figure 10 illustrates an example of a table in the form of a curve that relates an electrical control signal to a wavelength of a diffracted light beam;

[0038] - Figure 11 illustrates individual values ​​of the electrical control signal of the table illustrated in Figure 10, as well as the associated wavelengths and spectral width associated with these electrical control signal values;

[0039] - Figure 12 illustrates a dispersive element positioned in front of the imaging detector of the imaging device illustrated in Figure 1. In this case, the imaging device according to this disclosure is a spectrometer;

[0040] - Figure 13 illustrates an embodiment of an imaging method according to this disclosure.

[0041] In this disclosure, the terms before, upstream, after, and downstream are established according to a direction of light propagation established in the device according to this disclosure.

[0042] Device

[0043] Figure 1 illustrates an embodiment of an optical coherence tomography imaging device 100 according to this disclosure.

[0044] The imaging device 100 illustrated in Figure 1 is configured to image a sample 1, for example a biological tissue 1, positioned on a support 97. For this purpose, the device 100 includes a light source 20 arranged to emit an incident light beam F1 to illuminate the sample 1. In particular here, the light source 20 generates the incident light beam F1 in the direction of the sample 1.

[0045] In this disclosure, the light source 20 may be of various types, for example, it may be a laser source. For this purpose, the light source 20 may include a supercontinuum laser, for example, the "The SuperK COMPACT" white light laser, or the "Electro VIS" supercontinuum laser from leukos-laser, or the "Iceblink Supercontinuum Fiber laser" from Fyla, 450 nm-2300 nm, or a light-emitting diode or a plasma source, for example, laser-controlled. In another embodiment, the light source 20 is not a laser source but may include a thermal source, for example, comprising a heated filament or an electric arc.

[0046] The light source 20 is preferably a broad-spectrum light source with low temporal coherence. By broad spectrum, we mean that the spectrum of the light source extends over more than 300 nm, specifically here over 1000 nm. Typically, the spectrum of the light source 20 has a spectral width extending from the visible to the near-infrared, for example, between 500 nm and 2500 nm. Such a spectral width improves the axial resolution of the image obtained of the sample by the imaging device 100.

[0047] The imaging device 100 also includes a scanning module 30 positioned after the light source 20. As illustrated in Figure 1, the light source 20 and the scanning module 30 are two separate components. Specifically, "separate" here means that the light source 20 and the scanning module 30 are separate and independent of each other. Typically, the scanning module 30 is positioned outside the light source 20, specifically outside the laser cavity of the light source emitting the incident light beam F1 when the latter is a laser light source. This provides greater flexibility to the imaging module 100 and thus reduces maintenance costs. Furthermore, it can be seen that the incident light beam F1 emitted by the light source 20 propagates in a free field from the light source 20 to the scanning module 30.Free field means that the incident light beam F1 does not encounter any other element of the imaging device 100 before reaching the scanning module 30. Using free field propagation limits the modification of the wave propagation of the incident light beam F1. This thus limits optical aberrations and / or optical losses introduced by optical elements.

[0048] The scanning module is configured to form a diffracted light beam F2. For this purpose, the scanning module 30 includes an acousto-optic tunable filter 31. This acousto-optic tunable filter 31 is configured to perform spectral scanning over a spectral band encompassing wavelengths from the visible to the near-infrared, for example, wavelengths between 550 nm and 1050 nm. In particular, using wavelengths in the visible range improves the resolution of the acquired images, especially the transverse resolution of a three-dimensional image of the sample. This also results in more signal scattered by the sample (due to stronger light-matter interaction) and therefore a more intense interference beam, which improves the signal-to-noise ratio of the images acquired by the imaging device 100.

[0049] Typically, and advantageously, the 31 acousto-optic tunable filter can perform a spectrum sweep over a spectral band of at least 0.2 nm, or at least 1 nm, or at least 100 nm in less than 50 ps, ​​specifically here in less than 3 ps. This allows for rapid wavelength changes and complex modulation of the spectrum of light illuminating the interferometer. For example, the 31 acousto-optic tunable filter is a Brimrose TEAF5-0.575-1.00-MSD acousto-optic tunable filter.

[0050] In this disclosure, the acousto-optic tunable filter 31 is controlled (or driven) by an electrical control signal. This control signal is time-modulated, meaning it varies to achieve the desired result. In practice, this electrical control signal is a voltage. It is therefore not a radio frequency wave. Alternatively, this electrical control signal can be understood to be an electrical current, for example, by converting the electrical control signal from a voltage to a current using Ohm's law.

[0051] In practice, the electrical control signal drives the radio frequency antenna of the acousto-optic tunable filter 31. For this purpose, the control device 40 described below can include or be connected to a converter (here, an analog voltage-to-radio frequency converter) configured to convert the electrical control signal into a radio frequency signal. This converter can be directly integrated into the control module 40 described below or integrated into or connected to the scanning module 30. Thus, the control module (specifically via the electrical control signal generator (here, the DAQ module)) transmits or sends the electrical control signal to the analog-to-radio frequency converter to drive the crystal of the acousto-optic tunable filter. The radio frequency wave generated by the acousto-optic tunable filter 31 is a function of the value (or amplitude) of the electrical control signal.Thus, each value of the electrical control signal is associated with a unique radio frequency waveform. Furthermore, this allows for very rapid wavelength selection, unlike direct computer control of the radio frequency signal, which relies on serial communication and is inherently slower. This superior execution speed enables efficient spectral shaping over a short detector activation time.

[0052] The scanning module 30 is configured to diffract and spectrally disperse the incident light beam F1 using the tunable acousto-optical filter 31 and to select a length of the incident light beam F1 according to the electrical control signal so as to form the diffracted light beam F2.

[0053] In this disclosure, it is understood that the scanning module 30 is configured both to diffract the incident light beam F1 using the dynamic diffraction grating linked to the frequency variation of the acoustic wave generated by the acousto-optic tunable filter (the acoustic wave being driven by the electrical control signal) and to disperse the incident light beam F1 by separating (dispersing) the spectrum of the incident light beam F1 into different wavelengths according to the acoustic wave generated by the acousto-optic tunable filter 31. Thus, for the sake of simplicity, the light beam exiting the scanning module 30 is called the diffracted light beam F2.

[0054] Thus, it is understood that the scanning module 30 includes a spatial filter 33 configured to block a portion of the light beam dispersed by the acousto-optic tunable filter 31 and to select a wavelength from the spectrum of the incident light beam F1. The wavelength of the diffracted beam transmitted after this spatial filtering is controlled, specifically selected, by the electrical control signal. Indeed, by changing the frequency of the radio waves of the acousto-optic tunable filter 31 (controlled by the electrical control signal), the dispersed beam is shifted, and the wavelength passing through the filter changes, thus allowing it to be selected. In the following, the selected wavelength is denoted 70.

[0055] This spatial filter 33 comprises a transmission aperture or slot arranged after the acousto-optic tunable filter 31. This aperture may be circular. Alternatively, this aperture may be rectangular, having a first dimension oriented along the propagation of the acoustic wave and a second dimension oriented along an axis perpendicular to the first direction of acoustic propagation. The dimensions of this aperture or slot are predefined and depend on the acousto-optic tunable filter 31 and the scanning module 30.

[0056] In one embodiment, this opening or slot can be active, in the sense that its dimensions can vary. This can be done manually.

[0057] Advantageously, the scanning module 30 can be configured to diffract the incident light beam F1 to a diffraction order of norm equal to 1. Thus, following this example, the tunable acousto-optic filter 31 can diffract the incident light beam F1 to the order +1 or -1 (here +1).

[0058] Diffracting the incident light beam F1 according to the +1 or -1 order results in more efficient diffraction compared to diffraction according to higher or lower order norms. This notably improves the filtering properties of the incident light beam F1, specifically allowing for precise selection of the wavelength 70 of the diffracted light beam F2 while minimizing optical losses. The +1 order is preferred because it offers a better compromise between flexibility and the amount of signal retained (higher signal amplitude compared to orders other than +1).

[0059] It is therefore understood that this spatial filter 33 is configured to section off a single wavelength of a single diffraction order from the diffracted light beam. This spatial filter 33 is arranged to filter out diffraction order 0 and orders other than order +1, so that only the diffracted light beam of order +1 propagates from the output of the scanning module 30. Thus, the scanning module 30 forms, from the incident light beam F1, the diffracted light beam F2 at the selected wavelength X0.

[0060] In this disclosure, the electrical control signal varies over time to perform a spectral scan of the incident light beam F1. It is understood that the electrical control signal varies within a range of values ​​that allows it to perform a spectral scan of the incident light beam F1.

[0061] As will be described below, this spectral scan is modulated according to the electrical control signal. Therefore, it is understood that this spectral scan is not performed on a fixed spectral band of the spectrum of the incident light beam F1, but on a spectral band that can be modulated according to the selected values ​​of the electrical control signal.

[0062] For this purpose, the imaging device 100 also includes a control module 40 configured to output the electrical control signal, for example, via an electrical signal generator included in or coupled to the control module 40. Hereafter, this electrical signal generator corresponds to a DAQ, for example, the NI 9263 Analog Output Module, 4 channels, and is driven by the control module 40, + / -10 V, 16 bits. Thus, this analog output generator outputs the electrical control signal, which is therefore an analog signal.

[0063] Typically, here, the control module 40 drives the variation (or modulation) of the electrical control signal, which allows control of the spectral sweep performed by the scanning module 30. In practice, the control module 40 is configured to adjust (or select or modify) at least one value of the electrical control signal and an application time of the electrical control signal, for example, associated with at least one value. For example, the application time can be between 10 ps and 1 s.

[0064] Here, adjusting a value of the electrical control signal means selecting an instantaneous physical quantity of the electrical control signal. The value designates the level of the physical quantity (voltage or current) at a given instant. This value can, in particular, correspond to the amplitude of the electrical control signal (expressed, for example, in volts or amperes). The application time of this value corresponds to selecting the duration for which this amplitude is applied. Indeed, here, the tunable acousto-optic filter 31 generates an acoustic wave, which is a function of the electrical control signal, in order to form a Bragg grating within the tunable acousto-optic filter to diffract the incident light beam.It is thus understood that being able to select, modify, or adjust the value of the electrical control signal allows us to select the wavelength X0 of the diffracted light beam F2, and its application time allows us to control the emission time of this wavelength X0. This amplitude constitutes the analog setpoint which, after conversion, determines the acoustic frequency within the filter and therefore the selected central wavelength Âo. Consequently, adjusting the signal value is equivalent to modifying the amplitude of the voltage or current to dynamically control the spectral sweep.

[0065] The modulation of the electrical control signal is achieved by modifying the values ​​of the electrical control signal (i.e., by varying the amplitude of the electrical control signal) and / or their application times.

[0066] Thus, it is possible to perform a non-linear scan of the spectrum of the light source 20 by successively selecting any wavelength of the spectrum of the incident light beam. Here, this is implemented by selecting values ​​of the electrical control signal and their emission times.

[0067] Selecting different values ​​of the electrical control signal also allows you to select the spectral width AX of the wavelength of the diffracted light beam F2. Indeed, here, the spectral width associated with each wavelength X0 is a function of the electrical control signal. Thus, it is understood that in this disclosure, the spectral width associated with each wavelength selected by the scanning module 30 is not fixed; it is variable and depends on the value of the applied electrical control signal.

[0068] Furthermore, the control module 40 can be configured to generate a second electrical control signal 302, 304, 308, 310, 312 (an analog electrical signal that is not a radio frequency wave, but rather a voltage), allowing the image detector 120 to be controlled synchronously with the wavelength scan, as will be explained later. This second electrical control signal is referred to hereafter as the activation signal 302, 304, 308, 310, 312 of the image detector 120.

[0069] In this disclosure, the control module 40 includes a computer 41. A computer 41 is defined as any computing unit, processor, computer, or other electronic component capable of implementing a series of commands and / or calculations. This computer 41 typically includes a processor, memory, and various input and output interfaces.

[0070] Thanks to its input and output interfaces, the computer 41 is programmed to receive data, for example, in this case, a sequence of values ​​of the electrical control signal and their application times, in order to control the scanning of the incident light beam F1 by the scanning module 30. It is also programmed to control, more generally, any Human-Machine Interface that allows control of the spectral scanning of the incident light beam. It is thus understood that the scanning module 30 is computer-controlled.

[0071] In one embodiment, the control module 40 can be configured to adjust a value of the electrical control signal with an accuracy of 1*10' 3 , for example 1*10' 3 volt (V) to select wavelengths from the spectrum of the light source spaced less than 0.02 nm apart. (Alternatively, the accuracy can be 1*10') 2V to select wavelengths spaced less than 0.2 nm apart). Thus it is possible to select wavelengths that are very close spectrally.

[0072] Furthermore, here, the control module 40 is arranged and / or configured to select the wavelength AO of the diffracted light beam F2 with an accuracy of 1*10' 2 nm (Figure 11).

[0073] It is also understood that in this disclosure, being able to select the selected wavelength so precisely allows for a non-fixed scan step, which here varies according to the different values ​​of the control signal selected by the control module 40. Thus, the control module 40 can be configured to control the scanning of the spectrum of the incident light beam F1 with a spectral scan step that is a function of the value of the selected control signal.

[0074] It will now be described, using Figure 2, Figure 3, Figure 4, Figure 5 and Figure 6, Figure 7, Figure 8, examples of time-controlled signal modulation controlled by the control module 40 and their implications on the diffracted light beam F2.

[0075] To this end, Figure 2 illustrates an example of the variation of the control electrical signal 202 over time, controlled by the control module 40. In this figure, it can be seen that the control electrical signal 202 corresponds to a voltage that varies non-linearly over time. By non-linear, we mean that the control electrical signal does not exhibit a variation with a direct and proportional relationship over time. In this disclosure, the non-linearity applies both to the values ​​of the control electrical signal and / or to the duration of application of each value of the control electrical signal.

[0076] Typically, here we can see that the values ​​of the control signal 202 change discontinuously, with, for example, abrupt voltage jumps. Typically, there can be a difference of several volts between voltages V1 and V2. Furthermore, we can see that the variation of the electrical signal can increase (for example, between voltages V1 and V2) and then decrease between voltages V2 and V3. Thus, in this example, we go from a control signal value V2 to a control signal value V3 that is lower than V2. We can therefore understand that the control module 40 can be configured to scan the spectrum of the incident light beam discontinuously. The step size (Pi, where i is an index associated with the voltage index) of the change from one value of the control signal 202 to another is not constant. It is modulated by the control module 40. Therefore, P1, P2, and P3 are different here.

[0077] Figure 2 also shows that the values ​​of the control electrical signal 202 are applied irregularly, in that their application times (ti, where i is an index associated with the voltage index) can differ from one value of the control electrical signal 202 to another. Typically, we can see here that the application time t1 of the value V1 of the control electrical signal 202 is greater than the application times t2 and t3 of the value V2 and that of the value V3, while the application time t3 of the value V3 of the control signal is less than the application time t2 of the value V2.

[0078] Figure 3 illustrates the spectral variation over time of the diffracted light beam associated with the control signal 202 shown in Figure 2. Here, we can see that the spectrum of the diffracted light beam varies non-linearly, in the sense that the wavelengths selected by the control module 40 change discontinuously. In this disclosure, the spectral variation of the diffracted light beam is a function of the variation of the electrical control signal 202.

[0079] Figure 4 illustrates another example of the variation of the control signal 204 over time, implemented by the control module 40. Here, unlike in Figure 2, the control signal 204 varies linearly over time. Thus, in this example, the control module 40 modifies the value of the control signal 204 with a constant (voltage) step size, for example, a step size of 0.1 V. Furthermore, each value of the control signal can be applied with a constant application time, for example, between 10 ps and 1 s (here, 10 ps).

[0080] Figure 5 illustrates the spectral variation of the diffracted light beam controlled by the electrical control signal 204 shown in Figure 4. Here, we can see that the wave vector (inverse to the wavelength) of the diffracted light beam F2 varies proportionally to the variation of the electrical control signal 202. Indeed, in Figure 5, we can see that the spectrum of the diffracted light beam also evolves linearly.

[0081] Figure 6 illustrates another example of the variation of the control signal 206 over time by the control module 40. Here, we can see that the value of the control signal 206 varies linearly, meaning that the values ​​of the control signal 206 change with a similar step size (step size in constant voltages, P1 = P2 = P3 = P4 = etc.). However, in this case, the control module 40 modifies the application time of each value of the control signal 206. Typically, we can see that the voltages V1 and V2 are applied with the same application time (similar times t1 and t2), while the application time t3 of value V3 is greater than the application times t1 and t2 of values ​​V1 and V2.

[0082] Figure 8 illustrates another example of variation in the electrical control signal. Here, the control module 40 modulates the electrical control signal 208 over several time periods Tps (where s is an index referring to the time period number), here 4. Each time period (Tp1, Tp2, Tp3, Tp4) is associated with a temporally continuous variation of the electrical control signal 208i, 2082, 2083, 2084, i.e., without temporal interruption. Typically, there is at least one period, or time interval, here at least 1 ps, separating each time period, this interval being driven by the control module 40. Here, over each time period, the electrical control signal varies nonlinearly because the values ​​of the electrical control signal are applied with different application times. Figure 9 illustrates spectra obtained during the scanning of the spectrum of the incident light beam over four time periods.Here, each spectrum is associated with a variation of the control electrical signal 208i 2082, 2083, 2084, illustrated in Figure 8, during a given time period Tp1, Tp2, Tp3, Tp4. We can see that four different spectra are obtained, each extending over at least 50 nm. Indeed, in Figure 8, the control electrical signal 208 has been modulated in four different ways (here with different voltage values). We can also see that the activation signal is adapted to the modulation of the control electrical signal 208 at each time period Tp. Typically, for each time period Tp, the activation time Tap (where p is an index referring to the time period number) of the imaging detector 120 is less than the duration of the given time period Tp.

[0083] In this disclosure, to drive the electrical control signal, the control module 40 uses a table that relates the electrical control signal to a wavelength or uses a mathematical function that relates the electrical control signal to the wavelength selected by the control module. Thus, using such elements allows for very precise control of the spectral sweep with a large degree of freedom, since it is possible to select each value of the electrical control signal and its application time.

[0084] Figure 10 illustrates an example of a table that relates the electrical control signal to a wavelength, and the figure of the values ​​extracted from this table is shown in Figure 10. It can be seen that the electrical control signal is proportional to the wavelength selected by the scanning module. Furthermore, it can be seen that the wavelength selection by the scanning module is performed with an accuracy of 1*10⁻¹⁰⁰. 2nm. It is thus understood that in the present disclosure, the spectral sweep of the incident light beam F1 is fully adjustable in the sense that the control module can select any wavelength of the spectrum of the incident light beam F1 by adjusting the values ​​of the electrical control signal. In Figure 11, it can be seen that each wavelength selected X0 by the control module 40 is associated with a given spectral width AX, this spectral width therefore being different for each wavelength X0.

[0085] At the output of the scanning module 30, the diffracted light beam F2 then propagates into the imaging device 100.

[0086] The imaging device 100 also includes a multimode fiber 60 positioned after the scanning module 30, and optionally a lens 50 positioned between the scanning module 30 and the optical fiber 60 to focus the diffracted light beam F2 into the multimode optical fiber 60. This increases the beam coupling in the optical fiber 60. Such a lens 50 allows for better control of the diffracted light beam F2 and thus increases the power of the light beam exiting the multimode optical fiber 60 through the imaging device 100.

[0087] This 60 multimode optical fiber is designed to receive the diffracted light beam F2 and is necessary to adjust the spatial coherence of the diffracted light beam F2. To this end, the 60 optical fiber advantageously offers numerous spatial modes, and the output beam has a spatial coherence length shorter than the spatial coherence length of the incident light beam F1. The 60 multimode optical fiber thus enables the filtering of photons scattered by the sample by breaking the spatial coherence of the diffracted light beam F2.

[0088] Typically, here, the 60 multimode fiber has at least one hundred transverse modes to break the spatial coherence of the diffracted F2 light beam. Preferably, the multimode fiber has between 100 and 1,000,000 transverse modes, to minimize the spatial coherence of the diffracted F2 light beam through modal dispersion.

[0089] The multimode optical fiber 60 advantageously has a length greater than the temporal coherence length of the diffracted light beam F2 in order to minimize the speckle or mottling phenomenon of the light beam exiting the optical fiber 60 due to the modal dispersion introduced by the multimode optical fiber 60. In practice, here, the multimode fiber 60 has a length between 2 and 3 meters or between 100 and 200 meters (m) depending on the applications of the imaging device 100, as will be described below, particularly depending on the number of wavelengths selected during scanning. For example, if the set of wavelengths selected during scanning (over a given time period) forms a spectrum greater than or equal to 80 nm, preferably greater than or equal to 100 nm, the optical fiber can have a length between 2 m and 3 m.If the set of wavelengths selected during the scan (during a given time period) forms a spectrum less than or equal to 80 nm, the optical fiber 60 can have a length greater than or equal to 10 m. For example, for a spectrum less than or equal to 5 nm, the fiber has a length between 100 m and 200 m. It is therefore understood that for the study in the Fourier domain, the optical fiber 60 has a length between 100 m and 200 m, while for the time domain, this will depend on the number of wavelengths selected (i.e., the number of voltage values ​​applied to the scan module 30).

[0090] In this disclosure, the optical fiber length 60 is preferably greater than the temporal coherence length of the diffracted light beam F2.

[0091] In the imaging device 100, light propagates through the optical fiber 60. At the output of the multimode optical fiber 60, the light beam has a spatial coherence length shorter than the spatial coherence length of the dispersed light beam F2 entering the multimode optical fiber 60. For example, the spatial coherence length of the light beam at the output of the optical fiber 60 is at least thirty times shorter than the spatial coherence length of the dispersed light beam F2 entering the multimode optical fiber 60. For example, the coherence length of the light beam at the output of the optical fiber 60 is between 300 pm and 0.1 pm, for example, here on the order of 10 pm.

[0092] Following the multimode optical fiber 60, the imaging device 100 includes a tomography module 90. In this example, the tomography module 90 is an interferometric module. For this purpose, it includes an interferometer, for example, a Linnik interferometer. However, this tomography module 90 can, of course, include other types of interferometers, for example, a Michelson interferometer, or a Mach-Zehnder interferometer, etc.

[0093] The tomography module 90 introduced above is a full-field optical coherence tomography (OCT) module. This configuration allows for better transverse resolution and increased imaging rates compared to a scanning OCT configuration. Of course, in a variant (not shown), the tomography module 90 described in this disclosure can be a non-full-field OCT module, requiring a spatial scan along the x, y directions of sample 1, as described above or as illustrated in Figure 12a of the document "Optical Coherence Tomography - Application to Ophthalmology and Biofilm Imaging".

[0094] This tomography module 90 includes an optical device 80 arranged to collimate the light beam exiting the multimode optical fiber 60 and direct it towards the interferometer within the tomography module 90, specifically here towards the beam splitter described below. In the following, the light beam exiting the optical device 80 is referred to as the collimated light beam F3.

[0095] In this example, the optical device 80 includes an optical element, for example a collimating lens 81 or any other combination of optical elements, to collimate the light beam exiting the multimode optical fiber 60. Such a device optimizes the amount of light used in the imaging device 100 by recovering a maximum of light from the output of the multimode optical fiber 60. It thus allows the size of the light beam to be adjusted in the image plane of the imaging device and in its conjugate plane.

[0096] Optionally, the optical device 80 also includes a positioning means 82 for the light beam exiting the multimode optical fiber 60. Typically, this positioning means 82 is positioned after the collimating lens 81 and is configured to direct the collimated light beam F3 towards the tomography module 80. The positioning means 82 facilitates the implementation of the imaging device 100, for example, by limiting its compactness in a given direction. Alternatively, if the optical device 80 does not include a positioning means 82, it is understood that the output of the optical fiber 60 (here, its end oriented towards the tomography module 90) is chosen so as to limit the compactness of the imaging device 100.

[0097] This positioning means 82 is in this variant optically combined with optical elements of the tomography module 90 allowing for magnification.

[0098] As illustrated in Figure 1, this tomography module 90 also includes a reference arm 91 with a reference mirror 92. In this example, this reference arm 91 also includes a first optical element 93 arranged to achieve magnification, for example, an optical lens or objective lens with a magnification greater than 2, for example, 4, and a numerical aperture greater than 0.10, for example, 0.16. The first optical element 93 is arranged so that the reference mirror 92 is positioned in the focal plane of the first optical element 93, and to produce an image of the reference mirror 92 on the imaging detector 120. Alternatively, this optical element 93 can be positioned upstream of the beam splitter 101 described below to obtain the same result.

[0099] In this disclosure, the reference arm 91 may include means for controlling the position 94 of the reference mirror 92. These control means 94 are configured to change the position of the reference mirror 92 along at least one axial direction, here oriented along the z-direction aligned with the direction of propagation of the light beam arriving at the reference mirror 92. Of course, these position control means 94 may also change the position of the reference mirror 92 along three axial directions, for example, the x, y, and z directions. These position control means 94 may be manually or automatically controlled by means of a motor or a piezoelectric element controlled by the control device 40.To this end, these control means 94 may include a modulation device 95, for example a piezoelectric element, arranged to modulate the phase of the light beam arriving at the reference mirror 92. Furthermore, the reference arm 91 may include additional control means that allow the total length of the reference arm 91 to be modified by simultaneously moving the first optical element 93 and the reference mirror 92. These control means allow the position of the coherence volume in the sample to be adjusted, and the position corresponding to the zero path difference to be found.

[0100] The tomography module 90 illustrated in Figure 1 also includes an object arm 96 comprising the sample holder 97 which carries the sample 1 imaged by the imaging device 100. In various embodiments, the sample holder 97 can have different configurations adapted to the nature of the object to be imaged. In particular, the sample holder 97 can be configured to serve as a support or stabilizer, for example in the form of a chin rest or a forehead support, when the sample 1 is an eye (especially for in vivo imaging). Alternatively, the holder 97 can consist of a stage, a specimen holder, or an imaging cell arranged to receive a biological sample, such as biological tissue, a histological section, or a cell culture.

[0101] This object arm 96 optionally includes a second optical element 98 arranged to provide magnification, for example, an optical lens or objective lens with a magnification greater than 2, for example, 4, and a numerical aperture greater than 0.10, for example, 0.16. Typically, this second optical element 98 is similar to the first optical element 93. It is therefore understood that if the optical element 93 is positioned upstream of the beam splitter 101, then the tomography module 90 does not include an element 98. In this disclosure, the object arm 96 may include means for controlling the position 99 of the sample holder 97. These control means 99 are arranged to modify the position of the holder 97 along at least one axial direction, here oriented along the z-direction aligned with the direction of propagation of the light beam arriving at the sample 1.Of course, these position control means 99 can also modify the position of the support 97 of sample 1 in three spatial directions, for example in the x, y and z directions. These position control means 99 can be operated manually or automatically via a motor controlled by the control device 40.

[0102] The tomography module 90 shown in Figure 1 also includes a beam splitter 101 arranged to split the light beam from the optical fiber, here corresponding to the collimated light beam F3, into a reference light beam F4 in the reference arm 91 and an object light beam F5 in the object arm 96, and / or to recombine the reference light beam F4 and the object light beam F5 into an interference light beam F6. More specifically, the tomography module can include one or more beam splitters. Typically, in configurations implementing a Michelson, Linnik, Mirau, or Fizeau interferometer, a single beam splitter 101 can be used to split and recombine the light, whereas in the Mach-Zehnder configuration, two beam splitters can be used to split and recombine the light, respectively.

[0103] Typically, this 101 beam splitter features a 50 / 50 non-polarized beam splitter cube to reflect and transmit light equally. Of course, this ratio can be different; for example, it can be 40 / 60.

[0104] Optionally, as shown in Figure 1, the tomography module 90 also includes an additional optical element 102 positioned upstream of the beam splitter 101, i.e., before the collimated light beam F3 is split into the reference light beam F4 and the object light beam F5. This additional optical element 102 includes, for example, a lens and is arranged to focus the reference light beam F4 into the rear focal plane of the optical element 93 upstream of the reference mirror and the object light beam F5 into the rear focal plane of the optical element 98 upstream of the sample 1. In practice, this lens is therefore arranged so that its focal plane is positioned in the rear focal plane of the optical elements 93 and 98. This allows the light beam to be homogenized in the planes of the sample and the reference arm, notably by masking the heterogeneities of the beam exiting the multimode optical fiber 60.Alternatively, if the device does not include this additional lens 102, the optical fiber 60 is directly arranged so as to be conjugated with the sample 1.

[0105] The imaging device 100 also includes an imaging detector 120, which is optically conjugate to a focal plane located in sample 1. This imaging detector 120 is configured to record the interference beam and to form an image of sample 1 on the imaging detector 120 from the interference beam F6. It is therefore understood that the interference beam F6 captured by the imaging detector 120 has the wavelength selected by the scanning module 30. This imaging detector 120 can be a two-dimensional (2D) or one-dimensional (1D) detector. Typically, this imaging detector 120 includes a photodetector, such as a 2D or 1D photodiode.

[0106] In one embodiment, this image detector 120 is a two-dimensional image detector comprising a matrix of 1400 pixels x 1400 square pixels of 12 pm on a side (i.e. 2D photodiode), for example the Adimec Q-2HFW camera.

[0107] Alternatively, this imaging detector 120 may consist solely of a one-dimensional imaging detector 120, for example, a line of 1400 square pixels, 12 pm on a side, for instance, when the tomography module 90 is not full-field. In this case, at least one optical element is ideally added to render the light beams F3, F4, and F5 one-dimensional. Furthermore, a means for translating or rotating the light beam F5 is added to scan the position of the sample in the direction perpendicular to the line forming the beam F5. For example, an angular galvanometer scanner is inserted between the beam splitter 101 and the second element 98. This scanner is then arranged conjugate to the rear focal plane of the second optical element 98.This imaging detector 120 can also include a point imaging detector 120, for example a photodiode, when the tomography module 90 is not full-field and non-linear. In this case, the light beams F3, F4, and F5 are collimated so as to illuminate only one point of the sample at a time. A two-dimensional translation mechanism is then added to scan the sample's position in both directions. In this case, the tomography module can be similar to the one illustrated in Figure 13b of the document "Optical Coherence Tomography - Applications to Ophthalmology and Biofilm Imaging," by Olivier Thouvenin, but without the use of the diffraction grating.

[0108] Alternatively (Figure 12), as described in Figure 13b of the document mentioned in the paragraph above, the imaging device 100 can be a spectrometer. For this purpose, the imaging device 100 can include a diffractive element positioned upstream of the imaging detector 120 described above. Typically, in this case, this diffractive element can include a diffraction grating arranged to spectrally disperse the interference light beam F6. Here, the spectrum can be scanned over a wide spectral range in a time shorter than the sensor's exposure time Ta (sensor high state). This grating improves spectral resolution. It also increases the image acquisition time. For example, if the imaging detector operates at 1 kHz (1 ms exposure time Ta), 100 different wavelengths can be selected with the scanning module.The use of the dispersive element allows these wavelengths to be acquired, particularly in spectral-domain OCT (non-full-field OCT). In this embodiment, the imaging detector 120 is, for example, a one-dimensional (1D) detector, for example, a line of 1400 square pixels with sides of 12 pm oriented along the x or y axis arranged to capture the light beam dispersed by the spectrometer 151. Alternatively, the imaging detector 120 is, for example, a two-dimensional (2D) detector, for example, a pixel matrix comprising 1400x1400 square pixels with sides of 12 pm oriented to capture in one dimension the light beam dispersed by the spectrometer 151, and in the other a spatial information of the sample (spectral line OCT).

[0109] In this disclosure, the image detector 120 is configured to capture the interference light beam F6 at specific times. It is understood that the activation of the image detector 120 (i.e., recording) is controllable. This can be done manually, by manually activating the image detector 120, or controlled by a control device via the activation signal. This control device can be similar to the one described above. For this purpose, the image detector 120 can be connected to the control module 40. This activation signal is modulable in that it can be controlled by the control module 40. It is understood that the control module is also configured to adjust the activation of the image detector 120, specifically, in this case, the activation time, or activation duration Ta, of the image detector 120.In these examples, this activation time Ta is illustrated by a high state of the activation signal.

[0110] To precisely control the activation of the image detector 120, the imaging device 100 may include a synchronizing device 130 configured to synchronize the activation of the image detector 120 with the scanning of the spectrum of the incident light beam. This synchronizing device 130 may be directly integrated into the control module 40 or be a separate component of the control module 40, as illustrated in Figure 1. This synchronizing device 130 receives the activation signal and, depending on this signal, activates or deactivates the image detector 120. Here, "deactivated" means that no image or signal is captured by the image detector 120. For example, in this example, the activation signal is binary and has a high state associated with the activation of the image detector and a low state associated with the non-activation of the image detector 120 (Figure 2, Figure 4, Figure 6, Figure 7, Figure 8).Each high state exhibits an activation time Ta of the imager detector 120. This activation time is controlled by the control module. Therefore, the control module can be configured to adjust the activation time of the imager detector, for example, at each time period during which a spectral scan is performed or at given values ​​of the control signal. It is thus understood that the activation time Ta of each high state of the activation signal is controlled by the control module 40.

[0111] The tomography module 90 includes, in this imaging configuration (Linnik interferometer used in full-field optical coherence tomography) a third optical element 140, positioned in front of the imaging detector 120 and configured to focus the interference light beam F6 on the imaging detector 120, and to form the imager of the sample 1.

[0112] The imaging device 100 may also include a processing unit 41 configured to process a set of images of the sample 1. A processing unit 41 is understood to mean a computer, processor, or any computing unit configured to implement a sequence of commands. Typically, this processing unit 41 is included in the control module 40. Alternatively, the processing unit 41 may be a computing module separate from the control module 40.

[0113] As will be described below, this processing unit 41 can use different algorithms, for example a spectral processing algorithm in the Fourier domain or in the time domain to provide the depth image of sample 1.

[0114] Time Domain Optical Coherence Tomography

[0115] This section will describe an implementation of the 100 imaging device used in the time domain.

[0116] In this embodiment, the control module 40 modifies the value of the electrical control signal for at least one continuous (i.e., uninterrupted) time period in order to scan a portion of the spectrum of the incident light beam. Indeed, as described above, each value of the electrical control signal is associated with a wavelength.

[0117] Typically, in this embodiment, the synchronization device 130 is configured to synchronize the activation of the imager detector 120 with each time period. Typically, here, the activation of the imager detector 120 (high state of duration Ta) is synchronized with a variation (or modulation), without temporal interruption, of the electrical control signal so as to acquire an image of the sample at each time period (Figure 2, Figure 4, Figure 6, Figure 8).

[0118] In practice here, the image detector 120 is configured to integrate the interference light beam F6 at each time period, for a duration defined by the activation of the image detector 120. By "integrate," we mean that the image detector 120 records, collects, or captures the interference light beam F6. It is therefore understood here that the image detector 120 is continuously activated for at least the entire duration of the time period.

[0119] For example, Figures 2 and 4 show a variation curve of the activation signal 302, 304 of the imager detector 120 in the form of a rectangular signal having a high state when the imager detector 120 is activated and a low state (0V) when the imager detector 120 is not activated. In these figures, the imager detector 120 is activated throughout the variation of the electrical control signal in order to integrate all the wavelengths of the diffracted light beam F2. Thus, here, the electrical control signal varies over a time period Tp.

[0120] In these embodiments, the imager detector 120 captures a light signal that comprises all the interference light beams F6 that have been formed (individually via the values ​​of the electrical control signal). Thus, the longer the imager detector 120 integrates the interference light beam F6 over a longer time period, the greater the intensity of the captured signal.

[0121] It is understood that in this embodiment, the different wavelengths selected by the scanning module 30 are recombined during integration, which makes it possible to capture an integrated light signal having a wide spectral band since it corresponds to all the wavelengths selected by the scanning module 30 (see for example in figure 9 the spectrum of the integrated light signal on each time period).

[0122] In cases where the electrical control signal varies non-linearly over time (Figure 2 and Figure 6), the longer the application time of a control signal value, the greater the relative intensity of the signal captured by the imaging detector during the time period (integration period). Thus, it is possible to obtain a signal with a spectrum that is fully modulated according to the variation of the electrical control signal.

[0123] In this embodiment, the imaging detector 120 is arranged to provide one image of the sample per time period. Thus, in Figures 2, 4, and 6, only one image of the sample is formed by the imaging detector 120, whereas in Figure 8, four images are formed.

[0124] This image corresponds to a cross-sectional image of sample 1 at a given depth, with an axial resolution that depends on the width of the spectral band scanned by the scanning module 30. Thus, here, the image of sample 1 formed from the interference beam F6 corresponds to an image of the sample (a plane (x,y)) at a given depth (along the axial direction z). Typically, this plane is parallel to the surface of sample 1, and the depth of sample 1 is a function of the position of the reference mirror and / or the position of sample 1.

[0125] In practice, this image of sample 1 at a given depth can be obtained as described in the document "Methods and applications of full-field optical coherence tomography: a review", Ling Wang et al. Generally, in time-domain optical coherence tomography, the amplitude of the interference term is extracted (here by the processing unit 41), either digitally (for example via Hilbert transform), or by physical modulation of the path difference, for example in four-phase, five-phase, etc. modulation.

[0126] In this disclosure, multiple images of sample 1 can be obtained (for example, n images, where n is an integer greater than 1). These n images may correspond to the image of the same transverse layer of the sample. In this case, it is understood that the n images are associated with the same depth along the axial z-direction.

[0127] In this case, the n images are obtained by modulating, via the control device 40, the electrical control signal for n time periods in order to acquire n images of the sample 1, and for each time period, by activating the image detector 120 (via the activation signal) throughout the modulation of the electrical control signal.

[0128] Figure 8 illustrates another example of modulation of the electrical control signal and activation of the imaging detector 120. In Figure 8, the electrical control signal is modulated over several time periods, here four equivalent time periods Tp1, Tp2, Tp3, Tp4. It is understood that each time period can be different in terms of duration. We can see here that the activation signal of the imaging detector 312 is synchronized with each of the time periods. The interference signal F6 is integrated over each of the time periods. Typically, we can see here that four images (n = 4) of the sample are acquired by the imaging detector 120 for four time periods during which the electrical control signal varies.Therefore, it is understood that the control device 40 is configured to control the electrical control signal for n time periods in order to acquire n images of the sample 1 and activate the image detector 120 at each time period.

[0129] These images can be acquired for the same depth or for a different depth.

[0130] Thus, here, several images of one or more slices of the sample can be obtained (for example, n images, where n is an integer greater than 1) by configuring the control device 40 to scan different spectral bands for each time period. This yields a hyperspectral image, or a hyperspectral volume, allowing the study of how the reflectivity of the sample structures changes with wavelength. This enables a better characterization of the imaged structures, for example, determining their sizes and geometries. Typically, this is particularly advantageous when a spectrometer 151 is included in the imaging setup.

[0131] Optionally, here, as the tomography module 90 illustrated in figure 1 is a full-field optical coherence tomography module 90, a phase modulation can be performed on the light beam incident at the reference mirror 92 so as to introduce a phase shift between the reference arm 91 and the object arm 96. This modulation makes it easier to extract the interference term included in the interference light beam F6.

[0132] The imaging device 100 is also configured to provide a three-dimensional image of sample 1, that is, a depth image of sample 1 imaging an internal structure of sample 1, from several images of the sample. The processing unit 41 then processes all the sample images to provide the image of the internal structure of sample 1.

[0133] The information on the depth of the sample is obtained by acquiring a succession of images (n images, with n an integer greater than 1) of sample 1 at r different depths (with r an integer greater than 1, which can be equal to n or be different from n), which allows us to obtain r cross-sections of sample 1. In this case, we understand that we move the reference mirror and / or the sample support r times via the position control means 94, 99 described above to obtain at least one image of the sample at each position of the reference mirror and / or the sample support.

[0134] Thus, for each r position of the sample holder and / or the reference mirror, the electrical control signal can be modulated for at least one time period to acquire at least one image of sample 1 at a given depth r, by activating the imaging detector 120 at each time period. In this case, it will be possible to obtain either one image of the sample at each position r or one or more images at each position of the reference mirror and / or the sample holder.

[0135] All of these images at different depths can then be processed by the processing unit, for example using the method described in the document "Ultrahigh-resolution full-field optical coherence tomography", A. Dubois et al, to reconstruct a three-dimensional view of the internal structure of the sample.

[0136] Spectral domain optical coherence tomography

[0137] This section will describe an implementation of the 100 imaging device used in the Fourier domain.

[0138] In this embodiment, the synchronization device 130 described above is configured to synchronize the activation of the image detector 120 with specific values ​​of the electrical control signal, each activation being associated with a unique value of the electrical control signal. Thus, it is understood that the image detector 120 is configured to acquire an image of the sample 1 at each wavelength Δo (or central wavelength) selected by the scanning module 30.

[0139] Typically, in Figure 7, we can see that the activation signal 310 of the imager detector 120 is binary and consists of several rectangular pulses. A high state, associated with the activation of the imager detector 120, indicates that the imager detector 120 detects or records the interference light beam F6. A low state (here, 0V) indicates that the imager detector 120 does not detect the interference light beam F6. Thus, the imager detector 120 is synchronized so that each high state (here, 5V) is associated with a single wavelength X0, with a minimum spectral width AA (on the order of 3 to 8 nm here). Therefore, specific values ​​of the electrical control signal can be selected and / or entered via the human-machine interface. Typically, in Figure 7, values ​​have been selected by the control module 40.Alternatively, the selected values ​​can be those directly programmed or entered on the human-machine interface. Thus, for each selected control signal value, the activation time of the imager detector 120 (the high state duration Ta) is less than the application time t of the given activation signal value, in order to avoid wavelength overlap.

[0140] As can be seen in this figure, during each application time Ta of the electrical control signal (modulated by the control device 40), the value (in this case, the voltage) of the electrical control signal can vary (slightly), for example, linearly. Thus, for the same activation or exposure time Ta, it is possible to select several closely spaced wavelengths by varying the value of the electrical control signal during this activation Ta, as defined by the control module.

[0141] Thus, the control module 40 can be configured to select (for the same image) a plurality of wavelengths, preferably spectrally close, i.e., separated by less than 0.2 nm and, optionally, with a separation of at least 0.01 nm, during the same activation time of the imaging detector. This results in a broadening of the spectral width AX over this given activation time Ta. It is therefore understood that, in this context, the central voltage value (corresponding to the median or average value of the voltage variation over the exposure time Ta) of the electrical control signal defines the central wavelength Δo of the diffracted light beam over this given activation time Ta.

[0142] It is worth noting that the greater the number of wavelengths selected, the greater the spectral width AX, which increases signal accuracy and improves axial sampling in Fourier domain OCT. However, this spectral broadening AX can lead to a reduction in maximum imaging depth due to decreased frequency (or wavelength) resolution.

[0143] In this embodiment, controlling the activation of the image detector 120 advantageously allows for modifying the relative intensity of the signal captured by the image detector 120 for each wavelength. The longer the activation signal value is applied, the greater the relative intensity of the signal captured by the image detector at the wavelength defined by the control module 40. Thus, it is possible to obtain a signal with a spectrum that is fully modulated according to the variation of the electrical control signal.

[0144] Typically, here the scanning module 30 can scan all successive lengths of a spectral band of the incident light beam spectrum, but acquires a specific number of images by activating the imaging detector 120 at specific values ​​of the electrical control signal (as in Figure 7). Therefore, here, each image of the sample is associated with a wavelength ΔO. Each image of the sample 1 is thus a monochrome image.

[0145] All of these images l(x, y, k, with k = 2K / Δ,0) are stored in the memory of the processing unit 41 and concatenated to form an image stack in which each image l(x, y, k) is associated with a wavelength ΔO. This yields a multispectral image of the sample. It is possible to retrieve the depth information using Fourier processing and obtain a depth image of the sample, as described in the document "Optical Coherence Tomography - Applications to Ophthalmology and Biofilm Imaging" by Olivier Thouvenin. In this embodiment, the depth image of the sample can be obtained without moving the reference mirror and / or the position of the sample 1.

[0146] Method

[0147] An example of the implementation of an imaging method 1000 according to this disclosure will now be described with the aid of Figure 12. The method 1000 illustrated in Figure 13 is implemented by the imaging device 100 according to this disclosure, such as that described with the aid of Figure 1.

[0148] Method 1000 involves the emission E1 of an incident light beam F1 by the light source 20. As mentioned above, the incident light beam F1 emitted by the light source 20 is a temporally incoherent light beam with a broad spectral band, in this case exceeding 300 nm. The incident light beam F1 then propagates in a free field towards the scanning module 30.

[0149] Method 1000 also includes a spectral filtering step E2 by dispersion / refraction to select a wavelength of the incident light beam F1 and form a diffracted light beam F2 at the selected wavelength. This step allows for a spectral scan of the incident light beam F1 by diffracting and dispersing the incident light beam F1 multiple times in order to select several wavelengths ΔO.

[0150] This spectral filtering step E2 is carried out by the scanning module 30, in particular via the acousto-optical tunable filter 31, but is driven by the control module 40 via the electrical control signal as described above.

[0151] To this end, method 1000 also includes an application step E3 of the electrical control signal to drive the spectral filtering implemented by the scanning module 30. As explained above, the value of the control signal controls the diffraction of the incident light beam F1 and selects the wavelength of the diffracted light beam F2. Therefore, this step E3 includes a substep E31 for generating the electrical control signal, for example, using a voltage generator included in or connected to the control module 40, and then a substep E32 for adjusting the value of the control signal and / or the application time of this value before it is applied to the scanning module 30. It is understood that the adjustment step E32 can be repeated to obtain a time-varying electrical control signal.

[0152] The adjustment of the value of the electrical control signal and its application time is controlled by the control module 40. For this purpose, it is understood that it is sufficient to enter the values ​​of the electrical control signal and their associated application times via the human machine interface controlling the control module 40.

[0153] This human-machine interface or directly the control module 40 can use either a table that links values ​​of the electrical control signal to the selected wavelength or a mathematical function that links the selected wavelength to the electrical control signal.

[0154] Of course, in one variant, the adjustment (or selection) of the value of the electrical control signal and the time of application of this value can be set manually, for example via knobs connected to the control module 40.

[0155] The electrical control signal is applied to the scanning module 30 throughout the spectral scan of the incident light beam.

[0156] Method 1000 also includes a step E4 for adjusting the spatial coherence of the diffracted light beam F2 via the multimode optical fiber 60 described below. Typically, the diffracted light beam F2 propagates through the optical fiber 60 and its spatial coherence decreases during this propagation.

[0157] Method 1000 includes a step E5 for forming an interference beam F6 by propagating the light beam from the optical fiber output into the tomography module 90 described above. In this step E5, the light beam from the optical fiber 60 propagates into the tomography module 90 and, after its propagation (unfibered) into the interferometric module 90, forms the interference beam F6.

[0158] This F6 interference light beam then propagates towards the imaging detector 120.

[0159] Method 1000 includes a capture step E6 of the interference light beam by the imaging detector 120 and a formation step E7 of an image of the sample 1 from the interference light beam 1 by the imaging detector 120.

[0160] As described above, the E6 capture and E7 formation steps can be carried out in different ways.

[0161] For example, in the time-domain study, the imaging detector 120 is activated only once during a time period in which the spectral scan is performed by the scanning module 30. Here, this activation occurs throughout the entire time period. During this time period, the imaging detector 120 integrates the interference light beam. In other words, the capture step E6 is implemented throughout the entire spectral scan of the incident light beam. Therefore, here, the imaging detector 120 records or captures all the wavelengths selected by the scanning module 30 (i.e., all the interference light beams with the different selected wavelengths) and forms an image of the sample for each time period.

[0162] Thus, in the temporal case, the depth image of the sample can be obtained by repeatedly performing the steps of Method 1000 described above, that is, by repeating the emission step E1, the filtering step E2, the application step E3, the spatial coherence adjustment step E4, the interference beam formation step E5, the capture step E6, and the sample image formation step E7. In practice, Method 1000 is repeated, for example, n times to obtain n images of the sample and to form a depth image of the sample from these n images using the processing unit 41 described above, where n is an integer greater than 1, so as to form the depth image from at least two sample images.

[0163] Specifically, here, each image of sample 1 represents a plane of the sample positioned at a different depth z. Therefore, it is necessary to modify the position of the sample and / or the reference mirror from one image of the sample to another.

[0164] Alternatively, in the case of spectral domain analysis, the capture step is repeated k times during the time period. Thus, here, the imaging detector 120 is activated k times during the time period to form k images of the sample during the time period, where k is an integer greater than 1. The activation time of the imaging detector 120 during each of the k images can be modified by applying the control electrical signal driven by the unit 40 so as to modulate the relative intensity of each image and modulate the total spectrum (corresponding to the sum of the k images) arbitrarily. Typically, the synchronization module 130 synchronizes the activation of the imaging detector 120 to given (selected) values ​​of the control electrical signal (e.g., k given values) so as to obtain an image of the sample for each given value or selected values ​​of the control electrical signal.Since each value of the electrical control signal is associated with a wavelength or center wavelength (if a range of values ​​is used for an image), it is understood that several images of sample 1 are obtained here (k images), each image being associated with a wavelength selected or center wavelength by the scanning module 30. In this variant, the depth image of the sample is obtained by processing the k images according to algorithms in the Fourier domain. It is not necessary to modify the position of the reference mirror and the sample as described in the time domain.

[0165] The present invention is in no way limited to the embodiments described and represented, but a person skilled in the art will be able to make any variation in accordance with the invention.

Claims

34 DEMANDS 1. Optical coherence tomography imaging device (100) for imaging a sample (1) comprising: - a light source (20) to emit an incident light beam exhibiting a spectrum, - a scanning module (30) comprising an acousto-optical tunable filter (31) configured to spectrally diffract the incident light beam and select a wavelength of the incident light beam, the scanning module (30) providing a diffracted light beam at the selected wavelength, the acousto-optical tunable filter (31) being driven by an electrical control signal (202, 204, 208) that can be modulated to control a scan of at least a portion of the spectrum of the incident light beam, the selected wavelength being a function of the electrical control signal (202, 204, 208), said electrical control signal not being a radio frequency signal; - a multimode optical fiber (60) arranged to adjust the spatial coherence of the diffracted light beam; - an optical coherence tomography module (90) comprising: (i) a reference arm (91) equipped with a reference mirror (92) and an object arm (96) comprising a support (97) for the sample (1); ii) a beam splitter (101) arranged to split the light beam from the multimode optical fiber (60) into a reference light beam in the reference arm (91) into an object light beam in the object arm (96) and / or to recombine the reference light beam and the object light beam into an interference light beam; iii) an imaging detector (120) configured to record the interference light beam and to form, from the interference light beam, an image of the sample (1) on the imaging detector (120), the imaging device (100) comprising a control module (40) emitting the electrical control signal and configured to modify the following elements: - a value of the electrical control signal (202, 204, 208); - an application time of a value of the electrical control signal (202, 204, 208).

2. Imaging device (100) according to claim 1, wherein the control module (40) is configured to select the wavelength of the diffracted light beam with at least an accuracy enabling the selection of wavelengths spaced less than 0.2 nm apart, advantageously less than 0.01 nm.

3. Imaging device (100) according to any one of claims 1 to 2, wherein the electrical control signal (204, 208) is configured to vary in a manner35 linear over time during a time period, preferably between 5 ms and 15 ms.

4. Imaging device (100), according to any one of claims 1 to 3, wherein the control module (40) is configured to drive an activation of the imager detector (120) by means of a second electrical control signal (302, 304, 308, 310, 312), said second electrical control signal (302, 304, 308, 310, 312) being modulable.

5. Imaging device (100) according to any one of claims 1 to 4, wherein the control module (40) is configured to change the value of the electrical control signal (202, 204, 206, 208) for at least one time period, said imaging device (100) comprising a synchronizing device (130) configured to synchronize an activation of the imaging detector (120) during the time period, said imaging detector (120) being configured to integrate the interference light beam at each time period and provide an image of the sample (1) from the interference light beam integrated during each time period.

6. Imaging device (100) according to claim 5, comprising at least one of the following elements: - control means (99) of the position of the support (97) of the sample (1) to modify a position of the sample (1) according to at least one degree of freedom oriented along an axial direction normal to a surface of the sample (1); - control means (94) for the position of the reference mirror (92) to modify a position of the reference mirror (92) along at least one degree of freedom oriented along an axial direction normal to the reference mirror (92), and comprising - a processing unit (41) coupled to the imaging detector (120), the control device (40) being configured to control the electrical control signal (202, 204, 206, 208) for n time periods and to change the position of the sample (1) and / or the reference mirror (92) at each time period so that each time period is associated with a different position of the sample (1) and / or the reference mirror (92), the image detector (120) being configured to form an image of the sample (1) at each time period so as to form n images of the sample (1), n ​​being an integer greater than 1, the processing unit (41) being configured to process the n images of the sample (1) to provide a depth image of the sample (1).

7. Imaging device (100) according to any one of claims 1 to 6, wherein the control module (40) is configured to modify the value of the electrical control signal (202, 204, 206, 208) for at least one time period, the imaging device (100) comprising a synchronization device (130) configured to synchronize an activation of the imaging detector (120) to values ​​of the electrical control signal (202, 204, 206, 208) within at least one time period so as to record, at each activation, the interference light beam at a given wavelength and to provide an image of the sample (1) at each activation.

8. Imaging device (100) according to any one of claims 1 to 7, wherein the optical coherence tomography module (90) is a full-field optical coherence tomography module.

9. Optical coherence tomography imaging method for imaging a sample, said method comprising the following steps: - emission of a beam of light incident by a light source; - spectral filtering by diffraction to select a wavelength of the incident light beam in order to perform a spectral scan of the incident light beam, said filtering step being implemented by a scanning module comprising an acousto-optic tunable filter and forming a diffracted light beam at the selected wavelength; - application of an electrical control signal to drive the tunable acousto-optical filter so as to control a sweep of at least a portion of the spectrum of the incident light beam, said electrical control signal being modulable and not being a radio frequency electrical signal; - adjusting the spatial coherence of the diffracted light beam by propagation of the diffracted light beam in a multimode optical fiber; - formation of an interference light beam by propagation of the light beam at the output of the optical fiber in an optical coherence tomography module; - capture of the interference light beam by an imaging detector; - formation of an image of the sample from the interference light beam by the imaging detector, said application step comprising a selection of the following elements: - a value of the electrical control signal (202, 204, 208); - an application time of a value of the electrical control signal (202, 204, 208).

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