Device and method for integrating brillouin spectroscopy in optical fiber

Abstract

Optical device (1) for integrating Brillouin spectroscopy in optical fiber, wherein the device (1) comprises an optical assembly (2) having an input (23), a first output (24), and a second output (25), wherein the optical assembly (2) comprises a first single-mode optical fiber (3) configured to receive a light beam from the input (23) of the optical assembly (2) at a first end (4) and to transmit the light beam from the first end (4) to a second end (6) to illuminate a sample (5) at the first output (24) of the optical assembly (2), and further configured to collect light scattered by the sample (5) as a result of illumination at said second end (6) and to transmit the scattered light from the second end (6) to the first end (4), and an optical filter (7) coupled to the optical assembly (2) at the second output (25) to receive the scattered light coming from the first end (4) of the first optical fiber (3), wherein said optical filter (7) is defined by a transmission function periodic in the frequency domain for simultaneously suppressing the elastic Rayleigh background signal and the inelastic Brillouin signal due to the interaction of the sample (5) illumination light beam with the first optical fiber (3).

Description

[0001] Specto Sri Case: 061836 WO

[0002] DEVICE AND METHOD FOR INTEGRATING BRILLOUIN SPECTROSCOPY IN OPTICAL FIBER

[0003] TECHNICAL FIELD

[0004] The present invention relates to an optical device and a method for integrating Brillouin spectroscopy in optical fiber. Furthermore, the present invention relates to an apparatus for the acquisition of Brillouin spectra of a sample.

[0005] BACKGROUND ART

[0006] Brillouin spectroscopy is an advanced optical technique used to study the mechanical properties of materials by exploiting the inelastic scattering of light. When photons interact with spontaneous acoustic waves (acoustic phonons) in the material, Stokes and Anti-Stokes peaks are generated, shifted by approximately 1-20 GHz relative to the laser frequency. Analysis of the frequency and width of these peaks provides valuable information about the viscoelastic properties of the illuminated sample.

[0007] Unlike traditional methods, which require direct contact with the material and are limited to surface analysis, Brillouin spectroscopy uses light as a probe, avoiding any physical interaction. This approach enables three-dimensional analysis of biological systems, such as cells and tissues, with submicron optical resolution. Furthermore, given the close correlation between biomechanical properties and the onset of numerous diseases, including atherosclerosis, cancer, and glaucoma, this technology represents an innovative diagnostic platform for the biomedical field.

[0008] However, integrating this technique into an optical fiber-based system for biomedical endoscopic applications presents significant challenges. While the use of optical fibers allows the system to be more compact and flexible, enabling in situ spectroscopic analysis of materials otherwise inaccessible through static optical systems, such as microscopes, this integration introduces additional complexities. In addition to Rayleigh elastic scattering and Fresnel reflections, which form the elastic background typically several orders of magnitude stronger than the Brillouin light signal in static optical systems, using optical fibers to illuminate the sample generates a third, crucial contribution: spontaneous Brillouin scattering resulting fromSpecto Sri Case: 061836 WO

[0009] the interaction between the illuminating laser beam and the material constituting the fiber core, usually made of silica glass (SiCh) or, in some applications, polymeric materials.

[0010] Because the interaction occurs along extended fiber paths (typically >1 m), this fiber-generated background Brillouin signal is also orders of magnitude stronger than the Brillouin light signal from the sample itself. For this reason, using a single optical fiber to both illuminate the sample and collect the Brillouin light signal has not been feasible until now. Standard methods for suppressing the elastic light signal, such as gas cells containing iodine or rubidium that absorb light at the laser frequency, cannot simultaneously suppress the Brillouin signal generated by the fiber, since this signal is shifted in frequency, typically by >20 GHz relative to the laser frequency, depending on the fiber material and laser wavelength.

[0011] Indeed, the Brillouin frequency shift, given here by the expression v = (2n / )V, where V is the speed of sound and n is the refractive index of the sample, is inversely proportional to the wavelength . Previously, the Brillouin frequency shift from glass fiber light has been measured at approximately 25 GHz at a wavelength of 660 nm. However, biomedical applications may require longer wavelengths, up to 1565 nm, or shorter wavelengths, down to 400 nm, for industrial purposes.

[0012] Several approaches have been proposed to overcome the problem of inelastic background caused by the use of optical fibers. For example, Kabakova et al., in “Fiber-integrated Brillouin microspectroscopy: Towards Brillouin endoscopy", Journal of Innovative Optical Health Sciences, 10(06), 1742002, 2017, introduced a dual-fiber system to decouple the Brillouin signal from the illuminating fiber from that of the sample collected by a second fiber. However, using two optical fibers — one for illumination and the other for collection — results in high coupling losses and optical aberrations, compromising spatial resolution and drastically increasing Brillouin signal acquisition times. Other attempts, such as time-resolved probes, allow discrimination of the sample’s Brillouin signal through post-acquisition processing, but they require physical contact with the sample and local heating, thus limiting the practicality of the approach.

[0013] A further attempt to eliminate Brillouin scattering from the fiber involved the use of hollowcore fibers, which feature a hollow core. However, these fibers are fragile and incompatible with standard components, such as circulators and couplers, limiting their integration into established Brillouin spectroscopy systems.Specto Sri Case: 061836 WO

[0014] Accordingly, the present invention aims to provide a device and method for integrating Brillouin spectroscopy in optical fiber that is effective while allowing a compact and manageable configuration. Furthermore, the present invention aims to provide an optical apparatus capable of acquiring Brillouin spectra using a single optical fiber to both illuminate the sample and simultaneously collect the Brillouin light signal from the sample.

[0015] SUMMARY

[0016] These objectives are achieved by a device, a method, and an optical apparatus as defined in the claims at the end of this description.

[0017] In a first aspect of the invention, there is provided an optical device for integrating Brillouin spectroscopy in optical fiber, wherein the device comprises:

[0018] an optical assembly having an input, a first output, and a second output, wherein the optical assembly comprises a first single-mode optical fiber configured to receive a light beam from the input of the optical assembly at a first end and to transmit the light beam from the first end to a second end to illuminate a sample at the first output of the optical assembly, and is also configured to collect the light scattered by the sample following the illumination at said second end and transmit the scattered light from the second end to the first end; and

[0019] an optical filter coupled to the optical assembly at the second output for receiving the scattered light coming from the first end of the first optical fiber, wherein said optical filter is defined by a periodic transmission function in the frequency domain to simultaneously suppress the elastic Rayleigh background signal and the inelastic Brillouin signal due to the interaction of the light beam illuminating the sample with the first optical fiber.

[0020] In a second aspect of the invention, a method is provided for integrating Brillouin spectroscopy in optical fiber, wherein the method comprises:

[0021] receiving a light beam from an input of an optical assembly and transmitting said light beam via a first optical fiber from a first end to a second end of said first optical fiber; illuminating a sample at a first output of the optical assembly;

[0022] collecting the light scattered by the sample following the illumination at said second end and transmitting the scattered light from the second end to the first end; andSpecto Sri Case: 061836 WO

[0023] filtering the scattered light coming from the first end of the first optical fiber through an optical filter positioned at a second output of the optical assembly, wherein said optical filter is defined by a periodic transmission function in the frequency domain to simultaneously suppress the elastic Rayleigh background signal and the inelastic Brillouin signal due to the interaction of the light beam illuminating the sample with the first optical fiber.

[0024] In a third aspect of the invention, an apparatus is provided for acquiring Brillouin spectra of a sample, wherein the apparatus comprises:

[0025] a laser light source;

[0026] an optical device according to the first aspect; and

[0027] a spectral analyzer,

[0028] wherein the laser light source is coupled to the optical assembly and the spectral analyzer is coupled to the optical filter of the device.

[0029] Thanks to this device and method, it is possible to integrate Brillouin spectroscopy within a fiber-optic-based system. Furthermore, it is possible to obtain an optical apparatus for acquiring Brillouin spectra of a sample by illuminating the sample and collecting the scattered light signal using a single-mode optical fiber. The solution proposed herein aims to overcome the previously described limitations through the use of an optical filter characterized by a transmission function with periodic suppression peaks in the frequency domain. This filter allows the simultaneous suppression of the elastic Rayleigh background signal and the inelastic Brillouin signal generated due to the use of a single optical fiber. The integration of Brillouin spectroscopy in optical fiber enables extending this technique to applications previously not feasible, such as biomedical endoscopy, where the detection of the Brillouin spectrum can allow non-invasive clinical analyses and diagnoses, for example of cardiovascular diseases or tumors, without the need for markers and with high spatial resolution.

[0030] It is noted that, advantageously, the optical assembly of the device described herein employs a single standard single-mode optical fiber. This configuration offers several benefits. First, since the illumination volume of the sample, defined by the point spread function (PSF), coincides with the collection volume, it is possible to maximize the collection efficiency of the scattered Brillouin light signal, thereby maximizing the optical signal acquisition rate.

[0031] Furthermore, the use of a single-mode fiber, unlike previously developed systems that relied on multiple fibers for illumination and collection, enables extreme miniaturization of the system,Specto Sri Case: 061836 WO

[0032] paving the way for the development of Brillouin spectroscopy-based endoscopic devices for clinical applications. Such endoscopes can, for example, be used to characterize the stiffness of arteries in situ to analyze the viscoelastic properties of atherosclerotic plaques, where the probe size is a critical parameter to reach specific remote regions within the body.

[0033] In addition, unlike hollow-core fibers, standard single-mode fibers allow the system to be coupled with off-the-shelf optical components, such as microlens systems and circulators, enabling a more efficient implementation of the device both economically and in terms of performance.

[0034] These advantages represent a significant step forward compared to existing methods, opening the way to potential applications in biomedical endoscopy and advanced diagnostics.

[0035] These and other aspects of the present invention will become more apparent in light of the following description of some preferred embodiments described below.

[0036] BRIEF DESCRIPTION OF DRAWINGS

[0037] Fig. 1 shows a schematic representation of an optical device for integrating Brillouin spectroscopy in optical fiber according to an example.

[0038] Fig. 2 shows the device of Fig. 1 with a detail of the optical assembly connected to the first optical fiber.

[0039] Fig. 3 shows a flow diagram of a method for integrating Brillouin spectroscopy in optical fiber according to an example.

[0040] Fig. 4 shows an optical filter configuration according to an example based on a birefringent crystal.

[0041] Fig. 5 shows an optical filter configuration according to an alternative example based on a Mach-Zehnder interferometer.

[0042] Fig. 6 shows the operation of the optical filter illustrated in Figs. 4 and 5.

[0043] Fig. 7 shows an optical filter configuration according to an example based on an etalon.

[0044] Fig. 8 shows an optical filter configuration according to an alternative example based on an optical resonator.

[0045] Fig. 9 shows the operation of the optical filter illustrated in Figs. 7 and 8.Specto Sri Case: 061836 WO

[0046] Fig. 10 shows a schematic representation of an optical apparatus for acquiring Brillouin spectra of a sample according to an example.

[0047] Fig. 11 A shows a configuration of an optical fiber terminal according to an example.

[0048] Fig. 11B shows a configuration of an optical fiber terminal according to an alternative example. Fig. 12 shows a schematic representation of a spectral analyzer according to an example based on a VIPA optical element.

[0049] Fig. 13 shows a schematic representation of a spectral analyzer according to an alternative example based on a ring resonator.

[0050] Fig. 14 shows a schematic representation of an optical apparatus for acquiring Brillouin spectra of a sample according to another example.

[0051] DESCRIPTION OF THE EMBODIMENTS

[0052] Figure 1 illustrates a representative schematic of an optical device 1 for integrating Brillouin spectroscopy in optical fiber. The device 1 essentially comprises an optical assembly 2 including at least one optical fiber 3 (first optical fiber) and an optical filter 7 coupled to the optical assembly 2. The optical assembly 2 comprises an input 23, a first output 24, and a second output 25. The first optical fiber 3 is a single-mode fiber having a first end 4 and a second end 6, and is used to receive a light beam from a laser light source 8 (narrowband monochromatic) through the input 23 and to transmit the light beam from the first end 4 to the second end 6. The second end 6 of the first optical fiber 3 is positioned at the first output 24 of the optical assembly 2 so that the light beam is directed out of the optical assembly 2 to illuminate a sample 5. The sample is thus located at the output of the optical assembly 2 corresponding to the first output 24. Following illumination, the sample 5 emits scattered light, which is collected by the first optical fiber 3 through the first output 24. The first output 24 thus also serves as an input (second input of the optical assembly 2) to collect the scattered light from the sample 5 and transmit it within the first optical fiber 3 from the second end 6 to the first end 4. The double arrow shown inside the first optical fiber 3 indicates that the same optical fiber 3 is used to transfer light in both directions. The first optical fiber 3 can therefore be defined as a combined illumination and collection fiber.

[0053] The device 1 further comprises an optical filter 7 coupled to the optical assembly at the second output 25. The optical filter 7 is configured to receive the light scattered from the sample 5 that has passed through the first optical fiber 3 from the second end 6 to the first end 4. Since theSpecto Sri Case: 061836 WO

[0054] illumination light of the sample 5 passes through the first optical fiber 3, an additional inelastic Brillouin component is generated due to the interaction with the core of the fiber 3 itself. The optical filter 7 is configured to have a periodic transmission function in the frequency domain so as to simultaneously suppress the elastic Rayleigh background signal and the additional inelastic Brillouin signal, allowing only the Stokes and Anti-Stokes Brillouin peaks of the sample 5 to pass through.

[0055] It is noted that the properties of this periodic optical filter 7 apply to a light beam passing through any optical fiber having a core made of a specific material, for example silica glass. Reference is made here to the filtering of light that passes through the first optical fiber 3 and illuminates the sample 5. However, it is evident that if the first optical fiber 3 were coupled (in series) to one or more additional optical fibers, the optical filter 7 is configured to filter the light coming from all these fibers. Specifically, if all the optical fibers are made of the same material, the same additional inelastic Brillouin signal due to the interaction with the fiber material will be generated for each fiber. Therefore, the optical filter 7 will act identically on the signal passing through each optical fiber.

[0056] By coupling the periodic optical filter 7 to the optical assembly 2, it is possible to use a single optical fiber to both illuminate and simultaneously collect the Brillouin light signal coming from the sample, while preventing additional inelastic Brillouin signals caused by the interaction between the optical fiber and the illumination light from masking or disturbing the Brillouin signal originating from the sample 5.

[0057] It is noted that the first optical fiber 3, specifically the first end 4 of said fiber 3, is configured to receive light coming from the input 23 and to transmit the scattered light to the second output 25 (dashed lines in the figure).

[0058] At the first end 4, the first single-mode optical fiber 3 is configured to receive a light beam from the input 23 of the optical assembly 2 and, simultaneously, to transmit the scattered light from the sample 5 to the optical filter 7, which is positioned at the second output 25 of the optical assembly 2. This configuration does not imply that the first end 4 is directly connected to both the input 23 and the second output 25; rather, it indicates that the fiber is arranged to allow this dual signal path. Although no physical optical routing element or beam splitter is explicitly specified, it is clear and implicit that the described function requires an optical path organized in such a way as to allow both the entry of light and the collection of scattered light, as provided by the configuration of the device. In other words, the term “configured to” defines theSpecto Sri Case: 061836 WO

[0059] functional capability of the fiber to simultaneously handle propagation and collection of the signal, without requiring a direct physical connection between the input and output.

[0060] The coupling between the first optical fiber 3, the input 23, and the second output 25 can be achieved in various ways, for example through a system of lenses, mirrors, and / or additional optical fibers. Figure 2 shows a detail of a possible coupling configuration.

[0061] In one example, the optical assembly 2 may further include a second optical fiber 9 connected to the input 23 of the optical assembly 2 to transmit the light beam coming from said input 23, a third optical fiber 10 connected to the second output 25 of the optical assembly 2, and an optical signal handling element 11 connected to the first optical fiber 3 at the first end 4, to the second optical fiber 9, and to the third optical fiber 10, in order to direct the light beam from the second optical fiber 9 toward the first optical fiber 3, and to direct the scattered light collected by the first optical fiber 3 toward the third optical fiber 10. The optical signal handling element 11 can be a fiber optic coupler, capable of combining or splitting signals from different fibers. Alternatively, the optical signal handling element 11 can be a fiber optic circulator, which can route the signal in one direction among multiple ports connected to different optical fibers. Figure 3 shows a flow diagram of a method 100 for integrating Brillouin spectroscopy in optical fiber. In a first step (SI 01), the method 100 includes receiving a light beam from the input 23 of the optical assembly 2. The light beam is then transmitted through the first optical fiber 3 from a first end 4 to a second end 6. In a second step (SI 02), the method 100 includes illuminating the sample 5 to be analyzed. The illumination occurs at the first output 24 of the optical assembly 2. The light scattered by the sample 5 is then collected by the same first optical fiber 3 and transmitted back from the second end 6 to the first end 4. Finally, the method 100 includes the step of filtering (SI 04) the light scattered by the sample that has passed through the first optical fiber 3. The filtering is performed by an optical filter 7 positioned at the second output 25 of the optical assembly 2. In particular, the optical filter 7 is characterized by a periodic transmission function in the frequency domain to simultaneously suppress the elastic Rayleigh background signal and the inelastic Brillouin signal due to the interaction of the light beam illuminating the sample 5 with the first optical fiber 3.

[0062] An example of a periodic filter 7 for the simultaneous suppression of elastic and inelastic background signals is shown in Figure 4. The optical filter 7 comprises a first polarizer 12, a birefringent crystal 13 positioned downstream of the first polarizer 12, whose optical axis forms an angle 9 with respect to the transmission axis of the first polarizer 12, with 9 within the rangeSpecto Sri Case: 061836 WO

[0063] 0 < 9 < 7t / 2, an active element 14 positioned downstream of the birefringent crystal 13, which acts as a variable delay plate, and a second polarizer 15 positioned downstream of the active element 14.

[0064] The light collected by the optical filter 7 includes Rayleigh light signals (R), Brillouin signals from the fiber (BSiO2), and Brillouin signals from the sample (BsP). These signals are transmitted from a combined illumination and collection fiber (e.g., the first optical fiber 3) to the third optical fiber 10, passing through the periodic filter 7 with the same polarization state P. The signals first pass through the first polarizer 12 (Pl), which increases the degree of polarization and ensures a linearly polarized output.

[0065] The transmitted signal then passes through the birefringent crystal 13 (BC) of length L. The birefringence of this crystal is given by the difference An = |no - ne|, where no is the ordinary refractive index and neis the extraordinary refractive index, and the optical axis forms an angle 9 with the transmission axis of the first polarizer 12, with 0 < 9 < 7t / 2. The function of the birefringent crystal 13 is to introduce a relative phase delay Acp between the elastic and inelastic optical signals, thereby decoupling the polarization of the sample Brillouin signal from the polarization of the elastic and inelastic background signals.

[0066] Specifically, the birefringent crystal 13 is ideally designed so that its length and birefringence coefficient produce a Free Spectral Range (FSR), given by FSR = c / L n, equal to the Brillouin frequency shift of the fiber, FSR = vsi02, typically in the range 15 GHz < FSR < 35 GHz depending on the laser wavelength, preferably around 26.5 GHz ± 1.5 GHz for an excitation wavelength of 669 nm in a fiber with a silica core. This is because, as previously described, the Brillouin frequency shift, given by v = — V (where V is the sound velocity and n is the A

[0067] refractive index of the sample), is inversely proportional to the wavelength . Consequently, the FSR of the filter preferably satisfies the relation FSR = K / , where K ranges from 16,599 to 18,599 GHz nm, preferably about 17,599 GHz nm. This value may vary depending on the fiber core material, the concentration of dopants, operating temperature, and changes in the acoustic velocity of the material.

[0068] Since the birefringence An depends on the angle 9 of the birefringent crystal, the crystal can be conveniently rotated to adjust the FSR of the filter and compensate for any shifts in the Brillouin frequency of the fiber (vsio?) caused, for example, by temperature changes in the first fiber 3 due to the ambient environment.Specto Sri Case: 061836 WO

[0069] The light signal transmitted through the birefringent crystal 13 then passes through an active element 14, acting as a variable delay plate, for example a birefringent liquid crystal (LC). Its function is to transform the polarization state of the background signal exiting the birefringent crystal 13 into linear polarization. This allows the second polarizer 15 (P2), whose optical axis is orthogonal to the background polarization, to suppress the background signal, while the Brillouin signal from the sample is fully or partially transmitted.

[0070] Because the FSR of the birefringent crystal 13 is designed to have two suppression peaks with a spectral separation equal to the Brillouin frequency of the collection fiber core (the first optical fiber 3), it is therefore possible to suppress or significantly attenuate both the elastic Rayleigh and inelastic Brillouin background signals simultaneously.

[0071] An alternative example of the periodic filter 7 for simultaneous suppression of the elastic and inelastic background signals is shown in Figure 5. In this example, the periodic filter 7 comprises a Mach-Zehnder interferometer (MZI) 16, in particular integrated in waveguides. It should be noted, however, that this filter can also be implemented using a free-space optical system.

[0072] The light signal collected by the collection optical fiber (for example, from the first optical fiber 3 and transmitted to the third optical fiber 10) is split by a splitter element 29 (beam splitter, BS) into two arms of different lengths AL, namely a first path 32 in which a first light beam 31 propagates and a second path 34 in which a second light beam 33 propagates. In one of the two arms, for example the second path 34, a phase modulator 40 is inserted, such as a heater to which a current is applied, which is configured to finely shift the transmission function of the interferometer in the frequency domain.

[0073] The two interferometer arms then recombine through a combiner 30 (second BS) to generate interference between the two light beams 31 and 33. The frequency spacing between the destructive and constructive interference peaks is given by FSR = — . As in the previously

[0074]

[0075] described example of Figure 4, the MZI filter according to the present invention is configured such that FSR = vsi02, so that two adjacent orders of destructive interference can simultaneously suppress the elastic background at the laser frequency as well as the fiber contribution at the Brillouin frequency associated with the material of the optical fiber core. At the output of the combiner 30, there are conveniently two output signals 37 and 38 corresponding to two outputs 35 and 36. At the first output 35, the Brillouin signal of the sample is transmitted without the Rayleigh elastic component and without the additional inelasticSpecto Sri Case: 061836 WO

[0076] Brillouin component. The second output 36 is configured to propagate the second output signal 38, which is used by a closed-loop control system 41 to adjust the interference between the first light beam 31 and the second light beam 33 via the phase modulator 40, minimizing the Rayleigh elastic component and the additional Brillouin component in the first output signal 37.

[0077] In particular, the filter 7 may include an optical detector 39, in particular a photodetector, positioned at the second output 36 to measure the intensity of the second output signal 38, which is used by the closed-loop control system 41. The filter 7 may further include a hysteresis module 42 coupled to the closed-loop control system 41 to increase the stability and effectiveness of the control.

[0078] Figure 6 illustrates the underlying mechanism of the periodic filter 7 operation for the two examples shown in Figures 4 and 5. In particular, Figure 6 shows the light spectrum in the case of a fiber-integrated Brillouin system. The central Rayleigh peak occurs at the same frequency as the laser incident on the sample and is typically more than 8 orders of magnitude stronger than the Brillouin peaks of the sample 5, which are shifted in frequency by ±vSp, where the sign is due to the Doppler effect.

[0079] In addition to these two contributions, there is a third signal generated by Brillouin light scattering from the optical fiber, shifted in frequency by ±v_SiO2. Its intensity depends on the length of the collection optical fiber (i.e., the lengths of the first and third optical fibers 3 and 10) but is also typically several orders of magnitude greater than that of the Brillouin peaks from the sample 5. It should be noted that, since the stiffness of the fiber is much higher than that of biological samples, v_SiO2 > v_Sp.

[0080] This allows the construction of a periodic filter whose transmission function (dashed line) is sinusoidal, with a periodicity characterized by FSR = vsi02. Under this condition, once the filter is properly aligned in the frequency domain, it is possible to simultaneously suppress both the elastic and inelastic background signals while allowing transmission of the Brillouin inelastic signal originating from the sample 5, which has frequencies lying between those of the laser and the optical fiber.

[0081] According to a further example, the optical filter 7 can comprise a solid Fabry-Perot interferometer 17, i.e., an etalon, as shown in Figure 7. In this configuration, the optical filter 7 consists of an optical cavity defined by two highly reflective surfaces, for example two mirrors, with reflectivities defined by the parameters R1 and R2, i.e., a first reflective surface 43 and aSpecto Sri Case: 061836 WO

[0082] second reflective surface 44 arranged on two parallel planes. The medium separating the first surface 43 from the second surface 44 can be a material, such as a gas, whose refractive index n varies in response to an applied voltage or a temperature gradient.

[0083] The distance between the two mirrors 43 and 44 determines the optical path and, consequently, the phase delay (|) introduced on the transmitted or reflected beam. In this way, the etalon 17 can be considered a highly selective narrowband filter that eliminates spectral bands in reflection according to its FSR, given by FSR = — .

[0084] Similarly, by appropriately choosing the separation between the two mirrors 43 and 44 and the medium that separates them, it is possible to set an FSR equal to the Brillouin shift of the fiber FSR = vsi02), thereby allowing simultaneous suppression of both the elastic and inelastic background signals.

[0085] Figures 8A and 8B show a fourth example of a periodic optical filter 7. In this case, the optical filter 7 can comprise a ring resonator 18, in particular integrated in waveguides whose phase can be finely tuned through the application of a voltage. The ring resonator 18 can be implemented in two different configurations. In a first “add-drop” configuration (Figure 8A), the ring has an input BUS waveguide 45 and an output BUS waveguide 46, and a second waveguide for the drop channel 47. In a second “all-pass” configuration (Figure 8B), the ring has only a BUS waveguide for coupling the light signal 45 and for the pass channel 46. In both cases, the ring has a curvature radius R and a group refractive index ng such that it generates an FSR, given in this case by the expression FSR = - , equal to the Brillouin frequency

[0086]

[0087] shift of the fiber FSR = vsi02). Similar to the previous examples, this configuration allows suppression or attenuation of the elastic and inelastic background signals, while transmitting at least partially the Brillouin signal of the sample through the “pass” output channel 46.

[0088] Figure 9 illustrates the operating mechanism of the periodic filter 7 for the two examples of Figures 7 and 8. The light spectrum in Figure 9 is analogous to that of Figure 6, with the only difference that, in this case, the transmission function of the periodic filter 7 is no longer a sinusoidal function but rather a function characterized by narrower suppression peaks. This feature allows for further reduction of losses of the Brillouin signal from the sample transmitted through the periodic filter 7.

[0089] Figure 10 shows an apparatus 19 for acquiring Brillouin spectra of a sample using the optical device 1 described previously. According to one example, the apparatus may be an endoscope.Specto Sri Case: 061836 WO

[0090] The apparatus 19 comprises a laser light source 8, an optical device 1 for integrating Brillouin spectroscopy in optical fiber, and a spectral analyzer 20. The laser light source 8 is coupled to the optical device 1, in particular to the second optical fiber 9 of the optical group 2, while the spectral analyzer 20 is coupled to the optical filter 7 of the device 1.

[0091] The light beam can advantageously be emitted by a narrowband (<100 MHz) single-mode laser, which is coupled to an optical fiber, for example the second optical fiber 9, through a set of collection optics. The light beam passes through an optical signal management element 11, such as a fiber coupler, which directs the light from the laser 8 to another optical fiber, for example the first single-mode fiber 3, used to illuminate the sample 5 via an optical system (fiber terminal 26) at the output of the fiber, while simultaneously collecting the scattered light from the sample 5 through the same optical system.

[0092] The light collected by the same illumination fiber 3 is transmitted to a third optical fiber 10 via the coupler and spectrally analyzed by the detection modules 20. In one example, these three optical fibers 3, 9, 10 may be single-mode and polarization-maintaining. The optical signal to be analyzed is coupled via the third optical fiber 10 to an optical filter 7 characterized by a transmission function with periodic suppression peaks, whose spacing is defined by the Free Spectral Range (FSR).

[0093] The advantage of using this filter 7 is that, due to the periodicity of the suppression peaks, it is possible to simultaneously remove or attenuate the parasitic background signal generated by various elastic and inelastic contributions, such as Fresnel reflections, elastic Rayleigh scattering, and inelastic Brillouin scattering generated by the interaction of the laser beam with the illumination and collection fiber, i.e., the first optical fiber 3. Once the background signal is removed or attenuated, the Brillouin light signal transmitted by the optical filter 7 is analyzed by a high-resolution (<2 GHz) spectral analyzer 20 to resolve the Brillouin spectral bands and, consequently, derive the viscoelastic properties of the analyzed sample 5.

[0094] The apparatus 19 described can be coupled with standard imaging modules, for example Optical Coherence Tomography (OCT), to allow visualization of the sample and identification of regions of interest where the Brillouin probe acts to detect mechanical properties. For this purpose, a second laser source, in this case a non-monochromatic broadband light beam (for example, a superluminescent source (SLED) or a swept-source laser), can be coupled to the optical group 2 of the device 1 through at least one of the second optical fiber 9 or the first optical fiber 3 to illuminate the sample, for instance via the same fiber terminal 26.Specto Sri Case: 061836 WO

[0095] In this way, the interference signal reflected from the sample 5 can be collected through the same first optical fiber 3 using, for example, a Michelson or Mach-Zehnder interferometer. Also for the OCT imaging system, the signal can be collected by the same fiber 3. However, the detection of the signal in the time or frequency domain is decoupled from the Brillouin detection modules by means of, for example, a spectral or dichroic filter.

[0096] A computer 27 can be connected both to the optical filter 7 and the spectral analyzer 20 to manage the filter and analyze the data from the analyzer 20 and, optionally, from the OCT detection module.

[0097] As shown in the figure, the optical signal management element 11 can advantageously be a circulator. This element has the benefit of transmitting light from the laser 8 from the second optical fiber 9 to the first optical fiber 3, while transmitting light collected by the first optical fiber 3 to the third optical fiber 10 with very low losses due to crosstalk between the different channels. In this way, system efficiency can be optimized by directing all (or nearly all) of the illumination light onto the sample 5 to be analyzed, while maximizing the collection of the Brillouin light signal toward the periodic filter 7 and the spectral analyzer 20.

[0098] Figures 11 A and 1 IB illustrate two possible variations of the fiber terminal 26 for illumination and signal collection. In a first example, shown in Figure 11 A, illumination and collection are achieved via a microlens system 48 that collimates the divergent light beam exiting the first optical fiber 3 and focuses it onto the sample 5. This system can include structures to maintain a fixed working distance and protect the optics from potential impacts with the sample 5, which could otherwise damage them.

[0099] In a second example, shown in Figure 11B, the beam is focused orthogonally to the direction of the first optical fiber 3 using, for example, a system of lenses 48 and mirrors 54. This configuration can be particularly advantageous when the fiber-integrated system operates as an endoscope to measure the stiffness of arterial walls, for instance in cardiovascular diagnostics. In both cases, active elements such as electromagnetic coils can be used to achieve z-axis scanning of the illumination beam to reach different depths of the sample, or to allow the fiber terminal 26 to rotate about its axis to obtain a stiffness map of the sample, such as that of an artery. Such rotation can be achieved, for example, via controlled mechanical rotation of the optical fiber within a protective sheath, using a rotary j oint coupled to a motor, and with a distal terminal including an angled lens or a mirror capable of laterally deflecting the light beam, thereby enabling angular scanning of the surrounding environment.Specto Sri Case: 061836 WO

[0100] In one example, the spectral analyzer 20 comprises a Virtually Imaged Phased Array (VIPA) 21, as shown in Figure 12. The light transmitted by the periodic filter 7 passes through a fourth optical fiber 49. This fiber serves to carry the Brillouin signal from the sample, largely free of background light, to a spectral analyzer or spectrometer 20 based on a device known as a Virtually Imaged Phased Array (VIPA), which is a modified etalon. Like a standard etalon, the VIPA 21 has two highly reflective surfaces but additionally includes an anti -refl ection window at the input for coupling the light signal to be analyzed.

[0101] The spectrometer 20 is formed, for example, by a spherical lens 50 that collimates the beam exiting the fourth optical fiber 49, and a cylindrical lens 51 that focuses the Brillouin light beam into the VIPA 21 through its anti -refl ection input window. Inside the VIPA cavity, multiple internal reflections occur, and a Fourier lens 52 is used to perform a Fourier transform of the signal transmitted by the VIPA 21, generating the spectrum on a spatial plane where a camera 53, such as a CCD, EMCCD, or CMOS sensor, is placed to detect the Brillouin spectrum. Conveniently, the VIPA 21 has a Free Spectral Range (FSR) greater than that of the periodic filter, for example 20 GHz < FSR < 40 GHz, to fully capture the Brillouin spectrum of the sample 5.

[0102] In an alternative example, the spectral analyzer 20 comprises at least one ring resonator 22, as shown in Figure 13. Specifically, the spectral analyzer, or spectrometer, 20 can be formed by an “add-drop” type ring resonator 22. Unlike the ring resonator configuration used for the periodic filter 7, where the Brillouin signal exited through the ‘pass’ channel 46, in this case the Brillouin signal is detected via a detector at the output of the ‘drop’ channel 47 after performing a rapid scan of the ring resonator 22 using a phase modulator, for example a heater 55, to which a current is applied.

[0103] As in the previous example, the FSR of the ring resonator can conveniently be wider than that of the periodic filter, for instance 20 GHz < FSR < 40 GHz, in order to fully measure the Brillouin spectrum of the sample 5.

[0104] Figure 14 shows a schematic representation of a further configuration for integrating Brillouin spectroscopy in optical fiber according to one example. Compared to the configuration illustrated in Fig. 10, this configuration includes the insertion of a transformation polarization element 30 at the output of the illumination / collection fiber 3, i.e., at the second end 6 of the first optical fiber 3. This element 30 can be positioned either upstream or downstream of the terminal optical element 26. Specifically, the transformation polarization element 30 is anSpecto Sri Case: 061836 WO

[0105] element configured to generate orthogonal states between the illumination of the sample 5 and the collection of the light from the sample 5.

[0106] In one example, the transformation polarization element 30 can be a quarte wave (X / 4) plate or a Faraday rotator. However, any optical element that meets the polarization transformation criteria outlined earlier would also be suitable for this purpose.

[0107] As an example, the transformation polarization element 30 can be a quarte wave (X / 4) plate that is configured to convert the polarization of the illumination beam from linear to circular by appropriately orienting its optical axis with respect to the incident polarization. When the light scattered from the sample 5 is collected again by the same fiber 3 and passes back through the transformation polarization element 30, the polarization is reconverted into linear polarization with an orientation orthogonal to that of the illumination beam, as exemplarily shown in the diagrams at the bottom of the figure.

[0108] At the output of the third optical fiber 10, i.e., along the optical path of the third fiber 10 downstream of the optical signal management element 11, a polarization analyzer 31 can be placed with its transmission axis parallel to the polarization of the signal collected from the sample 5.

[0109] Since the background signal component originating from the optical fiber, including at least the Brillouin signal generated within the fiber itself and additional noise contributions resulting from the interaction between the excitation beam and the optical fiber, substantially retains the same polarization as the illumination beam, this background component has a polarization orthogonal to the signal collected from the sample 5. Consequently, this background component is suppressed or at least attenuated by the polarization analyzer 31, which is configured with its transmission axis orthogonal to the polarization associated with the background component. This configuration can be used as a replacement for the previously described periodic filter 7, or in combination with it to further increase the background suppression level, placing the polarization analyzer upstream of the periodic filter 7 input. In a further example, the input polarizer of the optical filter illustrated in Fig. 4 can also serve as this polarization analyzer.

[0110] A skilled person in the field, with the aim of meeting additional or contingent requirements, may make numerous further modifications and variations to the above-described device,Specto Sri Case: 061836 WO

[0111] method, and optical apparatus. All such modifications and variations are, however, included within the scope of protection of the present invention as defined by the appended claims.

Claims

Specto Sri Case: 061836 WOCLAIMS1. Optical device (1) for integrating Brillouin spectroscopy in optical fiber, wherein the device (1) comprises:an optical assembly (2) having an input (23), a first output (24) and a second output (25), wherein the optical assembly (2) comprises a first single-mode optical fiber (3) configured to receive a light beam from the input (23) of the optical assembly (2) at a first end (4) and to transmit the light beam from the first end (4) to a second end (6) to illuminate a sample (5) at the first output (24) of the optical assembly (2), and also configured to collect the light scattered from the sample (5) following the illumination at said second end (6) and transmit the scattered light from the second end (6) to the first end (4); and an optical filter (7) coupled to the optical assembly (2) at the second output (25) for receiving the scattered light coming from the first end (4) of the first optical fiber (3), wherein said optical filter (7) is defined by a periodic transmission function in the frequency domain to simultaneously suppress the elastic Rayleigh background signal and the inelastic Brillouin signal due to the interaction of the light beam illuminating the sample (5) with the first optical fiber (3).

2. Optical device (1) according to claim 1, wherein the optical assembly (2) further comprises a second optical fiber (9) connected to the input (23) of the optical assembly (2) for transmitting the light beam coming from said input (23), a third optical fiber (10) connected to the second output (25) of the optical assembly (2) and an optical signal handling element (11) connected to the first optical fiber (3) at the first end (4), to the second optical fiber (9) and to the third optical fiber (10) to direct the light beam from the second optical fiber (9) towards the first optical fiber (3), and to direct the diffused light collected by the first optical fiber (3) towards the third optical fiber (10).

3. Optical device (1) according to one of the preceding claims, wherein the optical filter (7) comprises a first polarizer (12), a birefringent crystal (13) positioned downstream of the first polarizer (12), whose optical axis has an angle 0 with respect to the transmission axis of the first polarizer (12) included within the range 0<9<7t / 2, an active element (14) positioned downstream ofSpecto Sri Case: 061836 WOthe birefringent crystal (13), which acts as a variable delay plate, and a second polarizer (15) positioned downstream of the active element (14).

4. Optical device (1) according to one of claims 1 to 2, wherein the optical filter (7) comprises a Mach-Zehnder interferometer(16).

5. Optical device (1) according to one of claims 1 to 2, wherein the optical filter (7) comprises a solid Fabry -Perot 20 interferometer (17).

6. Optical device (1) according to one of claims 1 to 2, wherein the optical filter (7) comprises a ring resonator (18).

7. Optical device (1) according to one of the preceding claims, wherein the free spectral range, FSR, of the optical filter (7) corresponds to the Brillouin frequency shift of the first optical fiber (3), in particular it is between 15 GHz and 35 GHz.

8. Optical device (1) according to one of the preceding claims, further comprising a transformation polarization element (30) at the second end (6) of the first single-mode optical fiber (3) and a polarizer analyzer (31) located along the optical path of the third optical fiber (10) downstream the optical signal handling element (11).

9. Optical device (1) according to claim 8, wherein the transformation polarization element (30) is a quarter wave plate or a Faraday rotator.

10. Apparatus (19) for the acquisition of Brillouin spectra of a sample (5), wherein the apparatus (19) comprises:a laser light source (8);Specto Sri Case: 061836 WOan optical device (1) according to one of the claims between 1 and 7, anda spectral analyzer (20),wherein the laser light source (8) is coupled to the optical assembly (2) and the spectral analyzer (20) is coupled to the optical filter (7) of the device (1).

11. Apparatus (19) according to claim 10, wherein:a. the spectral analyzer (20) comprises a virtually image phase array, VIPA (21); orb. the spectral analyzer (20) comprises at least one ring resonator (22).

12. Apparatus (19) according to claim 10 or 11, further comprising at least one optical coherence tomography (OCT) imaging module, configured to emit a second broadband beam of light coupled to the optical assembly (2) of the device (1) through at least one of the second optical fiber (9) and the first optical fiber (3), and to 20 collect the reflected interference signal from the sample (5) via the first optical fiber (3), using a Michelson or Mach-Zehnder interferometry, wherein the OCT and Brillouin signals are separated by means of a spectral or dichroic filter.

13. A method (100) for integrating Brillouin spectroscopy in optical fiber, wherein the method (100) comprises:receiving (S 101) a light beam from an input (23) of an optical assembly (2) and transmitting said light beam via a first optical fiber (3) from a first end (4) to a second end (6) of said first 30 optical fiber (3); illuminating (S102) a sample (5) at a first output (24) of the optical assembly (2);collecting (SI 03) the light scattered by the sample (5) following the illumination at said second end (6) and transmitting the scattered light from the second end (6) to the first end (4); and filtering (SI 04) the scattered light coming from the first end (4) of the first optical fiber (3) through an optical filter (7) positioned at a second output (25) of the optical assembly (2), wherein said optical filter (7) is defined by a periodic transmission function in the frequency domain to simultaneously suppress the elastic Rayleigh background signal and the inelastic21Specto Sri Case: 061836 WOBrillouin signal due to the interaction of the light beam illuminating the sample (5) with the first optical fiber (3).

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