WO2024194797A1

WO2024194797A1 - Optical filter and spectroscopy system employing the filter

Info

Publication number: WO2024194797A1
Application number: PCT/IB2024/052645
Authority: WO
Inventors: Giuseppe ANTONACCI; Cristian Angelo Manzoni; Dario POLLI; Giulio CERULLO
Original assignee: Specto S.R.L.
Priority date: 2023-03-20
Filing date: 2024-03-19
Publication date: 2024-09-26

Abstract

It is described an optical filter (100) configured to operate for Brillouin scattering and/or low-frequency Raman scattering spectroscopy applications and comprising: an input (IN) for a first radiation (R_B) having a first input linear polarization (p_B1) and a second radiation (R_R) having a second input linear polarization parallel (p_R1) to said first input linear polarization (p_B1); a birefringent medium (1) configured to reciprocally rotate the first (p_B1) and second (p_R1) input polarizations to obtain that the first radiation (R_B) has a first output polarization (p_B2) and the second radiation (R_R) has a second output polarization (p_R2) different from said first output polarization (p_B2). The filter further includes a polarizer (2; 7, 20) coupled to said birefringent medium (1) to transmit said first radiation (RB) having the first output polarization (p_B2) and reject said second radiation (R_R) having the second output polarization (p_R2).

Description

P7493PC00 “Optical filter and spectroscopy system employing the filter” *.*.*.*.* BACKGROUND Technical field The present invention relates to optical filters and particularly, to notch filters employable, as an example, for suppressing the dominant elastic background light (pump) affecting the inelastic light signal in Brillouin and low-frequency Raman spectroscopy measurements. Description of the Related Art Inelastic light scattering gives rise to spectral features that are indicative, for example, of the mechanical, chemical and structural properties of a material. As an example, Brillouin scattering arises when light interacts with thermal density fluctuations of matter and provides information about the material viscoelastic properties. In this process, most of the scattered light has the same frequency as the incident one (elastic Rayleigh scattering or Fresnel reflections). A small portion (~10- ¹²) of the incident light, however, is scattered at a different frequency (inelastic scattering) so that the resulting spectrum contains also peaks that are shifted by a few GHz from the elastic Rayleigh peak. The frequency shift and linewidth of the inelastic peaks provide useful information on the stiffness and viscosity of a material, yet the detection of such signal is challenging due to the dominant elastic Rayleigh (background) light that overwhelms the inelastic peaks. The small frequency shift and the weakness of the spontaneous inelastic Brillouin scattered light generally impose the use of a spectrometer with sub-GHz spectral P7493PC00 resolution and a high (>40dB) spectral contrast, where the spectral contrast is defined as the peak-to-background ratio of the spectrometer transmission function. Multipass Fabry-Perot (FP) interferometers are currently the gold-standard instruments to measure high- resolution spectra in this contest, providing a spectral contrast of >150dB (10¹⁵). Nevertheless, these interferometers require long (>10sec) data acquisition time as a consequence of the slow scanning mechanism that is needed to avoid mechanical drifts, limiting Brillouin spectroscopy mostly to single-point measurements on bulk materials. Recently, a high-throughput, non-scanning dispersive device known as Virtually Imaged Phased Array (VIPA) has been introduced. The VIPA is essentially a modified version of the traditional FP interferometer but with the main advantage of enabling a rapid detection of the Brillouin spectrum without moving parts. On the other hand, single-stage VIPA spectrometers have a limited spectral contrast of 30 dB (10³) and therefore cannot detect the Brillouin spectra in semi-transparent samples, where Rayleigh scattering is >10⁶ higher in intensity with respect to the Brillouin signal. Therefore, despite the high potential and the increasing demand, the use of Brillouin spectroscopy across biomedical labs and clinics has been fundamentally restricted as a consequence of the intrinsic limitations of the existing technology. The following documents describe contrast-enhancement methods: US7898656B2; US20200182694A1, WO2022031815A1; US11143555B2; WO2019064093A9. In addition to the development of spectrometers with large spectral contrast, several filtering methods have been proposed to suppress the dominant elastic Rayleigh P7493PC00 light that overwhelms the Brillouin signal. Some methods involve the use of interferometric schemes where the beam of light is split in two unbalanced paths to yield destructive interference. Examples of such methods are described in the following documents: - Fiore, A., Zhang, J., Shao, P., Yun, S.H. and Scarcelli, G., 2016. High-extinction virtually imaged phased array-based Brillouin spectroscopy of turbid biological media. Applied physics letters, 108(20), p.203701. - Lepert, G., Gouveia, R.M., Connon, C.J. and Paterson, C., 2016. Assessing corneal biomechanics with Brillouin spectro-microscopy. Faraday discussions, 187, pp.415-428. - Antonacci, G., Lepert, G., Paterson, C. and Török, P., 2015. Elastic suppression in Brillouin imaging by destructive interference. Applied Physics Letters, 107(6), p.061102. These schemes, however, are typically unstable against temperature fluctuations and frequency drift of the pump light and they require constant re-alignment by specialized users. Other methods, such as that described by document “Meng, Z., Traverso, A.J. and Yakovlev, V.V., 2014. Background clean-up in Brillouin microspectroscopy of scattering medium. Optics express, 22(5), pp.5410-5415”, involve the use of absorptive gaseous cells with absorption peak tuned to the pump frequency, yet they also cause significant attenuation of the Brillouin signal due to the presence of multiple close absorption lines of the gaseous cell medium that degrades the Signal to Noise Ratio (SNR) and the precision in localization of the Brillouin peaks. To address those issues, integrated optics have been recently proposed, as in document WO2020084466A1: yet the extinction ratio is still limited to <10dB. P7493PC00 Similar limitations are encountered in low-frequency (THz) Raman. Low-frequency Raman spectroscopy is a technique used to investigate the low-frequency vibrational modes of molecules. Unlike conventional Raman spectroscopy, which typically probes high frequency vibrations (approximately 10 to 100 THz, i.e. from 300 to 3000 cm^-1) in the visible or near-infrared range, low-frequency Raman focuses on the terahertz frequency range (approximately 0.1 to 10 THz, i.e. in the range of 10-300 cm^-1). This technique provides valuable insights into molecular dynamics, intermolecular interactions and structural properties of materials, including biomolecules, polymers, and solids. Low-frequency Raman spectroscopy is particularly useful for studying collective molecular motions and low-frequency phonons, offering unique advantages in various fields such as materials science, drug discovery, chemistry, and biophysics. Likewise Brillouin spectroscopy, excess amount of elastic Rayleigh scattering represents a major limitation also for low-frequency Raman spectroscopy. In fact, commercial optical filters and dichroic mirrors relying on multiple thin layers do not offer a bandwidth that is sufficiently narrow to remove the background light in the spectral region of interest. Recently, strong effort in the development of notch filters based on Bragg gratings have been made, yet these solutions typically do not offer a bandwidth smaller than 20 cm^-1, preventing the detection of low-frequency (shear) modes. Document US8125634-B1 describes an Additive Šolc Filter (ASF) assembly including: a light source and collimating lens providing multispectral light input to an additive Šolc filter. The filter of this document is employed for filtering high-frequency Raman inelastic scattering. A Šolc filter is also described in the document “Ivan P7493PC00 Šolc, "Birefringent Chain Filters," J. Opt. Soc. Am. 55, 621-625 (1965)”. Summary of the invention The present invention addresses the technical problem of providing an optical filter, of the type employable for suppressing dominant elastic background radiation associated to inelastic Brillouin and/or low-frequency Raman scattered radiation, that is alternative to the filters of the prior art and that overcomes at least partially the above-mentioned problems shown by the known filters. Particularly, the Applicant observes that a suitable optical filter that can be an effective solution to analyze simultaneously inelastic Brillouin and low-frequency Raman scattered light would be highly desirable to retrieve both mechanical and structural properties of materials at the same time. According to a first aspect the present invention relates to an optical filter as defined by claim 1. Particular embodiments of the optical filter are described by the dependent claims 2-9. In accordance with a second aspect, the present invention relates to a spectroscopy system as defined by claim 10. Particular embodiments of the spectroscopy system are defined by the dependent claims 11-13. Brief Description of the Drawings Further characteristics and advantages will be more apparent from the following description of a preferred embodiment and of its alternatives given as an example with reference to the enclosed drawings in which: Figure 1 shows a first example of an optical filter including a birefringent medium; Figure 2 schematically shows an example of polarization rotation/conversion undergone by a radiation P7493PC00 propagating along the birefringent material; Figure 3 shows an example of a transfer function of said optical filter, together with Rayleigh and Brillouin scattering peaks; Figure 4A schematically shows a further example of the optical filter employing a mirror and a polarizing beam splitter; Figure 4B schematically illustrates another example of the optical filter employing a closed feedback loop; Figure 5 shows a first example of a spectroscopy system employing said optical filter and a spectrometer; Figure 6 shows a second example of the spectroscopy system employing said optical filter and a VIPA etalon; Figures 7A and 7B refer to experimental results and show a transfer function of said optical filter; Figure 8 shows, by experimental results, the capability of said optical filter to suppress Rayleigh peak even in the case of extreme sample opacity; Figure 9 shows an example of a transfer function of said optical filter, together with Rayleigh, Brillouin and low-frequency Raman scattering peaks. DETAILED DESCRIPTION Figure 1 shows a first example of an optical filter 100 comprising a birefringent medium 1 and a first polarizer 2. The optical filter 100 can be configured to operate as a notch filter. As an example, the optical filter 100 can be employed to reject, from a first input radiation beam B1, the radiation corresponding to a Rayleigh scattering and transmit the radiation corresponding to a Brillouin scattering. Particularly, the optical filter 100 can be configured to operate in all the visible range, i.e. ^=400-700nm, and in the Near-Infrared (NIR) range, i.e. ^=700-1500nm. P7493PC00 The birefringent medium 1 comprises, at a first end, an input port IN (that can be considered the input of the filter 100) and, at a second end, an output port OU1. The birefringent medium 1, having a length L, is made by a birefringent material provided with an optical axis OA and can be a birefringent crystal (as represented in Figure 1) or a birefringent optical fiber. The birefringent crystal that can be employed for realizing the birefringent medium 1 is an anisotropic uniaxial material (e.g. Calcite, ^-BBO, YVO₄, Hg₂Cl₂) characterized by a privileged direction that is the optical axis OA. Particularly, the optical axis OA is orthogonal to the direction of propagation of the radiation. In such material, the ordinary (o) polarization of the radiation (i.e. the one normal to the plane containing the optical axis OA and the direction of propagation of the radiation) travels at speed c/no, while the extraordinary (e) polarization (i.e., parallel to the plane containing the optical axis OA and the radiation direction of propagation) travels at speed c/n_e, where n_o and n_e are the two refractive indexes and Δn = |n_o - n_e| ≠ 0 is the birefringence

coefficient. The first polarizer 2 is optically coupled to the output port OU1 of the birefringent medium 1 and has a transmission axis TA having a different orientation with respect to the optical axis OA of the birefringent medium 1. Particularly, the plane containing the optical axis OA and the direction of propagation of the radiation is rotated by 45° relative to the polarizer transmission axis TA. This type of first polarizer 2 is adapted to transmit electromagnetic radiation having polarization parallel to its transmission axis TA and block the radiation having P7493PC00 polarization perpendicular to its transmission axis TA. As an example, the first polarizer 2 can be a conventional linear thin film or a birefringent polarizer. One face of the optical polarizer 2 is oriented towards the output port OU1 of the birefringent medium 1 and the opposite face of the optical polarizer 2 defines an output OU of the optical filter 100. An example of operation of the optical filter 100 is described with reference to Figures 1, 2 and 3. The first input beam B1 enters the birefringent medium 1, consisting of at least one birefringent crystal, according to the example. The first input beam B1 comprises a first radiation R_B (in a first frequency band) having a first input linear polarization p_B1 and a second radiation R_R (in a second frequency band) having a second input linear polarization p_R1 parallel to said first input linear polarization p_B1. Particularly, first radiation R_B can be an inelastic signal (such as that generated by a Brillouin scattering on an illuminated sample) and lying in a band having center frequency f_B and center wavelength λ_B. The second radiation R_R can be an elastic signal (such as that generated by a Rayleigh scattering on the same sample) and lying in a band having center frequency f_R and center wavelength λ_R. In other words, the first radiation R_B and the second radiation R_R have, at the input port IN, different wavelengths but equal polarization state. Moreover, the optical axis OA of the birefringent material 1 is transversal (i.e. non parallel) with respect to the input linear polarizations p_B1 and p_R1. According to an example, the optical axis OA is rotated with respect to the input linear polarizations p_B1 and p_R1 by an angle α different from 0° and 90°. According to a P7493PC00 preferred example, the optical axis OA rotation angle is 45°. Moreover, considering the above situation, the first polarizer 2 is oriented so as to show the transmission axis TA rotated (particularly, orthogonal) with respect to the first input linear polarization p_R1 (or, equivalently, p_B1). The birefringent medium acts by rotating and modifying the first input polarizations p_B1 and the second input polarization p_R1. Figure 2 schematically shows, as an example, the rotations experienced by first input linear polarization p_B1 along the crystal, and the polarization state changes experienced by the second input linear polarization p_R1, which can assume, as an example, elliptical and circular polarization states. Reference is made to the configuration of Figure 1 where the optical axis OA forms an angle α of 45° with the two input linear polarizations p_B1 and p_R1. In Figure 1, the linear polarizations p_B1 and p_R1 and the optical axis OA are represented on a same plane (defined by a horizontal axis x and a vertical axis y, particularly, orthogonal to a propagation direction of the radiation). In this configuration, the ordinary and extraordinary polarization components (not represented in the drawings) of the first radiation R_B accumulate a relative phase delay φ = 2 ^/ ^ LΔn, where ^ is the wavelength ( ^ is ^_B for the first radiation R_B or ^_R for the second radiation R_R) and L the crystal thickness, as already defined. When φ = m 2 ^ (with m an integer number), the extraordinary polarization and the ordinary polarization are in phase, so that the radiation has the same linear polarization as the input one. On the other hand, when φ = (2m+1) ^, the ordinary polarization o and the extraordinary polarization e have a phase shift of 180°, so that the P7493PC00 output radiation has a polarization rotated by 90°. An analogous description is valid for the ordinary and extraordinary polarization components of the second radiation R_R. Since the first radiation R_B and the second radiation R_R have different wavelengths, it is possible to design L so that the birefringent module rotates by 90° the relative polarization state of one electric field with respect to that of the other. As a result, the first and the second radiations R_B and R_R exit the birefringent crystal 1 with orthogonal linear polarization states p_B2 and p_R2, as shown in figures 1 and 2. The above beam reaches the first polarizer 2. The first radiation R_B shows an output first linear polarization state p_B2 which is parallel to the transmission axis TA of the first polarizer 2 and therefore such radiation passes through the first polarizer 2 and is transmitted to the output OU of the optical filter 100. The second radiation R_R shows an output second linear polarization state p_R2 which is orthogonal to the second transmission axis TA of the first polarizer 2 and therefore such radiation is rejected (absorbed or reflected, according to the type of polarizer) by the first polarizer 2, and is then suppressed by the optical filter 100. With reference to the design of the birefringent medium 1 and the first polarizer 2 the following indication can be considered. The birefringent medium 1 acts such that it introduces an accumulated phase retardation φ_R to the second radiation R_R such that one of the following two conditions are satisfied: 1) φ_R = (2m+1) ^ with the first polarizer 2 having its transmission axis parallel to the second linear polarization p_R1, at the input port IN. P7493PC00 2) φ_R = m2 ^ with the first polarizer 2 having its transmission axis perpendicular to the second linear polarization p_R1, at the input port IN where m is an integer number. In order to maximize the transmission of the first radiation R_B, the relative phase retardation of the radiations R_B and R_R at the output of the birefringent medium 1 is Δφ = φ_R-φ_B = (2k+1) ^ (1) where k is an integer number. Such retardation Δφ is Δφ = 2 ^/c(f_R∆n_R - f_B∆n_B)L which, if ∆n_R = ∆n_B = ∆n, can be written as: Δφ = 2 ^/c ∆f ∆n L Where: φ_B is the phase delay for the first radiation R_B; φ_R is the phase delay for the second radiation R_R; f_B is the frequency of the first radiation R_B, f_R is the frequency of the second radiation R_R, ∆n_B is the birefringence coefficient for the first radiation R_B, ∆n_R is the birefringence coefficient for the second radiation R_R; ∆f=|f_R-f_B| is the difference between the frequency f_R and the frequency f_B. Condition (1) is obtained when the following relation is satisfied: L ∆n=(1/2+k)c/∆f (2) or equivalently L ∆n=λ²/(2 ∆λ)(1/2+k) (3) where: - L and ∆n=|n_e-n_o| have been already defined; - ∆λ =|λ_R-λ_B|, i.e. the difference between the wavelength λ_R of the second radiation R_R and wavelength λ_B P7493PC00 of the first radiation R_B; With an appropriate choice of the birefringence coefficient ∆n and/or of the length L of the birefringent medium 1, it is then possible to obtain an output state for which the relative difference between the polarizations of the first and second radiations is ideally 90°. It is noticed that the optical filter 100 above described with reference to the example of figure 1 (linear input polarizations and the particular illustrated relative orientations of the optical axis AO1 and transmission axis TA) refers to an ideal situation. In a non-ideal situation or according to another embodiment, the optical filter 100 can be so as that the birefringent medium 1 reciprocally rotates the first and second input polarizations (p_B1 and p_R1) to obtain output polarizations (p_B2 and p_R2) that are different from each other (i.e. they define different polarization states), even if they are not both linear or they are not orthogonal to each other. However, also in this further situation, the first polarizer 2 is oriented so as to reduce (i.e. suppress at least in part) the transmission of the second optical radiation R_R towards the output OU and transmit (at least in part) the first radiation R_B towards said output. Figure 3 refers to a situation where the first radiation R_B is the Brillouin scattered light and the second radiation R_R is central elastic Rayleigh peak. More particularly, Figure 3 shows the spectrum of the light scattered by a sample and composed by a central elastic Rayleigh peak P_RY with two sidebands: a Stokes inelastic Brillouin peak P_BS and an Anti-Stokes inelastic Brillouin peak P_AS. It is noticed that the Rayleigh peak P_RY is typically >10⁶ higher in intensity compared to the Brillouin signal. The dashed line represents an example of the sinusoidal P7493PC00 transmission transfer function TF of the optical filter 100. It is noticed that the crystal length L and birefringence coefficient ^n can be chosen to obtain a suitable Free Spectral Range (i.e. the distance between two transmission maxima in Figure 3) given by FSR=c/ ^nL that maximizes the transmission of the Brillouin peaks PB_{_}S and PB_AS. Figure 4A shows another embodiment of the optical filter 100 that further comprises: a second polarizer 4 (LP), an input polarizing beam splitter 20, a mirror 21 and an optional additional polarizer 22. The second polarizer 4 is optically coupled with an input of the input polarizing beam splitter 20 having a first output optically coupled with the input port IN of the birefringent medium 1 and a second output optically coupled with the additional polarizer 22. The output port OU1 of the birefringent medium 1 is optically coupled with the mirror 21. The second polarizer 4 (that can be structurally analogous to the first polarizer 2) is configured to receive a second input beam B_IN and transmit the first and second radiations R_B and R_R having parallel linear polarizations p_B1 and p_R1, correctly oriented with respect to the optical axis OA of the birefringent medium 1 (e.g. with an inclination of 45°). The input polarizing beam splitter 20 is configured to transmit the first beam B1 entering a first port 23 towards a second port 24 and transmit along different propagation directions, on the basis of polarization states, the radiation entering the second port 24. The input polarizing beam splitter 20 is a birefringent crystal polarizer, such as an example, a Glan-Thompson, Glan-Taylor or a Wollaston prism. In operation, the first and second radiations R_B and P7493PC00 R_R, entering the first port 23 of the input polarizing beam splitter 20 are transmitted towards the input port IN of the birefringent medium 1. In the birefringent medium 1, the first and second radiations R_B and R_R experience a polarization rotation/conversion as described before, exit said medium at the output port OU1, where the first and second radiations R_B and R_R show a first intermediate polarization state p_BI and a second intermediate polarization state p_RI. The first and second radiations R_B and R_R exiting the birefringent medium 1 are reflected by the mirror 21 towards the same output port OU1. The reflected radiations R_B and R_R pass again through the birefringent medium 1 and, exiting from the port IN, reach the input polarizing beam splitter 20, at a second port 24. The radiations R_B and R_R that have passed through the birefringent medium 1 for the second time, so assuming polarizations p_B2 and p_R2, respectively, enter the second port 24 of the input polarizing beam splitter 20. The input polarizing beam splitter 20 transmits the received radiations R_B and R_R along different propagation directions, on the basis of the different polarizations that they have assumed. Particularly, the second radiation R_R with second output polarization p_R2 is transmitted towards the first port 23 and the first radiation R_B with first output polarization p_B2 is transmitted towards a third port 25,(coupled with the additional polarizer 22) which may represent the output port OU of the optical filter 100. It is observed that the filtering function carried out by the first polarizer 2 of Figure 1 is carried out by the input polarizing beam splitter 20, according to the embodiment of Figure 4A. P7493PC00 It is noticed that by tuning the birefringent medium 1, it is possible to use the input polarizing beam splitter 20 as an analyzer to remove the second radiation R_R, having the same polarization as the first radiation R_B, and reflect at least part of the second radiation R_B having a polarization state different from the one of the first radiation R_R as a result of the phase delay induced by the birefringent medium 1. The additional polarizer 22 has the advantage of increasing the extinction ratio of the optical filter 100, given the typically limited extinction ratios of the polarizing beam splitters. It is noted that the configuration of figure 4A allows reducing the size of the birefringent crystal 1 by half, thus providing more flexibility to achieve the desired FSR. The fabrication of long birefringent crystals is a non-trivial task as it involves time-consuming and complex growth and cutting processes. Figure 4B schematically shows a further embodiment of the optical filter 100 that allows performing a fine tuning of the transfer function TF of said filter (as that shown in figure 3, as an example). In addition to the component shown in figure 1, the optical filter 100 as shown in Figure 4B comprises: the second polarizer 4 (optional), an adjustable retarder 5 (AR), a feedback module 6 (FB) and a polarizing beam splitter 7 (which replaces the first polarizer 2 of figure 1). The second polarizer 4 is optically coupled with the input port IN of the birefringent medium 1 and is analogous to the one described with reference to figure 4A. It is observed that the second polarizer 4 can be used also in the optical filter 100 of Figure 1, if necessary. Moreover, a half-wave plate (not shown in the figures) may P7493PC00 also be used before the second polarizer 4 to rotate the input polarization around the transmission axis of the second polarizer 4. Similarly, a quarter-wave plate may be used before the birefringent medium 1 to transform the linear polarization into an elliptical or circular polarization state. The adjustable retarder 5 is placed between the output port OU1 of the birefringent medium 1 and the polarizing beam splitter 7. The adjustable retarder 5 can be, as an example, an electrooptical retarder that introduces a phase retardation of at least 360° with the goal of maximizing the beam rejected by the polarizing beam splitter 7. Particularly, the adjustable retarder 5 can be a nematic liquid crystal, a pair of birefringent wedges or other electrically tunable birefringent crystals (e.g. Lithium niobate). The polarizing beam splitter 7 is a polarizer configured to transmit the first radiation R_B and the second radiation R_R along different propagation directions, on the basis of the different polarizations that they assume exiting the birefringent medium 1. Particularly, the rejected second radiation R_R is transmitted by the polarizing beam splitter 7 towards the feedback module 6. The feedback module 6 comprises a photodetector and a control circuit (not shown). The photodetector is configured to receive the second radiation R_R and provide a corresponding feedback signal to the control circuit. The control circuit is configured to receive the feedback (e.g. electrical) signal and produce a control signal S_C that is provided to the adjustable optical retarder 5 to adjust the variable phase delay introduced by said retarder in order to maximize the rejection of said second radiation R_R, so as to avoid that it is provided on the output OU of the P7493PC00 filter 100. The above-described closed-loop control allows tuning in real time the minimum of the sinusoidal transfer function TF at the frequency f_R, in turn stabilizing the optical filter 100 against temperature variations and light drifts. It is observed that the adjustable retarder 5 and the feedback module 6 of Figure 4 act as if they introduced a tuning of the birefringence coefficient ∆n and/or of the length L of the birefringent medium 1. This tuning allows setting the second radiation R_R in a linear polarization state so that the polarizing beam splitter 7, having the transmission axis TA crossed with respect to such polarization state, can suppress the spurious second radiation R_R while transmitting the weak first radiation R_B, i.e. enhancing its visibility with respect to the otherwise overwhelming linear background. In accordance with another embodiment, a closed-loop control analogous to the one of figure 4 is obtained without employing the adjustable retarder 5 but by using an adjustable birefringent medium 1 that is able to introduce a phase delay which can be adjusted depending on the control signal S_C. The adjustable birefringent medium 1 can operate according to an electro-optic effect or an acoustic-optic effect. With reference to all the embodiments of the optical filter 100, and as already mentioned, the birefringent medium 1 can be, as an example, a birefringent fiber. It is noticed that birefringent fibers have lower birefringence coefficients with respect to birefringent crystals. Nevertheless, it is easily possible to increase the length of the fiber to obtain the same retardation factor without compromising the filter throughput. Figure 5 describes, as an example, a first embodiment P7493PC00 of a spectroscopy system 200 employing the optical filter 100 (NF), which can be made according to any of the above described embodiments. The spectroscopy system 200 further comprises a support 8 for a sample 9 to be illuminated by a beam that can be produced by a laser 10 (LS), which can be internal or external to the spectroscopy system 200. As an example, the laser 10 is a single longitudinal mode laser with a narrow (<100 MHz) linewidth. Particularly, the laser 10 is optically coupled to the sample 9 by means of illumination optics 11 (IL), such as, for example, one or more lenses or objectives. The sample 9 can be, as an example, a cell or a tissue and is to be illuminated by a laser beam B_L produced by the laser 10. The spectroscopy system 200 may also include collection optics 12 (CL) and coupling optics 13 (CPL) which can include further lenses or mirrors. The coupling optics 13 is optically coupled to the collection optics 12 and to a first end of an optional single mode fiber 14 having a second end optically coupled to the input port IN of the optical filter 100. Preferably, the single-mode fiber 14 (when employed) is a polarization- maintaining fiber (PMF). The output of the optical filter 100 is optically coupled with a spectrometer 16 (SPM) via an interposed additional single mode fiber 15. The single-mode fibers 14 and 15 increase versatility and portability of the optical filter 100 and remove optical aberrations. Notwithstanding the above, the single-mode fibers 14 and 15 can be replaced by other optical coupling devices. In operation, the sample 9 is illuminated by the laser beam B_L and produces a scattered radiation B1 including the first radiation R_B (i.e. the inelastic scattering such as, P7493PC00 particularly, the Brillouin scattering) and the second radiation R_R (i.e. the elastic scattering – also called pump signal - such as, particularly, the Rayleigh scattering). The scattered radiation B1 is collected by the collection optics 12 and coupled into the polarization- maintaining single-mode fiber 14 using the coupling optics 13. The polarization-maintaining single-mode fiber 14 delivers the collected scattered radiation B1 to the optical filter 100 which performs, as explained before, a suppression of the second radiation R_R, i.e. the pump signal. The inelastic light R_B transmitted to the output OU is spectrally analyzed by a spectrometer 16 to measure its spectrum. It is noticed that inelastic scattering R_B contains spectral features that are indicative of the material mechanical, chemical and structural properties. As an example, Brillouin scattering arises when radiation (e.g., light) interacts with thermal density fluctuations of matter and provides information about the material viscoelastic properties. In this process, most of the light has the same frequency as the incident one. A small portion (e.g. ~10^-12) of the incident beam B_L, however, is scattered inelastically (first radiation R_B) so that the resulting spectrum contains peaks that are shifted by a few GHz from the elastic Rayleigh peak. It is noticed that the elastic background signal can be 10⁶ - 10¹² times higher than the Brillouin scattering. The frequency shift ^_B and linewidth ^_B (as can be noticed from the example of Figure 3) of the inelastic peaks P_AS and P_BS can be indicative of the stiffness and viscosity of the material of the sample 9, yet the detection of such signal is challenging due to the dominant elastic P7493PC00 background light P_RY that overwhelms the inelastic peaks P_AS and P_BS. Figure 6 shows an example of a second embodiment of the spectroscopy system 200 where the inelastic light R_B transmitted by the optical filter 100 is focused by a first lens 17 into a VIPA (Virtually Imaged Phased Array) etalon 18. A second lens 19 performs a Fourier transform of the output field, generating the spectrum at the focal plane where a CCD camera 20 acquires the signal. Experimental results Figures 7A and 7B refer to experimental results obtained using an exemplary version of the optical filter 100 having as birefringent medium 1 a YVO₄ crystal of L=32mm length. Figure 7A shows the experimental transfer function around ^=660nm. More particularly, Figure 7A illustrates the normalized intensity NI versus the wavelength ( ^) and the relative wavelength RW. The experimental transfer function exhibits the expected sinusoidal frequency dependence and associated FSR. Figure 7B summarizes the measured FSR for a range of visible and Near-IR wavelengths, demonstrating the capability of the optical filter 100 to be used in a broad spectral range. Figure 8 provides a proof-of-concept of the optical filter 100 capability to suppress the Rayleigh peak even in the case of extreme sample opacity. Different solutions of milk in water (different percentages indicated with letters A to H) have been probed in a backscattering Brillouin spectroscopy system (analogous to system 200 of figure 6) using a single-stage VIPA spectrometer as the detection unit. Letter A corresponds to pure milk (100%) P7493PC00 and letter H corresponds to pure water. The spectrometer results are diagrammed by the intensity INT (expressed in arb. u.) versus the frequency shift ^f. At increasing milk concentrations, elastic scattering increases as visually illustrated by the laser propagation inside the cuvette. On the other hand, the Rayleigh peak is almost completely suppressed by the filter up to a concentration of 0.1% (trend D), enabling the measurement of the Brillouin peaks even in pure milk sample (trend A). As a direct comparison, it was not possible to measure even the spectrum of pure distilled water without the filter, as a consequence of the specular elastic reflections of the system that overwhelm the Brillouin signal. As described above, the optical filter 100 is particularly adapted to remove the background light to simplify the detection of the Brillouin signal using standard spectrometers, enabling potential biomedical applications such as the diagnosis and monitoring of eye (e.g. glaucoma, keratoconus, presbyopia) and age-related (e.g. atherosclerosis, cancer, ALS) diseases. Notwithstanding the above, it is noticed that the optical filter 100 can be employed not only with reference to the Brillouin scattering but also for other applications. An example of a further application is the use of the optical filter 100 in low-frequency Raman spectroscopy, detecting Raman shifts in the range from 10 to 100 cm^-1, employed, particularly, to measure structural properties of matter. It is noticed that, with reference to all the embodiments described, said birefringent medium 1 can be designed to have a birefringence coefficient ^n and a P7493PC00 length L so as that the product L ^n satisfies the following relation: 10^-4 m <L ^n< 10^-1 m (4) The following relation can be considered for application of the filter 100 in Brillouin spectroscopy and microscopy: 5 10^-3 m <L ^n< 10^-1 m (5). The above relation (5) ensures that, for the case of Brillouin scattering, the elastically scattered Rayleigh component is suppressed while the inelastically scattered Brillouin light, with frequency shift Δf = 1-59 GHz (i.e. 0.1-2 cm^-1), is transmitted. It is observed that an optical filter 100 configured according to any of the embodiments herein described and according to the relation (5) can be effectively employed to operate for Brillouin scattering and/or low-frequency Raman scattering spectroscopy applications. Particularly, the Applicant has noticed that such an optical filter 100 can be used to effectively remove the elastically scattered Rayleigh from an inelastically radiation that comprises not only a Brillouin scattering radiation but also a low- frequency Raman radiation, in spite of a signal loss of approximately 3 dB for the transmitted low-frequency Raman radiation. Figure 9 schematically relates the operation of the optical filter 100 when configured in accordance with the relation (5). The central portion of Figure 9 is analogous to that shown in the above discussed Figure 3 and refers to the situation where the first radiation R_B is the Brillouin scattered radiation (GHz) and the second radiation R_R is the central elastic Rayleigh peak. The side portions of Figure 9 show the spectrum of the low-frequency (THz) Raman scattered radiation having a Stokes inelastic low-frequency Raman peak PRA-S and an P7493PC00 anti-Stokes inelastic low-frequency Raman peak PRE-AS to be transmitted. Notwithstanding the fact that the anti-Stokes inelastic low-frequency Raman peak PRE-AS has much larger frequency shift (typically 0.1 to 10 THz, or 3-300 cm^-1) than the inelastic Brillouin peaks PB-AS and PB-S (typically 3 to 30 GHz, or 0.1-1 cm^-1), the presence of the central elastic Rayleigh peak P_RY still represents a major obstacle to detect and analyze the low-frequency Raman spectral bands that have similar intensity compared to the Brillouin scattered radiation. The dashed line represents the same exemplary sinusoidal transmission transfer function TF of the optical filter 100 defined by the Free Spectral Range FSR=c/ ^nL that maximizes the transmission of the Brillouin peaks PB- S and PB-AS. Given the typically larger (>100 GHz, or >3 cm^-1) linewidths of the inelastic low-frequency Raman peaks, the sinusoidal transmission transfer function TF of the optical filter 100 acts as a high frequency intensity modulator involving a signal loss of approximately 50% but with no loss in spectral information, as schematically represented in figure 9. As a result, the optical filter 100 configured according to relation (5) can be an efficient solution to suppress the central Rayleigh peak P_RY enabling the detection of both the inelastic Brillouin and low-frequency Raman signals simultaneously, i.e. without need for a crystal length L readjustment. Alternatively, the following relation can be considered for application of said filter for low-frequency Raman: 10^-4 m <L ^n< 10^-3 m (6). With reference to all the above-described embodiments, it is observed that the birefringent crystal 1 can include two or more separated and optically coupled crystal P7493PC00 sections having a same single orientation of the optical axis, i.e. having planes containing the optical axis and the direction of propagation parallel to each other. One or more of such crystal sections can be configured to operate as a half-wave plate which rotates the linear polarizations or a quarter-wave plate which converts linearly polarized input radiation into circularly or elliptically polarized output radiation. It is noticed that the optical filter 100 shows several advantages over the prior art: - it shows a very high extinction ratio >70dB; - it is a common-path notch filter, i.e. there is no need to split the light into multiple optical paths; - it can be tuned to work at all visible and NIR wavelengths; - it can be ultracompact (L<100mm, lateral size down to less than 1cm x 1cm) involving standard polarizers; - it provides the possibility to introduce an active control loop (as shown in Figure 5) to stabilize the optical filter 100 to suppress the elastic scattering; - It introduces minimal insertion losses (<1dB) on the inelastic signal as all optical components are transparent with possibility to include an anti- reflection coating; - involving off-the-shelf polarizing components, it can be relatively inexpensive to produce. P7493PC00 Legend of the symbols of the drawings - optical filter 100 - birefringent medium 1 - first polarizer 2 - first input beam B1 - optical axis OA - input port IN - output port OU1 - output OU - transmission axis TA - first radiation R_B - first input linear polarization p_B1 - second radiation R_R - second input linear polarization p_R1 - first output linear polarization p_B2 - second output linear polarization p_R2 - second polarizer 4 - adjustable retarder 5 - feedback module 6 - polarizing beam splitter 7 - control signal S_C - spectroscopy system 200 - support 8 - sample 9 - laser 10 - illumination optics 11 - laser beam B_L - collection optics 12 - coupling optics 13 - single mode fiber 14 - additional single mode fiber 15 - spectrometer 16 - first lens 17 - VIPA etalon 18 P7493PC00 - second lens 19 - input polarizing beam splitter 20 - mirror 21 - first port 23 - second port 24 - third port 25 - first intermediate polarization state p_BI - second intermediate polarization state p_RI

Claims

P7493PC00 CLAIMS 1. Optical filter (100) configured to operate for Brillouin scattering and/or low-frequency Raman scattering spectroscopy applications comprising: an input (IN) for a beam (B1) comprising a first radiation (R_B) in a first frequency band having a first input linear polarization (p_B1) and a second radiation (R_R) in a second frequency band and having a second input linear polarization parallel (p_R1) to said first input linear polarization (p_B1); wherein the first radiation (R_B) corresponds to an inelastic scattering and the second radiation (R_R) corresponds to an elastic scattering or specular reflection; a birefringent medium (1) configured to propagate said beam (B1) and showing an optical axis (OA) having different orientation with respect to said first (p_B1) and second (p_R1) input linear polarizations; an output (OU1) for said first (R_B) and second radiations (R_R) exiting the medium (1); wherein said medium (1) is configured to reciprocally rotate the first (p_B1) and second (p_R1) input polarizations such that, at said output, the first radiation (R_B) has a first output polarization (p_B2) different from a second output polarization (p_R2) assumed by the second radiation (R_R); a first polarizer (2; 7, 20) coupled to said medium (1) and configured to transmit said first radiation (R_B) having the first output polarization (p_B2) and reject said second radiation (R_R) having the second output polarization (p_R2), wherein: said birefringent medium (1) is designed to have a birefringence coefficient ^n and a length L such that the product L ^n satisfies the following relation: 5 ^10^-3 m <L ^n< 10^-1 m. P7493PC00 2. Optical filter (100) according to claim 1, wherein said birefringent medium (1) is one of the following devices: birefringent crystal, birefringent optical fiber. 3. Optical filter (100) according to claim 1, wherein said first polarizer is one of the following devices: linear polarizer (2), polarizing beam splitter (7). 4. Optical filter (100) according to claim 1, further including a second polarizer (4) configured to receive an input beam (B_IN) and transmit said beam (B1) comprising the first and second radiations with the first and second input linear polarizations parallel to each other and rotated with respect to said optical axis (OA) of an angle different from 0° and 90. 5. Optical filter (100) according to claim 1, wherein: said first polarizer (7) is a polarizing beam splitter configured to transmit the second radiation (R_R) and the first radiation (R_B) along different propagation directions; and said optical filter (100) further comprises a feedback module (6) comprising: a photodetector configured to receive said second radiation (R_R) exiting said first polarizer (7) and provide a corresponding feedback electrical signal; a control circuit configured to receive the feedback electrical signal and produce a control signal (S_C); - an adjustable optical retarder (5) placed between said output (OU1) and the first polarizer (7) and configured to introduce a variable phase delay according to said control signal (S_C) in order to maximize the rejection of said second radiation (R_R). 6. Optical filter (100) according to claim 1, wherein said birefringent medium (1) is configured so as to cause that said first output polarization (p_B2) and said second output polarization (p_R2) are linear polarizations that are P7493PC00 orthogonal to each other. 7. Optical filter (100) according to claim 2, wherein said birefringent crystal is an anisotropic material selected from: Calcite, ^-BBO, YVO_4, Hg₂Cl₂. 8. Optical filter (100) according to claim 1, further including: a mirror (21) optically coupled to said output (OU1) to reflect back in the birefringent medium (1) said first (R_B) and second (R_R) radiations exiting the birefringent medium (1) to cause a further propagation of the first (R_B) and second (R_R) radiations in the birefringent medium (1) and obtain an output first radiation having said first output polarization (p_B2) and an output second radiation (R_R) having the second output polarization (p_R2); wherein: said first polarizer is an input polarizing beam splitter (20) optically coupled to the birefringent medium (1) to receive said output first (R_B, p_B2) and second (R_R, p_R2) radiations and separate the output first radiation from the output second radiation. 9. Optical filter (100) according to claim 1, wherein said birefringent medium (1) includes at least two separated and optically coupled sections; one or more of said separated sections are configured to operate as half-wave plate to rotates linearly polarized radiation or quarter-wave plate to converts linearly polarized radiation into circularly or elliptically polarized radiation. 10. A spectroscopy system (200) comprising: a laser source (10) configured to generate a laser beam (B_L); a support (8) configured to support a sample (98) to be irradiated by said incident beam (B_L) and generate by scattering a first radiation (R_B) and a second radiation (R_R); P7493PC00 an optical filter (100) made in accordance to at least one of the preceding claims and configured to transmit said first radiation (R_B) and reject said second radiation (R_R); a spectrometer (16) coupled to the optical filter (100) to receive and analyze said first radiation (R_B). 11. The system (200) of claim 10, wherein said a birefringent medium (1) shows a single optical axis (OA). 12. The system (200) of claim 10, wherein the first radiation (R_B) corresponds toa Brillouin inelastic scattering with a frequency shift included into the range 1-59 GHz. 13. The system (200) of claim 10, further including at least one of the following optical coupling devices: lens, mirror, single mode optical fiber, polarization-maintaining optical fiber, half-wave plate.