Optical filter and spectrometry system using the same

Abstract

There is described an optical filter (100) configured for Brillouin scattering and / or low frequency Raman scattering spectroscopy applications and comprising: an input port (IN) for a first radiation (RB) having a first input linear polarization (pB1) and a second radiation (RR) having a second input linear polarization (pR1) parallel to the first input linear polarization (pB1); a birefringent medium (1) configured to rotate a first input polarization (pB1) and a second input polarization (pR1) relative to each other such that the first radiation (RB) has a first output polarization (pB2) and the second radiation (RR) has a second output polarization (pR2) different from the first output polarization (pB2). The filter further comprises a polarizer (2; and a light source (7, 20) coupled to the birefringent medium (1) to transmit the first radiation (RB) having the first output polarization (pB2) and to remove the second radiation (RR) having the second output polarization (pR2).

Description

Technical Field

[0001] The present invention relates to optical filters, in particular notch filters, which can be used, for example, to suppress elastic background light (pump) that primarily affects inelastic optical signals in Brillouin and low-frequency Raman spectroscopy measurements. Background Art

[0002] Inelastic light scattering produces spectral signatures that are indicative of, for example, the mechanical, chemical and structural properties of a material.

[0003] For example, Brillouin scattering occurs when light interacts with thermal density fluctuations of a substance and provides information about the material's viscoelastic properties.

[0004] In this process, most of the scattered light has the same frequency as the incident light (elastic Rayleigh scattering or Fresnel reflection). However, a small portion (~10 -12 ) is scattered at a different frequency (inelastic scattering), so that the resulting spectrum also contains peaks that are offset by several GHz from the elastic Rayleigh peak.

[0005] The frequency shift and linewidth of the inelastic peak provide useful information about the material's stiffness and viscosity, but detecting this signal is challenging because the dominant elastic Rayleigh (background) light swamps the inelastic peak.

[0006] The small frequency shift and weak nature of spontaneous inelastic Brillouin scattered light typically require the use of a spectrometer with sub-GHz spectral resolution and high (>40 dB) spectral contrast, where spectral contrast is defined as the peak-to-background ratio of the spectrometer transmission function.

[0007] The multi-channel Fabry-Perot (FP) interferometer is currently the gold standard instrument for measuring high-resolution spectra in this competition, providing >150 dB (10 15 However, these interferometers require long data acquisition times (>10 seconds) due to the slow scanning mechanism required to avoid mechanical drift, which limits Brillouin spectroscopy to single-point measurements in bulk materials.

[0008] Recently, a high-throughput, non-scanning dispersive device called a Virtually Imaged Phased Array (VIPA) has been introduced. VIPA is essentially a modified version of the traditional FP interferometer, but its main advantage is the ability to rapidly detect Brillouin spectra without moving parts.

[0009] On the other hand, the single-stage VIPA spectrometer has a 30dB (10 3 ) due to the limited spectral contrast of the sample, it is impossible to detect the Brillouin spectrum of semi-transparent samples, where the intensity of Rayleigh scattering is 10 times higher than that of the Brillouin signal. 6above.

[0010] Therefore, despite its great potential and increasing demand, the use of Brillouin spectroscopy in biomedical laboratories and clinics is fundamentally restricted due to the inherent limitations of existing technologies.

[0011] The following documents describe contrast enhancement methods: US7898656B2; US20200182694A1; WO2022031815A1; US11143555B2; WO2019064093A9.

[0012] In addition to developing spectrometers with large spectral contrast, several filtering methods have been proposed to suppress the dominant elastic Rayleigh light that swamps the Brillouin signal.

[0013] Some methods involve the use of interferometric schemes, where the beam is split into two unbalanced paths to produce destructive interference. Examples of such methods are described in the following literature:

[0014] -Fiore, A., Zhang, J., Shao, P., Yun, SH, and Scarcelli, G., 2016. High-extinction virtually imaged phased array-based Brillouin spectroscopy ofturbid biological media. Applied physics letters, 108(20), p.203701.

[0015] -Lepert, G., Gouveia, RM, Connon, CJ and Paterson, C., 2016. Assessing corneal biomechanics with Brillouin spectro-microscopy. Faraday discussions, 187, pp. 415-428.

[0016] -Antonacci, G., Lepert, G., Paterson, C.and P., 2015. Elastic suppression in Brillouin imaging by destructive interference. Applied Physics Letters, 107(6), p.061102.

[0017] However, these schemes are generally unstable to temperature fluctuations and frequency shifts of the pump light, and they require constant recalibration by expert users.

[0018] Other methods, such as those described in the literature “Meng, Z., Traverso, AJ and Yakovlev, VV, 2014. Background clean-up in Brillouin microspectroscopy of scattering medium. Optics express, 22(5), pp. 5410-5415”, involve the use of an absorbing gas cell with an absorption peak tuned to the pump frequency, but they also lead to significant attenuation of the Brillouin signal due to the presence of multiple close absorption lines in the gas cell medium, which reduces the signal-to-noise ratio (SNR) and positioning accuracy of the Brillouin peak.

[0019] To address these issues, integrated optical devices have recently been proposed, as described in document WO2020084466A1: however, the extinction ratio is still limited to <10 dB.

[0020] Similar limitations are encountered in low-frequency (THz) Raman spectroscopy, a technique used to study low-frequency vibrational modes of molecules. This is in contrast to the high-frequency vibrations typically detected in the visible or near-infrared range (approximately 10 to 100 THz, or 300 to 3000 cm). -1 Unlike traditional Raman spectroscopy, low-frequency Raman spectroscopy focuses on the terahertz frequency range (about 0.1 to 10 THz, i.e. 10-300 cm -1 This technique provides valuable insights into the molecular dynamics, intermolecular interactions, and structural properties of materials, including biomolecules, polymers, and solids.

[0021] Low-frequency Raman spectroscopy is particularly useful for studying collective molecular motions and low-frequency phonons, and has unique advantages in various fields such as materials science, drug discovery, chemistry, and biophysics.

[0022] Similar to Brillouin spectroscopy, excessive elastic Rayleigh scattering is a major limitation of low-frequency Raman spectroscopy. In fact, commercial optical filters and dichroic mirrors that rely on multilayer thin films do not provide sufficiently narrow bandwidths to remove background light in the spectral region of interest. Recently, significant efforts have been made in the development of notch filters based on Bragg gratings, but these solutions generally do not provide bandwidths less than 20 cm. -1 bandwidth, making it impossible to detect low-frequency (clip) modes.

[0023] Document US8125634-B1 describes an additional filter (ASF) assembly, which includes: a light source and a collimating lens, for additional The filter provides multi-spectral light input and is used to filter out high-frequency Raman inelastic scattering.

[0024] Literature "Ivan "Birefringent Chain Filters," J. Opt. Soc. Am. 55, 621-625 (1965)" also describes filter. Summary of the Invention

[0025] The technical problem solved by the present invention is to provide an optical filter that can be used to suppress the main elastic background radiation associated with inelastic Brillouin and / or low-frequency Raman scattered radiation. It is an alternative to the filters in the prior art and at least partially overcomes the above-mentioned problems presented by the known filters.

[0026] In particular, the Applicant observed that a suitable optical filter that can serve as an effective solution for the simultaneous analysis of inelastic Brillouin scattered light and low-frequency Raman scattered light is highly desirable for simultaneously acquiring the mechanical and structural properties of materials.

[0027] According to a first aspect, the invention relates to an optical filter as defined in claim 1. Particular embodiments of the optical filter are described by the dependent claims 2-9.

[0028] According to a second aspect, the invention relates to a spectrometry system as defined in claim 10. Particular embodiments of the spectrometry system are defined by the dependent claims 11-13. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Further features and advantages will become more apparent from the following description of a preferred embodiment and its alternatives as examples with reference to the accompanying drawings, in which:

[0030] Figure 1 A first example of an optical filter including a birefringent medium is shown;

[0031] Figure 2 schematically illustrates an example of polarization rotation / conversion experienced by radiation propagating along a birefringent material;

[0032] Figure 3 Examples showing the transfer function of the optical filter and the Rayleigh and Brillouin scattering peaks;

[0033] Figure 4A Another example of an optical filter using a mirror and a polarizing beam splitter is schematically shown;

[0034] Figure 4B Another example of an optical filter employing a closed-loop feedback circuit is schematically shown;

[0035] Figure 5 A first example of a spectroscopy system employing the optical filter and spectrometer is shown;

[0036] Figure 6 A second example of a spectroscopy system employing the optical filter and the VIPA etalon is shown;

[0037] Figure 7A and Figure 7B is an experimental result showing the transfer function of the optical filter;

[0038] Figure 8 The experimental results show that the optical filter can suppress the Rayleigh peak even in the case of extremely opaque samples;

[0039] Figure 9 Examples of transfer functions of the optical filters are shown, along with Rayleigh, Brillouin, and low-frequency Raman scattering peaks. DETAILED DESCRIPTION

[0040] Figure 1 There is shown a first example of an optical filter 100 including a birefringent medium 1 and a first polarizer 2. The optical filter 100 may be configured to function as a notch filter.

[0041] For example, the optical filter 100 may be used to remove radiation corresponding to Rayleigh scattering from the first input radiation beam Bl and to transmit radiation corresponding to Brillouin scattering.

[0042] In particular, the optical filter 100 may be configured for use in all visible light ranges (ie, λ = 400-700 nm) and in the near infrared (NIR) range (ie, λ = 700-1500 nm).

[0043] The birefringent medium 1 comprises an input port IN at a first end (which can be considered as the input of the filter 100) and an output port OU1 at a second end. The birefringent medium 1 with a length L is made of a birefringent material having an optical axis OA, which can be a birefringent crystal (such as Figure 1 shown) or birefringent fiber.

[0044] Birefringent crystals that can be used to realize birefringent medium 1 are anisotropic uniaxial materials (e.g., calcite, α-BBO, YVO4, Hg2Cl2), characterized by a preferred direction of the optical axis OA. In particular, the optical axis OA is orthogonal to the propagation direction of the radiation. In such materials, the ordinary (o) polarization of the radiation (i.e., the polarization perpendicular to the plane containing the optical axis OA and the propagation direction of the radiation) is polarized at a rate of c / n opropagates, while the extraordinary (e) polarization (i.e. parallel to the plane containing the optical axis OA and the direction of radiation propagation) propagates at a speed of c / n e propagation, where n o and n e are two refractive indices, and Δn=|n o -n e |≠0 is the birefringence coefficient.

[0045] The first polarizer 2 is optically coupled to the output port OU1 of the birefringent medium 1 and has a transmission axis TA having an orientation different from the optical axis OA of the birefringent medium 1 .

[0046] In particular, the plane containing the optical axis OA and the radiation propagation direction is rotated by 45° relative to the polarizer transmission axis TA.

[0047] This type of first polarizer 2 is adapted to transmit electromagnetic radiation having a polarization parallel to its transmission axis TA and to block radiation having a polarization perpendicular to its transmission axis TA. For example, the first polarizer 2 may be a conventional linear film or a birefringent polarizer.

[0048] One surface of the optical polarizer 2 faces the output port OU1 of the birefringent medium 1, and the opposite surface of the optical polarizer 2 defines the output OU of the optical filter 100. Figure 1 、 Figure 2 and Figure 3 An operation example of the optical filter 100 is described.

[0049] According to this example, a first input beam B1 enters a birefringent medium 1 composed of at least one birefringent crystal. The first input beam B1 comprises a first input linear polarization p B1 The first radiation R B (in the first frequency band) and having the same linear polarization as the first input light p B1 The second input linear polarization parallel to p R1 The second radiation R R (in the second frequency band).

[0050] In particular, the first radiation R B It can be an inelastic signal (e.g. caused by Brillouin scattering on the illuminated sample) and is located at a center frequency f B and the central wavelength λ B within the frequency band.

[0051] Second radiation R R It can be an elastic signal (e.g. caused by Rayleigh scattering on the same sample) and is located at a center frequency f R and the central wavelength λ R within the frequency band.

[0052] In other words, the first radiation R B and the second radiation R R At the input port IN, they have different wavelengths but the same polarization state.

[0053] Furthermore, the optical axis OA of the birefringent material 1 is transverse (ie, non-parallel) to the input linear polarization p B1 and p R1 .

[0054] According to one example, the optical axis OA is relative to the input linear polarization p B1 and p R1 The rotation is by an angle α different from 0° and 90°. According to a preferred example, the optical axis OA is rotated by an angle of 45°.

[0055] Furthermore, considering the above situation, the first polarizer 2 is oriented to show relative to the first input linear polarization p R1 (or equivalently, p B1 ) is rotated (especially orthogonally) about the transmission axis TA.

[0056] The birefringent medium rotates and modifies the first input polarization p B1 and the second input polarization p R1 As an example, Figure 2 Schematically shows the first input linear polarization p B1 along the rotation experienced by the crystal, and the second input linear polarization p R1 The polarization state undergoes changes, which may take on elliptical and circular polarization states, as examples.

[0057] refer to Figure 1 The configuration in which the optical axis OA is aligned with the two input linear polarizations p B1 and p R1 Forming an angle α of 45°. Figure 1 In the linear polarization p B1 and p R1 and the optical axis OA are represented in the same plane (a plane defined by the horizontal axis x and the vertical axis y, in particular orthogonal to the radiation propagation direction). In this configuration, the first radiation R B The accumulated relative phase delay of the ordinary and extraordinary polarization components (not shown in the figure) LΔn, where λ is the wavelength (for the first radiation R B , λ is λ B , for the second radiation R R , λ is λ R ) and L is the crystal thickness, as defined above.

[0058] when When m is an integer, the extraordinary polarization and the ordinary polarization are in phase, so the output radiation has the same linear polarization as the input radiation. On the other hand, when When , the ordinary polarization o and the extraordinary polarization e have a phase shift of 180°, so the polarization of the output radiation is rotated by 90°.

[0059] A similar description applies to the second radiation R R The ordinary and extraordinary polarization components of the first radiation R B and the second radiation R R With different wavelengths, L can be designed so that the birefringence module rotates the relative polarization state of one electric field by 90° with respect to the other electric field.

[0060] As a result, the first radiation R B and the second radiation R R In orthogonal linear polarization states p B2 and p R2 Leaving the birefringent crystal 1, as Figure 1 and Figure 2 shown.

[0061] The above light beam reaches the first polarizer 2. The first radiation R B The output first linear polarization state p B2 The transmission axis TA is shown parallel to the first polarizer 2 , so this radiation passes through the first polarizer 2 and is transmitted to the output OU of the optical filter 100 .

[0062] Second radiation R R The output second linear polarization state p R2 It is shown orthogonal to the second transmission axis TA of the first polarizer 2 , so this radiation is removed by the first polarizer 2 (either absorbed or reflected depending on the type of polarizer) and then suppressed by the optical filter 100 .

[0063] With reference to the design of the birefringent medium 1 and the first polarizer 2 , the following indications may be considered.

[0064] The function of birefringent medium 1 is to transmit the second radiation R R Introducing cumulative phase delay Such that one of the following two conditions is met:

[0065] 1) At the input port IN, the transmission axis of the first polarizer 2 is parallel to the second linear polarization p R1 .

[0066] 2) At the input port IN, the transmission axis of the first polarizer 2 is perpendicular to the second linear polarization p R1 .

[0067] Where m is an integer. In order to make the first radiation R B The transmission of the birefringent medium 1 is maximized, and the output radiation R B and R R The relative phase delay is

[0068]

[0069] Where k is an integer. This delay for

[0070]

[0071] If Δn R =Δn B =Δn, then it can be written as:

[0072]

[0073] in:

[0074] is the first radiation R B Phase delay of

[0075] is the second radiation R R Phase delay of

[0076] f B is the first radiation R B frequency,

[0077] f R is the second radiation R R frequency,

[0078] Δn B is the first radiation R B The birefringence coefficient,

[0079] Δn R is the second radiation R R The birefringence coefficient of

[0080] Δf=|f R -f B | is the frequency f R and frequency f B difference.

[0081] Condition (1) is obtained when the following relationship is satisfied:

[0082] L Δn=(1 / 2+k)c / Δf (2)

[0083] or equivalently

[0084] L Δn=λ 2 / (2 Δλ)(1 / 2+k) (3)

[0085] in:

[0086] L and Δn=|n e -n o |Already defined;

[0087] Δλ=|λ R -λ B |, that is, the second radiation R R Wavelength λ R With the first radiation R B Wavelength λ B difference;

[0088] By appropriately choosing the birefringence index Δn and / or the length L of the birefringent medium 1 , an output state can be obtained in which the relative difference between the first and second radiation polarization is ideally 90°.

[0089] It should be noted that the above reference Figure 1 The optical filter 100 described by way of example (linear input polarization and specific illustrative relative orientations of the optical axis OA1 and the transmission axis TA) is an ideal case.

[0090] In a non-ideal situation or according to another embodiment, the optical filter 100 can be such that the birefringent medium 1 causes the first and second input polarizations (p B1 and p R1 ) are rotated oppositely to obtain output polarizations (p B2 and p R2 ) (i.e. they define different polarization states), even if they are not all linear or they are not orthogonal to each other.

[0091] However, in this further case, the first polarizer 2 is also oriented to reduce (ie at least partially suppress) the second optical radiation R R transmission towards the output OU and (at least partially) the first radiation R B Transmitted toward the output.

[0092] Figure 3 is the first radiation R B is the Brillouin scattered light, the second radiation R R is the case of a central elastic Rayleigh peak. More specifically, Figure 3 shows the spectrum of light scattered by the sample, which consists of a central elastic Rayleigh peak P with two sidebands RY Composition: Stokes inelastic Brillouin peak P BS and the anti-Stokes inelastic Brillouin peak P AS It is worth noting that compared with the Brillouin signal, the Rayleigh peak P RYThe intensity is usually 10 higher 6 above.

[0093] The dashed line represents an example of a sinusoidal transmission transfer function TF of the optical filter 100. It should be noted that the crystal length L and the birefringence index Δn can be selected to obtain a suitable free spectral range given by FSR=c / ΔnL (i.e. Figure 3 ), which maximizes the transmission of the Brillouin peaks PB_S and PB_AS.

[0094] Figure 4A Another embodiment of an optical filter 100 is shown, which further comprises: a second polarizer 4 (LP), an input polarizing beam splitter 20 , a mirror 21 and an optional additional polarizer 22 .

[0095] Second polarizer 4 is optically coupled to the input of input polarization beam splitter 20 having a first output optically coupled to input port IN of birefringent medium 1 and a second output optically coupled to additional polarizer 22. Output port OU1 of birefringent medium 1 is optically coupled to mirror 21.

[0096] The second polarizer 4 (which may be similar in structure to the first polarizer 2) is configured to receive the second input beam B IN , and transmits parallel linear polarization p having a correct orientation relative to the optical axis OA of the birefringent medium 1 B1 and p R1 The first and second radiation R B and R R (For example, tilted 45°).

[0097] The input polarizing beam splitter 20 is configured to transmit the first light beam B1 entering the first port 23 toward the second port 24 and transmit the radiation entering the second port 24 along different propagation directions based on the polarization state. The input polarizing beam splitter 20 is a birefringent crystal polarizer, such as a Glan-Thompson, Glan-Taylor, or Wollaston prism.

[0098] In operation, the first and second radiation R entering the first port 23 of the input polarization beam splitter 20 B and R R Transmitted toward the input port IN of the birefringent medium 1.

[0099] In the birefringent medium 1, the first and second radiation R B and R R After undergoing polarization rotation / conversion as described above, the first and second radiation R B and R R The first intermediate polarization state pBI and the second intermediate polarization state p RI .

[0100] The first and second radiation R leaving the birefringent medium 1 B and R R The light is reflected by the reflector 21 toward the same output port OU1.

[0101] Reflected radiation R B and R R The light passes through the birefringent medium 1 again, exits from the port IN, and reaches the second port 24 of the input polarization beam splitter 20 .

[0102] The radiation R that passes through the birefringent medium 1 for the second time B and R R (respectively present polarization p B2 and p R2 ) enters the second port 24 of the input polarization beam splitter 20.

[0103] The input polarization beam splitter 20 is based on the received radiation R B and R R The different polarizations present transmit them along different propagation directions.

[0104] In particular, with the second output polarization p R2 The second radiation R R transmits towards the first port 23 with a first output polarization p B2 The first radiation R B Transmitted towards the third port 25 (coupled to the additional polarizer 22 ), which may represent the output port OU of the optical filter 100 .

[0105] Observe that Figure 1 The filtering function performed by the first polarizer 2 is performed by Figure 4A The input polarization beam splitter 20 of the embodiment is implemented.

[0106] Note that by adjusting the birefringent medium 1, the input polarization beam splitter 20 can be used as an analyzer to remove the first radiation R B A second radiation R with the same polarization R , and the reflected phase retardation due to the birefringence medium 1 has the same value as the first radiation R R The second radiation R of different polarization state B at least part of.

[0107] Considering the typically limited extinction ratio of polarization beam splitters, the additional polarizer 22 has the advantage of increasing the extinction ratio of the optical filter 100. It is worth noting that Figure 4AThe configuration allows the size of the birefringent crystal 1 to be reduced by half, thus providing greater flexibility in achieving the desired FSR. The fabrication of long birefringent crystals is a difficult task as it involves time-consuming and complex growing and cutting processes.

[0108] Figure 4B Another embodiment of an optical filter 100 is schematically shown, which allows fine-tuning of the transfer function TF of the filter (e.g. Figure 3 shown as an example).

[0109] Apart from Figure 1 In addition to the components shown, Figure 4B The optical filter 100 shown further includes: a second polarizer 4 (optional), an adjustable retarder 5 (AR), a feedback module 6 (FB) and a polarization beam splitter 7 (replacing the Figure 1 The first polarizer 2 in the embodiment.

[0110] The second polarizer 4 is optically coupled to the input port IN of the birefringent medium 1 and is similar to the reference Figure 4A Description of the polarizer.

[0111] It is observed that, if desired, a second polarizer 4 may also be used Figure 1 optical filter 100. In addition, a half-wave plate (not shown) may 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 convert linear polarization into an elliptical or circular polarization state.

[0112] The adjustable retarder 5 is placed between the output port OU1 of the birefringent medium 1 and the polarization beam splitter 7. For example, the adjustable retarder 5 can be an electro-optical retarder that introduces a phase delay of at least 360° in order to maximize the light beam removed by the polarization beam splitter 7. In particular, 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).

[0113] The polarization beam splitter 7 is a polarizer configured to polarize the first radiation R B and the second radiation R R The different polarizations assumed upon leaving the birefringent medium 1 transmit them along different propagation directions.

[0114] In particular, the removed second radiation R R The polarization beam splitter 7 transmits the light toward the feedback module 6 .

[0115] The feedback module 6 includes a photodetector and a control circuit (not shown). The photodetector is configured to receive the second radiation R RThe control circuit is configured to receive the feedback (eg electrical) signal and generate a control signal S C , the control signal S C is provided to an adjustable optical retarder 5 to adjust the variable phase delay introduced by the retarder so as to maximize the second radiation R R is removed to avoid it being provided at the output OU of the filter 100.

[0116] The closed-loop control described above allows real-time adjustment of the sinusoidal transfer function TF at frequency f R The minimum value at φ , thereby stabilizing the optical filter 100 against temperature changes and light drift.

[0117] It is observed that the adjustable retarder 5 and the feedback module 6 of FIG4 act as if they introduce an adjustment of the birefringence index Δn and / or the length L of the birefringent medium 1. This adjustment allows the second radiation R R The polarization beam splitter 7 is set to a linear polarization state so that the transmission axis TA intersects this polarization state and can suppress the parasitic second radiation R R , while transmitting the weak first radiation R B , i.e. enhancing its visibility against the otherwise overwhelming linear background.

[0118] According to another embodiment, a closed-loop control similar to that in FIG. 4 is obtained by using an adjustable birefringence medium 1 capable of introducing a phase delay without using the adjustable retarder 5. The phase delay can be adjusted according to the control signal S C The tunable birefringence medium 1 can be operated according to the electro-optic effect or the acousto-optic effect.

[0119] With reference to all embodiments of the optical filter 100, as previously described, the birefringent medium 1 can be, for example, a birefringent optical fiber. Note that birefringent optical fibers have a lower birefringence coefficient than birefringent crystals. However, it is easy to increase the length of the optical fiber to achieve the same delay factor without affecting the filter throughput.

[0120] As an example, Figure 5 A first embodiment of a spectroscopy system 200 is described that employs an optical filter 100 (NF), which may be manufactured according to any of the embodiments described above.

[0121] The spectroscopy system 200 also includes a support 8 for a sample 9, which is illuminated by a beam generated by a laser 10 (LS), which can be internal or external to the spectroscopy system 200. For example, the laser 10 is a single longitudinal mode laser with a narrow (<100 MHz) linewidth. In particular, the laser 10 is optically coupled to the sample 9 via an illumination optic 11 (IL), such as one or more lenses or an objective lens.

[0122] For example, the sample 9 may be a cell or tissue, and the laser beam B generated by the laser 10 may be L Irradiation.

[0123] The spectroscopy system 200 may also include collection optics 12 (CL) and coupling optics 13 (CPL), which may include other lenses or mirrors.

[0124] Coupling optics 13 are optically coupled to collection optics 12 and a first end of an optional single mode fiber 14, a second end of which is optically coupled to input port IN of optical filter 100. Preferably, single mode fiber 14 (when employed) is a polarization maintaining fiber (PMF).

[0125] The output of optical filter 100 is optically coupled to spectrometer 16 (SPM) via an inserted additional single-mode fiber 15. Single-mode fibers 14 and 15 increase the versatility and portability of optical filter 100 and eliminate optical aberrations. However, single-mode fibers 14 and 15 can be replaced with other optical coupling devices.

[0126] In operation, the sample 9 is laser beam B L irradiation, and generates scattered radiation B1, which includes the first radiation R B (i.e. inelastic scattering, especially Brillouin scattering) and the second radiation R R (ie elastic scattering, also known as pump signal, such as in particular Rayleigh scattering).

[0127] The scattered radiation B1 is collected by collection optics 12 and coupled into a polarization-maintaining single-mode optical fiber 14 using coupling optics 13 .

[0128] The polarization-maintaining single-mode optical fiber 14 transmits the collected scattered radiation B1 to the optical filter 100. As described above, the optical filter 100 performs the second radiation R R (i.e., the pump signal) is suppressed. The inelastic light R transmitted to the output OU B The spectrometer 16 performs spectrum analysis to measure the spectrum.

[0129] It is worth noting that the inelastic scattering R BContains spectral features that indicate the mechanical, chemical, and structural properties of a material. For example, Brillouin scattering occurs when radiation (such as light) interacts with fluctuations in the thermal density of a substance and provides information about the material's viscoelastic properties. In this process, most of the light has the same frequency as the incident light.

[0130] However, the incident beam B L A small part (e.g. ~10 -12 ) is inelastically scattered (the first radiation R B ), so that the resulting spectrum contains peaks that are offset by several GHz from the elastic Rayleigh peak. It is worth noting that the elastic background signal can be 10 higher than the Brillouin scattering 6 -10 12 times.

[0131] Inelastic peak P AS and P BS Frequency shift ν B and line width Δ B (like Figure 3 The example shown in FIG) can indicate the stiffness and viscosity of the material of sample 9, but due to the main elastic background light P RY Submerge the inelastic peak P AS and P BS , detecting such signals is challenging.

[0132] Figure 6 An example of a second embodiment of a spectroscopy system 200 is shown, wherein the inelastic light R transmitted by the optical filter 100 B The light is focused by the first lens 17 into a VIPA (Virtual Imaging Phased Array) etalon 18 .

[0133] The second lens 19 performs Fourier transform on the output field, generating a spectrum at the focal plane where the CCD camera 20 acquires the signal.

[0134] Experimental results

[0135] Figure 7A and Figure 7B 1 and 2 are experimental results obtained for an exemplary version of the optical filter 100 using a YVO 4 crystal having a length of L=32 mm as the birefringent medium 1 .

[0136] Figure 7A The experimental transfer function near λ = 660 nm is shown. More specifically, Figure 7A The normalized intensity NI is shown as a function of wavelength (λ) and relative wavelength RW.

[0137] The experimental transfer function exhibits the expected sinusoidal frequency dependence and associated FSR.

[0138] Figure 7BThe measured FSRs in the visible and near-infrared wavelength ranges are summarized, demonstrating the capability of the optical filter 100 to be used over a wide spectral range.

[0139] Figure 8 A proof of concept that the optical filter 100 can suppress the Rayleigh peak even in the case of extreme sample opacity is provided. Using a single-stage VIPA spectrometer as the detection unit, a backscattering Brillouin spectroscopy system (similar to Figure 6 In the system 200 of FIG. 1 , different solutions of milk in water (denoted by letters A to H representing different percentages) are detected. Letter A corresponds to pure milk (100%) and letter H corresponds to pure water.

[0140] The spectrometer results are presented as a graph of the intensity INT (expressed in arbitrary units) versus the frequency shift Δf.

[0141] As the milk concentration increases, elastic scattering increases, as shown by the laser propagation in the test tube. On the other hand, the Rayleigh peak is almost completely suppressed by the filter down to a concentration of 0.1% (trend D), while the Brillouin peak can be measured even in pure milk samples (trend A).

[0142] As a direct comparison, without the filter, it is even impossible to measure the spectrum of pure distilled water because the specular elastic reflections of the system swamp the Brillouin signal.

[0143] As described above, the optical filter 100 is particularly suitable for removing background light to simplify Brillouin signal detection using a standard spectrometer, thereby enabling potential biomedical applications such as diagnosis and monitoring of eye diseases (e.g., glaucoma, keratoconus, presbyopia) and age-related diseases (e.g., atherosclerosis, cancer, ALS).

[0144] However, it should be noted that the optical filter 100 can be used not only for Brillouin scattering but also for other applications. Another example of application is the use of the optical filter 100 in low frequency Raman spectroscopy to detect 10 to 100 cm -1 Raman shifts in the wide range are particularly useful for measuring the structural properties of materials.

[0145] It should be noted that, with reference to all the embodiments described, the birefringent medium 1 can be designed to have a birefringence index Δn and a length L such that the product LΔn satisfies the following relationship:

[0146] 10 -4 m <L Δn< 10 -1 m (4)

[0147] For applications of the filter 100 in Brillouin spectroscopy and microscopy, the following relationship may be considered:

[0148] 5×10 -3 m <LΔn<10 -1 m(5)

[0149] The above relation (5) ensures that in the case of Brillouin scattering, the Rayleigh component of elastic scattering is suppressed, while the frequency shift Δf = 1-59 GHz (i.e. 0.1-2 cm -1 ) is transmitted.

[0150] It is observed that the optical filter 100 according to any of the embodiments described herein and configured according to relation (5) can be effectively used for Brillouin scattering and / or low-frequency Raman scattering spectroscopy applications. In particular, the applicant has noted that, despite a signal loss of approximately 3 dB for the transmitted low-frequency Raman radiation, such an optical filter 100 can be used to effectively remove elastic Rayleigh scattering from inelastic radiation that includes not only Brillouin scattering radiation but also low-frequency Raman radiation.

[0151] Figure 9 The operation of the optical filter 100 when configured according to relation (5) is schematically illustrated. Figure 9 The central part is similar to the one discussed above. Figure 3 The part shown is the first radiation R B is the Brillouin scattered radiation (GHz), the second radiation R R This is the case of a central elastic Rayleigh peak.

[0152] Figure 9 The two side parts of show the spectrum of low frequency (THz) Raman scattered radiation, which has a Stokes inelastic low frequency Raman peak PRA-S and an anti-Stokes inelastic low frequency Raman peak PRE-AS to be transmitted.

[0153] Although the frequency shift of the anti-Stokes inelastic low-frequency Raman peak PRE-AS (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 ) is much larger, but the central elastic Rayleigh peak P RY The presence of is still the main obstacle to the detection and analysis of low-frequency Raman spectral bands with similar intensity to Brillouin scattered radiation.

[0154] The dashed line represents the same exemplary sinusoidal transmission transfer function TF of the filter 100 defined by the free spectral range FSR=c / ΔnL, which maximizes the transmission of the Brillouin peaks PB-S and PB-AS. Considering that the line width of the inelastic low frequency Raman peak is usually large (>100 GHz or >3 cm -1), the sinusoidal transmission transfer function TF of the optical filter 100 acts as a high-frequency intensity modulator, involving a signal loss of about 50%, but no loss of spectral information, as Figure 9 Shown schematically.

[0155] As a result, the optical filter 100 configured according to the relation (5) can be a filter that suppresses the central Rayleigh peak P RY This is an effective solution that can simultaneously detect inelastic Brillouin signals and low-frequency Raman signals, i.e., there is no need to readjust the crystal length L.

[0156] Alternatively, for the application of the filter to low-frequency Raman, the following relationship can be considered:

[0157] 10 -4 m <LΔn<10 -3 m(6)

[0158] With reference to all of the above-described embodiments, it is observed that the birefringent crystal 1 can include two or more separate and optically coupled crystal portions whose optical axes have the same single orientation, i.e., have planes containing the optical axis and the propagation direction parallel to each other. One or more of such crystal portions can be configured to operate as a half-wave plate for rotating linear polarization or a quarter-wave plate for converting linearly polarized input radiation into circularly polarized or elliptically polarized output radiation. Notably, the optical filter 100 exhibits several advantages over the prior art:

[0159] - It shows a very high extinction ratio >70dB;

[0160] - It is a common path notch filter, i.e. it does not need to split the light into multiple light paths;

[0161] -It can be tuned to operate at all visible and near-infrared wavelengths;

[0162] - It can be ultra-compact (L < 100 mm, lateral dimensions less than 1 cm × 1 cm), involving standard polarizers;

[0163] - It provides the opportunity to introduce active control loops (such as Figure 5 ) to stabilize the optical filter 100 to suppress the possibility of elastic scattering;

[0164] - It introduces minimal insertion loss (<1dB) on inelastic signals because all optical components are transparent and can include anti-reflection coatings;

[0165] - Involves readily available polarization components, which can be produced at relatively low cost.

[0166] Reference Numeral Legend—Optical Filter 100

[0167] -Birefringent medium 1

[0168] -First polarizer 2

[0169] -First input beam B1

[0170] -Optical axis OA

[0171] -Input port IN

[0172] -Output port OU1

[0173] -Output OU

[0174] -Transmission axis TA

[0175] -First Radiation R B

[0176] -The first input linear polarization p B1 -Second Radiation R R

[0177] -The second input linear polarization p R1

[0178] -The first output linear polarization p B2

[0179] -The second output linear polarization p R2

[0180] - Second polarizer 4

[0181] -Adjustable delay 5

[0182] -Feedback Module 6

[0183] -Polarization beam splitter 7

[0184] -Control signal S C

[0185] -Spectroscopy System 200

[0186] -Support 8

[0187] -Sample 9

[0188] -Laser 10

[0189] - Illumination optics 11

[0190] -Laser beam B L

[0191] - Collection optics 12

[0192] - Coupling optics 13

[0193] -Single mode fiber 14

[0194] - Additional single mode fiber 15

[0195] -Spectrometer 16

[0196] -First lens 17

[0197] -VIPA etalon 18

[0198] -Second lens 19

[0199] - Input polarization beam splitter 20

[0200] -Reflector 21

[0201] -First port 23

[0202] - Second port 24

[0203] -Third port 25

[0204] - the first intermediate polarization state p BI - the second intermediate polarization state p RI

Claims

1. An optical filter (100) configured for use in Brillouin scattering and / or low-frequency Raman scattering spectroscopy applications, the optical filter (100) comprising: An input port (IN) for a light beam (B1) comprising a first input linear polarization (p B1 )'s first radiation (R B ) and a second frequency band having a polarization parallel to the first input line (p B1 ) of the second input linear polarization (p R1 )'s second radiation (R R );in The first radiation (R B ) corresponds to inelastic scattering, the second radiation (R R ) corresponds to elastic scattering or specular reflection; a birefringent medium (1) configured to propagate the light beam (B1) and having an optical axis (OA) exhibiting an angular polarization with respect to the first input linear polarization (p B1 ) and the second input linear polarization (p R1 ) have different orientations; For the first radiation (R B ) and the second radiation (R R ) exits the output port (OU1) of the medium (1); wherein the medium (1) is configured to cause the first input linear polarization (p B1 ) and the second input linear polarization (p R1 ) are rotated relative to each other so that at the output port, the first radiation (R B )’s first output polarization (p B2 ) is different from the second radiation (R R ) presents the second output polarization (p R2 ); A first polarizer (2; 7; 20) coupled to the medium (1) and configured to transmit a first output polarization (p B2 ) of the first radiation (R B ) and remove the second output polarization (p R2 ) of the second radiation (R R ), in: The birefringent medium (1) is designed to have a birefringence index Δ n and length L, so that the product LΔ n The following relationship is satisfied: 5×10 -3 m<LΔ n <10 -1 m。 2. The optical filter (100) according to claim 1, wherein the birefringent medium (1) is one of the following devices: a birefringent crystal, a birefringent optical fiber.

3. The optical filter (100) according to claim 1, wherein the first polarizer is one of the following devices: a linear polarizer (2), a polarization beam splitter (7).

4. The optical filter (100) according to claim 1, further comprising a second polarizer (4), wherein the second polarizer (4) is configured to receive the input light beam (B IN ) and transmits the light beam (B1), wherein the light beam (B1) includes first input linear polarization and second input linear polarization of first radiation and second radiation that are parallel to each other and rotated at angles different from 0° and 90° relative to the optical axis (OA).

5. The optical filter (100) according to claim 1, wherein: The first polarizer (7) is a polarizing beam splitter configured to transmit the second radiation (R R ) and the first radiation (R B );and The optical filter (100) further comprises a feedback module (6), wherein the feedback module (6) comprises: a photodetector configured to receive the second radiation (R R ) and provide corresponding feedback electrical signals; A control circuit configured to receive the feedback electrical signal and generate a control signal (S C ); An adjustable optical retarder (5) is located between the output port (OU1) and the first polarizer (7) and is configured to adjust the optical retarder according to the control signal (S C ) introduces a variable phase delay to maximize the second radiation (R R ) removal.

6. The optical filter (100) according to claim 1, wherein the birefringent medium (1) is configured to make the first output polarization (p B2 ) and the second output polarization (p R2 ) are linear polarizations that are orthogonal to each other.

7. The optical filter (100) according to claim 2, wherein the birefringent crystal is an anisotropic material selected from calcite, α-BBO, YVO4, and Hg2Cl2.

8. The optical filter (100) according to claim 1, further comprising: A reflector (21) optically coupled to the output port (OU1) for reflecting the first radiation (R B ) and the second radiation (R R ) is reflected back into the birefringent medium (1) so that the first radiation (R B ) and the second radiation (R R ) further propagates in the birefringent medium (1) and obtains the first output polarization (p B2 ) of the output first radiation and having the second output polarization (p R2 ) output second radiation (R R ); in: The first polarizer is an input polarization beam splitter (20) optically coupled to the birefringent medium (1) for receiving the output first radiation (R B , p B2 ) and the output second radiation (R R , p R2 ), and separating the output first radiation from the output second radiation.

9. An optical filter (100) according to claim 1, wherein the birefringent medium (1) comprises at least two optically coupled separate parts; one or more of the separate parts is configured to act as a half-wave plate to rotate linearly polarized radiation, or as a quarter-wave plate to convert linearly polarized radiation into circularly polarized radiation 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 (9) to be incident upon the light beam (B L ) and generates the first radiation (R B ) and the second radiation (R R ); An optical filter (100) made according to at least one of the preceding claims and configured to transmit said first radiation (R B ) and remove the second radiation (R R ); A spectrometer (16) coupled to the optical filter (100) for receiving and analyzing the first radiation (R B ).

11. The system (200) according to claim 10, wherein the birefringent medium (1) exhibits a single optical axis (OA).

12. The system (200) according to claim 10, wherein the first radiation (R B ) corresponds to Brillouin inelastic scattering with a frequency shift in the range of 1-59 GHz.

13. The system (200) according to claim 10, further comprising at least one of the following optical coupling devices: a lens, a mirror, a single-mode optical fiber, a polarization-maintaining optical fiber, a half-wave plate.

Images