Optical filter for spectroscopy

Abstract

An optical filter (1), in particular Mach-Zehnder interferometer, for uppressing or attenuating the Rayleigh elastic component of scattered or reflected light, said filter (1) being integrable within an active and / or passive photonic circuit and being positionable between a scattered light source (2) and a spectral analyser (3). The filter (1) comprises: at least one input (4; 52) for receiving the scattered light; at least one partitioning element (5) for dividing the scattered light into a first beam of light (6) travelling along a first path (7) and into a second beam of light (8), separated from the first beam of light (6), travelling along a second path (9), wherein the difference between the first path (7) and the second path (9), defined as the path difference (ΔL), is non-zero; at least one phase modulator (13) positioned at the first and / or second path (7, 9; a closed-loop control system (14) coupled to the phase modulator (13); a combiner element (10) for combining the first beam of light (6) output from the first path (7) with the second beam of light (8) output from the second path (9), and a first output (11) and a second output (12) coupled to the combiner element (10), wherein the first output (11) is configured to propagate a first output signal (15) where the Rayleigh elastic component of the scattered light is attenuated or suppressed as a result of a destructive interference of said Rayleigh elastic component between the first beam of light (6) and the second beam of light (8).

Description

[0001] Optical filter for spectroscopy

[0002] TECHNICAL FIELD

[0003] The present invention relates to an optical filter, in particular a Mach-Zehnder interferometer, for suppressing or attenuating the Rayleigh elastic component of scattered or reflected light . Said optical filter is integrable into at least one photonic circuit . Furthermore , the present invention relates to a spectrometer comprising said filter and an apparatus for Brillouin spectroscopy, microscopy or endoscopy comprising said spectrometer .

[0004] BACKGROUND ART

[0005] Brillouin spectroscopy is a known technique for optically measuring the mechanical properties of matter . Brillouin light scattering is an inelastic scattering process resulting from the interaction of light with spontaneous acoustic waves of matter ( acoustic phonons ) . Elastically scattered light (Rayleigh) from a biological system has the same frequency as the illumination beam . However, the spectrum shows two additional sidebands , usually referred to as Brillouin Stokes ( SB ) peaks and Brillouin Anti-Stokes (ASB ) peaks , slightly shi fted by 1-20 GHz from the Rayleigh frequency .

[0006] The frequency and line width of Brillouin peaks are indicative of the viscoelastic properties of a sample . In particular, the density and refractive index of a material being known, the frequency shi ft and the Brillouin line width of fer information on the elastic moduli characteri zing the illuminated material . Standard methods for measuring the mechanical properties of a material require physical contact with the sample , thus making these techniques invasive and limited to the topographic surface . In contrast , Brillouin microscopy or endoscopy uses light as a probe in order to avoid any form of contact . Brillouin spectroscopy probes the mechanical properties with an optical resolution below the micron (micrometer ) and, applied to microscopy, can perform a three- dimensional analysis of the volume of biological systems ( e . g . cells and tissues ) .

[0007] Since biomechanical properties are critically involved in various forms of pathologies such as atherosclerosis , cancer and glaucoma, for example , Brillouin spectroscopy has great potential to become a new diagnostic tool for the biomedical sector .

[0008] The main component of Brillouin spectroscopy, microscopy or endoscopy is the spectrometer which requires both sub-GHz spectral resolution and high spectral contrast . A high spectral contrast is for example required to measure opaque biological samples , where the elastically scattered light and reflections are several orders of magnitude higher than the weak Brillouin light signal . When the amount of elastic background light exceeds the contrast of the spectrometers , parasitic crosstalk signals emerge along the scattering axis , which obscure the less intense Brillouin peaks . As described by S . M . Lindsay et al . J. Phys . E 10, 150 (1977) , in Brillouin spectroscopy the sample is illuminated by a laser source and the scattered light is analysed spectrally using multiple Fabry-Perot interferometers ( FP ) positioned in tandem . Although FP interferometers and " etalons" have considerable contrast and spectral resolution, these usually require a long integration time ( dwell time ) ( > 1 sec ) which results in Brillouin spectroscopy limited to single point measurements , thus not allowing extension to an imaging technique .

[0009] In recent decades , a new type of etalon FP interferometer, known as the Virtual Imaged Phased Array (VIPA) , has been introduced . The use of VIPA spectrometers in Brillouin spectroscopy has decreased the data acquisition time from tens of seconds to about 100 msec using low optical power sources (< 10 mW) , while at the same time ensuring the use of Brillouin spectroscopy in a three-dimensional non-contact imaging mode to map the viscoelastic properties in the volume of biological systems . Although very ef ficient , conventional VIPA spectrometers are inherently limited by a spectral contrast of about 30 dB . To increase the spectral contrast in Brillouin microscopy, there are currently methods using di f ferent VIPA etalons (multi-stage VIPA) placed in cascade with each other [ G . Scarcelli et al . Opt . Express 19, 10913 (2011 ) ] . However, this results in a reduced ef ficiency of about 25% with associated increased image acquisition time . In addition, VIPA cascade spectrometers require a high number of optical components that make these systems expensive and di f ficult to align .

[0010] In addition, the attention necessitated by the manufacturing tolerance related to the parallelism and uni formity of the optical surfaces ( typically higher than 100X) requires high production costs . Although considerable ef forts have been made to employ Brillouin spectrometers , they still lack suf ficient spectral contrast to measure highly heterogeneous and opaque samples such as human tissues . To overcome these disadvantages , many filtering methods have been proposed in recent years with the aim of suppressing the elastic background light . However, the currently available solutions still rely on bulky components and wide interferometers which are particularly sensitive to the temperature and frequency shi ft of the laser, thus necessitating continuous realignment and recalibration . Recently, progress has been made in the area of stimulated Brillouin scattering in order to speed up data acquisition . However, this approach requires the use of high power lasers that are not practical in a context of in-vi vo measurements and clinical applications .

[0011] Document CN 105588587 B discloses a method and a device for extracting a spontaneous Brillouin scattering signal . In particular, a Mach-Zehnder interferometer consisting of the coupling of the ends of two optical fibres is used to attenuate the Rayleigh elastic component . However, the device described in this document is essentially employed for a sensing technique and is not suitable to be used as an optical filter for Brillouin spectroscopy, for example in a photonic integrated circuit .

[0012] It is an obj ect of the present invention to partially or totally overcome the above-mentioned drawbacks of the known systems and to provide optical devices , such as for example optical filters or spectrometers comprising said optical filters , which improve the suppression of the elastic component of the scattered light , which reduce the dependence of the performance on the dimensions of the structure made and which can automatically and in real time adapt the working point to environmental or signal variations . In particular, it is an obj ect of the present invention to provide optical devices that are capable of taking measurements on biological materials ( and possibly on non-biological materials ) with Brillouin spectroscopy, microscopy or endoscopy, from which it is possible to obtain mechanical or structural properties on the analysed samples .

[0013] DISCLOSURE OF THE INVENTION

[0014] Presented herein is an optical filter, a spectrometer comprising said optical filter and an apparatus comprising the spectrometer according to the independent claims . Embodiments of the filter and spectrometer are disclosed in the corresponding dependent claims .

[0015] In a first aspect of the invention, there is provided an optical filter for suppressing or attenuating the Rayleigh elastic component of scattered or reflected light , particularly employed for Brillouin spectroscopy . The optical filter is positionable between a scattered light source and a spectral analyser . The optical filter is integrable into an active and / or passive photonic circuit .

[0016] The optical filter comprises : at least one input for receiving scattered or reflected light ; at least one partitioning element for dividing the scattered light into a first beam of light travelling along a first path and a second beam of light , separated from the first beam of light , travelling along a second path, wherein the di f ference between the first path and the second path, defined as the path di f ference , is non-zero; at least one phase modulator positioned at the first and / or second path; a closed-loop control system coupled to the phase modulator ; a combiner element for combining the first beam of light output from the first path with the second beam of light output from the second path, and a first output and a second output coupled to the combiner element , wherein the first output is configured to propagate a first output signal where the Rayleigh elastic component of the light spectrum is attenuated or suppressed following destructive interference of said Rayleigh elastic component between the first beam of light and the second beam of light .

[0017] As mentioned above , the optical filter can advantageously be positioned between a scattered light source and a spectral analyser . However, its operation is independent of its position and is determined by its basic components .

[0018] Speci fically, the optical filter comprises : at least one input for receiving scattered or reflected light containing a Brillouin component and a Rayleigh elastic component ; at least one partitioning element for dividing the scattered light into a first beam of light travelling along a first path and a second beam of light , separated from the first beam of light , travelling along a second path, wherein the di f ference between the first path and the second path, defined as the path di f ference ( AL ) , is non-zero; at least one phase modulator positioned at the first and / or second path; a closed-loop control system coupled to the phase modulator ; a combiner element for combining the first beam of light output from the first path with the second beam of light output from the second path, and a first output and a second output coupled to the combiner element , wherein the first output is configured to propagate a first output signal where the Rayleigh elastic component of the light spectrum is attenuated or suppressed following destructive interference of said Rayleigh elastic component between the first beam of light and the second beam of light .

[0019] The optical filter can be a Mach-Zehnder interferometer (MZ I ) . In particular, the filter may consist of a single MZ I or of a plurality of MZ I s connected to each other, for example in series .

[0020] The use of an optical filter consisting of at least one MZ I improves the characteristics of an optical system of which it can be a part . In fact , the optical filter increases the degree of suppression of the elastic component and makes the manufacture of the system of which it is a part less complex and less expensive . In other words , this optical filter allows the use of a small spectral analyser, thus increasing the compactness of the total optical system .

[0021] An optical filter that uses at least one MZ I typically achieves 10 times greater performance in terms of extinction than a di f ferent optical filter, which uses for example optical ring resonators , thanks to the possibility of a 50%-50% balance of the amplitude of the input light on the two filter paths . As structurally configured, a ring resonator that functions as a filter has a very high margin of error by de fault due to the exponential trend of the degree of extinction as a function of the separation between the input waveguide and the ring resonator . This is reflected in a lower extinction than the optical filter using an MZ I . Furthermore , an optical filter consisting of at least one MZ I is more easily reproducible than a ring resonator filter since the latter is subj ect to a lower manufacturing reproducibility due to the limited resolution of the lithography process .

[0022] The MZ I filter also of fers the possibility of being used as a dynamic adaptive " closed-loop" filter, i . e . it allows the suppression peak to be automatically and continuously calibrated to the Rayleigh wavelength .

[0023] In particular, the two outputs of the optical filter allow the use of corresponding output signals that can be used for closed-loop control , solving the technical problem of aligning and maintaining the filter suppression bandwidth at the frequency of the light source that produces the scattered light ( e . g . the laser source ) . This leads to the following advantages : reduction of noise due to scattered Rayleigh light and specular reflections and thus a higher signal-to-noise of Brillouin peaks ; reduction of errors by automatically adj usting the system inputs , thus being able to contain overshooting; improving the stability of an unstable system due to laser frequency dri fts or thermal variations in the surrounding environment ; change in the sensitivity of the system; increased robustness with respect to disturbances external to the system; and reliable and reproducible performance .

[0024] Because the path di f ference is non- zero , within this filter it is possible to create an optical path di f ference between the two beams of light travelling within it . As the optical path difference increases, the so-called free spectral range (FSR) of the optical apparatus decreases. This is particularly advantageous in the case of Brillouin spectroscopy whose typical FSR values vary between 10 and 50 GHz. For this reason, the difference between the first path and the second path, that is, the path difference, of this optical filter is advantageously between 5 mm and 30 mm. Of course, other values, outside this range, may also be used for the path difference.

[0025] By using at least one phase modulator within at least one of the two paths, it is possible to modulate or adjust the position of the interference by acting on the voltage applied to the modulator. A shift of n is sufficient to ensure an adjustment range equivalent to a single FSR.

[0026] Therefore, this optical filter is particularly advantageous for being employed for Brillouin spectroscopy and for effectively suppressing the Rayleigh elastic component.

[0027] As mentioned above, the optical filter can be integrated within an active and / or passive, e.g. nanometric, photonic circuit. The photonic circuit can also be combined with an electronic circuit .

[0028] In one embodiment, the optical filter may be fully integrated within said active and / or passive photonic circuit. In one example, the circuit may be characterized by waveguides whose refractive index of the inner "core" material ( Sis^-silicon nitride) is greater than that of the cladding material (SiC>2- silicon oxide) . All the features described below relating to the optical filter that is integrable into the photonic circuit also apply to the optical filter directly integrated into said circuit . Therefore , unlike devices known in the literature , such as for example the device described in CN 105588587 B mentioned above which is based on the use of optical fibres , the optical filter according to the present invention is completely integrable ( or is integrated) in an ultra-compact silicon photonic chip making the final device more robust and stable with regard to mechanical movements and thermal fluctuations .

[0029] It is noted that although the photonic integrated circuits and the optical fibres are both waveguides , their operating principle is completely di f ferent . In fact , while the former are based on Maxwell ' s equations with the fundamental optical mode not strictly confined within the centre ( " core" ) of the waveguide , the optical fibres are instead based on a total internal reflection in which the optical mode is strictly confined within the " core" of the waveguide . Given the nature of optical fibres and their limitation in being able to achieve bend radii of less than a few tens of centimetres without the risk of a loss of ef ficiency or the breakage of the optical fibres themselves , devices using optical fibres are much more bulky, fragile and unstable than integrated photonic circuits .

[0030] Also with regard to the operation of the Mach-Zehnder interferometer, the optical filter according to the present invention behaves di f ferently from devices known in the literature . For example , again with reference to the aforementioned document CN 105588587 B, the two optical fibre paths representing the arms of the MZ I are initially identical . A phase modulation takes place through a piezoelectric element that in a sense " stretches" one of the two optical fibres . The first and the second path of the optical filter according to the present invention are instead di f ferent in length from the beginning and determine a path di f ference that always remains the same . In this case , a phase modulator ( e . g . a thermal resistor ) is used to vary the refractive index ( and not the length which is already di f ferent ) of the waveguide to obtain destructive interference of the Rayleigh component .

[0031] The device described in CN 105588587 B instead uses an optical isolator to block the Rayleigh light and provides a backward path of the Brillouin signal which therefore enters the MZ I twice , thus preventing a possible tandem or cascade configuration of several MZ I s . It is noted that no optical isolators are known in the literature that can be made by means of integrated photonic circuits .

[0032] In a second aspect of the invention there is provided a spectrometer for analysing the spectrum of scattered light . The spectrometer comprises : a main input for receiving scattered light ; at least one optical filter for suppressing or attenuating the Rayleigh elastic component of scattered or reflected light according to the first aspect of the invention, wherein said filter is coupled to the main input of the spectrometer ; and a spectral analyser for selecting and separating certain multiple frequency components of the scattered light , for measuring the intensity of the di f ferent frequency components and reconstructing the spectrum profile of the scattered light , wherein said spectral analyser is coupled to the first output of the filter, wherein at least the filter is constituted by a photonic integrated circuit .

[0033] By using the optical filter according to the present invention within a spectrometer, it is possible to suppress the Rayleigh elastic component of the scattered light more ef fectively and without the need for optical components . In particular, it is possible to couple the filter to a spectral analyser by increasing the signal-to-noise ratio of the Brillouin peaks .

[0034] In a third aspect of the invention there is provided an apparatus or equipment comprising the spectrometer according to the second aspect of the invention .

[0035] In particular, such an apparatus is suitable for Brillouin spectroscopy, microscopy or endoscopy . The apparatus may therefore further comprise a light source such as a laser in the visible field ( e . g . a 400- 650 nm laser ) or in the near infrared ( e . g . a 650- 1500 nm laser ) and a target of biological material . The light scattered by the interaction between laser and target is thus conducted in the spectrometer through the optical filter and is collected by the spectral analyser . The apparatus may further comprise one or more electronic calculators connected to the spectral analyser and / or the optical filter for managing the spectral analysis , acquiring the detected data, analysing it and displaying the results contained in the light spectrum information .

[0036] In a fourth aspect of the invention there is provided a filter module comprising a plurality of optical filters according to the first aspect connected to each other having a module input for receiving the scattered light , a first module output and a second module output , wherein the first output of a first filter of the plurality of filters is coupled to the input of a subsequent second filter . Advantageously, each filter of the plurality of filters is coupled to the next filter in such a way that only one output of said filter is coupled to only one input of the next filter . Preferably, the only filter output connected to the next filter input is positioned at the " cross" channel . The filter module is integrable into an active and / or passive , e . g . nanometric, photonic circuit . Alternatively, the filter module can be fully integrated into said active and / or passive photonic circuit .

[0037] Such a filter module serves to improve the cutting of the Rayleigh band from the scattered light spectrum . It is noted that , in general , with a plurality of optical filters according to the present description connected in series the transmission function remains the same . However, the attenuation ef fect of one of the components of the scattered light ( e . g . Rayleigh component ) is greater . In particular, the filter module described herein can generally be used when two signals are close to each other in terms of frequency but have di f ferent intensity . An optical filter as defined in this description, but even more a plurality of filters forming a filter module , is able to separate the two signals and to more ef fectively suppress one of said signals ( the one with the highest intensity) .

[0038] In a fi fth aspect of the invention, there is provided a system comprising the optical filter according to the first aspect or the filter module according to the fourth aspect coupled with at least one optical ring resonator .

[0039] In a further aspect of the invention there is provided the use of an optical integrated circuit comprising at least one filter according to the first aspect for suppressing or attenuating the Rayleigh elastic component of a scattered or reflected light , in particular for spectral analysis of opaque or semitransparent samples , or the use of an optical integrated circuit comprising a spectrometer according to the second aspect , or an apparatus according to the third aspect , for analysing the spectrum of scattered light , in particular for spectral analysis of opaque or semi-transparent samples .

[0040] These and other aspects of the present invention will become more apparent by reading the following description of some preferred embodiments disclosed below .

[0041] BRIEF DESCRIPTION OF THE FIGURES

[0042] Fig . 1 shows a schematic representation of the apparatus comprising the spectrometer and the optical filter according to one example .

[0043] Fig . 2 shows a schematic representation of the optical filter according to one example .

[0044] Fig . 3a-b show a schematic representation of two optical filters according to one example .

[0045] Fig . 4a-b show the operational principle of a Mach-Zehnder interferometer .

[0046] Fig . 5 shows a schematic representation of an optical filter according to one example .

[0047] Fig . 6 shows a schematic representation of an optical filter with a MIMO controller according to one example .

[0048] Fig . 7a-b show a schematic representation of a spectral analyser and its operation according to one example .

[0049] Fig . 8 shows a schematic representation of a spectral analyser according to one example .

[0050] Fig . 9 shows a schematic representation of a spectral analyser according to another example .

[0051] DETAILED DESCRIPTION OF SOME ILLUSTRATIVE PREFERRED

[0052] EMBODIMENTS Figure 1 shows a schematic drawing of an apparatus 37 for spectroscopy . The apparatus 37 may for example be a spectroscopy apparatus , microscopy apparatus , or Brillouin endoscopy apparatus , or an apparatus using a similar technique . The apparatus 37 may comprise a light source , for example a narrowband (<100 MHz ) longitudinal single mode laser 38 directed at a sample 40 ( target material ) through an illumination optics 39 . The sample 40 may be biological material . The laser beam light interacts with acoustic phonons and exchanges energy resulting in a scattered light signal that exhibits minimal deviation from the laser frequency ( 5- 30 GHz ) . Following the interaction between the laser beam and the sample 40 , the sample thus becomes a scattered light source 2 . The light scattered from the sample 40 is collected by a collection optics 41 and, via a fibre optic coupler 42 and a single-mode optical fibre 43 , is directed to a spectrometer 20 . The spectrometer 20 receives the scattered light through a main input 21 at which there are coupling means 44 for establishing a coupling with the fibre optical coupler 42 and the single-mode optical fibre 43 . The spectrometer 20 comprises an optical filter 1 for suppressing or eliminating some components of the scattered light ( e . g . the Rayleigh elastic component ) and a spectral analyser 3 for reconstructing and analysing the spectrum of scattered light . The spectral analyser 3 may advantageously have a resolution below GHz ( sub- GHz ) . By means of a computer system 51 , coupled for example to the spectrometer 20 , the acquired data can be analysed and appropriately displayed . It can be seen that the spectrometer 20 comprising the optical filter 1 and the analyser 3 can be fully integrated into a photonic chip e . g . made of silicon . For example , the light signal that is analysed spectrally can travel through waveguides with nanometric dimensions and a high refractive index contrast between the core material and the cladding material of the waveguide to allow small bend radii , for example between 0 . 1 mm and 2 mm . The light signal to be analysed can be coupled into the photonic chip via a single mode polari zation-maintaining optical fibre and a beam si ze converter ( spot-si ze converter ) such as a microlens , grid or optical limiter ( inverted taper ) . The photonic chip may be mounted and connected via external circuitry to a printed circuit to facilitate electrical adj ustment of the phase modulator 13 .

[0053] Figure 2 shows in detail the elements of the optical filter 1 positioned between the scattered light source 2 and the spectral analyser 3 . The filter may advantageously be integrated into a nanometric photonic circuit . The filter 1 comprises at least one input 4 , 52 for receiving the scattered light from the scattered light source 2 and two outputs 11 , 12 . In particular, the input 4 , 52 is positioned on one end of the filter 1 ( first end) and the two outputs 11 , 12 are positioned on the other end of the filter 1 , i . e . on the second end, which is opposite to the first end; a first output 11 configured to propagate a first output signal 15 and a second output 12 configured to propagate a second output signal 16 . The filter 1 further comprises a partitioning element 5 for dividing the scattered light into a first beam of light 6 which travels along a first path 7 and into a second beam of light 8 which travels along a second path 9 . The first beam of light 6 is then separated from the second beam of light 8 . The two beams of light meet at a combiner element 10 . In particular, the first and the second beam of light 6 , 8 are guided respectively along the first path 7 and the second path through an integrated photonic circuit and not through an optical fibre . In this way, it is possible to obtain very narrow bend radii, up to a few microns, compared to that of optical fibres (tens of centimetres) . This allows much more compact and therefore also more stable devices to be created. It is noted that an integrated photonic circuit based on, for example, a strip waveguide or a rib waveguide may be used.

[0054] The two paths 7, 9 have a different length so that the optical path of the first beam of light 6 is different from that of the second beam of light 8. According to Figure 2, the first path 7 has a greater length than the second path 9. Specifically, the difference between the length of the two paths is such that the FSR, defined as c / ngAL where ngis the group index of the waveguide, is between 10 GHz and 50 GHz so as to maximise the transmission of Brillouin peaks by suppressing the elastic Rayleigh backlight. The difference between the first path 7 and the second path 9 can be defined as the path difference AL.

[0055] Depending on the partitioning element 5 used, the intensity of the first beam of light 6 may be the same as or different from the intensity of the second beam of light 8.

[0056] In one example, the intensity of the first beam of light 6 is greater than the intensity of the second beam of light 8, and as shown in Figure 2, the first path 7 in which the first beam of light 6 travels is longer than the second path 9 in which the second beam of light 8 travels, i.e., the path difference AL is greater than zero. This serves to balance the difference between the propagation losses along the two paths. Of course, in an alternative configuration, the intensity of the first beam of light 6 can be less than the intensity of the second beam of light 8 and the first path 7 can be shorter than the second path 9 (the path difference AL is less than zero) . In another example, the intensity of the first beam of light 6 can be equal to the intensity of the second beam of light 8 with the propagation losses along the two waveguides being close to zero. In this case, the path difference (AL) is nonzero .

[0057] The first and second beams of light 6, 8 are combined at a combiner element 10 that is coupled to the first and second outputs 11, 12 to propagate the two output signals 15, 16. The combiner element 10 recombines the first beam of light 6 and the second beam of light 8 to generate interference. The combiner element, as well as the partitioning element, may consist of, for example, a multimode interferometer (MMI) or a directional coupler. The first and second output signals 15, 16 may be referred to as a first channel or "bar" (or "cross") channel output signal, respectively, and a second channel or "cross" (or "bar") channel output signal, wherein the "bar" (or "cross") channel is coupled to the spectral analyser 3. It is noted that the definition of "bar" signal and / or "cross" signal essentially depends on the position of the output channel with respect to the input channel. The signal is defined as a "cross signal" if it is located at the output channel opposite the input channel, while it is defined as a "bar signal" if it is located at the output channel positioned on the same side of the input channel.

[0058] The filter 1 further comprises at least one phase modulator 13 and a closed-loop control system 14 coupled to the phase modulator 13. The phase modulator 13 is positioned at the second path 9. However, the phase modulator 13 may additionally, or alternatively, be positioned at the first path 7. The phase modulator 13 serves to adjust the filter 1 and to induce destructive interference on the wavelength of the elastic Rayleigh signal .

[0059] It is noted that the phase modulator 13 serves to vary the phase of the optical signal crossing the first or second path 7 , 9 by varying the refractive index of the corresponding photonic integrated circuit in which the signal passes without however varying the length of the two paths 7 , 9 . In other words , the phase modulator 13 is an element that acts on the refractive index of the material used to transport the light along the first path 7 and / or the second path 9 . The phase modulator 13 does not comprise a piezoelectric element and does not alter the length of first path 7 and / or second path 9 . For example , the phase modulator 13 may comprise a heating element . In this case the phase modulation is a thermal modulation .

[0060] Through the closed-loop control system, Rayleigh signal suppression can be maximised . In particular, the control is established by acquiring and processing a feedback signal provided by one of the two output signals 15 , 16 , e . g . , the second output signal 16 , and adj usting the interference by the phase modulator 13 . This mechanism allows a continuous process of maximising the elastic Rayleigh background signal at one of the two output signals 15 , 16 , for example the second output signal 16 , and a simultaneous removal of such a background signal from the scattered light propagated at the other of the two output signals 15 , 16 , for example the first output signal 15 .

[0061] The first output 11 is configured to propagate the first output signal 15 where the Rayleigh elastic component of the scattered light is attenuated or suppressed as a result of a destructive interference of said Rayleigh elastic component between the first beam of light 6 and the second beam of light 8. Preferably, the first output signal 15 in which the Brillouin signal is transmitted occurs at the "cross" channel, i.e. at the channel opposite the input channel. This ensures greater visibility of the Brillouin signal as the extinction rate of the Rayleigh light component in the "cross" channel is higher than the "bar" channel due to the presence of spurious light that travels outside the waveguide along the "bar" channel.

[0062] That is, in an optical filter 1 forming a Mach Zehnder interferometer represented by a system comprising two inputs (i.e., a first input and a second input) in which scattered light is received at one of the first and second inputs and two outputs, the output at which the Brillouin signal is transmitted where the Rayleigh elastic component of the light spectrum is attenuated or suppressed is preferably opposite to the input into which the scattered light enters.

[0063] In one example, the optical filter 1 may comprise a beam of light converter (not shown in the figure) , in particular an optical limiter (inverted taper) , positioned at the input 4, 52. The converter serves to convert, i.e. to adapt, the size of the input beam of light when for example the optical filter 1 is integrated in a photonic circuit and is coupled to an optical fibre as mentioned above. The beam of light converter therefore allows the optical filter to be used when it is integrated into a photonic circuit since the propagation mode inside the latter is typically much smaller (on the order of hundreds of nanometers) than that of optical fibres (a few micrometers) .

[0064] A possible configuration of the optical filter 1 is shown in Figures 3A and 3B. It is noted that the optical filter 1 is based on a principle of interference between light signals that travel a different optical path. Thus, the filter 1 may be constituted by, or behave as, a Mach-Zehnder interferometer for suppressing the Rayleigh elastic component.

[0065] Figures 3A and 3B show how the interferometer comprises two inputs (e.g. first input 4 and second input 52 in which the scattered light is received in one of the first and second inputs) and two outputs (e.g. first output 11 and second output 12) and that the light is divided into two paths of different length in the area between the inputs 4, 52 and the outputs 11, 12. For example, an upper path may be defined (first path 7 in the upper portion shown in the figures) and a lower path may be defined (second path 9 in the lower portion shown in the figures) . According to the example of Figures 3A and 3B, the upper path (first path 7) is longer than the lower path (second path 9) . In particular, each output 11, 12 is located on the opposite end of each input 4, 52. Defining the "cross signal" as the signal that is located at the opposite output channel to the input channel, means that in this case the output channel is located at the output (e.g. 11) of one light path (e.g. the upper path) while the input channel is located at the input 52 of the opposite path (e.g. the lower path) , or vice versa. Defining the "bar signal" as the signal that is at the output channel positioned on the same side of the input channel, means that in this case the output channel is at the output (for example first output 11) of the same light path (for example the upper or lower path) associated with the input channel (for example at the input 4) .

[0066] According to one example, the output at which the Brillouin signal is transmitted where the Rayleigh elastic component of the light spectrum is attenuated or suppressed (e.g. the first output 11) is at an output channel of the first path 7 while the input at which the scattered light enters (e.g. the second input 52) is at an input channel of the second path 9.

[0067] Alternatively, the output at which the Brillouin signal is transmitted where the Rayleigh elastic component of the light spectrum is attenuated or suppressed (e.g. the second output 12) is at an output channel of the second path 9 while the input at which the scattered light enters (e.g. the first input 4) is at an input channel of the first path 7.

[0068] As shown in Figure 3A, the filter 1 comprises a first filter input 4 and a second filter input 52 for receiving scattered light from the sample 40, wherein the scattered light is received at one of the first and second inputs. The scattered light is divided by the partitioning element 5 into two beams of light 6, 8 which travel respectively in two distinct paths 7, 9. The partitioning element 5 can be a multimode interferometer (MMI) or a directional coupler. The difference in length between the first and the second path 7, 9, i.e. the path difference AL, determines the FSR, i.e. the distance in frequency between two adjacent orders of interference of the filter 1 understood as a Mach-Zehnder interferometer, by means of the relationship FSR=c / ngAL, previously defined. The interference between the first and the second beam of light 6, 8 occurs at the combiner element 10. Specifically, a first output signal 15 at the first output 11 is the result of destructive interference of the Rayleigh elastic component. This first output signal 15 represents a Brillouin signal free of elastic background noise. If the first output signal 15 is on the same side as the first input 4 which receives the light scattered by the light source 2, the first output signal 15 is at the output of the so-called "bar" channel. If, on the other hand, the scattered light is received at the second input 52, the first output signal 15 is located at the output of the " cross" channel , as opposed to the input channel . As mentioned, it is preferable that the output signal in which the Brillouin signal is transmitted is at the " cross" channel for the reasons stated above .

[0069] In one example , the second output 12 is configured to propagate a second output signal 16 , wherein the second output signal 16 is used by the closed-loop control system 14 to adj ust interference between the first beam of light 6 and the second beam of light 8 via the phase modulator 13 and minimise the Rayleigh elastic component of the first output signal 15 . In particular, the filter may comprise an optical detector 19 , in particular a photodetector, positioned at the second output 12 for measuring the intensity of the second output signal 16 which is used by the closed-loop control system 14 .

[0070] The closed-loop mechanism is then established by connecting the second output signal 16 to the optical detector 19 . Speci fically, the closed-loop control functionality Ci ( s ) aims to maximise the propagated signal s so as to minimise the Rayleigh component . The signal s , which may correspond to the signal 16 of Figure 3A, is defined as the sum of the elastic Rayleigh signal SRand the Brillouin signal sB, s=sR+sB. Therefore , by taking sR»sB, s=sR+sR=sR. The function Ci ( s ) can be used in a variety of electronics-based methods involving analogue-to-digital (AD) conversion and control functions based on the combination of proportional-integral-derivative ( RID) control models as well as non-linear control or inputoutput functions based on analytical or machine learning models . In other examples , the function Ci ( s ) can also be used for data processing of various kinds , e . g . optical , micromechanical , etc . According to one example , the filter 1 further comprises a hysteresis module 18 coupled to the closed-loop control system 14 to increase the stability and ef fectiveness of the control . In particular, hysteresis may be used in the same manner as Ci ( sR) .

[0071] Figure 3B shows the same configuration of the filter 1 of Figure 3A with the only di f ference relating to closed-loop control .

[0072] According to one example , the filter 1 further comprises an absorption means 17 positioned at the first output 11 for picking up a portion of the first output signal 15 which is used by the closed-loop control system 14 for adj usting the interference between the first beam of light 6 and the second beam of light 8 via the phase modulator 13 and minimising the first output signal 15 so as to filter the Rayleigh elastic component from the first output signal 15 . It is noted that the position between the first and second outputs 11 , 12 in Figure 3B is reversed with respect to the position of the same outputs in Figure 3A.

[0073] In this example , the closed-loop control is based on a C2 ( s ) functionality . In the event that the scattered light is received at the second input 52 , the control is carried out at the "bar" channel to partially absorb a small portion of the light signal s . In this case , the signal s may correspond to the signal 15 . The function C2 ( s ) is used to minimise the signal s at the "bar" channel so as to filter the Rayleigh component from the propagated Brillouin signal . The control is carried out by positioning the absorption means 17 in the vicinity of the waveguide , wherein the distance and the length of the means with respect to the waveguide determine the amount of light absorbed and converted into an electrical signal by means of , for example , electrodes . It is noted that this configuration is less ef ficient than that of Figure 3A since the control implies a partial attenuation of the Brillouin light signal and a lower accuracy in reading the elastic Rayleigh signal .

[0074] In one example , the closed-loop control system 14 can use the "dithering" technique . This technique allows the real-time extraction of the first derivative of the trans fer function of an optical device directly from the physical system and uses this derivative for control purposes . In particular, the signal strength from a photonic device can be either maximised or minimised by setting the signal of the first derivative to zero as this condition indicates stationary points of the trans fer function . This approach is particularly suitable in the case of a cascade architecture as the " set point" does not require any calibration and does not depend on the absolute amount of light reaching each device . Furthermore , in case of variations of the optimal working point , it is possible to understand the direction of the displacement from the sign of the derivative so as not to leave any ambiguity about the control action to be taken . Finally, variations in the dark readout baseline do not af fect the measurement allowing the dithering reading to be used ef fectively as an error signal of a robust closed-loop control system . In order to extract the derivative of the trans fer function, a small modulation signal may be superimposed on the voltage of the actuator controlling the optical device causing an oscillation of the trans fer function at the bias working point and a consequent modulation of the optical energy at the output . Furthermore, the dithering technique can also be ef fectively employed to distinguish the ef fect of multiple actuators while still using a single detector . This is very useful in case of devices that require the use of a plurality of heaters (modulating elements ) as in the case of Mach-Zehnder interferometers or in cascade structures where a plurality of actuators influence the intensity of the output signal , such as for example coupled (micro- ) optical ring resonator systems . In the speci fic case of the optical filter described, the dithering control is particularly useful as it allows the optimal phase term of the interferometers to be obtained by minimising the derivative first obtained through the modulation of the current of the actuators instead of maximising the Rayleigh signal . This allows greater precision to obtain destructive interference of the Rayleigh signal , while maximising the visibility and intensity of the Brillouin signal .

[0075] Figs . 4A and 4B show the operating principle of the filter 1 , speci fically the operating principle of the Mach-Zehnder interferometer . The trans fer function of the filter 1 ( change in intensity as a function of frequency) is characteri zed by a sinusoidal profile , in which the FSR - which corresponds to the period of the sine function - is ( ideally) such as to have a transmission window centred along the expected spectral range of the Brillouin peaks B and B' which are typically shi fted by 5-30 GHz from the Rayleigh peak R . These figures show an example in which the maximisation of the Rayleigh signal occurs at the "bar" channel while the minimisation of the Rayleigh signal occurs at the " cross" channel . It is noted that the trans fer function to the "bar" channel ( Fig . 4A) is opposite to that of the " cross" channel ( Fig . 4B ) so that the maximisation of the Rayleigh signal at the "bar" channel results in turn in a minimisation of the background light at the " cross" channel where only the Brillouin peaks are therefore transmitted . Thanks to the phase modulator, it is possible to adjust (tune) the frequency transmission function in such a way as to maximise the degree of extinction.

[0076] As mentioned above, a spectrometer 20 comprises a spectral analysis component for Brillouin spectrum analysis and a filtering component positioned between the spectral analysis component and a scattered light source which essentially serves to filter the Rayleigh elastic component. The filter component may comprise a single optical filter 1 as described above or a plurality of optical filters 1 so as to increase the effectiveness of the filtering effect. Because the optical filter is integrable / is integrated into nanoscale optical circuits, multiple optical filters can be integrated without substantially increasing the size of the device.

[0077] In one example, the spectrometer 20 comprises a plurality of filters li, I2,..., lnconnected together in series to form a filter module 22 having a module input 23 for receiving the scattered light, a first module output 24 and a second module output 25, the first module output 24 being coupled to the spectral analyser 3, wherein the first output Hi of a first filter li of the plurality of filters li, 12,..., lnis coupled to the input 42 of a subsequent second filter I2. This is shown in Figure 5. Advantageously, each filter of the plurality of filters li, I2,..., lnis coupled to the next filter in such a way that only one output (e.g. the first output lln) of said filter is coupled to only one input (e.g. the input 4n) of the next filter. Preferably, the only filter output connected to the next filter input is positioned at the "cross" channel.

[0078] Each optical filter li, I2,..., In can be a Mach-Zehnder interferometer. According to Figure 5, each optical filter li, I2,..., lnis connected to a photodetector 19i, 192,..., 19n, to a closed-loop controller 14 and optionally to a hysteresis module (not shown in the figure) for processing the data and sending the appropriate input voltage (V) to the corresponding phase modulator 13i, 132,..., 13n. Since realistic values of the extinction ratio of a single optical filter intended as a Mach- Zehnder interferometer do not exceed approximately 20 dB, placing a plurality N of optical filters (MZI) in cascade has the advantage of increasing the total extinction ratio by a factor N without, however, excessively affecting the size of the optical system. Furthermore, because the phase of each optical filter (MZI) is independently controllable so as to minimise the propagation of the Rayleigh elastic component, the resulting Brillouin signal entering the spectral analyser 3 is found to be free of spurious background light.

[0079] It is noted that the optical filters li, I2,..., lndescribed in Figure 5 show only one input 4i, 42,..., 4n. It is however evident that in reality each optical filter comprises a second input as clearly defined in Figures 3A and 3B. Therefore, all the characteristics defined for the optical filter 1 of Figures 3A and 3B apply to each of the plurality of optical filters li, I2,..., lnof Figure 5. The same applies to the optical filters depicted in Figure 6 described below.

[0080] Thus, the closed-loop control functionalities described in Figures 3A and 3B can be efficiently applied to the configuration comprising a filter module 22 having a plurality of optical filters li, I2,-, In- In one example, the closed- loop control system 14 includes a single-input, single-output, SISO, control system. In this case, each optical filter (MZI) li, I2,-, In and each associated signal s± is individually processed by the function Ci(si) . In another example , the closed-loop control system 14 includes a multiple-input multiple-output , MIMO, control system . As shown in Figure 6 , one or more manipulated variables may af fect the interactions of controlled variables in a speci fic circuit or all other control circuits . Compared to the S ISO system of Figure 5 , the MIMO system proces ses the input signals with a single CM( si ) function, guaranteeing a more precise and ef fective control of the optical filter at the expense of greater complexity . This type of control system allows dynamic systems to be incorporated, the costs of individual control actions to be considered, the ef fects of possible system-wide disturbances to be analysed and the limited sensitivity of the photodetector 19 to be overcome . Also in this case , it is possible to optionally insert a hysteresis module (not shown in the figure ) associated with the control system 14 for each phase modulator 13i, 132 , ..., 13n.

[0081] The spectral analyser 3 within the spectrometer 20 may be of di f ferent types . For example , Figures 7A, 7B, and 8 show a spectral analyser 3 consisting of at least one optical ring resonator while Figure 9 shows a spectral analyser 3 comprising a virtual imaged phased array (VIPA) element .

[0082] In one example , the spectral analyser 3 comprises at least one optical ring resonator 26 comprising an input waveguide 28 for receiving light from the first output 11 of the filter 1 , a closed-loop waveguide 29 , having an ef fective refractive index neff and coupled to the input waveguide 28 for selecting at least a certain frequency vresof the scattered light , an output waveguide 30 coupled to the closed-loop waveguide 29 for outputting the selected frequencies and a pass-through waveguide 31 coupled to the input waveguide 28 and the closed- loop waveguide 29 for outputting the unselected frequencies , and wherein the spectral analyser 3 further comprises a modulator element 27 coupled to the optical ring resonator 26 for modulating the ef fective refractive index neff and for scanning the di f ferent multiple frequency components by varying the optical path of the closed-loop waveguide 29 . This is shown in Figure 7A. In this case , the Brillouin signal devoid of the Rayleigh background signal deriving from the optical filter 1 or from the f ilter module 22 is partially coupled via the input waveguide 28 (BUS ) to the ring waveguide 29 by means of , for example , a directional coupler . The amount of coupling is such as to maximise the Q- factor of the ring and at the same time to avoid substantial signal transmission losses . To increase the coupling force , the ring may have an oblong shape ( " racetrack" configuration) in which the coupling region is elongated to overcome the limited lithographic resolution . The resonance of the ring can be quickly adj usted by the modulator element 27 ( such as a heating element or an opto-electric medium) and the light transmitted at the output waveguide 30 ( DROP ) can be acquired by a photodetector 45 . By scanning at least one value of FSR = X2 / ngL, it is possible to acquire the entire Brillouin spectrum, where L which is the travel length in the ring must be selected such that FSR>15 GHz . The value of the intensity of the frequency signal on the basis of the variation in voltage applied to the modulator element 27 is shown in Figure 7B .

[0083] Figure 8 shows the example of a spectrometer 20 wherein the spectral analyser 3 comprises a plurality of optical ring resonators 26i, 262 , . . . , 26narranged in a cascade , wherein the closed-loop waveguide 29i of a first optical resonator 26i is coupled with the closed-loop waveguide 292 of a subsequent second optical resonator 262 . The configuration of Figure 8 is needed when a single ring resonator 26 is not suf ficient to obtain a resolution below GHz ( sub-GHz ) due to losses (bending and propagation) within the single ring . Advantageously, the spectral analyser 3 further comprises at least two modulating elements 27A, 27Bcoupled to each optical ring resonator 26i, 262, ..., 26n. A phase modulator 27Anserves to bring the rings to the same resonance wavelength by accurately adj usting the applied voltage VRni. Once this condition has been met , a Brillouin spectrum can be acquired by rapidly scanning the resonance of all rings simultaneously by adj usting the voltage VRN2of the second phase modulator 27Bn.

[0084] In one example , the spectral analyser 3 comprises a single dispersive angular optical element (VIPA) 49 for dispersing an electromagnetic signal along the direction transverse to the direction of propagation of the output signal 15 . It is noted that the use of an optical dispersive device such as VIPA allows the Brillouin spectrum to be analysed without having to scan the spectrum . However, due to the dominant signal caused by Rayleigh elastic light , it is usually necessary to employ a plurality of VIPA elements in order to increase the spectral contrast in order to measure Bri llouin peaks otherwise covered by Rayleigh light . By using the optical filter 1 as described above , it is instead possible to use only a single dispersive angular optical element (VIPA) 49 , thus making the overall system more compact . In other words , given the filtering ef ficiency of the Rayleigh elastic component by the optical filter 1 ( or filter module 22 ) as described above , it is possible to reali ze a spectral analyser 3 , and consequently a spectrometer 20 , which is compact and has excellent performance for Brillouin spectroscopy . As shown in Figure 9 , the spectral analyser 3 is coupled to the optical filter 1 or to the f ilter module 22 via an input optical fibre 46 to receive the output signal of the filter in which the Rayleigh elastic component is suppressed or attenuated . For example , the input optical fibre 46 receives the first output signal 15 . The spectral analyser 3 further comprises a first lens 47 ( collimating lens ) for receiving and collimating the output signal 15 to be analysed and a second lens 48 ( focusing lens ) for focusing the collimated output signal 15 .

[0085] The spectral analyser 3 further comprises a dispersive angular optical element (VIPA) 49 coupled to the first and second lenses 47 , 48 for dispersing an electromagnetic signal along the direction transverse to the direction of propagation of the output signal 15 and a third lens 50 ( Fourier lens ) for converting the electromagnetic signal dispersed by the dispersive angular optical element 49 into a plurality of spectral peaks for measuring the spectrum of light scattered through a plurality of optical detectors , for example a CCD camera 51 . Although the advantage of using the optical filter 1 or the filter module 22 is to be able to employ a single VIPA device , it is also conceivable to consider a spectral analyser 3 having alternatively a plurality of these VIPA optical devices .

[0086] The optical filter 1 ( and filter module 22 ) as well as the spectrometer 20 and apparatus 37 described above have considerable potential in the biomedical field . In fact , these optical devices can be an excellent solution for integrating Brillouin microscopy into the analysis of mechanical properties of biological systems . Brillouin microscopy is a relatively new technique with growing interest in research and the biomedical industry . Using the optical filter 1 as described, it is therefore possible to obtain an ultra-compact spectral device , with a high contrast , with a resolution below GHz ( sub-GHz ) and with a high transmission ef ficiency that can be used in Brillouin microscopes for the diagnosis of diseases and the analysis of biomechanical processes . Furthermore , since the optical filter is integrable / is integrated in nanometric photonic circuits , the final device will have greater ease of use than conventional free-space optical-based filters as it has no physical elements to be aligned .

[0087] A person skilled in the art can perform several and further modi fications and variants to the filter 1 , to the spectrometer 20 and to the apparatus 37 disclosed above , in order to satis fy further and contingent needs , all said modi fications and variants being however included within the scope of protection of the present invention as defined by the appended claims .

Claims

CLAIMS1. Optical filter (1) , in particular Mach-Zehnder interferometer, for suppressing or attenuating the Rayleigh elastic component of scattered or reflected light, said filter (1) being integrable within an active and / or passive photonic circuit and being positionable between a scattered light source (2) and a spectral analyser (3) , wherein the filter (1) comprises: at least one input (4; 52) for receiving the scattered light; at least one partitioning element (5) for dividing the scattered light into a first beam of light (6) travelling along a first path (7) and a second beam of light (8) , separated from the first beam of light (6) , travelling along a second path (9) , wherein the difference between the first path (7) and the second path (9) , defined as the path difference (AL) , is non-zero; at least one phase modulator (13) positioned at the first and / or second path (7, 9) ; a closed-loop control system (14) coupled to the phase modulator ( 13 ) ; a combiner element (10) for combining the first beam of light (6) output from the first path (7) with the second beam of light (8) output from the second path (9) , and a first output (11) and a second output (12) coupled to the combiner element (10) , wherein the first output (11) is configured to propagate a first output signal (15) wherein the Rayleigh elastic component of the light spectrum is attenuated or suppressed following a destructive interference of said Rayleigh elastic component between the first beam of light (6) and the second beam of light (8) .

2. Filter (1) according to claim 1, wherein the second output (12) is configured to propagate a second output signal (16) , wherein the second output signal (16) is used by the closed- loop control system (14) to regulate the interference between the first beam of light (6) and the second beam of light (8) via the phase modulator (13) and to minimise the Rayleigh elastic component of the first output signal (15) .

3. Filter (1) according to claim 2, further comprising an optical detector (19) , in particular a photodetector, positioned at the second output (12) for measuring the intensity of the second output signal (16) which is used by the closed-loop control system (14) .

4. Filter (1) according to claim 1, further comprising an absorption means (17) positioned at the first output (11) for picking up a portion of the first output signal (15) which is used by the closed-loop control system (14) for regulating the interference between the first beam of light (6) and the second beam of light (8) by the phase modulator (13) and minimising the first output signal (15) so as to filter the Rayleigh elastic component from the first output signal ( 15) .

5. Filter (1) according to any of claims 1 to 4, wherein: a. the intensity of the first beam of light (6) is equal to the intensity of the second beam of light (8) and wherein the path difference (AL) is non-zero; or b. the intensity of the first beam of light (6) is greater than the intensity of the second beam of light (8) and wherein the path difference (AL) is greater than zero.

6. Filter (1) according to any of claims 1 to 5, further comprising a hysteresis module (18) coupled to the closed- loop control system (14) to increase the stability and effectiveness of the control.

7. Filter (1) according to any of claims 1 to 6, comprising a first input (4) and a second input (52) wherein the scattered light is received in one of the first and second inputs (4, 52) , and wherein: a. the output (11) at which the Brillouin signal is transmitted, where the Rayleigh elastic component of the light spectrum is attenuated or suppressed, is opposite to the input (52) in which the scattered light enters; and / or b. the filter (1) comprising a first end at the first and second input (4, 52) and a second end at the first and second output (11, 12) and wherein the first end is opposite to the second end.

8. Filter ( 1 ) according to any of claims 1 to 7, wherein: a. the output (11) at which the Brillouin signal is transmitted where the Rayleigh elastic component of the light spectrum is attenuated or suppressed is at an output channel of the first path (7) whereas the input (52) into which the scattered light enters is at an input channel of the second path (9) , or b. the output (12) at which the Brillouin signal is transmitted where the Rayleigh elastic component of the light spectrum is attenuated or suppressed is at an output channel of the second path (9) whereas the input (4) into which the scattered light enters is at an input channel of the first path (7) .

9. Filter (1) according to any of claims 1 to 8, further comprising a beam of light converter, in particular an optical limiter, positioned at the input (4; 52) .

10. Spectrometer (20) for analysing the spectrum of scattered or reflected light, comprising: a main input (21) for receiving the scattered light; at least one optical filter (1) for suppressing or attenuating at least the Rayleigh elastic component of scattered or reflected light according to any of claims 1 to 6, wherein said filter (1) is coupled to the main input (21) of the spectrometer (21) ; and a spectral analyser (3) for selecting and separating certain multiple frequency components of the scattered light, for measuring the intensity of the different frequency components and reconstructing the profile of the spectrum of the scattered light, wherein said spectral analyser (3) is coupled to the first output (11) of the filter (1) , wherein at least the filter (1) is constituted by a photonic integrated circuit.

11. Spectrometer (20) according to claim 10, comprising a plurality of filters (li, I2,..., ln) connected together in series to form a filter module (22) having a module input (23) for receiving the scattered or reflected light, a first module output (24) and a second module output (25) , the first module output (24) being coupled to the spectral analyser (3) , wherein the first output (Hi) of a first filter (li) of the plurality of filters (li, I2,..., ln) is coupled to the input (42) of a subsequent second filter (12) .

12. Spectrometer (20) according to claim 11, wherein: a. the closed-loop control system (14) comprises a singleinput, single-output, SISO, control system; or b. the closed-loop control system (14) comprises a multiple-input multiple-output, MIMO, control system.

13. Spectrometer (20) according to any of claims 10 to 12, wherein the spectral analyser (3) comprises at least one optical ring resonator (26) comprising an input waveguide (28) for receiving light from the first output (11) of the filter (l),a closed-loop waveguide (29) , having an effective refractive index neff and coupled to the input waveguide (28) for selecting at least a certain frequency vresof the scattered light a waveguide output (30) coupled to the closed-looped waveguide (29) for outputting the selected frequencies and a pass-through waveguide (31) coupled to the input waveguide (28) and the closed-loop waveguide (29) for outputting the unselected frequencies, and wherein the spectral analyser (3) further comprises a modulator element (27) coupled to the optical ring resonator (26) for modulating the effective refractive index neff and for scanning the different multiple frequency components through the variation of the optical path of the closed-loop waveguide (29) .

14. Spectrometer (20) according to claim 13, wherein the spectral analyser (3) comprises a single dispersive angular optical element (VIPA) (49) for dispersing an electromagnetic signal along the direction transverse to the direction of propagation of the output signal (15) .

15. Apparatus (37) for Brillouin spectroscopy, microscopy or endoscopy comprising the spectrometer (20) according to any of claims 10 to 14.

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