IT202200026169A1 - Optical filter for spectroscopy - Google Patents

Description

Optical filter for spectroscopy

TECHNICAL FIELD

The present invention relates to an optical filter, in particular a Mach-Zehnder interferometer, for suppressing or attenuating the Rayleigh elastic component of a scattered or reflected light. Said optical filter is integrable in 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.

STATE OF THE ART

Brillouin spectroscopy is a well-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 (Rayleigh) light 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 shifted by 1-20 GHz from the Rayleigh frequency.

The frequency and linewidth of the Brillouin peaks are indicative of the viscoelastic properties of a sample. In particular, given the density and refractive index of a material, the frequency shift and the Brillouin linewidth provide information on the elastic moduli characterizing the illuminated material.

Standard methods for measuring the mechanical properties of a material require physical contact with the sample, making these techniques invasive and limited to the topographic surface. In contrast, Brillouin microscopy or endoscopy uses light as a probe so as to avoid any form of contact. Brillouin spectroscopy probes mechanical properties with an optical resolution of the submicron (micrometer) and, applied to microscopy, can perform a three-dimensional analysis of the volume of biological systems (e.g. cells and tissues).

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

The main component of a Brillouin spectroscopy, microscopy or endoscopy is the spectrometer which requires both sub-GHz spectral resolution and high spectral contrast. High spectral contrast is for example required to measure opaque biological samples, where the elastically scattered light and reflections are several orders of magnitude larger 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 J.Phys. E 10, 150 (1977), in Brillouin spectroscopy the sample is illuminated by a laser source and the scattered light is spectrally analyzed using multiple Fabry-Perot (FP) interferometers positioned in tandem. Although FP interferometers and etalons have remarkable contrast and spectral resolution, they usually require a long integration time (> 1 sec) which results in Brillouin spectroscopy being limited to single point measurements, thus not allowing extension to an imaging technique.

In the last decades, a new type of FP etalon interferometer, known as Virtual Imaged Phased Array (VIPA), has been introduced. The use of VIPA spectrometers in Brillouin spectroscopy has brought the data acquisition time down from tens of seconds to about 100 msec using low optical power sources (< 10 mW), while enabling the use of Brillouin spectroscopy in a non-contact three-dimensional imaging mode to map the viscoelastic properties in the volume of biological systems. Although very efficient, conventional VIPA spectrometers are intrinsically limited by a spectral contrast of about 30 dB. To increase the spectral contrast in Brillouin microscopy, there are currently methods using several VIPA etalons (multi-stage VIPA) cascaded together [ Opt. Express 19, 10913 (2011)]. However, this is not possible with conventional VIPA spectrometers. results in a reduced efficiency of about 25% with an associated increase in image acquisition time. Furthermore, cascaded VIPA spectrometers require a high number of optical components which make these systems expensive and difficult to align.

In addition, the manufacturing tolerances related to parallelism and uniformity of optical surfaces (typically higher than 100?) impose high production costs. Although considerable efforts have been made to deploy Brillouin spectrometers, they still lack sufficient spectral contrast to measure highly heterogeneous and opaque samples such as human tissue. To overcome these drawbacks, several filtering methods have been proposed in recent years with the aim of suppressing elastic background light. However, current solutions still rely on bulky components and large interferometers that are particularly sensitive to temperature and laser frequency shift, thus requiring continuous realignment and recalibration. Recently, progress has been made in the field of stimulated Brillouin scattering to speed up data acquisition. However, this approach requires the use of high-power lasers that are not practical in the context of in vivo measurements and clinical applications.

The purpose of the present invention is to overcome in part or in full the above-mentioned drawbacks of known systems and to provide optical devices, such as 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 created and which can automatically and in real time adapt the operating point to environmental or signal variations. In particular, the purpose of the present invention is to provide optical devices which are able to carry out measurements on biological materials (and possibly on non-biological materials) with a spectroscopy, microscopy or Brillouin endoscopy, from which it is possible to obtain mechanical or structural properties on the analyzed samples.

DESCRIPTION OF THE INVENTION

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 the spectrometer are described in the corresponding dependent claims.

In a first aspect of the invention, an optical filter is provided for suppressing or attenuating the elastic Rayleigh component of a scattered or reflected light. The optical filter is positionable between a scattered light source and a spectral analyzer. The optical filter is integrable in an active and/or passive photonic circuit.

The optical filter includes:

at least one input for receiving the diffused or reflected light; at least one splitter element for dividing the diffused light into a first light beam traveling along a first path and a second light beam, separate from the first light beam, traveling along a second path, wherein the difference between the first path and the second path, defined as the path difference, 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 combining element for combining the first light beam exiting the first path with the second light beam exiting the second path, and

a first output and a second output coupled to the combining element,

wherein the first output is configured to propagate a first output signal where the elastic Rayleigh component of the light spectrum is attenuated or suppressed as a result of destructive interference of said elastic Rayleigh component between the first light beam and the second light beam.

The optical filter can be a Mach-Zehnder interferometer (MZI). In particular, the filter can consist of a single MZI or a plurality of MZIs connected together, for example in series.

The use of an optical filter consisting of at least one MZI improves the characteristics of an optical system of which it may be part. In fact, the optical filter increases the degree of suppression of the elastic component and makes the manufacturing of the system of which it is part less complex and less expensive. In other words, this optical filter allows the use of a spectral analyzer of reduced dimensions, thus increasing the compactness of the total optical system.

An optical filter that employs at least one MZI typically achieves performances greater than 10 times higher in terms of extinction compared to a different optical filter, which employs 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. Due to its structural configuration, a ring resonator that functions as a filter has by default a very high margin of error due to the exponential behavior of the degree of extinction as a function of the separation between the input waveguide and the ring resonator. This results in a lower extinction compared to the optical filter that employs an MZI. Furthermore, an optical filter consisting of at least one MZI is more easily reproducible than a filter with a ring resonator since the latter is subject to a lower manufacturing reproducibility due to the limited resolution of the lithography process.

The MZI filter also offers the possibility of being used as a "closed loop" adaptive dynamic filter, that is, it allows the suppression peak to be automatically and continuously calibrated at the Rayleigh wavelength.

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 to the frequency of the light source producing the scattered light (e.g. the laser source). This leads to the following advantages:

-reduction of noise due to Rayleigh scattered light and specular reflections and therefore a higher signal-to-noise ratio of the Brillouin peaks;

- error reduction by automatically adjusting system inputs thus containing overshooting;

- improving the stability of an unstable system due to laser frequency drifts or thermal variations in the surrounding environment;

- variation of system sensitivity;

- increased robustness to external disturbances to the system; and

- reliable and reproducible performance.

Since the path difference is non-zero, it is possible to create an optical path difference between the two light beams traveling through this filter. As the 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, where typical FSR values range between 10 and 50 GHz. For this reason, the difference between the first path and the second path, i.e. the path difference, of this optical filter is advantageously between 5 mm and 30 mm. Naturally, other values for the path difference outside this range can also be used.

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 ? is sufficient to guarantee a regulation range equivalent to a single FSR.

Therefore, this optical filter is particularly advantageous to be used for Brillouin spectroscopy and to effectively suppress the elastic Rayleigh component.

As mentioned above, the optical filter can be integrated into an active and/or passive photonic circuit, for example nanometric. The photonic circuit can also be combined with an electronic circuit.

In one embodiment, the optical filter can be completely integrated within said active and/or passive photonic circuit. In one example, the circuit can be characterized by waveguides whose refractive index of the internal material "core" (Si3N4-silicon nitride) is greater than that of the coating material "cladding" (SiO2-silicon oxide). All the characteristics described below relating to the optical filter that can be integrated within the photonic circuit also apply to the optical filter directly integrated into said circuit.

In a second aspect of the invention, a spectrometer is provided for analyzing the spectrum of a scattered light. The spectrometer comprises:

a main input for receiving the scattered light; at least one optical filter for suppressing or attenuating the Rayleigh elastic component of the 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 analyzer for selecting and separating certain multiple frequency components of the scattered light, for measuring the intensity of the different frequency components and reconstructing the spectral profile of the scattered light, wherein said spectral analyzer is coupled to the first output of the filter,

where at least the filter is made of a photonic integrated circuit.

By using the optical filter according to the present invention inside a spectrometer, it is possible to suppress the elastic Rayleigh component of the scattered light more effectively and without the need for optical components. In particular, it is possible to couple the filter to a spectral analyzer increasing the signal-to-noise ratio of the Brillouin peaks.

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.

In particular, this apparatus is suitable for spectroscopy, microscopy or Brillouin endoscopy. The apparatus may therefore also comprise a light source such as a laser in the visible range (for example a 400-650 nm laser) or in the near infrared range (for example a 650-1500 nm laser) and a target of biological material. The light scattered by the interaction between the laser and the target is thus conducted into the spectrometer passing through the optical filter and is collected by the spectral analyser. The apparatus may also comprise one or more electronic calculators connected to the spectral analyser and/or the optical filter to manage the spectral analysis, acquire the detected data, analyse them and display the results containing the information on the light spectrum.

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 the spectral analysis of opaque or semi-transparent 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 analyzing the spectrum of a scattered light, in particular for the spectral analysis of opaque or semi-transparent samples.

These and other aspects of the present invention will become clearer in light of the following description of some preferred embodiments described below.

Fig. 1 shows a schematic representation of the apparatus including the spectrometer and the optical filter according to an example.

Fig. 2 shows a schematic representation of the optical filter according to an example.

Fig. 3a-b show a schematic representation of two optical filters according to an example.

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

Fig. 5 shows a schematic representation of an optical filter according to an example.

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

Fig. 7a-b show a schematic representation of a spectrum analyzer and its operation according to an example.

Fig. 8 shows a schematic representation of a spectrum analyzer according to an example.

Fig. 9 shows a schematic representation of a spectrum analyzer according to another example.

Figure 1 shows a schematic drawing of a spectroscopy apparatus 37. The apparatus 37 may, for example, be a spectroscopy apparatus, a microscopy apparatus, or a Brillouin endoscopy apparatus or similar technique. The apparatus 37 may comprise a light source, for example a narrow-band (<100 MHz) longitudinal single-mode laser 38 directed at a sample 40 (target material) through an illumination optics 39. The sample 40 may be a biological material. The light from the laser beam interacts with acoustic phonons and exchanges energy resulting in a diffuse light signal that has a minimal deviation from the laser frequency (5-30 GHz). As a result of the interaction between the laser beam and the sample 40, the sample therefore becomes a source of scattered light 2. The scattered light from the sample 40 is collected by a collection optics 41 and, via a fiber optical coupler 42 and a single-mode optical fiber 43, is directed towards a spectrometer 20. The spectrometer 20 receives the scattered light through a main input 21 at which there are coupling means 44 to establish coupling with the fiber optical coupler 42 and the single-mode optical fiber 43. The spectrometer 20 comprises an optical filter 1 to suppress or eliminate some components of the scattered light (e.g. the elastic Rayleigh component) and a spectral analyzer 3 to reconstruct and analyze the scattered light spectrum. The spectral analyzer 3 can advantageously have a resolution below 1 GHz (sub-GHz). By means of a computer system 51, coupled for example to the spectrometer 20, the acquired data can be analyzed and appropriately displayed. It is noted that the spectrometer 20 comprising the optical filter 1 and the analyzer 3 can be completely integrated into a photonic chip, for example made of silicon. For example, the light signal to be spectrally analyzed 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 curvature radii, for example between 0.1 mm and 2 mm. The light signal to be analyzed can be coupled into the photonic chip via a polarization-maintaining single-mode optical fiber and a spot-size converter such as a microlens, a grating or an optical constrictor (inverted taper). The photonic chip can be mounted and connected via external circuits to a printed circuit board to facilitate the electrical adjustment of the phase modulator 13.

Figure 2 shows in detail the elements of the optical filter 1 positioned between the scattered light source 2 and the spectral analyzer 3. The filter can advantageously be integrated into a nanometric photonic circuit. The filter 1 comprises an input 4 for receiving the scattered light from the scattered light source 2 and two outputs 11, 12. 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 splitter element 5 for splitting the scattered light into a first light beam 6 that travels along a first path 7 and a second light beam 8 that travels along a second path 9. The first light beam 6 is then separated from the second light beam 8. The two light beams rejoin at a combining element 10. The two paths 7, 9 have different lengths so that the optical path of the first light beam 6 is different from that of the second light beam 8. According to Figure 2, the first path 7 has a greater length than the second path 9. Specifically, the difference between the lengths of the two paths is such that the FSR, defined as c/ng?L where ng is the group index of the waveguide, is between 10 GHz and 50 GHz so as to maximize the transmission of Brillouin peaks while suppressing elastic Rayleigh background light. The difference between the first path 7 and the second path 9 can be defined as the path difference ?L.

Depending on the dividing element 5 used, the intensity of the first light beam 6 may be equal to or different from the intensity of the second light beam 8.

In one example, the intensity of the first light beam 6 is greater than the intensity of the second light beam 8, and as shown in Figure 2, the first path 7 in which the first light beam 6 travels is longer than the second path 9 in which the second light beam 8 travels, i.e., the path difference ?L? is greater than zero. This serves to balance the difference in propagation losses along the two paths. Of course, in an alternative configuration, the intensity of the first light beam 6 may be less than the intensity of the second light beam 8, and the first path 7 may be shorter than the second path 9 (the path difference ?L? is less than zero).

In another example, the intensity of the first light beam 6 can be equal to the intensity of the second light beam 8 since the propagation losses along the two waveguides are close to zero. In this case, the path difference (?L) is different from zero.

The first and second light beams 6, 8 are combined at a combining element 10 which is coupled to the first and second outputs 11, 12 to propagate the two output signals 15, 16. The combining element 10 recombines the first light beam 6 and the second light beam 8 to generate interference. The combining element, as well as the dividing element, may consist, for example, of a multimode interferometer (MMI) or a directional coupler. The first and second output signals 15, 16 may be referred to respectively as the output signal to a first channel, or “bar” channel, and the output signal to a second channel, or “cross” channel, where the “bar” channel is coupled to the spectral analyzer 3.

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

By means of the closed-loop control system, it is possible to maximize the suppression of the Rayleigh signal. 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 at the “cross” channel, and by adjusting the interference via the phase modulator 13. This mechanism allows a continuous process of maximizing the elastic Rayleigh background signal at one of the two output signals 15, 16, e.g. the second output signal 16 at the “cross” channel, and a simultaneous removal of such background signal from the propagated scattered light at the other of the two output signals 15, 16, e.g. the first output signal 15 at the “bar” channel.

The first output 11 is configured to propagate the first output signal 15 where the elastic Rayleigh component of the scattered light is attenuated or suppressed as a result of destructive interference of said elastic Rayleigh component between the first light beam 6 and the second light beam 8.

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 an interference principle between light signals traveling along different optical paths. Therefore, the filter 1 can be constituted by, or behaves like, a Mach-Zehnder interferometer for the suppression of the elastic Rayleigh component.

As shown in Figure 3A, filter 1 includes a filter input 4 for receiving scattered light from sample 40. The scattered light is split by splitter element 5 into two light beams 6, 8 that travel in two distinct paths 7, 9, respectively. Splitter element 5 may be a multimode interferometer (MMI) or a directional coupler. The difference in length between the first and second paths 7, 9, i.e. the path difference ΔL, determines the FSR, i.e. the frequency distance between two adjacent interference orders of filter 1 intended as a Mach-Zehnder interferometer, via the relation FSR=c/ngΔL, defined previously. The interference between the first and second light beams 6, 8 occurs at combiner element 10. Specifically, a first output signal 15 at the first output 11 is the result of a destructive interference of the elastic Rayleigh component. This first output signal 15, at the output of the ?bar? channel, represents a Brillouin signal free of elastic background noise.

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 adjust the interference between the first light beam 6 and the second light beam 8 via the phase modulator 13 and minimize the Rayleigh spring component of the first output signal 15. In particular, the filter may comprise an optical detector 19, particularly a photodetector, positioned at the second output 12 to measure the intensity of the second output signal 16 that is used by the closed-loop control system 14.

The closed-loop mechanism is then established by connecting the second output signal 16 at the cross-channel to the optical detector 19. Specifically, the closed-loop control functionality C1(s) aims to maximize the propagated signal s so as to minimize the Rayleigh component at the bar channel. The signal s, which may correspond to signal 16 of Figure 3A, is defined as the sum of the elastic Rayleigh signal SR and the Brillouin signal sB, s=sR+sB. Therefore, with the assumption of sR>>sB, s=sR+sR?sR. The function C1(s) can be employed in a variety of electronics-based ways involving analog-to-digital (AD) conversion and control functions based on the combination of proportional-integral-derivative (PID) control models as well as as nonlinear or input-output control functions based on analytical or machine learning models. In other examples, the C1(s) function can also be used through data processing of different nature, 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 effectiveness of the control. In particular, the hysteresis can be used in the same way as C1(sR).

Figure 3B shows the same configuration as filter 1 in Figure 3A with the only difference being the closed loop control.

According to one example, the filter 1 further comprises an absorption means 17 positioned at the first output 11 to take a portion of the first output signal 15 which is used by the closed-loop control system 14 to regulate the interference between the first light beam 6 and the second light beam 8 via the phase modulator 13 and minimize 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 inverted with respect to the position of the same outputs in Figure 3A.

In this example, the closed-loop control is based on a C2(s) functionality. The control is performed 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 C2(s) function is used to minimize the signal s at the ?bar? channel so as to filter the Rayleigh component from the propagated Brillouin signal. The control is performed by positioning the absorption medium 17 in proximity to the waveguide, where the distance and length of the medium 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 efficient than the one in figure 3A since the control involves a partial attenuation of the Brillouin light signal and a lower accuracy in reading the elastic Rayleigh signal.

Figures 4A and 4B show the operating principle of filter 1, specifically the operating principle of the Mach-Zehnder interferometer. The transfer function of filter 1 (intensity variation as a function of frequency) is characterized by a sinusoidal profile, where the FSR? which corresponds to the period of the sine function? is (ideally) such as to have a transmission window centered along the expected spectral range of the Brillouin peaks B and B? which are typically shifted by 5-30 GHz from the Rayleigh peak R. The transfer function at the ?cross? channel (fig. 4A) is however opposite to that of the ?bar? channel (fig. 4B) so that the maximization of the Rayleigh signal at the ?cross? channel results in a minimization of the background light at the ?bar? channel where only the Brillouin peaks are transmitted. Thanks to the phase modulator ? It is possible to tune the frequency transmission function in such a way as to maximize the degree of extinction.

As previously mentioned, a spectrometer 20 comprises a spectral analysis component used for analyzing the Brillouin spectrum and a filter component positioned between the spectral analysis component and a scattered light source that serves essentially to filter the elastic Rayleigh component. The filter component may comprise a single optical filter 1 as described above or a plurality of optical filters 1 in order to increase the effectiveness of the filtering effect. Since the optical filter is integrable/integrated in nanometric optical circuits, it is possible to integrate multiple optical filters without this resulting in a substantial increase in the size of the device.

In one example, the spectrometer 20 comprises a plurality of filters 11, 12, ?,1n connected to each other 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 analyzer 3, wherein the first output 111 of a first filter 11 of the plurality of filters 11, 12, ?,1n is coupled to the input 42 of a subsequent second filter 12. This is shown in FIG. 5.

Each optical filter 11, 12,?,1n can be a Mach-Zehnder interferometer. According to Figure 5, each optical filter 11, 12,?,1n is connected to a photodetector 191, 192,?,19n, a closed-loop controller 14 and optionally to a hysteresis module (not shown in the figure) to process the data and send the appropriate input voltage (V) to the corresponding phase modulator 131, 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, cascading a plurality of N optical filters (MZI) has the advantage of increasing the total extinction ratio by a factor N without excessively affecting the size of the optical system. Furthermore, since the phase of each optical filter (MZI) is approximately 100 μm, the MZI can be cascaded with a plurality of N optical filters (MZI). independently controllable to minimize the propagation of the elastic Rayleigh component, the resulting Brillouin signal entering the spectral analyzer 3 is found to be free of spurious background light.

The closed-loop control functionality described in Figures 3A and 3B may be efficiently applied to the configuration comprising a filter module 22 having a plurality of optical filters 11, 12,?,1n. In one example, the closed-loop control system 14 comprises a single-input, single-output, SISO, control system. In this case, each optical filter (MZI) 11, 12,?,1n and each associated signal is individually treated by the Ci(si) function.

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 can influence the interactions of controlled variables in a specific loop or all other control loops. Compared to the SISO system of Figure 5, the MIMO system processes the input signals with a single CM(si) function, providing more precise and effective control of the optical filter at the expense, however, of greater complexity. This type of control system allows for the incorporation of dynamic systems, consideration of the costs of individual control actions, analysis of the effects of possible disturbances at the system level, and overcoming the limited sensitivity of the photodetector 19. Again, an optional hysteresis module (not shown in the figure) associated with the control system 14 can be inserted for each phase modulator 131, 132, ?, 13n.

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

In one example, the spectral analyzer 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 ring-shaped waveguide 29, having an effective refractive index neff and coupled to the input waveguide 28 for selecting at least a certain frequency ?res of the scattered light, an output waveguide 30 coupled to the ring-shaped waveguide 29 for outputting the selected frequencies, and a pass-through waveguide 31 coupled to the input waveguide 28 and the ring-shaped waveguide 29 for outputting the non-selected frequencies, and wherein the spectral analyzer 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 optical path variation of the closed ring waveguide 29. This is shown in Figure 7A. In this case, the background Rayleigh-free Brillouin signal from the optical filter 1 or the filter 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 maximize the Q-factor of the ring and at the same time avoid substantial signal transmission losses. To increase the coupling strength, the ring may have an oblong shape (racetrack configuration) in which the coupling region is elongated to overcome the limited lithographic resolution. The ring resonance may can be quickly adjusted by the modulator element 27 (e.g. a heating element or an opto-electric means) and the transmitted light at the output waveguide 30 (DROP) can be acquired by a photodetector 45. By scanning over at least one value of FSR = ?<2>/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 frequency signal intensity based on the voltage variation applied to the modulator element 27 is shown in Figure 7B.

Figure 8 shows an example of a spectrometer 20 where the spectral analyzer 3 comprises a plurality of optical ring resonators 261, 262, ?, 26n arranged in a cascade, where the closed ring waveguide 291 of a first optical resonator 261 is coupled with the ring waveguide 292 of a second subsequent optical resonator 262. The configuration of figure 8 is useful when a single ring resonator 26 is not sufficient to obtain a resolution below GHz (sub-GHz) due to the losses (curvature and propagation) within the single ring. Advantageously, the spectral analyzer 3 also comprises at least two modulator elements 27A, 27B coupled to each optical ring resonator 261, 262, ?, 26n. A phase modulator 27An is used to bring the rings to the same resonant wavelength by carefully adjusting the applied voltage VRn1. Once this condition is achieved, a Brillouin spectrum can be acquired by rapidly scanning the resonance of all rings simultaneously by adjusting the voltage VRN2 of the second phase modulator 27Bn.

In one example, the spectral analyzer 3 includes a single optical dispersive element (VIPA) 49 to disperse 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 the VIPA allows the Brillouin spectrum to be analyzed without having to perform a spectrum scan. However, due to the dominant signal due to elastic Rayleigh light, it is usually necessary to use a plurality of VIPA elements in order to increase the spectral contrast in order to measure the Brillouin peaks otherwise covered by the Rayleigh light. By using the optical filter 1 as described above, it is instead possible to use only a single optical dispersive element (VIPA) 49, thus making the overall system more compact. In other words, given the efficiency of the optical filter 1 (or filter module 22) in filtering the elastic Rayleigh component, it is possible to use a single optical dispersive element (VIPA) 49. it is possible to realize a spectral analyzer 3, and consequently a spectrometer 20, which is compact and has excellent performance for Brillouin spectroscopy.

As shown in Figure 9, the spectral analyzer 3 is coupled to the optical filter 1 or the filter module 22 via an input optical fiber 46 to receive the output signal of the filter in which the Rayleigh elastic component is suppressed or attenuated. For example, the input optical fiber 46 receives the first output signal 15. The spectral analyzer 3 further comprises a first lens 47 (collimating lens) for receiving and collimating the output signal 15 to be analyzed and a second lens 48 (focusing lens) for focusing the collimated output signal 15.

The spectral analyzer 3 further comprises an angle-dispersive 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 angle-dispersive optical element 49 into a plurality of spectral peaks for measuring the spectrum of the scattered light 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 that a single VIPA device can be used, it is also conceivable to consider a spectral analyzer 3 that alternatively has a plurality of these VIPA optical devices.

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

A technician in the field may make numerous further modifications and variations to the filter 1, the spectrometer 20 and the apparatus 37 described above, in order to satisfy further and contingent needs, all of which however fall within the scope of protection of the present invention as defined by the attached claims.

Claims (12)

1. Optical filter (1), in particular Mach-Zehnder interferometer, for suppressing or attenuating the elastic Rayleigh component of a diffused or reflected light, said filter (1) being integrable within an active and/or passive photonic circuit and positionable between a diffused light source (2) and a spectral analyzer (3), wherein the filter (1) comprises: at least one input (4) for receiving the diffused light; at least one splitter element (5) for dividing the diffused light into a first light beam (6) which travels along a first path (7) and into a second light beam (8), separate from the first light beam (6), which travels along a second path (9), wherein the difference between the first path (7) and the second path (9), defined as the path difference (?L), is different from 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 combining element (10) for combining the first light beam (6) exiting from the first path (7) with the second light beam (8) exiting from the second path (9), and a first output (11) and a second output (12) coupled to the combining element (10), wherein the first output (11) is configured to propagate a first output signal (15) where the elastic Rayleigh component of the light spectrum is attenuated or suppressed as a result of destructive interference of said elastic Rayleigh component between the first light beam (6) and the second light beam (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 adjust the interference between the first light beam (6) and the second light beam (8) via the phase modulator (13) and minimize 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) to measure 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) to take a portion of the first output signal (15) which is used by the closed-loop control system (14) to regulate the interference between the first light beam (6) and the second light beam (8) via the phase modulator (13) and minimize the first output signal (15) so as to filter the Rayleigh elastic component from the first output signal (15). 5. Filter (1) according to one of claims 1 to 4, wherein a. the intensity of the first light beam (6) is equal to the intensity of the second light beam (8) and where the path difference (?L) is different from zero; or b. the intensity of the first light beam (6) is greater than the intensity of the second light beam (8) and where the path difference (?L) is greater than zero. 6. Filter (1) according to one 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. Spectrometer (20) for analyzing the spectrum of a 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 a scattered or reflected light according to one of claims 1 to 6, wherein said filter (1) is coupled to the main input (21) of the spectrometer (21); and a spectral analyzer (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 spectrum profile of the scattered light, wherein said spectral analyzer (3) is coupled to the first output (11) of the filter (1), where at least the filter (1) is made of a photonic integrated circuit. 8. Spectrometer (20) according to claim 7, comprising a plurality of filters (11, 12,?,1n) connected to each other 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 analyzer (3), wherein the first output (111) of a first filter (11) of the plurality of filters (11, 12,?,1n) is coupled to the input (42) of a subsequent second filter (12). 9. Spectrometer (20) according to claim 8, wherein: a. the closed-loop control system (14) comprises a single-input, single-output control system, SISO; or b. the closed-loop control system (14) includes a multiple-input, multiple-output, MIMO control system. 10. Spectrometer (20) according to one of claims 7 to 9, wherein the spectral analyzer (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 ring-shaped waveguide (29), having an effective refractive index neff and coupled to the input waveguide (28) for selecting at least one given frequency ?res of the scattered light, an output waveguide (30) coupled to the ring-shaped waveguide (29) for the output of the selected frequencies and a pass-through waveguide (31) coupled to the input waveguide (28) and to the ring-shaped waveguide (29) for the output of the non-selected frequencies, and wherein the spectral analyzer (3) further comprises a modulator element (27) coupled to the optical ring resonator (26) to modulate the effective refractive index neff and to scan the different multiple frequency components by varying the optical path of the closed ring waveguide (29). 11. Spectrometer (20) according to claim 10, wherein the spectral analyzer (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). 12. Apparatus (37) for Brillouin spectroscopy, microscopy or endoscopy comprising the spectrometer (20) according to one of claims 7 to 11.