Abstract
Spectrometer for analyzing the spectrum of a Brillouin scattered light including input means receiving the scattered light, and selecting means for selecting and separating specific multiple frequency components of the scattered light. The selecting means has at least one main input, and at least an optical detector is coupled to the selecting means for measuring the intensity of the different frequency components and reconstructing the spectrum profile of the scattered light. The selecting means include an optical integrated circuit having at least one optical ring resonator of a first type having an input waveguide for receiving the light from the input means, a closed loop waveguide having an effective refractive index n.sub.eff, and an output waveguide. The selecting means further have at least a modulator element of the effective refractive index n.sub.eff coupled to the closed loop waveguide of the optical ring resonator of the first type.
Claims
1. Spectrometer for analyzing the spectrum of a Brillouin scattered light, comprising: input means for receiving the scattered light; selecting means coupled to the input means for selecting and separating specific multiple frequency components of the scattered light, wherein said selecting means comprise at least one main input; and at least an optical detector coupled to the selecting means for measuring the intensity of the different frequency components and reconstructing the spectrum profile of the scattered light, wherein the selecting means are composed by an optical integrated circuit having at least an input port, at least an output port and at least an pass port, said circuit comprising at least one optical ring resonator of a first type having an input waveguide for receiving the light, a closed loop waveguide having an effective refractive index n.sub.eff and coupled to the input waveguide for selecting at least a specific frequency ν.sub.res of the scattered light, an output waveguide coupled to the closed loop waveguide for the output of the selected frequencies and a pass waveguide coupled to the input waveguide and to the closed loop waveguide for the output of the not selected frequencies and wherein the selecting means further comprise at least a modulator element coupled to the closed loop waveguide of the optical ring resonator of the first type for modulating the effective refractive index n.sub.eff and scanning the different multiple frequency components through the variation of the optical path of the closed loop waveguide.
2. Spectrometer according to claim 1, wherein the closed loop waveguide of the optical ring resonator of the first type has the shape of a circumference having a radius R or has an oblong shape like an oval made of the conjunction of a central rectangular region with two semicircles at the opposite sides of said central region, wherein both semicircles have a diameter D=2R.
3. Spectrometer according to claim 2, wherein the radius R of the circumference or the semicircles of the closed loop waveguide of the optical ring resonator of the first type has a value comprised between 100.0 μm and 1.0 mm.
4. Spectrometer according to claim 1, wherein the optical integrated circuit comprises a core region where the light is propagated inside the waveguides and which is made of a material transparent in the field of the visible and of the near infrared, preferably in Si.sub.3N.sub.4, and a cladding region which is made of a material having a refractive index lower than that of the core region, preferably SiO.sub.2, which surrounds the core region.
5. Spectrometer according to claim 1, wherein the selecting means comprise a plurality of optical ring resonators of the first type arranged in cascade, wherein the closed loop waveguide of a first optical resonator is coupled with the closed loop waveguide of a second successive optical resonator and wherein between the closed loop waveguide of the first optical resonator and the closed loop waveguide of the second successive optical resonator are located detuning means.
6. Spectrometer according to claim 5, wherein the optical ring resonators of the first type arranged in cascade have am increasing or decreasing variable radius with a factor comprised between 0.9 and 0.999.
7. Spectrometer according to claim 1, wherein the selecting means comprise at least an optical ring resonator of a second type for suppressing or attenuating the elastic components of the scattered light, wherein said optical ring resonator of the second type is positioned between the input port of the optical integrated circuit and the optical ring resonator of the first type, wherein the optical ring resonator of the second type comprises a pass waveguide coupled to the input waveguide of the optical ring resonator of the first type.
8. Spectrometer according to any claim 1, wherein the distance between the closed loop waveguide and the input waveguide of the optical resonator of the first type has a value comprised between 50 nm and 500 nm.
9. Apparatus for the spectroscopy or microscopy or endoscopy of Brillouin comprising the spectrometer according to claim 1.
10. Method for analyzing the spectrum of a Brillouin scattered light, comprising: receiving the scattered light; selecting and separating specific multiple frequency components of the scattered light by conducting the scattered light in an input waveguide of at least one optical ring resonator of a first type; scanning the different multiple frequency components through at least a modulator element that modulates the effective refractive index n.sub.eff of a closed loop waveguide of said optical ring resonator of the first type through the variation of the optical path of the closed loop waveguide; and measuring the intensity of the frequency components and reconstructing the spectrum profile of the scattered light.
11. Method according to claim 8, further comprising the step of: suppressing or attenuating the elastic components of the scattered light by conducting the scattered light in an input waveguide of at least one optical ring resonator of a second type before selecting and separating the frequency components of the scattered light through the optical ring resonator of the first type.
12. Use of an optical integrated circuit comprising at least an optical ring resonator of a first type for measuring the intensity of different frequency components of a Brillouin scattered light and analyzing the spectrum of said scattered light, and selecting and separating specific multiple frequency components of the scattered light by conducting the scattered light in an input waveguide of at least one optical ring resonator of a first type, wherein the closed loop wave guide of the optical ring resonator of the first type has a radius R with a value comprised between 100.0 μm and 1.0 mm and the optical integrated circuit comprises an core region where light is propagated within waveguides and made of a transparent material in the visible and in the near infrared field, preferably in Si.sub.3N.sub.4 and a cladding region made of a material having a refractive index lower than that of the core region, which surrounds the core region.
13. Method for reducing the size of an optical system of Brillouin spectroscopy, microscopy or endoscopy for the analysis of the mechanical and structural properties of a sample, said optical system comprising at least one light source, preferably a laser, incident on said sample and generating a scattered light, an optical apparatus for receiving and measuring the scattered light spectrum, by acquiring data as a function of the frequency of said scattered light, and a computer system for analyzing said data, wherein the method comprises the step of replacing said optical apparatus with the spectrometer according to claim 1.
Description
[0054] These and other aspects of the present invention will become clearer after the following description of some preferred embodiments described below.
[0055] FIG. 1 shows a schematic representation of the spectrometer according to the present invention;
[0056] FIG. 2 shows a schematic representation of the apparatus according to the present invention;
[0057] FIG. 3 shows a schematic representation of an optical ring resonator;
[0058] FIG. 4a-b show a schematic representation of an optical ring resonator (a) and a cross-section thereof (b);
[0059] FIG. 5a-b show a schematic representation of the spectral profiles obtained at the output waveguide (a) and at the pass waveguide (b) of an optical ring resonator;
[0060] FIG. 6a-d show a schematic representation of two cascaded optical ring resonators at the drop port (a) and at the pass port (b) and the associated intensity profile plots as well as a configuration having two (c) or more than two (d) cascaded optical resonators according to the present invention;
[0061] FIG. 7 shows a schematic representation of a configuration having an optical ring resonator of the second type coupled with an optical resonator of the first type;
[0062] FIG. 8 shows a schematic representation of a configuration having a plurality of optical ring resonators of the second type coupled with a plurality of optical resonators of the first type;
[0063] FIG. 9 shows a schematic representation of a second configuration having a plurality of optical ring resonators of the second type coupled with a plurality of optical resonators of the first type arranged in parallel;
[0064] FIG. 10 shows a schematic representation of a third configuration having a plurality of optical ring resonators of the second type coupled with a plurality of optical resonators of the first type arranged in cascade and in parallel;
[0065] FIG. 11 shows a flow chart illustrating the method for analyzing the spectrum of the Brillouin scattered light according to the present invention;
[0066] FIG. 12 shows a schematic circuit of a single optical ring resonator having the oblong shape of an oval;
[0067] FIG. 13 shows a schematic representation of ring resonators of the first type arranged in cascade and having different diameters;
[0068] FIG. 14a-b show experimental data of the resonance frequencies of oblong-shaped optical ring resonators mounted on an integrated circuit in which a light signal enters the input port and leaves at the drop port;
[0069] FIG. 15a-b show experimental data of the resonance frequencies of oblong-shaped optical ring resonators mounted on an integrated circuit in which a light signal enters the input port and exits through the pass port.
[0070] FIG. 1 shows an embodiment of a spectrometer 10 according to the present invention. The Brillouin scattered light L comprising an elastic component (Rayleigh) and an inelastic component (Stoke and Anti-Stokes peaks) arises from the interaction between a light source and a target material. The scattered light L is led by input means 20 into the selection means 30 through a main input 31. The selection means 30 comprise at least one optical ring resonator 32 which is made of at least an input waveguide 33, a closed ring waveguide 34, an output waveguide 35 and a pass waveguide 36. Inside the selection means 30, the light L is led through the input waveguide 33 into the closed loop waveguide 34. Given the resonance process similar to that occurring in a Fabry-Perot interferometer, only some frequency components of the scattered light are selected and separated from one to the other. In more details, the multiple frequency components are separated at the output of the optical ring resonator 32 by a quantity known as FSR (free spectral range), which depends both on the value of the radius R of the optical ring resonator and on the value of the group index n.sub.g of the waveguide, which is a particular type of refractive index that takes into account the dispersion of light. In order to measure the frequency shift, for example of the Brillouin scattered light, from the main frequency of the laser, as well as the linewidth of the Stokes and Anti-Stokes peaks, the frequency components can be scanned by locally varying the effective refractive index of the ring waveguide 43 through the modulator element 37 which varies the refractive index n.sub.eff. The modulator element 37 is capable of performing a rapid scan (in the order of MHz and GHz) of the different frequency components through a variation of the optical path of the ring waveguide 34, providing a frequency tuning from the elastic Rayleigh light to the inelastic Stokes and Anti-Stokes light components, for example of the Brillouin scattered light. An optical detector 40, such as a photomultiplier or photodiode, is coupled to the output waveguide 35 to reconstruct the spectral profile of the frequency peaks during the scanning process. The variation of the n.sub.eff refractive index induced by the modulator 37 can take place in a discrete manner. An intensity value measured by the detector 40 corresponds to each discrete level of this variation of the n.sub.eff refractive index. In this way it is possible to reconstruct the entire spectral profile of the scattered light L coupled into the optical ring resonator 32.
[0071] The detector 40 can be integrated into the selection means 30 and being directly coupled to the output waveguide 35 of the optical resonator 32. Alternatively, the detector 40 can be an external and detachable element coupled to the selection means 30 through a main output 39 (as shown in FIG. 1). Finally, a modulator element 37 is coupled to the optical resonator 32, in particular to the closed ring waveguide 34.
[0072] FIG. 2 shows an apparatus 50 which comprises the spectrometer 10. The apparatus 50 can be, for example, an apparatus for Brillouin spectroscopy, microscopy or endoscopy or a similar technique. The apparatus 50 comprises a light source S, for example a laser source, which emits a light beam LB with a wavelength between 400 nm and 1500 nm propagating towards a target material T. The target T may be a biological material. The scattered light L is collected and analyzed by the spectrometer 10. By means of a computer system C, coupled to the spectrometer 10, the acquired data can be analyzed and appropriately displayed.
[0073] FIG. 3 shows a schematic circuit which integrates a single optical ring resonator of the first type 32. The scattered light L is led to the input waveguide 33 through the optical fiber coupler 20 and is indicated by an arrow at the “input” port of the circuit. A certain amount of light enters the ring waveguide 34 of the optical resonator 32 of radius R as a consequence of the presence of the evanescent field generated at the interface between the core region and the cladding region of the waveguide 34. As already anticipated previously, the coupling efficiency strictly depends on the distance d (shown in FIG. 4) of the ring resonator 32 from the input waveguide 33. Once the scattered light enters the closed ring waveguide 34, a constructive and destructive interference occurs, as described above. The dashed arrows in FIG. 3 represent the direction of the light path inside the closed ring waveguide 34. The light exiting the ring travels through the output waveguide 35, or at the “drop” port, to the optical detector 40. The non-selected light components by the closed ring waveguide 34 exit the ring through the pass waveguide 36 and reach the “pass” port of the circuit. The modulator element 37 is coupled to the closed ring waveguide 34 for adjusting the resonance frequency of the optical ring resonator 32. For example, the modulator element 37 can comprise a heating system to which a voltage V.sub.1 is applied.
[0074] FIG. 4b shows an exemplary configuration of the modulator element 37 inside a circuit (or chip) that integrates the optical resonator 32. Specifically, the modulator element 37 comprises a series of micro-heaters positioned on the upper side of the closed ring waveguide 34. It should be noted that FIG. 4b represents a cross-section of the circuit along the dotted line shown in FIG. 4a. The Cartesian axes shown in FIGS. 4a and 4b better clarify this concept. FIG. 4b further illustrates how the modulator element 37 is only located above the closed ring waveguide 34 to vary the optical path of said waveguide. The input 33, output 35 and pass 36 waveguides are located at a distance d from the ring waveguide 34. The modulating elements 37 or micro-heaters can be made of conductive materials such as titanium, platinum, nickel, chromium, or similar. The modulator elements 37 can be made of only one of these materials or a combination of two or more materials. Specifically, the circuit can comprise a core region 341 where the light propagates, for example realized in Si.sub.3N.sub.4 and which comprises the input 33, output 35, pass 36, and ring 34 waveguides. The circuit further comprises a cladding region 342 and 343, for example made of SiO.sub.2, as well as a substrate 344, for example made of silicon, on which the cladding region 342, 343 is located.
[0075] FIGS. 5a and 5b show the spectral profiles obtained at the “drop” port (FIG. 5a) and at the “pass” port (FIG. 5b). FIG. 5a shows the transmission of multiple elastic peaks (E.sub.n, E.sub.n+1) which are separated by one FSR (free spectral range) dependent on the value of the radius R of the optical ring resonator and on the effective refractive index n.sub.eff of the waveguide. FIG. 5a also shows the transmission of the inelastic Stokes and Anti-Stokes peaks (dotted line) for example of the Brillouin scattered light (B.sub.n, B.sub.n+1) arising along one FSR of the spectrometer. For a core region 341 realized with Si.sub.3N.sub.4 having a refractive index n˜2 and a cladding region 342, 343 made of SiO.sub.2 having a refractive index n˜1.4 and therefore with an effective refractive index n.sub.eff=1.4-1.8, for a ring radius R=(100-1000) μm, it is obtained a value of the FSR that is suitable for Brillouin spectroscopy (FSR=30-120 GHz). Using the same materials for the core region 341 and the cladding region 342, 343 but with a radius of value of R=(1-100) μm, it is obtained a FSR value suitable for the Raman THz spectroscopy (FSR=0.3-6.0 THz). The spectral contrast (SC), which is defined as the intensity ratio between the peak and the background, can assume, in the case of a single ring resonator of the first type 32, a value up to about 30 dB. On the other hand, the spectral resolution can be associated with the FWHM (Full-Width-at-Half-Maximum) bandwidth of the transmitted peaks and, in the case of Brillouin spectroscopy, assumes a value in the order of 100 MHz. As described above, in order to measure both the Brillouin frequency shift with respect to the main laser frequency and the bandwidth of the Brillouin peaks, it is necessary to scan the frequency components through the use of a modulator element that varies the effective refractive index n.sub.eff of the waveguide. A complete spectral profile, including at least one Rayleigh peak (E) and one Brillouin peak (B), can be reconstructed by detecting the output intensity values associated with the discrete n.sub.eff variation at the drop port using an optical detector.
[0076] At the pass port (FIG. 5b) it is obtained the inverse intensity profile with respect to that obtained at the drop port. This is the result of the energy conservation. A circuit with an optical ring resonator where the output port corresponds to the pass port is extremely useful as a narrow-band optical filter of certain wavelengths. The extinction ratio (ER) for a single-ring optical resonator is always 30 dB and depends on the optical losses of the ring.
[0077] To increase the spectral contrast of the spectrometer 10, it is possible to use two or more optical ring resonators of the first type 32 in cascade. For example, two identical optical ring resonators of the first type 32.sub.1 and 32.sub.2, each providing 20 dB in the spectral contrast and arranged in cascade at the drop port, work as an effective spectral analyzer providing a total contrast of 40 dB (FIG. 6a). In the same way, two identical ring resonators of the second type 38.sub.1 and 38.sub.2, each providing an extinction ratio of 20 dB and arranged in cascade at the pass port, work as an effective spectral filter providing a total extinction value of 40 dB (FIG. 6b). It can be noted that in the vertical cascade arrangement (FIG. 6a), i.e. at the exit of the drop port, the ring resonators of the first type 32.sub.1 and 32.sub.2 are displaced with respect to each other to avoid a resonant effect. The curved dashed arrows indicate the path of the light inside the waveguide. This means that a possible third ring resonator of the first type 32.sub.3 (not shown in the figure) would be shifted to the right with respect to the second ring resonator 32.sub.2 and located along the same vertical axis of the first ring resonator 32.sub.1 according to a zigzag configuration.
[0078] It is also noted that an intermediate BUS waveguide 32.sub.4 is located between the first ring resonator 32.sub.1 and the second ring resonator 32.sub.2.
[0079] FIG. 6c shows a configuration where two optical ring resonators of the first type 32.sub.1 and 32.sub.2 are coupled together by means of the respective closed ring waveguides 34.sub.1 and 34.sub.2 and the intermediate waveguide 32.sub.4. FIG. 6d shows the general case of a plurality of N said optical resonators. In other words, the limited spectral contrast of a single optical ring resonator can be increased by adding the spectral contrast contribution of the next resonator. The resonators in FIG. 6 show ring waveguides that have the same radius R. However, it is also possible to arrange the spectrometer 10 with optical ring resonators of different radii. Furthermore, each ring waveguide is coupled to an associated modulator element 37.sub.1, 37.sub.2, . . . 37.sub.N to which a voltage V.sub.1, V.sub.2, . . . V.sub.N is applied. The voltage may be different depending on the optical ring resonator for tuning a specific resonance frequency of the resonator and thus to avoid spectral broadening. This is necessary to compensate for small fab imperfections on the optical path length of the waveguides that could induce a slight shift in the resonance frequency for each ring resonator. It should be noted that the optical resonators of the first type 32 shown in FIGS. 6c and 6d are arranged one above the other only for graphic convenience but instead they must be intended as displaced as in the case of FIG. 6a.
[0080] FIG. 7 shows a schematic circuit which, besides a single optical ring resonator of the first type 32, also integrates a single optical ring resonator of the second type 38. In more details, the optical resonator of the second type 38 is located between the input port or circuit input and the optical resonator of the first type 32. FIG. 8 shows a configuration in which a plurality of optical ring resonators of the second type 38.sub.1, . . . 38.sub.M, are arranged between the input port of the circuit and a plurality of optical ring resonators of the first type 32.sub.M+1, . . . 32.sub.M+N. The optical resonator of the second type 38 operates in this configuration through the pass port as an optical filter to suppress the elastic component (Rayleigh) of the scattered light, the latter being typically a few orders of magnitude more intense than the inelastic components of the scattered light. By suppressing or attenuating this elastic component of light, the visibility of the Brillouin peaks can be significantly increased. Furthermore, this can prevent a saturation of the optical detector during the scanning process. In order to avoid any kind of interference between the light coming out of the drop port and the optical ring resonator of the second type 38, as shown in FIG. 7, the output waveguide 35 can be advantageously deviated from the direction of the optical ring resonator of the second type 38, thus assuming a curvilinear shape. This deviation is useful for preventing the elastic components of the scattered light rejected by the ring resonator of the second type 38 from being coupled again in the drop port at the ring waveguide 38.sub.2 of the second type 38, thus eliminating the advantages of the filter provided by the ring waveguide 38.sub.2 of said optical ring resonator of the second type 38.
[0081] In more details, the elastic light can be effectively suppressed or attenuated by fine tuning the voltage V.sub.M applied to each optical ring resonator of the second type 38.sub.1, . . . 38.sub.M. As a result, only the Brillouin signals will be detected at the drop port after the scan process by means of the modulating elements 37.sub.M+1, . . . 37.sub.M+N of the optical ring resonators of the first type 32.sub.M+1, . . . 32.sub.M+N arranged in cascade. It should be noted that the optical resonators of the first type 32 shown in FIG. 8 are arranged one above the other for pure graphic convenience but instead they must be conceived to be displaced vertically as in the case of FIG. 6a.
[0082] FIG. 9 shows an alternative schematic circuit which integrates a plurality of optical ring resonators of the first type 32.sub.M+1, 32.sub.M+2, . . . 32.sub.M+N arranged in parallel with a plurality of optical ring resonators of the second type 38.sub.1, . . . 38.sub.M. Unlike the previous configurations, the optical ring resonators of the first type 32.sub.M+1, 32.sub.M+2, . . . 32.sub.M+N are arranged in parallel so that each one is coupled to a different drop port. This distribution enables either through a suitable periodic modulation of the effective refractive index of each ring or through a variation of the radius of the rings to simultaneously select all the light frequency components distributed along an FSR. Consequently, a single optical detector 40.sub.1, 40.sub.2, . . . 40.sub.N is associated to each optical ring resonator of the first type 32.sub.M+1, 32.sub.M+2, . . . 32.sub.M+N. In this case, the modulators that act on the optical resonators of the first type 32.sub.M+1, 32.sub.M+2, . . . 32.sub.M+N are not used to perform a frequency scan, but rather to apply a periodic modulation so that each ring selects only one specific frequency at the output of its associated drop port, allowing other non-selected frequencies to pass through the pass port. In this way, while a single optical detector 40.sub.i would measure the intensity of a single light component with frequency vi, the subsequent optical detector 40.sub.i+1 arranged in parallel would measure the intensity of a single light component with frequency ν.sub.i+1=ν.sub.i+δν. Following this approach, all frequencies contained within a spectral range defined by at least one single FSR would be measured simultaneously. To exploit the effective spectral resolution of the spectrometer, the number N of optical ring resonators of the first type arranged in parallel ideally should be twice the finesse, defined as the FSR/FWHM ratio of the spectrometer. However, in order to enable a faster acquisition and to reduce the number of optical detectors being used (and thus the production cost of the on-chip spectrometer), this number N could also be less than 2*finesse.
[0083] FIG. 10 shows an alternative schematic circuit which integrates a plurality of optical ring resonators of the first type 32.sub.M+1, 32.sub.M+2, . . . 32.sub.M+N . . . 32.sup.P.sub.M+1, 32.sup.P.sub.M+2, . . . 32.sup.P.sub.M+N with a plurality of optical ring resonators of the second type 38.sub.1, . . . 38.sub.M. Unlike the previous configurations, this configuration comprises ring resonators of the first type arranged both in cascade and in parallel. In particular, while an optical detector at the output of the drop port of a series of cascaded resonators measures the intensity of a single light component with frequency vi, the next one measures the intensity of the following frequency component ν.sub.i+1=ν.sub.i+δν selected through the parallel series of cascaded resonators. In essence, the configuration illustrated in FIG. 10 differs from that of FIG. 9 because the contrast SC and the spectral resolution FWHM of the spectrometer can be increased by means of a plurality of ring resonators of the first type arranged in cascade, as already described for the configuration of FIG. 6. In a similar way to FIG. 9, the configuration described in FIG. 10 allows to acquire the entire spectrum of the scattered light without the need to perform a frequency scan. In this regard, the modulators present in this configuration have the only purpose on the one hand of adjusting the resonance frequency among the waveguides of the cascaded ring resonators, and on the other of introducing a periodic modulation of the effective refractive index (and therefore of the optical path) among the ring resonators arranged in parallel. It should be noted that the optical resonators of the first type 32 shown in FIG. 10 are arranged one above the other for pure graphic convenience but instead they must be intended to be displaced vertically and provided with corresponding intermediate BUS waveguides 324 as in the case of FIG. 6a.
[0084] FIG. 11 shows a flowchart of the method 100 for analyzing the spectrum of the Brillouin scattered light. The method 100 initially involves the step 102 of receiving the scattered light L. The Brillouin scattered light comprising an elastic component (Rayleigh) and an inelastic component (Stokes and Anti-Stokes peaks) arises from the interaction between the light source S and a target material T. The method 100 further involves the step of selecting and separating 104 certain multiple frequency components of the scattered light L by leading the scattered light L in an input waveguide 33 of at least one optical ring resonator of the first type 32. Next, the method 100 involves the scanning 106 of the different multiple frequency components by means of a modulator element 37 of the effective refractive index n.sub.eff of a closed ring waveguide 34 of said optical ring resonator of the first type 32 by varying the optical path of the closed ring waveguide 34. Finally, method 100 involves the steps of measuring 108 the intensity of the frequency components and reconstructing the spectrum profile of the scattered light L.
[0085] Before the step of selecting and separating 104 the frequency components of the scattered light L by means of an optical ring resonator of the first type 32, the method 100 can further involve the step of suppressing or attenuating 110 the elastic components of the scattered light L by leading the scattered light L in an input waveguide 381 of at least one optical ring resonator of the second type 38.
[0086] FIG. 12 shows a schematic circuit which integrates a single optical ring resonator of the first type 32 having the oblong shape of an oval as described above. Of course, this also applies to the ring resonator of the second type 38. The scattered light L, indicated with an arrow at the port “input” of the circuit, is led to the input waveguide 33 by means of the optical fiber coupler 20. It should be noted that the oval ring waveguide is constituted by a central rectangular region 34.sub.R joined at the sides by two semicircle regions 34.sub.C1 and 34.sub.C2 of radius R. In particular, if the central region 34.sub.R is defined by the sides L.sub.1 and L.sub.2, the diameter D=2R of each half-circle 34.sub.C1 and 34.sub.C2 coincides with one of the sides of the central region 34.sub.R, in particular with the side that does not form the oblong ring. According to the example shown in FIG. 12, the diameter D of each semicircle 34.sub.C1 and 34.sub.C2 coincides with the length of the side L.sub.2 of the central region 34.sub.R. The total length of the ring will therefore be determined by the sum of the two semicircles of radius R and of the two sides L.sub.1. A given amount of light enters the ring waveguide 34 of the optical resonator 32 following the formation of the evanescent field generated at the interface between the core region and the cladding region of the waveguide 34 (illustrated in details in the bottom figure showing a simulation of a light beam that is partially coupled into the ring waveguide from the input waveguide). Thanks to the oblong shape of the ring, the coupling efficiency is increased. Once the scattered light enters the closed ring waveguide 34, constructive and destructive interference takes place, as described above. The dashed arrows in FIG. 12 represent the direction of the light path inside the closed ring waveguide 34. The light coming out of the ring is led through the drop port 35 to the optical detector 40.
[0087] FIG. 13 shows a configuration in which ring resonators of the first type 32.sub.1 and 32.sub.2 are arranged in cascade and have different diameters from each other. For simplicity, the figure only shows two ring resonators. However, the same principle can be applied to a plurality of ring resonators of the first type. Furthermore, the figure shows a configuration of increasing diameter where a second ring resonator 32.sub.2 has a larger diameter with respect to that of a first ring resonator 32.sub.1. However, the same principle can also be applied for a decreasing diameter configuration. FIG. 13 highlights that this kind of configuration determines a total Free Spectral Range (FSR.sub.tot) given by the combination of the FSR.sub.1 and FSR.sub.2 associated with each ring. As a result, it is possible to extend the Free Spectral Range of the total system by combining ring resonators with variable diameters.
[0088] FIGS. 14 and 15 show the resonance frequencies measured for oblong-shaped optical ring resonators mounted on an integrated circuit. In particular, FIG. 14a shows an image of a resonator in which a light signal enters through the input port and exits at the ‘drop’ port. By changing the voltage applied to the modulator element 37 arranged above the ring waveguide 34, several peaks associated with the specific resonance frequencies of the ring are obtained in transmission (FIG. 14b). This process is similar to that of a Fabry-Perot interferometer used to scan the Brillouin light spectrum.
[0089] FIG. 15a shows an image of a resonator in which a light signal enters through the input port and exits through the ‘pass’ port. By changing the voltage applied to the modulator element 37 arranged above the ring waveguide 34, several dips associated with the specific resonance frequencies of the ring are obtained in transmission (FIG. 15b). These dips are used to suppress the (very intense) elastic light signal that would otherwise prevent the detection of Brillouin peaks. In this regard, a ring resonator arranged in this configuration works as a filter of the elastic light, thus increasing the visibility of Brillouin spectral peaks.
[0090] To the spectrometer, to the apparatus, to the method and to the use described above, a skilled person of the field, in order to satisfy further and contingent needs, may make several further modifications and variations, all of which are included in the scope of protection of the present invention as defined by the claims.
