Apparatus for spectroscopic analysis and method for spectroscopic analysis

Abstract

An apparatus (1) for spectral analysis of optical signals and a spectral analysis method using said apparatus, wherein the apparatus (1) comprises: an optical filtering stage (2) having an optical path (5); a photodetector (17) optically connected downstream of the optical filtering stage (2); an actuator (18) configured for changing an optical refractive index of at least one portion of the optical path (5); wherein the optical filtering stage (2) comprises a first Mach-Zehnder interferometer (6) and a first ring resonator (7), cascaded to each other to form a respective part of the optical path (5), and wherein a respective optical path difference (dL0) between a first (11) and a second optical branch (12) of the first Mach-Zehnder interferometer (6) is greater than zero, the first Mach-Zehnder interferometer (6) is optically connected to the rest of the optical path (5) via one and only one first port (13) of the input optical couplers (9) and one and only one second port (14) of the output optical couplers (10).

Description

Technical Field

[0001] The present invention relates to an apparatus for spectral analysis and a method for spectral analysis. Background Art

[0002] The present invention belongs to the field of photonics, namely a set of technologies and methods for optical signal generation, and / or transmission, and / or processing and / or reception.

[0003] The term "optical" refers to an electromagnetic radiation which does not necessarily strictly fall within the visible light frequency band (i.e., indicatively 400 - 700 nm), but more generally falls within a broader band than this visible light frequency band, for example including near infrared light (e.g., wavelengths between approximately 700 nm and approximately 2 μm).

[0004] In the field of photonics, apparatuses for spectral analysis (also known as OSA, from the English "optical spectrum analyzer") are known, which are used to analyze the spectrum of an optical signal. The expression "spectrum" generally refers to the power distribution of an optical signal (e.g., expressed in dBm or Watt), which is a function of wavelength or frequency.

[0005] Document US2017 / 0331550 A1 discloses an OSA which includes a modulator, an integrated optical filter, and a photodetector. Summary of the Invention

[0006] In the context of the above-mentioned apparatus for spectral analysis, the applicant has made the following considerations (hereinafter, when referring to the wavelength of an optical signal, similar considerations also apply to the frequency aspect).

[0007] First, the applicant believes that it is particularly advantageous to be able to determine the spectrum of an optical signal with high resolution, i.e., to be able to determine the light intensity of a plurality of optical sub-signals, each optical sub-signal including its respective wavelength sub-range, such as each wavelength sub-range being as narrow as possible (and the sum of the analyzed sub-ranges completely includes the wavelength range of the original optical signal). This enables obtaining a spectrum with the desired accuracy and which is as faithful as possible to the effective light intensity distribution per unit wavelength of the original optical signal.

[0008] To this end, it is advantageous to produce a device for spectral analysis that includes at least one optical filtering stage for an input optical signal, capable of filtering the optical signal to allow only the transmission (e.g., to a photodetector downstream of the optical filtering stage) of the portion of the optical signal corresponding to as narrow a wavelength sub-range as possible. This translates into the implementation of a device whose optical filtering stage is characterized by having an overall transfer function (e.g., obtained by multiplying the individual transfer functions of the optical filter components that implement the filtering stage), which includes a periodic repetition of (single) peaks having a desired amplitude (e.g., the half-height amplitude, also known as the FWHM): the smaller the amplitude, the higher the achievable resolution.

[0009] The wavelength range facing the above-mentioned peaks is called the passband.

[0010] Furthermore, as described above, since the peaks of the overall transfer function are periodically repeated, if two (or more) repetitions of the above-mentioned peaks are simultaneously within the frequency band of the optical signal (i.e., within the wavelength range contained in the optical signal), interference will be generated in the detected optical intensity because the intensity does not clearly correspond to a single, determined passband but rather to two (or more) passbands. This will lead to problems in determining the final spectrum that actually corresponds to the optical signal.

[0011] Therefore, the applicant has also found that it is particularly advantageous to obtain the overall transfer function of the optical filtering stage in such a way that the distance between two consecutive repetitions of the above-mentioned peaks (referred to as the free spectral range, also known as the FSR) is wide (wider than the entire frequency band of the signal).

[0012] Based on the foregoing considerations, the applicant has found that the devices known in the above-mentioned context have various problems and can therefore be improved in one or more aspects.

[0013] For example, in fact, the integrated optical filter of the device described in US2017 / 0331550 A1, which includes two ring resonators arranged in cascade, is affected by practical manufacturing problems, particularly those related to the level of construction quality (e.g., very small manufacturing tolerances) that the ring resonators must meet in order to achieve the desired (high) resolution and wide free spectral range. For example, a parameter that characterizes a ring resonator is the quality factor of the ring, also known as the Q factor, which represents the ratio between the free spectral range and the passband of the transfer function of the ring resonator. In other words, a ring resonator with a very high Q factor (usually 10 4 、10 5 or even higher) generally indicates that it has a reduced passband and a wider free spectral range.

[0014] However, a ring resonator with such a high Q-factor is not easy to produce and / or is not easy to find on the market, and is highly sensitive to manufacturing defects, making these components very fragile.

[0015] Therefore, the present applicant has faced the problem of implementing a device for spectral analysis having desired operating characteristics (e.g., in terms of resolution and free spectral range), and which is at the same time robust and / or has a simple structure and / or is easy to manufacture and / or operate.

[0016] According to the applicant, the above problem is solved by a spectral analysis device according to the appended claims and / or having one or more of the following features.

[0017] According to one aspect, the present invention relates to a device for spectral analysis of optical signals.

[0018] The device comprises:

[0019] - an optical filtering stage having an input port, an output port and an optical path extending from the input port to the output port;

[0020] - a photodetector optically connected to the optical filtering stage downstream of the output port;

[0021] - an actuator coupled to the optical filtering stage and configured to change the optical refractive index of at least a portion of the optical path of the optical filtering stage;

[0022] wherein the optical filtering stage comprises a first Mach-Zehnder interferometer (MZI) and a first ring resonator, the first Mach-Zehnder interferometer (MZI) and the first ring resonator being cascaded with each other to form a respective portion of the optical path,

[0023] wherein the first Mach-Zehnder interferometer comprises an input optical coupler, an output optical coupler and first and second optical branches connecting the input optical coupler to the output optical coupler,

[0024] wherein the respective optical path difference between the first and second optical branches of the first Mach-Zehnder interferometer is greater than zero,

[0025] and wherein the first Mach-Zehnder interferometer is optically connected to the remainder of the optical path of the optical filtering stage through one and only one first port of the input optical coupler and one and only one second port of the output optical coupler.

[0026] According to the applicant, the optical filtering stage includes an unbalanced Mach-Zehnder interferometer (i.e., one optical branch having a greater length than the other) and a ring resonator, allowing the transfer functions of these optical components to be combined in a particularly advantageous and synergistic manner, thereby allowing an overall transfer function of the optical filtering stage with desired characteristics in terms of resolution and free spectral range (i.e., high resolution and wide free spectral range) to be obtained.

[0027] On the one hand, in fact, through the unbalanced MZI, a transfer function with a single peak that is periodically repeated and with a desired free spectral range can be obtained (e.g., having a higher FSR compared to the typical frequency band of the optical signal analyzed by such a device, such as an optical signal for telecommunications).

[0028] On the other hand, the ring resonator allows a corresponding transfer function with peaks (also periodically repeated) characterized by a narrower amplitude (i.e., very narrow peaks) to be provided.

[0029] In this way, by combining the transfer functions corresponding to the above two optical components, an overall transfer function (which represents the product of the individual transfer functions) can be obtained, which combines the advantages of the individual transfer functions and is able to compensate for and mitigate their respective disadvantages with respect to each other, thereby obtaining an overall transfer function with a wide free spectral range and small-amplitude peaks (i.e., high resolution), while also obtaining many practical advantages in terms of the robustness and ease of implementation and / or operation of the optical filtering stage.

[0030] In fact, the MZI itself is not only a robust optical component and is less sensitive to manufacturing tolerances than the ring resonator, but its presence synergistically greatly relaxes the construction constraints on the ring resonator, so the ring resonator does not have to simultaneously have high resolution and a wide free spectral range (i.e., does not have to have a high Q factor), but only needs to (merely) have the desired resolution (i.e., a reduced peak amplitude, independent of its mutual distance), thereby fully exploiting the greater robustness and simplicity of the ring resonator in terms of manufacturing and / or searching for and / or using (since a resonator with only this characteristic is easier to manufacture in practice), and the resulting optical filtering stage.

[0031] Furthermore, the fact that the Mach-Zehnder interferometer is optically connected to the rest of the optical path of the optical filtering stage through one and only one first port of the input optical coupler and through one and only one second port of the output optical coupler (usually a port cross-configured with the first port used by the input optical coupler) enables the signal intensity obtained at the photodetector to directly represent the optical intensity of the optical signal within the passband, thereby limiting the subsequent reprocessing stages (such as software processing) of the photodetector signal, such as Fourier transform spectroscopy (FTS) operations, simplifying the operation of the device and / or reducing the computational workload.

[0032] Finally, the presence of an actuator, which is coupled to the optical filtering stage and is configured to change the optical refractive index of at least a part of the optical path of the optical filtering stage, allows the passband of the overall transfer function of the optical filtering stage to be dynamically changed, thus enabling the entire wavelength range of the optical signal to be scanned and, consequently, the complete spectrum of the optical signal to be analyzed.

[0033] The term "photodetector" refers to an optoelectronic component structured to convert a corresponding optical input signal into an electrical signal (current and / or voltage) representative of the optical intensity of the corresponding optical input signal.

[0034] The terms "upstream", "downstream", "inserted", "input", "output", "cascaded" refer to the propagation direction of the considered optical signal. The term "directly", when used in combination with "upstream", "downstream", etc., means that there are no other inserted elements (except for the connected optical waveguides).

[0035] The term "optical coupler" refers to an optical component capable of distributing the optical power entering a first port between a pair of second output ports. For example, an optical coupler includes a pair of first ports, a pair of second ports, and a pair of optical branches (e.g., semiconductor optical waveguides), each optical branch connecting a corresponding first port to a corresponding second port, where the two optical branches are optically coupled to each other at a corresponding optical coupling path between the corresponding first port and the corresponding second port. Examples of optical couplers are tunable directional optical couplers (also known as TDCs, from the English "tunable directional coupler"), power splitters, multimode interferometers (MMIs), Y-branch interferometers, star couplers, Mach-Zehnder interferometers (MZIs).

[0036] The present invention may have one or more of the following preferred features in the above aspects.

[0037] Preferably, the optical filtering stage includes one or more second Mach-Zehnder interferometers (different from the first Mach-Zehnder interferometer), which are cascaded with each other to respectively form corresponding parts of the optical path. Preferably, each second Mach-Zehnder interferometer includes a corresponding input optical coupler, a corresponding output optical coupler, and corresponding first and second optical branches connecting the corresponding input optical coupler to the corresponding output optical coupler, where the corresponding optical path difference between the corresponding first and second optical branches is greater than zero. Preferably, the differences in the corresponding optical paths of the first Mach-Zehnder interferometer and the one or more second Mach-Zehnder interferometers are different from each other. In this way, the transfer functions of each unbalanced MZI can be appropriately combined to further improve the characteristics of the overall transfer function of the optical filtering stage (e.g., in terms of resolution and / or FSR).

[0038] Preferably, each second Mach-Zehnder interferometer is optically connected to the respective remaining part of the optical path of the optical filtering stage through one and only one respective first port of a respective input optical coupler and one and only one respective second port of a respective output optical coupler. In this way, in terms of the direct relationship between the measured light intensity and the effective light intensity of the filtered part of the optical signal, introducing more MZIs does not interfere with the spectral analysis.

[0039] Preferably, the one and only one second port of the output optical coupler of each Mach-Zehnder interferometer is arranged in a crossed configuration relative to the one and only one first port of the input optical coupler of the respective Mach-Zehnder interferometer. The crossed configuration (or "crossed ports") means that the one and only one second port belongs to an optical branch of the Mach-Zehnder interferometer that is different from the optical branch to which the one and only one first port belongs (and vice versa for the "through ports"). In this way, even in the face of possible structural defects, robust operation of the respective MZI can be achieved. For example, a desired spectral blocking (i.e., the transfer function of the MZI is zero outside the respective bandwidth) can be achieved. In this way, the desired overall transfer function of the optical filtering stage can be obtained.

[0040] Preferably, the one or more second Mach-Zehnder interferometers include two, and / or no more than nineteen, more preferably no more than nine, and even more preferably no more than two second Mach-Zehnder interferometers. In this way, the complexity of the device and / or the number of components are limited. The applicant has also recognized that for a given achievable resolution and free spectral range, the presence of (at least) one ring resonator along the optical path synergistically limits the total number of unbalanced MZIs arranged in cascade in the optical filtering stage.

[0041] Preferably, the first ring resonator is the single ring resonator of the optical filtering stage. Thus, the device has a simple structure.

[0042] In one embodiment, the optical filtering stage includes one or more second ring resonators (different from the first ring resonator), which are cascade-connected to each other to respectively form the respective parts of the optical path. Preferably, the respective micro-rings of the first ring resonator and each second ring resonator all have different radii. In this way, the overall transfer function of the optical filtering stage is further improved in terms of resolution and / or free spectral range.

[0043] In one embodiment, the one or more second ring resonators include two, and / or no more than seven, more preferably no more than four second ring resonators. In this way, the structural complexity of the device is limited.

[0044] Typically, each ring resonator includes at least one micro-ring (in an embodiment, a series of cascaded micro-rings) inserted between and optically coupled to two optical waveguides, a corresponding input port and a corresponding drop port, the drop port being arranged on a different optical waveguide associated with the input port (also referred to as the "drop" port). Preferably, each ring resonator is optically connected to the respective remainder of the optical path (and possibly additional optical paths described below) only through the corresponding input port and the corresponding drop port. In this way, the resonator is suitably arranged to contribute to the overall transfer function of the filtering stage.

[0045] Preferably, the optical path difference between the first and second optical branches of the first Mach-Zehnder interferometer is a function of the optical signal frequency band. Preferably, the optical path difference is such that the transfer function of the first Mach-Zehnder interferometer has a free spectral range (FSR) that is substantially equal to or greater than the optical signal frequency band width. In this way, the entire frequency band of the optical signal can be analyzed.

[0046] Preferably, the respective optical path difference of each second Mach-Zehnder interferometer is a function of the optical path difference of the first Mach-Zehnder interferometer, and more preferably, it is equal to 2i times the optical path difference of the first Mach-Zehnder interferometer, where i is the order of the respective second Mach-Zehnder interferometer. In this way, the free spectral range of the respective transfer function of the ith second MZI is equal to half of the free spectral range of the transfer function of the Mach-Zehnder interferometer with index i - 1 (for i = 1, the free spectral range is half of the free spectral range of the first MZI, which can be assumed to correspond to i = 0). In this way, a condition can be obtained in a favorably simple manner that the maximum values of all the peaks (except one) of the transfer function of the ith MZI are located at the valleys of the transfer function of the MZI with index i - 1 on the wavelength axis. In this way, all the peaks (except one) suppress each other to achieve, in a simple manner, an overall transfer function having only one peak (corresponding to the coincident maximum points of the respective MZI transfer functions). This enables the optimization of the use of MZIs because, for a given final characteristic of the overall transfer function, the number of MZIs used is limited.

[0047] Preferably, the optical path difference of the first Mach-Zehnder interferometer is greater than or equal to 1 μm. In this way, the free spectral range FSR is comparable to the typical optical signal frequency band used in the telecommunications field (e.g., about 1520 - 1580 nm).

[0048] Preferably, the optical path difference of the first Mach-Zehnder interferometer (and possibly each second Mach-Zehnder interferometer) is less than or equal to 2 mm. In this way, the spatial expansion of the optical filtering stage is limited, which is beneficial to the overall size of the device (for example, for a specific use on a support with limited size, such as an integrated chip).

[0049] Preferably, the micro-ring radius of the first ring resonator is a function of the optical path difference of the first Mach-Zehnder interferometer. More preferably, the micro-ring circumference of the first ring resonator is equal to (2n + 1) times the optical path difference of the first Mach-Zehnder interferometer, where n is equal to the total number of second Mach-Zehnder interferometers. In one embodiment, the micro-ring circumference of each second ring resonator is equal to (2n + 1 + m) times the optical path difference of the first Mach-Zehnder interferometer, where m is equal to the order of the second ring resonator. In this way, the same considerations regarding the halving of the free spectral range of the MZI are valid, and similar advantages are also obtained in optimizing the use of the ring resonator.

[0050] Preferably, the one or more second Mach-Zehnder interferometers are arranged in a continuous row with the first Mach-Zehnder interferometer (i.e., they are arranged directly downstream of the previous one without interruption regardless of the total number of MZIs used). Preferably, the one or more second ring resonators and the first ring resonator are arranged in a continuous row. This simplifies the structure of the device.

[0051] Preferably, the input port of the optical filtering stage coincides with one and only one first port of the input optical coupler of the first Mach-Zehnder interferometer, and the output port of the optical filtering stage coincides with the drop port of the first ring resonator (or the last second ring resonator of the continuously arranged ring resonators). Preferably, the photodetector is directly optically connected to the output port of the optical filtering stage. In this way, the device is simplified.

[0052] Preferably, the device includes a preprocessing stage of the optical signal located upstream of the optical filtering stage (and the possible additional optical filtering stages). Preferably, the preprocessing stage includes a polarization separator for separating the second polarization component (e.g., transverse magnetic) of the optical signal from the first polarization component (e.g., transverse electric polarization component).

[0053] Preferably, the preprocessing stage includes a corresponding input port and corresponding first and second output ports (one for each of the first and second polarization components).

[0054] Preferably, the preprocessing stage includes a polarization rotator for rotating the polarization of the second polarization component of the optical signal (e.g., rotating by 90°). Rotating one of the polarization components (usually the transverse magnetic polarization component) allows the second polarization component to be given the same polarization direction as the first polarization component while retaining their respective optical intensities. In this way, the second polarization component can also be analyzed by the same analysis device without the need for appropriate modification and / or calibration and / or adjustment during use. In this way, the device has a wide range of versatility and is capable of analyzing the spectrum of an optical signal independently of its polarization (which usually varies with time).

[0055] In a first embodiment, the device includes an additional optical coupler (different from a Mach-Zehnder interferometer) inserted between the preprocessing stage and the optical filtering stage, wherein the first and second output ports of each preprocessing stage are (directly) optically connected to one (and only one) corresponding first port of the additional optical coupler, and wherein one and only one second port of the additional optical coupler is (directly) optically connected to the input port of the optical filtering stage (preferably directly optically connected to one and only one first port of the input optical coupler of the first Mach-Zehnder interferometer). Preferably, the device further includes first and second optical switches (e.g., variable optical attenuators or VOAs) inserted between the corresponding first and second output ports of the preprocessing stage and the corresponding first ports of the additional optical coupler. In this way, a structure is created in a simple and effective manner that is capable of selectively transmitting only one or the other polarization component (electric or magnetic) of the optical input signal to the optical filtering stage to analyze its corresponding spectrum.

[0056] Preferably, the continuously arranged Mach-Zehnder interferometers are directly arranged upstream of the continuously arranged ring resonators. In this way, the overall structure of the device is simplified.

[0057] In a second embodiment, the optical filtering stage includes a first 3 dB optical coupler (different from a Mach-Zehnder interferometer), the first 3 dB optical coupler forming a corresponding part of the optical path and being inserted between the first Mach-Zehnder interferometer (or, if present, the one or more second Mach-Zehnder interferometers) and the first ring resonator. Preferably, one (and only one) first port of the first 3 dB optical coupler is (directly) optically connected to the one and only one second port of the first Mach-Zehnder interferometer (or the corresponding output optical coupler of the second Mach-Zehnder interferometer arranged in the last position along the corresponding sequence). Preferably, one (and only one) second port of the first 3 dB optical coupler is (directly) optically connected to the input port of the first ring resonator. Preferably, the remaining second ports of the first 3 dB optical coupler are (directly) optically connected to the second output port of the preprocessing stage.

[0058] Preferably, the device includes an additional optical filtering stage, which includes an additional input port, an additional output port, and an additional optical path extending from the additional input port to the additional output port. Preferably, the additional optical filtering stage includes the first Mach-Zehnder interferometer (more preferably including each second Mach-Zehnder interferometer) shared with the optical filtering stage and an additional ring resonator (different from the first ring resonator and the one or more second ring resonators), which are cascaded with each other to form a corresponding part of the additional optical path. In this way, an optical filtering stage is created for each polarization component of the optical signal (propagating in opposite directions during operation), while the device remains compact due to the first MZI (and possibly each second MZI) being shared. The combination of the first MZI and the additional ring resonator allows obtaining the overall transfer function of the additional optical filtering stage, which has the same advantages as the related optical filtering stage described above.

[0059] In one embodiment, the additional optical filtering stage includes a plurality of additional ring resonators (including the additional ring resonator). Preferably, the additional ring resonator (or possibly the plurality of additional ring resonators) includes the features of one or more, more preferably all, of the first ring resonators (and possibly the one or more second ring resonators), with reference to the additional optical filtering stage as appropriate. In this way, the desired overall transfer function can be obtained.

[0060] Preferably, the device includes an additional photodetector, which is optically connected to the additional optical filtering stage downstream of the additional output port, more preferably to the drop port of the additional ring resonator.

[0061] Preferably, the additional optical filtering stage includes a second 3 dB optical coupler (different from the Mach-Zehnder interferometer and the first 3 dB optical coupler), the second 3 dB optical coupler forms a corresponding part of the additional optical path and is inserted between the first Mach-Zehnder interferometer and the additional ring resonator. Preferably, one (and only one) first port of the second 3 dB optical coupler is (directly) optically connected to the first output port of the preprocessing stage. Preferably, the remaining first ports of the second 3 dB optical coupler are (directly) optically connected to the input port of the additional ring resonator. Preferably, one (and only one) second port of the second 3 dB optical coupler is (directly) optically connected to the one and only one first port of the input optical coupler of the first Mach-Zehnder interferometer.

[0062] The optical connection characteristics of the above-mentioned first and second 3 dB optical couplers allow a closed-loop device architecture to be created in a simple structure manner and / or using a limited number of components, such that the first polarization component is guided to the optical filtering stage, while the second polarization component is simultaneously guided to the additional optical filtering stage in the opposite propagation direction. The applicant has found that although the two polarization components coexist simultaneously along the common part of the optical path created by the Mach-Zehnder interferometer and the additional optical path, they do not interfere with each other, thereby allowing their corresponding spectra to be obtained simultaneously. In this way, the device has versatility and practicality, and has a short spectral scanning period.

[0063] Preferably, the radius of each micro-ring of the first ring resonator (and optionally each second and additional ring resonator) is less than or equal to 500 μm. In this way, the size of the device is suppressed.

[0064] In the second embodiment, preferably, the device includes an optical isolator arranged upstream of the preprocessing stage. In this way, the unwanted signal residues generated due to the closed-loop architecture are suppressed.

[0065] Preferably, the actuator is also coupled to the additional optical filtering stage and is configured to change the optical refractive index of at least one part of the additional optical path of the additional optical filtering stage.

[0066] Preferably, the actuator includes a plurality of sub-actuators, and each sub-actuator is coupled to a corresponding Mach-Zehnder interferometer or a corresponding ring resonator (for changing the optical refractive index of the corresponding optical path part).

[0067] According to another aspect, the present invention relates to a method for spectral analysis of an optical signal having a frequency band. The method includes:

[0068] - providing a device for analysis according to any embodiment of the present invention;

[0069] - Introduce the optical signal as an input into the device;

[0070] - Tune the optical filtering stage (and possibly additional optical filtering stages) by the actuator such that the passband of the optical filtering stage (and possibly additional optical filtering stages) moves (continuously) along the frequency band of the optical signal;

[0071] - For each passband, acquire, via the photodetector (and possibly also via the additional photodetector), a corresponding signal representative of the optical intensity of the optical signal in the passband;

[0072] - Calculate the spectrum based on the corresponding signals representative of the optical intensity.

[0073] In this way, a spectrum is obtained with a desired resolution and a simple structure.

[0074] Preferably, the method further includes performing a deconvolution process on the corresponding signals representative of the optical intensity. Preferably, the deconvolution process includes:

[0075] - Determine the overall transfer function of the optical filtering stage (and possibly the additional optical filtering stage);

[0076] - Calculate the Fourier transform of the overall transfer function of the optical filtering stage (and additional optical filtering stage) (e.g., via the FFT algorithm);

[0077] - Calculate the Fourier transform of the corresponding signal representative of the optical intensity within the frequency band (e.g., via the FFT algorithm);

[0078] - For each passband, calculate the quotient between the Fourier transform of the corresponding signal representative of the optical intensity and the Fourier transform of the overall transfer function of the optical filtering stage (and additional optical filtering stage);

[0079] - Calculate the inverse Fourier transform of the quotient (e.g., using the FFT algorithm).

[0080] In this way, a desired resolution of the spectrum can be obtained. BRIEF DESCRIPTION OF THE DRAWINGS

[0081] Figure 1 A block diagram showing a first embodiment of a device according to the present invention is shown;

[0082] Figure 2 Is schematically shown Figure 1 The circuit form of the device;

[0083] Figure 3 A block diagram showing a second embodiment of a device according to the present invention is shown;

[0084] Figure 4 shows schematically Figure 3 the circuit form of the device;

[0085] Figure 5 graphically shows an example of the transfer function of the optical components of the optical filtering stage;

[0086] Figure 6 graphically shows Figure 5 an example of the overall transfer function obtainable from the transfer function of

[0087] Figures 7 - 10 graphically shows some simulation results of the analysis method according to the present invention. Detailed implementation mode

[0088] The features and advantages of the present invention will be further clarified by the following detailed description of some embodiments presented by way of non - limiting examples of the present invention with reference to the accompanying drawings (not to scale).

[0089] In the figure, the numeral 1 represents a device for optical signal spectral analysis.

[0090] The device 1 includes an optical filtering stage 2 ( Figure 1 and Figure 3 the closed dashed line shown in), which has an input port 3, an output port 4, and an optical path 5 extending from the input port to the output port.

[0091] The optical filtering stage 2 includes a first Mach - Zehnder interferometer 6 and a first ring resonator 7, which are cascaded with each other to form corresponding parts of the optical path 5.

[0092] Exemplarily, the optical filtering stage 2 further includes two second Mach - Zehnder interferometers 8, which are cascaded with each other to each form a corresponding part of the optical path (a total of three Mach - Zehnder interferometers). Exemplarily, the two second Mach - Zehnder interferometers 8 and the first Mach - Zehnder interferometer 6 are arranged in a continuous arrangement (i.e., one directly downstream of the previous one).

[0093] The first Mach - Zehnder interferometer 6, and each exemplary second Mach - Zehnder interferometer 8, includes a corresponding input optical coupler 9, 9', a corresponding output optical coupler 10, 10', and corresponding first 11, 11' and second optical branches 12, 12' connecting the corresponding input optical coupler 9 to the corresponding output optical coupler 10.

[0094] The first Mach-Zehnder interferometer 6, and each exemplary second Mach-Zehnder interferometer 8, are optically connected to the respective remaining portions of the optical path 5 of the optical filter stage through one and only one first port 13, 13' of the respective input optical couplers 9, 9' and through one and only one second port 14, 14' of the respective output optical couplers 10, 10'.

[0095] Exemplarily, one and only one second port 14, 14' of the output optical couplers 10, 10' of each Mach-Zehnder interferometer 6, 8 is arranged in a crossed configuration with respect to one and only one first port 13, 13' of the input optical couplers 9, 9' of the respective Mach-Zehnder interferometers (i.e., it belongs to different optical branches).

[0096] In an embodiment not shown, one and only one second port of the output optical coupler of each Mach-Zehnder interferometer is a through port.

[0097] The respective optical path differences dL0 between the first 11 and second optical branches 12 of the first Mach-Zehnder interferometer 6, and the respective optical path differences dLi between the first and second optical branches of each exemplary second Mach-Zehnder interferometer 8, are all greater than zero and different from each other.

[0098] Exemplarily, the optical path difference dL0 between the first and second optical branches of the first Mach-Zehnder interferometer 6 is a function of the optical signal frequency band to be analyzed (e.g., from about 1520 nm to about 1580 nm). More specifically, the optical path difference dL0 exemplarily causes the transfer function FDT0 ( Figure 5 ) to have a corresponding free spectral range that is substantially equal to the optical signal frequency band width (about 60 nm in the example).

[0099] Figure 5 Shows the trend of the transfer function FDT0 (expressed in dB) varying with wavelength (expressed in μm). In Figure 5 The free spectral range corresponds to the distance between two consecutive valleys of the transfer function FDT0.

[0100] Exemplarily, the respective optical path differences dLi of each second Mach-Zehnder interferometer 8 are a function of the optical path difference of the first Mach-Zehnder interferometer 6. Specifically, the optical path difference dLi of the i-th second Mach-Zehnder interferometer is exemplarily given by the formula dLi = 2idL0, where i = 1, 2. Exemplarily, thus dL1 = 2dL0 and dL2 = 4dL0.

[0101] Figure 5Also shown are the transfer functions FDT1 (i = 1) and FDT2 (i = 2) of two second Mach-Zehnder interferometers. It can be observed how the free spectral range of FDT1 (equal to the distance between two corresponding consecutive valleys) is halved with respect to the free spectral range of FDT0, and further how the free spectral range of FDT2 is in turn half of the free spectral range of FDT1. In this way, within the frequency band of the optical signal, one and only one wavelength value is obtained at which FDT0, FDT1, and FDT2 each have corresponding coincident peak repetitions (exemplarily, this value corresponds to approximately 1567 nm). As Figure 5 shown, the remaining repetitions of the FDT1 peaks are all at the valleys of FDT0, while the remaining repetitions of the FDT2 peaks are all arranged at the valleys of FDT0 or FDT1, thus obtaining a mutual suppression effect.

[0102] Exemplarily, the optical path differences of the Mach-Zehnder interferometers are all between 1 micron and 2 millimeters (including the extreme values).

[0103] Exemplarily, the first ring resonator 7 is a single ring resonator of the optical filtering stage 2, which exemplarily includes a single micro-ring, an input port 15, and a drop port 16.

[0104] Exemplarily, the first ring resonator 7 is optically connected to the respective remaining parts of the optical path 5 only through the input port 15 and only through the drop port 16.

[0105] Exemplarily, the input port 3 of the optical filtering stage 2 coincides with one and only one first port 13 of the input optical coupler 9 of the first Mach-Zehnder interferometer 6, while the output port 4 of the optical filtering stage 2 coincides with the drop port 16 of the first ring resonator 7.

[0106] Exemplarily, the micro-ring radius R of the first ring resonator 7 is a function of the optical path difference of the first Mach-Zehnder interferometer 6. More specifically, the micro-ring circumference of the first ring resonator 7 exemplarily equals 2n + 1 multiplied by the optical path difference dL0 of the first Mach-Zehnder interferometer, where n equals 2 (i.e., the total number of the second Mach-Zehnder interferometers 8). Exemplarily, 2πR = 8dL0 is obtained.

[0107] Figure 5 Shown is the transfer function of the first ring resonator FDTr: It is characterized by having peaks with decreasing amplitude and frequent repetitions within the optical signal frequency band. It can be observed that all repetitions of the FDTr peaks (except the one at 1567 nm) are arranged at the valleys of FDT0, FDT1, or FDT2, thus resulting in cancellation. This effect is simply achieved due to the relationship between the above-mentioned micro-ring radius R and dL0. In this way, exemplarily, Figure 6The overall transfer function FDTc of the optical filtering stage shown (obtained by the product of FTD0, FDT1, FDT2, and FDTr). It can be observed that, since there is only one peak repetition within the optical signal frequency band of interest, and due to the first ring resonator, how FDTc has the desired resolution (corresponding to the amplitude of the corresponding peak at 1567 nm), and due to the first MZI, how FDTc has the desired free spectral range.

[0108] In one embodiment (not shown), the optical filtering stage may include one or more second ring resonators (different from the first ring resonator), which are cascade-connected to each other (preferably arranged continuously with the first ring resonator) to each form a corresponding part of the optical path. Preferably, the corresponding micro-ring radii of the first ring resonator and the corresponding micro-ring radii of each second ring resonator are all different from each other.

[0109] Preferably, the corresponding perimeter of the micro-ring of each second ring resonator is equal to 2n + 1 + m multiplied by the optical path difference of the first Mach-Zehnder interferometer, where m is the (gradually increasing) order of the second resonator ring.

[0110] Device 1 includes a photodetector 17, which is optically connected to the optical filtering stage 2 downstream of the output port 4 (exemplarily, the photodetector 17 is directly optically connected downstream of the drop port 16).

[0111] Device 1 includes an actuator 18, which is coupled to the optical filtering stage 2 and is configured to change the optical refractive index of at least one part of the optical path 5 of the optical filtering stage.

[0112] For example, the actuator may utilize the thermo-optic effect, the electro-optic effect, or both. In one embodiment, the actuator may be configured to change the optical refractive index at one (or both) branches of one or more MZIs or ring resonators, for example as shown in the known art. In one embodiment, the actuator may be configured to directly change the optical refractive index of one or more MZIs directly on one or two layers of the corresponding optical coupler, for example as described in patent application numbers 102021000025160 and 102021000025166 filed in the name of the same applicant.

[0113] Exemplarily, device 1 includes a preprocessing stage 19 for the optical signal located upstream of the optical filtering stage. Exemplarily, the preprocessing stage 19 includes a polarization separator 20, which is used to separate the second polarization component TM (e.g., transverse magnetic) of the optical signal from the first polarization component TE (e.g., transverse electric polarization component).

[0114] Exemplarily, the preprocessing stage 19 includes a corresponding input port 21 and corresponding first 22 and second output ports 23 (for the first and second polarization components respectively).

[0115] Exemplarily, the preprocessing stage 19 further includes a polarization rotator 24, which is configured to rotate the polarization direction of the second polarization component TM of the optical signal (e.g., rotate by 90°).

[0116] In the first embodiment ( Figure 1 and 2 ), the device 1 includes an additional optical coupler 25 (different from the Mach-Zehnder interferometer), which is inserted between the preprocessing stage 19 and the optical filtering stage 2.

[0117] Exemplarily, the first 22 and second output ports 23 of the preprocessing stage 19 are directly optically connected to one and only one corresponding first port 26 of the additional optical coupler 25 respectively, and one and only one second port 27 of the additional optical coupler 25 is directly optically connected to the input port 3 of the optical filtering stage 2, more specifically, directly optically connected to one and only one first port 13 of the input optical coupler 9 of the first Mach-Zehnder interferometer 6.

[0118] Exemplarily, the additional optical coupler 25 is a tunable balanced Mach-Zehnder interferometer, and its splitting ratio can be adjusted between 0 and 100.

[0119] Exemplarily, the device 1 further includes first 28 and second optical switches 29 (e.g., variable optical attenuators or VOAs), which are inserted between the corresponding first 22 and second output ports 23 of the preprocessing stage and the corresponding first ports 26 of the additional optical coupler 25 respectively.

[0120] Exemplarily, the continuous arrangement of the Mach-Zehnder interferometers is directly arranged upstream of the first ring resonator 7, and the input port of the first ring resonator is directly optically connected to one and only one output port 14' of the output optical coupler 10' of the second Mach-Zehnder interferometer 8 arranged in the last position.

[0121] In the second embodiment ( Figure 3 and 4 ), the optical filtering stage 2 includes a first 3dB optical coupler 30 (different from the Mach-Zehnder interferometer), and the first 3dB optical coupler 30 forms a corresponding part of the optical path 5 and is inserted between the second Mach-Zehnder interferometer 8 arranged in the last position and the first ring resonator 7.

[0122] Exemplarily, one and only one first port 31 of the first 3 dB optical coupler 30 is directly optically connected to one and only one second port 14' of the output optical coupler 10' of the second Mach-Zehnder interferometer 8 arranged in the last position, one and only one second port 32 of the first 3 dB optical coupler 30 is directly optically connected to the input port 15 of the first ring resonator 7, and the remaining second ports 33 of the first 3 dB optical coupler 30 are directly optically connected to the second output port 23 of the preprocessing stage 19.

[0123] Exemplarily, the device 1 includes an additional optical filtering stage 40 (defined by the closed dashed line), which includes an additional input port 41, an additional output port 42, and an additional optical path 43 extending from the additional input port to the additional output port.

[0124] Exemplarily, the additional optical filtering stage 40 includes a first Mach-Zehnder interferometer 6 and two second Mach-Zehnder interferometers 8 shared with the optical filtering stage 2, and an additional ring resonator 44 (different from the first ring resonator 7), which are cascaded with each other to form corresponding parts of the additional optical path 43.

[0125] Exemplarily, the additional ring resonator 44 includes all the features of the first ring resonator 7 (referring to the additional optical filtering stage 40 as appropriate).

[0126] Exemplarily, the device 1 includes an additional photodetector 45, which is optically connected to the drop port 60 of the additional ring resonator 44.

[0127] Exemplarily, the additional optical filtering stage 40 includes a second 3 dB optical coupler 46 (different from the Mach-Zehnder interferometer and the first 3 dB optical coupler 30), and the second 3 dB optical coupler 46 forms a corresponding part of the additional optical path and is inserted between the first Mach-Zehnder interferometer 6 and the additional ring resonator 44.

[0128] Exemplarily, one and only one first port 47 of the second 3 dB optical coupler 46 is directly optically connected to the first output port 22 of the preprocessing stage 19, the remaining first ports 48 of the second 3 dB optical coupler 46 are directly optically connected to the input port 61 of the additional ring resonator 44, and one and only one second port 49 of the second 3 dB optical coupler is directly optically connected to one and only one first port 13 of the input optical coupler 9 of the first Mach-Zehnder interferometer 6.

[0129] Exemplarily, the micro-ring radii of the first ring resonator 7 and the additional ring resonator are less than or equal to 500 μm.

[0130] In a second embodiment, the device 1 exemplarily includes an optical isolator 50 arranged upstream of the preprocessing stage 19.

[0131] In one embodiment (not shown), the additional optical filtering stage may include a plurality of additional ring resonators (similar to the second ring resonator of the optical filtering stage).

[0132] Exemplarily, the actuator 18 is also coupled to the additional optical filtering stage 40 and is configured to change the optical refractive index of at least one part in the additional optical path of the additional optical filtering stage.

[0133] Exemplarily (not shown), the actuator includes corresponding sub-actuators for each Mach-Zehnder interferometer and each ring resonator (to change the optical refractive index of the corresponding optical path part). Exemplarily, each sub-actuator can be of a known type (such as an electric heater, an electrode pair conductively connected to an optical waveguide, etc.).

[0134] Exemplarily, the device includes semiconductor optical waveguides to connect the above-mentioned optical components. Exemplarily, the device can be fabricated on any semiconductor photonics platform, such as, for example, silicon, silicon-on-insulator, silicon nitride, indium phosphide, silicon carbide, gallium arsenide, lithium niobate, or any other semiconductor waveguide platform.

[0135] In use, the device 1 allows for the execution of a method for spectral analysis of an optical signal having a frequency band.

[0136] The method includes introducing the optical signal as an input into the device. Exemplarily, the optical signal is filtered by the optical filtering stage 2 (and possibly the additional optical filtering stage 40), and only the part of the optical signal corresponding to the filtering passband is allowed to be transmitted to the photodetector 17 (and possibly the additional photodetector 45).

[0137] Referring to the first embodiment of the above-mentioned device, the method exemplarily includes actuating one of the two optical switches 28, 29 to allow only one polarization component to be transmitted to the additional optical coupler 25 each time. Subsequently, the additional optical coupler 25 is adjusted accordingly to allow maximum through or cross transmission depending on the input polarization component to be transmitted to the optical filtering stage. For example, if it is required to transmit the TE polarization component to the optical filtering stage, the first switch 28 is activated to block the transmission of the TM component, and the additional optical coupler 25 is tuned to the cross configuration (such that the TE component passes through the additional optical coupler completely). The reverse also applies to the TM component. For example, it can be stipulated to analyze only the light intensity of the TE component within the optical signal frequency band and then perform the same operation on the TM component.

[0138] Referring to the second embodiment of the above-described apparatus, the method exemplarily includes simultaneously analyzing two polarization components, TE and TM, such that they simultaneously pass through the apparatus (specifically, three MZIs), and propagate in opposite directions. For example, the TE component reaches the second 3 dB optical coupler 46 after passing through the preprocessing stage 19, and is (partially) transmitted through the second port 49 to the optical filtering stage. Conversely, the TM component reaches the second 3 dB optical coupler 46 through the second port 49 after passing through the three MZIs, and is (partially) transmitted through the remaining first port 48 to the additional ring resonator 44. Mirroringly applicable to the first 3 dB optical coupler 30.

[0139] Therefore, the method includes adjusting the optical filtering stage (and possibly additional optical filtering stages) by the actuator 18 such that the passband of the optical filtering stage (and possibly additional optical filtering stages) is shifted along the frequency band of the optical signal (i.e., translated along the wavelength horizontal axis Figure 6 of the FDTc in Figure 6 An example of the shifted overall transfer function FDTct is shown, which corresponds to the FDTc of the optical filtering stage shifted as a result of tuning the optical filtering stage by the actuator 18. It can be seen how the passband of the optical filtering stage is exemplarily shifted in the direction of higher wavelength values.

[0140] For example, the optical filtering stage can be adjusted by actuating one or more sub-actuators associated with the MZIs 6, 8 and the first ring resonator 7 (similarly, for additional optical filtering stages, referring to the sub-actuators of the MZIs 6, 8 and the sub-actuators of the additional ring resonator 44). Exemplarily, the method can include a preliminary calibration phase of the sub-actuator operating points (e.g., by known calibration methods, and thus not described in detail).

[0141] Therefore, the method includes, for each passband, collecting, by a photodetector (and possibly also by additional photodetectors), a corresponding signal representative of the optical intensity of the optical signal in the passband.

[0142] Finally, the method includes calculating the spectrum of the optical signal based on the corresponding signals representative of the optical intensity as described above.

[0143] Figure 7 and 8 Some simulation results of the method according to the present invention are shown (shown only with reference to the TE component, independent of the embodiment of the apparatus). The vertical axis shows the optical intensity in dBm, while the horizontal axis shows the frequency in THz.

[0144] In Figure 7 the optical input signal IS is simulated as a Dirac delta function, while the remaining curves represent the collected signals AS (i.e., the set of all corresponding signals representative of the passband optical intensity).

[0145] In Figure 8 , the optical input signal IS is simulated as a wave of variable wavelength intensity, characterized by four consecutive distinct peaks and zero outside the corresponding frequency bands. The remaining curves are the corresponding acquired signals AS.

[0146] It can be observed that, in the examples of Figure 7 and 8 , each corresponding acquired signal AS represents the convolution between the spectrum of the input optical signal IS and the overall transfer function of the optical filtering stage FDTc.

[0147] To further improve the resolution of each acquired signal AS, the method exemplarily includes performing a deconvolution process on the acquired signal AS. The algorithm exemplarily includes the following steps:

[0148] - Calculate the Fourier transform of the overall transfer function FDTc of the optical filtering stage;

[0149] - Calculate the Fourier transform of the acquired signal AS (i.e., the corresponding signal representing the optical intensity within the optical signal frequency band);

[0150] - For each passband, calculate the quotient between the Fourier transform of the acquired signal AS and the Fourier transform of the overall transfer function;

[0151] - Calculate the inverse Fourier transform of the quotient.

[0152] Figure 9 And 10 show the results of applying the above deconvolution process to the acquired signals AS of Figure 7 and 8 , as well as the corresponding optical input signal IS (shown as a dashed line in Figure 9 ). The vertical axis shows the optical intensity expressed in dBm, while the horizontal axis shows the frequency expressed in THz.

[0153] It can be observed how the resolution of the acquired signal is improved with respect to the corresponding signal before applying the above algorithm.

[0154] In Figure 9 , the peak of the acquired signal is almost narrowed down to coincide with the input optical signal, while in Figure 10 , the acquired signal basically follows the input optical signal completely.

[0155] In particular, the applicant observes that, in the shown examples, due to the above deconvolution process, the resolution decreases from approximately 10 GHz ( Figure 7 and 8 ) to approximately 4 GHz ( Figure 9 and 10 ). The above description of the method and the deconvolution process similarly applies to additional optical filtering stages.

Claims

1. A device (1) for spectral analysis of optical signals, said device (1) comprising: - an optical filtering stage (2) having an input port (3), an output port (4) and an optical path (5) extending from said input port (3) to said output port (4); - a photodetector (17) optically connected to said optical filtering stage (2) downstream of said output port (4); - an actuator (18) coupled to said optical filtering stage (2) and configured to change the optical refractive index of at least a portion of said optical path (5) of said optical filtering stage (2); wherein said optical filtering stage (2) includes a first Mach-Zehnder interferometer (6) and a first ring resonator (7), said first Mach-Zehnder interferometer (6) and said first ring resonator (7) being cascaded with each other to form respective portions of said optical path (5), wherein said first Mach-Zehnder interferometer (6) includes an input optical coupler (9), an output optical coupler (10) and first (11) and second optical branches (12) connecting said input optical coupler (9) to said output optical coupler (10), wherein a corresponding optical path difference (dL0) between said first (11) and second optical branches (12) of said first Mach-Zehnder interferometer (6) is greater than zero, and wherein said first Mach-Zehnder interferometer (6) is optically connected to the remainder of said optical path (5) of said optical filtering stage (2) through one and only one first port (13) of said input optical coupler (9) and one and only one second port (14) of said output optical coupler (10).

2. The device (1) according to claim 1, wherein the optical filtering stage (2) comprises one or more second Mach-Zehnder interferometers (8) connected in cascade with each other to respectively form corresponding portions of the optical path (5), wherein each second Mach-Zehnder interferometer (8) comprises a corresponding input optical coupler (9'), a corresponding output optical coupler (10'), and corresponding first (11') and second optical branches (12') connecting the corresponding input optical coupler (9') to the corresponding output optical coupler (10'), wherein the corresponding optical path difference (dLi) between the corresponding first (11') and second optical branches (12') is greater than zero, wherein the differences between the optical paths of the first Mach-Zehnder interferometer (6) and the corresponding optical paths of the one or more second Mach-Zehnder interferometers (8) are all different from each other, wherein each second Mach-Zehnder interferometer (8) is optically connected to the corresponding remaining portion of the optical path (5) of the optical filtering stage (2) through one and only one corresponding first port (13') of the corresponding input optical coupler (9') and through one and only one second port (14') of the corresponding output optical coupler (10'), and wherein the one or more second Mach-Zehnder interferometers (8) comprise two and no more than nineteen second Mach-Zehnder interferometers.

3. The device (1) according to any one of the preceding claims, wherein the one and only one second port (14, 14') of the output optical coupler (10, 10') of each Mach-Zehnder interferometer (6, 8) is arranged in a crossed configuration with respect to the one and only one first port (13, 13') of the input optical coupler (9, 9') of the corresponding Mach-Zehnder interferometer, wherein the first ring resonator (7) is a single ring resonator of the optical filtering stage (2), and the actuator (18) comprises a plurality of sub-actuators, each sub-actuator being coupled to a corresponding Mach-Zehnder interferometer (6, 8) or a corresponding ring resonator (7).

4. The device (1) according to any one of the preceding claims, wherein the optical path difference (dL0) between the first (11) and second optical branches (12) of the first Mach-Zehnder interferometer (6) is a function of the frequency band of the optical signal, wherein the optical path difference (dL0) is such that the transfer function (FDT0) of the first Mach-Zehnder interferometer (6) has a free spectral range that is substantially equal to or greater than the width of the frequency band of the optical signal.

5. The apparatus (1) according to claim 2, wherein the respective optical path difference (dLi) of each second Mach-Zehnder interferometer (8) is a function of the optical path difference (dL0) of the first Mach-Zehnder interferometer (6) and is equal to 2i times the optical path difference (dL0) of the first Mach-Zehnder interferometer (6), where i is equal to the order of the respective second Mach-Zehnder interferometer (8), wherein the optical path difference (dL0) of the first Mach-Zehnder interferometer (6) is greater than or equal to 1 μm, wherein the optical path difference of the first Mach-Zehnder interferometer and the respective optical path difference (dLi) of each second Mach-Zehnder interferometer (8) are both less than or equal to 2 mm, wherein the radius (R) of the micro-ring of the first ring resonator (7) is a function of the optical path difference (dL0) of the first Mach-Zehnder interferometer (6), and wherein the circumference of the micro-ring of the first ring resonator (7) is equal to 2n + 1 times the optical path difference (dL0) of the first Mach-Zehnder interferometer (6), where n is equal to the total number of second Mach-Zehnder interferometers (8).

6. The apparatus (1) according to any one of the preceding claims, comprising a preprocessing stage (19) of the optical signal upstream of the optical filtering stage (2), wherein the preprocessing stage (19) comprises a polarization separator (20) for separating the second polarization component (TM) of the optical signal from the first polarization component (TE), wherein the preprocessing stage (19) comprises respective input ports (21), respective first (22) and respective second output ports (23), and wherein the preprocessing stage (19) comprises a polarization rotator (24) for rotating the polarization of the second polarization component (TM) of the optical signal.

7. The apparatus (1) according to claim 6, comprising an additional optical coupler (25) inserted between the preprocessing stage (19) and the optical filtering stage (2), wherein each of the first (22) and second output ports (23) of the preprocessing stage (19) is optically connected to a respective first port (26) of the additional optical coupler (25), wherein one and only one second port (27) of the additional optical coupler (25) is optically connected to the input port (4) of the optical filtering stage (2), and wherein the apparatus (1) further comprises first (28) and second optical switches (29) inserted respectively between the respective first (22) and second output ports (23) of the preprocessing stage (19) and the corresponding first ports (26) of the additional optical coupler (25).

8. The device (1) according to claim 6, wherein the optical filtering stage (2) comprises a first 3 dB optical coupler (30) which forms a respective part of the optical path (5) and is inserted between the first Mach-Zehnder interferometer (6) and the first ring resonator (7), wherein one and only one first port (31) of the first 3 dB optical coupler (30) is optically connected to the one and only one second port (14) of the output optical coupler (10) of the first Mach-Zehnder interferometer (6), wherein one and only one second port (32) of the first 3 dB optical coupler (30) is optically connected to the input port (15) of the first ring resonator (7), and wherein the remaining second ports (33) of the first 3 dB optical coupler (30) are optically connected to the second output port (23) of the preprocessing stage (19).

9. The device (1) according to claim 8, comprising an additional optical filtering stage (40) which comprises an additional input port (41), an additional output port (42) and an additional optical path (43) extending from the additional input port to the additional output port, wherein the additional optical filtering stage (40) comprises the first Mach-Zehnder interferometer (6) common to the optical filtering stage (2) and additional ring resonators (44) connected in cascade with each other to form respective parts of the additional optical path (43), wherein the device (1) comprises an additional photodetector (45) which is optically connected to the additional optical filtering stage (40) downstream of the additional output port (42), wherein the additional optical filtering stage (40) comprises a second 3 dB optical coupler (46) which forms a respective part of the additional optical path (43) and is inserted between the first Mach-Zehnder interferometer (6) and the additional ring resonators (44), wherein one and only one first port (47) of the second 3 dB optical coupler (46) is optically connected to the first output port (22) of the preprocessing stage (19), wherein the remaining first ports (48) of the second 3 dB optical coupler (46) are optically connected to the input port (61) of the additional ring resonators (44), wherein one and only one second port (49) of the second 3 dB optical coupler (46) is optically connected to the one and only one first port (13) of the input optical coupler (9) of the first Mach-Zehnder interferometer (6), wherein the device (1) comprises an optical isolator (50) arranged upstream of the preprocessing stage (19), and wherein the actuator (18) is further coupled to the additional optical filtering stage (40) and is configured to change the optical refractive index of at least one part of the additional optical path (43) of the additional optical filtering stage (40).

10. A method for spectral analysis of an optical signal having a frequency band, the method comprising: - providing the analysis device (1) according to any one of the preceding claims; - introducing the optical signal as an input into the device (1); - tuning the optical filtering stage (2) by means of the actuator (18) such that the passband of the optical filtering stage (2) moves along the frequency band of the optical signal; - for each passband, acquiring, by means of the photodetector (17), a respective signal representative of the optical intensity of the optical signal in the passband; - calculating the spectrum according to the respective signals representative of the optical intensity.

11. The method according to claim 10, further comprising deconvolving the signals representative of the optical intensity, wherein the deconvolution comprises: - determining the overall transfer function (FDTc) of the optical filtering stage (2); - calculating the Fourier transform of the overall transfer function of the optical filtering stage; - calculating the Fourier transform of the respective signals representative of the optical intensity within the frequency band; - for each passband, calculating the quotient between the Fourier transform of the respective signals representative of the optical intensity and the Fourier transform of the overall transfer function (FDTc) of the optical filtering stage (2); - calculating the inverse Fourier transform of the quotient.

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