WO2020058830A1 - Apparatus for optical investigation of photonic circuits - Google Patents

Abstract

A method and apparatus for non-destructive imaging of integrated photonic circuits are used to obtain time-resolved images of an integrated photonic circuit to be investigated on temporal scales of the order of tens of femtoseconds. The method and apparatus are based on the principle of optical gating wherein a probe beam is coupled into an input coupler (IC) of the circuit (IRC) and the resulting wide-field image of the circuit is temporally selected by a gate beam thanks to a sum-frequency generation process that allows to obtain time-resolved images of the circuit (I PC) with a temporal resolution of the order of few femtoseconds and with a spatial resolution of the order of 1 micrometer.

Description

DESCRIPTION OF THE INVENTION

TITLE

APPARATUS FOR OPTICAL INVESTIGATION OF PHOTONIC CIRCUITS TECHNICAL FIELD

[0001] The present invention concerns the technological field of optical devices for the characterization and diagnostics of integrated photonic circuits.

STATE OF THE ART

[0002] Integrated photonic circuits are devices that combine their photonic functionalities with the functionalities typical of integrated electronic circuits while carrying the information as a luminous signal with typical wavelength in the infrared range. Integrated photonic circuits can generate, guide and manipulate optical signals that can be used for a wide array of applications, ranging from sensing to medicine and telecommunications, to name a few. Additionally, integrated photonic circuits can be employed to realize both existing devices comprising different optical components (e.g., integrated laser sources, photodiodes, optical amplifiers, etc.) and novel devices similarly to what can be done with electronic circuits, but with significant energy savings and increased bandwidth.

[0003] Using devices and technologies based on integrated photonic devices is key to increase the efficiency of many applications in terms of speed and frequency bandwidth in said fields. This can be accomplished by either implementing integrated circuits instead of combining discrete elements mounted on different boards, or by creating new, multifunctional devices allowing to process the information optically rather than electronically. Additionally, as compared to traditional electronic devices, optical information processing benefits from an increased speed and bandwidth which makes them an ideal candidate for future development of high-throughput networks, optical computing or deep learning applications.

[0004] Naturally, the advent of new integrated optical devices will require that appropriate diagnostics techniques are also developed, in order to investigate the accuracy and fidelity of the manufacturing processes. [0005] In this respect, different techniques and apparatus are known that are targeted at the functional characterization and diagnostic of photonic systems.

[0006] For example, US 2009245322 A1 describes a procedure and relative apparatus for the investigation of a photonic circuit based on an applied signal that perturbs its temperature locally. These variations are analyzed in order to highlight the possible presence of defects. A downside of this approach is that it requires to perturb the photonic circuit.

[0007] Other known techniques and apparatuses for the investigation of photonic systems are based on the principle of Optical Time-Domain Reflectometry (OTDR), which is based on the analysis of light that is backreflected through an optical fiber.

[0008] For example, US 2017307474 A1 describes a OTDR analysis procedure based on cross- polarized probe signals which allows to evaluate local variations of birefringence by measuring the resonances obtained in reflection.

[0009] In US 2017343389 A1 an OTDR analysis is performed using repeaters and optical signal amplifiers along the optical fiber that is to be characterized, in order to compensate for intrinsic propagation losses and allow the analysis of long fibers.

[0010] A similar technique that is used for the characterization of optical systems is Optical Frequency Domain Reflectometry (OFDR), which is based on the Fourier domain of a broadband spectrum pulse.

[0011] For example, this technique is used in JP2016153768 where a procedure and device are described comprising an optical fiber and an array of Bragg gratings that allow to measure how deformations and temperature variations are distributed locally in the fiber.

[0012] OFDR is also employed in the method described in CN106872071 where two signals are collected, translated into the optical path length domain via a Fourier transform and then analyzed to reveal the presence of local mechanical twisting and temperature variation along the investigated optical fiber.

[0013] Both OTDR and OFDR methods and related devices are well suited for the characterization of optical fibers and, more generally, extremely long transmission lines, but are not suitable for the investigation of integrated photonic circuits which are typically miniaturized components with many branching connections and a network topology.

[0014] Concerning the field of pulsed signals, the time-gating technique is known as a mean to select a time window and resolve time-dependent signals by discharging the ones that fall outside the selected time window.

[0015] For example, US 5,451 ,785 describes and protect a method and relative apparatus to obtain a bi-dimensional wide field image representation of an opaque body behind a trans-illuminated scattering medium by time-gating and frequency domain transformation techniques. In said apparatus, the optical gating is obtained from a probe and a gate pulse by means of sum frequency generation. The time resolution is, however, only employed to select the ballistic component which bears the shadow of the interlaying obstacles and not to reconstruct the time evolution of a signal. Moreover, the apparatus is designed exclusively for use in transmission, since the ballistic shadow signal that is sought cannot be formed, for obvious reasons, in reflection. Finally, the apparatus and method described protect a configuration in which the non-linear component responsible for frequency conversion is placed in the focal plane of the collection lens, instead of in its image plane. If on the one hand this configuration allows, in principle, to obtain a two-dimensional representation of the shadow with more defined edges, therefore useful to better determine its shape, on the other hand this configuration is known to return bi- dimensional representations in which the intensity of light does not correspond exactly to that of the image to be reconstructed, but depends in a variable way on the position within the visual field, thus preventing a quantitative analysis, which is required instead for the functional characterization of a photonic circuit.

[0016] It is also known from scientific literature the application of a perturbative pump-probe technique to the study of integrated photonic circuits. In R. Bruck et al. ,“Device-level characterization of the flow of light in integrated photonic circuits using ultrafast photomodulation spectroscopy”, Nature Photonics, vol. 9, no. 1 , pp. 54-60, 2014, a method and apparatus for the study of integrated photonic circuits are described based on the perturbation of the investigated device based tightly focused pulsed beam. More in detail, a probe pulse in the infrared wavelength range is coupled to the investigated device and collected at its exit coupler to be analyzed at a spectrometer. A delay line is used to send a pump pulse on the optical circuit from above, perturbing it. Therefore, in this approach both the beams interact with the sample.

[0017] A downside of this technique is that it perturbs the investigated device. Also, because of the specific perturbation used, its application is limited to photonic circuits made of silicon. Another substantial disadvantage is represented by the fact that, in order to obtain a bi-dimensional representation of the studied component, it is intrinsically necessary to repeat the measurement point by point by performing a sequential scan of the area of interest.

SUMMARY OF THE INVENTION

[0018] The object of the present invention is that of proposing a method and related apparatus for the investigation of integrated photonic circuits by performing a time-resolved imaging characterization using an optical-gating technique.

[0019] Another object of the present invention is that of proposing a method and related apparatus for the analysis of integrated photonic circuits which offers a high temporal resolution.

[0020] An additional object of the present invention is that of proposing a method and related apparatus for the analysis of integrated photonic circuits presenting a spatial resolution comparable to that of common optical imaging techniques.

[0021] Another object of the present invention is that of proposing a method and related apparatus for the analysis of integrated photonic circuits combining a said high temporal resolution and good spatial resolution to provide precise diagnostics insight into the functional characterization of integrated photonic circuits.

[0022] Another object of the present invention is that of proposing a method and related apparatus for the analysis of integrated photonic circuits that does not have any requirement in terms of the constituent material of said photonic circuits nor of its substrate.

[0023] Another object of the present invention is that of proposing a method and related apparatus for the analysis of integrated photonic circuits that is non perturbative. [0024] Another object of the present invention is that of proposing a method and related apparatus for the analysis of integrated photonic circuits providing directly wide-field images, i.e., without requiring a sequential scanning read-out of the spatial representation of the photonic circuits, with reduced time for acquisition.

[0025] Another object of the present invention is to propose a method and relative apparatus for the analysis of integrated photonic circuits which allows to obtain accurate bi-dimensional representations from the quantitative point of view with respect to the levels of luminous intensity revealed at different points of the wide field of view.

[0026] Another object of the present invention is that of proposing a method and related apparatus for the analysis of integrated photonic circuits that is compatible with optical circuits presenting elements that are protruded vertically, i.e., with a non-planar topography which are not suitable for e.g., near-field characterization techniques.

[0027] Another object of the present invention is that of proposing a method and related apparatus for the analysis of integrated photonic circuits where the detected signal is inherently converted to a visible wavelength, whereas the probe signal is typically in the infrared range (e.g., the telecom C-band at 1550 nm). This allows to use more efficient and cost-effective detectors making the measurement procedure simpler and more accurate.

[0028] According to an aspect of the present invention, these objects are obtained using a non destructive non-perturbative imaging analysis technique of integrated photonic circuits where:

a pulsed optical probe beam with pulses duration of the order of tens of femtoseconds and wavelength in the infrared or near infrared range is coupled to an input coupler of the integrated photonic circuit to be investigated and collected to form a wide field representation of the whole region of interest of the photonic circuit;

said probe beam collected in a wide field optical configuration is temporally selected in the image plane defined by the output optical elements by means of an optical gating system where a gate beam that is synchronous to said probe beam is overlapped spatially to the latter in the volume of a non-linear crystal placed in the image plane of the output optical elements so that its area is illuminated in a substantially uniform way. The gate beam does not interact with the photonic circuit at any time before reaching the non-linear crystal, and covers a variable path length in such a way to impinge on the non-linear crystal overlapping to said wide-field image-bearing probe signal with a predetermined time delay in order to generate a temporally resolved image-bearing optical beam to select the time response;

the temporally resolved image-bearing optical beam is detected by an area sensor which is sensitive to the sum-frequency wavelength.

[0029] The fact that the optical gate beam is spatially extended so as to illuminate the non-linear crystal surface with almost uniform intensity together with the fact that the non-linear crystal is placed in the image plane of the output optical system allows to perform a quantitative analysis since in every point of the obtained bi-dimensional representation the luminous intensity corresponds faithfully to that of the image to be reconstructed.

[0030] Advantageously, the synchronous probe and gate beams have two different frequencies.

[0031] Advantageously, the integrated photonic circuit under investigation can be covered with high refractive index nanoparticles beforehand so that a negligible fraction of light can be scattered out of the optical circuits to be detected, without altering appreciably the transmission properties of the circuit.

[0032] More specifically, a sonicated suspension of Ti02 nanoparticles in isopropanol alcohol can be deposited with an appropriate concentration on top of the integrated photonic circuit.

[0033] Still advantageously, after the optical characterization of the integrated circuit, said particles can be removed straightforwardly by sonicating the photonic circuit itself.

[0034] According to an additional aspect of the present invention, the objects defined earlier are realized by means of an apparatus for non-destructive imaging analysis of integrated photonic circuits which comprises:

a pulsed laser source emitting a probe beam with infrared or near-infrared wavelength;

input optical elements suitable for coupling said beam into an input coupler of the integrated photonic circuit under investigation; output optical elements suitable for collecting the probe beam over a wide field of view, forming an image of the region of interest of said photonic circuit;

elements responsible for the temporal selection of said probe beam collected by the output optical elements over a time window of the order of tens of femtoseconds, said elements comprising: a pulsed laser source emitting a gate beam that is synchronous to the probe beam. Said gate beam covers a path length that does not interact with the photonic circuit under investigation,

optical coupling elements used to overlap spatially the gate and the probe beam as collected by said output optical elements;

elements used to vary the path length of the gate beam in order to finely tune the relative delay between the gate and probe beams;

elements responsible for the sum-frequency generation from the wide-field probe beam and the gate beam, resulting in a time-resolved signal beam having a frequency which the sum of the frequencies of said wide-field probe beam and said gate beam, said time-resolved beam carrying the wide-field bi-dimensional image of the photonic circuit under investigation, said elements responsible for the sum-frequency generation comprising a non-linear crystal placed in the image plane of an output optical system, said non-linear crystal being adapted to perform a time selection and an upward conversion of the frequency generating an optical signal whose frequency is the sum of the frequencies of said wide-field optical probe beam and of said gate beam spatially extended so as to illuminate the entire surface of said non-linear crystal with almost uniform intensity; and

detection means suitable for image acquisition at the wavelength of said frequency-upscaled time- resolved beam.

[0035] Additional properties peculiar to the apparatus are highlighted in the dependent claims.

[0036] The apparatus previously outlined for the analysis by imaging of integrated photonic circuits allows to perform a non-perturbative and non-destructive analysis and it allows to obtain a quantitative analysis thanks to the fact that gate beam which is spatially extended together with the non-linear crystal which is placed in the image plane of the output optical system allow to obtain a representation of the integrated photonic circuit under investigation in which at each point of the bi-dimensional representation the light intensity corresponds faithfully to that of the image to be reconstructed and, in addition, a spatial resolution of the order of 1 micrometer and a resolution of the order of a few femtoseconds are obtained. BREVE DESCRIZIONE DEI DISEGNI

[0037] The features and advantages of the present invention will be apparent from the description of preferred embodiments provided in the following, in accordance to what proposed in the claims, and with the integration of the attached diagrams and drawings, showing:

Fig. 1 : a block diagram of a preferred embodiment of the apparatus described in the present invention

Fig. 2: an illustrative integrated photonic circuit which can be investigated with the present invention

Fig. 3: a schematic representation of the results obtained according to the present invention on the integrated photonic circuit represented in Fig. 2.

DESCRIZIONE DELLE FORME REALIZZATIVE PREFERITE

[0038] With respect to Fig. 1 , the overall apparatus for non-destructive imaging analysis of integrated photonic circuits concerned by the present invention is indicated with 100. The apparatus 100 comprises a probe beam source, 10, which emits a probe beam, 20, with a wavelength in the infrared or near infrared range. Input optical elements, 30, direct the probe beam 20 to the integrated photonic circuit,

I PC, under investigation, focusing the probe beam 20 and coupling it to an input coupler IC of the photonic circuit I PC. Output optical elements 35, collect over a wide field of view the probe beam 20 emerging from photonic circuit I PC under investigation, which is carrying a two-dimensional image of the integrated photonic circuit itself. Elements responsible for the temporal selection, 50, are used to select a time-resolved signal from the probe beam 20 within a temporal range of the order of a few tens of femtosecond, which carries a wide-field two-dimensional representation as collected by the output optical elements 35. The elements responsible for the temporal selection 50 comprise a pulsed laser source, 51 , emitting the gate beam, 40, which is synchronous to the probe beam 20. The elements responsible for the temporal selection 50 comprise optical elements used to guide the gate beam 40 through an optical path that does not interact with the integrated photonic circuit I PC, before it reaches overlapping elements, 60, which are responsible of overlapping spatially the gate beam 40 onto the probe beam 20 which is collected over a wide field of view as a wide-field image beam 21 by the output optical elements 35. Path-length tuning elements, 70, are used to adjust the optical path length of the gate beam 40 in order to finely tune the relative temporal delay between the gate beam 40 and the probe beam 20 collected over a wide field of view as a wide-field image beam 21 , said gate beam 40 and said wide-field image beam 21 being subsequently overlapped spatially by means of the optical overlapping elements 60. Elements responsible for the sum-frequency generation, 80, are used to upscale the frequency of the probe beam in order to generate a temporally resolved beam whose frequency is the sum of the probe and gate beam frequencies, and which carries the two-dimensional wide field image of the integrated photonic circuit IPC under investigation. Elements responsible for image collection and detection, 90, which is sensitive to the wavelength of said time-resolved sum- frequency signal is finally placed to detect it.

[0039] In the embodiment shown in Fig. 1 the probe beam source 10 of the probe beam 20 is a pulsed Ti:Sa laser, 1 1 , which pumps a Parametric Optical Oscillator (OPO), 12, with optical pulses of duration of the order of few tens of femtoseconds, so that the latter generates the probe beam 20 which has conveniently a wavelength of 1550nm, which is particularly relevant for applications in the telecommunication field. In general, however, the OPO can emit pulses of tunable wavelength in the infrared or near infrared range. The group consisting of the pulsed laser 11 and the oscillator 12 generates also the gate beam 40. The polarization of the probe beam 20 is controlled using a half-wave plate, 31 , the probe beam is subsequently focused by an objective, 32, onto the input coupler IC of the integrated photonic circuit IPC under investigation. An infinity-corrected collection objective, 36, collects the probe beam 20 over the whole region of interest of the integrated photonic circuit IPC forming a wide-field representation that is transmitted as a wide-field image beam, 21 , through a dichroic mirror,

61. [0040] The gate beam 40 is collimated by a beam expander, 53, and overlapped spatially to the probe beam carrying the two-dimensional representation as the wide-field image beam 21 by means of a mirror 62 and a dichroic mirror 61. The optical path length of the gate beam 40 is varied by the path-length tuning elements 70 placed between the oscillator 12 and the beam expander 53.

[0041] The path-length tuning elements 70 comprise a motorized stage, 71 , which can translate along a certain distance range, 72. By controlling the length of the optical path traveled by the gate beam 40, the relative delay between the gate 40 and the probe beam 20 can be finely tuned. The path-length tuning elements 70 are such to allow a time-resolution of the order of ten femtoseconds sweeping an appropriate delay window, typically of the order of a few hundreds of picoseconds.

[0042] The wide-field image beam 21 and the gate beam 40 pass through a high-pass filter, 63, blocking possible ambient light in the visible range, and subsequently impinge onto a non-linear crystal, 64, being overlapped both spatially and temporally. The non-linear crystal 64 is responsible for the temporal selection through the generation of an optical signal which with a frequency given by the sum of the frequencies of the wide-field image beam 21 and the gate beam 40, which is to say a signal that is proportional to the convolution integral relative to the temporal overlap between the two signals. The beam expander 53 ensures that the gate beam is large enough to illuminate the whole surface of the non-linear crystal 64 with uniform intensity, which is required to have a uniform conversion efficiency of the non-linear crystal 64 where the upconverted image is formed, over the whole field of view.

[0043] A lens, 91 , or group of lenses suitable for image formation, an additional mirror, 92 and further filters, 93 transmit the time-resolved sum-frequency signal which preserves the two-dimensional representation of the integrated photonic circuit IPC under investigation, to an image sensor, 94, which is sensitive to the frequency of the time-resolved sum-frequency signal and therefore suitable for detecting and representing the two-dimensional image of the integrated photonic circuit IPC.

[0044] Conveniently, the wide-field image beam 21 and the gate beam 40 synchronous to said probe beam are generated at different frequencies, which allows to form the two-dimensional image in a configuration where said optical beams 21 , 40 propagate collinearly, which leads to a better quality of the final time-resolved images. Even if the beams are overlapped and collinear, in fact, it is still possible to separate the time-resolved sum-frequency signal via spectral filtering, for its frequency will be different from those of the second harmonic of the probe beam 20 and the gate beam 40.

[0045] Conveniently, for specific applications, an appropriate tube lens, 37, can be associated to the infinity-corrected collection objective 36 in order to focus and/or magnify the output image that is directed to the non-linear crystal.

[0046] By means of the previously described apparatus 100, an investigation method according to the present invention for the non-destructive analysis of integrated photonic circuits can be performed.

[0047] With respect to Fig. 1 , a method according to the present invention involves a probe beam 20, with pulses of duration of the order of tens of femtoseconds and wavelength in the infrared or near infrared range, being coupled to an input coupler IC of an integrated photonic circuit I PC under investigation and collected over a wide field region so that the resulting wide-field image beam 21 carries a two-dimensional representation of the region of interest of said photonic circuit. The wide-field image beam 21 is therefore resolved temporally by the elements responsible for the temporal selection 50. The elements responsible for the temporal selection 50 involve the generation of a gate beam 40 that is synchronous to the probe beam 20. The two beams are superimposed spatially in the volume of a non linear crystal 64. The gate beam 40 does never interact with the integrated photonic circuit I PC under investigation on its path to the non-linear crystal 64. The path length is varied so that the gate pulse overlaps in time with wide-field image beam 21 with a controlled time delay, which results in a time- resolved sum-frequency signal generate by the non-linear crystal 64, that is eventually detected by an appropriate area sensor. With respect to the relative delay between the probe beam 20 and the gate beam 40, the length of the path length traveled by the gate beam 40 is progressively swept in order to cover the time range where the temporal evolution of the image-bearing probe beam occurs. This is done recording a succession of consecutive time windows that are shifted by roughly 10 femtoseconds, which corresponds to the temporal resolution of the optical gating system described in this invention.

[0048] With respect to Figg. 2 and 3, typical results are described for purely illustrative purposes.

[0049] Fig. 2 shows an example of integrated photonic circuit IPC to be investigated, comprising an input coupler, IC, a main optical path, MP, two intermediate optical paths, IP1 , IP2, originating at the bifurcation of the main optical path MP, four output optical paths, OP1 , OP2, OP3, OP4, originating at the bifurcation of the intermediate optical paths IP1 and IP2, four output couplers OC1 , OC2, OC3, OC4 placed at the end of the output optical paths OP1 , OP2, OP3, OP4, an optical resonator, R, positioned along the first output optical path OP1 , and a dent, D, placed along the fourth output optical path OP4, due to a defect in the fabrication process.

[0050] Fig. 3 shows a diagram illustrating the temporal traces recorded by the image sensor 94 starting from the time-resolved signal relative to few regions of interest of the exemplary integrated photonic circuit of Fig. 2. The probe beam 20 is coupled to the input coupler IC of the integrated photonic circuit I PC under investigation. The temporal trace T1 is relative to the region of the first output coupler OC1 and shows a train of pulses with exponentially decreasing intensity resulting from the coupling with the optical resonator R along the output optical path OP1. The temporal traces T2 and T3 are relative to the regions of the second output coupler OC2 and the third output coupler OC3, respectively. Thanks to a temporal resolution that is much finer than the duration of the single pulse, the analysis allows to reveal that the two optical paths are different even though they should be nominally equal by design, highlighting the presence of small differences occurred during the fabrication process. The temporal trace T4, relative to the region where the dent D is present, allows to reveal its presence and position thanks to the light that the imperfection scatters away from the optical path. The temporal trace T5, relative to the region where the input coupler IC is, allows to observe the back-reflected optical signal that returns to the input coupler after a certain propagation time through the integrated photonic circuit.

[0051] The apparatus and the method of the present invention provide a tool to perform complex diagnostic analysis on photonic components with a spatial resolution comparable to that of traditional optical imaging, i.e. around 1 micrometer for the described embodiment, and with a temporal resolution of the order of a few femtoseconds. Moreover, the analysis is non-invasive in that it does not alter the integrated photonic circuits under investigation in any aspect.

[0052] In a variation of realization of a method according to the present invention, scattering nanoparticles are deposited the integrated photonic circuit IPC to be investigated. These particles have a high refractive index and can extract a tiny fraction of the optical signal that is propagating through said circuit, without altering appreciably its transmission properties.

[0053] More specifically, a suspension with an appropriate concentration of Ti02 nanoparticles in isopropanol alcohol is sonicated beforehand and drop-casted onto the integrated photonic circuit to be investigated.

[0054] The weak signal scattered out from the integrated photonic circuit due to the presence of the scattering nanoparticles is detected from the image sensor so that the two-dimensional representation contains an image of all the paths of the integrated photonic circuit I PC that have been traveled by an optical signal.

[0055] At the end of the analysis procedure the scattering particles can be removed from the integrated photonic circuit I PC by sonication, so that the temporary perturbation of the I PC circuit is perfectly reversible.

[0056] The description provided and the drawings it refers to represent only few of the possible embodiments of the invention. The objects of the invention, as well as the advantages that derive from them, are obtained also in case of different forms of realizations that a specialized technician will be able to implement without involving an inventing step and without leaving the scope of the appended claims.

Claims

1. Non-destructive non-perturbative imaging analysis method of integrated photonic circuits characterized in that:

- a pulsed probe beam (20) with pulses duration of the order of tens of femtoseconds and wavelength in the infrared or near infrared range is coupled to an input coupler (IC) of an integrated photonic circuit (I PC) and collected over a wide field of view as a wide-field image beam (21) carrying a two-dimensional representation of said integrated photonic circuit (IPC);

- the wide-field image beam (21) collected in a wide field optical configuration is temporally selected in the image plane defined by output optical elements (35) by means of elements responsible for the temporal selection (50) where a spatially extended gate beam (40) that is synchronous to said wide-field image beam (21) is overlapped spatially to the latter in the volume of a non-linear crystal (64) placed in the image plane of the output optical elements (35) so that its surface is illuminated in a substantially uniform way, said gate beam (40) not interacting with the photonic circuit (IPC) at any time before reaching said non-linear crystal (64), said gate beam (40) covering a variable path length in such a way to impinge on the non linear crystal (64) overlapping to said wide-field image beam (21) with a predetermined time delay in order to generate a temporally resolved image-bearing optical beam to select the time response;

- said temporally resolved image-bearing optical beam is detected by an image sensor (94) which is sensitive to said sum frequency.

2. Non-destructive imaging analysis method of integrated photonic circuits according to claims 1 characterized in that said integrated photonic circuit (IPC) is treated beforehand by depositing on its surface high refractive index nanoparticles in order to scatter out of the circuit a tiny fraction of the optical signal traveling through said circuit, without altering appreciably its optical transmission properties.

3. Non-destructive imaging analysis method of integrated photonic circuits according to the previous claim characterized in that said particles are made by Ti02 nanoparticles in an alcoholic suspension by means of sonication and then drop-casted with appropriate concentration on the surface of the integrated photonic circuit.

4. Apparatus for non-destructive non-perturbative imaging analysis of integrated photonic circuits characterized in that it comprises:

- A laser probe beam source (10) suitable for generating a probe beam (20) with wavelength in the infrared or near-infrared range,

Input optical elements (30) suitable for coupling said probe beam (20) to an input coupler (IC) of said integrated photonic circuit (IPC),

Output optical elements (35) suitable for collecting over a wide field of view the wide-field image beam (21 ) from said integrated photonic circuit (IPC), said beam representing a two- dimensional representation of the said integrated photonic circuit (IPC),

Elements responsible for the temporal selection (50) suitable for selecting a time frame of said wide-field image beam (21 ) in a time window of the order of a few tens of femtoseconds, said elements responsible for the temporal selection (50) comprising:

- A pulsed laser source (51 ) suitable for generating a gate beam (40) synchronous to said probe beam (20), said gate beam (40) traveling a path that does not interact said integrated photonic circuit,

Overlapping elements (60) suitable for overlapping spatially said gate beam (40) and said wide field image beam (21 ) collected by the output optical elements (35),

Path-length tuning elements (70) to vary the path length of said gate beam (40) in order to tune the temporal delay between said gate beam (40) and said probe beam (20),

Elements responsible for the sum-frequency generation (80) suitable for generating a time- resolved sum-frequency image-bearing signal from said wide-field image beam (21 ) and said gate beam (40), said time-resolved image-bearing signal having a frequency given by the sum of the frequencies of said wide-field image beam (21) and said gate beam (40), and preserving the wide-field image content carried by the wide-field image beam (21), said elements responsible for the sum-frequency generation (80) comprising a non-linear crystal (64) placed in the image plane of output optical elements (35), said non-linear crystal (64) being adapted to perform a time selection and an upward conversion of the frequency generating an optical signal whose frequency is the sum of the frequencies of said wide-field optical probe beam and of said gate beam spatially extended so as to illuminate the entire surface of said non linear crystal with almost uniform intensity, and

Elements responsible for image collection and detection (90) suitable for collecting and detecting signals at said sum frequency in order to represent the two-dimensional image of said integrated photonic circuit (IPC).

5. Apparatus for non-destructive non-perturbative imaging analysis of integrated photonic circuits according to the previous claim characterized in that it comprises a beam expander (53) provided to expand said gate beam (40) generated by said pulsed laser source (51) so that it ensures that the gate beam (40) is large enough to illuminate the whole surface of the non-linear crystal (64) with uniform intensity.

6. Apparatus for non-destructive non-perturbative imaging analysis of integrated photonic circuits according to the previous claim characterized in that said probe beam source (10) is a pulsed Ti:Sa laser (1 1) pumping a parametric optical oscillator (12) with pulses having a duration of few tens of femtoseconds so that the latter generates a synchronous pulsed probe beam (20), said group comprising said pulsed laser (11) and said oscillator (12) generating also said gate beam (40).

7. Apparatus for non-destructive non-perturbative imaging analysis of integrated photonic circuits according to claims 4, 5 or 6 characterized in that said output optical elements (35) comprise an infinity-corrected collection objective (36) that collects said probe beam (20) over a wide field region of interest of said integrated photonic circuit (IPC), transmitting said wide-field image beam (21) that carries a two-dimensional representation of said integrated photonic circuit (IPC).

8. Apparatus for non-destructive non-perturbative imaging analysis of integrated photonic circuits according to the previous claim characterized in that a tube lens (37) is included after the infinity- corrected collection objective (36).

9. Apparatus for non-destructive non-perturbative imaging analysis of integrated photonic circuits according to any of claim 4 or followings characterized in that said path-length tuning elements (70) comprise a motorized stage (71) which can translate over a certain distance range (72) so that the optical path length of said gate beam (40) is adjusted to tune the relative temporal delay between said gate beam (40) and said probe beam (20).

10. Apparatus for non-destructive non- perturbative imaging analysis of integrated photonic circuits according to any of claim 4 or followings characterized in that it comprises a dichroic mirror (61) adapted to overlap spatially said wide-field image beam (21) and said gate beam (40).