EP2074376A2

EP2074376A2 - Phase-sensitive low-coherence interferometry apparatus

Info

Publication number: EP2074376A2
Application number: EP07827702A
Authority: EP
Inventors: Andrea Melloni; Cristina Canavesi; Filippo Persia
Original assignee: Politecnico di Milano
Current assignee: Politecnico di Milano
Priority date: 2006-09-21
Filing date: 2007-09-19
Publication date: 2009-07-01
Also published as: WO2008035389A3; WO2008035389A2; ITMI20061792A1

Abstract

The invention relates to an interferometry apparatus (100) for measuring at least one parameter of a sample (S) comprising: first (S1) and second (S2) sources, respectively, of first (So1) and second (So2) electromagnetic radiations, having distinct wavelengths and time coherences; interference means (50) of said radiations to provide a combination electromagnetic signal (Soc2) of first (Soi1) and second (Soi2) interference electromagnetic signals between portions, of first and second radiations, respectively. The apparatus being characterized in that it further comprises: a device (PD) for converting the combination electromagnetic signal (Soc2) into a corresponding combined electric signal (Sce) and separation electronic means (F-B) of the combined electric signal (Sce) into first (Se1) and second (Se2) electric signals representative of said interference signals (Soi1, Soi2), respectively.

Description

DESCRIPTION Phase-sensitive low-coherence interferoiαetry apparatus.

The present invention relates to interferometry, particularly optical interferometry. Various optical interferometry techniques are known, such as: Optical Coherent Reflectometry (OCR), Optical Low-Coherence Interferometry (OLCI) , Optical Coherence Tomography (OCT) , Optical Doppler Tomography (ODT) , Optical Photoelastic Tomography (OPT) . Low-coherence optical interferometry can be used, for example, for non-invasive optical diagnostics and for the characterization of optical components and devices, optical fibers, materials, biological samples, and any element having such characteristics as to be optically observed.

Furthermore, low-coherence interferometry (like the OCR technique) can be used in the characterization of integrated, bulk, micro-optical and hybrid optical components, in applications such as the analysis of defects, transfer function characterization, tuning or trimming .

Optical Coherence Tomography, Optical Doppler Tomography are techniques typically used in biomedical applications, such as the diagnosis and characterization of neoplasia and disorders, diagnosis of the oral cavity cancer, investigations of vascular dynamics, ophthalmic imaging, biological tissue imaging. Optical Photoelastic Tomography is used, for example, to study optical fiber preforms. With reference to the low-coherence optical interferometry, according to a first technique, an individual light source is used which is sent to an interferometer having an arm in which a sample is placed to be analyzed, and a reference arm that is typically obtained with a mirror moving in free space and allowing to change the length of the reference path.

The light illuminating the reference arm and that illuminating the sample are combined with each other, and when the optical length of the reference arm equals the optical length of the arm including the sample, at less than the source coherence length, interference bands (i.e. an interferogram) are produced, which are acquired by means of a photodetector. By knowing the interferogram, some information can be gained on the sample composition.

An example of low-coherence optical interferometry apparatus of the type discussed above is described in US-A-2002/006794 in which an interferometer is proposed in a reflection configuration. Another example of low-coherence optical interferometry using an individual source is described in the article by Francesco Morichetti, Filippo Persia and Andrea Melloni "Characterization of ring-resonators by optical low-coherence interferometry", 12^th European Conference on Integrated Optics, Grenoble, 2005. In this article, a transmission interferometry scheme is described, which is partially made of optical fiber, and uses a translatable mirror in the reference arm and a balanced photodetector for converting the optical interference signal into an electric signal .

According to another OLCI technique, in addition to the low-coherence source, an additional coherent source is used which allows obtaining a low-coherence interferogram that is not affected by an inevitable irregular movement of the movable mirror of the reference arm. This additional coherent source is used, by means of the same interferometer in which the low- coherence radiation propagates, in order to produce another interferogram, due to which information is collected on the movement error of the translatable mirror that can be used to correct the low-coherence interferogram. This technique is known as the phase- sensitive low-coherence optical interferometry. Example of phase-sensitive low-coherence optical interferometers are described in the article by Carlos Palavicini et al. "Phase-sensitive optical low-coherence reflectometry technique applied to the characterization of photonic crystal fiber properties", OPTICS LETTERS, 30(4), 361-363, 2005 (which describes a reflection configuration) and in the article by Kazumasa Takada "Phase Error Measurement of an Arrayed-Waveguide Grating in the 1.3-μm Wavelength Region by Optical Low Coherence Interferometry" , IEEE PHOTONICS TECHNOLOGY LETTERS, 14(7), 956-967, 2002 (describing a transmission configuration) .

Conventional phase-sensitive low-coherence interferometers, such as those discussed in the articles mentioned above, carry out detections of the coherent source light and low-coherence source light using two respective photodetectors that must ensure high mutual synchronism, in order to allow a satisfactory correction of the low-coherence interferogram. Synchronism seems to be a very stringent requisite, and besides burdening the apparatus with structural complexity, it also leads to a limitation in the translation speed of the movable mirror of the reference arm which results in an extension of the measurement time . The object of the present invention is to provide an interferometry apparatus alternative to the conventional ones, and that, for example, at least partially overcomes the drawbacks set forth herein above with reference to the prior art.

This object is achieved by an apparatus in accordance with claim 1.

Alternative embodiments are as defined in the dependent claims 2 to 29. The object of the present invention is also the use of the apparatus in accordance with the invention in the application fields as defined in claim 30.

Further characteristics and advantages of the invention will appear from the description given below of preferred embodiments thereof, which are intended to be indicative and non-limiting examples, with reference to the annexed figures, in which:

- Fig. 1 schematically shows by functional blocks an interferometry apparatus according to a first exemplary embodiment of the present invention of the transmission type;

- Fig. 2 schematically shows a balanced photodetector for use in said apparatus; - Fig. 3 shows the behaviour of an electric signal acquired by means of said apparatus in the spatial domain;

- Fig. ₄a shows the spectrum of an electric signal acquired by means of said apparatus;

Fig. 4b shows the spectra of a low-coherence interferogram during a step prior to and during a step after a correction procedure;

- Fig. 5 shows an example, at least partially made of optical fiber, of the apparatus in Fig. 1;

- Fig. 6 schematically shows by functional blocks a further interferometry apparatus in accordance with a second exemplary embodiment of the present invention of the reflection type; - Fig. 7 shows an example of the apparatus in Fig. 6 at least partially made of optical fiber. Structural description

Fig. 1 shows a first exemplary embodiment of an interferometry apparatus 100 for measuring at least one parameter of a sample S. Particularly, it is a phase- sensitive low-coherence interferometry apparatus 100 that can be used in various applications, for example in the Optical Low-Coherence Interferometry (OLCI) .

For example, the interferometry apparatus 100 (for clarity, referred to herein below as the apparatus 100) can be used in the field of the optical component characterization (either integrated or not) and allows finding the defects thereof, measuring the modulation and phase transfer function (and measuring the group delay and chromatic dispersion) , calculating the double refraction and the group refractive index thereof, and the like. This apparatus 100 can be also used in the biomedical field to perform the characterization of biological materials, or to perform the characterization of non-biological materials, such as, for example, for calculating the refractive index of an optical fiber preform.

The apparatus 100 comprises a first source Sl of a first electromagnetic radiation Sol (according the example, at optical wavelength) having a first time coherence value, and a second source S2 of a second electromagnetic radiation So2 (for example, at optical wavelength) having a second time coherence value higher than said first value. Particularly, both sources Sl and S2 operate at different wavelengths.

Furthermore, the apparatus 100 comprises an interference block 50 of radiations Sol and So2, a device converting the electromagnetic signals into electric signals PD, an electric separation or filtering block F-B and, advantageously, a processing module P-M. In greater detail, the first source Sl is a low- coherence source, whereas the second source S2 is coherent. Several exemplary values of the time coherence of these two sources Sl and S2 will be provided herein below.

The interference block 50 is such as to provide a second optical signal Sco2 obtained by combining a first interference signal Soil between portions of the first radiation Sol and a second interference signal Soi2 between portions of the second radiation So2. According to the example in Fig. 1, the interference and propagation block 50 provides a transmission interferometry configuration. The interference block 50 can be made by different technologies such as, for example, by means of optical fiber pathways, waveguides, or it can be made in free space or by means of hybrid technologies .

The interference block 50 comprises, for example, an input device SC, for combining the optical radiations Sol and So2 into an individual optical beam, i.e. a first optical combined signal Scol. The input combination device SC is optically coupled to a device B for splitting the radiation onto a reference optical arm R-P and onto a measurement optical arm S-P, having respective outputs connected to said output combination device C.

For example, the input combination device SC, the division device B and the output combination device C can be provided by means of conventional optical fiber components (for example, an individual directional coupler can perform the function of both devices SC and B) , or by means of corresponding components_^ in integrated optics or in free space. The division device B is preferably configured such that the two arms R-P and S-P receive the same radiation intensity, however, other intensity rates are also possible.

The measurement arm S-P contains the sample S to analyze and the reference arm R-P includes a variable delay element R. Variable delay elements for use with the present invention are known to those skilled in the art, and for example, can be provided by means of one or more movable optical components. Particularly, a variable delay element R for use in the apparatus 100 is described below with reference to Fig. 5 and comprises a translatable reflecting prism.

With further reference to the time coherence values of the two sources Sl and S2, it should be noted that the second source S2 preferably has a time coherence higher than or equal to the maximum extension of the delay element R, i.e. the maximum dimension of the sample S to be measured. For example, for typical optical applications, the time coherence of the second source S2 is higher than 10 m; for the characterization of optical components, this time coherence of the second source S2 is higher than 1 m; for biomedical applications, it is higher than 10 cm.

The first source Sl has a lower time coherence than the coherence of source S2 and, particularly, it has a coherence ranging between 1 μm and 0.50 m or, preferably, between 1 μm and 10 cm or, more preferably, between 1 μm and 20 μm.

The output combination device C is such as to cause the interference between a portion of the first radiation Sol that passed through the reference arm R-P and another portion of the first radiation Sol that passed through the measurement arm S-P, thereby originating the first interference signal Soil.

In the output combination device C the interference also takes place between a portion of the second radiation So2 that passed through the reference arm R and another portion of the second radiation So2 that passed through the measurement arm S, thereby originating the second interference signal Soi2. Furthermore, the output combination device C is such as to combine the first interference signal Soil and the second interference signal Soi2 such as to provide the second optical combined signal Sco2 on an optical output port thereof. The output combination device C, that can be implemented in the various technologies mentioned above, has an operating band that is sufficiently broad as to allow the radiations to propagate to the output port at the wavelengths of both sources Sl and S2.

The conversion device PD is, according to the example, a photodetector comprising an input electromagnetic port being coupled to the output of the output combination device C and at least one output electric terminal .

This photodetector PD is used to convert the second optical combined signal Sco2 into a corresponding combined electric signal See. The photodetector PD can comprise an individual photodiode D being connected to the output of the combination device C.

Alternatively, the photodetector PD can be a balanced photodetector (see Fig. 2) including two photodiodes Dl and D2. In this case, the output combination device C is such as to have two separate outputs on which power portions of the same second optical combined signal Sco2 are provided. An optical input of each of the diodes Dl and D2 is coupled to a respective output of the combination device C, such as to receive these power portions of the second optical combined signal Sco2.

As shown in Fig. 2, each of the two photodiodes Dl and D2 has its own output electric terminal (for a respective converted electric signal) connected to a mixing point N, which is such as to provide said combined electric signal See, obtained according to a difference between the electric signals applied to the mixing point. The use of the balanced photodetector allows reducing the photodiode current noise and substantially cancelling the power fluctuations of the source .

The photodetector PD and, particularly, each of the photodiodes contained therein (according to the example, the photodiode D or the two photodiodes Dl and D2) , is such as to detect the wavelength of both sources Sl and S2.

The filtering electric block F-B allows, starting from the combined electric signal See, to separate a first electric signal SeI, representative of the first optical interference signal Soil, from a second electric signal Se2, representative of the second optical interference signal Soi2, due to the fact that the two sources Sl (low-coherence) and S2 (coherent) have different wavelengths, and hence different frequencies.

The filtering electric block F-B is of a numerical or analogue type. With the numerical type, the filtering block comprises, for example, an analogue-to-digital converter AD, a first transformation module TR-M which carries out the Fourier transform (for example, according to the known FFT algorithm, Fast Fourier Transformation) , a filtering module FLT and a second transformation module TR-M^"1 to carry out an inverse ^; Fourier transform.

The filtering module FLT can be a software or, in order to be implemented, an embedded filter can be used, such as a Digital Signal Processor (DSP) or a Field Programmable Gate Array (FPGA) programmed to implement a Finite Impulse Response Filter (FIR) or an Infinite Impulse Response Filter (IIR) , or a combination of a FIR filter and an IIR filter. Furthermore, in order to implement the filter FLT, microprocessors or microcontrollers or other digital processing systems can be used.

The analogue FLT filter can be a Sallen key filter, an LRC-ladder filter, an active or passive filter, or a resonant crystal filter. In order to implement the analogue FLT filter, heterodyne or superheterodyne detection schemes or lock-in can be used, which are tuned to the characteristic frequencies of the two sources Sl and S2. The processing module P-M allows processing the electric signals SeI and Se2, which result from the filtering block F-B, and obtaining information needed to determine the required low-coherence interferogram, from which the parameters of interest of sample S can be obtained.

Several teachings on the dimensioning of the apparatus 100 will be set forth below.

As regards the selection of the typology of the optical sources Sl and S2, it should be noted that the first low-coherence source Sl can be conveniently selected considering that, as it is known, the time coherence length thereof is reduced as the band increases .

Furthermore, the selection of the first low- coherence source Sl depends on the particular application. For example, a broadband source having a coherence length less than 200 μm and central wavelength comprised between 1500 and 1600 nm is suitable to characterize optical components working in the C+L band of optical communication. Sources having a wavelength ranging between 1250 and 1350 run are conveniently selected for bio-medical applications and in order to characterize optical components working in the 0 band of optical communication. The second source S2 of a coherent type, will advantageously have high time coherence length and, thus, a narrow band.

The link between the spectral band ΔfpmM, i.e. the full width half-maximum amplitude (or, in terms of wavelength, ΔX_FWJM) , and the coherence length L_a is given by

_ 41n2 c 41n2 λ² . .

JUc=¹ = (1)

In ISfFWHM 2π AXFWHM wherein λ is the central wavelength and c is the speed of light in vacuum. It is useful to select the coherence length L_c of the second coherent source S2 such as to be conveniently greater than the length by which the movable optical element included in the delay element R can move, within the reference arm R-P.

In order to make an optimum selection, the second coherent source S2 is selected such as to have a wavelength ranging within the band of the photodetector PD and, advantageously, such that the wavelength thereof is the farthest possible from the central wavelength of the first low-coherence source Sl. Furthermore, when the interference block 50 includes optical fiber tracts, the low-coherence Sl and coherent S2 sources are preferably selected such as to fall within the monomodal propagation condition. With reference to the delay element R made by means of a translatable prism, it should be observed that the greatest the translation thereof, the greatest the length on which the sample S can be characterized. Description of the operation In relation with the operation of the apparatus 100, the low-coherence Sl and coherent S2 optical sources are activated. The first Sol and second So2 optical signals are sent to the input combination device SC providing the first optical combined signal Scol. The first Sol and second So2 optical signals do not interfere in the first splitter B, in that they are signals having distinct wavelengths.

The first optical combined signal Scol is split by the splitter B into a first split signal SpI and second split signal Sp2 (Fig. 1) having, for example, equal powers. The first and second split signals SpI and Sp2 are then transmitted along the reference arm R-P and along the measurement arm S-P.

Within the measurement arm S-P, the first split signal SpI passes through the sample to be analyzed S, whereas, within the reference arm, the second split signal Sp2 travels through the variable delay element R. By acting on the variable delay introduced by the element R, i.e. causing the translatable prism to take various positions, the phase delays may be introduced, which will lead to the formation of interference bands, i.e. the desired interferogram.

By exiting the reference R-P and measurement S-P arms, the split signals SpI and Sp2 are re-combined within the second combination device C such that an interference will occur within those radiation portions at the same wavelength which result to be outphased. Thereby, the first interference signal Soil (at the wavelength of the low-coherence source Sl) and second interference signal Soi2 (at the wavelength of the coherent source S2) are formed, which are transmitted in the form of an individual optical beam, and i.e. the second optical combined signal Sco2.

The second optical combined signal Sco2 is detected by the photodetector PD (both of the type with an individual photodiode, and balanced type) , which provides the combined electrical signal See. It should be observed that the detection of the second optical combined signal Sco2 by means of a same individual photodetector PD causes the interference optical signals Soil and Soi2 to be simultaneously detected, and thus the corresponding electric signals thereof that are obtained from the detection are inherently synchronized. By way of example, Fig. 3 shows a progress (obtained during the testing) of the module of the combined electric signal See (analogue signal) , detected by the photodetector PD, as a function of the shifting ΔL of the movable optical element associated with the reference arm R-P. By observing Fig. 3, the overlapping of the low- coherence interferogram (with which the peak seen in the figure is associated) , (corresponding to the first interference electric signal SeI) and of the interferogram of the coherent radiation (corresponding to the second interference electric signal Se2) can be noted.

In accordance with the example in Fig. 1, the combined electric signal See is converted, in a conventional manner, by the conversion module AD into a digital signal. The first transformation module TR-M processes the samples obtained from the analogue-to- digital conversion and yields the Fourier transform of the combined electric signal See, i.e. the representation of the latter in the frequency or spectrum domains. Fig. 4a shows a spectrum of the signal acquired by the photodetector PD (modulus expressed in dB) from which the distinct spectral components are seen of the first interference electric signal SeI (relating to the interference for the first low-coherence source Sl) and of the second interference electric signal Se2 (relating to the interference for the second coherent source S2) . In the exemplary case as shown in Fig. 4a, the first interference electric signal SeI has a central frequency of about 13 kHz, whereas the second interference electric signal Se2 has a central frequency of about 15 kHz.

It can be demonstrated that the electric frequency f₀ of each of both electric signals SeI and Se2 is given by the following relationship: wherein v represents the speed (either even or variable over time) at which the movable optical element of the delay element R is translated and λ is the optical wavelength.

It should be noted that the optical component associated with the delay element R is moved at ideally even speed but, however, a movement irregularity is present, which produces an undesired modulation that broadens the spectrum. The selection of the spacing between the spectral components associated with the two sources Sl and S2 in the domain of the electric frequencies is such as to reduce or -avoid the fact that the spectra of both signals are excessively overlapped. The samples representative of the Fourier transform of the combined electric signal See are filtered by the numerical filter FLT such as to separate the samples corresponding to the spectrum of the first interference electric signal SeI from the samples corresponding to the spectrum of the second interference electric signal Se2.

It should be observed that the Applicant has noticed how a satisfactory separation of the spectra can be obtained, for example, for spectra overlapping by normalized amplitudes less than -20 dB or, preferably less than -30 dB.

The second transformation module TR-M^"1 carries out an inverse Fourier transform of the samples obtained from the filtering, such as to obtain samples (in the time domain) representative of the first interference electric signal SeI and i.e. representative of a first low-coherence interferogram, and separated samples representative of the second interference signal Se2 and hence, the coherent interferogram. Thereby, the processing module P-M has the two interferograms available, and can carry out various processing depending on the particular application. According to an example, the processing module P-M processes the second electric signal Se2, corresponding to the coherent interferogram, in order to obtain information on the error introduced by the irregularities in the translation of the optical element employed by the delay element R. This type of processing, known to those skilled in the art, is based, for example, on the consideration that the interference signal of a coherent source is a sinusoid, in the event of a linear translation of the reference arm. When, on the other hand, the movement of the delay element is irregular, it produces a deviation from the sinusoid and this deviation is the correction to be made.

Based on this information concerning the error, the processing module P-M processes the samples of the first low-coherence interferogram (SeI) such as to correct the latter thus obtaining a second interferogram, which is corrected of the errors due to the inevitable irregularity in the movement of the movable optical element. By way of example, Fig. 4b shows the normalized spectra of a low-coherence interferogram during a preceding step of the correction processing (dotted curve) and during a subsequent step of this processing (solid curve) . As may be seen in Fig. 4, the low-coherence spectrum has re-acquired, following to the processing, a Gaussian form, typical of the source Sl of a test set-up (which will be described below with reference to Fig. 5) by means of which the interferogram has been obtained.

It should be observed that the correction of the low-coherence interferogram is carried out with high precision, in that the synchronism of the detections of the two interference optical signals Soil and Soi2 is ensured by using the individual photodetector PD of the second optical combined signal Sco2. This high precision results in the possibility of adopting high translation speeds v for the movable optical element within the reference arm R-P, thereby allowing to carry out a particularly rapid acquisition.

After the corrected low-coherence interferogram has been obtained, the processing module P-M may further process the latter, with conventional modes, such as to calculate the desired parameters characterizing the sample S. As it is known to those skilled in the art, the low-coherence interferogram is a function of the spectrum of the low-coherence radiation Sol being used, as well of the transfer function of sample S, which completely describes the behaviour thereof . Further embodiments

Fig. 5 shows an exemplary embodiment of the apparatus 100 implemented according to a transmission configuration, at Mach-Zehnder interferometer. The Applicant has carried out tests using a similar configuration as that in Fig. 5 to measure the amplitude and phase response of optical components. It should be observed that the same numerals will be used throughout the figures of the present description in order to designate apparatuses, components, blocks, modules and elements in general, either equal or similar to each other . The apparatus 100 in Fig. 5 is mainly manufactured from optical fiber and comprises a first source Sl that can be implemented, for example, by a superluminescent diode having a central wavelength of 1570 nm, a band of 46 nm and a time coherence length of 45 μm. Other low- coherence sources that can be used are, for example, an Erbium-Doped Fiber Amplifier source (EDFA) , a Light Emitting Diode (LED) , an Amplified Spontaneous Emission source (ASE) , a supercontinuum generation device, Photonic Crystal fiber (PhC) , a halogen lamp. The second source S2 (i.e., the coherent source) is, in accordance with the example indicated above, a Distributed FeedBack laser (DFB) having a central wavelength of 1310 run, a band of about 0.01 pm and a time coherence length of about 50 m. According to other embodiments, the second source S2 is a Distributed Bragg Reflector (DBR) or an external cavity laser.

The functions of the first combination device SC and splitter B as shown in Fig. 1 are performed by a first fiber-made directional coupler CSl (of the 3 dB- type) having two inputs, each coupled to one of the sources Sl and S2, and two outputs, each coupled to one of the reference R-P and measurement S-P branches.

Advantageously, an input fiber of each of the reference R-P and measurement S-P arms comprises a respective polarization controller PCl and PC2, which allows improving the signal-noise ratio of the optical signal being detected.

The variable delay element R is provided, according to the example set forth above, by means of a prism PR (depicted as a mirror in Fig. 5) , which is mounted on a slide moving along a guide (both not shown) , and having at least one first surface suitable to receive the optical radiation outputted from the polarization controller PCl of the reference arm R-P, to which it is coupled by means of a first optical lens LSI. Alternatively to the prism PR, a mirror (for example metallic, dielectric or multi-layer) , a pentaprism, a corner cube or other element can be used, which is capable to reflect a suitable portion of the incident- signal .

At least one second surface of the prism PR is such as to receive the radiation reflected from the first surface of the prism, and reflect it, in turn, to a second optical lens LS2 coupled to a first fiber tract Fl.

During the test set-up carried out by the Applicant, the prism PR could be subjected to a shifting of max 30.6 cm, corresponding to a total travel in air of 61.2 cm. The two lenses being used, LSI and LS2, were graded index lenses (GRIN) though other types of lenses may alternatively be used. During this test, speed v at which the prism PR has been translated had an average value of 20 mπi/s. It should be noted that this value is at least one order of magnitude greater than that used according to the prior art.

According to an alternative embodiment, instead of the translatable prism PR, an optical fiber can be used, for example, to be elongated with a piezo-ceramic: thereby, the variable optical delay line is completely made from fiber.

The measurement arm S-P₇ including the sample S to be analyzed has a second tract of optical fiber F2 coupled to a respective input of a second fiber-made directional coupler CS2 (for example, of a 3 dB-type) having two outputs F3 and F4 that are coupled, by means of respective optical fiber tracts, to respective optical inputs of the photodiodes Dl and D2 of the balanced photodetector PD, such as that shown in Fig. 2. The first optical fiber tract Fl, connected to the second lens LS2, is coupled to another input of this second directional coupler CS2. The second fiber-made directional coupler CS2 carries out the functions of combination and interference of the above-described output combination device C (Fig. 1) . The fiber-made directional coupler CS2 can be coupled to the measurement arm R-P, for example, by means of lenses, collimators, objectives or other collimation or focalization systems.

The optical fibers for use with the interference block 50 are, for example, standard single mode fibers; however, for several applications, it is convenient to use polarization-maintaining fibers in the measurement arm S such as to be capable of measuring, for example, the response of orthogonal polarizations. Other conventional types of fibers can be also used.

The balanced photodetector PD used during the test illustrated in Fig. 5 had a band that allowed to acquire signals having a wavelength within the range 800-1700 nm, therefore, this photodetector perfectly acquired the radiation of the two sources Sl and S2 indicated above. A PD photodetector for use in the present invention is the photodetector Nirvana 2017 produced by NewFocus. The analogue-to-digital conversion function of the module AD in Fig. 1 is performed in the apparatus 100 in Fig. 5 by an acquisition circuit board DAB (commercially available) with a sampling frequency of 200000 samples/second, connected to an output electric terminal of the photodetector PD by means of a first electric connection Ll. The minimum sampling frequency acceptable meets the Nyquist sampling theorem, i.e. it is at least twice the maximum electrical frequency of the signal of the photodetector PD, i.e. for example, twice the ratio of the maximum speed at which the prism PR translates to the shortest wavelength between the two radiations Sol and So2 being used.

The operations performed by the transformation TR- M, filtering FLT, inverse transformation TR-M^"1 modules and processing module P-M can be carried out by a suitably programmed conventional computer, such as a personal computer 20, connected to the acquisition board DAB by means of a suitable transmission line L2. The personal computer 20 is also used for controlling a motor (not shown) which drives the translation of the slide to which the prism PR is mounted, and is, to the purpose, connected to the motor by means of a respective electric cable L3.

The operation of the apparatus 100 in Fig. 5 is similar to that described with reference to Fig. 1. The progressions of the signals shown in Fig. 3 and 4 have been acquired by means of the apparatus 100 made in accordance with what has been stated above with reference to the test set-up in Fig. 5 described above. Fig. 6 shows another exemplary embodiment of an interferometry apparatus 100' to be used in a similar manner as the apparatus 100, but implemented according to a reflection configuration.

As may be seen from a simple comparison between the apparatus 100 in Fig. 1 and that 100' in Fig. 6, a difference between these apparatuses is that in the latter the optical radiation does not pass through the sample S to analyze but is rather partially or totally reflected either at the interface thereof or therein. A splitter/coupler B/C plays the role of splitting the first optical combined signal Scol, received from the first combination device SC, into two portions (SpI and Sp2) that are transmitted to the reference R-P and measurement S-P arms. Furthermore, the device B/C has the task of combining the optical signal reflected by the sample S and that transmitted by the variable delay element R, thus causing the portions to interfere at the same wavelength and generating the interference optical signals Soil and Soi2. Fig. 7 shows an exemplary embodiment of the apparatus 100', which is mainly made from optical fiber and according to Michelson's configuration. This embodiment includes similar components as those in Fig. 5 but, as compared with the latter, it is also provided with an input coupler Cl (for example, made from optical fiber) which combines the first optical signal Sol with the second optical signal So2, thereby performing the functions of the input combination device SC, shown in Fig. 6. The split function carried out by the device B/C, described with reference to Fig. 6, is performed by the first directional coupler CSl which has an input fiber tract F connected to an output of the input coupler Cl and a second fiber tract F' acting as the output for the optical signal reflected by the sample S, connected to the tract of fiber F2, inputted to the second coupler CS2. The second coupler CS2 performs the function of combination of the device B/C in Fig. 6, for the formation of the second optical combined signal Sco2. As will be appreciated by those skilled in the art, in addition to the above-mentioned OLCI interferometry, the present invention can be also applied to other interferometry techniques, such as, for example: Optical Coherence Tomography (OCT) , Optical Doppler Tomography, Optical Photoelastic Tomography (OPT) , Optical Coherent Reflectometry (OCR) . Advantages

The teachings of the invention have considerable advantages. It should be noted that the use of an individual photodetector allows obtaining a highly accurate detection, since the latter is carried out in a certainly synchronized manner for the signals corresponding to the low-coherence and coherent optical radiations . Furthermore, with the teachings of the invention a greater precision can be obtained than that achievable with the conventional technologies using an optical separation of the coherent from the low-coherence radiations. In fact, the radiation of the coherent source travels through the same optical pathway as the radiation emitted by the low-coherence source and thus can "observe" any fluctuation, not only of the movable delay element in the reference arm, but also of any other component of the interference block (50) . It should be observed that, contrarily to what occurs in the prior art, as the synchronization of the two photodetectors (each dedicated to one of the radiations of the two sources) is not required, with the apparatus according to the invention a more rapid measurement can be carried out, since the movement speed of the movable optical element (when used) , which contributes to cause the phase delay, can be increased.

Furthermore, using the individual photodetector allows simplifying the structure of the interferometry apparatus and reducing the cost thereof, as optical filters for separating the two signals are not required.

To the embodiments of the device described above, those skilled in the art, aiming at satisfying contingent requirements, may carry out modifications, adjustments and replacements of elements with others functionally equivalent, without departing from the scope of the claims below.

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Claims

1. An interferometry apparatus (100; 100') for measuring at least one parameter of a sample (S) ; comprising: first (Sl) and second (S2) sources, respectively, of first (Sol) and second (So2) electromagnetic radiations, having distinct wavelengths and time coherences; interference means (50) of said radiations in order to provide an electromagnetic signal (Soc2) for combining first (Soil) and second (Soi2) interference electromagnetic signals between portions of the first and second radiations, respectively; characterized in that it further comprises: a device (PD) for converting the combination electromagnetic signal (Soc2) into a corresponding combined electric signal (See) ; electronic means (F-B) for separating the combined electric signal (See) into first (SeI) and second (Se2) electric signals representative of said interference signals (Soil, Soi2) , respectively.

2. The apparatus (100; 100') according to claim 1, further comprising a combination device (C; B/C; CS2) of the first (Soil) and second (Soi2) interference electromagnetic signals to provide the combination electromagnetic signal (Sco2) .

3. The apparatus (100; 100') according to claim 1, further comprising processing means (P-M; 20) that are connected to said separation means (F-B) for providing said at least one parameter of the sample based on the first and second electric signals.

4. The apparatus (100; 100') according to at least one of the preceding claims, wherein said interference means comprise: a measurement arm (S-P) to be coupled to said sample (S) ; a reference arm (R-P) including a variable delay- device (R) of portions of said first and second radiations .

5. The apparatus (100; 100') according to claim 4, wherein said interference means (50) are configured such as to provide the first interference signal (Soil) by means of interference between a portion of the first radiation (Sol) propagating in the measurement arm (S-P) and another portion of the first radiation propagating in the reference arm (R-P) .

6. The apparatus (100; 100') according to claim 5, wherein said interference means are further configured such as to provide the second interference signal (Soi2) by means of interference between a portion of the second radiation propagating in the measurement arm (S-P) and another portion of the second radiation propagating in the reference arm (R-P) .

7. The apparatus (100; 100') according to at least claim 4 , wherein said delay device comprises a movable optical element (PR) movable in various positions such as to obtain said first and second interference electromagnetic signals.

8. The apparatus (100; 100') according to claim 7, wherein said variable delay device includes an optical prism (PR) mounted to a slide and electromagnetically coupled to said reference arm (R-P) .

9. The apparatus (100; 100') according to at least one of the preceding claims, wherein said first radiation (Sol) is low-coherent and the second radiation (So2) is coherent; the apparatus being a phase-sensitive, low- coherence apparatus .

10. The apparatus (100; 100') according to at least one of the preceding claims, wherein at least one between the first and second radiations is an optical radiation.

11. The apparatus (100; 100') according to claim 10, wherein the first source (Sl) is a device belonging to the group comprising: a superluminescent diode, an Erbium-Doped Fiber Amplifier source (EDFA) , a Light Emitting Diode (LED) , an Amplified Spontaneous Emission source (ASE) , a supercontinuum generation device, a photonic crystal fiber PhC, a halogen lamp.

12. The apparatus (100) according to claim 10, wherein the second source is one of the following devices : a Distributed FeedBack laser (DFB) , a DBR reflector, an external cavity laser.

13. The apparatus (100; 100') according to at least one of the preceding claims, wherein the conversion device (PD) comprises at least one input electromagnetic port coupled to at least one corresponding output (F3, F4) of the combination device (C; CS2; B/C) and at least one electric output terminal.

14. The apparatus (100; 100') according to claim 13, wherein the converter device (PD) is a photodetector .

15. The apparatus (100; 100') according to claim 14, wherein the photodetector includes at least one first detecting photodiode (D; Dl) .

16. The apparatus (100; 100') according to claim 15, wherein the converter device is a balanced photodetector comprising the first photodiode (Dl) suitable to provide a first output signal and a second photodiode (D2) suitable to provide a second output electric signal and a mixing point (N) ; the mixing point providing said combined electric signal (See) obtained based on a difference between the first and second electric signals .

17. The apparatus (100; 100') according to at least one of the preceding claims, wherein said conversion device (PD) has an operating band including the wavelengths of said first (Sol) and second (So2) electromagnetic radiations .

18. The apparatus (100; 100') according to at least one of the preceding claims, wherein said interference means (50) are such as to implement a Mach-Zehnder transmission configuration.

19. The apparatus (100; 100') according to at least one of the preceding claims, wherein said interference means (50) are such as to implement a Michelson reflection configuration.

20. The apparatus (100; 100') according to claims 2 and 4, wherein said interference means (50) comprise at least one first fiber-made coupler (CSl) that is optically coupled to said sources (Sl, S2) to provide portions of an input optical combined signal (Scol) to said reference

(R-P) and measurement (S-P) arms; the combination device being another fiber-made coupler (CS2) having input fibers optically coupled to said reference and measurement arms.

21. The apparatus (100; 100') according to at least one of the preceding claims, wherein the interference means

(50) are made by means of one of the following technologies: optical fiber, free space, integrated optics, hybrid technology.

22. The apparatus (100; 100') according to at least one of the preceding claims, wherein said separation electronic means (F-B) are of a numerical type and can be made according to at least one of the following technologies: a processing software module; a Digital Signal Processor device (DSP) or a Field Programmable Gate Array device (FPGA) which are programmed to implement a Finite Impulse Response filter (FIR) or Infinite Impulse Response filter (IIR) ; a combination of a FIR filter and a IIR filter, a microprocessor or a microcontroller.

23. The apparatus (100; 100') according to at least one of the preceding claims, wherein said separation electronic means (F-B) include an analogue filtering module comprising one of the following analogue electric filters, Sallen key filter, active or passive LRC-ladder filter, resonant crystal filter; heterodyne or superheterodyne detection schemes or lock-in.

24. The apparatus (100; 100') according to claim 1, wherein said separation electronic means (F-B) comprise an analogue-to-digital conversion block (AD) suitable to receive said combined electric signal (See) in the analogue form and convert the same into a corresponding digital signal .

25. The apparatus (100; 100') according to claim 24, wherein said separation electronic means (F-B) further comprise a transformation module (TR-M) to generate digital samples representative of a Fourier transform of said combined electric signal in the digital form.

26. The apparatus (100; 100') according to claim 25, wherein said separation electronic means (F-B) further comprise a filtering software module for processing said samples representative of the Fourier transform and separating first digital samples representative of the Fourier transform of said first electric signal (Seel) from second digital samples representative of the Fourier transform of said second electric signal (Sce2) .

27. The apparatus (100; 100') according to claim 26, wherein said separation electronic means (F-B) further comprise a further transformation module (TR-M^"1) to process said first and second samples and provide in the digital form the first and second electric signals, corresponding respectively to a first interferogram of the first radiation and a second interferogram of the second radiation.

28. The apparatus (100; 100') according to claims 3 and 4, wherein the processing means (P-M) are configured such as to process the second electric signal (Se2) to obtain information on an error introduced by said variable delay element (R) ; furthermore, the processing means (P-M) are configured to process the first electric signal on the basis of said information and correcting the first interferogram.

29. The apparatus (100; 100') according to claim 7, wherein said movable optical element comprises at least one of the following optical devices: a reflecting device; a metallic, dielectric or multi-layer mirror; a pentaprism; a corner cube.

30. Use of the apparatus according to at least one of the preceding claims in at least one of the following fields: Optical Low-Coherence Interferonstry (OLCI), Optical Coherence Tomography (OCT) , Optical Doppler Tomography (ODT) , Optical Photoelastic Tomography (OPT) , Optical Coherent Reflectometry (OCR) .