Optical dispersion evaluation system

Abstract: An optical spectroscopy system, wherein a pulse of a radiation to be analysed is sent to a diffractive element (2) presenting high temporal chromatic dispersion, and information relevant to the radiation spectrum is obtained from pulse broadening in the time domain.

Description: The present invention refers to a method of spectral analysis of a light radiation comprising different wavelengths, wherein at least one pulse of said radiation is sent into a diffractive element of known and controlled characteristics, along which the different wavelengths propagate at different velocities so that a temporal dispersion of said wavelengths occurs, and said dispersion is evaluated to obtain the spectral information, and to an apparatus for carrying out the method comprising: a source of pulses of the radiation under test; a diffractive element receiving said pulses and emitting the different wavelengths in each pulse at different instants, thereby causing a temporal dispersion of the pulse itself; and detecting means receiving the pulses having undergone temporal dispersion. Optical spectroscopy systems commonly used to analyse a light radiation spectrum employ prisms or gratings which cause spatial dispersion of the various wavelengths present in the radiation emitted from the source. Wavelengths very closely spaced (e. g. wavelengths which differ by a few nanometers) are to be distinguished in certain cases, such as for instance in characterization of monochromatic sources with quite narrow band (e.g. a LED), in Raman or Brillouin spectroscopy etc. Very complex and hence expensive systems are to be used to obtain the resolutions necessary in these applications by spatial dispersion. For instance, a system described in the paper "Micro-Raman Studies of Fluoride Glass Optical Fibers", presented by J. A. Freitas, Jr., P. C. Pureza, I. D. Aggarwal and U. Strom at the 6th International Symposium on Halide Glasses, Clausthal-Zellerfeld, 1-5 October, 1989 collects and analyses the radiation scattered by a sample by means of a triple spectrometer. Other similarly expensive systems require objectives with very high focal length, which render the system rather cumbersome. In "Optical Fibers" by Takanori Okoshi, Academic Press, 1982, pages 71-78 is described the principle that different wavelengths in a pulse sent along an optical fibre suffer different delays, i.e. the pulse undergoes a temporal dispersion. However, there is no indication about a method and a device exploiting the temporal dispersion to carry out the spectral analysis of a non-monochromatic radiation, especially when the wavelengths in such radiation are very close to one another. In accordance with the invention a spectroscopy method and apparatus are provided which present high measurement sensitivity without resorting to complex and expensive optical equipments. The invention provides a method for spectral analysis of a light radiation, wherein a reference pulse, of known wavelength, is combined with the pulse of the radiation under test and is sent into the diffractive element together with the pulse of the radiation under test; the arrivals on a detector of the reference pulse and of the different wavelengths of the pulse of the radiation under test are recognized to evaluate dispersion; and the values of the dispersion are processed taking into consideration the characteristics of the diffractive element, previously stored, for obtaining the spectral information on the radiation under test. The invention also providse an apparatus for implementing the method, which further comprises a source of pulses of a reference radiation which are combined with the pulses of the radiation under test before being sent into the diffractive element, and in which said detecting means is associated with means evaluating said temporal dispersion by recognizing the arrivals of the reference pulse and of the different wavelengths in the pulse of the radiation under test on said detecting means and with a processing system having stored the dispersion characteristics of the diffractive element, controlling the emission of said pulses and obtaining the values of the wavelengths present in the radiation under test by comparing the temporal dispersion with the data relevant to the dispersion characteristics of the diffractive element. Advantageously, the diffractive element is an optical fibre with high dispersion. The invention will be better understood with reference to the annexed drawing, in which: Fig. 1 is a graph of the dispersion of an optical fibre; Figs. 2 and 3 schematically show two embodiments of an apparatus for carrying out the invention, relevant to two different applications; Fig. 4 is an example of spectrum to be analyzed by the apparatus shown in Fig. 3; and Fig. 5 is a schematic representation of a variant embodiment of the apparatus shown in Fig. 2. Before describing in details the apparatus according to the invention, the principles whereupon the invention is based will be shortly discussed. It is known that a radiation sent into an optical waveguide, e.g. an optical fibre, undergoes a dispersion, i.e. the various wavelengths in the radiation propagate at different velocities, so that a pulse emitted from a source with a certain linewidth and a duration practically equal to zero will "broaden" assuming a finite duration. Dispersion is basically due to three terms: material dispersion, guiding dispersion (due to refractive index difference between core and cladding) and modal dispersion: the last term is obviously missing in single-mode fibres. Overall dispersion has a behaviour shown in Fig. 1, whatever the type of fibre; the actual curve slope depends on the fibre type. Hence, by detecting at a detector the instants of arrival and disappearance of a pulse outgoing from the fibre and comparing them with the instants of arrival and disappearance of a reference pulse of known wavelength and linewidth, or by measuring the time intervals which separate successive peaks within the pulse, it is possible to immediately obtain information about the various wavelengths present in the pulse, starting from the fibre dispersion characteristics. It is evident that, whilst dispersion is a negative factor when using fibres as transmitting medium for optical communications, which factor must be minimized to allow high transmission rates, a high dispersion is instead an advantage in the present application, and the higher the dispersion, the less sensitive and hence the less expensive the apparatus to be used. An example of apparatus performing optical spectroscopy in time domain according to the invention is shown in Fig. 2 for the case of analysis of the light emitted by a source and in Fig. 3 for the case of Raman spectroscopy. In Fig. 2 the light to be analysed, emitted by a source 1, is coupled to an optical fibre trunk 2 by an optical system schematized by lens 3, after having been possibly combined with the light emitted by a reference source 20, as schematically indicated by mirror 21. Sources 1 and 20 are associated with conventional control units 4, allowing emission of light pulses of predetermined duration either by direct source modulation or through a modulator 22. Advantageously, wavelength λ R of source 20 is that at which the fibre has null dispersion, so that the pulse does not undergo distorsion. The light outgoing from fibre 2 is collected by another optical system schematically represented by lens 5, and sent to a detector 6. The electrical signal outgoing from detector 6 is collected by a temporal analysis device 7, e. g. a sampling oscilloscope, which measures the difference between the instants of arrival of the reference pulse and of the pulse emitted by source 1 and the duration of the latter. A processing device 8, comprising a memory storing the characteristics of fibre 2 (namely core and cladding refractive indices, numerical aperture and delay) for a wavelength range comprising the wavelengths emitted by source 1 under test and reference source 20, obtains the information about source 1 (e. g. the linewidth of same and hence its central wavelength) from the time information supplied by analysis device 7. Advantageously, processing device 8 can carry out the synchronization between analysis device 7 and source control means 4. In an exemplary embodiment of the invention, source 1 is a led with central emission wavelength 630 nm and linewidth of a few nanometers, and is controlled so as to emit pulses with duration 1 ns. Fibre trunk 2 is a trunk 1 Km long of monomode fibre of silica doped with 3.1% of GeO₂, whose dispersion characteristics are exactly as shown in Fig. 1. Such a fibre has a step refractive index profile and, at the source central wavelength, the refractive indices of core and cladding are 1.461 and 1.457 respectively and the numerical aperture is 0.12. That fibre has null dispersion at about 1500 nm (see Fig. 1) whilst dispersion in the range from 600 to 1000 nm varies from about 308 ps/km.nm to about 60 ps/km.nm at 1000 nm. The fibre characteristics are listed in the annexed table, wherein λ is the wavelength in nanometers, NA(λ) is the numerical aperture and T(λ) is the transit time, measured in picoseconds, along a fibre trunk 1 km long. In the spectral range of that source the delays introduced by fibre trunk 2 are of the order of some nanoseconds. Temporal analysis device 7 is a sampling oscilloscope capable of resolving time intervals of 20 ps. Said oscilloscope, associated with a photodiode 6 with 20 GHz band, allows wavelength resolutions of the order of about 0.15 nm in the source spectrum interval. The above example clearly shows the high resolution attained by the technique according to the invention. Furthermore, a sampling oscilloscope is much cheaper than a triple monochromator like that disclosed in the cited paper, and is also much less cumbersome than high-resolution single-monochromator systems. The optical fibre trunk has a negligible cost. It is hence evident that the invention supplies a sensitive measurement instrument of limited cost. Fig. 3 shows the apparatus in case of Raman spectroscopy measurements. The elements present also in Fig. 2 have been denoted by the same references as in that Figure. As known, Raman spectroscopy exploits the fact that a monochromatic radiation sent into a sample of material to be studied, produces a scattered radiation comprising not only a line at the source wavelength, but further pairs of lines, which are symmetrical with respect to the source line and are spaced therefrom by an extent which is typical of the material under test. In this application of the invention the light pulse emitted by source 1 is sent onto a sample 10 of the material to be analyzed, and the light laterally scattered by said sample is collected by a suitable optical system 11 and sent onto a filter 12 apt to isolate the useful spectrum portion, more particularly, to attenuate the peak corresponding to the incident radiation, which in this application carries out also the function of reference radiation. The scattered radiation is collected by fibre 2 and sent as before to detector 6, whose output signals are processed by sampling oscilloscope 7 and by processing device 8. Assuming that the spectrum portion at longer wavelengths than the source (the so-called Stokes lines, which have higher intensity than the corresponding Anti-stokes lines, which are symmetrical with respect to the source line) is used in the analysis, the spectrum to be analyzed comprises, as shown in Fig. 4, a main peak at the source wavelength and the Raman scattering peaks, of much smaller amplitude, the first of which is shifted by several nanometers (e. g. 5 to 20 nm in case of a test radiation with wavelength 500 nm) with respect to the source peak. The shift entity is characteristic of the sample material. Supposing that the same fibre and source as above are employed, that first scattering peak will then arrive at the detector with a delay of 1.5 to 6 ns with respect to the main peak, which delay is easily detectable by the described apparatus. This apparatus, by using an adequate pulse frequency, allows also study of the kinetics of a number of even fast events (e. g. crystal formation, growth, modification) through an evaluation of the variations undergone by the scattering peaks during the event: when a conventional monochromator is used, this would require employ of an array detector which further increases costs and, moreover, has much lower speed limits. In case very small time dispersions (e. g. of the order of the hundreds of picoseconds or less) are to be evaluated, which would require detectors with very wide band (up to the limits of the present technology), or in case very weak signal intensities are to be detected, the apparatus can be modified as shown in Fig. 5. Here detector 6 and temporal analysis device 7 of Figs. 2 and 3 are replaced by a detection and analysis system 16 which advantageously comprises an acousto-optical deflecting cell 17 driven by a signal with periodically varying frequency (e.g. a ramp signal) emitted by a generator 18, and a photodetector array 19, where the individual photodetectors supply output signals whose intensities depend i. a. on the time for which they receive the optical signal to be detected; e.g. the detectors may use charge coupled devices (CCD). Detector array 19 is placed in a plane at a predetermined distance L from cell 17. By this arrangement, the various wavelengths λ₁,λ₂ ... λ n in the signal emitted by source 1, which arrive at cell 17 at different instants, are differently deflected by said cell, depending on the value of the cell driving frequency at the instant they reach the cell. Hence the different wavelengths will be collected by different photodetectors in the array. Preferably, distance L and range ΔF of variation of the cell driving frequency are chosen so that the different wavelengths are distributed over the whole array surface. By duly synchronizing the pulse emission by source 1 and by reference source 20 with the variation of the cell driving frequency and by detecting the position on the array where the reference pulse arrives, taking into account difference ΔF between the extreme frequencies of the cell modulation frequency range, processing system 8 can obtain the wavelength present on each point of the array, since the difference between two wavelengths will be proportional to the distance between the detectors by which said wavelengths are received. By using a detector array receiving the different wavelengths at different points and at different instants the problems of the bandwidth of the unique sensor are eliminated. By using charge coupling devices or the like as detectors in the array, that is devices whose output signal is proportional to the integral in time of the input signal, it is possible to send towards detection and analysis system a train of identical pulses and to evaluate the output signal of the individual detectors at the end of the train. It is hence possible to evaluate a signal integrated in time, whose intensity is much higher than that obtainable by sending a unique pulse, and hence good results can be obtained from the spectral analysis also for very low intensities of the individual spectral peaks. It is clear the photodiode array can be replaced by a single photodiode, still using CCD devices and the like, which scans the signal emerging from the fibre in synchronism with the variation of the driving frequency of cell 17. Photodiode movements will be controlled by processor 8. In a variant of the apparatus shown in Fig. 5, acousto-optic cell 17 can be replaced by a so-called "streak camera". A streak camera is a device for analysing the amplitude of pulses having time-varying amplitudes, with resolutions of the order of the picoseconds: the photons of the light pulse to be analyzed are converted into an electron current, whose intensity is proportional to the instant pulse intensity and which is made to pass between deflecting plates controlled by a periodically varying voltage, so that the electrons will be collected by different points of a detector. The distance between two points is proportional to the difference between the emission instants of the respective electron current. In the present application the amplitude variability of the pulse sent to the device is due to the presence of different peaks corresponding to different wavelengths in the light pulse, which spread in time as an effect of the transit in the fibre. The detector can be a photoluminescent screen or an array of charge coupling devices. The operation is equivalent to that of the embodiment using an acousto-optic cell, since a spatial dispersion representing the time dispersion is obtained.

Claims: A method of spectral analysis of a light radiation comprising different wavelengths, wherein at least one pulse of said radiation is sent into a diffractive element (2) of known and controlled characteristics, along which the different wavelengths propagate at different velocities so that a temporal dispersion of said wavelengths occurs, and said dispersion is evaluated to obtain the spectral information, characterized in that a reference pulse, of known wavelength, is combined with the pulse of the radiation under test and is sent into the diffractive element (2) together with the pulse of the radiation under test; the times of arrival on a detector (6; 19) of the reference pulse and of the different wavelengths of the pulse of the radiation under test are determined to evaluate dispersion; and the values of the dispersion are processed taking into consideration the characteristics of the diffractive element (2), previously stored, for obtaining the spectral information on the radiation under test. A method as claimed in claim 1, characterized in that the wavelength of the reference pulse is a wavelength at which the diffractive element has null dispersion. A method as claimed in claims 1 or 2, characterized in that the difference between the instants of arrival at the detector (6) of the reference pulse and of the wavelengths of the pulse of the radiation under test is measured to evaluate dispersion. A method as claimed in any preceding claim, wherein the radiation under test is a radiation scattered by a sample of material onto which a monochromatic radiation has been sent and comprises a first spectral line at the same wavelength as said monochromatic radiation and further pairs of spectral lines symmetrical with respect to said first line, characterized in that the peak corresponding to said first line is exploited as reference pulse and the differences between the instants of arrival of the peaks corresponding to said first line and to the lines of the further pairs are measured. A method as claimed in any preceding claim, characterized in that the pulse of the radiation under test outgoing from the diffractive element is subjected also to spatial dispersion, by deflecting the different wavelengths by different angles, the angle of deflection of each wavelength depending on the delay with which the wavelength itself leaves the diffractive element (2) with respect to the reference pulse, and the wavelength values are obtained from the relative positions of the points of arrival of the different wavelengths and of the reference pulse on the detector (19). A method as claimed in claim 5, characterized in that a train of pulses of the radiation under test is sent into the diffractive element (2), the pulses emerging from the diffractive element (2) are collected by the detector (19) supplying an output signal integrated in time, and the integrated signal is evaluated at the end of the pulse train. A method as claimed in claims 5 or 6, wherein the radiation under test is a radiation scattered by a sample of material onto which a monochromatic radiation has been sent and comprises a first spectral line at the same wavelength as said monochromatic radiation and further pairs of spectral lines symmetrical with respect to said first line, characterized in that the peak corresponding to said first line is exploited as reference pulse and the relative positions of the points of arrival on the detector of the peaks corresponding to said first line and to the lines of the further pairs are determined to evaluate dispersion. A method as claimed in any preceding claim, characterized in that the diffractive element (2) is an optical fibre with high chromatic dispersion. Apparatus for carrying out the method according to any preceding claim, comprising: a source (1) of pulses of the radiation under test; a diffractive element (2) receiving said pulses and emitting the different wavelengths in each pulse at different instants, thereby causing a temporal dispersion of the pulse itself; and detecting means (6; 19) receiving the pulses having undergone temporal dispersion; characterized in that it further comprises a source (20) of pulses of a reference radiation which are combined with the pulses of the radiation under test before being sent into the diffractive element (2), and in that said detecting means (6; 19) is associated with means (7) evaluating said temporal dispersion by determining the times of arrival of the reference pulse and of the different wavelengths in the pulse of the radiation under test at said detecting means (6; 19) and with a processing system (8) having stored the dispersion characteristics of the diffractive element (2), controlling the emission of said pulses and obtaining the values of the wavelengths present in the radiation under test by comparing the temporal dispersion with the data relevant to the dispersion characteristics of the diffractive element. Apparatus as claimed in claim 9, characterized in that said diffractive element (2) is an optical fibre with high chromatic dispersion. Apparatus as claimed in claim 9 or 10, characterized in that said evaluating means (7) determines and supplies the processing system (8) with the values of the difference between the transit times in the fibre (2) of simultaneous pulses of the reference radiation and of the radiation under test. Apparatus as claimed in any of claims 9 to 11, characterized in that said evaluating means (7) is synchronized with control devices (4) of the source (1) so as to receive the signal outgoing from the detector (6) at predetermined instants only. Apparatus as claimed in any of claims 9 to 12, characterized in that it comprises, between the diffractive element (2) and the processing system (8), a detection and analysis system (16) including the detecting means (19) and further means (17, 18) for deflecting the different wavelengths of the radiation under test by different angles, depending on the instant at which the different wavelengths leave the diffractive element (2); and the detection and analysis system (16) detects the relative positions of the points of arrival on the detecting means (19) of the different wavelengths of the radiations under test and of the reference radiation, and the processing system (8) is arranged to obtain the spectral information from such relative positions. Apparatus as claimed in claim 13, characterized in that the deflecting means (17, 18) comprises an acousto-optical cell (17) driven by a variable frequency and a generator (18) of said variable frequency, and the processing system (8) synchronises the frequency variation with the emission of the pulses of the reference radiation and of the radiation under test. Apparatus as claimed in claim 13, characterized in that the deflecting means (17, 18) comprises a streak camera. Apparatus as claimed in any of claims 13 to 15, characterized in that the detecting means (19) comprises a detector array, in which different detectors receive different portions of the radiation under test and of the reference radiation. Apparatus as claimed in claim 16, characterized in that the source (1) of the radiation under test sends a train of pulses of such radiation towards the diffractive element (2) and each detector in the array is arranged to supply, at the end of the train, an output signal whose intensity is proportional to the integral in time of the intensity of the signal received.

Inventors: GREGO GIORGIO, GREGO, GIORGIO
Applicants: CSELT CENTRO STUDI LAB TELECOM, CSELT CENTRO STUDI E LABORATORI TELECOMUNICAZIONI S.P.A.
Application year: 1991
Main classification: G01J3
IPC codes: G01J3, G01N21, G01J11