FR2794858A1 - Optical spectrum analyzer, has input coupler connected to incoming wave guide or optical fibre, wave guides of incrementally increasing length, image production optics and means of photo detection - Google Patents

Abstract

The optical spectrum analyzer (32) comprises a phasar diffraction system (34) produced on an optical plate (16) and image formation optics (36). The diffraction system has an entry coupler (10) and a phase displacement system using waveguides of incrementally increasing length (14). After diffraction light passes through optical elements of collimation (42) and focussing (44) before arriving at a photodetector (38)

Description

TECHNICAL FIELD OF THE INVENTION The invention relates to an optical spectrum analyzer device with integrated optical diffraction grating comprising a diffraction grating realized in integrated optics and composed of a part of an input coupler connected to at least one input waveguide or an input optical fiber capable of injecting the optical signal, and secondly of a plurality of optical waveguides; wave connected to the input coupler and distributed with increasing lengths in a substantially constant increment to form a phase grating diffracting each wavelength of the optical signal in a different angular direction, and photodetection means for detecting the optical spectrum diffracted by the diffraction grating. PRIOR STATE OF THE ART The principle of the phasar-type diffraction grating in integrated optics is described in the documents of M. K. Smit, "New focusing and dispersive planar component based on an optical phased array", Electronics Letters, Vol. 24, No. 7, pp. 385-386 (1988) and A.R. Vellekoop, M. K. Smit, "Low-loss planar optical polarization splitter with small dimensions", Electronics Letters., Vol. 25, No. 15, pp. 946-947 (1989). US-A-5,002,350 and US-A-5,136,671 relate to a component for performing multiplexing and demultiplexing functions of the optical channels of a wavelength multiplexed network. This component is now known as "Phasar" (Europe), "Arrayed Waveguide Grating" (Japan) and "Dragone type design" (USA).

FIG. 1 shows such a state of the art of the Phasar component made in integrated optics, which is broken down into three parts. The input coupler 10 is a plane waveguide delimited by the ends of the input waveguides. 12 and the phase network 14. The starting points of the guides forming the phase network 14 occupy periodic positions on a circular arc of radius R and whose center is in the vicinity of the central input guide. The input waveguides 12 also end on a circular arc of radius RI2 and whose center is therefore in the middle of the axis formed by the central input guide and the central guide of the phase network 14 This arrangement of the input waveguides 12 and the phase network 14 constitutes the assembly of Rowland. The light injected into this structure by means of an input guide 12 is guided in the direction perpendicular to the integrated optical plate 16, and may diverge in the direction parallel to the phase grating 14 where each guide of wave collects a fraction of the incident wave. # The phase grating 14 consists of waveguides designed with increasing lengths in constant increments. Thus, the phase grating 14 serves the double function of guiding the light of the input coupler 10 to the output coupler 18, and of applying to the phase of the optical wave an incremental and increasing delay according to the guide number. in the network.

 # The output coupler 18 is identical to the input coupler 10. The ends of the guides of the phase grating 14 are arranged periodically on a circular arc of radius R and whose center is placed in the vicinity of the central outlet guide. Due to the transfer function applied by the phase grating and the periodic disposition of the end of the guides in the output coupler 18, this line constitutes a grating diffracting each wavelength in different angular directions. The arcuate arrangement of the guides of the phase grating 14 further ensures the focusing of these wavelengths at single points on the focal line 20 where the exit guides 22 are located.

The input waveguide 12 and output waveguide 22 are monomode, and the integrated optical plate is formed by a plane substrate, for example glass, silica or silicon, on the surface of which are integrated the various waveguides according to one of the well-known techniques in integrated optics, such as ion diffusion or CVD vapor deposition. The light is thus guided within a core material whose refractive index is greater than that of the substrate and the covering material. By definition in this document, a waveguide or simply a guide designates a structure that guides the light along the directions perpendicular and parallel to the integrated optical wafer while the guidance by a plane waveguide is performed only following the direction perpendicular to the plate. For multiplexing - wavelength demultiplexing applications in telecommunication networks, input 24 and output 26 optical fibers 26 are further attached to the input waveguides 12 and output waveguides 22, by the intermediate connectors 28, 30 arranged at the edge of the integrated optical wafer 16. The function of the demultiplexer of Figure 1 is to separate the mixed light radiation in the input optical fiber 24, so as to find the radiation lengths of waves @, j to i, 16 on the output guides 22 and the output optical fibers 26, 2616.

In the demultiplexer of US Pat. No. 5,002,350, a significant coupling of proximity between the guides of the set of integrated waveguides forming the phase grating is described, in order to minimize the optical losses. the output of the input coupler and the input of the output coupler. Such coupling is considered in US-A 5,136,671 as being unfavorable to the multiplexing sought if it is not taken into account in the design of the component.

According to the paper by H. Li, S. Zohng, X. Yang, Y.J. Chen and D. Stone, "Full coverage multichannel wavelength monitoring circuit using center-offset phased array waveguide grating", Electronic Letters, Vol. 34, No. 22, pp. 2149 2151 (1998), it is possible from the phasar component of FIG. 1 to constitute an optical spectrum analyzer by replacing the sheet of optical output fibers 26 by a strip of photodetectors. The number of measurement points of this device is then equal to the number of output guides. In the context of a wavelength multiplexed network supervision application, the analyzer resolution must be 5 to 10 times better than that of the multiplexing and demultiplexing components. The size of the integrated optical chips nevertheless limits the number of possible outputs for a phasar component. The solution proposed in the aforementioned document then consists in using a Vernier effect between several inputs of the component and the photodetector array. A spatial switching device is then necessary to successively select the different inputs.

Etching of a facet diffraction grating in an integrated optical wafer has also been considered. In the device described in PC Clemens, R. Mârz, A. Reichelt, and HW Schneider, "Flat field spectrograph in IEEE Photonics Letters, Vol.4, No. 8, pp. 886-887 (1992), the network The light is injected by means of an optical fiber, and is analyzed by means of an array of photodetectors.In practice, the etching of such a network is difficult. achieved through the core layers and overlap on a height of the order of 20 Nm The definition of the facets of the network and its diffraction efficiency are then strongly degraded.

The paper by H. Takahashi, S. Suzuki, K. Kato, I. Nishi, "Arrayed-waveguide grating for wavelength division multi / demultiplexer with nanometer resolution," published in Electronics. Letters., Vol. 26, N 2, pp. 87-88, (1990), proposes the use of a phasar diffraction grating in a spectral analysis device. Only the phase grating is made in integrated optics so that optical assemblies are necessary on the one hand, to inject the light in the guides of the network and on the other hand, to image the diffracting line of the network on a camera, important problem has not been addressed. At the output of the network, the light diverges strongly in the direction perpendicular to the integrated optical wafer. This results in a significant attenuation of the optical signal.

The polarization dependence of the phasar components results from the birefringence of the waveguides constituting the phase grating. In the case of an integrated optical process generating silica guides doped at a high temperature, the birefringence originates from the stressing of the optical layers by the substrate because of their different expansion coefficients. The two solutions to this problem are the insertion of a half-wave plate into the phase grating, or the adaptation of the expansion coefficient of the cover layer to that of the substrate. Such devices are described respectively in H. Takahashi, Y. Hibino, I. Nishi, "Polarization-insensitive arrayed-waveguide grating wavelength multiplexer on silicon", Opt. Lett., Vol. 17, No. 7, pp. 499-501, (1992) and SM Ojha, C. Cureton, T. Bricheno, S. Day, D. Mold, AJ Bell and J. Taylor, "Simple method of fabricating polarization-insensitive and very low crosstalk AWG grating devices ", Electronics Letters, Vol. 34, N 1, pp. 78 - 79, (1998).

Another problem with phasar components is their dependence on temperature which results in a displacement of demultiplexed wavelengths. This is the reason why most of the functional phasar components are associated with a temperature stabilization device using a heating resistor or a Pelletier effect module. A technological solution consisting of etching a prism in the phase grating, and filling it with silicone having a high thermo-optical coefficient, has also been described in the article Y. Inoue, A Kaneko, F. Hanawa, H. Takahashi, K. Hattori and S. Sumida, "A thermal silica-based arrayed waveguide grating multiplexer", El Lett., Vol. 33, No. 23, pp. 1945, 1946, (1997).

 OBJECT OF THE INVENTION The object of the invention is to provide a reliable optical spectrum analyzer device with a high wavelength dispersion, while being insensitive to polarization and at the temperature.

The device according to the invention is characterized in that - the output ends of the waveguides of the phase grating are staggered at substantially constant pitch along a diffraction line, - the integrated optical chip is cut at short distance from the diffraction line thus creating a transmission line from which the beam diverges, - optical imaging means are arranged outside the integrated optical wafer by being inserted between the line of diffraction emission and the photodetection means for forming the image of the optical spectrum diffracted by the diffraction line on the photodetection means.

As compared with the phasar component, this phasar diffraction grating is devoid of an output coupler whose focusing function is provided by the optical imaging means. This combination makes it possible to achieve a longer wavelength dispersion. In addition, the image formed is a line where the wavelengths of the optical signal occupy different positions and which adapts to the photodetection means.

According to a preferred embodiment, the optical imaging means are arranged outside the integrated optical wafer by being inserted between the transmission line and the photodetection means. The optical imaging means comprise a first optical collimation element intended to parallel the divergent light beam delivered by the transmission line in the direction perpendicular to the integrated optical wafer without modifying the direction of the beam diffracted in the plane of the integrated optical wafer and a second optical focusing element for focusing the output beam of said optical collimation element on the photodetection means in the directions parallel and perpendicular to the integrated optical wafer.

The collimating optical element is a cylindrical lens, and has an object focal line extending in alignment with the emission line. The height of the spot formed by the optical focusing element on the photodetection means in the direction perpendicular to the integrated optical wafer is determined by the mode diameter of the waveguides at the transmission line and following this same direction, and the magnification factor of the optical imaging means, said spot having a dimension smaller than the height of the photosensitive elements of the photodetection means. The optical collimation element may also be composed of a cylindrical mirror or of several cylindrical lenses in order to improve its imaging characteristics.

The design of the optical collimation element can be further simplified by attaching to the transmission line a mode adapter which widens the mode diameter in the direction perpendicular to the integrated optical wafer and thus decreases the divergence of the emitted beam. .

The optical focusing element may also comprise one or more lenses or a concave mirror, which have a symmetry of revolution around the optical axis to ensure the focusing of the beam both in the directions parallel and perpendicular to the wafer. integrated optics. The optical focusing element can also be broken down into two cylindrical optical focusing elements, crossed at 90 and in alignment along the directions parallel and perpendicular to the integrated optical wafer, which then separately focus the beam in these two directions. The cylindrical optical locating element focusing the beam in the plane of the integrated optical wafer or a part thereof can optionally be defined in the integrated optical element by forming a diffraction line in a circular arc.

The photodetection means consist of an array of photodetectors arranged in the image plane of the imaging means, for example linear InGaAs photodiodes distributed with a constant pitch and whose electrical signals representative of the incident optical power are multiplexed to form a signal video. They can also be composed of two strips of photodetectors arranged on either side of a beam splitter to cover different areas of the spectral range. Another variant provides an element made of integrated optics, having single-mode or multimode waveguides, whose input ends are arranged with a reduced pitch in the image plane of the imaging means, and whose output ends have a step expanded to connect a fiber web, or a strip of photodetectors.

The phase grating can be designed so that the length of the guides deviates from a strict periodicity in order to act on the transfer function applied to the phase of the wave. For the same reason, the position of the entry or exit ends of the guides can also deviate from a strictly periodic staggering. The diffraction line at the output of the phase grating may finally be rectilinear or form an arc of a circle. In the latter case, the diffraction line produces in part or in full the desired focusing effect in the plane of the integrated optical wafer to form the image of the optical spectrum diffracted by the diffraction line on the photodetection means . The diffraction line may also be advantageously confused with the emission line.

The input coupler can also be designed with several input guides and connected to additional input optical fibers intended for example to inject one or more reference wavelengths to ensure calibration at the level of the photodetection means. A switching device can also advantageously be associated with the different input optical fibers in order to select the signals of one of them and to implement a Vernier effect between these fibers and the photosensitive elements of the photodetection means which increases the resolution. overall optical spectrum analyzer device. According to another characteristic of the invention, an optical deflection correction element is associated with the optical imaging means to compensate for the polarization and / or temperature dependence of the waveguides of the diffraction grating. The optical deflection correction member is formed by a first prism birefringent material, in particular quartz, which is interposed either between the lens and the optical focusing element, or between the diffraction line of the wafer and the lens. A second prism made of material with a high thermo-optical coefficient, in particular silicon or silicone, may be associated with the optical imaging means to compensate for the sensitivity of the phase grating to the temperature.

BRIEF DESCRIPTION OF THE DRAWINGS Other advantages and features will emerge more clearly from the following description of an embodiment of the invention given by way of non-limiting example, and represented in the accompanying drawings, in which: FIG. 1 is a schematic view of an integrated optics phasar component providing wavelength multiplexing and demultiplexing functions in optical fiber telecommunication networks; Figures 1a and 1b are sectional views along the lines la-la and lb-1b of Figure 1; FIG. 2a represents a schematic perspective view of the optical spectrum analyzer according to the invention; FIGS. 2b and 2c are sectional views of FIG. 2a respectively in the plane of the integrated optical plate and perpendicular to this plane; FIG. 3 shows an enlarged view of the phasar diffraction grating used in the invention; FIG. 4a represents the diagram establishing the variation of the wavelength separating power as a function of the RIwX ratio characterizing the input coupler; FIG. 4b shows the variation of the wavelength separating power and the insulation as a function of the number N of phase network waveguides; FIG. 5 represents the response diagram of the analyzer with a laser line; FIGS. 6a-6f show various possible embodiments of the phasar diffraction grating according to the invention; FIG. 6g is a perspective view of a separator cube equipped with two photodetector arrays; FIG. 6h shows a variant of the analyzer in which the optical focusing element consists of two crossed cylindrical lenses; FIG. 6i shows another variant of the analyzer in which the photodetection means consist of an integrated optical element connected to a fiber mat or a photodetector strip; FIG. 6j is a variant of the analyzer with a single photodetector; FIG. 7 shows another variant of the analyzer with compensation for the polarization dependence; # Figure 8 shows another variant of the analyzer with compensation for temperature dependence.

DESCRIPTION OF THE INVENTION With reference to FIGS. 2a, 2b and 2c, the optical spectrum analyzer 32 is composed of a phasar-type diffraction grating 34 made on a wafer 16 in integrated optics, and an optical optic device. imaging 36 of the diffraction line and the transmission line on a photodetector array 38. Figures 2b and 2c are sections in the plane of the integrated optical plate 16 and perpendicular thereto, including the path of optical rays. The phasar-type diffraction grating 34 in integrated optics is more particularly shown in FIG. 3. The same reference numbers will be used to designate parts identical to those of the phasar component of FIG. 1.

The phasar diffraction grating 34 comprises the input coupler 10 and the phase grating 14 of the phasar component of FIG. 1, but does not have an output coupler 18. An input optical fiber 24 attached to a guide input 12 allows the injection of light into the input coupler 10. The latter is constituted by a plane waveguide, in which the light is guided in the direction perpendicular to the integrated optical wafer 16, and may diverge in the parallel direction so as to excite the waveguides forming the phase grating 14.

The ends of the waveguides of the phase grating 14 connected to the input coupler 10 are arranged periodically with the pinned pitch along a circular arc of radius R, and whose center 11 is in the vicinity of the end of the waveguide. central entrance. The waveguides of the phase grating 14 are composed of sections of straight and curved guides which are calculated on the one hand, so that the total length of the guides is increasing in constant increment <B> AL, </ B> and d on the other hand, so that their output ends extend in parallel and periodically terminate with the output step forming a straight line 40a, which is referred to herein as a diffraction line. The integrated optical wafer 16 is then cut at a short distance from this diffraction line 40a. The light diffracted by the diffraction line is thus guided by a plane waveguide portion and then diverges from a transmission line 40b. The diffraction and emission lines can be confused in the case where the cutting of the wafer is made along the diffraction line.

Mode adapters (funnel adiabatically varying the width of the guides) can also be arranged at the ends of the guides to improve the collection of light by the grating 14 in the input coupler 10, and the diffraction efficiency at the output of the network. The coupling of proximity between waveguides is mainly located at the ends of the network 14. It may be higher or lower depending on the confinement of the mode in the guide, and the values chosen for the step in and out. The guides then deviate and the coupling decreases to become negligible. At the output of the phase grating 14, the interference of the waves emitted by each of the waveguides constitutes the diffraction phenomenon. The interference is constructive in the angular direction 0 (relative to the normal of the diffraction line) for the wavelength k, when the Next relation is verified nAL <I> + </ I> ps () nite < I≥ </ I> m /, where n ,, denotes the effective phase index of the waveguides of the phase grating, and m is an integer representing the diffraction order of the grating 14.

où #.0 est la longueur d'onde diffractée dans la direction 6 = 0, dOldk est la dispersion en longueur d'onde du réseau 14, et n9 est l'indice effectif de groupe des guides d'onde. This relation is classically decomposed in the following way n, oOL <I≥ </ I> mAo

where # .0 is the diffracted wavelength in the 6 = 0 direction, dOldk is the wavelength dispersion of the grating 14, and n9 is the effective group index of the waveguides.

Cette relation démontre un premier intérêt de l'analyseur de spectre optique 32, par lequel un réseau de diffraction 34 de type phasar peut être conçu dans un ordre de diffraction élevé avec une efficacité de diffraction voisine de l'unité. La dispersion en longueur d'onde, proportionnelle au rapport mlp est donc plus élevée que celle d'un réseau de diffraction gravé ou holographique avec lesquels un ordre aussi élevé ne peut être atteint. La dispersion du dispositif est ensuite augmentée par l'élément optique de focalisation 44 dans le rapport de sa longueur focale. The optical imaging means 36 of the diffraction line 40a and the emission line 40b of the grating 14 on the photodetector strip 38 consist of a cylindrical lens 42 and a focussing optical element 44 of great focal length. The cylindrical lens 42 serves to make the diverging beam emitted by the waveguides parallel from the transmission line in the direction perpendicular to the integrated optical wafer 16. The objective focal line of the lens 42 is aligned with the emission line of the grating 14. The optical focusing element 44 then ensures the focusing of the beam in the two directions parallel and perpendicular to the wafer 16 on the photodetector array 38 which deliver an electrical signal representative of the optical spectrum. By designating the focal length of the optical focusing element 44 as F 1, the wavelength dispersion of the optical spectrum analyzer 32 on the photodetector strip 38 is given by

This relationship demonstrates a first interest of the optical spectrum analyzer 32, whereby a phasar diffraction grating 34 can be designed in a high diffraction order with a diffraction efficiency close to unity. The dispersion in wavelength, proportional to the ratio mlp is therefore higher than that of an etched or holographic diffraction grating with which such a high order can not be reached. The dispersion of the device is then increased by the focusing optical element 44 in the ratio of its focal length.

The device illustrated in FIGS. 2a and 3 has other advantages over the analyzers with an etched or holographic diffraction grating. Its reliability is superior because it does not include a mobile mechanical element and allows to consider a network supervision application for which the required life is 25 years. In addition, the incremental optical delay applied to the phase of the wave and the generator of the diffraction phenomenon is defined in the material of the waveguides, and not in the air as is the case for a diffraction grating. . This device is therefore independent of variations in the air index according to the measurement altitude, the atmospheric pressure, the temperature and the humidity. This feature simplifies the wavelength calibration procedure.

Les caractéristiques de l'analyseur de spectre optique 32 peuvent être calculées selon les formules précédentes. Le réseau de diffraction 34 de type phasar doit posséder un intervalle spectral libre (ISL = kolm) plus large que le domaine spectral à couvrir de manière à éviter le recouvrement des ordres adjacents

Un domaine de largeur 0k = 35 nm centré autour de la longueur d'onde #O = 1545nm peut donc être analysé au moyen d'un réseau de diffraction dont l'ordre de diffraction est égal à m=35 ce qui correspond à un pas d'accroissement de la longueur des guides du réseau égal à

et à une dispersion en longueur d'onde pour le réseau seul égale à (n9 = n,,)

L'élément optique de focalisation 44 doit ensuite être choisie pour atteindre la dispersibn en longueur d'onde voulue sur la barrette de photodétecteurs 38:

Dans la direction perpendiculaire à la plaquette 16 d'optique intégrée et à la barrette de photodétecteurs 38, le spot formé sur la barrette doit posséder une hauteur inférieure à celle des éléments photosensibles. La hauteur du spot s'obtient à partir du diamètre de mode des guides d'onde en sortie du réseau 14 (wy = 5Nm) multiplié par le facteur de grandissement (F/f) de l'ensemble lentille 42 cylindrique et élément optique de focalisation 44. Un facteur supplémentaire de 1.5 est appliqué pour tenir compte du profil d'intensité gaussien, et assurer une focalisation de 99% de l'énergie sur la hauteur (sy = 30Nm) des éléments photosensibles

soit une lentille 42 cylindrique de longueur focale <I>f ></I> 30mm Dans le plan de la barrette de photodétecteurs 38, le pouvoir séparateur en longueur d'onde est défini par la largeur à mi-hauteur de la réponse de l'analyseur de spectre optique 32 à une raie laser de largeur spectrale négligeable. Son évaluation fait intervenir le diamètre de mode wx du guide d'entrée au point de connexion au coupleur d'entrée 10, la longueur R (rayon de courbure de l'arc formé par les guides du réseau de phase du coupleur d'entrée 10, le nombre N de guides dans le réseau de phase 14 et la focale F de l'élément optique de focalisation 44. Finally, the problem of the dependence of the index of the material of the waveguides as a function of the temperature can simply be corrected by means of a measurement of the temperature of the network 14, and a calibration of the wavelengths according to of the temperature read, by mounting the network 14 on a temperature stabilization device using a heating resistor or a Pelletier effect module. A solution that will be described later uses a prism made of a material with a high thermo-optical coefficient. In practice, the wavelength division multiplexed optical fiber telecommunication networks deployed currently comprise up to 40 separate 0.8nm channels, and placed in the 1527.5nm - 1562.5nm band of erbium doped fiber amplifiers. The central wavelength and the width of this domain are thus respectively equal to a.0 = 1545nm, 0 #. In addition, an integrated optical technology of the PECVD type by vacuum deposition makes it possible to generate guides in doped silica with an effective index of the order of n ,. = 1.45 and typical mode diameters (diameter defined as 1 / e2 of the maximum intensity): wX = 9Nm in the plane of the wafer and <B> WY </ B> = 5pm in the direction perpendicular to the wafer. Given the deposition constraints of the overlay in this technology, two adjacent guides must be separated by the distance of 10 μm. The pitch chosen for the guides of the phase grating 14 to the connection to the input coupler 10 and to the diffraction line 40 of the grating results from this pentred condition = ps = p = 10Nm. Finally, linear arrays of photodetectors 38 of the type CCD, with InGaAs photosensitive elements having 600 pixels of length 26Nm are available in this window of wavelengths. The size of the photosensitive elements is s × = 20 Nm in the plane of the strip 38, and s s = 30 pm in the perpendicular direction. The wavelength dispersion of the device must be calculated in such a way that the spectral range λ 0 = 35 nm is spread over the length of the strip 38

The characteristics of the optical spectrum analyzer 32 can be calculated according to the preceding formulas. The phasar diffraction grating 34 must have a free spectral range (ISL = kolm) wider than the spectral range to be covered so as to avoid the overlap of adjacent orders

A domain of width 0k = 35 nm centered around the wavelength # 0 = 1545nm can therefore be analyzed by means of a diffraction grating whose diffraction order is equal to m = 35, which corresponds to a step increasing the length of the network guides equal to

and a wavelength dispersion for the single network equal to (n9 = n ,,)

The optical focusing element 44 must then be chosen to reach the desired wavelength dispersibn on the photodetector array 38:

In the direction perpendicular to the integrated optical plate 16 and to the photodetector strip 38, the spot formed on the strip must have a height less than that of the photosensitive elements. The height of the spot is obtained from the mode diameter of the waveguides at the output of the grating 14 (wy = 5Nm) multiplied by the magnification factor (F / f) of the cylindrical lens assembly 42 and optical element of 44. An additional factor of 1.5 is applied to take account of the Gaussian intensity profile, and to ensure a focus of 99% of the energy on the height (sy = 30Nm) of the photosensitive elements.

or a cylindrical lens 42 of focal length <I>f></I> 30mm In the plane of the photodetector array 38, the wavelength separating power is defined by the width at half height of the response of the photodetector. optical spectrum analyzer 32 with a laser line of negligible spectral width. Its evaluation involves the mode diameter wx of the input guide at the point of connection to the input coupler 10, the length R (radius of curvature of the arc formed by the guides of the phase grating of the input coupler 10 , the number N of guides in the phase grating 14 and the focal length F of the optical focusing element 44.

The input coupler assembly 10 and the focusing optical element 44 apply the nF / R magnification factor to the input guide mode diameter wx (n is the effective index of the plane waveguide forming the coupler The input coupler 10 thus intervenes in this calculation by the ratio R / wX In integrated optics, the mode diameter of a waveguide in the plane of the wafer 16 can be adjusted by means of a funnel which transforms the width of a guide adiabatically The length R of the input coupler 10 is defined during the design of the phase grating 14. Finally, the constraint of limiting the size of the phase grating 14 and the number N of the guides which compose it has the effect of widening the spectral response and of degrading the insulation in the vicinity of the diffracted wavelength by generating secondary intensity maximums.

FIG. 4a firstly establishes the relationship between the separating power and the ratio RIwx which characterizes the input coupler 10. A separation power of 0.2 nm is reached for the operating point R / wX = 1356 wX = 9Nm R = 12.2 mm The effect of the number N of grating guides on the separating power and the insulation (defined by the ratio between the intensity of the diffracted wavelength and the main secondary maximum) is given in Figure 4b. An insulation greater than 35 dB corresponds to a network comprising N = 301 guides. These parameters are those of the diffraction grating 34 of FIG. 3. The response of the optical spectrum analyzer 32 to a laser line is given in FIG. halfway width is 0.2nm, so that channels 0.8nm apart can easily be separated. In addition, the insulation, greater than 38 dB at a distance of 0.35 nm from the center of the channel, also makes it possible to measure the optical signal-to-noise ratio of the link. The power supervision, wavelength and signal-to-noise ratio of the 40-channel telecommunications network separated by 0.8 nm taken as an example, is therefore feasible with this type of spectral analysis device. From the basic configuration illustrated in FIGS. 2a and 3, it is possible to propose a large number of embodiments derived according to the applications envisaged, and described by way of example with reference to FIGS. 6a-6j.

In FIG. 6a, the input fiber 24 can be directly attached to the plane waveguide forming the input coupler 10 of the phasar type diffraction grating 34. The size of the grating 34 is thus reduced. In addition, the diameter wx of the mode injected into the input coupler 10 can be adapted outside the constraints of the integrated optical technology used for the network 34. A specific fiber, a mode adapter made in another technology of the invention. integrated optics or a gradient index lens can thus be connected to the input coupler 10.

FIG. 6b shows another way of reducing the size of the network 34, by folding the input coupler 10a into V. The light injected into the coupler 10a reaches the diffraction grating 34 after total reflection on a glass-air interface, if the angles of incidence are sufficiently large or reflective treated.

In Fig. 6c, a mode adapter 35 is coupled to the transmit line 40b for the purpose of enlarging the mode diameter of the waveguides 14 in the direction perpendicular to the integrated optical wafer 16. The design of the optical collimation element is simplified because the divergence of the beam is thus reduced. It may be for example a plane waveguide made in integrated optics by ion exchange in a glass substrate, whose mode diameter at the input end and in the direction of interest is equal to that of the guides of the emission line and that at the end of the output has been widened adiabatically by diffusing the dopants. FIG. 6d shows the realization of a curvilinear diffraction line 40a, in particular in a circular arc set back from the transmission line 40b. The beam diffracted in the plane of the integrated optical wafer 16 can thus be rendered convergent or divergent so that part or all of the focusing effect required in this plane can be generated at the diffraction grating of the phasar type.

With reference to FIG. 6e, the input coupler 10b may be designed with a plurality of input fibers 24, 24a, 24b. The input fiber 24 continues to inject the useful optical signals, and the spectral domains injected into the different inputs are superimposed on the photodetector array 38. One or more reference wavelengths located on either side of the main spectrum to be analyzed can thus be injected into the device via the input fibers 24a and 24b additional. According to one variant, the different inputs may also constitute separate measurement channels, provided that the spectral domains injected into each of the inputs do not overlap on the photodetector array 38.

In FIG. 6f, a phasar-type diffraction grating with several inputs also makes it possible to implement a Vernier effect between these inputs and the photodetectors of the array in order to improve the resolution. An optical fiber switching device 35 is necessary to successively select the different inputs 24, 24a, 24b. The total number of measurement points of the device is then equal to the number of inputs multiplied by the number of pixels of the photodetector array.

The resolution of the optical spectrum analyzer 32 can also be improved by making use of several photodetector arrays. Figure 6g shows the establishment of two bars 38a, 38b through a splitter cube 46 beam. The two bars 38a, 38b occupy staggered positions on two perpendicular faces of the separator cube 46 to cover different areas of the spectral range. This two-band device can also be used to analyze overlapping optical spectra that originate from two diffraction orders of the phase grating. Each bar must then be equipped with an optical filter that selects the order to be analyzed.

In FIG. 6h, the optical focusing element 42 is broken down into two cylindrical optical focusing elements, crossed at 90 and in alignment in the directions parallel and perpendicular to the integrated optical plate, which then separately focus the beam according to these two directions.

In FIG. 6i, the photodetection means is a new element 47 in integrated optics composed of monomode or multimode waveguides arranged with a reduced pitch in the image plane of the optical focusing element 44 and having an extended step output. so as to connect a sheet of fibers or a strip of photodetectors.

In FIG. 6j, it is also possible to use a single photodetector 48 at the focus of the optical focusing element 44. The photodetector 48 may be a photodiode, and the spectral analysis is then obtained with an angular deflection device 50 (For example a rotating mirror, or an acousto-optic modulator) placed between the cylindrical lens 42 and the optical focusing element 44.

où BGuide = n#P TE - n1PTM représente la biréfringence des guides d'onde formant le réseau 34 de type phasar. COMPENSATION OF SENSITIVITY TO POLARIZATION OF LIGHT According to FIG. 7, the specific configuration of the spectral analysis device makes it possible to provide a solution to the problem of the birefringence of waveguides and the dependence on the state of polarization. of the resulting light for a phasar diffraction grating. Due to the planar geometry of the integrated optical wafer 16, the two birefringence axes are parallel and perpendicular to the wafer, and correspond to the polarization directions TE (plane of the integrated optical wafer 16) and TM (direction perpendicular to the plate 16) of the light. The extreme variations of the diffraction direction are thus obtained for the directions of polarization TE and TM

where BGuide = n # P TE - n1PTM represents the birefringence of the waveguides forming the phasar type network 34.

With an optimized integrated optics method, the birefringence of the guides can be reduced to B = 10-4, a variation of the diffraction direction JE - 0TM = 0.0214. This represents an apparent displacement of diffracted wavelengths of 47.4 Nm or 0.1 nm on the photodetector array 38. The access to the light beam in the optical imaging means 36 makes it possible to insert a first prism 52 or other corrector optical deflection made in a birefringent material, and calculated to compensate for the effect of the birefringence waveguides of the phase grating 14. The quartz is generally chosen for this type of application. It is a uniaxial crystal (axis of symmetry of the crystal) whose extraordinary index does not (direction parallel to the optical axis of the crystal), and the ordinary index no (direction perpendicular to the optical axis crystal) are at the wavelength of 1541.4nm equal to ne = 1.53630, n = 1.52781 BQuarb = ne - no = 0.00849 Generally speaking, a prism deflects an incident beam towards its base according to the relation <I> D = </ I> (n-1) Where D is the deflection of the beam, n is the index of the prism and A is the apex angle of the prism.

Le prisme 52 peut être inséré à différentes positions dans les moyens optiques d'imagerie 36. Le prisme 52 illustré à la figure 7 est placé entre la lentille 42 cylindrique et l'élément optique de focalisation 44, mais il est clair qu'il peut aussi être inséré entre la plaquette 16 d'optique intégrée et la lentille 42 cylindrique. La déviation du faisceau lumineux au passage du prisme 52 est de D = 1.33 . If the optical axis of the quartz prism 52 is aligned with the plane of the integrated optical wafer 16 and the polarization direction TE, the beam deviation varies as a function of the polarization state of the light according to the DTE relation. <I> - </ I> Dnf <I≥ </ I> B2 ,,,, - <I> A </ I> where BQuanz = ne - no represents the birefringence of the quartz forming the prism 52. A beam diffracted in the direction 6 by the phasar type grating 34, and deviated from D by the prism 52 has the global angular direction 8 + D. The condition for the polarization dependence of the grating 34 to be exactly compensated by that of the prism 52 is therefore eTE <I> - </ I> eT "<I> + </ I> D <I> - </ I> DM <I≥ o </ I> This condition leads, on the one hand, to orient the base of the prism 52 on the side of the waveguide of greater length in the phasar-type grating 34, and on the other hand, to make the prism 52 with an apex angle equal to

The prism 52 can be inserted at different positions in the optical imaging means 36. The prism 52 illustrated in FIG. 7 is placed between the cylindrical lens 42 and the focusing optical element 44, but it is clear that it can also be inserted between the integrated optical wafer 16 and the cylindrical lens 42. The deviation of the light beam at the passage of the prism 52 is D = 1.33.

Dans le cas de guides d'optique intégrée en silice dopée déposée sur un substrat de silice, la dépendance de l'indice effectif des guides d'onde en fonction de la température (dn/dT : coefficient thermo-optique) est celle de la silice. De plus, la variation de longueur des guides du réseau est imposée par la dilatation du substrat dont l'épaisseur est très supérieure à celle des couches optiques

soit une dépendance angulaire à la température de 0.0024 / C ce qui correspond à un déplacement des longueurs d'onde diffractées de 5,4pm/ C ou 0.012nm/ C sur la barrette de photodétecteurs. Sur la figure 8, un deuxième prisme 54 réalisé dans un matériau avec un fort coefficient thermo-optique, peut être inséré dans les moyens optiques d'imagerie 36 pour compenser la sensibilité du réseau 34 à la température. Si 8 et D représentent la direction de diffraction et la déviation du prisme 54, la direction globale du faisceau est A + D, et la condition pour atteindre une dépendance nulle à la température est

avec

où n et A désignent l'indice de réfraction et l'angle au sommet du deuxième prisme 54. <U> COMPENSATION OF </ U> SENSITIVITY <U> A </ U> TEMPERATURE Another important problem of phasar networks is the dependence of the diffraction direction on a temperature variation.

In the case of integrated optics guides doped silica deposited on a silica substrate, the dependence of the effective index of the waveguides as a function of temperature (dn / dT: thermo-optical coefficient) is that of the silica. In addition, the variation in the length of the grating guides is imposed by the expansion of the substrate, the thickness of which is much greater than that of the optical layers.

or an angular dependence at the temperature of 0.0024 / C which corresponds to a displacement of diffracted wavelengths of 5.4pm / C or 0.012nm / C on the photodetector array. In FIG. 8, a second prism 54 made of a material with a high thermo-optical coefficient can be inserted into the optical imaging means 36 to compensate for the sensitivity of the grating 34 to the temperature. If 8 and D represent the diffraction direction and the deviation of the prism 54, the overall direction of the beam is A + D, and the condition for reaching a zero temperature dependence is

with

where n and A denote the refractive index and the apex angle of the second prism 54.

La condition d'une dépendance nulle à la température pour l'ensemble réseau @4 et prisme 54 implique que la base du prisme soit orientée du côté du guide de plus grande longueur dans le réseau 34 de type phasar. De plus, l'angle au sommet du prisme 54 doit être égal à

Comme précédemment, le deuxième prisme 54 peut être inséré soit entre la plaquette 16 d'optique intégrée et la lentille 42 cylindrique, ou entre la lentille 42 cylindrique et l'élément optique de focalisation 44. Du fait de l'indice élevé du silicium, des traitements antireflets sont nécessaires sur les deux faces du prisme 54. La déviation du faisceau au passage du prisme 54 est importante (D = 42.6 ), et doit être pris en compte dans la conception optique du dispositif. Given the spectral domain, the prism 54 can be made of silicon. It is a resistant material that has good mechanical properties whose thermo-optical coefficient is 13 times greater than that of silica

The condition of zero temperature dependence for the array @ 4 and prism array 54 implies that the prism base is oriented toward the longer guide side in the phasar array 34. In addition, the apex angle of the prism 54 must be equal to

As before, the second prism 54 may be inserted either between the integrated optical wafer 16 and the cylindrical lens 42, or between the cylindrical lens 42 and the optical focusing element 44. Because of the high index of silicon, anti-glare treatments are required on both sides of the prism 54. The deviation of the beam at the passage of the prism 54 is important (D = 42.6), and must be taken into account in the optical design of the device.

Avec ces paramètres, l'angle au sommet du prisme et la déviation moyenne du faisceau au passage du prisme sont respectivement égaux â A = 6.52 D = 3.26 Le coefficient thermo-optique étant de signe négatif, la base du prisme doit être disposée du côté du guide de plus courte longueur dans le réseau de phase.The use of a prism made of silicone is also conceivable to compensate for the temperature dependence. This material has both a thermo-optical coefficient greater than that of silicum and a lower refractive index which limits the deviation of the beam at the passage of the prism:

With these parameters, the apex angle of the prism and the average deviation of the beam at the passage of the prism are respectively equal to A = 6.52 D = 3.26 The thermo-optical coefficient being of negative sign, the base of the prism must be arranged on the side. of the shorter length guide in the live network.

 The design of the optical imaging means is simplified because of the smaller size of the prism and the weaker deviation of the beam at the passage of the prism.

Claims (16)

claims An optical optics grating device having an integrated optics diffraction grating comprising: a diffraction grating (34) made of integrated optics and composed on the one hand of an input coupler (10, 10a, 10b) connected to least an input waveguide (12) or an input optical fiber (24, 24a, 24b) capable of injecting the optical signal, and secondly a plurality of waveguides connected to the input coupler (10, 10a, 10b) and distributed with increasing lengths in substantially constant increment to form a phase grating (14) diffracting each wavelength of the optical signal in a different angular direction, - and means of photodetection (38, 38a, 38b) for detecting the diffracted optical spectrum by the diffraction grating, characterized in that - the output ends of the phase grating waveguides are staggered at substantially constant pitch along a line of diff (40a), - the integrated optical wafer (16) is cut away from the diffraction line (40a), thereby creating a transmission line (40b) from which the beam diverges, - optical means imaging devices (36) are arranged outside the integrated optical wafer (16) by being inserted between the emission line (40b) and the photodetection means (38, 38a, 38b) to form the image of the optical spectrum diffracted by the diffraction line (40a) on the photodetection means (38, 38a, 38b). Optical spectrum analyzer device according to claim 1, characterized in that the optical imaging means (36) comprise an optical collimation element (42) for paralleling the diverging light beam in the direction perpendicular to the wafer. integrated optical system provided by the transmission line (40b); and an optical focusing element (44) for focusing the beam of said collimating optical element (42) on the photodetection means (38, 38a, 38b). Optical spectrum analyzer device according to claim 2, characterized in that the optical collimation element (42) comprises one or more cylindrical lenses, and / or a cylindrical mirror, and has an object focal line extending in alignment with the transmission line (40b) of the diffraction grating (34). 4. Optical spectrum analyzer device according to one of claims 1 to 3, characterized in that the spot formed on the photodetection means (38, 38a, 38b) in the direction perpendicular to the wafer (16) of integrated optics is determined by the mode diameter of the waveguides at the transmission line 40b, and the magnification factor of the optical imaging means (36), said spot having a dimension substantially smaller than the height of the photosensitive elements photodetection means (38, 38a, 38b). 5. Optical spectrum analyzer device according to one of claims 1 to 4, characterized in that the diffraction line 40a is rectilinear and coincides with the transmission line 40b. 6. Optical spectrum analyzer device according to one of claims 1 to 4, characterized in that the diffraction line 40a is a circular arc and produces in part or in full the desired focusing effect in the plane of the wafer 16 of optical integrated to form the image of the optical spectrum diffracted by the diffraction line 40a on the photodetection means (38, 38a, 38b). 7. Optical spectrum analyzer device according to one of claims 1 to 6, characterized in that the input coupler (10, 10a, 10b) is a plane waveguide capable of guiding the light in the direction perpendicular to the insert (16) of integrated optics, said light being able to diverge in the parallel direction to excite the input ends of the waveguides of the phase grating (14), said ends being arranged with a substantially constant pitch following an arc of a circle and whose center (11) is located near the opposite end of the central entry guide. Optical spectrum analyzer according to one of claims 1 to 7, characterized in that the input coupler (10b) is additionally connected to additional input optical fibers (24a, 24b) for injecting reference wavelengths for calibrating at the photodetection means (38, 38a, 38b). 9. An optical spectrum analyzer according to claim 8, characterized in that a switching device (35) is associated with the various input optical fibers (24, 24a, 24b) for selecting the signals of one of the between them. 10. Optical spectrum analyzer device according to one of claims 1 to 9, characterized in that the photodetection means (38, 38a, 38b) consist of a photodetector array disposed in the image plane of the imaging means (36). ), and ensuring the reading of the optical spectrum. 11. Apparatus optical spectrum analyzer according to one of claims 1 to 9, characterized in that the photodetection means (38, 38a, 38b) comprise two strips of photodetectors (38a, 38b) arranged on either side of a beam splitter (46) for covering different areas of the spectral domain 12. An optical spectrum analyzer according to one of claims 1 to 9, characterized in that the photodetection means (38, 38a, 38b) comprise an integrated optical element (47) having single-mode waveguides. or multimodes, whose input ends are arranged with a reduced pitch in the image plane of the imaging means (36), and whose output ends have an enlarged step so as to connect a fiber web, or a bar photodetectors. 13. An optical spectrum analyzer according to one of claims 1 to 9, characterized in that - the photodetection means (38, 38a, 38b) comprise a single photodetector (48) arranged at the image focus of the optical element of focusing (44), - an angular deflection device (50) is associated with the optical imaging means (36) for performing the spectral analysis. Optical spectrum analyzer device according to one of claims 1 to 13, characterized in that an optical deflection corrector is associated with the optical imaging means (36) to compensate for the polarization dependence and / or the temperature of the waveguides of the diffraction grating (34). 15. Optical spectrum analyzer device according to claim 14, characterized in that the optical deflection corrector is formed by a first prism (52) of birefringent material, in particular quartz, which is associated with the optical means of imaging (36) to compensate for the sensitivity of the phase grating to the polarization state of the light. 16. Optical spectrum analyzer device according to claim 14, characterized in that the optical deflection corrector member comprises a second prism (54) made of material with a high thermo-optical coefficient, in particular silicon, which is associated with the optical optical means. imaging (36) to compensate for the sensitivity of the phase grating to temperature.