FR2861470A1 - OPTICAL SYSTEM WITH OPTICAL COMPONENTS IN FREE SPACE PRESERVED BY MEANS OF A BELT - Google Patents

Abstract

The invention relates to an optical system comprising at least two optical components. According to the invention, said components are held by a belt surrounding said optical system.

Description

Optical system with optical components in free space maintained by means

of a belt.

  FIELD OF THE DISCLOSURE The field of the invention is that of optical telecommunications. More specifically, the invention relates to a new technique for assembling the various components of an optical platform, intended in particular for the field of optical telecommunications.

  2. Solutions of the Prior Art In this field, various optical systems are implemented, which conventionally comprise a plurality of optical components, in particular passive optical components in free space, arranged relative to one another.

  In all these optical systems, and more particularly in free space, the alignment and spacing constraints of the optical components are satisfied by keeping the components relative to each other by means of a rigid support plate on which they are set. Such a support plate constitutes a common alignment reference for all components of the system.

  Thus, in European Patent Specification No. 1,126,294, the various components of the optical system in question are mounted on a silica support plate 670.

  3. Disadvantages of the Prior Art Designers and manufacturers of optical systems today have to cope with ever-increasing integration constraints: optical systems used in the telecommunications field must be smaller and flatter.

  A disadvantage of the technique of the prior art based support plate is that it does not meet this constraint integrability of optical systems. Indeed, the support plate is intended to give the optical system sufficient rigidity to avoid any relative displacement of the components relative to each other, and therefore any disturbance of the system, which is very sensitive to alignment constraints and spacing of the components.

  To be sufficiently rigid, the support plate must necessarily be thick, which leads to the design of optical systems 5 thick and bulky.

  More specifically, such optical systems are in particular intended to be inserted between the electronic boards of racks, and must therefore be of the lowest possible height, which does not allow the techniques of the prior art.

  4. OBJECTIVES OF THE INVENTION The object of the invention is notably to overcome these disadvantages of the prior art.

  More specifically, an object of the invention is to provide a technique for assembling components within an optical system that makes it possible to reduce the thickness of the system compared to the techniques of the prior art.

  Another objective of the invention is to propose such a technique that significantly increases the integrability of the optical system with respect to the prior art.

  The invention also aims to provide such a technique that is simple and inexpensive to implement.

  The invention also aims to provide such a technique that is suitable for all types of optical systems, including systems formed of passive optical components in free space.

  A secondary objective of the invention is to propose, according to such a technique, an optical system that does not exhibit residual aberration. 5. Key Features of the Invention These and other objects which will appear later are achieved by an optical system comprising at least two optical components.

  According to the invention, said components are held by a belt surrounding said optical system.

  Thus, the invention is based on a completely new and inventive approach to assembling components within an optical system. Indeed, instead of mounting the components on a thick and bulky support plate, the invention proposes to maintain the components within the system by means of a rigid belt surrounding the optical system. The optical system of the invention is therefore much thinner than the systems of the prior art, since the invention makes it possible to gain the thickness of the support plate in height.

  The optical system of the invention can therefore be ultra-thin, and meet the strong integration constraints of the field of optical telecommunications.

  Advantageously, at least one thin plate is fixed to said belt, so as to stiffen said system.

  "Thin" means less thick than the support plates used according to the techniques of the prior art (and in particular, for example, the silica plate referenced 670 in European Patent Document No. EP 1 126 294). Indeed, the thin plate of the present invention is intended to increase the rigidity of the system, already obtained through the belt. In contrast, the prior art support plates were the only elements providing rigidity to the system, and therefore necessarily needed to be thicker than the thin plate of the present invention.

  Preferably, such an optical system comprises two plates adjacent to said belt respectively forming an upper face and a lower face of said optical system, so as to seal said system.

  It is thus not necessary to place the optical system in a sealed housing, the seal being already ensured by the presence of the upper and lower plates mounted on the belt.

  According to a first advantageous characteristic of the invention, at least a portion of said belt and / or one of said processes ésf formed by one of said components.

  4 2861470 Thus reduces the total number of parts forming the optical system, and thus also its size.

  According to a second advantageous characteristic of the invention, said component forming a portion of said belt is a mirror.

  According to a third advantageous characteristic of the invention, said component forming said plate is an electrode.

  For example, in the case where the optical system is a modulator, one of the electrodes of this modulator can be integrated with one of the upper or lower plates of the assembly, to further reduce the number of parts constituting the system. We thus gain in integrability.

  Preferably, at least one of said components is a passive optical component in free space.

  The invention applies in fact more particularly to optical systems in free space, which must satisfy high alignment constraints.

  Advantageously, said belt has at least one opening and / or at least one transparent zone intended to allow the passage of an optical signal from and / or to at least one input / output block and / or at least one a block comprising a reconfigurable element with which said optical system is coupled.

  When the belt is made of an opaque material such as ceramic or metal, it is necessary to provide an opening for the passage of optical signals. On the other hand, when the belt is made of silica or standard glass or any other material that allows the optical signals to pass through, it may suffice to provide in the latter a transparent zone for the passage of signals.

  Preferably, said belt is made of a material with a low coefficient of thermal expansion (silica, ceramics, etc.) and / or in a material that compensates thermal variations of said components. For example, in an optical system comprising standard glass mirrors, the belt may also be made of standard glass.

  In the case of a dioptric system, which will be described in more detail later in this document, the thermal compensation of the lens focal length generally requires to be made in a material other than that of the lenses.

  According to a preferred embodiment, in which the invention proves to be particularly advantageous, the optical system is catadioptric and comprises astigmatism correction means and a diffraction grating.

  Preferably, said astigmatism correction means belong to the group comprising: a cylindrical corrector separated from said diffraction grating (illustrated more particularly in the following with reference to FIG. 5); a cylindrical corrector integrated in the surface of said diffraction grating (illustrated more particularly in the following with reference to FIGS. 6a and 6b); a biconical mirror (two different radii of curvature along the horizontal axis and the vertical axis); a digital hologram.

  Advantageously, said components of said catadioptric system are arranged symmetrically on either side of a pupil, so as to reduce the coma.

  According to another embodiment of the invention, such an optical system is with dioptric elements and at least a portion of said belt is made by means of a prism.

  The invention also relates to a method of assembling an optical system 25 as described above, comprising successive steps of: assembling said belt; assembling non-adjustable components of said optical system; assembling the adjustable components of said optical system.

  Preferably, such a method comprises at least one of the following steps: a step of assembling at least one sealing plate; an insertion step in said optical system of at least one polarization diversity module.

  6. List of Figures Other features and advantages of the invention will appear more clearly on reading the following description of a preferred embodiment, given as a simple illustrative and nonlimiting example, and the accompanying drawings, among others. which: Figure 1 shows a block diagram of a spectral equalizer in free space to which the present invention can be applied; FIG. 2 illustrates a beam splitting device serving as input to an interferometric system; FIG. 3 depicts the block diagram of a two-mirror and transmitting-array catadioptric system to which the present invention can be applied; Figure 4 shows the belt assembly diagram according to the invention of the optical system of Figure 3; FIG. 5 illustrates an example of an optical system with dioptric elements having a belt assembly according to the invention; FIGS. 6a and 6b show two views of a compact DSE at an input / output fiber, in which an electrode is integrated into a plate of the belt assembly; Figures 7a and 7b illustrate a compact DSE with separate input and output fibers; - Figures 8 to 11 illustrate the different assembly steps of an optical system according to the invention: - Figure 8 shows the assembly step of the belt; FIG. 9 describes the assembly of the non-adjustable elements of the optical system; FIG. 10 illustrates the assembly of the adjustable elements of the optical system; FIG. 11 shows a step of adding a polarization diversity module in the optical system of the invention.

  DESCRIPTION OF AN EMBODIMENT OF THE INVENTION The general principle of the invention rests on the maintenance of components within an optical system in free space by means of a rigid belt, which makes it possible to form a ultra-thin optical system. This general principle is particularly applicable to the realization of optical systems of catadioptric type.

  The various optical elements of the system are therefore no longer held by a support plate on which the elements are fixed but by a belt that surrounds the optical system. This makes it possible to gain the thickness of the support plate (necessarily thick to be rigid) over the height and opens the way to several additional integration possibilities: thin plates can be added above and below the system, and fixed to the belt, for example by gluing, to make the system waterproof. Input / output and reconfigurable component blocks with which the optical system is to be coupled can then be sealed to the system thus formed. The whole can then be placed in a case that is not necessarily waterproof; a mirror can be integrated in one of the walls of the belt to reduce the number of components of the system; - An electrode (necessary in the case of the use of a modulator for example) can be integrated into one of the upper or lower plates to further reduce the number of parts.

  These three possibilities can be implemented simultaneously to achieve an ultra-flat system.

  Different types of optical fibers (standard or extended core) can be used as input / output of the optical system and different modulators can be used for dynamic spectrum processing. In the case of a polarization-sensitive modulator, a block operating on each of the bias polarization-type polarization components can be integrated into the optical path and assembled on the belt, as illustrated later in relation to the polarization-sensitive modulator. figure 11.

  An example of an optical system 5 that can be mounted according to the belt-mounting technique of the invention, namely a spectral equalizer in free space, is presented in connection with FIG.

  The optical system strictly speaking is coupled to an input input block, which comprises either an optical fiber 10 which is used at the input and output of the system (according to the variant represented in FIG. 1), or two optical fibers for having a separate input from the output, a fiber followed by a beam splitter for input to an interferometric system.

  Such a device is illustrated in FIG. 2 and is described in more detail in the French patent document FR 03 09412 in the name of the same Applicant as the present patent application. It comprises, for example, a separator 4165 and two prisms 4166 and 4167 with total reflection. An incident beam 4162 originating from the fiber 10 is first separated into two beams by the separating plate 4165. Each of the beams is then reflected several times on one of the prisms 4166 or 4167. Thus, a fiber emulates, in this case. mounting, two secondary sources 4161 and 4162 virtual output respectively two bundles 4163 and 4164 parallel. After processing by the imager 42, the phase shifter 43 and the mirror 44, the device accepts, by reverse return, two beams whose spectral components are out of phase. The splitter plate 4165 then combines the two beams on the fiber 10 and / or the fiber 15 according to the phase shift value associated with each spectral component.

  In the system of FIG. 1, the optical fiber is preferably an extended core fiber (of the Gradissimo (registered trademark) type) but any standard fiber can be used. A dispersive network 12 is located in the Fourier plane of a first lens 11. The dispersive network is followed by a second lens 13. Such an optical system of passive components in free space 11, 12, 13 is also coupled to a block with reconfigurable component, namely a variable attenuator 14 operating in reflection.

  More generally, the invention applies preferentially to optical systems with passive components in free space, intended to be coupled to a reconfigurable component block performing a variable attenuation, such as a spatial modulator or a strip of variable attenuators. for example. Such an optical system then makes it possible to adapt the main spectral processing parameters (resolution, bandwidth, filter stiffness, etc.) of an optical signal WDM (for the English Wavelength Division Multiplexing) to the constraints associated with the introduction. the reconfigurable component block.

  In addition, when combined with an attenuation and / or variable phase spatial function, this optical device enables the generation of a whole range of dynamic functions (EDFA gain equalizer, blockers, Add & Drop, etc.) responding to different needs and uses in an optical network. Finally, by design, this device can be combined or integrated with other dynamic functions of the transport network or metropolitan (eg intelligent amplifier, coupling to a monitoring stage, etc.), which is today a need identified market.

  Such an optical system conventionally has two imaging planes (an object plane where is placed the input fiber of an input / output block and an image plane where the reconfigurable component block is placed) and an intermediate plane (confocal with two previous) where are introduced dispersive elements (prisms, networks ...) and spectral filters (apodization or stiffness filter etc.). In a configuration where the variable attenuator of the reconfigurable component block is in reflection, the object and image planes are conjugated and the input fiber can serve as an output fiber.

  FIGS. 3 and 4 show an example of a catadioptric system that can be mounted in a belt according to the invention.

  FIG. 3 illustrates the general principle of operation of such an optical system, which is coupled to an input / output block 20 and to a reconfigurable component block 21.

  Such an optical system is characterized by the use of two spherical mirrors 31, 32 off-axis so as to effect the coupling in a single pass in the system, or in two passes when the transmission device 20 is coupled with itself. (which is the case, for example, of an attenuator in reflection).

  The advantage of such an optical system is to be able to introduce different optical or electro-optical elements in the free space between the mirrors 31, 32, in particular in the Fourier plane, and thus to allow variations of the function (DSE (Dynamic Spectral Equalizer) versus DCE (Dynamic Channel Equalizer), for example) as well as being able to easily modify the characteristics of the dispersive element (such as an extension of the band of wavelength C towards the band C + L by example).

  FIG. 3 also shows the different imaging planes of such an optical system, namely the object plane 33, the image plane 34 and the intermediate plane of the system 35, comprising the point of symmetry 36 of the system optical.

  FIG. 4 more particularly illustrates the belt assembly of the optical system of FIG. 3. The various components of the optical system are held by means of a rigid belt 42, in which two openings 30, 302 are made, allowing optical coupling with the input / output block 20 and the reconfigurable component block 21. The spherical mirrors 31 and 32 are integrally mounted to the belt 42 by means of mechanical parts 43 and 44. Between the two mirrors 31 and 32, the beam optical crosses a diffraction grating 40, on which is affixed an astigmatism corrector 41, for example a cylindrical blade.

  In the assembly of FIGS. 3 and 4, a symmetrical arrangement of the mirrors 31, 32 makes it possible to correct the coma.

  The dispersive element 40 and the cylindrical surface 41 can be combined to reduce the number of components and thus facilitate the assembly of the system.

  The catadioptric system presented in connection with FIGS. 3 and 4 is particularly advantageous for meeting a goal of compactness and integration. Indeed, the compactness of such a system is obtained, on the one hand thanks to the belt assembly proposed by the present invention, and on the other hand by the use of catadioptric elements, preferably to the dioptric elements (such as illustrated for example in Figure 5).

  The belt-mounted catadioptric systems according to the invention are therefore a particularly advantageous solution to the problems of integrability and reduction of the height of the systems intended for optical telecommunications applications.

  An example of an optical system with dioptric elements that can be mounted according to the belt-mounting technique of the invention is now shown in connection with FIG.

  Such an optical system is, again, coupled to an input / output block 20 and to a reconfigurable component block 21. The optical system itself comprises two lenses 22 and 23, a diffraction grating 24 and a prism 25.

  The prism 25 also has a reflective surface 26, and forms, with the elements referenced 27 to 29, a belt for holding the various components 22, 23, 24 of the optical system.

  This belt also has two openings 301 and 302 for passing the optical signals to and from the input / output blocks 20 and reconfigurable component 21. Thus, the optical signals from the input / output block 20 pass through, through the opening 30, made in the belt assembly, the lens 22, and then are diffracted by the diffraction grating 24, before being reflected by the reflecting face 26 of the prism 25 incorporated in the belt. They then leave the optical system, via the opening 302, after having passed through the lens 23, towards the reconfigurable component block (for example a variable attenuator 14 of the type of that of FIG. 1).

  Note that the lenses 22, 23 are represented symbolically, because any type of lens can be used: single lens or doublet (broadband correction S + C + L possible), spherical or aspherical surface, lens with one or two surfaces diffractive, index gradient lenses, etc.

  FIGS. 6a and 6b illustrate an example of a belt-mounted optical system according to the invention, in which a thin stiffening plate has been fixed to the belt 71 on a lower face of the optical system. An electrode is further integrated with this lower plate, which then plays a dual role of holding and modulator electrode, which reduces the number of component parts of the optical system of the invention.

  The device of FIGS. 6a and 6b is thus very compact. This compactness is further increased by the fact that the mirror 70 is an integral part of the belt 71. The cylindrical surface of the astigmatism correction blade is integrated in the diffraction grating.

  More precisely, FIGS. 6a and 6b illustrate a compact DSE with a single input / output fiber 10.

  The belt assembly technique of the invention is also suitable for different input and output devices, as shown in Figs. 7a and 7b, which illustrate a compact DSE with separate input and output fibers 80.

  Even when the input and output optical fibers 80 are dissociated, the optical system of the invention remains very compact since it has overall dimensions of 30 mm x 41 mm x 12.5 mm.

  We will now describe in more detail, with reference to FIGS. 8 to 11, the various steps of mounting a assembled optical system in a belt, as described above.

  Step 1: Assembly of the belt In the example of Figure 8, the belt is formed of bars 50 to 53, which are pressed against a jig 54, then glued together. The configuration shown in FIG. 8, in which each bar 50 to 53 abuts on only one of its neighbors, makes it possible to overcome the length tolerance of the bars. The bar 53 has two openings or transparent areas 301 and 302 for the passage of the input and output optical beams.

  The arrows referenced 55 to 58 represent the forces applied to the belt elements to press them on the jig. These forces can be exerted at one or more points of each of the bars.

  The support elements of the optical components (not shown in FIG. 8) are then assembled outside the belt.

  The template 54 is then detached from the belt (50-53), possibly by cooling it if it is made of a material of coefficient of expansion greater than the coefficient of expansion of the material of the belt.

  Step 2: Assembly of the non-adjustable elements The non-adjustable optical elements (for example the prisms P1 and P2) and the supports of the optical elements (for example Cl and C2) are then assembled by gluing by applying a controlled force in quantity and direction .

  FIG. 9 illustrates an example of bonding a support element C2. Prisms P1 and P2, as well as the element C1 and the carrier 62 have already been assembled in the modulation system of Figure 9. To mount the element C2 in the last free corner of the belt the bars 50 and 51 of the belt are positioned in abutment against stops 60. The bearing faces 63 and 64 of the element C2 are then pressed against the corresponding surfaces of the bars 50 and 51. To ensure that the C2 element perfectly plate on the belt, it exerts a force 61 on C2, in the corner of the belt.

  During this second step, all the non-adjustable elements are assembled by gluing, brazing or welding.

  3rd step: Assembling the adjustable elements The partially constituted optical system is then placed on an adjustment tool, to assemble and glue the adjustable elements. In the particular case of the modulation system described here (see FIG. 10), these adjustable elements comprise a fiber carrier 65, mirrors M1 and M2 and a modulator 66. FIG. presence of a network 68. Once the adjustment is made, the adjustable elements are glued.

  Step 4: Assembling Sealing Plates Once all internal elements have been bonded, the sealing plates (not shown in the figures) can be glued to the underside and top of the structure.

  To minimize the stresses due to differential expansions, the sealing plates are preferably made of a material having the same coefficient of expansion as the belt.

  In a particular embodiment of the invention, the method used for the realization is UV bonding. According to the tolerances and the constraints of the envisaged product, the elements can also be assembled by gluing with other types of glues, by soldering or by glass soldering ("glass fritt").

  If the adhesives used are UV-curable adhesives, the belt must be made of UV-transparent material. Depending on the geometric tolerances and the cost objective, different transparent materials can be used: standard glass, fused silica, etc. If non-UV curable glue is used, non-transparent materials may be used: carbon fiber parts, metal, ceramic, etc. An optional step of mounting the optical system may include adding a polarization diversity module 67 in the optical path: for example, one may for example refer to the French patent document No. 03 01699 in the name of the same Applicant as this patent application.

  A simple way of making such an addition is illustrated in FIG. 11, in which the polarization diversity block 67 consists of a birefringent crystal and a half-wave plate.

  Such a step of adding a polarization diversity system can be done during the 2nd step (assembly of the non-adjustable elements) if this system consists of non-adjustable elements or during the 3rd step (assembly of the non-adjustable elements). adjustable elements) if it consists of adjustable elements.

Claims (16)

  An optical system comprising at least two optical components, characterized in that said components are held by a belt surrounding said optical system.   2. Optical system according to claim 1, characterized in that at least one thin plate is attached to said belt, so as to stiffen said system.   3. Optical system according to any one of claims 1 and 2, characterized in that it comprises two plates adjacent to said belt respectively forming an upper face and a lower face of said optical system, so as to seal said system .   4. Optical system according to any one of claims 1 to 3, characterized in that at least a portion of said belt and / or one of said plates is formed by one of said components.   5. Optical system according to claim 4, characterized in that said component forming a portion of said belt is a mirror.   6. Optical system according to any one of claims 4 and 5, characterized in that said component forming said plate is an electrode.   7. Optical system according to any one of claims 1 to 6, characterized in that at least one of said components is a passive optical component in free space.   8. Optical system according to any one of claims 1 to 7, characterized in that said belt has at least one opening and / or at least one transparent zone intended to allow the passage of an optical signal from and / or to at least one input / output block and / or at least one block comprising a reconfigurable element with which said optical system is coupled.   9. Optical system according to any one of claims 1 to 8, characterized in that said belt is made of a low thermal expansion coefficient material and / or a material compensating for thermal variations of said components.   10. Optical system according to claim 9, characterized in that said material belongs to the group comprising: silica; glass; metal; carbon fiber; ceramic.   11. Optical system according to any one of claims 1 to 10, characterized in that it is catadioptric and comprises astigmatism correction means and a diffraction grating.   12. The optical system as claimed in claim 11, characterized in that said astigmatism correction means belong to the group comprising: a cylindrical corrector separated from said diffraction grating; a cylindrical corrector integrated in the surface of said diffraction grating; - a biconical mirror; a digital hologram.   13. Optical system according to any one of claims 11 and 12, characterized in that said components of said system are arranged symmetrically on either side of a pupil, so as to reduce coma.   14. Optical system according to any one of claims 1 to 10, characterized in that it is dioptric elements and in that at least a portion of said belt is made by means of a prism.   15. A method of assembling an optical system according to any one of claims 1 to 14, characterized in that it comprises steps of: assembling said belt; assembling non-adjustable components of said optical system; assembling the adjustable components of said optical system.   16. The assembly method according to claim 15, characterized in that it comprises in Miter at least one of the following steps: - a step of assembling at least one sealing plate; a step of inserting into said optical system at least one polarization diversity module.