Classifications

• • G—PHYSICS
  • G02—OPTICS
  • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
  • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
  • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
  • G02F1/03—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on ceramics or electro-optical crystals, e.g. exhibiting Pockels effect or Kerr effect
  • G02F1/035—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on ceramics or electro-optical crystals, e.g. exhibiting Pockels effect or Kerr effect in an optical waveguide structure
  • G02F1/0356—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on ceramics or electro-optical crystals, e.g. exhibiting Pockels effect or Kerr effect in an optical waveguide structure controlled by a high-frequency electromagnetic wave component in an electric waveguide structure

Definitions

• Electro-optical modulator production method and block for implementation
• the present invention relates to the field of electro-optical modulators and in particular to integrated modulators based on lithium niobate and comprising an optical structure of the ach-Zehnder interferometer type, directional coupler or the like.
• the proposed invention can however be applied to modulators made of other materials such as lithium tantalate, III-V semiconductor materials (Gallium arsenide, Indium phosphide) as well as with polymers.
• the major application of fast optical modulators based on lithium niobate mainly concerns the field of digital telecommunications by optical fiber over long distance and at high speed.
• This type of known modulator comprises an optical chip whose typical dimensions are of the order of 5 cm in the direction of the length, 2 mm in the direction of the width and 0.5 mm in the direction of the height.
• the optical chip is generally inserted in a metal case whose thermal expansion properties are taken into account with respect to those of the chip and those of the input and output optical fibers.
• the optical chip based on electro-optical crystals of lithium niobate comprises an optical circuit towards its surface making it possible to guide the light according to a configuration of Mach-Zehnder interferometer, directional coupler, or, more generally, any other structure allowing modulation of an optical signal by an electrical signal.
• the optical circuit or light guide comprises two Y-shaped branches connected by two rectilinear arms.
• the light guide can be produced by thermal diffusion of a metal such as titanium or also by substitution between lithium ions and protons of an acid source, a method called proton exchange.
• a layer of dielectric material is deposited in thin thickness, about 1 ⁇ m.
• this layer dielectric for example made of silica, a so-called traveling wave electrode structure is produced.
• These electrodes can in particular be presented according to a coplanar waveguide architecture, with a central conductor and two lateral ground planes. These electrodes make it possible to convey an electrical modulation signal which interacts with the light propagating in the arms of the interferometer.
• the differential optical phase shift between the arms which is modulated by the electrooptic effect results in modulated light interference and results in a light signal whose intensity varies according to the modulation.
• the modulator therefore acts as an interface between the electrical signal and the light signal.
• the coplanar line must have low losses, be adapted in impedance to avoid parasitic reflections, and, moreover, the speed of the microwave wave in the line must be equal to the speed of the light wave in the optical guide (isochronism condition) to guarantee optimal operation over a wide frequency band.
• the width of the central conductor must be less than ten micrometers and the spacing of the ground planes must not exceed three times the width of the line for satisfy both the phase adaptation and impedance condition and maintain a high electric field inside the guide under a low potential difference.
• the modulators require a continuous control voltage in order to control the operating point of the device. This is generally ensured by taking a part of the light energy from the output fiber via a fiber coupler.
• the signal thus sampled is detected by an external photodiode connected to an electronic servo circuit connected either to a set of low frequency electrodes or to the microwave electrodes by means of a component called “polarization tee”.
• polarization tee a component that has the effect of making the assembly heavier.
• the optical chip as it is known is also characterized by the fact that the input and the output of the optical circuit are effected by a rectilinear guide section starting and ending on the opposite faces of the device. In return, the entry and exit of the microwave line is effected by one or each of the sides perpendicular to the optical input / output faces.
• the input / output optical fibers are conventionally bonded to the crystal by means of a polished ferrule.
• the optical fibers can be brazed on orifices of the housing so as to guarantee the hermeticity of the housing. This operation can be carried out by stripping the optical fiber from its sheath, by metallizing it and by performing soldering on the stripped part.
• the fact that the input and the output of the microwave signal is made perpendicular to the direction of propagation of the signals in the interaction region requires the use of a significant curvature of the electrodes.
• the two curvatures initially introduce propagation losses. They also cause an antenna effect with possible coupling on the substrate modes, in particular due to the anisotropy of the latter, resulting in attenuation of the high frequency signal and the appearance of undesirable absorption lines.
• the curvature must be specifically studied and the modes of substrates eliminated for example with a thinning of the crystal.
• the present invention aims to propose a solution to these various drawbacks by proposing a modulator with a particular structure and making it possible to implement a new method of assembling the optical fibers on the optical chip.
• the invention is based on an original approach to the encapsulation of modulators, with the aim of significantly improving their performance, in particular during very high frequency operation.
• the optical circuit integrates the curvatures conventionally introduced into the design of the microwave lines arranged on the fast optical modulator.
• the invention relates to an electro-optical modulator comprising a block of a crystal with electro-optical properties, the block being elongated lengthwise along a major axis and comprising a light guide extending between a guide entry light and a light guide output, an electrical modulation signal acting on an interaction zone of the block by coplanar electrodes in relation to the light guide via a dielectric layer, the electrodes being arranged on a elongated main face of the block and the interaction zone being substantially elongated along the major axis, the light to be modulated arriving via an input optical fiber coupled to the light guide at the level of the light guide entry on a first face of the block, the modulated light leaving the modulator by an output optical fiber coupled to the light guide light at the light guide outlet on a second side of the block.
• the first face and the second face are one and the same face parallel to the major axis, the light guide comprising two curvatures respectively between the light guide entry and a first end of the interaction zone and between a second end of the interaction zone and the light guide outlet.
• the light guide inlet and the light guide outlet are on the same face of the block which is parallel to the major axis of the block.
• the terms input and output are indicative because we consider bidirectional devices.
• the two curvatures are substantially symmetrical with one another,
• the input and output optical fibers pass through the housing by passages and are fixed for example by soldering on the housing,
• the optical fibers are coupled to the block by means of polished ferrules with a connection angle between the ferrule and the block intended to suppress parasitic reflections in the coupling zone
• the optical guide comprises a Mach-Zehnder interferometer
• the block has the following approximate dimensions: 5 cm along the major axis by 2 mm in width and 0.5 mm in height,
• the crystal is lithium niobate
• the crystal is lithium tantalate, - the crystal is based on l-V semiconductors (gallium arsenide, indium phosphide), - the crystal is based on polymers,
• the light guide is the result of thermal diffusion of a metal
• the metal is titanium
• the light guide is the result of a proton exchange by substitution of lithium ions and protons of an acid
• the dielectric layer is silica
• the dielectric layer has a thickness of approximately 1 ⁇ m
• the electrodes end with two distant electrode ends, the electrode ends ending along opposite edges on the main face,
• the electrodes end with two distant electrode ends, the electrode ends ending along the same edge on the main face, - each of the electrode ends is connected to a connector on the housing,
• one of the ends of the electrodes is intended to be connected to an electrical modulation signal generator via a connector on the housing and the other end of the electrodes is connected to an adaptation load
• an electronic amplification module is placed in the housing between a connector and one of the ends of the electrodes
• an electronic modulation and amplification module is placed in the housing between a connector and one of the ends of the electrodes
• the modulator further comprises means for adjusting its electrical operating point
• one of the operating point adjustment means consists of at least one additional electrode arranged on a section of the Mach-Zehnder interferometer and produced outside the interaction zone with coplanar electrodes,
• one of the means for adjusting the operating point is a feedback circuit comprising a photodiode intended to pick up a complementary light field transmitted by the interferometer in the block, - the photodiode is arranged on a lateral face of the block perpendicular to the long axis of said block,
• one of the operating point adjustment means is a feedback circuit comprising an electronic circuit
• the modulator further comprises means for adjusting its electrical operating point comprising a photodiode intended to pick up a complementary light field transmitted by the interferometer in the block, an electronic circuit and a set of feedback electrodes arranged on a section of the Mach-Zehnder interferometer and produced outside the interaction zone with coplanar electrodes,
• one of the operating point adjustment means is a feedback circuit comprising a photodiode intended to pick up a complementary light field transmitted by the interferometer in the block, an electronic circuit and a set of feedback electrodes arranged on a section of the Mach-Zehnder interferometer and produced outside the interaction zone with coplanar electrodes.
• the invention also relates to a method for producing an electro-optical modulator comprising a block of a crystal with electro-optical properties, the block being elongated in length along a major axis and comprising a light guide s' extending between a light guide input and a light guide output, an electrical modulation signal acting on a zone of interaction of the block by coplanar electrodes in relation to the light guide via a dielectric layer , the electrodes being arranged on an elongated main face of the block and the interaction zone being substantially elongated along the major axis, the light to be modulated arriving by an optical input fiber coupled to the light guide at the input of light guide on a first face of the block, the modulated light leaving the modulator by an optical output fiber coupled to the light guide at the level of the guide output e of light on a second face of the block in which one implements for the first face and the second face one and the same face parallel to the major axis and one realizes two curvature in the light guide respectively between the light
• the method of the invention can be combined with all the actions that can be envisaged in order to achieve the structural characteristics previously listed, such as, for example, the production of one or more additional electrodes for the production of one of the means for adjusting the point of operation.
• the invention finally relates to a block as it is implemented in the modulator according to one or more of the preceding characteristics.
• the invention allows a reduction in the size of the housing lengthwise, the simplification of the passage of the optical fibers through the openings of the housing in a single operation without excessive twisting of the fiber.
• the fibers being bonded on one side of the crystal, it is necessary to polish only one side of the substrate.
• the fibers emerging from only one side of the housing, the thermal expansion effects are exerted only on one side, avoiding traction on the micro-blocks glued on opposite sides, as is the case in the art prior.
• the preferred mode of light propagation being the extraordinary mode, more confined than the ordinary mode, the differences in curvature losses have a partially polarizing effect by attenuating the ordinary component, which is interesting depending on the intended application.
• the microwave or electrode lines can be designed without curvatures, which significantly improves the high frequency performance, on the one hand in terms of electrical losses, on the other hand as regards coupling by radiation on the substrate modes.
• the electromagnetic field remains confined between the line and the ground plane throughout the propagation. It is thus possible to eliminate the losses introduced by the curvatures of the electrodes in the propagating electrical signal and to eliminate the couplings by antenna effect on the substrate modes of the lithium niobate crystal. It is possible to integrate control electronics on the front face of the crystal block and a suitable output load, without disturbing the optical signal and without discontinuities in the propagation of the microwave electrical signal.
• a photodiode on the rear face of the block to ensure detection of the complementary light field transmitted by the interferometer and to provide a feedback allowing the control of the operating point of the modulator, this in a compact manner and without take light from the output optical fiber. It is also possible within the framework of the invention to produce a set of parallel electrodes for the application of a direct voltage intended for adjusting the operating point of the modulator.
• the advantage of the invention in terms of encapsulation lies in the fact that the entry faces exit must be the subject of a particularly precise polishing in order to achieve a sharp edge having an angle of 90 'without chips. According to the prior art, this operation must be carried out once for each face. In the proposed invention, the same face acting as an inlet face and an outlet face for the light, a single polishing operation is necessary, simplifying the technological manufacturing steps.
• This invention therefore provides a solution in the field of encapsulation in particular for the passage of optical fibers and for the reduction of the longitudinal size of the boxes.
• the invention makes it possible to significantly shorten the length of the housing, and therefore the size of the device.
• the contributions to the overall length of the device come from the length of the optical chip itself, increased by the length of two micro-blocks maintaining the fibers at input and output, by the lengths of fiber necessary for join the openings in the housing, and finally elements of rigid and flexible mechanical protection of the fiber, arranged outside the housing.
• the set can represent a total length of between 12 and 15 cm.
• the various contributions are supported by one of the sides of the housing.
• the lateral dimensions are therefore increased compared to the prior art, but the longitudinal dimensions are reduced to the minimum corresponding to the length of the optical chip itself.
• FIG. 1 of the prior art represents a modulator in a housing
• Figure 2 represents a first implementation of the invention
• Figure 3 represents a second implementation of the invention
• Figure 4 represents a third implementation of the invention
• FIG. 5 represents a fourth implementation of the invention
• FIG. 6 represents a fifth implementation of the invention
• FIG. 7 of the state of the art represents a boxing step
• FIG. 8 represents the same step for a modulator according to the invention.
• Figure 1 shows the current state of the art in the field of modulators in integrated optics on lithium niobate.
• the optical chip receives two optical fibers 2 'and 2 "input / output mounted on polished ferrules 3' and 3" respectively with an angle to overcome parasitic reflections and aligned with a guide light 4 scattered on the surface of block 1 of crystal. Note that it is necessary here to polish the two opposite faces of the block.
• the light guide is produced according to a Mach-Zehnder interferometer architecture. The light guide enters along the edge or face A of the block and the exit through the edge or face B. Coplanar electrodes 5 are aligned parallel to the pattern of the guide.
• the input 6 and the output 7 of the electrodes located on the edge or face C of the crystal, are aligned perpendicular to the main direction of propagation of the waveguide and connected to the electro-optical interaction zone by means of a curvature rotating at an angle of 90 '.
• a connector 8 mounted on the housing allows the supply of electrical modulation signal on the input of the electrodes.
• a second connector 9 is connected to the output of the electrodes 5 and allows the connection of a load adapted to the characteristic impedance of the line.
• the fibers are stripped behind the micro-block and brazed on the part allowing the fiber to pass out of the housing 10.
• the lithium niobate optical chip for example cut preferably in the direction Z of the crystal, comprises an active part of Mach-Zehnder interferometer, consisting of two sections of parallel rectilinear guides forming the arms of the 'interferometer.
• a Y junction is connected to each of these arms, at the input to divide and separate the incident light wave in half, and at the output to recombine the two phase-shifted waves by electro-optical effect by the electrical signal, thus allowing modulation of the light.
• these Y-shaped junctions are connected to rectilinear optical guide sections, ending on each opposite face of the crystal according to a sharp edge which allows the insertion of the light from a first side and extraction of the light from the opposite side.
• a sharp edge which allows the insertion of the light from a first side and extraction of the light from the opposite side.
• the optical guides constituting the integrated optical circuit are generally produced by depositing a metal strip, for example made of titanium, of given thickness, for example 70 nm and of fixed width, for example at 7 ⁇ m. An index gradient is obtained from this metal layer by thermal diffusion at high temperature for ten hours.
• the optical guide obtained supports only one mode at the working wavelength. Weakly confined, it allows effective compromise between the light field of the optical fiber and the light field guided by the diffused index gradient.
• FIG. 2 corresponds to the first version of the invention and represents the optical chip in the form of a block 1 of material with electro-optical properties inserted in its housing 10.
• the entry and exit of the guide light are on the same face D of the block 1.
• This is obtained by producing a curvature 11 'and 11 "at each end of the optical circuit in the block, the two curvatures being preferably symmetrical to one another as shown. that here, only one and the same side D of the block must be polished which simplifies the manufacturing operations.
• the micro-blocks 3 ′ and 3 "enclosing the optical fibers are attached and glued on one and the same side D and consequently the two fibers come out on the same side.
• This first approach allows the insertion of the fibers through the 12 'and 12 "inlet / outlet passages, drilled in the housing, in a single operation without excessive twisting of the fibers.
• the traction which may be exerted on the fibers due to the expansion of the materials is done in one direction.
• the microwave lines have a curvature in input and output.
• the major advantage of this approach is to considerably reduce the dimensions and dimensions of the housing in the direction of the length or major axis of the block.
• ferrules supporting the fibers are inclined. This corresponds to the need to introduce a small angle when inserting the light from the fiber to the guide and from the guide to the fiber. This angle is generally used to avoid reflections on the interfaces between diopters of different refractive indices, detrimental to the quality of the radiation and that of the modulation.
• the Snell-Descartes relations are used to choose the angle of incidence of the optical guide relative to that of the optical fiber.
• the fiber Taking into account the index of the lithium niobate crystal of the order of 2.14 at the wavelength of 1550 nm used in modern fiber communications systems, and the index of the silica of the fiber optics of the order of 1.46, if we consider an angle of 10 ° between the direction of the guide and the plane of out of the crystal, the fiber must be polished at an angle of 14.6 °. This 10 ° emergence angle limits the rotation of the curvature to 80 °. The losses in the curvatures being proportional to the angle traveled, the angle of emergence contributes for a few percent to the reduction of the optical losses.
• the Y-shaped junctions are connected to a section of optical guide of curved shape, drawing an arc of a circle, which allows the light to travel through an angle comprised for example between 80 ° and 90 °, and this with a radius of curvature of the order of 5 mm, typically.
• the orientation of the curvatures is such that it is possible to connect these curved guides to a section of short straight guide making it possible to connect each end of the optical circuit to a single lateral face of the crystal. In such a configuration, the light coming from an optical fiber held in a ferrule will enter through one face of the crystal to exit at the other end of the optical circuit towards the output fiber connected by a ferrule to the same face of the crystal.
• the optical losses by propagation in the curvatures can be reduced significantly by increasing the confinement of the light, in a localized way. For this, it suffices to increase in the curved regions the thickness of the titanium ribbon, for example 100 nm, and its width, for example 8 ⁇ m. This corresponds to an almost multimode operation for a straight guide which greatly reduces the losses when a curvature is introduced. This technological process can be called "double deposit".
• the diffusion profile of the extraordinary index used to make a low control voltage modulator is significantly different from that of the ordinary index.
• the ordinary index profile obtained after dissemination Titanium thermal is much less confined than that of the extraordinary index. Consequently, the optical propagation losses in the curvatures, even for a "double deposit" structure for the ordinary mode are significantly higher than those of the extraordinary mode.
• the introduction of curvature therefore has a partially polarizing effect on the incident radiation, which can be of interest in multiple applications.
• FIG. 3 is an alternative in which, with respect to the device of FIG. 2, the microwave lines deposited on the surface of the optical circuit do not have any curvatures and are connected at A and B to electronic connectors 8 and 9 d input / output inserted on two opposite sides of the housing.
• the optical circuits have curvatures 1 1 ′ and 1 1 ".
• the major advantage of this configuration without curvature in the power lines is, on the one hand, to limit losses, allowing operation at very high frequency, and, d on the other hand, to limit the couplings on the modes of substrates, which makes it possible to obtain a response without resonances at high frequency.
• a variant consists in carrying out a microwave line output load in a coplanar manner on an alumina substrate and welded to the end of said line, which makes it possible to dispense with the electrical output connector.
• FIG. 4 is an improved version of the version of FIG. 3 in which the housing includes amplification electronics 13 in the form of one or more semiconductor chips inserted between the electronic input connector 8 and the start of the microwave line of the optical chip by the face or edge A of the block.
• a suitable load 14 is connected to the other end of the line by the face or edge B of the block.
• Biasing or adjusting the operating point such structures can be produced by construction by integrating an optical phase shifter of 90 'between the arms, for example by ablation of the surface of the guide corresponding to one of the arms, or else also by the use of a polarization tee.
• an optical phase shifter of 90 'between the arms for example by ablation of the surface of the guide corresponding to one of the arms, or else also by the use of a polarization tee.
• the end of the microwave line has a curvature towards a suitable load, this after the interaction with the optical wave.
• the photodiode is placed on the path of the complementary light transmitted by the interferometer. This light comes from the coupling of light after recombination of the waves after propagation through the arms.
• the waves are in phase, they are coupled in the symmetrical fundamental mode of the optical guide which is transmitted by the fiber.
• the waves are in phase opposition, they are coupled in the asymmetric mode which is not guided and which is therefore a leakage mode.
• This light which propagates in a straight line can be exploited when it is detected through the exit face of the substrate as a free beam.
• a photodiode can be a fast photodiode intended to produce a feedback on the amplitude of microwave modulation, so as to optimize the error rate in a standard digital transmission.
• the microwave line ends in a curvature 15 after the region or zone of interaction with the optical wave. This end is connected to a suitable load 14 placed on one side.
• the end B of the block or substrate which is then released is used to place a photodiode 16 intended to capture the complementary light field transmitted by the interferometer in the substrate.
• the photodiode is intended for servo-control by an electronic circuit 17 for feedback of the operating point of the modulator by means of low frequency electrodes 18. This provides a very integrated version of the device.
• 6 is yet another version in which advantage is taken of the possibility of producing curvatures in the optical waveguides to introduce an offset in the Mach-Zehnder structure, so that a section of the Mach-Zehnder either outside the structure of coplanar electrodes 5 this thanks to curvatures 19, which makes it possible to apply a direct voltage for adjusting the operating point of the modulator using an additional electrode 20.
• Figure 7 shows how to insert optical fibers
• FIG. 8 shows the procedure for inserting the two optical fibers 2 into the passages 12 of the housing according to the proposed invention. This technique makes it possible to minimize the curvatures of the optical fibers and the stresses exerted.

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• Optical Modulation, Optical Deflection, Nonlinear Optics, Optical Demodulation, Optical Logic Elements (AREA)
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