Optical phase modulator with sinusoidal PN junction

Abstract

The object of the invention is a phase modulator including a modulation guide intended to guide die propagation of a light flow, said guide comprising a PN junction extending mainly along the main axis of propagation according to an oscillating continuous function. Advantageously, the oscillating continuous function is defined in such a way that the PN junction covers at least 50% of the light flow between the input and the output of the modulation guide. According to one possibility, the continuous function is sinusoidal. The object of the invention is also a switch and an intensity modulator each comprising a phase modulator.

Claims

1. A phase modulator for a light flow comprising: an input guide and at least one multimode modulation guide having an input optically coupled with an output of the input guide, an output and a main axis (y) of propagation along which a light flow is intended to pass through the at least one multimode modulation guide, the at least one multimode modulation guide further comprising: a P-doped zone and a N-doped zone designated in such a way as to define a PN junction generating a space charge region, a device configured to vary a voltage V applied between the N-doped and P-doped zones in such a way as to vary a phase of the light flow intended to propagate in the at least one multimode modulation guide, wherein the input guide is configured to excite various propagation modes in the at least one multimode modulation guide and in that the PN junction extends mainly along the main axis (y) of propagation, following an oscillating continuous function forming oscillations each defining a crest and in which the crests of two successive oscillations are separated by a distance P.sub.jPN.

2. The phase modulator according to claim 1, wherein the oscillating continuous function is defined in such a way that the space charge region covers at least X % of the light flow between the input and the output of the at least one multimode modulation guide, with X50.

3. The phase modulator according to claim 1, wherein the distance P.sub.jPN is such that P.sub.jPN300 nm.

4. The phase modulator according to claim 1, wherein the oscillating continuous function is periodic, having a period P.sub.jPN.

5. The phase modulator according to claim 1 wherein the input guide is configured in order for the light flow propagating in the at least one multimode modulation guide to have a power distribution oscillating in a periodic manner according to a period P.sub.flux, and the PN junction is configured in such a way as to vary P.sub.flux between a value P.sub.fluxE at the input of the guide and a value P.sub.fluxS at the output of the guide, said period P.sub.fluxS being dependent on the voltage V, with V varying between V.sub.min and V.sub.max during operation, the distance P.sub.jPN associated with the PN junction being chosen in such a way that:
k.sub.1.Math.P.sub.fluxEP.sub.jPNk.sub.2.Math.P.sub.fluxE
k.sub.3.Math.P.sub.FluxSP.sub.jPNk.sub.4.Math.P.sub.FluxS with k.sub.1=0.5 and k.sub.2=2, and with k.sub.3=0.5 and k.sub.4=2, for any value of V between V.sub.min and V.sub.max.

6. The phase modulator according to claim 1, wherein the light flow propagates in a dual-mode manner in the at least one multimode modulation guide according to a fundamental mode and a secondary mode and has a wavelength and a power distribution oscillating according to a period P.sub.flux such that:
P.sub.flux=/(n.sub.eff1n.sub.eff2) where n.sub.eff1 is an effective index associated with a fundamental mode and n.sub.eff2 is an effective index associated with a secondary mode, further satisfying n.sub.eff2.

7. The phase modulator according to claim 5, wherein V.sub.min=0 Volts and |V.sub.max|8V.

8. The phase modulator according to claim 1, wherein the at least one multimode modulation guide has an input and an output and P.sub.jPN is constant along the main axis (y) of propagation between the input and the output of the at least one multimode modulation guide.

9. The phase modulator according to claim 1, wherein the oscillating continuous function is periodic, with a period P.sub.jPN and wherein the at least one multimode modulation guide has an input and an output and the period P.sub.jPN varies along the main axis (y) of propagation between a period P.sub.jPNE at the input of the at least one multimode modulation guide and a period P.sub.jPNS at the output of the at least one multimode modulation guide, such that:
l.sub.1.Math.P.sub.jPNSP.sub.jPNEl.sub.2.Math.P.sub.jPNS with l.sub.1=0.8 and l.sub.2=1.2.

10. The phase modulator according to claim 1, wherein the phase modulator is configured for a light wave having a wavelength , such that P.sub.jPN.

11. The phase modulator according to claim 1, wherein the at least one multimode modulation guide is a dual-mode guide.

12. The phase modulator according to claim 1, wherein the input guide is a single-mode guide having an axis of propagation not aligned with the main axis (y) of propagation of the at least one multimode modulation guide.

13. The phase modulator according to claim 1, wherein the input guide is an output of a mode converter.

14. The phase modulator according to claim 1, wherein the phase modulator is configured for a light wave having a wavelength between 1530 mn1585 nm (C band) or 1260 nm1360 nm (O band).

15. The phase modulator according to claim 1, wherein a doping of the P-doped zone is less than 10.sup.18 cm.sup.3.

16. The phase modulator according to claim 1, wherein a doping of the N-doped zone is less than 2.10.sup.18 cm.sup.3.

17. The phase modulator according to claim 1, wherein said oscillating continuous function is sinusoidal.

18. The phase modulator according to claim 1, wherein said oscillating continuous function is triangular.

19. A switch comprising a phase modulator according to claim 1 and at least two output guides, the output of the at least one multimode modulation guide being optically coupled to inputs of the output guides.

20. An intensity modulator comprising at least: a phase modulator according to claim 1 and an output guide, the phase modulator further comprising a first guide segment configured to optically couple the output of the input guide to the input of the at least one multimode modulation guide, said first guide segment being further configured to optically couple the output of the at least one multimode modulation guide to an input of the output guide, a second guide segment extending from the output of the input guide to the input of the output guide.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The goals, objects, features and advantages of the invention will be clearer from the detailed description of embodiments of the latter that are illustrated by the following accompanying drawings in which:

(2) FIG. 1a is a perspective view of a phase modulator according to one embodiment of the present invention;

(3) FIG. 1b is a perspective view of a phase modulator according to another embodiment of the present invention;

(4) FIG. 2a is a top view of the phase modulator illustrated in FIG. 1a and showing a first space charge region;

(5) FIG. 2b is a top view of the phase modulator illustrated in FIG. 1a and showing a second space charge region;

(6) FIG. 3a is a top view of the phase modulator illustrated in FIG. 1a, coupled with an input guide, this view also illustrating a distribution of the light flow guided in the modulator;

(7) FIG. 3b is a top view of an input guide resulting from a mode converter;

(8) FIG. 4a is a top view of a switch according to one embodiment of the present invention;

(9) FIG. 4b is a top view of an intensity modulator according to one embodiment of the present invention.

(10) The drawings are given as examples and are not limiting to the invention. They are schematic representations of a principle, intended to facilitate the understanding of the invention, and are not necessarily on the scale of the practical applications. In particular, the thicknesses and dimensions of the various layers and portions of the modulators illustrated are not representative of reality.

DETAILED DESCRIPTION OF THE INVENTION

(11) Before starting a detailed review of embodiments of the invention, optional features that can optionally be used in combination or alternatively are mentioned below: the oscillating continuous function is defined in such a way that the PN junction covers at least X % of the fight flow between the input and the output of the modulation guide, with X50, preferably X60, preferably X70, preferably X80, preferably X90. the oscillating continuous function forms oscillations each defining a crest and the crests of two successive oscillations are separated by a distance P.sub.jPNP.sub.jPN can thus be qualified as a peak-to-peak distance in the case in which the crests form peaks. the distance P.sub.jPN corresponds to the shortest distance between two successive crests taken from all of the crests formed by the PN junction along the modulation guide. Preferably, P.sub.jPN is such that P.sub.jPN300 nm, preferably P.sub.jPN750 nm, and preferably P.sub.jPN1 m. This distance P.sub.jPN can be measured regardless of whether the function is periodic or non-periodic, with a constant or non-constant period over the length of the modulation guide. said continuous function is periodic, having a period P.sub.jPN. Thus, according to this embodiment, the distance P.sub.jPN between two successive crests is equal to the period of the oscillations. said continuous function is sinusoidal. said continuous function is triangular. the input guide is configured in order for the light flow propagating in the modulation guide to have an oscillating power distribution and this distribution to oscillate periodically according to a period P.sub.flux in the modulation guide. the power distribution of the light flow propagating through the modulation guide oscillates periodically according to a period P.sub.fluxE at the input of said guide and the PN junction is configured in such a way as to satisfy the following formula:
k.sub.1.Math.P.sub.fluxEP.sub.jPNk.sub.2.Math.P.sub.fluxE
with k.sub.1=0.5 and k.sub.2=2, preferably k.sub.1=0.7 and k.sub.2=1.5, preferably k.sub.1=0.8 and k.sub.2=1.2, preferably k.sub.1=0.9 and k.sub.2=1.1, and preferably k.sub.1=0.95 and k.sub.2=1.05. the period P.sub.fluxS(V) of the power distribution of the light flow at the output of the guide is dependent on the voltage V, with V varying between V.sub.min and V.sub.max during operation, and the PN junction is configured in such a way as to verify the following formula:
k.sub.3.Math.P.sub.FluxSP.sub.jPNk.sub.4.Math.P.sub.FluxS with k.sub.3=0.5 and k.sub.4=2, for any value of V between V.sub.min and V.sub.max. V.sub.min=0 Volts and |V.sub.max|8V, preferably |V.sub.max|5V and preferably |V.sub.max|2.5V. the modulation guide has an input and an output and P.sub.jPN is substantially, preferably strictly constant along the main axis (y) of propagation between the input and the output of the modulation guide. the modulation guide has an input and an output and P.sub.jPN varies along the main axis (y) of propagation between a period P.sub.jPNE at the input of a modulation portion of the modulation guide and a period P.sub.jPNS at the output of said modulation portion of the modulation guide, such that:
l.sub.1.Math.P.sub.jPNSP.sub.jPNEl.sub.2.Math.P.sub.jPNS with l.sub.1=0.8 and l.sub.2=1.2, preferably l.sub.1=0.9 and l.sub.2=1.1, and preferably l.sub.1=0.95 and l.sub.2=1.05. P.sub.jPN300 nm, preferably P.sub.jPN750 nm, and preferably P.sub.jPN1 m. the modulation guide is a muitimode guide, preferably dual mode. the modulator comprises an input guide having an output optically coupled to the modulation guide. the input guide is configured to excite various propagation modes in the modulation guide. the input guide is a single-mode guide having an axis of propagation not aligned with the main axis (y) of propagation of the modulation guide. the input guide is an output of a mode converter. the modulator is configured for a light wave having a wavelength between 700 nm and 3000 nm, and preferably 1530 nm1585 nm (C band) or 1260 nm1360 nm (O band). the modulator is configured for a light wave having a wavelength , and said oscillating continuous function of the PN junction is periodic with a period R.sub.jPN such that P.sub.jPN. the input guide is a multimode guide simultaneously guiding various propagation modes of the light flow. the doping of the P+ zone is less than 10.sup.18 cm.sup.3, preferably approximately 10.sup.17 cm.sup.3. the doping of the N+ zone is less than 2.10.sup.18 cm.sup.3, preferably approximately 2.10.sup.17 cm.sup.3.

(12) In order to determine the trajectory and the position of a light flow propagating in the modulation guide, a simulation can be carried out. For this, the following can for example be used: methods for calculating finite differences in the time domain (FDTD for the acronym Finite Difference Time Domain). The software Rsoft and Lumerical for example allows these calculations to be earned out. finite-elements methods (FEM for the acronym Finite Elements Method). The software Comsol for example allows such a method to be used. beam-propagation methods (BPM for the acronym Beam Propagation Method). The software Rsoft for example allows such a method to be used
In order to measure the amount of overlapping X between the space charge region and the light flow, the following procedure is used:

(13) The space charge region has spatial distributions of concentrations of free carriers, electrons and holes, respectively noted as N.sub.e(x,y,z) and N.sub.h(x,y,z), where y is the direction of propagation of the light.

(14) These distributions of concentrations of carriers generate local modifications of the core index n.sub.c, which is dependent on the material and on the wavelength. For silicon, around =1.55 m for example, the following relationship exists:
n.sub.c(x,y,z)=8.8.10.sup.22.Math.N.sub.e(x,y,z)8.5.10.sup.18.Math.(N.sub.h(x,y,z)).sup.0.8

(15) The amount of overlapping X of the ZCE can thus be expressed by the electromagnetic field E(x,y,z) of the light wave, by integrating over the cross-section S of the optical mode or of the superimposed optical modes in such a way that:

(16) $X=\frac{{}_s|n_c\left(x,y,z\right)||E\left(x,y,z\right){|}^2\mathrm{dxdz}}{{}_s\left|n_c\left(x,y,z\right)\right|\mathrm{dxdz},{}_s|E\left(x,y,z\right){|}^2\mathrm{dxdz}}$

(17) Conventionally, that is to say, in the mathematical sense of the term, a function is a relationship between a set of inputs and a set of outputs, with the property that each input is linked to exactly one output.

(18) In this sense, a distribution or a rectangular signal is not a function.

(19) Moreover, the continuity of a function is associated with the notion of continuum, the origin of which is geometric. The real functions defined over an interval and having a graph that can be traced without picking up the pencil are continuous.

(20) Thus, a sinusoidal function or a triangular function is continuous.

(21) However, a function that has jumps is discontinuous.

(22) In particular, a Heaviside function is not continuous.

(23) Hereinafter, an oscillating function means a function that is alternatively increasing and decreasing. An oscillating function can be periodic or non-periodic.

(24) In the present invention, modes for guiding a light wave will be indicated. A light wave is an electromagnetic wave comprising an oscillating electric field coupled with an oscillating magnetic field. When the light wave is confined in a waveguide or an optical fibre for example, it propagates according to various modes called transverse modes (because of the conditions at the limits imposed by the confinement), including: the TE modes (Transverse Electric according to the acronym) not having an electric field in the direction of propagation. The corresponding fundamental mode is noted as TE00, the corresponding secondary modes are noted as TE01, TE10, TE11, TE02, TE12 etc. the TM modes (Transverse Magnetic according to the acronym) not having a magnetic field in the direction of propagation. The corresponding fundamental mode is noted as TM00, the corresponding secondary modes are noted as TM01, TM10, TM11, TM02, TM12 etc.

(25) The light flow corresponds here to the power distribution resulting from the propagation modes, of the lightwave, excited in the modulation guide. In particular, the period of the light flow is different from the wavelength of the light wave.

(26) In the present invention, types of doping will be indicated. These dopings are non-limiting examples. The invention covers all the embodiments in which the dopings are inverted. Thus, if an embodiment mentions P doping for a first zone and N doping for a second zone, the present description thus describes, at feast implicitly, the inverse example in which the first zone has N doping and the second zone P doping.

(27) Conventionally, doping noted as P+ (respectively N+) means that this is P-type doping (respectively N-type doping) having a concentration of doping species that is greater than or equal to 1 atom of the doping species for less than 100000 atoms of the semiconductor. A doping noted as P++ (respectively N++) is a P-type doping (respectively N-type) having a concentration of doping species that is greater than or equal to 1 atom of the doping species for less than 1000 to 10000 atoms of the material forming the semiconductor layer.

(28) A first embodiment of a phase modulator according to the invention will now be described in reference to FIGS. 1a, 2a, 2b and 3.

(29) As illustrated in FIG. 1a, the modulator 1 comprises at least one modulation guide 2 configured to guide at least one optical mode of a light wave, preferably two, and modulate the phase of this guided wave.

(30) Without this being limiting, the modulation guide 2 can be for example a rib-shaped guide and have a width dimension W along the axis x between 200 nm and 3 m, a height dimension h along the axis z between 100 nm and 600 nm, preferably between 150 nm and 350 nm, and a length dimension L along the main axis of propagation (y) between 20 m and 4000 m, preferably between 50 m and 500 m.

(31) A height h of the rib, formed for example by partial etching, can be such that: h=h/2 or h=h/5 for example.

(32) These dimensions are preferably compatible with multimode guiding of the light wave, in particular for h=300 nm, h=150 nm, and W>450 nm.

(33) The modulation guide 2 is preferably a multimode guide, and preferably a dual-mode guide.

(34) It is preferably made of a semiconductor material having a contrast with the surrounding medium in terms of refractive index, in such a way as to confine the light wave in the guide. Hereinafter, the contrast in refractive index is simply called index contrast.

(35) The modulation guide 2 is preferably made of silicon.

(36) According to one possibility, the guide 2 can be made of germanium.

(37) It can be formed via at least one step of lithography and at least one step of etching for example, on a substrate 10. Preferably, the substrate 10 is also made of silicon.

(38) The modulation guide 2 comprises an input 1 and an output 22 through which the light wave enters and exits.

(39) It can preferably have a section called rib-shaped section (also called RIB) in the plane (xz) of the reference frame illustrated in the drawings.

(40) It can further have one or more surfaces 20 encapsulated by silica for example.

(41) The modulation guide 2 comprises at least one modulation portion 63 having a length L.sub.M along (y), said portion 63 comprising a PN junction 6.

(42) This modulation portion 83 can extend over the entirety of the modulation guide 2 from the input 21 to the output 22, in such a way that the length L.sub.M of the modulation portion is equal to the length L of the modulation guide.

(43) The length L of the modulation guide is measured in the main direction (y) between the input 21 and the output 22.

(44) Alternatively, as illustrated in FIG. 1b, this modulation portion 63 follows an input portion 23 of the modulation guide, said input portion 23 extending from the input 21 in the main direction (y), over a distance L.sub.E less than or equal to n times the length L of the modulation guide 2. Preferably, n is equal to 0.2, preferably n is equal to 0.1, preferably n is equal to 0.05, and preferably n is equal to 0.01.

(45) Similarly, an output portion 24 having a length L.sub.S (L.sub.Sn.Math.L) can also follow the modulation portion 63, said output portion 24 extending from the end of the modulation portion 63 to the output 22 of the modulation guide 2.

(46) The PN junction 6 is preferably an interface between a P+ doped zone 4 and an N+ doped zone 5. For example, the doping of these N+ and P+ zones 5, 4 can be carried out via ion implantation in a direction parallel to the axis (z). The PN junction 6 can thus extend in parallel to (z) over a depth d, taken along the axis (z), preferably between 200 m and 500 nm.

(47) The PN junction 6 also extends in (x) and in (y) (and thus in the plane xy) according to a curve of a continuous function.

(48) Preferably, the main dimension of the PN junction 6 is parallel to the axis (y) of propagation of the light wave.

(49) Advantageously, the PN junction 6 at least partially coincides with a light flow 3 associated with the light wave propagating inside the modulation guide 2, in such a way as to interact with said light flow 3.

(50) The propagation of the light wave in the modulation guide 2 comprises at least one fundamental mode TE00, and can comprise secondary modes.

(51) The input guide 210 and the modulation guide 2 are configured in such a way that the light flow 3 propagating in the modulation guide 2 results from a superposition of various excited modes.

(52) The secondary modes can be excited in the modulation guide 2, for example via an input guide 210 offset with respect to the main axis of propagation (y) of the modulation guide 2, or excited upstream of the modulation guide 2, that is to say, before the light flow 3 propagates in the modulation guide 2.

(53) The various excited modes propagating simultaneously in the modulation guide 2 preferably comprise symmetric and antisymmetric modes. For example, the input guide 210 and the modulation guide 2 are configured in such a way that two modes propagate simultaneously in the modulation guide 2, one being symmetric and the other being antisymmetric.

(54) Thus, the excited modes propagating simultaneously in the modulation guide 2 are superimposed in such a way as to form a light flow 3 having an oscillating optical power distribution in the modulation guide 2.

(55) The successive oscillations of this power distribution are separated from each other by a distance P.sub.flux.

(56) In particular, the optical power distribution of the guided modes TE00 and TE01 propagating along the main axis (y) have periodic oscillations having a period P.sub.flux along said axis (y). All the explanations that follow and that refer to the guided modes TE00 and TE01 also apply to the guided modes TM00 and TM01.

(57) The period of the oscillation P.sub.flux is such that:
P.sub.flux=/(n.sub.eff1n.sub.eff2):(1)

(58) where n.sub.eff1 is the effective index associated with the fundamental mode and f.sub.eff2 is the effective index associated with the secondary mode, furthermore satisfying n.sub.eff1>n.sub.eff2.

(59) The fundamental and secondary mode of propagation are partially dependent on the geometry of the guide 2. The effective indices associated with these propagation modes are dependent in fine on the geometry of the guide 2, and are in particular a function of the width W of the guide 2.

(60) The table below compiles results of finite-element modelling from the software Rsoft, for various widths W of the modulation guide 2:

(61) TABLE-US-00001 W = 0.6 m W = 0.7 m W = 0.8 m W = 0.9 m n.sub.eff1 3.061 3.082 3.098 3.110 n.sub.eff2 2.799 2.856 2.906 2.946 P.sub.flux 5 m 5.8 m 6.8 m 8 m

(62) The PN junction 6 is thus advantageously defined by the curve of an oscillating continuous function in the plane (xy).

(63) Thus, the P+ doped zone 4 and the N+ doped zone 5 are advantageously interdigital in the modulation guide 2.

(64) The function that defines the shape of the PN junction can in particular be periodic with a period P.sub.jPN, such that:
k.sub.1.Math.P.sub.fluxP.sub.jPNk.sub.2.Math.P.sub.flux:(2)
with k.sub.1=0.5 and k.sub.2=2, preferably k.sub.1=0.7 and k.sub.2=1.5, preferably k.sub.1=0.8 and k.sub.2=1.2, preferably k.sub.1=0.9 and k.sub.2=1.1, and preferably k.sub.1=0.95 and k.sub.2=1.05.

(65) This period P.sub.jPN can be an average period or a pseudo-period over the length L.sub.M of the modulation portion 63.

(66) As illustrated in FIG. 3a, said function can advantageously be sinusoidal, in such a way that the PN junction 6 has a good correspondence with the power distribution of the light flow 4 in the modulation guide 2.

(67) Alternatively, said function can be triangular.

(68) According to an embodiment that is only optional, the period P.sub.jPN can be substantially, preferably strictly constant along the main axis (y) of propagation over the modulation portion 63 of the modulation guide 2.

(69) In particular, the period P.sub.jPN can be greater than 300 nm, preferably P.sub.jPN750 nm, and preferably P.sub.jPN1 m.

(70) The modulator 1 can in particular be configured for a light wave having a wavelength between 700 nm and 3000 nm, and preferably 1530 nm1585 nm (C band) or 1260 nm1360 nm (O band).

(71) Thus, the periodic continuous function having a period P.sub.jPN defining the PN junction 6 can be such that P.sub.jPN.

(72) The dimensional constraints associated with the creation of such a PN junction 6 having a period P.sub.jPN are not strong. The creation of this PN junction 6 can thus be perfectly carried out with the conventional optical lithography techniques. The modulator 1 according to the invention can thus be obtained in an easy, reproducible and not very costly manner.

(73) As illustrated in FIGS. 2a and 2b, at the PN junction 6, a space charge region, noted as ZCE 60 hereinafter, extends on either side of the PN junction in a direction orthogonal to the direction tangent to the curve defining the PN junction 6.

(74) The ZCE 60 has a limit, also called border 61, at the P+ doped zone 4, and a border 62 at the N+ doped zone 5. The ZCE 60 is defined by the borders 61, 62.

(75) The ZCE 60 thus has a width W.sub.ZCE, preferably taken in a direction orthogonal to the direction tangent to the curve defining the PN junction 6, i.e. to the curves formed by the borders 61, 62. The widths W.sub.ZCE of two different ZCE 60s are referenced in FIGS. 2a and 2b.

(76) This space charge region is depleted of charge carriers.

(77) The depletion spatially modifies distributions of concentrations of free carriers, electrons and holes, respectively noted as N.sub.e(x,y,z) and N.sub.h(x,y,z).

(78) These distributions of concentrations of carriers generate local modifications of the core index n.sub.c.

(79) The variation in core index n.sub.c(x,y,z) is dependent on the material and on the wavelength of the light wave propagating in the guide 2.

(80) Silicon and =1.55 m for example give the following relationship:
n.sub.c(x,y,z)=8.8.10.sup.22.Math.N.sub.e(x,y,z)8.5.10.sup.18.Math.(N.sub.h(x,y,z)).sup.0.8

(81) The variation in effective index at a position y on the axis (y) of propagation of the light wave can thus be deduced through integration over the section S of the optical mode of propagation of the light flow 3:

(82) $\begin{array}{cc}{n}_{\mathrm{eff}}\left(z\right)=\frac{{}_s{n}_c\left(x,y,z\right)|E\left(x,y,z\right){|}^2\mathrm{dxdz}}{{}_s|E\left(x,y,z\right){|}^2\mathrm{dxdz}}& \left(3\right)\end{array}$

(83) E(x,y,z) being the electric component of the electromagnetic field of the light wave.

(84) The phase shift of the light over a length L.sub.M of propagation is thus written as:

(85) $\begin{array}{cc}=k_0{}_0^{L_N}{n}_{\mathrm{eff}}\left(y\right).Math.\mathrm{dy}\mathrm{Or}\text{:}& \left(4a\right)\\ =k_0{}_0^{L_N}\left[\frac{{}_s{n}_c\left(x,y,z\right)|E\left(x,y,z\right){|}^2\mathrm{dxdz}}{{}_s|E\left(x,y,z\right){|}^2\mathrm{dxdz}}\right].Math.\mathrm{dy}& \left(4b\right)\end{array}$

(86) If the depletion is periodic, and the light wave travels for N periods over a length L, this integral can be expressed through one period:

(87) $=k_{0}N{}_0^p\left[\frac{{}_s{n}_c\left(x,y,z\right)|E\left(x,y,z\right){|}^2\mathrm{dxdz}}{{}_s|E\left(x,y,z\right){|}^2\mathrm{dxdz}}\right].Math.\mathrm{dy}$ :(4c)

(88) Moreover, the level of coverage X of the ZCE 60 by the Sight flow 3 can be expressed as:

(89) $\begin{array}{cc}X=\frac{{}_s|n_c\left(x,y,z\right)||E\left(x,y,z\right){|}^2\mathrm{dxdz}}{{}_s\left|n_c\left(x,y,z\right)\right|\mathrm{dxdz},{}_s|E\left(x,y,z\right){|}^2\mathrm{dxdz}}& \left(5\right)\end{array}$

(90) The phase shift on which the principle of phase modulation of the modulator 1 is basedis therefore in particular proportional to the amount of overlapping X between the light flow 3 and the ZCE 60.

(91) Advantageously, the ZCE 60 is configured to cover the guided modes of the light wave in an optimal manner, in such a way as to obtain an efficient phase shift of the light wave propagating in the modulation guide 2.

(92) FIG. 3a illustrates an embodiment in which the ZCE 60 covers almost all of the light flow 3.

(93) In particular, the function defining the PN junction 8, and by extension the ZCE 60, is configured to have an amount of overlapping X with the light flow 3 between the input 21 and the output 22 of the modulation guide 2, with X50%, preferably X60%, preferably X70%, preferably X80%, preferably X90%.

(94) This correspondence between the power distribution of the light flow 3, that is to say, the position of the light flow 3 in the modulation guide 2, and that of the ZCE 60 along the main axis of propagation makes the phase shift particularly efficient.

(95) The light wave undergoes a phase shift via phase accumulation, in such a way that the corresponding light flow 3 has a period P.sub.FluxE at the input 21 of the modulation guide 2, at least partly on the input portion 23, and a period P.sub.fluxS at the output 22 of the modulation guide 2, at least partly on the output portion 24.

(96) The phase shift can preferably be such that P.sub.fluxE<P.sub.fluxS.

(97) In order to compensate a progressive offset of the fight flow 3 caused by the cumulative nature of the phase shift, the oscillating continuous function defining the PN junction 6 can have a period P.sub.jPN that varies along the main axis (y) of propagation between a period P.sub.jPNE at the input of the modulation portion 63 and a period P.sub.jPNS at the output of the modulation portion 63, such that:
l.sub.1.Math.P.sub.jPNSP.sub.jPNEl.sub.2.Math.P.sub.jPNS:(6)

(98) with l.sub.1=0.8 and l.sub.2=1.2, preferably l.sub.1=0.9 and l.sub.2=1.1, and preferably l.sub.1=0.95 and l.sub.2=1.05.

(99) The amount of overlapping X is thus advantageously increased.

(100) Moreover, since the amount of overlapping is increased, the length L of the guide, in its main direction, can be reduced. This compactness allows the bandwidth of the modulator 1 to be significantly increased by limiting the optical losses on the path of the light flow 3.

(101) The amount of overlapping can also vary according to the width W.sub.ZCE of the ZCE 60.

(102) The width of the ZCE 60 can be increased by applying an electric voltage V between the N+ and P+ doped zones, by reducing the concentrations of dopants N.sub.A and N.sub.D, or by a combination of these possibilities.

(103) The concentration of dopants N.sub.A in the P+ zone 4 is advantageously less than 10.sup.18 cm.sup.3, preferably approximately 10.sup.17 cm.sup.3. The concentration of dopants N.sub.D in the N+ zone 5 is advantageously less than 2.10.sup.18 cm.sup.3, preferably approximately 2.10.sup.17 cm.sup.3.

(104) These relatively low concentrations of dopants further allow the optical losses to be limited, and contribute to improving the bandwidth of the modulator 1.

(105) FIGS. 2a and 2b illustrate the same phase modulator. The ZCE 60 of the phase modulator, illustrated in FIG. 2a, has a width W.sub.ZCE significantly smaller than the width W.sub.ZCE of the ZCE 60 of the phase modulator illustrated in FIG. 2b.

(106) In order to compensate for a possible decentring of the ZCE 80 with respect to the PN junction 6 caused for example by the difference in effective doping between the P+ zone and the N+ zone 4, 5, the oscillating continuous function defining the PN junction 6 can advantageously have a main axis along (y) that is off centre, in a projection in the plane (xy), with respect to the central longitudinal axis of the modulation guide 2, and parallel to the latter.

(107) The phase shift can also be modulated by applying the electric voltage V.

(108) Advantageously, the modulator 1 comprises a device 45 configured to apply such a variable voltage V between the N+ and P+ doped zones 5, 4. This voltage varies during operation preferably between and V.sub.min and V.sub.max with V.sub.min=0 Volts and |V.sub.max|8V, preferably |V.sub.max|5V and preferably |V.sub.max|25V.

(109) In particular, V.sub.min and V.sub.max can be adapted according to the use for example, or for a wavelength or a particular band of wavelengths.

(110) V.sub.max can in particular correspond to the half-wave voltage V.sub. allowing a phase variation of to be induced in the light wave at the output 22 of the modulator.

(111) Since the period of the light flow 3 at the output 22 of the guide is proportional to the phase shift , it is also dependent on the voltage V and the PN junction 6 should be configured in such a way that:
k.sub.3.Math.P.sub.FluxS(, V)P.sub.jPNk.sub.4.Math.P.sub.FluxS(, V):(7)
with k.sub.3=0.05 and k.sub.4=2, for any value of V between V.sub.min and V.sub.max.

(112) The modulator 1 comprises zones of contacts 42, 52, intended to connect said device 45 to the N+ and P+ zones. In the N+ zone 5, and respectively the P+ zone 4, the contact zone comprises at least one metal layer deposited on an N++ doped zone 51, and respectively on a P++ doped zone 41.

(113) The metal layer(s) can comprise copper, titanium, aluminium, nickel, chromium in the form of a pure element or in an alloy. At least one intermediate layer (for example between the zones 41, 51 and the contacts 42, 52, respectively) can exist in order to reduce the resistance of the contacts, such a layer preferably being a Nickel-Silicon alloy, which is called siliconisation, Siliconisation is widely known in particular from MOS technologies.

(114) The doping of the N++ and P++ zones can be greater than 10.sup.19 cm.sup.3 in such a way as to ensure a low access resistance and good electric transport of the charges to the N+ and P+ zones. The N++ zone 51, and respectively the P++ zone 41, has a continuity with the M+ zone 5, and respectively the P+ zone 4.

(115) Advantageously, the surfaces of the M++ and P++ zones, and the free surface 20 of the modulation guide 2 can be in the same plane, in such a way as to limit the steps of the manufacturing method, in particular the steps of lithography.

(116) The N++ and P++ zones can thus have a step with respect to the respective portions 50,40 of the N+ and P+ zones insulating the modulation guide 2. The step can advantageously have a height h along the axis z in a similar way as the height h of the modulation guide 2.

(117) The contact zones are preferably parallel to the modulation guide 2, spaced apart from said guide by a distance along the axis (x) between 1 m and 20 m, preferably between 5 m and 15 m, and preferably between 1 m and 3 m.

(118) This small distance contributes to reducing the time it takes for the carriers to cross the PN junction 6 and allows the quickness of the modulator 1 to be improved.

(119) According to one possibility, the zones of contacts 42, 52 can comprise a plurality of independent contact portions.

(120) The modulator 1 can further comprise an input guide 210 having an output 2100 optically coupled with the modulation guide 2,

(121) This coupling can be direct or evanescent.

(122) Said input guide 210 can preferably be a single-mode guide having an axis of propagation offset, in a projection in the plane (xy), with respect to the main axis (y) of propagation of the modulation guide 2.

(123) This decentring allows the appearance of a plurality of modes in the modulation guide 2 to be promoted. In particular the decentring allows the modes TE00 and TE01 or the modes TM00 and TM01 to be excited, thus resulting in an oscillation of the peak of light intensity.

(124) Alternatively, the input guide 210 can be a multimode guide simultaneously guiding a plurality of excited modes.

(125) According to an advantageous possibility, the input guide 210 can comprise an output of a mode converter.

(126) FIG. 3b illustrates an embodiment of a multimode input guide 210 resulting from an output branch of a mode converter.

(127) Such a mode converter configured to form a multimode light flow 3 can use a multiplexing technology, called spatial-division multiplexing, SDM (from the acronym Spatial-Division Multiplexing).

(128) For example, this converter can comprise a power separator or a multimode interference device 30 MMI (acronym for MuitiMode Interference) allowing a first and a second single-mode (for example TE00) light wave propagating in a first and a second single-mode branch 301, 302, respectively, to be generated via spatial division using a single-mode (TE00) input light wave propagating in a single-mode input branch 300.

(129) The first single-mode branch 301 can be enlarged at a portion 301 in such a way as to form the input guide 210 configured to guide and/or hybridize for example two modes of propagation of a light flow.

(130) The first light wave can thus be transmitted in the input guide 210. It preferably propagates via a symmetrical propagation mode.

(131) The dimensioning of this enlarged portion 310 is in particular dependent on the input light wave, the modes to be guided and/or hybridised in the input guide 210, and the effective refractive indices associated with these modes.

(132) The reference Silicon muitimode photonic integrated devices for on-chip mode-division-multiplexed optical interconnects, D, Dai et al., Progress in Electromagnetics Research, Vol. 143, 773-819, 2013 gives details on various possible dimensionings.

(133) The second single-mode branch 302 preferably has a decoupling portion 321 and a coupling portion 322.

(134) The decoupling portion 321 is configured to prevent undesired reciprocal coupling between the light waves propagating in the input guide 210 and in the second branch 302, at least in the immediate vicinity of the enlarged portion 310. This decoupling portion 321 can be created by increasing the distance of spatial separation between the input guide 210 and the second branch 302.

(135) The coupling portion 322 is configured to transfer, via coupling, at least a portion of the second light wave into the input guide 210, in such a way as to excite for example an antisymmetric propagation mode TE01 of the light flow propagating in the input guide 210, in addition to the symmetric mode TE00.

(136) Such a converter for example allows the formation of a light flow having a dual-mode propagation in the input guide 210. This light flow can then be transmitted at the input 21 of the modulation guide 2.

(137) Other embodiments of a mode converter having a multi-mode branch that can be coupled at the input 21 of the modulation guide 2 of the modulator 1 are described in the reference Silicon multimode photonic integrated devices for on-chip mode-division-multiplexed optical interconnects, D. Dai et al., Progress In Electromagnetics Research, Vol. 143, 773-819, 2013, for example in FIGS. 25 and 27 of this document.

(138) In reference to FIGS. 4a and 4b, two particularly advantageous uses of the invention will now be described.

(139) FIG. 4a illustrates a switch 11 comprising at least: an input guide 210, at least two output guides 220, 221 and a phase modulator 1 according to the invention.

(140) The input 21 of the phase modulator 1 is preferably optically coupled to an output 2100 of the input guide and the output 22 of the phase modulator 1 is preferably optically coupled to the inputs 2200, 2210 of the output guides 220, 221.

(141) The input guide 210 and output guides 220, 221 are preferably made of silicon and can have at least one free surface.

(142) Their cross-sections can have a surface area two times less than the surface area of the cross-section of the modulation guide 2.

(143) The input guide 210 can preferably be a single-mode guide. Advantageously, it has an axis of propagation offset, in the plane (xy), with respect to the main axis (y) of propagation of the modulation guide 2, in such a way as to promote the appearance of a plurality of modes in the modulation guide 2.

(144) The output guides 220, 221 are preferably adjacent to each other at the coupling with the output 22 of the modulation guide 2. The sum of their respective cross-sections can advantageously be equal to the cross-section of the modulation guide 2.

(145) The output guides 220, 221 can preferably be made of silicon and/or sheathed in order to separate them optically.

(146) According to the voltage V applied to the phase modulator 1, the phase shift obtained promotes the injection of a secondary mode guided in the modulation guide 2 into one or the other of the output guides 220, 221. This arrangement advantageously allows an optical signal to be directed to one or the other of the output guides 220, 221. The switching is advantageously dependent on the quickness of the phase modulator 1 described in the first embodiment.

(147) FIG. 4b illustrates an intensity modulator 111 according to the invention. The intensity modulator 111 comprises at least one input guide 210, at least one output guide 220, at least a first and a second guide section 200, 201, also called guide segments, each extending from an output of the input guide to an input of the output guide. The intensity modulator 111 farther comprises at least one phase modulator 1 according to the invention. The phase modulator 1 is positioned on at least one out of the first and second guide segment 200, 201.

(148) The input 21 of the phase modulator 1 is preferably optically coupled to an intermediate output 2001 of the first guide segment 200 for example. The output 22 of the phase modulator 1 is preferably optically coupled to an intermediate input 2002 of the first guide segment 200 for example.

(149) Said first and second segment 200, 201 are preferably made of silicon and can be sheathed, that is to say, surrounded by silica for example.

(150) These guide segments 200, 201 can preferably be single-mode.

(151) The intermediate output 2001 of the first guide segment 200 advantageously has an axis of propagation offset with respect to the main axis (y) of propagation of the modulation guide 2, in such a way as to promote the appearance of a plurality of modes in the modulation guide 2.

(152) Alternatively, one and/or the other of the guide segments 200, 201 can be multimode.

(153) An optical signal entering the intensity modulator 111 can propagate in the first and second guide segment 200, 201 and be recombined in the output guide 220 of the intensity modulator 111.

(154) The portion of the optical signal propagating for example in the first segment 200 comprising the phase modulator 1 undergoes a phase shift with respect to the portion of the signal propagating for example in the second signal 201.

(155) The phase shift can induce interferences during the recombination of the two portions of the signal at the output guide 220, said interference producing a variation in intensity in the optical signal exiting the intensity modulator 111.

(156) Such an intensity modulator 111 can in particular be of the Mach-Zehnder type.

(157) The intensity modulator 111 is advantageously dependent on the compactness of the phase modulator 1 described in the first embodiment.

(158) According to one embodiment, the intensity modulator 111 only comprises a single phase modulator 1 positioned on one of the first and second guide segment 200, 201.

(159) According to an alternative embodiment, the intensity modulator 111 comprises a plurality of phase modulators 1. A phase modulator 1 can thus be positioned on each of the first and second guide segment 200, 201, in such a way that the intensity modulator is configured according to a configuration called push-pull configuration for example.

(160) The invention is not limited to the embodiments described above and extends to all the embodiments covered by the claims.

REFERENCES

(161) 1. Phase modulator 2. Modulation guide 20. Free surface 21. Input of the modulation guide 22. Output of the modulation guide 210. Input guide 2100. Output of the input guide 220, 221. Output guides 2200, 2210. Inputs of the output guides 200, 201. First and second segments 2001. Intermediate output 2002. Intermediate input 23. Input portion of the modulation guide 24. Output portion of the modulation guide 3. Light flow 4. P-doped zone 40. Portion of the P-doped zone 41. P++ doped zone 42. Electric contact 45. Voltage device 5. N-doped zone 50. Portion of the N-doped zone 51. N++ doped zone 52. Electric contact 6. PN junction 60. ZOE 61. 62. Borders of the ZOE 63. Modulation portion of the modulation guide