Electro-optical Mach-Zehnder modulator featuring an enhanced transmission RF coplanar line

Abstract

The invention relates to an electro-optical Mach-Zehnder modulator (1) comprising an input divider, first and second main waveguides (10p, 20p), and an output combiner (3). It also includes a first coplanar RF line (30) associated with the first main waveguide (10p). Finally, it includes a first additional waveguide (10a), not coupled to the input divider (2) and the output combiner (3), and having a positioning relative to a central track (33), dimensions, and doping profile symmetrical to those of the first main waveguide (10p), with respect to a plane of symmetry (Ps) orthogonal to the main plane (XY), parallel to the first main (10p) and additional (10a) waveguides, and passing through a center of the central track (33). Figure for the abstract: Fig. 2A

Description

Title of the invention: Mach-Zehnder electro-optical modulator comprising an enhanced transmission RF coplanar line technical field

[0001] The field of the invention is that of Mach-Zehnder electro-optical modulators comprising at least one coplanar RF line of the ground / signal / ground type (GSG, for Ground, Signal, Ground in English). PREVIOUS STATE OF THE ART

[0002] Mach-Zehnder electro-optical modulators (MZMs) are an essential component in telecom or datacom type optoelectronic systems. They allow information from an electrical source to be encoded into an optical carrier by varying its intensity or amplitude.

[0003] A Mach-Zehnder modulator typically comprises an input divider, two waveguides that form the modulator arms, and an output combiner. Thus, an incident optical wave of power Pin traveling in the input waveguide is split into two optical waves of power Pin / 2 in the modulator arms. Each arm is doped to form a pn junction, a pin junction, or even a MOS capacitor.

[0004] Generating an electric field in at least one of the arms allows its refractive index to be modified by electro-optical effect, and therefore the phase of the optical wave propagating through it. The intensity modulator, which is the Mach-Zehnder modulator, is thus composed of at least one phase modulator associated with one of the modulator arms. At the end of the arms, a combiner receives the two optical waves, which are out of phase with respect to each other. These waves then interfere constructively or destructively, depending on the phase difference between the two optical waves, to form the output optical wave, the amplitude of which is thus modulated.

[0005] To apply an electric field to the modulator arm, the Mach-Zehnder modulator (MZM) may include a coplanar RF waveguide (also called CPW), which is usually formed of three GSG (Ground, Signal, Ground) conductive tracks arranged above the arm. The central conductive track (Signal) carries the electrical signal, and the two lateral conductive tracks (Ground) are connected to ground. When the Mach-Zehnder modulator has only one coplanar RF waveguide, and therefore only one control voltage is applied, it has a so-called Single Drive configuration. Conversely, when it has two coplanar RF waveguides (one per arm) and that two control voltages are applied, it then presents a configuration known as Dual Drive.

[0006] Figures IA and IB are schematic and partial views, in top view and in cross-section along the cutting line AA, of a Mach-Zehnder modulator according to an example of the prior art.

[0007] The Mach-Zehnder modulator 1 therefore comprises a divider 2, two main waveguides lOp, 20p which form the arms of the modulator, and a combiner 3. The configuration shown is of the Dual Drive type, so that the modulator 1 comprises a coplanar RF line 30 associated with the main waveguide lOp, and a coplanar RF line 40 associated with the main waveguide 20p.

[0008] The main waveguide lOp has a ribbed waveguide shape, where a pn junction is located. The coplanar RF line 30 is formed of three coplanar conducting tracks: a central track 33 to which the electrical modulation signal is applied, and two lateral tracks 31, 32 connected to ground. The main waveguide lOp is located between tracks 31 and 33, which also provide the biasing of the pn junction.

[0009] It is known that the sizing of the main waveguide and the associated coplanar RF line can impact the performance of the Mach-Zehnder modulator. Therefore, there is a need for a Mach-Zehnder modulator with improved performance. Description of the invention

[0010] The invention aims to remedy at least in part the drawbacks of the prior art, and more particularly to propose a Mach-Zehnder modulator comprising at least one GSG type RF coplanar line which has improved performance.

[0011] For this purpose, the object of the invention is an electro-optical Mach-Zehnder modulator, comprising: an input divider; first and second waveguides called main waveguides, coupled to the input divider, of which at least the first main waveguide has a doping profile forming a semiconductor junction or a MOS capacitance, extending longitudinally in a principal plane of the modulator; an output combiner, coupled to the two main waveguides; and at least one first coplanar RF line, associated with the first main waveguide, formed of three conductive tracks, coplanar, and parallel to the first main waveguide: a central track and a first and a second lateral tracks located on either side of the central track, the first main waveguide being located between the central track and the first lateral track.

[0012] According to the invention, the modulator also includes a first additional waveguide: extending parallel to the first main waveguide; not coupled to the input divider and output combiner; located opposite and between the central track and the second side track; and having a positioning with respect to the central track, a dimensioning and a doping profile symmetrical to those of the first main waveguide, with respect to a plane of symmetry, orthogonal to the main plane, parallel to the first main and additional waveguides, and passing through a center of the central track.

[0013] Some preferred but not limiting aspects of this Mach-Zehnder modulator are as follows.

[0014] The central track can be symmetrical with respect to the plane of symmetry.

[0015] The lateral tracks can be symmetrical with respect to the plane of symmetry.

[0016] The first main and additional waveguides can be waveguides with rib.

[0017] The modulator may include, for the first main and additional waveguides, lateral parts extending from the ribs, made of the same doped semiconductor material as the ribs, and intended to bias the semiconductor junction or the mos capacitance, the lateral parts associated with the additional waveguide having a positioning, dimensioning and doping profile symmetrical to those associated with the first main waveguide with respect to the plane of symmetry.

[0018] The modulator may further comprise a second coplanar RF line associated with the second main waveguide, and a second additional waveguide: extending parallel to the second main waveguide; not coupled to the input divider and output combiner; located opposite and between a central track and a second lateral track of the second coplanar RF line; having a positioning with respect to the central track of the second coplanar RF line, a dimensioning and a doping profile symmetrical to those of the second main waveguide, with respect to a plane of symmetry, orthogonal to the main plane, parallel to the second main and additional waveguides, and passing through a center of the central track of the second coplanar RF line.

[0019] The first main and additional waveguides may each include a pn junction.

[0020] The main and additional waveguide(s) can be made from silicon. Brief description of the drawings

[0021] Other aspects, objects, advantages and features of the invention will become more apparent upon reading the following detailed description of preferred embodiments thereof, given by way of non-limiting example, and made with reference to the accompanying drawings in which:

[0022] [Fig.1A], already described, is a schematic and partial top view of a Mach-Zehnder modulator according to an example of the prior art;

[0023] [Fig.1B], already described, is a schematic and partial cross-sectional view of a part of the Mach-Zehnder modulator of [Fig.1A] along the section line AA;

[0024] [Fig.2A] is a schematic and partial top view of a Mach-Zehnder modulator according to one embodiment;

[0025] [Fig.2B] is a schematic and partial cross-sectional view of a part of the Mach-Zehnder modulator of [Fig.2A] along the section line AA;

[0026] [Fig.3] illustrates a transmission spectrum of a coplanar RF line of a Mach-Zehnder modulator similar to that of [Fig.1A] and IB;

[0027] [Fig.4] is a schematic and partial cross-sectional view of a part of the Mach-Zehnder modulator of [Fig.2A] along the section line AA;

[0028] [Fig.5A] is a schematic and partial top view of a Mach-Zehnder modulator according to an example of the prior art;

[0029] [Fig.5B] is a schematic and partial cross-sectional view of part of a Mach-Zehnder modulator according to an embodiment, which is similar to that of [Fig.5A] but which includes an additional waveguide;

[0030] [Fig.6] illustrates the transmission spectra of the RF coplanar line of the Mach-Zehnder modulators of [Fig.5A] and 5B.

[0031] DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

[0032] In the figures and throughout the description, the same reference numerals represent identical or similar elements. Furthermore, the various elements are not drawn to scale in order to enhance the clarity of the figures. Moreover, the different embodiments and variants are not mutually exclusive and may be combined. Unless otherwise indicated, the terms "approximately," "about," and "in the order of" mean within 10%, and preferably within 5%. Furthermore, the terms "between ... and ..." and equivalents mean that the limits are inclusive, unless otherwise stated.

[0033] Figures 2A and 2B are schematic and partial views, in top view ([Fig.2A]) and in cross-section along the cutting line AA ([Fig.2B]), of a Mach-Zehnder modulator 1 according to one embodiment.

[0034] Here and for the remainder of the description, we define a three-dimensional orthogonal XYZ direct frame of reference, where the X and Y axes form a plane parallel to the principal plane of modulator 1 in which waveguides 10, 20 extend, and where the Z-axis is oriented from the support layer towards the coplanar RF lines. In the following description, the terms "lower" and "upper" are understood to be relative to an increasing positioning along the +Z direction.

[0035] The Mach-Zehnder electro-optical modulator 1 (MZM) is adapted to modulate the amplitude of an incident optical wave. To this end, it comprises, at a minimum: a divider 2 to receive an incident optical wave and divide it into two optical waves; first and second principal waveguides lOp, 20p coupled to the divider 2; and a combiner 3 coupled to the two principal waveguides lOp, 20p to receive the optical waves and recombine them by interference. The principal waveguides lOp, 20p extend in a principal plane of the modulator 1 parallel to the XY plane.

[0036] The divider 2 receives the incident optical wave of power Pin and divides it into two optical waves of equal power Pin / 2 which then circulate in the main waveguides lOp, 20p. The combiner 3 is coupled to the two main waveguides lOp, 20p to receive the two optical waves which then recombine by interference.

[0037] At least the first main waveguide 1Op has a doping profile forming a pn junction (in the case of a depletion modulator), a pin junction (in the case of an injection modulator), or a MOS capacitor (charge-storing modulator). In this example, the doping profile forms a pn junction that extends along the longitudinal axis of the main waveguide 10p. The length of the pn junction combined with the length of the coplanar RF line 30 defines the modulation length of the main waveguide 10p.

[0038] The Mach-Zehnder modulator 1 includes at least one first coplanar RF line 30, adapted to apply to the first main waveguide lOp an electric field modifying its refractive index and thus modulating the phase of the optical wave which circulates there (in order to generate a phase shift with the optical wave circulating in the main waveguide 20p).

[0039] A "coplanar RF line" corresponds to an electrode configuration, here three parallel and coplanar tracks, made of an electrically conductive material, adapted to transmit the high-frequency electrical signal used to apply a modulation voltage to the modulator electrodes. The term "RF," for Radio Frequency, refers to the high frequencies at which the electrical signal oscillates: for example, from a few GHz to several tens of GHz, depending on the application. More precisely, the coplanar RF line consists of two lateral tracks (called Ground when connected to ground potential) and a central track (called Signal because it is powered by the RF electrical potential).

[0040] It is understood that the RF 30 coplanar line is of the GSG type, for Ground, Signal, Ground. Modulator 1 here has a Dual Drive configuration since it has two RF 30 and 40 coplanar lines, here of the GSGSG type. Alternatively, it can have a Single Drive configuration and therefore have only one RF 30 coplanar line.

[0041] The first coplanar RF line 30 is located opposite the first main waveguide lOp along a vertical axis Z. The three conductive tracks 31, 32, 33 extend longitudinally parallel to the first main waveguide 10p. The first and second lateral tracks 31, 32 are located, in an XY plane, on either side of the central track 33. The main waveguide lOp is located, in projection onto the XY plane passing through the conductive tracks 31, 32, 33, between the central track 33 and the first lateral track 31. The main waveguide lOp is therefore not located exactly perpendicular to the central track 33. Note that the first lateral track 31 is the one located between the two main waveguides lOp, 20p, but alternatively it could be the opposite lateral track (here the lateral track 32).

[0042] The inventors observed that the RF coplanar line 30, in the case of a Mach-Zehnder modulator similar to that of [Fig. 1A] and 1B, can exhibit degraded transmission properties. Indeed, the transmission spectrum of the RF coplanar line, that is, the evolution of the parameter S2i as a function of the frequency of the RF electrical signal, shows the presence of transmission dips at regular intervals. These transmission dips thus degrade the Mach-Zehnder modulator 1's ability to transmit data rapidly.

[0043] To improve the transmission properties of the RF coplanar line 30 and thus the performance of the Mach-Zehnder modulator 1, the latter includes a first additional waveguide 10a, associated with the first RF coplanar line 30. This additional waveguide 10a is not optically coupled to the divider 2 and the combiner 3, so that no optical waves travel through it. It extends parallel to the first main waveguide 1Op, and therefore also to the conductive tracks 31, 32, 33 of the RF coplanar line 30. Furthermore, it is located opposite and between (and thus projected onto the main plane) the central track 33 and the second lateral track 32.

[0044] Finally, the additional waveguide 10a has a positioning relative to the central track 33, a dimensioning and a doping profile symmetrical to those of the main waveguide lOp, with respect to a plane of symmetry Ps, orthogonal to the principal XY plane, parallel to the first main waveguide lOp and additional 10a, and passing through the central track 33 (and more precisely passing through the geometric center of the central track 33). The central track 33 is itself symmetrical with respect to the plane of symmetry Ps.

[0045] It appears that the presence of such an additional waveguide 10a, arranged next to the first main waveguide 1Op and associated with the same first coplanar RF line 30, makes it possible to symmetrize the environment seen by the electric field generated by the coplanar RF line 30, in terms of the spatial distribution of the refractive index and, more broadly, of the permittivity. Thus, the electric field has a symmetrical spatial distribution on either side of the central track 33, and the propagation modes of the electric field (here CPW mode, or even mode) travel along the coplanar RF line 30 in a symmetrical environment. They therefore exhibit the same transmission properties, which then results in a reduction of the transmission dips mentioned previously. The coplanar RF line thus exhibits better transmission properties, which also improves the performance of the Mach-Zehnder modulator.

[0046] Figure 3 illustrates a transmission spectrum of the RF coplanar line of a Mach-Zehnder modulator similar to that of Figures 1A and 1B. Here, unlike the modulator according to the invention, the modulator does not include an additional waveguide, so that the propagation mode of the electric field generated by the RF coplanar line does not propagate in the same environment depending on whether one is on one side or the other of the central track.

[0047] The Mach-Zehnder modulator is configured to put the transmission of the electrical signal according to the CPW propagation mode (also called even mode), for an impedance of 50 Q. The modulator here has a length of 2mm in a GSG configuration without symmetrization of the coplanar line.

[0048] As previously stated, the transmission spectrum exhibits transmission dips at regular intervals, possibly associated with Bragg interference. These dips may correspond to a power transfer from the desired propagation mode (here even mode) of the electric field to an undesired mode (here odd mode).

[0049] It therefore appears that, as the inventors have observed, the presence of an additional waveguide as defined above makes it possible to reduce or even eliminate the presence of these transmission dips, thus improving the performance of the coplanar RF line and therefore that of the Mach-Zehnder modulator.

[0050] Fig. 4 is a schematic and partial view of a portion of the Mach-Zehnder modulator 1 illustrated in Figs. 2A and 2B, in cross-section along section line AA. This Fig. 4 is identical to Fig. 2B and allows for a more detailed illustration of certain dimensional parameters of the modulator 1.

[0051] The Mach-Zehnder modulator 1 is here a component of a photonic chip that contains an integrated photonic circuit. This includes passive and / or active optical components such as waveguides, diodes, modulators, optically coupled multiplexers, etc... It includes a support layer on which the waveguides lOp, 20p, 10a, 20a of modulator 1 rest. In this example, the Mach-Zehnder modulator 1 is made in silicon technology, but other technologies are possible (LiNbO3 for example).

[0052] The modulator comprises a support layer 51 made of an electrically insulating material. This is in this case a buried oxide layer (BOX) of a silicon-on-insulator (SOI) substrate. The waveguides 1Op, 20p, 10a, 20a rest on the support layer and are made from a semiconductor layer 52 of a crystalline material, here monocrystalline silicon, which has a higher refractive index than the surrounding layers.

[0053] The main waveguide 1Op and the additional waveguide 10a are ribbed waveguides: they thus comprise a rib 1Ip, lia formed from a thinner base (slab). The pn junction is located at the rib. The waveguides 1Op, 10a each have lateral polarization portions of the pn junction.

[0054] Concerning the main waveguide lOp: - Rib 1 Ip has a width wn and a thickness en. The junction pn is located approximately in the center of the rib, but it could be located at the interface between the rib and one or the other of the lateral parts; - a first intermediate lateral part 12p. 1, extends from the rib 1 Ip, doped here of type p, of thickness epi less than en and of width Wpü; followed by a first lateral contact part 13p. 1, doped here p +, of thickness epci and of width wpci, in contact with which comes a conductor via 34 connected to the first lateral track 31; - a second intermediate lateral part 12p.2, extends from the other side of the rib 1 Ip, doped here of type n, of thickness epi and of width wpi2 (preferably equal to wpii); followed by a second lateral contact part 13p.2, doped here n+, of thickness epc2 and of width wpc2, in contact with which comes a via conductor 34 connected to the central track 33.

[0055] The additional waveguide 10a is associated with the coplanar RF line 30. It is positioned and dimensioned symmetrically to the main waveguide lOp with respect to the plane of symmetry Ps. This plane passes through the center of the central track 33, is orthogonal to the XY plane, and is parallel to the main waveguide lOp and the additional waveguide 10a. The additional waveguide 10a also has a doping profile (pn junction) symmetrical to that of the main waveguide 10p.

[0056] Regarding the additional waveguide 10a: - the rib lia has a width wn and a thickness in identical to those of the rib 11p. The junction pn is also located here at the center of the rib, but the doping profile is reversed with respect to that of the main waveguide lOp; - a first intermediate lateral part 12a.1, which extends from the rib lia, doped here of type p, of thickness epi and of width wpii; followed by a first lateral contact part 13a.l, doped here p+, of thickness epci and of width wpci, in contact with which comes a conductor via 34 connected to the second lateral track 32; - a second intermediate lateral part 12a.2, which extends from the other side of the rib 1 Ip, doped here of type n, of thickness epi and of width wpi2; followed by a second lateral contact part 13a.2, doped here n+, of thickness epc2 and of width wpc2, in contact with which comes a via conductor 34 connected to the central track 33. Note that the lateral contact parts 13a.2 and 13p.2 are coincident: it is the same portion of the semiconductor layer 52.

[0057] The coplanar RF line 30 rests on an encapsulation layer 53 made of an electrically insulating material having a refractive index lower than that of the semiconductor layer 52. The conductive tracks 31, 32, 33 are electrically connected to the respective lateral contact parts 13p.1, 13a.1 and 13a / p.2 by conductive vias 34 which pass through the encapsulation layer 53.

[0058] The central track 33 advantageously has two branches symmetrical with respect to the plane Ps, of width ws, extending from the via 34. Similarly, the lateral track 31 has an internal branch of width wg, extending from the via 34, oriented towards the central track 33, as does the lateral track 32. Thus, the central track 33 and its via 34 are advantageously symmetrical with respect to the plane of symmetry Ps. The vias 34 of the lateral tracks 31 and 32 are advantageously symmetrical with respect to the plane Ps. The internal branches of the lateral tracks 31 and 32 are also advantageously symmetrical with respect to the plane Ps.

[0059] Figures 5A and 5B are schematic and partial cross-sectional views of part of a Mach-Zehnder modulator, which does not include an additional waveguide in the case of [Fig.5A] (modulator according to the prior art), and which includes an additional waveguide in the case of [Fig.5B] (modulator according to the invention).

[0060] With reference to [Fig. 5A], the Mach-Zehnder modulator 1 comprises a main waveguide lOp having a pn junction, which is biased by a control voltage applied from the central 33 and lateral 31 tracks, via the parts lateral contact parts 13p.1, 13p.2 and intermediate lateral parts 12p.1, 12p.2. In this example, each conductive track is connected to the corresponding lateral contact part by several vias 34 connected by an intermediate connection track 35. The modulator 1 therefore does not have an additional waveguide 10a.

[0061] With reference to [Fig. 5B], the Mach-Zehnder modulator 1 is identical to that of [Fig. 5A], but includes an additional waveguide 10a in the sense of the invention, i.e., it has a positioning relative to the central track 33, a dimensioning and a doping profile symmetrical to those of the first main waveguide 1Op, with respect to the plane of symmetry Ps. Here, the lateral contact parts 13p.2 and 13a.2 are distinct.

[0062] The modulators of [Fig. 5A] and 5B are made of silicon technology. The support layer and the encapsulation layer are made of silicon oxide, and the waveguides 1Op, 10a as well as the side sections are made of crystalline silicon. The tracks 31, 32, 33, the vias 34 and the connecting tracks 35 are made of at least one metallic material.

[0063] Fig. 6 illustrates the transmission spectrum of the RF 30 coplanar line of the Mach-Zehnder modulators of Fig. 5A and 5B.

[0064] In the case of the asymmetric configuration (dashed line) of the modulator in [Fig. 5A], the spectral response shows the presence of several transmission dips in the spectral band from 0 to 32 GHz. As mentioned previously, the presence of these transmission dips arises from the asymmetric environment, in terms of permittivity, in which the propagation mode of the electric field generated by the coplanar RF line propagates.

[0065] In contrast, in the case of the symmetrical configuration (solid line) of the modulator in [Fig. 5B], the spectral response shows an absence of these transmission dips. This arises from the fact that the electric field mode propagates in a symmetrical environment on either side of the central track 33. Thus, the performance of the coplanar RF line, and therefore that of the modulator, is improved.

[0066] Specific embodiments have just been described. Various variants and modifications will be apparent to those skilled in the art.

Claims

Demands

1. Mach-Zehnder electro-optical modulator (1), comprising: • an input divider (2); • first and second main waveguides (1Op, 20p), coupled to the input divider (2), of which at least the first main waveguide (1Op) has a doping profile forming a semiconductor junction or a MOS capacitance, extending longitudinally in a principal (XY) plane of the modulator (1); • an output combiner (3), coupled to the two main waveguides (1Op, 20p);• at least one first coplanar RF line (30), associated with the first main waveguide (1Op), formed of three coplanar conducting tracks (31, 32, 33) parallel to the first main waveguide (1Op): a central track (33) and first and second lateral tracks (31, 32) located on either side of the central track (33), the first main waveguide (1Op) being located between the central track (33) and the first lateral track (31); • characterized in that it comprises a first additional waveguide (10a): • extending parallel to the first main waveguide (1Op); • not coupled to the input divider (2) and the output combiner (3); • located opposite and between the central track (33) and the second lateral track (32);• having a positioning with respect to the central track (33), a dimensioning and a doping profile symmetrical to those of the first main waveguide (lOp), with respect to a plane of symmetry (Ps), orthogonal to the main plane (XY), parallel to the first main waveguide (lOp) and additional (10a), and passing through a center of the central track (33).;

2. Modulator (1) according to claim 1, wherein the central track (33) is symmetric with respect to the plane of symmetry (Ps).

3. Modulator (1) according to claim 1 or 2, wherein the side tracks (31, 32) are symmetric with respect to the plane of symmetry (Ps).

4. Modulator (1) according to any one of claims 1 to 3, wherein the first main waveguide (1Op) and additional waveguide (10a) are ribbed waveguides (1 Ip, 1 la).

5. Modulator (1) according to claim 4, comprising, for the first main waveguide (1Op) and additional (10a), lateral parts extending from the ribs (1 Ip, 1 la), made of the same doped semiconductor material as the ribs, and intended to bias the semiconductor junction or the mos capacitance, the lateral parts associated with the additional waveguide (10a) having a positioning, dimensioning and doping profile symmetric to those associated with the first main waveguide (1Op) with respect to the plane of symmetry (Ps).

6. Modulator (1) according to any one of claims 1 to 5, further comprising a second coplanar RF line (40) associated with the second main waveguide (20p), and a second additional waveguide (20a): extending parallel to the second main waveguide (20p); not coupled to the input divider (2) and the output combiner (3); situated opposite and between a center track and a second lateral track of the second coplanar RF line (40); featuring a positioning with respect to the central track of the second coplanar RF line (40), a dimensioning and a doping profile symmetric to those of the second main waveguide (20p), with respect to a plane of symmetry, orthogonal to the main plane (XY), parallel to the second main waveguides (20p) and additional (20a), and passing through a center of the central track of the second coplanar RF line (40).

7. Modulator (1) according to any one of claims 1 to 6, wherein the first main waveguide (1Op) and additional waveguide (10a) each comprise a pn junction.

8. Modulator (1) according to any one of claims 1 to 7, wherein the main and additional waveguide(s) are made from silicon.

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