GB2558307A

GB2558307A - Waveguide array

Info

Publication number: GB2558307A
Application number: GB1622436.2A
Authority: GB
Inventors: Dell'orto Flavio; Balsamo Stefano; Vergani Paolo
Original assignee: Oclaro Technology Ltd
Current assignee: Lumentum Technology UK Ltd
Priority date: 2016-12-30
Filing date: 2016-12-30
Publication date: 2018-07-11
Anticipated expiration: 2036-12-30
Also published as: GB2558307B; GB201622436D0; WO2018122551A1; GB2558307A8; US11402673B2; CN110140080A; CN110140080B; US20190346706A1

Abstract

A radio frequency RF waveguide array that comprises a substrate with a plurality of optical waveguides, an electrical RF transmission line array is located on a face of the substrate and comprises a plurality of signal electrodes (Signal[1 4]) and a plurality of ground electrodes (Gnd[1 - 5]) wherein each signal electrode is positioned to provide a signal to two respective waveguides and at least one intermediate ground electrode is positioned between a pair of signal electrodes and each intermediate ground electrode includes a portion extending into the substrate. The downwardly extending portion acts to limit the spread of the electrical field and to reduce interference between adjacent field lines.

Description

(71) Applicant(s):

Oclaro Technology Pic (Incorporated in the United Kingdom)

Caswell, TOWCESTER, Northamptonshire, NN12 8EQ, United Kingdom (56) Documents Cited:

US 20100329600 A1 US 20100034496 A1

US 20100209040 A1 US 20020141679 A1 (58) Field of Search:

INT CL G02B, G02F

Other: EPODOC, WPI, Patent Fulltext (72) Inventor(s):

Flavio Dell'Orto Stefano Balsamo Paolo Vergani (74) Agent and/or Address for Service:

Marks & Clerk LLP

Fletcher House (2nd Floor), Heatley Road,

The Oxford Science Park, OXFORD, ΟΧ4 4GE, United Kingdom (54) Title of the Invention: Waveguide array

Abstract Title: A waveguide array in which a ground electrode extends downwardly into a substrate (57) A radio frequency RF waveguide array that comprises a substrate with a plurality of optical waveguides, an electrical RF transmission line array is located on a face of the substrate and comprises a plurality of signal electrodes (Signal[1 - 4]) and a plurality of ground electrodes (Gnd[1 - 5]) wherein each signal electrode is positioned to provide a signal to two respective waveguides and at least one intermediate ground electrode is positioned between a pair of signal electrodes and each intermediate ground electrode includes a portion extending into the substrate. The downwardly extending portion acts to limit the spread of the electrical field and to reduce interference between adjacent field lines.

Figure 4

At least one drawing originally filed was informal and the print reproduced here is taken from a later filed formal copy.

1/4

04 18

205 204

205 204 205

210

220

Figure 2

300

Figure 3

2/4

04 18

Figure 4

Figure 5

3/4

04 18

Figure 8B

4/4

900

Figure 9

04 18

WAVEGUIDE ARRAY

Field of the Invention

The invention relates to components for use in RF optical apparatus. In particular the invention relates to a waveguide array, for example for use in a dual parallel modulator.

Background

A dual parallel l/Q (in phase/quadrature) modulator typically has the structure schematically shown in Figure 1. The signal enters a splitter 101, which divides the signal into each channel of four Mach-Zehnder modulators 110, each of which comprises two waveguides 111, and an electrical RF (radio frequency) transmission line 112. The modulators apply the required modulation, and the signal is recombined by a combiner 102.

The Mach-Zehnder modulator has a cross section along A as shown in Figure 2. The waveguide section of the Mach-Zehnder modulator comprises a substrate 201, which contains waveguides 202. The RF transmission line array 203 is placed on one face of the substrate. Each RF transmission line comprises a signal electrode 204. Each signal electrode 204 has a ground electrode 205 on either side. The modulators may either be arranged x-cut (210), with the waveguides positioned below the gaps between the signal and ground electrodes (symmetrically around the signal electrode), or z-cut (220) with one waveguide beneath the signal electrode, and one beneath one of the ground electrodes. Normally, all modulators in an array would be the same type, but both z- and x-cut modulators are shown in Figure 2 for illustration. Ground electrodes are typically shared between adjacent RF transmission lines.

Summary

In accordance with one aspect of the present invention there is provided an RF waveguide array. The array comprises a substrate comprising a plurality of optical waveguides, each waveguide being elongate in a first direction. An electrical RF transmission line array is located on a face of the substrate and comprises a plurality of signal electrodes and a plurality of ground electrodes, each electrode extending in the first direction. Each signal electrode is positioned to provide a signal to two respective waveguides. The ground electrodes include at least one intermediate ground electrode positioned between each pair of signal electrodes. Each intermediate ground electrode includes a portion extending into the substrate.

Further aspects and preferred features are set out in claim 2 et seq.

Brief Description of the Drawings

Figure 1 is a schematic representation of a dual parallel l/Q (in phase/guadrature) modulator.

Figure 2 is a cross section through the modulator of Figure 1.

Figure 3 is a schematic illustration of electric field lines in the modulator of Figure 2. Figure 4 is a schematic representation of an RF waveguide array.

Figure 5 is a schematic representation of an alternative RF waveguide array.

Figure 6 is a schematic representation of a further alternative RF waveguide array. Figure 7 is a schematic representation of a further alternative RF waveguide array. Figures 8A and 8B are graphs illustrating the cross-talk and S21 curve of the RF transmission array.

Figure 9 is a schematic illustration of an RF waveguide array showing alternative arrangements for downwardly extending portions of ground electrodes.

Detailed Description “Length” and “along” are used herein to refer to distance in the direction of travel of the RF signals in the RF transmission lines - i.e. “out of the page” in Figure 2.

“Height”, “up”, and “down” are used herein to refer to distance in a direction perpendicular to the face of the substrate to which the RF transmission lines are attached - i.e. vertically in Figure 2, with “down” being towards the substrate.

“Width” and “across” are used to refer to distance in a direction perpendicular to both height and length - i.e. horizontally in Figure 2, unless otherwise specified.

Figure 3 shows the electric field line distribution of the RF waveguide array shown in Figure 2 (the substrate and waveguides are omitted for clarity). Signal electrodes are labelled 311, 312, 313, 314 from left to right, ground electrodes are labelled 321, 322, 323, 324, 325 from left to right. 321 and 325 are edge ground electrodes, 322, 323 and

324 are intermediate ground electrodes. As can be seen from the field lines 300, the signal from the signal electrode 312 extends through the ground electrodes 322 and 323, and to the nearest other signal electrodes 311 and 313. The wide electrical field distribution gives rise to high frequency losses - even in the case of a single transmission line - and the spread of the field to adjacent lines causes unwanted “crosstalk”, i.e. interference on one line caused by another.

An electrode structure for reducing the crosstalk is proposed below and shown in Figures 4, 5, 6 and 7. While the figures are shown for a z-cut transmission line array, the same principle applies for an x-cut array. In order to limit the spread of the electrical field, each ground electrode is provided with a downwardly extending portion, which extends into the substrate. This may be achieved by creating a “trench” in the substrate and covering the walls of the trench with metal (e.g. by sputtering) or filling the trench with metal. The metal which is used to cover or fill the trench may be the same metal as the electrodes, or a different metal.

Figure 4 shows an RF waveguide array having electrodes with a downwardly extending portion formed by filling a trench in the substrate with metal. Figure 5 shows a similar arrangement, with each ground electrode having a reduced height portion to save on material costs. Figure 6 shows an embodiment where electrodes having a reduced height portion are combined with a downwardly extending portion formed by sputtering metal into a trench of the substrate. The sputtering may take place before or after the main body of the ground electrode is applied to the substrate. Figure 7 shows an embodiment similar to figure 6, except that the metal used for the sputtering is not the same as that used for the rest of the electrode.

Providing the downwardly extending portion results in significant improvements to the cross-talk and S21 curve of the RF transmission line array, as shown in Figure 8A and B (where the arrow indicates increasing trench depth).

The downwardly extending portion may be in the centre of the ground electrode, i.e. equidistant from the transmission lines adjacent to the ground electrode, or it may be located off-centre. It may be that a downwardly extending portion is only provided in intermediate ground electrodes, i.e. in those which are between a pair of transmission lines (rather than at the edge of the array). Where there are multiple intermediate ground electrodes between a given pair of adjacent signal electrodes, the downwardly extending portion may be provided in one or more of them. As an example, if there are two intermediate ground electrodes between each transmission line, then each may have a downwardly extending portion which covers a wall of a trench between them, and the bottom of the trench may be non-conducting.

The downwardly extending portion of the electrode may be provided along the whole length of the electrode, or only along a portion or several portions of that length. This is shown schematically in Figure 9, which is a plan view showing the signal electrodes 910 and the ground electrodes 900-903. The locations of portions of the ground electrodes 900-903 which extend downwards are shown shaded. The nonintermediate ground electrodes 900 do not include portions extending downwards in this example. One ground electrode 901 has a portion extending downwards along the whole length of the electrode. The other ground electrodes 902 and 903 have several portions which extend downwards, each in a different portion of the length of the electrode. While each intermediate ground electrode is shown with a different arrangement in the figure, it is expected that all ground electrodes would use the same arrangement in practice - although the gaps between portions extending downwards may be offset.

The substrate is typically formed from lithium niobate (LiNbO₃), although it will be appreciated that this approach is not limited to LiNbO₃ and may be applicable to other materials, including semiconductors (such as, for example, indium phosphide or silicon). The trench may be formed by laser ablation, or any other suitable etching process. The performance improves with the depth of the downwardly extending portion, though there is obviously a practical limit close to the depth of the substrate. The trench depth may be, for example, at least 10 microns, at least 25 microns, at least 50 microns, or at least 100 microns. The downwardly extending portion of the ground electrode may be the same depth as the trench, or may extend only part-way down the trench.

An RF waveguide array incorporating downwardly extending portions in the ground electrode may be used as part of a dual parallel in-phase/quadrature, l/Q, modulator, where the waveguides associated with each signal electrode are configured to form a Mach-Zehnder modulator.

Claims

CLAIMS:

1. A radio frequency, RF, waveguide array, the array comprising:

a substrate comprising a plurality of optical waveguides, each waveguide being elongate in a first direction;

an electrical RF transmission line array located on a face of the substrate and comprising:

a plurality of signal electrodes; and a plurality of ground electrodes;

each electrode extending in the first direction;

wherein:

each signal electrode is positioned to provide a signal to two respective waveguides;

the ground electrodes include at least one intermediate ground electrode positioned between each pair of signal electrodes;

each intermediate ground electrode includes a portion extending into the substrate.

2. An RF waveguide array according to claim 1, wherein the portion extending into the substrate comprises a coating on a wall of a trench in the substrate.

3. An RF waveguide array according to claim 1, wherein the portion extending into the substrate comprises a portion which fills a trench in the substrate.

4. An RF waveguide array according to any preceding claim, wherein the portion extending into the substrate is made from a metal which is different to a metal of another portion of the intermediate ground electrode.

5. An RF waveguide array according to any preceding claim, wherein the portion extending into the substrate extends at least 10 microns into the substrate, more preferably at least 25 microns, more preferably at least 50 microns.

6. An RF waveguide array according to any preceding claim, wherein the portion extending into the substrate is positioned equidistant between each signal electrode of the respective pair of signal electrodes.

7. An RF waveguide array according to any preceding claim, wherein the portion extending into the substrate extends along the length of the intermediate ground electrode.

8. An RF waveguide array according to any of claims 1 to 6, wherein each intermediate ground electrode comprises multiple portions extending into the substrate, each of the portions extending along a discrete section of a length of the intermediate ground electrode.

9. A dual parallel in-phase/quadrature, l/Q, modulator comprising an RF waveguide array according to any preceding claim, wherein the waveguides associated with each signal electrode are configured to form a Mach-Zehnder modulator.

10. A method of manufacturing an RF waveguide array, the method comprising: providing a substrate comprising a plurality of pairs of optical waveguides, each waveguide being elongate in a first direction;

etching a plurality of trenches in a face of the substrate between each pair of optical waveguides, each trench being elongate in the first direction; coating walls of the trenches with a metal or filling the trenches with a metal; applying a plurality of signal electrodes to the face of the substrate, each signal electrode extending in the first direction and being positioned so as to provide a signal to a pair of respective waveguides;

applying a plurality of intermediate ground electrodes to the substrate, each intermediate ground electrode extending in the first direction, being located between two signal electrodes, and being coupled to the metal in the trenches, such that said metal forms a portion of the intermediate ground electrode extending into the substrate.

11. A method according to claim 10, wherein the step of coating walls of the trenches comprises sputtering the metal onto the walls of the trenches.

Intellectual

Property

Office

GB 1622436.2

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