Patent Description:
Travelling wave electro-optic modulators are well known in the field of electro-optic devices. During operation RF waves travel along transmission line electrode strips arranged on either side of optical waveguides. When these travelling waves reach the end of the electrode strips it is essential they are not reflected back along the electrode strips as this has an adverse effect on the operation of the device and on the electrical system driving it. A typical limit on normalised reflective RF power at the device input is -10dB. Scattering parameter S11 is conventionally used to denote this quantity.

It is known to terminate such electrode strips with a single resistive shunt path which is matched to the impedance of the electrode strips. This approach however has a number of drawbacks. The resistive element must be small enough to behave as a lumped element at the highest frequencies of interest. If integrated into the modulation device heat dissipation during operation must be properly managed. The presence of essentially a point heat source close to the optical waveguides can destabilise the operation of the electro-optic modulator. If external to the modulator such resistive elements need to be wire bonded to the ends of the electrode strips inevitably producing an RF discontinuity which causes reflections which rise with frequency. The addition of these external components to the electro-optic modulator increases the cost and complexity of manufacture of the electro-optic modulator.

<CIT>) discloses a Mach-Zehnder modulator which includes: first and second resistive elements each having first and second contact areas, the first and second contact areas of the second resistive element being arranged in a direction of a first axis, the first and second contact areas of the second resistive element being arranged in a direction of a second axis; a common conductor connecting the first contact areas of the first and second resistive elements with each other; first and second waveguide structures each including a waveguide portion extending in a direction of a third axis intersecting the first and second axes; a first signal conductor connected to the waveguide portion of the first waveguide structure and the second contact area of the first resistive element; and a second signal conductor connected to the waveguide portion of the second waveguide structure and the second contact area of the second resistive element.

The present invention seeks to overcome the problems of the prior art.

Accordingly, the present invention provides a travelling wave electro-optic modulator comprising.

The matched termination of the travelling wave electro-optic modulator according to the invention is a distributed structure. Heat can therefore be advantageously distributed over a wider area. Further, the matched termination can be manufactured entirely within the same foundry process as the rest of the device. The matched termination can therefore be included in the travelling wave electro-optic modulator during manufacture at no extra cost. Further, since the matched termination is manufactured integrally as part of the travelling wave electro-optic modulator there is no need to wire bond the matched termination to the waveguide strips. This eliminates any RF discontinuities and their associated reflection of RF travelling waves.

Preferably for at least one of the first and second electrodes the at least one portion is a T rail.

Preferably the serpentine strip is a metal strip.

Preferably the travelling wave electro-optic modulator further comprises a third electrode strip parallel to the first and second electrode strips.

Preferably the matched termination is further connected to the third electrode strip with the serpentine strip connecting the first, second and third electrode strips together.

Preferably the serpentine electrically conductive strip and backplane matching element are arranged in parallel spaced apart planes.

Preferably when viewed along a direction normal to the parallel spaced apart planes the serpentine electrically conductive strip and backplane matching element at least partially overlap.

Preferably each backplane arm is U shaped, at least a portion of the U overlapping with the serpentine electrically conductive strip when viewed along the direction normal to the parallel spaced apart planes.

Preferably at least one backplane arm comprises a backplane spur portion, at least a portion of the backplane spur portion overlapping with the serpentine electrically conductive strip when viewed along the direction normal to the parallel spaced apart planes.

Preferably the serpentine electrically conductive strip comprises a plurality of substantially parallel straight portions connected together by U shaped portions, at least some of the straight portions and U shaped portions overlapping the backplane matching element when viewed along the direction normal to the parallel spaced apart planes.

Preferably the serpentine electrically conductive strip and backplane matching element are configured such that the slope of the RF reflection coefficient as a function of frequency for one is substantially equal and opposite to that of the other at the point of crossover of the two.

Preferably the travelling wave electro-optic modulator further comprises an optical source connected to the optical waveguides.

Preferably the travelling wave electro-optic modulator further comprises an RF source connected to the first, second and third electrode strips.

Preferably the backplane matching element comprises an n doped AlGaAs portion arranged within the substrate.

The present invention will now be described by way of example only and not in any limitative sense with reference to the accompanying drawings in which.

Shown in <FIG> in schematic plan view is a travelling wave electro-optic modulator <NUM> according to the invention. The travelling wave electro-optic modulator <NUM> comprises an insulating or semiinsulating substrate <NUM>. Arranged on the substrate <NUM> are input and output optical waveguides <NUM>,<NUM> having first and second optical waveguides <NUM>,<NUM> extending therebetween. The first and second optical waveguides <NUM>,<NUM> are of substantially equal length. Arranged beneath the first and second optical waveguides <NUM>,<NUM> within the substrate <NUM> is a semiconductive backplane layer <NUM>. An optical source <NUM> is connected to the input optical waveguide <NUM>. Also arranged on the substrate are first and second parallel, spaced apart electrode strips <NUM>,<NUM>. The optical waveguides <NUM>,<NUM> are arranged between the first and second electrode strips <NUM>,<NUM> and extend parallel thereto as shown.

A third electrode strip <NUM> is also arranged on the substrate <NUM> parallel to the second electrode strip <NUM> and on the opposite side of the second electrode strip <NUM> to the optical waveguides <NUM>,<NUM>. Electrical connections <NUM> extend between the first and third electrode strips <NUM>,<NUM>.

A first end of each of the first, second and third electrode strips <NUM>,<NUM>,<NUM> is connected to an RF source <NUM>. The second opposite ends of each of the first, second and third electrode strips <NUM>,<NUM>,<NUM> is connected to a matched termination <NUM>.

The first electrode strip <NUM> comprises a plurality of electrically conductive portions <NUM> (in this embodiment T rails) which extend proximate to the first optical waveguide <NUM>. Similarly, the second electrode strip <NUM> comprises a plurality of electrically conductive portions <NUM> (again T rails) which extend to proximate to the second optical waveguide <NUM>.

In use the optical source <NUM> provides a coherent light beam to the input optical waveguide <NUM>. From here it is split approximately equally between the first and second optical waveguides <NUM>,<NUM>. The light travels along the first and second optical waveguides <NUM>,<NUM> before being recombined at the output optical waveguide <NUM>. If the light in the first optical waveguide <NUM> is in phase with the light in the second optical waveguide <NUM> at the point of recombination then the light in the two waveguides <NUM>,<NUM> combines constructively. However, if the light in the first optical waveguide <NUM> is not in phase with the light in the second optical waveguide <NUM> at the point of recombination then the light in the two waveguides <NUM>,<NUM> will not combine constructively. Accordingly, by controlling the phase relation between the light in the two waveguides <NUM>,<NUM> at the point of recombination one can control the intensity of the light output by the output optical waveguide <NUM>.

At the same time that the optical source <NUM> provides light to the input optical waveguide <NUM>, the RF source <NUM> provides an RF travelling wave to each of the first, second and third electrode strips <NUM>,<NUM>,<NUM>. The electromagnetic fields generated in the optical waveguides <NUM>,<NUM> by means of the T rails <NUM>,<NUM> interact with the light passing along the first and second optical waveguides <NUM>,<NUM> so altering the phase of the light in the first and second optical waveguides <NUM>,<NUM>. Accordingly, by supplying appropriate RF travelling waves to the first, second and third electrode strips <NUM>,<NUM>,<NUM> one can control the intensity of light output by the output optical waveguide <NUM>.

A problem arises when the RF travelling waves reach the end of the electrode strips <NUM>,<NUM>,<NUM>. If the travelling waves are reflected back towards the RF source <NUM> then these reflected travelling RF waves interfere with the incident RF travelling waves so negatively affecting the operation of the travelling wave electro-optic modulator <NUM>. In order to overcome this problem the travelling wave electro-optic modulator <NUM> comprises a matched termination <NUM> connected to the first, second and third electrode strips <NUM>,<NUM>,<NUM> and which has an impedance matched to the line impedance of the first, second and third electrode strips <NUM>,<NUM>,<NUM>, so eliminating any reflected travelling RF waves. The matched termination <NUM> is described in more detail below.

Shown in <FIG> is the travelling wave electro-optical modulator <NUM> of <FIG> in vertical cross section through line X of <FIG>. The substrate <NUM> comprises an insulating bottom GaAs layer <NUM>, an insulating bottom AlGaAs layer <NUM> arranged on the insulating bottom GaAs layer <NUM>, an insulating top GaAs layer <NUM> arranged on the insulating bottom AlGaAs layer <NUM> and an insulating top AlGaAs layer <NUM> arranged on the insulating top GaAs layer <NUM>. The first and second optical waveguides <NUM>,<NUM> are insulating AlGaAs and are arranged on the top GaAs layer <NUM> as shown. A portion <NUM> of the bottom AlGaAs layer <NUM> is n doped so that is it electrically semiconductive and forms the semiconductive backplane layer <NUM> arranged within the substrate <NUM> beneath the first and second optical waveguides <NUM>,<NUM> and extending therebetween.

The first, second and third waveguide strips <NUM>,<NUM>,<NUM> are arranged on the top AlGaAs layer <NUM>. The T rails <NUM>,<NUM> extend from the first and second electrode strips <NUM>,<NUM> onto the top faces of the optical waveguides <NUM>,<NUM> as shown.

<FIG> shows the ends of the first, second and third electrode strips <NUM>,<NUM>,<NUM> and the matched termination <NUM> connected thereto in perspective view. For clarity other elements of the travelling wave electro-optical modulator <NUM> are not shown.

The matched termination <NUM> comprises a serpentine, electrically conductive (in this case a metal) strip <NUM> arranged on the top AlGaAs layer <NUM>. A first portion of the serpentine strip <NUM> connects the first and second electrode strips <NUM>,<NUM> together. A second portion of the serpentine strip <NUM> connects the second and third electrode strips <NUM>,<NUM> together. The serpentine metallic strip <NUM> is typically a tri-metal evaporated structure of titanium, platinum and gold. The thickness of the serpentine metallic strip <NUM> is tightly controlled and hence is highly consistent. The serpentine metallic strip <NUM> comprises a plurality of straight portions <NUM> connected together by a plurality of U shaped portions <NUM>. The length of the straight portions <NUM>, to some degree, determines the resistance of the serpentine metallic strip <NUM> whilst the number of U shaped portions <NUM>, at least to some extent, determines its inductance.

The matched termination <NUM> further comprises a semiconductive backplane matching element <NUM> arranged within the substrate <NUM>. The backplane matching element <NUM> is formed by an n doped semiconductive portion of the bottom AlGaAs layer <NUM> surrounded at its edges by an insulating portion of the bottom AlGaAs layer <NUM>. The serpentine metallic strip <NUM> and the backplane matching element <NUM> are therefore arranged in parallel spaced apart planes separated by insulating material. The backplane matching element <NUM> is shaped as a plurality of semiconductive backplane plates <NUM> connected together by semiconductive U shaped backplane arms <NUM>.

The spatial relationship between the electrode strips <NUM>,<NUM>,<NUM> , the serpentine metallic strip <NUM> and backplane matching element <NUM> is best shown in <FIG> shows these elements viewed along a direction normal to the parallel spaced apart planes. For the serpentine metallic strip <NUM>, this extends beyond the ends of the electrode strips <NUM>,<NUM>,<NUM>. As for the backplane matching element <NUM> the backplane plates <NUM> are arranged underneath and spaced apart from the ends of the electrode strips <NUM>,<NUM>,<NUM> along the direction of view so that the electrode strips <NUM>,<NUM>,<NUM> and backplane plates <NUM> are capacitively coupled together. The backplane arms <NUM> extend beyond the ends of the electrode strips <NUM>,<NUM>,<NUM>. The backplane arms <NUM> and the serpentine metallic strip <NUM> are arranged such that when viewed along the direction normal to the parallel planes the backplane arms <NUM> and serpentine metallic strip <NUM> at least partially overlap in a coupling region <NUM>. In the coupling region <NUM> the serpentine metallic strip <NUM> is proximate to the backplane arms <NUM> and so the two are electrically coupled together.

<FIG> shows a similar view as <FIG> for a further embodiment of a travelling wave electro-optic modulator <NUM> according to the invention. Only the second and third electrode strips <NUM>,<NUM> are shown. This differs from the embodiment of <FIG> in that the backplane arm <NUM> comprises a backplane spur portion <NUM>. The backplane spur portion <NUM> overlaps with a portion of the serpentine metallic strip <NUM> which increases the size of the coupling region <NUM> and so increases the electrical coupling between the serpentine metallic strip <NUM> and the backplane matching element <NUM>.

Shown in <FIG> is the reflection coefficient as a function of frequency for a matched termination <NUM> comprising the serpentine metal strip <NUM> only, a matched termination <NUM> comprising the backplane matching element <NUM> only and a matched termination <NUM> comprising both the serpentine metal strip <NUM> and backplane matching element <NUM> according to the invention as described in <FIG>. These are curves A, B and C respectively.

Considering first the serpentine metallic strip <NUM> only, at DC and low frequencies the resistive behaviour of the serpentine metallic strip <NUM> dominates and so the reflectance is low. As the frequency increases however the inductive reactance of the serpentine metallic strip <NUM> dominates and the serpentine metallic strip <NUM> increasingly looks like an open circuit.

Considering the backplane matching element <NUM> only, the capacitive coupling between the backplane matching element <NUM> and the electrode strips <NUM>,<NUM>,<NUM> means that the backplane matching element <NUM> increasingly looks like an open circuit towards DC.

As can be seen from curve C a matched termination <NUM> according to the invention comprising both a serpentine metallic strip <NUM> and a backplane matching element <NUM> performs significantly better than either a serpentine metallic strip <NUM> alone or a backplane matching element <NUM> alone. This is due to the complex electromagnetic interaction between the serpentine metallic strip <NUM> and the backplane matching element <NUM> in the coupling region <NUM>.

Curve C is only one example of the behaviour of a matched termination <NUM> according to the invention. One can tune the behaviour of the matched termination <NUM> of the travelling wave electro-optic modulator <NUM> according to the invention by tuning the behaviour of either or both of the serpentine metallic strip <NUM> and the backplane matching element <NUM>. One can tune the behaviour of the serpentine metallic strip <NUM> by adjusting its geometry, in particular by changing the length of the straight sections <NUM> and the number of U sections <NUM>. One can tune the behaviour of the backplane matching element <NUM> by adjusting the area of the backplane plates <NUM> and/or the distance the backplane plates <NUM> extend along the electrode strips <NUM>,<NUM>,<NUM>. One can further tune the behaviour of the matched termination <NUM> by changing the size of the coupling region <NUM> again by changing the geometry of the serpentine metallic strip <NUM> or the geometry of the backplane arms <NUM> or by adding backplane spurs <NUM> to the backplane arms <NUM>. Generally speaking it is desired to tune the serpentine metallic strip <NUM> and the backplane matching element <NUM> so that the slope of the RF reflection coefficient as a function of frequency for one is substantially equal and opposite in sign to that of the other at the point of the crossover of the two (ie at the point where curves A and B cross).

Shown in <FIG> are the ends of the electrode strips <NUM>,<NUM> and matched termination <NUM> of a further embodiment of a travelling wave electro-optic modulator <NUM> according to the invention. This embodiment is similar to that described above except it comprises first and second electrode strips <NUM>,<NUM> only.

Claim 1:
A travelling wave electro-optic modulator (<NUM>) comprising
a substrate (<NUM>);
first and second parallel spaced apart electrode strips (<NUM>, <NUM>) arranged on the substrate (<NUM>);
first and second optical waveguides (<NUM>, <NUM>) arranged on the substrate (<NUM>), the optical waveguides (<NUM>, <NUM>) being positioned between the first and second electrode strips (<NUM>, <NUM>) and extending parallel thereto;
the first electrode strip (<NUM>) comprising at least one portion (<NUM>) extending proximate to the first optical waveguide (<NUM>);
the second electrode strip (<NUM>) comprising at least one portion (<NUM>) extending proximate to the second optical waveguide (<NUM>);
a semiconductive backplane layer (<NUM>) arranged within the substrate (<NUM>) and extending between the waveguides (<NUM>, <NUM>); and,
a matched termination (<NUM>) connected to the first and second electrode strips (<NUM>, <NUM>), the matched termination (<NUM>) comprising
(a) a serpentine electrically conductive strip (<NUM>) arranged on the substrate (<NUM>) and connecting the first and second electrode strips (<NUM>, <NUM>) together; and,
(b) a semiconductive backplane matching element (<NUM>), the backplane matching element (<NUM>) comprising a plurality of semiconductive backplane plates (<NUM>) connected together by at least one semiconductive backplane arm (<NUM>), the plates (<NUM>) and at least one backplane arm (<NUM>) being arranged within the substrate (<NUM>), the plates (<NUM>) being arranged proximate to the electrode strips (<NUM>, <NUM>) such that each electrode strip (<NUM>, <NUM>) is capacitively coupled to at least one backplane plate (<NUM>);
the serpentine electrically conductive strip (<NUM>) being arranged such that at least a portion of its length is proximate to at least one backplane arm (<NUM>) such that the two are electrically coupled together.