WO2002050606A1

WO2002050606A1 - Optical modulator_______________________________________________

Info

Publication number: WO2002050606A1
Application number: PCT/GB2001/005582
Authority: WO
Inventors: Eric Mark Kimber; Robert G. Walker
Original assignee: Bookham Technology Plc
Priority date: 2000-12-21
Filing date: 2001-12-18
Publication date: 2002-06-27
Also published as: GB0031241D0; GB2370369B; GB2370369A

Abstract

An optical modulator 1 has an optical input signal waveguide 2, first and second interferometric waveguide arms 6, 8, a modulated output signal waveguide 12, a splitter 4 for dividing the input optical signal between the first and second arms 6, 8, and a combiner 10 for re-combining the signals transmitted along each of the first and second arms 6, 8 into the output signal waveguide 12. The modulator 1 further has first and second electrodes 14, 16 each of a segmented structure having a plurality of elements 18, 20 each extending from a respective one of the first and second electrodes 14, 16. The elements 18, 20 of each of the first and second electrodes 14, 16 each have a first part 30, 32 positioned such as to have an electro optic effect on a signal transmitted through the corresponding one of the first and second arms 6, 8 and at least a proportion of the elements 18, 20 of the first electrode 14 each have a second part 34 positioned such as to have limited or no electro optic effect on a signal transmitted through the first arm 6.

Description

OPTICAL MODULATOR

The invention relates to optical modulators and in particular to modulators having the capability to introduce a controlled amount of chirp into a modulated output signal. The invention also relates to devices and networks incorporating such modulators

Optical modulators are extensively used in optical communications networks. At the modulator, an input optical signal from a suitable source, for instance, a semiconductor laser diode, may be modulated according to an electrical signal to produce an optical data signal for transmission over the network. Semiconductor Mach-Zehnder modulators are especially suitable for producing high bit rate modulated optical data signals for optical networks.

Mach-Zehnder modulators typically comprise a splitter for dividing an incoming optical signal on an input waveguide equally between two interferometric waveguide arms. Each arm is located within an electrode structure to which an RF signal is applied. Downstream of the arms is a suitable combiner for re-combining each of the signals transmitted through the arms into a single modulated output signal. Mach- Zehnder modulators may be fabricated in gallium arsenide with aluminium gallium arsenide and gallium arsenide for the waveguides.

The electrode structure of a Mach-Zehnder modulator is so arranged that the RF signal applied thereto has an electro optic effect on the two interferometric arms. As a result, a phase shift may be induced in the signals transmitted through the arms. According to the manner of handling the RF signal, for instance in a push-pull mode, the relative phase shifts induced in the signals in each of the arms can be such as to produce either constructive or destructive interference on re-combination. For example, if no voltage is applied to the electrode structure, no phase shift is induced in the signals passing through either of the arms and the signals will interfere constructively on recombining to provide a "1" or ON output. On the other hand, if the voltage is so applied as to induce a relative 180° phase shift between the two signals, destructive interference occurs on re-combination, which will dictate a "0" or OFF output. Hence, phase shift may be manipulated to produce an amplitude modulated output.

The electrode structure of a Mach-Zehnder modulator may be of a so-called lumped element type wherein an RF signal is applied to a single electrode which interacts electro optically with the two interferometric arms. It is also known to use electrodes termed segmented element electrodes wherein an electrode or one electrode for each arm is segmented into a number of elements, each extending from a respective location along the electrode and sampling the voltage at that location. Typically, there will be of the order of 20-200 elements per electrode. Each element has apart at a position adjacent to a corresponding one of the arms, at or over it, so as to electro optically interact with a length of the corresponding arm. The interaction between each element part and the length of the corresponding arm induces a phase shift in the signal transmitted through it. The total phase shift induced in the signal is the sum of the phase shifts contributed by each of the parts. Each element behaves as a capacitor, and the total capacitance presented to the RF signal is the sum of the individual capacitance contribution from each element.

In a balanced "chirp free" dual electrode modulator, each electrode may be segmented into an equal number of elements extending from correspondingly the same locations along each electrode and such that the elements contribute a periodical, symmetrical capacitance structure to the electrodes. The two electrodes may be back connected to provide a series push-pull configuration, and because of the periodical structure the RF signal is split equally in magnitude between the two electrodes.

Chirp is the term given to an amount of phase shift in a signal. In optical networks, the transmission of optical data signals over long distances, which is desirable to minimise the requirements for further processing, may be limited by effects such as dispersion. It is known that introducing chirp is effective in compensating for dispersion in long haul optical signals.

Narious Mach-Zehnder modulators have been proposed, which include the capability to dictate an amount of chirp in the modulated output signal for the purpose of overcoming such limitations as dispersion.

An object of the invention is to provide an optical modulator with the capability to introduce a controlled amount of chirp into a modulated output signal. A further object is to provide a device and network incorporating such an optical modulator.

According to a first aspect, the invention provides an optical modulator comprising an optical input signal waveguide, first and second interferometric waveguide arms, a modulated output signal waveguide, a splitter for dividing the input optical signal between the first and second arms, a combiner for re-combining the signals transmitted along each of the first and second arms into the output signal waveguide, first and second electrodes each of a segmented structure having a plurality of elements each extending from a respective one of the first and second electrodes, wherein the elements of each of the first and second electrodes each have a first part positioned such as to have an electro optic effect on a signal transmitted through the corresponding one of the first and second arms and wherein at least a proportion of the elements of the first electrode each have a second part positioned such as to have limited or no electro optic effect on a signal transmitted through the first arm.

Thus, the elements of the first electrode have an active first part, positioned at or over the first arm, and a passive second part, positioned alongside the first arm, of only limited electro optic effect, in the sense of having less of an effect if it were positioned at or over the first arm, or no electro optic effect. Each element of each of the first and second electrodes behaves as a capacitor. The capacitance of each element of the first electrode which has first and second parts is split between its parts. Hence the sum total capacitance of the second electrodes is at or over the second arm whereas the total capacitance of the first electrode is not at or over the first arm; it is split between the first and second parts. The electric field effect of each first part of each element is a product of the capacitance, and the electric field effect determines the electo optic effect of each part. Hence, the electro optic effect on each signal is the sum of the electro optic effect of the first parts at or over each arm. With the sum total capacitance of the first parts of the first electrode less than the sum total of the capacitance of the second parts, there will be an imbalance of electro optic effect achieved. This produces an overall phase shift imbalance in the output signal on recombination at the combiner. The degree of imbalance is dictated by the total number of elements and the split of the capacitance between the first and second parts of the first elements.

Preferably, each of the first and second electrodes presents the same capacitance to the RF signal. Such balancing of the capacitance structure is beneficial in terms of RF signal performance and facilitates the device design process. Further preferably, each element of the first electrode has the same capacitance of each element of the second electrode, so as to present a symmetrical capacitance structure.

Also preferably, all of the elements of the first electrode each have a second part.

The second parts could be positioned at or over a dummy waveguide or an additional land. Also further preferably, each of the second parts has the same dimensions as the corresponding first part, that is, the first part of the same element, although they could feasibly be wider or longer or both, or proportioned in any manner which achieves an appropriate split of properties between the first and second parts.

According to a second aspect, the invention provides an optical communications network device including an optical modulator comprising an optical input signal waveguide, first and second interferometric waveguide arms, a modulated output signal waveguide, a splitter for dividing the input optical signal between the first and second arms, a combiner for re-combining the signals transmitted along each of the first and second arms into the output signal waveguide, first and second electrodes each of a segmented structure having a plurality of elements each extending from a respective one of the first and second electrodes, wherein the elements of each of the first and second electrodes each have a first part positioned such as to have an electro optic effect on a signal transmitted through the corresponding one of the first and second arms and wherein at least a proportion of the elements of the first electrode each have a second part positioned such as to have limited or no electro optic effect on a signal transmitted through the first arm.

According to a third aspect, the invention provides an optical communications network in which optical signals are modulated using an optical modulator comprising an optical input signal waveguide, first and second interferometric waveguide arms, a modulated output signal waveguide, a splitter for dividing the input optical signal between the first and second arms, a combiner for re-combining the signals transmitted along each of the first and second arms into the output signal waveguide, first and second electrodes each of a segmented structure having a plurality of elements each extending from a respective one of the first and second electrodes, wherein each of the first and second electrodes each have a first part positioned such as to have an electro optic effect on a signal transmitted through the corresponding one of the first and second arms and wherein at least a proportion of the elements of the first electrode each have a second part positioned such as to have limited or no electro optic effect on a signal transmitted through the first arm.

The invention will now be described by way of example with reference to the drawings in which:

Figure 1 is a schematic plan view of the basic layout of an optical modulator according to the invention.

With reference to figure 1 , an optical network transmitter module indicated generally at 1 includes a modulator 3 and a laser diode source S. The module 1 may also include, for example, an etalon locker and a variable optical attenuator, but description of these or other well known components has been omitted as unnecessary for the purpose of understanding the present invention. An optical signal from a laser diode source S is input on an input optical waveguide 2 to a splitter 4. The input signal is divided by the splitter 4 between each of first and second interferometric waveguide arms 6, 8 respectively. The divided signals transmitted through each of the arms 6, 8 respectively are re-combined at combiner 10 to provide a modulated output signal on an output optical waveguide 12.

Each of the first and second interferometric waveguide arms 6, 8 has associated with it first and second electrodes 14, 16 respectively. Although not shown in the figure, the electrodes 14, 16 are each set above a virtual ground plane (not shown). Each of the first and second electrodes 14, 16 is capable of producing a phase shift in the signal transmitted along the corresponding arm 6, 8 respectively: The phase shifts produced by each of the electrodes 14, 16 are not equal and opposite so that there is a residual phase shift when the two signals are re-combined at the combiner 10.

Each electrode 14, 16 is segmented into a plurality elements 18, 20 respectively each of which extends transversely from the corresponding one of the first and second electrodes 14, 16. Only four elements 18, 20 are shown in the figure although in reality the modulator would have many more. Each of the elements 18, 20 has a first part 30, 32 respectively positioned at or over the corresponding arm 14, 16. Each of the elements 18 of the first electrode 14 has a second part 34 which is positioned alongside the first arm.

Each element 18, 20 behaves as a capacitor and the total capacitance presented to the RF signal carried by each electrode 14, 16 is the sum of the individual capacitances of each element 18, 20. The capacitance of each of the elements 18 of the first electrode 14 is the series sum of the individual capacitances of each of the first and second parts 30, 34. In the preferred embodiment illustrated, this sum is equal to the capacitance of each of the first parts 32 of each of the elements 20 of the second electrode 16. Hence, the segmented structure is symmetrical to the extent that each electrode 14, 16 has the same number of elements 18, 20 at correspondingly the same longitudinal positions along the other electrode 16, 14, each with the same capacitance value. Consequently, in the preferred embodiment illustrated, the electrodes 14, 16 have a periodical, symmetrical capacitance structure, with each electrode 14, 16 presenting the same capacitance to the RF signal.

The difference between the elements 18 of the first electrode 14 and the elements 20 of the second electrode 16 is their electro optic effectiveness. The total capacitance of each element 18 of the first electrode is split between the first and second parts 30, 34. Moreover, the first part 30 is located at or above the first arm 6 whereas the second part 34 is located alongside the arm 6. The first part 30 is active in the sense that its position gives rise to it having an electro optic effect on the signal transmitted through the arm 6. The second part 34 is passive because, in the illustrated embodiment, its position dictates that it will not have an electro optic effect on the arm 6. The extent of the electro optic effect achieved by each element 18 is dictated by the electric field effect of each element which is a product of the capacitance of that element 18. Hence, as only part of the capacitance of the element 18 of the first . electrode 14 is contributing to the electro optic effect, in comparison to each element 20 for which the total capacitance is contributing to the electro optic effect.

Accordingly, there is an imbalance between corresponding elements 18, 20 in each of the first and second arms 6, 8. Each of these imbalances adds together to generate a total greater overall electro optic effect in one arm 8 than in the other 6. Thus, there will be an imbalance in the phase shift induced in the signals transmitted in each of the arms 6, 8. In other words, on recombination, there will be a residual phase shift or chirp in the output signal.

Claims

1. An optical modulator comprising an optical input signal waveguide, first and second interferometric waveguide arms, a modulated output signal waveguide, a splitter for dividing the input optical signal between the first and second arms, a combiner for re-combining the signals transmitted along each of the first and second arms into the output signal waveguide, first and second electrodes each of a segmented structure having a plurality of elements each extending from a respective one of the first and second electrodes, wherein the elements of each of the first and second electrodes each have a first part positioned such as to have an electro optic effect on a signal transmitted through the corresponding one of the first and second arms and wherein at least a proportion of the elements of the first electrode each have a second part positioned such as to have limited or no electro optic effect on a signal transmitted through the first arm

2. An optical modulator according to claim 1 wherein each first part is positioned at or over a corresponding one of the first or second arms and each second part is positioned alongside the first arm.

3. An optical modulator according to claim 1 wherein at least one second part is positioned at or over a dummy waveguide or land.

4. An optical modulator according to claim 1 wherein at least one second part is of substantially the same or different dimensions to the or each corresponding first part.

5. An optical modulator according to claim 1 wherein all of the elements of the first electrode have a second part.

6. An optical modulator according to claim 1 wherein each of the first and second electrodes presents the same capacitance.

6. An optical modulator according to claim 1 wherein the capacitance of each element of the first electrode is equal to the capacitance of each element of the second electrode.

7. An optical modulator according to claim 1 wherein the capacitance of those elements of the first electrode which have a second part is split between the first and second parts.

8. An optical communications network device including an optical modulator comprising an optical input signal waveguide, first and second interferometric waveguide arms, a modulated output signal waveguide, a splitter for dividing the input optical signal between the first and second arms, a combiner for re- combining the signals transmitted along each of the first and second arms into the output signal waveguide, first and second electrodes each of a segmented structure having a plurality of elements each extending from a respective one of the first and second electrodes, wherein the elements of each of the first and second electrodes each have a first part positioned such as to have an electro optic effect on a signal transmitted through the corresponding one of the first and second arms and wherein at least a proportion of the elements of the first electrode each have a second part positioned such as to have limited or no electro optic effect on a signal transmitted through the first arm.

9. An optical communications network in which optical signals are modulated using an optical modulator comprising an optical input signal waveguide, first and second interferometric waveguide arms, a modulated output signal waveguide, a splitter for dividing the input optical signal between the first and second arms, a combiner for re-combining the signals transmitted along each of the first and second arms into the output signal waveguide, first and second electrodes each of a segmented structure having a plurality of elements each extending from a respective one of the first and second electrodes, wherein the elements of each of the first and second electrodes each have a first part positioned such as to have an electro optic effect on a signal transmitted through the corresponding one of the first and second arms and wherein at least a proportion of the elements of the first electrode each have a second part positioned such as to have limited or no electro optic effect on a signal transmitted through the first arm.