MULTI-MODE INTERFERENCE OPTICAL DEVICES

FIELD OF THE INVENTION

The present invention relates to integrated optical devices, and specifically relates to multi-mode interference (MMI) optical devices.

BACKGROUND

As telecommunications and data communications increasingly adopt optical fiber as a preferred communication medium, integrated electrical components, generally present in communications equipment, are being replaced by equivalent integrated optical components, including waveguides, optical amplifiers, splitters, etc.

To pass from optical fiber to integrated optical components, an optical signal is first received by a waveguide, often formed in the same medium on which the integrated optical components have been formed. The optical signal carried by a waveguide can be said to have at least one "mode", where a mode is one of number of possible patterns of propagating or evanescent electromagnetic fields that maintain their transverse field distribution during propagation. Each mode is characterized by frequency, polarization, electric field strength and magnetic field strength. The electromagnetic field pattern of a mode depends on the frequency of the signal and refractive indices, dielectric constants and geometry of the waveguide.

Multimode Interference (MMI) devices rely on a self-imaging property of multimode waveguides. Self-imaging is the property by which an input field pattern is reproduced in single or multiple images at periodic intervals along the propagation direction of the waveguide. This self-imaging is a result of the

(near quadratic) dependence of a propagation constant with the mode number.

An MMI coupler is an exemplary MMI device that consists of input waveguides and output waveguides separated by an MMI region. The MMI region supports a large number of modes that propagate with different phase velocities leading to periodic self-imaging. There may be multiple inputs to the MMI region and multiple outputs. The dimensions of the MMI region are selected based on the wavelength of the signal of interest and establish the nature of the optical signal on the outputs as a function of the signals on the inputs. For instance, an appropriately dimensioned MMI coupler may have a single input and divide the signal in that input such that the signal is split between two outputs.

MMI devices have become important components within the integrated optical circuits that require NxM power splitting structures, which distribute (not necessarily uniformly) the power of N inputs to M outputs. Exemplary integrated optical circuits that require power splitting structures include ring lasers, arrayed waveguide gratings, interferometric modulators and optical switches. A Mach-Zehnder modulator is another example of an integrated optical circuit that requires a splitting structure. The configuration of a power splitting structure in an interferometric modulator determines a power splitting ratio, where the power splitting ratio indicates a division of input power between a number of output powers, for instance, two output powers.

It would be advantageous to provide new devices for attenuating or modulating light and optical signals. Some known types of optical attenuators and modulators control the intensity of the propagated light by the mechanism of electro-absorption. Such an attenuator or modulator comprises a single- mode waveguide, with an electrode formed from a layer of metal deposited over the entire width of the waveguide along part of its length, and another electrode (e.g. a ground electrode) situated below the waveguide. Also, n and p doped regions of the device are situated on opposite sides of the waveguide adjacent to the respective electrodes. In use, light propagating along the

waveguide is attenuated or modulated by an electric field applied to the waveguide via the electrodes.

A typical profile of the absorption of light per unit length along the waveguide of a known attenuator or modulator is in the form of a decay curve, with a large peak at the input of the attenuator or modulator. Light absorption causes heat generation, and in some known devices the limiting design parameter has commonly been the amount of heat that can be locally dissipated out of the waveguide (principally into the substrate) at the input region of the device. Excess heat generation in the known devices can cause failure due to catastrophic optical damage ("COD") or at least reduced reliability (and thus reduced device lifetime) due to raised temperatures (causing enhanced deleterious diffusion of atoms within the structure, for example). In addition to COD, in known optical attenuator and modulator devices light absorption due to direct bandgap transitions commonly causes charge carriers (electrons and holes) to be generated. Even if heat is adequately managed, an accumulation of carriers (particularly holes) at high optical powers can cause undesirable effects, such as pattern dependent jitter.

Consequently, in order to maintain the optical absorption density in the input region of such known devices beneath the level at which damage or excessive carrier accumulation occurs, it is generally necessary to restrict the electrical voltage applied to the device and to make the device longer to compensate. (A typical length of such a device can be 100 to 500 μm.)

Thus, there is a need in the art for an improved device that overcomes the above problems, and which can (for example) be shorter, more reliable, and/or can be driven with a higher drive voltage than the prior art devices. The present invention seeks (among other things) to provide such a device.

SUMMARY

A first aspect of the present invention provides an optical device comprising a multi-mode interference (MMI) device having a single optical input and a single optical output, and a first electrode associated with the MMI device arranged to apply an electric field thereto, thereby to cause absorption of light propagating through the MMI device.

The invention has the advantage that by spreading the light within a multi-mode interference (MMI) device, the localized optical power absorption per unit plan area is reduced relative to a conventional single-mode waveguide device, thereby enabling a higher electrical drive voltage and/or current to be applied to the device and/or enabling the device to be shorter in length.

In some preferred embodiments of the invention, the optical device comprises an optical attenuator. More preferably, the device comprises a variable optical attenuator, and the absorption of light propagating through the MMI device is varied by varying the electric field applied to the MMI device. The electric field may advantageously be applied to the MMI device by a DC variable electric current supplied to the first electrode, for example.

Alternatively, the optical device according to the invention may comprise an optical modulator. Preferably, the electric field is applied to the MMI device by a modulating electric current supplied to the first electrode. Advantageously, the modulating electric current may be a radio frequency (RF) modulating electric current.

The first electrode preferably is situated on a surface of the MMI device.

The optical device preferably includes a substrate on which the MMI device is situated. It is generally preferred for the first electrode to be situated on a top surface of the MMI device remote from the substrate. The device may include a second electrode situated on the substrate, preferably on an opposite side of

the substrate to the MMI device. Advantageously, the electric field may be applied to the MMI device by the first and second electrodes.

In preferred embodiments of the invention, the MMI device is formed from semiconductor material. Preferably the device includes a doped region of semiconductor material situated adjacent to each electrode. For example, one of the doped regions may be an n-doped region, and the other doped region may be a p-doped region. An intrinsic region may be situated between the doped regions. Preferably, the electric field is applied by reverse-biasing the doped regions.

In some embodiments of the invention, the first electrode is situated on substantially the entire surface of the MMI device.

Although the spreading of the optical power within the MMI device reduces the optical power density and thus improves the thermal dissipation per unit length compared with a known single-mode waveguide device, the device will nonetheless still exhibit a peak in the absorption profile as a function of length close to the input of the MMI device, albeit at a reduced level. However, the present inventors have found that if a shaped first electrode is used, it is possible to further improve the performance of the device, by controlling the absorption of light within the MMI device.

Consequently, in some embodiments of the invention, the first electrode is situated on only part of the surface of the MMI device. For example, the first electrode may be absent from a part of the surface of the MMI device adjacent to the optical input and/or adjacent to the optical output. Additionally or alternatively, the first electrode may be absent from one or more parts of the surface of the MMI device between the optical input and the optical output. In this way, for example, the optical absorption per unit length along the length of the MMI device can be controlled by patterning the first electrode such that the optical overlap between light within the MMI device and the regions of the

MMI device that are driven by the first electrode varies. For example, the optical overlap may increase along the length of the device, thus substantially flattening the profile of the optical absorption per unit length (or at least making the profile flatter than it would have been).

Advantageously, the first electrode may be shaped in the form of a substantially symmetrical pattern comprising at least two substantially symmetrical portions thereof. For example, the MMI device may have a plane of symmetry oriented substantially perpendicular to the surface of the MMI device, and the plane of symmetry may also be a plane of symmetry for the first electrode. Preferably, the optical input and the optical output each lie on the plane of symmetry, at opposite ends of the MMI device. It should be appreciated that the light within the MMI device is concentrated at certain locations, due to it conforming to an interference pattern, and does not propagate along the MMI device with a uniform power density. The first electrode may have a shape substantially corresponding to a shape adopted by at least some regions of maximum intensity of light propagating through the MMI device in use. Consequently, the electrode may approximately "map" the profile of the light in at least these regions. Again, this patterning can be arranged so that the first electrode is absent from the region(s) of the very highest intensities of light, e.g. close to the optical input, thereby causing a flattening of the optical absorption profile.

Any such absence(s) of the electrode from the surface of the MMI device and/or any such patterning of the electrode on the surface of the MMI device, can have the advantage of enabling a reduction in the capacitance of the optical device, while substantially maintaining the effectiveness of the device, by maintaining (or at least minimizing any reduction in) the influence of the applied electric field on the light propagating through the MMI device. This advantage is particularly relevant where the optical device is a modulator, driven by a modulating electric current (potentially generating a high capacitance); it is less relevant where the optical device is an attenuator,

driven by a substantially DC variable electric current (potentially generating a low capacitance). The reduction in capacitance is advantageous because, without it, an electrode covering a whole MMI device would often be much larger and have a larger capacitance than that covering a single-mode waveguide. At high data modulation speeds (e.g. 10 and 40 Gbit/s etc.) this higher capacitance could become a significant problem.

Preferably the optical input of the optical device is a waveguide, especially a single-mode waveguide. The optical output may be a waveguide, especially a single-mode waveguide. The multi-mode interference (MMI) device is a multi-mode waveguide. The MMI device may, for example, be 2 to 4 times (e.g. approximately 3 times) wider than the widths of each of the input and output waveguides. For example, a single-mode input or output waveguide may have a width of approximately 2 μm, and the MMI device may have a width of approximately 6 μm. The MMI device and/or the input waveguide and/or the output waveguide may have any suitable form, but an example of a preferred form is that of a rib waveguide, comprising a substrate and a rib situated above the substrate.

It is to be understood that any aspect or feature of the invention may be an aspect or feature of any other aspect or feature of the invention.

Other aspects and features of the present invention will become apparent to those of ordinary skill in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

In the figures which illustrate example embodiments of this invention:

FIG. 1 (views (a) and (b)) illustrates an embodiment of an optical device according to the invention;

FIG. 2 illustrates another embodiment of an optical device according to the invention; and

FIG. 3 illustrates a further embodiment of an optical device according to the invention.

DETAILED DESCRIPTION

Figure 1 (views (a) and (b)) illustrates an embodiment of an optical device 900 according to the invention. The optical device 900 comprises a multi-mode interference (MMI) device 912 having a single optical input 908 and a single optical output 910. (The direction in which light propagates through the device 900, in use, is indicated by the arrow.) The input 908 and the output 910 are each single-mode waveguides, whereas the MMI device 912 is a multi-mode waveguide. A first electrode 904 is situated on substantially an entire top surface 916 of the MMI device 912 remote from a substrate 906 on which the MMI device is situated. A second electrode 902 is situated on the substrate 906, on an opposite side 914 of the substrate to the MMI device 912. (This opposite side 914 of the substrate 906 is the underside of the substrate as illustrated in Figure 1.) The first electrode 904 and the second electrode 902 are arranged to apply an electric field to the MMI device (i.e. the electric field is applied between the electrodes, across the depth of the MMI), thereby to cause absorption of light propagating through the MMI device in use. View (b) of Figure 1 is a partially exploded view, showing the first electrode 904 separated from the top surface 916 of the MMI device 912, for clarity.

The optical device 900 is formed from semiconductor material, that is, the substrate 906, the MMI device 912, and the input and output waveguides 908 and 910, are formed from semiconductor material. Preferably the device

900 includes a doped region of semiconductor material situated adjacent to each of the electrodes, i.e. at the top surface 916, and at the underside of the substrate 906. One of the doped regions may be an n-doped region, and the other doped region may be a p-doped region. An intrinsic region may be situated between the doped regions. Preferably, the electric field is applied by reverse-biasing the doped regions.

The optical device 900 illustrated in Figure 1 may be a variable optical attenuator or an optical modulator. For those embodiments in which the device 900 is a variable optical attenuator, the absorption of light propagating through the MMI device in use is varied by varying the electric field applied to the MMI device. The electric field may advantageously be applied to the MMI device by a DC variable electric current supplied to the first and second electrodes.

Alternatively, the optical device 900 illustrated in Figure 1 may be an optical modulator. In such embodiments of the invention, the electric field may be applied to the MMI device by a modulating electric current supplied to the first and second electrodes. Advantageously, the modulating electric current may be a radio frequency (RF) modulating electric current.

Figure 2 shows another embodiment of an optical device 1000 according to the invention. The device 1000 illustrated in Figure 2 is identical to the device 900 illustrated in Figure 1, except that the first electrode 1004 is situated on only part of the top surface 1006 of the MMI device. In particular, the first electrode is absent from a part of the surface of the MMI device adjacent to the optical input 908, and widens from a narrow front region 1008 to a rear region 1010 covering substantially the entire width of the MMI device 912. This widening out (or fanning out) of the electrode 1004 corresponds to a widening out (or fanning out) of the light as it propagates into and through a front region of the MMI device. In this way, the optical absorption per unit length along the length of the MMI device is controlled. In particular, the

absence of the first electrode 1004 from the MMI device adjacent to the optical input 908 avoids a large peak in the optical absorption profile where the light intensity is especially great (i.e. adjacent to the input). Also, because the first electrode 1004 widens out from the narrow front region 1008, the overlap between the electric field and the light increases along the length of the MMI device, thus substantially flattening the profile of the optical absorption per unit length (or at least making the profile flatter than it would have been).

Figure 3 is a plan view illustration of a further embodiment of an MMI device 912 of an optical device according to the invention. The optical device of Figure 3 is identical to the optical devices 900 and 1000 illustrated in figures 1 and 2, with the exception that the shape of the first electrode 1104 is different to the shapes of the first electrodes 904 and 1004. In the Figure 3 embodiment of the invention, the first electrode 1104 is absent from a part of the surface of the MMI device 912 adjacent to the optical input 908, and is also absent from a part of the surface of the MMI device adjacent to the optical output 910. Additionally, the first electrode 1104 is shaped in the form of a substantially symmetrical pattern comprising main portions 1110, 1112, 1114 and 1116, interconnected by portions 1118, 1120 and 1122. The MMI device 912 has a plane of symmetry A-A oriented substantially perpendicular to the surface of the MMI device, and the plane of symmetry is also a plane of symmetry for the first electrode 1104. The optical input 908 and the optical output 910 each lie on the plane of symmetry A-A, at opposite ends of the MMI device. With this arrangement, the first electrode 1104 has a shape substantially corresponding to a shape adopted by some regions of maximum intensity of the light propagating through the MMI device in use. Consequently, the electrode approximately "maps" the profile of the light in these regions.

The absences of the first electrode 1104 from the surface of the MMI device and the patterning of the first electrode on the surface of the MMI device, have the advantage of enabling a reduction in the capacitance of the

optical device, while substantially maintaining the effectiveness of the device, by substantially maintaining (or at least minimizing any reduction in) the influence of the applied electric field on the light propagating through the MMI device. The reduction in capacitance is advantageous because, without it, an electrode covering a whole MMI device would often be much larger and have a larger capacitance than that covering a single-mode waveguide. At high data modulation speeds (e.g. 10 and 40 Gbit/s etc.) this higher capacitance could become a significant problem.

Other modifications will be apparent to those skilled in the art and, therefore, the invention is defined in the claims.