DISPERSIVE OPTICAL DEVICE

Abstract: A dispersive optical device comprises an input optical coupler (20''); an input waveguide (10'') arranged to launch light into the input optical coupler at a launching position with respect to the input optical coupler; an output optical coupler; one or more output waveguides arranged to receive light from the output optical coupler; and a plurality of transmission waveguides (30'') arranged to receive light from the input optical coupler and direct it to the output optical coupler, the transmission waveguides generally converging towards the input optical coupler so that optical paths through the input optical coupler, which follow the axes of the transmission waveguides at their interface with the input optical coupler, intersect at an intersection point; in which the launching point is displaced from the intersection point in the direction of the axes of the input waveguide.

Description: DISPERSIVE OPTICAL DEVICE This invention relates to dispersive optical devices. Data transmission in optical fibres is generally limited by power loss and pulse dispersion. The advent of erbium-doped fibre amplifiers (EDFAs) has effectively removed the loss limitation for systems operating in the third optical communication window (around a wavelength of about 1.55μm (micrometer)), leaving pulse dispersion as a serious limitation, especially in future high-capacity multi-wavelength optical networks. More importantly, most fibre which has already been installed for telecommunication links (i.e. standard non- dispersion shifted fibre) exhibits a dispersion zero around 1.3μm and thus exhibits high (about 17ps/nm.km (picosecond per nanometre-kilometre)) dispersion around 1.55μm. Upgrading this fibre to higher bit rates involves the use of EDFAs and a shift in operating wavelength to 1.55μm where dispersion-compensation, using some sort of dispersive optical device or other technique becomes a necessity. Several techniques have been demonstrated including laser pre-chirping, mid- span spectral-inversion (phase-conjugation), the addition of highly-dispersive compensating fibre and chirped fibre gratings. In another aspect of the development of optical networks, a technology known as dense wavelength division multiplexing (DWDM) is being extensively investigated. DWDM employs many closely spaced optical carrier wavelengths, multiplexed together onto a single waveguide such as an optical fibre. The carrier wavelengths are spaced apart by as little as 50 GHz in a spacing arrangement defined by an ITU (International Telecommunications Union) channel "grid". Each carrier wavelength may be modulated to provide a respective data transmission channel. By using many channels, the data rate of each channel can be kept down to a manageable level, so avoiding the need for expensive very high data rate optical transmitters, optical receivers and associated electronics. It has been proposed that DWDM can make better use of the inherent bandwidth of an optical fibre link, including links which have already been installed. It also allows a link to be upgraded gradually, simply by adding new channels. However, one particularly advantageous feature of DWDM is that it allows all-optical routing and handling of telecommunications signals. To implement this aspect of DWDM technology, it is necessary to develop a new range of optical components such as switchers, cross-point networks, channel add-drop multiplexers, variable optical attenuators and so on. It has been proposed that so-called optical integrated circuits offer potential to meet these needs. As part of such a network, a dispersion compensation device based on optical integrated circuit technology becomes desirable. Dispersive optical devices based on arrayed waveguide gratings (AWGs) have been proposed, in which the path lengths of the large number of transmission waveguides linking input and output optical couplers have been varied, for example by heating. Such an arrangement is described in a conference paper by Parker et al, paper reference IWC2, at "Integrated Photonics Research", 12-15 July 2000, Canada. This invention provides a dispersive optical device comprising an input optical coupler; an input waveguide arranged to launch light into the input optical coupler at a launching position with respect to the input optical coupler; an output optical coupler; one or more output waveguides arranged to receive light from the output optical coupler; and a plurality of transmission waveguides arranged to receive light from the input optical coupler and direct it to the output optical coupler, the transmission waveguides generally converging towards the input optical coupler so that optical paths through the input optical coupler, which follow the axes of the transmission waveguides at their interface with the input optical coupler, intersect at an intersection point; in which the launching point is displaced from the intersection point in the direction of the axis of the input waveguide. The invention provides a new type of dispersive optical device based on an AWG. Instead of varying the path length of the many transmission waveguides linking the input and output optical couplers, the input coupler (e.g. a so-called "slab" or star coupler) is deliberately "defocused" by ensuring that the convergence of optical paths from the transmission waveguides does not coincide with the launching of light from an input waveguide into the input optical coupler. Preferably the convergence is close to the launching position, but displaced (i.e. at least having a component of displacement) along the axis of the input waveguide. This defocusing means that the optical path lengths from the launching point to each of the transmission waveguides are not the same. Because of the well- established principles of operation of an AWG and in particular the relationship between propagation in the transmission waveguide and output wavelength channels, this gives an overall path length variation across the device which varies with wavelength. In other words, the device provides a chirp or a dispersion. It has been calculated that a displacement of the launching point from the convergence of the optical paths by just 25 μm can give the device a chirp of 30ps/nm, although these results depend on various other parameters of the AWG device. Positive and negative chirps can be obtained in embodiments of the invention, and in some embodiments a variable chirp AWG can be achieved. The AWG is a reversible device in that either side can be connected as an input and the other side as the output. Accordingly, although the claims below define input and output couplers and waveguides, the device as defined could be reversed in sense so that the defocusing occurs at the output coupler (or indeed at both couplers). Embodiments of the invention will now be described with reference to the accompanying drawings, throughout which like parts are referred to by like references, and in which: Figure 1 is a schematic illustration of an arrayed waveguide grating (AWG) device; Figure 2 schematically illustrates the alignment of waveguides in a previously proposed AWG device; Figures 3 and 4 schematically illustrate an input slab waveguide of an AWG; and Figure 5 schematically illustrates an input slab to an AWG having a variable optical path length section. Figure 1 is a schematic illustration of an arrayed waveguide grating (AWG) device comprising an input waveguide 10, an input optical coupler 20, a set of transmission waveguides 30, an output optical coupler 40 and output waveguides 50. AWGs are described extensively in the literature. An AWG similar to that shown in Figure 1 (apart from features to be described with reference to Figures 3 to 5) is disclosed in US-A-5 136 671. The input optical coupler 20 and the output optical coupler 40 are so-called "slab" or "star" couplers each comprising a dielectric slab defining a free space region across which light can propagate. Figure 2 schematically illustrates the alignment of waveguides in a previously proposed AWG device. The transmission waveguides 30 are arranged so that, at the interface between the waveguides and the coupler 20, the axis of the waveguides converge towards an intersection point 60 coinciding with the position at which the input waveguide 10 launches light into the slab 20. This gives the potential of an equal path length between the end of the input waveguide 10 and the start of each of the transmission waveguides 30. Figures 3 and 4 schematically illustrate the alignment of waveguides in devices according to embodiments of the invention. In Figure 3, although the paths of the transmission waveguides' converge towards a convergence point 60', the end of the input waveguide 10' is displaced at a position 70, a distance δ from the intersection point 60'. Light leaving the input waveguide 10' starts to diverge before it reaches the slab 20'. This gives a non-equal path length for light propagating to each of the transmission waveguides 30'. A mechanical actuator 75 may be used, under the control of appropriate control electronics (not shown) to vary the displacement δ so as to provide a variable chirp device. The displacement need not be away from the slab, but could be into a recess in the slab. In Figure 4, the transmission waveguides 30" are arranged so that their paths intersect a distance δ inside the slab 20". Again this gives a slight defocusing and the desired chirp. Using an example set of parameters the distance δ is preferably under about 25μm to give a dispersion of 30ps/nm, although these results depend on other routing parameters of the AWG device. The defocusing, and so the chirp, can be in either sense, i.e. + and - chirping. Figure 5 schematically illustrates another approach in which a region 80 of the slab 20'" is etched away and replaced by for example a polymer or other material. An electrical heating and/or cooling element 90 is disposed over the replaced material section 80. If the replacement material has a higher (or even a different) rate of change of refractive index with respect to temperature (dn/dT) than that of the remainder of the slab 20'", the effective position of the intersection point with respect to the launching point of the input waveguide can be varied. Control electronics (not shown) can control the temperature of the replaced material to give the desired dispersion properties. Of course, the region 80 need not be rectangular. Suitable replacement materials for the region 80 include silicone resin, polysilioxane, halogenated silicone resin, halogenated polysilioxane, polyamides, polycarbonates or the like. The rate of change of refractive index for these materials with respect to temperature (dn/dT) is of the order of -1 x 10 "4 to -5 x 10 "5 per degree Celsius. This compares with a much smaller and positive dn/dT for typical glass materials of the order of +1 x 10 "5 . The much larger magnitude and opposite sense dn/dT for the polymer material means that the heating of the material 80 does not have to be completely localised to that element - in fact, depending on whether other polymer features requiring independent responses are formed on the same device, the entire device could even be heated or cooled to effect a temperature change of the core modifying element and so vary its response. In another embodiment, the surface of the slab 20 in Figure 2 could be polished and a high refractive index material moved very close to the surface. This will alter the optical path lengths within the slab and so achieve the desired chirping effect.

Claims: CLAIMS 1. A dispersive optical device comprising: an input optical coupler; an input waveguide arranged to launch light into the input optical coupler at a launching position with respect to the input optical coupler; an output optical coupler; one or more output waveguides arranged to receive light from the output optical coupler; and a plurality of transmission waveguides arranged to receive light from the input optical coupler and direct it to the output optical coupler, the transmission waveguides generally converging towards the input optical coupler so that optical paths through the input optical coupler, which follow the axes of the transmission waveguides at their interface with the input optical coupler, intersect at an intersection point; in which the launching point is displaced from the intersection point in the direction of the axis of the input waveguide. 2. A device according to claim 1, in which the input optical coupler is a slab coupler. 3. A device according to claim 2, in which the launching point is spaced apart from an input edge of the input optical coupler. 4. A device according to claim 3, comprising a mechanical actuator for varying the displacement between the launching point and the input edge. 5. A device according to claim 2, in which the transmission waveguides are disposed so that optical paths through the input optical coupler, which follow the axes of the transmission waveguides at their interface with the input optical coupler, intersect at an intersection point within the slab coupler. 6. A device according to claim 1 or claim 2, comprising a heating/cooling arrangement for heating and/or cooling at least a part of the input optical coupler so as to vary optical path lengths within the input optical coupler. 7. A device according to claim 6, in which at least a part of the input optical coupler is formed of a material having a rate of change of refractive index with temperature (dn/dT) which has the opposite sign to the dn/dT of another part of the input optical coupler. 8. A device according to claim 7, in which at least a part of the input optical coupler is formed of a polymer material. 9. A device according to any one of the preceding claims, in which the launching point is displaced from the intersection point by under +/- 50μm. 10. A dispersive optical device substantially as hereinbefore described with reference to Figures 1 and 3, Figures 1 and 4 and/or Figures 1 and 5 of the accompanying drawings.

Inventors: LAMING RICHARD IAN, LAMING, RICHARD, IAN
Applicants: ALCATEL OPTRONICS UK LTD, LAMING RICHARD IAN, ALCATEL OPTRONICS UK LIMITED, LAMING, RICHARD, IAN
Application year: 2001
Main classification: G02B6
IPC codes: G02B6, G02F1