Optical waveguide having an optically decoupled modulator region

An optical waveguide, such as a rib waveguide, having a first portion (7) to which a dopant (9) and/or a metal layer (10) is applied to enable an optical property of a second portion of the waveguide to be altered, the first portion (7) having a structure, e.g. being corrugated, the geometry of which is such as to prevent an optical wave being carried in the first portion (7). The dopant (9) and/or metal layer (10) can thus be positioned close to the second portion which carries the optical wave without causing perturbation, e.g. attenuation and/or polarization, of the optical wave.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The invention relates to an optical waveguide having a region to which
 interacting means, such as a dopant or electrode, are applied for
 controlling an optical property of the a waveguide.
 2. Background of the Related Art
 It is known to apply dopant to a portion of the waveguide, for instance to
 form a p-n or p-i-n diode across the waveguide for injecting charge
 carriers into the waveguide and thus altering the effective refractive
 index of the waveguide. By this means a phase modulator can be provided
 such as the phase modulator disclosed in WO95/08787. However, in designing
 such a device, a compromise has to be made between the desire to maximise
 the overlap between the charge carriers injected into the waveguide and
 the optical mode therein while minimising the attenuation caused by
 overlap between the optical mode and the doped regions.
 It is also known to apply a metal layer to a waveguide, e.g. to provide an
 electrical contact, or a heating element thereon. Again, there is a desire
 to position this as close as possible to the optical mode to maximise the
 effect of the electrical contact and/or heating but, on the other hand,
 the metal layer needs to be spaced from the optical mode so as to minimise
 absorption of one or both of its constituent TM or TE modes by tha metal
 layer.
 There thus remains a need to be able to apply interacting means, such as
 dopant or an ohmic contact, to a waveguide without the interacting means
 itself causing substantial perturbation, such as attenuation or
 polarization, of an optical wave carried by the waveguide.
 SUMMARY OF THE INVENTION
 The present invention aims to provide a solution to the problems associated
 with the related art.
 Thus, according to the present invention, there is provided an optical
 waveguide having a first portion to which interacting means are applied to
 enable an optical property of a second portion of the waveguide to be
 altered via said interacting means, the first and second portions each
 being formed of a light conducting material, the first portion having a
 structure the geometry of which prevents an optical wave being carried
 thereby, whereby the interacting means can be positioned in close
 proximity to the second portion without the interacting means itself
 causing a substantial perturbation of an optical wave carried by the
 second portion of the waveguide.
 The present invention may be achieved in whole or in part by an optical
 waveguide, comprising: (1) a first light conducting portion having a
 structure that inhibits an optical wave from propagating therein; (2) a
 second light conducting portion adapted to guide the optical wave; and (3)
 interacting means positioned on the first light conducting portion such
 that the interacting means can alter an optical property of the second
 light conducting portion, wherein the interacting means is positioned so
 that it does not directly and substantially perturb the optical wave when
 the optical wave is propagating through the second light conducting
 portion.
 The present invention may also be achieved in whole or in part by an
 optical waveguide, comprising: (1) a first waveguide portion; (2) a light
 guiding region within the first waveguide portion; (3) a second waveguide
 portion that inhibits light from propagating therein; and (4) a modulator
 having at least a portion that is positioned on or in the second waveguide
 portion, wherein the first waveguide portion, the second waveguide portion
 and the modulator are arranged such that the modulator can modulate an
 optical property of the light guiding region and such that, when light is
 guided by the light guiding region, the guided light is not substantially
 perturbed by the modulator.
 The present invention may also be achieved in whole or in part by an
 optical waveguide comprising: (1) a silicon substrate; (2) an insulating
 layer on the silicon substrate; (3) a silicon layer on the insulating
 layer shaped into a first waveguide portion and a second waveguide
 portion, wherein the second waveguide portion is adapted to inhibit light
 from propagating therein; (4) a light guiding region within the first
 waveguide portion; and (5) a modulator having at least a portion that is
 positioned on or in the second waveguide portion, wherein the first
 waveguide portion, the second waveguide portion and the modulator are
 arranged such that the modulator can modulate an optical property of the
 light guiding region and such that, when light is guided by the light
 guiding region, the guided light is not substantially perturbed by the
 modulator.
 Additional advantages, objects, and features of the invention will be set
 forth in part in the description which follows and in part will become
 apparent to those having ordinary skill in the art upon examination of the
 following or may be learned from practice of the invention. The objects
 and advantages of the invention may be realized and attained as
 particularly pointed out in the appended claims.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
 FIG. 1 shows a rib waveguide formed in a light conducting material 1
 (preferably a silicon layer), which is separated from a silicon substrate
 2 by an insulating layer 3 of silicon dioxide. Such waveguides are
 described further in WO95/08787 and the references given therein. A diode
 can be formed across the waveguide by providing a doped region on each
 side thereof, e.g. a p-doped region 4 on one side of the waveguide and a
 n-doped region 5 on the other side thereof, e.g. to form a phase
 modulator. The doped regions may be formed in the top surface of a slab
 region 1A of the light conducting material 1 on either side of the rib 1B
 formed therein as shown in FIG. 1. It should be noted that with this form
 of waveguide, the waveguide is provided not just by the rib 1B but also by
 the slab region 1A beneath the rib and, to some extent, the slab region on
 either side of the rib 1B. This is shown by the position of the optical
 mode which is depicted by dashed lines in FIG. 1. It will also be
 appreciated that an optical wave traveling along the waveguide also
 includes exponentially decaying portions extending both laterally and
 vertically from the concentrated part of the optical mode illustrated in
 FIG. 1.
 It should be noted that the devices shown in the Figures would usually have
 an oxide layer formed over the light conducting material [silcon layer] 1,
 but this is not shown in the Figures.
 It should be noted that the devices shown in the Figures would usually have
 an oxide layer formed over the silicon layer 1, but this is not shown in
 the Figures.
 The arrangement shown in FIG. 1 is preferred to an arrangement in which the
 doped regions are provided in the side faces of the rib 1B as, by
 positioning the doped regions in the slab regions 1A on either side of the
 rib 1B, overlap between the doped regions 4 and 5 and the optical mode is
 reduced. Nevertheless, part of the optical mode, and in particular the
 exponentially decaying part of the wave on either side thereof may still
 be attenuated to some extent by the presence of the doped regions 4 and 5.
 Furthermore, the positions of the doped regions 4 and 5 and the slab
 regions 1A on either side of the rib 1B leads to a less than optimal
 overlap between the injected charge carriers and the optical mode, as a
 significant proportion of the current between the doped regions 4 and 5
 passes beneath the concentrated area of the optical mode. The current
 paths from the p-doped regions 4 to the n-doped regions 5 are shown by
 arrows in FIG. 1. To improve the overlap between the injected carriers and
 the optical mode, it would be preferable to provide a doped region on top
 of the waveguide rib 1B, e.g. a p-doped region, and complementary doped
 regions in the slab regions on either side of the rib 1B, e.g. n-doped.
 However, a disadvantage of such a structure is that the overlap between
 the optical mode and the doped region on top of the rib 1B would introduce
 unacceptable attenuation of the optical mode.
 FIG. 2 also shows a rib waveguide formed in a silicon-on-insulator (SOI)
 chip, in this case with a metal layer 6A and oxide layer 6B formed
 thereon. The metal layer 6A may be used as a resistance heater to heat the
 waveguide, for example to form a thermal modulator. However, unless the
 oxide layer 6B is thick, the metal layer 6A absorbs a portion of the
 optical mode travelling in the waveguide, e.g. the TM mode, and thus
 causes polarization of the wave. If a thick oxide layer 6B is used, this
 greatly reduces the effect of the heater as the oxide 6B has a relatively
 low thermal conductivity.
 FIG. 3 shows a phase modulator according to an embodiment of the invention,
 which helps overcome the problems discussed in relation to the modulator
 shown in FIG. 1. This is achieved by modifying the structure of a light
 conducting material 1, preferably a silicon layer 1, with a rib portion 1B
 protruding therefrom with a first light conducting portion 21 or first
 portion 21 of the waveguide to which the dopant 9 is applied, in the
 example shown, this being the upper surface of the rib 1B, by forming
 corrugations 7 therein. The first light conducting portion of the
 waveguide being formed on a second light conducting portion 20 or a second
 portion 20 of the waveguide. The illustrated embodiment comprises four
 corrugations 7 with three grooves 8 therebetween. The geometry of the
 corrugations 7 is such that their effective refractive index is too low to
 allow the optical mode guided by the rib 1B to penetrate into the
 corrugations 7. The optical mode carried within the light guiding region
 50 of the second portion 20 of the rib waveguide is thus excluded from the
 corrugated region of the waveguide. The first doped regions 9 of the first
 dopant 9 can thus be formed in the corrugations 7 in close spacial
 proximity to the optical mode, but will not interact optically with the
 optical mode in the second portion 20 of the waveguide as the optical mode
 is excluded from tile corrugations 7. The first dopants 9 are in the first
 doped regions 9 provided in the corrugated portion of the waveguide and
 may be p-doped, and the second dopants 12, which may be n-doped in the
 second doped regions 12 are provided in the slab region on either side of
 the rib 1B. In this way, doping the top of the waveguide rib 1B can be
 achieved without introducing significant further loss and the current flow
 between the second doped regions 12 and the first doped regions 9 passes
 through a much greater proportion of the optical mode and so has a greater
 effect thereon.
 Although in the illustrated embodiments of the present invention a
 corrugated geometry is used for the structure of the first waveguide
 portion, it should be appreciated that other geometries that inhibit the
 optical mode from propagating in the first waveguide portion may be used
 without departing from the spirit and scope of the present invention.
 Alternatively, the n-doped regions may be provided in further corrugated
 regions (not shown) provided in the slab region on either side of the rib
 1B and/or on the side faces of the rib 1B, or may be provided on the
 underside of the waveguide.
 The doped regions are preferably formed by implantation as this enables the
 depth of penetration of the dopant into the silicon to be carefully
 controlled and thus kept low. Diffusion doping can, however, be used if a
 short drive in period is used. The majority of the dopant is preferably
 contained within the upper half, or less, of the height of the
 corrugations 7. Preferably, no dopant extends beyond the point where the
 corrugations 7 meet the waveguide rib 1B.
 The first doped region 9 can be contacted by a metal layer provided on the
 distal or upper surface of each corrugation 7 or a single metal layer 10
 may be provided across the upper surfaces of the corrugations 7 if the
 grooves 8 therebetween are filled with some other material of sufficiently
 low refractive index (not shown), such as silicon dioxide or a polymer
 such as polymethylmethacrylate (PMMA), which acts as a cladding rather
 than a light conductor. The metal layer 10 does not absorb part of the
 optical mode as the optical mode is effectively excluded from the
 corrugated region of the waveguide, as described above. The first doped
 regions 9 and the metal layer 10 can be referred to as modulators 30 or
 interacting means 30, as both the first doped regions 9 and the metal
 layer 10 modulate an optical property of the light guiding region and
 interact with the optical mode.
 In a similar manner, a thermal modulator can be formed as shown in FIG. 4,
 with a corrugated region on the upper surface of the rib 1B, with the
 grooves 8 filled by silicon dioxide or PMMA 8A, and a metal layer 11 in
 the form of a resistance heater applied across the upper surface of the
 corrugations. The thermal conductivity of the corrugated region is
 significantly improved compared to the arrangement shown in FIG. 2 in
 which an oxide layer (having poor thermal conductivity) is provided
 between the metal layer 6 and the rib 1B to reduce the perturbation caused
 by the presence of the metal layer.
 The rib waveguide 1B is preferably approximately 4 microns high measured
 from the silicon dioxide layer (and excluding the corrugations), and
 preferably approximately 4 microns wide. In this case, the three grooves 8
 are peferably approximately 0.35 to 1.0 microns wide and approximately
 0.35 to 1.0 microns deep. The size of the corrugations 7 preferably fall
 in the same range, with the grooves 8 and corrugations 7 preferably being
 of a similar width. The width and depth of the grooves, the number of
 grooves (and hence the number of corrugations) and the width of the
 corrugations will, however, depend on the dimensions of the waveguide and
 their geometry is selected so as to ensure substantially no, or very
 little, optical power is carried in the corrugated portion of the rib
 waveguide 1B. If the light to be guided by the rib waveguide 1B has a
 wavelength in the range 1.2 to 1.7 microns, the corrugations 7 preferably
 have a width of 0.5 microns or less.
 The corrugated region may be formed by etching grooves 8 in the upper
 surface of the rib 1B (which is preferably made slightly taller to allow
 for this). Alternatively, the corrugated region may be formed by growing
 or depositing the corrugations 7 on the upper surface of the rib 1B.
 The corrugated region should comprise at least one corrugation 7 or at
 least one groove 8 but preferably a plurality of corrugations 7 and/or
 grooves 8 are provided so the doped regions 9 are spread out rather than
 being too localised (which would lead to high current densities in a diode
 formed across the waveguide).
 The corrugations 7 and grooves 8 in the embodiments described above are
 shown to be parallel to the waveguide axis or optical axis 40 of the
 waveguide but in other embodiments this need not be so. Grooves 8 or
 corrugations 7 may, for instance, be formed across a waveguide as shown in
 FIG. 5, which shows a rib waveguide similar to that of FIG. 3 but with
 corrugations 14 extending across the upper surface of the rib 1B, which
 can be substantially perpendicular to the optical axis 40 of the
 waveguide. The upper portions 15 of the corrugations 14 may then be doped
 and/or a metal layer (not shown) may then be applied to the upper surface
 of the corrugated portion (with the grooves filled by silicon dioxide or
 some other material). It will be appreciated that grooves extending
 laterally across the waveguide reduce the effective refractive index of
 the corrugated region so light cannot travel therein. The dimensions and
 period of these grooves are preferably in the same range as for the
 embodiment described above in relation to FIG. 3.
 The above examples relate to a rib waveguide formed on a
 silicon-on-insulator chip. Similar arrangements may, however, be used with
 waveguides formed of other materials and other forms of waveguide, e.g. a
 slab waveguide, provided they permit the formation of a structure the
 geometry of which prevents light being carried thereby.
 Other structures having a geometry which excludes light from a region of
 the waveguide may also be used beside the corrugations described above,
 e.g. a lattice of grooves or of corrugations. The periodic nature of the
 structure may also be regular or irregular.
 The foregoing embodiments are merely exemplary and are not to be construed
 as limiting the present invention. The present teaching can readily
 applied to other types of apparatuses. The description of the present
 invention is intended to be illustrative, and not to limit the scope of
 the claims. Many alternatives, modifications, and variations will be
 apparent to those skilled in the art. In the claims, means-plus-functions
 are clauses are intended to cover the structures described herein as
 performing the recited function and not only structural equivalents, but
 also equivalent structures. For example, although silicon dioxide and
 polymethylmethacrylate (PMMA) may not be structural equivalents in that
 PMMA is a polymer, whereas silicon dioxide is not a polymer, in the
 environment of low refractive index materials, silicon dioxide and PMMA
 may be equivalent structures.