A PHOTONIC DEVICE
Field of the invention
The present invention relates generally to a photonic device and relates particularly, though not exclusively, to an optical modulator and optical attenuator.
Background of the Invention
An ever-increasing number of communication systems utilise an optical fibre as the communications link. The optical fibre is used to convey electromagnetic radiation such as light generated from a laser source. In such systems devices for modulating the laser light at very high speeds are necessary. However, one of the inherent problems in achieving rapid optical switching within silica-based optical fibres is the absence of any fast non-linear effects which can be effectively used. One approach has been to induce higher electro-optic coefficients through thermal poling and UV poling of silica, but this method has met with mixed success. Many other types of modulators are also known. For example, various types of electro-optic modulators have been proposed. The simplest type comprises a block of optically-active material sandwiched between a pair of electrodes. When a voltage, such as signal stream, is applied to the electrodes, an electric field is created within the active material which causes a change in refractive index of the material. Light passing through the material will experience a phase change in accordance with the applied signal. However, such devices require a relatively high electrical power source to drive them and are difficult to splice into an all-fibre communications system.
There is therefore a need for a low power, all-fibre photonic device which does not rely on non-linear effects in silica.
Summary of the Invention
According to a first aspect of the present invention there is provided a photonic device comprising:
• a waveguide portion for guiding a photonic signal and adapted to be spliceable between an input and an output waveguide,
• a means for controlling optical properties of the waveguide portion, the means comprising a material having a variable refractive index which is switchable between a first refractive index and a second refractive index characterising a first and a second state, respectively, of the device; whereby, in use, switching of the refractive index changes the radial intensity distribution of optical modes in the waveguide portion.
According to a second aspect of the present invention there is provided a photonic device comprising:
• a waveguide portion for guiding a photonic signal and comprising a core region and a cladding region and having substantially uniform sectional dimensions,
• a means for controlling optical properties of the waveguide portion, the means comprising a material having a variable refractive index which is switchable between a first refractive index and a second refractive index characterising a first and a second state, respectively, of the device;
• whereby, in use, switching of the refractive index changes the radial intensity distribution of optical
modes in the waveguide portion.
Preferred features of the Invention The waveguide portion preferably comprises an optical fibre portion. The waveguide portion of the photonic device according to the first aspect of the present invention preferably is coreless. For example, the waveguide portion may be composed of doped or undoped silica glass having uniform optical properties throughout its thickness.
The means for controlling optical properties of the waveguide portion may have any form, but preferably comprises an outer layer at least in part surrounding the waveguide portion.
In one embodiment of the present invention the photonic device, according to either the first or second aspect of the present invention, forms an optical modulator for modulating the photonic signal. In this case the device may be arranged such that, in use, switching of the refractive index changes the radial intensity distribution of optical modes in the waveguide portion in a manner such that the efficiency of propagation of the photonic signal through the modulator is greater in the first state than in the second state. In the first state the radial mode intensity distribution may be concentrated closer to a longitudinal axis of the waveguide portion than in the second state. The second refractive index preferably is greater than the first refractive index. The waveguide portion preferably is arranged such that the radial mode intensity distribution is at a maximum at or adjacent the longitudinal axis in the first state, and at a maximum at or adjacent an outer region of the waveguide
portion. The modulator most preferably is arranged such that in the second state the mode intensity distribution is at a maximum in the vicinity of the outer layer and is significantly lower in the vicinity of the longitudinal axis.
In another embodiment of the present invention the photonic device, according to either the first or the second aspect of the present invention, forms an optical attenuator. In this case the photonic device is arranged such that, in use, a level of attenuation of the photonic signal is controlled by the refractive index of the material comprised by the means for controlling optical properties of the waveguide portion. A change in the refractive index of the outer layer gives rise to a change in a radial distribution of mode intensities in the waveguide portion, which in turn causes an associated change in attenuation of the photonic signal. The means for controlling optical properties preferably comprises the outer layer and the attenuator preferably is arranged such that optical attenuation increases as the refractive index of the outer layer increases.
If the waveguide portion comprises a core region and a cladding region according to the second aspect of the invention, the core and the cladding regions may be arranged such that, in use, in the first state each optical mode is substantially confined to the core region and in the second state the intensity of modes is greatest outside the core region. The waveguide portion preferably is spliceable to an input and an output waveguide. The core and cladding regions of the waveguide portion preferably are arranged such that in the first state the mode intensity distribution is substantially the same as a mode intensity distribution of the photonic signal
propagating in the input waveguide.
The core region of the waveguide portion may comprise a region of increased refractive index relative to the cladding region, and may have either a graded or stepped index profile. Any other refractive index profile may be included in the waveguide portion which confines optical modes to the vicinity of the longitudinal axis in the first state. For example, the waveguide portion may be in the form of a Bragg fibre. The outer layer may comprise a layer of an electro-optic material which can be induced to change refractive index upon application of an electric field. The outer layer preferably also comprises electrodes arranged to activate the electro-optic layer. The outer layer may comprise a thin film of electro-optic material sandwiched between an upper electrode and a lower electrode. For example, the electro-optic material may comprise a layer of zinc oxide (ZnO) , zinc sulfide (ZnS) or a Perovskite titanate such as PZT. Alternatively, the photonic device may comprise a separate means for varying the refractive index of the outer layer, and may employ heat, mechanical energy, or acoustic effects to change the refractive index.
It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country.
Embodiments of invention will now be described, by way of example only, with reference to accompanying drawings.
Brief Description of the Drawings In the drawings -
Figure 1 shows a sectional view of an optical modulator in accordance with an embodiment of one aspect the present invention;
Figure 2 shows the refractive index profile and lowest order mode intensity distribution of the modulator shown in Figure 1 for (a) the "ON" state and (b) the "OFF" state; Figure 3 shows a sectional view of an optical modulator in accordance with an embodiment of another aspect of the present invention;
Figure 4 shows the refractive index profile and lowest order mode intensity distribution of the modulator shown in Figure 3 for (a) the "ON" state and (b) the "OFF" state; and
Figure 5 shows the refractive index profile and lowest order mode intensity distribution for a further embodiment of the other aspect of the invention for a modulator in (a) the "OFF" state and (b) the "ON" state.
Detailed Description of the Drawings
Referring to Figure 1, a modulator 10 comprising a coreless waveguide portion 110 in accordance with a first aspect of the present the invention is described. The waveguide portion is a fibre portion and is spliced between an input optical fibre 20 and an output optical fibre 30. The input and output optical fibre in this embodiment are standard single-mode fibres, having an input and output light-guiding cores 40, 50, respectively. A photonic signal to be modulated 60 propagates in the input core 40 with a wavelength of 1.55 micrometres. In this embodiment the fibre portion 110 is surrounded by an
outer layer 120 which functions to control optical properties of the fibre portion. In this embodiment, the fibre portion 110 has a diameter which is equal to the diameters of the input and output fibres 20, 30. The outer layer 120 comprises a film of an electro- optic material 130 sandwiched between an outer electrode 140 and a inner electrode 150. The outer and inner electrodes 140, 150 comprise thin films of an electrically-conductive material, such as gold, chromium, aluminium, or tungsten etc. The lower electrode 150 is situated at the interface between the fibre portion 110 and the film of electro-optic material 130. The lower electrode 150 should be at least partially transparent to the photonic signal to be modulated. In this embodiment, the lower electrode is formed from an ultra-thin metallic film of chromium approximately 3 nanometres in thickness. Such a film has a sufficient electrical conductivity to function as an electrode, while being sufficiently transparent to the photonic signal. As an alternative to metallic materials, the lower electrode 150 may be formed from a transparent conducting metal oxide, such as indium tin oxide. The film of electro-optic material 130 is included in the outer layer 120 in order to change the refractive index of the outer layer as required. However, it will be understood that the outer layer 130 may alternatively consist of any other material having a refractive index that can be changed. For example, the outer layer can alternatively comprise a material with a refractive index which may be controlled by heating, compression, or acoustic effects.
Figure 2A shows a refractive index profile 160 and two-dimensional plot of lowest order mode intensity distribution 170 for the modulator 10 shown in Figure 1
when in the "ON" state. It can be seen that in the "ON" state, the refractive index of the fibre portion 110 (which extends out to a radius of 60μm) is uniform across its diameter and is equal to the refractive index of the outer layer 120 (which extends from a radius of 60μm to 80μm) . In this state, the intensity of modes propagating in the fibre portion 110 is at a maximum along the centre of the fibre portion 110, and tapers off towards the outer layer 120. Such a distribution of mode intensities 170 allows a reasonable proportion of the original photonic signal 60 to propagate through the modulator 10. However, as the intensity distribution of optical modes within the fibre portion is much broader than the diameter of the output core 50, there are significant coupling losses when the photonic signal progresses into the output fibre portion.
In Figure 2B, the modulator 110 of Figure 1 is shown in the "OFF" state. In this state, a voltage is applied across the electrodes 140, 150 to generate an electric field therebetween and thus change the refractive index of the electro-optic film. The refractive index profile 180 in Figure 2B shows that the applied voltage increases the index of the outer layer by 0.01. The increase in refractive index has the effect of removing the original lowest order mode which, in the "OFF" state, had a maximum in the centre of the modulator. In contrast, the two- dimensional plot of mode intensities 190 in Figure 2B is at a maximum in or near the outer layer 120. In effect, the higher-index outer layer 120 "drains" light away from the centre of the fibre portion 110. Consequently, when the modulator is in this state, light is coupled much less efficiently into the output fibre portion 30 than when the mode intensity distribution is at a maximum in the centre
of the fibre portion.
Each of the modulators described above and below was designed using standard finite element methods to calculate the mode solutions at a wavelength of 1.55 micrometers. The dimensions and refractive indices of the outer layer and fibre portion were chosen such that in the "ON" state there is a lowest order propagating mode with a peak intensity at the centre of the fibre portion. On the other hand, the modulator was designed such that when in the "OFF" state, there does not exist any mode which has a peak intensity at the centre of the fibre portion. When the modulator is in the "ON" state, the modulator behaves as an optical fibre in which the core is absent i.e. a fibre which is wholly composed of cladding. Such a waveguide supports multiple propagation modes which are highly sensitive to changes in boundary conditions at the cladding-air interface. The present invention takes advantage of this sensitivity to boundary conditions by using the outer layer 120 to introduce a small refractive index change at the cladding-air interface and thereby disrupt the mode of propagation.
Referring to Figures 3 and 4, a second embodiment of an optical modulator 200 in accordance with a second aspect of the present invention will now be described. The same reference numerals are used where the corresponding features are similar to those in the first embodiment. In this embodiment, a fibre portion 210 comprises a longitudinal core region 220 surrounded by a cladding region 230, which is in turn surrounded by an outer layer 120. The outer layer 120 is the same as that described with reference to Figure 1. As shown in the refractive index profile 240 of Figure 4A, the core 220 has a higher refractive index than the cladding region
230. Also, the core 220 has a diameter which is equal to the input and output cores 40, 50 of the input and output fibre portions 20 and 30 between which the fibre portion is spliced. The plot of mode intensity distribution 250 in Figure 4A shows that the core of the fibre portion 210 has the effect of confining low order modes much closer to the centre of the fibre portion 210 than in the first embodiment 10 (see Figure 2A) . Thus, when the modulator 200 is in the "ON" state, a greater proportion of light is coupled from the input fibre 20 via the fibre portion 210 into the output fibre 30 than for the embodiment shown in Figure 1. Again, when the refractive index of the outer layer 120 is increased (see refractive index profile in 260 in Figure 4B) the distribution of mode intensities shifts away from the centre of the fibre portion 210 to a ring in the vicinity of the outer layer. Since there is almost no light in the centre of the fibre portion 210 when the modulator is in the "OFF" state, there is a sharp contrast between the "OFF" and "ON" states . A third embodiment of an optical modulator will now be described with reference to Figure 5. As can be seen in the refractive index profile 310 in Figure 5A, the fibre portion includes a core region 330 in the form of a plurality of coaxial layers 340 arranged radially in order of alternating refractive index so as to confine light by longitudinal Bragg reflection. In the "ON" state the mode intensity distribution 350 (Figure 5A) has a maximum in the centre of the core region 330. However, the Bragg structure in the core region 330 confines the lower order modes much more weakly than the modulator 200 shown in Figure 3 and consequently the distribution of modes is much broader. When the refractive index of the outer layer 120 is increased (see refractive index profile 360
in Figure 5B) , light is drained away from the core region and is concentrated in a ring located in or near the outer layer (see the mode intensity distribution 370 in Figure 5B) . While the above embodiments are described with reference to the "OFF" and "ON" states of the modulator, the modulator can also operate as an optical attenuator capable of attenuating a photonic signal by a predetermined amount. In this mode of operation, the refractive index of the outer layer is only adjusted to an extent sufficient to produce the required optical attenuation.
In each of the above embodiments, the fibre portion, with or without core region, is formed from a glass material. If the fibre portion comprises a core region, variations in refractive index in the core region may be present which is formed by doping the core region with an appropriate dopant such as germanium. However, it will be understood that the principles of the present invention may be applied to other optically-transmissive materials, such as polymers. Also, whilst the above embodiments have been described with reference to a single-core optical fibre, the present invention is equally applicable to modulating or attenuating a plurality of photonic signals propagating in multiple-core optical fibres.
Although the refractive index of each outer layer in the above embodiments is increased in order to put the modulator into the "OFF" state, the present invention also includes within its scope modulators and attenuators in which the outer layer decreases in refractive index relative to the fibre portion in the "OFF" state.
It will be appreciated by a person skilled in the art that numerous variations and/or modifications may be made
to the present invention in addition to the specific embodiments described without departing from the spirit or scope of the invention as broadly described. The means for controlling optical properties of the fibre portion may have any form for example it may comprise an array of elongate portions. In the most preferred from the means for controlling optical properties of the fibre portion comprises an outer layer that may surround either wholly or partially the circumference of the fibre portion in its entire length or in a fraction of its lenth.. The present embodiments are therefore to be considered in all respects illustrative and not restrictive.