CA2217691A1 - Fast switching asymmetric thermo-optical device - Google Patents

Abstract

The present invention refers to an asymmetric thermo-optical device comprising a waveguiding structure which comprises at least one input light path (1) and a first output light path (2) and a second output light path (3), the second output light path (3) having a smaller width than the first output light path (2), and the first output light path (2) being provided with a first heating element (4), characterised in that the second output light path (3) is provided with a second heating element (5). In a preferred embodiment of the invention the second heating element (5) is connected to a capacitor which is connected in parallel with the first heating element (4).

Description

.. , FAST SWITCHING ASYMMETRIC THERMO-OPTICAL DEVICE

The present invention is in the field of thermo-optical devices, more particularly, asymmetric thermo-optical devices.

Thermo-optical devices are known, e.g., from the description by Diemeer et al. in Journal of Lightwave Technology, Vol.7, No.3 (1989), pp 449-453. Their working is generally based on the phenomenon of the optical waveguide material employed exhibiting a temperature dependent refractive index (polarisation independent thermo-optical effect).
Such devices have been realised, int.al., in inorganic materials such as ion-exchanged glass and titanium-doped lithium niobate. The use of all-polymeric waveguides for thermo-optical devices has also been disclosed, an advantage thereof disclosed by Diemeer et al. being that a modest increase in temperature may result in a large index of refraction change. The device described by Diemeer is an all-polymeric planar switch. Switching is achieved by employing total internal reflection from a thermally induced index barrier. The device comprises a substrate (PMMA), a waveguiding structure (polyurethane varnish), and a buffer layer (PMMA), with the heating element being a silver stripe heater deposited by evaporation upon the buffer layer through a mechanical mask.

In Electronics Letters, Vol. 24, No. 8 (1988), pp 457-458 an optical switch is disclosed in which optical fibres are coupled using a single-mode fused coupler having a silicone resin cladding material provided on the coupling region. Switching is achieved by a thermally induced refractive index change of the silicone cladding.

In US 4,753,505 a thermo-optical switch is described comprising a layered waveguide in which the material having a temperature dependent refractive index is a polymer or glass.

In US 4,737,002 a thermo-optical coupler is described which may be formed using either optical fibres or integrated optics.

None of the above-mentioned publications describes an asymmetricthermo-optical device.

In SPIE Vol. 1560: Nonlinear Optical Properties of Organic Materials IV (1991), pp. 426-433 an asymmetric thermo-optical device is described. The disclosed device is a polarisation/wavelength insensitive polymeric switch comprising an asymmetric Y-junction. The switching properties are based on heat-induced refractive index modulations causing variations in the mode evolution in such asymmetric Y-junctions. The device comprises a glass substrate and a polymeric multilayer comprising an NLO polymer.
An asymmetric thermo-optical device comprises a waveguiding structure which comprises at least one input light path (1) and a first output light path (2) and a second output light path (3), the second output light path (3) having a smaller width than the first output light path (2), and the first output light path (2) being provided with a first heating element (4). As the first output light path has a greater width than the second output light path, the light travels through the first output light path in the default switching state. These kinds of asymmetric thermo-optical devices can be used when it is desired to have the switch in a well-defined default state in the zero power state. This is the case in protection or redundancy switches and switching matrices.

In principle, these asymmetric devices only need one heating element to switch the light to the second output light path. However, in the conventional asymmetric thermo-optical devices the switching time needed for getting back to the default setting is too long due to the free thermal diffusion of the heat from the heating element. The present invention provides an asymmetric thermo-optical device with a shorter switching time to the default setting.

To this end the invention consists in that in an asymmetric thermo-optical device of the type identified in the opening paragraph the second output light path (3) is provided with a second heating element (5).
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The reference numbers refer to Figures 1-3, which will be elucidated below.
After the light has been switched to the second (smaller) output light path, the second heating element (5) may be heated, so that the light will be switched back to the default state faster. In a preferred embodiment of the invention the electric power needed for this heat pulse in the second heating element is supplied by a capacitor which has been charged while heating the first heating element. This can be done by connecting the first heating element in parallel with said capacitor, while heating the first heating element for switching to the non-default state. Now the capacitor is charged. When switching back to the default state, the capacitor is connected to the second heating element: the capacitor is decharged through the second heating element. This induces a heat pulse in the second heating element. As the capacitor was already charged with energy while switching from the default state, the switching back needs no additional energy feed.
This has advantages, particularly in switching matrices.

A device according to the invention can be made using either optical fibres or integrated optics. Among these integrated optics, polymer thermo-optical devices are preferred because even a modest temperature change may give rise to a large change in refractive index.
Furthermore, polymers are more easily processable than inorganic material, for instance, they can be applied on any substrate. Thus, a W O96t33441 PCTAEP96/01636 substrate with high thermal conductivity, such as silicon, can be combined with polymeric material for the waveguiding structure which has a low thermal conductivity. This way, a device with a good localised thermal profile is provided.

An integrated thermo-optical device can be built up, e.g., as follows.
Under~eath the waveguiding structure is a support, e.g., a glass or silicon substrate. On the substrate the following successive layers can be identified: a lower cladding layer, a core layer (guiding layer), and an upper cladding layer. The cladding material may be an inorganic or a polymeric material. Said cladding layers have an index of refraction lower than that of the core layer. The core layer, which comprises the actual waveguiding design, may be made of inorganic or polymeric material. On top of the upper cladding are placed the heating elements.
In a thermo-optical switch with asymmetric layer build-up the cladding adjacent to the heating elements has a lower refractive index than the other cladding layer. This has the advantage of creating an increased refractive index contrast between the upper cladding and the core layer. Therefore, the upper cladding layer may be made thinner than usual. This increases the response time of the thermo-optical device to a temperature rise in either of the heating elements, further decreasing the switching time. Also, less power supply is needed. The asymmetric thermo-optical device according to the invention may advantageously have an asymmetric layer build-up.

When using a polymeric core layer, the use of polymeric cladding layers is preferred. In these all-polymeric devices the relevant physical properties such as Tc and thermo-optical effect are comparable. Also, the thermal expansion coefficients and thermal conductivities are approximately the same, providing a more stable device. The polymers used for these layers are so-called optical polymers.

The refractive index of the optical polymers used will generally be within the range of from 1.4 to 1.8, preferably of from 1.45 to 1.60.
When a thermo-optical device with asymmetric layer build-up is used, the refractive index contrast between the two cladding layers may vary.

Optical polymers are known, and the person of ordinary skill in the art will be able to choose polymers having the appropriate refractive indices, or to adapt the refractive indices of polymers by chemical modification, e.g., by introducing monomeric units that affect the refractive index. As all polymers exhibit a thermo-optical effect, basically any polymer having sufficient transparency for the wavelength used can be employed in the core of the waveguide component. Said transparency requirement also holds for the cladding.
Particularly suitable optical polymers include polyacrylates,polycarbonates, polyimides, polyureas, polyarylates.

A waveguiding structure according to the invention can be provided with a pattern of light paths in various manners. Methods to achieve this are known in the art. For example, it is possible to introduce such a pattern by removing portions of the slab waveguide, e.g., by means of wet-chemical or dry etching techniques (reactive ion etching, laser ablation), and to optionally fill the gaps formed with a material having a lower index of refraction. Or, e.g., photosensitive material that can be developed after irradiation may be used. In the case of a negative photoresist the photosensitive material is resistant to the developer after irradiation, and the portions of the material that were not subjected to irradiation can be removed. It is preferred to use a positive photoresist, and to define the channels by means of an irradiation mask covering the waveguide portions that will form the channels. The irradiated material then is removed using developer, after which a material of lower refractive index is applied.

It is more strongly preferred, however, to use a core material that allows defining a pattern of light paths without material having to be removed. Materials of this nature exist, e.g., those that will undergo chemical or physical conversion into a material having a different refractive index when subjected to heat, light, or UV radiation. In the cases where this conversion results in an increase in the refractive index, the treated material will be employed as core material for the waveguide channels. This can be carried through by employing a mask in which the openings are identical with the desired waveguide pattern. In the case of the treatment leading to a decrease of the refractive index, the treated material is suitable as a cladding material. In that case a mask as mentioned above is used, i.e., one that covers the desired waveguide channels. A particular, and preferred, embodiment of this type of core material is formed by polymers that can be bleached, i.e., of which the refractive index is lowered by irradiation with visible light or UV, without the physical and mechanical properties being substantially affected. To this end it is preferred to provide the slab waveguide with a mask that covers the desired pattern of waveguide channels, and to lower the refractive index of the surrounding material by means of (usually blue) light or UV radiation. Bleachable polymers have been described in, int. al., EP 358 476, EP 645 413, W0 94/01480, EP 350 112, and EP 350 113.

It is further preferred to employ NL0 polymers in the core, in order to have the possibility of making combined thermo-optical/electro-optical devices.

Optically non-linear materials, also called non-linear optical (NL0) materials, are known. In such materials non-linear polarisation occurs - 30 under the influence of an external field of force (such as an electric field). Non-linear electric polarisation may give rise to several optically non-linear phenomena, such as frequency doubling, Pockels effect, and Kerr effect. Alternatively, NL0 effects can be generated opto-optically or acousto-optically. In order to render polymeric NLO
materials NLO-active (obtain the desired NLO effect macroscopically), the groups present in such a material, usually hyperpolarisable sidegroups, first have to be aligned (poled). Such alignment is commonly effected by exposing the polymeric material to electric (dc) voltage, the so-called poling field, with such heating as will render the polymeric chains sufficiently mobile for orientation. NLO polymers are described in, int. al., EP 350 112, EP 350 113, EP 358 476, EP 445 864, EP 378 185, and EP 359 648.

Making a polymeric optical waveguide of the invention will generally involve applying a solution of the polymer used as the lower cladding to a substrate, e.g., by means of spincoating, followed by evaporating the solvent. Subsequently, the core layer, and the upper cladding layer, can be applied in the same manner. On top of the upper cladding the heating element will be placed, e.g., by means of sputtering, chemical vapour deposition, or evaporation and standard lithographic techniques. For fixation and finishing a coating layer may be applied on top of the entire structure, so as to allow better handling of the device. Alternatively, instead of a coating layer a glue layer may be used for fixation, after which the total structure can be finished by placing an object glass on it.
When making all-polymeric layered waveguide structures, it is advantageous to apply the individual layers in the form of cross-linkable polymers. These are polymers which comprise cross-linkable monomers or polymers which comprise so-called crosslinkers such as polyisocyanates, polyepoxides, etc. This makes it possible to apply polymers on a substrate and so cure the polymer that a cured polymeric network is formed that does not dissolve when the ~ 30 next layer is provided.

Suitable substrates are, int. al., silicon wafers, ceramic materials, or plastics laminates, such as those based on epoxy resin which may be reinforced or not. Suitable substrates are known to the skilled man.
Preferred are substrates that, by virtue of a high thermal conductivity, can function as a heat sink, since this can speed up the thermo-optical switching process considerably. For, considering that switching to, say, the "on" state can be achieved by heating the waveguide, reaching the "off" state will then require leaving the waveguide to cool. The preferred substrates in this respect are glass, metal, or ceramics, and particularly silicon.

To avoid loss of light through the lower cladding layer it is preferred to use a lower cladding layer which is made up of two sublayers, the lower of which (i.e., the one adjacent to the substrate) is a thin layer (e.g., about 3 ~m) having a lower index of refraction than the other sublayer (i.e., the one adjacent to the core layer). Thus, the actual waveguiding structure is "optically insulated" from the substrate. This is particularly important if the substrate is one chosen for its heat-dissipating properties rather than for its refractive index. For instance, silicon is an excellent heat sink but has a higher index of refraction than the layers making up the waveguide. This may lead to loss of light, due to radiation into the silicon substrate. The additional low index layer provides the certainty that all the light will propagate through the waveguide.
This considerably facilitates designing the layered waveguide. In order to not affect the thermal profile, the total thickness of the layered waveguide preferably is not affected by the presence of the additional low index layer. This can be realised in a simple manner by choosing polymeric materials over inorganic materials.

The heating elements will generally be made up of a thin film electric conductor, usually a thin metal film. Such a thermal energy generating live electric conductor can also be called "resistor wire" for short.
Of course, suitable thermal energy generating conductors are not restricted to the wire form.

The thermal energy generating live electric conductor, the resistor wire, may be a heating element known in itself from the field of thin-film technology, such as Ni, Ni/Fe or Ni/Cr. Alternatively, it is possible to employ as electric conductor those materials which in the field of electro-optical switches are known as the ones from which electrodes are made. These include noble metals, such as gold, platinum, silver, palladium, or aluminium, as well as those materials known as transparent electrodes, e.g., indium tin oxide. Nickel, chromium and gold are preferred.

If poled NL0 polymers are employed in the present waveguides, using heating elements that can function as an electrode makes it possible to combine thermo-optical and electro-optical functions in a single device.

In the case of the functions of the electrode and the resistor wire being combined, a surge can be realised in actual practice by, say, employing a feed electrode of relatively large diameter (low current density) followed by a segment having a comparatively small diameter.
A high current density will then be created in this narrow segment, so that heat is generated. Alternatively, it is possible to employ a material made up of two metals of different intrinsic resistance, and to vary either the thickness of the different metallisations or the composition of the material in such a way as to obtain the desired effect of a low current density, or a low intrinsic resistance upon supply, while a high current density or a comparatively high intrinsic resistance is displayed at the location where the thermo-optical effect is desired. By thus varying current densities it - is possible to obtain a thermo-optical effect locally.
In the case of NL0 polymers being employed, the heating element may be put to initial use during the alignment of the NL0 polymers.

Devices according to the invention can be used with advantage in optical communication networks of various kinds. Generally, the thermo-optical components either will be directly combined with optical components such as light sources (laser diodes) or detectors, or they will be coupled to input and output optical fibres, usually glass fibres.

The invention is further illustrated with reference to the following Figures 1, 2, and 3.

Figure 1 shows a schematic top view of a known asymmetric thermo-optical device.
Figure 2 shows a schematic top view of an asymmetric thermo-optical device according to the invention, showing the electrical configuration when switching from the default state to the non-default state.
Figure 3 shows the same asymmetric thermo-optic switch as in figure 2, but now the electrical configuration is shown when switching back from the non-default stae to thedefault state.

Figure 1 shows a schematic top view of a known asymmetric thermo-optical device comprising a waveguiding structure which comprises at least one input light path (1) and a first output light path (2) and a second output light path (3), the second output light path (3) having a smaller width than the first output light path (2), and the first output light path (2) being provided with a first heating element (4).

Figure 2 shows a schematic top view of an asymmetric thermo-optical device according to the invention in the switching state comprising a waveguiding structure which comprises at least one input light path (1) and a first output light path (2) and a second output light path (3), the second output light path (3) having a smaller width than the first output light path (2), the first output light path (2) being provided with a first heating element (4), and the second output light path (3) being provided with a second heating element (5). This Figure shows the electrical configuration when switching from the default state to the non-default state: When the light is switched from the default state by heating the first heating element (4), the capacitor (6) is simultaneously charged. When switching back to the default state, the capacitor is connected to the second heating element (5), so that the second heating element (5)is heated by decharging the capacitor (6).

Figure 3 shows a schematic top view of the same asymmetric thermo-optical device as in Figure 2. This Figure shows the electrical configuration when switching back to the default state: When switching back to the default state, the capacitor (6) is decharged, whereupon the second heating element (5) is heated.

Claims (4)

1. An asymmetric thermo-optical device comprising a waveguiding structure which comprises at least one input light path (1) and a first output light path (2) and a second output light path (3), the second output light path (3) having a smaller width than the first output light path (2), and the first output light path (2) being provided with a first heating element (4), characterised in that the second output light path (3) is provided with a second heating element (5).

2. An asymmetric thermo-optical device according to claim 1, comprising a capacitor (6).

3. An asymmetric thermo-optical device according to claim 1 or 2, characterised in that the asymmetric thermo-optical device is a polymeric device.

4. An asymmetric thermo-optical device according to claim 3, characterised in that not only the guiding layer but also the cladding layers are polymeric.