Optical waveguides – Optical fiber waveguide with cladding
Reexamination Certificate
1998-08-12
2001-01-23
Spyrou, Cassandra (Department: 2872)
Optical waveguides
Optical fiber waveguide with cladding
C385S037000, C385S130000, C385S132000
Reexamination Certificate
active
06178280
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to optical devices which include a waveguiding configuration.
2. Related Art
SUMMARY OF THE INVENTION
It has been recognised that the application of an electrical field to a glass can change the optical properties of the glass. In particular the electric field can change the refractive index of the glass. Since the refractive index affects the velocity of light in the medium changing the refractive index changes the velocity of light and this can be used to produce phase modulation. Phase modulation can be used to produce other effects, e.g. a Mach Zehnder configuration converts phase modulation into on/off modulation.
According to this invention an optical device includes a waveguiding configuration and an electrical capacitive configuration wherein the capacitive configuration comprises glass electrode regions and glass dielectric regions with electrical leads, e.g. metallic conductors, for applying electrical control signals to the electrode regions. The preferred metals for the conductors are Ni, Ti and Au. The waveguiding structure is configured so that the fields associated with optical signals propagating therein interact with dielectric regions. During the use of the device control signals are applied to the dielectric regions by means of the electrical leads. The control signals have the effect that temporary electric fields are applied to the dielectric regions. The fields change the optical properties of the dielectric regions and this affects the propagation of the optical signals.
Optical devices according to the invention can be considered as glass structures which comprise a waveguiding configuration superimposed upon an electrical capacitive configuration. Both configurations are implemented as a base glass composition, e.g. silica, with various additives to produce the different regions of the waveguiding configuration and the electrical capacitive configuration.
As is conventional, the waveguiding configuration may either be in fibre or, preferably, planar format. In either case the waveguiding configuration comprises a core formed of one or more glass compositions and a cladding comprising one or more glass compositions. In a conventional waveguide, it is desirable that the core has a uniform refractive index and the cladding has a uniform (lower) refractive index. In other words, for a conventional waveguide, all the glasses comprised in the core should have the same refractive index and all the glasses comprised in the cladding should have the same (lower) refractive index. This conventional waveguiding structure may be modified in devices according to the invention. For example, as will be discussed in greater detail below, it may be convenient for electrode regions and/or dielectric regions to extend into both the core and cladding. Since it is clearly convenient for any one region to have a constant composition it follows that any one region will, conveniently, have a constant refractive index. This further implies that there may be places where the refractive index of the core will be equal to the refractive index of the cladding. This is unconventional in waveguide design but, at least for small regions, the waveguiding properties are not substantially degraded and this is acceptable to achieve better electrical properties.
The electrical capacitive configuration comprises one or more glass regions having a very high electrical resistivity to constitute the dielectric region or regions and glasses having a lower electrical resistivity to constitute the electrode region or regions. Since the changes in optical properties occur primarily in the dielectric regions it is necessary that these regions be located where they will interact with the fields of optical signals propagating in the waveguide. The dielectric regions may either be placed in the claddings to interact with the evanescent fields of signals travelling in the core or a more direct interaction can be achieved when the dielectric regions are located in the core.
Planar devices according to the invention are made using a combination of deposition techniques and photolithography. There are several well known techniques for depositing glasses onto substrates in order to make planar devices, e.g. flame hydrolysis, chemical vapour deposition and sol-gel deposition. A complicated structure will require several stages of deposition interspersed with several stages of photolithography. It is also necessary to incorporate the electrical leads at a suitable stage of the process. For fibre configurations, it is appropriate to prepare a preform, e.g. by vapour phase deposition, which is then pulled into a fibre. In this case it is usually more convenient to attach the leads after the pulling.
The preparation of devices according to this invention includes, as well as the stages mentioned above, polling to enhance the electrical effects. The purpose of polling is to introduce permanent electric fields into the structure. The fields are “permanent” in that they remain for the lifetime of the device. Polling comprises applying high voltages, e.g. 10-100 kv, to the dielectric regions of the device. It is convenient to apply the polling voltage by means of the leads connected to the electrode regions.
It is believed that the high voltages applied during polling cause the migration of electrons within the glass structure. This creates electron depletion regions which effectively have a positive electrical charge and electron excess regions which effectively have a negative electric charge. The permanent fields exist between the depletion region and the excess region.
During the polling the structure is preferably irradiated, e.g. with ultraviolet light (e.g. within the wavelength range 190 nm to 270 nm). The irradiation enhances the electrical conductivity of the glasses and it is convenient to define the effect of the irradiation as “photo-conductivity”. The polling and irradiation may be carried out at ambient temperature but, in some cases, higher temperatures, e.g. 100° C. to 400° C., enhance the effect. It is also possible to achieve polling without irradiation, e.g. at high temperatures such as 100° C. to 400° C., but irradiation constitutes the preferred technique.
The polling applies high fields at a time when the electrical conductivity is enhanced and therefore electric charges (electrons) are more mobile. The result is that electric fields are produced in or near the dielectric regions. For example the fields are produced at or across the interfaces between the electrode regions and dielectric regions. When the enhancement is removed these fields remain and they become “permanent” as described above. In the use of the device control signals of 5-100 v are applied to the leads. The temporary fields produced by the control signals interact with the permanent fields to modify the optical properties of the dielectric region.
It is emphasised that the electrical conductivity is enhanced during the polling and the enhancement facilitates the movement of electric charges. When the enhancement is removed the electrical conductivities return to normal and the charges become trapped. It will be recognised that it is desirable to avoid conditions which will enhance the electrical conductivity after the polling has taken place. Unintended enhancement of the conductivity could allow the charges to leak and degrade the operational performance of the device. Thus it is advisable to avoid irradiation or heating of the device after polling. In other words, any shaping of the device which requires heating to high temperatures should be carried out before the polling. Temperatures which could result in charge leakage are higher than those to be expected in the normal use of telecommunications devices.
It is now convenient to discuss the chemical constitution of the preferred glasses which constitute the devices described above.
The major component of all the regions is silica, SiO
2
. More specifically all of the regions contain at least 70
Maxwell Graeme D
Smith Raymond P
Williams Douglas L
British Telecommunications public limited company
Curtis Craig
Nixon & Vanderhye P.C.
Spyrou Cassandra
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