Optical waveguides – Planar optical waveguide
Reexamination Certificate
1999-10-25
2001-05-01
Healy, Brian (Department: 2874)
Optical waveguides
Planar optical waveguide
C385S130000, C385S131000, C385S141000, C385S132000, C065S385000, C065S386000
Reexamination Certificate
active
06226433
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a planar optical waveguide and to a method of spatially selectively increasing the refractive index in a glass.
Planar optical waveguides of the type defined are known from an article by G. D. Maxwell entitled “Photosensitivity & Rare-Earth Doping in Flame Hydrolysis Deposited Planar Silica Waveguides”, Proceedings of the International Society for Optical Engineering (SPIE), Vol.2695, p. 16-29. It is a common feature of all the glass waveguides described therein that the photosensitivity of the glass used is increased by suitable measures. Photosensitivity refers to the property of a material to react to irradiation with light of a suitable wavelength by a change in its refractive index of greater or lesser duration. Utilizing this effect, regions having an increased refractive index compared to the adjoining regions are formed in the optical waveguides by locally limited irradiation with UV light. By appropriately arranging these regions, for example a Bragg grating can be formed in the planar optical waveguide. Bragg gratings play an important role in transmission devices based on the principle of optical wavelength multiplexing.
The silicate glass used to produce optical waveguides has an extremely low inherent photosensitivity. Therefore without special treatment only a very small change in the refractive index can be obtained when the glass is exposed to a UV laser. The above mentioned article by G. D. Maxwell specifies several measures whereby the photosensitivity can be increased.
One measure consists in exposing the glass to a reducing atmosphere during sintering. However this leads to increased optical losses in the sintered glass for the wavelengths normally used in optical transmission technology. In the case of another measure also used in optical fibres, the glass is exposed to a hydrogen atmosphere at high pressure. This process, which is also referred to as “hydrogen charging” extends over several days; moreover, although the photosensitivity thereby attainable is relatively high, it is not very durable. Therefore the change in refractive index inducible by means of the irradiation also is not durable, for which reason optical components produced in this way must be more frequently readjusted by repeated UW irradiation. In the case of another known measure known as “flame brushing” the glass is treated with a hot oxyhydrogen flame. An excess of hydrogen in the flame causes a reaction presumably comparable with that which occurs in the case of hydrogen charging. The relevant details are given for example in an article by S. Gujrathi entitled “Photosensitivity in Phosphorus-Doped Silica Glass and Optical Waveguides”, Appl. Phys. Lett. 65 (4), Jul. 25, 1994. However, similarly as in the case of hydrogen charging, here too the increase in photosensitivity is not very durable.
SUMMARY OF THE INVENTION
Therefore the object of the invention is to provide a planar optical waveguide of the type defined which does not have the described disadvantages associated with measures for increasing photosensitivity.
The planar optical waveguide according to the invention comprises at least one glass layer which has been produced by flame hydrolysis deposition of a hydrolytic glass initial product on a base and subsequent sintering of the glass initial product, and which comprises at least one region in which (1) the refractive index is locally increased compared to regions of the glass layer adjoining this region and (2) has the same chemical composition as the adjoining region and has a higher density than the adjoining regions. The invention is based on the recognition that hydrolytic glass initial products can be redensified after sintering by supplying heat without any change occurring in the chemical composition. The regions with a greater density have a higher refractive index than the other regions; the attainable change in refractive index &Dgr;n is in the order of 1.5·10
−3
. As this effect is not based on the principle of photosensitivity, no chemical reaction takes place when heat is supplied. In particular however, the method according to the invention dispenses with the need to achieve noteworthy photosensitivity by one of the above explained known measures. Consequently a planar optical waveguide produced by the method according to the invention also does not possess the disadvantages associated with these measures, for example higher optical losses or poor durability of the change in refractive index.
The invention can be applied both to pure glass waveguides, in which all the layers consist of glass, and to optical waveguides in which not all the layers consist of glass. JP-A-04238305 has for example disclosed a planar optical waveguide in which a part of the top layer is formed by a polymer having thermo-optical properties. Here and in the following “layer” is also to be understood as an arrangement formed by structuring a planar, cohesive layer. Thus, the glass layer containing the regions with an increased refractive index can also consist of a structured core layer, and thus a light-conducting planar arrangement of cross-members of approximately rectangular cross-section.
The invention can be effectively utilized wherever a change in refractive index is to be selectively induced. In interferometric arrangements (Mach-Zehnder interferometers or so-called arrayed waveguide gratings (AWG)) for example the optical phase position is finely tuned by a local change in refractive index. Up until the present time such fine tuning has been achieved via UV irradiation (UV trimming).
The invention can be applied particularly advantageously to optical waveguides with an integrated Bragg grating. This is formed for example by causing the beam of an infrared laser to pass through a phase mask onto the optical waveguide. The refractive index profile in the waveguide then corresponds to the diffraction pattern produced by the phase mask. In contrast to known optical waveguides with an integrated Bragg grating, here the refractive index profile is durable so that no impairment of the reflective properties occurs with increasing age.
It has also been shown that the described densification effect does not occur if the glass initial product is not of a hydrolytic but an oxidic origin. If an oxidic glass initial product of this kind is sintered, the density of the glass which forms is such that the glass can no longer be redensified by thermal treatment. If a core layer produced from a hydrolytic glass initial product is surrounded by glass layers produced from an oxidic glass initial product, the change in refractive index produced by the thermal effect can thus be limited to the core region of the waveguide as it is only here that redensification takes place.
The invention further relates to a method of spatially selectively increasing the refractive index in a glass layer, in particular a glass layer in a planar, optical waveguide of the type described above. In accordance with one aspect of the invention, the heat is preferably supplied by means of a laser as its beam collimation ensures a high degree of spatial precision in the heat supply. Preferably a laser with a long-wave operating wavelength (infrared laser) is used, as the UV lasers employed in known methods result in only a small temperature increase in the glass.
In a further preferred embodiment, the core is defined not by structuring a core layer but by locally supplying heat. The core is thus “burned” into the core layer in a type of scribing process. This dispenses with the core-structuring process step.
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patent: 5556442 (1996-09-01), Kanamori et al.
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patent: 5930437 (1999-07-01), Nakai et al.
patent: 0 702 525 A1 (1996-03-01), None
patent: 0 545 432 B1 (1996-07-01), None
patent: 0 810 454 A1 (1997-12-01), None
G.D Maxwell, “Photosensitivity & Rare-Earth Doping in F
Alcatel
Healy Brian
Sughrue Mion Zinn Macpeak & Seas, PLLC
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