Optical waveguide device

Details Optical waveguide device Optical waveguide device

2001-08-15

2004-01-13

Cherry, Euncha (Department: 2872)

Optical waveguides

Planar optical waveguide

C385S131000

Reexamination Certificate

active

06678452

ABSTRACT:

The invention relates to an optical device with reduced birefringence. More particularly it relates to an optical planar device in which the influence of the substrate on the stress-induced birefringence of the optical waveguide device is reduced by modification of the substrate from Underneath the waveguide element.
TECHNICAL FIELD AND BACKGROUND OF THE INVENTION
Integratable planar optical waveguide devices usually consist of a multilayer stack of glass-based materials fabricated onto a suitable substrate. In cases where glass-materials comprise the multi-layer structure, a silicon substrate can be used as the fabrication base, since it is cheap and its processing is well known. Upon application of thermal processing of the multilayer in the fabrication process (growth, diffusion, annealing), the different thermal expansion coefficients of silicon and the glasses lead to unacceptably high values of induced anisotropic stress within the optical guiding structure, notably near the waveguide core. The induced stress changes the propagation characteristics of the TE and TM optical polarizations. With other words, the stress-anisotropy causes birefringence, i.e. polarization-dependent refractive indices. As a practical example, the high refractive index contrast in silica-on-silicon waveguide technology with SiON core layers can be seen. Optical birefringence is a limiting factor in the use and scalability of SiON waveguide technology.
In the article “Characterization of Silicon-Oxynitride Films deposited by Plasma Enhanced CVD” by Claassen, v.d. Pol, Goemans and Kuiper in J. Electrochem. Soc.: Solid state science and technology, July 1986, pp 1458-1464 the composition and mechanical properties of silicon-oxynitride layers made by plasma-enhanced deposition using different gas mixtures are investigated. It is stated that the mechanical stress strongly depends on the amount of oxygen and hydrogen incorporated in the layer. Heat treatment at temperatures higher than the deposition temperature leads to a densification of the film due to hydrogen desorption and cross-linking.
In “Temperature dependence of stresses in chemical vapor deposited vitreous films” by Shintani, Sugaki and Nakashima in J. Appl. Phys. 51(8), August 1980, pp 4197-4205 its is shown that in vitreous silicate glass depending on deposition background pressure different components of tensile and compressive stress occur. Also a hysteresis of the stress is observed.
In “Stress in chemical-vapor-deposited SiO
2
and plasma-SiN
x
films on GaAs and Si” by Blaauw in J. Appl. Phys. 54(9), September 1983, pp 5064-5068 stress in films of CVD-SiO
2
and plasma-SiN
x
on GaAs is measured as a function of temperature. Different properties of the stress are observed depending on e.g. film thickness, doping and annealing parameters. “Stress in silicon dioxide films deposited using chemical vapor deposition techniques and the effect of annealing on these stresses” by Bhushan, Muraka and Gerlach in J. Vac Sci. Technol. B 8(5), September/October 1990, pp 1068-1074 deals with in situ measured stress as a function of annealing temperature. Different deposition techniques are investigated and in PECVD silica films on silicon substrates a change of the stress sign from tensile to compressive is observed with rising annealing temperature.
In U.S. Pat. No. 5,502,781, integrated optical devices which utilize a magnetostrictively, electrostrictively or photostrictively induced stress to alter the optical properties of one or more waveguides in the device are disclosed. The integrated optical devices consist of at least one pair of optical waveguides preferably fabricated in a cladding material formed on a substrate. A stress-applying material, which may be a magnetostrictive, electrostrictive or photostrictive material, is affixed to the upper surface of the cladding material near at least one of the optical waveguides. When the appropriate magnetic, electric or photonic field is applied to the stress applying material, a dimensional change tends to be induced in the stress applying material. The constrained state of the stress applying material, however, caused by its adhesion to the cladding material, causes regions of tensile and compressive stress, as well as any associated strains, to be created in the integrated optical device. By positioning one or more optical waveguides in a region of the device which will be subjected to a tensile or compressive stress, the optical properties of the stressed waveguide may be varied to achieve switching and modulation. Latchable integrated optical devices are achieved by utilizing a controlled induced stress to “tune” one or more waveguides in an integrated optical device to a desired refractive index or birefringence, which will be retained after the field is removed.
U.S. Pat. No. 4,358,181 discloses a method of making a preform for a high numerical aperture gradient index optical waveguide. Therein the concentration of two dopant constituents is changed during fabrication. Concentration of the first dopant, GeO
2
, is changed radially as the preform is built up in order to produce the desired radial refractive index gradient. The concentration of the second dopant, B
2
O
3
, is changed radially to compensate for the radial change in thermal expansion coefficient caused by the varying GeO
2
concentration. B
2
O
3
is added to the cladding layer to make the thermal expansion coefficient of the cladding equal to or greater than the composite thermal expansion coefficient of the core. The magnitude of residual tension at the inner surface caused by thermal expansion gradients is reduced and premature cracking of the preform is eliminated.
Disclosed in U.S. Pat. No. 4,724,316 is an improved fiber-optic sensor of the type in which a fiber-optic waveguide component of the sensor is configured to be responsive to an external parameter such that curvature of the fiber-optic waveguide is altered in response to forces induced by changes in the external parameter being sensed. The alteration of the curvature of the fiber-optic waveguide causes variations in the intensity of light passing therethrough, these variations being indicative of the state of the external parameter. The improvement comprises coating material covering the exterior portion of the fiber-optic waveguide, the coating material having an expansion coefficient and thickness such that distortion of the fiber-optic waveguide caused by thermally induced stresses between the coating material and the glass fiber is substantially eliminated. Also disclosed is a support member for supporting the curved fiber-optic waveguide, the support member and fiber-optic waveguide being configured and arranged to minimize the effects of thermal stress tending to separate the waveguide from the support member.
A reported method to reduce the induced stress within the optical guiding channel is described in U.S. Pat. No. 4,781,424, using the application of grooves adjacent to the channel in order to relieve the stress-component within the glass layers. U.S. Pat. No. 4,781,424 is related to a single mode optical waveguide having a substrate, a cladding layer formed on the substrate, a core portion embedded in the cladding layer, and an elongated member for applying a stress to the core portion or a stress relief groove for relieving a stress from the core portion in the cladding layer along the core portion. The position, shape and material of the elongated member or the groove are determined in such a way that stress-induced birefringence produced in the core portion in accordance with a difference in thermal expansion coefficient between the substrate and the single mode optical waveguide is a desired value. In all methods disclosed therein, the device is subjected to treatment from the upper side, i.e. the side where the waveguide structure is located. The disclosed method further employs a mask to define the grooves and a removal technique to produce the grooves. Both items lead lo significant additional processing work.
In EP 0 678 764 the fabrication o

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