Optical waveguides – Integrated optical circuit
Reexamination Certificate
1999-08-23
2001-11-13
Ngo, Hung N. (Department: 2874)
Optical waveguides
Integrated optical circuit
C385S037000, C385S131000, C385S132000, C385S147000, C359S900000
Reexamination Certificate
active
06317528
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to integrated optical (“planar”) Bragg gratings, and particularly to integrated optical Bragg gratings whose peak reflection wavelength is substantially insensitive to temperature variations.
2. Technical Background
A Bragg grating consists of an optical waveguide defining a pattern or stack of regions having alternating higher and lower refractive indices, so that light within a narrow wavelength band is reflected by the grating, and wavelengths outside the band are transmitted through it. A Bragg grating is an excellent narrow-band optical filter which may have a variety of applications, such as a wavelength multiplexer, a fiber laser mirror, a dispersion control device, or a sensor. For this reason, Bragg gratings are becoming increasingly important in optical communications.
An important characteristic of a Bragg grating is the stability of its peak reflection wavelength with respect to variations in temperature. The peak reflection wavelength of a silica fiber Bragg grating generally increases with temperature by about 10 picometers/° C., unless measures are taken to compensate for this shift. Stress may be used to provide such compensation. The peak reflection wavelength of a silica fiber Bragg grating tends to increase linearly with stress at approximately 0.1 picometers per psi of tensile stress. In order to stabilize the peak reflection wavelength of a fiber Bragg grating over a range of operating temperatures, various methods have been devised to vary the level of stress experienced by the grating as the temperature varies, so that the peak reflection wavelength is substantially temperature insensitive. As a rule of thumb, if the stress on a fiber Bragg grating decreases at a rate of about 120 psi/° C., the grating's peak reflection wavelength will be substantially invariant with respect to temperature.
One way of achieving temperature compensation is to attach a fiber Bragg grating that is under tension to a substrate that has a negative thermal coefficient of expansion. As the temperature increases, the substrate contracts, thereby relieving some of the tension in the grating. When the fiber Bragg grating and the substrate are properly designed, the effect that this reduction in the tension in the grating would have on the peak reflection wavelength is offset by the effect that the increase in temperature would have on the peak reflection wavelength in the absence of any change in tension.
There are several drawbacks to the current technology, however. First, state of the art products are bulky and may need to be housed in a hermetic package to prevent degradation of the substrate and/or the fiber-to-substrate bonds. Secondly, the manufacturing process of current devices is generally complicated and expensive. Thus, there remains a need for a compact fiber Bragg grating that is relatively insensitive to variations in temperature.
SUMMARY OF THE INVENTION
One aspect of the invention comprises a method for forming an integrated Bragg grating that includes providing a negative expansion substrate, forming a waveguide on the substrate by alternately depositing material on the substrate and relieving stress within the deposited material, and forming a Bragg grating in the waveguide. The Bragg grating has a peak reflection wavelength, and the waveguide and the substrate have a coefficient of expansion mismatch that is selected to substantially compensate for shifts in the peak reflection wavelength that would otherwise arise from changes in temperature. The coefficient of expansion mismatch may be advantageously greater than about 4 ppm/° C. The stress may be relieved by annealing the deposited material. In a preferred embodiment, forming a waveguide comprises forming a cladding layer on the substrate by alternately depositing a first material on the substrate and annealing the first material, and forming a core layer on the cladding layer by alternately depositing a second material on the cladding layer and annealing the second material, wherein the core layer has an index of refraction that is higher than the index of refraction of the cladding layer.
According to another aspect of the invention, an integrated planar Bragg grating comprises a negative expansion substrate and a planar waveguide having a Bragg grating therein. The Bragg grating has a peak reflection wavelength, in which the waveguide overlies and is in mechanical tension with the substrate. The coefficient of expansion mismatch between the planar waveguide and the substrate is greater than about 4 ppm/° C., and is selected to substantially compensate for shifts in the peak reflection wavelength arising from changes in temperature. In one preferred embodiment, the planar waveguide includes a cladding layer that overlies the substrate; and a core layer adjoining the cladding layer, in which the core layer has an index of refraction that is higher than the index of refraction of the cladding layer.
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Gadkaree Alka K.
Gadkaree Kishor P.
Alden Philip G.
Corning Incorporated
Douglas Walter M.
Ngo Hung N.
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