Temperature stable integrated optical device

Details Temperature stable integrated optical device Temperature stable integrated optical device

1998-07-23

2001-04-03

Bovernick, Rodney (Department: 2874)

Optical waveguides

Planar optical waveguide

C385S130000, C385S039000, C385S014000

Reexamination Certificate

active

06212323

ABSTRACT:

TECHNICAL FIELD
This invention relates to temperature stable integrated optical devices such as integrated silicon waveguides for use in optical circuits and the modulation of light within those circuits.
BACKGROUND ART
Interferometers are important elements of optical circuits, and are used, for example, to provide a wavelength selection function, routing functions, analysis functions, interrogation functions, switching etc. Such devices work by providing two pathways for the incoming light which are of slightly different optical length. The two pathways then recombine, at which point interference effects occur and wavelength selection, for example, is achieved.
A major difficulty with fabricating such interferometers is that silicon has a refractive index that varies with temperature, as do most optical materials. Thus, devices using imbalanced optical paths with a nominally fixed length will have a transmission characteristic which will vary with temperature.
This temperature variation is commonly alleviated either by using thermo-electric coolers or by heating the entire optical component to a specified temperature. Thermo-electric coolers are expensive, whilst it is often undesirable to heat the component.
DISCLOSURE OF INVENTION
According to the present invention, there is provided an integrated optical device comprising first and second associated optical pathways, at least the first pathway including at least two sub-sections of first and second different transmissive materials, the different materials exhibiting a different temperature dependence of refractive index, the lengths of the pathways and of sub-sections being selected such that the device is substantially temperature insensitive.
Preferably, through the use of two different materials, the invention allows the difference in thermal dependence between the two materials in the first pathway to be exploited so as to match the composite thermal dependence of the first pathway to the second.
Alternatively, it is possible for the second pathway to be of a single material, but it is usually easier to design the device if the second pathway also includes a sub-section of a different transmissive material.
Suitable materials are silicon and silicon nitride, although the present invention is not limited to that combination of materials. Likewise, simplicity of design can be achieved by including only two sub-pathways in the relevant pathway.
The preferred embodiment is likely to include two sub-sections on two pathways, and therefore the remainder of this Application will include description of such a device. However, the principle of temperature compensation disclosed herein can be employed if desired in more complex arrangements.
A preferred method of selecting the lengths of pathways and sub-pathways to achieve temperature insensitivity is to ensure that the ratio of the difference in length of sub-sections formed of the first material to the difference in length of sub-sections formed of the second material is equal to the ratio of the refractive index gradient with temperature of the second material to the refractive index gradient with temperature of the first material. This can also be expressed as in equation 7 below.
The device is preferably an interferometer. However, the invention is applicable to any device using two or more nominally fixed length pathways, such as an arrayed waveguide grating.
Other preferred features of the invention will be apparent from the following description and the subsidiary claims of the specification.


REFERENCES:
patent: 4923278 (1990-05-01), Kashyap et al.
patent: 5311279 (1994-05-01), Wendler
patent: 5611007 (1997-03-01), Wolf et al.
patent: 5703978 (1997-12-01), DiGiovanni et al.
patent: 5799118 (1998-08-01), Ogusu et al.
patent: 0 281 385 A2 (1988-09-01), None
International Search Report for International Application No. PCT/GB99/01946 (2 pages).
H. Tanobe et al. “Temperature Insensitive Arrayed Waveguide Gratings on InP Substrates”, IEEE Photonics Technology Letters, vol. 10, No. 2, Feb. 1, 1998, pp. 235-237.

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