Integrated optical device providing attenuation

Details Integrated optical device providing attenuation Integrated optical device providing attenuation

1999-05-06

2001-10-16

Healy, Brian (Department: 2874)

Optical waveguides

Accessories

Attenuator

C385S129000, C385S130000, C385S037000, C385S014000

Reexamination Certificate

active

06304710

ABSTRACT:

TECHNICAL FIELD
This invention relates to integrated optical devices such as integrated silicon waveguides for use in optical circuits.
BACKGROUND ART
Integrated optical devices can include silicon waveguides formed on the upper surface of a silicon wafer.
FIG. 1
shows such a construction, in which a rib waveguide
10
is formed on a layer of silicon
12
. The silicon layer
12
is silicon-on-insulator, having been grown epitaxially over a silica layer
14
within a silicon wafer
16
. The entire waveguide is coated for protective purposes with the silica layer
18
. As a result, light propagates within the waveguide
10
.
The actual distribution of optical energy is in fact within the zone
20
. This extends within the upstanding waveguide rib
10
, but is principally within the SOI layer
12
and does in fact extend slightly either side of the waveguide rib
10
.
Some stray light will inevitably be lost from the waveguide. This will normally propagate within the SOI layer
12
, being retained therein by internal reflection. Eventually it may be reflected into a receiver photodiode present on the chip, thus increasing the cross talk signal and decreasing the signal to noise ratio for the device as a whole. The performance of the device could therefore be improved by eliminating such stray light.
It is known to provide locally doped areas within the SOI layer. These act as absorbent areas for stray light, which is then dissipated as heat.
DISCLOSURE OF INVENTION
The present invention provides an integrated optical component comprising a light transmissive layer, the layer having at least one serration formed along an edge thereof, thereby to cause multiple internal reflection of light and hence attenuation thereof.
Such serrations will provide alternative angles for the internal light to be reflected, with minimised perpendicular reflections. Serrations also generate multiple reflections for the stray light, at least some of which will be lossy. These effects will reduce the proportion of stray light which is returned to the active region of the device.
It is clearly preferred that there are a plurality of such serrations along the edges of the light transmissive layer. It may be possible to design the layer such that the majority of the scattered light is captured by a smaller number of suitably located serrations. However, it is preferred if substantially all edges of the layer include serrations. The serrations can be substantially uniform. However, it may in particular designs be preferable to arrange the serrations non-uniformly, for example at a variety of angles.
As the angles subtended by the convergent sides of the serration decreases, so the likelihood of an incoming beam becoming trapped within the serration increases. During such trapping light beams will undergo multiple reflections, all of which will incur some attenuation. For this reason, it is preferred that this angle, hereinafter referred to as &agr;, is minimised. A preferred maximum is twice the critical angle of internal reflection of the material of the light transmissive layer (hereinafter &thgr;
c
). When &agr; is below this angle, any beam which succeeds in reflecting internally into the serration must be channelled towards the tip, its angles of incidence at successive reflections decreasing. Eventually, the angle of incidence may decrease sufficiently for near complete refraction to take place, coupling the beam out of the transmissive layer.
The usual material for the light transmissive layer is silicon, usually presented as silicon-on-insulator. A typical insulator is silica. The refractive index of silicon is approximately 3.5, giving a critical angle of approximately 17° (ignoring the effect of any protective layers of silica).


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patent: 0 397 337 (1990-04-01), None
patent: 0 595 080 A1 (1993-10-01), None
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