Tapered rib waveguide

A tapered rib waveguide tapering from a large, multi-mode optical waveguide to a smaller, single-mode optical waveguide, the tapered rib waveguide comprising two portions (4,5) formed of the same material: a lower portion (4) which tapers laterally from the large waveguide to the smaller waveguide and an upper portion (5) formed on the lower portion (4), which tapers to a point (or other form of termination), the dimensions of the two portions (4,5) being such that substantially all of a fundamental mode propagated in the large multi-mode waveguide is coupled to the smaller, single-mode waveguide.

TECHNICAL FIELD 
This invention relates to a tapered rib waveguide providing a transition 
from an optical waveguide of relatively large cross-section to an optical 
waveguide of smaller cross-section. 
BACKGROUND ART 
An integrated optical circuit employs optical waveguides to transmit light 
around the circuit and to connect to external optical waveguides such as 
optical fibres. The integrated optical waveguides and/or optical fibres 
may be of different cross-sectional size so there is a need for means to 
provide a transition from a waveguide of relatively large cross-section to 
a waveguide of smaller cross-section. A variety of devices have been 
proposed for this purpose but they tend to be relatively complex and/or 
difficult to fabricate. 
This invention aims to provide a tapered rib waveguide which is of 
relatively simple construction, is relatively easy to fabricate and which 
provides a low-loss transition from a large cross-section to a smaller 
cross-section. 
It should be noted that terms such as "upper", "lower", "laterally" etc. 
used herein are used in a relative sense, i.e. relative to the orientation 
of the apparatus shown in the accompanying drawing, and are not intended 
to imply any orientation with respect to the force of gravity. 
DISCLOSURE OF INVENTION 
According to a first aspect of the present invention, there is provided a 
tapered rib waveguide tapering from a large, multi-mode optical waveguide 
to a smaller, single-mode optical waveguide, the tapered rib waveguide 
comprising at least two portions formed of the same material: a lower 
portion which tapers laterally from the large waveguide to the smaller 
waveguide and an upper portion, formed on the lower portion, which tapers 
to a point or other form of termination, the dimensions of the two 
portions being arranged such that substantially all of a fundamental mode 
propagated in the large multi-mode waveguide is coupled to the smaller, 
single-mode waveguide. 
Preferred features of the invention will be apparent from the subsidiary 
claims of the specification and from the following description.

BEST MODE OF CARRYING OUT THE INVENTION 
The drawing shows a tapered rib waveguide tapering from a large multi-mode 
waveguide about 10 microns.times.10 microns to a smaller single-mode 
waveguide about 4.3 microns.times.4 microns. 
The illustrated waveguides are in the form of ribs formed on the upper 
surface of a silicon layer, e.g. of a silicon-on-insulator chip. The 
silicon-on-insulator chip is preferably manufactured from conventional 
wafers such as those used in Very Large Scale Integration (VSLI) 
electronic circuits. The paper "Reduced defect density in 
silicon-on-insulator structures formed by oxygen implantation in two 
steps" by J. Morgail et al, Appl. Phys. Lett, 54, p 526, 1989 describes 
the manufacture of suitable wafers. Methods of manufacturing rib 
waveguides on a silicon-on-insulator chip are described in the paper "Low 
loss single mode optical waveguides with large cross-section in 
silicon-on-insulator" by J. Schmidtchen et al, Electronic Letters, 27, p 
1486, 1991. 
The drawing shows the rib waveguides formed in the silicon layer 1 of such 
a chip and shows the oxide layer 2 separating the silicon layer 1 from a 
silicon substrate 3. A silicon dioxide cladding (not shown) is also 
usually provided over the rib. 
The tapered rib waveguide shown in the drawing comprises two portions: a 
lower portion 4 which tapers laterally from a width of about 10 microns to 
a width of about 4 microns over a length of about 1000 microns, and an 
upper portion 5, formed on the lower portion 4, which tapers from a width 
of about 10 microns to a point over a length of about 800 microns. The 
upper portion 5 thus tapers more rapidly than the lower portion 4 and in 
the embodiment shown has a triangular, wedge-like shape when viewed from 
above. Both portions are, however, designed to provide a substantially 
adiabatic taper. 
In another arrangement (not shown), both the upper and lower portions 5 and 
4 taper at the same angle so the sides of the two portions are parallel to 
each other. In this case, the wide end of the upper portion 5 is narrower 
than the wide end of the lower portion 4. 
The upper and lower portions 5 and 4 preferably taper at an angle (with 
respect to the axis of the waveguide) of less than 1 degree and preferably 
less than 0.5 of a degree. Such a gradual taper ensures that any losses 
caused by the taper are kept small. The upper portion 4 tapers to a point 
or to some other form of termination. In the arrangement shown in FIG. 1, 
the tapering section of the upper portion 5 is formed on top of the 
tapering section of the lower portion 4. 
The two portions 4 and 5, including the input facet 6, can be formed 
together by conventional lithography and dry etching as they are defined 
by vertical surfaces. They are formed of the same material and thus have 
the same refractive index. The two portions may be formed homogenously on 
the chip. Alternatively, the upper portion 5 may be formed separately and 
mounted on the lower portion 4. 
The upper faces of the upper and lower portions 4 and 5 are substantially 
parallel to the plane of the chip, i.e. the upper and lower portions 4 and 
5 each have a substantially constant height above the surface of the chip 
(7.2 and 1.5 microns, respectively, in the illustrated example). 
The tapered rib waveguide shown in the drawing thus provides a 
two-dimensional taper, the taper in the lateral direction being provided 
by an actual tapering of the waveguide and the taper in the vertical 
direction (i.e. perpendicular to the plane of the chip) being provided by 
means of the laterally tapered upper portion 5. The problems associated 
with forming a vertically tapered waveguide, i.e. defined by non-vertical 
surfaces, as used in some of the prior art, are thus avoided. 
The shape and dimensions of the tapered rib waveguide are such that if the 
large, multi-mode waveguide is excited in a fundamental mode only, this 
mode is forced down into the smaller, single-mode waveguide as the 
cross-sectional shape of the waveguide changes gradually from that of the 
larger waveguide to that of the smaller waveguide. The effective 
refractive index in the upper portion 5 is progressively reduced as it 
tapers. This forces the mode into the lower portion 4 of the waveguide. It 
should be noted that the tapered rib waveguide does not require a 
refractive index difference between the two portions or for the 
maintenance of single-mode operation of the larger waveguide. As indicated 
above, the larger 10 micron.times.10 micron waveguide is a multi-mode 
waveguide but if it is excited by a fundamental mode on its axis, e.g. 
from an optical fibre, the power in higher order modes is negligible. 
The large refractive index difference between the silicon waveguide and the 
silicon dioxide cladding (not shown) helps ensure that the effective 
refractive index of the upper portion 5 is suppressed sufficiently for the 
optical mode to be substantially entirely guided into the lower portion 4 
before the narrowest section thereof is reached. Thus, as indicated above, 
the upper portion 5 need not taper to a point so the difficulties of 
fabricating very narrow sections at the narrow end of the upper portion 5 
can be avoided. 
The dimensions of the two portions 4 and 5 are selected in accordance with 
conventional requirements of rib waveguides and with appropriate 
dimensions substantially all of a fundamental mode propagated in the large 
multi-mode waveguide is coupled to the smaller, single-mode waveguide. 
A typical example having dimensions as shown in the drawing provides a 
low-loss coupling (typically 0.1 dB or less) between the two waveguides. 
The tapered rib waveguide described herein may be used to provide a 
transition between a variety of sizes of waveguide and particularly from 
an optical fibre to a smaller integrated waveguide. The larger waveguide 
typically has cross-sectional dimensions in the range 7-12 microns wide by 
7-12 microns deep (and is thus compatible with conventional optical 
fibres) and the smaller waveguide typically has cross-sectional dimensions 
in the range 2-5 microns wide by 4-5 microns deep (and is thus compatible 
with conventional integrated optical waveguides). As shown in FIG. 1, the 
depth is measured from the oxide layer 2. 
As indicated above, the input facet 6 may be formed by dry etching, no saw 
cuts or polishing being necessary. The facet 6 may be perpendicular to the 
axis of the waveguide or may be angled. 
It will also be appreciated that the tapered rib waveguide can be used in 
either direction, i.e. from a large waveguide to a small waveguide or from 
a small waveguide to a large waveguide. 
The tapered rib waveguide described above is easily integrated with other 
components of an integrated optical circuit. An optical fibre (not shown) 
inputting light into facet 6 may be positioned in a V-groove (not shown) 
provided in the silicon-on-insulator chip. The position of the V-groove 
and of the tapered rib waveguide may be defined by a common lithographic 
step so they are automatically aligned with each other. 
The applicants' co-pending application no. PCT/GB96/01068 (Publication no. 
WO97/45234) describes a connection between an integrated optical waveguide 
and an optical fibre and is hereby incorporated herein. This earlier 
application describes a rib waveguide formed on a silicon-on-insulator 
chip, the rib waveguide and underlying layer of silicon dioxide 
overhanging the end of a V-groove formed in the chip in which an optical 
fibre is positioned so the end of the rib waveguide lies in close 
proximity with the end of the optical fibre, the overhang being required 
as the end of the V-groove is not formed perpendicular to the surface of 
the chip. 
The tapered rib waveguide structure described herein may be formed on such 
an overhang to provide a transition between a large, multi-mode waveguide 
for receiving light from the optical fibre and a smaller, single-mode 
integrated waveguide formed on the chip. 
The illustrated embodiment comprises a tapered rib waveguide formed as a 
straight waveguide. In some circumstances, however, it may be desirable to 
form a similar structure on a gradually curving waveguide. The tapered 
sides of the upper and lower portions 4 and 5 do not need to be straight 
but may also be curved, nor need they necessarily be symmetrical. 
As indicated above, the taper should be gradual. If the taper is too rapid, 
the losses will be higher. On the other hand, if the taper is too gradual, 
the device may be larger than is desirable. A compromise between these two 
requirements is usually necessary. 
In addition to tapering the lower portion 4, a tapered section of the 
silicon slab 7 may also be removed as shown by dashed lines in FIG. 1. 
Removal of the slab 7 adjacent the rib waveguide increases the refractive 
index difference between the waveguide and its surroundings and by 
gradually narrowing the gap 7, the effective refractive index difference 
is gradually reduced. 
The illustrated embodiment shows a tapered waveguide having two layers, 
i.e. the upper portion 5 and lower portion 4. However, it will be 
appreciated that further layers may be provided, e.g. an additional 
tapering portion on top of the upper portion 5, and that the tapering may 
occur in two or more stages at different positions along the waveguide. 
FIGS. 2A and 2B are schematic diagrams illustrating an example of a 
multi-stage tapered rib waveguide. A 10 microns high rib 10 initially 
tapers to a 7 microns high rib 11 which then tapers to a 4 microns high 
rib 12. FIG. 2B shows a cross-section taken across line a--a of FIG. 2A. 
By this means a 10.times.10 microns waveguide can be tapered down in two 
stages to a 2.times.4 microns waveguide, e.g. for coupling with a laser. 
The tapered portion 10 overlaps the tapered portion 11 so as to reduce or 
minimize losses due to light diverging as it is forced down from portion 
10 to portion 11 and, similarly, tapered portion 11 overlaps tapered 
portion 12. 
FIGS. 3-5 show further embodiments of a tapered waveguide, 
The arrangement shown in FIGS. 3A and 3B is similar to that shown in FIG. 1 
except that the tapered structure 13, comprising the upper and lower 
portions 5 and 4 and a substrate 14 on which these are formed, is a 
separate component which is mounted on a silicon-on-insulator chip 15 and 
aligned with a waveguide 16 thereon. This enables the tapered structure 13 
to be formed of a different material, e.g. silicon dioxide or silicon 
nitride, and enables it to be fabricated separately from the remainder of 
the optical circuit. 
The tapered structure 13 is mounted on the silicon-on-insulator chip 15 by 
locating surfaces 13A and 13B against corresponding surfaces 15A and 15B 
in a recess or cut-out section of the chip 15. A further pair of abutting 
surfaces (not shown) may be provided for locating it in a lateral 
direction relative to the waveguide 16. The tapered structure 13 may be 
secured in place by means of adhesive. 
A two-stage tapered waveguide may also be made in a similar manner as shown 
in FIGS. 4A and 4B. In this case, a separate tapered structure 17 is 
provided which comprises the upper tapered portion 5 on an untapered 
portion 18 and a substrate 19. The lower tapered portion 4 is formed on 
the silicon-on-insulator chip 15. The tapered structure 17 is mounted on 
and aligned with the silicon-on-insulator chip 15 in a similar manner to 
that described above. 
FIGS. 5A and 5B show a further arrangement in which a tapered structure 20 
of a different material is fabricated directly onto a silicon-on-insulator 
chip 15. FIGS. 5A and 5B are similar to FIGS. 3A and 3B but in this case 
the upper and lower portions 5 and 4 and substrate 14 are fabricated 
directly onto the chip 15. The tapered structure 20 may, for example, be 
formed of silicon dioxide deposited or grown on the silicon-on-insulator 
chip 15 or formed of a polymer material spun onto the chip. 
As the tapered structures 13, 17 and 20 are formed of a different material 
compared to the waveguide 16 (and lower portion 4 in FIG. 4), an 
anti-reflective coating is preferably provided between the two to help 
reduce back-reflection of light passing from the tapered structure into 
the silicon-on-insulator device. 
Although the examples described above are formed on a silicon-on-insulator 
chip, and although this has the advantages referred to above, a similar 
tapered rib waveguide can be formed on other types of chip.