Method and arrangement for coupling a wave guide to a component

Optical components intended for telecommunications purposes must be manufactured and mounted with a great degree of accuracy, since a component, such as an optochip, when mounted must be coupled optically, electrically, mechanically and thermally at the same time. In order to obtain a right-angled geometry when using surface-emitting or surface-detecting components, and to obtain a reduced optical travel path and accurate fixation of an optofiber, a reflective surface (12) which slopes at an angle of 45 degrees is arranged between a light-conducting core (16) and the active surface (10) of an optochip, and the light-conducting core has been placed closer to the reflective surface by bevelling the optofiber (9) and fitting the fiber in a V-groove and accurately fixating the fiber in the groove by means of a flat cover means (20). The aforesaid solutions relate generally to problems of a geometrical nature, but are able to provide important advantages in the arrangement of optical components in so-called optical micro-structures in comparison with earlier known techniques with regard to requirements of space, signal transmission performances and manufacturing costs.

FIELD OF THE INVENTION 
The present invention relates to a method and to an arrangement for 
coupling a waveguide to a light-emitting or light-detecting component, 
such as for coupling an optofibre to an optochip. 
DESCRIPTION OF THE PRIOR ART 
Optical components intended for telecommunications purposes are expensive. 
A considerable part of the cost of fabricating an optical component can be 
referred to the coupling established between an optochip, the 
light-emitting or light-detecting component, and an optofibre, the 
waveguide. Extremely high mechanical demands are placed on the criterion 
of "component alignment", the required precision in this regard lying in 
the order of 1/1000 mm. Advanced mounting methods and high-grade precision 
mechanics with elements made of special alloys are needed to meet this 
requirement, which is reflected in the cost. 
Fabrication can be simplified by using small silicon plates as 
micromechanical substrate carriers. Silicon has many advantages which 
provide the unique possibility of producing micromechanical structures 
that can be used for alignment purposes. The carriers can be processed in 
parallel, therewith enabling many "carrier chips", to be obtained from one 
silicon plate, which can result in low manufacturing costs. Silicon has 
also highly effective electrical and thermal properties, which are 
necessary in achieving functional mounting of the optochip. Finally, there 
is a wealth of experience with regard to silicon processing, such as 
electrode patterning and electrical coupling techniques. 
A large number of concepts and proposals have been put forward with regard 
to suitable geometry's for resolving the optochip-waveguide alignment 
problem. Many of these concepts and proposals utilize so-called V-grooves 
in silicon for positioning optofibres in desired places. Anisotropic 
etching methods afford extremely good dimension control of the V-groove, 
where the angle subtended by the walls of the V-form are defined by 
crystal planes in the silicon. For 100! silicon, where 100! denotes 
crystal orientation in relation to the normal vector of a silicon plate, 
it is possible to obtain an appropriate V-groove having a wall angle 
.alpha.=arcsine .sqroot.2/3.apprxeq.54.7 degrees. This angle is also 
obtained at the end of the groove. By metallizing the end of the groove, 
light from an optofibre placed in the V-groove can be reflected up onto a 
light detector. Conversely, light from a light-emitting component can be 
led into the optofibre. 
Mounting of an optochip is an art in itself, since the chip is connected 
optically, electrically, mechanically and thermally at the same time. 
Alignment can be achieved in several different ways. The most common 
method is self-alignment with solder, passive alignment with mechanical 
counterpressure or abutment surfaces, or a pick and place method. 
The first method utilizes the surface tension forces manifested in metal 
solder. With the aid of well-defined, adjacent and solder-wettable 
surfaces on both optochip and carrier, the surface tension forces in the 
molten solder are able to bring the optochip to a desired position on the 
carrier. As the temperature falls, the solder solidifies and affixes the 
optochip in its correct position. 
The second method is based on positioning the optochip in a desired 
position with the aid of micromechanical counterpressure or abutment 
surfaces on the carrier. These surfaces may be fabricated from silicon 
dioxide deposited on the carrier and thereafter patterned to form a corner 
into which the optochip fits. Good control of the position of the corner 
in relation to the waveguide and the active surface of the optochip in 
relation to its outer geometry enables good alignment to be achieved. 
This latter method utilizes alignment marks on carrier and optochip. These 
alignment marks enable the components to be orientated in a common 
co-ordinate system with great precision. In order to subsequently assemble 
the components, there is required a high-class mechanical process which 
will enable the components to be moved in the common co-ordinate system in 
a predetermined manner. All three methods require mounting precision in 
the micrometer range. Details that lie peripheral to these methods will 
not be discussed in this document, although their existence is a 
prerequisite for the suitability of the concept described in the 
following. 
SUMMARY OF THE INVENTION 
In order to obtain a right-angled geometry when using surface-emitting or 
surface-detecting components, and a reduced optical travel path and 
accurate optofibre fixation, a reflective surface that slopes at an angle 
of 45 degrees has been disposed between a light-conductive core and the 
active surface of an optochip. The light conductive core has been caused 
to lie closer to the reflective surface by bevelling the optofibre and is 
therewith also adapted to fit between a V-groove and a flat cover for 
accurate fixation of the fibre. The aforesaid solutions relate essentially 
to problems of a geometrical nature, but provide important advantages in 
comparison with earlier known techniques with regard to space, signal 
transmission performance and manufacturing costs, similar to the 
arrangement of optical components in so-called optical micro-structuring 
techniques.

DESCRIPTION OF PREFERRED EMBODIMENTS 
FIG. 1A-1D illustrate coupling of an optochip in accordance with an earlier 
known technique. A carrier material 2, which may be silicon, has been 
provided with a V-groove 3; see FIG. 1A. There has been placed in the 
groove an optofibre 4 and an optochip 1 has been placed over the free end 
of the optofibre; see FIG. 1B. When seen in cross-section, part of the 
optofibre 4 placed in the V-groove 3 lies outside the groove and carrier 
material; see FIG. 1C. 
As shown in FIG. 1D, the direction of the light is changed through roughly 
110 degrees subsequent to reflection in a mirror 5 on the carrier 
material. Consequently, this fact must be taken into account in order to 
achieve precise positioning of the chip/detector 1. On the other hand, 
this is not sufficient for a light emitting component. This is because the 
numerical aperture (acceptance angle) of a fibre will not permit 
excessively oblique incident light angles to be coupled into a single mode 
fibre, for instance. Consequently, a right-angled geometry based on a 
45-degree mirror would be desirable. This can be achieved by using an 
"obliquely sawn" 100! silicon block instead of "typical" 100! silicon. 
In practice, such so-called wafers are obtained by sawing a silicon block 
obliquely, more specifically at an angle of 9.7 degrees. If this is done 
correctly, the mirror that was earlier inclined at 54.7 degrees will now 
be inclined at 45 degrees (54.7-9.7). Another feature, evident from FIG. 
1D, is that the optical wavelength is relatively long between a 
light-conductive core 6 and an active detector surface, which may have a 
detrimental effect on the coupling efficiency. This is partly because the 
lower part 8 of the optofibre contacts the wall of the mirror, causing 
light to be delivered first to the mirror, and partly because in order to 
reach the detector the light must then pass along a path whose length 
corresponds to one fibre radius. 
The distance between an optofibre 9 and an active surface 10 on an optochip 
11 can be decreased by reducing a first part of the distance, by "cutting 
off" a mirror 12 on a carrier material 13 having a "vertical" wall 14; see 
FIG. 2A. This is achieved conveniently by sawing. The lower part of the 
mirror 12 is completely sawn away with the same technique as that used to 
separate microelectronic chips, wherein the vertical wall 14 may commence 
in the bottom 15 of a groove and terminate immediately beneath a fibre 
core 16 on the optofibre 9. The distance between the fibre core and a 
reflective point 17 on the mirror may be limited to about 10 micrometers. 
The vertical wall 14 may also serve as an opto fibre abutment surface 
therewith facilitating mounting of the fibre. 
The second part of the distance can be decreased by using a fibre whose 
outer diameter is smaller than normal, meaning that the diameter is 
smaller than 125 micrometers. Because it must still be possible to handle 
the fibre, the outer diameter or dimension may not be smaller than 60-80 
micrometers. The total wavelength between the core 16 and the active 
surface 10 will therefore be slightly longer than about 70-90 micrometers. 
With the intention of further decreasing said distance, the fibre 9 has 
been bevelled in the manner of a so-called D-fibre; see FIG. 2B. The 
D-fibre is, in principle, a typical single mode fibre although with a 
D-shaped cross-section, with the core 16 lying close to a flat side 18 of 
the original circular fibre. This particular fibre shape is obtained by 
sawing a preform, i.e. the "glass rod", constituting the original fibre 
material along its length at an appropriate distance from the core. The 
fibre retains the proportions of the pre-form when drawing out the fibre. 
This enables a fibre to be produced in which the distance between the flat 
side and the core centre is very short, less than 10 micrometers. By 
mounting a D-fibre in a V-shaped groove 9 with the flat side 18 of the 
D-fibre facing upwards , the total wavelength, fibre to optochip, can be 
kept beneath 20 micrometers. 
Something which is not evident from FIGS. 1A-D is the difficulty 
experienced in placing an optofibre in a V-groove with the optofibre 
remaining in abutment with the walls of the groove. The optofibre is most 
usually glued in the V-groove. However, the glue tends to lift the fibre 
out of position, with a negative effect on the desired mounting precision. 
Consequently, the use of an auxiliary means to hold the fibre in position 
during the gluing process would be desirable. One such auxiliary means may 
have the form of a cover means or lid 20 secured on top of the carrier 
material, such as the silicon carrier 13, such that the V-groove 19 and 
the cover means 20 together form a space having triangular capillaries 21, 
where the optofibre fits exactly and is therefore unable to change 
position during the gluing process; see FIG. 2B. FIG. 2C illustrates from 
above the fibre 9 fixed in position by the cover means 19 and coupled to 
the optochip 11. 
The cover means 20 may be provided conveniently by anodic bonding. An 
anodic bond is effected by placing together a substrate carrier and a 
cover means, which may be made of silicon or transparent glass, heating 
the assembly and applying an electric potential. Mobile ions produce a 
high field strength across a joint, where the electrostatic forces 
contribute towards creating durable bonds on an atomic scale. The strength 
of the joint is comparable with a strong glue joint. Bonding is preferably 
effected on a so-called wafer level, whereafter separate carriers can be 
sawn from the wafer. The carrier/cover assembly is configured to enable 
certain parts of the cover to be sawn away so as to provide a suitable 
optochip mounting surface. The combination of triangular capillaries 21 
and D-fibre 9 is particularly suitable, since it is otherwise difficult to 
ensure that the D-fibre will be affixed with its flat side upwards, since 
the D-fibre will only fit with the V-groove capillary when positioned 
correctly. 
The aforedescribed solutions provide an optically microstructure which is 
accurately fixated, has a short optical wavelength and is particularly 
suited for mounting on light-emitting or light-detecting optochips mounted 
on silicon carriers.