Note: Descriptions are shown in the official language in which they were submitted.
BACKGROUND OF THE INVENTION
1. Fleld of the invention.
This invention concerns the coupling between an
integrated optical wave-guide and a monomodal optical fiber.
Such fibres are used for optical telecommunica-
tions, because they allow signals to be propagated with pass-
bands of more than 1 gigahertz.
Optical telecommunications also make use of modu-
lators and switches, preferably of integrated optical cons-
truction. The integrated guides for such modulators and
switches have to be connected to telecommunication lines
made wi-th monomodal optical fibres t with minimum transmis-
sion loss caused by such connection.
2. Description of the prior art.
Methods for connecting a monomodal fibre and an
integrated waveguide in such a way as to ensure negligible
positioning errors have been described, for example, by
~.P. Hsu and A.F~ Milton, in the periodical Electronics
Letters, vol. 12, no. 16 of 5th August 1976. An improved
process for aligning a monomodal optical fibre and an
integrated wave-guide formlng part of an integrated opti-
cal circuit is characterized by the Eact the fibre is pla-
ced in a straight
- .
., ,.:j ~
33~
groove in a plate, which rests on one base, a second base is juxtaposed
beside the first, by placing flat slip surfaces against each other, the
integrated optical circuit being attached to the second base, the axes
and ends of the wave-guide and fibre are aligned by moving the fibre in
the groove and by sliding the first base relative to the second bases the
fibre, plate and first base being clamped together, and the two bases
being joined by an adhesive film covering the slip surfaces,suchadhesive
being applied while fluidg at any rate during the final step of alignment
operations.
However, the efficiency of connection depends on how well the modes
of light energy distribution in the fibre and wave-guide sections overlap.
On the one band, in the fibre, the core radius of which is only a
few microns for monomodal fibres, the electric field is distributed accord-
ing to a substantially Gaussian and circular pattern. The distance WO,
(distribution parameter) from the fibre axis at which the electric
field drops to 1/e of its value at the axis is also a few microns.
On the other hand, the electric field of a wave-guide with lateral
confinement, realised in integrated optics follows substantially a
Gaussian elliptical distribution, along two rectangular axes of the
cross-sectional plane. For a guide with rectangular cross-section, there
are two distribution parameters Wx and Wy, i.e. the distances along the
rectangular axes x and y at which the electric field drops tol/e of its
value at the centre of the guide.
This invention offers a method -to improve the coupling by
adapting such distribution modes in order to increase their mutual
overlapping, when (as is generally the case) WO is greater than ~ .
SUMMARY OF THE INVENTION
The invention concerns a method for coupling or connecting an
integrated optical wave-guide and a monomodal optical fibre, the monomodal
fibre propagating a ma;nly Gaussian distribution of the electric field
with a distribution parameter WO, and the integrated optical wave-guide
with lateral confinement propagating a mode of distribution of the
electric field with two separate parameters Wx and Wy on the cross-
sectional plane of the guide, this method being characterized by the
fact that it comprises a preliminary step during which the end portion
of the fibre and/or wave-guide is treated, in order to reduce the
diFference between WO and ~WxWy at the coupling section.
This preliminary treatment consists in stretching the end portion
of the fibre. Experience shows that if the fibre is heated for long
enough, doping agents will migrate, producing the necessary adaptation.
BRIEF DESCRIPTION OF THE DRAWINGS
Other features and advantages of the invention will ~ppear from
the following description, with reference to the accompanying figures:
- figure 1, showing a diagrammatical view of the junction of an
optical fibre and an integrated wave-guide;
- figure 2, showing electric field distributions in the wave-guide
and fibre;
- figure 3, showing an explanatory graph.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Figure 1 shows an integrated wave-guide G on a blade L, produced,
for example, by diffusing titanium on a lithium niobate substrate.
r - ~
3~
The core C of the monomotlal optical fibre F to be connected to
the guide G has a radius a.
Taking co-ordinates x and y in the input section of the guide G,
the electric field E1 (x~y) in the guide cross section can be generally
represented by the following equation:
E~(x,Y) = exp ( X2 + y2 ~
4WxWy Wx~ Wy2 ( 1 )
where Wx and Wy depend on the dimensions and refraction indexes of the
guide.
A distinction may be made between monomodal fibres in which the
refraction index varies continously and those in which it is subject to
abrupt variations.
For abruptly varying refractive indexes, if r is the distance to
the fibre axis z and n(r) is the refraction index:
n(r) = n(g) where r exceeds a
n(r) = n(g) + ~n where r is less than a
and ~n = n(c) - n(g) (2)
where n(g) and n(c) are the refraction indexes of the sheath and core,
and a the core radius; An is also known as the "index jump".
For continously-varying refraction indexes, n(r) is given by the
following equation, where it is assumed to be Gaussian:
n(r) = n(g) + ~n exp (~r2~a2) (3)
To simplify matters, the core radius will be referred to as a
! and the index variation as ~n for both types of fibres, since the same
laws of propagation apply for practical purposes to equations 2 and 3
above.
3~3
If the index variation is discontinous, it can be shown that, for
a fibre to be monomodal, V must be less than 2.4 (4)
with V = a2~ ~ 2n~n (5)
where ~ = is the wavelength.
The fundamental mode, which can be propagated only if equation 4
applies, is in practice such that:
E2(r) = 1l exp(-r2/W02) (6)
where E2(r) is the electric field, and WO, for Gaussian dlstribution,
is generally given by the equation:
~0 = a/(V~ (7)
where V is given by equation 5.
For an abrupt index variation, on the other hand, WO is given by
the equation: -
Wo = a
Ln(V2)
If it is assumed that the axis z of the fibre F is properly
centred relative to the guide Gi with negligible angular error, and also
that the end of the fibre F is more or less in contact with the guide
input, the coupling y between fibre and guide is given by the equation:
y = {I~E1(x~y)E2(x,y) dxdy}2 (8)
where E1(x,y) and E2(x,y) are standardized values for electric fields of
modes in the integrated guide G and fibre F, with integration on the
25 plane ~ in figure 1.
Equations 1, 6 and 8 above show that y is maximum where:
~0 = \/ WxWy
Figure 2 shows, in diagrammatical form, the curves CG and CF
5 corresponding to the points at which E~(x,y) and E2(x9y) are equal to
1/e of the maximum.
In this invention, the end portion of a monomodal optical fibre
with a given Wo(i) is treated in order to obtain a new distribution
parameter W to on the end surface such that:
W to = \/ WxWy
In one embodiment9 the increase in WO at the fibre end is obtained
by drawing a length 11, normally greater than 100um, of the end portionT.
Figure 3 shows the variation in W to . at the fibre end in relation
to $he radius of the core end a t , where the refraction index is subject
to Gaussian variation in accordance with equation 7.
In another embodiment, at the output WO is brought to W-to by
heating, which causes doping agents to migrate. For Gaussian distribution
of the index, this operation produces an increase in the core radius and
a reduction in An.
These two operations, which match the parameters by treating the
fibre, can be transposed to the integrated wave-guide, in order to obtain
the equation WO = ~ WxWy~ This may be done, for example, by migration
of doping agent on the end portion of a guide of uniform width, or by
gradual modification of the width of the end part of an integrated
wave-guide.
