Note: Descriptions are shown in the official language in which they were submitted.
~, 2172448
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Field of the Invention
The invention concerns an optical branch comprising a s.~ h~te and an integ.~l-
ed optical w-~veg~ide structure con.~i.sting of at least one waveguide for coupling glass
fibers thereto and at least two branching waveguides. Such branches are used e~peci~lly
in optical co~ nications systems.
~3c~ lion of the Prior Art
An optical branch is known from publication EP 0 277 390 B1, wherein optical
waveguides are integrated on a substrate, namely an input waveguide ext~ncling in a
straight line from one end of the s tbsll~e to the opposite end, with two c~c~-lin~
10 branches, each of which contains two limbs. This achieves a distribution of 1:5. In
addition, IOC modules with tree structures implemented by ~c~ding Y branches areknown, which have a distribution of 1:8 e.g. (EP 0 283 301 B1, Figure 34).
Since passive optical distributors have increasingly more ~ignific~nce in new
optical distribution nelwulh~, optical distributors with partition ratios of l:N or 2:N (e.g.
N = 2, 4, 8, 16, 32...) have been designed for coupling single-mode glass fibers (ELEC-
TRONICS Ll~l l~KS, Volume 26, No. 11 of May 24, 1990, pages 707 and 708, and
ELECTRONICS Ll~l l~KS, Volume 27, No. 23 of November 7, 1991, pages 2131-
2133).
The coupling between glass fibers and optical waveguides at the branch inlet is
20 problematic, particularly because of an unavoidable offset belween the glass fiber core
and the optical waveguide. As r~olled in the "IEEE JOURNAL OF LIGHTWAVE
TECHNOLOGY, Volume 10, No. 11 of November 1992, pages 1570-1573" and in
"OPTICS L~ KS, Volume 16, No. 5 of March 1, 1991, pages 309-311", the offset
itself leads to the undesirable excitation of higher-order modes in w~vegl~ides of the mode
field, which have been adapted to the glass fibers.
As quasi-guided modes, these higher-order modes can hltelÇele over relatively
long distances of several millimeters for example, with one or even several guided modes
in the optical waveguide, and thereby lead to an oscillatory line loss of the guided
2172448
mode(s) along the length of the optical path. This leads to a greater attenuation and, in
the branch areas, to unequal and polarization-dependent distribution of the optical output
to the individual branches as well.
Sullln~ of the Invention
The invention has the task of reducing the optical losses and the polarization-
dependent unequal distribution of light by means of an optical branch comprising a sub-
strate and an integrated optical waveguide structure consicting of at least one waveguide
for coupling glass fibers thereto and at least two branching waveguides, ch~te.;7P~d in
that before the branching point, the waveguide has a constriction. A configuration of the
10 branch includes constriction before each branching point if the wa~eguide structure is in
the form of c~cc~-lP~ Y branches.
The constriction can further comprise a first taper and a second taper, the first
taper reducing the width of the waveguide to a narrow waveguide section, and the second
taper subse~luently widening the narrow waveguide section again.
Furthermore, the entire waveguide structure between each branch stage and the
~eclive widening second taper can be formed by constricted wa~eguide sections.
The optical branch can further be configured so that the first taper is located
immediately at an input of the waveguide structure serving to couple a glass ffber thereto.
The optical branch can also be configured so that the second taper is located
20 immediately at an output of the waveguide structure serving to couple a glass fiber there-
to.
21724~8
Desc~iplion of the DMwinP
The invention is described in greater detail in the following by means of a draw-
ing which scl-e..,At;cally illustrates a configuration example of an optical branch.
Detailed Desc~ lion of the ~ler~lled Embodiment
In the drawing, the optical branch as a whole has the number 1. It comprises a
s.-bsl~te 2 with an inte~ led optical cGI~lponent, which in the configuration example has
the wdveguide structure of a single Y branch.
A waveguide 3 is grown from doped SiO" on the s~bs~l~te 2, which is made of
silicon e.g., and is separated from the latter by a buffer layer. The waveguide 3, which
10 is covered by another buffer layer, is 7 ~m thick for example, and its width is adapted to
the profile of a single-mode fiber. Waveguide cross sections of 7x7 ~m2 are typical. The
refractive index step between waveguide 3 and the buffer layers is 0.004 e.g. Of course,
other mAtenAl~ can be used to construct the op~ical branch 1, such as III-V semiconduc-
tors, quartz glass, lithium-niobate or lithium-tantalate.
The waveguide structure of the substMte 2 on the inlet side of the optical branch
1 compAses an optical waveguide 3, which is used to couple a glass fiber, a first taper 4
co~ ed thereto, and a n~lower waveguide section 5. A s.~l)s~uent second taper 6 is a
mirror-image of the first taper 4, which widens the narrow waveguide section 5 again,
e.g. to the width of the original optical waveguide 3. However, in the example illustrated
20 by the drawing, the second taper 6 widens even more to twice the width of one of the two
outgoing branches 7, each ~f which is as wide as the optical waveguide 3. If nece~A~y to
achieve a defined separation of the branches 7, a gap that inte.lu~ the optical
waveguides can be provided bet~een the second taper 6 and the branches 7 (not
illustrated).
With CA~Ca~1;ng Y branches, the diverging waveguides of each branch have
generally different lengths to the point from where each branch 7 continues with a suit-
able bend 8 at a distance and parallel to the neighboring branch 7.
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The improvement of the optical branch 1 plo~ellies compAses in pareicular
forcing the radiation of undesirable quasi-guided modes in the w~veguide very quickly
over a short path, thereby avoiding the effect of the coherent coupling. This is achieved
by constricting the waveguide 3 (waveguide section 5), which serves as an optical filter,
before it branches off (branches 7). It is imm~teri~l whether the optical wa~eguide struc-
ture consists of buried, planar integrated or strip lines. In the present configuration
example of the optical con-ponent, the constriction is achieved by nal,owing the width of
waveguide 3. However, as an altemative, the constriction can be a waveguide section
whose width has been reduced accordingly.
It was ~el~milled that the inlelrelence effect described above can take place inall disturbances of the waveguide; the constriction can thelc~ro~ be provided for each
optical col,-ponent which contains at least one waveguide that has been disturbed in some
manner, for example by a bend in the line or wherever undesil~ble modes occur; e.g. in
a coupling area between a laser that is integl~ed into the substrate and coupled to a
waveguide, or between other optical co-nponents. The waveguide 3 on the inlet side can
be omitted in this coupling area, as well as in the glass fiber coupling area, and the laser
or the glass fiber can be coupled directly to the base of the first taper 4. This ~..eas.lle
does not affect the intended effect of very quickly sul~lessing the quasi-guided modes,
but it allows the manufacture of optical branches 1 with short lengths.
If the source of the disturbance producing undesirable modes is the bend 8 in a
waveguide (branch 7), the constriction is located behind the bend in such a way, that the
constriction encompasses the bent waveguide. This allows the length of the constricted
linear waveguide to be shorter. With c~cading cG.--pollel-ts in general, the entire
waveguide area belweell the branching steps can be configured for a reduced wa~eguide
width, so that the entire waveguide path up to the widening taper of the next ~lanching
step is constricted and functions as an optical filter. This allows the manufactured length
to be shortened even further.
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If the second taper 6 forms the outgoing end of the waveguide structure, it can
be used to directly couple a glass fiber or another component, or a linear waveguide can
be connected to the taper (6), to which the glass fiber or another col..pollent is coupled.
The dimensions of the constriction depend on the design of the optical compo-
nent. Designs of 1300 nm and 1550 nm wavelengths are typical. In an optical branch 1
with integrated waveguides that are about 7 ,um wide, the tapers 4, 6 have a taper angle a
that is e.g. 0.1 to 1.0 with reference to the central axis of the waveguide 3, a
wdveguide section 5 that is between about 6.0 ~m and 6.5~m wide, and is applo~imately
lmm to 4 mm long, including the first taper 4.
