Indiffused optical waveguide structures in a substrate

Details Indiffused optical waveguide structures in a substrate Indiffused optical waveguide structures in a substrate

2001-06-12

2004-04-06

Healy, Brian (Department: 2874)

Optical waveguides

Planar optical waveguide

Thin film optical waveguide

C385S129000, C216S024000, C216S037000

Reexamination Certificate

active

06718110

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to a method of manufacturing an indiffused optical waveguide structure in a substrate, Furthermore, the invention relates to an indiffused optical waveguide manufactured by such a method and to various integrated optics devices using such indiffused waveguide structures. Such integrated optic devices may be for example an acousto-optical mode converter, an accusto-optical switch, an optical power splitter, a dual-output Mach-Zehnder modulator, a polarisation splitter and an electro-optical switch. In particular the invention relates to making an indiffused optical waveguide in birefringent substrate materials like LiNbO
3
. In such a birefringment substrate the refractive index neff, TE, TM for the quasi TE and quasi TM-modes in the waveguides will respectively have slightly different values with respect to the substrate index.
The manufacturing method and the waveguide structures according to the invention are superior to previously known waveguide structures in that they can be manufactured with higher dimensional precisions, for example, in order to keep the variations of the birefringence along the optical waveguides as small as possible resulting in an overall improved performance of integrated optic devices using such waveguide structures.
BACKGROUND OF THE INVENTION
FIG. 1
shows several optical waveguide structures applied to integrated optic devices, for example a raised stripe waveguide (
FIG. 1
b
), a rib waveguide or optical stripe-line (
FIG. 1
c
), a general channel waveguide (
FIG. 1
d
) or a ridge waveguide (
FIG. 1
e
).
Many integrated optics devices use the so-called diffused or embedded waveguide as shown in
FIG. 1
a.
Furthermore, the optical waveguide structures are not limited to any particular longitudinal geometry, i.e. any kind of straight or curved geometry as used in Y-junctions, polarising beam splatters etc. can be used.
FIG. 2
shows examples of such basic structures:
FIG. 2
a
: Y-junction,
FIG. 2
b
: WDM.-device;
FIG. 2
c
: star coupler and
FIG. 2
d
: polarising beam coupler.
Furthermore, the optical waveguide and substrate materials are not limited to any particular material, That is, generally the materials can comprise isotropic, anisotropic and birefringent materials. In particular, the usage of a birefringent substrate is essential for the manufacturing of acousto-optical devices. For example, the diffused channel waveguide of
FIG. 1
a
can comprise a substrate material of LiNbO
3
with a waveguide made by a titanium indiffusion.
Whilst theoretically the geometry of the waveguide within the substrate is determined on the basis of known diffusion profiles, due to the fabrication conditions such a theoretically calculated diffusion profile or waveguide geometry is never reached in practice. Furthermore, there is no easy means to measure the actual distribution of the waveguide cross section. Therefore, the operator performs numerous experiments to find out the manufacturing conditions such that the produced integrated optics device has a performance that fulfils the theoretically calculated requirements.
Therefore, due to the imperfections during the fabrication processes used for making the waveguides in
FIG. 1
(for example disuniformities in the titanium stripe dimensions, temperature gradients during diffusion, etc.) the effective waveguide birefringence varies locally over the wafer used for making a plurality of such devices at the same time and also as an averaged value from wafer to wafer. The applicant has found that the performance of single optical components (e.g. straight and curved waveguides) as well as more complex integrated optical devices, like an acousto-optical mode converter depends critically on the uniformity of the waveguide birefringence. Thus, the overall performance and reproducibility of acoustooptical devices strongly depends on the homogeneity and reproducibility of the fabrication processes.
Birefringence essentially means that the effective index (or the propagation constant) for (quasi) TE-modes and TM-modes is different and therefore the requirement of a small variation of birefringence means that the difference in propagation constants or the difference in refractive index An remains the same along the optical waveguide as much as possible. There is no known relationship between the extent of such a birefringence variation and the fabrication parameters and thus it is unpredictable how large such variations are.
The birefringence variations can have detrimental effects even in simple single waveguides. In integrated optics and also in distributed optical communication systems it is often desirable to switch the input polarisation of a TE-mode to the TM-polarisation and this can, for example, be performed by electro-optical couplers or by an acousto-optical mode converter. The latter device is based on the usage of a birefringent optical waveguide and if this waveguide has birefringent variations this will cause the performance of this device to deteriorate drastically.
Birefringence Variation in Acousto-Optic Devices
The detrimental effects of birefringent variation in the basic acousto-optical mode converter are explained with reference to FIG.
3
. The working principle of an integrated acousto-optical device e.g, on LiNbO
3
is based on a wavelength selective polarisation conversion between two copropagating optical waves polarised along the main birefringence axes of the LiNbO
3
-crystal i.e. between the “TM”- and “TE”-modes, Energy can be exchanged between these orthogonal polarisation modes when they get coupled by the off-diagonal elements in the dielectric tensor. This is possible for example by the electro-optic or photo-elastic effect as explained below. A surface acoustic wave, i.e. an elastic “Rayleigh-wave” in a photoelastic and piezoelectric material such as in LiNbO
3
is an ideal means of coupling due to its tunability in frequency and in power.
As shown in
FIG. 3
a straight monomodal waveguide of conventionally for example 7 &mgr;m is embedded in about a 100 &mgr;mm wide monomodal acoustic-waveguide (x-cut, y-propagating LiNbO
3
-crystal). Both optical waveguides and acoustic claddings are fabricated by a titanium indiffusion. Metalinterdigital transducers of a suitable configuration are deposited on top of the crystal at the beginning of the acoustic waveguide. By applying a RF-drive signal at the interdigital transducer electrode an acoustic wave is excited. The acoustic wave travelling along the interaction length induces the mode coupling for the optical polarisation modes. To define a certain conversion band width, the interaction length L is limited by an acoustic absorber.
A fundamental condition for energy transfer is the phase matching between the polarisation modes which results from the solution of the coupled wave equations. A conversion efficiency of 100% can only be achieved if the phase difference between the two optical modes (TE- and TM-modes) with different effective refractive indices is continuously compensated, which means a completely synchronous interaction along the interaction length. This synchronous interaction is essentially caused by means of an acoustic “Bragg”-grating having a pre-determined period and inducing a coupling between the “TE”- and “TM”-mode. The coupling effect is described by the following equation:
2
⁢
π
⁢
 
⁢
n
eff
,
TM
λ
-
2
⁢
π
⁢
 
⁢
n
eff
,
TM
λ
=
β
TM
-
β
TE
=
Δβ
=
2
⁢
π
Λ
a
⁢
 
⁢
c
(
1
)
Here n
eff,TM
and n
eff,TE
are the effective refractive indices for the (quasi) TE- and TM-modes, &bgr;
TM
, &bgr;
TE
are the propagation constants for the wavelength &lgr; (in vacuum) and &Lgr;
ac
is the wavelength of the acoustic ware (i.e. the periodicity of the perturbation of the dielectric censor induced for instance by a periodic electric field or a surface corrugation, i.e. the acoustic “Bragg”-grating Typically, the &Lgr;
ac
, is about 20-21 &mgr;m for &lgr;=1530-1570 mm. The propagation co

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