Optical waveguides – Directional optical modulation within an optical waveguide – Electro-optic
Reexamination Certificate
1999-09-14
2002-04-16
Lee, John D. (Department: 2874)
Optical waveguides
Directional optical modulation within an optical waveguide
Electro-optic
C385S040000, C385S131000
Reexamination Certificate
active
06374001
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an optical device, and particularly but not exclusively to a device for modulating radiation guided in a waveguide.
2. Discussion of Prior Art
Optical devices are well known in the prior art. They are described in a publication “Introduction to Semiconductor Integrated Optics” by H P Zappe (ISBN 0-89006-789-9, Artech House Publishers 1995). Optical devices for modulating radiation operate by exploiting optical properties of a modulating medium which are modifiable by external influences. One of the optical properties may include a refractive index. Induced changes in the refractive index may be anisotropic, where the medium becomes birefringent, or isotropic. There are many possible techniques for modulating the refractive index. These techniques are herewith described.
Refractive index changes may be induced in some optically transmissive materials by the application of an external mechanical force to them. This is referred to as a photo-elastic effect. Thermally induced refractive index changes are referred to as a thermo-optic effect.
Magnetically induced birefringence, referred to as a Faraday or magneto-optic effect, arises within some optically transmissive materials when subjected to a magnetic field. Factors such as magnetic flux density within the materials, a Verdet constant of the materials, composition of the materials and radiation propagation path length within the materials determine the magnitude of birefringence attainable.
Refractive index changes may be induced in some materials by application of an electric field to them. These refractive index changes occur due to both the Kerr and the Pockels effect. Refractive index changes arising from the Kerr effect are proportional to the Kerr constant of the materials and the square of the electric field applied to them. For the Pockels effect, refractive index changes are proportional to the applied electric field. The Pockels effect is only observed in crystalline materials comprising crystals which lack a centre of symmetry.
Refractive index changes may also be induced in some materials by introducing free charge carriers into them. Such changes are referred to as free carrier modulation or sometimes as a plasma dispersion effect. The free carriers modify both real and imaginary parts of the refractive index, thereby introducing both optical phase shift and optical absorption to optical radiation propagating through regions of these materials in which the carriers are present.
Silicon has a centro-symmetric crystalline structure and therefore does not exhibit the Pockels effect, except when high temperature poling is applied in which case a weak effect is obtained. This weak effect corresponds to a coefficient r of 10
−12
m V
−1
in equation [1] which describes a change in refractive index &Dgr;n as a function of silicon refractive index n
o
and applied electric field E:
Δ
n
=
1
2
n
0
3
rE
[
1
]
Silicon weakly exhibits the Kerr effect when very high strength electric fields are applied to it, for example refractive index changes of approximately 10
−4
are attainable for applied electric field strengths of 10
6
V m
−1
. In order to provide a practicable optical device for modulating radiation based upon a silicon waveguide, either the thermo-optic effect or the plasma dispersion effect have to be exploited. Operating bandwidths of devices relying on the thermo-optic effect in a silicon waveguide are restricted by relatively slow thermal dynamics of the waveguide, bandwidths of tens of kilohertz may be attained in practice for power inputs amounting to several Watts. Conversely, operating bandwidths of devices relying on the plasma dispersion effect in silicon waveguide are restricted by rapidity of removal and injection of charge carriers from a region thereof in which optical radiation propagates; such devices may provide operating bandwidths of several tens of megahertz in practice.
Optical radiation propagating within a homogeneous medium has an electric field vector of a magnitude E which varies spatially in the medium at an instance of time according to equation [2]:
E∝e
ikx
[2]
in which
k is a wavenumber of the optical radiation;
x is a distance in the medium; and
i is a square root of −1.
The wave number k in equation [2] is expressible as a product of a free-space wavenumber k
o
for the optical radiation and the refractive index n of the medium according to equation [3]:
E∝e
ink
o
x
[3]
In equation [3], the refractive index n may be expressed in terms of a real part n
r
and an imaginary part &agr; according to equation [4]:
n=n
r
+i&agr;
[4]
from which the magnitude of the electric field strength E is expressible according to equation [5]:
E∝e
in
r
k
o
x
e
−&agr;k
o
x
[5]
When the medium is silicon, injection of free carriers thereinto modifies both the real part n
r
and imaginary part &agr; of the refractive index n which are interrelated according to the Kramers-Kronig relationship which is expressed in equations [6] and [7]:
Δ
n
r
=
-
q
3
λ
2
4
π
2
c
3
n
r
ϵ
o
(
N
e
m
ce
2
μ
e
+
N
h
m
ch
2
μ
h
)
[
6
]
Δ
α
=
-
q
2
λ
2
8
π
2
c
2
n
r
2
ϵ
o
(
N
e
m
ce
+
N
h
m
ch
)
[
7
]
in which
c is the speed of light in vacuum;
&mgr;
e
is an electron mobility within silicon;
&mgr;
h
is an hole mobility within silicon;
m
ce
is an effective mass of a free electron within silicon;
m
ch
is an effective mass of a free hole within silicon;
q is the charge on an electron;
&lgr; is a wavelength of radiation propagating in the medium;
N
e
is a free electron concentration within the medium;
N
h
is a free hole concentration within the medium;
&Dgr;n
r
is a change in the real part n
r
;
&Dgr;&agr; is a change in the imaginary part &agr;; and
&egr;
o
is the permittivity of free space.
For optical radiation of 1 &mgr;m wavelength propagating in silicon, changes to the real part n
r
of approximately 10
−4
may be induced by charge carrier injection. Accompanying changes to the imaginary part are an order of magnitude smaller than this.
Prior art optical devices for modulating radiation based on a silicon waveguide generally exploit the plasma dispersion effect. Such devices employ a silicon p-i-n diode structure fabricated using standard silicon microfabrication techniques, for example epitaxial techniques for growing layers onto a wafer substrate. The structure comprises an electron acceptor doped p region, an intrinsic i region in the form of a rib and an electron donor doped n region. Optical radiation is confined to the intrinsic i region which functions as a waveguide. Charge carriers are injected into the intrinsic i region from the p and n regions when the p region is biased at a higher potential than the n region. The carriers modulate the refractive index of the waveguide.
The injected charge carriers induce a small phase change in the radiation propagating in the prior art devices. This phase change is converted into an amplitude change by incorporating at least one device into a Mach-Zehnder interferometer.
A first example of a prior art optical device is described in a patent specification U.S. Pat. No. 4,787,691. The device is designed for modulating and switching guided light in a waveguide. It comprises in sequence a silicon substrate base, a n+ doped influx silicon substrate, a low refractive index dielectric layer, a n-type crystalline silicon layer and a p+ doped silicon layer. The low index dielectric layer is etched during device fabrication to form a dielectric strip in the device. The n-type layer and p+ doped layer are etched during device fa
Bozeat Robert J
Nayar Vishal
Lee John D.
Nixon & Vanderhye P.C.
Rahll Jerry
The Secretary of State for Defence
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