No polarization dependent waveguide type optical circuit

Details No polarization dependent waveguide type optical circuit No polarization dependent waveguide type optical circuit

2001-01-17

2003-04-08

Epps, Georgia (Department: 2873)

Optical waveguides

Integrated optical circuit

C385S129000, C385S130000, C385S131000, C385S011000, C385S028000

Reexamination Certificate

active

06546161

ABSTRACT:

This application is based on Japanese Patent Application No. 2000-13029 filed Jan. 21, 2000, the content of which is incorporated hereinto by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a waveguide type optical circuit to be employed in construction of an optical communication system, an optical information processing system and so forth. More particularly, the invention relates to a technology effective in application for a no polarization wave dependent waveguide type optical circuit having no polarized wave dependency.
2. Description of the Related Art
Conventionally, associating with development of an optical communication technology, research and development has been made for various optical parts. Amongst, a waveguide type optical parts based on an optical waveguide on a flat substrate is the most important part. The reason is that the waveguides type optical part has a feature to be easily manufactured with high reproductivity achieving a precision less than or equal to an optical wavelength by a photolithographic technology and fine machining technology.
For example, a waveguide type Mach-Zehnder interferometer optical circuit is constructed with two optical couplers on the substrate and two connection waveguides connecting connecting these two optical coupler. By controlling optical path length difference and interference between two waveguide type phase condition, various functions can be realized. Such optical circuit has wide application fields, and has been put into practical use.
FIGS. 9A and 9B
show an example of the conventional waveguide type Mach-Zehnder interferometer.
FIG. 9A
is a plan view and
FIG. 9B
shows a section taken along line IXB—IXB in FIG.
9
A.
As shown in
FIG. 9A
, the Mach-Zehnder interferometer is constructed with a first input waveguide
23
and second input waveguide
24
formed in a clad
22
of a silicon substrate;
21
, a first directional coupler
25
formed by placing the first input waveguide
23
and the second input waveguide
24
to be proximal with each other, a first output waveguide
26
and a second output waveguide
27
, a second directional coupler
28
formed by placing the first output waveguide
26
and the second output waveguide
27
proximal with each other, first connecting waveguide
39
and second connecting waveguide
40
connecting the first directional coupler
25
and second directional coupler
28
, and a thermo-optic phase shifter
41
(thin film heater) As a material for forming the optical waveguide, a fused silica prepared by way of flame hydrolysis deposition is used. As shown in
FIG. 9B
, the section is that a core having a section of 7 &mgr;m×7 &mgr;m, for example the first connecting waveguide
39
and the second connecting waveguide
40
, is embedded at substantially center portion of a clad
22
of 50 &mgr;m thick deposited on the silicon substrate
21
. A difference of refraction indexes of the clad and the core is 0.75%.
As the first directional coupler
25
and the second directional coupler
28
, it is typical to employ 3 dB coupler set a branching ratio of 1:1. A light input from the input waveguide is equally divided by the first directional coupler
25
and propagated to the first connecting waveguide
39
and the second connecting waveguide
40
. The light propagated to the first connecting waveguide
39
and the second connecting waveguide
40
are combined to cause mutual interference in the second directional coupler
28
. The light combined by the second directional coupler
28
is variable of amont of light to be output to the first output waveguide
26
and the second output waveguide
27
respectively depending upon phase condition at that time. For example, when a light having a wavelength &lgr; from the first input waveguide
23
is equally divided by the first directional coupler
25
and combined in the second directional coupler
28
, if the phase error of two lights is 0 or an integer multiple of the wavelength &lgr;, the combined light is output from the second output waveguide
27
. On the other hand, when the phase error of the combined two lights is odd number multiple of the half-wavelength (&lgr;/2), the combined light is output from the first output waveguide
26
. Furthermore, when the phase error of the combined light is the intermediate condition of the condition set forth above, namely when the phase error is neighter 0, integer multiple of the wavelength &lgr; nor odd number multiple of the half wavelength, the light is output from both of the first output waveguide
26
and the second output waveguide
27
at a ratio depending upon the instantaneous condition.
Assuming an optical path length difference between the first connecting waveguide
39
and the second connecting waveguide
40
is &Dgr;L, the Mach-Zehnder interferometer, as shown in
FIG. 9A
, in which the optical path length-difference &Dgr;L is 0 or approximately half wavelength of the wavelength of the light may operate as an optical attenuator or an optical switch by providing a thin film header operating as thermo-optic phase shifter
41
on the first connecting waveguide
39
.
FIG. 10
is an illustration for explaining characteristics of the conventional Mach-Zehnder interferometer optical circuit, in which may operate thermo-optic phase shifter
41
in case of the optical path length difference &Dgr;L is 0. When thermo-optic phase shifter
41
may not operate, a light input from the first input waveguide
23
output from the second output waveguide
27
. Thus, when the optical path length is effectively increased by heating the first connecting waveguide
39
by operating the thermo-optic phase shifter
41
(thin film heater) to increase refraction index of the first connecting wave guide
39
by thermo-optic effect, a part of the incident light from the first input waveguide
23
is output to the first connecting waveguide
39
. When the optical path length difference &Dgr;L becomes half wavelength by adjusting a temperature of the first connecting waveguide
39
, all of the incident light from the first input waveguide
23
is output to the first output waveguide
26
. Thus, by variably adjusting the optical path length difference &Dgr;L of two connecting waveguides from 0 to have wavelength using the thermo-optic phase shifter
41
, it becomes possible to operate as the optical attenuator. On the other hand, by using the optical path length difference &Dgr;L of two connecting waveguides only at 0 and half wavelength, it can be operated as special optical switch. An electrical power required for switching by the optical switch is about 0.5 W in case the thin film heater of 5 mm length and 50 &mgr;m width. On the other hand, a temperature elevation of the thin film feature is about 30° C.
The Mach-Zehnder interferometer optical circuit having the optical path length difference &Dgr;L of two connecting waveguides greater than or equal to several &mgr;m, it can be operated as a wavelength filter.
FIG. 11
is a plan view showing a general construction of the asymmetric Mach-Zehnder interferometer optical circuit. The asymmetric Mach-Zehnder interferometer optical circuit is constructed with the first input waveguide
23
and second input waveguide
24
fabricated on the clad
22
on the silicon substrate
21
, a first directional coupler
25
formed by placing the first input waveguide
23
and the second input waveguide
24
to be proximal with each other,
1
o the first output waveguide
26
and a second output waveguide
27
, the second directional coupler
28
formed by placing the first output waveguide
26
and the second output waveguide
27
proximal with each other, first connecting waveguide
39
and second connecting waveguide
40
connecting the first directional coupler
25
and second directional coupler
28
. As a material for forming the optical waveguide, a fused silica prepared by way of flame hydrolysis deposition is used. As shown in
FIG. 9B
, the section is that a core having a section of 7 &mgr;m×7 &

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