Optical waveguides – With optical coupler – Particular coupling structure
Reexamination Certificate
2001-05-23
2004-01-27
Chowdhury, Tarifur R. (Department: 2871)
Optical waveguides
With optical coupler
Particular coupling structure
Reexamination Certificate
active
06684013
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates to the art of poling an optical waveguide device for use in the field of optical communications, and more particularly to an optical waveguide device to be subjected to optical poling based on the application of both an ultraviolet radiation and an electric field, a method of manufacturing such an optical waveguide device, and a method of optically poling such an optical waveguide device.
(2) Description of the Prior Art
One type of optical waveguide device is available as a waveguide optical switch. The waveguide optical switch comprises an optical substrate and an optical waveguide made of a given material that is disposed on the optical waveguide. The waveguide optical switch performs a switching action by changing the intensity of light or changing light paths when the waveguide optical switch is caused to change its refractive index by a thermo-optic effect or an electro-optic effect. Waveguide optical switches whose optical waveguides are made of quartz have recently been expected to find practical applications because they suffer a small loss, allow quartz waveguides to be fabricated together on one substrate, and can be connected for good matching to a single-mode fiber of quartz.
A specific optical switch using a quartz waveguide that has come into reality is a TO (Thermal Optical) switch as introduced by N. Takao, et. al., “Silica-Based Single-Mode Waveguides on Silicon and their Application to Guide-Wave Optical Interferometers”, J. Light Technol., VOL. 6, 1988, pp. 1003-1010. However, the introduced TO switch has a response speed of about 1 msec, and is not suitable for high-speed signal processing applications.
One waveguide optical switch that can possibly be used as a high-speed switch is a waveguide optical switch whose response speed is increased by the Pockels effect that is induced by thermal poling to apply a high voltage at an increased temperature. The Pockels effect is described in detail by P. G. Kazansky, et. al. “Pockels effect in thermally poled silica optical fibers”, Electronics Lett., Vol. 31, 1995, pp. 62-63.
The above article reports that the Pockels effect offers a response speed of 10 nsec or lower, allowing a high-speed switch operable at a frequency of 100 MHz or higher to be realized. However, the drive voltage for the high-speed switch is required to be 1 kV or higher because an electro-optical constant inducted by thermal poling has a small value of 0.05 pm/V or lower.
The Pockels effect can be enhanced by an optically pumped poling process which applies visible light or ultraviolet (UV) radiation while under an electric field. An article by T. Fujiwara, D. Wong, Y. Zhao, S. Fleming, S. Poole, and M. Sceats, Electron Lett., 31, 1995, 573 has reported that a high electro-optical constant of 6 pm/V is obtained by optically pumped poling.
Japanese laid-open patent publication No. 9-258151 discloses a waveguide optical switch based on optically pumped poling.
FIG. 1
of the accompanying drawings schematically shows the disclosed waveguide optical switch.
The waveguide optical switch shown in
FIG. 1
is a Mach-Zehnder interferometer waveguide optical switch which has two waveguides
112
,
113
, serving as Mach-Zehnder interferometer arms, disposed on Si substrate
111
, with thin film electrode
116
disposed on one of waveguides
112
. Waveguides
112
,
113
have ends coupled respectively to two input waveguides as input ports P
1
, P
2
by directional coupler
117
, and other ends coupled respectively to two output waveguides as output ports P
3
, P
4
by directional coupler
118
.
The illustrated Mach-Zehnder interferometer waveguide optical switch is poled as follows: While a laser beam having a prespecified wavelength, i.e., such a wavelength that will not cause a coupling in directional couplers
117
,
118
, is being introduced from input port P
1
, a voltage of a certain magnitude is applied between thin-film electrode
116
and Si substrate
111
. The laser beam introduced from input port P
1
is not coupled in directional coupler
117
, but propagated through waveguide
112
as one of the arms. After elapse of a predetermined time, the laser beam is turned off, and the voltage is dropped to 0 V, thus finishing the poling process.
The arm waveguides thus optically poled induces an electro-optic effect which allows the refractive index to change when an external electric field is applied. For example, the magnitude &Dgr;n of a change of the refractive index which is produced when an external electric field Eex is applied in a TM direction is expressed as follows:
&Dgr;n
TE
=(½)
r
1
n
TE
2
E
ex
&Dgr;
n
TM
=(½)
r
2
n
TM
3
E
ex
(see Nishihara, et. al., “optical integrated circuit” published by Ohm-sha). In the above equations, r
1
, r
2
represent electro-optic constants in the TE, TM directions, respectively, upon application of the external electric field in the TM direction, and n
TE
, n
TM
represent refractive indexes in the TE, TM directions, respectively. It will be seen from the above equations that the stronger the external electric field, the greater the change of the refractive index.
After the above poling process, a laser beam having a prespecified wavelength, i.e., such a wavelength that will cause a coupling in directional couplers
117
,
118
, is introduced from input port P
1
, and a voltage having a predetermined magnitude is applied between thin film electrode
116
and Si substrate
111
. The laser beam introduced from input port P
1
is coupled in directional coupler
117
, propagated through waveguides
112
,
113
, then coupled in directional coupler
118
, and propagated through the output waveguides of output ports P
3
, P
4
.
FIG. 2
of the accompanying drawings show how the intensities of output beams from output ports P
3
, P
4
change depending on the applied voltage. It can be seen from
FIG. 2
that the phase of the output beams changes in substantial proportion to the applied voltage V.
As described above, it is possible to increase electro-optic constants and lower drive voltages according to the optically pumped poling process. However, the waveguide optical switch disclosed in the above publication which is processed by the optically pumped poling process suffers the following shortcomings if a UV radiation is used as the pumping radiation:
When the waveguide optical switch is UV-poled by introducing the UV radiation from input port P
1
and applying a voltage of a predetermined magnitude between thin film electrode
116
and Si substrate
111
, the introduced UV radiation is propagated through a Ge-doped waveguide to a region of the waveguide
112
which is to be pumped. Before the UV radiation reaches the region of the waveguide
112
which is to be pumped, the propagated UV radiation is partly absorbed by the Ge-doped waveguide. Since the UV radiation is progressively attenuated as it travels through the waveguide, the disclosed waveguide optical switch cannot efficiently be UV-poled, and fails to provide a uniform electro-optic effect. In addition, the waveguide which has absorbed the UV radiation tends to be damaged or otherwise made defective.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an optical waveguide device which has waveguides less susceptible to damage upon being irradiated with a UV radiation and which can efficiently be UV-poled, a method of manufacturing such an optical waveguide device, and a method of optically poling such an optical waveguide device.
To achieve the above object, there is provided in accordance with the present invention an optical waveguide device comprising a waveguide whose refractive index changes can be controlled by an electro-optic effect and a guide waveguide for coupling or applying an ultraviolet radiation to a predetermined area of the waveguide. The waveguide may comprise first and second waveguides serving as respective arms of Mach-Zehnder interferometer, and the guide waveguide may be arranged to coup
Furukawa Akio
Hanada Tadahiko
Ofusa Naoki
Seki Yuko
Urino Yutaka
Chowdhury Tarifur R.
Kim Richard
NEC Corporation
Sughrue & Mion, PLLC
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