Optical waveguides – Temporal optical modulation within an optical waveguide – Electro-optic
Reexamination Certificate
2001-01-25
2003-12-23
Palmer, Phan T. H. (Department: 2874)
Optical waveguides
Temporal optical modulation within an optical waveguide
Electro-optic
C385S008000, C385S015000, C385S129000
Reexamination Certificate
active
06668103
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical modulator with a monitor for use in optical communications, and more particularly to a Mach-Zehnder interferometric optical modulator with a monitor which has two branched optical waveguides for causing light waves propagated therethrough to interfere with each other.
2. Description of the Related Art
Optical modulation principles are roughly classified into a direct modulation process wherein a laser diode as a light source is directly controlled to modulate a laser beam emitted thereby and an external or internal modulation process wherein a semiconductor laser beam is externally or internally modulated. The former modulation process is mainly used for low-rate optical communications at communication rates up to 10 Gbps and the latter modulation process is mainly used for high-rate optical communications at higher communication rate.
Optical modulators based on the external modulation principles include a Mach-Zehnder interferometric optical modulator. The Mach-Zehnder interferometric optical modulator is widely used as an external modulator particularly for ultra-high-rate optical communication systems because it can provide modulation characteristics which are stable against disturbance and have a good S/N ratio by canceling out in-phase noise components with the push-pull application of a drive voltage.
FIG. 1A
of the accompanying drawings shows a general Mach-Zehnder interferometric optical modulator. As shown in
FIG. 1A
, the Mach-Zehnder interferometric optical modulator has optical waveguide
82
embedded in the surface of optical substrate
81
having an electro-optic effect. Optical waveguide
82
includes input optical waveguide
82
a
divided into two optical waveguides
82
b
,
82
c
by a Y-shaped divider and output optical waveguide
82
d
combined from optical waveguides
82
b
,
82
c
by a Y-shaped coupler. The Mach-Zehnder interferometric optical modulator also has optical buffer layer
89
and traveling-wave electrode
84
in a certain pattern which are disposed on optical waveguides
82
b
,
82
c.
A single linearly polarized light beam applied to input optical waveguide
82
a
is divided by the Y-shaped divider into equal light beams which travel respectively through optical waveguides
82
b
,
82
c
. At this time, an electric field generated by applying a voltage to traveling-wave electrode
84
from high-frequency power supply
87
is applied vertically to optical waveguides
82
b
,
82
c
in opposite directions, as shown in
FIG. 1B
of the accompanying drawings. Because of the electric field thus applied, the refractive indexes of optical waveguides
82
b
,
82
c
are changed by the electro-optic effect of optical substrate
81
. The changes of the refractive indexes of optical waveguides
82
b
,
82
c
are equal in quantity, but opposite in sign. Therefore, the changes of the refractive indexes modulate the phases of the light beams in optical waveguides
82
b
,
82
c
in a push-pull manner. The light beams that are phase-modulated in optical waveguides
82
b
,
82
c
by ±&phgr;/2, respectively, are combined by the Y-shaped coupler into a light beam that travels through output optical waveguide
82
d
, which outputs the light beam from its output end. The output light beam changes by cos
2
(&phgr;/2) with respect to the total phase modulation &phgr;. For example, when the light beams traveling through optical waveguides
82
b
,
82
c
are combined in phase with each other (&phgr;=2n&pgr;), the output light beam is of a maximum output, and when the light beams traveling through optical waveguides
82
b
,
82
c
are combined in opposite phase with each other (&phgr;=(2n+1)&pgr;), the output light beam is of a minimum output (n=1, 2, 3, . . . ).
For optical intensity modulation, it is preferable to set an initial operating point of the Mach-Zehnder interferometric optical modulator shown in
FIG. 1A
to an intermediate point (&pgr;/2 phase) between the maximum and minimum outputs. To set such an initial operating point, there has been proposed an optical modulator design which is similar to the optical modulator shown in
FIG. 1A
except that it also has, as shown in
FIG. 2A
of the accompanying drawings, DC power supply
85
and bias circuit
86
in addition to high-frequency power supply
87
so as to be able to adjust the initial operating point. With the proposed optical modulator, in addition to the modulation signal (AC) voltage which is a drive voltage, a DC voltage for setting a bias is applied to the traveling-wave electrode
84
to change the refractive indexes of the optical waveguides due to the electro-optic effect of the optical substrate for thereby shifting the phase.
FIG. 2B
of the accompanying drawings shows output characteristics of the optical modulator shown in
FIG. 2A
at the time the DC voltage is 0 V.
The optical modulator shown in
FIG. 2A
is, however, disadvantageous in that it is unable to maintain stable modulation characteristics over a long period of time owing to time-dependent changes (DC drift) in the operating point. The DC drift often occurs if the optical substrate is made of lithium niobate crystal, for example.
In view of the above drawback, it has been proposed to detect a portion of the output light beam of the optical modulator as a monitor light beam, and supply the monitor light beam through a feedback loop to correct the applied voltage depending on a change in the electric field due to the DC drift. One proposed optical modulator with a monitor, which is disclosed in Japanese patent No. 2738078, is illustrated in
FIG. 3
of the accompanying drawings.
The optical modulator shown in
FIG. 3
is substantially similar to that of the optical modulator shown in
FIG. 2A
except that it has a structure for extracting a portion of the output light beam of the optical modulator as a monitor light beam and supplying the monitor light beam through a feedback loop. Those parts of the optical modulator shown in
FIG. 3
which are identical to those of the optical modulator shown in
FIG. 2A
are denoted by identical reference characters.
In
FIG. 3
, input signal power supply
90
comprises high-frequency power supply
87
, DC power supply
85
, and bias circuit
86
shown in
FIG. 2A
, and is arranged to be able to adjust the initial operating point with the DC bias. To input optical waveguide
82
a
, there is connected single-mode optical fiber
92
which guides a light beam emitted by semiconductor laser
91
into input optical waveguide
82
a
. Output optical waveguide
82
d
is connected to single-mode optical fiber
93
which is branched into single-mode optical fibers
95
,
96
by fiber coupler
94
. A modulated light beam, i.e., a signal light beam, output from output optical waveguide
82
d
is divided by fiber coupler
94
into light beams that travel respectively through single-mode optical fibers
95
,
96
, from which the light beams are output. The modulated light beams, i.e., signal light beams, output from single-mode optical fibers
95
,
96
are detected by respective photodetectors
97
,
98
. Photodetector
97
is a photodetector that belongs to a party with which to communicate. The photodetector
98
supplies its output signal to signal processor/controller
99
.
The modulated light beam output from single-mode optical fiber
93
is divided by fiber coupler
94
into a light beam that is detected by photodetector
97
and a light beam that is detected by photodetector
98
. Based on the light beam detected by photodetector
98
, signal processor/controller
99
detects a change in the operating point and controls input signal power supply
90
and sends the detected change to input signal power supply
90
via a feedback loop for thereby adjusting the DC bias in input signal power supply
90
so as to catch up to a change in the electric field due to a DC drift.
The publication referred to above also proposes an optical modulator capable of monitoring light radia
McGinn & Gibb PLLC
NEC Corporation
Palmer Phan T. H.
LandOfFree
Optical modulator with monitor having 3-dB directional... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Optical modulator with monitor having 3-dB directional..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Optical modulator with monitor having 3-dB directional... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3134248