Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2000-11-22
2002-06-04
Dang, Hung Xuan (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C385S001000, C385S002000, C385S003000, C385S039000, C385S040000, C385S041000
Reexamination Certificate
active
06400490
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical modulator used in an optical communication system, and particularly to a Mach-Zehnder optical modulator.
2. Description of the Related Art
In the field of optical communication systems that are capable of transmitting a large volume of information, further improvements in transmission rates are considered crucial, and high-speed modulators are considered to be key devices for improving transmission rates.
One optical modulator that is capable of high-speed modulation has an optical waveguide structure that uses a Mach-Zehnder interference system. Such a Mach-Zehnder optical modulator is frequently used particularly as an external modulator in ultra-high-speed communication systems not only because it is capable of canceling the in-phase component in the noise component by applying drive voltage in a push-pull mode, but is also stable with respect to disturbance and can obtain modulation characteristics featuring excellent S/N (signal-to-noise ratio).
In a Mach-Zehnder optical modulator, input light is split into two beams which each undergoes phase modulation and then are combined. In this way, modulation of light intensity is effected by mutual interference. A Mach-Zehnder optical modulator is normally constructed such that the phase difference between the two optical waveguides that propagate the split beams is 0 when voltage is not being applied. In this type of optical modulator, therefore, input light is outputted without change when the applied voltage is 0, and the intensity of the output light varies as a cosine curve as voltage is applied. When operating this type of optical modulator, however it is desirable for the radiated light intensity to vary linearly with respect to the intensity of the electric field that is applied from the outside. For this reason, the initial operating point in a Mach-Zehnder optical modulator is typically set to a position where the phase is shifted by &pgr;/2 radian (90 degrees).
Referring now to
FIG. 1A
, in which is shown the construction of a typical Mach-Zehnder optical modulator, imbedded optical waveguide
82
is provided in optical substrate
81
, which has an electro-optical effect. In optical waveguide
82
, input waveguide
82
a
is branched into two optical waveguides
82
b
and
82
c
by way of the Y-shaped branching portion, following which branch optical waveguides
82
b
and
82
c
are joined by way of the Y-shaped combining portion, thereby constituting a Mach-Zehnder interference system waveguide. Optical buffer layer
89
and a travelling wave electrode
84
of a prescribed pattern are further provided on branch optical waveguides
82
b
and
82
c.
In this optical modulator, linearly single polarized light that is applied to input waveguide
82
a
is equally divided at the Y-shaped branching portion and advances into optical waveguides
82
b
and
82
c.
At this time, electric fields that are generated in optical waveguides
82
b
and
82
c
by applying voltage to travelling wave electrodes
84
as shown in
FIG. 1B
are applied to optical waveguides
82
b
and
82
c
in mutually opposite vertical directions. As a result, the refractive indexes of each of optical waveguides
82
b
and
82
c
change due to the electro-optical effect of optical substrate
81
, the change in refractive index in each of optical waveguides
82
b
and
82
c
being equal in amount and acting oppositely according to the positive and negative sign. The phase modulation due to the change in refractive index thus works in a push-pull manner on optical waveguides
82
b
and
82
c.
The light waves that receive the phase modulation (±&phgr;/2) of these optical waveguides
82
b
and
82
c
are combined by the Y-shaped combining portion, mutually interfere, and then proceed to output waveguide
82
d
to be outputted from the output terminal. In this case, the intensity of the output light is altered by cos
2
(&phgr;/2) with respect to the total amount of phase modulation &phgr;. For example, when light waves that are guided by optical waveguides
82
b
and
82
c
are subject to combination/interference, output is at a maximum when the light waves are of the same phase (&phgr;=2n &pgr;) and at a minimum when of the opposite phase (&phgr;=(2n+1)&pgr;). In this case, n is an integer such as 0, 1, 2 . . .
Typically, when carrying out light intensity modulation in the Mach-Zehnder interference optical modulator shown in
FIG. 1A
, the initial operating point is preferably set to an intermediate position (&pgr;/2 phase state) between the maximum output and the minimum output. A device has therefore been proposed in which dc power supply
85
and bias circuit
86
are provided in addition to high-frequency power supply
87
as shown in
FIG. 2A
in order to allow regulation of the initial operating point. According to this configuration, a direct current (dc) voltage for setting bias is applied to travelling wave electrode
84
in addition to the modulation signal voltage (ac voltage), which is the driving voltage, whereby the refractive index of the optical waveguide changes due to the electro-optical effect of the optical substrate and the phase shifts.
FIG. 2B
shows the output characteristic in this optical modulator when the dc voltage is 0.
As another method in which the initial operating point of an optical modulator is adjusted to &pgr;/2 phase state, Japanese Patent Laid-open No. 297332/93 (JP, 05297332, A) discloses a device in which the two branch optical waveguides that constitute a Mach-Zehnder interference system are formed with slightly differing lengths, thus producing a difference in the lengths of the optical paths between these two optical waveguides that is on the optical wavelength order.
As yet another known method of adjusting the initial operating point of an optical modulator, the width of the two optical waveguides that constitute a Mach-Zehnder interference system may be varied in portions to produce a non-symmetrical shape, thereby bringing about a difference in the effective refractive index and shifting the initial operating point (Tsuchiya, Kubota, Seino; Papers of the 1992 Autumn Conference, Institute of Electronics, Information and Communication Engineers, C-171). In addition, Japanese Patent No. 2,564,999 (JP, 2564999, B1)(corresponding to Japanese Patent Laid-open No. 24327/92 (JP, 04024327, A)) discloses a method in which the two optical waveguides that constitute a Mach-Zehnder interference system are coupled by a 3-dB directional coupler to shift the optical output by &pgr;/2 radian.
Nevertheless, the above-described optical modulators of the prior art have the following problems:
In an optical modulator in which the adjustment of the initial operating point is performed by applying dc voltage in addition to the modulation signal voltage (ac voltage), a stable modulation characteristic cannot be maintained over a long time period due to change of the operating point with the passage of time, i.e., the dc drift phenomenon. This dc drift phenomenon is often observed when, for example, a lithium nobate (LiNbO
3
) crystal are used in the optical substrate. Moreover, the bias circuit that is necessary for applying the dc voltage takes up space and adds to the expense of the device. Furthermore, the initial operating point and phase of the output light of the optical modulator can be kept uniform by providing a feedback circuit for correcting the voltage to follow up change in the electric field due to the dc drift phenomenon, thereby stabilizing the modulation characteristic, but taking such measures not only increases cost but adds to the complexity of the circuit configuration.
Optical modulators in which physical differences in the optical paths are created by varying the length or shape of the two optical waveguides that constitute the Mach-Zehnder interference system allow open control for the drive circuit of each optical modulator and thus have real advantages because they have a simplified c
Dang Hung Xuan
Tra Tuyen
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