Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2001-02-12
2003-01-07
Dang, Hung Xuan (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S238000, C359S254000, C385S002000, C385S008000
Reexamination Certificate
active
06504640
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to optical modulators that are of interest to communication systems, and more particularly, to resonant optical modulators used in such systems.
2. Discussion of the Related Art
As the demand for high-speed and complex optical communication systems continues to grow, so too has the need for reliable high-speed devices needed for modulating optical signals traversing such systems. Optical modulators are of great interest in operating a fiber optic communication system in the range of 2.5 to 10 Gbps (Giga bits per second), and potentially to 40 Gbps or more. Of particular interest are modulators having low operating voltage and low optical and/or electrical losses that can reliably modulate optical signals transmitted through optical fiber or other optical media. Also of interest are modulator devices that can be integrated into optical circuits that may comprise a plurality of modulators and other related devices disposed on a common substrate.
There exist certain types of anisotropic materials of uniaxial crystal whose permittivities are directly proportional to an applied electric field and vary almost linearly with an applied electric field. This electrooptic property is known as the Pockels effect. Applying an electric field across an area occupied by a light signal in these types of uniaxial materials can modulate the light signal utilizing the electrooptic properties of the material. Because wave velocity is generally inversely proportional to the square root of the permittivity of the material in which the wave is propagating, a change in permittivity affects wave velocity within the electric field. In uniaxial crystal waveguides, this effect is advantageously used to shift a phase of the carrier wave traveling through the crystal and thus modulates the carrier wave phase.
In a simple form, a phase modulator can consist of a single channel optical waveguide formed within uniaxial material with electrodes disposed in such a way that an electric field applied across the channel modulates the phase of the carrier wave propagating within the channel. Another commonly used waveguide structure used for optical modulation is the Mach-Zehnder Interferometer (MZI), as illustrated in FIG.
1
. An MZI includes a waveguide channel
12
having two opposing Y junctions
10
a
and
10
b
joined by waveguide arms
12
a
and
12
b.
Waveguide
12
is formed within uniaxial material to exploit electrooptic effects, as described above. In the illustrated MZI, the waveguide junctions are symmetrical and operate as 50:50 power dividers.
FIG. 2
shows an optical modulator using an MZI having coplanar waveguide electrodes
22
-
26
formed over optical waveguide
12
. Electrodes
22
and
24
are supplied with a ground potential, while electrode
26
is supplied with an RF signal that terminates at impedance R
T
. In operation, when a carrier wave from a light source, for example a DFB laser, enters at optical waveguide input
14
, the carrier power is evenly split at the first Y junction
10
a
into the two light channels of the MZI arms
12
a
and
12
b.
By applying an electric field between the electrode
26
and ground electrodes
22
and
24
, oppositely oriented electric field vectors exist in the crystal, one in each MZI arm
12
a
and
12
b.
Consequently, the carrier light wave within each of the arms is complementarily phase shifted relative to one another in push-pull fashion. Light from each arm is then combined at Y junction
10
b
where constructive or destructive interference resulting from combining phase shifted carrier waves causes signal intensity modulation. When the total phase shift &thgr; between the carrier waves in arms
12
a
and
12
b
is such that &thgr;=&pgr;, light entering the device at 14 radiates into the substrate and results in zero channel output at 15.
Of uniaxial materials used to fabricate optical modulators, lithium niobate (LiNbO
3
) or lithium tantalate (LiTaO
3
) are popular substrate choices. LiNbO
3
is widely used due to its combination of low loss characteristics, high electrooptic coefficients, and high optical transparency in the near infrared wavelengths used for telecommunications. Its high Curie temperature (1100° C.-1180° C.) makes it practical for fabrication of optical waveguides because strip waveguides can be fabricated by means of Ti-indiffusion at temperatures near 1000° C.
LiNbO
3
wafers are available in three different crystal cuts (x-, y-, and z-cut).
FIGS. 3
a
and
3
b
respectively illustrate a cross-section of x-cut and z-cut LiNbO
3
substrates
11
. For the most pronounced electrooptic effect, the strongest component of the applied electric field is aligned with the z-axis of the crystal (because the z-axis has the highest electrooptic coefficient) to take advantage of the r
33
coefficient. On z-cut LiNbO
3
, vertical fields are used with a TM mode to take advantage of the r
33
coefficient. On x-cut, horizontal field electrodes and a TE mode utilize the r
33
coefficient.
As shown in
FIG. 3
a,
x-cut crystal substrates require placement of MZI arm
12
a
between electrodes
22
and
26
, and arm
12
b
between electrodes
26
and
24
.
FIG. 3
b
illustrates a z-cut crystal, where RF and ground electrodes must be placed directly over waveguide arms
12
a
and
12
b.
Thus, in both the x- and z-cut cases, applied electric fields from respective TE and TM modes of the RF input are aligned with the z-axis of the LiNbO
3
crystal. While not shown in
FIG. 3
b,
an insulation buffer film such as silicon dioxide or Al
2
O
3
may be used as a buffer to minimize z-cut LiNbO
3
optical losses that occur through TM mode absorption in the electrode metal. Buffer films are also beneficial to x-cut LiNbO
3
devices operating at high frequency.
LiNbO
3
modulators are used external to a source of an optical signal, unlike directly modulating a light source that provides an optical signal, such as a laser diode. External modulation avoids chirping (a time-dependent fluctuation of the wavelength in a modulated optical beam) and patterning effects inherent to directly modulated lasers, which is particularly important in digital applications requiring large extinction ratios.
LiNbO
3
modulators are widely used in digital applications to modulate a carrier wave using RF input in several modulation formats. Of particular interest are return-to zero (RZ) modulation formats. The RZ format has been employed in recent high-bandwidth terrestrial and submarine systems, especially those requiring long transmission distances. Dispersion managed soliton and other narrow-pulse transmission techniques can be considered specialized versions of RZ transmission.
Unlike the nonreturn-to-zero (NRZ) format, where binary data represented by a modulated carrier wave output maintains a high level when representing a “1” in a bit interval, in RZ coding of binary data, the output returns to a “zero” level for one or more portions of the bit interval. In the conventional NRZ pulse format, interaction between self-phase modulation (SPM) and group velocity dispersion (GVD) causes transfer of energy from the center of the pulse toward the pulse edges. Use of RZ format in a dispersion-managed system allows for balancing SPM and GVD, resulting in greater pulse-to-pulse consistency.
Recent high dense wavelength division multiplexed (DWDM) channel loading, increased bit-rate requirements of next-generation systems, and the desire to build wavelength-intelligent networks, have pushed the capabilities of the NRZ transmission to its limits. Thus, there remains a need in the art for external modulation devices capable of producing pulse forms necessary to transmit broad band optical signal data through optical fibers, and to alleviate the aforementioned problems associated with present optical communication systems.
SUMMARY OF THE INVENTION
The present invention has been made in view of the above circumstances and provides a resonant optical modulator for optical communication system
Codeon Corporation
Dang Hung Xuan
Morgan & Lewis & Bockius, LLP
Tra Tuyen
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