Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
1999-10-27
2001-09-11
Mai, Huy (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S254000, C359S261000, C359S298000
Reexamination Certificate
active
06288823
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention relates in general to the field of electrooptics, and in particular to electrooptic devices for the modulation of light.
BACKGROUND OF THE INVENTION
Interferometric electrooptic modulators fabricated in the substrate material lithium niobate (LN) are widely used in digital communication systems operating at 2.5 Gb/s and 10 Gb/s and in analog systems for cable television. Not only are modulator rise and fall times <10 ps achieved with this technology, but interferometric designs provide the chirp free performance needed for long-distance transmission. These devices utilize a traveling wave (TW) configuration in which the modulating microwave signal propagates in a strip line or coplanar waveguide on the surface of the insulating substrate in the same direction as the modulated light wave, as described by G. K. Gopalakrishnan et al. in
Journal of Lightwave Technology
, vol 12, pp. 1807-1818, 1994. Best performance for high speed or high bandwidth modulation is achieved if the velocity of the modulating radio frequency wave closely matches that of the modulated optical wave. Present practice for the highest bandwidths (>>1 GHz) is to use very thick (≈15-30 &mgr;m) electrodes to achieve velocity matching by increasing the microwave propagation speed to match that of the optical carrier.
In spite of recent commercial success, the present TW modulator technology still has some shortcomings. Electrical power required to drive the modulators at microwave frequencies is high (typically several hundred mW for a pi-radian phase retardation). This means that a medium power microwave amplification circuit is needed in each transmitter. In the case of analog transmission, the relatively low sensitivity of modulated power to applied voltage and the inherent nonlinearity in dependence of modulated power on applied voltage can adversely affect link dynamic range. Further, the requirement for very thick electrodes on the LN substrate substantially increases the fabrication cost of the modulator chip.
One approach to overcoming these shortcomings is to use a material which supports a stronger electrical/optical interaction. Ferroelectric materials such as strontium barium niobate (SBN) with much higher electrooptic coefficients than LN have been known for decades, and low-loss waveguides and GHz-bandwidth modulation have recently been demonstrated in such materials. However, it is well known that materials with such high electrooptic coefficients also have very large dielectric constants. This means that microwave propagation is very slow, so that prohibitively thick electrodes are needed for velocity matching by the conventional method.
SUMMARY OF THE INVENTION
In accordance with the present invention, a method and apparatus are provided for the modulation of light which substantially eliminate or reduce disadvantages and problems associated with prior methods and apparatuses.
In particular, the present invention makes use of a grating structure integrated with the waveguide to induce slow wave optical propagation in optical waveguides. The grating structure induces contradirectional coupling of light in the waveguide whereby the light bounces back and forth in the waveguide as it propagates through it. This causes the transmitted light to emerge from the waveguide at a later time than would be the case if the grating structure were not present. For the purposes of this invention, the phenomenon whereby the forward propagating optical wave is slowed due to the integrated grating structure is identified as “slow wave optical propagation.”
In one embodiment of the present invention, a device for modulating the phase of a light wave is provided. In this embodiment a phase modulator comprises a single mode optical waveguide on a substrate of an electrooptic material. A grating structure integrated with the waveguide results in slow wave optical propagation. The grating structure is formed as a corrugation on the surface of the waveguide or as a refractive index variation in the waveguide material. Electrodes on the surface of the substrate are disposed to produce an electric field in the slow wave propagation region of the waveguide in response to a voltage V(t) applied across the electrodes. The electric field causes a change in the refractive index of the material in and near the optical waveguide, resulting in a modulation of the phase of the forward propagating light wave. In a preferred embodiment, the electrodes are disposed to form a transmission line, such that an applied voltage signal V(t) produces a traveling electromagnetic wave which propagates in the same direction as the incident light wave. The desired result is that the velocity of the modulated light wave matches that of the modulating electromagnetic wave.
In another embodiment of the present invention, a device for modulating the intensity of a light wave is provided. In this embodiment an intensity modulator comprises a Mach Zehnder waveguide interferometer on a substrate of an electrooptic material. The interferometer consists of an input single mode optical waveguide section, a branching waveguide region whereby the input waveguide diverges into two parallel waveguide sections, and a second branching waveguide region whereby the two parallel waveguide sections converge to form an output single mode waveguide section. Grating structures integrated with the two parallel waveguide sections between the branches induce slow wave optical propagation. Electrodes on the surface of the substrate are disposed to produce electric fields in these slow wave propagation regions in response to a voltage V(t) applied across the electrodes. These electric fields are of substantially the same magnitude but opposite in sign in the two waveguide sections which support slow wave optical propagation. The electric fields cause changes in the refractive index of the material in and near the slow wave propagation regions, resulting in a modulation of the phase of the forward propagating light waves in each. The phase modulation is substantially the same in magnitude but opposite in sign in the two slow wave optical propagation regions. A light wave coupled into the input optical waveguide section and propagates through the Mach Zehnder waveguide interferometer experiences intensity modulation in response to the voltage V(t) due to optical interference of the phase modulated light waves in the waveguide sections which support slow wave optical propagation. In a preferred embodiment, the electrodes are disposed such that an input voltage signal applied to them propagates in the same direction as the incident light wave in the slow wave optical propagation regions. The desired result is that the speed of the phase modulated light wave in the slow wave optical propagation regions matches that of the modulating electromagnetic wave. Furthermore, in cases where the modulator is used in the transmission of analog signals, the grating structure is designed to produce a dependence of modulated optical power on applied voltage which is substantially more linear than the sinusoidal dependence characteristic of conventional Mach-Zehnder modulators.
An important advantage of the present invention is that slowing the speed of optical propagation in the electrical-optical interaction region enables velocity matching between the modulated light wave and a modulating radio frequency or microwave signal in materials in which the microwave propagation speed is normally much lower than the optical propagation speed. It is well known in the art that velocity matching is required for very high frequency modulation performance. Conventional high frequency modulators in lithium niobate (LN) substrates achieve velocity matching by speeding the microwave propagation through the use of very thick (15-30 &mgr;m) electrodes, as described by K. Noguchi et al. in
Electronics Letters
, vol. 34, pp. 661-663, 1998. In the present invention, velocity matching, as disclosed above and more fully hereafter, is achieved with t
Mai Huy
Shaffer & Culbertson LLP
Shaffer Jr. J. Nevin
Texas A&M University System
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