Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
1999-04-01
2001-01-30
Mack, Ricky (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C385S002000
Reexamination Certificate
active
06181456
ABSTRACT:
BACKGROUND OF THE INVENTION
Electrooptic materials, such as lithium niobate, are extremely useful in the telecommunications industry for modulating and demodulating signals carried by an optical carrier beam. For example, to modulate an optical beam, an electrode or a multi-electrode structure, such as a coplanar waveguide (CPW) electrode structure, exposes an optical waveguide disposed with an electrooptic substrate to a time-varying (typically RF) electric field. The RF field varies the index of refraction of the electrooptic material of the optical waveguide, changing the phase of the beam propagating along the waveguide, thus modulating the beam. As is known in the art, it is often advantageous to arrange such a modulator as an interferometer wherein the CPW electrode structure applies the RF electric field to two optical waveguide lengths in a “push-pull” fashion. Beams propagating along the two optical waveguide lengths are combined to interfere to produce a single optical output. Techniques known in the art for forming optical waveguides with electrooptic substrates include titanium indiffusion and an annealed proton exchange (APE™) technique.
Devices such as modulators and detectors are typically operated at a selected bias point. As is understood by those of ordinary skill in the art, depending on the circumstances and device configurations, a bias point can be selected such that the device operates within a particular linear range, at a minimal zero throughput, or at a half power point of optical output.
According to the prior art, electrooptic devices are typically biased by attempting to apply a known constant electric field to the optical waveguide(s) formed in or on the electrooptic substrate, such as by applying a fixed d.c. bias voltage to an appropriately located bias electrode. In some instances, the bias voltage can be applied to the some or all of the electrodes that apply the RF fields. Unfortunately, voltage biasing techniques must deal with the pheonomenon of bias drift. Although a constant voltage is applied to the biasing electrode, the actual electric field applied to the electrooptic optical waveguide varies, and the bias point of the device drifts. Physical impurities, crystal defects, and any causes of both trapped and mobile charges are considered to affect the bias stability of the device. In addition, because the optical waveguides are typically located near the surface of the electrooptic substrate, the crystal composition near the surface affects drift of the bias point via a variety of surface chemistry mechanisms. Even the method used for fabricating the waveguides, often involving indiffusion or proton exchange processes, can affect bias point drift, because these techniques modify the crystal structure. Bias point drift is a known problem and extensively discussed in the technical literature, particularly regarding lithium niobate, the most common electrooptic material used for optical devices.
In one approach to countering bias drift in interferometer-type modulators, the d.c. voltage is not fixed. A feedback circuit monitors the bias point, i.e. the phase or intensity of the output beam, and adjusts the bias voltage applied to the modulator. However, the voltage available for bias is typically limited, for example, to the “rail” voltage of 15 volts. It is possible that a feedback circuit, in tracking and correcting for drift, could “hit the rail,” that is, apply the full 15 volts, and to correct the drift will decrease the voltage by some step, essentially going to the next “fringe” of the interferometer. Such a “reset” is considered undesirable as it can result in lost data. Reset can usually be avoided by proper design, but it remains a concern, and compensating for the possibility of reset can complicate the biasing circuit design.
In some instances the optical devices can be manufactured to operate at a selected bias point. For example, the two optical waveguide lengths of an interferometer device can be fabricated having different physical lengths to introduce a selected phase difference between beams propagating along the lengths. This technique is effective and can increase cost, but may be limited to use in a specific application.
Accordingly, it is an object of the present invention to overcome one or more or the aforementioned drawbacks and disadvantages of the prior art.
This and other objects of the invention will in part appear hereinafter and in part be apparent to one of ordinary skill in light of the disclosure herein.
SUMMARY OF THE INVENTION
According to the invention the phase and/or the magnitude of an optical beam propagating along an optical waveguide length of an electrooptic device can be selected by controlling the temperature of the optical waveguide length. The selected phase or magnitude is stable and substantially drift free. The temperature of the optical waveguide length can be controlled via a heater for transferring thermal energy to the optical waveguide length. The heater can be disposed with a substrate with which the optical waveguide lengths are disposed. Advantageously, the present invention can be practiced simply and economically.
According to one aspect of the invention, an optical modulator includes an electrooptic substrate; an optical waveguide interferometer disposed with the electrooptic substrate and including first and second optical waveguide lengths for propagating beams for interfering to form an output beam; an electrode structure disposed with the substrate and having a center electrode and first and second ground electrodes, the electrode structure for exposing the optical waveguide lengths to time-varying electric fields produced between the center and ground electrodes for modulating the output beam; and means for providing a selected temperature difference between the temperature of at least a portion of the first optical waveguide length and the temperature of at least a portion of the second optical waveguide length for providing a selected bias point for operation of the modulator.
In another aspect, there is provided according to the invention an optical device that includes an electrooptic substrate having first and second optical waveguide lengths disposed with said substrate, said optical waveguide lengths for propagating first and second beams. Also provided are means for asymmetrically transferring thermal energy with said first and second optical waveguide lengths for providing a selected difference in temperature between at least a portion of said first waveguide length and a portion of said second waveguide length for providing one of a selected phase difference and a selected intensity difference between the first and second beams. A thermally conductive element can be disposed with the substrate and adjacent an optical waveguide length for facilitating the provision of the selected temperature difference. The thermally conductive element
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acts as a “heat spreader” to enhance the difference in temperature between the optical waveguide lengths.
In yet a further aspect, there is provided in accordance with invention an electrooptic optical device for providing stable control of the phase or intensity of an optical beam. The electrooptic optical device includes an electrooptic substrate; a waveguide length disposed with the substrate and for propagating the optical beam; and means for transferring thermal energy with the optical waveguide length for heating or cooling the waveguide length for varying the phase or intensity of an optical beam propagating along the optical waveguide length. The phase and intensity are substantially drift free as compared with control of the phase or intensity of the beam via a fixed d.c. voltage control approach.
The invention also includes methods practiced in accordance with the disclosure herein.
According to one feature, the invention includes a method of biasing an electrooptic modulator to have a selected phase or intensity. The electrooptic modulator includes at least first and second optical waveguide lengths disposed with an
Fritz Daniel J.
Kissa Karl M.
McBrien Gregory J.
Mack Ricky
McCormick Paulding & Huber LLP
Uniphase Telecommunications Products, Inc.
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