Coherent light generators – Particular beam control device – Tuning
Reexamination Certificate
2001-03-21
2004-11-09
Scott, Jr., Leon (Department: 2828)
Coherent light generators
Particular beam control device
Tuning
C372S019000, C372S029015, C372S032000, C372S098000, C372S102000
Reexamination Certificate
active
06816516
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
This invention pertains to error signal generation and servo systems usable for positional control of optical elements in optical systems. More particularly, the invention pertains to an error signal generation system for continuous, accurate positional control of a tunable element with respect to a coherent light beam.
2. Description of the Background Art
Fiberoptic telecommunications are continually subject to demand for increased bandwidth. One way that bandwidth expansion has been accomplished is through wavelength division multiplexing (WDM) wherein multiple separate data streams exist concurrently in a single optical fiber, with modulation of each data stream occurring on a different channel. Each data stream is modulated onto the output beam of a corresponding semiconductor transmitter laser operating at a specific channel wavelength, and the modulated outputs from the semiconductor lasers are combined onto a single fiber for transmission in their respective channels. The International Telecommunications Union (ITU) presently require channel separations of approximately 0.4 nanometers, or about 50 GHz. This channel separation allows up to 128 channels to be carried by a single fiber within the bandwidth range of currently available fibers and fiber amplifiers. Improvements in fiber technology together with the ever-increasing demand for greater bandwidth will likely result in smaller channel separation in the future.
Transmitter lasers used in WDM systems have typically been based on distributed feedback (DFB) lasers operating with a reference etalon associated in a feedback control loop, with the reference etalon defining the ITU wavelength grid. Statistical variation associated with the manufacture of individual DFB lasers results in a distribution of channel center wavelengths across the wavelength grid, and thus individual DFB transmitters are usable only for a single channel or a small number of adjacent channels. Continuously tunable external cavity lasers have been developed to overcome this problem.
The trend towards smaller channel separation and the advent of channel selectivity in transmitter lasers has given rise to a need for greater accuracy and control in the positioning of tunable elements associated with transmitter lasers. As tunable elements are configured for narrower channel separation, decreasing component tolerances and thermal fluctuation become increasingly important. Non-optimal positioning of tunable elements results in spatial losses and reduced transmitter output power.
SUMMARY OF THE INVENTION
The present invention relates to an error signal generation system and method for continuous and accurate tuning of a tunable element used in association with a coherent light source. In its most general terms, the invention comprises a coherent light beam with a fixed frequency or wavelength, a tunable element positioned in the light beam, and a detector, positioned in association with the light beam and tunable element, that is capable of generating an error signal indicative of a characteristic or property associated with the relationship of the element and the light beam. The invention also may comprise a tuning assembly operatively coupled to the tunable element and detector and configured to tune the tunable element according to the error signal generated by the detector.
The tunable element defines at least one characteristic or property with respect to the coherent light beam which is adjustable according to the error signal derived from the detector. For example, the tunable element may define a constructive interference fringe for the coherent light beam, with non-optimal tuning of the tunable element resulting in the constructive interference fringe being non-centered with respect to the light beam, resulting in spatial losses to the beam as it is transmitted through or reflected off the tunable element. The detector is positioned to detect such spatial losses and generate a corresponding error signal, which is usable to re-center the constructive interference fringe in the coherent light beam.
In certain embodiments the detector is a split detector, and non-optimal tuning of the tunable element such that the constructive interference fringe is not centered within the light beam will result in unequal amounts of optical power being detected by the two halves of the split detector. The voltage outputs of the detector halves are utilized to generate an error signal which corresponds to or is indicative of the optical power received by the different halves of the split detector. The error signal generated by the detector is used to tune the tunable element by adjusting a property of the tunable element by the tuning assembly so that the constructive interference fringe defined by the tunable element remains centered in the coherent light beam. In other embodiments the detector may comprise various types of multi-element detector, or a lateral effect detector.
The tunable element, in one embodiment, comprises a tapered or “wedge” etalon which may, for example, be in the form of an air gap between reflective surfaces of adjacent substrates, a single, solid substrate with tapered reflective surfaces, or a tapered thin film interference filter. The wedge etalon defines a constructive interference fringe for the wavelength of the coherent light beam. The tuning assembly may comprise a mechanical, electrical, piezoelectric or like system configured drive or translate the wedge etalon with respect to the light beam, or to apply a voltage, magnetic field, mechanical stress, or other effect which alters the characteristics of the wedge etalon. The tuning assembly may comprise, for example, a stepper motor that is configured to translate the wedge etalon such that the constructive interference fringe is moved with respect to the center of light beam.
The tunable element of the invention may alternatively comprise an air gap etalon embodied in a micro electrical mechanical system (MEMS) device wherein the air gap etalon is defined by parallel, reflective micro-machined silicon surfaces. The tuning assembly for this embodiment may comprise, for example, electrodes associated with the reflective surfaces of the air gap. The optical thickness of the air gap etalon is controlled by positioning one or both reflective surfaces according to voltage applied to the electrodes to vary the optical path length of the air gap. One or more of the electrodes are operatively coupled to the split detector, and the potential applied to the electrodes for control of the air gap spacing is responsive to the error signal derived from the split detector.
In another embodiment of the invention the tunable element comprises an electro-optic device having an effective optical path length or optical thickness that is adjustable according to an applied electric field, magnetic field, mechanical stress via gas pressure or other source, thermal, or non-linear optical effect. The electro-optic device may comprise, for example a substrate made of a electro-optic material such as a liquid crystalline material wherein the refractive index of the substrate can be varied by suitable application of voltage to the substrate. The tuning assembly in this case comprises electrodes associated with the electro-optic material substrate which are suitably positioned to control the refractive index of the etalon electro-optic substrate material.
In another embodiment the tunable element may comprise an air gap etalon defined by reflective surfaces which are movable via a piezoelectric material associated with the reflective surfaces wherein the air gap separation can be varied by suitable application of voltage to the piezoelectric material. The tunable element may comprise an air gap etalon with reflective surfaces that are movable via thermal control, using thermal expansion and contraction via heating and cooling of a spacer associated with the reflective surfaces to provide tuning.
In another embodiment of the invention the tunable element comp
Blakely , Sokoloff, Taylor & Zafman LLP
Intel Corporation
Jr. Leon Scott
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