Optical: systems and elements – Optical frequency converter
Reexamination Certificate
2000-12-27
2002-10-29
Lee, John D. (Department: 2874)
Optical: systems and elements
Optical frequency converter
C359S199200
Reexamination Certificate
active
06473222
ABSTRACT:
BACKGROUND
1. The Field of the Invention
This invention relates to signal processing of light waves and other electromagnetic radiation and, more particularly, to novel systems and methods for detection and use of coherent photonic signals in various applications.
2. Background
Coherence detection using interference is an important element of signal processing for optical signals. In general, when a signal is to be detected, the detection process relies on transmission and receipt of a signal having a value a substantial distance from a value of some base noise level. In order to detect a signal, some window of bandwidth at which the signal is expected to occur will be selected. In order to provide more channels of data, it is desirable to be able to narrow down the bandwidth that is required to receive a particular signal.
Broadcasting or transmitting a signal precisely, with a minimum of noise at other frequencies, is important. Likewise, filtering and detecting a received signal over a narrow band, despite any associated noise, is important for communication. Narrowing the bandwidth of operation of a receiving apparatus requires a filter. Such a filter requires, in the case of optical systems, detection of the coherence of a signal using interference, and thus the applicability of that signal to the frequency range of interest.
As the relative phase of two coherent signals changes, the difference between the constructive interference (CI) and destructive interference (DI) outputs of an interferometer reduces as the phase difference approaches 90 degrees. Thus, a dead spot exists when differential detection is used, and when the two signals are out of phase by 90 degrees. Thus, coherence detection is phase-sensitive. What is needed is a method and apparatus for phase-insensitive coherence detection.
BRIEF SUMMARY AND OBJECTS OF THE INVENTION
In view of the foregoing, it is a primary object of the present invention to provide a method and apparatus for phase-insensitive coherence detection. It is another object of the invention to account for the dead spot that occurs when the phase difference is 90 degrees. It is another object of the invention to avoid any dead spot in the bandwidth of a coherence detector by modifying the input to an interferometer. It is another object to avoid a dead zone or dead spot when the phase difference is 90 degrees by modifying the output of an interferometer.
Further objects of the invention include providing a phase and frequency insensitive detection of coherence in photonic signals. It is yet a further object of the invention to provide a sensor for telecommunications lines, for receiving photonic signals, narrowing the required bandwidth necessary for effective capture of a received signal. It is another object of the invention to provide various apparatus implementing coherence detectors therein, for example: spectrum analyzers, signal processors, and so forth. Another object is to expand bandwidth for greater selectivity.
Consistent with the foregoing objects, and in accordance with the invention as embodied and broadly described herein, a method and apparatus are disclosed in one embodiment of the present invention provide multi-domain, phase-compensated, differential-coherence detection of photonic signals for interferometric processes. Manufacture of devices holographically and repeatably is done with emulsion development in situ or with removablek, automatic registration structures connecting and registering holograms and photonic sources with respect to each other in a single frame.
Photonic or electronic post processing may include outputs from a cycling or rotation between differently phased complementary outputs of constructive and destructive interference. A hyper-selective, direct-conversion, expanded-bandpass filter may rely on an expanded bandpass for ease of filtering, with no dead zones for zero beat frequency cases. A hyper-heterodyning, expanded bandpass system may also provide improved filtering and signal-to-noise ratios. An ultra-high-resolution, broadband spectrum analyzer may operate in multiple domains, including complex “fingerprints” of phase, frequency, and other parameters.
The associated technologies of the invention may be used to produce extreme precision in multi-domain locking of sophisticated waveforms varying in several domains. Phase-masking techniques may provide phased arrays of complementary outputs over a broad band, such as may be implemented in a projected phase-mask, multiple phase interferometer. Topographic holographic imaging and projection techniques are enabled at very fine resolutions, while minimizing required information for systems such as holographic television. Phase-stabilization, modulation, compensation and the like are enabled by devices and methods in accordance with the invention, and may be servo-controlled.
Coherence detection may rely on an interferometer called a homodyne. A homodyne may require a single interferometer having sensors such as photodiodes, or other elements for detecting the light signal output, and forwarding a communications signal to a device. In a homodyne, adjustment typically provides for one sensor “detector” to receive energy from a region of destructive interference “DI” of two photonic beams. Another region may provide an area of constructive interference “CI” due to an interference pattern between the two photonic beams.
When two photonic inputs into an interferometer are coherent, two outputs provide a differential with respect to one another. If non-coherent light arrives as inputs, then outputs to the two sensors or detectors will lack the pronounced differential, and may effectively be non-differentiable. A differential detector for measuring the overall difference between the two signals received at the two sensors may thus determine if coherence exists. The existence of coherence can be used to indicate that a signal at a desired or expected frequency is arriving at the detectors to be processed.
Within contemplation is an embodiment of an apparatus in accordance with the invention in which a portion of a differential output provides a feedback signal to a servo circuit. This servo circuit controls an electrically driven or control phase-adjusting optical element in a photonic input pad leading to an interferometer. In one embodiment, the servo mechanism so constructed can change the phase of an input signal to avoid any dead spot near the 90 degree or quarter wave zone. As a result, any phase change that occurs between two inputs may be tracked by a servo in order that the differential output of an interferometer will be continuously adjusted to avoid any dead zone or dead spot condition.
In an alternative embodiment, the 90 degree or quarter-wave dead spot may be avoided in an output signal by providing at least two interferometers energized by a shared input signal. Accordingly, one input of one of the interferometers may be optically phase shifted so that at least one of the interferometers provides a differential output when the two inputs are coherent. The two differential outputs may then be combined into a single, phase-insensitive output.
In one embodiment, coherence detection may be implemented in a narrowband active optical filter or photonic active filter. A signal selection process may be useful in a demultiplexer, such as a wave division multiplexer (WDM) or a time division multiplexer (TDM). Coherence detection elements based on interference between a detected incoming signal, and a reference signal, may provide extremely narrowband selection allowing a significant increase in the channel-carrying capacity of an optical communication system.
In certain embodiments, a coherence detector implemented as a filter in a wavelength demultiplexing system may be used for precise wavelength measurements, thus forming a spectrum analyzer. A phase-insensitive method and apparatus for coherence detection is essential, and may be accomplished by splitting an input signal, and a reference signal, into a number of individual beams, each having su
Carter Robert L.
Hait John N.
Preston Ronald E.
Eastman Gary L.
Lee John D.
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