Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1999-11-03
2001-10-23
Epps, Georgia (Department: 2873)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200
Reexamination Certificate
active
06307655
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates generally to the field of signal processing, and more particularly to wideband spectral analysis using an optically implemented chirp transform.
BACKGROUND OF THE INVENTION
Wideband frequency analyzers may be used to analyze the spectral characteristics of a received electromagnetic signal. As a result, wideband frequency analyzers are useful in many different signal processing applications. For example, wideband frequency analyzers may be used in various types of communication systems, including satellite and microwave communication systems, radar systems, radar threat receiver systems, studio sound recording systems, and the like.
The analysis of a received electromagnetic signal generally includes determination of the frequency, or frequency band, associated with each signal, and may also include other signal characteristics, such as a transform function associated with the signal. Wideband frequency analyzers operate over a specific frequency range. The wideband frequency analyzer can detect and analyze any signal that has a frequency within the frequency range of the wideband frequency analyzer.
In multi-signal applications, the wideband frequency analyzer may receive a broadband signal that comprises multiple individual signals that are received simultaneously. Using the wideband frequency analyzer, the characteristics of each signal within the broadband signal may be analyzed. In particular, each signal within the broadband signal has a specific frequency, or frequency band, associated with the respective signal. For example, a communication system may utilize a broadband signal that contains hundreds or thousands of individual signals, with each individual signal comprising, for example, a data stream for a number of phone calls.
One important technique for accomplishing wideband spectral analysis employs transversal filters to implement the chirp transform. Typical wideband frequency analyzers implement the chirp transform with transversal filters built using either surface acoustic wave (SAW) devices or tapped, superconducting delay lines. Such wideband frequency analyzers have a limited frequency detection range. In particular, conventional SAW device wideband frequency analyzers are generally limited to a frequency bandwidth on the order of 500 MHZ. Conventional high temperature superconducting (HTS) tapped delay line wideband frequency analyzers are generally limited to a frequency bandwidth on the order of 4 Ghz. Further, conventional HTS device wideband frequency analyzers also have limited frequency resolution capabilities due to limitations on the filter delay times achievable with such devices. For example, conventional HTS device wideband frequency analyzers are generally limited to a frequency resolution on the order of 25 MHZ.
Many conventional wideband frequency analyzers also have a limited dynamic range that cannot be electronically adjusted. In particular, conventional SAW and HTS wideband frequency analyzers utilize fixed taps for apodization, i.e., amplitude weighting. The fixed taps do not allow the apodization to be electronically corrected to correct for signal irregularities, such as chirp and other non-linearities.
SUMMARY OF THE INVENTION
Accordingly, a need has arisen for an improved wideband frequency analyzer and method of performing wideband spectral analysis. The present invention discloses a wideband frequency analyzer and method of performing wideband spectral analysis that employs an optically implemented chirp transform to achieve multi-gigahertz bandwidths, fine frequency resolution, and electronically adjustable apodization.
According to one aspect of the present invention a wideband frequency analyzer includes a first electro-optic modulator, a second electro-optic modulator, and an optical transversal filter. The first electro-optic modulator is operable to modulate an electrical input signal, such as a wideband radio-frequency signal, onto an optical carrier signal to output a modulated optical signal. The second electro-optic modulator is operable to mix the modulated optical signal with an electrical chirp signal to output a chirped modulated optical signal. The optical transversal filter is operable to filter the chirped modulated optical signal to output an electrical output signal. The electrical signal output by the optical transversal filter represents the amplitude of the Fourier transform of the input signal. The first and second electro-optic modulators may comprise Mach-Zehnder modulators. The wideband frequency analyzer may further include an optical signal source such as, for example a laser, that supplies the optical carrier signal to the first electro-optic modulator and a chirp signal source that supplies the chirp signal to the second electro-optic modulator.
The optical transversal filter may comprise a tapped optical fiber delay line having a plurality of taps spaced apart from one another along the optical fiber delay line. The optical fiber delay line may comprise a single mode optical fiber. The taps may comprise fiber 1% (nominal) couplers, and each tap obtains a sample of the chirped modulated optical signal after a period of delay corresponding to the location of the tap along the optical fiber delay line. The taps are spaced apart from one another along the optical fiber delay line to sample the chirped modulated optical signal in such a way so as to act as a matched filter. In this regard, the spacing between successive taps along the optical fiber delay line may correspond to the positive and negative half-wave positions of the modulation envelope of the chirp signal modulated on the optical carrier signal.
The taps direct the samples to optical detectors that convert the samples to electrical signals comprising the output signal. The optical detectors may comprise PIN diodes that are reverse biased. There may be a separate optical detector associated with each tap. Electrical signals from the detectors associated with the first, third, fifth, etc. taps along the optical fiber delay line may be summed to produce a positive output signal. Likewise, the electrical signals from the detectors associated with the second, fourth, sixth, etc. taps along the optical fiber delay line may be summed to produce a negative output signal. The positive and negative output signals may then be summed to produce the output signal of the wideband frequency analyzer. However, if the phase coherence of the chirped modulated optical signal can be spoiled, only two optical detectors are necessary. One optical detector receives samples from the first, third, fifth, etc. taps and the other optical detector receives samples from the second, fourth, sixth, etc. taps.
The optical transversal filter may also comprise a branched optical fiber delay line including a plurality of optical branches. The chirped modulated optical signal is split among the optical branches to obtain samples of the chirped modulated optical signal at the ends of each optical branch after a period of delay corresponding to the length of each optical branch. Optical detectors disposed at the end of each branch receive the delayed samples of the chirped modulated optical signal from the ends of the optical branches and convert the samples to electrical signals comprising the output signal.
According to another aspect of the present invention, a wideband frequency analyzer includes an electrical signal mixer, an electro-optic modulator, and an optical transversal filter. The electrical signal mixer is operable to mix an electrical input signal, such as a wideband radio-frequency signal, with an electrical chirp signal to output a chirped input signal. The electro-optic modulator is operable to modulate the chirped input signal onto an optical carrier signal to output a chirped modulated optical signal. The optical transversal filter is operable to filter the chirped modulated optical signal to output an electrical output signal representing the amplitude of the Fourier transform of the input signa
Epps Georgia
Lockhead Martin Corporation
Marsh & Fischmann & Breyfogle LLP
Tra Tuyen
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