Optics: measuring and testing – By light interference – Having light beams of different frequencies
Reexamination Certificate
2000-02-17
2001-07-03
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By light interference
Having light beams of different frequencies
C359S199200
Reexamination Certificate
active
06256103
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to the field to optical measurements and measuring systems, and more particularly to a system and method for optical heterodyne detection of an optical signal.
BACKGROUND OF THE INVENTION
Dense wavelength division multiplexing (DWDM) requires optical spectrum analyzers (OSAs) that have higher spectral resolution than is typically available with current OSAs. For example, grating-based OSAs and autocorrelation-based OSAs encounter mechanical constraints, such as constraints on beam size and the scanning of optical path lengths, which limit the degree of resolution that can be obtained.
As an alternative to grating-based and autocorrelation-based OSAs, optical heterodyne detection systems can be utilized to monitor DWDM systems.
FIG. 1
is a depiction of a prior art optical heterodyne detection system. The optical heterodyne detection system includes an input signal
102
, an input waveguide
104
, a local oscillator signal
106
, a local oscillator waveguide
108
, an optical coupler
110
, an output waveguide
118
, a photodetector
112
, and a signal processor
116
. The principles of operation of optical heterodyne detection systems are well known in the field of optical heterodyne detection and involve monitoring the heterodyne term that is generated when an input signal is combined with a local oscillator signal. The heterodyne term coexists with other direct detection signals, such as intensity noise from the input signal and intensity noise from the local oscillator signal.
Optical heterodyne detection systems are not limited by the mechanical constraints that limit the grating based and autocorrelation based OSAs. The spectral resolution of an optical heterodyne system is limited by the linewidth of the local oscillator signal, which can be several orders of magnitude narrower than the resolution of other OSAs.
In order to improve the performance of optical heterodyne detection systems with regard to parameters such as sensitivity and dynamic range, it is best for the heterodyne signal to have a high signal to noise ratio. However, the signal to noise ratio of the heterodyne signal is often degraded by noise that is contributed by the direct detection signals, especially in the case where the input signal includes multiple carrier wavelengths. One technique for improving the signal to noise ratio of the heterodyne signal, as described in U.S. Pat. No. 4,856,899, involves amplifying the input signal before the input signal is combined with the local oscillator signal in order to increase the amplitude of the heterodyne signal. Although amplifying the input signal increases the amplitude of the heterodyne signal, the amplification also increases the intensity noise of the input signal and may not improve the signal to noise ratio of the heterodyne signal.
It is also important in optical heterodyne detection that the polarization of the input signal and the local oscillator signal are matched. In order to match the polarization of the local oscillator signal to the polarization of the input signal, the polarization state of the local oscillator signal may be controlled by a polarization controller
120
as indicated by the two loops in the heterodyne detection system of
FIG. 1. A
disadvantage of the optical heterodyne detection system of
FIG. 1
is that detection of the input signal is highly dependent on the polarization of the input signal.
A polarization diversity receiver can be incorporated into an optical heterodyne detection system to provide polarization independent signal detection. Although a polarization diversity receiver provides polarization independent signal detection, the polarization diversity receiver does not provide a way to separate the intensity noise from the heterodyne signal. In order to improve the performance of heterodyne detection systems, it is necessary to be able to clearly distinguish the heterodyne signal from the intensity noise.
In view of the prior art limitations in optical heterodyne detection systems, what is needed is an optical heterodyne detection system that generates a heterodyne signal with an improved signal to noise ratio.
SUMMARY OF THE INVENTION
A system for monitoring an optical signal includes an optical heterodyne detection system in which the intensity noise of the input signal and/or the intensity noise of the swept local oscillator signal is reduced before the input signal and the swept local oscillator signal are combined. An embodiment of the optical heterodyne detection system includes an intensity noise reducer for the input signal and/or an intensity noise reducer for the swept local oscillator signal, an optical coupling unit, at least two photodetectors, and may include a processor. Utilizing intensity noise reducers for the input signal and/or the swept local oscillator signal reduces the intensity noise that is detected by the photodetectors and improves the signal to noise ratio and the dynamic range of the heterodyne detection system. Optical intensity noise reduction can be accomplished utilizing various techniques and the particular technique utilized is not critical.
An embodiment of an optical heterodyne detection system includes an optical amplifier for amplifying the swept local oscillator signal. The optical amplifier increases the power of the local oscillator signal, thereby improving the signal to noise ratio and dynamic range of the heterodyne detection system. The intensity noise reducer for the local oscillator signal is preferably utilized when the optical amplifier is utilized in order to control the additional intensity noise that is contributed by the optical amplifier.
Because intensity noise reducers for both the input signal and the swept local oscillator signal may not be required on a single system, an embodiment of the system may include an intensity noise reducer for the input signal and no intensity noise reducer for the swept local oscillator signal. Conversely, an embodiment may include an intensity noise reducer for the swept local oscillator signal and no intensity noise reducer for the input signal.
In an embodiment, the optical heterodyne detection system includes an optical pre-selector connected to the output of the optical combining unit. The optical pre-selector has a passband that tracks the wavelength of the swept local oscillator signal. In an embodiment, the optical combining unit includes an optical coupler for combining the input signal and the swept local oscillator signal and for outputting light beams to corresponding photodetectors. In another embodiment, the optical combining unit includes an optical coupler and a polarizing beam splitter for splitting the combined optical signal into polarized portions that are output to corresponding photodetectors.
A method for monitoring an optical signal utilizing an optical heterodyne detection system involves reducing the intensity noise of the input signal and/or the swept local oscillator signal, combing the input signal and the swept local oscillator signal to generate a combined optical signal, and outputting light beams each including a portion of the combined optical signal, generating electrical signals in response to the light beams, and processing the electrical signals to determine an optical characteristic represented by the input signal.
The method for monitoring an optical signal utilizing optical heterodyne detection may involve additional steps. In one embodiment, the intensity noise of both the input signal and the swept local oscillator signal is reduced before the signals are combined. In an embodiment, the swept local oscillator signal is amplified before it is combined with the input signal. In another embodiment, the light beams are optically filtered before the electrical signals are generated. The optical filtering passes a wavelength band that corresponds to the wavelength of the swept local oscillator signal. The passband of the filtering is adjusted in real-time to track the changing wavelength of the swept local oscillator signal.
In a
Baney Douglas M.
Sorin Wayne V.
Agilent Technologie,s Inc.
Font Frank G.
Natividad Phil
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