Optics: measuring and testing – By light interference – Having light beams of different frequencies
Reexamination Certificate
2000-02-17
2001-07-10
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By light interference
Having light beams of different frequencies
C359S199200
Reexamination Certificate
active
06259529
ABSTRACT:
FIELD OF THE INVENTION
The invention relate 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 all 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 an input signal and a swept local oscillator signal are combined and output in at least two beams. The output beams are filtered by a filter having a passband which tracks the wavelength of the swept local oscillator signal. As the local oscillator signal sweeps across a wavelength range, filtering of the output beams is adjusted to track the wavelength of the local oscillator signal. Filtering the output beams to pass a wavelength band that tracks the wavelength of the swept local oscillator signal reduces the intensity noise contributed from light sources having wavelengths that are not near the wavelength of the local oscillator signal.
An embodiment of an optical heterodyne detection system includes an optical combining unit, an optical pre-selector, and two photodetectors. An input signal and a swept local oscillator signal are combined in the optical combining unit to create a combined optical signal. The optical combining unit includes two outputs for outputting two beams with each of the two beams including a portion of the combined optical signal. An optical pre-selector is optically arranged to the two outputs of the optical combining unit. The optical pre-selector has a passband that tracks the wavelength of the swept local oscillator signal. Two photodetectors are optically connected arranged to receive a different one of the two beams. The two photodetectors generate two electrical signals in response to the respective two beams.
In an embodiment, an intensity noise reducer is utilized to reduce the intensity noise of the input signal. In another embodiment, an intensity noise reducer is utilized to reduce the intensity noise of the swept local oscillator signal.
In an embodiment, the optical combining unit includes an optical coupler that has two outputs. Two portions of the combined optical signal are output from the two outputs of the coupler and are transmitted to the optical pre-selector. The two portions of the combined optical signal generate two electrical signals which can be processed to electronically reduce the intensity noise component of the combined optical signal. The resulting output signal is not independent of the polarization state of the input signal.
In another embodiment, the optical combining unit includes an optical coupler and a polarizing beam splitter. The optical coupler combines the input signal and the local oscillator signal and outputs a first beam of the combined optical signal. The polarizing beam splitter splits the first beam into two polarized beams. In another embodiment, the optical combining unit further includes a half-wave plate for shifting the polarization state of one of the two beams to match the polarization state of the other beam. The two polarized beams generate two electrical signals which can be processed to generate an output signal that is independent of the polarization state of the input signal.
A method for monitoring an optical signal utilizing an optical heterodyne detection system involves combining an input signal and a swept local oscillator signal to create a combined optical signal and outputting at least two beams of the combined optical signal. The at least two beams are then filtered to pass a wavelength band that tracks the wavelength of the swept local oscillator signal. Electrical signals are generated from the two filtered beams and the electrical signals are processed to determine an optical characteristic represented by the input signal.
In an embodiment of the method, the input signal and the swept local oscillator signal are combined to generate two instances of the combined optical signal and each of the two instances of the combined optical signal is output for filtering.
In another embodiment of the method, the combined optical signal
Baney Douglas M.
Sorin Wayne V.
Agilent Technologie,s Inc.
Font Frank G.
Natividad Phil
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