Optics: measuring and testing – By light interference – Spectroscopy
Reexamination Certificate
2001-09-28
2004-02-03
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
Spectroscopy
C356S484000
Reexamination Certificate
active
06687006
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to the field of optical measurements and measuring systems, and more particularly to a method and system for optical spectrum analysis that utilizes optical heterodyne detection.
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. For example, grating based OSAs can achieve a spectral resolution on the order of 100 picometers (pm). As an alternative to grating based and autocorrelation based OSAs, optical heterodyne detection systems can be utilized to monitor DWDM systems.
Optical heterodyne detection systems involve mixing an input signal with a local oscillator signal. Optical heterodyne detection systems can be utilized for optical spectrum analysis of an input optical signal by mixing the input signal with a local oscillator signal that is swept across a range of wavelengths or frequencies. Heterodyne based OSAs can achieve a spectral resolution on the order of 0.001 pm.
FIG. 1
depicts an example of a heterodyne based OSA that includes an optical coupler
110
that combines an input signal
102
from an input fiber
104
with a swept local oscillator signal
106
from a local oscillator source
105
via local oscillator fiber
108
. The combined input and local oscillator signals travel on an output fiber
118
and are detected by a heterodyne receiver
112
. A detector
130
within the heterodyne receiver converts optical radiation from the combined input and local oscillator signals into an electrical signal. Square law detection results in mixing of the combined input and local oscillator signals and produces a heterodyne beat signal at a frequency that is equal to the frequency difference between the input and local oscillator signals. The heterodyne beat signal is conditioned by a signal conditioner
132
and a data acquisition (DAQ) unit
134
generates digital heterodyne beat signal data from the conditioned heterodyne beat signal. The digital heterodyne beat signal data generated by the data acquisition unit is processed by a processor
116
to determine a characteristic of the input signal, such as frequency, wavelength, or amplitude. A characteristic of the input signal, such as a waveform or fringe pattern, can then be output to a display
120
.
FIG. 2
depicts an example heterodyne beat signal
224
that is generated in response to mixing of an input signal and a swept local oscillator signal using the heterodyne based OSA of FIG.
1
. The heterodyne beat signal is graphed with intensity on the vertical scale and time on the horizontal scale and the pattern formed by the heterodyne beat signal is referred to as a fringe pattern.
To provide enough data points so that the processor
116
can adequately resolve the fringe pattern, the data acquisition unit
134
must sample the fringe pattern at a rate that produces multiple samples of each fringe.
FIG. 2
depicts samples S
1
through S
N
that are obtained at a high enough sampling rate to adequately resolve the fringe pattern. One tradeoff to obtaining enough samples to adequately resolve a fringe pattern is that a large number of data points are generated and must be processed. For example, a 100 nm scan at a sweep rate of 40 nm/s and a sampling rate of 10 MHz will produce 25 million data points. Generating and processing large numbers of data points can add unwanted delay to the production of scan results.
One way to reduce the number of data points generated by a heterodyne based OSA is to reduce the scan width of the local oscillator signal. However, reducing the scan width of the local oscillator signal is not desirable because it limits the range of optical signals that can be detected by the OSA.
Alternatively, the number of data points generated during a scan can be reduced by reducing the sampling rate of the data acquisition unit. That is, a fewer number of samples of the fringe pattern are taken per unit of time. The sampling rate of the data acquisition unit can be reduced without reducing the scan width or the sweep rate of the local oscillator signal. Although reducing the sampling rate of the data acquisition unit without reducing the scan width or the local oscillator sweep rate reduces the number of data points generated per scan, reducing the sample rate also increases the possibility of not detecting the input signal during a given scan. For example, the input signal may not be detected if a sample is taken at or near a zero crossing or if the input signal passes between sampling events.
FIG. 3
depicts the example heterodyne beat signal
224
of
FIG. 2
, where the fringe pattern is sampled at a greatly reduced rate. In the example of
FIG. 3
, a first sample, S
1
, is taken before the appearance of the fringe pattern. With a reduced sampling rate, the next sample could be taken at or near any of the many zero crossings. For example, sample S
2A
is taken at a zero crossing and therefore no signal is detected during the sampling event. If a sample is taken at or near a zero crossing and no other samples of the fringe pattern are obtained because of the reduced sampling rate, the input signal will not be detected. In another scenario, if the sampling period is too long, or the fringe pattern is too short, the next sample could be taken after the appearance of the fringe pattern, such that the input signal is not detected. For example, sample S
2B
is taken after the appearance of the fringe pattern and therefore no signal is detected. Either way, a sampling rate that may allow the fringe pattern to go undetected in not desirable. While in some situations, it may be sufficient to simply detect the presence of an input signal without being able to fully resolve the fringe pattern of the signal, it is not desirable to allow the fringe pattern to go undetected.
In view of the limitations of prior art heterodyne based OSAs, what is needed is a heterodyne based optical spectrum analysis technique that reduces the volume of data generated per scan while maintaining a broad scan width and fast local oscillator sweep rate that does not jeopardize the resolution achievable through heterodyne based optical spectrum analysis.
SUMMARY OF THE INVENTION
A method and system for heterodyne based optical spectrum analysis involves mixing an input signal with a swept local oscillator signal to generate a heterodyne beat signal and then stretching the heterodyne beat signal before the signal is sampled. The heterodyne beat signal is stretched before it is sampled so that the signal can be reliably detected using a reduced sampling rate. Specifically, stretching the heterodyne beat signal involves extending the duration of the signal so that the signal can be detected with fewer samples per unit of time than an unstretched signal. The reduced sampling rate generates a smaller volume of data per scan that must be processed to generate scan results. Processing a smaller volume of data enables quicker generation of scan results. An advantage to stretching the heterodyne beat signal and sampling at a reduced rate is that a wide wavelength range can be quickly scanned to locate an unknown signal and then a more narrow scan, focused around the located signal, can be performed on an unstretched version of the heterodyne beat signal. The unstretched heterodyne beat signal can be sampled at a high enough rate to adequately resolve the fringe pattern of the heterodyne beat signal.
An embodiment of the invention is a system for optical spectrum analysis that includes a local oscillator source, an optical coupler, a heterodyne receiver, and a processor. The local oscillator source generates a swept local oscillator signal that sweeps across a range of frequencies. The optical coupler has a firs
Baney Douglas M.
Pering Richard D.
Agilent Technologie,s Inc.
Lyons Michael A.
Turner Samuel A.
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