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.
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 S1 through SN 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, S1, 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 S2A 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 S2B 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.
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 first input and a second input, the first input being optically connected to receive an input signal and the second input being optically connected to the local oscillator source to receive the swept local oscillator signal. The optical coupler has an output for outputting a combined optical signal that includes the input signal and the swept local oscillator signal. The heterodyne receiver has an input for receiving the combined optical signal from the optical coupler, a circuit for stretching a heterodyne beat signal that is generated by the heterodyne receiver in response to the combined optical signal, and an output for outputting heterodyne beat signal data that is generated in response to the stretched heterodyne beat signal. The processor utilizes the heterodyne beat signal data from the heterodyne receiver to generate an output signal that is indicative of an optical parameter of the input signal.
An embodiment of the stretcher circuit includes a diode in series with a capacitor and a resistor. Another embodiment of the stretcher circuit includes a diode in series with a capacitor and a switch.
The heterodyne receiver may include an analog processor for rectifying and applying a log function to the heterodyne beat signal and a data acquisition unit for sampling the stretched heterodyne beat signal to generate the heterodyne beat signal data. In an embodiment, the data acquisition unit samples the stretched heterodyne beat signal at a rate in the range of 20 kHz to 500 kHz.
Another embodiment of the system for optical spectrum analysis includes a high resolution data acquisition unit that samples the heterodyne beat signal at a rate in the range of 5 MHz to 100 MHz to generate high resolution heterodyne beat signal data. The high resolution heterodyne beat signal data can be used to generate a high resolution output signal that is indicative of an optical parameter of the input signal.
Another embodiment of the invention is a method for optical spectrum analysis that involves providing an input signal, providing a swept local oscillator signal that sweeps across a range of frequencies, and combining the input signal with the swept local oscillator signal to create a combined optical signal. The combined optical signal is then detected to generate a heterodyne beat signal. The heterodyne beat signal is stretched and heterodyne beat signal data is obtained from the stretched heterodyne beat signal. An output signal that is indicative of an optical parameter of the input signal is then generated from the heterodyne beat signal data.
In an embodiment of the method, stretching the heterodyne beat signal involves filling in gaps between peaks of the heterodyne beat signal and extending the duration of the heterodyne beat signal. The heterodyne beat signal may be rectified and logged before it is stretched.
In an embodiment, the stretched heterodyne beat signal is rapidly dropped after each sampling event.
In an embodiment, the heterodyne beat signal is sampled at a rate in the range of 20 kHz to 500 kHz.
In another embodiment of the method, high resolution heterodyne beat signal data is obtained from the heterodyne beat signal in addition to the data that is obtained from the stretched signal. A high resolution output signal that is indicative of an optical parameter of the input signal is generated from the high resolution heterodyne beat signal data. In an embodiment, the high resolution heterodyne beat signal data is obtained at a sampling rate in the range of 5 MHz-100 MHz.
Another embodiment of the invention involves obtaining a first set of heterodyne beat signal data using a first sampling rate and obtaining a second set of heterodyne beat signal data using a second sampling rate that is lower than the first sampling rate. Obtaining heterodyne beat signal data at two different sampling rates enables an OSA to perform relatively quick low resolution scans and slower high resolution scans.
A system for optical spectrum analysis that can obtain two sets of data includes a local oscillator source, an optical coupler, a heterodyne receiver, and a processor. In this system, the heterodyne receiver includes a low resolution data acquisition unit for outputting low resolution heterodyne beat signal data and a high resolution data acquisition unit for outputting high resolution heterodyne beat signal data. The processor utilizes the low resolution heterodyne beat signal data to generate a low resolution output signal and uses the high resolution heterodyne beat signal data to generate a high resolution output signal.
In an embodiment of the system, the low resolution data acquisition unit includes a circuit for stretching a heterodyne beat signal that is generated by the heterodyne receiver. In an embodiment, the stretcher circuit includes a diode in series with a capacitor and a resistor and in another embodiment, the stretcher circuit includes a diode in series with a capacitor and a switch.
In an embodiment, the low resolution data acquisition unit samples the stretched heterodyne beat signal at a rate in the range of 20 kHz to 500 kHz and the high resolution data acquisition unit samples the unstretched heterodyne beat signal at a rate in the range of 5 MHz to 100 MHz.
A method for optical spectrum analysis that includes high and low resolution capabilities involves providing an input signal, providing a swept local oscillator signal that sweeps across a range of frequencies, combining the input signal with the swept local oscillator signal to create a combined optical signal, detecting the combined optical signal to generate a heterodyne beat signal, obtaining low resolution heterodyne beat signal data from the heterodyne beat signal, and obtaining high resolution heterodyne beat signal data from the heterodyne beat signal. A low resolution output signal that is indicative of an optical parameter of the input signal is generated from the low resolution heterodyne beat signal data and a high resolution output signal that is indicative of an optical parameter of the input signal is generated from the high resolution heterodyne beat signal data.
In an embodiment of the method, obtaining the low resolution heterodyne beat signal data includes stretching the heterodyne beat signal. In a further embodiment, stretching the heterodyne beat signal includes filling in gaps between peaks of the heterodyne beat signal and extending the duration of the heterodyne beat signal.
In an embodiment of the method, the stretched heterodyne beat signal is sampled at a rate in the range of 20 kHz to 500 kHz to generate the low resolution heterodyne beat signal data and the unstretched heterodyne beat signal is sampled at a rate in the range of 5 MHz to 100 MHz to generate the high resolution heterodyne beat signal data.
In an embodiment of the method, the low resolution heterodyne beat signal data is obtained by sampling the stretched heterodyne beat signal at a slower rate than a sampling rate that is used to obtain the high resolution heterodyne beat signal data.
In an embodiment of the method, the low resolution output signal is generated before the high resolution output signal. In another embodiment, the high resolution heterodyne beat signal data is obtained simultaneously with the low resolution heterodyne beat signal data.
In another embodiment of the invention, high resolution heterodyne beat signal data is generated in response to the low resolution output signal. Specifically, in an embodiment, the sweep range used to obtain the high resolution heterodyne beat signal data is selected in response to a previously obtained low resolution output signal.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.
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