Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
  • H04B10/07—Arrangements for monitoring or testing transmission systems; Arrangements for fault measurement of transmission systems
  • H04B10/075—Arrangements for monitoring or testing transmission systems; Arrangements for fault measurement of transmission systems using an in-service signal
  • H04B10/079—Arrangements for monitoring or testing transmission systems; Arrangements for fault measurement of transmission systems using an in-service signal using measurements of the data signal
  • H04B10/0795—Performance monitoring; Measurement of transmission parameters
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
  • H04B10/07—Arrangements for monitoring or testing transmission systems; Arrangements for fault measurement of transmission systems
  • H04B10/075—Arrangements for monitoring or testing transmission systems; Arrangements for fault measurement of transmission systems using an in-service signal
  • H04B10/079—Arrangements for monitoring or testing transmission systems; Arrangements for fault measurement of transmission systems using an in-service signal using measurements of the data signal
  • H04B10/0795—Performance monitoring; Measurement of transmission parameters
  • H04B10/07953—Monitoring or measuring OSNR, BER or Q
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
  • H04B10/80—Optical aspects relating to the use of optical transmission for specific applications, not provided for in groups H04B10/03 - H04B10/70, e.g. optical power feeding or optical transmission through water
  • H04B10/85—Protection from unauthorised access, e.g. eavesdrop protection

Definitions

• the present invention is directed to a system and method for measuring in-band crosstalk in an optical communication system, and particularly for using such measurement in conjunction with other measurements to estimate bit error rate (BER).
• BER bit error rate
• Optical routing in optical communication systems requires a wavelength and polarization insensitive optical switch. Determining a bit error rate (BER) after each of these switches is useful for determining and maintaining the health of a WDM network.
• the BER is defined as the ratio of the number of erroneous bits received to the total number of bits received per second.
• Eye diagrams are a known technique to track channel power as a function of time. These diagrams are generated by plotting the received signal as a function of time, and then shifting the time axis by one bit interval and plotting again. The superimposed bits define most probable (constructive and destructive) interference events due to transmission in the channels adjoining the particular channel plotted. Thereby, the eye diagram depicts the worst-case impairment as measured by the greatest ordinate value clear of traces (by the vertical dimension of the clear space between a peak and a null).
• a system that is not excessively impaired shows clear discrimination between "l 's" and "O's" in a digital signal, with an "eye opening" in the center of the diagram.
• a truly unimpaired system is considered to have an eye opening of 1.0.
• ISI Inter-Symbol Interference
• monitoring the BER of a system is conducted using spectrum analyzers to look at the primary noise source, such as amplified spontaneous emission (ASE).
• ASE amplified spontaneous emission
• OSNR optical signal-to-noise ratio
• Efforts to find a metric of BER typically entail demodulating the transmitted signal, measuring the power spectral density (i.e., carrier signal power to noise floor), or channel sampling. While the accuracy of this inferential technique may increase with each additional accurate assessment of noise parameters, there is still a need to improve techniques for estimating the BER.
• the present invention is directed to measuring an additional metric of BER, which may be used to enhance the estimation of BER.
• a system for estimating in-band cross-talk in an optical communication system may include a selective element for separating a signal in a desired channel from a plurality of channels in the optical communication system; a filter, which passes the signal at a rate proportional to the time rate of change of a phase of an optical source generating the signal; a digital signal processor, which receives the signal from the filter and converts the signal into a frequency domain; and a spectrum analyzer, which measures at least one feature of the signal in the frequency domain to quantify the in-band cross-talk.
• a method for estimating in-band cross-talk in an optical communication system includes separating a signal in a desired channel from a plurality of channels in the optical communication system; passing the signal in proportion to the time rate of change of a phase of an optical source generating the signal; converting the signal into a frequency domain; and analyzing at least one feature of the signal in the frequency domain to quantify the in-band cross-talk.
• estimating bit error rate (BER) in an optical communication system includes a selective element, which separates a signal in a desired channel from a plurality of channels in the optical communication system; a filter, which passes the signal at a rate proportional to the time rate of change of a phase of an optical source generating the signal; a digital signal processor, which converts the signal into a frequency domain; a spectrum analyzer which measures at least one feature of the signal in the frequency domain to quantify the in-band cross-talk; and a post processor which combines at least one feature measured by the spectrum analyzer with at least one noise feature to estimate BER.
• a selective element which separates a signal in a desired channel from a plurality of channels in the optical communication system
• a filter which passes the signal at a rate proportional to the time rate of change of a phase of an optical source generating the signal
• a digital signal processor which converts the signal into a frequency domain
• a spectrum analyzer which measures at least one feature of the signal in the frequency domain to quantify the in-
• a method for estimating bit error rate (BER) in an optical fiber includes separating a signal in a desired channel from a plurality of channels in the optical communication system; passing the signal at a rate proportional to the time rate of change of a phase of an optical source generating the signal; converting the signal into a frequency domain; analyzing the signal in the frequency domain to quantify the in-band cross-talk; and combining at least one feature from the analyzing with at least one noise feature to estimate BER.
• Figure 1 is a block diagram of the system for measuring in-band cross-talk in accordance with an exemplary embodiment of the present invention.
• Figures 2a-2h are plots of gain versus frequency for varying levels of power in the low frequency spectrum measured according to an exemplary embodiment of the present invention.
• the present invention is directed to recognizing that one metric of BER is the in-band cross-talk.
• In-band cross-talk is an extraneous optical field, which interferes with the communications signal upon optical-to-electrical conversion resulting in noise having a spectrum, which falls within the electrical bandwidth of the receiver system.
• in-band cross-talk may be cross-talk within a single channel, which arises from any pair of back reflections generated in an optical communication system. In an optical communication system, if a signal is reflected twice, that erroneous signal is then traveling in the same direction as the desired as the desired signal and may interfere with the desired signal.
• the time delay between the input signal and the reflected signal may interfere, and this may lead to beating.
• the relative phase is random and temporally varying, leading to a time varying interference (e.g., beating).
• the importance of measuring in-band cross-talk has increased with the rise of optical networking, in which the network may be reconfigured by the flip of a switch.
• in-band cross-talk does not normally change the overall optical signal-to-noise ratio (OSNR), since the in-band cross-talk occurs in a much narrower spectrum than the OSNR and is not resolved in the OSNR measurements.
• OSNR optical signal-to-noise ratio
• in-band cross-talk normally will be concentrated in a spectral region in proportion to the time rate of change of the relative phases between the signal and the cross-talk components. For some devices such as semiconductor lasers, this type of noise will be most prevalent at low (e.g., radio) frequencies.
• the noise due to in-band cross-talk may be determined. While the absolute value of the in-band cross-talk is difficult to ascertain, the relative values may be useful in estimating the BER, especially when used with other metrics of BER to improve these estimates.
• the measurement of phase noise in an optical source is known from the study of laser noise.
• FIG. 1 A configuration for determining the low frequency features of in-band cross-talk in a single WDM channel in an optical communication system 28 according to an exemplary embodiment of the present invention is shown in Fig. 1.
• All incoming WDM channels on an optical waveguide such as an optical fiber 10 are passed through a selective element 12.
• the signals are incident on a photodetector 14.
• a single channel from the plurality of WDM channels is selected based on the corresponding illuminated pixel for the deflected wavelength or the location of the tunable filter.
• the optical communication system 28 incorporates an optical waveguide such as an optical fiber and/or a planar optical waveguide.
• an optical waveguide such as an optical fiber and/or a planar optical waveguide.
• the invention of the present disclosure may be used in optical communication systems incorporating other types of optical wavguides.
• the invention of the present disclosure may be used in optical communication systems, which include "free-space" portions as well. These free-space portions include, but are not limited to, micro-optic devices such as filters, isolators and switches.
• selective element 12 may be a dispersive element, or a tunable filter.
• the filter location may be dithered to ensure optimal channel overlap with the filter passband. This dithering frequency may then be filtered out by post-processing.
• the input signal is demultiplexed (and spatially separated) into component wavelengths by the selective element 12.
• grating 12 may be any conventional demultiplexer, such as a grating, a blazed grating, an arrayed waveguide grating, or a prism; a micro-optic based filter; a thin-film based filter; or a waveguide based filter such as a fiber Bragg grating (FBG).
• FBG fiber Bragg grating
• the signal from the selected channel is passed from the photodetector 14 through a low-noise pre-amplifier 16 and low frequency filter 18, which is illustratively an anti-aliasing filter.
• the low frequency filter 18 is selected in proportion to the time rate of change of the phase in the optical source (not shown) and according to well known radio frequency (rf) techniques.
• the signal is then sampled by an analog-to-digital converter (ADC) 20 at a frequency high enough to prevent signal degradation due to aliasing; illustratively this frequency is equal to or greater than the Nyquist frequency (f ⁇ ) of the previous analog filter 18.
• ADC analog-to-digital converter
• the dynamic range of these elements is illustratively greater than 30 dB.
• a digital signal processing (DSP) unit 22 recovers the low frequency signature across the phase noise spectrum of the optical source (e.g. laser) spectrum by converting the signal from the ADC 20 to the frequency domain via windowing and either a Discrete Fourier Transform (DFT) or a Fast Fourier Transform (FFT). If the levels of in-band cross-talk are relatively low (illustratively on the order of -30dB), additional signal averaging in the frequency domain with a finite impulse response (FIR) filter is usefully performed.
• DFT Discrete Fourier Transform
• FFT Fast Fourier Transform
• the resultant signal is then provided to a spectrum analyzer 24 which can be used to determine the magnitude, location, number and width of the in-band cross-talk features; particularly the peak of the spectrum, for a more accurate picture.
• the noise spectral density of the in-band cross-talk spectrum can be averaged over an appropriate frequency range and then compared with a spectrum outside this frequency range to estimate the contribution of the in-band cross-talk to the BER.
• the appropriate frequency range is determined by the speed with which the phase noise of the optical source changes. However, the lower frequencies of this range, where 1/f noise is prevalent, should not be included. An upper end should cut off well after any such noise is expected to be present.
• the appropriate frequency range may be from about % of where the phase noise maxima occur to about twice this frequency. This value is dependent on the phase noise spectrum of the source. Illustratively, for DFB lasers, this frequency range is on the order of approximately 50 MHz.
• the in-band cross-talk features may be combined with other measurements to provide a more accurate estimate of BER in a post processing unit (PPU) 26.
• the in-band cross-talk features may be combined with the received signal's power spectral density (PSD).
• PSD is the Fourier transform of the autocorrelation of the noise amplitude, i.e., the degree to which any the noise random variables at different times depend on one another. Additional information may be included in the PPU to increase the accuracy of the
• Such infoimation may include but is not necessarily limited to the ASE noise floor, the number of add/drops the channel has undergone, the width of the main lobe of the PSD, and the location of the wavelength band of the channel.
• the metric of the in-band cross-talk may be readily included with the other metrics to more accurately estimate BER.
• the effect of phase-to-intensity noise conversion by multiple reflection on Gigabit/sec DFB laser transmission systems is known.
• Figures 2a-2h illustrate the in-band cross-talk features measured by the illustrative system of Fig. 1. As can be seen therein, the in-band cross-talk features increase with increasing levels of power. For the plots shown in Figures 2a-2h, the data was processed with sixty-four averages to reduce the influence of other noises.

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• Physics & Mathematics (AREA)
• Electromagnetism (AREA)
• Computer Networks & Wireless Communication (AREA)
• Signal Processing (AREA)
• Computer Security & Cryptography (AREA)
• Optical Communication System (AREA)
• Monitoring And Testing Of Transmission In General (AREA)

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  • 2001-01-12 EP EP01942484A patent/EP1247356A2/de not_active Withdrawn
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