1. Field of the Invention
The present invention relates to a signal-to-noise ratio (SNR) measurer, and in particular, to an optical SNR measurer in a system using a wavelength division multiplexing (WDM) transmission scheme.
2. Description of the Related Art
Wavelength division multiplexing is one of optical signal transmitting techniques, in which a plurality of optical signals at different optical wavelengths are simultaneously propagated through a strand of optical fiber. Wavelength division demultiplexing in the wavelength division multiplexing transmission scheme refers to separation of an optical signal multiplexed on a strand of optical fiber into its constituent optical signals.
An optical SNR is a very significant factor in a wavelength division multiplexing optical transmitting device because it directly affects optical signal quality. U.S. Pat. No. 5,689,594 to Xiaoping Charles Mao entitled Multiple Wavelength Bidirectional Lightwave Amplifier discusses an eight wavelength multiple wavelength bidirectional lightwave amplifier which optimizes the signal-to-noise ratio of each wavelength by automatically compensating for any gain change in the wavelengths. U.S. Pat. No. 5,796,479 to Dennis Derickson et al. entitled Signal Monitoring Apparatus For Wavelength Division Multiplexed Optical Telecommunication Networks describes a detector array spectrometer which provides efficient use of detectors to simultaneously monitor wavelength, power, and signal-to-noise ratio of wavelength division multiplexing channels in optical telecommunication networks. The detector array spectrometer incorporates an angled diffraction grating to achieve compact size, while spatially separating component signals from each of the wavelength division multiplexing channels according to the channels' wavelengths. The component signals provided by the diffraction grating are incident on an array of split-detectors that conforms to the spatial separation of the component signals. While the split-detectors receive a signal from each wavelength division multiplexing channel, noise detectors positioned between adjacent split-detectors measure noise levels between wavelength division multiplexing channels. Each of two halves of each split-detector is equally illuminated by the component signal when the wavelength division multiplexing channel is operating at its designated channel wavelength. Output signals from the two halves are summed to monitor the power of the wavelength division multiplexing channel, while the difference between the output signals from the two halves indicates deviation from the designated wavelength division multiplexing channel wavelength. The ratio of the power in the wavelength division multiplexing channel to the noise level measured by the adjacently positioned noise detector monitors the SNR of the wavelength division multiplexing channel. U.S. Pat. No. 5,894,362 to Hiroshi Onaka et al. entitled Optical Communication System Which Determines The Spectrum Of A Wavelength Division Multiplexed Signal And Performs Various Processes In Accordance With The Determined Spectrum describes signal-to-noise ratio (SNR) detection in each channel of wavelength division multiplexed signal light. The SNR in each channel can be obtained by calculating the ratios between peak powers in each channel and noise components near the respective channels. Further, true signal power can be calculated by subtracting noise component power near each channel from the peak power of each signal in the corresponding channel. The operational conditions and optical output of the optical amplifier can be suitably set according to both the true signal power in each channel and the total optical power inclusive of noise power. More specifically, the power of pumping light is controlled according to the result of measurement of the total optical power inclusive of noise power, thereby making flat the gain characteristic of the optical amplifier (the characteristic representing the relation between gain and wavelength). Further, the optical output of the optical amplifier can be controlled so as to make constant the true signal power or the average of true signal powers in channels in the case that the number of the channels has been recognized. Control processing includes detecting a "first" signal-to-noise ratio of the wavelength division multiplexed signal light before the Wavelength division multiplexed light is amplified. The monitoring unit also detects a "second" signal-to-noise ratio of the wavelength division multiplexed signal light after the wavelength division multiplexed signal light is amplified. The monitoring unit then determines a noise figure from the ratio of the first signal-to-noise ratio to the second signal-to-noise ratio and, utilizing the noise figure, controls the optical amplifier so as to make constant the detected true signal power. Additionally, the control processing can also include controlling the optical amplifier in accordance with the determined noise figure to achieve a desired noise figure.
It is also known that in order to measure the optical SNR, the wavelength division multiplexing optical transmitting device typically uses a multichannel optical SNR measurer, due to the multiplexing of optical signals of plural signal channels at different wavelengths on a strand of optical fiber, and measures an optical SNR for each channel. For this operation, the optical SNR measurer is provided with a filter. This filter is virtually the same in function and configuration as a wavelength division demultiplexer for demultiplexing a wavelength division multiplexed optical signal into optical signals of signal channels in a wavelength division multiplexing optical transmitting device. The filter as well as the wavelength division demultiplexer is expensive.
As described above, the conventional wavelength division multiplexing optical transmitting device additionally uses a filter for separating an optical signal for each channel from a wavelength division multiplexed optical signal so that optical SNR can be measured, resulting in a cost increase.
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