Future optical networks are envisioned to be dynamically reconfigurable - the optical channels are frequently added/dropped or cross-connected in the optical layer using wavelength-division-multiplexing (WDM) technologies. For the efficient operation, administration, maintenance and provision (OAM&P) of these all-optical WDM networks, it is essential to monitor the performance of each signal at every optical node where the terminal equipment, add/drop multiplexer and optical cross-connects are placed. In this dissertation, we propose simple but efficient techniques for the monitoring of various performance parameters including optical frequencies, signal powers, and chromatic dispersion using pilot tones.
First, we have proposed and demonstrated a simple technique for monitoring the frequencies and optical powers of WDM signals simultaneously. The proposed technique could be implemented by using only two photodiodes, a fixed Fabry-Perot etalon filter, and an electronic circuit for A/D conversion and numerical processing with Fast Fourier Transform (FFT). To demonstrate the principle, we monitored the frequencies and optical powers of 16 WDM channels. The results showed that the frequencies and optical powers of multiple WDM channels could be monitored with accuracy better than ±3 GHz and ±0.5 dB, respectively. In addition, the proposed technique did not cause any serious degradation in the receiver sensitivity (< 0.5 dB). Using the pilot-tone based monitoring technique, we have also demonstrated an automatic frequency stabilization of multiple WDM lasers simultaneously. Unlike other stabilization techniques, the proposed technique could stabilize a large number of WDM channels using only one control loop. Thus, it is a cost-effective solution for the high-capacity WDM system with large number of channels. The results show that the proposed technique could stabilize the frequencies of 16 WDM channels within ±100 MHz.
When the pilot-tone based monitoring techniques are used in amplified WDM networks, their performances could be impaired by the slow dynamic property of Erbium-doped fiber amplifier (EDFA). Thus, to estimate the scalability of the pilot-tone based monitoring technique, we have developed a simple model to describe the effects of EDFA’s cross-gain modulation. This model was used to calculate the maximum numbers of EDFA spans and WDM channels that the pilot-tone based monitoring technique could support. The results show that the pilot tones in the range of ~100 kHz could be used for the performance monitoring of WDM signals in metro optical networks. For the use in long-haul WDM networks with large number of channels (>100 channels), the frequencies of pilot tones should be higher than hundreds of MHz (to suppress the ghost tones caused by stimulated Raman scattering) and the modulation indices should be smaller than 5 % (to maintain the pilot-tone induced power penalty within 0.5 dB).
We have also demonstrated a simple technique that can be used to monitor the chromatic dispersion in WDM networks using high-frequency pilot tone. When the high-frequency pilot tone propagates along optical fiber, the chromatic dispersion changes the phase difference between the optical carrier and its sidebands. As a result, the amplitude of the transmitted pilot tone could be changed by the chromatic dispersion. Thus, the chromatic dispersion could be monitored by measuring the amplitude of high-frequency pilot tone. The results show that, using the phase-modulated pilot tone, the chromatic dispersion of each channel could be monitored with accuracy better than 60 ps/nm even after the transmission over 640-km long non-zero dispersion shifted fiber. We have also showed that the first-order polarization-mode dispersion and chromatic dispersion could be monitored simultaneously by measuring the high-frequency signal components while scrambling its polarization. This technique could monitor the polarization-mode dispersion and chromatic dispersion of each WDM channel with accuracy better than ±2.5 ps and ±30 ps/nm, respectively.
