The polarization-nulling technique utilizes the different properties of optical signal and amplified spontaneous emission (ASE) noise for accurate monitoring of the optical-signal-to-noise ratio (OSNR) in dynamic optical networks. However, the performance of this technique is bound to be deteriorated if the signal is depolarized by polarization-mode dispersion and/or nonlinear birefringence or the ASE noise is partially polarized due to polarization-dependent loss (PDL) in the transmission link. The authors analyze these effects on the performance of the polarization-nulling technique and introduce several techniques to overcome these problems. These improved versions of the polarization-nulling techniques could monitor the OSNR with accuracy of better than plusmn1 dB, even when the differential group delay is as large as 60 ps. These techniques could also negate the effect of the signal depolarization caused by nonlinear birefringence in a highly nonlinear transmission link. The effect of the partially polarized ASE noise due to PDL is found to be not severe in most cases, as long as the PDL/span is smaller than 0.2 dB. To verify the possibility of using the polarization-nulling technique in real systems, the OSNR of the wavelength-division-multiplexed (WDM) signals transmitted through a 120-km-long aerial fiber link is measured for one week. No significant degradation in the monitoring accuracy is observed during this long-term measurement. In addition, the performance of the polarization-nulling technique in an ultralong-haul transmission link is evaluated by using a 640-km-long recirculating loop. The results show that this technique could accurately measure the OSNR in the transmission link longer than 3200 km. From these results, the authors conclude that the polarization-nulling technique is well suited for monitoring the OSNR in dynamic WDM networks

A Review of the Polarization-Nulling Technique for Monitoring Optical-Signal-to-Noise Ratio in Dynamic WDM Networks

We propose a simple self-polarization-stabilization technique for the wavelength-division-multiplexed passive optical network implemented with reflective semiconductor optical amplifiers (RSOAs) and self-homodyne coherent receivers. By placing a 45° Faraday rotator in front of the RSOA in the optical network unit, the state-of-polarization of the upstream signal becomes orthogonal to that of the linearly polarized seed light at the input of the coherent receiver regardless of the birefringence in the transmission link. Thus, we can achieve the polarization stability of the upstream signal at the input of the coherent receiver. We first implement a self-homodyne receiver by using the proposed self-polarization-stabilization technique and measure its sensitivity by using 2.5-Gb/s binary phase-shift keying signals in the laboratory. The result shows an excellent receiver sensitivity of -46.4 dBm. We also confirm the efficacy of the proposed technique in the transmission experiment over 68-km long link partially composed of installed (buried and aerial) fibers. No significant degradation in the receiver sensitivity is observed during the 10-h experiment despite the large polarization fluctuations occurred in these installed fibers.

Long-Reach Coherent WDM PON Employing Self-Polarization-Stabilization Technique

We report on the improved OSNR monitoring technique for WDM networks using the polarization-nulling method. Unlike the previous techniques based on the polarization, the proposed technique was not sensitive to PMD and nonlinear birefringence.

OSNR Monitoring Technique basde on Polavization - Nulling Method

We demonstrate a long-reach wavelength-division-multiplexed passive optical network (WDM PON) operating at the symmetric rate of 10.3 Gb/s. For the cost-effectiveness, we realize the upstream transmission by utilizing directly-modulated TO-can packaged reflective semiconductor optical amplifiers (RSOAs) and digital coherent receivers. In addition, to overcome the limited modulation bandwidth of this TO-can packaged RSOA (~2.2 GHz) and operate it at 10.3 Gb/s, we utilize the quadrature phase shift keying (QPSK) format and the electronic phase equalization technique. The result shows that we can extend the maximum reach of the 10.3-Gb/s RSOA-based WDM PON to ~80 km without using any remote amplifiers.

Long-reach 10-Gb/s RSOA-based WDM PON employing QPSK signal and coherent receiver

A comprehensive analysis and survey of the optical performance monitoring (OPM) is provided towards the deployment of translucent/transparent optical networks. OPM applications and technologies are reviewed for the different stages of the optical network life cycle: planning and provisioning, impairment mitigations, maintenance and failure location. Recommendations for the most relevant OPMs required to be integrated in translucent/transparent optical networks are provided.

Optical performance monitoring for translucent/transparent optical networks

Feature
Issue on Optical Performance Monitoring (OPM). We report on the
performance of an optical signal-to-noise ratio (OSNR) monitor with the
polarization-nulling technique. The results show that this monitor was robust to
polarization-mode dispersion and in-band cross talk. For a demonstration, we
monitored the OSNR of the optical signal transmitted through a 2×60 km long aerial
fiber link for one week and confirmed that the OSNR could be measured with accuracy
better than ±0.3 dB (when the OSNR is lower than 26 dB).

S. K. Shin

Papers

Performance of an OSNR monitor based on the polarization-nulling technique