S.M.R.M. Nezam

Monitoring and control of polarization-related impairments in optical fiber systems

Polarization-related impairments have become a critical issue for high-data-rate optical systems, particularly when considering polarization-mode dispersion (PMD) and polarization-dependent loss (PDL). In general, polarization effects are stochastic processes and can occur on short or long time scales. Therefore, control and monitoring of these effects may be crucial in any systems-level mitigation. This tutorial will highlight the following key issues: PMD and PDL, monitoring schemes, emulation of proper statistics, interaction, and applications of polarization scrambling.

Degree-of-polarization-based PMD monitoring for subcarrier-multiplexed signals via equalized carrier/sideband filtering

We demonstrate a method for differential-group-delay (DGD) and polarization-mode-dispersion (PMD) monitoring using the degree-of-polarization (DOP) in subcarrier-multiplexed (SCM) systems. Traditional SCM signal show very little DOP sensitivity to DGD/PMD due to the low modulation depth used for generating SCM signals. We use a narrow-band optical filter to equalize the power of the carrier and one of the sidebands by offsetting the filter from the carrier, enabling PMD and DGD monitoring and more than tripling the DOP sensitivity to DGD/PMD. Our technique is simple, uses only a single optical filter, and can be applied to both single- and double-sideband (SSB and DSB) SCM signals as well as single and multisubcarrier systems. Additionally, we show that our monitoring technique is robust to the chromatic dispersion-induced radio-frequency (RF) power fading effect seen in DSB SCM signals. Using this technique to enhance the DOP sensitivity to DGD/PMD and generate a feedback signal to a PMD compensator (PMDC), we obtain an 11-dB improvement in the 5% RF power tail.

Performance optimization of RZ data format in WDM systems using tunable pulse-width management at the transmitter

Tunable pulse-width management is one of the efficient methods to enhance the robustness of return-to-zero (RZ) data formats for long-haul transmission systems. We demonstrate both single channel and 4 /spl times/ 10-Gb/s wavelength-division-multiplexed performance optimization using tunable pulse-width management. Pure RZ single with tunable pulse-width is generated by changing the driving voltages on a phase modulator and the dispersion values of a tunable dispersion element simultaneously according to our simulation results. Varying the pulse width from 50 to 10 ps at the transmitter can almost double the transmission distance with 4% variation in the residual link dispersion.

First-order PMD monitoring for NRZ data using RF clock regeneration techniques

We propose and demonstrate two novel techniques for 10 Gb/s polarization-mode-dispersion (PMD) monitoring for NRZ signals that use a regenerated RF clock tone as a monitoring signal. Our techniques regenerate the RF clock tone that is usually absent after square-law detection in the electrical NRZ data spectrum (in the absence of dispersion). Our first technique uses a dispersive element in the monitoring tap-line to put the beat terms between the optical clock sidebands and the carrier in phase and thus regenerates the RF clock tone after detection. Our second technique involves the use of an optical filter that is centered at the bit rate frequency on either the upper or lower sideband of the optical spectrum, removing one of the sidebands and thus preventing the beating that normally cancels the RF clock tone. We show (theoretically, via simulation, and experimentally) the effect that PMD has on these regenerated RF clock tones. We also demonstrate PMD compensation at 10 Gb/s using these techniques for monitoring and show a 6-dB improvement in the 1% power penalty tail. Our techniques are simple, do not require modification at the transmitter, and can be applied to WDM systems via the use of a multichannel dispersive element or a tunable filter swept across all channels.

PMD monitoring by tracking the chromatic-dispersion-insensitive RF power of the vestigial sideband

We demonstrate a simple technique for polarization-mode-dispersion (PMD) monitoring that is insensitive to chromatic dispersion (CD). By monitoring the extracted radio-frequency clock power from the vestigial sideband, a CD-insensitive PMD feedback signal is realized. For a 10-Gb/s return-to-zero (RZ) system, experimental results show that the fluctuation of the feedback clock power due to CD is reduced from /spl sim/20 to /spl sim/3 dB. In the presence of all-order PMD, the distribution of the clock power is essentially unchanged as the inline CD varies. Simulation results for a 40-Gb/s RZ system are also presented.

Theoretical and experimental analysis of the dependence of a signal's degree of polarization on the optical data spectrum

We show theoretically and experimentally the relationship between a signals degree of polarization (DOP), all-order polarization mode dispersion (PMD), and the optical spectrum (and hence the data modulation format and pulse width), and that these effects must be taken into account when using the DOP for differential group delay (DGD) monitoring. We explain the theory behind how all-order PMD affects a signals DOP, and observe the pulse-width dependence for 10-, 20-, and 40-Gb/s return-to-zero (RZ) systems as the duty cycle changes. We then analyze and show (via simulation and experimentation) the effects of different data modulation formats (RZ, carrier-suppressed RZ, alternate-chirped RZ, and differential phase-shift keying) on the DOP in a DGD monitor. We conclude that the measurable DGD range and DOP sensitivity in DOP-based DGD monitors are dependent on a signals pulse width and the data modulation format. We also show the theory behind the effects of first- and second-order PMD on the maximum and minimum DOP.

Wide-dynamic-range DGD monitoring by partial optical signal spectrum DOP measurement

We propose a novel partial-optical-spectrum degree of polarisation (DOP) measurement that increases the pulse-width dependent differential group delay (DGD) monitoring range for 10, 20, and 40-Gbit/s RZ and sensitivity for 10-Gbit/s NRZ signals. The monitoring range for 20 and 40-Gbit/s RZ signals is extended to one bit time.

Enhancing the dynamic range and DGD monitoring windows in DOP-based DGD monitors using symmetric and asymmetric partial optical filtering

We propose and demonstrate a new type of degree-of-polarization (DOP)-based differential-group-delay (DGD) monitor using an optical filter such that the DGD monitoring range and DOP dynamic range are dramatically increased. We apply this technique to varying pulsewidth return-to-zero (RZ), carrier-suppressed RZ (CSRZ), and alternate-chirped RZ (ACRZ) signals and show that by optimally setting the position, bandwidth, and shape of a filter, we can double the DGD monitoring range compared to traditional DOP-based DGD monitors. Using our technique, the DGD monitoring ranges for 10, 20, and 40 Gb/s /spl sim/12.5-ps pulsewidth RZ signals are increased by 32, 33, and 12 ps, respectively. We also show that a narrow-band optical filter, offset from the center of the optical spectrum by the bit-rate frequency, can double the dynamic range of DOP-based DGD monitors for non-RZ (NRZ) signals.

Experimental and theoretical analysis of the optimum decision threshold for varying numbers of active users in a 2-D time-wavelength asynchronous O-CDMA system

We experimentally demonstrate the existence of an optimal decision threshold in an optical code-division-multiple-access (O-CDMA) system and show its relation to the number of active users. Our results show that tuning the decision threshold in an O-CDMA receiver to an optimal point based on the number of active users improves receiver sensitivity by > 3 dB. We also demonstrate a monitoring scheme to estimate the number of active users in an O-CDMA system using harmonics of RF clock tones.

PMD monitoring in WDM systems for NRZ data using a chromatic-dispersion-regenerated clock

We demonstrate a polarisation mode dispersion (PMD) monitor for NRZ signals that measures the effects of PMD on a chromatic-dispersion-generated clock tone. In NRZ transmission, no clock tone appears at a detector if the link chromatic dispersion is well compensated. In our method, we introduce fiber-Bragg based chromatic dispersion (negligible PMD) in the feedback loop of the PMD compensator to generate a 10 GHz clock tone within an NRZ signal.