Arne G. Striegler

NOLM-based RZ-DPSK signal regeneration

We present a nonlinear optical loop mirror (NOLM)-based 2R-regenerator setup, which is capable of regenerating signals modulated in phase-sensitive modulation formats. In a conventional NOLM, fluctuations of the signal amplitude are converted into phase fluctuations. Therefore, it is not suitable for regeneration of signals, modulated in formats such as differential phase-shift keying (DPSK) or duobinary. In this letter, we present a modified NOLM setup for 2R-regeneration taking return-to-zero DPSK as an example.

All-optical DPSK signal regeneration based on cross-phase modulation

We present a concept for all-optical differential phase-shift keying (DPSK) signal regeneration, which reduces amplitude jitter and preserves phase information. At first, we consider a configuration to regenerate optical signals based on spectral broadening by self-phase modulation (SPM). An analytical approach of the induced phase shift demonstrates why SPM is not suitable for DPSK signal regeneration. We derive conditions for DPSK signal regeneration and present a configuration based on cross-phase modulation. Amplitude jitter is reduced effectively while preserving DPSK phase information. Numerical simulations prove the results and demonstrate the performance of DPSK signal regeneration.

Analysis and optimization of SPM-based 2R signal regeneration at 40 gb/s

In order to maintain a signal quality and to enhance the robustness of long haul transmission, optical regenerators have been developed and investigated during the last few years. This paper provides a detailed analysis of self-phase modulation (SPM)-based 2R signal regeneration by an analysis of the spectrum of a single pulse. A model is presented to explain the principle of ghost-pulse suppression and amplitude-jitter reduction and to derive design rules to optimize signal regeneration at 40 Gb/s. Finally, the influence of the fiber dispersion on the regeneration is discussed.

Compensation of intrachannel effects in symmetric dispersion-managed transmission systems

This paper analyzes perturbations caused by nonlinear intrachannel effects and shows how to cancel them by symmetric configuration of transmission and dispersion-compensating fiber. From an analytical study of amplitude and timing jitter conditions are derived to reduce them effectively. Simulations are performed using different schemes of dispersion compensation and improvement and limits of regenerative symmetric links are also discussed. Finally, the paper presents a simple fiber-based configuration to reduce timing jitter in particular. The quality of existing transmission systems is improved.

Experimental investigation of a modified NOLM for phase-encoded signal regeneration

We experimentally investigate the amplitude and phase transfer characteristics of a modified nonlinear optical loop mirror (NOLM) with a directional attenuator (DA-NOLM) optimized for differential phase-shift keying signal regeneration. The results show that the phase relation is preserved in the setup and thus the DA-NOLM is suitable for amplitude regeneration of phase-shift-keyed signals

Fiber-based compensation of IXPM-induced timing jitter

We present a simple setup consisting of a short piece of fiber and a fiber grating to compensate timing shifts induced by intrachannel cross-phase modulation in a preceding transmission span. As accumulated dispersion with inverse sign causes inverse timing and energy shifts, timing and amplitude jitter can be balanced out. We demonstrate by numerical simulation the principle for a one-span setup. Furthermore, we proof the applicability of this fiber-based retiming structure at a data rate of 40 Gb/s in a transmission system of 960-km length. The results show that timing jitter is reduced dramatically and has potential for application as a inline retiming stage.

Extinction ratio improvement by an advanced NOLM setup

We present a nonlinear optical loop mirror (NOLM)-based setup which is optimized for regeneration of signals with a poor extinction ratio caused by, e.g., high powered ghost pulses. In a conventional NOLM, the improvement of the extinction ratio is limited by the input extinction ratio or insufficient amplitude jitter reduction. These limitations can be overcome by an additional directional phase shifter. According to numerical simulation, the extinction ratio can be improved from 6 dB up to several 10 dB.

Combination of all-optical 2R signal regeneration with fiber-based retiming

We present all-optical signal regeneration using fiber-based retiming to overcome limitation by intrachannel cross-phase modulation-induced timing jitter in 2R regenerated transmission systems. As timing shifts of the optical pulses depend on the sign of the accumulated dispersion, timing jitter can be balanced out by a short piece of fiber with inverse sign of accumulated dispersion as the preceding transmission span. Timing jitter is cancelled out completely. Remaining amplitude jitter and ghost pulses are reduced by 2R-regeneration based on self-phase modulation. Limitation by amplitude jitter of the retiming stage and limitation by timing jitter of the 2R-regeneration cancel out each other. Numerical simulations show that maximum transmission distance is enhanced significantly.

Innovative optical components and their impact on future optical transmission systems

Optical transmission systems made impressive progress in the field of system capacity, reach and flexibility. Beside all improvements achieved so far, signal distortion on the one hand side and noise accumulation on the other hand side are the limitations of optical transmission system performance. In terms of signal distortion besides advanced link design strategies optical regenerators are of high interest for signal conditioning along a fiber link. Our paper reviews signal regeneration techniques focusing on fiber nonlinearity based setups both in interferometric and non interferometric architecture. We analyze the impact of regenerator design parameters as fiber nonlinear parameters, regenerator input power and characteristics of filters on signal improvement and demonstrate e.g. the increase of system reach. In order to address the upcoming techniques of advanced modulation formats we introduce a fiber based regenerator scheme that shows attractive potential especially for phase modulated signals. Combining these new optical formats with regenerative methods is attractive for high performance systems. For the differential phase shift keying (DPSK) modulation format we investigate the potential of a cross phase modulation based type of regenerator in more detail. The availability of suitable regenerator components, especially highly nonlinear fibers is one of the challenges. Especially the new fiber types known as photonic crystal fibers can act as bases for new structures and thus will open the door for ultra high performance optical transmission systems.

All-optical DPSK-signal-regeneration based on a NOLM-setup

We present a novel concept for all-optical DPSK signal regeneration based on a directionally attenuated non linear loop mirror (DA-NOLM).