Kristian Cvecek

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.

Evaluation of nonlinear absorptivity in internal modification of bulk glass by ultrashort laser pulses

Thermal conduction model is presented, by which nonlinear absorptivity of ultrashort laser pulses in internal modification of bulk glass is simulated. The simulated nonlinear absorptivity agrees with experimental values with maximum uncertainty of ± 3% in a wide range of laser parameters at 10 ps pulse duration in borosilicate glass. The nonlinear absorptivity increases with increasing energy and repetition rate of the laser pulse, reaching as high as 90%. The increase in the average absorbed laser power is accompanied by the extension of the laser-absorption region toward the laser source. Transient thermal conduction model for three-dimensional heat source shows that laser energy is absorbed by avalanche ionization seeded by thermally excited free-electrons at locations apart from the focus at pulse repetition rates higher than 100 kHz.

2R-Regeneration of an RZ-DPSK Signal Using a Nonlinear Amplifying Loop Mirror

The performance of a nonlinear amplifying loop mirror as a 2R-regenerator for return-to-zero differential-phase-shift-keyed signals has been investigated experimentally. The measured power characteristics and phase functions show that the signal amplitude is regenerated while the signal phase is preserved in the setup. A significant eye-opening improvement and a negative power penalty of about 1.5 dB were obtained

Phase-preserving amplitude regeneration for a WDM RZ-DPSK signal using a nonlinear amplifying loop mirror.

We propose a modified nonlinear amplifying loop mirror (NALM) for phase-preserving 2R regeneration of wavelength division multiplexed (WDM) return-to-zero differential phase-shift-keyed signals. As proof of principle the regeneration capability of this NALM setup has been investigated experimentally for two 10 Gbit/s wavelength channels. A significant eye-opening improvement and a negative power penalty of 1.2 dB have been observed in both channels.

Sample preparation method for glass welding by ultrashort laser pulses yields higher seam strength

Glass welding by ultrashort laser pulses allows joining without the need of an absorber or a preheating and postheating process. However, cracks generated during the welding process substantially impair the joining strength of the welding seams. In this paper a sample preparation method is described that prevents the formation of cracks. The measured joining strength of samples prepared by this method is substantially higher than previously reported values.

Characteristics of laser absorption and welding in FOTURAN glass by ultrashort laser pulses

The nonlinear absorptivity of FOTURAN glass to ultrashort laser pulses is evaluated by experimental measurement and thermal conduction model at different parameters including energy and repetition rate of the laser pulse, translation speed and thermal properties of the sample. The mechanical strength of an embedded laser-melted sample and an overlapped weld sample is determined by a three-point-bending test and a shear test, respectively. The results are related to the average absorbed laser power Wab. We found the mechanical strength of an overlapped weld joint to be as high as that of the base material for low Wab, if the sample pair is pre-bonded to provide optical contact.

Optimization of a Nonlinear Amplifying Loop Mirror for Amplitude Regeneration in Phase-Shift-Keyed Transmission

We present the numerical optimization of the transmission characteristics of a nonlinear amplifying loop mirror for amplitude regeneration of phase-encoded optical transmission formats. Adjusting the splitting factor, the amplifier gain and the phase bias, minimal phase distortions can be achieved while strong amplitude fluctuations are regenerated. The limiting effects of noise from the built-in amplifier and of amplified Rayleigh backscattering are also discussed.

2R-Regeneration of an 80-Gb/s RZ-DQPSK Signal by a Nonlinear Amplifying Loop Mirror

The performance of a nonlinear amplifying loop mirror as a 2R-regenerator for an 80-Gb/s return-to-zero differential-quadrature-phase-shift-keyed signal has been investigated experimentally. A significant eye-opening improvement and a negative power penalty of up to 2.6 dB were obtained.

3.9-dB OSNR gain by an NOLM-based 2-R regenerator

We experimentally demonstrate bit-error-rate (BER) improvement of more than nine decades by an asymmetric nonlinear optical loop mirror (NOLM). This can be related to an optical signal-to-noise ratio (OSNR) gain of up to 3.9 dB with respect to the NOLM input OSNR at a bit rate of 40 GB/s. The principle of operation of NOLM-based 2-R regeneration with respect to BER improvement is investigated and it is experimentally shown that BER improvement cannot be detected directly at the regenerators output.

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