Round-trip time and dispersion optimization in a dual-wavelengthactively mode-locked Er-doped fiber laser including nonchirped fiberBragg gratings

Round-trip time and intracavity dispersion were optimized in a dual-wavelength actively mode-locked Er-doped fiber laser including two nonchirped fiber Bragg gratings. This optimization allowed to generate nearly transform-limited pulses as short as 16 and 13 ps at 1547 and 1562 nm, respectively, at a 3-GHz repetition rate only limited by the modulator bandwidth. In the experiment, the round-trip time difference between pulses at two wavelengths was adjusted using a fiber stretcher. Intracavity dispersion was managed at each wavelength separately     

Retrieving Optical Pulse Profiles Using a Nonlinear Optical Loop Mirror

A novel technique is proposed to characterize the power profile of short optical pulses using the transmission characteristic of a nonlinear optical loop mirror (NOLM). Under some assumptions, the power profile can be retrieved from pulse energy measurements at the NOLM output using a slow detection setup. As a fiber-based technique based on the Kerr effect, it does not require phase matching or tedious mechanical adjustments. The technique is able to characterize simultaneously pulse features from the picosecond to the nanosecond scale.     

Step-like all-optical decision function using nonlinear polarisation rotation in a Nonlinear Optical Loop Mirror and in a subsequent fibre section with output polarisation selection

We propose and study numerically a fibre-based scheme designed to generate a step-like optical decision function for ultrafast data processing applications. The setup includes two main sections disposed in series: a Nonlinear Optical Loop Mirror with an output polariser on the one hand, and a circularly birefringent fibre span with a quarter wave retarder at the input and a polariser at the output on the other hand. Both sections operate through nonlinear polarisation rotation, and the switching characteristic is controlled through the light ellipticity, which can be adjusted by setting properly the orientation of the input polarisation, and by rotating the wave retarder and the two polarisers. For proper adjustment of these four parameters, a step-like optical decision function with ～10 dB/dB steepness is predicted. In the frame of all-optical regeneration, the setup allows eliminating ～76% peak-to-peak amplitude fluctuations on the marks and ～30% relative power on the spaces. We believe that this architecture should be considered for the design of ultrafast transmission networks and in the frame of all-optical computing applications.     

Theoretical and experimental analysis of tunable Sagnac high-birefringence loop filter for dual-wavelength laser application

We present detailed investigations of the spectral dependencies of the transmission of a fiber optical loop mirror (FOLM) consisting of a coupler with output ports spliced at arbitrary angles to a high-birefringence (Hi-Bi) fiber. The application for dual-wavelength lasers is discussed. For this aim, the spectral dependence of the reflection is tuned by the temperature of the Hi-Bi fiber that allows a fine adjustment of the cavity loss for generated wavelengths. The ratio between maximum and minimum reflection can be adjusted by the twist angle of the fiber at the splices, which also provides useful possibilities for the adjustment of cavity losses. We used the twist and temperature variation of the Hi-Bi fiber to change the operation from single wavelength to stable dual-wavelength generation with either equal or unequal powers of wavelengths.     

Supermode noise of harmonically mode-locked erbium fiber laserswith composite cavity

The supermode noise of a harmonically mode-locked erbium-doped fiber laser with composite cavity is investigated both theoretically and experimentally. We propose a simple model based on the transfer function of the composite cavity. From this model, the frequencies of the cavity modes and their frequency-dependent losses are determined. A comprehensive experimental study of the composite cavity laser is carried out, covering a wide range of path-length differences between both arms of the fiber interferometer that form the composite cavity. Experimental results are in good agreement with our model. In particular, the path length difference determines periodic frequency windows within which supermodes of the composite cavity are observed. Outside these windows, supermodes are not detectable. Our results show quite remarkably that, although the number of supermodes is reduced with respect to the simple cavity, no measurable reduction of the overall supermode noise is obtained, contrary to what has recently been previously suggested     

An end to unemployment in Europe: from breakdown to breakthrough

Despite signs of another economic upturn, persistent unemployment in the fifteen countries comprising the EEC remains at a level that affects eighteen million individuals and their families. Statistically, it is clear that funds now doled out as unemployment compensation could, if channeled into the creation of valid jobs in the private and public sectors, end unemployment in Western Europe. This article proposes the establishment of a European Employment Trust as an intersectoral instrument to accelerate job creation within the EEC. The Trust, managed by fiduciaries who have earned the confidence of labor unions, employers' associations, and governments, would invest in enterprises that have demonstrated the capability of creating growth and profitability while increasing the number of persons they employ. Young people without requisite skills or experience, or those who wish to devote a year to environmental enhancement or the rescue and relief of populations suffering from natural disasters would be accepted as volunteers in a Conservation and Rescue Service, as described in an earlier volume of this Journal (1996, Vol. 18, No. 4, pp. 419–442). The combined programs offer Europe a “win-win” strategy that would restore its cohesion, self-confidence, and prosperity. The cumulative impact of the European Employment Trust and a Conservation and Rescue Service would terminate the scourge of unemployment without further burdens on government or taxpayer budgets, would not require the creation of additional bureaucracies, and would provide new opportunities for private investment and co-investment.     

Large signal-to-noise-ratio enhancement of ultrashort pulsed optical signals using a power-symmetric Nonlinear Optical Loop Mirror with output polarisation selection

A numerical study of a fibre-based scheme for the regeneration of ultrashort-pulsed optical signals is presented. The setup is made of a power-symmetric Nonlinear Optical Loop Mirror (NOLM) followed by a polariser. The NOLM operates through nonlinear polarisation rotation, and includes twisted, anomalous-dispersion fibre and a quarter-wave retarder. When the orientations of the linear input polarisation and of the output polariser are properly adjusted, the output energy characteristic flattens at high power, a property that can be used to eliminate large amplitude fluctuations on the logical ones of an optical signal. When the input pulse parameters closely match those of fundamental solitons or of stable elliptically polarised solitary waves, a wide and flat plateau is obtained, allowing the reduction of ～30% amplitude fluctuations to less than 1%. Very large amplitude fluctuations beyond 50% can also be reduced down to a few %. Although the output pulses are slightly chirped, they are free of pedestal, thanks to the zero low-power transmission of the NOLM, which also allows the simultaneous regeneration of logical zeros. We believe that this setup will be useful for the regeneration of highly degraded signals in future ultrafast transmission networks.     

Polarization-maintaining fiber Bragg gratings for wavelengthselection in actively mode-locked Er-doped fiber lasers

Fiber Bragg gratings written in polarization-maintaining fiber are proposed for wavelength selection in actively mode-locked Er-doped fiber lasers. Combined with single-polarization optical circulator, they form unidirectional transmission filters that can be incorporated in polarization-maintaining laser cavities. We have fabricated such a grating in hydrogen-loaded PANDA fiber and we have incorporated it in a polarization-maintaining actively mode-locked Er-doped fiber laser designed to generate soliton-like pulses. Dependencies of pulse duration and spectral width on average intracavity power were measured. The power range over which soliton-like pulses were generated without pedestals was found to be ultimately limited by the grating's bandwidth