Autor: EVGENY KUZIN
EVGENY KUZIN (2008)
In this work we present a novel design of an optical amplifier with high gain elaborated from erbium-doped fiber (EDF). Its purpose is the amplification of low power pulses from laser diodes. The high power of the output pulses allows the investigation of nonlinear processes in optical fibers. The optical amplifier consists of two stages. The first stage uses a reflective configuration where a signal is amplified twice in the same EDF. The fiber Bragg grating (FBG) was the main element to reflect the signal. The second stage works as a high power amplifier. The input pulse has duration in the range of 1 to 50 ns, with wavelength equal to 1549.1 nm. With input pulse power of 1.5 mW, the amplifier provides 70 W in the output pulse. The gain obtained by the amplifier was 47 dB.
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, anomalousdispersion 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.
We propose in this work a technique for short pulse profile retrieval based on the Kerr effect in a fiber nonlinear optical loop mirror (NOLM). Under some assumptions, the profile can be determined from the energy transfer characteristic of the pulses through the NOLM, which can be measured with a low-frequency detection setup. Two numerical approaches are considered, both relying on the resolution of a system of nonlinear algebraic
equations, and which differ in the way that the profiles are discretized. We show numerically that both approaches allow proper profile retrieval for a wide variety of pulse shapes. The two techniques are compared, and their advantages and drawbacks are discussed. The effect of the amplitude noise of the pulses is assessed, as well as the impact of an inaccurate knowledge of the NOLM transfer function, or of the energy transfer characteristic. The technique is demonstrated in the frame of the characterization of both ns and ps pulses.
We demonstrate theoretically and experimentally that efficient signal shaping operation can be obtained at moderate power by using the transmission characteristic of a power-symmetric nonlinear optical loop mirror (NOLM) including highly twisted fibre and operating through nonlinear polarisation rotation, when the circular polarisation state orthogonal to the input polarisation is selected at the NOLM output. By adjusting the angle of the quarter-wave retarder inserted in the loop, the phase bias of the transfer characteristic
can be adjusted precisely to enable proper signal shaping for moderate values of input power, remaining well below switching power. The tolerance of the procedure to deviations of the input polarisation from the ideal circular case is investigated numerically. We demonstrate experimentally the capabilities of this setup for both power equalisation and extinction ratio enhancement. Finally, we show that this setup is also useful to shape ultrashort optical pulses from the relaxation oscillations of a DFB semiconductor laser. In comparison with other NOLM-based techniques, the proposed approach allows to reduce by a factor of 8–10 the peak power required for pulse shaping, for the same fibre length and Kerr coefficient.
In this paper we show numerically that high-energy pulses can be obtained with a figure-eight Erbium-doped fiber laser with large normal net dispersion, and in which an anomalous-dispersion Nonlinear Optical Loop Mirror (NOLM) is used as the effective saturable absorber. One advantage of this configuration over the ring cavity is the possibility to adjust the length of the NOLM loop to avoid overdriving the saturable absorber. The ring section of the laser includes a bandpass filter to balance the combined effects of Kerr nonlinearity and normal dispersion. Strict polarization control is performed in the NOLM as well as in the ring section of the laser. The NOLM is a power-symmetric scheme whose switching relies on nonlinear polarization rotation. This architecture allows a precise control of the low-power NOLM transmission through the orientation of a quarter-wave retarder, whose adjustment is shown to be critical for stable pulsed operation. Pulse formation appears to depend critically on the filter width. If it is wide enough, ps pulses with a large positive linear chirp are produced. After dechirping outside the laser, nearly transform-limited pulses with durations down to 240 fs, energies up to 10 nJ and peak powers beyond 40 kW are predicted.
We propose a polarisation-maintaining NOLM switch design to be used as optical regenerator or wavelength converter in dense optical time-division multiplexed (OTDM) systems. The Sagnac loop is made of a piece of high birefringence fibre which is cut and crossspliced in the middle. If pump and probe polarisations are linear and aligned in the co-propagating direction, the cross-splice ensures that the counter-propagating probe beam will be orthogonal to the pump, so that the parasitic cross-phase modulation between counter-propagating beams is minimised. This architecture also allows easy control of the optical phase bias, through squeezing a short section of the fibre, without any other modification of the setup. The performances of the proposed architecture are studied analytically and numerically, and compared with those of conventional schemes. It appears that, although the proposed setup reduces the interaction between counter-propagating beams only by a factor 3, it yields an extinction ratio improvement of a factor 10 or higher in comparison with conventional schemes. If there is substantial walkoff between pump and probe, a 10-fold reduction of the relative intensity noise of the emerging signal is also obtained when the mark ratio of the incoming data is variable.
We propose and study numerically a fibre-based scheme designed to generate a step-like optical decisión 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.
We analyse numerically the capabilities of a power-symmetric nonlinear optical loop mirror (NOLM) in the ultrashort pulse regime for high-quality amplitude regeneration of an optical signal. The device, which operates through nonlinear polarisation rotation, includes twisted, anomalous-dispersion fibre and a quarter-wave retarder. For particular adjustments of the retarder orientation, and a circularly polarised input beam, the output energy characteristic flattens near the switching energy, a property that can be used to eliminate large amplitude fluctuations in an optical signal. The group velocity mismatch between polarisation components introduced by twist is mitigated by the interplay between anomalous dispersion and the nonlinear Kerr effect, although strong twist should be avoided as it still introduces substantial pulse distortion. Contrary to other designs, where a plateau characteristic requires a large power imbalance between the counter-propagating beams, both pulses in the present scheme can be simultaneously close to fundamental solitons, which allows a substantial widening of the plateau for particular pulse parameters. Good quality, nearly transform-limited pulses are obtained in this case at the NOLM output. The device is applicable for the regeneration of ultrafast data streams in which the signal-to-noise ratio is severely deteriorated.
Four procedures for simultaneous high-quality amplitude jitter reduction and extinction ratio enhancement of optical data streams are presented and studied using numerical simulations. They all rely on the use of a power-balanced NOLM, optionally followed by a polarizer. The setup can be operated in various regimes, leading to several switching characteristics with different merits in the frame of the proposed application. These are discussed and compared with the results obtained using other NOLM configurations.
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.