We analyzed the two coupled propagation equations in a circularly birefringent fiber. We found that the difference between perpendicular and parallel Raman gain results in energy transform from a slow to fast circularly polarized component. We have shown this effect analytically and by a numerical simulation. We analyzed and measured the polarization of solitons generated by modulation instability and we found that polarization ellipticity of solitons is distributed randomly; however, the ellipticity of polarization is close to the polarization of the input pulse and always is shifted toward the circular polarization. Therefore, if a circularly polarized pulse is launched at the fiber input then the produced solitons are also circularly polarized. In the experiment, we used SMF-28 twisted fiber by 6 turns/m; the fiber twist mitigated the random linear birefringence, and the fiber was pumped by 1-ns pulse. We have found that circular polarization of the pump pulse in the twisted fiber produces circularly polarized solitons with a high grade of polarization while in the fiber without twist the soliton polarization was random.
Common optical fibers are randomly birefringent, and solitons formatting and traveling in them are randomly polarized. However it is desirable to have solitons with a well-defined polarization. We analyzed the two coupled propagation equations in a circularly birefringent fiber. Our equations include SPM, XPM and the soliton self frequency shift. In a difference of previously published works we consider the polarization dependence of the Raman amplification. We have found that the difference between perpendicular and parallel Raman gain results in energy transform from slow to fast circularly polarized component. We have shown this effect analytically and by a numerical simulation. For analytical consideration we performed a transformation of equations which reduces them to a form of perturbed Manakov task. The perturbation method gives us equations for evolution of the polarization state of pulse which show that in a circularly birefringent fiber the cross–polarization Raman term leads to unidirectional energy transfer from the slow circularly polarized component to the fast one. The magnitude of this effect is determined by the product of birefringence and amplitudes of both polarization components. Thus, solitons with any initial polarization state will eventually evolve stable circularly polarized solitons. We also made numerical analysis of two coupled nonlinear Shrödinger equations using a split-step Fourier method. The parameters of a standard fiber were used with delay between left- and right- circular polarizations of 1 ps/km that corresponds to circular birefringence in a fiber twisted by 6 turns/m. Numerical analysis confirms the analytical approximation. We analyzed also the polarization of solitons generated by modulation instability. We used a 30-ps, 40-W pulse with noise imposed on them at the fiber input. We found that polarization ellipticity of solitons is distributed randomly; however the average polarization ellipticity is closer to the circular than the polarization ellipticity of the input pulse. In experiment we used 220-m SMF-28 fiber twisted by 6 turns/m. The fiber was pumped by 1-ns pulse. We have found that at circular polarization of the input pulse solitons at the fiber output have polarizations close to the circular.
We investigate optical switching based on stimulated Raman scattering. The circuit consists of two fiber stages connected in series through a bandpass filter. When the pump and signal are launched to the input, the pump is saturated because of the signal amplification in the first stage; the amplified signal is rejected by the filter, so that only low-power pump enters the second stage; and no signal pulses appear at the output. The second stage is fed by 1 mW power at signal wavelength. When the pump only enters at the input, it passes through the first stage without saturation, enters the second stage, and amplifies the signal entering this stage; strong signal pulses appear at the output. We use 2-ns pump pulses at 1528 nm and cw signal at 1620 nm. In the first stage, we use both fibers with normal and anomalous dispersion. In fibers with anomalous dispersion, pump saturation is affected by modulation instability. We find that the contrast may be improved using fibers with normal and anomalous dispersion connected in series in the first stage, provided there is appropriate selection of their lengths. The best contrast we achieve is 15 dB at 6 W pump peak power.
We experimentally investigate optical switching based on stimulated Raman scattering in optical fibers. The
experimental setup consists of two fiber stages connected in series with a spectral filter rejecting a signal inserted
between them. When both pump and signal are launched to the input, the pump is saturated because of the signal
amplification in the first stage; the amplified signal is rejected by the filter, so that only the low-power pump enters the
second stage and no signal pulses appear at the output. When pump only enters at the input, it passes through the first
stage without saturation, enters the second stage and amplifies the signal entering this stage; strong signal pulses appear
at the output. We used 2-ns pulses at 1528 nm as the pump and a cw 1620-nm diode as the signal source for the first as
well as for the second stages. In fibers with anomalous dispersion pump saturation was affected by modulation
instability. We found that the contrast (the ratio of energies) may be improved using fibers with normal and anomalous
dispersion connected in series in the first stage provided that the ratio between the lengths of the fibers with normal and
anomalous dispersion is appropriately selected. The best achieved contrast was 15 dB at peak pump power of 6 W.
We investigate optical switching based on stimulated Raman scattering. The circuit consists of two fiber stages
connected in series with a spectral filter rejecting a signal inserted between them. When both pump and signal are
launched to the input, the pump is saturated because of the signal amplification in the first stage; the amplified signal is
rejected by the filter, so that only the low-power pump enters the second stage and no signal pulses appear at the output.
Second stage is fed by 1-mW power at signal wavelength. When pump only enters at the input, it passes through the first
stage without saturation, enters the second stage and amplifies the signal entering this stage; strong signal pulses appear
at the output. The on-off contrast is deteriorated by the pulse shape because the pump saturation is observed in the
central part of pulses, by fiber GVD, etc. These effects were not considered before. We used 2-ns pulses at 1528 nm as
the pump and a 1620-nm cw as the signal. We used in the first stage both fibers with normal and anomalous dispersion.
In fibers with anomalous dispersion pump saturation was affected by modulation instability. We found that the contrast
may be improved using fibers with normal and anomalous dispersion connected in series in the first stage provided that
the ratio between the lengths of the fibers with normal and anomalous dispersion is appropriately selected. The best
achieved contrast was 15 dB at 6-W pump peak power.
Pulses propagating in the fiber with anomalous dispersion are broken up to the bunch of soliton. The
extraction of an individual soliton from the bunch can be used for soliton generation and also for investigation
of the process of the soliton formation. In this work we experimentally demonstrate that the NOLM allows
extraction of an individual soliton. Earlier we have shown numerically that the NOLM has high transmission
for the solitons with a range of durations while solitons with shorter and longer durations are rejected. The
range of the durations with high transmission depends on the NOLM length and also can be moved by
amplification of solitons before entering to the NOLM. In the experiment we launched 25-ps pulses with
about 10 W of power to the 500-m single mode fiber with dispersion equal to 20 ps/nm-km. As a result of the
pulse breakup, a bunch of solitons is formed at the fiber output. The resulting solitons are launched to the
EDFA and then to the NOLM made from the 40-m of the same fiber. The NOLM parameters are adjusted to
transmit the highest soliton in the bunch (about 50 W of power and 1 ps of duration according to theoretical
estimations). In the experiment we detected at the NOLM output a single pulse with duration of 1.46 ps and
autocorrelation function similar to that of the soliton. When a 1-km fiber was attached to the NOLM at the
fiber output we detected a soliton with duration of 0.9 ps.
We report numerically and experimentally analysis of optic fiber Sagnac interferometer and fine adjustment of cavity
loss by the use of the FOLM with a hi-bi fiber in the loop. Changes in transmittance profile amplitude and wavelength
shift are caused by the whirl effect in the connectors of a coupler with ports output connected to a birefringent fiber.
Also the experimental demonstration of dual wavelength operation of a fiber laser through fine adjustment of cavity loss,
using a Fiber Optical Loop Mirror (FOLM) with a high-birefringence fiber in the loop. The reflection and transmission
of the FOLM presents a sinusoidal wavelength dependence which can be shifted by controlling the temperature of the hibi
fiber. A temperature change of the hi-bi fiber by 0.1°C causes a measurable change in the ratio between the
reflectance for the wavelengths R(λ1)/R(λ2). Using this adjustment be able to change the generation mode from single
wavelength to stable dual wavelength generation with equal powers for λ1 and λ2 or to stable dual wavelength generation
with unequal powers at λ1 and λ2. The change of the ratio between the FOLM reflection R(λ1)/R(λ2) allows the
investigation of tolerance of dual wavelength generation on the ratio between cavity loss. Was found that for the switch
from a single wavelength emission at λ1 to single wavelength emission at λ2 the ratio R(λ1)/R(λ2) has to be changed by
the order of magnitude of 10-2. This value shows the tolerance of the dual wavelength laser to the cavity loss adjustment.
We investigated numerically the transmission of a set of solitons through a nonlinear optical loop mirror (NOLM) and
found that under some conditions a single soliton can be extracted. We analyzed the evolution of relatively long (20-50
ps) and no very strong (10 W) pulses. The results show that the input pulse duration and the amplification of the solitons
resulting from pulse breakup play important role to extract the highest quality pulse. The transmitted pulses correspond
to fundamental solitons with subpicosecond durations. We believe that the proposed method can be useful for the study
of nonlinear phenomena in optical fibers.
We report the two-stage single-pump configuration of an erbium-doped fiber amplifier, in which a Sagnac interferometer is introduced to reduce the most important contribution of amplified spontaneous emission (ASE) noise, providing significant improvement on the amplifier performance. A Sagnac interferometer, made from a high-birefringence fiber loop, is included between the first and second stages. It is designed to provide transmittance with a period of 46 nm that allows us to adjust the minimum transmission around 1530 nm (peak of ASE noise) and maximum transmission at 1550 nm (signal wavelength). For optimizing the configuration, we measure the erbium-doped fiber parameters and simulate the amplification of the signal along the fiber. In the experiment, a significant absorption coefficient for pump and signal is found. The absorption looks to be too strong for the background absorption, and we suppose that it may be caused at least partly by excited-state absorption (ESA). Including the absorption coefficient allows very good correspondence between simulation and experiment. Experimental results show that with a simple configuration, we obtain up to 53-dB amplification with only 73 mW of pump power.
In this paper, we theoretically and experimentally investigated a two-stage erbium-doped fiber amplifier (EDFA) with a
single-pump laser diode pumped at 980 nm in which a Sagnac interferometer filter is introduced to reduce amplified
spontaneous emission (ASE) providing significant improvement on the amplifier performance. The erbium-doped fiber
(EDF) parameters were measured in order to optimize parameters such as pump power, EDF length, ASE noise and
signal gain. The Sagnac interferometer filter was designed to provide a periodic transmittance of 46 nm that allows by
temperature to adjust the minimum transmission at 1530 nm (peak of the ASE noise) and maximum transmission at 1550
nm (signal wavelength). Experimental results show that with a simple configuration we obtained up to 53-dB
amplification with only 75 mW of pump power, which can be enhanced easily by 3 dB providing total amplification up
to more than 55 dB.
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