We present an experimental method for straight forward dual wavelength Erbium doped fiber linear cavity laser characterization based in laser line spectrum behavior due to the Hi-Bi FOLM transmission spectrum wavelength displacement by temperature variations in the fiber loop. The laser operation is for a single and dual mode, obtained through the adjustment of the cavity losses by the Sagnac interferometer spectrum wavelength displacement due to the temperature variation of the fiber loop. The method allow determine the laser operation from a single emission line and a two emission lines simultaneously through the Sagnac transmittance spectrum optical power variations measurement due to wavelength spectrum shifting for each laser wavelength generated separately and overlapping these obtained spectrums.
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.
Common optical fibers are randomly birefringent, and solitons traveling in them develop random polarization states upon propagation. 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 included the soliton self frequency shift. For our best knowledge this set of equation was analyzed for the first time. We performed a transformation of equations which reduces them to a form of perturbed Manakov task. The difference between our equations and the integrable Manakov case was considered as a perturbation. The perturbation method gives us an equations for evolution of the polarization state of pulse. The evaluation equation shows that in a circularly birefringent (twisted) 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 solved 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 a fiber twisted by 6 turns/m.
We report a tunable actively Q-switched fiber laser. A Fabry-Perot cavity is formed by a tunable fiber Bragg grating and two short-wave pass dichroic mirrors, (SWP-0-R1550-T1064-0525-C) and (SWP-45-RU1550- TU1064-PW-0525-C) incident angles, with high reflectivity (<99.5%) at 1550-nm and high transmission (<90%) at 1064-nm. The Er3+/Yb3+ double-clad fiber was pumped by a high power diode laser (JOLD-30-FC-12-976). The diode laser has output fiber with a core diameter and numerical aperture of 200-μm and 0.22, respectively. The Er3+/Yb3+ fiber has a core diameter of 7-μm and numerical aperture of 0.17. The pump was coupled into the doped fiber through the collimating aspherical lens with focal distance 18-mm, 45° dichroic mirror, focusing aspheric lens with focal distance of 8-mm, and a 7.5° cleave at one end of the doped fiber. The use of aspherical lenses allows reducing the spherical aberration caused by the large numerical aperture of the fibers and increasing the coupling efficiency for both pump and signal. An acousto-optic IR modulator with a diffraction efficiency of <60% at first order was inserted in the cavity. The pump power used in experiments was limited to a maximum power of 5-W. The laser was tunable over 5-nm in the wavelength region of 1550-nm with a wavelength selectivity of 0.8-nm from 1549 to 1544-nm. The Q-switch operation was achieved at pulse repetition rate of 120- kHz. Average output power was 1-W at 5-W of pump with a pulse duration of 530-ns.
We study experimentally the behavior of a dual wavelength Erbium doped fiber laser based on superimposed fiber Bragg gratings (SI-FBGs) and a loop mirror. The linear cavity is formed by a Hi-Bi Fiber Optical Loop Mirror (HBFOLM) and a SI-FBG. Three pairs of SI-FBGs were used to tune the dual wavelength laser at different wavelength ranges. The SIFBGs were placed on a mechanical mount that allow compression, both lines are shifted simultaneously. The separation of SI-FBGs is 5nm, and the set of three SI-FBGs permit to cover almost the entire spectrum of erbium from 1529nm to 1556nm approximately. The HBFOLM is used as a spectral tunable filter, which adjusts the losses by controlling the temperature in the Hi-Bi fiber, in order to stabilize the dual wavelength emission of the laser.
In this work we study experimentally a novel passively mode-locked erbium-doped figure-eight fiber laser based on a
polarization-imbalanced Nonlinear Optical Loop Mirror (NOLM). The NOLM operation strongly depends on the
polarization state at its input. In this experiment, the input polarization state is set to linear, and its orientation is
controlled through a half-wave retarder plate. The variation of the input polarization angle allows adjusting the NOLM
switching power over a wide range. In this work we show experimentally that this adjustment makes it possible to tune
the spectral bandwidth and the temporal properties of the generated pulses over a wide range.
Experimental studies of titania nanotubes (TiO2) and tungsten oxide nanospheres (WO3) as devices of saturable
absorption for a fiber laser in ring configuration to optical short-pulse generation are presented. A deposition technique,
based on optical pressure radiation generated from a coherent source at 1550 nm is used to deposit the nanostructured
materials. Since this nanomaterials can be deposited directly on the optical fiber, this proposal results very interesting for
applications of saturable absorbers. Experimental results, by using nanotubes TiO2 and nanospheres deposited on the
fiber as a saturable absorption device, show that the TiO2 nanotubes exhibit better saturable absorption properties than
WO3 nanospheres, generating pulses with a wavelength of 1550 nm, frequency of 10 MHz, temporally width of 4.5 ps
and an output power of 1 mW.
In this paper we show numerically how a Gires-Tournois etalons (DGTE) is used to flatten
the gain spectrum of an erbium-doped fiber amplifier. A broadband amplifier with uniform
gain over 40-nm with a residual gain <2-dB is presented.
We propose and study experimentally a novel passively mode-locked figure-eight fiber laser scheme based on a
polarization-imbalanced Nonlinear Optical Loop Mirror (NOLM). In contrast to conventional power-imbalanced
structures, the NOLM used in the proposed laser relies on a difference of nonlinear polarization rotation between the
counter-propagating beams to provide switching. In this experiment, the polarization state at the NOLM input is set to
linear. By controlling the polarization orientation at the NOLM input through a half-wave retarder plate, it is possible to
adjust the NOLM switching power. This property of the NOLM is attractive in the frame of a figure-eight laser. Firstly,
the switching power can be readily set to a value ensuring stable mode-locking operation, without having to cut into the
loop and modify the NOLM length. On the other hand, we observed that stable pulsed operation is maintained over a
certain range of the NOLM input polarization angle, whereas the pulse properties vary over that range. In particular, the
spectral width varies from 16 to 52 nm over that range. This spectral variation is associated with a variation of the pulse
temporal properties. This result can be explained by the fact that the input polarization angle allows controlling the
critical power of the NOLM, which in turn affects the pulses characteristics. The proposed device thus offers a
convenient way to adjust the pulses properties (in particular their spectral bandwidth and duration), simply by controlling
the angle of a wave retarder, a property which is attractive for some applications.
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