In this work a Q-switched mode-locked (QML) noise-like pulse (NLP) emission from a thulium-doped all-fiber laser with passive saturable absorption action through the nonlinear polarization rotation effect is demonstrated. QML-NLP pulses were obtained at 812 kHz repetition rate within a 11 μs QML envelope and pulse energy of 460 nJ. Central modelocked NLP sub-pulses were measured with 8.71 ns of width and 4.17 of peak power. The evolution of QML-NLP is experimentally investigated, and the results demonstrate the feasibility of an unconventional pulse operation regime of NLPs and provide insights into the dynamics of mode-locked fiber lasers.
Passively mode-locked fiber lasers are capable of generating a variety of optical pulses, depending of the pump power and cavity adjustments, which makes them attractive for research in multidisciplinary fields. In this paper, we analyze the experimental results of a fiber laser operating in passively mode-locked regime. The figure-eight laser cavity configuration uses 4 m of a co-doped Er/Yb double clad fiber, as a gain medium, pumped by a 976 nm diode laser. The cavity is completed by an isolator for ensure unidirectional propagation, two quarter-wave retarders (QWR), and a nonlinear optical loop mirror (NOLM) with 120 m of twisted standard single-mode fiber in the loop acting as saturable absorber. We used a couple of quarter-wave retarders (QWR) QWR1 at the NOLM input and QWR2 into the NOLM loop. By carefully adjusting of the QWRs the laser generates noise-like square pulses at the 1.54 μm range. With the increase of the pump power, the laser generates harmonics of the mode-locking fundamental mode with repetition frequency of 1.38 MHz, from which we can obtain the second, third and fourth harmonic in a controlled mode.
Simultaneous Q-switching and mode-locking (QML) pulse operation is demonstrated by using an in-fiber acousto-optic bandpass modulator (AOBM). The AOBM exhibits a high rejection efficiency of 28 dB for the non-resonant wavelengths, a broad modulation bandwidth of 0.9 nm, a modulation depth of 0.74 and easy implementation in an allfiber device. The effectiveness of the modulator is demonstrated in a rig cavity, mode locking is achieved by amplitude modulation of the transmitted light and matching the modulation period with the cavity round-trip time, simultaneously, the Q factor of the cavity is controlled by superimposing a rectangular wave modulation on the acoustic signal, with variable frequency and duty cycle, allowing a QML pulse operation. Experimental results demonstrate stable QML operation at 0.5 kHz repetition rate. A maximum energy of 1.4 μJ was achieved within a burst of 110 ps mode-locked sub-pulses contained in a 6.4 μs Q-switched envelope. These results demonstrate the potential of the AOBM to perform the double function of active mode locker and Q-switching device in an all-fiber laser.
We present a multi-wavelength ring fiber laser cavity based on modified loop fiber optic interferometer. Here, the interferometer is composed by three segments: two thin-core fiber (TCF) sections and one no-core fiber (NCF) section, these fibers are arranged as follows: TCF-NFC-TCF, then this structure is set into a fiber loop. Hence, by using a conventional ring laser cavity the proposed fiber optic loop is seed. Afterward, by controlling the modal fiber curvature and the ring cavity polarization state: single, dual and triple lasing modes can be achieved. The laser offers good power and wavelength stability, the maximal variations are 0.2 nm and 0.12 dB.
We report a linear cavity all-fiber passive Q-switched thulium-doped fiber laser operating at the 2 μm wavelength range. The laser configuration is based on a thulium-doped fiber used as a gain medium and an unpumped segment of holmium-doped fiber which acts as a fiber saturable absorber. The cavity is formed by a fiber optical loop mirror and the flat end facet of the holmium-doped fiber. The fiber segments as saturable absorber is a 1-m long single mode doubleclad holmium-doped fiber. Q-switched pulses are obtained at the wavelength of 2024.5 nm with a pulse width of 1.1 μs. The pulse repetition rate increases as a linear function of the applied pump power. The maximum pulse repetition rate of 100 kHz was obtained with a pump power of 2.4 W.
A polarization asymmetrical nonlinear optical loop mirror (NOLM) is investigated to perform pedestal-free optical pulses in a figure-eight laser (F8L). The results demonstrate that in the low-power regime the NOLM operates as a halfwave plate and the output polarization is orthogonal to the input one. However, at higher power level the polarization component parallel to the input appears, with a transmission that always begins from zero at low power, allowing the rejection of low-intensity components. Experimental results demonstrate that by employing this configuration we can obtain a contrast between the peak and continuous background higher that 40 dB.
The effect of a gradual reduction of the fiber diameter on the acousto-optic (AO) interaction is reported. The experimental and theoretical study of the intermodal coupling induced by a flexural acoustic wave in a biconical tapered fiber shows that it is possible to shape the transmission spectrum, for example, substantially broadening the bandwidth of the resonant couplings. The geometry of the taper transitions can be regarded as an extra degree of freedom to design the AO devices. Optical bandwidths above 45 nm are reported in a tapered fiber with a gradual reduction of the fiber down to 70 μm diameter. The effect of including long taper transition is also reported in a double-tapered structure. A flat attenuation response is reported with 3-dB stopband bandwidth of 34 nm.
The development of efficient in-fiber amplitude modulators that operate in the MHz frequency range has enabled some
recent advances in actively mode-locked all-fiber lasers. Our approach is based on the excitation of a standing acoustic
wave along the fiber, which modulates the coupling between modes at twice the frequency of the acoustic wave. Among
the remarkable features of these in-fiber modulators, we mention the high peak power damage, high modulation depth,
broad bandwidth, easy tunability in optical wavelength and low insertion losses. The in-fiber modulation is crucial for
the implementation of all-fiber compact and robust lasers. The experimental characterization of different cavity
configurations as a function of the radio frequency voltage that controls the modulator, the length of the active fiber, the
overall dispersion of the cavity, etc., has led to an improved operation of the lasers: output optical pulses of 34 ps
temporal width, 1.4 W peak power and 4.7 MHz repetition rate, at the emission wavelength of erbium.
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.
We show preliminary results of the implantation of a stabilized Michelson interferometer for the characterization of ultrasonic transducers. The detection system consists of a Michelson interferometer in which a feedback system has been integrated to set the interferometer at the linear region of detection and to compensate optical phase changes introduced by environmental vibrations; this allows us to work in the most sensible region of the system and obtain output signals proportional to the detected displacements (< 10 nm). The obtained results show that it is possible to detect ultrasonic displacements close to 0,2 nm in a bandwidth of 3 MHz using a He-Ne laser with an output power of 1 mW. Preliminary characterization of ultrasonic displacements generated by an ultrasonic transducer immersed in a water tank was performed using this interferometer.
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