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Nonlinear amplifying-loop mirror (NALM) and nonlinear optical loop mirror (NOLM) are two basic Sagnac interferometers widely used in mode-locked fiber lasers. Here we construct a variant by replacing the amplifier in an NALM with an absorber, which forms a type of nonlinear absorbing-loop mirror (NAbLM). The optical waves counter-propagating within an ideal NAbLM possess equal optical powers when they return and interfere inside the used fiber coupler owing to the equal absorptions. Meanwhile, their nonlinear phase shifts can still differ adequately if an asymmetric distribution of the absorber is employed. We theoretically predict that, in comparison to typical NALM and NOLM, the NAbLM can achieve relatively higher extinction ratio and larger modulation depth, resting on the unique equal-power interference. We then first used it as a saturable absorber in a holmium-doped fiber laser and achieved passively mode-locked operations in both normal and anomalous dispersion regimes. In the normal dispersion regime, h-shaped pulse was generated; whereas in the anomalous dispersion regime, both noise-like and dissipative-soliton-resonance-like pulses were achievable with polarization manipulation. NAbLM enables more options in nonlinear fiber optics especially when a high extinction ratio or large modulation depth is required. [ABSTRACT FROM AUTHOR]

Physical parameter sensing largely benefits the lifetime and operational costs of machines and has been widely used for machine fault detection. Herein, in this article, we developed a multinode sensor network, which is fully self-powered by harvesting mechanical vibration energy, to establish a machine fault detection system. A multilayered vibrational triboelectric nanogenerator (V-TENG) was designed to scavenge energy from working machines. Triggered by a vibration motion with the frequency of 8 Hz, the V-TENG can generate an output with power density of 3.33 mW/m3. With a power management module, the microcontrol unit integrated with sensors and a wireless transmitter can be continuously powered by the V-TENG to construct a self-powered vibration sensor node (SVSN). A supporting vector machine algorithm-based machine fault detection system was then established through a three-SVSN network by acquiring acceleration and temperature data from the working machine. Based on the system, different working conditions of the machine were recognized with an accuracy of 83.6%. The TENG-based SVSN for machine fault detection has demonstrated wide prospects in production monitoring, intelligent manufacturing, and smart factory. Moreover, the proposed self-powered sensor network has great potential and wide application in the era of distributed Internet of Things, artificial intelligence, and big data. [ABSTRACT FROM AUTHOR]

By incorporating a house-made microfiber knot resonator (MKR), mode-locking regime transformation in an erbium-doped fiber (EDF) laser was achieved. With typical nonlinear polarization rotation (NPR) as the mode-locker, ∼890 fs soliton pulses could be directly produced from the EDF laser. However, once the MKR was further incorporated in it, the emitted pulse duration fell into the nanosecond regime, and meanwhile the spectral bandwidth dramatically narrowed down around 0.05 nm. This resulted from that the longitudinal laser modes were greatly eliminated by the MKR. The MKR exhibits tightly-packed, extremely narrow transmission teeth, acting as a comb-like, sharp-rolloff bandpass filter. The mode elimination factor could reach ∼98.81%. Further manipulating the intra-cavity polarization state, the emission wavelength could be switchable from one of the transmission teeth of the MKR to another, due to the intrinsic polarization-sensitivity of the NPR. Our results demonstrated that MKR could be a useful device in fiber lasers to enable some artificially pre-defined spectral and temporal characteristics through pre-designing on the related parameters of the MKR. This could also provide a route to transform the temporal characteristics through tailoring the spectral profile, i.e., manipulating the permitted longitudinal laser modes, in fiber lasers. [ABSTRACT FROM AUTHOR]

We experimentally report the novel evolutions of dissipative-soliton-resonance (DSR) pulses with respect to the pump power in an all-normal-dispersion fiber laser based on a nonlinear optical loop mirror. By solely increasing the pump power, pulse breaking of the DSR pulses was observed apart from the typical pulse broadening at a fixed pulse peak power. Instead of arbitrarily broadening with the increase in the pump power or directly losing the mode-locking state, the process of pulse breaking was accompanied by the multipulse state and harmonic mode-locking pulses evolving from the original DSR pulse. In addition, to the best of our knowledge, a narrowing of the DSR pulse with the increase in the pump power was observed for the very first time. Further results show that these unusual evolutions of DSR pulses could be attributed to the changes in several laser parameters resulting from the increasing pump power under specific operating conditions. [ABSTRACT FROM AUTHOR]

We report on a type of peak-power-clamped passive Q-switching in a thulium/holmium co-doped fiber laser, for the first time to the best of our knowledge. Nonlinear polarization rotation was used as an artificial saturable absorber to initiate the Q-switched operation. The peak-power-clamping (PPC) effect was realized by incorporating a piece of high nonlinear fiber in the fiber resonator and launching high-enough pump power. Normal Q-switching behavior could be observed with the pump power within the range of 0.9–0.95 W, where the pulse peak power monotonically rose with the pump power increasing. However, once the pump power reached a threshold-like level, i.e., ∼1 W, the delivered Q-switched pulse peak power started to be clamped at a nearly constant value of ∼0.4 W, corresponding to an intracavity total peak power of ∼2.22 W. It indicates that, besides typical dissipative soliton resonances in mode-locked fiber lasers, PPC effect can also occur in the passively Q-switched regime with appropriate pump power and cavity nonlinearity managements. [ABSTRACT FROM AUTHOR]

We demonstrate what is believed to be the first dissipative-soliton-resonance mode-locking of a holmium-doped fiber (HDF) laser. A telecommunication band fiber source was used to pump a piece of thulium-doped fiber, resulting in 1.9- $\mu \text{m}$ band lasing, which was subsequently used to intra-cavity-pump another piece of HDF, resulting in lasing beyond $2~\mu \text{m}$. A nonlinear optical-loop mirror incorporating two pieces of high nonlinear fiber (HNLF) was employed as an artificial saturable absorber, and, meanwhile, to enhance the cavity nonlinearity. By varying the pump power from ~2.1 to ~4 W, the produced pulse could be tuned from ~4.85 to ~20.8 ns in duration while maintaining a nearly constant peak power, i.e., peak power clamping (PPC) effect. By further incorporating different lengths of HNLF, it was found that the longer the HNLF, the larger the pulse duration and the lower the pulse peak power, due to the expected stronger PPC effect. Meanwhile, spectral blue-shifting effect has occurred due to the increased fiber attenuation loss. Based on our preliminary experiments using the HDF laser as a test bed, a higher pulse energy at a longer wavelength could be expected with further enhanced pumping and gain-tailoring designs. [ABSTRACT FROM AUTHOR]

We report on repetition-rate-switchable and self-mode-locked pulses generation from a gain-switched thulium-doped fiber laser. Stable pulse trains were gain-switched by a home-made, pulsed Erbium/Ytterbium codoped fiber amplifier. The pulse repetition rate (PRR) of stable output pulse trains can be switched as 1/4, 1/3, and 1/2 times of that of the pump pulse train. Besides, the gain-switched mode-locked pulses were also achieved in the same cavity. By following three-stage thulium-doped fiber amplifiers, the average output power of the stable gain-switched pulses was scaled to 115 W with the PRR of 100 kHz, corresponding to a pulse energy of about 1.15 mJ. The amplification properties of the gain-switched mode-locked pulses were also characterized and discussed. [ABSTRACT FROM PUBLISHER]

We investigate errors in small-signal absorption spectra that result from re-emission in single-mode fibers with overlapping absorption and emission spectra. Experiments on Er-doped fibers and simulations of Er- and Yb-doped fibers show that the re-emission can severely distort the spectrum, especially the peak, under common measurement conditions, and underestimate the absorption by well over 10% already at 30-dB peak absorption, if only the source or the detector is spectrally filtered. Re-emission can then be the dominant source of errors. The error increases for higher absorption and higher fiber-NA. For sufficiently high NA, a significant error remains even in the limit of zero absorption and reaches 5% at the peak of a 0.46-NA Yb-doped fiber. Furthermore, in contrast to the high-absorption case, the error is larger at longer wavelengths than at the peak. Simultaneous filtering of both source and detector with 0.1-nm bandwidth reduces the re-emission error to ~1% or less up to 90-dB absorption. Then, detector noise or saturation errors are likely to dominate and render re-emission errors insignificant. A standard amplifier model is well suited to the simulations of the rich physics of single-mode-fiber absorption measurements. [ABSTRACT FROM AUTHOR]

We report a mid-infrared (MIR) spectral intensity-enhanced supercontinuum (SC) generation from a ZrF4-BaF 2-LaF3-AlF3-NaF fiber pumped by a nanosecond thulium-doped fiber laser. The spectral region covered from 1930 to 3500 nm with an average output power of 5.23 W. The average power ratio of the MIR SC for the wavelength > 2200 and > 2500 nm were 91.81% and 74.29% with respect to the total output power, respectively. The 1950-nm nanosecond seed pulses were generated from a long ring cavity based on semiconductor saturable absorber mode locker. The pulse repetition rate was 3.02 MHz with a tunable pulse duration of about 1.0 ns–3.9 ns. In addition, a 69.5-W short-wave infrared SC was obtained through a two-stage thulium-doped fiber amplifier with the spectrum extending from 1930 to 2500 nm. To the best of our knowledge, this is the first time such a spectral intensity-enhanced MIR SC directly pumped by a high-pulse-energy nanosecond thulium-doped fiber laser has been achieved. [ABSTRACT FROM PUBLISHER]

We study the sign inversion behavior of organic magnetoresistance (OMR) in organic thin-film devices, to elucidate the mechanisms governing the well-known OMR phenomenon. From the combination of a percolation theory with a magnetic field modulated bipolaron mechanism, we derive a model that accounts for OMR sign inversion (OMRSI) behavior. It exhibits how an applied magnetic field acts together with other factors (temperature, bias, and film thickness) on the device current. Under the framework of space-charge-limited current, we reproduce two kinds of OMRSI behavior. In the end, we discuss the influence of hyperfine field on OMRSI lines. [ABSTRACT FROM AUTHOR]