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The high repetition rate, widely tunable, picosecond mid-infrared laser plays an important role in various fields. In this paper, the single-pass optical parametric generator (OPG) pumped by Yb-fiber laser based on MgO: PPLN is proposed. The Yb-fiber laser is mode locked by a nonlinear amplifying loop mirror (NALM) with an all-polarization-maintaining (PM) structure. A maximum power of 6.2 W was obtained with a repetition rate of 15.8 MHz and pulse duration of 18 ps after amplification by cascaded fiber. Based on the principle of nonlinear frequency down-conversion, the tunable mid-infrared output wavelength of 2.76–3.98 μm was realized by varying the crystal grating periods and temperature. The maximum power was 500 mW at 3.22 μm, corresponding to a quantum conversion efficiency of 25%, and the relative power fluctuation was measured as 0.37% over 30 min.

Despite extensive applications ranging from optoelectronic devices to flexible devices, two-dimensional layered materials used as ultrafast optical modulation materials have suffered from limited operation wavebands due to their intrinsic bandgap. Here, we investigate the nonlinear optical (NLO) response of group IIIA/IVA monochalcogenides XTe (X = Ga and Ge), which exhibit a remarkable saturable absorption response at a broadband wavelength. By using a liquid phase exfoliation technique, we prepare XTe nanosheets with few-layer thickness in a deionized water solvent and investigate their NLO responses at 800 nm, 1.5 µm, and 2 µm, respectively. Both GaTe and GeTe nanosheets exhibit ultrafast saturable absorption at 800 nm with 32.4 μJ (48.4 µJ) incident intensity. After preparing microfiber-based nanosheet optical modulators, their saturable absorptions are measured at 1.5 µm and 2 µm. The GaTe-SA possesses a little bit higher modulation depth (42.3% at 1.5 µm/30.7% at 2 µm) than the GeTe-SA (30.5% at 1.5 µm/8.3% at 2 µm) at two wavelengths. The lifetime of GaTe and GeTe nanosheets is measured to be 2.8 ps and 3.3 ps, respectively. In addition, we further examine their saturable absorption via incorporating them into an erbium- and a thulium-doped fiber laser system and obtain stably passively mode-locked pulse trains, respectively. The distinct NLO properties indicate a large potential of XTe nanosheets in the development of an ultrafast optical mode locker with a wideband operation wavelength.

We demonstrate a simple Fe3O4 nanoparticles (FONPs) based Q-switched fiber laser that is able to generate stable nanosecond pulses with single-wavelength or multiwavelength regimes. In the single-wavelength lasing scheme, the fiber laser generates 613-ns pulse with average output power of 41.2 mW, pulse energy of 321.3 nJ, and signal-noise-ratio of 54.4 dB. In the multiwavelength lasing scheme, stable pulse is achieved with 18-wavelength channels contained within the 3-dB bandwidth range. Our results prove that the FONPs can act as an effective Q-switch for high-energy pulse generation.

A position determination method for low-coherence interference fringe in optical fiber sensing is proposed. This method can be used to measure displacement or length. An amplitude ratio curve is created from spatially modulated interference fringes. There is a one-to-one correspondence relationship between the peaks of the amplitude ratio curve and the extremes of the low-coherence interference fringe. The peak positions of the amplitude ratio curve only rely on the Fabry-Pérot (F-P) cavity length. In addition, the peaks of the amplitude ratio curve are very sharp, which can be easily identified. In the calculation of the amplitude ratio curve, the low-coherence interference fringe is spatially modulated by Pockels effects. The impacts of the modulated voltage on the sensing precision are analytically and numerically discussed. Compared with the envelop detection method, the position determination precision of low-coherence interference fringe is enhanced about 30 times by creating an amplitude ratio curve. This method can be widely employed in the sensing of all kinds of parameters that can be converted to displacement or length.

We demonstrated a fiber-optic sensor using spheroidal-cavity-overlapped fiber Bragg grating (FBG) for simultaneous temperature and strain sensing. The spheroidal cavity is fabricated through splicing two sections of the photon-sensitive fiber together and located in the center of the FBG to act as a Fabry-Pérot interferometer (FPI). Experimental results demonstrate that the temperature sensitivity of the FPI part of the sensor is 0.67 pm/°C at the range of -70 °C-20 °C, and its strain sensitivity is 3.76 pm/με at the range of 0-500 με, while those of the FBG part are 8.40 pm/°C and 1.40 pm/με, respectively. The sensitivity difference ensures good simultaneous measurement performance.

We establish an optical path difference (OPD) distribution model in a spatial scanned polarized low-coherence interferometry (LCI) considering the nonperpendicular incidence of light to explore the spatial frequency drift. Simulation shows that the OPD is no longer linearly distributed on a charged coupled device (CCD) and that the spatial frequency drifts approximately linearly as low coherent interference fringe (LCIF) shifts. A compensation process for spatial frequency domain analysis (SFDA)-based algorithm is proposed to avoid interference-order misidentification and consequent jump error. We verified our analysis and effectiveness of the proposed algorithm with an optical fiber Fabry-Pérot pressure sensing experiment. The measured spatial frequency drift agreed well with simulation results, and the interference-order misidentification was eliminated.

The development of simple and sensitive sensors with the capability of simultaneously detecting magnetic field intensity and its direction in three-dimensional (3D) space is a technical imperative for magnetic field detection. In this paper, we first demonstrated the mechanism of anisotropic manipulation of ferromagnetism nanoparticles (FMNPs) in a non-continuous magnetic fluid film with the aid of varying magnetic fields. Second, based on the anisotropic distribution of the FMNPs around an optical fiber, we fabricated a vector magnetometer with a direction error of ±1.9° and an intensity sensitivity of 222.0 pm/mT, respectively. The sensing mechanism relies on the magnetism-controllable effective refractive index modification of asymmetric cladding modes in an in-line fiber-optic Mach-Zehnder interferometer. Compared to the previously reported intensitybased magnetometers, the as-fabricated magnetometer also provides an avenue to monitor the vector direction of the magnetic field in 3D space. [ABSTRACT FROM AUTHOR]

We demonstrated a simple and batch-producible fiber-optic sensor based on hybrid Fabry-Pérot (FP) configuration for simultaneous pressure and temperature sensing. The proposed sensor head chips are batch fabricated by double-sided anodic bonding of a through-holes-array-structured glass wafer and two silicon wafers. The silicon-glass-silicon sandwich bonding structure constructs two serially connected low-finesse FP cavities naturally. The first cavity (FP1) is a silicon cavity and utilizes the refractive indices temperature dependence of silicon to achieve temperature sensing. The second cavity (FP2) is a vacuum cavity and employs a thin silicon diaphragm as a pressure sensing element. The reflection spectra exhibit hybrid interference fringes with different frequencies produced by silicon and vacuum cavity, and the temperature and pressure are simultaneously measured. Experiment results demonstrate that the pressure sensitivity of FP2 is 12.82 nm/kPa with a high linear pressure response over the range of 10-250 kPa, and the temperature sensitivity of FP1 is 142.02 nm/°C under the range of -20 °C-70 °C. [ABSTRACT FROM PUBLISHER]

We propose a direct and nondestructive self-referenced residual pressure measurement method for MEMS chip of fiber-optic pressure sensor. In the proposed method, two multimode fibers are placed at the center and the edge position of the sealed cavity. With scanning external pressure, two groups of cavity lengths data are obtained by using polarization low-coherence interference demodulator. The intersection point of their linear fitting curves indicates that the diaphragm is in flat condition and residual pressure is determined by the external pressure at this point. We successfully demonstrated the effectiveness of the residual pressure measurement method with measure error 0.049 kPa. [ABSTRACT FROM PUBLISHER]

A simple and effective method for polarized low-coherence interference is proposed in this letter. Despite the need to extract the coherence envelope, it sets an appropriate threshold to retrieve valid peaks or troughs from various fluctuations. Subsequently, we can utilize any valid peak or trough for demodulation. Experiments were carried out to demonstrate the validity and exceptionally good performance of this method. Experimental results have shown that the proposed method can achieve higher accuracy and stability when compared with the envelope peak detection method. [ABSTRACT FROM PUBLISHER]

We demonstrate an Er-doped fiber laser (EDFL) mode-locked by a MoS2 saturable absorber (SA), delivering a 256 fs, 2 nJ soliton pulse at 1563.4 nm. The nonlinear property of the SA prepared by magnetron sputtering deposition (MSD) is measured with a modulation depth (MD) of ∼19.48% and a saturable intensity of 4.14 MW/cm2. To the best of our knowledge, the generated soliton pulse has the highest pulse energy of 2 nJ among the reported mode-locked EDFLs based on transition metal dichalcogenides (TMDs). Our results indicate that MSD-grown SAs could offer an exciting platform for high pulse energy and ultrashort pulse generation. [ABSTRACT FROM AUTHOR]

Ultrafast pulse generation was demonstrated in thulium doped fiber laser mode locked by magnetron sputtering deposited Sb2Te3 with the modulation depth, non-saturable loss, and saturable intensity of 38%, 31.2%, and 3.3 MW/cm2, respectively. Stable soliton pulses emitting at 1930.07 nm were obtained with pulse duration of 1.24 ps, a 3-dB spectral bandwidth of 3.87 nm, an average output power of 130 mW, and signal-to-noise ratio (SNR) of 84 dB. To our knowledge, this is the first demonstration of Sb2Te3-based SA in fiber lasers at 2-μm regime. [ABSTRACT FROM AUTHOR]

Transition metal dichalcogenides (TMDs) have been successfully used as broadband optical modulator materials for pulsed fiber laser systems. However, the nonlinear optical absorptions of exfoliated TMDs are strongly limited by their nanoflakes morphology with uncontrollable lateral size and thickness. In this work, we provide an effective method to fully explore the nonlinear optical properties of MoSe2. Large-area and high quality lattice MoSe2 grown by chemical vapor deposition method was adopted as an optical modulator for the first time. The large-area MoSe2 shows excellent nonlinear optical absorption with a large modulation depth of 21.7% and small saturable intensity of 9.4 MW cm−2. After incorporating the MoSe2 optical modulator into fiber laser cavity as a saturable absorber, a highly stable Q-switching operation with single pulse energy of 224 nJ is achieved. The large-area MoSe2 possessing superior nonlinear optical properties compared to exfoliated nanoflakes affords possibility for the larger-area two-dimensional materials family as high performance optical devices. [ABSTRACT FROM AUTHOR]

We demonstrate an Er-doped fiber laser (EDFL) mode-locked by a MoS2 saturable absorber (SA), delivering a 256 fs, 2 nJ soliton pulse at 1563.4 nm. The nonlinear property of the SA prepared by magnetron sputtering deposition (MSD) is measured with a modulation depth (MD) of ∼19.48% and a saturable intensity of 4.14 MW/cm2. To the best of our knowledge, the generated soliton pulse has the highest pulse energy of 2 nJ among the reported mode-locked EDFLs based on transition metal dichalcogenides (TMDs). Our results indicate that MSD-grown SAs could offer an exciting platform for high pulse energy and ultrashort pulse generation. [ABSTRACT FROM AUTHOR]