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The pursuit of compact and integrated devices has stimulated a growing demand for multifunctional sensors with rapid and accurate responses to various physical parameters, either separately or simultaneously. Fluorescent fiber sensors have the advantages of robust stability, light weight, and compact geometry, enabling real-time and noninvasive signal detection by monitoring the fluorescence parameters. Despite substantial progress in fluorescence sensors, achieving multifunctional sensing in a single optical fiber remains challenging. To solve this problem, in this study, we present a bottom-up strategy to design and fabricate thermally drawn multifunctional fiber sensors by incorporating functional nanocrystals with temperature and pressure fluorescence responses into a transparent glass matrix. To generate the desired nanocrystal-in-glass composite (NGC) fiber, the fluorescent activators, incorporated nanocrystals, glassy core materials, and cladding matrix are rationally designed. Utilizing the fluorescence intensity ratio technique, a self-calibrated fiber sensor is demonstrated, with a bi-functional response to temperature and pressure. For temperature sensing, the NGC fiber exhibits temperature-dependent near-infrared emission at temperatures up to 573 K with a maximum absolute sensitivity of 0.019 K-1. A pressuredependent upconversion emission is also realized in the visible spectral region, with a linear slope of -0.065. The successful demonstration of multifunctional NGC fiber sensors provides an efficient pathway for new paradigms of multifunctional sensors as well as a versatile strategy for future hybrid fibers with novel combinations of magnetic, optical, and mechanical properties. [ABSTRACT FROM AUTHOR]

Due to large coherence length, high detection sensitivity and feasible design, a narrow-linewidth single-frequency laser in the blue spectral range is essential for underwater communication and high-resolution atomic/molecular spectroscopy. By exploiting the efficient frequency doubling of a 978 nm Yb-doped phosphate fiber laser in a fiber-coupled 10 mm long periodically poled LiNbO3, an all-fiber single-frequency blue laser at 489 nm with an ultra-compact and practical design is demonstrated. Over 68 mW of stable narrow-linewidth blue radiation is generated with a conversion efficiency of 30.3%. The estimated laser linewidth is less than 15 kHz, while the measured relative intensity noise is less than –139 dB Hz−1 at frequencies above 20 MHz. The obtained blue laser output is naturally stable with a power stability of ±1.5% over a period of 120 min. [ABSTRACT FROM AUTHOR]

An iterative method based on coupled mode theory and the steepest descent principle in a microwave waveguide is proposed in the optical waveguide for the design of a LP0m mode converter. After iterative design, a LP02-LP01 mode converter and a LP03-LP02 mode converter can achieve 99% conversion efficiency in our model. The 1dB bandwidth of a LP02-LP01 mode converter is measured as more than 130 nm. These optimum waveguides are simulated by RSoft software and indicate high conversion efficiency as well. Furthermore, we investigate the compatibility of the iterative method for mode converter design with initial waveguides of multiple taper shapes. The proposed method is promising for applications in mode division multiplexing systems. [ABSTRACT FROM AUTHOR]

The high order vector mode (HOVM) coupling mechanism is investigated based on the mode matching method (MMM). In the case of strong HOVM coupling where the weakly guiding approximation fails, conventional coupling analysis methods become invalid due to the asynchronous coupling feature of the horizontal and vertical polarization components of HOVM. The MMM, which uses the interference of the local eigenmodes instead of the assumptive modes to simulate the light propagation, is adopted as a more efficient analysis method for investigating HOVM coupling processes, especially for strong coupling situations. The rules of the optimal coupling length, coupling efficiency, and mode purity in microfiber directional coupler are firstly quantitatively analyzed and summarized. Different from the specific input modes, some special new modes would be excited at the output through the strong HOVM coupling process. The analysis of HOVM coupling mechanism based on MMM could provide precise and accurate design guidance for HOVM directional coupler and mode converter, which are believed to be fundamental devices for multi-mode communication applications. [ABSTRACT FROM AUTHOR]

An efficient low-noise, single-frequency 1033 nm master oscillator power amplifier (MOPA) Yb3+-doped phosphate fiber (YPF) laser system is demonstrated. A maximal output power of 612 mW with a laser linewidth of <1.2 kHz is obtained from a 3.7 cm long YPF. By suppressing the intensity noise of the short-cavity phosphate fiber oscillator with a semiconductor optical amplifier, a measured signal-to-noise ratio from the MOPA system of >65 dB and relative intensity noise (RIN) of <−120 dB Hz−1 at the relaxation oscillation frequency are achieved. Moreover, an RIN of <−152 dB Hz−1 is achieved for frequencies above 15.0 MHz, which approaches the shot noise limit. This laser system has applications in the fields of optical frequency standards and beam combining. [ABSTRACT FROM AUTHOR]

We present a compact passively Q-switched single-frequency fiber laser based on a 12-mm-long laboratory-built highly Er3+/Yb3+ codoped phosphate fiber (EYDPF) and a semiconductor saturable absorber mirror (SESAM). An effective cavity length of less than 20 mm ensures the stable single-frequency operation of the Q-switched fiber laser. By employing a SESAM for Q-switching, a single-pulse energy of more than 34.4 nJ is realized with the narrowest pulse duration of 95 ns, and the repetition rate of the Q-switched fiber laser reaches over 600 kHz. In addition, the optical signal-to-noise ratio of the output laser is as high as 68.0 dB. [ABSTRACT FROM AUTHOR]

We report an all-fiber high power nanosecond pulsed laser at a center wavelength of 1064 nm. Optimizing the coiling diameter of the active fiber, 188 W average power is achieved at a repetition rate of 40 kHz. The pulse width is measured as 101 ns, while the peak power can be estimated to 46.5 kW. [ABSTRACT FROM AUTHOR]

An all-fiber Yb-doped kHz-linewidth low-noise linearly polarized single-frequency master-oscillator power-amplifier (MOPA) laser with a stable CW output power of >52 W is demonstrated. By suppressing the intensity noise of the DBR phosphate fiber oscillator, the linewidth of MOPA laser is not noticeably broadened, and an ultra-narrow linewidth of <3 kHz is obtained. Furthermore, the low-noise behavior of MOPA lasers is investigated. A measured relative intensity noise of < −130 dB Hz−1 at frequencies of over 2 MHz, a phase noise above 1 kHz of <5 μrad/Hz1/2, and a signal-to-noise ratio of >63 dB are achieved. [ABSTRACT FROM AUTHOR]

Examines the effects of BO3/2 on lifetime of Er3+ in tellurite glass and the quenching mechanism of results from the BO3/2 addition based on nonradiative transition probability (NTP) and interactions between rare earth ions. Details of measured lifetime and quantum efficiency; Rise of concentration quenching and/or cooperative upconversion emission; Contribution of high NTP to the fluorescence quenching of Er3+ in the glasses.

A compact frequency-modulation Q-switched single-frequency fiber laser is demonstrated at 1083 nm. The short linear resonant cavity consists of a 12 mm long homemade Yb3+-doped phosphate fiber and a pair of fiber Bragg gratings (FBGs) in which the Q-switching and the frequency excursion is achieved by a tensile-induced period modulation. Over 375 MHz frequency-tuning range is achieved with a modulation frequency varying from tens to hundreds of kilohertz. The highest peak power of the output pulse reaching 6.93 W at the repetition rate of 10 kHz is obtained. [ABSTRACT FROM AUTHOR]

A compact several-hundred-Hz-linewidth single-frequency fiber laser based on the slow-light effect induced by a fiber-Bragg-grating Fabry–Perot (FBG-FP) filter is demonstrated. An FBG-FP filter is utilized for optical time delay, extending the efficient cavity length of the resonator. As a result, a slow-light short-linear-cavity fiber laser at 1550 nm with a linewidth less than 780 Hz is obtained. The relaxation oscillation frequency is around 234 kHz owing to the slow-light effect, and the relative intensity noise is less than −150 dB/Hz at 25 kHz. [ABSTRACT FROM AUTHOR]

We demonstrate an optoelectronic feedback method to simultaneously reduce the intensity and frequency noise of a single-frequency phosphate fiber laser for the first time. Based on the fact that the intensity and frequency noise in this laser mainly originate from the pump power fluctuations, we implement a simple and compact feedback loop and achieve a maximum noise reduction of over 20 dB for the intensity noise at frequencies of 0.3–3 kHz and 10 dB for the frequency noise at around 1 kHz. In addition, the laser instability is improved from ±1.7% to ±0.1%, and the laser line shape is observed to be compressed at the central part. [ABSTRACT FROM AUTHOR]

We demonstrate an experimental investigation on the linewidth of a single-frequency Yb-doped phosphate fiber laser through a loss-compensated recirculating delayed self-heterodyne (LC-RDSH) method at 1.0 μm. A resolution of 3 kHz is achieved by only using a 6 km fiber delay line in the linewidth measurement system. With optimal parameters of the LC-RDSH system, more than 16 orders of beat note are detected. By analyzing the behaviors of different orders of beat note, a broadening effect on the linewidth is observed with an increasing pump strength of the under-tested fiber laser. Associated with theoretical analysis, the frequency noise incorporated with the linewidth broadening is shown to be the attribution of the Lorentzian noise. Frequency noise of the laser is also measured, to verify the observed linewidth characteristics. To the best of our knowledge, this is the first time that the LC-RDSH has been applied to the 1.0 µm window. [ABSTRACT FROM AUTHOR]

High-efficiency single-frequency compact master-oscillator power amplifiers (MOPAs) based on core-pumped highly Yb-doped phosphate fibers (YPFs), several centimeters long, are investigated theoretically and experimentally. By using numerical modeling and testing different YPF lengths to optimize the amplifier configuration, a stable output of more than 1.06 W, at 1014 nm, was measured experimentally from a MOPA laser, with a 4.0 cm long YPF. An optical-to-optical conversion efficiency of 81.4%, a typical net gain of more than 20.5 dB, and an output power per unit length of up to 265 mW/cm were obtained with this laser system. The measured optical signal-to-noise ratio of the MOPA laser is higher than 62 dB and the estimated laser linewidth is less than 20 kHz, without obvious broadening or degradation after amplification. There is excellent agreement between the results of the simulations and the experiments. [ABSTRACT FROM AUTHOR]

In this work, we present a high-power, high-repetition-rate, all-fiber femtosecond laser system operating at 1.5 µm. This all-fiber laser system can deliver femtosecond pulses at a fundamental repetition rate of 10.6 GHz with an average output power of 106.4 W -- the highest average power reported so far from an all-fiber femtosecond laser at 1.5 µm, to the best of our knowledge. By utilizing the soliton-effect-based pulse compression effect with optimized pre-chirping dispersion, the amplified pulses are compressed to 239 fs in an all-fiber configuration. Empowered by such a high-power ultrafast fiber laser system, we further explore the nonlinear interaction among transverse modes LP01, LP11 and LP21 that are expected to potentially exist in fiber laser systems using large-mode-area fibers. The intermodal modulational instability is theoretically investigated and subsequently identified in our experiments. Such a high-power all-fiber ultrafast laser without bulky free-space optics is anticipated to be a promising laser source for applications that specifically require compact and robust operation. [ABSTRACT FROM AUTHOR]

Though widely applied in light-emitting diode (LED) lighting and X-ray detection, respectively, rare-earth ions doped phosphor and scintillating single crystals still face many challenges in practicalities, like aging of package material and difficulty to construct tiny array structure. Using phosphate glass as the host is quite beneficial for solving these problems while enabling distinguished luminescence. Therefore, Eu3+ and Tb3+ doped phosphate glasses which, respectively, show strong red/green emissions are researched. Superior thermal stabilities (78.69% and 89.35% emission intensities remain at 150 °C) and high internal quantum efficiencies (87.3% and 68.6%) are found to guarantee the excellent photoluminescence performances. The brilliant X-ray excited luminescence properties are demonstrated by general 19.49% and 34.74% integrated emission intensities of that of commercial Bi4Ge3O12 single crystal scintillator with the change of X-ray power. Meanwhile, Tb3+ doped scintillating fiber is drawn. Finally, excited by 380 nm LED, color manipulation and white light generation are achieved in LEDs with a structure of stacked emitting layers (mainly Eu3+ and Tb3+ doped phosphate glasses). Many of the as-constructed white LED’s performance parameters are much better than the commercial white LED. These results show that Eu3+ and Tb3+ doped phosphate glasses possess promising potential in LED lighting and X-ray detections. [ABSTRACT FROM AUTHOR]

For first-order topological semimetals, non-Hermitian perturbations can drive the Weyl nodes into Weyl exceptional rings having multiple topological structures and no Hermitian counterparts. Recently, it was discovered that higher-order Weyl semimetals, as a novel class of higher-order topological phases, can uniquely exhibit coexisting surface and hinge Fermi arcs. However, non-Hermitian higher-order topological semimetals have not yet been explored. Here, we identify a new type of topological semimetal, i.e., a higher-order topological semimetal with Weyl exceptional rings. In such a semimetal, these rings are characterized by both a spectral winding number and a Chern number. Moreover, the higher-order Weyl-exceptional-ring semimetal supports both surface and hinge Fermi-arc states, which are bounded by the projection of the Weyl exceptional rings onto the surface and hinge, respectively. Noticeably, the dissipative terms can cause the coupling of two exceptional rings with opposite topological charges, so as to induce topological phase transitions. Our studies open new avenues for exploring novel higher-order topological semimetals in non-Hermitian systems. [ABSTRACT FROM AUTHOR]

The article present a novel real-time frequency-encoded spatiotemporal (FEST) focusing technology. It mentions that FEST focusing uses a novel programmable two-dimensional optical frequency comb with an ultrafine line width to perform single-shot wavefront measurements, with a fast single-pixel detector; and a new avenue for high-speed wavefront shaping in the field of photonics.

Examines the effects of BO3/2 on lifetime of Er3+ in tellurite glass and the quenching mechanism of results from the BO3/2 addition based on nonradiative transition probability (NTP) and interactions between rare earth ions. Details of measured lifetime and quantum efficiency; Rise of concentration quenching and/or cooperative upconversion emission; Contribution of high NTP to the fluorescence quenching of Er3+ in the glasses.