Zhiyu Yan

Tunable and switchable dual-wavelength Tm-doped mode-locked fiber laser by nonlinear polarization evolution.

We propose and demonstrate a tunable and switchable dual-wavelength ultra-fast Tm-doped fiber laser. The tunability is based on nonlinear polarization evolution (NPE) technique in a passively mode-locked laser cavity. The NPE effect induces wavelength-dependent loss in the cavity to effectively alleviate mode competition and enables the multiwavelength mode locking. The laser exhibits tunable dual-wavelength mode locking over a wide range from 1852 to 1886 nm. The system has compact structure and both the wavelength tuning and switching capabilities can be realized by controlling the polarization in the fiber ring cavity.

High-power thulium fiber laser Q switched with single-layer graphene

We report high-power 2 μm Tm3+ fiber lasers passively Q switched by double-piece single-layer graphene transferred onto a glass plate. Through manipulating intracavity laser beam size and increasing pump ratios, an average power of 5.2 W is directly achieved from the laser oscillator with an optical-to-optical slope efficiency of 26%. The laser pulse energy can be as high as ∼18  μJ, comparable to that from actively Q-switched fiber lasers. The narrowest pulse width is 320 ns, and the pulse repetition rate can be tuned from tens of kilohertz to 280 kHz by changing the pump power. To the best of our knowledge, this is the highest average power and pulse energy, as well as the narrowest pulse width, from graphene-based Q-switched 2 μm fiber lasers.

High-power graphene mode-locked Tm/Ho co-doped fiber laser with evanescent field interaction

Mid-infrared ultrafast fiber lasers are valuable for various applications, including chemical and biomedical sensing, material processing and military applications. Here, we report all-fiber high-power graphene mode-locked Tm/Ho co-doped fiber laser at long wavelength with evanescent field interaction. Ultrafast pulses up to 7.8 MHz are generated at a center wavelength of 1879.4 nm, with a pulse width of 4.7 ps. A graphene absorber integrated with a side-polished fiber can increase the damage threshold significantly. Harmonics mode-locking can be obtained till to the 21th harmonics at a pump power of above 500 mW. By using one stage amplifier in the anomalous dispersion regime, the laser can be amplified up to 450 mW and the narrowest pulse duration of 1.4 ps can be obtained simultaneously. Our work paves the way to graphene Tm/Ho co-doped mode-locked all-fiber master oscillator power amplifiers as potentially efficient and economic laser sources for high-power laser applications, such as special material processing and nonlinear optical studies.

Experimental observation of ferrielectricity in multiferroic DyMn2O5

One of the major breakthroughs associated with multiferroicity in recent years is the discovery of ferroelectricity generated by specific magnetic structures in some magnetic insulating oxides such as rare-earth manganites RMnO3 and RMn2O5. An unresolved issue is the small electric polarization. Relatively large electric polarization and strong magnetoelectric coupling have been found in those manganites of double magnetic ions: magnetic rare-earth R ion and Mn ion, due to the strong R-Mn (4f-3d) interactions. DyMn2O5 is a representative example. We unveil in this work the ferrielectric nature of DyMn2O5, in which the two ferroelectric sublattices with opposite electric polarizations constitute the ferrielectric state. One sublattice has its polarization generated by the symmetric exchange striction from the Mn-Mn interactions, while the polarization of the other sublattice is attributed to the symmetric exchange striction from the Dy-Mn interactions. We present detailed measurements on the electric polarization as a function of temperature, magnetic field, and measuring paths. The present experiments may be helpful for clarifying the puzzling issues on the multiferroicity in DyMn2O5 and other RMn2O5 multiferroics.

Broadband Saturable Absorption of Graphene Oxide Thin Film and Its Application in Pulsed Fiber Lasers

Graphene oxide (GO) has been used for optical intensity manipulation and short pulse shaping. Most interestingly, it can be used as a broadband light absorber. Such property has been systematically investigated in this paper, in a broad wavelength range from 1 to 2-μm. We fabricated one type of GO/polyvinyl alcohol (GO/PVA) film and use it as a saturable absorber (SA) in Er-, Yb-, and Tm-doped fiber ring lasers, respectively, to build pulsed fiber lasers. It is demonstrated that broad range of mode-locking in the 1-μm and 1.5-μm regions can be obtained. In addition, Q-switching around wavelength of 1870 nm can be obtained in a Tm-doped fiber ring lasers. To the best our knowledge, this is the broadest wavelength regime in which one type of GO SA film can be used to build all-fiber pulsed ring lasers.

Switchable multi-wavelength Tm-doped mode-locked fiber laser.

We propose and demonstrate for the first time a switchable tri-wavelength Tm-doped ultra-fast fiber laser based on nonlinear polarization evolution (NPE) technique. The NPE effect induces wavelength-dependent loss in the cavity that changes the homogeneous broadening of the effective gain to become inhomogeneous. This inhomogeneous effective gain spectral profile enables the multi-wavelength mode locking. Binary control of three bits can be realized by controlling the polarization in the compact fiber ring cavity. Such switchable laser has potential applications in optical signal processing and communication.

Widely tunable Tm-doped mode-locked all-fiber laser.

We demonstrated a widely tunable Tm-doped mode-locked all-fiber laser, with the widest tunable range of 136 nm, from 1842 to 1978 nm. Nonlinear polarization evolution (NPE) technique is employed to enable mode-locking and the wavelength-tunable operation. The widely tunable range attributes to the NPE-induced transmission modulation and bidirectional pumping mechanism. Such kind of tunable mode-locked laser can find various applications in optical communications, spectroscopy, time-resolved measurement, and among others.

Mid-IR supercontinuum pumped by femtosecond pulses from thulium doped all-fiber amplifier

We present a mid-infrared (mid-IR) supercontinuum (SC) light source pumped by femtosecond pulses from a thulium doped fiber amplifier (TDFA) at 2 μm. An octave-spanning spectrum from 1.1 to 3.7 μm with an average power of 253 mW has been obtained from a single mode ZBLAN fiber. Spectral flatness of 10 dB over a 1390 nm range has been obtained in the mid-IR region from 1940 - 3330 nm. It is resulted from the enhanced self phase modulation process in femtosecond regime. The all-fiber configuration makes such broadband coherent source a compact candidate for various applications.

Unusual ferromagnetism enhancement in ferromagnetically optimal manganite La0.7−yCa0.3+yMn1−yRuyO3 (0≤y<0.3): the role of Mn-Ru t2g super-exchange

The eg-orbital double-exchange mechanism as the core of physics of colossal magnetoresistance (CMR) manganites is well known, which usually covers up the role of super-exchange at the t2g-orbitals. The role of the double-exchange mechanism is maximized in La0.7Ca0.3MnO3, leading to the concurrent metal-insulator transition and ferromagnetic transition as well as CMR effect. In this work, by a set of synchronous Ru-substitution and Ca-substitution experiments on La0.7–yCa0.3+yMn1–yRuyO3, we demonstrate that the optimal ferromagnetism in La0.7Ca0.3MnO3 can be further enhanced. It is also found that the metal-insulator transition and magnetic transition can be separately modulated. By well-designed experimental schemes with which the Mn3+-Mn4+ double-exchange is damaged as weakly as possible, it is revealed that this ferromagnetism enhancement is attributed to the Mn-Ru t2g ferromagnetic super-exchange. The present work allows a platform on which the electro-transport and magnetism of rare-earth manganites can be controlled by means of the t2g-orbital physics of strongly correlated transition metal oxides.

50-W 2-μm Nanosecond All-Fiber-Based Thulium-Doped Fiber Amplifier

We report a two-stage 2-μm Tm3+ fiber amplifier seeded by an acousto-optic modulator (AOM) modulated narrow-bandwidth pulsed laser. Maximum output average power is over 50 W (slope efficiency of ~58%) at 50 kHz repetition rate, providing record performance in nanosecond 2-μm all-fiber-based amplifiers. The maximum pulse energy and peak power are ~1.0 mJ and 10 kW, respectively. The pulse width can be tuned from tens of to hundreds of nanosecond through changing the seeds pump power or the modulation repetition rate. This high-power 2-μm Tm3+ fiber amplifier has an all-fiber-based configuration, providing high-degree stability and robustness. The laser emission wavelength, centered at ~1951 nm with a spectral width of ~1.4 nm, shows no obvious change with amplified power.