arXiv subjects

Pu Zhou

Publications and source records attributed to Pu Zhou.

31 records · Page 2Linked to original sources

arXiv2017

Spectral adjustment of a powerful random fiber laser

Random fiber laser (RFL) based on random distributed feedback and Raman gain has earned much attention in recent years. In this presentation, we demonstrate a powerful linearly polarized RFL with spectral adjustment, in which the central wavelength and the linewidth of the spectrum can be tuned independently through a bandwidth-adjustable tunable optical filter (BA-TOF). As a result, the central wavelength can be continuously tuned from 1095 to 1115 nm, while the full width at half-maximum (FWHM) linewidth has a maximal tuning range from ~0.6 to more than 2 nm. This laser provides a flexible tool for scenes where the temporal coherence property accounts, such as coherent sensing/communication and nonlinear frequency conversion. To the best of our knowledge, this is the first demonstration of a high power linearly polarized RFL with both wavelength and linewidth tunability.

physics.optics↗

arXiv2017

High power orbital-angular-momentum beam generation system based on coherent beam array combination technique

High power orbital-angular-momentum (OAM) beam has dominant advantages in capacity increasing and data receiving for free-space optical communication systems at long distance. Utilizing coherent combination of beam array technique and helical phase approximation by piston-phase-array, we have proposed a generation system of high power novel beam carrying OAM, which could overcome power limitation of common vortex phase modulator and single beam. We have experimentally implemented a high power OAM beam by coherent beam combination (CBC) of a six-element hexagonal fiber amplifier array. We show that the CBC technique utilized to control the piston phase differences among the array beams has a high accuracy with residual phase errors superior to λ/30. On the premise of CBC, we have obtained novel vortex beams carrying OAM of +/- 1 by applying an additional piston-phase-array modulation on the corresponding beam array. The experimental results approximately coincide with the theoretical analysis. This work could be beneficial to the areas that need high power OAM beams, like ultra-distance free-space optical communications, biomedical treatments, powerful trapping and manipulation under deep potential well.

physics.optics↗

arXiv2017

Self-started stable pulsing operation of random fiber laser

Unlike traditional fiber laser with defined resonant cavity, random fiber laser (RFL), whose operation is based on distributed gain and feedback via Rayleigh scattering and stimulated Raman scattering in long passive fiber, has fundamental scientific challenges in pulsing operation for its remarkable cavity-free feature. Here, we propose and experimentally realize the passively spatiotemporal gain modulation induced self-started stable pulsing operation of counter-pumped RFL. Thanks to the good temporal stability of employed pumping amplified spontaneous emission source and the superiority of this pulse generation scheme, stable and regular pulse train can be obtained. Furthermore, the pump hysteresis and bistability phenomena with the generation of high order Stokes light is presented and the dynamics of pulsing operation is discussed. This work extends our comprehension of temporal property of RFL and provides an effective novel avenue for the exploration of pulsed RFL with structural simplicity, low cost and stable output.

physics.optics↗

arXiv2017

High power highly stable passively Q-switched fiber laser based on monolayer graphene

We demonstrate a monolayer graphene based passively Q-switched fiber laser with three-stage amplifiers that can deliver over 80 W average power at 1064 nm. The highest average power achieved is 84.1 W, with pulse energy of 1.67 mJ. To the best of our knowledge, this is the first time for a high power passively Q-switched fiber laser in the 1 um range reported so far. More importantly, the Q-switched fiber laser operates stably during a week few-hours-per-a-day tests, which proves the stability and practical application value of graphene in high power pulsed fiber lasers.

physics.optics↗

arXiv2017

Incoherently pumped high-power linearly-polarized single-mode random fiber laser: experimental investigations and theoretical prospects

We present a hundred-watt-level linearly-polarized random fiber laser (RFL) pumped by incoherent broadband amplified spontaneous emission (ASE) source and prospect the power scaling potential theoretically. The RFL employs half-opened cavity structure which is composed by a section of 330 m polarization maintained (PM) passive fiber and two PM high reflectivity fiber Bragg gratings. The 2nd order Stokes light centered at 1178 nm reaches the pump limited maximal power of 100.7 W with a full width at half-maximum linewidth of 2.58 nm and polarization extinction ratio of 23.5 dB. The corresponding ultimate quantum efficiency of pump to 2nd order Stokes light is 89.01%. To the best of our knowledge, this is the first demonstration of linearly-polarized high-order RFL with hundred-watt output power. Furthermore, the theoretical investigation indicates that 300 W-level linearly-polarized single-mode 1st order Stokes light can be obtained from incoherently pumped RFL with 100 m PM passive fiber.

physics.optics↗

arXiv2016

Study on the power scaling and mode instability property in an all-fiber narrow-linewidth tapered fiber amplifier

We develop a high-power narrow-linewidth fiber amplifier based on a tapered Yb-doped fiber. In the experiment, the narrow end of the tapered fiber is used as the input port of both pump source and signal light to form a robust all-fiber configuration. The stimulated Brillouin scattering (SBS) is effectively suppressed owing to its gradually increased mode area as well as continuously changed Brillouin frequency shift dependent on the core diameter. As a result, an output power of 260 W without SBS and mode instability (MI) is obtained, with a slope efficiency of 71.3 % and a narrow linewidth of ~2GHz. However, the MI rather than SBS is observed when output power reaches 260 W. The features of MI are experimentally studied in detail. It is pointed out that, the MI seems the primary and durative limitation factor of the tapered fiber amplifier for high-power narrow-linewidth output, although it has advantage to reduce other nonlinear effect. To the best of our knowledge, this is the first detailed experimental study on MI in a tapered fiber. At last, some worthwhile discussion about the optimization of the tapered fiber and the whole system for both SBS and MI suppression is conducted based on the present and previous results.

physics.optics↗

arXiv2016

Influence of seed power on mode instabilities in high power fiber amplifiers

We use a semi-analytical model of stimulated thermal Rayleigh scattering to present theoretical study of seed power on mode instability with the presence of photodarkening. The behavior of mode instabilities as a function of seed power has been investigated. The nonlinear dependence of the threshold with lower seed powers is ascribed to the influence of gain saturation while the reduction behavior of the threshold with higher seed powers is put down to the effects of photodarkening, which agrees well with the experimental results.

physics.optics↗

arXiv2015

Study of dopant concentrations on thermal induced mode instability in high power fiber amplifiers

Dependence of mode instabilities (MI) on ytterbium dopant concentrations in high power fiber amplifiers has been investigated. It is theoretically shown that, by only varying the fiber length to maintain the same total small-signal pump absorption, the MI threshold is independent of dopant concentration. MI thresholds of gain fibers with ytterbium dopant concentration of 5.93X10^25/m3 and 1.02X10^26/m3 have been measured, which exhibit similar thresholds and agree with theoretical results. The result indicates that heavy doping of active fiber can be adopted to suppress nonlinear effects without decreasing MI threshold, which provides a method of maximizing the power output of fiber laser, taking into account the stimulated Brillouin scattering, stimulated Raman Scattering, and MI thresholds simultaneously.

physics.optics↗

arXiv2015

Study of wavelength dependence of mode instability based on a semi-analytical model

We present theoretical study of wavelength dependence of mode instability (MI) in high power fiber lasers, which employs an improved semi-analytical theoretical model. The influence of pump / seed wavelength and photodarkening on threshold has been studied. The results indicate promising MI suppression through pumping or seeding at an appropriate wavelength. Small amounts of photodarkening can lead to significant impact on MI.

physics.optics↗

arXiv2015

Influence of core NA on Thermal-Induced Mode Instabilities in High Power Fiber Amplifiers

We report on the influence of core NA on thermal-induced mode instabilities (MI) in high power fiber amplifiers. Influence of core NA and V-parameter on MI has been investigated numerically. It shows that core NA has larger influence on MI for fibers with smaller core-cladding-ratio, and the influence of core NA on threshold is more obvious when the amplifiers are pumped at 915nm. The dependence of threshold on V-parameter revealed that the threshold increases linearly as V-parameter decreases when V-parameter is larger than 3.5, and the threshold shows exponentially increase as V-parameter decreases when V-parameter is less than 3.5. We also discussed the effect of linewidth on MI, which indicates that the influence of linewidth can be neglected for linewidth smaller than 1nm when the fiber core NA is smaller than 0.07 and fiber length is shorter than 20m. Fiber amplifiers with different core NA were experimentally analyzed, which agreed with the theoretical predictions.

physics.optics↗

arXiv2015

Polarization Effects on Thermal-Induced Mode Instabilities in High Power Fiber Lasers

We present detailed studies of the effect of polarization on thermal-induced mode instability (MI) in ytterbium-doped fiber amplifiers. Based on a steady-state theoretical model, which takes both electric fields along the two principal axes into consideration, the effect of polarization effects on the gain of Stokes wave was analyzed, which shows that the polarization characteristics of the fiber laser have no impact on the threshold of MI. Experimental validation of the theoretical analysis is presented with experimental results agreeing well with the theoretical results, in which polarization-maintained and non-polarization-maintained fiber lasers with core/inner cladding diameter of 30/250um and core NA of 0.07 were employed. The MI threshold power is measured to be about 367~386W.

physics.optics↗

arXiv2014

Mitigating of Modal Instabilities in Linearly-Polarized Fiber Amplifiers by Shifting Pump Wavelength

We investigated the effct of pump wavelength on the modal instabilities (MI) in high power linearly-polarized Yb-doped fiber amplifiers. We built a novel semi-analytical model to determine the frequency coupling characteristics and power threshold of MI, which indicates promising MI suppression through pumping at an appropriate wavelength. By pumping at 915nm, the threshold can be enhanced by a factor of 2.36 as compared to that pumped at 976nm. Based on a high power linearly-polarized fiber amplifier platform, we studied the influence of pump wavelength experimentally. The threshold has been increased by a factor of 2 at 915nm, which agrees with the theoretical calculation and verified our theoretical model. Furthermore, we show that MI suppression by detuning the pump wavelength is weakened for fiber with large core-to-cladding ratio.

physics.optics↗

arXiv2014

1.3kW monolithic linearly-polarized single-mode MOPA and strategies for mitigating mode instabilities

We report on the high power amplification of 1064nm linearly-polarized laser in all-fiber polarization-maintained MOPA, which can operate at output power level of 1.3kW. The main amplifier was pumped with six 915nm laser diodes, and the slope efficiency is 65.3%. The beam quality (M2) was measured to be <1.2 at full power operation. The polarization extinction rate of the fiber amplifier was measured to be above 94% before mode instabilities (MI) sets in, which reduced to about 90% after the onset of MI. Power scaling capability of strategies for suppressing MI is analyzed based on a novel semi-analytical model, the theoretical results of which agree with the experimental results. It shows that mitigating MI by coiling the gain fiber is an effective and practical way in standard double-cladding large mode area fiber, and, by tight coiling of the gain fiber to the radius of 5.5cm, the MI threshold can be increased to 3 times higher than that without coiling or loose coiling. Experimental study has been carried out to verify the idea, which has proved that MI was suppressed successfully in the amplifier by tight coiling.

physics.optics↗