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Highlights The static friction coefficient and rolling friction coefficient of oat seeds were calibrated by the discrete element method. Two representative oat varieties were selected. The hollow cylinder method and sidewall collapse method were used together to reduce the test error. Abstract . Hulless and shelled oat are two types of oat with major differences in physical appearance. To study the contact parameters between the two different oat seed types, these parameters were delineated with the discrete element method and graphic image processing technology. Using plexiglass as the contact material, the experiments used two different angle of repose measurement methods—hollow cylinder and collapse sidewall devices, to perform bench and simulation experiments on the two different oats. Under different measurement methods, bench experiments measured the angles of repose of the two oat seed types at 33.19°, 33.82° and 22.45°, 23.57°; the static friction coefficient and rolling friction coefficient were the experimental factors, and the angle of repose was the experimental indicator in the simulation. The steepest climbing experiment determined the optimal range of the experimental factor, and the regression equation between the static friction coefficient, rolling friction coefficient and angle of repose was established by a quadratic orthogonal rotation combination experiment. Finally, the angles of repose measured by the bench experiment with the two different measurement methods were treated as target values, the coefficient of static friction and the coefficient of rolling friction were solved; the coefficient of static friction between hulless oats was 0.36, and the coefficient of rolling friction between hulless oats was 0.052; the coefficient of static friction between shelled oats was 0.24, and the coefficient of rolling friction between shelled oats was 0.036. The obtained contact parameters between seeds were input into EDEM, the simulation and bench experiment results were verified. The difference between the simulation results and the actual values was within 3%. The angle of repose of oats after calibration was close to the actual situation, and the calibration results had high reliability and provided a referencefor the measurement of contact parameters between other agricultural crop seeds. Keywords: Calibration, Contact parameters, Discrete element method, Oat.

We have numerically simulated the operation of the Fourier-domain mode-locked (FDML) fiber laser based on the wavelength reconstruction method instead of numerical solving the nonlinear Schrodinger equation. We studied the influences of the filter bandwidth and the relative time delay caused by the fiber chromatic dispersion on the instantaneous linewidth of the FDML fiber laser. The results show that the instantaneous linewidth broadens as the filter bandwidth and the relative time delay increase. When the filter has the bandwidth of 0.02 nm, the narrowest and broadest instantaneous linewidths are 0.024 and 0.042 nm, respectively. We give an understanding for the oscillation of the instantaneous linewidth of FDML. The presented result can be used to evaluate the performance achievable in the FDML fiber lasers.

The frequency doubling of femtosecond pulses from an Yb-doped fiber laser source was demonstrated in a PPKTP waveguide fabricated by femtosecond laser direct writing. The PPKTP waveguide contains a fixed period of 8.9 μm and the feomtosecond fundamental pulses have a central wavelength of 1044 nm. A maximum SHG power of 406 mW was produced, yielding a conversion efficiency of 5.6%. Numerical simulations were carried out to investigate the property of frequency doubling for femtosecond pulses. The results show that the SHG process proceeds even the quasi-phase-matching (QPM) condition is not well satisfied, which is significantly different from that of “long” pulses or CW light and is accorded with the experimental results.

Based on the nonlinear polarization rotation (NPR) technique and a fiber Mach–Zehnder interferometer (MZI), a fiber laser which can work in two different modes has been demonstrated. In one mode, it works as a continuous wave (CW) multiwavelength laser, and stable dual-, triple- and tetra-wavelength lasing with the wavelength spacing of 1.1 nm are obtained. In the other mode, it works as a passively mode-locked fiber laser, and the generated pulses have a repetition rate of 10.6 MHz, an FWHM width of 800 ps and the largest energy of 1 nJ. The multi-functional property makes this fiber laser a very convenient, cost effective and multi-purpose laser source.

We report on a all-fiber actively Q-switched laser based on a Mach-Zehnder interferometer (MZI). One of the arms of the MZI is bonded on a piezoelectric transducer (PZT). The path length difference between those two arms will change when a RF electric signal is applied drives the PZT, allowing control of the Q-factor of the cavity and, thus, performing active Q-switching. Using Er3+/Yb3+ co-doped fiber (EYDF) and a maximum pump power of 712 mW, Q-switch pulses with peak power of 2.2 W and pulse width of 1.2 μs were obtained. Furthermore, our result also shows that, to generate stable Q-switched-pulses, with different pump power, one must select different modulation frequency, this phenomenon is experimentally proved for the first time to our knowledge. The pulse repetition rate of the laser can be adjusted from few Hz up to 25 kHz. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 2183–2186, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22692

A new S-band double-pass (DP) fibre Raman amplifier (FRA) with the characteristics of improved gain and noise figure (NF) is demonstrated. By employing a high birefringence fibre loop mirror (HiBi-FLM) with a comb-like reflection spectrum as a signal reflector, the ASE noise of this DP-FRA greatly decreases compared with that of the conventional DP-FRA with the FLM. A maximum NF improvement of 6.2 dB and an average NF improvement of 4.9 dB over 40 nm (1480–1520 nm) bandwidth are realized. By using the FLM as a residual pump reflector to improve the gain of the amplifier, a maximum gain of 24 dB and an average NF of 4.3 dB are obtained in the same bandwidth.

Using a section of un-pumped Er/Yb co-doped fiber (EYDF) as a saturable absorber, Self-Q-switching and self-mode-locking pulses have been obtained in an all-fiber EYDF ring laser. Such laser is with the self-Q-switched pulse threshold of 135.22 mW, the repetition rate of approximately 22.2 kHz, and the pulse duration of ∼2.8 μs, respectively. The self-mode-locked threshold is 591.8 mW. By incorporating the saturable absorption in an un-pumped EYDF and a Mach–Zehnder interferometer, when the pump power is increased to 1242.9 mW, the continuous-wave (CW) mode-locking with the pulse width of 26 ns has also been demonstrated experimentally for the first time.

A novel interrogation of fiber Bragg grating (FBG) in a strain sensor, which is based on Er-doped fiber laser's transient regime, is reported. The applied strain on FBG changes the laser cavity loss, which in turn modifies the laser's transient regime. The strain value can be obtained by measuring the build-up time of the laser. This method translates wavelength shift information into time domain that can be accurately measured with low-cost electronic devices. In the experiment, the strain sensitivity can be adjusted by simply controlling the pumping level of the laser, a sensitivity of 1.66 μe/μs is achieved under fitful pumping level. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48:1904–1904, 2006; Published online in Wiley InterScience (www.interscience.wiley.com(. DOI 10.1002/mop.21773

A narrow line width, tunable, and highly efficient fiber laser based on Er3+/Yb3+ co-doped double-clad fiber with the maximum output power of 438 mW and a slope efficiency of ∼16% has been demonstrated. By using a fiber Bragg grating (FBG) as a narrow band reflector, 3-dB line width of the output laser could be as narrow as 0.04 nm over the whole tuning range, and by compressing or stretching the FBG, wavelength tunable range of 4.0 nm was realized. In the experiment, we also found that there existed an optimum splitting ratio of the output coupler, at which the maximum output power could reach ∼647 mW. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48: 1736–1739, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.21762

We propose a novel remote Raman pumps monitoring technique based on the pump-to-signal four-wave-mixing (FWM) in a 50-m long highly nonlinear dispersion-shifted photonic crystal fiber. The health information of the pumps can be reproduced to the new FWM signal accompanying the transmission signal through the pump-to-signal FWM. As a result, the modulation frequency limitation for cross gain modulation can be avoided effectively, and no degradation of the transmission system is induced by this technique.

Modulation instability induced by the cross-phase modulation in a decreasing dispersion fiber (DDF) is investigated. The expression of the dispersion relation is obtained when the fiber loss and the walk-off effect are taken into account. The results show that the gain spectrum of the DDF is much broader than that of the ordinary optical fibers, and the gain spectral width will increase with the propagation distance when the dispersion decreasing parameter is greater than the fiber loss coefficient. The results also show that a small walk-off effect will increase the range and the maximum value of the gain spectrum in the fiber.

We study the temporal characteristics of a high-energy Er/Yb-codoped fibre ring laser. By using cladding pumping and using a fibre Bragg grating (FBG) as a wavelength selector, laser oscillation with the maximum average output power of 425 mW and the slope efficiency of 19.7% has been obtained. Using a section of unpumped Er/Yb-codoped fibre as a saturable absorber, the device can operate in several different operation modes for different pump powers—such as stable self-Q-switched, self-mode-locked pulse, and continuous-wave (CW) mode-locked operation. As there is no active element inside the cavity, this laser potentially has many advantages over conventional pulsed fibre laser systems.

A hollow-core photonic crystal fiber Fabry-Perot interferometric (HC-PCF-FPI) strain sensing system is proposed and demonstrated. The sensing system has an excellent multiplexing capability owing to the wide free-spectral range of the HC-PCF-FPI structure. Moreover, with a simple fast Fourier transform demodulation method, the proposed strain sensing system could be potentially applied in structural health monitoring.

We proposed and demonstrated a room-temperature multiwavelength Yb-doped fiber ring laser with high stability based on a Mach-Zehnder interferometer by exploiting spatial hole burning and polarization hole burning effects, from which stable eight lasing wavelengths is obtained. The number of the lasing wavelengths (eight) is the most to our knowledge, and the wavelength spacing is 1.1 nm. Triple-, and dual-wavelength operation with the wavelength spacing of 1.1 nm or its integral multiples have been obtained by adjusting the intracavity polarization controllers. © 2008 Wiley Periodicals, Inc. Microwave Opt Technol Lett 50: 723–725, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.23206

We propose and demonstrate a novel room-temperature multiwavelength erbium-doped fiber laser (EDFL) with a 0.8-nm wavelength spacing based on four-wave mixing (FWM) in a length of high-nonlinear photonic crystal fiber and sampled-fiber Bragg grating. The FWM processes suppress the wavelength competition of the EDFL and increase the number of lasing wavelength. By adjusting the FWM efficiency, the number of the concurrent lasing wavelengths can be changed, and the peak power differences among the main oscillation wavelengths are less than 2.0 dB.

A diode-pumped Yb-doped double-clad fiber laser operating in a Q-switched regime is described. By exploiting pulsed pump light and stimulated Brillouin scattering of the gain fiber, a peak power of 105 kW has been demonstrated, which is an order of magnitude greater than that of conventional Q-switched fiber devices. Pulse energy of 0.5 mJ and pulse duration of 5 ns have been achieved.

We present in this Letter the second-harmonic generation of femtosecond pulses in double-line-written waveguides fabricated in periodically poled lithium niobate (PPLN) with femtosecond laser pulses. In a 10-mm-long sample, a normalized conversion efficiency of 12.6% W(-1) cm(-2) has been achieved for 40 fs pump pulses with the wavelengths centered at 1550 nm. Simulation results show that in PPLN waveguides the FWHM of wavelength tuning curve for 40 fs pump pulses is 42 nm, which is 15 times of that for 40 ps pump pulses.

Second harmonic generation of femtosecond pulses in PPLN waveguide written by femtosecond laser pulses was described both theoretically and experimentally. An average power conversion efficiency of 21.5% was obtained.

We present in this paper the fabrication and characterization of thermally stable double line waveguides in Z-cut periodically poled Lithium Niobate crystals. The waveguides were fabricated by using a femto-second laser, and utilized for second-harmonic generation. Our experiments have shown that a quasi-phase matching wavelength of 1548.2 nm, a tuning bandwidth of 2 nm, and a tuning temperature range of 150.4±1.6°C can be achieved.

In this paper, a novel Q-switched fiber laser made up of OptoCeramic electrooptic ceramics is reported. The ring laser is composed with an electrooptic ceramics modulator as Q-switched, a highly Yb-doped fiber as the gain medium, and pumped by a semiconductor laser of 976 nm wavelength. Q-switched is realized from modulating the loss, controlled by the material refractive index, which is changed with the voltage of OptoCeramic element. The laser system generates pulses about 100ns, with the repetition rate continuously adjusted from 3kHz to more than 40 kHz.

The numerical study of annealed proton exchanged (APE) waveguide in Z-cut periodically poled LiNbO 3 is presented using finite element method (FEM), compared with beam propagation method (BPM). Also given the analysis of the effects of different annealed time and annealed temperature on modes distribution.

In this paper, a novel double-pass bidirectional pump broadband L-band erbium-doped superfluorescent fiber source (SFS) is demonstrated for the first time. In this fiber source, the EDF is divided into two segments, one of which (EDF2) is bidirectional pumped by a 980nm laser diode through two wavelength division multiplexers (WDM), and the other one (EDF1) is arranged between the reflector and the first WDM. EDF1 is unpumped. The fiber length ratio of the EDF1 length to the total length is defined as R L =L EDF1 /(L EDF1 + L EDF2 ). The pump power ratio of forward to total pump power of EDF2 is defined as K=P forward /P total . The effects of the fiber length and pump power arrangement on the output characteristics of the L-band fiber source are simulated. With an appropriate pump power ratio K and an optimal fiber length ratio R L , broadband L-band erbium-doped SFS with flat output spectrum can be obtained. Additionally, the optimal fiber length ratio R L is also depended on the pump power ratio K. When K>0.4, the optimal R L tends to be changeless. When K=0.1, the optimal R L is 0 and widest flat spectrum is achieved with a 3-dB bandwidth of 63 nm (1540nm-1603nm).

A dual-wavelength narrow line width laser system, with ultra-narrow wavelength spacing, is proposed and demonstrated. The operation principle of the laser system is based on the pumped-induced thermal effects of the Er-Yb-doped fiber DBR laser. We designed and constructed a common Er-Yb-doped DBR fiber laser with proper parameters. With the pump power of the DBR laser increasing, the Bragg wavelengths and the bandwidths of the two gratings will change unequally. When pumped with 45mW, we obtain a stable dual wavelength lasing with the bandwidth of 0.014nm and the wavelength spacing of 0.055nm.© (2007) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The onset of supercontinuum generation in a photonics crystal fiber is investigated experimentally and numerically as a function of pump optical power with a femtosecond pulse. The effects of optical wave breaking of higher-order solitons, Stimulated Raman scattering (SRS), and coupling between the SRS and parametric four-wave mixing on supercontinuum generation are investigated. Good agreement between experiment and simulation is obtained.

Self similar mode locked fiber laser is studied based on a numerical model. By introducing a dimensionless factor k to characterize the pulse, the self similar pulse formation and its temporal and spectral changes are investigated throughout the cavity. The influences of all the elements inside the cavity on the pulse formation are also studied.

In an intermediate asymptotic propagation distance, pulse's self similar evolution is influenced by the initial pulse width and chirp, as a result pulses can not completely evolve into a parabolic pulse, which can affect the compression of the evolved pulse. The self similar evolution results of ultra-short pulse with different width and initial chirp coefficient are studied by numerical simulation. The study shows the wider the pulse is, the worse the self similar evolution and compression are; it also shows the initial chirp can greatly hold out the self similar evolution and compression, the larger the chirp and the wider the pulse, the worse the self similar evolution, and the influence of the positive chirp on the evolution is bigger than that of negative chirp.

A broadband source (1520-1620nm) with output power of more than 200mW is achieved using only a single-wavelength pumping scheme dependent on the combined effects of stimulated Raman scattering, parametric process, and Erbium-doped fiber amplification.

In this letter, a S-band discrete Raman fiber amplifier(RFA) with the maximum net gain of 18.3dB was obtained. Using the gain flattening filter based on a cascaded long-period fiber grating (LPFG), the gain profile of this RFA was flattened to within ±1dB in the wavelength range from 1 480 nm to 1 515 nm, and an average gain of 11.3dB was achieved.© (2006) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

In this paper, we review the recent works on high power cladding-pumped fiber lasers and amplifiers. And we will present some recent results on their applications such as Raman fiber laser and supercontinuum generation.

Picosecond soliton pulse train has been obtained from a passively mode locked erbium-doped ring fiber laser. The passive mode-locking mechanism that is at play in this laser relies on the modulational instability (MI) theory. By accurately adjusting the polarization setting of the circulating cavity light, the repetition rate can be tuned from 58 to 114GHz. Theoretical explanation is also given.

In this paper cw (continuous wave) pumped self-started, widely tunable frequency-shifted feedback Yb-doped double-clad fiber laser is experimentally studied. We observed 1GHz repetition rate with an output power 56.3mW ps pulses. The laser was capable of generating as short as 5ps pulses in the range of 1078.7nm -1140.2nm (more than 60nm). The ps pulses were self-started without any other external trigger. We used 77MHz RF signal AOM and a grating which not only forms external cavity to provide feedback, but also make the laser linewidth narrower.

We report the passive mode-locking at harmonics of the free spectral range (FSR) of the intracavity multi-channel filter in a fiber ring laser. The laser uses a sampled fiber Bragg grating (SFBG) with a free spectral range (FSR) of 0.8 nm, or 99 GHz at 1555 nm, and a length of highly nonlinear photonic crystal fiber with low and flat dispersion. Stable picosecond soliton pulse trains with twofold to sevenfold enhancement in the repetition rate, relative to the FSR of the SFBG, have been achieved. The passive mode-locking mechanism that is at play in this laser relies on a dissipative four-wave mixing process and switching of repetition rate is realized simply by adjustment of the intracavity polarization controllers.

An efficient and tunable Er3+/Yb3+ co-doped cladding pumped fiber ring laser operating in L-band is demonstrated. Rejecting the amplified spontaneous emission (ASE) of C-band by a fiber Bragg grating (FBG) into the same Er3+/Yb3+ co-doped fiber, the laser can operate in L-band stably. The insertion of the FBG can reduce the lasing threshold, and narrow the 3dB line-width of output laser. Using a two-section high-birefringence fiber loop mirror as a wavelength filter, the output wavelength can be tuned over 40nm range. The lasing threshold is about 48.80mW and the side mode suppression ratio is better than 51dB.

Large mode area (LMA) Yb-doped double clad fiber lasers with high power are theoretically and experimentally investigated. For a Fabry-Perot cavity structure, we show the dependence of output power on the output mirror reflectivity, coupled pump power and fiber length, respectively by using power propagation equations and rate equations. Stable CW operation of the fiber laser at 1064 nm with maximum power 4.6 W and slope efficiency 82.6% is achieved in the experiment.

In this paper, a novel all-fiber Q-switched single mode Er/Yb co-doped double clad fiber ring laser operating in 1548.11nm is reported, which exploits fiber Bragg grating Sagnac loop mirror as a wavelength-selective device. To obtain higher output power, the laser is pumped by six CW laser diodes operating at 976nm through six pigtails of a Tapered Fiber Bundle connected with the EYDF. When the pump power is 718.9mW, stable self Q-switched pulse series with pulse width of 2.8μs, repetition rate of 50kHz and peak power of 274.3mW are achieved. Self mode-locked pulse is also obtained in such experimental setup.

A tunable Er 3+ /Yb 3+ co-doped cladding-pumped all fiber ring laser is presented. Under the maximum pump power of 3594.5mW, the absorbed pump power of the fiber is measured to be 2737.37mW, the maximum output power of the fiber laser is 438mW, and the slope efficiency is greater than 15.9%. By using a fiber Bragg grating (FBG) as a wavelength filter, the linewidth of output laser is as narrow as 0.04nm by 3 dB, and by compressing or stretching the FBG, tuning range of 4.0nm is realized, the side mode suppression ratio is about 42dB. We also study the relationship between the output power and the splitting ratio of the output coupler, and it is found that there is an optimum splitting ratio of the output coupler at which the highest output power can achieve 647mW.

A high efficiency Q-switched and self-mode-locked Er 3+ /Yb 3+ co-doped fiber ring laser with a Mach-Zehnder interferometer (MZI) is proposed. The fiber laser with threshold of 126.84 mw and linewidth of 0.06nm has been demonstrated. The average power of 62.6mW, the peak power of 1231.8mW of the Q-switched giant pulses with the pulse duration of 2.6μs can be achieved. It is found that in order to generate Q-switched giant pulses, different repetition frequency must be selected as pumping power is changed. This phenomenon is observed for the first time to our knowledge. In the experiment, it is also found that stable passive Q-switch pulses can be observed, which waveform is not the same with the different pump power, although the arms lengths of the interferometer is not changed. When the absorbed pumped power is increased to 591.8mW, a self-mode-locking pulse can be formed and it is given a detailed theoretically illustration in this paper.

A novel remote pump monitoring scheme based on pump-to-signal four-wave mixing in a 50 m highly nonlinear dispersion shifted photonic crystal fiber is proposed in this paper.

The polarization degree of the two wavelengths involved in FWM can be decreased in the resonant spectra of stimulated Raman scattering, leading to polarization insensitive FWM being observed by using a continuous wave pump source.

The double-clad fiber laser is a novel one with high-efficiency. It has many advantages such as narrow line width and tunable, good beam quality and compact configuration. It can be widely used in optic communications, optic sensor, optic measurement, etc. In this paper, the principle and experimental results of double-clad Yb 3+ -doped fiber laser are reported. This kind of laser is pumped by a high power semiconductor laser array and coupled by a set of lens. The laser output of 209mW at 1.0872μm is obtained with a pump threshold of 192mW. The slope efficiency is 49%.

In this paper, Fiber Bragg Grating external cavity semiconductor laser and Blazed grating external cavity semiconductor laser are analyzed in theory. Some experiments on two types of Blazed Grating external cavity laser (Littrow and Littman set-up) are done, and some conclusions are also obtained. Two set-ups are compared in detail; at last the intending application of the laser is prospected.

A cw laser-diode-pumped Yb-doped double-clad fiber laser operating in a hybrid Q-switched regime was demonstrated. The output pulses had a duration as short as 4.2 ns, a tunable wavelength range from 1080.8 to 1142.7 nm, and a linewidth of less than 0.05 nm. Maximum peak power of approximately 175 kW and single-pulse energy of 1.57 mJ were obtained.

The dynamics of the establishment and stabilization of mode-locking have been investigated, and the ratio of the whole cavity length (L) to DBR length (L/sub DBR/) exerts a significant influence on harmonic mode locking when the facet reflectivities on the absorber and DBR sides of the laser diode are less than 30%. These results are in agreement with previous observations, and are reported here for the first time to the best of our knowledge. The mode-locked semiconductor laser diode length L, used in our simulations has gain section length (L/sub g/), absorber section length (L/sub l/), and DBR section length (L/sub DBR/). The gain and saturable absorber sections have the same modal index of 3.461, and are forward biased, and grounded or reverse biased respectively.

The output characteristics of linear cavity Yb-doped double-clad fiber laser have been studied theoretically and experimentally. Analytical expressions are evaluated for threshold pump power, output power and slope efficiency of the laser under steady state conditions. The expression of minimum of fiber length for designing a linear cavity Yb-doped double-cladding fiber laser has been given. Then, the Yb-doped double-clad fiber laser has been investigated experimentally. By exploiting various output coupler, output power, threshold pump power and the slop efficiency have been measured. Comparison between the experimental results and theoretical data has been performed. They are in good agreement with each other.

A compact Q-switched diode-pumped double-clad Yb3+-doped fiber laser was described. By using conventional Q-switching technique and exploiting nonlinearity in the core of the gain fiber, the laser operated at a repetition rate of 1.5kHz with an peak power over 13.3kW, pulse width of 5.2ns and pulse energy of 80 µ J. The output wavelength was tunable from 1080nm to 1119nm with a line width less than 0.2nm.

An all-fiber Yb-doped double-clad fiber laser based on distributed Bragg reflection (DBR) with fiber Bragg gratings (FBGs) is demonstrated. The single transverse-mode wavelength-stable output at 1 .06 µ m is up to 1.18W, with a slope efficiency of 68% and an optical-to-optical conversion efficiency of more than 50%

As well-known fact, the relation between the radius of TM00 mode of solid laser and 1/ft (the focal power of crystal) can be expressed with double U curves'1. In this paper, self-Q-switched of Nd:YVO4 solid laser of asymmetry flat-flat cavity was studied theoretically and experimentally, using this character. Self-Q-switched giant pulses with 200ns (FWHM) were observed in experiment. Pumped pulse width was 100µs. Peak power of laser pulse was several million times than the CW level. The experiment results were in good agreement with the theory. Giving the result of experiment, we offered a method to enhance the peak power of pulse.