Passive combination of multiple beams in an optical fiber via stimulated Brillouin scattering

We demonstrate passive combination of multiple beams in an optical fiber via stimulated Brillouin scattering (SBS). Four off-axis beams are combined in a long multimode optical fiber using a novel all-optical mount. The combined beam has the high spatial coherence properties of the LP(01) mode owing to the beam cleanup properties of SBS. The threshold for SBS when off-axis pumps are used is shown to be a factor of 13 times higher than for on-axis pump beams. We propose this method would improve the brightness of an array of fiber amplifiers.     

Multimode fiber laser beam cleanup based on stochastic parallel gradient descent algorithm

We present experimental research on multimode fiber laser beam cleanup based on a stochastic parallel gradient descent (SPGD) algorithm. The multimode laser is obtained by injecting a 1064 nm central wavelength single mode fiber laser into a multimode fiber and the system is setup by using phase only liquid crystal spatial light modulators (LC-SLM). The quality evaluation function is increased by a factor of 10.5 and 65% of the laser energy is encircled in the central lobe when the system evolves from open-loop into close-loop state. Experimental results indicate the feasibility of the multimode fiber laser beam cleanup by adaptive optics (AO).     

Single-mode Raman fiber laser based on a multimode fiber

Single-mode operation of a Raman fiber laser based on a multimode fiber has been demonstrated for what is to our knowledge the first time. Single-mode operation was achieved through the beam cleanup effect of stimulated Raman scattering in multimode fibers. The Raman laser, consisting of a 50-microm-core multimode fiber and a multimode Nd:YAG laser pump, produced an output beam corresponding to the fundamental mode of the fiber.     

High-power tunable single-frequency single-mode erbium:ytterbium codoped large-core fiber master-oscillator power amplifier source

We present a cladding-pumped single-frequency, single-mode erbium:ytterbium codoped fiber master-oscillator power amplifier source generating up to 151 W of continuous-wave output power at 1563 nm with 33% slope efficiency and 20 dB gain. This source was also tunable and had a stable operation range of 1546 to 1566 nm at an output power level in excess of 125 W. The doped fiber exploited a large-core design for improved power handling and mitigation of stimulated Brillouin scattering. There was no sign of having stimulated Brillouin scattering even at the highest power. Despite a large core (V = 12), the output beam was nearly diffraction limited (M2 = 1.1). The source showed slight rollover at over 100 W of output power because of the onset of emission from ytterbium, centered at 1060 nm.     

Review of photorefractive materials: an application to laser beam cleanup

Nonlinear photorefractive materials are well suited to record dynamic volume holograms using the wave mixing of coherent laser beams. The photo-induced index modulation is due to a space charge field which modulates the crystal refractive index. The basic phenomena and beam interactions are reviewed for different types of materials operating in the visible and near infrared. In particular, we outline their ability to amplify a low intensity signal beam due to the intensity transfer of a spatially multimode pump beam. We apply this interaction to the cleanup of a multimode large core fiber amplifier. This class of nonlinear materials contribute to extending the performances of laser sources for advanced applications of photonics. To cite this article: L. Lombard et al., C. R. Physique 8 (2007).     

High-energy and high-peak-power nanosecond pulse generation with beam quality control in 200-microm core highly multimode Yb-doped fiber amplifiers

We explored high-energy and high-peak-power pulse generation in large-core multimode fiber amplifiers, achieving what is to our knowledge the highest reported energies, up to 82 mJ for 500-ns pulses, 27 mJ for 50-ns pulses, and 2.4-MW peak power for 4-ns pulses at 1064 nm, using 200-microm-diameter and 0.062-N.A. core Yb-doped double-clad fiber amplifiers. The highly multimode nature of the fiber core was mitigated by use of a coiling-induced mode-filtering effect to yield a significant improvement in output-beam quality from M2 = 25 from an uncoiled fiber to M2 = 6.5 from a properly coiled fiber, with the corresponding reduction in number of propagating transverse modes from > or = 200 to < or = 20.     

Degradation of beam quality and depolarization of the laser beam in a step-index multimode optical fiber

The characteristics of a laser beam are altered during propagating through large-core multimode optical fibers. The distribution of modes excited by the input laser beam is modified by means of mode coupling on transmission through the fiber, leading to the degradation of beam quality and the depolarization of the delivered beam. The relationship between the beam quality factor (M²) of output beam from a large-core multimode fiber and the fiber length, as well as the relationship between the degree of polarization (V) of output beam from such a fiber and the fiber length, are introduced in this paper. When a laser beam was well launched into a large-core step-index multimode fiber, M² of the output beam was a compound tanh function of the fiber length. A linear polarization beam that well launched into such fiber suffered depolarization. The V of the output beam was an exponent function of fiber length. And the misalignment between beam axis and fiber axis made the beam quality degrade faster but made no difference of the utmost M² in the aligned and misaligned conditions. Also, the misalignment condition made the polarization of output beam degrade faster.     

多模光纤受激布里渊散射非相干组束的机理及理论模型

 基于布里渊散射耦合强度方程，分析了多模光纤受激布里渊散射非相干组束的机理，建立了其非相干组束的理论模型，并依据模型进行了理论计算。结果表明：在较长的多模光纤中，受激布里渊散射产生的Stokes光以LP­­₀₁模传输；其非相干组束过程是混合光分解成线偏振光的逆过程；计算的组束功率与T.H.Russel等人的实验结果相一致。最后，分析了组束光强的分布，表明了组束光强对模场尺寸的依赖关系，证明提高组束光强的有效措施是减小组束光的模场半径。     

Phase conjugating mirror combining a fused silica rod and a fiber

A phase conjugating mirror combining a fused silica rod and an optical fiber is presented. This configuration combines a large diameter fused silica rod with a high laser damage threshold and an optical fiber with a low stimulated Brillouin scattering (SBS) threshold. The composed SBS generator-amplifier construction, similar to the two-cell type makes a forced SBS process, in which the Stokes beam reflected by the fiber is injected into the fused silica rod. Using this combined phase conjugating mirror in a laser-diode-pumped laser master oscillator power amplifier system, we obtained 42% SBS reflectivity at most at 100 Hz repetition rate. At the same time, the inverse beam quality parameter M(2) is reduced from 3 to 1.6, while at 400 Hz repetition rate, M(2) is reduced from 4 to 1.7.     

Single-mode operation of a coiled multimode fiber amplifier

We report a new approach to obtaining single-transverse-mode operation of a multimode fiber amplifier in which the gain fiber is coiled to induce significant bend loss for all but the lowest-order mode. We demonstrated this method by constructing a coiled amplifier using Yb-doped, double-clad fiber with a core diameter of 25 microm and a numerical aperture of ~0.1 (V approximately 7.4) . When the amplifier was operated as an amplified-spontaneous-emission source, the output beam had an M(2) value of 1.09 +/- 0.09 ; when seeded at 1064 nm, the slope efficiency was similar to that of an uncoiled amplifier. This technique will permit scaling of pulsed fiber lasers and amplifiers to significantly higher pulse energies and peak powers and cw fiber sources to higher average powers while maintaining excellent beam quality.     

光纤移频分布式布里渊光纤传感技术

提出了一种利用布里渊光纤环形腔移频技术实现分布式光纤布里渊传感的方法.该方法基于布里渊光时域分析法原理,将一束单纵模运转激光器输出的激光分为两束;一束光入射布里渊光纤环形腔中产生窄线宽的受激布里渊散射光作为斯托克斯光,另一束光经过低频相位调制后作为泵浦光;斯托克斯光和泵浦光分别相向入射进入传感光纤,通过测量布里渊谱得到...     

激光二极管抽运电光调Q双通激光器中的光纤相位共轭

为了降低高重复频率脉冲固体激光器中热致光束畸变对光束质量的影响，在激光二极管抽运、电光调Q的Nd：YAG激光双通放大系统中对多模石英光纤相位共轭镜进行了实验研究。激光重复频率100Hz、脉宽20ns。实验结果表明：应用多模光纤能够显著改善放大级中热效应引入的波面畸变，双通获得近衍射极限的激光输出，并可大幅度压缩脉宽至7ns，达到了3：1的压缩比。在受激布里渊散射阈值附近，实验中观察到相位共轭光光斑剧烈闪烁，而随着注入光纤能量的增大，闪烁现象渐渐消失，输出激光能量的稳定度提高，在注入能量5．3mJ的情况下，获得了14％的能量不稳定度，此时的反射率和退偏率分别为32．6％和7％。     

光纤移频分布式布里渊光纤传感技术

提出了一种利用布里渊光纤环形腔移频技术实现分布式光纤布里渊传感的方法.该方法基于布里渊光时域分析法原理,将一束单纵模运转激光器输出的激光分为两束|一束光入射布里渊光纤环形腔中产生窄线宽的受激布里渊散射光作为斯托克斯光,另一束光经过低频相位调制后作为泵浦光|斯托克斯光和泵浦光分别相向入射进入传感光纤,通过测量布里渊谱得到光纤温度或应变.利用该方法可将十几GHz的微波频率转化为兆赫信号频率进行探测处理,仅需一台激光器,因此系统结构简单、成本低,还可减小激光器频率波动对测量准确度的影响.实验验证了该方法的可行性.     

Fiber phase conjugators at 1064-nm, 532-nm, and 355-nm wavelengths

Undoped multimode quartz fibers can be used as effective phase conjugators with high fidelity and reflectivity for improving the beam quality of lasers with high average power. Such phase conjugators based on stimulated Brillouin scattering (SBS) in undoped multimode quartz fibers with a core diameter of 100 microm and a length of more than 2 m were characterized at wavelengths of 1064, 532, and 355 nm. The power thresholds at these three wavelengths are 6.4, 3.3, and 3 kW, respectively, and the maximum SBS reflectivity is more than 50% in all cases, without a significant depolarization owing to the fiber. Direct measurement of the far-field fidelity, i.e., the phase-conjugated part of the reflected light in the far field at 1064 nm, results in a fidelity of more than 90%.     

Investigation and applications of all-fiber brillouin ring resonator lasers

We examine the theoretical and experimental behavior of Brillouin ring resonator lasers (BRRL), highlighting differences between Brillouin scattering in ring resonators and long fiber lengths. The theory, which assumes that the nonlinear interaction is welldescribed by two coupled equations for the optical waves, agrees well with measured data. We examine the single and multimode behavior of BRRL and show how this may be usefully implemented in microwave signal generation, in Stimulated Brillouin Scattering gain studies, and to realize a new form of all-fiber frequency shifter.     

Multimode to single-mode conversion of a Yb-doped fiber amplifier with a photorefractive Rh:BaTiO3 crystal

We present a technique to convert a multimode-transverse beam obtained from a multimode large core fiber amplifier into a single-mode beam by means of two-wave mixing in an infrared-sensitive Rh:BaTiO₃ crystal. Received: 26 March 2001 / Revised version: 27 March 2001 / Published online: 27 April 2001     

Laser beam combining and cleanup by stimulated Brillouin scattering in a multimode optical fiber

A new technique for combining low-power laser beams has been demonstrated by use of semiconductor diode lasers. The technique, which is appropriate for any single-longitudinal-mode laser, is based on stimulated Brillouin scattering (SBS) in long multimode optical fibers. It produces a clean Gaussian-like beam that corresponds to the fundamental fiber mode, irrespective of the profile of the pump. Coherent as well as incoherent combining was demonstrated, and conversion slope efficiencies as high as 67% and 83% were shown to be achievable for the single-pass and ring-cavity SBS geometries, respectively.     

Stimulated Brillouin Scattering Enhancement Factor Improvement in a 11.6-GHz-Linewidth 1.5-kW Yb-Doped Fiber Amplifier

The stimulated Brillouin scattering(SBS) threshold enhancement factor in a pure white noise linewidth broadening Yb-doped fiber amplifier(YDFA) with a short large mode area fiber is theoretically and experimentally studied.We demonstrate a 1064.08 nm,11.6 GHz linewidth,1.5kW output power YDFA with an SBS threshold enhancement of ~57(26 W SBS threshold with single frequency seed).The output beam is near-diffraction limited with a beam quality factor of M~2 = 1.15 and a slope efficiency of up to 87%.No SBS or stimulated Raman scattering effects are observed in the whole power range.Further power scaling is limited by the available pump power in our system.     

Effect of Brillouin slow light on distributed Brillouin fiber sensors

The effect of Brillouin slow light on distributed Brillouin fiber sensors (DBFSs) is studied. We demonstrate Brillouin slow light for a 1.2 ns pulse with peak powers (PS) from 3.3 to 56.2 mW on depletion of the pump power (PP) ranging from 1.3 to 83.2 mW in conventional optical fibers (SMF-28). Experiments show that, when pump power depletion is not negligible, for a given PP the Brillouin gain and delay time of a pulse decrease when PS increases in a long (> or =10 km) sensing fiber. The optimum pump beam depletion resulting from strong interaction of the pump and the probe in the fiber provides accurate temperature and strain information at a high spatial resolution. Our study reveals that at low PP the spatial resolution error caused by the pulse delay for a DBFS with centimeter spatial resolution is less than 5% of the pulse length.     

Watts-level, short all-fiber laser at 1.5 microm with a large core and diffraction-limited output via intracavity spatial-mode filtering

We report over 2 W of single spatial-mode output power at 1.5 microm from an 8-cm-long, large-core phosphate fiber laser. The fiber has a numerical aperture of approximately equal to 0.17 and a 25-microm-wide core, heavily doped with 1% Er(+3) and 8% Yb(+3). The laser utilizes a scalable evanescent-field-based pumping scheme and can be pumped by as many as eight individual multimode pigtailed diode laser sources at a wavelength of 975 nm. Nearly diffraction-limited laser output with a beam quality factor M2 approximately equal to 1.1 is achieved by use of a simple intracavity all-fiber spatial-mode filter. Both spectrally broadband and narrowband operation of the laser are demonstrated.