Forward and backward intensity noises induced by stimulated Brillouin scattering in optical fiber

The characteristic of intensity noise is degraded when stimulated Brillouin scattering (SBS) occurs in the fiber transmission systems. We use the localized fluctuating model to study SBS and obtain the curves of intensity fluctuations versus the single-pass gain. Corresponding experiments are also conducted. For the forward light, the relative intensity noise (RIN) dramatically increases at first and gradually stabilizes when the input power is above the SBS threshold. For the backward light, the RIN increases dramatically with the input power near the threshold. As the input power continues to increase, the RIN decreases quickly at first and subsequently decreases slowly. This observation is attributed to the lower frequencies.     

A simple model of suppressing stimulated Brillouin scattering in optical fiber with frequency-modulated laser

A simple model is developed to study the mechanism of stimulated Brillouin scattering（SBS） suppression with frequency-modulated laser in optical fiber. By taking into account the laser frequency distribution along the fiber induced by frequency modulation, the average effective Brillouin gain is calculated to determine the SBS threshold. Experimental results show agreement with the numerical analysis. The application for SBS suppression in interferometric fiber sensing system is also discussed in this paper. The results show that the maximum input power can be increased effectively by frequency modulation method.     

Dependence of stimulated Brillouin scattering in pulsed fiber amplifier on signal linewidth,pulse duration,and repetition rate

The dependencies of stimulated Brillouin scattering(SBS) threshold in pulsed fiber amplifiers on spectral linewidth,pulse duration,and repetition rate are measured and discussed.Experimental results show that the SBS threshold is highly related to spectral linewidth and pulse duration.Therefore,the power handling limitation in a pulsed fiber amplifier may be the average power in some cases and the peak power in others.     

A 168-W high-power single-frequency amplifier in an all-fiber configuration

We present a high-power,single-frequency,narrow linewidth fiber amplifier based on master oscillator power amplification chains in an all-fiber configuration.The effect of the delivery fiber on the maximum output power is studied.A home-made 1064-nm seed laser with a 20-kHz linewidth is boosted to 129 W,and limited by stimulated Brillouin scattering(SBS) when the delivery fiber is 1.2 m long.By shortening the delivery fiber length to 0.7 m,the SBS threshold is increased efficiently and the maximum output power rises to 168 W with an 82.9% power conversion efficiency.The experimental results indicate that the output power can be further raised by shortening the delivery fiber length and increasing the pump power.     

A high-efficiency Brillouin fiber ring laser

A high-efficiency Brillouin fiber ring laser is demonstrated using the standard single-mode fiber. The laser exhibits a 3.6-mW threshold. The output power is 22 mW with 40-mW pump power, and the maximum optical-to-optical efficiency is 5570. The output is single wavelength with a 3-dB linewidth of 5 MHz, and the interval of center frequency between the laser and the pump light is 11 GHz （0.088 nm）. It is shown that the stimulated Brillouin scattering threshold of ring resonator is lower and the energy transfer efficiency is higher than those in fiber.     

High OSNR and simple configuration dual-wavelength fiber laser with wide tunability in S+C+L band

A simple configuration dual-wavelength fiber laser,by combining the first-order Brillouin laser and the residual pump laser,is proposed and experimentally demonstrated.A 1 km long single-mode fiber is used as the stimulated Brillouin scattering gain medium pumped by a narrow linewidth tunable laser source(TLS).Through simply adjusting the TLS output power,power-equalized dual-wavelength lasing can be achieved with a high optical signal to noise ratio(OSNR) of 80 d B.With the good tunability of the TLS,the dual-wavelength fiber laser has a tunable range of ～130 nm,and simultaneously the beat frequency of the two lasing wavelengths can be tuned from 10.1875 to 11.0815 GHz with the tunable range of 0.8940 GHz.The high stability of the dualwavelength operation is experimentally verified by the measured beat frequency fluctuation of ≤6 MHz in 1 h and power fluctuation of ≤0.03 d B in 2 h.The temporal characteristics of the fiber laser are also investigated experimentally.The fiber laser will find good applications in fiber sensing and microwave photonics areas.     

Brillouin Lasing with a Reduced Self-Pulsing Characteristic Using a Short-Length Erbium-Doped Fiber as the Nonlinear Gain Medium

A single-wavelength Brillouin laser is demonstrated by using a 3-m-long erbium doped fiber （EDF） in a ring cavity. The EDF is used to provide both nonlinear and linear gains to generate a stimulated Brillouin scattering （SBS） and to amplify the generated SBS, respectively. The Brillouin erbium fiber laser （BEFL） operates at 1561.5 nm, where the operating wavelength is up-shifted by 0.08nm from the Brillouin pump. The operation wavelength is also tunable within 1560.6-1562.6nm. The BEFL Mso shows a self-pulsing characteristic with repetition of 66. 7kHz when the BP is set around the threshold pump power of 13roW. Compared to the conventional Brillouin fiber laser with a long cavity length, the proposed BEFL exhibits a significantly lower amplitude of pulse. This laser has many potential applications, such as in optical communication and sensors.     

A 330-W single-frequency retrievable multi-tone monolithic fiber amplifier

A single-frequency retrievable phase modulated multi-tone fiber amplifier is presented in theory and demonstrated in experiment. A multi-tone seed laser generated by a sine wave phase modulated single-frequency laser is employed for stimulated Brillouin scattering suppression in an all-fiber amplifier. A demodulation signal which is π phase shifted with respect to the modulation signal is used to retrieve the single-frequency laser from the multi-tone laser. In experiment, we first optimize the all-fiber master-oscillator power-amplifier. With this amplifier, we demonstrate a single-frequency retrievable multi-tone laser with 330-W output when driven by the multi-tone seed, while the ultimate output power is only 130 W when driven by the single-frequency laser. Then, we carry out an experiment for retrieving the single-frequency laser from the amplified multi-tone laser. Results indicate that the single-frequency laser can be retrieved with a sideband suppression of more than 20 dB. Retrieving an even higher power single-frequency laser is possible if a high power demodulator is available.     

Theoretical Analysis of the Critical Phenomena of a Brillouin Laser

The critical phenomena of a Brillouin laser are analyzed theoretically. The results show that the behavior of a Brillouin laser in the threshold region is a second-order phase transition. The critical point of the phase transition is the gain threshold of the Brillouin laser, and the order parameter is the amplitude of the Stokes component in stimulated Brillouin scattering. The critical slow-down phenomenon and the typical characteristics in phase transition are demonstrated. Further work on the combination of nonlinear optics and phase transition in the Brillouin laser may lead to a new view and findings that could be significant for both fields.     

Short distributed feedback fiber laser with unidirectional output for sensing applications

A short distributed feedback fiber laser with a nearly unidirectional output is fabricated and tested. The short fiber laser is made of a polarization-dependent phase-shift grating fabricated with a vertically polarized 244-nm ultraviolet (UV) laser. A single ∏ phase-shift is introduced to a 2-cm grating at a specified position by directly moving the phase mask during UV beam scanning. Test results show that the laser has a single polarization longitudinal mode with 2.6-mW pump threshold. The backward-to-forward output power ratio is approximately 30:1. The relative intensity noise is -88 dB/Hz, and the linewidth is approximately 10 kHz at 75-mW 980-nm pumping. The unidirectional output and short dimension of this short fiber laser make it very useful in sensing applications, especially in multiplexed sensing applications.     

Stimulated Brillouin scattering-induced phase noise in an interferometric fiber sensing system

Stimulated Brillouin scattering-induced phase noise is harmful to interferometric fiber sensing systems. The localized fluctuating model is used to study the intensity noise caused by the stimulated Brillouin scattering in a single-mode fiber. The phase noise structure is analyzed for an interferometric fiber sensing system, and an unbalanced Michelson interferometer with an optical path difference of 1 m, as well as the phase-generated carrier technique, is used to measure the phase noise. It is found that the phase noise is small when the input power is below the stimulated Brillouin scattering threshold, increases dramatically at first and then gradually becomes flat when the input power is above the threshold, which is similar to the variation in relative intensity noise. It can be inferred that the increase in phase noise is mainly due to the broadening of the laser linewidth caused by stimulated Brillouin scattering, which is verified through linewidth measurements in the absence and presence of the stimulated Brillouin scattering.     

远程干涉型光纤传感系统的非线性相位噪声分析

以远程干涉型光纤传感系统为背景, 研究了系统非线性相位噪声构成, 对各构成要素的具体影响进行了详细分析和综合评价, 简要讨论了噪声抑制方案. 研究表明, 系统相位噪声主要包括强度噪声转化而来的相位噪声、非线性效应引起激光 线宽展宽导致的相位噪声以及自相位调制和交叉相位调制引入的相位噪声. 受激布里渊散射和四波混频可引入强度噪声并转化为相位噪声, 对于探测带宽较窄的光纤传感系统, 四波混频引入的该部分噪声往往可以忽略. 受激布里渊散射、四波混频和调制不稳定性都可引起激光线宽展宽从而造成相位噪声的增大. 当系统信道数目较多时, 交叉相位调制对相位噪声的贡献不可忽略. 所得结论对远程干涉型光纤传感系统的实际应用具有重要的指导意义.     

Phase noise characteristics induced by stimulated Brillouin scattering in the presence of four-wave mixing in an interferometric fiber sensing system

Phase noise characteristics induced by stimulated Brillouin scattering (SBS) in the presence of four-wave mixing (FWM) in an interferometric fiber sensing system are investigated. It is found that the phase noise mainly results from SBS and the influence of FWM on the phase noise is negligible. It is due to that the channels are not perfectly equally spaced in the wavelength division multiplexing (WDM) system and the induced beat noise is filtered by the ??kHz/MHz photodetector. The phase noise caused by SBS rather than FWM should be focused on in the practical applications of interferometric fiber sensing systems.     

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.     

1.06kW 13GHz线宽全光纤激光器

分析了高功率光纤激光器中受激布里渊散射(SBS)效应的抑制方法。研究表明,利用宽带噪声源高速相位调制展宽光谱的方法对于抑制SBS十分有效,可实现kW级用于光谱组束的数10GHz高功率光纤激光子束。通过理论计算线宽与SBS阈值的关系,并分析噪声相位调制各参数对SBS阈值提升的影响,优化了光纤激光器设计参数。通过宽带噪声高速相位调制的方法,展宽单频种子源线宽至13GHz,通过两级预放大至10 W后,使用20/400μm掺Yb光纤最终实现了中心波长1064nm、线宽13GHz、最高功率1.06kW的激光输出,光束质量M21.2,光-光转换效率86%,实验过程未观测到模式不稳定性现象。进一步扩宽噪声源频带,加大调制深度,有望实现更高功率的窄线宽光纤激光输出。     

Numerical analysis of stimulated Brillouin scattering in high-power double-clad fiber lasers

A theoretical analysis of stimulated Brillouin scattering (SBS) in linear cavity Yb³⁺-doped double-clad fiber lasers is presented by solving the steady-state rate equations with the SBS. The effects of cavity length, fiber core diameter, input mirror reflectivity at Stokes wavelength, Yb³⁺ concentration and laser linewidth on the SBS are discussed. Numerical results show that the SBS threshold power can be improved by shortening the cavity length, using large mode area fiber, reducing the input mirror reflectivity at Stokes wavelength, lowering the Yb³⁺ concentration and broadening the laser linewidth, and the influence of the laser linewidth on the SBS threshold power is more noticeable than other system parameters.     

基于光子晶体光纤中受激布里渊散射的光载波抑制

设计了一种基于光子晶体光纤中受激布里渊散射的载波滤波器.该滤波器利用两个环形器和一段光子晶体光纤构成了一个环形腔,这种腔结构有效地降低了光纤中受激布里渊散射的阈值.理论分析了光子晶体光纤载波滤波器对提高光调制深度和射频增益的影响.利用25 m的高非线性光子晶体光纤作为受激布里渊增益介质,当入射载波功率为70 mW时,微波光子信号的射频增益为5.38 dB,实验结果与理论计算相吻合.     

S波段光纤拉曼放大器中级联受激布里渊散射串扰的实验研究

研究了光纤激光器前向抽运的S波段分布式光纤拉曼放大器中级联的受激布里渊散射(SBS)串扰现象。用窄光谱带宽(<100MHz)的可调谐激光二极管作为信号源,通过S波段分布式光纤拉曼放大器,当被放大的信号功率超过单模光纤受激布里渊散射的阈值时,出现了前向受激布里渊散射,这是传导声波布里渊散射在光纤放大器中放大的现象。随着拉曼放大器抽运功率的提高,在斯托克斯区,出现了两阶受激布里渊散射线,在实验中观测到偶数阶的受激布里渊散射谱线功率大于奇数阶的布里渊一瑞利散射线。当进一步增加拉曼放大器的抽运功率,出现了前向级联的多阶受激布里渊散射现象,拉曼放大器的增益下降,被放大的信号功率转换为受激布里渊散射,噪声变大。受激布里渊散射的串扰破坏了拉曼放大器的特性,使拉曼放大器无法在密集波分复用光纤传输系统中使用,因此需要严格地控制入纤的信号功率和放大器的抽运功率。在实验中还观测到在光纤拉曼放大器中被放大的信号光和受激布里渊散射线两侧的伴线。     

A 168-W high-power single-frequency amplifier in an all-fiber configuration

We present a high-power, single-frequency, narrow linewidth fiber amplifier based on master oscillator power amplification chains in an all-fiber configuration. The effect of the delivery fiber on the maximum output power is studied. A home-made 1064-nm seed laser with a 20-kHz linewidth is boosted to 129 W, and limited by stimulated Brillouin scattering (SBS) when the delivery fiber is 1.2 m long. By shortening the delivery fiber length to 0.7 m, the SBS threshold is increased efficiently and the maximum output power rises to 168 W with an 82.9% power conversion efficiency. The experimental results indicate that the output power can be further raised by shortening the delivery fiber length and increasing the pump power.