微弱光信号瞬态布里渊放大器的最佳工作点

为提高瞬态布里渊放大器的工作性能,利用包含噪声的瞬态布里渊放大理论模型数值模拟了放大器的信噪比、灵敏度、能量转换效率及信号放大率随指数增益系数G的变化规律,获得了放大器的最佳工作点G_opt.采用倍频Nd:YAG脉冲激光器,以CS₂和FC-72为非线性介质进行了实验验证.结果表明,抽运光脉冲相对于信号光脉冲延迟脉冲宽度的一半进入放大器时,G_opt可设在介质受激布里渊散射阈值增益G_th之上.对于共线型布里渊放大结构,G_opt为Gth的1.1—1.3倍;对于非共线型结构,G_opt可超过G_th1.3倍,实现近饱和放大.     

种子光与抽运光能量比对布里渊放大的影响

利用瞬态包含抽运抽空的受激布里渊散射理论模型对布里渊放大器进行了数值模拟,给出了布里渊放大器中能量提取效率、种子光放大率、种子光脉宽压缩率和抽运光脉宽压缩率随Stokes种子光与抽运光能量比的变化规律,并用实验进行了验证,理论与实验符合得较好 关键词： 布里渊放大器 提取效率 种子光与抽运光能量比     

抽运光参数对受激布里渊散射的影响

受激布里渊散射（SBS）系统中,具有一定带宽的抽运激光中会含有一定比例的Stokes散射光成分，此Stokes散射光成分经SBS介质后表面反射后，将形成种子光与抽运光联合入射，构成自种子光式SBS放大器.通过数值求解此SBS放大器型耦合波方程组，探讨了抽运光脉冲中所含Stokes散射光成分的比例、抽运激光波长、抽运光能量大小、入射的聚焦高斯光脉冲脉宽、相互作用时间等激光参数对SBS特征参数（Stokes散射光脉冲波形、材料内部最大应力的时间演化及空间分布、脉宽压缩效果、能量提取效率及Stokes散射光的共轭保真度）的影响.同时发现，SBS过程中产生的超声应力不仅会对SBS介质前表面造成破坏，还可能对焦点附近造成破坏；调整各激光参数还会使焦点附近优于前表面先破坏.数值模拟中采用的抽运光是聚焦高斯光束. 关键词： 受激布里渊散射 斯托克斯散射光 冲击应力 能量反射率     

水中受激布里渊散射脉冲的反常压缩

受激布里渊散射(SBS)具有脉冲压缩的特性,受激布里渊散射的脉冲宽度随着抽运能量的增大而变小,在水中可以达到几百皮秒的量级.本文在实验上观察到一种受激布里渊散射的脉冲宽度随抽运能量增大而变大的现象,这里称之为反常压缩.SBS的脉冲反常压缩和脉冲压缩与抽运光的强弱会聚情况有关.利用数值模拟,模拟了强弱会聚情况下抽运光在水中的传输规律,强弱会聚情况的抽运光对受激布里渊散射形成的有效增益长度不同:抽运光强会聚时有效增益长度短,形成SBS脉冲宽度的反常压缩;弱会聚时有效增益长度长,也就是正常的SBS脉冲压缩.     

大功率单频多芯光纤放大器中抑制受激布里渊散射的分析

针对多芯光纤完善了描述抽运光、信号光和Stokes信号的速率方程组.考虑了温差对受激布里渊散射的影响,利用有限元法求解温度分布方程组,分析了前向和后向抽运方式、对流系数、Stokes初始功率、光纤掺杂粒子密度和光纤长度对受激布里渊散射增益的影响.研究表明：后向抽运方式在抑制受激布里渊散射方面具有明显优势;减小对流系数有助于抑制受激布里渊散射;提高光纤掺杂密度能够加强抑制受激布里渊散射,同时也有助于提高光纤放大器的斜率效率.比较了在相同最佳光纤长度条件下,单芯和19芯光纤放大器的最高工作温度和受激布里渊散射 关键词： 光纤放大器 受激布里渊散射 大功率 有限元法     

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。     

抽运能量对受激布里渊散射光限幅特性的影响

采用布里渊噪声起源模型,数值模拟了受激布里渊散射介质CCl4对波长1064nm的Nd：YAG纳秒激光脉冲的传输特性及光限幅特性。入射抽运脉冲能量较低时,非线性介质对纳秒激光脉冲呈光学透明。入射抽运脉冲能量高于受激布里渊散射产生阈值后,透射脉冲峰值受限,脉宽压缩,能量趋于饱和,说明该光学系统同时具有光功率限幅和能量限幅的光限幅特性。利用理论模型模拟了如下光限幅参量：透射脉冲峰值功率、透射能量、能量透射率、脉宽压缩率依赖抽运光能量的变化关系。相应的理论模拟计算结果由实验进行验证,实验结果与理论模拟相符合。     

受激布里渊散射光限幅输出波形控制研究

数值模拟了种子场诱导受激布里渊散射光限幅过程的限幅输出波形特性.得到利用种子场控制限幅脉冲波形的规律：选取种子场脉冲宽度为抽运脉冲宽度的五倍，抽运脉冲相对于种子脉冲的延迟时间控制在与抽运脉冲宽度相当时，限幅输出波形最佳.限幅输出波形同时受抽运光功率影响，随抽运能量的增加，限幅脉冲功率不断下降，直至趋于0. 关键词： 受激布里渊散射 限幅输出波形 注入种子     

光学透明材料中瞬态SBS过程的数值研究

建立了描述噪声起振和Stokes种子光辅助起振的SBS（受激布里渊散射）理论模型，利用变步长有限差分方法数值求解了一维瞬态后向SBS耦合波方程组，得到了抽运光和散射光光强以及介质密度变化量的时空分布；研究了Stokes激光场对SBS反射率以及发生阈值的影响，发现Stokes种子光的存在大大减短了SBS过程的起振时间，并且在抽运光脉冲宽度固定的情况下使SBS的发生阈值大幅度降低；最后，探讨了瞬态受激布里渊过程对光学材料破坏的可能性并对今后的工作进行了展望. 关键词： 光学透明材料 受激布里渊散射 斯托克斯种子光 反射率     

利用染料片吸收控制受激布里渊散射功率限幅波形

理论研究了受激布里渊散射过程中功率限幅响应特性.采用高斯型入射脉冲，数值模拟了抽运参数(抽运峰值功率密度、抽运脉冲宽度)，介质参数(增益系数、声子寿命)和结构参数(聚焦透镜焦距、介质池长)等物理参数影响限幅输出波形中剩余峰的特性规律.对如何控制剩余峰进行了理论分析.研究表明，声子寿命较小、增益系数大的布里渊介质光限幅响应较好，声子寿命较大、增益系数小的布里渊介质光限幅响应较差；无法单纯通过控制布里渊介质参数来完全消除剩余峰.实验上采用染料片吸收剩余峰，获得接近平顶的限幅输出波形.     

Efficient Raman amplification of low divergent radiation in barium nitrate crystal

One stage first Stokes amplification by a factor of 1600 was demonstrated in a barium nitrate Raman amplifier. The amplified pulse energy was up to 63 mJ at pump energy of 208 mJ. M² factor of Stokes signal radiation was 1.5 while for amplified radiation 2.2. PACS 42.65.Dr     

Optical parametric chirped pulse amplification based on photonic crystal fibre

A compact two-stage optical parametric chirped pulse amplifier based on photonic crystal fibre is demonstrated.A 1064-nm soliton pulse is obtained in a home-made photonic crystal fibre (PCF) with femtosecond pulse pumping and then amplified to 2 mJ in an Nd:YAG regenerative amplifier.After the amplified pulses pass through the LBO crystal,the 532-nm double-frequency light with an energy of 0.8 mJ and a duration of over 100 ps at 10-Hz repetition rate is generated as a pump source in the following two-stage optical parametric amplification (OPA).The 850-nm chirped signal light gain from the stretcher is 1.5×10 4 in the first-stage OPA while it is 120 in the second-stage OPA.The total signal gain of optical parametric chirped pulse amplification (OPCPA) can reach 1.8×10 6.     

水中微弱光散射布里渊频谱选择性光放大研究

提出利用布里渊放大技术对水中微弱光散射信号进行频谱选择性光放大的方法.根据频率失谐布里渊放大理论模型，分析了Stokes种子光的线宽及频率失谐对布里渊放大的影响，研究了种子光放大率随水温及其功率密度变化的规律，理论和实验符合较好.研究表明，相同频率失谐时，线宽较大的种子光信号放大率更高.选取20℃的CS₂为放大介质，不同温度海水产生的Stokes种子光信号均能得到有效的布里渊放大.相同抽运光功率密度情况下，种子光的放大率随其功率密度的增加而减小，当种子光能量约为5μJ时，其放大率达10²以上.     

Picosecond optical parametric amplification of stimulated Raman as high peak-power source and ultra-sensitive preamplifier

We report the characteristics of the amplified stimulated Raman scattering (SRS) pulses generated in liquid benzene by a picosecond (ps) β-barium borate (BBO) optical parametric amplifier (OPA). When the OPA system was used as an energy amplifier for SRS pulses, with a pump energy of 2.4 mJ at 355 nm for the OPA, the maximum output energy of the amplified SRS was about 0.73 mJ for the signal and 0.18 mJ for the idler, the energy conversion efficiency was 30.4% from the pump beam to the amplified third order Stokes component at 635.1 nm. The total efficiency would be as high as 37.9% if the output of the idler is also included. The corresponding spectral line width of the amplified Raman pulse was 11.8 cm⁻¹ with a pulse width of 10.9 ps and a peak power of 67 MW. The OPA system was also used as an amplifier for very weak Raman signal, the slope gain factor of this amplifier was found to be as high as 4.1 × 10⁷ and the energy detection limit was as low as 14.8 aJ per pulse, or 48 photons at 635.1 nm, in particular. Such a detection limit corresponds to approximately 0.5 photons per pulse if the time-gate of the OPA is reduced to 150 fs and it is about the same as or even better than a recently report on the 0.75 photons detection limit for a 150-fs OPA of coherent signal at 800 nm.     

Parametric amplification in a microstructure fiber

We report an observation of parametric amplification with a gain greater than 30 dB in a 1.7-m microstructure fiber. It is found that there exists a peak pump power P_s. For peak pump powers less than P_s, the amplified twin (signal: longer wavelength, and idler: shorter wavelength) pulses are highly correlated, and the bandwidth of the amplified signal pulse broadens monotonically with pump power. For peak pump powers exceeding P_s, the correlation between the amplified twin pulses drops when the pump power increases, the bandwidth of the amplified signal pulse becomes narrower when the pump power increases, and the central wavelength of the amplified signal (idler) pulse shifts towards longer (shorter) wavelength, respectively, due to phase matching. PACS 42.65.Yj     

High-power tunable terahertz generation from a surface-emitted THz-wave parametric oscillator based on two MgO:LiNbO3 crystals

A high-powered tunable terahertz wave (THz-wave) has been parametrically generated via a surface-emitted THz-wave parametric oscillator (TPO) pumped by a multi-longitudinal-mode Q-switched Nd:YAG laser. The effective parametric gain length was enlarged by employing two MgO:LiNbO₃ crystals. The tunable THz-wave radiation from 0.8 to 2.8 THz was realized via varying phase-matching angle between the pump wave and the Stokes wave. The maximum THz-wave radiation was 173.9 nJ/pulse at 1.7 THz as the pump energy was 82 mJ, corresponding to an energy conversion efficiency of about 2.12 × 10⁻⁶ and a photon conversion efficiency of about 0.035%. The first-order, the second-order and the third-order Stokes waves were observed during the experiments.     

Compact high gain double-pass optical parametric chirped pulse amplifier

A novel double-pass noncollinear optical parametric chirped pulse amplifier based on an Yb³⁺-doped passively mode-locked fiber laser has been demonstrated in this paper. The signal was double-pass amplified in a single nonlinear crystal by a long pump pulse, and the signal and pump pulses of each pass were completely phase matched in the plane of the maximum effective nonlinearity. Net saturated gain of 2×10⁶ was achieved and the superfluorescence was suppressed by increasing the overlap time between the signal and pump pulses.     

Intracavity Raman conversion for 16-μm Stokes pulse generation in para-hydrogen

16-μm Stokes pulses were directly generated for the first time to our knowledge by an intracavity configuration for the para-hydrogen Raman laser. We have analyzed Stokes field growth using a focused gain model and designed a pump/Stokes cavity to satisfy CO₂ pump power and pulse duration requirements for Raman oscillation. The CO₂ laser oscillation with circular polarization was realized by seeding externally circularly polarized CO₂ radiation. An output energy of 2.4 mJ was obtained with the output coupler of 0.5% transmittance, which indicated that 420 mJ of Stokes pulse energy was stored inside the cavity. This suggests that a much higher energy can be extracted by the optimization of cavity parameters. Received: 18 November 1998 / Published online: 26 May 1999     

Continuous wave stimulated Raman scattering of benzene by fluorescence enhancement in hollow fused silica fiber

We present continuous wave stimulated Raman scattering (SRS) of benzene (C₆H₆) influenced by the fluorescent dye m‐cresol purple in a hollow fused silica fiber (HFSF). Because of the transmission loss of the HFSF filled with C₆H₆, the SRS occurs when the Stokes gain equals the transmission loss, with the loss taken at the Stokes wavelength. The 992 cm⁻¹ stimulated Stokes line has been obtained at the pump wavelength 658 nm, which cannot be obtained at 532 nm because the Stokes wavelength (562 nm) does not locate in the transmission loss. Also, the pump power is 35 mW with m‐cresol purple which is much lower than 800 mW without the dye. The profile of the 992 cm⁻¹ stimulated Stokes is changed, and the weak shoulders of the profile are amplified by fluorescence. These results are expected to be of relevance in applications on the tunable Raman laser at new wavelengths. Copyright © 2011 John Wiley & Sons, Ltd.     

Efficient rotational Raman conversion of XeCl laser radiation in H2

2 is investigated with a XeCl laser. It is shown that energy conversion to rotational Stokes radiation can be efficiently obtained by properly choosing the focusing geometry of pump radiation and the pressure of the Raman medium even at moderate pulse energies. Energy conversion to the first rotational Stokes at 313.8 nm with efficiencies as high as 38% is obtained with a circularly polarised XeCl pump beam of 10-mJ energy focused in 30 bar of H₂. The spectral and optical characteristics of the pump and the rotational Stokes radiation are analysed. Received: 29 May 1996/Revised version: 7 March 1997