Chirped-pulse four-wave Raman mixing in molecular hydrogen

Four-wave Raman mixing (FWRM) in molecular hydrogen was studied using chirped pump and Stokes pulses emitting at 802 and 1,203 nm, respectively. The group delay dispersion (GDD) of the anti-Stokes pulse was examined employing a frequency-resolved optical gating system at different GDDs of the pump and Stokes pulses (0 or ±1,000 fs²). As a result, the energy and the sign of GDD for the anti-Stokes pulse remained unchanged, when the pump and Stokes pulses had the GDD with the same sign. When the sign was not the same, the energy decreased and only the portion useful for resonant FWRM was converted into a Raman emission. This technique has a potential for use in compensation of dispersion by passing the negatively chirped high-order Raman sidebands through the optics with positive chirps in the spectral region from the deep-ultraviolet to the near-infrared, to generate multiple transform-limited Raman pulses and then to produce an ultrashort optical pulse by a Fourier synthesis of these Raman emissions.     

Subwatt threshold cw Raman fiber-gas laser based on H2-filled hollow-core photonic crystal fiber

We report on what is, to our knowledge, the first cw pumped Raman fiber-gas laser based on a hollow-core photonic crystal fiber filled with hydrogen. The high efficiency of the gas-laser interaction inside the fiber allows operation in a single-pass configuration. The transmitted spectrum exhibits 99.99% of the output light at the Stokes wavelength and a pump power threshold as low as 2.25 W. The study of the Stokes emission evolution with pressure shows that highly efficient Raman amplification is still possible even at atmospheric pressure. The addition of fiber Bragg gratings to the system, creating a cavity at the Stokes wavelength, reduces the Raman threshold power below 600 mW.     

Highly efficient Cherenkov radiation generation in the irregular point of hollow-core photonic crystal fiber

Highly efficient Cherenkov radiation(CR) is generated by the soliton self-frequency shift(SSFS) in the irregular point of a hollow-core photonic crystal fiber(HC-PCF) in our laboratory.The impacts of pump power and wavelength on the CR are investigated,and the corresponding nonlinear processes are discussed.When the average power of the 120 fs pump pulse increases from 500 mW to 700 mW,the Raman soliton shifts from 2210 nm to 2360 nm,the output power of the CR increases by 2.3 times,the maximum output power ratio of the CR at 539 nm to that of the residual pump is calculated to be 24.32:1,the width of the output optical spectrum at the visible wavelength broadens from 35 nm to 62 nm,and the conversion efficiency η of the CR in the experiment can be above 32%.     

基于空芯微结构光纤拉曼探针的实验研究

表面增强拉曼散射(SERS)技术可有效增强样品分子的拉曼信号,对生物分子检测具有较高的灵敏性,因此在生化方面有着许多潜在的应用.而将空芯微结构光纤与SERS技术相结合不仅能够远端实时、分布式地检测,同时还可以增加光场与待测物的有效作用面积,减少传统光纤探针无法避免的石英背景信号等问题.本文基于空芯微结构光纤进行SERS探针的制备及性能测试研究,利用真空物理溅射法在空芯光纤内镀纳米Ag膜,从而制备成SERS探针,通过实验检测不同浓度的罗丹明6G (R6G)酒精溶液的拉曼信号.结果表明,在探针的近端正面成功探测到了浓度低至10~(-9)mol/L的R6G拉曼信号,在探针的远端反面探测到的浓度可小于10~(-6)mol/L.该实验结果为研究高灵敏度的SERS探针提供了一种新的手段.     

Hollow-core photonic crystal fiber-optic probes for Raman spectroscopy

We have implemented a new Raman fiber-optic probe design based on a hollow-core photonic-crystal excitation fiber surrounded by silica-core collection fibers. The photonic-crystal fiber offers low attenuation at the pump radiation wavelength, mechanical flexibility, high radiation stability, and low background noise. Because the excitation beam is transmitted through air inside the hollow-core fiber, silica Raman scattering is much reduced, improving the quality of the spectra obtained using probes of this design. Preliminary results show that the new probe design decreases the Raman background from the silica by approximately an order of magnitude compared to solid-core silica Raman probes.     

Temperature effects on supercontinuum generation using a continuous-wave Raman fiber laser

We describe the effect of temperature variations on supercontinuum (SC) generation in optical fibers using a continuous-wave (CW) Raman fiber laser as a pump. We achieve supercontinuum generation by pumping only ∼2 W of power into a 7 km-long nonzero dispersion-shifted fiber (NZDSF) in the region of small anomalous dispersion. In these conditions, the supercontinuum builds up basically on modulational instability and Raman. At room temperature, the supercontinuum covers effectively the S, C and L transmission bands defined by the International Telecommunication Union (ITU). Temperature tuning of the fiber environment provides a means of tuning the fiber dispersion, and thus a means of changing the width and shape of the supercontinuum spectrum. We demonstrate a 27% increase in the 10-dB SC width. We believe that the application of this new tuning mechanism to other experimental configurations using pulsed sources might be used to produce extremely broad supercontinuums.     

压缩系数可控的光脉冲压缩器

提出了一种压缩系数可控的光脉冲压缩器.该脉冲压缩器主体包括梳状色散分布光纤（CDPF）和喇曼放大器两部分.梳状色散分布光纤由多段色散位移光纤和单模光纤拼接而成,具有喇曼增益介质的功能.对一给定的光脉冲序列,可以通过控制泵浦功率实现脉冲压缩器的压缩率可调.通过对非线性薛定锷方程的数值模拟,这一光脉冲压缩器的性能得到了验证.当光脉冲在CDPF中传输时,脉冲得到压缩并且被放大.对喇曼泵浦功率来改变可以得到不同的输出脉宽.通过优化CDPF的结构,得到了喇曼泵浦功率和压缩系数之间很好的线性关系.     

Nonlinear copropagation of two optical pulses of different frequencies in photonic crystal fibers

A theoretical investigation of the nonlinear copropagation of two optical pulses of different frequencies in a photonic crystal fiber is presented. Different phenomena are observed depending on whether the wavelength of the signal pulse is located in the normal or the anomalous dispersion region. In particular, it is found that the phenomenon of pulse trapping occurs when the signal wavelength is located in the normal dispersion region while the pump wavelength is located in the anomalous dispersion region. The signal pulse suffers cross-phase modulation by the Raman shifted soliton pulse and it is trapped and copropagates with the Raman soliton pulse along the fiber. As the input peak power of the pump pulse is increased, the red-shift of the Raman soliton is considerably enhanced with the simultaneous further blue-shift of the trapped pulse to satisfy the condition of group velocity matching. PACS 42.65.Tg; 42.81.Dp     

Generation of few-cycle laser pulses:Comparison between atomic and molecular gases in a hollow-core fiber

We numerically study the pulse compression approaches based on atomic or molecular gases in a hollow-core fiber.From the perspective of self-phase modulation(SPM), we give the extensive study of the SPM influence on a probe pulse with molecular phase modulation(MPM) effect. By comparing the two compression methods, we summarize their advantages and drawbacks to obtain the few-cycle pulses with micro- or millijoule energies. It is also shown that the double pump-probe approach can be used as a tunable dual-color source by adjusting the time delay between pump and probe pulses to proper values.     

Extended-range optical time domain-reflectometry system at 1.65 microm based on delayed Raman amplification

We describe both theoretical and experimental results obtained in an investigation of a new technique for increasing the dynamic range of 1.65-microm optical time domain-reflectometry (OTDR) systems. The technique utilizes delayed Raman amplification of a 1.65-microm signal pulse by a 1.53-microm pump pulse. Amplification occurs when the two pulses overlap, and this position is determined by the initial delay between the pulses and the fiber dispersion. An increase in dynamic range of 17.5 dB has been observed, and the OTDR backscattered Rayleigh signal was detected up to 100 km. No significant noise penalty is introduced as a result of the directionality of the Raman gain.     

Stokes amplification regimes in quasi-cw pumped hydrogen-filled hollow-core photonic crystal fiber

Pure rotational stimulated Raman scattering spectra containing nine strong spectral components were generated from a approximately 11 m long hollow-core photonic crystal fiber filled with hydrogen and pumped with nanosecond pulses having energies around 100-300 nJ. Observation of both transient and steady-state scattering threshold behavior is reported. Passage from the transient to the steady state is observed with a pulse as long as 14 ns. Convenient analytical expressions for energy and power threshold are deduced for the present configuration.     

Ultrahigh efficiency laser wavelength conversion in a gas-filled hollow core photonic crystal fiber by pure stimulated rotational Raman scattering in molecular hydrogen

We report on the generation of pure rotational stimulated Raman scattering in a hydrogen gas hollow-core photonic crystal fiber. Using the special properties of this low-loss fiber, the normally dominant vibrational stimulated Raman scattering is suppressed, permitting pure conversion to the rotational Stokes frequency in a single-pass configuration pumped by a microchip laser. We report 92% quantum conversion efficiency (40 nJ pulses in 2.9 m fiber) and threshold energies (3 nJ in 35 m) more than 1 x 10(6) times lower than previously reported. The control of the output spectral components by varying only the pump polarization is also shown. The results point to a new generation of highly engineerable and compact laser sources.     

Multiwavelength L-band fiber laser with bismuth-oxide EDF and photonic crystal fiber

A multiwavelength laser comb using a bismuth-based erbium-doped fiber and 50 m photonic crystal fiber is demonstrated in a ring cavity configuration. The fiber laser is solely pumped by a single 1455 nm Raman pump laser to exploit its higher power delivery compared to that of a single-mode laser diode pump. At 264 mW Raman pump power and 1 mW Brillouin pump power, 38 output channels in the L-band have been realized with an optical signal-to-noise ratio above 15 dB and a Stokes line spacing of 0.08 nm. The laser exhibits a tuning range of 12 nm and produces stable Stokes lines across the tuning range between Brillouin pump wavelengths of 1603 nm and 1615 nm.     

互注入锁定产生双波长可调谐光脉冲的实验研究

报道了两个增益开关调制的法布里-珀罗半导体激光器互注入锁定实验方案,可产生双波长可调谐光脉冲。该装置对两个结构基本相同的法布里珀罗半导体激光器做增益开关调制,产生多模光脉冲输出;然后利用两个光纤光栅作为滤波元件,通过调节可调谐光延迟线(VODL2),可使得双波长光脉冲在两个法布里-珀罗激光器之间相互注入锁定．从而在输出端得到高边模抑制比的双波长光脉冲输出。然后再通过应力作用在两个光纤光栅上以改变它们的反射波长和适当调整另一个可调谐光延迟线(VODLI)长度．可得到不同的双波长的光脉冲输出,而重复频率保持524．6MHz不变。在13．2nm调谐范围内边模抑制比高于25dB．系统稳定且波长调节方便。     

Raman continuum generation at 1.0–1.3 μm in passively mode-locked fiber laser based on nonlinear polarization rotation

We experimentally demonstrate cascaded Raman scattering continuum generation at 1.0–1.3 μm with pulse energy of 47 nJ by utilizing a commercially available all-normal-dispersion single-mode fiber (SMF) with a low threshold pump power. We achieve passively mode-locked all-fiber lasing by nonlinear polarization rotation technique. Continuous Raman gain is obtained by the same SMF in every roundtrip because of the ring-cavity configuration, which makes it possible to achieve high-order Raman Stokes waves by a short fiber length. Limited by the pump power, the maximum output average power and peak power are 87.4 and 339 mW, but it has great potential to be improved. These make it an ideal continuum source for many applications, such as optical coherence tomography.     

Fiber parametric oscillator for the 2 μm wavelength range based on narrowband optical parametric amplification

We describe a widely tunable synchronously pumped coherent source based on the process of narrowband parametric amplification in a dispersion-shifted fiber. Using an experimental fiber with a zero-dispersion wavelength of 1590 nm and pump wavelengths of 1530 to 1570 nm yields oscillations at 1970 to 2140 nm-the longest reported wavelength for a fiber parametric oscillator. The long-wavelength oscillations are accompanied by simultaneous short-wavelength oscillations at 1200 to 1290 nm. The parametric gain is coupled to stimulated Raman scattering. For parametric oscillations close to the Raman gain peak, the two gain processes must be discriminated from each other. We devised two configurations that achieve this discrimination: one is based on the exploitation of the difference in group delay between the wavelengths where Raman and parametric gain peak, and the other uses intracavity polarization tuning.     

Polarization-dependent efficient Cherenkov radiation at visible wavelengths in hollow-core photonic crystal fiber cladding

Efficient Cherenkov radiation (CR) is experimentally generated by a soliton self-frequency shift (SSFS) in a knot of hollow-core photonic crystal fiber (HC-PCF). When the angle of the half-wave plate is rotated from 0° to 45° , the Raman soliton shifts from 2227 to 2300 nm, the output power of the CR increases 8.15 times, and the maximum output power ratio of the CR at 556 nm to the residual pump is estimated to be 20:1. The width of the output optical spectrum at visible wavelengths broadens from 25 to 45 nm, and the conversion efficiency of the CR can be above 28%. Moreover, the influences of the pump polarization and wavelength on the CR are studied, and the corresponding nonlinear processes are discussed.     

Character analysis of slow light with communication waveband in fiber optical parametric amplifier

The comprehensive characters of delay time and delay bandwidth with distortion, signal wavelength dependence and delay bandwidth product in fiber optical parametric amplifier are investigated theoretically when the pump locates at anomalous dispersion region. The delay character of single pulse, data stream with certain bit pattern and pseudo random data stream are analyzed comprehensively. In this category of fiber optical parametric amplifier, the delay time and delay bandwidth product vary from signal wavelengths and bit rates. For the same nonlinear coefficient, pump power and fiber length, the diverse dependences of bit pattern in different signal wavelengths are found by numerical simulation. The impacts of pump power and fiber length on the delay character and distortion are also discussed. These relations are conductive to the design of slow light based on fiber optical parametric amplifier in telecommunication waveband.     

测量时间分辨拉曼谱实验装置的研制

测量时间分辨拉曼谱实验装置的研制王晶晶金榕周赫田邹英华（北京大学物理系人工微结构及介观物理国家重点实验室北京１００８７１）孙志勇（北京理工大学四系４３１教研室北京１０００８１）ＴｈｅＥｘｐｅｒｉｍｅｎｔａｌＳｅｔｕｐｆｏｒＴｉｍｅ－ＲｅｓｏｌｖｅｄＲ...     

Femtosecond pulse compression in a hollow-core photonic bandgap fiber by tuning its cross section

We present a numerical study of soliton pulse compression in a seven-cell hollow-core photonic bandgap fiber. We analyze the enhancement of both the compression factor and the pulse shape quality of 360 nJ femtosecond pulses at the wavelength of 800 nm by tuning the cross section size of the fiber. We use the generalized non-linear Schrödinger equation in order to modeled the propagation of light pulses along the fiber. Our numerical results show that output compressed pulses can be obtained, in a propagation length of 31 cm, with a compression factor of 5.7 and pulse shape quality of 77% for a reduction of 4.5% of the cross section size of the fiber. The predicted compression factor is 3 times larger than that experimentally obtained in such propagation length of the pulse in a hollow-core photonic bandgap fiber.