Generation of pure two-beam interference grating structures in an optical fiber with a femtosecond infrared source and a phase mask

Fiber Bragg gratings were fabricated in all-silica core fiber by focusing 125-fs 800-nm pulses with an 80-mm lens through a phase mask with 4.28-microm pitch onto a fiber sample. When the phase-mask-fiber separation was 5 mm the observed structure was clearly the result of two-beam interference between the +/- 1 orders. The elimination of the remaining 9 orders is a consequence of the walk-off experienced by the mask orders and the short duration of the femtosecond pulse. This effect is unique to the fabrication of Bragg gratings with femtosecond sources and would not be observed with a longer pulse duration or incoherent UV sources.     

Inscription of fiber Bragg gratings with femtosecond pulses using a phase mask scanning technique

We report on the fabrication of highly efficient fiber Bragg gratings (FBG) in non-photosensitive fibers based on nonlinear absorption of fs laser light. Up to 40 mm long gratings with a transmission of T=-25 dB at the Bragg reflection wavelength were obtained and their coupling constant determined by spectral analysis. Therefore, a phase mask scanning technique with appropriate control of the focus was established. PACS  42.60.-v; 42.62.-b; 42.65.Re; 42.79.Dj; 42.81.-i     

Multiple-beam interference fringes applied to investigate the index profile of the optical fiber dip

A numerical-based phase shift method is presented to study optical fibers having a dip in the refractive index profile at the center of the core. Mathematical expressions for the shape of multiple-beam Fizeau fringes crossing a graded-index optical fiber (GRIN) comprising a central graded-index dip or GRIN fiber with a constant refractive index dip, immersed in a silvered liquid wedge, are derived. The index profile parameters of the central dip are determined by comparing the theoretical fringe shape across the fiber cross-section and the experimental fringe shape obtained from micro-interferograms of the wedge interferometer.     

Generation of single intense short optical pulses by ultrafast molecular phase modulation

Pulses with durations below 4 fs have been generated using the method of ultrafast molecular phase modulation. A laser pulse shorter than the molecular vibrational or rotational period obtains spectral broadening during propagation along a hollow waveguide filled with previously impulsively excited Raman active gases. The induced time dependent phase, frequency, and frequency chirp are controllable by changing the delay between excitation and probe pulse within the molecular vibrational period.     

Dynamics of parabolic pulses in an ultrafast fiber laser

We report experimental and numerical results on the dynamics and propagation of parabolic pulses in a passively mode-locked ytterbium-doped fiber laser. Experimental data and numerical simulations are shown to match. Particular importance is attached to the pulse-shaping process in the different sections of the resonator. The paramount role of the gain fiber and saturable absorber in the laser is explicated.     

Noiselike pulses with a broadband spectrum generated from an erbium-doped fiber laser

An erbium-doped fiber laser that produces a train of intense noiselike pulses with a broadband spectrum and a short coherence length is reported. The noiselike behavior was observed in the amplitude as well as in the phase of the pulses. The maximum spectral width obtained was 44 nm. The high intensity and the short coherence length of the light were maintained even after propagation through a long dispersive fiber. A theoretical model indicates that this mode of operation can be explained by the internal birefringence of the laser cavity combined with a nonlinear transmission element and the gain response of the fiber amplifier.     

Shaping, propagation and characterization of ultrafast pulses in optical fibers

The characterization of medium- to low-energy shaped pulses at 1.55 7m through frequency-resolved optical gating (FROG) is illustrated. This capability enables the study of ultrafast pulse propagation through optical fibers. The phase dynamics detected furnishes insight on pulse evolution, specifically on soliton formation - a subject of great importance for telecommunication applications. The combination of shaping and propagation of ultrafast pulses in fibers is examined theoretically using an adaptive pulse-shaping model, based on genetic algorithms, that furnishes optimized pulse shapes for fiber propagation.     

High peak power optical pulses generated with a monolithic master-oscillator power amplifier

We present results on a monolithic semiconductor-based master-oscillator power amplifier (MOPA) combining a distributed-feedback (DFB) laser and a tapered amplifier on a single chip. The MOPA reaches an output power of almost 12 W at an emission wavelength around 1064 nm in continuous-wave operation. Pulses with a length of around 100 ps can be obtained either by injecting nanosecond current pulses into the tapered amplifier alone or into both the DFB laser and the tapered amplifier. In the latter case, pulses with a width of 84 ps, a peak power of 42 W, and a spectral width of 160 pm are generated.     

Hydrogen loading for fiber grating writing with a femtosecond laser and a phase mask

The threshold for the fabrication of fiber Bragg gratings with ultrafast 800-nm radiation and a phase mask was studied in SMF-28 and all-silica core fiber by use of 125-fs pulses. High-pressure molecular hydrogen loading (H2 loading) was observed to significantly lower the grating writing threshold in standard Ge-doped telecommunication fiber. No reduction was observed with all-silica core fiber. The index change appeared to be confined to the Ge-doped core region of the fiber. Gratings in H2-loaded SMF-28 had thermal annealing behavior similar to UV-induced gratings. Unlike UV-induced H2-loaded gratings, no absorption associated with Ge-OH defect formation was observed.     

光孤子脉冲在光纤放大器中的传播

本文建立了包括增益色散,受激喇曼散射,双光子吸收效应的理论模型,讨论了光孤子脉冲在光纤放大器中的传播.数值计算结果表明:在反常色散范围内,光孤子的放大是不稳定的.增益色散导致光脉冲对称分裂,而受激喇曼散射则导致不对称的分裂.在增益色散和受激喇曼散射的共同作用下可获得新的时域和频域特征.有限带宽的放大能抑制受激喇曼散射引起的自频移.本文对有啁啾的光孤子脉冲的放大也进行了分析.     

基于飞秒锁模光纤激光脉冲基频光的差频产生红外光梳

报道了一种基于飞秒锁模光纤激光脉冲基频光的光纤型差频产生(DFG)红外光梳及其研制技术.基于自主研制的重频锁定200 MHz飞秒锁模掺铒光纤激光器,经啁啾脉冲光纤放大与超连续谱产生技术,优化近零色散OFS光纤(型号:OFS-980-20)长度,结合可调延时线,获得了精准同步的基频双色脉冲;以GaSe为非线性晶体,利用光整流技术,产生了可在6—10μm范围内宽带调谐的DFG红外光梳,光梳最大光谱宽度可达1.3μm.这种光纤型远红外光梳可望在分子光谱精密测量等领域发挥重要作用.     

光纤水听器相位生成载波解调非线性因素的抑制方法

为了消除非线性因素对相位生成载波解调结果的干扰,降低相位生成载波解调过程中低通滤波器的频响特性对相位生成载波解调结果的影响,该文提出了一种基于扩展卡尔曼滤波椭圆参数估计的相位生成载波微分交叉相乘解调方法。该方法在综合考虑各种非线性因素对相位生成载波微分交叉相乘解调结果的影响的基础上,对传统的相位生成载波微分交叉相乘解调过程进行了改进。经过计算机仿真和实验验证,该文所提出的扩展卡尔曼滤波微分交叉相乘方法能够有效地抑制非线性因素对相位生成载波解调结果的干扰,相比传统的相位生成载波微分交叉相乘方法,其信噪比提高了35.0 d B,总谐波失真降低了30.7 dB,信噪谐波比提高了31.0 dB,且扩展卡尔曼滤波微分交叉相乘方法受解调过程中低通滤波器频响特性的影响较小。     

Noise-like pulses generated at high harmonics in a partially-mode-locked km-long Raman fiber laser

We present a 5-km-long Raman fiber laser that delivers pulses at high harmonics of the fundamental cavity repetition rate, up to 1 GHz. The observed nanosecond pulses that propagate in an anomalous dispersion regime possess a complex noise-like structure with a coherence time of around 1 picosecond.     

Large temporal narrowing of subnanosecond pulses in a low-birefringence optical fiber

A temporal shortening by a factor higher than 10 of subnanosecond pulses launched into a low-birefringence optical fiber is achieved by polarization filtering at the output. By means of the theoretical approach, in good agreement with experimental measurements, the impact of the nonlinear polarization rotation and of the coherent coupling phenomena is pointed out.     

Femtosecond spectroscopy of coherent anti-Stokes Raman scattering with frequency-tunable chirped pulses generated in a microstructure fiber

A new scheme of chirped-pulse femtosecond coherent anti-Stokes Raman scattering spectroscopy is proposed and experimentally implemented. A theory of this modification of coherent nonlinear spectroscopy is developed. We use this approach to show that a linear time-frequency mapping defined by linearly chirped pulses allows the spectra of nonlinear response of a medium to be measured by varying the delay time between the pump pulses. Microstructure fibers with a special dispersion profile are at the heart of the experimental implementation of this technique. Such fibers are ideally suited for the generation of frequency-tunable pulses with a smooth envelope and a controlled chirp. We present the results of experimental characterization of the envelope, spectrum, and chirp of anti-Stokes pulses generated in microstructure fibers by femtosecond Cr:forsterite-laser pulses. These frequency-tunable anti-Stokes pulses produced and shaped in microstructure fibers are then employed for coherent anti-Stokes Raman scattering spectroscopy of toluene solution.     

Phase-only sampled 45 channel fiber Bragg grating written with a diffraction-compensated phase mask

A novel phase-only sampled 45 channel fiber Bragg grating (channel spacing, 100 GHz) with high interchannel and intrachannel reflection uniformity and precise dispersion matching capability to an 80 km SMF-28 fiber is demonstrated. This grating is fabricated by the side-writing technique, in which phase information of continuous phase-only sampling is completely incorporated into a diffraction-compensated phase mask.     

Spectral amplitude and phase measurement of ultrafast pulses using all-optical differential tomography

We demonstrate a simple, all-optical, fiber-based method for characterizing the spectral amplitude and phase of ultrafast pulses using a differential tomographic measurement realized via four-wave mixing. The technique is applied to subpicosecond pulses in the C-band of the telecommunication spectrum. Characterization of amplified pulses and propagation through dispersive media is demonstrated and compared with autocorrelation measurements and calculated predictions. We show how our approach can be extended to larger bandwidths in similar systems, extending tomographic reconstruction of coherent fields to nearly an octave of bandwidth while maintaining a robust, waveguide-based geometry.     

Absolute phase effect in ultrafast optical responses of metal nanostructures

We predict that nonlinear ultrafast electron photoemission by strong optical fields and, potentially, other nonlinear optical responses of metal nanostructures significantly depend on the absolute (carrier–envelope) phase of excitation pulses. Strong enhancement of the local optical fields produces these responses at excitation intensities lower by order(s) of magnitude than for known systems. Prospective applications include control of ultrafast electron emission and electron injection into nanosystems. A wider class of prospective applications is the determination of the absolute phase of pulses emitted by lasers and atoms, molecules, and condensed matter at relatively low intensities. PACS  78.67.-n; 78.47.+p; 79.60.Jv; 73.20.Mf     

Formation of parabolic pulses in inhomogeneous fiber optical amplifiers

The conditions for forming pulses with parabolic envelope asymptotics in inhomogeneous fiber optical amplifiers are investigated. The profile of normal group-velocity dispersion, which ensures formation of parabolic pulses with a reduced rate of frequency modulation, is determined. It is shown by numerical simulation that the presence of chirp in the input pulse may cause narrowing of the spectrum at a certain energy.     

Selecting optical patterns with spatial phase modulation

Optical pattern selection by use of spatial phase modulation is investigated experimentally in a photorefractive feedback system. A feedback mirror with spatially periodic phase modulation is used for selection of different spatial patterns. Local phase modulation is used to create patterns with coexisting spatial symmetries. The experimental results are consistent with numerical simulations based on a model with a cubicly nonlinear medium.