Nonlinear optical compression of Er3+-fiber amplified1.5-μm laser diode pulses

Femtosecond optical pulses of 110-200-fs width have been produced using a gain-switched distributed-feedback semiconductor laser followed successively by a linear compression stage and a nonlinear compression stage. Analysis is focused on this last stage where pulses with peak powers corresponding to 10and 12-order solitons are fed at the fiber input end. Experimental results are well described using both the modified nonlinear Schrodinger equation and an accurate intensity and phase model of the gain-switched laser diode. Experiments are shown to be correctly described only if both intrapulse stimulated Raman scattering and third-order dispersion are taken into account. Guidelines are then given to optimize the nonlinear fiber compression using laser diodes and fiber amplifiers. The influence of the third-order dispersion in the fiber compressor is first evaluated. Second, the nonlinear self-phase modulation induced in the fiber amplifier is studied. It is shown to be the main factor limiting any further pulse shortening with this technique     

Power Scaling on Efficient Generation of Ultrafast Terahertz Pulses

We have investigated power scaling for the efficient generation of the broadband terahertz (THz) pulses. These THz short pulses are converted from ultrafast laser pulses propagating in a class of semiconductor electrooptic materials. By measuring the dependence of the THz output on the pump beam in terms of incident angle, polarization, azimuthal angle, and pump intensity, we have precisely determined the contributions made by optical rectification, drift of carriers under a surface or external field, and photo-Dember effect. When a second-order nonlinear material is pumped below its bandgap, optical rectification is always the mechanism for the THz generation. Above the bandgap, however, the three mechanisms mentioned earlier often compete with one another, depending on the material characteristics and pump intensity. At a sufficiently high pump intensity, optical rectification usually becomes the dominant mechanism for a second-order nonlinear material. Our analysis indicates that second-order nonlinear coefficients can be resonantly enhanced when a material is pumped above its bandgap. In such a case, the THz output power and normalized conversion efficiency can be dramatically increased. We have also analyzed how the THz generation is affected by some competing processes such as two-photon absorption.     

Optical signal inversion phenomena due to negative nonlinear absorption effect

A negative input-output characteristic was obtained in an erbium-doped yttrium aluminum garnet crystal which has multiple-level energy structure: absorption can occur between a ground level and an excited level (⁴I−⁴I) and between two other excited levels (⁴I−²H) in Er³⁺. It occurred at a wavelength between 786.6 and 788.4 nm by variation of the incident laser intensity in the intensity range of 60 nW/cm² to 100 W/cm². It is considered that an enhanced absorption occurs due to the excited-state absorption from ⁴I to ²H, its spectra matching the wavelength dependence of the incident laser In addition, optical signal inverter phenomenon was observed using a laser diode modulated at 10 MHz. By considering multiple excited-state absorption in detail, active functions such as an optical signal inverter could be derived from the negative nonlinear absorption effect. © 1997 Scripta Technica, Inc. Electr Eng Jpn, 120(4): 74–80, 1997     

Recent Advances in Optical Processing Techniques Using Highly Nonlinear Bismuth Oxide Fiber

We report our recent studies on nonlinear processing of optical signals using a 35-cm highly nonlinear bismuth oxide fiber (Bi-NLF). Our findings are based on self-phase modulation, cross-phase modulation, and four-wave mixing in the Bi-NLF. We demonstrate applications of the nonlinear techniques in optical signal regeneration, tunable optical delay, stabilization of multiwavelength laser source, tunable optical pulse generation, microwave photonic carrier frequency multiplication, and all-optical wavelength conversion.     

Optical parametric amplification of femtosecond ultraviolet laser pulses

Broad-band amplification of femtosecond laser pulses using the scheme of noncollinear optical chirped pulse parametric amplification is modeled. The effect of two-photon absorption at the pump wavelength was also taken into account. The signal pulses range from 220 to 410 nm with pump pulses at 267, 248, and 213 nm. The best four crystals chosen among 12 possible ones are BBO, KDP, CLBO, and LB4. In an experiment, 30-fs laser pulses at 400 nm were amplified in a BBO crystal pumped by 267 nm pulses, exhibiting a single pass gain of 3550. The gain was found spectrally flat within the available 17-nm bandwidth of the signal pulse.     

Theory of relativistic optical harmonic generation

We present a theoretical model of optical harmonic generation excited by laser beams sufficiently intense that relativistic effects are important. This model shows that, under relativistic conditions, third-harmonic generation can be excited with comparable efficiency by either linear or circularly polarized light. This result is to be contrasted with experience from traditional (nonrelativistic) nonlinear optics, where group-theoretical arguments show that third-harmonic emission cannot occur under circularly polarized excitation. These results are in good agreement with the observed polarization dependence of the third-harmonic emission reported recently in an experiment conducted under conditions such that relativistic effects are important. Our theoretical model also predicts that all even and odd harmonies of the fundamental laser frequency are emitted in the near-forward direction with an intensity that increases with that of the incident laser field     

Ultrafast all-optical switching in multiple-quantum-well Y-junctionwaveguides at the band gap resonance

An integrated Y-junction switch is used to demonstrate all-optical switching due to two mechanisms in GaAs-AlGaAs multiple quantum well structures at room temperature. Carrier-induced nonlinearities are compared to nonlinearities due to the optical Stark effect for ultrafast operation. In the first case, device geometry is exploited by a two-pulse switching process, whereby one control pulse turns the device on and a second control pulse turns the device off. Time gating of signal pulses within an 8 ps window was realized in our experiments. In the second case, the nonlinearity is ultrafast, and hence switching and recovery take place during the control pulse evolution. Consecutive switching events spaced 1.7 ps apart have been achieved. In our measurements, two-photon absorption played a significant role in the switching characteristics of the device. In order to access the carrier-induced nonlinearity, the pulse energy that is needed for switching (9 pJ in this case) results in a very high peak intensity that leads to two-photon absorption. This is observed as a strongly induced absorption exactly at the precise zero time delay between the control and signal pulses. In the second case, two-photon absorption competes directly with the optical Stark effect since both are instantaneous intensity dependent effects. Furthermore, the optical Stark effect appears to saturate at a low intensity level (0.9 pJ or 200 MW/cm² ); consequently only incomplete switching with a 2:1 contrast ratio was observed, whereas a 9:1 switching contrast was achieved with the carrier-induced nonlinearity     

High-Power All-Fiber-Based Narrow-Linewidth Single-Mode Fiber Laser Pulses in the C-Band and Frequency Conversion to THz Generation

Based on highly Er/Yb codoped phosphate fibers, we have implemented all-fiber-based narrow-linewidth single-mode (SM) pulsed fiber lasers in master oscillator and power amplifier configuration. Two approaches were used to achieve the narrow-linewidth pulsed fiber laser seeds: 1) an all-fiber-based active Q-switched fiber laser using an actuator and 2) a directly modulated single-frequency continuous-wave fiber laser. Both the fiber laser seed pulses from the two approaches have the transform-limited spectral linewidth. Based on a newly developed large-core SM highly Er/Yb codoped phosphate fiber, the peak power of SM pulses can be scaled to more than 50 kW with transform-limited linewidth and diffraction-limited beam quality. These high-power narrow-linewidth SM fiber laser pulses have been successfully used to generate coherent terahertz (THz) waves based on parametric processes in a nonlinear optical crystal. The peak power of this fiber-based THz source can reach 26.4 mW.     

Experimental and theoretical study of semiconductor laser dynamics due to filtered optical feedback

We report experimental results on the nonlinear dynamical response of a semiconductor laser subjected to time-delayed (>5 ns), frequency selective, optical feedback from a Fabry-Pe/spl acute/rot interferometer type of filter. Three regimes of interest, based on the relative value of the filter bandwidth with respect to the relevant laser parameters (relaxation oscillation frequency and external cavity mode spacing), are identified, viz. a wide filter case, an intermediate filter width case, and a narrow filter case. The dynamical response of the laser is shown to be quite different in each of these regimes. The principal results are 1) the laser's linewidth enhancement factor, coupled with the nonlinear response of the filter, can be exploited to induce nonlinear dynamics in the instantaneous optical frequency of the laser light on a time scale related to the time-delay of the feedback, 2) a mode mismatch effect which arises from a detuning between the filter center frequency and the nearest external cavity mode and manifests itself in a reduction of the maximum light available for feedback, and 3) a reduction in, or even disappearance of, relaxation oscillations in the laser dynamics when a filter of appropriate width is chosen. More generally, it is observed that certain dynamics that occur due to unfiltered optical feedback may be suppressed when the feedback light is spectrally filtered.     

基于激光诱导荧光法诊断大气压低温等离子体射流中OH自由基和O原子的时空分布

由于传统辐射光谱法无法对大气压低温等离子体射流中OH自由基和O原子进行定量检测,本文利用自主研制的纳秒脉冲激励针筒型等离子体射流装置,基于单光子和双光子激光诱导荧光法分别对OH自由基和O原子的时空分布进行诊断。结果发现,OH自由基和O原子的寿命时间分别为1ms和3ms,远大于脉冲放电持续时间;采用拟合衰减曲线法,得到OH自由基的绝对密度为1012~1013cm-3;发现离喷嘴口越远,OH自由基和O原子密度越低。然而,即使距离喷嘴口数cm的地方,仍然存在大量的OH自由基和O原子;OH自由基和O原子的密度随激励频率和脉冲电压幅值的增加而升高,随H_2O含量和O_2含量的升高而出现先增大后减小的趋势。其中,当氦气中H_2O含量为0.012%时,OH自由基密度达到最大值。当氦气中O2含量为0.5%时,O原子密度达到最大值。本文研究为调控和优化低温等离子体射流中OH自由基和O原子密度提供重要科学依据。     

Ytterbium-doped silica fiber lasers: versatile sources for the1-1.2 μm region

Ytterbium-doped silica fibers exhibit very broad absorption and emission bands, from ~800 nm to ~1064 nm for absorption and ~970 nm to ~1200 nm for emission. The simplicity of the level structure provides freedom from unwanted processes such as excited state absorption, multiphonon nonradiative decay, and concentration quenching. These fiber lasers therefore offer a very efficient and convenient means of wavelength conversion from a wide variety of pump lasers, including AlGaAs and InGaAs diodes and Nd:YAG lasers. Efficient operation with narrow linewidth at any wavelength in the emission range can be conveniently achieved using fiber gratings. A wide range of application for these sources can be anticipated. In this paper, the capabilities of this versatile source are reviewed. Analytical procedures and numerical data are presented to enable design choices to be made for the wide range of operating conditions     

Production and probing of atomic wavepackets with ultrafast laser pulses: applications to atomic and molecular dynamics

We demonstrate that atomic wavepackets can serve as sensitive detectors for investigating atomic and molecular dynamics. In concert with parametric four-wave mixing, the interference between coherent superpositions of atomic excited states produced by ultrafast (/spl sim/150 fs) pump and probe pulses provides a new and powerful tool with which fundamental processes, such as molecular dissociation and Rydberg-Rydberg atomic collisions, can be observed with the extraordinary sensitivity afforded by a coherent nonlinear optical process. Experiments are described in which the dissociation of an electronically excited molecule (Rb/sub 2/) and the distribution of atomic fragments into excited states spanning >10 000 cm/sup -1/ are observed. Also, resonant collisions between Rb atoms in the 7s and 5d states are detected by monitoring the shift in the frequency of an atomic wavepacket induced by the dipole-dipole interaction.     

Experimental observation of noise-induced synchronization of bursting dynamical systems

Can bursts in dynamical systems be synchronized by a weak, common, noise background? We observe large, uncorrelated bursts of intensity fluctuations in two almost identical erbium-doped fiber ring lasers, initiated by common injection of a weak, constant intensity optical signal. Significant synchronization of the bursts is obtained for noise and sinusoidal modulation of the injected light intensity. Measurements of the burst statistics and synchronization are presented.     

On-off laser delivery into a precise tissue area using smart tissue-activated fiber probes

A novel and simple concept for on-off switching laser radiation delivery into a precise tissue area using tissue-activated optical fiber probes is demonstrated. The authors present the operating principle and general optical features of the fiber-optic-based delivery technique. The basic idea includes the use of a single delivery fiber with a specially shaped angled tip. Because of the frustrated-total-internal reflectance caused by the refractive-index change of the surrounding medium, the angled fiber tip acts as a smart tissue-activated probe. It provides a safe way for laser delivery that includes only two states of tissue illumination: 1) off-state (no tissue illumination), when the fiber tip is out of the tissue area and the laser emission is backreflected due to total-internal-reflection and 2) on-state (maximum tissue illumination), when the fiber tip is on the absorbing tissue area and becomes "transparent" because of the frustrated-total-internal reflectance. Here, optical properties of tissue-activated fiber probes used for precise laser delivery are investigated both experimentally and theoretically by analyzing the backreflectance signal power. Optical fibers working in the visible and mid-infrared spectral regions with various geometrical parameters are used and a spatial resolution of 2 /spl mu/m is achieved when the fiber tip is moved toward the absorption tissue surface.     

Narrow linewidth, tunable Tm3+-doped fluoride fiberlaser for optical-based hydrocarbon gas sensing

Operation of an efficient continuous-wave (CW) thulium-doped fiber laser emitting at wavelength, λ=2.31 μm is reported. The fiber laser parameters are optimized with a view to ultimately producing a compact and efficient laser source for optical absorption based gas sensing. A number of fiber laser configurations are investigated to assess their suitability for narrow linewidth, tunable fiber laser operation emitting around λ=2.3 μm, which is a wavelength region of significant importance for hydrocarbon gas monitoring. Tuning ranges of 140 nm and linewidths of less than 210 MHz have been demonstrated with lasers with bulk external tuning grating. Preliminary hydrocarbon gas sensing investigation confirm the potential of this source for detection of ppb gas concentrations     

Fiber-optic tactile microsensor for detecting the position of thetip of a fiberscope

A tactile microsensor employing optical fibers has been developed and evaluated. The sensor is used for touch sensing to detect the position of the tip of a fiberscope inside a small tubule, such as a blood vessel. The sensor consists of bosses as feelers, which are split sections of the end of a polyurethane tube, a metal diaphragm as a mirror and optical fibers transmit light. The sensors are fabricated using silicon micromachining, such as anisotropic etching and excimer laser etching. The sensing principle is based on the detection of changes in optical reflective intensity from a diaphragm located between a boss and an optical fiber. The width and height of the boss are both 200 μm. For the transmitting source and signal light, a multimode optical fiber 50/125 μm (core/clad) in diameter is used. The results show that tactility measurements can be made easily and safely in blood vessels. This paper describes the structure, fabrication processes, and performance of the sensor. We also propose micromeasuring systems combined with the tactile sensor and a fiberscope of contact type     

2.5 Gbit/s光传输系统超长中继距离传输研究

介绍了限制2.5 Gbit/s光传输系统超长中继距离传输的4个主要因素：衰减、色散、信噪比、非线性效应,有针对性地提出了掺饵光纤放大器技术、拉曼放大器技术、遥泵放大器技术、色散补偿技术、前向纠错技术等多种克服技术,给出各种克服技术的性能参数,提出了工程中不同站距情况下各种技术的组合方案,总结了2.5 Gbit/s光传输系统工程应用时的最大中继距离。     

An optical fiber sensor for biofilm measurement using intensitymodulation and image analysis

The development of an online sensor to determine the fouling properties of aqueous process fluids is described. A plastic optical fiber with its cladding removed over a sensitized length measures the growth of biofilms in a closed loop water process system by evanescent field attenuation and intensity modulation. The sensor detects material build-up at the core-cladding interface by means of refractive index modulation. A theoretical model is developed showing that an increase in cladding refractive index reduces the intensity of light propagating in the fiber and attenuates the high order modes. The modulation mechanism of the sensor is demonstrated using a CCD camera and frame grabber to record the far field modal distribution of the fiber, when the outer modes are excited. The intensity distribution changes spatially in response to the biofilm deposit on the sensor, indicating evanescent field attenuation     

Nonlinear Dynamics of Semiconductor Lasers Under Repetitive Optical Pulse Injection

In this paper, nonlinear dynamics of semiconductor lasers under repetitive optical pulse injection are studied numerically. Different dynamical states, including pulsation and oscillation states, are found by varying the intensity and the repetition rate of the injection pulses. The laser is found to enter the chaotic pulsation (CP) states and chaotic oscillation (CO) states through individual period-doubling routes. Mapping and corresponding Lyapunov exponents of these dynamical states are plotted and examined in the parameter space. Moreover, the bandwidths of the chaos states found are investigated, where the bandwidths of the CP states observed at the strong injection regime are two to four times broader than the bandwidths of the CO states found at the weak injection regime. In this paper, frequency-locked states with different winding numbers, the ratio of the oscillation frequency, and the repetition frequency of the injection pulses are also studied. Both the cases for repetition frequency above and below the relaxation oscillation frequency are examined. The winding numbers of the frequency-locked states reveal a Devil's staircase structure, where a Farey tree showing the relations between the neighboring states is constructed.      

Effect of Gain-Dependent Phase Shift on Fiber Laser Synchronization

Recent experiments have demonstrated synchronization of fiber laser arrays at low and moderate pump levels. It has been suggested that a key dynamical process leading to synchronized behavior is the differential phase shift induced by the gain media. We explore theoretically the role of this effect in generating inphase dynamics. We find that its presence can substantially enhance the degree of inphase stability to an extent that could be practically important. At the same time, our analysis shows that a gain-dependent phase shift is not a necessary ingredient in the dynamical selection of the inphase state, thus, leading us to reconsider the essential mechanism behind inphase selection in fiber laser arrays.