Nonlinear conversion of Ti:sapphire laser wavelengths

We review a series of experimental investigations into the use of nonlinear optics to shift wavelengths generated by the tunable Ti:sapphire laser. We consider two basic approaches: harmonic generation (and the related process of sum frequency generation) to reach shorter wavelengths, and optical parametric generation to cover longer wavelengths. Both techniques have been aided by the development of two sets of nonlinear crystals, the borates BBO and LBO for harmonic processes, and the KTP family of materials for use with optical parametric generation. In combination we have used nonlinear techniques to produce tunable wavelengths ranging from 193-3000 nm     

Determination of XUV optical constants by reflectometry using ahigh-repetition rate 46.9-nm laser

We report the measurement of the optical constants of Si, GaP, InP, GaAs, GaAsP, and Ir at a wavelength of 46.9 nm (26.5 eV). The optical constants were obtained from the measurement of the variation of the reflectivity as a function of angle utilizing, as an illumination source, a discharge pumped 46.9-nm table-top laser operated at a repetition rate of 1 Hz. These measurements constitute the first application of an ultrashort wavelength laser to materials research     

Mechanism for self-synchronization of femtosecond pulses in atwo-color Ti:sapphire laser

An experimental and theoretical analysis of the nonlinear coupling mechanism between the two solitary pulses circulating in a two-color femtosecond laser is presented. Two operation regimes; synchronized; and nonsynchronized; and a hysteresis of the transition between the two regimes are clearly observed; while independent modelocking and tunability of the output pulse trains is found in both regimes. Pulses in the range from 15 to 100 fs are synchronized with a timing jitter below 2 fs. The combined effects of cross-phase modulation and negative group velocity dispersion are shown to be responsible for the strong pulse correlation in the synchronized regime. Our experimental observations are in agreement with numerical simulations, thus confirming the theoretical model     

Over 200-mW operation of single-lateral mode 780-nm laser diodeswith window-mirror structure

The high-power operation of the lateral mode stabilized 780-nm AlGaAs laser diode (LD) with the window-mirror structure has been achieved. The stable lateral mode operation up to 250 mW is realized. This is the highest power record among the narrow stripe LDs with the wavelength of 780 nm. This LD is suitable for the next generation high-speed (16-24×) CD-R/RW drives needing 200 mW class LDs     

Fabrication of photonic crystals in silicon-on-insulator using 248-nm deep UV lithography

We demonstrate wavelength-scale photonic nanostructures, including photonic crystals, fabricated in silicon-on-insulator using deep ultraviolet (UV) lithography. We discuss the mass-manufacturing capabilities of deep UV lithography compared to e-beam lithography. This is illustrated with experimental results. Finally, we present some of the issues that arise when trying to use established complementary metal-oxide-semiconductor processes for the fabrication of photonic integrated circuits.     

Frequency tripling of a Q-switched Cr:LiSAF laser to the UV region

Frequency tripling of a Q-switched, tunable Cr:LiSAF laser to the UV wavelength region is accomplished by a mixing scheme involving second harmonic generation in lithium triborate (LBO), followed by sum frequency generation in β-barium borate (BBO). The generated UV output is tunable between 260 nm and 320 nm     

Seeding of an eleven femtosecond optical parametric chirped pulse amplifier and its Nd/sup 3+/ picosecond pump laser from a single broadband Ti:Sapphire oscillator

We demonstrate direct simultaneous seeding of a few-cycle optical parametric chirped pulse amplifier (OPCPA) in the 700-1000-nm spectral range, and of a Nd:YLF amplifier emitting 30-ps pulses at 1053 nm by use of a chirped-mirror 6-fs Ti:sapphire oscillator. This approach of employing a single master oscillator to drive two power amplifiers simplifies the pump laser design and is applied to eliminate the timing jitter between the seed and the pump pulses in the OPCPA chain. We show that 10 mJ fundamental picosecond pump pulses with the intensity contrast in excess of 10/sup 4/ relative to the nanosecond Q-switched background can be achieved with the seed intensity available in the edge of the oscillator spectrum around 1053 nm. Cross-correlation measurements between the picosecond pump and femtosecond oscillator pulses reveal no traceable timing jitter between the OPCPA pump and seed pulses. The estimated long-term jitter of 0.3 ps is attributed to the thermal expansion of the cavity of the Nd:YLF regenerative amplifier.     

Higher order chirp compensation of femtosecond mode-lockedsemiconductor lasers using optical fibers with different group-velocitydispersions

Higher order chirp compensation of optical short pulses by using two types of optical fibers with different group-velocity dispersions was theoretically and experimentally investigated in detail. By optimizing the lengths of two types of optical fibers, both second- and third-order dispersion of chirped optical pulses were found to be simultaneously compensated. Pulse-compression experiments with chirped optical pulses from a mode-locked laser diode demonstrated using this technique attained nearly transform-limited, 500-fs pulse generation     

Passive optical limiting of picosecond-nanosecond laser pulsesusing highly nonlinear organic liquid cored fiber array

We present a detailed account of theoretical and experimental studies of nonlinear molecular photonic processes accompanying the propagation of short intense laser pulses through an organic liquid cored fiber array. The theory takes into account two-photon absorption, excited state absorption, and dynamical evolution of the molecular energy level populations and the laser intensity. The coupled dynamical equations for the molecular energy-level populations and laser intensity are solved numerically using experimentally observed molecular and optical parameters. Experimentally obtained optical limiting results are analyzed along with nonlinear absorption and pump-probe studies of other nonlinear optical processes occurring in the fiber core. It is found that in the nanosecond time regime, besides excited state and two-photon absorption, thermal and density effects also contribute significantly to the optical limiting performance of the constituent fiber     

SBN thin film growth by pulsed laser deposition and study of nonlinear optical properties

Abstract

Strontium barium niobate (Sr_xBa_1?xNb₂O₆) thin films were grown by PLD and characterized as to their structure, composition, and both linear and nonlinear optical properties. Attempted composition of very nearly x'0.61 was achieved, as shown within experimental error of RBS and PIXE techniques. Films deposited with high growth rates and at 700°C substrate temperature resulted polycrystalline on (001)-cut MgO and amorphous on fused silica, while those grown on MgO at lower growth rates were very well oriented with the substrate. A considerable blue shift in the absorption band edge was observed for the disordered SBN:61 films, compared to that for bulk SBN:61 crystal. A broad luminescence band was observed to occur, similar to that for bulk SBN. Degenerate four-wave-mixing techniques were used to study the nonlinear optical response of the amorphous films. A considerable enhancement, by 2 orders of magnitude, of the third order nonlinear susceptibility χ⁽³⁾ in transverse alignment was found to occur with respect to bulk values. A significant squeezing of the coherent response signal was observed in the SBN films.     

Modeling the optical properties of twisted bilayer photonic crystals

We demonstrate a photonic analog of twisted bilayer graphene that has ultra-flat photonic bands and exhibits extreme slow-light behavior. Our twisted bilayer ph...     

Chromatic dispersion measurements of long optical fibers by meansof optical ranging system using a frequency-shifted feedback laser

We describe the experimental results on the chromatic dispersion measurement of long optical fibers by means of optical ranging system using a wavelength-tunable frequency-shifted feedback (FSF) laser. The optical ranging system using an FSF laser has high spatial resolution of several centimeter over 10-km measurement range, and then the dependence of the group delay time of the lightwave in the optical fibers on the wavelength can be directly measured. The chromatic dispersion and the zero-dispersion wavelength of an 80-km-long dispersion shifted fiber was estimated, which is in good agreement with the result obtained by the phase method     

凝结水系统变频运行改造措施及效果

分析了三河发电有限责任公司2×300MW机组凝结水系统,阐述了凝泵变频改造过程中遇到的凝结水最小流量、凝结水压力、除氧器水位控制等问题的解决方案。通过变频运行方式与工频运行方式的比较,发现变频改造的节能效果显著,而且设备运行稳定。     

Characteristics of the optical resonator composed of the nonlinear directional coupler

This paper describes a simple method of analysis of an optical resonator composed of a nonlinear directional coupler. Assuming the coupler is made in semiconductor‐doped glass by the K⁺‐ion diffused technique, the transmission and reflection characteristics are calculated and demonstrated. The parameter dependence of the bistability appearing in the transmitted and reflected light is also presented. © 2004 Wiley Periodicals, Inc. Electr Eng Jpn, 150(3): 1–10, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20036     

Analysis of noise mechanisms limiting the frequency stability of microwave signals generated with a femtosecond laser

Excess phase noise is observed in the spectrum of the microwave signal extracted from a photodetector illuminated by a train of ultrashort light pulses from the femtosecond laser. This noise affects the stability of frequency transfer from optical to microwave domains with the femtosecond laser. Some contributions to the excess phase noise are related to intrinsic beam-pointing fluctuations of the femtosecond laser and optical power fluctuations of the detected light. These factors contribute to excess phase noise at the harmonics of the pulse repetition rate due to power-to-phase conversion in the photodetector, spatially dependent time delays, and photodiode nonlinearities that distort the pulse shape. With spatial filtering of the laser beam and active control of its power, the additional fractional frequency fluctuations of pulse repetition rate associated with the excess noise of the photodetection process were reduced from 6/spl middot/10/sup -14/ to approximately 3/spl middot/10/sup -15/ over 1 s of averaging. The effects of other noise mechanisms, such as laser shot noise and phase noise introduced by a microwave amplifier, were also examined but were found to be at a less significant level.     

A new strategy: A more valid determination of the nonlinear optical parameters for optoelectronic applications

Journal of Computational Electronics - This work is an attempt to discuss and suggest a new semiempirical approach, which may help in determining the nonlinear optical parameters based on the value...     

Linear and nonlinear optical properties of multi-layered spherical nano-systems with donor impurity in the center

In this study, the linear, third-order nonlinear and total absorption coefficients (ACs) of multi-layered quantum dot (MLQD) and multi-layered quantum anti-dot (MLQAD) with a hydrogenic impurity are calculated. The analytical and numerical solutions of Schrödinger equation for both MLQD and MLQAD, within the effective mass approximation and dielectric continuum model, are obtained. As our numerical results indicate, an increase in the optical intensity changes the total AC considerably, but the intensity range that leads to these changes is different for MLQAD and MLQD. It is observed that by changing the incident photon energy, the AC curves corresponding to MLQAD and MLQD are of different shapes and behaviors. The peak heights of AC curves corresponding to MLQAD are strongly affected by changing the core antidot radius and the shell thickness values, however in these cases no considerable changes are observed in peak heights of MLQD. Furthermore, in contrast to MLQAD, the photon energies corresponding to total AC peaks of MLQD are more affected by changing the confining potentials (CPs).     

Closing the loop for memory prosthesis: detecting the role of hippocampal neural ensembles using nonlinear models

A major factor involved in providing closed loop feedback for control of neural function is to understand how neural ensembles encode online information critical to the final behavioral endpoint. This issue was directly assessed in rats performing a short-term delay memory task in which successful encoding of task information is dependent upon specific spatio-temporal firing patterns recorded from ensembles of CA3 and CA1 hippocampal neurons. Such patterns, extracted by a specially designed nonlinear multi-input multi-output (MIMO) nonlinear mathematical model, were used to predict successful performance online via a closed loop paradigm which regulated trial difficulty (time of retention) as a function of the "strength" of stimulus encoding. The significance of the MIMO model as a neural prosthesis has been demonstrated by substituting trains of electrical stimulation pulses to mimic these same ensemble firing patterns. This feature was used repeatedly to vary "normal" encoding as a means of understanding how neural ensembles can be "tuned" to mimic the inherent process of selecting codes of different strength and functional specificity. The capacity to enhance and tune hippocampal encoding via MIMO model detection and insertion of critical ensemble firing patterns shown here provides the basis for possible extension to other disrupted brain circuitry.     

Long-term stabilization of the optical fiber phase control using dual PID

We propose an approach of long-term stabilization of optical fiber phase by controlling a piezo-based phase modulator and a Peltier component attached to the fiber via a phase-locked loop(PLL) circuit with dual proportional-integral- derivative(PID) adjustment. With this approach, we can suppress the fast disturbance and slow drifting of optical fiber to satisfy the requirements of optical phase long-term locking. In theory, a mathematical model of an optical fiber phase control system is established. The disturbance term induced by environment influence is considered into the PLL model. The monotonous and continuous changing environment disturbance will cause a steady-state error in this theory model. The experimental results accords well with the theory. The steady-state performance, adjusting time, and overshoot can be improved by using the dual PID control. As a result, the long-term, highly stable and low noise fiber phase locking is realized experimentally.