Travelling-wave operation of a tunable cw dye laser

A ring cavity configuration for a tunable cw dye laser is described. A substantial reduction of the output bandwidth of the ring laser with increase in power for unidirectional or traveling-wave operation has been demonstrated. The advantages of the ring configuration for single-frequency operation of cw dye lasers are explored.     

Unbalanced machinery vibration isolation with a semi-active pneumatic suspension

The problem of unbalanced machinery isolation is tackled in this paper. The proposed solution incorporates an air suspension that can be adapted depending on the turning frequency. The system is built with three main parts: an air spring, a reservoir and a connecting pipe. A model of the suspension excited by the unbalanced rotor is also shown in this paper. The properties of the system make it possible to use a configuration of the suspension (one pipe size) over a bandwidth range and another configuration (another pipe size) over the remaining bandwidth range. This idea is implemented with solenoid controlled valves and the results show significant improvements with respect to completely passive configurations.     

Vibration isolation of unbalanced machinery using an adaptive-passive magnetoelastic suspension

The problem of unbalanced machinery isolation is tackled in this paper. The proposed method incorporates a magnetoelastic suspension which can be adapted depending on the rotational frequency and is built with two main parts: a magnetoelastic rod and a magnetizing solenoid. The properties of the system make it possible to use one configuration of the suspension (demagnetized state of the rod) over a bandwidth range and another configuration (saturated state of the rod) over the remaining bandwidth range. In addition, three different magnetoelastic materials were tested and the results show significant improvements with respect to completely passive configurations. A theoretical model is also explored in order to make comparisons with the experimental results.     

Attosecond-resolution timing jitter characterization of free-running mode-locked lasers

Timing jitter characterization of optical pulse trains from free-running mode-locked lasers with attosecond resolution is demonstrated using balanced optical cross correlation in the timing detector and the timing delay configurations. In the timing detector configuration, the balanced cross correlation between two mode-locked lasers synchronized by a low-bandwidth phase-locked loop is used to measure the timing jitter spectral density outside the locking bandwidth. In addition, the timing delay configuration using a 325 m long timing-stabilized fiber link enables the characterization of timing jitter faster than the delay time. The limitation set by shot noise in this configuration is 2.2 x 10(-8) fs(2)/Hz corresponding to 470 as in 10 MHz bandwidth.     

Stable all-fiber photonic temporal differentiator using a long-period fiber grating interferometer

A novel configuration of all-fiber photonic temporal differentiator optimized for tens-of-gigahertz processing bandwidth is suggested and experimentally demonstrated. This device covers the gap in terms of processing bandwidth between the two previously-demonstrated all-fiber designs, namely the fiber Bragg grating-based differentiator (<20 GHz) and that based on a uniform long-period fiber grating (>100-200 GHz).     

Ultrawide-bandwidth gain in a coaxial dielectric Cherenkov maser

A coaxial waveguide partially filled with a dielectric as the slow-wave structure of a dielectric Cherenkov maser is investigated. The dispersion of the fundamental mode of this waveguide is very weak at phase velocities close to the velocity of light, and for this reason a very wide gain bandwidth is possible under conditions of an interaction with a relativistic electron beam. The dispersion equation for an infinitely thin tubular beam in a coaxial waveguide with a dielectric liner adjoining either the outer or inner conducting surface is derived. The gain bandwidth as a function of the parameters of the electron beam and the slow-wave system are investigated on the basis of numerical solution of the dispersion equation, and a comparison with similar dependences for the conventional configuration of a dielectric Cherenkov maser is made. The structural features of the coaxial configuration which enable novel approaches to the problems of matching the microwave signal at the entrance and exit of the system are discussed. Zh. Tekh. Fiz. 67, 66–72 (May 1997)     

Increasing the bandwidth of resonant gravitational antennas: the case of explorer

Resonant gravitational wave detectors with an observation bandwidth of tens of hertz are a reality: the antenna Explorer, operated at CERN by the ROG Collaboration, has been upgraded with a new readout. In this new configuration, it exhibits an unprecedented useful bandwidth: in over 55 Hz about its center operating frequency of 919 Hz the spectral sensitivity is better than 10(-20) Hz(-1/2). We describe the detector and its sensitivity and discuss the foreseeable upgrades to even larger bandwidths.     

Gain and gain-flatness improved photonic crystal fiber Raman amplifier based on a dual-pass amplification configuration with single pump laser

A gain and gain-flatness improved L-band dual-pass Raman fiber amplifier (RFA) utilizing a photonic crystal fiber (PCF) as gain medium is demonstrated. By introducing complementary gain spectra of typical forward and backward pumping single-pass RFA using the same PCF, we finally achieve average net gain level of 22.5 dB with a ±0.8 dB flattening gain in 20-nm bandwidth from 1595 nm to 1615 nm, which is rare in RFAs with only one single pump and no flattening filter. Compared with the single-pass pump configurations, gain level, flatness and bandwidth are greatly improved by using the dual-pass amplification configuration. The limitation of this configuration caused by multi-path interference (MPI) noise and stimulated Brillouin scattering (SBS) is also discussed.     

Crossed-beam superluminescent diode

We investigate a novel surface-emitting superluminescent diode configuration that uses two detuned grating outcouplers to suppress lasing. This device exhibits a shaped beam with a peak power of 1.5 W quasi-continuous wave with an 11 nm bandwidth centered on 970 nm.     

Experimental demonstration of variable-reflectivity signal recycling for interferometric gravitational-wave detectors

The results of an experimental demonstration of a benchtop Michelson interferometer with a variable-reflectivity signal mirror are presented. This variable reflectivity is achieved by employment of a second Michelson interferometer. The results are presented in the form of the frequency responses obtained from this configuration with a signal laser injection method. It is shown that the frequency response can be dynamically tuned with independent peak frequency and bandwidth control. Such a configuration gives a tunable frequency response and has an application as a flexible gravitational-wave detector.     

Flat broadband erbium-doped fiber ASE source based on symmetric nonlinear optical loop mirror

A new type of flat broadband Erbium-doped fiber (EDF) amplified spontaneous emission source is proposed and demonstrated. The intensity-dependent loss induced by a symmetric nonlinear optical loop mirror is used to flatten the spectrum and broaden the bandwidth. The 3 dB bandwidth over 50 nm has been obtained with 15 m EDF and 100 mW pump power based on the single-pass forward pump configuration. This spectrum flattening and broadening method is quite effective over a large range both in pump power and EDF length.     

The Fulde-Ferrell-Larkin-Ovchinnikov states in s and d-wave superconductor with spin-dependent bandwidth imbalance on square lattice

We study numerically the phase diagram for s and d-wave fermionic superfluidity/superconductivity with spin-dependent bandwidth imbalance on a two-dimensional square-lattice. We also investigate the spontaneous space symmetry breaking states at low temperatures by solving the Bogoliubov-de Gennes equations. It is found that, the spatial configuration of the order parameter,both the uni-directional Fulde-Ferrell-Larkin-Ovchinnikov(FFLO) states and the two-dimensional FFLO state may show up in the presence of finite spin-dependent bandwidth imbalance. Moreover, we calculate the spectra of local density of states, and the experimental proposals of observing such FFLO states are therefore suggested.     

一种高效率的L波段掺铒光纤ASE宽带光源

利用双程双向泵浦单级掺铒光纤的结构实现高效率的L波段掺铒光纤放大自发辐射输出,同时选择1480nm半导体激光器作为泵浦源,高掺杂铒光纤为增益介质,通过优化铒光纤长度,获得了在1566-1604 nm（38 nm）,自发辐射谱功率高于－16 dBm,总输出功率13.7 dBm的L波段掺铒光纤放大自发辐射光源.该光源结构相比于双程前向泵浦结构的L波段掺铒光纤放大自发辐射光源,其泵浦效率从11.8%提高到23.4%.     

120 nm Bandwidth Erbium-doped Fiber Amplifier

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Design of broadband microwave absorber utilizing FSS screen constructed with coupling configurations

The design of a frequency selective surface (FSS) screen constructed using a coupling configuration to implement a broadband microwave absorber is presented in this paper. The reflectivity representation recognizes the characterization of the absorber. Simulation and measurement results are presented and analyzed. A coupling FSS screen is introduced in order to obtain a better bandwidth and absorption. The bandwidth with the reflectivity below ?10?dB could get 5.45?C18?GHz, compared with the 6.28?C18?GHz of the absorber with square patches FSS, provided that the thickness of the absorber is 4?mm.     

120 nm Bandwidth Erbium-doped Fiber Amplifier

A new dual band erbium-doped fiber amplifier configuration that provides 120 nm of optical bandwidth is simulated. This configuration employs a split-band architecture in which optical signals are splitted using a 1550/1610 nm port filter into two independent sub-bands which then pass in parallel through separate branches of the optical amplifier. Each branch may be optimized for the sub-band that traverses it. The independent sub-bands are combined and flattened before output, resulting in a 120 nm bandwidth gain-flattened optical amplifier.     

Enhancing the low frequency sound absorption of a perforated panel by parallel-arranged extended tubes

This paper is concerned with the use of a perforated panel with extended tubes (PPET) to improve the sound absorption confined to low frequencies. In comparison with a micro-perforated panel (MPP), the sound absorption can be significantly improved by using the PPET at the expense of the bandwidth of the sound absorption. A particular configuration combining four parallel-arranged PPETs with different cavities is introduced to achieve a wider bandwidth of the sound absorption at low frequencies. The analysis is extended to the combination of three parallel-arranged PPETs and a MPP to further increase the bandwidth of the sound absorption. A theoretical model is described to predict the sound absorption coefficient and the simulated annealing method is introduced to the proposed absorbers, allowing optimization of the overall performance. The theory with experimental validations demonstrates that the proposed configurations offer a potential improvement of more than one octave in the bandwidth of the sound absorption at low frequencies.     

High-resolution cavity-tuned ringdown spectrometer using a narrow-bandwidth pulsed laser source

A novel method of cavity ringdown spectroscopy is proposed to achieve high spectral resolution with tunable narrow bandwidth pulsed lasers. We demonstrate a cavity-tuned ringdown configuration in which only a single cavity mode is kept excited near the carrier frequency of a narrow bandwidth pulse laser. This is done simply by making a cavity resonance actively track the frequency reference served by the cw injection seed of the pulsed laser source. We present the servo mechanism used in the cavity resonance tracking, reliable procedures for transverse mode matching, and the evidence of single longitudinal mode excitation. The spectrometer performance is tested to record weak molecular overtone features of acetylene around the wavelength of 570 nm, showing cavity tracking stability within 5-MHz uncertainty which overcomes the bandwidth limit of pulsed laser sources itself. PACS 42.62.Fi; 42.60.Da; 33.20.Kf     

Polychromatic optical parametric generation by simultaneous phase matching over a large spectral bandwidth

The parametric generation of broad spectral bandwidths by the use of suitable phase-matching geometries is reported. Greater than 100-nm simultaneous bandwidth in the visible is generated in a collimated signal beam from a novel, noncollinear phase-matching geometry in a beta-barium borate optical parametric oscillator, which is pumped by the collimated output of a Q -switched and frequency-tripled Nd:YAG laser. Dispersive cavity tuning of the optical parametric oscillator by use of a rotatable Littrow-mounted grating, with a static crystal and pump configuration, is also described. A tunable bandwidth of >100 nm is also demonstrated.     

Bandwidth enhancement and response flattening of cascaded sum- and difference-frequency generation-based wavelength conversion

We investigate bandwidth enhancement and response flattening of wavelength conversion based on cascaded sum-frequency generation and difference-frequency generation (SFG/DFG) in quasi-phase-matched periodically poled lithium niobate waveguides by use of two pump lights. Analysis shows that the conversion properties deeply depend on the pump settings. The bandwidth is efficiently enhanced by augmenting the pump wavelength difference. The critical bandwidth corresponding to the response fluctuation of 1 dB in a 3-cm-long waveguide reaches 80-90 nm, which is enough to cover the entire conventional-band and long-wavelength-band. The cascaded SFG/DFG bandwidth is about 24% broader than that of the cascaded second-harmonic generation and difference-frequency generation-based configuration with the same waveguide length. However, the response fluctuation is also enhanced together with the bandwidth enhancement. The method of pump detuning is presented to flatten the response fluctuation. The fluctuation is reduced by more than 1 dB and it can be further reduced by increasing the pump detuning value with a little efficiency penalty.