Prism-lens dispersive delay line

A new dispersive prism-lens delay line has been designed and experimentally tested, which is based on a direct control of the spectral phase of pulses. Advantages of the proposed device are the compact design and the possibility of controlling both magnitude and sign of dispersion.     

Highly dispersive mirror in Ta2O5/SiO2 for femtosecond lasers designed by inverse spectral theory

A highly dispersive mirror for dispersion compensation in femtosecond lasers is designed by inverse spectral theory. The design of a simple quarter-wave Bragg reflector can be modified by moving the poles in the optical impedance found in the photonic stop band. These spectral quantities are used as independent variables in the numerical optimization because they have no effect on the location of the photonic stop band, and so the design requirements to obtain a high reflectivity and a specific delay spectrum are decoupled. The design was fabricated by ion-beam sputtering. A group delay dispersion of -300 fs(2) was measured over a bandwidth of 28 nm, with a remaining reflectivity of greater than 99% in this range. The mirrors were used to make two Ti:sapphire lasers with 10- and 4-mm-long crystals, both of which generated near-transform-limited pulses of 35-fs duration. Because of the high dispersion of the mirrors, the laser cavities needed only five and three bounces from the mirrors, thus keeping reflection losses to a minimum.     

Measuring dispersion between two modes of an optical fibre using low-coherence spectral interferometry with a Michelson interferometer

Abstract

It is demonstrated experimentally that even if the spectral interference between two modes of an optical fibre excited by a low-coherence source is not resolved at its output by a spectrometer of a given resolving power, it is resolved in the Michelson interferometer configuration. In a tandem configuration of a dispersive Michelson interferometer and a two-mode optical fibre, the optical path difference (OPD) in the interferometer is adjusted close to the group OPD between modes to produce a low-frequency spectral modulation that can be processed. Thus, using the Fourier-transform method in processing the measured spectral modulations and subtracting the effect of the dispersive Michelson interferometer, the wavelength dependence of the group OPD between two modes of the optical fibre over a limited spectral region is obtained.     

Effect of group delay ripple on picosecond pulse compression schemes

We show experimentally, through autocorrelation and frequency-resolved optical gating measurements, that a simple dispersive fiber Bragg grating with group delay ripple approximately 10 ps peak-to-peak may be used effectively to stretch ultrashort optical pulses for linear amplification before recompression to a higher-power pulse. We further investigate, through simulations, the effect of group delay ripple on the pulses and show that there are regimes, defined by both ripple magnitude and ripple period as a function of wavelength, in which the pulses are nearly perfectly compressed. A map with contours of equal figures of merit indicates favorable regions of operation.     

Ultrawide spectral range group-velocity dispersion measurementutilizing supercontinuum in an optical fiber pumped by a 1.5 μmcompact laser source

Group velocity dispersion (GVD) measurement is presented utilizing supercontinuum (SC) white pulses generated in an optical fiber by 15 μm compact laser sources. This provides 1) ultrawide continuous spectral measurement range >600 nm from a single optical source without the use of interpolation formulae and 2) stable far-end measurements by the simultaneous multi-wavelength nature of the SC pulses. A novel method that is independent of the detector bandwidth is proposed which measures λ-dependent phase shifts of one of the Fourier components of a short pulse train. Fiber GVD's of unusual dispersion characteristics were measured using SC pulses extended over the spectral range of 1150-1770 nm. It is shown that fiber lengths of up to 130 km can be measured with a group delay resolution of 0.01 ps/km     

Automatic time delay optimization between the pump and seed pulses of a broadly tunable femtosecond optical parametric amplifier

We propose a scheme for optimizing the time delay between the pump and seed pulses of an optical parametric amplifier (OPA) over a large spectral range. The efficiency of this method is demonstrated for a femtosecond BBO parametric amplifier seeded with a white-light continuum pulse. The error signal used for intensity stabilization results from a modulation of the temporal delay between the pump and the continuum pulses and phase-sensitive detection of the amplified signal. It allows us to lock the delay to the position that maximizes the OPA gain.     

Pattern时延差编码水声定位的时延估计研究

基于Pattern时延编码体制,针对水下超短基线定位系统设计了呼叫信号的正、负调频时延编码结构,并提出利用二次互相关技术进行时延估计。该方法在一定程度上能够提高呼叫方的可检测信噪比,同时在复杂环境下对信道中包括直达波在内的多径信号能量进行累加,能够准确地完成峰值检测,进而达到提高时延估计精度的目的。海试实验结果表明二次相关法在不同信噪比条件下能有效地抑制界面反射产生的多径干扰。     

Temporal filtering for Montgomery self-imaging under dispersive transmission

We present what we believe is a new method to introduce self-imaging properties under dispersive transmission of single or multiple light pulses with different temporal characteristics. By properly performing a temporal filtering into a given input signal it can produce an output signal having a spectral content satisfying the Montgomery condition, thereby allowing self-imaging of this signal under further dispersive transmission. An array of fiber loops performs the filtering operation on the input signal. We show some numerical simulations with a single light pulse as an input signal to verify the feasibility of the method and demonstrate the effects of the several involved parameters on both the pulse shape and the noise level.     

Variable-Width Pulse Generation Using Avalanche Transistors

The extremely fast ionization process in semiconductors offers a means of generating pulses that have subnanosecond rise times and high peak powers. There are several important applications of these pulses which require the duration of the generated pulse to be variable. This report investigates three methods of producing variable-width pulses using transistors operating in the avalanche mode. The first circuit studied is used with a capacitor as the collector load. It produces pulse rise times of less than one nanosecond but has the disadvantage of a relatively slow RC discharge fall time. Distributed and lumped parameter delay lines were used in the second form of circuits studied. With the artificial delay line the pulse width can be controlled by opening the line at different sections whereas, with the cable, the length must be altered to change the pulse width. These circuits produced fast rise, flat top, and fast fall pulses. In the third method studied, a composite circuit is used in which the fast rise of an avalanche pulse is added to a slower rise, but variable duration pulse, generated by a saturating transistor. This method is shown to produce fast rise, variable-width pulses with overshoot and ringing easily held to less than five per cent.     

Resolution improvement with dispersion manipulation and a retrieval algorithm in optical coherence tomography

We propose and demonstrate what is to our knowledge a novel technique of improving the spatial resolution of an optical coherence tomography (OCT) system given a non-Gaussian light source spectrum. By using dispersive materials in the reference arm of the OCT system, the resultant dispersion variation led to a full-width at half maximum (FWHM) of the interference fringe envelope smaller than the Fourier transform-limited value of a Gaussian spectral shape with the same spectral FWHM, at the expense of significant tails. The effects of the tails, which would blur the OCT images, were tremendously reduced with a retrieval algorithm. Simulation results and processed OCT scanning images have shown the capability of the proposed technique.     

Femtosecond microscopy of surface plasmon polariton wave packet evolution at the silver/vacuum interface

A movie of the dispersive and dissipative propagation of surface plasmon polariton (SPP) wave packets at a silver/vacuum interface is recorded by the interferometric time-resolved photoemission electron microscopy with 60 nm spatial resolution and 330 as frame interval. The evolution of SPP wave packets is imaged through a two-path interference created by a pair of 10 fs phase correlated pump-probe light pulses at 400 nm. The wave packet evolution is simulated using the complex dielectric function of silver.     

数字示波器大触发延迟时间的校准方法

针对数字示波器的大触发延迟时间,提出了一种标称校准方法。使用正弦激励信号周期作为测量尺度标准,将大触发延迟时间表述成整数个正弦周期与一个小数个正弦周期的合成。小数周期部分用相位差法直接测量获得,而整数部分则通过最大允许误差极限已知的标称大触发延迟时间计算获得,最终将两部分时间合成,得大触发延迟时间测量结果。在数字示波器上所作的一组实验验证了本文方法的正确性与可行性。该方法可用于数字示波器大触发延迟时间的精确测量和计量校准。     

Ultra-high-speed phase-sensitive optical coherence reflectometer with a stretched pulse supercontinuum source

We demonstrate an ultra-high-speed phase-sensitive time-wavelength-domain optical coherence reflectometer with a stretched pulse supercontinuum source. A pulsed fiber laser operating at 10 MHz repetition rate was used to generate a pulsed supercontinuum of 30 ps pulse duration by using a nonlinear optical fiber. The supercontinuum pulses are stretched into 70 ns pulses with a highly dispersive fiber. With this stretched pulse source, we have built a phase-sensitive optical coherence reflectometer that measures the spectral interferogram of reflected light. By using the linear relation between the wavelength and the temporal position in a linearly chirped pulse, ultra-high-speed spectrum measurement can be obtained with this method in the time domain. We have demonstrated ultra-high-speed two-dimensional surface profiling for a standard image target and high-speed single-point monitoring for a fixed point under vibrational motion. It is shown that the measurement speed for the position of a single point can be as fast as 2.5 MHz, while the position accuracy can be better than 4.49 nm.     

Waveguide damage detection by the matching pursuit approach employing the dispersion-based chirp functions

If the wave mode used in guided wave non-destructive inspection is dispersive, reflected pulses from damaged parts may be significantly distorted due to wave dispersion. The main concern, in this case, is how to detect the reflected pulses in noisy signals, and to extract meaningful damage information from the detected pulses. However, current signal processing techniques used for guided wave inspection do not account for pulse dispersion, so the extracted information is often not so accurate. The objective of this study is to develop an efficient technique to deal with dispersed pulses for guided-wave nondestructive evaluation. Our idea is to model dispersed pulses by chirp functions of special form that can simulate up to quadratically varying group delay. To determine the parameters of the chirp functions approximating dispersed, reflected pulses, an adaptive matching pursuit algorithm is employed. Once the characterizing parameters are found, the damage location and extent can be estimated. The proposed method is tested with experimentally measured signals of longitudinal waves in a circular cylinder.     

Bichromatic, phase compensating interferometer based on prism pair compressors

We present a bichromatic prism pair interferometer (BPPI) for controlling the delay between laser pulses of two different frequencies propagating collinearly in a single beam. The BPPI is especially useful when working with ultrafast laser pulses because it intrinsically allows for independent control over the second-order dispersion experienced by the differently colored pulses. We use this control to demonstrate successful precompensation for blue (lambda approximately 390 nm) and UV (lambda approximately 260 nm) pulses that pass through 2.2 cm of dispersive material after the interferometer. The BPPI is extremely flexible and works with all frequencies from the UV to the near-infrared. We demonstrate this by describing measurements made with BPPIs configured for three different combinations of central frequencies.     

Fuel vapor measurements by linear Raman spectroscopy using spectral discrimination from droplet interferences

Vapor-phase measurements by linear Raman spectroscopy are performed in the vicinity of methanol droplets. Several types of interference by these droplets are identified and removed by appropriate filtering. This procedure, together with the phase-dependent spectral shift of the OH stretching vibration frequency, is proved to permit single-pulse linear Raman measurements of methanol vapor and nitrogen on a line with coexisting droplets. Laser-induced droplet breakdown is found to limit the applicable laser irradiance to approximately 2 GW/cm(2) and is avoided by use of a flash-lamp-pumped dye laser with high energy (1-7 J) and long pulses (1.5 mus).     

数字示波器大触发延迟时间的变频测量方法

针对数字示波器的大触发延迟时间,提出了一种变频测量方法。使用正弦激励信号周期作为测量尺度标准,将大触发延迟时间表述成整数个正弦周期与1个小数个正弦周期的合成。小数周期部分用相位差法直接测量获得,而整数的获取,通过相同的大触发延迟时间在不同激励信号频率下离散化时间点均具有整数个信号周期的特征而判定,最终以在极限延迟时间范围内不同频率下离散时间点最小周期的唯一性作为测量结果定量依据,并以此计算获得整数值。最后将两部分时间合成得大触发延迟时间测量结果。在数字示波器上所作的一组实验验证了该方法的正确性与可行性。该方法可用于数字示波器大触发延迟时间的精确测量和计量校准中。     

Investigations on the photoelectron conversion efficiency of NaI(Tl) counters

The photoelectron conversion efficiency of NaI(Tl) detectors has been measured by counting single photoelectron pulses of the detector output pulse train on a very fast storage oscilloscope. This result is compared with those obtained by the measurement of the achievable time resolution using the first photoelectron triggering technique.     

Spectrally resolved phase-shifting interference microscopy: technique based on optical coherence tomography for profiling a transparent film on a patterned substrate

Spectrally resolved white-light phase-shifting interference microscopy has been used for measurements of the thickness profile of a transparent thin-film layer deposited upon a patterned structure exhibiting steps and discontinuities. We describe a simple technique, using an approach based on spectrally resolved optical coherence tomography, that makes it possible to obtain directly a thickness profile along a line by inverse Fourier transformation of the complex spectral interference function.