Highly efficient TW multipass Ti:sapphire laser system

An efficient chirped-pulse amplification, Ti: sapphire laser system, has been developed using mainly domestic components. The gain-narrowing effect has been significantly overcome by shaping spectrum of seeding pulses. With a novel aberration-free stretcher and two stage multi-pass amplifiers, pulses with duration of 25 fs and 36-mJ energy have been obtained at 10 Hz repetition rate, using only less than 290 mJ green Nd:YAG pump energy. This corresponds to a 1.4 TW peak power and 32% main amplification efficiency. The energy stability of the laser systems is better than ± 3%.     

Imaging through flesh tissue using fs electronic holographic gating method

The experimental results of imaging through flesh tissue using fs electronic holographic gating method is reported. In the experiment, Ti: sapphire mode-locked laser is used as light source, of which the repetition rate is 100 MHz, central wavelength 800 nm, duration of pulse 20 fs, output power 80 mW. Tissue is a 7 mm thick chicken slice, and the imaged object is a metal wire with diameter of 0.5 mm. A general CCD is used to record holograms and a clear image of metal wire is obtained. Several relevant problems are discussed.     

5.4-TW/46-fs 10-Hz Ti:sapphire laser system

We have developed a compact Ti: sapphire laser system which can generate 5.4-TW pulses at 10-Hz repetition rate. Chirped-pulses are amplified to 450 mJ by a regenerative amplifier, a 4pass amplifier and a 3-pass amplifier. After compression, 250-mJ, 46-fs pulses are obtained     

Experimental studies of high order soliton compression effect and gain characteristics in femtosecond laser pulses Er3+-doped fiber amplifier

Seed laser pulses with average power of 146 μW and pulse duration of 480 fs were amplified to 14.5 mW. The pulse duration was compressed to 260 fs using 6 m high concentration E³⁺ _r -doped fiber under forward pumping. The amplified signal pulse energy was 0.691 nJ (corresponding to a peak power of 2 657.7 W) and the repetition rate was 20.84 MHz. Spectrum breakup was observed simultaneously. The spectrum of pulses amplified by 3 m E³⁺ _r -doped fiber remains a single peak under different pump power. The amplified pulse duration was compressed abnormally with the increasing pump power using the backward pumping; that is, the amplified pulses were compressed with the increasing pump power under low pump power. When the pump power reached 38 mW, the shortest amplified pulse duration was 309 fs. With further increase in pump power, the amplified pulses began broadening, accompanied by a single peak spectrum under different pump power.     

Experimental studies of high order soliton compression effect and gain characteristics in femtosecond laser pulses E3+r-doped fiber amplifier

Seed laser pulses with average power of 146 μW and pulse duration of 480 fs were amplified to 14.5 mW. The pulse duration was compressed to 260 fs using 6 m high concentration E3+r-doped fiber under forward pumping. The amplified signal pulse energy was 0.691 nJ (corresponding to a peak power of 2 657.7 W) and the repetition rate was 20.84 MHz. Spectrum breakup was observed simultaneously. The spectrum of pulses amplified by 3 m E3+r-doped fiber remains a single peak under different pump power. The amplified pulse duration was compressed abnormally with the increasing pump power using the backward pumping; that is, the amplified pulses were compressed with the increasing pump power under low pump power. When the pump power reached 38 mW, the shortest amplified pulse duration was 309 fs. With further increase in pump power, the amplified pulses began broadening, accompanied by a single peak spectrum under different pump power.     

Experimental studies of self-starting passive mode locking Er3+ -doped fiber ring soliton laser

The mechanism of femtosecond optical pulse generation in a self-starting Er³⁺-doped fiber ring soliton laser and experimental research results are discussed. Using the nonlinear polarization rotation effect of the fiber for sat-urable absorbers (and then self-amplitude modulation) which acts as the mode locking mechanism in an Er³⁺-doped fiber ring cavity laser, stable self-starting mode locking pulses have been generated. The shortest output pulse is 269 fs, with the central wavelength of 1,531 pm at the repetition rate of 21.37 MHz. The average output powen of the two terminators of the laser are 0.25 mW and 0.08 mW respectively. The threshold pump power which sustains the mode locking is 15 mW. Under high pump power, the laser works in a high order harmonic mode locking state. The mode locking pulse durations vs different cavity lengths are also studied. Project supported by Major Project of Chines: Academy of Sciences (No. KJ952-J1-705).     

Experimental studies of high order soliton compression effect and gain characteristics in femtosecond laser pulses Er 3+-doped fiber amplifier

Seed laser pulses with average power of 146 μW and pulse duration of 480 fs were amplified to 14.5 mW. The pulse duration was compressed to 260 fs using 6 m high concentration E³⁺ _r -doped fiber under forward pumping. The amplified signal pulse energy was 0.691 nJ (corresponding to a peak power of 2 657.7 W) and the repetition rate was 20.84 MHz. Spectrum breakup was observed simultaneously. The spectrum of pulses amplified by 3 m E³⁺ _r -doped fiber remains a single peak under different pump power. The amplified pulse duration was compressed abnormally with the increasing pump power using the backward pumping; that is, the amplified pulses were compressed with the increasing pump power under low pump power. When the pump power reached 38 mW, the shortest amplified pulse duration was 309 fs. With further increase in pump power, the amplified pulses began broadening, accompanied by a single peak spectrum under different pump power.     

Amplified spontaneous emission and its restraint in a terawatt Ti:sapphire amplifier

Amplified spontaneous emission (ASE) and its restraint in a femtosecond Ti: sapphire chirped-pulse amplifier were investigated. The noises arising from ASE were effectively filtered out in the spatial, temporal and spectral domain. Pulses as short as 38 fs were amplified to peak power of 1.4 MI. The power ratio between the amplified femtosecond pulse and the ASE was higher than 10⁶: 1.     

Amplified spontaneous emission and its restraint in a terawatt Ti: Sapphire amplifier

Amplified spontaneous emission (ASE) and its restraint in a femtosecond Ti: sapphire chirped-pulse amplifier were investigated. The noises arising from ASE were effectively filtered out in the spatial, temporal and spectral domain. Pulses as short as 38 fs were amplified to peak power of 1.4 MI. The power ratio between the amplified femtosecond pulse and the ASE was higher than 10⁶: 1.     

High-order harmonic generation in Ar and Ne with a 45fs intense laser field

Experimental results of high-order harmonic generation (HHG) in Ar and Ne gas driven with a 45fs Ti: sapphire laser are presented. The shortest-wavelength harmonic emission corresponding to the 91st order harmonic (8.63nm) is observed in argon. In neon, the harmonics up to order 131 (5.99nm) is also observed. The effects of gas density, laser intensity, free electron and the focusing geometry parameters of the laser beam on the process of harmonic generation are investigated. The direct experimental evidence that an increased electron density causes a degenerated harmonic radiation is obtained. Project supported by the Chinese Academy of Sciences and the High-Tech Project of China.     

Mechanical damage in polymers caused by laser pulses

The damage caused by Q-switched laser pulses and picosecond pulses in specimens of polymethyl methacrylate and polystyrene is described. The damage criterion depends on both characteristics of the laser pulse, i.e., energy and intensity. Nonlinear absorption effects are not observed when the energy threshold is crossed. It is shown that Q-switched and free-running laser pulses react with polymer specimens in essentially the same way. The induction period and rate of crack development have been established. The emission excited in the specimen by the laser pulse is investigated.Institute of Problems of Mechanics, Academy of Sciences of the USSR, Moscow, Physics Institute, Academy of Sciences of the USSR, Moscow, All-Union Scientific-Research Institute of Optico-physical Measurements, Moscow. Translated from Mekhanika Polimerov, No. 3, pp. 436–444, May–June, 1971.     

Parametric generation of even polarization harmonics in a layer with cubic nonlinearity

The article consider the response of a cubically nonlinear medium to a small-period laser pulse when the optical frequency is close to the linear oscillator frequency of the medium. We demonstrate the possibility of parametric generation of even harmonics in response to a high-intensity femtosecond pulse when its amplitude exceeds some critical value. The shape of the generated pulses is shown to depend on the absolute phase of the input pulse. The formation of a supercontinuum is observed when the applied small-period pulse has a certain duration and a certain intensity. __________ Translated from Prikladnaya Matematika i Informatika, No. 22, pp. 31–49, 2005.     

The essential spectrum of periodically stationary pulses in lumped models of short-pulse fiber lasers

In modern short-pulse fiber lasers, there is significant pulse breathing over each round trip of the laser loop. Consequently, averaged models cannot be used for quantitative modeling and design. Instead, lumped models, which are obtained by concatenating models for the various components of the laser, are required. As the pulses in lumped models are periodic rather than stationary, their linear stability is evaluated with the aid of the monodromy operator obtained by linearizing the round-trip operator about the periodic pulse. Conditions are given on the smoothness and decay of the periodic pulse that ensure that the monodromy operator exists on an appropriate Lebesgue function space. A formula for the essential spectrum of the monodromy operator is given, which can be used to quantify the growth rate of continuous wave perturbations. This formula is established by showing that the essential spectrum of the monodromy operator equals that of an associated asymptotic operator. Since the asymptotic monodromy operator acts as a multiplication operator in the Fourier domain, it is possible to derive a formula for its spectrum. Although the main results are stated for a particular experimental stretched pulse laser, the analysis shows that they can be readily adapted to a wide range of lumped laser models.     

Ein Elektronenvervielfacher als Pulslichterzeuger

Summary A 7-stage electron multiplier with grid control system is described with respect to construction and performance. A coaxial feedback cable permits the regeneration of pulses with a repetition frequency of from 10 MC to 0·1 MC and a pulse length of about 8×10^–9s. The luminescence of the dynodes was investigated and an identity in the duration of the light pulses and the electrical pulses has been found. The interpretation of the luminescence effects by assuming electro-fluorescence phenomena indicates applications to problems of stroboscopy and of high-speed computation and storage devices.     

Tunneldiode-Transistorschaltungen für die Pulstechnik

Summary Based on a novel tunneldiode-transistor configuration a pulse former and a full adder circuit are being derived. The pulse former shapes waveforms of repetition rates of up to 700 MHz into pulses of standardized amplitude and the full adder exhibits a carry signal delay of 1 nsec. Both a.c. and d.c. behaviour is analysed and compared to measured results.     

Spectral filtering for ultra-fast mode-locking in the normal dispersive regime

A theoretical model is developed which characterizes the physical process responsible for generating ultra-short, high-energy, mode-locked pulses in a normal GVD laser cavity with spectral filtering. Two physical parameters are critical to achieving optimal performance: the ratio of the filter bandwidth to the gain bandwidth and the placement of the output coupler in the laser cavity. The spectral filtering allows for the high-energy of typical normal GVD laser cavities to be maintained while the pulse intensity doubles and the FWHM reduces by an order of magnitude. This phenomenon is generic and ubiquitous to normal GVD mode-locking. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Dependence of the threshold laser pulses for polymethyl methacylate on specimen temperature and stress

It has been shown experimentally that heating the specimen to the softening point does not affect the value of the threshold laser pulse; stretching the specimen reduces the value of the threshold laser pulse in the case of millisecond pulses but has no effect in the case of nanosecond pulses. It is shown that the damage mechanism is not exclusively determined by thermal degradation or multiphoton photodegradation acting alone.Institute of Problems of Mechanics, Academy of Sciences of the USSR, Moscow. Translated from Mekhanika Polimerov, No. 6, pp. 997–1000, November–December, 1973.     

The AC Stark Effect, Time-Dependent Born–Oppenheimer Approximation, and Franck–Condon Factors

We study the quantum mechanics of a simple molecular system that is subject to a laser pulse. We model the laser pulse by a classical oscillatory electric field, and we employ the Born–Oppenheimer approximation for the molecule. We compute transition amplitudes to leading order in the laser strength. These amplitudes contain Franck–Condon factors that we compute explicitly to leading order in the Born–Oppenheimer parameter. We also correct an erroneous calculation in the mathematical literature on the AC Stark effect for molecular systems. Communicated by Christian Gérard. Submitted: August 15, 2005; Accepted: October 13, 2005     

Propagation Failure Along Myelinated Nerves

Propagation of traveling pulses in the myelinated Hodgkin–Huxley model is studied. The nerve impulse is a traveling wave with two components. At the Ranvier nodes, it behaves as a discrete traveling pulse. Wave motion through the internodal regions is then driven by this traveling pulse. We give analytical characterizations of the parameter ranges for which nerve impulses fail to propagate by exploiting time scale separation and the active node approximation, which reduces the dynamics of infinite fibers to the evolution of a few nodes. Simple recipes to predict the speed of the impulses and the widths of their peaks are also given. Predictions are in good agreement with the information provided by numerical simulations.     

Modeling of the material removal and heat affected zone formation in CFRP short pulsed laser processing

Short pulsed laser milling is a novel method to process the carbon fiber reinforced plastics (CFRP) which has bad machinability. This paper presents a numerical model studying the material removal mechanism of CFRP laser milling. It is confirmed by both the experiment and the simulation that laser ablation and mechanical erosion caused by the polymer pyrolysis are all involved in the material removal. Because the heating and cooling rate in short pulsed laser milling is high, ablation of two adjacent laser pulses almost has little influence on each other. By conducting the parametric analysis, it was found that the spacing distance under which the matrix between two adjacent laser pulses was completely degraded should be adopted to utilize the mechanical erosion effectively. Laser milling experiments of CFRP laminates were performed using a nanosecond pulsed laser system. The established model could predict the average ablation depth per scanning pass at an optimal spacing distance.