Generation of a single tunable ultrashort light pulse

A novel system for generating single tunable ultrashort light pulses of high power is described. The pulse train from a mode-locked flashlamp pumped dye laser passes through an amplifier, which is pumped by an N₂ laser. As gain is only available for a few nsec, only one pulse in the train gets amplified. The energy of the resulting single pulse is about 100 μJ.     

New single-pulse generation technique for distributed feedback dye lasers

The distributed feedback dye laser (DFDL) generates a train of picosecond pulses when pumped well above threshold. This DFDL emission can be quenched by injecting a laser pulse into DFDL. By proper timing of the quencher laser pulse, only the first DFDL pulse is generated while the successive pulses are suppressed. Operational characteristics and practical design considerations of such a quenched DFDL are given. With 2.5 ns long pump pulses from a N₂ laser, a shortest DFDL pulse of 17 ps was obtained at 380 nm.     

Ultralow-jitter passive timing stabilization of a mode-locked Er-doped fiber laser by injection of an optical pulse train

The pulse timing of a mode-locked Er-doped fiber laser was stabilized to a reference pulse train from a Cr:forsterite mode-locked laser by all-optical passive synchronization scheme. The reference pulses were injected into a ring cavity of the fiber laser by using a 1.3-1.5 mum wavelength-division multiplexer. The spectral shift induced by cross-phase modulation between copropagating two-color pulses realizes self-synchronization due to intracavity group-delay dispersion. The rms integration of timing jitter between the fiber laser pulse and the reference pulse was 3.7 fs in a Fourier frequency range from 1 Hz to 100 kHz.     

High-energy single pulse and multi-spike operation with a passive polymer Q-switch

Stable highly efficient polymer passive switch based on a polyurethane composite and organic dye BDN were developed for the neodymium lasers. The maximum energy of 0.98 J was obtained for a single pulse at passive Q-switching of these lasers. Repetition of single pulses at 50 Hz was possible. Repetitively pulsed operation with a peak output power up to 2 MW and an energy of train of nanosecond pulses 14.1 J were achieved in an yttrium aluminate laser for the first time. Variation of the initial switch transmittance made it possible to vary the pulse (spike) duration in the range 28–90 ns. The maximum pulse repetition rate in a train was 350 kHz when the Q-switching efficiency was 98%. The damage threshold of the investigated polymer matrix was achieved 14 and 18 J/cm² for single pulse duration (15 and 35 ns correspondingly) and 52 J/cm² in multi-spike generation regime (duration 80 ns).     

Generation of tunable picosecond pulses in a simple distributed feedback dye laser under N2-laser pumping

Simple possibility of single tunable picosecond pulse generation in a conventional inexpensive distributed feedback dye laser (DFDL) is demonstrated. It is shown that when a TEA N₂-laser is used for pumping no diffraction grating is necessary in the DFDL optical scheme for operation in the picosecond regime. Hence wide-range spectral tuning from 357 to 665 nm was easily achieved. When pumped with a 0.6 ns 40 kW pulse of the TEA N₂-laser the DFDL generated pulses as short as 7 ps with time-bandwidth product better than 0.6 and peak power ? 10 kW. At a repetition rate of 50 pps the shot-to-shot energy stability of a single picosecond pulse was ±18% at ±7% stability of the TEA N₂-laser.     

控制双激光脉冲的宽度提高N2分子的取向

提出了通过控制双激光脉冲宽度的方法来提高N₂分子取向程度. 利用数值方法求解了N₂分子刚性转子模型在双激光脉冲作用下的薛定谔方程,计算了双原子分子N₂在总强度固定的两束激光脉冲作用下,不同脉冲宽度对于N₂分子取向的影响. 研究结果表明,通过调整两束激光脉冲的宽度,选择合适的延迟时间能够有效提高N₂分子的取向程度. 关键词： 双激光脉冲 分子取向 脉冲宽度     

Parametric oscillation of high-power 3-THz pulse by synchronously pumped ZnGeP<Subscript>2</Subscript> crystal: Computer simulation

Possible parametric oscillation of 3-THz pulse at synchronous pumping of the ZnGeP₂ crystal by a train of short second-harmonic pulses from the CO₂ laser has been analyzed. Calculation shows that at changing laser pulse duration τ between 4 and 500 ps and correspondingly pumping energy density (0.5–3.5 J cm⁻²) THz pulse peak power varies from 3 to 70MW with maximum at τ =9 ps.     

High-energy long-pulse Nd:YAG picosecond laser

A long train of actively mode-locked pulses is obtained from a negative feedback controlled Nd:YAG laser. The 70s pulse train contains up to 0.1 J energy and the duration of the picosecond pulses is 120 ps. The laser is operated at a repetition rate of 20 Hz. ActiveQ-control of the cavity generates a short pulse train of duration 50 to 70 ns. An excellent peak-to-background intensity ratio of the correlation function for the picosecond pulses in the train of 5×10³ is reported.     

THz generation via optical rectification with ultrashort laser pulse focused to a line

We report on efficient THz pulse generation via optical rectification with femtosecond laser pulses focused to a line by a cylindrical lens. This configuration provides phase-matched conditions in the superluminal regime. 35 pJ THz pulses have been generated with this technique in a stoichiometric LiNbO₃ crystal pumped by 2 μJ femtosecond laser pulses at room temperature. An unusual superquadratic rise of the THz pulse energy with the laser pulse energy has been observed at high laser energies. This extraordinary energy dependence of the THz generation efficiency is explained by self-focusing of the laser beam in the crystal. Z-scan measurements and comparison of the THz pulse spectra created with laser pulses having different energies confirm this interpretation.     

Millimeter-wave pulse generation based on pulse reshaping using superstructure fiber Bragg grating

A novel optical approach is proposed to generate millimeter wave (MMW) pulse signal based on the pulse reshaping of superstructure fiber Bragg grating (SSFBG). In our scheme, one input pico-second Gaussian pulse is transformed into n Gaussian pulses by the SSFBG reshaping firstly, and then the pulse train is replicated to form a required frequency modulation MMW optical pulse envelope by the linear chirped fiber Bragg grating (LCFBG) or other highly dispersive element. The high-speed photodetector (PD) and band-pass filter can transform the MMW optical pulse into an MMW pulse signal ultimately. Depending on this scheme, MMW signals with frequency up to 10 GHz can be easily generated by the completed fiber components.     

Generation and detection of tunable ultrashort infrared and far-infrared radiation pulses of high intensity

We report on generation and detection of intense pulsed radiation with frequency tunability in the infrared and far-infrared spectral regions. Infrared radiation is generated with a transversally electrically excited high pressure CO₂ laser. A laser pulse of a total duration of about 300 ns consisted, due to self mode locking, of a series of single pulses, some with pulse durations of less than 450 ps and peak powers larger than 20 MW. Using these pulses for optical pumping of a Raman D₂O laser, trains of short far-infrared pulses with durations less than 400 ps were obtained. For detection a new ultrafast superconducting detector was used.     

Angular distributions of CH3I fragment ions under the irradiation of single pulse and trains of ultrashort laser pulses

The angular distribution of CH₃I is investigated experimentally using a single Fourier transform-limited laser pulse and a pulse train, where a 90-fs 800-nm linearly polarized laser field with a moderate intensity of 2.8×10¹³ W/cm² is used. The dynamic alignment is demonstrated in a single pulse experiment. Moreover, a pulse train is used to optimize the molecular alignment, and the alignment degree is almost identical to that with the single pulse. The results are analysed by using chirped femtosecond laser pulses, and it demonstrates that the structure of pulse train rather than its effective duration is crucial to the molecular alignment.     

Generation of a single attosecond pulse from an overdense plasma surface driven by a laser pulse with time-dependent polarization

The influence of time-dependent polarization on attosecond pulse generation from an overdense plasma surface driven by laser pulse is discussed analytically and numerically.The results show that the frequency of controlling pulse controls the number and interval of the generated attosecond pulse,that the generation moment of the attosecond pulse is dominated by the phase difference between the controlling and driving pulses,and that the amplitude of the controlling pulse affects the intensity of the attosecond pulse.Using the method of time-dependent polarization,a "single" ultra-strong attosecond pulse with duration τ≈ 8.6 as and intensity I ≈ 3.08 × 10 20 W·cm-2 can be generated.     

High frequency pulse trains from a self-starting additive pulse mode-locked all-fiber laser

In this paper, a coupled-cavity Er-doped fiber laser is experimentally developed and analyzed. The proposed scheme has the advantage of an all-fiber configuration. Two similar fiber Bragg gratings are employed as reflective components of the main cavity containing the gain medium. The second cavity is generated, in one side, by the reflective flat end of a standard fiber optic pigtail of variable length and, in the other, by one of the Bragg gratings belonging to the main cavity. Depending on the ratio between the lengths of both cavities, trains of stable and short pulses were obtained with a repetition frequency larger than the frequency of the main cavity. The repetition rate of the pulse trains experimentally obtained was as high as 780 MHz (15 times the main cavity frequency) and the pulse width was ∼110 ps. Prediction of the possible repetition rates for each cavities lengths ratio and the upgrading possibilities of this laser system are analyzed.     

Multiline short pulse amplification and compression in high gain CO2 laser amplifiers

Theoretical studies are presented for the amplification and compression of multiline short pulses of CO₂ laser radiation at 10.6 μ in a high gain, CO₂ laser amplifier. A method of efficiently generating high energy, subnanosecond pulses of CO₂ laser radiation is proposed utilizing a pulse tailoring technique recently disclosed by Figueira and Sutphin.     

Experiment on the optical tunneling process of electrons from a gold surface induced by mode-locked CO2 laser pulse trains

Using intense mode-locked CO₂ laser pulse trains we verified the occurrence of the laser-induced, new-type dynamic optical tunneling of electrons from a gold surface. The results are in accordance with the theoretical predictions and our previous experiments carried out for both atoms and gold surface using single longer (ns) CO₂ laser pulses. Unknown properties of the development of the CO₂ laser train are revealed based on our higher-order coherence detection of electron emission.     

Simultaneous generation and wavelength conversion of a pulse train from multi-wave mixing in an optical fibre

The insertion of three continuous-wave optical frequencies in a low-dispersion optical fibre resulted in the generation of a high-repetition-rate train of ultra-short pulses and its simultaneous wavelength conversion. Two of the frequencies were spaced by ∼0.17 THz and multi-wave mixing generated a frequency comb to which is associated a train of 1.6 ps pulses. Wave-mixing between the generated comb and the third input optical frequency then converts the pulse train into different wavelengths. The Split-Step Fourier method was applied to numerically simulate the generation/wavelength conversion and results are in good agreement with experiments.     

Coherent optical pulse evolution in a CO2 amplifier

The coherent reshaping of short duration (2–5 nsec) CO₂ laser pulses in a low-pressure (∽ 5 torr), longitudinal discharge CO₂ amplifier is experimentally studied in the linear regime for a variable number of gain lengths (αL?7). Single pulses grow considerably in duration as well as amplitude in agreement with theoretical considerations. Analysis of the observed pulse evolution is used to obtain the transverse relaxation parameter T₂. Zero-degree pulses {∫^+∞_-∞E ( z, t) dt = 0} are observed to terminate much of the long tail which occurs in single-pulse amplification. Off-resonant amplification of short-duration pulses is shown to lead to dramatic changes in the zero-degree pulse evolution. Numerical calculations relating to pulse amplification in the nonlinear regime for high-pressure CO₂ amplifiers are also presented.     

Development of ultra-short pulse VUV laser system for nanoscale processing

We have developed intense vacuum ultraviolet (VUV) radiation sources for advanced material processing, such as photochemical surface reactions and precise processing on a nanometer scale. We have constructed a new VUV laser system to generate sub-picosecond pulses at the wavelength of 126 nm. A seed VUV pulse was generated in Xe as the 7th harmonic of a 882-nm Ti:sapphire laser. The optimum conversion was achieved at the pressure of 1.2 Torr. The seed pulse will be amplified by the Ar₂^*\mathrm{Ar}_{2}^{*} media generated by an optical-field-induced ionization Ar plasma produced by the Ti:sapphire laser. We have obtained a gain coefficient of g=0.16 cm⁻¹. Our developing system will provide VUV ultra-short pulses with sub-μJ energy at a repetition rate of 1 kHz.