Single Ultrashort Pulse Research Articles

On the Determination of Spectral Properties of Non-stationary Radiation

The measurement of the time-dependent spectrum of non-stationary radiation is of interest, for instance, in the field of ultrashort-time spectroscopy using picosecond lasers. The incident non-stationary radiation is characterized by the time-dependent input spectrum. The electric field strength of the radiation in the exit plane of a grating spectrometer and of a prism spectrometer is analysed in terms of its dependence on time and on the spectral coordinate. The action of the grating spectrometer is regarded as a special case of diffraction at an arbitrary diaphragm. The intensity registered by a square-law detector as a function of time and the spectral coordinate (output spectrum) is calculated as a function of the input spectrum. The input spectrum can be determined from the output spectrum by mathematical operations for cases important in practise (very short light-pulses, intrinsically stationary radiation). Moreover, the conditions are investigated under which the output spectrum resembles the input spectrum, taking into account the influence of noise. Two examples of intrinsically stationary radiation (bandwidth-limited pulse, fluorescence of a two-level system) are studied in more detail. For experimental verification the ouput spectrum of a single ultrashort light pulse has been measured under the condition of different spectrometer time constants.

Use of various nonlinear crystals to measure the duration of single ultrashort pulses

An investigation is made of the feasibility of using various uniaxial and biaxial nonlinear crystals to measure the autocorrelation function of a single picosecond optical pulse using the second-harmonic beam method. General requirements are formulated for the wave interaction geometry in a crystal in order to maximize the temporal resolution and sensitivity of the autocorrelator. An analysis is made of LiIO3, KDP, KB5O84H2O and LiCHOOH2O crystals. The optimum interaction geometry is determined and the scanning duration and upper measurement limit are calculated for each crystal. Measurements of these quantities are reported for an LiIO3 crystal autocorrelator.

Dependence of the decrease in contrast ratios on the intensity of the laser pulse for two-photon fluorescence

The contrast ratio for two-photon fluorescence (TPF) is found to depend on the laser pulse intensity. The peak-to-background ratio is observed to decrease from the ideal value of 3 to about 2.1 as the intensity of the single Nd-glass laser ultrashort pulse is increased from 15 to 60 GW/cm2. Parametric analysis of the autocorrelation function for the pulse amplitude shows that this effect can be explained by two-photon attenuation of the laser pulse by the TPF medium.

Generation of single ultrashort pulses using dye solutions in a short resonator

Generation of single ultrashort pulses using dye solutions in a short resonator

Generation of frequency-tunable single ultrashort light pulses in an LiIO3crystal

Efficient parametric conversion of single ultrashort light pulses in an LiIO3 crystal was achieved. The parametric signal energy reached 200 μJ when the crystal was pumped with the second harmonic of a mode-locked YAG:Nd3+ laser. The wavelength of this signal could be tuned continuously in the range 0.66–2.75 μ. A Pikokhron image converter was used to show that the duration of the parametric signal decreased considerably to 2–3 psec, which was much less than the duration of the pump pulses.

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 N 2 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.

An optical discrimination amplifier

Measurements on an optical discrimination amplifier are presented which demonstrate its ability to increase the S/N ratio of single ultrashort pulses by 2 to 3 orders of magnitude. The amplifier consists of a series of amplifiers and dye cells such that for each dye-amplifier section the small-signal gain is less than one.

Structure of the spectrum of a single ultrashort pulse emitted by a neodymium-glass laser

An investigation was made of the spectra of ultrashort pulses, selected from different parts of a pulse train, and of the spatial distribution of the spectral components in a cross section of a single-mode laser beam. The spectra were found to be asymmetric with a predominance of the short-wavelength wing.

Generation of single ultrashort light pulses with the aid of a "combustible" mirror

The power density in the resonator of a self-mode-locked neodymium laser was limited by a mirror with a metal coating which was burnt through during the initial stage of the laser action. This limitation made it possible to increase considerably the initial density of the nonlinear absorber used in the resonator. In this way stable and sufficiently full mode locking was achieved. A considerable variation in the amplitudes of the ultrashort pulses at the beginning of a train was due to the use of a dense nonlinear absorber. This circumstance, in combination with the stoppage of the laser action on destruction of the mirror coating, enabled us to select reliably a single ultrashort pulse with a contrast of the order of 10.

GENERATION OF ULTRASHORT LIGHT PULSES BY MEANS OF LASERS

A review is given of the theory of the generation of ultrashort light pulses in lasers with bleachable filters. Attention is mainly concentrated on the statistical aspects of the formation of ultrashort light pulses in such lasers. The topics discussed include the formation of laser modes, the narrowing of the spectrum in the stage before the bleaching of the filter, the transformation of the field profile during the bleaching process, the statistics of the appearance of single ultrashort pulses, and the influence of the gain saturation on the time characteristics of the radiation. Several additional effects, which influence the final structure of the radiation, are considered. A quantitative theory is given of the widely used two-photon method for measuring ultrashort pulses and a basic shortcoming of this method is pointed out: it is not possible to find whether one or many pulses appear in a given period.