Experimental investigation of high-energy wave-breaking-free-pulse generation in bidirectional-pumping all-fiber laser

We report a passively mode-locked all-fiber laser with bidirectional pumping, emitting high-energy wave-breaking-free pulses. Experimental investigations show that forward and backward pump powers mainly contribute on the chirp and the nonlinear phase shift of pulses, respectively. Nonlinear chirp pulses with 50 nJ pulse energy, 340 ps duration at a 8.2 MHz repetition rate are directly emitted from the all-fiber laser pumped by two 550 mW, 977 nm laser diodes. The pulses cannot be compressed to near the transform limit, showing that the chirp of pulses is nonlinear.     

Chirped optical X-shaped pulses in material media

We analyze the properties of chirped optical X-shaped pulses propagating in material media without boundaries. We show that such ("superluminal") pulses may recover their transverse and longitudinal shapes after some propagation distance, whereas the ordinary chirped Gaussian pulses can recover their longitudinal width only (since Gaussian pulses suffer a progressive transverse spreading during their propagation). We therefore propose the use of chirped optical X-type pulses to overcome the problems of both dispersion and diffraction during pulse propagation.     

Multipass Amplification Using Optical Phase Conjugation by Stimulated Brillouin Scattering

Abstract

We built up a multipass amplifier for multiplication of infrared nanosecond pulses based on stimulated Brillouin scattering in acetone. Starting with a Q-switched Nd-doped yttrium aluminium garnet oscillator emitting pulses with less than 5 ns duration, we were able to generate a train of up to five pulses with a pulse shortening. Because only phase-conjugating mirrors have been used, we expect the pulses to have a high transverse homogeneity.     

Nontrivial polarization shaping of femtosecond pulses by reference to the results of dual-channel spectral interferometry

Adaptive shaping of time-dependent polarization pulses is performed by reference to the analyzed results of dual-channel spectral interferometry. The desired pulses can be generated only by use of such a polarization-characterization technique. We demonstrate the generation of shaped femtosecond pulses whose ellipticity increases at a constant rate. The relative error between the shaped pulse and the target pulse is less than 6% over the main part of the pulse. Shaped time-dependent polarization pulses have many potential applications.     

A design method of spatiotemporal optical pulse using level-set based time domain topology optimization

Recently, two emerging areas of photonics research, ultrafast photonics, and nanophotonics have started to come together. One of the main problems in this field is the precise control of spatial and temporal profiles of the optical pulses. In this paper, we propose a design method for user-specified spatiotemporal optical pulses using a level set-based time-domain topology optimization method. In the proposed method, the optimization problem is formulated based on time domain Maxwell equations so that the spatiotemporal optical pulses can be treated directly. The objective function is defined using the envelope information of the pulses, and an efficient way to calculate this information, based on calculations of the complex electromagnetic field, is introduced. A level set-based topology optimization method is applied to obtain optimized configurations. Using the proposed method, the spatiotemporal user-specified pulse profiles can be designed by modifying the structural details of the nanostructures through which the pulses propagate. As a simple example, we demonstrate that the optimized structures focus optical pulses into a single or multiple focal points with a user-specified pulse-width. The results show that the proposed method is able to design highly controlled spatiotemporal optical pulses by engineering the nanophotonic structure.     

Interferometric generation of parametrically shaped polarization pulses

We demonstrate the capabilities of the recently introduced interferometric parallel pulse shaper setup and present a method for fully tailoring the three-dimensional electrical field of femtosecond laser pulses. The possibility of producing parametric polarization pulses with arbitrary orientations and ellipticities in time is demonstrated with a selection of example pulses.     

Propagation of Bessel-X pulses in a hybrid photonic crystal

We report the propagation of Bessel-X pulses in a two-dimensional hybrid photonic crystal, investigated by the finite-difference time-domain method, in which broadband super-collimation and the propagation of self-collimated ultrashort pulses were reported. We first show the propagation of Bessel-X pulses in two-dimensional free space, whose transverse branches diverge rapidly with propagation. We then show that Bessel-X pulses propagate with their transverse and longitudinal shapes almost unchanged in the hybrid photonic crystal.     

Differences in the evolution of surface-microstructured silicon fabricated by femtosecond laser pulses with different wavelength

We experimentally investigate the differences in the evolution of surface-microstructured silicon fabricated by femtosecond laser pulses with different wavelength as a function of irradiated laser energy. The results show that when laser energy absorbed by the silicon material is the same, laser pulses with a shorter wavelength can form the surface-microstructured silicon with less laser energy, while the corresponding spike height is much lower than that of laser pulses with a longer wavelength. This is because the penetration depth of the laser pulses increases exponentially at the increase of the laser wavelength. Additionally, for two laser pulses with the certain wavelength and the certain absorption efficiency of silicon, the proportional relations between their formed spike height and irradiated laser energy should be determined. In particular, the average spike height is 3 times with 8 times corresponding energy for 800 nm laser pulses than that of 400 nm. These results are a benefit for the fast and optimum-morphology preparation of microstructured silicon.     

Counting signals from radioactivity-measurement systems

The original time-interval distribution of Poisson events, as for instance those originating from the decay of a radioactive source, is modified by the finite width of the electrical pulses. For the simple case of assumed rectangular pulses it is found that the available counting signals can be subdivided into single pulses and pile-up pulses (of a given order). The probability for observing a counting signal of order i is derived in a simple way and expressions for mean width and count rate are given. Confirmation has been obtained both by simulation and by experiments with a random-pulse generator.     

An experimental investigation of the stability of CMOS-logic integrated Schmitt triggers to ultra-short pulse overloads

The degradation of semiconductor CMOS structures due to the action of overloads in the form of ultra-short video pulses is investigated. It is shown that these video pulses in Schmitt triggers produce a change in the threshold voltages and a contraction of the area of uncertainty. In buffer elements, intense phase noise occurs, while in RC generators there is a change in the length and repetition frequency of the pulses generated. Experimental equipment for determining the critical parameters of the actions of ultra-short video pulses on a sample of digital microcircuits and CMOS structures is described.     

Transient response of superconducting Pb microbridges irradiated by picosecond laser pulses and its potential applications

We have observed voltage pulses having half-widths of less than 500ps generated by constant-current-biased superconducting Pb variable thickness microbridges driven normal by short (3-5ps) light pulses. This represents a first step in the effort to generate even shorter pulses, which according to our analysis of the Rothwarf-Taylor equations should be possible. The ultimate width should be equal to the phonon pair-breaking time, which for materials such as Nb can be as short as a few picoseconds. In addition to monitoring the voltage pulses directly, we have used a novel adoption of the optical autocorrelation technique having a time resolution limited only by the laser pulse width. It is pointed out that even shorter voltage pulses, and therefore greater potential for device applications, can be achieved by direct injection of quasiparticles.     

Diffraction of spatiotemporally localized X-wave pulses from a screen containing two rectangular slits

The diffraction of X-waves from a screen containing two slits is investigated. It is shown that when the peak of the incident X-wave hits the screen midway between the slits, two pulses are generated from each slit. These pulses follow laterally skewed trajectories relative to the direction of propagation of the incident X-wave along the central axis of the configuration. One of the two pulses converges on the central axis and the other diverges away from it. A geometrical construction explaining the behavior of these pulses is provided. It is shown that the trajectory of each radiated pulse can be deduced from the intersection of two curved wavefronts emanating from the two edges of each slit. The pulses converging on the central axis meet at a certain range, thus suggesting a novel focusing scheme.     

Mode-locked Yb-doped fiber laser with external compression to 89 fs in normal dispersion fiber

A practical ytterbium-doped mode-locked fiber source producing 89 fs pulses without an external bulk compensator was developed by investigating the pulse propagation dynamics in a mode-locked fiber laser with small average dispersion. Negatively chirped pulses are taken from the cavity and compressed in a standard output fiber resulting in high-quality pulses.     

Two-colour laser-induced electron dynamics in two-level quantum system

To attempt to control the quantum state of a physical system with a femtosecond two-colour laser field, a model for the two-level system is analysed as a first step. We investigate the coherent control of the two-colour laser pulses propagating in a two-level medium. Based on calculating the influence of the laser field with various laser parameters on the electron dynamics, it is found the electronic state can be changed up and down by choosing the appropriate laser pulses and the coherent control of the two-colour laser pulses can substantially modify the behaviour of the electronic dynamics: a quicker change of two states can be produced even for small pulse duration. Moreover, the oscillatory structures around the resonant frequency and the propagation features of the laser pulses depend sensitively on the relative phase of the two-colour laser pulses. Finally, the influence of a finite lifetime of the upper level is discussed in brief.     

Fiber-optic transmission of stretched pulses from a Q-switched ruby laser

Fiber-optic transmission of Q-switched ruby laser pulses is limited by fiber damage owing to the high laser-beam intensities. Pulse stretching with a semiconductor-based control circuit for the Pockels cell of the ruby laser to reduce the peak intensities is described. Pulses with durations from 200 ns to 1 μs and a coherence length of ~3 m were generated. These pulses were coupled into multimode optical fibers to investigate the transmission characteristics and the limits of transmittable pulse energies. Stretched pulses can be transmitted in quartz fibers with a 600-μm core diameter to pulse energies of 300 mJ, which is an increase by a factor of 4 compared with standard Q-switched pulses. It is expected that beam guiding of ruby laser pulses by fiber optics will significantly facilitate the use of holographic interferometry in technical applications such as vibration analysis.     

Diffraction characteristics of 10 fs laser pulses passing through an aperture

Results are presented on experimental and theoretical work performed to compare diffraction phenomena for ultrashort 10 fs pulses and continuous-wave propagation modes illuminating different-sized pinholes and slits. Results demonstrate that 10 fs pulses do not produce high-frequency diffraction like that produced with continuous-wave illumination. The diffraction through a 1 mm pinhole of temporally stretched pulses obtained by using fused silica plates whose frequency spectrum remains the same is compared with those of 10 fs pulses. The overall diffraction intensity profiles are, however, nearly identical in this case. The simulations of diffraction patterns for 100 fs, 10 fs, and 1 fs incident pulse were compared theoretically for different aperture sizes and frequencies. Calculations indicate that the lack of high-frequency diffraction for the mode-locked case is due to the broadband nature of the ultrashort laser pulses; i.e., the distribution of the frequency contained in the pulse ends up washing out when objects are illuminated with pulses of broad frequency content. The results of this work have important application in biomedical imaging and remote imaging applications, to name only a few.     

Complete characterization of optical pulses by real-time spectral interferometry

We demonstrate a simple method for complete characterization (of amplitudes and phases) of short optical pulses, using only a dispersive delay line and an oscilloscope. The technique is based on using a dispersive delay line to stretch the pulses and recording the temporal interference of two delayed replicas of the pulse train. Then, by transforming the time domain interference measurements to spectral interferometry, the spectral intensity and phase of the input pulses are reconstructed, using a Fourier-transform algorithm. In the experimental demonstration, mode-locked fiber laser pulses with durations of approximately 1 ps were characterized with a conventional fast photodetector and an oscilloscope.     

Optical limiting studies in a carbon-black suspension for subnanosecond and subpicosecond laser pulses

This paper presents results of measurements of short (0.3 ps, 0.2 ns, and 10 ns) laser pulse transmissions through a liquid suspension of fine carbon particles (named CBS for "carbon-black suspension") at input-pulse energies up to 10 mJ. The 10-ns pulses came from a Nd:YAG laser at 1064 nm, and the shorter pulses came from a Ti:sapphire laser at 800 nm. Limiting was observed with the 10-ns and the 0.2-ns laser pulses, but the 0.3-ps pulses produced white light and underwent the same level of attenuation in the solvent and in the CBS.     

Side effects of the use of cylindrical grooves in planar intrinsic germanium detectors

One technique for producing planar intrinsic germanium detectors involves machining a deep groove in the detector crystal just inside the outer cylindrical surface and running from one plane face almost to the other. Weak collection fields near the base of the groove can then be expected to lead to the appearance of some output pulses having very long rise times. Two detectors of this type have been examined. It was found that at low counting rates and in the absence of pulse shape discrimination slowly rising pulses were processed as if they formed a continuum of low energy pulses extending into regions where with a mono-energetic gamma source pulses would not normally be present. At high counting rates the smallest slow pulses, which escape the attention of the pile-up detection circuits, were found to give rise to lines with high energy tails, and to excess rate dependent line broadening.     

Pulsed Third Sound in the Presence of a Superfluid Flow

We report measurements of pulsed third sound in the presence of a uniform superfluid flow at T=0.89 K. The superfluid flow is created using a heater technique. For increasingly large values of power dissipated in the heater used to create the flow, the shapes of the received third sound pulses evolve. The pulses propagating parallel to the flow grow while the pulses propagating against the flow decrease. When the heater power is increased further such that film thinning effects occur, the pulses propagating antiparallel to the flow qualitatively resemble the derivative of the drive pulse. Similar effects were also observed in thin films at T=1.35 K. A possible explanation for the growth and decrease of the pulses will be discussed. The cause of the pulse shapes that resemble the derivative of the drive pulse is not understood.