Sequence Of Ultrashort Pulses Research Articles

Increasing the amplitude of ultrashort microwave pulses in a passive mode-locked oscillator by the method of double nonlinear filtering

We show the possibility of increasing the amplitude of ultrashort microwave pulses in a passive mode-locked oscillator in a circuit containing a chain of traveling wave tubes (TWTs) one of which operates in the amplification regime and the other two in the nonlinear Kompfner suppression regime. An increase in the number of nonlinear filtering stages makes it possible to enhance the filtering threshold level, below which low background noise is suppressed, while an almost complete transparency is maintained for peak amplitudes of the pulse signal. As a result, with an increase in the length of the amplifier, a linear growth in the peak amplitude of microwave pulses takes place because of the commutative extraction of energy from different electron fractions, along which the pulse is shifted due to the difference in its group velocity from the translational velocity of the particles. With an optimal selection of the parameters of the absorbers, the peak amplitude of the generated periodic sequence of ultrashort pulses can exceed the amplitude of stationary generation by more than an order of magnitude for given parameters of the TWT like current, voltage, and coupling impedance.

Soliton molecules in femtosecond fiber lasers: universal binding mechanism and direct electronic control

Sequences of ultrashort pulses form the basis of extremely precise laser applications ranging from femtosecond spectroscopy, to material microprocessing, to biomedical imaging. Dynamic patterns of temporal solitons—termed “soliton molecules”—inside mode-locked cavities provide yet unexplored means for generating reconfigurable arrangements of ultrashort pulses. Here, we demonstrate the external control of solitonic bound states in widespread erbium-doped fiber lasers via direct electronic modulation of the semiconductor pump source. This straightforward approach allows for switching between discrete soliton doublet states of picosecond separations, employing and relying on laser-intrinsic soliton interactions. We analyze the externally induced dynamics based on real-time switching data acquired by time-stretch dispersive Fourier transform spectroscopy and identify a universal bound-state formation mechanism different from broadly considered models. Owing to the ease of implementation and its intrinsic tunability, our control scheme is readily applicable to various laser platforms enabling, e.g., rapid multipulse measurements and tailored nonlinear light–matter interactions.

Effects of the induced seed pulse on generation of ultra-short pulse sequences by dark solitons

We study the impact of the seed pulse on ultra-short pulse sequences and narrow-band sources produced by pumping two input pulses that are identical but delayed one with another in a 10-mm long silica photonic crystal fiber with two ZDWs. The collision between the induced seed pulse and input pulses will increase the power of ultra-short pulse sequences in long-wavelength region. The darkness of dark solitons on the red side of pump is influenced by modulation frequencies and depths of the seed pulse. In particular, the energy red-shift of narrow-band sources can be obtained by tuning the parameters of the seed pulse. Besides, initial chirp also influences the generation of ultra-short pulse sequences by dark solitons. We have shown that it is effective to control the power of ultra-short pulse sequences by varying the parameters of the seed pulse.

Observation of the quasi-discrete supercontinuum at optical event horizon

Numerically study of the formation of the quasi-discrete supercontinuum (SC) with ultrashort pulse sequence in an optical analogue is reported in the paper. The quasi-discrete SC is produced by pumping an intense pulse and a probe on each side of the zero-dispersion wavelength (ZDW) in a fiber. The nonlinear reflection between them causes the emission of new frequencies and gives rise to the generation of ultrashort pulses. Isolated narrow-band sources occur via the spectra interference of the ultrashort pulses, forming the quasi-discrete SC. Ultrashort pulses with different width are generated by changing the central wavelength of probe. Besides, high energy conversion efficiency and maximum frequency shift caused by the soliton–DW interaction also can be manipulated by varying the central wavelength of probe and time delay of the input pulses. So wider supercontinuum and high intensity tunable ultrashort pulses sequence can be acquired.

Generation of tunable ultra-short pulse sequences in a quasi-discrete spectral supercontinuum by dark solitons

We explore how to acquire the tunable ultra-short pulse sequences in a quasi-discrete spectral supercontinuum (SC) via the formation of dark solitons in a fiber with two zero dispersion wavelengths (ZDWs). These dark solitons are produced by pumping two pulses in the normal dispersion that are identical but delayed one with respect to the other. Few-cycle pulses with high power as dual pumps experience temporal breakdown, resulting in a nearly-complete conversion of pump energy into two normal dispersion regions to form the ultra-short pulse sequences separated by dark solitons. The spectral interference of these generated ultra-short pulses gives rise to the isolated narrow-band sources, shaping a quasi-discrete spectral SC. Based on the combined effect of group-velocity dispersion and the initial time delay between dual pumps, the spectral width of narrow-band sources behaves in such a similar manner to the temporal width of ultra-short pulses that they are different in two normal dispersion regions. Moreover, they can be regulated considerably by tuning the time delay and pump power. Furthermore, the control of time delay and pump power can bolster the manipulation on the number of ultra-short pulses and narrow-band sources.

Tri-Comb and Quad-Comb Generation Based on a Multi-Dimensional Multiplexed Mode-Locked Laser

By introducing an additional comb beyond its dual-comb predecessor, tri-comb consisting of three combs with different mode spacing has shown its unique potential in several metrology areas such as nonlinear spectroscopy, frequency, and distance measurement. However, tri-comb generation based on three frequency-stabilized comb lasers would be overly complicated and costly for many potential applications. Previous studies had realized dual-comb generation from a single-cavity laser by leveraging multiplexing in one of the waveguiding dimensions of the optical pulses. Such sources with built-in passive mutual coherence could greatly reduce system complexity. Yet, squeezing more combs into one multiplexing dimension becomes increasingly difficult without significant sacrifice in performance, which could jeopardize the usefulness of such a scheme. In this paper, it is proposed and demonstrated that multi-comb generation could be realized by tapping into the multidimensional joint parameter space. Up to four wavelength/polarization multiplexed, asynchronous ultrashort pulse sequences can be generated with good stability from an all-fiber, mode-locked ring-cavity laser. This could offer an attractive alternative solution as the low-complexity light sources for field-deployable multi-comb metrology applications.

Generation of spectral wings and temporal ultra-short pulse sequence

We study the generation of supercontinuum (SC) with spectral wings and temporal ultra-short pulse sequence by pumping two identical time-delayed pulses in an all-normal dispersion fibre. The flat dispersion profile over the entire wavelength range of the generated SC provides an opportunity to divide these ultra-short pulses roughly evenly. By adjusting the time delay, the temporal width of these ultra-short pulses can be regulated considerably, which accelerates the development in numerous fields such as multi-photon microscopy and ultrafast spectroscopy. Besides, the temporal overlap of two pulses at different instantaneous frequencies leads to the emergence of spectral wings via four-wave mixing. By changing the input power and time delay, spectral wings with variable intensities and ranges can be emitted at different distances, which can expand the SC spectral bandwidth within a limited fibre length.

Generation of a Periodic Sequence of High-Power Ultrashort Pulses in a Chain of Coupled Backward-Wave and Traveling-Wave Tubes Operating in the Regimes of Amplification and Nonlinear Kompfner Suppression

We have studied the dynamics of an electronic microwave generator with backward-wave and traveling-wave tubes connected in the feedback circuit and operating in the regimes of amplification and nonlinear Kompfner absorption, respectively. It is shown that radiation generated in such a system can have the form of a periodic sequence of ultrashort pulses. The studied mechanism of pulse generation is analogous to the method of passive mode locking, which is widely used in laser physics.

Model of ultra-wideband signal transmission and reception using the pseudorandom carrier

We show the possibility of transmission and subsequent decoding of an ultra-wideband signal in an optical communication channel using a noise-like carrier. For an example, the simulation of the transmission of word encoded with Baudot telegraph code was realized. The necessary sequence of symbols was introduced into the noise-like carrier represented by a pseudo-random sequence of ultra short pulses. The possibility of the transmitted word decoding to a high degree of reliability was shown by calculation of auto- and cross-correlation functions. To some extent, the method can be treated as an analog of pulse-position modulation.

Generation of periodic high-power ultrashort pulse sequences in a chain of coupled traveling-wave tubes operating in the regimes of amplification and nonlinear Kompfner suppression

We have studied the dynamics of an electron microwave oscillator with a feedback loop containing two coupled traveling-wave tubes, the first operating in the regime of amplification and the second in the regime of nonlinear Kompfner absorption. It is established that oscillations generated in this system can take the form of a periodic sequence of ultrashort pulses. The proposed mechanism of pulse generation is analogous to the well-known method of passive mode locking.

Fibre laser with a subterahertz repetition rate of ultrashort pulses in the telecom range

We have investigated a new fibre laser configuration for the generation of ultrashort pulses at a repetition rate far exceeding the fundamental cavity frequency. The laser configuration includes a nonlinear amplifying mirror as an artificial saturable absorber for mode locking and a spectral comb filter for pulse separation stabilisation. Generation of trains and sequences of ultrashort pulses at a repetition rate tunable in the range has been demonstrated experimentally. The pulses generated by the laser have been shown to retain an ordered, equidistant structure on a nanosecond timescale.

Controlling the Radiation Parameters of a Resonant Medium Excited by a Sequence of Ultrashort Superluminal Pulses

We investigate the possibility of controlling the radiation parameters of a spatially periodic one-dimensional medium consisting of classical harmonic oscillators by means of a sequence of ultrashort pulses that propagate through the medium with a superluminal velocity. We show that, in the spectrum of the transient process, in addition to the radiation at a resonant frequency of oscillators, new frequencies arise that depend on the period of the spatial distribution of the oscillator density, the excitation velocity, and the angle of observation. We have examined in detail the case of excitation of the medium by a periodic sequence of ultrashort pulses that travel with a superluminal velocity. We show that it is possible to excite oscillations of complex shapes and to control the radiation parameters of the resonant medium by changing the relationship between the pulse repetition rate, the medium resonant frequency, and the new frequency.

Conversion of recoillessγradiation into a periodic sequence of short intense pulses in a set of several sequentially placed resonant absorbers

An efficient technique to produce a periodic sequence of ultrashort pulses of recoilless gamma-radiation via its transmission through the optically thick vibrating resonant absorber was demonstrated recently [Nature, 508, 80 (2014)]. In this work we extend the theoretical analysis to the case of a set of multiple absorbers. We consider an analytical model describing the control of spectral content of a frequency modulated gamma-radiation by selective correction of amplitudes and initial phases of some spectral components, using, respectively, the resonant absorption or dispersion of nuclei. On the basis of the analytical solutions we determine the ultimate possibilities of the proposed technique.

Fresnel lens for the generator of encoded sequences of ultrashort pulses. The spectral evidence of series of pulses formation

Fresnel lens is considered as the effective instrument for the transformation of a single femtosecond pulse into a series of pulses corresponding to some digital code. The theory describing transformation of spectrum of a finite series of equidistant pulses is developed and compared with experimental data received for two Fresnel lens patterns. Accordance of the measured and calculated spectra confirmed the basic idea of pulse transformation method.

The Lack of Resonance Problem in Coherent Control with Real-Time Time-Dependent Density Functional Theory.

We will show that adiabatic real-time TDDFT predicts a time- and energy-dependent electronic structure of molecules, which makes it hard to combine TDDFT with coherent control schemes that depend on resonance conditions. In this study, we use sequences of ultrashort pulses, separated by long intervals of field free evolution, to illustrate this phenomenon for two molecules and two functionals. In coherent control scenarios, long laser pulses, with many oscillation periods, excite the system continuously and, in this way produce, a time-dependent electronic structure.

Factorization of numbers with Gauss sums: II. Suggestions for implementation with chirped laser pulses

We propose three implementations of the Gauss sum factorization schemes discussed in part I of this series (Wölk et al 2011 New J. Phys. 13 103007): (i) a two-photon transition in a multi-level ladder system induced by a chirped laser pulse, (ii) a chirped one-photon transition in a two-level atom with a periodically modulated excited state and (iii) a linearly chirped one-photon transition driven by a sequence of ultrashort pulses. For each of these quantum systems, we show that the excitation probability amplitude is given by an appropriate Gauss sum. We provide rules on how to encode the number N to be factored in our system and how to identify the factors of N in the fluorescence signal of the excited state.

Two-level model of a superradiant laser with distributed feedback

Numerical one-dimensional simulation of the superradiance phenomenon in a sample of a two-level medium with cw pumping under the conditions of resonant Bragg backscattering of counterpropagating waves has been performed. A range of parameters is determined in which a superradiant laser generating a chaotic sequence of ultrashort high-power pulses can be implemented, and the properties of these pulses are studied.

Ultrafast coherent control and quantum encoding of molecular vibrations in D2+ using intense laser pulses

Intense, few-femtosecond pulse technology has enabled studies of the fastest vibrational relaxation processes. The hydrogen group vibrations can be imaged and manipulated using intense infrared pulses. Through numerical simulation, we demonstrate an example of ultrafast coherent control that could be effected with current experimental facilities, and observed using high-resolution time-of-flight spectroscopy. The proposal is a pump-probe-type technique to manipulate the D2+ ion with ultrashort pulse sequences. The simulations presented show that vibrational selection can be achieved through pulse delay. We find that the vibrational system can be purified to a two-level system thus realizing a vibrational qubit. A novel scheme for the selective transfer of population between these two levels, based on a Raman process and conditioned upon the delay time of a second control-pulse is outlined, and may enable quantum encoding with this system.

Reducing decoherence of the confined exciton state in a quantum dot by pulse-sequence control

We study the phonon-induced dephasing of the exciton state in a quantum dot excited by a sequence of ultrashort pulses. We show that the multiple-pulse control leads to a considerable improvement of the coherence of the optically excited state. For a fixed control time window, the optimized pulsed control often leads to a higher degree of coherence than the control by a smooth, single Gaussian pulse. The reduction of dephasing is considerable already for 2--3 pulses.

Reconfigurable Sampling of the Electric Field at the Reflecting Surface of Folded Arrayed Waveguide Grating Multiplexers

We describe the use of a reflective arrayed waveguide grating multiplexer in conjunction with a digital micromirror device to modulate the electric field distribution in the grating. Modulation of the electric field is accomplished by turning individual digital mirrors on and off. By imposing a reconfigurable grating superstructure on the digital mirror array, we can change the free-spectral range of the device and alter the overall transmission spectra. The experimental results are in excellent agreement with model calculations. This type of a device is expected to be useful in generation of reconfigurable sequences of ultrashort pulses.