Generation Of Few-cycle Pulses Research Articles

Generation of few-cycle laser pulses by coherent synthesis based on a femtosecond Yb-doped fiber laser amplification system

We demonstrate a coherent synthesis system based on femtosecond Yb-doped fiber laser technology. The output pulse of the amplification system is divided into two replicas and seeded into photonic crystal fibers of two parallel branches for nonlinear pulse compression. Because of the different nonlinear dynamics in the photonic crystal fibers, the compressed pulses show different spectra, which can be spliced to form a broad coherent spectrum. The integrated timing jitter between the pulses of two branches is less than one tenth of an optical cycle. By coherently synthesizing pulses from these two branches, 8 fs few-cycle pulses are produced.

Wavelength-tunable few-cycle pulses in visible region generated through soliton-plasma interactions.

We numerically investigate the generation of wavelength-tunable few-cycle pulses in the visible spectral region through soliton-plasma interactions. We found that in a He-filled single-ring photonic crystal fiber (SR-PCF), soliton-plasma interactions could shift the optical spectra of pulses propagating in the fiber to shorter wavelengths. Through adjusting the single pulse energy launched into the fiber, the central wavelength of these blueshifting pulses could be continuously tuned over hundreds of nanometers, while maintaining a high energy conversion efficiency of >57%. Moreover, we observed that during the nonlinear pulse propagation in the SR-PCF, soliton self-compression effects enhanced the plasma density in the fiber at high pulse energies, which could modulate the phase-matching condition of ultraviolet (UV) dispersive wave (DW) generation. Furthermore, we employed the recently-developed model to study numerically the loss and dispersion of the SR-PCF in its resonant and anti-resonant spectral regions, and demonstrated the remarkable influence of the core-cladding resonance on the process of soliton-plasma interactions. The numerical results demonstrated here pave the way to develop wavelength-tunable, few-cycle light sources in the visible region, which may have considerable application potential in pump-probe spectroscopy and strong-field physics.

Propagation of few-cycle pulses in a nonlinear medium and an integrable generalization of the sine-Gordon equation

The generalized sine-Gordon equation is obtained under the theoretical investigation of interaction of few-cycle pulses in a nonlinear medium modeled by a set of four-level atoms. This equation is derived without the use of the slowly varying envelope approximation and is shown to be integrable in the frameworks of the inverse scattering transformation method. Its solutions describing the propagation of the solitons and breathers and their interaction are investigated. In the case of different signs of the parameters of the equation considered, it is revealed, in particular, that the collision of solitons with opposite polarities can lead to an appearance of the short-living pulse having extraordinarily large amplitude, whose dynamics is similar to that of rogue waves. Also, the solitons of ``rectangular'' form and the breathers with rectangular oscillations exist in the case of the same signs of the parameters.

Generation of millijoule few-cycle pulses at 5 μm by indirect spectral shaping of the idler in an optical parametric chirped pulse amplifier

Spectral pulse shaping in a high-intensity midwave-infrared (MWIR) optical parametric chirped pulse amplifier (OPCPA) operating at 1 kHz repetition rate is reported. We successfully apply a MWIR spatial light modulator (SLM) for the generation of ultrashort idler pulses at 5 μm wavelength. Only bulk optics and active phase control of the 3.5 μm signal pulses via the SLM are employed for generating compressed idler pulses with a duration of 80 fs. The 80-fs pulse duration corresponds to less than five optical cycles at the central wavelength of 5.0 μm. The pulse energy amounts to 1.0 mJ, which translates into a peak power of 10 GW. The generated pulse parameters represent record values for high-intensity MWIR OPCPAs.

Deterministic generation of single soliton Kerr frequency comb in microresonators by a single shot pulsed trigger.

Kerr soliton frequency comb generation in monolithic microresonators recently attracted great interests as it enables chip-scale few-cycle pulse generation at microwave rates with smooth octave-spanning spectra for self-referencing. Such versatile platform finds significant applications in dual-comb spectroscopy, low-noise optical frequency synthesis, coherent communication systems, etc. However, it still remains challenging to straightforwardly and deterministically generate and sustain the single-soliton state in microresonators. In this paper, we propose and theoretically demonstrate the excitation of single-soliton Kerr frequency comb by seeding the continuous-wave driven nonlinear microcavity with a pulsed trigger. Unlike the mostly adopted frequency tuning scheme reported so far, we show that an energetic single shot pulse can trigger the single-soliton state deterministically without experiencing any unstable or chaotic states. Neither the pump frequency nor the cavity resonance is required to be tuned. The generated mode-locked single-soliton Kerr comb is robust and insensitive to perturbations. Even when the thermal effect induced by the absorption of the intracavity light is taken into account, the proposed single pulse trigger approach remains valid without requiring any thermal compensation means.

Laser beam deflector based generation of few-cycle electromagnetic pulses in a circular nonlinear medium

We study theoretically a novel possibility for the emission of few-cycle pulses via excitation of nonlinear oscillators arranged in a thin circular string by a spot of light moving along the string. Such excitation can be realized by a beam emitted from a source at some distance, when the beam rotates with high constant angular velocity. Here we analyze the possibility to realize such setup using ultrafast beam deflectors and a diffractive optical element (DOE). We show that the pulse duration, amplitude and spectrum can be controlled by the spot velocity, the resonance frequency of the medium as well as by the parameters of the DOE. Remarkably, the electric field area of the generated field does not depend on DOE characteristics. These theoretical predictions open novel opportunities in generation of few cycle pulses with controllable duration and waveshape.

Passive and hybrid mode locking in multi-section terahertz quantum cascade lasers

It is believed that passive mode locking is virtually impossible in quantum cascade lasers (QCLs) because of too fast carrier relaxation time. Here, we revisit this possibility and theoretically show that stable mode locking and pulse durations in the few cycle regime at terahertz (THz) frequencies are possible in suitably engineered bound-to-continuum QCLs. We achieve this by utilizing a multi-section cavity geometry with alternating gain and absorber sections. The critical ingredients are the very strong coupling of the absorber to both field and environment as well as a fast absorber carrier recovery dynamics. Under these conditions, even if the gain relaxation time is several times faster than the cavity round trip time, generation of few-cycle pulses is feasible. We investigate three different approaches for ultrashort pulse generation via THz quantum cascade lasers, namely passive, hybrid and colliding pulse mode locking.

Optical Rectification and Generation of Harmonics Under Condition of Propagation of Few-Cycle Pulses in the Birefringent Medium with Asymmetric Molecules

Without using the approximation of slowly varying envelopes, we investigate spectral transformations of vector few-cycle and quasi-monochromatic solitons under propagation in the birefringent medium of asymmetric molecules possessing permanent dipole moments. We show that when an ordinary pulse enters the medium, the self-modulation effect arises. As a result, the spectrum of the ordinary component undergoes significant changes from the violet shift and the generation of odd harmonics with close frequency satellites to the supercontinuum including zero frequency. The spectrum of the extra-ordinary few-cycle pulse also has the properties of a supercontinuum. When a quasi-monochromatic ordinary pulse enters the medium in the spectrum of an extra-ordinary component, the effect of optical rectification with simultaneous generation of the second harmonics is clearly manifested.

Generation and subsequent amplification of few-cycle femtosecond pulses from a picosecond pump laser

Using a new approach, in which generation of femtosecond pulses as short as a few field cycles is implemented directly from the radiation of a picosecond pump laser, pulses with the microjoule energy, the repetition rate 10 kHz, and the duration less than 26 fs are generated in the spectral range 1.3 − 1.4 μm. In the process of generating this radiation, use was made of a method providing passive phase stabilisation of the carrier oscillation of the electromagnetic field and its slow envelope. The radiation spectrum was converted into the range of parametric amplification in the BBO crystal by the broadband second harmonic generation; the pulse was parametrically amplified up to the microjoule level and compressed by chirped mirrors to a duration of 28 fs.

Simultaneous few-cycle pulse generation of the depleted pump and signal from an optical parametric amplifier

We generate few-cycle laser pulses from both the depleted pump and the signal of an optical parametric amplifier using self-phase modulation in a single hollow-core fiber. Despite the depleted pump’s poor spatial quality entering the fiber, it produces a field with less than 2.5 cycles. Moreover, we can simultaneously (same fiber) generate 4 cycle pulses from the signal. The coherent combination of both pulses allows for bandwidth spanning from 500 to 1500 nm. The geometry allows for minimal path differences of the two fields, paving a route for generating multi-octave, synthesized light transients centered in the near-infrared.

High-harmonic generation in ZnO driven by self-compressed mid-infrared pulses

Progress in attosecond science has relied on advancements in few-cycle pulse generation technology and its application to high-order harmonic generation. Traditionally, self-phase modulation in bulk solids has been used for the compression of moderate-energy pulses, additionally exhibiting favorable dispersion properties for mid-infrared (mid-IR) pulses. Here, we use the anomalous dispersion of Y3Al5O12 (YAG) to self-compress many-cycle pulses from a 50 kHz mid-IR OPA down to produce sub-three-cycle 10 μJ pulses and further use them to generate high-order harmonics in a ZnO crystal. In agreement with theoretical predictions, we observe a boost in the harmonic yield by a factor of two, and spectral broadening of above-gap harmonics, compared to longer driving pulses. The enhanced yield results from an increase in the intensity for the self-compressed pulses.

Influence of temperature on supercontinuum generation induced by femtosecond laser filamentation in NaCl solution

Supercontinuum generation is an important nonlinear phenomenon that occurs during the femtosecond laser filamentation in transparent medium, and its potential and promising applications like remote sensing, biomedical imaging and generation of few-cycle femtosecond pulses, etc. have aroused a great deal of interest. With the extensive and thorough theoretical simulation and experimental research of the supercontinuum generation in air, the mechanism of the supercontinuum induced by femtosecond laser filament in gaseous medium has become clear. However, the femtosecond laser filament-induced supercontinuum in liquid is still an open question. In this work, by taking NaCl solution for example, we investigate the influence of solution temperature on the supercontinuum induced by the femtosecond laser filamentation in solution. It is found that when the laser pulse energy is relatively low (e.g. 20 and 50 J), the influence of solution temperature on supercontinuum generation can be neglected. In contrast, when the laser pulse energy is relatively high (e.g. 200 J), with the increase of solution temperature, the supercontinuum generation shows a suppression tendency. The water molecules in NaCl solution are photo-ionized due to the high intensity of femtosecond laser filament, generating a great deal of oxygen (O2), hydrogen (H2) and water vapor (H2O), and thus forming bubbles that float upwards. In the case of lower pulse energy, the multi-photon ionization rate is low, therefore, only a few bubbles are generated, and they are small in size, which hardly affects the supercontinuum generation. In the case of higher pulse energy, a large number of bubbles can be observed in the NaCl solution, and their sizes become increasingly large when the temperature of NaCl solution increases. The generation of bubbles leads to the reflection and refraction of light, which inevitably influences the spectral intensity. Furthermore, the components (e.g. O2, H2 and H2O) in the bubbles also absorb the supercontinuum, which further lowers the spectral intensity. This work reveals that the main factors leading to the supercontinuum suppression in solution can be attributed to the generation of bubbles during femtosecond laser filamentation and the scattering and absorption of light caused by water vapor in bubbles. When we detect the components in solution via the femtosecond laser filament-induced supercontiunum, the influence of tempera-ture can be effectively eliminated by adjusting the incident pulse energy. Moreover, in the case of high pulse energy, the supercontinuum generation can be controlled by adjusting the solution temperature. This study is conducible to the application of supercontinuum as well as its generation.

Self-compression of 1.8-μm pulses in gas-filled hollow-core fibers*☆Project supported by the National Natural Science Foundation of China (Grant Nos. 61475169, 61521093, and 11127901), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB16), and the International Science and Technology Cooperation Program of China (Grant No. 2016YFE0119300).

We numerically study the self-compression of the optical pulses centered at 1.8- in a hollow-core fiber (HCF) filled with argon. It is found that the pulse can be self-compressed to 2 optical cycles when the input pulse energy is 0.2-mJ and the gas pressure is 500-mbar (1 bar=105 Pa). Inducing a proper positive chirp into the input pulse can lead to a shorter temporal duration after self-compression. These results will benefit the generation of energetic few-cycle mid-infrared pulses.

Tunable few-cycle pulses from a dual-chirped optical parametric amplifier pumped by broadband laser

We propose a dual-chirped optical parametric amplification (DC-OPA) scheme pumped by a broadband laser pulse. The pump pulse is spectrally broadened in a multi-plate system before amplifying the chirped seed in a BBO crystal. The system performance and phase-matching mechanism with different pump bandwidths are investigated thoroughly. It is found that the broadened pump bandwidth benefits the system most effectively when the pump and seed pulses are oppositely chirped. The idler bandwidth is nearly tripled in the broadband pumped system, supporting a transform-limited (TL) duration of 8.4fs (∼1.3cycles), meanwhile the energy bandwidth product of the idler is 72.6% higher. Furthermore, the idler wavelength is tunable between 1700nm and 2050nm, with sub-1.5-cycle TL duration and over 14% conversion efficiency. The proposed scheme provides a suitable approach for the generation of few-cycle pulses varying from near-infrared to mid-infrared regions.

Few-cycle pulse generation from noncollinear optical parametric amplifier with static dispersion compensation

We present a novel design of a few-cycle noncollinear optical parametric amplifier (NOPA) pumped by the second harmonic of a Ti:sapphire laser. A quasi-transform-limited sub-6fs pulse width was realized by static dispersion compensation with commercially available chirped mirrors. The performance of the NOPA was tested by performing transient absorption spectroscopy on sensory rhodopsin II, and we observe short-lived oscillatory components that are associated with the vibrational coherence from the isomerizing molecule in the excited electronic state.

Few-cycle pulse generation from compression of supercontinuum by optimizing initial chirp in ANDi-PCF

We proposed and demonstrated a few-cycle pulse generation from compression of supercontinuum (SC) pulse by using a simple grating pair compressor. An all-normal dispersion photonic crystal fiber (ANDi-PCF) with nearly zero flattened dispersion at 1550nm was chosen to generate a highly coherent SC. In such a fiber a highly coherent broadband SC with 20-dB wavelength from 1107nm to 2551nm is generated. It is found that the mechanism of spectral broadening is mainly dominated by self-phase modulation which results in the highly coherent SC generation. By using a grating pair compressor, an approximately 1.6-cycle pulse (8.4fs) but only with compression quality of 56.55% can be obtained from compression of the highly coherent SC optical pulse. By optimizing the initial chirp of the pump pulse, we have successfully compressed the SC pulse to less than 10fs with compression quality as high as 95.4%.

Spatiotemporal propagation dynamics of intense optical pulses in loosely confined gas-filled hollow-core fibers

We numerically study the propagation dynamics of intense optical pulses in gas-filled hollow-core fibers (HCFs). The spatiotemporal dynamics of the pulses show a transition from tightly confined to loosely confined characteristics as the fiber core is increased, which manifests as a deterioration in the spatiotemporal uniformity of the beam. It is found that using the gas pressure gradient does not enhance the beam quality in large-core HCFs, while inducing a positive chirp in the pulse to lower the peak power can improve the beam quality. This indicates that the self-focusing effect in the HCFs is the main driving force for the propagation dynamics. It also suggests that pulses at longer wavelengths are more suitable for HCFs with large cores because of the lower critical power of self-focusing, which is justified by the numerical simulations. These results will benefit the generation of energetic few-cycle pulses in large-core HCFs.

Generation of few-cycle laser pulses: Comparison between atomic and molecular gases in a hollow-core fiber**Project supported by the National Natural Science Foundation of China (Grant Nos. 11204328, 61221064, 61078037, 11127901, 11134010, and 61205208), the National Basic Research Program of China (Grant No. 2011CB808101), and the Natural Science Foundation of Shanghai, China (Grant No. 13ZR1414800).

We numerically study the pulse compression approaches based on atomic or molecular gases in a hollow-core fiber. From the perspective of self-phase modulation (SPM), we give the extensive study of the SPM influence on a probe pulse with molecular phase modulation (MPM) effect. By comparing the two compression methods, we summarize their advantages and drawbacks to obtain the few-cycle pulses with micro- or millijoule energies. It is also shown that the double pump-probe approach can be used as a tunable dual-color source by adjusting the time delay between pump and probe pulses to proper values.

Self-similar pulse evolution in a fiber laser with a comb-like dispersion-decreasing fiber.

We demonstrate an erbium fiber laser with self-similar pulse evolution inside a comb-like dispersion-decreasing fiber. We show numerically and experimentally that the comb-like dispersion-decreasing fiber works as well as an ideal one, and offers major practical advantages. The existence of a nonlinear attractor is verified by the invariant pulse chirp over a wide range of net cavity dispersion in experiments. The laser generates 1.3 nJ pulses with parabolic shapes and linear chirps, which can be dechirped to 37 fs. Comb-like dispersion-decreasing fiber should enable the generation of high-energy few-cycle pulses directly from a fiber oscillator.

Scalable broadband OPCPA in Lithium Niobate with signal angular dispersion

Angular dispersion of the signal beam is proposed for efficient, scalable high-power few-cycle pulse generation in LiNbO3 by optical parametric chirped-pulse amplification (OPCPA) in the 1.4 to 2.1µm wavelength range. An optimized double-grating setup can provide the required angular dispersion. Calculations predict 16.8fs (3 cycles) pulses with 13TW peak power. Further scalability of the scheme towards the 100-TW power level is feasible by using efficient, cost-effective, compact diode-pumped solid-state lasers for pumping directly at 1µm, without second-harmonic generation.