Amplification Of Ultrashort Pulses Research Articles

Six-orders-of-magnitude-spanning dispersion measurement via Kalman filtering-aided white-light interferometry.

Dispersion plays a great role in ultrafast laser oscillators, ultrashort pulse amplifiers, and many other nonlinear optical dynamics. Therefore, dispersion measurement is crucial for device characterization, system design and nonlinear dynamics investigation therein. In this work, we demonstrate a versatile approach, i.e., Kalman filtering-aided white-light interferometry, for group delay dispersion (GDD) characterization. Extended Kalman filter is adopted to track the cosine-like interferogram, and to eliminate the unintended bias and the envelope, providing a nearly ideal phase retrieval and GDD estimation. The measurement range could span from tens of fs2 to tens of ps2, with an uncertainty of about 0.1%, enabling precise GDD measurement for diverse optical components, ranging from a millimeter-thick glass slide to highly dispersive chirped fiber Bragg gratings. Benefited by the simplicity, convenient setup, and easy operation as well as relatively low cost, this approach would help photonic device characterization, dispersion management and nonlinear dynamics investigation in the laboratory and work plant.

Nd, Gd:SrF<sub>2</sub> crystal spectrum gain characteristic study on the broadband laser amplification

Spectral gain narrowing is one of the key factors affecting broadband amplification of ultrashort pulses. In this paper, the spectral gain characteristics in broadband amplification are studied theoretically and experimentally by using the characteristic of Nd,Gd:SrF<sub>2</sub> crystal, i.e. the emission spectrum has a certain width at the higher stimulated emission cross section. Through the numerical simulation, the evolution law of output spectrum of the laser gain medium under different spectral gain lineshapes and different gain values is studied in detail. Theoretical calculation shows that the spectral gain narrows obviously with the increase of gain values of the traditional Gaussian emission spectrum and increasing the spectral bandwidth at the maximum stimulated emission cross section can obviously suppress the spectral gain narrowing. Furthermore, the spectral gain narrowing characteristics of the Nd,Gd:SrF<sub>2</sub> crystal were studied experimentally. The Ф13mm×150mm Nd,Gd:SrF<sub>2</sub> crystal was used as the gain medium which was pumped by flash lamps in the experimental study. The experimental results show that the output spectra of Nd,Gd:SrF<sub>2</sub> crystals are not obviously narrowed when the input laser spectral width is 5nm (FWHM) and the gain is 140 times. The experimental results are consistent with the theoretical calculation and analysis. The crystal can work normally at the repetition rate of 0.2Hz and 1Hz in the experiment, but due to the influence of thermal effect, the gain will decrease to a certain extent with the increase of pump energy and repetition rate. The research results lay the foundation for the application of fluoride crystal in broadband chirped pulse amplification.

Accurate modeling of ultrafast nonlinear pulse propagation in multimode gain fiber

The nonlinear propagation of picosecond or femtosecond optical pulses in multimode fiber amplifiers underlies a variety of intriguing physical phenomena as well as the potential for scaling sources of ultrashort pulses to higher powers. However, existing theoretical models of ultrashort-pulse amplification do not include some critical processes, and, as a result, they fail to capture basic features of experiments. We introduce a numerical model that combines steady-state rate equations with the unidirectional pulse propagation equation, incorporating dispersion, Kerr and Raman nonlinearities, and gain/loss-spectral effects in a mode-resolved treatment that is computationally efficient. This model allows investigation of spatiotemporal processes that are strongly affected by gain dynamics. Its capabilities are illustrated through examinations of amplification in few-mode gain fiber, multimode nonlinear amplification, and beam cleaning in a multimode fiber amplifier.

Design of a large-mode-area tapered fiber amplifier for amplification of ultrashort laser pulses at 1535 nm wavelength

We design a W-type large-mode-area erbium doped tapered fiber with effective mode area varying from 1770 µm2 to 840 µm2with tolerance ± 0.0005 in refractive indices of different layers of fiber. The beam quality of the pulses is ensured by effective single-mode operation. We investigate the amplification of high power femtosecond pulses in this fiber by using nonlinear Schrodinger equation including gain term. It is solved using split-step Fourier method for both up-tapered (effective mode area increasing along the length) and down tapered (effective mode area decreasing along the length) fiber configurations at centered around 1535 nm wavelength. In both configurations of fiber, input pulse parameters like peak power, pulse width, and fiber length are calculated for maximum amplification with the least amount of pulse shape distortion. In down-tapered configuration, we numerically demonstrate the amplification of 1 kW peak power, 560 fs duration pulse to 3.6 MW peak power, 12 fs amplified pulse. In up-tapered configuration, we obtain the amplification of 2 kW, 565 fs input pulse to ∼7.9 MW peak power, ∼11 fs pulse. The fiber is useful for amplifying high-power laser pulses, and the resulting pulses can be used for laser processing, spectroscopy, and remote sensing.

Numerical Investigation of Polarization-Dependent Ultrashort Pulse Amplification in Erbium-Doped Fluoride Fibers

We have theoretically investigated the polarization-dependent ultrashort pulse amplification in erbium-doped fluoride fibers. The numerical model is based on the coupled generalized nonlinear Schrödinger equations and the rate equations. It is found that the Raman effect can be weakened by converting the seed polarization from linear to circular. Compared with the linearly polarized seed pulses, amplification of the circularly polarized seed pulses can generate Raman solitons with a larger pulse energy of the same central wavelength, although more pump power is needed. Furthermore, the amplification dynamics when no pulse splitting occurs have also been investigated. We demonstrate that amplification of the circularly polarized seed pulses allows the main pulse to accumulate more energy while maintaining a single-pulse profile. It is predicted that sub-two-cycle laser pulses at 2.8 μm with megawatt peak power and hundreds of nanojoules pulse energy can be directly generated from the erbium-doped fluoride fiber amplifiers. The investigations conducted in this paper can provide guidelines for the design of Raman-soliton-based tunable fiber laser systems and high-power few-cycle fiber laser systems.

High-energy 2.8-μm Ultrashort Pulses Generation in an Er:ZBLAN Fiber Amplifier

Compact, high-energy ultrashort pulsed fiber lasers in the 2.8 μm wavelength have attracted extensive interest in many scientific and industrial applications. Here, we demonstrated the generation of μJ-level ultrashort pulses from a quasi-all-fiberized Er:ZBLAN amplifier employing a 2.8-μm frequency-shifted soliton as a seed laser. With backward pumping, the pulse energy was enhanced to 0.282 μJ when the average output power was amplified to 1.023 W. Mechanisms for the generation and amplification of ultrashort pulses were theoretically revealed by numerical simulation, and further cascaded indium fluoride (InF3) fiber to obtain a 4.26-μm redshift soliton numerically. This compact fiber amplification system consisting of frequency-shifted-based seed pulses and fluoride fiber amplifiers will be of practical interest in applications.

Cleaning up of high-energy ultrashort pulses with saturable absorbers

Pulse energy transferred from the peak to wings is inevitable in the process of ultrashort optical pulse amplification due to the accumulation of high nonlinearity, which degrades the temporal pulse quality. The energy-related saturable absorbers are applied to remove the wings of high-energy pulses without causing a severe loss of the peak in this research. A cascaded-saturable-absorbers system is proposed, and the effects of the cascaded order, low-intensity loss coefficient, saturable fluence, relaxation time and input fluence distribution on the pulse wings are numerically investigated, respectively. The obtained results can provide a general guidance of design for high-energy ultrashort pulse cleaning.

Nd, Gd:SrF<sub>2</sub> crystal spectrum gain characteristic in broadband laser amplification

Spectral gain narrowing is one of the key factors affecting broadband amplification of ultrashort pulses. In this paper, the spectral gain characteristics in broadband amplification are studied theoretically and experimentally by using the characteristic of Nd,Gd:SrF<sub>2</sub> crystal, i.e. the emission spectrum that has a certain width at the higher stimulated emission cross section. Through the numerical simulation, the evolution law of output spectrum of the laser gain medium under different spectral gain lineshapes and different gain values is studied in detail. Theoretical calculation shows that the spectral gain is narrowed obviously with the increase of gain value of the traditional Gaussian emission spectrum, and that increasing the spectral bandwidth at the maximum stimulated emission cross section can obviously suppress the spectral gain narrowing. Furthermore, the spectral gain narrowing characteristics of the Nd,Gd:SrF<sub>2</sub> crystal are studied experimentally. The <i>Ф</i> 13 mm× 150 mm Nd,Gd:SrF<sub>2</sub> crystals are used as the gain medium which are pumped by flash lamps in the experimental study. The experimental results show that the output spectra of Nd,Gd:SrF<sub>2</sub> crystals are not obviously narrowed when the full width at half maximum (FWHM) of spectral width of the input laser is 5 nm and the gain is 140 times. The experimental results are consistent with the theoretical calculation and analysis. The crystal can work normally at a repetition rate of 0.2 Hz and 1.0 Hz in the experiment, but due to the influence of thermal effect, the gain will decrease to a certain extent with the increase of pump energy and repetition rate. The research results lay the foundation for the application of fluoride crystal in broadband chirped pulse amplification.

Watt-level 2.8 μm ultrashort pulse generation from an all-fiber Er:ZBLAN fiber amplifier

Compact and high-power 2.8 μm ultrashort pulse laser generated in a strictly all-fiberized system is a growing interest in the field of the mid-infrared high-power laser community. In this Letter, we demonstrated the generation of watt-level 2.8 μm ultrashort pulse laser from an all-fiber Er:ZBLAN amplifier by using the red-shifted Raman soliton pulse as the seed source. As the incident pump power increased to 8.8 W, the average output power and pulse energy of the 2.8 μm soliton pulses were boosted to 1.25 W and 26.02 nJ, respectively. To achieve the all-fiber configuration, the pump laser at 976 nm was coupled into Er:ZBLAN fiber by a home-made fluoride-fiber-based side-pump combiner (FFSPC). Numerical simulations were performed by solving the generalized nonlinear Schrödinger equation, which showed that the pulse duration of amplified pulses was limited by intracavity dispersion accumulation in Er:ZBLAN fiber amplifier, and the final pulse duration was estimated to be 5 ps. To the best of our knowledge, this is the first all-fiber structured 2.8 μm ultrashort pulse amplifier with an FFSPC. Our results may provide an effective approach for the generation of all-fiber high-power mid-infrared ultrafast lasers.

Three-dimensional modeling of intense unipolar THz pulses formation during their amplification in nonequilibrium extended Xe plasma channel

We develop a three-dimensional (3D) fully self-consistent model for analysis of an ultrashort THz pulse propagation and amplification in a nonequilibrium plasma channel formed in xenon by a femtosecond UV laser pulse. The model is based on the self-consistent solution of a second order wave equation in the cylindrical geometry and the kinetic Boltzmann equation for the electron velocity distribution function (EVDF) at different points of the spatially inhomogeneous nonequilibrium plasma channel. We analyze the wide range of plasma and seed pulse parameters and reveal the optimal regimes for producing high intensity outgoing THz fields as well as highly unipolar THz pulses within the proposed mechanism. It is demonstrated that the process of EVDF relaxation in plasma limits the amplification of THz pulses at the level of ∼10^{7}W/cm^{2}. Both focusing features of nonequilibrium plasma and the possibility of producing THz pulses with a high degree of unipolarity are confirmed for the case of 3D geometry.

Thin-disk multipass amplifier delivering sub-400 fs pulses with excellent beam quality at an average power of 1 kW

We present an improved multipass amplifier design, enabling the amplification of ultrashort pulses with excellent beam quality to more than 1 kW of average output power. 260 fs short pulses at an average power of 105 W and a repetition rate of 1 MHz were directly amplified up to an average power of 1033 W. The pulse duration at this power level was measured to be 388 fs assuming a Gaussian temporal shape. This corresponds to a peak power of 2.5 GW. The power stability was measured to be 0.16% RMS over a duration of more than two hours at a sampling rate of 2 Hz. High beam quality is proven with measured values of M x 2 = 1 .16 in the horizontal and M y 2 = 1 .19 in the vertical plane according to ISO Standard 11146.

Rare-earth doped mixed sesquioxides for ultrafast lasers [Invited

Sesquioxides are outstanding host materials for rare-earth doped laser gain media. Unfortunately, their very high melting points make it challenging for them to be fabricated in high quality. Recently, we demonstrated that some mixed sesquioxides exhibit significantly reduced melting temperatures compared to their constituents. This enables their growth by the established Czochralski method yielding rare-earth doped mixed sesquioxides of high optical quality. Due to their inhomogeneously broadened gain spectra caused by the intrinsic disorder, mixed sesquioxides are very promising for the generation and amplification of ultrashort pulses. To envisage the potential of this emerging class of gain materials, this paper reviews the spectroscopic as well as continuous wave and pulsed laser properties of crystalline and ceramic rare-earth doped mixed cubic sesquioxides of the form (Scx,Luy,Yz)2O3 with x + y + z = 1.

Self-consistent three-dimensional model of ultrashort terahertz pulse amplification in a laser-induced nonequilibrium plasma channel in xenon

The propagation and amplification of ultrashort terahertz pulses in a nonequilibrium photoionization plasma channel formed in xenon by a powerful femtosecond UV laser pulse is studied in three-dimensional geometry. The study is based on the self-consistent numerical integration of the second-order wave equation in cylindrical geometry and the Boltzmann kinetic equation in the two-term expansion to describe the electron velocity distribution function in the spatially inhomogeneous nonequilibrium plasma of the channel. Keywords: nonlinear optics, terahertz radiation, nonequilibrium plasma channel, numerical simulation.

Самосогласованная трехмерная модель усиления ультракороткого терагерцового импульса в лазерноиндуцированном неравновесном плазменном канале в ксеноне

The propagation and amplification of ultrashort terahertz pulses in a nonequilibrium photoionization plasma channel formed in xenon by a powerful femtosecond UV laser pulse is studied in In three-dimensional geometry. The study is based on the self-consistent numerical integration of the second-order wave equation in cylindrical geometry and the Boltzmann kinetic equation in the two-term eхpansion to describe the electron velocity distribution function in the spatially inhomogeneous nonequilibrium plasma of the channel.

Er:ZBLAN Fiber-based Ultra-short Pulse Generation, Amplification and Wavelength Tuning in Mid-IR and LWIR

Er:ZBLAN Fiber-based Ultra-short Pulse Generation, Amplification and Wavelength Tuning in Mid-IR and LWIR

Amplification of femtosecond and picosecond pulses using large mode area Erbium-doped fiber at 1530 nm wavelength

We present amplification of short and ultrashort laser pulses at 1530-nm center wavelength using a large mode area Er-doped fiber (LMA-EDF). The mode area of about 1700 μm2 tremendously increases the power handling capacity of the fiber and helps in achieving up to ∼ 17 dB/m gain with 10 W power of 980-nm wavelength pump. The LMA fiber used here is a 3-layer W-type fiber designed to have effective single mode operation with higher order mode discrimination and sufficient mode spacing between the first few modes to avoid any possible mode coupling. In particular, we numerically demonstrate the amplification of 300 fs, 1 kW peak power pulses to ∼ 20 fs, ∼2.6 MW peak power pulses in 1.5-m propagation distance, and of 50 ps, 1 kW peak power pulses to ∼ 65 ps, ∼26 MW peak power pulses in 3-m propagation distance of the LMA-EDF. The generated pulses would be useful in applications such as eye-safe remote sensing and 3D laser processing of silicon.

Inverse model for ultrashort pulse amplification in semiconductor optical amplifiers.

To describe ultrashort pulse amplification in semiconductor optical amplifiers (SOAs), several models have been developed that calculate the amplified output pulse as a function of the input. Because of the many processes at play in SOAs (band filling, carrier heating, spectral hole burning, two-photon absorption, and the associated free-carrier absorption), it is challenging to predict which input is needed to generate a targeted output. In this Letter, we construct a generic inverse SOA model that calculates the required input pulse including its shape and phase to obtain a desired output. This inverse model will enable a more efficient and well-targeted design of SOA-based photonic systems, while also allowing better quality and performance control.

Peculiarities of Propagation and Amplification of Ultrashort Terahertz Pulses in Strongly Nonequilibrium Plasma Channels Produced in Air by UV Femtosecond Laser Pulses during Multiquantum Ionization

The propagation of an ultrashort terahertz (THz) pulse in a nonequilibrium plasma channel produced by a UV femtosecond laser pulse in air is investigated theoretically. The analysis is carried out based on joint solution of the second-order wave equation and the Boltzmann kinetic equation in the binomial approximation for the electron velocity distribution function in the channel plasma. We assume that a THz pulse is quite weak and does not produce a reverse action on the electron energy spectrum in the channel plasma. It is shown that the plasma channel in air under a pressure of several atmospheres is a medium for effective amplification of ultrashort THz pulses in the frequency range up to several terahertz.

Comparative study on coherent noise in optical parametric and quasi-parametric chirped-pulse amplification

The coherent noise inherent to optical parametric chirped-pulse amplification (OPCPA) poses a fundamental limitation on the temporal quality of current ultrashort ultraintense lasers as well as their applications in strong-field physics research. Here we present a peer comparison of coherent noise between OPCPA and one of its alternatives — quasi-parametric chirped-pulse amplification (QPCPA). In the initial amplification stage termed as small-signal regime, the spectrum of chirped signal pulse in either OPCPA or QPCPA is exposed to the imperfections in pump pulse and optics, resulting in coherent noise pedestals in the time domain. While in the final amplification stage featured by amplifier saturation, the nonlinear growths of coherent noise become sharply different. We demonstrate that the non-parametric saturation behavior of QPCPA can greatly smooth the spectral distortions of signal, and thus bring down the coherent noise. Consequently, QPCPA scheme allows an improvement of pulse contrast by 2–3 orders of magnitude. The results presented in this paper provide a new perspective on high-contrast amplification of ultrashort pulses.

Research on amplification of ultrashort laser pulses at 1.03 µm in gain-managed nonlinearity regime

Research on amplification of ultrashort laser pulses at 1.03 µm in gain-managed nonlinearity regime