Chirped pulse monolithic fiber amplifier based on a newly developed tapered polarization maintaining Yb-doped fiber has been developed and optimized. A novel amplification regime in a relatively long (220 cm) tapered fiber of improved design, which has been theoretically predicted, allowed us to achieve an ultimate high peak power. In this regime, the signal propagates most of the fiber without amplification and growths very rapidly only in the last 80 cm of the tapered fiber, which has a mode field area of approximately 1000 μm2 near the output. We have demonstrated amplification of 20 ps chirped pulses centered at 1056-nm with spectral width of 20 nm to 0.7 MW peak power directly from the tapered fiber amplifier. The pulses had a diffraction limited quality (M2 ~ 1.124) and could be compressed down to 350 fs with 50% efficiency. In addition, amplification of narrow-band 9 ps pulses centered at 1064 nm to a peak power of 1.8 MW directly from the tapered fiber amplifier was demonstrated.
In this study, we propose a widely tunable in the 1.6-2.65 μm range femtosecond fiber laser source, generating high-quality sech-shaped pulses with the duration of order 100 fs. Experimental setup contains hybrid all-fiber Er/Tm pump laser generating 150 fs pulses of 2 nJ in Erbium (1.56 μm) channel and 125 fs pulses of 4 nJ in Thulium (2 μm) channel respectively. This laser source was coupled to a 50 cm piece of suspended-core microstructured TeO2-WO3- La2O3 glass fiber with launching efficiency of about 10%. We have observed Raman self-frequency shifting solitons in this fiber with maximum red shift of 2.25 μm for Erbium channel and 2.65 μm for Thulium channel. By varying energy of pump pulses, solitons can be tuned in broadband spectral region. We have made theoretical studies of nonlinear pulse dynamics in the tellurite fiber with carefully measured and calculated parameters. Numerical simulation is in a very good agreement with the experiment
We proposed and investigated a novel tapered ytterbium-doped fiber design. The tapered fiber has length of 2.1 m, single-mode thin end and thick end with core/clad diameter of about 50/430 µm. Master-oscillator power-amplifier monolithic fiber scheme have been developed using this fiber and 5 ps duration, 28 nm spectral width and 0.5 MW peak power pulses was achieved at the output. FROG measurements reveal linear chirp that allow one to expect decompression of these pulses down to sub-100 fs duration.
The use of double-clad fibers for short pulses amplification requires high active ions concentration in order to keep the active fiber length short. In the case of Er-doped fibers an increase of concentration leads to a significant drop of efficiency due to Er ions clustering. We have demonstrated through numerical simulation that efficiency of amplifiers based on double-clad P2O5-Al2O3-SiO2 (PAS) Er-doped fibers decreases slower with Er-concentration growth if compared with standard Al2O3-SiO2 fibers. In this paper, we present single-mode large-mode-area heavily Er-doped double-clad fiber based on PAS glass matrix for short pulses amplification. The developed PAS fiber has a 36 μm singlemode core and a small signal cladding absorption of 3 dB/m at 980 nm leading to an optimal fiber length in range of 5-8 m depending on the central wavelength. At first, an all-fiber nanosecond MOPA at 1560 nm was built using our PAS fiber as the final amplifier. We obtained 28 W of average output power (efficiency of 25 % with respect to the launched pump power at 976) limited by amplified spontaneous emission. Pulse energy of 1.5 mJ was achieved at pump power level of ~120 W. We believe that it is the first demonstration of mJ-energy level single-mode nanosecond fiber system. Then, direct amplification of 100-fs source was performed using this fiber. We obtained 12 nJ pulse energy and 100 kW of peak power from the fiber which is close to the record value for Er-doped fiber amplifiers.
The generation of attosecond pulses with an amplitude greatly exceeding the driving field of an ultrarelativistic laser
pulse at oblique irradiation of a solid target is investigated. We develop a universal model of the process, the so-called
relativistic electronic spring, which is different from the conventional concept of an oscillating mirror. It follows from
the model that there exists a parameter region where the energy conversion from the femto- to the attosecond regime is
maximal. Based on the study we propose a new concept of laser pulse interaction with a target having a groove-shaped
surface, which opens up the potential to exceed an intensity level of 1026 W/cm2 and observe effects due to nonlinear
quantum electrodynamics with upcoming laser sources.
The possibility to build up an optical source of femtosecond pulses that are smoothly tuned in the telecommunication range using a dispersion-decreasing fiber is demonstrated. The smooth tuning is based on the Raman frequency conversion of ultrashort pulses, which can be effectively tuned due to the compression mechanism for maintaining of a relatively high pulse intensity in the medium with a monotonically decreasing anomalous dispersion. The generation of a 90-fs soliton pulse whose wavelength is smoothly tuned in the wavelength range 1.55 - 1.85 μm is experimentally demonstrated.
We demonstrate both experimentally and theoretically that efficient frequency down-shifting of an optical soliton
pulse can be realized in a dispersion decreasing fiber. This effect relies on the Raman self-frequency shifting that
can occur with high efficiency due to the adiabatic compression of the pulse and therefore reaching higher intensities in a fiber with monotonically decreasing anomalous dispersion. A smooth frequency shifting of 90 fs soliton pulses in the 1.55-1.8 &mgr;m range is demonstrated.
Low frequency noise characteristics of light-emitting diodes with InAs quantum dots in GaInAs layer are investigated. Two noise components were found in experimental noise records: RTS, caused by burst noise, and 1/f Gaussian noise. Extraction of burst noise component from Gaussian noise background was performed using standard signal detection theory and advanced signal-processing techniques. It was found that Hooge's empirical relation applied to diodes by Kleinpenning is applicable to the electric 1/f noise of quantum dot diodes as well. Two different spectra decomposition techniques are used to obtain burst noise spectra. Bias dependences of burst and 1/f noise are compared. It is concluded that the RTS noise and 1/f noise have different physical origins in light-emitting diodes with quantum dots.
The bispectrum of the 1/f noise is investigated in the present work. For the Gaussian noise it equals zero. LEDs on self-organized InAs/GaAs quantum dots and laser diodes on In0.2Ga0.8As/GaAs/InGaP quantum wells made in Russia were tested. The voltage noise was analyzed in a wide interval of currents through the diodes. Estimates of the probability density function and semi-invariants of the noise have not revealed any appreciable deviations from the Gauss law. Noise spectra Sv(f)in the range 1 Hz - 20 kHz were analyzed. In most cases the frequency exponent γs of the spectrum is close to one, the Hooge’s parameter αH has magnitude of the order 10-4. The bispectrum Bv(f1,f2of the noise is a complex function of frequencies f1 and f2. Its absolute value is decreasing while moving from the beginning of the frequency plane Of1f2. The decrease along the bisector (f1 = f2 = f) follows the power law characterized by the frequency exponent γB ≈ 1.5 γs. The dependence of the "height" of |Bv(f,f)| on the current through the diode is qualitatively similar to this one for the spectrum. The power law describes these dependences, however the exponents are essentially different.
The two-dimensional quantum model is developed for describing the dynamics of an atom driven by strong few-optical-cycle laser pulse taking into account the magnetic component of exciting field. The conditions are found when magnetic field strongly suppresses high-energy photon production.
Recent progress in the technique of generation and amplification of ultrashort laser pulses has resulted in the production of pulse duration of the order of or even shorter than 10 fs [1,2]. The use of Ti:Sa active crystals and broad-band optical elements, perfect compensation of wave dispersion in laser cavities and thorough comprehension of mode locking at self-focusing nonlinearity (Kerr-lens-mode-locking) have provided pulses having only few (∼3) optical cycles. Application of such a remarkable instrument opens up new prospects for experimental research on the interaction of ultrashort pulses with matter. It is also evident that a time has come for a serious theoretical analysis of these interaction models. Our report addresses the problem on nonlinear spectrum conversion of high intensity ultrashort laser pulses during the process of rapid optically-induced gas ionization.
We have studied nonlinear interactions of optical pulses comprising a small number of optical cycles with quantum particle systems. For model intraatomic potentials we have monitored nonlinear medium responses with characteristic features for a variety of processes, including Kerr effect, harmonic generation and blue-shifting, atom excitation and ionization, and subfemtosecond XUV burst production. The efficiency and the hierarchy of these processes have been investigated.
We present and explore an idea of creating a source of extremely short (10-15 s) coherent radiation tunable in the XUV wavelength range. The underlying physical mechanism consists in nonlinear properties of rapidly ionized atoms to efficiently convert the spectra of laser radiation in the 1015 - 1016 W/cm2 intensity range. We demonstrate that under certain conditions the nonlinear effects of high-order harmonic generation, spectrum broadening and blueshifting can be simultaneously engaged and favorably combined. This mechanism offers an attractive possibility to enter the attosecond duration range by optimizing the process of high-order harmonic excitation at the ionizing fronts of ultrashort laser pulses.
Recent progress in powerful subpicosecond laser generation technology has stimulated interest in interaction of ultrashort electromagnetic pulses with nonlinear media. Being a classical example of a nonlinear medium and a traditional object of laboratory investigations, the plasma demonstrates an extreme variety in behavior under the action of the electromagnetic fields. Characteristic rise and relaxation times of nonlinear response can be varied by a few orders of magnitude depending on particle concentration, wavelength, external magnetic field, etc. Thus, the ultrashort pulse duration should be defined as a particular notion with respect to the phenomenon in question. In this paper we take interest in electromagnetic wave-plasma interaction processes faster than the relaxation of the nonlinear plasma response. In this context, we treat the interaction as essentially dynamical and the plasma as a medium with long-term memory properties. The nonsteady state of the nonlinear coupling and the plasma wake generation are two main features essential to the basic physical effects and applications.
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