The 19 reference contexts in paper A. Rudenkov S., V. Kisel E., A. Yasukevich S., K. Hovhannesyan L., A. Petrosyan G., N. Kuleshov V., А. Руденков С., В. Кисель Э., А. Ясюкевич С., К. Ованесьян Л., А. Петросян Г., Н. Кулешов В. (2018) “Регенеративный усилитель чирпированных фемтосекундных импульсов на основе кристалла Yb:CALYO для спектроскопии возбуждения-зондирования с высоким временным разрешением // Yb:CALYO-based femtosecond chirped pulse regenerative amplifier for temporally resolved pump-probe spectroscopy” / spz:neicon:pimi:y:2018:i:3:p:205-214

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    8322
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    Introduction Diode-pumped femtosecond laser sources with pulse repetition frequencies (PRF) of hundreds of kilohertz and pulse energies of tens microjoules are of practical importance for high temporal and spectral resolution measurements, precision micromachining, optical memory and biomedicine
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    [1]
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    . Extremely high average output powers up to the kilowatt level could be obtained by means of direct amplification of high repetition-rate oscillators. 1.1 kW of average power at 20 MHz repetition rate with 615 fs pulses were obtained using the Innoslab Yb:YAG concept [2]. 830 W trains of 640 fs pulses at 78 MHz were demonstrated by employing largemode area Yb-doped fiber
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    8613
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    Extremely high average output powers up to the kilowatt level could be obtained by means of direct amplification of high repetition-rate oscillators. 1.1 kW of average power at 20 MHz repetition rate with 615 fs pulses were obtained using the Innoslab Yb:YAG concept
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    [2]
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    . 830 W trains of 640 fs pulses at 78 MHz were demonstrated by employing largemode area Yb-doped fiber amplifiers [3]. Pulses at lower repetition frequencies, up to a few megahertz, with substantially higher energy and peak power are preferred for many applications.
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    8731
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    high average output powers up to the kilowatt level could be obtained by means of direct amplification of high repetition-rate oscillators. 1.1 kW of average power at 20 MHz repetition rate with 615 fs pulses were obtained using the Innoslab Yb:YAG concept [2]. 830 W trains of 640 fs pulses at 78 MHz were demonstrated by employing largemode area Yb-doped fiber amplifiers
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    [3]
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    . Pulses at lower repetition frequencies, up to a few megahertz, with substantially higher energy and peak power are preferred for many applications. These pulse trains can be generated conveniently with solid-state diodepumped regenerative amplifiers (RAs).
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    9187
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    These pulse trains can be generated conveniently with solid-state diodepumped regenerative amplifiers (RAs). Up to now, the highest average power RA systems are presented by thin-disk concept RAs. For example, up to 160 W average output power at 800 kHz PRF with 750 fs pulse duration were achieved in
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    [4]
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    . Up to 100 W average power at 400 kHz PRF with 800 fs pulse duration were reached in [5] employing Yb:YAG thin-disk active element. Despite thin-disk based regenerative amplifier systems demonstrate high average power, it should be noted that pulse duration of such systems is not less than 700 fs. 295 fs pulses were demonstrated for thin disk RA system that ap
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    9287
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    For example, up to 160 W average output power at 800 kHz PRF with 750 fs pulse duration were achieved in [4]. Up to 100 W average power at 400 kHz PRF with 800 fs pulse duration were reached in
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    [5]
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    employing Yb:YAG thin-disk active element. Despite thin-disk based regenerative amplifier systems demonstrate high average power, it should be noted that pulse duration of such systems is not less than 700 fs. 295 fs pulses were demonstrated for thin disk RA system that applied nonlinear pulse amplification regime but with substantially reduced output power (36 W) [6
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    9681
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    Despite thin-disk based regenerative amplifier systems demonstrate high average power, it should be noted that pulse duration of such systems is not less than 700 fs. 295 fs pulses were demonstrated for thin disk RA system that applied nonlinear pulse amplification regime but with substantially reduced output power (36 W)
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    [6]
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    . Another approach to implementation of RA systems is based on bulk regenerative amplifiers. The highest output power reported so far for bulk RAs is 42 W at 500 kHz pulse repetition frequency obtained in RA based on the active medium with high thermooptical properties – Yb:Lu2O3 [7].
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  7. Start
    9965
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    Another approach to implementation of RA systems is based on bulk regenerative amplifiers. The highest output power reported so far for bulk RAs is 42 W at 500 kHz pulse repetition frequency obtained in RA based on the active medium with high thermooptical properties – Yb:Lu2O3
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    [7]
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    . Relatively long amplified pulses of about 780 fs pulse duration obtained due to narrow gain bandwidth of Yb:Lu2O3. Substantially reduced pulse duration of 217 fs with relatively high output power of 28 W demonstrated at 500 kHz in RA based on crystal with wide gain bandwidth – Yb:CALGO [8].
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  8. Start
    10264
    Prefix
    Relatively long amplified pulses of about 780 fs pulse duration obtained due to narrow gain bandwidth of Yb:Lu2O3. Substantially reduced pulse duration of 217 fs with relatively high output power of 28 W demonstrated at 500 kHz in RA based on crystal with wide gain bandwidth – Yb:CALGO
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    [8]
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    . But the usage of active medium with wide gain bandwidth is not a sufficient condition for obtaining short amplified pulse duration due to the strong gain narrowing effect [9] that reduce amplified pulse spectral width and increase minimal transform-limited pulse duration.
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    10452
    Prefix
    Substantially reduced pulse duration of 217 fs with relatively high output power of 28 W demonstrated at 500 kHz in RA based on crystal with wide gain bandwidth – Yb:CALGO [8]. But the usage of active medium with wide gain bandwidth is not a sufficient condition for obtaining short amplified pulse duration due to the strong gain narrowing effect
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    [9]
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    that reduce amplified pulse spectral width and increase minimal transform-limited pulse duration. Several methods for overcoming the negative contribution of gain narrowing effect have been proposed [10, 11].
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    10669
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    usage of active medium with wide gain bandwidth is not a sufficient condition for obtaining short amplified pulse duration due to the strong gain narrowing effect [9] that reduce amplified pulse spectral width and increase minimal transform-limited pulse duration. Several methods for overcoming the negative contribution of gain narrowing effect have been proposed
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    [10, 11]
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    . The output power of about 21 W at 200 kHz PRF with 200 fs pulse duration is obtained with Yb:KGW dual crystal system [10]. Femtosecond laser pulses with duration as short as 97 fs with output power of 1.2 W at 50 kHz PRF were obtained with the Yb:CALGO RA system which demonstrates the possibility of sub-100 fs pulses amplification [11].
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  11. Start
    10796
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    Several methods for overcoming the negative contribution of gain narrowing effect have been proposed [10, 11]. The output power of about 21 W at 200 kHz PRF with 200 fs pulse duration is obtained with Yb:KGW dual crystal system
    Exact
    [10]
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    . Femtosecond laser pulses with duration as short as 97 fs with output power of 1.2 W at 50 kHz PRF were obtained with the Yb:CALGO RA system which demonstrates the possibility of sub-100 fs pulses amplification [11].
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    11016
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    Femtosecond laser pulses with duration as short as 97 fs with output power of 1.2 W at 50 kHz PRF were obtained with the Yb:CALGO RA system which demonstrates the possibility of sub-100 fs pulses amplification
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    [11]
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    . Despite the availability of femtosecond lasers providing wide spectral width pulses [12–14] based on Yb seed lasers a large number of RA systems described in the literature have narrower pulse spectral width not over than 15 nm.
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    11115
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    Femtosecond laser pulses with duration as short as 97 fs with output power of 1.2 W at 50 kHz PRF were obtained with the Yb:CALGO RA system which demonstrates the possibility of sub-100 fs pulses amplification [11]. Despite the availability of femtosecond lasers providing wide spectral width pulses
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    [12–14]
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    based on Yb seed lasers a large number of RA systems described in the literature have narrower pulse spectral width not over than 15 nm. And this also limits the amplified pulse spectral width and compressed pulse duration.
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    11743
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    Crystal growth Single crystals of Yb3+:CaYAlO4 (tetragonal structure, space group D174h-I4/mmm) doped with Yb3+(1.4 at. % and 3.5 at. %) were grown from stoichiometric melts by Czochralski method
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    [15]
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    . Yb3+ ions occupy Y3+ sites of the lattice with 9-fold coordination. Crystallization was carried out under an enclosed argon atmosphere using iridium crucibles (50 × 50 × 30 mm3) and seed crystals oriented along [110].
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    12985
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    Radiative lifetime was estimated by using crystalline powder immersed in glycerine suspension in order to eliminate radiation trapping effect caused by significant overlap of the absorption and emission bands
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    [16, 17]
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    . Measured lifetime for different weight content 207 208 of Yb3+(1.4 at. %):CaYAlO4 and Yb3+(3.5 at. %): CaYAlO4 crystalline powder in glycerine suspension are shown in Figure 2. Measured kinetics of luminescence decay for Yb3+:CaYAlO4 with 1.4 at. % and 3.5 at. % concentrations are shown in Figure 3.
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    13705
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    The radiative lifetime of the 2F5/2 manifold of Yb3+ions in CaYAlO4 was estimated to be (430 ± 15) μs. The stimulated emission cross sections (Figure 1) were calculated by use of integral reciprocity method
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    [18]
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    . Figure 1 – Absorption and stimulated emission crosssection spectra of the Yb3+:CaYAlO4 Figure 2 – Measured lifetime for different weight content of Yb3+:CaYAlO4 crystalline powder in glycerine suspension Figure 3 – Kinetics of luminescence decay 2F5/2 manifold of Yb3+-ions in CaYAlO4 The Yb3+:CaYAlO4 crystal exhibits broad stimulated emission cross-section spectra in
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    14472
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    The stimulated emission cross-section for σ-polarization demonstrates higher value of about ≈ 0.7·10-20 cm2 at 1030–1040 nm in comparison with ≈ 0.5·10-20 cm2 for π-polarization. Obtained spectroscopic properties of the Yb3+:CaYAlO4 crystal demonstrate good agreement with described in literature data
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    [19]
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    . Experimental setup The conceptual scheme of the system layout is shown in Figure 4. Figure 4 – Experimental setup of broad-band seeded Yb3+:CaYAlO4 chirped pulse RA Приборы и методы измерений 2018.
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    15814
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    The RA setup chosen for this experiment is quite common, employing a 40-mm-long double-BaB2O4 Pockels cell for pulse injection and ejection. Pulse repetition frequency (PRF) was chosen to be 200 kHz to prevent damage of the optical elements. «Off-axes» pump layout was used for longitudinal pumping of the active element
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    [20–22]
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    . Main advantage of such a pump scheme is that all the cavity mirrors have highly reflecting coating at (900–1100) nm. Maximum pump power was 25 W. 2mm-long a-cut Yb3+(3.5 at. %): CaYAlO4 crystal was used as a gain medium.
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    21955
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    Mathematical modelling To estimate the amplified pulse spectra limited mostly by gain crystal characteristics and gain narrowing effect mathematical simulation was made 211 b a b a with constant level of intracavity losses (≈ 5 %) for wide spectral range covering active crystal gain bandwidth. Simulation was based on the split-step Fourier method
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    [23]
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    . Gain curve was calculated by means of the absorption (ABS) and stimulated emission (SE) cross-section spectra under a certain population of the upper laser manifold of Yb3+ ions which corresponds to our experimental conditions.
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