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Femtosecond pulse width reduction upon second harmonic generation
High Energy Density Physics 33 (2019) 100701
Contents lists available at ScienceDirect
High Energy Density Physics journal homepage: www.elsevier.com/locate/hedp
Femtosecond pulse width reduction upon second harmonic generation N.G. Ivanov, V.F. Losev, M.V. Ivanov , S.V. Alekseev ⁎
T
Institute of High Current Electronics, Siberian Branch, Russian Academy of Sciences, Akademichesky ave. 2/3, 634055, Tomsk, Russia
ARTICLE INFO
ABSTRACT
Keywords: Fs-laser radiation Pulse duration Pulse compression Gratings
The results of second-harmonic generation (SHG) study for a pre-chirped radiation pulse of fundamental frequency with central wavelength of 950 nm and duration of 70 fs are presented. It is shown that providing of a small excess of group delay dispersion (GDD) ( ± 3200–4000 fs2) for fundamental frequency radiation in the femtosecond front-end compressor increases by 1.5–2 times of the second harmonic spectrum width. Compensation of the radiation pulses excess dispersion with broadened spectrum makes it possible to reduce the duration of second harmonic pulse approximately in proportion to the spectrum broadening and get minimum pulse duration of 35 fs.
1. Introduction At present, the formation of femtosecond radiation pulses in laser systems based on solid-state active media is realized. All these systems work in the infrared spectrum range (0.7–1 μm) [1–5]. Despite the wide range of applications of such laser beams, many applications require the femtosecond radiation pulses with a shorter wavelength (visible and ultraviolet spectrum range) [6,7]. The shorter wavelength of laser radiation is mainly obtained upon the conversion of infrared (IR) radiation pulses in nonlinear crystals (harmonic generation). In this case, the pulse duration, for example, of second harmonic (SH) in saturated mode is usually comparable with pulse width of IR radiation. Although under certain conditions, for example, in unsaturated mode, SH pulse duration may be √2 times shorter than the pump pulse due to quadratic dependence of its intensity concerning IR radiation intensity. Most common laser systems based on Ti:Sa crystal generate in range of 800 nm and have typical duration of 50–70 fs [1–12]. Shortening of pulse duration in such systems is not a trivial task and, as a rule, it is associated with decrease of laser pulse energy. Nevertheless, shortening of femtosecond laser pulse width always expands the area of its applications. In this regard, new methods research of SH pulse width reduction using standard Ti:Sa laser systems with pulse duration of 50–70 fs is relevant. Unfortunately, there are few works devoted to the study of conditions for SH spectral contour broadening. In [11] SH generation (SHG) in nonlinear KDP crystal for 70 fs first harmonic radiation pulse with very high peak intensity of 2 TW/cm2 was researched. The cubic nonlinearity of the frequency doubler leads to self- and cross-phase modulations of interacting fundamental and second harmonics waves. As a
⁎
result, SH spectrum is broadened and modulated and radiation pulse acquires a phase modulation and become not transform-limited at the exit of crystal. The acquired phase modulation can be partially compensated by, for example, mirrors with anomalous (negative) dispersion of group velocities (chirping mirrors). SHG in two nonlinear BBO crystals at low pump intensities (50 GW/ cm2) but in saturated mode was studied in [12]. Each crystal was tuned to different sides of the pump spectral band. Pump pulse (fundamental harmonic with λ = 800 nm) with a duration of 49 fs acquires a positive chirp in the output compressor of the laser system and 600 fs chirped pulse was converted in BBO crystals. As a result, SH spectrum was broadened almost twice. SH pulse with duration of 29 ps was obtained after compensating the excess positive group delay dispersion (GDD) in compressor on diffraction gratings (compression ratio relative to the fundamental frequency was 1.7). These studies show a possibility of SH spectrum broadening either as a result of cubic non-linearity of a crystal under high-intensity pump [11] or as a result of highly chirped pulse transformation in two crystals, resulting in a more than tenfold increase of its length. It is important to investigate other conditions for broadening the spectrum of SH radiation pulse and its consequent compression in order to shorten the pulse duration in comparison with fundamental frequency. In this paper, the conditions of spectrum broadening and compression of SH pulse at central wavelength of 475 nm in nonlinear KDP crystal using a weakly chirped pump radiation of fundamental frequency with intensity of 100 GW/cm2 are studied.
Corresponding author. E-mail address: [email protected] (M.V. Ivanov).
https://doi.org/10.1016/j.hedp.2019.100701 Received 12 March 2019; Received in revised form 18 July 2019; Accepted 19 July 2019 Available online 19 July 2019 1574-1818/ © 2019 Elsevier B.V. All rights reserved.
High Energy Density Physics 33 (2019) 100701
N.G. Ivanov, et al.
2. Experimental setup and research techniques The experiments were carried out on the femtosecond Ti:Sa laser complex. This complex is the front end of THL-100 multiterawatt laser system [13–23]. The laser complex consists of Ti:Sa master oscillator, stretcher, regenerative amplifier, two multipass amplifiers and diffraction gratings output compressor. The laser complex forms a Gaussian shape laser beam with a central wavelength of 950 nm, pulse duration of 100 ps (FWHM), and energy of 10–15 mJ. After compression of this radiation pulse in output compressor, a transform-limited radiation pulse has a duration of 70 fs and energy of 7–10 mJ. A small positive or negative excess of group velocity dispersion (chirp) can be added to pulse by changing the distance between the gratings of compressor relative to the position for a transform limited pulse. The pulse acquires a positive chirp with decreasing the distance between the diffraction gratings and acquires negative chirp with increasing the distance. Therefore, the duration of fundamental pulse increases in proportion to the magnitude of the excess dispersion. The 1.8 mm KDP crystal was used for SHG. Fused silica prism pair [24,25] and K8 glass block were used to compensate the excess positive and negative dispersion respectively. The spectrum of laser radiation in experiments was measured by ASP150C (Avesta-project) and Ocean Optics (HR4000CG-UVNIR) spectrometers. The pulse duration was measured by ASF-20 singlepulse autocorrelator (Avesta project) in sech2 approximation.
Fig. 2. Autocorrelation trace of the transform-limited (solid), negative chirped (dash) and compressed (dot) SH pulses.
3. Results and discussions At first stage, the spectral-limited radiation pulse of fundamental frequency was converted into SH. This pulse was obtained at the optimal distance between compressor gratings. KDP crystal was tuned to maximum efficiency of SHG. In result, SH pulse had a 6 nm spectrum width (FWHM), 56 or 66 fs pulse duration, and 2–3 mJ energy. Further, positive or negative chirp was created in the pulse of fundamental frequency radiation by changing the distance between the compressor gratings relative to an optimal distance and the parameters of SH were recorded. In case of space increase by 0.57 mm (negative chirp) SH spectrum width increased by 1.5 times (Fig. 1). At the same time, the pulse duration of SH was 82 fs (Fig. 2). Estimated negative GDD for this case was -3200 fs2. Further grating spacing increase did not enlarge the spectrum width, but led to modulation of the spectrum. To compensate the negative chirp a 1 cm thickness glass bulk was used. Autocorrelation functions for transform-limited, chirped pulses and SH pulse after compensating of the excess dispersion are shown in Fig. 2. Transformlimited, chirped and compressed pulse durations were 56, 82 and 37 fs respectively. Thus, the spectrum broadening of SH by 1.5 times with a negative chirp led to the pulse duration reduction by the same factor. In experiments with positive chirp the distance between gratings was reduced by 0.71 mm relatively to the optimal distance. Namely at this gratings position the widest spectrum of SH was recorded. The
Fig. 3. Spectrum of the transform-limited (solid) and positive chirped (dashed) SH pulses.
Fig. 4. Autocorrelation traces of transform-limited (solid) and compressed (dashed) SH pulses.
excess negative GDD for this case was 4000 fs2. At the same time, the half-width of the SH spectral contour almost doubled from 6.2 to 11.1 nm (Fig. 3). Further increase of the excess GDD in compressor led to the appearance of modulation in SH spectral contour. In this experiment, the SH transform-limited pulse duration was 66 fs (Fig. 4), and the chirped pulse duration was 102 fs. A prism pair with prism space of 0.9 m was used to compress this positively chirped pulse. The laser beam was aligned as close as possible to the prism top to reduce the positive dispersion of prism material. The autocorrelation functions of transform-limited pulse (solid) and compressed positive chirped pulse (dashed) of SH are shown in Fig. 4. The pulse duration of the transform-limited pulse decreased from 66 fs to 35 fs.
Fig. 1. Spectrum of the transform-limited (solid) and broadened pulses (dash). 2
High Energy Density Physics 33 (2019) 100701
N.G. Ivanov, et al.
SH spectrum broadening mechanism when a nonlinear crystal is pumped with a chirped pulse can be explained in accordance with the expression [12]:
v2 / v = (4
3( /
ch
)2)0.5
References [1] I.A. Begishev, M. Kalashnikov, V. Karpov, et al., J. Opt. Soc. Am. 21 (2004) 318–322. [2] R. Horlein, B. Dromey, D. Adams, et al., New J. Phys. 10 (2008) 083002. [3] R. Toth, J.C. Kieffer, S. Fourmaux, T. Ozaki, Rev. Sci. Instrum. 76 (2005) 083701. [4] Y. Wang, S. Wang, A. Rockwood, et al., Opt. Lett. 42 (2017) 3828–3831. [5] A. Curtis, C. Calvi, J. Tinsley, et al., Nat. Commun. 9 (2018) 1077. [6] L.D. Mikheev, V.I. Tcheremiskine, O.P. Uteza, M.L. Sentis, Progr. Quantum Electron. 36 (2012) 98–142. [7] L.D. Mikheev, V.F. Losev, High Energy and Short Pulse Lasers, InTech, Croatia, 2016, pp. 131–161 chapter 6. [8] S. Mironov, V. Lozhkarev, V. Ginzburg, Appl. Opt. 48 (11) (2009) 2051–2057. [9] N.G. Ivanov, V.F. Losev, V.E. Prokop'ev, K.A. Sitnik, Opt. Commun. 387 (2017) 322–327. [10] I.A. Begishev, M. Kalashnikov, V. Karpov, P. Nickles, J. Opt, Soc. Am. B. 21 (2) (2004) 318–322. [11] S. Mironov, et al., Quantum Electron. 41 (2011) 963. [12] N. Didenko, A. Konyashchenko, V. Pazyuk, S. Tenyakov, Opt. Commun. 282 (2009) 997–999. [13] S.V. Alekseev, A.I. Aristov, N.G. Ivanov, V.F. Losev, Laser Part. Beams 31 (01) (2013) 17–21. [14] S.V. Alekseev, A.I. Aristov, N.G. Ivanov, V.F. Losev, Quantum Electron. 43 (3) (2013) 190–200. [15] A.G. Yastremskii, N.G. Ivanov, V.F. Losev, Effect of the input laser pulse characteristics on the parameters of the XeF(C–A) amplifier in the THL-100 laser system, Quantum Electron. 46 (11) (2016) 982–988. [16] N.G. Ivanov, M.V. Ivanov, V.F. Losev, A.G. Yastremskii, Russian Phys. J. 7 (2016) 1–10. [17] V. Losev, N. Ivanov, L. Mikheev, Proc. SPIE 10173 (2017) 101731D-4. [18] V.F. Losev, S.V. Alekseev, M.V. Ivanov, et al., Proc. SPIE 10254 (2017) 1025415. [19] S.V. Alekseev, N.G. Ivanov, M.V. Ivanov, et al., Quantum Electron. 47 (3) (2017) 184–187. [20] S.V. Alekseev, M.V. Ivanov, N.G. Ivanov, et al., Russian Phys. J. 60 (8) (2017) 1346–1352. [21] V.F. Losev, S.V. Alekseev, M.V. Ivanov, et al., Proc. SPIE 11042 (2019) 110420. [22] A.G. Yastremskii, N.G. Ivanov, V.F. Losev, Quantum Electron. 49 (3) (2019) 205–209. [23] V.F. Losev, S.V. Alekseev, N.G. Ivanov, et al., Proceedings - International Conference Laser Optics 2018, 2018, p. 87. Article number 8435201. [24] R.L. Fork, O.E. Martinez, J.P. Gordon, Opt. Lett. 9 (5) (1984) 150–152. [25] J.A. Fulop, A.P. Kovacs, Z. Bor, Opt. Commun. 188 (2002) 365–370.
(1)
where ∆νω is the spectrum width of fundamental frequency, ∆ν2ω is the spectrum width of SH, τ is the transform–limited pulse duration of fundamental frequency, τch is the chirped pulse duration of fundamental frequency. According to the expression (1) in our case, SH spectrum should be broadened to 1.68 or 1.8 times for negative or positive chirp, respectively. 4. Conclusion Thus, the possibility of SH spectrum broadening in nonlinear crystal and the subsequent reduction of its pulse duration with a weakly chirped pumping of fundamental frequency radiation pulse has been studied. It was shown that the initiation of small excess GDD ( ± 3200–4000 fs2) in the femtosecond complex output compressor for fundamental frequency emission (λ = 950 nm) makes it possible to increase the width of SH spectrum. Thus, the presence of negatively or positively chirped radiation pulse increased the spectral width of SH by 1.5 or by 2 times respectively. Compression of radiation pulse with broadened spectrum allow shortening SH pulse duration approximately in proportion to the spectrum broadening. Minimum pulse duration of 35 fs at the wavelength of 475 nm was obtained which is 2 times shorter than the duration of fundamental frequency radiation pulse. Acknowledgment Russian Science Foundation (RSF) funded the reported study according to the research projects № 18-19-00009.
3
Contents lists available at ScienceDirect
High Energy Density Physics journal homepage: www.elsevier.com/locate/hedp
Femtosecond pulse width reduction upon second harmonic generation N.G. Ivanov, V.F. Losev, M.V. Ivanov , S.V. Alekseev ⁎
T
Institute of High Current Electronics, Siberian Branch, Russian Academy of Sciences, Akademichesky ave. 2/3, 634055, Tomsk, Russia
ARTICLE INFO
ABSTRACT
Keywords: Fs-laser radiation Pulse duration Pulse compression Gratings
The results of second-harmonic generation (SHG) study for a pre-chirped radiation pulse of fundamental frequency with central wavelength of 950 nm and duration of 70 fs are presented. It is shown that providing of a small excess of group delay dispersion (GDD) ( ± 3200–4000 fs2) for fundamental frequency radiation in the femtosecond front-end compressor increases by 1.5–2 times of the second harmonic spectrum width. Compensation of the radiation pulses excess dispersion with broadened spectrum makes it possible to reduce the duration of second harmonic pulse approximately in proportion to the spectrum broadening and get minimum pulse duration of 35 fs.
1. Introduction At present, the formation of femtosecond radiation pulses in laser systems based on solid-state active media is realized. All these systems work in the infrared spectrum range (0.7–1 μm) [1–5]. Despite the wide range of applications of such laser beams, many applications require the femtosecond radiation pulses with a shorter wavelength (visible and ultraviolet spectrum range) [6,7]. The shorter wavelength of laser radiation is mainly obtained upon the conversion of infrared (IR) radiation pulses in nonlinear crystals (harmonic generation). In this case, the pulse duration, for example, of second harmonic (SH) in saturated mode is usually comparable with pulse width of IR radiation. Although under certain conditions, for example, in unsaturated mode, SH pulse duration may be √2 times shorter than the pump pulse due to quadratic dependence of its intensity concerning IR radiation intensity. Most common laser systems based on Ti:Sa crystal generate in range of 800 nm and have typical duration of 50–70 fs [1–12]. Shortening of pulse duration in such systems is not a trivial task and, as a rule, it is associated with decrease of laser pulse energy. Nevertheless, shortening of femtosecond laser pulse width always expands the area of its applications. In this regard, new methods research of SH pulse width reduction using standard Ti:Sa laser systems with pulse duration of 50–70 fs is relevant. Unfortunately, there are few works devoted to the study of conditions for SH spectral contour broadening. In [11] SH generation (SHG) in nonlinear KDP crystal for 70 fs first harmonic radiation pulse with very high peak intensity of 2 TW/cm2 was researched. The cubic nonlinearity of the frequency doubler leads to self- and cross-phase modulations of interacting fundamental and second harmonics waves. As a
⁎
result, SH spectrum is broadened and modulated and radiation pulse acquires a phase modulation and become not transform-limited at the exit of crystal. The acquired phase modulation can be partially compensated by, for example, mirrors with anomalous (negative) dispersion of group velocities (chirping mirrors). SHG in two nonlinear BBO crystals at low pump intensities (50 GW/ cm2) but in saturated mode was studied in [12]. Each crystal was tuned to different sides of the pump spectral band. Pump pulse (fundamental harmonic with λ = 800 nm) with a duration of 49 fs acquires a positive chirp in the output compressor of the laser system and 600 fs chirped pulse was converted in BBO crystals. As a result, SH spectrum was broadened almost twice. SH pulse with duration of 29 ps was obtained after compensating the excess positive group delay dispersion (GDD) in compressor on diffraction gratings (compression ratio relative to the fundamental frequency was 1.7). These studies show a possibility of SH spectrum broadening either as a result of cubic non-linearity of a crystal under high-intensity pump [11] or as a result of highly chirped pulse transformation in two crystals, resulting in a more than tenfold increase of its length. It is important to investigate other conditions for broadening the spectrum of SH radiation pulse and its consequent compression in order to shorten the pulse duration in comparison with fundamental frequency. In this paper, the conditions of spectrum broadening and compression of SH pulse at central wavelength of 475 nm in nonlinear KDP crystal using a weakly chirped pump radiation of fundamental frequency with intensity of 100 GW/cm2 are studied.
Corresponding author. E-mail address: [email protected] (M.V. Ivanov).
https://doi.org/10.1016/j.hedp.2019.100701 Received 12 March 2019; Received in revised form 18 July 2019; Accepted 19 July 2019 Available online 19 July 2019 1574-1818/ © 2019 Elsevier B.V. All rights reserved.
High Energy Density Physics 33 (2019) 100701
N.G. Ivanov, et al.
2. Experimental setup and research techniques The experiments were carried out on the femtosecond Ti:Sa laser complex. This complex is the front end of THL-100 multiterawatt laser system [13–23]. The laser complex consists of Ti:Sa master oscillator, stretcher, regenerative amplifier, two multipass amplifiers and diffraction gratings output compressor. The laser complex forms a Gaussian shape laser beam with a central wavelength of 950 nm, pulse duration of 100 ps (FWHM), and energy of 10–15 mJ. After compression of this radiation pulse in output compressor, a transform-limited radiation pulse has a duration of 70 fs and energy of 7–10 mJ. A small positive or negative excess of group velocity dispersion (chirp) can be added to pulse by changing the distance between the gratings of compressor relative to the position for a transform limited pulse. The pulse acquires a positive chirp with decreasing the distance between the diffraction gratings and acquires negative chirp with increasing the distance. Therefore, the duration of fundamental pulse increases in proportion to the magnitude of the excess dispersion. The 1.8 mm KDP crystal was used for SHG. Fused silica prism pair [24,25] and K8 glass block were used to compensate the excess positive and negative dispersion respectively. The spectrum of laser radiation in experiments was measured by ASP150C (Avesta-project) and Ocean Optics (HR4000CG-UVNIR) spectrometers. The pulse duration was measured by ASF-20 singlepulse autocorrelator (Avesta project) in sech2 approximation.
Fig. 2. Autocorrelation trace of the transform-limited (solid), negative chirped (dash) and compressed (dot) SH pulses.
3. Results and discussions At first stage, the spectral-limited radiation pulse of fundamental frequency was converted into SH. This pulse was obtained at the optimal distance between compressor gratings. KDP crystal was tuned to maximum efficiency of SHG. In result, SH pulse had a 6 nm spectrum width (FWHM), 56 or 66 fs pulse duration, and 2–3 mJ energy. Further, positive or negative chirp was created in the pulse of fundamental frequency radiation by changing the distance between the compressor gratings relative to an optimal distance and the parameters of SH were recorded. In case of space increase by 0.57 mm (negative chirp) SH spectrum width increased by 1.5 times (Fig. 1). At the same time, the pulse duration of SH was 82 fs (Fig. 2). Estimated negative GDD for this case was -3200 fs2. Further grating spacing increase did not enlarge the spectrum width, but led to modulation of the spectrum. To compensate the negative chirp a 1 cm thickness glass bulk was used. Autocorrelation functions for transform-limited, chirped pulses and SH pulse after compensating of the excess dispersion are shown in Fig. 2. Transformlimited, chirped and compressed pulse durations were 56, 82 and 37 fs respectively. Thus, the spectrum broadening of SH by 1.5 times with a negative chirp led to the pulse duration reduction by the same factor. In experiments with positive chirp the distance between gratings was reduced by 0.71 mm relatively to the optimal distance. Namely at this gratings position the widest spectrum of SH was recorded. The
Fig. 3. Spectrum of the transform-limited (solid) and positive chirped (dashed) SH pulses.
Fig. 4. Autocorrelation traces of transform-limited (solid) and compressed (dashed) SH pulses.
excess negative GDD for this case was 4000 fs2. At the same time, the half-width of the SH spectral contour almost doubled from 6.2 to 11.1 nm (Fig. 3). Further increase of the excess GDD in compressor led to the appearance of modulation in SH spectral contour. In this experiment, the SH transform-limited pulse duration was 66 fs (Fig. 4), and the chirped pulse duration was 102 fs. A prism pair with prism space of 0.9 m was used to compress this positively chirped pulse. The laser beam was aligned as close as possible to the prism top to reduce the positive dispersion of prism material. The autocorrelation functions of transform-limited pulse (solid) and compressed positive chirped pulse (dashed) of SH are shown in Fig. 4. The pulse duration of the transform-limited pulse decreased from 66 fs to 35 fs.
Fig. 1. Spectrum of the transform-limited (solid) and broadened pulses (dash). 2
High Energy Density Physics 33 (2019) 100701
N.G. Ivanov, et al.
SH spectrum broadening mechanism when a nonlinear crystal is pumped with a chirped pulse can be explained in accordance with the expression [12]:
v2 / v = (4
3( /
ch
)2)0.5
References [1] I.A. Begishev, M. Kalashnikov, V. Karpov, et al., J. Opt. Soc. Am. 21 (2004) 318–322. [2] R. Horlein, B. Dromey, D. Adams, et al., New J. Phys. 10 (2008) 083002. [3] R. Toth, J.C. Kieffer, S. Fourmaux, T. Ozaki, Rev. Sci. Instrum. 76 (2005) 083701. [4] Y. Wang, S. Wang, A. Rockwood, et al., Opt. Lett. 42 (2017) 3828–3831. [5] A. Curtis, C. Calvi, J. Tinsley, et al., Nat. Commun. 9 (2018) 1077. [6] L.D. Mikheev, V.I. Tcheremiskine, O.P. Uteza, M.L. Sentis, Progr. Quantum Electron. 36 (2012) 98–142. [7] L.D. Mikheev, V.F. Losev, High Energy and Short Pulse Lasers, InTech, Croatia, 2016, pp. 131–161 chapter 6. [8] S. Mironov, V. Lozhkarev, V. Ginzburg, Appl. Opt. 48 (11) (2009) 2051–2057. [9] N.G. Ivanov, V.F. Losev, V.E. Prokop'ev, K.A. Sitnik, Opt. Commun. 387 (2017) 322–327. [10] I.A. Begishev, M. Kalashnikov, V. Karpov, P. Nickles, J. Opt, Soc. Am. B. 21 (2) (2004) 318–322. [11] S. Mironov, et al., Quantum Electron. 41 (2011) 963. [12] N. Didenko, A. Konyashchenko, V. Pazyuk, S. Tenyakov, Opt. Commun. 282 (2009) 997–999. [13] S.V. Alekseev, A.I. Aristov, N.G. Ivanov, V.F. Losev, Laser Part. Beams 31 (01) (2013) 17–21. [14] S.V. Alekseev, A.I. Aristov, N.G. Ivanov, V.F. Losev, Quantum Electron. 43 (3) (2013) 190–200. [15] A.G. Yastremskii, N.G. Ivanov, V.F. Losev, Effect of the input laser pulse characteristics on the parameters of the XeF(C–A) amplifier in the THL-100 laser system, Quantum Electron. 46 (11) (2016) 982–988. [16] N.G. Ivanov, M.V. Ivanov, V.F. Losev, A.G. Yastremskii, Russian Phys. J. 7 (2016) 1–10. [17] V. Losev, N. Ivanov, L. Mikheev, Proc. SPIE 10173 (2017) 101731D-4. [18] V.F. Losev, S.V. Alekseev, M.V. Ivanov, et al., Proc. SPIE 10254 (2017) 1025415. [19] S.V. Alekseev, N.G. Ivanov, M.V. Ivanov, et al., Quantum Electron. 47 (3) (2017) 184–187. [20] S.V. Alekseev, M.V. Ivanov, N.G. Ivanov, et al., Russian Phys. J. 60 (8) (2017) 1346–1352. [21] V.F. Losev, S.V. Alekseev, M.V. Ivanov, et al., Proc. SPIE 11042 (2019) 110420. [22] A.G. Yastremskii, N.G. Ivanov, V.F. Losev, Quantum Electron. 49 (3) (2019) 205–209. [23] V.F. Losev, S.V. Alekseev, N.G. Ivanov, et al., Proceedings - International Conference Laser Optics 2018, 2018, p. 87. Article number 8435201. [24] R.L. Fork, O.E. Martinez, J.P. Gordon, Opt. Lett. 9 (5) (1984) 150–152. [25] J.A. Fulop, A.P. Kovacs, Z. Bor, Opt. Commun. 188 (2002) 365–370.
(1)
where ∆νω is the spectrum width of fundamental frequency, ∆ν2ω is the spectrum width of SH, τ is the transform–limited pulse duration of fundamental frequency, τch is the chirped pulse duration of fundamental frequency. According to the expression (1) in our case, SH spectrum should be broadened to 1.68 or 1.8 times for negative or positive chirp, respectively. 4. Conclusion Thus, the possibility of SH spectrum broadening in nonlinear crystal and the subsequent reduction of its pulse duration with a weakly chirped pumping of fundamental frequency radiation pulse has been studied. It was shown that the initiation of small excess GDD ( ± 3200–4000 fs2) in the femtosecond complex output compressor for fundamental frequency emission (λ = 950 nm) makes it possible to increase the width of SH spectrum. Thus, the presence of negatively or positively chirped radiation pulse increased the spectral width of SH by 1.5 or by 2 times respectively. Compression of radiation pulse with broadened spectrum allow shortening SH pulse duration approximately in proportion to the spectrum broadening. Minimum pulse duration of 35 fs at the wavelength of 475 nm was obtained which is 2 times shorter than the duration of fundamental frequency radiation pulse. Acknowledgment Russian Science Foundation (RSF) funded the reported study according to the research projects № 18-19-00009.
3