- Email: [email protected]
Microjoule supercontinuum generation by stretched megawatt femtosecond laser pulses in a large-mode-area photonic-crystal fiber
Available online at www.sciencedirect.com
Optics Communications 280 (2007) 453–456 www.elsevier.com/locate/optcom
Microjoule supercontinuum generation by stretched megawatt femtosecond laser pulses in a large-mode-area photonic-crystal fiber A.V. Mitrofanov a, A.A. Ivanov
b,c
, M.V. Alfimov b, A.A. Podshivalov c, A.M. Zheltikov
a,c,*
a
c
Physics Department, M.V. Lomonosov Moscow State University, Vorob’evy gory, Moscow 119992, Russia b Center of Photochemistry, Russian Academy of Sciences, ul. Novatorov 7a, Moscow 117421, Russia Physics Department, International Laser Center, M.V. Lomonosov Moscow State University, Vorob’evy gory, Moscow 119992, Russia Received 14 February 2007; accepted 10 June 2007
Abstract Microjoule supercontinuum generation is demonstrated using a large-mode-area photonic-crystal fiber (PCF) pumped by an amplified stretched-pulse output of a mode-locked Cr:forsterite laser. A PCF with a mode area of 380 lm2 is employed to transform 300-fs Cr:forsterite laser pulses with a peak-power of a few megawatts into a supercontinuum radiation with a spectrum spanning from 700 to 1800 nm and a total energy of 1.15 lJ. Ó 2007 Elsevier B.V. All rights reserved. PACS: 42.65.Wi; 42.81.Qb
Recent advances in fiber-laser and fiber-frequency-conversion technologies make fiber-based light sources an attractive alternative to solid-state lasers in the vast area of laser science and technologies. In particular, supercontinuum generation in photonic-crystal fibers (PCFs) [1,2] has been the focus of intense recent research, opening new horizons in optical frequency metrology [3–5] leading to significant advances in the generation of few-cycle pulses with a controlled carrier-envelope phase [5], and allowing the creation of fiber-optic sources of broadband radiation for nonlinear spectroscopy [6–8], microscopy [9–11] and biomedical applications [12]. Small-core PCFs, typically operating in the regime of anomalous dispersion, show an excellent performance as supercontinuum sources ideally suited for nano- and subnanojoule input laser pulses. To accommodate higher input energies without the risk of laser-induced damage of the fiber material, PCFs with a larger core size are needed. Such large-mode-area * Corresponding author. Address: Physics Department, M.V. Lomonosov Moscow State University, Vorob’evy gory, Moscow 119992, Russia. Tel.: +7 95 939 5174; fax: +7 95 939 3113. E-mail address: [email protected] (A.M. Zheltikov).
0030-4018/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2007.06.062
(LMA) PCF components [13,14] have been used for the creation of high-power fiber-lasers [15,16] and amplification of a short-pulse fiber-laser output [17]. Su¨dmeyer et al. [18] have demonstrated an efficient spectral broadening of a submegawatt, subpicosecond thin-disk laser output in LMA PCFs, enabling a compression of 810-fs thin-disk laser pulses to a 33-fs pulse duration. Genty et al. [19] have employed LMA PCFs to transform nanosecond pulses from a Q-switched Nd:YAG laser into a supercontinuum radiation. Waveguide dispersion for large-core PCFs is typically weak, which limits the possibilities of fiber dispersion tailoring through fiber structure modifications. It becomes difficult, in particular, to achieve large shifts for the zero group-velocity dispersion (GVD) wavelength relative to the zero-GVD point in the bulk of the fiber material. As a result, the central wavelengths of many of the commonly used sources of high-energy ultrashort pulses are left in the regime of normal dispersion of silica large-mode-area PCF, making it difficult to generate broadband radiation through nonlinear-optical processes in the fiber. In this work, we show that a combination of Cr:forsterite laser sources with large-mode-area PCFs resolves the conflict
454
A.V. Mitrofanov et al. / Optics Communications 280 (2007) 453–456
Spectral intensity, arb. units
between the mode area and dispersion, allowing high-peakpower laser output to be efficiently transformed into supercontinuum radiation. This strategy offers much promise for the creation of high-peak-power fiber-format sources of broadband radiation for spectroscopic, microscopic, biomedical, and micromachining applications, as well as for the creation of front-end component for laser sources designed to deliver extremely high-peak-powers at their output. Large-mode-area PCFs used in our experiments were made of fused silica using a standard stack-and-draw technology [1]. For the fiber used in our experiments, the core diameter was about 22 lm with an effective mode area estimated as 380 lm2 for the fundamental mode. The fiber core was surrounded with four rings of air holes (see the inset in Fig. 1) with a diameter d 4.0 lm and a pitch K 11 lm. Such a fiber could support higher order guided modes of 1.24-lm Cr:forsterite laser radiation used in our experiments. With an appropriate fiber alignment, however, a robust fundamental mode of 1.24-lm radiation was observed at the output of the fiber in the low-peakpower regime, when nonlinear-optical effects were negligible. The zero-GVD wavelength for the PCF employed in our experiments is kz 1.27 lm, and the fiber nonlinearity c = 2pn2(kS) 1 (here, n2 is the nonlinear refractive index of the fiber material, k is the radiation wavelength, and S is the effective mode area) is about 0.4 km 1 W 1 at k = 1.24 lm. The laser system used in our experiments consisted of a Cr4+:forsterite master oscillator, a stretcher, an optical isolator, a regenerative amplifier, and a compressor [20]. The
10
3
2 1 1
800
1000
1200
1400
1600
1800
Wavelength, nm Fig. 1. Spectra of supercontinuum radiation from a 20-cm segment of a PCF with a mode area of 380 lm2 (shown in the inset). The input pulse width is 300 fs. The energy of laser radiation coupled into the fiber is (1) 0.15 lJ, (2) 0.98 lJ, and (3) 1.3 lJ. The input spectrum of the laser pulse is shown by the dashed line.
master oscillator, pumped with an ytterbium fiber-laser, generated 30–60-fs light pulses of radiation with a central wavelength of 1.25 lm at a repetition rate of 120 MHz. These pulses were transmitted through a stretcher and an isolator, to be amplified in a Nd:YLF-laser-pumped amplifier and recompressed to a pulse width of 90–400 fs with a maximum pulse energy up to 30 lJ at 1 kHz. To avoid selffocusing-induced damage of the fiber, the laser output was stretched, through a compressor adjustment, to a pulse width of approximately 300 fs. The energy of amplified Cr:forsterite laser pulses delivered to the fiber input through an objective was about 3 lJ. The laser energy fluence on the input end of the fiber was thus about 0.79 J/ cm2, remaining well below the laser damage threshold. A maximum of 1.3 lJ was coupled into the fundamental mode of the fiber, corresponding to a peak-power of P 4.3 MW. Although the input peak-power in our experiments was slightly higher than the lower-bound self-focusing threshold, a catastrophic self-focusing at this level of P was prevented by fiber dispersion [21], with a robust operation of PCFs provided and no sign of laser damage observed over many hours of measurements. The spectra of laser radiation transmitted through a 20cm piece of the large-mode-area PCF are presented by curves 1–3 in Fig. 1 in comparison with the spectrum of the input field (shown by the dashed line). For lower input energies (curve 1 in Fig. 1), PCF output spectra exhibit moderate broadening with well-pronounced Stokes and anti-Stokes sidebands, falling in the range of anomalous and normal dispersion, respectively. These spectral features are indicative of modulation instabilities [22] of the pump field whose central wavelength lies close to the zero-GVD wavelength, thus facilitating phase matching for Stokes and anti-Stokes sideband generation. For higher input energies (curves 2 and 3 in Fig. 1), isolated spectral components originating from the pump field and its Stokes and anti-Stokes sidebands merge together, giving rise to a broadband spectrum at the output of the fiber. In this regime, the output spectra display a powerful long-wavelength wing, stretching up to 1800 nm for an input energy of 1.3 lJ (curve 3 in Fig. 1), which indicates the significance of soliton self-frequency shifting phenomena, induced by the retarded part of the fiber nonlinearity. The visible part of the spectrum observed at the output of a 20-cm large-mode-area PCF is much less intense, carrying about 15% of the total energy of the fiber output, estimated as 1.15 lJ. As the short-wavelength part of the spectrum was generated in a mixture of guided modes, a considerable portion of the short-wavelength part of the spectrum (about 50%) was lost in our experiments in the course of propagation through leakage losses within the first 7–8 cm of the fiber, leading to a decrease in the total radiation energy from approximately 1.30 lJ (radiation energy coupled into the fiber) to about 1.15 lJ (radiation energy measured at the output of the 20-cm fiber). To visualize the temporal envelope of the PCF output, we employed cross-correlation frequency-resolved optical
A.V. Mitrofanov et al. / Optics Communications 280 (2007) 453–456
1
Spectral intensity, arb. units
gating (XFROG) technique [23,24] by mixing the PCF output with the amplified fundamental-wavelength Cr:forsterite laser radiation through sum-frequency generation in an LBO crystal. The spectrally resolved sum-frequency signal from the nonlinear crystal was measured as a function of the delay time s between the PCF output and the reference Cr:forsterite laser pulse, yielding sonograms of the light field at the output of the fiber. In Fig. 2, we present a typical XFROG trace measured for a 20-cm segment of PCF with an input pulse energy of 1.1 lJ. This trace features a distinct kink around the zero-GVD wavelength kz, separating regimes of normal and anomalous dispersion. For shorter wavelengths, k < kz, because of the normal dispersion, high-frequency components are delayed in time with respect to spectral components with lower frequencies. For longer wavelength, k > kz, well-resolved temporally isolated solitonic features are observed, indicating the significance of solitonic phenomena and the Raman-effectinduced soliton self-frequency shift [22] in the enhancement of the long-wavelength part of the supercontinuum spectrum at the output of the fiber. To quantify the enhancement provided by an LMA PCF for high-power supercontinuum generation relative to a bulk material, we compared the output spectra of the considered type of PCF with the spectral broadening of amplified Cr:forsterite laser pulses attainable in the bulk of fused silica. In the latter experiment, an amplified 300-fs, 1.24-lm Cr:forsterite laser output (dashed curve 1 in Fig. 3) was focused inside a silica plate into a spot with a diameter of about 20 lm, close to the core diameter of the PCF used in our experiments. To achieve maximum spectral broadening, the energy of laser pulses in these experiments was set equal to 8 lJ, which was slightly below the damage threshold of the silica plate. The thickness of the silica plate was
455
3
0.1
2 1 800
1000
1200
1400
1600
1800
Wavelength, nm Fig. 3. Radiation spectra of the amplified Cr:forsterite laser output (dashed line 1), a focused 300-fs, 8-lJ Cr:forsterite laser pulse spectrally broadened in a 2-cm silica plate (open circles), and a 300-fs, 1.3-lJ Cr:forsterite laser pulse transmitted through a 20-cm segment of the largemode-area PCF (filled circles).
chosen equal to 2 cm, which substantially exceeded the effective interaction length (a few millimeters for the above-specified experimental parameters), so that a further increase in the plate thickness lead to no increase in the output spectral width. With these subcritical for a silica plate parameters of incident laser pulses, output radiation spectra (open circles in Fig. 3) remained substantially narrower than the spectra of Cr:forsterite laser pulses with an input energy of 1.3 lJ and the same input pulse width transmitted through 20 cm of the large-mode-area PCF (filled circles in Fig. 3). We have thus demonstrated microjoule supercontinuum generation using a large-mode-area PCF pumped by an amplified stretched-pulse output of a femtosecond Cr:forsterite laser. This result highlights the potential of largemode-area PCFs as sources of high-peak-power broadband radiation for spectroscopic, microscopic, biomedical, and micromachining applications. Acknowledgements We are grateful to A.B. Fedotov, V.P. Mitrokhin, and I.V. Fedotov for valuable help in experimental work, F. Krausz and A. Apolonski for stimulating discussions, and K.V. Dukel’skii, A.V. Khokhlov, Yu.N. Kondrat’ev, A.V. Shcherbakov, and V.S. Shevandin for fabricating fiber samples. This study was supported in part by the Russian Foundation for Basic Research (Projects 06-02-16880 and 05-02-90566-NNS), INTAS (Projects Nos. 03-515037 and 03-51-5288), and CRDF Award No. RUP2-2695. References
Fig. 2. An XFROG trace of the PCF output measured for an input pulse energy of 1.1 lJ. The fiber length is 20 cm. The left- and right-hand ordinate axes represent the wavelength of the sum-frequency signal kSF and the wavelength of the PCF output kPCF, respectively.
[1] P.St.J. Russell, Science 299 (2003) 358. [2] J.K. Ranka, R.S. Windeler, A.J. Stentz, Opt. Lett. 25 (2000) 796. [3] R. Holzwarth, T. Udem, T.W. Ha¨nsch, J.C. Knight, W.J. Wadsworth, P.St.J. Russell, Phys. Rev. Lett. 85 (2000) 2264.
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[4] D.J. Jones, S.A. Diddams, J.K. Ranka, A. Stentz, R.S. Windeler, J.L. Hall, S.T. Cundiff, Science 288 (2000) 635. [5] Th. Udem, R. Holzwarth, T.W. Ha¨nsch, Nature 416 (2002) 233. [6] S.O. Konorov, D.A. Akimov, E.E. Serebryannikov, A.A. Ivanov, M.V. Alfimov, A.M. Zheltikov, Phys. Rev. E 70 (2004) 057601. [7] A.A. Ivanov, M.V. Alfimov, A.M. Zheltikov, Opt. Lett. 31 (2006) 3330. [8] D.A. Sidorov-Biryukov, E.E. Serebryannikov, A.M. Zheltikov, Opt. Lett. 31 (2006) 2323. [9] H.N. Paulsen, K.M. Hilligsøe, J. Thøgersen, S.R. Keiding, J.J. Larsen, Opt. Lett. 28 (2003) 1123. [10] H. Kano, H. Hamaguchi, Opt. Exp. 13 (2005) 1322. [11] B. von Vacano, W. Wohlleben, M. Motzkus, Opt. Lett. 31 (2006) 413. [12] I. Hartl, X.D. Li, C. Chudoba, R.K. Rhanta, T.H. Ko, J.G. Fujimoto, J.K. Ranka, R.S. Windeler, Opt. Lett. 26 (2001) 608. [13] J.C. Knight, T.A. Birks, R.F. Cregan, P.St.J. Russell, J.P. de Sandro, Electron. Lett. 34 (1998) 1347. [14] K. Furusawa, A. Malinowski, J. Price, T. Monro, J. Sahu, J. Nilsson, D. Richardson, Opt. Exp. 9 (2001) 714.
[15] W. Wadsworth, R. Percival, G. Bouwmans, J. Knight, P. Russell, Opt. Exp. 11 (2003) 48. [16] J. Limpert, T. Schreiber, S. Nolte, H. Zellmer, T. Tunnermann, R. Iliew, F. Lederer, J. Broeng, G. Vienne, A. Petersson, C. Jakobsen, Opt. Exp. 11 (2003) 818. [17] J. Limpert, A. Liem, M. Reich, T. Schreiber, S. Nolte, H. Zellmer, A. Tu¨nnermann, J. Broeng, A. Petersson, C. Jakobsen, Opt. Exp. 12 (2004) 1313. [18] T. Su¨dmeyer, F. Brunner, E. Innerhofer, R. Paschotta, K. Furusawa, J.C. Baggett, T.M. Monro, D.J. Richardson, U. Keller, Opt. Lett. 28 (2003) 1951. [19] G. Genty, T. Ritari, H. Ludvigsen, Opt. Exp. 13 (2005) 8625. [20] A.A. Ivanov, M.V. Alfimov, A.M. Zheltikov, Phys. Uspekhi 47 (2004) 687. [21] A.L. Gaeta, Phys. Rev. Lett. 84 (2000) 3582. [22] G.P. Agrawal, Nonlinear Fiber Optics, Academic, San Diego, 2001. [23] S. Linden, J. Kuhl, H. Giessen, Opt. Lett. 24 (1999) 569. [24] X. Gu, L. Xu, M. Kimmel, E. Zeek, P. O’Shea, A.P. Shreenath, R. Trebino, R.S. Windeler, Opt. Lett. 27 (2002) 1174.
Optics Communications 280 (2007) 453–456 www.elsevier.com/locate/optcom
Microjoule supercontinuum generation by stretched megawatt femtosecond laser pulses in a large-mode-area photonic-crystal fiber A.V. Mitrofanov a, A.A. Ivanov
b,c
, M.V. Alfimov b, A.A. Podshivalov c, A.M. Zheltikov
a,c,*
a
c
Physics Department, M.V. Lomonosov Moscow State University, Vorob’evy gory, Moscow 119992, Russia b Center of Photochemistry, Russian Academy of Sciences, ul. Novatorov 7a, Moscow 117421, Russia Physics Department, International Laser Center, M.V. Lomonosov Moscow State University, Vorob’evy gory, Moscow 119992, Russia Received 14 February 2007; accepted 10 June 2007
Abstract Microjoule supercontinuum generation is demonstrated using a large-mode-area photonic-crystal fiber (PCF) pumped by an amplified stretched-pulse output of a mode-locked Cr:forsterite laser. A PCF with a mode area of 380 lm2 is employed to transform 300-fs Cr:forsterite laser pulses with a peak-power of a few megawatts into a supercontinuum radiation with a spectrum spanning from 700 to 1800 nm and a total energy of 1.15 lJ. Ó 2007 Elsevier B.V. All rights reserved. PACS: 42.65.Wi; 42.81.Qb
Recent advances in fiber-laser and fiber-frequency-conversion technologies make fiber-based light sources an attractive alternative to solid-state lasers in the vast area of laser science and technologies. In particular, supercontinuum generation in photonic-crystal fibers (PCFs) [1,2] has been the focus of intense recent research, opening new horizons in optical frequency metrology [3–5] leading to significant advances in the generation of few-cycle pulses with a controlled carrier-envelope phase [5], and allowing the creation of fiber-optic sources of broadband radiation for nonlinear spectroscopy [6–8], microscopy [9–11] and biomedical applications [12]. Small-core PCFs, typically operating in the regime of anomalous dispersion, show an excellent performance as supercontinuum sources ideally suited for nano- and subnanojoule input laser pulses. To accommodate higher input energies without the risk of laser-induced damage of the fiber material, PCFs with a larger core size are needed. Such large-mode-area * Corresponding author. Address: Physics Department, M.V. Lomonosov Moscow State University, Vorob’evy gory, Moscow 119992, Russia. Tel.: +7 95 939 5174; fax: +7 95 939 3113. E-mail address: [email protected] (A.M. Zheltikov).
0030-4018/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2007.06.062
(LMA) PCF components [13,14] have been used for the creation of high-power fiber-lasers [15,16] and amplification of a short-pulse fiber-laser output [17]. Su¨dmeyer et al. [18] have demonstrated an efficient spectral broadening of a submegawatt, subpicosecond thin-disk laser output in LMA PCFs, enabling a compression of 810-fs thin-disk laser pulses to a 33-fs pulse duration. Genty et al. [19] have employed LMA PCFs to transform nanosecond pulses from a Q-switched Nd:YAG laser into a supercontinuum radiation. Waveguide dispersion for large-core PCFs is typically weak, which limits the possibilities of fiber dispersion tailoring through fiber structure modifications. It becomes difficult, in particular, to achieve large shifts for the zero group-velocity dispersion (GVD) wavelength relative to the zero-GVD point in the bulk of the fiber material. As a result, the central wavelengths of many of the commonly used sources of high-energy ultrashort pulses are left in the regime of normal dispersion of silica large-mode-area PCF, making it difficult to generate broadband radiation through nonlinear-optical processes in the fiber. In this work, we show that a combination of Cr:forsterite laser sources with large-mode-area PCFs resolves the conflict
454
A.V. Mitrofanov et al. / Optics Communications 280 (2007) 453–456
Spectral intensity, arb. units
between the mode area and dispersion, allowing high-peakpower laser output to be efficiently transformed into supercontinuum radiation. This strategy offers much promise for the creation of high-peak-power fiber-format sources of broadband radiation for spectroscopic, microscopic, biomedical, and micromachining applications, as well as for the creation of front-end component for laser sources designed to deliver extremely high-peak-powers at their output. Large-mode-area PCFs used in our experiments were made of fused silica using a standard stack-and-draw technology [1]. For the fiber used in our experiments, the core diameter was about 22 lm with an effective mode area estimated as 380 lm2 for the fundamental mode. The fiber core was surrounded with four rings of air holes (see the inset in Fig. 1) with a diameter d 4.0 lm and a pitch K 11 lm. Such a fiber could support higher order guided modes of 1.24-lm Cr:forsterite laser radiation used in our experiments. With an appropriate fiber alignment, however, a robust fundamental mode of 1.24-lm radiation was observed at the output of the fiber in the low-peakpower regime, when nonlinear-optical effects were negligible. The zero-GVD wavelength for the PCF employed in our experiments is kz 1.27 lm, and the fiber nonlinearity c = 2pn2(kS) 1 (here, n2 is the nonlinear refractive index of the fiber material, k is the radiation wavelength, and S is the effective mode area) is about 0.4 km 1 W 1 at k = 1.24 lm. The laser system used in our experiments consisted of a Cr4+:forsterite master oscillator, a stretcher, an optical isolator, a regenerative amplifier, and a compressor [20]. The
10
3
2 1 1
800
1000
1200
1400
1600
1800
Wavelength, nm Fig. 1. Spectra of supercontinuum radiation from a 20-cm segment of a PCF with a mode area of 380 lm2 (shown in the inset). The input pulse width is 300 fs. The energy of laser radiation coupled into the fiber is (1) 0.15 lJ, (2) 0.98 lJ, and (3) 1.3 lJ. The input spectrum of the laser pulse is shown by the dashed line.
master oscillator, pumped with an ytterbium fiber-laser, generated 30–60-fs light pulses of radiation with a central wavelength of 1.25 lm at a repetition rate of 120 MHz. These pulses were transmitted through a stretcher and an isolator, to be amplified in a Nd:YLF-laser-pumped amplifier and recompressed to a pulse width of 90–400 fs with a maximum pulse energy up to 30 lJ at 1 kHz. To avoid selffocusing-induced damage of the fiber, the laser output was stretched, through a compressor adjustment, to a pulse width of approximately 300 fs. The energy of amplified Cr:forsterite laser pulses delivered to the fiber input through an objective was about 3 lJ. The laser energy fluence on the input end of the fiber was thus about 0.79 J/ cm2, remaining well below the laser damage threshold. A maximum of 1.3 lJ was coupled into the fundamental mode of the fiber, corresponding to a peak-power of P 4.3 MW. Although the input peak-power in our experiments was slightly higher than the lower-bound self-focusing threshold, a catastrophic self-focusing at this level of P was prevented by fiber dispersion [21], with a robust operation of PCFs provided and no sign of laser damage observed over many hours of measurements. The spectra of laser radiation transmitted through a 20cm piece of the large-mode-area PCF are presented by curves 1–3 in Fig. 1 in comparison with the spectrum of the input field (shown by the dashed line). For lower input energies (curve 1 in Fig. 1), PCF output spectra exhibit moderate broadening with well-pronounced Stokes and anti-Stokes sidebands, falling in the range of anomalous and normal dispersion, respectively. These spectral features are indicative of modulation instabilities [22] of the pump field whose central wavelength lies close to the zero-GVD wavelength, thus facilitating phase matching for Stokes and anti-Stokes sideband generation. For higher input energies (curves 2 and 3 in Fig. 1), isolated spectral components originating from the pump field and its Stokes and anti-Stokes sidebands merge together, giving rise to a broadband spectrum at the output of the fiber. In this regime, the output spectra display a powerful long-wavelength wing, stretching up to 1800 nm for an input energy of 1.3 lJ (curve 3 in Fig. 1), which indicates the significance of soliton self-frequency shifting phenomena, induced by the retarded part of the fiber nonlinearity. The visible part of the spectrum observed at the output of a 20-cm large-mode-area PCF is much less intense, carrying about 15% of the total energy of the fiber output, estimated as 1.15 lJ. As the short-wavelength part of the spectrum was generated in a mixture of guided modes, a considerable portion of the short-wavelength part of the spectrum (about 50%) was lost in our experiments in the course of propagation through leakage losses within the first 7–8 cm of the fiber, leading to a decrease in the total radiation energy from approximately 1.30 lJ (radiation energy coupled into the fiber) to about 1.15 lJ (radiation energy measured at the output of the 20-cm fiber). To visualize the temporal envelope of the PCF output, we employed cross-correlation frequency-resolved optical
A.V. Mitrofanov et al. / Optics Communications 280 (2007) 453–456
1
Spectral intensity, arb. units
gating (XFROG) technique [23,24] by mixing the PCF output with the amplified fundamental-wavelength Cr:forsterite laser radiation through sum-frequency generation in an LBO crystal. The spectrally resolved sum-frequency signal from the nonlinear crystal was measured as a function of the delay time s between the PCF output and the reference Cr:forsterite laser pulse, yielding sonograms of the light field at the output of the fiber. In Fig. 2, we present a typical XFROG trace measured for a 20-cm segment of PCF with an input pulse energy of 1.1 lJ. This trace features a distinct kink around the zero-GVD wavelength kz, separating regimes of normal and anomalous dispersion. For shorter wavelengths, k < kz, because of the normal dispersion, high-frequency components are delayed in time with respect to spectral components with lower frequencies. For longer wavelength, k > kz, well-resolved temporally isolated solitonic features are observed, indicating the significance of solitonic phenomena and the Raman-effectinduced soliton self-frequency shift [22] in the enhancement of the long-wavelength part of the supercontinuum spectrum at the output of the fiber. To quantify the enhancement provided by an LMA PCF for high-power supercontinuum generation relative to a bulk material, we compared the output spectra of the considered type of PCF with the spectral broadening of amplified Cr:forsterite laser pulses attainable in the bulk of fused silica. In the latter experiment, an amplified 300-fs, 1.24-lm Cr:forsterite laser output (dashed curve 1 in Fig. 3) was focused inside a silica plate into a spot with a diameter of about 20 lm, close to the core diameter of the PCF used in our experiments. To achieve maximum spectral broadening, the energy of laser pulses in these experiments was set equal to 8 lJ, which was slightly below the damage threshold of the silica plate. The thickness of the silica plate was
455
3
0.1
2 1 800
1000
1200
1400
1600
1800
Wavelength, nm Fig. 3. Radiation spectra of the amplified Cr:forsterite laser output (dashed line 1), a focused 300-fs, 8-lJ Cr:forsterite laser pulse spectrally broadened in a 2-cm silica plate (open circles), and a 300-fs, 1.3-lJ Cr:forsterite laser pulse transmitted through a 20-cm segment of the largemode-area PCF (filled circles).
chosen equal to 2 cm, which substantially exceeded the effective interaction length (a few millimeters for the above-specified experimental parameters), so that a further increase in the plate thickness lead to no increase in the output spectral width. With these subcritical for a silica plate parameters of incident laser pulses, output radiation spectra (open circles in Fig. 3) remained substantially narrower than the spectra of Cr:forsterite laser pulses with an input energy of 1.3 lJ and the same input pulse width transmitted through 20 cm of the large-mode-area PCF (filled circles in Fig. 3). We have thus demonstrated microjoule supercontinuum generation using a large-mode-area PCF pumped by an amplified stretched-pulse output of a femtosecond Cr:forsterite laser. This result highlights the potential of largemode-area PCFs as sources of high-peak-power broadband radiation for spectroscopic, microscopic, biomedical, and micromachining applications. Acknowledgements We are grateful to A.B. Fedotov, V.P. Mitrokhin, and I.V. Fedotov for valuable help in experimental work, F. Krausz and A. Apolonski for stimulating discussions, and K.V. Dukel’skii, A.V. Khokhlov, Yu.N. Kondrat’ev, A.V. Shcherbakov, and V.S. Shevandin for fabricating fiber samples. This study was supported in part by the Russian Foundation for Basic Research (Projects 06-02-16880 and 05-02-90566-NNS), INTAS (Projects Nos. 03-515037 and 03-51-5288), and CRDF Award No. RUP2-2695. References
Fig. 2. An XFROG trace of the PCF output measured for an input pulse energy of 1.1 lJ. The fiber length is 20 cm. The left- and right-hand ordinate axes represent the wavelength of the sum-frequency signal kSF and the wavelength of the PCF output kPCF, respectively.
[1] P.St.J. Russell, Science 299 (2003) 358. [2] J.K. Ranka, R.S. Windeler, A.J. Stentz, Opt. Lett. 25 (2000) 796. [3] R. Holzwarth, T. Udem, T.W. Ha¨nsch, J.C. Knight, W.J. Wadsworth, P.St.J. Russell, Phys. Rev. Lett. 85 (2000) 2264.
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