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All-photonic-crystal-fiber coherent black-light source Bo-Wen Liu,1 Minglie Hu,1 Si-Jia Wang,1 Lu Chai,1 Chingyue Wang,1 Neng-li Dai,2 Jing-Yan Li,2 and Aleksei M. Zheltikov3,* 1
Ultrafast Laser Lab, School of Precision Instruments and Optoelectronics Engineering, and Key Laboratory of Information Technical Science of Ministry of Education, Tianjin University, 300072 Tianjin, China 2 Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, China 3
Physics Department, International Laser Center, M. V. Lomonosov Moscow State University, 119992 Moscow, Russia *Corresponding author:
[email protected] Received September 8, 2010; revised October 11, 2010; accepted October 12, 2010; posted October 22, 2010 (Doc. ID 134659); published November 23, 2010 Photonic-crystal fibers (PCFs) are widely recognized as efficient sources of white-light supercontinuum. Here, we show that a combination of an ultrashort-pulse ytterbium PCF laser with a highly nonlinear PCF enables all-fiber generation of ultrashort pulses of coherent UV radiation in the black-light spectral region with all the key processes, such as lasing, amplification, and conversion of the amplified PCF-laser output to the UV range, taking place in specifically designed fiber components. © 2010 Optical Society of America OCIS codes: 190.4370, 190.7110.
White-light supercontinuum generation is a spectacular phenomenon and a powerful technology [1–3] that enables new approaches in optical metrology [4], time-resolved spectroscopy [3], ultrafast photonics [5], nonlinear-optical microscopy [6], and neuroimaging [7]. Spectral broadening of short-pulse optical fields into a white-light supercontinuum becomes possible as a result of an intricate interplay of nonlinear-optical effects [1–3], including self-phase modulation, four-wave mixing, soliton self-frequency shift, and soliton-instability-induced dispersive-wave generation. Highly nonlinear waveguides, including photoniccrystal fibers (PCFs) [8,9] in particular, provide an ideal tool for the generation of white-light waveforms carefully tailored for a specific application. Here, we show that PCF technologies enable the creation of all-fiber sources of black light, also referred to as Wood’s light [10], i.e., sources of long-wavelength UVA radiation emitting no or only a little visible light. In a nonlinear fiber, UV radiation, as shown in earlier work [11–16], can be generated through third-harmonic generation (THG). PCFs for the THG transformation of ultrashortpulse output of mode-locked solid-state Ti:sapphire [11,12,14] and Cr:forsterite lasers [13,15,16] have been demonstrated. THG in highly nonlinear fibers has been shown to involve unusual physical scenarios and effects that are not observed in THG in nonlinear crystals [17]. In particular, THG by a solitonic pump field has been identified as a significant process enhancing the shortwavelength part of the white-light supercontinuum PCF output [16] and allowing phase-matched high-order harmonic generation in gas-filled hollow-core PCFs [18]. In this Letter, we demonstrate that a highly nonlinear PCF combined with an ultra-short-pulse PCF-laser source enables the generation of ultrashort pulses of coherent UV radiation in the black-light spectral region with all the key processes, such as lasing, amplification, and THG, taking place in specifically designed fiber components. Experiments were performed with a pump laser source consisting of a fiber oscillator and a fiber amplifier both based on ytterbium large-mode-area (LMA) PCFs [19]. The pump laser system delivered light pulses with a central wavelength of 1060 nm, a pulse width of 260 fs, and an average power of up to 600 mW at a pulse 0146-9592/10/233958-03$15.00/0
repetition rate of 50 MHz. The pump pulses were launched into a highly nonlinear PCF with a core diameter of 2 μm (shown in Fig. 1). With an appropriate adjustment of the input fiber end, the pump beam was coupled into an HE21 -type mode of the PCF. The zero group-velocity dispersion wavelength for this mode, according to finite-element analysis, is 1:01 μm. The pump field thus tends to generate solitons as it propagates through the PCF. As the solitons undergo a continuous redshift due to the Raman effect as a part of their evolution in the fiber, they sweep over phase-matching resonances with high-order modes (insets in Fig. 2) of the third harmonic (TH), generated in the fiber due to its third-order nonlinearity. The main features of multimode THG with a soliton pump have been identified in [15,16]. Briefly, the electric field in the soliton is represented as A ¼ ψðξÞ exp f−iωs t þ i½βn ðωs Þ þ qzg, where ξ ¼ t − z=vg is the retarded time, βn is the propagation constant of the nth waveguide mode, z is the propagation coordinate, ωs and vg are the central frequency and the group velocity of the soliton, ψðξÞ is the soliton pulse shape, q ¼ γP=2, γ is the nonlinear coefficient, and P is the soliton peak
Fig. 1. Scanning electron microscopy image of the highly nonlinear PCF. © 2010 Optical Society of America
December 1, 2010 / Vol. 35, No. 23 / OPTICS LETTERS
power. With a Fourier-integral representation A ¼ R ^ FðωÞ expf−iðωs − ωÞt þ i½βn ðωs Þ þ q − ðωs − ωÞ=vg zgdω, ^ where FðωÞ is the Fourier transform of the soliton spectrum, we can define the soliton propagation constant (see also [15]) as βsol ðωÞ ¼ βn ðωs Þ þ q þ ðω − ωs Þ=vg . The propagation constant of the dispersive-wave TH emitted in the mth guided mode is written as βTH ð3ωÞ ¼ βm ð3ωÞ. With the effective mode index for the dispersive-wave TH defined as nm ð3ωÞ ¼ βTH ð3ωÞc=ð3ωÞ, the phase-matching condition for the generation of a dispersive-wave TH by a soliton pump is given by nm ð3ωÞ ¼ βTH ð3ωÞc=ð3ωÞ, where nsol ðωÞ ¼ βsol ðωÞ=k is the effective mode index for the soliton, k ¼ 2π=λ, and λ is the radiation wavelength. The dashed curves in Fig. 2 represent the effective mode indices nm of high-order PCF modes of the TH field (shown in the insets to Fig. 2) calculated with the finiteelement method as functions of the wavelength in the TH spectral range, shown by the lower abscissa axis. The solid curve shows the effective mode index nsol of the soliton pump as a function of the pump wavelength (shown in the upper abscissa axis). Phase matching is achieved for THG at the wavelengths λ where the solid curve, representing nsol ðλÞ, crosses one of the dashed curves, representing nm ðλ=3Þ for one of the TH modes. The spectrum of the TH PCF output measured in our experiments features well-resolved narrowband peaks (Figs. 2 and 3). The central frequencies of these peaks correlate well with nsol ðλÞ ¼ nm ðλ=3Þ phase-matching resonances (Fig. 2). Pump pulses with higher input peak powers P 0 give rise to a larger number of solitons in the PCF [20] and induce stronger soliton frequency shifts [see the insets in Figs. 3(a)–3(c)], leading to the generation of a larger number of TH modes at the output of the
Fig. 2. (Color online) Spectrum of the TH (filled circles connected by a solid curve) generated in a 1:5 m PCF by 8 nJ 260 fs ytterbium PCF-laser pulses. The dashed curves labeled as dispersive waves show the effective mode indices nm of high-order PCF modes (simulated field intensity profiles of these modes are shown in the insets) as functions of radiation wavelength in the short-wavelength (TH) spectral range (the lower abscissa axis). The solid curve shows the effective mode index nsol of the soliton pump in the fundamental PCF mode as a function of the pump wavelength (the upper abscissa axis). Phase matching is achieved at the wavelengths λ where the solid curve, representing nsol ðλÞ, crosses one of the dashed curves, representing nm ðλ=3Þ.
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fiber and increasing the total energy of the TH fiber output. For an average laser power p0 ≈ 110 mW, the spectrum of pump pulses measured at the output of the highly nonlinear PCF features a single redshifted peak [inset in Fig. 3(a)], indicating formation of only one soliton. The TH spectrum in this regime is dominated by a peak centered at 350 nm [Fig. 3(a)], which is generated by the frequency components of the pump field close to the central wavelength of the laser output. As the laser power is increased, generating a larger number of solitons in the PCF [insets in Figs. 3(a) and 3(b)], the TH spectra
Fig. 3. (Color online) Typical spectra of the TH output of the highly nonlinear PCF. The pump pulses delivered by the ytterbium PCF laser have a pulse width of 260 fs, a repetition rate of 50 MHz, and average power of (a) 110, (b) 230, and (c) 510 mW. The spectra of pump pulses transmitted through the PCF are shown in the insets.
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become more crowded [Figs. 3(a) and 3(b)]. With p0 ≈ 510 mW, the laser pulses generate multiple solitons, whose spectra merge into a supercontinuum at the output of the highly nonlinear PCF [inset in Fig. 3(c)], giving rise to a manifold of peaks in TH output spectra [Fig. 3(c)] at the frequencies where the solitonic pump is phase matched with the waveguide modes of the TH (Fig. 2). In this regime, the highest intensity TH component is observed at 383 nm. Although the pump–TH mode overlap for this TH mode is a factor of 1.2 smaller than that for the 371 nm TH component, fiber dispersion and soliton frequency shift deceleration at longer wavelengths [3] provide a large pump–TH interaction length for the 383 nm mode, enhancing frequency conversion to this component. A typical pulse width of the multimode TH output of a 1 m PCF was estimated as 1:2 ps, with the output pulse widths of individual TH modes varying, according to our simulations, from 210 to 480 fs. Because of fiber dispersion, phase matching between the pump and its TH can be satisfied only within a limited bandwidth. In a simple slowly varying envelope approximation model of THG in a highly nonlinear fiber [17], this −1 −1 −1 bandwidth is given by δω ∝ ju−1 p − uh j l , where l is the propagation path and up and uh are the group velocities of the pump and TH pulses. A broadband pump with a bandwidth Δωp can thus give rise to an equally broadband TH only within a propagation path shorter than −1 −1 −1 ju−1 p − uh j Δωp . Beyond this path (a few centimeters in our experiments), coherent buildup of the TH can occur only within a limited bandwidth, decreasing as l−1 , which gives rise to narrow TH peaks at the output of long nonlinear PCFs. In this regime, the pump-to-TH conversion efficiency lowers, since a substantial fraction of the pump spectrum misses THG phase matching, but the total energy of the TH output continues to grow with an increase in the fiber length. With a 600 mW LMA PCFlaser output coupled into a 60 cm section of highly nonlinear PCF with a coupling efficiency of 40%, the maximum power of the multimode TH fiber output achieved in our experiments was estimated as 1 mW, corresponding to a pulse energy of 20 pJ, a pump-to-TH conversion efficiency of 0.4%, and a laser-output-to-TH conversion efficiency of 0.17%. With a 1:5 m section of highly nonlinear PCF, spectral peaks with a bandwidth of less than 0:5 nm were produced in the UVA range [Figs. 3(a)–3(c)]. This narrowband UV radiation source offers much promise for high-resolution microspectroscopy, including UV-resonance-enhanced coherent anti-Stokes Raman scattering microscopy, selective photoexcitation of biomolecules, such as DNA, as well as numerous other applications, including forensics, disinfection, air purification, protein analysis [21], DNA sequencing [22], drug detection, UV-light therapy, photolithography, detection of biohazard species, and analysis of minerals. The black-light PCF output spectra in Figs. 2 and 3 also perfectly fit the absorption spectra of blue, cyan, and green fluorescent proteins and their variants, suggesting an attractive fiber-format source for a multicolor excitation of fluorescent proteins functioning as noninvasive fluorescent markers in living cells [23], e.g., for a simultaneous detection of parallel activities in networks of living neurons [7].
This work was supported in part by the State Key Development Program for Basic Research of China, the National Natural Science Foundation of China (NSFC), the Key Project of Chinese Ministry of Education, the Foundation for the Author of National Excellent Doctoral Dissertation, the Program for New Century Excellent Talents in University, and the Federal Program of the Russian Ministry of Education and Science. References 1. R. R. Alfano, The Supercontinuum Laser Source: Fundamentals with Updated References, 2nd ed. (Springer, 2006). 2. Supercontinuum Generation in Optical Fibers, J. M. Dudley and J. R. Taylor, eds. (Cambridge U. Press 2010). 3. A. M. Zheltikov, Phys. Usp. 49, 605 (2006). 4. T. Udem, R. Holzwarth, and T. W. Hänsch, Nature 416, 233 (2002). 5. E. Goulielmakis, S. Koehler, B. Reiter, M. Schultze, A. J. Verhoef, E. E. Serebryannikov, A. M. Zheltikov, and F. Krausz, Opt. Lett. 33, 1407 (2008). 6. E. R. Andresen, V. Birkedal, J. Thøgersen, and S. R. Keiding, Opt. Lett. 31, 1328 (2006). 7. L. V. Doronina, I. V. Fedotov, A. A. Voronin, O. I. Ivashkina, M. A. Zots, K. V. Anokhin, E. Rostova, A. B. Fedotov, and A. M. Zheltikov, Opt. Lett. 34, 3373 (2009). 8. P. St. J. Russell, Science 299, 358 (2003). 9. J. C. Knight, Nature 424, 847 (2003). 10. R. W. Wood, J. Physiol. 9, 5 (1919). 11. J. K. Ranka, R. S. Windeler, and A. J. Stentz, Opt. Lett. 25, 796 (2000). 12. F. G. Omenetto, A. Taylor, M. D. Moores, J. Arriaga, J. C. Knight, W. J. Wadsworth, and P. S. J. Russell, Opt. Lett. 26, 1158 (2001). 13. A. N. Naumov, A. B. Fedotov, A. M. Zheltikov, V. V. Yakovlev, L. A. Mel’nikov, V. I. Beloglazov, N. B. Skibina, and A. V. Shcherbakov, J. Opt. Soc. Am. B 19, 2183 (2002). 14. A. Efimov, A. J. Taylor, F. G. Omenetto, J. C. Knight, W. J. Wadsworth, and P. St. J. Russell, Opt. Express 11, 910 (2003). 15. E. E. Serebryannikov, A. B. Fedotov, A. M. Zheltikov, A. A. Ivanov, M. V. Alfimov, V. I. Beloglazov, N. B. Skibina, D. V. Skryabin, A. V. Yulin, and J. C. Knight, J. Opt. Soc. Am. B 23, 1975 (2006). 16. A. B. Fedotov, A. A. Voronin, E. E. Serebryannikov, I. V. Fedotov, A. V. Mitrofanov, A. A. Ivanov, D. A. SidorovBiryukov, and A. M. Zheltikov, Phys. Rev. E 75, 016614 (2007). 17. A. M. Zheltikov, Phys. Rev. A 72, 043812 (2005). 18. E. E. Serebryannikov, D. von der Linde, and A. M. Zheltikov, Opt. Lett. 33, 977 (2008). 19. B. W. Liu, M. L. Hu, X. H. Fang, Y. Z. Wu, Y. J. Song, L. Chai, C. Y. Wang, and A. M. Zheltikov, Laser Phys. Lett. 6, 44 (2009). 20. J. Herrmann, U. Griebner, N. Zhavoronkov, A. Husakou, D. Nickel, J. C. Knight, W. J. Wadsworth, P. St. J. Russell, and G. Korn, Phys. Rev. Lett. 88, 173901 (2002). 21. J. A. Madsen, D. R. Boutz, and J. S. Brodbelt, J. Proteome Res. 9, 4205 (2010). 22. M. Margulies, M. Egholm, W. E. Altman, S. Attiya, J. S. Bader, L. A. Bemben, J. Berka, M. S. Braverman, Y.-J. Chen, Z. Chen, S. B. Dewell, L. Du, J. M. Fierro, X. V. Gomes, B. C. Godwin, W. He, S. Helgesen, C. H. Ho, G. P. Irzyk, S. C. Jando, M. L. I. Alenquer, and T. P. Jarvie, Nature 437, 376 (2005). 23. M. Chalfie, Y. Tu, G. Euskirchen, W. W. Ward, and D. C. Prasher, Science 263, 802 (1994).
