Generation of an air laser at extended distances by femtosecond laser filamentation with telescope optics Chenrui Jing,1,3 Haisu Zhang,1,3 Wei Chu,1 Hongqiang Xie,1,3 Jielei Ni,1 Bin Zeng,1 Guihua Li,1,3 Jinping Yao,1 Huailiang Xu,2,4 Ya Cheng,1,5 and Zhizhan Xu1,* 1
2
State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, P.O. Box 800-211, Shanghai 201800, China State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, China 3 Graduate School of Chinese Academics of Science, Beijing 100039, China 4 [email protected] 5 [email protected] * [email protected]
Abstract: We present the generation of self-induced-white-light-seeded lasing action of nitrogen molecules in air by a Ti:sapphire femtosecond laser (800nm, 5.5mJ) and demonstrate that such lasing action is strongly influenced by external focusing conditions. It is found that the self-seeded lasing signal of N2+ at ~391 nm decreases dramatically by orders of magnitude and ultimately disappears when the focal length of an external lens increases from 0.5 m to 1 m. By using a telescope, it is shown that such limitation can be overcome and the 391 nm lasing can be controlled to occur at remotely designated distance, providing a possibility for practical applications in standoff spectroscopy. ©2014 Optical Society of America OCIS codes: (190.7110) Ultrafast nonlinear optics; (260.5950) Self-focusing.
References and links 1.
Q. Luo, W. Liu, and S. L. Chin, “Lasing action in air induced by ultra-fast laser filamenttation,” Appl. Phys. B 76(3), 337–340 (2003). 2. J. Yao, B. Zeng, H. Xu, G. Li, W. Chu, J. Ni, H. Zhang, S. L. Chin, Y. Cheng, and Z. Xu, “High-brightness switchable multi-wavelength remote laser in air,” Phys. Rev. A 84, 051802 (2011). 3. D. Kartashov, S. Ališauskas, A. Baltuška, A. Schmitt-Sody, W. Roach, and P. Polynkin,“Remotely pumped stimulated emission at 337 nm in atmospheric nitrogen,” Phys. Rev. A 88, 041805 (2013). 4. J. Ni, W. Chu, C. Jing, H. Zhang, B. Zeng, J. Yao, G. Li, H. Xie, C. Zhang, H. Xu, S. L. Chin, Y. Cheng, and Z. Xu, “Identification of the physical mechanism of generation of coherent N2+ emissions in air by femtosecond laser excitation,” Opt. Express 21(7), 8746–8752 (2013). 5. H. Zhang, C. Jing, J. Yao, G. Li, B. Zeng, W. Chu, J. Ni, H. Xie, H. Xu, S. L. Chin, K. Yamanouchi, Y. Cheng, and Z. Xu, “Rotational coherence encoded in an “Air-Laser” spectrum of nitrogen molecular ions in an intense laser field,” Phys. Rev. X 3, 041009 (2013). 6. Y. Liu, Y. Brelet, G. Point, A. Houard, and A. Mysyrowicz, “Self-seeded lasing in ionized air pumped by 800 nm femtosecond laser pulses,” Opt. Express 21(19), 22791–22798 (2013). 7. T. Wang, J. Ju, J. F. Daigle, S. Yuan, R. Li, and S. L. Chin, “Self-seeded forward lasing action from a femtoscond Ti:sapphire laser filament in air,” Laser Phys. Lett. 10(12), 125401 (2013). 8. W. Chu, G. Li, H. Xie, J. Ni, J. Yao, B. Zeng, H. Zhang, C. Jing, H. Xu, Y. Cheng, and Z. Xu, “A self-induced white light seeding laser in a femtosecond laser filament,” Laser Phys. Lett. 11(1), 015301 (2014). 9. G. Point, Y. Liu, A. Brelet, A. Houard, and A. Mysyrowicz, “Lasing of ambient air with microjoule pulse energy pumped by a multi terawatt IR femtosecond laser,” Opt. Lett. submitted to. 10. S. L. Chin, Femtosecond Laser Filamentation (Springer, 2010). 11. S. Henin, Y. Petit, J. Kasparian, J. P. Wolf, A. Jochmann, S. D. Kraft, S. Bock, U. Schramm, R. Sauerbrey, W. M. Nakaema, K. Stelmaszczyk, P. Rohwetter, L. Wöste, C. L. Soulez, S. Mauger, L. Bergé, and S. Skupin, “Saturation of the filament density of ultrashort intense laser pulse in air,” Appl. Phys. B 100(1), 77–84 (2010). 12. Z. Jia, J. Liu, Z. Wang, J. Ju, X. Lu, Y. Jiang, Y. Leng, X. Liang, W. Liu, S. L. Chin, R. Li, and Z. Xu, “Femtosecond laser filamentation with a 4J/60fs Ti:Sapphire laser beam: multiple filaments and intensity clamping,” Laser Phys. 20(4), 886–890 (2010).
#204121 - $15.00 USD Received 3 Jan 2014; revised 20 Jan 2014; accepted 27 Jan 2014; published 3 Feb 2014 (C) 2014 OSA 10 February 2014 | Vol. 22, No. 3 | DOI:10.1364/OE.22.003151 | OPTICS EXPRESS 3151
13. F. Théberge, W. Liu, P. T. Simard, A. Becker, and S. L. Chin, “Plasma density inside a femtosecond laser filament in air: Strong dependence on external focusing,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 74(3), 036406 (2006). 14. W. Liu, F. Théberge, J.-F. Daigle, P. T. Simard, S. M. Sarifi, Y. Kamali, H. Xu, and S. L. Chin, “An efficient control of ultrashort laser filament location in air for the purpose of remote sensing,” Appl. Phys. B 85(1), 55–58 (2006). 15. J. Ni, W. Chu, H. Zhang, C. Jing, J. Yao, H. Xu, B. Zeng, G. Li, C. Zhang, S. L. Chin, Y. Cheng, and Z. Xu, “Harmonic-seeded remote laser emissions in N₂-Ar, N₂-Xe and N₂-Ne mixtures: a comparative study,” Opt. Express 20(19), 20970–20979 (2012). 16. J. H. Marburger, “Self-focusing: theory,” Prog. Quantum Electron. 4, 35–110 (1975). 17. Q. Luo, S. A. Hosseini, W. Liu, J. F. Gravel, O. G. Kosareva, N. A. Panov, N. Aközbek, V. P. Kandidov, G. Roy, and S. L. Chin, “Effect of beam diameter on the propagation of intense femtosecond laser pulses,” Appl. Phys. B 80, 35–38 (2005). 18. J.-F. Daigle, O. Kosareva, N. Panov, M. Bégin, F. Lessard, C. Marceau, Y. Kamali, G. Roy, V. P. Kandidov, and S. L. Chin, “A simple method to significantly increase filaments’ length and ionization density,” Appl. Phys. B 94(2), 249–257 (2009).
1. Introduction Recently, several groups have reported observations of lasing actions of nitrogen molecules or nitrogen molecular ions during filamentation of intense femtosecond laser pulses in air [1–5]. The phenomenon has attracted rapidly increasing interest for its obvious potential in standoff spectroscopy and remote sensing. In particular, the air laser induced by tunnel/multiphoton ionization of nitrogen molecules with infrared femtosecond light shows a clear self-seeding effect, enabling much higher gains than that of the backward ASE (amplified spontaneous emissions) lasers and unique wavelength tenability [2]. Several works reported more recently further demonstrate that lasing from nitrogen ions (i.e., N2+) can be induced in ambient air with only 800 nm laser beams, which utilize self-generated white light continuum as the seed [6–8]. Based on this approach, G. Point et al have shown lasing emissions at 428 nm with a maximum output energy of 2.5μJ and a maximum conversion efficiency of 3.5 × 10−5 at the distance of 1 m using 800 nm femtosecond pulses with peak powers up to 4 TW [9]. To enable practical standoff spectroscopy application with such air laser, there is an urgent need to further extend the distance of the location where lasing action takes place. However, as it will be presented in this paper, it is found that the generation of self-seeded air lasing could not be extended to a far distance by simply increasing the focal length of an external focusing lens, and as a result no such lasing actions could occur in the filament produced with a collimated and freely propagated femtosecond laser beam. This may be mainly hampered by the early self-focusing, and consequently, filamentation of intense laser pulses well before the geometric focal point [10]. Owing to the well-known intensity clamping effect, the intensity inside the filament core is clamped to 5 × 1013 W/cm2 in air and it becomes difficult to achieve high ionization density at long distances with a freely propagated beam even if the femtosecond pump laser systems can provide higher pulse energies, which generally lead to the generation of multiple filaments [11, 12]. In this Letter, we attempt to extend the generation of lasing action to a remotely designated distance by changing the external focusing conditions, which can control the filament location and enable promotion of ionization density in the filament [13]. The main idea is to use a telescope as the focusing system [14], which enlarges the incident beam, and thus the hot spots in the beam, thereby avoiding the early self-focusing as compared with the case of using a single focal lens. In our experiment, we show that such scheme allows us to generate air lasers with 50 fs, 5.5mJ laser pulses at distances ~2 m, and ~3 m; whereas for the case of using single focal lenses, air laser can only be generated at a remote distance in the range from 0.8 to 1 m. 2. Experiments setup The experimental setup is illustrated in Fig. 1. A Ti:sapphire laser system (Legend Elite Duo, Coherent Inc.) delivers 1 kHz linearly polarized laser pulses with a center wavelength at 800 nm, a maximum pulse energy of:15mJ. The beam diameter at 1/e2 is ~8mm. The laser beam
#204121 - $15.00 USD Received 3 Jan 2014; revised 20 Jan 2014; accepted 27 Jan 2014; published 3 Feb 2014 (C) 2014 OSA 10 February 2014 | Vol. 22, No. 3 | DOI:10.1364/OE.22.003151 | OPTICS EXPRESS 3152
passes through a telescope composed of a concave lens (f1 = −0.2 m) and a convex lens (f2 = 0.5 m), whose effective focal length fe can be adjusted through changing the distance between these two lenses. The diameter of the initial beam is first enlarged by the concave lens, which is then focused by the convex lens to generate a filament in ambient air, as shown by the inset (a) of Fig. 1 as an example. The pulse energy is 5.5mJ measured after the convex lens, which is much higher than the critical power of 5-10 GW in air [10]. For comparison, the laser beam has also been directly focused in air to generate a filament (see the inset (b) of Fig. 1) by a single convex lens with different focal lengths. After the filament, the forward beam is first reflected by a wedge to attenuate the light intensity, and then further reflected by a dichroic mirror with a high reflectivity at 800 nm and a high transmittance at 400 nm to isolate the air lasing signals from the fundamental pump laser. A fused silica lens is used to image the lasing signal onto the entrance slit of a grating spectrometer (Shamrock 303i, Andor).
Fig. 1. Schematic of the experimental setup. Insets: filament profiles captured with a digital camera with (a) a telescope and (b) a single focal lens. Geometric focal positions in (a) and (b) are indicated by the vertical dashed line.
3. Results and discussion Figure 2(a) shows the forward spectrum in air generated by a single focusing lens with the focal length of f = 0.5 m (i.e., f/# = 62.5) in the spectral range of 378-433 nm. The data are accumulated over 2000 laser shots. The pulse energy after the focal lens is 5.5mJ. It can be seen in Fig. 2(a) that a clear and strong narrow-band emission at ~391 nm appears on the spectrum, which corresponds to the P branch band head of vibrational transition (0-0) between the excited B2u+ state and the ground X2g+ state of N2+. This strong 391 nm emission has been ascribed to the self-induced-white-light seeded lasing action during the filamentation of femtosecond laser pulses in air [6–8]. The white light (supercontinuum) in the spectrum results from self-phase modulation and self-steepening effects during the filamentation of femtosecond laser pulses in air [10]. To further extend the distance of the lasing action in air, the simplest way one may expect is to generate the filament by using longer focal lenses. Therefore, the focal lens of f = 0.5 m is replaced by the lenses with f = 0.8 (i.e., f/# = 100) and 1 m (i.e., f/# = 125), whereas the input laser parameters are kept constant. Surprisingly, it is found that the 391 nm lasing emission drops significantly. The intensity of the 391 nm lasing signal is reduced by about 3 orders of magnitude when the focal length increases from f = 0.5 m to 0.8 m (Fig. 2(b)), and it totally
#204121 - $15.00 USD Received 3 Jan 2014; revised 20 Jan 2014; accepted 27 Jan 2014; published 3 Feb 2014 (C) 2014 OSA 10 February 2014 | Vol. 22, No. 3 | DOI:10.1364/OE.22.003151 | OPTICS EXPRESS 3153
disappears with only the white light background in the spectrum when the lens of f = 1 m is used (Fig. 2(c)). All the signal intensities of the spectra shown in Figs. 2(a)-2(c) are calibrated to same scale for comparison. Note that the strong white-light modulation in Fig. 2(c) may be caused by the interference between the pulses induced by pulse splitting during filamentation of the pump laser in air, which is strongly dependent on the plasma length [10]. Likewise, no lasing actions could occur under the same experimental conditions for the case of further loose focusing, and eventually the free propagating (i.e. unfocused) Gaussian beam as well.
Fig. 2. Forward spectra of lasing in air at 391nm obtained with different single focusing lenses: (a) f = 0.5 m; (b) f = 0.8 m; (c) f = 1 m.
The phenomenon of the fast drop of the lasing signal as a function of external focal length may be explained as follows. Based on the previous understanding of the air lasing mechanism [15], the laser intensity can be well described by a small signal gain equation, Ilaser = Iseed egL, where L stands for the length of the gain medium (i.e., the filament length in our experiment), Iseed is the seed pulse intensity (i.e., the white light intensity in our experiment), and g is the gain coefficient of the medium. In the case of a focused beam by a lens with the focal length of f, the gain medium length can be expressed as L = f 2 / (f + Lc) [14], where Lc is the self-focusing distance for a parallel Gaussian beam and keeps constant as a result of the fixed input laser parameters in our case [16]. Therefore, the filament length, i.e., the gain medium length increases when the lens with longer focal lengths is used. In addition, it is well known that the white light develops progressively along the filament, thereby resulting in a stronger intensity when the filament becomes longer. Therefore, the employment of a longer focusing lens would contribute positively to Iseed and L. As a consequence, when the laser beam becomes loosely focused, the gain coefficient of the medium for the lasing generation must be strongly decreased. The gain coefficient g = δ21 × Δn depends on the stimulated emission cross section δ21, which is a constant at a specific lasing wavelength in our case, and the population inversion Δn. Because of the clamping effect, when a lens with longer focal length is used, the intensity in the filament will keep almost the same with the clamped value, but the plasma density in the filament could be reduced significantly [13]. Therefore, the influence of the external focusing on the gain coefficient mainly results from the variation of the plasma density. As a consequence, a higher plasma density is required for the generation of lasing action in the air filament. This is consistent with our previous observations as we have found that for efficiently generating air lasing a tighter focusing than that required by initiating filamentation is necessary [2,4] Based on the above analysis, it is reasonable to expect that techniques developed previously for enhancing the plasma density in the femtosecond filament [14, 17, 18] would be of benefits to the generation of the air lasing. Hence, a telescope system (see Fig. 1), which can enlarge the incident beam and thus the hot spots in the beam, thereby avoiding the early self-focusing as compared with the case of using a single focal lens, is chosen in the current study. The purpose of choosing such a focusing geometry is also to meet the need for generating air lasing at
#204121 - $15.00 USD Received 3 Jan 2014; revised 20 Jan 2014; accepted 27 Jan 2014; published 3 Feb 2014 (C) 2014 OSA 10 February 2014 | Vol. 22, No. 3 | DOI:10.1364/OE.22.003151 | OPTICS EXPRESS 3154
extended distances since the telescope optics has been proved to enable an efficient control of filament location [14]. The latter can be seen in the insets (a) and (b) of Fig. 1, in which the filament (i.e., the plasma channel) profiles generated respectively by the telescope system with a 1 m effective focal length and a 1 m single lens are captured from the side of the filaments by a digital camera (D40, Nikon). When the single focusing lens is employed, the self-focusing position is brought toward the lens from the vicinity of geometrical focus with a filament length of ~80 mm. However, when using the telescope system, the length of the filament is shortened to ~8 mm, but with a higher plasma density as can be seen from the fluorescence signal intensity.
Fig. 3. Forward spectra of lasing in air at 391nm obtained with the telescope system: (a) fe = 2 m; (b) fe = 3 m.
Figure 3(a) shows the forward spectrum in air generated by the telescope system with the effective focal lens of fe = 2 m (f/#≈75). All the laser parameters are kept the same as those used in the single lens measurements. It can be seen in Fig. 3(a) that with the telescope system, the strong and narrow-band lasing line at 391 nm appears on the spectrum generated at the extended distance of 2 m. According to the empirical self-focusing distance of the collimated Gaussian beam given by Lc = 0.367ka2/{[P/Pcr −0.852]2-0.0219}1/2 [16], with k being the wave number, a the beam radius, P the laser power and Pcr the critical power of the medium for self-focusing, it can be seen that the self-focusing distance Lc is proportional to the square of the beam radius. As a result, the enlarged input beam by the telescope would result in a longer Lc, and thus a shorter filament length L in the vicinity of focus position according to the equation of L = f 2 / (f + Lc). Due to the small beam radius at the focus, the energy in background reservoir may get more effectively used and also multiple filaments can be merged to the geometrical position, leading to a higher plasma density, thereby enhancing the lasing signal at 391 nm. It can also be seen from Fig. 2(b) that a peak at about 380 nm appears when the focal length becomes 0.8 m, which corresponds to the vibrational transition (0-2) between the excited states C3πu and B3πg of N2. This peak may be attributed to amplified spontaneous emission (ASE) according to the above-mentioned transition [1]. Since the distance between the two lenses in the telescope can be changed continuously, controllable formation of filaments at different locations can be achieved. In Fig. 3(b), the forward spectrum with the effective focal length of fe = 3 m (f/#≈125) is shown, in which the lasing line at 391nm can still be observed clearly, but the intensity of the lasing signal decreases. This trend is similar to that observed with the single lens focusing scheme, and can be easily understood as follow. To make the effective focal length longer, the distance between the two lenses f1 and f2 in the telescope should be shortened, leading to a decrease of the beam radius on the second lens. This leads to the longer filament length, and in turn the decrease in the plasma density. Due to the limited space of our laboratory and laser energy, the experiment for measuring the 391 nm lasing at distances longer than 3 m cannot be carried out. Nevertheless, it can be expected that the distance of lasing action in air should be further #204121 - $15.00 USD Received 3 Jan 2014; revised 20 Jan 2014; accepted 27 Jan 2014; published 3 Feb 2014 (C) 2014 OSA 10 February 2014 | Vol. 22, No. 3 | DOI:10.1364/OE.22.003151 | OPTICS EXPRESS 3155
extended, in a scalable manner, by using larger telescope systems combined with high-peak-intensity (e.g., TW or PW level) femtosecond laser systems, which have been built and routinely in operation world widely today. In the previous experiments of generation of forward air lasers with femtosecond pump sources at 800 nm wavelength [6–8], usually lasing at 428 nm wavelength can be more easily initiated. However, in our current experiment under different focal conditions, the lasing at 391 nm wavelength dominates. The mechanisms behind the competition between these two lasing lines have not completely clarified so far. Nevertheless, we propose a tentative model as follow. It is known that the molecular vibrational transition probability is governed by Franck-Condon factor, which means a higher transition probability for the 391 nm emission from B2u+ (ν’ = 0) to X2g+ (ν = 0) than that for the 428 nm emission from B2u+ (ν’ = 0) to X2g+ (ν = 1). Because the 391nm and 428nm lasing signals originate from transitions between the same upper energy level but different lower energy levels, whenever the white light spectrum can be extended to the blue side for covering the 391 nm, the generation of the 391 nm laser will be more efficient. This condition is fulfilled in our current experiment, because tighter focusing has been used which generates more plasmas and stronger spectral shift toward the blue side of the white light. This would provide stronger seed for facilitating generation of lasing at the 391 nm wavelength. 4. Summary In summary, we have systematically investigated the generation of self-seeded air lasing action of nitrogen molecular ions at significantly extended distances by femtosecond laser filamentation in air with telescope optics. Our experimental observations demonstrate that such air laser is strongly dependent on the external focusing conditions and that the usage of the telescope optics can benefit the generation of the self-seeded lasing action at extended distances. Despite its simplicity, this technique offers the potential of long-distance generation of the air laser, which will have important implication for remote sensing applications. Acknowledgments This work is supported by the National Basic Research Program of China (Grants No. 2011CB808102 and No. 2014CB921300), National Natural Science Foundation of China (Grants No. 11127901, No. 11134010, No. 61221064, No. 61275205, No. 61235003 and No. 11204332), the Program of Shanghai Subject Chief Scientist (11XD1405500), the Open Fund of the State Key Laboratory of High Field Laser Physics (SIOM), and the Fundamental Research Funds of Jilin University.
#204121 - $15.00 USD Received 3 Jan 2014; revised 20 Jan 2014; accepted 27 Jan 2014; published 3 Feb 2014 (C) 2014 OSA 10 February 2014 | Vol. 22, No. 3 | DOI:10.1364/OE.22.003151 | OPTICS EXPRESS 3156
2
State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, P.O. Box 800-211, Shanghai 201800, China State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, China 3 Graduate School of Chinese Academics of Science, Beijing 100039, China 4 [email protected] 5 [email protected] * [email protected]
Abstract: We present the generation of self-induced-white-light-seeded lasing action of nitrogen molecules in air by a Ti:sapphire femtosecond laser (800nm, 5.5mJ) and demonstrate that such lasing action is strongly influenced by external focusing conditions. It is found that the self-seeded lasing signal of N2+ at ~391 nm decreases dramatically by orders of magnitude and ultimately disappears when the focal length of an external lens increases from 0.5 m to 1 m. By using a telescope, it is shown that such limitation can be overcome and the 391 nm lasing can be controlled to occur at remotely designated distance, providing a possibility for practical applications in standoff spectroscopy. ©2014 Optical Society of America OCIS codes: (190.7110) Ultrafast nonlinear optics; (260.5950) Self-focusing.
References and links 1.
Q. Luo, W. Liu, and S. L. Chin, “Lasing action in air induced by ultra-fast laser filamenttation,” Appl. Phys. B 76(3), 337–340 (2003). 2. J. Yao, B. Zeng, H. Xu, G. Li, W. Chu, J. Ni, H. Zhang, S. L. Chin, Y. Cheng, and Z. Xu, “High-brightness switchable multi-wavelength remote laser in air,” Phys. Rev. A 84, 051802 (2011). 3. D. Kartashov, S. Ališauskas, A. Baltuška, A. Schmitt-Sody, W. Roach, and P. Polynkin,“Remotely pumped stimulated emission at 337 nm in atmospheric nitrogen,” Phys. Rev. A 88, 041805 (2013). 4. J. Ni, W. Chu, C. Jing, H. Zhang, B. Zeng, J. Yao, G. Li, H. Xie, C. Zhang, H. Xu, S. L. Chin, Y. Cheng, and Z. Xu, “Identification of the physical mechanism of generation of coherent N2+ emissions in air by femtosecond laser excitation,” Opt. Express 21(7), 8746–8752 (2013). 5. H. Zhang, C. Jing, J. Yao, G. Li, B. Zeng, W. Chu, J. Ni, H. Xie, H. Xu, S. L. Chin, K. Yamanouchi, Y. Cheng, and Z. Xu, “Rotational coherence encoded in an “Air-Laser” spectrum of nitrogen molecular ions in an intense laser field,” Phys. Rev. X 3, 041009 (2013). 6. Y. Liu, Y. Brelet, G. Point, A. Houard, and A. Mysyrowicz, “Self-seeded lasing in ionized air pumped by 800 nm femtosecond laser pulses,” Opt. Express 21(19), 22791–22798 (2013). 7. T. Wang, J. Ju, J. F. Daigle, S. Yuan, R. Li, and S. L. Chin, “Self-seeded forward lasing action from a femtoscond Ti:sapphire laser filament in air,” Laser Phys. Lett. 10(12), 125401 (2013). 8. W. Chu, G. Li, H. Xie, J. Ni, J. Yao, B. Zeng, H. Zhang, C. Jing, H. Xu, Y. Cheng, and Z. Xu, “A self-induced white light seeding laser in a femtosecond laser filament,” Laser Phys. Lett. 11(1), 015301 (2014). 9. G. Point, Y. Liu, A. Brelet, A. Houard, and A. Mysyrowicz, “Lasing of ambient air with microjoule pulse energy pumped by a multi terawatt IR femtosecond laser,” Opt. Lett. submitted to. 10. S. L. Chin, Femtosecond Laser Filamentation (Springer, 2010). 11. S. Henin, Y. Petit, J. Kasparian, J. P. Wolf, A. Jochmann, S. D. Kraft, S. Bock, U. Schramm, R. Sauerbrey, W. M. Nakaema, K. Stelmaszczyk, P. Rohwetter, L. Wöste, C. L. Soulez, S. Mauger, L. Bergé, and S. Skupin, “Saturation of the filament density of ultrashort intense laser pulse in air,” Appl. Phys. B 100(1), 77–84 (2010). 12. Z. Jia, J. Liu, Z. Wang, J. Ju, X. Lu, Y. Jiang, Y. Leng, X. Liang, W. Liu, S. L. Chin, R. Li, and Z. Xu, “Femtosecond laser filamentation with a 4J/60fs Ti:Sapphire laser beam: multiple filaments and intensity clamping,” Laser Phys. 20(4), 886–890 (2010).
#204121 - $15.00 USD Received 3 Jan 2014; revised 20 Jan 2014; accepted 27 Jan 2014; published 3 Feb 2014 (C) 2014 OSA 10 February 2014 | Vol. 22, No. 3 | DOI:10.1364/OE.22.003151 | OPTICS EXPRESS 3151
13. F. Théberge, W. Liu, P. T. Simard, A. Becker, and S. L. Chin, “Plasma density inside a femtosecond laser filament in air: Strong dependence on external focusing,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 74(3), 036406 (2006). 14. W. Liu, F. Théberge, J.-F. Daigle, P. T. Simard, S. M. Sarifi, Y. Kamali, H. Xu, and S. L. Chin, “An efficient control of ultrashort laser filament location in air for the purpose of remote sensing,” Appl. Phys. B 85(1), 55–58 (2006). 15. J. Ni, W. Chu, H. Zhang, C. Jing, J. Yao, H. Xu, B. Zeng, G. Li, C. Zhang, S. L. Chin, Y. Cheng, and Z. Xu, “Harmonic-seeded remote laser emissions in N₂-Ar, N₂-Xe and N₂-Ne mixtures: a comparative study,” Opt. Express 20(19), 20970–20979 (2012). 16. J. H. Marburger, “Self-focusing: theory,” Prog. Quantum Electron. 4, 35–110 (1975). 17. Q. Luo, S. A. Hosseini, W. Liu, J. F. Gravel, O. G. Kosareva, N. A. Panov, N. Aközbek, V. P. Kandidov, G. Roy, and S. L. Chin, “Effect of beam diameter on the propagation of intense femtosecond laser pulses,” Appl. Phys. B 80, 35–38 (2005). 18. J.-F. Daigle, O. Kosareva, N. Panov, M. Bégin, F. Lessard, C. Marceau, Y. Kamali, G. Roy, V. P. Kandidov, and S. L. Chin, “A simple method to significantly increase filaments’ length and ionization density,” Appl. Phys. B 94(2), 249–257 (2009).
1. Introduction Recently, several groups have reported observations of lasing actions of nitrogen molecules or nitrogen molecular ions during filamentation of intense femtosecond laser pulses in air [1–5]. The phenomenon has attracted rapidly increasing interest for its obvious potential in standoff spectroscopy and remote sensing. In particular, the air laser induced by tunnel/multiphoton ionization of nitrogen molecules with infrared femtosecond light shows a clear self-seeding effect, enabling much higher gains than that of the backward ASE (amplified spontaneous emissions) lasers and unique wavelength tenability [2]. Several works reported more recently further demonstrate that lasing from nitrogen ions (i.e., N2+) can be induced in ambient air with only 800 nm laser beams, which utilize self-generated white light continuum as the seed [6–8]. Based on this approach, G. Point et al have shown lasing emissions at 428 nm with a maximum output energy of 2.5μJ and a maximum conversion efficiency of 3.5 × 10−5 at the distance of 1 m using 800 nm femtosecond pulses with peak powers up to 4 TW [9]. To enable practical standoff spectroscopy application with such air laser, there is an urgent need to further extend the distance of the location where lasing action takes place. However, as it will be presented in this paper, it is found that the generation of self-seeded air lasing could not be extended to a far distance by simply increasing the focal length of an external focusing lens, and as a result no such lasing actions could occur in the filament produced with a collimated and freely propagated femtosecond laser beam. This may be mainly hampered by the early self-focusing, and consequently, filamentation of intense laser pulses well before the geometric focal point [10]. Owing to the well-known intensity clamping effect, the intensity inside the filament core is clamped to 5 × 1013 W/cm2 in air and it becomes difficult to achieve high ionization density at long distances with a freely propagated beam even if the femtosecond pump laser systems can provide higher pulse energies, which generally lead to the generation of multiple filaments [11, 12]. In this Letter, we attempt to extend the generation of lasing action to a remotely designated distance by changing the external focusing conditions, which can control the filament location and enable promotion of ionization density in the filament [13]. The main idea is to use a telescope as the focusing system [14], which enlarges the incident beam, and thus the hot spots in the beam, thereby avoiding the early self-focusing as compared with the case of using a single focal lens. In our experiment, we show that such scheme allows us to generate air lasers with 50 fs, 5.5mJ laser pulses at distances ~2 m, and ~3 m; whereas for the case of using single focal lenses, air laser can only be generated at a remote distance in the range from 0.8 to 1 m. 2. Experiments setup The experimental setup is illustrated in Fig. 1. A Ti:sapphire laser system (Legend Elite Duo, Coherent Inc.) delivers 1 kHz linearly polarized laser pulses with a center wavelength at 800 nm, a maximum pulse energy of:15mJ. The beam diameter at 1/e2 is ~8mm. The laser beam
#204121 - $15.00 USD Received 3 Jan 2014; revised 20 Jan 2014; accepted 27 Jan 2014; published 3 Feb 2014 (C) 2014 OSA 10 February 2014 | Vol. 22, No. 3 | DOI:10.1364/OE.22.003151 | OPTICS EXPRESS 3152
passes through a telescope composed of a concave lens (f1 = −0.2 m) and a convex lens (f2 = 0.5 m), whose effective focal length fe can be adjusted through changing the distance between these two lenses. The diameter of the initial beam is first enlarged by the concave lens, which is then focused by the convex lens to generate a filament in ambient air, as shown by the inset (a) of Fig. 1 as an example. The pulse energy is 5.5mJ measured after the convex lens, which is much higher than the critical power of 5-10 GW in air [10]. For comparison, the laser beam has also been directly focused in air to generate a filament (see the inset (b) of Fig. 1) by a single convex lens with different focal lengths. After the filament, the forward beam is first reflected by a wedge to attenuate the light intensity, and then further reflected by a dichroic mirror with a high reflectivity at 800 nm and a high transmittance at 400 nm to isolate the air lasing signals from the fundamental pump laser. A fused silica lens is used to image the lasing signal onto the entrance slit of a grating spectrometer (Shamrock 303i, Andor).
Fig. 1. Schematic of the experimental setup. Insets: filament profiles captured with a digital camera with (a) a telescope and (b) a single focal lens. Geometric focal positions in (a) and (b) are indicated by the vertical dashed line.
3. Results and discussion Figure 2(a) shows the forward spectrum in air generated by a single focusing lens with the focal length of f = 0.5 m (i.e., f/# = 62.5) in the spectral range of 378-433 nm. The data are accumulated over 2000 laser shots. The pulse energy after the focal lens is 5.5mJ. It can be seen in Fig. 2(a) that a clear and strong narrow-band emission at ~391 nm appears on the spectrum, which corresponds to the P branch band head of vibrational transition (0-0) between the excited B2u+ state and the ground X2g+ state of N2+. This strong 391 nm emission has been ascribed to the self-induced-white-light seeded lasing action during the filamentation of femtosecond laser pulses in air [6–8]. The white light (supercontinuum) in the spectrum results from self-phase modulation and self-steepening effects during the filamentation of femtosecond laser pulses in air [10]. To further extend the distance of the lasing action in air, the simplest way one may expect is to generate the filament by using longer focal lenses. Therefore, the focal lens of f = 0.5 m is replaced by the lenses with f = 0.8 (i.e., f/# = 100) and 1 m (i.e., f/# = 125), whereas the input laser parameters are kept constant. Surprisingly, it is found that the 391 nm lasing emission drops significantly. The intensity of the 391 nm lasing signal is reduced by about 3 orders of magnitude when the focal length increases from f = 0.5 m to 0.8 m (Fig. 2(b)), and it totally
#204121 - $15.00 USD Received 3 Jan 2014; revised 20 Jan 2014; accepted 27 Jan 2014; published 3 Feb 2014 (C) 2014 OSA 10 February 2014 | Vol. 22, No. 3 | DOI:10.1364/OE.22.003151 | OPTICS EXPRESS 3153
disappears with only the white light background in the spectrum when the lens of f = 1 m is used (Fig. 2(c)). All the signal intensities of the spectra shown in Figs. 2(a)-2(c) are calibrated to same scale for comparison. Note that the strong white-light modulation in Fig. 2(c) may be caused by the interference between the pulses induced by pulse splitting during filamentation of the pump laser in air, which is strongly dependent on the plasma length [10]. Likewise, no lasing actions could occur under the same experimental conditions for the case of further loose focusing, and eventually the free propagating (i.e. unfocused) Gaussian beam as well.
Fig. 2. Forward spectra of lasing in air at 391nm obtained with different single focusing lenses: (a) f = 0.5 m; (b) f = 0.8 m; (c) f = 1 m.
The phenomenon of the fast drop of the lasing signal as a function of external focal length may be explained as follows. Based on the previous understanding of the air lasing mechanism [15], the laser intensity can be well described by a small signal gain equation, Ilaser = Iseed egL, where L stands for the length of the gain medium (i.e., the filament length in our experiment), Iseed is the seed pulse intensity (i.e., the white light intensity in our experiment), and g is the gain coefficient of the medium. In the case of a focused beam by a lens with the focal length of f, the gain medium length can be expressed as L = f 2 / (f + Lc) [14], where Lc is the self-focusing distance for a parallel Gaussian beam and keeps constant as a result of the fixed input laser parameters in our case [16]. Therefore, the filament length, i.e., the gain medium length increases when the lens with longer focal lengths is used. In addition, it is well known that the white light develops progressively along the filament, thereby resulting in a stronger intensity when the filament becomes longer. Therefore, the employment of a longer focusing lens would contribute positively to Iseed and L. As a consequence, when the laser beam becomes loosely focused, the gain coefficient of the medium for the lasing generation must be strongly decreased. The gain coefficient g = δ21 × Δn depends on the stimulated emission cross section δ21, which is a constant at a specific lasing wavelength in our case, and the population inversion Δn. Because of the clamping effect, when a lens with longer focal length is used, the intensity in the filament will keep almost the same with the clamped value, but the plasma density in the filament could be reduced significantly [13]. Therefore, the influence of the external focusing on the gain coefficient mainly results from the variation of the plasma density. As a consequence, a higher plasma density is required for the generation of lasing action in the air filament. This is consistent with our previous observations as we have found that for efficiently generating air lasing a tighter focusing than that required by initiating filamentation is necessary [2,4] Based on the above analysis, it is reasonable to expect that techniques developed previously for enhancing the plasma density in the femtosecond filament [14, 17, 18] would be of benefits to the generation of the air lasing. Hence, a telescope system (see Fig. 1), which can enlarge the incident beam and thus the hot spots in the beam, thereby avoiding the early self-focusing as compared with the case of using a single focal lens, is chosen in the current study. The purpose of choosing such a focusing geometry is also to meet the need for generating air lasing at
#204121 - $15.00 USD Received 3 Jan 2014; revised 20 Jan 2014; accepted 27 Jan 2014; published 3 Feb 2014 (C) 2014 OSA 10 February 2014 | Vol. 22, No. 3 | DOI:10.1364/OE.22.003151 | OPTICS EXPRESS 3154
extended distances since the telescope optics has been proved to enable an efficient control of filament location [14]. The latter can be seen in the insets (a) and (b) of Fig. 1, in which the filament (i.e., the plasma channel) profiles generated respectively by the telescope system with a 1 m effective focal length and a 1 m single lens are captured from the side of the filaments by a digital camera (D40, Nikon). When the single focusing lens is employed, the self-focusing position is brought toward the lens from the vicinity of geometrical focus with a filament length of ~80 mm. However, when using the telescope system, the length of the filament is shortened to ~8 mm, but with a higher plasma density as can be seen from the fluorescence signal intensity.
Fig. 3. Forward spectra of lasing in air at 391nm obtained with the telescope system: (a) fe = 2 m; (b) fe = 3 m.
Figure 3(a) shows the forward spectrum in air generated by the telescope system with the effective focal lens of fe = 2 m (f/#≈75). All the laser parameters are kept the same as those used in the single lens measurements. It can be seen in Fig. 3(a) that with the telescope system, the strong and narrow-band lasing line at 391 nm appears on the spectrum generated at the extended distance of 2 m. According to the empirical self-focusing distance of the collimated Gaussian beam given by Lc = 0.367ka2/{[P/Pcr −0.852]2-0.0219}1/2 [16], with k being the wave number, a the beam radius, P the laser power and Pcr the critical power of the medium for self-focusing, it can be seen that the self-focusing distance Lc is proportional to the square of the beam radius. As a result, the enlarged input beam by the telescope would result in a longer Lc, and thus a shorter filament length L in the vicinity of focus position according to the equation of L = f 2 / (f + Lc). Due to the small beam radius at the focus, the energy in background reservoir may get more effectively used and also multiple filaments can be merged to the geometrical position, leading to a higher plasma density, thereby enhancing the lasing signal at 391 nm. It can also be seen from Fig. 2(b) that a peak at about 380 nm appears when the focal length becomes 0.8 m, which corresponds to the vibrational transition (0-2) between the excited states C3πu and B3πg of N2. This peak may be attributed to amplified spontaneous emission (ASE) according to the above-mentioned transition [1]. Since the distance between the two lenses in the telescope can be changed continuously, controllable formation of filaments at different locations can be achieved. In Fig. 3(b), the forward spectrum with the effective focal length of fe = 3 m (f/#≈125) is shown, in which the lasing line at 391nm can still be observed clearly, but the intensity of the lasing signal decreases. This trend is similar to that observed with the single lens focusing scheme, and can be easily understood as follow. To make the effective focal length longer, the distance between the two lenses f1 and f2 in the telescope should be shortened, leading to a decrease of the beam radius on the second lens. This leads to the longer filament length, and in turn the decrease in the plasma density. Due to the limited space of our laboratory and laser energy, the experiment for measuring the 391 nm lasing at distances longer than 3 m cannot be carried out. Nevertheless, it can be expected that the distance of lasing action in air should be further #204121 - $15.00 USD Received 3 Jan 2014; revised 20 Jan 2014; accepted 27 Jan 2014; published 3 Feb 2014 (C) 2014 OSA 10 February 2014 | Vol. 22, No. 3 | DOI:10.1364/OE.22.003151 | OPTICS EXPRESS 3155
extended, in a scalable manner, by using larger telescope systems combined with high-peak-intensity (e.g., TW or PW level) femtosecond laser systems, which have been built and routinely in operation world widely today. In the previous experiments of generation of forward air lasers with femtosecond pump sources at 800 nm wavelength [6–8], usually lasing at 428 nm wavelength can be more easily initiated. However, in our current experiment under different focal conditions, the lasing at 391 nm wavelength dominates. The mechanisms behind the competition between these two lasing lines have not completely clarified so far. Nevertheless, we propose a tentative model as follow. It is known that the molecular vibrational transition probability is governed by Franck-Condon factor, which means a higher transition probability for the 391 nm emission from B2u+ (ν’ = 0) to X2g+ (ν = 0) than that for the 428 nm emission from B2u+ (ν’ = 0) to X2g+ (ν = 1). Because the 391nm and 428nm lasing signals originate from transitions between the same upper energy level but different lower energy levels, whenever the white light spectrum can be extended to the blue side for covering the 391 nm, the generation of the 391 nm laser will be more efficient. This condition is fulfilled in our current experiment, because tighter focusing has been used which generates more plasmas and stronger spectral shift toward the blue side of the white light. This would provide stronger seed for facilitating generation of lasing at the 391 nm wavelength. 4. Summary In summary, we have systematically investigated the generation of self-seeded air lasing action of nitrogen molecular ions at significantly extended distances by femtosecond laser filamentation in air with telescope optics. Our experimental observations demonstrate that such air laser is strongly dependent on the external focusing conditions and that the usage of the telescope optics can benefit the generation of the self-seeded lasing action at extended distances. Despite its simplicity, this technique offers the potential of long-distance generation of the air laser, which will have important implication for remote sensing applications. Acknowledgments This work is supported by the National Basic Research Program of China (Grants No. 2011CB808102 and No. 2014CB921300), National Natural Science Foundation of China (Grants No. 11127901, No. 11134010, No. 61221064, No. 61275205, No. 61235003 and No. 11204332), the Program of Shanghai Subject Chief Scientist (11XD1405500), the Open Fund of the State Key Laboratory of High Field Laser Physics (SIOM), and the Fundamental Research Funds of Jilin University.
#204121 - $15.00 USD Received 3 Jan 2014; revised 20 Jan 2014; accepted 27 Jan 2014; published 3 Feb 2014 (C) 2014 OSA 10 February 2014 | Vol. 22, No. 3 | DOI:10.1364/OE.22.003151 | OPTICS EXPRESS 3156
