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Wavelength control of random polymer fiber laser based on adaptive disorder Zhijia Hu,* Pengfei Gao, Kang Xie, Yunyun Liang, and Haiming Jiang School of Instrument Science and Opto-electronics Engineering, Hefei University of Technology, Hefei 230009, China *Corresponding author: [email protected] Received October 20, 2014; accepted November 7, 2014; posted November 14, 2014 (Doc. ID 224999); published December 10, 2014 We demonstrate the realization of two different kinds of random polymer optical fiber lasers to control the random lasing wavelength by changing the disorder of polymer optical fibers (POFs). One is a long-range disorder POF based on copolymer refractive-index inhomogeneity, and the other is a short-range disorder POF based on polyhedral oligomeric silsesquioxanes scattering. By end pumped both disorder POFs, the coherent random lasing for both is observed. Meanwhile, the random lasing wavelength of the short-range disorder POF because of a small scattering mean-free path has been found to be blue shifted with respect to the long-range disorder POF, which will give a way to control the random lasing wavelength. © 2014 Optical Society of America OCIS codes: (140.3510) Lasers, fiber; (160.4236) Nanomaterials; (160.5470) Polymers; (290.5850) Scattering, particles. http://dx.doi.org/10.1364/OL.39.006911
The random laser (RL) is an unconventional type of laser, in which an optical feedback is realized by multiple light scattering in disorder systems that contain optical gain medium [1]. On the basis of the scattering strength, the disorder can fall into two types: (1) long-range disorder with a large scattering mean-free path (ls ), which can be formed owing to refractive-index fluctuations, δnr , caused by long-range inhomogeneity in the disorder medium; and (2) short-range disorder caused by strong light scatters resulting in small ls [2]. Since Letokhov predicted RLs [3], RLs have generated significant interest among researchers because of their unique properties, such as miniaturization and the absence of the requirement for a cavity formed by stationary mirrors [4]. However, the non-directional and high threshold characters of the traditional RL systems have largely limited their application. Recently, the effects of one-dimensional confinement on the lasing properties of a classical RL system have received increasing attention, bringing about the birth of random fiber lasers (RFLs) [5]. Then a coherent RFL based on nanoparticles (NPs) scattering in an extremely weakly scattering regime was reported [6,7], which reduces the laser threshold and increases the emission directionality. Furthermore, a stabilized coherent laser action based on laser dye PM597doped one-dimensional disorder polymer optical fiber (POF) has been obtained by the weak optical multiple scattering of the polyhedral oligomeric silsesquioxanes (POSS) NPs in the core of the POF in situ formed during polymerization, which was enhanced by the waveguide confinement effect [8]. The disordered POF is considered to be a promising candidate in microphtotonic/ nanophotonic integrated systems and fiber sensors because of the low cost and the mechanical flexibility [9]. The reported RFLs are based almost completely on short-range disorder with NPs scattering, and the long-range disorder RFLs have not been reported. The absence of a well-defined cavity to trap light [1], however, results in an unpredictable random emission spectrum. Therefore, controlling the random lasing is a vexing problem. Note that alternative techniques to tune the random laser emission by actively shaping the optical 0146-9592/14/246911-04$15.00/0
pump [10], the spatial pump profile [11], adjusting the amount of absorption of emission light [12], or varying scattering coefficient determined by Mie resonances in mono-disperse polystyrene microspheres have been reported in the literature [13]. In this Letter, we have demonstrated different random polymer fiber lasers to control the random lasing wavelength by changing the disorder of the POFs. One type is a long-rang disorder POF based on refractive-index fluctuations caused by long-range inhomogeneity in the disorder copolymer medium, the core material of which is laser dye PM597 doped-poly(methyl methacrylateco-benzyl methacrylate) [poly(MMA-co-BzMA] copolymer. The other is a short-range disorder POF, the core material of which is PM597 doped-poly(methyl methacrylate-co-benzyl methacrylate-co-methacrylisobutyl polyhedral oligomeric silsesquioxanes) [poly(MMAco-BzMA-co-MMAPOSS)]. The cladding materials of the two kinds of disorder POFs are poly(methyl methacrylate-co-butyl acrylate) and poly(MMA-co-BA). By end pumping the short-range disorder POF, the coherent random lasing has been observed, which has been reported in our previous work [8]. Interestingly, the coherent random lasing has also been observed by end pumping the long-range disorder POF. Meanwhile, the random lasing wavelength of the short-range disorder POF has been found to be blue shifted with respect to the long-range disorder POF, which will provide a method to control the random lasing wavelength that always is a challenge in the field of random lasers. The short-range disorder POF is fabricated using the “Teflon Technique” [14]. First, the distilled MMA and BA with the weigh ration of ∼4∶1, the initiator lauroylperoxide (LPO) (0.5 wt. %), and chain transfer agent 1-butanethiol (0.3 wt. %) in an appropriate concentration with regard to the total amount of organic monomers are mixed in the Teflon tube. The diameter of the tube is 2 cm, and a thin Teflon string is used to be fixed properly in the center of the tube. Polymerization is performed in a thermal bath where its temperature is raised to 50°C from 30°C by 5°C/day and then increased up to 80°C in steps of 10°C/day to make sure that the © 2014 Optical Society of America
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monomers are polymerized fully. Hence, the hollow POF preform is obtained. Second, the hollow POF preform is then filled with the PM597 laser dye (0.14 wt. %) and the core monomers composed of MMA, BzMA, and POSS NPs with the weigh ration of ∼5.7∶1∶2, which are polymerized using an initiator LPO (0.5 wt. %) and a chain transfer agent 1-butanethiol (0.3 wt. %) with regard to the total amount of organic monomers in the same heat process as mentioned above. Finally, the PM597 laser dye dopeddisorder POF preform is drawn to POF in a custom-made fiber drawing tower at 170°C. The diameter of the POF can be varied by adjusting the feed rate of the POF preform and the draw rate of the fiber. In the same way, the long-range disorder POF is obtained, and the components are PM597, MMA, and BzMA with a weigh ration of 0.14:85:15. Figure 1 shows the optical microscope cross-sectional images and fluorescence images for the two types of disorder POFs. It can be seen that the core (cladding) diameter of the two types of POFs are ∼30 3 750 10 μm from an optical microscope image. The refractive index of the core (cladding) material for the short-range disorder POF is 1.4955 (1.4780), satisfying the total internal reflection condition. In addition, the refractive index of the core (cladding) material for the long-range disorder POF is 1.5037 (1.4780). The disorder POFs (8 cm long) are measured by end pumping. The schematic of the entire experimental setup for the optical measurements of the POFs is similar to the Ref. 6. The 532 nm output of a Q-switched Nd:YAG laser (pulse duration of 10 ns, repetition rate of 10 Hz) is used to pump the random system. The pump pulse energy is controlled by a Glan prism. The emitted light is collected by a fiber spectrometer (QE 65000, ocean optics, resolution ∼0.4 nm, and integration time of 100 ms) after a 560 nm long-wave pass filter. Figures 2(a) and 2(b) show the schematic and TEM image of POSS-based NPs in the core of short-range disorder POF. The NPs can be formed through the dipole–dipole interaction of POSS cores and disperse
Fig. 1. Optical microscope (a) cross-sectional image and (b) fluorescence image of short-range POF, as well as (c) cross-sectional image and (d) fluorescence image of longrange POF.
Fig. 2. (a), (b) Schematic and TEM image of POSS-based NPs in the core of short-range POF. (c) The fluorescence emission spectra of the PM597 (0.14 wt. %) doped two types of disorder POFs.
effect of polymer chains during the polymerization process [15]. Finally, it can be seen that the POSS NPs with an average size of 150 nm are well dispersed in the core of the short-range disorder POF. Figure 2(c) shows the fluorescence emission spectra of the PM597 (0.14 wt. %) doped two types of disorder POFs. It can be seen that the fluorescence emission peaks of the PM597 doped two types of disorder POFs both located in 574 nm. Figure 3(a) shows the evolution of emission spectra recorded for the PM597 (0.14 wt. %)-doped short-range disorder POF with POSS NPs (22.9 wt. %). At the lower pump energy (25 μJ), only a broad spontaneous emission profile centered at ∼577.0 nm with full width at halfmaximum (FWHM) of ∼11.7 nm can be observed. At pump energies above ∼51 μJ, the multimode lasing on top of a globally narrowed amplified spontaneous emission (ASE) spectrum is obtained, whose FWHM is reduced greatly (e.g., ∼0.8 nm for the ∼577.5 nm peak at ∼113 μJ). Typically, a set of such sharp spectral features indicates coherent lasing resonances [1]. The inset of Figure 3(a) plots the main peak emission intensities as a function of the pump energies for the short-range disorder POF system. It can be seen that there is a nonlinear dependence of the main peak emission intensity on pump energy, which points to a lasing action, and the threshold is determined to be ∼51 μJ. Figure 3(b) shows the evolution of emission spectra recorded for the PM597 (0.14 wt. %)-doped long-range disorder POF. Under the low pump energy (45 μJ), we also observe only spontaneous emission profile. When pump energy increases above 45 μJ, the multimode lasing on top of a globally narrowed ASE spectrum as well is obtained,
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Fig. 3. (a) Evolution of emission spectra recorded for the PM597 (0.14 wt. %)-doped short-range disorder POF and (b) long-range disorder POF. The inset plots the main peak emission intensities as a function of the pump energy for both disorder POF systems. [In both cases, the spectral intensity increases by increasing the pump energy.]
which also indicates coherent lasing resonances. The FWHM of ∼580.5 nm peak at ∼150 μJ is reduced to ∼0.9 nm. The inset of Fig. 3(b) plots the main peak emission intensities as a function of the pump energies for the long-range disorder POF system. It can be seen that there is a nonlinear dependence of the main peak emission intensity on pump energy, which points to a lasing action, and the threshold is determined to be ∼62 μJ. The slope of random lasing for the short-disorder polymer optical fiber is larger than that for the long-range disorder, which is caused by the short-disorder polymer optical fiber having stronger disorder. Figure 4 shows the emission spectra recorded for the PM597 doped POFs without (long-range disorder) and with (short-range disorder) POSS NPs at a pump energy of 96 μJ. The main peak of random laser for the shortrange disorder POF (577.4 nm) is found to be blue-shifted 3 nm with respect to that of the long-range disorder POF (580.4 nm). The scattering mean-free path ls of the shortrange disorder POF at 577 nm is calculated to be 910 μm [9] in our previous work based on ls 1∕ρσ s , where ρ is the particle density and σ s the scattering cross section of the POSS NPs, which is caused by the NPs scattering. The scattering of the long-range disorder POF originates from refractive-index fluctuations, δnr , in the effective
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Fig. 4. Emission spectra recorded for the PM597 doped POFs without long-range disorder and with short-range POSS NPs at a pump energy of 96 μJ.
refractive-index caused by inhomogeneity in the core of the POF. This type of long-range disorder actually encourages RL because it tends to trap light in random resonators with refraction index nr δnr , higher than the average refraction index, nr , of the copolymer [16]. However, this disorder type cannot be quantified by the coherent backscattering technique [17], but the scattering mean-free path of the long-range disorder POF is larger than that of the short-range disorder POF. The scattering mean-free path is proportional to the wavelength. A shorter mean-free path means more scattering events and consequently a longer (and more random) total path in the POF system. The blue-shift effect observed here can be understood by considering the fact that shorter wavelengths are more amplified than larger wavelengths, and the original non-lasing shorter wavelengths should also laser preferentially. Therefore, at a certain value of the pump energy, this effect produces a blue-shifted sharp peak for the short-range disorder POF with respect to the long-range disorder POF at the border wavelength. To gain further insight into the blue-sifted effect of our POF systems, we have performed the peak emission wavelength as a function of the pump energy for the two disorder POF systems, as shown in Fig. 5. It can be seen that the lasing peak wavelength of long-range POF locates in the range of 577.6–588.9 nm when the pump energy increases from 18.1 to 149.9 μJ. It is worth
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Fig. 5. Random lasing emission wavelength as a function of the pump energy for the two disorder POF systems.
noting that the lasing peak wavelength of the short-range disorder POF overall blue-shift to the range of 568.7– 580.7 nm when the pump energy increases from 14.7 to 112.5 μJ. The blue-shift effect for short-range POF has also been observed, even with a different pump energy, which echoes well to the aforementioned mechanism with a constant pump energy. The lasing emission wavelength of RFLs can be controlled by using different types of POFs (e.g., short-range/long-range disorder POF). Further, the disorder can be well controlled by selecting a different weight of POSS. Therefore, the research would pave a way to control the random lasing wavelength of the RFLs, which always is a challenge in the field of RLs. To verify the excited random cavities caused by the disorder of our two different range disorder POF systems, we have performed a power Fourier transform (PFT) analysis for the emission spectrum of the two different range disorder POFs at ∼96 μJ in Fig. 3, as shown in Fig. 6. The PFT of the emission spectrum (in k 2π∕λ space, λ being the emission wavelength) from a welldefined laser cavity yields peaks at Fourier components pm mnLc ∕π, where m is the order of the Fourier harmonic, n the refractive index of the gain medium, and Lc is the cavity path length [18]. Based on the above relationship and n 1.4955 for the core material of the short-range disorder POF, the first sharp peak in the PFT spectra (i.e., the fundamental Fourier component pm1 10.3 μm) gives the Lc value of ∼21.6 μm. In the same way, the Lc value of the long-range disorder POF is calculated to be ∼63.7 μm based on pm1 63.7 μm
Fig. 6. PFT of the corresponding emission spectra for (a) short-range and (b) long-range POF at a pump energy of ∼96 μJ shown in Fig. 3(a).
and n 1.5037 for the core material of the long-range disorder POF. Interestingly, the Lc of the long-range disorder POF is larger than that of the short-range disorder POF, which can be well explained by the fact that the short-range disorder POF has a stronger disorder than the long-range disorder POF. Therefore, the Lc value offers further proof that the disorder of the short-range disorder POF is stronger than that of the long-range disorder POF, which thus echoes the aforementioned blue-shift effect for the short-range POF. To summarize, we have reported two types of random polymer fiber lasers by end pumping two different disorder POFs. One type is a long-range random polymer fiber laser based on POSS NPs scattering. The other is a shortrange random polymer fiber laser based on copolymer refractive index inhomogeneity. The lasing wavelength of the short-range disorder POF exhibits blue-shift effect compared to long-range disorder. It is possible that the ability to control the lasing wavelength can be extended by using different disorder POFs. We envision that the different disorder random polymer fiber laser may open a window to control random fiber lasers. The authors appreciate the financial support from National Natural Science Foundation of China (Grants Nos. 11404087, 51405126, 60588502) and Science research start-up funding of Hefei University of Technology No. 407-037133. References 1. H. Cao, Waves Random Media 13, R1 (2003). 2. A. Tulek, R. C. Polson, and Z. V. Vardeny, Nat. Phys. 6, 303 (2010). 3. V. S. Letokhov, JETP Lett. 5, 212 (1967). 4. D. S. Wiersma, Nat. Phys. 4, 359 (2008). 5. C. J. S. de Matos, L. de S. Menezes, A. M. Brito-Silva, M. A. M. Gámez, A. S. L. Gomes, and C. B. de Araújo, Phys. Rev. Lett. 99, 153903 (2007). 6. Z. J. Hu, H. J. Zheng, L. J. Wang, X. J. Tian, T. X. Wang, Q. J. Zhang, G. Zou, Y. Chen, and Q. Zhang, Opt. Commun. 285, 3967 (2012). 7. Z. Hu, Q. Zhang, B. Miao, Q. Fu, G. Zou, Y. Chen, Y. Luo, D. G. Zhang, P. Wang, H. Ming, and Q. J. Zhang, Phys. Rev. Lett. 109, 253901 (2012). 8. Z. J. Hu, B. Miao, T. Wang, Q. Fu, D. G. Zhang, H. Ming, and Q. J. Zhang, Opt. Lett. 38, 4644 (2013). 9. V. D. Ta, R. Chen, L. Ma, Y. J. Ying, and H. D. Sun, Laser Photon. Rev. 7, 133 (2013). 10. N. Bachelard, S. Gigan, X. Noblin, and P. Sebbah, Nat. Phys. 10, 426 (2014). 11. N. Bachelard, J. Andreasen, S. Gigan, and P. Sebbah, Phys. Rev. Lett. 109, 033903 (2012). 12. R. G. S. El-Dardirya and A. Lagendijk, Appl. Phys. Lett. 98, 161106 (2011). 13. S. Gottardo, R. Sapienza, P. D. García, A. Blanco, D. S. Wiersma, and C. López, Nat. Photonics 2, 429 (2008). 14. G. D. Peng, P. L. Chu, Z. J. Xiong, T. W. Whitbread, and R. P. Chaplin, J. Lightwave Technol. 14, 2215(1996). 15. Y. C. Sheen, C. H. Lu, C. F. Huang, S. W. Kuo, and F. C. Chang, Polymer 49, 4017 (2008). 16. R. C. Polson, M. E. Raikh, and Z. V. Vardeny, C.R. Physique 3, 509 (2002). 17. R. C. Polson and Z. V. Vardeny, Phys. Rev. B 71, 045205 (2005). 18. R. C. Polson, G. Levina, and Z. V. Vardeny, Appl. Phys. Lett. 76, 3858 (2000).
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Wavelength control of random polymer fiber laser based on adaptive disorder Zhijia Hu,* Pengfei Gao, Kang Xie, Yunyun Liang, and Haiming Jiang School of Instrument Science and Opto-electronics Engineering, Hefei University of Technology, Hefei 230009, China *Corresponding author: [email protected] Received October 20, 2014; accepted November 7, 2014; posted November 14, 2014 (Doc. ID 224999); published December 10, 2014 We demonstrate the realization of two different kinds of random polymer optical fiber lasers to control the random lasing wavelength by changing the disorder of polymer optical fibers (POFs). One is a long-range disorder POF based on copolymer refractive-index inhomogeneity, and the other is a short-range disorder POF based on polyhedral oligomeric silsesquioxanes scattering. By end pumped both disorder POFs, the coherent random lasing for both is observed. Meanwhile, the random lasing wavelength of the short-range disorder POF because of a small scattering mean-free path has been found to be blue shifted with respect to the long-range disorder POF, which will give a way to control the random lasing wavelength. © 2014 Optical Society of America OCIS codes: (140.3510) Lasers, fiber; (160.4236) Nanomaterials; (160.5470) Polymers; (290.5850) Scattering, particles. http://dx.doi.org/10.1364/OL.39.006911
The random laser (RL) is an unconventional type of laser, in which an optical feedback is realized by multiple light scattering in disorder systems that contain optical gain medium [1]. On the basis of the scattering strength, the disorder can fall into two types: (1) long-range disorder with a large scattering mean-free path (ls ), which can be formed owing to refractive-index fluctuations, δnr , caused by long-range inhomogeneity in the disorder medium; and (2) short-range disorder caused by strong light scatters resulting in small ls [2]. Since Letokhov predicted RLs [3], RLs have generated significant interest among researchers because of their unique properties, such as miniaturization and the absence of the requirement for a cavity formed by stationary mirrors [4]. However, the non-directional and high threshold characters of the traditional RL systems have largely limited their application. Recently, the effects of one-dimensional confinement on the lasing properties of a classical RL system have received increasing attention, bringing about the birth of random fiber lasers (RFLs) [5]. Then a coherent RFL based on nanoparticles (NPs) scattering in an extremely weakly scattering regime was reported [6,7], which reduces the laser threshold and increases the emission directionality. Furthermore, a stabilized coherent laser action based on laser dye PM597doped one-dimensional disorder polymer optical fiber (POF) has been obtained by the weak optical multiple scattering of the polyhedral oligomeric silsesquioxanes (POSS) NPs in the core of the POF in situ formed during polymerization, which was enhanced by the waveguide confinement effect [8]. The disordered POF is considered to be a promising candidate in microphtotonic/ nanophotonic integrated systems and fiber sensors because of the low cost and the mechanical flexibility [9]. The reported RFLs are based almost completely on short-range disorder with NPs scattering, and the long-range disorder RFLs have not been reported. The absence of a well-defined cavity to trap light [1], however, results in an unpredictable random emission spectrum. Therefore, controlling the random lasing is a vexing problem. Note that alternative techniques to tune the random laser emission by actively shaping the optical 0146-9592/14/246911-04$15.00/0
pump [10], the spatial pump profile [11], adjusting the amount of absorption of emission light [12], or varying scattering coefficient determined by Mie resonances in mono-disperse polystyrene microspheres have been reported in the literature [13]. In this Letter, we have demonstrated different random polymer fiber lasers to control the random lasing wavelength by changing the disorder of the POFs. One type is a long-rang disorder POF based on refractive-index fluctuations caused by long-range inhomogeneity in the disorder copolymer medium, the core material of which is laser dye PM597 doped-poly(methyl methacrylateco-benzyl methacrylate) [poly(MMA-co-BzMA] copolymer. The other is a short-range disorder POF, the core material of which is PM597 doped-poly(methyl methacrylate-co-benzyl methacrylate-co-methacrylisobutyl polyhedral oligomeric silsesquioxanes) [poly(MMAco-BzMA-co-MMAPOSS)]. The cladding materials of the two kinds of disorder POFs are poly(methyl methacrylate-co-butyl acrylate) and poly(MMA-co-BA). By end pumping the short-range disorder POF, the coherent random lasing has been observed, which has been reported in our previous work [8]. Interestingly, the coherent random lasing has also been observed by end pumping the long-range disorder POF. Meanwhile, the random lasing wavelength of the short-range disorder POF has been found to be blue shifted with respect to the long-range disorder POF, which will provide a method to control the random lasing wavelength that always is a challenge in the field of random lasers. The short-range disorder POF is fabricated using the “Teflon Technique” [14]. First, the distilled MMA and BA with the weigh ration of ∼4∶1, the initiator lauroylperoxide (LPO) (0.5 wt. %), and chain transfer agent 1-butanethiol (0.3 wt. %) in an appropriate concentration with regard to the total amount of organic monomers are mixed in the Teflon tube. The diameter of the tube is 2 cm, and a thin Teflon string is used to be fixed properly in the center of the tube. Polymerization is performed in a thermal bath where its temperature is raised to 50°C from 30°C by 5°C/day and then increased up to 80°C in steps of 10°C/day to make sure that the © 2014 Optical Society of America
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monomers are polymerized fully. Hence, the hollow POF preform is obtained. Second, the hollow POF preform is then filled with the PM597 laser dye (0.14 wt. %) and the core monomers composed of MMA, BzMA, and POSS NPs with the weigh ration of ∼5.7∶1∶2, which are polymerized using an initiator LPO (0.5 wt. %) and a chain transfer agent 1-butanethiol (0.3 wt. %) with regard to the total amount of organic monomers in the same heat process as mentioned above. Finally, the PM597 laser dye dopeddisorder POF preform is drawn to POF in a custom-made fiber drawing tower at 170°C. The diameter of the POF can be varied by adjusting the feed rate of the POF preform and the draw rate of the fiber. In the same way, the long-range disorder POF is obtained, and the components are PM597, MMA, and BzMA with a weigh ration of 0.14:85:15. Figure 1 shows the optical microscope cross-sectional images and fluorescence images for the two types of disorder POFs. It can be seen that the core (cladding) diameter of the two types of POFs are ∼30 3 750 10 μm from an optical microscope image. The refractive index of the core (cladding) material for the short-range disorder POF is 1.4955 (1.4780), satisfying the total internal reflection condition. In addition, the refractive index of the core (cladding) material for the long-range disorder POF is 1.5037 (1.4780). The disorder POFs (8 cm long) are measured by end pumping. The schematic of the entire experimental setup for the optical measurements of the POFs is similar to the Ref. 6. The 532 nm output of a Q-switched Nd:YAG laser (pulse duration of 10 ns, repetition rate of 10 Hz) is used to pump the random system. The pump pulse energy is controlled by a Glan prism. The emitted light is collected by a fiber spectrometer (QE 65000, ocean optics, resolution ∼0.4 nm, and integration time of 100 ms) after a 560 nm long-wave pass filter. Figures 2(a) and 2(b) show the schematic and TEM image of POSS-based NPs in the core of short-range disorder POF. The NPs can be formed through the dipole–dipole interaction of POSS cores and disperse
Fig. 1. Optical microscope (a) cross-sectional image and (b) fluorescence image of short-range POF, as well as (c) cross-sectional image and (d) fluorescence image of longrange POF.
Fig. 2. (a), (b) Schematic and TEM image of POSS-based NPs in the core of short-range POF. (c) The fluorescence emission spectra of the PM597 (0.14 wt. %) doped two types of disorder POFs.
effect of polymer chains during the polymerization process [15]. Finally, it can be seen that the POSS NPs with an average size of 150 nm are well dispersed in the core of the short-range disorder POF. Figure 2(c) shows the fluorescence emission spectra of the PM597 (0.14 wt. %) doped two types of disorder POFs. It can be seen that the fluorescence emission peaks of the PM597 doped two types of disorder POFs both located in 574 nm. Figure 3(a) shows the evolution of emission spectra recorded for the PM597 (0.14 wt. %)-doped short-range disorder POF with POSS NPs (22.9 wt. %). At the lower pump energy (25 μJ), only a broad spontaneous emission profile centered at ∼577.0 nm with full width at halfmaximum (FWHM) of ∼11.7 nm can be observed. At pump energies above ∼51 μJ, the multimode lasing on top of a globally narrowed amplified spontaneous emission (ASE) spectrum is obtained, whose FWHM is reduced greatly (e.g., ∼0.8 nm for the ∼577.5 nm peak at ∼113 μJ). Typically, a set of such sharp spectral features indicates coherent lasing resonances [1]. The inset of Figure 3(a) plots the main peak emission intensities as a function of the pump energies for the short-range disorder POF system. It can be seen that there is a nonlinear dependence of the main peak emission intensity on pump energy, which points to a lasing action, and the threshold is determined to be ∼51 μJ. Figure 3(b) shows the evolution of emission spectra recorded for the PM597 (0.14 wt. %)-doped long-range disorder POF. Under the low pump energy (45 μJ), we also observe only spontaneous emission profile. When pump energy increases above 45 μJ, the multimode lasing on top of a globally narrowed ASE spectrum as well is obtained,
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Fig. 3. (a) Evolution of emission spectra recorded for the PM597 (0.14 wt. %)-doped short-range disorder POF and (b) long-range disorder POF. The inset plots the main peak emission intensities as a function of the pump energy for both disorder POF systems. [In both cases, the spectral intensity increases by increasing the pump energy.]
which also indicates coherent lasing resonances. The FWHM of ∼580.5 nm peak at ∼150 μJ is reduced to ∼0.9 nm. The inset of Fig. 3(b) plots the main peak emission intensities as a function of the pump energies for the long-range disorder POF system. It can be seen that there is a nonlinear dependence of the main peak emission intensity on pump energy, which points to a lasing action, and the threshold is determined to be ∼62 μJ. The slope of random lasing for the short-disorder polymer optical fiber is larger than that for the long-range disorder, which is caused by the short-disorder polymer optical fiber having stronger disorder. Figure 4 shows the emission spectra recorded for the PM597 doped POFs without (long-range disorder) and with (short-range disorder) POSS NPs at a pump energy of 96 μJ. The main peak of random laser for the shortrange disorder POF (577.4 nm) is found to be blue-shifted 3 nm with respect to that of the long-range disorder POF (580.4 nm). The scattering mean-free path ls of the shortrange disorder POF at 577 nm is calculated to be 910 μm [9] in our previous work based on ls 1∕ρσ s , where ρ is the particle density and σ s the scattering cross section of the POSS NPs, which is caused by the NPs scattering. The scattering of the long-range disorder POF originates from refractive-index fluctuations, δnr , in the effective
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Fig. 4. Emission spectra recorded for the PM597 doped POFs without long-range disorder and with short-range POSS NPs at a pump energy of 96 μJ.
refractive-index caused by inhomogeneity in the core of the POF. This type of long-range disorder actually encourages RL because it tends to trap light in random resonators with refraction index nr δnr , higher than the average refraction index, nr , of the copolymer [16]. However, this disorder type cannot be quantified by the coherent backscattering technique [17], but the scattering mean-free path of the long-range disorder POF is larger than that of the short-range disorder POF. The scattering mean-free path is proportional to the wavelength. A shorter mean-free path means more scattering events and consequently a longer (and more random) total path in the POF system. The blue-shift effect observed here can be understood by considering the fact that shorter wavelengths are more amplified than larger wavelengths, and the original non-lasing shorter wavelengths should also laser preferentially. Therefore, at a certain value of the pump energy, this effect produces a blue-shifted sharp peak for the short-range disorder POF with respect to the long-range disorder POF at the border wavelength. To gain further insight into the blue-sifted effect of our POF systems, we have performed the peak emission wavelength as a function of the pump energy for the two disorder POF systems, as shown in Fig. 5. It can be seen that the lasing peak wavelength of long-range POF locates in the range of 577.6–588.9 nm when the pump energy increases from 18.1 to 149.9 μJ. It is worth
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OPTICS LETTERS / Vol. 39, No. 24 / December 15, 2014
Fig. 5. Random lasing emission wavelength as a function of the pump energy for the two disorder POF systems.
noting that the lasing peak wavelength of the short-range disorder POF overall blue-shift to the range of 568.7– 580.7 nm when the pump energy increases from 14.7 to 112.5 μJ. The blue-shift effect for short-range POF has also been observed, even with a different pump energy, which echoes well to the aforementioned mechanism with a constant pump energy. The lasing emission wavelength of RFLs can be controlled by using different types of POFs (e.g., short-range/long-range disorder POF). Further, the disorder can be well controlled by selecting a different weight of POSS. Therefore, the research would pave a way to control the random lasing wavelength of the RFLs, which always is a challenge in the field of RLs. To verify the excited random cavities caused by the disorder of our two different range disorder POF systems, we have performed a power Fourier transform (PFT) analysis for the emission spectrum of the two different range disorder POFs at ∼96 μJ in Fig. 3, as shown in Fig. 6. The PFT of the emission spectrum (in k 2π∕λ space, λ being the emission wavelength) from a welldefined laser cavity yields peaks at Fourier components pm mnLc ∕π, where m is the order of the Fourier harmonic, n the refractive index of the gain medium, and Lc is the cavity path length [18]. Based on the above relationship and n 1.4955 for the core material of the short-range disorder POF, the first sharp peak in the PFT spectra (i.e., the fundamental Fourier component pm1 10.3 μm) gives the Lc value of ∼21.6 μm. In the same way, the Lc value of the long-range disorder POF is calculated to be ∼63.7 μm based on pm1 63.7 μm
Fig. 6. PFT of the corresponding emission spectra for (a) short-range and (b) long-range POF at a pump energy of ∼96 μJ shown in Fig. 3(a).
and n 1.5037 for the core material of the long-range disorder POF. Interestingly, the Lc of the long-range disorder POF is larger than that of the short-range disorder POF, which can be well explained by the fact that the short-range disorder POF has a stronger disorder than the long-range disorder POF. Therefore, the Lc value offers further proof that the disorder of the short-range disorder POF is stronger than that of the long-range disorder POF, which thus echoes the aforementioned blue-shift effect for the short-range POF. To summarize, we have reported two types of random polymer fiber lasers by end pumping two different disorder POFs. One type is a long-range random polymer fiber laser based on POSS NPs scattering. The other is a shortrange random polymer fiber laser based on copolymer refractive index inhomogeneity. The lasing wavelength of the short-range disorder POF exhibits blue-shift effect compared to long-range disorder. It is possible that the ability to control the lasing wavelength can be extended by using different disorder POFs. We envision that the different disorder random polymer fiber laser may open a window to control random fiber lasers. The authors appreciate the financial support from National Natural Science Foundation of China (Grants Nos. 11404087, 51405126, 60588502) and Science research start-up funding of Hefei University of Technology No. 407-037133. References 1. H. Cao, Waves Random Media 13, R1 (2003). 2. A. Tulek, R. C. Polson, and Z. V. Vardeny, Nat. Phys. 6, 303 (2010). 3. V. S. Letokhov, JETP Lett. 5, 212 (1967). 4. D. S. Wiersma, Nat. Phys. 4, 359 (2008). 5. C. J. S. de Matos, L. de S. Menezes, A. M. Brito-Silva, M. A. M. Gámez, A. S. L. Gomes, and C. B. de Araújo, Phys. Rev. Lett. 99, 153903 (2007). 6. Z. J. Hu, H. J. Zheng, L. J. Wang, X. J. Tian, T. X. Wang, Q. J. Zhang, G. Zou, Y. Chen, and Q. Zhang, Opt. Commun. 285, 3967 (2012). 7. Z. Hu, Q. Zhang, B. Miao, Q. Fu, G. Zou, Y. Chen, Y. Luo, D. G. Zhang, P. Wang, H. Ming, and Q. J. Zhang, Phys. Rev. Lett. 109, 253901 (2012). 8. Z. J. Hu, B. Miao, T. Wang, Q. Fu, D. G. Zhang, H. Ming, and Q. J. Zhang, Opt. Lett. 38, 4644 (2013). 9. V. D. Ta, R. Chen, L. Ma, Y. J. Ying, and H. D. Sun, Laser Photon. Rev. 7, 133 (2013). 10. N. Bachelard, S. Gigan, X. Noblin, and P. Sebbah, Nat. Phys. 10, 426 (2014). 11. N. Bachelard, J. Andreasen, S. Gigan, and P. Sebbah, Phys. Rev. Lett. 109, 033903 (2012). 12. R. G. S. El-Dardirya and A. Lagendijk, Appl. Phys. Lett. 98, 161106 (2011). 13. S. Gottardo, R. Sapienza, P. D. García, A. Blanco, D. S. Wiersma, and C. López, Nat. Photonics 2, 429 (2008). 14. G. D. Peng, P. L. Chu, Z. J. Xiong, T. W. Whitbread, and R. P. Chaplin, J. Lightwave Technol. 14, 2215(1996). 15. Y. C. Sheen, C. H. Lu, C. F. Huang, S. W. Kuo, and F. C. Chang, Polymer 49, 4017 (2008). 16. R. C. Polson, M. E. Raikh, and Z. V. Vardeny, C.R. Physique 3, 509 (2002). 17. R. C. Polson and Z. V. Vardeny, Phys. Rev. B 71, 045205 (2005). 18. R. C. Polson, G. Levina, and Z. V. Vardeny, Appl. Phys. Lett. 76, 3858 (2000).
