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Random distributed feedback fiber laser pumped by an ytterbium doped fiber laser
G Model IJLEO-54364; No. of Pages 3
ARTICLE IN PRESS Optik xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
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Random distributed feedback fiber laser pumped by an ytterbium doped fiber laser Lulu Chen, Yingchun Ding ∗ Department of Physics, Beijing University of Chemical Technology, No. 15 Beisanhuan East Road, Chaoyang District, Beijing 100029, China
a r t i c l e
i n f o
Article history: Received 17 July 2013 Accepted 9 January 2014 Available online xxx Keywords: Random fiber lasers Rayleigh scattering Raman amplification
a b s t r a c t A random distributed feedback fiber laser operating at 1115 nm has been demonstrated experimentally in standard communication optical fibers by using a LD-pumped Yb-doped fiber laser as the pump source. We have studied the effect of different fiber spans on this new type of random fiber laser output power. It is shown that the generation power is the highest up to 198 mW in a 50 km fiber span. The slope efficiency is more than 28.7%. Stable, high-power continuous-wave (CW) lasing can be generated when the pump power is 3.6 W. The threshold power has also been calculated which well proves a random fiber laser operating via Rayleigh scattering, amplified through the Raman scattering. © 2014 Elsevier GmbH. All rights reserved.
1. Introduction Recently, there has been a great deal of interest in random lasers (RLs) without any optical cavity. Random lasing was predicted theoretically by Letokhov [1,2] more than thirty years ago and discovered experimentally in the past decades [3–5] where light was generated in an amplified disordered medium without any point reflection mirrors. In a random laser, the multiple-scattering plays an important role and the interference in the multiple-scattering process determines the mode structure. So random lasers are ‘mirror-less’ but not ‘mode-less’ [2]. Random lasers have many advantages, such as simple technology (no optical cavity) and low production costs. However, some characteristics of random lasers, such as complex features in emission spectra and angular dependence, can be retained. One-dimensional random fiber lasers (RFLs) [6] were proposed to improve the performance of RLs. In 2007, de Matos et al. reported the first random fiber laser. Subsequently, RFLs have attracted more and more attention. In April, 2010, the concept of a fiber laser based on a random distributed feedback provided by a naturally present Rayleigh backscattering which is captured by the fiber waveguide was proposed and implemented. Turitsyn et al. [7] named it the random distributed feedback fiber laser (RDFBFL). Compared to traditional lasers, RDFBFLs achieve gain through distributed stimulated Raman Scattering and achieve optical feedback through the random distributed Rayleigh backscattering. It is well-known that
∗ Corresponding author. E-mail address: [email protected] (Y. Ding).
Rayleigh backscattering can occur when light is propagating in the fiber core owing to the microinhomogeneities of refractive index along the whole fiber. The Rayleigh backscattering coefficient is quite small, only å ∼ 10−5 km−1 , but such weak random distributed feedback could be great enough to overcome lasing loss in a very long fiber with distributed Raman amplification. Different systems have been studied such as the RDFBFL operating at telecommunication transparency window (∼1.55 m) [7] and the RDFBFL operating at the short-wavelength window (<1.2 m) [8]. Furthermore, multi-wavelength [9] and widely tunable RDFBFLs [10] have also been demonstrated. RDFBFLs open potential applications in nonlinear optics, laser display and optical sensing and so on. Approaches based on different amplification mechanism have been proposed to form random fiber laser systems during the past years [11–13]. In the paper [7], stationary directional laser generation was achieved in a two-arm scheme. A standard communication fiber span of 83 km was pumped from its center in opposite directions by two equal-power 1455 nm Raman fiber lasers. The output power as high as 150 mW and the slope efficiency up to 15% was obtained from each fiber end, meaning that the total slope efficiency was up to 30%. In our paper, we experimentally realize a RDFBFL operating at 1115 nm by using an ytterbium doped fiber laser which is pumped by a 975 nm LD as the pump source. It is very difficult for shorter wavelength of the random fiber laser because of the higher loss in the optical fiber. Thus the study of the short-wavelength RDFBFL is one of the key contents. We establish a one-arm scheme in which the fiber is pumped from one side using one pump laser only. A standard single-mode fiber span is used as the random laser
http://dx.doi.org/10.1016/j.ijleo.2014.01.081 0030-4026/© 2014 Elsevier GmbH. All rights reserved.
Please cite this article in press as: L. Chen, Y. Ding, Random distributed feedback fiber laser pumped by an ytterbium doped fiber laser, Optik - Int. J. Light Electron Opt. (2014), http://dx.doi.org/10.1016/j.ijleo.2014.01.081
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2
Fig. 1. RDFBFL configuration.
medium. Then we study the effect of different fiber spans on the RDFBFL output power. The result shows that the generation power is the highest in a 50 km fiber span. The laser can generate 198 mW output power with slope efficiency more than 28.7% (single side output). In a word, this kind of RDFBFLs has clear advantages, such as short wavelength, simple technology, high output power and low costs.
Fig. 3. YDFL output spectrum with a narrow line-width.
2. Experimental setup The RDFBFL configuration is shown in Fig. 1. The scheme consists of a LD, a span of about 4.5 m ytterbium doped fiber, two fiber Bragg gratings (FBGs) including a high reflectivity (R > 90%) FBG and a high transmittance (R > 30%) FBG at 1064 nm, a 1064/1115 nm WDM and a span of standard single mode fiber (SMF). The core and cladding diameters of SMF are 9 m and 125 m. The ytterbium doped fiber laser (YDFL) operating at 1064 nm which is pumped by a 975 nm LD providing up to 7.3 W is used as the pump source and provides Raman gain with the output power of 4.28 W. Conversion efficiency from the LD laser to the YDFL reaches 85.7%, seen in Fig. 2. Fig. 3 shows the YDFL output spectrum near 1064 nm with a narrow line-width. This means that the pump source has a good beam quality which is important for the random lasing generation. The 1064/1115 nm WDM is used as the port of random fiber laser output. The long enough SMF is used as the gain medium. Angled cleaves are used at the right end of fiber span to eliminate 4% Fresnel reflection and ensure that the feedback is contributed only by the Rayleigh scattering. 3. Results and discussion We have measured the RDFBFL powers by implementing a set of SMF with fiber lengths: L = 30 km, 40 km, 50 km, 60 km and 70 km, shown in Fig. 4.
Fig. 2. YDFL output power versus the LD pump power.
Fig. 4. RDFBFL power as a function of the LD pump power at different fiber spans.
Note that the RDFBFL power is much different in spite of nearly the same threshold pump power and the similar linear growth of the generated output power above threshold under different conditions. It is shown that both the laser output power as high as 198 mW and slope efficiency up to 28.7% are the highest inside a 50 km SMF span. Because the longer the fiber span is, the higer the fiber loss is, the lasing power does not show a linear growth with the fiber length. The relationship between laser power and fiber length is shown in Fig. 5. In contrast with the paper [8], these parameters are obviously improved. Fig. 4 also shows lasing with a threshold
Fig. 5. RDFBFL power as a function of different fiber spans (black curve is theoretical calculation result).
Please cite this article in press as: L. Chen, Y. Ding, Random distributed feedback fiber laser pumped by an ytterbium doped fiber laser, Optik - Int. J. Light Electron Opt. (2014), http://dx.doi.org/10.1016/j.ijleo.2014.01.081
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pump power of ∼3.6 W. Above ∼3.6 W, the setup starts to generate radiation close to 1115 nm which agrees well with 13 THz [14] SRS shift in the silica fiber. The detailed threshold estimation is as follows by considering the whole span as a multitude of cavities of various lengths. The equation [7] for the generated power reads as dP ± = ∓˛s P ± ± gR (PP+ + PP− )P ± ± εP ∓ dz
(1)
Here PP± (z) are pump power and P ± (z) are Stokes power, while + and − refer to forward and backward propagation, respectively. gR ≈ 0.35 W−1 km−1 is Raman gain coefficient, ˛s ≈ 0.115 km−1 is Stokes loss coefficient, ε ≈ 10−5 km−1 is Rayleigh scattering coefficient. The generation starts when total gain overcomes total losses. The gain/loss balance equation [7] reads as
L/2
dz exp −2˛z + 2gR
ε 0
z
Pp (s)ds = 1
(2)
0
where L is the fiber length, Pp (z) = P0 exp(−˛P |z|), P0 is an input pump power, ˛P ≈ 0.138 km−1 is pump loss coefficient. Based on this equation, we can get the generation threshold pump power Pth ≈ 2.9 W lower than the measured threshold 3.6 W. The difference could be caused by the higher loss of WDM. Also, the difference could be atrributed to the estimated fiber coefficients with real ones. It is concluded that the theoretical mode proposed in paper [7] can be applied to shorter wavelength and random fiber lasers are based on Rayleigh scattering and Raman amplification. A new wavelength is provided for the applications of random fiber lasers. Experimental evidence has been provided that the random lasing is due to Rayleigh scattering rather than WDM’s influence and residual reflectance by the fiber facet. First, we have measured the spectra of Yb-doped fiber laser in two cases (with and without WDM) and the result shows that there is no change. Second, rightangled cleaves are used at the right end of 50 km SMF span and the output power is about 100 mW lower than 198 mW. Fig. 5 clearly presents the effect of different fiber spans on the RDFBFL output power. Notice that the laser power has the maximum at L = 50 km. For L < 50 km, the output power grows gradually with increasing fiber span. Once passing 50 km, it starts to decrease rapidly. It is very important for us to select the optimal fiber length for maximum laser power. Finally, we have characterized temporal behaviors and optical spectrum of the generated lasing inside a 50 km fiber span, seen in paper [15]. Below or near the generation threshold, an unstable curve with many random spikes can be observed in our oscilloscope. Whereas, when the pump power approaches the threshold, the impact of Rayleigh feedback grows rapidly. Especially well above the threshold, the laser starts to operate in a stable regime with strongly suppressed fluctuations. When the pump power is above threshold, the optical spectrum is stabilized and smooth, showing two narrow (full-width at halfmaximum, FWHM, about 1 nm) laser lines localized near 1115 nm, which is similar to that in the paper [7]. However, there is no better theory and numerical simulation to explain this phenomenon now and the reason is not very clear. To explore the modes of random fiber lasers, new physical mechanisms should be considered, for example, four-wave mixing effect and dispersion effect. So more attempts need to be made to solve the problem which has pioneering significance of the development of random fiber lasers.
3
4. Conclusion Summarized as follows, our one-arm research shows that a RDFBFL operating at 1115 nm and using a LD-pumped Yb-doped fiber laser as the pump source has been demonstrated experimentally and different fiber spans have a great effect on the RDFBFL power. It is shown that a 50 km fiber span is more suitable to be used as the gain medium. The output power of 198 mW with a slope efficient up to 28.7% (single side output) and the threshold pump power of ∼3.6 W inside a 50 km fiber span have been generated. Then the threshold power has been calculated by using the coupled-wave equation. The calculated value well demonstrates a random fiber laser operating via Rayleigh scattering and amplified through the Raman effect. The dynamic change over the time domain has also been observed. Well above the threshold, lasing performs with stable, high-power continuous-wave (CW) output. These can be attributed to Rayleigh scattering feedback and Raman amplification, detailed in paper [16]. In a word, these laser characteristics show the potential of this novel type of laser and promising applications in different fields such as laser physics, the theory of disordered systems, specklefree [17], fiber optics, nonlinear science, and remote sensing [8]. Thus, we also anticipate that new applications and technologies will keep emerging from the study of the physics of RDFBFLs and shorter-wavelength RDFBFLs will be realized. Acknowledgments This work was supported by the National Natural Science Foundation of China under the Grant No. 60978006 and the Beijing Natural Science Foundation under the Grant No. 4122055. References [1] V.S. Letokhov, Generation of light by a scattering medium with negative resonance absorption, Sov. Phys. JETP 26 (1968) 835–840. [2] D.S. Wiersm, The physics and applications of random lasers, Nat. Phys. 4 (2008) 359–367. [3] N.M. Lawandy, R.M. Balachandran, A.S.L. Gomes, E. Sauvain, Laser action in strongly scattering media, Nature 368 (1994) 436–438. [4] D.S. Wiersma, A. Lagendijk, Light diffusion with gain and random lasers, Phys. Rev. E 54 (1996) 4256–4265. [5] H. Cao, Y.G. Zhao, S.T. Ho, E.W. Seelig, Q.H. Wang, R.P.H. Chang, Random laser action in semiconductor powder, Phys. Rev. Lett. 82 (1999) 2278. [6] C.J.S. de Matos, L.S. Menezes, A.M. Brito-Silva, M.A. Martinez Gamez, A.S.L. Gomes, C.B. Araujo, Random fiber laser, Phys. Rev. Lett. 99 (2007) 153903. [7] S.K. Turitsyn, S.A. Babin, A.E. El-Taher, et al., Random distributed feedback fibre laser, Nat. Photon 4 (2010) 231–235. [8] I.D. Vatnik, D.V. Churkin, S.A. Babin, S.K. Turitsyn, Cascaded random distributed feedback Raman fiber laser operating at 1.2 m, Opt. Express 19 (2011) 18486–18494. [9] A.E. El-Taher, P. Harper, S.A. Babin, D.V. Churkin, E.V. Podivilov, J.D. AniaCastanon, S.K. Turitsyn, Effect of Rayleigh-scattering distributed feedback on multiwavelength Raman fiber lasergeneration, Opt. Lett. 36 (2011) 130–132. [10] S.A. Babin, A.E. El-Taher, P. Harper, E.V. Podivilov, S.K. Turitsyn, Tunable random fiber laser, Phys. Rev. A 84 (2011) 021805R. [11] H. Ahmad, M.Z. Zulkifli, M.H. Jemangin, S.W. Harun, Distributed feedback multimode Brillouin–Raman random fiber laser in the S-band, Laser Phys. Lett. 10 (2013) 055102. [12] N. Lizárraga, N.P. Puente, E.I. Chaikina, Single-mode Er-doped fiber random laser with distributed Bragg grating feedback, Opt. Express 17 (2009) 395–404. [13] Z.J. Hu, H.J. Zheng, L.J. Wang, X.J. Tian, T.X. Wang, Q.J. Zhang, G. Zou, Y. Chen, Q. Zhang, Random fiber laser of POSS solution-filled hollow optical fiber by end pumping, Opt. Commun. 285 (2012) 3967–3970. [14] G.P. Agrawal, Fiber-optic communication systems, 3rd ed., Wiley-Interscience, New York, 2002. [15] R.X. Teng, Y.C. Ding, L.L. Chen, Random fiber laser operating at 1115 nm, Appl. Phys. B 111 (2013) 169–172. [16] D.V. Churkin, S.A. Babin, A.E. El-Taher, P. Harper, S.I. Kablukov, V. Karalekas, J.D. Ania-Castanon, E.V. Podivilov, S.K. Turitsyn, Raman fiber lasers with a random distributed feedback based on Rayleigh scattering, Phys. Rev. A 82 (2010) 033828. [17] B. Redding, M.A. Choma, H. Cao, Speckle-free laser imaging using random laser illumination, Nat. Photonics 29 (2012) 355–359.
Please cite this article in press as: L. Chen, Y. Ding, Random distributed feedback fiber laser pumped by an ytterbium doped fiber laser, Optik - Int. J. Light Electron Opt. (2014), http://dx.doi.org/10.1016/j.ijleo.2014.01.081
ARTICLE IN PRESS Optik xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Random distributed feedback fiber laser pumped by an ytterbium doped fiber laser Lulu Chen, Yingchun Ding ∗ Department of Physics, Beijing University of Chemical Technology, No. 15 Beisanhuan East Road, Chaoyang District, Beijing 100029, China
a r t i c l e
i n f o
Article history: Received 17 July 2013 Accepted 9 January 2014 Available online xxx Keywords: Random fiber lasers Rayleigh scattering Raman amplification
a b s t r a c t A random distributed feedback fiber laser operating at 1115 nm has been demonstrated experimentally in standard communication optical fibers by using a LD-pumped Yb-doped fiber laser as the pump source. We have studied the effect of different fiber spans on this new type of random fiber laser output power. It is shown that the generation power is the highest up to 198 mW in a 50 km fiber span. The slope efficiency is more than 28.7%. Stable, high-power continuous-wave (CW) lasing can be generated when the pump power is 3.6 W. The threshold power has also been calculated which well proves a random fiber laser operating via Rayleigh scattering, amplified through the Raman scattering. © 2014 Elsevier GmbH. All rights reserved.
1. Introduction Recently, there has been a great deal of interest in random lasers (RLs) without any optical cavity. Random lasing was predicted theoretically by Letokhov [1,2] more than thirty years ago and discovered experimentally in the past decades [3–5] where light was generated in an amplified disordered medium without any point reflection mirrors. In a random laser, the multiple-scattering plays an important role and the interference in the multiple-scattering process determines the mode structure. So random lasers are ‘mirror-less’ but not ‘mode-less’ [2]. Random lasers have many advantages, such as simple technology (no optical cavity) and low production costs. However, some characteristics of random lasers, such as complex features in emission spectra and angular dependence, can be retained. One-dimensional random fiber lasers (RFLs) [6] were proposed to improve the performance of RLs. In 2007, de Matos et al. reported the first random fiber laser. Subsequently, RFLs have attracted more and more attention. In April, 2010, the concept of a fiber laser based on a random distributed feedback provided by a naturally present Rayleigh backscattering which is captured by the fiber waveguide was proposed and implemented. Turitsyn et al. [7] named it the random distributed feedback fiber laser (RDFBFL). Compared to traditional lasers, RDFBFLs achieve gain through distributed stimulated Raman Scattering and achieve optical feedback through the random distributed Rayleigh backscattering. It is well-known that
∗ Corresponding author. E-mail address: [email protected] (Y. Ding).
Rayleigh backscattering can occur when light is propagating in the fiber core owing to the microinhomogeneities of refractive index along the whole fiber. The Rayleigh backscattering coefficient is quite small, only å ∼ 10−5 km−1 , but such weak random distributed feedback could be great enough to overcome lasing loss in a very long fiber with distributed Raman amplification. Different systems have been studied such as the RDFBFL operating at telecommunication transparency window (∼1.55 m) [7] and the RDFBFL operating at the short-wavelength window (<1.2 m) [8]. Furthermore, multi-wavelength [9] and widely tunable RDFBFLs [10] have also been demonstrated. RDFBFLs open potential applications in nonlinear optics, laser display and optical sensing and so on. Approaches based on different amplification mechanism have been proposed to form random fiber laser systems during the past years [11–13]. In the paper [7], stationary directional laser generation was achieved in a two-arm scheme. A standard communication fiber span of 83 km was pumped from its center in opposite directions by two equal-power 1455 nm Raman fiber lasers. The output power as high as 150 mW and the slope efficiency up to 15% was obtained from each fiber end, meaning that the total slope efficiency was up to 30%. In our paper, we experimentally realize a RDFBFL operating at 1115 nm by using an ytterbium doped fiber laser which is pumped by a 975 nm LD as the pump source. It is very difficult for shorter wavelength of the random fiber laser because of the higher loss in the optical fiber. Thus the study of the short-wavelength RDFBFL is one of the key contents. We establish a one-arm scheme in which the fiber is pumped from one side using one pump laser only. A standard single-mode fiber span is used as the random laser
http://dx.doi.org/10.1016/j.ijleo.2014.01.081 0030-4026/© 2014 Elsevier GmbH. All rights reserved.
Please cite this article in press as: L. Chen, Y. Ding, Random distributed feedback fiber laser pumped by an ytterbium doped fiber laser, Optik - Int. J. Light Electron Opt. (2014), http://dx.doi.org/10.1016/j.ijleo.2014.01.081
G Model IJLEO-54364; No. of Pages 3
ARTICLE IN PRESS L. Chen, Y. Ding / Optik xxx (2014) xxx–xxx
2
Fig. 1. RDFBFL configuration.
medium. Then we study the effect of different fiber spans on the RDFBFL output power. The result shows that the generation power is the highest in a 50 km fiber span. The laser can generate 198 mW output power with slope efficiency more than 28.7% (single side output). In a word, this kind of RDFBFLs has clear advantages, such as short wavelength, simple technology, high output power and low costs.
Fig. 3. YDFL output spectrum with a narrow line-width.
2. Experimental setup The RDFBFL configuration is shown in Fig. 1. The scheme consists of a LD, a span of about 4.5 m ytterbium doped fiber, two fiber Bragg gratings (FBGs) including a high reflectivity (R > 90%) FBG and a high transmittance (R > 30%) FBG at 1064 nm, a 1064/1115 nm WDM and a span of standard single mode fiber (SMF). The core and cladding diameters of SMF are 9 m and 125 m. The ytterbium doped fiber laser (YDFL) operating at 1064 nm which is pumped by a 975 nm LD providing up to 7.3 W is used as the pump source and provides Raman gain with the output power of 4.28 W. Conversion efficiency from the LD laser to the YDFL reaches 85.7%, seen in Fig. 2. Fig. 3 shows the YDFL output spectrum near 1064 nm with a narrow line-width. This means that the pump source has a good beam quality which is important for the random lasing generation. The 1064/1115 nm WDM is used as the port of random fiber laser output. The long enough SMF is used as the gain medium. Angled cleaves are used at the right end of fiber span to eliminate 4% Fresnel reflection and ensure that the feedback is contributed only by the Rayleigh scattering. 3. Results and discussion We have measured the RDFBFL powers by implementing a set of SMF with fiber lengths: L = 30 km, 40 km, 50 km, 60 km and 70 km, shown in Fig. 4.
Fig. 2. YDFL output power versus the LD pump power.
Fig. 4. RDFBFL power as a function of the LD pump power at different fiber spans.
Note that the RDFBFL power is much different in spite of nearly the same threshold pump power and the similar linear growth of the generated output power above threshold under different conditions. It is shown that both the laser output power as high as 198 mW and slope efficiency up to 28.7% are the highest inside a 50 km SMF span. Because the longer the fiber span is, the higer the fiber loss is, the lasing power does not show a linear growth with the fiber length. The relationship between laser power and fiber length is shown in Fig. 5. In contrast with the paper [8], these parameters are obviously improved. Fig. 4 also shows lasing with a threshold
Fig. 5. RDFBFL power as a function of different fiber spans (black curve is theoretical calculation result).
Please cite this article in press as: L. Chen, Y. Ding, Random distributed feedback fiber laser pumped by an ytterbium doped fiber laser, Optik - Int. J. Light Electron Opt. (2014), http://dx.doi.org/10.1016/j.ijleo.2014.01.081
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pump power of ∼3.6 W. Above ∼3.6 W, the setup starts to generate radiation close to 1115 nm which agrees well with 13 THz [14] SRS shift in the silica fiber. The detailed threshold estimation is as follows by considering the whole span as a multitude of cavities of various lengths. The equation [7] for the generated power reads as dP ± = ∓˛s P ± ± gR (PP+ + PP− )P ± ± εP ∓ dz
(1)
Here PP± (z) are pump power and P ± (z) are Stokes power, while + and − refer to forward and backward propagation, respectively. gR ≈ 0.35 W−1 km−1 is Raman gain coefficient, ˛s ≈ 0.115 km−1 is Stokes loss coefficient, ε ≈ 10−5 km−1 is Rayleigh scattering coefficient. The generation starts when total gain overcomes total losses. The gain/loss balance equation [7] reads as
L/2
dz exp −2˛z + 2gR
ε 0
z
Pp (s)ds = 1
(2)
0
where L is the fiber length, Pp (z) = P0 exp(−˛P |z|), P0 is an input pump power, ˛P ≈ 0.138 km−1 is pump loss coefficient. Based on this equation, we can get the generation threshold pump power Pth ≈ 2.9 W lower than the measured threshold 3.6 W. The difference could be caused by the higher loss of WDM. Also, the difference could be atrributed to the estimated fiber coefficients with real ones. It is concluded that the theoretical mode proposed in paper [7] can be applied to shorter wavelength and random fiber lasers are based on Rayleigh scattering and Raman amplification. A new wavelength is provided for the applications of random fiber lasers. Experimental evidence has been provided that the random lasing is due to Rayleigh scattering rather than WDM’s influence and residual reflectance by the fiber facet. First, we have measured the spectra of Yb-doped fiber laser in two cases (with and without WDM) and the result shows that there is no change. Second, rightangled cleaves are used at the right end of 50 km SMF span and the output power is about 100 mW lower than 198 mW. Fig. 5 clearly presents the effect of different fiber spans on the RDFBFL output power. Notice that the laser power has the maximum at L = 50 km. For L < 50 km, the output power grows gradually with increasing fiber span. Once passing 50 km, it starts to decrease rapidly. It is very important for us to select the optimal fiber length for maximum laser power. Finally, we have characterized temporal behaviors and optical spectrum of the generated lasing inside a 50 km fiber span, seen in paper [15]. Below or near the generation threshold, an unstable curve with many random spikes can be observed in our oscilloscope. Whereas, when the pump power approaches the threshold, the impact of Rayleigh feedback grows rapidly. Especially well above the threshold, the laser starts to operate in a stable regime with strongly suppressed fluctuations. When the pump power is above threshold, the optical spectrum is stabilized and smooth, showing two narrow (full-width at halfmaximum, FWHM, about 1 nm) laser lines localized near 1115 nm, which is similar to that in the paper [7]. However, there is no better theory and numerical simulation to explain this phenomenon now and the reason is not very clear. To explore the modes of random fiber lasers, new physical mechanisms should be considered, for example, four-wave mixing effect and dispersion effect. So more attempts need to be made to solve the problem which has pioneering significance of the development of random fiber lasers.
3
4. Conclusion Summarized as follows, our one-arm research shows that a RDFBFL operating at 1115 nm and using a LD-pumped Yb-doped fiber laser as the pump source has been demonstrated experimentally and different fiber spans have a great effect on the RDFBFL power. It is shown that a 50 km fiber span is more suitable to be used as the gain medium. The output power of 198 mW with a slope efficient up to 28.7% (single side output) and the threshold pump power of ∼3.6 W inside a 50 km fiber span have been generated. Then the threshold power has been calculated by using the coupled-wave equation. The calculated value well demonstrates a random fiber laser operating via Rayleigh scattering and amplified through the Raman effect. The dynamic change over the time domain has also been observed. Well above the threshold, lasing performs with stable, high-power continuous-wave (CW) output. These can be attributed to Rayleigh scattering feedback and Raman amplification, detailed in paper [16]. In a word, these laser characteristics show the potential of this novel type of laser and promising applications in different fields such as laser physics, the theory of disordered systems, specklefree [17], fiber optics, nonlinear science, and remote sensing [8]. Thus, we also anticipate that new applications and technologies will keep emerging from the study of the physics of RDFBFLs and shorter-wavelength RDFBFLs will be realized. Acknowledgments This work was supported by the National Natural Science Foundation of China under the Grant No. 60978006 and the Beijing Natural Science Foundation under the Grant No. 4122055. References [1] V.S. Letokhov, Generation of light by a scattering medium with negative resonance absorption, Sov. Phys. JETP 26 (1968) 835–840. [2] D.S. Wiersm, The physics and applications of random lasers, Nat. Phys. 4 (2008) 359–367. [3] N.M. Lawandy, R.M. Balachandran, A.S.L. Gomes, E. Sauvain, Laser action in strongly scattering media, Nature 368 (1994) 436–438. [4] D.S. Wiersma, A. Lagendijk, Light diffusion with gain and random lasers, Phys. Rev. E 54 (1996) 4256–4265. [5] H. Cao, Y.G. Zhao, S.T. Ho, E.W. Seelig, Q.H. Wang, R.P.H. Chang, Random laser action in semiconductor powder, Phys. Rev. Lett. 82 (1999) 2278. [6] C.J.S. de Matos, L.S. Menezes, A.M. Brito-Silva, M.A. Martinez Gamez, A.S.L. Gomes, C.B. Araujo, Random fiber laser, Phys. Rev. Lett. 99 (2007) 153903. [7] S.K. Turitsyn, S.A. Babin, A.E. El-Taher, et al., Random distributed feedback fibre laser, Nat. Photon 4 (2010) 231–235. [8] I.D. Vatnik, D.V. Churkin, S.A. Babin, S.K. Turitsyn, Cascaded random distributed feedback Raman fiber laser operating at 1.2 m, Opt. Express 19 (2011) 18486–18494. [9] A.E. El-Taher, P. Harper, S.A. Babin, D.V. Churkin, E.V. Podivilov, J.D. AniaCastanon, S.K. Turitsyn, Effect of Rayleigh-scattering distributed feedback on multiwavelength Raman fiber lasergeneration, Opt. Lett. 36 (2011) 130–132. [10] S.A. Babin, A.E. El-Taher, P. Harper, E.V. Podivilov, S.K. Turitsyn, Tunable random fiber laser, Phys. Rev. A 84 (2011) 021805R. [11] H. Ahmad, M.Z. Zulkifli, M.H. Jemangin, S.W. Harun, Distributed feedback multimode Brillouin–Raman random fiber laser in the S-band, Laser Phys. Lett. 10 (2013) 055102. [12] N. Lizárraga, N.P. Puente, E.I. Chaikina, Single-mode Er-doped fiber random laser with distributed Bragg grating feedback, Opt. Express 17 (2009) 395–404. [13] Z.J. Hu, H.J. Zheng, L.J. Wang, X.J. Tian, T.X. Wang, Q.J. Zhang, G. Zou, Y. Chen, Q. Zhang, Random fiber laser of POSS solution-filled hollow optical fiber by end pumping, Opt. Commun. 285 (2012) 3967–3970. [14] G.P. Agrawal, Fiber-optic communication systems, 3rd ed., Wiley-Interscience, New York, 2002. [15] R.X. Teng, Y.C. Ding, L.L. Chen, Random fiber laser operating at 1115 nm, Appl. Phys. B 111 (2013) 169–172. [16] D.V. Churkin, S.A. Babin, A.E. El-Taher, P. Harper, S.I. Kablukov, V. Karalekas, J.D. Ania-Castanon, E.V. Podivilov, S.K. Turitsyn, Raman fiber lasers with a random distributed feedback based on Rayleigh scattering, Phys. Rev. A 82 (2010) 033828. [17] B. Redding, M.A. Choma, H. Cao, Speckle-free laser imaging using random laser illumination, Nat. Photonics 29 (2012) 355–359.
Please cite this article in press as: L. Chen, Y. Ding, Random distributed feedback fiber laser pumped by an ytterbium doped fiber laser, Optik - Int. J. Light Electron Opt. (2014), http://dx.doi.org/10.1016/j.ijleo.2014.01.081