Tunable random distributed feedback fiber laser operating at 1 μm Xueyuan Du, Hanwei Zhang, Xiaolin Wang, and Pu Zhou* College of Optoelectronic Science and Engineering, National University of Defense Technology, Changsha 410073, China *Corresponding author: [email protected] Received 29 October 2014; revised 25 December 2014; accepted 26 December 2014; posted 6 January 2015 (Doc. ID 225982); published 29 January 2015
A tunable random distributed feedback (RDFB) fiber laser operating at 1 μm is presented, which consists of a piece of ytterbium-doped fiber, a manual tunable filter and a 1-km-long single-mode fiber that provides RDFB. The wavelength of the RDFB fiber laser can be tuned continuously from 1040 to 1090 nm with a linewidth of about 0.4 nm. Power output maintains good flatness in the 50 nm tuning range, with fluctuations within 1 dB. It is the first reported tunable RDFB fiber laser working at 1 μm as far as we know. © 2015 Optical Society of America OCIS codes: (140.3600) Lasers, tunable; (140.3510) Lasers, fiber; (290.5870) Scattering, Rayleigh; (140.3490) Lasers, distributed-feedback. http://dx.doi.org/10.1364/AO.54.000908
1. Introduction
The concept of a random laser has drawn a great deal of attention in recent years, as random lasers are able to generate coherent light without a welldefined cavity by exploiting multiple scattering in an amplifying disordered medium [1]. Random lasers have a range of potential applications including medical applications, compact light sources, spectroscopic monitoring devices, and illumination materials [2–4]. Different types of random lasers differ due to the material choice for the disordered medium. In 2010, a random distributed feedback (RDFB) fiber laser based on Raman amplification and distributed Rayleigh scattering feedback in a single-mode fiber (SMF) was reported [5]. Fiber is now a remarkable and practical realization of a disordered medium providing positive feedback in the random laser regime. Random fiber lasers (RFLs) with various features, such as high-power laser generation [6–9], noise and gain optimization [10,11], multiwavelength
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generation [12–14], narrow linewidth outputs [15,16] and tunable lasers [17,18], have been demonstrated. A tunable RFL or multiwavelength generation takes advantage of the modeless characteristics that suppress multimode interactions and the relatively wide gain bandwidth in RFLs, which can be used in imaging, communications, sensing, and other fields. Current research has made certain achievements in the tunable random fiber laser field. Babin et al. introduced a scheme of a tunable RDFB fiber laser by implementing a fiber-pigtailed tunable filter in the middle of the symmetric configuration and acquired ∼2.2 W laser output with 30% efficiency and a 40 nm tuning range [18]. Tunable multiwavelength generation concerning the spectral width of each individual line was also demonstrated in [14], where a Mach–Zehnder interferometer based on two longperiod fiber Bragg gratings (FBGs) and a Fabry– Perot filter was used to generate a multiwavelength laser with a tuning range of ∼10 nm and an individual line spectral width of 0.034 nm. Most recently, Wang et al. proposed a tunable Er-doped fiber laser based on RDFB backward Rayleigh scattering in a 20 km SMF with a tunable fiber Fabry–Perot interferometer [17], which realized a tuning range of
about 40 nm with a linewidth of ∼0.04 nm and pump efficiency of 14%. The above setups all demonstrated random laser generation in 1.5 μm spectral bands. It is likely that the same lasing concept can be realized in a broader spectral range. Since a fiber laser working at 1 μm possesses the capability of free communication via atmospheric windows and high-power laser output potential, we demonstrate in this paper a fiber laser based on RDFB operating at a 1 μm spectral band, which has not been studied before. Among the currently reported RFLs, there are mainly three kinds of system structures. The most widely recognized structure is based on the original report [5], in which no extra gain regime is introduced, except for the Raman gain and RDFB provided by the passive fiber. Another kind of “half-opened” cavity structure was proposed by adding a high-reflectivity (HR) FBG to one side of the random cavity, in order to reduce the laser threshold [8,10,19]. There is also a kind of implementation based on a combination of active and passive fiber, and RDFB fiber laser structures based on a combination of Er-doped fiber and SMF were reported [17,20]. In this paper, we adopt the third kind of structure to propose tunable lasing around 1 μm based on Yb-doped fiber (YDF) and RDFB from SMF. Here we intend to use the YDF instead of Raman gain to generate a 1 μm RDFB fiber laser, which has more potential in power scaling. The generated RDFB fiber laser can be tuned continuously from 1040 to 1090 nm (a 50 nm tuning range), while the measured threshold for pumping power is around 1.5 W. Clear signs for the modeless characteristics can be seen both from the spectrum and the time domain. The pump efficiency is about 20% when the stable 1 W laser output is met. 2. Experimental Setup
The experimental setup for a tunable 1 μm YDF fiber laser based on RDFB is shown in Fig. 1. The pump source is a 976 nm laser diode (laser diode), which has a maximum output power of around 10 W. The pumping light is injected into the gain medium of a 5-m-long Yb-doped double-clad fiber whose absorption coefficient is 4 dB/m, through the common port of a 2 1 × 1 pump combiner using the cladding– pump method. The filter included in the setup is made out of a ruling fiber Bragg grating and can be tuned from 1020 to 1090 nm through a mechanical knob. A wideband 50/50 coupler is placed between the filter and the YDF. The common port of the
Fig. 1. Schematic diagram of experimental setup. P, port; YDF, Yb-doped fiber; TFB, 2 1 × 1 pump combiner; SMF, singlemode fiber; RDFB, random distributed feedback; OSA, optical spectrum analyzer; OPM, optical power meter; OSC, oscilloscope.
coupler link with the YDF and the two 50% output ports connect both sides of the filter, forming a kind of “tunable grating.” The signal port of the combiner joins a 1-km-long passive fiber (SMF with a core diameter of 6 μm) acting as a “random mirror” to provide feedback via backward Rayleigh scattering. Thanks to the half-cavity structure, the threshold pump power is reduced. Laser output is measured from the other port of the SMF by using an optical spectrum analyzer, an oscilloscope, and an optical power meter. All the free ends of the cavity are angle cleaved to 8° in case of Fresnel reflection. Gain is generated through population inversion of the Yb ions as the pumping light is injected into the doped fiber. The laser output rises from distributed Rayleigh feedback in the long SMF and is amplified by the YDF gain. When passing through the “tunable grating,” the spectrum is reflected selectively. More and more resonances occur when the YDF provides efficient gain with increasing pump laser, which overcomes the total loss inside the cavity. By turning the mechanical knob on the filter, we can tune the output laser within a certain wide range. 3. Results and Discussion
The system begins to generate laser nearly above a 1 μm wavelength when the pump power is high enough. Figure 2 shows the output power dependence of the generated RDFB fiber laser on the pump power. A clear power reduction of the output laser around a 1040 nm wavelength is observed. The reason may be the relatively long YDF, which causes reabsorption of the laser operating at 1040 nm and results in an efficiency decrease. Most laser generations up to the 1 W level have threshold pump values around 1.5 W and a slope efficiency of ∼25% when tuning from 1050 to 1090 nm. The laser output spectra are measured when the output wavelength is tuned with a step of 10 nm from 1040 to 1090 nm, as shown in Fig. 3, where the pump power values are 3.11, 3.69, and 5.61 W, respectively. The FWHM linewidth is as narrow as ∼0.4 nm, which is narrower than that generated by Raman gain, which is often several nanometers [5]. We attribute the narrow linewidth to the combined effect of the tunable filter and the coupler. Although the coupler is the broadband type, it still exhibits selectivity on the transmission spectrum. The tunable
Fig. 2. Laser output power dependence on pump power, while tuning in 10 nm steps from 1040 to 1090 nm. 1 February 2015 / Vol. 54, No. 4 / APPLIED OPTICS
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Fig. 4. Output spectra of a laser working at 1060 nm under different pump power values.
propagation. When the RDFB fiber laser is working near the threshold pump power, massive burrs occur on the output spectrum. And when the pump power increases, the spectrum becomes smooth and stable. The flattening process in Fig. 4 agrees with the spectral characteristic of random laser generation reported in previous literature [17]. And thanks to the absence of longitudinal modes caused by RDFB
Fig. 3. Laser spectra of different tuning wavelengths measured from pump power (a) 3.11 W, (b) 3.69 W, and (c) 5.61 W.
filter functions as a HR FBG in this half-opened cavity structure. It also introduces a restricted spectral linewidth according to its operating mechanism. Analyzing the output spectra above, we see the maximum power fluctuation is only 1 dB, which is very stable in a wide wavelength range. It is worth mentioning that amplified spontaneous emission (ASE) occurs when the tuning wavelength is around 1040 nm or below this level. Self-Q-switching takes place because of the combination of stimulated Brillouin scattering (SBS), stimulated Raman scattering (SRS), and ASE effects and is related to the intracavity loss and feedback strength [21]. The occurring ASE action combined with the SBS effect causes the emergence of following spikes and the reduction in power output [22]. Fixing the wavelength at 1060 nm, we measure the output spectra of the laser from different pump power values, and the result is shown in Fig. 4. As the pump power increases, a clear flattening pattern can be observed as well as the nonlinear spectral broadening effect caused by the long SMF fiber 910
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Fig. 5. Normalized time domain when pump power is (a) 1.5 W (threshold), (c) 2.44 W, (e) 3.11 W, (g) 5.61 W and corresponding Fourier transform image when pump power is (b) 1.5 W (threshold), (d) 2.44 W, (f) 3.11 W, (h) 5.61 W.
lasing, no self-induced laser line sweeping occurs without intracavity mode interference [23]. We also measure the time domain characteristics of the laser output through an oscilloscope with a bandwidth of 1 GHz and a detector with a bandwidth of 5 GHz, while keeping the laser wavelength at 1060 nm. Figure 5 shows the normalized time domain and corresponding Fourier transform image when the pump power changes from the threshold (1.5 W) to a higher level. From the above time domain characteristics and Fourier transform images, we find the varying pattern from obvious fluctuations to smooth features when the pump power exceeds the threshold value and continues to increase. The temporal behavior corresponds with the characteristics proposed in previous literature [5,10]. The stable time domain indicates that no self-pulsing or multimode interactions take place in the RDFB laser structure when no extra end face feedback is introduced [24]. Recent research suggests that the observed improved power equalization between the generated lines in the RDFB cavity is certainly defined by unique features of Rayleighscattering-based RDFB: spectral selectivity with near-Gaussian statistics [22] in the absence of longitudinal modes [5]. According to this and also based on the previous results, the observed modeless characteristics that can suppress multimode interactions on both the spectra and the time domain can help demonstrate the generation of a RDFB fiber laser. 4. Conclusion
In this paper, we introduced a tunable YDF laser based on RDFB working near a 1 μm wavelength. The use of YDF as a gain medium instead of other rare-earth-doped fibers or Raman gain has resulted in good performance with high efficiency, narrow linewidth, and a wide tunable range. The laser can be tuned continuously from 1040 to 1090 nm with wattlevel output. The ability to maintain good flatness throughout the 50 nm tuning range with maximum power fluctuations within 1 dB shows good power scaling potential for meeting a higher power level. The next step of our research should be focusing on generating a higher-power RDFB fiber laser while maintaining stable operation and a large tuning range. This work was supported by the National Natural Science Foundation of China under Grant No. 61322505 and Program for New Century Excellent Talents in University. References 1. D. S. Wiersma, “The physics and applications of random lasers,” Nat. Phys. 4, 359–367 (2008). 2. D. S. Wiersma, M. P. van Albada, and A. Lagendijk, “Random laser?” Nature 373, 203–204 (1995). 3. H. Cao, “Review on latest developments in random lasers with coherent feedback,” J. Phys. A 38, 10497–10535 (2005). 4. B. Redding, M. A. Choma, and H. Cao, “Speckle-free laser imaging using random laser illumination,” Nat. Photonics 6, 355–359 (2012).
5. S. Turitsyn, S. Babin, A. E. El-Taher, P. Harper, D. V. Churkin, S. I. Kablukov, J. D. Ania-Castañón, V. Karalekas, and E. V. Podivilov, “Random distributed feedback fiber laser,” Nat. Photonics 4, 231–235 (2010). 6. I. D. Vatnik, D. V. Churkin, and S. A. Babin, “Power optimization of random distributed feedback fiber lasers,” Opt. Express 20, 28033–28038 (2012). 7. H. Zhang, P. Zhou, H. Xiao, and X. Xu, “Efficient Raman fiber laser based on random Rayleigh distributed feedback with record high power,” Laser Phys. Lett. 11, 075104 (2014). 8. H. Zhang, H. Xiao, P. Zhou, X. Wang, and X. Xu, “Random distributed feedback Raman fiber laser with short cavity and its temporal properties,” IEEE Photon. Technol. Lett. 26, 1605–1608 (2014). 9. Z. Wang, H. Wu, M. Fan, L. Zhang, Y. Rao, W. Zhang, and X. Jin, “High power random fiber laser with short cavity length: theoretical and experimental investigations,” IEEE J. Sel. Top. Quantum Electron. 21, 900506 (2015). 10. D. V. Churkin, S. A. Babin, A. E. El-Taher, P. Harper, S. I. Kablukov, V. Karalekas, J. D. Ania-Castanon, E. V. Podivilov, and S. K. Turitsyn, “Raman fiber lasers with a random distributed feedback based on Rayleigh scattering,” Phys. Rev. A 82, 033828 (2010). 11. S. A. Babin, E. I. Dontsova, and S. I. Kablukov, “Random fiber laser directly pumped by a high-power laser diode,” Opt. Lett. 38, 3301–3303 (2013). 12. A. E. El-Taher, P. Harper, S. A. Babin, D. V. Churkin, E. V. Podivilov, J. D. Ania-Castanon, and S. K. Turitsyn, “Effect of Rayleigh-scattering distributed feedback on multiwavelength Raman fiber laser generation,” Opt. Lett. 36, 130–132 (2011). 13. A. M. R. Pinto, O. Frazao, J. L. Santos, and M. Lopez-Amo, “Multiwavelength Raman fiber lasers using Hi-Bi photonic crystal fiber loop mirrors combined with random cavities,” J. Lightwave Technol. 29, 1482–1488 (2011). 14. Y. Y. Zhu, W. L. Zhang, Y. J. Rao, Z. N. Wang, and X. H. Jia, “Tunable multi-wavelength fiber laser based on random Rayleigh back-scattering,” IEEE Photon. Technol. Lett. 25, 1559–1561 (2013). 15. M. Peng, X. Bao, and L. Chen, “Observation of narrow linewidth spikes in the coherent Brillouin random fiber laser,” Opt. Lett. 38, 1866–1868 (2013). 16. S. Sugavanam, N. Taraso, X. Shu, and D. V. Churkin, “Narrowband generation in random distributed feedback fiber laser,” Opt. Express 21, 16466–16472 (2013). 17. L. Wang, X. Dong, P. P. Shum, and H. Su, “Tunable erbiumdoped fiber laser based on random distributed feedback,” IEEE Photon. J. 6, 1501705 (2014). 18. S. A. Babin, A. E. El-Taher, P. Harper, E. V. Podivilov, and S. K. Turitsyn, “Tunable random fiber laser,” Phys. Rev. A 84, 21801–21805 (2011). 19. W. L. Zhang, Y. J. Rao, J. M. Zhu, Z. X. Yang, Z. N. Wang, and X. H. Jia, “Low threshold 2nd-order random lasing of a fiber laser with a half-opened cavity,” Opt. Express 20, 14400– 14405 (2012). 20. W. L. Zhang, S. W. Li, R. Ma, Y. J. Rao, Y. Y. Zhu, Z. N. Wang, X. H. Jia, and J. Li, “Random distributed feedback fiber laser based on combination of Er-doped fiber and single-mode fiber,” IEEE J. Sel. Top. Quantum Electron. 21, 900406 (2015). 21. A. V. Kir’yanov, Y. O. Barmenkov, and M. V. Andres, “An experimental analysis of self-Q-switching via stimulated Brillouin scattering in an ytterbium doped fiber laser,” Laser Phys. Lett. 10, 055112 (2013). 22. A. A. Fotiadi and R. V. Kiyan, “Cooperative stimulated Brillouin and Rayleigh backscattering process in optical fiber,” Opt. Lett. 23, 1805–1807 (1998). 23. A. V. Kir’yanov and N. N. Il’ichev, “Self-induced laser line sweeping in an ytterbium fiber laser with non-resonant Fabry-Perot cavity,” Laser Phys. Lett. 8, 305–312 (2011). 24. D. Toral-Acosta, A. Martinez-Rios, R. Selvas-Aguilar, A. V. Kir’yanov, G. Anzueto-Sanchez, and V. M. Duran-Ramirez, “Self-pulsing in a large mode area, end-pumped, doubleclad ytterbium-doped fiber laser,” Laser Phys. 24, 105107 (2014).
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A tunable random distributed feedback (RDFB) fiber laser operating at 1 μm is presented, which consists of a piece of ytterbium-doped fiber, a manual tunable filter and a 1-km-long single-mode fiber that provides RDFB. The wavelength of the RDFB fiber laser can be tuned continuously from 1040 to 1090 nm with a linewidth of about 0.4 nm. Power output maintains good flatness in the 50 nm tuning range, with fluctuations within 1 dB. It is the first reported tunable RDFB fiber laser working at 1 μm as far as we know. © 2015 Optical Society of America OCIS codes: (140.3600) Lasers, tunable; (140.3510) Lasers, fiber; (290.5870) Scattering, Rayleigh; (140.3490) Lasers, distributed-feedback. http://dx.doi.org/10.1364/AO.54.000908
1. Introduction
The concept of a random laser has drawn a great deal of attention in recent years, as random lasers are able to generate coherent light without a welldefined cavity by exploiting multiple scattering in an amplifying disordered medium [1]. Random lasers have a range of potential applications including medical applications, compact light sources, spectroscopic monitoring devices, and illumination materials [2–4]. Different types of random lasers differ due to the material choice for the disordered medium. In 2010, a random distributed feedback (RDFB) fiber laser based on Raman amplification and distributed Rayleigh scattering feedback in a single-mode fiber (SMF) was reported [5]. Fiber is now a remarkable and practical realization of a disordered medium providing positive feedback in the random laser regime. Random fiber lasers (RFLs) with various features, such as high-power laser generation [6–9], noise and gain optimization [10,11], multiwavelength
1559-128X/15/040908-04$15.00/0 © 2015 Optical Society of America 908
APPLIED OPTICS / Vol. 54, No. 4 / 1 February 2015
generation [12–14], narrow linewidth outputs [15,16] and tunable lasers [17,18], have been demonstrated. A tunable RFL or multiwavelength generation takes advantage of the modeless characteristics that suppress multimode interactions and the relatively wide gain bandwidth in RFLs, which can be used in imaging, communications, sensing, and other fields. Current research has made certain achievements in the tunable random fiber laser field. Babin et al. introduced a scheme of a tunable RDFB fiber laser by implementing a fiber-pigtailed tunable filter in the middle of the symmetric configuration and acquired ∼2.2 W laser output with 30% efficiency and a 40 nm tuning range [18]. Tunable multiwavelength generation concerning the spectral width of each individual line was also demonstrated in [14], where a Mach–Zehnder interferometer based on two longperiod fiber Bragg gratings (FBGs) and a Fabry– Perot filter was used to generate a multiwavelength laser with a tuning range of ∼10 nm and an individual line spectral width of 0.034 nm. Most recently, Wang et al. proposed a tunable Er-doped fiber laser based on RDFB backward Rayleigh scattering in a 20 km SMF with a tunable fiber Fabry–Perot interferometer [17], which realized a tuning range of
about 40 nm with a linewidth of ∼0.04 nm and pump efficiency of 14%. The above setups all demonstrated random laser generation in 1.5 μm spectral bands. It is likely that the same lasing concept can be realized in a broader spectral range. Since a fiber laser working at 1 μm possesses the capability of free communication via atmospheric windows and high-power laser output potential, we demonstrate in this paper a fiber laser based on RDFB operating at a 1 μm spectral band, which has not been studied before. Among the currently reported RFLs, there are mainly three kinds of system structures. The most widely recognized structure is based on the original report [5], in which no extra gain regime is introduced, except for the Raman gain and RDFB provided by the passive fiber. Another kind of “half-opened” cavity structure was proposed by adding a high-reflectivity (HR) FBG to one side of the random cavity, in order to reduce the laser threshold [8,10,19]. There is also a kind of implementation based on a combination of active and passive fiber, and RDFB fiber laser structures based on a combination of Er-doped fiber and SMF were reported [17,20]. In this paper, we adopt the third kind of structure to propose tunable lasing around 1 μm based on Yb-doped fiber (YDF) and RDFB from SMF. Here we intend to use the YDF instead of Raman gain to generate a 1 μm RDFB fiber laser, which has more potential in power scaling. The generated RDFB fiber laser can be tuned continuously from 1040 to 1090 nm (a 50 nm tuning range), while the measured threshold for pumping power is around 1.5 W. Clear signs for the modeless characteristics can be seen both from the spectrum and the time domain. The pump efficiency is about 20% when the stable 1 W laser output is met. 2. Experimental Setup
The experimental setup for a tunable 1 μm YDF fiber laser based on RDFB is shown in Fig. 1. The pump source is a 976 nm laser diode (laser diode), which has a maximum output power of around 10 W. The pumping light is injected into the gain medium of a 5-m-long Yb-doped double-clad fiber whose absorption coefficient is 4 dB/m, through the common port of a 2 1 × 1 pump combiner using the cladding– pump method. The filter included in the setup is made out of a ruling fiber Bragg grating and can be tuned from 1020 to 1090 nm through a mechanical knob. A wideband 50/50 coupler is placed between the filter and the YDF. The common port of the
Fig. 1. Schematic diagram of experimental setup. P, port; YDF, Yb-doped fiber; TFB, 2 1 × 1 pump combiner; SMF, singlemode fiber; RDFB, random distributed feedback; OSA, optical spectrum analyzer; OPM, optical power meter; OSC, oscilloscope.
coupler link with the YDF and the two 50% output ports connect both sides of the filter, forming a kind of “tunable grating.” The signal port of the combiner joins a 1-km-long passive fiber (SMF with a core diameter of 6 μm) acting as a “random mirror” to provide feedback via backward Rayleigh scattering. Thanks to the half-cavity structure, the threshold pump power is reduced. Laser output is measured from the other port of the SMF by using an optical spectrum analyzer, an oscilloscope, and an optical power meter. All the free ends of the cavity are angle cleaved to 8° in case of Fresnel reflection. Gain is generated through population inversion of the Yb ions as the pumping light is injected into the doped fiber. The laser output rises from distributed Rayleigh feedback in the long SMF and is amplified by the YDF gain. When passing through the “tunable grating,” the spectrum is reflected selectively. More and more resonances occur when the YDF provides efficient gain with increasing pump laser, which overcomes the total loss inside the cavity. By turning the mechanical knob on the filter, we can tune the output laser within a certain wide range. 3. Results and Discussion
The system begins to generate laser nearly above a 1 μm wavelength when the pump power is high enough. Figure 2 shows the output power dependence of the generated RDFB fiber laser on the pump power. A clear power reduction of the output laser around a 1040 nm wavelength is observed. The reason may be the relatively long YDF, which causes reabsorption of the laser operating at 1040 nm and results in an efficiency decrease. Most laser generations up to the 1 W level have threshold pump values around 1.5 W and a slope efficiency of ∼25% when tuning from 1050 to 1090 nm. The laser output spectra are measured when the output wavelength is tuned with a step of 10 nm from 1040 to 1090 nm, as shown in Fig. 3, where the pump power values are 3.11, 3.69, and 5.61 W, respectively. The FWHM linewidth is as narrow as ∼0.4 nm, which is narrower than that generated by Raman gain, which is often several nanometers [5]. We attribute the narrow linewidth to the combined effect of the tunable filter and the coupler. Although the coupler is the broadband type, it still exhibits selectivity on the transmission spectrum. The tunable
Fig. 2. Laser output power dependence on pump power, while tuning in 10 nm steps from 1040 to 1090 nm. 1 February 2015 / Vol. 54, No. 4 / APPLIED OPTICS
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Fig. 4. Output spectra of a laser working at 1060 nm under different pump power values.
propagation. When the RDFB fiber laser is working near the threshold pump power, massive burrs occur on the output spectrum. And when the pump power increases, the spectrum becomes smooth and stable. The flattening process in Fig. 4 agrees with the spectral characteristic of random laser generation reported in previous literature [17]. And thanks to the absence of longitudinal modes caused by RDFB
Fig. 3. Laser spectra of different tuning wavelengths measured from pump power (a) 3.11 W, (b) 3.69 W, and (c) 5.61 W.
filter functions as a HR FBG in this half-opened cavity structure. It also introduces a restricted spectral linewidth according to its operating mechanism. Analyzing the output spectra above, we see the maximum power fluctuation is only 1 dB, which is very stable in a wide wavelength range. It is worth mentioning that amplified spontaneous emission (ASE) occurs when the tuning wavelength is around 1040 nm or below this level. Self-Q-switching takes place because of the combination of stimulated Brillouin scattering (SBS), stimulated Raman scattering (SRS), and ASE effects and is related to the intracavity loss and feedback strength [21]. The occurring ASE action combined with the SBS effect causes the emergence of following spikes and the reduction in power output [22]. Fixing the wavelength at 1060 nm, we measure the output spectra of the laser from different pump power values, and the result is shown in Fig. 4. As the pump power increases, a clear flattening pattern can be observed as well as the nonlinear spectral broadening effect caused by the long SMF fiber 910
APPLIED OPTICS / Vol. 54, No. 4 / 1 February 2015
Fig. 5. Normalized time domain when pump power is (a) 1.5 W (threshold), (c) 2.44 W, (e) 3.11 W, (g) 5.61 W and corresponding Fourier transform image when pump power is (b) 1.5 W (threshold), (d) 2.44 W, (f) 3.11 W, (h) 5.61 W.
lasing, no self-induced laser line sweeping occurs without intracavity mode interference [23]. We also measure the time domain characteristics of the laser output through an oscilloscope with a bandwidth of 1 GHz and a detector with a bandwidth of 5 GHz, while keeping the laser wavelength at 1060 nm. Figure 5 shows the normalized time domain and corresponding Fourier transform image when the pump power changes from the threshold (1.5 W) to a higher level. From the above time domain characteristics and Fourier transform images, we find the varying pattern from obvious fluctuations to smooth features when the pump power exceeds the threshold value and continues to increase. The temporal behavior corresponds with the characteristics proposed in previous literature [5,10]. The stable time domain indicates that no self-pulsing or multimode interactions take place in the RDFB laser structure when no extra end face feedback is introduced [24]. Recent research suggests that the observed improved power equalization between the generated lines in the RDFB cavity is certainly defined by unique features of Rayleighscattering-based RDFB: spectral selectivity with near-Gaussian statistics [22] in the absence of longitudinal modes [5]. According to this and also based on the previous results, the observed modeless characteristics that can suppress multimode interactions on both the spectra and the time domain can help demonstrate the generation of a RDFB fiber laser. 4. Conclusion
In this paper, we introduced a tunable YDF laser based on RDFB working near a 1 μm wavelength. The use of YDF as a gain medium instead of other rare-earth-doped fibers or Raman gain has resulted in good performance with high efficiency, narrow linewidth, and a wide tunable range. The laser can be tuned continuously from 1040 to 1090 nm with wattlevel output. The ability to maintain good flatness throughout the 50 nm tuning range with maximum power fluctuations within 1 dB shows good power scaling potential for meeting a higher power level. The next step of our research should be focusing on generating a higher-power RDFB fiber laser while maintaining stable operation and a large tuning range. This work was supported by the National Natural Science Foundation of China under Grant No. 61322505 and Program for New Century Excellent Talents in University. References 1. D. S. Wiersma, “The physics and applications of random lasers,” Nat. Phys. 4, 359–367 (2008). 2. D. S. Wiersma, M. P. van Albada, and A. Lagendijk, “Random laser?” Nature 373, 203–204 (1995). 3. H. Cao, “Review on latest developments in random lasers with coherent feedback,” J. Phys. A 38, 10497–10535 (2005). 4. B. Redding, M. A. Choma, and H. Cao, “Speckle-free laser imaging using random laser illumination,” Nat. Photonics 6, 355–359 (2012).
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