Volume Bragg grating narrowed high-power and highly efficient cladding-pumped Raman fiber laser Jun Liu,1,2 Weichao Yao,1 Chujun Zhao,3 Deyuan Shen,1,* and Dianyuan Fan4 1
School of Physics and Electronic Engineering, Jiangsu Normal University, Xuzhou 221116, China
2
SZU-NUS Collaborative Innovation Center for Optoelectronic Science & Technology, Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China 3 4
College of Physics and Microelectronic Science, Hunan University, Changsha 410082, China
Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China *Corresponding author: [email protected] Received 9 September 2014; revised 30 October 2014; accepted 31 October 2014; posted 4 November 2014 (Doc. ID 222671); published 3 December 2014
High-power and highly efficient operation of a single-mode cladding-pumped Raman fiber laser with narrow lasing bandwidth is demonstrated. The spectral narrowing was realized by an external cavity containing a volume Bragg grating with a center wavelength of 1658 nm. A maximum output power of 10.4 W at 1658.3 nm with a spectral linewidth (FWHM) of ∼0.1 nm was obtained for the launched pump power of 18.4 W, corresponding to a slope efficiency of 109% with respect to the launched pump power. Lasing characteristics of free-running operation are also evaluated and discussed. © 2014 Optical Society of America OCIS codes: (140.0140) Lasers and laser optics; (140.3510) Lasers, fiber; (140.3550) Lasers, Raman; (140.3570) Lasers, single-mode. http://dx.doi.org/10.1364/AO.53.008256
1. Introduction
Raman fiber laser sources, benefiting from the broad Raman gain spectrum, are much more wavelengthagile than their counterparts based on rare-earth (RE) doped fibers. Typically, laser emissions in the 1.62–1.72 μm eye-safe wavelength range, which cannot be covered by the traditional RE-doped fiber lasers, can be easily achieved through the stimulated Raman scattering (SRS) effect in silica fibers [1–5]. However, for many applications the requirement for considerable flexibility in operating wavelength is also accompanied by the need for high-efficiency and high output-power together with a good beam quality, which is often difficult to achieve in conventional 1559-128X/14/358256-05$15.00/0 © 2014 Optical Society of America 8256
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core-pumped single-mode Raman devices with a relatively small core owing to the lack of high-power single-mode pump sources. Despite the pump power, limitation can be alleviated by combining multiple single-mode pumps to some extent, but the ultimate lasing performance sometimes suffers from the nonlinear effects and optical damages instead. An alternative to core-pumping Raman fiber lasers is to use multi-mode gain fiber which can be pumped with high-power low-brightness laser sources. Favorably, the beam clean-up effect of SRS in multi-mode fibers (i.e., fundamental or lower-order transverse mode of the Stokes beam experiences preferential amplification relative to neighboring modes in a multimode fiber) was experimentally demonstrated. A near diffraction-limited Stokes laser beam in Raman fiber lasers based on multi-mode fibers was also achieved [6–8]. Nevertheless, the degree of beam cleanup
experienced by the Stokes beam is highly sensitive to the overlap between the pump beam and the lowerorder modes of the fiber [6]. Thus this advantageous property depends greatly on the appropriate modal excitation of fundamental mode in the multi-mode fiber and needs fine adjustment and adequate environmental stability. In addition, the laser-beam quality will be degraded increasingly with the outputpower level, which further limits the power scaling. Cladding-pumping of Raman fiber lasers offers a promising route to allow scaling to high power levels without decreasing efficiency or degrading beam quality. A single-mode operation of cladding-pumped Raman fiber lasers was demonstrated at ∼1.6 and ∼1 μm in [1] and [9], respectively. In the claddingpumping regime, the power injected into the fundamental mode of the Stokes can be excited and controlled more accurately and easily through the confinement of the specific single-mode core relative to the large multi-mode core structures. The lasing spectral bandwidths in both experiments were larger than ∼2 nm for output power of several watt levels due to the lack of an efficient spectrum narrowing element and the strong nonlinear effects such as four-wave mixing resulting from the high intensity in the fiber core. In this paper we report on a high-power and highly efficient operation of a single-mode cladding-pumped Raman fiber laser. By optimizing the Raman gain fiber length, a maximum output power of 10.4 W at 1658.3 nm with a spectral linewidth (FWHM) of ∼0.1 nm was obtained, corresponding to a slope efficiency of 109% with respect to the launched pump power. A volume Bragg grating (VBG), which used widely efficient wavelength selection components in high-power fiber lasers [10–12], was employed to act as a one cavity mirror in the cladding-pumped Raman fiber laser and to narrow the Stokes laser output spectrum simultaneously. To the best of our knowledge this is the narrowest linewidth with the highest efficiency from cladding-pumped Raman fiber lasers. 2. Experiment and Results
Considering the particular interest for CO2 sensing applications combined with a potential pump source for optical parametric oscillators in the dual midinfrared lasing scheme where the signal wavelength of ∼3 μm and idler wavelength of ∼4 μm can be generated simultaneously [13], the cladding-pumped Raman fiber lasers were designed to operate at ∼1658 nm. Therefore, we need a pump source operating at 1545 nm to excite the first Stokes at 1658 nm in the germanosilicate fiber with the Raman frequency shift peaked at 440 cm−1 . The double-clad Raman fiber (DCRF) was fabricated at CorActive using a standard modified chemical-vapor deposition process. This fiber had a concentric germanosilicate core and inner cladding with different germanium concentration to enhance the Raman gain in the core section, surrounded by a
circular pure silica out cladding. The core and inner cladding diameter is 11 and 26 μm, with the corresponding numerical aperture of 0.14 and 0.22, respectively. To guarantee an efficient pump to first-order Stokes power conversion and to suppress the secondorder Stokes emission simultaneously, the DCRF design satisfied the area ratio limitation relation between the inner cladding and core dimensions [14]. The core has an estimated cutoff wavelength of 1630 nm. The measured propagation losses in the core and inner cladding at the pump wavelength of 1545 nm are 0.63 and 0.55 dB/km, respectively. The experimental schematic diagram is shown in Fig. 1. The pump source was an in-house constructed Er,Yb co-doped fiber laser, the operating wavelength of which was locked at 1545 nm using a VBG. More details about the pump source can be found in [15]. The output beam from the pump source was freespace launched into the inner cladding of the DCRF with the aid of a set of lenses. Both end sections of the DCRF were carefully mounted in water-cooled V-groove heat sinks, and the remaining part was coiled on a fan-cooled disk. The Raman laser cavity consists of a perpendicularly cleaved fiber-end facet (providing 3.6% Fresnel reflection and acting as the output coupler) and, at the opposite end, by a simple external cavity comprising an antireflection-coated collimating lens and a high-reflection (HR) plane mirror over the band from 1500 to 1700 nm or a VBG. Different external cavity configurations were employed depending on the desired mode of operation (i.e., free-running or fixed operating wavelength). The Stokes laser output was extracted by means of a dichroic mirror with high transmission at the pump wavelength and high reflectivity over the 1.6–1.8 μm band aligned at a small angle tilt between the coupling lenses. In the free-running (i.e., without wavelength selection) operation regime, we first evaluated the lasing characteristics of Raman fiber lasers with different gain fiber lengths and thus determined the optimized Raman fiber length. The HR plane mirror in the external cavity acted as one cavity mirror of the lasing feedback in the free-running configuration. Stokes laser output powers as a function of the launched pump power for different Raman fiber lengths are shown in Fig. 2. We can see that the 1.2 km Raman fiber laser had the best lasing performance in terms of output power and slope efficiency (defined as the slope of the laser output-power curve
Fig. 1. Experimental schematic of cladding-pumped free-running and fixed-wavelength Raman fiber lasers. DCRF, double-clad Raman fiber; HR/VBG, high reflection or volume Bragg grating; EYDFL, Er,Yb co-doped fiber laser; DM, dichroic mirror. 10 December 2014 / Vol. 53, No. 35 / APPLIED OPTICS
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Fig. 2. Output power as a function of launched pump power for different Raman fiber lengths.
versus the launched pump power). A maximum output power of 13.2 W at ∼1658 nm with a slope efficiency of 120% was obtained for the launched pump power of 18.4 W. The corresponding opticalto-optical conversion efficiency is 71.7%. In the experiment, the Raman fiber laser with longer effective fiber length tends to have a lower pump threshold, which is in good agreement with our simulated results. The Stokes laser output spectra for the freerunning Raman fiber lasers with different fiber lengths, shown in Fig. 3, were measured at the maximum launched pump power of 18.4 W using an optical spectrum analyzer (AQ6375, Yokogawa). The output spectra are all composed of two discrete broadband peaks with center wavelengths located at ∼1658 and ∼1671 nm, corresponding to the two Raman gain peaks of germanosilicate fibers with a Raman frequency shift of 440 and 490 cm−1 , respectively. Actually, the laser output spectrum only exhibits the single 440 cm−1 high-frequency Raman peak (at ∼1658 nm) when the first-order Stokes begins to oscillate. The first-order Stokes output power at ∼1658 nm starts to increase with the pump power.
Fig. 3. Output spectra recorded at maximum launched pump power of 18.4 W for different Raman fiber lengths. 8258
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While the laser emission at ∼1658 nm grows powerful enough, the 490 cm−1 low-frequency Raman peak will be amplified through the Raman amplification process with the ∼1658 nm laser emission acting as the pump. It is expected that the 490 cm−1 lowfrequency Raman peak (at ∼1671 nm) will dominate in the first Stokes laser spectrum when the output power increases to as much as several tens of watts. In addition, for the Raman fiber laser with the longest fiber length (2.1 km), the second-order Stokes signal at ∼1790 nm appears in the output spectrum due to the low lasing threshold. According to the lasing performance in terms of output power and slope efficiency as a function of the Raman fiber length, the 1.2 km DCRF was selected as the gain fiber in the fixed-wavelength operation regime. A reflective VBG was included in the external cavity in place of the HR plane mirror to lock the operating wavelength at the 440 cm−1 Raman gain peak. The VBG (Optigrate Corp.) was designed to have a center wavelength of 1658 nm with peak reflectivity of 99.9% and spectral selectivity of 0.33 nm. It had a thickness of 9 mm with its clear aperture of 5 × 5 mm2 and was mounted in a copper heat sink with a layer of indium foil (0.1 mm in thickness) to ensure good heat dissipation. The focal length of the collimating lens in the external cavity was chosen to be 15 mm to make full use of the VBG aperture. In addition, the fiber end adjacent to the external cavity was angle cleaved at ∼8° to suppress the broadband feedback and second Stokes generation from the two uncoated fiber facets. Stokes laser output power from the 1.2 km fixedwavelength Raman fiber laser as a function of launched pump power is shown with squares in the Fig. 4. A maximum output power of 10.4 W at 1658.3 nm was generated for the launched pump power of 18.4 W, corresponding to a slope and conversion efficiency of 109% and 56.5% with respect to the launched pump power, respectively. The lasing performance of the 1.2 km free-running Raman fiber laser is also shown in Fig. 4 for comparison. It can be seen
Fig. 4. Output powers as a function of launched pump power in free-running and fixed-wavelength operating regimes for the 1.2 km fiber.
Fig. 5. Beam propagation and intensity profile (far field) in fixed wavelength operating regime.
that the relatively higher output power and slope efficiency achieved in the free-running Raman fiber laser for the same launched pump power is attributed to the higher pump intensity in the fiber core resulting from the residual pump being retro-reflected back into the cavity by the HR mirror after a single pass. It is worth noting that both configurations have exhibited a very high slope efficiency of larger than ∼1 at the powers we measured owing to the large Raman gain in the custom DCRF. However, it should be expected that the slope efficiencies are going to moderate and rollover due to pump depletion at higher pump power, which is characteristic of the nonlinear nature of the SRS [3]. The output powers were all linearly dependent on the launched pump power, suggesting that there is considerable scope to scale up the output power by simply increasing the pump power. The maximum output-power fluctuations remained below ∼2%, which is attributed to the external disturbances such as mechanical vibration. The laserbeam quality for the fixed-wavelength Raman fiber laser at the maximum output power was measured by a beam profiler (Nanoscan, Photo Inc.). Figure 5 shows the measured beam radii at different positions along the beam propagation direction. By fitting the measured data with a hyperbolic curve, the beam propagation factors (M2 ) were calculated to be ∼1.22 and ∼1.23 in the x and y directions, respectively. The inserted image in Fig. 5 is the two-dimensional beam profile of the laser output, which is neardiffraction-limited. The output spectrum for the fixed-wavelength Raman fiber laser was measured and shown in Fig. 6, which was much narrower with a FWHM bandwidth of ∼0.1 nm recorded at a resolution of 0.05 nm compared with that of the free-running counterpart. It is worth noting that the lasing linewidth was also narrower than the spectral selectivity of 0.33 nm of the VBG we used. This can be explained by the mode competition in the cavity due to the unflat-top reflectivity curve of the VBG, as a result of which only the wavelengths with lowest loss survived. In addition, no second-order Stokes signal was observed in the
Fig. 6. Change 2st to 2nd in fig. 6. Laser output spectrum of the 1.2 km fixed-wavelength Raman fiber laser in comparison with that of the free-running counterpart. Inset: the free-running output spectrum in logarithmic scale.
fixed-wavelength operation regime even at the maximum pump power; however, there appears to be a signal of second-order Stokes at ∼1790 nm of ∼30 dB lower than the magnitude of the first-order Stokes laser at ∼1658 nm in the free-running output spectrum (see the inset of Fig. 6). 3. Conclusion
In conclusion, we reported on a high-power and highly efficient operation of a single-mode cladding-pumped Raman fiber laser. Lasing characteristics of the freerunning cladding-pumped Raman fiber lasers with different fiber lengths were first evaluated and optimized Raman gain fiber length was obtained. Output power up to 13.2 W with slope efficiency of ∼120% was generated in the free-running configuration, limited only by the launched pump power. By employing a VBG as the spectral narrowing element, the output spectral was narrowed to ∼0.1 nm (FWHM) at 1658.3 nm with a maximum output power of 10.4 W, corresponding to a slope efficiency of 109% with respect to the launched pump power. The claddingpumping regime was proved to be an efficient approach for the output-power scaling of Raman fiber lasers. We greatly appreciate Prof. Y. Feng’s theoretical and experimental guidance from the Research Center of Space Laser Information Technology, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences. This work is supported by the National Natural Science Foundation of China (NSFC 61177045), the Research Fund for the Doctoral Program of Higher Education of China (RFDP: 20110071110016) and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). References 1. C. A. Codemard, P. Dupriez, Y. Jeong, J. K. Sahu, M. Ibsen, and J. Nilsson, “High-power continuous-wave claddingpumped Raman fiber laser,” Opt. Lett. 31, 2290–2292 (2006). 10 December 2014 / Vol. 53, No. 35 / APPLIED OPTICS
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2. J. Liu, D. Shen, H. Huang, C. Zhao, X. Zhang, and D. Fan, “High-power and highly efficient operation of wavelengthtunable Raman fiber lasers based on volume Bragg gratings,” Opt. Express 22, 6605–6612 (2014). 3. Y. Feng, L. R. Taylor, and D. B. Calia, “150 W highlyefficient Raman fiber laser,” Opt. Express 17, 23678–23683 (2009). 4. S. A. Babin, D. V. Churkin, S. I. Kablukov, M. A. Rybakov, and A. A. Vlasov, “All-fiber widely tunable Raman fiber laser with controlled output spectrum,” Opt. Express 15, 8438–8443 (2007). 5. E. M. Dianov, I. A. Bufetov, M. M. Bubnov, M. V. Grekov, S. A. Vasiliev, and O. I. Medvedkov, “Three-cascaded 1407-nm Raman laser based on phosphorus-doped silica fiber,” Opt. Lett. 25, 402–404 (2000). 6. K. S. Chaing, “Stimulated Raman scattering in a multimode optical fiber: evolution of modes in Stokes waves,” Opt. Lett. 17, 352–354 (1992). 7. S. H. Baek and W. B. Roh, “Single-mode Raman fiber laser based on a multimode fiber,” Opt. Lett. 29, 153–155 (2004). 8. N. B. Terry, K. T. Engel, T. G. Alley, and T. H. Russell, “Use of a continuous wave Raman fiber laser in graded-index multimode fiber for SRS beam combination,” Opt. Express 15, 602–607 (2007).
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9. C. A. Codemard, J. Ji, J. K. Sahu, and J. Nilsson, “100 W CW cladding-pumped Raman fiber laser at 1120 nm,” Proc. SPIE 7580, 75801N (2010). 10. J. W. Kim, P. Jelger, J. K. Sahu, F. Laurell, and W. A. Clarkson, “High-power and wavelength-tunable operation of an Er,Yb fiber laser using a volume Bragg grating,” Opt. Lett. 33, 1204–1206 (2008). 11. F. Wang, D. Y. Shen, D. Y. Fan, and Q. S. Lu, “Spectrum narrowing of high power Tm: fiber laser using a volume Bragg grating,” Opt. Express 18, 8937–8941 (2010). 12. X. Zhang, F. Gao, J. Feng, K. Zou, B. Xiong, and X. Yuan, “Wavelength combining with volume Bragg gratings in photo-thermorefractive glasses,” Chin. Opt. Lett. 12, 030901 (2014). 13. M. Wang, L. Zhu, W. Chen, and D. Fan, “Efficient all- solidstate mid-infrared optical parametric oscillator based on resonantly pumped 1.645 μm Er:YAG laser,” Opt. Lett. 37, 2682–2684 (2012). 14. J. Ji, C. A. Codemard, M. Ibsen, J. K. Sahu, and J. Nilsson, “Analysis of the conversion to the first Stokes in claddingpumped fiber Raman amplifiers,” IEEE J. Sel. Top. Quantum Electron. 15, 129–139 (2009). 15. J. Liu, D. Shen, H. Huang, X. Zhang, L. Wang, and D. Fan, “High-power and narrow-linewidth Er, Yb fiber laser locked by a volume Bragg grating-pair,” IEEE J. Quantum Electron. 50, 88–91 (2014).
School of Physics and Electronic Engineering, Jiangsu Normal University, Xuzhou 221116, China
2
SZU-NUS Collaborative Innovation Center for Optoelectronic Science & Technology, Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China 3 4
College of Physics and Microelectronic Science, Hunan University, Changsha 410082, China
Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China *Corresponding author: [email protected] Received 9 September 2014; revised 30 October 2014; accepted 31 October 2014; posted 4 November 2014 (Doc. ID 222671); published 3 December 2014
High-power and highly efficient operation of a single-mode cladding-pumped Raman fiber laser with narrow lasing bandwidth is demonstrated. The spectral narrowing was realized by an external cavity containing a volume Bragg grating with a center wavelength of 1658 nm. A maximum output power of 10.4 W at 1658.3 nm with a spectral linewidth (FWHM) of ∼0.1 nm was obtained for the launched pump power of 18.4 W, corresponding to a slope efficiency of 109% with respect to the launched pump power. Lasing characteristics of free-running operation are also evaluated and discussed. © 2014 Optical Society of America OCIS codes: (140.0140) Lasers and laser optics; (140.3510) Lasers, fiber; (140.3550) Lasers, Raman; (140.3570) Lasers, single-mode. http://dx.doi.org/10.1364/AO.53.008256
1. Introduction
Raman fiber laser sources, benefiting from the broad Raman gain spectrum, are much more wavelengthagile than their counterparts based on rare-earth (RE) doped fibers. Typically, laser emissions in the 1.62–1.72 μm eye-safe wavelength range, which cannot be covered by the traditional RE-doped fiber lasers, can be easily achieved through the stimulated Raman scattering (SRS) effect in silica fibers [1–5]. However, for many applications the requirement for considerable flexibility in operating wavelength is also accompanied by the need for high-efficiency and high output-power together with a good beam quality, which is often difficult to achieve in conventional 1559-128X/14/358256-05$15.00/0 © 2014 Optical Society of America 8256
APPLIED OPTICS / Vol. 53, No. 35 / 10 December 2014
core-pumped single-mode Raman devices with a relatively small core owing to the lack of high-power single-mode pump sources. Despite the pump power, limitation can be alleviated by combining multiple single-mode pumps to some extent, but the ultimate lasing performance sometimes suffers from the nonlinear effects and optical damages instead. An alternative to core-pumping Raman fiber lasers is to use multi-mode gain fiber which can be pumped with high-power low-brightness laser sources. Favorably, the beam clean-up effect of SRS in multi-mode fibers (i.e., fundamental or lower-order transverse mode of the Stokes beam experiences preferential amplification relative to neighboring modes in a multimode fiber) was experimentally demonstrated. A near diffraction-limited Stokes laser beam in Raman fiber lasers based on multi-mode fibers was also achieved [6–8]. Nevertheless, the degree of beam cleanup
experienced by the Stokes beam is highly sensitive to the overlap between the pump beam and the lowerorder modes of the fiber [6]. Thus this advantageous property depends greatly on the appropriate modal excitation of fundamental mode in the multi-mode fiber and needs fine adjustment and adequate environmental stability. In addition, the laser-beam quality will be degraded increasingly with the outputpower level, which further limits the power scaling. Cladding-pumping of Raman fiber lasers offers a promising route to allow scaling to high power levels without decreasing efficiency or degrading beam quality. A single-mode operation of cladding-pumped Raman fiber lasers was demonstrated at ∼1.6 and ∼1 μm in [1] and [9], respectively. In the claddingpumping regime, the power injected into the fundamental mode of the Stokes can be excited and controlled more accurately and easily through the confinement of the specific single-mode core relative to the large multi-mode core structures. The lasing spectral bandwidths in both experiments were larger than ∼2 nm for output power of several watt levels due to the lack of an efficient spectrum narrowing element and the strong nonlinear effects such as four-wave mixing resulting from the high intensity in the fiber core. In this paper we report on a high-power and highly efficient operation of a single-mode cladding-pumped Raman fiber laser. By optimizing the Raman gain fiber length, a maximum output power of 10.4 W at 1658.3 nm with a spectral linewidth (FWHM) of ∼0.1 nm was obtained, corresponding to a slope efficiency of 109% with respect to the launched pump power. A volume Bragg grating (VBG), which used widely efficient wavelength selection components in high-power fiber lasers [10–12], was employed to act as a one cavity mirror in the cladding-pumped Raman fiber laser and to narrow the Stokes laser output spectrum simultaneously. To the best of our knowledge this is the narrowest linewidth with the highest efficiency from cladding-pumped Raman fiber lasers. 2. Experiment and Results
Considering the particular interest for CO2 sensing applications combined with a potential pump source for optical parametric oscillators in the dual midinfrared lasing scheme where the signal wavelength of ∼3 μm and idler wavelength of ∼4 μm can be generated simultaneously [13], the cladding-pumped Raman fiber lasers were designed to operate at ∼1658 nm. Therefore, we need a pump source operating at 1545 nm to excite the first Stokes at 1658 nm in the germanosilicate fiber with the Raman frequency shift peaked at 440 cm−1 . The double-clad Raman fiber (DCRF) was fabricated at CorActive using a standard modified chemical-vapor deposition process. This fiber had a concentric germanosilicate core and inner cladding with different germanium concentration to enhance the Raman gain in the core section, surrounded by a
circular pure silica out cladding. The core and inner cladding diameter is 11 and 26 μm, with the corresponding numerical aperture of 0.14 and 0.22, respectively. To guarantee an efficient pump to first-order Stokes power conversion and to suppress the secondorder Stokes emission simultaneously, the DCRF design satisfied the area ratio limitation relation between the inner cladding and core dimensions [14]. The core has an estimated cutoff wavelength of 1630 nm. The measured propagation losses in the core and inner cladding at the pump wavelength of 1545 nm are 0.63 and 0.55 dB/km, respectively. The experimental schematic diagram is shown in Fig. 1. The pump source was an in-house constructed Er,Yb co-doped fiber laser, the operating wavelength of which was locked at 1545 nm using a VBG. More details about the pump source can be found in [15]. The output beam from the pump source was freespace launched into the inner cladding of the DCRF with the aid of a set of lenses. Both end sections of the DCRF were carefully mounted in water-cooled V-groove heat sinks, and the remaining part was coiled on a fan-cooled disk. The Raman laser cavity consists of a perpendicularly cleaved fiber-end facet (providing 3.6% Fresnel reflection and acting as the output coupler) and, at the opposite end, by a simple external cavity comprising an antireflection-coated collimating lens and a high-reflection (HR) plane mirror over the band from 1500 to 1700 nm or a VBG. Different external cavity configurations were employed depending on the desired mode of operation (i.e., free-running or fixed operating wavelength). The Stokes laser output was extracted by means of a dichroic mirror with high transmission at the pump wavelength and high reflectivity over the 1.6–1.8 μm band aligned at a small angle tilt between the coupling lenses. In the free-running (i.e., without wavelength selection) operation regime, we first evaluated the lasing characteristics of Raman fiber lasers with different gain fiber lengths and thus determined the optimized Raman fiber length. The HR plane mirror in the external cavity acted as one cavity mirror of the lasing feedback in the free-running configuration. Stokes laser output powers as a function of the launched pump power for different Raman fiber lengths are shown in Fig. 2. We can see that the 1.2 km Raman fiber laser had the best lasing performance in terms of output power and slope efficiency (defined as the slope of the laser output-power curve
Fig. 1. Experimental schematic of cladding-pumped free-running and fixed-wavelength Raman fiber lasers. DCRF, double-clad Raman fiber; HR/VBG, high reflection or volume Bragg grating; EYDFL, Er,Yb co-doped fiber laser; DM, dichroic mirror. 10 December 2014 / Vol. 53, No. 35 / APPLIED OPTICS
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Fig. 2. Output power as a function of launched pump power for different Raman fiber lengths.
versus the launched pump power). A maximum output power of 13.2 W at ∼1658 nm with a slope efficiency of 120% was obtained for the launched pump power of 18.4 W. The corresponding opticalto-optical conversion efficiency is 71.7%. In the experiment, the Raman fiber laser with longer effective fiber length tends to have a lower pump threshold, which is in good agreement with our simulated results. The Stokes laser output spectra for the freerunning Raman fiber lasers with different fiber lengths, shown in Fig. 3, were measured at the maximum launched pump power of 18.4 W using an optical spectrum analyzer (AQ6375, Yokogawa). The output spectra are all composed of two discrete broadband peaks with center wavelengths located at ∼1658 and ∼1671 nm, corresponding to the two Raman gain peaks of germanosilicate fibers with a Raman frequency shift of 440 and 490 cm−1 , respectively. Actually, the laser output spectrum only exhibits the single 440 cm−1 high-frequency Raman peak (at ∼1658 nm) when the first-order Stokes begins to oscillate. The first-order Stokes output power at ∼1658 nm starts to increase with the pump power.
Fig. 3. Output spectra recorded at maximum launched pump power of 18.4 W for different Raman fiber lengths. 8258
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While the laser emission at ∼1658 nm grows powerful enough, the 490 cm−1 low-frequency Raman peak will be amplified through the Raman amplification process with the ∼1658 nm laser emission acting as the pump. It is expected that the 490 cm−1 lowfrequency Raman peak (at ∼1671 nm) will dominate in the first Stokes laser spectrum when the output power increases to as much as several tens of watts. In addition, for the Raman fiber laser with the longest fiber length (2.1 km), the second-order Stokes signal at ∼1790 nm appears in the output spectrum due to the low lasing threshold. According to the lasing performance in terms of output power and slope efficiency as a function of the Raman fiber length, the 1.2 km DCRF was selected as the gain fiber in the fixed-wavelength operation regime. A reflective VBG was included in the external cavity in place of the HR plane mirror to lock the operating wavelength at the 440 cm−1 Raman gain peak. The VBG (Optigrate Corp.) was designed to have a center wavelength of 1658 nm with peak reflectivity of 99.9% and spectral selectivity of 0.33 nm. It had a thickness of 9 mm with its clear aperture of 5 × 5 mm2 and was mounted in a copper heat sink with a layer of indium foil (0.1 mm in thickness) to ensure good heat dissipation. The focal length of the collimating lens in the external cavity was chosen to be 15 mm to make full use of the VBG aperture. In addition, the fiber end adjacent to the external cavity was angle cleaved at ∼8° to suppress the broadband feedback and second Stokes generation from the two uncoated fiber facets. Stokes laser output power from the 1.2 km fixedwavelength Raman fiber laser as a function of launched pump power is shown with squares in the Fig. 4. A maximum output power of 10.4 W at 1658.3 nm was generated for the launched pump power of 18.4 W, corresponding to a slope and conversion efficiency of 109% and 56.5% with respect to the launched pump power, respectively. The lasing performance of the 1.2 km free-running Raman fiber laser is also shown in Fig. 4 for comparison. It can be seen
Fig. 4. Output powers as a function of launched pump power in free-running and fixed-wavelength operating regimes for the 1.2 km fiber.
Fig. 5. Beam propagation and intensity profile (far field) in fixed wavelength operating regime.
that the relatively higher output power and slope efficiency achieved in the free-running Raman fiber laser for the same launched pump power is attributed to the higher pump intensity in the fiber core resulting from the residual pump being retro-reflected back into the cavity by the HR mirror after a single pass. It is worth noting that both configurations have exhibited a very high slope efficiency of larger than ∼1 at the powers we measured owing to the large Raman gain in the custom DCRF. However, it should be expected that the slope efficiencies are going to moderate and rollover due to pump depletion at higher pump power, which is characteristic of the nonlinear nature of the SRS [3]. The output powers were all linearly dependent on the launched pump power, suggesting that there is considerable scope to scale up the output power by simply increasing the pump power. The maximum output-power fluctuations remained below ∼2%, which is attributed to the external disturbances such as mechanical vibration. The laserbeam quality for the fixed-wavelength Raman fiber laser at the maximum output power was measured by a beam profiler (Nanoscan, Photo Inc.). Figure 5 shows the measured beam radii at different positions along the beam propagation direction. By fitting the measured data with a hyperbolic curve, the beam propagation factors (M2 ) were calculated to be ∼1.22 and ∼1.23 in the x and y directions, respectively. The inserted image in Fig. 5 is the two-dimensional beam profile of the laser output, which is neardiffraction-limited. The output spectrum for the fixed-wavelength Raman fiber laser was measured and shown in Fig. 6, which was much narrower with a FWHM bandwidth of ∼0.1 nm recorded at a resolution of 0.05 nm compared with that of the free-running counterpart. It is worth noting that the lasing linewidth was also narrower than the spectral selectivity of 0.33 nm of the VBG we used. This can be explained by the mode competition in the cavity due to the unflat-top reflectivity curve of the VBG, as a result of which only the wavelengths with lowest loss survived. In addition, no second-order Stokes signal was observed in the
Fig. 6. Change 2st to 2nd in fig. 6. Laser output spectrum of the 1.2 km fixed-wavelength Raman fiber laser in comparison with that of the free-running counterpart. Inset: the free-running output spectrum in logarithmic scale.
fixed-wavelength operation regime even at the maximum pump power; however, there appears to be a signal of second-order Stokes at ∼1790 nm of ∼30 dB lower than the magnitude of the first-order Stokes laser at ∼1658 nm in the free-running output spectrum (see the inset of Fig. 6). 3. Conclusion
In conclusion, we reported on a high-power and highly efficient operation of a single-mode cladding-pumped Raman fiber laser. Lasing characteristics of the freerunning cladding-pumped Raman fiber lasers with different fiber lengths were first evaluated and optimized Raman gain fiber length was obtained. Output power up to 13.2 W with slope efficiency of ∼120% was generated in the free-running configuration, limited only by the launched pump power. By employing a VBG as the spectral narrowing element, the output spectral was narrowed to ∼0.1 nm (FWHM) at 1658.3 nm with a maximum output power of 10.4 W, corresponding to a slope efficiency of 109% with respect to the launched pump power. The claddingpumping regime was proved to be an efficient approach for the output-power scaling of Raman fiber lasers. We greatly appreciate Prof. Y. Feng’s theoretical and experimental guidance from the Research Center of Space Laser Information Technology, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences. This work is supported by the National Natural Science Foundation of China (NSFC 61177045), the Research Fund for the Doctoral Program of Higher Education of China (RFDP: 20110071110016) and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). References 1. C. A. Codemard, P. Dupriez, Y. Jeong, J. K. Sahu, M. Ibsen, and J. Nilsson, “High-power continuous-wave claddingpumped Raman fiber laser,” Opt. Lett. 31, 2290–2292 (2006). 10 December 2014 / Vol. 53, No. 35 / APPLIED OPTICS
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