August 1, 2014 / Vol. 39, No. 15 / OPTICS LETTERS
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100 W-level Tm-doped fiber laser pumped by 1173 nm Raman fiber lasers Xiong Wang, Pu Zhou,* Hanwei Zhang, Xiaolin Wang, Hu Xiao, and Zejin Liu College of Optoelectronic Science and Engineering, National University of Defense Technology, Changsha 410073, China *Corresponding author: [email protected] Received April 21, 2014; accepted June 10, 2014; posted June 12, 2014 (Doc. ID 210584); published July 18, 2014 We present a high power and high efficiency Tm-doped fiber laser (TDFL) pumped by two high power Raman fiber lasers (RFLs) at 1173 nm. The output power of the TDFL reached 96 W with slope efficiency of 0.42. The central wavelength located at 1943.3 nm with a 3 dB bandwidth of 0.1 nm. Higher output power can be achieved if more RFLs are employed to pump the TDFL. This is to our knowledge the first demonstration with 100 W-level output power achieved in TDFLs around their ∼1200 nm absorption band pumped by RFLs, which indicates a promising and powerful pump scheme to achieve higher power output in TDFLs. © 2014 Optical Society of America OCIS codes: (140.3070) Infrared and far-infrared lasers; (140.3410) Laser resonators; (140.3510) Lasers, fiber; (140.3550) Lasers, Raman. http://dx.doi.org/10.1364/OL.39.004329
Tm-doped fiber lasers (TDFLs) have been widely investigated in recent decades and have grabbed more and more attention due to their broad emission range covering from ∼1700 nm to ∼2100 nm, which enables TDFLs as promising laser source in various applications [1–7]. The energy levels of Tm3 ions are complex, which determines the complicated and various lasing processes in TDFLs pumped at different wavelengths [8]. There are three main absorption bands of Tm3 ions (∼790 nm, ∼1200 nm, and ∼1600 nm), which enable three corresponding pump schemes. The fast development of laser diodes (LDs) around 790 nm makes high power TDFLs feasible, and the cross-relaxation (CR) process in heavily Tm-doped fiber (TDF) guarantees favorable pump efficiency. Thus, the output power of TDFLs has been improved tremendously and the records of TDFLs; for instance, 608 W single frequency output [9] and 1 kW broadband output [10], are achieved employing ∼790 nm LDs. However, the high concentration of Tm3 ions in TDFLs (which is introduced in order to enhance the CR process) and the intrinsic high quantum defect (which is derived from the large frequency difference between the pump and lasing light) jointly result in inevitable high thermal load when pumped by ∼790 nm LDs. Furthermore, the brightness of ∼790 nm LDs is far below that of 9 × ×nm LDs for the time being; thus, the output power of TDFLs is still far below that of Yb-doped fiber lasers (YDFLs) [11]. One of the key techniques for high power YDFLs is tandem pumping, which employs YDFLs as a pump source to improve the pump efficiency, and single mode output with 10 kW average power has been achieved [12]. As for TDFLs, researchers have also tried to scale up the output power employing a similar pump scheme. The Er-doped or Er/Yb-codoped fiber lasers (EDFLs or EYDFLs) lasing near 1600 nm can be employed as pump sources with remarkable quantum efficiency [13], but the efficiency and output power of EDFLs or EYDFLs are not very high. A newly presented tandem pump scheme, which employs a 1908 nm TDFL as pump source for a 2005 nm TDFL, can improve the slope efficiency to >90% with W-level output power [14], but further endeavors should be paid to 0146-9592/14/154329-04$15.00/0
realize higher power output, since the cladding reabsorption of 1908 nm laser in TDF is challenging. The ∼1200 nm pump band covers the emission and Raman scattering wavelengths of YDFLs, and moderate pump efficiency of TDFLs can be achieved in this band [15–19]. Although exited-state absorption (ESA) effect may limit the quantum efficiency to a certain extent, slope efficiency of 0.51 has been demonstrated using 1150 nm LDs [18]. The relatively favorable slope efficiency can be attributed to the CR process similar with the process in ∼790 nm pump band. However, the output power of TDFLs pumped at ∼1200 nm band is limited to several watts, since the powers of LDs at ∼1200 nm band are relatively low and the powers of YDFLs and Raman fiber lasers (RFLs) employed in the previous demonstrations are also not very high. In this Letter, we present a TDFL pumped at ∼1200 nm band employing high power RFLs at 1173 nm [20,21], which can provide sufficient pump power for TDFLs with high efficiency and high output power. Moreover, the absorption cross section of Tm3 ions at 1173 nm is moderate and favorable. In our experiment, 100 W-level TDFL has been achieved, which we believe is the highest output power of TDFLs pumped at ∼1200 nm band as far as we know. The slope efficiency is about 0.42, and the CR process and lasing near 2 μm may deplete the Tm3 ions at upper excited states and, hence, mitigate the upconversion process, which guarantees the favorable pump efficiency. Figure 1 shows the experimental setup of the TDFL pumped by two 1173 nm RFLs. Each of the 1173 nm RFLs consisted of three 976 nm LDs, a 7 × 1 pump combiner (only three ports were employed), a pair of 1120 nm fiber Bragg gratings (FBGs, 99% and 50%), a length of 5 m Ybdoped fiber with core diameter of 10 μm and inner cladding diameter of 125 μm (10∕125), a pair of 1173 nm FBGs (99% and 50% for RFL 1, 99% and 75% for RFL 2) and a length of single mode 10∕125 passive fiber (PF, 125 m for RFL 1 and 100 m for RFL 2). Although the reflectivity of the FBG and the length of the PF will influence the efficiency of RFLs seriously, we had to use the above FBGs and PFs due to limited experimental equipment. The two © 2014 Optical Society of America
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Fig. 1. Experimental setup of the TDFL (bottom) pumped by two 1173 nm RFLs (top). HR FBG, high reflectivity fiber Bragg grating; YDF, Yb-doped fiber; PF, passive fiber; OC FBG, output coupler fiber Bragg grating; RFL, Raman fiber laser; TDF, Tm-doped fiber.
RFLs were combined by a 7 × 1 pump combiner and the light was launched into the TDFL directly, which consisted of a high reflectivity FBG (HR FBG), a length of 6.5 m 25∕250 double cladding TDF, and an output coupler FBG (OC FBG). The HR FBG’s central wavelength was located at 1943 nm with a bandwidth of 1.5 nm and reflectivity of 99%, and the OC FBG had the matched central wavelength with bandwidth of 0.5 nm and reflectivity of 20%. The cladding absorption of the TDF at 1173 nm was about 2.5 dB∕m. The fusion splice joint of the TDF and OC FBG was covered with high-refractivity gel to dump the unabsorbed pump laser. The TDF was placed on a water-cooled heat sink to avoid thermal damage. The output end of the OC FBG was angle-cleaved to prevent the unwanted feedback induced by Fresnel reflection. The output power was measured by a power meter and the spectrum was monitored by an optical spectrum analyzer (OSA) with a resolution of 0.05 nm. Due to the difference between the available 976 nm LDs’ output powers, the output powers of the two 1120 nm YDFLs were 154 and 180 W, respectively. The maximum output powers of the two 1173 nm RFLs both surpassed 110 W, as shown in Fig. 2. Actually, there was a little residual 1120 nm laser in the output light of the RFLs, although the proportions were only 0.01 and 0.06, respectively. Since the absorption cross section of Tm3
Fig. 2. Output power data of the 1173 nm RFLs pumped by 1120 nm YDFLs. Inset: normalized linear spectra of the 1173 nm RFLs.
ions at 1120 nm is only about one half of that at 1173 nm, and the power of the residual 1120 nm laser is very low, we pick out the 1120 nm laser’s power when considering the pump performance of the TDFL. Thus, the plotted output power data of the 1173 nm RFLs in Fig. 2 are revised according to their spectra to make the results accurate. The slope efficiencies of the RFLs are 0.83 and 0.70, respectively, against the launched 1120 nm pump power. The difference of the slope efficiencies mainly comes from the parameters of the fiber components (FBGs and PFs as aforementioned) being not exactly the same. But the increasing trends of the RFLs’ output power are almost the same. The data of the TDFL’s output power versus 1173 nm pump power are shown in Fig. 3. The lasing threshold is nearly 14 W, and the maximum output power reaches 96 W when the 1173 nm pump power is about 234 W. The slope efficiency is 0.42 against the launched pump power, and the corresponding quantum efficiency is about 0.7, which are slightly lower those in Ref. [18]. The efficiency differences can be attributed to the distinctness between the active fibers’ parameters, and the slope efficiency in our experiment can be further improved by optimizing the parameters of the TDFL. No power saturation phenomenon is observed, the output power is only limited by available pump power, and higher output power can be achieved by increasing the pump power. The main obstacle of the TDFL’s pump scheme at ∼1200 nm is the ESA effect, as shown in Fig. 4. The simplified energy level diagram of the TDFL can be found in Refs. [17,18] and we replotted it here. The laser at 1173 nm not only pumps the Tm3 ions to the upper lasing level (ground state absorption, GSA), but also enables ESA processes from the 3 F4 level to the 3 F2;3 level (ESA 1) and from the 3 H4 level to the 1 G4 level (ESA 2). The consequent upconversion fluorescence emission (∼480 nm and ∼810 nm) due to the ESA processes will consume the pump energy with unfavorable and deleterious waste heat. Upconversion with blue light emission (∼480 nm) was observed in the experiment even when the pump power was below the lasing threshold. However, the upconversion process might not reduce the pump efficiency much when the pump power is beyond the threshold. Actually, the absorption cross sections of the ESA 1 and ESA 2 processes at 1173 nm are relatively low [17]
Fig. 3. Output power data of the TDFL pumped by 1173 nm RFLs.
August 1, 2014 / Vol. 39, No. 15 / OPTICS LETTERS 1G
4
ESA 2
480 nm
3F 2,3 3H
4
ESA 1 3H
CR
810 nm
5
3F 4
GSA @ 1173 nm 3H
Fig. 4. RFLs.
1943 nm
6
Energy level sketch of the TDFL pumped by 1173 nm
and strong ESA is avoided to a great extent. Note that the absorption cross sections of ESA 1 and ESA 2 at 1120 nm are much higher than that at 1173 nm; hence, the 1120 nm laser in this experiment can play a rather limited influence on increasing the output power of the TDFL. On the contrary, the 1120 nm laser may even enhance the ESA processes and reduce the overall pump efficiency. That is another reason that we picked out the 1120 nm laser’s power when examining the aforementioned TDFL’s performance. Furthermore, the cumulation of Tm3 ions on the upper lasing level (3 F4 ) will stagnate when the lasing process at 1943 nm starts; thus, the ESA 1 and ESA 2 processes will not be enhanced further when more of the 1173 nm pump laser is launched into the oscillator. Moreover, the ESA may be even reduced somewhat when the lasing threshold is reached, compared with that of no lasing process [17]. Thus, we can expect a higher efficiency and a better performance of this 1173 nm pump scheme with optimized parameters. If the TDF amplifier and master oscillator power amplifier (MOPA) configuration are employed, higher output power near 2 μm can be anticipated employing this pump scheme. In addition, the CR process may improve the performance of the TDFL, but the process might not be very intense since the slope efficiency is moderate.
Fig. 5. Spectrum of the TDFL. Inset: spectrum in large scale.
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The spectrum of the TDFL was depicted in Fig. 5. The amplified spontaneous emission (ASE) noise is well suppressed by more than 25 dB. The length of active fiber is optimized to make the oscillator’s ASE band cover the central wavelength of FBGs. The lasing wavelength is located at 1943.3 nm with a 3 dB bandwidth of 0.1 nm. In conclusion, we demonstrate a high power and high efficiency TDFL pumped by two 1173 nm RFLs. The slope efficiency reaches 0.42, and the output power reaches 96 W. The upconversion process is generated but does not reduce the pump efficiency much since the absorption cross sections of the ESA processes at 1173 nm are relatively low. This is to our knowledge the first demonstration of 100 W-level TDFL pumped by RFLs at ∼1200 nm, which indicates a promising and powerful method for further scaling up the output power of TDFLs. Higher pump efficiency can be expected by optimizing the fiber parameters and employing the amplifier configuration, and higher output power can be achieved by launching more pump power into the oscillator and using the MOPA configuration. This work was supported by the Graduate Student Innovation Foundation of National University of Defense Technology (Grant No. B130704), National Natural Science Foundation of China (Grant No. 61322505), Program for New Century Excellent Talents in University and Hunan Provincial Innovation Foundation for Postgraduate. The authors would like to thank Dr. Jinyong Leng for his helpful support. References 1. P. F. Moulton, G. A. Rines, E. V. Slobodtchikov, K. F. Wall, G. Frith, B. Samson, and A. L. G. Carter, IEEE J. Sel. Top. Quantum Electron. 15, 85 (2009). 2. S. D. Jackson, A. Sabella, and D. G. Lancaster, IEEE J. Sel. Top. Quantum Electron. 13, 567 (2007). 3. S. D. Jackson, Nat. Photonics 6, 423 (2012). 4. Y. Tang, C. Huang, S. Wang, H. Li, and J. Xu, Opt. Express 20, 17539 (2012). 5. J. Liu, K. Liu, F. Tan, and P. Wang, IEEE J. Sel. Top. Quantum Electron. 20, 3100306 (2014). 6. J. Li, Z. Sun, H. Luo, Z. Yan, K. Zhou, Y. Liu, and L. Zhang, Opt. Express 22, 5387 (2014). 7. Z. Li, S. U. Alam, Y. Jung, A. M. Heidt, and D. J. Richardson, Opt. Lett. 38, 4739 (2013). 8. S. D. Jackson and T. A. King, J. Lightwave Technol. 17, 948 (1999). 9. G. D. Goodno, L. D. Book, and J. E. Rothenberg, Opt. Lett. 34, 1204 (2009). 10. T. Ehrenreich, R. Leveille, I. Majid, K. Tankala, G. Rines, and P. Moulton, Proc. SPIE 7580, 758016 (2010). 11. D. J. Richardson, J. Nilsson, and W. A. Clarkson, J. Opt. Soc. Am. B 27, B63 (2010). 12. E. Stiles, in Proceedings of the 5th International Workshop on Fiber Lasers (2009), pp. 4–6. 13. M. Meleshkevich, N. Platonov, D. Gapontsev, A. Drozhzhin, V. Sergeev, and V. Gapontsev, in Lasers and ElectroOptics, 2007 and the International Quantum Electronics Conference (CLEOE-IQEC) (IEEE, 2007), pp. 1. 14. D. Creeden, B. R. Johnson, S. D. Setzler, and E. P. Chicklis, Opt. Lett. 39, 470 (2014). 15. A. Taniguchi, T. Kuwayama, A. Shirakawa, M. Musha, K. Ueda, and M. Prabhu, Appl. Phys. Lett. 81, 3723 (2002).
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16. D. C. Hanna, M. J. McCarthy, I. R. Perry, and P. J. Suni, Electron. Lett. 25, 1365 (1989). 17. Y. H. Tsang, D. J. Coleman, and T. A. King, Opt. Commun. 231, 357 (2004). 18. S. D. Jackson, F. Bugge, and G. Erbert, Opt. Lett. 32, 2873 (2007).
19. K. S. Kravtsov, I. A. Bufetov, O. I. Medvedkov, E. M. Dianov, M. V. Yashkov, and A. N. Gur’Yanov, IEEE J. Quantum Electron. 35, 586 (2005). 20. H. Zhang, H. Xiao, P. Zhou, X. Wang, and X. Xu, IEEE Photon. J. 5, 1501706 (2013). 21. Y. Feng, L. R. Taylor, and D. B. Calia, Opt. Express 17, 23678 (2009).
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100 W-level Tm-doped fiber laser pumped by 1173 nm Raman fiber lasers Xiong Wang, Pu Zhou,* Hanwei Zhang, Xiaolin Wang, Hu Xiao, and Zejin Liu College of Optoelectronic Science and Engineering, National University of Defense Technology, Changsha 410073, China *Corresponding author: [email protected] Received April 21, 2014; accepted June 10, 2014; posted June 12, 2014 (Doc. ID 210584); published July 18, 2014 We present a high power and high efficiency Tm-doped fiber laser (TDFL) pumped by two high power Raman fiber lasers (RFLs) at 1173 nm. The output power of the TDFL reached 96 W with slope efficiency of 0.42. The central wavelength located at 1943.3 nm with a 3 dB bandwidth of 0.1 nm. Higher output power can be achieved if more RFLs are employed to pump the TDFL. This is to our knowledge the first demonstration with 100 W-level output power achieved in TDFLs around their ∼1200 nm absorption band pumped by RFLs, which indicates a promising and powerful pump scheme to achieve higher power output in TDFLs. © 2014 Optical Society of America OCIS codes: (140.3070) Infrared and far-infrared lasers; (140.3410) Laser resonators; (140.3510) Lasers, fiber; (140.3550) Lasers, Raman. http://dx.doi.org/10.1364/OL.39.004329
Tm-doped fiber lasers (TDFLs) have been widely investigated in recent decades and have grabbed more and more attention due to their broad emission range covering from ∼1700 nm to ∼2100 nm, which enables TDFLs as promising laser source in various applications [1–7]. The energy levels of Tm3 ions are complex, which determines the complicated and various lasing processes in TDFLs pumped at different wavelengths [8]. There are three main absorption bands of Tm3 ions (∼790 nm, ∼1200 nm, and ∼1600 nm), which enable three corresponding pump schemes. The fast development of laser diodes (LDs) around 790 nm makes high power TDFLs feasible, and the cross-relaxation (CR) process in heavily Tm-doped fiber (TDF) guarantees favorable pump efficiency. Thus, the output power of TDFLs has been improved tremendously and the records of TDFLs; for instance, 608 W single frequency output [9] and 1 kW broadband output [10], are achieved employing ∼790 nm LDs. However, the high concentration of Tm3 ions in TDFLs (which is introduced in order to enhance the CR process) and the intrinsic high quantum defect (which is derived from the large frequency difference between the pump and lasing light) jointly result in inevitable high thermal load when pumped by ∼790 nm LDs. Furthermore, the brightness of ∼790 nm LDs is far below that of 9 × ×nm LDs for the time being; thus, the output power of TDFLs is still far below that of Yb-doped fiber lasers (YDFLs) [11]. One of the key techniques for high power YDFLs is tandem pumping, which employs YDFLs as a pump source to improve the pump efficiency, and single mode output with 10 kW average power has been achieved [12]. As for TDFLs, researchers have also tried to scale up the output power employing a similar pump scheme. The Er-doped or Er/Yb-codoped fiber lasers (EDFLs or EYDFLs) lasing near 1600 nm can be employed as pump sources with remarkable quantum efficiency [13], but the efficiency and output power of EDFLs or EYDFLs are not very high. A newly presented tandem pump scheme, which employs a 1908 nm TDFL as pump source for a 2005 nm TDFL, can improve the slope efficiency to >90% with W-level output power [14], but further endeavors should be paid to 0146-9592/14/154329-04$15.00/0
realize higher power output, since the cladding reabsorption of 1908 nm laser in TDF is challenging. The ∼1200 nm pump band covers the emission and Raman scattering wavelengths of YDFLs, and moderate pump efficiency of TDFLs can be achieved in this band [15–19]. Although exited-state absorption (ESA) effect may limit the quantum efficiency to a certain extent, slope efficiency of 0.51 has been demonstrated using 1150 nm LDs [18]. The relatively favorable slope efficiency can be attributed to the CR process similar with the process in ∼790 nm pump band. However, the output power of TDFLs pumped at ∼1200 nm band is limited to several watts, since the powers of LDs at ∼1200 nm band are relatively low and the powers of YDFLs and Raman fiber lasers (RFLs) employed in the previous demonstrations are also not very high. In this Letter, we present a TDFL pumped at ∼1200 nm band employing high power RFLs at 1173 nm [20,21], which can provide sufficient pump power for TDFLs with high efficiency and high output power. Moreover, the absorption cross section of Tm3 ions at 1173 nm is moderate and favorable. In our experiment, 100 W-level TDFL has been achieved, which we believe is the highest output power of TDFLs pumped at ∼1200 nm band as far as we know. The slope efficiency is about 0.42, and the CR process and lasing near 2 μm may deplete the Tm3 ions at upper excited states and, hence, mitigate the upconversion process, which guarantees the favorable pump efficiency. Figure 1 shows the experimental setup of the TDFL pumped by two 1173 nm RFLs. Each of the 1173 nm RFLs consisted of three 976 nm LDs, a 7 × 1 pump combiner (only three ports were employed), a pair of 1120 nm fiber Bragg gratings (FBGs, 99% and 50%), a length of 5 m Ybdoped fiber with core diameter of 10 μm and inner cladding diameter of 125 μm (10∕125), a pair of 1173 nm FBGs (99% and 50% for RFL 1, 99% and 75% for RFL 2) and a length of single mode 10∕125 passive fiber (PF, 125 m for RFL 1 and 100 m for RFL 2). Although the reflectivity of the FBG and the length of the PF will influence the efficiency of RFLs seriously, we had to use the above FBGs and PFs due to limited experimental equipment. The two © 2014 Optical Society of America
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Fig. 1. Experimental setup of the TDFL (bottom) pumped by two 1173 nm RFLs (top). HR FBG, high reflectivity fiber Bragg grating; YDF, Yb-doped fiber; PF, passive fiber; OC FBG, output coupler fiber Bragg grating; RFL, Raman fiber laser; TDF, Tm-doped fiber.
RFLs were combined by a 7 × 1 pump combiner and the light was launched into the TDFL directly, which consisted of a high reflectivity FBG (HR FBG), a length of 6.5 m 25∕250 double cladding TDF, and an output coupler FBG (OC FBG). The HR FBG’s central wavelength was located at 1943 nm with a bandwidth of 1.5 nm and reflectivity of 99%, and the OC FBG had the matched central wavelength with bandwidth of 0.5 nm and reflectivity of 20%. The cladding absorption of the TDF at 1173 nm was about 2.5 dB∕m. The fusion splice joint of the TDF and OC FBG was covered with high-refractivity gel to dump the unabsorbed pump laser. The TDF was placed on a water-cooled heat sink to avoid thermal damage. The output end of the OC FBG was angle-cleaved to prevent the unwanted feedback induced by Fresnel reflection. The output power was measured by a power meter and the spectrum was monitored by an optical spectrum analyzer (OSA) with a resolution of 0.05 nm. Due to the difference between the available 976 nm LDs’ output powers, the output powers of the two 1120 nm YDFLs were 154 and 180 W, respectively. The maximum output powers of the two 1173 nm RFLs both surpassed 110 W, as shown in Fig. 2. Actually, there was a little residual 1120 nm laser in the output light of the RFLs, although the proportions were only 0.01 and 0.06, respectively. Since the absorption cross section of Tm3
Fig. 2. Output power data of the 1173 nm RFLs pumped by 1120 nm YDFLs. Inset: normalized linear spectra of the 1173 nm RFLs.
ions at 1120 nm is only about one half of that at 1173 nm, and the power of the residual 1120 nm laser is very low, we pick out the 1120 nm laser’s power when considering the pump performance of the TDFL. Thus, the plotted output power data of the 1173 nm RFLs in Fig. 2 are revised according to their spectra to make the results accurate. The slope efficiencies of the RFLs are 0.83 and 0.70, respectively, against the launched 1120 nm pump power. The difference of the slope efficiencies mainly comes from the parameters of the fiber components (FBGs and PFs as aforementioned) being not exactly the same. But the increasing trends of the RFLs’ output power are almost the same. The data of the TDFL’s output power versus 1173 nm pump power are shown in Fig. 3. The lasing threshold is nearly 14 W, and the maximum output power reaches 96 W when the 1173 nm pump power is about 234 W. The slope efficiency is 0.42 against the launched pump power, and the corresponding quantum efficiency is about 0.7, which are slightly lower those in Ref. [18]. The efficiency differences can be attributed to the distinctness between the active fibers’ parameters, and the slope efficiency in our experiment can be further improved by optimizing the parameters of the TDFL. No power saturation phenomenon is observed, the output power is only limited by available pump power, and higher output power can be achieved by increasing the pump power. The main obstacle of the TDFL’s pump scheme at ∼1200 nm is the ESA effect, as shown in Fig. 4. The simplified energy level diagram of the TDFL can be found in Refs. [17,18] and we replotted it here. The laser at 1173 nm not only pumps the Tm3 ions to the upper lasing level (ground state absorption, GSA), but also enables ESA processes from the 3 F4 level to the 3 F2;3 level (ESA 1) and from the 3 H4 level to the 1 G4 level (ESA 2). The consequent upconversion fluorescence emission (∼480 nm and ∼810 nm) due to the ESA processes will consume the pump energy with unfavorable and deleterious waste heat. Upconversion with blue light emission (∼480 nm) was observed in the experiment even when the pump power was below the lasing threshold. However, the upconversion process might not reduce the pump efficiency much when the pump power is beyond the threshold. Actually, the absorption cross sections of the ESA 1 and ESA 2 processes at 1173 nm are relatively low [17]
Fig. 3. Output power data of the TDFL pumped by 1173 nm RFLs.
August 1, 2014 / Vol. 39, No. 15 / OPTICS LETTERS 1G
4
ESA 2
480 nm
3F 2,3 3H
4
ESA 1 3H
CR
810 nm
5
3F 4
GSA @ 1173 nm 3H
Fig. 4. RFLs.
1943 nm
6
Energy level sketch of the TDFL pumped by 1173 nm
and strong ESA is avoided to a great extent. Note that the absorption cross sections of ESA 1 and ESA 2 at 1120 nm are much higher than that at 1173 nm; hence, the 1120 nm laser in this experiment can play a rather limited influence on increasing the output power of the TDFL. On the contrary, the 1120 nm laser may even enhance the ESA processes and reduce the overall pump efficiency. That is another reason that we picked out the 1120 nm laser’s power when examining the aforementioned TDFL’s performance. Furthermore, the cumulation of Tm3 ions on the upper lasing level (3 F4 ) will stagnate when the lasing process at 1943 nm starts; thus, the ESA 1 and ESA 2 processes will not be enhanced further when more of the 1173 nm pump laser is launched into the oscillator. Moreover, the ESA may be even reduced somewhat when the lasing threshold is reached, compared with that of no lasing process [17]. Thus, we can expect a higher efficiency and a better performance of this 1173 nm pump scheme with optimized parameters. If the TDF amplifier and master oscillator power amplifier (MOPA) configuration are employed, higher output power near 2 μm can be anticipated employing this pump scheme. In addition, the CR process may improve the performance of the TDFL, but the process might not be very intense since the slope efficiency is moderate.
Fig. 5. Spectrum of the TDFL. Inset: spectrum in large scale.
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The spectrum of the TDFL was depicted in Fig. 5. The amplified spontaneous emission (ASE) noise is well suppressed by more than 25 dB. The length of active fiber is optimized to make the oscillator’s ASE band cover the central wavelength of FBGs. The lasing wavelength is located at 1943.3 nm with a 3 dB bandwidth of 0.1 nm. In conclusion, we demonstrate a high power and high efficiency TDFL pumped by two 1173 nm RFLs. The slope efficiency reaches 0.42, and the output power reaches 96 W. The upconversion process is generated but does not reduce the pump efficiency much since the absorption cross sections of the ESA processes at 1173 nm are relatively low. This is to our knowledge the first demonstration of 100 W-level TDFL pumped by RFLs at ∼1200 nm, which indicates a promising and powerful method for further scaling up the output power of TDFLs. Higher pump efficiency can be expected by optimizing the fiber parameters and employing the amplifier configuration, and higher output power can be achieved by launching more pump power into the oscillator and using the MOPA configuration. This work was supported by the Graduate Student Innovation Foundation of National University of Defense Technology (Grant No. B130704), National Natural Science Foundation of China (Grant No. 61322505), Program for New Century Excellent Talents in University and Hunan Provincial Innovation Foundation for Postgraduate. The authors would like to thank Dr. Jinyong Leng for his helpful support. References 1. P. F. Moulton, G. A. Rines, E. V. Slobodtchikov, K. F. Wall, G. Frith, B. Samson, and A. L. G. Carter, IEEE J. Sel. Top. Quantum Electron. 15, 85 (2009). 2. S. D. Jackson, A. Sabella, and D. G. Lancaster, IEEE J. Sel. Top. Quantum Electron. 13, 567 (2007). 3. S. D. Jackson, Nat. Photonics 6, 423 (2012). 4. Y. Tang, C. Huang, S. Wang, H. Li, and J. Xu, Opt. Express 20, 17539 (2012). 5. J. Liu, K. Liu, F. Tan, and P. Wang, IEEE J. Sel. Top. Quantum Electron. 20, 3100306 (2014). 6. J. Li, Z. Sun, H. Luo, Z. Yan, K. Zhou, Y. Liu, and L. Zhang, Opt. Express 22, 5387 (2014). 7. Z. Li, S. U. Alam, Y. Jung, A. M. Heidt, and D. J. Richardson, Opt. Lett. 38, 4739 (2013). 8. S. D. Jackson and T. A. King, J. Lightwave Technol. 17, 948 (1999). 9. G. D. Goodno, L. D. Book, and J. E. Rothenberg, Opt. Lett. 34, 1204 (2009). 10. T. Ehrenreich, R. Leveille, I. Majid, K. Tankala, G. Rines, and P. Moulton, Proc. SPIE 7580, 758016 (2010). 11. D. J. Richardson, J. Nilsson, and W. A. Clarkson, J. Opt. Soc. Am. B 27, B63 (2010). 12. E. Stiles, in Proceedings of the 5th International Workshop on Fiber Lasers (2009), pp. 4–6. 13. M. Meleshkevich, N. Platonov, D. Gapontsev, A. Drozhzhin, V. Sergeev, and V. Gapontsev, in Lasers and ElectroOptics, 2007 and the International Quantum Electronics Conference (CLEOE-IQEC) (IEEE, 2007), pp. 1. 14. D. Creeden, B. R. Johnson, S. D. Setzler, and E. P. Chicklis, Opt. Lett. 39, 470 (2014). 15. A. Taniguchi, T. Kuwayama, A. Shirakawa, M. Musha, K. Ueda, and M. Prabhu, Appl. Phys. Lett. 81, 3723 (2002).
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16. D. C. Hanna, M. J. McCarthy, I. R. Perry, and P. J. Suni, Electron. Lett. 25, 1365 (1989). 17. Y. H. Tsang, D. J. Coleman, and T. A. King, Opt. Commun. 231, 357 (2004). 18. S. D. Jackson, F. Bugge, and G. Erbert, Opt. Lett. 32, 2873 (2007).
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