Laser oscillation of Yb3+:Er3+ co-doped phosphosilicate microsphere [Invited] Tianjiao Wu,1 Yantang Huang,1,* Jing Huang,1 Yu Huang,1 Peijin Zhang,1 and Jing Ma1,2 1
College of Physics and Information Engineering, Fuzhou University, Fuzhou 350108, China 2
e-mail: [email protected]
*Corresponding author: [email protected] Received 25 March 2014; revised 11 June 2014; accepted 16 June 2014; posted 18 June 2014 (Doc. ID 208760); published 17 July 2014
A fiber-taper-microsphere-coupled system was used to research the characteristics of laser oscillation and upconversion luminescence of Yb3 :Er3 co-doped phosphosilicate (YECP) microspheres. The YECP microspheres were fabricated by melting the end of phosphosilicate filaments. Single- and multimode laser oscillation at 1535–1565 nm within the C-band were obtained. In addition, the output power of the single-mode laser at 1545.5 nm can be as high as 48.98 μW, which was achieved under pump power of 9.63 mW, and the side-mode suppression ratio was 51.49 dB. Upconversion fluorescence of Er3 at 521, 532, and 544 nm also were measured, and the pump power dependence was studied. The fluorescence intensity was lower than that of Yb3 :Er3 co-doped silica and oxyfluoride glass ceramic microspheres. Moreover, the physical mechanism of upconversion suppression and laser oscillation enhancement observed in our experiment was presented, which is beneficial to the preparation of rare-earth-doped microcavity lasers. © 2014 Optical Society of America OCIS codes: (140.3510) Lasers, fiber; (140.3570) Lasers, single-mode; (140.3613) Lasers, upconversion; (140.3945) Microcavities; (160.5690) Rare-earth-doped materials; (230.4555) Coupled resonators. http://dx.doi.org/10.1364/AO.53.004747
1. Introduction
Optical microcavities have been studied for several years due to their unique characteristics and various applications [1,2]. The special whispering gallery modes (WGMs) of microsphere resonators usually bring about small volumes and an ultrahigh quality factor. Thus the pump power required for laser emission and nonlinear effects can be greatly reduced [3,4]. In the past decades, microcavities also have been widely applied in quantum optics [5], quantum electrodynamics [6], microsensors, and many other areas [7–9]. Rare-earth-ions-doped material usually works as a gain medium for microlasers. Among all the rareearth ions, Yb3 ion is one of the best candidates
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for a laser device, owing to its simple energy-level structure, high quantum efficiency, and larger absorption cross section than Er3 at around 980 nm. Er3 ion with abundant energy-level structures has a strong emission band at around 1550 nm, which is an appropriate wavelength within the low-loss communication window. Yb3 co-doped with Er3 ion also can act as a sensitizer. Compared with Er3 , it can absorb pump photons more efficiently and transfer to Er3 by collision, which realizes the population inversion of Er3 and produces a stronger laser in a microcavity. In previous researches, Er3 doped∕ Yb3 :Er3 co-doped materials already have been studied by means of implantation [10], sol-gel method [11], and so forth. The substrates are mainly made up of germanate glass [12], tungstate [13], phosphate glass [14], or silicate glass [11]. Yang and Vahala have reported a single-mode laser with an output power of 10 μW [15] in an Er3 -doped silicate microresonator. 20 July 2014 / Vol. 53, No. 21 / APPLIED OPTICS
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Dong et al. have fabricated a silicate microsphere covered with Yb3 :Er3 co-doped phosphate, and the single-mode microlaser had a maximum output power of only 15 μW [16]. In this paper, a fiber-taper-microsphere-coupled system was adopted to investigate the laser oscillation properties of Yb3 :Er3 co-doped phosphosilicate (YECP) microspheres. Single-mode lasing can be obtained at 1545.5 nm with a threshold of 1.83 mW, an output power maximum of 48.98 μW, and side-mode suppression ratio of 51.49 dB. Furthermore, multimode lasing also can be obtained near 1530 nm, which is within the eye-safe waveband. Moreover, upconversion fluorescence emissions of Er3 also were obtained at 521 nm (4 H11∕2 → 4 I15∕2 ), 532 nm (4 H11∕2 → 4 I15∕2 ), and 544 nm (4 S3∕2 → 4 I15∕2 ), and the upconversion was suppression. Under the condition of using the same structure of fiber taper, the experimental data is relatively optimized in this experiment. 2. Experiments
YECP glass is composed of 55.93P2 O5, 3.57Al2 O3 , 15Na2 CO3 , 20SiO2 , 5Yb2 O3 , and 0.3Er2 O3 . Wellmixed powders were put into a covered corundum crucible, transferred to an electric resistance furnace, and heated for 60 min at 1450°C; then the molten glass was immediately drawn into glass filaments with a rod glass. Under the effect of surface tension, the glass filaments of YECP were melted into microspheres with the voltaic arc of electrode discharge [17]. The diameter of YECP microspheres was controlled within a range from tens to hundreds of micrometers mainly by adjusting discharge intensity and frequency. The diameter of microspheres also was determined by that of glass filaments. Optical fiber tapers were fabricated by melting and drawing a standard single-mode fiber [18], which was used as a coupler—not only for launching pump power into the microspheres but also for coupling out the emitting laser [19,20]. Figure 1 reveals the
Fig. 1. Fabrication process of the YECP microsphere. 4748
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Fig. 2. Diagram of measurement setup.
fabrication process of the YECP microsphere. The schematic of the measurement setup is shown in Fig. 2. A three-axis manual control stage was adopted to couple the YECP microsphere tangent with a tapered fiber, and the pump source was a 976 nm semiconductor laser (the maximum of output power is 12 mW). Figure 2(a) presents a photomultiplier (PMT) grating spectrometer with measurement range from 200 to 660 nm; Fig. 2(b) is of an optical spectrum analyzer (OSA) (the measurement range is from 600 to 1700 nm). The inset in Fig. 2 is the coupling picture taken by a CCD camera. 3. Results and Discussions A. Laser Oscillation
As is known, WGMs are formed by repeated total internal reflection of light, which is eventually constrained on the surface of the microsphere. WGMs can be described by three mode numbers: radial mode number n, angular mode number l, and azimuthal mode number m [21]. When n is the minimum and m ≈ l, optical field distribution near the surface of the microsphere is approximate to the equator, and the field is compressed into the smallest spaces. In order to couple light into the microsphere efficiently, the evanescent wave field mode of the taper fiber must match with the WGMs of the microsphere. When the YECP microsphere is adequately coupled with the taper fiber, a single-mode laser can be achieved. Otherwise, a multimode laser can be observed [16]. In the experiment, single- and multimode lasing have been measured. As shown in Fig. 3, the YECP microsphere with the diameter of 52.5 μm produced a single-mode lasing at 1545.5 nm, with a threshold of about 1.83 mW. When the excitation power reached 9.63 mW, the output power of the single-mode laser could reach 48.98 μW, the FWHM of the single-mode laser was 0.17 nm, and the side-mode suppression ratio could be as high as 51.49 dB. To explore the characteristics of the lasing oscillation, we have collected the output power versus the absorbed power of the single-mode lasing at 1545.5 nm (Fig. 4). In the diagram, the threshold is about 1.83 mW, which is limited by the concentration of Er3 and Q value of the YECP microsphere.
Fig. 3. Single-mode laser oscillation with pump power of 9.63 mW; inset describes the single-mode laser at 1545.5 nm in detail.
Fig. 5. Multimode laser oscillation spectrum from 1535 to 1565 nm under pump power of 5.41 mW.
Fig. 6. Schematic of energy-level diagram of Er3 and Yb3 ions.
Fig. 4. Output power versus the absorbed power for the singlemode lasing at 1545.5 nm.
The concentration of Er3 should not be much higher; otherwise, lasing could not be obtained due to quenching. As Fig. 5 shows, by using the tested microsphere with a diameter of 104 μm, a multimode laser from 1535 to 1565 nm was obtained with the pump power of 5.41 mW. With the increase of pump power, the multimode lasing remains modulated by the basic morphology of the microsphere. With a view to elaborate on the laser oscillation process, Fig. 6 presents a schematic of the energylevel diagram of Yb3 and Er3 ions. During the downconversion laser oscillation process, Yb3 ion works as an energy transporter for Er3 ion in an indirect way. First, Yb3 ion absorbs pump photons and transfers the energy to Er3 ion by collision (Yb3 ion transits to the ground state by nonradiative transition), which makes Er3 ion transit to 4 I11∕2 (4 I11∕2 is a metastable level). Subsequently,
under the influence of a high phonon energy field, the electrons in the excited states of Er3 ion similarly transit from the metastable level to 4 I13∕2 in the form of nonradiative transition. Finally, Er3 ion transits to the ground state by radiative transition. The 4 I13∕2 → 4 I15∕2 transition of Er3 results in the lasing oscillation at around 1550 nm in the YECP microsphere [22,23]. In this process, the effectiveness of SiO2 instead of Pb2 O5 can more easily help to deduce the multiphonon relaxation (4 I11∕2 → 4 I13∕2 ) and enhance laser emission; the high phonon energy contributes to more excited state Er3 ions at a lower level (4 I13∕2 ). As a consequence, Er3 ions for generating upconversion fluorescence were reduced, and laser oscillation was enhanced. B. Upconversion Luminescence
Figure 7 shows the upconversion luminescence spectra within the range of 510–580 nm of an YECP microsphere whose diameter is 152 μm. We have set the PMT grating spectrometer parameters: V 700 V, g 4 (V is voltage, g is gain coefficient). The 2 H11∕2 → 4 I15∕2 transition of Er3 results in the emissions at 521 and 532 nm, and the 544 nm emission is due to the 4 S3∕2 → 4 I15∕2 transition [24]. The relationship between the upconversion luminescence 20 July 2014 / Vol. 53, No. 21 / APPLIED OPTICS
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I15∕2 has decreased. Therefore, under the environment of high phonon energy, most of the excited electrons are involved in the nonradiative process instead of the upconversion luminescence, which was thus restrained. In this experiment, cooperative luminescence and laser oscillation of Yb3 cannot be observed [19]. 4. Conclusion
Fig. 7. Luminescence spectra from 510 to 580 nm of the YECP microsphere.
intensity and the pump power of a 976 nm laser is also described by log–log plot in Fig. 8. The linear fitting slopes for 521, 532, and 544 nm are, respectively, 2.00, 1.99, and 1.84, which prove a double-photon process. As is well known, the major factor that affects the efficiency of rare-earth fluorescence is the maximal phonon energy of the host material [25,26]. The larger the maximal phonon energy of the host material is, the higher the rate of nonradiative relaxation and the lower efficiency of fluorescence. So the upconversion efficiency in the phosphosilicate material is lower than that of low phonon energy materials (such as SiO2 [27] and fluoride oxide [28]). Moreover, it is much lower than that in oxyfluoride glass ceramic [29]. In YECP with high phonon energy, the high transition rate from the metastable state 4 I11∕2 to 4 I13∕2 contributes to a large number of particles populated in the 4 I13∕2 state used for producing a downconversion laser. In contrast, there are fewer particles in the metastable 4 I11∕2 state with a shorter lifetime, which means the particles used for the transition to the ground-state
Fig. 8. Pump power dependence of upconversion luminescence intensity at 976 nm in the YECP microsphere. 4750
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In conclusion, we have reported YECP microspheres pumped at 976 nm. Single-mode and multimode downconversion lasing in the communication band were successfully demonstrated, and the output power of the single-mode laser could be as high as 48.98 μW. Green upconversion luminescence from 521 to 544 nm also was observed, which proved to be a two-photon process. Moreover, the physical mechanism of upconversion restraint and laser oscillation enhancement was analyzed. References 1. K. J. Vahala, “Optical microcavities,” Nature 424, 839–846 (2003). 2. Y. T. Huang, C. L. Guo, R. Y. Bao, and X. H. Wang, “Design and fabrication of a silica optical micro-kayak cavity on a silicon chip,” Chin. Opt. Lett. 11, 052201 (2013). 3. L. Yang, T. Carmon, B. Min, S. M. Spillane, and K. J. Vahala, “Erbium-doped and Raman microsphere on a silicon chip fabricated by the sol-gel process,” Appl. Phys. Lett. 86, 091114 (2005). 4. S. M. Spillane, T. J. Kippenberg, and K. J. Vahala, “Ultralowthreshold Raman laser using a spherical dielectric microcavity,” Nature 415, 621–623 (2002). 5. M. Cai, O. Painter, and K. J. Vahala, “Observation of critical coupling in a fiber taper to a silica-microsphere whisperinggallery mode system,” Phys. Rev. Lett. 85, 74–77 (2000). 6. S. M. Spillane, T. J. Kippenberg, K. J. Vahala, K. W. Goh, E. Wilcut, and H. J. Kimble, “Ultrahigh-Q toroidal microresonators for cavity quantum electrodynamics,” Phys. Rev. A 71, 013817 (2005). 7. N. Q. Nguyen, N. Gupta, T. Ioppolo, and M. V. Ötügen, “Whispering gallery mode-based micro-optical sensors for structural health monitoring of composite materials,” J. Mater. Sci. 44, 1560–1571 (2009). 8. P. B. Li, S. Y. Gao, and L. L. Fu, “Quantum-information transfer with nitrogen-vacancy centers coupled to a whisperinggallery microresonator,” Phys. Rev. A 83, 054306 (2011). 9. J. P. Laine, C. Tapalian, B. Little, and H. Haus, “Acceleration sensor based on high-Q optical microsphere resonator and pedestal antiresonant reflecting waveguide coupler,” Sens. Actuators A 93, 1–7 (2001). 10. P. V. dos Santos, M. V. D. Vermelho, E. A. Gouveia, M. T. de Araújo, A. S. Gouveia-Neto, F. C. Cassanjes, S. J. L. Ribeiro, and Y. Messaddeq, “Blue cooperative luminescence in Yb3+doped tellurite glasses excited at 1.064 μm,” J. Chem. Phys. 116, 6772–6776 (2002). 11. X. Orignac, D. Barbier, X. M. Du, R. M. Almeida, O. McCarthy, and E. Yeatman, “Sol-gel silica/titania-on-silicon Er/Yb-doped waveguides for optical amplification at 1.5 μm,” Opt. Mater. 12, 1–18 (1999). 12. M. Ajroud, M. Haouari, H. Ben Ouada, H. Mâaref, A. Brenier, and B. Champagnon, “Energy transfer processes in (Er3+-Yb3+)codoped germanate glasses for mid-infrared and up-conversion applications,” Mater. Sci. Eng., C 26, 523–529 (2006). 13. N. V. Kuleshov, A. A. Lagatsky, V. G. Shcherbitsky, V. P. Mikhailov, E. Heumann, T. Jensen, A. Diening, and G. Huber, “CW laser performance of Yb and Er, Yb doped tungstates,” Appl. Phys. B 64, 409–413 (1997). 14. D. L. Veasey, D. S. Funk, P. M. Peters, N. A. Sanford, G. E. Obarski, N. Fintaine, M. Young, A. P. Peskon, W.-C. Liu,
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S. N. Houde-Walter, and J. S. Hayden, “Yb/Er-codoped and Ybdoped waveguide lasers in phosphate glass,” J. Non-Cryst. Solids 263–264, 369–381 (2000). L. Yang and K. J. Vahala, “Gain functionalization of silica microresonators,” Opt. Lett. 28, 592–594 (2003). C. H. Dong, Y. F. Xiao, Z. F. Han, G. C. Guo, X. S. Jiang, L. M. Tong, C. Gu, and H. Ming, “Low-threshold microlaser in Er:Yb phosphate glass coated microsphere,” Photon. Technol. Lett. 20, 342–344 (2008). P. J. Zhang, Y. Huang, C. L. Guo, and Y. T. Huang, “Study of cascaded Raman scattering laser in silica microsphere pumped by 976 nm laser,” Acta Phys. Sin. 62, 24207 (2013). C. L. Guo, Y. Huang, P. J. Zhang, and Y. T. Huang, “Study of laser emission from Er3+-doped silica microsphere pumped by 976 nm light,” Chin. J. Las. 40, 0302004 (2013). Y. T. Huang, Y. Huang, P. J. Zhang, and C. L. Guo, “Ultralowthreshold laser and blue shift cooperative luminescence in a Yb3+ doped silica microsphere,” AIP Advances 4, 027113 (2014). Y. T. Huang, P. J. Zhang, Y. Huang, and C. L. Guo, “Triple and double photons absorption process and down-conversion laser emitting investigation of Er-ion doped microsphere,” J. Mod. Phys. 4, 1622–1626 (2013). J. C. Knight, G. Cheung, F. Jacques, and T. A. Birks, “Phasematched excitation of whispering-gallery-mode resonances by a fiber taper,” Opt. Lett. 22, 1129–1131 (1997).
22. M. Cai, O. Painter, and K. J. Vahala, “Fiber-coupled microsphere laser,” Opt. Lett. 25, 1430–1432 (2000). 23. E. Desurvire and J. R. Simpson, “Evaluation of 4I15/2 and 4I13/2 Stark-level energies in erbium-doped aluminosilicate glass fibers,” Opt. Lett. 15, 547–549 (1990). 24. H. T. Sun, C. L. Yu, Z. C. Duan, L. Wen, J. J. Zhang, L. L. Hu, and S. X. Dai, “Optical transitions of Er3+ and Yb3+ ions in novel oxyfluoride bismuth-germanium glass: observation of up-conversion,” Opt. Mater. 28, 448–452 (2006). 25. X. B. Yu, X. Zhao, C. M. Li, and H. Lin, “Infrared spectrum estimation for maximum phonon energy in optical glasses,” J. Dalian Polytechnic University 27, 155–157 (2008). 26. M. Shi, Y. G. Zhao, Q. Qian, D. D. Chen, and Q. Y. Zhang, “Role of PbO substitution by Bi2O3 on 1.47 μm luminescence properties of Tm3+/Tb3+ doped Bi2O3–GeO2–Ga2O3 glass,” J. Alloys Compounds 499, 126–130 (2010). 27. M. Cai and K. Vahala, “Highly efficient hybrid fiber taper coupled microsphere laser,” Opt. Lett. 26, 114–116 (2001). 28. B. Chen, H. Wang, W. Qin, W. Cao, W. Xu, and S. Huang, “Fluoride-oxide glass for high efficiency upconversion from IR to green,” Spectro. Spectral Anal 20, 257–259 (2000). 29. J. Huang, Y. T. Huang, T. J. Wu, Y. Huang, P. J. Zhang, and C. L. Guo, “The study of luminescent properties of Er3+/Yb3+ co-doped oxyfluoride glass ceramic microsphere with the tapered fiber-microsphere coupling system,” Acta Phys. Sin. 63, 127802 (2014).
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College of Physics and Information Engineering, Fuzhou University, Fuzhou 350108, China 2
e-mail: [email protected]
*Corresponding author: [email protected] Received 25 March 2014; revised 11 June 2014; accepted 16 June 2014; posted 18 June 2014 (Doc. ID 208760); published 17 July 2014
A fiber-taper-microsphere-coupled system was used to research the characteristics of laser oscillation and upconversion luminescence of Yb3 :Er3 co-doped phosphosilicate (YECP) microspheres. The YECP microspheres were fabricated by melting the end of phosphosilicate filaments. Single- and multimode laser oscillation at 1535–1565 nm within the C-band were obtained. In addition, the output power of the single-mode laser at 1545.5 nm can be as high as 48.98 μW, which was achieved under pump power of 9.63 mW, and the side-mode suppression ratio was 51.49 dB. Upconversion fluorescence of Er3 at 521, 532, and 544 nm also were measured, and the pump power dependence was studied. The fluorescence intensity was lower than that of Yb3 :Er3 co-doped silica and oxyfluoride glass ceramic microspheres. Moreover, the physical mechanism of upconversion suppression and laser oscillation enhancement observed in our experiment was presented, which is beneficial to the preparation of rare-earth-doped microcavity lasers. © 2014 Optical Society of America OCIS codes: (140.3510) Lasers, fiber; (140.3570) Lasers, single-mode; (140.3613) Lasers, upconversion; (140.3945) Microcavities; (160.5690) Rare-earth-doped materials; (230.4555) Coupled resonators. http://dx.doi.org/10.1364/AO.53.004747
1. Introduction
Optical microcavities have been studied for several years due to their unique characteristics and various applications [1,2]. The special whispering gallery modes (WGMs) of microsphere resonators usually bring about small volumes and an ultrahigh quality factor. Thus the pump power required for laser emission and nonlinear effects can be greatly reduced [3,4]. In the past decades, microcavities also have been widely applied in quantum optics [5], quantum electrodynamics [6], microsensors, and many other areas [7–9]. Rare-earth-ions-doped material usually works as a gain medium for microlasers. Among all the rareearth ions, Yb3 ion is one of the best candidates
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for a laser device, owing to its simple energy-level structure, high quantum efficiency, and larger absorption cross section than Er3 at around 980 nm. Er3 ion with abundant energy-level structures has a strong emission band at around 1550 nm, which is an appropriate wavelength within the low-loss communication window. Yb3 co-doped with Er3 ion also can act as a sensitizer. Compared with Er3 , it can absorb pump photons more efficiently and transfer to Er3 by collision, which realizes the population inversion of Er3 and produces a stronger laser in a microcavity. In previous researches, Er3 doped∕ Yb3 :Er3 co-doped materials already have been studied by means of implantation [10], sol-gel method [11], and so forth. The substrates are mainly made up of germanate glass [12], tungstate [13], phosphate glass [14], or silicate glass [11]. Yang and Vahala have reported a single-mode laser with an output power of 10 μW [15] in an Er3 -doped silicate microresonator. 20 July 2014 / Vol. 53, No. 21 / APPLIED OPTICS
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Dong et al. have fabricated a silicate microsphere covered with Yb3 :Er3 co-doped phosphate, and the single-mode microlaser had a maximum output power of only 15 μW [16]. In this paper, a fiber-taper-microsphere-coupled system was adopted to investigate the laser oscillation properties of Yb3 :Er3 co-doped phosphosilicate (YECP) microspheres. Single-mode lasing can be obtained at 1545.5 nm with a threshold of 1.83 mW, an output power maximum of 48.98 μW, and side-mode suppression ratio of 51.49 dB. Furthermore, multimode lasing also can be obtained near 1530 nm, which is within the eye-safe waveband. Moreover, upconversion fluorescence emissions of Er3 also were obtained at 521 nm (4 H11∕2 → 4 I15∕2 ), 532 nm (4 H11∕2 → 4 I15∕2 ), and 544 nm (4 S3∕2 → 4 I15∕2 ), and the upconversion was suppression. Under the condition of using the same structure of fiber taper, the experimental data is relatively optimized in this experiment. 2. Experiments
YECP glass is composed of 55.93P2 O5, 3.57Al2 O3 , 15Na2 CO3 , 20SiO2 , 5Yb2 O3 , and 0.3Er2 O3 . Wellmixed powders were put into a covered corundum crucible, transferred to an electric resistance furnace, and heated for 60 min at 1450°C; then the molten glass was immediately drawn into glass filaments with a rod glass. Under the effect of surface tension, the glass filaments of YECP were melted into microspheres with the voltaic arc of electrode discharge [17]. The diameter of YECP microspheres was controlled within a range from tens to hundreds of micrometers mainly by adjusting discharge intensity and frequency. The diameter of microspheres also was determined by that of glass filaments. Optical fiber tapers were fabricated by melting and drawing a standard single-mode fiber [18], which was used as a coupler—not only for launching pump power into the microspheres but also for coupling out the emitting laser [19,20]. Figure 1 reveals the
Fig. 1. Fabrication process of the YECP microsphere. 4748
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Fig. 2. Diagram of measurement setup.
fabrication process of the YECP microsphere. The schematic of the measurement setup is shown in Fig. 2. A three-axis manual control stage was adopted to couple the YECP microsphere tangent with a tapered fiber, and the pump source was a 976 nm semiconductor laser (the maximum of output power is 12 mW). Figure 2(a) presents a photomultiplier (PMT) grating spectrometer with measurement range from 200 to 660 nm; Fig. 2(b) is of an optical spectrum analyzer (OSA) (the measurement range is from 600 to 1700 nm). The inset in Fig. 2 is the coupling picture taken by a CCD camera. 3. Results and Discussions A. Laser Oscillation
As is known, WGMs are formed by repeated total internal reflection of light, which is eventually constrained on the surface of the microsphere. WGMs can be described by three mode numbers: radial mode number n, angular mode number l, and azimuthal mode number m [21]. When n is the minimum and m ≈ l, optical field distribution near the surface of the microsphere is approximate to the equator, and the field is compressed into the smallest spaces. In order to couple light into the microsphere efficiently, the evanescent wave field mode of the taper fiber must match with the WGMs of the microsphere. When the YECP microsphere is adequately coupled with the taper fiber, a single-mode laser can be achieved. Otherwise, a multimode laser can be observed [16]. In the experiment, single- and multimode lasing have been measured. As shown in Fig. 3, the YECP microsphere with the diameter of 52.5 μm produced a single-mode lasing at 1545.5 nm, with a threshold of about 1.83 mW. When the excitation power reached 9.63 mW, the output power of the single-mode laser could reach 48.98 μW, the FWHM of the single-mode laser was 0.17 nm, and the side-mode suppression ratio could be as high as 51.49 dB. To explore the characteristics of the lasing oscillation, we have collected the output power versus the absorbed power of the single-mode lasing at 1545.5 nm (Fig. 4). In the diagram, the threshold is about 1.83 mW, which is limited by the concentration of Er3 and Q value of the YECP microsphere.
Fig. 3. Single-mode laser oscillation with pump power of 9.63 mW; inset describes the single-mode laser at 1545.5 nm in detail.
Fig. 5. Multimode laser oscillation spectrum from 1535 to 1565 nm under pump power of 5.41 mW.
Fig. 6. Schematic of energy-level diagram of Er3 and Yb3 ions.
Fig. 4. Output power versus the absorbed power for the singlemode lasing at 1545.5 nm.
The concentration of Er3 should not be much higher; otherwise, lasing could not be obtained due to quenching. As Fig. 5 shows, by using the tested microsphere with a diameter of 104 μm, a multimode laser from 1535 to 1565 nm was obtained with the pump power of 5.41 mW. With the increase of pump power, the multimode lasing remains modulated by the basic morphology of the microsphere. With a view to elaborate on the laser oscillation process, Fig. 6 presents a schematic of the energylevel diagram of Yb3 and Er3 ions. During the downconversion laser oscillation process, Yb3 ion works as an energy transporter for Er3 ion in an indirect way. First, Yb3 ion absorbs pump photons and transfers the energy to Er3 ion by collision (Yb3 ion transits to the ground state by nonradiative transition), which makes Er3 ion transit to 4 I11∕2 (4 I11∕2 is a metastable level). Subsequently,
under the influence of a high phonon energy field, the electrons in the excited states of Er3 ion similarly transit from the metastable level to 4 I13∕2 in the form of nonradiative transition. Finally, Er3 ion transits to the ground state by radiative transition. The 4 I13∕2 → 4 I15∕2 transition of Er3 results in the lasing oscillation at around 1550 nm in the YECP microsphere [22,23]. In this process, the effectiveness of SiO2 instead of Pb2 O5 can more easily help to deduce the multiphonon relaxation (4 I11∕2 → 4 I13∕2 ) and enhance laser emission; the high phonon energy contributes to more excited state Er3 ions at a lower level (4 I13∕2 ). As a consequence, Er3 ions for generating upconversion fluorescence were reduced, and laser oscillation was enhanced. B. Upconversion Luminescence
Figure 7 shows the upconversion luminescence spectra within the range of 510–580 nm of an YECP microsphere whose diameter is 152 μm. We have set the PMT grating spectrometer parameters: V 700 V, g 4 (V is voltage, g is gain coefficient). The 2 H11∕2 → 4 I15∕2 transition of Er3 results in the emissions at 521 and 532 nm, and the 544 nm emission is due to the 4 S3∕2 → 4 I15∕2 transition [24]. The relationship between the upconversion luminescence 20 July 2014 / Vol. 53, No. 21 / APPLIED OPTICS
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I15∕2 has decreased. Therefore, under the environment of high phonon energy, most of the excited electrons are involved in the nonradiative process instead of the upconversion luminescence, which was thus restrained. In this experiment, cooperative luminescence and laser oscillation of Yb3 cannot be observed [19]. 4. Conclusion
Fig. 7. Luminescence spectra from 510 to 580 nm of the YECP microsphere.
intensity and the pump power of a 976 nm laser is also described by log–log plot in Fig. 8. The linear fitting slopes for 521, 532, and 544 nm are, respectively, 2.00, 1.99, and 1.84, which prove a double-photon process. As is well known, the major factor that affects the efficiency of rare-earth fluorescence is the maximal phonon energy of the host material [25,26]. The larger the maximal phonon energy of the host material is, the higher the rate of nonradiative relaxation and the lower efficiency of fluorescence. So the upconversion efficiency in the phosphosilicate material is lower than that of low phonon energy materials (such as SiO2 [27] and fluoride oxide [28]). Moreover, it is much lower than that in oxyfluoride glass ceramic [29]. In YECP with high phonon energy, the high transition rate from the metastable state 4 I11∕2 to 4 I13∕2 contributes to a large number of particles populated in the 4 I13∕2 state used for producing a downconversion laser. In contrast, there are fewer particles in the metastable 4 I11∕2 state with a shorter lifetime, which means the particles used for the transition to the ground-state
Fig. 8. Pump power dependence of upconversion luminescence intensity at 976 nm in the YECP microsphere. 4750
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