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Ytterbium-doped Y2O3 nanoparticle silica optical fibers for high power fiber lasers with suppressed photodarkening
Optics Communications 283 (2010) 3423–3427
Contents lists available at ScienceDirect
Optics Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / o p t c o m
Ytterbium-doped Y2O3 nanoparticle silica optical fibers for high power fiber lasers with suppressed photodarkening S. Yoo a,⁎, M.P. Kalita a, A.J. Boyland a, A.S. Webb a, R.J. Standish a, J.K. Sahu a, M.C. Paul b, S. Das b, S.K. Bhadra b, M. Pal b a b
Optoelectronic Research Centre, University of Southampton, Southampton So17 1BJ, United Kingdom Fiber Optics Laboratory, Central Glass and Ceramic Research Institute, CSIR, Kolkata 70032, India
a r t i c l e
i n f o
Article history: Received 29 January 2010 Received in revised form 27 April 2010 Accepted 27 April 2010 Keywords: Optical fiber Fiber laser Fiber material Laser material Fiber fabrication Fiber characterization
a b s t r a c t We report efficient laser demonstration and spectroscopic characteristics of a Yb-doped Y2O3 (or Y3Al5O12) nanoparticle silica fiber developed by conventional fiber fabrication technique. The spectroscopy study evidences modification in the environment of Yb ions by the Y2O3 nanoparticles. As a result, photodarkening induced loss is reduced by 20 times relative to Yb-doped aluminosilicate fibers. The fiber is suitable for power scaling with good laser slope efficiency of 79%. © 2010 Elsevier B.V. All rights reserved.
1. Introduction
2. Yb-doped nanoparticle based silica fiber fabrication
As the ytterbium-doped silica fiber laser make a breakthrough in power scaling to kilowatt level [1], the efforts to improve the host material properties become much more important. In particular, photodarkening observed in Yb-doped fibers is host material dependent. The photodarkening in Yb-doped fibers induces permanent excess loss in the pump and signal bands of Yb-doped fibers [2,3], which degrades the laser efficiency. The induced loss is much more pronounced when the Yb3+ ions are incorporated in aluminosilicate host than in phosphosilicate host [4,5] and also it is found to be temperature dependent. [6] Thus, the photodarkening appears to be controllable by modifying the host composition. In this paper, we investigate Yb-doped Y2O3 (or Y3Al5O12) nanoparticles in a silica-rich matrix, as an alternative to the Yb in a ‘standard’, such as aluminium or phosphorous co-doped, silica host for use in high power fiber lasers. It is expected that the Y2O3 nanoparticles within a silica host will modify the Yb environments, which influences host material dependent processes such as photodarkening and rare-earth solubility.
We incorporated Yb into thermally induced Y2O3 (or Y3Al5O12) nanoparticles within the SiO2–Al2O3–P2O5–Li2O–BaO core glass of an optical fiber preform by a standard MCVD-solution doping technique [7,8]. Our solution composition for making of optical preform consists of 0.025(M)YbCl 3 ·6H 2 O + 0.125(M) AlCl 3 ·6H 2 O + 0.06(M) YCl3·6H2O + 0.05(M) LiNO3 + 0.005(M) BaCl2·2H2O in (4:1) ethanol–water mixture. During the preform collapsing stage, which is performed at high temperature (N2000 °C), the Yb:Y2O3 (or Yb: Y3Al5O12) nanoparticles are phase-separated into the silica-rich core. We performed electron probe microanalyses (EPMA) measurement to quantify the composition in the fabricated preform. Fig. 1 summarizes the measured composition across preform core. The average doping levels within the central core region of optical fiber preform consists of Al2O3:0.9 mol%, Y2O3:0.5 mol%, Yb2O3:0.13 mol%, P2O5:0.3 mol%, BaO:0.05 mol% evaluated based on EPMA results. Here we are not able to detect the doping level of Li2O by EPMA due to low molecular weight of lithium atom. The D-shaped fibers were drawn in double clad structure from the preforms in 125 μm inner-cladding diameter. Fig. 2 represents refractive index profile of the nanoparticle fibers with crosssectional view in the inset. The fiber retained uniform index profile longitudinally. The background loss of the fibers was measured with high resolution optical time-domain reflectometer (OTDR) from Luciol. The background loss at 1285 nm varied between 40 and 400 dB/km, depending on the core composition and size of the
⁎ Corresponding author. Tel.: +44 23 8059 9254. E-mail address: [email protected] (S. Yoo). 0030-4018/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2010.04.093
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S. Yoo et al. / Optics Communications 283 (2010) 3423–3427
Fig. 1. The distribution of the doping level of (a) Al2O3, (b) Y2O3, (c) Yb2O3, (d) P2O5 and (e) BaO within the core region of optical fiber preform measured based on electron probe microanalyses (EPMA).
nanoparticles. Inner-cladding absorption at 976 nm was in the range of 4–10 dB/m. The presence of Y2O3 nanoparticles in the fiber was confirmed under transmission electron microscope (TEM) measurement as shown in Fig. 3. The size contribution was found to be in the 5.0– 8.0 nm region. Dark spots in the TEM image illustrate phase-separated region. The energy dispersive X-ray (EDX) spectra in Fig. 3 reveal that yttrium and ytterbium are dominant in phase-separated particles whilst they are sparse in the non-phase-separated region. In addition, the intensity of aluminium in phase-separated particles is much lower than that in non-phase-separated region. Thus, we concluded that majority of the ytterbium is located in the phase-separated yttria-rich region than the aluminium dominated non-phase-separated region. The nature of the phase-separated particles is found to be noncrystalline confirmed from their electron diffraction pattern as shown in the inset of Fig. 3. More detailed characteristics and fabrication of the fibers will be reported in somewhere else [9].
Fig. 2. Refractive index profile of Yb-doped nanoparticle silica fiber. Inset: Crosssectional view of the fiber with dimension measurement marked by white horizontal lines.
3. Spectroscopic properties of the Yb-doped nanoparticle silica fibers The spectroscopic properties of the prepared fibers were investigated and compared to a Yb-doped aluminosilicate (Yb:Al) fiber fabricated in-house. The Yb2O3 concentration in the Yb:Al fiber is ∼0.13 mol%. A fiber-coupled single mode laser diode at 915 nm was used as a pump source. The pump fiber was spliced to a fiber under test and the other end of the test fiber was angle cleaved to suppress any feedback. Index matching fluid was applied to the output end to further suppress the undesired feedback. The fluorescence was captured by placing one end of a multi-mode fiber to the side of the test fibers. An InGaAs photodetector was connected to the other end of the multi-mode fiber to record fluorescence decay time. The 915 nm laser diode was modulated externally by an acousto-optic modulator in the course of the lifetime measurement. We used less than 5 cm long fibers to avoid amplified spontaneous emission and reabsorption. Fig. 4 shows the measured decay time with the nanoparticle fiber and the Yb:Al fiber. The decay time was determined at the point where the intensity drops to 1/e of its original value. Both decay curves were well fitted with single exponential form. The decay time of the nanoparticle fiber was recorded as ∼860 μs which is close to that of Al:Yb fiber. The fitting goodness was better than 0.9996. We determined the spectral shape of fluorescence and absorption of the nanoparticle fiber. An optical spectrum analyser (OSA) replaced the photodetector in the lifetime measurement setup and the fluorescence was monitored against wavelength with the external modulation off. The Yb absorption spectrum was determined based on the cladding absorption using a white light source. Fig. 5 shows measured absorption and fluorescence spectra for the nanoparticle fiber compared to those of Yb:Al fiber. Each peak was scaled to unity to make a spectral shape comparison. We see that the spectral shape of Yb absorption in the nanoparticle fiber is modified against the Yb:Al counterpart, which again indicates the different environments for the Yb ions. However, the line shapes do not follow the Yb-doped Y2O3 ceramic [8] due to the glassy nature of the nanoparticles as confirmed in the TEM analysis [10].
S. Yoo et al. / Optics Communications 283 (2010) 3423–3427
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Fig. 3. TEM analyses of Yb-doped nanoparticle silica optical fiber sample. (a) Electron diffraction pattern and (b) EDX spectra on and out of the particles.
4. Laser performance of the Yb-doped nanoparticle fiber The Yb-doped nanoparticle fibers were tested under laser configuration. A D-shaped fiber in double clad structure was pulled in 400 μm inner-cladding diameter. The large inner-cladding diameter was chosen to enable an efficient pump launch from the high power pump diodes. The experimental arrangements are schematically shown in Fig. 6. The fiber was end-pumped by a 975 nm laser diode. Pump launch end of the fiber was cleaved perpendicularly to the fiber axis to provide 4% Fresnel reflection for the laser cavity. At the other end, a high reflective mirror (100%) at signal band was used to close
Fig. 4. Fluorescence decay time of Yb-doped nanoparticle silica fiber and Yb:Al fiber.
the laser cavity. Most of the launched pump beam was absorbed through the 5 m long fiber. The output power linearly increased with the launched pump power. The output reached 85 W for a launched pump power of 120 W, representing good slope efficiency of 76% with respect to the launched pump power [11]. We further investigated the lasing bands in nanoparticle fibers. Fibers with 125 μm inner-cladding diameter were placed in a freerunning linear 4%–4% laser cavity test bed and the output spectrum was recorded by an OSA. We found that both lasing wavelength and
Fig. 5. Spectral shapes of absorption (solid line) and fluorescence (dashed line) of Ybdoped nanoparticle silica fiber (red) and Yb:Al fiber (black).
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S. Yoo et al. / Optics Communications 283 (2010) 3423–3427
Fig. 6. Experimental arrangement for the laser performance characterization. LD: Laser Diode, DM: Dichroic Mirror, FUT: Fiber under test.
3 dB bandwidth are pump power dependent. Fig. 7(a) represents the pump power dependent of laser spectra in 0.2 nm of OSA resolution. In a 4 m long fiber, the oscillation started at 1057 nm with total 13 dB of pump power absorption. As we increased the pump power, another band appeared at longer wavelength, ∼ 1070 nm. The laser operated at two wavelength bands at ∼ 1050 and 1070 nm with a gap between. More intense pump power filled the gap and made up a broad band oscillation from 1040 to 1075 nm. The observed pump power dependent of laser operation band is different as compared to Yb-doped silica fibers in conventional hosts. However, it did not compromise the laser efficiency. The fiber provided laser efficiency of 79% with respect to the launched pump power as shown in Fig. 7(b). Identification of the cause of this behavior is under progress.
5. Photodarkening measurement As noted earlier, the pump induced photodarkening in Yb-doped fibers has been recognized as a bottleneck for power scaling in many
Fig. 7. (a) Pump power dependent lasing band in Yb-doped nanoparticle silica fiber in 4 m length (b) Laser performance of 4 m long Yb-doped nanoparticle silica fiber.
applications. It was found that the induced loss is proportional to the inversion level of the Yb3+ ions. Host material dependence was also reported and phosphosilicate can suppress the photodarkening in a significant amount compared to the aluminosilicate counterpart [4,5]. As the spectroscopic properties of the nanoparticle fiber indicate modification in the surrounding environments of the Yb ions, we expect different behavior of photodarkening in the fibers. The photodarkening of the fibers was evaluated by monitoring the transmitted probe power at 633 nm through the test fibers under 977 nm irradiation. The photodarkening measurement setup is presented in Fig. 8. We used fiber-coupled single mode 977 nm laser diode as a pump source. The output end of the pump fiber was spliced to one port of wavelength-division multiplexing (WDM) coupler and the pump beam was delivered to the test fiber by splicing the output end of the WDM coupler and the test fiber. A He–Ne laser at 633 nm was used as a probe beam which coupled to the test fiber through the WDM coupler. The probe beam propagated the same direction as the pump beam. The output end of the test fiber was spliced to another WDM coupler to separate the pump and probe beam. The probe beam was chopped by mechanical chopper and the output power was detected by photodiode and lock-in amplifier after passing through the monochromator. We used 1 cm of the test fiber to suppress unwanted amplified spontaneous emission. The pump input power was maintained to provide ∼ 35% of population inversion of Yb3+ throughout the fibers. We carried the photodarkening measurement with the Yb-doped nanoparticle fiber and the Yb:Al fiber. Both fibers were in 125 μm diameter. The small signal absorption in both fibers was around 3 dB/m. The temporal characteristics of the transmitted probe power are represented in Fig. 9. The photodarkening induced loss is significantly reduced in the Yb-doped nanoparticle fiber. When we fitted the measured results with stretched exponential form [4], we found that the saturated induced loss in Yb-doped nanocrystalline fiber reduced by 20 times compared to the aluminosilicate counterpart.
Fig. 8. Photodarkening induced loss measurement setup. FUT: Fiber under test, LD: Laser diode, WLS: White light source.
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fiber relative to conventional Yb-doped aluminosilicate fibers, which further favors power scaling. This class of fibers will keep the advantage of the mechanical properties of silica glass, whereas the surrounding of rare-earth ions can be engineered by varying the nanoparticle compositions. Acknowledgements This work was supported by the Royal Society, UK and CSIR, India under the joint collaborative research programme (2007/R2). References
Fig. 9. Temporal characteristics of transmitted power at 633 nm for Yb-doped nanoparticle fiber and Yb:Al fiber.
6. Conclusions In conclusion, we reported Yb-doped Y2O3 nanoparticle fibers fabricated by a conventional MCVD-solution doping technique. The spectroscopy study indicated modification of the local environment of Yb ions by the Y2O3 nanoparticles. The fiber was suitable for power scaling for high power fiber lasers with good slope efficiency of 79%. The laser oscillation band was pump power dependent and we could realize 35 nm 3 dB bandwidth covering from 1040 to 1075 nm. Moreover, we found suppressed photodarkening in the nanoparticle
[1] Y. Jeong, J.K. Sahu, D.N. Payne, J. Nilsson, Opt. Express 12 (2004) 6088. [2] J.J. Koponen, M.J. Söderlund, H.J. Hoffman, S.K.T. Tammela, Opt. Express 14 (2006) 11539. [3] S. Yoo, C. Basu, A.J. Boyland, C. Stone, J. Nilsson, J.K. Sahu, D. Payne, Opt. Lett. 32 (2007) 1626. [4] A.V. Shubin, M.V. Yashkov, M.A. Melkumov, S.A. Smirnov, L.A. Bufetov, E.M. Dianov, CLEO EU, 2007 CJ3-1-THU. [5] S. Jetschke, S. Unger, A. Schwuchow, M. Leich, J. Kirchhof, Opt. Express 16 (2008) 15540. [6] S. Yoo, A.J. Boyland, R.J. Standish, J.K. Sahu, Electron. Lett. 46 (2010) 233. [7] J.E. Townsend, S.B. Poole, D.N. Payne, Electron. Lett. 23 (1987) 329. [8] E. Bourova, B. Billancourt, S. Blanchandin, F. Leplingard, J. Lasas, US-patent, US 7355788 B2, 2008. [9] M.C. Paul, S. Bysakh, S. Das, S.K. Bhadra, M. Pal, J.K. Sahu, S. Yoo, M.P. Kalita, A.J. Boyland, (submitted for publication). [10] J. Ballato, T. Hawkins, P. Foy, B. Kokuoz, R. Stolen, C. McMillen, M. Daw, Z. Su, T.M. Tritt, M. Dubinskii, J. Zhang, T. Sanamyan, M.J. Matthewson, J. Appl. Phys. 105 (2009) 053110. [11] J.K. Sahu, M.C. Paul, M.P. Kalita, A. Boyland, C. Codemard, S. Yoo, A. Webb, R.J. Standish, J. Nilsson, S. Das, S.K. Bhadra, M. Pal, A. Dhar, R. Sen, CLEO EU, 2009 CJ2.1 WED.
Contents lists available at ScienceDirect
Optics Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / o p t c o m
Ytterbium-doped Y2O3 nanoparticle silica optical fibers for high power fiber lasers with suppressed photodarkening S. Yoo a,⁎, M.P. Kalita a, A.J. Boyland a, A.S. Webb a, R.J. Standish a, J.K. Sahu a, M.C. Paul b, S. Das b, S.K. Bhadra b, M. Pal b a b
Optoelectronic Research Centre, University of Southampton, Southampton So17 1BJ, United Kingdom Fiber Optics Laboratory, Central Glass and Ceramic Research Institute, CSIR, Kolkata 70032, India
a r t i c l e
i n f o
Article history: Received 29 January 2010 Received in revised form 27 April 2010 Accepted 27 April 2010 Keywords: Optical fiber Fiber laser Fiber material Laser material Fiber fabrication Fiber characterization
a b s t r a c t We report efficient laser demonstration and spectroscopic characteristics of a Yb-doped Y2O3 (or Y3Al5O12) nanoparticle silica fiber developed by conventional fiber fabrication technique. The spectroscopy study evidences modification in the environment of Yb ions by the Y2O3 nanoparticles. As a result, photodarkening induced loss is reduced by 20 times relative to Yb-doped aluminosilicate fibers. The fiber is suitable for power scaling with good laser slope efficiency of 79%. © 2010 Elsevier B.V. All rights reserved.
1. Introduction
2. Yb-doped nanoparticle based silica fiber fabrication
As the ytterbium-doped silica fiber laser make a breakthrough in power scaling to kilowatt level [1], the efforts to improve the host material properties become much more important. In particular, photodarkening observed in Yb-doped fibers is host material dependent. The photodarkening in Yb-doped fibers induces permanent excess loss in the pump and signal bands of Yb-doped fibers [2,3], which degrades the laser efficiency. The induced loss is much more pronounced when the Yb3+ ions are incorporated in aluminosilicate host than in phosphosilicate host [4,5] and also it is found to be temperature dependent. [6] Thus, the photodarkening appears to be controllable by modifying the host composition. In this paper, we investigate Yb-doped Y2O3 (or Y3Al5O12) nanoparticles in a silica-rich matrix, as an alternative to the Yb in a ‘standard’, such as aluminium or phosphorous co-doped, silica host for use in high power fiber lasers. It is expected that the Y2O3 nanoparticles within a silica host will modify the Yb environments, which influences host material dependent processes such as photodarkening and rare-earth solubility.
We incorporated Yb into thermally induced Y2O3 (or Y3Al5O12) nanoparticles within the SiO2–Al2O3–P2O5–Li2O–BaO core glass of an optical fiber preform by a standard MCVD-solution doping technique [7,8]. Our solution composition for making of optical preform consists of 0.025(M)YbCl 3 ·6H 2 O + 0.125(M) AlCl 3 ·6H 2 O + 0.06(M) YCl3·6H2O + 0.05(M) LiNO3 + 0.005(M) BaCl2·2H2O in (4:1) ethanol–water mixture. During the preform collapsing stage, which is performed at high temperature (N2000 °C), the Yb:Y2O3 (or Yb: Y3Al5O12) nanoparticles are phase-separated into the silica-rich core. We performed electron probe microanalyses (EPMA) measurement to quantify the composition in the fabricated preform. Fig. 1 summarizes the measured composition across preform core. The average doping levels within the central core region of optical fiber preform consists of Al2O3:0.9 mol%, Y2O3:0.5 mol%, Yb2O3:0.13 mol%, P2O5:0.3 mol%, BaO:0.05 mol% evaluated based on EPMA results. Here we are not able to detect the doping level of Li2O by EPMA due to low molecular weight of lithium atom. The D-shaped fibers were drawn in double clad structure from the preforms in 125 μm inner-cladding diameter. Fig. 2 represents refractive index profile of the nanoparticle fibers with crosssectional view in the inset. The fiber retained uniform index profile longitudinally. The background loss of the fibers was measured with high resolution optical time-domain reflectometer (OTDR) from Luciol. The background loss at 1285 nm varied between 40 and 400 dB/km, depending on the core composition and size of the
⁎ Corresponding author. Tel.: +44 23 8059 9254. E-mail address: [email protected] (S. Yoo). 0030-4018/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2010.04.093
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S. Yoo et al. / Optics Communications 283 (2010) 3423–3427
Fig. 1. The distribution of the doping level of (a) Al2O3, (b) Y2O3, (c) Yb2O3, (d) P2O5 and (e) BaO within the core region of optical fiber preform measured based on electron probe microanalyses (EPMA).
nanoparticles. Inner-cladding absorption at 976 nm was in the range of 4–10 dB/m. The presence of Y2O3 nanoparticles in the fiber was confirmed under transmission electron microscope (TEM) measurement as shown in Fig. 3. The size contribution was found to be in the 5.0– 8.0 nm region. Dark spots in the TEM image illustrate phase-separated region. The energy dispersive X-ray (EDX) spectra in Fig. 3 reveal that yttrium and ytterbium are dominant in phase-separated particles whilst they are sparse in the non-phase-separated region. In addition, the intensity of aluminium in phase-separated particles is much lower than that in non-phase-separated region. Thus, we concluded that majority of the ytterbium is located in the phase-separated yttria-rich region than the aluminium dominated non-phase-separated region. The nature of the phase-separated particles is found to be noncrystalline confirmed from their electron diffraction pattern as shown in the inset of Fig. 3. More detailed characteristics and fabrication of the fibers will be reported in somewhere else [9].
Fig. 2. Refractive index profile of Yb-doped nanoparticle silica fiber. Inset: Crosssectional view of the fiber with dimension measurement marked by white horizontal lines.
3. Spectroscopic properties of the Yb-doped nanoparticle silica fibers The spectroscopic properties of the prepared fibers were investigated and compared to a Yb-doped aluminosilicate (Yb:Al) fiber fabricated in-house. The Yb2O3 concentration in the Yb:Al fiber is ∼0.13 mol%. A fiber-coupled single mode laser diode at 915 nm was used as a pump source. The pump fiber was spliced to a fiber under test and the other end of the test fiber was angle cleaved to suppress any feedback. Index matching fluid was applied to the output end to further suppress the undesired feedback. The fluorescence was captured by placing one end of a multi-mode fiber to the side of the test fibers. An InGaAs photodetector was connected to the other end of the multi-mode fiber to record fluorescence decay time. The 915 nm laser diode was modulated externally by an acousto-optic modulator in the course of the lifetime measurement. We used less than 5 cm long fibers to avoid amplified spontaneous emission and reabsorption. Fig. 4 shows the measured decay time with the nanoparticle fiber and the Yb:Al fiber. The decay time was determined at the point where the intensity drops to 1/e of its original value. Both decay curves were well fitted with single exponential form. The decay time of the nanoparticle fiber was recorded as ∼860 μs which is close to that of Al:Yb fiber. The fitting goodness was better than 0.9996. We determined the spectral shape of fluorescence and absorption of the nanoparticle fiber. An optical spectrum analyser (OSA) replaced the photodetector in the lifetime measurement setup and the fluorescence was monitored against wavelength with the external modulation off. The Yb absorption spectrum was determined based on the cladding absorption using a white light source. Fig. 5 shows measured absorption and fluorescence spectra for the nanoparticle fiber compared to those of Yb:Al fiber. Each peak was scaled to unity to make a spectral shape comparison. We see that the spectral shape of Yb absorption in the nanoparticle fiber is modified against the Yb:Al counterpart, which again indicates the different environments for the Yb ions. However, the line shapes do not follow the Yb-doped Y2O3 ceramic [8] due to the glassy nature of the nanoparticles as confirmed in the TEM analysis [10].
S. Yoo et al. / Optics Communications 283 (2010) 3423–3427
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Fig. 3. TEM analyses of Yb-doped nanoparticle silica optical fiber sample. (a) Electron diffraction pattern and (b) EDX spectra on and out of the particles.
4. Laser performance of the Yb-doped nanoparticle fiber The Yb-doped nanoparticle fibers were tested under laser configuration. A D-shaped fiber in double clad structure was pulled in 400 μm inner-cladding diameter. The large inner-cladding diameter was chosen to enable an efficient pump launch from the high power pump diodes. The experimental arrangements are schematically shown in Fig. 6. The fiber was end-pumped by a 975 nm laser diode. Pump launch end of the fiber was cleaved perpendicularly to the fiber axis to provide 4% Fresnel reflection for the laser cavity. At the other end, a high reflective mirror (100%) at signal band was used to close
Fig. 4. Fluorescence decay time of Yb-doped nanoparticle silica fiber and Yb:Al fiber.
the laser cavity. Most of the launched pump beam was absorbed through the 5 m long fiber. The output power linearly increased with the launched pump power. The output reached 85 W for a launched pump power of 120 W, representing good slope efficiency of 76% with respect to the launched pump power [11]. We further investigated the lasing bands in nanoparticle fibers. Fibers with 125 μm inner-cladding diameter were placed in a freerunning linear 4%–4% laser cavity test bed and the output spectrum was recorded by an OSA. We found that both lasing wavelength and
Fig. 5. Spectral shapes of absorption (solid line) and fluorescence (dashed line) of Ybdoped nanoparticle silica fiber (red) and Yb:Al fiber (black).
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S. Yoo et al. / Optics Communications 283 (2010) 3423–3427
Fig. 6. Experimental arrangement for the laser performance characterization. LD: Laser Diode, DM: Dichroic Mirror, FUT: Fiber under test.
3 dB bandwidth are pump power dependent. Fig. 7(a) represents the pump power dependent of laser spectra in 0.2 nm of OSA resolution. In a 4 m long fiber, the oscillation started at 1057 nm with total 13 dB of pump power absorption. As we increased the pump power, another band appeared at longer wavelength, ∼ 1070 nm. The laser operated at two wavelength bands at ∼ 1050 and 1070 nm with a gap between. More intense pump power filled the gap and made up a broad band oscillation from 1040 to 1075 nm. The observed pump power dependent of laser operation band is different as compared to Yb-doped silica fibers in conventional hosts. However, it did not compromise the laser efficiency. The fiber provided laser efficiency of 79% with respect to the launched pump power as shown in Fig. 7(b). Identification of the cause of this behavior is under progress.
5. Photodarkening measurement As noted earlier, the pump induced photodarkening in Yb-doped fibers has been recognized as a bottleneck for power scaling in many
Fig. 7. (a) Pump power dependent lasing band in Yb-doped nanoparticle silica fiber in 4 m length (b) Laser performance of 4 m long Yb-doped nanoparticle silica fiber.
applications. It was found that the induced loss is proportional to the inversion level of the Yb3+ ions. Host material dependence was also reported and phosphosilicate can suppress the photodarkening in a significant amount compared to the aluminosilicate counterpart [4,5]. As the spectroscopic properties of the nanoparticle fiber indicate modification in the surrounding environments of the Yb ions, we expect different behavior of photodarkening in the fibers. The photodarkening of the fibers was evaluated by monitoring the transmitted probe power at 633 nm through the test fibers under 977 nm irradiation. The photodarkening measurement setup is presented in Fig. 8. We used fiber-coupled single mode 977 nm laser diode as a pump source. The output end of the pump fiber was spliced to one port of wavelength-division multiplexing (WDM) coupler and the pump beam was delivered to the test fiber by splicing the output end of the WDM coupler and the test fiber. A He–Ne laser at 633 nm was used as a probe beam which coupled to the test fiber through the WDM coupler. The probe beam propagated the same direction as the pump beam. The output end of the test fiber was spliced to another WDM coupler to separate the pump and probe beam. The probe beam was chopped by mechanical chopper and the output power was detected by photodiode and lock-in amplifier after passing through the monochromator. We used 1 cm of the test fiber to suppress unwanted amplified spontaneous emission. The pump input power was maintained to provide ∼ 35% of population inversion of Yb3+ throughout the fibers. We carried the photodarkening measurement with the Yb-doped nanoparticle fiber and the Yb:Al fiber. Both fibers were in 125 μm diameter. The small signal absorption in both fibers was around 3 dB/m. The temporal characteristics of the transmitted probe power are represented in Fig. 9. The photodarkening induced loss is significantly reduced in the Yb-doped nanoparticle fiber. When we fitted the measured results with stretched exponential form [4], we found that the saturated induced loss in Yb-doped nanocrystalline fiber reduced by 20 times compared to the aluminosilicate counterpart.
Fig. 8. Photodarkening induced loss measurement setup. FUT: Fiber under test, LD: Laser diode, WLS: White light source.
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fiber relative to conventional Yb-doped aluminosilicate fibers, which further favors power scaling. This class of fibers will keep the advantage of the mechanical properties of silica glass, whereas the surrounding of rare-earth ions can be engineered by varying the nanoparticle compositions. Acknowledgements This work was supported by the Royal Society, UK and CSIR, India under the joint collaborative research programme (2007/R2). References
Fig. 9. Temporal characteristics of transmitted power at 633 nm for Yb-doped nanoparticle fiber and Yb:Al fiber.
6. Conclusions In conclusion, we reported Yb-doped Y2O3 nanoparticle fibers fabricated by a conventional MCVD-solution doping technique. The spectroscopy study indicated modification of the local environment of Yb ions by the Y2O3 nanoparticles. The fiber was suitable for power scaling for high power fiber lasers with good slope efficiency of 79%. The laser oscillation band was pump power dependent and we could realize 35 nm 3 dB bandwidth covering from 1040 to 1075 nm. Moreover, we found suppressed photodarkening in the nanoparticle
[1] Y. Jeong, J.K. Sahu, D.N. Payne, J. Nilsson, Opt. Express 12 (2004) 6088. [2] J.J. Koponen, M.J. Söderlund, H.J. Hoffman, S.K.T. Tammela, Opt. Express 14 (2006) 11539. [3] S. Yoo, C. Basu, A.J. Boyland, C. Stone, J. Nilsson, J.K. Sahu, D. Payne, Opt. Lett. 32 (2007) 1626. [4] A.V. Shubin, M.V. Yashkov, M.A. Melkumov, S.A. Smirnov, L.A. Bufetov, E.M. Dianov, CLEO EU, 2007 CJ3-1-THU. [5] S. Jetschke, S. Unger, A. Schwuchow, M. Leich, J. Kirchhof, Opt. Express 16 (2008) 15540. [6] S. Yoo, A.J. Boyland, R.J. Standish, J.K. Sahu, Electron. Lett. 46 (2010) 233. [7] J.E. Townsend, S.B. Poole, D.N. Payne, Electron. Lett. 23 (1987) 329. [8] E. Bourova, B. Billancourt, S. Blanchandin, F. Leplingard, J. Lasas, US-patent, US 7355788 B2, 2008. [9] M.C. Paul, S. Bysakh, S. Das, S.K. Bhadra, M. Pal, J.K. Sahu, S. Yoo, M.P. Kalita, A.J. Boyland, (submitted for publication). [10] J. Ballato, T. Hawkins, P. Foy, B. Kokuoz, R. Stolen, C. McMillen, M. Daw, Z. Su, T.M. Tritt, M. Dubinskii, J. Zhang, T. Sanamyan, M.J. Matthewson, J. Appl. Phys. 105 (2009) 053110. [11] J.K. Sahu, M.C. Paul, M.P. Kalita, A. Boyland, C. Codemard, S. Yoo, A. Webb, R.J. Standish, J. Nilsson, S. Das, S.K. Bhadra, M. Pal, A. Dhar, R. Sen, CLEO EU, 2009 CJ2.1 WED.