Continuously-tunable Cherenkov-radiation-based detectors ...
Tunable Laser in Ytterbium-Doped ${\rm Y}_{2}{\rm O}_{3}$ Nanoparticle Optical Fibers
6
IEEE Proof IEEE PHOTONICS TECHNOLOGY LETTERS 1 Tunable Laser in Ytterbium-Doped Y 2 O 3 Nanoparticle Optical Fibers Kok-Sing Lim, Seongwoo Yoo, Mukul Chandra Paul, Harith Ahmad, Mrinmay Pal, Shyamal Kumar Bhadra, and Jayanta Kumar Sahu Abstract— We investigate tunability of fiber lasers in 1 ytterbium-doped Y 2 O 3 nanoparticle fibers. A net emission 2 cross section constructed from the spectroscopic measurements 3 reveals the feasibility of tunable laser operation in the fiber. 4 Broad tuning ranges of 1040–1108 nm and 1025–1090 nm are 5 demonstrated in 6- and 3-m-long fibers, respectively, with 76% 6 of maximum efficiency. 7 Index Terms— Fiber laser, fiber material, optical fiber, 8 ytterbium-doped fiber. 9 I. I NTRODUCTION 10 T IGHT confinement of optical beam attained in an optical 11 fiber over a distance favors high gain efficiency, which 12 has brought out many studies in silica fiber development 13 toward fiber based light sources such as rare-earth doped fibers 14 [1]–[4], glass-ceramic incorporated fiber [5], [6], transition- 15 metal doped fiber [7]–[9], and nanoparticle fiber [10], [11], 16 to name a few. Recent development of ytterbium (Yb)-doped 17 Y 2 O 3 nanoparticle fiber was found power scalable above 80 W 18 with excellent efficiency of 76% which is comparable to 19 conventional Yb-doped fibers [12]. Interestingly, the Yb-doped 20 nanoparticle fiber exhibits low photodarkening (PD) achieved 21 by modifying Yb environment with the nanoparticles [11]. Ce 22 co-doping is also helpful for reducing the photodarkening. [13] 23 General approaches to suppress the PD include host material 24 modification from Al:Yb to P:Yb [14] or P:Al:Yb [15]. The 25 material modification approach, however, invokes complicacy 26 in the fiber manufacturing due to the high vapor pressure 27 of phosphorus and, more importantly, it restricts the range 28 of laser operation below 1070 nm due to the spectroscopic 29 cross-sections of Yb in the phosphorous rich site [16]. 30 In contrast, the laser wavelength in Yb:Al fibers can extend 31 to 1178 nm [17]. 32 As the Yb in nanoparticle fiber exhibits similar shape of 33 absorption and emission as the Yb in aluminosilicate host [10], 34 Manuscript received December 29, 2011; revised January 25, 2012; accepted January 26, 2012. Date of publication February 3, 2012. K.-S. Lim and H. Ahmad are with the Photonic Research Centre, University of Malaya, Kuala Lumpur 50603, Malaysia (e-mail: [email protected]; [email protected]). S. Yoo is with the School of Electrical and Electronic Engineering, Nanyang Technological University, 637553, Singapore (e-mail: [email protected]). M. C. Paul, M. Pal, and S. K. Bhadra are with the Fiber Optics and Photonics Division, Central Glass and Ceramic Research Institute-CSIR, Kolkata 70032, India (e-mail: [email protected]; [email protected]; [email protected]). J. K. Sahu is with the Optoelectronic Research Centre, University of Southampton, Southampton SO17 1BJ, U.K. (e-mail: [email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2012.2186437 [11], the Yb-doped nanoparticle fiber is expected to have a 35 broad range of laser operation similar to the Yb:Al fiber laser 36 in conjunction with PD suppression. 37 Here we report on efficient tunable laser realized in the 38 Yb-doped Y 2 O 3 nanoparticle fiber by tuning laser wavelength 39 with external blazed grating in a Fabry-Perot linear cavity. The 40 laser performance in the tuning range is characterized in terms 41 of efficiency and threshold pump power. 42 II. FIBER CHARACTERISTICS 43 In the fabrication of Yb-doped Y 2 O 3 fiber, a 44 standard MCVD-solution doping technique was used 45 [1], [18]. To incorporate Yb:Y 2 O 3 nanoparticles into the 46 fiber, an alcoholic-water solution of a mixture of appropriate 47 strength of YbCl 3 .6H 2 O, AlCl 3 .6H 2 O, YCl 3 .6H 2 O, LiNO 3 48 and BaCl 2 .2H 2 O was used for soaking of the deposited 49 multiple porous phospho-silica layers at preform making 50 stage. During the sintering process, the Yb:Y 2 O 3 nanoparticles 51 were thermally induced in the SiO 2 -Al 2 O 3 -P 2 O 5 -Li 2 O-BaO 52 core glass. Then, the phase-separation of the nanoparticles in 53 the silica rich core glass was achieved by thermal treatment 54 of the collapsed preform at temperature >1200 °C The 55 preform was later drawn into a D-shaped double clad fiber 56 with a core diameter of 14 μm and a core NA of ∼0.14. 57 The nanoparticles still remain in the fiber even after fiber 58 drawing process. The presence of the nanoparticles after 59 the fiber drawing process was confirmed through the TEM 60 analysis [11]. The background loss of the fiber was measured 61 as 60 dB/km at 1285 nm. 62 Gain spectrum of the Yb-doped nanoparticle fiber is 63 attainable from net emission cross-section, σ net . To obtain 64 σ net , we first determined absorption cross-section from the 65 Yb-doped nanoparticle fiber. The absorption spectrum of the 66 fiber was measured with a white light source and an optical 67 spectrum analyzer. We quantified concentration of the Yb 68 ions as 1.03 × 10 26 m −3 in a core section of a preform from 69 the electron probe microanalyses measurement. Then the 70 absorption cross-section spectrum was determined by scaling 71 the absorption spectrum according to the concentration and 72 overlap factor between optical beam and doped area in a 73 fiber. The maximum absorption cross-section at 976 nm is 74 1.35 × 10 −24 m 2 which is smaller than that in the Yb:Al 75 (2.40 × 10 −24 m 2 ). We use the McCumber relation [19] 76 to define emission cross-section spectrum. Finally, the σ net 77 of the Yb-doped nanoparticle fiber is calculated from the 78 1041–1135/$31.00 © 2012 IEEE
Transcript of Tunable Laser in Ytterbium-Doped ${\rm Y}_{2}{\rm O}_{3}$ Nanoparticle Optical Fibers
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IEEE PHOTONICS TECHNOLOGY LETTERS 1
Tunable Laser in Ytterbium-Doped Y2O3Nanoparticle Optical Fibers
Kok-Sing Lim, Seongwoo Yoo, Mukul Chandra Paul, Harith Ahmad, Mrinmay Pal,Shyamal Kumar Bhadra, and Jayanta Kumar Sahu
Abstract— We investigate tunability of fiber lasers in1
ytterbium-doped Y2O3 nanoparticle fibers. A net emission2
cross section constructed from the spectroscopic measurements3
reveals the feasibility of tunable laser operation in the fiber.4
Broad tuning ranges of 1040–1108 nm and 1025–1090 nm are5
demonstrated in 6- and 3-m-long fibers, respectively, with 76%6
of maximum efficiency.7
Index Terms— Fiber laser, fiber material, optical fiber,8
ytterbium-doped fiber.9
I. INTRODUCTION10
T IGHT confinement of optical beam attained in an optical11
fiber over a distance favors high gain efficiency, which12
has brought out many studies in silica fiber development13
toward fiber based light sources such as rare-earth doped fibers14
[1]–[4], glass-ceramic incorporated fiber [5], [6], transition-15
metal doped fiber [7]–[9], and nanoparticle fiber [10], [11],16
to name a few. Recent development of ytterbium (Yb)-doped17
Y2O3 nanoparticle fiber was found power scalable above 80 W18
with excellent efficiency of 76% which is comparable to19
conventional Yb-doped fibers [12]. Interestingly, the Yb-doped20
nanoparticle fiber exhibits low photodarkening (PD) achieved21
by modifying Yb environment with the nanoparticles [11]. Ce22
co-doping is also helpful for reducing the photodarkening. [13]23
General approaches to suppress the PD include host material24
modification from Al:Yb to P:Yb [14] or P:Al:Yb [15]. The25
material modification approach, however, invokes complicacy26
in the fiber manufacturing due to the high vapor pressure27
of phosphorus and, more importantly, it restricts the range28
of laser operation below 1070 nm due to the spectroscopic29
cross-sections of Yb in the phosphorous rich site [16].30
In contrast, the laser wavelength in Yb:Al fibers can extend31
to 1178 nm [17].32
As the Yb in nanoparticle fiber exhibits similar shape of33
absorption and emission as the Yb in aluminosilicate host [10],34
Manuscript received December 29, 2011; revised January 25, 2012; acceptedJanuary 26, 2012. Date of publication February 3, 2012.
K.-S. Lim and H. Ahmad are with the Photonic Research Centre, Universityof Malaya, Kuala Lumpur 50603, Malaysia (e-mail: [email protected];[email protected]).
S. Yoo is with the School of Electrical and Electronic Engineering, NanyangTechnological University, 637553, Singapore (e-mail: [email protected]).
M. C. Paul, M. Pal, and S. K. Bhadra are with the Fiber Optics and PhotonicsDivision, Central Glass and Ceramic Research Institute-CSIR, Kolkata 70032,India (e-mail: [email protected]; [email protected]; [email protected]).
J. K. Sahu is with the Optoelectronic Research Centre, University ofSouthampton, Southampton SO17 1BJ, U.K. (e-mail: [email protected]).
Color versions of one or more of the figures in this letter are availableonline at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LPT.2012.2186437
[11], the Yb-doped nanoparticle fiber is expected to have a 35
broad range of laser operation similar to the Yb:Al fiber laser 36
in conjunction with PD suppression. 37
Here we report on efficient tunable laser realized in the 38
Yb-doped Y2O3 nanoparticle fiber by tuning laser wavelength 39
with external blazed grating in a Fabry-Perot linear cavity. The 40
laser performance in the tuning range is characterized in terms 41
of efficiency and threshold pump power. 42
II. FIBER CHARACTERISTICS 43
In the fabrication of Yb-doped Y2O3 fiber, a 44
standard MCVD-solution doping technique was used 45
[1], [18]. To incorporate Yb:Y2O3 nanoparticles into the 46
fiber, an alcoholic-water solution of a mixture of appropriate 47
strength of YbCl3.6H2O, AlCl3.6H2O, YCl3.6H2O, LiNO3 48
and BaCl2.2H2O was used for soaking of the deposited 49
multiple porous phospho-silica layers at preform making 50
stage. During the sintering process, the Yb:Y2O3 nanoparticles 51
were thermally induced in the SiO2-Al2O3-P2O5-Li2O-BaO 52
core glass. Then, the phase-separation of the nanoparticles in 53
the silica rich core glass was achieved by thermal treatment 54
of the collapsed preform at temperature >1200 °C The 55
preform was later drawn into a D-shaped double clad fiber 56
with a core diameter of 14 μm and a core NA of ∼0.14. 57
The nanoparticles still remain in the fiber even after fiber 58
drawing process. The presence of the nanoparticles after 59
the fiber drawing process was confirmed through the TEM 60
analysis [11]. The background loss of the fiber was measured 61
as 60 dB/km at 1285 nm. 62
Gain spectrum of the Yb-doped nanoparticle fiber is 63
attainable from net emission cross-section, σ net . To obtain 64
σ net , we first determined absorption cross-section from the 65
Yb-doped nanoparticle fiber. The absorption spectrum of the 66
fiber was measured with a white light source and an optical 67
spectrum analyzer. We quantified concentration of the Yb 68
ions as 1.03 × 1026 m−3 in a core section of a preform from 69
the electron probe microanalyses measurement. Then the 70
absorption cross-section spectrum was determined by scaling 71
the absorption spectrum according to the concentration and 72
overlap factor between optical beam and doped area in a 73
fiber. The maximum absorption cross-section at 976 nm is 74
1.35 × 10−24 m2 which is smaller than that in the Yb:Al 75
(2.40 × 10−24 m2). We use the McCumber relation [19] 76
to define emission cross-section spectrum. Finally, the σ net 77
of the Yb-doped nanoparticle fiber is calculated from the 78
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2 IEEE PHOTONICS TECHNOLOGY LETTERS
Fig. 1. Net emission cross sections, σnet , of Yb-doped nanoparticle fiber(red) and Yb:Al fiber (black) at 5% (solid line) and 10% (dashed line) ofpopulation inversion.
Fig. 2. Experimental setup of the tunable laser. LD: laser diode. M: mirror.DM: dichroic mirror. FUT: fiber under test. HR: high reflection. HT: hightransmission. λs : signal wavelength. λp : pump wavelength.
obtained cross-sections based on the expression below:79
σnet (λ) = [σe(λ) + σa(λ)]n2 − σa(λ) (1)80
where σ e, σ a are the emission and absorption cross sections81
respectively and n2 is fraction of excited ions, averaged82
over the fiber. The spectrally resolved σ net of Yb-doped83
nanoparticle fiber is compared to Yb:Al fiber in Fig. 1.84
We use cross-sections of Yb:Al from [20]. Note that the85
net emission cross-section linearly scales to optical gain86
spectrum. In the figure, we present σ net at 5% and 10% of87
population inversions which are possible for cladding pumped88
Yb-doped nanoparticle fiber lasers. The σ net of Yb in the89
nanoparticle possesses similar spectral profile as that of the90
Yb:Al. This indicates feasibility of broadband laser oscillation91
although the bandwidth of the positive σ net in the Yb in the92
nanoparticle is slightly narrower than the Yb:Al. The σ net of93
the Yb doped nanoparticle fiber is more narrowed in the 10%94
inversion level than in the 5% inversion, compared to the95
Yb:Al. Thus, it is conclusive that the Yb-doped nanoparticle96
fiber can be used as a tunable gain medium with slightly97
narrower operation bandwidth than the Yb:Al fiber laser. We98
note that the low values of cross-sections of the Yb in the99
nanoparticle contribute to lower σ net than that of Yb:Al.100
III. LASER EXPERIMENTS101
Fig. 2 shows the schematic diagram of the tunable laser102
setup. The fiber was pumped by a 975 nm laser diode through103
Fig. 3. Laser efficiencies of the (a) 6-m and (b) 3-m Yb-doped nanoparticlesilica fibers at different signal wavelengths. η: efficiency. The insets in (a) and(b) show the respective overlaid spectra of the output lasers. For better clarityof viewing, the peak of each spectrum is offset to 0 dBm. The maximumoutput and threshold power of the (c) 6-m-long and (d) 3-m-long fibers.
one end of the fiber via an aspheric lens, a reflective mirror 104
and a dichroic mirror with high transmission (HT) at signal 105
and high reflection (HR) at pump wavelength. The pump input 106
fiber end was flat-cleaved to provide ∼4% Fresnel reflection 107
to the laser cavity. The signal wave exits from the other end of 108
the fiber and passes through another aspheric lens. It is then 109
reflected by another dichroic mirror with HR at the signal, 110
and HT at the pump wavelength to a blazed diffraction grating 111
with 600 groove/mm and blaze wavelength at 1000 nm. The 112
grating was mounted on a rotation stage to provide tunable 113
wavelength feedback to the fiber. This end of the fiber was 114
angle-cleaved to reduce the Fresnel reflection to the cavity. 115
The transmitted pump power through the dichroic mirror could 116
be measured to estimate the pump absorption of the fiber. 117
Diffraction efficiency of the grating is approximately 70% 118
at 1 μm. 119
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LIM et al.: TUNABLE LASER IN Yb-DOPED Y2O3 NANOPARTICLE OPTICAL FIBERS 3
The performance of the tunable Yb-doped nanoparticle laser120
is presented in Fig. 3. The 6 m Yb-doped nanoparticle fiber121
permits 10 dB of pump absorption and marks slope efficiency122
of 44–76% in the tuning range of 1040–1108 nm as shown123
in Fig. 3(a). The highest efficiency is 76% at 1063 nm124
with the maximum output power of 3.54 W which is pump125
power limited. This efficiency is comparable to 79% efficiency126
obtained in the free-running laser configuration [11]. The127
efficiencies at the end wavelengths of the tuning range are128
54% and 44% at 1040 nm and 1108 nm, respectively. The129
maximum output power inversely follows the threshold power130
as shown in Fig. 3(c).131
As a comparison, the tuning range obtained in 3 m fiber132
shifts to the shorter wavelength in 1025–1090 nm due to its133
low pump absorption of 5 dB. The highest efficiency is 50%134
at 1050 nm with the highest output power of 2.11 W. The135
laser efficiencies at the ends of the tuning range are 39%136
at 1025 nm and 33% at 1090 nm. The relatively low laser137
efficiency results from incomplete pump absorption. We note138
here that our tuning range is narrower than the reported tunable139
Al:Yb fiber lasers [21], which complies with the net emission140
cross-sections in Fig. 1.141
IV. CONCLUSION142
We have demonstrated an efficient tunable laser in Yb-doped143
Y2O3 nanoparticle fibers. Under the influence of different144
pump absorption, the laser tuning range covers 1040–1108 nm145
and 1025–1090 nm for 10 and 5 dB of pump absorptions146
respectively. The combined tuning range is as broad as 83nm,147
comparable to tunable Yb:Al fiber lasers. The observed tunable148
operation is in agreement with the net emission cross-section149
of the fiber.150
REFERENCES151
[1] S. Poole, D. Payne, R. Mears, M. Fermann, and R. Laming, “Fabri-152
cation and characterization of low-loss optical fibers containing rare-153
earth ions,” J. Lightw. Technol., vol. 4, no. 7, pp. 870–876, Jul.154
1986.155
[2] Y. Jeong, J. Sahu, D. Payne, and J. Nilsson, “Ytterbium-doped large-core156
fiber laser with 1.36 kW continuous-wave output power,” Opt. Express,157
vol. 12, no. 25, pp. 6088–6092, Dec. 2004.158
[3] S. Yoo, et al., “Analysis of W-type waveguide for Nd-doped fiber laser159
operating near 940 nm,” Opt. Commun., vol. 247, nos. 1–3, pp. 153–162,160
2005.161
[4] P. F. Moulton, et al., “Tm-doped fiber lasers: Fundamentals and power 162
scaling,” IEEE J. Sel. Topics Quantum Electron., vol. 15, no. 1, pp. 163
85–92, Jan. 2009. 164
[5] S. Yoo, U.-C. Paek, and W.-T. Han, “Optical properties of the optical 165
fiber containing Co2+ doped ZnO Al2O3 SiO2 glass-ceramics,” J. Non- 166
Cryst. Solids, vol. 303, no. 2, pp. 291–295, 2002. 167
[6] W. Blanc, et al., “Fabrication of rare earth-doped transparent glass 168
ceramic optical fibers by modified chemical vapor deposition,” J. Amer. 169
Ceram. Soc., vol. 94, no. 8, pp. 2315–2318, Aug. 2011. 170
[7] V. Felice, B. Dussardier, J. K. Jones, G. Monnom, and D. B. Ostrowsky, 171
“Chromium-doped silica optical fibres: Influence of the core composition 172
on the Cr oxidation states and crystal field,” Opt. Mater., vol. 16, nos. 1– 173
2, pp. 269–277, 2001. 174
[8] V. V. Dvoyrin, V. M. Mashinsky, and E. M. Dianov, “Efficient bismuth- 175
doped fiber lasers,” IEEE J. Quantum Electron., vol. 44, no. 9, pp. 834– 176
840, Sep. 2008. 177
[9] S. Yoo, M. P. Kalita, J. Nilsson, and J. Sahu, “Excited state absorption 178
measurement in the 900–1250 nm wavelength range for bismuth-doped 179
silicate fibers,” Opt. Lett., vol. 34, no. 4, pp. 530–532, 2009. 180
[10] A. V. Kir’yanov, et al., “Fabrication and characterization of new Yb- 181
doped zirconia-germano-alumino silicate phase-separated nano-particles 182
based fibers,” Opt. Express, vol. 19, no. 16, pp. 14823–14837, 2011. 183
[11] S. Yoo, et al., “Ytterbium-doped Y2O3 nanoparticle silica optical fibers 184
for high power fiber lasers with suppressed photodarkening,” Opt. 185
Commun., vol. 283, no. 18, pp. 3423–3427, Sep. 2010. 186
[12] J. K. Sahu, et al., “Ytterbium-doped nanostructured optical fibers for 187
high power fiber lasers,” in Proc. CLEO 2009, p. 1, paper CJ2_1. 188
[13] P. Jelger, M. Engholm, L. Norin, and F. Laurell, “Degradation-resistant 189
lasing at 980 nm in a Yb/Ce/Al-doped silica fiber,” J. Opt. Soc. Amer. 190
B, vol. 27, no. 2, pp. 338–342, 2010. 191
[14] J. K. Sahu, et al., “488 nm irradiation induced photodarkening study 192
of Yb-doped aluminosilicate and phosphosilicate fibers,” in Proc. Conf. 193
Lasers Electro-Opt./Quantum Electron. Laser Sci. Conf. Photon. Appl. 194
Syst. Technol., 2008, pp. 1–3, paper JTuA27. 195
[15] S. Jetschke, S. Unger, A. Schwuchow, M. Leich, and J. Kirchhof, 196
“Efficient Yb laser fibers with low photodarkening by optimization of 197
the core composition,” Opt. Express, vol. 16, no. 20, pp. 15540–15545, 198
Sep. 2008. 199
[16] A. S. Kurkov, “Oscillation spectral range of Yb-doped fiber lasers,” 200
Laser Phys. Lett., vol. 4, no. 2, pp. 93–102, Feb. 2007. 201
[17] M. P. Kalita, et al., “Multi-watts narrow-linewidth all fiber Yb-doped 202
laser operating at 1179 nm,” Opt. Express, vol. 18, no. 6, pp. 5920– 203
5925, 2010. 204
[18] M. C. Paul, et al., “Yb2O3-doped YAG nano-crystallites in silica-based 205
core glass matrix of optical fiber preform,” Mater. Sci. Eng.: B, vol. 175, 206
no. 2, pp. 108–119, Nov. 2010. 207
[19] D. E. McCumber, “Einstein relations connecting broadband emission 208
and absorption spectra,” Phys. Rev., vol. 136, no. 4A, pp. A954–A957, 209
Nov. 1964. 210
[20] H. Injeyan and G. D. Goodno, High-Power Laser Handbook. New York: 211
McGraw-Hill, 2011, pp. 420–424. 212
[21] J. Nilsson, J. A. Alavarez-Chavez, P. W. Turner, W. A. Clarkson, C. C. 213
Renaud, and A. B. Grudinin, “Widely tunable high-power diode-pumped 214
double-clad Yb3+-doped fiber laser,” in Proc. Adv. Solid State Lasers, 215
vol. 26. 1999, pp. 1–3, paper WA2. 216
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IEEE PHOTONICS TECHNOLOGY LETTERS 1
Tunable Laser in Ytterbium-Doped Y2O3Nanoparticle Optical Fibers
Kok-Sing Lim, Seongwoo Yoo, Mukul Chandra Paul, Harith Ahmad, Mrinmay Pal,Shyamal Kumar Bhadra, and Jayanta Kumar Sahu
Abstract— We investigate tunability of fiber lasers in1
ytterbium-doped Y2O3 nanoparticle fibers. A net emission2
cross section constructed from the spectroscopic measurements3
reveals the feasibility of tunable laser operation in the fiber.4
Broad tuning ranges of 1040–1108 nm and 1025–1090 nm are5
demonstrated in 6- and 3-m-long fibers, respectively, with 76%6
of maximum efficiency.7
Index Terms— Fiber laser, fiber material, optical fiber,8
ytterbium-doped fiber.9
I. INTRODUCTION10
T IGHT confinement of optical beam attained in an optical11
fiber over a distance favors high gain efficiency, which12
has brought out many studies in silica fiber development13
toward fiber based light sources such as rare-earth doped fibers14
[1]–[4], glass-ceramic incorporated fiber [5], [6], transition-15
metal doped fiber [7]–[9], and nanoparticle fiber [10], [11],16
to name a few. Recent development of ytterbium (Yb)-doped17
Y2O3 nanoparticle fiber was found power scalable above 80 W18
with excellent efficiency of 76% which is comparable to19
conventional Yb-doped fibers [12]. Interestingly, the Yb-doped20
nanoparticle fiber exhibits low photodarkening (PD) achieved21
by modifying Yb environment with the nanoparticles [11]. Ce22
co-doping is also helpful for reducing the photodarkening. [13]23
General approaches to suppress the PD include host material24
modification from Al:Yb to P:Yb [14] or P:Al:Yb [15]. The25
material modification approach, however, invokes complicacy26
in the fiber manufacturing due to the high vapor pressure27
of phosphorus and, more importantly, it restricts the range28
of laser operation below 1070 nm due to the spectroscopic29
cross-sections of Yb in the phosphorous rich site [16].30
In contrast, the laser wavelength in Yb:Al fibers can extend31
to 1178 nm [17].32
As the Yb in nanoparticle fiber exhibits similar shape of33
absorption and emission as the Yb in aluminosilicate host [10],34
Manuscript received December 29, 2011; revised January 25, 2012; acceptedJanuary 26, 2012. Date of publication February 3, 2012.
K.-S. Lim and H. Ahmad are with the Photonic Research Centre, Universityof Malaya, Kuala Lumpur 50603, Malaysia (e-mail: [email protected];[email protected]).
S. Yoo is with the School of Electrical and Electronic Engineering, NanyangTechnological University, 637553, Singapore (e-mail: [email protected]).
M. C. Paul, M. Pal, and S. K. Bhadra are with the Fiber Optics and PhotonicsDivision, Central Glass and Ceramic Research Institute-CSIR, Kolkata 70032,India (e-mail: [email protected]; [email protected]; [email protected]).
J. K. Sahu is with the Optoelectronic Research Centre, University ofSouthampton, Southampton SO17 1BJ, U.K. (e-mail: [email protected]).
Color versions of one or more of the figures in this letter are availableonline at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LPT.2012.2186437
[11], the Yb-doped nanoparticle fiber is expected to have a 35
broad range of laser operation similar to the Yb:Al fiber laser 36
in conjunction with PD suppression. 37
Here we report on efficient tunable laser realized in the 38
Yb-doped Y2O3 nanoparticle fiber by tuning laser wavelength 39
with external blazed grating in a Fabry-Perot linear cavity. The 40
laser performance in the tuning range is characterized in terms 41
of efficiency and threshold pump power. 42
II. FIBER CHARACTERISTICS 43
In the fabrication of Yb-doped Y2O3 fiber, a 44
standard MCVD-solution doping technique was used 45
[1], [18]. To incorporate Yb:Y2O3 nanoparticles into the 46
fiber, an alcoholic-water solution of a mixture of appropriate 47
strength of YbCl3.6H2O, AlCl3.6H2O, YCl3.6H2O, LiNO3 48
and BaCl2.2H2O was used for soaking of the deposited 49
multiple porous phospho-silica layers at preform making 50
stage. During the sintering process, the Yb:Y2O3 nanoparticles 51
were thermally induced in the SiO2-Al2O3-P2O5-Li2O-BaO 52
core glass. Then, the phase-separation of the nanoparticles in 53
the silica rich core glass was achieved by thermal treatment 54
of the collapsed preform at temperature >1200 °C The 55
preform was later drawn into a D-shaped double clad fiber 56
with a core diameter of 14 μm and a core NA of ∼0.14. 57
The nanoparticles still remain in the fiber even after fiber 58
drawing process. The presence of the nanoparticles after 59
the fiber drawing process was confirmed through the TEM 60
analysis [11]. The background loss of the fiber was measured 61
as 60 dB/km at 1285 nm. 62
Gain spectrum of the Yb-doped nanoparticle fiber is 63
attainable from net emission cross-section, σ net . To obtain 64
σ net , we first determined absorption cross-section from the 65
Yb-doped nanoparticle fiber. The absorption spectrum of the 66
fiber was measured with a white light source and an optical 67
spectrum analyzer. We quantified concentration of the Yb 68
ions as 1.03 × 1026 m−3 in a core section of a preform from 69
the electron probe microanalyses measurement. Then the 70
absorption cross-section spectrum was determined by scaling 71
the absorption spectrum according to the concentration and 72
overlap factor between optical beam and doped area in a 73
fiber. The maximum absorption cross-section at 976 nm is 74
1.35 × 10−24 m2 which is smaller than that in the Yb:Al 75
(2.40 × 10−24 m2). We use the McCumber relation [19] 76
to define emission cross-section spectrum. Finally, the σ net 77
of the Yb-doped nanoparticle fiber is calculated from the 78
1041–1135/$31.00 © 2012 IEEE
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2 IEEE PHOTONICS TECHNOLOGY LETTERS
Fig. 1. Net emission cross sections, σnet , of Yb-doped nanoparticle fiber(red) and Yb:Al fiber (black) at 5% (solid line) and 10% (dashed line) ofpopulation inversion.
Fig. 2. Experimental setup of the tunable laser. LD: laser diode. M: mirror.DM: dichroic mirror. FUT: fiber under test. HR: high reflection. HT: hightransmission. λs : signal wavelength. λp : pump wavelength.
obtained cross-sections based on the expression below:79
σnet (λ) = [σe(λ) + σa(λ)]n2 − σa(λ) (1)80
where σ e, σ a are the emission and absorption cross sections81
respectively and n2 is fraction of excited ions, averaged82
over the fiber. The spectrally resolved σ net of Yb-doped83
nanoparticle fiber is compared to Yb:Al fiber in Fig. 1.84
We use cross-sections of Yb:Al from [20]. Note that the85
net emission cross-section linearly scales to optical gain86
spectrum. In the figure, we present σ net at 5% and 10% of87
population inversions which are possible for cladding pumped88
Yb-doped nanoparticle fiber lasers. The σ net of Yb in the89
nanoparticle possesses similar spectral profile as that of the90
Yb:Al. This indicates feasibility of broadband laser oscillation91
although the bandwidth of the positive σ net in the Yb in the92
nanoparticle is slightly narrower than the Yb:Al. The σ net of93
the Yb doped nanoparticle fiber is more narrowed in the 10%94
inversion level than in the 5% inversion, compared to the95
Yb:Al. Thus, it is conclusive that the Yb-doped nanoparticle96
fiber can be used as a tunable gain medium with slightly97
narrower operation bandwidth than the Yb:Al fiber laser. We98
note that the low values of cross-sections of the Yb in the99
nanoparticle contribute to lower σ net than that of Yb:Al.100
III. LASER EXPERIMENTS101
Fig. 2 shows the schematic diagram of the tunable laser102
setup. The fiber was pumped by a 975 nm laser diode through103
Fig. 3. Laser efficiencies of the (a) 6-m and (b) 3-m Yb-doped nanoparticlesilica fibers at different signal wavelengths. η: efficiency. The insets in (a) and(b) show the respective overlaid spectra of the output lasers. For better clarityof viewing, the peak of each spectrum is offset to 0 dBm. The maximumoutput and threshold power of the (c) 6-m-long and (d) 3-m-long fibers.
one end of the fiber via an aspheric lens, a reflective mirror 104
and a dichroic mirror with high transmission (HT) at signal 105
and high reflection (HR) at pump wavelength. The pump input 106
fiber end was flat-cleaved to provide ∼4% Fresnel reflection 107
to the laser cavity. The signal wave exits from the other end of 108
the fiber and passes through another aspheric lens. It is then 109
reflected by another dichroic mirror with HR at the signal, 110
and HT at the pump wavelength to a blazed diffraction grating 111
with 600 groove/mm and blaze wavelength at 1000 nm. The 112
grating was mounted on a rotation stage to provide tunable 113
wavelength feedback to the fiber. This end of the fiber was 114
angle-cleaved to reduce the Fresnel reflection to the cavity. 115
The transmitted pump power through the dichroic mirror could 116
be measured to estimate the pump absorption of the fiber. 117
Diffraction efficiency of the grating is approximately 70% 118
at 1 μm. 119
IEEE
Proo
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LIM et al.: TUNABLE LASER IN Yb-DOPED Y2O3 NANOPARTICLE OPTICAL FIBERS 3
The performance of the tunable Yb-doped nanoparticle laser120
is presented in Fig. 3. The 6 m Yb-doped nanoparticle fiber121
permits 10 dB of pump absorption and marks slope efficiency122
of 44–76% in the tuning range of 1040–1108 nm as shown123
in Fig. 3(a). The highest efficiency is 76% at 1063 nm124
with the maximum output power of 3.54 W which is pump125
power limited. This efficiency is comparable to 79% efficiency126
obtained in the free-running laser configuration [11]. The127
efficiencies at the end wavelengths of the tuning range are128
54% and 44% at 1040 nm and 1108 nm, respectively. The129
maximum output power inversely follows the threshold power130
as shown in Fig. 3(c).131
As a comparison, the tuning range obtained in 3 m fiber132
shifts to the shorter wavelength in 1025–1090 nm due to its133
low pump absorption of 5 dB. The highest efficiency is 50%134
at 1050 nm with the highest output power of 2.11 W. The135
laser efficiencies at the ends of the tuning range are 39%136
at 1025 nm and 33% at 1090 nm. The relatively low laser137
efficiency results from incomplete pump absorption. We note138
here that our tuning range is narrower than the reported tunable139
Al:Yb fiber lasers [21], which complies with the net emission140
cross-sections in Fig. 1.141
IV. CONCLUSION142
We have demonstrated an efficient tunable laser in Yb-doped143
Y2O3 nanoparticle fibers. Under the influence of different144
pump absorption, the laser tuning range covers 1040–1108 nm145
and 1025–1090 nm for 10 and 5 dB of pump absorptions146
respectively. The combined tuning range is as broad as 83nm,147
comparable to tunable Yb:Al fiber lasers. The observed tunable148
operation is in agreement with the net emission cross-section149
of the fiber.150
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