Parameter
Unit
Symbol
Value
Remarks
Thulium Cone. m-^ 3.2 10^^ Specification.
Core diam. )im 4.4 Specification.
NA 0 .17 Specification.
Background lo ss @ 800 nm dB/m 0.1 Specification.
Signal stim ulated em ission cr o ss section
m" 2 10'''’ M easured (ch. 2)
Signal ESA cross-section m^ 0.9 lO ""^ M easured (ch. 2)
Abs. c ro ss section 8 00 nm m"
a 1.9 lO '"'’ [15]
A bs cro ss section 1.064 )im m" 3 10''*' M easured (ch. 2)
m" 9 10'^^ M easured (ch. 2)
m" 2.5 10'^^ [17]
C onfinem ent factor 800nm 0.52 Measured.
C onfinem ent factor 1 .064)im
0.94 Estimated [18].
S p o n ta n eo u s em ission rate s-^ As,
As2 As4 A 4, A42 A2I 56 4 .7 21 4 .0 138.3 61 5 .6 75.1 123.8
U sed for the rate equations [13]
Figure 6 .11 sh ow s the calculated gain as a function o f the tw o pump pow ers. The gain measured for 3 0 m W o f Ti:Sapphire and 60 mW o f N d:Y ag is 7.2 dB and the calculated value is 7 .0 4 dB w hile with 60 mW o f Ti:Sapphire the m easured gain is 12.5 dB as show n in fig. 6 .4 w h ile the calculated gain is 13.2 dB, sh ow ing good agreem ent. For a giv en Ti:Sapphire power, a fast increase in gain with N d:Y A G pow er is seen, due to the depletion o f the terminating level, after which the gain is alm ost independent o f the N d :Y A G p ow er due to the sm all G S A absorption at 1.064 )j.m.
Gain
0.2 0.18 0.16 0.14 w 0.12 0> O 0.1 CL TO 0 08 >- 0.06 o CO 0.04 0.02 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2TiiSapphire Power, [W]
Figure 6.11: Calculated gain for different values of the two pump powers.
The transition between these two regimes can be seen as a maximum in the PCE. At a given Ti:Sapphire power, the PCE will sharply rise with the increase o f the Nd:YAG power and then as the terminating level is bleached the PCE will decrease. The contour of maximum PCE (fig. 6.12) gives the optimum pump power combination and can be approximated by an empirical quadratic equation given the best ratio between
Ti:Sapphire power and YAG power as:
^Nd-Yag TiSapphire-
We have defined the PCE as the number of stimulated emitted photons normalised by the total number of pump photons (eq. 6.2); considering only the two strong absorption mechanism the theoretical limit will be 50%, however a more reasonable figure
observed in a real amplifier [19] is around 30% which is close to the limit reached by PCE for Ti;Sapphire power above 150 mW and YAG power above 75 mW. A longer fibre length is not needed since the theoretical PCE performance can be achieved in the short fibre and longer fibre will reduce the obtainable theoretical limit due to the scattering loss associated with the two pumps and the signal.
PC E
0.2 0.18 0.16 o 0.14 t . ' 0 .1 2 0) o 0.1 CL 0.08 > - 0.06 oo 0.04 0.02 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2Ti:Sapphire Pow er, [W]
Figure 6.12: Calculated PCE for different values of the two pump powers. The maximum value for the PCE with respect to the pumps is shown as a dashed line.
In figure 6.13 we report the expected gain within the fibre length for the two pump powers. Starting from the left hand side of fig. 6.13 it is possible to calculate the 800 nm power required in order to obtain the maximum PCE and the gain obtained as a function of the length of the active fibre. The method can be used for different pumping schemes and can be tailored in order to fulfil different types o f conditions and fibre geometry.
0.5 1.0 1.5 2.0 2.5 3.0 3.5
Fibre Length, [m]\ Figure 6.13: Gain predicted within the fibre for the two pumps when the power for the
Nd:YAG is correlated to the TirSapphire by the empirical relationship 6.9.
6.8 Conclusions
W e have constructed a 4 m length thulium doped fibre am plifier pum ped by a N d:Y A G (1.0 64 |im ) laser and a Ti:Sapphire ( 800 nm) laser with the intention o f optim ising the operation o f the am plifier for high signal gain at a w avelength above 1.49 )xm.
Experim ental observations suggest the best pump w avelength is located at 800 nm for the Ti:Sapphire and practical signal operation o f - 1 0 dBm does not considerably affect the gain at 1.49 )J,m.
It has been demonstrated that the transient gain is connected to the lifetim e o f the *G4^ ^ H6 transition, w hile low cross talk at saturated operation is expected.
High signal operations require the m axim um photon conversion efficien cy , so w e have d evelop ed a m odel, not lim ited to the sm all-signal gain, in order to find the operation points w here the m axim um PCE is obtained for the low est optical p ow er o f the tw o pum ps involved.
For a giv en value o f the Ti:Sapphire p ow er w e established that the corresponding value for the N d:Y A G pow er ob eys to an em pirical quadratic relationship.
T he gain within the fibre length o f the am plifier has been calculated so the required length in order to ach ieve the desired gain can be found.
6.9 References
[1] F. Roy, "Recent advances in thulium-doped fiber amplifiers," presented at Optical Fiber Communication Conference and Exhibit, 2002. OFC 2002, 2002. [2] R. Caspary, U. B. Unrau, and W. Kowalsky, "Recent progress on S-band fiber amplifiers," presented at Transparent Optical Networks, 2003. Proceedings of 2003 5th International Conference on, 2003.
[3] V. P. Gapontsev, D. V. Gapontsev, N. S. Platonov, O. Shkurihin, V. Fomin, and S. Ferin, "2 kW CW ytterbium fiber laser with record diffraction-limited
brightness," presented at CLEO Europe 2005, Munich (DE), 2005.
[4] C. Alegria, Y. Jeong, C. Codemard, J. K. Sahu, J. A. Alvarez-Chavez, L. Fu, M. Ibsen, and J. Nilsson, "83-W single-frequency narrow-linewidth MOPA using large-core erbium-ytterbium Co-doped fiber," Photonics Technology Letters, IEEE, vol. 16, pp. 1825-1827, 2004.
[5] B. Bourliaguet, F. Emond, S. Mohrdiek, A.-C. Jacob-Poulin, P.-Y. Cortes, and J. Lauzon, "Thulium-doped fibre amplifier using 1055 nm laser diode pumping configuration," Electronics Letters, vol. 38, pp. 447-448, 2002.
[6] S. Aozasa, H. Masuda, H. Ono, T. Sakamoto, T. Kanamori, Y. Ohishi, and M. Shimizu, "1480-1510 nm band Tm-doped fibre amplifier with high power conversion efficiency of 42%," Electronics Letters, vol. 37, pp. 1157-1158,
2001
.[7] B. Pedersen, W. J. Miniscalco, and S. A. Zemon, "Evaluation of the 800 nm pump band for erbium-doped fiber amplifiers," Lightwave Technology, Journal of, vol. 10, pp. 1041-1049, 1992.
[8] R. I. Laming, J. E. Townsend, D. N. Payne, F. Meli, G. Grasso, and E. J. Tarbox, "High-power erbium-doped-fiber amplifiers operating in the saturated regime,"
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[9] T. Rasmussen, A. Bjarklev, O. Lumholt, M. Obro, B. Pedersen, J. H. Povlsen, and K. Rottwitt, "Optimum design of Nd-doped fiber optical amplifiers,"
Photonics Technology Letters, IEEE, vol. 4, pp. 49-51, 1992.
[10] E. Desurvire, "Analysis of transient gain saturation and recovery in erbium- doped fiber amplifiers," Photonics Technology Letters, IEEE, vol. 1, pp. 196- 199,1989.
[11] B. M. Walsh and N. P. Barnes "Comparison of Tm:ZBLAN and Tm:silica fiber lasers; Spectroscopy and tunable pulsed laser operation around 1.9 |im," Applied
[12] S. C. Fleming, "Crosstalk in 1.3 |lm praseodymium fluoride fiber amplifiers," Lightwave Technology, Journal of, vol. 14, pp. 66-71, 1996.
[13] I. Joindot and F. Dupre, "Spectral hole burning in silica-based and in fluoride- based optical fibre amplifiers," Electronics Letters, vol. 33, pp. 1239-1240,
1997.
[14] C. R. Giles and E. Desurvire, "Modeling erbium-doped fiber amplifiers," Lightwave Technology, Journal of, vol. 9, pp. 271-283, 1991.
[15] T. Kasamatsu, Y. Yano, and T. Ono, " 1.49-|im-band gain-shifted thulium-doped fiber amplifier for WDM transmission systems," Lightwave Technology, Journal o f vol. 20, pp. 1826-1838, 2002.
[16] F. F. Ruhl, "Figures of merit for doped fibre amplifiers in 1300 nm and 1550 nm windows," Electronics Letters, vol. 27, pp. 1605-1607, 1991.
[17] T. Komukai, T. Yamamoto, T. Sugawa, and Y. Miyajima, "Upconversion pumped thulium-doped fluoride fiber amplifier and laser operating at 1.47 |J.m," Quantum Electronics, IEEE Journal of, vol. 31, pp. 1880-1889, 1995.
[18] T. J. Whitley and R. Wyatt, "Alternative Gaussian spot size polynomial for use with doped fiber amplifiers," Photonics Technology Letters, IEEE, vol. 5, pp. 1325-1327, 1993.
[19] T. Kasamatsu, Y. Yano, and T. Ono, "Gain-shifted dual-wavelength-pumped thulium-doped fiber amplifier for WDM signals in the 1.48-1.51-|im wavelength region," Photonics Technology Letters, IEEE, vol. 13, pp. 31-33, 2001.