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Fluoride glass optical fiber amplifiers

Fluoride glasses were seen to be potentially excellent hosts for RE doping as early as 1978, when these hosts were being viewed as solid-state laser materials.^2,3 One particularly attractive feature of the fluoride glass hosts is the large bandwidth of the emission spectra compared to the oxide glasses. In the excellent review article by Miniscalco,¹ he shows the normalized emission band for Er³⁺ in oxide and fluoride glass hosts. His data is reproduced in Fig. 10.4, where it can be clearly seen that the bandwidth is much larger for the heavier fluorohafnate compared to the silicate glasses. However, the emission spectra and bandwidths shown in Fig. 10.4 may be influenced by reabsorption, so instead it is more valuable to look at the gain bandwidth. The gain bandwidth refers to the bandwidth of the actual gain that can be expected in a high-gain amplifier. The data in Table 10.1, taken from Miniscalco, shows that the FWHM bandwidth at 1550 nm for which the gain remains above some value, say 20 dB or more, can be significantly greater for the fluoride compared to the silica glasses.

Figure 10.4  1500-nm emission spectra for different glass hosts doped with Er³⁺. Note the variation in shape and linewidth for the various host glasses.¹ [Reprinted with permission.]

HMFG has been dopedwith a wide range of RE ions, including themore important Pr³⁺, Er³⁺, Ho³⁺, Tm³⁺, and Nd³⁺ ions. Spirit et al.⁸ doped a 7.1-µm- core fluorozirconate fiber with 500 ppm Er³⁺ to amplify light in the 1550-nm band. A 1485-nm pump laser was used, and the length of the Er-doped fiber was 5 m. Using this fiber amplifier they were able to achieve a gain of 18 dB from 1530 to 1565 nm. This range compares favorably with the 40-nm range from an Er-doped, Al co-doped silica fiber. The only advantage to the fluoride host is that the gain spectrum is flatter than it is for silica hosts. The efficiency for the Er-doped ZBLAN or silica fiber hosts is about the same.

Table 10.1 1500-nm bandwidths for some Er³⁺-doped glasses.

While the results of Spirit et al. are encouraging, they are not appreciably better than those for Er-doped silica fibers. Furthermore, silica has much more favorable physical properties than fluoride glass, and, therefore, recent research on fluoride fiber amplifiers has turned from 1550 nm to the important, shorter- wavelength telecom band at 1300 nm where EDFAs do not operate. Probably the most important ion for fiber amplification at 1300 nm is Pr³⁺. The energy level diagram for Pr³⁺ is shown in Fig. 10.5.³ The important transition for amplification is ¹G₄ -> ³H₅, but the ¹G₄ metastable state is largely depopulated by nonradiative (phonon) transitions; therefore, the quantum efficiency of this transition is only a few percent. While this system is not as efficient as the EDFAs, it nevertheless is a promising 1300-nm OFA. Ainslie et al.³ have doped ZBLAN fibers with 1000 ppm Pr³⁺ ions and studied their performance at 1300 nm. The gain bandwidth for a 24-m-long fiber longitudinally illuminated with a 1.047-µm 300-mW pump source is given in Fig. 10.6.³ The peak of the gain curve is at 1300 nm and the 3-dB bandwidth is 20 nm. This bandwidth is compatible with current telecom wavelengths.

Figure 10.5 Energy level diagram for Pr³⁺ for emission into the 1300-nm bands. [Reprinted from Ref. 3, Copyright 1995, with permission from Elsevier.]

Figure 10.6 Small signal gain for Pr³⁺-doped fluoride glass fiber amplifier with a 300-mW pump input power. [Reprinted from Ref. 3, Copyright 1995, with permission from Elsevier.]

Not all RE-doped fluoride fibers have applications as amplifiers for telecommunications systems. Fiber lasers were made from Nd³⁺-doped HMFG in 1987 by Brierly and France.⁴ These fiber lasers operated at 1.05 µm. A more efficient fiber laser was reported a year later by Brierly and France⁵ when they used Er-doped fluorozirconate fibers. The Er-doped MM fiber, 75 cm in length, was pumped by 476.5-nm radiation from an Ar ion laser. The cw output from the ⁴I_11/2 -> ⁴I_13/2 transition gives lasing action at 2.7 µm. Allen et al.⁶ also achieved lasing action at the same wavelength for Er:ZBLAN fibers. In their case they used a much more convenient 792-nm laser diode to pump the fiber. The maximum output power was about 2 mW and the efficiency was 8%. Finally, a more efficient and powerful fluoride fiber laser was reported by Percival et al.⁷ using a fluoride fiber co-doped with Ho³⁺ and Tm³⁺. Lasing action takes place from a transition in Ho. The Tm ions are pumped by diode laser light between 820 and 830 nm and then the energy is transferred to the Ho ions. The maximum cw output power was 250 mW at 2.024 µm, with a maximum slope efficiency of 60%.

References

1. W. J. Miniscalco, "Erbium-doped glasses for Fiber Amplifiers at 1500 nm," J. Lightwave Tech., Vol. 9, pp. 234-250 (1991).
2. M. Poulain, M. Chanthanasinh, and J. Lucas, "New fluoride glasses," Mat. Res. Bull., Vol. 12, pp. 151-156 (1977).
3. B. J. Ainslie, S. T. Davey, D. Szebesta, J. R. Williams, M. W. Moore, T. Whitley, and R. Wyatt, "A review of fluoride fibres for optical amplification," J. Non-Cryst. Solids, Vol. 184, pp. 225-228 (1995).
4. M. C. Brierley and P. France, "Neodymium-doped fluorozirconate fibre laser," Electron. Lett., Vol. 23, pp. 815-817 (1987).
5. M. C. Brierley and P. France, "Continuous wave lasing at 2.7 microns in an erbium-doped fluorozirconate fibre," Electron. Lett., Vol. 24, pp. 935-937 (1988).
6. R. Allen, L. Esterowitz, and R. J. Ginther, "Diode-pumped single-mode fluorozirconate fiber laser from the 4I11/2 to the 4I13/2 transition in erbium," Appl. Phys. Lett., Vol. 56, pp. 1635-1637 (1990).
7. R. M. Percival, D. Szebesta, S. T. Davey, N. A. Swain, and T. A. King, "Thulium sensitized holmium-doped cw fluoride fibre laser of high efficiency," Electron. Lett., Vol. 28, pp. 2231-2232 (1992).
8. D. M. Spirit, G. R. Walker, P. W. France, S. F. Carter, and D. Szebesta, "Characterization of diode-pumped erbium-doped fluorzirconate fibre optical amplifier," Electron. Lett., Vol. 26, pp. 1218-1220 (1990).