ASAHI GLASS CO., LTD : Research & Development/R&D Library/Vol.52/Paper 1

Paper1. Bi2O3 - based Glass for S - band Amplification

Seiki Ohara*, Naoki Sugimoto*, Yuki Kondo*, Katsuhiro Ochiai*, Yutaka Kuroiwa*,
Yasuji Fukasawa*, Takeshi Hirose*, Hideaki Hayashi* and Setsuhisa Tanabe**

　S - band amplification characteristics of Bi2O3 - based thulium (Tm³⁺) doped glass fiber (Bi - TDF) were investigated. Bi2O3 - based glass was doped with Tm³⁺ and melted using a conventional method. Emission spectra of the ³H4 - ³F4 were measured by pumping at a wavelength of 792 nm using a laser diode (LD). Full width of half maximum (FWHM) of the emission is 1.4 times and 1.1 times broader than that of fluoride glass and tellurite glass, respectively. Moreover, the emission peak shifted towards longer wavelengths as compared with fluoride and tellurite glasses. Single mode Bi - TDFs with Tm³⁺ concentrations of 2000 ppm, 3900 ppm and 6500 ppm were fabricated and evaluated with fusion splicing to SiO2 fibers. Gain profiles were measured with bi - directional pumping using 1047 nm Yb fiber lasers. The gain - peaks observed around 1470 nm shifted towards longer wavelengths with increasing Tm³⁺ concentration. Gain properties of Bi - TDF with Tm³⁺ concentrations of 2000 ppm and 3900 ppm were improved by additional pumping at the wavelength of 1560 nm. A maximum gain over 10 dB was obtained using a fusion spliced Bi - TDF with only 1 m of fiber.


目次
	1. Introduction
	2. Experimental
	3. Results and Discussion
	4. Conclusion

1.Introduction

　Wavelength division multiplexing (WDM)system has been adopted to meet rapidly increasing data traffics. Optical amplifiers are required to transmit data over long distances and to compensate for the insertion loss of processing devices. Erbium doped fiber amplifiers (EDFAs)comprise an important part in WDM systems because of their practical use in C - band (1530 ～1560 nm)or L - band (1570 ～1600 nm). Broadening the amplifiable band is important to increase transmittance capacity, thus, tellurite and bismuthate based glasses are proposed as alternatives to silicate glass^{(1) - (3)} . For example, in a separate study it was shown that Bi2O3 - based erbium doped fibers (Bi - EDF)with erbium concentration of 6500 ppm showed more than 9 dB gain in C+L (1530 - 1610 nm)band using only 22 cm fiber length ⁽²⁾.
　To increase the number of WDM channels in addition to C - band and L - band, thulium (Tm³⁺) doped fibers (TDF)have been studied for S band amplification ^(4)-(7) . The energy difference between the ³H4 level, which is the initial level for emission, and the ³H5 level, which is just one level lower is small. Therefore, non - radiation decay originated from phonon relaxation occurs easily in TDF. Thus, silicate fiber is difficult to use for TDF because of its large phonon energy (～1100 cm^－1) . On the other hand, fluoride glass has low phonon energy (～500 cm^－1) , so it has been investigated for its possible use in TDF. Fluoride based TDF shows gain bandwidth around 1460 - 1490 nm using 1.05 μm pumping. However, gain shift to longer wavelength (1480 ～1510 nm)is required to obtain seamless gain bandwidth from C - band region. To meet this requirement, dual wavelength [ (1.05 μm +1.5 μm) or (1.4 μm +1.5 μm)] pumping ⁽⁵⁾⁽⁶⁾or high concentration doping⁽⁷⁾ of Tm³⁺ have been proposed.
　Tellurite glasses for S - band amplification are also being investigated, because of the advantages of high stability, easy fabrication and broader gain characteristics⁽⁸⁾. Bi2O3 - based host glasses are also being investigated in the Bi2O3 - GeO2 and Bi2O3 - SiO2 systems, which show broadband emission as compared with fluoride and tellurite glasses⁽⁹⁾ .
　On the other hand, fusion splicibility of TDF to conventional SiO2 fibers is a practical requirement to obtain mechanical reliability, low loss and low cost. Bi2O3 - based glass is fusion - spliceable to silica fibers using a conventional fusion splicing machines⁽³⁾, while, fluoride and tellurite fibers are difficult to fusion - splice.
　In this paper, we report the broadband emission property of Tm³⁺ in Bi2O3 - based glass and S - band amplification characteristics of Bi - TDF.

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2. Experimental

　Tm - doped Bi2O3 - based glasses were prepared by a melting method. Glass samples were melted at 1150 ℃ in air, cast into a mould, and annealed at the glass transition temperatures.　The obtained bulk glass samples were polished to measure absorption and emission spectra. We fabricated single mode Bi - TDFs with Tm³⁺ concentration of 2000 ppm, 3900 ppm and 6500 ppm. Refractive index of core glass was 2.02 and numerical aperture (NA)was 0.2 at 1310 nm.
　Emission spectra of the ³H4 － ³F4 transitions of bulk samples were measured with pumping at a wavelength of 792 nm using a LD. Amplification characteristics were evaluated using the experimental set - up shown in Fig.1. Figure 1 shows the experimental set - up for the measurement of the gain spectra and amplified spontaneous emission (ASE) pumping at the wavelength of 1047nm ^(a) , and the 1560 nm +1047 nm^(b) . For 1560 nm pumping, Bi2O3 - based erbium doped fiber amplifier (Bi - EDFA)with Er³⁺ concentration of 3200 ppm was used.　Bi - TDF was fusion - spliced to SiO2 fibers.

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3. Results and Discussion

3 .1 Spectroscopy of Tm³⁺ - doped Bi2O3 - based glass

　Figure 2 . shows Tm³⁺ energy levels. Figure 3 shows the typical absorption spectrum of Tm³⁺ - doped Bi2O3 - based glass. Absorption peaks corresponding to the ³F4, ³H5 and ³H4 transitions from the ³H6 level appear at 1650 nm, 1210 nm and 790 nm, respectively. When ³H4 level is directly excited by 792 nm LD, the energy transition occurs as ³H4 → ³F4 → ³H6. The ³H4 → ³F4 transition energy corresponds to S - band. The band emission spectrum of Tm³⁺ doped Bi2O3 - based glass was compared to those of fluoride and tellurite based glasses as shown in Fig.4. 　 Table 1 shows peak wavelength and FWHM. Bandwidth around 1450 nm for Tm³⁺ - doped Bi2O3 - based glass is broader than those of fluoride and tellurite glass, and the peak shifts to longer wavelength than those of the other hosts. FWHM of Bi2O3 - based glass is 1.4 and 1.1 times broader than those of fluoride glass and tellurite glass, respectively. This property indicates that Bi2O3 - based glass is more suitable for S - band amplification.

3 .2 Bi2O3 - based Tm³⁺ doped fiber

3 .2 .1 Absorption and ASE spectra

　Figure 5 shows the transmission spectrum of Bi - TDF with Tm³⁺ concentration of 2000 ppm. The absorption peaks are at 1650 nm and 1210 nm, which are the same as those of the bulk glass. Absorption coefficient at 1210 nm was 1.1 dB/cm.
Figure 6 shows ASE spectra of Bi - TDF with different concentrations of Tm³⁺ , bi - directionally pumped by 1047 nm Yb fiber - laser with pumping power of 250 mW+250 mW. Emission intensity was normalized at peak intensity. Emission band shifted to longer wavelengths with increasing Tm³⁺concentration.　Cross relaxation between the ³H4 →³F4 and the ³H6 → ³F4 becomes easy to occur as theTm³⁺ concentration increases. This causes low population inversion between the ³H4 level and the ³F4 level, and as a result, emission spectrum shifts to longer wavelengths.

3 .2 .2 Characteristics of Bi - TDF

　Figure 7 shows the internal gain profiles of Bi - TDF with Tm³⁺concentrations of 2000 ppm, 3900 ppm and 6500 ppm bi - directionally pumped at 1047 nm. The fiber lengths were 2.0 m for 2000 ppm, 1.0 m for 3900 ppm and 0.60 m for 6500 ppm. The
number of Tm ions is almost same in these fibers. Pumping power was 760 mW (380 mW +380 mW), the input signal power was 0 dBm.
　The gain - profile shapes clearly differ changing the Tm³⁺concentration. The peak at 1470 nm with Tm³⁺concentration of 2000 ppm shifts to 1500 nm with an increase in Tm³⁺ concentration up to 6500 ppm.
　We obtained 2dB - gain from 1450 nm to 1510 nm with 3900 ppm, and from 1470 nm to 1520 nm with 6500 ppm. So we can reduce the space between S and C band. On fluoride glass, the gain band is difficult to extend to 1520nm.
　Figure 8 (a) shows the internal gain as a function of pumping power at 1047 nm. Tm³⁺ concentration was 3900 ppm and input signal power was 0 dBm. Even at the pumping power of 1000 mW, no saturation was observed. This indicates that we can obtain more gain by improving the pumping scheme or fiber length. Figure 8 (b) shows the internal gain as a function of pumping power of 1560 nm added to 1047 nm pumping of 760 mW. Bi - EDF was used for additional 1560 nm pumping. Slope efficiency of 1560 nm pumping was 0.062dB/mW. It was almost 5 times than that of 1047 nm pumping. Figure 9 shows the gain profiles with and without 1560 nm pumping. Pumping power was 760 mW at 1047 nm and 12 mW at 1560 nm. With additional pumping at 1560 nm the gain was improved, but the gain profile did not change clearly.
　Figure 10 shows the dependence of the gain with the input signal power at 1450 - 1530 nm with pumping power of 760 mW at 1047 nm. The gain does not decline even at the signal power of 0 dBm.
　Figure 11 shows the gain profiles of Bi - TDF with concentrations with input signal power of 0 dBm. Max Internal Gain of 12dB was obtained for only 1.0 m long Bi - TDF with Tm³⁺ concentration of 3900 ppm. Above 10dB gain was observed from 1450 nm to 1500 nm with Tm³⁺ concentration of 2000 ppm, and from 1460 nm to 1510 nm with Tm concentration of 3900 ppm.

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4. Conclusion

The Bi2O3 - based Tm³⁺ doped glass fiber (Bi - TDF) was investigated for S band amplification. The Bi - TDF showed broadband gain from 1450 nm to 1520 nm. Peak wavelength shifts from 1470nm to 1500 nm with increasing Tm concentration. The peak gain above 10 dB was obtained with only 1.0 m long Bi - TDF at 1460 - 1510 nm. Moreover, the Bi - TDF was fusion - spliceable to conventional SiO2 fibers. These characteristics indicate that Bi - TDF is a very promising candidate for S - band amplifiers.

－References －

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(7)	S. Aozasa, H. Masuda, H. Ono, T. Sakamoto, T. Kanamori, Y. Ohishi and M. Shimizu, "1480 - 1510 nm - band Tm doped fiber amplifier (TDFA)with a high power conversion efficiency of 42%", OFC2001, PD1
(8)	S. Shen, M. Naftaly, A. Jha and S. J. Wilson, "Thulium - doped tellurite glasses for S - band amplification", OFC2001, TuQ6
(9)	S. Tanabe, T. Tamaoka, T. Hanada, Y. Kondo, N. Sugimoto and S. Ito, "Spectral properties of Tm³⁺ - doped glasses for S - band amplifier", OAA2001, OWB4

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