Yb3+-codoped tellurite glass fiber

JOURNAL OF RARE EARTHS, Vol. 26, No. 6, Dec. 2008, p. 915

Fabrication and gain performance of Er3+/Yb3+-codoped tellurite glass fiber* DAI Shixun (), XU Tiefeng (), NIE Qiuhua ( ), SHEN Xiang ( ), WANG Xunsi ( ) (College of Information Science and Engineering, Ningbo University, Ningbo 315211, China) Received 10 September 2007; revised 21 May 2008

Abstract: Er3+/Yb3+-codoped TeO2 -ZnO-BaO-La2O3 tellurite glass fiber was fabricated by rotation and rod-in-tube technologies. The thermal stability and optical refractive index of the core and cladding glasses were determined by DTA and optical coupler, respectively. The average background loss of tellurite glass fiber was 1.8 dB/m at 1310 nm. Optical microscopy and field emission scanning electron microscope (FESEM) were used to study structural characteristics of preforms and optical fibers. The main loss of tellurite glass fiber could be attributed to scatter centre due to core-cladding interface defects. The amplifier performance of tellurite glass fiber was investigated by pumping with 980 nm laser diode (LD). The gain coefficient and maximum signal gain were 0.21 dB/mW and 10 dB, respectively, for a pumping power of 120 mW. Gains exceeding 5 dB were obtained over 30 nm bandwidth from 1535 to 1565 nm. The minimum noise figure was 4.8 dB at 1557 nm. Keywords: tellurite glass; glass fiber; erbium; gain; rare earths

Recently, great interest is being developed in Er3+ doped tellurite glasses, which can be used for 1.5 μm active optical devices such as fiber-optical amplifiers[1–6] and planer waveguide[7]. Er3+-doped tellurite glass fibers can offer broad amplification bandwidth and high radiative transition efficiencies[1–6]. They are now promising candidates for using in fabricating novel optical amplifiers as a means of extending the transmission bandwidth of optical fibers beyond the range available from conventional Er3+-doped silica fiber, such as Er3+-doped tellurite fiber amplifier (EDTFA) for C-band and L-band[1–6]. The 980 nm laser diode (LD) was often used as the pumping source to realize 1.5 μm amplification with broadband gain, high power output, and good noise figure instead of 1480 nm LD pumping[8]. In order to improve the pumping efficiency of 980 nm LD, Yb3+ was used mostly as a sensitizer since it exhibits an intense broad absorption cross-section between 850 and 1080 nm, whereas Er3+ ions have weak absorption at this band range[9]. Recently, the present authors reported the characterization of ASE from Er3+-single doped tellurite fiber[10]. In this present investigation, a kind of Er3+/Yb3+-codoped tellurite fiber based on TeO2-ZnO-BaO-La2O3 glasses was fabricated by rotation and rod-in-tube technologies. The amplifier performance of the tellurite glass fiber was investigated.

1 Experimental The glasses used in the present study were prepared from

reagent grade TeO2, ZnO, BaCO3, La2O3, Yb2O3, and Er2O3. The cladding and core glass compositions were: 70TeO2and 71TeO2-18.8ZnO-7BaO20ZnO-7BaO-3La2O3, 2.4La2O3-0.4Yb2O3-0.2Er2O3, respectively (in molar fractions). Properly mixed powders of cladding and core compositions were melted separately in platinum crucible at 800 ºC. The glasses obtained were annealed to room temperature. Samples were cut and polished to a size of 10 mm ×10 mm ×1 mm for spectrum measurements. The refractive index of core and cladding bulk glasses was measured using SAIRON-SPA4000 Prism coupler at 632.8, 1310, and 1532 nm, respectively. Differential thermal analysis (DTA) was carried out on bulk samples using SEIKO-TG/DTA 6300 thermal analyzer in N2 atmosphere with a scan rate of 10 ºC/min. The flowchart of the preform fabrication process is shown in Fig.1. Rotation and rod-in-tube methods were adopted to fabricate tellurite glass preforms. Fiber drawing was finished at home-built tower equipped with automated preform feeder and capstan driving. The average background loss was measured using the cutback method, and the measuring wavelength was at 1310 nm. An optical microscope equipped with JVC CCD camera was used for measuring the core and cladding diameters of the fabricated fiber. The microstructural characteristics of core-cladding interface were observed using FEI SIRION 200 Field Emission Scanning Electron Microscopy (FESEM). The absorption spectrum of the core glass was recorded in the range of 300–1650 nm using PERKIN-

Foundation item: Project supported by the Science and Technology Department of Zhejiang Province (2006C21082) and K.C.Wong Magna Fund in Ningbo University Corresponding author: DAI Shixun (E-mail: [email protected]; Tel.: +86-574-87600947)

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JOURNAL OF RARE EARTHS, Vol. 26, No. 6, Dec. 2008

ELMER-LANBDA 950UV/VIS/NIR spectrophotometer. The experimental setup of gain performance of the fiber is shown in Fig.2. The Er3+/Yb3+-codoped tellurite glass fiber is pumped by single mode 980 nm fiber-pigtailed LD, with a maximum output power of 240 mW. The pump lights are coupled into the tellurite fiber through a fused wavelength division multiplexer (WDM) combining 980 and 1550 nm with 1.8 dB insert loss. A tunable laser source (TLS) is used as the signal source with wavelength range from 1520 to 1570 nm and the output power range from –30 dBm to +7 dBm. The isolator is connected between the TLS and the WDM. The output signal of TLS is coupled into the tellurite fiber through the WDM. The amplified signal is input to the optical spectrum analyzer (Ando 6317B). The isolators ensure that the signal is along the right directions.

Fig.1 Flowchart of preform fabrication

Fig.2 Schematic diagram for measuring the gain performance of Er3+/Yb3+-codoped tellurite glass fiber

2 Results and discussion Table 1 shows the refractive indexes of cladding and core glasses at 632.8, 1310, and 1532 nm, respectively. The numerical aperture (NA, NA =

2

2

ncore − ncladding

) of tellurite glass

fiber was about 0.34 at 1310 nm. Differential thermal analysis (DTA) was carried out on these samples to determine the variation of crystallization temperature between the core and cladding glasses. The DTA traces of core and cladding glasses are shown in Fig.3 and are annotated with the glass transition temperatures (Tg) and the onset crystallization temperature (Tx). The difference between Tx and Tg (ΔT= Tx–Tg) for the core and cladding glasses are 100 and 98 ºC, Table 1

respectively. Therefore, it was concluded that core and cladding glasses have relatively good thermal stabilities. In order to study in detail the structural characteristics of preforms and optical fibers fabricated by rotational casting and rod-in-tube methods, optical microscopy (OM) and FESEM were used. The resulting fiber had a cladding diameter of 125 μm and a core diameter of D≈4 μm. Fig.4 shows the magnified cross-section of the tellurite glass fiber.

Refractive index of cladding and core glasses Refractive index

Glass composition (mol%)

@632.8/

@1310/

@1532/

nm

nm

nm

Cladding : 70TeO2-20ZnO-7BaO-3La2O3

2.0130

1.9134

1.9118

Core: 71TeO2-18.8ZnO-7BaO-2.4La2O3-

2.0174

1.9431

–

0.4Yb2O3-0.2Er2O3

Fig.3 DTA traces of core and cladding glasses

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DIA S X et al., Fabrication and gain performance of Er3+/Yb3+-codoped tellurite glass fiber*

Fig.4 Optical photomicrograph of Er3+-doped tellurite glass fiber

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preform was drawn into fiber, the core usually cracked due to high stress at the core-cladding interface. Another reason is likely due to contamination on either tube surface or shaped preform surface prior to drawing. Therefore, the main loss of the fiber can be attributed to scatter at centre due to defects at core-cladding interface. The net gain at 1550 nm versus pumping power is shown in Fig.6. The input signal power was about –30 dBm. The signal gain increased linearly as the pump increased and no saturation phenomenon appeared until 120 mW. As indicated in Fig.6, the threshold pumping power is about 60 mW in our tellurite glass fiber at a length of 30 cm. The gain coefficient and the maximum signal gain were 0.21 dB/mW and 10 dB, respectively, for a pumping power of 120 mW. Fig.7 shows the small signal gain and noise figure spectra of the tellurite-based EDFA for a pumping power of 240 mW. Due to the limitation of signal light in the range of 1520 to 1570 nm, the peak gain was about 16 dB at 1556 nm, and gains exceeding 5 dB were obtained over a bandwidth of 30 nm from 1535 to 1565 nm. The minimum noise figure was 4.8 dB at 1557 nm, and a noise figure < 7.5 dB was obtained from 1533 to 1570 nm.

Fig.6 Net gain at 1.55 μm (line is drawn as a guide)

Fig.5 FESEM images of the core-cladding interface (a) Magnified by 5000 times; (b) Magnified by 40000 times

A fine and well-defined core-cladding interface can be clearly seen, showing good chemical compatibility at the interface. The average background loss measured by cutback method was 2.1 dB/m at 1310 nm. Fig.5 shows FESEM images of the core-cladding interface. It is evident that there were some small defects (holes, cracks) at the interface. The fiber-drawing process can also be regarded as a reheating process [11]. Since the core and cladding are differently doped, their viscosities and thermal expansion coefficients are also different. When the glass

Fig.7 Small signal gain and noise figure spectra of tellurite-based EDFAs

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3 Conclusion Er3+/Yb3+-codoped tellurite glass fiber with a core diameter of 14 μm was fabricated. The fiber loss was about 2.1 dB/m at a wavelength of 1310 nm, which was mainly attributed to scatter centers due to defects between core and cladding interface. The gain coefficient and the maximum signal gain were 0.21 dB/mW and 10 dB, respectively, for a pumping power of 120 mW. Gains exceeding 5 dB were obtained over 50 nm bandwidth from 1535 to 1565 nm. The minimum noise figure was 4.8 dB at 1557 nm.

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Avanesov A G, Zhorin V V, Okhrimchuk A G, Shestakov A V. Study of energy transfer from Yb3+ to Er3+ in rareearth silicates and borates. Journal of Luminescence, 1997, 72-74: 942. [9] Hu Y, Jiang S, Sorbello G, Luo T, Ding Y, Hwang B, Kim J, Seo H, Peyghambarian N. Numerical analyses of the population dynamics and determination of the upconversion coefficients in a new high erbium-doped tellurite glass. Journal of the Optical Society of America B: Optical Physics, 2001, 18(12): 1928. [10] Dai S, Zhang J, Li S, Hu L, Jiang Z. Characterization of amplified spontaneous emission from Er3+ -doped tellurite fibre. Chin. Phys. Lett., 2004, 21(2): 329. [11] Wang J, Prasad S, Kiang K, Pattnaik R, Toulouse J, Jain H. Source of optical loss in tellurite glass fibers. Journal of NonCrystalline Solids, 2006, 352(6-7): 510.

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