Fabrication of highly nonlinear optical fibers with tellurite glass core and phosphate glass cladding

Optical Materials 34 (2012) 1795–1803

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Fabrication of highly nonlinear optical fibers with tellurite glass core and phosphate glass cladding Hoang Tuan Tong ⇑, Chihiro Kito, Takenobu Suzuki, Yasutake Ohishi Research Center for Advanced Photon Technology, Toyota Technological Institute, 2-12-1 Hisakata, Tempaku-ku, Nagoya 468-8511, Japan

a r t i c l e

i n f o

Article history: Received 9 April 2012 Received in revised form 2 May 2012 Accepted 6 May 2012 Available online 15 June 2012 Keywords: Tellurite glass Chromatic dispersion control Highly nonlinear fibers

a b s t r a c t We perform a systematic investigation of phosphate glasses containing alkaline earth, alkali and mixedalkali oxides proposed as cladding glasses for tellurite-glass core to realize highly nonlinear optical fibers with tailored chromatic dispersion. The effects of glass composition on the optical and thermal properties of phosphate glasses are studied in this work. The phosphate glasses containing alkali oxides exhibit transmission windows in the range of 0.2–3.5 lm and refractive indices as low as 1.500 at 1544 nm. The coefficient of thermal expansion, viscosity, glass transition temperature, deformation temperature and crystallization temperature of those glasses are optimized to allow successful fiber drawing with tellurite-glass cores. We propose the 40P2O5–30ZnO–20Na2O–10K2O mol% (PZNK) mixed-alkali phosphate glass as a suitable cladding material for the 78TeO2–5ZnO–12Li2O–5Bi2O3 mol% (TZLB) tellurite-glass used as a core material since they have nearly identical thermal characteristics. A successful fabrication of step-index optical fibers composed of TZLB and PZNK glasses and numerical calculations in chromatic dispersion and nonlinear coefficient are demonstrated. The large refractive index difference of 0.49 between TZLB and PZNK glasses confirms the high nonlinearity and freely-tailored chromatic dispersion profiles of these new fibers. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Tellurite glasses which have high refractive index, wide transmission range from visible to infrared region [1,2], good thermal stability, chemical durability, less toxic [3] and low glass transition temperature [4] are of great interest to photonics applications. As these glasses possess high nonlinear refractive index, they have been considered as potential candidates for high nonlinear optical fibers [5,6]. Over the past decade, development of microstructured optical fibers (MOFs), which consist of a small core surrounded by periodic arrangement of air holes, has been a key to achieve higher nonlinearity and more flexible chromatic dispersion control [7]. High performance of four wave mixing [8], supercontinuum (SC) generation [9–14] and other nonlinear effects have been demonstrated by using tellurite MOFs. These effects are useful for many optical applications including wavelength conversion, broadband optical amplification, phase conjugation, squeezing and frequency metrology, multi-wavelength optical source, optical coherence tomography, pulse compression and optical frequency metrology. However, it is difficult to control the total chromatic dispersion of tellurite optical fibers due to their large material dispersions. Thus, much effort has been devoted to modify the waveguide ⇑ Corresponding author. E-mail address: [email protected] (H.T. Tong). 0925-3467/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2012.05.008

dispersion in tellurite MOFs in order to compensate the material dispersion. Dispersion engineering in tellurite MOFs by changing fiber structures such as core and air hole diameters was reported by Liao et al. [9,10,15]. Recently, the dispersion profiles have been further improved by using hybrid microstructured optical fibers (HMOFs) with large refractive index contrast between fiber core and cladding. A HMOF composed of different tellurite glasses as core and cladding materials was demonstrated by Duan et al. [16]. Although the refractive index contrast in that tellurite– tellurite HMOF was just about 0.11 at 1.55 lm, a remarkable chromatic dispersion profile and enhanced SC generation were reported. Similarly, the HMOF was employed to control the chromatic dispersion in chalcogenide-core optical fibers with tellurite claddings as reported in the Refs. [11,17]. For step-index fibers, a method to obtain chromatic dispersion control by using two or three glasses with large refractive index contrast has been proposed by Poletti et al. [18]. Those mentioned approaches suggested that an efficient chromatic dispersion control in optical fibers is achievable by using high refractive index contrast between fiber core and claddings. To enhance tunability of chromatic dispersion profile in tellurite-core fibers, there is a demand on searching for low refractive index glasses as suitable cladding materials. As well known, pure silica has low refractive index of 1.46 at 1.55 lm, but it has a very high processing temperature [19,20] far above from those required for drawing soft glass materials such as tellurite

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glasses. Among soft glasses, phosphate glasses have high mechanical strength, good thermal characteristics [21] and low glass transition temperature [22,23] which would be suitable cladding materials for tellurite-core HMOFs. The purpose of our work is to develop novel highly nonlinear tellurite optical fibers with phosphate glasses as cladding materials. We have investigated the optical and thermal properties of phosphate glasses containing alkaline earth, alkali and mixed-alkali oxides. Those phosphate glasses possess infrared transparency up to 3 lm, low refractive index slightly over 1.500 at 1.55 lm, high thermal stability, high thermal expansion coefficient of 2.13  105 °C1 and suitable viscosity for drawing fiber with tellurite-glass core. The 40P2O5–30ZnO–20Na2O–10K2O mol% (PZNK) glass is proposed as a promising cladding candidate for the 78TeO2–5ZnO–12Li2O–5Bi2O3 mol% (TZLB) glass due to their similar thermal characteristics. We have demonstrated step-index optical fibers composed of TZLB core and PZNK cladding with the refractive index difference of 0.49. Numerical calculations have been performed to clarify the chromatic dispersion and nonlinear coefficient. The large refractive index difference between TZLB core and PZNK cladding confirms the potential chromatic dispersion control and the high nonlinearity of the new fiber. 2. Experimental 2.1. Glass synthesis Phosphate glasses containing P2O5, ZnO, alkaline earth, alkali and mixed-alkali oxides such as MgO, CaO, BaO, Li2O, Na2O and K2O were prepared by conventional melt-quenching method. Their compositions are shown in Table 1. The analytic grade powders used as raw materials were weighed and well mixed in a glove box before melting to avoid moisture contamination. A covered Pt crucible containing the mixture of oxides was melted at 900 °C in an electrical furnace for an hour under an oxygen flow. Subsequently, the melt was cast into a preheated mold and kept at 300 °C for 3 h to release residual stress. An X-ray diffractometer (Shimadzu LabX XRD-6100) with Cu Ka radiation source (k = 1.5405 Å) was used to investigate the glass stability. 2.2. Measurements of glass properties Glass samples were first cut and polished to meet the requirements of optical measurements. The sample thickness was about 1.5 mm. A UV/Vis/Nir Spectrometer (Perkin Elmer, Lamda 900) and an FT-IR spectrometer (Perkin Elmer, Spectrum 100) were used to measure transmission spectra of glasses in the range of 200– 3300 nm and 2.5–7 lm, respectively. A prism coupler system (Metricon, 2010) was employed to measure the glass refractive indices at 633, 974, 1320 and 1544 nm. The glass transition (Tg) and crystallization (Tx) temperatures were determined by a differential scanning calorimetry (Rigaku, Thermo Plus DSC 8270). About 30 mg powder placed in a platinum pan was heated to 800 °C at a rate of 10 K/min under a nitrogen gas

atmosphere. The same amount of Al2O3 powder was used as a reference sample. We employed a thermal mechanical analysis (Rigaku, Thermo Plus TMA 8310) to measure the coefficient of thermal expansion (CTE) and viscosity. The samples were prepared in form of cylindrical rods with the diameter of 5 mm. Their length was 15 mm for CTE measurements and 4 mm for viscosity measurements, respectively. The CTE value of glass was determined from the linear part of the TMA curve which was obtained by heating sample from the room temperature to a temperature just above the deformation temperature (Td) at a rate of 10 K/min. The CTE value, a, was defined by.

1 l2  l1 l0 T 2  T 1

a¼ 

ð1Þ

where l0, l1, l2 are the length of sample at room temperature, at temperature T1 and at temperature T2, respectively. The glass viscosity at a certain temperature, g, which was measured by penetration method, was given by

g¼

Wt k

ð2Þ

where t is the period of time that the tip penetrated 50-mm depth from the sample surface under a constant stress. The stress, W, is defined as load per unit of area. The constant coefficient, K, corresponds to tip characteristics. Viscosity measurements at four different temperatures were carried out to determine the temperature dependence of glass viscosity. 2.3. Fiber fabrication Fabrication techniques based on extrusion, rod in tube, ultrasonic drilling and stacking have been developed for soft glass optical fibers with complex microstructures [9]. In this work, we fabricated step-index optical fibers by rod in tube method. Phosphate-glass tubes were first obtained by rotational casting method. A tellurite-glass rod was inserted into a phosphate-glass tube then they were elongated together to obtain a cane with a smaller core diameter. This procedure was repeated twice before the final cane was inserted into the third cladding tube and drawn into fiber. 3. Results 3.1. Optical properties Fig. 1 depicts transmission spectra of studied tellurite and phosphate glasses. As shown in Fig. 1a, TZLB glass exhibits a transmission window from 0.4 up to 6.5 lm with maximum transmittance at approximately 75%. It can be seen from Fig. 1b–e that the transmission windows of phosphate glasses containing alkaline earth, alkali and mixed-alkali oxides range from 0.2 to 3.2 lm. The strong OH absorption at 3–3.2 lm determines the long-wavelength absorption edges of those glasses.

Table 1 Glass names and compositions. Name

40P2O5–40ZnO–20MO/R2O 40P2O5–(60  x)ZnO–(x)R 40P2O5–35ZnO–(25  x)Li2O–(x)K2O 40P2O5–35ZnO–(25  x)Na2O–(x)K2O 40P2O5–30ZnO–(30  x)Na2O–(x)K2O

Composition (mol%) P2O5

ZnO

MO/R2O

40 40 40 40 40

40 60  x 35 35 30

20 x 25  x 25  x 30  x

R02 O

x x x

with with with with with

M = Mg, Ca, Ba; R = Li, Na, K R = Li, Na, K; x = 25, 30 R = Li; R0 = K; x = 0, 5, 10, 15, 20, 25 R = Na; R0 = K; x = 0, 5, 10, 15, 20, 25 R = Na; R0 = K; x = 0, 5, 10, 15, 20, 25, 30

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(a) 100 TZLB

Transmittance (%)

90 80 70 60 50 40 30 20 10 0

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0

Wavelength (µm)

Transmittance (%)

90 80 70 60

(c) 100 40P2O5-40ZnO-20MgO 40P2O5-40ZnO-20CaO 40P2O5-40ZnO-20BaO

50 40 30 20 10 0

90

Transmittance (%)

(b) 100

80 70 60

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0

Wavelength (µm)

(e) 100 40P2O5-30ZnO-30Li2O

90

40P2O5-30ZnO-30Na2O 40P2O5-30ZnO-30K2O

50 40 30 20 10 0

Transmittance (%)

Transmittance (%)

80 70 60

40P2O5-40ZnO-20K2O

20 10 0

Wavelength (µm)

90

40P2O5-40ZnO-20Na2O

50 40 30

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0

(d) 100

40P2O5-40ZnO-20Li2O

40P2O5-30ZnO-20Na2O-10K2O 40P2O5-30ZnO-15Na2O-15K2O

80

40P2O5-35ZnO-15Na2O-10K2O

70

40P2O5-35ZnO-10Na2O-15K2O

60 50 40 30 20 10 0

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0

Wavelength (µm)

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0

Wavelength (µm)

Fig. 1. Transmission spectra of: (a) TZLB glass, phosphate glasses contain, (b) 20 mol% alkali earth oxides, (c) 20 mol% alkali oxides, (d) 30 mol% alkali oxides, and (e) mixedalkali oxides.

The refractive indices of alkali phosphate glasses measured at four wavelengths of 633, 974, 1320 and 1544 nm are shown in Fig. 2. The refractive index at 1544 nm decreased from 1.581 to 1.500 according to the compositional changes in the line BaO > CaO > Li2O > MgO > Na2O > K2O as shown in Fig. 2a. Except for phosphate glasses containing MgO and Li2O, those doped with alkaline earth oxides had higher refractive index than the others. Fig. 2b–e shows the alkali oxide concentration dependence of phosphate glass refractive index. The refractive index decreased with increasing alkali oxide concentration from 20 to 30 mol%. When Li2O and Na2O were partially replaced by K2O, the refractive index continued to decrease as shown in Fig. 2c–e. It reached 1.500 at 1544 nm when K2O concentration was 30 mol%. 3.2. Thermal properties Compositional dependence of glass transition temperature (Tg), deformation temperature (Td) and coefficient of thermal expansion

(CTE) of phosphate glasses containing various alkali oxides are shown in Figs. 3–5. Fig. 3 indicates the behaviors of Tg, Td and CTE values with changes in the alkaline earth and alkali oxides while P2O5 and ZnO concentration remained unchanged. Tg and Td values of phosphate glasses decreased when MgO was replaced by CaO, BaO, Li2O, Na2O and K2O. The decrease was much more significant for alkali oxides (Li2O, Na2O and K2O) than for alkaline earth oxides (MgO, CaO and BaO). Fig. 3a shows Tg dropped down from 455 to 432 °C by replacing MgO with CaO and BaO then decreased from 321 to 303 °C for Li2O, Na2O and K2O containing glasses. Similarly, a decrease in Td from 503 to 352 °C by changing alkaline earth and alkali oxides was observed in Fig. 3b. On the other hand, CTE increased from 0.691  105 up to 1.63  105 °C1 when MgO was replaced by CaO, BaO, Li2O, Na2O and K2O as seen in Fig. 3c. The compositional dependence of Tg, Td and CTE values is shown in Fig. 4 for phosphate glasses with changes in concentration of alkali oxides and ZnO. The solid square markers show single-alkali

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1.610

(b)

40P2O5-40Z nO-20MgO

1.610

40P2O5-40ZnO-20Li2O

1.600

40P2O5-40Z nO-20CaO

1.600

40P2O5-35ZnO-25Li2O

1.590

40P2O5-40Z nO-20Ba O

1.590

40P2O5-30ZnO-30Li2O

1.580

40P2O5-40Z nO-20Li 2O

1.580

40P2O5-40ZnO-20Na2O

1.570

40P2O5-40Z nO-20Na 2O

1.570

40P2O5-35ZnO-25Na2O

Refractive Index

Refractive Index

(a)

40P2O5-40Z nO-20K 2O

1.560 1.550 1.540 1.530

40P2O5-35ZnO-25K2O

1.540

40P2O5-30ZnO-30K2O

1.530 1.520

1.510

1.510

1.500

1.500 1.490 600

900

1200

40P2O5-40ZnO-20K2O

1.550

1.520

1.490

40P2O5-30ZnO-30Na2O

1.560

1500

600

Wavelength (nm)

Refractive Index

(c)

900

1200

1500

Wavelength (nm)

1.610

40P2O5-35ZnO-25Li 2O

1.600

40P2O5-35ZnO-20Li2O-5K2O

1.590

40P2O5-35ZnO-15Li2O-10K 2O

1.580

40P2O5-35ZnO-10Li2O-15K 2O 40P2O5-35ZnO-5Li2O-20K2O

1.570

40P2O5-35ZnO-25K 2O

1.560 1.550 1.540 1.530 1.520 1.510 1.500 1.490

600

900

1200

1500

Wavelength (nm)

(d) 1.540

1.540

40P2O5-35ZnO-25Na 2O 40P2O5-35ZnO-20Na2O-5K 2O

1.535

40P2O5-30ZnO-30Na2O 40P2O5-30ZnO-25Na2O-5K 2O

1.535

40P2O5-30ZnO-20Na2O-10K 2O

40P2O5-35ZnO-15Na2O-10K 2O 40P2O5-35ZnO-10Na2O-15K 2O 40P2O5-35ZnO-5Na2O-20K 2O

1.525

40P2O5-35ZnO-25Na2O

1.520 1.515 1.510

40P2O5-30ZnO-15Na2O-15K 2O

1.530

Refractive Index

Refractive Index

1.530

40P2O5-30ZnO-10Na2O-20K 2O

1.525

40P2O5-30ZnO-5Na2O-25K 2O 40P2O5-30ZnO-30Na2O

1.520 1.515 1.510

1.505

1.505

1.500

600

900

1200

1500

Wavelength (nm)

1.500 600

900

1200

1500

Wavelength (nm)

Fig. 2. Refractive index phosphate glasses contain: (a) alkaline earth and alkali oxides, (b) alkali oxides, (c) 40P2O5–35ZnO–(25  x)Li2O–(x)K2O, (d) 40P2O5–35ZnO– (25  x)Na2O–(x)K2O, and (e) 40P2O5–30ZnO–(30  x)Na2O–(x)K2O.

phosphate glasses containing Li2O. The solid triangles and solid circles are for single-alkali phosphate glasses containing Na2O and K2O, respectively. As shown in Fig. 4a, Tg decreased in the line of Li2O > Na2O > K2O for any concentration of alkali oxides. An increase in Li2O, Na2O and K2O concentration from 20 to 30 mol% causes a decrease in Tg from 321 to 272 °C. Phosphate glasses containing Li2O and Na2O showed a slight Tg decrease, while a significant decrease from 303 to 272 °C was observed for those

containing K2O. As shown in Fig. 4b, Td values decreased with increasing concentration of alkali oxides in glasses from 20 to 30 mol% and decreasing concentration of ZnO from 40 to 30 mol%, simultaneously. An increase in CTE value with increasing alkali oxide concentration is depicted in Fig. 4c. CTE values increased in the line of Li2O < Na2O < K2O. The highest measured CTE value was 2.07  105 °C1 for the 40P2O5–30ZnO–30K2O glass. It can be seen from Figs. 3 and 4 that phosphate glasses con-

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(a)

520

(a) 360

480

340

40P2O5-(60-x)ZnO-(x)Na 2O 40P2O5-(60-x)ZnO-(x)K 2O

320

o

T g ( C)

o

Tg ( C)

440

40P2O5-(60-x)ZnO-(x)Li2O

400

300

360 280

320 280 MgO

CaO

BaO

L i2O

Na2O

260

K 2O

20

Composition

(b)

(b) 390

520

440

o

T d ( C)

o

30

400

40P2O5-(60-x)ZnO-(x)Li2O

380

40P2O5-(60-x)ZnO-(x)Na 2O

370

40P2O5-(60-x)ZnO-(x)K 2O

480

Td ( C)

25

Concentration (mol%)

360 350 340

360

330 320

MgO

CaO

BaO

L i2O

Na2O

K 2O

320

Composition

20

25

30

Concentration (mol%)

(c)

1.8

(c)

2.4 40P2O5-(60-x)ZnO-(x)Li2O

1.4

o

1.2

-5

CTE x 10 (1/ C)

2.2

-5

o

CTE x 10 (1/ C)

1.6

1.0 0.8

40P2O5-(60-x)ZnO-(x)Na 2O 40P2O5-(60-x)ZnO-(x)K 2O

2.0 1.8 1.6 1.4

0.6

MgO

CaO

BaO

L i2O

Na2O

K 2O

Composition Fig. 3. Effect of alkali oxides on: (a) glass transition temperature, (b) deformation temperature and (c) thermal expansion coefficient of 40P2O5–40ZnO–20MO/R2O mol% phosphate glasses contain alkaline earth oxides (MO) and alkali oxides (R2O).

taining 30 mol% K2O had lower Tg, Td and higher CTE values than other glasses. Thermal properties of mixed-alkali phosphate glasses and TZLB glass are compared in Fig. 5. The K2O concentration dependences of Tg, Td and CTE values are shown in Fig. 5a–c. The solid diamond markers show TZLB glass. The solid squares correspond to mixed-alkali phosphate glasses containing (25  x)Li2O–(x)K2O. The solid circles and the solid triangles correspond to mixed-alkali phosphate glasses containing (25  x)Na2O–(x)K2O and (30  x)Na2O(x)K2O, respectively. As shown in Fig. 5a, the phosphate glasses without K2O had Tg value as high as 316 °C. The Tg values decreased as low as 268 °C when K2O concentration increased up to 30 mol%. For any K2O concentration, Tg value decreased in the order of (25  x)Li2O–(x)K2O > (25  x)Na2O– (x)K2O > (30  x)Na2O–(x)K2O. By changing the alkali oxides and ZnO concentration, Tg values of phosphate glasses could be controlled from 268 to 316 °C. Fig. 5b shows Td values of TZLB glass and of three alkali phosphate glasses series. In each series, the phosphate glasses

1.2

20

25

30

Concentration (mol%) Fig. 4. Effect of alkali oxide concentration on: (a) glass transition temperature, (b) deformation temperature, and (c) thermal expansion coefficient of phosphate glasses.

containing mixed-alkali oxides (Li2O–K2O or Na2O–K2O) had lower Td values than the glasses containing only Li2O, Na2O or K2O. It can be noticed that the 40P2O5–30ZnO–(30  x)Na2O–(x)K2O series had lower Td values (from 309 °C to 334 °C) than other series. When the K2O concentration varied from 0 to 30 mol%, a minimum Td value of 309 °C was obtained by the 40P2O5–30ZnO–15Na2O– 15K2O glass. However, an opposite trend can be observed for CTE values. The CTE values shown in Fig. 5c increased with increasing the K2O concentration from 0 to 25 mol%. The 40P2O5–30ZnO– 5Na2O–25K2O glass had highest CTE value of 2.13  105 °C1. It is noticed that the 40P2O5–35ZnO–(25  x)Li2O–(x)K2O and 40P2O5–35ZnO–(25  x)Na2O–(x)K2O series had lower CTE values than TZLB glass. As can be seen from Fig. 5, the 40P2O5–30ZnO– 20Na2O–10K2O (PZNK) glass had low Tg of 276 °C, low Td of 312 °C and high CTE of 2.03  105 °C1. These values are very close to those of TZLB glass (275 °C, [24] 306 °C and 1.96  105 °C1, respectively). In addition, PZNK glass is thermally stable without any apparent crystallization peak on its DSC curve as depicted in Fig. 6.

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H.T. Tong et al. / Optical Materials 34 (2012) 1795–1803

40P2 O5 -30ZnO-30Na 2O

40P2O5-35ZnO-(25-x)Li 2O-(x)K 2O

320

40P2O5-35ZnO-(25-x)Na 2O-(x)K 2O

310

exo.

(a)

40P2 O5 -30ZnO-25Na 2O-5K 2O

40P2O5-30ZnO-(30-x)Na 2O-(x)K 2O

40P2 O5 -30ZnO-20Na 2O-10K 2 O

Heat flow (a.u)

o

Tg ( C)

TZLB

300 290 280

40P2 O5 -30ZnO-15Na 2O-15K 2O 40P2 O5 -30ZnO-10Na 2O-20K 2O 40P2 O5 -30ZnO-5Na 2O-25K 2O 40P2 O5 -30ZnO-30K 2O

270 5

10

15

20

25

30

TZLB endo.

0

K2O concentration (mol%)

(b)

100 380

300

400

500

600

700

o

Temperature ( C)

40P2O5-35ZnO-(25-x)Na2O-(x)K2O 40P2O5-30ZnO-(30-x)Na2O-(x)K2O

360

Fig. 6. Effect of glass composition on DSC curves of mixed-alkali phosphate glasses.

TZLB

o

T d ( C)

200

40P2O5-35ZnO-(25-x)Li2O-(x)K2O

(a)

340

300

0

5

10

15

20

25

30

TZLB

Log

320

K 2O concentration (mol%)

(c)

2.8 40P2O5-35ZnO-(25-x)Li2O-(x)K 2O 40P2O5-35ZnO-(25-x)Na2O-(x)K 2O

2.4

40P2O5-30ZnO-(30-x)Na2O-(x)K 2O

2.2

TZLB

-5

o

CTE x 10 (1/ C)

2.6

o

Temperature ( C)

(b) 10.2

2.0 1.8 1.6

0

5

10

15

20

25

30

TZLB

K 2O concentration (mol%) Fig. 5. Effect of potassium oxide concentration on: (a) glass transition temperature, (b) deformation temperature, and (c) coefficient of thermal expansion of mixedalkali phosphate glasses.

Log

1.4 1.2

10.2 TZLB 40P2O5-35ZnO-25K2O 10.0 40P2O5-35ZnO-5Na2O-20K2O 9.8 40P2O5-35ZnO-10Na2O-15K2O 9.6 40P2O5-35ZnO-15Na2O-10K2O 9.4 40P2O5-35ZnO-20Na2O-5K2O 9.2 40P2O5-35ZnO-25Na2O 9.0 8.8 8.6 8.4 8.2 300 310 320 330 340 350 360 370 380 390 400 410 420

TZLB

10.0 40P2O5-30ZnO-30K2O 40P2O5-30ZnO-5Na2O-25K2O 9.8 40P2O5-30ZnO-10Na2O-20K2O 9.6 40P2O5-30ZnO-15Na2O-15K2O 9.4 40P2O5-30ZnO-20Na2O-10K2O 9.2 40P2O5-30ZnO-25Na2O-5K2O 9.0 40P2O5-30ZnO-30Na2O 8.8 8.6 8.4 8.2 300 310 320 330 340 350 360 370 380 390 400 410 420 o

Temperature ( C) Fig. 7a and b shows the effect of alkali oxide concentration and the effect of temperature on viscosity of mixed-alkali phosphate glasses. The lines in Fig. 7 indicate temperature dependence of glass viscosity. The glasses whose viscosity lines exist in higher temperature range than that of TZLB glass are much more viscous than TZLB glass. It is obvious from Fig. 7 that viscosities of phosphate glasses containing mixed-alkali oxides are closer to TZLB viscosity which makes them more suitable for fiber drawing with TZLB glass. 3.3. Fabrication of TZLB–PZNK step-index optical fibers An XRD measurement of PZNK glass was performed before fiber fabrication to ensure its thermal stability. The PZNK glass annealed at 350 °C for 4 h did not show any apparent crystallization peak in XRD result as shown in Fig. 8. Subsequently, a step-index optical fiber composed of TZLB glass core and PZNK glass cladding was fabricated by rod-in-tube method as described in Section 2. The fiber was successfully drawn at 315 °C. When compared with the

Fig. 7. Effect of alkali oxide concentration on viscosity of mixed-alkali phosphate glasses: (a) 40P2O5–35ZnO–(25  x)Na2O–(x)K2O and (b) 40P2O5–30ZnO– (30  x)Na2O–(x)K2O mol%.

temperature at which the PZNK glass was annealed to test its thermal stability, this fiber drawing temperature is 35 °C lower. It means the fiber can be drawn without crystallization. The fiber outer and core diameters were about 125 and 1 lm, respectively. The cross-section image of the TZLB–PZNK step-index optical fiber is shown in Fig. 9. The smooth boundary between fiber core and the cladding glasses inferred that the fiber can potentially have low loss performance. 4. Discussions 4.1. Optical properties Both TZLB and phosphate glasses show wide transmission windows from UV to mid-infrared region with high transmittance, see

H.T. Tong et al. / Optical Materials 34 (2012) 1795–1803

1801

Intensity (a.u)

as 0.5. This value is favorable for further development of engineering chromatic dispersion since zero-flattened chromatic dispersion of step-index optical fibers can only be achieved by high refractive index contrast between fiber core and cladding [18]. The mixed-alkali phosphate glasses, therefore, become promising cladding candidates for highly nonlinear tellurite-core optical fibers. 4.2. Thermal properties

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80

(deg) Fig. 8. XRD pattern of PZNK glass annealed at 350 °C for 4 h.

Fig. 9. Cross-section of step-index optical fiber composed of TZLB core and PZNK cladding.

in Fig. 1. The transmission ranges of phosphate glasses are slightly dependent on the alkali oxide concentration and extend towards longer wavelengths by increasing the radius and the concentration of alkali ions. The IR edges at 3–3.5 lm are caused by the OH group absorption [1]. Suppression of OH contamination could be achieved by using dry oxygen and nitrogen flow during melting process to broaden the glass transmission range. As shown in Fig. 2, the refractive index of alkali phosphate glass depends on the alkali oxide concentration. An increase in the alkali oxide concentration causes a decrease in the refractive index of glass. This tendency can be explained according to the Lorentz– Lorenz equation [21]. In general, the refractive index of oxide glasses decreases with decreasing ionic polarizability and with increasing molar volume. The ionic polarizability depends on the ionic size and charge. As shown in Fig. 2, the effect of alkaline earth ionic size on the refractive index is significant. When Ba2+ ions were substituted by Ca2+ or Mg2+ ions, the refractive index decreased due to decreasing ion radius. On the other hands, substitution of K+ by Na+ or Li+ ions resulted in increasing refractive index, although the ionic radius decreased from K+ to Na+ and Li+ ions, respectively. This is because the molar volume effect on refractive index of glass is significant for alkali ions and the molar volume decreases from K+ to Na+ and Li+ ions. As shown in Fig. 2c–e, the refractive index of mixed-alkali phosphate glasses measured at 1544 nm decrease to as low as 1.500. Since the refractive index of TZLB at 1544 nm is 2.005, the refractive index difference between TZLB and mixed-alkali phosphate glasses can be as large

As shown in Figs. 3–5a, the decrease in Tg of phosphate glasses was observed with increasing in alkali oxide concentration and with alkali ion substitution according to Mg2+(0.95) > Ca2+(0.69) > Ba2+(0.51) > Li+(0.45) > Na+(0.35) > K+(0.27) where the values in brackets refer to cationic field strength (CFS) of cations in glasses [21]. Since Tg can be defined as the temperature where a solid starts to behave as a viscoelastic solid, it strongly depends on the strength of glass network. The low cationic field strength results in low strength of glass network and leads to a decrease in Tg. In addition, alkaline earth and alkali oxides are considered as glass modifiers. They introduce non-bridging oxygens (NBOs) which do not form network bonds into the glass network. The lack of network bonds makes the glass network weaker and causes a decrease in Tg values. It is assumed from Fig. 5a that for mixed-alkali phosphate glasses, the decrease in Tg value is due to an increase in the concentration of low field strength cations, when the K2O concentration further increases. The Tg values of phosphate glasses containing Li2O are higher than the others due to high CFS of Li+ ions. As seen in Fig. 5a, phosphate glasses containing two alkali oxides can reach lower Tg. This feature is attributed to the mixed-alkali effect stated in Refs. [22,25–28]. As shown in Figs. 3–5b, Td values of studied phosphate glasses have similar trend to Tg values, which could be explained analogously by the effect of CFS and NBOs concentration. The mixed-alkali effect on Td values was confirmed in Fig. 5b, since lower Td values were obtained for mixed-alkali phosphate glasses containing two alkali oxides Li2O–K2O and Na2O–K2O, respectively. When the glass network becomes weaker, glasses would easily tend to expand their volume with an increase in temperature. This feature explains the behavior of CTE values when alkali oxide concentration was changed. It is contrary to the behaviors of Tg and Td values when CTE values increased with a decrease in the strength of glass network as shown in Figs. 3–5c. Fig. 7 shows viscosity of mixed-alkali phosphate glasses and TZLB glass in a logarithmic scale. Practically, the fabrication of TZLB-based optical fiber is successfully obtained from 310 to 320 °C. In this temperature range, 40P2O5–35ZnO–(25  x)Na2O– (x)K2O glasses are much more viscous than TZLB as can be seen in Fig. 7a. Those glasses need higher temperatures than 330 °C in order to start fiber drawing. However, these high temperatures can deform the fiber structures and reduce the quality of the TZLB hybrid fibers since viscosity of tellurite-glass is sensitive to temperature change. By increasing the concentration of mixed-alkali oxides to 30 mol%, the viscosities of phosphate glasses were shifted towards the viscosity range of TZLB glass. As can be inferred from Fig. 7b, the fiber drawing temperature of phosphate glasses containing 10Na2O–20K2O, 15Na2O–15K2O and 20Na2O–10K2O became similar to that of TZLB, which makes them good candidates for successful fabrication of hybrid microstructured optical fibers composed of TZLB core and mixed-alkali phosphate cladding. Deformation temperature, thermal expansion coefficient and viscosity of glasses are important parameters for thermal properties of optical fibers. Large difference in those parameters may cause residual stresses that damage the fiber structures during or after fiber drawing process. In addition, high quality optical fibers also require good choices of core and cladding glasses which have excellent thermal stability against crystallization. A value of DT,

H.T. Tong et al. / Optical Materials 34 (2012) 1795–1803

-1

2500

-1

given by DT = Tx  Tg, larger than 100 °C usually suggests acceptable glass stability [1,29]. However, glasses without Tx are more favorable, since the fiber drawing process may undergo several heating cycles. Although crystallization may easily occur for the phosphate glass series 40P2O5–30ZnO–(30  x)Na2O–xK2O at the temperatures higher than 350 °C as shown in Fig. 6, we can conclude that the 40P2O5–30ZnO–20Na2O–10K2O (PZNK) glass is thermally stable since its DSC curve showed no apparent crystallization peak. The thermal stability of PZNK was confirmed by the result of XRD measurement in Fig. 8 showing no crystallization even after the glass was annealed at 350 °C for 4 h.

Nonlinear coefficient (W km )

1802

2000 1500 1000 500 0 0.0

4.3. Fabrication of TZLB–PZNK step-index optical fibers

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

Core diameter (µm) The possibility of adjusting the thermal properties such as Tg, Td, CTE and viscosity of phosphate glasses by tuning the mixed-alkali oxide concentration has been discussed in the aforementioned investigation. The PZNK glass with similar thermal properties of TZLB glass would be a promising cladding material for fabricating highly nonlinear optical fibers with TZLB core. Since refractive index of PZNK is 1.511 at 1544 nm, the refractive index difference between PZNK and TZLB is 0.49. This large refractive index difference is advantageous to realize large waveguide dispersion to compensate material dispersion of tellurite-glass core. To confirm this prediction, we demonstrated a step-index optical fiber composed of TZLB core and PZNK cladding based on rod-in-tube method. During fiber drawing process, the diameter of TZLB core could be controlled. The smooth interface between TZLB core and PZNK cladding could be observed from the cross-section image in Fig. 9. We analyzed the chromatic dispersion and nonlinear coefficient of the TZLB–PZNK step-index optical fibers using the full-vector finite element method with the perfectly matched layer boundary condition. Chromatic dispersion calculations with changes in core diameters are presented in Fig. 10. The red line shows material dispersion profile of TZLB glass which has the zero material dispersion wavelength at 1850 nm. It was found that when the diameter of fiber core varied from 0.8 to 6.0 lm, the zero dispersion wavelength could be tailored flexibly from 1150 up to 1850 nm. In addition, the simulation in Fig. 11 shows nonlinear coefficient of TZLB–PZNK step-index optical fibers at 1.55 lm. The nonlinear coefficient, c, can be calculated by

2p c¼  k

RR 1

n ðx; yÞjFðx; yÞj4 dxdy 1 2 RR 2 2

ð3Þ

ð jFðx; yÞj dxdyÞ

where F(x, y)is the profile of the field and n2(x, y) is the nonlinear refractive index distribution, which is 5.9  1019 m2/W for TZLB glass. The calculated nonlinear coefficient at 1.55 lm

Dispersion (ps/nm/km)

200

Fig. 11. Core diameter dependence of nonlinearity coefficient in TZLB–PZNK stepindex optical fibers.

varies from 500 to 2500 W1 km1. It could be as high as 2537 W1 km 1 when the core diameter of TZLB–PZNK step-index optical fiber is 0.85 lm. This value is nearly 250 times larger than the nonlinear coefficient of silica fiber reported in Ref. [30]. From this view point, we confirmed the highly nonlinearity of optical fibers composed of TZLB core and PZNK cladding glasses. 5. Conclusions We have demonstrated a systematic study on phosphate glasses containing alkaline earth, alkali and mixed-alkali oxides aiming at the fabrication of highly nonlinear tellurite-based optical fibers. The composition of mixed-alkali phosphate glasses could be adjusted by changing alkali oxides and their concentration to achieve low refractive index and similar thermal characteristics of TZLB tellurite-glass. The 40P2O5–30ZnO–20Na2O–10K2O (PZNK) glass with favorable Tg, Td, CTE and viscosity is proposed as a cladding material for TZLB-core optical fibers. The step-index optical fiber composed of TZLB core and PZNK cladding with refractive index difference of 0.49 was successfully fabricated having good controllability of fiber outer and core size during the fiber drawing process. Through numerical simulation in our work for TZLB–PZNK step-index optical fibers, we have shown that their total chromatic dispersion could be well-controlled, their zero dispersion wavelength could be shifted from 1150 to 1850 nm and their nonlinear coefficient could reach 2537 W1 km1 with suitable fiber core diameter. The high nonlinearity and freely-tailored chromatic dispersion profiles of TZLB–PZNK optical fibers are proposed to realize ultimate performance of nonlinear effects including four-wavemixing or supercontinuum generation. We believe that this material choice may play an important role in many future developments of highly nonlinear hybrid microstructured optical fibers.

100

Acknowledgement 0

This work was supported by MEXT, the Support Program for Forming Strategic Research Infrastructure (2011–2015).

-100 TZLB D 0.8 µm D 0.9 µm D 0.94 µm D 1.0 µm D 1.2 µm D 1.4 µm D 1.6 µm

-200 -300 -400 800

1000

1200

1400

1600

1800

D 2.0 µm D 3.0 µm D 4.0 µm D 5.0 µm D 6.0 µm

2000

2200

Wavelength (nm) Fig. 10. Simulation of tailoring chromatic dispersion by changing core diameter of TZLB–PZNK step-index optical fibers. The red line indicates TZLB material dispersion.

References [1] R. Stepien, R. Buczynski, D. Pysz, I. Kujawa, A. Filipkowski, M. Mirkowska, R. Diduszko, J. Non-Cryst. Solids 357 (2011) 873–883. [2] Aoxiang Lin, Aidong Zhang, Elizabeth J. Busong, Jean Toulouse, Opt. Exp. 17 (2009) 16716–16721. [3] Xian Feng, Wei H. Loh, Joanne C. Flanagan, Angela Camerlingo, Sonali Dasgupta, Periklis Petropoulos, Peter Horak, Ken E. Frampton, Nicholas M. White, Jonathan H.V. Price, Harvey N. Rutt, David J. Richardson, Opt. Exp. 16 (2008) 13651–13656. [4] Amit Belwalkar, Wojciech Z. Misiolek, Jean Toulouse, J. Non-Cryst. Solids 356 (2010) 1354–1358.

H.T. Tong et al. / Optical Materials 34 (2012) 1795–1803 [5] H. Desirena, A. Schulzgen, S. Sabet, G. Ramos-Ortiz, E. de la Rosa, N. Peyghambarian, Opt. Mater. 31 (2009) 784–789. [6] Rajan Jose, Guanshi Qin, Yusuke Arai, Yasutake Ohishi, Jpn. J. Appl. Phys. 46 (2007) 651–653. [7] Xian Feng, Francesco Poletti, Angela Camerlingo, Francesa Parmigiani, Periklis Petropoulos, Peter Horak, Giorgio M. Ponzo, Marco Petrovich, Jindan Shi, Wei H. Loh, David J. Richardson, Opt. Fiber. Technol. 16 (2010) 378–391. [8] Guanshi Qin, Xin Yan, Chihiro Kito, Meisong Liao, Takenobu Suzuki, Atsushi Mori, Yasutake Ohishi, J. Appl. Phys. 107 (2010) 43108. [9] Meisong Liao, Xin Yan, Guanshi Qin, Chitrarekha Chaudhari, Takenobu Suzuki, Yasutake Ohishi, Opt. Exp. 17 (2009) 15481–15490. [10] Meisong Liao, Chitrarekha Chaudhari, Guanshi Qin, Xin Yan, Takenobu Suzuki, Yasutake Ohishi, Opt. Exp. 17 (2009) 12174–12182. [11] Meisong Liao, Chitrarekha Chaudhari, Guanshi Qin, Xin Yan, Chihiro Kito, Takenobu Suzuki, Yasutake Ohishi, Morio Matsumoto, Takashi Misumi, Opt. Exp. 17 (2009) 21608–21614. [12] Rogerio F. Souza, Marcio A.R.C. Alencar, Jandir M. Hickmann, Renata Kobayashi, Luciana R.P. Kassab, Appl. Phys. Lett. 89 (2006) 171917. [13] Guanshi Qin, Meisong Liao, Chitrarekha Chaudhari, Xin Yan, Chihiro Kito, Takenobu Suzuki, Yasutake Ohishi, Opt. Lett. 25 (2010) 58–60. [14] Guanshi Qin, Xin Yan, Chihiro Kito, Meisong Liao, Takenobu Suzuki, Atsushi Mori, Yasutake Ohishi, Opt. Lett. 35 (2010) 136–138. [15] Meisong Liao, Xin Yan, Guanshi Qin, Chitrarekha Chaudhari, Takenobu Suzuki, Yasutake Ohishi, J. Non-Cryst. Solids 356 (2010) 2613–2617. [16] Zhongchao Duan, Meisong Liao, Xin Yan, Chihiro Kito, Takenobu Suzuki, Yasutake Ohishi, Appl. Phys. Exp. 4 (2011) 72502.

1803

[17] Chitrarekha Chaudhari, Takenobu Suzuki, Proc. SPIE 7728 (2010) 7728101– 7728109. [18] Francesco Poletti, Xian Feng, Giorgio M. Ponzo, Marco N. Petrovich, Wei H. Loh, David J. Richardson, Opt. Exp. 19 (2011) 66–80. [19] Xian Feng, Arshad K. Mairaj, Daniel W. Hewak, Tanya M. Monro, J. Lightwave Technol. 23 (2005) 2046–2054. [20] J.S. Wang, E.M. Vogel, E. Snitzer, Opt. Mater. 3 (1994) 187–203. [21] Yang Li, Jianhu Yang, Xu Shiqing, Guonian Wang, Hu Lili, J. Mater. Sci. Technol. 21 (2005) 391–394. [22] H.W. Guo, X.F. Wang, Y.X. Gong, D.N. Gao, J. Non-Cryst, Solids 356 (2010) 2109–2213. [23] P.Y. Shih, Mate. Chem. Phys. 80 (2003) 299–304. [24] G. Senthil Murugan, Takenobu Suzuki, Yasutake Ohishi, Appl. Phys. Lett. 86 (2005) 221109. [25] Yong Gao, Chem. Phys. Lett. 417 (2006) 430–433. [26] C. Karlsson, A. Mandanici, A. Matic, J. Swenson, L. Borjesson, J. Non-Cryst. Solids 307–310 (2002) 1012–1016. [27] M.A. Samee, A.M. Awasthi, T. Shripathi, Shashidhar Bale, Ch. Srinivasu, Syed Rahman, J. Alloys. Compd. 509 (2011) 3183–3189. [28] Gregory Tricot, Bertrand Revel, Sebastian Wegner, J. Non-Cryst. Solids 357 (2011) 2708–2712. [29] J. Massera, A. Haldeman, D. Milanese, H. Gebavi, M. Ferraris, P. Foy, W. Hawkins, J. Ballato, R. Stolen, L. Petit, K. Richardson, Opt. Mater. 32 (2010) 582– 588. [30] F. Poli, F. Adami, M. Foroni, L. Rosa, A. Cucinotta, Selleri, Appl. Phys. B 81 (2005) 251–255.