Cadmium-containing fluoride glasses

J O U R N A L OF

Journal of Non-Crystalline Solids 161 (1993) 198-201 North-Holland

NON-CRYSTALI~E SOLIDS

Refractive index measurement of the TeX glasses H . L . M a a, X . H . Z h a n g a, j. L u c a s a, R. Iizuka b, T. Y a m a g i s h i b a n d T. Y a m a s h i t a b a Laboratoire des Verres et Cdramiques, Universitd de Rennes, Campus de Beaulieu 35042 Rennes, France b Non-Oxide Glass Co. Ltd, 668, lwahara, Minamiashigara-shi, Kanagawa, 250-01 Japan

Refractive index measurements have been performed as a function of wavelength and as a function of temperature for different tellurium halide based glasses (the TeX glasses). The dispersion properties have been discussed and compared with chalcogenide glasses. It has been indicated that a small modification of the glass composition can lead to an important change of the refractive index. These results are interesting for the design of fibres having a core-cladding structure. As far as the d n / d T is concerned, this thermal-optical parameter is almost the same for different glasses in the same family.

1. Introduction The refractive index is an important parameter for the design of optical components such as prisms, windows and optical fibres. It is also necessary to determine this optical parameter as a function of wavelength and as a function of temperature. For traditional chalcogenide glasses, the refractive index and material dispersion have been reported and some data of the thermal coefficient of the refractive index dn/dT are also available for some compositions [1,2]. For chalcohalide glasses, it has been reported that the refractive index decreases when a halogen (Br 2 or 12) is introduced into the chalcogenide glasses [3]. No data on material dispersion or dn/dT are available for these glasses. The TeX glasses are a new family of IR transmitting materials based on the glass-forming ability of the binary system T e - X (X = C1, Br, I). They are interesting optical materials because of their wide optical window ranging from ~ 1 to 20 p.m. Good quality optical fibres, thin films and lenses have been obtained from these stable

Correspondence to: Dr X.H. Zhang, Laboratoire de Verres et de C6ramiques, URA CNRS 1496, Universit6 de Rennes, Campus de Beaulieu, 35042 Rennes, France. Tel: + 33 99 28 62 62. Telefax: +33 99 28 16 00. Please complete

glasses against crystallization and against corrosion [4]. In this paper we report the refractive index of several T e X glasses as a function of wavelength and as a function of temperature. These data are compared with those of chalcogenide glasses.

2. Experiment

2.1. Sample preparation The preparation of the TeX glasses has been described elsewhere in detail [4,5]. All starting elements are treated by appropriate techniques: Te is etched by an HBr + Br 2 solution; Se and As are heated under vacuum at 250 and 350°C, respectively. Br 2 and 12 are purified by distillation or sublimation. The starting elements are then put into a silica tube which is evacuated and sealed. The mixture in the silica tube is heated and homogenized in a rocking furnace at a temperature of 300-600°C depending on the composition. Glasses are obtained by cooling the silica tube containing the melt in air. The samples for refractive index measurements are annealed at a temperature near Tg for about 2 h and then cooled at a rate of 10°C/h to room temperature. The obtained glass rods are

0022-3093/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved

199

H.L. M a et al. / Refractive index measurement o f the T e X glasses

thermocouple

X•x

N~i

mirror /

Fig. l. Schematic principle of refractive index m e a s u r e m e n t .

Table 1 Refractive index as a function of wavelength for four TeX glasses at 25°C Wave-

Refractive index

length (~m)

Te3SesBr 2 Te2Se4lAs3 " "

Te2Se3iAs4

TezSelAs 6

3 4 5 6 7 8 9 10 10.6 11

2.7030 2.6990 2.6932 2.6886 2.6880 2.6863 2.6834 2.6799 2.6776 2.6748

2.9035 2.8979 2.8918 2.8885 2.8840 2.8807 2.8785 2.8741 2.8718 2.8702

3.0489 3.0436 3.0382 3.0345 3.0302 3.0262 3.0237 3.0203 3.0178 3.0162

2.8435 2.8385 2.8357 2.8334 2.8295 2.8267 2.8244 2.8222 2.8205 2.8194

then cut into prisms and polished before the measurements. The Te3SesBr 2 glass and three glasses of the family Te2Sev_xIAs ~ have been chosen for the measurements.

ments. The refractive index values are very different for different compositions.

2.2. Refractive index measurements

3.2. Refractive index as a function of temperature

The set-up for the refractive index measurement is mainly composed of a light source, a monochromator, a goniometer and a detector. The principle of the m e a s u r e m e n t is schematized in fig. 1. Only the light arriving perpendicular to the mirror is reflected back along the exact same optical path as the incident light. The incidence angle for which this condition is satisfied is then precisely measured and the refractive index is given by n = sin 0 / s i n i (see fig. 1). By heating the prism, it is possible to determine the thermal coefficient of the refractive index d n / d T .

Figure 2 shows the refractive index of Te3SesBr 2 glass as a function of t e m p e r a t u r e from 30 to 65°C at the wavelengths of 3, 5, 7 and 10.6 txm. It can be noted that n increases linearly with the temperature, making it possible to determine the thermal coefficient of the refractive index d n / d T . The values of d n / d T are listed in table 2.

2.71

at 3 ~m

ex

0

2.70

at5 lsm

"0

3. Results

.c_

(D .~

2.69

,.

2.68

3.1. Refractive index as a function of wavelength The m e a s u r e m e n t s have been performed at 25°C for Te3Se5Br 2 glass and for the glass family Te2Se 7 xlASx with x = 3, 4 and 6. The results are collected in table 1 which shows the refractive index, n, as a function of wavelength, A, for four T e X glasses. It can be seen that the refractive index is relatively high and n d e c r e a s e s slowly when A increases in the region of the measure-

at 7 m m

at 10.6

Ism

2.67 20

30

40

50

60

70

Temperature (°C) Fig. 2. Refractive index of the Te3SesBr 2 glass as a function of temperature. The curves are a fit of the data to the function n = C i + B i T , where C i and B i are constants for each wavelength.

H.L. Ma et al. / Refractive index measurement of the TeX glasses

200

Table 2

can be described by eq. (1). The constants A, B, C, D and E can be determined by using the least-squares method. It is then possible to calculate the material dispersion:

d n / d T of Te3SesBr 2 glass at different wavelengths Wavelength (Ixm):

3 6.5

d n / d T (× 10 -4 K - l ) :

5 7.6

7 7.4

10.6 8.8

n(A)

The same measurements have been performed

=A/A4+B/A2+C+DA2+EA

4.

In our case, only 10 data are available for determining the five constants A - E . Consequently, the accurancy of the material dispersion calculation should be low. We prefer to calculate the index of dispersion v between 3-5 ~m and 8-11 ~m by using eq. (2). The values for TeX glasses are listed in table 4 and there is no obvious relation between the composition and vs_lv The values of between 150 and 250 are comparable with those of the chalcogenide glasses

at 5 and 10.6 p.m for three glasses of the family Te2SeT_xlAS x with x = 3, 4 and 6. The results and the values of d n / d T are collected in table 3. For comparison, the values of d n / d T are respectively 0.7 × 10 -4 K -1 for Ge30As13Se57 and 1.5 X 10 - 4 K -1 f o r G e 3 0 A s 1 3 S e 2 7 T e 3 0 [2].

4. Discussion

[21:

In the region of transparency, the relationship between the refractive index and the wavelength

V l _ 2 ~___[1(?11 qt_ n2 ) _ 1 ] / ( / / 1

_ n2).

(2)

Table 3 Refractive index of three glasses of the family Te2Se7_xlASx as a function of temperature

Composition

Temperature

n at5 ~m

n at 10.6 ~ m

2.8357 2.8373 2.8396 2.8402 2.8418

2.8205 2.8211 2.8233 2.8244 2.8267

d n / d T = 1.5 × 10 _4 K -~

( d n / d T = 1.5× 10 -4 K -a

2.8918 2.8929 2.8946 2.8957 2.8974

2.8718 2.8735 2.8752 2.8763 2.8774

( d n / d T = 1.4× 10 -4 K 1

( d n / d T = 1.4× 10 -4 K -1

3.0382 3.0404 3.0420 3.0436 3.0452

3.0178 3.0205 3.0221 3.0237 3.0248

( d n / d T = 1.7× 10 -4 K - I

( d n / d T = 1.7× 10 -4 K -1

(°C) Te2Se4IAs 3

24 35 45 55 65

TezSe3IAs 4

25 35 45 55 65

Te2SeIAs 6

24 36 45 55 65

Table 4 Index of dispersion between 3-5 Ixm and 8-11 0,m for four TeX glasses Te3SesBr2

Te2Se4IAs 3

(1)

Te2Se3IAs4

Te2Selms 6

W3-5

1'8-11

1"3-5

1"8-1l

~3-5

1"8-11

V3-5

~8-11

173

146

236

250

162

179

191

202

201

H.L. Ma et al. / Refractive index measurement o f the TeX glasses

cladding glasses calculated by eq. (3) is about 0.007:

3.10 ,.-

3.05

NA= x

">~

~

2.95 2.90

n"

An : 0 . 2

3.00

2.85

/10,6

gm

2.80 2

x in TozSe 7.xlASx Fig. 3. Relation between the refractive index and the glass composition. The curves are a fit of the data to the function n = A i + B i x , where A i and B i are constants for each wavelength. In table 1, it is interesting to note that the refractive index of the glass family Te2Se7_xlAS x can change considerably with the composition. As an example, the refractive index at the wavelength of 10.6 ~ m is changed from 2.8205 for the Te2SenlAs 3 to 3.0178 for the T e z S e l A s 6 glass. In order to find the relationship between the refractive index, n, and the composition, n is plotted against x in Te2Sev_xlAS x (fig. 3). Considering the composition error due to the preparation and purification, it is found that n increases linearly with x. This knowledge is particularly interesting when a glass with a specific refractive index is needed. It can also be noted that n increases rapidly with the composition. So, it is possible to choose, for fibres with a core-cladding structure, two glasses having very similar compositions. For example, if the fibre has a numerical aperture (NA) of 0.2 at 10.6 Ixm and if Te2Se4IAs3 is chosen as its base composition, the refractive index difference, ~n, between the core and

This refractive index variation can be obtained by modifying very slightly the glass composition with Ax = 0.1 in TeiSeT_xIAS x. Such a composition variation will induce a change of the thermal expansion coefficient lower than 0.03 × 10 -5 K - 1 [6]. Therefore, the core glass and the cladding glass could be perfectly compatible.

5. C o n c l u s i o n

The refractive index of several T e X glasses has been determined as a function of wavelength and as a function of t e m p e r a t u r e for different glasses. The data on d n / d T have been obtained and the dispersion has been discussed. It has been found that the refractive index increases considerably in a linear way with the As concentration in the Te2SeT_xlAS ~ glasses. This knowledge is particularly interesting for the optical design. It has been determined that the core and cladding glasses of a optical fibre can be obtained by modifying very slightly the composition, making these two glasses perfectly compatible.

References

[1] P. Klocek and L. Colombo, J. Non-Cryst. Solids 93 (1987) 1. [2] J.A. Savage, Infrared Optical Materials and Their Antireflection Coatings, (Hilger, Bristol, 1985) p. 83. [3] J. Heo and J. Mackenzie, J. Non-Cryst. Solids 111 (1989) 29. [4] J. Lucas and X.H. Zhang, J. Non-Cryst. Solids 125 (1990) 1. [5] H.L. Ma, X.H. Zhang and J. Lucas, J. Non-Cryst. Solids 135 (1991) 49.