(1)

Broadband spectral set-up

An experimental set-up was built for measuring the spectral loss of the unclad (mono-index) Te glass fibers in the region 4 – 20 μm by the fiber cut-back technique. The components of this set-up are described in Table 1.

Table 1

Components of broadband spectral set-up

Some pictures of the set-up are presented in Fig. 2.

Fig. 2.

Top view of broadband spectral set-up (top) with details, showing the fiber (on Al substrate) in front of the pinhole (bottom left) and the output of the fiber close to the detector window (bottom right)

(2)

CO₂ laser set-up

The attenuation measurements at 10.6 μm using the cut-back method are performed with a pulsed CO₂ laser (SYNRAD J48-1SW). The maximum output power is 10W, the beam size is about 3 mm and the pulse frequency is 5 kHz. The output optical power can be tuned by changing the duty cycle. The set-up is sketched in Fig. 3.

Fig. 3.

Pulsed CO₂ laser set-up for fiber attenuation measurements at 10.6 μm

Spectral attenuation of TeGeGal fibers

The transmission of two TeGeGal fiber samples, approximately 5 cm long, was measured using the broadband spectral set-up. One fiber sample was prepared from an untreated glass and another sample from a similar glass composition, after an additional chemical purification procedure to remove (oxygen) impurities.

The corresponding spectra are presented in Fig. 4.

Fig. 4.

Comparison of spectral transmission of a 5 cm fiber of purified and non-purified (untreated) TeGeGaI mono-index fiber

It can be seen in Fig. 4 that the TeGeGaI fiber has the highest transmission in the 12 – 13 μm region. Besides, part of the absorption of the TeGeGaI fiber in the region 15 – 20 μm is extrinsic, probably due to oxygen related impurities. By the application of proper purification methods, the transmission in the region 15 – 20 μm can be increased.

Next, the spectral attenuation of a 9 cm long sample of the purified TeGeGaI fiber was determined by cutting back this fiber piece in 3 steps to about 3 cm.

The measured average spectral attenuation is presented in Fig. 5.

Fig. 5.

Averaged spectral attenuation (dB/cm) of a purified TeGeGaI fiber at room temperature determined by fiber cut-back method

A minimum in the attenuation of 0.4 dB/cm was found between 12 and 13 μm. The attenuation between 16 and 20 μm increases from a value around 1.5 dB/cm to 3 dB/cm.

Although the measured values for the spectral loss are still relatively high, these results prove that the purified TeGeGaI fiber transmits light in the region between 15 and 20 μm through short fiber samples. This performance can probably be improved by advanced glass manufacturing processing and improved purification methods.

Attenuation of high Te fibers at 10.6 μm

The attenuation at 10.6 μm of fiber samples of about 50 cm long of the glass types TeGeGaI, TeGeI and TeGeSe was determined by cutting back these fibers in many steps to about 10 cm. After each cut the transmitted light intensity through the remaining fiber length was measured.

The results of the fiber cut-back measurements with the CO2 laser set-up are summarized in Table 2.

Table 2

Attenuation at 10.6 μm of different high Te-glass fiber types (unclad), made from purified glasses

The measured loss value at 10.6 μm of 0.46 dB/cm for the TeGeGaI glass fiber corresponds reasonably well with the measured values between 10 and 11 μm in Fig. 5. Apparently, the optical loss at the CO2 laser wavelength (10.6 μm) of the glass fiber types TeGeI and particularly TeGeSe is considerably lower.

This observation was confirmed by other spectral measurements, not reported here, which showed that the TeGeSe glass fibers have a minimum attenuation between 10 and 11 μm, so slightly shifted to shorter wavelengths compared to TeGeGaI fibers. The origin of the differences in the optical loss between the different fiber types (intrinsic/extrinsic absorptions, glass quality, crystals in the fiber, etc.) is still under study.