5. Fiber-optic temperature sensor with crystalline sensitive element

Preliminary analysis of the probable variants of crystalline materials which can be used as temperature-sensitive elements indicated the possibility of using base materials of the A3B5 (GaAs, GaP), which today are the basic semiconductor crystals for modern microelectronics [19, 20].

From the practical point of view, the interest in these materials is due to their wide application in the elements of infrared optics, television equipment, and fiberoptic communication. These materials are resistant to radiation, which makes their practical application under conditions of radioactive exposure possible, and the

Fiber-Optic Temperature Sensors with Chalcogenide Glass and Crystalline Sensing Element DOI: http://dx.doi.org/10.5772/intechopen.89207

possibility of anatomy of radioactive defects opens the possibility of their multiple use in extreme radiation conditions. In addition, the optical properties of these crystals are immune to high-frequency electromagnetic fields.

All of this indicates a possibility of effective application of A3B5 crystals as sensitive elements for amplitude-type FOS for temperature, pressure, and other physical quantities.

For modeling parameters of primary measuring converter, GaP crystals doped with Zn (GaP:Zn) were used. These crystals are used to construct different types LEDs [21].

For experimental studies of optical transmittance, special samples were made. The methods of making are as follows: crystal plates were cut out of solid material and polished on abrasive powders, then polished on diamond paste of different consistency, and electropolished to a high class of roughness. As a result, we got plates with a thickness of 300 microns and high-quality surface.

Figure 4 shows a picture of one of the crystal GaP:Zn. This plate was used in our optical studies.

Results of these studies, in the form of direct records, were made on a specialized optical installation.

According to the transmission spectrum, which is shown in Figure 5, the optical absorption spectra at different temperatures, shown in Figure 6, were calculated.

Optical absorption in GaP:Zn crystals according to the literature data [22] is indirect banded, which determines the angle of absorption dependence from wavelength. For the convenience of determining the width of the bandgap at different temperatures, the dependence α<sup>2</sup> = f (hv) was constructed (Figure 7).

Subsequent processing on the computer obtained experimental results in the form of transmission spectra of GaP:Zn crystal at various temperatures which is shown in Figure 8. It can be seen from Figure 8 that with increasing of temperature, the transmittance dependence shifts to greater wavelengths and overall bandwidth drops. This allowed us to determine the wavelength position which will determine the controlled level of transmission at given temperature. It was most convenient to use a λ = 600 nm, which was determined by the level of transmission at a fixed temperature. The results of the analysis of the temperature shift of the transmission level, determined by the spectra in Figure 8, are presented in Figure 9.

Dependence of the absorption coefficient α in non-triangular semiconductors to which GaP belongs is described by the dependence:

Figure 4. Photograph of GaP:Zn crystal used in optical studies.

Figure 5. GaP:Zn crystal spectrum at different temperatures T (K).

Figure 6.

Dependence of absorption of GaP:Zn crystals at different temperatures T (K).

Figure 7. Dependence of absorption of GaP:Zn crystal at different temperatures T (K).

Fiber-Optic Temperature Sensors with Chalcogenide Glass and Crystalline Sensing Element DOI: http://dx.doi.org/10.5772/intechopen.89207

Figure 8. Spectrum of GaP:Zn crystal at different temperatures T (K).

Figure 9. Temperature dependence of transmission τλ on λτ = 600 nm for the GaP:Zn crystal.

$$\alpha = \alpha\_0 \left[ \mathbf{h} \mathbf{v} - \mathbf{E}\_{\mathfrak{g}} \right]^\frac{1}{2} \tag{6}$$

This means that when α<sup>2</sup> dependence is built, it is possible to obtain the expression:

$$\alpha^2 = \alpha\_0^2 \left[ \mathbf{h} \mathbf{v} - \mathbf{E}\_\mathbf{g} \right] \tag{7}$$

From which it is easy to obtain the value of the width of the indirect bandgap Eg by extrapolating to the zero of the linear part of the dependence α<sup>2</sup> = f (hv).

Analyzing Figure 8, we can select the required operating wavelength to demonstrate the practical use of GaP:Zn crystals as a temperature-sensitive element of a fiber-optic temperature sensor. As a result we obtained temperature dependence of transmission of GaP:Zn crystal (Figure 9).

It can be seen from Figure 9 that the passage of the GaP:Zn plate at a controlled wavelength λ = 600 nm linearly decreases in the temperature range from 300 to 400 K. This, not directly, indicate that the expected and calculated full-cycle photocurrent will also change with temperature change. That suggests the possibility of using GaP:Zn crystals as temperature-sensitive element of a fiber-optic temperature sensor.
