DD285418A5 - FIBER OPTIC TEMPERATURE SENSOR - Google Patents

Abstract

The invention relates to a fiber optic sensor for temperature measurement. The invention can be used when temperatures by non-electrical means, for. B. in the vicinity of strong electromagnetic interference fields or increased risk of explosion to be measured. The sensor has high sensitivity even when using broadband LEDs. It is characterized in that a compensator crystal of another material is combined with a sensor crystal such that the thickness ratio of the two crystals is reciprocal to the birefringence ratio of these crystals and the temperature dependency of birefringence of the two crystals is greatly different. Temperature sensor, fiber optic; Birefringence change, temperature dependent; Crystal; Kompensatorkristall}

Description

good measuring sensitivity is (n, - n _o ) l as large as possible, so that at a spectral bandwidth of about 50 nm, the phase differences Δφ, usually vary by several 2n. This opens up after passing through the crystal for the different spectral components quite different polarization states, which in turn results in a temperaturabhängigenÄndorungvonn - n _o nurzu low intensity changes of the measurement signal behind the analyzer. For polarization-optical investigations compensators are known (zs Soleil-Babinet Compensator), the total phase difference between two orthogonal waves after passage through a birefringent crystal greatly reduced by a second, identical crystal is mounted almost the same thickness behind the first, wherein the optical Axes of the two Krislalle are rotated by 90 ° to each other. As a result, defined polarization states are also obtained for broadband light sources at the output. Of course, such an arrangement is not suitable as a temperature sensor, since it would also compensate for the temperature-dependent birefringence changes of the two crystals. The variation of the phase differences Δφι for the different spectral components must therefore be compensated without appreciably influencing the temperature dependence of the birefringence of the overall arrangement. This is possible according to the invention by using a compensator crystal made of a different material, wherein the phase compensation must be valid

I (n, - n _o ) i · I i = »I (n, -n _o ) n · In |. (2)

Here are (n, - n ₀ ), and I, birefringence and thickness of the sensor crystal and (n, - n _o ) n and Iu- birefringence and thickness of the Kompensatorkristalls. The amount characters in Eq. (2) are because both materials may be both positively and negatively birefringent, and the compensation can be achieved in any case by appropriate orientation of the two crystals to each other.

In the choice of the two birefringent materials is added as an additional requirement that the Kompensatormaterial does not deteriorate the measuring sensitivity, d. h., that either

d | (n - n ") i · I ₁ ] d [(n - n") n In) .

dT'dT

must apply, d. h., That the Kompensatorkristall compared to the sensor crystal should have a negligible temperature-dependent Doppeibrechungsänderung or that it should be soin kind that the measuring effect is enhanced.

embodiments

The overall structure of the sensor shows the figure. In the collimated beam path between feed fiber 1 and discharge fiber 2, a sensor crystal 3 is arranged between crossed or parallel polarizers so that its optical axis is inclined by 45 ° to the transmission direction of polarizer 5 and analyzer 6. Thus, the perpendicularly polarized light components in the crystal (neat LiNbOa, or else BiI ₂ SiOw, are particularly suitable as sensor material, especially for congruently grown LiNbOj, a very large temperature-dependent birefringence change of

 CIT

The present between neat and extremely polarized wave at the output of the sensor crystal 3 phase difference is almost completely compensated by a located behind the sensor crystal Kompensatoi crystal 4. As Kompensatorkristall z. B. quartz with a very small birefringence change of

-1-10- ⁶ K- '.

dT

Since LiNbO _{3 is} a negatively birefringent crystal with n, - n _o = -0.08 and quartz is a positively birefringent crystal with

n, - n ₀ = * +0.009, the optical axes of the two crystals must be aligned parallel to each other and the thickness ratio liiNMvsio, = * 1: 9 for compensation.

To achieve greater compactness, it is also possible to use suitable crystals with high birefringence,

whereby the compensator Krista'ldicken can be significantly lower. Rutile would be possible with n, - n ₀ = 0.28, so that in this case the thickness ratio

luNbo,: lfluui ^β must be 3.5: 1.

In the event that the sensor and Kompensatorkristall have the same direction birefringence (both positively or negatively birefringent), the optical axes of the two crystals must be rotated by 90 ° to each other, so that the effect of the phase difference compensation can be achieved.

Claims (1)

Fiber optic sensor for temperature measurement based on the temperature-dependent birefringence change of crystals, characterized in that a Kompensatorkristall of another material is combined with a known sensor crystal such that the thickness ratio of the two crystals is reciprocal to the birefringence ratio of these crystals and the temperature dependence of the Birefringence of the two crystals is very different. For this 1 page drawing Field of application of the invention The invention relates to a temperature sensor in which the temperature dependence of the birefringence of a crystal is utilized. The light used for the measurement is supplied to the sensor head via optical waveguides or led away from it after passing through the crystal. The sensor can be used in optical signal transmission systems with single-mode or mohrmodigen optical waveguides, if z. B. Temperatures are measured by non-electrical means in an environment of strong electromagnetic interference fields or with high risk of explosion. Characteristic of the known state of the art There are known fiber optic temperature sensors, which are based on very different principles of action. So z. As the temperature-dependent photoluminescence of solid materials (DE-OS 3036682, EP 0006530), the temperature-dependent refractive index change of liquids (DE-OS 2941677), the temperature-dependent band edge shift of materials (EP 0006530, EP 0210719) and the temperature-dependent birefringence change of optical crystals, such as , B. LiNbO ₃ (EP 0014848) described. In the case cited, the sensor head contains, in addition to the birefringent crystal, two polarizers and lenses for coupling or coupling in the light from the feed or the discharge fiber. The change of the polarization state of the light through passage through the birefringent crystal is converted by the second polarizer into an amplitude change which serves as a measurement signal. To ensure sufficient sensitivity, the light path through the crystal must be correspondingly large. This makes it necessary to use light sources with a low spectral bandwidth (EP 0014848), so that after passing through the crystal there is still a defined polarization state. In conjunction with multi-mode Lichtwellonleitern but i. general broadband LED (Δλ ™ 50 nm), which do not meet this requirement. In order to be able to exploit the sensor principle of the temperature-dependent double orientation change in this case as well, a Michelson interferometer was used for the analysis of the spectrally dependent optical physical difference of the orthogonal modes on the receiving side (technical measurement, 54th year "issue 7/8, 1987, pp. 304-309). This solution is of course very complex, the benefits of simplicity and compactness are lost. Object of the invention The aim of the invention is a fiber optic sensor for temperature measurement on the basis of the temperature-dependent birefringence change of crystals, in the retention of high sensitivity should also be used for the use of broadband LED. The production costs should not be increased and the evaluation of the measurement signal should not be complicated. Explanation of the essence of the invention The invention has for its object to provide a fiber optic temperature sensor based on birefringent materials, in which the temperature-dependent birefringence change is used and can be used without significant restrictions for broadband LED. The solution of this object succeeds with a fiber optic sensor for temperature measurement on the basis of the temperature-dependent birefringence change of crystals erfinJungsgemäß characterized in that a Kompensatorkristall of another material is combined with a per se known sensor crystal such that the thickness ratio of the two crystals is reciprocal to the birefringence of the Crystals behälf and the temperature dependence of the birefringence of the two crystals is very different. When using broadband light sources, the problem generally arises that the different spectral components λ; strongly different phase differences Δφ, - corresponding to the orthogonal methods 2n (n. - n _o ) l
α, - as it passes through the birefringent crystal. Here, n "n _o" extraordinary refractive index of the crystal, "I" geometric path length of the light through the crystal, λ "spectral portion of the wavelength. To achieve a