Temperature Measurement – Optical Frequency Domain Reflectometry (OFDR)

by Annika Hangleiter (winter semester 2020/21)

Temperature measurement based on light - This article describes a temperature measurement method in which the interferometry of light is analyzed. The aim is to detect the reflected light waves in a glass fiber when the fiber changes in space due to heating. This is a non-destructive method for measuring temperature and strain in small and hard-to-reach areas. The strain measurement is based on the same principle, but is not discussed in this article.

Introduction

Temperature is an important parameter used in all disciplines of natural science and engineering and consequently also in non-destructive testing.

As already mentioned in the article: `Temperature – Basics and measurement´, temperature is a description of a physical and chemical macroscopic state in matter. In temperature measurement several scales and units exist, the most commonly used is Celsius (°C); furthermore: Fahrenheit (°F) and especially in science: Kelvin (K).

For measuring the temperature there are several different common methods available: liquid-in-glass thermometer, gas thermometer, thermocouples, thermistors, resistance temperature detectors (RTD), pyrometers, Langmuir probes, infrared thermometer and other thermometers that includes the innovative technology of fiber optical thermometer. ^[1]

The methods are distinguished between non-contact measurement by using emitted radiation of the observed body and the contact measurement which are connected to the measured object. In the case of a contact measurement they mostly working on low levels of electrical current or voltage. Under these conditions, electric and magnetic influences have a significant impact. In applications where electrical disturbances occur or where a non-contact measurement method is inaccurate or even impossible, optical measurement is an alternative. In this case the principles of blackbody radiation, luminescence in crystals, refractive index phenomenon in the Fiber Bragg Gratings or the phase shift of two coherent beams in interferometric sensors are an option. ^[2]

As already described in the chapter `Physical Background’ of the article `Raman Spectroscopy & Microscopy´ from F. Bachmaier, all optical measurement methods are based on the interaction of electromagnetic radiation with matter and their emission, absorption, fluorescence or scattering phenomena.

A comprehensive overview about the Fiber Bragg Grating (FBG) in an optical fiber based sensor is given in the article `Distributed Strain and Temperature Sensing: Laser and Fiber Bragg Gratings´ from S. Wörle. In this article the Fiber Bragg Grating sensors, for distributed strain and temperature determination, is based on the Brillouin backscattering.

The article from M. Krapp about `Faser-Bragg-Gitter Sensoren´ describes using the FBG for monitoring buildings. The FBG sensors opens up the possibility to measure structural changes if there is for example a local change in length in the building. The optical measurement system shows the strain along the optical fiber in that area where the Bragg grating is introduced into the glass fiber core. The article also points out different fields of application of the optical measurement with FBGs.

The principle of optical frequency domain reflectometry (OFDR) is assigned to the previous topics because it also works with the backscatter phenomena in a fiber optic, optionally with and without FBG. The OFDR is based on Rayleigh back scattering.

Basic conceptualities

Optical Fiber - structure

The structure of an optical fiber is shown in figure1. A single glass fiber, the core (usually 9 µm in diameter), is embedded into another glass structure, the cladding. The glass core and the cladding have different refractive indices. The refractive index is important to determine the traveled path of a light wave in the fiber. Due to the fact that the glass fiber has to have a higher refractive index than the cladding, a total reflection of the light wave occurs at the boundary between core and cladding. This allows the light wave to move through the fiber optic core in a certain mode. The coating protects the fiber from breaking when the fiber is bent in different angles. Nevertheless, the fiber is still very sensitive to sharp bending, especially if the fiber becomes wrinkled. ^[3]

There are two different kinds of fibers: single mode and multimode fibers. In a multimode fiber, different light waves with a specified wavelength can be transmitted in different angles at the same time. In a single mode fiber, only a single wave with a specific angle can be transmitted in the core. The diameter of the core of a single-mode fiber is smaller than the wavelength of the inserted light so only one mode can be transmitted. A so called mode is a bright-dark-ratio which occurs with the interference of the transmitted light. ^[4]

Coherence

If the frequency and the waveform are identical and the phase difference is constant between two wave sources, the sources are referred to as perfectly coherent. The Coherence is an ideal property of waves that enables stationary interference in time and space. Two waves are coherent if they have a constant relative phase. The coherence is measured by the interference visibility, a size of the interference fringes relative to the input waves is displayed. ^[5,6]

Swept-wavelength laser

A swept-wavelength laser is a tunable laser source that allows to periodically tune an optical wavelength over a broad spectral range. With such a swept-wavelength source, interference signals at individual wavelengths can be measured sequentially with high spectral resolution. The spectrally resolved data acquisition is key for frequency-domain ranging which  offers significantly higher sensitivity than the time-domain ranging method. ^[7]

Principle of the optical frequency domain reflectometry (OFDR)

There are fundamentally three reflectometric techniques suitable for fiber-based applications: optical time domain reflectometry (OTDR), low coherence domain reflectometry (OLCR) and coherent optical frequency domain reflectometry (OFDR). The differences in the methods result from the conflicts of interest between range, resolution, speed, sensitivity and accuracy. The OLCR has a very high sensitivity but only a limited range (< 5 m). The OTDR, has a longer range (several km) and a lower resolution. The OFDR falls between them, with a range from tens to hundreds of meters and a millimeter-range resolution. ^[8]

The OFDR is based on swept-wavelength coherent interferometry. The light from a tunable laser source (TLS) is used to interrogate fiber optic sensors. Therefore, the physical changes of the optical fiber during an external interruption such as a temperature difference or an elongation of the fiber itself, a measurable change of how the light is scattered is created. By comparing the scattering light of a sensor to a reference measurement of the fiber that was recorded at a known strain state, one can determine the physical state of the fiber at the time of measurement. Rayleigh backscatter in optical fibers is caused by incidental fluctuations in the index profile along the fiber length. When observing the amplitude of light scattered along the fiber, a highly repeatable profile is observed that is a unique and static property of a given fiber. This technique enables steady and practical distributed temperature and strain measurements with millimeter-range spatial resolution and with strain and temperature resolutions as fine as 1 μm and 0.1°C. ^[9]

The Measurement Interferometer and the Trigger Interferometer uses the Mach-Zehnder interferometer which switches and recombine the light by using a 3 dB coupler. The two paths between the couplers are of different lengths. One of the paths is for the measurement and one for the reference. ^[4] The measurement interferometer absorbs the backscattering light amplitude from one end of the fiber. The light is measured as a function of optical frequency by tuning the laser over its linewidth. The Computer shown in figure 2 analyzes the signal from the detector (S and P). Therefore a low-frequency spectrum analyzer performing with the Fourier transform simultaneously recovers the backscattered waves from all points along the fiber. In the case of the LUNA ODiSI-B the minimum distance between the points was 1 mm. In detail, the Fourier transform is used for conversion from the time domain to the space domain. Eickhoff and Ulrich give in their paper a deeper explanation of the Fourier transform and the equation of the backscattering signal. ^[10]

The following provides a brief description of the components in an OFDR device:

Measurement Interferometer

In the measurement Interferometer the Mach-Zehnder interferometer within the included optical fiber is shown. The measurement principle of this Mach-Zehnder interferometer is to compare the coherent intensity of the reached TLS-light between the measuring and the reference paths. ^[8,9]

Trigger Interferometer

Is used to trigger the data acquisition at the `S´ and `P´ detectors. This interferometer is also based on the Mach-Zehnder interferometer principle. The trigger interferometer is mainly for monitoring phase errors during laser tuning. ^[8,9,11]

Polarization Beam Splitter (PBS)

The PBS splits the reference field into two orthogonal states. This polarization diversity technique is used to weaken signal fading due to misalignment of the interfering measurement and local-oscillator fields. With this polarization element, changes in the state of polarization of the measurement field can be monitored. The polarization diversity receiver can be used in an OFDR system to track changes in the polarization state of light propagating through a birefringent component. ^[8,9]

Analog to Digital Converter (ADC)

Interference bands, in relation to the complex reflectivity of the optical fiber can be observed in the ADC if the TLS is tuned. Also, fringes from the trigger interferometer are used for the data consumption at the `S´ and `P´ light detectors. This triggered-acquisition technique reduce tuning errors of the laser that would otherwise adversely affect the data. ^[8,9] The ADC represents a high-speed digital data acquisition card. ^[11]

OFDR Technology in practical use

This chapter gives a brief overview of an experiment that was used to compare the measurement of temperature with the OFDR with thermocouples. The comparison was made using the LUNA ODiSI-A system. This system isn´t the latest system from LUNA, but it works quite well with the OFDR technology.

Figure 3 shows a setup with the measurement device ODiSI-A from LUNA in the middle and a heating chamber of a rheometer in the upper-right photo. The optical fiber and the thermocouple are introduced through an opening in the chamber (photo number 7). A heating chamber of a rheometer was used because the oven can heat up to 600 °C. All the other heating devices that are available for the experiment, weren´t heating precisely or high enough. In the detailed views 6, 7 and 8, the thermocouples are wrapped in Kapton tape and the optical fiber is protected with a plastic tube (outside the heating chamber at room temperature) and a stainless steel tube (inside the heating chamber) are shown. The protection with the plastic tube is necessary to prevent bending and wrinkling of the fiber also so that the fiber could be easily seen by the operator or others working with the system. With the stainless steel tube a decoupling of the temperature measurement from strain measurement is realized. ^[3]

The principal focus of this experiment, was the discussion: can we measure temperature with a non-FBG fiber? And how can we decoupling the temperature measurement from the strain impact, during the heating. Furthermore, we take a look on the polyamide coating, what happens with the coating when it degraded at temperatures over 450 °C. Is than a further measurement possible with the degraded coating?  Summarizing it turned out that there is a linear deviation from the thermocouples while heating and cooling. Then we showed that a termination with a coreless fiber instead of an FBG is just as suitable for temperature measurement. And a burned-out coating tends to break at a high sensitivity when the fiber is touched and thus bent at a very sharp angle. ^[3]

References

[1]       Wikipedia. Temperature measurement. [April 03, 2021]; Available from: https://en.wikipedia.org/wiki/Temperature_measurement#:~:text=One%20of%20the%20most%20common,the%20volume%20of%20the%20fluid.

[2]       Mikolajek M, Martinek R, Koziorek J, Hejduk S, Vitasek J, Vanderka A et al. Temperature Measurement Using Optical Fiber Methods: Overview and Evaluation. Journal of Sensors 2020;2020:1–25.

[3]       Hangleiter A. Methodenentwicklung zur faseroptischen Temperaturerfassung in der Fused Filament Fabrication. 01th ed. München; 2020.

[4]       Bludau W. Lichtwellenleiter in Sensorik und optischer Nachrichtentechnik. Berlin, Heidelberg: Springer Berlin Heidelberg; 1998.

[5]       Andersson E, Öhberg P. Quantum Information and Coherence. Cham: Springer International Publishing; 2014.

[6]       Mandel L, Wolf E. Optical coherence and quantum optics. Cambridge: Cambridge Univ. Press; 2008.

[7]       Greenbaum E, Drexler W, Fujimoto JG. Optical Coherence Tomography: Technology and Applications. Berlin, Heidelberg: Springer Berlin Heidelberg; 2008.

[8]       Soller BJ, Gifford DK, Wolfe MS, Froggatt ME, Yu MH, Wysocki PF. Measurement of localized heating in fiber optic components with millimeter spatial resolution. In: Optical Fiber Communication Conference, 2006 and the 2006 National Fiber Optic Engineers Conference, OFC 2006: Anaheim, California, 5 - 10 March 2006, Anaheim, CA, USA, 3/5/2006 - 3/10/2006.

[9]       LUNA Technologies. Optical Distributed Sensor Interrogator Model ODiSI-B: User´s Guide ODiSI-B. 5th ed. Blacksburg, VA 24060; 2017.

[10]     Eickhoff W, Ulrich R. Optical frequency domain reflectometry in single‐mode fiber. Appl. Phys. Lett. 1981;39(9):693–5.

[11]     Soller B J, Wolfe M Froggatt M E. Polarization Resolved Measurement of Rayleigh Backscatter in Fiber-optic Components. In: Optical Fiber Communication Conference, 2006 and the 2006 National Fiber Optic Engineers Conference, OFC 2006: Anaheim, California, 5 - 10 March 2006, Anaheim, CA, USA, 3/5/2006 - 3/10/2006.