Temperature fiber-optic sensor with ZnO ALD coating

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Introduction
Fiber-optic sensors have been developed and improved upon for over a decade.Due to their versatility, they are used in numerous fields, such as industry, science or medicine [1][2][3][4].Optimization of measurement parameters plays a significant role in development of the fiber-optic sensors.While planning measurements, selection of the sensor is crucial element, depending on their purpose and conditions, in which they will be performed.Based on the type of sensor, diversity of designs and parameters can be optimized: adjustable cavity length, structure modification [5][6][7][8], as well as metrological properties, such as: resolution, precision, sensitivity, accuracy [9,10].Many researchers contribute to determine properties and parameters of various materials and structures [11,12].
This study investigates sensing abilities of the microsphere-based fiber-optic sensor with a 100 nm ZnO ALD coating during temperature measurements.

Materials and Methods
Measurements were performed using a sensor made of a standard single-mode telecommunication optical fiber (SMF-28, Thorlabs Inc., Newton, NJ, USA) with a microsphere structure produced at the end of the fiber, using fiber-optic splicer (FSU975, Ericsson, Sweden).Obtained microsphere has diameter of 245 µm.After the manufacturing of the microsphere, the ZnO coating of 100 nm thickness was deposited on its surface by Atomic Layer Deposition (ALD) method.Detailed description of the deposition process is presented elsewhere [13,14].
To assess quality of the structure and the deposited ZnO ALD coating of 100 nm thickness, it was then investigated under Scanning Electron Microscope (SEM, Phenom XL G2, Thermo Fisher Scientific, Waltham, MA, USA), which results are shown in Figure 1.The image shows the device with a magnification of 1000x and it can be seen, the structure exhibit excellent roundness.Furthermore, the presence of ZnO coating is apparent.
Moreover, metrological properties of the sensor were validated by performing experimental measurements.During investigation, sensor is placed in temperature calibrator (ETC-400A, Ametek, Berwyn, PA, USA), which was increased from 100°C to 300°C, with a 10°C step.Temperature was stabilized for 3 minutes, at each step, allowing the sensor to adjust to altered conditions.The measurements were executed using a light source with a center wavelength of 1310 nm ±20 nm (SLD-1310-18-W, FiberLabs Inc., Fujimino, Japan).The signal was propagated through a 2:1 50/50% optical coupler (G657A, CELLCO, Kobylanka, Poland) to the sensor head coated with a 100 nm ZnO ALD coating, which is highly reflective, allowing the wave to superpose, therefore inciting interference as shown in Figure 2. By obtaining interference, the integrity of the structure can be monitored, ensuring the sensor is not damaged.Reflected signal is then collected by Optical Spectrum Analyzer (OSA, Ando AQ6319, Yokohama, Japan).Depending on the position of the spectral peak of the signal, temperature can be determined.

Results and Discussion
This section presents results, which were acquired from the measurements performed with the setup shown above.
Figure 3 shows normalized values of the measured signal response for the microsphere-based sensor with a 100 nm ZnO ALD coating, at 100°C and 300°C to preserve readability of the plot.By rising the temperature, spectral peak of the reflected signal shifts toward lower values of the wavelength.The envelope, however, remains the similar for each temperature.In addition, interference fringes visible in the Figure, inform about the integrity of the sensor head structure, which allows to monitor its condition in real-time.Dependence of the peak wavelength position on the temperature can be observed in Figure 4.Moreover, theoretical linear fit is also presented, as well as coefficient R 2 , which equals 0.99176, was determined to confirm fitness of the obtained data to the theoretical model.Furthermore, the results presented in Figure 4 allowed to calculate the sensitivity of the microsphere-based sensor with a 100 nm ZnO ALD coating -0.019 nm/°C.The spectrum changes its peak wavelength position when the temperature is altered.The higher the temperature, the spectrum shift is constant throughout whole range of roughly 2 nm per 100°C.By following linear regression, it is possible to determine the position of the reflected signal peak for each measured temperature.

Conclusion
Microsphere-based sensors are ideal for long-term and remote measurements of parameters such as temperature or refractive index due to their ability to constantly monitor the integrity of the sensor head.Presented study a 100 nm ZnO ALD coating on the surface of a microsphere-based fiberoptic sensor for temperature measurements.Selection of an optimal coating is crucial for long-term and remote measurements.While devising the measurements, it is important to select proper parameters of the fiber-optic sensor coating for optimal efficiency.Sensor with a 100 nm ZnO ALD coating exhibits close match between measurement data and theoretical linear fit, which is confirmed by R 2 coefficient of over 0.99.The sensitivity of the sensor with a 100 nm coating equals 0.019 nm/°C.Additionally, for microsphere-based sensor with 100 nm ZnO ALD coating, changes of temperature can be observed based on the spectral shift, which coincides with rise of the temperature.The sensor also indicates its proper operation by inciting interference.

Figure 1 .
Figure 1.SEM image of the microsphere-sensor with a 100 nm ZnO ALD coating.Magnification of 1000x

Figure 2 .
Figure 2. Principle of operation of a microsphere-based fiber-optic sensor.

Figure 3 .
Figure 3. Normalized measured response of the reflected signal for the microsphere-based sensor with 100 nm ZnO ALD coating at: a) 100°C and b) 300°C.

Figure 4 .
Figure 4. Dependence of the spectral shift of a reflected signal on the temperature.