A fiber extrinsic Fabry-Pérot interferometer (EFPI) assisted with iron V-groove for temperature measurement is proposed and investigated by means of both numerical simulation and experiment for the first time to our best knowledge. The main temperature sensing component is acted by the iron V-groove whose coefficient of linear thermal expansion (CLTE) is much higher than that of the silica glass. Two fibers are stuck to the V-groove with two glued points, respectively. Maximum sensitivity of 260.7 nm/°C, which is the highest value for a fiber interferometric sensor up to now, has been achieved experimentally. It is worth noting that the temperature sensitivity of this sensor can be improved limitlessly via implementing a smaller gap size of the EFPI, longer distance between the two glued points or material with higher CLTE of the V-groove, theoretically.
© 2015 Optical Society of America
In recent years, significant progress has been made in the application of many fiber-optic sensors for various ambient parameters detecting, such as refractive index, strain, temperature, displacement/distance, liquid level, and so on. Most of these sensors can be divided into two types: one is the intensity-based sensors, and the other one is the wavelength/frequency-based sensors. Comparing to the intensity-based sensors, the latter, which can avoid the disturbance of power fluctuation of the optical source, are more stable in the application. In the modern industrial automatic system, lots of wavelength/frequency-based sensors have been used for temperature measurement mostly owning to their advantages of high sensitivity, low cost and high stability. The most common types of wavelength/frequency-based sensors are the fiber gratings and fiber interferometers, both of them have been investigated and applied for several decades. In general, the sensitivities of fiber gratings are about 10 pm/°C [1, 2], mainly due to their intrinsic coefficient of linear thermal expansion (CLTE) of silica glass. What’s more, maximum sensitivities of some fiber gratings written on the special optical fibers are only tens [3–6] or hundreds of picometers  per degree.
The reason why fiber can serve as the medium for temperature sensing is that the ambient temperature has effect on the effective refractive index and length of the fiber. The changing of both fiber parameters mentioned above in an interferometer will cause changing of the optical paths difference (OPD) between the two paths of light, which is the fundamental of most fiber temperature interferometric sensors. Therefore, the rates of change of the two elements affected by variational temperature are the main influencing factors for the sensitivity of a interferometric fiber sensor. Comparatively speaking, structures of the interferometric sensors are more flexible and diverse, such as Mach-Zehnder interferometers (MZI), Michelson interferometers (MI), Fabry-Pérot interferometers (FPI), Sagnac interferometers (SI) and multiple modes interferometers (MMI). Moreover, their sensitivities have the potential to be improved via lots of novel designs owning to their various structures. In line fiber MZI is a kind of typical structure for multiple parameters measurement including temperature. On the one hand, the transmission constants of two paths of light can be affected by the ambient temperature; on the other hand, the dynamic temperature also results in variational length of the inserted fiber between two single mode fibers (SMF). As a consequence, the optical path difference between two paths of light is related to the ambient temperature, which is the fundamental of many MZIs proposed up till now [8, 9]. Their sensitivities can easily reach dozens or hundreds of picometers per degree [10–12], even the magnitude of nanometers per degree also has been realized by virtue of some novel structures [13, 14]. In addition, many other types of fiber temperature sensors including FPI [15–20], MI [21–23] and SI [24–27] whose sensitivities are almost the same as that of the MZI mentioned above in magnitudes, also have been demonstrated. Maximum sensitivity of only −26.0 nm/°C is obtained by applying a complex selective-filling photonic crystal fiber SI  which is rather complex in its manufacturing operation and will raise the cost of the whole sensing system. Generally speaking, most temperature sensitive fiber interferometers rely upon the length changes of the fibers. However, the CLTE of the fibers are not so large, and the sensing fibers are relative short in order to get visible and discernable interference fringes. In this way, the ambient temperature will cause very limited influence of the OPD; as a result, the sensitivities of the sensors are not large enough and have little potential to be improved.
For one thing, absolute value of the sensitivity is an extremely important parameter for the sensor; for another, sensitivity per gauge length unit (SPGLU) of the sensor should also be taken into account for improving sensitivity and reducing sensor size. Table 1 shows the comparison of sensitivity, sensor size and SPGLU of several representative temperature fiber sensors. It can be seen that temperature sensitivities of the fiber gratings, generally under the value of 100 pm/°C, are related to the CLTE of the fiber materials. That is why the polymer grating can gain higher sensitivity than silica grating. Moreover, longer length of the fiber grating can introduce higher extinction ratio rather than higher sensitivity. In comparison, sensors based on interferometers can easily obtain high SPGLU owing to their high sensitivities and small sizes. Especially, the temperature sensitivity of magnitude of nm/°C has been demonstrated by means of micro-cavity based FPI. However, their sensitivities, which are limited by the rate of change in cavity length with the temperature variation, cannot be improved by introducing longer cavities. On the contrary, shorter cavity is of great benefit to higher sensitivity but is difficult to produce; furthermore, too short cavity will lead to huge free spectrum range which acquires the expensive ultrawide band detector. Similarly, other types of interferometric fiber sensors also have the same problem that there is less room to improve the sensitivities by utilizing the existing structures.
In this paper, we propose and demonstrate a new extrinsic Fabry-Pérot interferometer (EFPI) structure for ambient temperature measurement. Two SMFs and the iron V-groove are stuck together with glue, and an air gap between the two fiber end faces is formed. Larger distance between the two glued points and smaller air gap size can bring about higher sensitivity of the EFPI in its temperature detecting via the numerical simulation. Furthermore, the experimental result shows a linear relationship between the wavelength shift of the transmission spectra and the variational temperature. Maximum sensitivity of 260.7 nm/°C, which is achieved by applying gap size of 4.8 μm and distance of 5.2 cm between the two glued points, is at least four orders of magnitude larger than that of the normal silica fiber grating based sensors . Furthermore, the proposed sensor also provides a large value of SPGLU as shown in Table 1, which means that it has the advantage of acquiring high sensitivity with small sensor size.
2. Design and simulation
The schematic of the proposed temperature sensor based on EFPI is shown in Fig. 1. Two aligned SMFs are fixed in an iron V-groove and a gap size of l between the end faces is formed. Two fibers and the V-groove are glued together by adhesive, and the glued points are within the distance of L away from each other. In order to avoid additional duration induced by the V-groove before the sensor reaches thermal equilibrium, the experimental V groove function is implemented by two parallel iron capillaries in close proximity to each other and the bottom iron plate instead of a commercial massive V-groove. The iron capillaries are within the diameter of 350 μm and thickness of 100 μm, thickness of the iron plate is 150 μm. The proposed V-groove also has the advantage of saving production cost and facilitating size adjustment of the sensor. One fiber pigtail of the EFPI is connected to one port of an optical directional coupler (ODC); the other one is cut with a certain angle to avoid optical reflection at this fiber end face. For the same purpose, adjacent fiber pigtail of the ODC is also processed with a certain angle of the end face. The rest two ports of the ODC are connected to the broadband source (BBS) and optical spectrum analyzer (OSA), respectively.
The EFPI is consisted by the end faces of the two fibers, and both end faces provide optical reflectivities of 4% according to the principle of Fresnel reflection. The reflectivities are so low that the light of multiple reflections by both end faces can be ignored, thus only two main paths of light should be taken into account. The transmission spectrum of the proposed EFPI can be given as ,29]30] and silica glass (5.6e-7/°C) , respectively. Sensitivity is a very important parameter for a sensor, thus the following discussions mainly focus on the influences of various factors for temperature sensitivity of the proposed EFPI based sensor.
2.1 The influence of the selected dip for sensitivity
In this subsection, we fix the parameters as αFe = 12e-6/°C, Lorigin = 5 cm and lorigin = 5 μm, respectively. By applying Eqs. (1)-(3), the transmission spectra with various temperature values are obtained as shown in Fig. 2(a). It can be seen that all dips of the transmission spectra shift towards the long wavelength with the increase of temperature. In Fig. 2(b), it gives the wavelength shifts with different selected dips, from which we can conclude that the amount of wavelength shift of the dip at longer wavelength is larger than that of the dip at shorter wavelength. That is to say, the selected dip at longer wavelength is helpful to gain higher sensitivity.
2.2 The influence of original gap size (lorigin) for sensitivity
From Eq. (2) and (3), it can be seen that the lorigin is related to ΔΦ. In other words, the original gap size lorigin is a potential influencing factor for sensitivity of the sensor. Here, we assume that Lorigin = 5 cm is satisfied to study the influence of lorigin for the sensor sensitivity. In addition, maximum sensitivity with a certain gap size can be achieved at the longer wavelength, thus we only consider a single dip for instance. According to Figs. 3(a)-3(c), one can see the transmission spectra with the increase of temperature at various lorigin, and the wavelength shifts are shown in Fig. 3(d). The simulation result indicates that larger gap size will cause greater value of the wavelength shift in a certain temperature steps. In order to enhance the temperature sensitivity, a smaller gap size of the EFPI should be implemented according to the above analysis.
2.3 The influence of the V-groove material for sensitivity
One novel configuration of the proposed fiber sensor is the separated external sensitive component whose CLTE is much higher than that of the optical fiber material. The parameters of Lorigin = 5 cm and lorigin = 5 μm are satisfied in this subsection, and the CLTE of several typical materials including iron are taken into account for discussing the influence of CLTE of the V-groove. αSi = 2.6e-6/°C, αFe = 12e-6/°C, αAu = 14e-6/°C, αAg = 19e-6/°C and αAl = 23e-6/°C are the corresponding CLTEs of silicon, iron, gold, silver and aluminum, respectively . Thus, the transmission spectra of the proposed EFPIs based on V-grooves made of various materials are presented as shown in Figs. 4(a)-4(e), and Fig. 4(f) expresses the wavelength shifts of dips. These results indicate that the V-groove made of material with higher CLTE is beneficial to the proposed EFPI to enhance its temperature sensitivity.
2.4 The influence of distance between the two glued points (Lorigin) for sensitivity
As shown in subsection 2.3, the ambient temperature can be sensed by the external V-groove whose CLTE is a very important influencing factor of the proposed sensor for temperature measurement, and the other important factor equal to the CLTE of the V-groove is the distance between the two glued points. Here, we assume that the V-groove is made of pure iron. Therefore, αFe = 12e-6/°C, lorigin = 5 μm and αSiO2 = 5.6e-7/°C are taken into account in studying the influence of Lorigin. The simulation results are shown in Fig. 5 from which it can be seen that the wavelength shift of the EFPI with a larger Lorigin is greater than that of the EFPI with smaller Lorigin at the same temperature dynamic range. The implication is that the sensitivity of the proposed EFPI can be improved by applying larger distance between the two glued points.
3. Experimental result and analysis
The experimental microscopic image of the proposed EFPI is shown in Fig. 6, in which two aligned SMFs are placed in the iron V-groove with the gap size of 4.8 μm between the two fiber end faces. α-cyanaloc acrylic resin adhesive is applied to gluing both the SMFs with the V-groove, and the distance of 5.2 cm is implemented between the two glued points. In addition, the ODC with center wavelength of 1550 nm is used for connecting the BBS (KOHERAS, superK uersa), the OSA (YOKOGAWA, AQ6375) and the proposed EFPI as shown in Fig. 1. Tunable temperature environment is provided by the temperature test chamber (WT1180, WEISS).
The transmission spectra of the proposed sensor are shown in Fig. 7(a). It can be seen that the curves present the constant envelop of approximate parabola, which is caused mainly by the intrinsic transmission characteristic of the ODC. By cutting off the fiber between ODC and the EFPI, the independent transmission spectrum of ODC can be obtained as shown in Fig. 7(b). The unsmooth drastic fluctuation of the curves at wavelength of about 1400 nm is related to the uneven spectrum of the BBS. By eliminating the influence of ODC, the treated transmission spectra can be expressed as shown in Fig. 8, from which we can conclude that all the dips shift towards the long wavelength.
The experimental results as shown in Fig. 9 express the fact that the dip with longer wavelength can gain higher temperature sensitivity, which agrees with the simulation result. Furthermore, maximum linear sensitivity of 260.7 nm/°C achieved by dip A at the temperature range of 25.2-28.2 °C, to our best knowledge, is the highest measured value up to now. In addition, the sensitivity can be further improved by applying V-groove with higher CLTE material, smaller gap size of the EFPI and larger distance between the two glued points. In addition, the SPGLU of 5013 pm/°C/mm can be obtained by applying the proposed sensor, which could bear comparison with some micro interferometric sensors. It signifies that this kind of structure can be implemented with smaller size as well as super high sensitivity.
In order to verify the capacity of resisting disturbance of the sensor, the constant temperature experiment has been performed for 100 minutes with interval of 5 minutes as shown in Fig. 10. It can be seen that the wavelength fluctuation of the dip are within 6.9 nm, which leads to a temperature resolution of 0.026°C. Actually, the tunable resolution of the temperature test chamber is 0.1°C, so that the temperature fluctuation below 0.1°C cannot be detected. Nevertheless, this part of temperature fluctuation is lumped together with the ambient perturbation in the experiment, which will bring about large error between the measured value and the actual one due to the super high temperature sensitivity of the proposed sensor. Under this condition, the temperature fluctuation has become the major source which causes measurement error of the resolution. It can be expected that the actual resolution of the proposed sensor should be much smaller than the current measure value of 0.026°C. The measurement error can be further reduced by employing higher resolution temperature test device.
We have demonstrated a novel fiber temperature sensor based on EFPI assisted with iron V-groove. Four main influencing factors for the proposed EFPI sensitivity have been investigated according to the numerical simulation in section 2. The analysis expresses that the sensitivity can be improved via applying selected dip with longer wavelength, V-groove based on material with higher CLTE, smaller gap size of the EFPI and larger distance between the two glued points. As a matter of fact, the improvement of the first three influencing factors can enhance the sensitivity by a large margin, but it is still subject to the restricted of the selected wavelength range, the CLTE of actual material and minimum gap size of the EFPI. Comparatively speaking, the distance between the two glued points has the unlimited capacity for enhancing the sensitivity, theoretically. These factors can be jointly used for improving the property of the proposed sensor, but not mutually exclusive. In the experiment, the gap size of about 4.8 μm and distance of 5.2 cm between the two glued points are implemented for temperature sensing in the range of 25.2-28.2 °C. The experimental result shows that the wavelength shifts present the good linear relationships with the dynamic temperatures, and maximum sensitivity of 260 nm/°C has been achieved. What’s more, the proposed temperature sensor has the potential to reach higher sensitivity with the simple structure.
This work is jointly supported by the National Natural Science Foundation of China (61177069), and the National Natural Science Foundation of Beijing (4154081).
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