1. Introduction

Distributed optical fiber sensors (DOFSs) have been widely investigated for several decades with the growth of social and industrial needs for real-time monitoring. In general, DOFSs based on optical reflectometry technology can be categorized into two different classes: the truly DOFSs and the quasi-distributed Fiber Bragg Grating (FBG)-based sensors [1]. The former work on the basis of intrinsic scattering of fibers (Raman, Brillouin and Rayleigh). Because of the inhomogeneity of the medium, the backscattering occurs when an electromagnetic wave is launched into an optical fiber [2]. Since the scattering signal within fiber is modulated by the external perturbations, the truly DOFSs can be achieved by measuring the variation of the modulated signal, using techniques such as optical time-domain reflectometry (OTDR) or optical frequency-domain reflectometry (OFDR). Each kind of DOFSs has specific features and advantages, but the specific drawbacks are also inevitable. For example, Raman DOFSs based on OTDR are developed to sense the ambient temperature and have no cross-sensitivity with other environment variables. However, the sensing distance and resolution are limited because of the very weak signal arisen from the thermally-activated spontaneous Raman scattering [3, 4]. Brillouin DOFSs mark the beginning of the simultaneous measurement of strain and temperature as a result of the longitudinal Brillouin frequency profile of these two variables [5–8]. Two categories have been developed, namely: Brillouin optical time domain reflectometer (BOTDR) [9] and Brillouin optical time domain analysis (BOTDA) [10]. Whilst both BOTDR and BOTDA inherently provide longer measurement distance than Raman DOFSs, the spatial resolution of roughly 1 m restricts their applications in ultra-precision localization. Rayleigh DOFSs using OTDR are achieved by monitoring the variations of the signal intensity of the Rayleigh backscattering [11]. A sudden change can be detected when the optical fiber under test (FUT) suffers an external perturbation. It has the same problem with the BOTDR and BOTDA in regard to the spatial resolution and the temperature-strain cross-sensitivity. Therefore, OTDR-based DOFSs are suitable for sensing over long distance with metric spatial resolution. A variety of methods have been studied to improve the spatial resolution of OTDR, greatly increasing the complexity and cost of the sensor. In this case, OFDR is developed for the goal of high spatial resolution over a limited sensing range [12, 13]. Rayleigh DOFSs are currently the most studied systems using OFDR, in which the backscattering pattern presents a spectral shift when the FUT is subjected to an external stimulus (like temperature) [14–16]. By measuring the interference fringes of the Rayleigh scattering light from a tunable laser source and a static reference fiber in frequency domain, the external perturbation can be detected and localized. However, the requirement for the specific laser source increase the system cost. Compared to Rayleigh DOFSs, the quasi-distributed FBG-based sensors possess much higher backscattering light coupling efficiency, leading to a higher signal-to-noise ratio (SNR) [1, 17]. Both OFDR and OTDR can be applied in the system, and the latter is more expensive due to the demands of ultrafast light modulation and detection. Besides, the fabrication of a long and homogenous FBG is also a challenge to be deal with.

DOFSs are an attractive alternative to multiplexed point sensors, simplifying the installation and readout of the sensing system. They can be combined with the early warning system, forming a real-time link between the local sensing probes and decision makers, to reduce potential structure failure along with casualties and property losses. As already mentioned, DOFSs based on OTDR, with their extended measurement range more than 1 km, are generally applied in large structures, such as pipelines, tunnels and so on [18]. In some applications require a better spatial resolution, the OFDR-based DOFSs are usually used, which offer a more cost-effective way with a limited sensing range less than 100 m. For leakage detection and localization, early warning DOFSs are mainly adopted in applications of long-distance pipeline, ensuring the safety of transportation [19]. However, with the continuous development of micro-electronics and other high technology industries, the demand for local detection of condensation or liquid leak around the expensive equipment and facilities has increased dramatically. The sensing range are limited in a few meters with a spatial resolution of centimeter level. Thus, a DOFS with rapid response needs to be designed, maybe sacrificing the sensing range and spatial resolution. Since the existing DOFSs cannot directly measure the change of the refractive index (RI), a certain time is required to convert the leaking events to the variations of stress or temperature [20], greatly affecting the real-time measurement. Besides, the complex algorithm also influences the response ability.

In recent decades, polymer optical fibers (POFs) have attracted a considerable amount of focus for a number of attractive features, including low cost, high flexibility and large core diameter. Compared with the glass optical fiber (GOF)-based sensors, the POF-based counterparts possess the following significant cost advantages in areas from the raw material cost, the processing and connection cost, and the maintenance cost. In addition, with the development of LED technology, the LED-based FLBs have realized commercialization, which is cost-effective and can be customized according to requirements. Thus, considering the cost and flexibility, the POF and LED technologies can be combined to achieve an early warning system for protecting the expensive equipment and facilities.

In this paper, a quasi-distributed liquid leakage (QDLL) sensor is proposed, which is based on the side-coupling effect between a polymer optical fiber (POF) and a flexible lamp belt (FLB) with low-power surface-mounted device (SMD) light-emitting diodes (LEDs). The lights from the LED are side-coupled into the POF through a structural imperfection on the outer side of its core, forming a series of sensing probes along the POF. The side-coupling ratio is directly modulated by the RI of the coupling medium. Thus, the proposed DOFS can detect the change of RI in real time by monitoring the output signal intensity from the two ends of the POF, increasing the fast-response ability. In addition, the leakage localization is achieved by the LED scanning sequence of the lamp belt. As the prices of POF and FLB are decreased by the fast development, the proposed sensor based on intensity detection is more cost-effective compared with the existing local QDLL sensors. The side-coupling ratio of different structural imperfections are compared. The experimental results show that the proposed QDLL sensor can directly detect the variation of refractive index of the coupling medium, of which the measurement range and the spatial resolution are 1 and 0.1 m, respectively. The analysis and results presented in this paper constitute the first demonstration of a novel QDLL sensor, providing the cornerstone to further develop it into other sensing applications.

2. Principle

As we know, the design of a DOFS always involves two elements: the sensing unit for the measurand and the localization method. Various sensing technologies based on different sensitive units have been investigated, and the localization methods mainly use OTDR or OFDR. Here a new localization method based on external-source coupling has been introduced.

2.1. Leakage localization method

The schematic diagram of the proposed QDLL sensor is shown in Fig. 1, which consists of a POF, a FLB constituted by SMD LEDs mounted on a flexible printed circuit (FPC) in a belt shape, and two optical power meters. Figures 1(a) and 1(b) show the front and vertical view of the sensor, respectively. A series of structural imperfections, served as side-coupling structures, are processed on one side of the POF. Each side-coupling structure corresponds to a LED, forming a liquid leakage sensing probe. The propagation paths of the lights from the LED are changed by the side-coupling structure, which make a part of light rays propagate along the fiber instead of direct transmission out of the POF and can be called the side-coupling power. The side-coupling power is modulated by the RI of the coupling medium between the side-coupling structure and LED, as will be explained later. Two optical power meters are connected to the both ends of the POF to monitor the side-coupling power of each sensing probe. According to the scanning sequence, the power meter next to the first illuminated side-coupling structure is called the backward power meter, and another one is called the forward power meter. As depicted in Figs. 1(c) and 1(d), when the FLB scans in a certain sequence, an output pulse sequence is achieved at both ends of the POF by the optical power meters. If a liquid leakage incident occurs around the sensing probe, its corresponding pulse intensity will mutate due to the RI change of the coupling medium. And the location of the leakage incident is easily obtained by the position of the changed pulse in its sequence, which is the localization principle of the proposed QDLL sensor.

 

Fig. 1 Schematic diagram: (a) front view and (b) vertical view of the proposed sensor. Output pulse sequence corresponding to the leakage sensing probe of (c) the backward power meter and (d) the forward power meter.

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2.2. Leakage sensing principle

Compared with the existing QDLL sensors based on optical fiber sensing technology, the proposed sensor offers the advantage of directly measurement of RI variation, improving the real time of the system. The sensing principle is described as the side-coupling induction technology (SCIT) [21], which means the liquid leakage sensing can be achieved by measuring the variation of the side-coupling power modulated by the RI of coupling medium. As shown in Fig. 2, the liquid leakage sensing probe, consisting of a LED and a side-coupling structure, is displayed. The shape of the side-coupling structure is rectangle both in the front view and vertical view. For the multimode POF (SK80, Mitsubishi) we have chosen, the coupling scheme can be analyzed by geometric optics because the core diameter of 1960 µm is much larger than the wavelength of the coupled lights. Furthermore, the depth of the structural imperfection is also much larger than the wavelength. Thus, the evanescent field coupling is not taken into consideration here.

 

Fig. 2 Schematic diagram of the sensing probe with a single-structure imperfection.

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Figure 2 shows the configuration of the sensing probe, of which the side-coupling structure is a single-structure imperfection. The luminescent part of the LED is the PN junction diode, and each LED is composed of three diodes encapsulated with transparent organic silicone. The organic silicone acts as a lens that modulates the angular distribution of luminous intensity. If the top of the lens is flat, the corresponding viewing angel is bigger compared with the one of the pointed resin lens. n₁ is the RI of the organic silicone, and the light emitting efficiency of the LED can also be improved by selecting a suitable RI. The three PN junction diodes are arranged in a shape of an inverted equilateral triangle, and the rectangular imperfection is aligned with the two diodes above. The lights from the LED are partly coupled into the POF via the Fresnel reflection on the silicone-environment interface and the environment-core interface. The RI of the organic silicone, n₁, is approximately 1.53. n₂ is the RI of the coupling medium (air or liquid) which represents the ambient medium between the LED and the side-coupling structure. n₃ and n₄ are the RI of the core and cladding, respectively.

For an arbitrary light ray from the PN junction diode incident on the silicone-environment interface, the total internal reflection occurs with a critical angle ${\alpha }_c={\mathit{\text{sin}}}^{-1}\frac{n_2}{n_1}$. When the coupling medium changes from air (n=1.00) to water (n=1.33), the critical angle increases, and more light rays are refracted from the organic silicone to the coupling medium. A portion of the refracted lights rays are then coupled into the POF. A typical light ray incident on the environment-core interface is shown in Fig. 2. According to the Snell′s law, the refraction angle, θ_t1, can be deduced as follows:

The signal-to-noise ratio (SNR) is one of the important indicators in designing a sensor. To achieve a better SNR, the multi-structure imperfection, aiming to increase the intensity of the output pulses, is introduced and illustrated in Fig. 3. Compared with the single-structure imperfection, more light rays with different angles are produced by the multi-structure imperfection through increasing the vertical reflective surfaces, resulting in more refracted light rays into the POF. Thus, the SNR is improved with the increase of the side-coupling ratio.

 

Fig. 3 Schematic diagram of the sensing probe with a multi-structure imperfection.

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3. Sensor fabrication and experiment setup

Figure 4 shows the experimental setup of the proposed QDLL sensor, mainly composed of a FLB (CSO-DD-503, CSO core electronic) and its controller, a POF with side-coupling structures, and two optical power meters (PM100USB with S151C, Thorlabs). During our experiments, a 2 m-long FLB was selected. Each LED serves as a side-coupling light source with a central wavelength of 625 nm and an output power of 0.2 W. These LEDs are driven by the LED driver ICs (SM16703P) which are also mounted on the FLB. The FLB is controlled by a FLB controller, the core of which is a FPGA chip, and can work in different luminescence modes. The two optical power meters with a resolution of 0.1 nW were used to obtain the output pulses from the sensitive probes along the POF. When the coupling medium in one sensitive probe changes, the intensity of corresponding pulse increases in real time. Thus, the liquid leakage incident can be detected by the intensity variation of the output pulse before and after the leakage.

 

Fig. 4 Experimental setup of the proposed QDLL sensor.

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The intensity of the output pulse is affected by the side-coupling ratio between the LED and the side-coupling structure along the POF, which can be improved by adjusting the geometric shape of the side-coupling structure. During the experiments, two kinds of side-coupling structures were compared first. A total length of 0.5 m of POF was used, and the side-coupling structure was processed in the middle of the fiber. Then the POF and the FLB were glued together by the glass tape, assuming that the distance between the LED and the POF, d, was zero. The forward and backward power meters connected to the POF can achieve an output pulse at every scanning sequence, respectively. The higher the intensity of the output pulse without leakage, the better the SNR of the sensor. And the difference of the pulse intensity with and without leakage represents the sensitivity of the sensor. The sensor with single-structure imperfection was described as Sensor 1, and the one with multi-structure imperfection was described as Sensor 2. Through multiple practical experiments, it had been proved that the side-coupling ratio improves with the increase of the side-coupling contact area and the depth of the imperfection. However, if the contact area is too large or the depth is too deep, the POF will break more easily, and the losses of the side-coupling power propagating to both ends of the fiber will increase when passing through other side-coupling structures. When the location resolution is constant, excessive losses will reduce the measurement range of the proposed sensor. Thus, the length, width and height of the side-coupling structure for Sensor 1 and Sensor 2 are outlined in Table 1, which come from the trade-off between the side-coupling ratio and the optical attenuation of the structural imperfection. The POF we selected is a naked fiber, which is only composed of the core and cladding layer. Therefore, the POF does not need pretreatment and can be processed directly. The side-coupling structure of Sensor 1 was fabricated by the UV nanosecond laser, and the one of Sensor 2 was drilled by a microbit with a diameter of 0.5 mm fixed on a common numerical control (CNC) machine tool. Both fabricating methods are simple, and the latter is more cost-effective. The liquid leakage tests were achieved by a dropper which adds liquid between the LED and the side-coupling structure. Water was selected as the tested liquid. For each sensor, ten experiments were performed, and the lowest side-coupling power was recorded for comparison. The temperature dependence of the sensor with higher side-coupling ratio was then investigated and selected for subsequent QDLL experiment.

Table 1. Characteristics of the proposed sensors

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To validate the detection and localization abilities of the proposed QDLL sensor, a total length of 1.6 m of POF was used. The sensing area started at where is 30 cm away from one end of the fiber and had 1 m in length, with multi-structure imperfection spaced at every 0.1m. Thus, there were 11 liquid leakage sensing probes of the proposed sensor. During the experiments, the side-coupling structures of the POF were illuminated by the FLB one by one, which means there were 11 output pulse at every scanning sequence got from the output power meters. The liquid leakage tests of every sensing probe were conducted ten times, and the lowest intensity and variation for leakage of the output pulse were recorded for comparison.

4. Results and discussion

As depicted in Fig. 5, the liquid response of Sensor 1 and Sensor 2 were investigated. The black column represents the output pulse before leakage, and the red one represents the output pulse after leakage. In theory, the forward and backward power meters should have same output pulse. However, a slight difference is inevitable due to the inconsistency in the structure processing or the direction of visible light interference. The side-coupling ratio (SCR) and intensity variation of output pulse for leakage (IVOPL) of the sensor can be described by $\eta =\frac{P_s}{P_L}\times 100\%$ and $C=\frac{P_s{ }^{\prime }-P_s}{P_s}\times 100\%$, where P_L is the output power of the LED, and P_s and P_s′ are the output power of the liquid leakage sensing probe before and after leakage, respectively. Thus, the SCR and IVOPL of Sensor 1 are 0.0052 % and 15.3 %. And the SCR and IVOPL of Sensor 2 are 0.0086 % and 20.1 %. It is obvious that the sensor with multi-structure has better performances which will be used in the proposed QDLL sensor.

 

Fig. 5 Liquid leakage response of the proposed sensor with different side-coupling structure: (a) backward power meter; (b) forward power meter.

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As the LED luminous efficacy decreases with the increase of temperature [22] and the thermos-optic coefficient of POF is relative high compared with the one of GOFs [21], the temperature dependence of Sensor 2 was studied. The sensitive probe was fixed at a 2 mm distance from a heating plate. A thermometer was introduced to monitor the ambient temperature of the probe. In Fig. 6, the intensity of output pulses from both power meters exhibit a linear change of approximately 0.96 µW when the ambient temperature varies from 35 to 70 °C. For sensor 2, the temperature sensitivity is about 0.027 µW/°C.

 

Fig. 6 Temperature dependence of the sensor with one multi-structure imperfection.

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Figure 7 shows the liquid leakage response of the proposed QDLL sensor. As the side-coupling power of one sensing probe attenuates when it passes through other sensing probes, the intensity of the output pulse decreases with the increase of distance between the power meter and the sensing probe. For each output pulse, the signal intensity increases when the coupling medium changes from air to water. The IVOPL of these eleven output pulses varies between 17 and 25 %. Therefore, the leakage alarm threshold of IVOPL for the proposed QDLL sensor can be set at 15 %. When the IVOPL of one output pulse exceeds the threshold, it indicates a liquid leakage incident occurs, and the location of the leakage point can be determined by the position of the pulse in the output sequence.

 

Fig. 7 Liquid leakage response of the proposed QDLL sensor.

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The proposed QDLL sensor has a measurement range of 1 m and a spatial resolution of 0.1 m, which can be improved by designing a side-coupling structure with a higher side-coupling ratio and lower optical attenuation ratio. Not only the structural imperfection, special structures such as fiber Bragg gratings or long-period fiber gratings are also the direction of our research. As the sensor shows a linear temperature dependence, a similar QDLL sensor can be introduced to eliminate the effect of temperature. Through proper encapsulation, the coupling medium is the air and does not change with the surrounding medium. In addition to RI of the coupling medium, the side-coupling ratio is also affected by the shape of the side-coupling structure, angle of the light source, distance between the LED and the POF and other parameters. Thus, our follow-up work will lay emphasis on improving the side-coupling ratio further and apply this DOFS in other sensing fields such as structural monitoring or temperature warning.

5. Conclusion

A QDLL sensor based on a FLB and a POF with plenty of side-coupling structures is proposed and experimentally demonstrated. Each side-coupling structure along the POF is illuminated by the SMD LED one by one, and an output pulse sequence is obtained from the power meter under a scanning sequence. The side-coupling ratio between the LED and the side-coupling structure is modulated by the RI of the coupling medium. When the coupling medium changes from air to liquid, the intensity of the output pulse increases. The experimental results show that the liquid leakage incident can be detected by monitoring the IVOPL, and the position of the changed pulse in its output sequence represents the location of the leakage. The side-coupling ratio of the sensor with multi-structure imperfection is larger than the one with single-structure imperfection. The positioning method based on the FLB is novel, and the fabrication process is simple and cost-effective. The proposed QDLL sensor can be bent and fixed on any surface, which has simple structure, fast response and low cost.