Design of a Hollow-Core Photonic Crystal Fiber Based Edible Oil Sensor

Abstract

This work proposes and statistically analyzes a hexagonal-shaped hollow-core photonic crystal fiber-based edible oil sensor in the terahertz (THz) range. The suggested sensor’s performance was assessed by means of Comsol Multiphysics, a finite element method-based commercial tool. The simulation results demonstrate that the suggested sensor has more than 99% relative sensitivity for different types of edible oils at 1.6 THz under ideal geometric conditions. Furthermore, the suggested sensor exhibits low confinement loss, high numerical aperture and effective area at optimal geometry and operational conditions. The proposed sensor is realizable using conventional production procedures and its superior sensing qualities may make it a key component of real-world oil detection systems.

1. Introduction

Edible oils are dietary fats that play a vital role in the human body, satiating nutritional demands, promoting development and ensuring healthy brain and nerve function, as well as the endocrine environment. Soybean oil, derived from the seeds of the soybean plant, is one of the most widely used cooking oils in the world. Since it is a kind of vegetable oil, it offers several health advantages, particularly for the skin, bone, brain and heart. It also includes omega-3 fatty acids and polyunsaturated fatty acids, both of which are beneficial to heart health, cognitive function and immunity [1]. This frying oil also includes vitamin k, which is essential for blood contraction, bone metabolism management and bone mass maintenance [2,3]. On the other hand, as soybean oil also includes omega-6 fatty acids, it might have certain adverse consequences when used excessively in food preparation [4]. Unfortunately, a few fraudulent companies offer contaminated oil, resulting in a number of adverse health risks. As a result, physicians increasingly advise and encourage cardiac patients and the general public to utilize other vegetable and seed oils for cooking. Sunflower oil, mustard oil, olive oil, coconut oil and other regularly used cooking oils have insignificant health effects compared to soybean oil. However, since the colors of the cooking oils are almost identical, it is difficult to distinguish between them. Low-cost cooking oils (palm oil) are sometimes disguised as high-cost oils (mustard oil, sunflower oil, etc.) by adding color or chemical compounds that are toxic to humans. As a result, detecting pure edible oil is critical for reducing health risks and maintaining a healthy lifestyle.

Photonic Crystal Fiber (PCF) has created new possibilities for enhancing photonic instruments for sensing & telecom applications since its invention in 1996 [5]. This type of glass fiber allows for a wide range of optical qualities to be realized in its design, including infinite single-mode operation, a larger effective core area, higher transparency, design flexibility and low loss [6,7,8,9]. These distinguishing characteristics have resulted in the development of cutting-edge technologies to address conventional theme issues such as the launch and dissemination of telecommunications. Similarly, PCFs were also developed to establish a better generation of active and passive optical devices for the telecommunications industry [10,11]. On the contrary, owing to their high sensitivity and small size, PCF-based sensor devices were used in a variety of potential real-world applications, such as chemical sensing, biological tissue diagnosis, cancer cell detection, etc. [5,12,13,14].

Due to its multiple prospective applications, a small region in the electromagnetic spectrum (0.1 to 10 THz) has recently gained widespread attention and is termed the terahertz (THz)radiation band [15,16]. Because this radiation band is between the infrared and microwave regions, it is often employed with no negative effects on humans or the environment [17]. As air lacks the absorbent qualities of this spectral range, it was first utilized as a medium for transmitting THz signals. Despite this, major issues such as dispersion, isolation, and transmitter-receiver alignment occurred when transmitting THz signals over long distances [18]. Several waveguides, such as parallel plate waveguides, metal waveguides, hollow-core waveguides, and PCFs, have been proposed to provide effective signal transmission throughout this spectral range [19,20,21]. THz PCF waveguide has higher transmission qualities than other waveguides because it absorbs less energy [22,23]. As a result, PCFs become a crucial component in THz signal’s transmission and these waveguides were recommended as sensors for various applications [24,25,26,27]. PCFs are frequently divided into three types: solid core [8], porous core [9,23] and hollow-core [18,21].

Hollow-core PCF, on the other hand, is the finest choice for application purposes since it provides the greatest field of connectivity between the analyzer and the radiation. Recently, numerous alternative supported waveguides have been offered and examined to work out the change of fluids inside the THz range [23]. Sultana et al., for example, developed a sensor device that supported a perforated core PCF [24]. Within the THz frequency band, this arrangement displays an exceptional capacity to detect variations in the ethanol mixture. The PCF-based sensor has a greater relative sensitivity of 68.87% at 1 THz and a minimum loss of 6.799 × 10⁻¹² cm⁻¹, according to numerical calculations. In 2018, Islam et al. published a hollo core Zeonex-based PCF with rectangular air holes for detecting various liquids in the THz frequency band [28]. The numerical study of this design demonstrates extremely exceptional sensing sensitivity of 96.69%, 96.97%, and 97.2% for three different liquids, i.e., water, ethanol, and benzene, respectively.

In 2019, Bellal et al. proposed and studied a hollow-core PCF for detecting blood components, utilizing Zeonex as the base material and rectangular holes inside the core and cladding region [26]. The authors looked at how efficiently this setup could detect water, white and red blood cells, hemoglobin, and plasma, at 2 THz. The simulation results revealed that this detector may reach very high relative sensitivity ranging from 89.14% to 93.50% with a minimal loss on the order of 10⁻¹⁴ cm⁻¹. In 2021, Eid et al. proposed a similar type of senor [29] which offered better sensitivity and loss profile than Ref. [30]. Ferdous et al. published an oil sensor in 2022 that can detect the quantity of adulteration in liquid fuel [31]. According to the researchers, their suggested sensor verified high relative sensitivity, low loss, and large numerical aperture in the THz domain at optimum structural conditions. On the contrary, hitherto a massive number of one-dimensional PCF-based sensors have been proposed in order to detect liquid samples by using the surface plasmon resonance (SPR) technique [32,33,34]. In this method, a layer of solid metal or composite material is used as a detection layer and, for a particular refractive indexed sample, a sharp loss peak is found at a specific wavelength. Although both types of sensors offer excellent characteristics, the fabrication of three-dimensional PCF-based sensors is less complicated than that of single-dimensional sensors.

This paper proposes and quantitatively investigates a hybrid structured hexagonal hollow-core PCF-based oil detector in the THz domain for the detection of various food oils such as sunflower oil, mustard oil, olive oil, palm oil, and coconut oil. The innovative hybrid structural cladding provides a compact waveguide, allowing the proposed sensor to have outstanding sensing capabilities. This PCF offers extremely high relative sensitivity of 99%, ultra-low confinement loss of 10⁻¹² dB/m as well as very low bulk material loss of 0.0034 cm⁻¹ for single geometric and operating conditions. The suggested sensor’s guiding and sensing properties are investigated and explained in the corresponding section over a broad range of geometric and operational parameters.

2. Design of the Proposed Sensor

As illustrated in Figure 1, the suggested hollow-core PCF has a single hexagonal-shaped air-hole in the middle of its construction. The hexagonal core will allow additional samples to be placed in that location, increasing the sensing capabilities. The diameter of the core is D, which is the distance between the core’s opposing corners. The proposed structure’s cladding region was created utilizing semicircular pentagonal holes with a radius of R and a fiber radius of R1 where R = 2.2D and R1 = 2.3D. These values are chosen by trial and error since the suggested sensor has desirable features under these conditions. The distance between the core and closest cladding arm as well as the consecutive cladding air holes are denoted by p where p = 0.15D. All geometric parameters are connected to only one factor, i.e., D, to eliminate fabrication complexity. Finally, at the outer area of the fiber, a circular perfectly matched layer (PML) is inserted, whose job is to reduce back-reflection by absorbing light that seeps from the core at the cladding’s outer surface. Zeonex was employed as the background material for the claimed oil detector because it has the lowest loss and has a consistent index of refraction (n = 1.53) in the range of 0.1 to 2 THz. In the THz frequency spectrum, Zeonex has the highest refractive index and transparency, hence it is used as the sensor’s substance.

3. Simulation Results and Discussion

The proposed waveguide’s electric field distribution is investigated using the FEM, with the core filled with the sample under test (SUT).

Table 1. Refractive indices of different oil samples under test at room temperature [35].

Oil Type	Refractive Index
Sunflower oil	1.472
Mustard oil	1.470
Olive oil	1.466
Coconut oil	1.463
Palm oil	1.454

depicts the refractive index information of different oil samples at room temperature. A THz light source, a finely produced PCF sensor, an optical detector, a spectrum analyzer, and a display unit are the main parts of this kind of sensing system. For maximum accuracy, an isotonic solution of the testing sample and a laser light source with a narrow bandwidth is required. The sample will be placed in the core using any popular method before the light source is turned on. The photodetector will receive the light beam after it has propagated through the waveguide and the analyzer will analyze the amount of power and the effective refractive index of the received light. Finally, a computer will compute the proposed sensor’s relative sensitivity and other guiding parameters using the necessary mathematical equations. Figure 2a–c shows the electrical field distribution for the x-polarization mode when the proposed sensor is filled with sunflower, mustard, and olive oils, in that order. All of the figures showed that the light is highly restricted within the core, which is required for high sensitivity.

The Beer-Lambert law explains that the strength of radiation-matter interaction determines the sensitivity of the oil adulteration sensor. This working principle is used for the suggested sensor, where the measurements rely on the changes in the absorption coefficient at a specific frequency, as shown in Equation (1).

$$I\left(f\right)=I_0\left(f\right)e^{-r{a}_m{l}_c}$$

(1)

where I(f) denotes the intensity of the radiation when the THz waveguide is filled with the SUT, I₀(f) is the intensity without the presence of SUT, r is the relative sensitivity of the sensor, α_m is the absorption coefficient and l_c is the length of the waveguide. Relative sensitivity is the most important characteristic since it represents the sensor’s ability to detect changes in the SUT. The following Equation (2) can be used to calculate the relative sensitivity of the proposed sensor [21].

$$r=\frac{n_r}{n_{eff}}X$$

(2)

where n_r denotes the real part of the refractive index (RI) of the analyte targeted to be sensed and n_eff represents the effective RI of the guided mode respectively. It is worth noting that the guided mode’s effective RI might be quite sensitive to changes in the SUT characteristics. The amount of light signal that interacts with the testing analyte is expressed as $X$ (power fraction), making it straightforward to calculate the power that flows through the center area which interacts with the SUT. The power fraction is often calculated using Equation (3) [21]:

$$X=\frac{{\displaystyle \underset{sample}{\int }{R}_e\left(E_x{H}_y-E_y{H}_x\right)}}{{\displaystyle \underset{total}{\int }{R}_e\left(E_x{H}_y-E_y{H}_x\right)}}\times 100$$

(3)

where E and H, respectively, denote the electric field and magnetic field of the propagating signal, and, finally, the subscripts x and y designate the polarization in the x- and y-axis. In Equation (3), the denominator performs the combination of the real part (R_e) of the whole power over the whole fiber dimensions and the numerator performs a similar operation for only the portion of the fiber where the sample is located, i.e., within the core region. The performance of our proposed oil adulteration sensor is analyzed as a function of the geometry of the structure as well as the frequency. Initially, the relative sensitivity of the sensor is observed as a function of the diameter of the hexagonal core, which is tuned from 270 µm to 370 µm. The investigation is carried out at an operating frequency of 1.6 THz. Figure 3a shows the relative sensitivity for various core diameters. This graphical illustration indicates that the relative sensitivity rises slowly as the core diameter increases. The reason for this trend is that, as the diameter of the core increases, so does the amount of detected analyte inside the core. As a result, more radiations interact with the analyte, increasing the relative sensitivity of the proposed sensor. The total fiber dimensions, on the other hand, are governed by the core size and the larger and bulkier structure. As a result, a tradeoff between the expected parametric value and the fiber dimensions is considered with 340 µm as the ideal core diameter. The relative sensitivity is higher and the confinement loss is satisfactory for this core diameter. At ideal conditions, the relative sensitivities of our proposed sensors; i.e., sunflower, mustard, olive, palm, and coconut oil are, respectively, 99.65%, 99.60%, 99.55%, 99.50%, and 99.35%, as shown in Figure 3a. This happens since sunflower oil has the highest refractive index and coconut oil has the lowest. In addition, sunflower oil has the highest relative sensitivity and coconut oil has the lowest.

The relative sensitivity of the proposed sensor at a core diameter of 340 µm for various operating frequencies is shown in Figure 3b. The relative sensitivity grows quickly from 0.8 to 1.3 THz, then slowly beyond that and, after 1.6 THz, the relative sensitivity for all oil types is nearly constant. A higher frequency electromagnetic wave tends to traverse a higher indexed zone, hence more light propagates through the high indexed sample, thereby increasing the sensitivity. To summarize, the suggested sensor has a relative sensitivity of over 99% for all samples, which is higher than any other reported results in the literature to the best of our knowledge.

The loss profile of the suggested oil detector is now examined for different conditions. Effective material loss (EML) and confinement loss (CL) are the two main forms of losses that occur in all types of PCF-based sensors. The first happens due to the presence of solid material in the waveguide, while the second arises due to power absorption by the cladding air holes around the core. The optical waveguide’s EML and CL may be calculated using the following mathematical expressions [23,31].

$${\alpha }_{eff}=\frac{{\left(\raisebox{1ex}{${\epsilon }_0$}\!\left/ \!\raisebox{-1ex}{${\mu }_0$}\right.\right)}^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}{\displaystyle {\int }_{A_{mat}}n}{\alpha }_{mat}{\left|\mathrm{E}\right|}^{2}d\mathrm{A}}{2{\displaystyle \underset{All}{\int }{S}_{z}dA}}$$

(4)

$${\alpha }_{CL}=8.686\times \frac{2\pi f}{c}\mathrm{Im}(n_{eff})$$

(5)

where the frequency of the operational EM signal is denoted by f, the velocity of light in a vacuum is denoted by c, the loss coefficient of the substance material is indicated by α_mat, and the electric field is identified by E and Im(n_eff) denotes the imaginary component of the effective RI of the traveling wave. The EML of the proposed sensor for various core diameters and operation frequencies is shown in Figure 4a,b, respectively. These figures show that EML reduces with an increasing core diameter at 1.6 THz as the expanded core allows less resistivity to propagate through the core. As a result, a limited amount of light is captured by the solid substance, and the loss is reduced. The EML for different oil samples at optimal core diameter varies approximately from 0.032 cm⁻¹ to 0.034 cm⁻¹, which is quite small. Again, oil with a higher refractive index has a lower EML since it allows more light to pass through it at a constant operating condition than oil with a lower refractive index. Similar types of characteristics are shown by the sensor for different operating frequencies as shown in Figure 4b. At higher frequencies, reduced light travels through the low-indexed cladding region and therefore a lesser amount of light is trapped by the solid material between the core and cladding. This is the reason why the loss is lower for higher frequencies. At the 1.6 THz range and 340 µm core diameter, the EML of the reported sensor is less than 0.0034 cm⁻¹, which is lower than the previously reported works.

Figure 5a,b demonstrate the CL profile of the proposed oil sensor for various core diameters and operating frequencies, respectively. In both situations, the loss reduces when the x-axis parameter is raised, since increasing the core width and frequency allows more light to enter the core. At optimum core diameter (340 µm) and operating frequency (1.6 THz), the confinement loss of the proposed sensor is 1.91 × 10⁻¹² dB/m, 1.93 × 10⁻¹² dB/m, 2.62 × 10⁻¹² dB/m, 3.3 × 10⁻¹² dB/m and 1.31 × 10⁻¹¹ dB/m for sunflower, mustard, olive, palm, and coconut oil, respectively.

As stated earlier, the EML and CL are the two primary losses in PCF-based sensors. Figure 6 depicts the overall loss produced by the proposed sensor. At optimum conditions, the total losses offered by the sensor for different kinds of oil samples are 0.0131 dB/cm, 0.0134 dB/cm, 0.0139 dB/cm, 0.0141 dB/cm, and 0.0145 dB/cm, respectively, which are better than the previously reported works in Refs. [21,22,23,27,28,29,30,31].

The numerical aperture (NA) of the proposed sensor is discussed in this section as it is a very significant property of a sensor and determines the greatest permissible cone of incident light that would pass through the optical waveguide. The greater the NA, the higher the probability of the light reaching the core and increasing the likelihood of light-analyte interaction within the fiber. The mathematical expression to calculate the NA of any PCF-based sensor is given by the following equation [31].

$$NA=\frac{1}{\sqrt{1+\frac{\pi A_{eff}{f}^2}{c^2}}}\approx \frac{1}{\sqrt{1+\frac{\pi A_{eff}}{{\lambda }^2}}}$$

(6)

where A_eff is the effective area of the fiber, which represents the actual area through which the light propagates, and f is the frequency of the incident light beam.

For ideal structural conditions, the relation between the NA and the operating frequency is graphically shown in Figure 7. At higher operating frequency f, light confinement becomes more condensed, resulting in a reduced effective area. According to Equation (6), NA and A_eff are inversely proportional, i.e., NA decreases as f increases. The figure also shows that the NA of this suggested oil detector is about 0.37 for various types of oil samples at 1.6 THz.

Finally, we studied the effect of varying effective area of our proposed photonic crystal fiber-based sensor, which provides useful information about the actual region across which light signal propagates. The effective area of the PCF is calculated by using the following Equation (7).

$$A_{eff}=\frac{{\left[{\displaystyle \int I\left(r\right)rdr}\right]}^2}{{\left[{\displaystyle \int I^r\left(r\right)dr}\right]}^2}$$

(7)

where $I\left(r\right)={\left|E_t\right|}^2$ is the electric field distribution of the proposed fiber sensor. Figure 8 depicts the effective area of the photonic crystal fiber for the optimal core diameter as the frequency is changed from 0.8 THz to 2 THz. The figure indicates that the effective area is inversely related to the operating frequency. The high-frequency electromagnetic signal is tightly confined within a small region of the fiber sensor. At ideal operating conditions, the effective area of the proposed sensor is around 7.1 × 10⁻⁸ m² for all oil samples, which is very practical.

A comparison table (

Table 2. Comparison of sensing and guiding characteristics of PCF-based sensors.

Ref	Year	Sensing Sample	Relative Sensitivity (%)	Confinement Loss (dB/m)	Numerical Aperture
[28]	2018	Benzene Ethanol Water	97.20 96.97 96.69	8.80 × 10−12 5.13 × 10−12 2.41 × 10−11	--- --- ---
[30]	2019	RBC Hemoglobin WBC Plasma Water	93.50 92.41 91.25 90.48 89.14	1.80 × 10−12 6.13 × 10−12 2.15 × 10−11 5.85 × 10−11 8.93 × 10−11	--- --- --- --- ---
[21]	2020	Benzene Ethanol Water	98.50 98.20 97.60	2.34 × 10−12 5.98 × 10−12 9.51 × 10−11	--- --- ---
[20]	2021	RBC Hemoglobin WBC Plasma Water	95.80 95.00 93.60 92.50 91.40	3.80 × 10−11 1.13 × 10−11 2.15 × 10−10 6.25 × 10−10 8.3 × 10−9	0.38 0.38 0.38 0.38 0.38
This work	2022	Sunflower oil Mustard oil Olive oil Coconut oil Palm oil	99.65 99.60 99.55 99.50 99.35	1.91 × 10−12 1.93 × 10−12 2.62 × 10−12 3.3 × 10−12 1.31 × 10−11	0.37 0.37 0.37 0.37 0.37

) is tabulated to reflect the sensing and guiding characteristics of the proposed sensor with the recently proposed PCF-based sensors. The table highlights that the proposed sensor is better than the recently proposed chemical/liquid sensors in terms of relative sensitivity by a good margin. Finally, the possible fabrication methodology of the proposed sensor is investigated.

The cross-sectional depiction in Figure 1 shows that this sensor is hybrid in structure, having a hexagonal-shaped core. Furthermore, the manufacturing complexity is reduced since the entire core and cladding holes are reliant only on one constant (D). Researchers have suggested and produced a variety of hexagonal-shaped photonic crystal fibers with high precision using various approaches [27,28,29,30,31]. Moreover, different asymmetric structured PCFs have been fabricated in the laboratory by using various stacking approaches. For example, six-hole suspended core fiber [36] and high birefringence suspended core fiber [37] were fabricated in the laboratory and the electronic spectroscopic view is shown in Ref. [36]. If we take a close look at those fibers, then it is clear that the fabricated PCFs are quite identical to our simulated PCF. So, it is clearly understood that the stack and draw method can be used to fabricate a hybrid PCF with acceptable accuracy. Now, if we consider the structure of our proposed PCF with the previously reported PCFs, then this proposed fiber could be produced in the laboratory with low complexity. The optimum fiber dimensions of the proposed PCF are almost 1.4 mm, whereas the fibers in Ref. [36] are less than 1 mm, which indicates that the proposed sensor can be fabricated easily in the laboratory. That is the reason that we recommend the stack-and-draw approach to manufacture these specialty fibers, since all types of PCFs can be made in the laboratory with sufficient precision using this method.

4. Conclusions

Pure and nutritious edible oil helps people live a secure and healthy life. However, some unethical dealers sell low-grade oil to make extra money at the expense of consumers’ health. Therefore, it is crucial to assess the quality of edible oil before human consumption. Therefore, a completely new structural PCF is suggested in this research to detect various food oils utilizing THz signals. Based on the numerical computations, the proposed sensor exhibits a relative sensitivity of 99.65%, 99.60%, 99.55%, 99.50%, and 99.35% for sunflower, mustard, olive, palm, and coconut oil, respectively, at 1.6 THz. Furthermore, under ideal geometric conditions, the fiber achieves minimal confinement loss of about 10⁻¹³ dB/m, a low effective material loss of 0.0035 cm⁻¹ and a high numerical aperture of 0.37. We strongly believe that the proposed sensor will be a competitive device in the field of oil detection applications due to its high sensitivity and simple structure.