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Optical waveguide-based sensors are gaining popularity due to their cost-effectiveness, compact size, and high sensitivity. There are two primary techniques for designing these sensors: changes in effective refractive index or changes in the evanescent field ratio. The absorption property, specifically the evanescent field-based photonic sensing approach, is preferred over effective index-based sensing. This preference arises from the fact that the change in effective index is relatively smaller when the sensing material is present, compared to the change in the evanescent field. The absorption of light depends on the dimension and material of the sensor device, the operating wavelength, and the material being sensed. The percentage of evanescent field/light in the upper cladding/slot region of the photonic waveguide changes during its interaction with the molecules of the sensing material. Therefore, it is crucial to design photonic sensors with waveguides that have a relatively larger evanescent field in the upper cladding region. In slot waveguides, light is predominantly confined to the slot region, resulting in a higher evanescent field value. As the sensitivity of the photonic sensor depends on the percentage of evanescent field/light in the upper cladding region, the slot waveguide offers significantly higher sensitivity compared to other photonic waveguide structures.
Department of Electronics and Communication Engineering, SRM Institute of Science and Technology Kattankulathur, India
*Address all correspondence to: veerc@srmist.edu.in
1. Introduction
Optical waveguide-based photonic sensors have gained significant popularity due to their advantageous characteristics, making them increasingly preferred in diverse applications. One key factor contributing to their popularity is their cost-effectiveness, making them an economically viable choice for various industries. The ultra-compact size of these sensors further enhances their appeal, allowing for integration into smaller devices and systems. One noteworthy attribute of optical waveguide-based photonic sensors is their high sensitivity. This heightened sensitivity makes them particularly suitable for applications where precise and accurate measurements are crucial. In fields such as biomedical research, chemical detection, and gas detection, where minute changes in parameters can be of great significance, these sensors offer a reliable and efficient solution. The miniaturization of sensors is a particularly intriguing aspect of this technology. Unlike traditional bulky sensors, optical waveguide-based photonic sensors can be seamlessly incorporated into a small chip area. This compact design not only facilitates easy integration into various devices but also opens up new possibilities for the development of smaller, more portable sensor systems. Another advantage lies in the ease of fabrication, owing to the compatibility of these sensors with complementary metal-oxide-semiconductor (CMOS) technology. The CMOS compatibility streamlines the manufacturing process, making it more efficient and cost-effective. This compatibility also allows for the integration of photonic sensors with existing electronic systems, creating synergies between optical and electronic components [1]. Designing a photonic sensor involves a keen observation of variations in either the effective refractive index or the evanescent field ratio. These parameters play a crucial role in the sensor’s ability to detect changes in the surrounding environment or the material being sensed. There are two primary approaches for photonic sensing: effective index-based sensing and absorption property-based sensing. In the context of these approaches, the absorption property-based photonic sensing method is often favored. This preference stems from the recognition that, in the presence of the material or sample under consideration for sensing, the change in the effective refractive index is relatively smaller compared to the change in the evanescent field. The dominant factor contributing to this change in the evanescent field is the absorption of light by the sensing material. The evanescent field is an electromagnetic field that extends beyond the surface of the waveguide or sensor. Its variation is particularly sensitive to changes in the surrounding medium. When the sensing material interacts with this field, the absorbed light induces a noticeable alteration in the evanescent field, making it a reliable indicator of changes in the environment. This sensitivity to absorption phenomena allows for a more accurate and responsive detection mechanism. The magnitude of light absorption depends on several factors, including the dimensions and material composition of the sensor device, the operating wavelength of the light used, and the characteristics of the material to be sensed. The choice of these parameters influences the sensor’s efficiency and specificity in capturing changes in the absorption properties of the surrounding medium [2, 3].
In the mid-infrared range, the authors of [4, 5, 6] have proposed the concept of an evanescent field-based photonic gas sensor employing strip, rib, and slot photonic waveguides. During its contact with the molecules of the sensing material, the photonic waveguide’s upper cladding/slot region’s percentage of light or evanescent field varies. As a result, the waveguides in the top cladding region of the photonic sensors must be built with a greater evanescent field. The slot waveguide, at the expense of a larger propagation loss [7, 8, 9, 10], has the highest evanescent field ratio in the slot (upper cladding) region when compared to other photonic waveguide architectures, as the authors in [11, 12] have shown. Slot waveguides represent a distinctive type of waveguide design where light is primarily confined within a narrow slot region. This characteristic imparts unique advantages to slot waveguides, especially concerning their evanescent field properties. The evanescent field is a critical component in photonic sensors, and in the context of slot waveguides, its magnitude is notably higher compared to other waveguide structures [13, 14, 15, 16, 17, 18, 19]. The confinement of light within the slot region is a defining feature of slot waveguides. This results in a more significant evanescent field, which is the electromagnetic field extending beyond the core of the waveguide. In practical terms, the higher evanescent field in slot waveguides means that a larger portion of the light is near the waveguide’s surface, making it more responsive to changes in the surrounding environment. However, it is essential to acknowledge that slot waveguides may incur propagation losses. To address these losses, a compensatory strategy involves using a light source with relatively high input power. This higher power input helps counterbalance the losses experienced during the propagation of light through the waveguide. While this compensatory measure may require more power, it contributes to maintaining the overall efficiency of the photonic sensor system. The significance of the evanescent field in photonic sensors cannot be overstated. The sensitivity of these sensors relies heavily on the percentage of evanescent field or light present in the upper cladding region, where interactions with the external environment occur. In the case of slot waveguides, the inherently higher evanescent field values translate into a sensor with significantly heightened sensitivity.
In the contemporary landscape, Micro-Electro-Mechanical System-based sensors have gained widespread acceptance as essential electronic devices [20, 21, 22, 23, 24]. However, a promising counterpart, known as the Micro-Opto Mechanical System, has emerged as a compelling technology [1, 25, 26]. This amalgamation of features positions the Micro-Opto Mechanical System as a technology with immense potential, raising expectations that it may play a dominant role in the global sensor market in the near future. The distinguishing feature of the Micro-Opto Mechanical System lies in its ability to merge photonics and micro-machining. This convergence results in a sensor system that harnesses the benefits of both domains. High sensitivity is a particularly noteworthy advantage of the Micro-Opto Mechanical System, making it adept at capturing subtle changes in the environment or the parameters being measured. Additionally, the ease of fabrication associated with the Micro-Opto Mechanical System facilitates cost-effective manufacturing processes, contributing to its attractiveness for widespread adoption. The synergy between the Micro-Opto Mechanical System and CMOS enhances the overall efficiency and functionality of the sensor devices, making them more versatile and adaptable to various applications. Anticipating the future trajectory, it is plausible to envision Micro-Opto Mechanical System-based sensor devices occupying a substantial share of the global sensor market. This projection is rooted in the Micro-Opto Mechanical System’s inherent advantages, including high sensitivity, ease of fabrication, and compatibility with CMOS technology. Moreover, beyond the capacitive sensing approach commonly associated with Micro-Electro-Mechanical Systems, the literature showcases various photonic sensors based on optical interference methods [27, 28, 29]. These sensors leverage optical interference phenomena to precisely measure deflection, providing accurate information about the exerted pressure. This diversified approach in photonic sensing methods contributes to the expanding capabilities of Micro-Opto Mechanical System -based sensors, further solidifying their potential dominance in the sensor market.
2. Working principles of optical waveguide-based sensor
The sub-wavelength dimensions of dielectric waveguides contribute to a fascinating phenomenon in the form of a small fraction of electromagnetic (EM) modes that extend beyond the waveguide itself. This extension of electromagnetic fields beyond the waveguide is referred to as the evanescent field. The significance of this evanescent field, quantified by the evanescent field ratio, becomes pivotal in understanding the behavior of light propagation along the waveguide. Several factors influence the evanescent field, including the cross-sectional geometry of the waveguide in relation to the wavelength of the propagating light and the optical properties of the materials involved [30]. The evanescent field, because of its sub-wavelength nature, plays a crucial role in applications such as gas sensing. Gas sensing is particularly demanding due to the typically low absorption coefficients associated with most gases. In the context of gas sensing with dielectric waveguides, achieving a high evanescent field ratio becomes essential. A high evanescent field ratio indicates that a substantial portion of the electromagnetic modes propagating along the waveguide extend outside its physical boundaries. The importance of a high evanescent field ratio in gas sensing is closely tied to the fact that the intrinsic damping of the evanescent field should be kept low. In other words, the evanescent field should be capable of penetrating into the surrounding medium, and its energy should not be rapidly attenuated. This characteristic is crucial for effective gas sensing because when an absorbing medium such as a gas is introduced in the vicinity of the waveguide, the evanescent field is absorbed by the gas. This absorption attenuates the guided wave within the waveguide, leading to a reduction in intensity at the output port of the waveguide [31].
2.1 Design and modeling
The sensor test structures can be considered as silicon slot waveguides of approximately 1 cm in length, specifically engineered for the purpose of gas sensing. To enhance their functionality, taper structures and launch pads have been incorporated at both ends of the waveguides. Notably, a grating coupler is positioned on the launch pad to facilitate the coupling of mid-infrared radiation (MIR) from an external MIR source into the waveguide. Additionally, the grating coupler serves the purpose of efficiently extracting the light from the waveguide, directing it toward a detector for further analysis. The detailed design of this grating coupler, employing two-dimensional finite element models, has been elucidated in [32]. The schematic representation in Figure 1 provides an overview of the measurement configuration employed. It includes a gas cell housing the test chip with the silicon waveguide structures. The gas cell is an integral component of the experimental setup, allowing controlled exposure of the sensor to different gas environments for testing purposes. The schematic captures the essential elements of the setup, depicting the waveguide, taper structures, launch pads, and the grating coupler. The gas cell within the configuration serves as a controlled environment where the gas sensing experiments are conducted. The integration of the grating coupler allows for the efficient interaction of mid-infrared radiation with the waveguide, enabling precise and selective sensing. The out-coupling of light onto a detector facilitates the measurement of the guided wave, providing valuable data related to the absorption characteristics of gas in the mid-infrared spectrum.
Figure 1.
Schematic representation of the measurement setup.
2.2 Absorption-based sensing
Sensing mechanisms relying on the absorption properties of materials are generally favored over those based on the effective refractive index. This preference arises from the observation that changes in the effective refractive index have a relatively minor impact compared to alterations in the evanescent field, primarily induced by absorption phenomena. In absorption-based sensing, the evanescent field engages with the molecules of the sensing element, leading to the absorption of a certain amount of power by these molecules, depending on the wavelength. Consequently, designing photonic sensors involves the use of waveguides with larger evanescent fields, thereby ensuring higher sensitivity, particularly in the near-IR region. The sensitivity can be quantified using Eq. (1) [8, 12].
Sensitivitys=−ηELexp−ηECL−αwvgLE1
where, η, E,C,αwvg, and L are the evanescent field fraction, absorption coefficient, concentration, intrinsic waveguide loss, and length of the waveguide respectively. The evanescent field fraction is defined as the amount of power in the upper etched cladding region, interacting with the sensing environment.
2.3 Evanescent field ratio
The evanescent field ratio holds significant importance in the development of devices and sensors based on evanescent field absorption. This parameter is defined as the proportion of power in the upper cladding region, engaged with the environment or material intended for sensing, relative to the total incident power on the device. The evanescent field ratio is predominantly influenced by the geometric characteristics of the photonic waveguide and operating wavelength. The evanescent field ratio can be expressed by Eq. (2) below, highlighting its dependence on specific aspects of the waveguide’s design and the wavelength employed.
η=∬evanS→.n→dxdy∬TotalS→.n→dxdyE2
where, η is the evanescent field ratio, n is the normal vector to the waveguide cross-section and S is the Pointing vector of the mode field in the waveguide.
2.4 Propagation loss
In addition to the evanescent field ratio, the analysis of sensor performance also hinges on the propagation loss within the waveguide. The propagation loss is a crucial factor influenced by the geometrical characteristics of the waveguide and is particularly dependent on the imaginary part of the effective refractive index. This relationship is mathematically expressed in Eq. (3) below [27, 28, 29]. Therefore, understanding and controlling waveguide geometries and the associated imaginary component of the effective refractive index is pivotal in comprehending and optimizing the performance of sensor designs.
LossdB=10×log10e×4π×ImneffλE3
where, Imneff is the imaginary part of the effective refractive index, and λ is the operating wavelength. From Eq. (3), it is clear that for a fixed wavelength, the propagation loss varies only with Imneff. The slot waveguide usually offers higher evanescent field ratio values at the cost of higher propagation loss, which can be overwhelmed by the enhanced incident power.
2.5 Optimum length of waveguide
Moreover, the optimum length (Lopt), which can be found for a photonic sensor or device by solving the equation dsdL=0, from Eq. (1) above, is the waveguide length at which the sensitivity is maximal and beyond that the sensitivity starts to erode due to waveguide loss. Consequently, it can be stated as shown in the following Eq. (4),
3. Design and sensing performance of optical waveguide
3.1 Waveguide design
Normally, three distinct types of waveguides—namely strip, rib, and slot waveguides—can be designed for the purpose of absorption-based gas sensing in the mid-infrared (mid-IR) region, as illustrated in Figure 2. The waveguides are crucial components in the context of detecting gases, particularly emphasizing their application in the absorption of infrared radiation. The parameters width and height of the waveguide, respectively, provide key dimensions for the characterization of the waveguide structure. In the case of rib waveguides, an additional parameter rib layer, is introduced, representing the thickness of the dielectric slab layer. For slot waveguides, the slot gap between two high-index arms is a crucial parameter. These specifications are fundamental in tailoring the waveguide geometry to optimize its performance for gas sensing applications [12, 33, 34]. As a practical example, the chapter delves into the design of a specific waveguide intended for the absorption line of CH4 at a wavelength of 3.31 μm in the mid-IR range. This wavelength selection aligns with the characteristic absorption behavior of CH4 molecules in the mid-IR spectrum. By strategically configuring the dimensions and structure of the waveguide, the aim is to enhance its effectiveness in facilitating the absorption of infrared radiation, thereby enabling precise and sensitive gas sensing capabilities. This design approach emphasizes the importance of customization and optimization of waveguide parameters to suit the specific absorption characteristics of the target gas, showcasing the versatility and adaptability of waveguides for mid-IR gas sensing applications. The simulated η values will be used in the case study of CH4 sensing in the next section. Since the gas in a slot waveguide will interact with the evanescent field at the waveguide’s two sidewalls as well as on top, both quasi-TE and quasi-TM modes are interesting for field augmentation. To meet the electrical displacement continuity requirement, the dominating electrical field for the quasi-TE mode is in the horizontal direction, and it will be strengthened at the gas interface with the sidewalls (Figures 3-5).
Figure 2.
The measurement setup along with waveguide configuration and optical fibers, with input and output light.
Figure 3.
Diagram displaying a dielectric waveguide’s side view, illustrating the electric field distribution of the fundamental quasi-TE and quasi-TM modes.
Figure 4.
Schematics of the evanescent absorption gas sensor: (a) cross-sectional view and (b) side view.
Figure 5.
Different types of possible waveguide structures: (a) strip, (b) rib (c) slot (d) partial-strip-loaded slot, and (e) full-strip-loaded slot waveguide.
3.2 Influence of waveguide dimension on evanescent field ratio
The slot waveguide is recognized as a prominent structure for implementing evanescent field-based photonic sensor devices, owing to its notable feature of high evanescent field in the slot region. This phenomenon makes the slot waveguide particularly effective for applications that rely on the interaction of the evanescent field with external substances or analytes, such as in photonic sensing devices. The efficacy of the evanescent field in the slot region is intricately tied to the specific dimensions and structure of the waveguide. The amount of evanescent field present in the slot region is a critical factor that directly influences the performance and sensitivity of the sensor. Therefore, it becomes imperative to carefully examine and understand the impact of variations in the waveguide’s dimensions on the evanescent field ratio for all three considered slot waveguide structures. The term evanescent field ratio refers to the concentration of the electric field within the waveguide structure, particularly in the slot region. This distribution is crucial for enhancing the interaction between the waveguide and the external environment, facilitating improved sensitivity in sensing applications. By visualizing and analyzing the effects of changes in waveguide dimensions on the electric field distribution, researchers and practitioners gain insights into how alterations in the structural parameters influence the sensor’s performance. This understanding is pivotal for optimizing the design of slot waveguides to achieve desired sensing characteristics and to tailor the sensor’s response to specific applications. Hence, the chapter underscores the importance of investigating the intricate relationship between waveguide dimensions and evanescent field distribution to advance the development of efficient and reliable evanescent field-based photonic sensor devices.
In the investigation of conventional slot waveguides, the parameter ‘W’ has been systematically varied within the range of 450–650 nm for two distinct slot gaps (G) of 80 and 120 nm, as illustrated in Figure 6a. The graphical representation indicates a noticeable decrease in the percentage of evanescent field ratio with the initial increase in the width of the waveguide. Remarkably, for a slot gap of 80 nm, a sudden surge in evanescent field ratio can be observed at a waveguide width of 510 nm, reaching its maximum value (0.3) around the width of 520 nm. Prior to the width of 510 nm, light confinement predominantly occurs in the side arms (Ge) or lower-cladding regions, whereas after the width of 520 nm, light propagation primarily takes place through the slot region. Beyond the width of 520 nm, the confinement of light in the slot region gradually diminishes with increasing arm widths. A similar trend is identified for a slot gap of 120 nm, with light confinement in the slot region initiating at the width of 530 nm, peaking at width of 540 nm, before gradually decreasing. Extensive simulation analysis reveals that achieving light confinement in the slot region is challenging for lower values of slot gap, potentially resulting in light leakage into the upper/lower-cladding and high-index regions. Notably, substantial light propagation is observed for a slot gap value around 80 nm, leading to significant evanescent field values. In the context of evanescent field analysis for different slot waveguide structures, the thickness (‘t’) of the germanium (Ge) layer is varied up to 140 nm [35]. This exploration aims to identify the optimal Ge-layer thickness that maximizes the evanescent field ratio in the slot region. The analysis considers two slot gaps of 80 and 120 nm, presented in Figure 6b and c, respectively, for two different types of slot waveguides i.e., partial-strip-loaded and full-strip-loaded. Notably, similar trends in the graph have been reported in recent literature, emphasizing the variations in the confinement factor in slot waveguides. Other studies by different authors have also showcased analogous transitions in their results concerning the confinement factor of slot waveguides [36, 37, 38].
Figure 6.
Evanescent field ratio variations at two distinct slot gaps with thickness in (a) typical slot waveguide, (b) partial-strip-loaded slot waveguide, and (c) full-strip-loaded slot waveguide [35].
Building upon the insights gained from the analysis of conventional slot waveguides and the meticulous simulations involving partial-strip-loaded and full-strip-loaded slot waveguides, the study focuses on a fixed waveguide width value of 510 nm. In Figure 6b, dedicated to the partial-strip-loaded slot waveguide, the maximum levels of the evanescent field are observed at approximately 30.22 and 28.31% for slot gaps of 80 and 120 nm, respectively. These peaks occur at specific thickness values, namely 10 and 60 nm. Similarly, Figure 6c, dedicated to the full-strip-loaded slot waveguide, presents the variations in evanescent field ratio concerning thickness. Maximum evanescent field ratio values of about 31.4 and 28.77% are noted for the same respective slot gaps, accompanied by corresponding thickness values of 0 and 40 nm. Interestingly, following the attainment of the highest evanescent field values in both different types of slot waveguides namely partial-strip-loaded and full-strip-loaded, an increase in thickness leads to a subsequent decrease in the evanescent field. It is noteworthy that the identified values of width and thickness for the considered slot waveguides align with the configurations that yield the maximum evanescent field in the slot region. However, beyond just the evanescent field, the propagation loss plays a pivotal role in determining the optimal dimensions of waveguides. Striking a balance between achieving higher sensitivity and minimizing propagation loss is crucial, and these considerations are further delved into in the subsequent subsections below.
3.3 Influence of waveguide dimension on propagation loss
Propagation loss is intricately linked to the choice of materials and the geometric characteristics of photonic waveguides. Building upon the preceding analysis, there is an expectation that depositing a Ge-layer on CaF2 may lead to a reduction in propagation loss in slot waveguide structures. To explore the impact of variations in waveguide dimensions on propagation loss, the width of the slot waveguide is once again systematically adjusted, ranging from 450 to 650 nm, for slot gaps (G) of 80 and 120 nm. Through simulation analysis, the imaginary part of the effective refractive index can be derived. Subsequently, using these imaginary values of effective refractive indices values for different slot waveguide dimensions in Eq. (3), the propagation loss in slot waveguides can be determined. In Figure 7a, the relationship between propagation loss and width of conventional slot waveguide is illustrated. As the width increases, there is a noticeable decrease in propagation loss. However, as highlighted in the preceding subsection, for slot gap of 80 nm, substantial light confinement in the slot region commences at a waveguide width of 510 nm, reaching its maximum at a waveguide width of 520 nm. Consequently, the propagation loss begins to increase at waveguide width 510 nm, peaking at a waveguide width 520 nm, before experiencing a continuous decrease. This suggests that in slot waveguides, achieving high light evanescent field comes at the expense of elevated propagation loss. Moreover, to strike a balance between low propagation loss and significant light confinement, the choice of waveguide width becomes crucial. For instance, at a waveguide width of 550 nm with a slot gap of 80 nm, the evanescent field decreases by only 3%, accompanied by a substantial reduction in propagation loss, nearly 7 dB/cm compared to that at waveguide width of 520 nm. Similarly, for a slot gap of 120 nm, selecting a waveguide width of 560 nm results in a reduction in the evanescent field and corresponding propagation loss by around 2.4% and 7.6 dB/cm, respectively, when compared to a waveguide width of 540 nm. This underscores the importance of judiciously selecting the waveguide width to achieve optimal trade-offs between propagation loss and light confinement, particularly in the context of varying slot gaps.
Figure 7.
Propagation loss variations versus thickness at slot gaps of 80 nm and 120 nm for (a) conventional slot waveguide, (b) partial-strip-loaded slot waveguide, and (c) full-strip-loaded slot waveguide [35].
Furthermore, in the analysis of propagation losses within different slot structures, i.e., partial-strip-loaded and full-strip-loaded, a fixed waveguide width value of 510 nm is maintained, while the thickness of the waveguide layer can be systematically varied up to 140 nm. This examination is conducted for slot gaps (G) of 80 and 120 nm, as illustrated in Figure 7b and c, respectively. The results clearly indicate that for a slot gap of 80 nm, the propagation loss consistently decreases with the incremental rise in the thickness layer, exhibiting a smooth trend without any abrupt changes in propagation loss observed at intermediate thickness layer values, as noted in the case of a slot gap of 120 nm. Similar to the earlier analysis, the careful selection of waveguide bottom layer thickness becomes paramount to achieving a significant evanescent field with low propagation loss. Figure 7 highlights that both waveguide width and thickness values contribute to the reduction in propagation loss, with substantially lower propagation loss apparent for smaller values of slot gap. Furthermore, in terms of propagation loss, the full-strip-loaded type structure outperforms the partial-strip-loaded and notably surpasses the conventional slot waveguide. This suggests that both different slot waveguide structures offer viable options for the design of gas sensors based on the absorption property of sample material in photonic waveguide, emphasizing their potential suitability and superior performance in such applications.
3.4 Sensitivity analysis for methane gas
To enhance the sensitivity of photonic waveguides, it is imperative to maximize the evanescent field while minimizing propagation loss. However, in typical slot waveguides, there exists a trade-off between achieving a higher evanescent field and keeping propagation loss low. Striking a balance between these two factors is crucial to attain a satisfactory level of sensitivity. Utilizing Eq. (3), sensitivity values for all three slot waveguide structures can be estimated, considering an absorption coefficient of 960.96 L/Mol-cm [35] to quantify device sensitivity. Figure 8a–c portray the variations in sensing capabilities concerning waveguide lengths for different evanescent field ratio values in the slot region, propagation losses, and gas concentrations across all different slot waveguide structures. Specifically, for a slot gap of 80 nm in Figure 8a, the sensitivity variations are illustrated for three distinct evanescent field ratios (0.22, 0.27, and 0.28) obtained for the different slot waveguide structures, respectively. These analyses are conducted while maintaining a fixed propagation loss of 3 dB/cm.
Figure 8.
Sensitivity as a function of waveguide lengths for the different waveguide structures: (a) at a fixed propagation loss of 3 dB/cm and slot gap of 80 nm, (b) at a fixed evanescent field ratio of 0.28 and (c) four distinct methane gas concentrations for a full-strip-loaded slot waveguide at slot gap of 80 nm and [35].
The figure distinctly illustrates that among the three different structures of slot waveguide, the full-strip-loaded slot waveguide exhibits the highest sensing ability. However, the sensitivities of both partial-strip-loaded and full-strip-loaded slot waveguides are closely aligned, given their similar evanescent field values at a fixed propagation loss of 3 dB/cm. Furthermore, the influence of varying waveguide lengths can also be discussed in Figure 8b, considering different propagation losses (approximately 14, 7, and 5 dB/cm) achieved for different slot waveguide structures. In this analysis, a fixed evanescent field ratio of 0.30 and slot gap of 80 nm has been maintained. Notably, while maintaining identical evanescent field values for all slot waveguide structures, their propagation losses differ due to distinct design structures. Additionally, the partial-strip-loaded and conventional slot waveguides exhibit reduced sensing abilities by 5–15 and 20–40 L/Mol, respectively, compared to the full-strip-loaded slot waveguide. Consequently, from Figure 8a and b, it is evident that sensitivity increases with rising propagation loss and decreasing evanescent field value, underscoring the superior performance of the full-strip-loaded slot waveguide. Similarly, Figure 8c analyses the sensitivity variations in terms of waveguide lengths for a full-strip-loaded slot waveguide at different concentrations of methane gas (50, 100, 500, and 1000 ppm). The figure reveals that sensitivity decreases with an increase in gas concentrations. This phenomenon is attributed to the heightened interaction of gas molecules with the evanescent field as the concentration rises, leading to increased light/evanescent field absorption and subsequently elevated losses, thereby decreasing sensitivity. However, for a specific gas concentration, sensitivity initially rises, reaches its peak at an optimum length, and then starts to decline beyond this optimal length.
3.5 Comparison for optical detection of different gas
The primary emphasis of the current research lies in the development of the sensing element dedicated to a light source with a specific wavelength of 3.31 μm. However, it is crucial to acknowledge that the effectiveness of photonic sensors is not solely dependent on the sensing element; the photodetector also plays a pivotal role in the overall design and analysis of such sensors. In the context of methane gas sensing devices, the focus shifts toward the implementation of a PbTe type of photodetector. This photodetector is identified as a suitable choice for detecting light with a wavelength of 3.31 μm emanating from the sensor device. The unique advantage of PbTe photodetectors lies in their capability for monolithic integration, meaning they can be seamlessly incorporated into the sensor’s structure. The integration of PbTe photodetectors contributes to the reliability of the sensing system, particularly in the detection of mid-infrared (mid-IR) light waves. This reliability is vital for applications like methane gas sensing, where accuracy and precision in detecting specific wavelengths are essential. Therefore, by focusing on both the sensing element and the choice of a suitable photodetector, the research aims to enhance the overall performance and reliability of photonic sensors, particularly in the targeted detection of 3.31 μm wavelength light in methane gas sensing applications [39].
Optical detectors for gas sensing, aimed at measuring I(l), have demonstrated successful implementation across a spectrum of gases including acetone (C3H6O), ammonia (NH3), carbon dioxide (CO2), formaldehyde (CH2O), nitric oxide (NO), carbon monoxide (CO), methane (CH4), and methanol (CH3OH). Diverse methodologies have been employed to construct gas sensors utilizing optical detection, as outlined in Table 1. These approaches encompass designs featuring tube-like gas cells established between face-to-face configured emitters and detectors, dome-like gas cells with planar configured emitters and detectors, open cells, cavity-enhanced cells, and waveguides utilizing evanescent-field interaction. The selection of light sources for these gas sensors has been varied, encompassing Micro-Electro Mechanical System heaters, LEDs, distributed feedback lasers (DFBs), and quantum cascade lasers (QCLs). Concurrently, an array of detectors, including photodiodes, thermopiles, pyroelectric detectors, and photoconductive detectors, has been utilized in these optical gas sensing configurations. The versatility in the design and components employed underscores the adaptability of optical gas sensors across a wide range of gases and applications.
The sensing performance of optical waveguides refers to the ability of these waveguides to effectively and accurately detect changes or variations in the surrounding environment, typically with respect to optical properties. Optical waveguides are structures designed to confine and guide light, enabling the transmission of optical signals with minimal loss. Evaluating the sensing performance involves assessing how well these waveguides can capture, transmit, and respond to optical signals in the context of specific applications, such as sensing various physical or chemical parameters. Key factors contributing to the sensing performance of optical waveguides include sensitivity. Sensitivity refers to the waveguide’s ability to detect small changes in the surrounding environment, often expressed as the minimum detectable change in a parameter of interest. Moreover, the effectiveness of optical waveguides in sensing applications is influenced by their design, materials, and the specific detection mechanisms employed. Waveguides can be engineered to interact with external factors such as, pressure or chemical concentrations, allowing for a diverse range of sensing applications. Different types of waveguides, including strip and slot photonic waveguides, may exhibit variations in their sensing capabilities based on their inherent characteristics. The sensing performance of optical waveguides encompasses their ability to sensitively, accurately, and promptly detect changes in the optical environment. This property makes them valuable components in a wide array of sensing applications, ranging from environmental monitoring to biomedical diagnostics and gas sensing analysis.
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Written By
Veer Chandra
Submitted: 04 March 2024Reviewed: 27 March 2024Published: 26 July 2024
Open access peer-reviewed chapter - ONLINE FIRST
