Multi-Channel Simultaneous Measurement of Cu(II) and Zn(II) Metal Ions Using Multimode Interference Based Sensors Encompassed in Fiber Ring Laser

In this research, a novel fiber-optic multimode interference based multi-channel transmissive sensors integrated into a fiber laser loop (FRL) for the detection of <bold><inline-formula><tex-math notation="LaTeX">$\text{Cu}^{2+}$</tex-math></inline-formula></bold> and <bold><inline-formula><tex-math notation="LaTeX">$\text{Zn}^{2+}$</tex-math></inline-formula></bold> metal ions in aqueous solution is proposed and experimentally corroborated. The multi-channel configuration includes single mode-claddingless- graded index multimode- single mode (SCGS) fiber structure and single mode- claddingless-few mode-single mode fiber (SCFS) combination in parallel for tracing <bold><inline-formula><tex-math notation="LaTeX">$\text{Cu}^{2+}$</tex-math></inline-formula></bold> and <bold><inline-formula><tex-math notation="LaTeX">$\text{Zn}^{2+}$</tex-math></inline-formula></bold> metal ions simultaneously and implemented in FRL setup to enhance the optical sensing characteristics. The sensing probe utilises the self-imaging effect to detect and measure the concentrations of two different metal ions. The SCGS structure has achieved a sensitivity of <inline-formula><tex-math notation="LaTeX">$4.347 \times 10^{-4}$</tex-math></inline-formula> nm/nM and a coefficient of determination <inline-formula><tex-math notation="LaTeX">$\mathrm{R}^{2}$</tex-math></inline-formula> value of 0.9922. The SCFS combination, on the other hand, has attained a sensitivity of <inline-formula><tex-math notation="LaTeX">$4.358 \times 10^{-4}$</tex-math></inline-formula> nm/nM and a <inline-formula><tex-math notation="LaTeX">$\mathrm{R}^{2}$</tex-math></inline-formula> value of 0.99277. The proposed sensing configuration holds promise for multi-point sensing in bio and chemical applications, particularly in real-time scenarios.


Multi-Channel Simultaneous Measurement of Cu(II) and Zn(II) Metal Ions Using Multimode Interference
Based Sensors Encompassed in Fiber Ring Laser Index Terms-Multimode interference, copper ions concentration, zinc ions concentration, multi-channel RI sensing, few mode fiber, fiber ring laser cavity.

I. INTRODUCTION
H EAVY metals are a group of metallic elements charac- terized by their high atomic weights and relatively high densities.Industrial activities have resulted in the entry of high levels of heavy metals into the surroundings via air, water, and soil.Heavy metals including lead (Pb), mercury (Hg), Zinc (Zn), copper (Cu), and arsenic (As) provide a significant danger to both individuals and the environment [1].Hence, the prompt and precise identification of metal ions has emerged as a significant concern [2].
Recently, there have been numerous proposals and demonstrations of different types of metal ion sensors.Traditional metal ion sensors adopt electrochemical methods [3].However, electrochemical sensors typically have a relatively low sensitivity, making them unsuitable for detecting metal ions at low concentrations.An alternative type of metal ion sensor uses metal ion markers stuck in matrix materials.The optical properties of The authors are with the School of Electrical Engineering, Vellore Institute of Technology, Vellore 632014, India (e-mail: anuradha.natarajan@vit.ac.in; ssivabalan@vit.ac.in).
Digital Object Identifier 10.1109/JPHOT.2024.3365422 the features, such as their colour [4], absorbance [5], [6], and fluorescence intensities [7], [8], can be employed to identify the metal ions in aqueous solutions.Nevertheless, the research stated above possesses intrinsic limitations as their signals are susceptible to variations in intensity induced by light pathways or sources.
In recent times, there has been considerable interest in the advancement of miniaturised fiber-optic metal ion sensors on account of their intrinsic benefits, including portability, affordability, resistance to electromagnetic interference, and dependability for in-vivo assessments.Various fiber optic sensing methods, such as fiber Bragg grating (FBG) sensors [9], evanescent field effect sensors [10], surface plasmon resonance (SPR)-based sensors [11], interferometric sensors [12], and fluorescence labelling sensors [13], have demonstrated excellent performance in the measurement of metal ions.
Among these reported fiber-optic sensors, the optical fiber interferometer has gained considerable attention in metal ion sensing due to its remarkable sensitivity, high multiplexing capabilities, and compact design.In the recent past, Bobo Gu et al. [14] demonstrated a fiber-optic sensor for the detection of metal ions using a thin-core fiber modal interferometer (TCFMI) coated with poly4-vinylpyridines (P4VP) and poly acrylic acid (PAA) via hydrogen bonding on the side surface.For instance, Raghunandhan et al. [15] developed an interferometric sensor for detecting Ni(II) metal ions using a no-core fiber (NCF) inserted between single-mode fibers (SMF) and functionalized with chitosan (CS)/(PAA) self-assembled polyelectrolyte layers.Following this, Ghufran Mohammed Jassam et al. [16] reported an SPR-based Mach-Zehnder interferometer sensor for Zn(II) detection using tapered photonic crystal fiber (PCF) between two multi-mode fibers (MMFs).In a recent study, Gengsong Li et al. [17] proposed a fiber-optic interferometer sensor to detect the concentration of Pb(II) ions, which comprises a few-mode fiber (FMF) with NCF structure, cladded with a smart hydrogel membrane and employs a cascading FBG for temperature monitoring.The aforementioned research focuses solely on single-point metal ion sensing.However, in the pharmaceutical, chemical, biological, and medical fields, the ability of fiberoptic interferometric-based metal ion sensors with multiplexing capability is crucial in detecting multiple heavy metal ions simultaneously or performing multi-point sensing.
In the line of multi-point sensing, Cennamo et al. [18] developed a multi-channel chemical sensor using SPR in a POF to detect two different metal ions in broadband configuration.The SPR sensing platform was created by removing the POF's cladding around half its circumference and coating it with a thin gold film containing a particular binding receptor capable of detecting Cu(II) and Fe(III) metal ions.Although multi-channel measurement is carried out for the aforesaid sensing structure, simultaneous detection of two separate metal ions has not been implemented.Moreover, the sensing device has inherent limitations due to sensitive film coatings that reduce linearity and measuring precision.Recently, the multimode interference (MMI)-based fiber optic interferometric sensors using single mode-multimode-single mode (SMS) structure have attracted attention due to their flexible design, easy fabrication and high multiplexing capability [12], [14], [17], [19].As of yet, there has been a lack of proposals on multi-point sensing for the simultaneous detection of heavy metal ions using fiber-optic sensors.Consequently, there is a prospect to incorporate MMIbased fiber-optic metal ion sensors equipped with multi-point simultaneous sensing flexibility inherent in simple manufacturing procedures to circumvent the drawbacks associated with delicate film coatings and thereby enhance the optical sensing characteristics.
In this research proposal, to the best of the author's understanding, the first novel optical fiber multi-channel transmissive sensor operates on the principle of the MMI effect, integrated into a fiber ring laser (FRL) cavity with enhanced optical performance traits has been reported for the simultaneous low-level detection of Cu 2+ and Zn 2+ metal ions.The proposed concept is the inaugural instance of conducting multi-channel simultaneous measurement in FRL for metal ions sensing.This research intends to utilize two SMS structures of varying lengths and multimode fibers, coupled in a multi-channel (parallel) configuration in a ring laser loop to simultaneously detect and analyze the optical properties of two distinct metal ions.The two SMS structures include SCGS (single mode-cladding lessgraded index-single mode) fiber combination for Cu 2+ metal ions detection and SCFS (single mode-cladding less-few modesingle mode) fiber combination for Zn 2+ metal ions detection.The SCFS fiber combination is the novel fiber sensor reported for the first time for metal ion sensing.In the experimental investigation, the SCGS sensor has demonstrated a sensitivity of 4.347 × 10 −4 nm/nM in detecting Cu 2+ metal ions within the concentration range of 100 nM to 1000 nM, with a R 2 value of 0.9922.Similarly, the SCFS sensor has achieved a sensing accuracy of 4.358 × 10 −4 nm/nM in detecting Zn 2+ metal ions within the same concentration range, with a R 2 value of 0.99277.The enhanced sensing performance is attributed to the utilization of combinational multimode fibers in the sensing structure and the implementation of the sensor in FRL.

II. DESIGN AND WORKING PRINCIPLE OF THE SENSOR
The schematic representation of the fiber optic transmissive sensing structures involved in the multi-channel configuration for simultaneously analyzing two distinct metal ions is depicted in Fig. 1.The two different sensing structures proposed in this article have been experimentally studied.They are essentially SMS combination-based fiber structures involved in the parallel configuration, which facilitates the multiplexing capability of the sensing system.The two distinct SMS structures for analyzing two different metal ions are the SCGS configuration, termed sensor 1, and the SCFS configuration, represented as sensor 2. In this context, the MMF refers to the combined portion of CLF and GIMF for sensor 1 and the merged combination of CLF and FMF for sensor 2, which are pigtailed between two SMFs to form the fundamental SMS structures.To enhance the sensitivity of the CLF, the acrylate coating that covers the core is eliminated.This action effectively admits the leakage of the cladding modes, thereby enabling direct contact between the CLF and the SRI.
The significant difference in core diameter between the output SMF and the CLF causes coupling loss at the fusion points.A fiber specification must account for the compromised core diameter between the CLF and the output SMF.This will aid in limiting signal coupling loss at the splicing joint, resulting in increased visibility of spectral fringes and a significant boost in output power at the output of the sensing system.Henceforth, for sensor 1, the GIMF fiber, when paired with CLF, effectively minimizes coupling loss [20] and dispersion loss [21] by combining most of the higher-order modes that escape due to core mismatch between CLF and the output SMF.The combination of CLF along with FMF in sensor 2 improves the modes stability for the output transmission spectrum, as the fiber supports only a few lower-order modes to transmit through, and compensates for the variation in core diameter between the CLF and the SMF at the output stage, resulting in a significant reduction in coupling loss [22], [23].
The fundamental operating principle of the sensing structures is based on the MMI effect.The operation of an MMI-based SMS sensor is based on a simple concept that has been explained elsewhere [24], [25], [26].The essential element is a precise length segment of MMF patched between two SMFs.The light thrown into the MMF from the lead-in SMF will excite many guided modes supported by the MMF, and as the modes propagate, they will produce images of the input field along the MMF axis, termed the self-imaging effect.When the accumulated phase difference between the modes is an integer multiple of 2π, such that the modes recombine in phase, it leads to the replication of the input image and its peak wavelength is given by [27], The sign λ 0 denotes the wavelength of light in a vacuum, whereas n MMF and D MMF represent the refractive index and core diameter, respectively, in MMF.L MMF represents the measurement of the length of the MMF.Thus, the operating wavelength of the sensing structure depends on the MMF specifications, including the RI of the MMF (thermo-optic effect), the effective length of the MMF as stated in [28], and the core diameter of the MMF.
The (1) demonstrates that a certain MMF length corresponds to a specific peak wavelength, resulting in the highest transmission level through the MMI sensor structure.Deviation from this particular wavelength results in the formation of the image either prior to or after the MMF-SMF interface, resulting in a decrease in the coupling to the output SMF.Consequently, when an extensive range of wavelengths is transmitted through an MMI device, it generates a response that acts as a band pass filter.Thus, the SMS structures utilised in the multi-channel architecture of the laser sensing system will serve dual purposes as both a sensor and a wavelength tuner within the lasing system.For the proposed sensing structure, the peak operating wavelength, λ 1 , of sensor 1 depends on the CLF and GIMF fiber parameters, and it is given by [29], The variables n CLF and D CLF represent the refractive index and diameter of the fundamental mode, respectively.The variable L CLF represents the length of the CLF used.In this context, L GIMF denotes the specific length of the GIMF that has been employed for analysis, whereas L T represents the total length of the combined CLF and GIMF.The refractive index of the core of GIMF is denoted as n GIMF , whereas the diameter of the GIMF is marked as D GIMF .The peak wavelength of operation, λ 2 , for sensor 2 which incorporates CLF and FMF as the MMF in the SMS structure, can be determined by [29], In this context, L CLF denotes the required length of the CLF, n CLF denotes the fundamental mode's RI, and D CLF denotes its core diameter.The length of the FMF utilised for analysis is denoted as L FMF in this context, and the total length of the CLF and FMF combination used in sensing structure 2 is L T .In the case of an FMF fiber, n FMF represents the refractive index and D FMF represents the core diameter.
The Equations ( 2) and ( 3) demonstrate that the CLF, GIMF, and FMF exhibit a linear wavelength response in terms of their change in length.The effective RI and the core diameter of the MMFs determine the slope of this response.Consequently, the sensitivity of the sensor structures and the optimum working wavelength are determined by the MMF specifications that are chosen.

III. CONSTRUCTION AND EVALUATION OF THE PROPOSED SENSING ARRANGEMENT IN A MULTI-CHANNEL SETUP
The proposed sensor configuration comprises an SCGS sensor and an SCFS sensor coupled in parallel.The SCGS fiber combination (sensor 1) consists of a segment of CLF fused to a short length of GIMF sandwiched between two SMFs.The CLF chosen has a length of 8.8 cm with a core RI of 1.4507.The sensing structure employs the standard SMF provided by Corning, which has a core diameter of 8 μm and a cladding diameter of 125 μm.The refractive indices of the core and cladding are denoted as 1.4525 and 1.4468, respectively.The CLF is combined with the GIMF, which has a core refractive index of 1.4585, a length of 4.5 cm, and core and cladding diameters of 62.5 μm and 125 μm, respectively.This combination is inserted between the SMFs.The splicing process is performed using the fusion splicer (specifically the FUJIKURA 80-S Fusion Splicer) to develop the sensing structure 1.In the same way, the fabrication of SCFS arrangement (sensor 2), which is implemented in parallel combination along with SCGS structure, is carried out by incorporating FMF with a length of 2.8 cm and core, cladding RI of 1.4504, 1.4336 respectively with the core, cladding diameter of 50 μm and 125 μm is fusion spliced with CLF of length 8.8 cm having the core RI of about 1.4507 and core diameter of 125 μm is then pigtailed in-between two similar SMFs.The SCFS fabrication and fusion splicing process uses the identical SMF dimensions as the SCGS sensor.
Extensive experimental tests with different fiber combinations and self-imaging distances have revealed the sensing length of SCGS and SCFS fiber structures.A variety of fiber combinations are investigated, including SMF with CLF concatenated with GIMF or FMF.The authors thoroughly evaluated the procedure used to optimise the self-imaging length of the SCGS sensing structure in their earlier work, as documented in [30].For the SCFS sensor, various self imaging distances of CLF and FMF are analysed and length optimization is carried out based on the spectral characteristics obtained.The research concluded that the optimal self-imaging lengths for CLF, GIMF, and FMF are 8.8 cm, 4.5 cm, and 2.8 cm, respectively.As a result, the two sensing structures are finalised based on the concluded selfimaging distances.The suggested sensing structures have an optimum self-imaging length, which results in reduced coupling loss during transmission, increased optical signal-to-noise ratio (OSNR), stable output of the lasing system, and a limited spectral bandwidth.Once the sensing structures have been fabricated, they are assembled in a multi-channel format utilising two 50:50 couplers that are implemented for analysis.Furthermore, the performance of the constructed device is assessed by integrating it into the FRL cavity [31] and by utilising a conventional broadband source [32].The output power spectrum obtained in conventional broadband configuration for the individual SCGS (sensor 1) structure, SCFS (sensor 2) structure, and the multi-channel operation of both the sensors are shown in Fig. 4(a)-(c) and the characteristic performance of the individual spectrum is compared with the output of the multi-channel spectrum.The output spectrum for the SCGS sensor is measured at around 1021.98 nm in the typical broadband experimental analysis, with an output transmission power of −35.505 dB.On the other hand, the peak spectrum for the SCFS sensor is detected at 1027.03 nm with a peak power of −35.487 dB.The transmission loss for SCGS and SCFS sensing structures are 2.669 dB and 2.6676 dB, respectively.For the multi-channel configuration of sensors in open loop broadband configuration, the peaks are obtained at 1021.92 nm, with a power received of about −36.885 dB and 1027.18nm, with the acquired power of −36.137 dB.Thus, it is evident that the multi-channel analysis of the sensing combination, which consists of two different sensors, closely matches the power spectrum of individual sensing in conventional broadband configuration with stable fringe visibility and enhances OSNR.
The parallel sensing arragement is executed in a laser ring configuration and the visual representation of its block diagram is depicted in Fig. 2 and the schematic experimental setup is presented in Fig. 3.This configuration aims to optimize the sensor's functionality concerning output spectral power, signal precision, and transmission bandwidth, all while reducing the coupling loss in-between the fiber components.The FRL configuration utilizes a continuous wave laser diode (CWLD) manufactured by Lumics, more precisely, the LU0975M500 model, renowned for its capabilities at a wavelength of 975 nm and a peak power output of 600 mW.Installing a pump protection filter element at the output of the LD, has effectively impeded the reversal of light transmission back into it.A one-meter-long Yb-doped single-clad fiber (YB 406, Coractive) is linked to the pump laser through a 980/1064 nm wavelength division multiplexer (WDM).This gain fiber typically exhibits a gain spectrum spanning from 1010 nm to 1120 nm, with a peak at 1030 nm when stimulated with a 975 nm wavelength.Connecting the gain fiber output to a polarization-independent isolator ensures unidirectional transmission of the laser light.
The optical signal from the isolator is linked to the sensing device in a parallel multi-channel arrangement.This sensing arrangement includes two arms, one housing the SCGS structure and the other housing the SCFS sensing structure.A 50:50 coupler is employed to establish a connection between the two distinct sensors, and the output of this combined arrangement is subsequently merged using another 50:50 coupler.The incoming light from the 50:50 coupler is coupled to a 22 GHz photodetector (EOT, ET3600F) via a 90:10 coupler to quantify the transmission spectrum's maximal power.The other port of the 90:10 coupler ensures the closed connection for the FRL cavity by connecting to the WDM another port.Furthermore, it has been associated with an optical spectrum analyzer (OSA, YOKOGAWA AQ6370D) powered by a 0.02 nm precision to investigate the spectral attributes.
The proposed sensing framework in parallel multi-channel configuration is analysed in FRL and the output power spectrum is compared with the individual sensor analysed in FRL configuration, and the power spectrum obtained is captured through OSA is represented in Fig. 5(a)-(c) with the inset of sensor 1 and sensor 2 schematic representation.
The output peak is observed at a wavelength of 1027.2 nm with a peak power of −20.45 dB for sensor 1, as shown in Fig. 5(a).For sensor 2, the spectrum is measured at 1027.74 nm, and the power obtained is −26.546dB, as depicted in Fig. 5(b).In a multi-channel setup of sensors based on FRL, two peaks are detected at frequencies of 1027.024nm and 1027.788nm, displayed in Fig. 5(c).The highest power levels recorded for these peaks are −20.406dB and −27.225 dB, respectively.The two extremes are achieved within a narrow bandwidth of 1 nm, exhibiting a highly steady output spectrum.The power obtained in peak B is comparatively less than peak A because of the two distinct sensing structures, which utilise SCGS in one arm and SCFS in another arm for the parallel configuration, and their optical performances are varied based on the fiber characteristics.The GIMF in sensor 1 has the profile of less coupling loss.Thus, there is an enhanced power at the output Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.  of the lasing system.Sensor 2 involves FMF, which supports only a few lower order dominant modes, from the CLF to couple through; there is a decrease in output power compared to sensor 1, leading to a stabilised spectral output.Evidently, the wavelength measured for the individual spectrum in FRL configuration closely matches the wavelength obtained in the parallel detection of multi-channel realisation incorporated in FRL implementation.

IV. ANALYSIS OF EXPERIMENTAL FINDINGS AND DISCUSSION
The performance attributes of the sensing structures are evaluated through the utilisation of a conventional broadband open loop setup and FRL configuration, subsequent to the fabrication of two unique metal ion sensors in a multi-channel configuration applying fiber-optic structures.The sensor 1, which includes the SCGS structure, is characterised for Cu 2+ metal ions, and sensor 2, which consists of an SCFS fiber combination, is realised for Zn 2+ metal ions.For the sensor 1 realisation, copper nitrate solution Cu(NO3)2 is used and for the sensor 2, Zinc nitrate Zn(NO3)2 solution is used for the experimental analysis.During the investigation, metal ion concentrations of 200 nM, 400 nM, 600 nM, 800 nM, and 1000 nM are taken, and the performance characteristics are examined by submerging the sensing structure in the corresponding analyte solution.

A. Broadband Open Loop Experimental Analysis
In the Broadband setup, a Cu(NO3)2 solution is used to examine Sensor 1, and a Zn(NO3)2 solution is used to analyse Sensor 2. The interference spectra acquired from the OSA reveal the changes in wavelength and intensity.When environmental RI exceeds the CLF's Core RI, the discrepancy in effective RI decreases.Consequently, the transmission peak in the input spectrum shifts to a region with shorter wavelengths.A decrease in the SRI relative to the CLF's core RI will increase the effective RI difference.The apex of the transmission will then relocate to the region comprising longer wavelengths due to the redshift that occurs in the interference spectrum.The output spectrum is acquired for six distinct concentrations of the solution, with each concentration increased by 200 nM.Fig. 6(a) depicts the plotted graph for the output spectrum in open loop configuration with simultaneous testing of metal ions.
The concentration change of 200 nM of Cu 2+ and Zn 2+ resulted in a wavelength shift of 0.07 nm in sensor 1 and 0.04 nm in sensor 2, respectively.In both sensors, the average peak power shift was measured to be 0.12 dB and 0.05 dB, respectively.The linear regression computed for the conventional broadband arrangement is displayed in Fig. 6(b).The highest sensitivity achieved for sensor 1, which was tested with a Cu 2+ solution holding the SCGS structure, was measured at approximately 1.06 × 10 −3 nm/nM.The sensitivity of the Zn 2+ sensor, which employs the SCFS structure, was calculated to be approximately 3.534 × 10 −4 nm/nM.

B. FRL Configuration Experimental Analysis
Initially, sensor 1 is tested for five different concentrations of Cu(NO3)2, with the step size of 200 nM and sensor 2 maintained at a constant level, and the shift in wavelength for sensor 1 obtained in OSA is noted.The experimental analysis is carried out at the room temperature of (∼25 • C) throughout the examination.The transmission spectrum obtained for every change in concentration is plotted and depicted in Fig. 7(a) with the linearity fitting obtained in Fig. 7(b).
The wavelength shift noted for sensor 1 with the Cu(NO3)2 solution to test the concentration of Cu 2+ metal ions is about 0.05 nm with a peak shift of about −0.18 dB.Fig. 7(b) shows a linear dependency between the change in concentration and the shift in wavelength.The R 2 value calculated is about 0.98482.
Further, sensor 2 undergoes testing using five distinct concentrations of Zn(NO3)2.The concentration is incremented by 200 nM each time.Meanwhile, sensor 1 remains consistent, and the resulting change in wavelength for sensor 1 is recorded using OSA.The experimental evaluation is conducted at a constant temperature of (∼25 • C) throughout the examination.The transmission spectrum is plotted for each change in concentration and displayed in Fig. 8(a), while the linearity fitting is performed in Fig. 8(b).The Zn(NO3)2 solution caused a wavelength shift of around 0.08 nm in sensor 2, indicating a change in the concentration of Zn 2+ metal ions.Additionally, the peak shift was measured to be around −0.12 dB.Fig. 8(b) demonstrates a clear correlation between the shift in concentration and the change in wavelength, indicating a linear relationship and the computed R 2 value is around 0.99637.
In addition, the multi-channel configuration of sensors is tested simultaneously with five different concentrations of Cu(NO3)2 and Zn(NO3)2 solution with the step interval of 200 nM and the performance characteristics are noted.The experimental evaluation is carried out under isothermal conditions, with a constant temperature of (∼25 • C) maintained during the investigation.The transmission spectrum has been plotted for each variation in concentration and presented in Fig. 8(a), while the linearity fitting is depicted in Fig. 8(b).The Cu(NO3)2 metal ion solution and the Zn(NO3)2 metal ion solution in sensor 1 and sensor 2, respectively, resulted in a wavelength shift of approximately 0.217 nm and 0.135 nm, corresponding to the change in concentration of Cu 2+ and Zn 2+ metal ions.In addition, the average peak shift was quantified to be around −0.05 dB and −0.02 dB for both peaks in the transmission spectrum.Fig. 8(b) exhibits a noticeable correlation between the difference in concentration and a shift in wavelength, suggesting a linear relationship.The calculated R 2 coefficient for the Cu 2+ metal ion sensor is approximately 0.99922, and the Zn 2+ metal ions sensor is 0.99277.
A comparison table for the shift in wavelength and the sensitivity obtained for both the sensors for the simultaneous variation of Cu 2+ metal ions and the Zn 2+ metal ions using the SCGS and SCFS fiber optic structure is shown in Table I.The difference in analyte concentration taken for analysis for both the metal ions is 200 nM.The sensitivity measured for the shift in wavelength for different concentrations is illustrated.The highest shift is achieved at 400 nM concentration.For Cu 2+ metal ions, the shift measured is 0.1837 nm, and the sensitivity of 9.1 × 10 −4 nm/nM is achieved.For Zn 2+ metal ions, the highest sensitivity is reached for 600 nM concentration and measured as 4.88 × 10 −4 nm/nM with a shift of 0.0977 nm is observed.
In order to assess the stability of the sensing probe in a multichannel configuration for detecting two different metal ions in a FRL arrangement, the spectral performance of the sensor was measured over a period of 40 minutes, with measurements taken every 5 minutes.The resulting graph is displayed in Fig. 10.During the 40-minute investigation, there were no significant fluctuations in the wavelength shift or peak power variation.The smallest observed wavelength shift is 0.02 nm, and the minimum Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.power variation is −0.3 dB.To avoid cross sensitivity difficulties caused by strain effect, the sensing probe is securely packed within a cylindrical container and sealed using adhesive material, ensuring no external disruptions.The tiny sensors that use SMS technology are not affected by cross-sensitivity issues because the influence of temperature is quite insignificant [12].Hence, it is evident that the suggested sensing arrangement generates a stable interference spectrum, making it an ideal choice for real-time application.

V. CONCLUSION
In this paper, a multi-channel sensor system based on the multimode interference effect in a SMS based optical fiber combination, for simultaneous detection of Cu 2+ and Zn 2+ incorporated in FRL, is presented.In one channel, SCGS fiber structure combination is utilized for the detection of Cu 2+ metal ions in the range of 100 nM to 1000 nM with the sensing accuracy of 4.347 × 10 −4 nm/nM and R 2 obtained is 0.9922.On the other channel SCFS fiber combination is used as the sensing probe to detect the Zn 2+ metal ions in the sensing interval of 100 nM to 1000 nM and the sensing sensitivity received is 4.358 × 10 −4 nm/nM with the R 2 value of 0.99277, and the multi-channel sensing configuration is implemented in FRL for the enhanced sensing performance.This sensing structure is proven for the multiplexing capability to detect different metal ions in FRL configuration with increased sensing performance and stabilised transmission spectrum.Thus, the proposed sensor structure is well-suited for in-situ and multi-point sensing applications.
Anuradha N and Sivabalan S Abstract-In this research, a novel fiber-optic multimode interference based multi-channel transmissive sensors integrated into a fiber laser loop (FRL) for the detection of Cu 2+ and Zn 2+ metal ions in aqueous solution is proposed and experimentally corroborated.The multi-channel configuration includes single modecladdingless-graded index multimode-single mode (SCGS) fiber structure and single mode-claddingless-few mode-single mode fiber (SCFS) combination in parallel for tracing Cu 2+ and Zn 2+ metal ions simultaneously and implemented in FRL setup to enhance the optical sensing characteristics.The sensing probe utilises the self-imaging effect to detect and measure the concentrations of two different metal ions.The SCGS structure has achieved a sensitivity of 4.347 × 10 −4 nm/nM and a coefficient of determination R 2 value of 0.9922.The SCFS combination, on the other hand, has attained a sensitivity of 4.358 × 10 −4 nm/nM and a R 2 value of 0.99277.The proposed sensing configuration holds promise for multi-point sensing in bio and chemical applications, particularly in real-time scenarios.

Fig. 2 .
Fig. 2. Schematic block representation of parallel sensor implemented in FRL configuration.

Fig. 3 .
Fig. 3. Schematic experimental setup for the proposed multi-channel sensing structure in FRL configuration.

Fig. 6 .
Fig. 6.Metal ion sensors implemented in conventional broadband configuration.(a) Wavelength shift observed simultaneously for the change in concentrations of Cu 2+ and Zn 2+ metal ions solution.(b) Linear response of the sensors for different concentrations of Cu 2+ and Zn 2+ metal ions.

Fig. 7 .
Fig. 7. Metal ion sensors implemented in FRL configuration.(a) Wavelength shift observed in sensor 1 corresponds to the variation in concentrations of Cu 2+ metal ions.(b) Linear response of the sensor for different concentrations of Cu 2+ metal ions.

Fig. 8 .
Fig. 8. Metal ion sensors implemented in FRL configuration.(a) Wavelength shift observed in sensor 2 corresponds to the variation in concentrations of Zn 2+ metal ions.(b) Linear response of the sensor for different concentrations of Zn 2+ metal ions.

Fig. 9 .
Fig. 9. Metal ion sensors implemented in FRL configuration.(a) Wavelength shift observed in both the sensors corresponds to the simultaneous variation in concentrations of Cu 2+ and Zn 2+ metal ions.(b) Linear response of the sensors for different concentrations of Cu 2+ and Zn 2+ metal ions.

Fig. 10 .
Fig. 10.Time response recorded for the output power spectrum in FRL configuration.