Tapered PCF Mach-Zehnder interferometer based on Surface Plasmon Resonance (SPR) for estimating concentration Toxic metal ions (Lead)

In this work, Mach-Zehnder interferometer (MZI) based Surface-Plasmon-Resonance (SPR), is created by fusing two regular single-mode �bers with a photonic crystal �ber (PCF) (SMF -PCF-SMF) for estimating the concentration and refractive index of Toxic lead ion. A little part (4cm) of Photonic crystal �ber is manufactured in the middle of the sensor, and then the tapering process is completed, the gold metal is then deposited at a thickness of about (40nm). Toxic lead ion are placed on gold coated core of a Photonic crystal �ber. The calculated the signal to noise ratio, sensitivity, resolution and Figures of merit are approaching SNR is 0.09, S 714 nm/RIU, Resolution is 2.8x10 -3 and the FOM is5.368 is a chivied.


Introduction
High-performance optical ber communications and other optical devices have relied heavily on advancements in ber production technology during the previous two decades.As a result, new ber design approaches that are better suited to speci c applications have emerged.Photonic crystal bers (PCFs) are a new design method that is being employed in a variety of applications such as ber communication systems, ber lasers, and sensing applications [1].Unlike traditional optical bers, the PCF has unique properties such as low dispersion, high birefringence, and lower nonlinearity.All of these qualities stem from its one-of-a-kind architecture, which involves threading a series of air-lled holes along the length of the PCF to increase single-mode con nement inde nitely [2].When speci c resonance conditions are met, Surface Plasmon's (SPs) are collective free electron oscillations that propagate along a metal-dielectric interface.[3,4].Because of its extraordinarily sensitive performance, SPR has received a lot of attention.A coupling metal-deposited prism with a narrow base is utilized in the traditional Kretschmann SPR setup, and SPs are stimulated in the presence of a metal-dielectric interaction of incoming light at a speci ed angle.The traditional SPR con guration, on the other hand, is bulky and unsuitable for remote sensing, limiting its large-scale fabrication for real-time applications [5].On the surface Plasmons, an optical device such as a diffraction grating, optical ber, or a high refractive index prism is utilized [6].Optical sensors based on SPR are used in a variety of elds including life, electrochemistry, environmental safety, sciences, chemical sciences, and biomedical diagnostics [7].For ambient refractive index sensing, PCFs' modal and light guidance features are intriguing.Fiber optic Mach-Zehnder sensors have been employed in a variety of sensing applications.Mach-Zehnder sensor in SMF-PCFSMF arrangement has been demonstrated for RI sensor [8].MZI sensor with tapered PCF proposed by Li et al. sensing with a high refractive index, although the sensor is only 2.4 cm long [9].In many applications, sensors with reduced size and localized sensing are required.The sensor length is said to be kept to a minimum by acid etching the PCF [10].In this paper, a SMF-PCF-SMF construction is described as a unique optical ber SPR.By sputtering a 40 nm thick gold layer onto the sensing area, a high refractive index (RI) sensitivity and concentration of Lead ions were achieved.

Surface Plasmon Resonance (Spr)
SPR refers to the electromagnetic wave-driven resonance oscillation of conduction electrons in an insulating metal interface.Photons or electrons can both stimulate SPR.When the collective oscillation of surface electrons in a metal equals the propagation constant of incident light, their momenta match, and resonance occurs.As a result, Changes in the refractive index are very sensitive to Surface Plasmon Resonance [11].

Performance Characteristics
The performance qualities that will be examined are resolution, sensitivity, Figures of merit, and Signal to noise ratio.Sensitivity is connected with a change in the refractive index per unit change in the resonance wavelength of the sensing area in spectral interrogation, and it's possible to describe it as [12]: The SPR spectral curve's width is inversely proportional to the Figure of Merit (FOM), and the Signal to Noise Ratio (SNR) can be written as [12]: The smallest variation in refractive index which a sensor can detect is known as its resolution, it can detect, expressed as [13]:

The Experimental Set-up
The following is the experimental setup for determining the spectrum of light transferred: a ray of light with the wavelength of λ = 510-532 nm from a Green laser, PCF (Thorlabs Co.), SMF, coupler, and a spectrophotometer which connected to a computer for data recording, a sensing PCF interferometer, and Lead concentration.Figures 1 (a) and 1 (b) demonstrate, for the Photonic crystal ber based on SPR, a photo of the experimental set-up and a plan, respectively.

Fabrication Of The Mach-zehnder Interferometer Structure
Splicing PCF between two the SMFS and after that tapering it by using a ber fusion splicer (The Shinho S16) created the PCF interferometer .Because they have the same diameter, we used the PCF (ESM-12, NKT) and the SMF Single Mode Fiber (SMF-28) for fewer fusion losses.The cross-section of the PCF [ ] utilized in the experiment is shown in Figure (2a).The ends of the two Single Mode Fibers were rst spliced to a PCF.Following fusion, The Photonic Crystal Fiber's air holes about the ends were entirely compressed, as illustrated in Figure (2b).Using hydro uoric acid (HF), after then, the PCF in the center would be pulled into a taper.Because of its instability, the traditional heat-and-draw technique was discontinued.The fusion splicer, on the other hand, can precisely regulate the motor.Before drawing, PCF must have its polymer coating removed.The tapered Photonic Crystal Fiber (PCF) is illustrated in Figure (2c) after multiple trials.

The Interference Properties Of The Mach-zehnder Interferometer
Based On Tapered Of Photonic Crystal Fiber Figure 3 shows the sensor production process in two steps.The basic Mach-Zehnder Interferometer (MZI) is constructed by splicing a short stub of Photonic Crystal Fiber (PCF) between ordinary SMFs, as illustrated in Fig. 3 (a).The PCF is tapered in the second phase, while both sides' SMFs remain constant.A short portion of PCF collapses during splicing, diffracting the fundamental core mode.When incident light interacts with the initial interface (Single Mode Fiber and Photonic Crystal Fiber), before recombining at the second interface, it permeates across both the PCF core and cladding, as shown in Fig. 3 (b) (SMF and PCF) The cladding modes' propagation properties are controlled by the Refractive Index difference at the cladding ambient contact because the PCF is smaller than the core.The cladding modes can approach the ambient solution much more closely, giving the sensor good sensitivity to variations in the surrounding Refractive Index RI.
Because the energy reduces as the resonance wavelength increases, the sensor medium's RI increases, and the resonance wavelength shifts to the longer wavelength side after a sharp dip (red shift), as seen in  for each sample, the sensor varies the dip position and width of each SPR response curve, and the extent to which the gradient position changes increases as the refractive index rises.Because it depends on the change of the λ res , the change of IR, and the spectral curve's width, With changes in the refractive index and resonance wavelength of the sensing area, the performance characteristics that depend on the surface plasmon resonance curve width, shifting value, and dip position vary.
Table (1) represents the experimental performance characteristics for a gold-layered sensor that was manufactured and simulated ,table (2) shows the concentration and Refractive Index values for each lead sample at different the resonance wavelength s.As the refractive index rises, the resonance wavelengths, and the concentration of the samples rises with it, due to the sharp wavelength changes to the red.
Table (1) The gold sensor's experimental performance parameters are shown.

Metal
Sensitivity

Conclusion
This study shows how a Mach-Zehnder interferometer (MZI) sensor may be built by fusion splicing a PCF between two standard SMF using the SPR technique to estimate the concentration and refractive index of toxic metal ions in Lead samples.This paper showed the (SPR) response curve for several Lead samples, as well as a dip in the resonance location.The resonant wavelength changes with each Lead sample.For each change in RI and then for each concentration of samples, the value of the resonance wavelength will change.To our knowledge, this is the rst time a high-sensitivity sensor for monitoring several harmful metal ions has been developed.The effective performance parameters for SMF-PCF-SMF such as the sensitivity is approaching 714 nm/RIU, SNR is 0.09, FOM is 5.368 and the Resolution is (2.8x10 − 3 ) are chivied.For SMF-PCF-SMF, the refractive index as a function of sucrose /water solution concentration.On this graph, the equation y=0.0015x+1.3299 is used to t the curve Figure 5 shows the resonance wavelength as a function of the refractive index for a gold-coated sensor.On this gure, the equation y=0.0014x+0.6333 is used to t the curve.

Figure ( 6
Figure (6) explain the surface Plasmon resonance for a manufactured sensor with a gold layer at different refractive indexes of toxic ions Lead in water (sensing medium).With a variable refractive index

Table ( 2
)Concentration and refractive index values for different Lead resonance wavelengths.