Design of High Performance Triangular Fiber Bragg Grating Temperature Sensor using OptiSystem and OptiGrating Software

This paper presents on the design of high performance triangular Fiber Bragg Grating (FBG) as temperature sensor. This triangular FBG is designed and simulated using OptiGrating Software. The Triangular FBG design is exported to OptiSystem Software. In the OptiSystem software, the overall triangular FBG temperature sensor system is designed and simulated. It found that the triangular FBG has higher temperature sensitivity which is 37.5pm/°C than the standard FBG which is around 13pm/°C. It also has high linearity of 99.2% which specifies the consistency of wavelength shift as the temperature is increased. It also has higher resolution due to its sharp peak and smaller bandwidth.


1.
Introduction Optical fiber sensors based on fiber Bragg gratings (FBG) have attracted a lot of attention in recent years, and they are now becoming an industry standard for a wide range of applications in important industries including transportation, energy, safety, security, and medicine.In terms of tolerance to electromagnetic interference, dynamic response, multiplexing, and embedding capabilities, they implement multiple sensors, making them ideal for smart structures and structural health monitoring [1][2].Fiber Bragg gratings (FBGs) are extensively utilized in a broad range of applications, spanning from optical communications to optical sensing, and have a long history [3].FBG based sensors have shown to be an effective option for monitoring tasks in hazardous environments.Because of its self-referencing capabilities, large-scale multiplexing capabilities, and tolerance to electromagnetic interference, FBG is being recognized as an essential sensor technology.Wavelength division multiplexing (WDM) and time-division multiplexing (TDM) techniques have been widely applied to FBG arrays.However, they have restrictions in terms of optical bandwidth and low signal-to-noise ratio (SNR) which will limit the resolution and multiplexing abilities.WDM is a technique using a sequence of high reflectivity narrow bandwidth FBGs with different center wavelengths and suitable frequency spacing.A broadband laser source is used to probe these FBGs [4].In contrast, TDM uses a single narrowband laser source to interrogate a sequence of FBGs with identical center wavelengths.The TDM FBGs typically have low reflectivity along the optical cable.Using an optical to electrical converter, the reflected signals from the FBGs are transformed to electrical signals [5].WDM is a popular technique due to its intuitionistic wavelength demodulation method.With a limited bandwidth of less than 100 nm for broadband light sources, typically only tens of sensors can be effectively multiplexed in one fiber.In TDM, equal wavelength FBG sensors with different time delay between reflected pulses along the fiber are detected.Thus, it is more potential for higher number of multiplexing sensors [5].In sensor applications, the FBG is employed in the sensor head to detect changes such as strain and temperature [6].
The combination of OptiSystem, Optiwave's integrated solutions, and the inclusion of OptiGrating expands the application potential of FBGs and enhances their use in fiber optic sensing applications [7].Using OptiGrating, FBG can be designed and characterized.The integration of OptiGrating and OptiSystem allows a wider application scenario [8].The design and construction of FBG have lately gotten a lot of interest in the field of fiber optics.Recently, it has been proved that a new class of continuous evolutionary algorithms, known as covariance matrix adapted evolution strategy (CMAES), is successful in handling strongly linked multimodal optimization issues.In the optimization process, CMAES creates new population members by sampling from a probability distribution that was computed earlier in the process.Researchers in [9] studied two different design issues in order to illustrate the efficiency of the CMAES algorithm in the design of triangular FBG filters.The design of an FBG filter with a particular bandwidth is taken into consideration.
In this project, triangular FBG is designed using OptiGrating software and the design is exported to OptiSystem software to validate the design.In OptiSystem software, the overall setup for FBG temperature sensor is designed and simulated.By deploying triangular FBG temperature sensor, it is found that the triangular FBG sensor has better performance in terms of resolution and sensitivity.

2.
Research Methodology In this paper, the design of triangular FBG is simulated using OptiGrating software.The OptiGrating software was used to construct and construct triangular FBG sensors under a variety of temperature conditions.The developed FBG is exported as spectrum files that were readable by the OptiSystem tool, allowing them to be used in the sensor setup that was designed using OptiSystem software.The OptiGrating technique is utilized to create triangular FBG sensors with a bandwidth of 0.2 nm, a total chirp of 2 nm, a length of 50mm, and an index modulation of 9.1x10 -0.05 .After conducting the simulation using OptiGrating software, the design in exported to OptiSystem software.By varying the temperature, the FBG reflection spectrum is observed both using the OptiGrating and Optisystem software.
Figure 1 shows the FBG grating parameters that has been used for this design.Figure 2 shows the reflection and transmission spectrum of the FBG that has been simulated using Optigrating software.The blue line shows the triangular FBG reflection spectrum while the red line shows the transmission spectrum of the FBG. Figure 3 shows the grating index change versus the length of the FBG.This design is exported to OptiSystem software to be tested as FBG temperature sensor.A comparison of the linearity response of both of the developed FBG sensors as well as their sensitivity to the applied physical measurand was made.When it comes to temperature sensors, the first FBG, which is centered at 1549 nm, is devoted to non-temperature sensor measurements, and will be referred to as FBG1, while the second FBG, which is centered at 1550 nm and is exposed to temperature fluctuations, will be referred to as FBG2. Figure 2 shows the Grating Index change of the triangular FBG.The FBG temperature simulation model is represented in Figure 4 which includes white light source with wavelengths ranging from 1549 nm to 1553 nm, an optical circulator, single-mode optical fibers with lengths of 1 km and two triangular FBG sensors.The optical sources, each with a power of 5 dBm on average, are launched into the optical fiber through the circulator, where the reflected signal from the two FBG sensors, both designed by OptiGrating, is directed to the optical spectrum analyzers for analysis and decoding of the response signal.The two FBG1 and FBG2 that have been created by OptiGrating, as previously indicated in the preceding part, are uploaded into OptiSystem for further processing.In order to distinguish between the cross-sensitivity of the measurand, the FBG1 is assumed to have no temperature sensor applied, whereas the FBG2 is considered to have a temperature sensor applied.

Result and Discussion
The simulation results are obtained using OptiGrating and OptiSystem softwares.The temperature is varying in the OptiGrating software and the FBG reflection spectrum is observed in OptiGrating and OptiSystem software.Figure 5 shows the wavelength shift versus the applied temperature.The blue line graph shows the simulation result of the wavelength shift from the triangular FBG when various temperature is applied.From both the theoretical and simulation results, it is found that the wavelength increases as the temperature increase.The simulation result shows higher sensitivity of 37.5pm/°C than the standard FBG which is around 13pm/°C.It also has high linearity of 99.2% which specifies the consistency of wavelength shift as the temperature is increased.Figure 5 shows the reflection spectrum of normal FBG temperature sensor as the temperature increase from 30°C to 150°C.The uniform FBG sensor has wider bandwidth as compared to the triangular FBG which will limit the resolution of the temperature sensor.Figure 6 shows the simulation result of the reflection spectrum of the triangular FBG as the temperature increase.Triangular FBG has sharp peak which will produce higher resolution of temperature sensor as compared to uniform or normal FBG.

Conclusion
In conclusion, triangular FBG as temperature sensor has been successfully designed and simulated.Triangular FBG as temperature sensor has been designed using OptiGrating and the design is exported to OptiSystem software.In the OptiSystem software, the FBG temperature setup is designed and simulated.It is found that the triangular FBG has higher resolution and higher sensitivity of 37.5pm/°C than the standard FBG which is around 13pm/°C.It also has high linearity of 99.2% which specifies the consistency of wavelength shift as the temperature is increased.

Figure 1 .
Figure 1.Grating parameters used in OptiGrating software

Figure 2 . 3 .
Figure 2. Reflectivity and transmittivity Figure 3. Grating Index Change versus length of the triangular FBG centered at 1549nm

Figure 4 .
Figure 4. FBG Interrogation scheme as temperature sensor whereas WLS is White Light Source, SMF is Single Mode Fiber, OSA is Optical Spectrum Analyzer and FBG is Fiber Bragg Grating.

Figure 5 .
Figure 5.Comparison between theory and simulation of FBG wavelength shift for various temperature