The effect of temperature to the reflected signal of PMMA FBG in FTTH network

The need for an early detection of disaster has grown vital because of the rise in natural disaster events occurred in Malaysia recently, such as fire, landslides, and floods. Thus, an early warning system could somehow reduce the seriousness of disaster. Due to this, Polymer Optical Fiber (POF) has garnered a lot of interest, because of its promising features especially for becoming a sensing device. This paper study the effectiveness of polymer FBG sensor in Fiber-To-The Home (FTTH) network. The simulation was done using OptiSystem7.0 software. The changes of power(dBm) due to the temperature changes was tabulated. PMMA has shown a significant change in power with averagely 1.123 dBm drop for every 10°C increase of temperature. This indicates that PMMA have promising characteristics to become the main material for fabrication of FBG sensor.


Introduction
With recent extreme weather in Malaysia, natural disaster has happened more frequent and unpredicted.Disasters like floods, fires, and landslides have claimed lives while ruining infrastructure and residential buildings.A study has indicate that flood, typhoon, and epidemic disasters significantly reduce the income of the nation [1].This aftermath of the natural disaster was huge and fatal due to the incompetency of the early detection system.
Due to this, the possibility of using the existing fiber optics network as a sensing device has been raised.The capabilities of FBG sensor in has been studied extensively.It has been studied that the FBG can provide temperature sensitivity of 0.108 nm/ C. [2] [3].However, the test did not show the changes in power (dBm) of the fiber.By monitoring power changes (dBm) it would be much easier to predict changes in real time.Furthermore, the test was done for a single FBG unit and not in the FTTH network.Thus, this research will simulate the effect of polymer FBG in FTTH network and analyze the effect of temperature change to the reflected signal power (dBm).

Poly-methyl methacrylate (PMMA)
PMMA is the most often used polymer optical fiber material due to its high quality, transparency, low cost, and ease of processing [4].In addition, the recycled PMMA can be used for the fabrication of optical fiber [5] and thus reduce the cost of fabrication and promotes sustainability.
In addition to that, the ability to dope PMMA for the development of fiber-based devices such as lasers and amplifiers, as well as the opportunity to improve photosensitivity for effective fiber Bragg  [6] .The key advantage of adopting PMMA polymer is that it is extremely temperature sensitive [7].The fundamental downside of PMMA is its high attenuation losses, which can reach 100 dBm in the 1300-1650 nm wavelength interval [8], which is commonly employed in optical communication applications.

Simulation design
The design for the sensing system uses FTTH network.The FBG sensor is located at each of the customer-end (ONU).The FBG is being connected in series configurations.The transmitted signals are 1310nm, 1490nm, and 1550nm for triple play service and a white light with 1625nm wavelength is being transmitted for the purpose of monitoring.This simulation uses 20km fiber cables which then ended up at the customer Optical Network Unit (ONU).The change of temperature is being simulated by changing the refractive index of the FBG and the reflected power is monitored.

Results and Discussion
Based on the simulation results of temperature ranges from 20°C to 100°C, the reflected signal power is showing a decreasing trend.This indicates that the higher the temperature, the lower the reflected power of PMMA FBG.In general, when a polymer's temperature rises, its refractive index decreases.This behavior can be explained by the polymer's thermal expansion and the resulting changes in density and molecular interactions.The refractive index values are important because they will affect the reflected power of the FBG, which will then give the indication of temperature changes.
The thermo-optic coefficient can be used to quantify the temperature dependency of the refractive index.The change in refractive index per unit change in temperature is represented by thermo-optic coefficient.Different polymers can have varied thermo-optic coefficient values, and it is frequently stated for a specific temperature range.
The reflected power is monitored by Optical Spectrum Analyzer (OSA).This reflected power can be monitored in real-time, so early prediction of fire (in specific location) can be done by installing PMMA FBG at each ONU location.However, we discovered that the reduction in power loss does not correspond to a drop in temperature.For example, the power reduction from 20°C to 30°C is 1.331dBm, while it is just 0.983dBm from 30°C to 40°C.This shows us that the relationship of refractive index with temperature is not perfectly linear.Nonetheless, this is not a severe issue because a monitoring system should capture any changes and any little difference should be clearly noticeable.

Conclusions
PMMA has been investigated for use as a sensing element in FBG.Based on the simulation results, we believe that any decrease in reflected power should provide us with an early warning of a potential fire at a specific location (customer ONU).It is concluded that PMMA exhibits a considerable change in the FBG reflected signal power, with a reduction of 1.123dBm(average) for every 10°C increase in temperature.This is consistent with the study done by Beadie et al. [9] where the PMMA had shown the same behavior.This suggests that PMMA has great potential to become the primary material for the development of FBG sensors.In addition to that, Ren et al. [10] has experimented the possibilities to further enlarge the refractive index changes, which will then increase the sensing sensitivity.Nonetheless, it is important to note that the temperature dependency of the refractive index is not uniform for all polymers and might vary based on factors such as polymer composition and molecular structure.

Figure 1 .
Figure 1.Simulation of POFBG sensing in FTTH network using OptiSystem7.02.1.Simulation of PMMA as FBGIn order to simulates PMMA as an FBG material in Optisystem7.0, the refractive index of PMMA for certain temperature values is tabulated based on the study done by M Kovacevic et.Al[3].This study provides the data of refractive index vs temperature changes of PMMA polymer.

Figure2.
Figure2.The relationship between PMMA refractive index and temperature.

Table 1 .
The FBG reflected signal power reaction against Temperature changes