Sensitivity Comparison of Refractive Index Transducer Optical Fiber Based on Surface Plasmon Resonance Using Ag, Cu, and Bimetallic Ag–Cu Layer by Zakaria, Rozalina, Zainuddin, Nur Aina'a Mardhiah, Raya, Sofiah Athirah, Alwi, Siti Anis Khairani, Anwar, Toni, Sarlan, Aliza, Ahmed, Kawsar, Amiri, Iraj Sadegh