tailieunhanh - Advances in Optical Amplifiers Part 14

Tham khảo tài liệu 'advances in optical amplifiers part 14', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 376 Advances in Optical Amplifiers Kashyap 2010 . FBGs are reflective type filters with slightly periodic refractive index modulation running along fiber axis in the fiber core. Incident wavelengths are reflected when the Bragg conditions ẦB neffA where ẦB is the Bragg wavelength neff is the effective modal index and A is the grating period are satisfied and otherwise they are transmitted. The typical grating period is around pm the reflection bandwidth is around nm the reflectivity is larger than 99 20 dB and the insertion loss is less than dB for reflective applications at the C-band optical communication window. Environmental stability for fiber gratings was originally a big issue but can be controlled now by a suitable annealing process and an appropriate package. FBGs are usually fabricated by inscribing the periodical intensity of UV lights onto the photo-sensitive fiber core to induce the permanent periodical refractive index change. The photosensitivity of the fiber core is mainly caused by formation of color center or densification and increase in tension. Various laser light sources have been used to induce refractive index changes in optical fibers. The commonly used pulse lasers are KrF 248 nm ArF 193 nm and Ti Sapphire 800 nm while the commonly used continuous wave laser is the frequency-doubled Ar-Ion laser 244 nm . Under the intensities of 100-1000 mJ cm2 the amount of induced refractive index change in germanium doped optical fibers is around 10-5-10-3. Higher index changes can be achieved by hydrogen loading in high pressure Hill et. al. 1997 . Fig. 1. Various applications of fiber Bragg gratings. Kashyap 2010 Establishing the easy-realized FBGs technology is promising and very useful in various photonic industries. Several fabrication schemes have been proposed for FBG inscription by forming interference light fringes including the common-used phase mask method and holographic method. In the phase mask technique the two first .