In t h~s paper, for the first to our knowledge, we have performed EDFA gain spectrum flattering effectively based on long period fiber grating (LPG), which is fabricated by moving fibedphase maskscanning beam techmque. The results show that the flat band-with of the related EDFA exceeding 30nm and the fluctuation is less than 0.6d.B.
I IntroductionEDFA (Erbium-Doped Fiber Amplifier) is essential in the near all-optical communication, especially in WDM networks. It can am?liQ the optical signal as well as own the characters of low noise and polarization senselessness. However, its un-flat gain spectrum (see Fig. 1) results in the power difference between channels[I]. In the technology of WDM+EDFAs, this become more serious because cascaded EDFAs have to be employed in the rather long &stance transmission systems. To realize the flat gain spectrum of EDFA has aroused great interest in recent years, mainly because of the unprecedented increase in the demand for transmission capacity The useable bandwidth of the signal spectrum can be significantly increased by using EDFA with a flattened gain spectrum [2] . There are two basic ways to flatten the gain spectrum: one is to tailor the material propernes of the erbiumdoped fiber. and the other is to use filler designed to approximate the inverse characteristics of the gain spectrum [3]. LPG is a type of transmission grating based on the principle of the coupling the p d e d fundamental mode to the fonvard propagating lossy cladding modes. so the LPG have found an important role in the EDFA gain spectrum flattering. In this paper, we use LPG made by our research group to flatten the EDFA gain spectrum, the results show that the gain spectrum bandwith is more than 30nm and with a fluctuation less than 0.6d.B I1 Moving fibedphase mask-scanning beam technique for fabricating LPGs Unlike short period uniform fiber Bragg grating (FBG), the LPG is a type of transmission grating, its period is generally several hundred um. The refractive index perturbation of LPG have a much greater periodicity than the wavelength of the light, which wallows this kind of grating to be easily manufactured [4 J. The phase mask approach to produce FBGs is attractive because it allows fiber gratings to be written with greatly relaxed tolerances on the coherence of the writing beam, and with better repeatability. One drawback, however, has been that the grating wavelength is dictated by the period of mask, with separate masks required for difference wavelengths. Some effort has gone into malang the phase mask approach more flexible, e.g. by using a magnifying lens to alter the fiber Bragg wavelength [j]. The use of a scanning writing beam has enabled the fabrication of long fiber gratings through modulation of the scanning beam [6]. Our experimental setup designed for writing the LPGs is similar to the above scanning writing beam configuration but whereby the fiber is slowly moved relative to the phase mask as the UV beam is scanning. Here, a key technique we considered and used, is the clamp employ...
