732 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 19, NO. 10, MAY 15, 2007 Optically Tunable Long-Period Fiber Grating on an Er Fiber A. Quintela, M. A. Quintela, C. Jauregui, and J. M. Lopez-Higuera, Senior Member, IEEE Abstract—An optically tunable long-period grating (LPG) on an erbium-doped fiber (EDF) is experimentally demonstrated. It is based on the core refractive index changes induced by the pump power on an LPG written in the EDF core. With an optimized de- sign of the LPG, a shift of 5.5 nm of the attenuation band wave- length is obtained with a pump power of 100 mW at 1480 nm. Index Terms—Erbium-doped fiber (EDF), long-period grating (LPG), optical tuning, pump power. I. INTRODUCTION L ONG-PERIOD fiber gratings (LPGs) can be obtained from a periodic perturbation of the core refractive index of the optical fiber with a period typically in the range of 100–2000 m. External parameters (temperature, mechanical stress, bending, among others) may modify the period of the LPG and/or the differential refractive index of the core and cladding modes. Therefore, changes in the central wavelengths of the attenuation bands are induced. Using the sensitivity to these external parameters, their wavelength tuning capacity can be employed to design fiber-optics components, such as fiber sensors, narrowband loss filters, including tunable filters, modulators of optical radiation, etc. [1]. For a long time, the development of optically tunable components has been an important aim for the scientific and technical communities. For this reason, an optically tunable LPG is proposed for the first time in this letter, where the central wavelength shifts of the attenuation band are optically induced. When the pump power is launched into an erbium-doped op- tical fiber (EDF), signal gain control can be obtained and, in addition, induced core refractive index changes can also be ob- served. This feature of the EDF has been theoretically studied and experimentally measured [2]–[4]. Based on this, property signal processing, fiber lasers, and amplified lightwave commu- nication systems [5], [6] devices and subsystems can be devel- oped. Moreover, EDFs are usually codoped with germanium and LPGs can be written in this fiber due to their UV photosen- Manuscript received December 12, 2006; revised February 20, 2007. This work was supported in part by the Spanish Government under the TEC2004- 05936-C02 and TEC2005-08218-C02-02 Projects. A. Quintela, M. A. Quintela, and J. M. Lopez-Higuera are with the Photonics Engineering Group, University of Cantabria, 39005 Santander, Spain (e-mail: quintela@unican.es). C. Jauregui is with the Optoelectronics Research Centre, University of Southampton, Southampton SO17 1BJ, U.K. Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2007.895889 sitivity. Tuning capacity from the optical-induced refractive index changes in an EDF LPG can, potentially, be obtained. In this letter, the optical tuning of an EDF LPG from the pump power level is for the first time, to our knowledge, experimen- tally demonstrated. II. THEORY In an LPG, the resonant coupling between the fundamental mode and one of the cladding modes is achieved when (1) is satisfied (1) where is the effective refractive index of the core propaga- tion mode, is the refractive index of the -cladding mode, is the resonance wavelength associated to the -cladding mode, and is the LPG period. In EDFs, through the Kramers–Kroning relations [7], a non- linear change of the core refractive index takes place as a con- sequence of the change in the absorption spectrum under the optical pumping. An almost wavelength-independent offset of the refractive index is also included in the overall change of the refractive index. Although several mechanisms have been sug- gested to explain this offset, it is commonly accepted that the absorptions in the UV are more significant [8], [9]. Their value is correlated with the level of erbium-doping concentration as have been suggested by other authors [10]. Assuming two energetic levels in an EDF, the core refractive index changes induced by the pump power are given by [10] (2) where is the doping concentration, is the confinement factor of the signal, is the signal wavelength, is the refrac- tive index, is the lifetime of the metastable state, is the pump wavelength absorption, is the EDF length, is the pump power, is the threshold pump power, and is the lineshape function, which is defined by (3) where is the full-width at half-maximum of the transition, is the optical frequency, and is the resonance frequency. According to (3), it can be observed that pumping power changes can produce corresponding variations in the refractive index. On the other hand, the variation of the EDF refractive 1041-1135/$25.00 © 2007 IEEE