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In this paper,we introduce a new model, of long-period fiber grating, by taking the value of ambient refractive index greater than the refractive cladding index, where the difference between them must equal(≈0.2).The results show the change in power attenuation coefficient, where it increases with the increase in refractive ambient index. The power attenuation coefficient shift shows a dramatic change of a sharp increase from 0.00dB to ≈ 0.03788dB and 0.00dB to ≈ 0.04675dB and 0.00dB to ≈ 0.06875 dB. Here this model is used as sensor for shifting in input signal (15nm, 5nm). Key Word: Long Period Fiber Grating, single mode fiber optical. لاسرلإا فيط تحازإ راسكنا مماعم رييغتب ا طيحمن ةرتفنا وذ يفيهنا زسحمنا يف يجراخنا تهيوطنا قازرنا دبع سـنوي ثارــف Furat_al2006@yahoo.com. ثاُجيشبنا تسذُه ىسق مصىًنا تعياج تـصلاخنا جرىًَ اُيذق ثحبنا ازه ٍف آ ذَذج آ ساسكَلاا مياعي تًُق زخا للاخ ٍي ,تهَىطنا ةشخفنا ور ٍفُهنا صضحًنا ٍي طُحًهن ٍجساخنا نا صضحًنا ةششقن ساسكَلاا مياعي ٍي شبكا ه تهَىطنا ةشخفنا ور ٍفُ ( ٌواسَ اًهُُب قشفنا ٌىكَ ثُحب , 2.0 )≈ . جئاخُنا جُُب ا شُغخنا ٍف مصاحن ٍُهىح مياعي ةسذقنا طُحًهن ساسكَلاا مياعي ٍف ةداَضنا عي دادضَ ٍُهىخنا مياعي ٌإ ثُح ، ٍجساخنا قهن ساسكَلاا مياعي ٍي ًهعأ ٌىكح ٍخنا ةشش ساسكَلاا مياعي عي بساُخَ ٍُعي يذي ًٍض ٍكنو ةششقن ا صضحًن ( ٍي دادضَ dB 2.22 ًنإ ≈ dB 2.23000 و 0.00dB ًنإ ≈ 0.04675dB و 0.00dB ًنإ ≈ 0.06875dB ) . ىح ذقن واذخخسا جرىًُنا ازه سسحخًك تحاصلان لاخدلإا ةساشأ ٍف 5nm) .(15nm, Received: 24 – 11 2010 Accepted: 7 – 6 2011 Al-Rafidain Engineering Vol.20 No. 2 March 2012 132 1-Introduction: Fiber Optic Gratings or Fiber Bragg Gratings (FBG) were first reported in 1978 by Hill et al. However, such devices only attracted the researcher’s attention in1989, when new production techniques allowed their use with optical communication wavelengths. Several methods exist for FBG production, among them the direct phase mask writing and phase mask interferometers stand out[1,2]. A new class of fiber grating called Long Period Fiber Grating (LPFG) was demonstrated by Vengsarkar et al in 1996.The name is due to the refractive index change periodicity from 100μm to 700μm, about 100 times larger than the values employed for FBG formation. This difference makes possible the use of amplitude masks instead of phase masks, resulting in lower manufacturing costs when compared to the FBG production’s costs. Besides this, the LPFG presents other surpassing characteristics such as low insertion losses, low back reflection, relatively simple fabrication, and a high sensitivity to changes in physical external parameters. These features made the LPFG outstanding devices for application such as band rejection filters and gain equalizing filters in optical communication, beyond its wide applicability as a fiber optic sensor. A long-period fiber grating, which couples light from fundamental guided core mode into co-propagating cladding modes at various wavelengths. The LPFG have also been used as gain-flattening filter and as optical fiber polarizer. Unlike FBG the cladding mode configuration of the LPFG is extremely sensitive to the refractive index of the medium surrounding the cladding, thus allowing it to be used as an ambient index sensor. The wavelength at which the coupling from core to cladding modes takes place is directly dependent on the difference between the core and cladding indices, the dimensions of the core and cladding and grating period, and any change in these values can shift the transmission spectral profile [1, 2, and 3]. 2-Long-Period Fiber Grating(LPFG): Long period gratings are fiber optics based devices made up of periodic changes in core’s refractive index [2]. Photo induced long-period fiber gratings (LPFG) with periods m LLPG  3 2 10 10   . LPG is transmission grating in which the coupling is between forward propagation core and cladding modes, propagation in the same direction. Typically in a single mode fiber (see figure 1) an LPFG couples the fundamental guided core mode to a copropagating cladding mode at a coupling (or resonance) wavelength [1, 2].

Transmission Spectrum Shifting By Varying Ambient Refractive Index in LPFG