Abdul-Razak: Transmission Spectrum Shifting By Varying Ambient Refractive Index ----- 131 Transmission Spectrum Shifting By Varying Ambient Refractive Index in LPFG

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].


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 L LPG  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]. The excited cladding mode attenuates in the coated fiber part after the grating, which results in the appearance of resonance loss in the transmission spectrum.
The interaction of one mode of a fiber with other modes is commonly described with the help of coupled-mode theory in which only two modes are supposed to be nearly phasematched and capable of resonant coupling. Based on this theory, quantitative information about the coupling coefficients and spectral properties of fiber grating can be obtained [4, 5, 6, and 7].

3-Results and discussion:
Here, we use matlab program version 6.5 to simulate and design the transmission characteristics of an LPFG coupler are the normalized energies of the core and cladding modes, respectively, h is the coupling coefficient , d is The normalized frequency.
At the exact resonance d=0 gives a sinusoidal law of the energy exchange, showing a possibility of mutual energy transfer from one mode to another[1]: The effective indices of the cladding modes clad eff n are dependent on the cladding index and the index of the surrounding (outer) ambient environment ( am n )(see fig.1). This means that the nth cladding mode coupling wavelength will change as the index of the surrounding environment changes. Cladding modes are most accurately calculated using three-layer modes for case of clad clad eff am n n n   and irradiated part within one grating period =0.5e -3 m [1]. In this paper we focus on modeling of the leaky configuration (i.e. when n am > n clad ).
In the two-layer model, the power attenuation coefficient of the first order cladding mode being calculated over a wide range of n am . Figure (3) shows that the most sensitive regions are when n am is close to n clad and when n am changes from 1.279 to 1.489 (i.e. ∆n≈ 0.2). The power attenuation coefficient shift shows a dramatic change from a sharp increase of 0.00 dB to a sharp of 0.03788 dB. This "switching property ", in this sensitive region, has a potential application in optical communications and optical sensors(i.e. work region of our LPFG model will be only in sensitive region). Fig.(4) shows the same effect result but with change to the core refractive index, cladding refractive index and surrounding refractive index (n am >n clad ).From the equation (5)[1], show the relationship between input signal wavelength and index ambient :- Where A is fiber core area(1-10)µm 2 . From the above equation we can design our the LPFG modeling in order to be used as a sensor. Our model consists of index ambient (1.4 to 1.48) range from, that is larger than cladding index (i.e. ∆n≈ 0.2 ). Figs(5,6,7,8)   From the figs and the table above notice the difference between cladding refractive index and surrounding refractive index must be ∆n≈ 0.2, i.e. working region will be in sensitive region ( eff amb N > clad N ).we will use our model as a sensor(shift transmission spectrum profile). Figs.(9,11,13) illustrate the input signal power of LPFG. Figs.(10,12,14) present the shift process for transmission spectrum (output signal power of LPFG).    . In our modeling we focused on surrounding refractive index, to take any point from transmission spectrum we notice the shift value to the wavelength ≈ 15nm that equal 10* eff amb N ≈ 16.1.(example: figs.(9,10) the value at the wavelength 1510nm in input signal become the same value at shift wavelength 1525nm in output signal that shift wavelength equal ≈ 15nm ). Therefore, we will control on the shift wavelength to the input signal LPFG (rang fiber optical 1500nm to 1600nm) through the surrounding refractive index, in the other word LPFG is working as sensor.
The We take another example in shift transmission spectrum through taking another model from effective surrounding refractive index to power attenuation coefficient. Figs. (15, 16) show effective index ambient on power attenuation coefficient, we notice a dramatic change and sensitive region and power attenuation coefficient value at eff amb N ≈ 1.44.
From the figs.(15,16) and the table(3) notice the difference between cladding refractive index and surrounding refractive index must be ∆n≈ 0.2, i.e. working region will be in sensitive region ( eff amb N > clad N ). we will use another model as a sensor (shift transmission spectrum profile).  5 1.51 1.52 1.53 1.54 1.55 1.56 1.57 1.58 1.59 1.6 x 10  5 1.51 1.52 1.53 1.54 1.55 1.56 1.57 1.58 1.59 1.6 x 10   Figs.(18,20) presents the shift process for transmission spectrum profile(output signal power of LPFG).   5 1.51 1.52 1.53 1.54 1.55 1.56 1.57 1.58 1.59 1.6 x 10  5 1.51 1.52 1.53 1.54 1.55 1.56 1.57 1.58 1.59 1.6 x 10 . In another modeling we notice the shift value to the wavelength ≈ 5nm that equal 3.5* eff amb N ≈ 5.04.(example: figs.(17,19) the value at the wavelength 1585nm in input signal become the same value at shift wavelength 1590nm in output signal that shift wavelength equal ≈ 5nm ). Therefore, we will control on the shift wavelength to the input signal LPFG through the surrounding refractive index, in the other word LPFG its working as sensor. The

4-Conclusions:
In this paper, We have presented the LPFG model by changing the ambient refractive index higher than cladding refractive index (n am >n clad ). The results may come up with many points: 1. The first model (figs.5, 6, 7, 8) works over a wide range (1.4-1.6).i.e. many inputs at the same time. 2. The second model (figs 15, 16) works over a wide range (1.2-1.4).i.e. many inputs at the same time. 3. is presented effective ambient index to LPFG working where the difference between cladding refractive index and surrounding refractive index in our two modeling must be equal to ∆n ≈ 0.2. 4. In first modeling, the best working point, is when power attenuation coefficient is of a higher value ≈ 0.03788 dB.  5 1.51 1.52 1.53 1.54 1.55 1.56 1.57 1.58 1.59 1.6 x 10 In second modeling, the best working point, is when power attenuation coefficient is of a higher value ≈ 0.04675 dB and ≈ 0.06875 dB. 6-LPFG its working as sensor, therefor the input signal to this device is shifted, depending on the value of surrounding refractive index. In our two modeling shift values of the wavelength are equal 10* eff amb N ≈ 16.1 and 3.5* eff amb N ≈ 5.04.
7-In two our modeling notice power attenuation coefficient dependent on core index and clad index not only on surrounding refractive index, therefore, the shift value to input signal depends on this parameters, any change to this parameters causes change in the shift value of LPFG input signal wavelength. The work was carried out at the college of Engineering. University of Mosul