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eISSN: 2576-4543

Physics & Astronomy International Journal

Research Article Volume 3 Issue 2

Formation and characterization of non uniform reversible optical fiber gratings with single and double sided loading

Sunita P Ugale

KK Wagh Institute of Engineering Education and Research, Nashik, India

Correspondence: Sunita P Ugale, KK Wagh Institute of Engineering Education and Research, Nashik, India

Received: July 24, 2017 | Published: March 14, 2019

Citation: Ugale SP. Formation and characterization of non uniform reversible optical fiber gratings with single and double sided loading. Phys Astron Int J. 2019;3(2):66-68. DOI: 10.15406/paij.2019.03.00159

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Abstract

In this paper the formation and characterization of non uniform single side loaded and double side loaded optical gratings is done. The non uniform long period reversible optical grating is practically induced in communication grade single mode fiber. In reversible optical fiber gratings the fiber is subject to periodical stress, which results in alternated regions under compression and stretching that modulate the refractive index via the photo elastic effect. The spectral responses of single side loaded optical grating is then compared with double side loaded optical grating.

Keywords: reversible gratings, non uniform grating, single sided loading, double sided loading

Introduction

Long period fiber grating (LPFG) can be realized with permanent modification of fiber such as UV induced refractive index changes and etching or temporary alternation of fiber propagation characteristics. LPFG by UV light exposure with amplitude phase mask are popular, but their spectra can hardly be tuned once they have been fabricated, which may limit applications of LPFG. The temporary or reversible grating can be implemented through the application of acoustic wave to the fiber1,2 or periodical loading onto the fiber.3 Reversible optical fiber gratings need neither a special fiber nor an expensive writing device for fabrication. These gratings also offer advantages of being simple, inexpensive, erasable, reconfigurable, and also give flexible control of transmission spectrum. Depending upon refractive index profile and grating period variation, gratings are of different types. The grating period can be uniform or graded, and either localized or distributed in structure. Uniform optical fiber grating yields highly undesirable side-lobes due to the sharp boundaries of the grating.4 A well-discussed method to reduce these side lobes is to apodize the grating coupling strength along the grating by gradually tapering the refractive index modulation amplitude to zero at both end of the grating. Periodic refractive index perturbations in an optical grating with single sided loading could be primarily stress induced, since the points of stress would be those where the plate presses the fiber against the grooved plate. While in an optical grating with double sided loading refractive index perturbation is due to the periodic microbend.

Mathematical analysis

Non uniform (apodized and chirped) gratings are modeled mathematically in the following section.4 Consider a bare three-layer step-index fiber that has a core with an index of 1.45 and a radius of 4.15μm, a cladding with an index of 1.444 and a radius of 62.5μm. The surrounding medium is air. A non uniform grating in this fiber is divided into a number of concatenated uniform grating sections, and the coupling strength, period and the resonance wavelength are allowed to vary from section to section. It has following effective index variation along the core of a fiber as shown in Figure 1.

Figure 1 Multisection Apodized LPFG with (a) varying average index along the length of grating    (b) varying period along the length of grating.4

Δ n ( z ) = Δ n d c ( z ) + Δ n a c ( z ) sin 2 π z Λ + φ MathType@MTEF@5@5@+= feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaqcfaOaeuiLdq KaamOBaiaacIcacaWG6bGaaiykaiabg2da9iabfs5aejaad6gadaWg aaqaaiaadsgacaWGJbaabeaacaGGOaGaamOEaiaacMcacqGHRaWkcq qHuoarcaWGUbWaaSbaaeaacaWGHbGaam4yaaqabaGaaiikaiaadQha caGGPaGaci4CaiaacMgacaGGUbWaaeWaaeaadaWcaaqaaiaaikdacq aHapaCcaWG6baabaGaeu4MdWeaaiabgUcaRiabeA8aQbGaayjkaiaa wMcaaaaa@565A@ (1)

Where z is distance along fiber 0     z     L MathType@MTEF@5@5@+= feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaqcfa4aaeWaae aaqaaaaaaaaaWdbiaaicdacaqGGaGaeyizImQaaeiiaiaadQhacaqG GaGaeyizImQaaeiiaiaadYeaa8aacaGLOaGaayzkaaaaaa@40B9@ , L is grating period, L is grating length, Δ n d c z MathType@MTEF@5@5@+= feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaqcfaieaaaaaa aaa8qacqqHuoarcaWGUbWdamaaBaaabaWdbiaadsgacaWGJbaapaqa baWaaeWaaeaapeGaamOEaaWdaiaawIcacaGLPaaaaaa@3DC1@ is background index variation, Δ n a c z MathType@MTEF@5@5@+= feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaqcfaieaaaaaa aaa8qacqqHuoarcaWGUbWdamaaBaaabaWdbiaadggacaWGJbaapaqa baWaaeWaaeaapeGaamOEaaWdaiaawIcacaGLPaaaaaa@3DBE@ is grating amplitude variation, φ MathType@MTEF@5@5@+= feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaqcfaieaaaaaa aaa8qacqaHgpGAaaa@385E@ is a constant phase. The dc component Δ n d c z MathType@MTEF@5@5@+= feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaqcfaieaaaaaa aaa8qacqqHuoarcaWGUbWdamaaBaaabaWdbiaadsgacaWGJbaapaqa baWaaeWaaeaapeGaamOEaaWdaiaawIcacaGLPaaaaaa@3DC1@ can be positive or negative, while the ac amplitude Δ n ac z MathType@MTEF@5@5@+= feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaqcfaieaaaaaa aaa8qacqqHuoarcaWGUbWdamaaBaaabaWdbiaadggacaWGJbaapaqa baWaaeWaaeaapeGaamOEaaWdaiaawIcacaGLPaaaaaa@3DBE@ is taken to be positive. Δ n dc z MathType@MTEF@5@5@+= feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaqcfaieaaaaaa aaa8qacqqHuoarcaWGUbWdamaaBaaabaWdbiaadsgacaWGJbaapaqa baWaaeWaaeaapeGaamOEaaWdaiaawIcacaGLPaaaaaa@3DC1@ and Δ n ac z MathType@MTEF@5@5@+= feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaqcfaieaaaaaa aaa8qacqqHuoarcaWGUbWdamaaBaaabaWdbiaadggacaWGJbaapaqa baWaaeWaaeaapeGaamOEaaWdaiaawIcacaGLPaaaaaa@3DBE@ are slowly varying functions of z. The grating has 8 sections with lengths z1, z2, z3, z4 and z4, z3, z2, z1 MathType@MTEF@5@5@+= feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaqcfaieaaaaaa aaa8qacaWG6bGaaGymaiaacYcacaqGGaGaamOEaiaaikdacaGGSaGa aeiiaiaadQhacaaIZaGaaiilaiaabccacaWG6bGaaGinaiaabccaca WGHbGaamOBaiaadsgacaqGGaGaamOEaiaaisdacaGGSaGaaeiiaiaa dQhacaaIZaGaaiilaiaabccacaWG6bGaaGOmaiaacYcacaqGGaGaam OEaiaaigdaaaa@5077@ with different periods. Over the spectral range of interest, LP01 core mode couples to 2, LP0m (m>1) cladding modes. A nominal resonance wavelength of ideal apodized LPFG that has Δ n dc z =0 MathType@MTEF@5@5@+= feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaqcfaieaaaaaa aaa8qacqqHuoarcaWGUbWdamaaBaaabaWdbiaadsgacaWGJbaapaqa baWaaeWaaeaapeGaamOEaaWdaiaawIcacaGLPaaacqGH9aqpcaaIWa aaaa@3F81@ is given by following equation,

λ 0 = N 01 ( λ 0 ) N 0 m ( λ 0 ) Λ MathType@MTEF@5@5@+= feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaqcfaOaeq4UdW 2aaSbaaeaacaaIWaaabeaacqGH9aqpdaWadaqaaiaad6eadaWgaaqa aiaaicdacaaIXaaabeaacaGGOaGaeq4UdW2aaSbaaeaacaaIWaaabe aacaGGPaGaeyOeI0IaamOtamaaBaaabaGaaGimaiaad2gaaeqaaiaa cIcacqaH7oaBdaWgaaqaaiaaicdaaeqaaiaacMcaaiaawUfacaGLDb aacqqHBoataaa@4B43@ (2)

Where N01 and N0m are the effective indices of LP01 and LP0m modes of fiber in absence of grating. In presence of average index variation effective indices and grating periods in different sections are in general different, which results in different resonance wavelengths. The resonance wavelength in ith section is

λ 0i = N 01 ( λ 0i ) N 0mi ( λ 0i ) Λ MathType@MTEF@5@5@+= feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaqcfaOaeq4UdW 2aaSbaaeaacaaIWaGaamyAaaqabaGaeyypa0ZaamWaaeaacaWGobWa aSbaaeaacaaIWaGaaGymaaqabaGaaiikaiabeU7aSnaaBaaabaGaaG imaiaadMgaaeqaaiaacMcacqGHsislcaWGobWaaSbaaeaacaaIWaGa amyBaiaadMgaaeqaaiaacIcacqaH7oaBdaWgaaqaaiaaicdacaWGPb aabeaacaGGPaaacaGLBbGaayzxaaGaeu4MdWeaaa@4EFB@           (3)

λ 0i MathType@MTEF@5@5@+= feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaqcfaieaaaaaa aaa8qacqaH7oaBpaWaaSbaaeaapeGaaGimaiaadMgaa8aabeaaaaa@3A4C@  is larger or smaller than λ0 depends on sign of dc index change and sign of modal dispersion factor γ.

λ 0 i = λ 0 + γ Δ n d c i ( η 01 η 0 m ) Λ MathType@MTEF@5@5@+= feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaqcfaOaeq4UdW 2aaSbaaeaacaaIWaGaamyAaaqabaGaeyypa0Jaeq4UdW2aaSbaaeaa caaIWaaabeaacqGHRaWkcqaHZoWzcqqHuoarcaWGUbWaaSbaaeaaca WGKbGaam4yaiaadMgaaeqaaiaacIcacqaH3oaAdaWgaaqaaiaaicda caaIXaaabeaacqGHsislcqaH3oaAdaWgaaqaaiaaicdacaWGTbaabe aacaGGPaGaeu4MdWeaaa@4FCB@ (4)

γ= 1Λ( d N 01 dλ d N 0m dλ ) MathType@MTEF@5@5@+= feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaqcfaOaeq4SdC Maeyypa0ZaamWaaeaacaaIXaGaeyOeI0Iaeu4MdWKaaiikamaalaaa baGaamizaiaad6eadaWgaaqaaiaaicdacaaIXaaabeaaaeaacaWGKb Gaeq4UdWgaaiabgkHiTmaalaaabaGaamizaiaad6eadaWgaaqaaiaa icdacaWGTbaabeaaaeaacaWGKbGaeq4UdWgaaiaacMcaaiaawUfaca GLDbaaaaa@4CB8@   (5)

Where

η 01 &  η 0m MathType@MTEF@5@5@+= feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaqcfaieaaaaaa aaa8qacqaH3oaApaWaaSbaaeaapeGaaGimaiaaigdaa8aabeaapeGa aiOjaiaabccacqaH3oaApaWaaSbaaeaapeGaaGimaiaad2gaa8aabe aaaaa@3F15@  are confinement factor of LP01 and LP0m mode in core area.

Δ n dc (z)= a 0 A(z) Δ n ac (z)= a 1 A(z) MathType@MTEF@5@5@+= feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGceaqabeaajuaGcq qHuoarcaWGUbWaaSbaaeaacaWGKbGaam4yaaqabaGaaiikaiaadQha caGGPaGaeyypa0JaamyyamaaBaaabaGaaGimaaqabaGaamyqaiaacI cacaWG6bGaaiykaaGcbaqcfaOaeuiLdqKaamOBamaaBaaabaGaamyy aiaadogaaeqaaiaacIcacaWG6bGaaiykaiabg2da9iaadggadaWgaa qaaiaaigdaaeqaaiaadgeacaGGOaGaamOEaiaacMcaaaaa@502E@         (6)

Where A (z) is normalized apodization profile function, a0 & a1 are amplitudes of corresponding index changes. In an apodized LPFG having dc index variation, the spread of the resonance wavelength depends on value of modal dispersion factor γ. For a typical fiber, γ is positive for low-order cladding modes and negative for high-order cladding modes [5]. For a positive dc index change, a positive γ gives red shift in resonance wavelength i. e. a red shift is observed for low order cladding modes. While for a negative dc index change, a negative γ gives blue shift in resonance wavelength i. e. a blue shift is observed for high order cladding modes.6

Characterization of non uniform gratings

Hardened steel block of 7x2x1cm (L x W x T) with periodical corrugations on L x W plane were prepared. Circular grooves of 50% duty cycle, 800μm in depth and varying period (600-2400μm) were fabricated with mechanical processes. A commonly used communication fiber (corning SMF-28) is used in experimentation. The experimental set up is as shown in Figure 2. Single sided loading is done; the bottom plate is plain, while the upper plate is specially designed corrugated plate. For characterization the light from broadband superluminacent LED with output power of -8dBm, center wavelength 1530nm and bandwidth of 69nm was passed through the grating under test and the transmitted signal was analyzed with the help of optical spectrum analyzer covering the wavelength range from 1250nm to 1650nm. Coupling coefficient can be adjusted through changing the pressure applied on the fiber. Because of photo-elastic effect and microbend effect, the pressure controls the refractive index modulation depth, and changes the coupling state between the core and the cladding modes.37 Another way to obtain apodization is by changing the pressure distribution along the grating length.8 Respective changes in ac and dc index are shown in Figure 1.

Figure 2 Experimental set up for characterization of single sided loading of fiber.

Double sided loading in reversible gratings

Fill In an optical grating with double sided loading refractive index perturbation is due to the periodic microbend as shown in Figure 3. The experimental set up is as shown in Figure 4. The bottoms as well as the upper plate are identical corrugated plates with 1200µm plain corrugations. For characterization the light from broadband superluminacent LED was passed through the grating under test and the transmitted signal was analyzed with the help of optical spectrum analyzer covering the wavelength range from 1250nm to 1650nm.The positions and depths of transmission spectral depressions varied with pressure applied on grating as shown in Figure 5 and Figure 6.

Figure 3 Fiber deformations in (a) single sided loading (b) double sided loading for π relative phase between two plates (c) double sided loading for 0 relative phase between two plates (crest to crest matching).

Figure 4 Fiber deformations in (a) single sided loading (b) double sided loading for π relative phase between two plates (c) double sided loading for 0 relative phase between two plates (crest to crest matching).

Figure 5 Transmission spectrum of reversible LPFG (L=1200μm) with π relative phase.

Figure 6 Transmission spectrum of reversible LPFG (L=1200μm) with 0 relative phase.

Conclusion

In double sided loading the positions and depths of transmission spectral depressions varied with pressure applied on grating. Another interesting observation in double sided grating is the appearance of new peaks. In this configuration, one clearly induces microbends in the fiber, which is different from the case where the fiber is pressed with a flat surface on one side. When the two periodical structures are exactly in phase i. e. crests are exactly matched, all spectral depressions start gradually disappearing (broadened and reduced amplitude) with increase in load.

Acknowledgments

This work is partially supported by the Department of Science and Technology of India.

Conflict of interest

The author declares there is no conflict of interest.

References

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