Theoretically And Experimentally Study On Photorefractive Spatial Dark Solitons Splitting

Photorefractive (PR) spatial solitons is the result of PR effect balanced thediffraction. Since the prediction and the observation of PR spatial solitons, it hasattracted more attention in the formation and characters of PR spatial solitons owingto the potential application in beam steering and manipulating light by light, all-optical switching, optical interconnects and optical nonlinear devices et al.In this paper, I theoretically studied the multiple incoherent gray screeningsolitons splitting and incoherent screening-photovoltaic dark (gray) solitons splittingby using of coherent density method. I investigate the time-dependent coherentphotovoltaic spatial solitons splitting under open-circuit conditions using beampropagation method. In addition, I experimentally study the incoherent photovoltaicsolitons splitting in LiNbO3:Fe crystal without background light. The detailedconclusions as follow:1. Experimental study on incoherently photovoltaic solitonsA dark stripe is generated when the partially spatially incoherent beam passesthrough amplitude mark without background illumination. The dark stripe is imagedinto the front face of the crystal. The even-number sequence of incoherent darkphotovoltaic solitons is generated. The experimental results indicate that theincoherent photovoltaic soliton sequence can be generated in non-instantaneousnonlinearity photorefractive photovoltaic crystal and that the number of solitons isdependent on the initial input width of the dark stripe and the coherence length of thebeam.For a fixed beam’s spatial coherence, the number of solitons is dependent to theinitial input width of the dark stripe. Only a Y-junction solitons pair can be observedwhen the input width is small. With the increasing of the initial input width, theeven-number sequence of dark photovoltaic solitons is generated. The more solitonsare formed for wider initial input dark stripe, which is similar to coherent solitonsplitting. When more solitons are generated, the separations between adjacent dark solitons become smaller and the grayness of solitons is grayer as their transversedistance from the center increases. In addition, the coherence length of the incoherentbeam affects the number of solitons splitting. The bigger the coherence length of thebeam, the smaller the input width of the dark stripe needed.2. Incoherent screening grey solitonsWe study the propagation properties and spatial coherence of multiple incoherentscreening grey soliotns in biased photorefractive media by using the coherent densitymethod. The numerically studies results demonstrate that even-number sequence ofmultiple incoherent gray screening solitons is possible.We find that when the external voltage is small, only an incoherent Y-junctionscreening soliton is generated, corresponding to the lowest order in the even-numberincoherent screening soliton sequence. When the external voltage is increased, theincoherent gray beam splits into an even-number sequence of multiple gray screeningsoliton. For a fixed beam’s grayness, increasing the external voltage and the full widthhalf maximum of the beam’s intensity can produce such an even-number sequence ofmultiple incoherent gray screening solitons. Moreover, for a fixed full width at halfmaximum of the initial input beam’s intensity, increasing the external voltage anddecreasing the beam’s grayness still produces such an even-number sequence ofmultiple incoherent gray screening solitons.In addition, we find that the coherence length of multiple incoherent grayscreening solitons becomes higher around the soliton regions and that the smaller theminimum intensity of multiple incoherent gray screening solitons, the bigger itscorrelation length. We also find that the correlation length is changeless far away fromthe solitons.3. Incoherent dark photovoltaic solitonsWe study the formation of the multiple dark photovoltaic solitons splitting inphotorefractive photovoltaic nonlinear media under open-circuit condition by using ofbeam propagation method. The simulated results indicate that a single dark stripe onan otherwise uniform optical beam can be split into odd (or even) number sequence ofdark photovoltaic solitons in the photovoltaic photorefractive nonlinear media according to different initial conditions.We find that for a given physical system, the formation of multiple darkphotovoltaic solitons is depended on the initial input conditions as well as on the fullwidth at half maximum (FWHM) of the input beam’s intensity. On one hand, anodd-phase input beam always tends to split into the odd-number sequence of multipledark photovoltaic solitons, whereas the even-phase input beam tends to split into theeven-number sequence of multiple dark photovoltaic solitons. On the other hand,when the initial input width of the dark stripe is small, it only can form a foundationalsolitons or a Y-junction solution pair. With the increasing of the input width of thedark stripe, it can split into multiple dark photovoltaic solitions. The wider the inputwidth of the dark stripe, the more the number of solitons is generated. When moresolitons are generated, the separations between adjacent solitons become smaller andthe visibility of the solitons far away from the center decrease.4. Incoherent screening-photovoltaic solitonsWe study the propagation properties and the spatial coherences of the incoherentdark and gray screening-photovoltaic spatial solitons in biased photorefractive-photovoltaic crystals by using the coherent density method. The results of numericalsimulation demonstrate that the incoherent screening-photovoltaic dark (or gray)solitons can be generated in biased photorefractive photovoltaic crystal.For the incoherent screening-photovoltaic spatial dark solitons, formation of it isdependent on the initial input conditions as well as the initial input width of the darkstripe. Under the odd or even initial conditions, the odd or even number sequence ofincoherent screening-photovoltaic solitons is generated throught increasing theintensity FWHM of the optical beam. The wider the input width of the dark stripe, themore the number of incoherent dark screening-photovoltaic solitons is generated.When the more incoherent solitons are generated, the separations between adjacentdark solitons become smaller and the solitons far away from the center become lessvisible. For a given physical system and splitting number, the separations betweenadjacent dark solitons decrease with the increasing of width of the dark stripe.For the incoherent gray screeing-photovoltaic solitons, its formation is effected by the incoherent beam’s grayness and the FWHM of the incoherent beam’s intensity.Under even initial conditions, if the incoherent beam’s grayness or FWHM of theincoherent beam intensity is small, only a Y-junction soliton is generated. Withincreasing of the grayness of the incoherent beam or FWHM of the incoherent beam’sintensity, the incoherent multiple gray screening-photovoltaic solitons can be formedin the biased photorefractive-photovoltaic crystal.The coherence length of the incoherent beam is invariable far away from thesoliton region; however the coherence length is tempestuously changed, in particular,the coherence length of the incoherent beam becomes higher within the center solitonpair, with a depression an the center.In addition, we study the characters of the dark and gray incoherent screening-photovoltaic spatial solitons in biased photorefractive-photovoltaic crystals. We foundthat it translates into the dark incoherent screening soliton splitting when the bulkphotovoltaic effect is neglectable and the dark incoherent photovoltaic solitonsplitting when the external bias field is absent.