Patent Application: US-35026909-A

Abstract:
an optical router for all - optical control over the propagation direction of optical pulses , comprising : a non - uniform one - dimensional photonic crystal receiving a plurality of input optical pulses , comprising : at least one first region used to obtain bragg solitons ; at least one second region in which non - linear interaction between two sufficiently adjacent solitons is obtained ; and at least one third region used to de - couple resulting after the interaction pulses outside the one - dimensional photonic crystal &# 39 ; s grating ; and a plurality of sufficiently temporally separated optical pulses launched towards said one - dimensional photonic crystal from either of its sides , such that the number of pulses de - coupled from at least one of the sides of the grating is different in case when interaction between the pulses occurs inside the grating , from the case when no interaction between pulse occurs inside the grating .

Description:
in the following detailed description of various embodiments , reference is made to the accompanying drawings that form a part thereof , and in which are shown by way of illustration specific embodiments in which the invention may be practiced . it is understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention . interaction between bragg solitons with the same polarization changes the frequencies of the interacting solitons . similar effect occurs in a standard fiber [ 11 ]. however , in fbgs , the high frequency selectivity of the grating can be used for utilizing the frequency changes in order to obtain logical operators . the lengths of the proposed devices can be about 15 cm , two orders of magnitude shorter then soliton - based optical devices in regular fibers [ 1 ]. the maximum power needed to operate the devices can be on the order of hundreds of watts . the gates can be cascaded in order to obtain complicated logical operators . the proposed devices operation is not sensitive to the phases of the input solitons . fig1 a and 1b illustrate the device operation for the and gate ( fig1 a ) and the not gate ( fig1 b ). the arrows represent the propagation directions of optical pulses in the fiber . the vertical lines arrays represent fbg . a “ one ” state is determined by the presence of a soliton . for the and gate , the input solitons are launched from the same side . when a single soliton is launched into the device , it is back - reflected . however , when two solitons with approximately the same parameters are launched , one of the solitons is transmitted while the other soliton is back - reflected . therefore , a “ one ” state , represented by the transmitted soliton , is obtained only when two solitons are launched . unlike the device reported in refs . [ 5 ], [ 6 ], the device of the invention operates with solitons that have the same polarization and hence it does not require using polarizers . the device of the invention is based on a soliton interaction that increases the frequency of one of the solitons and hence enables its transmission through the grating bandgap . this effect does not require that the input solitons will be launched at the same time , as needed in former works in order to strongly shift the bandgap frequency . [ 5 ], [ 6 ] therefore , an and operation is obtained even when the two solitons remain well separated along the whole device . in case of the not gate , two solitons are launched from opposite sides of the device . one of the solitons is the signal while the other soliton is the control soliton or the clock . the output signal is defined by the existence of pulse that exits the grating side , where the signal soliton was launched . therefore , the input and the output signals propagate in opposite directions . when the clock soliton is launched without the signal soliton , the clock is transmitted through the device and hence a “ one ” state is obtained . when two solitons are launched , no wave exits at the grating side where the signal was launched and hence a “ zero ” state is obtained . the and gate , made of a chirped fbg , is divided into three regions as shown in fig2 a . in region ( i ), the chirp of the grating is used to slow - down the soliton velocity . [ 8 ] when a single soliton enters the device it is back - reflected . when two solitons are launched , the spatial distance between the solitons decreases and a strong interaction is obtained in region ( i ). the frequency of one of the solitons is increased above bragg region of the grating and hence it overcome the bandgap and is able to be transmitted through the device . the frequency of the other soliton is decreased and hence it is back - reflected . region ( iii ) is used to obtain an output soliton with parameters similar to those of the input soliton . the chirp in this region has the same magnitude but has an opposite sign compared to that in region ( i ). simulations show that there is a need to add another uniform grating region ii , in order to stabilize the transmitted soliton . the not gate is a chirped fbg divided into three sections , as shown in fig2 b . the signal and the clock pulses are slowed down in regions i and iii , respectively . the chirp in these regions is designed to allow the transmission of the clock soliton when the signal soliton is not launched . when the two pulses are simultaneously launched , a strong interaction is obtained in region ii because of the slow velocities of the counter - propagating pulses in this region . in order to increase the duration of the soliton interaction , the shape of the chirp in region ii is designed to form an effective potential well for the solitons . in order to break the symmetry of the device for the two pulses , the frequency of the signal soliton is chosen to be slightly higher then the frequency of the clock soliton . due to the interaction , the frequency of the clock soliton is decreased . therefore , when both of the solitons are launched , the signal soliton is transmitted through the device while the clock soliton is trapped inside region ii and will be eventually absorbed and smeared . the propagation of pulses inside a fbg can be analyzed using the coupled - mode equations [ 3 ]: ± i ∂ z u ± + iv g − 1 ∂ n u ± + κu ∓ + γ (| u ± | 2 + 2 | u ∓ | 2 ) u ± + σ ( z ) u ± = 0 , ( 1 ) where u ± is the slowly varying amplitude of the forward (+) and the backward (−) propagating waves , σ ( z ) is the chirp parameter , κ is the grating amplitude , γ is the non - linear coefficient , v g = c / n eff is the fiber group velocity , and n eff = 1 . 45 is the effective refractive index . we solve the coupled - mode equations using the method described in ref . [ 9 ]. the lengths of the grating regions for the and gate , are l 1 = l 3 = 5 : 11 cm , l 2 = 2 : 34 cm , and hence the total grating length is equal to 12 . 55 cm . in the case of the not gate , the lengths of the grating regions are l 1 = l 3 = 5 . 03 cm , l 2 = 2 . 34 cm , and hence the total grating length is equal to 12 . 39 cm . the grating parameters in both cases are : κ = 9000 m − 1 and σ ( z ), is a linear function of z in regions i and iii with a slope of − 888 . 594 and + 888 . 594 m − 2 respectively . in case of the and gate σ ≡− 45 : 41 m − 1 in region ii . in case of the not gate σ ( z ) in region ii has a full - period cosine profile with a minimum value of − 44 . 7 m − 1 at both ends of region ii and a maximum value of − 40 . 7 m − 1 at the middle of region ii . in order to demonstrate the absorption and the smearing of the trapped soliton , we assume that the fiber loss coefficient is equal to α = 0 . 023 m − 1 . the input solitons had a full width at half maximum ( fwhm ) of 18 . 85 ps and a peak power of 3 . 02 kw . the frequency offset of the input solitons for the and gate and for the clock soliton of the not gate , relative to the local bragg frequency at the grating entrance , was equal to 297 . 48 ghz . the frequency offset of the signal soliton in the not gate was equal to 297 . 53 ghz . hence all of the input solitons were located outside the grating bandgap . by using the method described in ref . [ 7 ], the required input solitons can be formed from input pulses with a peak power of only 340 w and a fwhm of 640 ps . the results of the simulations , shown in fig3 a , 3 b and fig4 a , 4 b , demonstrate the operation of the two gates as described above . fig3 a , 3 b also show that the output pulses may experience oscillations . however , we have verified that the amplitude oscillation does not prevent cascading of two and gates . the minimal spatial separation between the peaks of the two solitons during the interaction , shown in fig3 b , was about 1 cm while the fwhm of the input solitons was equal to 0 . 39 cm . therefore , we could study separately the frequency change of each soliton during the interaction . the results showed that the frequency of the forward propagating soliton at the entrance to region ii was about 0 . 2 ghz higher when two solitons were launched , compared to the case when only a single soliton was launched . we have verified that the increase in the forward soliton frequency enabled its transmission through region ii of the grating . the energy q and the normalized velocity { tilde over ( ν )} of the pulses can be calculated using the moments : the integration was performed for each pulse separately over a spatial window that was equal to about 3 . 5 of the spatial fwhm of the input solitons . we have verified that during the interaction the two pulses maintained their hyperbolic - secant profiles . therefore , we used the connections for solitons : [ 8 ] and ν ={ tilde over ( ν )}, where ({ tilde over ( δ )}, ν ) are the soliton parameters as defined in ref . 3 . the frequency shift of the soliton relative to the local bragg frequency is given by ω =( 1 − ν 2 ) − 0 . 5 κcos ({ tilde over ( δ )}) v g . [ 3 ] the absolute shift in the soliton carrier frequency is equal to δf = ω − σ ( z ) v g − ω 0 , where ω 0 is the initial detuning of the soliton relative to the local bragg frequency . the frequency shift as a function of the solitons location when a single soliton is launched and when two solitons are launched is shown in fig5 . fig5 shows that the interaction between the solitons changes their frequencies . during the interaction the frequency of forward soliton at the entrance to region ( ii ) is about 0 . 2 ghz higher when two solitons are launched , compared to the case when only a single soliton is launched . the increase in the forward soliton frequency enables its transmission through region ( ii ) of the grating . to verify that the change of 0 . 2 ghz in the soliton frequency can transmit the forward soliton through the grating , we have simulated the propagation of a single soliton through the grating for several different initial central frequencies . we have found out that when the initial central frequency of the soliton , shown in fig3 a , was increased by more than 0 . 05 ghz , the soliton was transmitted through the grating . the interaction depends on the relative phase between the two solitons . fig6 shows the interaction when the relative phase was increased by π compared to the case shown in fig3 b . depending on the relative phase , the interference between the solitons may increase or decrease the intensity in the spatial region between the pulses . hence , the bandgap in that region may be shifted towards or shifted away from the solitons carrier frequency due to kerr effect . in the first case the solitons experience a repulsive force . therefore , the frequency and the velocity increase for the forward soliton and decrease for the trailing soliton . hence , the two solitons remain separated as shown in fig3 b . in the second case , the solitons attract each other . hence , the frequency and the velocity decrease for the forward soliton and increase for the trailing soliton . therefore , the two solitons overlap on time during the interaction and the trailing soliton is transmitted through the grating , as shown in fig6 . we have simulated the device behavior for different relative phases between the input solitons , that were uniformly distributed in the region [ 0 , 2π ]. we have found that although the waveforms evolved differently during the interaction , a single soliton was back - reflected , while when two solitons were launched , one of the solitons was transmitted . the carrier frequency of the input solitons could be changed in the region [− 150 , 50 ] mhz compared to the carrier frequency used in fig3 a and 3b . when two gates are cascaded , the bandgap of the second gate should be slightly up - shifted compared to that of the first gate in order to take into account the frequency change of the soliton transmitted from the first gate . the above analysis indicated that if the soliton frequency is increased only slightly the transmission of the and gate becomes very strong . alternatively , if the grating pitch is changed by an extremely small amount ( tenths of pico - meter in the above described setup ), the grating transmissivity can be dramatically changed . the grating pitch change can be induced by various physical changes in the environment of the fiber , such as physical strain , temperature change , humidity change ( effectively , through the propagating mode propagation constant ) etc . accordingly , the described device can sense extremely small changes in the environment and translate these changes into drastic changes in the transmission intensity . the typical measurement setup to detect such changes would include a grating similar to that introduced above , a pulsed , stable laser source that would generate pulses train for repetitive launching of bragg solitons at a sufficiently stable frequency and a detector on the transmission side of the grating . the laser frequency and the grating pitch can be tuned one towards the other , both by using laser frequency tuning and the grating strain tuning by some piezoelectric element . the grating should be put in such way , so that the physical force under the measurement would apply on it . the detector would measure the optical intensity at the output of the grating . when the applied physical force becomes lower or higher of some threshold value , a drastic change in grating transmission would be detected by the sensor . σ ( z ) is proportional to the changes in the average refractive index along a single period of the grating . a similar effect of the grating on the soliton propagating can be obtained if instead of decreasing and increasing the average refractive index along sections i and iii respectively , the local grating pitch is decreased in section i and increased in section iii , assuming the case of positive non - linearity γ & gt ; 0 . another option is to keep the average refractive index and the grating pitch constant along the grating , only changing the modulation depth of the grating , increasing it along section i , and decreasing it along section iii ( appodization ). the above gate and sensor rely on propagation of slow bragg solitons . since the losses in fbgs are much higher then the losses in untreated fiber , the slowly propagating bragg solitons can experience a sufficient attenuation while propagating through the grating . due to the losses the propagating soliton can break and couple to dispersive waves . accordingly , we propose a method to overcome the attenuation of slow bragg solitons in the above mentioned gate and sensor and also for other applications not described here . the idea is to write the fbg for bragg solitons propagation into an erbium doped fiber , similarly to the distributed feedback ( dfb ) lasers . here , however , our interest is not to use the grating as a laser amplifying cavity , but only to achieve a sufficient distributed gain to overcome the losses . as a result we do not restrict ourselves to phase - shifted gratings and to optical frequencies well inside the bandgap . soliton interaction depends on the relative phase between the interacting solitons . we have simulated the behavior of the and and the not gates for 10 different initial relative phases between the solitons , uniformly distributed in the region [ 0 , 2π ]. we have found that although the waveforms evolved differently during the interaction , correct operation of the gates was maintained . fig3 a and 3b also show that the transmitted and the back - reflected pulses experience oscillations in their amplitude as was observed in previous work [ 8 ]. many alterations and modifications may be made by those having ordinary skill in the art without departing from the spirit and scope of the invention . therefore , it must be understood that the illustrated embodiment has been set forth only for the purposes of example and that it should not be taken as limiting the invention as defined by the following invention and its various embodiments . therefore , it must be understood that the illustrated embodiment has been set forth only for the purposes of example and that it should not be taken as limiting the invention as defined by the following claims . for example , notwithstanding the fact that the elements of a claim are set forth below in a certain combination , it must be expressly understood that the invention includes other combinations of fewer , more or different elements , which are disclosed in above even when not initially claimed in such combinations . a teaching that two elements are combined in a claimed combination is further to be understood as also allowing for a claimed combination in which the two elements are not combined with each other , but may be used alone or combined in other combinations . the excision of any disclosed element of the invention is explicitly contemplated as within the scope of the invention . the words used in this specification to describe the invention and its various embodiments are to be understood not only in the sense of their commonly defined meanings , but to include by special definition in this specification structure , material or acts beyond the scope of the commonly defined meanings . thus if an element can be understood in the context of this specification as including more than one meaning , then its use in a claim must be understood as being generic to all possible meanings supported by the specification and by the word itself . the definitions of the words or elements of the following claims are , therefore , defined in this specification to include not only the combination of elements which are literally set forth , but all equivalent structure , material or acts for performing substantially the same function in substantially the same way to obtain substantially the same result . in this sense it is therefore contemplated that an equivalent substitution of two or more elements may be made for any one of the elements in the claims below or that a single element may be substituted for two or more elements in a claim . although elements may be described above as acting in certain combinations and even initially claimed as such , it is to be expressly understood that one or more elements from a claimed combination can in some cases be excised from the combination and that the claimed combination may be directed to a sub - combination or variation of a sub - combination . insubstantial changes from the claimed subject matter as viewed by a person with ordinary skill in the art , now known or later devised , are expressly contemplated as being equivalently within the scope of the claims . therefore , obvious substitutions now or later known to one with ordinary skill in the art are defined to be within the scope of the defined elements . the claims are thus to be understood to include what is specifically illustrated and described above , what is conceptually equivalent , what can be obviously substituted and also what essentially incorporates the essential idea of the invention . 1 . kyu h . ahn , m . vaziri , brandon c . barnett , guy r . williams , x . d . cao , mohammed n . islam , b . malo , kenneth o . hill , and d . q . chowdhury , j . lightwave technol ., 14 , 8 ( 1996 ) 2 . c . r . menyuk , opt . lett ., 12 , 614 ( 1987 ); j . opt . soc . am . b 12 , 392 ( 1988 ). 3 . c . m . de sterke and j . e . sipe , “ gap solitons ”, in progress in optics xxxiii , e . wolf ed ., ( elsevier , amsterdam , 1994 ), pp . 203 - 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