the preferred embodiments seek to overcome timing limitations by means of applying pump control techniques to stimulate the decay of the free carriers , and thus reducing the limiting performance factor . some embodiments further allow for complete remote access / control of the device via optical fibre , with no need for localised power supplies or controlling circuitry . modelling the proposed design requires solving the stimulated and spontaneous emission rate equations . the model is based on assuming a second pump can only cause stimulated emissions and no absorption . this simplification is substantially valid for silicon as the absorption process would require the absorption of a phonon which not very probable if the pump energy is close to the band gap energy . the spontaneous rate equation is given by where a 21 is the einstein a coefficient and equals to the reciprocal of the carrier life time , and n ( t ) is the free carrier density . solving the above differential equation yields the following result : where n ( 0 ) is the initial free carrier density . fig1 shows example carrier density decay dynamics 10 by assuming an initial carrier density of 1 . 92 a . u , and a carrier lifetime of 100 ns . where p ( v ) is the energy density , defined as the intensity of the optical source divided by the speed of light , the intensity is defined as the pump power divided by the cross section waveguide area , and b 21 is the einstein b coefficient , which is expressed as : where p is the electric dipole moment , ε 0 is the permittivity of free space , and h is planks constant . solving the above differential equation yields the following result : this equation is plotted in fig2 over time for pump powers of 1 mw 11 and 2 mw 12 . for comparison , the spontaneous emission curve 10 is also plotted . fig2 shows that the rate of stimulated emission increases with optical pump power 11 , 12 . it shows that if stimulated emission occurs inside the optical device , the free carrier lifetime can be reduced , resulting in faster tuning performances . this analysis can be used to validate a dual pumping structure , where the first pump emits photons of energy that induces high absorption rates , and the second pump emits photons of energy which induces high stimulated emission rates . the need for this structure arises from the indirect band gap structure of the silicon material , where the excited free carriers quickly decay to the bottom of the conduction band after excitation . this means that in order for the optical pump to induce stimulated emissions , its emitted photons can be of less energy when compared to the first pump . this change in pump energy ( e ) is achieved by changing the operating frequency of the pump source , in a relationship governed by e = v , where v is the operating frequency of the pump source . the preferred embodiments provide a new tuning method which provides a technique to achieve stimulated emission of free carriers , based on carrier injection , using dual pumps . the design consists of two propagating optical pump beams , in which one pump stimulates the excitation of free carriers , and the second pump stimulates the decay of free carriers . this solves the problem of lengthy free carrier lifetimes , and consequently improves the tuning performance of integrated optical circuits such as optical switches , modulators and delay lines . it is possible to stimulate emission within both direct and indirect band gap materials via incident photons . depending on the impurity level of indirect band gap materials , one can either stimulate the emission from the bottom of the conduction band , or from the recombination centre . different material impurities will have different carrier lifetimes at the recombination centre , and this should be taken into account when implementing this design on a particular material . unidirectional waveguide pumping design : turning to fig3 , there is illustrated a first design 30 , based on coupling the pump signals from pumps 31 , 32 to an optical integrated circuit component 34 or its surrounding material from the same input waveguide or input port . the second pump is additionally sent through a delay line 33 , in order to adjust the pulse timing and also insure stimulated emissions occurs for the part of the spectrum limited by the free carrier lifetime . a controlled probe signal 35 can be directed through the integrated circuit in either the same or different directions relative to the pump signals . fig4 illustrates a similar arrangement 40 to fig3 , with the probe signal 41 inserted in a different direction to fig3 . different pumping signals are also available . for example , a bidirectional waveguide pumping scheme is illustrated 50 in fig5 , with the two pump signals 51 , 52 coupled to the optical integrated circuit component or its surrounding material from different input waveguides or input ports in opposed directions . the probe signal can be launched into the integrated circuit in either the same or different directions relative to the pump signals . fig6 illustrates a bidirectional waveguide pumping scheme with the probe signal coupling to the same port as pump 1 , and fig7 illustrates a bidirectional waveguide pumping scheme with the probe signal coupling to the same port as pump 2 further , different illumination schemes can be used . fig8 illustrates an alternative illumination pumping scheme based on using the pump sources 81 , 82 to illuminate the integrated circuit 83 or its surroundings . the pump signals can either be illuminating the circuit from above , below , the side or any combination thereof . fig9 illustrates illumination and waveguide pumping scheme based on sending one of the pump signals 92 to the integrated circuit 94 via an input waveguide , and using the second pump 91 to illuminate the device . the waveguide or illumination pump can be either the absorption or stimulated emission pumps , providing that both the absorption and stimulated emission pumps are being implemented in the design . fig1 illustrates a dual pump implementation structure 110 . here , the signal from a single optical pump source 111 is split into two paths , where the first path functions as the absorption pump , and the second path functions as the stimulated emission pump . wavelength conversion 112 can be used to maximise the absorption / stimulated emission rates of the respective pumps . an optical amplifier / attenuator 113 , 114 can be used in one or both paths to vary the optical power between the two signals . optical delay lines 115 , 116 can also be used in one or both of the arms for synchronisation purposes . the proposed pump design can be used to replace the two optical pump sources in all presented designs . pulse shaping using both the dual and single pump structures , whereby the modulated optical pump pulse can cause higher rates of stimulated emission . pump pulse modulation can be achieved by using linear filters , such as a finite - impulse - response ( fir ) filter where the pump pulse undergoes several optical taps , each encountering differing delays and magnitude alterations before being combined at the output . the pump pulse modulation scheme can be applied to all presented designs . fig1 shows a single pump shaping scheme and fig1 shows a dual pump pulse modulation scheme . the multiple taps can also be achieved by using multiple laser sources , where the combination of multiple pump sources achieve a particular pulse spectrum . fig1 illustrates a first design of an all optical modulator that can be achieved by controlling the modulation of a probe signal in a dual pump structure . the probe 134 and the pump signals 132 , 133 are combined and coupled into a ring resonator 131 . based on the free carrier variations induced by the pump signals , the optical path length , or equivalently , the phase delay of the probe signal propagating through the ring resonator 131 will differ , resulting in either constructive or deconstructive interference at the output . the dual pump structure modulates the probe signal faster than current tuning methods , and either turns it on or off at the output ( or any magnitude in - between ). fig1 illustrates an optical switch design 140 based on using a ring resonator 141 in an add / drop filter configuration , where the probe signal 142 can either be switched to the through port 143 or the drop port 144 based on the carrier density . if the probes round trip phase shift is an integer multiple of 2π , constructive interference will occur and the signal will be switched to the drop port . otherwise , if the probe signal is an integer multiple of π , deconstructive interference will occur and the probe signal will be switch to the through port . it can be seen that the preferred embodiments provide a system and method for the rapid response of an optical system utilising two pumping frequencies . one for inducing photon absorption in a material and the second for inducing stimulated photon emission . in addition to other experiments , computer simulations were conducted in order to validate the dual pump design . the vpi photonics transmission and component maker software was used to provide a platform on which non - linear optical waveguides can be simulated by applying formulas associated to the two - photon - absorption simulations . fig1 illustrates the results of the simulation for a single pump structure response ( 161 ), and the dual pump structure response for differing values of frequency for the carrier density output . an all - pass filter using a single ring resonator was used as a testing circuit in the simulation . the frequencies tested include 133 thz ( 165 ), 132 . 99 thz ( 163 ) 132 . 98 thz ( 164 ) and 132 . 97 thz ( 162 ). the plot highlights the effect of frequency variations of the second pump . the second peak 166 occurs as a result of the instantaneous pump power being too high , resulting in an increased probability of absorption . this high intensity is as a result of the pump signal approaching a resonate wavelength . the results are presented in fig1 , showing a different time scale and highlighting to date a best speed improvement achieved 161 via the addition of data points . these points are used to calculate the improvement in the tuning performance . the tuning time for the different curves to decay to a free carrier magnitude of 3 . 528 × 10 24 was measured for the different plots . a calculation was carried out to determine the speed improvement of the dual pump design . calculation results show the single pump tuning speed , dual pump tuning speed are 67 ps and 29 ps respectively indicating a significant tuning speed improvement . a further simulation was carried out utilising the main non - linear waveguide component parameters as set out in the table below : the effect of different pulse duration was tested , in order to test the hypothesis that pump pulses of longer and low magnitude tails will lead to faster tuning dynamics by way of higher stimulated emission rates . this concept was investigated on both the single and dual pump designs . to test whether modulating the pump signal has any influence on the tuning performance , both the single pump and the dual pump structures were tested with pulse shapes mimicking the natural decay rate of exited free carriers . the pump signal was as illustrated in fig1 , with a pulse shape denoted 171 . the tail of this pulse shape was also then slightly extended 172 in order to see if this will cause more stimulated emissions . fig1 compares the two pulse shapes used in the simulation , where a second curve 171 being a slightly lengthened version of the curve 172 . fig1 illustrates a schematic diagram 180 of the components used to simulate the single pump modulation design in the vpi software . the emission frequency of pump 1 ( 181 ) is 193 . 09 thz . fig1 illustrates the tuning performance 190 for the output carrier density for the single pump design , where the curve 191 corresponds to the longer modulated pump signal , and the curve 192 corresponds to the shorter modulated pump signal . fig2 illustrates that the longer modulated signal induces slower tuning performances . this is thought to occur because the extended tail of the longer signal is being absorbed to cause stimulated emissions . the absorption process is occurring at a higher probability in this scenario because the excited free carriers are decaying to lower energy values in the conduction band , and thus , population inversion is not achieved for the higher energy region corresponding to the incident pump photon energy . it is theorised that higher rates of stimulated emission can occur if the dual pumping structure is used , where the pump inducing stimulated emission can have its photon energies design to be equivalent to the band gap energy . this concept was tested via simulations . it is also theorised that changing the frequency of pump 1 ( 181 , fig1 ) to 132 . 98 thz may also improve the rates of stimulated emission , as this will result in the incident photons having an energy equal to the band gap energy . however , this may render the initial absorption process more difficult , as the free carriers will need to additionally absorb a phonon in order to enter the higher energy state . the previous simulation was repeated for this new frequency value , for both the longer and shorter pump spectrums . fig2 illustrate the carrier density output spectrum , with the curve 201 illustrating the longer modulated pulse and the curve 202 illustrating the shorter modulated pulse . the result of fig2 show that the slightly longer pump pulse caused a substantial improvement in the tuning performance of the design , highlighting that is it possible for the tail of the single pump design to stimulate the emission of free carriers . however , it also shows that the peak carrier density magnitude for the longer modulated pump signal occurs earlier in time , and has a smaller peak magnitude when compared to the shorter pulse . this is thought to occur because the tail of the pulse is already causing some stimulated emissions before the peak carrier density value has been reached . this may be a problem if the higher peak carrier density magnitude was required , in order to induce a particular resonate shift . fig2 illustrates a screenshot 210 of the tuning measurements taken for the above design . the measurements were used in the calculations to establish a speed improvement of the design . the tuning performance of the shorter tailed pump pulse 212 is calculated as 0 . 066 ns the tuning performance of the longer tailed pump pulse 211 is calculated as 0 . 021 ns thus , the overall speed improvement of the modulated signal , comparing the longer and shorter tailed pulses is calculated as 3 . 19 this highlights that by modulating the single pump power spectrum to have a longer tail can yield tuning performances which is more than 3 times faster when compared to pump power spectrums having shorter tails . to prevent the reduction in peak free carrier magnitude event in the single pump modulation simulation , a dual pumping structure was tested , where the delay of the second pump can be precisely controlled , limiting the stimulated emission process to the switching off of part of the spectrum . fig2 illustrates the dual pump arrangement 220 , with first 221 and second 222 pump signals being provided . the emission frequency of the second pump 222 was 132 . 98 thz . fig2 illustrates the pulse spectrums used for pump 1 ( 231 ) and pump 2 ( 232 ), highlighting the different pulse shapes used in the simulation . fig2 illustrates the tuning performance 240 of the design for the carrier density output , where the dashed line 241 corresponds to the single pump tuning method , and the line 242 corresponds to the the shorter modulated pump signals and the line 243 corresponds to the longer modulated pump signals . fig2 illustrates that the longer modulating signal 243 of pump 2 is causing higher rates stimulated emission , resulting in a faster tuning performance . this is thought to occur because the longer tail of the pump pulse 243 results in the presentation of more incident photons when compared to the shorter pulse 242 . this leads to higher rates of stimulated emission . therefore , by optimising the pump pulse shape , higher rates of stimulated emission can be achieved over a larger range of the carrier density spectrum . based on using the dual pumping structure arrangement an alternative design is to utilise a fibre bragg grating . fig2 illustrates one such arrangement 250 . the outputs from two optical pumps 251 , 252 is time delayed 254 before being fed through circulator 256 . at one port of the circulator is attached a fiber bragg grating ( fbg ) 255 . the fbg 255 provides the correct output spectrum of a filter . generating free carriers will red shift the filter spectrum , while stimulating the decay of free carriers will blue shift the filter spectrum . an optical filter 257 is used at the output of the circulator to filter out the remaining pump signals from the output spectrum . this design can also be used to generate a real time optical fourier transform . reference throughout this specification to “ one embodiment ”, “ some embodiments ” or “ an embodiment ” means that a particular feature , structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention . thus , appearances of the phrases “ in one embodiment ”, “ in some embodiments ” or “ in an embodiment ” in various places throughout this specification are not necessarily all referring to the same embodiment , but may . furthermore , the particular features , structures or characteristics may be combined in any suitable manner , as would be apparent to one of ordinary skill in the art from this disclosure , in one or more embodiments . as used herein , unless otherwise specified the use of the ordinal adjectives “ first ”, “ second ”, “ third ”, etc ., to describe a common object , merely indicate that different instances of like objects are being referred to , and are not intended to imply that the objects so described must be in a given sequence , either temporally , spatially , in ranking , or in any other manner . in the claims below and the description herein , any one of the terms comprising , comprised of or which comprises is an open term that means including at least the elements / features that follow , but not excluding others . thus , the term comprising , when used in the claims , should not be interpreted as being limitative to the means or elements or steps listed thereafter . for example , the scope of the expression a device comprising a and b should not be limited to devices consisting only of elements a and b . any one of the terms including or which includes or that includes as used herein is also an open term that also means including at least the elements / features that follow the term , but not excluding others . thus , including is synonymous with and means comprising . as used herein , the term “ exemplary ” is used in the sense of providing examples , as opposed to indicating quality . that is , an “ exemplary embodiment ” is an embodiment provided as an example , as opposed to necessarily being an embodiment of exemplary quality . it should be appreciated that in the above description of exemplary embodiments of the invention , various features of the invention are sometimes grouped together in a single embodiment , fig ., or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects . this method of disclosure , however , is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim . rather , as the following claims reflect , inventive aspects lie in less than all features of a single foregoing disclosed embodiment . thus , the claims following the detailed description are hereby expressly incorporated into this detailed description , with each claim standing on its own as a separate embodiment of this invention . furthermore , while some embodiments described herein include some but not other features included in other embodiments , combinations of features of different embodiments are meant to be within the scope of the invention , and form different embodiments , as would be understood by those skilled in the art . for example , in the following claims , any of the claimed embodiments can be used in any combination . in the description provided herein , numerous specific details are set forth . however , it is understood that embodiments of the invention may be practiced without these specific details . in other instances , well - known methods , structures and techniques have not been shown in detail in order not to obscure an understanding of this description . similarly , it is to be noticed that the term coupled , when used in the claims , should not be interpreted as being limited to direct connections only . the terms “ coupled ” and “ connected ,” along with their derivatives , may be used . it should be understood that these terms are not intended as synonyms for each other . thus , the scope of the expression a device a coupled to a device b should not be limited to devices or systems wherein an output of device a is directly connected to an input of device b . it means that there exists a path between an output of a and an input of b which may be a path including other devices or means . “ coupled ” may mean that two or more elements are either in direct physical or optical contact , or that two or more elements are not in direct contact with each other but yet still co - operate or interact with each other . thus , while there has been described what are believed to be the preferred embodiments of the invention , those skilled in the art will recognize that other and further modifications may be made thereto without departing from the spirit of the invention , and it is intended to claim all such changes and modifications as falling within the scope of the invention . for example , any formulas given above are merely representative of procedures that may be used . functionality may be added or deleted from the block diagrams and operations may be interchanged among functional blocks . steps may be added or deleted to methods described within the scope of the present invention .