Abstract:
An optical processing system providing a rapid optical response, the system including: a first optical material sensitive to an effective refractive index change under photon absorption; a first optical pump for optically pumping the first optical material at a first frequency so as to cause the first optical material to undergo an effective refractive index change by means of photon absorption; a second optical pump for optically pumping the first optical material at a second frequency so as to cause the first optical material to undergo a rapid second refractive index change by means of stimulated emission.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to the field of high speed optical tuning, and, in particular, discloses a high speed pumping system for an optical device. 
       REFERENCES 
       [0000]    
       
         [1] X. Y. Tong Chen, Thomas Huang, and Robert A. Minasian, “Multiple-bipolar-tap tunable spectrum sliced microwave photonic filter,”  Optics Letters , vol. 35, pp. 3934-3936 2010. 
         [2] M. Lipson, “Guiding, modulating, and emitting light on Silicon-challenges and opportunities,”  Lightwave Technology, Journal of , vol. 23, pp. 4222-4238, 2005. 
         [3] D. F. Qianfan Xu, and Raymond G. Beausoleil, “Silicon micro ring resonators with 1.5-μm radius,”  Optics Express , vol. 16, pp. 4309-4315, 2008. 
         [4] P. D. H. Wim Bogaerts, Thomas Van Vaserenbergh, Katrien De Vos, Shankar Kumar Selvaraja, Tom Claes, Pieter Dumon, Peter Bienstman, Dries Van Thourhout, Rod Baets, “Silicon Microring Resonators,”  Laser Photonics Review, Rev  6, pp. 47-73, 2012. 
         [5] C. Manolatou and M. Lipson, “All-optical silicon modulators based on carrier injection by two-photon absorption,”  Lightwave Technology, Journal of , vol. 24, pp. 1433-1439, 2006. 
         [6] M. H. K. Hao Shen, Li Fan, Lin Zhao, Yi Xuan, Jing Ouyang, Leo T. Varghese, and Minghao Qi, “Eight-channel reconfigurable microring filters with tunable frequency, extinction ratio and bandwidth,”  Optics Express , vol. 18, pp. 18067-18076, 2010. 
         [7] Q. S. Xu, Bradley; Pradhan, Sameer; Lipson, Michal, “Micrometer-scale silicon electro-optic modulator,”  Nature Publishing Group , vol. 435, pp. 325-7, 2005. 
         [8] T. A. Ibrahim, W. Cao, Y. Kim, J Li, J. Goldhar, P. T. Ho, et al., “All-optical switching in a laterally coupled microring resonator by carrier injection,”  Photonics Technology Letters, IEEE , vol. 15, pp. 36-38, 2003. 
         [9] G. V. William S. Fegadolli, Xuan Wang, Felipe Valini, Luis A. M. Barea, Jose E. B. Oliveira, Newton Frateschi, Axel Scherer, Vilson R. Almeida, and Roberto R. Panepucci, “Reconfigurable silicon thermo-optical ring resonator switch based on Vernier effect control,”  Optics Express , vol. 20, pp. 14722-14733, 2012. 
         [10] Z. Xiaoliang, M. Wang, K. Bergman, H. L. R. Lira, L. Lian-Wee, and M. Lipson, “FPGA controlled microring based tunable add-drop filter,” in  Optical Interconnects Conference,  2013  IEEE,  2013, pp. 102-103. 
         [11] V. R. B. Almeida, Carlos A; Panepucci, Roberto R; Lipson, Michal, “All-optical control of light on silicon chip,”  Nature Publishing Group , vol. 431, pp. 1081-1084, 2004. 
       
     
       BACKGROUND 
       [0013]    Any discussion of the background art throughout the specification should in no way be considered as an admission that such art is widely known or forms part of common general knowledge in the field. 
         [0014]    All optical signal processing has the benefit of encompassing a large bandwidth, being low loss and immune to electromagnetic interference [1]. Current solutions to realize all optical signal processors count on the ability to optically tune devices such as phase shifters, modulators, delay lines, filters or switches. It is a current trend to manufacture these devices based on silicon photonics to establish integrated circuits compatible with current CMOS technology [2]. Photonic circuitry based on silicon optical waveguides offers advantages such as small physical footprint [3], high light confinement factor [4], low radiation propagation losses [4], and being compatible with current CMOS microchips and fabrication processes. 
         [0015]    The process of tuning silicon based photonic integrated circuits can be achieved by exciting free carriers in the silicon waveguide. Changes to the free carrier concentration result in changing in the materials refractive index, and consequently, changes the optical path length of the light as it travels through the waveguide. However, a weak refractive index dependence on the free carrier concentration in silicon renders tuning difficult [5], and has typically required large amounts of power in order to excite enough carriers to achieve the desired refractive index change. This problem can be abated by using a ring resonator structure, which is characterised by strong light confinement and strong wavelength dependence on the transmission spectrum [5]. For example, due to these factors, a change of 10 −3  in the refractive is enough to achieve an 80% decrease in the output optical intensity using the design proposed in [5]. Changing the free carrier concentration of the ring resonator has the effect of shifting the frequency spectrum, and hence, changes the resonate wavelengths. Various methods have been presented to achieve controlled tunability in ring resonator based integrated photonic circuits including micro-heaters [6], PN-junction [7], or an optical pump [8]. These methods are discussed as follows: 
         [0016]    Micro-Heaters Based Tuning: 
         [0017]    Micro-heaters are placed at discrete locations above or around each of the ring resonators. As current is passed to the heaters, heat is transferred to the ring resonators, causing the excitation of free carriers [9]. The switching time of this method is slow, in the order of microseconds [6]. Faster tuning performances can be achieved by using a field-programmable gate array controller to control the magnitude of current passed to the heaters [10]. Rise times of 4 μs, followed by fall times of 30 μs can be achieved. 
         [0018]    Single Optical Pump Based Tuning: 
         [0019]    An optical pump pulse laser source is used to excite free carriers via an absorption process. The optical pump source is either coupled to the ring evanescently by directing it along an input waveguide [8] or by positioning the source above the integrated circuit, and directly illuminating the ring resonator [11]. Both methods are effective at exciting free carriers, with rise times as fast as 1 ps being experimentally achieved [11]. However, the fall time of the tuning method is limited to 400 ps [5], as a result of lengthy free carrier lifetimes. This limits the overall tuning performance. Reducing the free carrier lifetime would offer the advantage of speeding up the fall times of the device and improve the tuning performance. Current methods of achieving this including shrinking the cavity size, manipulating surface passivation and ion implantation [5]. Using these techniques, a fall time as fast as 100 ps has been achieved [5], which is still inferior when compared to the rise time performance. 
         [0020]    PN Junctions Based Tuning: 
         [0021]    Surrounding the ring resonator with highly doped p- and n-regions and applying bias voltage across the junction can provide tunability of ring resonators [7]. When a forward bias voltage is applied, it will excite electrons to their higher energy levels, while a reverse bias voltage stimulates the decay of electrons to their original lower energy state. The tuning performance of the above design is dependent on the input bias voltage. Using a peak to peak voltage bias of 3.3V (−1.85V to +1.45V), experimental results show a rise time of 10 ns and fall times of 1.5 ns [7]. Increasing the peak to peak voltage to 6.9V (−2.8V to +4.1V) yields an improvement in the tuning performance, with the design achieving a rise time of 200 ps and a fall time of 150 ps [7]. 
         [0022]    It can be seen therefore that the performance of micro-heater based tuning is limited by slow thermal dynamics, and the usage of PN-junctions is inefficient in injecting free carriers. Moreover, both techniques lack the capability of remote control. The usage of an external optical pump to stimulate carrier injection to change the material properties of the device provides the possibility of achieving tunability via remote control. However, the tunning speed of this approach is fundamentally limited by the lengthy free carrier lifetime. 
       SUMMARY OF THE INVENTION 
       [0023]    It is an object of the invention, in its preferred form to provide an improved form of rapid optical tuning of optical circuits. 
         [0024]    In accordance with a first aspect of the present invention, there is provided an optical processing system providing a rapid optical response, the system including: a first optical material sensitive to an effective refractive index change under photon absorption; a first optical pump for optically pumping the first optical material at a first frequency so as to cause the first optical material to undergo an effective refractive index change by means of photon absorption; a second optical pump for optically pumping the first optical material at a second frequency so as to cause the first optical material to undergo a rapid second effective refractive index change by means of stimulated emission. 
         [0025]    The first optical material can form part of an integrated circuit such as a microring resonator and the optical pumps can act to control the resonant frequencies of operation of the microring resonator. 
         [0026]    In some embodiments, there can further be provided an optical probe signal input, input to the first optical material, with the first optical material acting to filter the optical probe signal input to produce an optical output signal. 
         [0027]    In some embodiments, at least one of the optical pumps can be interconnected to the optical material by means of a waveguide. In some embodiments, at least one of the optical pumps projects its optical output to the optical material by means of free space projection. In some embodiments, the output of at least one of the optical pumps undergoes intermediate frequency conversion before being projected through the first optical material. The output of at least one of the output pumps can be filtered before being projected through the first optical material. 
         [0028]    In some embodiments, the pump decay of the second optical pump can be modulated to increase the speed of stimulated emission. This can be provided by the length of the pump decay being increased. The first and second optical pump can be connected to a relative delay element so as to provide a fixed delay between the first and second pumping. The relative delay element comprises a fiber bragg grating. 
         [0029]    In accordance with a further aspect of the present invention, there is provided a method of controlling the rate response of an optically tuned system, the method including the steps of: (a) providing a first optical material sensitive to an effective refractive index change under stimulated emission; (b) pumping the first optical material with a first pump frequency to induce an effective refractive index change due to optical absorption; (c) pumping the first optical material with a second pump frequency to induce a rapid change in the effective refractive index of the first optical material by stimulated emission. 
         [0030]    The method can further include the step of: (d) projecting an optical probe signal through the first optical material, and the first optical material acting to filter the optical probe signal in accordance with a current refractive index state of the optical material. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0031]    Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: 
           [0032]      FIG. 1  illustrates a graph of the free carrier density variation occurring due to spontaneous emissions; 
           [0033]      FIG. 2  illustrates a graph comparing spontaneous emission, with stimulated emission with pump powers of 1 mW and 2 mW; 
           [0034]      FIG. 3  illustrates schematically a Unidirectional Waveguide Pumping Scheme with Unidirectional Probe Signal; 
           [0035]      FIG. 4  illustrates schematically a Unidirectional Waveguide Pumping Scheme with a Probe; 
           [0036]      FIG. 5  illustrates schematically a Bidirectional Waveguide Pumping Control Scheme; 
           [0037]      FIG. 6  illustrates schematically an alternative waveguide pumping control scheme; 
           [0038]      FIG. 7  illustrates schematically an alternative waveguide pumping control scheme; 
           [0039]      FIG. 8  further illustrates schematically an alternative waveguide pumping control scheme; 
           [0040]      FIG. 9  illustrates schematically a Combination of Waveguide and Illumination Pumping Scheme; 
           [0041]      FIG. 10  illustrates schematically a Wavelength Conversion Pumping Scheme; 
           [0042]      FIG. 11  illustrates schematically a Single Pump Pulse Modulation Scheme; 
           [0043]      FIG. 12  illustrates schematically a Dual Pump Pulse Modulation Scheme; 
           [0044]      FIG. 13  illustrates schematically a Ring Resonator Based All Optical Modulator; 
           [0045]      FIG. 14  illustrates schematically a Ring Resonator Based All Optical Switch; 
           [0046]      FIG. 15  illustrates a graph of dual pumping at different frequencies; 
           [0047]      FIG. 16  illustrates a graph of the Dual Pumping Simulation results shown for various different Pump 2 frequencies; 
           [0048]      FIG. 17  illustrates a Schematic Diagram for the Single Pump Modulation Tests 
           [0049]      FIG. 18  a schematic block diagram of the layout of a Single Pump Modulation Test; 
           [0050]      FIG. 19  illustrates the Single Pump Modulation Test for carrier density output; 
           [0051]      FIG. 20  illustrates the Single Pump Modulation Test for Carrier Density Output using a pump frequency of 132.98 THz; 
           [0052]      FIG. 21  illustrates the Single Pump Modulation Test for Carrier Density Output using a pump frequency of 132.98 THz showing the data points used for calculations; 
           [0053]      FIG. 22  illustrates a Schematic for Dual Pump tuning of a ring resonator based on two photon absorption; 
           [0054]      FIG. 23  illustrates the power spectrum for the Dual Pump Modulation Test; 
           [0055]      FIG. 24  illustrates the Dual Pump Modulation Test for Carrier Density; and 
           [0056]      FIG. 25  illustrates a Fibre Bragg Grating Structure using a Dual Pumping Structure. 
       
    
    
     DETAILED DESCRIPTION 
       [0057]    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. 
         [0058]    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 
         [0000]    
       
         
           
             
               
                 dN 
                  
                 
                   ( 
                   t 
                   ) 
                 
               
               dt 
             
             = 
             
               
                 - 
                 
                   A 
                   21 
                 
               
               × 
               
                 N 
                  
                 
                   ( 
                   t 
                   ) 
                 
               
             
           
         
       
     
         [0059]    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: 
         [0000]        N ( t )= N (0)× e   −A     21     t  
 
         [0000]    where N(0) is the initial free carrier density.  FIG. 1  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. 
         [0060]    The stimulated rate equation is given by the following formula: 
         [0000]    
       
         
           
             
               
                 dN 
                  
                 
                   ( 
                   t 
                   ) 
                 
               
               dt 
             
             = 
             
               
                 - 
                 
                   B 
                   21 
                 
               
               × 
               
                 p 
                  
                 
                   ( 
                   v 
                   ) 
                 
               
               × 
               
                 N 
                  
                 
                   ( 
                   t 
                   ) 
                 
               
             
           
         
       
     
         [0000]    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: 
         [0000]    
       
         
           
             
               B 
               21 
             
             = 
             
               
                 π 
                  
                 
                   
                      
                     p 
                      
                   
                   2 
                 
               
               
                 3 
                  
                 
                   ɛ 
                   0 
                 
                  
                 
                   h 
                   2 
                 
               
             
           
         
       
     
         [0000]    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: 
         [0000]        N ( t )= N (0)× e   −B     21     p(v)t  
 
         [0061]    This equation is plotted in  FIG. 2  over time for pump powers of 1 mW  11  and 2 mW  12 . For comparison, the spontaneous emission curve  10  is also plotted. 
         [0062]      FIG. 2  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. 
         [0063]    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. 
         [0064]    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. 
         [0065]    A number of different embodiments are possible. These can include: 
         [0066]    Unidirectional waveguide pumping design: Turning to  FIG. 3 , 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. 
         [0067]      FIG. 4  illustrates a similar arrangement  40  to  FIG. 3 , with the probe signal  41  inserted in a different direction to  FIG. 3 . 
         [0068]    Different pumping signals are also available. For example, a bidirectional waveguide pumping scheme is illustrated  50  in  FIG. 5 , 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. 
         [0069]    The probe signal can be launched into the integrated circuit in either the same or different directions relative to the pump signals.  FIG. 6  illustrates a Bidirectional Waveguide Pumping Scheme with the Probe Signal Coupling to the Same Port as Pump 1, and  FIG. 7  illustrates a Bidirectional Waveguide Pumping Scheme with the Probe Signal Coupling to the Same Port as Pump 2 
         [0070]    Further, different illumination schemes can be used.  FIG. 8  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. 
         [0071]      FIG. 9  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. 
         [0072]      FIG. 10  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. 
         [0073]    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.  FIG. 11  shows a single pump shaping scheme and  FIG. 12  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. 
       Applications 
       [0074]      FIG. 13  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). 
         [0075]      FIG. 14  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. 
         [0076]    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. 
         [0077]    Simulation Results 
         [0078]    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. 
         [0079]      FIG. 15  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 ). 
         [0080]    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. 
         [0081]    The results are presented in  FIG. 16 , 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. 
         [0082]    A further simulation was carried out utilising the main non-linear waveguide component parameters as set out in the table below: 
         [0000]    
       
         
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Parameter 
                 Value 
                 Unit 
               
               
                   
                   
               
             
             
               
                   
                 Waveguide Length 
                 4.81935 × 10 −5   
                 M 
               
               
                   
                 Waveguide Effective Index 
                 2 
               
               
                   
                 Kerr Coefficient 
                 0 
                 m 2 /W 
               
               
                   
                 Two Photon Absorption 
                  10 −9   
                 m/W 
               
               
                   
                 Coefficient 
               
               
                   
                 Recombination Process 
                 LINEAR 
               
               
                   
                 Gain Model 
                 LOGARITHMIC 
               
               
                   
                 Gain Shape 
                 PARABOLIC 
               
               
                   
                 Peak Gain Frequency 
                 132.99 × 10 12   
                 Hz 
               
               
                   
                 Gain Bandwidth 
                   0.1 × 10 12   
                 Hz 
               
               
                   
                 Initial Carrier Density 
                    1 × 10 24   
                 m −3   
               
               
                   
                   
               
             
          
         
       
     
         [0083]    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  FIG. 17 , 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.  FIG. 17  compares the two pulse shapes used in the simulation, where a second curve  171  being a slightly lengthened version of the curve  172 . 
         [0084]      FIG. 18  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. 
         [0085]      FIG. 19  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. 
         [0086]      FIG. 20  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. 
         [0087]    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. 
         [0088]    It is also theorised that changing the frequency of pump 1 ( 181 ,  FIG. 18 ) 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.  FIG. 20  illustrate the carrier density output spectrum, with the curve  201  illustrating the longer modulated pulse and the curve  202  illustrating the shorter modulated pulse. 
         [0089]    The result of  FIG. 20  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. 
         [0090]    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. 
         [0091]      FIG. 21  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. 
         [0092]    The tuning performance of the shorter tailed pump pulse  212  is calculated as 0.066 ns 
         [0093]    The tuning performance of the longer tailed pump pulse  211  is calculated as 0.021 ns 
         [0094]    Thus, The overall speed improvement of the modulated signal, comparing the longer and shorter tailed pulses is calculated as 3.19 
         [0095]    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. 
         [0096]    Dual Pump Modulation Test 
         [0097]    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. 
         [0098]      FIG. 22  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. 
         [0099]      FIG. 23  illustrates the pulse spectrums used for pump 1 ( 231 ) and pump 2 ( 232 ), highlighting the different pulse shapes used in the simulation. 
         [0100]      FIG. 24  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.  FIG. 24  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. 
         [0101]    Fiber Bragg Grating Filter Embodiment 
         [0102]    Based on using the dual pumping structure arrangement an alternative design is to utilise a Fibre Bragg Grating.  FIG. 25  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. 
         [0103]    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. 
       Interpretation 
       [0104]    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. 
         [0105]    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. 
         [0106]    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. 
         [0107]    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. 
         [0108]    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. 
         [0109]    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. 
         [0110]    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. 
         [0111]    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. 
         [0112]    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.