Patent Application: US-80274501-A

Abstract:
a hybrid optical transmission system comprising an optical transmitter and receiver . the transmitter includes an input for receiving an encoding signal and an encoder arranged to encode an optical signal with any one of a plurality of encoding signatures according to the encoding signal . the optical receiver comprises a grating decoder connected to receive the encoded optical signal from the input , the grating decoder incorporating a decoding signature complementary to a matched one of the encoding signatures so as to decode the encoded optical signal when encoded with the matched one of the encoding signatures . a hybrid system is thus provided that uses grating decoders at the receiver in combination with active drive - signal - based encoders at the transmitter . in this way , flexibility can be retained for the transmitter hardware , whereas the advantages of grating decoders can be exploited at the receivers .

Description:
[ 0064 ] fig1 ( a ) shows an experimental set up according to a first embodiment used to test the hybrid approach of the invention . the first embodiment provides a 10 - gbit / s all - optical code generation and recognition system based on a hybrid approach of optical fiber delay line and superstructure fiber bragg grating technologies . a 10 ghz regeneratively mode locked erbium fiber ring laser ( efrl ) producing 2 ps pulses is used as the source . the operating wavelength of the laser is 1554 nm corresponding to the center wavelength of the ssfbg . the pulses are launched into the optical fiber delay line encoder through a splitter to generate the desired code . as the decoder , a ssfbg also operating as a matched filter [ 7 ], is used to recognize the encoded sequence . the resulting pulse form after reflection from the decoder grating is measured and analyzed using both a fast photodiode / scope (˜ 25 ghz bandwidth ) and an autocorrelator (& lt ; 100 fs resolution ). the optical fiber delay line encoder consists of four parallel fiber delay arms , where each delay is set according to a 7 - chip m - sequence amplitude code ( 1110010 ). each delay τ corresponds to 6 . 4 ps , hence a chip rate of 160 gchip / s , and the encoded sequence has a total duration of 44 . 8 ps . the m - sequence code is chosen so that the autocorrelation features upon decoding will have a single dominant , well - defined peak with low level cross correlation features . a polarization controller ( pc ) in each delay line is used to align the individual pulses to a single polarization axis . the delayed pulses are then recombined and launched into a polarization maintaining ( pm ) isolator to confirm the same polarization state of the composite delayed pulses . the 7 - chip amplitude modulated ssfbg decoder was written using our continuous grating writing technique as reported in [ 8 ]. it has a total length of 4 . 62 mm with an absolute reflectivity of 4 %. the individual chip duration is 6 . 4 ps corresponding to a chip rate of 160 gchip / s . [ 0068 ] fig1 ( b ) shows the measured reflectivity profile of the ssfbg decoder ( solid line ) and shows good agreement with the theoretical calculation ( dashed line ). in order to quantify the quality of the fiber delay line encoder , we performed a series of code generation experiments to examine the temporal characteristics of the pulse forms generated from the fiber delay line . these results then were compared to the pulse form generated by using a ssfbg as the decoding element . [ 0070 ] fig2 ( a ) shows the temporal response of the fiber delay line encoder as measured using the shg autocorrelator as well as the direct electronic measurements on the oscilloscope with ˜ 25 ghz spectral bandwidth . the measured autocorrelation and oscilloscope traces of encoded signals are found to be in good agreement with the theoretical predictions confirming not only the formation of the correct code patterns but also the desired individual chip separation of 6 . 4 ps . [ 0071 ] fig2 ( b ) shows the equivalent results of the pulse forms on reflection from the ssfbg decoder ( in this case used as the encoder ) with evidence of good qualitative coincidence between experiment and theory . to characterize the full hybrid system , the two different schemes of encoding / decoding ( delay line encoder : ssfbg decoder , ssfbg encoder : ssfbg decoder ) then were put together and the results of the code recognition analysed and compared . in the case of the fiber delay line encoder , fine control of both the phase and polarization is required to obtain the optimum results . this is because each delayed pulse experiences a random phase change , which can lead to changes in the form of the output recognition pattern [ 2 ]. [ 0073 ] fig3 ( a ) shows a direct comparison of the shg autocorrelations of the code recognition signature of the 7 - chip unipolar code against the theoretical predictions . good agreement is found between them as shown . the well - defined code recognition peak was found to have a pulse width of 12 . 4 ps . despite the limitation of oscilloscope bandwidth , we were able to obtain a single peak deconvolved decoded pulse form with good agreement to the theoretical calculations . [ 0074 ] fig3 ( b ) shows for comparison the equivalent shg autocorrelation traces obtained in a non - hybrid system with ssfbg encoder and decoder . in conclusion , we have demonstrated both experimentally and theoretically that ssfbgs can be used to recognize codes generated from another pattern generating scheme , in this case fiber delay lines . these results illustrate that the ssfbg approach is compatible with other technologies for code generation . our experiment constitutes the first demonstration of a hybrid all - optical encoder - decoder system , an approach that could prove a necessary solution for reconfigurable point - to - point optical correlation systems . [ 0076 ] fig4 shows an experimental setup according to a second embodiment . an optical source 10 in the form of a distributed feedback ( dfb ) laser diode ( ld ) is connected to an encoder unit 20 . the encoder unit 20 has an electrical input 22 for receiving an encoding signal . this input is split by a control unit 23 into amplitude and phase components . one output , conveying the amplitude component , is connected to an electro - absorption modulator ( eam ) 24 to define the code length and / or associated amplitude modulation . a second output is connected to a phase modulator ( pm ) 26 via a variable delay line 28 which is used to encode associated phase information associated with the code . the above - described components collectively form a transmitter ( tx ) part of the system . the transmitter tx is linked to a receiver part ( rx ) of the system by a transmission line 30 which leads to an optical circulator 40 for directing the transmitted signal to a ssfbg 42 arranged in reflection with the circulator 40 to decode the signal by applying the reverse functional manipulation to that applied at the transmitter by the encoder unit 20 . finally , a photodiode ( pd ) 44 is shown arranged at the receiver for converting the decoded optical signal into an electrical signal , as may sometimes be required . [ 0077 ] fig5 experimental setup in a third embodiment . an optical source 50 in the form of a distributed feedback ( dfb ) laser diode ( ld ) is provided . the source 50 is driven with a drive current from an input 52 . in this embodiment , the drive current is used to directly modulate and encode the optical signal as it is generated . as is widely known in the art , that this kind of direct driving of a laser diode is prone to result in significant nonlinearities in the signal modulation as a result of the nonlinear nature of the gain spectrum of the laser diode , and its fast gain dynamics . consequently , this approach is often avoided . however , in the present embodiment , this approach is deliberately taken and the flexibility provided by a ssfbg is used to cancel out , or at least compensate for , the non - linearities . namely , a ssfbg is arranged to receive the output from the source 50 via a circulator 54 . the ssfbg provides decoding as in the previous embodiments , but in addition incorporates a filtering function to compensate for nonlinearity / distortion that results from the application of the drive signal to the laser diode . this approach can be generalized so that the same structure as shown in the figure could be used , where the grating solely has the function of filtering out the nonlinearities . in this case , the grating could be provided at the transmitter for correcting the signal generated by the directly driven laser . note also that since an ocdma system is ordinarily a linear optical system it is also possible to envisage hybrid ocdma systems such as those described herein in which an ssfbg system is used in the transmitter and an alternative decoding technology is used within the receiver . thus according to a further aspect of the invention there is provided a grating decoder based transmitter in combination with a reconfigurable receiver based on active decoding , for example using delay lines . 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