Over recent years, optical code-division multiple-access (OCDMA) systems have been investigated forlocal area networks (LANs), and have received sig-nificant attention for their ability to increase thenumber of active users and provide multiple-user ac-cess over the same bandwidth. In OCDMA systems,each user is typically assigned a unique signature (orcode), which is spread in time and/or frequency anddesigned to minimize the interchannel interferenceand to maximize the spectrum utilization. The keycomponents of OCDMA systems are the encoder andthe decoder that perform all-optical code generationand data recognition, respectively [1]. A code or se-quence of pulses referred to as “chips” is attributed toeach user to encode its data bits. The encoded dataare then broadcast into the network and recognizedonly by the matched decoder.
Various OCDMA implementation techniques havebeen proposed, including direct-sequence encoding[1], wavelength encoding [2–4], spectral-phase encod-ing [5], spectral-amplitude coding [6], two-dimensional (2D) encoding [7], and hybrid coding ap-proaches [8]. Most wavelength-encoding OCDMAcoding designs have so far been based on using opti-cal filters, such as fiber Bragg gratings (FBGs). How-
ever, OCDMA systems based on such designs requireN-1 encoders for each user (N is the number of usersin the network) as well as an optical switch to selectthe proper encoder. The use of an optical switch andFBGs that can be tuned in real time makes suchOCDMA systems impractical because it limits theflexibility, accuracy, and variety of the encoding–decoding operation.
In this Letter, we propose a wavelength-encodedOCDMA encoder–decoder structure employing anFBG array that spectrally and temporally slices thebroadband input pulse into several chips in conjunc-tion with an opto-very-large-scale-integrated (VLSI)processor [9] that generates codewords through digi-tal phase holograms. The proof-of-concept of the pro-posed OCDMA encoder–decoder structure is demon-strated at 2.5 Gbits/s.
The structure of the proposed OCDMA encoder–decoder is shown by the experimental setup in Fig. 1.The encoder of a transmitter consists of an array ofequally spaced FBGs of different Bragg wavelengths.The FBG array spectrally slices an incoming broad-band optical pulse into several chips of differentwavelengths. These chips are equally spaced by achip interval corresponding to the round-trip propa-
gation between two adjacent Bragg gratings. It is im-portant that, to avoid chip collision, all the chipsshould exit the FBG array before the next pulse islaunched. Subsequently, all the chips are routed to a1 mm diameter collimator and launched to a 1200line/mm grating plate. The latter spreads the wave-length components of the collimated beam along dif-ferent directions and maps them onto the active win-dow of the first opto-VLSI processor (encoder). Theopto-VLSI processor comprises an array of liquidcrystal (LC) cells independently driven electronicallyby a VLSI circuit that generates multiphase digitalholographic diffraction gratings capable of steeringand/or shaping optical beams. The steering capabilityof opto-VLSI processors was previously reportedin [9].
Each wavelength component incident onto theopto-VLSI processor is assigned a pixel block loadedwith an optimized digital phase hologram so that theoptical power falling on that pixel block is indepen-dently either reflected back along its incident opticalpath, and hence coupled into the fiber collimator withminimum attenuation, or appropriately steered offtrack, so its power is not coupled back into the fibercollimator; thus a codeword can be generated. Thewavelength components coupled into the fiberthrough the collimation lens are routed through anoptical circulator and amplified using an erbium-doped fiber amplifier (EDFA). The encoded signal isthen transmitted into the network.
At the receiver side, the encoded signal is routed toan FBG array similar to that of the transmitter butarranged in a reversed order. The receiver’s FBG ar-ray realigns all chips into a single-pulse time slot.Subsequently, the signal is routed to a fiber collima-tor and the collimated optical beam is launched to-ward a diffraction grating plate, which spreads thewavelength components along different directionsand maps them onto the active window of the decod-ing opto-VLSI processor. It is important to note thatto retrieve the original signal, the second opto-VLSIprocessor must be loaded with digital phase holo-grams similar to those of the first opto-VLSI proces-sor. The decoded signal is coupled back into the fibercollimator and routed through an optical circulator toan EDFA that amplifies the optical signal before de-tection by the photoreceiver. When the digital-phaseholograms of the encoder and decoder are matched, ahigh-peak autocorrelation waveform is generated,which can be recognized using a threshold detector.Otherwise, a low-amplitude cross-correlation func-tion is generated, indicating a mismatch between thecodewords of the encoder and the decoder.
To prove the principle of the proposed structure, alow-coherent amplified spontaneous emission (ASE)source was launched into an electro-optic modulator(EOM) and intensity-modulated with a non-return-to-zero periodic pattern “10,000” at 12.5 Gbits/s (thisis equivalent to a 2.5 Gbits/s return-to-zero patternwith a 20% duty cycle). The modulated light waslaunched into an optical fiber having an array of fivefiber FBGs equally spaced, with center-to-center
spacing of 10 mm. All FBGs had identical reflectivi-
ties of 90% and Bragg wavelengths of �1=1551.6 nm,�2=1553 nm, �3=1554.5 nm, �4=1556.1 nm, and �5=1557.5 nm, respectively.
Both 1D opto-VLSI processors used in the experi-ments had 1�4096 pixels and 256 phase levels, with1 �m pixel size, and 0.8 �m spacing. They were con-trolled by using LABVIEW software to generate opti-mized digital holograms that selectively steer the in-cident wavelength components along arbitrarydirections. The 256 voltage levels applied to the indi-vidual pixels were between 0 and 2 V.
The first opto-VLSI processor was configured togenerate a codeword employing �1, �2, and �3. There-fore, the wavelengths �1, �2, and �3 were coupledback into the collimator, while the wavelengths �4and �5 were substantially attenuated (steered away).Figure 2(a) shows the schematic of the phase holo-grams loaded on the first opto-VLSI processor. Figure2(b) shows the measured spectrum of the optical sig-nal coupled back into the collimator. Figure 3(a)shows the encoded signal that was detected by ahigh-speed photodiode. The encoded signal was thenamplified by an EDFA and sent to the decoder, whichhad an FBG array similar to that of the encoder, butarranged in a reversed order. After reflection off thesecond FBG array, the time delays (introduced by thefirst FBG array) between the chips carried by the dif-ferent wavelengths were eliminated. Subsequently,the various wavelengths were routed to a 1 mm di-ameter collimator, and the collimated optical beamwas launched toward a 1200 line/mm grating surfacethat spread the wavelengths along different direc-tions and mapped them onto the active window of thesecond opto-VLSI processor (decoder), which wasloaded with the same phase holograms as those of thefirst opto-VLSI processor. The coupled-back opticalsignal was detected by a high-speed photodetectorand monitored by using a high-speed digital oscillo-scope.
Fig. 2. (Color online) (a) Digital phase holograms loadedon the first and second opto-VLSI processor (encoder) gen-erating a codeword employing �1, �2, and �3. (b) Measured
When the first and second opto-VLSI processorshad the same phase hologram configuration, theoriginal input pulse was recovered, generating ahigh-peak autocorrelation function as shown in Fig.3(b). Figure 3(c) shows the eye diagram of an error-free transmission. Note that a mismatch between theencoder and decoder digital phase holograms resultsin a low-intensity output pulse. To prove this, thedigital phase holograms loaded into the pixels blocksof the first opto-VLSI processor (encoder) were leftunchanged (i.e., coupling the wavelength components�1, �2, and �3) while the second opto-VLSI processor(decoder) was reconfigured with a codeword assignedto couple back the wavelength components �2, �4, and�5. In this case, a low-amplitude cross-correlationoutput signal was obtained, as shown in Fig. 3(d).The results shown in Figs. 3(a)–3(d) demonstrate theprinciple of the proposed wavelengths-encodedOCDMA encoder–decoder structure.
The performance analysis of wavelength-encodedOCDMA systems was recently reported by Rochetteet al. [2], who showed that multiuser interference(MUI) is the main factor degrading the performanceof OCDMA systems. For the proposed OCDMA struc-ture, employing five FBGs allows only ten users to beaccommodated when three wavelengths are em-ployed for coding. An additional FBG increases thenumber of users to 20.
It is important to note that if an FBG array is notemployed in the encoder, the opto-VLSI processoralone is able to generate wavelength codewords; how-ever, in this case, all wavelength components occupya single bit slot, making the OCDMA system suscep-tible to crosstalk. Therefore, the use of an FBG arrayis essential in the encoding process to enable thecodeword to be spread over many bit slots, thus re-ducing the interference and improving the security ofthe encoding.
Finally, it is important to note that with current
Fig. 3. (a) Encoder output generating a codeword employs �codewords were matched. (c) Eye diagram of an error-free tcodewords were not matched.
opto-VLSI fabrication processes, the active window of
the opto-VLSI processor can practically be as large as20 mm�20 mm, thus enabling up to 32 differentwavelengths to be processed simultaneously.
We have proposed and experimentally demon-strated a 2.5 Gbits/s wavelength-encoded OCDMAencoder–decoder structure employing opto-VLSI pro-cessors. The attractive feature of the structure is thatit enables N encoders at each station to be replacedwith a single opto-VLSI processor in conjunction withan FBG array. Codewords have been synthesized atthe encoder and at the decoder using computer-generated digital phase holograms, and a high-peakautocorrelation function at the decoder has success-fully been observed when the digital phase holo-grams loaded on the opto-VLSI processors of the de-coder and encoder are matched. In “no match”scenarios, low-peak cross-correlation functions havebeen detected and easily recognized from autocorre-lation functions.
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2, and �3. (b) Decoder output when the decoder and encodermission. (d) Decoder output when the decoder and encoder
