IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 23, NO. 9, MAY 1, 2011 591

Novel Reconfigurable Two-DimensionalCoherent Optical En/Decoder Based on Coupled

Micro-Ring ReflectorXu Wang, Senior Member, IEEE, and Zhensen Gao, Student Member, IEEE

Abstract—We propose a novel reconfigurable optical en/decoderto generate and recognize two-dimensional (2-D) optical codes forcoherent optical-code-division-multiple-access (OCDMA) applica-tion. The proposed device is based on cascaded coupled micro-ringreflectors, which can enable simultaneous tuning of the fast wave-length hopping and spectral phase encoding code patterns. Thecoding performance is verified by simulation.

Index Terms—Coupled micro-ring reflector, optical-code-divi-sion-multiple-access (OCDMA), optical en/decoder.

I. INTRODUCTION

O PTICAL code division multiple access (OCDMA)technique has attracted intensive research over the past

decade, mainly due to its unique advantages of high speedall-optical signal processing, fully asynchronous transmissionwith low latency access, simplified network management,potentially improved security and etc [1], [2]. Generally, theOCDMA system can be classified into incoherent or coherentOCDMA according to the operation principle, or accordingto the working dimensions as one dimensional (1-D), ortwo-dimensional (2-D) OCDMA [2]. Optical en/decoder is thekey device that determines the system performance. Variousoptical en/decoders have been developed for either 1-D or 2-DOCDMA en/decoding, such as fiber optical delay line (FODL),planar lightwave circuits (PLC), spatial light modulator (SLM),and superstructured FBG (SSFBG) [3].However, there have been very few reports on 2-D coherent

optical en/decoding technique, although the 2-D coherenten/decoding can potentially enhance the capacity and securityof OCDMA system [2], [4]. Recently, time domain spectralphase en/decoding scheme using Fiber-Bragg-Grating (FBG)array and high speed phase modulator has been demonstratedfor 2-D coherent en/decoding [4]. This is a very flexible schemethat can rapidly reconfigure the spectral phase code. But thetemporal code is fixed by the pattern of FBG array, whichlimits its flexibility and the number of available codes. Alsopractically, it is very difficult to fabricate long FBG array

Manuscript received December 14, 2010; revised January 23, 2011; acceptedFebruary 12, 2011. Date of publication February 17, 2011; date of current ver-sion April 13, 2011. This work was supported by the Open Project of State KeyLaboratory on Integrated Optoelectronics, Institute of Semiconductors, ChineseAcademy of Sciences.The authors are with the School of Engineering and Physical Sci-

ences, Heriot-Watt University, Edinburgh, EH14 4AS, U.K. (e-mail:x.wang@hw.ac.uk; zg27@hw.ac.uk).Color versions of one or more of the figures in this letter are available online

at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/LPT.2011.2116155

Fig. 1. (a) Schematic diagram of proposed optical en/decoder; (b) 2-D opticalcode; and (c) coupled micro-ring reflector.

(consisting of FBGs with different central wavelengths) withwavelength order precision.On the other hand, optical micro-ring resonator based inte-

grated photonic devices has witnessed significant progress inboth the design and fabrication. Optical en/decoder based onmicro-ring resonator for spectral phase en/decoding has alsobeen demonstrated, which is very compact, rapidly reconfig-urable, and with high frequency resolution and accurate phasecontrol [5].In this letter, we propose a novel compact and integrated op-

tical en/decoder based on coupled micro-ring reflectors for 2-Dcoherent OCDMA. The proposed device is very flexible to gen-erate and recognize the 2-D coherent optical code and can si-multaneously reconfigure the wavelength hopping and spectralphase coding patterns using a single device.

II. PROPOSED 2-D OPTICAL EN/DECODER

Fig. 1(a) shows the configuration of the proposed device,which consists of several pairs of identical coupled micro-ringreflectors. In Fig. 1(a), five pairs are used as an example. Eachpair is composed of two weakly coupled ring resonators thatare both coupled to a bus optical waveguide [6]. A heater islaid over every single ring resonator to change the effective

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592 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 23, NO. 9, MAY 1, 2011

Fig. 2. (a) Dependence of and to achieve single peakreflection. (b) Reflective spectrum for different errors.

refractive index by using thermal-optic effect to tune the res-onant wavelength. Carrier injection or electro-optic effect canalso be applied under the p-i-n diode structure to enable highspeed electrical tuning [7], [8]. A phase shifter (PS) is placedon the bus waveguide between two adjacent pairs of coupledreflectors to adjust the relative phase shift of the reflected spec-tral component according to the optical code pattern: phaseshift for symbol “ 1”, or 0 phase shift for symbol “1”. The in-terval between every two reflectors is corresponding to timedelay , where c is the light velocity, n is the effectiverefractive index. is chip duration of the optical code.The working principle of this device is illustrated in Fig. 1.

The 2-D coherent optical code to be generated in this exampleis shown in Fig. 1(b). The wavelength hopping code (WHC) is

and the spectral phase code (SPC) is (0, ,, 0,0). An ultra-short optical pulse with a broadband spectrumis coupled into the bus waveguide via either an optical circularor coupler for integration with other photonic devices. By prop-erly tuning the resonant wavelength of the first pair of coupledreflectors, the spectral component of is on resonance. istherefore reflected back and output from the waveguide, whilethe other wavelengths will transmit through thestructure. Similarly, the spectral component of is reflectedback from the second reflector after experiencing a relative timedelay of T and has relative phase shift adjusted by the first PS( phase shift as light pass through it twice). The remainingspectral components of are operated in the samemanner by the rest reflectors and the output signal from thewaveguide is encoded by the 2-D optical code. Both the WHCand SPC can be reconfigured simultaneously by the heaters andPSs, respectively. The decoder is exactly the same structure asthe encoder but with the reverse wavelength and phase pattern.As shown in Fig. 1(c), each frequency bin can be generated

by one pair of coupledmicro-ring reflector. To realize fast wave-length hopping, single peak and flat top spectral response ofeach frequency bin is required which can be achieved by prop-erly optimize the ring-bus and ring-ring couplingcoefficient . Coupled mode theory can be used toanalyze each coupledmicro-reflector [6]. Fig. 2(a) shows the de-pendence of with to achieve single peakoperation represented by the solid line with circles. Because ofthe inevitable fabrication error in practical, the coupling coef-ficient may deviate from the ideal value, which induces eitherspectrum splitting when the is smaller than the targetvalue, or reduced reflectivity if is larger than the op-timized value. The reflective spectrum for differenterrors is depicted in Fig. 2(b), from which one can

Fig. 3. (a) Peak reflectivity and 3-dB bandwidth versus for dif-ferent error of . (b) Peak reflectivity versus ring loss for different

.

see that single peak reflection with high reflectivity can only beachieved for appropriate .To investigate the reflection performance in presence of

fabrication error, Fig. 3(a) plots the peak reflectivity (PR)and 3 dB bandwidth (BW) versus for different

. As can be seen from Fig. 3(a), the PR dramaticallydecreases for small when the error isintroduced, while the BW is similar for both cases. The ringloss is another unnegligible parameter when designing themicro-resonator. Fig. 3(b) shows the peak reflectivity versusring loss for different , from which one can see thatthe larger the , the higher ring loss can be tolerated inthe structure. For a ring loss of 2-dB/cm, to achieve a toleranceof 3-dB degradation of the PR, the should be largerthan 0.05. On the other hand, large correspondsto large BW as shown in Fig. 3(a), which may reduce thechip number for a given bandwidth-limited optical pulse. Inaddition, when the is greater than 0.1, theis approximately 1 to keep single peak operation, which makesit quite sensitive to the coupling distance error, indicatingthe tradeoff for choosing the and corresponding

.

III. ENCODING/DECODING PERFORMANCE

The en/decoding performance is investigated by injecting aGaussian-shaped optical pulse with pulse width of 2 ps andcenter wavelength of 1550.64 nm into the structure. Theen/decoder has five pairs of coupled micro-ring reflectors. Boththe two rings in each reflector have a ring radius of 50 m [6],and the and are chosen as 0.08 and 0.56to achieve a BW of 0.16 nm for each frequency bin. The chiprate is set as 15 GHz/chip corresponding to mm. Thering loss and straight waveguide propagation loss are assumedas 2 dB/cm and 0.5 dB/cm, respectively. The free spectral range(FSR) is 2.5 nm that allows the wavelength tuning within 5 nmspectral range, so the out-of-band noise shall directly transmitthrough the whole structure. Smaller radius can enlarge the FSRbut may introduce higher loss which can be reduced by highindex contrast for strong optical confinement [5]. III-V semi-conductors like GaAs-Al Ga As that can be engineered viaadjustment of the composition x to achieve a refractive index of2.9 3.4 can be used for this purpose [8], [9]. In the simulation,the effective refractive index is chosen as 3 to get the resonantwavelength around 1550 nm. The high effective refractive indexcan also decrease the straight bus waveguide length to re-duce the loss. By using transfer matrix method [10] consideringdifferent WHC and SPC for the cascaded coupled micro-ring

WANG AND GAO: NOVEL RECONFIGURABLE 2-D COHERENT OPTICAL EN/DECODER 593

Fig. 4. (a) Encoded spectrum for WHC {1, 2, 3, 4, 5} without SPC. (b) En-coded spectrum for WHC {5, 1, 4, 2, 3} and SPC { 1, 1, 1, 1, 1}. (c) and(d) Corresponding encoded waveforms for (a) and (b), respectively.

Fig. 5. (a) Auto-/cross-correlation signals with (i) correct and (ii–iv) incorrectcodes. (b) P/W and P/C versus the channel spacing.

reflectors, the light propagation through the whole en/decodercan be analyzed by Matlab programming. Fig. 4(a) shows theencoded spectrum for WHC of {1, 2, 3, 4, 5} without spec-tral phase pattern, and the corresponding encoded waveform isshown in Fig. 4(c). The simultaneous fast wavelength hoppingand spectral phase encoding are also verified by applying a 2-Dcoherent code with WHC {5, 1, 4, 2, 3} and SPC { 1, 1, 1,1, 1}, where the encoded spectrum and waveform are shownin Fig. 4(b) and (d), respectively. The dip and ripple existing inthe encoded spectrums indicate the interference between adja-cent channels due to the crosstalk. Note from Fig. 4(a) that thereflectivity of the frequency bins is decreased gradually sincethe final pulse experiences much more losses, which will in-duce pulse broadening of the decoded pulse and degradationof the peak power ratio (P/C) between auto-/cross-correlationsignals. To achieve unity reflectance, besides reducing the loss,one should also make the reflectivity slightly different for eachreflector or nonuniformly allocate the spectrum to compensatefor the loss imbalance. To effectively recover the original pulse,both the WHC and SPC are indispensible for the decoder.Fig. 5 (a-i) depicts the auto-correlation signal with high peak

and needle-like pulse, while for the case of incorrect SPC (ii),incorrect WHC (iii) and both the SPC and WHC are incor-rect (iv), the decoded signal spread in time domain with lowpeak power, which verifies the feasibility of 2-D coherent en/de-coding using the proposed device. The sidelobes of the auto-cor-relation signal in Fig. 5 can be ascribed to the crosstalk be-

tween two adjacent frequency bins and the relative large channelspacing. The dependence of the autocorrelation peak to max-imum wing ratio (P/W) and P/C with the channel spacing isillustrated in Fig. 5(b), from which one can see that both theP/W and P/C will be degraded for too large or too small channelspacing. When the channel spacing is too large, the spectrumbetween two adjacent channels can not be effectively utilizedresulting high sidelobes in the decoded waveforms, while fortoo small channel spacing, the interference between differentchannels will be strengthened and the effective code length willbe decreased inducing the degradation of P/W and P/C. Sincethe P/C is of the main concern for general OCDMA systems,the channel spacing should be adjusted to a magnitude com-parable to the bandwidth to guarantee the en/decoding perfor-mance. As 2-D optical codes can be generated by the proposeddevice, the number of possible codes (or code cardinality) is

and thus it can support more active userscomparing to that of a 1-D transmission device with only spec-tral phase pattern. In a practical OCDMA system with mul-tiple-access-interference (MAI) noise, the usable code set maybe reduced. To scale this technique by increasing the code lengthat the cost of chip size, smaller ring with low waveguide lossand crosstalk is desirable for achieving large FSR and coveringmore frequency bins to enhance the performance.

IV. CONCLUSION

A novel optical en/decoder composed of coupled micro-ringreflectors is proposed for two-dimensional coherent OCDMAapplication. By optimizing the rings-bus and ring-ring couplingcoefficients, a flat-top reflective spectral response can be formedfor the micro-ring reflector. The coding performance of recon-figurable wavelength hopping and spectral phase encoding hasbeen investigated by simulation. The proposed micro-ring re-flector based optical en/decoder can be integrated with otherphotonic devices, and decreasing the size would enable a com-pact, flexible and programmable 2-D coherent OCDMA system.

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