1802 OPTICS LETTERS / Vol. 28, No. 19 / October 1, 2003

Optical microwave filter based on spectral slicing by use ofarrayed waveguide gratings

Daniel Pastor, Beatriz Ortega, José Capmany, Salvador Sales, Alfonso Martinez, and Pascual Muñoz

Optical Communications Group, Departamento de Comunicaciones and IMCO2 Research Institute,Universidad Politécnica de Valencia, Camino de Vera s/n, Valencia 46022, Spain

Received April 3, 2003

We have experimentally demonstrated a new optical signal processor based on the use of arrayed waveguidegratings. The structure exploits the concept of spectral slicing combined with the use of an optical disper-sive medium. The approach presents increased f lexibility from previous slicing-based structures in termsof tunability, reconfiguration, and apodization of the samples or coefficients of the transversal optical filter.© 2003 Optical Society of America

OCIS codes: 060.0060, 060.2330, 060.4550, 060.2340.

Many different approaches to optical processingfor f iltering tasks over the radio-frequency (rf),millimeter-wave and microwave bands have beendemonstrated in the past few years by use of typi-cal finite impulse response structures1 –6 and morerecently by direct f iltering in the optical domain,employing specially designed fiber Bragg gratingstructures.7 Here we focus on the f irst set of ap-proaches (finite impulse response structures). Oneset of these structures is based on using only one lasersource1,2 and had yielded promising results regardingfilter shape and tunability. This approach presentsan intrinsic limitation resulting from the coherentbeating that limits the maximum separation betweenrf bands to hundreds of megahertz. The use of an ar-ray of tunable lasers to construct a transversal f ilter incombination with a dispersive optical component hasalso been proposed3,4 as a way to avoid the coherenceproblem, allowing for very low time delays betweensamples (high sample rate) and therefore a veryhigh free spectral range (FSR). This approach alsoprovides the maximum f lexibility with regard to theapodization or weighting of the samples. The maindisadvantage is the relatively high cost if the numberof sources increases. The last possibility is to use theslicing technique over a broadband optical source toobtain a sampled optical spectrum that could replacethe laser array.5,6 This last approach also presentsits own limitations, related mainly to poor f lexibility,since it relies on fixed optical filters to accomplishthe source slicing. In this Letter we demonstrate anoptical processor based on the slicing technique thatuses arrayed waveguides (AWGs) and that relaxes thef lexibility restriction of this technique, opening newpossibilities for f lexible and tunable devices.

The structure of the proposed and tested optical pro-cessor is depicted in Fig. 1. From left to right we canfind two broadband optical sources in cw operation,a SuperLED (SLED; centered around 1555 nm witha 40-nm bandwidth), and an erbium-doped fiber am-plifier (EDFA). We combine these two sources to ob-tain a nearly f lat spectrum in the wavelength rangeof interest. The following optical elements are two1 3 40 channel AWGs with equal spectral responseplaced symmetrically. The AWGs were originally de-signed for dense wavelength-division multiplexing ap-

0146-9592/03/191802-03$15.00/0 ©

plications with 0.4-nm bandwidth with the standardITU spacing between channels of 0.8 nm. Figure 1shows one of the sliced optical spectra that was em-ployed; 12 channels were used (12 output channels ofthe f irst AWG connected to the corresponding 12 inputchannels of the second AWG). We amplif ied the slicedoptical spectrum to overcome some of the excess lossesof AWGs (approximately 12 dB). The slicing of theoptical spectrum could also be done by means of aset of fiber Bragg gratings in a serial configurationplaced at the same fiber and using only one circula-tor. This feasible slicing structure, although it pro-duced less optical loss, also presented difficulties forslice-by-slice control of power and therefore for recon-figuration. The cw sliced spectrum was then modu-lated by an electro-optical modulator (EOM). The rfsignal for the modulation proceeded from a networkanalyzer that provided the final response of the opti-cal f ilter. Finally, before the detector was applied themodulated signal goes through a 23-km coil of stan-dard single-mode fiber that played the role of a dis-persive optical element, providing the basic time delaybetween each spectral slice. It can be observed thatthe structure behaves as a traditional transversal op-tical rf f ilter based on a dispersive optical device3,4 butemploying an optical source based on a sliced broad-band optical signal instead of a laser array. A similarapproach was experimentally proved in Ref. 5, but inthat study a Fabry–Perot optical f ilter was employedto obtain the spectrum. The main drawback of theFabry–Perot approach was its intrinsic lack of f lexi-bility because of the impossibility of adjusting the FSRof the Fabry–Perot filter or to adjust the amplitude

Fig. 1. Optical transversal f ilter structure. SOA, semi-conductor optical amplif ier.

2003 Optical Society of America

October 1, 2003 / Vol. 28, No. 19 / OPTICS LETTERS 1803

of each resonance on an individual basis. The pro-posed structure overcomes these limitations becausethe slices can be switched on or off or can be adjustedindependently (see the intermediate element betweenthe AWGs in Fig. 1).

Figure 2 shows the rf normalized response forthe f irst case of spectrum slicing shown in Fig. 1.In the f irst case, alternate channels were chosenfrom 1543.7 to 1561.3 nm up to a total number of12 bands (samples), 1.6 nm apart. The rf responsepresents a band spacing of 1.56 GHz that correspondsto the inverse of the basic time delay between thetransversal contributions ��640 ps�. These datavalues agree with the dispersion features of the fibercoil �400 ps�nm. The first rf passband presents atotal width of �125 MHz at 23 dB and �200 MHzat 210 dB (see the inset of Fig. 1). The main-to-secondary-lobe ratio (MSLR) measured at the f irstresonance is 15 dB. It has to be pointed out thatthe lobe at zero frequency does not appear in Fig. 2because the network analyzer employed has a lowerspectral cutoff frequency of 130 MHz. Another impor-tant feature of the measurement in Fig. 2 is the fadingof the spectral response as the rf frequency increases.This behavior is due to the interaction between thedispersion of the f iber coil and the nonzero opticalbandwidth of each spectral slice (0.4 nm). This inter-action is manifested as a low pass f iltering effect thatweights the ideal response. Other phenomena to beobserved from the results in Fig. 2 are the progressiveincrease of the rf lobe bandwidth and the decrease ofthe MSLR in the second, third, fourth, and subsequentresonances. This particular phenomenon is due tothe second-order dispersion S �S � dD�dl� or thegroup-velocity dispersion (GVD) slope present on thestandard single-mode fiber (SSMF). To verify thiswe carried out numerical simulations of the structureunder typical values of S � 0.06 ps�nm2 km and GVD400 ps�nm (23 km of standard single-mode fiber) andwith the corresponding sliced trace of the spectrum.Figure 3(a) shows the measured rf response (thickcurve) and the simulated rf response (thin curve withopen circles). In this case one of each two channelswas removed from the previous case (only six slices),resulting in a 3.2-nm wavelength spacing. As canbe observed, the agreement is almost perfect for thefirst lobes and very good for the higher-order lobeswithout any other types of numerical corrections.The FSR decreased by a factor of 2 to 780 MHz, andthe MSLR decreased several decibels to 12 dB in thefirst lobe region. The bandwidth of the passbandsis still 125 MHz at 23 dB as the theory predicts,because the number of transversal taps decreased(by a factor of 2) and the FSR decreased by the samefactor. Figure 3(b) shows the simulated normal-ized decay envelope for a f ixed GVD of 400 ps�nmand second-order dispersion S � 0 to separate thephenomena. Different slice widths from bandwidth�nm� � �0.1 0.6 nm� were used. Also, the detail ofthe rf response for an 11-slice transversal filter of0.4 nm is represented. Note that the envelope isnot dependent on the FSR or the number of samplesof the transversal f ilter, so the results shown are

general and applicable to all the f ilters with thesame GVD �ps�nm� 3 BW �nm� product. The rffrequency, which shows 3-dB decay with respect tof � 0, can be obtained by application of the expressionf3dB �GHz� � 431��BW �nm� 3 GVD �ps�nm��. Thisformula is exactly valid for our specific case (i.e., twocascaded AWGs and therefore a squared Gaussianshape of the slice). Other optical f ilters could verifyslightly different coefficients in the numerator.

Other rf reconfiguration possibilities can be still ex-ploited. For example, rf tuning of the bands to higherfrequencies can be achieved by reduction of the wave-length separation of the slices. To measure this re-duction, all the channels of the AWGs from 1542.9 to1561.3 nm were connected. The wavelength spacingwas the minimum achievable, 0.8 nm, and the numberof slices was 24. The resultant sliced spectrum andthe rf response are shown in Fig. 4. There is a clearlimitation of tunability because of the restricted num-ber of channel sets used on the slicing (slicing spacing0.8, 1.6, 2.4 nm, etc.), and the optical channel position

Fig. 2. Normalized rf amplitude for the sliced spectrumcase of Fig. 1. Inset, precise measurement of the f irstpassband.

Fig. 3. (a) Normalized rf amplitude: measured (thickcurve) and simulated (thin curve with circles). Sixslices spaced 3.2 nm apart. FSR, 780 MHz. (b) Decayenvelope as a result of dispersion and slice width.

1804 OPTICS LETTERS / Vol. 28, No. 19 / October 1, 2003

Fig. 4. Normalized rf response for 24 slices (1542.9–1561.3 nm), with 0.8-nm wavelength spacing. FSR,3.1 GHz; MSLR, 14 dB; 3-dB bandwidth, �125 MHz.Q factor, �24.8.

Fig. 5. rf response for six apodized slices, spaced 3.2 nmapart. FSR, 780 MHz; 3-dB bandwidth, �200 MHz;MSLR, 15 dB.

of the AWGs cannot be tuned easily. A certain degreeof continuous tunability (or in a smaller step basis)can be achieved by use of variable or switched disper-sive devices as different f iber coil lengths or tunablelinearly chirped gratings, although in the latter casethe available optical bandwidth will be limited by thegrating bandwidth.

In Fig. 4 the optical spectrum is represented inlinear units that provides clearer information aboutthe power nonuniformity as a result of nonuniformlosses in connectors and polarization-dependent lossesof the AWG and the EOM. In one application casethese two factors may be better controlled. The FSRwas 3.1 GHz, with a MSLR of 14 dB, and the lobe3-dB bandwidth was maintained near 125 MHz. Oneof the most important features of the proposed struc-ture is the possibility of adjusting the amplitude ofeach spectral slice independently by applying opticalloss or gain to each separate connection betweenthe AWGs. This can be done manually or automati-cally, employing electronically controlled variableattenuators that are already available for power equal-ization in dense wavelength-division multiplexingapplications. We manually adjusted the power of thespectral slices, inserting losses at the connection array.The sliced spectrum of six slices and the rf measuredresponse are shown in Fig. 5. The shape followed bythe power adjustment was nearly triangular at the

edges. As the theory predicts, the MSLR decreaseswith respect to the case of six nonapodized slices inFig. 3(a). Also, the main lobe increases in bandwidthto 200 MHz. Note how the response in the lattercase presents an almost periodic and very predictiveshape, not only for the f irst lobe but also for the restof the lobes up to the f ifth. The periodic behavioris altered or weighted only by the always-presentlow-pass filtering effect mentioned above. This clearincrease of the response quality was due to the morecareful adjustment made during the measurement ofthe apodization shape, contrary to the previous cases(Figs. 2–4), in which power f luctuations up to 2 dBwere allowed in a more relaxed measurement. Ingeneral, proper control of the modulated amplitudesof the different signal taps at the detector is verycritical if a high MSLR must be obtained. The mainsource of uncertainty in this sense was the fact thatthe polarization dependence of the EOM was polarizedas a result of the superLED light.

In summary, the proposed structure has greatpotential because (1) the spectrum slices can be in-dependently adjusted or switched on or off by opticalcomponents as electronically operated attenuatorsdeveloped for dense wavelength-division multiplexing,providing fast (milliseconds) tunability and reconfigu-rability and (2) the structure is a cheaper approachthan laser array structures, especially when the num-ber of contributions increases. State of the art AWGsallow an extremely high number of channels8 withdifferent channel separation. The main deleteriousphenomenon is the low-pass filtering effect of the dis-persion combined with the broadband slices that occurmainly for high-order bands and that can be overcomewith lower GVD �ps�nm� 3 BW 3 �nm� products.

The authors acknowledge the f inancial support ofthe Spanish Comisión Interministerial de Ciencia yTecnologia project and TIC2001-2969-C02-01 and theEuropean projects NEFERITITI IST-2001-32786 andLABELS IST-2001-37435. D. Pastor’s e-mail addressis dpastor@dcom.upv.es.

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