An opto-very-large-scale-integrated (opto-VLSI)-based tunable optical filter structure is demonstrated. Filter tunability is achieved by reconfiguring the holographic diffraction grating of an opto-VLSI processor, allowing virtually any type of filter response to be synthesized. A proof-of-concept tunable notch filter with wavelength span of 7 nm is experimentally verifie

Alameh, Kamal

Poh, Chung Kiak

Wang, Zhenglin

English

Research Online @ ECU

Edith Cowan University Research Online ECU Publications Pre. 2011 2006 Demonstration of Tunable Optical Notch Filter Using 1-D Opto-VLSI Processor Chung Kiak Poh Edith Cowan University Kamal Alameh Edith Cowan University Zhenglin Wang Edith Cowan University Follow this and additional works at: https://ro.ecu.edu.au/ecuworks  Part of the Engineering Commons 10.1109/LPT.2006.875339 This is an Author's Accepted Manuscript of: Poh, C. K., Alameh, K. , & Wang, Z. (2006). Demonstration of Tunable Optical Notch Filter Using 1-D Opto-VLSI Processor. IEEE Photonics Technology Letters, 18(11), 1234-1236. Available here © 2006 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. This Journal Article is posted at Research Online. https://ro.ecu.edu.au/ecuworks/2076 1234 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 11, JUNE 1, 2006Demonstration of Tunable Optical Notch FilterUsing 1-D Opto-VLSI ProcessorChung-Kiak Poh, Kamal Alameh, Senior Member, IEEE, and Zhenglin WangAbstract—An opto-very-large-scale-integrated (opto-VLSI)-based tunable optical filter structure is demonstrated. Filtertunability is achieved by reconfiguring the holographic diffractiongrating of an opto-VLSI processor, allowing virtually any type offilter response to be synthesized. A proof-of-concept tunable notchfilter with wavelength span of 7 nm is experimentally verified.Index Terms—Opto-very-large-scale-integrated (opto-VLSI)processor, phase hologram, tunable notch filter.I. INTRODUCTIONTUNABLE optical filters are versatile devices that have ap-plications in optical code-division multiple-access [1], [2],variable optical attenuators [3], space to wavelength switches[4], and dispersion compensators [5]. In particular, tunable op-tical notch filters are key components for spectral equalizationin wavelength-division-multiplexing (WDM) optical networks[6], where the nonflat gain spectrum of erbium-doped fiber am-plifiers (EDFA) leads to a power imbalance of the WDM chan-nels, especially if many EDFAs are cascaded [7]. A dynamicgain-equalizer is also of great interest because the gain spectrumof the EDFA depends on its signal input power. Among poten-tial technologies used for tunable notch filtering are mechan-ical switches [8], acoustooptic filters [9], and Fabry–Pérot fil-ters [10]. Although these filter structures can tune the notch fre-quency, their ability to change the notch shape factor is limited.In this letter, we report a dynamic optical notch filteremploying an opto-very-large-scale-integrated (opto-VLSI)processor that generates dynamic digital diffraction gratingsto perform adaptive spectral attenuation through optical beamsteering. We experimentally demonstrate a tunable optical notchfilter by reconfiguring the beam steering phase holograms ofa one-dimensional (1-D) opto-VLSI processor. A computeralgorithm is developed which optimizes the steering phase holo-grams that couple specific spectral wavebands to the output fiberport thus realizing a notch response with variable central wave-length and depth. A tuning range of 7 nm is demonstrated with anotch bandwidth of 1.2 nm and less than 1-dB passband ripples.II. THEORYThe key component of the tunable optical filter is the 1-Dopto-VLSI processor. The opto-VLSI processor comprises anarray of liquid crystal (LC) pixels that can be independentlyManuscript received November 16, 2005; revised February 25, 2006.The authors are with the WA Centre of Excellence for MicrophotonicSystems, Edith Cowan University, Joondalup, WA 6027 Australia (e-mail:chungp@student.ecu.edu.au; k.alameh@ecu.edu.au).Digital Object Identifier 10.1109/LPT.2006.875339addressed by a VLSI circuit to reconfigure a multiphase holo-gram capable of beam steering and/or shaping [8]. By incor-porating a thin quarter-wave plate layer between the LC andthe VLSI backplane, a polarization-insensitive opto-VLSI pro-cessor can be realized, allowing optical beam steering with po-larization-dependent loss as low as 0.5 dB [9].The application of a periodic sequence of voltage ramps ofperiod across the opto-VLSI array aperture steers the incidentlight beam to an angle (relative to the normal) given by thegeneral grating equation [10](1)where is the incident wavelength in free space, is theangle of incidence relative to the normal, is the gratingperiod, is the number of pixels within the grating period, andis the center-to-center spacing between the pixels.The steering angle depends on the periodicity (and sign)of the applied voltage ramp. Current LC materials suffer fromfringing field and resistance to rapid change in orientation andas a consequence, a portion of the incident wavefront is steeredto the wrong angle, resulting in a loss in the correct directionand high sidelobes [10]. The overall steering efficiencyis given by [10](2)where is the width of the flyback region. It is important tonote that the overall steering efficiency can be increased by in-creasing the number of pixels within a grating period. However,this reduces the maximum steering angle as obvious from (1).III. TUNABLE OPTICAL FILTER ARCHITECTUREThe architecture of the tunable optical filter is as shown inFig. 1. It consists of a circulator, fiber collimator, a diffractiongrating, and a 1-D opto-VLSI processor. The diffraction gratingmaps the different wavebands of the collimated input opticalsignal onto different pixel blocks on the active area of the opto-VLSI processor. By driving each pixel block with the appro-priate hologram, a waveband in that region can either be steeredback along the incidence path, thus coupling it into the fibercollimator with minimum attenuation, or deliberately steered“off-track” so that its power is partially coupled back into thefiber collimator leading to a high optical attenuation for thatwaveband component. By optimizing the phase holograms ofthe pixel blocks, one can control the overlapping between the1041-1135/$20.00 © 2006 IEEEPOH et al.: DEMONSTRATION OF TUNABLE OPTICAL NOTCH FILTER USING 1-D OPTO-VLSI PROCESSOR 1235Fig. 1. Reconfigurable optical notch filter architecture. The pixels arepartitioned into pixel blocks; each has a different phase hologram that steersincident wavebands from  to  back into the collimator.coupled waveband leading to a notch filter response with lowpassband ripples.IV. RESULTS AND DISCUSSIONAn erbium amplified spontaneous emission source was usedto generate a broadband input optical signal of uniform spec-trum, which was routed, through the circulator, to the fiber col-limator and collimated at 1-mm diameter, as shown in Fig. 1.The diffraction grating used in the experiments was a blazedgrating of 1200 lines/mm and the 1-D opto-VLSI processor had1 4096 pixels. The maximum voltage applied to the pixelswas 5 V. The opto-VLSI processor used in the experimentshad a low reflectivity and a low fill factor resulting in a mea-sured insertion loss of about 6–11 dB depending on the steeringangle. The measured two-way fiber coupling losses were around2 dB. A program was written in LabVIEW to generate and sendcommand voltages in the form of 1-D image to the opto-VLSIprocessor to generate optimized steering holograms for the in-cident wavebands. The program allowed the user to create asmany input channels as required, with each channel indepen-dently controlled in terms of position, width, and phase profile.The default (or initial) phase hologram used for the real-timeoptimization procedure was a blazed grating. The steered wave-bands were coupled into the fiber and routed via the circulator tothe output port and monitored by an optical spectrum analyzer.The first part of the experiment focused on the generation ofa flat passband filter. Multiple independent holograms with dif-ferent widths and periods were optimized in real-time to syn-thesize a flat passband response with 7-nm bandwidth and aripple level less than 1 dB. This is shown in Fig. 2(b) as graphwhose corresponding hologram is shown in Fig. 2(a). Thelimited bandwidth was due to the limited active area of theopto-VLSI processor, which was around 6 mm. It was foundthat the width of a pixel block influences the reflected powercoupled into the fiber collimator and hence controls the attenu-ation of the waveband associated to that pixel block. It was alsofound that the position of the pixel block relative to the center ofthe associated waveband influences the bandwidth and sidelobesof the coupled wavebands. Note that, by using an opto-VLSIprocessor with wider active window or diffraction grating withlower groove frequency, a broader tuning wavelength span canbe achieved.Fig. 2. (a) Holograms in the form of 1-D digital image for the flat-passbandand the various notch filter responses. H : Hologram for the flat passband;H : Hologram for the notch filter with 10-dB attenuation;H : Hologram for thenotch filter with 15-dB attenuation; andH : Hologram for the notch filter with20-dB attenuation. (b) Various filter responses that correspond to input hologramfrom H to H . The notches are centered around 1529.5 nm.Fig. 3. (a) H : Hologram used to generate the 10-dB notch filter centeredat 1528.1 nm. Changes in the hologram relative to H are highlighted. (b) A10-dB-deep notch filter response centered at 1528.1 nm generated usinghologram H .The second part of the experiments concentrated on thedemonstration of a reconfigurable notch filter response. Anotch was inserted at 1529.5 nm and was attenuated by 10, 15,1236 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 11, JUNE 1, 2006Fig. 4. (a) Holograms used to generate the flat notch responses. HologramH is included for comparison. H : Hologram for the flat notch with 3-dBattenuation and a 3-dB bandwidth of 2 nm; andH : Hologram for the flat notchwith 8-dB attenuation and a 3-dB bandwidth of 1.5 nm. (b) Flat notch responsewith variable attenuation generated using hologramH andH .and 20 dB with respect to the passband level while keeping theripple level to within 1 dB, as shown in Fig. 2(b), where graphsare labeled according to their corresponding phase hologramsto that are shown in Fig. 2(a). It is obvious from Fig. 2(a)and (b) that the notch depth can be controlled by changing thephase profile of a small pixel group [highlighted in Fig. 2(a)]while keeping the rest of the hologram intact.Since the passband was constructed by a series of indepen-dent phase holograms, it was also possible to have the notchcentered at any wavelength (within the passband limit) with ar-bitrary notch bandwidth and depth, as shown in Fig. 3(b) whosecorresponding phase hologram is shown in Fig. 3(a). Alsohighlighted for comparison is the hologram that was used togenerate the flat passband response.Fig. 4(a) and (b) shows the holograms that generate flat notchresponses of bandwidth 2 nm centered around 1531.5 nm.The flat notch depth could be varied from 3 to 8 dB simplyby adjusting the input phase hologram of the pixel blockscorresponding to the flat notch region [Fig. 4(a)]. This validatesthe potential of opto-VLSI technology to generate arbitraryoptical notch filter responses.V. CONCLUSIONWe have demonstrated a reconfigurable optical notch filterbased on a 1-D opto-VLSI processing. The ability to control thebandwidth, central frequency, and depth of the notch filter hasbeen demonstrated over a 7-nm wavelength span with less than1 dB of passband ripples.REFERENCES[1] A. Weiner, “Femtosecond pulse shaping using spatial light modulators,”Rev. Sci. Instrum., vol. 71, pp. 1929–1960, 2000.[2] H. P. Sardesai, C. C. Chang, and A. M. Weiner, “A femtosecond codedivision multiple access communication system test bed,” J. Lightw.Technol., vol. 16, no. 11, pp. 1953–1964, Nov. 1998.[3] J. E. Ford and J. A. Walker, “Dynamic spectral power equalization usingmicro-opto-mechanics,” IEEE Photon. Technol. Lett., vol. 10, no. 10, pp.1440–1442, Oct. 1998.[4] D. T. Fuchs, C. R. Doerr, V. A. Aksyuk, M. E. Simon, L. W. Stulz,S. Chandrasekhar, L. L. Buhl, M. Cappuzzo, L. Gomez, A. Wong-Foy,E. Laskowski, E. Chen, and R. Pafchek, “A hybrid MEMS-waveguidewavelength selective crossconnect,” IEEE Photon. Technol. Lett., vol.16, no. 1, pp. 99–101, Jan. 2004.[5] T. Sano, T. Iwashima, M. Katayama, T. Kanie, M. Harumoto, M. Shige-hara, H. Suganuma, and M. Nishimura, “Novel multichannel tunablechromatic dispersion compensator based on MEMS and diffractiongrating,” IEEE Photon. Technol. Lett., vol. 15, no. 8, pp. 1109–1110,Aug. 2003.[6] K. Maru, K. Tanaka, T. Chiba, H. Nonen, and H. Uetsuka, “Dynamicgain equalizer using proposed adjustment procedure for thermoopticphase shifters under the influence of thermal crosstalk,” J. Lightw.Technol., vol. 22, no. 6, pp. 1523–1532, Jun. 2004.[7] S. Wada, S. Abe, Y. Ota, B. Reichman, T. Imura, and C. A. Daza, “Vari-able gain equalizer using magneto-optics,” in Optical Fiber Communi-cation Conf. Exhibit (OFC), 2002, pp. 324–326.[8] R. Giles, V. Aksyuk, and B. Barber, “A silicon MEMS optical switchattenuator and its use in lightwave subsystems,” IEEE J. Sel. TopicsQuantum Electron., vol. 5, no. 1, pp. 18–25, Jan. 1999.[9] J. L. Jackel, J. L. Goodman, M. S. Baran, and J. E. Tomlinson,“Acousto-optic tunable filters (AOTFs) for multiwavelength opticalcross-connects: crosstalk considerations,” J. Lightw. Technol., vol. 14,no. 6, pp. 1056–1066, Jun. 1996.[10] D. Sadot and E. Boimovich, “Tunable optical filters for dense WDMnetworks,” IEEE Commun. Mag., vol. 36, no. 12, pp. 50–55, Dec. 1998.[11] M. Raisi, S. Ahderom, K. E. Alameh, and K. Eshraghian, “Opto-VLSImultiband tunable optical filter,” Electron. Lett., vol. 39, no. 21, pp.1533–1534, Oct. 2003.[12] I. G. Manolis, T. D. Wilkinson, M. M. Redmond, and W. A. Cross-land, “Reconfigurable multilevel phase holograms for Optical switches,”IEEE Photon. Technol. Lett., vol. 4, no. 6, pp. 801–803, Jun. 2002.[13] P. McManamon, T. A. Dorschner, and D. L. Corkum, “Optical PhasedArray Technology,” IEEE Proc., vol. 84, no. 2, pp. 268–298, Feb. 1996.

Demonstration of Tunable Optical Notch Filter Using 1-D Opto-VLSI Processor

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Demonstration of Tunable Optical Notch Filter Using 1-D Opto-VLSI Processor

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