Optical Interference Suppression using MicroPhotonic RF Filter Structure

Transcription

1 Edith Cowa Uiversity Research Olie ECU Publicatios Pre Iterferece Suppressio usig MicroPhotoic Filter Structure Rog Zheg Edith Cowa Uiversity Kamal E. Alameh Edith Cowa Uiversity Zhegli Wag Edith Cowa Uiversity /COMMAD This coferece paper was origially published as: Zheg, R., Alameh, K.E., & Wag, Z. (005). Iterferece Suppressio usig MicroPhotoic Filter Structure. Proceedigs of 004 Coferece o Optoelectroic ad Microelectroic Materials ad Devices. (pp ). Brisbae. IEEE. Origial article available here 005 IEEE. Persoal use of this material is permitted. Permissio from IEEE must be obtaied for all other uses, i ay curret or future media, icludig repritig/republishig this material for advertisig or promotioal purposes, creatig ew collective works, for resale or redistributio to servers or lists, or reuse of ay copyrighted compoet of this work i other works. This Coferece Proceedig is posted at Research Olie.

2 Iterferece Suppressio usig MicroPhotoic Filter Structure Rog Zheg, Kamal E. Alameh ad Zhegli Wag Cetre for MicroPhotoic Systems Electro sciece Research Istitute, Edith Cowa Uiversity, Joodalup, Perth, Australia Abstract--The impact of laser coherece oise o covetioal photoic filters is ivestigated i this paper. I additio, a ew MicroPhotoic adaptive filter structure is proposed, which ca simultaeously suppress the phased-iduced itesity oise caused by optical iterferece. Results show that the coherece legth of the laser light sigificatly degrades the frequecy respose of a photoic trasversal filter, whereas the MicroPhotoic filter has the capability of geeratig arbitrary trasfer fuctio with o phase-iduced itesity oise. Keywords: laser coherece, phase-iduced itesity oise, photoic sigal processor. I. INTRODUCTION The ability of photos to carry high iformatio capacity, preserve their eergy for a loger time tha electros, ad pass ear oe aother without geeratig crosstalk, has drive the use of fibres for adaptive widebad sigal processig [1-3]. Photoic sigal processors based o fibre optical cavities have bee widely used for the realizatio of trasversal filters through true-time delay geeratio [4-7]. However, these filter structures have two fudametal problems, amely: They geerate substatial phase-iduced itesity oise, caused by the log coherece legth of laser sources. Their trasfer characteristics caot be arbitrarily recofigured iterferece is a commo problem ecoutered i photoic sigal processig systems utilizig log coherece legth lasers. I such systems, delayed -modulated optical sigals, which may be origiated by multiple reflectios, coectors, or splices i the optical systems, ca be coverted to itesity oise that falls withi the bad, hece it caot be filtered out. The aalysis of phase-iduced itesity oise (PIIN) has previously bee studied i details [8-1]. Tur et al. have theoretically ad experimetally ivestigated the phased-iduced itesity oise power spectrum of a fibre recirculatig loop [8]. They showed that this oise sigificatly degrades the performace of a photoic sigal processor ad limits its highfrequecy operatio rage. Moslehi has obtaied a uiversal expressio for the phase-iduced itesity oise of a geeral optical sigal processor [11], [1]. Although both aalyses have accurately evaluated the iterferece, they have both igored the iteractio betwee the iterferece caused by the optical carrier ad the modulatig sigal itself. I this paper, we ivestigate the oise performaces of a covetioal 4-tap photoic filter, icludig the iterferece itroduced by optical carrier ad the modulatio sigal, ad propose a ew MicroPhotoic adaptive iterferece mitigatio filter structure, which ca simultaeously suppress the optical iterferece ad sythesise arbitrary frequecy resposes. The proposed MicroPhotoic structure itegrates a VCSEL array, a D ultra-widebad photoreceiver array ad a multi-cavity optical substrate, ad ca achieve arbitrary, highresolutio filter trasfer characteristics, with o phaseiduced itesity oise. Results show that the coherece legth of the laser light sigificatly degrades the frequecy respose of a photoic trasversal filter, whereas the MicroPhotoic filter has the capability of geeratig arbitrary trasfer fuctio with o phase-iduced itesity oise II. CONVENTIONAL PHOTONIC FILTER The architecture of the covetioal photoic filter is show i Figure 1. The laser output is itesity modulated by a trasverse electro-optic amplitude modulator, amplified by a Erbium-Doped Fibre amplifier (EDFA) ad fed ito a 1:N optical power splitter. The split sigals are delayed ad the recombied with a N:1 optical combier ad the combied sigal is detected by a photodetector. To ivestigate the impact of the laser coherece legth o the respose of the trasversal filter, we assume a DFB laser source of a typical Lorezia-shape spectrum modulated by a siusoid. The electric field of the iput -modulated optical sigal is give by: jω 0t E i = [ 1 + M cos ω t] e ε (1) where M is the modulatio idex, ad (t) is the slowly varyig evelop of the -modulated optical field, which is give by: /05/$ IEEE 01 COMMAD 04

3 j ε ε ω ω t = ~ ( ) e dω () Usig Wieer-Khitchie theorem, the autocorrelatio fuctio of optical field ca be defied as: Γ ( τ ) = Γ ( τ ) = ε ( t ) ε ( t τ ) (3) where <> represet a esemble, or time-averagig process. Without loss of geerality, we assume that (t) is ormalized such that (0) = 1. () ca be expressed i terms of the badwidth ±, the coherece legth of the laser source l coh, is give by: l coh ~ π c / δω (4) For the covetioal photoic otch filter show i Figure 1, the output field E out (t) ca be expressed as: M jω 0 t M j ( ω 0 + ω ) t M j ( ω 0 ω ) t E out = k ε ( t ) e + e + e (5) = 0 This shows that the output field E out (t) impigig o the photodetector has three spectral compoets at 0, ad 0 ± The photodetector curret is give by: I = A ε ε ( t τ ) (6) Whe the detectio respose time d is sufficietly short, 1/ d 0, the detected curret has three frequecy compoets, amely: dc; ad, which are referred to as the dc, ad secod-harmoic currets, respectively. The dc ad secodharmoic currets are usually filtered out, ad will ot be discussed here. The respose ca be expressed i terms of its amplitude A ad phase fuctios, j( ω t+φ ) I = Ae (7) For a optical source of Loretzia lie shape spectrum, the autocorrelatio fuctio of optical field is give by: δωτ Γ ( τ ) = e (8) Substitutig Equatio 8 ito Equatio 6 yields, N N δω ( m ) T A = kkme cos[( m ) ω0t ][cos( mω ) + cos( ω T )] = 0 m= 0 N N δω ( m ) T + kkme cos[( m ) ω0t ][si( mω ) + si( ω T )] (9) = 0 m= 0 where N is the umber of the ports of the splitter, T is the uit delay time for each optical path, k i is the couplig ratio for each brach of the optical splitter ad lossless. III. N k i i = 1 MICROPHOTONIC FILTER = 1, if the splitter is MicroPhotoic architectures utilizig true-time-delay methods ca perform broadbad sigal processig that caot be achieved by the covetioal photoic filter structure described i Figure 1. CW source Electro- optic Modulator iput sigal Amplifier Amplifier 1:4 Splitter L L+m L+4m L+6m HP 8150 Network Aalyzer 4:1 Combier Photodetector Fibre Delay Lies output sigal Figure 1. The cofiguratio of covetioal photoic filter Figure illustrates the priciple of MicroPhotoic sigal processig. A electrical-to-optical coversio array coverts the iput sigal ito may -modulated collimated optical beams, which are delayed directly i the optical domai via the optical delay geerator that geerates a large umber of delayed optical beams. The delayed optical beams are photodetected ad appropriately post-amplified by a photoreceiver array to produce a delayed sigal with appropriate amplitude. By combiig the delayed sigals, a FIR trasversal filter characteristic is sythesized, which depeds o the amplitudes ad the delay times of the combied sigals. modulated Beams Iput Sigal Electricalto- Coversio Delay Geerator Delayed Beams D Photoreceiver Array Delayed sigals Figure. Priciple of MicroPhotoic sigal processig The architecture of the MicroPhotoic filter is show i Figure 3. The low-oise amplifier (LNA) pre-amplifies the iput sigal to compesate for subsequet splittig loss ad also to boost the modulatio efficiecy of the VECSEL array (where the first E stads for exteral ). The splitter equally splits the iput sigal ito N sigals, which modulate the N elemets of the 1 N VECSEL array itegrated o the VECSEL/photoreceiver chip. The VECSEL array ca geerate high optical power per elemet while maitaiig fudametal mode oscillatio. The diffractive optical elemet (DOE) is a thi film that collimates ad routes the Gaussia beams geerated by the VECSEL array. Each -modulated collimated optical beam geerated by a VECSEL elemet propagates withi the optical substrate ad udergoes several reflectios i a cavity whose legth defied by oe of the mirrors assiged to that VECSEL elemet ad the DOE. Every time a beam hits the DOE, a small 0

4 fractio (5-10%) of the power of that beam is trasmitted through the DOE for detectio ad amplificatio by a elemet of the widebad photoreceiver array that is itegrated o the VECSEL/photoreceiver chip, while the remaiig large fractio is reflected ad routed to for subsequet delayed photodetectio. A combier adds (or subtracts) the amplified photocurrets to geerate the output sigal. To easily comprehed the ability of the MicroPhotoic filter to recofigure its trasfer characteristic, we examie the impulse respose of the filter by drivig the iput with a impulse. I this case, each VECSEL elemet of the VECSEL array geerates a optical impulse of power P 0. The optical impulse propagatig iside a optical cavity of legth L m is sampled at a time icremet of 4 L m + d / υ, where d is the spacig betwee the photoreceiver elemets, ad is the speed of light iside the cavity. The power of the reflected impulse after photodetectios is P 0 R ad that of the trasmitted impulse is P 0 (1 - R)R, where R is the reflectivity of the DOE. For example, if R = 90% ad P 0 = 10 mw, the the power of the optical impulse detected by the first photoreceiver elemet is 0.1 mw while the power of optical impulse detected by the 16 th photoreceiver elemet is 0. mw. By combiig the arbitrarilyweighted curret impulses, a adaptive Fiite Impulse Respose (FIR) trasversal filter is realized. The frequecy respose of the filter is give by: N 1 N 1 4L + d m H ( ω ) = RK P R R G j VCSEL 0 (1 ) m, exp ω (10) m= 0 = 0 v where R is the photoreceiver resposiveess, G m, is the curret gai of the photoreceiver elemet (m,), ad K VCSEL = OMD / I is the modulatio respose of the VCSEL elemets, with OMD is the optical modulatio depth ad I is the iput curret modulatig the VCSEL elemet. The weights of the curret impulses geerated by the photoreceiver elemets associated with a optical cavity ca be chaged by adjustig the gais of those photoreceiver elemets. IV. RESULTS Figure 4 shows the simulated ormalized phase-iduced itesity oise versus the laser coherece legth for a 4- tap covetioal photoic otch filter. The uit legth of fibre delay lie was assumed to be 100 mm. It is show that whe the coherece legth of the laser is reduced from 10 m to 40 mm, the oise is reduced sigificatly over the bad. Figure 5 shows the ormalized maximum phase-iduced itesity oise versus the legth of the fibre icremet delay, for a laser coherece legth of 10 m. By icreasig the fibre icremet delay, the maximum PIIN is reduced. Therefore, to attai a low PIIN, ad hece a stable filter respose, the coherece legth of the laser source must be smaller tha the delay icremet i the filter structure. However, the cetre frequecy of the filter is iversely proportioal to the legth of the delay icremet. Therefore, for a give laser coherece legth, the PIIN ca oly be reduced i low-frequecy (or log delay icremet) filter structures. Figure 4 Normalized phase-iduced itesity versus the laser coherece legth. The legth of delay:100mm Widebad D Photoreceiver Array Diffractive Elemet (DOE) Substrate Combier out i Mirrors Collimated Beams VECSEL Array Figure 3. Architecture of the MicroPhotoic filter Splitter LNA Figure 5. Normalized maximum phase-iduced itesity versus delay legth correspodig to uit delay time T Computer simulatios were carried out to geerate a tuable iterferece mitigatio filter with miimum passbad ripples by recofigurig the gais of the photoreceiver amplifiers of the MicroPhotoic filter. Results were focused o recofigurig the 03

5 photoic structure to realise multiple bad otch filter objectives. Figure 6 shows the frequecy resposes for a 16- cavity MicroPhotoic filter employig a photoreceiver array. The gai profiles were optimized to tue a otch respose at cetre frequecies aroud 4.75 GHz while sythesizig two fixed otches at 0.7 GHz ad 8.75 GHz. The legth of the shortest cavity was assumed to be 1 cm. V. CONCLUSION The impact of optical coherece o the performace of optical fibre based FIR filter has bee studied i detail. Photoic sigal processors usig log-coherece-legth optical sources geerate phase-iduced itesity oise which causes substatial istability to the resposse. To suppress the phase-iduced itesity oise i the photoic filter, a ovel MicroPhotoic broadbad iterferece mitigatio processor that geerates true-time delays to perform fiite impulse respose trasversal filterig, has bee preseted. The ovel MicroPhotoic structure itegrates a photoreceiver array, a Vertical Cavity Surface Emittig Laser (VCSEL) array, ad a multi-cavity optical substrate to realize a low-cost adaptive widebad iterferece mitigatio filter. Experimet results demostrated the geeratio of may true-time delayed optical beams ad computer simulatio results verified the capability of the MicroPhotoic processor to realize a high-resolutio, multibad tuable iterferece mitigatio filter. REFERENCES Figure 6. Three-bad MicroPhotoic filter resposes. Tuig aroud 4.75 GHz. Figure 7 shows a spot array geerated by lauchig four, 500m diameter collimated optical beams ito a custom-made optical cavity of legth 0 mm ad mirrors of reflectivities 95% ad 100%. This proves the capability of the MicroPhotoic filter to geerate a spot array of delayed -modulated beams. It is obvious from Figure 7 that the beams diverge as they propagate withi the cavities, ad that a DOE film is critical for geeratig loger delays. This issue will be addressed i future publicatios. [1] K.P.Jackso, S.A.Newto, B.Moslehi, M.Tur, C.C.Cutler, J.W.Goodma, ad H.J.Shaw, fiber delay-lie sigal processig, Microwave Theory ad Techiques,IEEE Trasactios o., vol. MIT-33, o., pp , [] R. Miasia, Photoic sigal processig of high-speed sigals usig fiber gratigs, fiber techology, vol. 6, o., pp , 000. [3] M. Y. Frakel ad R. Esma, Fiber optical tuable microwave trasversal filter, Photoics Techology Letter, IEEE, pp , [4] A. Seeds, Microwave photoics, Microwave Theory ad Techiques,IEEE Trasactios o, vol. 50, o. 3, pp , 00. [5] N. You ad R. Miasia, A ovel high-q optical microwave processor usig hybrid delay-lie filters, Microwave Theory ad Techiques, IEEE Trasactios o, vol. 47, o , pp , [6] J. Capmay, D. Pastor, ad B. Ortega, New ad flexible fiber-optic delaylie filters usig chirped bragg gratigs ad laser arrays, Microwave Theory ad Techiques, IEEE Trasactios o, vol. 47, o. 7, pp , [7] D. Pastor, J. Capmay, S. Sales, P. Muoz, ad B. Ortega, Recofigurable fiber-optic-based filters usig curret ijectio i multimode lasers, Photoics Techology Letters, IEEE, vol. 13, o. 11, pp , 001. [8] M. Tur., B. Mashi, ad J. W.Goodma, Theory of laser phase oise i recirculatig fiber optical delay lies, Lightwave Techology, Joural of, vol. LT-3, o. 1, pp. 0 31, [9] J. L.Gimlett ad N. K.Cheug, Effects of phase -to- itesity moise coversio by multiple reflectios o gigabit-per-secod dfb laser trasmissio systems. Lightwave Techology, Joural of, vol. 7, o. 6, pp , [10] P. K. Pepeljugoski ad K. Y.Lau, Iterferce reductio i fiber optic liks by superpositio of high frequecy modulatio, Lightwave Techology, Joural of, vol. 10, o. 7, pp , 199. [11] B. Moslehi, Aalysis of optical phase oise i fiber-optic systems employig a laser source with arbitrary coherece time, Lightwave Techology, Joural of, vol. 4, o. 9, pp , [1] B. Moslehi, Noise power spectra of optical two-beam iterferometers iduced by the laser phase oise, Lightwave Techology, Joural of, vol. 4, o. 11, pp , Figure spot array geerated withi a custom-made 0mm log optical substrate 04