Analysis of mach zehnder modulator response to fiber dispersion in radio over fiber at 60 GHz for multigigabit wireless transmission

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1 Indian Journal of Pure & Applied Physics Vol. 54, Septeber 06, pp Analysis of ach zehnder odulator response to fiber dispersion in radio over fiber at 60 GHz for ultigigabit wireless transission Shankar Duraikannan a *, Siti Barirah Ahad Anas a, Borhanuddin Mohd Ali a, Zuraidah Zan a, Vinesh Thiruchelva b & Pooria Varahra c a Wireless and Photonics Network (WIPNET) Resarch Centre, Departent of Coputer and Counication Systes Engineering, Faculty of Engineering, University Putra Malaysia, UPM Serdang, Selangor, Malaysia b Faculty of Coputing Engineering and Technology, Asia Pacific University, Bukit Jalila, Kuala Lupur 57000, Malaysia c Departent of Electronic Engineering, Maynooth University, Maynooth Co Kildare, Ireland Received 9 July 05; revised July 06; accepted August 06 Radio over fiber (RoF) at 60 GHz band is a proising technology for future wireless ulti-gigabit transission in personal, local and wide area networks. With coherent optical orthogonal frequency division ultiplexing (CO-OFDM) technique adopted in RoF, to achieve ultigigabit speed of transission, the fiber dispersive effect on the odulated output of Mach Zehnder odulator (MZM) of the RoF syste play a critical role in liiting the perforance of the syste. This paper briefly reviews the ulti-gigabit wireless transission at 60 GHz that indicates the need for radio over fiber architecture, followed by the analysis of MZM response in a dispersive fiber it leads to the power degradation of the transitted signal over distance. The results for varied bias voltages (i.e., for large and sall chirp) at finite extinction ratio indicate that the power degradation due to fiber dispersion at any distance of the fiber can be controlled by appropriate choice of the chirp. Furtherore the results indicate that for a personal and local area networks at 60 GHz, which extends up to few hundred eters the power degradation due to fiber dispersion, can be reduced significantly by varying the chirp without any additional dispersion copensation technique. Keywords: Radio over fiber, Fiber dispersion, Mach Zehnder odulator, IEEE80.ad, WiGig, Multigigabit transission Introduction In present scenario the whole world is treading towards internet of things; where everyone wants to connect everything, counicate inforation related to everything, fro everywhere, which lead to the increase in the nuber of interconnected ultigigabit transission speed gadgets inside an office or a house. Also, bandwidth hungry services and applications such as DVB-S, Ultra HDTV, wireless display and docking and other interactive services that require the transission of uncopressed videos over wireless networks have driven the wireless networks to evolve to higher band fro.4 GHz and 5 GHz. The data rate of the future especially, in personal and local area network ay reach 0 Gb/s. A huge copetition exists across the wave standards of high capacity, which are capable of catering the ultigigabit transission needs. The ost recent standard 80.ad of WiGig consortiu leads Multi Gigabit wireless standards (MGWS) over the other *Corresponding author (E-ail: itzdshankar@yahoo.in) transission standards. WiGig recoends the usage of 60 GHz band, the characteristics of which deands the aid of RoF technology for a long distance transission. Multigabit Wireless Transission at 60 GHz The 80.ad deals with the radio technologies at 60 GHz unlicensed frequency, which is capable of supporting data transission rate up to 7 Gbps which is 0 ties faster than IEEE 80.n. The channelization of 60 GHz band is typically around 7 GHz, as listed in Table. The band as shown in Fig. extends fro 57 GHz to 66 GHz with the iddle two channels available Channel nuber Table GHz Band channelization Low frequency (GHz) Center frequency (GHz) High frequency (GHz) Nyquist Roll-off bandwidth factor (MHz) A A A A

2 57 INDIAN J PURE & APPL PHYS, VOL 54, SEPTEMBER 06 around the world. The specification supports 7 Gbps transission speed with OFDM and 4.6 Gbps over single carrier. The spectru even though has an advantage of low ultipath ipairent; the coverage range is liited due to high channel attenuation as shown in Fig.. Bea foring technique is adopted to enable ultigigabit counication extended up to 0 distance. But still the spectru suffers a high penetration loss across the walls as shown in Fig. 3. Thereby the coverage of the spectru is liited inside a roo 3. To enhance the coverage of 60 GHz across the roos of a building or an enterprise a ixed architecture of radio and fiber is adopted widely. The ixed architecture of radio and fiber technology is classified into three broad categories, baseband over fiber, IF over fiber and radio over fiber (RoF). Radio over fiber is widely adopted at 60 GHz, as it enlarges the radio signal coverage and also caters the deand, for iunity to electrical interferences, high linearity, low RF attenuation, high data rates, siple structure RAU and centralized network control and anageent. The conceptual representation of the RoF syste is shown in Fig. 4. The input RF signal odulates the intensity of the laser light at the head end which is transitter over fiber to the reote antenna units at any distance fro the head end. The optical signal is converted to back RF signal at the reote antenna units enabling the wireless transission at the site 4-8. The RoF syste eploys a diverse category of odulation techniques, either direct odulation or external odulators such as Mach Zehnder odulators (MZM). The external odulation is Fig. Worldwide spectru of 60 GHz band Fig. 3 Penetration loss in db of 60 GHz passing through differen aterials Fig. Path loss of.4 GHz, 5 GHz and 60 GHz RF signal Fig. 4 Conceptual representation of RoF syste

3 DURAIKANNAN et al: MACH ZEHNDER MODULATOR RESPONSE TO FIBER DISPERSION IN RADIO 573 adopted over the direct odulation due to high speed, high spectral resolution and tunable characteristics 6-8. In a MZM the intensity of the light traveling through one or both ars of the Mach Zehnder odulator is odulated by the RF signal which introduces a phase shift exploiting the Pockels effect. The interference of the two waves at the output of the MZM results in the intensity odulation depending on the phase shift induced by the RF signal 4-8. In each path the phase odulation is equal with different sign, then the output is purely intensity odulation without any incidental phase odulation or siply the output is chirp free. The MZM can be directly odulated by the RF signal which leads to the generation of two side bands. The double side band DSB is affected by chroatic dispersion which leads to phase dispersion and in turn a destructive interference if the phase difference is at the photo detector. Therefore, a single side band generation is preferred which is generated by a dual drive MZM odulator 7,8. Insertion loss, power consuption, nonlinearities and dispersion effects of the dual drive MZM odulator influence the perforance of the RoF systes. 3 Analysis of MZM Response in a Dispersive Fiber The dual drive MZM is widely used odulator for illieter wave generation in RoF systes. By varying the chirp or the applied drive signals at the ars the optical signal can be transitted to a longer distance without dispersion copensation. There are several odels proposed by researches for analysis of MZM response in a dispersive fiber. The odel proposed by Oliveira et al 9 is considered for the analysis as the odel has shown an coparable results with the other odels proposed especially by Devaux et al 0. The scheatic of the proposed odel is shown in Fig. 5. Two novel expression of the optical intensity at the dispersive ediu are adopted for the atheatical analysis of MZM RoF syste at 60 GHz frequency 0. The MZM light wave electric field is given as: j ( ) ( ) ω ( ) t j j t t E t e e e c OUT... () where c is the frequency of the optical carrier, (t) and (t) are the change of phase in the ars of MZM due to the applied electric field given as: π i ( t) v i ( t) V π...() where V is the half voltage, v i (t) is the voltage signals at the ars of MZM and /V = is the index change per volt. If the odulator is biased at quadrature and if sinusoidal voltages of sae frequency of two different peak aplitudes V and V are applied at the two ars and the bias voltage V b applied at one of the ar of the MZM, then the phase change at the two ars of the odulator is: π ( t) ηv sin(ω t ) ( t ) η V sin(ω t )...(3) Applying Eq. (3) and applying Euler s forula in Eq. (), the electrical field of the MZM can be expressed as the su of Bessel s function series: ( Jo (η V ) jjo (η V jω t E ( t) ( J (η V ) jj (η V sin((n)ω t) e c OUT n n n n ( J (η V ) jj (η V cos(nω t) n n...(4) The atheatical analysis can be done using the all four sideband or the first two sidebands. The electrical field output of the MZM with all four side band is expressed as: jω t E ( t) E E sin(ω t) E cos(ω t) e c OUT 4 SB DC ω ω where, Fig. 5 Scheatic representation of the MZM odel for analysi of dispersive effect E DC ( J o ) jj o

4 574 INDIAN J PURE & APPL PHYS, VOL 54, SEPTEMBER 06 E ω ( J n ) jj n n ω ( J n ) jj n n E...(5) The paraeters E DC, E and E is the agnitude of the carrier and the side bands. The RF beat signal at the photo detector is proportional to the optical intensity given as: I 4 SB ( t ) E E DC E E cos( cos( EDC E E E 3 L) L... (6) where L is the fiber length, is the propagation constant and E is the phase. The optical intensity of the SB odel is: fiber. The phase shift introduced depends on the fiber length, RF frequency and the dispersion paraeter. This phase shift introduces a phase difference which akes the RF frequency side bands to beat which further results in RF power degradation. Figures 6 and 7 are the 4SB and SB odel RF power degradation as the function of the fiber length for large chirp odel and for the central frequency of the four channels of 60 GHz band. It can be noticed that both the odels provide an appreciable results copared to the benchark paper by Devaux et al 0. Figures 9 and 0 show the RF power degradation as the function of fiber length for sall chirp in 4SB and SB odel. Coparing Fig. 6 with Fig. 9 and Fig. 7 with Fig. 0, it is evident that with the change in the bias voltage at the ars the dispersive effect of the fiber can be altered at any distance of the fiber. Thereby the ( ) cos(θ θ β ω ) SB t E E L... (7) I ω DC ω EDC E 4 Results and Discussion The nuerical analysis of the 4SB and SB MZM odulator is perfored with the odel defined in Eqs. (6) and (7), for the central frequencies of the four channels of IEEE 80.ad. The half wave voltage V is 5 V, the voltage V at ar of MZM is V and the voltage V at ar is - and.4 V for sall chirp and large chirp odel, respectively. The siulation results clearly indicate that the chroatic dispersion effect shifts the phase of the carrier and the sidebands propagating through the Fig. 7 Degradation of RF power with distance for large chirp SB MZM Fig. 6 Degradation of RF power with distance for large chirp 4SB MZM Fig. 8 Degradation of RF power with odulation frequency f fo large chirp configuration at L = 50 k

5 DURAIKANNAN et al: MACH ZEHNDER MODULATOR RESPONSE TO FIBER DISPERSION IN RADIO 575 Fig. 9 Degradation of RF power with distance for sall chirp 4SB MZM Fig. 0 Degradation of RF power with distance for sall chirp SB MZM dispersive effect can be iniized without any additional dispersion copensation. Figures 8 and which illustrate the RF power degradation with frequency at a finite distance of the fiber length, iply that for any defined length of the fiber with appropriate choice of bias voltage and the frequency will further reduce the effect of fiber dispersion. Further the results clearly indicate that the power degradation as a function of chirp pronounces with increase in the RF frequency. Also the power degradation due to the dispersion is insignificant in personal and local area counication networks which extend few hundred eters and have a great ipact on the perforance with increase in the length of the fiber. Fig. Degradation of RF power with odulation frequency f for sall chirp configuration at L = 50 k 5 Conclusion An extensive analysis of channelization and spatial characteristics of 60 GHz is presented in this paper. The analysis of the RoF transission schees highlights the significance of MZM based external odulation technique for RoF at 60 GHz. The siulation results indicate that the power degradation of the optical signal due to fiber dispersion varies with distance and chirp or ar voltages of the Mach Zehnder odulator. Also the results indicate that for a personal and local area networks at 60 GHz which extends up to few eters the power degradation due to fiber dispersion can be reduced significantly by varying the chirp or ar voltage of the MZM. Furtherore, either by varying the chirp or with adoption of appropriate fiber dispersion copensation techniques like, usage of dispersion copensated fibers, for the reduction of dispersive effect, the intensity response of the MZM can be transitted to any distance with low path loss, thereby paving way to the application of 60 GHz signal in a wide area networks for ulti-gigabit transission. References Guilloroy J, Meyer S, Siaud I, Marie A, Uler-Moll, Charbonnier B, Pizzinat A & Algani C, IEEE Veh Technol Mag, 5 (00) 30. Wi-Fi Alliance, Wi gig and the future of sealess connectivity, Septeber Xu H, Kukshya V & Rappaport T S, IEEE J Select Areas Coun, 0 (00) Wake D, Nkansah A & Goes N J, J Lightwave Technol, 8 (00) Pleros N, Vyrsokinos K, Tsagkaris K & Tselikas N D, J Lightwave Technol, 7 (009) 957.

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