1.25 Gbits Over 50 m Step-Index POF Transceiver

1.25Gbit/s Over50m Step-Index Plastic Optical Fiber Using a Fully Integrated Optical Receiver With an Integrated EqualizerMohamed Atef,Member,IEEE,Robert Swoboda,and Horst Zimmermann,Senior Member,IEEEAbstract—A single-chip optical receiver with an integrated equalizer is used to achieve a high performance gigabit transmis-sion over step-index plastic opticalfiber(SI-POF).The integrated equalizer can compensate for different POF lengths up to50m. The integrated optical receiver is fabricated in a low-cost silicon 0.6m BiCMOS technology and has a power consumption of 100mW.Real-time transmission at data rates of1.8Gbit/s over 20m SI-POF and1.25Gbit/s over50m SI-POF with high sensi-tivities and BER of is achieved.The optical transmitter is based on an edge emitting laser.Index Terms—Equalization,integrated optical receiver, large-area integrated photodiode,step-index plastic optical fiber(SI-POF).I.I NTRODUCTIONP LASTIC OPTICAL FIBER(POF)provides benefits com-pared to glass opticalfiber(GOF).POF possesses a greater flexibility and resilience to bending,shock and vibration,and is easier in handling and connecting.These features make the total cost of a POF link less expensive.There is no need for expen-sive connectors like used for GOF.It can be easy installed and connected by nonexpert persons,so the high costs for an expert technician can be saved.The operation in the visible range is possible allowing an inherent eye-safety.These advantages make POF very attractive for use in short-range optical communication as in media oriented system transport(MOST),industrial control,and within in-building networks[1],[2].The PMMA SI-POF has the lowest bandwidth and the highest attenuation among multimodefibers.This small band-width(40MHz/100m)limits the maximum data rate which can be transmitted through step-index(SI)POF[2].A target data rate of1.25Gbit/s over50m SI-POF was specified by European Telecommunications Standards Institute[3].To increase the transmission length over the high attenuation PMMA POF(0.14dB/m at650nm)highly sensitive optical receivers with large area photodiode have to be used[4],[5].Manuscript received June28,2011;revised October04,2011,November30, 2011;accepted December05,2011.Date of publication December13,2011; date of current version January18,2012.M.Atef and H.Zimmermann are with Vienna University of Tech-nology,Institute of Electrodynamics,Microwave and Circuit Engineering, 1040Vienna,Austria(e-mail:mohamed.abdelaal@tuwien.ac.at;horst. zimmermann@tuwien.ac.at).R.Swoboda is with A PICs Electronics Development GmbH,1200Vienna, Austria.Color versions of one or more of thefigures in this paper are available online at .Digital Object Identifier10.1109/JLT.2011.2179520Using complex modulation formats like multilevel pulse am-plitude modulation(M-PAM)or discrete multitone(DMT)can help to solve the limited bandwidth of the SI-POF[6],[7].A high linearity optical receiver is needed for M-PAM and DMT. This needs a sophisticated design for an automatic gain con-trol transimpedance amplifier(AGC-TIA)to achieve the re-ceiver’s high linearity over a wide input optical power range [8],[9].There is a need for more circuits at the transmitter part to generate these complex modulation formats and also at the re-ceiver to decode these complex signals to binary.The circuitry at the M-PAM and DMT receivers and transmitters increase the system complexity,cost,and power consumption.To obtain a cheap SI-POF optical receiver system with less complexity and lower power consumption,we should return to a binary signal.The approach is to use a binary signal and an equalizer to compensate for the small bandwidth of the SI-POF. The binary approach with an equalizer is easier to implement and needs a simpler optical receiver compared to M-PAM.In this paper we introduce a single-chip fully integrated op-tical receiver with an integrated POF-equalizer.The integration of large-area photodiodes,TIA,POF equalizer and50driver on a single chip enhances the performance and lowers the cost for giga-bit communication over SI-POF.The paper will be organized as follows.Thefirst section dis-cusses the state of the art for SI-POF equalization for giga-bit transmission.The second section describes the main blocks of the presented integrated optical receiver.The third section will speak about the adjustable equalizer which was integrated with the optical receiver.The fourth section will present the exper-imental results and discussions of the results.The last section will be the conclusion.II.E QUALIZED GIGA-B IT T RANSMISSION O VER SI-POF There are two approaches for a SI-POF equalizer in the liter-ature.Thefirst is to use a pre-equalization,and the second is to use an adaptive post-equalizer.Passive pre-equalization was used in[10]to reach a data rate (DR)of1.25Gbit/s over50m SI-POF for binary NRZ modu-lation.A650nm edge emitting laser diode,an optical receiver with an800m diameter PIN photodiode(PIN-PD)and a sepa-rate TIA were used[4].The measured sensitivity was16dBm at a bit error ratio(BER)of.Laser source pre-equalization(peaking)lowers the modula-tion depth of the emitted light;this reduces the effective power per pulse compared with rectangular pulses without peaking. This is at the expense of the system power budget.Also,if the frequency response changes,as a result of different lengths of the POF or bends in thefiber,the result will be too much or too0733-8724/$26.00©2011IEEEFig.1.Vertical structure of a PIN photodiode in BiCMOS technology.little compensation,therefore the BER will increase.In[11]a data rate of1Gb/s over50m POF was achieved by using an adaptive decision feedback equalizer(DFE)and forward error correction(FEC)which were fully implemented in an FPGA. The measured sensitivities for BER were13.5dBm using resonate cavity light emitting diode(RC-LED)and 18.5dBm with a VCSEL.The adaptive post-equalization can overcome the changes of the frequency response resulting from different POF lengths or POF bending.An integrated adaptive post-equalizer for SI-POF is needed. Thefirst integrated POF equalizer was introduced in[12].A discrete optical receiver(Hamamatsu S5052800m PIN-PD and28V reverse bias,Maxim3266TIA)was used.The output of the optical receiver was fed to an integrated equalizer chip.A data rate of1.25Gbit/s was achieved over50m SI-POF using 650nm RC-LED.The BER was.All of the above optical receivers use external POF equal-izers;even the integrated equalizer reported in[12]still needed a separate optical receiver.III.I NTEGRATED O PTICAL R ECEIVERA PIN photodiode with400m diameter is integrated within the presented optical receiver chip.The anode of the photo-diode is formed by the substrate and is connected to ground, see Fig.1.The cathode of the photodiode is formed by an source/drain region.Because of the use of an anti-reflection coating a responsivity of A/W could be achieved at an optical wavelength of nm.The PIN photodiode has a bandwidth of720MHz.The PIN-PD structure used here was introduced in[13].Two PIN-PDs are integrated with the optical receiver.One is active to collect the optical power and the other one is shielded to balance the differential transimpedance amplifier input.The differential topology is preferred for the TIA because of its high immunity against power supply and common-mode noise,see Fig.2.An automatic gain control(AGC)sets the gain of the TIA to avoid overloading of the TIA and to increase the maximum received optical power to1mW(0dBm).The AGC makes the TIA operate in the linear region of opera-tion to prevent the limiting effect of the TIA at high input optical power.This linear operation of the AGC-TIA is required for the POF equalizer in the next stage to work properly.The TIA has a3dB bandwidth of622MHz and an input referred noise current of about170nA.The outputs of the dif-ferential AGC-TIA arefed to the integrated POF equalizer.The Fig.2.Circuitry of the presented optical receiver.Fig.3.Circuitry of the presented equalizer cell.output signal of the POF equalizer is amplified by the post am-plifier(see Fig.2),which is a limiting amplifier.The last stage in the optical receiver is a50output driver which provides impedance matching to the measurement equipment.The op-tical receiver uses a single3.3V supply and consumes100mW. The test chip is fabricated in0.6m BiCMOS technology and occupies an area of1.44mm0.91mm.IV.I NTEGRATED SI-POF E QUALIZERThe used equalizer in Fig.3has two control inputs.Thefirst control signal()controls the high passfilter corner frequency and the second control signal()controls the low-frequency gain.The high cut-off is controlled by varying the MOS capacitors and.The additional passive resistoris used to give a proper minimum low-frequency gain to pre-vent an undesired too low low-frequency gain of when is off.A similar equalizer in pure CMOS was presented by[14] to equalize for the copper cable limited bandwidth for gigabit transmission using several equalizer stages.Here we introduce a BiCMOS version with the bipolar transistors and re-sulting in a larger gain than in[14].For POF applications,a single-stage equalizer will be enough for POF lengths up to50m.This is because the POF lengths up to50m show a frequency response similar to afirst order lowFig.4.Measured transfer function of single stage equalizer at different control signalvalues.Fig.5.Block diagram of the measurement setup.pass filter [7].The measured transfer function of the presented single-stage equalizer is illustrated in Fig.4.It is clear from Fig.4that by increasingthe corner fre-quency increases.This high pass filter effect of the equalizer can compensate for the low pass filter behavior of the SI-POF.In the presented design we aimed to reduce the number of con-trol signals from two to one (only ).The values of the resistorsand are selected to achieve a high gain.is held constant to obtain the required low fre-quency gain.the effect of on the low-frequency gain will be neglected and the only is just done via .This gives a simple and effective control for the equalizer which is suf fi-cient to equalize different POF lengths of up to 50m.V .E XPERIMENTAL R ESULTSFor the frequency response measurements a network analyzer (HP 8753E)was used to modulate a 655nm laser diode (LD).The PMMA SI-POF (POF1.100.B22M from Luceat)with a corediameter of 1mm andwas butt coupled to the 655nm LD.The optical power after the POF was received by the inte-grated optical receiver also using butt coupling.The block dia-gram for the measurement setup is shown in Fig.5.The output electrical power of the receiver measured as function of fre-quency for different equalized SI-POF lengths (20m and 50m)using the presented optical receiver is shown in Fig.6.The re-ceived electrical power is proportional to the square of the op-tical power,due to the square-law effect of the PD [2].So,the 3dB optical bandwidth will be the 6dBelectrical band-width.The 20m POF has an equalized bandwidth of 750MHzFig.6.Equalized transfer function for 20m and 50m SI-POF.Fig.7.Measured BER for the equalized POF.(a)20m SI-POF at DR 1.25Gbit/sand 1.8Gbit/s.(b)50m SI-POF at DR 1Gbit/s and 1.25Gbit/s.Fig.8.Measured equalized 50m-POF transfer function for different received average optical power.and a 50m POF has an equalized bandwidth of 700MHz.This bandwidth is suf ficient for giga bit transmission over SI-POF.For BER measurement the 655nm LD was modulated by a1.8Gbit/s or a 1.25Gbit/s signal atand 5dBm optical power was butt coupled to a 20m and a 50m SI-POF,respectively.Fig.9.Eye diagrams measured at the output of the integrated optical receiver with the integrated equalizer for a binary signal with at: (a)1.8Gbit/s over20m SI-POF with an average received optical power of16dBm.(b)1.8Gbit/s over20m SI-POF with an average received optical power of8dBm.(c)1.25Gbit/s over50m SI-POF with an average received optical power of16dBm.(d)1.25Gbit/s over50m SI-POF with an average received optical power of8dBm.The received data stream from the integrated optical receiverwas compared with the original transmitted signal by bit-error counting using a bit-error rate tester(BERT).The measured BER in dependence on the received optical power at different equalized POF lengths are illustrated in Fig.7.1.25Gbit/s and 1.8Gbit/s can be transmitted over20m SI-POF with sensitivity of19.6dBm and16.5dBm,respectively,with a BER of and a PRBS with a length of.1Gbit/s and1.25Gbit/s can be transmitted over50m SI-POF with sensitivity of dBm and15.5dBm,respectively,at BER of and a PRBS with the length of.The average optical power received by the integrated photodiode is measured by an on-chip circuitry(actually the photocurrent is measured and the optical power is calculated with the known responsivity (0.52A/W at660nm)of the integrated photodiode).The transmitted average optical power from the655nm LD is5dBm.The maximum average optical power received by the optical receiver after50m POF is8dBm.The13dB loss from the LD to the integrated optical receiver comes from the POF attenuation(7dB after50m)and the cou-pling loss of the1mm core diameter POF to the0.4mm diam-eter PIN-PD(6dB).By using the presented single-chip optical receiver with the integrated equalizer a sensitivity of15.5dBm for1.25Gbit/s transmitted over50m POF is reported.There will be a7.5dB optical power margin()for 1.25Gbit/s transmission over50m SI-POF.The transfer function of the equalized50m SI-POF is shown in Fig.8for different optical power values(15dBm and 8dBm).The transfer function in Figs.6and8indicates that the integrated equalizer works successfully for gigabit transmission over50m SI-POF and at different input optical power levels.The measured eye diagrams after20m SI-POF with a data rate of1.8Gbit/s,PRBS and a received average Fig.10.Measured eye diagrams for data rate 1.25Gbit/s with over50m SI-POF with three circular bendings with12mm bending radius.optical power of16dBm and7dBm are illustrated in Fig.9(a)and(b),respectively.The measured eye diagrams after50m SI-POF with a data rate of1.25Gbit/s,PRBSand a received average optical power of16dBm and8dBm are illustrated in Fig.9(c)and(d),respectively.It is clear from Fig.9that1.25Gbit/s transmission over50m POF and 1.8Gbit/s transmission over20m POF is possible with high sensitivity and at different input optical power levels.A data rate of1.25Gbit/s was transmitted over50m SI-POF with three circular bendings in the POF near to the laser source with a radius of12mm.The optical signal from the POF was received and equalized by the integrated optical receiver.The output eye diagram of the equalized signal is shown in Fig.10.The bending increases the modal dispersion effect be-cause more modes are exited and therefore the system band-width is reduced and the jitter is increased(see Fig.10).It is clear from the eye diagram in Fig.10that the effect of POF bending still can be equalized to make1.25Gbit/s over 50m POF possible.VI.C ONCLUSIONThis paper presents a fully integrated optical receiver with an integrated SI-POF equalizer.Thefirst-order low-passfilter characteristic of the SI-POF was compensated by the integrated single-stage equalizer to achieve gigabit transmission over dif-ferent SI-POF lengths.The target data rate of1.25Gbit/s over 50m SI-POF was achieved with a7.5dB optical power margin. The presented integrated optical receiver with an integrated adjustable equalizer is attractive for gigabit transmission over SI-POF due to its high performance and low costs.R EFERENCES[1]P.Polishuk,“Plastic opticalfibers branch out,”IEEE Commun.Mag.,vol.44,no.9,pp.140–148,Sep.2006.[2]O.Ziemann,J.Krauser,P.E.Zamzow,and W.Daum,POF Hand-book—Optical Short Range Transmission Systems,2nd ed.Berlin,Heidelberg:Springer,2008.[3]Access,Terminals,Transmission and Multiplexing(ATTM);PlasticOptical Fibre System Specifications for100Mbit/s and1Gbit/s,ETSI TS105175-1V1.1.1(2010-01),Jan.2010[Online].Available:/WebSite/homepage.aspx[4]F.Tavernier and M.Steyaert,“A high-speed POF receiver with1mmintegrated photodiode in180nm CMOS,”presented at the Proc.36thmun.(ECOC),2010,P2.04.[5]R.Swoboda and H.Zimmermann,“2.5Gbit/s silicon receiver OEICwith large diameter photodiode,”Electron.Lett.,vol.40,no.8,pp.505–507,2004.[6]M.Atef,R.Swoboda,and H.Zimmermann,“Gigabit transmission overPMMA step-index plastic opticalfiber using an optical receiver formultilevel communication,”presented at the36th Eur.Conf.Exhib.mun.(ECOC),Torino,Italy,2010,Paper6.02.[7]S.C.J.Lee,F.Breyer,S.Randel,R.Gaudino,G.Bosco,A.Bluschke,M.Matthews,P.Rietzsch,H.P.A.van den Boom,and A.M.J.Koonen,“Discrete multitone modulation for maximizing transmissionrate in step-index plastic opticalfibers,”J.Lightw.Technol.,vol.27,no.11,pp.1503–1513,Jun.1,2009.[8]M.Atef,W.Gaberl,R.Swoboda,and H.Zimmermann,“An integratedoptical receiver for multilevel data communication over plastic opticalfiber,”J.Analog Integr.Circuits Signal Process.,vol.67,no.1,pp.3–9,2011.[9]M.Atef,R.Swoboda,and H.Zimmermann,“Optical receiver for mul-ticarrier modulation in short-reach communication,”Electron.Lett.,vol.46,no.3,pp.225–226,Feb.2010.[10]R.Krugloy,S.Loquai,O.Ziemann,J.Vinogradov,and C.Bunge,“Channel capacity of step-index polymer opticalfibers:Experi-ments and simulation with realistic parameters,”in Proc.Opt.FiberCommun./Nat.Fiber Opt.Eng.Conf.(OFC/NFOEC),2010,PaperJWA12.[11]A.Nespola,S.Straullu,P.Savio,D.Zeolla,J.C.R.Molina,S.Abrate,and R.Gaudino,“A new physical layer capable of record gigabittransmission over1mm step index polymer opticalfiber,”J.Lightw.Technol.,vol.28,no.20,pp.2944–2950,Oct.15,2010.[12]C.Zerna,J.Sundermeyer,A.Fiederer,N.Verwaal,B.Offenbeck,andN.Weber,“Integrated PAM2decision feedback equalizer for gigabitethernet over standard SI-POF using red LED,”in Proc.36th Eur.mun.(ECOC),2010,Paper We.6.B.4.[13]M.Förtsch,H.Dietrich,and H.Zimmermann,“Complete low-cost625Mbit/s opticalfiber receiver in0.6m BiCMOS technology,”in Proc.SPIE Opt.Fiber.Appl.,2005,vol.pp.59520R1–59520R6.[14]J.-S.Choi,M.-S.Hwang,and D.-K.Jeong,“A0.18um CMOS3.5-Gb/s continuous-time adaptive cable equalizer using enhancedlow-frequency gain control method,”IEEE J.Solid-State Circuits,vol.39,no.3,pp.419–425,Mar.2004.Mohamed Atef(M’07)received the B.Sc.and M.Sc.degrees in electrical en-gineering,electronics and communications from Assiut University,Egypt,in 2000and2005,respectively,and the Ph.D.degree in2010from the Institute of Electrodynamics,Microwave and Circuit Engineering,Vienna University of Technology,Vienna,Austria.From2006to2007,he was a Researcher in the Department of Microelec-tronics,Czech Technical University,Prague,working on the optical prosperities of quantum dots.He has authored or coauthored more than24scientific publica-tions.His current research interests are in the area of optoelectronic integrated circuit design and short-reach communication over plastic opticalfiber. Robert Swoboda was born in Vienna,Austria,in1970.He received the Dipl. Ing.degree in2001and the Ph.D.degree in2007,both from the Vienna Uni-versity of Technology,Austria.From2001to2005,he was with the Institute of Electrical Measurements and Circuit Design,Vienna University of Technology.In2007,he joined A PICs Electronics Development GmbH,Vienna,Austria.He is author and coauthor of more than40scientific publications.His majorfields of interest are analogue high frequency circuit design in general,optoelectronic integrated circuits and circuit theory.Horst Zimmermann(SM’02)received the Ph.D.degree in engineering from the Fraunhofer Institute for Integrated Circuits(IIS-B),Erlangen,Germany,in 1991.He was an Alexander-von-Humboldt Research-Fellow at Duke University, Durham,NC,where he worked on diffusion in Si,GaAs,and InP.In1993, he joined the Chair for Semiconductor Electronics at Kiel University,where he lectured in optoelectronics and worked on optoelectronic integration.Since 2000,he has been a Professor for electronic circuit engineering at Vienna Uni-versity of Technology,Austria.He is author of the two books Integrated Sil-icon Optoelectronics(Springer,2000)and Silicon Optoelectronic Integrated Circuits(Springer,2004),as well as coauthor of Highly Sensitive Optical Re-ceivers(Springer,2006).Furthermore,he is author and coauthor of more than 350scientific publications.His main interests are in design and characterization of analog deep-sub-micron and nanometer CMOS circuits as well as optoelec-tronic integrated CMOS and BiCMOS circuits.。