Jensen, Jesper Bevensee

Rodes Lopez, Roberto

Tafur Monroy, Idelfonso

Zibar, Darko

Coherent detection of directly modulated 1550nm VCSELs in 5Gbit/s bidirectional 40km SSMF PON-links is presented. Receiver sensitivity of –37.3dBm after transmission is achieved with 30dB system margin, corresponding to 1:1024 passive powersplitting

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Downloaded from orbit.dtu.dk on: Dec 20, 2017Coherent Detection for 1550 nm, 5 Gbit/s VCSEL Based 40 km Bidirectional PONTransmissionJensen, Jesper Bevensee; Rodes Lopez, Roberto; Zibar, Darko; Tafur Monroy, IdelfonsoPublished in:Optical Fiber Communication Conference and Exposition (OFC/NFOEC), 2011 and the National Fiber OpticEngineers ConferencePublication date:2011Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Jensen, J. B., Rodes Lopez, R., Zibar, D., & Tafur Monroy, I. (2011). Coherent Detection for 1550 nm, 5 Gbit/sVCSEL Based 40 km Bidirectional PON Transmission. In Optical Fiber Communication Conference andExposition (OFC/NFOEC), 2011 and the National Fiber Optic Engineers Conference (pp. OTuB2). OpticalSociety of America.Coherent Detection for 1550 nm, 5 Gbit/s VCSELBased 40 km Bidirectional PON TransmissionJesper Bevensee Jensen, Roberto Rodes, Darko Zibar and Idelfonso Tafur MonroyDTU Fotonik, Technical University of Denmark, Oersteds Plads Building 343, Kgs. Lyngby, DK-2800 Denmarkjebe@fotonik.dtu.dkAbstract: Coherent detection of directly modulated 1550nm VCSELs in 5Gbit/s bidi-rectional 40km SSMF PON-links is presented. Receiver sensitivity of –37.3dBm aftertransmission is achieved with 30dB system margin, corresponding to 1:1024 passive power-splitting.© 2010 Optical Society of AmericaOCIS codes: 060.4510, 140.7260, 060.1660.1. IntroductionIn next generation passive optical networks (PONs) for fiber to the home deployment (FTTH), bidirectional trans-mission of symmetric uplink/downlink data capacity will be increasingly important due to the growth of intrinsicallybidirectional services such as on-line gaming and peer-to-peer networks. Moreover, for point-to-multipoint PONs,there is a desire to increase the reach as well as the amount of users serviceable from a single central office. Indeed, apassive splitting ratio of 1000 has been proposed for future PONs [1], a number unreachable by present PON technol-ogy without the introduction of e.g erbium doped fiber amplifiers (EDFAs) and dispersion compensating fiber (DCF)in the field; a solution which would be problematic in the access networks due to complexity and power consumptionissues. Moreover, an EDFA/DCF based architecture does not lend itself well to single-fiber bidirectional transmission.An alternative solution is to introduce coherent detection in access networks. By employing coherent detection, thereceiver sensitivity can be significantly increased [2], dispersion compensation can be performed in the digital domain[3], and support for advanced modulation formats can be provided [4]. Moreover, as the current PON architecture basedon passive optical transmission and splitting can be maintained, graceful migration can be performed by upgradingterminal equipment only with no changes in the optical fiber transmission path. Due to the cost sensitivity of opticalaccess system, however, present day coherent technologies based on external modulation needs to be simplified.In recent years, a number of breakthroughs within the area of long wavelength vertical cavity surface emitting lasers(VCSELs) have resulted in the emerge of 1550 nm VCSELs [5]. Due the cost-effectiveness and low drive voltage,these are attractive candidates for future PONs. Coherent detection of VCSEL based signals have been presented [6],albeit only with the employment external I/Q modulators; a solution which to some degree counteracts the cost andenergy efficiency of VCSELs.We present what is, to the best of our knowledge, the world’s first demonstration of a bidirectional coherent detectionPON link employing directly modulated (DM) VCSELs operating at 1550 nm wavelength as transmitters. Our systemoperates at 5 Gbit/s in both directions, and transmission is over a single, un-amplified 40 km standard single modefiber (SSMF) link with no use of optical dispersion compensation. Receiver sensitivity at the 2×10−3 limit for forwarderror correction (FEC) was better than –37 dBm, corresponding to an improvement of more than 17 dB over directdetection using the same DM-VCSEL transmitter.2. Experimental SetupA block diagram of the bidirectional transmission system and optical spectra (a) and of the coherent/digital signalprocessing (DSP) reciver (b) is shown in Fig. 1.A pulse pattern generator directly modulates two uncooled VCSELs (VCSEL1 at 1550.8 nm center wavelengthfor downstream and VCSEL2 at 1548.3 nm center wavelength for upstream) with a 5 Gbit/s pseudo random binarysequence (PRBS) with a word length of 215−1. Driving voltage is 1 V peak-peak, and bias current of the VCSELs isset to 12.4 mA for VCSEL1 and 15.6 mA for VCSEL2. Adjustment of bias current and drive voltage is optimized bymaximizing the extinction ratio while minimizing the overshoot. The extinction ratio of the VCSELs at the transmitter       OTuB2.pdf   OSA/OFC/NFOEC 2011 TuB2.pdf©Optical Society of AmericaPattern GeneratorVCSEL140 kmSSMFVCSEL2CoherentReceiver2CoherentReceiver1Pattern GeneratorCirculator CirculatorVCSEL11550.8 nmLO peak0.1 nm/div.LO peakVCSEL11548.3 nm0.1 nm/div.(a) SetupDSP Receiver90 HybridLODSOPCPCiIiQPowerMeter20 dBVOAEnvelope detectionSynchronizationCD compensationThreshold gatingFrom fiber(b) Coherent ReceiverFig. 1. Experimental setup of the bidirectional system (a) and the coherent/DSP receiver (b). VOAis a variable optical attenuator, DSO is a digital storage oscilloscope with a samplig rate of 40GSamples/s, LO is the local oscillator laser, and PC is polarization controllers.is 6.8 dB measured with an oscilloscope. The different center wavelength of the two VCSELs is due to bias currentdependence of a VCSELs emission wavelength. As the two VCSELs were not completely identical, and the biascurrent was optimized for identical extinction ration and overshoot for the two VCSELs, it was not possible to achieveexactly identical optical spectrum and center wavelength for the two VCSELs. The optical power spectra of the twoVCSELs combined with the local oscillator (LO) laser are shown as inserts in Fig. 1 (a).The two VCSELs are connected to opposite ends of a 40 km SSMF transmission span via optical circulators sothat the received signal from VCSEL1 can be received at the site of VCSEL2 and vise versa, while the two signalsare simultaneously counter-propagating in the fiber. Launch power into the fiber is 0.6 dBm in both directions; fiberattenuation is 7.7 dB; and total chromatic dispersion is 640 ps/nm.At the receiver input, the signal is attenuated, and the receiver input power is measured with a calibrated opticalpower meter through a –20 dB directional coupler. After the coupler, the signal is mixed with light from a free runningexternal cavity laser (ECL) local oscillator (LO) in a 90◦ optical hybrid, and detected by two sets of balanced photode-tectors. The received II and IQ components of the signal are stored by a digital storage oscilloscope with a samplingrate of 40 Gsamples/s.The coherent detection is performed using intra-dyne detection, where the LO laser has a small frequency offset withrespect to the transmitter laser center frequency. The narrow linewidth LO laser can be identified within the broaderVCSEL spectra in Fig. 1 (a).Demodulation of the data is performed by off-line digital signal processing (DSP), consisting of synchronization ofthe received bits, chromatic dispersion (CD) compensation, envelope detection by squaring and low pas filtering, anddecision gating. Due to the chirp induced by the direct modulation of the VCSEL, optimum performance is achievedby over-compensating by 425 ps/nm. This residual chirp-compensation is independent of fiber length (the same valuewas used back to back and after transmission), and is therefore not expected to be a problem in system installations.The effect of random frequency offset between the VCSELs and LO laser does not effect the performance, as the DSPperform envelope detection of the in-phase and quadrature components of the received field.3. ResultsThe measured BER of the coherently detected signals before (B2B) and after bidirectional transmission over 40 kmSSMF is plotted in Fig. 2 (a) together with the measured BER after direct detection of VCSEL1 with no counterpropagating signal. Back to back receiver sensitivity at the FEC limit of 10−3 for the coherent detection case is−37.7 dBm for VCSEL1 and –39.3 dBm VCSEL2. After 40 km transmission with the counterpropagating signal, thereceiver sensitivity is –37.3 dBm and –37.7 dB for VCSEL1 and VCSEL2 correponding to transmission penalties of0.4 dB and 1.6 dB. In order to benchmark these results against conventional direct detection unidirectional systems,the BER of VCSEL1 is measured in this manner. The receiver sensitivities for VCSEL1 employing direct detection are–23.6 dBm B2B and –19.8 dBm after transmission, corresponding to a transmission penalty of 3.8 dB. The increasedtransmission penalty using direct detection is due to chromatic dispersion, which for the coherent detection case wascompensated by DSP. This is confirmed by inspection of the direct detection eye diagrams in Fig. 2 (b) which showsevere waveform distortion after transmission. The low transmission penalty of the coherently detected signals showsthat the chromatic dispersion has been successfully compensated in the DSP. The improvement in receiver sensitivitydue to coherent detection for VCSEL1 is 13.1 dB B2B and 17.5 dB after 40 km SSMF.       OTuB2.pdf   OSA/OFC/NFOEC 2011 TuB2.pdf-40 -35 -30 -25 -20 -1565432 VCSEL 1 - B2B VCSEL 1 - 40 km SSMF VCSEL 2 - B2B VCSEL 2 - 40 km SSMF DD - B2B DD - 40 km SSMF-log(BER)Receiver input power [dBm](a) BERB2B40 km SSMF(b) Eye diagramsFig. 2. Measured BER back to back (B2B) and after 40 km SSMF bidirectional transmission. Directdetection (DD) direction, single transmission using VCSEL1 is included for reference.As the launch power is 0.6 dBm, and total fiber loss is 7.7 dB, the presented bidirectional coherent system providesa system margin above 30 dB, corresponding to a passive spitting ratio of 1024. This is a dramatic improvement overthe direct detection margin of 12.7 dB corresponding to a passive splitting ratio of 18. Alternatively, as chromaticdispersion is compensated using DSP, the improved system margin could be utilized to extend the reach to e.g. 80 km.Assuming an 80 km link loss of 15.4 dB, the residual margin of 22.3 dB would be able to accommodate the loss of a1:128 power splitter.4. ConclusionWe have presented experimental results from what we believe to be the worlds first demonstration of bidirectional40 km SSMF link employing coherent detection with free running local oscillators and un-cooled directly modulatedVCSEL transmitters. Through the use of digital signal processing for demodulation and dispersion compensation, re-ceiver sensitivities of –37.3 dBm downstream and –37.7 dBm upstream was achieved with no use of optical amplifiersor optical dispersion compensation. At a launch power of 0.6 dBm and 7.7 dB fiber loss, a system margin of 30 dBwas achieved, corresponding to the loss of a record passive splitting ratio of 1024.The reported experiments demonstrate the potential for coherent detection in bidirectional access links employingdirectly modulated VCSEL transmitters. Future work include the investigation of the feasibility of using low-costsolutions for LO lasers, and upgrade to 10 Gbit/s bit rate.References1. S. Smolorz, H. Rohde, P. Ossieur, C. Antony, P. D. Townsend, T. De Ridder, B. Baekelandt, X. Z. Qiu, S. Ap-pathurai, H.-G. Krimmel, D. Smith and A. Poustie, “Next generation access networks: PIEMAN and beyond”,International Conference on Photonics in Switching (PS) (2009).2. G. Lachs, S. M. Zaidi and A. K. Singh, “Sensitivity enhancement using coherent heterodyne detection”, J.Lightw. Tech., Vol. 12, No. 6 (1994).3. S. J. Savory, “Electronic signal processing in optical communications”,in Proceedings of SPIE Vol. 7136 (2008).4. X. Zhou and J. Yu,“Advanced coherent modulation formats and algorithms: higher-order multi-level coding forhigh-capacity system based on 100 Gbps channel”, in Optical Fiber Communication Conference (OFC) (2010).5. E. Kapon and A. Sirbu, “Long-wavelength VCSELs: Power-efficient answer”, Nature Photonics 3, 27 - 29(2009).6. Sang-Yuep Kim, N. Sakurai, H. Kimura, H. Hadama, “VCSEL-based coherent detection of 10-Gbit/s QPSKsignals using digital phase noise cancellation for future optical access systems”, in Conference on Optical FiberCommunication (OFC) (2010).       OTuB2.pdf   OSA/OFC/NFOEC 2011 TuB2.pdf

Coherent Detection for 1550 nm, 5 Gbit/s VCSEL Based 40 km Bidirectional PON Transmission

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Coherent Detection for 1550 nm, 5 Gbit/s VCSEL Based 40 km Bidirectional PON Transmission

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