Optical transmission systems over Plastic Optical Fiber (POF) at high bit rate

Optical transmission systems over Plastic Optical Fiber (POF) at high bit rate Politecnico di Torino, 13 Sept. 2007 Daniel Cárdenas OptCom Group Photonlab Dipartimento di Elettronica Politecnico di Torino - ITALY

Outline 2 What is Plastic Optical Fiber (POF)? POF & GOF Why POF? Where? POF impairments to be overcome High bit rate over POF, how high? The EU-STREP POF-ALL 100 Mb/s media converter prototype Description -> DSP techniques Performance Current and future work Conclusions

Optical fibers: GOF 3 Glass Optical fiber (GOF) has a very high performance: Very high bandwidth -> TRX @ 160 Gb/s!, up to Terabit/s!! Very low attenuation -> Ideal transmission media for very long distances (ten- hundreds of km) Standardized for backbone cabling BUT: too expensive for very short reach applications What is a very short reach app? Why is so expensive there?

GOF 4 Very short reach applications: Horizontal cabling Industrial sector apps. In-house networks/ domotic apps. Automotive/airplanes/ships cabling Last part (towards the final user) in access networks The high cost in these applications is related to the fiber small diameter: Core: 10 μm SMF, 50/65 μm MMF n 0 n 1

GOF cost 5 Accurate alignment needed -> Packaging and pigtailing costs are high Care with the handling of the fiber Technically skilled personnel and suitable tools needed -> Installation costs are high for massive penetration in these sectors In-house/domotic networks: fibers should run into already deployed structure. Ideal if there were a self-installing system

Numerical Aperture Alignment issues are also related to the angular accuracy required to connect two optoelectronic devices 6 Fiber acceptance angle θ 0 n 1 θ 1 Unguided ray Guided ray n 2 Numerical Aperture Acceptance Angle NA = θ acc n 2 2 1 n2 n1 2Δ where : Δ = 1 2 2 = 1 sin n1 n2 sin 1 n 1 ( n 2Δ ) n n 1 2

Numerical Aperture 7 Typical values in GOF: NA: 0.2 0.3 => 11-17 degrees of acceptance angle In very short reach applications, a higher acceptance angle is desired, and thus a higher NA

What s POF? 8 Select another material that Allows high diameter Allows high NA Mechanically robust Inexpensive 1963, DUPONT -> fibers made with the polymer: poly-metil metha-acrylate (PMMA). Patent taken by Mitsubishi. 1980, Growing interest in POF for illumination 1990, POF for datacom

Why PMMA-POF? 9 With plastic material, the diameter can be significantly increased without introducing excessive bending problems Ex: 1-mm Step Index PMMA-POF -> Core: 980 um Different indexes of refraction can be obtained. Large numerical aperture, and consequently large acceptance angles, are feasible Typical values for standard PMMA fibers Core: n core =1.49 Cladding: n cladding =1.40 Numerical aperture: NA=0.5-0.55 Acceptance angle: >30 degrees

Why PMMA-POF? 10 Today: Considered as a candidate for optical media in very short reach applications. (High NA and large diameter) Allow self-installation. Low installation costs. Inexpensive material.

POF Current applications 11 Standardized in Automotive sector MOST (Media Oriented System Transport) BMW, Daimler Chrysler, etc IEEE 1394 considers PMMA POF as a transmission medium S200: 200 Mb/s, 40 50 m S400: 400 Mb/s, 20 30 m Outdoor video surveillance Lighting (classical application) Analog video, Low bandwidth applications Proposed for industrial applications <100 Mb/s, <100 m

POF Current and future applications - datacom 12 Current commercial media converters based on PMMA POF: 10 Mb/s (Ethernet) up to 200 m 100 Mb/s (Fast Ethernet) up to 80m Domotic networks Very high bit rate (> 1Gbit/s?) over short distances (10-20 meters) For datacom and domotic applications: More work to be done. Lot of research on GI-POF (graded index perfluorinated POF) for the last mile Japan: www.koike.appi.keio.ac.jp TuE, Netherlands

PMMA - POF 13 Mechanically robust, but What about the performance? How high is the bit rate? Where is it used? Future trends?

Outline 14 What is Plastic Optical Fiber (POF)? POF & GOF Why POF? Where? POF impairments to be overcome High bit rate over POF, how high? The EU-STREP POF-ALL 100 Mb/s media converter prototype Description -> DSP techniques Performance Current and future work Conclusions

PMMA POF Characteristics 15 PMMA attenuation, db/km Green wavelength >75 db/km Red wavelength >130 db/km *Silica glass fibers at 1550 nm: 0.2 db/km

POF Attenuation 16 PMMA POF attenuation: ~0.08dB/m, 520 nm - green wavelength (visible) Nevertheless, interesting for short reach & low-cost applications Others: Perfluorinated POF Intrinsic attenuation as low as GOF (theoretically), and anyway much lower than PMMA More expensive than PMMA Ex: 125 μm core (500 μm cladding), diameter advantage is lost Nevertheless, they are gaining more interest nowadays

POF is multimodal Step index optical fiber is single mode if: V 2.405, where : π d λ NA Where: d is the core diameter NA is the numerical aperture λ 0 is the used optical wavelength V is an adimensional parameter, usually called the normalized frequency parameter V = 0 17 If λ 0 =650 nm (red) and NA=0.4, the single mode condition requires to satisfy: d 1.2 μm

POF is highly multimodal -> low bandwidth 18 Each mode has a different equivalent traveling speed of the z-axis due to the different angles multimodal dispersion Mode #2 z Mode #1 We remember that a mode in a fiber can be interpreted as a ray traveling along the fiber with a given angle

Multimodal dispersion 19 Input signal (isolated 1 ) -T b 0 x(t) T b t Modal decomposition x a (t) x b (t) x c (t) Mode a t Mode b t Mode c t Output signal x(t) t T out >T b

How to increase POF bandwidth? 20 Multimodal dispersion problem can be greatly reduced using graded-index (GI) fibers n 1 Mode 1 Mode 2 Index of refraction profile n 2 Mode 1 follows a longer path than Mode 2, but sees lower index of refraction, and thus travel faster

POF 21 Perfluorinated GI-POF -> similar to GOF? They still need to show a real cost advantage with respect to GOF 1mm PMMA SI-POF The competitor is copper, not GOF When compared to copper, the following plus are relevant Total immunity to EMI Smaller diameter (compared to coaxial and also UTP) Smaller weight

Design considerations 22 100 Mb/s media converter Long link (>200m) PMMA-SI-POF 1 mm diameter High attenuation Low bandwidth

Outline 23 What is Plastic Optical Fiber (POF)? POF & GOF Why POF? Where? POF impairments to be overcome High bit rate over POF, how high? The EU-STREP POF-ALL 100 Mb/s media converter prototype Description -> DSP techniques Performance Current and future work Conclusions

EU STREP POF ALL. 24 POF ALL shall develop a technology to allow delivery of 100+ Mb/s symmetrically to residential users at costs far lower than existing alternatives. Other partners use QAM, DMT Our work -> WP1 @ ISMB 100 Mb/s (Fast Ethernet) PMMA-SI-POF (1 mm) Simple multilevel modulation format Link > 200m?

POF-ALL organization 25

PMMA-SI-POF 26 High attenuation (0.08 db/m @520nm-green λ) Low bandwidth! ~19 MHz /200m ~10 MHz /300m

POF impairments 27 With such a low bandwidth, how can we get a reliable transmission at 100 Mb/s over 200m+? => Digital techniques: Simple multilevel modulation format => 8-PAM FEC => Reed Solomon Pre- and adaptive equalization => LMS However, it means redundancy 100Mb/s + redundancy => 120 Mb/s After 8-PAM => Baud rate: 40 Mbaud

Media converter: Global architecture 28 The design of the media-converter involves: an analog link to and from the UTP (Ethernet) some signal processing (FPGA) an optical link to and from the POF (proprietary) UTP 10/100 BaseT PHY MII Interface FPGA SIGNAL PROCESSING Proprietary Symbol Serial Interface POF PHY POF

Transmitter implementation 29 Digital Signal Processing blocks implemented inside a FPGA 00101 8b/9b (propietary) FEC encoder RS(511,479) 8-PAM modulator linecoding Preequalizer (FIR) LED nonlinearity compensation D/A converter LED driver Analogue optoelectronics LED POF Legend: Implemented Being implemented GREEN LED

Receiver implementation 30 POF Photodiode and TIA AGC A/D converter Legend: Implemented Being implemented Analogue optoelectronics Clock Recovery Adaptive equalizer FIR DLMS 8-PAM Demod FEC decoder 8b/9b decoder Digital Signal Processing blocks implemented inside a FPGA

Pre- and post-equalization 31 Pre-equalization filter: Raised cosine spectrum with 80% roll-off. 20 tap FIR filter Adaptive post-equalization: LMS -> Least Minimum Square 10-tap FIR filter Automatically switching from the blind algorithm to the smoother conventional decision directed (DD) algorithm

The experimental setup 32 BER Monitoring 8-PAM modulator 00101 Bit pattern generator Bit error detector Preequalizer (FIR) FPGA board 8-PAM Demod LED nonlinearity compensation D/A converter A/D converter Adaptive equalizer Optoelectronics and POF

Pre-equalizer (200m POF) 33 BER Monitoring 8-PAM modulator 00101 Bit pattern generator Bit error detector Preequalizer (FIR) FPGA board 8-PAM Demod LED nonlinearity compensation D/A converter A/D converter Adaptive equalizer Optoelectronics and POF After 200m, 40 Mbaud

Experimental results: Post-equalizer only 34 Post-equalizer After 225m of POF, (no pre-eq)

Experimental results: Pre- and post-equalizer 35 Post-equalizer After 250m of POF, (with pre-eq filter designed for 200m)

Experimental results: BER vs Optical power 36 Error free after 200 meters of POF (received optical power is -16dBm) With the use of RS FEC a system margin of 6 db is expected (assuming FEC failure level around 10-3 ) 2dB penalty when only adaptive post equalization is used (reference level 10-8 ) Worse performances when pre-equalization is coupled to adaptive equalization: lower extinction ratio of the received optical signal.

Experimental results: BER vs distance 37 A maximum distance of 275 meters is demonstrated using standard POF connectors and patch cables of different length. Adaptive equalization + Pre-equalization 200 m 225 m 250 m 275 m ERROR FREE 10-8 10-5 10-2 Adaptive equalization only 200 m 225 m 250 m 275 m ERROR FREE 10-8 10-6 10-3

Current work 38 Ethernet interfacing FEC and global architecture implementation CDR Hybrid CDR (external VCO) Digital Timing Recovery

Outline 39 What is Plastic Optical Fiber (POF)? POF & GOF Why POF? Where? POF impairments to be overcome High bit rate over POF, how high? The EU-STREP POF-ALL 100 Mb/s media converter prototype Description -> DSP techniques Performance Current and future work Conclusions

Summary / Conclusions 40 We introduced well known digital techniques to counteract physical imparments of POF. Pre-equalization Post-equalization LED nonlinearity compensation The target 100 Mb/s over 200+ meters was accomplished. Error free measurements after 200m POF With FEC coding, a longer link is feasible Last stage of full prototype implementation

Summary / Conclusions 41 Previous experience with an old prototype: 10 Mb/s over 425m SI-POF Commercial media converters, up to 200m. FEC: RS 8b10b Data recovery (Oversampling) Binary transmission No equalization

Acknowledgments 42 This work was supported by the EU IST-FP6-STREP project n. 027549 POF-ALL WEB site: www.ist-pof-all.org For any info regarding the project: info@ist-pof-all.org To contact the coordinator Dr. Roberto Gaudino E-mail: gaudino@polito.it