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Finisar Presents: Optics, Optical Fibers, and Transceivers Optics 101 March 2011 © 2011 Finisar Corporation Optics, Optical Fiber, & Transceivers Part 1 – Optics Reflection & Refraction Index of refraction Part 2 – Optical Fiber Fiber construction Multi mode fiber Single Mode fiber © 2011 Finisar Corporation 2 Optics, Optical Fiber, & Transceivers Part 3 - Optical p Transceiver Transceiver module architecture & construction Transmitter Receiver Optical sub-assemblies (OSA) Tx / Rx performance figures of merit © 2011 Finisar Corporation 3 Optics, Optical Fiber, & Transceivers Part 4 –Power Budget, g , References Power (link) budgets References © 2011 Finisar Corporation 4 Why Fiber Optics? Bandwidth is a compelling reason 1970’s – Copper cable, 672 simultaneous data streams, with 2 km spacing between amplification points Today – With a single fiber, in excess of 130,000 simultaneous data streams, with 60 km spacing between amplification points © 2011 Finisar Corporation 5 Why fiber optics? Low loss - Flat attenuation (loss) across frequency Electromagnetic immunity - No radiation or absorption of electromagnetic energy Light weight - Copper co-ax cable can weigh 9 times as much as fiber cable Small size - A smaller diameter fiber cable provides more bandwidth than a larger copper cable Safety - No electrical hazard hazard, no spark potential Security - Extremely difficult to “fiber tap” © 2011 Finisar Corporation 6 Part 1 – Optics Reflection and Refraction © 2011 Finisar Corporation 7 Optics - Reflection & Refraction “Speed of light” = 3 x 105 km/sec (186,000 miles per second) in a vacuum Light travels at different speeds in different materials; speed is material dependent Different wavelengths of light travel at different speeds in the same material; speed is wavelength dependent Light traveling at an angle from one material to a different material changes direction This change of direction is known as refraction © 2011 Finisar Corporation 8 Optics - Refraction n1 n2 Light traveling at an angle from one material to a different material changes direction © 2011 Finisar Corporation 9 Optics - Refraction n1 n2 Light traveling at an angle from one material to a different material changes direction For a given material, different wavelengths of light travel at diff different t speeds; d speed d iis wavelength l th d dependent d t © 2011 Finisar Corporation 10 Optics - Index of Refraction Index of refraction n, n = c/v, where: c = velocity l it off light li ht iin ffree space v = velocity of light in a specific material Index of refraction for selected media: Material Index (n) Light Velocity (km/s) 1.0 300,000 1.0003 300,000 Water 1.33 225,000 Fused Quartz 1.46 205,000 Glass 1.5 200,000 Silica 1.52 198,000 Vacuum Air © 2011 Finisar Corporation 11 Optics – Refraction and Reflection greater than critical angle For n1 greater than n2: Incident light at the critical angle is not refracted into material i l n2 2 critical angle n1 n2 Incident light greater than the critical angle does not pass into material n2, but is reflected within material n1 © 2011 Finisar Corporation 12 Optics - Reflection Example: θc = 80.6° 80 6° n1 = 1.48, n2 = 1.46 θc = arcsin (1.48 / 1.46) n1 θ2 = 90° n2 θc = 80.6° 80 6° © 2011 Finisar Corporation 13 Optics - Reflection Where index of refraction n1 is greater than n2, total internal reflection will occur if θ is greater that the “critical angle” n1 n2 © 2011 Finisar Corporation 14 Optics - Reflection Example Cladding n2 Core n1 Cladding n2 © 2011 Finisar Corporation 15 Part 1 – Q&A Q: Why is fiber so attractive? A: Bandwidth Q: Name one advantage of fiber over copper, besides bandwidth A: Low loss, EM immunity, light weigh, small size, no electrical hazard, secure Q: What is the speed of light in a vacuum? A: 300,000 km/hr or 186,000 miles per second Q: Light traveling thru material n1, which is greater than n2, does not refract from n1 into n2, but reflects back into n1. What is this called? A: Total internal reflection © 2011 Finisar Corporation 16 Part 2 – Optical Fiber Fiber construction Multi mode fiber Single mode fiber © 2011 Finisar Corporation 17 Optical Fiber Concentric layers Core Cladding Jacket The index of refraction, n, of the core (n1) is greater than the cladding (n2) Jacket Cladding Core © 2011 Finisar Corporation Cross-section view 18 Optical Fiber Two p primary y types yp of fiber Multi mode fiber Single mode fiber © 2011 Finisar Corporation 19 Multi mode fiber Core is 50 μm to 62.5 μm, cladding is 125 μm (point of reference: human hair is about 100 μm) Multiple M lti l light li ht modes d propagate t thru th the th fiber. fib Diff Differentt modes d ttravell different paths, some longer than others, resulting in a spreading of the light pulse - modal dispersion Cladding Cl ddi core cladding M d l di Modal dispersion i iis a performance f lilimiting iti ffactor t iin multi lti mode d fib fiber © 2011 Finisar Corporation 20 Dispersion – pulse spreading Input pulses are separate and distinct Output pulses exhibit pulse spreading, leading to pulse overlapping; pulses become indistinguishable from each other Bandwidth of the fiber decreases as dispersion p increases Input pulses 1 2 Output pulses 3 Spreading caused by dispersion 1 © 2011 Finisar Corporation 2 3 Signal loss 21 Single mode fiber Core is approx. 9 μm, cladding is 125 μm Core is sized proportionately (9 μm) to the wavelength (1100 nm and above) as to only propagate one mode efficiently cladding core cladding Modal dispersion does not exist in single mode fiber © 2011 Finisar Corporation 22 Dispersion in single mode fiber Chromatic (material) dispersion Chromatic dispersion is the result of different wavelengths traveling at different speeds 1310 nm 1309.5 nm Chromatic (material) dispersion is a performance limiting factor in single mode fiber 1310.5 nm Laser emission © 2011 Finisar Corporation 23 Attenuation Attenuation is the loss of optical power as light travels thru the fiber Varies with wavelength g Constant across frequency (unlike copper cable) To minimize attenuation, use a source (laser) that emits in the low-loss region of the fiber 8 6 4 2 Attenuation (d dB/km) Atten nuation (d dB/km) multi mode fiber single mode fiber 4 3 2 1 850 1310 Wavelength (nm) 850 © 2011 Finisar Corporation 1310 Wavelength (nm) 24 Attenuation – two primary contributors Scattering Loss off optical L ti l energy d due tto iimperfections f ti iin fib fiber Light becomes multi-directional Absorption Impurities in the fiber absorb optical energy © 2011 Finisar Corporation 25 Summary – fiber, dispersion, and attenuation Multi mode fiber Single mode fiber Modal dispersion is the performance limiting factor Chromatic (material) dispersion exists, but is not significant Multi mode fiber is used for short distance links (up to 500 meters at 850 nm and Gigabit frequencies) due to bandwidth limitation caused b modal by d l di dispersion i Modal dispersion does not exist Chromatic (material) dispersion is a performance limiting factor Single mode fiber is used for long distance links (up to 80+ km), at 1310 nm and 1550 nm Multi mode and single mode fibers and transceivers are generally not interchangeable © 2011 Finisar Corporation 26 Part 2 – Q&A Q: What are the two primary types of optical fiber? A: Multi mode and single mode Q: What color is the jacket of single mode fiber? A: Yellow Q: How many modes propagate thru a single mode fiber? A: One Q: In what fiber does modal dispersion become a performance limiting factor? A: Multi mode fiber Q: What happens if n1 is LESS THAN n2? A The light refracts into the cladding, A: cladding and does not propagate thr thru the core © 2011 Finisar Corporation 27 Part 3 – Optical Transceivers Module architecture Transmitter Optical sub assemblies Transmitter characteristics Transmitter eye pattern Receiver Receiver characteristics & eye pattern © 2011 Finisar Corporation 28 Basic Module Architecture © 2011 Finisar Corporation 29 Transceiver Architecture Differential signaling used to minimize EM emissions to prevent: Crosstalk – coupling of (e.g.) Tx signal to Rx circuit in module, which reduces sensitivity of receiver EMI – electromagnetic interference affecting the customer’s box; undesirably high emissions from the transceiver Laser Driver IC converts differential input signal into a current capable of driving the laser TOSA converts electrical signal to light (laser) and couples the light into optical fiber via lens © 2011 Finisar Corporation 30 TOSA Architecture TOSA Port Cross-Section Fiber Bore Coupling Lens Fiber Stop Header/Can Assembly fitted with window (aka “Laser”) Laser Die (sealed inside can) Electrical Leads (die is wire bonded to the tops of leads inside can) © 2011 Finisar Corporation 31 Laser characteristics light L current I I th © 2011 Finisar Corporation 32 Laser driver output current time t current swing g current I © 2011 Finisar Corporation 33 Laser driver and laser performance L Tx data “1” P Tx = fiber-coupled optical power Pave = average optical power ave data “0” 0” Ith I t I © 2011 Finisar Corporation 34 Laser output as digital optical signal L Tx data “1” P L 1 1 1 ave data “0” 0” 0 Ith 1 I 0 0 941 ps bit period for 1.0625 1 0625 Gb/s 0 Time t I © 2011 Finisar Corporation 35 Building an eye pattern L Tx data “1” P L ave data “0” 0” Ith I Time t I © 2011 Finisar Corporation 36 Building an eye pattern L Tx data “1” P L ave data “0” 0” Ith I Time t I © 2011 Finisar Corporation 37 Transmitter eye pattern Pave OMA Jitter Average power (Pave) Optical modulation amplitude (OMA, in μW) Jitt (i Jitter (in ps)) © 2011 Finisar Corporation 38 Transmitter eye pattern Jitter Jitter Deterministic jitter– caused by dispersion in fiber fiber, and inadequate bandwidth of transceiver components Random jitter– caused by thermal noise, shot noise in components © 2011 Finisar Corporation 39 Transmitter eye pattern - mask Objective – no “mask mask hits hits” © 2011 Finisar Corporation 40 Transmitter Characteristics Tx Power = fiber-coupled optical power Optical Modulation Amplitude (OMA) = difference in power between “1” and “0” bits “Eye Eye Diagram” Diagram = useful mathematical construct for assessing digital signal quality, comprising 1000s (or more) of bits overlapped in time to one bit period Jitter = “thickness” of 0-to-1 and 1-to-0 transitions in eye diagram; an indication of instability in the bit period Eye Mask = keep out zone in eye diagram for error free transmission © 2011 Finisar Corporation 41 ROSA Architecture Optical signal from fiber enters ROSA Lens couples p light to photodiode (detector) Pre-Amp IC (TIA) boosts signal from detector Post amp IC (on the pcba) further boosts signal and provides host de device ice with ith the differential o output tp t voltage oltage signal © 2011 Finisar Corporation 42 Receiver Characteristics Rx Amplitude – peak to peak voltage output Rx Jitter – analogous to Tx jitter Signal Detect / Loss of Signal (SD / LOS) BER (Bit Error Rate) = # bits in error per # bits transmitted. E.g., 1 error in 10^12 bits means BER = 10^12 Sensitivity = minimum received optical power that yields BER required by application © 2011 Finisar Corporation 43 Receiver electrical eye pattern Rx amplitude Jitter Rx amplitude (mV) Rx jitter (ps) © 2011 Finisar Corporation 44 Part 3 – Q&A Q: The transceiver has two interfaces – what are they? A: Electrical and optical Q: True or false - the OSA’s contain a telescoping lens A: False – they contain a ball lens Q: True or false – the laser is “always on” A: True. True The low signal point has the laser operating above Ith. Q: T or F -The optical output signal is the inverse of the laser driver output signal A: False Q: How much jitter is enough? A: Jitter is a detrimental attribute – the less, less the better © 2011 Finisar Corporation 45 Part 4 –Power Budget, References Power budget g References © 2011 Finisar Corporation 46 Power budget The difference between minimum transmitter output and minimum receiver sensitivityy Tx output power: Max = -4 dBm Mi = -9.5 Min 9 5 dB dBm Rx sensitivity: Max = 0 dBm Mi = -18 Min 18 dB dBm Transceiver power budget is -9.5 - (-18) = 8.5 dB From this we subtract all the link losses, and need to have margin remaining © 2011 Finisar Corporation 47 Power budget Fiber (100m) Fiber (100 m) Switch Splice Server Bandwidth check: (using multi mode fiber @ 850 nm & 1 Gb/sec) – 500 MHz/km / 1000 Mhz = 500 m ; we’re ok at 200 m Losses: Fiber: 2.4 2 4 dB/km; @ 200m = Splice: 2 connectors @ .3 dB each = .5 5 dB .3 dB .6 dB Total = 1.4 dB Conclusion: p power budget g of 8.5 dB leaves plenty p y of margin g © 2011 Finisar Corporation 48 Power budget Fiber (500m) Fiber (500 m) Switch Splice Server Bandwidth check: (using multi mode fiber @ 850 nm & 1 Gb/sec) – 500 MHz /1000 m x 1000 m = 1000 meters in example – link is bandwidth limited!! Max frequency q y is 500 MHz! Losses: Fiber: 2.4 dB/km; @ 500m = Splice: 2 connectors @ .3 dB each = Total = 1.2 dB .3 dB .6 dB 2.1 dB Conclusion: Link is bandwidth limited; loss is irrelevant unless we operate the link at 500 M instead of 1Gig © 2011 Finisar Corporation 49 Power budget To achieve longer links Use single mode fiber – bandwidth is limited not by modal dispersion, but by chromatic dispersion Use U a llaser with ith narrow spectral t l width, idth which hi h results lt in low chromatic dispersion. Attenuation & power budget become the link limiting factors up to roughly 80km Use a receiver having greater sensitivity © 2011 Finisar Corporation 50 Power budget Typical 850 nm multi mode transceiver specifications Tx average power -7 7 dBm Rx sensitivity -21dBm Power budget (link budget) 14 dB Loss (dB) Power Remaining (%) 10 10.0 20 1.0 30 0.1 © 2011 Finisar Corporation 51 Power budget Longer link budget transceivers (using DFB lasers, APD d t t ) detectors) Tx average power 0 dBm Rx sensitivity -30 30 dBm Power budget (link budget) 30 dB Loss (dB) Power Remaining (%) 10 10.0 20 1.0 30 0.1 © 2011 Finisar Corporation 52 References Schelto’s Physical Page www.schelto.com IEEE / Fibre Channel – Physical Interface Standard www.t11.org GBIC standard ftp://playground.sun.com/pub/OEmod/ Tutorials,, industryy information www.lightreading.com g g search for “tutorials” Technician's Guide to Fiber Optics – Third Edition Donald J. Sterling, Jr.,, Delmar Publishers Finisar Corporation www.finisar.com © 2011 Finisar Corporation 53 Part 4 – Q&A Q: What is the recommended maximum distance for a multi mode link operating at 850 nm and 1.0625 Gb/s? A: 500 meters Q: How much light (of the 100% optical signal output) does a fully utilized 30dB link budget use? A: Close to 99.9% Q True Q: ue o or False: a se MAXIMUM U receiver ece e se sensitivity s t ty is s key ey to a g greater eate po power e budget A: False Q: Where is a great place to shop for your optical transceiver needs? A: Finisar Q: Have you found this session informative? A You A: Y decide d id and d llett us kknow!! © 2011 Finisar Corporation 54
