Post on 13-Feb-2017
Finisar Presents: Optics, Optical Fibers, and Transceivers
Optics 101
© 2011 Finisar Corporation
March 2011March 2011
Optics, Optical Fiber, & Transceivers
Part 1 – OpticsReflection & RefractionIndex of refraction
Part 2 – Optical FiberFiber constructionMulti mode fiberMulti mode fiberSingle Mode fiber
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Optics, Optical Fiber, & Transceivers
Part 3 - Optical TransceiverpTransceiver module architecture & constructionTransmitterReceiverReceiverOptical sub-assemblies (OSA)Tx / Rx performance figures of merit
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Optics, Optical Fiber, & Transceivers
Part 4 –Power Budget, Referencesg ,Power (link) budgetsReferences
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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
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Why fiber optics?
Low loss - Flat attenuation (loss) across frequencyElectromagnetic immunity - No radiation or absorption ofElectromagnetic immunity No radiation or absorption of electromagnetic energyLight weight - Copper co-ax cable can weigh 9 times as much as fiber cableas fiber cableSmall size - A smaller diameter fiber cable provides more bandwidth than a larger copper cableSafety No electrical hazard no spark potentialSafety - No electrical hazard, no spark potentialSecurity - Extremely difficult to “fiber tap”
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Part 1 – Optics
Reflection and Refraction
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Optics - Reflection & Refraction
“Speed of light” = 3 x 105 km/sec (186,000 miles per second) in a vacuumsecond) in a vacuumLight travels at different speeds in different materials; speed is material dependentDifferent wavelengths of light travel at different speeds in the same material; speed is wavelength dependentLight traveling at an angle from one material to a differentLight traveling at an angle from one material to a different material changes directionThis change of direction is known as refraction
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Optics - Refraction
n1n2n2
Light traveling at an angle from one material to a different material changes direction
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material changes direction
Optics - Refraction
n11
n22
Light traveling at an angle from one material to a differentLight traveling at an angle from one material to a different material changes directionFor a given material, different wavelengths of light travel at diff t d d i l th d d t
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different speeds; speed is wavelength dependent
Optics - Index of Refraction
Index of refraction n, n = c/v, where:l it f li ht i fc = velocity of light in free space
v = velocity of light in a specific material
Index of refraction for selected media:
Material Index (n) Light Velocity (km/s)Vacuum 1.0 300,000Air 1.0003 300,000Water 1.33 225,000Fused Quartz 1.46 205,000Glass 1.5 200,000Silica 1.52 198,000
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Optics – Refraction and Reflection
For n1 greater than n2:
critical anglegreater than critical angle
n1n1
Incident light at the critical angle is not refracted into
i l 2 n2n2material n2
Incident light greater than the critical angle does not pass into material n2, but is reflected within material n1
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Optics - Reflection
Example:θ 80 6°
n1n1 = 1.48, n2 = 1.46 θ2 = 90°
θc = 80.6°
n2θc = arcsin (1.48 / 1.46)
θ 80 6°θc = 80.6°
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Optics - Reflection
Where index of refraction n1 is greater than n2, total internal reflection will occur if θ is greater that theinternal reflection will occur if θ is greater that the “critical angle”
nn1
n2
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Optics - Reflection Example
Cladding n2
Core n1Core n1
Cladding nCladding n2
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Part 1 – Q&A
Q: Why is fiber so attractive?A: BandwidthA: Bandwidth
Q: Name one advantage of fiber over copper, besides bandwidthA: 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
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A: Total internal reflection
Part 2 – Optical Fiber
Fiber constructionMulti mode fiberMulti mode fiberSingle mode fiber
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Optical Fiber
Concentric layers CoreCoreCladdingJacket
The index of refraction, n, of the core (n1) is greater than the cladding (n2)
CladdingJacket
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Core Cross-section view
Optical Fiber
Two primary types of fiberp y ypMulti mode fiberSingle mode fiber
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Multi mode fiber
Core is 50 μm to 62.5 μm, cladding is 125 μm (point of reference: human hair is about 100 μm)M lti l li ht d t th th fib Diff t d t lMultiple light modes propagate thru the fiber. Different modes travel different paths, some longer than others, resulting in a spreading of the light pulse - modal dispersion
Cl ddiCladdingcorecladding
M d l di i i f li iti f t i lti d fib
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Modal dispersion is a performance limiting factor in multi mode fiber
Dispersion – pulse spreading
Input pulses are separate and distinctOutput pulses exhibit pulse spreading, leading to pulse overlapping; pulses become indistinguishable from each otherBandwidth of the fiber decreases as dispersion increasesp
1 2 3Input pulses
Spreading caused by dispersion
Output pulses
Signal lossSpreading caused by dispersion1 2 3
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Single mode fiber
Core is approx. 9 μm, cladding is 125 μmCore 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
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Modal dispersion does not exist in single mode fiber
Dispersion in single mode fiber
Chromatic (material) dispersion Chromatic dispersion is the result of differentChromatic dispersion is the result of different wavelengths traveling at different speeds
1310 nmChromatic (material) dispersion is a
1309.5 nm
dispersion is a performance limiting factor in single mode fiber
1310.5 nm
fiber
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Laser emission
Attenuation
Attenuation is the loss of optical power as light travels thru the fiberVaries with wavelengthgConstant across frequency (unlike copper cable)To minimize attenuation, use a source (laser) that emits in the low-loss region of the fiber
Atten 8
multi mode fiber
Atten 4 single mode fibernuation (d
6
4
uation (d3
2dB/km
) 2
4
dB/km
) 1
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Wavelength (nm)850 1310
Wavelength (nm)850 1310
Attenuation – two primary contributors
ScatteringL f ti l d t i f ti i fibLoss of optical energy due to imperfections in fiberLight becomes multi-directional
AbsorptionImpurities in the fiber absorb optical energy
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Summary – fiber, dispersion, and attenuation
Multi mode fiberModal dispersion is the performance limiting factorChromatic (material) dispersion exists, but is not significantMulti mode fiber is used for short distance links (up to 500 meters at 850 nm and Gigabit frequencies) due to bandwidth limitation caused b d l di iby modal dispersion
Single mode fiberModal dispersion does not existChromatic (material) dispersion is a performance limiting factorSingle 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
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Part 2 – Q&A
Q: What are the two primary types of optical fiber?A: Multi mode and single modeA: Multi mode and single mode
Q: What color is the jacket of single mode fiber?A: YellowA: Yellow
Q: How many modes propagate thru a single mode fiber?A: OneA: One
Q: In what fiber does modal dispersion become a performance limiting factor?A: Multi mode fiberA: Multi mode fiber
Q: What happens if n1 is LESS THAN n2?A The light refracts into the cladding and does not propagate thr the core
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A: The light refracts into the cladding, and does not propagate thru the core
Part 3 – Optical Transceivers
Module architectureTransmitterOptical sub assembliesTransmitter characteristicsTransmitter characteristics Transmitter eye pattern ReceiverReceiver characteristics & eye pattern
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Basic Module Architecture
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Transceiver Architecture
Differential signaling used to minimize EM emissions to prevent:prevent:
Crosstalk – coupling of (e.g.) Tx signal to Rx circuit in module, which reduces sensitivity of receiverEMI electromagnetic interference affecting theEMI – electromagnetic interference affecting the customer’s box; undesirably high emissions from the transceiver
Laser Driver IC converts differential input signal into aLaser Driver IC converts differential input signal into a current capable of driving the laserTOSA converts electrical signal to light (laser) and couples the light into optical fiber via lensthe light into optical fiber via lens
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TOSA Architecture
Fiber BoreTOSA Port
Cross-Section
Coupling LensFiber Stop
Header/Can Assemblyfitted with windowLaser Die fitted with window
(aka “Laser”)(sealed inside can)
Electrical Leads (die is wire bonded to the tops of leads inside can)
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Laser characteristics
light L
current II th
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Laser driver output current
time t
current swingg
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current I
Laser driver and laser performance
data “1”L
Tx = fiber-coupled optical powerTx
data “0”
Pave
Pave = average optical power
data 0”
IIth
t
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I
t
Laser output as digital optical signal
data “1”
LTx
L1 111
data “0”
Pave
data 0”
IIth Time
0 0 0 0
941 ps bit period for 1 0625 Gb/sfor 1.0625 Gb/s
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t
Building an eye pattern
data “1”
LTx
L
data “0”
Pave
data 0”
IIth Time
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t
Building an eye pattern
data “1”
LTx
L
data “0”
Pave
data 0”
IIth Time
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t
Transmitter eye pattern
Pave
OMA
Average power (Pave)
Jitter
Average power (Pave)
Optical modulation amplitude (OMA, in μW)
Jitt (i )
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Jitter (in ps)
Transmitter eye pattern
JitterJitter
Deterministic jitter– caused by dispersion in fiber andDeterministic jitter– caused by dispersion in fiber, and inadequate bandwidth of transceiver componentsRandom jitter– caused by thermal noise, shot noise in components
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components
Transmitter eye pattern - mask
Objective no “mask hits”
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Objective – no mask hits
Transmitter Characteristics
Tx Power = fiber-coupled optical powerOptical Modulation Amplitude (OMA) = difference in power between “1” and “0” bits“Eye Diagram” = useful mathematical construct forEye Diagram = useful mathematical construct for assessing digital signal quality, comprising 1000s (or more) of bits overlapped in time to one bit periodJitter = “thickness” of 0-to-1 and 1-to-0 transitions in eye diagram; an indication of instability in the bit periodEye Mask = keep out zone in eye diagram for error freeEye Mask = keep out zone in eye diagram for error free transmission
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ROSA Architecture
Optical signalOptical signal from fiber enters ROSALens couples plight to photodiode (detector)
Pre-Amp IC (TIA) boosts signal from detector
Post amp IC (on the pcba) further boosts signal and provides host de ice ith the differential o tp t oltage signal
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host device with the differential output voltage signal
Receiver Characteristics
Rx Amplitude – peak to peak voltage outputRx Jitter – analogous to Tx jitterSignal Detect / Loss of Signal (SD / LOS)BER (Bit Error Rate) = # bits in error per # bitsBER (Bit Error Rate) = # bits in error per # bits transmitted. E.g., 1 error in 10^12 bits means BER = 10^-12Sensitivity = minimum received optical power that yields BER required by application
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Receiver electrical eye pattern
Rx amplitude
Jitter
Rx amplitude (mV)
Rx jitter (ps)
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Part 3 – Q&A
Q: The transceiver has two interfaces – what are they?A: Electrical and opticalA: Electrical and optical
Q: True or false - the OSA’s contain a telescoping lensA: False – they contain a ball lensA: False they contain a ball lens
Q: True or false – the laser is “always on”A: True The low signal point has the laser operating above IthA: 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 signalA: FalseA: False
Q: How much jitter is enough?A: Jitter is a detrimental attribute – the less the better
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A: Jitter is a detrimental attribute the less, the better
Part 4 –Power Budget, References
Power budgetgReferences
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Power budget
The difference between minimum transmitter output and minimum receiver sensitivityy
Tx output power:Max = -4 dBmMi 9 5 dB
Rx sensitivity:Max = 0 dBm
Mi 18 dBMin = -9.5 dBm Min = -18 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
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Power budget
Switch Splice ServerFiber (100m)Fiber (100 m)
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 dB/km; @ 200m = 5 dBFiber: 2.4 dB/km; @ 200m .5 dBSplice: .3 dB 2 connectors @ .3 dB each = .6 dB
Total = 1.4 dB
Conclusion: power budget of 8.5 dB leaves plenty of margin
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p g p y g
Power budget
Switch Splice ServerFiber (500m)Fiber (500 m)
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 is 500 MHz!q y
Losses:Fiber: 2.4 dB/km; @ 500m = 1.2 dBSplice: .3 dB 2 connectors @ .3 dB each = .6 dBTotal = 2.1 dB
Conclusion: Link is bandwidth limited; loss is irrelevant unless we operate the link at 500 M instead of 1Gig
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Power budget
To achieve longer links Use single mode fiber – bandwidth is limited not by modal dispersion, but by chromatic dispersion U l ith t l idth hi h ltUse a laser with narrow spectral width, which results in low chromatic dispersion. Attenuation & power budget become the link limiting factors up to roughly 80kmUse a receiver having greater sensitivity
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Power budget
Typical 850 nm multi mode transceiver specificationsTx average power 7 dBmTx average power -7 dBmRx sensitivity -21dBm Power budget (link budget) 14 dBPower budget (link budget) 14 dB
Loss (dB) Power Remaining (%)Loss (dB) Power Remaining (%)10 10.0
20 1.0
30 0.1
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Power budget
Longer link budget transceivers (using DFB lasers, APD d t t )detectors)
Tx average power 0 dBmRx sensitivity 30 dBmRx sensitivity -30 dBm Power budget (link budget) 30 dB
Loss (dB) Power Remaining (%)10 10.0
20 1.0
30 0.1
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References
Schelto’s Physical Page www.schelto.comIEEE / Fibre Channel – Physical Interface Standard www.t11.orgGBIC standard ftp://playground.sun.com/pub/OEmod/Tutorials, industry information www.lightreading.com search for , y g g“tutorials”Technician's Guide to Fiber Optics – Third Edition Donald J. Sterling, Jr., Delmar Publishers,Finisar Corporation www.finisar.com
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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?budget use?
A: Close to 99.9%
Q: True or False: MAXIMUM receiver sensitivity is key to a greater power budgetQ ue o a se U ece e se s t ty s ey to a g eate po e budgetA: False
Q: Where is a great place to shop for your optical transceiver needs?A: Finisar
Q: Have you found this session informative?A Y d id d l t k !
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A: You decide and let us know!