광전송기술 기초 통신망관리팀 U-Gov 연구반

2 Decibels (dB): unit of level (relative measure) X dB is 10 -X/10 in linear dimension e.g. 3 dB Attenuation = = Standard logarithmic unit for the ratio of two quantities. In optical fibers, the ratio is power and represents loss or gain. Decibels-milliwatt (dBm) : Decibel referenced to a milliwatt X mW is 10  log 10 (X) in dBm, Y dBm is 10 Y/10 in mW. 0dBm=1mW, 17dBm = 50mW Wavelength ( ): length of a wave in a particular medium. Common unit: nanometers, m (nm) 300nm (blue) to 700nm (red) is visible. In fiber optics primarily use 850, 1310, & 1550nm Frequency ( ): the number of times that a wave is produced within a particular time period. Common unit: TeraHertz, cycles per second (Thz) Wavelength x frequency = Speed of light  x = C 용어의 정의

3 Attenuation = Loss of power in dB/km The extent to which lighting intensity from the source is diminished as it passes through a given length of fiber-optic (FO) cable, tubing or light pipe. This specification determines how well a product transmits light and how much cable can be properly illuminated by a given light source. Chromatic Dispersion = Spread of light pulse in ps/nm-km The separation of light into its different coloured rays. ITU Grid = Standard set of wavelengths to be used in Fibre Optic communications. Unit Ghz, e.g. 400Ghz, 200Ghz, 100Ghz Optical Signal to Noise Ration (OSNR) = Ratio of optical signal power to noise power for the receiver Lambda = Name of Greek Letter used as Wavelength symbol ( ) Optical Supervisory Channel (OSC) = Management channel 용어의 정의 (Cont.)

4 감쇄 ; 소스로 부터 나오는 빛의 세기가 주어진 길이의 코어를 지남에 따라 줄어드는 것. “ 이 숫자는 장치가 빛을 얼마나 잘 전달 할 수 있느냐 ” 와 “ 이 숫자는 케이블이 광원을 얼마나 잘 전달하느냐 ” 에 대한 특성에 사용된다. 분산 ; 서로 다른 Colored-ray 에 대한 분리 OSNR ; 광파워와 잡음 파워에 대한 비율 주파수 ; 주어진 시간에 그려지는 사이클 ( 주기 ) : Tera Hz 는 초당 10 에 12 승 만큼의 사이클을 나타냄. Wavelength 는 특정 매질에서 보여지는 파 (wave) 의 길이 광원의 속도는 주파수 x Wavelength

5 dB versus dBm dBm used for output power and receive sensitivity (Absolute Value) dBm 은 파워 수신이나 수신감도 처럼 Fix value dB used for power gain or loss (Relative Value) DB 는 파워 이득이나 손실 처럼 상대 value

6 dBmmWdBmmWdBmmWdBmmW dBm versus mW

7 Bit Error Rate ( BER) BER is a key objective of the Optical System Design BER 은 시스템 디자인의 핵심 중에 하나이다. Goal is to get from Tx to Rx with a BER < BER threshold of the Rx 전송할때 수신단에서 BER 임계치 내에서 수신할 수 있도록 하는것이 광전송의 목표이다. BER thresholds are on Data sheets BER 의 임계치는 대계 장치 생산 시 데이터 시트로 나타내어 진다. Typical minimum acceptable rate is 10 –12 전송에서 이야기하는 가장 전형적인 Bit Error Rate 는 10 에 –12 이다.

8 Optical Budget Optical Budget is affected by: Fiber attenuation Splices Patch Panels/Connectors Optical components (filters, amplifiers, etc) Bends in fiber Contamination (dirt/oil on connectors) Basic Optical Budget = Output Power – Input Sensitivity P out = +6 dBmR = -30 dBm Budget = 36 dB

9 Glass Purity Propagation Distance Need to Reduce the Transmitted Light Power by 50% (3 dB) Window Glass1 inch (~3 cm) Optical Quality Glass10 feet (~3 m) Fiber Optics9 miles (~14 km) Fiber Optics Requires Very High Purity Glass

10 Attenuation Dispersion Nonlinearity Waveform After 1000 KmTransmitted Data Waveform Distortion It May Be a Digital Signal, but It’s Analog Transmission Fiber Fundamentals

11 Attenuation: Reduces power level with distance Dispersion and Nonlinearities: Erodes clarity with distance and speed Signal detection and recovery is an analog problem Analog Transmission Effects

12 Analog Transmission Effects Loss Attenuation DispersionDistortion Nonlinearity New Frequencies Gain Amplification & Noise CauseEffect

13 CladdingCore Coating Fiber Geometry An optical fiber is made of three sections: The core carries the light signals The cladding keeps the light in the core The coating protects the glass

14  n2n2 n1n1 Cladding  Core Intensity Profile Propagation in Fiber Light propagates by total internal reflections at the core-cladding interface Total internal reflections are lossless Each allowed ray is a mode

15 n2n2 n1n1 Cladding Core n2n2 n1n1 Cladding Core Different Types of Fiber Multimode fiber Core diameter varies 50 mm for step index 62.5 mm for graded index Bit rate-distance product >500 MHz-km Single-mode fiber Core diameter is about 9 mm Bit rate-distance product >100 THz-km

16 Light Ultraviolet (UV) Visible Infrared (IR) Communication wavelengths 850, 1310, 1550 nm Low-loss wavelengths Specialty wavelengths 980, 1480, 1625 nm UV IR Visible 850 nm 980 nm 1310 nm 1480 nm 1550 nm 1625 nm 125 GHz/nm Wavelength:  (nanometers) Frequency:   (terahertz) C =  x Optical Spectrum

17 Optical Attenuation Specified in loss per kilometer (dB/km) 0.40 dB/km at 1310 nm 0.25 dB/km at 1550 nm Loss due to absorption by impurities 1400 nm peak due to OH ions EDFA optical amplifiers available in 1550 window 1310 Window 1550 Window

18 Optical Attenuation Wavelength (nm) ~ 200 ppb OH OH Peaks “First Window” “Third” “Second” Rayleigh scattering  1 4 ( ) Attenuation db Km [ ] Infra-red Absorbtion

19 T T P i P 0 Optical Attenuation Pulse amplitude reduction limits “how far” Attenuation in dB Power is measured in dBm: Examples 10dBm 10 mW 0 dBM 1 mW -3 dBm 500 uW -10 dBm 100 uW -30 dBm 1 uW )

20 Polarization Mode Dispersion (PMD) Single-mode fiber supports two polarization states Fast and slow axes have different group velocities Causes spreading of the light pulse Chromatic Dispersion Different wavelengths travel at different speeds Causes spreading of the light pulse Types of Dispersion

21 Affects single channel and DWDM systems A pulse spreads as it travels down the fiber Inter-symbol Interference (ISI) leads to performance impairments Degradation depends on: laser used (spectral width) bit-rate (temporal pulse separation) Different SM types Interference A Snapshot on Chromatic Dispersion

22 60 Km SMF-28 4 Km SMF Gbps 40 Gbps Limitations From Chromatic Dispersion t t Dispersion causes pulse distortion, pulse "smearing" effects Higher bit-rates and shorter pulses are less robust to Chromatic Dispersion Limits "how fast“ and “how far”

23 Combating Chromatic Dispersion Use DSF and NZDSF fibers (G.653 & G.655) Dispersion Compensating Fiber Transmitters with narrow spectral width

24 Dispersion Compensating Fiber Dispersion Compensating Fiber: By joining fibers with CD of opposite signs (polarity) and suitable lengths an average dispersion close to zero can be obtained; the compensating fiber can be several kilometers and the reel can be inserted at any point in the link, at the receiver or at the transmitter

25 Transmitter Dispersion Compensators Dispersion Shifted Fiber Cable Cumulative Dispersion (ps/nm) Total Dispersion Controlled Distance from Transmitter (km) No Compensation With Compensation Dispersion Compensation

26 How Far Can I Go Without Dispersion? Distance (Km) = Specification of Transponder (ps/nm) Coefficient of Dispersion of Fiber (ps/nm*km) A laser signal with dispersion tolerance of 3400 ps/nm is sent across a standard SMF fiber which has a Coefficient of Dispersion of 17 ps/nm*km. It will reach 200 Km at maximum bandwidth. Note that lower speeds will travel farther.

27 Polarization Mode Dispersion Caused by ovality of core due to: Manufacturing process Internal stress (cabling) External stress (trucks) Only discovered in the 90s Most older fiber not characterized for PMD

28 Polarization Mode Dispersion (PMD) The optical pulse tends to broaden as it travels down the fiber; this is a much weaker phenomenon than chromatic dispersion and it is of little relevance at bit rates of 10Gb/s or less nxnx nyny Ex Ey Pulse As It Enters the Fiber Spreaded Pulse As It Leaves the Fiber

29 Combating Polarization Mode Dispersion Factors contributing to PMD Bit Rate Fiber core symmetry Environmental factors Bends/stress in fiber Imperfections in fiber Solutions for PMD Improved fibers Regeneration Follow manufacturer’s recommended installation techniques for the fiber cable

30 SMF-28(e) (standard, 1310 nm optimized, G.652) Most widely deployed so far, introduced in 1986, cheapest DSF (Dispersion Shifted, G.653) Intended for single channel operation at 1550 nm NZDSF (Non-Zero Dispersion Shifted, G.655) For WDM operation, optimized for 1550 nm region – TrueWave, FreeLight, LEAF, TeraLight… Latest generation fibers developed in mid 90’s For better performance with high capacity DWDM systems – MetroCor, WideLight… – Low PMD ULH fibers Types of Single-Mode Fiber

31 The primary Difference is in the Chromatic Dispersion Characteristics Different Solutions for Different Fiber Types SMF (G.652) Good for TDM at 1310 nm OK for TDM at 1550 OK for DWDM (With Dispersion Mgmt) DSF (G.653) OK for TDM at 1310 nm Good for TDM at 1550 nm Bad for DWDM (C-Band) NZDSF (G.655) OK for TDM at 1310 nm Good for TDM at 1550 nm Good for DWDM (C + L Bands) Extended Band (G.652.C) (suppressed attenuation in the traditional water peak region) Good for TDM at 1310 nm OK for TDM at 1550 nm OK for DWDM (With Dispersion Mgmt Good for CWDM (>8 wavelengths)

32 Fiber Sample 28 Dec 2005 Multi Mode Fiber Single Mode Fiber

33 Fiber Sample 28 Dec 2005 Dispersion Shifted Fiber Non-Zero DSF

34 The 3 “R”s of Optical Networking A Light Pulse Propagating in a Fiber Experiences 3 Type of Degradations: Loss of Energy Loss of Timing (Jitter) (From Various Sources) Loss of Timing (Jitter) (From Various Sources) t t s Optimum Sampling Time t Phase Variation Shape Distortion Pulse as It Enters the FiberPulse as It Exits the Fiber

35 Re-Shape DCU The 3 “R”s of Optical Networking (Cont.) The Options to Recover the Signal from Attenuation/Dispersion/Jitter Degradation Are: Pulse as It Enters the FiberPulse as It Exits the Fiber Amplify to Boost the Power t t s Optimum Sampling Time t Phase Variation Re-Generate O-E-O Re-gen, Re-shape and Remove Optical Noise t t s Optimum Sampling Time Phase Re-Alignment