CD / PMD Fundamentals Jean-Sébastien Tassé Product Line Manager Contributions from Gwennael Amice and Francis Audet August 2014 What is dispersion? • Dispersion = pulse broadening or pulse spreading as light propagates in a fiber. • In other words, dispersion increases pulse duration. Impact of dispersion • Dispersion causes pulses to broaden which might lead to pulses that overlap. • In extreme cases, we get inter-symbol interference (ISI) and higher Bit Error Rate. Dispersion consequences Dispersion CD/PMD Pulse Broadening Bit Errors and BERT issues Significant delays and expense in installation and commissioning SLA Penalties Inability to operate at high speed (extreme) Two main types of dispersion • Chromatic Dispersion (CD): • Different wavelengths travel at different velocities Pulse Spreading • Polarization Mode Dispersion (PMD): • Two polarization modes travel at different velocities Pulse Spreading CD Fundamentals Chromatic Dispersion (CD) • Different wavelengths travel at different speeds in a fiber, causing pulse broadening • Limits how fast and how far a signal will travel • Higher bit rates are less robust t z,t Effects of Chromatic Dispersion Low chromatic dispersion High chromatic dispersion Facts about chromatic dispersion • • • • Chromatic dispersion is linear. It increases proportionally with distance. Does not change over time. Since CD properties are set during manufacturing, there is no FAIL in the field. Values measured are used to adjust the dispersion compensation modules and to select the optimum set of transmitter/receiver. Chromatic dispersion • Total chromatic dispersion is the sum of • Material dispersion + • Waveguide dispersion Be-an-Expert Training Program Cause 1: Material Dispersion • White light is composed of all colors. • Different colors travel at different speeds in fiber because the index of refraction depends on the wavelength. • Just like a prism decomposes white light because the angle of refraction depends on the index of refraction, which depends on the wavelength. Be-an-Expert Training Program Cause 2: Waveguide Dispersion • Different index profiles of the fiber (i.e. the waveguide) affect the fiber CD properties. Be-an-Expert Training Program Cause 2: Waveguide Dispersion Be-an-Expert Training Program Chromatic Dispersion (ps/nm-km) CD for different fiber types + +7 dispersion unshifted G.652 + non-zero dispersion dispersion shifted G.653 non-zero dispersion shifted G.655 (nm) CD key measurements: 1. CD at 1550 nm Example: G.652 fiber is ~ +17 ps/nm/km at 1550 nm. 2. Zero dispersion wavelength Example: G.652 fiber has zero dispersion at ~ 1310 nm. CD example • A pulse is sent in a SMF28 fiber (CD=17ps/[email protected]) • Source is DFB laser: • Linewidth = 7pm, central wavelength = 1550 nm. • WDM network is 600 km long. • Pulse duration t1 and t2: • t1: depends on the opto-electronics • t2 = t1 + (17ps/nm.km * 7pm * 600 km) = t1 + 71.4ps!! t1 t2 • This means that the pulse duration increased by 71.4 ps due to CD. How to compensate? CD ? 20 18 16 14 What compensation should you apply? 12 1525 Be-an-Expert Training Program 1550 1565 CD accumulation with distance CD Distance Be-an-Expert Training Program CD: Good Compensation CD Distance CD: Bad Compensation CD Distance Be-an-Expert Training Program CD compensation • If measured accurately, CD can be compensated for with dispersion compensation modules (DCM). No Compensation With Compensation OC192 Delay Threshold Delay Length TX DCM DCM RX Fiber characterization and CD • Regardless if the services are to be deployed on a new build or existing fiber network, a series of measurements must be performed to certify that the link is appropriate for use. • The series of measurements performed is known as “fiber characterization”. • The specific requirements are identified by the ITU-T SG15, ref: G.650.3. • CD is one of the measurements specified in this standard. Tolerance to CD versus Bit Rate • NRZ pulses (on-off keying) • 1 dB power penalty Bit rate CD Tolerance CD Tolerance (km of G.652) 2.5Gbits (OC48/STM16) 18468 ps/nm 970 km 10Gbits (OC192/STM64) 1193 ps/nm 63 km 40 Gbits 75 ps/nm 4 km 100 Gbits 12 ps/nm 0.6 km Maximum reachable distance • Maximum reachable distance for NRZ pulses (on-off keying) without CD compensation as a function of bit rate and fiber type PMD Fundamentals Definition of polarization • Light is electromagnetic wave composed of • Electric Field Vector E • Magnetic Field Vector H • Propagating in time and in space in the z direction • Oscillating in the x-z and y-z planes • Polarization is a property of an electromagnetic wave that describes the orientation of its oscillation. Polarization Mode Dispersion (PMD) • Pulses travel at different speeds depending on the polarization. This is called PMD. If PMD = 0: polarization vectors travel at same speed. If PMD ≠ 0: polarization vectors do not travel at same speed. Differential Group Delay (DGD) • • • • Fast axis: polarization axis such that the pulse travels the fastest. Slow axis: polarization axis such that the pulse travels the slowest. Fast and slow axis are called principal states of polarization. Delay between fast and slow axis is called Differential Group Delay (DGD). T t fast axis slow axis z,t PMD and DGD DGD is specific to one wavelength and it varies over time. PMD: average of DGD, for all wavelengths. PMD and DGD • • • • Probability density function: distribution of all DGD values. DGD varies over time, but PMD is more stable. At a specific time, 0 ps < DGD < DGD max. Typically, DGD max = 3 times PMD. Visualizing PMD • Let’s visualize a light pulse traveling into a fiber and segment it into 9 quadrants. Visualizing PMD • Fiber section: • Light pulse: Visualizing PMD If we transmit 1-0-1: 1 0 1 With PMD, this becomes: 1 0 1 The « 1 » is dimmer, the « 0 » can have light: BER PMD causes Geometric Internal Stress External Stress Environmental constraints Wind (aerial fibers) Heat Bend PMD causes • Asymmetries in fiber core geometry and/or stress distribution create local fiber birefringence. • A "real" fiber is a randomly distributed addition of these local birefringent portions. Older fibers exhibit worse PMD Source: John Peters, Ariel Dori, and Felix Kapron, Bellcore Fiber characterization and PMD • Regardless if the services are to be deployed on a new build or existing fiber network, a series of measurements must be performed to certify that the link is appropriate for use. • The series of measurements performed is known as “fiber characterization”. • The specific requirements are identified by the ITU-T SG15, ref: G.650.3. • PMD is one of the measurements specified in this standard. PMD thresholds: how much PMD causes failures? • • • • • Instantaneous DGD (DGDinst): DGD at specific time. DGDmax: max DGD a system can suffer from without outage. Therefore, outage occurs when DGDinst > DGDmax. However, PMD analyzers measure PMD, not DGDinst. Relationship between PMD and outage probability is given below for 1 dB OSNR penalty. DGDmax to PMD ratio Probability that DGDinst > DGDmax Time per year that DGDinst > DGDmax 2.5 1.5 x 10-3 13.1 h 3.0 4.2 x 10-5 22 min 3.2 9.2 x 10-6 5 min 3.5 7.7 x 10-7 24 s 3.8 5.1 x 10-8 1.6 s 4.0 7.4 x 10-9 0.23 s DGDmax thresholds • Systems fail when DGDinst > DGDmax • Status: refers to whether this value has been adopted by standards, or is at the proposal stage. Data rate Modulation format DGDmax Status 2.5G SDH/SONET NRZ 120 ps Adopted 10G SDH/SONET NRZ or OTN 30 ps Adopted 40G NRZ-DPSK (noncoherent) 8 ps Proposal 40G RZ-QPSK (noncoherent) 9 ps Proposal 40G DP-QPSK (coherent) 75 ps Proposal 100G NRZ (noncoherent) 2.9 ps Proposal 100G DP-QPSK (coherent) 27 ps Proposal PMD threshold example • Let’s do an example of PMD threshold calculation. • First, you need to know the data rate and modulation format of the channel. Let’s assume 40G RZ-QPSK. • Second table says DGDmax = 9 ps. • According to the first table, that means if the measured PMD is 3.6 ps (DGDmax/PMD ratio = 2.5), then it should be expected that outages due to PMD will occur during 13.1 hours in a given year. • Similarly, if the measured PMD is 3 ps (DGDmax/PMD = 3), then it should be expected that outages due to PMD will occur during 22 minutes in a given year. • The PMD threshold therefore depends on the acceptable outage probability, as determined by each service provider. • A DGDmax/PMD ratio of 3 is commonly chosen for determining the PMD threshold. Questions?