Technological Training

Polarization Mode Dispersion (PMD) in Optical Communications

Description:

Polarization Mode Dispersion, PMD, is a parameter of great importance in modern optical communications. It imposes limitations on both analog (CATV) and digital (telecommunication) optical systems. System designers need to understand well the subject of PMD for them to specify fibers and other components. On the other hand, fiber and cable manufacturers must develop products that satisfy the increasingly demanding specifications. Also, installation techniques have to be reviewed to guarantee that the PMD stays below acceptable levels. On the other hand, international bodies such as ITU, TIA, IEC and EU-COST have been conducting several studies, intending to create technical recommendations related to PMD.

PMD arises from the fact that even in current fibers, which have excellent geometry and little internal stress, there exists a small level of birefringence. Birefringence causes pulse spreading due to the slight difference between the propagation velocities of the two orthogonal polarization states that constitute the light signal. Due to fiber movements, temperature variations and other environmental variations, these two polarization states couples strongly. This makes PMD to be wavelength dependent and strongly affected by the environment. Thus, PMD varies randomly and is best characterized through statistical measurements.

PMD imposes a limit in the information transmission capacity of long distance optical telecommunication systems. For instance, a 400 km 10 Gbit/s standard fiber link demands fiber with PMD <0.5 ps km-1/2. In the case of DS (Dispersion Shifted) fibers or higher transmission rates, a PMD < 0.1 ps km-1/2 should be specified. Analog optical communications (CATV) is also affected by PMD, which introduces distortions and adds noise to the signal. Several newly installed CATV systems have required rework due to unfortunate combinations of these parameters.

A key element in keeping PMD under control is its proper measurement. This allows quality control and introduction of improvements in the fiber and fiber cable manufacturing processes. It allows also to evaluate installed links and to investigate special cares needed during the link design and installation techniques. Several methods have been proposed to measure PMD. Unfortunately, there are no standardized methods or agreement on which are best qualified for laboratory or field use. This is very much due to the statistical nature of PMD, which makes it difficult of being clearly measured.

In this course, we explore all aspects related to PMD. Initially, we describe its origins and statistical characteristics associated to its dependence with geometric and environment variations of the fiber. Next, we describe the influence of the link length and the effects of PMD in analog optical systems (CATV) and digital telecommunication optical systems. The interaction of PMD with polarization dependent loss and gain (PDL and PDG) of the system components are discussed as well. The different existing measuring methods, their limitations and accuracies are presented and compared. Standardization studies by the ITU, TIA, IEC and EU-COST on measuring techniques and recommended PMD values are also shown. A detailed survey of the measuring techniques best recommended for laboratory (fiber manufacturing) and field (fiber installation) use is presented, followed by discussions about and the commercially available instruments for such purpose. Measurement procedures and special cares during system design are discussed. The influence of the fiber and fiber cable manufacturing processes and of the installation techniques on the PMD levels are shown. PMD values for different fibers and optical devices are presented, with special emphasis for the comparison between conventional fibers and dispersion shifted fibers.

This course is intended for telecommunication and CATV optical system designers, for engineers and technicians involved in the manufacturing process of fibers and fiber cables, and for fiber cable installation technicians.

Course Content:

Fundamentals of Polarized Light
Modal birefringence in single-mode optical fibers and birefringence measurement
Mode coupling and mode coupling length
Polarization maintaining and HiBi Fibers
Polarization Ellipse
Poincare Sphere
Stokes Parameters
Polarization measurements and measurement of the Stokes Parameters (fundamentals of Polarimetry)
The Poincare Sphere for different polarization states

Linear, Circular, Elliptical
Varying the wavelength (frequency) of light
In the presence of random mode coupling

The origins of PMD
Definitions in the PMD world, DGD (Differential Group Delay), PSP (Principal States of Polarization), etc.
Distinction between PMD and Chromatic Dispersion
Polarization fluctuations and the PMD dependence with environmental conditions such as temperature, tension, vibration, etc.
Statistical Characteristics of PMD
PMD characteristics of:
- Discrete optical components, short lengths of fibers
- HiBi Fibers
- Long lengths of fibers (i.e., in the presence of strong mode coupling)

PMD dependence on the fiber link length: linear variation and variation with the square root

PDL (Polarization Dependent Loss) and PDG (Polarization Dependent Gain) of system components:

Definition
Concatenation of PDL and PDG
PDL / PDG measurements

PMD effects in optical communication systems:

PMD in analog/CATV optical systems (noise and distortions introduced in the system)
PMD in digital telecom optical systems (performance deterioration and BER fluctuation)
PDL and PDG and its interplay with PMD in analog/digital optical systems

PMD Measurement Techniques:

Interferometry (IF)

Bulk and fiber interferometric set-ups

Fourier-transformed interferometry

Considerations on limitations, measurement accuracy and implementation cost of method

Measurement examples (discrete optical components, HiBi fibers, short/long lengths of fibers, cables, fiber amplifiers/amplified links)

Wavelength Scanning or Fixed Analyzer (WS or FA)

Technique description

Fourier-transformed wavelength scanning (FTWS or FTFA)

Considerations on limitations, measurement accuracy and implementation cost

Measurement examples using WS and FTWS techniques

- The polarimetric techniques for PMD measurement ...

Poincare Sphere (PS)

State of Polarization (SOP): Combining PS + WS

Jones Matrix Eigen-Analysis (JME)

Jones matrix representation of transmission characteristics of two-port optical device

Technique description

Considerations on limitations, measurement accuracy and implementation costs

Measurement examples

Comparison of PMD measurement techniques

Laboratory X field measurements
Measurement accuracy, limitations and implementation costs
Standardization process: ITU, COST 241 and TIA round-robin studies

The importance of establishing a PMD measurement procedure for reliable and repeatable results and quality control in the fabrication process
PMD typical measured values of spooled fiber, cabled fiber and installed cables
PMD acceptable standards and the system designer special care with future upgrades
PMD control in the fiber fabrication and cabling processes
Conventional fiber VERSUS dispersion shifted (DS) fiber: PMD characteristics
PMD of fiber amplifiers and amplified optical links
Polarization OTDR: PMD measurement along the fiber link
Commercially available equipments for PMD measurements:

*Hewlett Packard
*Photonetics
*RPA (Instrument Systems Optishe Messtechnik)
*JDS Fitel
*EG&G
*Profile Optishe
*GAP / EXFO
*Santec
*Others: NOI, IPTH, Oyokoden
Techniques used
Specifications, accuracy, limitations
Applicability (laboratory X field use)
Future proof (since standards may change)
Possibility of PDL / PDG measurement
Commercial prices
Choosing a measurement technique
Indications on the best purchase choice

Summary and conclusions
Discussions

Duration: 8 Hours

Instructor: Dr. Sérgio Barcelos

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