Types of Single Mode Fibers

Types of Single Mode Optical Fiber

   We’ve already discussed how single-mode fiber is used for essentially all long-reach fiber applications.But there are also several different types of SMF.

The most common types are:
• “Standard” SMF (ITU-T G.652) A.K.A. SMF-28
• Low Water Peak Fiber (ITU-T G.652.C/D)
• Dispersion Shifted Fiber (ITU-T G.653)
• Low-Loss Fiber (ITU-T G.654)
• Non-Zero Dispersion Shifted Fiber (ITU-T G.655)
• Bend Insensitive Fiber (ITU-T G.657)

these are the different fibers defined theoretically, however the below mentioned fibers are the one you see often in the field

“Standard” Single-Mode Fiber (G.652)  

 The original and mode widely deployed fiber is Frequently called “SMF-28” or SMF. SMF-28 is actually a product name from Corning.this fiber is Optimized for the 1310/1550nm band.the Lowest rate of dispersion occurs in this 1310 band but Attenuation is lower at 1550nm, but more dispersion there.
 

Low Water Peak Fiber (G.652.C/D)

   Modified G.652 is designed to reduce water peak.Water peak is a high rate of attenuation at certain frequencies due to OH- hydroxyl molecule within the fiber.these type of fiber give very good raman gain.

Dispersion Shifted Fiber (ITU-T G.653)

    This type of fiber is a n attempt to improve dispersion at 1550nm.The rate at which chromatic dispersion occurs changes across different frequencies of light.The natural point of zero dispersion occurs at 1300nm.But this is not the point of lowest attenuation. DSF shifts the point of zero dispersion to 1550nm.But this turned out to cause big problems.Running DWDM over DSF causes non-linear interactions.The notable example is called Four Wave Mixing where 3 equally spaced wavelengths interact to produce a 4th wavelength.As a result, this fiber is rarely used today.
This type of fiber is hugely deployed in Japan.

Non-Zero Dispersion Shifted Fiber

    They have similar concept to Dispersion Shifted Fiber but the zero point is moved outside of the 1550nm band. This leaves a small amount of dispersion, but avoids the non-linear cross-channel interactions cause by DSF. To manage dispersion, NZDSF comes in 2 types, they are NZD+ and NZD-, with opposite dispersion “slopes”.One spreads the 1550nm band out and the other compresses it in the opposite direction.By switching between the two slopes, the original signal can be maintained even over extremely long distances.this is what is done in subsea cable

Other Single-Mode Fiber Types

the other type of single mode fibers are 

ELEAF, TWRS, TWS, Teralight etc

 

Dispersion of different Fiber types

Optical Power in dB

What is Optical Power? 

• The brightness (or “intensity”) of the light. 

• As light travels through fiber, some of the energy is lost. 

• Either absorbed by the glass particles, and converted to heat;

• Or scattered by microscopic imperfections in the fiber. 

• This loss of intensity is called “attenuation”. 

• We typically measure optical power in “Decibels” 

• A decibel (dB, 1/10th of a Bel) is a logarithmic-scale unit expressing the relationship between two values.
• The decibel is a “dimensionless-unit”, meaning it does NOT express an actual physical measurement on its own. 

Optical Power and the Decibel

A decibel itself is simply a ratio between values! 

• 0 dB is no change, +3 dB is double, -3 dB is half, etc.
• To express an absolute value (i.e. an actual light level), it must be compared to a known reference value. 

In optical networking, this is typically a “dBm”.

• That is, a decibel relative to 1 milliwatt (mW) of power. 
• 0 dBm is 1 mW, 3 dBm is 2 mW, -3 dBm is 0.5mW, etc.
• So what does this make 0mW? Negative Infinity dBm.
• Confusion between dB and dBm is one of the most common mistakes when working with optical networks! 19 Optical Power and the Decibel 

So why do we measure light with Decibels? • Light, like sound, follows the “inverse square” law.

• The signal is inversely proportional to the distance squared.
• After a signal travels distance X, and loses half of its intensity.
• After it travels another distance X, it loses half again.
• Thus after 2X only 25% remains, after 3X only 12.5% remains, etc.

Using a logarithmic scale simplifies the calculations.

• A 3dB change is approximately half/double the original signal. 
• In the example above, there is a 3dB loss per distance X. 
• At distance 2X there is 6dB of loss, at distance 3X it is 9dB, etc.
• Using a logarithmic scale “cancels out” the exponential loss, giving us a linear system that lets us use simple math (like addition and subtraction) when calculating losses. 20 Decibel to Power Conversion Table 

Frequently asked questions about Optical networks

Many students and engineers asks this below questions

1.What is Coherent Transmission?

As mobile networks advance towards data networks, intelligent terminals are widely used, and new services, such as IPTV,eCommerce and cloud computing,online gaming are increasing rapidly, the transmission capacities of legacy networks have to be improved. The system supports high-speed transmission with the use of advanced modulation formats like ePDM-16QAM, ePDM-QPSK, ePDM-BPSK, and coherent detection technologies to meet the high-speed transmission requirements on OSNR, CD, PMD, and nonlinear effects. It offers ultra-large bandwidth (400G, 200G, 100G and 40G).

2.What is ROADM?

It’s the abbreviation of reconfigurable optical add/drop multiplexer. With ROADM technology, flexible optical-layer grooming is available. The ROADM reconfigure wavelengths by blocking or cross-connecting the wavelengths. It changes the static allocation of the resource to flexible and dynamic allocation.

The main technology of RAODM is WSS. The WSS module on a WSS board splits a colored light signal into multiple parallel monochromatic signals and adjusts the optical power of each monochromatic signal. The WSS module directs each monochromatic signal into the corresponding multiplexer using its 1xN optical switch. The WSS module then multiplexes the monochromatic signals into one signal for further transmission. In this way, a monochromatic signal can be transmitted out of the board through any port.

3.What is ASON?

It’s the abbreviation of Automatically Switched Optical Network. ASON introduces Generalized Multiprotocol Label Switching (GMPLS) control plane to achieve dynamic connection management, automatic discovery, protection & restoration, and CAPEX & OPEX reduction. The control plane of ASON complies with Link Management Protocol (LMP), Link Management Protocol (OSPF), and Resource Reservation Protocol-Traffic Engineering (RSVP-TE) protocols.

ASON can transport services of different Service Level Agreements (SLAs) based on customers’ requirements. The SLA divides services into various levels according to the service protection capability.

There two kinds of ASON, optical-layer ASON and electrical-layer ASON. The optical-layer ASON, which is also known as WSON, is based on flexible ROADM with the using of WSS technologies to implement colorless, directionless and contentionless applications. As to electrical-layer ASON, there two types of electrical-layer ASON: OTN ASON and SDH ASON. Electrical-layer ASON is based on optical-layer server trails. The OTUk link or VC link may be inconsistent with the physical topology. OTN ASON can be easily deployed on standard OTN networks to improve the network reliability.


NB:I will describe each of the above terminology in another post

Polarization Mode Dispersion

Introduction

      There are three fundamentally different dispersive phenomena in optical fiber, of which polarization mode dispersion (PMD) is the most complex. In digital multimode fiber systems, a light pulse separates into multiple spatial paths or modes. Each component reaches the receiver at a slightly different time as shown in the figure broadening the received pulse. Single-mode fiber solves the differential mode delay problem, allowing data rates to be increased until chromatic dispersion — the variation of propagation speed with wavelength — produces unacceptable pulse spreading. The amount of chromatic dispersion that a system can tolerate is inversely proportional to the square of the bit rate because an increased data rate means not only a wider spectrum and increased spreading, but also narrower bit slots that are more sensitive to the spreading of neighboring pulses. 

Three Different types of dispersion in fiber

     When chromatic dispersion is compensated — typically to a small but nonzero value in dense wavelength division multiplexed (DWDM) systems — the bit rate can be increased until it is limited by the third dispersive effect, PMD. Every network exhibits two slightly different propagation delays that correspond to different input polarization's. Some of the pulse energy experiences the longer delay and the rest of the energy experiences the shorter delay. As with the other dispersive effects, the result is a broadening of the received pulse. 

      PMD is considerably more subtle and interesting than this, however, and the topic accounts for a rapidly growing body of technical literature. This article will explore the origins, statistical character, measurement and mitigation of first-order polarization mode dispersion. 

Properties of polarized light

     The electric and magnetic fields of a lightwave fluctuate at right angles to one another in the plane perpendicular to the direction of propagation like in the figure below. PMD in single-mode optical fiber originates with noncircularity of the core  Fiber birefringence has two components. Form birefringence is a basic characteristic of any oval waveguide. Stress birefringence — generally dominant — is induced by the mechanical stress field that is set up when the fiber is drawn to other than a perfectly circular shape. Over short lengths, fiber birefringence splits the input pulse into linear slow and fast polarization modes, behaving like a linearly birefringent crystal. The corresponding difference in propagation time is called the differential group delay (DGD), expressed in picoseconds (1 ps = 10-12 s). Together, the differential group delay and the orthogonal polarization modes are the fundamental manifestations of first-order PMD. 

Lightwave Fluctuating between electric and magnetic field

   Given the extremely weak birefringence of telecom fiber, mode coupling is easily induced in the fiber by the mechanical forces arising from spooling, cabling or installation. 

 Core noncircularity is the root of PMD in single-mode fiber.

     The differential group delay at a given wavelength and time is called the instantaneous differential group delay. The average value of the DGD over wavelength is called the PMD delay. The average DGD divided by the square root of fiber length is called the PMD coefficient. 

How much can be tolerated?

    Digital transmission systems are designed to tolerate 10 to 15 percent of a bit period of average differential group delay, or 10 to 15 ps for a 10 Gb/s system. The average differential group delay of long routes of legacy fiber is often greater than this limit and in particularly severe cases can exceed 100 ps. 

     New optical fiber generally exhibits an average differential group delay in the range of 0.05 to 0.10 ps/km1/2. 

PMD mitigation

      The development of PMD mitigation techniques is driven by the upgrade of legacy fiber links to 10+ Gb/s. Any mitigation scheme must account for random changes in the differential group delay and principal states of polarization. One approach is to eliminate pulse spreading by coupling the transmitter output to a single input principal state of polarization of the link. Drawbacks are the need for specialized hardware at both transmitter and receiver and the delay of the feedback loop, which is twice the length of the link. 

    PMD can also be electrically mitigated by means of an equalizer circuit installed following the receiver photodetector. The detected signal is split into several paths to be differentially delayed and scaled, then recombined to squeeze the pulse back to a narrower shape. This technique has a history in microwave communications. PMD mitigation is receiving wide research attention and field trials have been run on several methods. 

Summary 

   When chromatic dispersion is compensated, PMD becomes a bit-rate limiting factor in digital fiber optic communications systems. The high PMD of many legacy fibers calls for measurement of the installed fibers and motivates the development of PMD mitigation. Specifications for components and fibers are tightening, and an understanding of polarized light and its interaction with hardware has become a key success factor for component manufacturers

Simple Optical Fiber Calculations

Introduction

Here i will try to describe about the simple Optical Fiber calculations done in the field.

Attenuation:
1) For an SMF fiber with attenuation coefficient 0.25 dB/Km, what is the attenuation for 100 Kms of SMF fiber?

attenuation coefficient= 0.25 dB/Km

length of the Fiber= 100 Km

Attenuation for 100 Km SMF fiber = Attenuation Coeffiteint x Length of the Fier
                                                          = 0.25 x 100
                                                          = 25 dB

Attenuation is usually represented in dB

Dispersion:

The Chromatic Dispersion of a fiber is expressed in ps/(nm*km), representing the differential delay, or time spreading (in ps), for a source with a spectral width of 1 nm traveling on 1 km of the fiber. It depends on the fiber type, and it limits the bit rate or the transmission distance for a good quality of service.
For a standard SMF fiber, the dispersion coeffitient is  17 ps/(nm*km)
Here also calculations can be done by direct multiplication,

ie for a 100Km standard SMF, the Residual Dispersion will be = Length of the Fiber x Dispersion Coeffitient
                                                                                                     = 100 x 17 = 1700 ps