Test Equipment used in Optical fiber maintenance

Testing Parameters in Optical Network

Following are the testing parameters used in optical network.

Trans optical power
•Receive optical power
•OSNR
•Dispersion (Read about dispersion here)
•Wavelength
•Wavelength deviation
•Fiber loss (Read about Fiber loss here)
•Connector loss
•Splicing loss

Optical Power Meter (OPM)

•OPM measure the average optical power out of an optical fiber.
•Power meters are calibrated at the typical wavelengths used in fiber optics, 850, 1300 and 1550 nm.
•Some meter uses Auto wavelengths Recognition feature.

Optical Light Source(OLS)

•OLS along with Optical Power Meter is used to make measurements of optical loss or attenuation in fibers and connectors.
•It should be compatible with the type of fiber in use (singlemode or multimode with the proper core diameter) and the wavelength.
•Sources may be either LED's or LASERS.
•Sometimes optical power meters are combined with an Optical Light Source (OLS) or Visual Fault Locator (VFL).

OTDR

•Optical Time Domain Reflectometer (OTDR) is used for estimating the fiber’s length and overall attenuation, including splice and mated-connector losses and to locate faults, such as breaks.
•OTDR functions by injecting a series of optical pulses into the fiber under test, using LASER.
•It requires access to only one end of the fiber.

Fiber Trace at OTDR

Optical Spectrum Analyzer (OSA)

•Optical signal monitoring on live channels.

•Able to analyze

–Optical Power

–Optical wavelength

–OSNR of each wavelength

–Spectral width of optical source

Visual Fault Locator

•Visual Fault Locator is used to inject the visible light to visually trace fiber.

•It covers the range where OTDRs are not useful.

•This is used for buffered fiber and even jacketed single fiber cable if the jacket is not opaque to the visible light.

Dispersion in Optical Fiber / Notes on Dispersion

Dispersion in Optical Fiber

Various wavelengths of the signal have different propagation velocities. Dispersion is the spreading out of a light pulse in time as it propagates down the fiber. Dispersion limits the information carrying capacity of a fiber. It is measured as ps/nm.km

Types of Dispersion

–Intermodal dispersion
–Material dispersion
–Waveguide dispersion.
( Also read about losses in optical fiber )

Intermodal Dispersion:

•Each mode enters the fiber at a different angle, take different path, travel different distance and arrive at different time at the fiber output.
•Multimode fibers have many different light modes since they have much larger core size.
•The light pulse spreads out in time which can cause signal overlapping.

Material Dispersion

•Dependence of refractive index of the material on wavelength.
•The velocity of light through a fiber depends on its wavelength.
•Different wavelengths travel at different velocity due to refractive index, hence the different propagation causes material dispersion.
•Material dispersion at 1300nm for silica is zero.

Waveguide Dispersion

•Distribution of light between core and cladding is a function of the wavelength travelling in waveguide.
•Light ray that travels in the cladding travels faster than that in the core causes waveguide dispersion.
•Single mode fiber suffers from waveguide dispersion as it has small core diameter

Chromatic Dispersion

•Combined effect of Material and Waveguide Dispersion

Polarization Mode Dispersion (PMD)

•Asymmetry, due to Mechanical and thermal stresses during fiber manufacturing introduces small index of refraction differences for the two polarization states.
•Polarization Mode Dispersion (PMD) a form of modal dispersion, causes broadening of the input pulse due to a phase delay between input polarization states.

Effect of Dispersion

•Spreading of pulses which can cause Inter Symbol Interference ‘ISI.
•Detection of individual pulse is not easy at receiver.
•Poor BER performance.
•Limit the communication distance.
•Limit the transmission rate.

Losses in Optical Fiber

Signal Degradation in Optical Fiber :

• Attenuation and dispersion determine the maximum distance an optical signal can be transmitted before the receiver is unable to detect it.
• The attenuation and dispersion of a fiber are wavelength dependant.

Transmission Losses in Fibers:

Transmission loss or attenuation of the signal in an optical fiber is measured in dB/km.

Types of Losses

–Material Absorption

–Rayleigh scattering

–Waveguide imperfections

–Radiative loss

Material Absorption

•Intrinsic Absorption:

–Intrinsic absorption losses correspond to absorption by fused silica.

–intrinsic material absorption for silica in the wavelength range 0.8~1.6um is below 0.1dB/km.

•Extrinsic Absorption:

–Extrinsic absorption is related to losses caused by impurities within silica.

–The main source of extrinsic absorption silica fibers is the presence of water vapors.

Rayleigh Scattering

•Silica molecules move randomly in the molten state and freeze in place during fiber fabrication.

•Density fluctuation lead to random fluctuations of the refractive index.

•Light scattering in such a medium is known as Rayleigh scattering.

Waveguide Imperfections

•Imperfections at the core-cladding interface, such as random core-radius variations, can lead to losses.

•This has been taken good care of in optical fiber manufacturing and the core radius is made sure not to vary significantly along the fiber length.

Radiative Losses

•Radiative losses occur whenever an optical fiber undergoes a bend of finite radius of curvature.

•Fibers can be subjected to two types of bends.

–Macroscopic bending: 

Bends having diameter smaller than the specified by manufacturer e.g. when a fiber cable turns a corner.

–Microscopic bending: 

Random microscopic bends of the fiber axis that can arise when the fibers are incorporated into cables, e.g. deformation of axis.

Gap- Loss :

•Gap-loss happens when there is a space, breakage, between fiber connection.

•Light can cross this gap, but spreads out and is weakened and diffused

Types of Fiber

Optical fiber is classified based on material used, mode & refractive index.

Step Index Optical Fiber:

• In the step index fiber, the refractive index of the core is uniform throughout.
• Different light modes in a step-index multimode fiber follow different lengths along the fiber in zigzag way
• The arrival of different modes of the light at different times causes Inter Modal Dispersion.
• 50 mm core for step index.

Graded Index Optical Fiber:

• Graded-index fiber’s refractive index decreases gradually away from its center.
• Light rays follow sinusoidal paths.
• Reduces inter modal dispersion.
• 62.5 mm core for graded index.

Single Mode Fiber:

• In a single mode fiber only one mode can propagate through the fiber.
• Core of SM fiber is smaller (8.3 to 10 microns). Common size is 9um.
• Laser is used as light source for single mode fiber.
• Used in long distance communication upto100 Gbit/s data in DWDM.
• 1310 or 1550 nm wavelength is used in SM fiber with higher data rate.

Multimode Fiber:

• Multimode fibers allow a large number of modes.
• A multimode optical fiber has larger core (50 to 100um). Most common size is 50um or 62.5um.
• DWDM is not normally used on multi-mode fiber.
• 850 or 1300 nm wavelength is used in multimode fiber.
• LED is used as a light source in multimode fibers .

Practically used Optical Fibers:

• MM Step index: 62.5/125 µm
• MM Graded index: 50/125 µm
• SM Step index: 9/125 µm
• Single mode graded index is not practically used

Fiber Design

Construction of Fiber

An optical fiber consists of a core, cladding and buffer (a protective outer coating). The cladding guides the light along the core by using the method of total internal reflection. In the glass fiber core and cladding are made of high quality silica glass.

Principle of Optical Communication

Optical fiber communication works on the principle of Total Internal Reflection.

Total Internal Reflection

– For all angles of incidence greater than the critical angle, the incident ray will get reflected back into the denser medium itself. This phenomenon is called total internal reflection.

Wavelengths in Optical Fiber

Normally we use light wavelengths around 850, 1310 and 1550 nm for fiber optics with glass fibers. These wavelengths are falling in the infrared region and having less attenuation. 

Optical Fiber as Telecommunication Medium

Physical path between transmitter and receiver is called transmission medium, it may be wireline or wireless.

Wireless (unguided) media

–Microwave radio.

–Communication satellites

•Wireline (guided) medium

–Copper wires

–Fiber-optic cables.

Fiber Medium Vs Microwave medium

Optical Fiber Communication

Transmitting information from one place to another by sending pulses of light (wavelength) through an optical fiber. Modulation used: Light Intensity Modulation

Advantages of Optical Fibers

• Very high information carrying capacity
• Less attenuation (order of 0.2 db/km)
• Small in diameter and size & light weight
• Greater safety and immune to EMI & RFI, moisture & corrosion
• It is dielectric in nature so can be laid in electrically sensitive surroundings
• Difficult to tap fibers, so secure
• No cross talk and disturbances

Disadvantages of Optical Fibers

• The terminating equipment is still costly as compared to copper equipment.
• It is delicate so has to be handled carefully
• High Cost
• Last mile is still not totally fiberised due to costly subscriber premises equipment
• Optical fiber splicing is a specialized technique and needs expertly trained manpower.
• The splicing and testing equipments are very expensive as compared to copper equipments.

Applications of Optical Fiber

• Long distance communication: Backbones
• Broadband services
• Computer data communication (LAN, WAN etc..)
• Medical Industry
• Military application