Công nghệ sợi quang, phụ kiện đấu nối

Introduction to Optical Fiber
Công nghệ sợi quang, phụ kiện đấu nối
Sưu tầm: Phan van Tue
Principles
Fiber Basics
Measurement Units
Sources and Power meters
Connector Types
Cleaning and Inspection
PRINCIPLES
Advantages of Fiber Optics
Noise immunity
Electromagnetic interferences (EMI) or radio frequency interferences (RFI) have no influence on transmission.
Low-loss attenuation
Singlemode loss as low as 0.2 dB/km (4.5%)
Multimode loss around 1 dB/km (30%)
High bandwidth
Transmission rates going up to 40 Gb/s (OC-768)
Advantages of Fiber Optics
Small size
A fiber is the size of human hair (125 mm).
A cable containing 12 pairs of optical fiber, 1.4 cm in diameter, is equivalent to a cable containing 900 pairs of copper wire that has an 8 cm diameter.
Light weight
Copper cable 900 pairs  8000 kg/km
Optic fiber cable 12 pairs  88 kg/km
Advantages of Fiber Optics
Transmission security
No energy radiation  no detection, hard to find
Intrusion on the link creates loss  intruders will be detected.
Short-circuit free/no fire hazard
No electrical energy inside the fiber, therefore no risk of short circuit, no sparks, and no heat. Ideal for hazardous environments.
Environmental temperature
Fiber can operate in a wide temperature range ( -40oC/100oC).
Electromagnetic Spectrum
Speed of light in a vacuum=299 792 458 m/s
Wavelength=Speed of light / Frequency
l (nm)=c (m/s) / f (Hz)
Electromagnetic Spectrum
Optical fiber domain
850 nm  353 000 GHz
1650 nm  182 000 GHz
Units
Micrometers (mm) - 10-6 m
Nanometers (nm) - 10-9 m
Mega - 106
Giga - 109
Tera - 1012
Peta - 1015
Pico - 10-12
Light Properties
The light consists of
Electric Field - E
Magnetic Field - H
Traveling in time (t) along the axis of propagation (Z)
Refractive Index

The speed of light = c = 299793 km/s under vacuum

Each material that can transmit light has it’s own index of refraction represented by n


Example


n = c vac / c mat

In a given index of refraction, the speed of light gets slower
Speed of light in water ≈ 225 000 km/s
Speed of light in fiber optic ≈ 215 300 km/s

Snell and Fresnel
Reflection
When a light beam I hits a material with a different index of refraction, a portion of the beam is reflected R
The angle of this reflected beam is the same as the incident beam
Cladding
Core
I
R
θi
θR
n1
n2
θi = θR
Snell and Fresnel
Refraction
For the same light beam hitting a different material, another portion is refracted
This occurs when a the light goes through a material with a different index of refraction
The angle of this beam changes because the speed of propagation changes
Cladding
Core
I
R
T
θi
θr
θR
n1 sin(θi) = n2 sin(θr)
n1
n2
Snell and Fresnel
Refraction and reflection
The more perpendicular the incident light beam is, less reflection you will have
I
R
T
I
R
T
I
R
T
Total Internal Reflection
Using Snell’s law: sin(r) = 1.5 sin(i)
Remember that the sine of an angle cannot be greater than 1.

As soon as a certain incident angle is reached, the refraction cannot occur. The light is reflected, and this phenomenon is called
total internal reflection

This angle is called the
critical angle (qc)
Snell and Fresnel
Critical angle (Total internal reflection)
There is a certain angle where 100% of the light is reflected and no light is refracted, we call this angle, the critical angle
Fiber optics use this concept to propagate light
Cladding
Core
I
R1
R2
θC
θR
FIBER BASICS
Fiber Optic
Core
Glass index n1
Cladding
Glass index n2
Coating
Acrylate, teflon, polyimide
Fiber Optic
Core diameter = 9, 50, 62.5 µm
Cladding diameter = 125 µm
Coating diameter = 250 µm
Fiber Fabrication
Preform fabrication
Fuel
Ex. : Silicium and germanium ultra pure vapors
Pressure
Soot deposited on the rod
Rod
1st step, Core
2nd step, Cladding
Fiber Fabrication
Fiber tower
The preform is melted while rotating in the oven. The glass draws down in the tower to create a fine thread identical to the preform but hundreds of time smaller. This thread becomes the fiber
The glass hardens and is controlled by a monitor placed in the tower
The coating is applied and the fiber is spooled on a reel
Preform
Oven
Diameter Monitor
Coating
Winding drum
Fiber Types
There are 2 fiber optics types in telecommunications
Singlemode
Multimode
9/125 (µm)
Core
50/125 (µm)
Cladding
Core
Core
Cladding
Cladding
62.5/125 (µm)
ITU-T G.652D
For telecommunications applications
Fiber Types
Fiber optics identifications are always written the same way
Diameter core / Diameter cladding / Diameter coating


9/125/250 (µm) 200/220/400 (µm)






62.5/125/250 (µm) 800/830/1000 (µm)
Coating
Cladding
Core
(Singlemode)
(Multimode)
(Specialty fiber)
(Specialty fiber)
Telecommunications
low-speed, short-distance applications
Fiber Optic
Jackets are made to protect the fiber against crush, bending and tension
Bare fiber (with acrylate): 250µm
PVC 900µm jacket (0.9mm)
PVC 2mm jacket
PVC 3mm jacket (Simplex)
PVC 3mm jacket (Duplex)
Fiber Types
A color code exists to identify different maintype fiber optics
Singlemode 9/125 Yellow
Multimode 62.5/125 Orange
Multimode 50/125 10Gbit/s Aqua
Multimode 100/140 Green
TIA/EIA-598 color code for multifiber cables
This color code is a sequence with blue being 1 and aqua being 12
We use the same sequence to identify sub-groups
1. Blue
2. Orange
3. Green
4. Brown
5. Slate
6. White
7. Red
9. Yellow
11. Pink
8. Black
12. Aqua
10. Violet
12 fibers cable
Fiber Types
Difference in the light propagation between singlemode and multimode fibers
Multimode
NA
Pulse
Pulse
X kilometers
NA
Loss
dB/Km
Pulse
X kilometers
Singlemode
Fiber Types
Fiber Types
Major Fiber Types

Multimode fiber: 50/125 µm and 62.5/125 µm
50/125 µm allows higher transmission rate than 62.5/125µm
Used in LAN networks
Lower data speed than singlemode fibers, due to modal dispersion
Normally used inside buildings

Singlemode fiber
Size = 8.6 to 9.5/125 µm
Applications = Long-haul, access, metropolitan and high-speed networks
Outside plant installations
MEASUREMENT UNITS
Measurement Units
dBm
The dBm is use to measure the output power of a light source
Laser source
Detector
Fiber optic
Instrument reading

- 3.50 dBm
We know that the laser’s output is -3.50 dBm
Measurement Units
mW
How to convert the dBm in mW

dBm = 10*log(mW)

The laser seen on previous page was emitting -3.50dB

-3.50 dBm so 0.45 mW
Measurement Units
dB (relative power)
dB is the difference between 2 power measurements
Take the -3.50 dBm laser shown previously
-3.50 dBm
-4.25 dBm
-3.50 dBm
Laser output = -3.50 dBm
The is an event on the fiber and the detector reads -4.25 dBm
To calculate this difference:
(-3.50 dBm) – (-4.25 dBm) = 0.75 dB
We have lost 0.75 dB

So the insertion loss is -0.75dB
Reflectance (-dB)
Will come from abrupt changes in the IOR:
Fiber break, mechanical splice, bulkheads, connectors, etc.
We use the term « reflectance » when speaking of the amount of energy returned by specific points within the network
Expressed as a negative value

Few examples:
Connector reflectance: -45dB
Mechanical splice reflectance: -45dB
Connector reflectance: -55dB
Reflectance (-dB)
Simplified formula
Reflectance [dB] = Preflected [dBm] - Pincident [dBm]

The higher is the negative value, the better it is

Example:

Connector 1 = -40dB
Connector 2 = -50dB

Q: Which one is better?
A: Connector no.2 is better and returns less energy towards the transmitter
Optical Return Loss (ORL) (dB)
Comes from the amount of energy lost within components and fiber due to back reflections

We use the term « ORL » when speaking of the amount of energy returned by a section or an entire link

Expressed as a positive value
Optical Return Loss (ORL) (dB)
Few examples:
Section 2 & 3 ORL = 45dB
Link ORL = 35dB
Optical Return Loss (ORL) (dB)
Simplified formula
ORL [dB] = Pincident [dBm] - Preflected [dBm]

The higher is the positive value, the better it is

Example:

Section 1 ORL = 40dB
Section 2 ORL = 30dB

Q: Which one is better?
A: Section no.1 is better and returns less energy towards the transmitter
Measurement Units
dB/Km

Standard unit used to express attenuation.
Attenuation results from absorption, scattering, microbending, macrobending, connections, and discontinuities.
It is one of the main performance limitations. It plays a major role in determining the maximum transmission distance between a transmitter and a receiver.
Attenuation (dB/Km)
Wavelength dependent
Fiber type dependent
Intrinsic Absorption in Optical Fiber
The absorption is cause by the fiber impurities








Each time a ray of light strikes an impurity, part of it’s energy is absorbed by this impurity, part of it’s energy is reflected
Laser
Cladding
Light beam
Core
Impurity
Scattering
There is scattering each time a ray hits an impurity and part of it is reflected in the fiber







Each impurity contributes in a accumulation of reflections and this results in a attenuation of the signal on a certain distance





Source
Light ray
Impurities
Macrobendings
Macrobendings
-3.50 dBm
-4.75 dBm
Critical angle not respected
Loss of -1.25 dB
The minimal radius of curvature for singlemode is 3 cm
Microbendings
Microbendings
-3.50 dBm
-3.75 dBm
Microbendings are created during fabrication or due excessive bending
They create very minimal loss but can deteriorate the signal with time
Loss -0.25 dB
Discontinuities the IOR
Reflectance
Ray of Light
Reflectance
Splice
Each time a ray of light meets a discontinuity, a part of the light is reflected to the source. This phenomenon is called reflectance.
Fusion Splicing
The fusion splice is made by melting together the ends of two fibers. It is a permanent connection.
Advantages
Low loss
Typical = 0.05 dB
Max = 0.1 dB
Very stable in time

Disadvantages
Permanent
Requires expensive equipment
Fusion Splice
When the core alignment occurs, a small offset can cause loss in the splice








Splicing 2 different core size fiber can cause loss
Mechanical Splice
Advantages:
Inexpensive
Quick

Disadvantages:
Greater loss than fusion splice and more fragile
Will create a reflection
Reflectance will increase over time
Loss

Loss in dB
0,01
0,05
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
1
2
3
4
5
6
7
8
9
10
20
30
40
50
60
Optical power loss
99,8
98,9
% of remaining
power
97,7
95,5
93,3
91,2
89,1
87,7
85,1
83,2
81,1
79,4
63,1
50,1
39,8
31,6
25,1
19,9
15,8
12,6
10,0
1,0
0,1
0,01
0,001
0,0001
Connector
Splice
-3.00dBm
-6.00dBm
-6.00dBm
-9.00dBm
-9.00dBm
-6.90dBm
-13.00dBm
50/50 splitter
50/50 splitter
80/20 splitter
SOURCES AND POWER METERS
LED, LASER AND VCSEL
The three main emitter devices:
LED
Light
Emitting
Diode
LASER
Light
Amplification by
Stimulated
Emission of
Radiation Diode
VCSEL
Vertical
Cavity
Surface
Emitting
Laser
Launching Conditions in MM Fibers:
LED vs VCSEL
Launching conditions will determine which propagation modes will be present in the optical fiber.
Optical Fibers and Sources
Different Sources for Different Needs
0.33 dB/km @ 1310 nm
0.50 dB/km @ 1383 nm (water peak)
0.30 dB/km @ 1490 nm
0.19 dB/km @ 1550 nm


Multimode sources: 850nm
(LED or VCSEL)
Multimode sources: 1300nm (LED or VCSEL)
Singlemode sources: 1550 nm (LASER)
Singlemode sources: 1310nm (LASER)
Singlemode sources: 1550 nm (LASER)
An optical source has a « pigtailed » output therefore can be used on a specific fiber type (SM or MM)
Optical Fibers and Power Meters
Power Meters Can Measure MM and SM





Multimode window
Singlemode window
Singlemode and multimode window
A power meter has no physical contact between the detector window and the connector therefore it can measure MM and SM fiber outputs
Any fiber between 8 and 62.5 µm of core size can be measured as long as it respects the numerical aperture for the power meter detector
CONNECTOR TYPES
Connectors
Polishing types:
UPC type
UPC connectors are « flat » polished
Most of UPC singlemode connectors are blue
Flat Polished connectors includes:
PC
SPC
UPC
Connectors
Single mode flat polish connectors:
Connectors
Polishing types:
APC type
Most of FTTH connectors including the FDH are Angle Polished Connectors (APC)
The OptiFit from Corning is SC/APC type inside
Most of APC connectors are green
APC is used in single mode applications
APC connectors will yield low reflections which are needed for video signals on FTTH.
Connectors
Single mode angle polish connectors:
Connectors
Connectors are polished to transmit light
UPC and APC polish are not compatible
UPC
Ultra physical polish
PC
Physical polish
(Multimode)
Angle 8º
APC
Angle physical polish
Conic ferrule
APC
Angle physical polish
Step ferrule
Angle 8º
Connectors
!!! Warning !!!
Angle Polished Connectors cannot be connected with Flat Polished Connectors!
Connectors
Multi mode flat polish connectors:
Connectors
In a connection, a portion of the light is reflected back in the fiber, reflectance
The APC was introduce with his 8 degree angle to send the reflection in the cladding
Connector Typical Specifications
Main connector specifications:
Bulkhead Adaptors
A bulkhead allows 2 connectors to transmit light
Hybrid bulkhead allows 2 different types of connectors to mate


Zirconia
Bronze
Connector Cleaning & Inspection
What causes loss
An inappropriate mating sleeve



Dirty bulkhead



Dirty ferrules
CLEANING AND INSPECTION
Connector Cleaning & Inspection
Inspection techniques:
A microscope or fiber probe can be used to inspect connectors
A microscope acts as a magnifying glass. If you inspect a connector on a live fiber, permanent damage can be done to your eyes!
Using a fiber probe, such as EXFO’s FIP-400, is the safest was to inspect a connector:
Connector Cleaning & Inspection
Cleaning Techniques:
The best way to clean connectors can be done by following these easy steps:
Clean the outside of the ferrule with a wet pad
Clean the ferrule using a dry pad
Inspect the connector using a microscope or fiber probe
If the connector is still dirty, repeat the 2 previous steps with a wet pad
Dirty ferrule
Clean ferrule
Connector Cleaning & Inspection
Bad cleaning results
Broken surface
Connector Cleaning & Inspection
Cleaning Techniques:
To clean detector windows, special care must be taken:
Do not push on detector windows, the glass window can break!
If you use compressed air, always blow at an angle; The air pressure can break the glass window!

Cleaning steps:
Using a wet pad or cleaning tip, gently wipe the detector window
Using a dry pad or cleaning tip dry out the window