Fiber Optics

1
Fiber Optics

• Southern Methodist University
• EETS7320
• Fall 2007
• Lecture 8A
• Slides only. (No notes.)

2
Electromagnetic Fields in Fiber Optics

• EM field is inherently internal to the fiber core
• Total Internal Reflection occurs at boundary
  between two dielectric materials with different
  dielectric constant (?) values (also described as
  different index of refraction n)
• Minimal external electromagnetic field prevents
  most attempts at wire tapping and minimizes
  crosstalk effects
• Reflection also occurs at boundary between metal
  (copper/brass or silver plating on copper), but
  reflection at dielectric boundary is almost
  perfectly efficient
• In one type of fiber (single mode) the wave power
  flows substantially parallel to the axis of the
  fiber (difficult to describe by means of
  geometric optics picture)
• Very similar to wave propagation in hollow metal
  waveguides
• Microwave waveguide is a hollow rectangular or
  elliptical cross section metal tube
• Power flow vector zig-zags between walls by
  multiple reflection in multi-mode fibers

3
Major differences in signal loss

• Reflection at metal surface loses 1 of power,
  reflects 99. 100 successive reflections leave
  only 36 of original power.
• Reflection at a dielectric surface loses only
  0.00001 of power, reflects 99.99999. 100
  reflections leave 99.99900000495 of original
  power.

4
Geometric Optics Description

• The ability of optical fiber to guide infra-red
  light (electromagnetic waves) with low power
  losses can be explained most easily, although
  only approximately, via concepts of geometric
  optics
• The full explanation, particularly for single
  mode fiber, requires a wave description
• The total internal reflection at the interface
  between high and low index glass is almost 100
  power efficient, much better than metallic
  mirrors (silver, aluminum, etc.)
• Over a million internal reflections occur in a km
  of multi-mode fiber, with loss of only 0.5 dB,
  because each reflection is over 99.99885
  efficient
• metal surface reflection, at 97 efficiency,
  would produce 130,000 dB loss in a geometrically
  similar 1 km length.

5
Snells Law

• Demonstration with glass of water

no1/co??o?o vacuum (or air) n11/c1??1?o
lower index medium n21/c2??2?o higher index
medium Snells law n2Sin(D) n1Sin(F)
Incident ray power is partly in reflected ray,
partly in refracted ray.
Angle of Reflected Ray R
Angle of Refraction F
RD and Sin(R)Sin(D)
Line perpendicular to interface at point where
ray intersects interface.
Angle of Incident Ray D
Material with lower dielectric constant ??,
faster wave speed, c1, smaller index n1.
Material with higher dielectric constant ??,
slower wave speed, c2, larger index n2.
6
Critical (Brewsters) Angle

• Demonstration with glass of water

Brewsters angle B n2Sin(B)
n1Sin(90º) so Sin(B) n1/n2 Any angle D?B
produces total internal reflection.
Incident ray power is totally in reflected ray.
This situation is called total
internal reflection.
Angle of Reflected Ray R
Hypothetical surface wave (zero power)
Angle of Refraction F 90º
Line perpendicular to interface at point where
ray intersects.
Angle of Incident Ray D?B
D
evanescent standing wave or reflected ray
described by wave theory
Material with larger index n2.
Material with lower index n1.
7
Total Internal Reflection

• When angle of incidence is beyond B, 100 of
  optical power is reflected internally
• some sources measure angle from the perpendicular
  line rather than from the interface, so
  inequality is stated differently
• When you (or a fish) go under a smooth water
  surface (e.g., a swimming pool), you can see up
  into the air only inside a circle. Outside that
  circle, you see only reflections from the surface.

B
Location of your (underwater) eye
8
Wave Phenomena

• Wave treatment of light gives a more
  comprehensive explanation than geometric ray
  treatment
• Explains evanescent wave, which travels away and
  back to/from interface in lower index medium, but
  has exponentially decreasing amplitude with
  distance.
• Evanescent optical wave can be recaptured by
  nearby high index material. This is basis of some
  non-contact fiber light measuring instruments
• Single mode fiber best explained by wave analysis
• Wave propagates along central axis without
  geometric reflections from interface between core
  and cladding,
• optical wave intensity at edges of core is lower
  because wave components are not in phase there
  (edge of first Fresnel zone)

9
Structure of Optical Fiber

• Manufactured by diffusing dopant (alloy)
  chemicals into inside of a 10 cm (outside
  diameter) hollow tube to modify the chemical
  composition, and thus modify the refractive
  index
• Tube is then heated and pulled. This collapses
  the hole in the center while the tube shrinks in
  diameter to a thin fiber.
• Plastics are used as well as glass (but seldom
  for telecom fiber) by running high index plastic
  fiber through liquid plastic to coat it with
  lower index plastic cladding.
• Single mode silica glass fiber today 0.1 dB/km
  power loss (thus up to 100 km fiber spans between
  repeaters)
• Research on exotic glass compositions
  (chalcogenide glass, etc.) promises even lower
  loss, but other negative aspects (like large
  changes in n due to small temperature changes)
  have prevented widespread use of chalcogenide
  fiber.
• Chalcogenide glass formulations contain some of
  the following elements As, Ge, P, Sb, Ga, Al,
  Si, etc.

10
Typical Fiber Structure

• Many fibers may be gathered in a protective
  covered cable, with steel or kevlar plastic
  rope (not shown) incorporated for pulling
  strength.

Plastic protective jacket, prevents
mechanical damage to outside surface of fiber.
Can be removed for splicing by cutting or
dissolving. Typically color coded for
identification of each fiber.
Lower index glass cladding
typical light ray
High index glass core
11
Multi-mode (Step-index), Graded Index, Single Mode

• Cross sectional views ( should be circles)
• Multi-mode Graded Index Single Mode

non-circularity of images is an artifact of
computer artwork software.
125??m
80??m
10??m
Accurate alignment less needed for splicing.
Higher loss. Major time dispersion of short
optical pulses due to different geometric paths.
Less used today, but historically important.
Accurate alignment less needed for splicing.
Higher loss. Reduced dispersion due to lower wave
speed in central rays, higher wave speed (lower
index) in outer part of core. Used for last
mile and service drops, with single mode
reserved for long runs.
Accurate alignment needed for splicing. Best low
loss. Most widely used fiber type for long spans.
12
Single Mode Fiber Propagation

• Single mode cant be described accurately by
  geometric (ray tracing) optics
• Parallel flat mirror model shows essential
  principles of single mode fiber operation,
  without the complication of circular geometry
• Infra-red power is focused in a narrow beam due
  to the combination of direct and reflected rays
• radiation that would otherwise spread to the
  sides is cancelled due to destructive
  interference with reflected image radiation
• Rays from side reflections are inverted in
  electric field polarity for each reflection
• Two mirrors in the model are separated by about
  10 wavelengths, similar to single mode
  dimensions. Image sources of alternating polarity
  appear to be present on both sides of the true
  source.

13
Plane Mirror Model
Red light-source symbols are in phase with real
source. Green light-source symbols are 180º or
?/2 out of phase with real source.
(Wave propagation direction.)
Real light source. All others are images.
d approx 6?, about 8 µm for 1300 nm ?
Two plane mirrors represent total
internal reflection surfaces of fiber core. True
length of mirrors in propagation direction is
actually very long.
back side of this Mirror is drawn in black.
Distant graph papershows brightness (power
intensity) vs. distance off the center line of
single mode fiber. Most of the power is
concentrated in the center. Off-center power is
smaller due to destructive interference between
real light sources and Reflected image light
sources.
Center line of fiber
14
Mode Descriptions

• Single mode power density is almost uniform
  across the 8 µm core diameter
• The major power flow is directly along the center
  axis
• Multi-mode or graded index fiber supports an
  optical ray that actually has less than 8 µm
  diameter. This ray can only propagate by
  reflecting diagonally from side to side of the
  80-100 µm diameter core
• The larger diameter of the core produces a
  tighter internal optical beam
• To launch the beam diagonally into the core, the
  phase angle of the entering beam is intentionally
  different at different points across the
  diameter.
• This situation is an example of the production of
  a narrower beam from a wider array of radiating
  sources.
• So-called smart antennas or adaptive beam
  forming in cellular base stations make use of
  the same principle

15
Infra-red Electro-Optic Converters

• Semiconductor light emitting diodes (LEDs)
  normally produce electro-magnetic radiation from
  electrons as they cross the diode junction during
  forward current conduction.
• The change in the electron energy is proportional
  to the frequency of the light emitted
  ?Ehfrequency.
• Optical power (brightness) is proportional to
  electric current.
• The energy change (color) can be controlled by
  the type and amount of alloy materials used on
  the P and N sides of the diode junction.
  Different materials have distinct energy
  differences between energy levels at which the
  electrons may stay.
• The wavelength of the light produced is inversely
  proportional to the frequency f (or to the energy
  change)
• Change in electron energy ?E hf hc/?, where
  wavelength ?c/f
• Wavelength in optical region of spectrum now
  usually measured in nanometers. 1 nm10-9 m.
• Older unit of wavelength, Ångstrom Å10-10 m
  (0.1 nm)

Plancks constant h6.62510-34 Ws2 (or
joulesec) speed of light c3108 m/s
16
Wavelength and Color Names
Ultra-violet
UV, IR not visible to human eyes
Infra-red
blue
green
red
400nm
500nm
600nm
700nm
850nm
1300nm
1550nm

• Wavelength (and color) can be controlled by
  type and amount of dopants (alloy materials)
  used to make the P and N sides of the light
  emitting diode.
• LEDs with infra-red output used as electro-optic
  (EO) converters for step or graded index fibers
• Light emitting diodes (LEDs) with visible light
  output are also used for indicator lights, etc.
• Construction of two parallel semi-reflecting
  surfaces on the diode with proper spacing
  relative to desired wavelength produces
  enhancement of one wavelength, yielding almost
  monochromatic LASER radiation (laser diode --
  LD), used for single-mode fiber
• Proper efficient coupling of light into the fiber
  core is a major design consideration as well (not
  discussed here)

850, 1300 and 1550 nm are local loss minima in
the fiber transmission spectrum.
These wavelengths often used for fiber systems.
17
Wavelength Refers to in-Air Measurements

• Wavelength of the same color optical signal is
  shorter inside glass or plastic
• This is the result of slower wave speed in glass
  or plastic (compared to air or vacuum)
• The frequency (cycles per second or Hz) of the
  wave is the same in air or in glass or plastic
• For a solid with index of refraction 1.3, the
  wavelength of a wave is 30 longer in air
• Measurements of wavelength are typically made in
  air

18
Infra-red Detectors

• A reverse-biased (negative voltage on the anode
  electrode) semiconductor diode has a normally
  very small so-called leakage current that
  increases due to higher temperature or due to
  illumination of the junction with light of
  appropriate wavelength
• Electric current only occurs when conduction
  electrons, which are moveable, are present in the
  junction region
• Light transfers energy to relatively immovable
  valence band electrons, causing them to change
  their electronic energy level (and their
  wavelength) so that they can move through the
  atomic lattice in the conduction band range of
  energy levels
• Photovoltaic cells are large flat junctions
  optimized for sunlight (solar power cells)
• Photodetectors for fiber are small and optimized
  for infra-red
• Frequency of detected light must be higher than
  ?Evc/h, where ?Evc is the energy change between
  valence and conduction band energy levels. Light
  at lower frequencies (longer wavelengths)
  produces no conduction band electrons and thus no
  signal-related current
• Amount (in mA or µA) of signal current is
  proportional to the brightness (power level) of
  the light (and to the number of moving electrons
  per second)

19
Avalanche Photo-Diodes (APD) Detectors

• For higher sensitivity to very low light power
  levels, avalanche photo-diodes are used
• High dopant concentrations and large negative
  power supply voltage produce a high electric
  field strength in the center region of the
  semiconductor junction of an APD.
• This produces high acceleration of conduction
  electrons when such electrons are produced by
  radiation absorption, giving these electrons high
  kinetic energy.
• High kinetic energy electrons strike other
  valence electrons, transferring energy to them
  and thus producing more conduction band electrons
  
• This is a chain reaction, like a rock slide
  avalanche on a mountainside! It produces multiple
  conduction electrons per light photon, rather
  than only one electron per photon
• The result is much higher current for a low light
  brightness
• Unfortunately also more dark current due to
  thermally produced conduction band electrons
• APDs are the mainly used with long fiber spans,
  single mode fiber applications

20
Preferred Infra-red Wavelengths

• 850 nm wavelength (short-haul LEDs)
• 1300 nm wavelength
• 1550 nm wavelength
• Each is a local minimum of glass transmission
  loss (see graph of loss vs. wavelength in
  Bellamy, p. 385 or other sources) or was a
  convenient wavelength in terms of historically
  available LED technology (850 nm in 1970s, 1980s)
• Higher absorption at intermediate wavelengths is
  due to atomic and molecular vibration resonances
  in the silica fiber
• Some combinations of atoms (OH hydroxyl ion
  pairs, etc.) oscillate, absorbing and
  re-radiating (scattering) IR light in all
  directions

21
Fiber Optic Transmission

• Electro-Optic (EO) Converter
• Light emitting Diode (LED) lower bit rates
• Laser Diode (LD) higher bit rates, used for long
  hauls, nearly monochromatic optical spectrum
• Opto-electric (OE) Detector
• Photo Detector Diodeshort hauls
• Avalanche Photo Diode better sensitivity, long
  hauls
• Binary Transmission Coding On-Off
• Single wavelength data rates over 1.6 Gb/s are
  feasible with single mode fiber, but requires
  higher electric power, more costly electronic
  components.
• So-called wavelength division multiplexing (WDM)
  increases total data rate by transmitting two or
  more independent bit streams on different optical
  wavelengths (different colors).
• Different wavelength optical signals (at or near
  1300 and 1550 nm) can be transmitted
  simultaneously, separated at detectors by
  appropriate filters. Filters cause additional
  optical power loss. Practical WDM systems mostly
  use erbium doped fiber amplification (EDFA laser
  amplification)
• Long/short haul refers to source-to-detector
  distance.

22
Optical Repeaters

• In general, optical repeaters perform functions
  similar to electrical digital repeaters the
  Three Rs
• Regenerate (amplify, compensate for power loss)
• Reshape (correct pulse wave-shape for distortions
  due to time dispersion)
• Retime (correct for jitter)
• OEO Repeaters
• Historical optical repeaters use an OE detector,
  electrical amplification and pulse shaping, and a
  EO LED or LD to transmit the repeated pulse
  stream into the next span.
• Only one wavelength can be processed by a single
  OEO repeater.
• Many optical and electronic components and some
  manual adjustment at installation time are
  required. This is a relatively complicated and
  costly device.
• A simpler type of all-optical repeater,
  particularly one that amplifies all the infrared
  wavelengths that are present, is desirable

23
EDFA Direct Optical Amplification

• Erbium-doped fiber amplifier (EDFA) is an
  Infra-Red LASER (Light Amplification by
  Stimulated Emission of Radiation) which converts
  shorter wavelength IR source (pump) power into
  greater power at signal wavelength(s).
• Advantages
• Simpler, uses less components than
  electro-optics, particularly for multiple
  wavelengths on same fiber (WDM)
• Amplifies many different optical wavelength
  signals present in WDM (present and future as
  well)
• Compensates for optical losses due to filters and
  optical combiners used in WDM
• But optical amplification does not correct
  timing or wave shape (two of the 3 Rs)

24
LASER Amplification

• Individual lower energy electrons absorb light
  wave energy and change their spatial electric
  charge configuration (they move to a higher
  energy level) at random time intervals
• Individual higher energy level electrons radiate
  light wave energy at random time intervals. When
  light waves are present at a frequency
  corresponding to the normal radiation frequency
  (f?E/h) for that downward energy change, the
  number of electrons that randomly radiate per
  unit time is increased. This increase is called
  stimulated emission.
• To continue amplifying, there must be a ready
  supply of high energy electrons. Electrons are
  continuously pumped up from a still lower
  energy level to the high energy level by constant
  irradiation using a much higher frequency,
  shorter wavelength optical source.
• Arthur L. Schawlow (1921-1999) and Charles H.
  Townes first built an amplifier using stimulated
  emission for amplifying microwaves. Gordon Gould
  is also credited with theoretical invention of
  the LASER in the patent office. Same method later
  applied to visible and infra-red light. Schawlow
  and Townes received Nobel prize with others.
• Terminology
• MASER (Microwave Amplification via Stimulated
  Emission of Radiation)
• LASER (Light Amplification via Stimulated
  Emission of Radiation)

25
Why Erbium?

• Erbium atoms have three important energy levels.
  The top two levels differ by an energy difference
  ?E E3-E2 corresponding to 1300 nm wavelength,
  the desired wavelength of the amplified signal.
  The lowest of the three energy levels differs
  from the top energy level (E3-E1 ) by an amount
  corresponding to the pumpsignal wavelength.
• A section of glass fiber made with Erbium doping
  is spliced into the signal-carrying fiber. This
  Erbium section is continuously illuminated with a
  pump optical signal of wavelength corresponding
  to E3-E1
• Radiative energy level transitions occur from
  level E3 down to E2 in proportion to the incoming
  (signal) light power level. The outgoing light
  power level is stronger.
• Electrons eventually fall in energy from level E2
  back to E1 as well, but produce light of a
  different wavelength than the 1300 nm wavelength
  used for optical signals. These other wavelengths
  are eventually absorbed by colored filters that
  only substantially pass 1300 nm infra red light.

E3
Radiation
E2
E1
Pump Action (sche- Matic)
26
Future Possibility Coherent Detectors

• Present fiber power detectors respond with
  current proportional to instantaneous optical
  power (brightness)
• No such thing as negative brightness
• Sophisticated modulation methods (used for
  audio/radio frequencies) are not feasible today
• So-called phase sensitive detection would
  permit discrimination between positive (in phase)
  and negative (180 degree or out of phase) wave
  amplitude, both of which have the same power
  level.
• This would permit phase modulation, QAM, other
  highly efficient modulation methods which
  transmit many bits per symbol, as are used for
  modems, radio systems, etc.
• For a pure sine wave, combination with a locally
  generated sine wave before photo-detection
  produces greater sensitivity, opens the
  possibility of phase modulation, etc..
• Requires ability to precisely control the phase
  of a local optical source at the detector -- very
  difficult to achieve!