3. Introduction to Fiber Bragg Grating
• The Fiber Bragg Grating (FBG) is a fiber optic passive component
exhibiting basic functional attributes of reflection and filtering.
• FBG’s are relatively simple to manufacture, small in dimension, low cost
and exhibit good immunity changing ambient conditions and EM
radiation.
• FBG’s have replaced bulk optic mirrors and beam splitters in equipment
which increases system stability and portability.
4. Fiber Bragg Grating: THEORY
• The photosensitivity of a fiber is its capability to change locally
its refractive index when it is irradiated by a UV light.
• Photosensitivity allows to realize Fiber Bragg Grating because
spatial periodic irradiation of the fiber leads to periodic
refractive index variation
5. Fiber Bragg Grating: THEORY
1978 – Hill et. all
• Phenomenon of photosensitivity in optical fibers
• Exposed Ge-doped core fibers to intense light at 488 or 514 nm
• Induced permanent refractive index changes to the core.
6. Fiber Bragg Grating: THEORY
• FBG is a longitudinal periodic variation of the index of
refraction in the core of an optical fiber.
• The spacing of the variation is determined by the wavelength
of the light to be reflected.
Bragg
Bragg
7. Fiber Bragg Grating: THEORY
The Bragg Condition is:
B = 2L neff
• The grating reflects the light at the Bragg wavelength (B)
• B is a function of the grating periodicity (L) and effective index (neff).
• Typically; B= 1.5 mm, L = 0.5mm
8. Fiber Bragg Grating: APPLICATIONS
1. Fiber Bragg Grating for dense wavelength division
multiplexing
• With the help of the fiber Bragg grating we can filter out the particular
wavelengths from the system as shown in the Fig:
Fig:MUX and DEMUX in DWDM
9. Fiber Bragg Grating: APPLICATIONS
2. FBG for OADM
• An Optical add drop multiplexer is a device that used in WDM systems for
multiplexing and routing different channels of light into or out of an optical fiber.
• Add/remove here refer to the ability of the device to add one or more new channels
to an existing multi-wavelength WDM signal and to remove one or more channels,
passing those signals to another path of network.
11. Fiber Bragg Grating: APPLICATIONS
3. Fiber Bragg Grating Sensors
• Strain and temperature simultaneously change the Bragg wavelength.
• We clearly see that strain and temperature have the same effect to shift the Bragg
wavelength. These are thus not separable in a single grating.
• By using, at the same location, two gratings with different sensitivities, it is possible
to simultaneously extract strain and temperature.
12. Fiber Bragg Grating: APPLICATIONS
Fig:Operation of the Fiber Bragg Grating sensor
13. Fiber Bragg Grating: APPLICATIONS
4.Distributed Feedback Lasers(DFB)
• In DFB laser,the optical grating is applied over the entire region
which is pumped.
• Used to provide single-frequency semiconductor optical
sources.
Fig:DFB laser
15. OPTICAL CROSS CONNECT
• Channel cross-connecting is a key function in most communications
systems.
• In electronic systems, the electronic cross-connecting fabric is constructed
with massively integrated circuitry and is capable of interconnecting
thousands of inputs with thousands of outputs.
• The same interconnection function is also required in many optical
communications systems
• An optical cross-connect (OXC) is a device used by telecommunications
carriers to switch high-speed optical signals in a fiber optic network
16. TYPES OF OPTICAL CROSS CONNECTION
• Purely electronic
• Purely optical
• Hybrid of both optical and electronic
17. Purely electronic Switching
• All the input optical signals are converted into electronic
signals after they are demultiplexed by demultiplexers.
• The electronic signals are then switched by an electronic
switch module.
• Finally the switched electronic signals are converted back into
optical signals by using them to modulate lasers and then the
resulting optical signals are multiplexed by
optical multiplexers onto outlet optical fibers.
18. Optical switching
• Switching optical signals in an all-optical device is the second
approach to realize an OXC.
• Optical signals are demultiplexed, then the demultiplexed
wavelengths are switched by optical switch module.
• After switching, the optical signals are multiplexed onto output
fibers by optical multiplexers.
19. Optical and Electronic switching
• In such a switch architecture, there is a switch stage which
consists of an optical switch module and an electronic
switch module.
• In most cases, the optical switch module is preferred.
• When the optical switch module's switching interfaces are
all busy or an optical signal needs signal
regeneration through an OEO conversion process, the
electronic module is used.
20.
21. Types of Optical Switches
• Different principles and technologies of optical switch are
different characteristics and suitable for different occasions.
• Depending on its fabrication technology, some of the
optical switch are:
1. Opto-mechanical optical switch,
2. MEMS (Micro-Electro-Mechanical Systems,
3. Liquid crystal optical switch.
22. Opto-mechanical optical switch
• These devices achieve switching by moving fiber or other
bulk optic elements by means of stepper motors or relay
arms.
• The stepper will move a mirror that guides the light from
the input to the desired output.
• The input and output light beams are collated and
“matched” within the switching device.
• This allows for low optical loss, and allows distance between
the input and output fiber without deleterious effects.
23. Opto-mechanical optical switch
• Switching time is in the 10-100 ms range.
• The defects of traditional mechanical optical switch are the
long time to turn on or turn off, also devices have more bulky
compared to other alternatives.
• However, with development of technology, the new micro-
mechanical devices overcome this.
• The beams are moved within the device to make sure the
switched connection from the input to the output.
25. MEMS Optical Switch
• MEMS stands for Micro-Electro-Mechanical Systems.
• It is an advanced technology of optical switch and currently
attracted a wide attention in the world.
• MEMS optical switches use a micro-mirror to reflect a light
beam.
• Mirrors can be tilted to any angles.
• The direction that the light beam is reflected can be changed by
adjusting the angle of the mirror, which allows the input light to
be connected to any out port.
26. MEMS Optical Switch
• N or 2N mirrors accomplish non-blocking switching.
• It is a compact optical switch which connects optical channels by
redirecting incoming optical signals into the selected output fibers.
• The switching state is highly stable against environmental variations of
temperature and vibration .
• MEMS optical switch is compact, lightweight and easy to expand as well
as the combination of the advantages of mechanical optical switch and
waveguide optical switch while overcomes their defects.
• It has now been widely used in industry.
28. Liquid crystal optical switch
• The switching speed of liquid crystal optical switch can reach
the degree of sub-microsecond.
• The working principle of the liquid crystal optical switch is
based on polarization control.
• One path, light is reflected by the polarization while the other
path, light can go through.
32. Optoelectronic integrated circuits (OEIC)
• Monolithic Integration of optical and electronic semiconductor
devices
• Hybrid technology in which photonic component (such as laser,
modulator, photodetector) co-exists as same chip as a
functional electronic circuit(e.g. Laser diode)
• Compound semiconductors such as GaAs-based and InP-based
alloy semiconductors are used.
*monolithic: composed of both active and passive components formed into a single chip.
33. Optoelectronic integrated circuits (OEIC)
• Two types of OEIC exist
– One is integration of light emitting devices (example: LD) and driving
FET circuits
– The other is integration of optical detection device like PD and
electronic circuits for amplification and signal processing
• OEICs, integrate optical devices was originally developed for
communication system and electronic devices was developed
for the signal processing system
38. Fabrication
Structural strategy for the fabrication of OEIC onto an InP substrate
Basic steps : Epitaxial growth
Waveguide etching
Passivation and planarization
Metallization and interconnect
40. Features of OEICs
• High Speed operation
– OEICs consists electronics circuit such as drivers or amplifiers which assists
in speeding the signal processing
• Multichannel Light signal processing
– OEICs not only emit and receive signals but also process them. Large scale
integration prove useful in multichannel light signal processing systems
such as optical LANs and the optical interconnection of computer systems.
• Small size, high reliability and low cost
– Their small size, high reliability and low cost will be a distinct advantage
when OEICs are used in subscriber system
41. Features of OEICs
• High Speed operation
– OEICs consists electronics circuit such as drivers or amplifiers which assists
in speeding the signal processing
• Multichannel Light signal processing
– OEICs not only emit and receive signals but also process them. Large scale
integration prove useful in multichannel light signal processing systems
such as optical LANs and the optical interconnection of computer systems.
• Small size, high reliability and low cost
– Their small size, high reliability and low cost will be a distinct advantage
when OEICs are used in subscriber system.
42. APPLICATIONS OF OEIC
• Digital Transmission
• Analog Transmission
• Switching
• Fiber Optic Gyroscopes
• WDM Systems
• Optical Storage and Display
44. Waveguide
• Waveguide is a structure that guides waves such as
electromagnetic waves with minimal loss by restricting
expansion to one or two dimension i.e. it confines
electromagnetic energy and channels it from one point to
another.
• It only carries or propagate signals above a certain frequency,
known as cut-off frequency.
45. Optical Waveguide
• An optical waveguide is a spatially homogeneous structure for
guiding light, i.e. for restricting the spatial region in which light can
propagate.
• Central to integrated optics is the concept of guiding light in
dielectric waveguide structure
• The dimension of dielectric waveguide is comparable to wavelength
of guided light i.e. size of the waveguide determines its operating
frequency.
• A dielectric waveguide confines light to the core of the waveguide
structure by reflecting power back towards the core that would
otherwise diffract or propagate away
47. Optical Waveguide
• Common technique to reflect power back towards the
waveguide employs 100% total internal reflection from the
boundary of high index core and low index cladding
• As light propagates down the axis of such waveguide structure,
the core region has larger retardation and resembles a
continuous focusing of light
• The structure used to illustrate this phenomenon is symmetric
slab wave guide
48. Basics of Slab Waveguide
• The core of the region is called film which has the refractive
index of n1
• The film is deposited on a layer called substrate and has the
refractive index of n2
• The cladding on the film is called superstrate and has refractive
index n3
• When n2=n3 it is symmetric slab waveguide
• When n2 is different from n3 it is asymmetric slab waveguide
49. Basics of Slab Waveguide
• This type of wave guide supports finite number of guided
modes as well as infinite numbers of radiation mode
• In order to achieve this n1>n2≥n3
50. Symmetric Slab Waveguide
• It is composed of three layers of homogeneous dielectrics
• Analysis of slab structure is done using Ray-optics approach
• In Ray optics, two types of phase changes
One while reflection
Other while travelling
• Steady state field has well defined phase at each point
51. Symmetric Slab Waveguide
• After propagation and two reflections, phase rejoins itself with
an integral multiple of 2𝜋 phase shift
• Give core thickness, limited number of propagation angles
exists in the core
53. Symmetric Slab Waveguide
• Critical angle depends on index of refraction of materials which
may vary according to wavelength of light
• Thicker and higher index waveguide core admits larger no. of
propagation angles
• Upon total internal reflection phase shift occurs also known as
Goos-Hanchen shift
• Slab waveguide employs TIR from an abrupt index
discontinuity for confinement
54. Symmetric Slab Waveguide
• Critical angle depends on index of refraction of materials
which may vary according to wavelength of light
• Thicker and higher index waveguide core admits larger no.
of propagation angles
• Upon total internal reflection phase shift occurs also known
as Goos-Hanchen shift
• Slab waveguide employs TIR from an abrupt index
discontinuity for confinement
55. Symmetric Slab Waveguide
• Critical angle depends on index of refraction of materials which
may vary according to wavelength of light
• Thicker and higher index waveguide core admits larger no. of
propagation angles
• Upon total internal reflection phase shift occurs also known as
Goos-Hanchen shift
• Slab waveguide employs TIR from an abrupt index
discontinuity for confinement
56. Symmetric Slab Waveguide
• TE and TM modes of symmetric slab is never cutoff
• For guided modes symmetric waveguides are either even or
odd in field distribution
• The number of guided modes supported in slab waveguides
depends on thickness, wavelength, indices of refraction
n1,n2,n3
57. Strip Waveguide
• A strip waveguide is basically a strip of layer confined between
cladding layers
• Strip waveguides are planar waveguide (guides light in one
direction) only
• A central chip with high refractive index is bordered by material
with smaller indices
• This results in total internal reflection at the lateral(sides) interface
and thus forming guided modes( a mode whose field decays
monotonically in transverse direction everywhere external to the
core)
58. Strip Waveguide : Rectangular Waveguide
• The simplest case of strip waveguide is rectangular
waveguide
• A rectangular waveguide supports TM and TE modes but not
TEM mode as it has only one conductor
59. Strip Waveguide: Rectangular Waveguide
• TEM mode exists only in two conductors and it is not possible in
waveguide
• EM wave consists of electric field and magnetic field propagating
in same direction but perpendicular to each other. In principle
mode i.e. TEM mode, electric field and magnetic field are
perpendicular (transverse) to the direction of propagation
• In TE mode, electric field is transverse to direction of
propagation
• In TM modes, magnetic field is transverse whereas electric field
is in the direction of propagation
60. Cut-off Frequency
Here m and n are integers which define wave modes.
a and b are dimensions of rectangular waveguide.
61. Dominant mode
• From 6.56 GHz≤ 𝑓 ≤ 13.12 GHz, only TE10 mode can propagate
• In TE10 electric field is perpendicular to direction of
propagation and also it is equal to λ/2 across broad dimension
and less than λ/2 across narrow dimension.
62. Comments
• What is relative index in Bragg grating?
– In a Bragg grating refractive index varies as it reflects particular wavelength
of light and transmits all others.
• What is a gyroscope?
– It is a device used for measuring or maintaining orientation or angular
velocity.
• Relationship between refractive index and thickness
– Refractive index= Real thickness/Apparent thickness (though it is not a
general formula, it can be used)
