BSC HONS. INDUSTRIAL PHYSICS/
BSC HONS. MEDICAL PHYSICS/
BSC HON. ELECTRONICS &
TELECOMMUNICATION TECHNOLOGY
HIPH201/HMPH201: Applied
Optics and Photonics
HETT201: Lasers & Fibre Optics
Prof E. Mashonjowa
Dept of Space Science & Applied Physics, UZ
LECTURE 7: OPTICAL DEVICES AND
SYSTEMS
Optical Instruments
Optical Instruments
• Analysis generally involves
the laws of reflection and refraction:
realm of Geometric optics
• To explain certain phenomena,
the wave nature of light must be used
The Camera
• Is an optical instrument
• Components:
– Opaque, light-tight box
– Converging lens
• Produces a real image
Object O
– CCD (or Film) behind the lens
• Receives the image
digital
analog
I
Real
Inverted
M<0
I smaller
than O
|M | < 1
Digital Camera
• Image is formed on
an electric device
CCD: Charge-Coupled Device
CMOS: Complementary
Metal-Oxide
Semiconductor
• Both convert the image
into digital form
• The image can be stored
in the camera’s memory
Camera Operation
• Proper focusing leads to sharp images
The lens-to-film distance q depends on:
the object distance p and on the focal length
• The shutter is a mechanical device
that is opened for selected time intervals
f of the lens
• Most cameras have an aperture
of adjustable diameter D to control
the intensity of the light reaching the film
Small-diameter aperture: only light from the central portion
reaches the film => spherical aberration is minimized
Camera Operation, Intensity
• Light intensity: the rate at which energy is received
by the film per unit area
• The intensity is proportional to the area of the lens,
~ D2
• The brightness of the image formed on the film
depends on the light intensity
– Depends on both the focal length f
~1/f
and the diameter of the lens D:
Intensity
2
Camera, f-numbers
• The ƒ-number of a camera is the ratio of the
focal length of the lens to its diameter
f:D
f
ƒ-number = f/D
The ƒ-number is given
a description
of the lens “speed”
• A lens with a low f-number
is a “fast” lens
Can take pictures
in low-light conditions
QQz: this lens, fast or slow?
Camera, f-numbers, Cont.
• The lowest ƒ-number setting:
the aperture wide open,
the maximum possible lens area in use
• Simple “point-and-shoot” cameras:
a fixed focal length and
a fixed aperture size,
with an ƒ-number of about 11
The high value of ƒ allows for
a large depth-of-field
=> no need to focus the camera
=> no good pictures in low-light conditions
The Eye
• The normal eye focuses light
and produces a sharp image
• Essential parts of the eye:
Cornea – light passes through
this transparent structure
Aqueous Humor – clear liquid
behind the cornea
• The pupil
A variable aperture
An opening in the iris
• The crystalline lens
Most of the refraction takes place at the outer surface of the eye,
where the cornea is covered with a film of tears
The Eyes – Parts, Cont.
• The iris is the colored
portion of the eye
– It is a muscular diaphragm that
controls pupil size
– Regulates the amount of light
entering the eye
by dilating the pupil
in low-light conditions
and contracting the pupil
in high-light conditions
– The f-number of the eye is
from about 2.8 to 16
The Eye – Operation
• The cornea-lens system focuses light
onto the back surface of the eye,
called the retina
– The retina contains receptors
of two types, rods and cones
– These structures send impulses
via the optic nerve
to the brain
The brain converts these impulses
into our conscious view of the world
Conditions of the Eye
• A mismatch between
the focusing power of the lens-cornea system
and the length of the eye, d
d
• Eyes may be
Nearsighted
Light rays reach the retina after they converge to form an image
Farsighted
Light rays reach the retina before they converge to form an image
Farsightedness
Light rays reach the retina before they converge to form an image
• Also called hyperopia
• The image focuses behind the retina
• Can usually see far away objects clearly,
but not nearby objects
Correcting Farsightedness
• A converging lens placed in front of the eye
can correct the condition
• The lens refracts the incoming rays more
toward the principle axis before entering the eye
– This allows the rays to converge and focus on the retina
Nearsightedness
Light rays reach the retina after they converge to form an image
• Also called myopia
• In axial myopia: the lens is too far from the retina
• In refractive myopia: the lens-cornea system is
too powerful
Correcting Nearsightedness
• A diverging lens can be used to correct the condition.
• The lens refracts the rays away from the principle axis
before they enter the eye
– This allows the rays to focus on the retina.
Simple Magnifier
• A simple magnifier consists
of a single converging lens
This
device
• … is used to increase
the apparent size of an object
• The size of an image formed on the retina
depends on the angle subtended by the eye
The Size of a Magnified Image
• When an object is placed at the near point,
the angle subtended is θo
– The near point is about
• When the object is placed near the focal
point of a converging lens,
the lens forms a virtual,
upright, and enlarged image h’
Compound Microscope
• A compound microscope consists of two lenses
– Gives greater magnification than a single lens
– The objective lens has a short focal length, ƒo< 1 cm
– The ocular lens (eyepiece) has a focal length, ƒe, of a few cm
Compound Microscope, Cont.
• The lenses are separated by a distance L
L >> ƒo , ƒe
is much greater than either focal length
• The approach to analysis is the same
as for any two lenses in a row:
– The image formed by the first lens
becomes the object for the second lens
• The image seen by the eye, I2,
is virtual, inverted and very much enlarged
Magnifications of the Compound
Microscope
• The lateral magnification of the microscope is
• The angular magnification
of the eyepiece
of the microscope is
• The overall magnification of the microscope
is the product of the individual
magnifications
Other Considerations
with a Microscope
• The ability of an optical microscope to view
an object depends on the size of the object, d,
relative to the wavelength λ of the light used
to observe it.
– Example, you could not observe
an atom
with visible light:
d  0.1 nm
λ 500 nm
Electronatomic-forcetunnelingmicroscopy
Telescopes
• Two fundamental types of telescopes
– Refracting telescope uses a combination of lenses
– Reflecting telescope uses a curved mirror and a lens
In both types, two optical elements in a row:
the image of the first element becomes
the object of the second element
Refracting Telescope
• The objective forms
a real, inverted image I1
• The image is near the focal
point of the eyepiece Fe
• The two lenses are
separated by the distance
ƒo + ƒe about the length
of the tube
• The eyepiece forms an
enlarged, inverted image I1
of the distant object
Angular Magnification of a Telescope
• The angular magnification depends on the focal
lengths of the objective and eyepiece:
• Angular magnification is only important
for observing nearby objects
• Very distant objects such as stars
still appear as small points of light
• Main goal of astronomy telescopes: Collect Light!
Disadvantages of Refracting Telescopes
• Large diameters are needed
More Light!
to study distant objects
• Large lenses are difficult and expensive
to manufacture
• The weight of large lenses leads
to sagging which produces aberrations
Reflecting Telescope
• Helps overcome some of the disadvantages
of refracting telescopes
– Replaces the objective lens with a mirror
– The mirror is often parabolic
to overcome spherical aberrations
• In addition, the light never passes through glass
(except a small eyepiece)
– Reduced chromatic aberrations
Reflecting Telescope,
Newtonian Focus
• The incoming rays are
reflected from the mirror and
converge toward point A
– At A, a photographic plate or
other detector can be placed
• A small flat mirror, M, reflects
the light toward an opening
in the side and passes into
an eyepiece
Examples of Telescopes
• Reflecting Telescopes
Largest in the world are D= 10 m
Keck telescopes
on Mauna Kea in Hawaii
altitude 4145 m
• Refracting Telescopes
Largest in the world is
Yerkes Observatory
in Wisconsin
• Has a 1 m diameter
The Hubble Space Telescope
• D = 2.4-meter observe in
the near-UV, visible, and near-IR spectra
• Enables us
to see
deep
into space
•
And further back in time!
Its scientific successor is scheduled to be launched in 2018
The RadioAstron,
“biggest-ever” telescope
• a Russian orbital
D = 10-meter parabolic
• Collecting data on clouds
of water molecules found
in the discs of galaxies
Application of Photonics
-Optical Communication
• Telecommunications Communications over “long
distances”
• Information is encoded as signals
• Signals are transmitted through a medium
• Signals are directed to recipients
• Many technologies
– Electronic, radio, microwave
– Fiber-optic, atmospheric optical
What is communicated?
• Data
– Telegrams
– Computer data, telemetry, etc.
– Internet , files, web pages
•
•
•
•
Voice: telephone, radio
Video: sound and images
All are encoded as signals
Systems are converging
Signal Structure
• Signal modulates a carrier wave
– Carrier wave is at much higher frequency
• Amplitude modulation changes carrier
intensity
– Standard in fiber optics, AM radio
• Frequency modulation changes carrier
frequency
– Standard for FM radio, television
Modulation formats
Signal Format and Structure (1)
Signal Format and Structure (2)
Bandwidth
• Information per unit time
– Frequency in Hertz (cycles per second)
– Bits per second
• Depends on signal source and format
– HDTV is highest video, analog NTSC lower
– Stereo music more than telephone audio
• Capacity depends on transmission medium
• May vary with length of medium
Multiplexing
•
•
•
•
•
•
•
•
Combines two or more signals
Multiple signals sent over one path
Dates back to telegraph
Reduces costs
Type
Frequency-division
Time-division
Wavelength-division
Frequency-division multiplexing
Time-division multiplexing
Wavelength-division multiplexing
Transmission distance
• Key figure of merit
• Depends on
– Transmitter power
– Receiver sensitivity
– Attenuation
• May vary with signal bandwidth
– Copper attenuation increases with frequency
Networks & Connectivity
• Network distributes signals
• Types of connectivity
– Point to point
– Point to multipoint (broadcast)
– Switched
– Networked
Fiber-optic communication
• is a method of transmitting information from
one place to another by sending light through
an optical fiber.
• The light forms an electromagnetic carrier
wave that is modulated to carry information.
Fiber-optic communication
• The process of communicating using fiberoptics involves the following basic steps:
– Creating the optical signal using a transmitter,
– relaying the signal along the fiber, ensuring that
the signal does not become too distorted or weak,
and
– receiving the optical signal and converting it into
an electrical signal.
How does fiber optic transmit light?
Optical Fiber: Advantages
• Capacity: much wider
bandwidth (10 GHz)
• Crosstalk immunity
• Immunity to static
interference
– Lightning
– Electric motor
– Florescent light
• Higher environment immunity
– Weather, temperature, etc.
Optical Fiber: Advantages
• Safety: Fiber is non-metalic
– No explosion, no chock
• Longer lasting
• Security: tapping is difficult
• Economics: Fewer repeaters
– Low transmission loss (dB/km)
– Fewer repeaters
Remember: Fiber is non– Less cable
conductive
Hence, change of magnetic
field has No impact!
Optical Fiber: Disadvantages
•
•
•
•
•
•
Higher initial cost in installation Interfacing cost
Strength
Lower tensile strength
Remote electric power
More expensive to repair/maintain
Tools: Specialized and sophisticated
Copper vs fiber bandwidth
Optical fiber communication:
Architecture
Optical Fiber Architecture –
Components
Global network
Components of global network
• Submarine cables
– High capacity intercontinental
– Shorter, regional cables
•
•
•
•
National backbone networks
Regional networks
Local networks
Satellites play minor role
Network nodes
• Network nodes Present in long-haul, regional and
metro
• Hubs or terminal points
– Ends of cables where signals are switched and reorganization
– Signals typically regenerated
– Signals may be broken down to slower data rates or
multiplexed to higher rates
• Add/drops
– Only a few optical channels are added/dropped
Optical Communication Systems
Fibre Structure
Types of optical fibre
Dispersion
Absorption Losses in Optic Fibre
Fibre Alignment Impairments
Optical Fibre System