UNIT - VUNIT - V

LASERS&

FIBER OPTICS

LASERS

LASER means…Light Amplification by Stimulated Emission of Radiation

Definition:

The light sources having High intensity High monochromatic High directional High coherence are called LASERS.

Principle of LASER:

Stimulated emission of radiation Note:

LASER came out from MASER technology.

(Microwave Amplification by Stimulated Emission of Radiation)

Importance of LASERS:

High Directional (strictly parallel)

Ordinary light

LASER (Angular spread 10 micro radians)

Explanation:

A LASER beam of 10cm in diameter whenbeamed at the moon’s surface 3,84,000km away is not more than 5km wide.

High Monochromatic

Explanation:

A LASER produces strictly a single wavelength of light.

i.e. The line width associated with laser beams are extremely narrow.

High Intensity

Explanation:LASER is highly intense than an ordinary light.Hence, it is used in welding (or) cutting the hard substances which involve reaching high temperatures.

Note: Due to the above reason LASER is called as a material less knife

High Coherence

Explanation:LASER is highly coherence, even we usetwo independent sources of lasers we get interference pattern.

Note: Coherence pattern can explain in two ways.

Temporal coherence Coherence property can be explain w.r.t. time is known as temporal coherence.

Example:An ordinary light temporal coherence is ≈ 10 - 9 Sec, Where as He-Ne gas laser is 2x10 - 3 Sec.

Spatial coherence

Coherence property can be explain w.r.t. distance is known as spatial coherence.

Example:An ordinary light spatial coherence is only 3cm, Where as He-Ne gas laser is about 600km.

Conditions to achieve LASER action There must be stimulated emission of radiation.

There must be a population inversion. There must be an arrangement to get multiple reflections without any reflection losses. (This is true in certain lasers only)

Einstein quantum theory of radiationAccording to Einstein, if radiation at a frequency corresponding to the energy difference (E2-E1) fallson the atomic system it can be interact in three distinct ways. As…..

Absorption Spontaneous Emission Stimulated Emission

AbsorptionWhen a photon of energy (hѴ = E2 -E1 ) interacts with an electron, the photon will disappear and an electron will move to the upper energy state.

This process is called as absorption.

Explanation:E2

E1

Absorption

Note:The absorption rate will be proportional to the number of electrons present in that state.

hѴ

Spontaneous EmissionAn electron is in its upper state and no radiation is present, then after certain mean time an electron moves of its own to the lower energy state emitting a photon of energy in this process.

This process is called as spontaneous emission.

Explanation:E2

E1

Spontaneous emission

Note:This emission was happened because not triggered by any outside influence.

hѴ

Stimulated EmissionAn electron makes a transition from E2 to E1 a radiation whose photon’s energy equal to (E2 – E1 ) is incident on the excitedelectron and stimulates it to emit radiation. Then the electron moves to its lower energy state and there are now two photons, called stimulated emission.

This process is called as stimulated emission.

Explanation:E2

E1

Stimulated emission

Note:This emission was happened because due to by the external triggering.

hѴ hѴhѴ

Conclusion: The emitted photon in stimulated process is in every way identical to the triggered photon.

i.e. the emitted photon has same in energy, direction & phase with triggered photon.

This process is very important to achieve the laser beam.

Metastable state:It is the excited energy state of an atomic system whose life time of the electron in that state will be very large is called metastable state.

Explanation:According to Heisenberg’s uncertainty principle,

∆E . ∆t > h/4π-(OR)

∆t α 1/∆E – (1)

∆E – width of the energy level where,

∆t – life time of the electrons in that state

Note: From eq(1) we can say that, the life time of the electrons in the metastable state will be very large due to narrow energy level & hence can easily achieve the population inversion.

i.e. For ordinary state -- 10 - 8 S ≈ 10 – 9 S (1 nano sec) For meta stable state -- 10 - 3 S (1 mill sec)

Population inversion( N2 > N1 )The state of the atomic system at which the number of electrons in the higher (OR) excited state is more than the number of electrons in the lower state (OR) ground state is called the population inversion.

Note: Population inversion can be achieved by pumping of electrons from lower energy level to higher energy level in the metastable state.

Explanation:

E1N1

E2N2

N1 N2 >

E1N1

E2N2

N2 N1 > (Population inversion)

(Normal condition)

Pumping process of electrons

Optical Pumping

Electric discharge

In-Elastic atom-atom collisions

Chemical reactions

Direct conversion Injection current…..

Difference between Spontaneous Emission & stimulated emission(Normal light & Laser)

Stimulated Spontaneous Emission of light photon takes place immediately without any inducement.

Emission of a light photon is by inducement of a photon having energy equal to the emitted photon energy.

Polychromatic radiation. Strictly monochromatic radiation. Incoherent radiation High coherent radiation Less directional High directional Less intense High intense More angular spread Less angular spread

Homo - JunctionA PN-junction is formed by a single crystalline material such that the basic material has been the same on both sides of the junction iscalled homo-junction.

( Direct recombination takes place in between electron & hole )

Drawbacks Threshold current density is very large Only pulse mode output is obtained Output has large beam divergence Poor coherent & stability Electromagnetic field confinement is poor

Hetero - JunctionThe material on one side of the junction differs from that on the other side of the junction is called hetero-junction.

( Direct recombination of electron & hole is not possible )

Advantages Low threshold current density Output is continuous High output power Narrow beam, high coherent, monochromotic & stable Long life time of the device

Einstein co-efficients

Emission

Where,

E1 – energy of ground levelE2 – energy of excited levelN1 – number of electrons in ground levelN2 – number of electrons in excited level

Absorption

E1N1

E2N2

hѴ hѴ

Theory:From figure the number of absorptions/unit time/unit volume can be expressed as

N1 B12 u(ω) --- (1)Where,

B12 – proportional constant

u(ω) – radiant energy/unit volume

The rate of spontaneous emission represents as

N2 A12 --- (2)

Similarly the rate of stimulated emission represented as

N2 B21 u(ω) --- (3)Where,

A12 , B12 , B21 – Einstein co-efficients

Therefore, under equilibrium condition,

The number of upward transitions = number of downward transitions

N1 B12 u(ω) = N2 A21 + N2 B21 u(ω) i.e.(OR)

u(ω) =

-- (4) A21

N1 B12 - B21 N2

From Boltzmann distribution law, we can write as

N1

N2

= e

(E2 – E1)/ KT

(OR)

N1

N2

= e

ђω/ KT -- (5)

Where,

K – Boltzmann constant

Substituting the eq(5) in eq(4), we get,

u(ω) =

-- (6) A21

B12 - B21 e

ђω/ KT

From Plank’s radiation law, the energy densityof radiation is given by

u(ω) =

-- (7) e

ђω/ KT

ђ ω3μ3 π2c3

1

- 1Where,

μ – refractive index of the mediumc – velocity of light

Comparing the eqs(6) ,(7) & B12 = B21 = B, we get,

A21

B21

= ђ ω3μ3

π2c3

-- (8)

Finally, the ratio of the number of spontaneous to stimulated emissions is given by

ђω/ KT

A21

B21

=

-- (9) u(ω)

e

- 1

Note:Einstein co-efficients method gives more clarity about absorption, spontaneous & stimulated emission.

Types of lasers Ruby laser

He - Ne gas laser Carbon dioxide (co2) laser Semiconductor diode laser

Ruby laser

Ruby is a crystal of Al2O3(Corundum)with Cr2O3 .

It was the first laser demonstrated by T.Maiman in 1960.

(i.e. In Ruby some of Al +3 atoms have been replaced with 0.05% Cr +3 ions ) Finally, Ruby is a single crystal of Al2O3 with 0.05% of chromium(doped).

Colour of the Ruby laser depends on the proportion of Cr +3 ions.

Construction

Capacitor

C

Supply

Ruby rod

electrodeelectrode

Partially reflector

Total reflectorhѴ

Xenon flash lamp(N2>N1)Optical pumping

Working

Ground level

E1

E2

Mete stable state

λ = 4000 0A λ = 6000 0A (Optical pumping)

Figure shows 3-energy level Ruby laser

hѴ

λ = 6943 0A (Deep red color Output)

(Non-radiative transitions)

A photon of λ = 6000 0A (green) on absorption excites chromium atom from the ground level to the E1 level.

A photon of λ = 4000 0A (blue) can take an atom from the ground level to the exited level E2 .

In meta stable state the life time of the carriers are 105 times more than E1 & E2 levels.

Since the meta stable state has very long life, the number of charges in this state keeps increasing & the population inversion from ground state to meta stable is achieved.

The lasing action is triggered by the spontaneously emitted photons. This results in stimulated emission of other identical photons & an output is in pulse form of wavelength 69430A.

Applications: Used in holography (3D- image) Used in LIDAR (Light Detecting & Ranging) Used in remote sensing Used in Ophthalmology (treatment to detached retina) Used in drilling small areas…

He - Ne gas laser It was the first continuous laser discovered by Ali Javan in USA.

Light with high monochromacity, directional, coherence.

But the output power is generally in the order of few milliwatts.

Construction

ElectrodesPartially silvered mirrorFully silvered mirror

Brewster’s window

He – Ne gas

Laser-Professionals.com

Courtesy of Metrologic, Inc.

It consists of a discharge tube made of quartz with a diameter of 1.5cm & length of nearly 1 meter

The tube is filled with a mixture of Helium & Neon gases at the pressure of Neon 0.1mm of Hg and Helium 1mm of Hg. ( i.e. He-Ne is at 10:1 ratio )

Flat quartz plates which function as Brewster windows are sealed to the tube at both of its ends.

Two mirrors- one is fully reflected and the other is partially reflector are fixed on either side of the tube.

Working

Figure shows He – Ne gas energy levelsHe gas

Ne gas

F1

F2

F3

E1

E2

E3

E4

E5E6

Elastic collision

Energy

Inelastic collision

Spontaneous emission ( λ = 6000 0A)

(λ = 3390 0A) Non-radiant

(λ = 1150 0A) Non-radiant

(λ = 6328 0A)

collision

hѴ

When an electric discharge is set up in the tube collisions between the electrons and ions raise the He atoms to the levels F2 & F3 are meta stable states. Ne has two meta stable states E4 & E6 at nearly the same energy levels as F2 & F3. He atoms excite them to levels E4 & E6 . Thus the population inversion is achieved. Spontaneous emission photon triggers laser action in any of the three transitions shown in above figure.

This photon travels through the gas mixture and it reflects by the mirror ends until it stimulates excited Ne atoms. The Ne atoms then drop down from the laser levels to the level E2 through spontaneous emission.

From the level E2 , the Neon atoms are brought back to the ground state through collisions with the walls. The transition emit radiation laser beam of 6328 0A wavelength.

Wavelengths of 3.39 μm & 1.15 μm are in the invisible region

Merits: It is a first continuous laser.

It gives high stability, directional, monochromatic & a pure spectrum.

Applications: It is useful in making holograms & interferometric experiments. It is used to read the bar coding & image processing. In medicine this acts as an aiming laser which is normally used to identify the spot where the laser surgery has to be performed. It is used in determining the size of tiny particles.

Carbon dioxide(co2)laser

An Indian engineer C.K.N. Patel designed this laser.

It is a pulsed & continuous laser.

It is a powerful laser.(high intensity laser)

In CO2 laser the transitions take place between the vibrational energy levels of the carbon dioxide molecules.

Carbon dioxide molecule is a linear tri-atomic molecule with three simple normal modes of vibrations are possible. Those are...

Symmetric stretch mode Bending mode

Asymmetric stretch mode

Symmetric stretch modeIn this mode, the oxygen atoms move symmetrically about the carbon atom such that both the C-O bonds either stretch (or) contract simultaneously as shown in below figure.

Carbon atom

Fig. Symmetric stretching (100 )

Oxygen atom Oxygen atom

Bending mode

In this mode, the oxygen atoms move normal to the C-O bond causing the bending of the bonds as shown in below figure.

Carbon atom

Fig. Bending mode(020)

Oxygen atom Oxygen atom

Asymmetric stretch modeIn this mode, the oxygen atoms move such that whenone C-O bond stretches, the other C-O bond contracts.Which is shown in below figure.

Carbon atom

Fig. Asymmetric stretching (001 )

Oxygen atom Oxygen atom

Construction He N2 CO2

Vacuum Pump

NaC

l Win

dow

Mirr

or

LASER

Tube is filled with a mixture of CO2 , Nitrogen & Helium gases in the ratio of 1:2:3 respectively.

It made up of long , narrow, cylindrically shaped gas discharge tube as shown in figure, with a 2.5 diameter & length of about 5meter

CO2 molecule consists of a central atom with two oxygen atoms attached one on either side. CO2 molecule has three independent vibration oscillations already we discussed earlier.

Nitrogen helps to increase the population of atoms in the upper level of CO2 , while helium helps to depopulate the atoms in the lower level of CO2 & also to cool the discharge tube.

The discharge is produced by D.C. excitation.

At the ends of the tube sodium chloride Brewster windows are placed & silicon mirrors coated with aluminum for proper reflection.

The output power can be increased by increasing the diameter of the tube.

Working

F1

Excited level

Nitrogen gas CO2 gasE1

E2

E3

E4

E5

λ = 10.6 μm λ = 9.6 μm(Radioactive decay)

( 020 )

Collision decay

( 010 )

Nitrogen atoms areexited with CO2 atoms

Energy transfer

( 100 )

( 001 )

When a discharge is passed through the tube, the nitrogen molecules are excited and are raised to higher excited state.

The excited energy of nitrogen molecules is transferred to carbon-dioxide molecules through collisions and carbon-dioxide molecules are raised to their excited vibrational energy level E5 (001) from their ground state.

The energy level ‘E5’ is a metastable state energy level. Hence there is population inversion.

Stimulating photons of wavelength 10.6 μm and 9.6 μm induce the CO2 molecules to undergo stimulated emission by laser transitions from E5 to E4 giving laser wavelength of 10.6 μm and from E5 to E3 , wavelength of 9.6 μm.

Since the laser transition from E5 to E4 has higher gain than from E5 to E3 , the laser usually oscillates at 10.6 μm.

The CO2 molecules from E4 and E3 are returned to their ground through fast decay and diffusion.

Applications:

It is applicable in medical field as Neurosurgery, microsurgery, treatment of liver, lungs & also in bloodless operations.

It is widely used in open air communication.

It is also have wide applications over military field…

Semiconductor diode laser

It is also called as a diode laser.

The wavelength of the emitted light depends upon the energy band gap of the material.

It is always operated in forward bias only.

Laser diode is highly doped.

Depletion region (Active region)

P Type

P-Type: Ga As doped with Germanium.N-Type: Ga As doped with Tellurium

N TypeElectrons

Holes

VF

The active medium is a PN- junction diode made from a single crystalline material.

i.e. Gallium Arsenide in which P-region is doped with germanium & N-type with Tellurium.

The refractive index of GaAs is high, hence it acts as optical resonator so that the external mirrors are not needed. .

The upper & lower electrodes fixed in the P & N region helps for the flow of current to the diode while biasing.

WorkingInjection

Of electrons

VB

CBEF

EF

P - Type N - Type

Injection Of

holesActive region

Semiconductor laser diode works similar to LED except here stimulated process takes place.

Laser diode is very highly doped, when it is in forward biased electrons & holes flow from P-side & N-side respectively. Then in the junction region both electrons & holes are in high concentrations, which means the population inversion.(direct conversion)

( i.e. Excess population can be maintained by carrier injection, the pumping source being the current passing through the semiconductor )

When a single electron moves from the conduction band to valence band it releases energy as a photon.

This photon stimulates another electron to fall into the valence band producing a second photon by stimulated emission.

The minimum current necessary to start stimulated emission is called the Threshold current.

Note: Threshold current is not for doping, it is for a electron coming from higher energy level to lower energy level.

To achieve threshold current, opposite faces of the PN- junction must be flat and parallel.

Advantages: Semiconductor lasers are small in size, compact, light in weight, low cost & more efficient. They can be tuned over considerable wavelength range by varying temperature & doping. They are very important light sources in fiber optic communication systems. Semiconductor laser has provided spectroscopists with a very useful tunable light source….

Disadvantage: Semiconductors are extremely sensitive to temperature changes.

( i.e. With a slight increase in temperature causes an appreciable increase in forward current & it goes change in frequency of emitted light. )

Applications of lasers In communication

more amount of data can sent because of large band width.

more channels with out trapping of signals.

Highly directional means greater potential use in space crafts & submarines….

In atmospheric science

i.e. to measure atmospheric pollutants, Ozone concentration, water vapor concentration.

LIDAR ( light detection & ranging ) to study about atmospheric features.

In computers In LAN, data transfer from one computer to other for short time. During reading & recording the data on CD’s

Laser printing……

In industries Welding, cutting & making the holes with small diameters. To measure distance to making maps by surveyors. Blast holes in hard materials like diamond, hard shell….

In scientific research To prepare isotopes of uranium. To create plasma this may be help the scientists to control nuclear fusion reaction. Hologram in optical signal processing. To produce some chemical reactions.

In defense To target enemy air plane (or) ship & to determine its distance. To destroy enemy aircrafts & missile. As war weapon. To find out of velocity of moving object.

In medicine To remove diseased body tissues. Retinal detachment by eye specialist. Ruby – ophthalmology & dermatology. He-Ne – diagnostic application……

Argon & co2 lasers are used in liver and lungs treatment.

To elimination of moles & tumors on skin tissues….

In entertainment In chemistry

In photography……

FIBER OPTICS

Principle:

Total internal reflection

Total internal reflection takes place due to following conditions

Light should travel from denser medium to rarer medium

The angle of incidence should be greater than the critical angle(ѲC ) of the medium.

Critical Angle( ѲC )

The value of the incident angle of the medium at which the angle of refraction is 900 is called as the critical angle.

Explanation

n2 n1

n1 n2 >

i

Ѳ

Fig-1

i < ѲC

Refracted ray

Reflected ray

n2

n1

n1 n2 >

Ѳ c

Fig-2

i = ѲC

critical ray

Reflected ray

n2

n1

n1 n2 >

Fig-3

i > ѲC

TIR i

Applying the Snell’s law in figure-2 (i = ѲC ), we get,

n1 sin Ѳ1 = n2 sin Ѳ2 n1 sin Ѳc = n2 sin 900

Since, Ѳ1 = Ѳc & Ѳ2 = 900 Therefore,

Ѳc = sin-1 (n2 /n1 ) sin Ѳc = n2 / n1

Since, n1 – refractive index of denser medium n2 – refractive index of rarer medium Ѳc - critical angle

Construction of fiber

Protective jacket Cladding ( n2 )

Core ( n1 )

Core(n1)A typical glass fiber consists of a central coreglass whose thickness is order of 50μm.

Actually, the light is propagated through this core.

Cladding(n2)Core is surrounded by a cladding made of a glass of slightly lower refractive index than core’s refractive index.

n1 > n2

Note: Cladding is formed by the addition of small amount

of boron, germanium (or) phosphorus in the outer layer to decrease the refractive index.

Cladding is necessary to provide proper light guidance. i.e. to retain the light wave with in the core.

Cladding provides high mechanical strength and safety to the core from scratches..

The overall diameter of core & cladding is about 125 to 200 μm.

Protective jacket

It is made up of plastic which protects the fiber from moisture & abrasion, also provides necessary toughness & tensile strength.

So that the fiber optic cable with stands without any brittleness during hard pulling, bending, stretching (or) rolling even though the fiber is made from brittle glass (or) silica glass(Sio2).

Light propagation through fiber

O

B

Cladding(n2)

Cladding(n2)

Core(n1)

Axis of the fiberC

Incident ray

Ѳ0

Ѳ1

Ѳ1

Acceptance Angle( Ѳ0 )The maximum angle of the fiber with a light ray can enter into the fiber and still goes under total internal reflection.Note:The core is referred as acceptance cone.

Theory:Let n0 is the refractive index of the surrounding medium, then applying the Snell’s law at point O in the above figure. we get,

n0 sin Ѳ0 = n1 sin Ѳ1

sin Ѳ0 = n1 / n0 sin Ѳ1

(OR)

sin Ѳ0 = n1 / n0 √1- cos2 Ѳ1 (1)

In the above figure at point B angle of incidence = (900 - Ѳ1 )

Applying the Snell’s law, we get,

n1 sin (900 – Ѳ1 ) = n2 sin 900

(OR)

cosѲ1 = n2 / n1 (2)

Substituting eq(2) in eq(1), we get,

sin Ѳ0 = n1 / n0 √1- n22 / n1

2

sin Ѳ0 = n1 / n0 √n12- n2

2 / n12

(OR)sin Ѳ0 = √n1

2- n22 / n0 (3)

If the fiber is situated in air, then n0 = 1 Therefore, eq(3) becomes as

sin Ѳ0 = √n12- n2

2 (OR)

Ѳ0 = sin-1 √n12- n2

2

(4)

Numerical Aperture (NA)

The sine of the acceptance angle of the fiber is called numerical aperture.i.e. NA = sin Ѳ0 = √n1

2- n22 / n0

If the fiber is situated in air, then n0 = 1

(5)NA = sin Ѳ0 = √n12- n2

2

Note:NA gives the light collecting efficiency of the fiber.

Types of fiber: Step index fiber

Graded index fiber

Note:In step & graded index fiber the signal can be send in single mode & multi mode transmission.

Step index fiber

Cladding(n2)

Cladding(n2)

Core(n1)

Axis of the fiber

Incident light ray

Figure - Light propagation in single mode step index fiber

Cladding(n2)

Cladding(n2)

Core(n1)

Axis of the fiber

Incident light rays

Figure - Light propagation in multi mode step index fiber

Properties of step index fiber

The refractive index is uniform through the core & cladding, but core’s refractive index is slightly more than the cladding refractive index.

This results in a sudden increase in the value of refractive index from cladding to core. Thus its refractive index profile takes the shape of a step.

Radial distance

Refractive index

Core ( n1 )

Cladding ( n2 )

The light rays are propagating in the form of meridional rays , which will cross the fiber axis during every reflection at the core – cladding boundary & propagating in a zig – zag manner.

In multimode step index fiber the different incident angle rays travel different distances. Hence, high angle rays arrive latter than the low angle rays. So the signal pulses are broadened out & distortion takes place.

But this distortion does not take place in single mode step index fiber.

Graded index fiber

Cladding(n2)

Cladding(n2)

Core(n1)

Axis of the fiber

Figure - Light propagation in single mode graded index fiber

Incident light ray

Cladding(n2)

Cladding(n2)

Core(n1)

Axis of the fiber

Figure - Light propagation in multi mode graded index fiber

Incident light rays

Properties of Graded index fiber The refractive index of the core is made to vary in the parabolic manner such that the maximum refractive index is present at the center of the core.

Cladding refractive index is constant. But the core refractive index is maximum along the axis & decreases parabolically towards the cladding interface, till it attains a constant value. Hence, according to Snell’s law a ray entering into the fiber, it bends away from the normal. Because the continuously lowering refractive index of the core.

Radial distance

Refractive index Core ( n1 )

Cladding ( n2 )

The light rays are propagating in the form of helical rays , which will not cross the axis at any time & are propagating around the fiber axis in a helical (OR) spiral manner.

Signal distortion is very low because of self focusing effect. Here the light rays travel at different speeds in different paths of the fiber because the refractive index varies through out the fiber.

As a result light rays near the outer edge travel faster than the light rays near the center of the core.

In effect light rays are continuously refocused as they travel down the fiber and almost all the rays reach the exit end of the fiber at the same time due to the helical path of the light propagation. Hence, the distortion is less in this fiber.

Attenuation in optical fiber

Attenuation is the loss of power of the light signal that occurs during its propagation through the fiber. Attenuation mechanisms are 3 types. Absorption Scattering Radiative losses

Absorption: Absorption of light during propagation occurs due to the impurities present & imperfections in the fiber material. Scattering: In fiber the disorder structure of glass will make some variations in the refractive index inside the fiber. As a result if light is passed through the atoms in the fiber, a portion of the light is scattered known as Rayleigh scattering.

Rayleigh scattering loss α 1/λ4

Radiative losses Radiative loss occurs in fibers due to bending of finite radius of curvature in optical fibers.

The types of bends are Macroscopic bend Microscopic bend

Applications of optical fibers: In communication

Transmitter Receiver

Drive circuit

Photo detector

Light source

Amplifier Signal restorer

Electrical signal LED PIN diode

Electrical signal Optical

fiber

Figure: Block diagram of optical fiber communication system

An optical fiber communication link consists of three important parts.

Transmitter

Optical fiber

Receiver

Transmitter

Transmitter consists of a light source usually laser diode (OR) LED along with its associated drive circuit . Transmitter receives electric input signal in the form of digital pulses from encoder & converts them into optical pulses.

The electrical pulses modulate the intensity of the light source.

Optical fiber

Optical fiber acts as the wave guide to transmit the optical pulses towards the receiver.

Receiver

The photo detector in the receiver end receives the optical pulses from the fiber & converts them into electrical pulses. The photo detector can be either an avalanche photo diode(APD) OR a positive-intrinsic- negative(PIN) diode.

An amplifier amplifies the received signal. Thus the original electrical signal is obtained.