TERM PAPER

ECE-201

TOPIC : - LASER DIODE

DOA: 16.03.2010

DOR: 5.04.2010

DOS: 5.05.2010

SUBMITTED TO: - SUBMITTED BY: -

Mr. TARUN AGGARWAL RISHABH SHARMA
SECTION: - D-6901
ROLL No.:- D-6901-B29
D.O.S. : - 5-05-2010
Reg. No. :-10901842

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 UNDER TAKING

I declare that this term paper is my individual work. I have not copied it from any
student or from any other source except where due acknowledgement is made explicitly in
the text, nor has any part been written for me by another person.

Roll no. B-29

Name – Rishabh Sharma

Course instructor- Mr Tarun Aggarwal

Section-D-6901

ACKNOWLEDGEMENT
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 I take this opportunity with much pleasure to thank all the people who have helped me
through the course of my journey towards this term paper. I sincerely thank my supervisor,
Mr. Tarun Aggarwal, for her guidance, help and motivation. Apart from the subject of my
term paper, I learnt a lot from him, which I am sure, will be useful in different stages of my
life. I would like to express my gratitude to the other members of my section for their help
and support to encourage me and to boosting my confidence.

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Contents

CHAPTER 1
1.1 INTRODUCTION................................................................................4
1.2 HISTORY.............................................................................................5
1.3 PN-JUNCTION DIODE.......................................................................5
1.4 LASER DIODE AND ITS WORKING...............................................7
1.5 LASER DIODE PARAMETERS.........................................................9
1.6 TYPES OF LASER DIODES...............................................................10
1.7 LASER SAFETY..................................................................................9
1.8 APPLICATION OF LASER DIODE...................................................10
1.9 REFRENCES........................................................................................13

1.1 INTRODUCTION

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 Laser diodes (also called .injection lasers.) are in effect a specialised form of LED. Just
like a LED, they are a form of P-N junction diode with a thin depletion layer where electrons
and holes collide to create light photons, when the diode is forward biased. The difference is
that in this case the active part of the depletion layer (i.e., where most of the current flows) is
made quite narrow, to concentrate the carriers. The ends of this narrow active region are also
highly polished, or coated with multiple very thin reflective layers to act as mirrors, so it
forms a resonant optical cavity. The forward current level is also increased, to the point
where the current density reaches a critical level where a carrier population inversion occurs.
This means there are more holes than electrons in the conduction band, and more electrons
than holes in the valence band or in other words, a very large excess population of electrons
and holes which can potentially combine to release photons. And when this happens, the
creation of new photons can be triggered not just by random collisions of electrons and holes,
but also by the influence of passing photons. Passing photons are then able to stimulate the
production of more photons, without themselves being absorbed. So laser action is able to
occur: Light Amplification by Stimulated Emission of Radiation. And the important thing to
realise is that the photons that are triggered by other passing photons have the same
wavelength, and are also in phase with them.

Fig 1.1 - How a typical horizontal laser chip is mounted in its package, with the built-in monitoring diode able to sense
its rear output.

1.2 HISTORY

The first laser diode was invented by a team of researchers at General Electric in 1962
but many other research teams also contributed to their initial development. The common

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heterojunction laser diode was created from the combined research of Herbert Kroemer and
Zhores Alferov, who were both awarded the Nobel Prize in 2000.

1.3 PN-JUNCTION DIODE

When an n-type semiconductor is joined to a p-type, a p-n junction diode is formed.

Separately, the two semiconductors are electrically neutral. When joined, a few electrons
near the junction diffuse from the n-type into the p-type semiconductor, where they fill
the holes. The n-type is left with a positive charge, and the p-type acquires a negative
charge. Thus a potential difference is established, with the n-side positive relative to the
p-side, and this prevents the further diffusion of electrons.
If a battery is connected to the diode with the positive terminal to the p-side and the
negative terminal to the n-side, as in (a) below, the externally applied voltage opposes the
internal potential difference and current will flow, as long as the external voltage is great
enough (about 0.3 V for Ge, and 0.6 V for Si). The positive holes in the p-type
semiconductor are repelled by the positive terminal of the battery and the electrons in the
n-type are repelled by the negative terminal of the battery. The holes and electrons meet
at the junction, and the electrons jump over and fill the holes. A current is flowing.
Meanwhile, the positive terminal of the battery is continually pulling electrons of the
p-side, forming new holes, and electrons are being supplied by the negative terminal at
the n-side. Consequently, a large current flows through the diode. The diode is said to
be forward biased.

Fig 1.2:- showing diode in reverse and forward bias

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 When the diode is reversed biased, as in (b) above, the holes in the p-side are attracted
to battery's negative terminal and the electrons in the n-side are attracted to the positive
terminal. The holes and electrons don't meet at the junction, and no current flows.
A graph of current verses voltage for a typical diode is shown below.

Fig 1.3:- Graph showing the current in reverse and forward bias.

The large increase in reverse current evident in Figure 1.3 is the result of junction
breakdown. It occurs when the reverse voltage reaches a critical value.

1.4 LASER DIODE AND ITS WORKING

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 Laser is the short hand form of amplification by stimulated emission of radiation. A laser
emits radiation of essentially one wavelength, or a very narrow band of wavelength. This
mean that the light has the single colour. Laser light is referred to a coherent light as opposed
to light made up of wide band of wave lengths, which is termed as incoherent. laser diode is
shown in fig 1.4.

(a) (b)

Fig 1.4:- (a) showing laser diode (b) showing internal part of laser diode.

This unique property of light produced by laser is that the emission is in the form of a
very narrow beam without significant divergence. The light waves have sufficient energy to
weld metals or to destroy cancerous growth. it can also be used to precise measurements, to
guidance of industrial machinery, to optical fibre communication technologies.
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 In led, source of light is the energy emitted by electrons which re combines with holes. In
case of LED, the light is incoherent i.e. it is made of a wide spectrum of wavelengths.

In a laser the atoms are stuck by photons which are extremely equal to photons energy
when recombination occurs. This triggers energy emission and results in two identical
photons for each recombination: the incident photons and the emitted photons. The photons
produce further emission of similar photons, which in turn generate more similar photons.
The result is the emission of energy in the form of a beam of coherent light. LEDs can be
designed so as to produce coherent light with a very narrow bandwidth.

The light generating process of a laser diode is similar to that of LEDs and the material
used is often the same. The difference is that the laser diode uses a much smaller junction
area and the concentration of injected carriers is much higher.

The active region of laser diode is enclosed by two aluminium enriched layers of lower
refractive index to act as optical reflectors. Because of confinement caused by these
reflectors, light can only be exit from front or back faces of the laser diodes. These faces are
transparent and form the resonant cavity for the light. At the certain current density within the
active region of the junction, the optical gain of the proton generated exceeds the losses from
the faces and the operating mode of the junction changes from the random type of the LED
output to an organized, coherent, stimulated emission of a laser. The threshold at which this
change occur is between 50mA and 150mA, depending upon the laser diode material. The
amount of emitted light decreased with increase in temperature, so that the laser diode must
be kept cool with the use of heat sink and other mechanical schemes. Laser diodes can
produce large amount of optical output power, with a very narrow output spectrum and hence
little dissipation and overlap. They produced the very tightly directed beam with small
numerical aperture which can be directed into fibre with little loss. This made them satiable
for use with the thin monomade fibres. Laser diodes can operate at rate exceeding GHz but
need more complex circuitry to derive than that of LEDs.

Because the high energy density, a laser beam can be quite dangerous. Eye protection
must be worn when working with these devices.

1.5 LASER DIODE PARAMETERS

Perhaps the key parameter for a laser diode is the threshold current (ITH), which is
the forward current level where lasing actually begins to occur. Below that current level the

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device delivers some light output, but it operates only as a LED rather than a laser. So the
light it does produce in this mode is incoherent. Another important parameter is the rated
light output (Po), which is the highest recommended light output level (in mill watts) for
reliable continuous operation. Not surprisingly there is an operating current level (IOP) which
Corresponds to this rated light output . There also the corresponding current output from the
feedback photodiode, known as the monitor current level (Im). Other parameters usually
given for a laser diode are its peak lasing wavelength, using given in nanometres (nm); and
its beam divergence angles (defined as the angle away from the beam axis before the light
intensity drops to 50%), in the X and Y directions (parallel to, and normal to the
Chip plane).

1.6 LASER SAFETY

Although most of the laser diodes used in electronic equipment have quite low optical output
levels. Typically less than 5mW (mill watts). Their output is generally concentrated in a
relatively narrow beam. This means that it is still capable of causing damage to a human or
animal eye, and particularly to its light-sensitive retina. Infra-red (IR) lasers are especially
capable of causing eye damage, because their light is not visible. This prevents the eye’s
usual protective reflex mechanisms (iris contraction, eyelid closure) from operating.
So always take special care when using devices like laser pointers, and especially when
working on equipment which includes IR lasers, to make sure that the laser beam cannot
Enter either your own, or anyone else eyes. If you need to observe the output from a laser,
either use protective filter goggles or use an IR-sensitive CCD type video camera.
Remember that eye damage is often irreversible, especially when it damage to the retina.

1.7 TYPES OF LASER DIODES

The simple laser diode structure, described above, is extremely inefficient. Such
devices require so much power that they can only achieve pulsed operation without damage.
Although historically important and easy to explain, such devices are not practical.

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1.8.1 DOUBLE HETROSTUCTURE LASER

In these devices, a layer of low band gap material is sandwiched between two high
band gap layers. One commonly-used pair of materials is gallium arsenide (GaAs) with
aluminium gallium arsenide (AlxGa (1-x) As). Each of the junctions between different band
gap materials is called a heterostructure, hence the name "double heterostructure laser" or DH
laser. The kind of laser diode described in the first part of the article may be referred to as a
homo junction laser, for contrast with these more popular devices.

1.8.2 QUANTUM WELL LASER

If the middle layer is made thin enough, it acts as a quantum well. This means that the
vertical variation of the electron's wave function, and thus a component of its energy, is
quantized. The efficiency of a quantum well laser is greater than that of a bulk laser because
the density of states function of electrons in the quantum well system has an abrupt edge that
concentrates electrons in energy states that contribute to laser action.

Lasers containing more than one quantum well layer are known as multiple quantum well
lasers. Multiple quantum wells improve the overlap of the gain region with the optical
waveguide mode.

1.9 APPLICATION OF LASER DIODE

Fig 1.5:- showing application of laser in drilling.

Laser diodes can be arrayed to produce very high power (continuous wave or pulsed)
outputs. Such arrays may be used to efficiently pump solid state lasers for inertial
confinement fusion or high average power drilling or burning applications.

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 Laser diodes are numerically the most common type of laser, with 2004 sales of
approximately 733 million diode lasers, as compared to 131,000 of other types of lasers.

Laser diodes find wide use in telecommunication as easily modulated and easily
coupled light sources for fibre optics communication. They are used in various measuring
instruments, such as rangefinders. Another common use is in barcode readers. Visible lasers,
typically red but later also green, are common as laser pointers. Both low and high-power
diodes are used extensively in the printing industry both as light sources for scanning (input)
of images and for very high-speed and high-resolution printing plate (output) manufacturing.
Infrared and red laser diodes are common in CD players, CD-ROMs and DVD technology.
Violet lasers are used in HD DVD and Blue-ray technology. Diode lasers have also found
many applications in laser absorption spectrometry (LAS) for high-speed, low-cost
assessment or monitoring of the concentration of various species in gas phase. High-power
laser diodes are used in industrial applications such as heat treating, cladding, seam welding
and for pumping other lasers, such as diode pumped solid state lasers.

Many applications of diode lasers primarily make use of the "directed energy"
property of an optical beam. In this category one might include the laser printers, bar-code
readers, image scanning, illuminators, designators, optical data recording, combustion
ignition, laser surgery, industrial sorting, industrial machining, and directed energy
weaponry. Some of these applications are emerging while others are well-established.

Laser medicine: medicine and especially dentistry have found many new applications
for diode lasers. The shrinking size of the units and their increasing user friendliness makes
them very attractive to clinicians for minor soft tissue procedures. The 800 nm - 980 nm units
have a high absorption rate for haemoglobin and thus make them ideal for soft tissue
applications, where good haemostasis is necessary.

2.0 REFERENCES

1. Introduction is taken from book LED/laser

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2. Fig 1.1 is taken from book LED/laser
3. Laser diode is taken from textbook J.B.Gupta.
4. www.wikipedia.com
5. Rest of the figures is taken from wikipedia

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