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Q1.What Is a Laser ?

Answer:
The term "laser" stands for "light amplification by stimulated emission of radiation."
It usually denotes a device for this purpose or the light beam produced by it.

<The basic structure of a laser system is shown below.>

The laser system generally consists of
(a) an active medium that can amplify light
(b) reflectors to return light.

The light is amplified in the active medium while it reciprocates between two reflectors.
Laser beams are part of this amplified light extracted from the reflectors.

Q2.How Does Laser Light Compare with Other Light ?

Answer:
(a) Light from an incandescent lamp (incoherent radiation)
(1) No directionality
(2) A mixture of various wavelengths
(3) Incoherent phases (temporal relations)
(4) Light waves do not prevail long

<Light from electric lamp>

(b) Features of laser light
(1) High directionality
(2) Single wavelength
(3) Coherent phases
(4) Light waves prevail long

<Laser light (coherent radiation)>

Q3.Are There Various kinds of Lasers ?

Answer:
There are soild-state, gas, liquid, and semiconductor lasers.
Their typical example and applications are listed below.

Q4.How Do Semiconductors Emit Light ?

Answer:
Electrons in atoms or molecules have different values of energy that are not continuous.

To raise an electron at a low energy level to a higher level, some energy must be given to it by light or an electric current.

An electron at a high energy level is unstable and tends to transit to a lower energy level.
When it does, it emits light of a wavelength proportional to the energy level difference.
This light emission occurs in two ways: spontaneous emission and stimulated emission.

In (a) above, electron at the E2 level transit to the E1 level independently of each other, thus without coherent temporal relations (phase) among them when they emit light.
In (b) on the other hand, under influence of the incident light, light is emitted with the same wavelength and the same phases as those of the incident light.

Q5.What is The Difference between LED and Laser Diode ?

Answer:
The light-emitting diode (LED) and the laser diode (LD) are both composed of semiconductor p-n junctions.
When they receive electric current, they emit light, as particles (holes) with positive changes join with particles (electrons) with negative charges.


The light-emitting diode produces a spontaneously emitted light.
As shown above, the directions, wavelengths, and temporal relations of emitted are incoherent.
The laser diode mainly emits light by stimulated emission and the wavelengths and temporal relations of produced light are coherent.
Also, two laser facets opposite to each other serve as reflecting mirrors, permitting the emitted light to possess high directionality.

Q6.What is The Basic Structure of Laser Diodes ?

Answer:
Laser mainly emits light by stimulated emission.
To ease the occurrence of stimulated emission, electrons initially must be held at high energy levels.
To achieve this requirement, the laser diode generally is a sandwiched structure (double-hetero junction) with a light emitting layer (active layer) set between two other layers (cladding layers) with larger energy level differences.
Also the inner active layer has a greater refractive index than outer cladding layers, permitting light to be confined effectively.
To conserve power consumption, currents are concentrated by providing narrow current channels in a striped arrangement.

Gain guided laser vs index guided laser

The index guided laser also has an active layer sandwiched horizontally between outer layers having larger energy level differences, permitting light to be confined effectively in a direction horizontal with the active layer.

Compared with the gain guided laser, the index guided laser provides a stable horizontal transverse mode.

Q7.What Advantages Do Laser Diodes Have ?

Answer:
Laser diode have the following advantages over other types of laser:
(1) Smaller size
(2) Higher efficiency with lower power requirements
(3) Higher response to input current
Specifically, visible laser diode possess the features shown in the table below, when compared with He-Ne gas lasers.

Q8.What Are Characteristics of Laser Diodes ?
(1) What Are Forward Current-Optical Output Power Characteristics ?

Answer:
Current flowing through a laser diode and the optical output power from it have the following characteristic relation.

With its optical output power remaining below Pth the laser produces a spontaneously emitted light.
As its optical output power rises over Pth the laser generates laser oscillation (stimulated emission).
The current with which laser oscillation starts is termed the threshold current lth.
Toshiba defines the term lth as forward current at the intersection of the X-axis and the extension of the forward current-optical output power line indicating laser oscillation.
The forward current with which the specified optical output power P0 can be obtained is termed the operating current lop.


(2) What Is the Lasing Wavelength ?

Answer:
Observation of the spectrum of a laser diode in oscillation reveals many wavelengths that slightly differ from each other.
This state is termed the longitudinal mode.
This results from the fact that many standing waves exist between the reflectors.

Among the many wavelengths, the one with a maximum intensity is termed the peak lasing wavelength λp
Laser diodes can be classified into two types:
Multi-mode lasers generating oscillation in many longitudinal modes,
and single-mode lasers generating oscillation in a single longitudinal mode.

(3) What Are Near-and Far-Field Patterns ?

Answer:
Light intensity distribution obtained on a facet is termed a "near-field pattern."
On the other hand, light from the facet becomes an expanded radiation, as shown below.
The intensity distribution, which has been measured on the raduation mentioned above at a distance from the facet horizontally with and perpendicular to the active layer, is termed a "far-field pattern."
The full angle at half maximum of the far-field pattern is termed a beam divergence.
The spread angles horizontal and perpendiculat to the active layer are represented by θ1 and θ2 respectively.