laser4

Chapter IV

LASER

Light Amplification by Stimulated Emission of Radiations

Lecture 4.5

Books:(1) Optics, 3

edition: Ajoy Ghatak, McGraw-Hill

Comapnies (chapter 23)

Here we shall discuss about:

_{Neodymium-Based Lasers: ND-YAG Laser}

CO2 Laser

Neodymium-Based Lasers: ND-YAG Laser:

The Nd:YAG laser (YAG stands for yttrium aluminum garnet

Which is Y₃Al₅O₁₂) is a important

solid-state laser systems in which the energy levels of the neodymium ion take part in laser emission.

The laser emission occurs at λ₀ ≈ 1.06 μm.

Since the energy difference between the lower laser level and the ground level is 0.26 eV, the ratio of its population to that of the ∼ ground state at room temperature

(T= 300 K) is

.

Thus the lower laser level is almost unpopulated

and hence inversion is easy to achieve.

The main pump bands for excitation of

the neodymium ions are in the 0.81 and 0.75 μm wavelength regions and pumping

Typical neodymium ion concentrations used are 1.38 × 10∼ 20 cm-3. The spontaneous lifetime corresponding

to the laser transition is 550 μs and the emission line corresponds to homogeneous broadening and has a width v 1.2×10∼ 11 _{Hz which}

corresponds to λ 4.5Å.∼

With the availability of high-power compact and efficient

semiconductor lasers, efficient pumping of Nd ions to upper laser level can be accomplished using laser diodes. This leads to very compact diode pumped Nd -based lasers.

Diode laser pumping is simpler than lamp pumping and also

produces much less heat in the laser medium leading to increased

overall efficiency. Since the laser diode output is narrow band unlike a normal lamp, the output at 808 nm can be efficiently used for

Typical output powers of 150 W are commercially available. In fact an intracavity second-harmonic generator can efficiently convert the laser wavelength to 532 nm (the second harmonic of 1064 nm of Nd:YAG) leading to very efficient green lasers.

In a CO2 laser one uses the transitions occurring between

different vibrational states of the carbon dioxide molecule.

Figure shows the

carbon dioxide molecule consisting of a central carbon atom with two oxygen atoms

attached one on either side. Such a molecule can vibrate in the three independent

modes of vibration shown in Fig.

These correspond to the symmetric stretch, the bending, and the asymmetric stretch modes. Each of these modes is characterized by a definite frequency of vibration.

According to basic quantum mechanics these vibrational

degrees of freedom are quantized, i.e., when a molecule vibrates in any of the modes it can have only a discrete set of energies. Thus if we call ν1 the frequency corresponding to the

Figure 2. shows the various vibrational energy levels taking part in the laser transition.

The laser transition at 10.6 μm

occurs between the (001) and (100) levels of carbon dioxide.

The excitation of the carbon dioxide molecules to the

long-lived level (001) occurs both through

collisional transfer from nearly resonant excited nitrogen molecules and also from the cascading down of carbon dioxide molecules

The CO2 laser possesses an extremely high efficiency of 30%. ∼ This is because of efficient pumping to the (001) level and also because all the energy levels involved are close to the ground level. Thus the atomic quantum efficiency which is the ratio

of the energy difference corresponding to the laser transition to the energy difference of the pump transition, i.e.,

Output powers of several watts to several kilowatts can be obtained from CO2 lasers. High-power CO2 lasers find

applications in materials processing, welding,

hole drilling, cutting, etc., because of their very high output

power. In addition, the atmospheric attenuation is low at 10.6 μm which leads to some applications of CO2

Semiconductor Laser:

_{The semiconductor or diode lasers are the smallest of all the} known lasers; they have a size of a fraction of a millimeter. The laser consists of a semiconducting crystal, such as gallium

arsenide, lead selenide, etc, with parallel faces at the ends to serve as partially reflective mirrors.

A semiconductor, as the name implies, is half-way between a conductor and an insulator (non-metal), so far as its electrical conductivity is concerned.

The semiconducting materials containing gallium and arsenic compounds have been found to generate infrared rays when the current is passed through them. This implies that these

semiconductors convert electrical energy into photons. But, these were ordinary incoherent light rays and were not

However, when the gallium arsenide crystal is through it, the laser action does take place. Many semiconductors serve as laser

materials and they have been made to 'lase' under the

stimulation of electricity instead of light which is used for the other solid-state lasers.

There are two types of semiconductors, viz., n-type and p-type. To understand the functioning of these devices, it is necessary to know the nature of the electronic energy states in a

semiconductor. A typical semiconductor has bands of allowed energy levels separated by

In an intrinsic semiconductor, there are just enough

electrons present to fill the uppermost occupied energy band (valence band) leaving the next higher band (conduction

band) empty. In an n-type semiconductor, a small amount

of impurity is added intentionally so that the material is made to have an excess of electrons, which thus becomes negative. On the other hand, by adding a different type of

impurity in a p-type semiconductor, the material can be made to have an excess of holes

The photons travelling through the junction region stimulate more electrons during the transition, releasing more

photons in the process. The laser action takes place along the line of the junction. Due to the polished ends of the block, the stimulated emission grows

enormously and a beam of coherent light is emitted from one of the two ends gallium arsenide laser, a continuous beam of a few militates power is easily obtained.

The semi conducting lasers are also called junction lasers or junction diode lasers because they produce laser energy at the junction of two types of impurities in a semiconductor. They are also called injection lasers because electrons are injected into the junction

The semiconductor laser consists of a tiny block (about one square millimetre in area) of gallium arsenide (Fig). When the p- and n-type layers are formed in an intimate contact, the interface becomes a p-n junction. When direct current is applied across the block, the electrons move across the junction region from the n-type material to the p-type material, having excess of

holes. In this process of dropping of the electrons into the holes,

The semiconductor lasers, being simple in construction and light In weight with compact units and requiring little auxiliary equipment, are very suitable for applications where high powers are not

required. They are primarily used in the area of communication in which the near-infrared laser beams can be transmitted over long distances through low loss optical fibres. In addition, they have found a large market as reading devices for

compact disc players.