RUBY LASER

1. It is the first solid state three level laser invented by T H Maiman in the year 1960.

2. It produces a high output power in the order of Mega Watts with ten nanoseconds

pulse duration.

3. Pumping Method used in this laser is Optical Pumping Scheme.

CONSTRUCTION:

1. It consists of a long cylindrical ruby rod which is made up of Aluminum Oxide

(Al2O3) and is doped with 0.05% of Cr2O3.

2. Due to the presence of Cr3+ ions the ruby rod is appears as pink colour. This suggest that there is

a strong absorption in the visible region.

3. The length of the ruby rod is about 10cm and diameter is 0.5cm.

4. The end faces of the rod are grounded and polished such that the end faces are exactly

parallel to each other.

5. One of the ends is silvered for 100% reflection and the another end is silvered for nearly 90%

Reflection.

6. A helical Xenon flash lamp is surround the ruby rod and is connected to a high voltage trigger

pulse (~20kV).

7. During the process, a large amount of heat is produced. So, the system is cooled with the help of

a coolant (water) circulating around the ruby rod.

WORKING:

1. Laser action will be takes place between the energy levels of the Cr3+ ions. Al2O3 is an insulator

and the introduction of Cr3+ ions results in additional energy levels with in the band gap.

2. The Chromium ions having three active energy levels known as E1 - Ground State, E2- Meta

stable state and E3- Higher Excited State.

3. E3 state is fairly wide and hence can accept a wide range of wavelengths. It has short life time.

4. In this the lasing action occurs between E2 and E1.

5. When ruby rod is irradiated with flash lamp, Chromium ions absorb the light of wavelength at

around 5600Ao (5000-6000Ao) which will be either green or blue color.

6. As a result the ions transferred to higher excited state E3 from Ground state E1.

7. From this level (E3) the ions will go down to Meta stable state (E2) in a non-radiative transition.

This energy is transferred to the crystal vibrations and changed into heat.

8. Since the life time of the E2 level is in the order of milliseconds, Chromium ions remain in this

level for longer duration.

9. So population inversion takes place between Meta stable state (E2) and Ground State (E1).

10. The spontaneously emitted initial photons would travel in all the directions, of these, those

travelling parallel to the axis of the rod would be reflected at the ends and pass many times

through the amplifying medium and stimulate the atoms in Meta Stable state.

11. As a result stimulated emission takes place and chromium ions translate from E2 to E1.

12. This transition gives rise to the emission of light of wavelength 6943Ao.

13. The output of this laser consists of a series of laser pulses for duration of milliseconds or less and

the diameter of the beam is 1 mm to 25mm.

APPLICATIONS: RUBY laser used in

1. Pulsed holography 2. Trimming resistors and IC masks. 3. Drilling high quality holes 4.In military, Target detectors and range finders (LIDAR)

LIMITATIONS:

1. Efficiency of RUBY laser is about 1% and it is less monochromatic, Less Directional.

2. The Xenon flash lamp operates only for few seconds. So meta stable state get depleted very rapidly.

He-Ne LASER

1. He-Ne laser was the first four level laser built by Ali Javen, W Bennett and D Herriot in 1961.

2. It produces a continuous output power varies from 0.5mW to 10mW.

3. The pumping method used in this laser is Electric discharge method.

CONSTRUCTION:

1. It consist of a long narrow quartz discharge tube is filled with He and Ne gas mixture in the

ration of 10:1.

2. The laser action takes place between in the energy levels of Neon atoms

3. Helium atoms helps to achieve the population inversion by imparting their energy to Neon

atoms

4. Two electrodes are fixed near the ends of the tube to pass electric discharge through the gas.

5. Two optically plane mirrors are fixed at the two ends of the tube normal to its axis.

6. One of the mirror if fully silvered so that nearly 100% reflection takes place and the other end is

partially silvered so that 1% of the light incident on it will be transmitted.

WORKING:

1. There are three active energy levels in Helium atom namely; He1, He2 and He3 of these levels

He2 and He3 are meta stable states. (Life time 10-4 & 5x10-6 sec respectively)

2. In Neon atom there exist six active energy levels namely; Ne1, Ne2, Ne3, Ne4, Ne5 and Ne6 of

these levels Ne4 and Ne6 are Meta stable states.

3. When a voltage of about 1000 V applied between electrodes, then electric discharge takes place

through the gas tube. The free electrons accelerated towards the anode and in their journey

they collide with ground state Helium atoms and excite them to He2 and He3 Meta stable

states. In these states the atoms remain relatively long time. N

4. From the energy level diagram it can be seen that the energy levels of He2 & He3 of Helium are

very close to the energy levels of Ne4 & Ne6 in Neon atoms.

5. The Helium atoms in the energy levels of He2 & He3 may make collisions with the Neon atoms

present in the Ground state and excite them to Ne4 and Ne6 levels.

6. The resonance transfer of energy takes place from He to Ne atoms because Ne4 & Ne6 energy

levels of Ne atom are very close to He2 & He3.

7. As the energy exchange continues, the population inversion will be achieved between Ne6 and

(Ne5, Ne3) groups of levels & also between Ne4 and Ne3 levels in Ne.

8. Transition between Ne6 to Ne4 and Ne5 to Ne3 levels are forbidden by quantum mechanical

selection rules (Δl=±l).

Various Possible Transitions in Ne energy levels:

a. The transition from Ne6 to Ne5 levels give rise to radiation of wavelength 3.30µm which is in

Infrared region.

b. Transition from Ne6 to Ne3 gives rise to visible radiation of wavelength 6328Ao.

c. Transition from Ne4 to Ne2 gives rise to a wavelength of 1.15µm which is in Infrared region.

d. Transition from Ne3 to Ne2 gives rise to a wavelength of 6000Ao

Finally Ne atoms de-excite to the Ground state by making collision with wall of the discharge tube.

The emitted photon from the levels Ne6 to Ne3 reflects back & forth by reflectors and stimulates

other excited Neon atoms causes to emit a fresh photon in phase with the stimulated photon.

This process is continued and a laser beam builds up in the tube. When the beam becomes

sufficiently intense, a portion of it escapes from the partially reflected end.

APPLICATIONS:

1. Laser Printing, bar code reading, Interferometers, as pointing and directional reference beams

and used in three dimensional recording of objects called Holography.

2. For precise measurements over a long distance

3. For Surveying.

SEMICONDUCTOR LASER Fermi level: It is the energy level with 50% probability of electron occupancy in a given material at any temperature other than absolute zero. Or, it is the top most filled electron energy level at absolute zero in the given material. Direct band gap semiconductors: In the energy band diagram of a semiconductor, with wave vector (k) on x-axis, if the minima of the conduction band valley falls at the same k value as that of the maxima of the peak of valence band, then the given semiconductor is said to be a direct band gap semiconductor. Ex: GaAs, InP, AlGaAs, etc. compound semiconductors made from III and V group elements or by combining II and VI group elements of the periodic table. Indirect band gap semiconductors: If the peak and valleys respectively of the valence and conduction bands fall at two different k values, then it will be an indirect band gap semiconductor. Ex: silicon and germanium (elemental semiconductors) Advantages of Direct band gap semiconductors over indirect band gap semiconductors:

1. Possibility for direct recombination of hole with electron with simultaneous emission of radiation. (Radiative recombination)

2. Possibility for increase or decrease of energy band gap and hence choice for obtaining desired wavelength of radiation.

Degenerate and non-degenerate semiconductors:

If the Fermi level of a given semiconductor exists within the forbidden energy gap, not closer than 3kBT from both valence and conduction band edges, then it will be a Non – Degenerate Semiconductor. If the Fermi level of a given semiconductor either penetrates into the conduction or valence band or falls within a gap of 3KBT from any of the conduction(EC) or valence band(EV) edges (within the forbidden gap, Ec - 3kBT or EV+3kBT), then the semi conductor is said to be a Degenerate Semiconductor. The Homojunction Laser: In the Homojunction laser, a degenerate semiconductor like GaAs is used for making the pn junction. Hence the junction is formed by same material with two different doping materials, one for p-type and the other for n-type. The donor and acceptor concentrations are so large (1018 atoms/cm3) that the Fermi levels fall in the valence band for the p-type, EFp, and in the conduction band for the n type, EFn. When a junction is formed, and if no voltage is applied, the band structure will be as shown in Fig, where the two Fermi energies are seen to be the same. When a forward bias voltage is applied, the band structure becomes as shown in Fig below. The two Fermi levels become separated by EG eV. We see from this figure that, in the junction region, electrons are

injected into the conduction band (from the n-type region) while holes are injected into the valence band (from the p-type region). Thus, under appropriate values of current density, the laser threshold condition can be reached. The electron can penetrate into the p-type material and becomes a minority carrier thus recombining with a hole. The penetration depth, d, of the electron in GaAs is 1µm. the thickness of junction is only 0.1 µm. Hence, the recombination process takes place up to 1µm distance. But the junction (active region) is only 0.1 µm. Due to this most of the injected current energy is wasted in the production of unwanted heat away from the junction. A typical configuration of a p-n junction laser is shown in Fig, the shaded region corresponding to the active layer. The diode dimensions are very small (some hundreds of microns). To provide feedback for laser action, two parallel end faces are prepared, usually by cleavage along crystal planes. Often the two surfaces are not provided with reflective coatings. The refractive index of a semiconductor is very large (3.6 for GaAs), there is already a sufficient high reflectivity (32% for GaAs) from the Fresnel reflection at the semiconductor-air interface. Thickness of the active region in the direction perpendicular to the junction is 1µm. Because of diffraction the transverse dimension of the laser beam in this direction 5µm which is significantly larger than the active region. A Homojunction laser has a very high threshold current density at room temperature J=105 A/cm2 which prevents the laser from operating in continuous wave mode at room temperature (without suffering destruction in a very short time!). There are two main reasons for this high threshold value: (1) the thickness of the active region is quite large and the threshold current, being proportional to the volume of the active medium, is proportional to this thickness. (2) The laser beam has comparatively large transverse dimensions, extends considerably into the p and n regions, where it is strongly absorbed. Given the above reasons, Homojunction lasers could only operate continuous wave at cryogenic temperatures (typically at liquid nitrogen temperature 77 K). Contact of the diode with liquid nitrogen helps to give a very efficient cooling. The Heterostructure Laser: The limitations discussed above are prevented by the Heterostructure laser. The figure shows a Heterostructure laser, where the active medium (hatched area) consists of GaAs and InGaAsP. The active medium is a thin layer 0.1–0.2µm of either GaAs or of the quaternary alloy InGaAsP. For the two cases considered here the p and n sides are made of Al0.3Ga0.7As and InP, respectively. When properly optimized the room-temperature threshold current-density of such a diode structure can be reduced to 103 A/cm2.Hence cw room temperature operations feasible. This strong reduction of threshold current density is due to the combined effect of two circumstances: (1) The refractive index of the active layer is 3.6 for GaAs and is significantly larger than that of the p-side and n-side layers (3.4 for Al0.3Ga0.7As), thus providing a guiding structure (total internal reflection). This means that the laser beam will now be mostly confined to the active layer region, (photon-confinement). (2) The band gap EG of the active layer 1.44 eV in GaAs is significantly smaller than that of the cladding layers, EG=1.8 eV for Al0.3Ga0.7As. Energy barriers are thus formed at the two junction planes thus effectively confining the injected holes and electrons within the active layer. Thus, for a given current density, the concentration of holes and electrons in the active layer is increased and therefore the gain is increased.

PUMPING MECHANISMS

There are several ways of pumping a LASER. Most commonly used methods are

(a) Optical Pumping:

In this method, light source is used to supply luminous energy.

Most of the energy comes in the form of short

flashes of Light.

This method was first used by Maiman in his

solid state Ruby laser.

The laser material is simply placed inside a

helical Xenon flash lamp of the type used in photography.

(b) Electric Discharge method:

This type of pumping method is mostly preferred in

gaseous lasers.

The electric field typically several kV/m, causes the

electrons emitted by the cathode to accelerate

towards an anode

Some of these electrons will collide with the atoms

of the active medium, ionize the medium and raise

it to the higher energy levels. This produces population inversion.

Ex : Argon laser.

(c) Inelastic-atom-atom-collisions:

This type of pumping method is used in gas lasers consisting of two species of

atoms.

The electrical discharge raises one type of atoms to their excited state and these

atoms collide inelastically

with another type of atoms and excited them to corresponding energy levels.

Thus Population inversion is achieved.

Ex: Helium-Neon Laser.

(d) Direct conversion method:

This type of method is used in semiconductor pn

junctions, electrons

and holes are made to recombine across the

depletion region by

applying a forward bias to the semiconductor

diode.

Electrons and holes recombine to emit radiation.

Thus direct conversion of electrical energy into radiation occurs in light emitting

diodes and semiconductor lasers.

(e) Chemical reactions method:

In chemical laser the energy comes from a chemical reaction without any need for

other energy source. Hydrogen can react with fluorine to produce hydrogen fluoride

according to the relation

H2+F2 -> 2HF +heat

This reaction generates enough heat to pump a Co2 laser.