SEMINAR ON : CO2 & N2 LASER

GUIDE : PROF. (DR.) PRAMOD GOPINATH

BY: KESHAV KUMAR JHAM.TECH (1 S T YEAR), OE & LT

International School of Photonics

IntroductionThe co2 laser was one of the earliest gas laser

invented by Kumar Patel of Bell Labs in 1964 is one of the most useful lasers.

Carbon dioxide gas lasers are the highest power continuous wave lasers that are currently available. They are also quite efficient, the ratio of output power to pump power can be as large as 20%.

The co2 laser produces a beam of infrared light ie. This laser is not visible with bare eyes, its principle wavelength bands centring around 9.6 and 10.6 micron.

Principle of CO2 LasersFor the laser action two points are

important one is the population inversion between the two levels and second increased density of the incident radiation.

Principle of a CO2 laser is transition between vibrational states of the same electronic state by achieving population inversion between these states.

Vibrational states of CO2 molecule

Laser Structure and Discharge Mechanism

It consist of discharge tube of size 2.5cm in diameter and 5.0cm in length. Two optically plane and parallel mirrors, one is completely reflector and another is partially reflector.

The discharge is filled with a mixture of CO2, N2 and He gases at a pressure of few mm of mercury.

A high value of DC voltage is used for electric discharge in the tube due to which CO2 molecules breaks into CO & O so periodic replacement of gas is required, O2 can corrode the electrodes.

Discharge Diagram

Working of CO2 Laser

The vibrational and rotational modes of the CO2 can’t be excited themselves by photons ie. by optical pumping.

When a voltage is placed across the gas, electrons collide with the N2 molecules and excite them to their vibrational levels.

These vibrational levels happen to be at an energy very close to the energy of the asymmetric vibrational states in the CO2 molecules. Now the excited N2 molecule populates the asymmetric vibtarational states in the CO2 molecules through collisions.

Energy Transition Diagram

Energy Transition Diagram

Working of CO2 laser

Te infrared output of the lasers is the result of transitions between rotational states of the CO2 molecule of the first asymmetric vibrational mode ( 0 0 1 ) to rotational states of both the first symmetric stretch mode ( 1 0 0 ) and second bending mode ( 0 2 0).

TEA CO2 Laser

These laser systems having gas pressure of 1 ATM or more to obtain higher energy output per unit volume of gas.

In longitudinal discharge scheme, operation at high pressure is extremely difficult because extremely high voltage is required to ionize the gas and to start discharge process.

There is always a chance to form arc between electrodes in longitudinal discharge if gas is not completely ionized, it is due to longer distance between the electrodes.

Transverse ExcitationThese shortcomings has been avoided in

transversely excited lasers as two electrodes are parallel to each other over the length of the discharge separated by few centimetres or more.

Before discharge to take place a form of pre-ionization is used to ionize the space between electrodes uniformly and thereby to fill it with electrons. By this pre-ionization a uniform discharge will take place throughout its length otherwise narrow arcing can occur, we can understand it as lightning between electrodes.

•UV spark is used for pre-ionizing the gas.

Now on the application of high voltage across electrodes lasers are produced. One of the advantage of this configuration is many Joules of energy can be produced for each litre of discharge volume.

This configuration has resulted in some of the highest energy pulsed lasers yet produced.

Transverse Excitation Atmospheric

Application

The advantage of using a laser for these applications is that very intense heating source can be applied to a very small area.

Because of the high power CO2 laser are frequently used in industrial applications for cutting and welding.

They are also very useful in surgical procedures because water absorbs this frequency of light very well.

Some examples of medical uses are laser surgery, skin resurfacing(laser facelifts).

Nitrogen Lasers

The nitrogen laser is gas discharge laser that produces ultraviolet laser output at 337.1nm.

This laser produced from N2 mlecules which involves a change in both electronic and vibrational energy levels.

Pressure which is applied on N2 molecule ranges from 20torr(1torr =1/760mm of Hg) to atmospheric pressure in a sealed chamber.

• The laser has high gain and thereby produces a highly multimode output that is useful for pumping dye lasers.

N2 lasers produce peak power outputs of up to 100KW over a duration of 10ns corresponding to a pulse energy of 100mJ per pulse.

With development of excimer lasers that produces several hundred miliJoules per pulse at a much higher efficiency, N2 laser as seen a significant decrease in its usage in recent years.

Laser Structure and Excitation Mechanism

Excitation of N2 laser is same that is used in TEA CO2 lasers. There is transverse electrodes in which N2 is filled at high pressure. The complete setup comes in a chamber and transverse electrodes and gas accommodation is sealed.

The upper laser level lifetime is very short only 40ns, so an extreme rapid pumping is required which is done via direct pumping from N2 ground state.

• The lower laser level lifetime is much longer 10micro seconds w.r.t. Lifetime of upper laser level, so this N2 laser is self terminating type i.e. N2 molecules get relaxed by their own without any external agent.

The laser output duration ranges from 2 to 20ns.

A high reflecting rear and 4% reflecting mirror is used as feedback to continue stimulated emission.

The discharge system and other constructional features are same as that of CO2 TEA laser.

Efficiency

The wall plug efficiency of N2 laser is very low, typically 0.1% or less, though nitrogen laser with efficiency 3% has been reported.

Application of N2 Lasers

Transverse optical pumping of dye lasers.Measurement of air pollution.

THE END Thank You