Light amplification by stimulated emission of radiation

A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation.

1. DATTA MEGHE COLLEGE OF PHYSIOTHERAPY DR.SHWETA PANCHBUDHE ASSOCIATE PROFESSOR (DMCOP)

2. LASER MACHINE

3. LASER LASER is an acronym or light amplification by stimulated emission of radiation. Radiation is the process by which energy is propagated through space The common characterisctics of all forms of radiant energy are:

4. They are produced by applying electrical or other forces to various forms of matter They all may be transmitted without the support of a sensible medium Their velocity of travel is equal in a Vacuum but may vary within different media

5. The direction of propagation is normally a straight line. They undergo reflection, deflection, and absorption by the media through which they travel. The low power laser is used in physical rehabilitation for pain control and soft tissue injury.

6. Low intensity laser therapy (LILT) is the application of light to injuries and lesions to stimulate healing in tissues. It is believed to resolve inflammation , reduce pain, increased speed, quality and strength of tissue repair and resolve infection.

7. CHARACTERISTICS OF LASER EMISSION MONOCHROMATICITY: Ordinary light consists of many wavelength, commonly known as VIBGYOR i.e violet ,indigo, blue, green, yellow, orange and red which merge to form white light But laser consists of one wavelength only, in a therapeutic unit, the band is 6328A UNITS The laser of 6328A units is of bright red color.

8. COHERENCE Because the wavelength of ordinary light are so variable and do not match in waveforms, frequencies or shapes. There is much scrambling of waveforms, cancellations and reinforcement of individuals waves and interference in the production of energy

9. This factor minimize the power of ordinary light as an energy source. The identical wavelength and forms that comprise laser light cause it to be greatly amplified. Since the waves and troughs of the radiation are reinforced because they are parallel and in line with each other they are termed coherent.

10. COLLIMATION As a consequence of spatial coherence lasers remains in a parallel beam because the radiations do not diverge the energy even if propagated over long distance

11. TYPES OF LASER RUBY LASER This consists of a small synthetic ruby rod made of aluminium oxide A helical xenon flash tube, wound around it gives an intense flash of white light

12. Both end of the rod are made flat and silvered one end being totally reflecting and the other partially transparent so that some radiations can be emitted This brief light pulse (0.5)ms excites the ruby molecules and raises many electrons to higher levels which they occupy for very short average time before falling to the metastable level where they remain for much longer average time.

13. Thus for a time there are many electrons in the metastable level than the ground level and so population inversion occurred. When the transition from the metastable to ground state does occur a photon with a wavelength of 694.3nm is emitted.

14. This photon would have exactly the right energy to raised state electron to the metastable level and be reabsorbed. But there are relatively few ground state electrons the photon is much more likely with other metastable electrons causing them to return to the ground state and so emitting an identical photon

15. The process rapidly accelerates as more and more photons are released i.e stimulated emission of radiation occurs. The photons having a wavelength of 694.3nm which is of course red light are reflected up and down the short ruby rod rapidly increasing the effect

16. Thus all the energy stored in the ruby molecules is released in a very brief time as a pulse of red light of identical photons & single wavelength of coherent radiations This emerges from the rod at the partially transparent end.

17. HELIUM –NEON LASERS Heliun neon lasers consists of a long tube containing these natural gases at low pressure surrounded by a flashgun tube as described for the ruby laser. Excitation of these atoms leads to a different energy level between them and the transfer of energy giving off a photon of wavelength equivalent to the energy gap.

18. The photons are reflected to and fro along the tube , giving rise to further photon emission and emerging as a narrow beam from the partially transparent end. Helium neon lasers give radiation in the red visible region at 632.8nm

19. The output is usually applied to the tissues via an optical guide-a fibreoptic cable-the end of which is held in contact with the tissues Some energy losses in the glass fibre of the cable and the laser beam may diverge to some extent as it emerges at the end of the optical fibre.

20. DIODE LASERS These are specialized light emitting diodes based on semiconductor p-n junctions They are of various kinds involving gallium aluminium arsenide . In these electrons can flow more readily in one direction than in the other.

21. The electrons are excited by the application of a suitable electrical potential and their occupation of holes in the crystals lattice arrangement mat lead to the emission of a photon The photons are reflected to and fro and emitted as a laser beam from one partially transparent end

22. These are conveniently small, relatively cheap and robust devices . By varying the ratio of gallium to aluminium such devices can be built to emit specific wavelength Semiconductor laser diodes can give either a continuous or a pulsed out

23. Continuous wave diode lasers are usually of relatively low power. Alternatively they can be pulsed electronically On some machines the pulse frequency can be varied.

24. MECHANISM To obtain the laser a tube filled with a gaseous mixture of helium and neon is stimulated electrically to emission levels. Within the highly reflective polished walls of the tube the molecules reverberate the carrom off the walls in a highly agitated state building energy

25. When a critical level is reached the flow of the energy literally bursts, through the semi-silvered front end of the tube and is channelled along an optic fibre to the beam applicator

26. MODES CONTINUOUS BEAM The continuous mode is recommended for acute pain and fresh wounds PULSE BEAM The pulse mode has been found more effective with chronic conditions Pulse models vary from 1 to 80 pulses per second