Lasers hasnain sayed
- 1. LASERS 1 Mode Locking General principle of laser action Population Inversion Cavity and Mode characteristics Q-switching SAYED MOHD HASNAIN 19MSC008 JAI HIND COLLEGE
- 2. General principle of LASER action LASER - Light Amplification by Stimulated Emission of Radiation Highly Directional, Intense, Monochromatic and Coherent Optical Sources Invented by “Theodore H. Maiman” – 1960. Lasers have also revolutionized research in physical chemistry. Using lasers, chemists can measure the spectra and photochemical dynamics of molecules with high spectral or time resolution. The generation of laser light depends on the rates at which the excited atoms or molecules decay back to their ground states . The essential feature of laser action is positive-feedback. 2
- 3. Induced absorption Spontaneous Emission Induced or Stimulated Emission 3
- 4. Population Inversion • Requirements – i. Existence of Metastable Excited state. ii.Existence of greater population in the Metastable Excited state • Because at thermal equilibrium the opposite is true, it is necessary to achieve a population inversion in which there are more molecules in the upper state than in the lower 4
- 5. Three-level LASER transition The molecule is excited to an intermediate state I, which then gives up some of its energy nonradiatively and changes into a lower state A; the laser transition is the return of A to the ground state X. The I ← X transition is stimulated with an intense flash of light in the process called pumping. The conversion of I to A should be rapid, and the laser transitions from A to X should be relatively slow. • Disadvantage – Difficult to achieve Population Inversion 5
- 6. Four Level LASER transition The arrangement adopted in a four-level laser have the laser transition terminate in a state A′ other than the ground state. Because A′ is unpopulated initially, any population in A corresponds to a population inversion, and we can expect laser action if A is sufficiently metastable. Moreover, this population inversion can be maintained if the A′ ← X transitions are rapid. • Advantage – Population Inversion can be maintained 6
- 7. Cavity and Mode characteristics Region between two mirrors that reflects the light back and forth. The only wavelength that can be sustained should satisfy the equation 𝒏 ∗ 𝝀 𝟐 = 𝑳 Where, n is an integer 𝝀 is wavelength L is length of the cavity Not all wavelengths that can be sustained by the cavity are amplified by the laser medium (many fall outside the range of frequencies of the laser transitions), so only a few contribute to the laser radiation. These wavelengths are the resonant modes of the laser. 7 Full mirror
- 8. Photons with the correct wavelength for the resonant modes of the cavity and the correct frequency to stimulate the laser transition are highly amplified. One photon might be generated spontaneously, and travel through the medium. It stimulates the emission of another photon, which in turn stimulates more The cascade of energy builds up rapidly, and soon the cavity is an intense reservoir of radiation at all the resonant modes it can sustain. Only photons that are travelling strictly parallel to the axis of the cavity undergo more than a couple of reflections, so only they are amplified, all others simply vanishing into the surroundings. Hence, laser light generally forms a beam with very low divergence. 8
- 9. Q-switching • LASER can be operated in pulses to avoid Overheating • Q-switching - Obtain giant pulse formation by which laser can be made to produce pulsed output beam • The aim of Q-switching is to achieve a healthy population inversion in the absence of the resonant cavity, then to plunge the population-inverted medium into a cavity, and hence to obtain a sudden pulse of radiation. • In practice, Q-switching can give pulses of about 5 ns duration. 9
- 10. Mode Locking The technique of mode locking can produce pulses of picosecond duration and less.(10-12 to 10-15 seconds) • Range of duration is A laser radiates at a number of different frequencies. • Normally, these modes have random phases relative to each other. However, it is possible to lock their phases together. Then interference occurs to give a series of sharp peaks, and the energy of the laser is obtained in short bursts. • In a laser with a cavity of length 30 cm, the peaks are separated by 2 ns. If 1000 modes contribute, the width of the pulses is 4 ps. Mode locking is achieved by varying the Q-factor of the cavity periodically at the frequency c/2L 10
- 11. References P . W. Atkins, Physical Chemistry, Oxford University Press, 6th edition, 1998 LASER Fundamentals, Book by William T. Silfvast LASERS- McQuarrie & Simon.pdf 11