B.SC.II PAPER-B (OPTICS and LASERS)

1. B.SC.IIPAPER-B (OPTICS and LASERS) Submitted by Dr. Sarvpreet Kaur Assistant Professor PGGCG-11, Chandigarh

2. Unit-IV Lasers and Fiber optics

3. Laser Types • According to the active material: solid-state, liquid, gas, excimer or semiconductor lasers. • According to the wavelength: infra-red, visible, ultra-violet (UV) or x-ray lasers.

4. Solid-state Laser • Example: Ruby Laser • Operation wavelength: 694.3 nm (IR) • 3 level system: absorbs green/blue • Gain Medium: crystal of aluminum oxide (Al2O3) • with small part of atoms of aluminum is replaced • with Cr3+ ions. • Pump source: flash lamp • The ends of ruby rod serve as laser mirrors.

5. How Ruby laser works?

6. 1. High-voltage electricity causes the quartz flash tube to emit an intense burst of light, exciting some of Cr3+ in the ruby crystal to higher energy levels. 2. At a specific energy level, some Cr3+ emit photons. At first the photons are emitted in all directions. Photons from one Cr3+ stimulate emission of photons from other Cr3+ and the light intensity is rapidly amplified.

7. 3. Mirrors at each end reflect the photons back and forth, continuing this process of stimulated emission and amplification. 4. The photons leave through the partially silvered mirror at one end. This is laser light.

8. Gas Laser • Example: Helium-neon laser (He-Ne laser) • Operation wavelength: 632.8 nm • Pump source: electrical discharge • Gain medium : ratio 5:1 mixture of helium and neon gases

9. He-Ne laser

10. Apparatus(CO2 laser) Water Power Meter Chopper Voltage Supply Laser Mirror Cathode Anode Grating w/Motor Air Pump Gas Tank To drain

11. Theory • When lasing begins at just under 15,000 Volts, electrons bombard the N2, but they cannot radiate. • They excite the CO2 molecules to vibrational states, because their energies are very close to the required level for the asymmetric vibrational state in CO2

13. More Theory • The CO2 laser is characterized by the vibrational and rotational transition states in the CO2 molecule, and the molecules act like simple harmonic oscillators in three distinct ways.

15. Yet More Theory • A molecule in a vibrational state also has associated with it many rotational states. Those states have degeneracy 2J+1 and account for the miniature peaks in the data acquired. • To see all of these miniature peaks, a chopper must be added to avoid lock-in (Milloni)

16. Semiconductor laser Semiconductor laser is a laser in which semiconductor serves as photon source. Semiconductors (typically direct band-gap semiconductors) can be used as small, highly efficient photon sources.

17. Semiconductor Lasers • Laser diode is similar in principle to an LED. • What added geometry does a Laser diode require? An optical cavity that will facilitate feedback in order to generate stimulated emission. Fundamental Laser diode: 1. Edge emitting LED. Edge emission is suitable for adaptation to feedback waveguide. 2. Polish the sides of the structure that is radiating. 3. Introduce a reflecting mechanisn in order to return radiation to the active region. 4.Drawback: low Q due to excessive absorption of radiation in p and n layers of diode. Remedy: Add confinement layers on both sides of active region with different refractive indexes.Radiation will reflect back to active region.

18. Laser Diodes 5. Polishing of the emitting sides of the cavity. A considerable percentage of the radiation is reflected back alone from the difference in reflective indexes of the air-AlGaAs interface. Therefore mirror coating not necessary. Note: radiation propagates from both sides of the device. What function can a photodiode provide in the process? It is attached to the inactive side to serve as a sensor for the power supply in order to provide an element of control of the laser output.

20. Laser Diodes • Lasing occurs when the supply of free electrons exceeds the losses in the cavity. • Current through the junction and the electron supply are directly proportional. must be exceeded before laser action occurs. • Drawback of laser diode: Temperature coefficient.Threshold current increases with temperature. Possible shutdown. Remedy:1. Cooling mechanism. (cooling mount) 2. Constant current power supply with photodetector.

21. Laser Diode Action (intrinsics) Refer to diagram of degenerately doped direct bandgap semiconductor pn junction. Degenerate doping- where fermi level is ( ) on P-side is in the valence band (VB)and on the N-side is in the conduction band (CB). • Energy levels up to the the fermi level are occupied by electrons. • When there is no applied voltage the fermi level is continuous across the diode ( ) .

23. Laser Diode (intrinsics) • Space charge layer (SCL) is very narrow. • Vo (built in voltage) prevents electrons in CB (n+-side) from diffusing into CB of p+-side. • There is a similar barrier preventing hole diffusion from p+ to n+ sides. • Assuming an applied voltage (ev) greater than the bandgap energy, are now separated by ev. • eV diminishes barrier potential to 0 allowing electrons to flow into SCL and over to p+-side to establish diode current.

24. Laser Diodes (intrinsics) • A similar reduction in barrier potential for holes from p+-side to n+-side occurs. • Result  SCL no longer depleted.

25. Laser Diode (Population Inversion) Refer to Density of States. • More electrons in the CB at energies near Ec than electrons in VB near Ev. • This is the result of a Population Inversion in energiesnear EC and EV. • The region where the population inversion occurs develops a layer along the junction called an inversion layer or active region.

27. Laser Diode (stimulated emission) An incoming photon with energy of will not see electrons to excite from due to the absence of electrons at . The photon can cause an electron to fall down from . The incoming photon is stimulating direct recombination.

28. Laser Diode (stimulated emission) • The region where there is more stimulated emission than absorption results in Optical gain. • Optical gain depends upon the photon energy and thus wavelength (see density of states). Summary: *Photons with energy > Eg but < cause stimulated emission. *Photons with energy > are absorbed.

29. Laser Diode (pumping) What is the impact of a temperature increase on Photon energy? The Fermi-Dirac function spreads the energy distributions of electrons in the CB to above and holes below in the VB. Result: a reduction in optical gain. *Optical gain depends on which depends on applied voltage. In turn this depends on diode current.

30. Laser Diode (pumping) An adequate forward bias is required to develop injection carriers across a junction to initiate a population inversion between energies at and energies at . What is the pumping mechanism used to achieve this? Forward diode current. The process is called injection pumping.

32. Laser Diode (optical cavity) In addition to population inversion laser oscillation must be sustained. • An optical cavity is implemented to elevate the intensity of stimulated emission. (optical resonator) • Provides an output of continuous coherent radiation. • A homojunction laser diode is one where the pn junction uses the same direct bandgap semiconductor material throughout the component (ex. GaAs) See slide 3.

33. Laser Diode (optical cavity) The ends of the crystal are cleaved to a flatnessand the ends polished to provide reflection. • Photons reflected from cleaved surface stimulate more photons of the same frequency. • The of radiation that escalates in the cavity is dependant on the length L of the cavity.(resonant length) • Only multiples of ½ exist.

35. LaserDiode (modes) Separation between the potential modes that can develop, or allowed wavelengths, can be determined by the equation in the previous slide as . =>the output spectrum of the laser diode depends upon the nature of the optical cavity and optical gain versus wavelength. Note: lasing radiation occurs when optical gain in the medium can overcome photon losses from the cavity which requires diode current to exceed a threshold current . Light that exists below is due to spontaneous emission. Incoherent photons are emitted randomly and device behaves like an LED.

36. Laser Diodes(output) Lasing oscillations occur when optical gain exceeds photon losses and this is where optical gain reaches threshold gain at . • This is the point where modes or resonant frequencies resonate within the cavity. • The polished cavity ends are not perfectly reflecting with approximately 32% transmitting out of cleaved ends. • The number of modes that exist in the output spectrum and their magnitudes depend on the diode current.

37. Applications of laser • 1. Scientific a. Spectroscopy b. Lunar laser ranging c. Photochemistry d. Laser cooling e. Nuclear fusion

38. Applications of laser • 2 Military a. Death ray b. Defensive applications c. Strategic defense initiative d. Laser sight e. Illuminator f. Rangefinder g. Target designator

39. Applications of laser • 3. Medical a. eye surgery b. cosmetic surgery

40. Applications of laser • 4. Industry & Commercial a. cutting, welding, marking b. CD player, DVD player c. Laser printers, laser pointers d. Photolithography e. Laser light display