Unit V

1. Unit V Lasers

2. Objectives • Characteristics, construction and working principle of He-Ne, and semiconductor laser • Definition of stimulated and spontaneous emission • Einstein coefficients, Requisites for laser system and conditions for laser action • Applications of Laser in welding, cutting, drilling and measurement of atmospheric pollutants • Holography – recording and reconstruction of 3-D images and applications

3. Introduction • Laser - Light Amplification by Stimulated Emission of Radiation • Albert Einstein In 1917, predicted that there are two kinds of light emissions, namely spontaneous and stimulated emission • Charles Towner In 1960 - Stimulated emission for first time at Microwave frequencies – MASER • Theodore Maiman – LASER • Ali Javanand his co-workers (1962)- He-Ne Laser

4. Characteristics of Laser Characteristics of lasers which are different from ordinary incoherent light are : • Directionality • High intensity • Monochromacityand • High degree of coherence

5. Directionality • The directionality of the laser beam is generally expressed in terms of full angle beam divergence which is twice the angle that the outer edge of the beam makes with the axis of the beam. • The outer edge is defined as a point at which the intensity (I) of the beam drops to 1/e times its value at the centre. • The full angle divergence in terms of minimum spot size of radius w0 is given by  = 1.27 λ / 2w0 • If a1 and a2 are the diameters of laser radiation at distances d1and d2from a laser source respectively, then the angle of beam divergence in degrees is given by  = (a2 – a1) / 2(d2 – d1)

6. Intensity • A laser emits light radiation into a narrow beam, and its energy is concentrated in a small region. This concentration of energy both spatially and spectrally accounts for the great intensity of lasers. Monochromatic • The light from a laser source is highly monochromatic compared to light from a conventional incoherent monochromatic source. The monochromacityis related to the wavelength spread of radiation given by • Δλ = (-c/f2) Δf

7. Coherence Spatial Coherence (Transverse coherence): how far apart two sources or two portions of the same source can be located in a direction transverse to the direction of observation and still exhibit coherent properties over a range of observation points Temporal Coherence (Longitudinal coherence): the coherence of two waves at two separate locations along the propagation direction of the two beams Coherence length (lc): Distance up to which two sources of light maintain the same phase. The relationship between coherence length, wavelength and wavelength spread is given by

8. Principle of production of Laser • Induced Absorption Atom + Photon ==> Atom*

9. Spontaneous Emission • Atom* => Atom + Photon

10. Stimulated Emission Atom* + Photon => Atom + 2 Photon

11. Einstein’s coefficients (expression for energy density) • Induced absorption per unit time = N1u(f)B12 • Spontaneous emission per unit time = N2A21 • Stimulated emission per unit time= N2u(f)B21 At thermal equilibrium, the number of upward transitions per unit time must be equal the number of downward transitions per unit time i.e., N1u(f)B12= N2A21 + N2u(f)B21

12. u(f) = N2A21 / [N1B12 - N2B21] -- (5.7) Dividing throughout by N2B21 , Eq. 5.7 becomes u(f) = [A21 / B21] / [(N1B12/N2B21) – 1] -- (5.8) According to Boltzmann distribution law, the number of atoms N1and N2in energy states E1and E2 in thermal equilibrium at temperature Tare given by ---- (5.9) where hf = E2 – E1

13. Using Eqn. (5.9) in Eqn. (5.8) we get, u(f) = [A21 / B21] / The above equation is the expression for energy density in terms of Einstein’s coefficients. Comparing the above equation with the Planck's radiation formula given by Comparing above two equations we get

14. Requisites of a Laser system • Population Inversion: – No of atoms in excited state should be larger than ground state • Pumping: Population inversion can be achieved by exciting the laser medium with a suitable form of energy • Optical pumping • Electrical discharge • Inelastic atom-atom collision • Direct conversion • Chemical reactions

15. Components & Condition for Laser action • (i) The pumping source • (ii) The active medium • (iii) The optical resonator

16. He-ne Laser • First continuous wave (CW) laser • Invented by AliJavan and his co-workers in 1961 • Four level laser and usually constructed to operate in the red at 632.8 nm

17. P- and N-type Semiconductors • In the compound GaAs, each gallium atom has three electrons in its outermost shelland each arsenic atom has five. When a trace of an impurity element with two outer electrons, such as zinc, is added to the crystal. The result is the shortage of one electron from one of the pairs, causing an imbalance in which there is a “hole” for an electron but there is no electron available. This forms a p-type semiconductor. • When a trace of an impurity element with six outer electrons, such as selenium, is added to a crystal of GaAs, it provides on additional electron which is not needed for the bonding. This electron can be free to move through the crystal. Thus, it provides a mechanism for electrical conductivity. This type is called an n-type semiconductor.

18. Semiconductor laser-Gallium Arsenide Laser • Active medium is formed by semiconducting material • Fabry-Perot cavity

21. Applications of Laser • Laser welding : High-power lasers such as CO2, Ruby laser, Nd:YAG lasers are important lasers used for accurate welding of materials. Applications: • electronics and microelectronics which require precise and accurate spot welding of very thin wires of 10-11m thickness or welding of two thin films together. • Advantages : (a) shorter duration for welding (b) welding can be done in regions adjacent to heat sensitive areas without affecting those elements (c) welding in inaccessible areas like inside a glass envelope can be done. • Materials used for laser welding Carbon and stainless steels • Aluminium and its alloys • Nickel and its alloys • Titanium and its alloys • Copper and its alloys

22. Laser cutting • For high precision cutting, high power lasers such as a carbon dioxide laser or Nd:YAG laser are used. • The efficiency of laser cutting may be improved by using a gas jet co-axial with the laser • Using a 200 W CO2 laser with oxygen jet, it is possible to cut a stainless steel sheet of 0.13 cm thickness at a rate of 0.8 m/min. Examples: low-carbon steel, stainless steel, titanium

23. Advantages of Laser cutting • Cutting edges are tight and parallel • Reduced Heat Affected Zone • Possibility to operate on complex profiles and reduced curving radius • Absence of mechanical distortion of the laser worked piece • No influence of the hardness of the material • No problems to cut materials previously coated

24. Laser drilling • Drilling of holes in different substances such as metals, ceramics, plastics, cloth, paper, glass etc., is another important application of the laser. • In conventional methods for drilling holes, less than 250 μm diameter is very difficult and some times it leads to the breakage of materials. • Using laser, even in the hardest materials holes of 10 μm diameter may be drilled easily and precisely • The pulsed Nd:YAG lasers, Ruby lasers and CO2 CW lasers are normally used for this purpose. • Hard and brittle materials (like diamond, ceramics, stainless steel, ferrites), and softer materials (like rubber, glass, plastics) have been processed alike with lasers

25. Measurement of atmospheric pollutants Laser is a very useful tool for the measurement of the concentrations of various atmospheric pollutants such as • Hydrocarbons like Alkanes, olefins (emitted during non-complete combustion of oil-derivative products), • CO (carbon monoxide, constituting around 2/3 of all volatile poisons originating from motor exhaust fumes), • NO2 and SO2(emitted during combustion of fossil fuels mainly coal), and • Particulate matter (any solid or liquid matter dispersed in the atmosphere) such as dust and smoke.

26. Measurement of atmospheric pollutants • Most of the techniques used for this purpose of studying pollutants are based on the principle of light absorption or scattering. Principle : • The primary influence of the atmosphere on laser beam is through scattering and absorption. Both processes cause an attenuation of the beam according to Beer’s law : where I is the intensity of the optical beam after transmission over a distance R, is the atmospheric extinction coefficient, and I0 is the initial intensity of the beam

27. Rayleigh scattering is due to particles in the atmosphere, such as molecules or fine dust, that are much smaller than the optical wavelength 𝛌, of the laser. • Mie scattering is associated with larger particles such as aerosols whose size is on the order of 𝛌. • Rayleigh and Mie processes are elastic scattering, i.e., the scattered light is the same wavelength as the incident laser beam. • Raman scattering is an inelastic interaction of the optical beam involving excitation of energy levels of a molecule and re-radiation at a different wavelength.

28. Measurement Techniques • The laser techniques, which uses the principle of light scattering by pollutants is called Light Detection and Ranging (LIDAR). • In this technique, a pulsed laser beam is used as the source of light and the light scattered back by the atmospheric sample is detected by means of a photodetector. • The principle of LIDAR is very much similar to that of RADAR except that microwave radiation is replaced by laser radiation. • It is mainly useful in knowing the distribution of atmospheric pollutants in different vertical sections and in monitoring their variations. • This technique allows environmental agencies to measure concentrations of harmful gases such as SO2 and NO2. • LIDAR technique is also used to measure Ozone concentrations in the lower troposphere and for climate monitoring.

29. The second technique uses the principle of absorption of laser beam by pollutants. The presence of specific gases/pollutants in the atmosphere can be detected using absorption spectroscopy techniques. A laser beam is transmitted through polluted sample and the attenuation of intensity of light due to absorption in the sample is detected and recorded. Each pollutant absorbs light of characteristic wavelengths and from the absorption spectrum, its existence can be determined. • A third technique uses Raman scattering to detect the pollutants. The Raman scattering involves scattering of laser light by gas molecules accompanied by a shift in the wavelength of scattered light. Raman shifts are characteristic of each molecular species present in the atmosphere. Hence, analysis of back scattered laser light reveals the constituents of the pollutant.

30. Holography • Holography is a technique for recording and reproducing an image of an object without the use of lenses • Discovered by Dennis Gabor • This technique uses the principle of interference to record both the amplitude and phase of a wave/particle The permanent record of the interference pattern on the recording medium is called a hologram. A hologram can be viewed from different directions to reveal different sides, and from various distances to reveal changing perspective.

31. Principle of Holography The process of generating holograms consists of two stages : (i) Recording of the hologram, and (ii) Reconstructing the image

32. Applications of holography • Holographic interferometry The technique called double exposure holographic interferometry and is proved to be highly useful in nondestructive testing of objects. In this technique, the undisturbed object is first recorded on the photographic plate with an exposure to a reference wave. Then, the object is stressed and is recorded on the same photographic plate through a second exposure along with the same reference wave. After this double exposure, the hologram is developed. If the hologram is now illuminated by a reconstruction wave, there would emerge from the hologram two object waves - one corresponding to the unstressed object and the other corresponding to the stressed object.

33. Acoustical holography • Light waves cannot propagate considerable distances in dense liquids and solids where as sound waves can propagate through them. Therefore a three dimensional acoustical hologram can be recorded using ultrasonic waves initially and then visible light can be used for reconstruction of the visual image of an opaque object. By viewing such hologram in visible light the internal structure of the object can be observed. Such techniques will be highly useful in the fields of medicine and technology. As sound waves can propagate through dense liquids and solids, acoustical holography has an advantage in locating underwater submarines etc., and study of internal body organs.

34. Data storage • A large amount of information such as 1012 bits can be stored in a cubic cm of a volume hologram. These memories have long lifetime because a small mechanical damage to the portion of a hologram will not erase the stored information.

35. Three-dimensional photography • Holographic technique is used in the production of a three-dimensional photograph, with the distance and orientation of each point of the object recorded in the image.

36. Microscopy • Holography can be used in microscopy to obtain a magnified image of an object. In such cases the recording is done with light of smaller wavelength and reconstruction of the image is done with light of longer wavelength. Smaller areas in an object can be examined in greater detail. This has great potential in observing micro-objects such as blood cells, amoebas, cancer affected tissues etc.

37. Holographic optical elements (HOE) • Holography can be used to make diffraction gratings known as holographic optical elements (HOEs). They function like a lens. They are less efficient than ruled gratings. HOEs are compact, lightweight and inexpensive. The scanners used for reading the bar code on the items in super market are made by HOEs.

38. Character recognition • Photolithography: Holography is used in the production of photographic masks used to fabricate microelectronic integrated circuits.