2 ans.docx
- 1. the name suggests semiconductor diode laser is basically a pn-junction diode. They are very compact and efficient lasers. They are electrically pumped. The dimensions of these diodes are less than a millimeter. The semiconductor diode laser emits light of wavelength ranging from 6000 A in red region to 16000 A in near infra-red region of the spectrum. To avoid scattering of carriers and light the semiconductor crystal must be defect free. Construction A semiconductor laser is made by forming a junction between p and n type materials in the same host lattice so as to form a pn - junction. Fig (3.18) shows a tiny block of gallium arsenide (GaAs). Electrical Metal Contact Contact Polished End P ype ntype Electrical Junction 1m Contact Laser Output Fig: 3.18 Semiconductor Diode Laser The interface between p and n type regions forms a pn -junction. The p side contains excess of holes andn side has excess of electrons. A layer of nmetal is given on the top as well as bottom face to provide extermal voltage to the laser. The two faces perpendicular to the laser axis are polished parallel to each other to constitute a Fabry-Perot resonator. Working The energy band diagram of a forward bias pn-junction with extremely heavy doping is shown in Fig. (3.19). With the number of electrons donated by the pentavalent impurity atoms entering the conduction band of n region, the Fermi level Ern shifts up and similarly due to increase in holes in the valance band of p region the Fermi level EFp moves down. The forward bias drives the electrons and holes into the depletion region. This is a transition region in which recombination of electrons and holes takes place that leads to emission of radiation. LASER 123 InveTe Region - Electrons h hv Holes E n - Type Junction p- lype Fig: 3.19 Energy Level Diagram of Semiconductor Laser At low forward current population inversion does not occur and only a small part of the carmiers undergo recombination and the radiation emitted is spontaneous. The output wavelength has a wide line-width. Such non-laser like spontancous emission is the source of radiation trom a light-emitting diode (LED). As the forvard current is increased further. more and more electrons as well as holes are pumped into the active region and a time is reached when population inversion is achieved and the spontaneous emission gives way to the stumulated emission. The pho1ons travelling through the junction region stimulate more electrons to undergo recombination with holes. The laser action takes place along the line of the junction. Due to polished ends of the block, the stimulated emission grows enormously and beam of coherent light is emitted from one of the two ends. The line-width of the output narrows dramatically. With a gallium arsenide (GaAs) laser, a continuous beam of a few miliwatt can be obtained. A disadvantage of gallium arsenide (GaAs) laser is that it emits light at a wavelength of 8400 A in near infrared region of the electromagnetic spectrum which is invisible. By doping the gallium arsenide with phosphorus (GaAsP) and operating at liquid nitrogen temperatures In conventional photography a two dimensional picture is produced by a lens. What is recorded is the intensity of light received from three a dimensional object. But no phase information is recorded on photographic negative. Holography produces a three dimensional view of the object or scene without the use of any image forming lens. In holography both intensity and phase of the light wave
- 2. is recorded. English physicist Dennis Gabor who invented holography in 1947 coined the name for this uncanny technique from "holos' and 'gramma', meaning in Greck, "the whole message'. Holography got a real boost after the invention of laser. Gas lasers are best suited for holography because they emit coherent continuous laser light. 8M Holography is a method of reproducing three-dimensional images without cameras or lenses using photographic film and coherent light. Recording a hologram make a hologram, a beam splitter is uscd to split light from single coherent laser source into two beams namely a reference beam and an object beam. As shown in Fig. (3.20) the object beam falls upon the object and on reflection incident on the photographic plate. The reference beam is incident on the photo plate directly. The two beams form interference patterns on the photographic plate thus forming the hologram. The fine speckled pattern on the photo plate contains information characteristic of the wave fronts themselves, rather than of light intensities as in normal photography. Laser Beam Mirror Reference Bc