Operation and Construction of Blue Diode Lasers | PHYSICS 325, Lab Reports of Physics

Download Operation and Construction of Blue Diode Lasers | PHYSICS 325 and more Lab Reports Physics in PDF only on Docsity! Operation and Construction of Blue Diode Lasers Ben Payne Optics 325 April 21, 2006 Blue diode lasers are currently being experimentally produced. First a brief lesson on how lasers operate is given, then how a blue diode laser works. Finally the uses are talked about. Operation and Construction of Blue Diode Lasers If one is interested in the properties of light, lasers offer a full spectrum of possible investigations from classical optics to quantum mechanics. Lasers model classical light rays very well for use with demonstrations of lenses but are produced by quantum mechanical processes. The monochromatic coherent light is useful in many applications, ranging from personal DVD and CD players to spectroscopy to more experimental uses such as fusion (Hecht, 598). We will look at how lasers work, then the operation of laser diodes, but not what happens to the beam after it leaves the apparatus. Before one starts to look at the development of the laser, it is instructive to first know what a laser is. When the device was first used in 1954 it was called Microwave Amplification by Stimulated Emission of Radiation or MASER. After that it was hypothesized that the same process could somehow produce visible light. The acronym was then changed to LASER around 1965 after the development of just such a device. Now the acronym has become so common that we call the device a laser, and if not in the visible spectrum then it is a microwave laser or x-ray laser. What does this name actually mean? The Light refers to the visible light that is emitted, as does the Radiation part. The Amplification portion notates the process of increasing a signal. The Stimulated Emission is the important, defining part of the process. The alternative is spontaneous emission, which produces incoherent light. By using stimulated emission all the light is in-phase both temporally and spatially, which is also known as coherence. The light is also of the same wavelength, which is observed as monochromatic light. Next we will look at how this is accomplished. There are various methods of pumping, but laser diodes use an electrical discharge in order to “pump” the energy state up. Now that we have a basic understanding of the physics behind lasers we can look at diode lasers. Laser diodes are different from gas and liquid lasers as they use semiconductors to produce light. Semiconductors are solid, mass producible, and not as fragile as the alternatives. This makes them excellent candidates for commercialization. In order to manufacture a diode laser, one simply layers different semiconductors. The hard part is knowing what materials to layer in what thicknesses, how well one can keep the substances pure and homogeneous, and the difficult techniques of putting layers down on top of other materials. Semiconductors are used because they have “band gaps:” a gap in energy levels between the next higher level for electrons and their current level. When these electrons are pumped up and then fall back into the hole they left, light is emitted. The motivation to use semiconductors comes from the fact that the diode is used for the pumping process. Diodes prevent current from flowing in one direction but permit it in the other. The semiconducting diodes used are either n-type, (negative from too many electrons) or p-type, (positive from the absence of electrons) both are a result of adding impurities, commonly called doping. Layering n-type and p-type semiconductors gives a diode. The holes and excess electrons are excited by electricity and combine to produce light. To figure out how thick the layers of semiconductor have to be, look at how deep the electrons will permeate the p-type material to access available holes. The diffusion equation is given by Svelto on page 397 as *d D τ= where d = penetration depth of electron, D = diffusion coefficient, and  = electron-hole recombination time. As an example, Gallium Arsinide (GaAs) has a penetration depth d of approximately 10-6 m. The reason to switch from GaAs to Gallium Nitride (GaN) is because the the band gap is 3.4 eV, which corresponds to 365 nm (Johnson, page 31). By adding other materials the wavelength changes slightly to blue or blue-green. The elements used come from the third group and fifth group of the Periodic table and are called III-V compounds. The specific materials used for blue lasers are Indium, Aluminum, Gallium, and Nitrogen, which produce 480 to 520 nm light. (See figure 3, the electromagnetic spectrum.) In order to use GaN as the active material one has to grow it so that it is a thin film. To do this Silicon was initially used as a substrate. In manufacturing one had to lay down thin layers of semiconductor at temperatures greater than 1000°C (steel melts at around 1370°C) in a process called metal-organic chemical vapor deposition (MOCVD). The GaN is grown on a sapphire surface. The two materials are slightly different, as specified by their “lattice constant.” Since the structures are different by 16%, cracks become present in the GaN. These cracks heat up when energy is added in the pumping process, which destroys the material. This problem was overcome using “buffer layers” of aluminum nitride (AlN) on the sapphire, as developed by Isamu Akasaki in 1986 (Johnson, page 32). The primary developer of the first blue nitride laser was Shuji Nakamura at Nichia Chemical Industries in Japan in 1995, now at the College of Engineering at the University of California at Santa Barbara. At first he was able only to pulse the laser (due to the material instabilities), but by 1998 Nakamura was able to produce a continuous wave (CW) laser with a projected lifetime of 10,000 hours, or more than a year. The physical construction of diode lasers now uses multiple layers of semiconductors to produce multiple quantum wells in devices called double heterostructure lasers. These potential wells are in the active material and provide a place for the electrons and holes to combine. The active material has dimensions such that the device acts as a waveguide, and the edges are either rough or smooth to utilize the index of refraction in such a way as to only allow stimulated light in one direction: out. Figure 2: diagram of laser construction. Dimensions used by Nakamura are (4x450)*10-6 m There are electrical inputs on the upper and lower surfaces, and the active material is sandwiched between two semiconductors. When current is introduced it pumps electrons up (gives them a higher energy state). When there are more electrons in the higher state than the lower state, a population inversion occurs and energy is released. Some of the energy is light, which is reflected until it is intense enough to leave the laser.