DPSS lasers

Wednesday, 31 January 2007

A laser is a device which utilizes the transitions between energy levels of atoms or molecules to amplify or generate light. Laser light has gained acceptance as a controllable light source not only in scientific research but also in many fields of everyday life. This is primarily due to the fact that a laser provides a monochromatic light source with coherent radiation which can be focused very well due to the small divergence. When an electron makes a transition from a higher energy level to a lower energy level, a photon, the elementary quantity of radiant energy, is emitted. The emitted photon travels in the same direction and is in the same phase as the incoming photon. When the stimulated emission in a laser involves only a single pair of energy levels, the resultant output beam has a single frequency or wavelength and is thus approximately monochromatic. Lasers are commonly used in industrial processes, for example, to cut and weld metals and other substances, particularly in the automotive, aerospace, appliance and shipbuilding industries. Lasers may also be useful for rock drilling for mining and/or oil and gas exploration purposes. Lasers have been produced from a variety of materials and in all phases: liquid, gas, plasma and solid-state. Liquid lasers most commonly contain of a gain medium of organic dye salts such as derivatives of Rhodamine or fluorescein which are dissolved in solvents such as methanol or water. The gain medium is excited by light produced typically by flashlamps or another laser. A significant challenge with such liquid lasers is the corrosive nature of the solvents which requires special handling both the preparation and use of the laser medium. When initially developed, these systems were initially pumped by flashlamps. By using recently developed laser diode arrays as pump sources, the pump light can match the absorption spectrum thereby enabling a resurgence of interest in this type of liquid laser. Solid-state lasers contain a gain medium which is comprised of a lasing ion contained in a crystal or amorphous matrix. The most common lasing species are based on rare-earth elements such as neodymium, erbium, ytterbium, etc. The laser properties of the gain medium are determined by the interaction of the local crystal field with the field of the ion itself. This interaction determines the specific energy levels of the ion and their width.

Solid-state laser systems are characterized in that they have a solid-state laser gain medium which converts energy from an optical pump source to a coherent output laser beam. A typical solid-state laser includes a laser cavity formed by two opposing mirrors, a solid state gain medium situated within the laser cavity, and an optical pump source for pumping the gain medium. The pump source can be one of many available energy-producing systems such as flash lamps or semiconductor laser diodes. The energy produced by the pump source is incident upon the laser medium and absorbed by the laser medium. Laser-diode pumping uses semiconductor lasers to replace incoherent optical pump sources such as flash lamps or arc lamps. Most of the recent growth in the solid-state laser (SSL) industry can be attributed to introduction of laser diodes as the source of energy because diodes are generally very efficient in converting electric energy into the pump light and usually deposit only a limited amount of waste heat into the laser gain medium. A laser diode is a light-emitting device based on a light amplification effect. Light is amplified when it passes inside a laser diode. A laser diode has a resonator. The resonator has a frequency response, while the light amplification effect has another frequency response. The overlapping area of two frequency responses will determine the lasing frequency. A laser diode pumped solid state laser produces laser light by pumping a resonator of a laser material with light from laser diodes, exciting atoms or molecules within the laser material. The ends of the laser material may be polished and covered with a highly reflective coating to form an oscillator cavity, reflecting light back and fourth through the laser material. This provides the feedback for optical amplification, resulting in a beam of laser light. Laser diodes emit a light beam with an elliptical shape. A laser diode array contains many individual laser diodes. Each diode emits its own laser beam from the emitting surface, which together form an elliptical light beam. Compared to the conventional pumping sources, e.g., linear flash lamps or arc lamps, the laser diode promises higher energy efficiency, lower heat generation and longer life. Laser diode pump sources have high electrical-to-optical conversion efficiency, and the narrow-band spectral output of laser diodes can be chosen to closely match the absorption bands of solid-state laser materials. As a result, heat loads in diode-pumped solid-state lasers are significantly lower than for the flashlamp-pumped solid-state lasers that have been largely supplanted by diode-pumped lasers for applications requiring compactness and high efficiency. Diode-pumped solid-state (DPSS) lasers are hence replacing a significant fraction of existing lasers in a variety of applications, including coherent radars, global sensing from satellites, communications, medical uses, micro-machining and miniature visible sources for digital optical storage. Diode pumped lasers are particularly useful in that diode pumps are power efficient, all solid-state and long lived. This results in laser systems that are lighter, more efficient and typically not water cooled, as compared to similar flashlamp pumped solid-state lasers.

Diode-pumped solid-state laser systems consist of several subsystems: a power supply to run the pump diodes, the pump diodes themselves, the laser head, and a harmonic conversion device to generate the visible or UV radiation. The lasing portions of the semiconductor diodes, typically PN junctions, are positioned near a laser medium, such as a crystal, so that laser energy from the semiconductor diodes is directed into the medium, either directly or by lenses. When "pumped" by the laser energy from the diodes, the energy excitation levels build up within the atomic structure of the medium. A typical green DPSS laser consists of a laser diode (LD) that comprises a LD casing, a heat sink mounted within the LD casing, a semiconductor chip mounted on the heat sink and an output window, which emitting a pumping radiation for exciting a lasing medium. A lens system is mounted within a casing for focusing of the pumping radiation. Diode-pumped solid-state lasers are usually classified into two types based on the pump scheme, i.e., the transverse (side) pumped and the longitudinal (end) pumped type, respectively. In the side pumped type, the diode laser array shines transversely on the side of a solid laser rod or bar; this direction is normal to the axis of the solid-state laser cavity. Diode pumps that are used for end pumping are usually either directly collimated and focused into the gain medium, or the pump light is introduced from the end of an optical fiber. Arrays or single emitters, used in this fashion, have sufficient brightness or beam quality to enable efficient end-pumping. The end pumping type of DPSS laser has efficiency and an advantage in forming the radiated light in beam mode because the lasing area of the laser material overlapping the radiating area of the laser light is large. With end pumping, the diode array focuses light onto the end of a laser rod or bar, in a direction parallel to the optical axis of the resonator. However, the end pumping type of DPSS laser has a predetermined limitation in a large output of the laser power owing to a heat lens phenomenon. The side-pumped systems, on the other hand, have little difficulty in generating high powers, but usually suffer from having a low optical gain and from the difficulty of achieving mode matching in conjunction with a compact-cavity design. This is due to the distribution of pump light over a larger volume.

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