Coherent light generators – Particular beam control device – Mode locking
Reexamination Certificate
1998-04-16
2002-02-12
Davie, James W. (Department: 2881)
Coherent light generators
Particular beam control device
Mode locking
C372S010000, C372S083000
Reexamination Certificate
active
06347101
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention.
The present invention relates to the optimized coupling of pump radiation into the gain medium of a laser and to systems including such lasers. Particular aspects of the present invention relate to the use of one or more arrays of semiconductor diodes to side pump a high absorption, high gain solid state gain medium such as neodymium yttrium vanadate.
2. Description of the Related Art.
Solid-state lasers are a class of lasers that use as a gain medium a crystalline, glass or other solid material as a host for an optically excitable material, such as an ion of a rare earth element like neodymium. The crystalline, glass or other solid host material acts as a matrix fixing the optically excitable material in place. For illustrative purposes, it is convenient to consider a common solid-state laser system that uses yttrium aluminum garnet (YAG) doped with neodymium ions as a gain medium. This system is usually identified as Nd:YAG. Application of pump radiation to the Nd:YAG solid-state gain medium excites Nd
3+
dopant ions within the YAG host material to metastable states that have lifetimes which allow the population of the states to become inverted in such a manner that optical transitions of the dopant ions from the excited metastable states can be used to establish laser action. Some solid-state laser systems use a broad-band optical radiation source such as a xenon flash-lamp or other type of flash-lamp to provide pump radiation to the gain medium to excite the dopant ions to their desired metastable states. Flash-lamps have the advantages of high available power levels and comparatively low cost. Nevertheless, the use of broad-band, poorly-focused pump radiation is undesirable because pump radiation is absorbed in portions of the gain medium other than within the resonant modes of the laser. Pumping portions of the gain medium that are not within the resonant modes does not result in laser action and generates heat that must be dissipated, and so can limit the range of usable pump intensities.
A second undesirable aspect of the inefficiency of flash-lamp excitation is that the broadband radiation of flash-lamps is not well matched to the task of pumping the dopant ions within the resonant modes to the desired excitation levels. That is, within flash-lamp pump radiation there are many photons with more energy than is necessary to excite the neodymium ions of the solid-state gain medium to their desired metastable states. When these too energetic photons are absorbed by the dopant ions, the excess optical energy absorbed by the neodymium ions beyond what is necessary to place the neodymium ions in their desired metastable state is dissipated at least partially through thermal excitation of the solid-state gain medium. In other words, the excess energy within the radiation absorbed into the resonant modes heats the gain medium. To a lesser extent, a similar phenomenon occurs for photons within the radiation that are insufficiently energetic to excite the neodymium ions to their desired states. Materials suitable as solid-state gain media tend to have imperfect crystal structures with comparatively broad absorption bands and with significant levels of defect-mediated absorption for comparatively low-energy photons. The fraction of the flash-lamp radiation absorbed that is insufficiently energetic to excite the dopants to their desired metastable states is also typically dissipated at least partially through heating of the gain medium.
For a variety of reasons, dissipating heat and avoiding unnecessary heat generation are important considerations for solid-state gain media. Solid-state lasers are generally used in high power applications, with corresponding high levels of input pumping power and high levels of heating of the gain medium. The cooling of solid-state gain media, whether using coolant based refrigerators or using solid-state refrigerators, is not wholly satisfactory because solid-state gain media are generally poor thermal conductors so that the interiors of solid-state gain media are in poor thermal communication with the surfaces through which cooling may occur. There is consequently a limit to the rate at which heat can be extracted from the pumped portion of the gain medium by cooling. Thus, the inherent properties of solid-state gain media and solid-state lasers are such that heat will build up in the gain media of these lasers. To make matters worse, solid-state gain media tend to be particularly ill suited to excessive heating conditions. Solid-state gain media are typically susceptible to mechanical failure under high thermal stress. Consequently, heating of the solid-state gain medium is frequently a limit on system performance.
The various problems associated with flash-lamp pumping of solid-state gain media have historically limited the applications for which solid-state lasers were considered appropriate. A major advance for the practical application of solid-state lasers proceeded from the maturation of semiconductor diode lasers as excitation sources for pumping the solid-state gain media. Through the use of sophisticated manufacturing techniques, semiconductor lasers are now available operating at wavelengths that are very close in energy to the excitation energies most suitable for many solid-state laser systems. These semiconductor lasers exhibit desirable mode quality and operate at sufficiently high energies to be useful for pumping solid-state gain media. Commercially available semiconductor lasers with output powers of twenty watts or greater generally consist of a linear array of discrete laser diode emitters formed on a single chip. These semiconductor laser bars can be made to have any of a variety of desirable output wavelengths and, for example, can have an output wavelength near 808 nanometers, which is convenient for pumping a solid-state laser system like Nd:YAG. Furthermore, these diode lasers are tunable during use over a sufficient range (±3 nanometers) to allow the diode laser bar to be matched to a particular solid-state gain medium. Tuning of semiconductor lasers is accomplished by adjusting the operating temperature of the semiconductor laser. The availability of semiconductor laser output wavelengths matched to the energy levels being pumped is important because it allows the efficient pumping of the solid-state gain medium with very little of the pump radiation directly generating heat within the gain medium. Energy level efficient pumping of the gain medium significantly reduces the amount of heat generated in the solid-state gain medium due to imprecise pumping.
Although semiconductor lasers can efficiently pump solid-state gain media, the maximum power output of semiconductor lasers is lower than is desirable for many solid-state laser applications. Consequently, it is important to collect as much of the semiconductor laser's pump light as is possible within the gain volume of the solid-state gain medium to take advantage of as much of the semiconductor laser's output intensity as is practical. The commonly used Nd:YAG gain medium is not a strong absorber of the wavelength of light most appropriate to pumping the neodymium ions within the YAG matrix. When a semiconductor laser is used to side pump a Nd:YAG gain medium, a substantial fraction of the pump light can pass entirely through the relatively small cross-sectional dimensions of the Nd:YAG gain medium without being absorbed. It is consequently very desirable that the pump light output by the semiconductor laser be directed along the longest axis of solid-state gain media like Nd:YAG and that the optical axis of the resonant cavity extend along a direction nearly parallel to that longest axis to increase the amount of semiconductor laser pump light that is absorbed within the resonant modes of the gain medium. Because of this, many solid-state lasers pumped by semiconductor lasers have used what is called an “end-pumped” configuration. Typical solid-state lasers include a resonant cavity define
Hemmati Hamid
Hug William F.
Partanen Jouni P.
Wu Xingkun
3-D Systems, Inc.
D'Alessandro Ralph
Davie James W.
Inzirillo Gioacchino
Wright William H.
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