Coherent light generators – Particular beam control device – Mode discrimination
Reexamination Certificate
2001-07-24
2004-08-31
Scott, Jr., Leon (Department: 2828)
Coherent light generators
Particular beam control device
Mode discrimination
C372S064000, C372S066000, C372S006000, C372S102000, C372S103000
Reexamination Certificate
active
06785304
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to pumped solid-state lasers, amplifiers and methods for using same and, particularly, when such pumping is provided by one or more laser diodes. The invention can be used in high power diode pumped lasers for applications such as materials processing. The invention can also be used in low power diode pumped lasers for applications such as marking, cutting, drilling, machining, and communications. The invention can also be used in amplifiers for amplifying laser beams. The invention offers particular advantages for amplifying pulsed laser beams such as those produced by q-switched and/or mode locked lasers.
2. Background Art
Most solid state laser applications benefit from the use of laser sources which have high beam quality, high efficiency, and high reliability, and which are low in cost. When compared to lamp-pumped, solid state lasers, LPL's, diode-pumped, solid state lasers, DPL's, offer significant advantages in terms of beam quality, efficiency, and reliability, but their cost effectiveness is hampered by the high cost of laser diodes.
Typically, the pump diodes are the single most expensive component in a diode-pumped solid-state laser. The diode cost may be minimized by utilizing a DPL design with a high optical-to-optical conversion efficiency (the percentage of output power from the pump diodes which is converted to useful output from the DPL). For a higher optical-to-optical efficiency, a lower diode pump power is required to achieve a given output power. High optical-to-optical efficiency also benefits the overall system efficiency and helps improve system reliability. Using diodes without beam conditioning optics (microlenses or fiber coupling) also helps reduce the cost of the pump diodes. Diodes with integrated microlenses or fiber coupling are significantly more expensive and are lower in efficiency because 10% to 20% of the diode output is normally lost in the beam conditioning optics. Additionally, utilizing diodes with simple packages can minimize the cost of the pump diodes. Typically, if the diodes must be packed very close together, expensive micro-channel heatsinks must be used. DPL designs that use diode bars individually mounted or stacked with a wide bar-to-bar spacing may benefit from the ability to use lower cost diode packages. In order to minimize the cost of the pump diodes, the ideal DPL design should have a high optical-to-optical conversion efficiency, should not require beam conditioning optics for the pump diodes, and should permit the use of diodes with simple packaging.
A variety of laser crystals and glasses may be used as the gain medium for DPL's. The most commonly used crystal for high power DPL's is Neodymium doped Yttrium Aluminum Garnet, Nd:YAG. YAG is a synthetic crystal with good thermal, mechanical, and optical properties. When doped with about 1% atomic Nd, it exhibits a number of strong four-level lasing transitions. The strongest line is at 1064 nm. Commercially available laser diodes at 808 nm and 880 nm are typically used to pump Nd:YAG.
For most types of lasers, and, in particular, for solid-state lasers, thermal effects in the gain medium hamper achieving high beam quality during high output power operation. In solid-state lasers, the gain medium is normally pumped throughout its volume and cooled on one or more surfaces. This volume heating and surface cooling leads to thermal gradients in the gain medium. These thermal gradients cause stress gradients in the gain medium because thermal expansion in the hotter part of the gain medium is constrained by the cooler part of the gain medium. Because the refractive index of the gain medium is dependent on both temperature and stress, the thermal and stress gradients in the gain medium create refractive index gradients. Light traveling in the gain medium perpendicular to these gradients will experience focusing effects because the refractive index gradient makes the gain medium act as a gradient index lens. Achieving high output power and high beam quality simultaneously requires taking some steps to minimize the impact of these effects on the laser performance.
Many different DPL designs have been developed in the effort to achieve high power, high beam quality, high efficiency, high reliability, and low cost. The most common configuration is the rod-geometry DPL. In a rod-geometry DPL, the gain medium is shaped as a cylinder. It is pumped either through its side surface or through its end surface(s) and is cooled on its side surface. The beam propagates along the axis of the rod.
In rod-geometry solid-state lasers the thermal gradients are radial and light traveling down the length of the rod is focused. The strength of this “thermal lensing” is directly proportional to the pumping power. This thermal lensing limits the beam quality of high power, rod-geometry solid-state lasers making them a poor choice for high power, high beam quality applications. Rod-geometry DPL's are relatively simple to build, can be designed using diodes without beam conditioning optics and have reasonable efficiencies. Rod-geometry DPL's are currently available at kilowatt average power levels. An exemplary rod-geometry DPL is generally indicated at
10
in FIG.
1
. The DPL
10
includes a laser diode stack
12
and lenses
14
which focus pump beams
16
through apertures formed in a tube
17
. The focused light travels through cooling water in a flow tube
18
and into a YAG rod
20
.
Numerous alternative solid-state laser geometries have been developed which use gain media with different shapes, beam paths, pumping arrangements, and cooling techniques in order to achieve high power operation at a high beam quality level. These designs include zigzag slab lasers, thin disk lasers, and planar waveguide lasers. Each of these designs utilizes cooling of a flat surface on the gain medium to produce a thermal gradient that is one-dimensional.
Zigzag slab lasers use a gain medium that is rectangular in cross section transverse to the beam propagation direction. The longer of the two opposing surfaces of the rectangle is cooled while the adjacent faces are uncooled. This establishes a one dimension thermal gradient perpendicular to the two cooled faces. Pumping can be either through the cooled faces or the uncooled faces. The beam path through the active medium follows a zigzag path making multiple reflections off the two cooled faces. The zigzag path has the effect of averaging the thermal gradient seen by any part of the laser beam such that thermal lensing is eliminated to first order. Second order effects still tend to hamper the beam quality at high power. The beam quality is typically different in the zigzag direction and the transverse direction. DPL's of this type typically require pump laser diodes
44
to be packed close together in order to minimize the required length of the gain medium. The precision required in the fabrication of the slab itself makes it significantly more expensive than a rod of comparable length. Several companies offer high power zigzag slab DPL's with power levels as high as 3 kW. TEMoo output powers from zigzag slab DPL's have been limited to about 100W. A diagram of this design is shown in
FIG. 2
wherein cooling water is indicated at
22
.
Thin disk lasers use a disk-shaped piece of laser crystal that has a diameter much larger than its thickness. It is cooled on one of its large flat surfaces. The cooled surface acts as a mirror in the beam path of the laser and the beam is amplified as it passes through the disk before and after reflection from the mirrored surface. Because the beam is traveling in the same direction as the thermal gradient in the laser crystal there is, in principle, no thermal lensing. Again, second order thermal effects are the ultimate limitation to beam quality at high power.
This type of laser was originally developed for fusion research using lamp pumping of one of the large faces. More recently, a version of t
Brooks & Kushman P.C.
GSI Lumonics Inc.
Jr. Leon Scott
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