Coherent light generators – Particular pumping means – Pumping with optical or radiant energy
Reexamination Certificate
2001-03-29
2003-05-27
Ip, Paul (Department: 2828)
Coherent light generators
Particular pumping means
Pumping with optical or radiant energy
Reexamination Certificate
active
06570902
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to optically-pumped lasers, and more particularly, to optically-pumped lasers that have a low pump energy absorption per unit length such as may result from a low pump absorption cross-section and/or a low doping concentration of pump absorptive ions.
2. Description of Related Art
An optically-pumped laser includes a gain medium and an optical pump source that supplies optical pump radiation to the gain medium, where it is converted into a laser emission. Many early optically-pumped lasers utilized high intensity arc lamps that were formed into any suitable shape, such as a linear shape or a helically wrapped configuration. Although these sources emit high intensity light, they are “low radiance” (i.e. they emit over a very large solid angle), and therefore efficiency was greatly improved by using pump cavities to collect and redirect the pump light to illuminate the laser medium. Accordingly, the laser medium generally was configured into a long, thin cylinder with a large side surface area, and its laser axis length was oriented to match the length of the arc lamp that pumped it. Early pump cavities utilized highly reflecting surfaces surrounding both the arc lamp and the laser medium, which collected and redirected the pump light through the side of the laser medium in a multi-pass configuration. Many versions of these so-called “side-pumped” or “transverse-pumped” optical pump cavity geometries were developed and used in laser products.
However, side-pumped laser configurations have many problems, such as conflicting requirements for cooling the laser medium through the medium side surfaces, suppressing parasitic pump cavity oscillations, and controlling the spatial distribution of the optical pump power within the gain medium, while still maintaining a high pump energy absorption efficiency. These problems are particularly difficult when the side-pumped laser medium has a low absorption of pump energy per transverse pass. In order to obtain high pumping efficiency in such low absorption media, the pump energy must be retained by the pump cavity and redirected back through the laser medium many times. When the pump energy losses resulting from the successive interactions with the cooling interface, parasitic suppression, and pump cavity optical systems become excessive, efficient optical pumping of the side-pumped laser medium cannot be achieved and high power laser operation may become impossible.
In more recent times high radiance pump sources such as lasers and diode laser arrays have been developed and utilized as pump sources for many laser media. Because the light from a high radiance source is emitted over a much smaller solid angle than from an extended lamp source, a high radiance pump source can be optically configured into a narrow beam by an optical system. In “end-pumped” or “longitudinal-pumped” configurations, the beam is introduced into the laser medium through one end and then travels along the laser axis down the length of the gain medium together with the laser emission. In some mode matched embodiments of end pumped lasers, the transverse optical pump radiation profile is matched to the desired transverse mode profile of the laser. In embodiments in which most of the pump energy is absorbed during transit along the round trip length of the laser medium (which is usually much larger than the two-pass transverse width of the laser medium), a pump cavity may not be required to attain high pumping efficiency.
However, significant problems render high power operation difficult to achieve in end-pumped configurations. For example, excessive optical power intensities are created due to the fact that end-pumped lasers have a common propagation axis for both the pump and extracted laser beams, combined with the fact that a typical laser diode pump beam has a highly non-uniform transverse intensity distribution. Attempts to design an efficient, practical high power end-pumped laser have encountered problems such as excessive optical power intensity due to the combination of the intensities of the pump and extracted laser beams, severe thermally-induced medium distortion, excessive doping concentration or medium length constraints on design optimization, increased laser resonator optical losses resulting from complex multi-wavelength optical coatings, and spatially non-uniform pumping distributions. These problems are particularly severe for embodiments in which the laser medium exhibits a high pump saturation flux, such as may result from a low pump absorption cross-section.
In addition to these problems, which can limit the performance and increase the complexity of end-pumped laser embodiments, an additional problem arises for an important class of laser media. Specifically, Òthree-levelÓ or Òquasi-three levelÓ laser media exhibit substantial performance benefits when the product of the medium dopant concentration and the active volume is minimized, i.e., when the dopant concentration is low by current standards. This concentration dependence arises due to the requirement that a substantial upper laser level population density must be maintained to overcome the equilibrium lower laser level population density. One such example of a concentration dependent laser material is ytterbium-doped yttrium aluminum garnet (ÒYb:YAGÓ), which has been identified to have potential for use in high power lasers. In order to take advantage of such concentration-sensitive media, end-pumped configurations utilizing diode lasers as the pump source have been proposed. However, such end-pumped configurations have encountered the above-discussed problems, which have restricted scaling them to high average power levels.
In order to quantify and compare the performance limitations of the prior art side pumping and end pumping concepts applied to concentration sensitive laser materials and to provide one measure of performance improvement over prior art which is afforded by the utilization of the present invention, it may be useful to define a concentration figure of merit, F
c
, for the laser medium, which is obtained by dividing the minimum optical pump radiation focal spot area, A
f
, by the product of the medium laser axis end area, A, the medium dopant concentration, N
o
, the pump absorption cross-section, &sgr;
p
, and the effective end-pumping medium length, L. Symbolically:
F
c
=A
f
/(
AN
o
&sgr;
p
L
). Eq. 1
As used herein, the value of F
c
provides a quantitative measure of the highest efficiency and lowest waste energy that can be realized from a given configuration of a concentration-sensitive medium. Generally, a higher F
c
indicates that the laser is more efficient, while a lower F
c
indicates that the laser is less efficient.
Prior to the conception and reduction to practice of the present invention, after consideration of all prior art shortcomings, it is believed that end-pumped configurations have provided the most efficient and scaleable configurations of concentration-sensitive media lasers such as Yb:YAG. In order to maintain a near optimum level of pump energy absorption in such configurations, the quantity N
o
&sgr;
p
L in the above equation must be approximately equal to 1. Since the minimum value for A is equal to A
f
, it is apparent that for end-pumping embodiments the largest possible value of the concentration figure of merit is equal to 1, which requires that the optical pump radiation completely fill the medium volume. Such optimum end-pumped configurations can be impractical to implement, and generally must be compromised to a lower F
c
value. Furthermore, side-pumped embodiments that use external re-entrant pump cavities have a low pump energy absorption efficiency and/or an F
c
value significantly lower than 1.0.
SUMMARY OF THE INVENTION
In order to overcome the limitations of the prior art, the present invention provides an optically-pumped laser having a gain medium configured to provide a low loss, three-dimensional integrated optica
Alkov Leonard A.
Ip Paul
Lenzen, Jr. Glenn H.
Nguyen Dung T
Raufer Colin M.
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