Optical: systems and elements – Optical amplifier – Particular pumping type
Reexamination Certificate
2000-05-30
2001-07-31
Hellner, Mark (Department: 3662)
Optical: systems and elements
Optical amplifier
Particular pumping type
C359S342000
Reexamination Certificate
active
06268956
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical amplifier for use, for example, in a high powered solid state laser and more particularly to an optical amplifier which includes an elongated slab of a solid state lapsing material, for example, a rare earth doped yttrium-aluminum-garnet (YAG) crystal and a plurality of diode arrays for exciting the solid state lapsing material to a relatively high energy metastable state, wherein the pumping light is coaligned with the amplified light resulting in relatively long absorption lengths and thus higher overall efficiencies, making the configuration particularly suitable for optical amplifiers which use solid state lapsing materials which have a relatively low absorption length, such as Yb and Tm.
2. Description of the Prior Art
Optical amplifiers are known which include an elongated generally rectangular or square slab of a lapsing material, such as a rare earth doped yttrium-aluminum-garnet (YAG) crystal. The slabs are formed with a generally rectangular or square cross section area defining a pair of opposing end faces and four lateral faces. Material for the slab is selected to have a relatively high index of refraction. The slab is cooled with a cooling medium having a relatively low index of refraction. This change in the index of refraction at the slab coolant interface results in incident light beams directed to one end face of the slab being totally internally reflected through the slab in a zig-zag manner. As such, the optical amplifiers with such a configuration have become known as zig-zag amplifiers. Examples of solid state lasers utilizing such zig-zag amplifiers are disclosed in U.S. Pat. Nos. 4,730,324; 4,852,109 and 5,305,345.
In order to excite the solid state lapsing material to a relatively high energy metastable state, various pumping sources, such as diode arrays are used, for example, as disclosed in U.S. Pat. Nos.: 4,852,109; 4,949,346; 4,984,246; 5,271,031; 5,305,345; 5,317,585 and 5,351,251. In many known optical amplifiers, the pumping sources are configured such that the light from the pumping source is directed along a lateral face of the slab in a direction generally perpendicular to the longitudinal axis of the slab. Examples of optical amplifiers with such a configuration are disclosed in U.S. Pat. Nos.: 4,127,827; 4,852,109; 5,271,031; 5,305,345; 5,646,773 and 5,651,021. Commonly owned U.S. Pat. No. 5,900,967 discloses a configuration where a plurality of diode arrays are directed along the lateral faces of the slab. The system disclosed in '434 application utilizes diode arrays directed generally orthogonal to a longitudinal axis of the slab as well as diode arrays which are directed at an angle relative to the lateral faces to provide a general uniform energy distribution in the slab. Such configurations, known as side pumped configurations, unfortunately limit the absorption length of the pumping light to just a few millimeters. When such side pump configurations are used with optical amplifiers which use a solid state lapsing material having a relatively low absorption coefficient, such as Yb and Tm doped materials, a relatively low absorption efficiency and, thus a lower overall efficiency results. Thus, there is a need for optical amplifiers capable of relatively longer absorption lengths resulting in relatively higher overall efficiencies.
SUMMARY OF THE INVENTION
Briefly, the present invention relates to an optical amplifier which includes an elongated slab of solid state lapsing material, such as a rare earth doped yttrium-aluminum-garnet garnet (YAG) slab. In order to provide a relatively increased absorption length and thus a higher overall efficiency, the optical amplifier in accordance with the present invention incorporates end pumping in which the pumped light is coaligned with the amplified light resulting in relatively longer absorption lengths and higher overall efficiencies. The coaligned pumped sources are directed to lateral faces of the slab which include footprints or windows, formed from anti-reflection coatings at the pump wavelength. In order to cause internal reflection of the pump beam along the lapsing axis, the end faces are formed at about a 45° angle relative to the longitudinal axis which causes the pumped light to be reflected within the slab along longitudinal axis. In order to confine the absorption of the pumped light to the center portion of the slab, the slab may be formed from a composite material with the opposing end portions of the slab formed from an undoped host material while the center portion of the slab along the longitudinal axis is formed from a doped host material. Such a configuration provides relatively low residual thermal lensing with virtually no birefringence. In one embodiment, the pump light from the diode arrays is coupled to the slab by way of lenses or lens ducts. In an alternate embodiment, the pump light is coupled to the slab by way of optical fibers. In yet another embodiment, the pump light and laser beams are interchanged forming a low-loss straight through slab with an end pumped architecture.
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Hoefer Carolyn S.
Injeyan Hagop
Palese Stephen P.
Hellner Mark
Keller Robert W.
TRW Inc.
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