Coherent light generators – Particular pumping means – Pumping with optical or radiant energy
Reexamination Certificate
2000-09-08
2004-03-09
Thomas, Tom (Department: 2815)
Coherent light generators
Particular pumping means
Pumping with optical or radiant energy
C372S072000, C372S075000
Reexamination Certificate
active
06704341
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Endeavor
The present invention relates to a lasers, and more particularly to a diodepumped laser with an improved pumping system.
2. State of Technology
Solid-state lasers pumped by high-power laser diodes have been used in a wide range of applications. Conventional diode-pumped solid state lasers can be classified as end-pumped and side-pumped lasers.
An example of a side-pumped laser is described in U.S. Pat. No. 5,978,407 to Jim J. Chang, et al, issued Nov. 2, 1999. This patent describes a compact, side-pumped laser pump cavity that uses non-conventional optics for injection of laser-diode light into a laser pump chamber. The laser pump cavity includes a plurality of elongated light concentration channels. The light concentration channels are compound parabolic concentrators (CPC) which have very small exit apertures so that light will not escape from the pumping chamber and will be multiply reflected through the laser rod.
In U.S. Pat. No. 4,924,474 to Shigenoria Yagi, et al, issued May 8, 1990, a laser device with high oscillation efficiency is shown. The laser device comprises a laser medium, a semiconductor light-emitting element for exciting the laser medium, a reflecting surface surrounding the outer periphery of the laser medium along its optical axis, and an opening provided in a part of the reflecting surface for causing light from the semiconductor light-emitting element to be incident upon the laser medium. The laser device is improved further by providing a polarizing plate and a light guide between the light-emitting element and the laser medium and by deviating the optical axis of excitation light from the central axis of the laser medium. The system described in the patent uses diode light sources which are discrete in the longitudinal direction and makes no provision to concentrate the light from these diodes. The discrete characteristic imposes nonuniformities in the deposition of the pump light decreasing the beam quality of the output laser beam and the lack of optical concentrators limits the number of diode sources which can excite the lasing medium thereby limiting the output power of the device. Another feature of the system described in the patent is that the lasing rod is cemented to a heat radiator which also serves as the reflector for the pump cavity. Although this is a convenient way to couple light into the lasing media, the heat removal capabilities are not great and this device would not scale to high average powers.
In an attempt to circumvent the problem of pump light re-entering the diode cavity through the opening in the pump cavity, an “optical diode” is provided. The problem with light re-entering the diode structure is that the subsequent diode heating shifts the wavelength of the diode light out of the absorption bands of the laser medium. This optical diode consists of a polarizing and quarter wave plate placed between the diode and the pump cavity and leads to higher cost and lower efficiency.
In U.S. Pat. No. 5,033,058 to Louis Cabaret, et al, issued Jul.16, 1991, a rod laser with optical pumping from a source having a narrow emitting area is shown. Pumping laser diodes are disposed around a reflector tube having narrow windows through which the emitting areas of the diodes look. The tube leaves an angular gap around a rod to be pumped and a cooling liquid flows in said gap. In this device the discrete nature of the diodes in the longitudinal direction has been eliminated in favor of a line source of diodes but again there is no optical concentrator. The lack of an optical concentrator limits the number of diodes which can surround the laser rod thereby the scaling to high average power. Since there is no optical concentrator which can also increase the angular distribution of the diode light, the absorption of the diode light in the laser rod will be nonuniform causing poor beam quality at high average power.
A problem plaguing both end-pump and side-pump configurations is that of inhomogeneous pumping. The side-pump laser has to balance De Beer's law absorption that tends to deposit the power closer to the surface closest to the emitter against the loss of absorption efficiency encountered when a lightly doped rod is used to permit deeper penetration.
In addition, diode light concentration using conventional optics such as cylindrical lenses often limits the number of emitters that can be circled around the rod. The end-pump laser using an imaging device to demagnify the emitting diode array results in imprints of the diode array image at the end of the rod. Guiding inside curved surfaces of the rod, leads to a repeating sequence of these images. Inevitably, both geometries end up with hot and cold spots. This limits the average output attainable because of wavefront distortion and depolarization.
To resolve these issues, a side-pumped scheme with a novel pump cavity configuration that can improve laser performance has been developed as described in and claimed in this patent application. An advance in laser technology will be provided by a system that enables more uniform laser pumping in both the transverse and longitudinal axes and highly effective recycle of pump radiation, leading to improved laser performance. It will also be an advance in laser technology to provide a system that effectively widens the acceptable radiation wavelength of the diodes, resulting in a more reliable laser performance with lower cost.
SUMMARY OF THE INVENTION
In accordance with the present invention, a laser pumping system is provided with a goal of improving the efficiency and reducing the cost of diodepumped solid-state lasers.
U.S. Pat. No. 5,978,407 to Jim J. Chang, et al, issued Nov. 2, 1999, incorporated herein by reference, describes a compact, side-pumped laser pump cavity that uses non-conventional optics for injection of laser-diode light into a laser pump chamber. The laser pump cavity includes a plurality of elongated light concentration channels. The light concentration channels are compound parabolic concentrators (CPC) which have very small exit apertures so that light will not escape from the pumping chamber and will be multiply reflected through the laser rod. This patent emphasizes the need for improvement in injection of laser-diode light into a laser pump chamber.
The present invention provides a laser having an elongated lasing medium within an elongated pumping chamber. The laser contains at least one diode for producing diode radiation and at least one compound parabolic concentrator light channel for transporting the diode radiation into the pump chamber and into the lasing medium. The compound parabolic concentrator light channels have reflective side walls and reflective end walls. In the preferred embodiment the reflective end walls have curved surfaces. A flow tube with light diffusion properties is located between said lasing medium and said at least one light channel.
In the preferred embodiment of the present invention, quasi-three-dimensional CPC light channels are employed to side-pump a laser rod enclosed in a laser pump chamber. In the previous Chang patent, the CPC was effective in channeling the pump light in the dimension transverse to the laser rod axis but did little to channel the pump light along the rod axis. In the invention described and claimed in this patent application, the pump radiation from the laser diodes is delivered to the pump chamber by the light channels. The pump radiation that enters the pump chamber is then distributed by the pump chamber to achieve uniform pumping of a laser rod in both the transverse and longitudinal directions. The surfaces of the pump chamber are constructed by either surface or volume diffusers. The design of the pump chamber enables efficient trapping the pump light once it enters the cavity. This leads to efficient recycling of unused pump radiation for improved laser performance. This improved pump system is expected to result in a more cost-effective diode-pump laser system.
Additional aspects, advantages, and featu
Diaz José R
Scott Eddie E.
The Regents of the University of California
Thomas Tom
Thompson Alan H.
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