Coherent light generators – Particular pumping means – Pumping with optical or radiant energy
Reexamination Certificate
2001-08-21
2003-08-19
Ip, Paul (Department: 2828)
Coherent light generators
Particular pumping means
Pumping with optical or radiant energy
C372S070000
Reexamination Certificate
active
06608852
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to solid state lasers, and particularly to a side-pumped, diode-pumped solid state laser which is side-pumped by a “semi-closely coupled” diode array.
2. Discussion of the Related Art
The side-pumped, diode-pumped solid state laser (side-pumped DPSSL) field can be divided into sub-fields based on how the otherwise highly divergent diode radiation is coupled into the laser rod. Some of these sub-fields would include: (a) using optics such as a cylindrical lens or elliptical mirror, (b) using an optical waveguide such as a reflective cavity or fiber; and (c) closely coupling the diode(s) to the rod. The invention according to the preferred embodiment would be most closely fit among these sub-fields into sub-field (c), although the diode array of the invention may be considered to be “semi-closely coupled” and thus not fit into any of the sub-fields (a)-(c), instead being in a new sub-field (d) of the side-pumped DPSSL field. Thus, the invention is considered to form a new sub-field relating to semi-closely coupled diode array side-pumping of solid state lasers.
Japanese patent no. JP 5-259540 discloses a side-pumped DPSSL wherein the rod is disposed within a diffuse reflector or condenser. A diode array emits radiation that enters the condenser for absorption by the rod via a narrow slit which guides the diode radiation toward the rod. A gel or liquid such as water surrounds the rod filling a spacing between the rod and a second solid state medium. The slit is quite narrow, e.g., on the order of the width of an active region of the laser diode, and the diode array is disposed right on the slit. The light emitted by the diode array undergoes several reflections from the walls defining the extent of the slit which reduces the efficiency of the device.
In the close-coupled arrangement described at U.S. Pat. No. 5,774,488, the rod is enclosed into a temperature conductive reflector, and the pump radiation is introduced through a narrow slit in the reflector. The problem with this embodiment is its complexity of manufacturing, since the rod must fit tightly into the metal reflector. Additionally, any difference in thermal expansion of the rod and reflector may cause mechanical stress. Also, pump radiation from the diode must pass through a long narrow slit (channel) in the metal reflector, thus suffering multiple reflections and, therefore, losses.
U.S. Pat. Nos. 5,521,936, 5,033,058, and 6,026,109 disclose water-cooled solid state lasers using closely coupled side-pumping diode arrays. The pumping laser diodes are disposed close to the rod in order that the rod remains in the path of the substantial portion of the divergent radiation, as it is not contemplated that rays missing the rod on the first pass will be subsequently redirected towards the rod to be absorbed on a second or later pass. U.S. Pat. No. 5,870,421 patent differs in that it discloses to use side-pumping optical fibers. Using optical fibers adds substantial manufacturing cost.
In addition, the alternative configurations described in U.S. Pat. Nos. 5,521,936, 5,033,058, and 6,026,109 include a rod which is cooled with a water jacket enclosed into a flow tube, with the diodes and reflector disposed outside the flow tube, similarly to JP 5-259540. Here, a disadvantage is that the wall thickness of the flow tube adds significant distance between the rod and reflector. In configurations using a diffuse reflector, this leads to increased losses of the pump light, resulting in reduced efficiency.
On the other hand, specular reflectors are not able to produce the same level of uniformity of the pump radiation as the diffuse reflectors. For example, Hanson, et al., citation below, disclose a three-bar diode array spaced somewhat from a large, approximately &pgr;/2 opening to a solid state laser cavity. Ajer et al., citation below, disclose a closely-coupled side-pumping diode array which pumps the rod through a slit-like opening. Each of the cavities disclosed by Hanson, et al. and Ajer, et al. include highly reflective inner surfaces, and the intensity distributions of the pumping diode radiation within the rods lack homogeneity.
Generally, pumping with a diode array from one direction can leads to a cylindrical intensity distribution (as shown, for example, in the paper by Hanson, et al.). This gives rise to a cylindrical thermal lens in the rod, which, in turn, results in an astigmatic output beam of the laser. This can be extremely disadvantageous in applications such as micro-machining of nozzles in car fuel injectors and ink-jet printers, where a hole roundness of better than a few percent is desired. To improve circularity, some of the mentioned above sources describe alternative arrangements which use pumping radiation from several (two or more) directions. The problem with this approach, however, is that laser diodes tend to age differently, which destroys the intensity balance over the lifetime of diodes. It is desired to have a pump chamber wherein the intensity distribution of the pump radiation inside the rod is uniformly distributed with substantially circular symmetry.
In addition to side-pumped solid-state lasers, there are numerous commercial lasers utilizing end-pumping. In such lasers, the radiation from the pump diodes is collinear or nearly collinear with the laser beam. An advantage of end pumping is its high efficiency, mainly due to the good spatial overlap of the pump beam with the generated laser beam. However, this configuration is limited to lower output power, typically around 10W or less. The reason for that is that the pump radiation is absorbed in a small volume which leads to significant thermo-optical distortions. On the other hand, side-pumping allows a distribution of pump energy over a longer active medium, thus increasing the pumped volume and allowing high scalability of the output power. It is recognized in the present invention that care should be taken to maintain a uniform distribution of the pump intensity in the gain volume, which is an object of the present invention.
The following summarizes the features of the desired configuration, as recognized in the present invention. First, the rod should be preferably cooled with water. Second, the reflector should be of diffuse type and placed close to the rod. Third, the pump radiation should come from one source, but still produce a circularly symmetrical intensity distribution inside the rod.
SUMMARY OF THE INVENTION
In view of the above, a side-pumped diode-pumped solid state laser device is provided including an elongated housing having an elongated cavity defined therein and further having an elongated opening defined between the cavity and the exterior of the housing, a solid state rod disposed within the cavity and surrounded by a cooling fluid, a cover seal outside the housing and sealably covering the opening and thereby enclosing the cavity, the cover seal being formed of a material that is at least substantially transparent to pumping radiation at a predetermined pumping wavelength, a diode array proximate to the cover seal for emitting the pumping radiation that traverses the cover seal and the opening to be absorbed by the rod to excite laser active species within the rod, and a resonator including the rod disposed therein for generating a laser beam.
A side-pumped diode-pumped solid state laser device is further provided including an elongated housing having an elongated cavity defined therein and further having an elongated opening defined between the cavity and the exterior of the housing, a solid state rod disposed within the cavity, a cover seal outside the housing and sealably covering the opening and thereby enclosing the cavity, the cover seal being formed of a material that is at least substantially transparent to pumping radiation at a predetermined pumping wavelength, a diode array proximate to the cover seal for emitting the pumping radiation that traverses the cover seal and the opening to be absorbed by the rod to excite laser active s
Govorkov Sergei V.
Wiessner Alexander Oliver Wolfgang
Al Nazer Leith
Ip Paul
Lameda Physik AG
Stallman & Pollock LLP
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