Coherent light generators – Particular beam control device – Optical output stabilization
Reexamination Certificate
2002-03-07
2004-11-23
Wong, Don (Department: 2828)
Coherent light generators
Particular beam control device
Optical output stabilization
C372S029020
Reexamination Certificate
active
06822985
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The present invention relates to a laser-diode-pumped solid-state laser apparatus and a status diagnostic method of the apparatus. Particularly, the present invention relates to a highly reliable laser-diode-pumped solid-state laser apparatus and a status diagnostic method of the apparatus, which are capable of performing degradation diagnosis for a laser diode simply and easily.
2. Description of the Related Art
A so-called LD (laser diode) pumped solid-state laser, which has a laser diode (hereinafter, abbreviated as LD) with higher absorption efficiency to a laser medium than a lamp and is small, highly efficient and has a long lifetime as a pumping light source, has drawn attention in recent years as a method of pumping light from a solid-state laser medium such as an Nd:YAG. Particularly, an LD-pumped solid-state laser apparatus has been developed in recent years, which emits a laser output reaching a kilowatt by using a few hundred LD's in one resonator.
It is believed that the LD has a more than ten times longer lifetime than the lamp and the LD can be continuously used for as much as 10,000 hours. However, the lifetime is an average and output from some LD's reduces after a few thousand hours, and it is difficult to completely recognize and remove them at initial LD selection. Further, since the LD reduces its lifetime considerably due to disturbances and changes in the external environment, such as static electricity, electric surges from a power source, return light, dust, gas and condensation, it is necessary to detect a quantity of a light pumped from the LD and know a degree of its degradation by some means in order to improve the reliability of the laser apparatus and to deal with a failure quickly.
Accordingly, in the LD for low output operation at 10 W or less, a photo-detector disposed in an LD package detects the energy of the pumping light that slightly leaks from a high reflection surface (rear surface) opposite to an emission surface (front surface) of an LD chip, and it is used for controlling a pumping light quantity or detecting degradation of the LD.
Further, a method has conventionally been used in which a laser oscillation light emitted outside the resonator from an output mirror that composes a solid-state laser resonator is partially split or the photo-detector measures the energy of the oscillation light leaked from a mirror other than the output mirror, thus controlling the laser output or detecting the degradation of LD.
Furthermore, a method has also been proposed, which detects fluorescence intensity or fluorescence distribution in a direction along a laser oscillation optical axis or on its extension. FIG.
1
and
FIG. 2
show the method described in Japanese Patent Laid-Open (unexamined) No. 2000-269576. A method is proposed in which a monitoring mirror splits fluorescence emitted from a solid-state laser rod along the laser oscillation optical axis, a CCD camera transforms its pumping distribution into an image for observation, and a drive current for each LD is adjusted individually based on the image to unify the pumping distribution. The prior art will be described as follows.
In the conventional example,
FIG. 2
shows a following method. When setting a value of the drive current for each of LD's
102
to
107
, excitation is performed first in the state where the mirror of the solid-state laser resonator is removed, variable resistors
122
to
127
adjust the values so as to unify the pumping distribution while a CCD camera
130
observes the pumping distribution, and the resonator mirror is attached again. Further, as shown in
FIG. 1
, a method is also proposed that a mirror
128
is inserted in the resonator only when measuring the fluorescence distribution in the state where the resonator mirrors
120
,
121
are attached, and the CCD camera
130
observes the fluorescence from a laser rod
101
through the mirror
128
.
Furthermore, this conventional example describes another method of making a drive power source
117
for LD small, simplifying wirings, and adjusting the drive current for each of the LD's
102
to
107
, in which all LD's are connected in series and driven by one power source
117
, and variable resistors
122
to
127
are connected to each of the LD's
102
to
107
in parallel, and thus controlling a current value that flows in each LD.
However, an LD chip for high output operation at 20 W or more, which has been widely marketed, has the emission surface of about 1 cm in length. Accordingly, a wide detector is required to detect all the light from the width of 1 cm by the photo-detector provided in a rear position, and output from the photo-detector saturates unless a neutral-density filter is provided before the photo-detector because much light quantity leaks from the rear surface of the chip. As a result, since the configuration for light detection becomes complicate and its cost also increases, the LD for high output operation is not regularly provided with a mechanism for detecting the light quantity in its package in many cases. Moreover, in the case of a configuration called a stack where the LD's are laminated for a semiconductor layer with a narrow gap in a perpendicular direction, photo-detection function is not regularly provided due to little space for providing the photo-detector on the rear surface. For this reason, it has been impossible to directly detect the degradation of LD in the case of a high output solid-state laser apparatus using the high output LD's.
Optical output that can stably operate the LD device having the emission surface of 1 cm in length is from 20 W to the maximum level of about 80 W. Accordingly, a plurality of the LD's need to be used in the case of the high output laser apparatus that requires higher excitation. For example, there is a case where a few tens to a few hundred or even more LD's are used as a pump source with respect to a solid-state laser medium to obtain the laser output in the kilowatt class from the solid-state laser apparatus. When a very large number of LD's are simultaneously used, the number of power sources becomes large if each LD device is individually provided with the drive power source, a wide installation volume is needed, the wirings become complicate, and thus leading to low operation efficiency. Therefore, a few to a few tens of LD's are driven by connecting them electrically in series as a group regularly.
As described, when a large number of LD's are driven in series, it is impossible to increase the drive current for a particular LD to compensate the output reduction of the LD even if individual LD is provided with the photo-detection function and the function can detect the output reduction of the particular LD among the LD's, because a plurality of LD's are electrically connected in series. For example, when compensating the output of the particular LD by increasing the drive current, the output of the other LD's having no output reduction becomes larger than an initial value. Thus, the pumping light output of the whole group increases remarkably, which may lead to increase of the solid-state laser output exceeding an initially set value. In other cases, excitation balance with the LD's of another group becomes unstable, which may conversely lead to reduction or instability of the solid-state laser output or quality reduction of an emission beam. Moreover, when each of a few tens of LD's is provided with the photo-detector, wirings and circuits for controlling them become very complicate, and thus increasing the apparatus cost.
In addition, as a well-known conventional method, it is possible to detect laser oscillation light quantity taken outside the solid-state laser resonator to know indirectly the degree of LD degradation. However, the oscillation light quantity form the resonator does not reduce only due to the LD degradation but also reduces considerably due to alignment slipp
Mukaihara Katsuji
Tsunekane Masaki
Jackson Cornelius H.
Laserfront Technologies, Inc.
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