Solid-state laser oscillator

Details Solid-state laser oscillator Solid-state laser oscillator

2000-06-21

2003-05-20

Ip, Paul (Department: 2828)

Coherent light generators

Particular active media

Plural active media or active media having plural dopants

C372S068000

Reexamination Certificate

active

06567453

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a solid-state laser oscillator using solid-state laser rods. More particularly, this invention is concerned with a solid-state laser oscillator having a high average-power transverse single-mode resonator which uses Nd:YAG rods to be excited by a high average-power laser diode.
2. Description of the Related Art
An active medium in a transverse single-mode oscillator is excited by an exciting light source such as a laser diode or a flash lamp. The excited active medium or any optical component is used to achieve transverse single-mode laser oscillation. This results in a transverse single-mode output. There are many approaches to exciting of an active medium. One of the approaches is end exciting.
End exciting is an exciting method for exciting a laser medium which is placed substantially along the optical axis of a resonator. The exciting is performed from the end of the laser medium, wherein a laser diode is mainly used for exciting. Exciting light output from the laser diode that is placed substantially along the optical axis of the resonator is incident substantially perpendicularly on an end of an active medium coated with an antireflection coating that is non-reflective to light having the same wavelength as the wavelength of the exciting light. The light is then absorbed into the active medium. The active medium is thus excited. The resonator consists of the excited active medium, total reflection mirrors, a partial reflection mirror, and an arbitrary optical component. The total reflection mirrors are located ahead of and behind the active medium, placed substantially along the optical axis of the resonator, and have a property of totally reflecting light having the same wavelength as the wavelength of laser light. The partial reflection mirror reflects part of the light having the same wavelength as the wavelength of the laser light. The excited active medium has electrons of high energy states made a transition to a lower energy state that is a stable state. At this time, photons are emitted. The total reflection mirrors and partial reflection mirror included in the resonator cause the photons to orbit. This stimulated emission performed by the active medium causes laser light of a specified wavelength to be amplified. Part of the laser light is emitted from the partial reflection mirror.
According to the end exciting method, the directivity of the laser diode is utilized in order to excite the solid-state laser medium so that transverse single-mode resonant laser beam alone will be propagated. Consequently, transverse single-mode laser oscillation is achieved highly efficiently. However, an output being generated from a single stripe in the laser diode is limited because the end of the laser diode is destroyed. For providing a high-power output, the number of laser diodes must be increased. This leads to deterioration in the directivity of the laser diode. Consequently, it becomes hard to excite the solid-state laser medium so that the transverse single-mode resonant laser beam alone will be propagated. Furthermore, since exciting light is converged on a microscopic area on the end of the solid-state laser medium, the power density of the exciting light is generally high. When the average power of exciting light is raised, the solid-state laser medium may be thermally destroyed with the exciting light. Existing transverse single-mode oscillators adopting the end exciting method have therefore been limited to applied fields in which low average-power laser light is needed.
High average-power solid-state lasers therefore adopt side exciting. Side exciting is an exciting method for exciting an active medium in a direction perpendicular to the optical axis of a resonator using an exciting source such as a laser diode or a flash lamp.
A transverse mode dependent on a laser medium is determined with resonance conditions. A transverse single mode is considered as a sort of pattern exhibited by laser light whose beam radius of the cross section of the laser light with the ray axis thereof as a center is the smallest. Low-order and high-order transverse modes are considered as sorts of patterns exhibited by laser light having larger beam radii. Now, assume that the laser medium itself is thought to serve as a mode selection aperture. If the size of an excited laser medium is equivalent to the beam radius of the laser light exhibiting the transverse single mode, the high-order transverse modes are not selected but the transverse single mode is selected. In contrast, when the size of the excited laser medium is larger than the beam radius of the transverse single-mode laser light, a high-order transverse mode is selected. At this time, laser oscillation is achieved to generate laser light exhibiting a multi-mode that is a combination of a plurality of waveguide modes including the transverse single mode, low-order modes, and high-order modes. A laser output is therefore multi-mode laser light. Multi-mode laser light is poorer in directivity than the transverse single-mode laser light. As the multi-mode laser light is propagated, it spreads widely. The multi-mode laser light is characterized in that when an attempt is made to converge the multi-mode laser light on a lens or the like, the cross section of the multi-mode laser light is not narrowed. Compared with a transverse single-mode laser, therefore, a multi-mode laser is of little worth for the purposes of configuring laser equipment that utilizes propagation of laser light or of performing machining with converged laser light.
For producing transverse single-mode light highly efficiently, the beam radius of transverse single-mode light propagated in a resonator must be equivalent to the size of a laser medium.
For improving the average power of transverse single-mode laser light, a laser medium must be excited with high-power exciting light. When the laser medium is excited with high average-power light, heat is generated in the laser medium due to the exciting light. Generation of heat optically distorts the laser medium. The thermal distortion leads to a loss of laser light orbiting within a resonator while being amplified. A gain of laser light to be produced by the resonator increases proportionally to the power of exciting light. However, as long as transverse single-mode laser oscillation is concerned, when the magnitude of thermal distortion is small, a loss stemming from thermal distortion increases in proportion to the square of the magnitude of thermal distortion. When the laser medium is excited with high-power exciting light, the loss increases more greatly than an increase in the gain of laser light produced by the resonator. The maximum power of laser light is therefore limited. For efficiently performing laser oscillation so as to generate high average-power transverse single-mode light, it is necessary to minimize the thermal distortion of the laser medium.
In general, a rod-shaped solid-state laser medium is referred to as a solid-state laser rod. When the solid-state laser rod is excited, heat is generated. The solid-state laser rod is therefore cooled with a coolant placed by the side thereof. Heat is distributed over the cross section of the solid-state laser rod. This results in a difference in temperature causing distribution of refractive indices. In particular, when the solid-state laser rod is realized with an isotropic medium made of an isotropic crystal of yttrium aluminum garnet (Y
3
Al
5
O
12
) with an atom of neodymium (Nd) appended thereto (hereinafter Nd.YAG), the solid-state laser rod acts as a convex lens (which may be referred to as a heat lens) relative to laser light. As the power of exciting light is raised, the heat lens effect is intensified (the focal length of the solid-state laser rod gets shorter).
When a laser medium brings about any kind of birefringence, a heat lens effect exerted from the laser medium also provides a doublet lens effect. For designing a resonator capable of generating trans

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