Solid state laser

The solid state laser comprises the laser medium 11 for absorbing the pumping light 12 from the LD 10 and generating or amplifying the light having a predetermined wavelength and a heat sink comprising the first block 15a, the second block 15b and the third block 15c for aligning the laser medium 11, cooling the laser medium 11 and reflecting the pumping light 12. The laser medium 11 has an arcuate light incident surface A and a flat reflection portion. The pumping light 12 having a diffusing angle is converted into parallel light by the light incident surface A, thereby pumping the laser medium 11 uniformly. The flat reflection portion makes it easy to align laser medium 11 having anisotropic characteristic and helps to efficiently pump the laser medium 11 by reflecting the reflection light substantially in parallel with the incident light.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a solid state laser, more specifically to a solid state laser pumped by a laser diode (abbreviated as LD below) as a pumping light source. Such solid state laser is excellent in both pumping efficiency and pumping distribution as well as easy in aligning the crystal axis even in an anisotropic laser medium.

2. Description of Related Art

LD-pumped solid state lasers using a LD as a pumping light source attract considerable attention as compact, highly efficient and longer lifetime laser light sources as compared to lamp-pumped solid state lasers. In particular, performance and quality of a LD have been improved significantly in recent years, thereby accelerating practical applications of such LD-pumped all solid state lasers. Such all solid state semiconductor lasers extend lifetime to 10,000 hours as compared to 1,000 hours of conventional lamp-pumped semiconductor lasers. Additionally, since the absorbing wavelength of the solid state laser medium and the oscillation wavelength of the LD are equal to each other in the LD-pumped solid state lasers, laser crystals can be pumped highly efficiently. High quantum efficiency helps to decrease thermal lens effect, thereby significantly improving the beam quality. As for pumping methods, various types of solid state lasers have been proposed and made by utilizing the so-called side surface pumping method in which the LD is disposed in parallel with the side surface of the light axis of the oscillating light from the elongate solid state laser medium.

Widely known as the laser media to be utilized for industry-use solid state lasers are Nd:YAG crystals and Nd:YVO4crystals. Nd:YAG crystals feature high mechanical strength and excellent thermal conductivity, but are not suitable for oscillation at high pulse repetition rate over 30 kHz because of longer fluorescent lifetime. Additionally, Nb:YAG crystals are easy to handle because they have no crystal axis characteristic, thereby exhibiting no dependence to the polarizing characteristic of the pumping light in pumping efficiency. On the other hand, Nb:YVO4crystals are inferior to Nb:YAG crystals in mechanical strength and thermal conductivity but have shorter fluorescence lifetime and higher induced emission cross sectional area than Nb:YAG crystals, thereby making them more suitable material for oscillating at higher pulse repetition rate. However, since Nb:YVO4crystals have a single crystal axis, when used for laser oscillation, they not only oscillate in polarized in a particular orientation but also exhibit anisotropy in light absorbing characteristic. Since LDs tend to oscillate generally in polarized in a particular orientation as described hereinabove, the following Reference Patent 1 discloses that high pumping efficient can be achieved by choosing the orientation of polarization with respect to the crystallization.

Reference Patent 1

An example of the side surface pumping type of LD-pumped solid state laser is disclosed in the following Reference Patent 2.

Reference Patent 2

As shown inFIG. 6, the laser comprises a reflection layer64for enclosing the circumference of a cylindrical laser medium61along its optical axis and an opening63in a part of the reflection layer64for directing a pumping light62from a LD60into the laser medium61. The reflection layer64covers almost the entire outer surface of the laser medium61except a small portion for directing the pumping light62from the LD60. In this way, the pumping light62from the LD60is entrapped inside the laser medium61for efficient absorption of the pumping light62in the laser medium61, thereby improving efficiency of oscillation energy.

As an example of the LD-pumped solid state laser for fixing the laser medium for proper alignment of the crystal axis of the laser medium in such a manner not to focus the reflection light onto a single point, there is proposed one using a rectangular rod shaped laser medium as shown in FIG.7. The LD-pumped solid state laser comprises a reflection layer74covering the outer circumference of a rectangular rod shaped laser medium71along its optical axis and an opening73formed in a part of the reflection layer74for allowing a pumping light72from an LD70to enter the laser medium71. Most of the outer surface of the laser medium71is covered by the reflection layer74except a small portion for receiving the pumping light72from the LD70. In this way, the pumping light72from the LD70is entrapped inside the laser medium71for efficient absorption of the pumping light by the laser medium71, thereby improving efficiency of oscillation energy. The use of rectangular Nd:YVO4crystal as the laser medium74enables to align the orientation of the optical electric field of the pumping light72in parallel with the c-axis of the Nd:YVO4crystal that is cut along the a-axis.

An example of a solid state laser capable of uniformly pumping the laser medium without using a focusing optical system is disclosed in, for example, the following Reference patent 3.

Reference Patent 3

As shown inFIG. 8, the solid state laser utilizes the particular feature of a laser medium81that exhibits a predetermined refractive index with respect to the pumping light. The laser medium81is disposed adjacent to a source of pumping light82and the laser medium81has a convex surface portion86at the side toward the pumping light source, thereby reducing the diffusing angle of the pumping light82without using a focusing optical system.

Also, an invention for effectively returning the pumping light not absorbed by a solid state laser rod (laser medium) back into the solid state laser rod is disclosed in the following Reference Patent 4.

Reference Patent 4

Unfortunately, however, the solid state laser as disclosed in the Reference Patent 4 and shown inFIG. 9is a multiple coaxial construction comprising a solid state laser rod1, a gap for cooling medium2, a cooling tube3and a cylindrical member4for providing a reflection surface. Accordingly, any pumping light not absorbed by the solid state laser rod1passes through the gap for the cooling medium2and the cooling tube3for at least twice, thereby attenuating the pumping light and failing to effectively utilize the reflection light.

However, all of the above mentioned conventional LD-pumped solid state lasers have a common problem that the laser medium can not be uniformly pumped by the LD.

For example, in the example as shown inFIG. 6, since the reflection layer64is circular, the reflection light tends to be focused on a single point, thereby not uniformly pumping the laser medium61by the LD (See FIG.10). On the other hand, in the example as shown inFIG. 7, although the rectangular reflection surface helps to distribute the reflected light over the entire area rather than focusing on a single point, the dispersing angle of the pumping light72from the LD70tends to cause diffusion in pumping distribution that is weaker at the outer side of the laser medium71, thereby causing high dimension spatial mode. As a result, the beam quality may be deteriorated as mentioned in the following Reference Patent 5. It is preferable that the laser medium is pumped by the pumping light in a parallel manner.

Reference Patent 5

Other problems associated with the conventional solid state lasers include difficulty in aligning the crystal axis of the laser medium and incapability of uniformly and efficiently pumping the laser medium.

For example, in the solid state laser as shown inFIG. 6, it is difficult to align the crystal axis of the laser medium61because it is cylindrical. In the solid state laser as shown inFIG. 7, the laser medium71is rectangular which is easy to align the crystal axis, but the pumping light72from the LD70tends to largely diffuse in the laser medium71, thereby not uniformly pumping it. In the solid state laser as shown inFIG. 8, the laser medium81is convex in one surface but flat in the opposite surface on which a mirror is disposed. As a result, a part of the pumping light82passes through the output mirror84, thereby not efficiently pumping the laser medium81. Additionally, no attention is paid to the relationship between the crystal axis of the laser medium81and the pumping light82.

SUMMARY OF THE INVENTION

Taking the above problems of the conventional solid state lasers into consideration, it is therefore an object of the present invention to provide a solid state laser having excellent pumping efficiency and distribution of the pumping light and yet easy to align the crystal axis even in the laser medium having anisotropic property.

The solid state laser according to the present invention is directed to one comprising a solid state laser medium, a pumping light source disposed at one side of the solid state laser medium for pumping the same and a reflection portion for recovering any pumping light from the pumping light source that is not absorbed by the solid state laser medium. The solid state laser is characterized in that the solid state laser medium has the surface facing the pumping light source as a light incident surface formed in such a manner that the pumping light is in parallel in the solid state laser medium, and the surface facing the reflection portion is flat so as to return the reflected light from the reflection portion into the solid state laser medium in a parallel manner. In this particular construction, the diffusing pumping light from pumping light source is converted into parallel light inside the laser medium and any light not absorbed by the laser medium in a first pass is reflected back into the solid state laser medium, thereby improving both pumping efficiency and distribution. Also, it simplifies alignment of the crystal axis by making the flat surface of the solid state laser medium in contact with the reflection portion.

The solid state laser according to the present invention having the above construction exhibits various advantages. It is simple by not using any optical component, enables to pump the laser medium uniformly and in high density, and provides highly efficient and high beam quality laser oscillation. Additionally, in case of using anisotropic laser medium having different performances depending on the axial directions, the laser medium can be pumped efficiently because the laser medium can be aligned easily by using the flat surface facing the reflection portion.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

Now, a presently preferred embodiment of the present invention will be described in detail by reference to the accompanying drawings.

FIG. 1is a cross section view of the LD-pumped solid state laser according to an embodiment of the present invention (referred to as the embodied solid state laser below) in the plane perpendicular to the direction of laser oscillation axis.FIG. 2shows a perspective view of the laser medium to be used in the embodied solid state laser.FIG. 3shows a trace of pumping light when the embodied solid state laser is pumped.

The embodied solid state laser comprises a LD10, a laser medium11and a heat sink15comprising a first block15a, second block15band a third block15c.

The LD10used as the pumping light source12is 40 watts in output, 40 degrees in total diffusing angle (half-amplitude-width) and 800 nm in wavelength. Note that 1064 nm thick Ar coating and optical polishing are made on the laser oscillation edge surface for reducing loss of laser oscillation.

The laser medium11is generally D-shaped in cross section in perpendicular to the direction of laser oscillation axis. In other words, the surface facing the LD10that defines an incident surface A is arcuate, while the surface opposite to the incident surface A is flat. It can be said that the incident surface A is made convex similar to that of a convex lens. Furthermore, the laser medium11is formed with a reflection layer14except the portion opposed to the LD10which is the pumping light source. The reflection layer14on the flat portion constitutes a reflection surface B. The laser medium11is a crystal, more concretely Nd:YVO4that is cut in the a-axis and doped with Nd of 0.5 at %. As shown inFIG. 2, the c-axis of the crystal of the laser medium11is parallel with the flat portion. The laser medium11is a rod of 1.5 mm in diameter and 12 mm in length with a part of the side surface cut away in flat at the location 0.5 mm toward the center. The laser medium11absorbs the pumping light12from the LD10for generating and amplifying the light having a predetermined wavelength. Such materials as Nd:YAG, Nd:YLF, Nd:GdVO4, Yb:YAG, Yb:YLF, Yb:GdVO4, Er:YVO4, Er:YAG, Er:YLF, Re:GdVO4, Ho:YVO4, Ho:YAG, Ho:YLF, Ho:GdVO4, Ti:AlO3or the like may be utilized as the laser medium11.

The first block15a, the second block15band the third block15cthat are made from, for example, copper are disposed in such a manner to enclose the laser medium11. Additionally, there is formed a slit13between the first block15aand the second block15bso that the pumping light12from the LD10is inputted into the laser medium11. The third block15cis formed with a flat portion16to be used for alignment of the crystal axis of the laser medium11by abutment of its flat portion against such flat portion16. Although not shown in the drawings, cooling water is forced to circulate in each of the blocks for indirectly cooling the laser medium11and the LD10. Also, in order to improve the cooling efficiency, the contact portion with the laser medium11is well polished to remove unevenness, thereby enhancing adherence between the laser medium11and the heat sink. Additionally, the inner surfaces of the first block15a, the second block15band the third block15care well polished to form mirror surfaces and a highly reflecting film coating such as an Au coating is preferably applied thereto.

In the embodied solid state laser having the above mentioned construction, when the pumping light12is inputted onto the incident surface A of the laser medium11of D-shaped rod at an output end at the distance of 0.5 mm, the total diffusing angle of the pumping light can be reduced to 5 degrees or less. This means that the pumping light12is substantially parallel and the optical axis of the pumping light12passes through the center of the laser medium11. The pumping light12propagates into the laser medium11in substantially parallel while being absorbed by the laser medium11. About 30% of the pumping light12which is not absorbed by the laser medium11in the first propagation is reflected by the flat reflection surface B in substantially parallel with the incident light, thereby being substantially superimposed with the incident light to pump the laser medium11and significantly increasing the pumping efficiency.

Since the polarization surface of the pumping light12is parallel with the Y axis inFIG. 1, the polarization surface of the pumping light12is parallel with the c-axis of the laser medium11by abutting the flat surface portion of the laser medium11against the third block15c. Accordingly, the absorbing rate of the pumping light12by the laser medium11and the induced emission cross section area of the laser oscillation are maximized, thereby achieving efficient laser oscillation even if Nd:YVO4having anisotropic characteristic is utilized as the laser medium11. Now, anisotropy of the Nd:YVO4crystal will be described by way of a concrete example. If the laser medium made from an Nd:YVO4crystal is rotated 90 degrees from the crystal axis that can be pumped in the maximum efficiency, absorption rate of the pumping light (in the range of about 808 nm) is significantly decreased by about ¼˜⅕ and the induced emission cross section area is also decreased by about ¼˜⅕. However, in the present embodiment, since the side surface of the laser medium11is made partially flat against which the flat surface portion16of the third block15cabuts, thereby easily aligning the crystal axis with the axial direction to pump in the maximum efficiency.

As apparent from the above description, according to the embodied solid state laser, the incident surface A of the laser medium11is shaped so that the diffusing pumping light12from the LD10is converted into parallel light, thereby enabling to pump the laser medium uniformly and in high density with a simple construction that requires no optical system for converting the pumping light12into parallel light. Moreover, since almost the entire outer surface of the laser medium11is covered with the reflection layer14, the pumping light12can be entrapped inside the laser medium11, thereby enabling to efficiently pump the laser medium. Additionally, since a part of the side surface of the laser medium11is made flat, it is easy to align the crystal axis. As a result, it is possible to efficiently pump the crystal having anisotropic characteristic such as, for example, Nd:YVO4. This means that the embodied solid state laser can perform laser oscillation in high efficiency and high beam quality by using either anisotropic or isotropic laser medium.

It is to be noted that the shape of the laser medium is not limited to the particular shape as shown inFIG. 2, i.e., D-shape in cross section in perpendicular to the direction of oscillation axis. Similar advantages can be enjoyed as long as the laser medium has the incident surface for converting the diffusing incident light into parallel light and the reflection surface is flat. FIG.4andFIG. 5show modifications of the laser medium different from the one that is shown in FIG.1and FIG.2. In the laser medium41as shown inFIG. 4, both upper and lower surfaces of the D-shaped rod as shown inFIG. 2are cut off. In the laser medium51as shown inFIG. 5, the incident surface inFIG. 4is replaced by symmetrical upper and lower flat surfaces, thereby providing generally pentagonal cross section as a whole. In FIG.4andFIG. 5, all constituent elements other than the laser media41and51are identical to those in FIG.1.

Although the presently preferred embodiment of the present invention has been described hereinabove in greater detail, it is to be noted that the present invention should not be restricted to such embodiment. Various modifications may be made easily by a person having an ordinary skill in the art without departing from the scope and spirit of the present invention.