KR100348998B1 - Solid-state laser module with diffusive cavity pumped by radially positioned laser diodes having line shape emitters - Google Patents

Abstract

The present invention relates to a solid state laser generator that realizes high-efficiency laser generation by laterally exciting a solid state laser gain medium such as Nd: YAG using a diode laser. To this end, in the present invention, a diode beam is incident on the excitation internal reflector containing a gain medium directly through a narrow slit without using expensive leveling and focusing optics. By properly maintaining the distance between the diode emitting surface and the gain medium and adjusting the concentration of Nd ions in the gain medium, the diode beam absorbed directly and the beam after several diffuse reflections are absorbed at an appropriate ratio, which dramatically improves the uniformity of the laser beam. Improved. And using a special shape and material to facilitate the cooling of the diode using a heat radiating fin for cooling, and the structure of the laser rod and the diode laser is simultaneously cooled through the blocks at both ends, it has a very simple shape. Blocks at both ends are processed with duralumin, which is light in weight and has excellent mechanical properties, greatly reducing the weight of the device and facilitating mass production. A ceramic coating is applied to the inner surface of the block so that distilled water used as cooling water is not contaminated. Doing.

Description

Solid-state laser module with diffusive cavity pumped by radially positioned laser diodes having line shape emitters

The present invention relates to a diode-excited solid state laser device. Diode-excited solid-state lasers are devices that generate lasers by exciting solid-state laser gain media such as Nd: YAG, Nd: YVO ₄ , and Yb: YAG using high-power diode lasers. The device uses a diode-excitation method, which increases energy conversion efficiency by 10 times compared to the method using an excitation light, and allows the laser to oscillate stably over a long period of time. There are two methods of exciting a diode laser: the cross-section excitation method and the side excitation method. In the cross-sectional excitation method, the direction of the diode laser beam and the optical axis of the laser coincide, but in the lateral excitation method, they are perpendicular to each other. In general, the cross-sectional excitation method has a very good laser oscillation efficiency and the spatial distribution of the beam is close to TEM ₀₀ mode. However, in the single-sided excitation method, the number of available diodes is limited, so they are used for low power lasers of 20 W or less. The side-excitation method allows a large number of diode lasers to be placed on the side, so it is suitable for high-power lasers of several kW class, and high-power lasers mainly use this method.

There are two kinds of diode lasers used for solid state laser excitation. When the increase of peak power is important, pulse type is used, and when average power is important, continuous oscillation type is used.

The shape of the gain medium is a round rod shape and a thin slab shape. The rod shape has the advantage of easy processing. The slab type is difficult to process, but it is used to generate high quality laser by compensating for the change of thermal refraction. Only one or two sides of the slab type can be used here, and the shape and appearance of the diode laser can be used easily. When using a laser gain medium, excitation can be performed in any direction on the side, but the beam spread of the diode laser should be considered.

The light emitting surface of the diode laser is long and very narrow in one direction, so that the divergence angle of the output beam is very large in the narrow direction, but not very large in the wide direction. Therefore, the longitudinal direction of the diode laser coincides with the direction of the laser crystal. In addition, in the narrow width direction, the excitation laser beam from the diode laser spreads very quickly, so that the cross section of the beam becomes larger than the diameter of the gain medium unless it is very close to the gain medium. have.

Various methods have been invented to solve this disadvantage. The most common method is to place collimating optics in front of the diode laser output plane to reduce the divergence angle. (US Patent 5,548,608) This method is very effective for delivering diode output to relatively distant laser gain media. The optical system is expensive and there is a disadvantage that the output is reduced due to the optical system.

The multilayer diode laser is well developed because it is easy to generate a large power of several kW with a simple structure. However, the light emitting surface is so wide that it is not suitable for use in effectively exciting a laser gain medium having a narrow cross section. A cusp-shaped reflective surface is used to collect the sunlight on a large surface and to generate a laser. Inventing a device that delivers diode output to the laser gain medium by using this, it was intended to utilize a large output laser having a wide surface. (T. Brand, Compact 170W continuous wave diode pumped Nd: YAG rod laser with a cusp-shaped reflector, Optics letters, Vol 20, No. 17, 1776 (1995)). In order to utilize this device, the position of the laser gain medium must be accurately aligned at the position where the diode output light is focused, so that it is difficult to align and to process the reflective surface. In addition, since the area of the diode laser is quite large, and the surface is directly exposed to the reflector, the diode laser beam reflected back has a high risk of damaging the diode and shortening its lifetime, and the diode laser occupies a large portion of the reflector, thereby increasing the excitation efficiency. This has the disadvantage of deteriorating.

Since the diode laser beam is rapidly diverging, there is a method of confining it using a thin glass plate and transferring it to near the gain medium. (S. Konno et al. : YAG rod laser ", Appl. Phys. Lett. 70 , 2650 (1997)).

This method is very difficult to deliver diode output to thin glass plates, and the glass plates have a weak impact and are not suitable for mass production. A similar method is to collect the diode output into the fiber and then place the fiber end near the laser gain medium (D. Golla et al. 62-W CW TEM00 Nd: YAG laser side-pumped by fiber coupled diode). laser, Opt. lett., Vol. 21, 210 (1996))

A method of delivering the laser power directly to the gain medium has also been proposed. Diode laser power is transmitted from the outside to the cylindrical glass tube through which the coolant flows. The outer wall of the cooling water pipe is reflective coated, and the diode beam is transmitted through the narrow groove not having the reflective coating. At this time, considering the divergence angle of the diode laser, the diameter of the laser gain medium and the divergent diode laser beam are arranged to be the same. (Louis Cabaret et al., Rod laser with optical pumping from a source having a narrow emitting area, US Patent, 5,033,058).

The diode beam delivered near the laser gain medium absorbs a significant amount because the wavelength matches well with the absorption wavelength of the laser medium. However, beams that are not absorbed and transmitted or scattered on the surface of the laser gain medium must be reflected and used again. Typical reflectors for diode beams include metals such as silver and gold and diffuse reflectors. In the above-described invention, Konno et al. Used a diffuse reflector made of ceramic material, Golla et al. Used a gold coated glass tube, and Brand et al. Used a gold coated metal surface. When a metal reflector is used, the uniformity of the excitation beam is lowered due to the effect of the excitation beam reflecting back on the curvature reflecting surface and focusing on the gain medium.

Both methods are commonly used in flashlight excited lasers. In flashlight excitation lasers, the light sources are arranged together in the same reflector internal space, and the structure is very different from the invention in the diode excitation method in that they are located at the confocal point of the ellipse or at a very close distance (Dan Bar-Joseph, Laser pumping cavity, US Patent 4,858, 243 (1989) and WW Buchman, Laser pumping cavity with polycrystalline powder coating, US Patent 3,979,696 (1976).

SUMMARY OF THE INVENTION An object of the present invention is to excite a solid gain medium using a diode laser and to excite with high efficiency in consideration of the oscillation characteristics of the diode. To this end, a diffuse reflector reflecting surface is used to enable uniform energy excitation in the laser gain medium to improve processing characteristics. A straight slit was applied to the reflector and a diode laser was brought close to the slit so that the diode laser was absorbed by the gain medium without passing through the slit. The Nd ion concentration of the gain medium was adjusted and the distance between the gain medium and the diode laser was optimized to optimize the direct and indirect absorption of the diode laser in the gain medium. In order to effectively stabilize the laser output, the diode laser was cooled by increasing the cooling area by using a cooling fin with an icicle cross section for effective cooling of the diode laser, which is the most important factor. On the other hand, the cooling water flows through the blocks at both ends of the laser generator to cool the laser gain medium and the diode laser at the same time. The blocks at both ends are processed with duralumin, which is light and have excellent mechanical properties, greatly reducing the weight of the device and facilitating mass production. The inner surface of the block has a ceramic coating to prevent the distilled water used as cooling water from being contaminated.

1: A cross-sectional view of an apparatus for exciting a laser gain medium using a long straight diode laser.

2: Path where a diode laser beam excites a laser gain medium.

3: Experimental results showing that the spatial distribution of laser power is very uniform.

4: Spatial distribution of the uneven laser output when the Nd concentration of the laser gain medium becomes 0.9%.

5: A structure for supporting a laser gain medium and flowing coolant without using a sleeve.

6: Path where the coolant cools the laser gain medium and the diode laser simultaneously.

7: Assembly view of the laser generating device.

<Explanation of symbols for main parts of drawing>

1: diode laser 2: cooling plate

3: coolant 4: slit

5: cooling water pipe 6: cooling water

7: laser gain medium 8: diffuse reflector

9: path of diode laser beam 10: fixed rod

11: Blocks at both ends 12: Wedge-shaped laser gain medium support

13 laser gain medium dust cover 14 dust cover

15: icicle shaped cooling fin cross section

According to the present invention, a plurality of diode lasers 1 having a long straight shape are arranged around the circumference in the direction of the laser gain medium 7, and at the same time, the laser gain medium 7 is excited from the side to produce the laser. The apparatus for oscillating is described.

The diode laser 1 has a narrow and long linear light emitting surface and has a characteristic of spreading with a very large divergence angle of about 40 ° in the short axis direction and about 10 ° in the short axis direction. The present invention efficiently delivers the laser beam having a large divergence angle to the laser gain medium 7 having a cylindrical structure in the diffuse reflector 8 to obtain high efficiency by uniformly exciting the laser gain medium 7. At the same time, a spatially uniform laser beam distribution is obtained. In addition, by using the special icicle cooling fins 15, the heat generated from the diode laser 1 and the laser gain medium 7 is effectively discharged. Prevent damage.

The device cross section of the invention is shown in FIG. The diode laser 1 may be an assembled object commercialized by connecting a diode chip and an electric wire in advance. (US Patent 5,913, 108) The assembled diode laser is operated for a long time without being damaged only when cooled. In addition, since the oscillation wavelength of the diode laser changes as the temperature rises, sufficient cooling and temperature stabilization are necessary to stably oscillate the laser. The diode laser is therefore attached to the chiller 3 which is cooled with water. Inside the cooling device 3, a cooling fin 15 having an icicle-shaped cross section extends in the longitudinal direction in which the cooling water flows, thereby effectively transferring heat generated by the diode laser 1 to the cooling water. The cooling fins 15 located directly under the light emitting shaft of the diode laser (1) have a particularly high height for better cooling. The material of the cooling fins 15 is oxygen free copper or beryllium (Be) having good thermal conductivity. Contained copper can be used. The process by which the diode beam is absorbed in the laser gain medium 7 is shown in FIG. 2. The diode laser beam 9 is absorbed by the laser gain medium 7 through the slit 4, the coolant pipe 5 and the coolant 6. The shape of the slit 4 is a long rectangle similar to the output surface of the diode, and the width and length of the slit 4 are processed larger and longer than the diode emitting surface in consideration of the beam spread of the diode laser. However, if the slit 4 is too large, the reflected beam leaks out and the efficiency is lowered, so that the distance between the slit 4 and the diode emitting surface is considered appropriately. The beam that has not been absorbed by the laser gain medium 7 and has passed or passed through the laser gain medium 7 is scattered in the plane of the diffuse reflector 8. The scattered laser beam 9 is again absorbed in the laser gain medium 7 or reflected once again in another diffuse reflection surface and then absorbed in the laser gain medium 7. For diffuse reflection, polymer spectra is used as the material of the reflector. Spectralone has a reflectance of at least 99% at 808 nm. It is also possible to use reflectors of different materials with the same shape. Copper or brass with good thermal conductivity are processed into the same shape and then corroded to weak acid to give diffuse reflection properties. When gold plating is applied to the outer wall of the processed object, the reflector 8 has a very high reflectance.

The internal structure of the diffuse reflector 8 device is cylindrical symmetrical. The laser medium is located in the center of the diffuse reflector 8 structure so that the directly absorbed diode laser 1 beam and the beam reflected from the diffuse reflecting surface are symmetrically absorbed. In addition, the ratio of directly absorbed beams and diffusely reflected beams is appropriate so that uniform excitation is achieved over the front face of the laser gain medium 7. Uniform excitation distribution is necessary to minimize the thermal lens effect and the birefringence effect due to heat. Reducing this effect is very important for high efficiency oscillation and the present invention has a structure of a reflector that enables oscillation of a laser having a uniform distribution.

The use of the diffuse reflector 8 in the present invention is an important process for inducing an excited state with a uniform distribution. In the diffuse reflection process, the diode beam loses its initial orientation and is directed in any direction. In the direct absorption process, the beam is absorbed by a beam having a predetermined direction, and thus is not evenly absorbed across the front end surface of the laser gain medium 7. As a result, the distribution of the absorbed beam is in a state where the gain medium toward the excitation direction is further excited to directly reflect the effect of the distribution of the excitation diode beam. If the excitation direction is greatly increased in more than five directions, the excitation uniformity by direct absorption will increase with the increase of the excitation direction, but the manufacturing cost increases due to the complexity of the device, and the effective area of the reflector is due to the area occupied for delivering the diode beam. This reduces the overall energy conversion efficiency and reduces the economics.

In the present invention, the direction of the laser to deliver the diode beam is limited to three places for the convenience of assembly and reduction of the manufacturing cost.

In order to uniformly excite while sending the diode beam in three directions, the distribution according to the angle of the diode laser is considered. Diode beam output has more than 90% power distribution within an angle of 40 degrees.

Specific specifications of each site used for uniform excitation in the present invention are as follows.

Specifications of the laser gain medium 7; Diameter: 3mm, length: 59mm, concentration of Nd: 0.6%

The size of the diffuse reflector 8; Inner diameter; 8mm

The size of the slit 4; Length: 34 mm, Width: 1.2 mm, Depth 0.7 mm

The specification of the cooling water pipe 5; Outer diameter: 6 mm, inner diameter: 4 mm

When the diameter of the laser gain medium 7 is 3 mm, the divergence angle of the diode laser 1 beam and the angle incident on the laser gain medium 7 at the diode laser 1 emitting surface substantially coincide. Uniformity is very poor when the directly incident beam is absorbed in the first pass. If the Nd concentration of the laser gain medium 7 is high, absorption is concentrated only on the portion closest to the diode incidence plane, so that even when the laser is oscillated, the spatial distribution of the laser beam is uneven and the efficiency is also lowered. Therefore, in order to increase the uniformity of the laser, the concentration of Nd ions was lowered to 0.6%. In this way, the beam which is not first absorbed by the laser gain medium 7 is reflected back at the external diffuse reflection surface and is absorbed again evenly. If the concentration is smaller than this, the uniformity is further improved but the efficiency is lowered. With this concentration, even excitation can be achieved by sending a diode beam in three directions.

When the diameter of the laser gain medium 7 is 2 mm, the angle incident on the laser gain medium 7 at the diode laser emission surface is about 20 degrees such that only about half of the diode output passes directly through the laser gain medium 7. The remaining diode beam passes through the diffuse reflection surface and then passes through several times to be absorbed. Therefore, even if the concentration of Nd ions is increased to about 0.7%, uniform excitation is achieved, which is very advantageous for high frequency oscillation in the TEM ₀₀ mode, and enables highly efficient excitation.

When the Nd: YAG rod having a diameter of 3 mm and an Nd ion concentration of 0.6% was excited, a laser beam having a very uniform spatial distribution as shown in FIG. 3 was obtained. When the concentration of Nd ions increases to 0,9%, the uniformity is very low as shown in FIG. Due to the uniform space distribution, the slope efficiency became very good, reaching 40% to 50%. Therefore, it has been proved that high efficiency is possible even with a simple structure as in the present invention. In addition, even if the device was operated for several hundred hours or more, the oscillation of the output was very stable, within 2%.

There are additional devices and functions that achieve high efficiency in the present invention. The surface of the cooling water pipe 5 of FIG. 1 is coated with a dielectric thin film that transmits light of a laser wavelength without reflection, so that a loss is small even when a diode beam is advanced several times. In the inner surface of the cooling water pipe 5, the contact with the cooling water and the difference in refractive index is small, so that the reflection is small.

One of the important devices to achieve high efficiency is to have diffuse reflections at both ends of the laser gain medium (7) so that they are scattered during diffuse reflection process inside the diffuse reflector (8). It is in shortened. The reflecting surfaces at both ends are separated from the diffuse reflection body 8 on the center where the slit 4 is processed, or processed integrally. Integral type is easy to assemble because the structure is simple, there is an advantage that the separation is not easy for vibration from the outside, but the processing is difficult, the separate type is more easy to process but more complex assembly.

One of the important devices to get a stable output is in the structure without the sleeve at both ends, as shown in FIG. If a sleeve having an inner diameter slightly larger than the diameter of the laser gain medium 7 is used to support the laser gain medium 7, the laser gain medium 7 may be caused by vibrations caused by turbulent flow of coolant or external vibration. It vibrates finely and causes oscillation of the output because the oscillation path of the laser beam is shaken. In the present invention, the laser gain medium 7 directly supports the blocks at both ends, and has a very stable structure.

Blocks 11 at both ends in FIG. 6 also serve to distribute the cooling water simultaneously. Since the coolant 6 flows through the wedge-shaped laser gain medium support 12 in which the inlet becomes narrower, the flow rate of the coolant 6 flows to the surface of the laser gain medium 7 very quickly and causes an increase in pressure. It also creates turbulence at the surface of the laser gain medium 7 to induce effective cooling.

Cooling water can be introduced into one or both ends of the block 11 at both ends, so that the cooling water can be connected and flowed very easily. Both ends of the blocks are connected to each other with a support rod so that the support is mechanically and firmly supported.

In addition, the blocks at both ends are invented so that the cooling water can flow not only in the laser gain medium 7 but also in the diode laser cooling plate 2 at the same time. Cooling fins 15 having an icicle-shaped cross section were attached to the cooling plate of the diode laser in parallel with the direction in which the cooling water flows. As a result, the flow of the cooling water 6 is smoothed to provide a structure in which cooling is efficiently performed without a drop in pressure. In this way, a very simple and simple connection path for the coolant 6 can be provided to effectively cool the entire apparatus.

7 shows the assembled cross section of the laser. In the assembly diagram, the blocks 11 at both ends have the largest weight in the laser device, so they are processed with duralumin material for weight reduction. Since the laser device is cooled using distilled water, the surface is converted to ceramic to prevent contamination of the distilled water. As a result, the laser can be stably oscillated for a long time without exchanging distilled water. In addition, the laser gain medium dust cover 13 and the dust cover 14 were used to prevent dust from entering the laser gain medium 7 and the diode laser 1.

As described above, the present invention can achieve simple laser beam transmission and uniform laser gain medium excitation without using an optical system by using a method of exciting a laser gain medium inside a diffuse reflection body through a slit of a diode laser. In addition, by effectively removing the heat of the laser diode using a high-efficiency laser diode cooling fins, a high-efficiency high-performance laser generator capable of generating a laser stably for a long time despite the change in the coolant temperature is possible.

Claims (3)

In the apparatus for exciting a laser gain medium 7 using a linear diode laser 1, A plurality of diode lasers (1) positioned in the longitudinal direction of the laser optical axis on the same circumference as a light source for exciting the laser gain medium (7), A slit 4 of suitable size for passing the beam out of the diode laser 1, A plurality of cylindrical diffuse reflectors 8 positioned opposite the diode lasers 1 for diffusely reflecting the beams of the diode lasers 1 exiting through the slits 4, A fixed rod 10 for fixing the plurality of cylindrical full reflection bodies 8, A cooling plate 15 constituting a body and a cooling fin 15 having an icicle-shaped cross section for cooling the diode laser 1; A wedge-shaped laser gain medium support 12 for directly fixing the laser gain medium 7 without using a sleeve; Both ends of the block 11 coated with ceramic in a lightweight duralumin material that allows the cooling water 3 for cooling the diode laser to flow simultaneously in the direction of the cooling gain 15 of the laser gain medium and the icicle cross section, and Using a plurality of linear diode lasers arranged radially, characterized by a structure in which the cooling plate 2 and the blocks 11 at both ends installed to cool the diffuse reflector 8 and the diode laser 1 are integrally manufactured. Solid state laser generator. The method of claim 1, Cylindrical diffuse reflector (8) is a solid laser generator using a plurality of radially arranged linear diode laser (1) characterized in that the material is made of a polymer such as spectralon with excellent reflectance. The method of claim 1, Cooling fin 15 having an icicle cross section is made of a solid-state laser generator using a plurality of linear diode lasers (1) arranged radially, characterized in that the material is made of oxygen-free copper or copper containing beryllium (Be) .