JP2004191963A - Wavelength conversion method and wavelength conversion apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a UV laser beam suitable for a precision fabrication by enhancing the conversion efficiency of the laser beam when the wavelength of the laser beam is converted to generate harmonic waves at <355 nm wavelength in a wavelength conversion element consisting of a lithium tetraborate single crystal. <P>SOLUTION: The wavelength conversion element is heated while controlling and kept at a specified temperature (e.g. 200°C) in the temperature range from 200 to 450°C. The temperature accuracy is controlled to ±0.3°C, preferably ±0.1°C. The two-photon absorption in the lithium tetraborate single crystal decreases by heating the wavelength conversion element. Therefore, even when the intensity of the incident light is increased, transmittance for the laser beam hardly decreases, which improves the conversion efficiency for the laser beam. As heat generation caused by two-photon absorption is suppressed, a problem that the refractive index changes to degrade phase matching can be avoided, loss or an unstable state in the output can be prevented and the quality of the beam can be maintained. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

  The present invention provides a wavelength conversion method for converting the wavelength of a laser beam with a wavelength conversion element made of a nonlinear optical crystal for the purpose of mainly generating an ultraviolet laser, and heating and controlling the wavelength conversion element to a constant temperature. And a heating mechanism for performing the heating.

In recent years, in this type of wavelength conversion, a method using a wavelength conversion element made of a lithium tetraborate single crystal (Li ₂ B ₄ O ₇ ) having excellent chemical stability and laser damage resistance has been attracting attention. According to this method, various harmonics can be obtained from various fundamental waves based on the principle of the second harmonic and the principle of the sum frequency. For example, based on the principle of the second harmonic, a second harmonic (green light having a wavelength of 532 nm) of an Nd: YAG laser can be made incident on a wavelength conversion element to generate a fourth harmonic (ultraviolet light having a wavelength of 266 nm). Alternatively, based on the principle of the sum frequency, the fundamental wave of the Nd: YAG laser and its second harmonic are simultaneously made incident on the wavelength conversion element to generate a third harmonic (ultraviolet light having a wavelength of 355 nm), The fundamental wave and its fourth harmonic can be simultaneously incident on the wavelength conversion element to generate a fifth harmonic (ultraviolet light having a wavelength of 213 nm).

  However, since the nonlinear optical constant of the lithium tetraborate single crystal is as small as 0.16 pm / V, there is a disadvantage that conversion efficiency in wavelength conversion is low. Therefore, conventionally, the low conversion efficiency has been compensated by condensing incident light with a lens to generate a high-output ultraviolet laser.

  However, when the harmonics generated by wavelength conversion by the wavelength conversion element made of the lithium tetraborate single crystal are those having a wavelength of less than 355 nm (fourth harmonic, fifth harmonic, etc.), the lithium tetraborate single crystal 2 Since photon absorption occurs and heat is generated inside the wavelength conversion element, the refractive index changes and the phase matching property is lost. As a result, output loss or instability occurs, or beam quality deteriorates, which is not suitable for precision processing such as drilling of printed circuit boards and scribing of various electronic components.

  In view of such circumstances, the present invention provides a wavelength conversion method capable of increasing the conversion efficiency of a laser beam and generating a harmonic having a wavelength of less than 355 nm such as an ultraviolet laser suitable for precision processing. Aim.

  In the present invention, attention has been paid to the fact that a lithium tetraborate single crystal has a property of reducing two-photon absorption by heating.

That is, the present invention according to claim 1 controls the heating of the wavelength conversion element when the wavelength conversion element made of lithium tetraborate single crystal converts the wavelength of the laser beam to generate a harmonic having a wavelength of less than 355 nm. Then, it is configured to be maintained at a specific temperature within a temperature range of 200 to 450 ° C. with a predetermined temperature accuracy. Further, the present invention according to claim 2 is configured such that the temperature accuracy of the wavelength conversion element is within ± 0.3 ° C. Furthermore, the present invention according to claim 3 is configured such that the temperature accuracy of the wavelength conversion element is within ± 0.1 ° C.

  By adopting such a configuration, the two-photon absorption of the lithium tetraborate single crystal is reduced by heating the wavelength conversion element, so that even if the intensity of the incident light is increased, the transmittance of the laser beam is hardly reduced, and It acts to suppress heat generation due to photon absorption.

  According to a fourth aspect of the present invention, there is provided a wavelength conversion device for performing wavelength conversion by injecting a laser beam into a wavelength conversion element made of a nonlinear optical crystal, wherein an outer surface of the wavelength conversion element excluding an incident end face and an emission end face is provided. A heating block that heats the wavelength conversion element from the outside with a heater, a temperature sensor that detects the temperature of the heating block, and a temperature that controls the heating block to a constant temperature based on the temperature detected by the temperature sensor. When the length of the wavelength conversion element is L, the width and height of the wavelength conversion element are set to L / 4 or less, and the heating block is connected to the incident end face of the wavelength conversion element. And it is configured to protrude L / 3 or more in the length direction from the emission end face.

  According to a fifth aspect of the present invention, there is provided a wavelength conversion device for converting a wavelength by inputting a laser beam to a wavelength conversion element made of a nonlinear optical crystal, wherein an outer surface of the wavelength conversion element is covered, and the wavelength is converted by a heater. A heating block that heats the conversion element from the outside, a temperature sensor that detects the temperature of the heating block, and a temperature controller that controls the heating block to a constant temperature based on the temperature detected by the temperature sensor, and An optical path through which a laser beam passes is formed on a surface of the heating block on the optical axis on the entrance side and the exit side.

  Further, according to the present invention as set forth in claim 6, the heating block includes a metal material having a high thermal conductivity such as copper having a built-in heater along a length direction, a ceramic covering the outside of the metal material, The present invention according to a seventh aspect of the present invention is configured by applying a heat-resistant plating process to the surface of the metal material.

  With such a configuration, the wavelength conversion element can be uniformly and accurately heated in the length direction, thereby eliminating the temperature distribution of the wavelength conversion element and achieving stable wavelength conversion efficiency. Obtainable.

  As described above, according to the present invention, the two-photon absorption of the lithium tetraborate single crystal is reduced by heating the wavelength conversion element. Since the transmittance of the beam is hardly reduced and the heat generated by two-photon absorption is suppressed, the conversion efficiency of the laser beam is increased, and at the same time, the harmonics having a wavelength of less than 355 nm such as an ultraviolet laser suitable for precision processing are increased. A wavelength conversion method that can be generated can be provided.

  According to the present invention, the temperature distribution of the wavelength conversion element can be eliminated, and the wavelength conversion efficiency can be stably maintained over the entire crystal. Wavelength conversion can be performed.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  In this embodiment, a block-shaped wavelength conversion element made of a lithium tetraborate single crystal is heated and controlled to be maintained at a specific temperature within a temperature range of 200 to 450 ° C. At this time, the temperature accuracy is set to ± 0.3 ° C (preferably ± 0.1 ° C). For example, when the specific temperature is 300 ° C., the wavelength conversion element is kept at 299.7 to 300.3 ° C. (preferably 299.9 to 300.1 ° C.). Then, a second harmonic (green light having a wavelength of 532 nm) of an Nd: YAG laser is incident on the wavelength conversion element while maintaining the wavelength conversion element at a specific temperature with a predetermined temperature accuracy. Then, when the second harmonic laser beam passes through the wavelength conversion element, the wavelength is reduced by half, and the fourth harmonic (ultraviolet light having a wavelength of 266 nm) is emitted and generated.

  When this fourth harmonic is generated, the wavelength conversion element is kept at a specific temperature, so that the two-photon absorption of the lithium tetraborate single crystal constituting the wavelength conversion element is reduced. Therefore, even if the intensity of the incident light is increased, the transmittance of the laser beam does not easily decrease, and the conversion efficiency of the laser beam is improved. Further, since heat generation due to two-photon absorption is suppressed, it is possible to avoid a problem in which the refractive index changes and phase matching is lost, prevent output loss and instability, and maintain beam quality. Therefore, precision processing such as drilling of a printed circuit board can be performed with the ultraviolet laser thus obtained.

  In the above-described embodiment, the second harmonic of the Nd: YAG laser is used as the light to be incident on the wavelength conversion element, and the fourth harmonic (ultraviolet light having a wavelength of 266 nm) is generated based on the principle of the second harmonic. However, in the present invention, the type of incident light or outgoing light (generated light) and the principle of wavelength conversion are not limited as long as a harmonic having a wavelength of less than 355 nm is generated. For example, using a fundamental wave of an Nd: YLF laser and a fourth harmonic thereof (ultraviolet light having a wavelength of 262 nm) as an incident light, a fifth harmonic wave (ultraviolet light having a wavelength of 209 nm) is generated based on the principle of the sum frequency. In this case, the present invention can be applied.

  In order to confirm the effects described above, wavelength conversion was performed using the fourth harmonic (ultraviolet light having a wavelength of 262 nm) of a Nd: YLF laser having a repetition rate of 30 kHz and 10 kHz, and the temperature of the wavelength conversion element was set to a temperature of 25 to 300 ° C. It was examined how the two-photon absorption coefficient β changes when it is changed within the range. The results are shown graphically in FIG. In this graph, the horizontal axis represents the temperature of the wavelength conversion element, and the vertical axis represents the two-photon absorption coefficient β. As is clear from the graph of FIG. 1, when the wavelength conversion element is heated, the two-photon absorption coefficient β decreases exponentially, and in the temperature range of 200 to 450 ° C., the two-photon absorption coefficient β significantly decreases. Since the two-photon absorption is proportional to the two-photon absorption coefficient β, the two-photon absorption is significantly reduced in the temperature range of 200 to 450 ° C.

  Further, when the temperature of the wavelength conversion element is room temperature (about 20 ° C., indicated as “RT” in the graph), 100 ° C., 200 ° C., and 300 ° C., the transmittance of the laser beam changes with the intensity change of the incident ultraviolet laser. We examined how it changed. The result is shown by a graph in FIG. In this graph, the horizontal axis represents the intensity of the incident ultraviolet laser, and the vertical axis represents the transmittance of the laser beam. As is clear from the graph of FIG. 2, at any temperature, the transmittance of the laser beam decreases as the intensity of the incident light increases, but especially at a high temperature of 200 ° C. or higher, the intensity of the incident light increases. It was also found that the transmittance of the laser beam did not decrease so much.

  Further, wavelength conversion is performed by using a second harmonic (green light having a wavelength of 523 nm) of a Nd: YLF laser having a repetition rate of 10 kHz and an output of 20 W, and the temperature of the wavelength conversion element is set to room temperature (about 20 ° C .; RT), 100 ° C., 200 ° C., 300 ° C., and 385 ° C., how the conversion efficiency and the output power change with the change in the output of the incident light. The results are shown graphically in FIGS. In the graph of FIG. 3, the horizontal axis represents the output of the incident light, and the vertical axis represents the conversion efficiency. On the other hand, in the graph of FIG. 4, the horizontal axis represents the output of the incident light, and the vertical axis represents the output power. As is clear from the graphs of FIGS. 3 and 4, in the state at room temperature where the wavelength conversion element is not heated, the upper limit of the conversion efficiency is only about 7% due to the influence of two-photon absorption (see FIG. 3). Attempts to generate an output of .1 W or more became unstable (see FIG. 4). On the other hand, when the wavelength conversion element was heated, the effect of two-photon absorption was reduced, and at 385 ° C., the conversion efficiency reached about 15% (see FIG. 3), and an output of about 3 W was generated (see FIG. 3). (See FIG. 4).

  The specific output of the wavelength conversion element was set to 200 ° C., and the output of the ultraviolet laser in the vicinity of the specific temperature was measured. The results are shown in a graph in FIG. In this graph, the horizontal axis represents the temperature of the wavelength conversion element, and the vertical axis represents the output of the ultraviolet laser. As is clear from the graph of FIG. 5, when the temperature of the wavelength conversion element deviates from the specific temperature (200 ° C.) to both the positive and negative sides, the generation output of the ultraviolet laser tends to sharply decrease. When the temperature is within 3 ° C., the variation of the output of the ultraviolet laser is within 5%, and when the temperature accuracy is within ± 0.1 ° C., the variation of the output of the ultraviolet laser is within 1%. Was.

  Next, an embodiment of a suitable heating mechanism for accurately heating the wavelength conversion element will be described with reference to FIGS.

  FIG. 6 shows a wavelength converter of the present embodiment, which includes a wavelength converter 1, a heating block 2 for heating the wavelength converter 1, and a temperature controller 3 for heating and controlling the heating block 2 to a constant temperature. The heating block 2 and the temperature control unit 3 constitute the above-described heating mechanism.

  The heating block 2 includes a cylindrical copper material 5 containing two ceramic heaters 4, 4, a heat insulating ceramic 6 provided so as to cover the outside of the copper material 5, and a material that covers the outside of the ceramic 6, for example. , Teflon or the like, and a wavelength conversion element 1 (four-hole) in which a rectangular space portion is provided in the center of the copper material 5 in the length direction and the end face is a square prism. (Lithium oxide single crystal).

  The above-mentioned two ceramic heaters 4, 4 are arranged at the upper and lower positions of the wavelength conversion element 1 along the length direction thereof, and each energization terminal is connected to the temperature control unit 3. A temperature sensor 8 is attached near the center of the copper material 5 in the length direction, and its output terminal is connected to the above-mentioned temperature control unit 3. The temperature controller 3 controls energization of the ceramic heaters 4 and 4 based on the temperature detected by the temperature sensor 8 so that the heating block 2 has a constant temperature.

  Further, in the present embodiment, the copper material 5 is formed as a combined structure of a pair of half-shaped copper materials 5a and 5b having an L-shaped cross section to facilitate the production and assembly of the heating block 2 and to form the copper materials 5a and 5b. For example, the surface is subjected to a heat-resistant plating process such as Ni plating to prevent oxidation of the copper material 5 and to prevent generation of dust or the like that hinders passage of a laser beam.

  By the way, as shown in FIG. 6, the heating block 2 of the present embodiment has the length dimension longer than the wavelength conversion element 1 and both ends of the heating block 2 are closer to the end faces 1 a and 1 b of the wavelength conversion element 1. It has a structure that protrudes by a certain length.

  As described above, in the wavelength conversion element, when a temperature distribution occurs in the length direction (that is, in the optical path direction), the refractive index changes in the crystal and the wavelength conversion efficiency decreases, as described above.

  The above-described projecting structure of the heating block 2 obtains an effect of blocking the outside air, suppresses heat dissipation at both end faces (the input end face 1a and the output end face 1b) of the wavelength conversion element 1 that directly contacts the outside air, and This is a measure to minimize the temperature distribution in the length direction. Thereby, stable wavelength conversion efficiency can be obtained regardless of the length direction of the crystal, the wavelength conversion element 1 can be effectively used over the entire length, and efficient wavelength conversion can be performed. .

In the present embodiment, the following temperature test was performed to confirm the effect of the projecting structure of the heating block 2. The heating temperature of the wavelength conversion element 1 at this time is set to 400 ° C.
FIG. 8 shows the temperature distribution in the optical axis direction of the crystal with respect to the amount of protrusion (X) of the heating block 2, and the horizontal axis represents the distance of the crystal in the optical axis direction (the longitudinal center of the crystal is assumed to be 0 mm). The vertical axis represents temperature.
Note that, as the wavelength conversion element 1, a lithium tetraborate single crystal having a crystal length and width (D) of 12 mm and a length (L) of 60 mm is used. The copper material 5 of the heating block 2 has a thickness of 10 mm. And the amount of projection (X) from the crystal end faces 1a and 1b was set to four types of 0 mm (no projection), 10 mm, 20 mm, and 30 mm.

As is apparent from FIG. 8, when the protrusion amount (X) is 10 mm or less, heat release at the crystal end faces becomes remarkable, and a temperature distribution occurs in the optical axis direction near the crystal end faces 1a and 1b. It is understood that when the protrusion amount (X) is set to 20 mm or more, the temperature distribution does not occur, and the crystal can be heated uniformly over the entire length direction.
Since the protrusion amount (X) of 20 mm corresponds to 1/3 of the crystal length (L) of 60 mm, a preferable heating mechanism that does not generate a temperature distribution in the crystal is realized. (X) is to be secured at least 1/3 or more of the crystal length (L).

FIG. 9 shows a temperature distribution in the optical axis direction with respect to the width (D) of the crystal. Represents temperature.
The copper material 5 used had a plate thickness of 10 mm, and each protrusion (X) from the crystal end faces 1a and 1b was 20 mm. The wavelength conversion element 1 is made of a single crystal of lithium tetraborate having a length (L) of 60 mm, and has four kinds of vertical and horizontal widths (D) of 12 mm, 20 mm, 30 mm, and 40 mm.

As is clear from FIG. 9, when the vertical / horizontal width (D) of the crystal is 20 mm, a slight temperature distribution occurs in the optical axis direction near the crystal end face, and when the width (D) is 12 mm or less, the temperature becomes lower. It turns out that distribution does not occur. The temperature distribution occurred at a width of 20 mm because the opening area on the end face side of the wavelength conversion element 1 became too large due to the increase in the thickness, the outside air blocking effect by the protruding structure was weakened, and the crystal end was affected by the outside air. This is because it is easy to receive.
Since the width (D) of 20 mm corresponds to one third of the crystal length (L) of 60 mm, the longitudinal and lateral widths of the crystal are determined by the present invention in order to eliminate the temperature distribution in the length direction of the crystal. The length (L) was set to 1/4 or less.

  As described above, based on the results of FIGS. 8 and 9, in the present invention, the vertical and horizontal widths (D) of the wavelength conversion element 1 are set to １ ／ or less of the crystal length (L), and the heating block 2 is In addition, the wavelength conversion element 1 is made to protrude from the incident end face 1a and the output end face 1b in the length direction by at least １ ／ of the crystal length (L).

FIG. 7 shows another embodiment of the heating block 2.
The present embodiment is different from FIG. 6 in that the entire crystal including the incident end face 1a and the outgoing end face 1b of the wavelength conversion element 1 is covered with the heating block 2, and the laser beam is placed on the optical axis of the end face of the heating block 2. A hole 9 serving as an optical path passing therethrough is formed.

However, it is necessary that the size of the hole 9 is at least as small as possible so that the angle of the crystal can be adjusted. The smaller the hole 9 is, the better the effect of blocking the progress of the laser beam is. Further, in this embodiment, a space is provided between the crystal end face and the heating block end face, but a structure in which both are closely contacted may be used.
FIG. 7 shows only the structure of the copper material 5 constituting the heating block 2, and the structure of each heat insulating material covering the same is omitted because it is the same as that of FIG. 6.

  As described above, by adopting a structure in which the end face of the crystal is also covered with the heating block 2, the effect of blocking the outside air is remarkably improved, and although not shown in the graph, the protrusion amount (X) of the heating block 2 and the vertical and vertical A temperature distribution in the length direction of the wavelength conversion element 1 can be prevented without being affected by the width (D).

5 is a graph showing the effect of the temperature of the wavelength conversion element on the two-photon absorption coefficient. 6 is a graph showing the relationship between the intensity of the incident ultraviolet laser and the transmittance for each temperature of the wavelength conversion element. 6 is a graph showing the relationship between the output of incident light and the conversion efficiency for each temperature of the wavelength conversion element. 6 is a graph showing the relationship between the output of the incident light and the output power for each temperature of the wavelength conversion element. 4 is a graph showing the effect of the temperature accuracy of the wavelength conversion element on the output of an ultraviolet laser. 1 shows a configuration of a wavelength conversion device according to the present invention, in which (a) is a front view and (b) is a side view. It is a sectional side view which shows the structure of the copper material which comprises a heating block. 5 is a graph illustrating a temperature distribution in a direction of an optical axis of a wavelength conversion element with respect to a protrusion amount of a heating block. 5 is a graph illustrating a temperature distribution in an optical axis direction with respect to a width of a wavelength conversion element.

Explanation of reference numerals

DESCRIPTION OF SYMBOLS 1 Wavelength conversion element 1a Incident end face 1b Outgoing end face 2 Heating block 3 Temperature controller 4 Heater (ceramic heater)
6 Ceramics 7 Resin material 8 Temperature sensor 9 Optical path (hole)

Claims (7)

When a laser beam is wavelength-converted by a wavelength conversion element made of lithium tetraborate single crystal to generate a harmonic having a wavelength of less than 355 nm,
A wavelength conversion method, wherein the wavelength conversion element is controlled to be heated and maintained at a specific temperature within a temperature range of 200 to 450 ° C. with a predetermined temperature accuracy. The wavelength conversion method according to claim 1, wherein the temperature accuracy of the wavelength conversion element is within ± 0.3 ° C. The wavelength conversion method according to claim 1, wherein the temperature accuracy of the wavelength conversion element is within ± 0.1 ° C. In a wavelength conversion device that performs wavelength conversion by injecting a laser beam into a wavelength conversion element made of a nonlinear optical crystal,
A heating block that covers an outer surface of the wavelength conversion element excluding an incident end face and an emission end face and heats the wavelength conversion element from the outside with a heater, a temperature sensor that detects the temperature of the heating block, and detection of the temperature sensor Based on the temperature, having a temperature control unit to control the heating block to a constant temperature,
When the length of the wavelength conversion element is L, the vertical and horizontal widths of the wavelength conversion element are L / 4 or less, and the heating block is longer than the entrance end face and the exit end face of the wavelength conversion element. A wavelength conversion device characterized by projecting L / 3 or more in the direction. In a wavelength conversion device that performs wavelength conversion by injecting a laser beam into a wavelength conversion element made of a nonlinear optical crystal,
A heating block that covers the entire surface of the wavelength conversion element and heats the wavelength conversion element from the outside with a heater, a temperature sensor that detects the temperature of the heating block, and the heating block based on the temperature detected by the temperature sensor. A wavelength conversion device having a temperature control unit for controlling the temperature to a constant temperature, and an optical path through which a laser beam passes is formed on a surface on an optical axis on an entrance side and an exit side of the heating block. The heating block is composed of a metal material having a high thermal conductivity such as copper having a built-in heater along the length direction, a ceramic covering the outside of the metal material, and a heat-resistant resin material covering the outside of the ceramic. The wavelength conversion device according to claim 4, wherein the wavelength conversion device is a cylindrical body. 7. The wavelength converter according to claim 6, wherein a heat-resistant plating process is performed on a surface of the metal material.

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