JP6514430B2 - Wavelength conversion optical device, wavelength conversion element and laser device - Google Patents

Description

  The present invention relates to a wavelength conversion optical device, a wavelength conversion element, and a laser device.

  Conventionally, Patent Document 1 is known as a wavelength conversion optical device using a single pulse. In this wavelength conversion optical device, as shown in FIG. 7, a circular beam of a fundamental wave is a quasi phase matching (Quasi Phase Matching) element (hereinafter referred to as a QPM element) 3 comprising a wavelength conversion element having a periodically poled structure. And the QPM element 3 obtains a circular beam close to the perfect circle of the second harmonic (SHG).

Also, QPM devices using nonlinear optical effects of ferroelectrics are known. The QPM device forms a periodically poled region inside a ferroelectric crystal such as lithium tantalate LT or LiNbO _{3 in} order to obtain the desired light.

  In addition, the creation method of a polarization inversion area | region is described in patent document 2, for example. When wavelength conversion of laser light is performed by a QPM device, a circular beam is generally used.

International Publication No. 2008050802 JP 2007-3885 A

  However, since the wavelength conversion element also has light absorption, when the wavelength conversion element is driven at a high power density, the efficiency may decrease due to the temperature rise, and the wavelength conversion element may be broken. In addition, when the power density of the fundamental wave is reduced to prevent the efficiency from being reduced or the element being broken, the conversion efficiency is proportional to the power density, so the efficiency is reduced.

  The object of the present invention is to provide a wavelength conversion optical device, a wavelength conversion element, and a laser device capable of preventing the efficiency reduction and destruction of the wavelength conversion element.

In order to solve the above problems, a wavelength conversion optical device according to the present invention forms a beam deformation element that deforms a circular beam of a fundamental wave into an elliptical beam of a fundamental wave and a periodically poled structure, and And a quasi phase matching element for converting an elliptical beam of the deformed fundamental wave into an elliptical beam of the second harmonic , wherein the beam shape at the output end face of the quasi phase matching element is elliptical and a perfect circle of the same area And the beam cross-sectional area / perimeter ≦ 0.56 .

  According to the present invention, since the beam incident on the quasi phase matching element is an elliptical beam, heat radiation becomes easy, so that the temperature rise of the wavelength conversion element is less likely to occur. Therefore, it is possible to provide a wavelength conversion optical device capable of preventing the efficiency reduction and destruction of the wavelength conversion element.

It is a figure explaining the principle of the wavelength conversion optical apparatus of this invention. It is a figure which shows the relationship of the ratio of an ellipticity and beam cross-sectional area and circumference. FIG. 2 is a block diagram of a wavelength conversion optical device of Example 1; FIG. 6 is a block diagram of a wavelength conversion element of Example 2; FIG. 7 is a view showing modes in a ridge waveguide formed in the wavelength conversion element of Example 2. FIG. 16 is a diagram showing modes in a ridge waveguide formed in the wavelength conversion element of Example 3. It is a block diagram of the conventional wavelength conversion optical apparatus.

  Hereinafter, embodiments of a wavelength conversion optical device, a wavelength conversion element, and a laser device of the present invention will be described in detail based on the drawings.

  The present invention is characterized in that the beam incident on the wavelength conversion element is an ellipse. Since the elliptical beam is easier to dissipate heat than the circular beam, the temperature rise of the wavelength conversion element is less likely to occur.

  The reason is described with reference to FIG. First, the circular beam shown in FIG. 1A and the elliptical beam shown in FIG. 1B have the same cross-sectional area. When these beams are compared, if the power density is the same, the amount of heat generated due to absorption is the same as it is proportional to the cross-sectional area.

  The heat escapes to the surroundings as shown by the arrows, so the longer the circumferential length, the better the heat dissipation. The oval shape is longer than the circle, so heat dissipation is good. Therefore, the temperature rise of the wavelength conversion element is less likely to occur.

  In addition, when the elliptical beam and the circular beam have the same beam cross-sectional area, the horizontal axis represents the ellipticity, and the vertical axis the beam cross-sectional area / peripheral length (normalized with the maximum value for the same beam cross-sectional area) It is shown in FIG.

  The beam cross-sectional area / peripheral length is maximum (that is, the lowest heat dissipation) in the case of a perfect circle, and the ellipticity decreases with distance from 1. That is, it can be seen that the heat dissipation is improved by the elliptical beam.

  Thus, if the beam is elliptical or other non-round figure, for example, rectangular, then the heat dissipation is improved and a high SHG output is obtained.

  In addition, when driving the wavelength conversion element with high output, thermal saturation and destruction often occur particularly due to absorption near the emission end face of SHG light. For this reason, it is effective that the beam of SHG light has a shape such as an ellipse or a rectangle at the emission end face.

  It is known that the SHG reaching power of the QPM device is significantly increased when the thermal resistivity of the wavelength conversion crystal is 0.56 (thermal conductivity is 1.8). In this case, the thermal resistivity is a comparison of SLT and Mg: LN.

  Of the thermal resistance of the wavelength conversion element, the component contributed by the wavelength conversion crystal is largely affected by the point extending just outside the beam from the beam, and this is equivalent to the circumferential length of the beam (beam cross section / Approximately proportional to the circumference). That is, it is considered that the SHG reaching power of the QPM device is significantly increased if the beam cross-sectional area / periphery is 0.56 times as large as that of a true circle of the same area.

The conditions that satisfy the above are
In the case of an ellipse, the major axis / minor axis 7.6 7.6
In the case of a rectangle, long side / short side 8 8
It is.

  FIG. 3 is a block diagram of a laser device including the wavelength conversion optical device of the first embodiment. The laser device shown in FIG. 3 includes a semiconductor laser 10, a convex lens 1, a concave lens 2, a QPM element 3, a concave lens 4, and a convex lens 5.

  The semiconductor laser 10 outputs a laser beam consisting of a circular beam of the fundamental wave to the convex lens 1. The convex lens 1 and the concave lens 2 form an optical system A, and transform the circular beam of the fundamental wave into an elliptical beam. A circular wavelength of 1.064 μm is used as a circular beam of the fundamental wave.

  The QPM element 3 wavelength-converts the elliptical beam of the fundamental wave from the optical system A into an elliptical beam of SHG. The QPM element 3 is made of PPMgSLT, and the optical axis coincides with the crystal X axis. PPMgSLT is a quasi phase matching type wavelength conversion device in which a periodically poled structure is formed on a lithium tantalate MgO having a stoichiometric composition doped with MgO and an SLT substrate. The polarization is periodically reversed in the same direction, and the crystal Z axis is arranged to coincide with the polarization direction of the incident beam. The periodic polarization inversion period is such that a fundamental wave with a wavelength of 1.064 μm is converted into SHG light with a wavelength of 0.532 μm at a desired temperature.

  The concave lens 4 and the convex lens 5 form an optical system B, and transform the elliptical beam of SHG from the QPM element 3 into a circular beam of SHG.

  As described above, according to the wavelength conversion optical device of the first embodiment, the circular beam of the fundamental wave is transformed into an elliptical beam by the convex lens 1 and the concave lens 2 and the elliptical beam is incident on the QPM element 3, so heat radiation becomes easy. Therefore, the temperature rise of the wavelength conversion element is less likely to occur. Therefore, it is possible to provide a wavelength conversion optical device capable of preventing the efficiency reduction and destruction of the wavelength conversion element.

  FIG. 4 is a block diagram of the wavelength conversion element of the second embodiment. FIG. 4 shows a cross-sectional view of the QPM element 3a which is a wavelength conversion element. The QPM element 3 a has a side wall 31 having a sloped recess, a bottom 32, and a trapezoidal ridge waveguide 33 formed between the side wall 31 and the bottom 32. A circular beam of the fundamental wave is incident on the QPM element 3 a and guided to the trapezoidal ridge waveguide 33.

  Here, the ridge width of the side wall portion 31 is W, the groove depth is d, the thickness is t, and the inclination angle is θ. In the trapezoidal ridge waveguide 33, as shown in FIG. 5, a substantially elliptical fundamental mode of SHG light is formed. In FIG. 5, the ridge width W is 4.7 μm, the groove depth d is 4 μm, the thickness t is 4.7 μm, and the inclination angle θ is 21 °.

  Thus, the ridge waveguide 33 is provided in the QPM element 3a, and the ridge waveguide 33 can convert the fundamental wave into the fundamental mode of the SHG ellipse. Therefore, since the heat radiation becomes easy, the temperature rise of the wavelength conversion element hardly occurs. Therefore, the efficiency reduction and destruction of the wavelength conversion element can be prevented.

  FIG. 6 is a view showing modes in a ridge waveguide formed in the wavelength conversion element of Example 3. The example shown in FIG. 6 is characterized in that the side wall portion of the QPM element has a vertically standing recess and has a rectangular ridge waveguide.

  Therefore, as shown in FIG. 6, a substantially rectangular fundamental mode of SHG light is formed in the rectangular ridge waveguide. In FIG. 6, the ridge width W is 4.7 μm, the groove depth d is 4 μm, the thickness t is 4.7 μm, and the inclination angle θ is 0 °.

  Thus, the QPM element is provided with a ridge waveguide, and this ridge waveguide can convert the fundamental wave into the rectangular fundamental mode of SHG. Therefore, since the heat radiation becomes easy, the temperature rise of the wavelength conversion element hardly occurs. Therefore, the efficiency reduction and destruction of the wavelength conversion element can be prevented.

  The present invention is applicable to a semiconductor laser device.

1, 5 convex lens 2, 4 concave lens 3, 3a QPM element 10 semiconductor laser 31 side wall 32 bottom 33 ridge waveguide

Claims (4)

A beam deformation element that deforms a circular beam consisting of a true circle of the fundamental wave into an elliptical beam of the fundamental wave;
A quasi phase matching element forming a periodically poled structure and converting an elliptical beam of the fundamental wave deformed by the beam deformation element into an elliptical beam of a second harmonic;
Equipped with
The quasi phase is beam shape elliptical on the emission end surface of the matching element, a wavelength conversion optical system, characterized in that 0.56 times the beam cross-sectional area / perimeter ≦ compared to the true circle having the same area . A waveguide forming a periodically poled structure and converting a circular beam consisting of a perfect circle of the fundamental wave into an elliptical fundamental mode of the second harmonic,
The wavelength conversion element, wherein the second harmonic oval fundamental modes at the exit end face, which is the 0.56 beam cross-sectional area / perimeter ≦ compared to a true circle having the same area. A waveguide forming a periodically poled structure and converting a circular beam consisting of a perfect circle of the fundamental wave into a rectangular fundamental mode of the second harmonic;
The wavelength conversion element, wherein the rectangular fundamental mode of the second harmonic at the exit end face, which is the 0.56 beam cross-sectional area / perimeter ≦ compared to a true circle having the same area. A semiconductor laser,
A wavelength conversion optical device according to claim 1;
The semiconductor laser is characterized in that laser light is incident on the beam deformation element as a circular beam of the fundamental wave.

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