JP2012216637A - Apparatus for converting wavelength of laser beam - Google Patents

Abstract

PROBLEM TO BE SOLVED: To inhibit the intensity unevenness of a laser beam after wavelength conversion.SOLUTION: An apparatus for converting the wavelength of a laser beam is provided. A laser resonator 11 comprises a laser medium 15, an end mirror 14 allowing an excited laser beam 12 incident upon the laser medium 15 to pass through and reflecting a laser beam 13 emitted from the laser medium 15 by stimulation with high reflectance, and an output mirror 16 allowing the laser beam 13 emitted from the laser medium 15 by stimulation to pass through. A pair of wedge-shaped nonlinear optical crystals 17a and 17b are disposed at a point on the path of the laser beam 13 from the laser medium 15 to the output mirror 16 so that a plane 17aa of incidence of the laser beam 13 and a plane 17bb of emission of a laser beam 19 after wavelength conversion are parallel to each other and the gap between the wedge-shaped nonlinear optical crystals 17a and 17b is constant. Means is provided for adjusting the crystal length by moving at least one of the wedge-shaped nonlinear optical crystals 17a and 17b while maintaining the aforementioned conditions. As a result, the intensity unevenness can be inhibited while the wavelength conversion efficiency is constantly maintained at the maximum.

Description

  The present invention relates to an apparatus for converting the wavelength of laser light in a laser oscillator or an optical parametric oscillator (hereinafter referred to as “OPO”) apparatus.

  For example, a laser resonator, which is a component of a laser oscillator, has a configuration in which a laser medium is disposed on a path of laser light confined in a laser resonator between an end mirror and an output mirror.

  When the laser light output from the laser resonator having such a configuration is incident on the nonlinear optical crystal and the wavelength of the laser light is converted, the nonlinear optical crystal has the energy of the incident laser light and the laser light after wavelength conversion. It absorbs a part and generates heat.

  As a result, the temperature distribution in the nonlinear optical crystal changes and phase matching mismatch occurs (see FIG. 5), or reverse conversion occurs in which the wavelength-converted laser light returns to the laser light of the original wavelength. The wavelength conversion efficiency in the nonlinear optical crystal is reduced. FIG. 5 is a diagram showing changes in the light intensity of the original laser beam (solid line) and the converted laser beam (broken line) due to the temperature rise inside the crystal.

  In addition, when the conditions such as the energy and repetition of the laser light incident on the nonlinear optical crystal are changed, there is a problem that the laser light after wavelength conversion does not always have the maximum crystal length.

  Therefore, for example, as shown in FIG. 6, the exit surface 5a of the nonlinear optical crystal 5 provided between the laser medium 3 of the laser resonator 1 and the output mirror 4 is tilted, and the tilt of the tilted exit surface 5a is changed. A technique has been proposed in which the maximum wavelength efficiency is always obtained by controlling (for example, Patent Document 1). In FIG. 6, 2 indicates an end mirror, 6 indicates excitation laser light, 7 indicates laser light stimulated and emitted from the laser medium 3, and 8 indicates laser light after wavelength conversion.

  However, when the exit surface of the nonlinear optical crystal is tilted and the tilt is controlled, the nonlinear optical crystal has a tilted cross section, which causes a problem of unevenness in intensity of the laser light after wavelength conversion. . In order to suppress the intensity unevenness generated in the laser light after wavelength conversion, the irradiation condition of the laser light incident on the nonlinear optical crystal is limited.

JP-A-6-177465

  The problem to be solved by the present invention is to always obtain the maximum wavelength efficiency by tilting the exit surface of the nonlinear optical crystal and controlling the tilt of the exit surface in wavelength conversion of laser light. The technology is that unevenness in intensity occurs in the laser light after wavelength conversion.

The wavelength conversion device for laser light of the present invention is:
In order to suppress the intensity unevenness generated in the laser light after wavelength conversion while always maximizing the wavelength efficiency,
A wavelength conversion device for laser light that converts the wavelength of laser light with a nonlinear optical crystal,
A laser medium that is excited to stimulate and emit laser light;
An end mirror that transmits the excitation laser light incident on the laser medium and reflects the laser light stimulated and emitted from the laser medium with a high reflectance;
A laser resonator composed of an output mirror that transmits laser light stimulated and emitted from the laser medium;
Two wedge-shaped nonlinear optical crystals are paired in the middle of the path of the laser beam from the laser medium to the output mirror. The laser beam incident surface and the wavelength converted laser beam exit surface are as parallel as possible. And the gap between the two wedge-shaped nonlinear optical crystals is arranged in a state as constant as possible,
The main feature is that a means for adjusting the crystal length by moving at least one of the two wedge-shaped nonlinear optical crystals forming the pair while maintaining the above state is provided.

  In the present invention having the above-described configuration, the crystal length of the two wedge-shaped nonlinear optical crystals forming a pair can be varied while arranging the laser light incident surface and the wavelength-converted laser light exit surface as parallel as possible. By doing so, it is possible to suppress the decrease in wavelength conversion efficiency due to the change in the refractive index due to the temperature gradient in the optical axis direction of the laser light, and to always maximize the wavelength conversion efficiency.

  In addition, the laser light incident surface and the laser light emission surface after wavelength conversion are as parallel as possible, and the gap between the two wedge-shaped nonlinear optical crystals is as constant as possible, The intensity unevenness of the laser light after wavelength conversion can be suppressed.

  The two wedge-shaped nonlinear optical crystals constituting the pair of the present invention may be arranged in the path of the laser beam output from the laser resonator. In this case, it is necessary to provide a mirror for removing the fundamental wave in the path of the laser light after wavelength conversion on the output side of the two wedge-shaped nonlinear optical crystals forming a pair.

  In the present invention, the effective crystal length of the two wedge-shaped nonlinear optical crystals forming a pair is arranged so that the laser light incident surface and the wavelength-converted laser light exit surface are as parallel as possible. By making the thickness variable, it is possible to suppress unevenness in strength while always maximizing wavelength conversion efficiency.

It is the figure which showed schematic structure of the wavelength converter of the laser beam of 1st this invention arrange | positioned in a laser resonator. It is a figure explaining the crystal | crystallization length in the wavelength converter of the laser beam of this invention, (a) is a figure which showed the positional relationship of two wedge-shaped nonlinear optical crystals in a normal state, (b) is compared with (a). The positional relationship when the crystal length is short, (c) shows the positional relationship when the crystal length is longer than (a). It is the figure which showed schematic structure of the wavelength converter of the laser beam of 2nd this invention. It is the figure which showed schematic structure of the wavelength converter of the laser beam of 3rd this invention arrange | positioned in a laser resonator. It is the figure which showed the change of the optical intensity of the original laser beam by the temperature rise inside a crystal | crystallization, and the laser beam after conversion. It is the figure which showed schematic structure of the wavelength converter of the conventional laser beam.

  The purpose of the present invention is to suppress the intensity unevenness generated in the laser light after wavelength conversion while always maximizing the wavelength efficiency, and to convert the laser light incident surface and the wavelength conversion of two paired wedge-shaped nonlinear optical crystals. This was realized by making the crystal thickness variable while arranging the laser light emission surfaces as parallel as possible.

Hereinafter, the wavelength conversion device for laser light according to the present invention will be described in detail with reference to FIGS.
1 and 2 are diagrams for explaining a laser beam wavelength conversion apparatus according to the first aspect of the present invention.

  In FIG. 1, reference numeral 11 denotes a laser resonator that is excited by incident excitation laser light 12 and stimulates and emits laser light 13. From the incident side of the excitation laser light 12, an end mirror 14, a laser medium 15, and an output mirror 16. Arranged in order.

  Among these, the laser medium 15 is excited by the excitation laser light 12 transmitted through the end mirror 14 and stimulates and emits the laser light 13. In the case of a laser oscillator, the laser crystal is used. In the case of an OPO device, the laser medium 15 is nonlinear. An optical crystal is employed.

  The end mirror 14 not only transmits the excitation laser beam 12 incident on the laser medium 15 but also has an effect of reflecting the laser beam 13 stimulated and emitted from the laser medium 15 with high reflectivity. The output mirror 16 transmits the laser beam 13 stimulated and emitted from the laser medium 15 with a predetermined transmittance.

The wavelength converter according to the present invention is arranged in the middle of the path of the laser beam 13 from the laser medium 15 to the output mirror 16, and is composed of a pair of wedge-shaped nonlinear optical crystals 17a and 17b. . As these wedge-shaped nonlinear optical crystals 17a and 17b, BBO (β-BaB ₂ O ₄ ), LBO (LiB ₃ O ₅ ), KTP (KTiOPO ₄ ), DKDP (KD ₂ PO ₄ ) and the like are employed.

  The wedge-shaped nonlinear optical crystals 17a and 17b include an incident surface 17aa of the wedge-shaped nonlinear optical crystal 17a on the laser medium 15 side (hereinafter referred to as one) and a wedge-shaped nonlinear optical crystal 17b on the output mirror 16 side (hereinafter referred to as the other). The emission surface 17bb is installed in parallel. Further, the gap between the emission surface 17ab of one wedge-shaped nonlinear optical crystal 17a and the incident surface 17ba of the other wedge-shaped nonlinear optical crystal 17b is set to be constant.

  The allowable range of the parallelism of the gap depends on the allowable angle of the wedge-shaped nonlinear optical crystals 17a and 17b to be used, and is desirably suppressed to ¼ or less (10% of the maximum output) of the allowable angle. In addition, it is desirable to adjust the allowable range of the gap so that the beam divergence angle is 0.5% or less of the beam diameter. For example, if the beam diameter is 5 mm and the beam divergence angle is 1 mrad, the maximum is 25 mm.

  In addition, in the present invention, for example, both wedge-shaped nonlinear optical crystals 17a and 17b are changed from the normal state shown in FIG. 2 (a) to the wavelength conversion output as shown in FIG. 2 (b) while maintaining the above state. The crystal length can be shortened or the crystal length can be increased as shown in FIG.

  As a means for adjusting the crystal length, for example, as shown in FIG. 1, one wedge-shaped nonlinear optical crystal 17a is installed on the uniaxial stage 18a, and can be moved in the direction of the other wedge-shaped nonlinear optical crystal 17b. What is necessary is just to set it as such.

  The wavelength-converted laser light 19 after moving on one wedge-shaped nonlinear optical crystal 17a is observed on, for example, the monitor 18b, and the output is fed back to the control unit 18c to optimize the crystal length according to the change in the wavelength-converted output. Adjust it. The optimum crystal length is adjusted by, for example, changing the crystal length and searching for the maximum output point. The relationship between the output and the crystal length can be known from FIG.

  By the way, if the gap between the exit surface 17ab of one wedge-shaped nonlinear optical crystal 17a and the incident surface 17ba of the other wedge-shaped nonlinear optical crystal 17b is too large, the beam diameter changes due to the divergence angle of the laser beam, and the light intensity decreases. Wavelength conversion efficiency will change. Further, due to the gap, the energy of the laser beam 13 incident on one wedge-shaped nonlinear optical crystal 17a and the wavelength-converted laser beam 19 emitted from the other wedge-shaped nonlinear optical crystal 17b is lost due to surface reflection. Therefore, it is desirable to make the gap as narrow as possible.

  In order to eliminate the loss of energy, reflection of the two wavelengths of the laser beam 13 and the laser beam 19 on the exit surface 17ab of one wedge-shaped nonlinear optical crystal 17a and the incident surface 17ba of the other wedge-shaped nonlinear optical crystal 17b is prevented. It is desirable to apply a film (AR coating).

  Further, as shown in FIG. 3, the wedge-shaped nonlinear optical crystals 17a and 17b are arranged in a case 20 filled with a liquid or grease-like refractive index matching agent having substantially the same refractive index as the wedge-shaped nonlinear optical crystals 17a and 17b. You may do it. In this case, it is possible to reduce the reflection of the laser beam 13 and the laser beam 19 that occur on the surfaces of the wedge-shaped nonlinear optical crystals 17a and 17b, thereby increasing the conversion efficiency. Note that reference numeral 20 a in FIG. 3 denotes windows provided in the incident portion of the laser beam 13 and the emitting portion of the laser beam 19 in the case 20.

  In the present invention having the above-described configuration, for example, the wedge-shaped nonlinear optical crystals 17a and 17b are arranged while the incident surface 17aa of one wedge-shaped nonlinear optical crystal 17a and the exit surface 17bb of the other wedge-shaped nonlinear optical crystal 17b are arranged in parallel. The crystal length can be changed by moving as shown in FIGS. 2B and 2C from the normal state of FIG.

  As described above, the adjustment of the crystal length is performed by observing the output of the laser light 19 after wavelength conversion and feedback-controlling the crystal length with the monitor 18b provided at the rear stage of the laser resonator 11.

  Therefore, while maintaining the crystal length, it is possible to suppress the decrease in wavelength conversion efficiency due to the change of the refractive index due to the temperature gradient in the optical axis direction of the laser light 13 and to always maximize the wavelength conversion efficiency. Become. Further, the relative position relationship between the wedge-shaped nonlinear optical crystals 17a and 17b makes it possible to keep the intensity of the laser light 19 after wavelength conversion uniform.

  When the incident surface 17aa of the paired wedge-shaped nonlinear optical crystal 17a and the exit surface 17bb of the other wedge-shaped nonlinear optical crystal 17b are not parallel, the conversion efficiency is drastically reduced when the following allowable range is exceeded. Since unevenness in strength occurs after conversion, it is desirable to arrange them as parallel as possible.

  For example, in the case of BBO, the angle is 0.37 mrad · cm (0.021 degrees) when the wavelength of 532 nm becomes 266 nm after conversion. This is a range in which the output after conversion is halved (full width at half maximum), and is 0.01 degrees when the crystal length is 5 mm.

  Accordingly, the angle when the incident surface 17aa of one wedge-shaped nonlinear optical crystal 17a and the exit surface 17bb of the other wedge-shaped nonlinear optical crystal 17b are not parallel is ¼ or less of the allowable angle width (10% of the maximum output). Is preferably within the allowable range.

  BBO is a crystal having a particularly narrow angle tolerance, and the angle tolerance (532 nm → 266 nm) in the case of CLBO is 1.08 mrad · cm, and the tolerance is about three times that of BBO.

  The present invention is not limited to the above examples, and it goes without saying that the embodiments may be appropriately changed within the scope of the technical idea described in each claim.

  For example, in the above example, the two wedge-shaped nonlinear optical crystals 17a and 17b that are paired are shown inside the laser resonator 11, but as shown in FIG. It may be installed in.

  However, as shown in FIG. 4, when two wedge-shaped nonlinear optical crystals 17a and 17b are paired in the path of the excitation laser light 12 output from the laser resonator 11, the wedge-shaped nonlinear optical crystal 17b It is necessary to provide a mirror 21 for removing the fundamental wave in the path of the laser light 20 after wavelength conversion on the output side.

  In the above example, the crystal length is controlled by observing the output of the laser light 19 after wavelength conversion with the monitor 18b. Instead, the wedge-shaped nonlinear optical crystals 17a and 17b are controlled. A temperature sensor may be provided to measure the temperature of the wedge-shaped nonlinear optical crystals 17a and 17b, and control may be performed based on the average value of the measured temperatures.

  In the above example, only one wedge-shaped nonlinear optical crystal 17a is moved. However, the other wedge-shaped nonlinear optical crystal 17b is moved, and both wedge-shaped nonlinear optical crystals 17a and 17b are moved. good.

DESCRIPTION OF SYMBOLS 11 Laser resonator 12 Excitation laser beam 13 Laser beam 14 End mirror 15 Laser medium 16 Output mirror 17a, 17b Wedge-shaped nonlinear optical crystal 17aa, 17ba Incident surface 17ab, 17bb Output surface 18a Uniaxial stage 18b Monitor 18c Control unit 19 After wavelength conversion Laser light 21 mirror

Claims (3)

A wavelength conversion device for laser light that converts the wavelength of laser light with a nonlinear optical crystal,
A laser medium that is excited to stimulate and emit laser light;
An end mirror that transmits the excitation laser light incident on the laser medium and reflects the laser light stimulated and emitted from the laser medium with a high reflectance;
A laser resonator composed of an output mirror that transmits laser light stimulated and emitted from the laser medium;
Two wedge-shaped nonlinear optical crystals are paired in the middle of the path of the laser beam from the laser medium to the output mirror. The laser beam incident surface and the wavelength converted laser beam exit surface are as parallel as possible. And the gap between the two wedge-shaped nonlinear optical crystals is arranged in a state as constant as possible,
A laser wavelength conversion device comprising means for adjusting the crystal length by moving at least one of the two wedge-shaped nonlinear optical crystals forming a pair while maintaining the above state. . A wavelength conversion device for laser light that converts the wavelength of laser light with a nonlinear optical crystal,
A laser medium that is excited to stimulate and emit laser light;
An end mirror that transmits the excitation laser light incident on the laser medium and reflects the laser light stimulated and emitted from the laser medium with a high reflectance;
Two wedge-shaped non-linear optical crystals that form a pair in the path of the laser beam output from the laser resonator constituted by the output mirror that transmits the laser beam stimulated and emitted from the laser medium are incident on the laser beam. The surface and the laser light emission surface after wavelength conversion are as parallel as possible, and the gap between the two wedge-shaped nonlinear optical crystals is as constant as possible,
And providing a mirror for removing the fundamental wave in the path of the laser light after wavelength conversion on the output side of the two wedge-shaped nonlinear optical crystals forming the pair, and at least of the two wedge-shaped nonlinear optical crystals forming the pair A laser light wavelength conversion device comprising means for adjusting the crystal length by moving either one of the above while maintaining the above state.   A monitor for observing the wavelength-converted laser beam, and a control unit that feeds back the output of the laser beam observed by the monitor and adjusts the crystal length to an optimum according to a change in the wavelength-converted output. The wavelength conversion device for laser light according to claim 1 or 2.

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