JP3412901B2 - Laser oscillator - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser oscillator,
In particular, the present invention relates to a laser oscillator that generates a second harmonic using laser light from a solid-state laser using trivalent neodymium (Nd) ions as light-emitting ions using a nonlinear crystal. 2. Description of the Related Art Solid-state lasers are industrially used in large quantities because of their small size, high output, and easy maintenance. When this laser light is used in the visible region, conversion using non-linearity with respect to light of a solid medium is often used. The conversion that doubles the frequency of the light is called second harmonic generation. For example, a wavelength of 1.06 μm obtained from a solid-state laser using trivalent neodymium (Nd) ions
Near infrared light of 0.53μ
m green light. In recent years, this conversion is expected to be applied to a reading light source for magneto-optical recording. As the nonlinear crystal used here, a crystal whose conversion performance varies depending on the medium, has a large nonlinear optical constant (d), and is capable of generating harmonics near room temperature is used. In the above-mentioned near infrared light conversion of 1.06 μm, for example, potassium titanium phosphate (KTiOPO ₄ : KTP) is often used. [0004] Lithium niobate (LiNbO ₃ ) crystal is a promising nonlinear material because it has a higher nonlinear optical constant (d) than the above-mentioned potassium titanate phosphate, and a low-cost and high-quality crystal is manufactured industrially. However, 1.
The phase matching temperature for the generation of the second harmonic of 06 μm is equal to or lower than room temperature, and the laser light in the visible region easily induces a change in the refractive index, particularly the extraordinary refractive index, making it difficult to generate a harmonic. Light damage occurs and cannot be used industrially. For this reason, lithium niobate is doped with divalent impurity ions to improve the optical damage characteristics. For example, it has been proposed to use magnesium ions (Literature: DA Bryan et al., A
ppl. Phys. Lett. 44 (1984) 8
47). It is said that this magnesium ion can be doped in a considerably large amount, and that the photodamage property is greatly improved. However, for the generation of the second harmonic of 1.06 μm near-infrared light, the phase matching temperature exceeds 100 ° C. in a magnesium ion concentration range effective for photodamage, for example, 5 mol% (WM Young et al.). al., JA
ppl. Phys. 69 (1991) 7372), it is not preferable to generate a compact green light because the crystal temperature control device becomes large and the surrounding electronic equipment is heated. [0006] Although alignment by angle adjustment is also possible,
A so-called walk-off phenomenon in which the beam trajectories of the input light and the output light are different occurs (JL Nightgale et al., Pro.
c. SPIE. 681 (1986) 20), 0.53μ
Since the conversion efficiency of green light of m is lowered, lithium niobate doped with such a magnesium ion is not preferable. SUMMARY OF THE INVENTION An object of the present invention is to convert a laser beam obtained from a solid-state laser using trivalent neodymium ions into a phase when a visible second harmonic is generated by a nonlinear crystal. It is an object of the present invention to provide a laser oscillator using lithium niobate which can be performed at a matching temperature near room temperature and has excellent optical damage characteristics of a nonlinear crystal. The above objects of the present invention can be attained by doping a fixed amount of scandium ion into a lithium niobate crystal which is a nonlinear crystal. That is, the present invention relates to a method for producing a solid-state laser using trivalent neodymium ions having a wavelength of 1.0 to 1.0.
In a laser oscillator that generates a second harmonic by phase-matching a 1 μm laser beam with a nonlinear crystal, the nonlinear crystal is a lithium niobate crystal containing 0.4 to 1.0 % by weight of scandium ions , and the lithium niobate is a lithium niobate crystal. crystal
Has a temperature control unit, and the phase matching temperature is 20 ° C to 40 ° C.
A laser oscillator characterized by being near room temperature . Hereinafter, the present invention will be described in detail with reference to the drawings. 1A and 1B are schematic views showing an example of the laser oscillator according to the present invention. In the figure, 1 is a solid-state laser using trivalent neodymium ions, 2 is a mirror, 3
Denotes a nonlinear crystal, 4 denotes a mirror, 5 denotes a semiconductor laser, and 6 denotes an oscillation material (crystal) using neodymium. In FIG. 1A, laser light (near-infrared light) of 1.0 to 1.1 μm, for example, 1.06 μm is obtained from a solid laser 1 using trivalent neodymium ions. Solid-state lasers using this trivalent neodymium ion include Nd: Y ₃ Al ₅ O ₁₂ (YAG), Nd: YVO
₄ (YVO) or the like is used. This laser light is guided to the nonlinear crystal 3 via the mirror 2. The mirror 2 totally reflects the laser beam of 0.53 μm. In the present invention, a lithium niobate crystal doped with scandium ions is used as the nonlinear crystal. The concentration of scandium ions is 0.4 to 2.0% by weight. Further, the operating temperature is preferably 55 ° C. or less, more preferably 4 ° C. or less, so as not to adversely affect peripheral silicon electronic devices.
0 ° C. or less is desirable. For this purpose, an appropriate concentration range varies depending on the wavelength of the fundamental wave of the solid-state laser, but is 0.7 to 1.0% by weight when the fundamental wave is 1.064 μm.
When the fundamental wave is 1.09 μm, the content is 1.0 to 1.5% by weight. If the concentration of scandium exceeds 2.0% by weight, good quality crystals cannot be produced, which is not preferable. In order to obtain scandium ion-doped lithium niobate, a raw material of lithium niobate crystal such as lithium carbonate and diniobium pentoxide and a predetermined amount of scandium oxide are mixed and formed into a green compact. A lithium niobate crystal doped with scandium ions is obtained by using a pulling method such as a Czochralski method, a floating zone method, a Bridgman method or a liquid phase epitaxy method. Among these methods, the Czochralski method can be used most preferably. Further, scandium oxide may be added to the surface of the lithium niobate single crystal not doped with scandium ions by ion plantation or sputtering, and doped with scandium ions by a diffusion process such as heat treatment. This method is a preferable method in consideration of the fact that scandium is a remarkably expensive element and local doping when manufacturing an optical device. As described above, the laser light is applied to the nonlinear crystal 3
And a second harmonic of, for example, 0.53 μm is generated. The light including green light of 0.53 μm, which is the second harmonic, is split by the mirror 4. That is,
The laser light of 1.06 μm is reflected by the mirror 4 and has a thickness of 0.1 μm.
The 53 μm green light partially transmits through the mirror 4. Next, in FIG. 1B , a laser beam of 0.8 μm is oscillated by the semiconductor laser 5, and the laser beam is guided to the oscillation material (crystal) 6 using neodymium via the mirror 2. The mirror 2 totally reflects the lights of 0.53 μm and 1.06 μm. In the oscillation material (crystal) 6 using neodymium, part of the 0.8 μm laser light becomes 1.06 μm laser light.
As an oscillation material (crystal) using neodymium, Nd: Y
₃ Al ₅ O ₁₂ (YAG), Nd: YVO ₄ (YVO), N
d: YLiF ₄ , Nd: glass or the like is used. Next, the laser light transmitted through the oscillation material (crystal) 6 using neodymium is phase-matched by the nonlinear crystal 3 to generate, for example, a second harmonic of 0.53 μm. The nonlinear crystal used here is a lithium niobate crystal doped with scandium ions as described above. The light including green light of 0.53 μm as the second harmonic is split by the mirror 4. That is,
The 0.8 μm and 1.06 μm laser beams are
The green light of 0.53 μm is partially transmitted through the mirror 4. As described above, conventionally, the phase matching temperature of a laser beam of 1.06 μm is lower than room temperature in an undoped lithium niobate crystal, and 5 mol% is added in a magnesium ion-doped crystal which is said to have excellent photodamage characteristics.
Compared to the reported temperature of 20 ° C., the lithium niobate crystal doped with scandium ions in the above range can perform phase matching near room temperature, which can be easily controlled by a Peltier device or the like. Furthermore, this temperature has a large scandium ion concentration dependency, and by manufacturing a crystal whose concentration is controlled, it is possible to manufacture an element that matches the desired temperature of phase matching. Further, this crystal has excellent photodamage characteristics. The inventors of the present invention have already reported that scandium-doped lithium niobate crystals have the best photodamage characteristics (Reference: JK Yaman).
amoto et al. , J. et al. Appl. Phys.
Lett, 41 (1992) 2156). However, the nonlinear performance of the lithium niobate crystal doped with scandium ions and the phase matching temperature characteristic of 1.06 μm were not known at all. The present invention has been accomplished as a result of detailed studies on a scandium ion-doped lithium niobate crystal having excellent performance in light damage characteristics. Hereinafter, the present invention will be described in detail based on examples and the like. Example 1 A lithium niobate crystal containing scandium ions was grown. The raw materials are lithium carbonate (LiCO ₃ ), niobium pentoxide (Nb ₂ O ₅ ), and scandium oxide (Sc ₂ O ₃ ).
Was used. Each burning raw material was grown after calculation. The crystal was grown by the harmonic heating type Czochralski method. All growth directions are C-axis, and each crystal has a size of 20φ × 60Lm without cracks or sub-grains.
m, and the crystal had a strong transparency. The ratio between lithium carbonate and niobium pentoxide was 0.97 (molar ratio). The grown crystal contained 0.5% by weight of scandium. Each of these crystals has a size of 5 × 5 × 10 m
m (z, y, x axis directions)
Optical polishing was performed on two 5 × 5 surfaces including the axis, and the phase matching temperature was measured using the experimental device of FIG. In FIG.
The same reference numerals as those in FIG. 1 indicate the same components, 7 is a lens, 8
Denotes a temperature control unit, 9 denotes a lens, 10 denotes a heat retaining tank, 11 denotes a cut filter, 12 denotes a photomultiplier, and 13 denotes a damping device. In FIG. 2, laser light (near infrared light) of 1.0 to 1.1 μm, for example, 1.06 μm is obtained from the solid-state laser 1 using trivalent neodymium ions. Next, the laser light is phase-matched by the nonlinear crystal 3 to generate, for example, a second harmonic of 0.53 μm. The nonlinear crystal 3 is provided with a temperature control unit 8 to keep the phase matching temperature constant. The laser light of 1.06 μm and the green light of 0.53 μm as the second harmonic pass through the lens 9 and are split by the mirror 4. That is, the 1.06 μm laser light is reflected by the mirror 4 and introduced into the damping device 13. On the other hand, the green light of 0.53 μm was transmitted through the mirror 4 and partially transmitted by the cut filter 11 to 1.06 μm.
Capture the laser light. The 0.5 obtained in this way
The 3 μm green light is introduced into the photomultiplier 12 . This measurement is based on the optical axis (z) of lithium niobate.
In this case, the light source laser is incident in the x-direction perpendicular to the axis (so-called non-critical phase matching), and the walk-off phenomenon described above does not occur in principle. In the figure, 1.06
The YAG laser emitting a laser beam of μm was Q-switched, and repeatedly oscillated at 10 Hz with a pulse width of 10 nanoseconds. The non-linear crystal (lithium niobate crystal doped with scandium ion) is surrounded by a copper plate which is a good conductor, and the copper plate is heated from -10 ° C to about 50 ° C.
The electronic control element which is the temperature control unit 8 up to around ℃ , for example
For example, it is heated and cooled by a Peltier element, and is further heated on a small resistance heater . Around the nonlinear crystal system, the phenomenon that water in the room adheres to the crystal
In order to avoid this, that is, to remove fluctuations in the output due to room temperature , it is further covered with a heat retaining tank 10 . The output green light was detected by the photomultiplier 12 . FIG. 3 shows the relationship between the output of green light and the phase matching temperature obtained from the measuring element in contact with the crystal in the embodiment. The example shows that phase matching at room temperature (21 ° C.) is possible. FIG. 4 shows the relationship between the phase matching temperature and the scandium ion concentration in the crystal. Comparative Examples 1-2 Undoped lithium niobate crystals (Comparative Example 2) and 5
A mol% magnesium ion-doped lithium niobate crystal (Comparative Example 3) was grown in the same manner as in Example 1. At that time, the mixing ratio of the raw materials was the same as in Example 1. However, magnesium ions were added at 4.5 mol%. A sample having the same shape as in Example 1 was processed from the obtained crystal, optically polished, and the phase matching temperature was measured. As a result, the results obtained at about 5 ° C. and about 120 ° C. were obtained for the undoped lithium niobate crystal and the 5 mol% magnesium ion-doped lithium niobate crystal, respectively. The temperature could not be determined exactly because the temperature was different from the value in the above-mentioned literature. However, in the case of undoped lithium niobate crystal, the moisture in the room adhered to the crystal and the output fluctuated. This is considered to be due to the fact that the 5 mol% magnesium ion-doped lithium niobate crystal emits heat from the crystal so strongly that the true temperature (near the laser beam path) cannot be determined. As described above, the laser oscillator of the present invention enables phase matching near room temperature. Details
Alternatively, in the nonlinear crystal used in the present invention, the setting of the phase matching temperature can be controlled using a temperature control unit, for example, an electronic control element such as a Peltier element. This can be downsized compared to hot wire heating elements such as hot plates,
Further, there is an advantage that precise electronic control of temperature is possible. In addition, the phenomenon of adhesion of moisture from the room at the time of temperature matching can be avoided. This phenomenon can contribute to downsizing and cost reduction when applied to private equipment. Further, there is little problem of optical damage of the nonlinear crystal.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view showing an example of a laser oscillator according to the present invention. FIG. 2 is a schematic diagram showing an experimental apparatus used in the examples. FIG. 3 is a graph showing the relationship between the output of green light and the phase matching temperature in Example 1. FIG. 4 is a graph showing a relationship between a phase matching temperature and a scandium ion concentration in a crystal. [Explanation of Signs] 1: Solid laser using trivalent neodymium ion, 2:
Mirror 3: nonlinear crystal 4: mirror 5: semiconductor laser 6: oscillation material (crystal) using neodymium.

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Claims (1)

(57) [Claims 1] A laser beam having a wavelength of 1.0 to 1.1 μm obtained from a solid-state laser using trivalent neodymium ions is phase-matched by a nonlinear crystal and a second harmonic is generated. In the generated laser oscillator, the nonlinear crystal is a lithium niobate crystal containing scandium ions in an amount of 0.4 to 1.0 % by weight , and the lithium niobate crystal has a temperature control unit.
A laser oscillator having a phase matching temperature around room temperature of 20C to 40C .