JP3398980B2 - Laser light generator - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser light generator, and more particularly to a laser light generator for generating a laser light by making a semiconductor laser enter a solid-state laser resonator as an excitation light source.

[0002]

2. Description of the Related Art It has been proposed to efficiently perform wavelength conversion by utilizing a high power density inside a resonator. For example, an external resonance type SHG (second harmonic generation), an SHG using a nonlinear optical crystal element inside a laser resonator, and the like have been tried.

As an example of the second harmonic generation type in the laser resonator, there is known one in which a laser medium and a nonlinear optical crystal element are arranged between at least one pair of reflecting mirrors constituting the resonator. In the case of this type of laser light generator, the nonlinear optical crystal element in the resonator is arranged so that the second harmonic laser light is phase-matched with the fundamental laser light, so that the second harmonic laser light is efficiently generated. Can be taken out.

As a method of realizing the above phase matching, a type I or type II phase matching condition is established between the fundamental laser light and the second harmonic laser light. In other words, the principle of type I phase matching is to use the ordinary ray of the fundamental laser light to generate a phenomenon in which two photons polarized in the same direction produce one photon having a frequency doubled. To do. On the other hand, type II phase matching is to make two fundamental polarizations that are orthogonal to each other enter a nonlinear optical crystal element so that the phase matching conditions can be established for each of the two polarizations. The fundamental wave laser light is divided into an ordinary ray and an extraordinary ray inside the nonlinear optical crystal element to cause phase matching with the extraordinary ray of the second harmonic laser light.

However, when the second harmonic laser light is to be generated using the type II phase matching condition, the intrinsic polarization of the fundamental laser light is changed every time the fundamental laser light repeatedly passes through the nonlinear optical crystal element. Since the phase changes, the generation of the second harmonic laser light may not be stably continued.

That is, every time the fundamental laser light generated in the laser medium repeatedly passes through the nonlinear optical crystal element due to the resonance operation, the phases of orthogonal natural vibrations (that is, p-wave component and s-wave component) are shifted. Then, in each part of the resonator, it becomes impossible to obtain a steady state in which the fundamental wave laser lights efficiently reinforce each other, so that a strong resonance state (strong standing wave) cannot be formed.
As a result, the conversion efficiency of the fundamental wave laser light into the second harmonic laser light may be deteriorated, and noise may be generated in the second harmonic laser light.

Therefore, the applicant of the present application filed Japanese Patent Application Laid-Open No. 1-220.
In Japanese Patent Publication No. 879, in a laser light source configured to generate a second harmonic laser beam by a non-linear optical crystal element, a birefringent element such as a quarter-wave plate is inserted in the resonance optical path of a fundamental wave laser beam. By doing so, a laser light source is proposed which stabilizes the second harmonic laser light emitted as the output laser light.

An example of a laser beam generator using the quarter-wave plate of the birefringent element will be described with reference to FIG. 5, which is a schematic block diagram thereof. In this case, a semiconductor laser 1 as an excitation light source, a solid-state laser medium 2 such as YAG that emits light when excited by laser light from the semiconductor laser 1, and the emission from the solid-state laser medium 2 as a fundamental wave light such as an optical second harmonic wave. A non-linear optical crystal element 3 such as KTP (KTiOPO ₄ ) for generating a laser beam is provided, and reflecting means 4 and 5 are provided before and after the solid-state laser medium 2 and the non-linear optical crystal element 3, respectively, and a laser resonance is provided therebetween. The container 6 is configured.

A quarter-wave plate (QWP) 7 made of, for example, a quartz plate, that is, a birefringent element is arranged in the laser resonator 6.

Reference numeral 8 designates a laser beam of the semiconductor laser 1 as a resonator 6
It is a condenser lens system for converging and entering the solid-state laser medium 2 inside.

In the illustrated example, the surface of the QWP 7 on the incident side of the laser light from the semiconductor laser 1 of the excitation light source, that is, the excitation light, shows no reflection at the wavelength of the laser light, for example, 810 nm, and is a solid-state laser. The light emitting laser light from the medium 2, that is, the reflection means 4 made of a coating that exhibits high reflection of the fundamental wave to the nonlinear optical crystal element 3, for example, 1064 nm, is formed, and is opposite to the fundamental wave incident side of the nonlinear optical crystal element 3. This is a case where the reflecting means 5 is formed on the surface on the side thereof, which has a high reflectance with respect to the fundamental wave and a high transmittance with respect to, for example, the second harmonic generated from the nonlinear optical crystal element 3.

According to such a configuration, the semiconductor laser 1
When the laser light from is incident on the solid-state laser medium 2, the laser light of 1064 nm, which is the fundamental wave of the nonlinear optical crystal element 3, is emitted, and is incident on the nonlinear optical crystal element 3. Since the laser light that becomes a wave is reflected by both reflecting means 4 and 5, it reciprocates between them and resonates.

As shown in FIG. 6, the quarter wave plate has an index of refraction n in the extraordinary light direction on the inner surface perpendicular to the light propagation direction.
Direction of _{e (3)} is a predetermined azimuthal angle relative to the direction of the nonlinear optical crystal element 3 of the extraordinary light direction refractive index n _{e (3)} theta, e.g. theta = 4
The optical axis position is set so as to be inclined by 5 °.

In the above structure, when the fundamental wave laser light passes through the resonance optical path and passes through the nonlinear optical crystal element 3, second harmonic laser light is generated, and the second harmonic laser light passes through the means 5. Then, the output laser light L is transmitted.

In this state, each light beam forming the fundamental wave laser light has an azimuth angle θ with respect to the nonlinear optical crystal element 3.
By passing through the birefringent element (that is, the quarter wavelength of the fundamental wave) QWP7 set in the direction inclined by = 45 °, the power of the laser light in each part of the resonator 6 is stabilized to a predetermined level. . This is because, when the fundamental wave laser light generated in the laser medium 2 is resonated so as to pass through the nonlinear optical crystal element 3 to generate the type II second harmonic laser light, the fundamental wave laser light is orthogonal to each other. The coupling due to the sum frequency generation between the two intrinsic polarization modes
The oscillation is stabilized by suppressing it by WP7.

[0016]

However, in the laser light generator having such a structure, the return light to the semiconductor laser as the excitation light source becomes a problem.

That is, in the laser light generator shown in FIG. 5, the laser light from the semiconductor laser 1 as the excitation light source that is incident on the QWP 7 has the wavelength of this laser light on the surface on the semiconductor laser 1 side. On the other hand, since the anti-reflection coating is applied, the light is hardly reflected here, but it is reflected by the surface 7s of the QWP 7 opposite to this surface and becomes a return light noise to the semiconductor laser 1.

It is known that this noise is caused by the phase relationship between the emitted light of the semiconductor laser and the returned light, and can be suppressed when the polarizations of these two lights are orthogonal to each other.

In the above configuration, the laser light from the semiconductor laser 1 is reciprocated in the QWP 7 in the process of being reflected by the surface 7s of the QWP 7 and returning to the semiconductor laser 1. By this, the returning light noise can be avoided. I can't.

That is, the semiconductor laser 1 as the excitation light source oscillates with, for example, horizontal linearly polarized light.
Is a quarter wavelength plate for the emission wavelength of the solid-state laser medium 2 of 1064 nm as described above, and since the intrinsic polarization of the crystal forming the QWP 7 and the polarization direction of the pumping semiconductor laser light are different, The phase delay of the light does not become π / 2 and returns to the semiconductor laser 1 as elliptically polarized light. Therefore, the two waves of the emitted light from the semiconductor laser 1 and the reflected light interfere with each other and the noise of the semiconductor laser increases. To do.

The present invention solves the problem of return light noise in a semiconductor laser as such a pumping light source.

[0022]

According to the present invention, a semiconductor laser as an excitation light source, a solid-state laser medium that emits light when excited by laser light from the semiconductor laser, and a laser resonator positioned before and after a fixed laser medium are provided. In a laser light generator having a reflecting means constituting the laser resonator and a birefringent element arranged in the laser resonator, return light returning to the inside of the semiconductor laser between the laser resonator and the semiconductor laser. The return light wave plate for adjusting the amount of phase delay of the return light is provided so that the polarization direction of is orthogonal to the polarization direction of the laser light emitted from the semiconductor laser.

According to the present invention, in this structure, the nonlinear optical crystal element is arranged in the laser resonator.

Furthermore, the present invention provides each of the above-mentioned configurations,
The wavelength plate for return light has both surfaces coated with a non-reflective coating for the laser light from the semiconductor laser.

Furthermore, in the present invention, the principal axis of the return light wave plate is made to coincide with the principal axis of the crystal of the birefringent element in the laser resonator.

Further, in the present invention, in each of the above-mentioned configurations, the surface of the return light wavelength plate on the semiconductor laser side is inclined from the surface orthogonal to the optical axis.

[0027]

[0028]

According to the present invention, by providing the return light wave plate having a required phase delay amount between the semiconductor laser and the laser resonator, the emitted light of the semiconductor laser and the polarized light of the return light are made orthogonal to each other. Therefore, the interference of both lights can be avoided, and the return light noise of the semiconductor laser light can be avoided or improved.

[0029]

EXAMPLE Referring to FIG. 1, a laser beam from a solid-state laser having a semiconductor laser as an excitation light source is used as a fundamental wave and an SHG (optical second harmonic generation device) by a non-linear optical crystal element to a second harmonic laser beam. An embodiment of a laser light generator (hereinafter referred to as SHG laser) for generating a laser beam will be described.

In this case, as described with reference to FIG. 5, the semiconductor laser 1 as the pumping light source, the solid laser medium 2 such as YAG which emits light by the pumping by the laser light from the semiconductor laser 1, and the solid laser medium 2 For example, KTP (KTi) that generates the second harmonic of light using the emitted light as the fundamental wave light
A non-linear optical crystal element 3 such as OPO ₄ ) is provided, and reflecting means 4 and 5 are provided before and after the solid-state laser medium 2 and the non-linear optical crystal element 3, and a laser resonator 6 is provided between them.
Make up.

In the laser resonator 6, a quarter wave plate (QWP) 7 made of, for example, a quartz plate, that is, a birefringent element is arranged.

Reference numeral 8 is a resonator 6 for the laser light of the semiconductor laser 1.
It is a condenser lens system for converging and entering the solid-state laser medium 2 inside.

The reflecting means 4 is non-reflective with respect to the wavelength of this laser light, for example, 810 nm, on the surface of the QWP 7 on the incident side of the laser light from the semiconductor laser 1 of the excitation light source, that is, the excitation light, and is a solid-state laser. The emission laser light from the medium 2, that is, the fundamental wave to the nonlinear optical crystal element 3, for example, is coated with a coating showing high reflection with respect to 1064 nm.

The other reflecting means 5 has a high reflectance for the fundamental wave on the surface of the nonlinear optical crystal element 3 opposite to the fundamental wave incident side, and is generated from the nonlinear optical crystal element 3, for example. It is constructed by coating with a high transmittance for the second harmonic.

Further, particularly in the present invention, a return light wave plate 11 having a required phase delay amount is arranged between the semiconductor laser 1 as a pumping light source and the resonator 6.

In the illustrated example, the return light wavelength plate 11 is arranged between the semiconductor laser 1 and the condenser lens system 8, but the arrangement is not limited to this arrangement position.

Both sides 11a of the return light wave plate 11 are also provided.
And 11b apply a non-reflective coating to the laser light of the semiconductor laser 1 to avoid generation of return light to the semiconductor laser 1.

Furthermore, in order to more completely avoid the generation of return light from the return light wave plate 11 to the semiconductor laser 1, the surface 11 of the return light wave plate 11 on the semiconductor laser 1 side.
Tilt a about 0.5 degrees.

The phase delay amount of the return light wave plate 11 is such that the polarization direction of the reflected return light from the surface 7s opposite to the incident side of the laser light from the semiconductor laser 1 of the pumping light source of the QWP 7 is semiconductor. The phase delay amount is orthogonal to the oscillation laser beam from the laser 1.

Specifically, the return light wave plate 11 is formed of, for example, a single crystal plate having a thickness of about 0.5 mm at the center. In this case, Sellmei
The thickness and the cutting angle are determined by the equation of r), and the phase delay amount of the return light wave plate 11 is determined.

FIG. 2 shows the relationship between the thickness of the crystal and the amount of phase delay. In this case, the cutting angle with respect to the c-axis is 16.2 °.

Further, FIG. 3 shows the relationship between the cut-out angle of the crystal with respect to the c-axis and the phase delay amount, and in this case, the thickness is 0.5 °.

As is clear from these relationships, the amount of phase delay can be selected by selecting the thickness and the cutout angle.

Now, assuming that the QWP 7 has a rotation angle of 45 ° and the solid-state laser medium 2 is YAG, the wavelength is 1064.
Since the amount of phase delay is π / 2 with respect to nm, 0.668 is one way for the wavelength of 810 nm of the semiconductor laser 1.
becomes π. Therefore, if the phase delay amount of the return light wave plate 11 is 0.832π, the return light wave plate 11 and the QWP 7 are
The amount of phase delay in both round trips is 3π, and the respective polarizations of the returning light from the surface 7s of the QWP 7 to the semiconductor laser 1 and the oscillation light of the semiconductor laser 1 are orthogonal to each other.

FIG. 4 shows a comparative example using the SHG laser having the configuration shown in FIG. 5 when the return light wave plate is not provided, and an SHG when the return light wave plate 11 shown in FIG. 1 according to the present invention is provided. The noise characteristics of the laser are shown by a broken line and a solid line, respectively. As is clear by comparing these curves, the SHG laser according to the present invention is improved in noise.

In the example of FIG. 1, the present invention is applied to an SHG laser, but it does not depend on the SHG laser, that is, in FIG. The present invention can also be applied to a solid-state laser using a laser as an excitation light source.

[0047]

As described above, according to the present invention, the wavelength plate for returning light is provided between the semiconductor laser as the pumping light source and the laser resonator, so that the semiconductor laser, in particular, the birefringent element is provided. For example, it is possible to effectively reduce the noise due to the returning light from the quarter-wave plate, so that the benefit is extremely large when it is used as a light source for various lights or magneto-optical recording / reproducing.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an example of a laser light generator according to the present invention.

FIG. 2 is a diagram showing a relationship of a phase delay amount with respect to a crystal thickness.

FIG. 3 is a diagram showing a relation of a phase delay amount with respect to a crystal cutting angle.

FIG. 4 is a noise measurement curve diagram.

FIG. 5 is a configuration diagram of a conventional laser light generator.

FIG. 6 is an explanatory diagram of an azimuth angle of a birefringent element.

[Explanation of symbols]

1 Semiconductor laser 2 Solid-state laser medium 3 Non-linear optical crystal element 4 reflection means 5 Reflection means 6 resonator 7 Wave plate for return light 11 1/4 wave plate (QWP)

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-1-241879 (JP, A) JP-A 1-220879 (JP, A) JP-A-3-185403 (JP, A) JP-A 5- 243650 (JP, A) JP-A-3-49278 (JP, A) JP-A-3-150888 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) H01S 3/00-3 / 30 H01S 5/00-5/50

Claims (5)

(57) [Claims] 1. A semiconductor laser as an excitation light source, a solid-state laser medium that emits light when excited by a laser beam from the semiconductor laser, and a reflection unit that is located before and after the fixed laser medium to form a laser resonator. In the laser light generator in which the birefringent element is arranged in the laser resonator, the polarization direction of the return light returning to the semiconductor laser is between the laser resonator and the semiconductor laser. A return light wavelength plate for adjusting the phase delay amount of the return light is provided so as to be orthogonal to the polarization direction of the laser light emitted from the semiconductor laser. 2. The laser light generator according to claim 1, wherein a nonlinear optical crystal element is arranged in the laser resonator. 3. The laser light generator according to claim 1, wherein both surfaces of the return light wave plate are provided with a non-reflective coating for the laser light from the semiconductor laser. 4. The laser beam according to claim 1, 2 or 3, wherein the principal axis of the return light wave plate is aligned with the principal axis of the crystal of the birefringent element in the laser resonator. Generator. 5. The return light wave plate according to claim 1, 2, 3 or 4, wherein a surface of the return light wave plate on the semiconductor laser side is tilted from a surface orthogonal to an optical axis. The laser light generator described.