JPH0645669A - End face excitation type solid laser - Google Patents

Abstract

PURPOSE:To facilitate optical alignment by reducing the number of parts of an end face excitation type solid laser for miniaturization. CONSTITUTION:A laser resonator is constituted of the surface at the side of an optical fiber 1 of a solid laser crystal 2 and an output mirror 3. The end face of the optical fiber 3 is adhered to or brought closer to the solid laser crystal 2 so that the light axis of the laser resonator matches that of the optical fiber 1. An excitation light 4 is connected and waveguided from the end face at the opposite side of the optical fiber 3 to excite the solid laser 2 and to obtain an output.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser device, and more particularly, to an edge-pumped solid-state laser using a semiconductor laser as a pumping light source.

[0002]

2. Description of the Related Art A conventional edge-pumped solid-state laser is shown in FIG.
As shown in (3), the excitation light emitted from the excitation light source such as the semiconductor laser 10 is condensed by the lens 9 and irradiated on the solid-state laser crystal 2 of the solid-state laser to be excited. Alternatively, the laser light emitted from the excitation light source is coupled to the optical fiber by the lens to guide the optical fiber, and the laser light emitted from the end face of the optical fiber is focused again by the lens before being used for the excitation. Regarding the semiconductor laser pumped solid-state laser, "Laser Research, Vol. 17, No. 10 (198
9), pp. 695-705 ”.

[0003]

As a pumping light source for this type of solid-state laser, a semiconductor laser which is relatively small and has a considerably large optical output is often used. When the laser light emitted from the semiconductor laser is condensed by a lens and used for exciting the solid-state laser, the resonator of the solid-state laser and the semiconductor laser must be integrated. In general, a semiconductor laser for exciting a solid-state laser has a large output and consumes a large amount of power, so that the heat generated by it cannot be ignored. Therefore, it is necessary to use a heat radiating means such as a heat radiating plate, an air cooling fan, and a Peltier element for cooling, and the size of the light emitting portion of the solid-state laser becomes large. In order to construct this type of laser, at least a semiconductor laser, a lens,
Four elements, a solid-state laser crystal and an output mirror, were required, and optical alignment of them required high technology and time.

By a method of coupling laser light emitted from a semiconductor laser to an optical fiber with a lens to guide the optical fiber, and collecting laser light emitted from an end face of the optical fiber with a lens and then using it for excitation, The former problem can be solved. However, the latter problem still remains.

It is an object of the present invention to provide an edge-pumped solid-state laser in which optical alignment is easier and a light emitting portion is smaller than in the prior art.

[0006]

An edge-pumped solid-state laser according to the present invention includes a solid-state laser crystal in a laser resonator, wherein at least one surface of the solid-state laser crystal constitutes a mirror of the laser resonator, and a laser resonance In the end-pumped solid-state laser that pumps the solid-state laser by irradiating the solid-state laser from the outside of the laser resonator along the optical axis of the resonator, an optical fiber for guiding the pump light is provided, and the optical fiber The end face on the output side of (1) is brought into close contact with or at least close to the solid-state laser crystal, and the solid-state laser is excited by the excitation light emitted from the end face.

In the edge pumped solid-state laser,
It is characterized in that a secondary nonlinear optical crystal for generating a high frequency is provided in the resonator.

Further, the beam spot radius at the position of the solid-state laser crystal determined from the structure of the laser resonator is W ₀ , the refractive index of the solid-state laser crystal is n ₁ , the absorption coefficient at the wavelength of the pumping light is α, When the core diameter of the optical fiber is 2r _C , the numerical aperture is NA, and the distance between the solid-state laser crystal and the optical fiber is d ₀ , r _C + d ₀ (NA) / {1- (NA) ² } ^1/2 <W ₀ and r _C + d ₀ (NA) / {1- (NA) ² } ^1/2 + (2 / α) × (NA) / {n ₁ ² − (NA) ² } ¹ It is characterized by satisfying ^{/ 2} > W ₀ .

Further, in the edge-pumped solid-state laser, an a-axis cut Nd: YV is used as the solid-state laser crystal.
Using O ₄ , the beam spot radius at the position of the Nd: YVO ₄ crystal determined from the configuration of the laser resonator is W ₀ , the refractive index of the Nd: YVO ₄ crystal is n ₁ , and the core diameter of the optical fiber is Is 2r _C , the numerical aperture is NA,
When the distance between the Nd: YVO ₄ crystal and the optical fiber is d ₀ , r _C + d ₀ (NA) / {1- (NA) ² } ^1/2 <W ₀ and r _C + d ₀ (NA ) / {1- (NA) ² } ^1/2 +1 (mm) × (NA) / {n ₁ ^2- (NA) ² } ^1/2 > W ₀ .

[0010]

When the end face excitation type solid state laser is excited, the absorption efficiency is improved by absorbing the excitation light in the oscillation region in the solid state laser crystal. The beam radius in this solid-state laser crystal changes depending on the radius of curvature of the output mirror and the cavity length. FIG. 4 shows the relationship between the resonator length and the beam spot radius at the position of the plane mirror in an optical resonator in which one mirror is a plane and the other mirror has a curvature.
The wavelength of light was 1064 nm. The radius of curvature r is used as a parameter, and the value of r is changed from 10 mm to 1000 mm. The edge-pumped solid-state laser has a cavity length of 10 mm.
The beam spot radius at the position of the plane mirror is approximately 50 to 300 μ in this range.
m. Conventionally, light from a semiconductor laser is guided directly or once through an optical fiber, and then light is condensed by a lens to excite a solid-state laser.

When an optical fiber is used, it can be excited simply by bringing its end face close to the solid-state laser crystal. FIG. 3 shows a state in which the optical fiber is brought close to the solid-state laser crystal 2. It is assumed that the core radius of the optical fiber 1 is r _C and the numerical aperture is NA. The refractive index of the solid-state laser crystal is n
_Set to ₁ . Further, the distance between the end face of the optical fiber 1 and the solid-state laser crystal is d ₀ , and the beam spot radius of the excitation light on the crystal end face is W ₁ . The beam spot radius in the solid-state laser crystal for oscillation of the solid-state laser is W ₀ , and the oscillation region of the solid-state laser is shown by a broken line. When most of the excitation light is absorbed in this oscillation region, the excitation efficiency improves. The excitation light guided through the optical fiber is generally emitted from the end face with an emission angle of 10 ° or more. Therefore, when the optical fiber is closely contacted with or close to the solid-state laser crystal for excitation, it is necessary to absorb the excitation light in a short region near the end face of the solid-state laser crystal before the excitation light spreads. If a solid-state laser crystal having a low absorptance is used, it will not be sufficiently absorbed in the oscillation region and will be absorbed in a portion that does not contribute to oscillation. However, when using a solid-state laser crystal with high absorption,
It is possible to absorb most of the energy within the oscillation region.

FIG. 5 shows the relationship between the crystal length and the absorption rate by the absorption coefficient α.
Is shown as a parameter. Nd: YAG, which has been often used as a solid-state laser crystal, has an Nd concentration of 1 atom. % Of about 810
Since the absorption coefficient in the vicinity of nm is about 0.4 mm ⁻¹ and the crystal length is required to be 4 mm or more to absorb the excitation light by 80% or more, it is not suitable for use in the laser of the present invention. . Regarding the absorption coefficient of Nd: YVO _{4 in} the vicinity of 810 nm, the normally used Nd concentration is 1 atom. %, When the electric field of the excitation light is parallel to the c-axis of Nd: YVO ₄ , the crystal length is about 3 mm ⁻¹ , and it is possible to absorb about 80% of the excitation light with a crystal length of about 0.5 mm. is there. Therefore, an appropriate optical fiber with a core diameter or numerical aperture NA,
It is possible to effectively absorb light in the oscillation region by using an output mirror having an appropriate radius of curvature or adjusting the cavity length.

In order to improve the pumping efficiency, the beam spot radius W ₁ of the pumping light at the crystal end face must be smaller than the beam spot radius W ₀ of the solid-state laser oscillation. When this is expressed by an equation, r _C + d ₀ tan θ ₀ <W ₀ (1). If the absorption of the pumping light is too concentrated in the center of the oscillation region of the solid-state laser, not only the transverse mode of the solid-state laser is affected but also the output itself is reduced. Therefore, the absorption of the excitation light is {1-exp (-
2)} × 100% (about 86%), so that the crystal length is d
₁ and the beam spot of the excitation light at that position is W
When it is set to ₂ , at least W ₂ needs to be larger than W ₀ . When expressed by an equation, r _C + d ₀ tan θ ₀ + d ₁ tan θ ₁ ≦ W ₀ (2) between d ₁ and α

Since it is necessary to establish the relationship of exp (-αd ₁ ) = exp (-2), and the relationship of NA = sin θ ₀ sin θ ₀ = n ₁ sin θ ₁ is satisfied, the formula (1) and the formula ( 2) can be expressed as follows.

R _C + d ₀ (NA) / {1- (NA) ² } ^1/2 <W ₀ (3) r _C + d ₀ (NA) / {1- (NA) ² } ^1/2 + (2 / Α) × (NA) / {n ₁ ² − (NA) ² } ^1/2
> W ₀ (4) It is possible to obtain higher oscillation efficiency by satisfying the expressions (3) and (4).

As a solid-state laser crystal, a-axis cut Nd:
Consider the case of using YVO ₄ . The optical fiber generally does not preserve the polarization, and the laser light guided through the optical fiber is unpolarized. The absorption coefficient is about 3 mm ⁻¹ for excitation light having an electric field parallel to the c-axis, and about 1.5 mm ⁻¹ for excitation light having an electric field perpendicular to the c-axis. Therefore, the crystal length d ₁ for absorbing about 83% of the excitation light is about 1 mm. Therefore, the condition of Expression (4) can be expressed as follows.

R _C + d ₀ (NA) / {1- (NA) ² } ^1/2 +1 (mm) × (NA) / {n ₁ ² − (NA) ² } ^1/2 > W ₀ (5) Therefore, when the a-axis cut Nd: YVO ₄ is used in the present invention, the oscillation efficiency is increased by satisfying the conditions of the expressions (3) and (5).

Further, according to the present invention, a coupling optical system such as a lens, which is conventionally used for focusing light in front of a solid-state laser crystal, is not required, and the number of parts is reduced. Therefore, optical alignment is easier than before. become.

[0019]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows a first embodiment of the present invention. The output mirror 3 has 80% to the oscillation wavelength of the solid-state laser.
The coating is applied so as to have a high reflectance of about 98%. The two faces of the solid-state laser crystal 2 are polished in parallel, and a coating is applied so that the output mirror 3 has a low reflectance and the opposite side has a reflectance close to 100% with respect to the oscillation wavelength of the solid-state laser. The highly reflective surface of the solid-state laser crystal 2 and the output mirror 3 constitute a laser resonator. One end face of the optical fiber 1 is brought into close contact with the solid-state laser crystal 2 so that the optical axis of the laser resonator and the optical axis of the optical fiber 1 coincide with each other. Light 4 from an excitation light source such as a semiconductor laser is coupled and guided from the end face on the opposite side of the optical fiber 1 to excite the solid-state laser. As the solid-state laser crystal, a crystal having a high absorptivity such as Nd: YVO ₄ or LNP (LiNd (PO ₃ ) ₄ ) is used.

FIG. 2 shows a second embodiment of the present invention. First
The major difference from the above embodiment is that a secondary nonlinear optical crystal 6 for wavelength conversion is provided in the laser resonator. The nonlinear optical crystal 6 is made of KTP (KTiOPO ₄ ), BBO
Any material such as (β-BaB ₂ O ₄ ), KNbO ₃ or the like that can perform phase matching for wavelength conversion, such as second harmonic generation using the oscillation light of the solid-state laser as a fundamental wave, may be used. Another difference from the first embodiment is that the reflectance of the output mirror 3 is close to 100% with respect to the oscillation wavelength and is low with respect to its second harmonic. Is. By doing so, the oscillation light of the solid-state laser, which becomes the fundamental wave in wavelength conversion, is confined in the laser resonator, the power density inside the laser resonator is increased, and the wavelength conversion efficiency is improved.

In the present embodiment, one of the mirrors constituting the laser resonator is formed by coating one end face of the solid-state laser crystal, and the output mirror is independently provided. It is also possible to coat the both end faces of the laser crystal to form the resonator, or to form the resonator by coating one end face of the solid-state laser crystal and one end face of the nonlinear optical crystal in the second embodiment. Needless to say.

[0023]

As described in detail above, according to the present invention,
It is possible to provide an edge-pumped solid-state laser in which optical alignment is easier than in the past and the light emitting portion is smaller. In addition, the number of lenses used can be reduced, which leads to cost reduction and is very effective in industry.

[Brief description of drawings]

FIG. 1 is a perspective view for explaining an embodiment of the present invention.

FIG. 2 is a perspective view for explaining a second embodiment of the present invention.

FIG. 3 is a diagram for explaining the operation of the present invention in which an optical fiber is brought close to a solid-state laser crystal and excited.

FIG. 4 is a diagram showing a relationship between a cavity length and a beam radius of an optical resonator including a plane mirror and a concave mirror.

FIG. 5 is a diagram showing a relationship between crystal length and absorptance.

FIG. 6 is a diagram showing an example of a conventional edge-pumped solid-state laser.

[Explanation of symbols]

 1 optical fiber 2 solid-state laser crystal 3 output mirror 4 pumping light 6 nonlinear optical crystal 9 lens 10 semiconductor laser

Claims (3)

[Claims] 1. A laser resonator includes a solid-state laser crystal, at least one surface of the solid-state laser crystal constitutes a mirror of the laser resonator, and the laser resonator includes the mirror along the optical axis of the laser resonator. In an end-pumped type solid-state laser that pumps a solid-state laser by irradiating pumping light from the outside on the solid-state laser side, an optical fiber for guiding the pumping light is provided, and the end face on the output side of the optical fiber is closely attached to the solid-state laser crystal. An edge-pumped solid-state laser, characterized in that a solid-state laser is excited by excitation light emitted from the end face at least in close proximity. 2. The edge-pumped solid-state laser according to claim 1, wherein the beam spot radius at the position of the solid-state laser crystal determined from the configuration of the laser resonator is W ₀ , and the refractive index of the solid-state laser crystal is n. ₁ , the absorption coefficient at the wavelength of the pumping light is α, and the core diameter of the optical fiber is 2
r _C , the numerical aperture is NA, and the distance between the solid-state laser crystal and the optical fiber is d ₀ , r _C + d ₀ (NA) / {1- (NA) ² } ^1/2 <W ₀ and r _C + d ₀ (NA) / {1- (NA) ² } ^1/2 + (2 / α) × (NA) / {n ₁ ² − (NA) ² } ^1/2 > W 0 Edge-pumped solid-state laser. 3. The edge-pumped solid-state laser according to claim 1, wherein a-axis cut Nd: YVO ₄ is used as the solid-state laser crystal, and the Nd: determined from the configuration of the laser resonator. The beam spot radius at the position of the YVO ₄ crystal is W ₀ , the refractive index of the Nd: YVO ₄ crystal is n ₁ , the core diameter of the optical fiber is 2r _C , the numerical aperture is NA, and the Nd: YVO ₄ crystal is When the distance between the optical fibers is d ₀ , r _C + d ₀ (NA) / {1- (NA) ² } ^1/2 <W ₀ and r _C + d ₀ (NA) / {1- (NA ) ² } ^1/2 +1 (mm) x (NA) / {n ₁ ^2- (NA) ² } ^1/2 > W ₀ is satisfied.