JPH0774417A - Solid-state laser device using optical fiber - Google Patents

Abstract

PURPOSE:To improve input output characteristic by combining resonator mode and excitation light mode of a solid-state laser device effectively. CONSTITUTION:A light projection edge face 1c of an optical fiber 1 which propagates excitation light from a semiconductor laser 4 to laser medium 2 is formed to a lens form and its focal distance is in a range of three to ten times as large as a radius of a fiber core. Laser medium is arranged in contact or in proximity to a light projection end of optical fiber. A core radius of optical fiber is about the same as a resonator mode radius in the neighborhood of an excitation light injection part.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a solid-state laser device in which pumping light is guided by an optical fiber and is incident on a laser medium.

[0002]

2. Description of the Related Art FIG. 13 shows a conventional solid-state laser device using an optical fiber as a pumping light guide means. Semiconductor laser 4
The pumping laser light from is conducted to the optical fiber 1. Since the excitation light emitted from the optical fiber 1 has a certain spread, it is converged by the optical system 11 such as a convex lens and made incident on the laser medium 2. The laser medium 2 has a function of amplifying light by stimulated emission. The amplified light resonates between the output mirror 5 and the total reflection mirror 3 set on the side of the laser medium 2 on which the light emitted from the optical fiber enters.

Further, in order to reduce the size of the device by reducing the number of parts and to facilitate the adjustment of the optical axis and the focus, the light emitting end of the optical fiber 1 is formed in a convex lens shape (1c), and other parts are formed. Excitation light is used as laser medium 2 without using an optical system.
Japanese Patent Application Laid-Open No. 2-1.
It is known from the publication No. 50087 (FIG. 12).

[0004]

However, the configuration of FIG. 12 has the following characteristics.

Light passing through the optical fiber 1 is reflected by the lens portion 1.
For c, since it is a surface light source having the same diameter as the lens diameter, the aberration will be larger than when an optical system is used.
The lens portion 1c at the light emitting end of the optical fiber is equivalent to a plano-convex lens, and since the surface light source is on the lens side of the object-side focal point of the lens, the characteristic of the optical fiber illumination system is that it is a diverging system. Since it is a large surface light source, there are three points that the substantial numerical aperture (NA) changes depending on the curvature of the lens.

Therefore, there are the following inconveniences.

(A) In the state of light emitted from an optical fiber, if the focal length of the lens is too short, the converging range of the excitation light from the end face of the optical fiber is short, and the excitation light diverges largely immediately after the condensing. Further, when the curvature of the lens becomes large, it passes near the outer periphery of the core of the optical fiber, so that the excitation light flux immediately before emission has a large incident angle with respect to the emission end face, and the excitation light flux is not incident on the laser medium 2 side, Many light beams are reflected by the surface of the emission end face 1c and return to the optical fiber. Therefore, the ratio of the excitation light from the semiconductor laser 4 that is absorbed by the resonator mode portion of the laser medium 2 (hereinafter referred to as the absorption ratio) is naturally low, and the excitation light mode is efficiently coupled to the resonator mode. The ratio (hereinafter, referred to as coupling efficiency) also decreases. Here, the resonator mode refers to a state of distribution of resonating laser light.

On the contrary, if the focal length is too long, the lens portion 1
Since c is a divergent system, the excitation light does not collect and diverges gently, so the difference between the intensity of the excitation light near the center of the optical fiber and the intensity of the excitation light passing near the outer periphery of the core of the optical fiber is small. Therefore, the curve of the excitation light mode draws a gentle curve, and the coupling efficiency is greatly reduced when the shape of the resonator mode curve is extremely different.

(B) Further, if the spot size of the resonator mode (hereinafter, simply referred to as the resonator mode radius) near the excitation light entering the laser medium 2 is too large as compared with the core radius of the optical fiber, resonance occurs. Although the amount of pumping light that is taken into the cavity mode is large, the spread of the pumping light incident on the laser medium becomes smaller compared to the spread of the laser light distribution in the resonator, so the resonator mode diameter was set to an appropriate size. The binding efficiency is lower than in the case.

On the other hand, if the resonator mode diameter is too small, the amount of pumping light taken into the resonator mode is small, so the absorption rate decreases, and the coupling efficiency is not high.

Due to these characteristics, it is difficult to efficiently couple the pumping light mode showing the optical fiber emission light state in the laser medium to the resonator mode of the solid-state laser. In order to improve the coupling efficiency, it is necessary to numerically specify the focal length of the lens, the core radius of the optical fiber, the mode diameter of the resonator, the relative positional relationship of each component, etc. in consideration of these characteristics. However, since there are many factors that influence the coupling efficiency, it is not easy to combine these with appropriate values to obtain the maximum coupling efficiency.

The present invention aims to provide a solid-state laser device having a high coupling efficiency, focusing on the improvement of the coupling efficiency which has not been a problem in the prior art.

[0013]

In order to achieve such an object, the present invention provides a light source for generating excitation light, and a light source for guiding the excitation light from the light source to a laser medium. The optical fiber has an emission end face formed in a lens shape and a laser medium that receives excitation light from the optical fiber. The focal length of the lens portion of the light emission end face of the optical fiber is 3 to 10 times the radius of the optical fiber core. The laser medium is arranged in close contact with or close to the light emitting end of the optical fiber, and the core radius of the optical fiber is the same as the cavity mode radius near the pumping light incident part of the laser medium. It is characterized by the degree.

[0014]

With these conditions, a laser device having a high coupling efficiency in which the pumping light mode is efficiently coupled to the resonator mode of the solid-state laser is established. That is, even if the spread of the optical fiber output light is relatively large, the spot size of the pumping light mode is close to the pumping light mode of the laser medium 2 because the output end face of the optical fiber is close to or in close contact with the input end surface of the laser medium. Is still small, there is no need to consider the spread of the excitation light.

Further, in this case, by limiting the focal length, it is possible to suppress the spread of the excitation light mode and increase the absorption rate.

By making the resonator mode diameter to be approximately the same as the optical fiber core diameter, the coupling efficiency of the laser device is increased. That is, the resonator mode and the distribution state of the excitation light are about the same,
Both can be efficiently combined. These three conditions need to be met at the same time.

[0017]

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

[Embodiment 1] A first embodiment of the present invention is shown in FIG.
Shown in.

In FIG. 1, the exit end of a step index type multimode optical fiber is spherically polished and formed into a lens shape to guide the pumping light of a semiconductor laser, and the pumping light is guided to an end facet pumping type solid-state laser resonator. This is an example when light is excited.

Excitation light 8 emitted from the semiconductor laser 4
Is incident on the optical fiber 1 mainly composed of the core portion 1a and the cladding portion 1b, and travels while reflecting inside the core portion 1a.

The emission end face of the optical fiber 1 is spherically polished with a radius of curvature r in a limited range described later.

The emission end face 1c of the optical fiber 1 is close to or in contact with the incidence end face 3 of the laser medium 2. It is desirable to make contact with each other in terms of efficiency, but since contact may cause damage to the reflecting mirror 3 provided at one end of the laser medium, it is possible to place them in close proximity within a range that does not affect efficiency.

The excitation light is condensed to the vicinity of the focal length f of the plano-convex lens 1c, and spreads greatly thereafter. That is, the focal length represents the approximate distance until the spread of the excitation light changes greatly, and the degree of condensing / spreading.

The limited range of the focal length f is 3 times or more and 10 times or less of the fiber core radius dc.

The radius of curvature r of the convex portion of the plano-convex lens is expressed by the following formula with the focal length f and the refractive index n of the lens, and the curvature of the lens is also limited.

-R = f (1-n) On the other hand, the solid-state laser resonator is composed of a total reflection mirror 3 formed on one end surface of the laser medium 2 and an output mirror 5.
In order to improve laser input / output characteristics, the output mirror 5 is often provided with a curvature. The size of the radius of curvature of this output mirror,
The distance between the total reflection mirror 3 and the output mirror 5 determines the resonator mode.

In this case, since the output mirror 5 has a curvature, the mode diameter dl of the resonator mode 6 becomes the smallest in the vicinity of the total reflection mirror 3 which is the coupling portion with the excitation light, and the optical fiber 1 The core diameter dc is set to be approximately the same.

Typical specifications of the step index type multimode optical fiber are: core radius dc = 0.2 mm;
The core part has a refractive index n = 1.5 and NA = 0.2.

In this case, the focal length f of the plano-convex lens formed at the exit end of the optical fiber is 3 of the core radius of the optical fiber.
Since it is set to be not less than 10 times and not more than 10 times, the range that f can take is as follows.

0.6 mm ≦ f ≦ 2.0 mm From the above equation, the range of the radius of curvature r of the convex portion of the lens is as follows. 0.3 mm ≦ r ≦ 1.0 mm Then, the exit end face of the optical fiber is brought close to or in contact with the entrance end face of the laser medium. When they are brought close to each other, the allowable range of the distance between the laser medium and the emission end face of the optical fiber differs depending on the size of the focal length f.

Since the resonator mode radius dl is the same as the core diameter of the optical fiber, it is set to dl = 0.2 mm. Here, a widely used YAG laser crystal is used as the laser medium.

A high-power semiconductor laser having a general wavelength spread is used as an excitation light source. The absorption coefficient of the excitation light for the YAG laser crystal is 0.6 mm ^-1 .

Here, the reason why the range should be limited to this range will be explained together with theoretical calculation.

With the above settings derived from the present invention, in a cylinder of a YAG laser crystal having a cavity mode radius dl,
The ratio η at which the pumping light propagating in the fiber is absorbed in the cavity mode part of the laser medium is numerically calculated. The resonator mode volume was calculated with the main part being 1 / e ² or more of the maximum value. The resonator mode radius d1 is set to d1 = 0.2 mm, which is the same as the core diameter dc of the optical fiber, and the emission end face of the optical fiber is close to or in contact with the laser medium. The result is shown in FIG. When the focal length f is short, the absorption rate is small. If the absorption rate is small, the binding efficiency also decreases.

When the focal length f is large, the absorption ratio is still relatively large. However, even if the absorption ratio is large, if the focal length f is too large, even the light absorbed by the laser medium is in the resonator mode. The coupling efficiency does not increase because it is emitted outside the cavity and does not contribute to resonance.

Next, FIG. 2B shows the relationship between the coupling coefficient Cp (coefficient representing the degree of coupling between the resonator mode and the excitation mode) and the focal length f of the lens. The coupling coefficient is one of the indexes showing the quality of the coupling efficiency, and its calculation formula is as follows, where Az is the effective cross section of the excitation light at which the absorption of the excitation light by the laser medium is 50%. Become.

Cp = η / Az As can be seen from FIG. 2 (b), the coupling coefficient becomes maximum when the focal length f of the lens is around 1.0 MM, and the coupling coefficient is large both when the focal length f is short and when it is long. descend.

FIG. 3 to FIG. 6 show the fiber emission states of the excitation light in the air at each focal length. The emission state of the excitation light is calculated by the law of refraction (Snell's law) of the traveling direction of light when the lens portion is specified at a certain focal length.

Since the emission state of light in air was calculated, when the emission end face of the optical fiber is in contact with the incident end face of the laser medium, the actual focal position in the laser medium is the refractive index of the laser medium at the focal length in air. Corresponding to the position multiplied by, the spread is different, but how the excitation light mode changes with each focal length has a similar relationship.

FIG. 3 shows a state in which the excitation light is emitted from the fiber at a focal length f of 0.4 mm, which is twice the radius of the optical fiber core, which is outside the above-mentioned setting range. You can see that the light is condensed at.

The excitation light passing near the outer circumference of the fiber core is not emitted by being reflected by the end face 1c because of the size of the curvature of the lens portion 1c.

Further, the divergence of the excitation light in the distance is quite large.

As shown in FIG. 2A, the absorption rate is low at about 48%. Further, as shown in FIG. 2B, the coupling coefficient is as small as about 3.1 mm ^-1 .

FIG. 4 shows a state in which the excitation light is emitted from the fiber when the focal length f is within the range of the above setting and the focal length f is 0.6 mm.

Excitation light has a long condensing range,
The effective spot diameter is also small.

According to the same calculation as above, the absorption rate is about 76.
% And f = 0.4 mm are larger than the case. The coupling coefficient also increases slightly to 4.4 mm ^-1 .

FIG. 5 shows the focal length f = within the above setting range.
The fiber output state of the excitation light at 1.0 mm is shown.

The spot diameter is about the same as the fiber core diameter, but the beginning of divergence is distant, and it is sufficient if the absorption rate of the laser medium is 0.6 mm ^-1 .

With the same calculation, the maximum absorption rate of about 85% is obtained (FIG. 2 (a)). The maximum coupling coefficient is 7.5 mm ^-1 (Fig. 2 (b)).

FIG. 6 shows the focal length f = within the range of the above setting.
This is a state where the excitation light is emitted from the fiber at 1.6 mm.

The excitation light travels to a spot diameter approximately the same as the lens diameter up to a certain degree, and thereafter diverges, so that the spot is no longer smaller than the diameter of the core of the optical fiber core, but the absorption rate is about 83%. is there. Also, the coupling coefficient is considerably reduced, but it is close to the case of f = 0.6 mm C
p = 4.2 mm ⁻¹ .

FIG. 7 shows the case where the focal length f = 2.0 mm, which is the upper limit of the range set above. The excitation light is gently diverged from the vicinity of the lens, and the emitted light is wider than the lens diameter. Although the absorption rate is still 80%, the coupling coefficient is small at about 3.7 mm ^-1, and the coupling efficiency is low.

FIGS. 8 and 9 show the cases where the focal lengths f = 4.0 mm and f = 140 mm, which are out of the range of the above setting. The spread of the emitted light diverges further from the suburbs of the lens than when f = 2.0 mm, and the emitted light spreads larger than the lens diameter. Therefore, the decrease in the absorption rate is not so remarkable as in the case where f is less than 0.6. However, even if the light absorbed in the laser medium is emitted to the outside of the resonator mode, there are many cases. The coefficient drops below 2.8 mm ^-1 (Fig. 2
(B).

As described above, when the focal length is about 5 times the radius of the optical fiber core (f = 1.0 mm), the absorption ratio and thus the coupling efficiency are the highest, and 10 times (f =
It can be said that the boundary point where the above effects can be expected is around 2.0 mm) and 3 times (f = 0.6 mm).

Further, when the emission end face of the optical fiber is moved away from the incidence end face of the laser medium, the curve of FIG. 2 has a lower absorption rate peak and a lower coupling efficiency.

Therefore, the improvement of the coupling efficiency can be achieved only when the two conditions of limiting the focal length and arranging the optical fiber emitting end face close to or in close contact with the laser medium incident end face as described above are satisfied. .

Furthermore, even if the above two conditions are satisfied, when the resonator mode diameter is made larger than the optical fiber core diameter, the curve of FIG. 2 shifts upward, but the pumping light density with respect to the resonator mode diameter is Since the size becomes smaller, the coupling efficiency decreases.

Conversely, when the resonator mode diameter is reduced, the excitation light density increases, but the curve in FIG. 2 shifts downward,
After all, the coupling efficiency is reduced.

Therefore, if the resonator mode diameter and the optical fiber core diameter are set to the same degree after satisfying the above two conditions, the coupling efficiency can be kept high.

Although the step index type multimode optical fiber is used in this embodiment, the present invention is not limited to this, and a graded type optical fiber may be used.

[Embodiment 2] A second embodiment of the present invention is shown in FIG.
It shows in 0. This is a multi-edge pumped solid-state laser device.

The parts having the same structures as those in the first embodiment are designated by the same reference numerals.

The focal length of the lens portion of the optical fiber, the size of the resonator mode radius, and the like are the same as those in the embodiment of FIG. The light emitted from the optical fiber resonates between the output mirror 5 and the total reflection mirror 3 after passing through the 1064 mm high reflection mirrors provided on both sides of the laser medium. In this embodiment, since the pumping lights incident from a plurality of optical fibers resonate in the same resonator mode, the total pumping light amount increases and a high output laser light with high coupling efficiency can be extracted.

[Embodiment 3] A third embodiment of the present invention is shown in FIG.
Shown in 1. The same configurations as those in the first embodiment will be described with the same reference numerals. This is a configuration in which the lens portion at the exit end of the optical fiber is replaced with an optical element having a planar lens function in the first embodiment.

This optical element is a Fresnel lens, a phorogram lens, a gradient index lens, or the like. In the case of this lens, it is relatively easy to remove the aberration of the lens, so that the pumping high mode can be efficiently coupled to the resonator mode of the solid-state laser.

[0066]

According to the present invention, the coupling efficiency of pumping light with respect to the resonator mode can be increased.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a solid-state laser device using an optical fiber according to a first embodiment of the present invention,

FIG. 2A is a graph showing the relationship between the focal length of the lens portion of the light emitting end surface of the optical fiber and the absorption rate of the laser device, and FIG. 2B is the graph of the lens portion of the light emitting end surface of the optical fiber. A graph showing the relationship between the focal length and the coupling coefficient of the laser device,

FIG. 3 is a diagram showing a fiber emission state of excitation light in air at a focal length f = 0.4 mm,

FIG. 4 is a diagram showing a fiber emission state of excitation light in air at a focal length f = 0.6 mm,

FIG. 5 is a diagram showing a fiber emission state of excitation light in air at a focal length f = 1.0 mm,

FIG. 6 is a diagram showing a fiber emission state of excitation light in air at a focal length f = 1.6 mm,

FIG. 7 is a diagram showing a fiber emission state of excitation light in air at a focal length f = 2.0 mm,

FIG. 8 is a diagram showing a fiber emission state of excitation light in air at a focal length f = 4.0 mm,

FIG. 9 is a diagram showing a fiber emission state of excitation light in air at a focal length f = 140 mm,

FIG. 10 is a configuration diagram of a solid-state laser device for pumping multi-end facet light according to another embodiment of the present invention,

FIG. 11 is an end facet type solid-state laser resonance in which an optical element having a lens function on a plane (Fresnel lens, phorogram lens, gradient index lens, etc.) is formed at the exit end of the optical fiber in another embodiment of the present invention. Configuration diagram of the vessel,

FIG. 12 is a basic configuration diagram of a laser device in which an emission end of an optical fiber of a conventional example is formed on a lens,

FIG. 13 is a configuration diagram of a conventional optical fiber coupled solid-state laser.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Optical fiber 1a ... Core part 1b ... Clad part 1c ... Plano-convex lens 2 ... Laser medium 3 ... Total reflection mirror 4 ... Semiconductor laser 5 ... Output mirror 6 ... Resonator mode 7 ... Excitation light mode 8 ... Excitation light 9 ... 1064mm High reflecting mirror 10 ... Fresnel lens 11 ... Lens

Claims (1)

[Claims] 1. A light source that generates excitation light, a laser medium that has a resonator and that excites laser light in a resonator mode by the incidence of the excitation light, and the light source that is disposed between the light source and the laser medium. In the solid-state laser device using an optical fiber having an optical output end face for guiding excitation light from the laser medium to the laser medium as a lens part and mainly including an optical fiber formed of a core and a clad, the optical fiber The focal length of the lens portion of the light emitting end surface of the optical fiber is in the range of 3 to 10 times the radius of the optical fiber core, and the laser medium is arranged in close contact with or close to the light emitting end of the optical fiber. A solid-state laser device using an optical fiber, wherein the core radius is about the same as the cavity mode radius near the pumping light incident part of the laser medium.