JPH07235717A - Laser amplifier - Google Patents

Abstract

PURPOSE:To provide a laser amplifier of improved gain and amplification. CONSTITUTION:Laser light emitted from a laser oscillator 31 is corrected in its expansion angle with respect to an optical axis through a convex lens 32. The corrected expansion angle is corrected in response to excitation light power to a solid laser medium 33b. Laser light transmitted through the convex lens enters the excited solid laser medium 33b with the corrected expansion angle. Accordingly, even though the laser amplifier is influenced by a thermal lens effect of the excited solid laser medium, a volume of the solid laser medium can be effectually utilized compared with the cases where parallel laser light enters and where laser light is incident with the expansion angle of the laser light incident on the solid laser medium 33b for an incident side focusing position of the excited solid laser medium 33b.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser amplifying device having a laser oscillator for oscillating a laser and a solid laser medium for amplifying laser light emitted from the laser medium. For example, the present invention relates to a laser amplifying device that obtains high-power pulsed laser light with high repetition in a unit time.

[0002]

2. Description of the Related Art Conventionally, in a laser device, in order to obtain a high output, a plurality of laser media are cascaded and used. That is, in the pulse laser amplifying device shown in FIG. 7, the pulse laser oscillator 50 composed of the solid-state laser medium includes the resonance mirrors 50a and 50a.
b and the pumping device 50c are provided, and pulsed excitation light is applied from the pumping device 50c to emit pulsed laser light. The emitted laser light enters a laser amplifier 51 for amplification, which is cascaded coaxially with the pulse laser oscillator 50. The laser amplifier 51 for amplification is made of a cylindrical laser medium having the same characteristics as the pulse laser oscillator 50. Then, the incident pulsed laser light is further excited by the excitation light from the pumping device 51a to be output as amplified pulsed laser light. The laser amplifier 51 is not limited to the case of using only one stage, but may be of a plurality of stages.

In such a laser amplifying device, in order to obtain high output pulsed laser light by repeating oscillation at a high rate (that is, increasing the oscillation frequency) in a unit time, the laser amplifier 5 is used.
It is necessary to increase the power supplied to one pumping device 51a. However, as the power increases, a phenomenon called a thermal lens effect of the solid-state laser medium occurs. This is because the pumping light from the pumping device diffuses in the solid-state laser medium and is converted into heat, but is cooled only from the outer surface side. That is, this causes a temperature distribution in which the temperature gradually decreases from the central axis of the solid-state laser medium toward the outer surface. Since the refractive index of this solid-state laser medium increases as the temperature rises, the solid-state laser medium has the same characteristics as the rod lens.

When a laser beam is incident on a solid-state laser medium having such a thermal lens effect, the laser beam is converged toward the central axis as it propagates in the laser medium. Therefore, since the laser light passes through only a part of the solid medium, only a part of the energy of the excitation light diffused in the solid laser medium is added to the laser light. Further, the energy density becomes large in the portion where the converged laser light passes, and the gain saturation effect of the laser amplifier occurs. For the above reasons, there is a problem in that the gain cannot be increased and thus the energy of the output laser light does not increase so much.

In order to solve this problem, Japanese Patent Laid-Open No. 57-
In No. 58379, an optical system is provided between the laser oscillator and the laser amplifier. Then, by using this optical system, the beam diameter of the laser light when entering the laser amplifier is corrected in advance to 0.95 times the diameter of the solid laser medium of the amplifier, thereby effectively using the solid laser medium. I am trying to

[0006]

However, in the conventional laser amplifying device described in the above publication, there is no disclosure about the divergence angle of the laser beam incident on the laser amplifier. Here, the "divergence angle of the laser beam" is the angle formed by the light rays at the peripheral edge of the laser beam. Therefore, there is a problem that it cannot be a fundamental measure against the problem of laser beam convergence.

That is, assuming that the divergence angles of the incident laser light are parallel, some effects can be obtained when the total length of the solid-state laser medium is short (see FIG. 8A, the laser light passes through). The portion is shown by hatching.) When the total length of the solid-state laser medium becomes long, the influence of the thermal lens effect becomes large, and it is not possible to prevent the laser light from converging near the central axis on the emission end side (Fig. 8 (b)
reference). In addition, even if the total length of the solid-state laser medium is short,
Even when the thermal lens effect is large, it is similarly impossible to prevent the laser light from converging in the vicinity of the central axis (see FIG. 8C). Therefore, the amplification factor cannot be increased.

Further, when the divergence angle of the incident laser beam is made extremely large, as shown schematically in FIG. 9, the outer peripheral portion of the laser beam (the vertical hatched portion in FIG. 9) is formed by the inner peripheral surface of the solid laser medium. ) Is broken. That is, the inner peripheral surface of the solid-state laser medium functions as a diaphragm to absorb or diffuse the broken laser beam and prevent the laser beam from being emitted from the emission end. Therefore, the amplification factor cannot be increased. In this case, it is also possible to reduce the diameter of the incident beam so that such laser beam shading does not occur (see the dotted line in FIG. 9). However, since this is equivalent to reversing the arrangement of the solid-state laser medium when the divergence angle of the incident beam is made parallel, the solid-state laser medium cannot be effectively used and the amplification factor is also increased. I can't.

When the divergence angle of the incident laser beam is made negative, as shown in FIG. 10, the laser beam only converges near the central axis earlier, and the amplification factor cannot be increased at all.

Therefore, in view of the above problems, an object of the present invention is to provide an optical system for correcting the divergence angle of an incident beam entering a laser amplifier, and to enter the laser beam according to the strength of the thermal lens effect and the total length of the solid laser medium. It is an object of the present invention to provide a laser amplifying device capable of appropriately correcting a divergence angle of a beam and sufficiently utilizing a volume of a laser medium to obtain maximum energy or power.

[0011]

The present invention adopts the following means in order to solve the above problems.

<Summary of the Invention> In order to solve the above-mentioned problems, a laser amplifying apparatus according to the present invention includes a laser oscillator (10) and a laser oscillator (10) as shown in the principle diagram of the invention of FIG. ), The solid-state laser medium (11) for amplifying the laser light emitted from the solid-state laser medium (11) by transmitting the inside thereof, and the solid-state laser medium (11) is arranged on the incident side and injected into the solid-state laser medium (11). And an optical system (21) for correcting the divergence angle of the laser light emitted from the laser oscillator (10) according to the power of the excitation light.

The present invention can be implemented in various forms as shown below.

<Range of divergence angle> First, when the focal length of the excited solid-state laser medium (11) is f and the radius of the solid-state laser medium (11) is r, the optical system (21). The solid-state laser medium (1
The divergence angle with respect to the optical axis of the laser light at the time of entering 1) is in the range of 0.35 r / f or more, and the divergence angle at which the loss when the excitation energy is not applied to the solid-state laser medium is 10% is 3 It may be set in a range of 5 mrad to 5 mrad. By doing so, it is possible to obtain an amplified output that is 90% or more of the amplified output value at the spread angle at which the amplified output value reaches a peak. Especially, the upper limit is 20 mrad
In this case, it is possible to obtain an amplified output value that is about the same as the spread angle of 0.35 mrad, which is the lower limit.

<Number of amplification stages> In addition, the solid-state laser medium (1
n) may be arranged in a plurality of stages, and the laser light emitted from the laser oscillator (10) may be superimposed and amplified a plurality of times. In this case, the laser light emitted from the first-stage solid-state laser medium (11) is further incident on the optical system (22) to correct the divergence angle, and the second-stage solid-state laser medium (1) is corrected.
It is incident on 2). This can be repeated n times to finally obtain the output of this laser amplification device. In this way, the incident side of the optical system (22) does not have to be the laser oscillator (10). A larger amplification output can be obtained by forming the solid-state laser medium (1n) in multiple stages.

<Laser oscillator> This laser oscillator (1
0) may be a pulse laser oscillator or a continuous wave laser oscillator. Further, as this laser oscillator, any type of laser oscillator can be used. For example, a solid-state laser oscillator, a gas laser oscillator, a dye laser oscillator, and a semiconductor laser oscillator.

<Optical system> As the optical system, a lens or a mirror can be used. If a lens is used, it is not necessary to bend the optical axis, so that the beam shape is not distorted. When using a lens, a convex lens or a concave lens can be used. When a convex lens is used, it may be converged on the exit side and then diffused. A rod lens can also be used as the lens. Further, it may be a single lens or a compound lens.
With the compound lens, it is easy to correct the aberration of the optical system. However, since the laser light is coherent, even a single lens can withstand use. When using a compound lens, it may be a zoom lens. With this configuration, the divergence angle can be easily corrected even when the pumping power to be applied is variable.

<Solid-State Laser Medium> As the solid-state laser medium, YAG, ruby, glass or the like can be used.
The solid-state laser medium may be excited by applying excitation light from a flash lamp, or by applying energy by another method.

[0019]

The divergence angle of the laser beam emitted from the laser oscillator (10) with respect to the optical axis is corrected by the optical system. With this divergence angle, the laser light transmitted through the optical system enters the excited solid-state laser medium (11). Therefore, even if the solid-state laser medium is influenced by the thermal lens effect of the excited solid-state laser medium, the solid-state laser medium is incident when parallel laser light is incident or from the incident side focal point of the excited solid-state laser medium (11). The volume of the solid-state laser medium can be effectively used as compared with the case where the laser light is incident at the divergence angle incident on (11). Therefore, the gain can be improved and the amplification factor can be increased.

[0020]

Embodiments of the present invention will now be described with reference to the drawings. FIG. 2 is a schematic diagram of a laser amplification device according to an embodiment of the present invention. As shown in FIG. 2, the laser amplification apparatus according to this embodiment includes a pulse laser oscillator 31 that emits pulsed laser light, and a convex lens 32 that is coaxially arranged on the emission side of the pulse laser oscillator 31. It is composed of a laser amplifier 33 coaxially arranged on the emitting side of the convex lens 32.

This pulse laser oscillator 31 is, for example, N
d: YAG (neodymium-doped yttrium.
It is composed of a solid laser medium 31a made of an aluminum garnet rod, a total reflection mirror 31b and a partial reflection mirror 31c for resonance, and a flash lamp 31d as a pumping device for exciting the solid laser medium 31a. By applying power to the flash lamp 31d to cause it to emit light, the solid-state laser medium 31
When a is excited, laser light oscillation can be obtained. At this time, the laser light can be pulsed by a method such as chopping the optical path in which the laser light resonates. The laser light emitted from the pulse laser oscillator 31 diverges and emits at a constant angle determined by the intensity of the electric power supplied to the flash lamp 31d.

The focal length and position of the convex lens 32 are set so that the beam waist of the laser light emitted from the pulse laser oscillator 31 is farther than the focal point on the incident side of the optical system 32. By setting in this way, the laser light incident on the convex lens 32 is
Converge on the output side of. Then, the emitted laser light forms an image (beam waist image) once before the incident surface of the laser amplifier 33, and then diverges again. The laser light thus diverged enters the laser amplifier 33 with a constant spread angle.

The laser amplifier 33 comprises a solid-state laser medium 33a, a flash lamp 33b, and a reflecting tube 33c. The solid-state laser medium 33a is Nd: YA
It consists of a G rod. The flash lamp 33b is used for pumping to excite the solid-state laser medium 33a, and Xe
(Xenon) tube. The reflecting cylinder 33c covers the outer circumferences of the solid-state laser medium 33a and the flash lamp 33b and efficiently collects the excitation light from the flash lamp 33b on the solid-state laser medium 33a. Then, as shown in the transverse sectional view of FIG.
It has a double-structured elliptical cross-sectional shape with the central axis of 3a as a common focal point. Flash lamps 33b and 33b are arranged at the other two focal positions of the reflecting cylinder 33c. Moreover, the inner surface thereof is a reflecting surface plated with gold.

Then, the medium 33a is excited by the excitation light from the flash lamp 33b, and the solid-state laser medium 3 is excited.
The laser light incident on 3a is amplified. Then, the amplified laser light is emitted from the emission end of the solid-state laser medium 33a.

When power is applied to the flash lamp 33b and excitation light is added to the solid laser medium 33a, a part of the excitation light is converted into heat in the solid laser medium 33a. As a result, a thermal lens effect is produced, and it becomes equivalent to a convex lens (hereinafter referred to as “equivalent convex lens”) 36 having a focal length f.

This focal length is unique as these functions depending on the input power (Pin) to the laser amplifier 33 (flash lamp 33b), the length (L) of the laser medium, and the refractive index (n) of the laser medium. Set to. Therefore, by defining the divergence angle θ of the laser light incident on the laser amplifier 33 as a function of f, the divergence angle θ can be determined in consideration of these values. The formula for determining the spread angle θ will be described below.

First, the function for determining f will be described. FIG. 4 is an explanatory diagram showing the relationship between the solid-state laser medium 33a and the equivalent convex lens 36. In FIG. 4, the equivalent convex lens 36 is shown as a thick lens, but in reality, a lens having no thickness, that is, a lens in which the principal planes on the incident side and the principal plane on the exit side are coincident with each other is assumed to be virtual. Then, the focal length f is calculated.

In FIG. 4, due to the thermal lens effect, the solid-state laser medium 33a is equivalent to a convex lens having an entrance surface 34 and an exit surface 35 indicated by a chain line. The principal plane on the incident side and the principal plane on the exit side of this convex lens are indicated by H and H '.
The focal length of this convex lens, that is, the distance x from the focal point F to the principal plane H is expressed by the following equation (1).

X = A / Pin ... (1) Here, A is the reflection tube 33 c and the flash lamp 33.
It is a constant that depends on b and is a value that can be experimentally obtained. The unit of x is mm and the unit of Pin is kW. As described above, the input power (Pin) and the focal length (x) are generally in inverse proportion.

In general, the position of the principal plane H is located inside the incident end surface I of the solid-state laser medium 33a by y = L / 2 · n. Therefore, considering the equivalent convex lens 36 arranged at the central position of the solid-state laser medium 33a, the equivalent convex lens 3
The focal length f of 6 is displaced by Z shown in FIG. Therefore,
From f = x + z = xy + L / 2, it is shown by the following equation (2).

F = A / Pin + L / 2 · (1-1 / n) (2) Here, the unit of f is mm. When θ = r / f (3) is obtained from f thus obtained, this θ passes through the periphery of the equivalent convex lens 36 when the point light source is placed at the focal point F.
It corresponds to the divergence angle of the light incident on. Where r is
This is the radius of the solid-state laser medium 33a. Therefore, θ = r /
When a laser beam is incident on the solid-state laser medium 33a at a divergence angle of f, it becomes parallel light on the emission side. However, the use volume of the solid-state laser medium 33a decreases when parallel light is emitted on the emission side, as described in FIG. 9 in the conventional problem. Therefore, it is necessary to multiply this θ = r / f by an appropriate correction factor.

An example of an experiment carried out to obtain this appropriate correction coefficient will be described below. First ask for A. First,
A YAG crystal containing 0.8 at% of Nd is used as the solid-state laser medium 33a, and its diameter (2 × r) is 10 mm,
The length (L) was 195 mm. Then, the He—Ne laser light is made into parallel light having a diameter of 10 mm, is incident on the solid-state laser medium 33a, and the input power (Pin) and the focus position (F ′) at that time are measured. This solid laser medium 33a
The focal length (x) can be obtained as a value obtained by adding the distance y = L / 2 · n to the distance (w) between the emission end surface O of the solid-state laser medium 33a and the focal position F ′. The result of the experiment, P
It was determined that in = 5.0 (kW) and x = 1184 (mm). Therefore, from the equation (1), this solid-state laser medium 33
The constant A of a was A = 5920. In addition, this A is usually about 3000 to 7000. In the two experiments described below, the focal length (f) is calculated using the value of A thus obtained.

(1) When Pin = 15 kW A = 5920, L = 195 mm, n = 1.82, Pin
= 15 is substituted into the equation (2), f = 438.60 mm
To get Next, from a diameter of 10 mm, r = 5 mm and f
Is substituted into the equation (3), θ = r / f = 11.4 mra
get d. In order to obtain the optimum correction coefficient for multiplying θ, some values around θ = 11.4 mrad were sampled and the output after amplification was measured. In order to optimize the measurement results, the power of the incident laser light was fixed at 0.8 kW and the beam diameter at the time of incidence was 9 m.
fixed to m.

The results of this measurement are shown in FIG. In addition, this measurement result is plotted in FIG. In FIG. 6, the horizontal axis represents the spread angle (mrad), and the vertical axis represents the output after amplification (kw).

As is apparent from FIG. 6, the divergence angle is 3.
A high amplified output can be obtained in the range of 99 to 20 mrad. That is, in the case of this parameter (Pin = 15), a peak value (1.15 kw) occurs at the spread angle of 15 mrad. When the divergence angle is in the above range, 0.91 times (1.05 kw) or more amplified output can be obtained with respect to the peak value. Therefore, θ = r / f
It was found that effective and highly efficient laser amplification can be performed by setting the divergence angle in the range of 0.35 times or more and 20 mrad or less.

(2) When Pin = 20 kW A = 5920, L = 195 mm, n = 1.82, Pin
= 20 is substituted into the equation (2), f = 339.93 mm
To get Next, from a diameter of 10 mm, r = 5 mm and f
Is substituted into the equation (3), θ = r / f = 14.7 mra
get d. In order to obtain the optimum correction coefficient for multiplying θ, some values around θ = 14.7 mrad were sampled and the output after amplification was measured. In order to optimize the measurement results, the power of the incident laser light was fixed at 0.8 kW and the beam diameter at the time of incidence was 9 m.
fixed to m.

The results of this measurement are shown in FIG. In addition, this measurement result is plotted in FIG. As is clear from FIG. 6, a high amplified output can be obtained in the range where the divergence angle is 5.15 to 20 mrad. That is, in the case of this parameter (Pin = 20), the peak value (1.29 kw) occurs at the spread angle of 14 mrad. If the divergence angle is in the above range, it is possible to obtain an amplified output 0.91 times (1.18 kw) or more with respect to the peak value. Therefore, it was found that effective and highly efficient laser amplification can be performed by setting the divergence angle in the range of 0.35 times the angle θ = r / f and 20 mrad or less.

(3) When Pin = 25 kW A = 5920, L = 195 mm, n = 1.82, Pin
= 25 into the formula (2), f = 280.73 mm
To get Next, from a diameter of 10 mm, r = 5 mm and f
Is substituted into the equation (3), θ = r / f = 17.8 mra
get d. In order to obtain the optimum correction coefficient for multiplying θ, some values around θ = 17.8 mrad were sampled and the output after amplification was measured. In order to optimize the measurement results, the power of the incident laser light was fixed at 0.8 kW and the beam diameter at the time of incidence was 9 m.
fixed to m.

The results of this measurement are shown in FIG. In addition, this measurement result is plotted in FIG. As is clear from FIG. 6, a high output after amplification can be obtained in the range where the divergence angle is 6.23 to 20 mrad. That is, in the case of this parameter (Pin = 25), the peak value (1.45 kw) occurs at the spread angle of 14 mrad. When the divergence angle is in the above range, it is possible to obtain an amplified output 0.91 times (1.32 kw) or more the peak value. Therefore, it was found that effective and highly efficient laser amplification can be performed by setting the divergence angle in the range of 0.35 times the angle θ = r / f and 20 mrad or less.

(4) When Pin = 32 kW A = 5920, L = 195 mm, n = 1.82, Pin
= 32 into the formula (2), f = 228.93 mm
To get Next, from a diameter of 10 mm, r = 5 mm and f
Is substituted into the equation (3), θ = r / f = 21.8 mra
get d. In order to obtain the optimum correction coefficient for multiplying θ, some values around θ = 21.8 mrad were sampled and the output after amplification was measured. In order to optimize the measurement results, the power of the incident laser light was fixed at 0.8 kW and the beam diameter at the time of incidence was 9 m.
fixed to m.

The results of this measurement are shown in FIG. In addition, this measurement result is plotted in FIG. As is clear from FIG. 6, a high amplified output can be obtained in the range of the divergence angle of 7.63 to 20 mrad. That is, in the case of this parameter (Pin = 32), the peak value (1.62 kw) occurs at the spread angle of 14 mrad. When the divergence angle is in the above range, it is possible to obtain an amplified output 0.93 times (1.50 kw) or more with respect to the peak value. Therefore, it was found that effective and highly efficient laser amplification can be performed by setting the divergence angle in the range of 0.35 times the angle θ = r / f and 20 mrad or less.

(5) Pin = 0 kw In any of the input power values, good post-amplification power is generated in the range where the spread angle is 20 mrad or less. That is, the upper limit of the divergence angle does not depend on the focal length f. That is, it is considered that the solid laser medium 33a itself causes a loss in a range where the divergence angle is large.
In order to confirm this, the output after amplification was measured without supplying power to the laser amplifier 33.

The results of this measurement are shown in FIG. In addition, this measurement result is plotted in FIG. As is clear from FIG. 6, the incident angle (15 mra) at which the loss is 10% (0.72 kw) with respect to the power (0.8 kw) of the incident laser light.
d) 5 mrad wide divergence is just 20 mrad
It turns out that The power of the incident laser light (0.
Even if the incident angle (15 mrad) at which the loss is 10% (0.72 kw) with respect to 8 kw) and the spread angle (20 mrad) is large by 5 mrad, the efficiency can be improved.

The power and position of the convex lens 32 are determined so that the emitted laser light falls within the range determined above. Then, as shown in the above experimental examples, good amplification characteristics can be obtained.

[0045]

According to the laser amplifying apparatus of the present invention described above, the divergence angle of the incident beam is corrected according to the pumping light power, so that the maximum energy or power can be obtained by fully utilizing the volume of the solid-state laser medium. Obtainable.

[Brief description of drawings]

FIG. 1 is a principle diagram showing the principle of the present invention.

FIG. 2 is a schematic diagram showing a configuration of a laser amplification device according to an embodiment of the present invention.

FIG. 3 is a vertical sectional view taken along line IV-IV in FIG.

FIG. 4 is an explanatory diagram showing the relationship between the solid-state laser medium and the equivalent convex lens in FIG.

FIG. 5 is a list of experimental results according to an embodiment of the present invention.

FIG. 6 is a graph diagram in which the experimental results shown in FIG. 5 are plotted.

FIG. 7 is a schematic diagram of a conventional laser amplification device.

FIG. 8 is a diagram showing a case where a laser beam incident on a laser amplifier is collimated in a conventional laser amplification device.

FIG. 9 is a diagram showing a case where the divergence angle of the laser light incident on the laser amplifier is increased in the conventional laser amplification device.

FIG. 10 is a diagram showing a case where the divergence angle of the laser light incident on the laser amplifier is made negative in the conventional laser amplification device.

[Explanation of symbols]

 31 laser oscillator 32 convex lens 33a solid-state laser medium

Claims (3)

[Claims] 1. A laser oscillator, a solid-state laser medium that amplifies laser light emitted from the laser oscillator by transmitting the inside thereof, and a solid-state laser medium disposed on the incident side of the solid-state laser medium. An optical system for correcting the divergence angle of laser light emitted from the laser oscillator according to the magnitude of the power of pumping light input. 2. When the focal length of the pumped solid-state laser medium is f and the radius of the solid-state laser medium is r, the laser is incident on the solid-state laser medium corrected by the optical system. The divergence angle of light is 0.
The laser amplifying device according to claim 1, wherein the laser amplifying device is set to 35 r / f or more and 20 mrad or less. 3. The laser amplifying apparatus according to claim 1, wherein a plurality of stages of the solid-state laser medium are prepared, and the optical system is arranged between the solid-state laser media.