JP7048101B2 - Laser amplifier - Google Patents

Description

The present invention relates to an amplifier for a solid-state laser such as a YAG laser.

Conventional solid-state laser amplifiers mainly use the W-PASS excitation method (see, for example, Patent Documents 1 and 2) that mainly utilizes polarization characteristics, and the Z-type, V-type, ring-shaped diagonal multiple reflection, or zigzag multiple reflection method. It can be classified into a method (see, for example, Patent Documents 3 and 4) in which the excitation distance is lengthened and the gain is increased by incidenting the inside of the laser amplification medium diagonally a plurality of times. Since the amplification efficiency of any amplifier is not good simply by passing it through the amplification medium, in the W-PASS excitation method, the polarized beam is rotated 90 degrees by the wave plate, reflected, and reciprocated in the amplification medium, which is nearly doubled. The amplification efficiency is increased.
Further, in the zigzag multiple reflection method, the amplification factor is increased by increasing the number of excitations a plurality of times.

As an example, FIG. 7 shows the "optical amplification system" disclosed in Patent Document 1.
FIG. 7 is a reference of FIG. 4 in Patent Document 1 as it is, and shows an optical system of an optical amplification system. A master laser 61 that emits signal light and an optical amplifier 62 that amplifies the signal light are provided, and the optical amplifier 62 is composed of an Nd-doped vanadate mixed crystal 63, a lens 64, and an excitation output light 65 from an excitation semiconductor laser array. To.
Between the master laser 61 and the optical amplifier 62, there are half-wave plates 66, 69, 72, polarization beam splitters 67, 70 that reflect or transmit light depending on the polarization direction, Faraday rotators 68, 71, and lenses 73, 74. 75 are arranged as shown in the figure.
The light emitted from the optical amplifier 62 passes through the lens 76, is reflected by the prism 77, and then enters the optical amplifier 62 again. After that, it passes through the lenses 78, 79, 80, and the half-wave plate 81, is reflected by the phase conjugate mirror (PCM) 82, and then is incident on the optical amplifier 62 again. The phase conjugate mirror (PCM) 82 is composed of an Nd-doped vanadate mixed crystal 83 and lenses 84 and 85. The light incident on the optical amplifier 62 again passes through the polarization beam splitter 70 and exits through the same path.
The laser amplifier disclosed in Patent Document 1 can amplify a picosecond laser by a W-PASS excitation method utilizing polarization characteristics. The basic principle of the laser amplifier of Patent Document 1 is that the polarized laser light oscillated from the master laser 61 is reflected by the polarizing beam splitter 70 at 90 degrees, and the laser light amplified by the optical amplifier 62 is reflected at 90 degrees using the half-wave plate 72. The light is input to the polarization beam splitter 70 again after changing the polarization direction, but since the polarization direction is rotated by 90 degrees, the light passes through as it is and is output.
However, in Patent Document 1, it is difficult to make the polarizing beam splitters 67 and 70 having strong yield strength, and the coat of the polarizing beam splitter is damaged at high peak power, so that the output power is limited. Further, since the degree of polarization of the polarization beam splitter 70 is not 100%, the polarization beam splitter 70 returns at a right angle and becomes return light to damage the LD. Therefore, an isolator composed of half-wave plates 66 and 69 and a polarizing beam splitter 67 is required.

Originally, the principle of the amplifier was simple, but many optical components were required to prevent damage, and it had to be complicated.
Moreover, it could not be amplified unless it was a polarized laser. Further, when the size becomes complicated and the size is increased, the output decreases due to a slight tilt of the mirror. A system was needed, and it had to be even larger.
Therefore, in order to solve the above problems, such as "laser medium unit, laser amplifier and laser oscillator and cooling method" disclosed in Patent Document 3 and "resonator and laser device" disclosed in Patent Document 4. , A method of obliquely incident on a laser amplification medium and causing V-shaped reflection in the laser amplification medium can be easily considered.

Japanese Unexamined Patent Publication No. 2009-290030 Japanese Unexamined Patent Publication No. 9-181378 Japanese Unexamined Patent Publication No. 2014-022568 Japanese Unexamined Patent Publication No. 2017-139344

However, when the incident laser light that is V-reflected as disclosed in Patent Document 3 and Patent Document 4 is excited, there is a problem that the beam shape is greatly distorted as shown in FIGS. 8 to 10.
The reason why the amplified laser light is distorted will be described below with reference to FIGS. 8 to 10.
In FIG. 8A, the incident laser light 87 is V-shaped reflected on the HR (enhanced reflection film coating) surface of the gain medium 86 for amplification, and is emitted as an amplified emitted laser light 88. When the excitation laser light 90 having the V-shaped reflected center line 89 as the optical axis is larger than the incident laser light 87, as shown in FIG. 8 (b), in the BB line cross section of FIG. 8 (a). Since the circular cross section 91 of the incident laser light 87 and the circular cross section 92 of the emitted laser light 88 are amplified in a deviated state, the laser light distorted by the emitted laser light 88 is amplified as shown in FIG. 8 (c). 93 is added to form a distorted elliptical shape. In addition to FIG. 8A, FIGS. 8B, 8C, and 9 and 10 also show that the upward direction when the paper surface is viewed from the front in FIG. 8A is the x direction. It is also illustrated. Further, the surface indicated by the reference numeral AR shown in FIGS. 8 to 10 means an antireflection film coated surface.
FIG. 9A is a conceptual diagram showing a state of V-shaped reflection when the irradiation diameter of the excitation laser light 90 is smaller than the irradiation diameter of the incident laser light 87. As shown in FIG. 9B showing the beam intensity distribution of the emitted laser light, the emitted laser light 88 has a distorted shape in which the central portion is sharpened in terms of beam intensity. The P-axis in the cross section of FIG. 9B is an axis indicating the intensity of the output in the CC cross-section, and indicates that the beam output is stronger in the upward direction of the paper surface (that is, in the direction toward the arrow of the P-axis). It is a thing. Further, the same components as those in FIG. 8 are designated by the same reference numerals.

Next, in FIG. 10A, the excitation laser light 90 and the emission laser light 88 are arranged coaxially with the excitation laser light 90 stopped obliquely incident in order to improve the appearance. In this case, the incident laser light 87 and the excitation laser light 90 have a superimposing portion 94 shown as a hatching region in the figure.
In this superimposing portion 94, as shown in FIG. 10 (b), the beam mode of the emitted laser light 88 having a right-angled triangular shape and further having a rectangular laser intensity distribution is rectangular as shown in FIG. 10 (c). The laser intensity distribution is 95 as it is amplified. Therefore, the laser intensity distribution 96 of the superposed portion 94 in FIG. 10 (b) and the laser intensity distribution 95 in FIG. 10 (c) are combined to obtain a distorted laser intensity distribution as shown in FIG. 10 (d).
As described above with reference to FIGS. 8 to 10, V-shaped reflection could not be amplified without distortion regardless of the positional relationship.

The present invention has been made in response to such conventional circumstances, and while adopting a V-shaped reflection mechanism, it enables amplification similar to vertical reflection, has less distortion of emitted laser light, and has a simple structure and high efficiency. The purpose is to provide a laser amplifier.

In order to achieve the above object, in the laser amplifier of the first invention, both end faces facing each other are parallel-polished, an antireflection film (HR coating) is formed on one end face, and an antireflection film (AR) is formed on the other end face. The first amplification gain medium on which the coating) is formed and the incident laser light introduced from the outside are reflected at the first end as the first incident laser light, and the reflection of the first amplification gain medium. By incidenting the incident laser light on the other end face on which the preventive film is formed at a predetermined angle, the first incident laser light is reflected by the one end face on which the enhancer reflection film is formed, and the other end face is again reflected. A first reflector emitted as a first emitted laser light amplified from, and a first excitation laser superimposed on a V-shaped optical path formed by the first incident laser light and the first emitted laser light. It has a first excitation laser light introduction portion that introduces light from the outside into the one end face of the first amplification gain medium, and the predetermined angle is the first of the first reflector. It is characterized by an angle that realizes a state in which the first emitted laser light is in contact with the end portion or a state in which the first emitted laser light is slightly in contact with the end portion.
In the first invention configured as described above, the first incident laser light is reflected by the first end portion of the first reflector to reflect the other end face (antireflection film) of the first amplification gain medium. Is incident on the amplification gain medium at a predetermined angle from the surface on which the The emitted laser light of 1 acts so as to be in a state of being in contact with the first end of the first reflector or a state of being slightly in contact with the first end of the first reflector.
That is, the first incident laser light reflected by the first reflector is incident from the other end face of the first amplification gain medium and reflected by one end face of the first amplification gain medium. Such an angle is set as a predetermined angle so that the angle formed by the first incident laser light and the first emitted laser light becomes extremely small when the light is emitted from the other end face as the first emitted laser light again. By arranging the first reflector to achieve the angle, the first incoming laser light and the first emitted laser light are in contact with the first end of the first first reflector or slightly. It acts so that it is in a state of being separated by a distance that does not touch.

Further, in the laser amplifier according to the second invention, in the first invention, the first reflector includes an edge portion composed of two surfaces sandwiching a sharp angle, and the first end portion is the edge portion. The incident laser light, which is formed as an end portion of the surface and is introduced from the outside, is reflected at the end portion on one side of the two surfaces constituting the edge, and the predetermined angle is the other of the first reflector. It is characterized by an angle that realizes a state in which the first emitted laser beam is in contact with the end portion on the one surface side or a state in which the first emitted laser light is separated by a distance that is slightly in contact with the end portion.
In the second invention configured in this way, in addition to the action of the first invention, the edge portion composed of the two surfaces sandwiching the sharp angle of the first reflector is the two surfaces of the edge portion. The first incident laser light reflected at the end on one side of the light incidents on the other end face of the amplification gain medium, is reflected by one end face of the amplification gain medium, and is again the first from the other end face. When emitted as laser light, it acts so as to be in contact with the end portion on the other one surface side of the edge portion or in a state of being separated by a distance slightly not in contact with the end portion.

The laser amplifier according to the third invention has the outer diameter of the first emitted laser beam as _out in the first invention or the second invention, and is the one of the first gain medium for amplification. When the distance from the end face to the first end of the first reflector is L.
d _out / 2L <0.04 (1)
It is characterized by being.
In the third invention configured as described above, in addition to the action of the first or second invention, by satisfying the above equation (1), the beam diameter of the first emitted laser beam is greatly deformed. It works so as not to cause it.

Further, the laser amplifier according to the fourth invention can adjust the angle of incidence of the first incident laser light on the first reflector in any one of the first inventions to the third invention. It is characterized by having a mirror.
In the fourth invention thus configured, in addition to the action of any one of the first to third inventions, the mirror adjusts the angle of incidence of the first incident laser beam on the first reflector. It works as much as possible.

Further, the laser amplifier according to the fifth invention includes a beam expander in front of the other end face of the first amplification gain medium in any one of the first to fourth inventions. It is characterized by that.
In the fifth invention configured as described above, in addition to the action of any one of the first to fourth inventions, the beam expander acts to increase the beam diameter of the first incident laser beam. ..
The beam expander is in front of the other end face of the first amplification gain medium, but this "front" broadly means the front stage or upstream, not only immediately before the other end faces, but also from the outside. It is a concept including from the place where the incident laser light introduced is incident on the laser amplifier of the present invention to the front of the other end face of the first growth gain medium.

Further, in the laser amplifier according to the sixth invention, in any one of the first to fifth inventions, the first excitation laser light introduction unit includes a bundle of optical fibers. It is characterized in that a switch is provided so that a first excitation laser beam can be introduced from a part or all of the bundle.
In the sixth invention thus configured, in addition to the action of any one of the first to fifth inventions, a switch that can be introduced from a part or all of the bundle of optical fibers is the first. It acts to adjust the output distribution of the emitted laser light of.

Further, the laser amplifier according to the seventh invention generates harmonics in front of the other end face of the first amplification gain medium in any one of the first to sixth inventions. It is characterized by being provided with a nonlinear optical element.
In the seventh invention configured as described above, in addition to the action of any one of the first to sixth inventions, the nonlinear optical element that generates harmonics converts the wavelength of the first incident laser light. It works like that.

Further, in the laser amplifier according to the eighth invention, in the first invention, the facing end faces are parallel-polished, a polyreflecting film (HR coating) is formed on one end face of the both end faces, and the other end face is formed. The first to nth amplification gain media having an antireflection film (AR coating) formed on the surface and the emitted laser light of the (n-1) th are reflected at the nth end as the nth incident laser light. By making the nth amplification gain medium incident on the other end face on which the antireflection film is formed at a predetermined nth angle, the nth incident laser light is used for the nth amplification. The first to first emitted laser light that is reflected by the one end face on which the augmentation reflection film of the gain medium is formed and is again emitted as the nth emitted laser light amplified from the other end face of the nth amplification gain medium. The n-th excitation laser light superimposed on the V-shaped optical path formed by the n-reflector, the n-th incident laser light and the n-th emission laser light is transmitted from the outside to the n-th amplification gain medium. It has an nth excitation laser light introduction portion to be introduced into one end face, and the nth emission laser light is in contact with the nth end portion of the nth reflector at the nth predetermined angle. It is an angle that realizes a state or a state separated by a distance that does not slightly touch, and n is an integer of 2 or more, and the first amplification gain medium, the first reflector, and the first excitation laser light introduction. From the unit, the nth amplification gain medium, the nth reflector, and the nth excitation laser light introduction unit use the emitted laser light of the (n-1) as the nth incident laser light. , It is characterized in that it is configured to be amplified in multiple stages.
In the eighth invention configured as described above, in addition to the action of the first invention, the first reflector, the excitation laser light introduction unit, the nth reflector from the amplification gain medium, and the excitation laser light introduction. By providing a gain medium for amplification, the incident laser light incident from the outside is used as the first incident laser light, and the first emitted laser light is used as the second incident laser light, which is sequentially amplified up to n stages. Acts to do.

According to the laser amplifier of the first invention, the gain medium for amplification is provided by arranging the first reflector so that the angle between the first incident laser light and the first emitted laser light becomes extremely small. Since one end face can be reflected in a V shape with an extremely small angle, amplification similar to that of vertical reflection is possible, and a highly efficient and simple double-pass end face excitation amplifier with less distortion can be realized.

According to the laser amplifier of the second invention, in addition to the effect of the first invention, by providing the edge portion having a sharp angle, the light is reflected on one surface side of the edge portion of the first reflector. The incident laser light of 1 and the first emitted laser light emitted so as to be in contact with or slightly in contact with the other one-sided end of the edge portion of the first reflector. It is possible to make the angle formed smaller than that of the first invention. Therefore, since one end face of the gain medium for amplification can be reflected in a V shape with an extremely small angle, amplification similar to vertical reflection is possible, and the double-pass end face excitation with less distortion, higher efficiency, and simpleness is possible. An amplifier can be realized.

According to the laser amplifier of the third invention, in addition to the effect of the first or second invention, the beam diameter of the first emitted laser light is not significantly deformed, so that the amplification efficiency can be improved. be. Therefore, by satisfying the equation (1), the effect that the energy loss due to the first reflector is almost eliminated is exhibited.

According to the laser amplifier of the fourth invention, in addition to the effect of any one of the first to third inventions, the angle of the first incident laser light incident on the first reflector is adjusted by the mirror. Therefore, it is possible to take a margin regarding the arrangement and orientation of the first reflector.

According to the fifth invention, the laser amplifier, in addition to the effect of any one of the first to fourth inventions, the beam diameter of the first incident laser light can be increased, so that the beam of the first emitted laser light can be increased. The diameter can be increased, and therefore the numerical aperture can be increased.

According to the sixth invention, the laser amplifier, in addition to the effect of any one of the first to fifth inventions, the heat distribution can be adjusted by adjusting the output distribution of the first emitted laser light. Therefore, heat damage can be suppressed even at the processing point due to the emitted laser beam.

According to the seventh invention, the laser amplifier, in addition to the effect of any one of the first to sixth inventions, a non-linear element is in front of the other end face when emission laser light of different wavelengths is required. Since it is provided, it acts in a round trip, and it is a highly efficient wave.

According to the laser amplifier of the eighth invention, in addition to the effect of the first invention, it is possible to obtain high-energy emitted laser light by amplification by multiple stages from one stage to n stages.

(A) is a block diagram of the laser amplifier according to the first embodiment of the present invention, and (b) is a partial block diagram showing the configuration of the circular portion indicated by reference numeral A in (a). (A) is a block diagram of an excitation optical system for explaining an evaluation experiment of a laser amplifier according to a first embodiment of the present invention, and (b) is a graph showing a laser intensity distribution of incident laser light, (c). ) Is a graph showing the beam intensity distribution of the emitted laser light. (A) is a block diagram of the excitation optical system of the laser amplifier according to the second embodiment of the present invention, (b) is a graph showing the beam intensity distribution of the incident laser light, and (c) is the beam of the emitted laser light. It is a graph which shows the intensity distribution. (A) is a block diagram of the excitation optical system of the laser amplifier according to the third embodiment of the present invention, (b) is a graph showing the beam intensity distribution of the incident laser light, and (c) is the beam of the emitted laser light. It is a graph which shows the intensity distribution, and (d) is a conceptual diagram which shows the cross-sectional structure of the optical fiber of the laser amplifier which concerns on 3rd Embodiment. It is a block diagram of the laser amplifier which concerns on Example 4 of this invention. It is a block diagram of the laser amplifier which concerns on Example 5 of this invention. It is a block diagram of the laser amplifier which concerns on the prior art. (A) is a conceptual diagram for explaining a phenomenon when V-shaped reflection is performed by a gain medium for amplification in a laser amplifier according to a prior art, and (b) and (c) are lines BB in (a). It is a conceptual diagram for demonstrating the beam intensity distribution of the emitted laser beam in the cross-sectional view of an arrow. (A) is a conceptual diagram for explaining a phenomenon when V-shaped reflection is performed by a gain medium for amplification in a laser amplifier according to a prior art, and (b) is a cross section taken along the line CC in (a). It is a conceptual diagram for demonstrating the beam intensity distribution of the emitted laser light of. (A) is a conceptual diagram for explaining a phenomenon when V-shaped reflection is performed by a gain medium for amplification in a laser amplifier according to a prior art, and (b), (c), and (d) are emitted laser beams, respectively. It is a conceptual diagram for explaining the beam intensity distribution of.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

FIG. 1A is a configuration diagram of a laser amplifier according to a first embodiment of the present invention, and FIG. 1B is a partial configuration diagram showing a configuration of a circular portion indicated by reference numeral A in FIG. 1A.
In FIG. 1A, the laser amplifier 1 introduces the pre-amplification laser light 2 which is the incident laser light introduced from the outside as the incident laser light 4 through the concave lens 3 which is the entrance to the laser amplifier 1. After that, the incident laser light 4 is reflected toward the prism 9a by the reflection mirror 8 via the convex lens 7 housed in the lens barrel 6, and further reflected by the prism 9a as a reflector. The incident laser light 12 after being reflected by the prism 9a is incident from the end face (hereinafter, simply referred to as AR end face) on which the antireflection film (AR coating) of the amplification gain medium 15 fixed to the base 14 is formed. It is reflected on the end face (hereinafter, simply referred to as HR end face) on which the antireflection film (HR coating) is formed, and is emitted again from the AR end face as the emitted laser light 26.
In the gain medium 15 for amplification in FIG. 1A, the AR end face is indicated by the reference numeral AR, and the HR end face is indicated by the reference numeral HR. The same applies to the subsequent drawings.
The amplification gain medium 15 is fixed to the base 14 around the rotation shaft 16 so that it can be tilted and adjusted by a screw 17. A lens barrel 20 containing two combined lenses 21a and 21b is arranged on the HR end face side of the gain medium 1 for amplification, and the lens barrel 20 is connected to an optical fiber 18 for introducing an excitation laser light 25. It functions as an introduction part of the excitation laser light 25.

On the AR end surface side of the gain medium 15 for amplification, a prism 9a having a laser reflecting surface formed on the upper side in the figure among the edge portions formed by two surfaces sandwiching an acute angle is provided on the optical axis 27 of the emitted laser light 26. It is located in the vicinity.
On the other hand, on the optical axis 5 of the incident laser beam 4 before amplification, a concave lens 3 fixed to the main body of the laser amplifier 1 and a convex lens 7 built in a lens barrel 6 movable back and forth are arranged, and further an optical axis. The reflection mirror 8 is diagonally arranged on the extension line of 5.
According to this configuration, the pre-amplification laser light 2 becomes the incident laser light 4 enlarged to a predetermined diameter by the concave lens 3 and the convex lens 7, and after being reflected by the reflection mirror 8, the incident laser light 12 is used as the edge portion of the prism 9a. It is reflected by the end 28a of the laser reflecting surface 32a formed on the upper side of the above, and is incident on the AR end surface of the gain medium 15 for amplification. Here, the lower surface 31a of the edge portion of the prism 9a in contact with the emitted laser light 26 may or may not shield the emitted laser light 26, that is, the emitted laser light 26 may or may not be in contact with the end portion 28a. It is parallel-polished with respect to the emitted laser beam 26 so as to be separated from the distance.

On the other hand, in the excitation laser light 25 incident on the HR end face of the amplification gain medium 15, the beam waist portion (indicated by the reference numeral _dw ) focused by the two combination lenses 21a and 21b is inside the amplification gain medium 15. Then, the incident laser light 12 is amplified by superimposing the incident laser light 12 reflected in a V shape at a slight angle. That is, the optical axis 13 of the incident laser light 12 and the optical axis of the excitation laser light 25 intersect at the HR end face of the amplification gain medium 15, and the optical axis 27 of the emitted laser light 26 and the optical axis of the excitation laser light 25 coincide with each other. ing.
It should be noted that these optical axis adjustments are adjusted to positions where the prism 9a and the emitted laser beam 26 are in contact with each other without being obstructed by the screw 17 which adjusts the rotation of the reflection mirror 8 and the gain medium 15 for amplification horizontally.
Although the prism 9a has been described as a reflector having an edge portion, as shown in FIG. 1 (b), the incident laser light is emitted from the end portion 28d of the laser reflecting surface 32b of the normal flat plate reflecting mirror 29 instead of the prism 9a. The lower surface 31b and the laser reflecting surface 32b of the flat plate reflecting mirror 29 are provided on two surfaces with a sharp angle so as to reflect 12 so that the end portion 28d and the emitted laser light 26 are separated from each other by a distance that is in contact with or slightly in contact with each other. It is self-evident that even if it is polished into an edge shape such as that formed from, it has the same effect. Specifically, the lower surface 31b and the laser reflecting surface 32b are formed in an edge shape by polishing the lower surface 31b of the flat plate reflecting mirror 29 so as to be parallel to the emitted laser light 26.
Further, although the amplification gain medium 15 is excited from the optical fiber 18, it is self-evident that it can be amplified even if it is directly excited from an excitation laser diode (LD) (not shown).

Next, the adjustment method will be described with reference to FIG. 1 (a) in such a configuration.
Since the actual laser light has a characteristic of gradually increasing, it is extremely difficult to adjust so that the prism 9a and the optical axis 27 of the emitted laser 26 are in contact with each other. Further, it is extremely difficult to make the emitted laser light 26 and the prism 9a in contact with each other so that the focused position of the excited laser light 25, that is, the position where the beam waist _dw and the V-shaped reflection in the amplification gain medium 15 are aligned. Further, not only the fixed position of each optical component varies, but also the incident laser beam 4 varies in the vertical direction in the drawing, so that it is more difficult to adjust.
In the present invention, in order to simplify such difficult adjustment as much as possible, the vertical tilting direction indicated by reference numeral 10 and the left-right tilting direction indicated by reference numeral 11 with respect to the incident laser beam 4 incident on the prism 9a (in FIG. 1A). A reflection mirror 8 that can be finely adjusted (perpendicular to the paper surface) has been added.

In such a configuration, first, the optical axis adjusting method will be described in the order of adjustment.
(1) The incident laser light 4 magnified to a predetermined diameter by the concave lens 3 and the convex lens 7 for aligning the optical axes of the pre-amplification laser light 2 to be amplified and the excitation laser light 25 is moved up and down in the left-right tilt direction 11. It is reflected by the reflection mirror 8 having the fine adjustment mechanism of the tilting direction 10, reflected by the end portion 28a of the edge portion of the prism 9a, and incident on the AR end face of the amplification gain medium 15, the excitation laser light 25 and the amplification gain medium. The tilt angle of the reflection mirror 8 is adjusted so that it comes to a position where it is superimposed on the incident laser light 12 reflected in a V shape inside the 15. At that time, since the prism 9a protrudes outward from the end portion 28a of the edge portion or enters the inside, the position of the prism 9a is slightly moved back and forth (left and right in the drawing) so as to come to the end portion 28a of the edge portion. adjust.
(2) Beam diameter matching of the incident laser light 12 and the excitation laser light 25 The lens barrel 20 containing the two combined lenses 21a and 21b is moved in the front-rear direction (left-right direction in the figure), and the optical fiber 18 is further emitted. The beam waist _dw is adjusted inside the amplification gain medium 15 so that the distance d between the end face 19 and the combined lenses 21a and 21b also moves back and forth and the emitted laser light 26 has the maximum power. This is called mode matching.
(3) Alignment between the prism 9a and the emitted laser beam 26 Since the emitted beam diameter changes by mode matching, it is necessary to align the prism 9a with the emitted laser beam 26. Since the amplification gain medium 15 is fixed to the base 14 that can be tilted and adjusted by the screw 17 around the rotation shaft 16, the screw 17 is moved back and forth (up and down in the figure) to adjust the tilt of the base 14.
(4) Optimization adjustment The optimum position can be adjusted by repeating the adjustments (1) to (3) above so that the emitted laser beam 26 has the maximum power.
In the text explaining the above adjustment, the emitted laser beam 26 is adjusted to a position in contact with the end portion 28a of the edge portion of the prism 9a, but it may be kept at a distance away from the prism 9a. FIG. 1 shows a state in which the distance is slightly non-contacting.

(Evaluation test)
When the diameter of the emitted laser beam 26 is _out , the distance from the HR end face of the gain medium 15 for amplification to the end 28a of the edge portion of the prism 9a is L, and the condition represented by the equation (1) is satisfied.
d _out / 2L <0.04 (1)
It was confirmed by the evaluation test that a good amplifier having high amplification efficiency can be constructed with no large deformation in the beam diameter and almost no loss due to the prism 9a.
The evaluation experiments carried out below will be described with reference to FIGS. 2 (a) to 2 (c).
FIG. 2A is a block diagram of an excitation optical system for explaining an evaluation experiment of a laser amplifier according to the first embodiment of the present invention, and FIG. 2B is a graph showing a laser intensity distribution of incident laser light. (C) is a graph showing the laser intensity distribution of the emitted laser light.
The excitation optical system including the lens barrel 20, the optical fiber 18, and the gain medium 15 for amplification has a beam waist d _w = 1.8 mm, and the combined lenses 21a and 21b and the lens barrel 20 have a beam waist length of about 2 to 3 mm. The position was fine-tuned. At this time, the measured effective excitation length L _eff = about 10 mm.
The diameter of the emitted laser beam 26 was set to d _out = 3 mm and L = 100 mm. In this case, d _out / 2L = 0.015, which is smaller than 0.04, so that the equation (1) is satisfied. The amount of optical axis deviation α due to V-shaped reflection is shown below.
α = d _out / 2 * (Leff / L) = 3/2 * (10/100) = 0.15 mm
Therefore, only 0.15/3 = 5% beam distortion was generated with respect to d _out = 3 mm, and it was possible to maintain a substantially circular beam. The beam diameter _din incident on the amplification gain medium 15 was set to 1.8 mm.

Next, the reason why the emitted laser light 26 amplified by the amplification gain medium 15 increases from 1.8 mm to 3 mm will be described with reference to FIGS. 2 (a) to 2 (c).
Since the beam waist d _w = 1.8 mm of the excitation laser light 25 is adjusted to be the same as the beam di _in of the incident laser light 12, the amount of optical axis shift at the effective excitation length Leff is further as described above. It becomes 0.15 mm. Therefore, the beam diameter d _out of the emitted laser beam 26 is calculated to be 1.8 + 0.15 = 1.95 mm.
However, since the excitation laser light 25 is a focused beam, the incident beam diameter dp to the amplification gain medium 15 is slightly larger than dw as shown in FIG. 2 (c). Due to the characteristics of the laser, the beam diameter di of the incident laser light has a slight intensity at the base 30 as shown _in FIG. 2 (b), so that the base 30 is amplified and the beam diameter is as large as 3 mm.

Next, the evaluation experiment conditions and results are shown.
(Conditions) Wavelength 1064 nm Amplification gain medium 6.35 mm diameter x 50 mm length,
Nd 0.7% content (dope)
808 nm excitation power 1.09 KW, excitation time 450 μs,
Laser energy of incident laser light = 24.5mJ
Pulse width 1ns x 4 pulses (result) Laser energy of emitted laser light No superimposition of excitation laser light = 21.62mJ
At the same time of superimposition = 123.2mJ
Loss = (24.57-21.62) /24.57 = 12%
Amplification rate = 123.2 / 24.57 = Approximately 5 times Peak power = 123.2mJ / (1ns x 4 pulses) = 30.8MW
As shown above, despite the small size and simplicity of the present invention, the loss of the laser amplifier is extremely small at 12%, the amplification factor is as large as 5 times, and it can withstand the generation of giant pulses with a peak power of 30.8 MW. Was also confirmed.
Further, since an optical component having a low yield strength such as a polarizing beam splitter (PBS) is not required, it can be further amplified by increasing the diameter of the incident beam or the excitation beam.
Further, since the beam expander by the combination of the concave lens 3 and the convex lens 7 whose beam diameter can be changed without the polarization dependence of the incident beam can be arbitrarily changed to the required size, it can also be used as a general-purpose laser amplifier.
In this embodiment, the concave lens 3 and the convex lens 7 are provided on the front stage (upstream) side of the reflection mirror 8 as the beam expander, but the beam expander is a place where the laser light 2 before amplification is incident on the laser amplifier 1. That is, it may be provided at any position as long as it is between the inlet of the laser amplifier 1 and the front of the AR end face of the amplification gain medium 15.
Conventionally, there is no general-purpose laser amplifier, and it was individually designed by a special advanced laser engineer as a set with an oscillator, but with this invention, it is possible to easily increase the intensity of various laser lights even if you are not an advanced laser engineer. become. We believe that this generalization will provide great convenience in laser utilization and contribute to the development of lasers.

Next, Example 2 according to the present invention will be described with reference to FIGS. 3 (a) to 3 (c). FIG. 3A is a block diagram of an excitation optical system of a laser amplifier according to a second embodiment of the present invention, FIG. 3B is a graph showing a beam intensity distribution of incident laser light, and FIG. 3C is an emitted laser light. It is a graph which shows the beam intensity distribution of.
In FIG. 3A, the beam waist diameter d _w 2 of the excitation laser light 25 is smaller than the incident beam diameter di _in , and the incident beam diameter d _p2 of the excitation laser light 25 on the HR end face of the gain medium 15 for amplification is also an incident laser. The beam diameter of the light 12 is smaller than the _diin .
In this case, since the base of the incident laser light 12 is not amplified, the beam diameter of the emitted laser light 26 can be made the same as the beam diameter of the incident laser light 12. Then, as a matter of course, the loss increases and the amplification factor also decreases, but it is possible to obtain an emitted laser beam 26 having a good beam mode, that is, a beam intensity distribution close to that of Gaussian.
Further, as shown in FIG. 3B, by changing the beam mode of the excitation laser light 25 to a top hat shape, the beam mode of the incident laser light 12 can be stored in a similar shape. Further, when the beam mode of the excitation laser light 25 is set to a Gaussian shape, the amplification factor is as large as 5 times, and the amplified emitted laser light 26 can also be made to have a Gaussian shape. By this principle, the beam quality can be improved.

Next, Example 4 according to the present invention will be described with reference to FIGS. 4A to 4C.
4 (a) to 4 (c) show the excitation optical system (a) and the excited state (b) when the optical fiber 18 is a ring-shaped light guide body such as a 7 combiner as shown in FIG. 4 (d). It is a figure which showed the beam mode (c) of the emitted laser light 26. In these figures, the beam waist diameter of the excitation laser light 25 and the incident beam diameter to the amplification gain medium 15 are dw3 and dp3, respectively.
When the peak power is KW class excitation, the central portion may be thermally damaged, but the central portion can be prevented from being thermally damaged by not exciting the central fiber 35 of the 7 combiner. Further, the emitted laser light 26 amplified at this time also has a ring shape, and when the emitted laser light 26 is focused, the ring beam is superimposed, and the central portion close to the top hat as shown in FIG. 4C is formed. A dented light-collecting spot can be obtained. Since the heat distribution is close to that of a top hat, even at the processing point, uniform processing that does not easily cause heat damage to the central part can be performed.
Further, by adding a beam expander having a variable magnification, the beam size ratio between the incident laser light 12 and the excitation laser light 25 can be changed, so that various arbitrary laser beam modes and beam diameters described above can be created.
Furthermore, since the configuration is not affected by polarization characteristics such as the absence of PBS, it is possible to mode-match laser light with arbitrary modification characteristics that customers want to amplify to any beam diameter, making it a simple and general-purpose laser. Can provide an amplifier,
Further, even for a flash lamp-pumped YAG laser having no polarization characteristic, not only can it be amplified if the oscillation wavelength of YAG is 1064 nm, but also fiber laser light having a wavelength close to 1064 nm can be amplified.

Next, Example 4 according to the present invention will be described with reference to FIG.
FIG. 5 is a block diagram of the laser amplifier according to the fourth embodiment of the present invention. The same components as those shown in the previous figures are designated by the same reference numerals.
In FIG. 5, the second harmonic generation (SHG) element 36 is subjected to the second harmonic generation (SHG) element 36 before being reflected in the vicinity of the end portion 28a of the edge portion of the prism 9a and incident on the AR end face of the amplification gain medium 15 as short as 10 to 20 mm. By arranging the amplification gain medium 15 in the vicinity of the AR end face, the second harmonic generation (SHG) element 36 is passed back and forth, so that highly efficient wavelength conversion can be performed.

Next, Example 5 according to the present invention will be described with reference to FIG.
FIG. 6 is a block diagram of the laser amplifier according to the fifth embodiment of the present invention. The same components as those shown in the previous figures are designated by the same reference numerals. Further, in the laser amplifier 41 disclosed in FIG. 6, a reflection type right-angle prism 9b, a second gain medium for amplification 44, a lens barrel 49, and an optical fiber 47 are newly added to the laser amplifier 1 in FIG. It has a structure like that.
According to this configuration, the incident laser beam 12 is reflected in the vicinity of the end 28b of the edge portion composed of two surfaces sandwiching the right angle of the right-angle prism 9b, and then the AR of the gain medium 15 for amplification fixed to the base 14 is AR. It is incident on the end face.
On the other hand, the emitted laser light 26 is reflected near the end portion 28b of the edge portion of the right-angle prism 9b and then reflected near the end portion 28c of the edge portion of the prism 9c to serve as the second incident laser light 42. , Is incident on the AR end face of the additional second amplification gain medium 44 fixed to the second base 43. A lens barrel 49 containing two combined lenses 50a and 50b is arranged on the HR end face side of the second gain medium 44 for amplification, and the lens barrel 49 is an optical fiber 47 for introducing the excitation laser light 51. Is connected to. The excitation laser light 51 is guided from the outside to the inside of the lens barrel 49 from the emission end face 48 via the optical fiber 47.
The second amplification gain medium 44 can be tilted and adjusted with a screw 46 around the rotation shaft 45, similarly to the amplification gain medium 15 of the first embodiment. When a plurality of stages of the gain medium for amplification are provided, it may not be possible to provide a screw on the main body of the laser amplifier 41 depending on the base of the gain medium for amplification as in the base 14 shown in FIG. A means to enable it is provided separately.
The excitation laser light 51 incident from the HR end face is incident from the AR end face inside the second amplification gain medium 44, is reflected in a V shape at a slight angle at the HR end face, and is emitted from the AR end face again. The second incident laser beam 42 is amplified by superimposing the second incident laser beam 42 on the incident laser beam 42. Then, the second incident laser light 42 is output as an emitted laser light 52 having an optical axis indicated by reference numeral 53.
In this embodiment, the laser light is amplified by two stages of the amplification gain medium 15 and the second amplification gain medium 44, but the number of stages is increased by providing the prism, the amplification gain medium, and the excitation laser light in a plurality of stages. It is easy to obtain a higher energy laser beam.
Further, in this embodiment, the prism that reflects the incident laser light 4 to be the incident laser light 12 and further reflects the emitted laser light 26 to be the second incident laser light 42 is a single right angle prism 9b. Instead of this, a prism (reflector) that reflects the incident laser light 4 to be the incident laser light 12 and a prism (reflector) that reflects the emitted laser light 26 to be the second incident laser light 42 are separately provided. Needless to say, it may be provided.

As described above, the invention according to claim 1 to 8 of the present invention can be widely used as an amplifier for a solid-state laser used for various purposes.

1 ... Laser amplifier 2 ... Laser light before amplification 3 ... Concave lens 4 ... Incident laser light 5 ... Optical axis 6 ... Lens tube 7 ... Convex lens 8 ... Reflection mirror 9a-9c ... Prism 10 ... Vertical tilt direction 11 ... Left and right tilt direction 12 ... Incident laser light 13 ... Optical axis 14 ... Base 15 ... Gain medium for amplification 16 ... Rotating axis 17 ... Screw 18 ... Optical fiber 19 ... Emission end face 20 ... Lens barrel 21a, 21b ... Combination lens 25 ... Excitation laser light 26 ... Emission laser Optical 27 ... Optical axis 28a-28d ... End 29 ... Flat plate reflecting mirror 30 ... Base 31a, 31b ... Surface 32a, 32b ... Laser reflecting surface 35 ... Central fiber 36 ... Second harmonic generation (SHG) element 41 ... Laser amplifier 42 ... Second incident laser light 43 ... Second base 44 ... Second amplification gain medium 45 ... Rotating shaft 46 ... Screw 47 ... Optical fiber 48 ... Ejecting end face 49 ... Lens tube 50a, 50b ... Combination lens 51 ... Excitation laser light 52 ... Emission laser light 53 ... Optical axis 61 ... Master laser 62 ... Optical amplifier 63 ... Nd-doped vanadate mixed crystal 64 ... Lens 65 ... Excitation output light from an excitation semiconductor laser array 66 ... Half-wave plate 66 67 ... Polarized beam splitter 68 ... Faraday rotor 69 ... Half-wave plate 70 ... Polarized beam splitter 71 ... Faraday rotor 72 ... Half-wave plate 73 ... Lens 74 ... Lens 75 ... Lens 76 ... Lens 77 ... Prism 78 ... Lens 79 ... Lens 80 ... Lens 81 ... Half-wave plate 82 ... Phase conjugate mirror (PCM) 83 ... Nd-doped vanadate mixed crystal 84 ... Lens 85 ... Lens 86 ... Gain medium for amplification 87 ... Incident laser light 88 ... Emission laser light 89 ... V-shaped reflection Center line 90 ... Excitation laser light 91 ... Circular cross section 92 ... Circular cross section 93 ... Distorted laser light 94 ... Overlapping part 95 ... Laser intensity distribution 96 ... Laser intensity distribution AR ... AR end face HR ... HR end face

Claims (6)

A first amplification gain in which both end faces facing each other are polished in parallel, an antireflection film (HR coating) is formed on one end face of the end faces, and an antireflection film (AR coating) is formed on the other end face. With the medium
A predetermined angle with respect to the other end face on which the antireflection film of the first amplification gain medium is formed by reflecting the incident laser light introduced from the outside as the first incident laser light at the first end portion. The first incident laser light is reflected by the one end face on which the hyperreflective film is formed, and is emitted again as the first emitted laser light amplified from the other end face. Reflector and
The first excitation laser light superimposed on the V-shaped optical path formed by the first incident laser light and the first emitted laser light is introduced from the outside into the one end face of the first amplification gain medium. It has a first excitation laser light introduction portion, and the predetermined angle is separated from the state where the first emitted laser light is in contact with the first end portion of the first reflector or at a distance slightly not in contact with the first end portion. It is the angle that realizes the state ,
When the outer diameter of the first emitted laser beam is _out , and the distance from the one end surface of the first amplification gain medium to the first end of the first reflector is L. To,
d _out / 2L <0.04
A laser amplifier characterized by being . The first reflector includes an edge portion composed of two surfaces sandwiching a sharp angle, the first end portion is formed as an end portion of the edge portion, and the incident laser light introduced from the outside is the above-mentioned. The predetermined angle is reflected by the end portion on one surface side of the two surfaces constituting the edge portion, and the predetermined angle is the first emitted laser light with respect to the end portion on the other one surface side of the first reflector. The laser amplifier according to claim 1, wherein the laser amplifier has an angle that realizes a state in which light is in contact with the laser or a state in which light is slightly away from the laser amplifier. The laser amplifier according to claim 1 or 2 , further comprising a mirror capable of adjusting the angle of incidence of the first incident laser light with respect to the first reflector. The laser amplifier according to any one of claims 1 to 3 , wherein a beam expander is provided in front of the other end face of the first gain medium for amplification. The laser amplifier according to any one of claims 1 to 4 , wherein a nonlinear optical element that generates harmonics is provided in front of the other end face of the first gain medium for amplification. For the first amplification, the facing end faces are ground in parallel, an antireflection film (HR coating) is formed on one end face of the end faces, and an antireflection film (AR coating) is formed on the other end face. The nth amplification gain medium and the (n-1) emission laser light are reflected from the gain medium as the nth incident laser light at the nth end to prevent the reflection of the nth amplification gain medium. By making the nth incident laser light incident on the other end face on which the film is formed at a predetermined nth angle, the nth incident laser light is directed to the nth amplification gain medium, the one on which the antireflection film is formed. From the first reflector to the nth reflector, which is reflected by the end face of the nth and is emitted again as the nth emission laser light amplified from the other end face of the nth amplification gain medium.
The nth excitation laser light superimposed on the V-shaped optical path formed by the nth incident laser light and the nth emitted laser light is introduced from the outside into the one end surface of the nth amplification gain medium. It has the first excitation laser light introduction section to the nth excitation laser light introduction section, and the nth predetermined angle is the nth emission to the nth end portion of the nth reflector. It is an angle that realizes a state in which the laser beam is in contact with the laser beam or a state in which the laser beam is slightly in contact with the laser beam.
The outer diameter of each of the nth emitted laser light from the first emitted laser light is defined as _dout , and the first reflector of the first reflector from the one end surface of the first gain medium for amplification. When each of the distance to the end portion or the distance from the one end surface of the nth amplification gain medium to the nth end portion of the nth reflector is L.
d _out / 2L <0.04
And
The n is an integer of 2 or more, and from the first amplification gain medium, the first reflector, the first excitation laser light introduction unit, the nth amplification gain medium, the nth. The reflector and the nth excitation laser light introduction unit are configured to amplify the emitted laser light of the (n-1) th (n-1) in a plurality of stages as the nth incident laser light. Item 1. The laser amplifier according to Item 1.

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