JPH0918071A - Aberration control of zigzag slab laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a mechanism which is easily applied to a zigzag slab laser to compensate a laser beam in a zigzag slab for its ununiuform wavefronts which intersect a direction of zigzag. SOLUTION: A zigzag slab laser 10 is equipped with a zigzag slab 12 which generates a light beam whose wavefront is ununiuform in a certain direction. The light beam is directed to a one-dimensional aberration corrector 32 which corrects the light beam on aberration in a certain direction so as to generate a light beam possessed of uniform wavefronts. The one-dimensional aberration corrector 32 comprises a liquid crystal spatial optical modular equipped with linearly arranged liquid crystal cells 36.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates generally to zig-zag slab lasers, and more particularly to one-dimensional correction for correcting aberrations in one direction of a light beam emitted from a zig-zag slab of a laser. Container (one-dimension
zigzag slab laser with al corrector).

[0002]

BACKGROUND OF THE INVENTION High power lasers, especially high power solid state lasers, are important devices for many military, industrial and commercial applications. These applications include cutting and perforating various materials, photolithography, space communications and medical supplies.
The highest power laser applications require that the laser beam produced by the laser have high beam quality, i.e. minimal beam aberration. Aberrations in the phase of the laser beam reduce the uniformity of the wavefront of the beam and thus reduce the ability of the beam to be tightly focused. In other words, the beam wavefront uniformity must be flat for the beam to be accurately focused. When the beam's ability to focus tightly decreases, the beam's focus intensity (foc
al intensity) and focus accuracy are reduced. For example, phase aberrations that cause non-uniformity of the wavefront of the laser beam used to drill small diameter holes obscure the spot size of the focused beam, creating non-uniform holes. An attractive type of solid state laser for high power applications is what is referred to in the art as a zigzag slab laser. This type of laser is a high-brightness solid-state laser (HI) equipped with a zigzag amplifier filed on November 5, 1993.
GH BRIGHTNESS SOLID-STATE LASER WITH ZIG-ZAG AMPLI
No. 08 / 148,758 entitled "FIER)", which is assigned to the assignee of the present application and incorporated herein by reference. Zigzag slab lasers include at least one zigzag slab of lasing medium, such as yttrium-aluminum-garnet (YAG) crystals, for generating and / or amplifying a laser beam.

Zigzag slabs are blocks of lasing media with two relatively closely spaced parallel sidewalls. A light beam introduced into or generated within the zigzag slab is reflected back and forth between the parallel sidewalls as the beam propagates through the slab. next,
A strong light beam is emitted from the end face of the zigzag slab.
Since the zigzag slab laser of the zigzag laser produces a high-intensity light beam, it is necessary to cool the slab. As a result of the heat generated by the light beam and the cooling of the slab, there are temperature gradients in the zigzag slab that cause different refractive indices of the lasing medium. The different indices of refraction within the slab cause the phase of the light beam to change differently at different positions, which results in a non-uniform wavefront of the beam. Since the beam is reflected back and forth from the parallel side walls of the slab, it passes through a high proportion of the slab areas with different refractive indices. This movement produces an averaging effect in the direction of the zigzag path of the beam that smooths the wavefront of the beam, thus eliminating phase aberrations in this direction. Therefore, the phase aberration of the wavefront generally exists only in the direction orthogonal to the zigzag optical path of the beam.

Various ideas have been proposed in the art to correct the non-uniformity of the wavefront of a zigzag slab laser in the direction transverse to the zigzag path of the beam. As one example, it has been proposed to provide a zigzag slab that, in addition to reflecting the beam from the slab sidewall, also reflects the light beam from the top and bottom surfaces of the slab perpendicular to the slab sidewall.

[0005]

However, in general, the top and bottom surfaces of the zig-zag slab are not located close to each other, which results in a minimum of the beam in that direction as it propagates through the slab. Causes a number of reflections. The zigzag laser of US patent application Ser. No. 08 / 148,758 uses a phase conjugation technique to reduce non-uniformity of the beam wavefront. This technique guides an optical beam emitted from a zigzag slab amplifier to a phase conjugate mirror, passes the beam back in the opposite direction through the slab amplifier, and corrects aberrations in the opposite direction of the original zigzag optical path through the slab. Is. However, this technique of correcting wavefront non-uniformity is complex. What is desired is a mechanism that can be easily implemented in a zigzag slab laser and that compensates for wavefront non-uniformity across the zigzag direction of the beam in the slab. Therefore, it is an object of the present invention to provide such a mechanism.

[0006]

According to the teachings of the present invention,
A zigzag slab laser is disclosed that substantially eliminates the wavefront non-uniformity of the light beam generated by the zigzag slab of the laser in a direction orthogonal to the zigzag optical path of the light beam within the zigzag slab. The zigzag slab laser of the present invention has a one-dimensional aberration corrector that corrects the phase aberration of the light beam emitted from the zigzag slab of the laser in the direction orthogonal to the zigzag optical path of the light beam in the slab. In one embodiment, the one-dimensional corrector is a liquid crystal spatial light modulator comprising a linear array of liquid crystal cells, the liquid crystal cells varying the index of refraction of the cells to compensate for wavefront non-uniformity. Selectively activated. Zigzag lasers may have various forms of oscillators and amplifiers associated with a one-dimensional corrector for correcting phase aberrations of the light beam. Other objects, advantages and features of the present invention will become apparent from the following description and claims taken in conjunction with the accompanying drawings.

[0007]

The following description, which describes a preferred embodiment of a zigzag slab laser with a zigzag slab and a one-dimensional aberration corrector, is merely illustrative in nature and is not intended to limit the invention and its application in any way. Absent. FIG. 1 shows a zigzag slab laser 10 according to an embodiment of the present invention. The zigzag slab laser 10 has a zigzag slab amplifier 12, which is a block of a suitable crystalline optical lasing material, such as a yttrium-aluminum-garnet (YAG) crystal. Of course, any crystalline material known in the art suitable for zigzag slabs can be adapted for the purposes of the present invention. FIG. 2 is a plan view of the slab amplifier 12.
The slab amplifier 12 has a proximal end surface 14 and a distal end surface 16 that are substantially parallel to each other. The slab amplifier 12 further has closely spaced side walls 18, 20 which are parallel to each other and inclined with respect to the end faces 14, 16. The slab amplifier 12 has a top surface 22.
And bottom surface 24, which are parallel to each other and at right angles to the sidewalls 18, 20, and at right angles to all surfaces that complete the slab amplifier 12.

Light beam 26 is introduced into the proximal end face 14 of slab amplifier 12 from a suitable light source, such as a master oscillator (not shown in FIG. 1). The light beam 26 impinges on the end face 14 at an appropriate angle of incidence, which causes the sidewall 1
One of the side walls 8 or 20 (here, the side wall 18)
Is refracted or directed toward. In this example, the end face 1
The angle of incidence of the beam 26 is substantially perpendicular to the end face 14 so as to reduce the loss due to reflection at the interface with 4. However, one of ordinary skill in the art will appreciate that other angles of incidence are applicable. In this embodiment, the light beam 2
End face 1 so that 6 faces the side wall 18 and impacts the amplifier slab 12 at right angles to the end face 14.
4, 16 are inclined with respect to the side walls 18, 20. Due to the index of refraction of the slab amplifier 12, the light beam 26 within the slab amplifier 12 is reflected back and forth between the sidewalls 18, 20 and amplified within the amplifier 12 as a zigzag beam 28 as shown. The zigzag beam 28 is then amplified by the light beam 30 amplified from the distal end face 16 of the slab amplifier 12.
Is issued as. The operation of zigzag slab amplifiers is well understood in the art. The light beam 28 is amplified inside the slab amplifier 12 by a lasing process. A suitable energizing light source (not shown), such as a flash lamp or diode, may be located outside the slab amplifier 12 and adjacent one or both sidewalls 18, 20. The energizing light source acts to raise the level of electrons associated with the atoms of the crystalline material that make up the slab amplifier 12 to an excited state. When the electron returns to its unexcited state, light is emitted from the atom, increasing the intensity of beam 28.
A large amount of heat accumulates in the slab amplifier 12 due to the absorption process of light into the atoms of the slab 12 and the emission process of light from the atoms, so that a cooling source such as a water channel (not shown) is provided on the side wall. It is provided adjacent to 18 and 20.
U.S. patent application Ser. No. 08 / 148,758 discloses a zigzag amplifier with a diode pump source and a water cooled channel.

The heat generated by the lasing process and the cooling effect of the cooling source causes a temperature gradient across the slab amplifier 12. Due to this temperature gradient, there are different indices of refraction in various regions within the slab amplifier 12. The different indices of refraction affect the propagation characteristics of the light beam 28 within the slab amplifier 12 such that the wavefront of the beam 28 is not uniform, ie the wavefront is not flat. This means that different wavefront regions of beam 28 have different phases that cause beam aberrations. Zig-zag slab designers generally prefer parallel sidewalls 1 to be effective in reducing temperature gradients within the slab amplifier 12.
Try to bring 8, 20 closer together. This design attempt has had some success in reducing temperature gradients. However, for high output and high precision applications,
Such temperature gradients cause unacceptable wavefront non-uniformities. This design of the zigzag slab amplifier eliminates wavefront aberrations in one direction as a result of the zigzag path of the light beam 28. More specifically, because the optical path of beam 28 intersects back and forth between sidewalls 18, 20, there is an averaging effect created by beam 28 passing through the expanded region of slab amplifier 12. This averaging effect is
8 to correct the wavefront aberration in the direction of the zigzag optical path.

In the example shown in FIG. 1, the wavefront aberration is corrected in the X direction. However, there is still wavefront non-uniformity in the direction orthogonal to the zigzag path of beam 28 (ie, the Y direction). Aberrations in this direction are not corrected even by the zigzag averaging effect. Thus, the beam 30 still has wavefront non-uniformity in this direction, which has an unacceptable effect on beam quality for many applications. The zigzag slab amplifier 12 shown in FIGS. 1 and 2 is a slab amplifier that amplifies the light beam 26 introduced into the amplifier 12. The slab amplifier 12 may be a zigzag slab resonator or oscillator that produces a laser beam. As an example of a resonator, the proximal end face 14 is wholly or partially mirrored and the distal end face 1 is
6 is partially mirrored. The light beam 28 is reflected back and forth between the end faces 14, 16 along the zigzag optical path of the beam 28 as the light is amplified by the lasing process. As a modification, the end face 1 is provided outside the slab amplifier 12.
Mirrors may be placed adjacent 4 and 16 to reflect the light beam 28 back and forth within the amplifier 12. Once the beam intensity reaches a certain level, the light beam 30 will emerge from the partially mirrored end face 16. Whether the zigzag slab amplifier 12 is used as a resonator or as an amplifier does not affect the scope of the invention, as will be described later.

In accordance with an embodiment of the present invention, a one-dimensional aberration corrector 32 is provided for correcting non-uniformity of the wavefront of beam 30 in one direction (Y direction in this example). The beam 30 is introduced into a corrector 32, which emits a beam 34 which is uniform in both the X and Y directions and thus has no phase aberration. Since the corrector 32 only needs to correct one-dimensional aberration, the corrector 32
Two-dimensional adaptive optical element (t
It is a very simple device compared to wo-dimensional adaptive optics. In the illustrated example, the one-dimensional aberration corrector 32 is
A liquid crystal device having a liquid crystal cell 36 in a linear array. The cells 36 in a linear array are arranged as shown, that is, the cells 36 are stacked in the Y direction and each cell 36 is continuous in the X direction. If desired, electrical control 38
Provides an electrical control signal to cell 36 to electrically adjust the index of refraction of cell 36. By selectively changing the index of refraction of different regions of the corrector 32 so as to smooth the wavefront of the beam, different parts of the light beam 30 are made independent of other parts of the light beam. Be changed. The operation of electrically controlling the refractive index of various regions of a liquid crystal display is well known in the art. By selectively examining the output beam 34 for wavefront homogeneity by suitable optics such as an interferometer (not shown), the index of refraction of the individual cells 36 is selectively controlled in the Y direction. Beam 3
The uniformity of the 0 wavefront can be adjusted. If the slab amplifier 12 is a zig-zag slab resonator, the compensator 32 can be located either inside or outside the resonator.

The one-dimensional aberration corrector 32 may be any device suitable for selectively varying various regions of the beam wavefront in a single direction. For example, the corrector 32 uses M
eadowlark Optics (L, Colorado)
ongmont) commercially available spatial light modulator (Sha
It may also be an addressable liquid crystal phase modulator such as peShifter ™. This spatial light modulator
By altering the characteristics of selected frequencies of light, the temporal profile of the light beam can be shaped and provides high resolution, high speed and low cost. Of course, other one-dimensional aberration correctors can be used for other applications. For example, mechanically deformable piezoelectric driven mirrors well known to those skilled in the art may be used. This type of deformable mirror results in low resolution, high speed and high cost. Also, other types of custom-made polishing compensators (custo
m ground corrector plates) can also be used.
3-5 show various embodiments of the present invention including a zigzag slab combined with a one-dimensional corrector. FIG. 3 is a plan view showing a laser 40 having an oscillator 42 and a one-dimensional aberration corrector 44 according to the present invention. The oscillator 42 is a zigzag slab resonator in the form of a slab amplifier 12,
Laser beam 46 having wavefront phase aberration in only one direction
In order to generate, the end face of this resonator can be mirrored or a zigzag slab resonator with another external mirror can be provided. In this embodiment, the corrector 44 is
The aberration in the uncorrected direction is corrected to generate the corrected laser beam 48.

FIG. 4 shows another embodiment of a laser 50 including an oscillator 52, an amplifier 54 and a one-dimensional aberration corrector 56 of the present invention. The amplifier 54 may be provided with a zigzag slab amplifier in the form of the slab amplifier 12. In this embodiment, the oscillator 52 may be of the same type as the oscillator 42, and the amplifier 54 may be configured to provide additional optical amplification for high power output. The zigzag slab resonator in oscillator 52 and the zigzag slab amplifier in amplifier 54 correct for coplanar wavefront aberrations. Therefore, the optical laser beam 58 emitted from the amplifier 54 has an uncorrected unidirectional aberration. One-dimensional corrector 56
Corrects the aberrations in this direction to obtain the corrected beam 60
Occurs. FIG. 5 is a plan view showing a laser 62 with an oscillator 64 in the form of the oscillators 42, 52, an amplifier 66 in the form of an amplifier 54, and a one-dimensional corrector 68 of the present invention. The laser 62 folds a fold optic that reverses the optical path of the uncorrected beam 72 emitted from the amplifier 66.
ng optics) 70, which returns the light beam to the amplifier 66 for further amplification. In this example, the light beam passes through the corrector 68 twice to produce a corrected beam 74. In this embodiment, the corrector 68 is placed in various positions, including the illustrated position 76, to provide the uncorrected beam 72.
Can be corrected.

The above description merely discloses an exemplary embodiment of the present invention. Various modifications will readily occur to those skilled in the art from the above description, the accompanying drawings and the appended claims without departing from the spirit and scope of the present invention.

[Brief description of the drawings]

FIG. 1 is a perspective view showing a zigzag slab laser including a one-dimensional aberration corrector according to an embodiment of the present invention.

FIG. 2 is a plan view showing a zigzag slab of the zigzag slab laser of FIG.

FIG. 3 is a plan view showing a laser having a one-dimensional aberration corrector according to another embodiment of the present invention.

FIG. 4 is a plan view showing a laser having a one-dimensional aberration corrector according to another embodiment of the present invention.

FIG. 5 is a plan view showing a laser having a one-dimensional aberration corrector according to still another embodiment of the present invention.

[Explanation of symbols]

 10 Zigzag Slab Laser 12 Zigzag Slab Amplifier 26 Optical Beam 28 Zigzag Beam 30 Amplified Optical Beam 32 One-Dimensional Aberration Corrector 36 Liquid Crystal Cell 38 Electric Control Device 42 Oscillator 54 Amplifier 70 Folding Optical Element

Claims (18)

[Claims] 1. A primary device responsive to a light beam from the optical amplification device, the optical amplification device producing a light beam having a substantially uniform beam wavefront in a first direction. A laser comprising a source corrector, the one-dimensional corrector correcting for non-uniformity of a wavefront of a light beam in a second direction substantially perpendicular to the first direction. 2. The optical amplification device comprises a zigzag slab with two parallel side walls closely spaced from each other,
The laser of claim 1, wherein the light beam also propagates through a zig-zag slab and is reflected back and forth between the sidewalls during amplification before being emitted from the slab. 3. The one-dimensional corrector is a spatial light modulator having a linear cell array, and the spatial light modulator comprises a cell for correcting non-uniformity of the wavefront in the second direction. The laser of claim 1, comprising means for selectively controlling the index of refraction. 4. The laser according to claim 1, wherein the one-dimensional corrector is selected from the group consisting of a liquid crystal light modulator, a deformable mirror and a correction plate. 5. The laser according to claim 2, wherein the slab forms part of an optical cavity. 6. The laser of claim 5, wherein the one-dimensional corrector is located in a cavity. 7. The laser of claim 5, wherein the one-dimensional corrector is located outside the resonator. 8. The optical amplifying device comprises an yttrium-aluminum-garnet (YAG) crystal.
A laser according to claim 1. 9. The optical amplifying device is an oscillator, the oscillator comprising a first zigzag slab with two parallel sidewalls, the light beam propagating through the first zigzag slab. The laser of claim 1, wherein the laser is reflected back and forth between the side walls in an optical path, and the first direction is a zigzag optical path direction. 10. A zigzag slab amplifier further comprising a zigzag slab amplifier responsive to a light beam from an oscillator before the one-dimensional corrector responds to the light beam, the zigzag slab amplifier comprising two parallel beams. A second zig-zag slab with a side wall, the light beam propagating through the second zig-zag slab and reflected from the side wall of the second zig-zag slab in a zig-zag optical path, and the light beam in the first zig-zag slab. 10. The laser of claim 9, wherein the zigzag optical path of is in the same direction as the zigzag optical path of the light beam in the second zigzag slab. 11. A folding optical element further comprising reversing the direction of the light beam after the light beam has passed through the one-dimensional corrector, wherein the folding optical element directs the light beam to the primary beam. The laser of claim 10, returning through the original corrector and the zigzag slab amplifier. 12. A zigzag slab amplifier further comprising a zigzag slab amplifier responsive to the light beam after the light beam propagates through the one-dimensional corrector, the zigzag slab amplifier having two parallel sidewalls. A second zigzag slab provided, wherein a light beam propagates through the second zigzag slab and is reflected in a zigzag light path from a sidewall of the second zigzag slab, and a zigzag light path of the light beam in the first zigzag slab. Are in the same direction as the zigzag optical path of the light beam at the second zigzag slab, and the laser further comprises a folding optical element, which folds the optical beam of the light beam after passing through the slab amplifier. 10. The laser of claim 9, which reverses direction and causes the folding optical element to return a light beam through the amplifier and the one-dimensional corrector. 13. A zigzag slab made of a light amplifying material, the slab having a first end face, a second end face, a first sidewall and a second sidewall, the first and second sidewalls being substantially Are parallel and the first end face is responsive to a light beam introduced into the slab, the light beam propagating through the zigzag slab and before being emitted as an amplified light beam from the second end face of the slab. Reflected back and forth between the first side wall and the second side wall in the zigzag path, and the amplified light beam has a substantial wavefront homogeneity in the direction of the zigzag path and is amplified from the zigzag slab. A one-dimensional corrector responsive to the modulated light beam, the one-dimensional corrector of the amplified light beam in a direction orthogonal to the zigzag path for producing a light beam with wavefront uniformity. Compensates for non-uniformity of wavefront Zig-zag slab amplifier. 14. The one-dimensional corrector is a spatial light modulator having cells in a linear array, the spatial light modulator for correcting non-uniformity of a wavefront in a direction perpendicular to a zigzag optical path. The amplifier of claim 13, further comprising means for selectively controlling the index of refraction of the cell. 15. The one-dimensional corrector is a liquid crystal light modulator,
Selected from the group consisting of deformable mirrors and correction plates,
The amplifier according to claim 13. 16. Providing a zigzag slab with two parallel sidewalls spaced apart from each other, and propagating a light beam through the zigzag slab so as to reflect back and forth from the sidewalls of the zigzag slab. A step of emitting a light beam from the zigzag slab so that the light beam has a substantially uniform beam wavefront in the zigzag light path direction; a step of providing a one-dimensional aberration corrector; and a zigzag light path direction. Guiding the light beam from the zigzag slab to the one-dimensional corrector to correct the uniformity of the wavefront of the light beam in the orthogonal direction. 17. The step of preparing the one-dimensional aberration corrector comprises preparing a spatial light modulator having cells in a linear array, wherein the spatial light modulator has a direction perpendicular to a zigzag optical path. 17. The method of claim 16, wherein the indices of refraction of the different cells are separately and selectively controlled to correct the uniformity of the wavefront at. 18. The step of preparing the zigzag slab comprises preparing an optical resonator provided with the zigzag slab, and the step of preparing the one-dimensional corrector corrects the resonator one-dimensionally. Consists of providing a vessel,
The method of claim 16.