JP2003344883A - Sbs reflection mirror and high-repetition-pulse laser system using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To output a pulse laser beam which has high efficiency and high mean power with high stability. <P>SOLUTION: The pulse laser beam oscillated by an oscillation part 1 is amplified by a preamplifier part 2 into an output to have a saturation gain at a main amplification part 7, and the output is made incident on a beam shaping part 4 through an optical isolator 3. The incident laser beam is magnified by a beam magnifier 11 of the beam shaping part and shaped by an apodizer 12 into a flat beam profile. A 90° rotator 15 makes polarizing directions of the beams passing through main amplifiers 14 and 16 of the main amplification part orthogonal. A multi-SBS cell constituting a phase conjugate mirror 8 rotates a laser beam incident on a plurality of SBS (stimulated Brillouin scattering) cells by a rotary prism while synchronizing its speed with the repetition of the pulse laser beam. The polarizing directions of the main amplification parts are made orthogonal by a Faraday rotator 18 and an outgoing beam output is taken out by a polarizer 6. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention generally transmits a laser beam and reflects an incident laser beam when the energy of the laser beam reaches a threshold value.
in scattering (Stimulated Brillouin scattering) cell SBS reflector and high repetition pulse laser system using the same, in particular, SBS reflection capable of obtaining a highly stable reflected wavefront when operating at high average power The present invention relates to a mirror and a high repetition pulse laser system using the same.

[0002]

2. Description of the Related Art Conventionally, one SBS cell is used as a phase conjugate reflecting mirror in a high repetition pulse laser system using this type of SBS reflecting mirror.
There is one disclosed in Japanese Patent No. 63262.

For example, as shown in FIG. 5, a repetitive pulse laser beam generated by a pulse laser oscillator 101 is amplified by an optical amplifier 102, passes through a polarizer 103, and enters a quarter wavelength plate 104. Quarter-wave plate 10
The outgoing beam of 4 is condensed by the condenser lens 105,
It is incident on the phase conjugate mirror 106 of the SBS cell.

The SBS is usually a laser beam that is transmitted, and at the moment when a certain threshold value is reached, most of the incident laser beam is rapidly and completely internally and with the same polarization plane. This is a phenomenon of reflection. SBS
The cell is a substance that causes such a phenomenon, namely SBS.
A medium, such as carbon disulfide (CS ₂ ), SB
If the S medium is a fluid, it is contained in a transparent container such as glass. That is, the SBS cell forms a phase conjugate mirror 106 in which the incident beam returns in exactly the same optical path.

The quarter-wave plate 104 rotates the polarization direction by 90 degrees by reciprocating the laser beam. Further, the polarizer 103 transmits while limiting the polarization direction of the laser beam sent from the pulse laser oscillator 101, while it is reflected by the phase conjugate mirror 106 of the SBS cell and the polarization direction is rotated by 90 degrees and returned. Reflect the beam off the optical axis.

The laser beam reflected by the polarizer 103 is
The light is reflected by the reflecting mirror 107 of the light guiding means, enters the optical amplifier 108, is amplified by the optical amplifier 108, and is output as a beam to the outside.

[0007]

DISCLOSURE OF THE INVENTION The conventional SBS described above
In a high repetition pulsed laser system using a reflector, S
One SBS cell is used as a BS reflector.

The SBS cell operates stably as the average power of the incident laser beam is lower than the value determined by the absorption limit to the medium and the pulse energy is large enough to make the reflectance high. Therefore, the laser pulse repetition cannot be greater than the average power divided by the pulse energy, which limits the laser pulse repetition.

To avoid this limitation, one SBS
There is a method of relaxing the thermal constraint by moving the focused spot position in the cell with a rotating prism. However, as long as the cross-sectional area of the cell is not increased, the repetition is limited depending on the circumferential distance of the condensing point that can be moved by the rotating prism, the condensing diameter and the thermal relaxation of the medium.

That is, in the high repetition pulse laser system using one SBS cell as described above, there is a problem that the average power corresponding to the repetition frequency and the absorption of the SBS medium are limited.

An object of the present invention is to solve the above problems and provide an SBS reflector capable of providing a pulse laser beam having high efficiency and high average power, and a high repetition pulse laser system using the SBS reflector. It is to be.

[0012]

In the high repetition pulse laser system using the SBS reflector according to the present invention, the SBS
A plurality of SBS cells are used as the reflecting mirror. For this reason, the prism is provided with an SBS cell that forms one optical axis that is incident / reflected by the SBS reflecting mirror into a plurality of optical axes, and that receives and reflects the laser beam one by one at the tip of each of these optical axes.

As a result, the power of one SBS cell and the absorption capacity of the medium can be increased multiple times, so that a pulsed laser system having high efficiency and high average power can be provided.

The plurality of SBS cells can be easily arranged by arranging their optical axes in parallel. In such an arrangement, the folding mirrors should be attached to each SBS.
Prepare for the cell. The folding mirror refracts each of the plurality of beams formed by the prism into parallel beams corresponding to each SBS cell.

It is also desirable from the standpoint of forming the folding mirror that the plurality of SBS cells are arranged so that the incident surface of the laser beam is annular with respect to the optical axis incident on the SBS reflecting mirror. Therefore, the prism is formed by forming the optical axis of the SBS reflecting mirror into a plurality of optical axes formed on the conical surface.
The reflected wave is emitted to the BS cell.

The prism in this structure is preferably a rotating wedge prism that rotates so as to sequentially connect a plurality of optical axes to each SBS cell in synchronization with the repeating pulsed laser beam that is incident and reflected. Further multiple SBSs
Forming the cells at equal intervals and the prisms corresponding to the SBS cells forming the optical axis on the conical surface at evenly spaced angles is useful for using repetitive pulse and rotating prisms.

As a result, all the energy of the laser beam incident on the SBS reflector can be supplied to each SBS cell.

Further, since the plurality of SBS cells are composed of a plurality of different media, it is possible to provide an SBS reflector capable of multiwavelength oscillation in which the wavelength of each pulse can be controlled.

A specific high repetition pulse laser system using the SBS reflecting mirror according to the present invention comprises an oscillating section, a preamplifying section, a main amplifying section, a polarizer, and a phase conjugate mirror. The oscillator oscillates a high repetition laser pulse train and emits it as a beam. The preamplifier section receives the beam emitted from the oscillating section, amplifies it to a predetermined required energy, and emits it. The main amplification section forms a round-trip path in which the output beam of the pre-amplification section is received at one end, amplified, emitted from the other end, and the reflected wave of the emitted beam enters from the other end and exits from the one end. And amplifies to the saturation amplification level. The polarizer transmits the outgoing beam of the preamplifier section and makes it incident on one end of the main amplifying section, and reflects the reflected wave emitted from the one end to output the beam to the outside. The phase conjugate mirror is a reflecting mirror having a multi-SBS cell structure having a plurality of SBS cells described above, and reflects a beam emitted from the other end of the main amplification unit to the other end. In addition, a reflector having a multi-SBS structure has a plurality of SBs for each laser pulse that is input and reflected.
The S cells are sequentially switched one by one.

The main amplifying section is inserted and arranged between the two main amplifiers and corrects depolarization between the two main amplifiers.
At least one set of serial arrangements with a 0 degree rotator,
A divergence angle adjuster that corrects the beam after amplification into parallel light is provided at the other end, and a Faraday rotator is provided between the main amplification unit and the phase conjugate mirror to make the polarization direction of the reflected beam incident. It is orthogonal to the beam by 90 degrees.

Further, it is preferable that the high repetition pulse laser system further includes a beam expander, an apodizer, and a spatial filter as a beam shaping unit. The beam expander receives the outgoing beam from the preamplifier unit and expands the beam diameter. The apodizer flattens the profile of the beam expanded by the beam expander and outputs it. The spatial filter maintains the beam profile emitted from the apodizer and transfers it to the main amplification unit. As a result, pulse efficiency can be improved and average power can be increased.

[0022]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a functional block diagram showing an embodiment of the present invention. In the high repetition pulse laser system using the SBS reflector shown in FIG. 1, a front-end oscillator (hereinafter abbreviated as an oscillator) 1, a CW excitation preamplifier (hereinafter abbreviated as a preamplifier) 2, an optical An isolator 3, a beam shaping unit 4, an optical system 5, a polarizer 6, a main amplification unit 7, and a phase conjugate mirror 8 are provided.

The oscillator 1 oscillates a high-quality, high-repetition laser pulse train. The preamplifier section 2 excites the continuous wave (CW) output of the oscillating section 1 and amplifies it so that the main amplifying section 7 can take out sufficient energy. The optical isolator 3 is provided for the purpose of preventing the return beam from the main amplification unit 7 from going back to the oscillation unit 1.

The beam shaping section 4 includes a beam expander 11 for expanding the beam diameter, and an apodizer 1 using, for example, a serrated aperture for flattening the beam profile.
2 and the spatial filter 13 that transfers the beam profile at the position of the apodizer 12 to the main amplification unit 7. Details will be described later with reference to FIG.

The optical system 5, a part of which is shown in the figure, is a component necessary for propagating a laser beam, and is composed of a reflecting mirror, a prism and the like. The polarizer 6 saccharifies the beam amplified by the preamplifier unit 2 and shaped by the beam shaping unit 4 and outputs the saccharified beam to the main amplifying unit 7, while reflecting the reflected wave from the phase conjugating mirror 8 incident from the main amplifying unit 7. The amplified beam is output to the outside.

The main amplifier 7 is provided between the CW pump main amplifiers (hereinafter abbreviated as main amplifiers) 14 and 16 and the main amplifiers 14 and 16 for amplifying a high repetition pulse train at a saturation amplification level. It is composed of a 90-degree rotator 15 for correcting depolarization between 16 and a divergence angle adjuster 17 for correcting the converging property of the beam after amplification and making it parallel light. The main amplifying section 7 shown in the figure has one set of the main amplifiers 14 and 16 and the 90-degree rotator 15, but a plurality of sets can be arranged in series to increase the amplification degree.

The phase conjugating mirror 8 is composed of an SBS reflecting mirror having a multi SBS cell structure for reflecting the laser beam in order to realize double pass amplification in the main amplifying section 7.
The Faraday rotator 18 makes the polarization direction of the laser beam reflected by such an SBS reflector orthogonal to 90 degrees.

Next, a multi-SBS reflector system in which the phase conjugate mirror 8 in FIG. 1 is formed by multi-SBS cells will be described with reference to FIGS.

The illustrated multi-SBS reflector system is composed of a rotating wedge prism (hereinafter referred to as a rotating prism) 21, a folding mirror 22, a condenser lens 23, and an SBS cell 24.

The rotating prism 21 makes the optical axes of the laser beams incident and reflected by the phase conjugate mirror 8 incident on the respective incident surfaces of a plurality of SBS cells 24 which are annularly arranged at equal intervals.
In addition, they are formed on the optical axis of the laser beam reflected from each and the direction is changed. The folding mirror 22 includes a plurality of SBSs.
The rotating prism 2 is provided corresponding to each cell 24.
The laser beam between 1 and the SBS cell 24 is reflected to direct the laser beam. Multiple condenser lenses 23
Each is provided in the vicinity of each SBS cell 24 and focuses the laser beam on the SBS cell 24. Single SBS
The cell 24 has a cylindrical shape filled with a liquid medium such as a fluorine-based inert liquid such as fluorocarbon that acts as a phase conjugate mirror.

Each of the plurality of SBS cells 24 shown in the drawing is arranged in an annular shape with their central axes parallel to each other, and has a configuration in which the overall appearance is cylindrical. Therefore, the same number of condenser lenses 23 as the SBS cells 24 form a ring-shaped surface in the vicinity of one end surface of the SBS cells 24, and the corresponding folding mirror 22.
A laser beam parallel to each is formed. The folding mirrors 22 of the same number as the condenser lens 23 form a conical optical axis so that the parallel laser beams are condensed at one point by the rotation of the rotating prism 21 on the surface that has passed through the rotating prism 21. Therefore, the folding mirror 22 is
It is arranged in a shape like an opening of a funnel opened to the S cell 24 side.

The rotating prism 21 rotates in synchronization with the pulse of the pulse laser beam to sequentially deliver the pulse of the pulse laser beam incident on the annular multi SBS cell to each SBS cell 24. Therefore, the total energy of the pulsed laser beam incident on the rotating prism 21 is sequentially distributed to and reflected by the single SBS cell 24.

In the above embodiment, by using the rotating prism, in order to simplify the rotation control and the structure of the multi-SBS cell and its peripheral elements, the annular structure and the arrangement of the folding mirror are arranged by the parallel arrangement of the SBS cells. Although shown and described, there is no need to use a folding mirror when the optical axis of each SBS cell is aligned with each of the multiple optical axes formed by the rotating prism.

Further, as long as the prism can form a plurality of optical axes corresponding to the optical axes of the laser beams that are incident / reflected as a phase conjugate mirror, the optical axes are incident / reflected on a plurality of SBS cells. It is not necessary to form the SBS cell of the above.

Next, referring also to FIGS. 1 to 4,
The operation function of the system shown in FIG. 1 will be described.

The laser beam output having a high beam quality and a high repetition laser pulse train oscillated by the oscillating unit 1 shown in FIG. 1 is amplified by the preamplifier unit 2 to an output such that the main amplifying unit 7 has a saturation gain. . At this time, the upper level life of the laser medium used in the preamplifier unit 2 is T
In the case of seconds, if the repetition of the pulsed laser beam to be amplified is set higher than “1 / T” pps, the energy stored in the upper level of the preamplifier section 2 can be efficiently extracted.

Since the main amplifiers 14 and 16 are used as double-pass amplifiers in the main amplifying section 7, there is a possibility that the amplifiers may be coupled with each other to cause parasitic oscillation, or the return light may cause return oscillation.
May disturb the performance of. In order to prevent this, the laser beam emitted from the preamplifier unit 2 enters the beam shaping unit 4 via the optical isolator 3.

The laser beam “A” incident on the beam shaping unit 4 is expanded by the beam expander 11 to be a laser beam “B” as shown in FIG. Then, the apodizer 12 shapes the spatial distribution of the laser beam “B” so as to obtain a flat beam profile in the central portion and generates a laser beam “C”. The spatial filter 13 and the optical system 5 are arranged so that this beam profile is imaged at the entrance of the main amplification unit 7. The high repetition pulse laser train is amplified by the main amplifiers 14 and 16 of the main amplifier 7. This amplification is similar to that of the preamplifier unit 2, and the main amplifier 14,
When the upper level life of the laser medium used in 16 is T seconds, the repetition of the pulsed laser beam for amplification is "1.
If it is higher than / T "pps, the energy stored in the upper level of the main amplifiers 14 and 16 can be efficiently extracted. Moreover, since the spatial beam profile is flat, the volume utilization efficiency of the laser medium can be increased.

The laser medium is YAG (Yttrium A)
When thermal depolarization such as that of a luminum garnet crystal occurs, depolarization can be canceled by making the polarization directions passing through the respective main amplifiers 14 and 16 orthogonal by the 90 ° rotator 15. Further, the thermal distortion of the laser medium due to the CW excitation is reflected by the phase conjugate mirror 8 composed of the multi-SBS cell while preserving the incident wavefront, and the laser medium that has been subjected to the same thermal distortion is caused to go backward. It is canceled by.
The polarization directions after double-pass amplification in the main amplification section 7 are made orthogonal by the Faraday rotator 18, and the beam output to the outside is taken out by the polarizer 6. Multi S shown in FIG.
The BS reflector system has SBs arranged at a plurality of equal intervals.
The rotating prism 21 receives the laser beam incident on the S cell 24.
, And the speed is synchronized with the repetition of the pulsed laser beam.

The direction of the beam is corrected by the folding mirror 22 and is condensed in the SBS cell 24 by the condenser lens 23. The medium in the SBS cell 24 acts as a phase conjugate mirror, stores the incident wavefront, and travels in the opposite direction. SBS
In the case of a multi SBS cell system in which the number of cells 24 is m, the incident laser power shared by each SBS cell 24 can be reduced to “1 / m”, and the SBS cells 24 become thermally unstable. The average output can be extended m times.

In the above description, the oscillator is the front-end oscillator, but a pulse laser oscillator may be used, and the amplifier is a CW pump amplifier, but a pulse pump amplifier may be used. The number of amplifiers and the location of the amplifiers are not limited to the above description. Further, the medium used for the SBS cell is assumed to be a fluorocarbon such as “C ₆ F ₁₄ ” or “C ₈ F ₁₈ mixed with C ₈ F ₁₆ O at a ratio of 1/5”. Although a liquid medium such as an inert liquid is used, carbon disulfide (CS ₂ ) or tetrachloride (eg TiCl 4, S
Any material can be used as long as it is a substance that causes SBS, such as a liquid made of nCl ₄ or CCl ₄ ), a gas such as CH ₄ or SF ₅ , or a solid such as quartz.

In the above description, the high repetition pulse laser system has been described, but the present invention can be applied to all laser systems.

As described above, in the above description, the functional blocks shown in the drawings are referred to, but the distribution by dividing and merging functions, the plurality of functional blocks and the locations where the functional blocks are disposed, the shapes of constituent elements and the arrangement thereof are changed. Is free as long as the above function is satisfied, and the above description does not limit the present invention.

[0045]

As described above, according to the present invention, the pulse laser beam is distributed to the plurality of SBS cells, and the pulses are sequentially moved. Therefore, the following effects can be obtained. Can be expected.

The first effect is that the average output of the high repetition pulse laser can be extended in proportion to the number of multi SBS cells.

That is, when a liquid medium such as a fluorine-based inactive liquid such as fluorocarbon is used as the medium of the SBS cell, the average power of the incident laser is lower than the value determined by the absorption limit of the medium, and the pulse power is higher. If the energy is large enough to make the reflectance high, the stable operation is achieved. Therefore, the repetition cannot be greater than the average power divided by the pulse energy, which limits the repetition of the laser. In order to avoid this limitation, there is a method of moving the focused spot position by a rotating prism in one SBS cell to relax the thermal constraint.
Unless the cell cross-sectional area is increased, the repetition is limited by the circumferential distance of the converging point and the condensing diameter that can be moved by the rotating prism, and the thermal relaxation of the medium. However, according to the present invention, there is no such concern. Therefore, a pulsed laser beam having high efficiency and high average power can be output with high stability.

The second effect is that the load per SBS cell can be reduced. That is, deterioration of the medium can be suppressed, long life can be expected, and maintainability is improved.
Only deteriorated cells can be replaced.

The third effect is that multiwavelength oscillation can be achieved by changing the medium of SBS for each cell. Therefore, the wavelength of each pulse can be controlled, and it can be utilized as a wavelength tunable laser.

[Brief description of drawings]

FIG. 1 is a functional block diagram showing an embodiment of the present invention.

FIG. 2 is a perspective view showing an embodiment of the phase conjugate mirror in FIG.

FIG. 3 is a component correspondence diagram corresponding to one optical axis in FIG.

FIG. 4 is a diagram illustrating beam shaping in FIG.

FIG. 5 is a functional block diagram showing a conventional example.

[Explanation of symbols]

1 Front-end oscillator (oscillator) 2 CW excitation preamplifier section (preamplifier section) 3 Optical isolator 4 Beam shaping section 5 Optical system 6 Polarizer 7 Main amplification section 8 Phase conjugate mirror 11 beam expander 12 Apotizer 13 Spatial filter 14, 16 CW pump main amplifier (main amplifier) 15 90 degree rotator 17 Spread angle adjuster 18 Faraday Rotator 21 Rotating wedge prism (rotating prism) 22 folding mirror 23 Condensing lens 24 SBS cells

Continued front page    (71) Applicant 502176786             Masanori Yamanaka             25-E205, Ishimaru 3-chome, Minoh City, Osaka Prefecture (71) Applicant 502194230             Eiji Yoshida             4-20-12 Hiyoshidai, Takatsuki City, Osaka Prefecture (72) Inventor Hiroyuki Kubomura             5-7 Shiba 5-1, Minato-ku, Tokyo NEC Corporation             Inside the company (72) Inventor Yasukazu Izawa             2-8-417, Hashiuchi, Ibaraki, Osaka (72) Inventor Masahiro Nakatsuka             1425-78 Midorigaoka, Ikoma City, Nara Prefecture (72) Inventor Masanori Yamanaka             25-E205, Ishimaru 3-chome, Minoh City, Osaka Prefecture (72) Inventor Eiji Yoshida             4-20-12 Hiyoshidai, Takatsuki City, Osaka Prefecture F-term (reference) 2K002 AB27 AB31 AB40 CA17 HA24                 5F072 HH07 JJ07 KK06 QQ06 QQ11                       SS06

Claims (10)

[Claims] 1. An SBS (Simulated Bri)
a reflection mirror using a stimulated Brillouin scattering cell, the prism forming one first optical axis into a plurality of second optical axes that enter and reflect the reflection mirror, and the second light. An SBS reflecting mirror having a multi-SBS structure including a plurality of SBS cells which are provided corresponding to respective axes and which reflect and reflect a laser beam by the second optical axis. 2. The plurality of SBS cells arranged with their optical axes parallel to each other, and the third parallel light provided corresponding to each of the SBS cells and reflecting / reflecting a laser beam to / from the SBS cells. An SBS reflecting mirror, comprising: a folding mirror that reflects a laser beam that forms an axis to form the second optical axis. 3. The plurality of SBS cells according to claim 1, wherein the incident surface of the laser beam is arranged in an annular shape with respect to the first optical axis, and the prism has the second optical axis. Rotation that is formed on a conical surface and that rotates so as to sequentially connect a plurality of second optical axes to the first optical axis in synchronization with the repeated pulsed laser beam that is reflected and reflected by the first optical axis. SB characterized by being a wedge prism
S reflector. 4. The SBS reflector according to claim 3, wherein the plurality of SBS cells are composed of a plurality of SBS cells of different media. 5. The plurality of SBS cells according to claim 3, wherein the plurality of SBS cells are arranged in an annular shape at equal intervals, and the prism forms the second optical axis on the conical surface at equal intervals. An SBS reflector. 6. A main amplifier which forms a oscillated high repetition laser pulse train into a laser beam, amplifies the laser beam to a required energy, transmits it through a polarizer, and then reflects it by a phase conjugate mirror to form a round trip path therebetween. A reflected wave amplified to a saturation amplification level by a section is taken out from the polarizer as an output, and the phase conjugate mirror is an SBS (Stimul).
aged Brillouin Scatterin
g: stimulated Brillouin scattering) A high-repetition pulse laser system using a plurality of cells and having a multi-SBS cell structure in which the SBS cells are sequentially switched for each incoming / reflecting laser pulse. 7. The SB according to claim 6, wherein the plurality of SBS cells of the phase conjugate mirror have a plurality of different media.
A high-repetition pulse laser system characterized by being composed of S cells. 8. An oscillating section that oscillates a high repetition laser pulse train and emits it as a beam, a preamplifier section that receives the outgoing beam of the oscillating section, amplifies and emits the energy up to a predetermined required energy, and the preamplifier at one end. And outputs the reflected wave of the emitted beam from the other end to form a reciprocal path in which the reflected wave of the emitted beam enters from the other end and exits from the one end to be amplified to a saturation amplification level. A main amplifying unit, and a polarizer that transmits the outgoing beam of the preamplifying unit, enters the one end of the main amplifying unit, and reflects the reflected wave emitted from the one end to output the beam to the outside. A phase conjugate mirror that reflects a beam emitted from the other end of the main amplification unit to the other end, wherein the phase conjugate mirror is an SBS (Stimulate).
d Brillouin Scattering (Stimulated Brillouin Scattering) using a plurality of cells, the SBS cell is sequentially switched for each laser pulse to be reflected and reflected.
A highly repetitive pulse laser system characterized by being a reflector having a BS cell structure. 9. The structure according to claim 8, wherein the main amplification section is arranged in series with at least one pair of two main amplifiers and a 90-degree rotator that is provided between the main amplifiers and corrects depolarization between the two main amplifiers. And a divergence angle adjuster for correcting the beam after amplification into parallel light at the other end, and further incident the polarization direction of the reflected beam provided between the main amplification unit and the phase conjugate mirror. High-repetition-pulse laser system comprising a Faraday rotator that makes the beam 90 degrees orthogonal to each other. 10. The beam expander according to claim 8, which receives the beam emitted from the preamplifier unit and expands the beam diameter, and the apodizer which shapes the profile of the beam expanded by the beam expander to be flat and emits it. And a spatial filter for maintaining the beam profile emitted from the apodizer as it is and transferring it to the main amplification unit.