Coherent light generators – Particular resonant cavity – Specified cavity component
Reexamination Certificate
1999-11-04
2003-01-21
Ip, Paul (Department: 2828)
Coherent light generators
Particular resonant cavity
Specified cavity component
C372S093000, C372S105000, C372S107000, C359S334000
Reexamination Certificate
active
06510170
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a laser system consisting of an oscillator and laser amplifier, and using double or multiple passes in the amplifier for achieving efficient high energy amplification. More particularly, the present invention is concerned with systems and methods utilizing reflecting means including a phase conjugate mirror for achieving high energy and efficient multipassage amplification.
BACKGROUND OF THE INVENTION
High power lasers are of a design including a master oscillator and a power amplifier. Usually, in order to efficiently extract the power stored in the amplifier rod, two or more amplifying passes are needed. In such a system, the beam passes through the amplifier in two directions, and polarization output coupling is required for extracting the laser beam.
FIG. 1
(prior art) illustrates a laser system with an oscillator/amplifier and a phase conjugate mirror. This known system is arranged in a manner utilizing a faraday isolator rotator
2
placed between an oscillator
4
and amplifier
6
to divert the reflected beam from the oscillator. The laser beam from oscillator
4
passes through polarizer
8
, interposed between the oscillator
4
and isolator/rotator
2
as a linearly polarized beam toward the faraday isolator/rotator
2
. The faraday isolator/rotator
2
rotates the angle of polarization by 45°, and a second polarizer
10
, which is in position to pass the laser beam in the new direction of polarization, routes the beam toward amplifier
6
. The beam, amplified as it passes through the amplifier
6
, is circularly polarized by a quarter wave plate
12
and focused into a non-linear medium by a focusing lens
14
before entering a phase conjugate mirror
16
. The phase conjugate mirror
16
retroreflects the input laser beam back through the lens
14
and converts it back to linear polarization but rotated 90° with respect to incident polarization by the quarter wave plate
18
. The double amplified laser beam
18
which is the output of amplifier
6
is reflected by polarizer
10
out of the laser system. The fraction of the beam that has been affected by the amplifier birefringence will pass through the polarizer to the faraday isolator/rotator
2
, which rotates its linear polarization by 45°, and polarizer
8
reflects it out of the system in a different direction
20
from the direction of the output laser beam
18
.
When the direction of the beam is toward the oscillator
4
, there is a risk that a part of the laser beam will return and enter into the oscillator, thereby affecting the performance of the laser, or even damaging optical elements of the oscillator. This problem is even more acute when solid state lasers are used, as a result of thermal effects. At high average input powers, the non-uniform temperature distribution in the amplifier rod induces significant birefringence via thermally induced stresses. Such thermal birefringence can lead to strong depolarization of the laser radiation, so that a significant fraction of the amplified beam goes into the oscillator. In order to prevent same, an optical isolator must be inserted between the oscillator and the amplifier. The isolator diverts the fraction of the beam which has the wrong polarization away from the oscillator, but in a direction different from the original beam direction, resulting in reduction of the laser's efficiency.
Several solutions to the thermally induced problem of birefringence have been proposed over the years, but none has ever performed reliably enough, or been deemed sufficiently practical, to receive widespread acceptance in the solid-state laser community. An example of such a solution, called the “Scott & dewitt scheme,” is illustrated in FIG.
2
.
In this prior art scheme as shown in
FIG. 2
, the partially depolarized beam from the first amplifier
6
passes through a rotator
22
, which rotates the polarization of the beam by 90° and passes the rotated beam through the second amplifier
24
. To the extent that the two amplifier rods are identical, the birefringence induced by the rod of the first amplifier
6
is canceled by the rod of the second amplifier
24
. At the output of amplifier
24
, the beam is linearly polarized in a direction rotated by 90° from the output beam of the oscillator
4
. The beam polarization is made circularly by a quarter wave plate
12
, and enters the phase conjugate mirror
15
via focusing lens
14
. The phase conjugate mirror
16
retroreflects the input laser beam back through the lens
14
. The beam is turned back to linear polarization but rotated at 90° with respect to the incident polarization by quarter wave plate
12
and is amplified by the two amplifiers
24
and
6
. The induced birefringence is reduced in the same way as in the first pass. The twice amplified laser beam output from amplifier
6
is reflected by the polarizer
10
and exits the laser system as beam
18
after it has been polarization rotated by 90°.
The above-described scheme attempts to eliminate birefringence effects by placing a 90° rotator (plate
22
) between two identical laser rods of amplifiers
6
and
24
. The idea is that, to the extent that the two rods are identical, the birefringence induced by one rod is canceled by the other. However, the success of this solution varies in consideration of heating non-uniformities and other practical issues related to the requirement that the rods be identical; for example, if, for any reason, one of the amplifier's rods has to be changed, both rods must be changed at the same time, to assure identicality. In addition, this system requires the use of two amplifiers.
SUMMARY OF THE INVENTION
It is therefore a broad object of the present invention to ameliorate the disadvantages of the prior art systems and to provide a laser amplification system and method for efficiently effecting multiple pass amplification.
It is a further object of the present invention to provide a system and method for effecting an unidirectional, multiple pass amplification of a laser beam through one or more amplifiers.
In accordance with the present invention, there is therefore provided a laser system for producing a high energy amplified laser beam output from an oscillator producing a wave front of low energy laser beam, said system comprising at least one amplifier positioned to receive said low energy laser beam for amplification via a first polarizer; a second polarizer, positioned along the axis of the amplified beam at the output side of said amplifier, for allowing a first fraction of the beam to pass therethrough and for reflecting a second fraction of said beam from an output surface of said polarizer, said second fraction constituting the output of the system; a retroreflector, associated with a quarter wave plate, oriented to receive said first fraction and to reflect it back toward said second polarizer; reflecting means for reflecting said reflected first fraction toward said first polarizer to be reflected toward said amplifier for further amplification and to be reflected off the output surface of said polarizer together with said second fraction.
The invention further provides a laser system for producing a high energy amplified laser beam output from an oscillator producing a wave front of low energy laser beam, said system comprising at least a first and a second amplifiers disposed in non-axial relationship to each other, said first amplifier being positioned to receive said low energy laser beam for amplification via a first polarizer; reflecting means for reflecting the amplified beam of said first amplifier toward the input side of said second amplifier; a second polarizer, positioned along the axis of said second amplifier at the output side thereof, for allowing a first fraction of the beam to pass therethrough and for reflecting a second fraction of said beam from an output surface of said second polarizer, said second fraction constituting the output of the system; and a retroreflector associated with a quarter wave plate, oriented to receive sa
Ravnitzki Gad
Zafrani Nisim
ELOP Electro-Optic Industrial Ltd.
Jackson Cornelius H.
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