Optical: systems and elements – Optical amplifier – Multiple pass
Reexamination Certificate
2002-01-15
2004-03-02
Hellner, Mark (Department: 3663)
Optical: systems and elements
Optical amplifier
Multiple pass
C359S337000
Reexamination Certificate
active
06700698
ABSTRACT:
The present invention relates to optical amplifier systems and in particular to solid state and gas optical amplifier systems in which a light beam to be amplified is passed repeatedly through an amplifier medium in order to improve the gain of the amplifier system.
An optical amplifier is a basic component of a laser system and is used in almost every aspect of laser technology. Since lasers were first developed, optical amplifiers have been used to increase the output energies from laser oscillators. However, it is generally difficult to extract energy efficiently from an optical amplifier and efficient energy extraction usually requires the input of high energy light beams, so that the level of net gain is modest. This is a particular problem in amplifier systems which amplify a continuous wave light source.
In order to achieve both high gain and efficient energy extraction, cascades of progressively larger amplifiers have been used. However, this results in relatively expensive multi-component amplifier systems which can have complicated optical geometries in which it can be difficult to accurately align optical components making up the system.
An alternative way of improving gain and energy extraction is to pass a light beam two or more times through the amplifier medium of an amplifier system, optionally using non-linear phase conjugation to compensate for aberrations such as thermal distortions caused by pumping of the medium. A first example has been developed and is described by N. F. Andreev, S. V. Kuznetsov, O. V. Palashov, G. A. Pasmanik and E. A. Khazanov in “Four-pass YAG:Nd laser amplifier with compensation for aberration and polarisation distortions of the wavefront” in Soviet Journal of Quantum Electronics, Volume 22, pages 800-802 (1992). This document describes a system in which a beam is passed through an amplifier four times by using polarisation multiplexing and phase conjugation. This is the maximum number of passes that can be achieved by this approach because they exploit the two orthogonal polarisations and the two possible directions of collinear propagation and they separate beams by means of polarisers. A second example of such a system has been developed and is discussed in a paper by C. B. Dane, L. E. Zapata, W. A. Newman, M. A. Norton, L. A. Hackel entitled “Design and Operation of a 150 W Near Diffraction Limited Laser amplifier with SBS Wavefront Correction, published in IEEE Journal of Quantum Electronics Vol. 31, p148, January 1995. According to this system a light beam is repeatedly directed into an amplifier medium successively at different angles. The problem with this approach is that the optical geometry of the amplifier system is quite complicated and that the system is specific to pulsed laser systems and to a particular class of slab amplifier which can be used in a 9- and 11-bounce mode. Also, this type of amplifier system requires careful alignment of the optical components which make up the system.
WO95/22187 discloses an amplifier system in which a light beam is passed through an amplifier medium four times and which compensates for depolarisation of the light beam in the amplifier medium. The light from the amplifier medium is rotated in phase through 90° before being passed back into the amplifier medium. A relay imaging telescope is used in the arrangement for applying the phase rotation, to image the light beam from the amplifier through a polarisation rotator and onto a reflector and then back onto the amplifier medium. Oppositely directed beams overlap at all points along the path until they are separated by their polarisation which limits the number of passes which can be made through the amplifier medium without causing the system to oscillate. This limits the gain of the amplifier system disclosed in WO95/22187.
The present invention seeks to overcome some of the problems discussed above by providing a high gain multi-pass laser amplifier system in which a light beam can be repeatedly passed through the amplifier medium of the system using a simple geometry in which it is relatively easy to align the optical components of the system in order to achieve a high gain and efficient energy extraction. The present invention also seeks to provide an optical amplifier system which is generic in that it can be applied to a large number of amplifier mediums. The present invention also seeks to provide a multi-pass amplifier suitable for continuous wave (cw) applications as well as for pulsed operation.
According to the present invention there is provided a multi-pass optical amplifier system comprising:
an amplifier medium;
at least one relay imaging telescope for imaging light from the amplifier medium onto a primary light directing optical component and for imaging light directed back from a primary light directing optical component into the amplifier medium so that light is re-passed through the amplifier medium, and
a phase conjugate mirror arrangement for intercepting light between passes of the amplifier medium to generate a phase conjugate reflection of the light incident on it.
The use of phase conjugation in combination with relay imaging enables high gain operation with significant levels of energy extraction from the amplifier medium. The phase conjugate mirror is preferably located so that light enters it after having passed through the amplifier medium some predetermined number of times. It produces a beam which is conjugate in phase to the beam incident on it, and this conjugate beam retraces the path of the incident beam back through the system to the input point. At this point it has high energy and a spatial phase which is conjugate to the phase of the input beam, so that the high energy beam is of good optical quality. For a phase conjugate mirror, such as a stimulated Brouillon scattering (SBS) cell, which has a threshold power for the input light below which no conjugate beam is generated, the phase conjugate mirror will act as a shutter or an isolator. Thus, gain reduction due to the effects of low power amplified stimulated emissions (ASEs) generated within the system which have an amplitude below the threshold of the phase conjugate mirror is prevented. For pulsed operation of the system the phase conjugate mirror acts as a shutter that only opens when the input beam arrives. This is because the threshold of the phase conjugate mirror is only exceeded when the light pulse is incident on it. In the case of a continuous wave (cw) input beam, the phase conjugate mirror acts as a spectral filter, so that only light in the same spectral bandwidth as the input beam is reflected and so as such it also acts as an inhibitor of ASEs. This is ause only light in the spectral bandwidth of the input beam will exceed the threshold of the phase conjugate mirror.
Preferably, the system is arranged such that a light beam from the amplifier medium which is re-imaged onto the amplifier medium is spatially separated from itself, ie. it does not overlap itself, at at least one point in its path between passes of the amplifier medium to enable it to be intercepted by at least one additional light directing optical component.
The present invention also provides a multi-pass optical amplifier system comprising an amplifier medium and at least one relay imaging telescope for imaging light from the amplifier medium onto a primary light directing optical component and for imaging light directed back from the primary light directing optical component into the amplifier medium, wherein the system is arranged such that a light beam from the amplifier medium which is re-imaged onto the amplifier medium is spatially separated from; itself, ie. it does not overlap itself, at at least one point in its path between passes of the amplifier medium to enable it to be intercepted by at least one additional light directing optical component. This arrangement may include a phase conjugate mirror arrangement as described above for intercepting light between passes of the amplifier medium to generate a phase conjugate reflection of the light incident
Hellner Mark
Nixon & Vanderhye P.C.
QinetiQ Limited
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