Coherent light generators – Particular beam control device – Mode discrimination
Reexamination Certificate
2001-01-05
2004-11-02
Wong, Don (Department: 2828)
Coherent light generators
Particular beam control device
Mode discrimination
C372S009000, C372S018000, C372S028000, C372S029023, C372S092000, C372S098000, C372S099000
Reexamination Certificate
active
06813286
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to optical resonators generally and to those used in lasers as specific examples of such resonators.
BACKGROUND OF THE INVENTION
In general, the intensity distribution of output light from optical resonators, especially those used in high power lasers with large apertures, is not uniform but has the form of a pattern of either spots or rings, or a combination of these. Such distributions of output light result from the fact that the light inside an optical resonator is distributed in similar patterns termed modes. The spots and rings of this intensity pattern are so distributed that adjacent spots and adjacent rings have opposite phases (&pgr; phase shift).
When only the fundamental mode (TEM
00
) exists in a resonator, the output light distribution consists of only a single spot, and this mode is considered optimal. However, when higher order modes exist, the light distribution contains multiple spots or rings, so that the focusability and intensity of the output beam decreases. Such a decrease is undesirable for most applications involving laser light.
In order to improve the characteristics of the output light, external phase elements can be introduced into the optical path of the output beam in order to provide controlled phase or amplitude changes to any cross-sectional part of the beam. Examples of such phase elements are presented in the article “How phase plates transform and control laser beams” by Lee W. Casperson, published in Laser Focus World, May 1994, 223-228. In this article, the phase elements are designed to ensure that all portions of the output laser beam are in phase, thereby improving the far field intensity distribution.
With the introduction of a phase element outside the laser, the usual far field distribution of spots is replaced by a single, intense, on-axis spot surrounded by a pattern of much fainter spots. By this means, the intensity of light distribution resulting from a single high order mode can be increased by a factor of ten or more. Unfortunately, such an increase can only occur when the output light distribution results from a single stable mode.
Several methods have been proposed in order to obtain a specific, stable, high order mode, so the total laser output power is high, and the stability is such that the distribution of the beam can be improved by means of an external phase element. These methods have had limited success. For example, in the article “Single-mode selection using coherent imaging within a slab waveguide CO
2
laser” published in Applied Physical Letters, Vol. 60, 2469-2471 (1992), K. M. Abramski, H. J. Baker, A. D. Colly and D. R. Hall propose the insertion of a wire grid into the laser resonator for selecting a specific high order mode. Unfortunately, the grid and the adjacent resonator mirror are susceptible to damage through heating of the grid by the inter-cavity flux of energy, particularly during mirror alignment.
Another example involves the use of a conical resonator mirror to obtain a single higher order mode. This method was published by M. Rioux, P. A. Belanger and M. Cormier in the article “High-order circular-mode selection in a conical resonator” in Applied Optics, Vol. 16, 1791-1792 (1977). Unfortunately, it is difficult to predict and obtain a specific distribution of the higher order mode output laser beam in order to design the correct external phase element. In particular, it is difficult to match the orientation of the required external phase element to that of the output light distribution.
In order to obtain a single, stable, lowest order mode TEM
00
, an aperture must be inserted into the resonator. When the aperture is small, only the TEM
00
mode with the narrowest field distribution propagates, while higher order modes are eliminated because they are absorbed by the aperture. In general a laser operating with a pure TEM
00
mode has optimal beam quality, but relatively low output power.
In summary it is appreciated that the prior art shows how external phase elements can improve the quality of the output light distribution from an optical resonator, such as a laser. Unfortunately, this improvement cannot be practically implemented with most lasers, especially these operating in multi-mode, so the introduction of external phase elements are indeed limited to very special cases. Furthermore, wires and conical elements used for controlling the modes inside the optical resonator are not practical and tend to reduce the power of the output beam. Intra-cavity apertures, though widely used reduce the laser power output significantly.
The laser design limitations described above are applicable to both gas and solid state lasers. In addition, there are a number of mode design problems specific to solid state lasers, which limit their performance. Solid state lasers are particularly susceptible to a problem known as thermal lensing of the laser rod, which constitutes the gain material in such lasers. Thermal lensing occurs when the shape or the optical properties of the laser rod change because of the high thermal stresses set up within the rod at high input power levels. The optical elements in a solid state laser are designed to create an optimal mode shape while taking into account thermal lensing of the gain medium at the output power levels at which the laser is designed to operate. However, if the output power of the laser is changed, the thermal lensing of the gain medium changes, and the optical resonator no longer supports the optimal mode shape. Furthermore, in extreme cases the optical resonator becomes unstable and the output power of the laser drops to zero.
The input power levels to the gain medium over which the optical resonator remains stable define the dynamic range of the laser. It has been shown by Vittorio Magni in the article “Resonators for solid state lasers with large volume fundamental mode and high alignment stability” published in Applied Optics, Vol.25, 107-117 (1986) that the dynamic range of a solid state laser is inversely proportional to the width of the Gaussian mode (
0
,
0
) inside the gain medium. The maximum output power is proportional to the width of this mode. Therefore, the dynamic range is inversely proportional to the maximum output power from the laser. This means that when designing a laser, a compromise must be made between the maximum output power of the laser and its dynamic range. This significantly limits the laser's usefulness.
Another problem which limits the performance of solid state lasers arises from the optical distortions set up in the mode structure because of the birefringent nature of the lasing rod. The birefringence introduces areas of circular polarization into the mode because of the different phase shifts introduced along the different crystal axes, and thereby degrades the quality of the laser output beam.
SUMMARY OF THE INVENTION
The present invention seeks to provide an improved optical resonator with an improved output beam.
There is thus provided in accordance with a preferred embodiment of the present invention an optical resonator comprising reflector elements and with at least one discontinuous phase element disposed between the reflector elements. The reflector elements may be full reflectors or output couplers.
The optical resonator may be a passive or active optical resonator. If the optical resonator is an active optical resonator, it may be embodied in a laser, such as an axial laser or a ring laser. The resonator may be a stable or an unstable resonator.
The discontinuous phase element may be embodied in at least one reflector or output coupler or may be positioned adjacent to an optical element or may be placed inside the resonator at a defined point which images onto itself by reflection from one of the above optical components of the resonator.
The discontinuous phase element may provide discontinuous phase change as well as one or more of angular, linear or radial phase change.
The optical resonator may also comprise an external discontinuous phase element o
Danziger Yochay
Friesem Asher A.
Oron Ram
Christie Parker & Hale LLP
Rodriguez Armando
Wong Don
Yeda Research and Development Co. Ltd.
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