Optical: systems and elements – Optical amplifier – Particular active medium
Reexamination Certificate
2000-12-14
2003-11-11
Hellner, Mark (Department: 3663)
Optical: systems and elements
Optical amplifier
Particular active medium
C372S041000
Reexamination Certificate
active
06646793
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to optical and other electromagnetic systems. More specifically, the present invention relates to high gain and high etendue optical amplifiers for use with lasers and other devices.
2. Description of the Related Art
Optical amplifiers are used in many applications. Optical amplifiers are used as gain elements in lasers and in a number of other optical systems such as amplifiers per se in Master Oscillator Power Amplifiers (MOPAs), the amplifying component of loop phase conjugator systems, and as the nonlinear medium in optical four-wave mixing devices. In these and other applications, high gain at large etendue is often the enabling and most important design consideration. Étendue is defined as the product of the cross-sectional area of a beam and its full angular extent.
The prior art for optical amplifiers can be divided into two categories: guided and unguided. With unguided amplifiers, étendue is maximized by adjusting the transverse-to-longitudinal dimension ratio, since the angular field of view of such an amplifier is, to within factors of order unity that depend on the details of the geometry, proportional to the transverse dimension divided by the length. Unfortunately, high gain and high etendue in these conventional, non-guided, optical amplifiers are inherently conflicting requirements. This is due to the fact that although the gain coefficient is directly proportional to the amplifier's length, the étendue is inversely proportional. Although the gain of these conventional optical amplifiers is in textbook theories limited by amplified spontaneous emission, in practice it is typically limited by the onset of parasitic oscillations involving reflections from the boundaries of the gain medium or other nearby surfaces.
These limits are addressed by guided amplifiers, which make higher gains and étendue available. In this context, it is important to point out that unguided amplifiers have historically had the advantage that optical images can be propagated through them, whereas guided amplifiers scramble the optical information. In optical systems where phase conjugation is used, however, this issue is effectively circumvented, and the advantages of the guided amplifiers come to the foreground.
Guided amplifiers use total internal reflection (TIR) from the interface of their optically finished boundaries to the surrounding air, water, bonding, or other material. Both rods and slabs have been used for this purpose. Fiber amplifiers also fall into the category of being guided amplifiers, but high-étendue devices have not been fabricated. In any event, the performance of guided devices can also be limited by parasitic oscillation. Recent art, for example, uses a polished cylindrical rod as a light guide in an end-pumped optical amplifier (in this art the guiding was desired only for the pump, not the gain wavelength). The gain achievable from this device was reported to be limited, however, by parasitic oscillation around its circumference.
Hence, there is a need in the art, for applications such as optical phase conjugation and others, for a system or method for providing optical amplification with high étendue and high gain while suppressing parasitic oscillation.
SUMMARY OF THE INVENTION
The need in the art is addressed by the optical amplifier and method of the present invention. Generally, the inventive amplifier includes a first crystal with a first index of refraction and a second crystal bonded to the first crystal about an axis of propagation and having a second index of refraction. The first index is slightly higher than the second index such that light through the first crystal is totally internally reflected.
In the illustrative embodiment, the first crystal is a square-cross-sectioned parallelepiped of Yb:YAG, having an index of refraction of approximately 1.82, and the second crystal is several pieces of sapphire with an index of approximately 1.78, surrounding the Yb:YAG. Light is guided along the long dimension of the parallelepiped. The invention is, in its preferred embodiment, a light guide fabricated out of crystalline materials, diffusion bonded together. If the core of the light guide is doped with laser ions, high gain amplifiers made be designed and operable over a large étendue. With a judicious choice of the laser crystal and cladding materials, shape, and bonding technique, the guided amplifier is much less susceptible to parasitic oscillation than amplifiers constructed in accordance with the teachings of the present art.
The diffusion bonded structure guides both the lasing wavelength and the optical pumping beam, which may be supplied by a laser diode array. Alternatively, if the pumping beams have insufficient brightness to be launchable into the core, they may be guided by the cladding. In either case, this low loss guiding down a path several absorption depths long is conducive to efficient pumping, making possible high gain with quasi-three or four level ions, not achievable with the present art.
The purpose in configuring the gain medium as a light guide is to take advantage of the fact that the étendue then depends only on the guide's transverse size and numerical aperture, and not on its length. Therefore, the gain and étendue can both be increased.
As mentioned above, the gain of practical optical amplifiers is limited by the onset of parasitic oscillation. Large étendue makes guided amplifiers particularly susceptible to parasitic oscillation. The present invention can better achieve high gain in a guided amplifier because, in the best implementations, it uses a polygonal (preferably square or rectangular) rather than a circular guide cross section. Also, the cladding refractive index is selected to limit the guide-to-cladding total internal reflection (TIR) angle to disallow parasitic modes.
Further, in a particular embodiment broad band antireflection (AR) coatings and/or end caps are used to inhibit guide-end-reflection-involved modes. Although the illustrative embodiment is a diffusion-bonded composite crystal, a functional embodiment of the invention could be made of glass. Indeed a functional embodiment could be fabricated of any transparent solids bonded together in any way (such as index-specific optical cements) that maintains the desired guiding properties and which will not overheat or damage at the optical power levels involved.
A further advantage is that the cladding leads to more effective cooling of the doped core than would be possible if a gain element of the core's size were directly cooled by impingement or conventional forced-convection cooling. In this type of cooling, the &Dgr;T between the surface being cooled and the coolant is directly proportional to the heat per area and inversely proportional to a heat transfer coefficient. The total temperature drop between the core and the coolant is smaller for the clad device. This is because although additional &Dgr;T is necessarily introduced via the finite thermal conductivity of the cladding, the cladding simply having larger surface area more than compensates for the difficulties involved in obtaining a correspondingly larger heat transfer coefficient at the solid-liquid interface.
Another advantage of the present invention is that it makes end pumping of low absorption cross-section materials, such as Yb:YAG possible. It allows use of a long piece of material, in either a core-pumped or a cladding-pumped geometry. Parasitic oscillation is prevented by selection of the relative indices of the core and cladding to disallow parasitic oscillation for the particular core cross sectional shape selected. Previous end-pumping schemes have failed because of the parasitic oscillation issue.
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patent: 6160824 (
Betin Alexander A.
Bruesselbach Hans W.
Sumida David S.
Alkov Leonard A.
Hellner Mark
Lenzen, Jr. Glenn H.
Raufer Colin M.
Raytheon Company
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