Coherent light generators – Miscellaneous
Reexamination Certificate
2001-11-30
2004-11-09
Lee, Wilson (Department: 2828)
Coherent light generators
Miscellaneous
C372S033000, C372S059000
Reexamination Certificate
active
06816536
ABSTRACT:
BACKGROUND
1. Field of the Invention
This invention relates generally to the fields of optics and lasers, and more particularly to protecting sensitive optical elements from alteration or damage due to exposure to trace atmospheric species during shipping, storage or use.
2. Description of the Related Art
Many optics and laser systems employ sensitive optical materials that may be damaged by exposure to trace elements or components in the surrounding environment. In particular, certain materials that are of great interest because of their unique optical properties are prone to interact with a surrounding atmosphere to an extent that is sufficient to change the material's physical structure, resulting in subsequent degradations in the material's performance. This presents a problem for in situ protection of these materials in both everyday use, shipping and storage. For some materials, even ambient levels of water vapor of a few percent by mass may pose a threat, while for other materials, certain chemical species in the surrounding environment such as organic molecules may be an issue.
Borate crystals are an example of one class of materials that may suffer deterioration in performance upon mere exposure to ambient environment, such as air. This is because the crystals are hygroscopic, and can chemically react with absorbed water molecules. Such reactions can cause undesirable alterations in the crystals' optical and physical properties. Examples of borate crystals include BBO (&bgr;-BaB
2
O
4
), LBO (LiB
3
O
4
) and CLBO (CsLiB
6
O
10
), all of which found considerable use in the nonlinear conversion of light from infrared and visible lasers into the UV spectral range. These crystals differ by their optical and physical properties including the degree to which they are innately prone to absorbing water vapor from a surrounding environment. Among these crystals, CLBO has become the crystal of choice for harmonic conversion to wavelengths shorter than 300 nm because of fortuitous combination of optical properties and nonlinear parameters. It is currently considered to be the most suitable material for fourth and fifth harmonic generation of laser light from infrared Nd-doped lasers at high powers. CLBO is however, also particularly hygroscopic. The literature on the deleterious effects of moisture on CLBO includes the article by Taguchi et al in Advanced Solid State Lasers, C. R. Pollock and W. R. Bosenberg, eds. OSA Vol. 10, pp.19 (1997), where correlations were described between the rate of surface hydration and induced cracking and refractive index changes. Such humidity dependent characteristics are generally considered to be highly detrimental to reliable and long term use and operation of CLBO in high power laser systems.
Other examples of known hygroscopic crystals are the so-called ADP-isomorphs, such as KD
2
PO
4
(KD*P), NH
4
D
2
PO$ (AD*P) and CsD
2
AsO
4
(CD*A). These crystals are commonly used in electro-optic light modulators or in non-linear frequency conversion. Among this class of materials, CD*A is known to be particularly sensitive to humidity levels.
Several techniques have been employed to protect strongly hygroscopic materials such as CLBO, CD*A and the like, from the deleterious effects associated with water vapor absorption. They include special coatings, hermetic sealing with gas purging and operation at elevated temperatures.
Anti-reflective coatings are traditionally applied to crystalline materials used in laser systems to prevent Fresnel losses. Yet, coatings are also useful as a barrier to prevent water vapor or oxygen molecules from permeating the crystal, and causing disadvantageous changes in the physical structure and attendant thermal and optical properties thereof. Use of protective films on crystals subjected to UV radiation was described, for example, in U.S. Pat. No. 5,862,163. Such coatings were useful for crystals such as BBO, which is not highly hygroscopic or reactive. Coatings have proven more problematic for more reactive crystals such as CD*A and CLBO, because the process of applying the coatings can itself precipitate further damage mechanisms at the interface between the crystal and the coatings. Furthermore, coatings become increasingly susceptible to damage as the wavelength of the light becomes shorter, an acute problem for CLBO which is most often used to convert light into the deep UV. For highly sensitive materials, in particular, there is often a negative trade-off between the requirement that the film be thick enough to prevent water permeability, yet be thin enough to avoid damage. Thus, in and of themselves, coatings may not be sufficient to provide the needed protection from moisture and other potentially harmful gas species.
Another protective measure involves use of clean room preparation and assembly techniques to seal the enclosure containing the sensitive material followed by purging with a high purity gas that is inert with respect to chemical interactions that may change the material's physical or optical properties. Such sealing and purging methods using most commonly Nitrogen or an inert element as a purge gas, are well known in the art of laser fabrication, assembly and maintenance, and purging is often used as a standard procedure for example, during field service, following the replacement or repair of optical elements contained within the laser. However, when the optical element is extremely fragile and is especially sensitive to contamination even by trace gas components, special precautions must be taken to provide sufficient protection against ambient environment. For example, the cell containing the material may be hermetically sealed, to prevent any potential for leakage or contamination, in which case a single charge of gas can be used. Alternatively, the purge gas may flow continuously or at intermittent intervals, in which case the chamber containing the material must still be tightly sealed against the external environment, and a complete gas pumping and purification system must be further provided as part of the assembly, with a ready supply of purge gas maintained at all times.
While effective in providing a degree of protection against contamination, techniques of purging and tight sealing present a number of significant practical disadvantages. In particular, tight, or, in the extreme case, hermetic sealing of the chamber does not allow for any ready access to the optical material for the purposes of adjustment, inspection or replacement. In addition, if windows that are transparent to deep UV light must be provided as part of the sealed cell, as is the case for example, for CLBO used in a harmonic conversion module, there is a substantial risk of damage to the window material, especially at higher powers. Combined with the limited accessibility, the need to design so as to avoid damage to windows limits the design flexibility of the entire system containing the material.
Complete gas sealing also has the complication that altitude or other atmospheric changes as may be encountered during shipping can produce undesirable forces on the mechanical system containing the cell, leading to potential misalignments which are not readily corrected. Considerable cost, bulk and complexity are also added to the system, whether a hermetically sealed cell is utilized or a complete purge system is included, even as the overall reliability and longevity of the entire system may be compromised by a potential for catastrophic failure should the purge unexpectedly fail, or the gas charge dissipate. This failure mode is especially problematic when shipping a device in which CLBO or a similarly sensitive material is a component. By land, sea or air, practicality demands that devices be unattended for extended times.
Whenever the purge is not operating, or and/or the tight sealing is compromised due, for example, to extreme temperature and pressure variations, the crystal may be left insufficiently protected and may become momentarily exposed to undesirable humidity or other contaminant levels.
Julliard Karin
Petersen Alan B.
Utter Steve
Heller Ehrman White & McAuliffe LLP
Lee Wilson
Nguyen Dung
Spectra Physics Inc.
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