Optical: systems and elements – Light interference – Produced by coating or lamina
Reexamination Certificate
2002-03-12
2004-09-14
Robinson, Mark A. (Department: 2872)
Optical: systems and elements
Light interference
Produced by coating or lamina
C359S580000, C356S454000
Reexamination Certificate
active
06791758
ABSTRACT:
INTRODUCTION
This invention relates to new optical etalons and to methods of making and using them, as well as to devices incorporating such etalons.
BACKGROUND
Etalons are ubiquitous in optical systems, such as optical sensors, optical communication systems, etc. The basic Fabry-Perot etalon can be designed and produced to have a sharp response at resonant frequencies, which makes them suitable as optical filters, such as passband filters. They also give a variable amount of dispersion, and so have been suggested for possible use as dispersion compensators. Thus Fabry-Perot etalons are a basic building block in a number of different optical elements, i.e., in optically functional components or devices. Such devices may be active or passive and may be employed in a system (or adapted to be employed in a system) to pass or transmit a selective wavelength or band of wavelengths or periodic set of wavelength bands. Exemplary optical elements in which etalons are used include optical sensors, and filters, e.g., band pass filters, single channel filters, and other wavelength selective filter devices such as wavelength division multiplexers, and dispersion compensators and other components of optical communication systems.
Fabry-Perot etalons typically comprise precisely parallel, selectively transmissive surfaces, such as thin films, i.e., partially reflective mirrors or surfaces on opposite sides of an integral number of half waves distance or gap between them, forming the etalon's cavity. The thin film and cavity characteristics determine the optical properties of the etalon. That is, the spectral characteristics of the etalon are generally determined by the reflectivity of the mirrors or surfaces and by the optical thickness of the cavity. Such etalons have long been produced, for example, by sputter deposition of film stacks of alternating layers of materials, i.e., a high refractive index material alternating with a low refractive index material, to form a mirror coating which is transmissive of selected wavelengths. Two such mirror coatings sandwich a sputter-deposited cavity layer between them. Sputtering or other physical deposition of the relatively thick cavity layer is time consuming and, therefore, adds substantial time and cost to the production of such etalons. The result is undesirably high cost of production for such etalons.
It has long been a recognized problem in this industry, that producing etalons having desired properties can be difficult and expensive. In addition, there are industry-recognized problems associated with producing structurally robust etalons having desired, precise optical properties. Prior known etalons have employed various designs, such as the etalons used in the interferometric optical devices of U.S. Pat. No. 6,125,220 to Copner et al. In the interleaver/de-interleaver devices of Copner et al, two glass interferometric end plates are separated by a spacer region where the etalon is formed. The spacer region is an air gap having a predetermined dimension. In adjustable Fabry-Perot devices, such as disclosed in U.S. Pat. No. 5,283,845 to Ip, tuning of the center wavelength of the spectral passband of an etalon is achieved by varying the effective cavity length (spacing) between two end plates carrying Fabry-Perot thin film reflectors. More specifically, in Ip a piezo actuator is used, extending between the two end plates. By varying the electric power applied to the piezo actuator, the axial length of the actuator can be varied, and thus the gap between the end plates varied. As alternatives to piezo-electric actuators, the tuning mechanism may include liquid crystals, temperature, pressure, and other mechanisms. It is a disadvantage that adjustable etalons as in Ip involves considerable assembly complexity and cost. Also, maintaining strict parallelism between the end plates can present additional difficulties.
The prior known optical etalons, as noted above, fail to fully meets the needs of many applications, especially for optical elements intended for optical communication systems, precision sensors, etc.
It is an object of the present invention to provide etalons addressing some of the deficiencies of the prior known technologies. It is a particular object of at least certain preferred embodiments, to provide improved etalons and methods of making same, and optical elements incorporating such etalons. Additional objects and aspects of the invention and of certain preferred embodiments of the invention will be apparent from the following disclosure and detailed description.
SUMMARY
In accordance with a first aspect, an etalon comprises a planar bulk optic having first and second parallel, selectively transparent surfaces. The bulk optic comprises an optically transparent body and a wedge correcting coating (referred to here generally as a “wedge coating”) on at least one of the two surfaces of the optically transparent body. The wedge coating, further described below, establishes high precision parallelism of the selectively transparent surfaces of the etalon. The bulk optic is a solid, optically transparent (at the wavelength or wavelengths of interest) body whose thickness, i.e., the dimension between the selectively transparent, parallel surfaces, including the wedge coating, defines the cavity spacing. In particular, the bulk optic, including the wedge coating, will typically have an optical thickness equal to an integral number of half-waves for the wavelength(s) of interest. In preferred embodiments the selectively transparent surfaces are thin film coatings comprising, for example, a film stack of alternating high and low refractive index oxides or a metal thin film in accordance with known thin film technologies.
The thickness of the wedge coating varies progressively across the etalon. That is, the thickness of the wedge coating, viewed in cross-section in at least one plane orthogonal to the parallel, selectively transparent surfaces of the etalon, has a thickness that increases (or decreases in the opposite direction) continuously, typically approximately linearly, to compensate for non-parallelism, or “wedge”, in the underlying body of the bulk optic. As described further below, the bulk optic can be diced from a wafer on which a wedge coating and the two thin film coatings have been deposited by magnetron sputtering, ion beam sputtering or other known deposition techniques. Preferably, surface polishing is performed to first polish the wafer, for example, a silica wafer suitable for optical filter production, to parallelism within 1 to 2 arc seconds and wavefront error of less than {fraction (1/50)} (2.0%) of a wave at the wavelength of interest. For an etalon intended for use as or in an optical element in an optical telecommunication system, the wavefront error will preferably be less than {fraction (1/50)} of a wave at 1550 nm. Low wavefront error can be understood in this context to mean that the thickness of the bulk optic, i.e., the distance between the two opposite surfaces of the bulk optic, is substantially linearly variable and, hence, controllable or correctable by the wedge coating in accordance with the present disclosure. Preferably, for etalons suitable for use in optical communication elements, the wedge coating brings parallelism of the opposite surfaces of the bulk optic body from the 1 to 2 arc seconds of wedge mentioned above to less than 0.1 arc seconds, most preferably less than 0.01 arc seconds.
It is a significant advantage that the etalons disclosed here can employ a bulk optic, comprising the optically transparent body and the wedge coating, to define the cavity spacing of the etalon. Substantial cost savings and production simplification can be realized in accordance with at least certain preferred embodiments of the etalon. Further, robust and accurate etalons can be achieved using production techniques whose application will be readily understood by those skilled in the art given the benefit of this disclosure.
In accordance with a method aspect of the present disclosure, the wedge coating
Amari Alessandro
Cierra Photonics Inc.
McDermott Peter D.
Robinson Mark A.
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