Patent Publication Number: US-8986922-B1

Title: Adjusting optical properties of optical thin films

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 11/127,558, now U.S. Pat. No. 7,901,870, entitled “Adjusting Optical Properties of Optical Thin Films,” filed on May 12, 2005 in the name of Michael L. Wach, which claims the benefit of priority to U.S. Provisional Patent Application Ser. No. 60/570,258, entitled “Adjusting Optical Properties of Optical Thin Films,” and filed May 12, 2004. The entire contents of each of the above identified priority documents and specifically U.S. patent application Ser. No. 11/127,558 and U.S. Provisional Patent Application Ser. No. 60/570,258 are hereby incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to optical devices and more specifically to adjusting an optical property, such as refractive index, of an optical thin film, thin film optical filter, or thin film optical system by an application of short-wavelength light energy or other energy. 
     BACKGROUND OF THE INVENTION 
     Optical thin films are useful for a wide range of optical applications such as antireflective (“AR”) coatings, high-reflective (“HR”) coatings, dielectric mirrors, and thin film interference filters. Compact size and environmental stability are two properties of optical thin films that have stimulated their deployment in modern applications including optical communications, lighting, vision, instrumentation, medical devices, and display systems. Optical thin films typically manipulate light by interference, which is an additive or subtractive process in which the amplitudes of two or more overlapping light waves systematically attenuate or reinforce one another. This interference can provide polarization, wavelength-selective transmission and reflection, beam splitting, or various other effects on a light beam, according to the design of the thin film and its interaction with adjacent features in an environment of an optical system. 
     Harnessing the interference phenomenon with an optical thin film relies on precise control of the optical thin film&#39;s physical dimensions and material properties. A slight variation in the thickness of an optical thin film can significantly impact the thin film&#39;s optical performance. Similarly, anomalies in an optical thin film&#39;s material can cause unwanted variation or degradation in optical performance. This susceptibility to performance variation usually imposes tight tolerances on a process, such as a vacuum deposition process, that fabricates optical thin films. 
     Vacuum deposition processes can be difficult to control to a level that is sufficient to produce consistent optical thin films for applications that require high performance. For example, the yield for fabricating twenty five gigahertz (“GHz”) thin film band-pass filters for dense wavelength division multiplexing (“DWDM”) can be low. In one technique for controlling a thin film deposition process, an optical instrument monitors the deposition process by observing the buildup of thin film material in a region of a deposition chamber. Relative to the total surface area of optical thin film generated in a deposition batch, the monitored region may be relatively small. While the monitored region of thin film may provide acceptable optical performance, the unmonitored regions may exhibit performance characteristics that are out-of-specification in relation to their spatial separation from the monitored region. In certain circumstances, the acceptability rate of the output of a thin film optical filter batch can be less than twenty-five percent. Since conventional processes for producing optical thin films typically lack a provision for adjusting an optical property of an optical thin film in a controlled fashion after deposition, optical filters that do not meet acceptability standards of optical performance are frequently discarded as scrap. 
     Optical thin films that meet performance specifications and that are not discarded as scrap are often deployed in optical systems as discrete components, wherein the optical thin film adheres to a substrate or is freestanding, for example supported only on its edges. As a deposition substrate, a plate of glass or other optical material provides an optical thin film with structural support that facilitates handling and placement into the optical system. Integrating an optical thin film into an optical component that passively or actively manipulates light provides an alternative configuration that can deliver two or more optical manipulations in a single, integrated component. This integration can reduce the total size of an optical system, improve reliability, and streamline assembly. Optical components that are compatible with integrated thin film optical filters or other optical thin films include lens, lasers, optical amplifiers, gradient index lenses, optoelectronic components, optical fibers, detectors, displays, and planar light guide circuits (“PLCs”), for example. 
     While offering certain benefits, depositing thin films onto such optical components often imposes tight tolerances upon the fabrication process of the optical thin film. The optical characteristics of conventional thin films are typically defined upon thin film formation. Those characteristics or properties are typically not readily modified following thin film formation in a controlled fashion by conventional processes. Thus, depositing an out-of-tolerance optical thin film on an optical component can result in discarding or scrapping the integrated unit. Scrapping integrated optical assemblies is problematic and wasteful as optical components are often expensive, and convention deposition processes frequently provide a low yield of optical thin films having acceptable or desirable optical performance. For example, depositing a stack of optical thin films onto an end face of an optical fiber combines a filter function with a waveguide function into a single, integrated optical element for which defective performance is undesirable. 
     As an alternative to depositing thin film filters onto fiber end faces, Bragg gratings can provide integrated filtering in waveguides such as optical fibers and PLCs. Such a Bragg grating comprises an undulated refractive index disposed in the path of light propagating in the waveguide. That is, the waveguide exhibits a periodic fluctuation in the refractive index of its core and/or surrounding cladding. The modal field of light guided in the waveguide interacts with that core or cladding material 
     Illuminating a PLC or a fiber-optic waveguide with a pattern of ultraviolet (“UV”) light corresponding to a desired refractive index undulation “writes” the Bragg grating into the waveguide. The term ultraviolet or UV, as used herein, refers the region of the electromagnetic spectrum or light spectrum having a wavelength shorter than approximately 450 nanometers. Etching a corrugated, surface relief pattern into the waveguide is an alternative approach to forming a Bragg grating in a waveguide. Conventional Bragg gratings have been etched into the surface of glass PLCs that have ion-exchanged waveguides. An overcoat of optical material such as silicon dioxide or silicon oxynitride environmentally seals the exposed grating and the waveguide to prevent contamination that can compromise optical performance of the system. Conventional uses for this PLC system include grating-based stabilization of external cavity lasers and grating-based optical add drop multiplexing (“OADM”). The optical properties of the etched and the UV-written type of Bragg gratings, in PLCs or in optical fibers, can be changed conventionally through exposure of UV light and/or thermal energy. 
     While the optical properties of conventional Bragg gratings are changeable via conventional methods after forming the primary grating features, Bragg gratings exhibit attributes that can be undesirable in many applications. Relative to thin film optical filters, Bragg gratings are often expensive and susceptible to optical drift resulting from temperature changes and other environmental influences. In comparison to the physical thickness of a thin film filter, light typically propagates through a long physical distance of a Bragg grating to achieve an acceptable level of light manipulation. Also, in a band-pass configuration, thin film filters typically provide more desirable optical performance characteristics than Bragg gratings offer. Furthermore, whereas Bragg gratings are ordinarily embedded in select optical materials that are amenable to the grating-generation process, thin films can be applied to a wide variety of optical materials, substrates, and components, with minimal impact on the substrate. 
     In addition to passive optical components such as optical fibers, optical thin films have been applied to the facets of optoelectronic components such as semiconductor gain media devices, including semiconductor lasers and semiconductor optical amplifiers (“SOAs”). Applying a HR coat to one facet of a Fabry-Perot laser and an AR coat to the opposite facet and placing a wavelength-selective reflector, such as a fiber Bragg grating, in front of the AR-coated facet can establish an external cavity laser system that emits monochromatic light in the format of a single longitudinal mode. The proper function of this external cavity laser system typically requires the AR coat to provide a high level of performance. That is, the AR coat should minimize the reflection of light from the facet to a level that does not degrade the optical performance of the laser system. Achieving a suitable level of suppression of the facet reflection based on conventional technology can be challenging. Exacerbating the problem, the refractive indices of most semiconductor gain media are significantly higher than air, the typical media surrounding the facet. 
     A conventional approach to applying an AR coating to a laser, SOA, or other semiconductor gain media, entails depositing AR thin film layers onto a batch of Fabry-Perot laser dies or similar components, for example in a bar form, in a deposition chamber. An electrical supply in the chamber may deliver current during the deposition process to one of the laser dies in the batch. Instrumentation coupled to the laser monitors the laser&#39;s active or dynamic response to the application of the AR coat, thus inferring the optical performance of the AR coat. An operator can adjust deposition parameters in the chamber during the deposition process in an attempt to maximize AR performance, by controlling the refractive index of one or more layers and/or controlling the thickness of one or more layers. This conventional process typically suffers from several drawbacks that adversely impact yield and AR performance. Due to deposition variations associated with the spatial position of each laser die in the chamber, the AR coating on the monitored laser die may differ from the AR coatings on the other laser dies in the batch. An undesirably large number of optical thin film layers may be needed to achieve a specified level of AR performance to overcome lack of refractive index control in each layer. Also, coating a laser die with an out-of-spec AR coat can result in scrapping or discarding the laser die, which is wasteful or financially undesirable. 
     While maintaining deposition consistency in a coating chamber is desirable for applications such as applying AR coats to laser dies, physical thickness variation can also be purposely introduced to a thin film during the deposition process. The thickness variation can cause a corresponding variation in the manner in which light interacts with the coat. Although optical thin films that have uniform optical properties in a dimension parallel to the surface of the optical thin film are suited to many applications, other applications benefit from thin films with optical properties that vary in this dimension. 
     An optical thin film system that exhibits a spectral performance that varies in a dimension perpendicular to the thickness of the optical thin film offers utility for certain optical system applications. For example, a thin film optical filter can have a pass band that varies in a graded manner along the plane of the filter. Positioning an array of optical-fiber-coupled gradient index lenses adjacent to the filter plane filters the light associated with each lens according to each lens&#39;s spatial position on the filter. 
     A conventional process for forming such a filter with graded spectral properties entails varying the deposition rate across the optical filter&#39;s substrate during the deposition process. Grading the flux of particles across the substrate during the deposition process creates each thin film layer with a physical profile of varying thickness. That is, more deposited material yields greater physical thickness at various locations of each layer. A filter fabricated with this process has thin film layers of graded physical thickness. Since physical thickness of layers can correlate with the spectral position of a filter&#39;s pass band position, a graded physical thickness can yield a thin film optical filter that has different spectral properties at different physical locations on the filter. 
     This conventional approach to graded thin film filters has disadvantages for many applications. Since the spectral properties are defined in the deposition process, there is typically no provision for modifying those properties in a controlled manner after the deposition process is complete. Yield problems can result from inadequate control of the deposition process, for example. Also, the process typically produces thin films and thin film optical filters that have varying thickness, so the outer surface of such a thin film optical filter slopes relative to the substrate surface to which the stack of thin films adheres. 
     To address these representative deficiencies in the thin film art, what is needed is a capability for adjusting one or more optical properties of an optical thin film, such as an optical thin film in a thin film optical filter. Further a capability is needed to impart an optical thin film with a pattern, such as a refractive index or spectral pattern that varies in a graded fashion in a dimension parallel to the thin film&#39;s surface. Such capabilities would enhance the precision with which an optical thin film manipulates light and facilitate the cost-effective utilization of optical thin film technology in numerous applications. 
     SUMMARY OF THE INVENTION 
     The present invention supports adjusting an optical property of an optical film, such as an optical thin film. In an aspect of the present invention, an optical thin film can manipulate light that is incident upon at least one of the film&#39;s surfaces. That is, the optical thin film can manipulate light rays that are incident on the optical thin film parallel to or otherwise at an angle of less than 90 degrees with respect to the thickness of the optical thin film. The optical thin film can include a layer of material situated between two surfaces or interfaces at the boundaries of the optical thin film. These two surfaces, which can be abrupt surfaces or graduated material interfaces, can each reflect a portion of the light that is incident on the optical thin film. The reflected portions of light from each surface can interact or interfere with one another as wave phenomena, resulting in additive or subtractive interference. 
     In one aspect of the present invention, applying energy, such as thermal energy or ultraviolet light, to an optical thin film can adjust an optical property of the thin film and can alter at least some aspect of the optical thin film&#39;s interference of light. An application of energy can adjust the refractive index of the optical thin film or the speed that light propagates in the optical thin film. An energy application can also cause a change in the optical thickness of the optical thin film. 
     In another aspect of the present invention, a chemical, additive, moiety, or agent of an optical thin film can increase the sensitivity of the optical thin film to an application of ultraviolet light. The agent can be nitrogen, hydrogen, germanium, or other material. The composition of the optical thin film can include silicon dioxide and/or silicon oxynitride. The optical thin film can be a dielectric thin film. 
     In yet another aspect of the present invention, a method for fabricating an optical thin film can include producing at least one optical thin film layer in a deposition chamber. The resulting optical thin film layer can be a layer in an antireflective coating on a laser or other optical device or a layer in an optical filter. Exposing the layer to a dose of ultraviolet light can alter the refractive index of the optical thin film layer. Subjecting the layer to hydrogen can enhance, increase, heighten, stimulate, cause, or promote the layer&#39;s response to the dose. 
     In yet another aspect of the present invention, a thin film optical system, which can be a single thin film layer or a stack of optical thin film layers, can have a variation in an optical property along the plane of the layer. That is, a material of an optical thin film can exhibit a variation or a pattern in an optical property along a direction that follows the plane of the optical thin film or that is perpendicular to the thickness of the optical thin film. 
     In yet another aspect of the present invention, a thin film optical filter can reflect or transmit light based on the color or wavelength of light, which can be visible light or light that is outside the range of human visual perception. The thin film optical filter can have filtration characteristics that vary across the surface of the optical thin film. That is, the optical thin film can preferentially transmit or reflect one color of light in one spatial area of the filter and preferentially transmit or reflect another color of light in another spatial area. 
     The discussion of optical thin films presented in this summary is for illustrative purposes only. Various aspects of the present invention may be more clearly understood and appreciated from a review of the following detailed description of the disclosed embodiments and by reference to the drawings and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  illustrate an optical thin film adhering to a substrate in accordance with an exemplary embodiment of the present invention. 
         FIGS. 1C and 1D  illustrate plots of physical and optical properties of regions the optical thin film of  FIGS. 1A and 1B  in accordance with an exemplary embodiment of the present invention. 
         FIG. 2  illustrates a stack of optical thin films on a substrate in accordance with an exemplary embodiment of the present invention. 
         FIG. 3  illustrates an optical thin film undergoing an optical property adjustment in accordance with an exemplary embodiment of the present invention. 
         FIG. 4A  illustrates a spectral plot of a thin film optical filter in accordance with an exemplary embodiment of the present invention. 
         FIG. 4B  illustrates spectral plots of a thin film optical filter resulting from an optical property adjustment in accordance with an exemplary embodiment of the present invention. 
         FIG. 5  illustrates spectral transmission plots of a thin film optical filter adjusted to match a wavelength grid in accordance with an exemplary embodiment of the present invention. 
         FIG. 6  illustrates spectral transmission plots of an adjustment to a thin film optical filter to alter the ripple in the pass band in accordance with an exemplary embodiment of the present invention. 
         FIG. 7  illustrates spectral plots of a thin film optical filter&#39;s response to an optical property adjustment in accordance with an exemplary embodiment of the present invention. 
         FIG. 8  illustrates spectral plots of the group delay of a thin film optical filter in response to an adjustment of an optical property in accordance with an exemplary embodiment of the present invention. 
         FIG. 9  illustrates a fiber optic device that includes a thin film optical filter having an adjusted optical property in accordance with an exemplary embodiment of the present invention. 
         FIG. 10  illustrates light incident on an optical thin film in accordance with an exemplary embodiment of the present invention. 
         FIG. 11  illustrates plots of the optical thickness of an optical thin film before and after undergoing an adjustment of its optical properties in accordance with an exemplary embodiment of the present invention. 
         FIG. 12  illustrates plots of the optical thickness of an optical thin film before and after undergoing an adjustment of its optical properties in accordance with an exemplary embodiment of the present invention. 
         FIG. 13  illustrates plots of the optical thickness of an optical thin film before and after undergoing an adjustment of its optical properties in accordance with an exemplary embodiment of the present invention. 
         FIG. 14  illustrates a system that adjusts an optical property of an optical thin film by an application of short wavelength light in accordance with an exemplary embodiment of the present invention. 
         FIG. 15  illustrates a laser-based system that adjusts an optical property of an optical thin film by an application of short wavelength light in accordance with an exemplary embodiment of the present invention. 
         FIG. 16  illustrates a system that adjusts an optical property of an optical thin film by an application of short wavelength light to the thin film in conjunction with monitoring an optical response of the optical thin film in accordance with an exemplary embodiment of the present invention. 
         FIG. 17  illustrates an optical interface for applying light to an optical thin film to adjust optical properties of the thin film in conjunction with monitoring an optical response of the optical thin film in accordance with an exemplary embodiment of the present invention. 
         FIG. 18  is a functional block diagram illustrating adjusting an optical property of an optical thin film under computer control in accordance with an exemplary embodiment of the present invention. 
         FIG. 19  is a functional block diagram illustrating closed loop feedback control of a process for adjusting an optical property of an optical thin film in accordance with an exemplary embodiment of the present invention. 
         FIG. 20  is a flow chart illustrating steps in a process for adjusting an optical property of an optical thin film in accordance with an exemplary embodiment of the present invention. 
         FIG. 21  is a flow chart illustrating steps in a process for adjusting an optical property of an optical filter in accordance with an exemplary embodiment of the present invention. 
         FIG. 22  illustrates a system for imparting a pattern into an optical thin film by an application of thermal energy in conjunction with monitoring the distribution of thermal energy in accordance with an exemplary embodiment of the present invention. 
         FIG. 23  illustrates a system for imparting a pattern into an optical thin film by an application of thermal energy in conjunction with monitoring an optical response of the optical thin film in accordance with an exemplary embodiment of the present invention. 
         FIG. 24  illustrates a system for imparting a pattern into an optical thin film by an application of radiant thermal energy in accordance with an exemplary embodiment of the present invention. 
         FIG. 25  is a flow chart illustrating steps in a process for varying the refractive index of an optical thin film in a graded pattern in accordance with an exemplary embodiment of the present invention. 
         FIG. 26  is a flow chart illustrating steps in a process for imparting an optical thin film with a pattern in accordance with an exemplary embodiment of the present invention. 
         FIG. 27  illustrates a system for adjusting the reflectivity of an AR coating on a laser in accordance with an exemplary embodiment of the present invention. 
         FIG. 28  is a flow chart illustrating steps in a process for adjusting an optical property of an AR coating on a laser in accordance with an exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     The present invention is directed to adjusting one or more optical properties of an optical thin film. Adjusting an optical property of an optical thin film can facilitate efficient and cost-effective fabrication of optical systems such as thin film optical filters that manipulate light based on thin film interference. An optical thin film adjustment can also create spatially specific optical properties or patterns in an optical thin film. 
     Turning now to discuss each of the drawings presented in  FIGS. 1-27 , in which like numerals indicate like elements through the several figures, an exemplary embodiment of the present invention will be described in detail. 
       FIGS. 1A and 1B  illustrate a thin film optical system  100  having an optical thin film  110  adhering to a substrate  120  in accordance with an exemplary embodiment of the present invention. A Cartesian coordinate system, having an x-axis  140 , a y-axis  150 , and a z-axis  130 , illustrates the relative orientation of the optical thin film  110  and substrate  120  in the optical system  110 . The x-axis  140  and the y-axis  150  are parallel to the major surfaces  170 ,  180  of the optical thin film  110 , while the z-axis  130  traverses the thickness  160  of the optical thin film  110 . Those major surfaces  170 ,  180 , whether planar or contoured in a non-planar form, can be referred to as faces of the optical thin film  110 . That is, the z-axis  130  is perpendicular to the planar interface  170  between the optical thin film  110  and the substrate  120 , while the x-axis  140  and the y-axis  150  lie in or along the plane of this interface  170 . 
     The exemplary substrate  120  is a volume of optical material taking the form of a slab with a smooth planar surface  170  that adheres to the optical thin film  110 . In this configuration, the substrate  120  provides mechanical support and physical stability for the optical thin film  110 . Although  FIG. 1  illustrates the optical thin film  110  in a planar configuration, the optical thin film  110  can alternatively have a contour, for example conforming to a convex, cylindrical, or concave surface of a lens or other surface, in accordance with exemplary embodiments of the present invention. 
     Functionally, a refractive index differential at the surface interface boundary  170  between the material of the optical thin film  110  and the material of the substrate  120  causes reflection of light  125  propagating through the thickness  160  of the optical thin film  110 , generally along the z-axis  130 . The outer surface  180  of the optical thin film  110 , opposite the substrate interface  170 , is also reflective to light  125  propagating through the thickness  160  of the optical thin film  110 . The outer surface&#39;s reflectivity arises from the refractive index differential between the material of the optical thin film  110  and the surrounding media  185 . That is, the inner surface  170  and the outer surface  180  of the optical thin film  110  individually reflect light  125  propagating through the thickness  160  of the optical thin film  110 . The degree of reflectivity of each of these surfaces  170 ,  180  is a function of the refractive index differential at or across each surface  170 ,  180  and the angle of the incident light  125  relative to the z-axis  130  and other potential factors, such as the polarization of the light  125 . The light  125  propagating through the optical thin film  110  may travel parallel with the z-axis  130 . Alternatively, the light  125  may be incident on the optical thin film  110  at an angle, such as an acute angle, with respect to the z-axis  130 . 
     In one exemplary embodiment of the present invention, the thickness  160  of the optical thin film  110  is less than approximately ten wavelengths of the light  125  that the optical thin film  110  is operative to manipulate. In one exemplary embodiment of the present invention, the thickness  160  of the optical thin film  110  is less than approximately five wavelengths of the light  125  that the optical thin film  110  is operative to manipulate. In one exemplary embodiment of the present invention, the thickness  160  of the optical thin film  110  is less than approximately three wavelengths of the light  125  that the optical thin film  110  is operative to manipulate. In one exemplary embodiment of the present invention, the thickness  160  of the optical thin film  110  is less than approximately one wavelength of the light  125  that the optical thin film  110  is operative to manipulate. In one exemplary embodiment of the present invention, the thickness  160  of the optical thin film  110  is approximately one fourth the wavelength of the light  125  that the optical thin film  110  is operative to manipulate. In one exemplary embodiment of the present invention, the thickness  160  of the optical thin film  110  is greater than approximately one nanometer and less than approximately ten microns. 
     The surrounding medium  185 , which is in contact with the outer surface  180  of the optical thin film  110 , can be a range of one or more gaseous, liquid, or solid materials. In one exemplary embodiment of the present invention, the medium  185  is space, essentially void of matter. That is, the thin film  110  can operate in a vacuum environment. In one exemplary embodiment of the present invention, the medium  185  is a gas, such as air, nitrogen, hydrogen, helium, oxygen, or a mixture of gases. In one exemplary embodiment of the present invention, the medium  185  is another optical thin film layer, such as a layer of a thin film interference filter. In one exemplary embodiment of the present invention, the optical thin film  110  adheres to this medium  185  as well as to the substrate  120 . In one exemplary embodiment of the present invention, this medium  185  is a liquid such as water, optical matching gel, matching fluid, a biological fluid, or hydrocarbon. Such biological fluids can include blood, saliva, cerebral spinal fluid (“CSF”), secretions, urine, or milk, any of which can be either in a processed or a natural form. The medium  185  can also be glass, a plastic, a rubber, a composite, an inhomogeneous matrix, a resin, or an epoxy, any of which can be in a solid or viscous state. In addition to passive materials, the medium  185  can be an active material such as a semiconductor detector, optically active material, electrically active material, or optical gain medium. The medium  185  can also be a biological composition such as a matrix of cells, tissue, tumorous material, muscular tissue, for example. In general, the medium  185  is not limited to a specific material, or any of the materials or compositions discussed herein. 
     In one exemplary embodiment of the present invention, the medium  185  is sealed in a hermetic environment. The medium  185  can be the hermetic internal environment of an electronic, optical, or optoelectronic package, for example a receiver, transmitter, or receiver module for optical networking. In one exemplary embodiment of the present invention, the medium  185  resides in a sealed environment that is not hermetic, such as in a water-tight enclosure. 
     The refractive indices of the medium  185 , the optical thin film  110 , and the substrate  120  can shift temporarily in response to an environmental change such as stress or operating temperature or alternatively remain stable with minimal influence to such influences. In one exemplary embodiment of the present invention, one or more of these refractive indices can respond to a control, which can be, without limitation, an electrical, magnetic, optical, or electromagnetic field, signal, or wave. 
     Interference of light results from additive or subtractive interaction between reflection at the outer surface  180  of the optical thin film  110  and reflection at the inner surface  170  of the optical thin film  110 . When these two reflections are in phase with one another, the amplitudes constructively add. Alternatively, when the reflections are out of phase with respect to one another, the amplitudes can destructively subtract. Such constructive and destructive interference can provide a wide assortment of optical effects. Exemplary effects include filtering, polarizing, and dispersing light, among others. 
     In one exemplary embodiment of the present invention, the operability of the optical thin film  110  to manipulate light  125  by thin film interference is a function of or is related to one or more of: the thickness  160  of the optical thin film  110  in relation to the wavelengths of manipulated light  125 ; the spatial relationship or physical separation between the outer surface  180  and the inner surface  170 ; the refractive indices of the substrate  120 , the optical thin film  110 , and the surrounding medium  185 ; the angle of the light  125  with respect to the z-axis  130 ; and the polarization of the light  125 . 
     In one exemplary embodiment of the present invention, interference of the optical thin film  110  is operable to transmit light of specific wavelengths and reflect light that is not transmitted. In one exemplary embodiment of the present invention, the optical thin film  110  minimizes the reflectivity of light  125  incident on the substrate  120 , effectively countering the tendency of the refractive index differential between the substrate  120  and the surrounding medium  185  to reflect light. This antireflective property can be either intentionally wavelength selective or operable across a purposely broad span of wavelengths. In one exemplary embodiment of the present invention, the optical thin film  110  generates or heightens reflectivity in the interface between these two media  120 ,  185 . 
     In one exemplary embodiment of the present invention, the optical thin film  110  functions in a bidirectional manner, providing essentially equal manipulation of light  125  traveling in either a positive direction or a negative direction with respect to the z-axis  130 . That is, the light  125  can travel through the optical thin film  110  either from the outer surface  185  to the inner surface  170  or from the inner surface  170  to the outer surface  185 . 
     The optical thin film  110  in the exemplary optical system  100  illustrated in  FIGS. 1A and 1B  adheres to a substrate  120  that is a plate of optical material, such as glass, silica, sapphire, or silicon. In this configuration, supporting and stabilizing the optical thin film  110  is the primary function of the substrate  120 , rather than manipulating light such as collimating, beam steering, or focusing light. 
     In another exemplary embodiment of the present invention, the substrate  120  is a component, such as a gradient index lens or an optical fiber, that provides light manipulation, such as collimating light or guiding light. Exemplary passive components that can be substrates include diffractive elements, holographic lenses, concave lenses, convex lenses, cylindrical lenses, Fresnel lens, PLCs, prisms, circulators, isolators, lens arrays, ball lenses, micro-optic components, nano-optic elements, planar micro-lens arrays, ion-exchanged components, displays, interconnects, crystals, lenslets, lenticulars, diffusers, micro-fluidic components, or other passive components known to those skilled in the art, according to exemplary embodiments of the present invention. 
     In addition to passive manipulation, the substrate  120  can actively manipulate light. That is, the substrate  120  can be a vertical cavity surface emitting laser (“VCSEL”), distributed feedback (“DFB”) laser, SOA, erbium doped fiber amplifier (“EDFA”), erbium doped waveguide amplifier (“EDWA”), charge coupled device (“CCD”), light emitting diode (“LED”), avalanche photodiode (“APD”), indium gallium arsenide (“InGaAs”) detector, optical modulator, germanium detector, sensor, or other active component known those skilled in the art, in accordance with exemplary embodiments of the present invention. 
     In one exemplary embodiment of the present invention, light travels in the optical thin film  110  along the plane of the optical thin film  110 , rather than through the thickness  160  of the optical thin film  110  as illustrated in  FIG. 1 . That is, light can either propagate through the optical thin film  110  at an acute angle with respect to the z-axis  130  or through the optical thin film  110  generally parallel to the plane defined by the x-axis  140  and the y-axis  150 . In one exemplary embodiment of the present invention, the optical thin film  140  waveguides light. In one exemplary embodiment of the present invention, the optical thin film  140  is etched, for example in an inductively coupled plasma (“ICP”) process, to form a structure that waveguides light. This waveguide structure can provide single mode light propagation. 
     The thickness  160  of the optical thin film  140  can be of an appropriate dimension to support single mode propagation. Such thickness  160  is related to the wavelength of the single mode light and the refractive index differential between the optical thin film  110  and the surrounding materials  120 ,  185 , which can function as waveguide cladding. The relationships between these parameters are well known to those skilled in the art and available to manipulate each parameter to generate specific optical effects. In one exemplary embodiment, the thickness  160  of the optical thin film  110  is approximately nine microns, and the mode field of the single mode light guided there through is approximately ten microns for a wavelength in the range of approximately 1310 to 1550 nanometers (“nm”). 
     In one exemplary embodiment of the present invention, a silicon photonic device comprises the optical film  110 . The film  110 , which may be thin, thick, or of arbitrary thickness, and the silicon photonic device can be a monolithic structure or a unitary structure or multiple structures fastened/attached to one another. In such an embodiment, the film  110  can either conduct light though one or both of its faces  170 ,  180  or between/along those faces  170 ,  180  in a waveguide manner as discussed above. The silicon photonic device can comprise a lasing device that comprises silicon, a silicon optical amplifier (“SiOA”), a silicon-based modulator, an attenuator comprising silicon, a silicon-based detector, a silicon-based emitter, and/or an optically-pumped silicon amplifying device, to name a few examples. Manipulating or changing the refractive index of an optical film  110  of a silicon photonic device can manipulate or change the optical function, performance, or characteristics of that device, for example adjusting it to comply with a performance specification. 
     The eight documents listed immediately below disclose exemplary silicon photonic devices that can comprise an optical film, such as the optical film  110 , whose optical properties can be adjusted using the technology, methods, processes, teachings, or invention(s) discussed herein. That is, according to exemplary embodiments of the present invention, the optical properties of the optical films or materials of the systems disclosed in the below eight documents can be adjusted to yield a corresponding performance adjustment in those systems. The films disclosed in those eight documents can be adapted to facilitate adjustment, for example by exposing the device&#39;s material to hydrogen to enhance UV sensitivity. Further, films or materials that are adjustable can be added to or integrated with the devices disclosed in those documents. The disclosures of the following eight documents are hereby incorporated by reference:
     1) “ A Continuous - Wave Raman Silicon Laser ,” by Haisheng Rong, Richard Jones, Ansheng Liu, Oded Cohen, Dani Hak, Alexander Fang, and Mario Paniccia,  Nature  3346, Mar. 2, 2005. Available at www.nature.com/nature and at www.intel.com.   2) “ An All - Silicon Raman Laser ,” by Haisheng Rong, Ansheng Liu, Richard Jones, Oded Cohen, Dani Hak, Remus Nicolaescu, Alexander Fang, and Mario Paniccia,  Nature , Volume 433, Jan. 20, 2005. Available at www.nature.com/nature and at www.intel.com.   3) “ Silicon Shines On ,” by Jerome Faist,  Nature  Volume 433, Feb. 17, 2005. Available at www.nature.com/nature and at www.intel.com.   4) “ Continuous Silicon Laser, Intel researchers create the first continuous silicon laser based on the Raman effect using standard CMOS technology ,” by Sean Koehl, Victor Krutul, and Mario Paniccia, published by Intel Corporation as a white paper, 2005. Available at www.intel.com.   5) “ Intel&#39;s Research in Silicon Photonics Could Bring High - speed Optical Communications to Silicon ,” by Mario Paniccia, Victor Krutul, and Sean Koehl, published by Intel Corporation as a white paper, February 2004. Available at www.intel.com.   6) “ Silicon Photonics ,” by Mike Salib, Ling Liao, Richard Jones, Mike Morse, Ansheng Liu, Dean Samara-Rubio, Drew Alduino, and Mario Paniccia,  Intel Technology Journal , Volume 08, Issue 02, May 10, 2004. Available at www.intel.com (http://developer.intel.com/technology/itj/index.html).   7) “ Introducing Intel&#39;s Advances in Silicon Photonics ,” by Mario Paniccia, Victor Krutul, Sean Koehl, published by Intel Corporation as a white paper, February 2004. Available at www.intel.com.   8) “ Intel Unveils Silicon Photonics Breakthrough: High - Speed Silicon Modulation ,” by Mario Paniccia, Victor Krutul, and Sean Koehl,  Technology@Intel Magazine , February/March 2004. Available at www.intel.com.   

     Referring now back to  FIG. 1B , the optical thin film  110  can be operative to manipulate light  125  of various wavelengths regions. In one exemplary embodiment of the present invention, the optical thin film  110  manipulates visible light  125  between about 400 nm and about 700 nm. In one exemplary embodiment of the present invention, the optical thin film  110  manipulates light  125  in the near infrared region between about 700 nm and about 3500 nm. In one exemplary embodiment of the present invention, the optical thin film  110  manipulates light between about 700 nm and about 900 nm. In one exemplary embodiment of the present invention, the optical thin film  110  manipulates UV light  125 . In one exemplary embodiment of the present invention, the optical thin film  110  manipulates light  125  at typical single-mode optical networking wavelengths, in the region between about 1200 nm and about 1750 nm. In one exemplary embodiment of the present invention, the optical thin film  110  manipulates light  125  in one or more spectral regions that provide low-loss transmission over optical fibers. Such low-loss spectral regions can be windows of low water absorption, such as about 1310 nm and about 1550 nm. The thickness  160  of the optical thin film  110  can be selected to provide specific manipulation effects of light  125  or electromagnetic radiation. 
     In one exemplary embodiment of the present invention, the optical thin film  110  is an element in a sensor system and is operative to guide light in a direction generally parallel to the x-y plane  140 ,  150 . The outer surface  180  of the optical thin film  110  provides the sensing interface. Light propagating in the optical thin film  110  interacts with the medium  185  that is adjacent this sensing interface. 
     In one exemplary embodiment of the present invention, the optical thin film  110  is deposited in a deposition process, which can be ion plating, ion assisted deposition (“IAD”), ion sputtering, plasma assisted deposition, or magnetron sputtering. Ion plating can be carried out with evaporation and/or with plasma. The deposition process can be a vacuum process, conducted in a deposition chamber at a pressure of less than one atmosphere. These deposition processes can be reactive, for example reactive ion beam sputtering. Intrpclucing nitrogen gas into the deposition chamber while sputtering silicon dioxide can form a film of silicon oxynitride in a reactive manner. Alternatively, a silicon target can be sputtered while introducing oxygen and nitrogen into the deposition chamber. 
     In one exemplary embodiment of the present invention, a physical vapor deposition (“PVD”) process such as evaporation or sputtering forms the optical thin film  110 . In one exemplary embodiment of the present invention, electron-beam (“e-beam”) evaporation or dual e-beam evaporation forms the optical thin film  110 . In one exemplary embodiment of the present invention, e-beam IAD beam ion assisted deposition forms the thin film. Reactive e-beam IAD can form the optical thin film by introducing nitrogen into the deposition chamber during e-beam IAD using a silicon dioxide target in a reactive process that can be a stoichiometric process. 
     In one exemplary embodiment of the present invention, direct current (“DC”) sputtering or radio frequency (“RF”) sputtering forms the optical thin film  110 , which can also be carried out in a reactive manner. The optical thin film  110  can also be formed with pulsed laser deposition. The optical thin film  110  can also be printed or spun on to a substrate  120  or formed with a sol gel process. 
     In one exemplary embodiment of the present invention, the optical thin film  110  is formed with epitaxial growth, such as liquid phase epitaxy (“LPE”), molecular beam epitaxy (“MBE”), vapor phase epitaxy (“VPE”). In one exemplary embodiment of the present invention, the optical thin film  110  is formed with a chemical vapor deposition (“CVD”) process such as atmospheric pressure chemical vapor deposition (“APCVD”), low-pressure chemical vapor deposition (“LPCVD”), very low-pressure chemical vapor deposition (“VLPCVD”), plasma-enhanced chemical vapor deposition (“PECVD”), laser-enhanced chemical vapor deposition (“LECVD”), metal-organic chemical vapor deposition (“MOCVD”), or electron-cyclotron resonance chemical vapor deposition (“EPCVD”). The above list of processes for forming optical thin films  110  in accordance with various embodiments of the present invention is an exemplary, rather than an exhaustive, list. 
     Turning now to  FIG. 1C , this figure illustrates a graph  155  with plots  155 ,  175  of physical and optical properties of a region of the optical thin film  110  of  FIGS. 1A and 1B  in accordance with an exemplary embodiment of the present invention. This graph  155  illustrates an exemplary variation of optical thickness  175 , which is an exemplary optical property, along the plane of the optical thin film  110 . 
     As illustrated in the plot  155 , the physical, or geometric, thickness  160  of the optical thin film  110  is uniform along the x-axis  140 . That is, the spatial separation  160  between the outer surface  180  and the inner surface  170  of the optical thin film  110  is essentially constant across the surface  170  of the substrate  120 , which follows the plane of the x- and y-axes  140 ,  150 . As described above, this physical thickness  160  can have a range of actual numerical values but is typically less than approximately five times the wavelength of the light  125  that the optical thin film  110  manipulates with thin film interference. 
     In addition to a physical thickness  160 , the optical thin film  110  has an optical thickness  175  that is a function of the refractive index of the material of the optical thin film  110  and the geometric or physical thickness  160 . The refractive index, or index of refraction, of a material is the speed of light in a vacuum divided by the speed of light in the material. Since light propagates more slowly in ordinary materials than in a vacuum, the refractive index of an ordinary material is greater than one, often between one and four. The optical thickness parameter  175  is physical thickness  160  multiplied by refractive index. The optical thickness  175  of a section of optical material, such as an optical thin film  110 , is the material section&#39;s physical thickness  160  multiplied by the material section&#39;s refractive index. Since refractive index is greater than one for normal optical materials, a section of ordinary material typically has an optical thickness  175  that is greater than its corresponding physical thickness  160 . 
     The plots  160 ,  175  of  FIG. 1C  illustrate a variation in the optical thickness  175  of the optical thin film  110  along the x-axis  140 . Although the illustrated variation in optical thickness  175  is a linear variation, the variation can be nonlinear or conform to other patterns. That is, the optical thickness  175  of the optical thin film  110  varies uniformly from one value to another value between two spatial regions displaced along the x-axis  140 . 
     The refractive index of the optical thin film  110  varies in correspondence to the variation of the optical thickness  175 . That is, the refractive index of the thin film material varies across the surface of the optical thin film  110 , thereby providing a gradient pattern of optical thickness  175 . 
     The refractive index  175  of the material of the optical thin film  110  can also vary along the y-axis  150  (not illustrated in  FIG. 1C ). Alternatively, the optical thin film  110  can have a refractive index  175  that varies in the x-dimension  140  but is essentially constant in the y-dimension  150 . 
     While the graph  155  illustrates a variation in optical thickness  175 , this optical property is exemplary, and the present invention supports variations in other optical properties across the optical thin film  110 . In one exemplary embodiment of the present invention, the optical thin film  110  provides a variation in the phase of light transmitted or reflected there through, such as a variable phase shift that is correlated to refractive index, across the surface of the optical thin film  110 . 
     In one exemplary embodiment of the present invention, the refractive index of the optical thin film  110  varies along the z-axis  130  in addition to varying along the x-axis  140 . For example the optical thin film  110  can be a layer in a rugate optical filter with a refractive index variation along the plane of the optical thin film  110 . A rugate filter can have a blended interface between the thin film layers of the filter. 
     Turning now to  FIG. 1D , this figure illustrates a graph  178  of physical and optical properties  160 ,  176 ,  177  of a region the optical thin film  110  of  FIG. 1A  in accordance with an exemplary embodiment of the present invention. This graph  178  illustrates an exemplary variation of physical thickness  160  along the plane of the optical thin film  110  and an offsetting variation in refractive index  176  corresponding to the physical thickness variation  160 . The plot  178  is overlaid with a cross section view of the optical thin film  110 , wherein the cross section is effectively the plot of physical thickness  160 . The variation in physical thickness  160  can result from a manufacturing defect or an intentional slope, for example. Such slope in the physical thickness of the layers in a thin film optical filer is sometimes called “wedge” and can be problematic for imaging applications, such as Raman or fluorescence microscopy, that entail transmitting an image through the filter. While the slope in thin film physical thickness  160  can distort the image, wedge can be corrected by increasing or adjusting the refractive of the thin film layers in a pattern  176  that offsets the deviation in physical thickness  160 . 
     The optical thickness  177  that results from the physical thickness profile  160  and the refractive index profile  176  is essentially uniform. That is, a region of the optical thin film  110  can have an unintentional slope in physical thickness  160  and an intentional refractive index variation  176  that offsets the optical effect of this geometric slope  160  and thereby provides a uniform optical thickness  177  along the direction of the geometric slope  160 . Adjusting the refractive index  176  of a region of an optical thin film  110  that has an unintentional variation in physical thickness  160  facilitates compensating for manufacturing defects. 
     Turning now to  FIG. 2 , this figure illustrates a stack of optical thin films  220 ,  230 ,  240  on a substrate  120  in accordance with an exemplary embodiment of the present invention. The optical system  200  of the thin film stack and the substrate  120  can be a thin film optical filter, such as a DWDM filter or a laser-rejection filter for laser-Raman spectroscopy. As an optical filter, the optical system  200  can be a high pass filter, a low pass filter, a band pass filter, or a notch filter. Alternatively, the optical system  200  can provide gain compensation, gain flattening, chromatic dispersion compensation, group delay correction, spectrally selective delay in an optical network or other optical manipulation based on interference of light interacting with each of the thin film layers  220 ,  230 ,  240 , the interfaces between these thin film layers  220 ,  230 ,  240 , and the substrate  120 . In one exemplary embodiment of the present invention, this optical system  200  is an element in a frequency locking system, such as an etalon-based “locker,” for a telecommunication application. One or more of the thin film layers  220 ,  230 ,  240  in the stack has an adjusted optical property, such as refractive index. 
     The thin film layers  220 ,  230 ,  240  are deposited on a substrate  120 . The substrate can be glass, BK-7 glass, silicate, fused silica, silicon, or other optical material that is generally transparent to the wavelengths of the light that the optical system  200  manipulates. 
     The stack includes thin film layers  220 ,  230  of alternating refractive index disposed face-to-face or adjacent one another. That is, the layers denoted with the reference number “ 220 ” are high refractive index, while the layers denoted with the reference number “ 230 ” are relatively low refractive index. In one exemplary embodiment of the present invention, the material compositions of the high refractive index layers  220  and the low refractive index layers  230  include tantalum pentoxide (Ta 2 O 5 ) and silicon dioxide respectively. In one exemplary embodiment of the present invention, the composition of at least one of the layers  220 ,  230  includes silicon oxynitride. In one exemplary embodiment of the present invention, the composition of at least one of the thin film layers includes diamond, such as diamond-like carbon, magnesium fluoride (MgF 2 ), dielectric material, silicon, titanium dioxide (TiO 2 ), aluminum oxide (Al 2 O 3 ) or other oxide. In one exemplary embodiment of the present invention, the composition of at least one of the thin film layers  220 ,  230  includes germanium (Ge). In one exemplary embodiment of the present invention, at least one of the alternating refractive index thin film layers  220 ,  230  is an essentially pure optical material. The packing density of the alternating refractive index layers  220 ,  230  is typically greater than 95 percent. The physical properties of these layers  220 ,  230  approach that of bulk material. 
     The stack of optical thin film layers  220 ,  230 ,  240  includes two spacer layers  240 , thus providing a multi-cavity interference device. That is, the thin film stack includes a plurality of cavities that function as an etalon. The spacer layers  240  are each disposed between two banks  250  of layers  220 ,  230  of alternating, high-low refractive index material. In one exemplary embodiment of the present invention, the composition of one or more of the spacer layers  240  includes silicon dioxide or other dielectric material. In one exemplary embodiment of the present invention, the composition of one or more of the spacer layers  240  includes silicon oxynitride. In one exemplary embodiment of the present invention, the composition of one or more of the spacer layers  240  includes germanium and/or hydrogen. In one exemplary embodiment of the present invention, each of the spacer layers  240  is deposited as an essentially pure optical material. The packing density of the spacer layers  240  is typically greater than 95 percent. The physical properties of these layers  240  approach that of bulk material. 
     The adjusted optical property can be graduated along the x-axis  140  so that the optical property has one value at a first location along the x-axis  140  and another value at a second location along the x-axis  140 . That is, the optical property can follow a profile or pattern along the plane of one or more layers  220 ,  230 ,  240 . Alternatively, the adjusted optical property can be uniform along the x-axis  140 . That is, the adjusted optical property can have essentially the same value along the x-axis  140 . In one exemplary embodiment of the present invention, the adjusted optical property is refractive index of the material of one or more layers  220 ,  230 ,  240 . 
     In one exemplary embodiment of the present invention, banks  250  of high-index layers  220  and low-index layers  230  are composed of tantalum pentoxide and silicon dioxide respectively and the spacer layers  240  are composed of silicon oxynitride. This configuration facilitates preferentially adjusting the refractive index of the spacer layers  240  with respect to the other layers  220 ,  230 . In one exemplary embodiment of the present invention, the high index layers  220  are tantalum pentoxide; the low-index layers  230  are composed of magnesium fluoride; and the spacer layers  240  are composed of silicon oxynitride. Preferentially adjusting the refractive index of one or more spacer layers  240  in a thin film optical filter with respect to the other layers  220 ,  230  facilitates adjusting the optical properties of the filter in a controlled manner, for example tuning the center wavelength of a DWDM band-pass filter. 
     In one exemplary embodiment of the present invention, the layers  220 ,  230 ,  240  are formed with e-beam IAD. The high-index layers  220  and low-index layers  230  are composed of tantalum pentoxide and silicon dioxide and are deposited on an optical substrate  120  using tantalum pentoxide and silicon dioxide targets in sequence for each respective layer  220 ,  230 . When the deposition process progresses to the spacer layer  240 , the process activates the silicon dioxide target and adds nitrogen to the deposition chamber, thus forming a silicon oxynitride spacer layer that is more receptive to UV adjustments of refractive index than the other layers. After forming the silicon oxynitride spacer layer  240 , the deposition process shuts off the nitrogen supply, eliminates the nitrogen from the deposition chamber, and returns to depositing silicon dioxide and tantalum pentoxide, with only minimal or trace concentrations of nitrogen in the high-index layers  220  and low-index layers  230  of the layer bank  250 . 
     In one exemplary embodiment of the present invention, the stack of thin films  200  is a thin film optical filter or other multi-layer interference device and is formed by ion beam sputtering. At least one of the layers is composed of silicon oxynitride and is formed by dual ion beam sputtering. This silicon oxynitride layer can be represented as SiO x N y  and can have a pre-adjustment refractive index between approximately 1.5 and 2.0, depending on the relative values of “x” and “y.” 
     Turning now to  FIG. 3 , this figure illustrates an optical thin film  320  undergoing an optical property adjustment in accordance with an exemplary embodiment of the present invention. The optical thin film  320  of the optical system  300  adheres to a substrate  120 , which facilitates handling and provides structural support for the optical thin film  320 . The refractive index optical property of the optical thin film  320  responds to UV light  330  directed thereto. That is, exposing the optical thin film  320  to UV light  330  adjusts the refractive index of the optical thin film material. 
     The composition of the optical thin film  320  includes germanium (Ge)  320 , silicon dioxide (SiO 2 ), and, in at least one phase of the refractive index adjustment process, hydrogen (H 2 )  340  and UV light  330 . The Ge can take the form of GeO (germanium monoxide) or GeO 2  (germanium dioxide), or a combination thereof. In one exemplary embodiment of the present invention, the composition of the optical thin film  320  is approximately equivalent to the composition of the core of standard, telecommunication-grade single-mode optical fiber, such as the optical fiber marketed by Corning Incorporated of Corning N.Y. under the product designation SMF-28. The composition is approximately three percent (3%) Ge—O 2  (one germanium atom—two oxygen atoms). 
     A four-stage process adjusts the refractive index of the optical thin film  320 . The four stages are, at a high level, subjecting the optical thin film  320  to hydrogen  340 , exposing the optical thin film  320  to 157-nm UV light  330 , thermally annealing the optical thin film  320 , and exposing the optical thin film  320  to 248-nm UV light  330 . 
     The optical thin film  310  is exposed to hydrogen  340  in a vessel pressured to approximately 1900 pounds per square inch (“psi”) for approximately five days. During this time, the hydrogen  340  diffuses into the optical thin film  310 . The hydrogen exposure time for thicker optical thin films is typically longer than the exposure time for thinner optical thin films. 
     The area of the optical thin film  320  undergoing refractive index adjustment is exposed to a beam of UV light  330  from a 157-nm F 2  (fluorine) laser. Lambda Physik of Fort Lauderdale, Fla. supplies suitable lasers, such as the company&#39;s Lambda Physik  220  product line. The beam of 157 nm laser light  330  is approximately 20 mm by 2.45 millimeters. The beam can also be shaped to coincide with an intended pattern of refractive index adjustment. 
     Placing the optical thin film  320  in an argon-flushed vessel during the illumination process eliminates the potential for absorption of the UV illumination light  330  by transmission in air. In one exemplary embodiment of the present invention, the optical thin film  310  is exposed to UV illumination  330  while the optical thin film  310  is in a hydrogen environment, thus avoiding any out-diffusion loss of hydrogen  340  from the optical thin film  320  prior to UV-light exposure. The optical thin film  310  receives pulses of laser light  330  having approximately 15 nanoseconds duration, cycled at 10 to 100 Hertz (“Hz”), or cycles per second. The fluency is approximately 3 mJ/cm 2  (three milli-joules per centimeter squared). 
     Annealing the optical thin film  320  at approximately 150° C. (one hundred fifty degrees Centigrade) for approximately 24 hours drives off residual hydrogen  340  from the optical thin film material. 
     A second beam of laser light  330  of approximately 248 nm and approximately 400 mJ/cm 2  is applied to the optical thin film  320  for approximately five minutes. A KrF (krypton fluorine) laser emits the beam at approximately 20 Hz. GSI Lumonics, Inc. of Billerica Mass. supplies suitable lasers under the PULSEMASTER trade name. 
     The effect of the 248-nm exposure is related to the dosage of the 157-nm exposure. In one exemplary embodiment of the present invention, the optical thin film  320  is exposed to a total of approximately 12 J/cm 2  of the 157-nm light  330  and approximately 5 kJ/cm 2  of 248-nm light  330 . In one exemplary embodiment of the present invention, the 157-nm exposure is between 200 and 300 J/cm 2  and is followed by an approximately 6-minute exposure to the 248-nm light  320 . In one exemplary embodiment of the present invention, the optical thin film  320  is exposed to approximately 2100 J/cm 2  of 157 nm laser light  330 . 
     In one exemplary embodiment of the present invention, the optical thin film  320  exhibits OH defects, such as Si—OH defects (not illustrated) or Ge—OH defects (not illustrated) following refractive index adjustment. Such defects can be nanometer-scale, for example smaller than 100 nm, 500 nm 1,000 nm, 10 microns, or in a range thereof. In one exemplary embodiment of the present invention, the optical thin film  320  includes oxygen deficient centers, which increase the index of refraction, as a result of a UV treatment. In one exemplary embodiment of the present invention, the optical thin film  320  includes Si—OH as a result of interaction between the infused hydrogen  340  or other agent and the dose of UV light  330 . Thus, the film  320  can comprise nano-scale structures, features, centers, or particles uniformly dispersed throughout that are individually or collectively associated with instances or occurrences of the agent, the energy dose, and/or the refractive index change. 
     In one exemplary embodiment of the present invention, the optical thin film  320 , with approximately 3% Ge—O 2  as described above, is deposited on the optical substrate  120  in a vacuum deposition process. In another exemplary embodiment of the present invention, the optical thin film  320  is grown on a silicon substrate  120 . That is, the substrate  120  is silicon and the optical thin film  320  is composed of an oxide layer on the surface of the silicon. 
     In another exemplary embodiment of the present invention, the optical thin film  320  is formed by mechanically processing a boule or preform of optical fiber material. That is, the optical thin film  320  can be formed by processing a blank or rod of the fiber optic material that is ordinarily drawn into optical fiber in a process conducted in a drawing tower. The stock of fiber optic material can be temporarily attached, for example in a jig configuration, to a base substrate and ground down through mechanical grinding and polishing. Alternatively, the material can be thinned with chemical, plasma-based, ion-based etching conducted in a vacuum environment or ICP etching. 
     The compensation of the optical thin film  320  can have a specific concentration within a range of concentrations of Ge—O 2 , for example between 0.25 percent and 15 percent. In one exemplary embodiment, the optical thin film  320  is essentially pure silicon dioxide, with only trace levels of Ge  320 , prior to hydrogen treatment and UV exposure. 
     In one exemplary embodiment of the present invention, the optical thin film  320  includes boron and Ge  320 . The boron is co-doped with approximately 12% Ge  320  in SiO 2 . 
     In one exemplary embodiment of the present invention, a silicon dioxide optical thin film  320  composed of approximately 20 mole percent GeO 2  and approximately 2700 parts per million (“ppm”) Er +3  is dosed with 248 nm light  330  from a KrF excimer laser with a pulse duration of approximately 10 ns. Tuilaser AG of Munich Germany supplies a suitable laser under the product name Braggstar  500 . The laser delivers approximately 12 mJ of energy at 200 Hz and a pulse fluence of approximately 180 mJ/cm 2 . Exposing the optical thin film  320  to this light  330  in a dose lasting a time period in the range between approximately thirty seconds and approximately 10 minutes generates a refractive index adjustment. Shaping the beam from this laser with various lens configurations alters the intensity pattern of delivered light  330  and the resulting pattern of refractive index variation. In one exemplary embodiment of the present invention, this optical thin film  320  can be treated with hydrogen  340  prior to the dose of UV light  330 . 
     Excimer lasers typically exhibit a relatively low coherence length and provide a high peak energy level, which in some circumstance can damage or stress an optical thin film  320 . Other UV laser sources can be used to adjust the refractive index of an optical thin film  320 . For example, a frequency doubled argon ion laser exhibits a higher coherence length and a lower peak power than is typical of excimer lasers. The relatively low peak power of such a laser can minimize the potential for the laser light to damage an optical thin film  320 , by overheating, cracking, chipping, etc. 
     In one exemplary embodiment of the present invention, a silicon dioxide optical thin film  320  has approximately 15 mole percent GeO 2 , 1.1 mole percent of Sn, and approximately 100 ppm of Er +3 . This codoped optical thin film  320  is subjected to a dose, measure, treatment, amount, or beam of 248 nm light  330  with a pulse duration of approximately 10 ns, delivering approximately 12 mJ at 200 Hz and a pulse fluence of approximately 180 mJ/cm2. The dose lasts approximately five to ten minutes, depending on the level of refractive index adjustment desired. In one exemplary embodiment of the present invention, this optical thin film  320  can be treated with hydrogen  340  prior to the dose of UV light  330 . 
     In one exemplary embodiment of the present invention, a silicon dioxide optical thin film  320  has approximately 15 mole percent GeO 2 , 5000 ppm of Sn +3 , and approximately 500 ppm of Er +3 . This optical thin film  320  is subjected to a dose of 248 nm light  330  with a pulse duration of approximately 10 ns, delivering approximately 12 mJ at 200 Hz and a pulse fluence of approximately 180 mJ/cm2. The dose lasts approximately five to ten minutes, depending on the level of refractive index adjustment desired. In one exemplary embodiment of the present invention, this optical thin film  320  can be treated with hydrogen  340  prior to the UV light treatment. 
     In one exemplary embodiment of the present invention, the thin film  320  illustrated in  FIG. 3  is deposited on the substrate  120  as essentially pure silicon dioxide, which typically includes small quantities of silicon monoxide and various impurities. Placing the optical thin film  320  in a vessel having a hydrogen environment of approximately 2000 psi for three days loads the optical thin film  320  with hydrogen  340 , which can act as a catalyst. The vessel includes a sapphire window that is transparent to UV light  330 . A dose of UV light  330 , transmitted through this window, interacts with the optical thin film  320  and adjusts the refractive index. The dose of UV light  330  can be delivered in a pulsed format or as a continuous beam. Annealing the optical thin film  320  in a vacuum or air environment following the UV treatment drives out residual hydrogen  340 , stabilizes the refractive index, and avoids a significant, gradual index change over time when deployed in an application. Placing the optical thin film  320  in an oven having a temperature between approximately 150° C. and 500° C. can provide adequate annealing conditions. 
     In one exemplary embodiment of the present invention, the optical thin film  320  is composed of fused silica with a GeO 2  mole-fraction concentration between 0.12 and 0.16. In one exemplary embodiment of the present invention, the optical thin film  320  is composed of vacuum deposited silica with a GeO 2  mole-fraction concentration between 0.02 and 0.16. In one exemplary embodiment of the present invention, the GeO 2  mole-fraction concentration varies across the surface of the optical thin film  320  from a level that approaches zero to a level of approximately 2.0. In one exemplary embodiment of the present invention, the optical thin film  320  is composed of silicon dioxide with a GeO 2  mole-fraction concentration greater than 0.04. 
     In one exemplary embodiment of the present invention, the optical thin film layer  320  is composed of silicon oxynitride (SiO x N y ) grown by a hybrid deposition based on the combination of pulsed laser deposition of silicon in an oxygen background together with a plasma based nitrogen source. Controlling the partial pressure of nitrogen with respect to oxygen in the deposition chamber controls the nitrogen content in the optical thin film  320 . 
     In one exemplary embodiment of the present invention, the optical thin film  320  is produced by reactive magnetron sputtering of a silicon target in a variable mixture of oxygen and nitrogen, which are the reactive gasses. The resulting optical thin film  320  is composed of silicon oxynitride (SiO x N y ). Adjacent optical thin film layers in a stack configuration as illustrated in  FIG. 2  can comprise varying concentrations of oxygen and/or nitrogen, which impacts the refractive index. That is the values of “x” and “y” in the SiO x N y  can vary in each layer. As such, the sensitivity or responsiveness of each layer to UV light  330  can also vary with this composition. In one exemplary embodiment of the present invention, the concentrations of silicon and nitrogen are graduated or smoothly varying along the thickness of the optical thin film  320 . In this configuration, the optical thin film  320  can be a layer in a rugate filter and can be deposited in a single deposition chamber, without interruption of the plasma in the chamber. 
     In one exemplary embodiment of the present invention, the optical thin film  320  is amorphous silicon oxynitride with at most a trace level of Ge  320  deposited on a glass substrate  120  with electron beam physical vapor deposition (“EB-PVD”) at a low temperature. 
     In one exemplary embodiment of the present invention, the optical thin film  320  is formed by reactive RF sputter deposition. Sputtering a silicon nitride target in an oxygen environment forms the optical thin film  320  with a composition of silicon oxynitride. Varying the flow rate of oxygen in the deposition chamber controls the refractive index of the optical thin film  320  between approximately 1.46 and 2.3. The RF power can be approximately 500 watts and the refractive index of the deposited material can vary in a linear manner with respect to the oxygen flow rate. With a sputtering gas having approximately ten percent (10%) oxygen and ninety percent (90%) argon, adjusting the gas flow rate between approximately nine standard cubic centimeters per minute (“sccm”) and twenty one sccm can produce a corresponding and essentially linear control of refractive index between approximately 1.8 and 1.5. 
     In one exemplary embodiment of the present invention, the optical thin film  320  is a single-layer antireflective coating and has a composition of silicon oxynitride generated in an ion-beam sputter deposition system. Such film can be void of Ge  320 . The optical thin film  320  can be a stoichiometric layer, formed in a reactive ion-beam sputtering process and having a high density and essentially no so-called columnar structures visible in scanning electron microscopy analysis. Such columnar structures, which are typically oriented along the thickness of imperfect optical thin films, are generally recognized in the art as being associated with physical defects or imperfections. The substrate  120  can be a semiconductor material including a semiconductor laser facet or an optical amplifier facet. After adjustment with UV light  330 , the optical thin film  320  can impart a reflectivity of less than 10 −4  to the facet. Alternatively, the optical thin film  320  can be a high reflective coating on a laser facet, for example on the back facet of a laser die or similar semiconductor amplifying device. 
     In one exemplary embodiment of the present invention, the optical thin film  320  is formed with a physical vapor deposition process based on RF sputtering, which can include dual frequency RF sputtering. Several options are available to control the refractive index of the optical thin film  320  during the deposition process. Adjusting the deposition temperature controls this refractive index so that an increase in deposition temperature can increase the refractive index of the optical thin film  320 . Increasing the RF power applied to the target during the deposition process also can increase the refractive index. Adding a reactive gas to the sputtering chamber modifies the chemical composition of the deposited material of the optical thin film  320  and imparts the film  320  with a corresponding change in refractive index. Furthermore, using a target material in a specific oxidation state can control the refractive index of the optical thin film  320  during the deposition process. Following deposition, the optical thin film  120  can receive UV light  330 , for example as a dose or at a measured level. 
     The RF sputtering method is also applicable to depositing pure materials and mixed materials including rare earth dopants. Adding a reducing gas such as hydrogen to the chamber while the optical thin film  320  is forming can increase the refractive index of this film  320 , while refractive index can be decreased by adding an oxidizing gas such as oxygen. Replacing argon as the sputtering gas with approximately two percent (2%) hydrogen (H 2 ) in argon increase the refractive index by approximately two percent (2%) or more. In one exemplary embodiment of the present invention, a portion of this hydrogen remains in the optical thin film  320  after the deposition is complete and provides the hydrogen  340  that promotes refractive index adjustment by UV light  330 . 
     Replacing a portion of the argon gas that is often present in RF sputtering environment with nitrogen can adjust the composition and refractive index of the optical thin film  320 . For example replacing approximately thirty three percent (33%) of such argon with nitrogen while sputtering a silicon dioxide (SiO 2 ) target yields approximately seven percent (7%) increase in refractive index. An optical thin film  320  formed in this manner is believed to contain SiO x N y . 
     In one exemplary embodiment of the present invention, such nitrogen is introduced in the deposition chamber during the formation of a wave division (“WDM”) filter, DWDM filter, or course wave division multiplexing (“CWDM”) filter having layers of tantalum pentoxide and silicon dioxide. Injecting nitrogen into the chamber during the deposition of one or more silicon dioxide layers, such as a spacer layer in a multi-cavity filter causes those layers to contain SiO x N y . Evacuating the nitrogen from the chamber following formation of such layer minimizes the level of SiO x N y  in subsequent silicon dioxide layers. 
     In one exemplary embodiment of the present invention, a silicon monoxide (SiO) target is sputtered in an argon environment to produce an optical thin film  320  with a refractive index of slightly above 2. Altering the composition of the sputtering environment can lower the refractive index to approximately 1.75. 
     The Examples 1, 2, 3, and 4 below, which are not offered as limitations, further describe forming the optical thin film  320  based on a RF sputtering process in accordance with exemplary embodiments of the present invention. Additional details regarding these examples and forming the optical thin film using RF sputtering can be found in U.S. Pat. No. 6,506,289 to Demaray et al, issued on Jan. 14, 2003, the full disclosure of which is hereby incorporated by reference. 
     RF Sputtering Deposition Example 1, Deposition of SiO 2    
     An AKT 1600 series PVD production reactor (Applied Komatsu Technology, Santa Clara, Calif.) with modifications for RF application with ceramic tile targets can be used for RF sputter deposition of SiO 2 . A wide-area target of dimension 550 mm by 650 mm is fabricated from four quartz tiles, each 4 mm thick, Corning code 9780 glass (Corning Inc. Elmira, N.Y.). The tiles are finished to a smooth surface, chemically cleaned, rinsed with hot deionized water, dried, and sputter coated with several microns of chrome. The chrome-coated sides are bonded to a thin plate of titanium. The titanium backing plate is prepared for bonding by bead blasting, chemical cleaning, and plasma coating with silicon. The tiles and the backing plate are heated to approximately 180° C., and regions are coated with a layer of liquid indium. The tiles are placed on the backing plate with separation less than about 0.02 inches between each tile and from the edges of the region exposed to plasma. 
     A 150 mm p-type silicon wafer substrate is placed in the center of a glass carrier sheet having dimensions of approximately 400 mm by 500 mm. A power level of approximately 800 watts is applied to the target at 13.56 MHz. A race-track shaped magnet of approximate dimension 150 mm by 600 mm is swept over the face of the target at a rate of 4 seconds per one-way scan, or equivalently 8 seconds per complete cycle. That is, the sweeping rate is approximately eight Hz. The substrate temperature is uniformly held at approximately 40° C. The sputter gas is essentially pure argon, for example 99.99999% argon, at a flow rate of 60 sccm. The target-to-substrate distance is approximately 6.5 cm. Deposition efficiency is approximately 0.8 angstroms-kW-seconds. Film thickness and index of refraction are measured at five equally spaced points over the full face of the wafer using a FilmTek 4000 interferometer, which is available from Scientific Computing International of Carlsbad, Calif. The refractive index of the resulting optical thin film  320  at 1550 nm can be 1.437998, plus or minus approximately 0.001297 (0.09%). Physical thickness  160  of this optical film  320  can be approximately 9227.66 nm with a non-uniformity of approximately 6.8%. Table 1 presents exemplary results of this exemplary process in the “A” row. 
     RF Sputtering Deposition Example 2, Deposition of SiO 2  with Index Modification 
     SiO 2  films can be deposited by processes analogous to that described in RF Sputtering Deposition Example 1 above, varying deposition temperature, applied power, and process gas. Exemplary results are tabulated in Table 1 below. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 SiO 2  Thickness and 
               
               
                 Refractive Index as a Function of Deposition Conditions 
               
            
           
           
               
               
               
               
               
               
            
               
                 Ex- 
                   
                   
                 Sputter 
                 Refractive 
                 Physical 
               
               
                 emplary 
                 Power 
                 Temp. 
                 Gas/Flow 
                 Index 
                 thickness (nm) 
               
               
                 Entry 
                 (watts) 
                 ° C. 
                 (sccm) 
                 (1550 nm)* 
                 (% nonuniformity) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 A 
                 800 
                 40 
                 Ar/60 
                 1.437998 
                 9227.66 
               
               
                   
                   
                   
                   
                 (0.001297) 
                 (6.8%) 
               
               
                 B 
                 800 
                 150 
                 Ar/60 
                 1.440923 
                 3133.25 
               
               
                   
                   
                   
                   
                 (0.001979) 
                   
               
               
                 C 
                 800 
                 400 
                 Ar/60 
                 1.450126 
                 9295.86 
               
               
                   
                   
                   
                   
                 (0.000726) 
                 (4.8%) 
               
               
                 D 
                 1200 
                 150 
                 Ar/60 
                 1.448610 
                 9200 
               
               
                   
                   
                   
                   
                 (0.000976) 
                   
               
               
                 E 
                 800 
                 150 
                 2% H 2  in 
                 1.462198 
                 1287.15 
               
               
                   
                   
                   
                 Ar/60 
                 (0.001809) 
                   
               
               
                 F 
                 800 
                 150 
                 N 2 /20, 
                 1.580249 
                 608.87 
               
               
                   
                   
                   
                 Ar/40 
                 (0.008346) 
                   
               
               
                 G 
                 1400 
                 150 
                 N 2 /20, 
                 1.548439 
                 2354.80 
               
               
                   
                   
                   
                 Ar/40 
                 (0.006499) 
                   
               
               
                 H 
                 800 
                 400 
                 Ar/60 
                 1.450036 
                 9295.84 
               
               
                   
                   
                   
                   
                 (0.000702) 
                 (4.8%) 
               
               
                   
               
               
                 *Tables 1, 2 and 3 list exemplary standard deviations for the refractive indices. 
               
            
           
         
       
     
     In exemplary embodiments of the present invention corresponding to the exemplary entries in these tables, the refractive indices those entries can be adjusted after the deposition steps are complete to improve the refractive index standard deviation. This refractive index adjustment can proceed with the application of a dose of UV light  330  or other energy in accordance with one or more of the post-deposition adjustment processes described herein. That is, the variation in the refractive index of a sample, produced with the RF processes discussed in Examples 1, 2, 3, and 4 can be reduced after the thin film  320  is formed in the deposition chamber, for example after the film  320  is removed from the chamber. 
     In exemplary embodiments of the present invention corresponding to the exemplary entries in Table 1, the optical thickness of one or more of the optical thin films described in the entries is adjusted to compensate for the listed variation in physical thickness  320 . That is, post-deposition adjustment of the refractive index of the optical thin film  320  in one or more areas having a deviation in physical thickness  160  provides an optical thickness that is more uniform, smooth, flat, or uninterrupted than the physical thickness  160 . 
     RF Sputtering Deposition Example 3, Deposition of SiO and Erbium Doped SiO 
     Target tiles of SiO can be prepared from a powder of SiO by low temperature isostatic pressure. The tiles are cut and bonded to a backing plate. Mixed oxide tiles used to deposit erbium doped SiO are prepared by mixing powdered Er 2 O 3  and SiO in a ratio of 2 molar cation percent erbia. SiO and Er doped SiO films are deposited in a similar manner as described in Examples 1 and 2 above. Exemplary refractive index and physical thickness  160  are tabulated for SiO in Table 2 and for Er doped SiO (SiO:Er) with an Er concentration of approximately 2×10 20  Er atoms/cm 3 , (erbium atoms per cubic centimeter) in Table 3. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 SiO Thickness and 
               
               
                 Refractive Index as a Function of Deposition Conditions 
               
            
           
           
               
               
               
               
               
               
            
               
                 Ex- 
                   
                   
                 Sputter 
                 Refractive  
                 Physical  
               
               
                 emplary 
                 Power 
                 Temp. 
                 Gas/Flow 
                 Index 
                 thickness (nm) 
               
               
                 Entry 
                 (watts) 
                 ° C. 
                 (sccm) 
                 (1550 nm)* 
                 (% nonuniformity) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 J 
                 1000 
                 150 
                 Ar/60 
                 2.084500 
                 691.78 
               
               
                 K 
                 1000 
                 150 
                 N 2 /10, 
                 1.736693 
                 1000.96 
               
               
                   
                   
                   
                 Ar/50 
                 (0.010250) 
                   
               
               
                 L 
                 1000 
                 150 
                 N 2 /25, 
                 1.740680 
                 770.08 
               
               
                   
                   
                   
                 Ar/50 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 SiO:Er Thickness and 
               
               
                 Refractive Index as a Function of Deposition Conditions 
               
            
           
           
               
               
               
               
               
               
            
               
                 Ex- 
                   
                   
                 Sputter 
                 Refractive 
                 Physical  
               
               
                 emplary 
                 Power 
                 Temp. 
                 Gas/Flow 
                 Index 
                 thickness (nm) 
               
               
                 Entry 
                 (watts) 
                 ° C. 
                 (sccm) 
                 (1550 nm)* 
                 (% nonuniformity) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 M 
                 1000 
                 150 
                 Ar/60 
                 2.132870 
                 791.35 
               
               
                 N 
                 1000 
                 150 
                 N 2 /10, 
                 1.740480 
                 1501.04 
               
               
                   
                   
                   
                 Ar/50 
                 (0.017838) 
                   
               
               
                 O 
                 1000 
                 150 
                 N 2 /25, 
                 1.750910 
                 1400.11 
               
               
                   
                   
                   
                 Ar/50 
                   
                   
               
               
                 P 
                 1000 
                 150 
                 N 2 /50, 
                 1.792790 
                 786.78 
               
               
                   
                   
                   
                 Ar/25 
                   
                   
               
               
                 Q 
                 800 
                 400 
                 O 2 /3, 
                 1.454825 
                 1159.50 
               
               
                   
                   
                   
                 Ar/57 
                 (0.005425) 
               
               
                   
               
            
           
         
       
     
     RF Sputtering Deposition Example 4, Single and Dual Frequency RF Sputter Deposition of Silica 
     An AKT 1600 series PVD production reactor and wide area target as described in Example 1 can be used to create optical thin films  320  following Example 4. Exemplary high frequency (13.56 MHz) and low frequency (about 350 kHz) process powers are listed along with surface roughness and refractive index (RI) of the deposited optical thin films  320  tabulated Table 4 below. Depositions are conducted at Ar flow rates of 40 sccm and at or near room temperature, except as noted below. Refractive index at a wavelength of 1.5 microns can measured using a Film Tek 4000 instrument. Average surface roughness, R a , can be characterized via Atomic Force Microscopy (“AFM”) measurements using a NanoScope III 5000 instrument (Digital Instruments, Veeco Metrology Group, Santa Barbara, Calif.). 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Average Surface Roughness and Refractive  
               
               
                 Index of RF Sputtered Silica 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Exem- 
                 HF  
                 LW  
                   
                   
                   
                 LF/HF 
               
               
                 plary 
                 Power 
                 Power 
                 Ra 
                   
                 Total 
                 Power 
               
               
                 Entry 
                 (kW) 
                 (kW) 
                 (nm) 
                 RI 
                 Power 
                 Ratio 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 A 
                 2.3 
                 — 
                 2.988 
                 1.4492 
                 2.300 
                 0 
               
               
                 B 
                 2.3 
                 — 
                 2.804 
                 1.4494 
                 2.300 
                 0 
               
               
                 C 
                 2.3 
                 — 
                 3.412 
                 1.4473 
                 2.300 
                 0 
               
               
                 D 
                 2.0 
                 0.350 
                 1.818 
                 1.4538 
                 2.350 
                 0.175 
               
               
                 E 
                 2.0 
                 0.350 
                 1.939 
                 1.4533 
                 2.350 
                 0.175 
               
               
                 F 
                 2.0 
                 0.350 
                 2.007 
                 1.4547 
                 2.350 
                 0.175 
               
               
                 G 
                 2.0 
                 0.350 
                 2.571 
                 1.4520 
                 2.350 
                 0.175 
               
               
                 H 
                 1.7 
                 0.600 
                 1.729 
                 1.4560 
                 2.300 
                 0.353 
               
               
                 I 
                 1.7 
                 1.000 
                 1.445 
                 1.4617 
                 2.700 
                 0.588 
               
               
                 J 
                 3.0 
                 0.525 
                 2.359 
                 1.4542 
                 3.525 
                 0.175 
               
               
                 K** 
                 2.0 
                 0.350 
                 3.419 
                 1.4523 
                 2.350 
                 0.175 
               
               
                 L# 
                 3.0 
                 0.525 
                 4.489 
                 1.4449 
                 3.525 
                 0.175 
               
               
                   
               
               
                 **Deposition temperature approximately 225° C. 
               
               
                 #Ar flow rate 120 sccm. 
               
            
           
         
       
     
     Films deposited with a single frequency RF process (Table 4 entries A-C) typically have average surface roughness values in the range of 2.8 to 3.4 nm while the dual frequency process can produce films with systematically lower average surface roughness of between 1.4 and 2.6 nm. Keeping other process conditions the same, increasing the ratio of low frequency to high frequency power can decrease surface roughness. Refractive index typically can have the opposite proportional dependence on power ratio, such that increasing the low frequency power contribution results in optical thin films  320  with higher refractive index. The higher refractive index material has the lower average surface roughness. Thus, in similar processes, one thin film layer  320  can be produced by using dual frequency deposition without use of an dopant to modify the index, while using only the high frequency component produces a material of lower refractive index in another thin film layer  320 . These two layers  320  can be in a stack of optical thin film layers in a thin film optical filter or other multi-layer interference system. 
     PECVD Deposition Process Example 5 
     In one exemplary embodiment of the present invention, a PECVD process using SiH 4 , N 2 O, and NH 3  as precursor gases produce an optical thin film  320 . That is, an optical thin film  320  can be grown using silane, ammonia, and nitrous oxide as reactant gases in a PECVD process. Such film  320  can be deposited at 350° C., 13.56 MHz RF and 1 Torr pressure by varying the flow rates of N 2 O and NH 3  gases. The resulting refractive indices of the optical thin film layers  320  can be in the range between 1.47 and 2.0. 
     The refractive indices of the resulting optical thin films  320  can respond to an application of UV light  330  in a manner that varies according to process conditions. For example, the sensitivity of an optical thin film  320  can increase with increased nitrogen in the thin film material. 
     The net chemical reaction pathway for this process, carried out at approximately 350° C. and under RF, is believed to be:
 
SiH 4 +N 2 O+NH 3 →SiO x N y H z (solid)+H 2 O(gas)+N 2 (gas).
 
     Holding the flow rate of silane fixed at 180 sccm and varying the flow rates of N 2 O and NH 3 , as illustrated in Table 5 below, can control the refractive index of the optical thin film  320  as formed. 
     
       
         
           
               
             
               
                 TABLE 5 
               
               
                   
               
               
                 Exemplary Process Parameters for 
               
               
                 Forming Silicon Oxynitride Film with PECVD 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 Silane (2% SiH 4 /N 2 ) Flow Rate 
                 180 sccm 
               
               
                   
                 NH 3  Flow Rate 
                 Vary (0, 15, 30 sccm) 
               
               
                   
                 N 2 O Flow Rate 
                 Vary (20-450 sccm) 
               
               
                   
                 Process Pressure 
                 1000 mTorr 
               
               
                   
                 RF Power 
                 10 W 
               
               
                   
                 Temperature 
                 350° C. 
               
               
                   
                 Deposition Time 
                 15 minutes 
               
               
                   
                   
               
            
           
         
       
     
     Index of refraction and thickness of the samples can be measured with an ellipsometer using a wavelength of approximately 632.8 nm. Characterization can be carried out by varying the N 2 O flow and holding NH 3  flow fixed at three different values. The resulting refractive index of the optical thin films  320  resulting from this process can be between approximately 1.67 and 1.47. The refractive index can be also extended to approximately 2.0. The film&#39;s refractive index can decrease in correlation with increasing oxygen, probably because oxygen has higher chemical reactivity than nitrogen. Additionally, increasing the flow rate of ammonia can increase the refractive index of the optical thin film  320 . 
     With silane flow rate at 180 sccm and the ammonia flow rate at approximately zero sccm, varying the nitrogen flow rate from approximately 25 sccm to approximately 75 sccm yields thin film materials with refractive indices that vary respectively between approximately 1.55 and approximately 1.473. Further varying the flow rate of nitrogen from approximately 75 sccm to approximately 300 sccm produces minimal further shift in the refractive index of the resulting material. 
     With silane flow rate at 180 sccm and the ammonia flow rate at 15 sccm, varying the nitrogen flow rate from approximately 25 sccm to approximately 75 sccm yields materials with refractive indices that vary respectively between approximately 1.67 and approximately 1.565. Varying the flow rate of nitrogen from approximately 75 sccm to approximately 300 sccm can reduce the refractive index to approximately 1.485. Further varying the flow rate of nitrogen from approximately 300 sccm to approximately 450 sccm reduces the refractive index to approximately 1.48. 
     With silane flow rate at 180 sccm and the ammonia flow rate at 30 sccm, varying the nitrogen flow rate from approximately 50 sccm to approximately 300 sccm yields materials with refractive indices that vary respectively between approximately 1.625 and approximately 1.495. 
     The film thickness values  160  from these exemplary processes can vary roughly between 4300 angstroms and 3000 angstroms. Increasing nitrous oxide flow rate can increase the film&#39;s growth rate. The deposition rate typically decreases with an increase in ammonia flow rate. These properties can be attributed to the oxygen&#39;s greater affinity for reacting with silane. As the reactive oxygen concentration increases, the reactive oxygen can begin to dominate over nitrogen in the chemical reactions of the PECVD process. With increased nitrogen concentration in the optical thin film  320 , the film&#39;s growth rate typically decreases as the probability of nitrogen-related bondings increases. By varying the relative flow rates of oxygen and nitrogen, a thin film layer  320  can have a gradient of material characteristics along the film&#39;s thickness  160 . For example, the material near one side  170  of an optical thin film  320  can be oxygen rich, while the material near the opposite side  180  of the optical thin film  320  can be nitrogen rich. Varying the oxygen and nitrogen richness in this manner can induce spatially selective refractive index adjustment upon exposure to a dose of UV light  330 . 
     Additional details regarding PECVD processes for forming silicon oxynitride films in general, and the processes of Example 5 specifically, can be found in a Master Degree Thesis by Feridun Ay, submitted in September 2000 to Bilkent University entitled “ Silicon Oxynitride Layers for Applications in Optical Waveguides.”   
     Turning now from Example 5 to a discussion that is not specific to that example, a silicon oxynitride optical thin film  320  formed in a PECVD process, such as one of the exemplary processes described herein, can have residual hydrogen  340  in the optical thin film  320  that can be driven off by annealing. In one exemplary embodiment of the present invention, the residual hydrogen serves to enhance the index of refraction adjustment achieved with a UV treatment  330 . That is, the optical thin film  320  can undergo a refractive index adjustment by UV exposure  330  prior to an annealing step, so that the residual hydrogen  340  promotes this adjustment. Alternatively, the optical thin film  320  can be annealed prior to receiving a dose of UV light  330 . In this case, the optical thin film  320  can be either subjected to an exposure of hydrogen gas  340  or treated with UV light  330  without such exposure. 
     In one exemplary embodiment of the present invention the optical thin film  320  has a composition represented by the formula Si 1-x Ge x O 2(1-y) N 1.33y  wherein “x” is approximately between 0.05 to 0.6, and “y” is approximately between 0.14 and 0.74. The refractive index of the optical thin film  320  resulting from this composition can be approximately from 1.6 to 1.8, variable with composition, process conditions of film formation, heat treatment, and exposure to UV light  330 . The substrate  120  can be glass, silicon, or another optical or optoelectronic material. 
     Such optical thin films  320  can be formed with a PECVD process using a parallel plate reactor with a heated stationary platen, a low frequency (375 kHz) RF generator and matching network, and a gas manifold supplying silane, germane (germanium hydride, GeH 4 ), nitrous oxide, ammonia, and nitrogen into the process chamber through a showerhead nozzle that uniformly distributes the reactive gasses. 
     Additional process conditions and other details regarding forming the optical thin film  320  with a composition of silicon, germanium, oxygen, and nitrogen can be found in U.S. Pat. No. 6,449,420 to Akwani et al, issued on Sep. 10, 2000, the full disclosure of which is hereby incorporated by reference. 
     In one exemplary embodiment of the present invention, a frequency doubled copper vapor laser supplies light  330  of approximately 255 nm for dosing optical films  320  for refractive index adjustment. The average power can be between 500 and 1000 mW, with a repetition rate of approximately 6 kHz, pulse energy of less than approximately 0.2 mJ and a coherence length of approximately 40 millimeters (“mm”). The dose time can be set to correspond to a desired level of refractive index adjustment, but is typically between 2 seconds and 30 seconds. The laser beam can be expanded, thereby reducing the flux density of delivered light  330 , as required to avoid damaging the optical thin film  320  by excessive intensity. In one exemplary embodiment of the present invention, the dose parameters are: a laser beam power density of approximately 55 W/cm 2 ; an energy density per pulse of approximately 0.009 J/cm 2 ; and an application time of approximately 90 seconds. Oxford Lasers Inc, of Littleton Mass. provides suitable lasers in the company&#39;s FBG600 product line. A copper vapor laser can also emit approximately 600 mW of 255 nm light  330 , 600 mW of 271 nm light  330 , or 250 mw of 289 nm light  330 . 
     In one exemplary embodiment of the present invention, a UV lamp of the style sometimes used for curing epoxies supplies UV energy  330  for adjusting the refractive index of an optical thin film  320 . The UV lamp can output up to 40 watts per square centimeter of light  330  having a wavelength from 280 nm to 450 nm. Alternatively a UV lamp can output approximately 7 watts per square centimeter of light  330  at approximately 365 nanometers. Dymax Corporation of Torrington, Conn. supplies such lamps. 
     In one exemplary embodiment of the present invention, a xenon lamp supplies light  330  for adjusting an optical property of an optical thin film  320 . The xenon source can supply approximately 1000 watts per square centimeter of peak power or 100 watts per square centimeter of average power per pulse. In one exemplary embodiment of the present invention, the light intensity is increased by concentrating the light energy beam via lenses and/or waveguides until sufficient refractive index adjustment is achieved. That is, an appropriate level of light intensity to achieve a desired refractive index adjustment can be achieved by varying the light intensity until acceptable results are achieved. Xenon Corporation of Woburn Mass. supplies xenon lamps. 
     In one exemplary embodiment of the present invention, a deuterium light source supplies a beam of light  330  to the optical thin film  320  to adjust an optical property. The source can output light  330  between approximately 175 nm and 400 nm, providing approximately 400 watts of light  330 . The beam can be condensed or expanded according to the desired adjustment. Oriel Instruments of Stratford Conn. supplies suitable deuterium lamp products, such as the company&#39;s product sold as Model 66135 High Power Deuterium Source. 
     In one exemplary embodiment of the present invention, a tunable UV laser outputting light  330  between approximately 200 nanometers and 400 nanometers supplies light  330  for adjusting the refractive index of the optical thin film  320 . The wavelength of the UV light  330  is selected according to the desired refractive index adjustment. Tuning the laser to a specific wavelength facilitates dosing specific thin film layers  320  in a thin film optical filter with UV light  330 , thus preferentially treating such layers  320  with respect to other layers  320  in the filter. At a specific tuned wavelength, the UV light may resonant within the filter, thereby intensifying. SpectraPhysics of Mountain View, Calif. supplies tunable UV lasers. 
     Those skilled in the art will appreciate that in certain instances the optical thin film  320  in conjunction with surrounding materials, such as other adjacent optical thin films, generates an interference pattern of the UV dose. The interference pattern and the corresponding field intensities of UV light  330  internal to the optical thin film  320  can be modeled with software such as the software product named TFCalc, which is commercially available from Software Spectra Inc. of Portland, Oreg. The effect of an optical cavity in a stack of thin films  320  to heighten the intensity of delivered UV light  330  can also be addressed empirically. In one exemplary embodiment of the present invention, the wavelength of the UV light  330 , as well as the accompanying parameters, of the UV dose are determined by adjusting each of these parameters and adopting the parameters that yield particularly beneficial or optimal results. For example, in some circumstances blue light generates a desirable level of refractive index adjustment. 
     The Thin Film Center Inc. of Tucson Ariz. provides products and services that can be useful in modeling the interference-related interactions between UV light  330  and a system of thin films  320 , such as a DWDM thin film optical filter. The company&#39;s design and analysis package, marketed under the product name “The Complete Macleod” can be a useful tool. 
     The software products of Software Spectra Inc, the Thin Film Center, and other suppliers of analytical tools for optical coatings, can support modeling the result of adjusting the refractive index of specific thin film layers  320  in an optical system having a stack of thin film layers  320 . Such software can assist a designer in identifying the adjustment parameters of the spacer layers in a thin film interference filter that will yield a desired effect on the total performance of the filter. For example, trimming the refractive index of a spacer layer to a specific refractive index, in accord with design calculation from a software tool, can adjust the group delay parameters of the filter. Yet another useful coating design and analysis tool is the software product known under the trade name FILMSTAR and available from FTG Software Associates of Princeton, N.J. 
     The present invention can include multiple optical thin film layers  320 , such as a stack of film layers  320 , individually interacting with the light (not shown) of an end-use application such as light having digital information coded thereon, wherein the light interactions from each individual layer  320  are, collectively, additive or subtractive upon one another. One or more such layers can embody certain functions described herein and illustrated in the examples, compositions, tables, functional block diagrams, and appended flow charts. However, it should be apparent that there could be many different ways of implementing aspects of the present invention in optical films, and the invention should not be construed as limited to any one optical thin film configuration. Further, a skilled optical engineer would be able to create such thin films without difficulty based on the exemplary functional block diagrams, flow charts, and associated description in the application text, for example. 
     Therefore, disclosure of a particular design of a stack of optical thin film layers is not considered necessary for an adequate understanding of how to make and use the present invention. The inventive functionality of any multilayer aspects of the present invention will be explained in more detail in the following description in conjunction with the remaining figures illustrating the functions, compositions, applications, and processes. 
     In one exemplary embodiment of the present invention, the optical thin film  320  is a layer in a thin film interference filter that is intended to manipulate light during normal operation in a wavelength region outside of the UV region of the electromagnetic spectrum. For example, this optical thin film  320  can be a layer in a band-pass filter intended to filter light in the 1550 wavelength region, which is in the near infrared, for DWDM optical networking. The spectral characteristics of the band-pass filter in the UV region of the spectrum are typically insignificant to the optical network, which usually operates towards the opposite end of the optical spectrum, such as in the near infrared. 
     The UV spectral characteristics of the optical filter can be adapted to facilitate the effect of UV light  330  to interact with the optical materials of the filter and thereby promote an adjustment in refractive index of these materials. In one exemplary embodiment, the thin film optical filter exhibits low reflectivity to UV light  330  and thereby transmits the UV light  330  that adjusts the refractive index of the filter&#39;s optical materials. In one exemplary embodiment of the present invention, the thin film optical filter exhibits high reflectivity to the UV light  330  that adjusts the refractive index of these optical materials. In one exemplary embodiment of the present invention, the thin film optical filter resonates UV light  330  to increase the intensity of the UV light field to which the optical materials of the filter are exposed during a refractive index adjustment. That is, the band-pass filter meets the performance specifications of the DWDM application while providing a resonant cavity or etalon that generates a standing wave of UV light  330  upon directing UV light  330  to the filter for adjusting an optical property of the optical thin film  320 . This standing wave increases the effective flux of UV light  330  in one or more layers in the optical thin film  320  to a higher level than the flux of UV light  330  external to the filter and emanating from the UV source. In one exemplary embodiment of the present invention, the design of the thin film optical filter heightens the UV light flux in one or more specific layers  330 , thereby increasing the refractive index adjustment in specific layers  330  relative to the other layers in the filter. For example, one or more spacer layers  330  can be targeted for a refractive index adjustment. Selecting a specific layer for a refractive index adjustment can facilitate precise adjustment of a specific optical property of a thin film optical filter or other optical device having two or more optical thin film layers  320 . 
     Turning now to  FIG. 4A , this figure illustrates a graph  400  with a spectral plot  410  of a thin film optical filter in accordance with an exemplary embodiment of the present invention. The thin film optical filter can have a structure corresponding to the stack of optical thin films  200  illustrated in  FIG. 2  and described above. The spectral plot  410  of the thin film optical filter characterizes transmission as a function of wavelength. The illustrated thin film optical filter response is representative of a band-pass filter that transmits light having a specific spectral range and reflects light outside of that spectral range. This filter can have other spectral regions (not shown) that also transmit light, such as the UV light  330  that adjusts an optical characteristic of this filter. The thin film optical filter may be used for optical networking as a WDM, CWDM, or DWDM filter, for example. 
     Adjusting the refractive index of one or more layers in the thin film optical filter induces, causes, or results in corresponding adjustment in an optical property of the thin film optical filter. The adjustment can alter the filter&#39;s spectral sensitivity to the angle of incident light, the center wavelength of the filter, or another optical property. 
     Turning now to  FIG. 4B , this figure illustrates a graph  450  of spectral plots  410 ,  460 ,  470  of a thin film optical filter resulting from an optical property adjustment in accordance with an exemplary embodiment of the present invention. The filter responds to a dose of UV light  330  with a shift in the center wavelength of the filter. The adjustment can trim an optical filter so that it has a center wavelength that meets a specification. After starting at plot  410  following forming the filter, the application of a first level of UV light  330  can shift the filter spectral characteristics so that it exhibits plot  470 . An additional dose of UV light  330  can further shift the spectral characteristics to plot  460 . 
     In one exemplary embodiment of the present invention, a dose of UV light  330  shifts the center wavelength of the filter towards the blue, rather than the red. That is, UV light  330  can either shift the spectral characteristics of a filter to a higher or lower wavelength, depending on the design of the filter, the materials of the layers, the deposition conditions, and the parameters and conditions associated with the application of energy to the filter. 
     By varying the composition of the layers of a thin film optical filter, the spacer layers, the high index layers, or all of the layers in the filter can be selected for a refractive index adjustment. The spectral shift can be described the following equation:
 
Δλ/λ=α(ΔOT/OT).
 
In this equation, the lambda symbol “λ” refers to wavelength, “OT” refers to optical thickness, and alpha “α” has a value of one when all of the layers in the filter undergo an equivalent adjustment in optical thickness and a value less than or equal to one when the spacer layer or all of the high-index layers in the thin film optical filter undergo an equivalent optical thickness adjustment.
 
     For a triple-cavity thin film optical filter nominally operative at 1550 nm telecommunication wavelength with 41 layers, with alternating layers composed of silicon and silicon dioxide, adjusting the optical thickness of the spacer layers by approximately 0.1% can be expected to yield a spectral shift towards the red (higher wavelength) of approximately 1.45 nm with minimal change in the shape of the spectral response. Adjusting the optical thickness of the high-index layers, including the spacer layers, in this optical filter by approximately 0.1% can be expected to yield a spectral shift towards the red of approximately 1.47 nm without significant shift in the shape of the filter&#39;s spectral characteristics. 
     Turning now to  FIG. 5 , this figure illustrates a graph  500  of spectral transmission plots  525 ,  550 ,  575  of a thin film optical filter adjusted to match a wavelength grid in accordance with an exemplary embodiment of the present invention. After forming the layers of the filter in a deposition chamber, the resulting filter stock is treated with UV light  330  so that the center wavelengths of sections of the filter are aligned with the DWDM grid, which can be a 12.5 MHz grid, a 25 MHz grid, a 50 MHz grid, 100 MHz grid, or other grid spacing. That is, each spectral plot  525 ,  550 ,  575  exemplifies a section of filter stock that is optically trimmed to match the wavelength specifications of a DWDM channel. In this manner, a single deposition batch can yield filters for multiple DWDM channels. 
     Turning now to  FIG. 6 , this figure illustrates a graph  600  of spectral transmission plots of an adjustment to a thin film optical filter to alter ripple in the pass band in accordance with an exemplary embodiment of the present invention. Prior to UV treatment, the thin film optical filter can exhibit a ripple or non-uniformity in the pass band section of the filter&#39;s plot  630 . After treatment with UV light  330 , the filter can have a spectral transmission plot  660  that is smoother, more uniform, or exhibits a flatter pass band. That is, a treatment of UV light  330  can improve the transmission characteristics of a thin film optical filter by minimizing ripple or improving pass band transmission. 
     In one exemplary embodiment of the present invention, the ripple characteristics of a thin film optical filter can be improved by selecting a UV wavelength that will preferentially treat one or more thin film layers. For example, tuning a tunable UV laser to a wavelength that preferentially exposes a defective layer to resonant UV light  330  can improve the defect. Ripple defects or non-uniformities sometimes occur due to a disruption or interruption in a filter deposition process that impacts a single layer. If the ripple is due to a layer having a reduced physical thickness, the layer can be treated with UV light  330  to increase the optical thickness of that layer. In one exemplary embodiment of the present invention, a UV dose or treatment is delivered along the plane of the filter, using positional placement to select one or more specific layers for corrective treatment. 
     Turning now to  FIG. 7 , this figure illustrates a graph  700  of spectral plots  725 ,  750 ,  775  of a thin film optical filter&#39;s response to an optical property adjustment in accordance with an exemplary embodiment of the present invention. As the solid curve  750  indicates, the thin film optical filter has a pass band, which is a contiguous spectral region between approximately 1545.43 nm and 1545.55 nm in which light transmits through the thin film optical filter. Out of the spectral region of the pass band, the thin film optical filter rejects the transmission of light by reflecting light. 
     For most filtering applications, the pass band of the thin film optical filter should be flat, with a high level of transmission and minimal or controlled ripple. The thin film optical filter should also provide a minimal level of light rejected within the pass band for many applications. Rejected light, particularly if the thin film optical filter reflects the rejected light, can cause problems for an application. For example, reflected light within a pass band can mix with the reflected light outside the pass band and become interference or stray light that can degrade the signal-to-noise ratio of an optical communication system, optical instrumentation system, or other optical system. Consequently, a high degree of stray light rejection (or a high level of transmission) within the pass band of a thin film optical filter is typically desirable. 
     The graph  700  includes a curve  725  illustrating an exemplary rejection of a thin film optical filter as formed within a thin film deposition system. This curve  725  shows a hump in the rejection spectrum  725  in the region of the thin film optical filter&#39;s pass band. Treating the thin film optical filter with a dose of UV light  330  can improve the level of rejection as illustrated by exemplary curve  775 . That is, the performance of a filter having the curve  725  can be improved by UV exposure that transforms the curve  725  to the curve  775 . In one exemplary embodiment of the present invention, thin film optical filters that exhibit out-of-tolerance rejection characteristics undergo a UV treatment and/or a thermal treatment to improve the rejection characteristics. 
     Turning now to  FIG. 8 , this figure illustrates a graph  800  of spectral plots  875 ,  850  of the group delay of a thin film optical filter in response to an adjustment of an optical property in accordance with an exemplary embodiment of the present invention. The graph  800  includes a transmission plot  825  of a band-pass thin film optical filter overlaid upon the group delay plots  875 ,  850 . 
     The upper group delay curve  850  illustrates an exemplary group delay of a thin film optical filter following producing the filter in a deposition chamber. The lower group delay curve  875  illustrates an exemplary improvement in the group delay that can result from dosing the thin film optical filter with UV light  330 . That is, treating a thin film optical filter with UV light  330  can improve or modify the filter&#39;s group delay, for example reducing or controlling the overall group delay and/or flattening the group delay profile in a spectral region adjacent or within the filter&#39;s pass band as exemplified by the improvement between the upper group delay curve  850  and the lower group delay curve  875  in the graph  800 . 
     A treatment of UV light  330  and/or thermal energy can also improve a thin film optical filter&#39;s dispersion, which is related to group delay. Whereas group delay is typically measured in the units of picoseconds (“ps”), dispersion is typically measured in picoseconds per nanometer (“ps/nm”). That is dispersion can be the derivative, with respect to nanometers, of group delay. 
     Improving the dispersion or group delay of a thin film optical filter can enhance the filter&#39;s performance in a high-speed optical networking application, for example an environment of transmitting data at 10 Gigabit per second, 40 Gigabits per second, or at a higher rate. For example, a thin film optical filter that provides a high level of dispersion or group delay performance can offer a desirable bit error rate performance to an optical communications network. Similarly, improved group delay can relax the laser specifications for an optical network application. 
     Thus, a treatment of energy can control the residence times of photons having different colors in an optical device. That is, an energy treatment can provide a device that operates over a span of wavelengths with defined or controlled levels of delay for light of those wavelengths. 
     In one exemplary embodiment of the present invention, the chromatic dispersion characteristics of a multilayer thin film optical system can correct or compensate for chromatic dispersion. For example, controlling the dispersion or group delay of a multilayer thin film optical system to achieve a desired spectral profile can be more beneficial than minimizing those optical properties. A treatment of UV light  330  can adjust or trim the dispersion or group delay spectral profile of a thin film system to meet a target specification. The resulting thin film optical system can compensate for chromatic dispersion of optical signals occurring on a span of fiber, in an optical amplifier, or laser cavity, for example. In other words, a dose of UV light  330  can control the chromatic dispersion of an optical thin film device so that the device can be placed in series with other devices that chromatically disperse light and so that the aggregate chromatic dispersion is flat. 
     Turning now to  FIG. 9 , this figure illustrates a fiber optic device  900  that includes a thin film optical filter having an adjusted optical property in accordance with an exemplary embodiment of the present invention. More specifically, the device  950  can be used as an add-drop optical filter assembly  900  for optical communication. 
     One single mode optical fiber  910  delivers multi-color light to an internal lens (not shown) such as a gradient index lens. The lens, which is inside the device&#39;s housing  950  collimates this multicolor light and delivers it to an internal thin film optical band-pass filter (not shown). The pass band of the filter transmits light, delivering the transmitted light to another lens, opposite the first lens, that focuses the transmitted light into a drop optical fiber  930 , which is protected by a strain relief boot  960 . The drop optical fiber  930  transmits the drop light, which transmits through the filter&#39;s pass band, to another device, such as a communication device in an optical communication network. The optical filter within the assembly  900  reflects the out-of-band light from the ingress optical fiber  910  to the egress optical fiber  920 . 
     The UV treatment improves performance of the assembly by centering the pass band of the filter, reducing the group delay, flattening the pass band, increasing the rejection, or adjusting one or more other optical properties of the thin film optical filter within the device. 
     While the UV treatment can be applied prior to integrating the filter with the device, in one exemplary embodiment of the present invention, UV treatment proceeds following the device&#39;s assembly. If the assembled device  900  does not meet a performance specification, then UV light  330  can be delivered to the optical thin film through one or more of the optical fibers  910 ,  920 ,  930  of the assembled device  900 . 
     An optical network, such as a SONET, access, storage, local area network (“LAN”), Internet protocol (“IP”), or other network can comprise the filter  900 . That network can carry a wide range of voice, data, video, or other communications. 
     Turning now to  FIG. 10 , this figure illustrates light  1010 ,  1020 ,  1030  incident on an optical thin film  110  in accordance with an exemplary embodiment of the present invention.  FIG. 10 , along with  FIGS. 11 ,  12 , and  13  are described in reference to the optical thin film system  100  illustrated in  FIG. 1  and discussed above. Nevertheless, those skilled in the art appreciate that the  FIGS. 10-13  can be related to other optical thin films, such as a multilayer system of optical thin films, for example as illustrated in  FIG. 2 . 
     The first surface  180  of the optical thin film  110  reflects a portion of the light  1010  normally incident on that surface  180 . The second surface  170  also reflects a portion of the light  1020  normally incident on that surface  170 . The light waves reflected off of these surfaces  170 ,  180  interfere with one another resulting in net transmission and reflection of light  1030 . The interference can be either constructive or destructive depending on the phase relationship between the two reflected waves. The thickness of the optical thin film  160 , the refractive index of the optical thin film, and the wavelength impact the relative phase of these reflected light waves. An energy treatment can adjust the manner in which the film  110  of  FIG. 10  interacts with and creates the various light rays depicted in that Figure. 
     Turning now to  FIG. 11 , this figure illustrates a graph  1100  of plots  1125 ,  1150  of optical thickness of an optical thin film  110  before and after undergoing an adjustment of its optical properties in accordance with an exemplary embodiment of the present invention. The optical thin film  110  has a physical thickness  160  that is essentially uniform across at least one dimension, as illustrated by the x-axis  140 . The physical thickness  160  undergoes negligible change as a result of a UV treatment. That is, the plot  160  of physical thickness can exemplify the physical thickness prior to a UV treatment, after a UV treatment, or both before and after a UV treatment. 
     Prior to the UV treatment, the optical thin film  110  has an optical thickness  1125  that is essentially uniform along the x-axis  140 . Those skilled in the art appreciate that an essentially uniform thin film  110  can have various minute levels of surface roughness, such as surface roughness that has negligible impact on the optical characteristics of the optical thin film  110 . 
     Treating the optical thin film  110  with a gradient of UV light  330  causes a corresponding gradient in the post-treatment optical thickness  1150  of the optical thin film  110 . A UV light beam can be swept across the optical thin film  110  at an accelerating speed so that one area of the optical thin film  110  receives a higher dose of UV light  330  than another area. That is, a UV light beam can be moved across the surface of the optical thin film  110  at a variable rate. When the speed of the beam is slow, the optical thin film  110  receives a higher dose of UV light  330  than when the speed of the beam is fast. After the UV treatment, the optical thin film  110  exhibits a pattern of optical thickness  1150  that slopes along the x-axis  140 . 
     In one exemplary embodiment of the present invention, a treatment of energy other than UV light  330  causes the gradient in the optical thickness  1150 . Treating the optical thin film  110  with a gradient or other pattern of thermal energy can also cause a corresponding gradient in optical thickness  1150 . Other forms of energy can also be applied to the optical thin film  110  following deposition to impact an optical property of the optical thin film  110 . Exemplary forms of such energy include ultrasonic energy, electrical energy, magnetic energy, stress, and stress relief. 
     Turning now to  FIG. 12 , this figure illustrates a graph  1200  of plots of the optical thickness  1225 ,  1250  of an optical thin film  110  before and after undergoing an adjustment of its optical properties in accordance with an exemplary embodiment of the present invention. The physical thickness  160  of the optical thin film  110  is essentially constant along the x-axis  140 . Prior to UV treatment, the optical thin film  110  has a spatially specific reduction in optical thickness  1225  as indicated by the dip in the graph  1225 . That is, a region of the optical thin film  110  has a decreased refractive index that can be due to an anomaly in, problem with, or interruption of the deposition process. Directing a UV light beam or field to that area of the film can increase the refractive index and corresponding optical thickness  1250  of this region and correct the defect. Following the UV treatment, the optical thickness  1250  of the optical thin film can be essentially uniform across the x-axis  140  of the optical thin film  110 . 
     In one exemplary embodiment of the present invention, a lens composed of one or more of the UV sensitive materials discussed herein is treated with UV light  330  in order to correct surface defects in the lens. For example a plano-convex lens can be formed from optical stock composed of silicon dioxide and germanium, silicon oxynitride, and/or hydrogen using grinding and polishing process steps known to those skilled in the art. The surface profile of the planar side or the convex side can have a spatially specific surface imperfection such as a slight depression. Treating the spatially specific surface imperfection with UV light  330  can slightly raise the refractive index of the material in the area of the surface imperfection and correct or partially correct the defect. 
     Turning now to  FIG. 13 , this figure illustrates a graph  1300  of plots  1325 ,  1350  of the optical thickness of an optical thin film  110  before and after undergoing an adjustment of its optical properties in accordance with an exemplary embodiment of the present invention. The optical thin film  110  has a sloped physical thickness  1300 . Prior to UV treatment, the optical thickness  1325  of the optical thin film is graded. Following UV treatment, the optical thickness  1350  is essentially constant. That is, the UV treatment can compensate for a variation in optical thickness. 
     Turning now to  FIG. 14 , this figure illustrates a system  1400  that adjusts an optical property of an optical thin film  110  by an application of short wavelength light  1420 ,  1440  in accordance with an exemplary embodiment of the present invention. A lamp (not shown) in a lamp housing  1410  outputs UV, violet, and blue light in a circular beam  1420 . A cylindrical lens  1430  forms the circular beam  1420  into a line-type beam  1440  and projects the resulting linear pattern of light  1440  onto a thin film optical system  100 , such as an optical thin film  110  attached to a substrate  120 . The optical thin film  110  can be a single layer of optical thin film material or a stack of optical thin film layers such as one or more layers in multi-cavity thin film optical filter. 
     The light source  1410  and light delivery optics  1430  are stationary, while a motion control  1450  moves the thin film optical system  100  with respect to the focal point  1440  of the light dose. A stepper motor, piezoelectric system, manual station, or other motion control apparatus known in the art can implement the motion control  1450 . 
     The motion control  1450  can move the thin film optical system  100  to control the regions of the optical thin film  110  that receive the light treatment. A shutter (not shown) can be closed until the motion control  1450  appropriately positions the thin film optical system  100  so that a select area of the optical thin film  110  receives an appropriate dosage. 
     The motion control  1450  can also move the thin film optical system  100  while the beam  1440  remains focused on the optical thin film  110 . The rate of motion can determine the level of dosage that each region of the optical thin film  110  receives as it passes below the focused beam  1440 . Such motion can impart a gradient profile or other pattern onto an optical thin film  110 . 
     Turning now to  FIG. 15 , this figure illustrates a system  1500  that adjusts an optical property of an optical thin film  110  by an application of short wavelength light in accordance with an exemplary embodiment of the present invention. A UV laser  1520  couples UV light into an optical fiber  1510 , which carries this light to a distal lens assembly  1510 . 
     The lens assembly  1510  shapes the light output by the optical fiber  1510  and delivers the shaped light to the optical thin film  110  in an appropriate format. The lens assembly  1510  can control the size of the light beam delivered to the optical thin film  110  to provide an adequate intensity of light to each illuminated area of the optical thin film  110 . The optical thin film  110  is attached to a substrate  120 , thus forming a thin film optical system  100  that, although illustrated as a single layer of optical thin film, can be a stack of optical thin films of with varying refractive index. 
     Controls  1530  on the laser  1520  facilitate adjusting various parameters of the laser such as power level, pulse duration, pulse profile, duty cycle, wavelength, and the monochromaticity of the laser light. 
     A motion control function  1540  provided by an assembly (not shown) moves the optical fiber  1510  and the light delivery head  1510  in at least two dimensions across the surface of the optical thin film  110  and the underlying substrate  120 . The motion  1540  defines the regions of the optical thin film  110  that receive a dose of UV laser light. The motion can also control the dosage level that one or more areas of the optical thin film  110  receive. 
     Turning now to  FIG. 16 , this figure illustrates a system  1600  that adjusts an optical property of an optical thin film  110  by an application of short wavelength light  1575  to the thin film  110  in conjunction with monitoring an optical response of the optical thin film  110  in accordance with an exemplary embodiment of the present invention. 
     The system  1600  includes a laser power supply  1650  that drives a laser  1655 , which delivers a beam of UV light  1675  to a thin film optical system  100 . The thin film optical system  100  includes an optical thin film  110  attached to a substrate  120 . The thin film optical system  100  can have a single thin film layer  110  or a plurality of thin films, such as a thin film optical filter. 
     An optical spectrum analyzer  1670  monitors the response of the thin film optical system  100  to the UV light  1675 . Agilent Technologies, Inc. of Palo Alto, Calif. supplies suitable instruments for characterizing the spectral transmission characteristics of optical thin films  110  that are intended for optical networking applications. Other types of spectrum analyzers, spectrographs, and spectrometer systems can be substituted for the illustrated optical spectrum analyzer  1670 . In an alternative exemplary embodiment of the present invention, the optical property adjustment system  1600  includes an instrument that measures the refractive index of the optical thin film  110  in place of the optical spectrum analyzer  1670 . 
     A test light source  1640 , which can be a tunable laser or a broadband light source, supplies test light (not shown) through an optical fiber  1610  and a collimating lens assembly  1630  to the thin film optical system  100 . The test light transmits through the thin film optical system  1000  to a complementary lens assembly  1630  that captures and focuses the test light into a return optical fiber  1620 . The return optical fiber  1620  transmits the test light to the optical spectrum analyzer  1670 , which analyzes the test light  1670  and displays a plot  1580  of the transmission characteristics of the thin film optical system  120 . 
     The optical spectrum analyzer  1670  can display optical properties  1580  of the thin film optical system  120  before, during, or following an adjustment of an optical property. The UV laser light  1675  can increase the temperature of the thin film optical system  100  and cause a temporary shift in the spectral measurements of the thin film optical system  100 . A delay between dosing the thin film optical system  120  with UV light  1675  and acquiring spectral data from the thin film optical system  120  gives the temperature of the optical thin film  110  time to stabilize and cool down from any heating that may have occurred as a result of the laser beam  1675 . 
     The optical spectrum analyzer  1670  can display a transmission spectrum  1580  of a thin film optical band-pass filter to indicate the center wavelength of the pass band of the filter. Measuring the center wavelength with the optical spectrum analyzer  1670  prior to treating the filter with UV light  1675  can determine a treatment dose. After a portion of the UV dose is delivered, the optical spectrum analyzer  1670  can acquire additional spectra to determine the result of the UV treatment. Depending on the impact of the initial treatment, the laser  1655  can deliver the balance of the dose, reduce the dose, increase the dose, or deliver a new dose to match the center wavelength of the filter with a center wavelength specification. If the filter is to undergo accelerated aging or annealing, the target spectrum can be offset from the final target specification to account for spectral shifting during annealing or accelerated aging. That is, the target adjusted spectrum in a UV treatment station  1600  can be offset from the final target spectrum to account for any known changes in the spectrum that occur following the UV treatment. 
     A motion control  1540  moves the thin film optical system  100  with respect to the UV treatment beam  1675  and the test light to control the areas of the optical thin film  110  that interact with the UV beam  1675 . A computer system  1665  controls the motion control  1540  as well as the other components of the system  1600 . The computer system  1665  includes data storage from a hard drive that logs the spectral data from the optical spectrum analyzer  1670  for quality control, statistical analysis, and historical trending. The computer system  1665  adjusts the laser  1655  via the laser power supply  1650  based on the optical spectrum analyzer&#39;s acquired spectrum to control the dose of the UV treatment. That is, the optical spectrum analyzer  1670  monitors the UV treatment process for closed loop feedback control through process control software (not shown) in the computer system  1665 . The process control software can include a control algorithm, such as a proportional integral derivative (“PID”) controller or a proportional integral (“PI”) controller. The controller can increase the UV dosage if the rate of change in the optical property is too low or decrease the dosage if the rate of change is too high, for example. 
     Turning now to  FIG. 17 , this figure illustrates an optical interface  1700  for applying light to an optical thin film  110  to adjust optical properties of the thin film  110  in conjunction with monitoring an optical response of the optical thin film  110  in accordance with an exemplary embodiment of the present invention. The interface  1700  can be included in a computer-based system that adjusts an optical thin film  110 , such as the system  1600  illustrated in  FIG. 16  and discussed above. 
     The optical interface  1700  delivers UV treatment light  1675  and test light (not shown) along an essentially common axis. A pair of dichroic filters  1750 , each sandwiched within a block of optical material  1775  at an approximate 45° angle, reflect the UV treatment light  1675  and transmit the test light. The test light passes from the delivery optic  1630  of the upper optical fiber  1610  and through the dichroic filters  1750  and the optical thin film  110  and its substrate  120 . A collection optic  1630  captures the test light and focuses it into a collection optical fiber  1620  for transmission to a monitoring system such as an optical spectrum analyzer  1670 . 
     The upper dichroic filter  1750  directs the UV treatment beam  1675  along essentially the same optical path in the thin film optical system  100  as the test light. The lower dichroic filter  1750  diverts the UV beam  1675  to a light trap  1725 , which receives and attenuates the beam. The collinear configuration of directing UV treatment light  1675  and test light along a common path in the thin film optical system  100  facilitates adjusting an optical property of the thin film optical system  100  in a precisely controlled manner. 
     Turning now to  FIG. 18 , this figure is a functional block diagram  1800  illustrating adjusting an optical property of an optical thin film  110  under computer control in accordance with an exemplary embodiment of the present invention. The computer  1665  receives a target  1860  from an operator, supervisory control system, production planning schedule, or other source. The target  1860  specifies the desired endpoint of a UV adjustment process, such as a refractive index, a center wavelength of a band-pass filter, the cut-on wavelength of a high-pass filter, the dispersion of a dispersion compensator, the group delay of a dielectric filter, or the attenuation profile of a gain flattening filter, for example. 
     The computer  1665  controls the energy source  1875  that provides energy to the optical thin film  110 . The energy source can be a laser source, heat source, or other source of energy that causes an adjustment  1825  to an optical property of an optical thin film  110 . A measurement  1850  of the optical property adjustment  1825  or a parameter linked to the adjustment  1825  is fed to the computer  1665 . Based on this measurement  1850 , the computer  1665  controls the energy source  1875  to cause equalization between the thin film measurement  1850  and the target  1860 . That is, the adjustment  1825  can be equal to the deviation, thereby causing the post-adjusted optical property to equal the target. If the measurement  1850  is under the target  1860 , the computer can control the energy source to increase the energy dosage, for example. 
     Turning now to  FIG. 19 , this figure is a functional block diagram  1900  illustrating closed loop feedback control of a process for adjusting an optical property of an optical thin film  110  in accordance with an exemplary embodiment of the present invention. 
     The process block  1925  is an adjustment process whereby energy directed to an optical thin film  110  adjusts an optical property of the film  110 . The target  1860 , which can be a set point, is the desired optical property of the optical thin film resulting from the optical property adjustment. A feedback loop  1975  carries one or more monitored parameters from the process to a summation junction  1960 . The summation junction  1960  subtracts the target  1860  from the monitored parameter  1975 , thereby determining the deviation or “error” between the desired optical parameter  1860  and an actual, monitored optical parameter  1975 . 
     A controller  1950  accepts this error signal  1930  as an input to a control algorithm (not shown) within the controller. The control algorithm computes an input  1980  to the process  1925  that is intended to bring the output of the process  1925  to the target  1860 . In other words, the controller  1950  controls the process  1925  to minimize the error signal  1930  and thereby cause equalization between the monitored optical property  1975  and the target optical property  1860 . 
     In an example of a UV laser  1655  adjusting the center wavelength of a thin film optical filter, the target  1860  may be an International Telecommunication Union (“ITU”) grid wavelength for a 25 GHz DWDM band-pass optical filter. The monitored signal  1975  can be a measurement of the filter&#39;s center wavelength of the filter&#39;s pass band from an optical spectrum analyzer  1670 . The error signal  1930  can be the difference between the desired center wavelength and the measured wavelength. The controller  1950  can determine or estimate a UV dosage to bring the center wavelength of the filter into equalization with the desired center wavelength. The controller  1950 , via a serial or parallel output from a computer that is fed into a laser power supply or beam shutter can control the dose of UV light that the optical thin film  110  receives. The optical spectrum analyzer  1670  provides feedback  1975  to the controller  1950  via the summation junction  1960  in the form of a measurement of the center wavelength of the filter. Based on the measured center wavelength, the controller can continue or terminate the treatment or can increase or decrease the dosage as needed to adjust the center wavelength so that it coincides with the target. 
     The controller  1950  can be implemented in software using a variety of algorithms. Exemplary control algorithms include neural network, deadbeat, predictive, PID, PI, Smith predictor, bang-bang, and Kalman filter control algorithms. 
     The present invention can include multiple computer programs embodying certain functions described herein and illustrated in the examples, functional block diagrams, and appended flow charts. However, it should be apparent that there could be many different ways of implementing aspects of the present invention in computer programming, and the invention should not be construed as limited to any one set of computer program instructions. Further, a skilled programmer would be able to write such computer programs without difficulty based on the exemplary functional block diagrams, flow charts, and associated description in the application text, for example. 
     Therefore, disclosure of a particular set of program code instructions is not considered necessary for an adequate understanding of how to make and use the present invention. The inventive functionality of any programming aspects of the present invention will be explained in more detail in the following description in conjunction with the remaining figures illustrating the functions and program flow and processes. 
     Certain steps in the processes described below must naturally precede others for the present invention to function as described. However, the present invention is not limited to the order of the steps described if such order or sequence does not alter the functionality of the present invention. That is, it is recognized that some steps may be performed before or after other steps or in parallel with other steps without departing from the scope and spirit of the present invention. 
     Turning now to  FIG. 20 , this figure is a flow chart illustrating steps in a process  2000 , entitled Adjust Film, for adjusting an optical property of an optical thin film  110  in accordance with an exemplary embodiment of the present invention. At Step  2010  a thin film deposition process such as a PECVD, sputtering process, or other thin film formation process described herein or known in the art deposits, grows, creates, or forms an optical thin film  110  on an optical substrate  120 . In one exemplary embodiment of the present invention, the optical thin film  110  has a thickness  160  between approximately 10 nm and 10 microns. 
     At Step  2015  an operator or a material handling system, robot, or other machine removes the optical thin film  110  from the deposition chamber and places the optical thin film  110  along with the substrate  120  in a fixture. At Step  2020  a spectrometer, optical spectrum analyzer, or refractive index monitor system tests the optical performance of the optical thin film  110  by acquiring an optical spectrum, measuring the refractive index, or otherwise analyzing the thin film  110 . Otsuka electronics Ltd. of Osaka, Japan supplies refractive index instruments that are acceptable for one exemplary embodiment of the present invention. Step  2020 , like the other steps in Process  2000  can proceed under automatic computer control in one exemplary embodiment of the present invention. 
     At inquiry Step  2025 , a computer program or operator determines if the optical thin film  110  is within a specification, such as a telecommunications standard or a manufacturing tolerance. If the optical thin film  110  is acceptable, then Adjust Film  2000  bypasses applying UV light  330  to the optical thin film  110  and, at Step  2075  performs a quality control analysis. This quality control can include additional optical performance analysis and physical characteristics such as physical size of the substrate  120 . 
     At inquiry Step  2080 , a computer algorithm or operator determines if the optical thin film  110  passes the quality control of Step  2075 . If the optical thin film  110  passes quality control, then at Step  2085 , a manual or automatic assembly process integrates the optical thin film  110  into an optical system. Exemplary optical system include add-drop filter assemblies, multiplexer cards, DWDM system, synchronous optical network (“SONET”) hardware, spectroscopic probes, Raman instrumentation, fluorescent microscopes, endoscopes, catheters, and flat panel displays. 
     At Step  2090 , a company installation team or other organization deploys the integrated optical system of Step  2090  into an application. Exemplary applications include SONET networks, telecommunications networks, local area networks, metropolitan area networks, service area networks, biomedical systems, living organisms (for example a catheter or other medical instrument), process analyzers, chemical process control systems, or other applications involving light. Following Step  2090 , Adjust Film  2000  ends. 
     Returning to inquiry Step  2025 , a determination at Step  2025  that the optical thin film  110  is out-of-specification results in the execution of Step  2030 , rather than Step  2075 . At Step  2030 , a light source directs radiation, such as UV and/or violet/blue light, to the optical thin film  110 . At Step  2035  an instrument, such as a spectrum analyzer  1670 , monitors the response of the optical thin film  110  to the directed light. At Step  2040 , a controller or human adjusts the dose of light based on the monitored response of the delivered light  330 . 
     At Step  2045 , a predictive algorithm predicts the end point optical performance of the optical thin film  110 . The predictive algorithm can compensate for accelerated aging, normal aging that occurs following deploying the optical thin film  110  in an application, annealing, relaxing stress in the optical thin film  110 , effects of assembling the optical thin film  110  in an optical device, cooling the optical thin film  110  to a normal operating temperature, or other effect. Step  2045  can include a linear projection, multivariate projection, statistical analysis, or other computation. 
     At inquiry Step  2050 , a computer program or manual calculation determines if the predicted performance of Step  2045  is acceptable. If the predicted performance is not acceptable, Adjust Film  2000  iterates Steps  2030 ,  2035 ,  2040 , and  2045  until the predicted performance is acceptable. 
     When inquiry Step  2050  determines that the predicted performance is acceptable, an operator or automatic computer control program stops the application of UV light  330  and waits a period of time for the temperature of the optical thin film  110  to stabilize. At Step  2060 , an optical spectrum analyzer  1670  or other instrument tests the optical performance of the optical thin film  110 . 
     At inquiry Step  2065  a computer program or operator determines if the results of the test at Step  2060  indicate acceptable optical performance. If the optical performance is not acceptable, Adjust Film  2000  returns to Step  2030  and continues processing the optical thin film  110  from that step  2030 . 
     If inquiry Step  2065  determines that the performance is acceptable, then an application of heat accelerates the aging of the optical thin film  110  at Step  2070  to minimize drift in the optical properties of the optical thin film  110  in an end-use application. Placing the optical thin film  110  in an oven at a temperature between 150° C. and 350 C.° can implement the accelerated aging for inorganic thin films. Films with organic content, such as plastic or polymer films, may also aging under appropriate conditions that may include reduced temperatures. Following Step  2070 , Adjust Film  2000  executes Steps  2080 ,  2085 , and  2090  as described above and then ends. 
     Turning now to  FIG. 21  this figure is a flow chart illustrating steps in a process  2100 , entitled Adjust Filter, for adjusting an optical property of an optical filter in accordance with an exemplary embodiment of the present invention. 
     At Step  2105 , an operator, production planner, engineer, manager, or computer program selects one or more filter parameters for adjustment. Exemplary optical parameters of a filter that can be selected for adjustment include, without limitation, group delay, temperature sensitivity, angular sensitivity, polarization mode dispersion, polarization dependent loss, center wavelength, wavelength accuracy, ITU grid accuracy, channel spacing, dispersion, wavelength dispersion, phase shift, pulse residence time, insertion loss, transmission loss, insertion loss ripple, spectral ripple, spectral flatness, loss, scattering loss, bandwidth, transmission channel rejection bandwidth, clear channel pass band, optical density, edge steepness, channel cross talk, adjacent channel isolation, reflection isolation, rejection, and reflectivity. 
     At Step  2110 , an instrument having a capability of measuring the parameter selected at Step  2105  acquires a measurement of the selected parameter from a sample thin film optical filter. Alternatively, an instrument can measure a parameter that is related to the selected parameter. For example, the selected optical parameter might be the bit error rate (“BER”) of a communication signal passed through a thin film optical filter, and the instrument might measure group delay. 
     At Step  2115 , the sample optical filter receives a test dose of photonic or thermal energy from a laser, arc lamp, light bulb, or other illumination source or a dose of thermal energy from an oven, radiant heat source, thermal conduction system, or other heat source. 
     At Step  2120 , the instrument retests the sample optical filter to determine the change in the parameter resulting from the test dose of energy. At Step  2125 , a computer program or a person determines a specification center point for the selected filter parameter. 
     Analysis at Step  2130  determines typical variability or statistical variance of the filter parameter. For example, a deposition process may yield thin film optical band-pass filters having an in-batch variation in the center of the pass band of approximately less than 0.25 nanometers for 90% of the filters of the batches produced over the last six months. That is, 90% of the filters from a typical deposition batch may have a center wavelength that is within 0.25 nanometers of a fabrication target. 
     Inquiry Step  2135  directs Process  2100  to Step  2140  if the filter parameter increases in response to the energy dose or to Step  2135  if the filter parameter decreases in response to the energy dose. 
     At Step  2140  a person or computer program determines a fabrication target for a thin film optical filter that undershoots or is below the specification center according to the manufacturing variability. For example, a 1550 nm wavelength might be desired as the center wavelength of the exemplary band-pass filter discussed above in reference to Step  2130 . In this example, a fabrication target could be 1550 minus 0.25 nanometers, which is 1549.75 nanometers. 
     At Step  2145 , the person or computer program determines a fabrication target for a thin film optical filter that overshoots, rather than undershoots, the specification center in accordance with the manufacturing variability. 
     At Step  2150 , a deposition process fabricates thin film optical filters using the target specification resulting from Step  2140  or  2145 , as applicable. Continuing with the example discussed above in reference to Steps  2130  and  2140 , the filter deposition process targets 1549.75 nanometers as the target. Historical process variation predicts that the batch will yield approximately 90% of its filter output having a center wavelength between 1549.50 nm and 1550 nm. 
     At Step  2160 , an optical spectrum analyzer, spectrometer, or other instrument tests the filter parameter of the fabricated filters. Inquiry step  2165  determines if the filters are within specification. Returning to the example, the specification might stipulate that the filters have a center wavelength between 1549.75 and 1550.25. Many of the filters from the deposition process can be expected to meet this specification based on the target selection and the historical process variability. 
     For the filters that are within specification, an application of heat such as an annealing stabilizes the filters in Step  2180 . Depending on the deposition process, Step  2180  can be skipped, particularly for the filters meeting a performance specification without need a post-deposition adjustment. 
     For the filters that are out-of-specification, a person or computer program computes an estimate of a corrective dose of energy at Step  2170  based on the response of the sample filter to the test dose of energy, in accord with Steps  2115  and  2120 , which are described above. An application of a dose of energy at Step  2175 , based on the corrective estimate, adjusts the filters. Following Step  2175 , Process  2100  repeats Step  2160  and subsequent steps to determine if the energy dose brought the filter into specification. When the filters are within specification, annealing at Step  2180  stabilizes the filter and Process  2100  ends. 
     Turning now to  FIG. 22 , this figure illustrates a system  2200  for imparting a pattern into an optical thin film  110  by an application of thermal energy in conjunction with monitoring the distribution of thermal energy in accordance with an exemplary embodiment of the present invention. 
     A mounting fixture  2275  holds the thin film optical system  100  in position. The fixture includes a base plate  2230  fabricated of thermally conductive material such as copper or fused silica. A resistive heating element  2240 , powered by an electrical source (not shown), at one end of the base plate  2230  supplies heat to the assembly  2275 . Cooling water  2250  flowing though a hole  2260  in the opposite end of the base plate  2230  carries thermal energy away from the base plate  2230 . The heating element  2250  and the cooling water  2250  produce a thermal gradient across the base plate  2230 . That is the temperature of the base plate  2230  varies in a controlled manner from a high temperature to a relatively low temperature across the base plate  2230   
     In one exemplary embodiment of the present invention, the temperature varies from approximately 500° C. to approximately 400° C. across the base plate  2230 . In one exemplary embodiment of the present invention, the temperature varies from approximately 600° C. to approximately 300° C. across the base plate  2230 . In one exemplary embodiment of the present invention, the temperature varies from approximately 350° C. to approximately 200° C. across the base plate  2230 . In one exemplary embodiment of the present invention, the temperature varies from approximately 450° C. to approximately 250° C. across the base plate  2230 . 
     The base plate  2230  transfers heat to the thin film optical system  100  to deliver a gradient pattern of thermal energy to the optical thin film  110 . A thermal imaging camera  2210  coupled to a computer  1665  provides a contour-style image that shows the temperature gradient that is applied to the thin film optical system. That is, the computer  1665  displays a thermal image  2220  of the thin film optical system  100  that can be used for manual or automatic control of the energy supplied by the resistive element  2240  and removed by the water  2250 . 
     The pattern of thermal energy delivered to the thin film optical system  100  imposes a corresponding optical property pattern to the thin film optical system  100 . In one exemplary embodiment of the present invention, the system  2200  imparts an optical thin film  110  with a monotonic variation in refractive index across the surface of the optical thin film  110 . In one exemplary embodiment of the present invention, the system  2200  imposes a positional shift in a thin film optical filter. For example, the system  2200  adjusts a thin film optical band-pass filter so the filter&#39;s center wavelength varies across the filter in an essentially linear pattern. 
     Turning now to  FIG. 23 , this figure illustrates a system  2300  for imparting a pattern into an optical thin film  110  by an application of thermal energy in conjunction with monitoring an optical response of the optical thin film  110  in accordance with an exemplary embodiment of the present invention. The system  2275 , includes the fixture  2275  described above in reference to  FIG. 22 , with a base plate of thermally conductive material that is optically transparent, such as sapphire or fused silica. 
     A light source  1640  supplies light through the thin film optical system  100  and the fixture  2275  via a fiber optic cable  1610  and a light delivery optic  1630 . A light collection optic  1630  and another fiber optic cable  1620  carry transmitted light to an optical spectrum analyzer  1670  which displays a spectrum  1580  of thin film optical system  100 . 
     A feedback control algorithm or manual control can adjust the heat dosage, either the heat intensity or the gradient heat pattern, to impart a desirable pattern of an optical property to the thin film optical system  100 . 
     Turning now to  FIG. 24 , this figure illustrates a system  2400  for imparting a pattern into an optical thin film  110  by an application of radiant thermal energy  2400  in accordance with an exemplary embodiment of the present invention. 
     A radiant heat source  2420  outputs radiant heat  2460  towards the thin film optical system  100 , including the optical thin film  110  and the substrate  120  to which it is attached. A neutral density filter  2475  or neutral density wedge blocks the radiant heat  2460  in a pattern corresponding to the optical property adjustment. The neutral density filter, which can be an infrared blocking filter, blocks, rejects, or reflects incident radiant heat in a graduated or graded pattern so that the optical thin film  110  receives a dose of thermal energy that varies in an essentially linear manner across the surface of the optical thin film  110 . A shield  2440  blocks the radiant energy  2460  from interacting with unwanted regions of the optical thin film  110 . That is, the shield  2440 , which can be formed of reflective metal, localizes the application of radiant heat  2460  to specific regions of the optical thin film  110 . 
     Turning now to  FIG. 25 , this figure is a flow chart illustrating steps in a process  2500 , entitled Vary Refractive Index, for varying the refractive index of an optical thin film  110  in a graded pattern in accordance with an exemplary embodiment of the present invention. 
     At Step  2510 , a deposition process forms an optical thin film  110  with an essentially uniform physical thickness. At Step  2520 , an energy deliver device, such as a laser, radiant heat source, convention heat source, or conductive heat source, applies photonic or thermal energy to the optical thin film  110  in a controlled manner. The energy delivery device varies the level of energy across the surface of the optical thin film  110 . The level of energy delivery can be varied by intensity, total energy delivery, or time of energy delivery across the surface of the optical thin film  110 . 
     At Step  2530 , the delivered energy alters the refractive index of the optical thin film  110  in correspondence to the spatial pattern of the delivered energy. That is, the delivered energy alters the film&#39;s refractive index perpendicular to the film&#39;s thickness  160 , along the plane of the optical thin film  110 . 
     At Step  2540 , the refractive index variation imparts the optical thin film  110  with an optical thickness that varies along the plane of the thin film  110 , perpendicular to the thickness  160 , while the physical thickness  160  of the optical thin film  110  remains essentially uniform. Following Step  2540 , which can occur contemporaneously with Step  2530 , Process  2500  ends. 
     Turning now to  FIG. 26 , this figure is a flow chart illustrating steps in a process  2650 , entitled Pattern Coating Process, for imparting an optical thin film  110  with a pattern in accordance with an exemplary embodiment of the present invention. 
     At Step  2655 , a deposition process such as one of the exemplary deposition processes described herein, fabricates an optical coating  110  having a composition such as one of the exemplary compositions described herein. 
     At Step  2660 , an engineer, operator, or other individual or a program specifies a desired optical property pattern for a coating. At Step  2665 , an energy source, for example a UV light source or thermal energy source, generates a pattern of energy corresponding to the desired optical property pattern. 
     At Step  2670  an algorithm, program, or person estimates a time and intensity needed to achieve a desired optical pattern. In one exemplary embodiment of the present invention, the estimate is derived by subjecting an optical thin film  110  to an initial dose known not to permanently adjust the optical property and then increasing the dose until a permanent adjustment results. In one exemplary embodiment of the present invention, determining the dose estimate includes varying various parameters, such as optical wavelength, of the dose. In one exemplary embodiment of the present invention, determining the dose includes empirical steps. 
     At Step  2675 , a person, computer, or program estimates the decay in the optical pattern that will occur as the coating ages during usage in an application. Aging can induce an exponential change, which can in totality be either negligible or significant, in an optical property of an optical thin film  110  over time. That is, the rate of change in the optical property of the optical thin film  110  increasingly lessens over time. 
     At Step  2680  a person or computer program prescribes or estimates an energy dosage that is expected to yield a desired pattern of an optical property taking into account aging-induced changes that are predicted to occur over time. For example, if aging causes a 0.1% reduction in the optical property, then the dosage can be prescribed to cause the optical property to overshoot a desired endpoint by 0.05% in order to pre-compensate for the anticipated 0.1% reduction. 
     At Step  2685 , delivering a patterned dose of energy to the optical thin film  110  imparts the optical thin film  110  with a patterned or spatially specific optical property. At Step  2690 , the optical thin film  110  undergoes a heat treatment to provide accelerated aging in order to decrease the rate of change in the optical property of the optical thin film  110  over the time that the optical thin film  110  operates in its intended or end-use application. 
     At Step  2695 , the optical thin film  110  performs its intended function in an end application. For example, the optical thin film  110  can filter, reflect, delay, or interfere with light in a communication or instrumentation system. Pattern Coating  2650  ends following Step  2695 . 
     Turning now to  FIG. 27 , this figure illustrates a system  2700  for adjusting the reflectivity of an AR coating  2760  on a laser  2710  in accordance with an exemplary embodiment of the present invention. The system  2700  includes an optical spectrum analyzer  1670  and a UV laser  1655 . An AR coat  1760  on a semiconductor laser  2710 , such as a Fabry-Perot laser die, or a silicon-based lasing cavity serves to defeat reflection from the end facet of this laser  2710  to which the AR coat  2760  adheres. 
     An electrical power supply (not shown) drives the semiconductor laser  2710  with current. The semiconductor laser  2710  responds to the current by outputting light in a pattern of longitudinal modes  2750 . A lens assembly  2740  collects light  2730  output from the semiconductor laser  2710  and focuses that light  2730  into an optical fiber  1620  that carries the light  2730  to the optical spectrum analyzer  1670 . The optical spectrum analyzer  1670  analyzes the light  2730  and displays a spectrum  2750  of the light  2730 . The longitudinal modes  2750  of the semiconductor laser  2710  appear as a comb-shaped pattern  2750  in the spectrum  2750  on the optical spectrum analyzer  1670 . 
     Adjusting the refractive index of the AR coating  2760  to an optimal or beneficial value or to a desirable range increases the coating&#39;s effectiveness in suppressing reflection from the laser&#39;s end facet. That is, setting the AR coating&#39;s refractive index to a value that matches the design of the coating increases the AR coating&#39;s performance. The AR coating&#39;s performance is typically sensitive to the coating&#39;s refractive index, especially if the thin film coating  2760  is a single-layer coating. 
     For a given current injected into the semiconductor laser  2710  via a laser power supply, the height of the comb in the displayed spectrum  2750  can indicate the effectiveness of the optical thin film coating  2760  in suppressing facet reflection. That is, achieving a target refractive index can reduce the facet reflection which in turn reduces the height of the comb pattern  2750  in the displayed spectrum  2750  on the optical spectrum analyzer  2750 . 
     Those skilled in the art appreciate that various methods of determining the effectiveness of an AR coating  2760  on a semiconductor laser  2710  are known in the art. In one exemplary embodiment of the present invention, the current to the semiconductor laser  2710  is increased until the laser  2710  crosses the lasing threshold and thus begins lasing. Current needed to induce lasing, that is threshold current, can be an indicator of the AR coating&#39;s performance. The higher the current that a semiconductor laser die can accept without lasing, the better the performance of the AR coat  2760  in suppressing facet reflection. 
     A UV laser  1655  delivers UV light  1575  to the AR coat  2760  of the semiconductor laser  2710  to adjust the refractive index of the AR coat  2760  to a value that reduces facet reflection to an acceptable or minimal level. The UV laser  1655  doses the AR coat  2760  with UV light  1575  until the semiconductor laser  2710  exhibits acceptable performance. Acceptable performance can be determined by evaluating the spectral pattern  2750  on the optical spectrum analyzer  1670 , by monitoring the threshold current of the semiconductor laser  2710 , or by other method known in the art. In one exemplary embodiment of the present invention, the UV laser  1655  applies UV trimming light  1575  to the facet coating  2760  with a grating or other spectrally selective reflector disposed adjacent to the facet so that the semiconductor laser  2710  operates as an external cavity laser. 
     Turning now to  FIG. 28 , this figure is a flow chart illustrating steps in a process  2800 , entitled Adjust Antireflective Coating, for adjusting an optical property of an AR coating  2760  on a laser  2710  in accordance with an exemplary embodiment of the present invention. At Step  2810 , a laser fabrication process fabricates mode expanded Fabry-Perot lasers for semiconductor gain media. Such laser fabrication processes are well known in the art. Suppliers of laser dies include: Sacher Lasertechnik Group of Marburg, Germany; Covega Corporation of Jessup Md.; and Princeton Lightwave of Cranbury, N.J. 
     Those skilled in the art will appreciate that one or more steps of Process  2800  can be applied to silicon-based optical devices that amplify or manipulate light, such as a silicon photonic device, following the teaching and disclosure presented herein. 
     At Step  2815 , an operator or a machine places a batch of semiconductor gain media  2760  in a deposition chamber in an orientation that provides a deposition coat on the facets of the gain media. At Step  2820 , the deposition process coats the gain media facets with a high-density AR coating  2760  using a high-energy process. In one exemplary embodiment of the present invention, an e-beam IAD process deposits a coating  2760  of silicon oxynitride on the facets and the resulting coat has a packing density that exceeds 95%. 
     At Step  2825 , an operator or machine removes the coated gain media  2710  from the deposition chamber. At Step  2830  an operator or machine mounts a sample gain medium  2710 , for example a Fabry-Perot laser die  2710 , in a test jig and energizes the sample with current. 
     At Step  2840  an instrument  1670  monitors the threshold current, spectral output, facet reflectivity, or other parameter from of the energized sample gain medium  2710 . At inquiry Step  2845  an operator or computer program determines if the performance of the AR coat  2760  is acceptable based on the test of Step  2840 . If the performance is not acceptable, then at Step  2850  a UV light source  1655  doses the coated facets of the batch of gain media  2710  with UV light  1570  to adjust the refractive index of the coating  2760 . Following Step  2850 , Process  2800  iterates Step  2840  and  2845  until the performance is acceptable. 
     When inquiry Step  2845  determines that the performance is acceptable, at Step  2860  assembly personnel and/or assembly machinery mount the coated gain media  2710  from the batch in subassemblies suitable for mounting in a product such as a laser module. Typical subassemblies can include an AR coated laser mounted on a substrate or carrier with electrical leads. 
     At Step  2870 , operators, technicians, or automated testing equipment tests each subassembly. At Step  2875  a computer program or test personnel determines if the tested subassemblies comply with a specification. If the tested subassemblies are not within specification, then at Step  2880  a UV light source  1655  doses the out-of-tolerance AR coatings  2760  with UV light  1575  while the subassemblies remain assembled. Treated subassemblies undergo testing again at Step  2870  and further UV treatments as required until each assembly meets specification. Assemblies or individual gain medium  2710  that are not responsive to UV treatment are removed from the process  2800 . 
     When Step  2875  indicates that each subassembly meets specification, at Step  2885  assembly personnel and/or assembly machinery integrate a Bragg grating with each subassembly, positioning the grating adjacent the coated facet in an external cavity configuration. Following Step  2885 , Process  2800  ends. 
     In summary, the present invention can provide optical property adjustments to optical films, including optical thin films, thin film optical filters, and antireflective coatings. 
     From the foregoing, it will be appreciated that the present invention overcomes the limitations of the prior art. From the description of the exemplary embodiments, equivalents of the elements shown herein will suggest themselves to those skilled in the art, and ways of constructing other embodiments of the present invention will suggest themselves to practitioners of the art. Therefore, the scope of the present invention is to be limited only by the claims below.