Field of the Invention The invention relates to methods of cleaning materials and substrates for optical devices using acid and oxidizer. Background Optical materials generally operate based on material properties including desired transmission, refraction, and reflection of electromagnetic (e.g., visible) wavelengths. These properties are enhanced by the use of exceedingly clean surfaces of devices, by which or through which radiation is transmitted, refracted, or reflected. Additionally, various surfaces of optical devices can include coatings or adhesives designed for specific interaction of the surface with radiation or other surfaces of the device or another device. These surfaces also benefit from extreme cleanliness to improve the bond of a coating or adhesive, as well as optical properties. Thus, ever-improved surface cleanliness is always a goal for optically-active surfaces of optical devices. Optically-active surfaces of optical components come into contact with a large number and variety of different materials during processing to a final product or device. These may include acids, bases, abrasives, solvents, and adhesives, among other useful processing material. For instance, a thermoplastic adhesive such as a "blocking wax" may be used to temporarily bond a substrate to another piece, for processing the substrate. These adhesives and other materials used for processing must eventually be removed. Other contaminants can also interfere with performance of optically active surfaces, such as dust, human detritus, fingerprints, and the like. Any such residue or contaminant mat is not completely removed, and remains at a surface of an optical material, can strongly affect the optical properties and performance of the optical material. In specific optical devices, residual surface contamination can produce a variety of detrimental effects. Surface contamination can produce optical scattering in the performance of a mirror. In highly precise mirror applications such as dielectric mirrors used in gyroscopes, this scattering can affect performance in the form of cavity loss and angle random walk (ARW). In passive or active surface waveguides, surface contamination can also result in surface scattering and losses. In optical sealing applications, surface contamination can interfere with optical contact (i.e., the adhering of two sufficiently clean and close-fitting surfaces without the use of cement.) Interface strength relates to Van der Waals forces between the atoms of the two respective surfaces, and as such, any amounts of contamination can reduce this interaction and dramatically lower interfacial strength. In vacuum applications, surface contamination can be volatile, allowing for redisposition of the contaminant and re-contamination of the device or vacuum chamber. Thus, while negative affects of residual surface contamination in optical devices and optical device processing are manifold, many materials used in processing optical devices are not easily removed following their usefulness. An example of a class of materials that can be especially difficult to remove is organic materials such as the organic (e.g., polymeric thermoplastic) adhesives. Standard blocking waxes used for temporarily mounting a substrate during processing such as coring, grinding, lapping, polishing, or other processing steps, can be very difficult to remove. A standard method for removing these materials is with solvents, e.g., in a heated solvent bath or solvent vapor degreaser. The limited solubility of these waxes in organic solvents, however, results in incomplete cleaning of the substrate surfaces. The nature of the cleaning process, of dissolving the organic material in a solvent in which many substrates are cleaned, also allows for re- precipitation or re-depositing of the organic material on a substrate surface, leaving a residue. There is continuing need for effective methods of cleaning optical substrates, especially for methods of cleaning organic materials (e.g., contaminants) from optically- active surfaces. Summary The invention provides methods for cleaning surfaces useful in optical devices, with a cleaning material that includes acid and oxidizer. For years, sulfuric acid and hydrogen peroxide have been used together in wet bath processes to clean semiconductor wafers, to remove photosensitive, reactive polymeric materials from semiconductor wafer surfaces, the photosensitive polymeric materials being photosensitive, reactive, positive and negative photoresist materials used in masking or patterning semiconductor wafers. The usefulness and potential advantages of using the same cleaning materials to remove non-photosensitive materials from optically functional surfaces, however, has not previously been appreciated. Thus, according to methods of the invention, organic materials such as non- photosensitive polymeric materials, are removed from optically active surfaces using acid and oxidizer. Photosensitive refers to materials that are sensitive or responsive (reactive) to light or other electromagnetic energy. In certain embodiments of the invention, the inventive cleaning processes can be shown to result in more effective cleaning of organic surface materials and contaminants, from an optical substrate, compared to past processes used for cleaning optically active surfaces. More specifically, exemplary processes of removing adhesive materials from an optical surface can produce a level of surface cleanliness that equals or exceeds levels achieved by presently-used industrial methods, such as methods that involve the use of organic solvents to dissolve an adhesive from a surface. Surface cleaning effectiveness can be measured and compared objectively by optical (e.g., microscopic) examination of a cleaned optically active surface, for residual organic materials such as adhesives, dust, human detritus, fingerprints, etc. Cleaning effectiveness can also be measured and compared by measuring the performance of an optical device or instrument that is prepared to use the cleaned optical substrate. Still further, cleaning effectiveness can be measured and compared based on objective measurements of optical properties such as transmissivity, reflectivity, or refraction. For mirrors specifically, surface contamination can be detected based on reflectivity performance of a mirror surface in terms of surface scattering. Where methods of the invention result in better cleaning of substrate surfaces, a natural consequence of commercially manufactured products using the cleaned optical substrates, is a reduction in waste and improvement in yields of the substrates and of products prepared from the substrates, as well as better performance. An aspect of the invention relates to a method of removing non-photosensitive organic material from an optical substrate. The method includes contacting the optical substrate with a cleaning material comprising oxidizer and acid. Another aspect of the invention relates to a method of removing organic adhesive from an optical substrate. The method includes providing an optical substrate having an organic adhesive material on a surface, providing an oxidizer, providing an acid, combining the oxidizer and acid to provide a cleaning material, and immersing the optical substrate in the cleaning material. In another aspect, the invention relates to a method of processing an optical substrate. The method includes providing an optical substrate, adhering the optical substrate to processing equipment using organic adhesive, processing a surface of the substrate while the substrate is adhered to the processing equipment, removing the processed substrate from the processing equipment, and removing the adhesive from the substrate by contacting the substrate with a cleaning material comprising oxidizer and acid. Description An "optical substrate," or simply "substrate" as used herein, is an article, piece, or other object or structure that includes an optically active surface that performs an optical function as part of the substrate or when included as part of a larger optical device. An optically active surface can be a surface that exhibits one or more of optically useful reflectivity, transmissivity, or refractivity, within the optical spectrum. Optically active surfaces may be prepared from materials of known utility in such applications, such as transmissive and highly transmissive materials including glass and quartz, optionally coated at one or more surfaces; highly reflective materials such as polished metals and metal alloys which may optionally be coated; other reflective materials such as a substrate (e.g., polymeric, ceramic, metal, or otherwise) coated at a surface with a reflective or otherwise optically active material such as a metal, metal alloy or metal oxide, etc. The material that includes the optically active surface may be the only component of an optical substrate, or that material may be included with other materials that may be either functional or structural. While silicon wafers can exhibit reflectivity, silicon wafers by themselves are not considered to be optical substrates as the term is used herein, e.g., because silicon wafers do not by themselves perform an optical function. Still, optical substrates processed according to the invention may include silicon and related substances, e.g., to perform an electronic or structural role, if the silicon is in addition to a material that has an optically active surface and that performs an optical function. An optical substrate may be a substrate at any of various stages of preparation or use, e.g., a finished (e.g., functional) optical substrate, a partially finished optical substrate, or an optical substrate installed and used in an optical device. For instance, the cleaning process may be used to clean optical substrates during their manufacture, i.e., to clean "in- process" optical materials or substrates, as the materials or substrates are being prepared to a form that will function within a larger optical component or optical device. Alternately, the process may be used to clean an optical substrate subsequent to manufacture and finishing of the substrate, such as intermittently during use through a useful life of an optical device. In general, optical devices include an optical material that is one or more of optically transmissive, reflective, or refractive, of electromagnetic radiation. According to the invention, such optical materials and their surfaces can be useful in highly precise electro or electro-mechanical instruments such as gyroscopes, lasers, phase and amplitude optical modulators, etc. These instruments may provide highly precise and accurate measurements. As such, they can be prepared from optical substrates processed according to the invention to include extremely clean surfaces. The methods of the invention allow for these optically active surfaces to be processed to extreme levels of cleanliness, which allows for the subsequent preparation of assembled optical devices that perform with yet additional precision and accuracy. Some specific examples of optical devices and optical substrates include articles and assemblies that are designed and used to act upon electromagnetic radiation within the optical spectrum, such as visible, infrared, or ultraviolet light, e.g., laser light, in a way that causes the radiation to be desirably reflected, transmitted, refracted, oriented, polarized, rotated, or focused, or otherwise optically affected. The optical spectrum may broadly be considered to be the electromagnetic spectrum between the wavelengths of the vacuum ultraviolet at 0.001 μm and the far infrared at 100 μm, with a large amount of optical applications being within the 0.1 to 2 micron regime. Specific examples of optically active surfaces are those surfaces associated with mirrors, waveguides, optical filters, lenses, optic wedges (e.g., birefπngent wedges), polarizers, beam splitters, prisms, acousto-optic and electro-optic modulators, lasers, optical amplifiers, and the like. Optically active surfaces do not include surfaces of devices intended primarily to function as electronic or microelectronic devices, such as surfaces of semiconductor materials and devices, although optical substrates and optical devices prepared according to the invention may include or be used with electronic or microelectronic materials and devices. A specific example of a reflective optical substrate is a mirror or a mirror backside. Typical mirrors can include a smoothly polished solid material such as a glass material, a polymeric (e.g., plastic) material, or a polished metal, coated with a coating (e.g., a metal film or a dielectric multilayer coating or "multilayer stack") that functions as the reflective mirror surface. A mirror may additionally include other layers or coatings, e.g., at the reflective surface, such as an antireflective coating. Mirrors can preferably reflect light in the visible or near visible spectrum at a desired reflectivity, depending on the application. Specific examples of mirrors include mirrors used in lasers, transducers, and gyroscopes. Other optical substrates that in general can benefit from an extremely clean optically active surfaces may include: a waveguide structure, e.g., a structure having the ability to guide optical energy; an optical filter structure that may selectively transmit or blocks a range of wavelengths; a lens structure; a polarizer; abeam splitter, e.g., an optical device that is useful for dividing a beam of electromagnetic radiation into two or more separate beams of desired magnitudes; a beam combiner; etc. Certain more specific examples of optical substrates that may produce added benefit from exceptionally clean optically active surfaces include ball lenses, gradient index lenses, waveguide optical amplifier (e.g. erbium- doped waveguide amplifiers (EDWA)), glass-based passive splitters, modulators such as those based on lithium niobate (lithium niobate modulators), and applications using ruled gratings (e.g., ruled grating substrates). Surface contaminants that can be located at a surface of an optical substrate, and removed or cleaned from a surface according to the invention, include many materials that are used in processing such substrates, as well as other contaminants that can become present due to handling or contact with atmospheric conditions. Processes described herein can be particularly effective to remove organic materials from optical substrates, including non-photosensitive polymeric organic materials (e.g., adhesives, thermoplastic materials); fingerprints, human detritus; dust; etc. One specific example of a class of organic materials that can be present as surface contamination on an optically active surface is the class of organic materials that useful as adhesives, including thermoplastic adhesives, cured or curable adhesives, pressure sensitive adhesives, thermosetting adhesives, etc. Relatively more specific examples of organic adhesive materials can include organic thermoplastic materials such as waxes that can be melted and solidified to allow their use as a structural adhesive. Some of these waxes are referred to as "blocking waxes," which generally include hydrophobic organic materials made of ingredients that include a linear, optionally substituted or branched, relatively long- chain saturated hydrocarbon portion. These materials are known in the arts of processing optical substrates, and generally include a variety of waxes or pitches, e.g., lanolin-based wax, beeswax-based wax, pine or balsam-based pitches. Examples of specific temporary mounting adhesive that has been used in optical substrate processing include Crystalbond™ 509 (from Aremco Products Inc., Valley Cottage NY), and the Crystalbond™ line of products, which also include thermoplastic organic polymers. Such adhesive materials are often used in processing an optical substrate to temporarily mount the substrate to processing equipment for processing steps, e.g., processing a surface of the substrate by machining, slicing, drilling, dicing, coring, grinding, lapping, polishing, and the like. Following the processing step during which the adhesive is useful, the adhesive must be removed with minimum residue left at the substrate surface. According to the invention, a surface of an optical substrate is cleaned to remove organic material or residue from the surface by contacting the substrate with a cleaning material (e.g., cleaning solution) that contains both acid and oxidizing agent. Without being bound by theory, a possible mechanism for removal of organic material from a substrate surface, by acid • * and oxidizer, may be that the acid can break down the organic material, and the oxidizing agent can react with (and thereby reduce) the reaction product to further break down the reaction product to carbon dioxide. The acid can be any acid that can be effective in a cleaning material, in combination with oxidizing agent, to remove organic material from a substrate surface, e.g., by breaking down an organic material at a substrate surface. The acid may be of any effective concentration or pH, and may be organic or inorganic. A strong acid (essentially 100 percent ionized) such as sulfuric acid may be useful in certain applications, while other organic acids, other strong acids, or inorganic acids, alone or in combination and optionally concentrated or non-concentrated, can also be useful. Examples may include nitric acid and hydrochloric acid. The selection of a specific type and concentration of acid can be affected by various factors relating to a specific process, such as the amount and type of oxidizer used in the process, compatibility of an acid with the optical substrate (e.g., materials of the substrate such as metal layers), timing and temperatures of exposure of the substrate to the cleaning material, etc. For certain embodiments, sulfuric acid may be useful. The oxidizer can be any useful oxidizing agent, which, according to the present description refers to a compound (not including the acid) that acts as an oxidizing agent when used according to a process as described, in combination with an acid, to remove organic material from an optically active surface. Some oxidizing agents are compounds that contribute an oxygen atom during reaction with another compound. An example of such an oxidizing agent is hydrogen peroxide. Other examples of oxidizing agents include ozone, oxygen (O2), elemental fluorine, metal nitrates, potassium permanganate, sodium hypochlorate, sodium dichromate, sodium perborate, among still others. Hydrogen peroxide may be particularly desirable in certain applications, because hydrogen peroxide has the advantage of producing only water as a reaction product. An aqueous hydrogen peroxide solution may be used, for example, at a useful concentration, e.g., up to a concentrated solution of 85 or 90 weight percent hydrogen peroxide. Hydrogen peroxide solutions of such high concentrations may be dangerous and difficult to handle, and present safety hazards due to their reactivity and flammability. Due to these factors, therefore, dilute solutions of hydrogen peroxide may be used instead, while still being effective for use in the processes of the present description. Examples of dilute aqueous hydrogen peroxide solutions may have hydrogen peroxide concentrations of below 50 percent by weight, e.g., below 40 percent by weight, or in the range from 25 to 40 weight percent, with concentrations of approximately 30 or 31 weight percent being typical. The acid and oxidizing agent can be combined to form a cleaning material (e.g., solution) that is useful according to the invention, to remove organic material from an optical substrate surface. The relative amounts of acid and oxidizing agent that are used can be any desired and useful amounts. According to certain embodiments of the invention, wherein the cleaning material used is sulfuric acid and the oxidizer used is hydrogen peroxide, these materials may be included in a volume ratio of sulfuric acid (anhydrous) to hydrogen peroxide (anhydrous) the range from 3:1 to 10:1, e.g., in the range from 3:1 to 5:1. The cleaning material may be prepared by combining oxidizer and acid, using adequate safety precautions. Alternately, combinations of oxidizer and acid materials are available commercially. One example of such a commercially available product is the Nanostrip line of products available from Cyantek, Fremont CA, presently including Nanostrip and Nanostrip 2X. These are solutions that contain sulfuric acid and hydrogen peroxide in what is referred to as a stabilized form. The invention does not depend on any particular mechanism of action by the acid, the oxidizer, or their combination, in cleaning an optical surface. The process described herein is useful and can according to at least some embodiments exhibit desired advantages, without regard to its mechanism of action. Still, without being bound by theory, the use of a cleaning material that contains acid and oxidizer is believed to be particularly effective in removing organic materials (e.g., contaminant) from a surface, based on the overall ability of the acid and oxidizer to react with and break down the organic material. In general, the acid, oxidizer, and organic material combine to produce carbon dioxide gas, indicating breakdown of the organic material by the acid and oxidization of carbon from the initial reaction, by the oxidizing agent. This could occur with the acid reacting with and breaking down an organic material to a degree that allows oxidation of a reaction product by the oxidizer. Possibly more specifically, reaction of the organic material and the acid can result in carbon (e.g., elemental carbon) that can be oxidized by the oxidizing agent to produce carbon dioxide. Advantageously, the organic material does not simply dissolve in the cleaning material, as is the action of solvent-bath cleaning processes. Consequently, as opposed to solvent-bath processes, the organic material cannot simply re-precipitate or otherwise become re-deposited at another substrate surface to re-contaminate the surface. If desired, a cleaning material as described herein may include other ingredients in addition to the acid and oxidizer, as desired and useful. These may include ingredients that may improve the effectiveness of the cleaning process or that may otherwise facilitate or add to the efficiency of a larger-scale process. Such ingredients may include a surfactant; organic solvent; diluents such as water or organic materials; stabilizers (e.g., peroxydisulfuric acid as described, e.g., in United States Patent No. 6,294,145); buffering agents, etc.; in desired amounts. According to the invention, the cleaning material is brought into contact with a substrate surface to remove an organic material. This may be done in any useful fashion, such as by applying the cleaning material to the substrate by a spray or a brush, or by immersing the substrate in a bath containing the cleaning material. Optionally, the method may include some form of agitation of the substrate, the cleaning material, or the organic material. Examples of such agitation can include mechanical action such as scrubbing or brushing; movement such as rotation or oscillation of a substrate; movement such as rotation or oscillation a cleaning material spray; sonic agitation; etc.; as desired and effective. One specific example is the use of a heated tub that contains the cleaning material, and into which substrates are dipped or immersed for cleaning. Oxidizer (and acid) can be continuously or periodically added as needed to maintain a desired concentration of each in the cleaning material. Optionally, the process can include heat energy to facilitate cleaning. Thus, the process can be performed at any temperature, including room temperature or elevated temperatures. Either of a substrate, a cleaning material, or both, may be heated. Useful processing temperatures, e.g., substrate or cleaning material temperature) can be above or below room temperature; e.g., in the range from room temperature (70F) to an elevated temperature such as 250 F. Exemplary elevated temperatures can be in the range from 180 to 220 F. According to certain specific embodiments of processes of the invention, a cleaning method can advantageously be performed a room temperature or within a nearby temperature range, e.g., from 60 to 80 degrees Fahrenheit, to reduce the cost and complication of heating to the cleaning material or substrate. The amount of time for contacting the substrate with cleaning material can be any effective amount of time. The specific timing for any particular process can depend on various specific factors such as the substrate and substrate constituents; the amount of organic material to be removed from a substrate (e.g., the process can be effective for cleaning full coverage of an organic adhesive, or for minute amounts (small areal fraction of contamination coverage); the type of organic material; the concentrations and relative concentrations of acid and oxidizer; the type of contact and cleaning apparatus being used, such as whether a spray or bath process is used and whether mechanical or ultrasonic agitation is used; the use of a heated substrate or cleaning material; among other factors. A significant factor can be the amount of organic material to be removed. Exemplary timing may be in the range of seconds, minutes, hours, with timing designed to be in the range of 1 minute to 1 hour often being useful. Certain preferred methods of removing small amounts of organic surface impurities (e.g., adhesives or waxes) can be designed to require relatively short periods of time, such as less than 5 minutes, e.g., from 30 seconds to 3 minutes. Methods of removing larger amounts of organic impurities may take longer, e.g., up to 60 minutes. As only one specific example of useful process features according to the invention, an optically transmissive substrate (e.g., a BK7 or Zerodur® substrate) with alternating high/low refractive index dielectric (oxide) films, can be cleaned using a 4:1 mixture of sulfuric acid to hydrogen peroxide, for 10 minutes at 60-lOOC (140F to 212F), to remove from trace amounts to full coverage of Crystalbond adhesive or polishing waxes/pitches. After the cleaning process, a substrate may be processed as desired. Optionally, a cleaned substrate may be rinsed in deionized water to remove any residue, then dried by any desired drying method such as spin-drying, blow-drying using a clean dry gas, etc. Other embodiments of this invention will be apparent to those skilled in the art upon consideration of this specification or from practice of the invention disclosed herein. Various omissions, modifications, and changes to the principles and embodiments described herein may be made by one skilled in the art without departing from the true scope and spirit of the invention which is indicated by the following claims.