CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to U.S. provisional patent application No. 60/276,538 filed Mar. 15, 2001, the contents of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     Optical fibers and components to manipulate optical signals are becoming pervasive elements of modern telecommunications networks. 
     An optical fiber confines light signals within a narrow inner core that allows the light signals to propagate long distances within the fiber. A single-mode fiber, for example, typically has an inner core diameter on the order of eight microns. At some point, such optical signals exit the fiber for downstream processing. Thus, the light signals are “coupled” from the optical fiber to subsequent optical components, e.g., lenses, switches, detectors, mirror arrays, amplifiers, etc. Similarly, light signals are coupled into optical fibers from similar such components as well as sources. To facilitate such coupling, focusing elements such as a lens (e.g., a gradient index lens or ball lens) can be positioned relative to the optical fiber to collimate light emerging from, or focus light into, the narrow fiber core. Optimizing the efficiency of such coupling typically require precise positioning and alignment of the respective optical components. This is true not only of optical fibers and coupling lenses, but optical components in general. 
     SUMMARY 
     In general, in one aspect, the invention features a method for attaching a surface of a first optical element to a surface of a second optical element. The method includes: providing a bonding glass on at least one of the surfaces, wherein the bonding glass is selected to match the refractive indices of the first and second optical elements at the surfaces over a first range of wavelengths and absorb optical energy to a greater extent than that of the optical elements over a second range of wavelengths different from the first range of wavelengths; positioning the surfaces proximate one another; directing optical energy to the bonding glass through at least one of the optical elements at a wavelength in the second range of wavelengths, wherein the optical energy is sufficient to melt the bonding glass without deforming the optical elements; and allowing the melted bonding glass to solidify and fuse the proximately positioned surfaces. 
     In general, in another aspect, the invention features an optical assembly including: a first optical component having a first surface; a second optical component having a second surface; and a bonding glass fusing the first surface to the second surface, wherein the bonding glass is selected to match the refractive indices of the first and second optical elements at the surfaces over a first range of wavelengths, and wherein the bonding glass is selected to absorb optical energy to a greater extent than that of the optical elements over a second range of wavelengths different from the first range of wavelengths such that optical energy directed to the bonding glass through at least one of the optical elements at a wavelength in the second range of wavelengths can melt the bonding glass without deforming the optical elements. 
     Other aspects, features, and advantages of the invention will be apparent from the following detailed description and from the claims. 
    
    
     DESCRIPTION OF DRAWINGS 
     FIGS. 1A,  1 B,  1 C, and  1 D illustrating step for attaching an optical fiber to a micro-optic component to form an integrated optical assembly. 
    
    
     DETAILED DESCRIPTION 
     The invention features a monolithic optical assembly in which a first optical element (e.g., an optical fiber) is fused to a second optical element (e.g., a coupling lens) by a bonding glass. The monolithic structure is rugged and maintains the alignment between the first and second elements following their fusion. The invention also features a method for attaching the two optical elements into the monolithic assembly by melting the bonding glass. 
     The bonding glass is selected to match the refractive indices of the first and second elements over a desired range of wavelengths (“the first wavelength range”). For example, the desired wavelength range may be in the near infrared (NIR) around about 1.3 to about 1.6 microns, which is typical of transmission wavelengths for optical telecommunications. When such index matching is achieved, there is no back reflection of light signals in the desired wavelength range at the bonding glass interface. Thus, for example, when one of the elements is an optical fiber, it is not necessary to lap the exposed face of the fiber at an angle to prevent collinear back reflection from that face. 
     On the other hand, in a second wavelength range (e.g., in ultraviolet region) the bonding glass is selected to absorb optical radiation more strongly than either of the first or second optical elements. Moreover, the bonding glass can be selected to have a low melt temperature. 
     Thus, the two elements can be attached to one another by applying the bonding glass as a thin film to one or both of the optical elements, bringing the two elements close to one another at the thin film(s), and using the thin film to fuse the elements together. Laser light in the second wavelength range (e.g., UV light) is transmitted through at least one of the optical elements to the bonding glass, which absorbs that laser light and melts. When the laser light is turned off, the bonding glass solidifies, thereby bonding the two elements together into the monolithic assembly 
     Before the bonding glass is melted and then solidified, the alignment of the two elements can be adjusted based on the throughput efficiency, back reflection reduction, pointing or other optical evaluation technique which is directed through the elements at a wavelength in the first wavelength range (e.g., near-IR light). 
     In preferred embodiments, the bonding glass is inorganic. In other words, it does not include organic components such as epoxy, which may cause outgasing concerns and/or have low damage thresholds. 
     In the following subsequent description, we describe attaching a micro-optic coupling lens to an optical fiber to make a monolithic assembly. The teachings herein can also be extended to attaching any other micro-optic component to an optical fiber, or more generally, to attaching any one optical element to any other optical element, including, for example, fiber arrays and lens arrays. 
     Referring to FIG. 1A, an optical fiber  100  includes an inner core  102  and an outside cladding  104 . Typically, core  102  is made from fused silica. Furthermore, for single mode optical fibers, inner core  102  has a diameter on the order of one to two microns. Optical fiber  100  is to be fused with a microlens  110  having a plano-surface  112  and a curved surface  114  to form a micro-optic assembly. Microlens  110  is also typically made of fused silica. 
     Plano surface  112  includes a thin film layer of bonding glass  120  to facilitate the fusion of the fiber  100  and lens  110 . The bonding glass can be formed on surface  112  by sputtering, thermal deposition, sol-gel deposition, implantation, ion exchange or diffusion or any other common thin film deposition technique. Bonding glass  120  is selected to form a low-melt temperature, glass interface that matches the refractive indices of fiber core  102  and lens  110  over a range of wavelengths (“the first range of wavelengths”) that includes those wavelengths desired for transmission through fiber  100  and lens  110 . For example, the first wavelength range can include wavelengths in the near infrared, e.g., around about 1.3 and/or around about 1.55 microns, which is common for telecommunications applications. 
     Bonding glass  120  is also selected to strongly absorb light in a second wavelength range for which fiber core  102  and lens  110  absorb less strongly, if at all. Thus, when light in the second wavelength range is transmitted through optical fiber  100  and into lens  110 , only bonding glass  120  substantially absorbs that light. For example, when fiber core  102  and lens  110  are made from fused silica, the second wavelength range may be in the ultraviolet (e.g., below about 400 nm). 
     Furthermore, bonding glass is selected to have a low-melt temperature, so that it melts and fiber core  102  and lens  110  do not melt or deform, when a sufficient amount of light in the second wavelength range is transmitted to bonding glass  120 . In preferred embodiments, the bonding glass does not include any organic components, which tend to cause low damage thresholds in the ultraviolet region. Organic components can also cause environmental concerns by outgasing. 
     Referring to FIG. 1B, fiber  100  and lens  110  are brought into contact with one another at bonding glass  120 , or brought near enough to one another to fuse when bonding glass  120  is melted. A source  130  of laser light at a wavelength in the first wavelength range directs light into fiber core  102  and through to lens  110 . Because bonding glass is selected to index match wavelengths in the first wavelength range, the laser light travels from fiber  100  into lens  110  with substantially no reflections. Whether or not that light emerges from lens  110  collimated, however, depends on the relative transverse positions of fiber  100  and lens  110 . A detector  140  can be positioned to monitor the collimation of the light, e.g., near-IR light, emerging from lens  110 . Based on the detector response, the transverse positions of fiber  100  and lens  110  can be adjusted to optimize such collimation. For example, fiber  100  and lens  110  can each be positioned in an adjustable chuck whose position is adjusted in response to the detector measurement to better align fiber  100  and lens  110  with one another. In alternative embodiments, the positions of source  130  and detector  140  can be reversed so that fiber  100  and lens  110  are aligned with one another based on the efficiency of light coupled into fiber core  102  from lens  110 . In either case, the alignment can be performed by a user or under servo control in an automated fiber alignment scheme. 
     Referring to FIG. 1C, after fiber  100  and lens  110  are aligned with one another, a second laser source  160  of light at a wavelength in the second wavelength range directs light into lens  110  and onto bonding glass  120 . Because that light is in the second wavelength range, bonding glass  120  absorbs that light and melts. Moreover, because fiber  100  and lens  110  are aligned with one another, lens  110  focuses that light directly onto the region of bonding glass  120  adjacent fiber core  102 , thereby increasing the light intensity at that region to better melt the bonding glass and ultimately provide a bond between the fiber core (e.g., a silica fiber core) and the lens (e.g., a silica lens). In other embodiments, the light used to melt the bonding glass may be directed to bonding glass  120  through fiber  100  rather than, or in addition to, through lens  110 . In such cases, the light energy from fiber  100  is necessarily directed to the region of bonding glass  120  adjacent fiber core  102 . 
     Moreover, in preferred embodiments, the second wavelength range is selected to be one where fiber core  102  and lens  110  are substantially transparent. Thus, optical energy from source  160  is only absorbed by bonding glass  120  and not elsewhere in the optical assembly. More generally, however, the second wavelength range is selected such that even if fiber core  102  and lens  110  absorb optical energy in that range, bonding glass  120  will melt in response to that optical energy before fiber core  102  and lens  110  deform. 
     After source  160  has delivered an amount of optical energy sufficient to melt the bonding glass between fiber core  102  and lens  110 , it is turned off. Referring to FIG. 1D, bonding glass  120  resolidifies to attach fiber core  102  to lens  110 . As is necessary, source  140  and detector  150  (shown in FIG. 1B) can be used to further optimize the alignment of fiber core  102  and lens  110  during the melting and resolidifying of the bonding glass. The resulting optical assembly  200  is an integrated structure having an optimized alignment and a substantially uniform refractive index for transmission wavelengths in the first wavelength range, i.e., the desired transmission wavelengths for the assembly. 
     Accordingly, the method, and particularly, the selection of bonding glass  120  and melting light source  160 , allow energy to be absorbed in the optical assembly primarily at the interface where the elements need to be attached, and not elsewhere. Thus, it permits alignment of the fiber to the lens with substantially no preheating and minimizes the chance of heat-induced asymmetric temperature gradients that may result in misalignments before solidification and fusing. 
     As described above, the method can be extended to optical components other than an optical fiber and a coupling lens. Moreover, the surfaces of the optical components to be attached need not be flat. For example, the surfaces could be convex or concave or wedged provided that they can be brought into sufficient proximity to one another to be bonded by the bonding glass after it is melted and resolidified. Furthermore, the surfaces to be attached could include surface features or markings to guide their alignment. Also, in additional embodiments, the bonding glass can be applied to one surface, the other surface, of both surfaces. For example, referring again to FIG. 1A, the bonding glass could be applied to the face of fiber core  102  in addition to, or instead of, piano surface  112  of lens  110 . 
     In many embodiments, the optical components to be attached (e.g., optical fiber  100  and lens  110 ) will be made from fused silica. For such embodiments, particularly suitable bonding glasses are: PFK 85 and CaFK95, available from SUMITA OPTICAL GLASS, INC. (4-7-25, Haigaya, Urawa, Saitama 338-8565, Japan); FK-3 and FK-54, available from Schott Glass Technologies (400 York Ave, Duryea Pa. 18642); FCD-100 and FCD-10, available from HOYA CORPORATION (2-7-5 Naka-Ochiai, Shinjuku-ku, Tokyo 161-8525 Japan); and FSL-3, SFPL-53 and SFPL-52, available from Ohara Glass (15-30 Oyama 1-Chome, agamihara-Shi Kanagawa, 229-1186, Japan). Also, fluorine-containing borophosphate glass may be suitable. See, e.g., “Easily melting glass for assembly of optical fiber into connectors”, SPIE Vol. 2290, pp 366-377, 1994. 
     Such materials have a refractive index that matches or very closely matches that of fused silica at wavelengths used for optical fiber communications (i.e., near-IR wavelengths) and transmits at these wavelengths with little or no attenuation. They also absorb UV wavelengths below 400 nm more strongly than fused silica and have a melt temperature less than that of fused silica. Thus, when exposed to a sufficient amount of UV radiation, they melt, and the fused silica components are substantially unaffected. A suitable source for such UV radiation is a frequency-doubled, Argon ion laser, which would operate at about 244 nm. 
     It is understood that while the invention has been described in conjunction with the detailed description thereof, the foregoing description is intended to illustrate and not limit the scope of the invention, which is defined by the scope of the appended claims. 
     Other aspects, advantages, and modifications are within the scope of the following claims.
The invention features a method for attaching a surface of a first optical element to a surface of a second optical element. The method includes: providing a bonding glass on at least one of the surfaces, wherein the bonding glass is selected to match the refractive indices of the first and second optical elements at the surfaces over a first range of wavelengths and absorb optical energy to a greater extent than that of the optical elements over a second range of wavelengths different from the first range of wavelengths; positioning the surfaces proximate one another; directing optical energy to the bonding glass through at least one of the optical elements at a wavelength in the second range of wavelengths, wherein the optical energy is sufficient to melt the bonding glass without deforming the optical elements; and allowing the melted bonding glass to solidify and fuse the proximately positioned surfaces.