Patent Description:
The present disclosure generally relates to methods for bonding optical elements to substrates and, more particularly, methods for directly bonding optical elements to substrates using a laser beam and optical assemblies comprising optical elements bonded to substrates.

Benefits of optical communication include extremely wide bandwidth and low noise operation. Because of these advantages, optical fiber is increasingly being used for a variety of applications, including, but not limited to, broadband voice, video, and data transmission. Connectors are often used in data center and telecommunication systems to provide service connections to rack-mounted equipment and to provide inter-rack connections. Accordingly, optical connectors are employed in both optical cable assemblies and electronic devices to provide an optical-to-optical connection wherein optical signals are passed between an optical cable assembly and an electronic device.

Optical devices, such as optical connectors, may include optical elements secured to a substrate. These optical elements should be precisely located on a substrate so that they may be optically coupled to a mated optical device so that optical signals may be propagated between the two devices. Commonly, precise V-groove substrates are employed to precisely locate the optical elements. However, such V-groove substrates having sub-micron tolerances are costly to produce and significantly increase the cost of optical devices.

<CIT> discloses a method of bonding a cover glass to a semiconductor substrate having conductors thereon. The cover glass and the semiconductor substrate are placed in a relatively high voltage field and heated to induce ion drift in the glass and improved conductivity in the substrate. Additional localized heating softens the cover glass in the vicinity of the conductors permitting the cover glass to flow around the conductors and to be drawn into contact and bonded with the substrate.

<CIT> relates to assemblies, optical connectors, and methods for bonding optical fibers to a substrate using a laser beam. In one embodiment, a method of bonding an optical fiber to a substrate includes directing a laser beam into the optical fiber disposed on a surface of the substrate, wherein the optical fiber has a curved surface and the curved surface of the optical fiber focuses the laser beam to a diameter that is smaller than a diameter of the laser beam as it enters the optical fiber. The method further includes melting, using the laser beam, a material of the substrate at a bond area between the optical fiber and the surface of the substrate such that the optical fiber is bonded to the surface of the substrate.

<CIT> discloses an optical interconnect that comprises a metallized optical fiber electrostatically bonded to a thin film of an alkali-containing glass which is itself bonded to a planar surface of a semiconductive or conductive substrate. Another optical interconnect comprises an optical fiber having a thin film of an alkali-containing glass deposited thereon, wherein the fiber is electrostatically bonded to a planar surface of a semiconductive or conductive substrate. A process of bonding an optical fiber to a semiconductive or conductive substrate includes contacting the fiber with the substrate, applying a DC potential to the fiber-substrate combination, slowly heating the combination to a maximum temperature between <NUM> and <NUM>. , maintaining the combination at the maximum temperature for a few minutes, cooling the combination to room temperature, and removing the DC potential.

<CIT> discloses techniques for forming fiber devices that engage fibers to a substrate with similar material properties. A semiconductor template may be used to define positions and orientations of the fibers.

The invention is defined in the independent claim to which reference should now be made.

Embodiments of the present disclosure are directed to methods for bonding one or more optical elements to a substrate using a laser beam, as well as optical connectors and assemblies resulting from said methods. The optical element is a curved element, such as a GRIN lens, a micro-lens or an optical fiber, that acts as a cylindrical lens to focus the laser beam into the substrate. The focused laser beam directly bonds the optical element to the substrate by melting the surface of the substrate material and/or the optical element material. Thus, the optical element is bonded to the substrate using a laser bonding process that produces less residual stress in the bond area than by bonding methods that melt the large volume of the material of the substrate and/or the optical element. The cylindrical lens provided by the curved optical element may eliminate the need to have a complicated optical delivery system to locally tightly focus the laser beam into the substrate material. In other words, the cylindrical lens provided by the curved optical element allows for using low numerical aperture focusing optics having larger window of focusing.

It is to be understood that both the foregoing general description and the following detailed description are merely exemplary, and are intended to provide an overview or framework to understanding the nature and character of the claims. The accompanying drawings are included to provide a further understanding, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments, and together with the description serve to explain principles and operation of the various embodiments.

Embodiments described herein are directed to methods for directly bonding optical elements to substrates using a laser beam. Embodiments of the present disclosure enable optical elements, which may have a curved shape, to be precisely aligned, and directly bonded to, a flat substrate without the use of expensive V-groove substrates. For effective optical communication between the optical connector and the optical channels of the photonic integrated circuit assembly (or other optical assembly), optical elements, such as optical fibers, should be aligned to the optical channel of the photonic integrated circuit assembly with sub-micron accuracy. Precision V-groove substrates having grooves to align and precisely place optical elements such as optical fibers onto a substrate are commonly used. However, inclusion of an additional precision component adds significant cost to the assembly.

Embodiments of the present disclosure provide for a fixed attachment procedure that does not rely on the use of alignment substrates, such as V-groove substrates, as part of the optical assembly.

Laser welding techniques that weld optical fibers to substrates by melting significant volume of the material of the optical fibers and/or the substrates may result in flaws or cracks in the optical fibers and/or the substrate, thereby reducing the strength of the bond. Such melting of the material of the optical fibers and/or the substrate may also shift the position of the optical fibers, which may cause misalignment between the optical fibers and optical elements to which the optical fibers are to be optically coupled. Further, melting of the material of the optical fibers and/or the substrate at the bond areas creates residual stress, which increases with temperatures reached during laser welding.

Embodiments of the present disclosure provide for a fixed attachment procedure that relies on lower laser power for softening the material of the optical elements and/or the substrate and/or an absorbent film, thereby creating bond areas that have low residual stress and that are substantially free of flaws and/or voids.

Particularly, embodiments of the present disclosure bond optical elements to substrates by low-temperature laser processes that result in bonding of the optical elements and/or the substrate.

Because of the low processing temperature to form the one or more bond areas at the optical elements, the bond areas have low residual thermal stress, are substantially free of flaws or cracks, and are longer than bond areas form by high power laser melting processes. Thus, embodiments provide robust bonding areas that resist detachment due to external forces.

Further, the laser bonding techniques described herein may bond optical elements in place during active or passive alignment when assembling optical components. For example, epoxy may cause optical elements to shift causing misalignment during curing. Epoxy also takes time to cure, thereby slowing down the assembly process. The laser bonding techniques do not suffer from misalignment and shifting, and do not require waiting for epoxy to cure before moving on to subsequent processing steps. Such laser bonding techniques may be particularly useful in the assembly of photonics components which typically require active alignment (e.g., by use of a vision system) and fixing the alignment between components accurately and quickly.

Optical elements as used herein encompasses optical components capable of propagating optical signals. Optical elements described herein may include curved optical elements, such as, without limitation, optical fibers, gradient-index (GRIN) lenses, optical fiber stubs, cylindrical waveguides, and convex lenses. A "curved optical element" according to this disclosure is an optical element that includes at least one curved outer surface intended to be bonded to a substrate.

Various embodiments of methods for bonding optical elements to substrates using a laser and assemblies comprising a plurality of optical elements bonded to a substrate are described in detail herein.

Referring now to <FIG>, a partial perspective view of a substrate <NUM> with a plurality of optical elements <NUM> (shown in <FIG> as a plurality of optical fibers) bonded thereto is schematically depicted. It should be understood that the optical elements <NUM> shown in <FIG> may also be configured as other curved optical elements, such as gradient-index (GRIN) lenses, optical fiber stubs, cylindrical waveguides, convex lenses, and concave lenses.

As an example and not a limitation, the substrate <NUM> and the plurality of optical elements <NUM> may be incorporated into an optical connector (e.g., a fiber optic connector), as illustrated schematically in <FIG>. For example, the optical connector <NUM> may include a housing <NUM> and the substrate <NUM>, and at least a portion of the optical elements <NUM> may be located in the housing <NUM>. It should be understood that embodiments described herein are not limited to optical connectors. The optical elements and substrate assemblies may be incorporated into other optical devices.

The example substrate <NUM> depicted in <FIG> comprises a first surface <NUM>, a second surface <NUM> opposite the first surface <NUM> and at least one edge <NUM> extending between the first surface <NUM> and the second surface <NUM>. The substrate <NUM> may be made of any material that absorbs the wavelength of the laser beam. For example, the material present at the first surface <NUM> may be in a range from <NUM>% to <NUM>% absorbing at the wavelength. As a non-limiting example, the material of the substrate <NUM> may be a dielectric material or a high-resistivity crystalline material, such as semiconductor materials. Non-limiting semiconductor materials include silicon, geranium, and silicon carbide, each of which may be doped or undoped.

Other materials may also be used for the substrate <NUM>. As additional non-limiting examples, laser-wavelength absorbing amorphous material such as glasses may be used as materials for the substrate <NUM>. Materials such as glass-ceramics having both an amorphous phase and one or more crystalline phases may also be utilized for the substrate <NUM>. Further, laser-wavelength transparent glasses and glass ceramics may be used when an absorbing film <NUM> (<FIG>) is applied to the first surface <NUM> of the substrate <NUM>. This absorbing film <NUM> enables the laser beam to be absorbed to heat the interface between the first surface <NUM> and an optical element <NUM>. It should be understood that the absorbing film <NUM> does not need to be utilized in embodiments where the material of the substrate <NUM> is absorbing at the wavelength of the laser beam. In some embodiments, the absorbing film <NUM> may be metal such that it is electrically conductive and assists in the electrostatic affixing process described below. The absorbing film <NUM> can also be an inorganic electrically insulating film. In this case, an electrically conducting substrate can be used. Either or both of should be capable of absorbing the laser beam. Non-limiting glass materials include alkaline earth boro-aluminosilicate glass (e.g., as manufactured and sold under the trade name Eagle XG® by Corning Incorporated of Corning, New York) and alkali-aluminosilicate glass (e.g., as manufactured and sold by Corning Incorporated of Corning, New York under the trade name Gorilla® Glass).

The thickness of the substrate <NUM> is not limited by this disclosure. The thickness of the substrate <NUM> may be any thickness as desired for the end-application of the optical element <NUM> and substrate <NUM> assembly.

The material of the optical element <NUM> should be transparent to the wavelength of the laser beam as described below, in a range from <NUM>% to <NUM>% absorbing at the wavelength of the laser beam. Non-limiting example materials for the optical element <NUM> include glass, glass-ceramics with scattering losses < <NUM>-<NUM>%, and crystal materials. Non-limiting glass materials include alkaline earth boro-aluminosilicate glass (e.g., as manufactured and sold under the trade name Eagle XG® by Corning Incorporated of Corning, New York) and alkali-aluminosilicate glass (e.g., as manufactured and sold by Corning Incorporated of Corning, New York under the trade name Gorilla® Glass), as well as optical fibers.

The plurality of optical elements <NUM> are bonded to the first surface <NUM> of the substrate <NUM> by one or more laser bonding processes as described in detail below. If needed, the optical elements <NUM>, if configured as optical fibers, are stripped of any jacket or outer layers to remove organic material. Although <FIG> depicts four optical elements <NUM>, it should be understood that any number of optical elements <NUM> may be bonded to a surface of the substrate <NUM>. It should also be understood that the optical elements <NUM> may be bonded to the second surface <NUM> (<FIG>), or both the first surface <NUM> and the second surface <NUM>.

As noted above, the optical elements <NUM> may be fabricated from fused silica. The optical elements <NUM> have a round shape in cross section. However, the optical elements <NUM> may be elliptical in shape, semi-spherical in shape, or have any curved surface. As described in more detail below, the optical elements <NUM> may have at least one curved surface that focuses a laser beam to a smaller size at the contact area between the optical element <NUM> and the first surface <NUM> of the substrate <NUM>.

Each optical element <NUM> is bonded to the first surface <NUM> of the substrate <NUM> at one or more bond areas <NUM> (also called a bond area or an additional bond area) along the length of the optical element <NUM>. It is noted that the bond areas <NUM> are denoted by ellipses in <FIG>. As described in detail below, the bond areas <NUM> are regions of the first surface <NUM> of the substrate <NUM> where the optical element <NUM> is bonded to the first surface <NUM> of the substrate <NUM> by the elevated temperature provided by a laser beam. As stated above, the optical element <NUM> and the substrate may each be comprised of a crystalline material or an amorphous material. The material of the optical element <NUM> has an optical element change temperature, which is a melting point of the optical element <NUM> material when it is fabricated from a crystalline material and a softening point of the optical element <NUM> material when it is fabricated from an amorphous material. Similarly, the material of the substrate <NUM> has a substrate change temperature, which is a melting point of the substrate <NUM> material when it is fabricated from a crystalline material and a softening point of the substrate <NUM> material when it is fabricated from an amorphous material.

To create the bond areas <NUM>, the laser beam heats the interface between the optical element <NUM> and the substrate <NUM> to a temperature that is higher than the lowest of the optical element change temperature and the substrate change temperature. Thus, when both the optical element <NUM> and the substrate <NUM> are made from crystalline materials, the laser beam heats the interface to a temperature that is higher than the lowest melting point of the optical element <NUM> and the substrate <NUM>. When both the optical element <NUM> and the substrate are made from amorphous materials, the laser beam heats the interface to a temperature that is higher than the lowest softening point of the optical element <NUM> and the substrate <NUM>. When the optical element <NUM> and the substrate <NUM> are a combination of a crystalline material and an amorphous material, the laser beam heats the interface to a temperature that is higher than the lowest of the melting point of the crystalline material or the softening point of the amorphous material.

The bond areas <NUM> secure the optical element <NUM> to the first surface <NUM>. It is noted that, in some embodiments, heating of a contact area <NUM> (<FIG>) between optical element <NUM> and the first surface <NUM> of the substrate <NUM> may be provided by application of electromagnetic energy (e.g., microwaves) rather than a laser beam to bond the optical elements <NUM> to the substrate <NUM>.

Any number of bond areas <NUM> may be provided along the length of the optical element <NUM>. Bonding the optical elements <NUM> to the surface of the substrate <NUM> may eliminate the need for adhesives or organic materials, such as epoxy, to secure the optical elements <NUM> to the substrate <NUM>. However, in some embodiments, adhesive is applied such that the resulting assembly has additional strength and rigidity during a solder reflow process. The assembly of the substrate <NUM> and the optical elements <NUM> may be subjected to elevated temperatures of a solder reflow process without movement of the optical elements <NUM> because the laser welding process keeps the optical elements in place. The laser welding provides accurate placement of the optical elements <NUM>, and eliminates the need for costly V-groove substrates for placement of the optical elements <NUM>.

Referring now to <FIG>, an example process for bonding curved optical elements <NUM>, such as optical fibers, to a substrate <NUM> is schematically illustrated. One or more curved optical elements <NUM> are disposed on the planar first surface <NUM> of the substrate <NUM> such that a contact area <NUM> is defined by contact between the curved surface of the optical element <NUM> and the first surface <NUM>. The contact area <NUM> generally extends along the length of the optical element <NUM> that it is in contact with the first surface <NUM>.

To enable bonding between the optical elements <NUM> and the substrate <NUM>, there should be substantially no gaps between the optical elements <NUM> and the first surface <NUM> (i.e., the contact area <NUM> should continuously extend along the length of the optical elements <NUM>). In embodiments according to the claimed invention, the optical elements <NUM> are electrostatically affixed to the first surface <NUM> of the substrate <NUM>. A charged substrate <NUM> and/or charged optical elements <NUM> causes the optical elements <NUM> to be attracted to the first surface <NUM>, thereby causing good contact therebetween and removing gaps between the optical elements <NUM> and the first surface <NUM> of the substrate <NUM>. Electrostatic charging allows for lower laser power and intensity for bonding the optical elements <NUM> to the substrate.

In some embodiments, the substrate <NUM> and/or optical elements <NUM> are charged by a plasma treatment process. Any known plasma treatment process may be utilized. The plasma treatment process removes organic contamination from the planar first surface <NUM> of the substrate <NUM>, which results in surface activation of the planar first surface <NUM>. The activated first surface <NUM> enables the optical elements <NUM> to be electrostatically affixed thereto. A non-limiting example of a device for effecting the plasma treatment process is the Plasma Wand sold by PlasmaEtch, Inc. of Carson City, Nevada.

After the optical elements <NUM> are electrostatically affixed to the first surface <NUM> of the substrate <NUM>, the one or more optical elements <NUM> are locally heated by a laser beam at desired bond areas to directly bond the one or more optical elements <NUM> to the first surface <NUM>. Referring now to <FIG>, a laser beam <NUM> for bonding is schematically illustrated. In the illustrated embodiment, the laser beam <NUM> is produced by a laser source <NUM> that provides an astigmatically shaped Gaussian laser beam <NUM> having a line focus <NUM> that is capable of being incident across a plurality of optical elements <NUM>. The line focus <NUM> enables multiple optical elements <NUM> to be directly bonded by one pass of the laser beam <NUM>. The laser source <NUM> may include any known or yet-to-be-developed optical components capable of generating the astigmatically shaped laser beam <NUM>, such as, without limitation, the use of one or more cylindrical lenses. In another embodiment, the laser source <NUM> may be capable of scanning a collimated laser beam rapidly back and forth across the plurality of optical elements <NUM> to achieve a similar effect as the astigmatically shaped laser beam <NUM> illustrated by <FIG>. The laser source <NUM> is translated in a direction parallel to the optical elements <NUM> as indicated by arrow A. Alternatively, the optical elements <NUM> and the substrate <NUM> may be translated and the laser source <NUM> may remain stationary, or both the laser source and the optical elements <NUM> and the substrate <NUM> may be translated simultaneously in opposite directions.

As illustrated by <FIG>, the laser source <NUM> may be continuously operated to produce a continuous laser beam <NUM> to form a continuous bond area along the optical elements <NUM> in direction A. As stated above, the laser beam <NUM> has a wavelength such that optical elements <NUM> are transparent to the laser beam <NUM> and the substrate <NUM> absorbs the laser beam <NUM>. As examples and not limitations, silicon is absorbing to wavelengths up to approximately <NUM>, and fused silica is transparent up to approximately <NUM>. As a non-limiting example, the laser beam <NUM> may be a green laser (<NUM> - <NUM>, including endpoints) or an ultraviolet laser. The laser beam <NUM> should have a power and an energy distribution to raise the temperature of the optical elements <NUM> and the first surface <NUM> at the contact areas <NUM> to a temperature above the surface softening/melting temperature of the respective materials (i.e., an optical element change temperature and a substrate change temperature as described above). The estimated laser beam fluence when welding fused silica optical fibers to a silicon substrate wherein the optical fibers electrostatically affixed to the silicon substrate by the methods disclosed herein is approximately <NUM> J/cm<NUM> (or <NUM> W/cm<NUM> intensity). For comparison, the estimated laser beam fluence when welding fused silica optical fibers to a silicon substrate without electrostatically affixing the optical fibers to the silicon substrate by melting the material is approximately <NUM> J/cm<NUM> (or <NUM> kW/cm<NUM> intensity). Thus, a much lower powered laser beam <NUM> is possible when bonding the optical fibers to the silicon substrate by charging the silicon substrate and/or the optical fibers.

As illustrated by <FIG> and stated above, the example optical element <NUM> has a curved surface, and has a generally circular shape. The shape of the optical element <NUM> enables the optical element <NUM> to act as a cylindrical lens that focuses an incident laser beam <NUM> at the contact area <NUM> without a complicated optical delivery system. The optical element <NUM> that receives the laser beam <NUM> focuses the laser beam <NUM> to a focused line (when using an astigmatically shaped laser beam) or a focused diameter at the contact area <NUM> that is smaller than the size of the laser beam <NUM> as the laser beam <NUM> enters the optical element <NUM> (i.e., at the upper surface <NUM> of the optical element <NUM>). The reduction in size of the laser beam causes the first surface <NUM> to be heated quickly and provide the formation of a bond area <NUM> (<FIG>) proximate the contact area <NUM> (<FIG>).

As illustrated by <FIG>, the heat generated by the laser beam <NUM> at the contact area <NUM> is enough to cause a bond between the optical element <NUM> and the first surface <NUM> of the substrate <NUM>. The temperature at the bond area <NUM> should be more than the melting temperature or the softening temperature of the optical element <NUM> and the substrate <NUM>.

When the astigmatically shaped laser beam <NUM> is continuously operated, there is a single continuous bond area <NUM> at the contact area <NUM> between the optical element <NUM> and the first surface <NUM> of the substrate <NUM>. However, the astigmatically shaped laser beam <NUM> may be sequentially turned on and off as it travels in direction A, which results in individual bond areas <NUM> along the optical element <NUM>, such as is shown in <FIG>.

Another method to electrostatically affix one or more optical elements <NUM> to the first surface <NUM> (and/or the second surface <NUM>) of the substrate <NUM> is by applying a voltage between the substrate <NUM> and the cover substrate <NUM>, as shown by <FIG>, and according to the claimed invention. In the illustrated embodiment, a cover substrate <NUM> having a bottom surface <NUM> with one or more grooves <NUM> (e.g., V-grooves) is applied over at least one optical element <NUM> and the first surface <NUM> of the substrate <NUM> such that the optical elements <NUM> are disposed within one or more grooves <NUM>. The grooves <NUM> allow for precise positioning of optical elements <NUM> on the first surface <NUM> on the x- and z-axes. When a plurality of optical elements <NUM> are utilized, a plurality of grooves <NUM> provide precise spacing between adjacent optical elements <NUM>, which may be beneficial in fiber-array connector applications.

The cover substrate <NUM> is electrically conductive and thus may be made of any electrically conductive material. The cover substrate <NUM> may include one or more windows <NUM> configured as openings through which the laser beam <NUM> may pass through to be incident on the one or more optical elements <NUM>. The cover substrate <NUM> may also be an insulating material that is coated with an electrically conductive film or coating.

When the substrate <NUM> is fabricated from a dielectric material or has an electrically conductive absorbing film <NUM> on the first surface <NUM>, no additional electrical conductors are needed. However, when non-electrically conducting glasses or glass ceramics are used without an electrically conductive absorbing film <NUM> for the substrate <NUM>, the substrate <NUM> may be positioned on an electrically conductive support plate <NUM>.

Referring now to <FIG>, a voltage V is applied between the substrate <NUM> and the cover substrate <NUM>. The voltage V should be enough to electrostatically affix the optical elements <NUM> to the first surface <NUM> of the substrate <NUM> to enable bonding. As stated above, electrically charging the substrate <NUM> draws the optical elements <NUM> close to the first surface <NUM> to reduce gaps therebetween. As an example and not a limitation, the voltage V may be greater than or equal to <NUM> V.

A laser beam <NUM> as described above may be translated in a direction parallel to the optical elements <NUM> as indicated by arrow A to produce a continuous bond area or a sequence of bond areas <NUM> as shown in <FIG> along the length of the optical elements. The laser beam <NUM> passes through the one or more windows <NUM> of the cover substrate <NUM> to directly bond the optical elements <NUM> to the substrate <NUM>. The cover substrate <NUM> may be removed after the bonding process.

In some embodiments, the substrate <NUM> is electrostatically charged using both an initial plasma treatment and application of a voltage using the cover substrate <NUM> as shown in <FIG>. The plasma treatment provides surface activation on the first surface <NUM> (and/or the second surface <NUM>) while the applied voltage V further charges the substrate <NUM>.

The laser beam <NUM> is not limited to an astigmatically shaped laser beam or a rapidly scanned laser beam that is scanned in the x-axis and z-axis directions. In some embodiments, the laser beam <NUM> may be a round laser beam focused to a beam spot. <FIG> schematically depicts a top-down view of optical elements 110A-110E electrostatically affixed on a first surface <NUM> of a substrate <NUM>. The laser beam <NUM> and/or substrate <NUM> is then moved (or translated) in a first direction (e.g., direction A) that is transverse to a longitudinal axis OA of the optical elements 110A-110E such that the laser beam passes over the optical elements 110A-110E to form bond areas <NUM>. In the example of <FIG>, the direction B of the laser beam <NUM> is perpendicular to the longitudinal axis OA of the optical elements 110A-110E. However, embodiments are not limited thereto and the laser direction can be at an angle that is different from <NUM> degrees with respect to the optical elements 110A-110E. It is noted that the laser beam <NUM> may be translated relative to the substrate <NUM>, or the substrate <NUM> may be translated relative to the laser beam <NUM>.

The laser beam <NUM> sequentially traverses and directly bonds multiple optical elements 110A-110E as it travels along direction B in a first pass 122A. As the laser beam <NUM> enters an optical element 110A-110E, it is focused as described above and creates a bond area <NUM>.

As shown by <FIG>, multiple passes 122A-122D of the laser beam <NUM> may be performed to weld the optical elements 110A-110E (e.g., optical fibers) to the substrate <NUM> at multiple bond areas <NUM> along the length of the optical elements 110A-110E. For example, a position of the laser beam <NUM> or the substrate <NUM> may be shifted by a distance d in a direction A parallel to the longitudinal axis OA of the optical elements 110A-110E after completion of a pass (e.g., the first pass 122A) to translate in a second direction to perform a subsequent pass (e.g., the second pass 122B) that may also be transverse to the longitudinal axis A of the optical elements 11A-110E. The distance d is not limited by this disclosure and may depend on the desired number of bond areas <NUM> desired for each optical element 110A-110E. The locations of the bond areas <NUM> should be spaced far enough apart to prevent proximity of weld lines, which may lead to excessive stress and cracking.

After shifting the position of the laser beam <NUM> or the substrate <NUM>, the laser beam <NUM> or the substrate <NUM> is again translated traverse to the longitudinal axis OA of the optical elements 110A-110E. In <FIG>, a fourth pass 122D is not yet complete as the laser beam <NUM> approaches a third optical element 110C. As a non-limiting example, the translation speed of the laser beam <NUM> with respect to the substrate <NUM> is in the range of about <NUM>/s to <NUM>/s, including endpoints.

Referring now to <FIG>, a microscope image of a plurality of optical elements <NUM> configured as SMF-<NUM>® optical fibers manufactured by Corning, Incorporated bonded to a first surface <NUM> of a substrate <NUM> configured as a silicon substrate is provided. The microscope image of <FIG> was taken by disposing an index matching fluid on the first surface <NUM> of the substrate <NUM> and then placing a glass substrate on top of the optical elements <NUM> such that the optical elements <NUM> and the index matching fluid was disposed between the substrate <NUM> and the glass substrate. In this manner, the optical elements <NUM> and their contact areas <NUM> become visible in the microscope image.

The laser beam used to weld the optical fibers was a single-mode mode <NUM> wavelength laser beam having a power of <NUM>-<NUM> W that was scanned in a manner as shown in <FIG>. The translucent areas in the image are the locations where the laser beam heated the contact area between the optical elements <NUM> and the substrate <NUM>. Further, the dark areas are modified regions <NUM> where material is ablated and ejected such that the optical elements are not in contact with the first surface <NUM> of the substrate <NUM>. Therefore, the modified regions <NUM> where material is ablated appear dark in the microscope image.

The reduced laser beam power and therefore the reduced laser beam fluence is such that the first surface <NUM> of the substrate <NUM> was not modified (i.e., surface modification) at areas of the first surface <NUM> outside of the optical elements <NUM> despite the laser beam being incident on the first surface <NUM> in these regions in the laser scanning pattern shown in <FIG>. As used herein, the term "surface modification" is defined as more than <NUM> localized deviation from the original characteristic surface profile of the optical element or substrate, which is detected by a surface profilometer having a resolution in the vertical (orthogonal to the surface) direction less than <NUM>. Examples of such profilometers are: optical interferometers (e.g., optical interferometers sold by Zygo of Middlefield, CT and Keyence America of Itasca, IL), confocal optical profilometers (e.g., confocal optical profilometers sold by Keyence and Carl Zeiss AG of Jena, Germany), and stylus profilometers (e.g., stylus profilometers sold by KLA Corporation of Milpitas, California). Accordingly, only the areas of the first surface <NUM> of the substrate <NUM> where an optical element <NUM> is present are modified. This results in a more robust substrate <NUM> and overall optical assembly.

The bond areas <NUM> in the microscope image of <FIG> are the translucent regions that are between pairs of ablated regions <NUM>. The individual bond areas <NUM> extend beyond individual pairs of ablated regions <NUM>, thereby providing long bond areas that increase the strength of the overall bond between the optical elements <NUM> and the substrate <NUM> over bonding techniques that melt material. The locations where the laser beam heats the contact area <NUM> may be precisely placed such that there is a continuous bond area <NUM> along the length of the optical element <NUM>. However, the locations where the laser beam heats the contact area <NUM> should not be too close to one another to prevent significantly overlapping bond areas <NUM>, which may cause increased residual stress and cracking. <FIG> is a microscope image of the substrate of <FIG> with the optical elements <NUM> configured as optical fibers removed from the first surface <NUM>. As shown by <FIG>, the bond areas <NUM> traces are indicative of a cohesive nature of bonding.

Optical assemblies comprising the substrate <NUM> and the optical elements <NUM> bonded thereto may be incorporated into any number of larger devices depending on the application. As an example and not a limitation, the substrate <NUM> and the plurality of optical elements <NUM> configured as optical fibers of a fiber ribbon <NUM> may be incorporated into an optical connector <NUM> (e.g., a fiber optic connector), as illustrated schematically in <FIG>. For example, the optical connector <NUM> may include a housing <NUM> and the substrate <NUM> and at least a portion of the optical elements <NUM> may be located in the housing <NUM>. End faces of the optical elements <NUM> may be exposed at a mating face <NUM> of the housing <NUM> to be optically coupled to a mated optical assembly or component. It should be understood that embodiments described herein, and useful to understand the claimed invention, are not limited to optical connectors. The optical elements and substrate assemblies may be incorporated into other optical devices, such as photonic integrated circuits, for example.

It should now be understood that embodiments described herein are directed to methods of bonding optical elements to substrates using a low-temperature, low-stress laser bonding process. The methods described herein include electrostatically affixing the optical elements to the substrates to reduce gaps therebetween, which enables the components to be bonded to one another using laser beaming having a power that is less than what is needed when not electrostatically affixing optical elements to a substrate.

Claim 1:
A method of bonding at least one curved optical element (<NUM>) to a substrate (<NUM>), the method comprising:
applying a cover substrate (<NUM>) over the at least one curved optical element (<NUM>) and the surface of the substrate (<NUM>), the cover substrate (<NUM>) comprising a window (<NUM>),
electrostatically affixing the at least one curved optical element (<NUM>) to a surface of the substrate (<NUM>) by applying a voltage between the cover substrate and the substrate (<NUM>); and
directing a laser beam into the at least one curved optical element (<NUM>), the laser beam passing through the window (<NUM>), wherein:
a material of the at least one curved optical element (<NUM>) has an optical element change temperature,
wherein the optical element change temperature is a melting point of the curved optical element material if fabricated from a crystalline material, and a softening point of the curved optical element material if fabricated from an amorphous material;
a material of the substrate (<NUM>) has a substrate change temperature, wherein the substrate change temperature is a melting point of the substrate material if fabricated from a crystalline material and a softening point of the substrate material if fabricated from an amorphous material;
the laser beam heats an interface between the at least one curved optical element (<NUM>) and the substrate (<NUM>) to a temperature that is higher than a lowest temperature of the optical element change temperature and the substrate change temperature, thereby forming a bond between the at least one curved optical element and the substrate at a bond area, and
the laser beam has a fluence that does not modify the substrate at regions of the substrate that are outside of the at least one curved optical element (<NUM>);
wherein the at least one curved optical element (<NUM>) is a GRIN lens, a micro-lens or an optical fiber.