1. Field of the Invention The present invention relates generally to a method for fabricating a multi- component optical device, and particularly to a method for removing impurity particles from polymer adhesives used in fabricating multi-component optical devices.

2. Technical Background Organic polymers are being used in many photonic applications in which high transparency at a specific wavelength or wavelengths is required. In applications where high pulsed laser power can be experienced, such as in an optical amplifier, glass has been the material of choice. Currently, techniques such as fusion splicing and laser welding are being used to connect fiber and other glass components. However, as the design of photonic devices become ever more complex, simple and less expensive alternatives are being sought.

In one approach that has been considered, polymers were used as adhesives to bond various waveguide and photonic components together. Polymers are attractive materials because they have optical and mechanical properties that allow them to be used as bonding agents in the optical path. However, some designers and researchers

have found that polymer adhesives were undesirable because of their low laser damage threshold. Under certain conditions, pulses and transients on the order of 100MW/cm2 to 100GW/cm2 have been experienced leaving the adhesive in a charred and damaged condition. It has been determined that one of the main causes of polymer adhesive failure under these conditions was the presence of highly absorbing impurity particles.

The damage was initiated at the site of the impurities. The impurity particles absorbed at least five to ten times as much energy at 1550 nm than did the surrounding medium, depending of course, on the actual composition of the impurity particle. Thus, it was imperative to find a method of removing these impurity particles if the use of polymer adhesives in the path of high-powered lasers was to become feasible.

In another approach, a method of purifying polymers that has proved successful has been to filter low-viscosity monomers prior to polymerization. This method has produced polymers that have a higher laser damage threshold. Figure 1 is a chart showing the relationship between laser threshold damage (Ed) of polymer adhesives and the pore size of the filter used to remove the impurity particles in the adhesive. As Figure 1 clearly shows, when the pore size of the impurity particles were reduced from approximately 16 pm to approximately 0.22 llm, the damage resistance of the polymer adhesive was improved by a factor of about 3.5. Further improvement could be realized if particles smaller than 0.22 pm could be removed. However, the improvements that can be realized using the filtration technique are limited. One limitation of filtration is that it is extremely difficult to filter out particles smaller than 0.2 microns. Secondly, as the size of the particles become smaller, the pressure and heat required to filter these particles out, becomes prohibitive. Finally, if the viscosity of the adhesive precursor is too high, filtration is no longer feasible.

Thus, the development of a polymer optical path adhesive resistant to damage from high-powered lasers would be highly desirable. Such an adhesive would provide several design and fabrication advantages over fusion splicing, laser welding or other techniques currently used to connect glass components.

SUMMARY OF THE INVENTION Accordingly, the present invention discloses a method of removing impurity particles from polymer precursor materials by using a high g-force centrifugation

technique as an alternative to filtering. This technique offers advantages over current polymer adhesive methods in that it is capable of removing impurity particles having a diameter greater than or equal to 0.1 microns, yielding approximately a ten-fold increase in the laser damage threshold. Further, the present invention does not require high pressure and temperature to remove particles in the 0.1 microns range and is not limited by the viscosity of the liquid precursor.

One aspect of the present invention is a method of fabricating an optical device used for transmitting light. The optical device includes a plurality of optical components each having an optical path. The method of fabricating an optical device includes the following steps. Providing an adhesive precursor. Applying a high-gravitational centrifugal force to the adhesive precursor to remove light scattering impurity particles to thereby produce a purified adhesive. Each of the plurality of optical components is bonded to at least one other of the plurality of optical components with the purified adhesive to form the optical device.

In another aspect, the present invention includes an optical device for transmitting a light signal generated by a high powered laser. The optical device includes a plurality of optical components. Each of said plurality of optical components has an optical path traversed by the light signal. A purified adhesive is applied to a portion of each of the plurality of optical components in the optical path. The purified adhesive bonds each of the plurality of optical components to at least one other of the plurality of optical components. The purified adhesive has no impurity particles greater than or equal to 0.1 go.

In another aspect, the present invention includes a method of transmitting a light signal generated by a high powered laser through an optical device. The optical device includes a plurality of optical components, wherein each of the plurality of optical components has an optical path. The method of transmitting a light signal includes the step of applying a purified adhesive to a portion of each of the plurality of optical components in the optical path. The purified adhesive bonds each of the plurality of optical components to at least one other of the plurality of optical components and has no impurity particles greater than or equal to 0.1 um. The method of transmitting a light signal also includes directing the light signal generated by the high powered laser into the optical device such that the light signal traverses the optical path.

Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.

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

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a chart showing the relationship between laser damage threshold of polymer adhesives and the size of impurity particles; Figure 2 is a chart showing the improvement in laser damage threshold realized by the present invention; and Figure 3 is a perspective of a fabrication detail of a multi-component optical device.

DETAILED DESCRIPTION According to the present invention, a method of removing impurity particles from liquid polymer precursor materials prior to polymerization or crosslinking is disclosed. The method involves spinning the precursor liquid in a high g-force centrifuge at a predetermined rate and for a predetermined time to achieve a desired particle separation. Polymers made from purified precursors using this centrifuge technique are resistant to damage from high-power laser pulses and their transients.

This is accomplished by separating impurity particles of approximately 0.1 microns and larger from the precursor material. Figure 2 is a chart showing the improvement in

laser damage threshold realized by the present invention. Both the first and second embodiments of the present invention, which are discussed below, have a laser damage threshold that is approximately ten times that of polymer adhesives developed using other techniques currently being employed.

The relationship between the sedimentation velocity, viscosity, particle density in the Stokes region of flow can be made by using the following equation. <BR> <BR> <BR> <BR> <BR> <BR> <P> Vc=--------------'(1)<BR> <BR> <BR> <BR> 18, u Where vc is the sedimentation velocity in a centrifugal field, Pc is the density of the impurity particle, pm is the density of the liquid precursor medium, Dp is the diameter of the impurity particle, co = angular velocity of the centrifuge, r is the radius of the centrifuge, and u is the viscosity of the liquid precursor medium. One of ordinary skill in the art will recognize from equation (1) that the rotation rate and the spin time will vary depending on the viscosity of the liquid precursor and the density difference between the particles and the liquid precursor. Generally, for low viscosity precursors, shorter times are required. As the viscosity of the precursor increases, the spin time increases accordingly. Generally, the viscosity of the adhesive precursors will range from 1 00cps to 2,000cps, but the present invention should not be construed as being limited to that. It should be noted that the present invention is particularly useful when the liquid precursor is too viscous to be filtered. According to equation (1), the greater the angular velocity, the faster the settling velocity will be. It should also be noted that the effectiveness of the centrifuge method depends on the density difference between the particles and the liquid. There is a direct correlation between the density difference and the settling velocity. The method is less effective when the density difference is low, and more effective when the density difference is large. When the density difference is low, the settling velocity is also low and the process takes longer to complete.

In a first embodiment of the present invention, a one-part liquid adhesive precursor material is pipetted into an even number of holding tubes. The only requirement of the one-part adhesive is that it is a liquid. The holding tubes are placed

in a holding fixture in the centrifuge. The centrifuge is operated at a predetermined spin rate for a predetermined amount of time based on the capabilities of the centrifuge and equation (1). Once the spinning is complete and particle separation has been achieved, the liquid is removed from the upper half of the centrifuge tube with a syringe or pipette to extract the purest liquid from the tube. Light scattering impurity particles greater than or equal to 0.1 pm remain in the bottom of the centrifuge tube and are discarded.

The centrifuge may be of any suitable well-known type. Fixed-angle-head-rotor type centrifuges are suitable because they are capable of producing the high G-forces required for this application. If an ultracentrifuge is employed, G-forces on the order of 50,000 to 100,000 can be achieved. There are more expensive centrifuges on the market capable of 1,000,000 G-forces. Using this centrifuge the spin time can be reduced to a matter of minutes.

As embodied herein and depicted in Figure 3, optical device 10 is a multi- component device including a first component 20 and a second component 40. These components are bonded together using the adhesive 30 which was purified using the process described above. As depicted in Figure 3, the purified adhesive 30 is applied to the components in the optical path 50 of device 10. Subsequently, the adhesive is cured.

The optical components 20 and 40 may be of any suitable well-known type, but there is shown by way of example, a waveguide 20 being bonded to a GRIN lens 40.

The purified adhesive 30 can also be used to bond one fiber to another fiber. The adhesive can also be used to bond a fiber to a planar waveguide, or to a grin lens. One of ordinary skill in the art will recognize that any optical components can be bonded together using the adhesive 30, without the adhesive deteriorating from high-power laser damage.

The adhesive precursor may be of any suitable well-known type. The reactive precursor can have any mono-functional or multi-functional groups attached for chemical cross-linking purposes. The one-part adhesive systems used in the first embodiment are photo-curable, heat-curable, or RTV curable adhesives. For example, a low weight polymer (oligomer) material with functional groups attached for chemical cross-linking can be used as the photo-curable adhesive precursor. The photocurable

precursor consists of a solution of the uncured liquid precursor and a photo-initiator.

When exposed to light energy, the photo-initiator material reacts with the liquid causing the solution to cure. The functional groups can include epoxy, epoxy acrylate, or epoxy methacrylate end groups. On the other hand, low molecular weight organic fluorinated or non-fluorinated precursors with epoxy, cycloaliphatic epoxy, or acrylate functional end groups can be employed. The precursor used can also be a low molecular weight liquid monomer prior to polymerization. Further, the precursor could also be an inorganic polymer such as a silicone oligomer with two or more cycloaliphatic epoxy functional groups. This material is suitable for both photocurable and RTV methods of curing. The number and type of functional groups can be varied based on the contemplated end use of the optical device. Precursors having one, two or more functional groups can be employed to obtain the desired mechanical and optical properties.

Example A centrifuge tube with a 2ml capacity is filled with the precursor. The precursor consists of an epoxy precursor with a viscosity of 100cp and a density of 0.9638 g/cm3.

Polymer impurity particles are dispersed in the liquid. According to the principles of the present invention, particles of 0. 1 llm in diameter or greater, are to be removed from the precursor liquid. The density of the 0.1 llm polymer particle is 1.05g/cm3. The height of the fluid in the tube measures 1.5 cm from the bottom of the tube to the meniscus of the fluid. Thus, the maximum distance a particle would have to settle is 1.5cm if it travels from the meniscus to the bottom. By knowing the distance the particle must travel, e. g. 1.5cm, the spin time required to precipitate the particle to the bottom of the tube can be calculated. Spinning at a rate of 13, 500rpm translates to an angular velocity of 1,414 rads/s. The radius of the centrifuge is 9.5cm. Equation (1) is then used to calculate a settling velocity equal to 0.091 x 10-6 cm/s. At this rate, the particle will settle 1.5 cm in 46 hours. Once the spinning is complete and particle separation has been achieved, the purified liquid is extracted from the tube. Applying the adhesive to the portion of the components in the optical path and joining them bonds the components. Thereafter, light energy is applied to the adhesive to effect photo-curing.

In a second embodiment of the present invention, a two-part liquid adhesive precursor material is employed. In a two-part system, the liquid precursor and the curing agent are purified separately in accordance with the methods discussed above.

Thus, the centrifuging process described above is performed twice. After light scattering impurity particles greater than or equal to 0.1 um have been removed from both materials, the two parts are mixed and applied to the optical components.

The adhesive precursor and curing agent in the two-part system may be of any suitable well-known type. The reactive precursor can have any mono-functional or multi-functional groups attached for chemical cross-linking purposes. These precursors discussed above with respect to first embodiment of the present invention can also be used in the two-part system of the second embodiment of the present invention.

Hardening agents such as amine or anhydride can be used.

As embodied herein and depicted in Figure 3, optical device 10 is a multi- component device including a first component 20 and a second component 40. These components are bonded together using the adhesive 30 which was purified using the process described above. As depicted in Figure 3, a mixture 30 of the purified adhesive and the hardening agent is applied to portions of the components in the optical path 50 of device 10 and cured. The two-part system must be applied to the optical components immediately after mixture because the hardening agent will react with the precursor soon thereafter. The mixture can be cured at room temperature or can be cured by applying heat. Note that the application of thermal energy to the adhesive will reduce the curing time. As discussed above with respect to the first embodiment, one of ordinary skill in the art will recognize that any optical components can be bonded together using the two-part adhesive system, without the adhesive deteriorating from high-power laser damage.

Because the same precursor material used in the first embodiment can be used in the second embodiment, a second example illustrating the purifying technique for a liquid precursor would be identical and thus, will not be repeated. As discussed above, equation (1) will also be utilized to determine the spin rate and spin time of the amine or anhydride-hardening agent. This step of course, is not needed in a one-part system.

It will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the spirit and

scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.

While the preferred embodiments of the above method and optical devicelO have been described in detail with reference to the attached drawing Figures, it is understood that various modifications, variations, and adaptations may be made in the method and optical device 10 by those skilled in the art without departing from the spirit and scope of the appended claims. Thus, it is intended that the present invention cover those modifications, variations, and adaptations provided they come within the scope of the appended claims and their equivalents.