Abstract:
A coating and a method for protecting a flexible region of an optical device that generates different output optical signals based upon whether the region is flexed. The coating is applied to at least a portion of the region. The coating has a relaxation time that does not substantially affect the different output optical signals transmitted through the region while the region is being flexed and then unflexed.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is generally related to optical component manufacturing and more particularly to protective coatings used in manufacturing optical components. 
     2. Technical Background 
     Variable optical attenuators (VOAs), 1×2 switches, and 2×2 switches are non-limiting examples of photonic devices which use a multiclad coupler. In these applications, the coupler requires a protective coating at the taper region to protect the coupler from breakage during the normal handling associated with assembly of the devices as well as during the functioning of the device. In the devices listed above, the coupler is flexed to attenuate the light signal propagating in the device. Any properties of the coating that degrade the optical signal are undesirable. Thus, the application of the coating must not negatively impact the attenuation response of the coupler after it is incorporated into the device. 
     FIG. 1 depicts an exemplary variable optical attenuator (VOA)  20  which uses a multiclad coupler  22  and a servomotor  26 . Coupler  22  includes an input fiber  28  and two output fibers  30  and  32 . First output fiber  30  is the output of VOA  20  and second output fiber  32  acts as a “dead-end” lead. An optical signal passes from input fiber  28  to either first output  30  or second output  32  through taper region  24  which couples the light signal from fiber  30  to fiber  32 . Flexing coupler  22  at taper region  24  by different amounts via servomotor  26  causes more or less of the light signal to be transmitted to the dead-end fiber  32 . The amount of flexing controls the attenuation of the signal. Thus, tapered region  24  functions as a commutator. 
     FIG. 2 depicts an optical step response  10  that was generated by an optical switch having a coupler without a coating in the tapered region. As depicted in FIG. 1, the tapered region is moved between a first unflexed position to a second flexed position, at the time of switching, T sw . The first position corresponds to a signal transmission state  12  wherein the insertion loss is approximately zero. The second state corresponds to a signal attenuation state  14  wherein the insertion loss is approximately 19.3 dB. Note that the plot of the insertion loss as depicted in FIG. 1 a  is a square-wave. The insertion loss in both the first state and the second state is substantially constant. This is a desired response. Unfortunately, the coupler represented by FIG. 2 does not have a coating. It is unprotected and susceptible to breakage. 
     In one approach that has been taken, couplers have been coated with a cationic ultraviolet (UV) curable epoxy system. FIG. 3 depicts the insertion loss response  10  of the switch of FIG. 2 having a coupler that is coated with the cationic UV epoxy. Again, the tapered region is moved between a first unflexed position to a second flexed position, at the time of switching, T sw . IL swc  is the peak insertion loss of the coated coupler at the time of switching (T sw ). IL swc  overshoots the insertion loss IL swu  of the uncoated coupler in the attenuation state. IL swu  is used a reference insertion loss value. Peak insertion loss IL swc  is followed by hysteresis  12 , which is the decay of the peak insertion loss IL swc  to IL swu . IL Δsw =¦IL swc −IL swu ¦ and represents the absolute value of the difference between the peak insertion loss of the coated coupler at the time of switching and the insertion loss of the uncoated coupler in the second state. As shown in FIG. 3, IL Δsw =23 dB−19.3 dB=3.7 dB. This formula is used to accommodate a coating material that generates a peak insertion loss IL swc  that undershoots IL swu . 
     It is useful to measure hysteresis  12  in terms of its decay time T D . The decay time T D  is a measure of the time it takes for peak insertion loss IL swc  to decay to IL swu . As depicted in FIG. 3, the cationic ultraviolet (UV) curable epoxy system produces transients that have a decay time T D  lasting approximately 14 seconds. As depicted, the decay of the transient hysteresis continues for several minutes. In more rigorous terms, T D  is defined as T D =T 1 −T sw , wherein T sw  is the time at which the coupler is switched from the first state to the second state, and T 1  is the time at which peak insertion loss IL swc  decays to IL D . IL D =(0.27)IL Δsw =(0.27)¦IL swc −IL swu ¦, which represents an exponential decay over time. 
     When the device is commutated from the second position to an unflexed first position at time T usw , a second hysteresis  16  is generated. The analysis discussed above with respect to hysteresis  12  can be used to analyze hysteresis  16 . As depicted, its decay time will also last several minutes. Both hysteresis  12  and hysteresis  16  are undesirable and illustrate the unwanted transients produced by the coating immediately after switch commutation. Another drawback to the cationic ultraviolet (UV) curable coating is that it is colorless. It is difficult to determine that the coating has been applied. 
     What is needed is a protective coating that does not generate the unwanted optical transients and hysteresis produced by earlier approaches. An optical device is needed that settles into a quiescent state immediately after commutation. Furthermore, the protective coating should include a tinted material. Since clarity is important, the tinted material should allow internal areas in the coupler to be viewed through the coating. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the aforementioned disadvantages as well as others. In accordance with the teachings of the present invention, the coating protects optical devices without generating unwanted optical side effects during flexing. The coating adheres readily to the glass of the waveguide component. The coating has a tint so that it can be readily ascertained that the coating has been applied, but also has sufficient clarity so that the internal areas in the component may be viewed. In one embodiment a UV coating cures to a tack free state in air so that a nitrogen blanket is not required during the cure. A solvent based coating such as a lacquer can also be used. The coating also does not degrade when exposed to relatively severe environmental conditions. 
     One aspect of the present invention is an optical device for directing a light signal. The optical device includes a commutation region movable between a first position corresponding to a signal transmission state, and a second position corresponding to a signal attenuation state. A protective coating is disposed on the commutation region that does not substantially introduce insertion loss transients when the commutation region is moved between the first position and the second position. 
     In another aspect, the present invention includes a method of directing a light signal in an optical device having a first output, and a commutation region movable between a first position corresponding to a signal transmission state, and a second position corresponding to a signal attenuation state. The method includes the steps of applying a protective coating onto the commutation region. Directing a light signal into the optical device. Moving the commutation region from the first position to the second position to thereby attenuate the light signal in the first output, whereby the protective coating does not substantially produce insertion loss transients in the optical device. 
     In yet another aspect, the present invention includes a method of fabricating an optical device, the optical device having a commutation region movable between a first position corresponding to a signal transmission state, and a second position corresponding to a signal attenuation state. The method including the steps of providing a coating material. Applying the coating material to the commutation region, wherein the coating material does not substantially produce insertion loss transients when the commutation region is moved between the first position and the second position. 
     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 
     FIG. 1 is a perspective view of a variable optical attenuator having a tapered region, in accordance with the present invention; 
     FIGS. 2 is a plot of insertion loss with respect to time, illustrating the optical response of an optical device without a coating; 
     FIGS. 3 is a plot of insertion loss with respect to time, illustrating the optical response of an optical device having a cationic ultraviolet (UV) curable epoxy coating; 
     FIG. 4 is a detail view of a multiclad coupler having a coating in accordance with the present invention; 
     FIG. 5 a flowchart depicting the steps associated with applying the coating of the present invention to an exemplary optical component; and 
     FIGS. 6 is a plot of insertion loss with respect to time, illustrating the optical response of an optical device having a coating in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the present preferred embodiments of the invention, an example of which is illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. An exemplary embodiment of the optical device of the present invention is shown in FIG. 1, and is designated generally throughout by reference numeral  20 . 
     In accordance with the invention, the present invention for a protective coating for optical components includes a commutation region that is flexed between a first position and a second position. The first position corresponds to a signal transmission state having negligible insertion loss. The second position corresponds to a signal attenuation state. One of ordinary skill in the art will recognize that the device can be flexed over a range of positions depending on the required degree of attenuation. A protective coating is disposed on the commutation region that does not substantially alter an insertion loss characteristic of the optical device when the commutation region is moved between the first position and the second position. In other words, any transients or hysteresis generated by the coating during commutation have a duration of less than one second. 
     As embodied herein and depicted in FIG. 1, a perspective view of a variable optical attenuator  20  is shown. In accordance with the present invention, coupler  22  has a protective coating disposed on tapered region  24 . The protective coating of the present invention protects coupler  22  from breakage due to handling during device assembly, and during flexure when the device is commutated. As discussed above, the protective coating does not generate the unwanted optical transients and hysteresis of earlier approaches. One of ordinary skill in the art will recognize that the protective coating compositions of the present invention is not limited to the VOA depicted in FIG. 1, but rather includes all types of photonic components that may undergo flexing. For example, a 2×2 switch commutated by a flexing motion can use the protective coating compositions of the present invention. 
     FIG. 4 is a detail view of the type of multiclad coupler  22  that is used in the optical device depicted in FIG.  1 . As embodied herein and depicted in FIG. 4, coupler  22  has a flexible acrylate coating  40  that is disposed on tapered region  24 . Flexible acrylate coating  40  has a thickness  42 . In this embodiment, coating thickness  42  is approximately 25 microns and coupler thickness  44  is approximately 65 millimeters. 
     EXAMPLE 
     The invention will be further clarified by the following example which is intended to be exemplary of the invention. The protective coating of the present invention is a low viscosity, low modulus, flexible acrylate coating that is cured in air to a tack free state. The coating is based on an acrylate oligomer and monomers that are available from such suppliers as Sartomer Corporation. The oligomer is CN 966 180 which is an aliphatic urethane acrylate oligomer blended in an 80:20 ratio with propoxylated neopentyl glycol diacrylate monomer in order to reduce the viscosity of the oligomer. This oligomer is a highly flexible material which provides the low modulus desired in the switch application while the flexibility enhances the adhesion of the coating to the coupler. The coating composition of the present invention as depicted in FIG. 4 is shown in Table 1: 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Material 
                 Level (phr) 
                 Exemplary Supplier 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 CN 966 I80 
                 30 
                 Sartomer Corporation 
               
               
                   
                 SR-501 
                 30 
                 Sartomer Corporation 
               
               
                   
                 SR-9003 
                 40 
                 Sartomer Corporation 
               
               
                   
                 Irgacure 1850 
                 3 
                 Ciba Corporation 
               
               
                   
                 Irganox 1035 
                 2 
                 Ciba Corporation 
               
               
                   
                 Resiflow LG-99 
                 2 
                 Estron Chemical 
               
               
                   
                 A-174 Silane 
                 4 
                 OSi Corporation 
               
               
                   
                 Triphenylphosphine 
                 5 
                 Aldrich Chemical 
               
               
                   
                 PCB-1 
                 3 
                 Corning ICA Lab 
               
               
                   
                   
               
               
                   
                 (Note: “phr” indicates parts per hundred resin).  
               
             
          
         
       
     
     SR-9003 is propoxylated neopentyl glycol diacrylate. This difunctional monomer serves as a reactive diluent. SR-501 is propoxylated trimethylolpropane triacrylate and provides a fast cure response. These monomers&#39; properties include water resistance, abrasion resistance and good adhesion. 
     The remainder of the formulation includes various additives with different functions. Irgacure 1850 from Ciba is a photoinitiator. It is a 50:50 blend of bis (2,6-dimethoxybenzoyl)-2,4,4-trimethyl pentyl phosphine oxide and 1-hydroxy-cyclohexyl-phenyl-ketone. Its function is to absorb the UV light and initiate the polymerization reaction by generating free radicals. Irganox 1035 is also supplied by Ciba and is an antioxidant used to protect the coating against thermal yellowing and degradation. Resiflow LG-99 is an acrylate functional flow and wetting agent from Estron Chemical. It provides good wetting to the glass as well as aids in the leveling of the coating after application which helps to ensure a smooth even layer. 
     The A-174 is a methacrylate functional silane coupling agent from OSi Corporation. It provides increased adhesion to the glass and is particularly useful for maintaining adhesion after high humidity exposure. Triphenylphosphine, from Aldrich, has been added to overcome the oxygen inhibition at the surface of the coating when cured in air. This additive allows the coating to be cured to a tack free state in air without having to resort to the use of a nitrogen blanket to provide an inert atmosphere, thus making the use of this coating in a production environment easier. 
     The PCB-1 is a Corning developed pigment dispersion. The function of the dispersion is to provide enough color so that the coating is visible after application. The use of a very small particle size transparent (&lt;0.5 micron) pigment helps to ensure that the coating retains the clarity needed for viewing the air lines (Note: some pigments have particle sizes this small but are opaque pigments, for example, TiO 2 ). The composition for this dispersion is provided in Table 2: 
     
       
         
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Material 
                 Level (phr) 
                 Exemplary Supplier 
               
               
                   
               
             
             
               
                 Hostaperm Blue B2G 
                 20 
                 Clariant 
               
               
                 SR-9003 
                 80 
                 Sartomer Corporation 
               
               
                 Disperbyk 164 (60% solids) 
                 15 
                 BYK Chemie 
               
               
                   
               
             
          
         
       
     
     The pigment is a copper pthalocyanine blue pigment (CI 15:3) from Clariant. Disperbyk 164 is a proprietary polymeric dispersing agent supplied by BYK Chemie at a 60% solids loading in butyl acetate. The function of the dispersing agent is to provide steric stabilization of the pigment particles once they are dispersed into the monomer. Finally, the monomer used in the dispersion is the SR-9003. It was chosen for its relatively low surface tension which results in good pigment wetting. One of ordinary skill in the art will recognize that the present invention is not limited to the above described pigment dispersion, but can also include the use of any commercially supplied pigment dispersion or dye that achieves the tint requirement. For example, a dispersion source from US Colors and Coatings can also be used. 
     The modulus of elasticity of coating  40  is approximately 3.1×10 8  Pa (modulus at 25° C.), and the viscosity range of coating  40  is 200 to 600 centipoises (cps). One of ordinary skill in the art will recognize that alternate embodiments of the present invention include modulus of elasticity values of 4.0×10 8  Pa (modulus at 25° C.) and higher. Furthermore, one of ordinary skill in the art will recognize that the present invention is not limited to these values, but rather such values vary depending on application and other physical properties such as creep, and stress relaxation. 
     Coating  40  performed well under environmental testing and did not exhibit degradations such as delamination from the glass, flaking, adhesion problems, or peeling. Overall, coating  40  of the present invention is a low viscosity, low modulus, flexible acrylate coating that cures rapidly to a tack free state in air. 
     As embodied herein and depicted in FIG. 5, a process for applying coating  40  to coupler  22  is shown. The coating application process starts at step  50  wherein a brush is used to apply coating to the taper region of a coupler. The coating is cured at step  52 . In the preferred embodiment, the coating is cured for approximately 90 seconds with a UV light source, such as with a Lessco Superspot UV light. The coating is placed under a Bondwand for preferably approximately 30 minutes at step  54 . Finally, the coating is exposed to 125° C. for 4 hours during thermal postcure of the funnel adhesive at step  56 . One of ordinary skill in the art will recognize that the present invention is not limited only the above-described parameters but includes operating the process with parameters sufficient to achieve the intended effect. For example, in alternate embodiments, the protective coating of the present invention are a solvent based coatings or lacquers. 
     As discussed above, FIG. 2 depicts the optical step response  10  of an optical device  20  that uses a coupler without a coating in the tapered region. As embodied herein and depicted in FIG. 6, a plot of insertion loss with respect to time, illustrating the response  82  of an optical device having coating  40  in accordance with the present invention is disclosed. FIG. 6 illustrates the advantages of the present invention, and in particular the substantial reduction of the hysteresis effect by the coating of the present invention. 
     Using the terminology developed above, FIG. 6 shows a peak insertion loss IL swc  of the coated coupler at time T sw  of approximately 17.03 dB. As shown, IL swc  overshoots the insertion loss IL swu  of the uncoated coupler (16.93 dB) by 0.1 dB. Thus, IL Δsw =¦IL swc −IL swu ¦=0.1 dB. In terms of decay time T D , the optical device employing coating  40  of the present invention exhibited a decay time T D  of approximately 0.9 seconds. 
     One of ordinary skill in the art will understand that there are numerous applications and implementations for the present invention. For example, the coating formulation of the present invention includes any type of UV curable formulation cured by either a free radical or cationic polymerization mechanism. For example, the coating formulation of the present invention includes such materials that cure cationically as epoxies and vinyl ethers. Other coating formulations include free radical cationic hybrids so that the advantages inherent in both chemistries can be realized. Also, the present invention includes using UV cure/thermal hybrids. The UV cure portion provides for a quicker process cure while the thermal portion cures during the 4 hour 125° C. thermal postbake (i.e., during step  56  of FIG.  5 ). One of ordinary skill in the art will also recognize that lacquer or solvent based coatings including thermoset or thermoplastic polymer formulations are within the scope of the present invention. 
     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.