Abstract:
An optical package comprises an optical element (e.g., a filter), a reflective surface, an input optical fiber and an output optical fiber. A light signal travels through the input fiber and through the element where it is shaped or modified a first time. The shaped light signal is reflected by the reflective surface and is again transmitted through the element where it is shaped or modified a second time. The twice-shaped light signal then travels out through the output fiber. The package thereby utilizes the element two times. The package is useful in wavelength division multiplex (WDM) telecommunication systems and other light processing systems.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to optical systems and, in particular, to an optical packaging design for optical filters, isolators and the like.  
           [0003]    2. Technical Background  
           [0004]    There is considerable interest in the field of optics, particularly relating to the area of telecommunication systems. Optical fibers are the transmission medium of choice for handling the large volume of voice, video, and data signals that are communicated over both long distances and local networks. Much of the interest in this area has been spurred by the significant increase in communications traffic which is due, at least in part, to the Internet. Important components of fiber optic networks are the optical filters concatenated into the wavelength division multiplexing (WDM) modules, optical isolators, and similar devices which modify, shape, and block light signals. These devices may be subjected to various thermal and mechanical stresses/during assembly, production, environmental testing, and operations. It is critical to the operation of the network that these devices function reliably over their projected 20 to 25 year service life. Further, these devices represent a significant portion of the cost of a network. Therefore, it is desirable to reduce the cost of these important components.  
           [0005]    An example of a typical filter device is illustrated schematically in FIG. 1. The device functions as follows. A light signal  11   a  travels through optical fiber  12   a  which is positioned in a capillary of input glass ferrule  13   a . The signal  11   a  exits the fiber  12   a  and travels through input collimating lens  14   a  where the signal is collimated into parallel rays and directed to thin film filter  15   b  which is deposited on a glass substrate  15   a . Filter  15   b  modifies the light signal  11  as the signal travels through filter  15   b . The signal  11  then travels through the output collimating GRIN lens  14   b  where the signal is directed to the output fiber  12   b.    
           [0006]    The typical filter package is further illustrated in the cross-section view of FIG. 2. In addition to the components shown in FIG. 1, there is shown the insulating glass sleeves  21   a  and  21   b , metal sleeves  22   a  and  22   b , outer metal enclosure  23 , and solder or weld joints  24   a  and  24   b.    
           [0007]    While these packages can function well, there are two areas which must be continually improved upon. These are cost and reliability. These devices continue to be expensive due to the numerous parts required and the high cost of some components. As can be seen in the figures, the device has multiple identical components. For example, there are two ferrules  13   a  and  13   b , two collimating GRIN lenses  14   a  and  14   b , two insulating glass sleeves  21   a  and  21   b , two metal sleeves  22   a  and  22   b , and two solder or weld joints  24 . All of these components are not only costly, but they also result in time and labor costs to assemble these precision devices. Further, an increased number of components generally reduces yield while increasing the failure rate. Of particular concern are the solder or weld joints  24  which create a hermetic or near hermetic seal for the device. If either one of these joints  24  fail, it increases the chance of a device failure. Also, the most significant cost of the device is the filter element itself. A single filter may cost several hundred dollars. A device, system, or method to reduce the costs and improve reliability would be a significant advantage.  
           [0008]    Finally, any package design should be adequate not only to mechanically protect the fragile optical components but also to compensate for and minimize the thermally induced shift in spectral performance.  
           [0009]    The continuing goal, therefore, is to find ways to reduce costs and improve quality and reliability of optical filtering packages. It is also a goal to design a package that is simple in construction and miniaturized.  
         SUMMARY OF THE INVENTION  
         [0010]    To address the goals stated above, the inventive optical package increases reliability and reduces cost, labor, and size. The invention achieves these goals by significantly reducing the parts required to make an optical package. The invention eliminates the need for half of the collimating GRIN lenses, half of the glass ferrules, half of the insulating sleeves, half of the metal sleeves, and half of the solder or weld joints. In addition, the innovative design reduces the size of the package by approximately one half and also reduces the cost of filters used in the package for some applications.  
           [0011]    The invention achieves these reductions using a new design which includes an optical filter film and a reflective coating deposited on a substrate. A light signals enters the package through an input fiber. The light signal impinges on the filter where it is spectrally shaped or modified. A portion of the signal passes through the filter and is reflected by the reflector. The signal then passes back through the filter a second time where it is spectrally shaped or modified again. The shaped signal exits the package through the output fiber. Using this design, both the incoming light signal and the outgoing light signal travel through virtually the same components and thereby eliminate the need for the output collimating lens  14   b , glass ferrule  13   b  and surrounding insulating and encapsulating components of FIG. 2. When passed twice through the same thin film coating the spectral function of the output light signal becomes steeper. This allows the use of a cheaper filter. For example, it is possible to use a 4-cavity system instead of a 5-cavity system.  
           [0012]    The invention achieves the reduced component count by depositing a reflective coating onto the filter substrate and adding a capillary to the input glass ferrule. Both of these changes are low cost modifications.  
           [0013]    In addition, only one solder or weld joint is needed. The new outer metal sleeve has one closed end and one open end, therefore only the open end needs to be hermetically sealed with solder or weld. This is a significant advantage for increasing manufacturing yield and product reliability since the operation of soldering and welding is a high-risk operation. The high temperatures associated with soldering or welding induce thermal mismatch stress, or in the glass optical components and insulating and encapsulating units. These stressed cause the repositioning of optical components, lowering its optical and mechanical performance, and even resulting in possible damage to the delicate components. In addition, the solder, flux or weld material may contaminate optical components. The invention reduces the risk associated with these factors by reducing the number of solder or weld joints.  
           [0014]    Another advantage is reduced cost of the filter. Optical filters often comprise four to ten dozen layers of dielectric films deposited onto a substrate. The cost of the filter increases non-linearly with the number of layers. Put another way, a filter requiring 100 layers costs more than twice as much as a filter requiring only 50 layers. Using the invention, a 50 layer filter can perform the function of a 100 layer filter since the light signal passes through the filter twice. This significantly reduces filter cost. Moreover, the low-cavity filters having limited applications in the transmitted (prior-art) devices, can be used to replace a better performing and more expensive higher-cavity systems. In another aspect, the reflector is a diffraction grating type that splits the incoming light signal into several reflected signals of different wavelengths.  
           [0015]    It is clear that the invention is a significant improvement over the prior art. Further, those skilled in the art recognized that the invention is not limited to use with optical filters. Other optical devices, such as isolators, may also be used in the invention.  
           [0016]    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 in the detailed description which follows, the claims, as well as the appended drawings.  
           [0017]    It is to be understood that the foregoing description is exemplary of the invention only and is intended to provide an overview for the understanding of the nature and character of the invention as it is defined by the claims. The accompanying drawings are included to provide a further understanding of the invention and are incorporated and constitute part of this specification. The drawings illustrate various features and embodiments of the invention which, together with their description serve to explain the principals and operation of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    [0018]FIG. 1 is a schematic diagram of a prior art filter package;  
         [0019]    [0019]FIG. 2 is a cross-section view of a prior art filter package;  
         [0020]    [0020]FIG. 3 is a schematic diagram of the preferred embodiment of the invention;  
         [0021]    [0021]FIG. 4 is a cross-section view of the preferred embodiment of the invention;  
         [0022]    [0022]FIGS. 5A through 5E illustrate five configurations of filters and reflectors according to the invention;  
         [0023]    [0023]FIG. 6 is a cross-section view of a package embodied with a metal enclosure; and  
         [0024]    [0024]FIG. 7 is a cross-section view of a package embodied with a glass enclosure. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0025]    Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numerals will be used throughout the drawings to refer to the same or like parts.  
         [0026]    An exemplary embodiment of the package of the present invention is shown in FIG. 3, and is designated generally throughout by reference numeral  30 .  
         [0027]    The invention will first be described referring to a schematic diagram and then referring to a cross-section view of the preferred embodiment. The most common use for the invention is as a component in a telecommunications network and therefore the following discussion will describe the invention in relation to a telecommunications application.  
         [0028]    Referring first to FIG. 3 there is shown a schematic diagram of the invention. A light signal  11   a  enters the package  30  through input optical fiber  12   a . Light signal  11  is preferably a conventional optical signal having wavelengths in, for example the C-band or possibly in the S-band or L-band. Optical fibers  12  are preferably conventional single mode optical fibers commonly used in telecommunications applications. Fibers  12  are stripped of their polymer coating, positioned inside the capillaries of glass ferrule  31  and epoxy bonded. The end face of the formed fiber-ferrule is polished to 8° and coated with an anti-reflection (AR) coating.  
         [0029]    Light signal  11   a  exits input fiber  12   a  and enters collimating lens  14  which directs the light beams to optical filter  15   b . Lens  14  is preferably a graded index (GRIN) collimating lens. Light signal  11   a  is spectrally modified by filter  15   a . Filter  15   b  is preferably a gain-flattening filter (GFF) similar to the type of filter commonly used in combination with an optical amplifier. However, the invention is also useful with other types of filters and optical elements. The spectrally modified light signal next passes through the transparent substrate  15   a  which is preferably glass. The filter  15   b  is deposited on the first surface of substrate  15   a  in a conventional manner.  
         [0030]    A reflective coating is deposited on the second surface of substrate  15   a  and forms reflector  32 . Reflector  32  is comprised of a metal coating, refractive film or reflection grating suitable for reflecting the wavelengths of interest. Note that glass substrate is wedged (approximately 1.5-2.5°) to provide the coupling of the reflected wavelength of choice to output fibers and to discriminate the other wavelength.  
         [0031]    The modified light signal is reflected by reflector  32  and is transmitted back through substrate  15   a  and through filter  15   b  where it is again spectrally modified. The twice modified light signal  11   b  is then transmitted back through GRIN lens  31  to output optical fiber  12   b  which guides the light signal  11   b  to the communications network.  
         [0032]    A benefit of the invention is that the light signal  11  is transmitted through filter  15   b  two times. Therefore, a filter comprising fewer number of dielectric films may perform the desired higher order gain flattening operation. Consequently, the filter  15   b  should be less expensive. The preferred embodiment will now be discussed in relation to a cross-section view.  
         [0033]    [0033]FIG. 4 is a cross-section view of the preferred embodiment. In addition to the elements described in FIG. 3, there is shown the insulating glass sleeve  21 , metal sleeve  22 , and filter holder  41 . The package is assembled as follows.  
         [0034]    The dual-capillary glass ferrule  42  and collimating GRIN lens  14  are embedded into insulating glass tube  21 , which is protected by a metal, glass, or ceramic sleeve  22 . The optical path consists of two (input and output) optical glass fibers  12   a  and  12   b  inserted and bonded into the ferrule  42  to produce a fiber-ferrule sub-assembly, a collimating (GRIN or aspheric) lens  14 , and a spectral shaping GFF  15   b  on the first surface of substrate  15   a . Lens  14  and fiber-ferrule  42  are sequentially positioned and have matching-angle polished 8 degree facets. The lens  14  collimates the light emitted from the input optical fiber  11   a  into parallel rays, which impinge upon the filter  15   b . The filter  15   b  splits the collimated light into two beams. One beam is spectrally modified (shaped) in the filter refractive films then reflected from the reflective coating  32  back to the filter refractive films, where they are again spectrally shaped and finally coupled through the lens  14  into the output optical fiber  11   b . The second light beam split by the filter  15   b  is reflected by filter  15   b . The second beam is generally of no interest and is absorbed by the components of the package or transmitted through it in the case of the glass enclosure. However, it is possible to direct this second beam to a second output fiber if there is a desire to conserve the signal. The optical components are assembled and aligned, so the reflected light beam or signal is collimated and insertion loss (IL) is minimized.  
         [0035]    Structural design and bonding are important factors affecting optical performance of fiber ferrules. Therefore, the preferred ferrule  42  comprises a pair of capillaries that allow stripped fibers (i.e. core and cladding only) to be inserted and positioned symmetrically and in parallel inside the ferrule  42 . The ferrule  42  is positioned on an axis with the lens  14  such that lengths of the input and output optical paths are nearly equal.  
         [0036]    The inserted fibers  11  are epoxy bonded inside the capillaries using heat-curable adhesive. The adhesive preferably has high Young&#39;s modulus (E&gt;100,000 psi), moisture-resistance, bond thickness about 1-2 μm, and moderate-to-high thermal expansion coefficient (α=40-60 10 −6  per degrees Celsius). A suitable product is 353ND EPO-TEK epoxy adhesive available from Epoxy Technology, Billerica, Mass.  
         [0037]    Once prepared, the fiber-ferrule  42  is aligned and assembled with the lens  14 . The end-face surfaces of lens  14  and fiber-ferrule  42  are coated with an anti-reflection (AR) film. Both the ferrule  42  and lens  14  are axially aligned and bonded to the interior of insulating glass sleeve  21 , which is in turn bonded inside metal sleeve  22 .  
         [0038]    The GFF filter  15   b  comprises a plurality of dielectric films with a high refractive index and, adjacent to the substrate, a reflective metal or other dielectric coating or a reflection grating, to achieve cascaded filtering of the reflected light signal and to discriminate (if necessary) some wavelengths.  
         [0039]    The filter  15   b , along with the wedged (approximately 1.5-2.5°) substrate  15   a , is bonded to filter holder  41 . Filter holder  41  may be either metal (shown in FIG. 4) or glass (shown in FIG. 7). In the case of a tubular glass filter holder, the holder glass should be UV- and IR-transparent and thermally matching the lens glass. The filter  15   b  is bonded to the end-face of holder  41 . In other words, its coefficient of thermal/expansion should be about 9×10 −6  or 10×10 −6  . Both UV and heat-curable epoxy adhesives may be used in this bond. The bond thickness is preferably maintained and limited to 4-6 μm. A UV/heat-curable, low-shrinkable and high-modulus adhesive with a coefficient of thermal expansion close to the lens glass and glass holder is preferred to bond the filter holder  41  with the lens  14 . To cure the adhesive inside a UV transparent glass holder, the UV light is guided and transmitted to the bond lines through the lateral surface of the holder  72  (in FIG. 7). To cure the adhesive inside a metal glass holder, the UV light is transmitted through slots or apertures formed in the side of the holder to allow UV light to enter. The duration of the UV exposure is inversely proportional to the minimal transmissivity of the glass holder on the G, H, and I bands of the UV spectrum of a mercury lamp.  
         [0040]    Highly expandable glasses (e.g. WG 320 or typical GRIN lens glass), moderately expandable alloys (e.g. 17-4 PH stainless steel), and a low-expandable adhesive filled with highly concentrated particles (e.g. EMI 3410 epoxy adhesive containing UV and heat-sensitive curing initiators available from Electronic Materials, Inc., of Breckenridge, Colo.) form an appropriate combination of materials for the lens  14 , holder  41  and adhesives.  
         [0041]    The adhesive for securing both the filter  15   b  to the filter holder  41  and for securing the holder  41  to the GRIN lens  14  should be thermally matched. A low-shrinkable and high-modulus adhesive, such as EMI 3410, with a coefficient of thermal expansion matching the adherent glass substrate  15   a  and metal holder  41 , is used to minimize the mismatched stresses in these bonds. The glass filter  15   b , including the glass substrate  15   a , is bonded to the metal holder  41 , which includes an aperture  41   a  through the center for the passage of light. The filter  15   b  (including substrate  15   a ) is positioned into the holder  41  and the filter  15   b  is bonded to the holder  41 . The filter holder  41  includes a lens aperture that telescopically overlaps the cylindrical GRIN lens  14  leaving sufficient space to allow for micro-tipping (approximately 2.5°) of the holder  41  relative to the lens  14  if active alignment is required. The holder  41  therefore has two opposite flat surfaces. The first one is perpendicular to the axis of the lens  14  and interfaces with the frontal face of the lens  14 . The opposite surface, which interfaces and bonds with the filter  15   b , is machined with a suggested tilt not exceeding about 2 degrees from the perpendicular to the axis of the GRIN lens  14 . This allows reducing the total tilt of the holder  41  to achieve optical alignment. Another purpose for the tilting of the filter  15   b  is to achieve the desired filtering characteristic according to the filter&#39;s desired angle of incidence (AOI).  
         [0042]    A simpler, but less accurate, method of mounting the filter  15   b  is to eliminate the filter holder  41  and bond the filter  15   b  directly to the end face of the lens  14  with a thin layer of optically transparent adhesive.  
         [0043]    Active alignment is preferred to minimize insertion loss. An alignment station allows for rotation of the filter holder  41  around the GRIN lens  14  and for tipping and tilting (tip-tilt) the filter  15   b  in two reciprocally perpendicular planes to the axis of the lens  14 . When a desired alignment is achieved, the filter holder  41  is adhesively bonded to the lens  14  to retain the alignment.  
         [0044]    Turning to FIGS. 5A through 5E there are illustrated various embodiments for configuring the filter  15   b , the substrate  15   a , and the reflector  32 . FIG. 5A shows the preferred embodiment having the filter  15   b  deposited on the first surface  15   c  of substrate  15   a . The reflective coating  32  is deposited on the second surface  15   d  of substrate  15   a . The light signal  11  passes through filter  15   b  where it is spectrally modified or shaped a first time. The signal  11  is reflected by reflector  32  and then again passes through filter  15   b  where it is spectrally modified or shaped a second time. An advantage of this configuration is the relative ease of depositing filter  15   b  and reflector  32  on the surfaces of the substrate  15   a.    
         [0045]    A second configuration is shown in FIG. 5B where both the reflector  32  and the filter  15   b  are both deposited on the first surface of the substrate. The reflector  32  is deposited on the substrate and next the layers of the filter  15   b  are deposited on top of the reflector  32 . This configuration may be more difficult to implement due to the increased number of layers deposited on one surface and also has difficulties associated with the coupling.  
         [0046]    The third configuration uses two substrates and two filters and is illustrated in FIG. 5C. A first filter  15   b  is deposited on the first surface of the first substrate  15   a . A second filter  51  is deposited on the second surface of substrate  15   a . This may be useful if there are too many layers of dielectric material to be easily deposed on a single surface or if some layers do not adhere well to other layers. The reflector  32  is deposited on the first surface of the second substrate  52  and then bonded to the second filter  51  with a thin layer of optically transparent adhesive. Reflector  32  is preferably comprises gold or a gold alloy applied to a thickness of about 150 nm. However, those skilled in the art understand that other suitable reflective materials may also be used. This configuration also allows use of two differently performing commercial thin film filters.  
         [0047]    The fourth configuration deposits all filters and reflectors on the first surface of the first substrate  15   a  and is shown in FIG. 5D. The reflector  32  is first deposited onto the substrate  15   a  followed by the first filter  15   b  and finally the second filter  51  is deposited on top of the first filter  15   b . This configuration may be preferred in some applications such as when the filters and reflector require a relatively small number of layers of films and coatings, but provide a small separation between the input and reflected beams.  
         [0048]    The configuration shown in FIG. 5E includes the reflection grating film  53  applied to the frontal surface of the second substrate  52 . The gratings selectively reflect and split different wavelengths that can be coupled into the output optical fibers  12   b . FIG. 5E illustrates this by showing an input signal of wavelengths λ 1 -λ 3  that is split into three signals of wavelengths λ 1 , λ 2 , and λ 3 . The three output signals are then coupled to three separate output optical fibers such as output optical fiber  12   b . In this case, a multi-capillary ferrule can be used to separate and couple all reflected wavelengths. This ferrule should have separations between capillaries that provide thermally independent operations. First substrate  15   a  and second substrate  52  are illustrated by a gap for illustrative purposes only. Preferably first substrate  15   a  and grating  53  are bonded together.  
         [0049]    As mentioned above in the summary, an advantage of the invention is that the filters modify or shape the light signal two times as opposed to only one time in the prior art. This allows either improved shaping or the use of less powerful and therefore less expensive filters to achieve the same results.  
         [0050]    Referring now to FIGS. 6 and 7 there are two basic packaging techniques for encapsulation of the assembly shown in FIG. 4. The first technique is shown in FIG. 6. An outer metal enclosure  61  houses the assembly from FIG. 4. A low-temperature solder  62  is used to encapsulate the metal sleeve  22  to the interior of the metal enclosure  61 . The assembled and soldered ferrule  42 , GRIN lens  14 , insulating glass tube  21 , and metal enclosure  61  experience residual thermal stresses due to the contraction mismatch of the materials used. In order to minimize and maintain these stresses, a high compliance bond is suggested and an RTV silicone adhesive, such as DC 577 or CV 32000, may be used. As shown in FIG. 6, the length of the solder pool is limited to 50% of the length of the metal sleeve  22 . This prevents chemical (through flux) and thermal conduction contamination of the filter and minimizes repositioning of the GRIN lens  14  and filter  15   b  due to thermal stresses. Since only one end of the package is soldered, this near hermetic package may have twice the reliability of prior art packages.  
         [0051]    The assembly of FIG. 4 may also be bonded inside a tubular UV-transmissible glass enclosure having a fused or closed end as shown in FIG. 7. In this embodiment the thermally matching glass enclosure  71  is bonded with adhesive  72  to the insulating glass sleeve  21  and the metal sleeve  22  previously shown in FIG. 4 is not needed. An adhesive such as EMI 3410 is a suitable choice. The ferrule  42  and lens  14  assembly is inserted into the glass enclosure and UV “tacked” to hold the positions. A final heat cure secures the assembly inside of the glass enclosure  71 .  
         [0052]    The glass filter holder  73  is preferably formed form a glass tube or rod and has a closed end for bonding to the filter  15   b  and substrate  15   a . Glass holder  73  also has a lens aperture  73   a  of sufficient dimensions to fit over lens  14  and allow for micro-tilting of the holder  73 . Filter  15   b  is bonded to holder  73  with a thin layer of optically transparent adhesive  74 . With this solution, the filter holder  73  can be made from a glass that thermally matches the glass of the GRIN lens  14 . This solution provides thermal compatibility of all component and enclosure units and substantially reduces the mismatch stresses. The UV and IR transparent tubular units also allow the unused wavelength to pass out of package and, therefore, improve its performance, particularly in the case of the reflected gratings.  
         [0053]    In addition to the previously mentioned advantages, the enclosure materials used in the invention are inexpensive, the thermo-mechanical behavior of the materials is well understood and can be predictable. Finally, the package does not require higher precision machining than the prior art.  
         [0054]    It will become apparent to those skilled in the art that various modifications to the preferred embodiment of the invention as described herein can be made without departing from the spirit or scope of the invention as defined by the appended claims.