Abstract:
Designs of 2-port optical devices are disclosed. The optical devices so designed are amenable to small footprint, enhanced impact performance, lower cost, and easier manufacturing process. The optical device comprises at least two collimators, each of the collimators including: a single-mode fiber, and a multi-mode graded index fiber, aligned with the single-mode fiber in a way that the multi-mode graded index fiber and the single-mode fiber are spliced together, wherein the multi-mode graded index fiber is designed to have a predefined length.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is related to the provisional application, No. 61/519,577, entitled “Mini-collimator for 2-port optical devices”, filed on May 26, 2011, which is hereby incorporated by reference for all purposes. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention is generally related to the area of optical communications. In particular, the invention is related to optical devices, modules or assemblies as pluggable devices processing light beams and the making thereof. The optical devices modules or assemblies include, but may not be limited to, 2-port pluggable fiber based WDM filters, pluggable optical fiber isolator, and miniature optical collimators. 
     2. The Background of Related Art 
     Optical fiber is an ideal waveguide than can transmit light between two ends thereof with very low loss to the light. However, a lot of optical functions available in free-space optics could be impractical or more expensive to be realized in the fiber media. For example, fiber collimators are widely used in fiber optic devices to couple light between fiber and free-space. As the characteristics of the light propagating in the fiber and in the free-space are usually very different, the interface between them are usually very lossy which makes a lot of applications impractical. With a collimator, one can achieve light coupling at low loss and a relatively long working distance in free-space to insert one or more free-space components in order to realize optical functions with complexity such as optical WDM, optical isolator, optical attenuator, and etc. 
     Currently, two types of collimators are widely used in telecom applications: C-lens collimators  100  and GRIN lens collimators  102  as demonstrated in  FIG. 1  ( a ) and  FIG. 1(   b ). Both approaches place a fiber facet at or around the focal point of a lens so that a light at the other side of the lens is collimated into paralleled beam to achieve a long working distance. Usually, a small angle (8°) is introduced at the interface between the fiber and the lens in order to manage the return loss to an acceptable level (usually 50-60 dB). As the C-lens and GRIN lens are both fabricated by bulk optical fabrication techniques, such as cutting, polishing and etc., it is difficult to achieve compact sizes (e.g., &lt;1 mm). As the optical devices nowadays go smaller and smaller, which causes more and more complexities within the same dimension restrictions, these types of optical collimators of  FIGS. 1  ( a ) and ( b ) limit the size of the optical devices. 
     Accordingly, there is a great need for such optical modules being made small, and at the same time, the collimators so designed are amenable to small footprint, broad operating wavelength range, enhanced impact performance, lower cost, and easier manufacturing process. Moreover, an emerging need for telecommunication devices calls for pluggable devices. Such needs have been witnessed by the active devices such as fiber optical transceivers and transponders that take a variety of form factors such as SFP, XFP, QSFP, CFP, etc. As the growth matures in active optical devices, passive optical devices such as optical WDM filters, multiplexers, attenuators, switches, isolators, will also need to match up to be pluggable in order to help more demand in planning and deployment flexibility. To facilitate such a pluggability need, fiber collimator dimension often is the limiting factor in order to shrink a fully functional passive device of interest to the necessary dimension. 
     SUMMARY OF THE INVENTION 
     This section is for the purpose of summarizing some aspects of the present invention and to briefly introduce some preferred embodiments. Simplifications or omissions in this section as well as in the abstract and the title may be made to avoid obscuring the purpose of this section, the abstract and the title. Such simplifications or omissions are not intended to limit the scope of the present invention. 
     In general, the present invention pertains to improved designs of optical devices, particularly for pluggable optical passive devices that need to use fiber collimators. As for key building blocks of such pluggable fiber optic passive devices, a compact fiber optic collimator is disclosed. According to one aspect of the present invention, two pieces of dissimilar fibers, a single mode fiber and a multi-mode graded index fiber, are aligned and spliced together. As the multi-mode graded index fiber (MM-GIF) works very similar to the GRIN lens, it can collimate a light beam with a properly designed multi-mode (MM) fiber length. In one embodiment, the length for optimal collimation is designed to be substantially close to (2n−1)*0.25*P, where n is an integer and P is the pitch of the MM-GIF which is determined by the fiber index distribution. 
     The optical collimators so designed in accordance with the present invention are amenable to small footprint, enhanced impact performance, lower cost, and easier manufacturing process. Various embodiments of the present invention may be used in many areas such as optical communications and devices and may be implemented in many ways as a subsystem, a device or a method. According to one embodiment, the present invention is an optical device having two ports, the optical device comprises at least two collimators, each of the collimators including: a single-mode fiber, and a multi-mode graded index fiber, aligned with the single-mode fiber in a way that the multi-mode graded index fiber and the single-mode fiber are spliced together, wherein the multi-mode graded index fiber is designed to have a predefined length. 
     In one embodiment, the predefined length of the multi-mode graded index fiber is defined to be substantially close to (2n−1)*0.25*P, where n is an integer and P is a pitch of one end of the multi-mode graded index fiber. In one embodiment, the length of such a proposed ultra-compact 2-port device may be as short as 20 mm including the ferrules. Compared to the equivalents with the prior art technology, it has great advantage in sizes in addition to similar optical performance, thermal stability and even lower cost compared to that for the prior art 2-port devices. Due to the size being compact, such a collimator can be inserted into standard ceramic ferrules with hollow structure in one embodiment. 
     When such a pair of mini-collimators are integrated in a 2-port device, a fiber optic pluggable device can be formed with different types of free-space components. An example of the free-space components includes, but may not be limited to, a thin-film filter (high-pass, low-pass, band-pass, etc.) and an isolator. Also, multiple components may be inserted in series between two such collimators for applications with higher complexity or to achieve better optical performance. Due to the fabrication tolerance of the mini-collimator, a light beam comes out from or into the collimator may have a small angle)(&lt;2° with respect to the axis thereof and it may be off-center (&lt;0.1 mm). In one embodiment, the holder may be designed to allow small angular or/and translational offset of the two collimators during an alignment process in order to compensate for the fabrication errors. After alignment, the two ferrules can be fixed to the holder with epoxy or other methods. After the assembly, the other side of the ferrules can be polished to PC, APC, UPC or other surface type, so that it is ready to mate with other ferrules. 
     Different types of free-space components may be used in the 2-port device. An example of the free-space components includes, but may not be limited to, a thin-film filter (high-pass, low-pass, band-pass, etc.), an isolator. Also, multiple components may be inserted in series between the collimators for applications with higher complexity or to achieve better optical performance. Due to the fabrication tolerance of the mini-collimator, a light beam comes out from or into the collimator may have a small angle)(&lt;2° with respect to the axis thereof and it may be off-center (&lt;0.1 mm). In one embodiment, the holder should be designed to allow small angular or/and translational offset of the two collimators during an alignment process in order to compensate for the fabrication errors. After alignment, the two ferrules will be fixed to the holder with epoxy or other methods. After the assembly, the other side of the ferrules can be polished to PC, APC, UPC or other surface type, so that it is ready to mate with other ferrules. 
     Many objects, features, and advantages of the present invention will become apparent upon examining the following detailed description of an embodiment thereof, taken in conjunction with the attached drawings 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where: 
         FIG. 1A  and  FIG. 1B  shows an exemplary C-lens collimator and an exemplary GRIN lens collimator, respectively; 
         FIG. 2  shows such a collimator according to one embodiment of the present invention; 
         FIG. 3  shows an exemplary collimator with an offset to the axis of the two fibers with straight cut at the end of the MM-GIF; 
         FIG. 4  shows a collimator may be manufactured by aligning two different fibers and bonding them together after alignment by epoxy or other method; 
         FIG. 5  shows a cross-section view of such a 2-port device employing two mini-collimators, each is preassembled with a ferrule; 
         FIG. 6  shows a cross-section view of another 2-port device employing two mini-collimators, each is preassembled with a ferrule; 
         FIG. 7  shows a cross-section view of an exemplary package design according to a predefined form factor; and 
         FIG. 8  shows a cross-section view of an exemplary package design according to another predefined form factor. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The detailed description of the present invention is presented largely in terms of procedures, steps, logic blocks, processing, or other symbolic representations that directly or indirectly resemble the operations of optical devices or systems that can be used in optical networks. These descriptions and representations are typically used by those skilled in the art to most effectively convey the substance of their work to others skilled in the art. 
     Reference herein to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. 
     According to one embodiment of the present invention, a novel optical collimator in compact size and good optical performance is disclosed.  FIG. 2  shows such a collimator  200  according to one embodiment of the present invention. As shown in the figure, two pieces of dissimilar fibers, a single mode fiber  202  and a multi-mode graded index fiber  204  are aligned and spliced together. As the multi-mode graded index fiber (MM-GIF) works very similar to the GRIN lens, it can collimate the light beam with a proper designed multi-mode (MM) fiber length. In one embodiment, the length for optimal collimation is designed to be substantially close to (2n−1)*0.25*P, where n is an integer and P is the pitch of the MM-GIF which is determined by the fiber index distribution. 
     The commonly used MM-GIFs are usually with a core size of 50 um or 62.5 um and an outer diameter being 125 um. Due to the small core size (i.e., the lens aperture), the working distances of such a collimator with these MM-GIFs are usually limited to sub-mm which are normally not practical to make optical devices to accommodate other free-space component(s). 
     According to one embodiment, an MM-GIF with large core and small numerical aperture (NA) is desired to increase the working distance. Calculation indicates that a working distance of 3.9 mm can be achieved using an MM-GIF with core diameter of 200 um and NA of 0.22. The working distance is sufficient for a 2-port device with a few components (such as thin-film filter) inserted between a pair of such mini-collimators. 
     According to one embodiment, to manufacture this kind of collimator is to splice two different fibers together. The diameters of the two fibers may be different, in which case specialty fiber splicer is needed. The advantages of this fabrication technique include: 1) reflection at the interface between the two different fibers are usually low as they are in physical contact and the refractive indexes of them are substantially close; 2) an MM-GIF can be cleaved into a designed length after being spliced, thus avoiding the difficulty of preparing and handling a fiber segment with very short length (usually &lt;5 mm); 3) good thermal reliability is expected as the two fibers are attached together. 
     To manage the return loss, the tip of the MM-GIF can be prepared to an angle by cleaving or polishing. Unlike a typical angle of 8° created to reduce the reflection in a prior art collimator, an angle of 1-2° or less with respect to the axis thereof is sufficient to sustain a high back-reflection or return loss performance. An alternative is to offset the axis of the two fibers with straight cut at the end of the MM-GIF as shown in  FIG. 3 . Our simulation shows an 8 um axial offset is sufficient to sustain a high back-reflection or return loss performance. 
     According to another embodiment, other than splicing, a collimator  400  as shown in  FIG. 4  may be manufactured by aligning two different fibers and bonding them together after alignment by epoxy or other method. As there is a gap between the two fibers, a high return loss is realized at the interface. In one embodiment, the index matching material of the two fibers or 8° cut angle on bother ends of the fibers are used, which is also illustrated in  FIG. 4 . 
       FIG. 5  shows a cross-section view of such a 2-port device  500  employing two mini-collimators  502  and  504 , each is preassembled with a ferrule  506  or  508 . If needed, free-space component(s) may be inserted between the two mini-collimators  502  and  504 , and can be attached to either one of the mini-collimators  502  and  504  or to the ferrule thereof or other structure. In one embodiment, the two ferrules  506  and  508  will be fixed on a common holder like the one  510  in  FIG. 5  after the two are aligned to achieve the optimal optical performance. 
       FIG. 6  shows a cross-section view of another 2-port device  600  employing two mini-collimators  602  and  604 , each is preassembled with a ferrule  606  or  608 . As shown in  FIG. 6 , two mini-collimators  602  and  604 , and free-space component(s) are attached together after alignment. Sometimes, a spacer is used to ensure good optical coupling between the mini-collimators  602  and  604  by separating them with an optimized working distance. After the mini-collimator pair is assembled, it is inserted to the ferrule pair and attached to the holder  610 . In this design, the hole diameter in the ferrules are slightly bigger in order to make room for the assembled mini-collimator pair due to tolerance and materials (such as epoxy) to attach the parts together. Similar to the design shown in  FIG. 5 , the ferrules can be polished and used to mate with other ferrules as well. 
     According to one embodiment, a 2-port device as disclosed in the present invention may be packaged into a device with a predefined form factor similar to a fiber adaptor with designed plastic housing(s). According to one embodiment,  FIG. 7  shows a cross-section view of an exemplary package design. Two pieces of plastic housing are designed to make the male-female type adaptors. It may also be packaged with different plastic or metal housings into other types (male-male, female-female) of adaptor form-factors depending on the applications.  FIG. 8  shows a cross-section view of an exemplary package design according to another form factor. 
     While the present invention has been described with reference to specific embodiments, the description is illustrative of the invention and is not to be construed as limiting the invention. Various modifications to the present invention can be made to the preferred embodiments by those skilled in the art without departing from the true spirit and scope of the invention as defined by the appended claim. Accordingly, the scope of the present invention is defined by the appended claims rather than the forgoing description of embodiments.