Abstract:
Optical alignment apparatus includes a first element mounting a first lens and a light source and a second element mounting a second lens and a light receiving structure. The first lens is placed a first distance from the light source and is constructed to collimate light received from the light source. The first and second elements are mounted relative to each other to position the second lens a third distance from the first lens and to receive the collimated light from the first lens. The second lens is positioned a second distance from the light receiving structure to focus the collimated light on the light receiving structure. The first and second lens are constructed so that the first and second distances are dependent upon each other and the third distance is independent of the first and second distances.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Patent Application Serial No. 60/275,000 filed Mar. 12, 2001 entitled OPTICAL/ELECTRICAL MODULE. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to optical-to-electrical and electrical-to-optical modules. 
     More particularly, the present invention relates to optical alignment features in such modules. 
     BACKGROUND OF THE INVENTION 
     In optical-to-electrical and electrical-to-optical (hereinafter “optoelectric”) modules used in the various communications fields, one of the most difficult problems that must be solved is the efficient transmission of light between a light. generating device and an optical fiber or, alternatively, the transmission of light from the optical fiber to a light receiving device. Here it will be understood by those skilled in the art that the term “light” is a generic term which includes any electromagnetic radiation that can be modulated and transmitted by optical fibers or other optical transmission lines. Because optical fibers and the active regions of light generating devices and light receiving devices are very small, alignment of an optical fiber with a light generating device or a light receiving device is difficult and can be very work intensive and time consuming. 
     For example, one method used to align an optical fiber with a light generating device or a light receiving device is called active alignment. In this process a light is introduced at one end of the optical fiber and the other end is moved adjacent the active area of an operative light receiving device, while monitoring the output of the light receiving device, until a maximum output signal is received. Alternatively, an operative light receiving device is attached to one end of an optical fiber and the other end is moved adjacent the active area of an operative light generating device until a maximum output signal is received. In both instances the amount of time and effort required to obtain the optimum alignment is extensive. 
     In a perfect system, all of the light generated passes directly into an optical fiber and all of the light exiting an optical fiber is directed onto an active surface of a light receiving device. However, in the real world much of the generated light travels outwardly in a direction to miss the optical fiber and some of the light impinging on the optical fiber is reflected back into the light generating device. Much of the cause of this outwardly or misdirected light comes from poor alignment along the Z axis (the axis of light propagation) as well as misalignment in the X and Y axes (defining a plane perpendicular to the direction of light propagation). The outwardly or misdirected light can impinge on adjacent devices to produce unwanted cross-talk within the system. Also, the reflected light can be directed back into the light generating device or the optical fiber and can interfere with generated light to produce unwanted and troublesome modes or frequencies. Also, the loss of light through misdirection and/or reflection means that additional power must be used to produce sufficient light to transmit between various devices, thus reducing efficiency. 
     It would be highly advantageous, therefore, to remedy the foregoing and other deficiencies inherent in the prior art. 
     Accordingly, it is an object the present invention to provide new and improved optical alignment features. 
     Another object of the present invention is to provide new and improved optical alignment features which reduce time and effort in alignment procedures. 
     And another object of the present invention is to provide new and improved optical alignment features which improve the efficiency of optical systems. 
     Still another object of the present invention is to provide new and improved optical alignment features which allow the use of a variety of components and component materials. 
     SUMMARY OF THE INVENTION 
     Briefly, to achieve the desired objects of the present invention in accordance with a preferred embodiment thereof, provided is optical alignment apparatus which includes a first element mounting a first lens and a light source and a second element mounting a second lens and a light receiving structure. The first lens is placed a first distance from the light source and is constructed to collimate light received from the light source. The first and second elements are mounted relative to each other to position the second lens a third distance from the first lens and to receive the collimated light from the first lens. The second lens is positioned a second distance from the light receiving structure to focus the collimated light on the light receiving structure. The first and second lens are constructed so that the first and second distances are dependent upon each other and the third distance is independent of the first and second distances. 
     In general, the light source is one of a laser, a light emitting diode, a light communicating optical fiber, or any other source of light for communication and the light receiving structure is any device that converts light energy into electrical energy, such as a photo-diode, a PIN diode, or one end of a light communicating optical fiber having such a: device positioned at the other end. The lens are constructed and positioned so that the first distance between the light source and the first lens determines a major portion of the optical power of the apparatus, so that the second lens can be formed of a low tolerance molded plastic part. Also, in some embodiments one of the first and second lenses can be formed to direct, impinging light received along a first axis, at an angle to the first axis and may include, for example, a curved reflecting surface. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and further and more specific objects and advantages of the invention will become readily apparent to those skilled in the art from the following detailed description of a preferred embodiment thereof, taken in conjunction with the drawings in which: 
     FIG. 1 is a sectional view of a simplified optoelectric module in accordance with the present invention; 
     FIG. 2 is a simplified schematic view of a lens system in a standard module; 
     FIG. 3 is a simplified schematic view of a lens system in the optoelectric module of FIG. 1; 
     FIG. 4 is a simplified schematic view of Z-axis adjustment apparatus; 
     FIG. 5 is a simplified schematic view representing a medium change, including air, in the passage of light; 
     FIG. 6 is a simplified schematic view representing a medium change in accordance with the present invention; and 
     FIG. 7 is a sectional view of another embodiment of a simplified optoelectric module in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention provides new and improved alignment features for telecommunication and data communication apparatus and the like and in particular for optoelectric modules. Turning to FIG. 1, a sectional view is illustrated of a simplified optoelectric module  10  in accordance with the present invention. As stated above, the term “optoelectric” is used herein to denote the fact that module  10  can be either an optical-to-electrical or electrical-to-optical module. 
     It will be understood by those skilled in the art that modules of the type discussed herein generally include a pair of channels, one of which receives electrical signals, converts the electrical signals to optical (light) beams by way of a laser or the like and introduces them into one end of an optical fiber, which then transmits the modulated optical beams to external apparatus. The second channel of the module receives modulated optical beams from an optical fiber connected to the external apparatus, conveys the modulated optical beams to a photo diode or the like, which converts them to electrical signals. In the following description, the apparatus and methods can generally be used in either of the channels but, since the optical portions of the two channels are substantially similar, only one channel will be discussed with the understanding that the description applies equally to both channels. Also, throughout this disclosure the term “laser” is intended to denote any light source including, for example, a laser, a light emitting diode, the end of a light communicating optical fiber, etc. and light receiving structures are intended to include any one of a photodiode, a pin diode, an end of a light communicating optical fiber, etc. 
     Module  10  of FIG. 1 includes a receptacle element or assembly  11  and an optoelectric element or package  12  aligned and affixed together, as will be disclosed in more detail below. Receptacle assembly  11  is designed to receive an optical fiber  14  in communication therewith, in a manner that will become clear presently. In the preferred embodiment, optical fiber  14  is a single mode fiber (the use of which is one of the major advantages of the present invention) including a glass core  15  and a cladding layer  16 . Receptacle assembly  11  includes an elongated cylindrical ferrule  20  defining a fiber receiving opening  21  at one end and a mounting flange  22  at the opposite end. 
     Progressing from the end of module  10  which defines opening  21  toward the end defining flange  22 , ferrule  20  has two radially outwardly directed steps  32  and  33 . Step  32  provides a surface or stop for the mounting of an optical spacer  35  and step  33  provides a surface or a stop for the positioning of an optical lens assembly  36 . In this preferred embodiment, lens assembly  36  is formed of plastic and may be, for example, molded to simplify manufacturing of module  10 . It should be understood that the term “plastic” is used herein as a generic term to describe any non-glass optical material that operates to transmit optical beams of interest therethrough and which can be conveniently formed into lenses and the like. Similarly, the term “glass” is defined as any material that is substantially temperature insensitive (i.e., stable throughout the operating temperature of the module), such as glass, crystalline material, or semiconductor material (e.g. silicon, oxides, nitrides, some ceramics, etc.). For example, in most optical modules used at the present time the optical beams are generated by a laser that operates in the infra-red band and any materials that transmit this light, including some oxides and nitrides, come within this definition. 
     Lens assembly  36  defines a central opening for the transmission of light therethrough which extends from an end  37  to an opposite end  38 . A lens  39  is integrally formed in the central opening a fixed distance from end  37 . Lens assembly  36  is frictionally held in place within ferrule  20  and holds spacer  35  fixedly in place. Also, lens  39  is spaced a fixed and known distance from spacer  35 . In this preferred embodiment, optical fiber  14  is inserted into ferrule  20  so that glass core  15  buts against spacer  35 , which substantially reduces or suppresses return reflections. 
     Optoelectric package  12 , in this simplified embodiment, includes a base or support plate  40  with an optoelectric device, such as a laser  45 , mounted on the upper surface and positioned to transmit light generated therein substantially perpendicular to the surface of support plate  40 . Alternatively, laser  45  could be a photodiode or the like. A lens  48  is mounted in a housing  50  and spaced a fixed distance, designated d, from laser  45 . It will be understood that housing  50  can be designed to be mounted over or adjacent the optoelectric device or could be manufactured as an integral component of the optoelectronic device. Here it will be understood that housing  50  fixes lens  48  relative to laser  45  so as to accurately position it relative to laser  45  or accurately determine the distance d. 
     Optoelectric package  12  is affixed to receptacle assembly  11  with flange  22  of ferrule  20  butting against the surface of support plate  40 . Ferrule  20  and support plate  40  are fixedly attached together by some convenient means, such as welding, gluing, etc. Here it should be noted that laser  45  can be enclosed by this process in a hermetically sealed chamber, if desired. However, a hermetic seal is not necessary in many embodiments in which the laser or photodiode used is either separately sealed or is not sensitive to atmospheric conditions. Further, during the assembly process, optoelectric package  12  and support plate  40  are adjusted until lenses  39  and  48  are substantially optically aligned so that light from laser  45  is directed into core  15  of optical fiber  14 . 
     Turning now to FIG. 2, a standard optical system is illustrated schematically for purposes of explaining the background. In this system, a laser  52  supplies light through a single lens  53  to an optical fiber  55 . Generally, as is understood in the art, for maximum efficiency, lens  53  should focus the light from laser  52  on the end surface of the core of optical fiber  55 . Because of this requirement, any changes in distance between lens  53  and optical fiber  55  changes the focus spot and, hence greatly changes the light introduced into optical fiber  55 . Also, the optical coupling efficiency of the system is affected by the distance between laser  52  and lens  53 . Thus positioning of the components is critical. and very little manufacturing tolerance is allowed. This is especially true when dealing with single mode optical fibers which have a core diameter less than ten microns. 
     Referring to FIG. 3, a simplified schematic view is illustrated of the lens system in module  10  of FIG. 1 in accordance with the present invention. In this lens system, lens  48  receives any divergent light from laser  45  and collimates the light so that it is directed to lens  39 . Lens  39  focuses the impinging collimated light onto the end of core  15  of optical fiber  14 . Lens  48  is fixedly positioned adjacent laser  45  and lens  39  is fixedly positioned adjacent an end of optical fiber  14 . Note that lens  48  is illustrated as a common lens for purposes of simplifying this explanation but virtually any type of lens or lens system could be utilized. Here it will be understood that fixing lens  48  to laser  45  accurately positions it relative to laser  45 . Also, fixing lens  39  to optical fiber  14  accurately positions it relative to optical fiber  14 . Because these are short distances (on the order of microns), they can be determined relatively accurately. However, the distance between lenses  39  and  48  is less critical, which provides substantially relaxed tolerances for module  10  and for the assembling thereof. The distance between lenses  39  and  48  is not critical because the light is collimated and slight variances in position simply produce a small amount of light loss. 
     Because of the use of the two lens concept, the distance d on the optical axis between laser  45  and lens  48  can generally be used to determine the most desirable distance between lens  39  and the end of optical fiber  14 . The distance d is a few microns and can be relatively easily maintained constant (i.e. a very small tolerance) between manufactured modules. Also, the distance d is a good indication of the most desirable distance between lens  39  and optical fiber  14 , because this distance affects the optical power of the system. Thus, by measuring the distance d in any specific module a desired distance between lens  39  and optical fiber  14  can be easily calculated. 
     Referring additionally to FIG. 4, lens  39 , spacer  35 , and optical fiber  14  are illustrated in a simplified and enlarged schematic diagram. Here it can be seen that in this embodiment by adjusting the thickness x of spacer  35  the distance between lens  39  and optical fiber  14  can be easily adjusted. The thickness x can be adjusted in several different ways. In one method, optimum spacing between lens  39  and optical fiber  14  is determined in advance, generally by measuring the distance d between laser  45  and lens  48 . A spacer with the correct thickness is then provided and placed in the module prior to assembly. In a second method, several thinner spacers (generally with some predefined thicknesses or steps of thicknesses) are provided and the optimum distance is achieved by selecting and affixing a plurality of thinner spacers together and placing them in the module prior to assembly of the module. Using this system, optimum light out of optical fiber  14  can be measured to determine the optimum distance x. 
     Turning now to FIG. 5, a simplified schematic view is illustrated representing a medium change, including air, in the passage of light from a laser  60  to the light inlet of an optical system, represented by a single lens element  62 . In FIG. 5, laser  60  is spaced from lens element  62  with air or space in between. Because of the change in refractive indices between the material of laser  60  and air, light waves have a tendency to spread widely so that lens element  62  must be relatively powerful to collimate the beam. 
     Referring additionally to FIG. 6, a simplified schematic view is illustrated representing a medium change in accordance with the present invention. In the optical system of FIG. 6, a plastic glob  66  is introduced around the end of a laser  65  and encompasses the inlet side of a lens element  67 . Thus, the change in refractive index is from laser  65  to plastic glob  66 . Plastic glob  66  can be selected so that it has a refractive index closer to the refractive index of laser  65 . Thus, there is less spreading of the light as it leaves laser  65  and lens element  67  can perform the required collimating with much less optical power. It can readily be seen that any improvement in refractive index produced by plastic glob  66 , over air, provides a reduction in the required power of lens element  67  and simplifies the manufacture and cost of the device. Lens  67  can be mounted a fixed distance from laser  65  by plastic glob  66  alone or in conjunction with additional structure. 
     Turning now to FIG. 7, another embodiment is illustrated of a simplified optoelectric module  10 ′ in accordance with the present invention. In this embodiment, components similar to the embodiment illustrated in FIG. 1 are designated with similar numbers and a prime is added to all numbers to indicate the different embodiment. Also, similar components with similar positions will not be described again in detail. In this embodiment, the optoelectric device will again be described as a laser  45 ′ for convenience but it could be a photodiode or the like, as explained in detail above. 
     Laser  45 ′ is mounted to one side of the optical axis Z, defined by,optical fiber  14 ′ and lens  39 ′. A lens block  46 ′ is constructed to define a lens  48 ′ with a curved reflecting surface designed to direct impinging light, received at an angle to the Z axis, along the Z axis and to collimate the impinging light. Lens block  46 ′ can be molded from plastic, including lens  48 ′, or it can be formed to fixedly mount a glass curved reflecting surface in a fixed position relative to laser  45 ′. The distance d is again the distance between laser  45 ′ and the curved reflecting surface forming lens  48 ′. 
     In a slightly different embodiment, lens block  46 ′ can be formed with a lens  49 ′ in the surface perpendicular to the Z axis. Lens  49 ′ can be included in addition to the curved reflecting surface. Also, lens  49 ′ can be included in lieu of curving the reflecting surface in some specific applications. However, if lens  49 ′ is included in lieu of the curved reflecting surface, then the distance d includes the distance from laser  45 ′ to the reflecting surface and the distance from the reflecting surface to lens  49 ′. Generally, the preferred embodiment is to include the curved reflecting surface as lens  48 ′ and not to include lens  49 ′, since this construction reduces the number of components. 
     Accordingly, new and improved optical alignment features are disclosed which substantially reduce time and effort in alignment procedures and which improve the efficiency of optical systems. Because a pair of lenses are incorporated that are fixed relative to a light source and a light receiving structure, respectively, the distance along the Z axis between the pair of lenses is not critical. Also, because the light traveling between the pair of lenses is collimated, slight variances in the lateral position (X and Y axes) of the lenses is much less critical, since such variances simply produce a small amount of light loss. Thus, manufacturing tolerances can be substantially reduced, substantially reducing manufacturing time, labor, and costs. Further, the new and improved optical alignment features allow the use of a variety of components and component materials. 
     Various changes and modifications to the embodiments herein chosen; for purposes of illustration will readily occur to those skilled in the art. To the extent that such modifications and variations do not depart from the spirit of the invention, they are intended to be included within the scope thereof which is assessed only by a fair interpretations of the following claims.