Abstract:
Systems and devices using diffractive and optionally refractive elements to launch an optical signal with a controlled amplitude and phase distribution into a multimode optical fiber for improved modal dispersion are provided. More particularly, optical ports, transmitter optical subassemblies (TOSAs), and transceivers use integrated diffractive and optionally refractive elements to launch a helically propagating light distribution, also known as a vortex launch. One embodiment includes a monolithic transparent port that generally includes: a lens on a first surface for receiving and collimating or focusing an optical signal; and a diffractive surface pattern for receiving the optical signal from the lens and launching the optical signal into a multimode optical fiber with a controlled intensity and phase distribution. Alternatively, the lens can be added as a separated component or omitted altogether while the diffractive surface pattern is formed on either the laser or fiber receptacle sides of the port.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. Provisional Application No. 60/585,216, filed Jul. 2, 2004, which is incorporated by reference herein in its entireties. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. The Field of the Invention  
         [0003]     The present invention relates to the field of optical communications. More particularly, the present invention relates to systems and methods using diffractive and optionally refractive surfaces to launch an optical signal into a multimode optical fiber with a controlled spatial intensity and phase profile.  
         [0004]     2. The Relevant Technology  
         [0005]     Computer and data communications networks continue to develop and expand due to declining costs, improved performance of computer and networking equipment, the remarkable growth of the internet, and the resulting increased demand for communication bandwidth. Such increased demand occurs within and between metropolitan areas as well as within communications networks. Moreover, as organizations have recognized the economic benefits of using communications networks, network applications such as electronic mail, voice and data transfer, host access, and shared and distributed databases are increasingly used as a means to increase user productivity. This increased demand translates into a need for higher speed communications links for which fiber optics is particularly well suited.  
         [0006]     Through fiber optics, digital data in the form of light signals is formed by light emitting diodes or lasers and then propagated through a fiber optic cable. Such light signals allow for high data transmission rates over distances for which electrical links are poorly suited. Other advantages of using light signals for data transmission include their resistance to electromagnetic radiation that interferes with electrical signals; fiber optic cables&#39; ability to prevent light signals from escaping, as can occur with electrical signals in wire-based systems; and light signals&#39; ability to be transmitted over great distances without the signal loss typically associated with electrical signals on copper wire.  
         [0007]     One important device for fiber optic communications is the laser. Generally, a laser is a light source that produces, through stimulated emission, coherent, near monochromatic light. The emitted laser light can be modulated to provide optical signals that can be transmitted over great distances. In this manner, an electrical signal is converted to an optical signal for data transmission. The optical signal is, in turn, received and converted back to an electrical signal by a receiver such as a monitor photodiode. A transceiver is an optical device that includes both a laser (as part of a transmitter) and a photodiode (as part of a receiver).  
         [0008]     The optical signal can be coupled to and from either multimode or single-mode fiber. The term “mode” refers to an electromagnetic wave traveling in a fiber or other waveguide which has a particular spatial field and phase distribution and which travels at a characteristic velocity. A small core optical fiber, for example 8-9 microns, can carry only a single-mode and is therefore termed single-mode fiber. Such a fiber is well suited for large transmission distances because all of the light travels with a well defined velocity. A larger core diameter fiber, for example 62.5 microns, can propagate more than one mode of light and is therefore termed multimode fiber. Multimode fiber is best suited to shorter transmission distances, for example within local area networks systems, while single-mode fiber is best suited to longer transmission distances such as long-distance telephony and cable television systems.  
         [0009]     Single mode fiber has advantages in that single-mode fiber allows for a higher bandwidth-distance capacity to transmit information because it can retain the fidelity of each light pulse over longer distances and it exhibits no dispersion caused by velocity differences between multiple modes. Single-mode fiber also enjoys lower fiber attenuation than multimode fiber. Thus, more information can be transmitted per unit of time. As a result, single mode is often preferred for optical communications. Nevertheless, multimode fiber has some advantages that caused it to be used in a large number of shorter distance applications, generally less than 2 km and usually less than 500 m. Such distances form the majority of connections in local area networks (LANs) and similar storage area networks (SANs). First, earlier fiber optic links at relatively low data rates of 100 Mb/s or less were based on very low cost LED (light emitting diode) sources. The highly multimode nature of a typical LED&#39;s output makes it impossible to couple a useful portion of the light into a single mode fiber. Thus, relatively large core (50-62.5 um diameter) multimode fibers were used to collect a reasonably larger fraction of the LED output. Additionally, and perhaps even more importantly at the time, the mechanical tolerances needed to make fiber optic connectors and the corresponding receptacles on fiber optic transceivers for single mode fiber were very expensive and made them impractical for the cost sensitive short data links.  
         [0010]     For these reasons, multimode fiber became, and to a large extent remains, the practical standard for fiber optic cabling within typical office buildings and the like. This has led to a large infrastructure of legacy multimode fiber and a corresponding desire to use this fiber for newer higher speed links.  
         [0011]     One of the limitations on the bandwidth distance product of a multimode fiber link (that is the maximum data that can be transmitted over a given link distance), is caused by differences in propagation velocity of the different modes of a multimode fiber. These differences, usually referred to as modal dispersion, cause a pulse representing a single data bit to spread in time and interfere with adjacent pulses causing what is known as ISI or inter-symbol interference, which will degrade the quality of the received signal and ultimately cause a link to become unusable. In an ideal multimode fiber, the differences in modal velocity are very small and the bandwidth distance product is actually very high (as high as 10 GHz*km). In real fibers, however, manufacturing imperfections in the refractive index profile of the fiber lead to a much larger range of modal velocities and limit the modal bandwidth to 160-500 MHz*km depending on factors such as the optical wavelength used. One particularly common imperfection of multimode fiber index profiles is a significant peak or dip in the refractive index at the center of the fiber core, which tends to lead to large differences in the velocity of the lowest order modes relative to the remaining modes.  
         [0012]     As the need for higher data rates in LANs and SANs increased, the problems of modal dispersion became a significant limitation. This limitation became a particular problem during the development of Gigabit Ethernet (which uses signaling rates of 1.25 Gb/s) in 1998, and threatened to significantly limit the distances. See D AVID  C UNNINGHAM ET AL. , G IGABIT  E THERNET  N ETWORKING  (June 1999), incorporated herein by reference. The solution adopted for Gigabit Ethernet involved controlling the optical launch into the multimode fiber to avoid launching into modes that lead to particularly low bandwidth.  
         [0013]     One approach to reducing modal dispersion in multimode fiber is to launch a small spot off-center into the fiber. This launches a reduced set of optical modes, and in particular tends to avoid modes which are strongly affected by imperfections in the fiber core as well as at the edges of the optical fiber. This is commonly performed by a “mode-conditioning” patch cable external to the fiber optic transceiver where the transceiver provides a single mode launch and the mode conditioning patchcord consists of a length of single mode fiber joined to a length of multimode fiber with the desired lateral offset. Unfortunately, this is a relatively expensive solution requiring additional hardware that adds an incremental cost to the optical system in which it is employed and requires an increased degree of care by the end user to make sure the patchcord is employed in links that require it.  
         [0014]     Another approach to conditioned launch is to launch light in a ring shaped pattern where the intensity is small at the center and edges of the fiber. This general approach was standardized during the development of 10 Gigabit Ethernet to ensure usable distances on links based on 850 nm multimode laser sources. This approach specified a test of the optical power distribution at the entrance to the span of multimode fiber which set an upper bound on the power within a particular small radius and a lower bound on the power within a much large radius.  
         [0015]     At the time of this writing, a standard is under development to allow transmission of 10 gigabit/second signals over legacy multimode fiber links with distances of up to 300 m. In this case, even with good control of the launch into the multimode fiber, the ISI is still too large for simple receivers to function well. The solution being developed in this standard is to use electronic dispersion compensation techniques in the receiver of the system to compensate for the ISI. As it turns out, even with practical degrees of electronic dispersion compensation, careful control of the launch into the fiber is still needed, and more significantly, the criteria for what is an acceptable launch is somewhat different than in the case of simpler receivers.  
         [0016]     Another prior art approach to launching an optical signal into an optical fiber includes adding a discreet insert to a transmitter or transmitter optical subassembly (TOSA). In this approach, an optically transparent insert with a diffractive pattern is placed in the optical path in the TOSA. The diffractive element launches the optical signal with a controlled intensity and phase distribution. One particularly useful launch is one where the amplitude distribution is in a donut form and the phase varies periodically around the azimuth of the fiber. Such a launch will excite modes which travel in a helical trajectory, when thought of in the ray picture, and are particularly well suited for avoiding imperfections in the core of the optical fiber. This launch has been referred to in the literature as a “vortex launch,” and an optical element which generates this launch as a “vortex lens.” Further details regarding a “vortex launch” and similar methods are disclosed in U.S. Pat. No. 6,530,697 B1, filed Jun. 11, 1999, U.S. patent Publication No. US2003/0142903 A1, filed Nov. 12, 2002, and U.S. Pat. No. 6,349,159 B1, each of the foregoing being incorporated herein by reference in their entireties. More generally, the term “vortex launch” is used herein to refer to any launch which results in a substantially spiral or helical propagation in the optical fiber. The term “vortex lens” is used herein to refer to an optical element such as a diffractive surface which will generate a vortex launch. This approach has several distinct disadvantages, however. For example, the fabrication of an additional part adds an incremental cost to the TOSA. In addition to the cost of fabricating the part, there is an assembly cost in that the discreet diffractive grating part has to be carefully aligned during assembly of the TOSA. Optical systems are a very competitive industry and each such cost decreases the competitiveness of a product.  
         [0017]     Accordingly, it would represent an advance in the art to provide less expensive methods and systems to reduce modal dispersion in existing multi-mode fibers and thereby improve the transmission of single mode optical signals over multimode fiber.  
       BRIEF SUMMARY OF THE INVENTION  
       [0018]     The present invention relates to the use of diffractive and refractive elements to launch an optical signal with a controlled amplitude and phase distribution into a multimode optical fiber. More particularly, embodiments of the invention relate to low cost ports, transmitter optical subassemblies (TOSAs), and transceivers that use integrated diffractive and optionally refractive elements to launch an optical signal with a controlled amplitude and phase distribution into a multimode optical fiber.  
         [0019]     Accordingly, a first example embodiment of the invention is a monolithic optical port for launching an optical signal into multimode fiber. The monolithic optical port generally includes a first surface on the optical port, the first surface comprising a diffractive surface pattern formed thereon, wherein the diffractive surface pattern is configured for receiving an optical signal and launching the optical signal into a multimode optical fiber with a controlled intensity and phase distribution.  
         [0020]     Another example embodiment of the invention is a transmitter optical subassembly (TOSA) for converting an electrical signal into an optical signal and then coupling the optical signal to an optical fiber. The transmitter optical subassembly generally includes: a light emitter; and a monolithic optical port, the optical port comprising: a fiber receptacle, wherein an optical path is situated between the light emitter and the fiber receptacle; and a diffractive surface pattern configured for receiving an optical signal from the light emitter and launching the optical signal into an optical fiber seated in the fiber receptacle, wherein the optical signal is launched into the optical fiber with a controlled amplitude and phase distribution.  
         [0021]     Yet another example embodiment of the invention is a transmitter optical subassembly which generally includes: an optoelectronic package comprising: a light emitter and a lens for receiving the optical signal from the laser and directing the optical signal onto an optical fiber seated in a fiber receptacle; and a transparent monolithic optical port comprising: the fiber receptacle, wherein an optical path is situated between the lens and the fiber receptacle; and a diffractive surface pattern monolithically formed with the optical port and configured for receiving an optical signal from the light emitter and launching the optical signal into a multimode optical fiber seated in the fiber receptacle, wherein the optical signal is launched into the multimode optical fiber with a controlled intensity and phase distribution.  
         [0022]     In each of the foregoing example embodiments, the light modifying surface is preferably a diffractive optical element that launches the optical signal into the fiber in a helical or vortex distribution. In addition, the light modifying surface can be located on either the fiber-adjacent side or the light emitter adjacent side of the optical port as desired.  
         [0023]     These and other objects and features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]     To further clarify the above and other advantages and features of the present invention, a more particular description of the invention will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. It is appreciated that these drawings depict only typical embodiments of the invention and are therefore not to be considered limiting of its scope. The invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:  
         [0025]      FIG. 1A  illustrates a perspective view of a transmitter optical subassembly according to one embodiment of the invention;  
         [0026]      FIG. 1B  illustrates a sectional view of the transmitter optical subassembly of  FIG. 1A ;  
         [0027]      FIG. 1C  illustrates a blowup view of a portion of the transmitter optical subassembly of  FIG. 1A ; and  
         [0028]      FIG. 2  illustrates a sectional view of another transmitter optical subassembly according to yet another embodiment of the invention;  
         [0029]      FIG. 3  illustrates a side view of an optical transceiver according to one embodiment of the invention;  
         [0030]      FIG. 4  illustrates another optical transceiver according to one embodiment of the invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0031]     The present invention relates to the use of diffractive and refractive elements to launch an optical signal off-center into a multimode optical fiber. More particularly, embodiments of the invention relate to low cost ports, transmitter optical subassemblies (TOSAs), and transceivers that use integrated diffractive and optionally refractive elements to launch the optical signal with a controlled amplitude and phase distribution.  
         [0032]     Reference will now be made to the figures wherein like structures will be provided with like reference designations. It is understood that the drawings are diagrammatic and schematic representations of presently preferred embodiments of the invention, and are not limiting of the present invention nor are they necessarily drawn to scale.  
         [0033]     In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be obvious however, to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known aspects of transceivers and transmitter optical subassemblies have not been described in particular detail in order to avoid unnecessarily obscuring the present invention.  
         [0034]     Referring now to  FIGS. 1A-1C , one embodiment of a TOSA  100  according to the invention is therein depicted. More particularly,  FIG. 1A  provides a perspective view of TOSA  100 ,  FIG. 1B  provides a sectional view of TOSA  100  to show the inside of TOSA  100 , and  FIG. 1C  provides an exploded view of one portion of TOSA  100 . Embodiments of the invention are suitable for use in connection with a variety of data rates, for example as about 1 Gbps, about 2 Gbps, about 4 Gbps, and 10 about Gbps, or higher.  
         [0035]     As best depicted in  FIG. 1B , TOSA  100  generally comprises optical port  102  and an optoelectronic package such as a Transistor Outline (“TO”) can  104 . It should be noted that at higher data rates such as 10 gigabits per second, the normal electrical interface of a TO-can may be modified but that other features such as the overall size and manufacturing process are retained. In one embodiment, optical port  102  is an optically transparent piece of molded plastic. The optical port  102  includes a nosepiece  105  having a fiber receptacle  106  for receiving a fiber ferrule. At the end of fiber receptacle  106  is a fiber stop  108  for setting the position of the fiber ferrule. Just past fiber stop  108  is optionally an open space  110 .  
         [0036]     On the optical port surface adjacent to open space  110  but opposite fiber stop  108  is a diffractive surface  112  directly patterned into the plastic surface, as best illustrated in  FIG. 1C . In other words, diffractive surface  112  is positioned on the side of the optical port  102  adjacent to the fiber receptacle. A diffractive surface is in general a microstructured pattern of reflecting or transmitting features which can be configured through the pattern, spacing and profile of its features to transform an incident amplitude and phase distribution into a very wide range of output amplitude and phase patterns. In a preferred embodiment, the diffractive surface  112  is designed to function as a vortex lens. The diffractive surface  112  is preferably a monolithic section of the optical port  102  that is formed as part of the molding process that forms the optical port  102 . Diffractive surface  112  can alternatively be formed as a monolithic section of the optical port  12  by, for example, a secondary embossing process or other formation techniques known in the art. One of the key advantages of the monolithic integration of the diffractive surface into the plastic port is that it has a fixed alignment relative to the fiber receptacle  106  and thus the fiber, as well as with the refractive lens  114 . This is achieved by careful positioning of the diffractive surface master within the mold for the port, and eliminates the need for active alignment of the diffractive element on a part to part basis. Alternatively, a separately formed diffractive element can also be assembled into TOSA  100  at the indicated position on optical port  102 .  
         [0037]     In another embodiment of the invention, an effectively designed refractive surface may be used in lieu of diffractive surface  112  or in lieu of other diffractive surfaces as disclosed herein. The design and implementation of refractive surfaces in optical systems is well known to those skilled in the art. Such refractive surfaces, in view of the inventive teachings and disclosure herein, can be used to achieve the launch of an optical signal into an optical fiber in a controlled amplitude and phase distribution.  
         [0038]     Also part of optical port  102  is lens  114 , which is positioned on the side of the optical port  102  adjacent to the laser, or the laser side. As with diffractive surface  112 , lens  114  is preferably a molded part of optical port  102 . In the depicted embodiment, diverging light  116  generated by light emitter  118  is received by lens  114  and relayed as collimated or focused light  120  to diffractive surface  112 . Diffractive surface  112  bends the path of the light such that it diverges  122  and is received by a fiber seated in fiber receptacle  106  with a controlled amplitude and phase distribution. For example, in a preferred embodiment, the light can be directed to be received by the fiber to result in a vortex launch.  
         [0039]     Lens  114  is not necessarily implemented as an integrated part of optical port  102 , however. In one variation of this and other embodiments, for example, lens  114  can be omitted in the event that a diffractive surface is positioned to receive a sufficient volume of the light emitted by light emitter  118  and modeled to effectively direct the light to the optical fiber in the desired pattern. That is, the diffractive surface can integrate the lens focusing function along with the function of producing a specific amplitude and phase distribution such as a vortex launch. In other variations of the invention, lens  114  is provided as a discreet component within TOSA  100 , for example as part of the TO-can  104 .  
         [0040]     TO-can  104  is preferably a hermetically sealed structure that houses light emitter  118 . Such devices are widely used in the field of optoelectronics and details regarding their construction and operation are well known to those skilled in the art. Inside TO-can  104  is a light emitter  118 . Light emitter  118  is preferably a laser diode such as a vertical cavity surface emitting laser (VCSEL), a Fabry-Perot Laser, or a distributed feedback (DFB) laser. Of course other light emitters capable of generating an optical signal at the required modulation rates may be compatible with various embodiments of the invention.  
         [0041]     Another part of TO-can  104  is an optically transparent can window  124  that is positioned to permit passage therethrough of light from light emitter  118  to optical port  102 , more specifically for example to lens  114 . TO-can also includes structures to enable the TO-can to communicate power and electrical signals with other electronic structures. For example, pins  126  are configured for this purpose. It should be noted that other electrical interfaces to the TO-can are commonly employed for higher data rates such as 10 gigabits per second.  
         [0042]     In summary, in order for TOSA  100  to launch an optical signal into an optical fiber seated in fiber receptacle  106 , an electrical signal is communicated to the TOSA  100  through pins  126 . Electronic circuitry in TO-can  104  directs light emitter  118  to generate an optical signal. The optical signal diverges as it is emitted and passes through can window  124  and reaches optically transparent optical port  102 . A molded lens in optical port  102  preferably receives the optical signal and directs it towards the optical fiber. Before the signal reaches the optical fiber, however, the optical signal is redirected by diffractive surface  112  so that the optical signal is launched into the optical fiber in a controlled amplitude and phase distribution.  
         [0043]     Referring now to  FIG. 2 , another embodiment of a TOSA  200  according to the invention is depicted. Similar to TOSA  100  in  FIGS. 1A-1C , TOSA  200  includes an optical port  202  and a TO-can  204 . Optical port  202  includes nosepiece  206 , fiber receptacle  208 , fiber stop  210 , and open space  212 , each as described above with respect to optical port  102 . In contrast to the illustrated optical port  102 , however, this embodiment includes diffractive surface  214  on the laser side of the optical port  202 , not the fiber side, and omits the lens from its molded construction.  
         [0044]     In the depicted embodiment, a lens  216  is positioned where can window  124  is positioned in a hermetically sealed TO-can  104 . In the depicted embodiment lens  216  is a ball lens. Thus, lens  216  is in the optical path of the optical signal generated by light emitter  220 . Of course, as with the previously discussed embodiments, the lens can be either omitted or positioned in different locations in the TOSA. For example, lens  216  could be positioned further inside the TO-can  204  or between TO-can  204  and optical port  202 , if desired. A variety of different lens structures could be used in this or other embodiments of the invention, including for example: ball lenses, conventional lenses with spherical surfaces, aspheric lenses, or very low cost lenses formed by the thermal reflow of a glass shot. In addition, a microlens assembly as disclosed in U.S. Patent Provisional Application No. 60/483,740, filed Jun. 30, 2003, incorporated herein by reference in its entirety, can also be suitably employed with embodiments of the invention.  
         [0045]     TO-can  204  is also structured generally similarly to TO-can  104  except that, in this example, an optional lens  216  is incorporated in TO-can  204 . As previously mentioned, lens  216  is positioned where can window  124  is positioned in TO-can  104 .  
         [0046]     Accordingly, in operation an electrical signal is relayed to TO-can  204  via pins  218 . The electrical signal is converted to an optical signal  222  by light emitter  220 . This optical signal  222  diverges as it leaves the light emitter  220  and is then optionally received by lens  216 . Lens  216  relays the optical signal onto diffractive surface  214  as a converging or collimated optical signal. As previously discussed, diffractive surface  214  next launches the optical signal onto an optical fiber seated in fiber receptacle  208  with a controlled amplitude and phase distribution.  
         [0047]     Turning now to  FIG. 3 , details are provided concerning the use of an exemplary optical transceiver  300  in various exemplary operating environments. In particular,  FIG. 3  is a side view of the optical transceiver  300  having a housing  310  that includes a pair of opposing vertical walls (only wall  311  is depicted) as well as a top cover  313  and bottom cover  317 . TOSA  315  is designed in line with the embodiments of the invention as disclosed herein and therefore includes diffractive and/or refractive surfaces to launch an optical signal with a controlled amplitude and phase distribution into an optical fiber. A ROSA is also included in transceiver  300  but is not depicted as it is positioned behind TOSA  315 .  
         [0048]     Exemplary implementations of the transceiver substrate  325  include various components, circuits and devices  340  which are mounted to the transceiver substrate. The transceiver substrate  325  also includes a connector  326  exemplarily implemented as an array of electrical connection pins  326 A.  
         [0049]     Similar to housing  310 , the housing  330  includes a pair of opposing vertical walls as well as a top cover and a bottom cover. In some implementations, the housings  310  and  330  are integrated with each other so that a single housing is defined. As further indicated in  FIG. 3 , the housing  310  further defines a cavity  350  wherein the TOSA  315  and ROSA are substantially disposed. In general, the cavity  350  at least partially defines a receptacle for receiving a fiber optic connector to be attached to one or both of the TOSA  315  and the ROSA. Various types of types of optical cable interfaces can be used in optical transceivers according to the invention.  
         [0050]     As part of ongoing efforts to reduce the size of optical transceivers and other components, manufacturing standards such as the small form factor (“SFF”), small form factor pluggable (“SFP”), and 10-gigabit small form factor pluggable (“XFP”) standards have been implemented in the industry. With reference to  FIG. 4 , the depicted transceiver module  400  is an XFP transceiver module, which is a 10-Gigabit XFP Form-Factor Pluggable Module for use in telecommunications networks, local area networks, metro area networks, storage area networks, wide area networks, and the like. It will appreciated by those skilled in the art that this is simply one compatible design for an optical transceiver, and other transceiver designs can incorporate aspects of the invention to launch an optical signal off-center into a multimode optical fiber. XFP transceivers are designed to occupy one-fifth of the space and dissipate one-half the power of prior 10 Gb/s modules. The transceiver modules constructed according to the invention can be compatible with the XFP MSA standards, for example including those set forth in the 10 Gigabit Small Form Factor Pluggable Module adoption draft specification Revision 4.0 published by the XFP Multi Source Agreement (MSA) Group on Apr. 13, 2004 (xfpmsa.org), which is incorporated herein by reference, and can also be compatible with future revisions or final XFP MSA specifications that will be adopted in the future.  
         [0051]     As depicted in  FIG. 4 , XFP transceiver module  400  includes TOSA  402  for generating an optical signal based on a received electrical input and receiver optical subassembly (“ROSA”)  404  for receiving an optical signal and generating an electrical output to relay the signal. TOSA  402  is designed in line with the embodiments of the invention as disclosed herein and therefore includes diffractive and/or refractive surfaces to launch an optical signal off-centered into an optical fiber.  
         [0052]     ROSA  404  may include, for example, a PIN photodiode for converting an optical signal received by ROSA  404  into an electrical signal. Of course, optical receivers that are compatible with embodiments of the invention may use other forms of photodiodes.  
         [0053]     Within a transceiver module, each optical subassembly generally includes electrical connections to various additional components such as a transceiver substrate, sometimes embodied in the form of a printed circuit board (“PCB”). Accordingly,  FIG. 4  depicts PCB  406 , first flexible circuit  408  and second flexible circuit  410  for relaying electrical signals.  
         [0054]     The depicted flexible circuits  408 ,  410  are an optional method to communicate electrical signals to and from the TOSA  402  and ROSA  404  to PCB  406  in the transceiver module  400 . First flexible circuit  408  interconnects TOSA  402  and PCB  406  while second flexible circuit  410  interconnects ROSA  404  and PCB  406 . In addition, the depicted embodiment uses pins for connecting TOSAs and ROSAs to the flexible circuits. Further details regarding the use of flexible circuits in optical transceivers are provided in U.S. patent application Ser. No. 10/409,837, filed Apr. 9, 2003, incorporated herein by reference in its entirety.  
         [0055]     The PCB can include multiple active circuitry components particularly designed to drive or handle electrical signals sent to or returning from one or more of the electrically-attached TOSAs and ROSAs. Accordingly, such a PCB will usually include a number of electrical transmission lines with the one or more TOSAs and ROSAs. Such connections may include “send” and “receive” data transmission lines for each TOSA and ROSA, one or more power transmission lines for each TOSA and ROSA, and one or more diagnostic data transmission lines for each TOSA and ROSA.  
         [0056]     Other components of a transceiver module may include a bail release and LC cable receptacles for receiving and securely attaching LC cables to each of TOSA  402  and ROSA  404 .  
         [0057]     The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.