Patent Publication Number: US-2005121736-A1

Title: Receiver optical subassembly

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The invention generally relates to an optical transmission module, and in particular relates to a receiver optical subassembly.  
      2. Related Art  
      In the trend of high transmission-rate communication, optical data transmission is being fast developed, and the related fiber optic products are getting more and more important. An optical data transmission system applying optical fiber media requires high transmission rate, low noise and of precise alignment of high quality optical transmission modules in order to meet user&#39;s needs. The key technologies for optical transmission modules are high-speed components, optical subassembly, alignment package and so on.  
      Currently, the optical transceivers are mainly of surface emitting and surface receiving elements, such as vertical cavity surface emitting lasers (VCSEL). The package of the optical transceiver has to provide good alignment of the transceiver and the lens in order to get precise and efficient transmission. A receiver optical subassembly in the optical transmission module has to meet the requirements of high precision, high transmission rate and low noise. Conventional receiver optical module is composed of metallic pad substrate and surface mounted capacitors. Other components are connected through wire mounting. In U.S. Pat. No. 5,610,395, a photoreceiver module uses a parallel plate capacitor (metal-insulator-metal, MIM capacitor) to replace the aforesaid substrate in order to reduce the package size and to simplify the steps of production.  
      Under the requirement of high coupling efficiency of laser source and optical fiber, the requirement of alignment is getting stricter. Currently, most optical communication elements are processed by active alignment that a light source passes into the module for executing semi-automatic mechanical alignments of relative positions of the components. The facilities and processes are rather complicated and time-consuming. In order to reduce the alignment cost, a passive alignment, such as that disclosed by U.S. Pat. No. 6,576,888, can be applied. A light-receiving module of the patent consists of a mount substrate, an optical fiber, a semiconductor photoreceiver, a mount member, and a signal processing semiconductor element. The mount substrate is placed between the pair of arm portions of the mount member and carries the optical fiber and the semiconductor photoreceiver. Because the optical fiber is located in an etched groove formed on the mount substrate, the precision of the groove greatly influences the alignment; also, the package gets a larger size, and the package process is more complicated.  
      Because the manufacturing process influences the construction and alignment precision of an optical subassembly, in order to improve the quality and yield rate of the product, the requirements on the process and facilities are getting stricter, and the costs are getting higher. Therefore, it is a demand to get an inventive optical subassembly package that meets the requirements of high transmission rate, low noise, effective component alignments, low cost and high yield rate.  
     SUMMARY OF THE INVENTION  
      The object of the invention is to provide a receiver optical subassembly having an integrated package and performing effective alignment of optical transceiver elements. The optical subassembly has high performance and small size.  
      A receiver optical subassembly for transforming received optical signal into electrical signal according to the invention includes a substrate, a photoreceiver and a transimpedance amplifier. There are high-speed traces formed on the substrate. The positive and negative pads of the photoreceiver are coplanar and connected to the traces through flip chip so as to reduce the parasitic impedance effect and improve the high-speed performance of the optical subassembly. The transimpedance amplifier is electrically connected to the traces via flip chip, wire bonding or other methods. The photoreceiver and the transimpedance amplifier are connected via the high-speed traces formed on the ceramic substrate.  
      The invention further includes a base and a metal case for packing aforesaid substrate, photoreceiver and transimpedance amplifier. The base includes a plurality of electrical pins including a ground pin. The substrate has grounding pad electrically connects with the ground pin for reducing crosstalk and other interferences generated by high frequency signals. The metal case and the base form a shield for the substrate, the photoreceiver and the transimpedance amplifier. The grounding pin connects to the metal case for good shielding. Electrical signals are connected outwards via the electrical pins passing through the bottom of the base.  
      In order to improve the coupling efficiency, the light-receiving surface of the photoreceiver is formed with a micro lens. In accompany with some external coupling elements, the beam coupling system is decreased with requirements of alignment precision and improved with coupling efficiency. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The invention will become more fully understood from the detailed description given hereinbelow. However, this description is for purposes of illustration only, and thus is not limitative of the invention, wherein:  
       FIG. 1  is a constructional view of a photoreceiver and a transimpedance amplifier in an embodiment of the invention;  
       FIG. 2  is a constructional view of a substrate and a base in an embodiment of the invention; and  
       FIG. 3  is an application view of an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      In the invention, the anode and cathode of the photoreceiver are coplanar (i.e. back-received photodidoe) and connected to the traces so as to reduce the parasitic impedance effect, improve the high speed performance of the optical subassembly and reduce the package size.  
      As shown in  FIG. 1 , a constructional view of a photodiode and a transimpedance amplifier in an embodiment of the invention, the photodiode  110  and the transimpedance amplifier  130  are electrically connected to the high-speed traces  120  formed on surface of the ceramic substrate  100 . The positive and negative pins of the photodiode  110  are coplanar and connected to the high-speed traces  120 . The transimpedance amplifier  130  is electrically connected to the high-speed traces  120  via flip chip, wire bonding or other methods. The high-speed traces  120  connected between the photodiode  110  and the transimpedance amplifier  130  surround with the ground pads  140 . The ground pads  140  also electrically connect downwards to the ground pin of the base via some grounding through holes  141  of the ceramic substrate  100  in order to reduce crosstalk. The ceramic substrate  100  can be formed with an alignment key so that the photoreceiver can be easily aligned.  
      A base  200  can be made with a grounding structure for improving the performance of the optical subassembly. The constructional view of the substrate  100  and the base  200  of the receiver optical subassembly is shown in  FIG. 2 . The ceramic substrate  100  is mounted on the base  200 . The base  200  includes a plurality of electrical pins  210  including a ground pin  211 . The ceramic substrate  100  has grounding pads  140  electrically connect with the ground pin  211  for reducing crosstalk and other interferences generated by high frequency signals. In order to lower the requirements of alignment precision, the light-receiving surface of the photodiode  110  is formed with a micro lens  111 . The bottom of the photodiode  110  is connected through tin balls  112  to the ceramic substrate  100 . The ceramic substrate  100  connects to the base  200  through a conductive via hole  142 . The electrical pins  210 , except the ground pin  211 , of the base  200  are shielded with glass  212 . A metal case  220  (referred to  FIG. 3 ) and the base  200  form a shield for the substrate, the photoreceiver and the transimpedance amplifier  130 . The grounding pin  211  electrically connects to the base  200 , the substrate  100  and a metal case for good shielding.  
      The metal case  220  incorporates the grounding pin of the base  200  for shielding the substrate  100 , the photodiode  110  and the transimpedance amplifier  130 . A coupling structure of application with an optical fiber connector is illustrated in  FIG. 3 . The grounding pin  211  is connected to the metal case  220  for good shield. The electrical signals are connected outwards via the grounding pin  211  passing through the bottom of the base  200 . In accompany with an external coupling elements  310  with the micro lens  111  of the photodiode  110 , the beam coupling system lowers the requirements of alignment precision and improves the coupling efficiency. One end of the optical fiber connector  300  is coupled to the metal case  220 . A light-receiving portion  221  of the metal case  220  is a transparent piece or a lens for incidental beams to pass to the photodiode  110 . The other end of the optical fiber connector  300  receives the optical fiber  310 . A first beam coupler  230  is mounted between the optical fiber  310  and the light-receiving portion  221  of the metal case  220  so that the incidental beam comes from the optical fiber  310  to the beam coupler  230 , the light-receiving portion  221  and the photodiode  110  sequentially. A second beam coupler (not shown in the drawing) may also be mounted on the light-receiving portion  221  of the metal case  220  in order to couple or collect the beam. The optical fiber connector  300  can also be coupled with the metal case to form a better shield.  
      The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.