Abstract:
An optical transmission module comprising at least a photoelectric element, a printed circuit board and a flexible printed circuit board is provided. The flexible printed circuit board electrically connects the photoelectrical elements and the printed circuit board. At least a ground plane is formed on the flexible printed circuit board to reduce crosstalk and electromagnetic interference. Transmission lines and impedance formed on the flexible printed circuit board solve the problems of signal distortion and impedance mismatch. The flexible printed circuit board also serves as a flexible interface to solve prior problems of connection stress between the photoelectric element and the printed circuit board.

Description:
This Non-provisional application claims priority under 35 U.S.C. § 119(a) on patent Application No. 092129665 filed in Taiwan on Oct. 24, 2003, the entire contents of which are hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention generally relates to an optical transmission module, and in particular relates to an optical transmission module using a flexible printed circuit board as a connecting interface between the photoelectric element and the circuit board. 
   2. Related Art 
   In the field of optical data processing, optical transmitter/receiver transmits the data signal of processed information through an emitter into optical signal. The optical signal transferred through optical fiber to a remote photoelectric receiver where the optical signal is further converted into electrical signal so as to accomplish data communication. 
   Some prior art optical data transmission modules are disclosed in patents such as U.S. Pat. No. 5,742,480 and U.S. Pat. No. 5,513,073. 
   In U.S. Pat. No. 5,742,480, a flexible printed circuit board is used to carry the circuit of whole module. The integrated circuit element is soldered directly on the flexible printed circuit board. The complicated control circuit and photoelectric element are all mounted on the board. The photoelectric element is fixed on the board through holes. In order to align the optical axis, an almost right angle bend for the flexible printed circuit board is required. 
   In U.S. Pat. No. 5,513,073, an optical transmission module mainly includes a flexible printed circuit board, heat spreader and thermal isolator. The flexible integrated circuit is similar to U.S. Pat. No. 5,742,480 that a complicated control circuit and photoelectric element is used; holes and almost a right angle bend are required for optical axis alignment. 
   The U.S. Pat. No. 5,742,480 and U.S. Pat. No. 5,513,073 have some possible problems. For example, the multiple-layer flexible board used in U.S. Pat. No. 5,742,480 increases the manufacturing difficulty. The circuits on the flexible board of the two patents are very complicated that make the fabrication and assembly difficult. The connections between the optical members and the flexible printed circuit board require holes that easily cause signal distortion, parasitic effects and fail to achieve high data rate transmission. Further, the bending of the flexible printed circuit board easily causes unreliable signal. The signals at the transmitter and the receiver are easy to be influenced. No countermeasures are provided in the prior arts. 
   Another prior art of optical transmission module is illustrated in  FIG. 1 . The photoelectric element  10  has a signal pin  11  and a non-signal pin  12  connecting to the integrated circuit  40  on the main circuit board  30  via connecting points  60  and transmission lines  41 ,  42 . The photoelectric element  20  has a signal pin  21  and a non-signal pin  22  connecting to the integrated circuit  50  on the main circuit board  30  via connecting points  60  and transmission lines  51 ,  52 . The connectors in this construction have problems of stress during connecting and decrease signal fidelity and cause interference to the receiver. 
   As described above, the optical transmission modules of prior arts have problems of lower signal fidelity and interference between transmitter and receiver. 
   SUMMARY OF THE INVENTION 
   The object of the invention is to provide an optical transmission module that can prevent from signal fidelity reduction caused by photoelectric transformation through the transmission interface, and can improve the quality of data transmission. A flexible printed circuit board connects the photoelectric element and the main circuit board. The flexible printed circuit board includes grounding to prevent crosstalk and to prevent from problems of electromagnetic emission and interference. 
   The flexible printed circuit board and the coupling circuit effectively solve the problems of high data rate signal loss and mismatch between the photoelectric element and the circuit board. The flexible printed circuit board also solves the problem of connection stress between the circuit board and the photoelectric element and allows the bending based on mechanical design. 
   Four embodiments of the invention are provided in the following description. A first embodiment of optical transmission module according to the invention includes at least a photoelectric element for optical and electrical signal transformation, and a flexible printed circuit board connecting the photoelectrical element and a circuit board. The pins of the photoelectrical element are soldered or fixed through other means to the flexible printed circuit board. There are transmission lines connecting to the circuit board. 
   In a second embodiment of the invention, a passive element is included on the flexible printed circuit board for impedance matching. 
   In the first and second embodiments, all the pins are connected to the circuit board through transmission lines. In a third and a fourth embodiments, only a pin is connected to the printed circuit board. 
   An optical transmission module according to the invention can reduce the signal fidelity reduction caused by photoelectric transformation through the transmission interface so as to improve the quality of data transmission. Through improvements of photoelectric elements, such as photoelectric elements packed by the currently most popular TOCAN (Transistor Outline CAN) method, and the interface between printed circuit boards, a high data rate optical transmission can be achieved without the need of expensive, bulky or new package methods. 
   Under considerations of fabricability and less cost increase, an optical transmission module of the invention can be well integrated with mechanical design to further improve the reliability. The module can solve the problems of electromagnetic emission and interference, signal crosstalk and so on, and enhance the performance of the whole module. 

   
     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 prior art optical transmission module; 
       FIG. 2  is a constructional view of a first embodiment optical transmission module of the invention; 
       FIGS. 3A to 3D  are alternative sectional side views of a first embodiment of the invention; 
       FIG. 4  is a constructional view of a second embodiment of the invention; 
       FIG. 5  is a constructional view of a third embodiment of the invention; 
       FIG. 6  is a constructional view of a fourth embodiment of the invention; 
       FIG. 7  is an eye diagram of conventional optical transmission module under data rate of 2.488Gbps; 
       FIG. 8  is an eye diagram of an optical transmission module of the invention under data rate of 2.488Gbps; 
       FIG. 9  is an eye diagram of conventional optical transmission module under data rate of 3.125Gbps; 
       FIG. 10  is an eye diagram of an optical transmission module of the invention under data rate of 3.125Gbps; 
       FIG. 11  is an eye diagram of conventional optical transmission module at the receiver side caused by interference; and 
       FIG. 12  is an eye diagram of an optical transmission module of the invention at the receiver side. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2  shows a first embodiment of the invention. Two photoelectric elements  10  and  20  are used. One works as a transmitter and the other as a receiver; or all as transmitter or receiver. Of course, there can only be one photoelectric element. 
   In the prior art, the pins of photoelectric elements are directly connected to printed circuit board. In the invention, the pins are electrically connected through an interface of flexible printed circuit board. 
   Pins of photoelectric element can be generally classified as signal pins and non-signal pins. The signal pins are for transmitting or receiving data. The non-signal pins are for power input or grounding. 
   For the photoelectric element  10 , signal pins  11  and  12  are electrically connected to the flexible printed circuit board  71  via connecting points  63  and  64  respectively. A transmission line  711  connects the connecting point  63  and a connecting point  61 . A transmission line  712  connects the connecting point  64  and a connecting point  62 . The connecting points  61  and  62  connect the flexible printed circuit board  71  to the printed circuit board  30 . There are transmission lines  41  and  42  connecting the signal to the integrated circuit  40 . The signal pins  11  and  12  are located on a signal transmission plane of the flexible printed circuit board  71 . 
   For the photoelectric element  20 , signal pins  21  and  22  are electrically connected to the flexible printed circuit board  72  via connecting points  67  and  68  respectively. A transmission line  721  connects the connecting point  67  and a connecting point  65 . A transmission line  722  connects the connecting point  68  and a connecting point  66 . The connecting points  65  and  66  connect the flexible printed circuit board  72  to the printed circuit board  30 . There are transmission lines  51  and  52  connecting the signal to the integrated circuit  50 . The signal pins  21  and  22  are located on a signal transmission plane of the flexible printed circuit board  72 . 
   The number and positions of pins in  FIG. 2  are only for description and not for limitation of the applications. The number of printed circuit board  30  can be accommodated to the number of photoelectric element. For example, a printed circuit board for receiver and a printed circuit board for transmitter. The layers of the flexible printed circuit boards  71  and  72  can be designed based on pins of the photoelectric elements. 
     FIGS. 3A to 3D  are alternative sectional side views of an optical transmission module of the invention. In  FIG. 3A , the flexible printed circuit board  71  includes a signal transmission plane  71 A. The flexible printed circuit board  73  includes a ground plane  73 B. The signal pin  11  of the photoelectric element connects to the signal transmission plane  71 A. The non-signal pin  13  connects to the ground plane  73 B. The flexible printed circuit boards  71  and  73  locate on different planes of the printed circuit board  30 . 
   In the  FIG. 3A  arrangement, the ground plane  73 B of the flexible printed circuit board  73  isolates external electromagnetic interference. However, the signal pin  11  is sensible to external electromagnetic interference. Therefore, as shown in  FIG. 3B , the signal transmission plane  71 A locates at the bottom of the flexible printed circuit board  71  so that the signal pin  11  is isolated from interference. The isolation can further be improved by applying a ground plane  71 B on top of the flexible printed circuit board  71 . 
   The position of the non-signal pin  13  can be located on a ground plane  73 B on upper plane of the flexible printed circuit board  73  as shown in  FIG. 3C . Therefore, the pins are isolated from electromagnetic interference by the ground planes  71 B,  73 B. 
     FIG. 3D  illustrates an arrangement of multiple pins. The flexible printed circuit board  71  includes a signal transmission plane  71 A and a ground plane  71 B. The flexible printed circuit board  73  includes a signal transmission plane  73 A and a ground plane  73 B. The signal pin  11  is connected to the signal transmission plane  71 A. The signal pin  14  is connected to the signal transmission plane  73 A. The non-signal pin  13  is connected to the ground plane  73 B. Therefore, the pins are isolated from external electromagnetic interference by the ground planes  71 B,  73 B. 
   On the flexible printed circuit board, there are transmission lines with impedance elements for making impedance matching between the printed circuit board  30  and the photoelectric elements  10  and  20 . The flexible printed circuit board and the photoelectric elements are connected by flat contact such that the flexible printed circuit board can be extended from said pins and parallel to the optical axis of the photoelectric elements. No sharp bend occurs during assembly so as to increase the reliability of the module. The construction is simple, easy to be fabricated and lower of cost. 
   The embodiment of the invention uses flexible printed circuit board and ground planes to isolate or reduce interferences between the transmitter and the receiver. The signal transmission can be located on different planes of the flexible printed circuit board so that the transmitter and the receiver are isolated by the space so as to reduce radio data rate interference and obtain better quality of signal. 
   The good transmission lines on the flexible printed circuit board gives continuous impedances for the pins of the photoelectric element so that the signals are fully transmitted; a high quality data transmission is obtained, and radio data rate interference is reduced. 
   Referring to  FIG. 4 , another embodiment of the invention has a passive element  80  soldered on the data transmission plane for impedance matching of the photoelectric element  20 . The passive element  80  connects via a connecting point  69 , a transmission line  713  to a connecting point  63  of a pin  11 ; and connects via a connecting point  70 , a transmission line  711  to a connecting point  61  on the printed circuit board  30 . A connector  64  for another pin  12  connects via transmission line  712  to a connector  62  on the printed circuit board  30 . Opposing to the signal transmission plane, there is a ground plane at the opposite side. 
   The crosstalk between the receiver and the transmitter in the optical transmission module of the invention is prevented, and a higher quality of signal and better performance of high data rate transmission are obtained. The performance can be checked through eye diagrams. 
   The eye diagrams at the transmitter side are first compared.  FIG. 7  is an eye diagram under 2.488Gbps data rate of a conventional module with photoelectric elements having 4 mm length pins.  FIG. 8  is an eye diagram of the invention under same conditions. It is apparent that the invention has a great improvement. 
     FIG. 9  is an eye diagram under 3.125Gbps data rate of a conventional module with TOCAN photoelectric elements having 4 mm length pins.  FIG. 10  is an eye diagram of the invention under same conditions. It is apparent that the invention has a great improvement when working under a higher data rate. 
   Nest, the eye diagrams of signal interference at the receiver side are compare using a standard light signal to check the receiver sensitivity. Three test conditions are used: first, no interfering signal to the receiver; second, using conventional TOCAN packaged photoelectric element for the transmitter; and third, using flexible printed circuit board for pin connection of the transmitter. When data rate is 2.488Gbps, the sensitivity of no interfering signal condition is −22.157dBm; the sensitivity of conventional package condition is −16.108dBm; and the sensitivity of flexible printed circuit board condition is −18.698dBm. The total is +3.5dBm. When data rate is 3.125Gbps, the sensitivity of no interfering signal condition is −20.468dBm; the sensitivity of conventional package condition is −14.314dBm; and the sensitivity of flexible printed circuit board condition is −17.085dBm. The total is +2.7dBm. The reference transmitter power is −4.588dBm. The interfered transmitter power is −5.448dBm. 
     FIG. 11  and  FIG. 12  are respective eyes diagrams under 2.488 Gbps data rate of conventional optical transmission module and the invention at the receiver side caused by interference. From  FIG. 11 , the conventional module, we can see that Jitter p-p=67 ps. From  FIG. 12 , the invention, we can see that Jitter p-p=52 ps. 
   The optical transmission module of the invention mainly solves the problems of bad signals and signal interference. From the aforesaid descriptions and illustrations we can see that the invention really solves the aforesaid problems when working with high data rate. Besides that, there are further advantages as follows:
     1) Improving signal continuity and photoelectrical transformation;   2) Lower cost;   3) Easy assembly with direct soldering;   4) Improving reliability of assembly;   5) Reducing the connection stress between the photoelectric element and the printed circuit by using a flexible printed circuit board;   6) Capable of modification from an original design instead of a total redesign;   7) Easy fabrication of the simple circuit of flexible printed circuit board;   8) Better signal transmission lines to reduce signal distortion and impedance incontinuity so as to obtain signal continuity;   9) Good grounding to prevent from electromagnetic interference;   10) Good shielding to reduce signal crosstalk and improve the circuit stability;   11) Simple signal transmission lines on the flexible circuit board control the impedance in an easy way; and   12) No need of large bending on the flexible printed circuit board so as to improve reliability.   

   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.