Abstract:
A fiber waveguide optical subassembly uses the multi-mode fiber to increase the alignment tolerance between the active optical element and the waveguide. The filter is thinner to lower the dispersion due to the optical coupling gap. The subassembly further combines the optical bench to achieve passive positioning. Therefore it reduces the cost and enhances the transmission rate.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention generally relates to an optical subassembly applicable to passive optical network (PON) and fiber-to-the-home (FTTH) systems, and in particular relates to a fiber waveguide optical subassembly.  
       BACKGROUND OF THE INVENTION  
       [0002]     The bi-directional optical transceiver commonly used in passive optical network (PON) and fiber-to-the-home (FTTH) systems are mainly of two types, duplexer and triplexer. The duplexer processes both data and voice signals, while the triplexer processes data, voice and video signals. As the increasing demands of digital and analog video transmission, conventional duplexer cannot meet the market requirements, and triplexer becomes the trend of optical communication in the future.  
         [0003]     A duplexer optical subassembly mainly includes a light emitter and a light receiver. The light emitter is mainly a laser diode; the light receiver is mainly a photo diode. Whatever an emitter or a receiver, the optical path requires alignment. In the emitter side, there are alignment problems between the laser diode and the single-mode fiber or planar waveguide. Because the alignment tolerance of a single-mode fiber or planar waveguide is just 1 or 2 microns, it is easy to be misaligned and poor coupling, and the power output and quality of transmission is turned down. Generally, there are two manners to improve alignment. The first is an active alignment of lighting the laser diode and aligning through coupling. It can achieve precise alignment, however it costs much. The second is a passive alignment of applying an alignment key for assembly. It greatly saves assembly cost, however the process is difficult to be approached.  
         [0004]     In a triplex optical transceiver, at the light emitter side, the laser beam from the laser diode has to pass through several splitters and filters before getting into a single-mode fiber. Because the laser beam transmitting in a free space diffuses in accordance with its transmission distance, it encounters a problem that the final laser beam coupled to the single-mode fiber is less. In order to solve the problem, micro lenses have to be used for increasing the numerical aperture. However, using micro lens increases the cost and the complication of assembly.  
         [0005]     For a duplex optical transceiver, there are commonly planar waveguides or tubular waveguides available. A planar waveguide optical subassembly includes three major optical coupling interfaces: the laser diode with a planar waveguide, the planar waveguide with another planar waveguide via a filter, and the planar waveguide with a single-mode fiber. The three interfaces all encounter problems of optical misalignment. Different types of waveguide further have problems of field mismatch. Therefore, it is hard to improve the coupling efficiency of the whole unit. Tubular waveguide optical subassembly mainly uses lens to solve the problem of free space optical misalignment. The alignment tolerance is compensated by lens. However, the coupling efficiency is still low and the lens increases the cost.  
         [0006]     Only tubular optical subassembly is used in a triplex optical transceiver. The tubular light emitter and tubular light receiver increase the cost of the optical components. Further, the tubular components have larger dimensions that increase the coupling length in free space and cause lower efficiency and light dispersion in the transmission.  
         [0007]     As described above, whatever for a planar waveguide optical subassembly or a tubular optical subassembly, the cost of the assembly in the coupling interface is hard to be reduced.  
       SUMMARY OF THE INVENTION  
       [0008]     The object of the invention is to provide a fiber waveguide optical subassembly to increase the alignment tolerance between the active optical element and the waveguide. The filter is thinner to lower the dispersion caused by the optical coupling gap. It reduces the cost and enhances the transmission rate.  
         [0009]     In order to achieve the aforesaid object, the fiber waveguide optical subassembly of the invention includes an optical bench, a light emitter, a first optical transmission element, a splitter, a light detector and a second optical transmission element. The optical bench supports all the optical elements. The first optical transmission element has relative front end and rear end. The front end couples with the light emitter. The rear end links to one end of the splitter. Another end of the splitter connects with the second optical transmission element. The light detector locates on one side of the splitter.  
         [0010]     The light emitter outputs light signal through the first optical transmission element, the splitter and the second optical transmission element. The input light signal passes through the second optical transmission element; reflected by the splitter and enters the light detector. The first optical transmission element increases the alignment tolerance between the light emitter and the waveguide. It prevents from additional alignment process, reduces the cost and enhances the transmission rate.  
         [0011]     In particular, the first and the second optical transmission element maybe is a multi-mode optical fiber or a planar waveguide. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     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:  
         [0013]      FIG. 1  is a first embodiment of duplexer optical subassembly of the invention;  
         [0014]      FIGS. 2A, 2B  are descriptive views of optical path in the first embodiment of duplexer optical subassembly of the invention;  
         [0015]      FIGS. 3A and 3B  are perspective views of a second embodiment of duplexer optical subassembly of the invention;  
         [0016]      FIG. 4  is a first embodiment of triplexer optical subassembly of the invention;  
         [0017]      FIGS. 5A, 5B  are descriptive views of optical path in the first embodiment of triplexer optical subassembly of the invention; and  
         [0018]      FIGS. 6A and 6B  are perspective views of a second embodiment of triplexer optical subassembly of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]     The fiber waveguide optical subassembly according to the invention is applicable to light transceivers and mainly includes: an optical bench, a light emitter, a first optical transmission element, a splitter, a light detector and a second optical transmission element. The first optical transmission element maybe is a multi-mode optical fiber or a planar waveguide, and the second optical transmission element maybe is a single-mode optical fiber, a multi-mode optical fiber or a planar waveguide.  
         [0020]     The following descriptions relate to general applications of duplexer and triplexer comprising the multi-mode optical fiber and the single-mode optical fiber.  
         [0021]     As shown in  FIG. 1 , a duplexer optical subassembly of the invention includes an optical bench  10 , a light emitter  30 , a multi-mode optical fiber  20 , a splitter  40 , a light detector  50  and a single-mode optical fiber  61 . The optical bench  10  includes two fiber grooves  11 ,  12  (such as V-shape grooves), an emitter groove  31  and a splitter groove  41  for supporting all the optical components. The multi-mode optical fiber  20  positioned in the fiber groove  11  has a front end  21  and a rear end  22 . The light emitter  30  is positioned in the emitter groove  31 . The front end  21  of the multi-mode optical fiber  20  couples with the light emitter  30 . The splitter  40  located in the splitter groove  41  couples with the rear end  22  of the multi-mode optical fiber  20 . The single-mode optical fiber  61  located in the fiber groove  12  couples with the splitter  40  and connects outward. The light detector  50  locates aside the splitter  40 . There can be a ball lens (not shown) located between the light emitter  30  and the front end of the multi-mode optical fiber  20 .  
         [0022]     The optical bench  10  is made of semiconductor material, polymer or metal. The light emitter  30  is an edge-emitting laser diode or a surface-emitting laser diode. As shown in  FIG. 2A , the light emitter  30  outputs light signal  91  through the multi-mode optical fiber  20 , the splitter  40  and the single-mode optical fiber  61 . The splitter  40  can be a film filter having thickness around 20 to 100 micrometers. As shown in  FIG. 2B , when downloading light signals, the input light signal  92  enters the single-mode optical fiber  61 , reflected by the splitter  40  and enters the light detector  50 . The multi-mode optical fiber  20  provides an alignment tolerance around +/−10 micrometers. Therefore, a length among 0.2 to 10 millimeters is applicable. The single-mode optical fiber  61  can be replaced by a multi-mode or another kind of optical fiber for a shorter distance local network transmission.  
         [0023]     As shown in  FIG. 1 , a monitor  70  is further installed behind the light emitter  30  for monitoring the emission of the emitter  30 . The light emitter  30  mainly provides a forward light toward the multi-mode optical fiber  20 , however, a little part of light emits backward. Therefore, a reflective surface  71  reflects the backward light to the monitor  70  for the monitoring function. As shown in  FIG. 2B , the light detector  50  is also mounted upon a reflective surface  51  for receiving the input light signal  92  reflected by the reflective surface  51 .  
         [0024]     To prevent from difficulties of fabricating the reflective surfaces  51 ,  71  on the optical bench  10 , a second embodiment is provided as shown in  FIG. 3A  and  FIG. 3B . A monitor carrier  72  formed with a reflective surface  71  carries the monitor  70 . The same, a detector carrier  52  formed with a reflective surface  51  carries the light detector  50 . Therefore, the optical bench  10  is not needed for being machined with the reflective surfaces  51 ,  71 ; but only to be mounted with the monitor carrier  72  and the detector carrier  52 .  
         [0025]     On the other hand, the invention may further comprise a third optical transmission element. The third optical transmission element is a multi-mode optical fiber described below, of course the third optical transmission element may is a planar waveguide. The first embodiment of triplexer optical subassembly of the invention is shown in  FIG. 4 . It has a similar construction to the duplexer optical subassembly described above, but further having a second multi-mode optical fiber  62  located in a fiber groove  13 ; and a third splitter  42  located in a third splitter groove  43 . The second multi-mode optical fiber  62  has a front end  621  coupled with the splitter  40 ; and a rear end  622  coupled with the third splitter  42 . In the drawing, besides the fiber groove  13  and the third splitter groove  43 , other optical components, such as the emitter groove  31 , the splitter groove  41 , the light detector  50 , a third light detector  80  and monitor  70 , are applied in the same way.  
         [0026]     As shown in  FIG. 5A , the output light signal  91  passes through the splitter  40  and the single-mode optical fiber  61 . As shown in  FIG. 5B , when downloading light signals, the input light signal  92  enters the single-mode optical fiber  61 , reflected by the splitter  40  and enters the second multi-mode optical fiber  62 . A part of the input light signal  92  passes the third splitter  42  and a part of it reflects in accordance with the wavelength of the input light signal  92 . Therefore, the input light signal  92  is separated to the light detector  50  and the third light detector  80 .  
         [0027]     The same, there is a reflective surface  81  under the third light detector  80 . In order to prevent difficult machining, in a second embodiment of triplexer optical subassembly of the invention as shown in  FIGS. 6A and 6B , a second detector carrier  82  formed with a reflective surface  81  carries the third light detector  80 . The rest construction is the same as that of duplexer optical subassembly described above and will not be further described herein.  
         [0028]     By suitably applying multi-mode optical fiber, the optical subassembly of the invention has the following advantages:  
         [0029]     1) Larger tolerance in optical alignment to achieve passive positioning;  
         [0030]     2) Thinner optical coupling spacial gap to lower the dispersion;  
         [0031]     3) Higher coupling efficiency to increase optical output of the subassembly; and  
         [0032]     4) Inexpensive optical fiber to lower the manufacturing cost.  
         [0033]     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.