Patent Publication Number: US-2007104426-A1

Title: Bi-directional optical transceiver

Description:
CLAIM OF PRIORITY  
      This application claims priority under 35 U.S.C. § 119 to an application entitled “Bi-directional Optical Transceiver,” filed in the Korean Intellectual Property Office on Nov. 10, 2005 and assigned Serial No. 2005-107517, the contents of which are incorporated herein by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates generally to a bi-directional optical transceiver module, and in particular, to an optical transceiver module including a reflective semiconductor light source.  
      2. Description of the Related Art  
      Optical communication networks have become available on the market as a means for quickly and securely providing bulk information to a plurality of subscribers. In recent optical communication networks, Fiber To The Home (FTTH) for providing a communication service to home of each of subscribers has been popularized. In particular, a wavelength division multiplexing passive optical network (WDM-PON) is capable of providing bulk data to each subscriber with high security by assigning an unique wavelength to each subscriber.  
       FIG. 1  is a block diagram of a conventional bi-directional optical transceiver  100  according to the prior art. As shown, the conventional bi-directional optical transceiver  100  includes a wavelength selection filter  150  for splitting first and second optical signals, a semiconductor light source  110  for generating the first optical signal, an optical detector  130  for detecting the second optical signal, a monitoring optical detector  140  for monitoring the magnitude of the first optical signal, first to third lens systems  101 ,  102 , and  103 , and an optical fiber  120 .  
      A reflective semiconductor optical amplifier used in a wavelength locking method includes a front surface coated with a non-reflective layer and a rear surface is coated with a high reflective layer. Alternatively, a Febry-Perot laser can be used for the semiconductor light source  110 . A photo diode can be used for the monitoring optical detector  140 , which detects the magnitude of some light that has passed the high reflective layer, and can estimate the magnitude of the first optical signal from the detection.  
      The first lens system  101  is located between the semiconductor light source  110  and the wavelength selection filter  150 , collimates the first optical signal generated by the semiconductor light source  110 , and inputs the collimated first optical signal to the wavelength selection filter  150 . The third lens system  103  is located between the optical fiber  120  and the wavelength selection filter  150 , converges the first optical signal into one end of the optical fiber  120 , collimates the second optical signal output from the optical fiber  120 , and inputs the collimated second optical signal to the wavelength selection filter  150 .  
      The second lens system  102  is located between the wavelength selection filter  150  and the optical detector  130  and converges the second optical signal reflected by the wavelength selection filter  150  into the optical detector  130 .  
      However, wavelength-locking light sources have a problem in that a ratio of the intensities of light output from the front and rear surfaces does not have a linearly proportional correlation according to the intensity of light input from the outside to induce a wavelength-locking optical signal. That is, due to a difference between asymmetrical reflection ratios of the high reflective layer and the non-reflective layer of the conventional light source, the magnitude of the first optical signal cannot be correctly monitored from the intensity of light passing through the high reflective layer.  
     SUMMARY OF THE INVENTION  
      Accordingly, the present invention provides a bi-directional optical transceiver module for correctly monitoring the magnitude of an optical signal generated by a wavelength locking semiconductor light source.  
      Further, the present invention provides a miniaturized bi-directional optical transceiver module.  
      In one embodiment, there is provided a bi-directional optical transceiver comprising: an optical fiber for transmitting and receiving first and second optical signals; a transmitter module for generating the first optical signal; a receiver module for detecting the second optical signal; a tap filter for splitting a portion of the first optical signal; a monitor module for monitoring the magnitude of the portion of the first optical signal split by the tap filter; and a wavelength selection filter, which is located between the tap filter and the optical fiber, inputs the first optical signal output from the tap filter into the optical fiber, and inputs the second optical signal output from the optical fiber into an optical detector.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which:  
       FIG. 1  is a block diagram of a conventional bi-directional optical transceiver according to the prior art;  
       FIG. 2  is a cross-sectional view of a bi-directional optical transceiver according to an embodiment of the present invention;  
       FIG. 3  is an exploded perspective view of the bi-directional optical transceiver of  FIG. 2 ;  
       FIGS. 4A  to  4 C are a cross-sectional view, a plan view, and a perspective view of a filter supporting member of  FIG. 2 ;  
       FIGS. 5A and 5B  are a perspective view and a cross-sectional view of a housing of  FIG. 2 ; and  
       FIGS. 6A and 6B  are diagrams for explaining the variation of a current of an optical signal detected by a monitor module according to the variation of temperature in accordance with an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION  
      Now, embodiments of the present invention will be described herein below with reference to the accompanying drawings. For the purposes of clarity and simplicity, well-known functions or constructions are not described in detail as they would obscure the invention in unnecessary detail.  
       FIG. 2  is a cross-sectional view of a bi-directional optical transceiver  200  according to an embodiment of the present invention.  FIG. 3  is an exploded perspective view of the bi-directional optical transceiver  200  of  FIG. 2 .  
      Referring to  FIGS. 2 and 3 , the bi-directional optical transceiver  200  includes an optical fiber  260  for transmitting and receiving first and second optical signals λ a  and λ b , a transmitter module  210  for generating the first optical signal λ a , a receiver module  220  for detecting the second optical signal λ b , a tap filter  204  for splitting a portion (dotted arrow) of the first optical signal λ a , a monitor module  230  for monitoring the magnitude of the first optical signal portion split by the tap filter  204 , a wavelength selection filter  205  located between the tap filter  204  and the optical fiber  260 , first to third lens systems  201 ,  202 , and  203 , a housing  240 , and a filter supporting member  250 .  
      The transmitter, receiver, and monitor modules  210 ,  220 , and  230  have a TO-CAN structure, wherein the transmitter module  210  is inserted into a relevant hole of the housing  240 , and the receiver and monitor modules  220  and  230  are located in parallel at the side of the housing  240  that can be applied to small-form-factor (SFF) or small-form-factor-pluggable (SFP).  
      The transmitter module  210  includes a light source for generating the first optical signal λ a . For the light source, a semiconductor light source can be used, wherein one surface from which the first optical signal λ a  is output and of which a non-reflective layer is coated on, and the other surface of which a high reflective layer is coated on. Furthermore, for the semiconductor light source, a reflective semiconductor optical amplifier, which can be used in a wavelength-locking method, or a Febry-Perot laser can be used.  
      Each of the receiver and monitor modules  220  and  230  is an optical detection device, i.e., a photo diode and can detect an optical signal having a relevant wavelength.  
      The tap filter  204  and the wavelength selection filter  205  are located to have a slope of predetermined degrees from a certain normal line perpendicular to the traveling path of the first optical signal λ a , thereby more effectively splitting a portion of the first optical signal λ a , or changing the path of the second optical signal λ b  in a desired direction. The tap filter  204  splits a portion of the first optical signal λ a  generated by the transmitter module  210  and inputs the split first optical signal λ a  into the monitor module  230 . The tap filter  204  is an edge type having an angle of incidence of 45° and may use a filter for passing 95% of the input first optical signal λ a  to the wavelength selection filter  205  and for reflecting the remaining 5% of the input first optical signal λ a  to the monitor module  230 .  
      Referring to  FIGS. 5A and 5B , at the both ends of the housing  240 , one end of the transmitter module  210  and one end of the optical fiber  260  are inserted to face each other. The filter supporting member  250  has a hollow cylindrical shape and is inserted into a hollow portion  245  of the housing  240  to allow the first optical signal λ a  to travel.  
       FIGS. 4A  to  4 C are a cross-sectional view, a plan view, and a perspective view of the filter supporting member  250  shown in  FIG. 2 . As shown, the filter supporting member  250  includes a first surface  251  facing the transmitter module  210 , a second surface  252  facing the optical fiber  260 , and arrangement keys  253  and  254  extended from the both ends. The tap filter  204  is fixed to the first surface  251  so that the incident surface of the tap filter  204  has a slope of predetermined degrees from the traveling path of the first optical signal λ a , and the wavelength selection filter  205  is fixed to the second surface  252 . The arrangement keys  253  and  254  are extended from the first and second surfaces  251  and  252 , and v-shaped grooves can be formed to fix the tap filter  204  and the wavelength selection filter  205  in the boundary portions between the arrangement keys  253  and  254  and the first and second surfaces  251  and  252 .  
      The first lens system  201  converges the first optical signal λ a  split by the tap filter  204  into the monitor module  230 , and the second lens system  202  converges the second optical signal  4  reflected by the wavelength selection filter  205  into the receiver module  220 . The third lens system  203  converges the first optical signal λ a  into one end of the optical fiber  260 , collimates the second optical signal λ b , and outputs the collimated second optical signal λ b  to the wavelength selection filter  205 . The first to third lens systems  201 ,  202 , and  203  may use a non-spherical lens. The first to third lens systems  201 ,  202 , and  203  are inserted into relevant holes  242 ,  243 , and  244  of the housing  240 .  
      The optical fiber  260  is located at the opposite end of a hole  241  into which the transmitter module  210  is inserted, outputs the first optical signal λ a  to the outside of the bi-directional optical transceiver  200 , and outputs the second optical signal λ b , which is input from the outside of the bi-directional optical transceiver  200 , to the wavelength selection filter  205 . The optical fiber  260  may have a slope of 8° from a certain normal line perpendicular to the traveling path of the first and second optical signals λ a  and λ a  in order to minimize a coupling loss due to the reflection at one end thereof through which optical signals are input/output.  
      The wavelength selection filter  205  is located between the optical fiber  260  and the tap filter  204 , outputs the first optical signal λ a , which is input from the tap filter  204 , to the optical fiber  260 , and reflects the second optical signal λ b , which is input from the optical fiber  260 , to the receiver module  220 .  
      The monitor module  230  can include a photo diode for detecting the first optical signal λ a  split by the tap filter  204 .  
       FIGS. 6A and 6B  are diagrams for explaining the variation of power of the first optical signal λ a  according to the variation of a current of an optical signal detected by the monitor module  230  with respect to the variation of temperature in accordance with the embodiment of the present invention.  
      In particular,  FIG. 6A  is a diagram for explaining the variation of power of the first optical signal λ a  according to the variation of a current of a rear-surface-monitored optical signal with respect to the variation of temperature according to the prior art.  FIG. 6B  is a diagram for explaining the variation of power of the first optical signal λ a  according to the variation of a current of a front-surface-monitored optical signal with respect to the variation of temperature according to the embodiment of the present invention.  
      If it is assumed that the detected current is 28 μA (the bold solid line parallel to the y-axis) in  FIG. 6A ,  FIG. 6A  shows a difference of around 1.9 dB between the power of the first optical signal λ a  at 25° C. and the power of the first optical signal λ a  at 70° C. If it is assumed that the detect current is 83 μA (the bold solid line parallel to the y-axis) in  FIG. 6B ,  FIG. 6B  shows a difference of around 0.41 dB between the power of the first optical signal λ a  at 25° C. and the power of the first optical signal λ a  at 70° C.  
      In  FIGS. 6A and 6B , the power can be transformed to dB using “power variation (dB)=10×log 10  (comparison power/reference power).” That is, in  FIG. 6A , the power variation can be calculated by 1.938 dB=10 log 10 (0.63 mW/1 mW). The reference power denotes power at the lowest temperature, i.e., 25° C., among the tested temperatures, and the comparison power denotes power at 70° C. under the same conditions. In  FIG. 6B , the power variation can be calculated in the same method. That is, in  FIG. 6B , the power variation can be calculated by 0.4096 dB=10 log 10 (0.91 mW/1 mW).  
      As a result, according to the embodiment of the present invention, by monitoring the variation of the magnitude of an optical signal obtained by splitting a portion of an output optical signal, the variation of a characteristic of the optical signal can be stably monitored even if the variation of temperature occurs. Accordingly, a bi-directional optical transceiver can correctly monitor the magnitude of a wavelength-locking optical signal regardless of the intensity of light input to induce the wavelength locking. In addition, the bi-directional optical transceiver can be applied as a miniaturized SFF or SFP type by placing monitor and receiver modules adjacently in parallel.  
      While the invention has been shown and described with reference to a certain preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.