Patent Publication Number: US-7218829-B2

Title: Optical transmission module

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an optical transmission module and method which directly intensity-modulate a semiconductor laser to carry out optical transmission, and more particularly to an optical transmission module connected to a transmission path, which includes a reflection point having a predetermined reflection coefficient, and an optical transmission method for setting a reflectivity condition of the optical module for carrying out optical transmission. 
   2. Description of the Background Art 
   Conventionally, through the rapid spread of the Internet, the transmission capacity of data communication networks has been increased. While the transmission capacity has been increased, middle- and long-distance systems, typified by metro networks, have become bottlenecks of the communication networks, increasing the need for techniques and optical devices which economically realize faster middle- and long-distance systems. As for short-distance data transmission, through standardization activities of 10-Gigabit-Ethernet, etc., reduction in size and cost of optical devices is actively sought. 
   As a light emitting device, which is a key device for determining an optical transmission efficiency, a distributed feedback semiconductor laser is widely used. Semiconductor lasers, which are light emitting devices for use in middle- and long-distance transmission through single mode fibers, achieve a satisfactory transmission quality and broadband characteristics because of their narrow spectrum characteristics. In the case of using an ordinary semiconductor laser together with a transmission path which includes a reflection point, it is necessary to provide the semiconductor laser with an optical isolator component for the purpose of achieving a satisfactory system efficiency, while reducing interference noise. For example, the optical isolator component is composed of a combination of a Faraday element having a magneto-optic effect and a polarizer. Even if the semiconductor laser is not provided with the optical isolator component, it is possible to reduce interference noise in a multiplexed electrical signal. However, in the case of ordinary middle- or long-distance high-speed baseband optical transmission, where a narrow spectrum width is required, an interference noise level is increased, and therefore the usefulness of such a semiconductor laser is low. 
   Hereinbelow, a conventional optical transmission/reception system including an optical isolator component is described.  FIG. 6  is a block diagram showing a schematic structure of the optical transmission/reception system, and  FIG. 7  is a diagram used for explaining how an optical signal propagates through the optical transmission/reception system shown in  FIG. 6 . 
   In  FIG. 6 , the conventional optical transmission/reception system includes a drive unit  101 , a semiconductor laser  102 , an aspherical lens  103 , an optical isolator  104 , a transmission path  105 , and an optical receiver  106 . If an input signal Si is inputted to the drive unit  101 , the drive unit  101  converts the inputted signal into a modulation signal Sm having a predetermined extinction ratio to directly intensity-modulate the semiconductor laser  102 . The semiconductor laser  102  performs electrical/optical conversion on the modulation signal Sm to generate signal light Ls 1 . The signal light Ls 1  is outputted from the semiconductor laser  102  and collected by, for example, the aspherical lens  103  to be coupled through the optical isolator  104  to an input end of the transmission path  105 . After propagating through the transmission path  105 , the signal light Ls 1  is outputted as signal light Ls 2  from an output end of the transmission path  105 , and received by the optical receiver  106 . 
   Here, a reflection point X having a predetermined reflection coefficient is formed on the transmission path  105  due to influences of connection portions with optical connectors, etc. As shown in  FIG. 7 , the signal light Ls 1  having been inputted to the transmission path  105  propagates on the transmission path  105 , and is partially reflected at the reflection point X. A portion of the signal light Ls 1  that has not been reflected at the reflection point X is inputted as direct light Ld into the optical receiver  106 . On the other hand, reflection light Lr, which has been reflected at the reflection point X on the transmission path  105  to propagate in a direction opposite to that of the signal light Ls 1 , is re-reflected by an output end surface, a laser active region, etc., of the semiconductor laser  102 . Thereafter, the re-reflected reflection light (multiplexed reflection light) Lr is outputted as the signal light Ls 2  together with the direct light Ld from the output end of the transmission path  105  to the optical receiver  106 . 
   However, the reflection light Lr, which has been reflected at the reflection point X on the transmission path  105  to propagate in the direction opposite to that of the signal light Ls 1 , is weakened by the optical isolator  104 , making it possible to prevent the reflection light Lr from being re-reflected by the output end surface, the laser active region, etc., of the semiconductor laser  102 . That is, it is possible to prevent the multiplexed reflection light Lr from occurring and being outputted to the optical receiver  106 . Accordingly, it is possible to prevent interference noise from occurring in the optical receiver  106  due to detection of the direct light Ld and the multiplexed reflection light Lr from the signal light Ls 2 . 
   Japanese Laid-Open Patent Publication No. 2000-47069 discloses a semiconductor laser module in which an output end surface of a semiconductor laser makes an angle with optical fibers. In the semiconductor module, the angle between the output end surface of the semiconductor laser and the optical fibers prevents the reflection light from being re-reflected by the output end surface, a laser active region, etc., of the semiconductor laser. 
   Regarding the optical transmission/reception system including the optical isolator  104 , the cost of the optical isolator  104  is high, and therefore it is difficult to reduce the cost of the optical transmission module including the optical isolator  104 . 
   As for the semiconductor laser module disclosed in Japanese Laid-Open Patent Publication No. 2000-47069, high accuracy is required for adjusting optical coupling conditions with the angled optical fibers. That is, there is a difficulty in optically coupling the semiconductor laser with the optical fibers, resulting in an increase in cost of the optical transmission module. 
   SUMMARY OF THE INVENTION 
   Therefore, an object of the present invention is to provide an optical transmission module and method capable of suppressing occurrence of multiplexed reflection light, thereby making it possible to easily carry out an optical coupling operation at low cost. 
   The present invention has the following features to attain the object mentioned above. It should be noted that reference numerals, etc., in parentheses merely indicate correspondence with a preferred embodiment which will be described further below for the purpose of better understanding of the present invention, and do not restrict the scope of the present invention. 
   An optical transmission module ( 10 ) of the present invention transmits signal light (Ls 1 , Ls 3 ) to a transmission path ( 5 ). The optical transmission module includes a drive unit ( 1 ,  11 ), a signal light output unit ( 2 ,  12 ), and a loss generator ( 4 ,  18 ). The drive unit outputs a modulation signal (Sm). The signal light output unit performs electrical/optical conversion on the modulation signal outputted from the drive unit to generate signal light with a predetermined extinction ratio (B), and outputs the generated signal light. The loss generator attenuates light propagating between the signal light output unit and the transmission path by a predetermined attenuation factor (C), thereby attenuating the signal light outputted from the signal light output unit by the predetermined attenuation factor, and outputs the attenuated signal light to the transmission path. 
   The predetermined attenuation factor of the loss generator may be set based on at least an extinction ratio of the signal light output unit. In the case where the transmission path has formed thereon a reflection portion (a reflection point X) having a predetermined reflection coefficient (γ1), if the predetermined reflection coefficient of the reflection portion is A, the extinction ratio of the signal light output unit is B, a signal Q factor of the signal light is Q, a tolerable penalty for a bit error rate is D (Ds), and the predetermined attenuation factor of the loss generator is C, the attenuation factor C is set by the following expression (5): 
   
     
       
         
           C 
           ≈ 
           
             
               B 
               
                 
                   ( 
                   
                     B 
                     - 
                     1 
                   
                   ) 
                 
                 · 
                 Q 
               
             
             · 
             
               
                 
                   
                     1 
                     - 
                     
                       10 
                       
                         
                           - 
                           D 
                         
                         / 
                         5 
                       
                     
                   
                   
                     2 
                     · 
                     A 
                   
                 
               
               . 
             
           
         
       
     
   
   The optical transmission module may further include an output light detection unit ( 13 ), a reflection light detection unit ( 14 ), and a comparison unit ( 15 ). The output light detection unit detects an intensity of output light from the signal light output unit. The reflection light detection unit receives a portion of reflection light (Lr) outputted from the transmission path to the signal light output unit, and detects an intensity of the reflection light. The comparison unit outputs a first control signal (Cs 1 ) for controlling a bias level and modulation amplitude of the modulation signal outputted by the drive unit ( 11 ) and a second control signal (Cs 2 ) for controlling the attenuation factor of the loss generator (the variable loss generator  18 ), based on the intensity of the output light detected by the output light detection unit and the intensity of the reflected light detected by the reflection light detection unit. In this case, the drive unit generates and outputs the modulation signal having a bias level and modulation amplitude in accordance with the first control signal outputted from the comparison unit. The signal light output unit performs electrical/optical conversion on the modulation signal outputted from the drive unit to generate signal light with an extinction ratio corresponding to the bias level and the modulation amplitude of the modulation signal, and outputs the generated signal light. The loss generator adjusts the attenuation factor in accordance with the second control signal outputted from the comparison unit, and attenuates the light propagating between the signal light output unit and the transmission path by the adjusted attenuation factor. 
   The comparison unit may output the second control signal, such that an intensity ratio (ε) of the intensity of the reflection light detected by the reflection light detection unit to the intensity of the output light detected by the output light detection unit becomes a predetermined value, and may output the first control signal for adjusting the bias level and the modulation amplitude of the modulation signal such that the extinction ratio of the signal light output unit becomes constant. Specifically, the comparison unit outputs the second control signal such that the intensity ratio becomes a value based on the extinction ratio of the signal light output unit. For example, in the case where the intensity ratio is ε, the extinction ratio of the signal light output unit is B, a signal Q factor of the signal light is Q, and a tolerable penalty for a bit error rate is D, the comparison unit outputs the second control signal such that the intensity ratio ε is obtained as follows: 
   
     
       
         
           ɛ 
           = 
           
             
               
                 { 
                 
                   B 
                   
                     
                       ( 
                       
                         B 
                         - 
                         1 
                       
                       ) 
                     
                     ⁢ 
                     Q 
                   
                 
                 } 
               
               2 
             
             ⁢ 
             
               
                 ( 
                 
                   
                     1 
                     - 
                     
                       10 
                       
                         D 
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                         5 
                       
                     
                   
                   2 
                 
                 ) 
               
               . 
             
           
         
       
     
   
   An optical transmission method of the present invention transmits signal light from an optical transmission module to a transmission path. The optical transmission method includes the steps of: outputting a modulation signal; performing electrical/optical conversion on the modulation signal to generate signal light with a predetermined extinction ratio and outputting the generated signal light; and attenuating light propagating between the optical transmission module and the transmission path by a predetermined attenuation factor, thereby attenuating the signal light by the predetermined attenuation factor, and outputting the attenuated signal light to the transmission path. 
   The predetermined attenuation factor may be set based on at least the predetermined extinction ratio. For example, in the case where the transmission path has formed there on a reflection portion having a predetermined reflection coefficient, if the reflection coefficient of the reflection portion is A, the extinction ratio is B, a signal Q factor of the signal light is Q, a tolerable penalty for a bit error rate is D, and the predetermined attenuation factor is C, the attenuation factor C is set as follows: 
   
     
       
         
           C 
           ≈ 
           
             
               B 
               
                 
                   ( 
                   
                     B 
                     - 
                     1 
                   
                   ) 
                 
                 · 
                 Q 
               
             
             · 
             
               
                 
                   
                     1 
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                       10 
                       
                         
                           - 
                           D 
                         
                         / 
                         5 
                       
                     
                   
                   
                     2 
                     · 
                     A 
                   
                 
               
               . 
             
           
         
       
     
   
   Alternatively, the modulation signal may be generated so as to have a bias level and modulation amplitude determined based on an intensity of the signal light and an intensity of at least a portion of reflection light reflected by the transmission path, the electrical/optical conversion may be performed on the modulation signal to generate signal light with an extinction ratio determined by the bias level and the modulation amplitude, and the predetermined attenuation factor may be adjusted based on the intensity of the signal light and the intensity of the reflection light. 
   The predetermined attenuation factor may be adjusted such that an intensity ratio of the intensity of the reflection light to the intensity of the signal light becomes a predetermined value, and the bias level and the modulation amplitude of the modulation signal may be adjusted such that the extinction ratio becomes constant. Specifically, the predetermined attenuation factor is adjusted such that the intensity ratio becomes a value based on the extinction ratio. For example, in the case where the intensity ratio is ε, the extinction ratio is B, a signal Q factor of the signal light is Q, and a tolerable penalty for a bit error rate is D, the predetermined attenuation factor is adjusted such that the intensity ratio ε is obtained as follows: 
   
     
       
         
           ɛ 
           = 
           
             
               
                 { 
                 
                   B 
                   
                     
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                         B 
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                       10 
                       
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                   2 
                 
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               . 
             
           
         
       
     
   
   In the optical transmission module of the present invention, the loss generator, which has an attenuation factor determined based on an extinction ratio of a modulation signal, is provided to attenuate reflection light in a transmission path, thereby making it possible to provide a low-cost optical transmission module, which ensures a practically satisfactory efficiency, without providing an optical isolator component. Also, in the optical transmission module, the intensity of light is adjusted by the loss generator. Thus, it is possible to realize the optical transmission module using an optical mounting technique with such accuracy as employed for an ordinary module. 
   In the case where the attenuation factor of the loss generator is set based on an extinction ratio, etc., the attenuation factor can be determined based on a known value(s), making it possible to readily set the attenuation factor of the loss generator. 
   Also, in the case where the attenuation factor of the loss generator and a bias level and modulation amplitude of a modulation signal are set based on the intensity of output light and the quality of reflection light, the attenuation factor can be automatically adjusted so as to attenuate the reflection light in the transmission path. Also, it is possible to stabilize an output of the optical transmission module. 
   Also, in the case where the intensity ratio of the intensity of the reflection light to the intensity of the output light is adjusted so as to become a value based on, for example, an extinction ratio, the attenuation factor can be set so as to be consistent with a target value based on a known value. Thus, it is possible to readily set the target value of the attenuation factor of the loss generator, making it possible to realize a low-cost optical transmission module, which ensures a practically satisfactory efficiency, without providing an optical isolator component. 
   Also, the optical transmission method of the present invention achieves an effect similar to that achieved by the above-described optical transmission module. 
   These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram showing a schematic structure of an optical transmission/reception system including an optical transmission module according to a first embodiment of the present invention; 
       FIG. 2  is a diagram used for explaining how an optical signal propagates through the optical transmission/reception system shown in  FIG. 1 ; 
       FIG. 3  is a block diagram showing a schematic structure of an optical transmission/reception system including an optical transmission module according to a second embodiment of the present invention; 
       FIG. 4  is a schematic view showing an exemplary structure of a variable loss generator  18  shown in  FIG. 3 ; 
       FIG. 5  is a schematic view showing another exemplary structure of the variable loss generator  18  shown in  FIG. 3 ; 
       FIG. 6  is a block diagram showing a schematic structure of the optical transmission/reception system; and 
       FIG. 7  is a diagram used for explaining how an optical signal propagates through the optical transmission/reception system shown in  FIG. 6 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   First Embodiment 
   Referring to  FIGS. 1 and 2 , an optical transmission/reception system including an optical transmission module according to a first embodiment of the present invention is described.  FIG. 1  is a block diagram showing a schematic structure of the optical transmission/reception system including the optical transmission module according to the first embodiment, and  FIG. 2  is a diagram used for explaining how an optical signal propagates through the optical transmission/reception system shown in  FIG. 1 . 
   In  FIG. 1 , the optical transmission/reception system includes a drive unit  1 , a semiconductor laser  2 , an optical guide  3 , a loss generator  4 , a transmission path  5 , and an optical receiver  6 . The drive unit  1 , the semiconductor laser  2 , the optical guide  3 , and the loss generator  4  compose an optical transmission module  10   a  according to the first embodiment. 
   The drive unit  1  receives an input signal Si, and converts it into a modulation signal Sm having a predetermined extinction ratio B to directly intensity-modulate the semiconductor laser  2 . The semiconductor laser  2  performs electrical/optical conversion on the modulation signal Sm to generate signal light Ls 1 . The signal light Ls 1  is outputted from the semiconductor laser  2  to be optically coupled to the optical guide  3 . The loss generator  4  is provided between the optical guide  3  and the transmission path  5  to attenuate light propagating therethrough at a predetermined attenuation factor C. For example, the loss generator  4  is composed of an existing optical attenuator or a material having a low transmittance. The signal light Ls 1  having been optically coupled to the optical guide  3  experiences attenuation by the attenuation factor C within the loss generator  4  before being transmitted from the optical transmission module  10   a  to an input end of the transmission path  5 . Then, after propagating through the transmission path  5 , the signal light Ls 1  is outputted as signal light Ls 2  from an output end of the transmission path  5 , and received by the optical receiver  6 . The optical receiver  6  detects the signal light Ls 2  outputted from the transmission path  5 . 
   Here, it is assumed that a reflection point X having a predetermined reflection coefficient γ1 is formed on the transmission path  5  due to influences of connection portions with optical connectors, etc. As shown in  FIG. 2 , the signal light Ls 1  having been inputted to the transmission path  5  propagates on the transmission path  5 , and is partially reflected at the reflection point X. While a portion of the signal light Ls 1  that has not been reflected at the reflection point X is inputted as direct light Ld into the optical receiver  6 , the signal light Ls 1  propagating on the transmission path  5  is partially reflected at the reflection point X with the reflection coefficient γ1 to propagate, as reflection light Lr, on the transmission path  5  in a direction opposite to that of the signal light Ls 1  (i.e., a direction toward the semiconductor laser  2 ). 
   The reflection light Lr reflected at the reflection point X propagates on the transmission path  5  in the above-mentioned opposite direction, and is attenuated by the attenuation factor C within the loss generator  4 . The attenuated light is outputted through the optical guide  3  to the semiconductor laser  2 . The semiconductor laser  2  has a limited number of reflection points having a predetermined reflection coefficient γ2 on an output end surface of a semiconductor laser element and a distribution mirror within an active region thereof. Accordingly, multiple reflection occurs at the reflection points of the semiconductor laser  2 , so that the reflection light Lr having been outputted toward the semiconductor laser  2  is re-reflected at the reflection points with the reflection coefficient γ2 toward the optical guide  3 . Then, the reflection light Lr is further attenuated by the attenuation factor C within the loss generator  4  before propagating through the transmission path  5  (as indicated by broken lines in  FIG. 2 ). Then, the reflection light (multiplexed reflection light) Lr multiplexed with the direct light Ld is outputted as signal light Ls 2  together with the direct the light Ld from an output end of the transmission path  5  to the optical receiver  6 . Accordingly, the optical receiver  6  detects the direct light Ld and the multiplexed reflection light Lr from the signal light Ls 2 . 
   In the case where frequency noise is RIN(f), a signal bandwidth of a baseband signal is Bw, and a line width of the semiconductor laser is Δν, the following expression (1) gives a total amount N of relative intensity noise caused to occur within the signal bandwidth when the optical receiver  6  carries out square-law detection of direct light Ld and multiplexed reflection light Lr, which is generated between two reflection points (with reflection points γ1 and γ2). 
                 N   =         ∫   0   Bw     ⁢       RIN   ⁡     (   f   )       ⁢           ⁢     ⅆ   f         =       4   π     ⁢     Arctan   ⁡     (     Bw     Δ   ⁢           ⁢   v       )       ⁢   γ   ⁢           ⁢     1   ·   γ     ⁢           ⁢   2               (   1   )               
In particular, where Δν&lt;&lt;Bw, the expression (1) is transformed into the following expression (2).
   N≈ 2·γ1·γ2  (2) 
   As is apparent from the above expression (2), the total amount N of the relative intensity noise is determined by a product of the reflection coefficients γ1 and γ2 of the two reflection points, i.e., an optical intensity ratio between the direct light Ld and the multiplexed reflection light Lr. Here, the loss generator  4  with the attenuation factor C is provided between the transmission path  5  and the optical guide  3 , so that the multiplexed reflection light Lr is caused to pass through the loss generator three times, making it possible to reduce the multiplexed reflection light Lr by C 2  as compared to the direct light Ld. This allows an optical transmission element to increase an intensity ratio of the direct light Ld to the multiplexed reflection light Lr, making it possible to reduce the amount of interferential noise occurrence in the optical receiver  6 . In this case, the total amount N of the relative intensity noise can be obtained by the following expression (3).
 
 N≈ 2·γ1·γ2 · C   2   (3)
 
   The transmission quality of the optical transmission/reception system can be given based on a penalty D for a transmitter bit error rate. If a signal Q factor is Q, the transmission quality is given by the following expression (4). 
                 D   ≈         -   5     ·   Log     ⁢       {     1   -       (     B     B   -   1       )     2       }     ·     Q   2     ·   N               (   4   )               
In the case where a tolerable penalty for ensuring a practically sufficient efficiency in the optical transmission/reception system is Ds, a reflection coefficient at the reflection point X is γ1, the extinction ratio of the modulation signal Sm is B, the signal Q factor is Q, and total reflection occurs at a reflection point in the semiconductor laser  2  (i.e., γ2=1), the attenuation factor C of the loss generator  4  can be obtained by the following expression (5).
 
                 C   ≈       B       (     B   -   1     )     ·   Q       ·         1   -     10       -   Ds     /   5             2   ·   γ     ⁢           ⁢   1                   (   5   )               
By providing the loss generator  4  having the attenuation factor C obtained by the above expression (5), it is made possible to satisfy the tolerable penalty Ds, thereby ensuring the practically satisfactory efficiency in the optical transmission/reception system.
 
   As such, in the optical transmission module according to the first embodiment, a loss generator, which has an attenuation factor determined based on an extinction ratio of a modulation signal, is provided for attenuating reflection light in a transmission path, thereby making it possible to provide a low-cost optical transmission module, which ensures a practically satisfactory efficiency, without providing an optical isolator component. Also, in the optical transmission module, the intensity of light is adjusted by the loss generator. Thus, it is possible to realize the optical transmission module using an optical mounting technique with such accuracy as employed for an ordinary module. 
   Second Embodiment 
   Referring to  FIG. 3 , an optical transmission/reception system including an optical module according to a second embodiment of the present invention is described. In the optical transmission module described in the first embodiment, a loss generator having a predetermined attenuation factor is provided for maintaining the transmission quality related to multiple reflection noise. However, signal light outputted from the optical transmission module is attenuated by the loss generator. The optical transmission/reception system described in the second embodiment stabilizes an output from the optical transmission module.  FIG. 3  is a block diagram showing a schematic structure of the optical transmission/reception system. 
   In  FIG. 3 , the optical transmission/reception system includes a drive unit  11 , a semiconductor laser  12 , an output light detection unit  13 , a reflection light detection unit  14 , a comparison unit  15 , a variable loss generator  18 , the optical guide  3 , and the transmission path  5 . The comparison unit  15  includes first and second control units  16  and  17 . The drive unit  11 , the semiconductor laser  12 , the output light detection unit  13 , the reflection light detection unit  14 , the comparison unit  15 , the variable loss generator  18 , and the optical guide  3  compose an optical transmission module  10   b  according to the second embodiment. Note that an optical signal transmitted through the transmission path  5  is received by an optical receiver which is not shown. The optical guide  3  and the transmission path  5 , which are the same as those described in the first embodiment, are denoted by the same reference numerals, and the detailed descriptions thereof are omitted here. 
   The drive unit  11  receives an input signal Si, and converts it into a modulation signal Sm, which has a predetermined extinction ratio B and is biased based on a first control signal Cs 1  outputted from the first control unit  16 , for the purpose of directly intensity-modulating the semiconductor laser  12 . The semiconductor laser  12  performs electrical/optical conversion on the modulation signal Sm to generate signal light Ls 3 . Then, the signal light Ls 3  is outputted from a front end surface of the semiconductor laser  12 , and optically coupled to a signal light input end of the optical guide  3 . The intensity of light outputted from a rear end surface of the semiconductor laser  12  is detected by the output light detection unit  13 . The output light detection unit  13  outputs a result of the detected intensity of the outputted light to each of the first and second control units  16  and  17 . 
   The variable loss generator  18  is provided between the optical guide  3  and the transmission path  5  for attenuating light propagating therethrough by an attenuation factor C based on a second control signal Cs 2  outputted from the second control unit  17 . Hereinbelow, exemplary structures of the variable loss generator  18  are described with reference to  FIGS. 4 and 5 .  FIG. 4  is a schematic view showing an exemplary structure of the variable loss generator  18 .  FIG. 5  is a schematic view showing another exemplary structure of the variable loss generator  18 . 
   In the exemplary structure of the variable loss generator  18  shown in  FIG. 4 , a liquid crystal element is provided such that an optical axis between the optical guide  3  and the transmission path  5  passes therethrough. The liquid crystal element of the variable loss generator  18  is composed of a pair of transparent electrode layers  181  and  182  and a liquid crystal layer  183 . The transparent electrode layers  181  and  182  are provided on opposite surfaces of the liquid crystal layer  183 . The transparent electrode layer  181  is adapted to be supplied with a second control signal Cs 2 . The transparent electrode layer  182  is grounded. Light transmittance of the liquid crystal layer  183  is changed based on the second control signal Cs 2  outputted from the second control unit  17 , thereby controlling the amount of loss of light transmitted through the variable loss generator  18  (i.e., the attenuation factor C of the variable loss generator  18 ). 
   In the exemplary structure of the variable loss generator  18  shown in  FIG. 5 , an optical filter  185  is provided such that the optical axis between the optical guide  3  and the transmission path  5  passes therethrough. The optical filter  185  is mounted on a moving element  184  such as an actuator. The optical filter  185  is movable in a predetermined direction (e.g., a direction perpendicular to the optical axis; a direction indicated by M in  FIG. 5 ) in accordance with the second control signal Cs 2  applied to the moving element  184 . The optical filter  185  is formed such that an optical attenuation factor is increased/decreased stepwise in the moving direction. Alternatively, the optical filter  185  may be composed of a plurality of optical filters provided in the moving direction, which are gradually different in optical attenuation factor. The moving element  184  moves the optical filter  185  in accordance with the second control signal Cs 2  outputted from the second control unit  17 , thereby controlling the amount of loss of light transmitted through the variable loss generator  18  (i.e., the attenuation factor C of the variable loss generator  18 ). 
   The signal light Ls 3  having been optically coupled to the optical guide  3  is attenuated by the variable loss generator  18  with the attenuation factor C, and then transmitted from the optical transmission module  10   b  to the input end of the transmission path  5 . The signal light Ls 3  propagating through the transmission path  5  is outputted from the output end of the transmission path  5 , and received by an optical receiver (not shown). 
   Here, it is assumed that a reflection point X with a predetermined reflection coefficient γ1 is formed on the transmission path  5  due to influences of connection portions with optical connectors, etc. Accordingly, the signal light Ls 3  having been inputted to the transmission path  5  is partially reflected at the reflection point X while propagating on the transmission path  5 . A portion of the signal light Ls 3  that has not been reflected at the reflection point X is inputted as direct light into the optical receiver. The signal light Ls 3  propagating on the transmission path  5  is partially reflected at the reflection point X with the reflection coefficient γ1 to propagate, as reflection light Lr, along the transmission path  5  in a direction opposite to that of the signal light Ls 3  (i.e., a direction toward the variable loss generator  18 ). 
   The reflection light Lr having been reflected at the reflection point X propagates through the transmission path  5  in the opposite direction to be attenuated by the variable loss generator  18  with the attenuation factor C before being outputted from a signal light input end of the optical guide  3 . The intensity of the reflection light Lr having been outputted from the signal light input end of the optical guide  3  is partially detected by the reflection light detection unit  14 . The reflection light detection unit  14  outputs a result of the detected intensity of the reflection light to the second control unit  17 . Although  FIG. 3  shows that the semiconductor laser  12  is arranged in a direction oblique to the optical axis of the optical guide  3  and the reflection light detection unit  14  is arranged on the optical axis, the present invention is not limited to such arrangements. The semiconductor laser  12  and the reflection light detection unit  14  can be arranged in other positions so long as the signal light Ls 3  outputted from the semiconductor laser  12  is inputted into the signal light input end of the optical guide  3  and the reflection light Lr outputted from the signal light input end is partially inputted into the reflection light detection unit  14 . 
   In the comparison unit  15 , the first control unit  16  and the second control unit  17  generate a first control signal Cs 1  and a second control signal Cs 2  based on results of light intensities detected by the output light detection unit  13  and the reflection light detection unit  14 . The first control signal Cs 1  generated by the first control unit  16  is outputted to the drive unit  11 , while the second control signal Cs 2  generated by the second control unit  17  is outputted to each of the variable loss generator  18  and the first control unit  16 . Hereinbelow, operations of the first and second control units  16  and  17  will be described. 
   The second control unit  17  controls the attenuation factor C of the variable loss generator  18  such that a ratio of the intensity of output light detected by the output light detection unit  13  to the intensity of output light detected by the reflection light detection unit  14  becomes such a value as to ensure a practically satisfactory efficiency in the optical transmission/reception system. In the case where a tolerable penalty for ensuring the practically satisfactory efficiency in the optical transmission/reception system is Ds, the reflection coefficient of the reflection point X is γ1, the extinction ratio of the modulation signal Sm is B, the signal Q factor is Q, and the reflection light Lr outputted from the signal light input end of the optical guide  3  is totally reflected to return to the signal light input end of the optical guide  3 , the second control unit  17  controls the attenuation factor C such that the ratio ε (the intensity of reflection light detected by the reflection light detection unit  14 /the intensity of output light detected by the output light detection unit  13 ) satisfies the following expression (6). 
                 ɛ   =           kC   2     ·   γ     ⁢           ⁢   1     =           {     B       (     B   -   1     )     ⁢   Q       }     2     ⁢     (       1   -     10     Ds   /   5         2     )       ∝   B               (   6   )               
The second control unit  17  outputs the second control signal Cs 2 , such that the ratio ε constantly satisfies the above expression (6), thereby carrying out feedback control of the attenuation factor C of the variable loss generator  18 . The ratio ε is set such that the tolerable penalty Ds is satisfied, and therefore the above control operation by the second control unit  17  constantly maintains the transmission quality related to multiple reflection noise. As is apparent from the above expression (6), the ratio ε is set based on the known extinction ratio B of the drive unit  11 .
 
   The first control unit  16  controls the drive unit  11 , such that light is outputted with consideration of attenuation by the variable loss generator  18 , based on the attenuation factor C controlled by the second control unit  17  and the intensity of output light detected by the output light detection unit  13 . Specifically, the first control unit  16  controls a bias level and modulation amplitude of the modulation signal Sm based on the attenuation factor controlled by the second control unit  17 , and outputs the first control signal Cs 1  to control the drive unit  11  such that the extinction ratio B determined by the bias level and the modulation amplitude of the modulation signal Sm becomes constant. The above control operation by the first control unit  16  maintains the transmission quality related to multiple reflection noise, thereby stabilizing the intensity of signal light outputted from the optical transmission module  10   b.    
   As such, in the optical transmission module according to the second embodiment, the loss generator having an attenuation factor obtained based on an extinction ratio of a modulation signal is provided to attenuate reflection light in a transmission path, thereby making it possible to realize a low-cost optical transmission module, which ensures a practically satisfactory efficiency, without providing an optical isolator component. Also, in the optical transmission module, the attenuation factor is automatically adjusted so as to attenuate reflection light in the transmission path, based on the intensity of reflection light detected in the transmission path and the extinction ratio of the modulation signal. Moreover, in the optical transmission module, the amount of light emission and the attenuation factor are detected to control the output light intensity such that light is outputted with consideration of attenuation, thereby stabilizing the output of the optical transmission module. 
   While the invention has been described in detail, the foregoing description is in all aspects illustrative and not restrictive. It is understood that numerous other modifications and variations can be devised without departing from the scope of the invention.