Abstract:
The present invention provides an optical data link capable of compensating the fluctuation due to the tracking error. The data link of the invention provides a look-up-table in which a plurality of control data, such as the bias and modulation current and the loop gain of the auto-power-control loop to make the average power and the extinction ratio of the optical output from the data link, not from the laser diode within the data link constant in the preset values, is stored in connection to temperatures. During the operation, the controller within the data link reads out these data to control the laser diode.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to an optical data link, in particular, the optical data link having a digitally controlled optical transmitting module.  
       RELATED PRIOR ART  
       [0002]     In general, the optical transmitting module, a semiconductor laser diode (LD) in the optical transmitting module is controlled in an optical output thereof by an automatic power control (APC). A photodiode (PD) monitors the optical output from the LD and the bias and modulation currents supplied to the LD is controlled so as to maintain the optical output thereof constant based on a signal output from the PD. Such technique has been disclosed, for example Japanese patent application published as 2003-218460.  
         [0003]     A recent optical data link usually provides a function to communicate with the host system, in which the host system may read out the information that relates to the operation of the data link including the optical output power of the LD therefrom, which has been ruled in, for example, a multi source agreement (MSA) for the SFF-transceiver (SFF-8472 Specification for Diagnostic Monitoring Interface for Optical Xcvrs Rev. 9.5 &lt;URL: ftp://ftp.Seagate.com/sff/SFF-8472.PDF&gt;.  
         [0004]     On the other hand, the optical output from the data link is extracted through an optical coupling system that connects the LD to an optical connector provided in the data link. The conventional APC loop using the PD is primarily based on the optical output from the LD, not through the optical coupling system, to adjust the bias and modulation currents for the LD. Since the optical coupling system shows the temperature dependence in the coupling efficiency thereof, primarily due to a positional deviation of the optical elements in the coupling system, the magnitude of the optical output from the data link may change as the temperature of the data link changes. Accordingly, even the APC loop keeps the optical output of the LD constant, the optical output from the data link via the coupling system fluctuates as the temperature changes. This phenomenon is called as the tracking error. The present invention is to provide an optical data link that overcomes the tracking error.  
       SUMMARY OF THE INVENTION  
       [0005]     One aspect of the present invention relates to a method for controlling an optical output of an optical data link. The data link comprises an LD, an optical coupling system, a PD, and a controller. The coupling system guides light emitted from the laser diode to an outside of the optical data link. The photodiode monitors the light of the laser diode. The controller, constituting the APC loop cooperating with the LD and the PD, provides a control look-up-table (LUT) in which information to make the average power and the extinction ratio of the optical output from the data link, not the light emitted from the LD, constant and another information to adjust the loop gain of the APC. This information is stored within the controller in connection with temperatures of the data link.  
         [0006]     The method according to the invention comprises steps of: (a) setting a target temperature in the controller, (b) reading information from the LUT to make the average power and the extinction ratio of the output from the data link constant, and setting current supplied to the LD based on this information, (c) reading another information to adjust the loop gain and setting the loop gain based on the another information, and (d) starting the APC loop.  
         [0007]     Since the controller first sets the bias and modulation currents such that the average power and the extinction ratio of the optical output from the data link are maintained, and secondly sets the loop gain of the APC loop, the tracking error can be cancelled. The APC loop may includes steps of: (i) comparing the average power of the light emitted from the LD with a reference value, (ii) revising the target temperature by adding a temperature difference obtained by multiplying a specific coefficient by this comparison result to the present target temperature, (iii) reading new information form the LUT in connection with the revised target temperature, and (iv) setting new current condition to the LD. A new set of the information to make the average power and the extinction ratio of the optical output from the data link constant is always read out from the LUT during the APC operation. Accordingly, the present data link may cancel the tracking error even when the average power of the light emitted from the laser diode changes.  
         [0008]     Even when the LUT provides the information in connection with sparse temperatures and the new target temperature does not match the temperatures saved within the LUT, the controller may interpolate or extrapolate the information and may obtain the new set of information corresponding to the new target temperature. Accordingly, the controller is unnecessary to provide a large size of the LUT.  
         [0009]     The loop gain of the APC loop may be adjusted by setting the conversion efficiency of the photodiode. The photodiode generates a photocurrent corresponding to the average power of the light emitted from the LD, and converts this photocurrent into a voltage signal. The loop gain of the APC loop may be adjusted by changing this conversion efficiency from the photo current into the voltage signal. The PD may provide a load resistor connected in serial thereto. The photocurrent may flow in this load resistor. Therefore, to change the resistance of this load resistor may change the conversion efficiency of the PD.  
         [0010]     Another aspect of the present invention relates to a method for manufacturing the optical data link. The method may comprise: (a) shutting the APC loop off and setting an ambient temperature of the data link to a preset temperature, (b) setting the bias and modulation currents for the LD such that the optical output from the data link shows a predetermined average power and a predetermined extinction ratio, (c) setting the conversion ratio of the PD to match the average power of the light directly from the LD with a reference value; (d) iterating the processes (b) and (c) as changing the preset temperature; and (e) creating the LUT by arranging the bias and modulation currents and the conversion ratio against the preset temperatures.  
         [0011]     The present method for manufacturing the data link, the optical output from the data link via the optical coupling system, not directly from the LD, are monitored and the bias and modulation currents are adjusted to make the average power and the extinction ratio of the optical output constant. Accordingly, the bias and modulation currents thus obtained compensate the tracking error.  
         [0012]     When the preset temperatures are sparse, the extrapolation or the interpolation of the bias and modulation currents and the conversion ratio may create a plurality of new set of data with a dense interval of temperatures.  
         [0013]     Still another aspect of the present invention relates to an optical data link. The data link comprises a LD, a PD, an optical coupling system, and a controller. The coupling system guides the light emitted from the LD to an outside of the data link, and may include a condenser lens. The coupling system may cause the tracking error because the optical coupling efficiency between the LD and an optical connector, provided in the data link to extract the light therefrom, that constitute the coupling system fluctuates as the temperature varies due to a positional deviation of members including the coupling system. The controller, which constitutes the APC loop cooperating with the LD and the PD, includes the LUT that saves a plurality of paired data of bias and modulation currents and a plurality of information relating to the APC loop in connection with temperatures. The paired bias and modulation currents are decided such that the average power and the extinction ratio of the optical output from the data link constant, while the APC loop information adjusts the loop gain thereof.  
         [0014]     The paired data of the bias and modulation currents is determined to make the average power and the extinction ratio of the optical output from the data link constant, not from the LD. Accordingly, the tracking error caused by the positional deviation due to the temperature varying of the optical coupling system may be cancelled. The present invention may be applicable no matter what the optical coupling system is constituted.  
         [0015]     Other features and advantages of the present invention will be apparent from the accompanying drawings and from the detailed description that follows below. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0016]      FIG. 1  is a schematic diagram showing the optical data link according to the present invention;  
         [0017]      FIG. 2  explains the structure of the look-up-table (LUT) storing the bias and modulation levels;  
         [0018]      FIG. 3  explains the structure of the LUT storing the resistance of the variable resistor;  
         [0019]      FIG. 4  schematically shows the testing of the data link at the factory; and  
         [0020]      FIG. 5  is a flow chart for the testing of the data link.  
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0021]     Next, preferred embodiments of the present invention will be described as referring to accompanying drawings. In the explanation of drawings and in the specification, the same symbols or numerals will refer to the same element without overlapping explanations.  
         [0022]      FIG. 1  is a schematic diagram of an optical data link  10  according to the present invention. As shown in  FIG. 1 , the data link  10  configures to be connected with the host system  15 . The data link  10  provides input terminals,  41  and  42 , to receive signals therein from the host system  15 . These terminals,  41  and  42 , receive signals, E IN+  and E IN− , which are complementary to each other.  
         [0023]     The data link  10  provides a transmitting optical subassembly (hereinafter denoted as TOSA)  12  that outputs light O OUT  to the outside of the data link  10  and includes a laser diode  14  (LD), an optical coupling system  16 , an optical connector  18 , and a photodiode (PD).  
         [0024]     The LD  14  optically couples with the optical connector via the coupling system  18 . The light O LD  emitted from the LD, probably from a front facet of the LD, couples with the optical connected  18  via the coupling system  16 , and outputs to the outside from the optical connector  18  as an optical output O OUT . The optical connector  18  operates as an optical output port of the data link  18 , and the coupling system guides the light emitted from the LD  14  to this output port. Mating an optical plug provided in a tip of an external optical fiber, the optical output O OUT  may be transmitted in the optical communication system via the optical fiber  21 .  
         [0025]     Both the anode of the LD  14  and the cathode of the PD  20  are connected to a power supply Vcc. The power supply Vcc biases the LD  14  in forward, while biases the PD  20  in reverse. The PD  20  detects light emitted from the LD  14 , generally the light emitted from the rear facet of the LD  14  when the LD  14  is an edge-emitting type, and generates a photo current corresponding to the magnitude of the monitored light, which generally correlates with the front facet light O LD . Since the response of the PD  20  used in the TOSA  12  is generally inferior to that of the LD  14 , the PD  20  outputs the photo current corresponding to an average of the light O LD .  
         [0026]     The cathode of the LD  14  connects to a driver  22 , which is called as an LD-Driver. The LD-Driver  22 , receiving the complementary signals, E IN+ and E IN− , drives the LD  14  to output the light O LD  to follow this complementary data. The LD-Driver  22  provides a bias current I B , which is a DC current, and a modulation current I M  to the LD  14 . The modulation current I M  is modulated by the complementary signals, E IN+  and E IN− , received via respective coupling capacitors,  23  and  24 . The LD-Driver  22  superposes this modulation current I M  with the bias current I B , and provides thus merged currents to the LD  14 . The LD  14  is driven by these bias current I B  and modulation current I M , and output the signal light O LD  modulated by the modulation current I M .  
         [0027]     The LD-Drive  22  connects to the controller  28  via two digital-to-analog converters (hereinafter denoted as the D/A-C),  25  and  26 . The controller  28  stabilizes the optical output O LD  of the LD  14  by carrying out the APC. The APC compares the optical output O LD  monitored by the PD  20  with a reference, and adjusts the bias and modulation currents, I B  and I M , depending on this comparison. The controller  28  sets analogue signals corresponding to the bias I B  and modulation currents I M in the LD-Driver  22  via two D/A-Cs,  25  and  26 . The LD-Driver  22  adjusts the magnitude of the currents, I B  and I M , as responding these signals from the controller  28 .  
         [0028]     The controller  28  connects a temperature sensor  31 , which is typically a thermistor, to monitor an internal temperature of the data link  10 . The internal temperature reflects or corresponds to the temperature of the LD  14 . The temperature sensor generates an analog signal corresponding to this internal temperature and sends the sensed signal to the controller  28  via the A/D-C  29 . The A/D-C  29  converts this analog signal into a digital value V T , which will be called as a temperature-monitor signal.  
         [0029]     The controller  28  further connects a variable resistor  32  via an A/D-C  30 . The variable resistor  32  is put between the anode of the PD  20  and the ground. The controller  28  may adjust the resistance of this variable resistor  32 .  
         [0030]     The photo current generated by the PD  20  flows in the variable resistor  32  and causes a voltage drop depending on the resistance R of the variable resistor  32 . Thus, this voltage drop reflects the photo current from the PD  20 . The variable resistor  32  operates as a load resistor for the PD  20 . As described later, the resistance R of the variable resistor  32  is adjusted by the controller  28  based on the LUT  54  created within the non-volatile memory in the controller.  
         [0031]     The A/D-C  30  connects to a node  50  between the PD  20  and the resistor  32 . The A/D-C  30  converts the analog signal thus caused in the resistor  32  into a corresponding digital value V P  to output to the controller  28 . The signal V P  reflects the photo current from the PD  20  and, consequently, corresponds to the optical output O LD  from the LD  14 . The V P  will be called as a power-monitor signal.  
         [0032]     The controller  28  carries out the APC based on this power-monitor signal V P . That is, the LD  14 , the PD  20 , the resistor  32 , the controller  28 , and the LD-Driver  22  constitutes a feedback loop, by which the output power from the LD  14  is controlled based on the power-monitor signal V P  from the PD  20  with a variable loop gain determined by the resistance R of the resistor  32 . The resistance R or the variable resistor  32  may be determined by the controller based on the temperature-monitor signal V T .  
         [0033]     The controller  28  also connects to the host system  15  via a serial port  43 . The controller  28  receives commands and, by responding the commands, sends data regarding to the operation of the data link  10  via the serial port  43  to the host system  15 . Such data sent to the host system  15  is typically the temperature-monitor signal V T  and the power-monitor signal V P .  
         [0034]     The controller  28  also provides a RAM  27  and a ROM  33 . The RAM  27  is a primary memory for the controller  28  to execute the tasks such as the APC. The RAM  27  stores the power-monitor signal V P  and the temperature-monitor signal V T , while the RCM  33  sets the LUT  53  illustrated in  FIG. 2 . The LUT  53  includes a plurality of paired digital values, B and M, each corresponding to the modulation I M  and bias I B  currents, respectively, in connection with various and different temperatures, V T1 , V T2 , . . . , V TN . In  FIG. 2 , the bias and modulation levels, B and M, are subscripted with the same symbol as those or the temperatures from V T1  to V TN , which indicate the inner temperature of the data link  10 . The temperatures from V T3  to V TN  may set with a constant width, for instance 2° C. The paired values of the bias and modulation level, B i  and M i , are defined such that, when the inner temperature of the data link  10  becomes the value V T1 , the data link  10  outputs the light O OUT  with a predefined average power and an extinction ratio as stopping the APC loop by supplying the bias and modulation currents, I B  and I M , which corresponds to the bias and modulation levels, B i  and M i .  
         [0035]     The controller may further provide outer ROMs,  34  and  35 , such as electrical erasable and programmable read only memory (EEPROM) that are re-writable from the controller  28 . The first EEPROM  34  sets another LUT  54  illustrated in  FIG. 3 . The LUT  54  stores the resistance R 1 , R 2 , . . . R N  in connection with the temperatures V T1 , V T2 , . . . V TN . These values from R 1  to R N  are defined such that, when the inner temperature of the data link  10  becomes the corresponding value T i  and the bias and modulation levels, B i  and M i , linked to the temperature T i  are set, the power-monitor signal V P  becomes a prescribed value. As shown in later in the present specification, this prescribed value is served as a reference value V R  for the APC loop. The APC loop compares the power-monitor signal V P  with the reference value V R , namely, with this prescribed value, and adjusts the bias and modulations currents, I B  and I M , based on the comparison. The values or the resistance, R 1  to R N , are measured at the delivery inspection in advance to the shipment.  
         [0036]     The host system may instruct the controller  28  via the serial port  43  to write the data into the nonvolatile memories, from  33  to  35 . The controller may behave as a memory controlling circuit to rewrite the nonvolatile memories,  33  to  35 , by responding to the command from the host system  15 .  
         [0037]     Next, the operation of the data link  10  will be described. Starting the data link  10 , the controller  28  sets the predefined initial temperature V TINT  in advance to the operation of the APC. The initial temperature V TINT  is one of the temperatures from V T1  to V TN  stored in the LUTs,  53  and  54 , and the controller generally selects the room temperature, 25° C. The controller  28  reads out the bias and modulation levels, B INT  and M INT , in connection with the initial temperature V TINT  from the LUT  53 , and the initial resistance RIB from the LUT  54  Subsequently, the controller  28  adjusts the bias and modulation currents, I B  and I M , according to the corresponding levels, B INT  and M INT , and sets the resistance R INT  of the variable resistor  32 .  
         [0038]     Thus, two levels, B INT  and M INT , and the resistance R INT  are read out from the LUTs,  53  and  54 , for the data link  10  to show the predefined the average power and extinction ratio. The bias and modulation levels, and the resistance to maintain the extinction ratio of the data link  10  depend on the inner temperature of the data link  10 . Accordingly, the bias and modulation levels and the resistance, by which the desired extinction ratio and the average power are set, are obtained in advance to the practical operation of the data link  10  at several temperatures and stores within the LUTs,  53  and  54 . Selecting two levels, B i  and M i , and the resistance R i  from the LUTs,  53  and  54 , corresponding to the preset temperature T INT , the initial condition of the APC loop may be determined.  
         [0039]     Subsequently, the controller  28  starts the APC loop and adjusts the bias and modulation levels to match the power-monitor signal V P  with the predefined reference value V R . The APC loop is one type of a closed loop operation that compensates not only the temperature dependence of the LD  14  but also the temporal degradation thereof.  
         [0040]     The APC loop by the controller  28  will be described in detail. The controller  28  adjusts the target temperature of the LD  14  based on the comparison between the power-monitor signal V P  and the reference value V R . In one embodiment, the controller  28  may add the product of the difference between the power-monitor signal V P  and the reference value V R  multiplied by a specific co-efficient to the present target temperature. The controller  28  reads out the bias and modulation levels, B j  and M j , and the resistance R j  each corresponding to the revised target temperature T j  from the LUTs,  53  and  54 . Assuming the difference between the power-monitor signal V P  and the reference value V R  is          V, the specific co-efficient is k[T/V], and the present target temperature is T i , the revised target temperature T j  becomes; 
 
 T   j   =kV+T   i . 
 
 The controller  28  reads out the new bias and modulation levels, B j  and M j , from the LUT  53  and the revised resistance R j  from the LUT  54 , each corresponds to the revised target temperature T j , and sets these readout values in respective D/A-Cs,  25 ,  26  and  32 . When the interval of the temperatures set in the LUTs,  53  and  54 , is rough or sparse such as 2° C., and the revised target temperature does not get on the temperatures in the LUTs,  53  and  54 , it may be applicable to operate the APC loop by the bias and modulation levels and the resistance corresponding to a temperature closest to the target temperature T j , or to calculate the levels and the resistance by the interpolation or the extrapolation for the data in the LUTs,  53  and  54 . 
 
         [0041]     The controller  28  sets the revised bias level V Bj  and the revised modulation level V Mj  in the D/A-Cs,  25  and  26 , respectively, and sets the revised resistance R j  in the variable resistor  32 . These levels, V Bj  and V Mj , are provided to the LD-Driver  22  to adjust the bias and modulation currents, I B  and I M , respectively. The signal R j  sent from the controller  28  sets the resistance of the variable resistor  32  defines the conversion gain of the photo current within the APC loop.  
         [0042]     When the power-monitor signal V P  is enough greater than the reference value V R , the bias level smaller than the present bias level is selected from the LUT  53 , while the power-monitor signal V P  is far smaller than the reference V R , the bias level higher than the present level is selected. Thus, the optical output O OUT  from the data link  10  is stabilized. Since the pair of bias and modulation levels in the LUT  53  and the resistance in the LUT  54  are so set that not only the average optical output power but also the extinction ratio be substantially constant, the extinction ratio of the optical output O OUT  can be also stabilized.  
         [0043]     The data within the nonvolatile memory,  33  to  35 , are set before the shipment of the data link  10  by the manufacturer.  FIG. 4  shows a schematic configuration to set the data within the memories,  33  to  35 . Using a signal generator  60 , an optical power meter  62 , and an external controller  64  may carry out the adjustment at the factory.  
         [0044]     Two input terminals,  41  and  42 , of the data link  10  are connected to the output of the signal generator  60  to receive two test signals, E IN+  and E IN− , which may be, for example the pseudo random signals complementary to each other. The LD-Driver  22  drives the LD  14  based on this test signal to output the light O LD . This light O LD  may couple to the optical connecter  18  via the optical coupling system  16 .  
         [0045]     The optical power meter  62 , which is placed outside of the data link  10 , is a type of an optical detector coupled with the optical connector  18 . The power meter  62  receives the optical output O OUT  and generates an electrical signal V PE  corresponding to this optical output O OUT . This signal V PE  is sent to the external controller  64 . The PD  20  within the data link  10  detects the output O LD  directly from the LD  14 , while the optical power meter  62  monitors the light O OUT  output through the optical connector  10 .  
         [0046]     The external controller  64  provides an interface connected to the serial port  43  of the data link  10  and a memory  65 . This external controller  64  may send commands to the controller  28  of the data link  10  via the serial port  43  to adjust the bias and modulation levels and the resistance R. Also, the external controller  64  may start or stop the APC loop operated by the controller  28 .  
         [0047]     Next, the method for storing the paired data of the bias and modulation levels and the resistance into the LUTs,  53  and  54 , will be described as referring to  FIG. 5  that is a flow chart showing the procedure to get paired data of the bias and modulation levels.  
         [0048]     The process shown in  FIG. 5  obtains data necessary to operate the APC loop as the loop is halted. Specifically, the bias and modulation levels are so adjusted that, as the inner temperature of the data link  10  is sequentially set at the plurality of preset temperatures, the optical output O OUT  from the datalink shows the predetermined average power and extinction ratio under respective preset temperatures. As an example, the preset temperatures are 25, −10 and 60° C., respectively. As shown in  FIG. 5 , the external controller  64  stops the APC loop within the data link  10  as step S 502 , and the inner temperature is set to be one of these preset temperatures at step S 504 . During the inner temperature of the data link is adjusted, the external controller  64  receives the temperature-monitor signal V T  from the controller  28  within the data link  10  via the serial interface, and decides whether the inner temperature of the data link  10  becomes stable at the preset temperature.  
         [0049]     Next, the external controller  64  adjusts the bias and modulation levels to obtain the predetermined average power and extinction ratio at step S 506 . In this step, the signal generator  60  outputs the test signal to the data link  10 , and the data link  10  generates the optical output O OUT  based on this test signal. This optical output O OUT  is detected by the power meter  62 , and the external controller  64 , receiving from the result V PE  from the power meter  62 , evaluates the average power and the extinction ratio of the optical output O OUT , and adjusts the bias and modulation levels to match with the predetermined conditions.  
         [0050]     Subsequently, the external controller  64  adjusts the resistance R of the variable resistor  32  at step s 507  such that the output O LD  from the LD  14 , which is detected by the PD  20 , becomes the reference V R . When the average power and extinction ratio of the optical output O OUT  from the data link  10  under the APC loop being halted, the optical output O LD  from the LD  14  does not always match with the reference V R  because the optical output O OUT  is affected by the temperature characteristic of the optical coupling system  16 . This temperature dependence of the optical coupling system is generally called as the tracking error. When the power-monitor signal V P  shifts from the reference V R , the APC loop operates to cancel this shift by adjusting the bias and modulation levels. Consequently, the average power and extinction ratio of the optical output O OUT  from the data link deviates from the predetermined value. To compensate this tracking error, the resistance R of the variable resistor  32  is adjusted to match the power-monitor signal V P , which corresponds to the optical output O LD  from the LD  14 , with the reference value V R  as the LD  14  is operated by the bias and modulation levels adjusted at the step S 506 .  
         [0051]     Thus, the average power and extinction ratio of the optical output O OUT  of data link  10  matches with the predetermined conditions and the power-monitor signal V P  showing the optical output O LD  of the LD  14  matches with the reference value V R  through the sequence of steps from S 502  to S 506  at the preset temperature. However, the gain of the APC loop is changed because the resistance R of the variable resistor  32  is adjusted.  
         [0052]     The bias and modulation levels and the resistance thus obtained are stored in the memory  65  in connection with the preset temperature V T  at step S 508 . Subsequently, the inner temperature of the data link  10  is set to the next preset temperature at step S 504 , another bias and modulation levels and another resistance are obtained by the same procedure described above at step S 506 , and thus obtained levels and resistance are stored in the memory  65  at step S 508 . The same procedure as those described above will be iterated until the preset temperatures are exhausted at step S 510 .  
         [0053]     Completing the adjustment of the bias and modulation levels and the resistance under the whole preset temperatures, the paired bias and modulation levels and the resistance obtained through the adjustments above are sent to the nonvolatile memory within the data link  10  to build the LUTs,  53  and  54 . In the present embodiment, the extrapolation or the interpolation of the data stored in the memory  65  may create a plurality of new set of the data at temperatures different to the prescribed one. Thus, the plurality paired bias and modulation levels, B 1  to B n  and M 1  to M n , respectively, and the plurality of resistance, R 1  to R n , are may be obtained in connection with the plurality of the inner temperatures of the data link  10 . These sets of the paired bias and modulation levels, B i  and M i , and the resistance R i  controls the average power and the extinction ratio of the optical output O OUT  at the inner temperature T i  of the data link  10 . The external controller  64  creates the first LUT  53  where the paired bias and modulation levels are set in connection with the temperature, and the second LUT  54  where the resistance is set in connection with the temperature. The controller  64  transfers these LUTs to the controller  28  via the serial port  43 , and sends a command to the controller  28  to write the LUTs,  53  and  54 , onto the nonvolatile memories,  33  and  34 , respectively. Although the present embodiment constructs two LUTs in the independent memories,  33  and  34 , the bias levels, B 1  to B n , the modulation levels, M 1  to M n , and the resistance, R 1  to R n , may be gathered in connection with the temperatures, T 1  to T n , within the signal LUT.  
         [0054]     Although the present invention has been thus described based on the embodiment and accompanying drawings, the present invention is not restricted to those embodiments. For example, although two LUTs store the data in connection to the same temperatures, each LUT does not always refer the same temperatures. Moreover, the embodiment concentrates on the data link having only the optical transmitting function. The function of the present invention may apply to the optical transceiver that provides not only the optical transmitting function but also the optical receiving function.  
         [0055]     Although the embodiment above described selects one pair of the bias and modulation levels in the LUT  53 , the various method to select these two levels may be applicable to the present invention. For example, the controller  28  may set the revised bias and modulation levels corresponding to the revised temperature by the interpolation or the extrapolation of the data stored in the LUT  53 . Even such method is applied to set the revised condition, the optical output O OUT  from the data link  10  may maintain the predetermined average power and the extinction ration, because the entire data in the LUT  53  gives the predetermined conditions.