Abstract:
The optical transmitter according to the present invention is capable of reducing wavelength fluctuations of a semiconductor laser (laser diode) owing to the temperature dependence of the light reception sensitivity of the monitoring photodiode. The optical transmitter comprises an optical transmitting module, an LD driver, a temperature sensor, a memory, a compensation circuit, and a controller. The optical transmitting module has a laser diode and a photodiode that monitors the light from the laser diode mounted thereon. The compensation circuit obtains the parameter corresponding with the temperature of the photodiode sensed by the temperature sensor from the memory and uses the parameter to compensate the signal which is then supplied to the controller. The controller controls the driving current of the LD driver on the basis of the difference between the compensated signal and the reference level.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to an optical transmitter.  
         [0003]     2. Related Background Art  
         [0004]     An optical transmitter comprises an optical transmitting module of a type known as the so-called coaxial type. The coaxial-type optical transmitting module contains a thermoelectric controller (TEC) for keeping the temperature of the LD (Laser Diode) constant and is described in a Japanese Patent Application Published as No. 2003-142766A, for example. The optical transmitting module installs an LD on the heat-absorbing plate of the TEC and a monitoring photodiode (referred to simply as a ‘PD’ (photodiode) hereinbelow) that monitors light emitted from the LD.  
         [0005]     Further, in order to achieve the low power consumption, a PD with a large heat capacity is provided in a different portion from the heat-absorbing plate in the TEC-installing optical transmitting module. However, in such optical transmitting module, the optical sensitivity of the PD varies depending on the ambient temperature. As a result, the driving current of the LD fluctuates and the wavelength of the output light of the LD varies.  
       SUMMARY OF THE INVENTION  
       [0006]     The optical transmitter according to the present invention comprises an optical transmitting module that installs an LD and a PD, a temperature sensor that monitors the temperature of the PD, a memory for storing compensation parameters, a compensation circuit for compensating the output of the PD on the basis of the compensation parameters, and a controller for supplying current to the LD based on a comparison between the compensated output and the predetermined reference signal. The compensation parameters are stored in a look-up table in the memory against the temperature. The temperature of the PD is monitored by the temperature sensor, the compensation parameters are read from the memory based on the temperature thus monitored, and the output of the PD is compensated.  
         [0007]     Alternatively, the optical transmitter according to the present invention comprises an optical transmitting module that contains an LD and a PD, a temperature sensor that monitors the temperature of the PD, a reference generator that generates a reference level, a memory that stores compensation parameters, a reference signal generator for generating a reference signal, and a controller that adjusts the current supplied to the LD based on the difference between the output of the PD and the reference signal. Here, the reference signal generator reads compensation parameters from the memory based on the temperature of the PD monitored by the temperature sensor and sends a reference signal that is compensated by the compensation parameters to the controller.  
         [0008]     The LD shifts the wavelength of the emitted light toward longer wavelengths due to the self-heating when excess current is supplied. With the optical transmitter according to the present invention, the output of the PD is compensated by the compensation parameters stored in the memory and, accordingly, even in a high-temperature in which the optical sensitivity of the PD lowers, the true magnitude of the optical output from the LD can be determined, an excess current can be prevented from being supplied to the LD, and a red-shift of the wavelength of the emitted light can be prevented. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  shows the configuration of the optical transmitter according to the first embodiment of the present invention;  
         [0010]      FIG. 2  is a perspective and partially exploded view of the optical transmitting module according to the embodiment of the present invention;  
         [0011]      FIG. 3  is a graph showing the temperature dependence of the optical sensitivity of the PD;  
         [0012]      FIG. 4  shows the relationship of the temperature of each part in the optical link against the ambient temperature; and  
         [0013]      FIG. 5  shows the configuration of the optical transmitter according to a second embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0014]     In the following, preferred embodiments of the present invention will be described in detail with reference to the drawings hereinbelow. The same numerals are assigned to the same or equivalent parts in the respective drawings.  
       First Embodiment  
       [0015]      FIG. 1  is a block diagram of an optical transmitter according to a first embodiment of the present invention. The optical transmitter  10  shown in  FIG. 1  comprises an optical transmitting module  12 , a TEC driver  16 , an LD driver  18 , a controller  20 , a compensation circuit  22 , a reference generator  24 , a current monitor  26 , a temperature sensor  28 , and a look-up table (storage means)  30 .  
         [0016]      FIG. 2  is a perspective and partially exploded view of the optical transmitting module according to the embodiment of the present invention. As shown in  FIG. 2 , the optical transmitting module  12  comprises an LD  40 , a first carrier  42 , a thermistor  44 , a thermoelectric controller (‘TEC’ hereinbelow)  46 , a PD carrier  48 , a PD  50 , and a housing  52 . The optical transmitting module of  FIG. 2  comprises a coaxial-type housing comprising a stem  52   a  and a cap  52   b  as the housing  52 . The LD  40 , the PD  50 , and the TEC  46  are hermetically sealed in the space formed by the stem  52   a  and the cap  52   b.    
         [0017]     The LD  40  is mounted on the upper plate  46   a  of the TEC  46  via a plurality of carriers  42   a ,  42   b , and  42   c . The TEC  46  is comprised of an upper plate  46   a  that operates as a heat absorber, a lower plate  46   c  that operates as a radiating fin, and a plurality of Peltier elements  46   b  interposed between the upper and lower plates. In the case of the TEC  46  of this embodiment, the size of the lower plate  46   c  is larger than that of the upper plate  46   a  and the PD  50  is mounted on the lower plate  46   c  of the TEC  46  via the PD carrier  48 .  
         [0018]     The LD driver  18  comprises a bias current source  18   a , a modulator  18   b , and a modulation current source  18   c . The bias current source  18   a  supplies a bias current IBIAS to the LD  40 . The modulator  18   b  receives an RF signal that is input to the input terminal  18   d  and modulates the current switching. The modulation current source  18   c  supplies a modulation current IMOD to the LD  40  via the modulator  18   b.  The magnitude of the modulation current IMOD supplied by the modulation current source  18   c  is controlled by a control signal CTRL sent from the controller  20 . The controller  20  supplies the control signal CTRL to the modulation current source  18   c  based on the current output from the PD  50  in order to keep the optical output of the LD 40  constant. Hence, the controller  20  outputs a control signal that reflects the difference between the signal from the compensation circuit  22  and the predetermined reference level from the reference generator to the modulation current source  18   c.    
         [0019]     Here, the temperature dependence of the optical sensitivity of the PD  50  will be described.  FIG. 3  is a graph that shows the temperature dependence of the optical sensitivity of the PD.  FIG. 3  shows the temperature dependence of the optical sensitivity for various wavelengths. The PD used here is a PD in which the optical sensitivity for the light of a wavelength of 1550 nm (C-band) is substantially independent on the temperature. As shown in  FIG. 3 , for a PD with the optical sensitivity substantially independent on the temperature in the C-band, some temperature dependence of the optical sensitivity may occur for the light with a wavelength of 1625 nm (L band). Specifically, the optical sensitivity decreases at low temperatures.  
         [0020]     Therefore, in an optical transmitter that converts photocurrent from the PD into a monitored signal (in a voltage form) and inputs the monitored signal to a controller, the LD  40  is applied as one for the L band and, when the ambient temperature changes, specifically changes to a low temperature, a phenomenon that the optical output from the LD  40  becomes small is appeared due to the temperature characteristic of the PD  50  and an excess modulation current is supplied to the LD  40  in order to compensate the reduction of the optical output. To provide an excess current to the LD  40  causes the temperature increase of the LD  40  and, as a result, the wavelength of the output light from the LD 40  shifts. More specifically, the wavelength of the output light from the LD  40  shifts toward longer wavelengths.  
         [0021]     In order to compensate this phenomenon, the optical transmitter  10  according to the present embodiment is configured such that a compensated signal for compensating the monitored signal is output to the controller in accordance with the temperature of the PD  50 . Specifically, as shown in  FIG. 1 , the current monitor  26 , the temperature sensor  28 , and the look-up table (‘LUT’ hereinbelow)  30  are connected to the compensation circuit  22 .  
         [0022]     The current monitor  26  comprises a current-to-voltage converter and an analog-to-digital converter (A/D-C). In the current monitor  26 , the photocurrent output from the PD  50  is first converted to a voltage signal by the current-to-voltage converter. The voltage signal is output to the compensation circuit  22  via the A/D-C. The temperature sensor  28  monitors the temperature of the PD  50 . The temperature sensor  28  outputs a voltage signal corresponding to the temperature to the compensation circuit  22 . In this embodiment, the temperature sensor  28  can be mounted outside the optical transmitting module  12 .  
         [0023]      FIG. 4  shows the relationship between the temperature of the respective elements of the optical link in which the optical transmitter  10  is mounted and the ambient temperature outside the optical link. In  FIG. 4 , the left-hand vertical axis ‘link temperature’ denotes the temperature inside the optical link and outside the optical transmitting module  12  and the right-hand vertical axis ‘TOSA (PD) temperature’ denotes the temperature of the PD  50 . Further, the temperature of the PD  50  is the temperature of the stem  52   a  of the optical transmitting module  12 . The PD  50  is mounted on the stem  52   a  via the lower plate  46   c  and therefore the temperature of the stem  52   a  and the temperature of the PD  50  substantially match.  
         [0024]     As shown in  FIG. 4 , even when the ambient temperature outside the optical link varies, only a few degree centigrade in the temperature is appeared between an area that mounts the PD and other areas within the optical link, although about 5° C. increase is apparent in the temperature of the optical link outside the optical transmitting module, namely, within the optical link, with respect to the temperature of the PD due to the heat generation by electronic parts installed within the optical link. Such difference can be ignored after compensating the voltage signal based on the temperature characteristic of the PD in  FIG. 3 .  
         [0025]     Returning now to  FIG. 1 , the LUT  30  is comprised of a CPU and a memory and stores parameters against the temperature of the PD  50 . When setting a certain temperature as the reference temperature, assuming the optical sensitivity of the PD  50  at the reference temperature as ηS and assuming the optical sensitivity of the PD  50  at a temperature T other than this reference temperature as ηT, the parameters can be written as ηS/ηT. The compensation circuit  22  obtains the parameter ηS/ηT corresponding to the temperature T monitored by the temperature sensor  28  from the LUT  30  and generates a compensated signal by calculating the monitored signal I multiplexed by the parameter ηS/ηT. The compensated signal is output to the controller  20  and the difference between the compensated signal and reference level is fed back to the modulation current source  18   c  by the controller  20 .  
         [0026]     With the optical transmitter  10 , the monitored signal is compensated based on the temperature of the PD  50 . Hence, an excess modulation current is not supplied to the LD  40  (caused by the temperature dependence of the optical sensitivity of the PD  50 .) As a result, the wavelength shifts of the LD  40  are reduced. In addition, because the temperature sensor  28  can be provided outside the optical transmitting module  12 , which results on a freely selection of the temperature sensor  28 .  
       Second Embodiment  
       [0027]     The optical transmitter according to the second embodiment of the present invention will be described hereinbelow.  FIG. 5  is a block diagram of the optical transmitter of the second embodiment of the present invention. Although the monitored signal has been compensated by the optical transmitter  10  according to the first embodiment, the reference level that is output by a reference generator  24 B is compensated by the optical transmitter  10 B shown in  FIG. 5  based on the temperature of the PD  50 . The points by which the optical transmitter  10 B differs from the optical transmitter  10  will be described hereinbelow.  
         [0028]     As shown in  FIG. 5 , in the optical transmitter  10 B, the current monitor  26  is directly connected to the controller  20  and the controller  20  directly obtains the signal IMON. Furthermore, the temperature sensor  28  and LUT  30 B are connected to the reference generator  24 B. The LUT  30 B stores parameters that correspond to the temperature of the PD  50  as described previously. Assuming that the optical sensitivity of the PD  50  at the reference temperature is ηS and the optical sensitivity of the PD  50  at a certain temperature T is ηT, the parameter is then ηT/ηS.  
         [0029]     The reference generator  24 B obtains the parameter ηT/ηS that corresponds to the temperature T monitored by the temperature sensor  28  from the LUT  30 B, calculates the predetermined reference level V multiplexed by the parameter ηS/ηT, and determines the corrected reference. The controller  20  feeds back the difference between the corrected reference and the signal I to the modulation current source  18   c . Thus, even when the reference level can be varied depending on the temperature of the PD  50 , the wavelength shifts of the LD  40  due to the temperature dependence of the optical sensitivity of the PD  50  can be reduced.  
         [0030]     The concepts of the present invention has been illustrated and described as referring to the preferred embodiments. However, the present invention is not limited to the specified configurations of the embodiments. For example, the compensating parameter in the first embodiment may be ηT/ηS and, in this case, the control signal is generated by dividing the signal IMON and the parameter ηT/ηS. While, the compensating parameter in the second embodiment may be ηS/ηT and, in this case, the corrected reference is determined by a predetermined reference level V divided by the parameter ηS/ηT. Moreover, although only the modulation current was controlled in the embodiments above, a bias current can also be controlled in addition to the modulation current.