Patent Publication Number: US-2007121683-A1

Title: Direct control of extinction ratio and optical modulation amplitude for fiber transmitters

Description:
BACKGROUND OF THE INVENTION  
       FIG. 1  shows a typical optical coupling portion of a optical coupling portion of a fiber optic transmitter  100  of the prior art. Light from a laser  101  of the optical coupling portion of a fiber optic transmitter  100  is coupled into a fiber  103 . A monitor PIN photodiode  105  detects light power generated by the laser  101  to provide feedback to the laser  101  in order to keep the light power coupled into the fiber  103  constant over the lifetime of the device and over temperature.  
      In the case of an FP/DFB laser, the monitor PIN photodiode  105  usually detects the light  104  emitted at the back facet of the laser as shown in  FIG. 1 .  
      Alternatively, in the case of a VCSEL, a beam splitter (not shown) is placed in the path of the laser beam, thus redirecting a fraction of light  106  to the monitor PIN photodiode  105 .  
      The monitor PIN photodiode  105  generates a monitor PIN photodiode current  107  which is directly related to the light emitted by the laser and the amount of optical power launched into the fiber  103 . The monitor PIN photodiode current  107  flows to an average light power controller  109  which tunes a bias current  111  driving the laser  101 . In this way the bias current  111  driving the laser  101  is tuned using feedback provided by the monitor PIN photodiode current  107 , thereby resulting in a monitor PIN photodiode current  107  that is stable over time and temperature. The average light power controller  109  includes a laser driver IC  113  which controls the bias current  111 .  
      The targeted PIN photodiode current is usually set at the production stage. If the monitor PIN photodiode current  107  drops below this level during use, the average light power controller  109  increases the bias current  111 . On the other hand, if the monitor PIN photodiode current  107  rises above this level during use, the average light power controller  109  decreases the bias current  111 .  
      This control loop can either be fully analog, digital or a hybrid of both.  
       FIG. 2  shows a portion of another typical optical coupling portion of a fiber optic transmitter  200  of the prior art. In this transmitter  200 , a controller  209 , in addition to tuning a bias current  211 , also tunes a modulation current  213  to keep the Optical Modulation Amplitude (OMA) and/or the Extinction Ratio (ER) constant over time and temperature. A temperature sensor  215  can also be inserted in the control loop to provide temperature feedback. The temperature control is usually complex because of the interdependence of the bias and modulation currents  211 ,  213  introduced by a laser driver IC  214 . The temperature behavior of a device changes from part to part and is usually measured at the production stage by operating the module at various temperatures. The results of these measurements are then used to set the parameters of the control loop, either by placement of analog components, by setting digital parameters, or a combination of both.  
      The Optical Modulation Amplitude (OMA) and Extinction Ratio (ER) of a signal are important parameters that are used in specifying the performance of optical links used in digital communication systems. The OMA directly influences the system bit error ratio (BER). With an appropriate point of reference (such as average power), OMA can be directly related to ER.  
      For bi-level optical signaling schemes, such as nonreturn-to-zero (NRZ), only two discrete optical power levels are used. The higher level represents a binary one, and the lower level represents a zero. The symbol P 1  represents the high power level and the symbol P 0  represents the low power level. Using these symbols a number of useful terms and relationships can be mathematically defined.  
      OMA is defined as the difference between the high and low levels, which can be written mathematically as: 
 
 OMA=P 1− P 0. 
 
      Average power is simply the average of the two power levels, i.e., 
 
 Pav =( P 1+ P 0)/2. 
 
      ER represents the extinction ratio, which is the ratio between the high and low power levels, and is given by: 
 
 ER=P 1 /P 0. 
 
      Through algebraic manipulation it can be shown that the OMA, Pav and Re are related by the equation: 
 
 OMA= 2 Pav ( ER− 1)/( ER+ 1). 
 
      The methodology of the prior art controls the ER/OMA indirectly using the temperature, which requires an additional tolerance margin to be put in place. For this algorithm to work, the effect of laser aging on the slope efficiency has to be estimated and “programmed” into the control loop.  
      It would be desirable to allow the ER/OMA to be measured directly, thereby improving yield and reducing test and laser programming time.  
     SUMMARY OF THE INVENTION  
      A fiber optic transmitter and the method of using the fiber optic transmitter includes a laser supplied by an input current and which produces a light beam coupled into a fiber. A photodiode detects the waveform of the light beam. A processor performs the steps of sampling the waveform and detecting peaks and valleys of the detected light beam waveform. The processor also tunes the input current based on the relative values of the detected peaks and valleys of the detected light beam waveform. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  shows an optical coupling portion of a optical coupling portion of a fiber optic transmitter of the prior art which includes tuning of the laser bias current.  
       FIG. 2  shows an optical coupling portion of a fiber optic transmitter of the prior art which includes tuning of both the laser bias and modulation currents.  
       FIG. 3  shows one embodiment of an optical coupling portion of a fiber optic transmitter of the present invention using random sampling.  
       FIG. 4  shows another embodiment of an optical coupling portion of a fiber optic transmitter of the present invention using sample and hold.  
       FIG. 5  shows another embodiment of an optical coupling portion of a fiber optic transmitter of the present invention including a temperature sensor. 
    
    
     DETAILED DESCRIPTION  
      The present invention allows the ER and OMA of the fiber transmitter to be measured directly, thereby improving yield and reducing test and laser programming time.  FIG. 3  shows one embodiment of an optical coupling portion of a fiber optic transmitter  300  for controlling ER and OMA. The laser  101  emits light to the fiber  103  and to a high-speed PIN photodiode  301  in the monitor path. Here “high-speed” means that the PIN photodiode is fast enough to follow the modulated light signal. This is to be compared to the low speed PIN photodiodes  105  used in the monitor path of the prior art fiber optic transmitters which average the modulated optical signal. The signal from the high-speed PIN photodiode  301  is sent to a pre-processor  303 . In one embodiment, the pre-processor  303  scales/converts the monitor current from the PIN photodiode  301  into a signal that can be digitized by a digital processor  305 . In the particular example of  FIG. 3 , the high-speed PIN photodiode  301  emits a monitor current  302 , which is then scaled and converted by the pre-processor  303  into the signal  304 .  
      The digital processor  305  samples the incoming signal  304  randomly over a time interval using an ADC (Analog-to-Digital Converter)  307 . The sampling interval used is determined by the required accuracy of the ER and OMA, the average sampling interval, and the speed of the modulated signal. The sampling interval is long enough to guarantee that at least one peak and valley of the modulated signal is captured. The digitized points captured from the signal  304  are shown in plot  308 . Next, the digital processor  305  uses a peak and valley detector  309  to determine the peaks and valleys from among the digitized points of the plot  308 .  
      A calculation section  311  of the digital processor  305  then calculates the ER, OMA and/or Pav from the peaks and valleys. Taking the ratio of the peak and valley values gives the ER. Taking the difference between the peak and valley values gives the OMA. Taking the sum of the peak and valley values and dividing two gives the average level of power (Pav).  
      A current tuning section  313  tunes the bias current Ibias  211  and the modulation current Imod  213  based on the values calculated by the calculation section  311  to thereby drive the laser  101  and keep the Optical Modulation Amplitude (OMA) and/or the Extinction Ratio (ER) substantially constant over time and temperature.  
      Another embodiment of the present invention is illustrated in  FIG. 4 . In the illustrated portion of a optical coupling portion of a fiber optic transmitter  400 , the preprocessor  403  uses analog circuitry to sample and hold values of the signal peaks and valleys. These values are then sent to the digital processor  305  and processed. The sub-sections of the digital processor  305  are similar to those illustrated in the embodiment of  FIG. 3 . This embodiment has the advantage that no random sampling over the timer interval is required, thus speeding up the control algorithm. For this reason, the calculation of new lbias and Imod values can be reduced to simple increment or decrement steps depending on the incoming signal values.  
      A optical coupling portion of a fiber optic transmitter  500  illustrated in  FIG. 5  shows another embodiment of the optical coupling system including a temperature sensitive device or temperature sensor  501  that feeds a signal to the control logic  503  and to the digital processor  305 , thereby allowing the temperature sensitivity of the optical coupling system to be directly compensated for as well. This feedback facilitates the use of plastic optical components, which exhibit a strong temperature dependent behavior as compared to conventional glass optics. The temperature  501  measures the ambient temperature.  
      The present invention counters any disturbing effects on the average launched power, ER and OMA introduced by temperature shifts of the laser or the optical coupling system. The changes in the laser temperature/optical elements will usually be slow (&lt;1/300 Hz) once the product has warmed up.  
      Although specific embodiments of the invention have been described and illustrated, the invention is not to be limited to the specific forms or arrangements of parts so described and illustrated. The scope of the invention is to be defined by the claims appended hereto and their equivalents.