Patent Application
20070121683
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
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.
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 P1 represents the high power level and the symbol P0 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=P1−P0.
Average power is simply the average of the two power levels, i.e.,
Pav=(P1+P0)/2.
ER represents the extinction ratio, which is the ratio between the high and low power levels, and is given by:
ER=P1/P0.
Through algebraic manipulation it can be shown that the OMA, Pav and Re are related by the equation:
OMA=2Pav(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.
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.
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.
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
A optical coupling portion of a fiber optic transmitter 500 illustrated in
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 (<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.
