Designers can use a photodiode in conjunction with a power-control feedback loop to compensate a laser driver circuit for temperature effects and the effects of the laser's aging. But a photodiode's response can vary by as much as 40%, so the system needs additional compensation. That can be achieved with a digital resistor that varies the power set-point in response to temperature.
For example, the MAX3740 VCSEL laser driver and DS1859 dual temperature-controlled digital resistor are popular choices for SFP and SFF fiber-optic systems, due to their small size and high integration level. A monitoring photodiode and the driver's on-board power-control amplifier create an automatic power-control (APC) loop.
A resistor between the MAX3740 reference pin (REF) and the power-monitor photodiode pin (MD) sets the photodiode current. The APC loop then drives the laser diode to the intensity that delivers that current. However, the problem with this approach is that the control voltage is too low. The nominal voltage at MD is 1.6 V and the nominal voltage at REF is 1.8 V, leaving only 0.2 V across the resistor to set the photodiode current.
Digital resistors such as those in the DS1859 can have minimum resistances as high as 1 kΩ , which results in a maximum current of only 200 µA. The current-versus-resistance function is very nonlinear, with poor resolution at high currents. You can add a fixed resistor between REF and MD to raise the maximum current, but the adjustment range is still only 200 µA. Nor does the fixed resistor improve the nonlinearity and resolution. A graph of photodiode current versus the DS1859 resistance range in a circuit with an 806-Ω series resistor shows the response biased up by 248 µA (Fig.1, lower trace).
The solution to these problems is to allow the resistor between REF and MD (R1) to set the maximum photodiode current (Fig. 2). Then, subtract a current proportional to the DS1859 resistance. The subtracted current comes from the op-amp output, which "steals" current from the photodiode through R2. The op amp shown was chosen for its small size (SC70 package) and low cost. It runs off the same +3.3-V power supply as the digital resistor and laser driver.
The op amp generates a voltage (VO) proportional to the value of MD (REF MD) and DS1859's value. That voltage generates a current through R2 proportional to the difference between the voltages at VO and MD. Because the effects at MD cancel out, the current through R2 depends only on (REF MD), a stable 0.2 V, and the DS1859 value.
Current through the photodiode equals the current through R1 (803 µA) minus the current through R2. Thus, the photodiode current is a linear function of the potentiometer value (Fig. 1, again). With appropriate resistor values, this circuit works with any value potentiometer and provides current over any range. Its only limitation is the current-drive capability of the op amp. The design calculations for this circuit are available here.
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