For the reverse situation, the analysis is similar. The optical output power level has decreased. So Q1 is turning on, more current flows through the laser. The amplifier A2, comparator, R5, R9, and D1 form an alarm circuit. The comparator is adjusted to turn on LED D1 when the current through the laser reaches a critical value.
Lasers are very sensitive to being overdriven.
Occasionally, the laser will appear to be normal but after the transmitter is turned off and allowed to cool, the laser will produce very low power when turned on again. Laser analog transmitter block diagram with temperature control and alarm circuits. To prevent laser degradation due to temperature and to maintain a stable operation point for bias current, a temperature-control circuit is added as we have mentioned previous. The thermistor for sensing temperature is in the bridge circuit as shown in Figure 3. As the temperature of the thermistor changes, amplifier At will amplify the difference, which will drive the power amplifier.